Patent Application
20070193665
The present invention relates to steel plate excellent in machineability and in toughness and weldability, in particular relates to steel plate having a plate thickness of 4 to 100 mm or so and a tensile strength of 570 to 720 MPa or so and a method of production of the same. The steel plate produced by this method of production can be used for ships, bridges, buildings, marine structures, pressure vessels, line pipe, and other welded structures in general, in particular is effective in use if fields requiring drilling, surface machining, and other machining work when fabricating a structure.
As steel plate used for a welded structure, high strength and also, as weldability, the suppression of weld fractures and a high weld heat affected zone toughness are usually required. In a steel material with a tensile strength of 570 MPa or more, it has been possible to achieve both high strength and weldability by keeping the amount of addition of alloy elements to a minimum and converting the main structure forming the steel bainite or martensite. However, when fabricating a building, bridge, ship, or other structure, drilling, surface machining, and other machining processes are involved. When bainite or martensite forms the main structure, the productivity of the work falls due to the increased frequency of replacement or resharpening of tools along with tool life, the drop in the machining speed due to the increase in machining resistance, etc. and, as a result, the structure increases in fabrication costs. For example, Japanese Unexamined Patent Publication No. 9-310117 tries to achieve both a high strength and weldability with a relatively low alloy elements by making the structure mainly bainite. However, since the steel has a structure mainly comprised of hard bainite, the machineability is poor and the cost required for machining work is high.
In a welded structure, the steel plate used for the main parts, for example, the webs and flanges, are welded and are subject to relatively large stress at the time of use, so require excellent weldability and toughness. On the other hand, there are parts which are not welded and parts where a high toughness is not required. For example, the tie plates etc. used when bolting main structures of bridges desirably have a toughness of a performance of an extent satisfying the standards, but do not have to have a level of the same extent as the main structures. If using steel plate having bainite or martensite as the main structures for these parts, since the machineability is poor, the machining work takes time so the structure greatly increases in fabrication costs.
To improve the machineability, in particular to extend the tool life and reduce the machining resistance, it is known that addition of S is effective. However, when adding a large amount of S, the matrix toughness falls and the weldability falls. As opposed to this, a technique for achieving both an improvement of the machineability by the addition of S and securing of the weldability is disclosed in Japanese Unexamined Patent Publication No. 6-184695. However, the weldability secured here only eliminates preheating and suppresses weld fractures. The welded zone toughness and the matrix toughness are low. Therefore, the use of such steel for welded structures is not possible. Further, a technique for achieving both machineability and matrix toughness is disclosed in Japanese Unexamined Patent Publication No. 2000-87179. By controlling the form of the sulfides by addition of Ca and Mg, the anisotropy of the matrix toughness is improved, but the absolute value is low and further the weldability is poor, so the use of such steel for a welded structure is not possible.
The machineability also depends on the configuration of the microstructure. It is known that rather than a structure mainly comprised of bainite or martensite, a ferrite and pearlite or a ferrite and bainite structure is excellent. For example, Japanese Unexamined Patent Publication No. 7-54100, Japanese Unexamined Patent Publication No. 7-109518, and Japanese Unexamined Patent Publication No. 7-166235 disclose steels with ferrite and bainite structures. Further, Japanese Unexamined Patent Publication No. 2000-63988, Japanese Unexamined Patent Publication No. 2000-63989, Japanese Unexamined Patent Publication No. 2000-282172, and Japanese Unexamined Patent Publication No. 2001-214241 disclose steels with prescribed fractions of ferrite. Steel plate with microstructures of ferrite and bainite and steel plates securing certain ferrite fractions are qualitatively excellent in machineability to steel mainly comprised of bainite or martensite, but the absolute margin of improvement cannot be said to be sufficient enough to improve the productivity in drilling or surface machining in the process of fabrication of a welded structure. Further, said art all call for large amounts of addition of alloy elements, so the toughness and weldability are low and the use of such steel for welded structures is not possible. From the above, it is not possible with the current art to produce steel plate having a 570 MPa or higher tensile strength and a high toughness, weldability, and machineability.
The present invention provides steel plate excellent in machineability and in toughness and weldability or extremely excellent in machineability and having a toughness of an extent enabling application to welded structures, having a plate thickness of 4 to 100 mm or so, and having a tensile strength of a level of 570 to 720 MPa or so and a method of production of the same. It has as its gist the following:
(1) Steel plate excellent in machineability and in toughness and weldability characterized in that the steel comprises, by mass %, a steel composition comprising:
(2) Steel plate excellent in machineability and in toughness and weldability, characterized in that the steel comprises, by mass %, a steel composition as set forth in (1) wherein:
X1 expressed by X1=C+(Mn+Cu+Cr)/20+Si/30+Ni/60+Mo/15+V/10+5×B is 0.24 or less,
(3) Steel plate excellent in machineability and in toughness and weldability, characterized in that the steel comprises, by mass %, a steel composition as set forth in (1) wherein:
(4) Steel plate excellent in machineability and in toughness and weldability as set forth in any of (1) to (3), characterized in that said steel further comprises, by mass %, one or more of:
(5) A method of production of steel plate excellent in machineability and in toughness and weldability characterized by heating a steel slab or cast slab having a steel composition as described in (1) and having an X1 expressed by X1=C+(Mn+Cu+Cr)/20+Si/30+Ni/60+Mo/15+V /10+5×B of 0.24 or less, then rough rolling it by a total reduction rate of 30% to 95%, then finish rolling it by a first pass bite temperature of a T1(° C.) expressed by T1=35 ln(X2/2)−25√t+1070, X2=(Si/5+Mo+Cr/2)/Mn, to 720° C. and a total reduction rate of 30% to 95%, starting water cooling after the end of the rolling at a flow rate of 0.2 m3/m2·min to 5.0 m3/m2·min, and ending the water cooling at 600° C. or less, where t is the plate thickness.
(6) A method of production of steel plate excellent in machineability and in toughness and weldability as set forth in (5), characterized in that in the water cooling started after the end of the rolling, an average cooling rate from a water cooling start temperature to over 650° C. is 1° C./s to 5° C./s and an average cooling rate from 650° C. to a water cooling stop temperature is 10° C./s to 100° C./s.
(7) A method of production of steel plate excellent in machineability and in toughness and weldability characterized by heating a steel slab or cast slab having a steel composition as described in (1) and having an X1 expressed by X1=C+(Mn+Cu+Cr)/20+Si/30+Ni/60+Mo/15+V/10+5×B of 0.24 or less, then rough rolling it by a total reduction rate of 30% to 95%, finish rolling it by a total reduction rate of 30% to 95%, then air cooling it, starting water cooling when the steel plate surface temperature is T2(° C.) expressed by T2=910−310×C−80×Mn−20×Cu−15×Cr−55×Ni−80×Mo+0.0006t2−0.56t−8.6 to 650° C. by a flow rate of 0=2 m3/m2·min to 5.0 m3/m2·min, and ending the water cooling at 500° C. or less, where t is the plate thickness.
(8) A method of production of steel plate excellent in machineability and in toughness and weldability characterized by heating a steel slab or cast slab having a steel composition as described in (1) and having an X1 expressed by X1=C+(Mn+Cu+Cr)/20+Si/30+Ni/60+Mo/15+V/10+5×B of 0.24 or less, then rough rolling it by a total reduction rate of 30% to 95%, finish rolling it by a total reduction rate of 30% to 95%, air cooling to 500° C. or less, reheating the steel plate to a T3(° C.) expressed by T3=0.0017t2+0.17t+730 to 850° C., then starting the water cooling, and ending the water cooling at 500° C. or less, where t is the plate thickness.
(9) A method of production of steel plate excellent in machineability and in toughness and weldability characterized by heating a steel slab or cast slab having a steel composition as described in (2) and having an X1 expressed by X1=C+(Mn+Cu+Cr)/20+Si/30+Ni/60+Mo/15+V/10+5×B of 0.24 or less and an X2 expressed by X2=(Si/5+Mo+Cr/2)/Mn of 0.15 to 10.0, then rough rolling it by a total reduction rate of 30% to 95%, then finish rolling it by a first pass bite temperature of a T4(° C.) expressed by T4=35 ln(X2/2)-251t+1100 to an Ar3 point by a total reduction rate of 30% to 95%, then speedily starting water cooling after the end of the rolling at a flow rate of 0.2 m3/m2·min to 5.0 m3/m2·min, and ending the water cooling at 600° C. or less, where t is the plate thickness.
(10) A method of production of steel plate excellent in machineability and in toughness and weldability as set forth in (9), characterized in that in the water cooling started after the end of the rolling, an average cooling rate from a water cooling start temperature to over 650° C. is 1° C./s to 5° C./s and an average cooling rate from 650° C. to a water cooling stop temperature is 10° C./s to 100° C./s.
(11) A method of production of steel plate excellent in machineability and in toughness and weldability characterized by heating a steel slab or cast slab having a steel composition as described in (2) and having an X1 expressed by X1=C+(Mn+Cu+Cr)/20+Si/30+Ni/60+Mo/15+V/10+5×B of 0.24 or less and an X2 expressed by X2=(Si/5+Mo+Cr/2)/Mn of 0.15 to 10.0, rolling it, cooling it to 500° C. or less, reheating the steel plate to 900° C. to 1050° C., water cooling it by an average cooling rate of 1° C./s to 100° C./s, and ending the water cooling at 500° C. or less.
(12) A method of production of steel plate excellent in machineability and in toughness and weldability characterized by heating a steel slab or cast slab having a steel composition as described in (2) and having an X1 expressed by X1=C+(Mn+Cu+Cr)/20+Si/30+Ni/60+Mo/15+V/10+5×B of 0.24 or less and an X2 expressed by X2=(Si/5+Mo+Cr/2)/Mn of 0.15 to 10.0, rough rolling it by a total reduction rate of 30% to 95%, finish rolling it by a total reduction rate of 30% to 95%, then air cooling it to an Ar3 point to a temperature lower than the Ar3 point by 150° C., starting water cooling at a flow rate of 0.2 m3/m2·min to 5.0 m3/m2·min, and ending the water cooling at 500° C. or less.
(13) A method of production of steel plate excellent in machineability and in toughness and weldability characterized by heating a steel slab or cast slab having a steel composition as described in (2) and having an X1 expressed by X1=C+(Mn+Cu+Cr)/20+Si/30+Ni/60+Mo/15+V/10+5×B of 0.24 or Less and an X2 expressed by X2=(Si/5+Mo+Cr/2)/Mn of 0.15 to 10.0, then rough rolling it by a total reduction rate of 30% to 95%, finish rolling it by a total reduction rate of 30% to 95%, then air cooling it to 500° C. or less, reheating the steel plate to 730° C. to less than 900° C., then water cooling it, and ending the water cooling to 500° C. or less.
(14) A method of production of steel plate excellent in machineability and in toughness and weldability characterized by heating a steel slab or cast slab having a steel composition as described in (3) and having an X1 expressed by X1=C+(Mn+Cu+Cr)/20+Si/30+Ni/60+Mo/15+V/10+5×B of 0.24 or less and an X2 expressed by X2=(Si/5+Mo+Cr/2)/Mn of 0.15 to 10.0, then rough rolling it by a total reduction rate of 30% to 95%, then finish rolling it by a first pass bite temperature of a T4(° C.) expressed by T4=35 ln(X2/2)−25√t+1100 to an Ar3 point by a total reduction rate of 30% to 95%, then speedily starting water cooling at a flow rate of 0.2 m3/m2·min to 5.0 m3/m2·min, and ending the water cooling at 600° C. or less, where t is the plate thickness.
(15) A method of production of steel plate excellent in machineability and in toughness and weldability as set forth in (14), characterized in that in the water cooling started after the end of the rolling, an average cooling rate from a water cooling start temperature to over 650° C. is 1° C./s to 5° C./s and an average cooling rate from 650° C. to a water cooling stop temperature is 10° C./s to 100° C./s.
(16) A method of production of steel plate excellent in machineability and in toughness and weldability characterized by heating a steel slab or cast slab having a steel composition as described in (3) and having an X1 expressed by X1=C+(Mn+Cu+Cr)/20+Si/30+Ni/60+Mo/15+V/10+5×B of 0.24 or less and an X2 expressed by X2=(Si/5+Mo+Cr/2)/Mn of 0.15 to 10.0, rolling it, cooling it to 500° C. or less, reheating the steel plate to 900° C. to 1050° C., water cooling it by an average cooling rate of 1° C./s to 100° C./s, and ending the water cooling at 500° C. or less.
(17) A method of production of steel plate excellent in machineability and in toughness and weldability characterized by heating a steel slab or cast slab having a steel composition as described in (3) and having an X1 expressed by X1=C+(Mn+Cu+Cr)/20+Si/30+Ni/60+Mo/15+V/10+5×B of 0.24 or less and an X2 expressed by X2=(Si/5+Mo+Cr/2)/Mn of 0.15 to 10.0, rough rolling it by a total reduction rate of 30% to 95%, finish rolling it by a total reduction rate of 30% to 95%, then air cooling it to an Ar3 point to a temperature lower than the Ar3 point by 150° C., starting water cooling at a flow rate of 0.2 m3/m2·min to 5.0 m3/m2·min, and ending the water cooling at 500° C. or less.
(18) A method of production of steel plate excellent in machineability and in toughness and weldability characterized by heating a steel slab or cast slab having a steel composition as described in (3) and having an X1 expressed by X1=C+(Mn+Cu+Cr)/20+Si/30+Ni/60+Mo/15+V/10+5×B of 0.24 or less and an X2 expressed by X2=(Si/5+Mo+Cr/2)/Mn of 0.15 to 10.0, rough rolling it by a total reduction rate of 30% to 95%, finish rolling it by a total reduction rate of 30% to 95%, then cooling it to 500° C. or less, reheating the steel plate to 730° C. to 900° C., water cooling it, and ending the water cooling at 500° C. or less.
(19) A method of production of steel plate excellent in machineability and in toughness and weldability as set forth in any one of (5) to (18) characterized in that said steel slab or cast slab further contains, by mass %, one or more of:
According to the present invention, by making the structure forming the steel a composite structure of soft ferrite and hard bainite and martensite and [1] strictly defining the ferrite fraction of the front and rear surfaces of the steel plate having a particularly great effect on tool wear, [2] adjusting the balance of steel ingredients so as to enable a great reduction of the machining resistance occurring at the time of machining at a high temperature, or [3] further raising the amount of addition of S in a range not causing a drop in toughness, it becomes possible to provide steel plate provided with a machineability of a high level not achievable with steel plate for welded structures in the past, excellent in strength, toughness, and weldability, and having a tensile strength of 570 to 720 MPa or so and a method of production of the same. The invention has high industrial value.
The present invention will be explained in detail below.
The inventors engaged in intensive studies on a method of production of steel plate described in (1) of the present invention having a plate thickness of 4 to 100 mm or so, a matrix strength of 570 to 720 MPa or so, and excellent in matrix toughness, weldability, and machineability. As a result, they discovered that by making a composite structure of a soft structure mainly comprised of ferrite and a hard structure mainly comprised of bainite and martensite the main structure of the steel, strictly defining the ferrite fraction of the front and rear surfaces of the steel plate having a large effect on tool wear since it corresponds to the start and end point of the machining work, strictly defining the method of production requiring water cooling, etc., the machineability is greatly improved while securing the strength and matrix toughness and weldability.
Note that the “weldability” referred to in present invention indicates both weld fractures and weld heat affected zone toughness. The more difficult the occurrence of weld fractures or the higher the weld heat affected zone toughness, the better the weldability. On the other hand, the “machineability” indicates the tool life, machining resistance, and chip control. The longer the tool life, the lower the machining resistance, and the easier the chip control, the better the machineability.
The most important thing in the steel plate described in (1) of the present invention is the formation of a large amount of ferrite near the surface of the steel plate. In the machining process, the surfaces of the steel plate correspond to the start and end points. A large load is applied to the tool, so the machining in these regions has an extremely large effect on the tool life or machining resistance and chip control at the subsequent machining. That is, by making the structure at the top and rear surfaces of the steel plate a composite structure of a soft structure and hard structure, the soft part easily deforms at the time of starting and time of ending the machining of the workpiece by the tool, while stress concentrates near the interface between the soft part and hard part. As a result, the machining is started and ended by extremely small plastic deformation. Due to this, the tool life becomes longer, the machining resistance falls, and chip control becomes easy. The inventors evaluated the machineability for various steel plate with different distributions of ferrite in the plate thickness direction and discovered that it is desirable to define as values representing the distribution of ferrite in the plate thickness direction, in the case of steel plate having a plate thickness of 4 mm to less than 10 mm, the three locations of the locations exactly ¼, ½, and ¾ of the plate thickness inside the steel plate from the top surface of the steel plate (hereinafter called the “t/4 part”, “t/2 part”, and “3t/4 part”) and, in the case of steel plate having a plate thickness of 10 mm to 100 mm, the five locations of the locations of the above three locations plus the locations exactly 2 mm inside the steel plate from the top surface and rear surface of the steel plate surface (hereinafter called the “top surface 2 mm part” and “rear surface 2 mm part”).
In the case of steel plate having a plate thickness of 4 mm to less than 10 mm, the inventors discovered that when the t/4 part and 3t/4 part have a ferrite fraction of 30% or more and the t/2 part has a ferrite fraction of 20% or more, the machineability becomes good. On the other hand, when even one of the t/4 part, t/2 part, and 3t/4 part has a ferrite fraction over 90%, the strength greatly falls. From this, the inventors defined the ferrite fraction for the t/4 part and 3t/4 part as 30% to 90% and the ferrite fraction for the t/2 part as 20% to 90% for steel plate having a plate thickness of 4 mm to less than 10 mm. Note that when the t/4 part, t/2 part, and 3t/4 part have ferrite fractions of 50% or more, the machineability is greatly improved, so preferably the t/4 part, t/2 part, and 3t/4 part have ferrite fractions defined as 50% to 90%.
In the case of steel plate having a plate thickness of 10 mm to less than 100 mm, when the top surface 2 mm part and the rear surface 2 mm part have ferrite fractions of 30% or more and the t/4 part, t/2 part, and 3t/4 part have ferrite fractions of 20% or more, the machineability becomes good. On the other hand, if even one of the top surface 2 mm part, rear surface 2 mm part, t/4 part, t/2 part, or 3t/4 part has a ferrite fraction over 90%, the strength greatly falls. From this, the inventors defined the ferrite fraction of the top surface 2 mm part and rear surface 2 mm part as 30% to 90% or less and the ferrite fraction of the t/4 part, t/2 part, and 3t/4 part as 20% to 90% for steel plate having a plate thickness of 10 mm to less than 100 mm. Note that when the top surface 2 mm part, rear surface 2 mm part, t/4 part, t/2 part, and 3t/4 part have a ferrite fraction of 50% or more, the machineability is greatly improved, so preferably the top surface 2 mm part, rear surface 2 mm part, t/4 part, t/2 part, and 3t/4 part have a ferrite fraction of 50% to 90%.
Here, the method of measurement of the ferrite fraction is defined. The measurement is conducted for the plane parallel to both the rolling direction and plate thickness direction (hereinafter called the “L plane”). Avoiding the ends of the steel plate in the width direction and the spans of locations inside the ends in the width directions by exactly the lengths corresponding to the plate thickness, test pieces of the entire thicknesses were taken from locations near the center part of the steel plate in the width direction as much as possible, polished at the L planes, and etched by Nital. After this, the L planes were observed under an optical microscope. A magnification of 500× is preferable. Observation was performed using an eyepiece with a net pattern. The number of lattice points corresponding to ferrite were counted. The fraction (percentage) of lattice points corresponding to ferrite in all of the Lattice points is defined as the ferrite fraction. The measurement was conducted for a minimum of 10 fields for each location. The amount of displacement from one field to the next field was held constant. Here, when a location was difficult to judge as being ferrite or another phase, it was counted as 0.5. Note that regarding the criteria for judgment of ferrite at the time of measurement, the “ferrite” referred to in the present invention generally indicates ferrite called bulk ferrite, polygonal ferrite, equiaxial ferrite, etc. and does not include acicular ferrite formed at a lower temperature. However, even with bulk ferrite, depending on the control of the austenite before transformation, sometimes anisotropy occurs in the growth direction and bulk ferrite having a form long in the rolling direction is produced. This is included in the ferrite in the present invention.
Further, to obtain an excellent weldability and toughness, the amounts of addition of the alloy elements have to be adjusted. When X1 expressed by X1=C+(Mn+Cu+Cr)/20+Si/30+Ni/60+Mo/15+V/10+5×B is 0.24 or less, the weld fractures can be greatly reduced. Not only this, the toughness and weld heat affected zone toughness also become excellent. Therefore, X1 is defined as 0.24 or less. Note that when X1 is 0.21 or less, this effect appears more remarkably, so preferably X1 is made 0.21 or less. Note that the C, Mn, Cu, Cr, Si, Ni, Mo, V, and B when calculating the X1 are all amounts of addition expressed by mass %.
Further, the inventors engaged in intensive studies on the method of production of steel plate described in (2) of the present invention having a plate thickness of 4 to 100 mm or so, a matrix tensile strength of 570 to 720 MPa or so, and excellent in all of machineability, matrix toughness, and weldability. As a result, they discovered that by making a composite structure of a soft structure mainly comprised of ferrite and a hard structure mainly comprised of bainite and martensite the main structure of the steel, by strictly defining the balance of the amounts of addition of Si, Cr, Mo, and Mn in the steel ingredients, by strictly defining the method of production mainly comprised of temperature control in a method of production requiring water cooling, etc., the machineability is improved while securing the strength, matrix toughness, and weldability.
Further, the inventors engaged in intensive studies on the method of production of steel plate described in (3) having a plate thickness of 4 to 100 mm or so and a matrix strength of 570 to 720 MPa or so, extremely excellent in machineability, having a toughness of an extent enabling use for welded structures. As a result, they discovered that by making a composite structure of a soft structure mainly comprised of ferrite and a hard structure mainly comprised of bainite and martensite the main structure of the steel, strictly defining the balance of the amounts of addition of Si, Cr, Mo, and Mn among the steel ingredients, increasing the amount of addition of S in a range not greatly reducing the toughness, strictly defining the method of production mainly comprised of temperature control in a method of production requiring water cooling, etc., the machineability is greatly improved while securing the strength and matrix toughness.
Note that the “weldability” referred to in the present invention indicates both weld fractures and weld heat affected zone toughness. The more difficult the occurrence of weld fractures or the higher the weld heat affected zone toughness; the better the weldability. On the other hand, the “machineability” indicates the tool life, machining resistance, and chip control. The longer the tool life, the lower the machining resistance, and the easier the chip control, the better the machineability.
The most important requirements for realizing excellent machineability in the present invention are the following two points in the steel plates described in (2) and (3) of the present invention, but in the steel plate described in (3) of the present invention, there is further the later explained point (third point). First, the first point is to make the structure of the steel plate a composite structure mainly comprised of soft ferrite and hard bainite and martensite. In particular, since steel plate having a plate thickness of 4 to 100 mm or so is covered, it is important that broad locations in the plate thickness direction become a soft and hard composite structure. By controlling the structure in this way, the soft part easily deforms at the time of machining, while stress concentrates near the interface of the soft parts and hard parts and thereby ductile fracture is promoted and as a result machining proceeds with extremely small plastic deformation. Due to this, the tool life becomes longer, the machining resistance falls, and chip control becomes easy. Even in the case of a composite structure mainly comprised of soft ferrite and hard bainite and martensite, if the soft ferrite fraction becomes lower than 30%, the machineability greatly falls, while if over 90%, the strength becomes insufficient, so the ferrite fraction is defined as 30% to 90% and the balance is defined as being mainly comprised of bainite and martensite. Further, when the ferrite fraction is 45% or more, the machineability is further excellent, so preferably the ferrite fraction is defined as 45% to 90% and the balance is defined as being mainly bainite and martensite. Further, when the ferrite fraction is 60% or more, the machineability is remarkably excellent, so more preferably the ferrite fraction is defined as 60% to 90% and the balance is defined as being mainly bainite and martensite. Note that the hard structure is mainly comprised of bainite and martensite, but even when partially including pearlite, acicular ferrite, and other inclusions, in the range defined by the present invention, the machineability does not deteriorate and remains excellent.
The ferrite fraction defined above is measured by observation of the microstructure under an optical microscope. The measurement plane is made the plane formed by the rolling direction and plate thickness direction (hereinafter called the “L plane”). The measurement locations in the plate thickness direction were made, in the case of a plate thickness of 8 mm or less, the three locations of the locations exactly ¼, ½, and ¾ of the plate thickness inside the steel plate from the top surface of the steel plate (hereinafter called the “t/4 part”, “t/2 part”, and “3t/4 part”) and, in the case of a plate thickness of over 8 mm, the three locations of the t/4 part, t/2 part, and 3t/4 part of the plate thickness direction plus the locations exactly 2 mm inside the steel plate from the top surface and rear surface of the steel plate surface (hereinafter called the “top surface 2 mm part” and “rear surface 2 mm part”). The line segments connecting the measurement points are designed to be parallel to the plate thickness direction. The measurement locations in the width direction avoid the ends of the steel plate in the width direction and the spans of locations inside the ends in the width directions by exactly the lengths corresponding to the plate thickness. The measurements are made at locations close to the center part in the width direction as much as possible. The measurements are preferably performed at magnifications of 100× to 500×. An eyepiece with a lattice is used for measurement by the point count method. The average value of the ferrite fractions at all of the measurement locations is used as the ferrite fraction in the present invention. Note that regarding the criteria for judgment of ferrite at the time of measurement, the “ferrite” referred to in the present invention generally indicates ferrite called bulk ferrite, polygonal ferrite, equiaxial ferrite, etc. and does not include acicular ferrite formed at a lower temperature. However, even with bulk ferrite, depending on the control of the austenite before transformation, sometimes anisotropy occurs in the growth direction and bulk ferrite having a form long in the rolling direction is produced. This is included in the ferrite in the present invention.
Next, the second point of the requirements for realizing excellent machineability is as follows: A composite structure of a soft structure mainly comprised of ferrite and a hard structure mainly comprised of bainite and martensite is, as explained above, excellent in machineability, but with this alone, the machineability is not necessarily sufficient for the drilling or surface machining etc. in fabrication of welded structures. It is necessary to optimize the ratio of the amounts of addition of specific alloy elements assuming a composite structure of a soft structure and a hard structure. Specifically, the rates of addition of the amount of Mn, the amount of Si, the amount of Cr, and the amount of Mo are strictly defined. Boring, surface machining, and other machining are in a sense fracture phenomenon of cut materials by tools at a high temperature and high strain rate. How much the energy required for this can be reduced is important, so it is necessary to make the difference in strength between the soft part and hard part at a high temperature large. If the amount of addition of Mn is large, the amount of solution strengthening of the soft ferrite becomes large and the difference in strength of the hard part and soft part is reduced, so the amount of addition of Mn is preferably low. On the other hand, increases in the amounts of addition of Si, Cr, and Mo contribute to the increase in the ordinary temperature strength of the hard part mainly comprised of bainite and martensite and raise the resistance of the hard part to a drop in strength at a high temperature, so have the effect of increasing the difference in strength between the soft part and hard part. The inventors used steel ingots of ingredients with amounts of addition of Mn, Si, Cr, and Mo changed in various ways to produce steels of composite structures of soft structures and hard structures and studied their machineability and ingredient balance and as a result discovered that if the X2 expressed by X2=(Si/5+Mo+Cr/2)/Mn is less than 0.15, the absolute level of the machineability is insufficient while conversely if the X2 is over 10.0, the weldability greatly falls. Accordingly, in the present invention, the X2 expressed by X2=(Si/5+Mo+Cr/2)/Mn is defined as 0.15 to 10.0. Note that when the value of X2 is 0.3 or more, the machineability is further improved, so preferably X2 is made 0.3 to 10.0. Further, when the value of X2 is 0.4 or more, the machineability is improved more remarkably, so more preferably X2 is made 0.4 to 10.0. Note that the Si, Mo, Cr, and Mn when calculating X2 are all amounts of addition expressed by mass %. In the present invention, Cr and Mo are important elements, but are added as need after consideration of the alloy costs. When Cr and Mo are not added, the value of said X2 is calculated from the amounts of Si and Mn.
In addition to the above most important two requirements for achieving excellent machineability in the steel plate described in (3) of the present invention as explained above, the third point in the requirements for achieving further improved machineability is that it is important to add as large an amount of S as possible to a range riot causing the weldability and toughness to greatly fall. S has the function of reducing the machining resistance and extending the tool life by the effect of MnS as a source of stress concentration. If the amount of addition is over 0.01%, the improvement in the machineability becomes remarkable, while if over 0.035%, the toughness and weldability both fall, so the amount of S is defined as over 0.01% to 0.035%.
The above summarizes the most important requirements for achieving excellent machineability in steel plates described in (2) and (3) of the present invention, but regarding the first point, that is, when the structure is complicated or an extremely fine grain structure, it is sometimes difficult to define a composite structure of a soft structure and a hard structure from observation under an optical microscope. In the present invention, instead, the method of judgment of a composite structure by the micro Vickers hardness is defined in combination. The micro Vickers hardness requires a smaller measurement area than the Vickers hardness, so in the case of a composite structure, the measurement value greatly fluctuates depending on the structure. In particular, a region mostly comprised of ferrite becomes low in hardness. It is possible to use the ratio of number of measurement points with a low hardness to define a composite structure of a soft structure and a hard structure. The inventors ran various structures through micro Vickers hardness tests and clarified the range of micro Vickers hardness giving an excellent machineability. As a result, when the ratio of micro Vickers hardness of 190 HV or less is 20% or more, the machineability is excellent, so the ratio of micro Vickers hardness of 190 HV or less is made 20% or more. Further, when the ratio of micro Vickers hardness of 180 HV or less is 20% or more, the machineability is excellent, so preferably the ratio of micro Vickers hardness of 180 HV or less is made 20% or more. Further, when the ratio of micro Vickers hardness of 170 HV or less is 20% or more, the machineability is further excellent, so more preferably the ratio of micro Vickers hardness of 170 HV or less is made 20% or more. Note that when the ratio of micro Vickers hardness of 170 HV or less is 40% or more, the machineability is further improved, so more preferably the ratio of micro Vickers hardness of 170 HV or less is made 40% or more.
The “micro Vickers hardness” referred to in the present invention is the value measured by the method defined in JIS Z 2244. Another method of measurement besides that defined in the standards will be explained in detail here. The test force is made 0.09807N. The measurement plane is made the L plane. The measurement locations in the plate thickness direction are made, in the case of a plate thickness of 8 mm or less, the three locations of the t/4 part, t/2 part, and 3t/4 part, and, in the case of a plate thickness of over 8 mm, five locations including also the top surface 2 mm part and rear surface 2 mm part. The line segments connecting the measurement points are designed to become parallel to the plate thickness direction. The measurement locations in the width direction avoid the ends of the steel plate in the width direction and the spans of locations inside the ends in the width directions by exactly the lengths corresponding to the plate thickness. The measurements are made at locations close to the center part in the width direction as much as possible. The measurements are conducted at intervals of 100 μm as shown in
The above definition is important for improving the machineability, but to secure the strength, toughness, and weldability, the following definitions become necessary.
First, to secure a tensile strength of 570 MPa or more, the Vickers hardness has to be defined. If the Vickers hardness drops below 165 HV, securing a tensile strength of 570 MPa or more becomes difficult, while if over 300 HV, the weldability greatly falls, so the Vickers hardness is defined as 165 HV to 300 HV.
The Vickers hardness referred to in the present invention is the value measured by a method defined in JIS Z 2244. Another method of measurement besides that defined in the standards will be explained in detail here. The test force is made 98.07N. The measurement plane is made the L plane. The measurement locations in the plate thickness direction are made, in the case of a plate thickness of 8 mm or less, the three locations of the t/4 part, t/2 part, and 3t/4 part, and, in the case of a plate thickness of over 8 mm, five locations including also the top surface 2 mm part and rear surface 2 mm part. The line segments connecting the measurement points are designed to become parallel to the plate thickness direction. The measurement locations in the width direction avoid the ends of the steel plate in the width direction and the spans of locations inside the ends in the width directions by exactly the lengths corresponding to the plate thickness. The measurements are made at locations close to the center part in the width direction as much as possible. The measurements are conducted at five points or more for each location and the average value of the locations is calculated. In the case of a plate thickness of 8 mm or less, the average value of three locations is calculated, while in the case of a plate thickness of over 8 mm, the average value of five points is calculated. This is used as the Vickers hardness.
Further, to obtain an excellent weldability and toughness, the amounts of addition of the alloy elements have to be adjusted. When the X1 expressed by X1=C+(Mn+Cu+Cr)/20+Si/30+Ni/60+Mo/15+V/10+5×B is 0.24 or less, not only are weld fractures greatly reduced, but also the toughness and weld heat affected zone toughness become excellent, so X1 is defined as 0.24 or less. Note that when X1 is 0.21 or less, this effect appears more remarkably, so preferably X1 is made 0.21 or less. Note that the C, Mn, Cu, Cr, Si, Ni, Mo, V, and B when calculating X1 are all amounts of addition expressed by mass %.
Below, the ranges of the alloy elements of the steel plate of the present invention will be defined.
The following, unless particularly indicated to the contrary, is common for the steel plates described in (1), (2), and (3) of the present invention.
C is an element required for securing strength, so the amount of addition is made 0.005% or more. However, on the other hand, an increase in the amount of C invites a drop in matrix toughness and a drop in weldability due to the formation of coarse precipitates, so the upper limit is made 0.2%. Note that if the amount of C is 0.07% or more, securing a tensile strength of 570 MPa or more becomes easy, while if 0.14% or less, the toughness and weldability become more excellent, so preferably the amount of C is made 0.07% to 0.14%.
Si is an extremely important element in the present invention. It is an element effective for increasing the strength and at the same time improving the machineability and for causing the formation of ferrite in a broad range of plate thickness in a method of production predicated on water cooling after rolling and then transforming the balance to mainly bainite and martensite to obtain a composite structure mainly comprised of soft ferrite and hard bainite and martensite. To achieve this effect, addition of 0.01% or more is necessary. Addition of over 1% causes the weldability to drop, so the amount of addition is made 0.01% to 1%. Note that to achieve the above effect more remarkably, addition of 0.2% or more is effective. On the other hand, with 0.55% or less, the weldability becomes extremely excellent, so preferably the amount is made 0.2% to 0.55%.
Mn is an element effective for increasing strength. To achieve the tensile strength of 570 MPa or more covered by the present invention, even at a minimum, 0.01% or more must be added, but conversely if adding over 2%, the weldability falls. Therefore, the amount is defined as 0.01% to 2%. Further, if adding Mn in an amount over 1.4%, the machineability falls, so from the viewpoint of the machineability, 1.4% or less is preferable. Therefore, in the steel plate described in (2) and (3) of the present invention, the amount is defined as 0.01% to 1.4%.
P is an impurity element and preferably has a low amount of addition. Addition over 0.02% causes a drop in the matrix ductility, toughness, and weldability, so the amount is made 0.02% or less.
S is an important element in the present invention and is positively added to improve machineability.
Addition of S causes formation of MnS which acts as a local source of stress concentration so additionally improves the machineability. This effect becomes greater the greater the amount of addition of S, but addition over 0.035% causes the matrix toughness to drop sharply, so the upper limit is defined as 0.035%.
Note that when reducing the amount of addition of S, the effect of improvement of machineability by S becomes smaller, but the matrix toughness and weldability are improved. Therefore, the amount of addition of S is preferably made large when stressing machineability and made small when conversely stressing matrix toughness and weldability.
That is, in the steel plate described in (2) of the present invention, the addition of S has no bearing in the mechanism of improvement of machineability, so a lower amount of addition is preferable. Addition over 0.01% causes the matrix toughness to drop due to the formation of MnS, so 0.01% or less is defined. Note that if the amount of S is 0.006% or less, the matrix toughness is more improved, so preferably the amount of S is defined as 0.006% or less.
Further, in the steel plate described in (3) of the present invention, addition of S functions to reduce the machining resistance and extend the tool life due to the effect of MnS as a source of stress concentration. If the amount of addition is over 0.01%, the improvement in the machineability becomes remarkable, while if over 0.035%, the toughness and weldability both fall, so the amount of S is made over 0.01% to 0.035%.
Al is an element effective as a deoxidizing material. The amount of addition is made 0.001% or more. However, on the other hand, an increase of the amount of Al invites a drop in the matrix toughness, so the upper limit is made 0.1%.
N is an impurity element. Addition over 0.01% causes the matrix toughness and weldability to drop, so 0.01% or less is defined.
Mo is an extremely important element in the present invention and can be added as needed after considering the cost. It is an element effective for increasing the strength and at the same time improving the machineability and for causing the formation of ferrite in a broad range of plate thickness in a method of production predicated on water cooling after rolling and then transforming the balance to mainly bainite and martensite to obtain a composite structure mainly comprised of soft ferrite and hard bainite and martensite. To achieve this effect, addition of 0.01% or more is necessary. Addition of over 1% causes the weldability to drop, so the amount of addition is made 0.01% to 1%. Note that to achieve the above effect more remarkably, addition of 0.1% or more is effective, so preferably the amount is made 0.1% to 1.0%.
Cr is an extremely important element in the present invention and may be added as needed after consideration of the cost. It is an element effective for increasing the strength and at the same time improving the machineability and for causing the formation of ferrite in a broad range of plate thickness in a method of production predicated on water cooling after rolling and then changing the balance to mainly bainite and martensite to obtain a composite structure mainly comprised of soft ferrite and hard bainite and martensite. To achieve this effect, addition of 0.01% or more is necessary. Addition of over 1% causes the weldability to drop, so the amount of addition is made 0.01% to 1%. Note that to achieve the above effect more remarkably, addition of 0.1% or more is effective, so preferably the amount is made 0.1% to 1.0%.
In the present invention, Nb, Ti, and V are also important elements. Nb, Ti, and V are elements effective for increasing the strength by precipitation strengthening etc. and for improving the toughness by making the structure finer and are added as needed from the viewpoint of securing the strength and toughness. The inventors evaluated the tool life when drilling steel plate comprised of a soft and hard composite structure strengthened by these elements. As a result, they discovered that even with a soft and hard composite structure, if the amount of precipitation strengthening is large, the difference in hardness of the soft part and hard part is reduced and the drill life falls. If any of the amounts of addition of Nb, Ti, and V exceeds 0.1%, the machineability remarkably falls. On the other hand, with addition of less than 0.001%, the effect of increase of strength is not obtained. Therefore, the amounts of addition of Nb, Ti, and V are made 0.001% to 0.1%. Note that when the amounts of addition of Nb, Ti, and V are 0.05% or less, 0.04% or less, and 0.05% or less, the drop in machineability accompanying the increase in strength is particularly small, so preferably the amounts of addition of Nb, Ti, and V are made 0.05% or less, 0.04% or less, and 0.05% or less.
Cu, Ni, and B are added as needed from the viewpoint of securing the strength. Cu is an element effective for securing strength. With addition of less than 0.005%, the effect is small, while addition of over 1% causes the weldability to drop, so the range is made 0.005 to 1%. Ni is added as needed to secure strength. With addition of less than 0.01%, the effect is small, while addition of over 2% causes the weldability to drop, to the range is made 0.01 to 2%. B is an element effective for increasing hardenability. The amount of addition is made 0.0002% or more. However, on the other hand, an increase of the amount of B invites a drop in the matrix toughness due to formation of coarse precipitates, so the upper limit is made 0.005%.
Addition of one or more of a REM, Ca, Zr, and Mg can improve the matrix toughness and weld heat affected zone toughness through control of matrix inclusions, increased fineness of the heated austenite of the weld heat affected zone, and formation of transformation nuclei from inside the grains, so these are added as necessary. To achieve this effect, addition of 0.0005% or more of any or REM, Ca, Zr, or Mg is necessary. On the other hand, if overly added, the sulfides and oxides become coarser and the matrix toughness and ductility are lowered, so the upper limit value is made 0.1% for REM and 0.02% for Ca, Zr, and Mg.
Note that in melting the steel of the present invention, the present invention is not impaired in effect in any way even if the O, Zn, Sn, Sb, Te, Ta, W, Pb, Bi, etc. which may dissolve out from the materials used including the added alloys or the furnace materials during melting and enter the steel as unavoidable impurities is 0.005% or less.
Next, the methods of production of steel plate described in (1) of the present invention will be explained. Roughly classified, there are three methods of production. The first method of production (method of production 1) is a method performing rolling at a relatively low temperature, then speedily water cooling, the second method of production (method of production 2) is the method of air cooling until the formation of ferrite after rolling and then continuing with water cooling, and the third method of production (method of production 3) is the method of heating again when the temperature of the rolled steel plate falls, then water cooling. In each case, to form ferrite in a broad range in the plate thickness direction and secure a high ferrite fraction near the steel plate surface, it is necessary to strictly control the temperature in accordance with the plate thickness.
Note that when producing the steel plate described in (1), a steel slab or cast slab having the steel composition described in (1) of the present invention, that is, C, 0.005 to 0.2%, Si: 0.01 to 1%, Mn: 0.01 to 2%, P: 0.02% or less, S: 0.035% or less, Al: 0.001 to 0.1%, N, 0.01% or less, and the balance of iron and unavoidable impurities, where X1 expressed by X1=C+(Mn+Cu+Cr)/20+Si/30+Ni/60+Mo/15+V/10+5×B is 0.24 or less, can be used.
First, the first method of production (method of production 1), that is, the method of rolling at a relatively low temperature, then speedily water cooling and the method of production described in (5) of the present invention will be explained. In this method of production, strict definition of the rough rolling, finish rolling, and water cooling are important.
The rough rolling is important for making the austenite finer by recrystallization and stably forming ferrite and as a result improving the machineability. If the total reduction rate of the rough rolling becomes less than 30%, the ferrite is not stably formed, while if over 95%, the productivity greatly falls, so the total reduction rate of the rough rolling is defined as 30% to 95%. Note that when the total reduction rate of the rough rolling is 50% or more, the machineability is improved more, so preferably the total reduction rate of the rough rolling is made 50% to 95%. Further, when the total reduction rate of the rough rolling is 80% or more, the machineability is improved still more, so more preferably the total reduction rate of the rough rolling is made 80% to 95%. The temperature for the rough rolling may be freely set so long as the condition of the finish rolling temperature is satisfied. Note that the “total reduction rate of the rough rolling” is the plate thickness before rough rolling minus the plate thickness after rough rolling divided by the plate thickness before rough rolling expressed as a percentage.
The finish rolling is important for actively utilizing the dislocations introduced in the unrecrystallization temperature range in various manners so as to promote the formation of ferrite and make it finer and as a result improve the machineability, toughness, and weldability. The inventors produced steel plates of various alloy ingredients and plate thicknesses by this method of production and evaluated them for machineability, weldability, and matrix toughness. As a result, they confirmed that when making the first pass bite temperature of the finish rolling a temperature of not more than T1 (° C.) expressed by T1=35 ln(X2/2)−25√t+1070, X2=(Si/5+Mo+Cr/2)/Mn, ferrite is formed in a broad plate thickness direction and the machineability, weldability, and toughness are all excellent. Accordingly, the first pass bite temperature of the finish rolling is defined as not more than T1 (° C.) expressed by T1=35 ln(X2/2)−25√t+1070, X2=(Si/5+Mo+Cr/2)/Mn. Note that Si, Mo, Cr, and Mn indicate amounts of addition expressed by mass %, while t is the thickness (mm) of the steel plate. If the first pass bite temperature of the finish rolling is less than 720° C., working of the ferrite will cause the matrix toughness and machineability to greatly fall, so the first pass bite temperature of the rolling is given a lower limit of 720° C. Note that if making the first pass bite temperature of the finish rolling 40° C. lower than T1, the machineability is improved more remarkably, so preferably the first pass bite temperature of the finish rolling is made a temperature 40° C. lower than T1. Further, if making the first pass bite temperature of the finish rolling 80° C. lower than T1, the machineability is improved still more remarkably, so more preferably the first pass bite temperature of the finish rolling is made a temperature 80° C. lower than T1. If the final pass bite temperature of the finish rolling is less than 700° C., working of the ferrite will cause the matrix toughness and machineability to greatly fall, while if over T1 (° C.), the ferrite will not be produced in a broad range in the plate thickness direction, so the final pass bite temperature of the finish rolling preferably has a lower limit of 700° C. and an upper limit of T1 (° C.). The total reduction rate is also important for the finish rolling. If less than 30%, ferrite is not formed over a broad range of the plate thickness, while conversely if over 95%, the productivity greatly falls, so the total reduction rate of the finish rolling is defined as 30% to 95%. Further, if the total reduction rate of the finish rolling is 60% or more, the ferrite is formed more stably and the machineability is improved, so preferably the total reduction rate of the finish rolling is made 60% to 95% or less.
Note that in the present invention, the rolling performed by a rough rolling machine is deemed rough rolling, while the rolling performed by a finish rolling machine is deemed finish rolling. If performing rough rolling and finish rolling by the same rolling machine, when there is a clear temperature setting dividing the rolling into a first half and a second half, the first half of rolling is deemed the rough rolling and the second half of rolling is deemed the finish rolling. When there is no clear temperature setting or when there are two or more temperature settings, all of the rolling passes after and including the rolling pass where the temperature of the steel plate surface before the start of that rolling pass became 950° C. or less are deemed the finish rolling. The “first pass bite temperature of the finish rolling” indicates the temperature measured at the surface of the steel plate before the first reduction by the finish rolling. The final pass bite temperature of the finish rolling indicates the temperature measured at the surface of the steel plate before the final reduction by the finish rolling. Note that steel plate surface temperature can be measured for example by using a radiant thermometer.
Next, the water cooling will be explained. Water cooling is effective for securing 570 to 720 MPa or so of tensile strength, securing strength with low alloy content and thereby improving the weldability, and further making the structure finer and thereby improving the matrix toughness. If the flow rate at the time of water cooling falls below 0.2 m3/m2·min, the strength falls, while if over 5.0 m3/m2·min, the ferrite is no longer stably formed in a broad range in the plate thickness direction and the machineability falls, so the flow rate at the time of water cooling is defined as 0.2 m3/m2·min to 5.0 m3/m2·min. If the end temperature of the water cooling exceeds 600° C., the residual austenite after ferrite formation will not transform at a low temperature and the strength will fall, so the end temperature of the water cooling is made 600° C. or less. Here, the “end temperature of the water cooling” means the maximum value of the steel plate surface temperature measured after waiting for heat recovery after water cooling. After water cooling, air cooling is performed.
In the water cooling performed after the end of rolling, by changing the first half and second half cooling rates, it is possible to form the ferrite more stably, so this technique may be adopted if necessary. By making the cooling rate of the first half defined by the water cooling start temperature to over 650° C. 1° C./s to 5° C./s and making the second half cooling rate of the second half defined by 650° C. to the water cooling end temperature 10° C./s to 100° C./s, steel plate with an even more excellent machineability and an equal or better strength can be produced. The cooling rate in the first half of the cooling is lowered so as to increase the amount of production of ferrite and make the C in the untransformed austenite more concentrated and thereby lower the transformation temperature of the bainite etc. formed in the second half cooling, while the cooling rate in the latter half is raised so as to make the transformation temperature of the untransformed austenite as low as possible. Note that the temperatures and cooling rates in this two-stage cooling are made the temperatures measured at the steel plate t/4 part and the average cooling rates calculated based on those values and can be measured using a spare sample of a steel plate in which a thermocouple is embedded and performing water cooling simulating actual water cooling.
Below, other preferable conditions in the first method of production (method of production 1) will be explained. Before the rough rolling and finish rolling, the steel slab or cast slab is heated. If the heating temperature is less than 900° C., part of the structure from before the heating will remain untransformed, so the material will become nonhomogeneous, while if the heating temperature exceeds 1350° C., the austenite will become coarser, the final structure will also become coarser, the matrix toughness will greatly fall, and also the formation of ferrite will be suppressed and the machineability will fall, so the heating temperature is preferably made 900° C. to 1350° C. The water cooling is to be started very speedily after the end of the finish rolling. For example, it is preferably started within 200 from the end of the finish rolling. This is because if the time until the start of the water cooling exceeds 200 s, the dislocations introduced by the rolling will be reduced by recovery, ferrite will not be stably formed in a broad range in the plate thickness direction, and the machineability will fall. Here, the “end of the finish rolling” means the point of time when the frontmost part of the steel plate is reduced at the final pass of the finish rolling, while the “start of the water cooling” means the point of time when the frontmost part of the steel plate reaches the water cooling facility and the water cooling starts. Further, the water cooled, then air cooled steel plate may be heat treated if necessary. For example, it may be tempered from the viewpoint of improving the matrix toughness.
Next, the second method of production (method of production 2), that is, the method of air cooling until formation of ferrite after rolling, then water cooling, and the method of production described in (7) of the present invention will be defined. The heating is similar to the first method of production. The temperature of the rough rolling can be freely set, but if the total reduction rate of the rough rolling is less than 30%, the toughness greatly falls, while if over 95%, the productivity greatly falls, so the total reduction rate of the rough rolling is defined as 30% to 95%. The temperature of the finish rolling is not defined like in the first method of production and can be any condition. If the total reduction rate of the finish rolling is less than 30%, the toughness greatly falls, while if over 95%, the productivity greatly falls, so the total reduction rate of the finish rolling is defined as 30% to 95%. After the heating, rough rolling, and finish rolling end, air cooling is performed. During the air cooling, ferrite is formed, then water cooling is performed. The inventors investigated steels of various ingredients by changing the steel plate surface temperature when shifting from air cooling after finish rolling to water cooling in various ways and discovered that when the steel plate surface temperature when shifting to water cooling is T2 (° C.) expressed by T2=910−310×C−80×Mn−20×Cu−15×Cr−55×Ni−80×Mo+0.0006t2−0.56t−8.6 or less, ferrite is formed in a broad range in the plate thickness direction and the machineability is improved, while when the steel plate surface temperature falls below 650° C., the strength greatly falls. Accordingly, the steel plate surface temperature when shifting to water cooling is defined as T2 (° C.) expressed by T2=910−310×C−80×Mn−20×Cu−15×Cr−55×N−80×Mo+0.0006t2−0.56t−8.6 to 650° C. Here, the “steel plate surface temperature when shifting to water cooling” means the steel plate surface temperature measured before water cooling. C, Mn, Cu, Cr, Ni, and Mo indicate the amounts of addition of the corresponding elements (mass %), while t indicates the plate thickness (mm). If the flow rate at the time of water cooling falls below 0.2 m3/m2·min, the strength falls, while if over 5.0 m3/m2·min, the ferrite is no longer stably formed in a broad range in the plate thickness direction and the machineability falls, so the flow rate at the time of water cooling is defined as 0.2 m3/m2·min to 5.0 m3/m2·min. If the end temperature of the water cooling exceeds 500° C., the residual austenite after formation of ferrite will not transform at a low temperature and the strength will falls, so the end temperature of the water cooling is made 500° C. or less. Here, the “end temperature of water cooling” means the maximum value of the steel plate surface temperature measured after waiting for heat recovery after water cooling. After water cooling, air cooling is performed. Further, the water cooled, then air cooled steel plate may be heat treated if necessary. For example, it may be tempered from the viewpoint of improving the matrix toughness.
Next, the third method of production (method of production 3), that is, the method of heating again after the temperature of the steel plate falls after rolling will be defined. The heating before the rolling is the same as the first method of production. The temperature of the rough rolling may be freely set, but if the total reduction rate of the finish rolling is less than 30%, the toughness will greatly fall, while if it exceeds 95%, the productivity greatly falls. Accordingly, the total reduction rate of the finish rolling is defined as 30% to 95%. The temperature of the finish rolling is not defined as in the first method of production and can be made any condition. If the total reduction rate of the finish rolling is less than 30%, the toughness will greatly fall, while if over 95%, the productivity greatly falls, so the total reduction rate of the finish rolling is defined as 30% to 95%. After the heating, rough rolling, and finish rolling, the steel plate is cooled to 500° C. or less by any means, then is again heated. The inventors investigated the reheating temperature by changing it in various ways. If the reheating temperature is less than T3 (° C.) expressed by T3=0.0017t2+0.17t+730 or if it is over 850° C., sufficient strength cannot be obtained, so the reheating temperature is defined as T3 (° C.) expressed by T3=0.0017t2+0.17t+730 to 850° C. After reheating, water cooling is performed. If the flow rate at the time of water cooling falls below 0.2 m3/m2·min, the strength falls, while if over 5.0 m3/m2·min, the ferrite is no longer stably formed in a broad range and the machineability falls, so the flow rate at the time of water cooling is defined as 0.2 m3/m2·min to 5.0 m3/m2·min. If the end temperature of the water cooling exceeds 500° C., the residual austenite after ferrite formation will not transform at a low temperature and the strength will fall, so the end temperature of the water cooling is made 500° C. or less. Here, the “end temperature of the water cooling” means the maximum value of the steel plate surface temperature measured after waiting for heat recovery after water cooling. After water cooling, air cooling is performed.
Next, the methods of production of steel plate of (2) and (3) of the present invention will be explained. Roughly classified, there are four methods of production. Methods of production 4 to 7 and 4′ to 7′ will be described for the steel plates. Further, the methods of production of steel plate described in (2) and (3) of the present invention are the same in all four methods of production in all conditions other than the steel compositions of the steel slabs or cast slabs, so in the following explanations, the explanations will be given for the steel plates described in (2) and (3). The first method of production (methods of production 4, 4′) is the method of water cooling speedily after rolling, the second method of production (methods of production 5, 5′) is the method of heating again after the temperature of the steel plate falls after rolling, then water cooling, the third method of production (methods of production 6, 6′) is the method of air cooling until the formation of ferrite after rolling, then water cooling, and the fourth method of production (methods of production 7, 7′) is the method of heating again to the two-phase region after the temperature of the steel plate falls after rolling, then water cooling.
Note that when producing the steel plate described in (2) of the present invention, a steel slab or cast slab having the steel composition described in (2) of the present invention, that is, having the steel composition described in (1) wherein Mn: 0.1% to 1.4%, S: 0.01% or less, having X1 expressed by X1=C+(Mn+Cu+Cr)/20+Si/30+Ni/60+Mo/15+V/10+5×B of 0.24 or less, and having X2 expressed by X2=(Si/5+Mo+Cr/2)/Mn of 0.15 to 10.0 is used, further, when producing the steel plate described in (3) of the present invention, a steel slab or cast slab having the steel composition described in (3) of the present invention, that is, having the steel composition described in (1) wherein Mn: 0.1% to 1.4%, S: over 0.01% to 0.035%, having X1 expressed by X1=C+(Mn+Cu+Cr)/20+Si/30+Ni/60+Mo/15+V/10+5×B of 0.24 or less, and having X2 expressed by X2=(Si/5+Mo+Cr/2)/Mn of 0.15 to 10.0 is used,
First, the first method of production (methods of production 4, 4′) among the methods of production of steel plate described in (2) and (3), that is, the method of starting water cooling speedily after rolling, and the methods of production described in (9) and (14) of the present invention will be explained. In this method of production, the rough rolling, finish rolling, and water cooling become important.
First, the rough rolling will be explained. Rough rolling is important from the viewpoint of increasing the fineness of the austenite through recrystallization and thereby promoting the formation of ferrite. If the total reduction rate of the rough rolling is less than 30%, the ferrite is not stably formed, while if over 95%, the productivity greatly falls, so the total reduction rate of the rough rolling is defined as 30% to 95%. Further, if making the total reduction rate of the rough rolling 50% or more, the ferrite is formed more stably and the machineability is further improved, so preferably the reduction rate in the rough rolling is made 50% to 95% Further, if making the total reduction rate of the rough rolling 80% or more, the ferrite is still more stably formed and the machineability still more improved, so more preferably the reduction rate of the rough rolling is made 80% to 95%. The rough rolling bite temperature and the steel plate surface temperature before the final pass may be freely set so long as the condition of the finish rolling temperature is satisfied. Note that the “total reduction rate of the rough rolling” is the plate thickness before rough rolling minus the plate thickness after rough rolling divided by the plate thickness before rough rolling expressed as a percentage.
The finish rolling is important for stably forming ferrite in the method of production using water cooling. The lower the temperature of the rolling, the higher the density of dislocations introduced per unit reduction rate and the more recovery of dislocations in the middle of transport between rolling passes or from the rolling machine to the water cooling facility is suppressed, so formation of ferrite can be promoted. The behavior of formation of ferrite is heavily influenced by the alloy ingredients, so the finish rolling temperature has to be defined in relation to the ingredients. In general, in thick gauge steel plate having a tensile strength of 570 MPa or more, with the method of production of water cooling speedily after rolling, formation of ferrite is difficult. To achieve this, an extremely low finish rolling temperature becomes necessary and the productivity falls, but in the present invention, it is newly discovered that production is possible without a drop in productivity in a range of ratios of ingredients of Si, Mn, Mo, and Cr defined for improving the machineability. The inventors studied the optimal first pass bite temperature of the finish rolling for steels of various ingredients and discovered that when the first pass bite temperature of the finish rolling is T4 (° C.) expressed by T4=35 ln(X2/2)−25√t−1100 or less, ferrite is stably formed. Accordingly, the first pass bite temperature of the finish rolling is defined as T4 (° C.) or less. Here, X2, as already shown, is the value calculated by X2=(Si/5+Mo+Cr/2)/Mn, and t is the plate thickness (mm). The formula of T4 includes the item of plate thickness because the greater the final plate thickness, the more the reduction rate at the rolling generally falls, so the increasing coarseness of the recrystallization grain size and drop in density of remaining dislocations cause the formation of ferrite to be suppressed and thereby low temperature rolling to become necessary. Note that if the first pass bite temperature of the finish rolling is set to 40° C. lower than T4, the machineability is improved more remarkably, so preferably the first-pass bite temperature of the finish rolling is made a temperature 40° C. lower than T4 or less. Further, if making the first pass bite temperature of the finish rolling 80° C. lower than T4, the machineability is improved still more remarkably, so more preferably the first pass bite temperature of the finish rolling is made a temperature 80° C. lower than T4 or less. Note that if the first pass bite temperature of the finish rolling is lower than the Ar3 point, the increase in hardness due to working of the ferrite will cause the machineability to fall, so the lower limit of the first pass bite temperature of the finish rolling is defined as the Ar3 point. The final pass bite temperature of the finish rolling is preferably given a lower limit of a temperature 100° C. lower than the Ar3 point or more and an upper limit of T4+50 (° C.) from the viewpoint of greatly suppressing any drop in machineability accompanying working of the ferrite.
Note that in the present invention, the rolling performed by a rough rolling machine is deemed rough rolling, while the rolling performed by a finish rolling machine is deemed finish rolling. If performing rough rolling and finish rolling by the same rolling machine, when there is a clear temperature setting dividing the rolling into a first half and a second half, the first half of rolling is deemed the rough rolling and the second half of rolling is deemed the finish rolling. When there is no clear temperature setting or when there are two or more temperature settings, all of the rolling passes after and including the rolling pass where the temperature of the steel plate surface before the start of that rolling pass became 950° C. or less are deemed the finish rolling. The “first pass bite temperature of the finish rolling” indicates the temperature measured at the surface of the steel plate before the first reduction by the finish rolling. The “final pass bite temperature of the finish rolling” indicates the temperature measured at the surface of the steel plate before the final reduction by the finish rolling. The Ar3 point cannot be directly measured, but can be estimated by thermo-mechanical treatment simulating actual production conditions while measuring the expansion curve. Note that steel plate surface temperature can be measured for example by using a radiant thermometer.
The total reduction rate of the finish rolling is important from the viewpoint of the stable formation of ferrite. If the total reduction rate of the finish rolling is 30% or more, the ferrite is stably formed and thereby the machineability is improved. On the other hand, if the total reduction rate of the finish rolling exceeds 95%, the productivity greatly falls. Accordingly, the total reduction rate of the finish rolling is defined as 30% to 95%. Note that making the total reduction rate of the finish rolling 60% or more improves the machineability more, so preferably the total reduction rate of the finish rolling is made 60% to 95% or less. Note that the “total reduction rate of the finish rolling” is the plate thickness before finish rolling minus the plate thickness after finish rolling divided by the plate thickness before finish rolling expressed as a percentage.
Next, the conditions of the water cooling will be explained. Water cooling is important for simultaneously achieving an improvement in the machineability through stable formation of ferrite and formation of a structure with a balance of mostly bainite and martensite by low temperature transformation, an improvement of matrix toughness by increasing the fineness of the grain size, and improvement of the weldability through securing strength with a low alloy content. If the flow rate at the time of water cooling falls below 0.2 m3/m2·min, the strength falls, while if over 5.0 m3/m2·min, the ferrite is no longer stably formed and the machineability falls, so the flow rate at the time of water cooling is defined as 0.2 m3/m2·min to 5.0 m3/m2·min. If the end temperature of the water cooling exceeds 600° C., the residual austenite after ferrite formation will not transform at a low temperature and the strength will fall, so the end temperature of the water cooling is made 600° C. or less. Here, the “end temperature of the water cooling” means the maximum value of the steel plate surface temperature measured after waiting for heat recovery after water cooling. After water cooling, air cooling is performed.
Further, the water cooling is preferably started speedily after the end of the finish rolling. This is because if the time from the end of the finish rolling to the start of the water cooling becomes long, the dislocations introduced by the rolling will be reduced by recovery, ferrite will not be stably formed, and the machineability will fall. Note that specifically the water cooling is preferably started within 200 s from the end of the finish rolling. Here, the “end of the finish rolling” means the point of time when the frontmost part of the steel plate is reduced at the final pass of the finish rolling, while the “start of the water cooling” means the point of time when the frontmost part of the steel plate reaches the water cooling facility and the water cooling starts.
In the water cooling performed after the end of rolling, by changing the first half and second half cooling rates, it is possible to form the ferrite more stably, so this technique may be adopted if necessary. By making the cooling rate of the first half defined by the water cooling start temperature to over 650° C. 1° C./s to 5° C./s and making the second half cooling rate of the second half defined by 650° C. to the water cooling end temperature 10° C./s to 100° C./s, steel plate with an even further improved machineability and an equal or better strength can be produced. The cooling rate in the first half of the cooling is lowered so as to increase the amount of production of ferrite and make the C in the untransformed austenite more concentrated and thereby lower the transformation temperature of the bainite etc. formed in the second half cooling, while the cooling rate in the latter half is raised so as to make the transformation temperature of the untransformed austenite as low as possible. Note that the temperatures and cooling rates in this two-stage cooling are made the temperatures measured at the steel plate t/4 part and the average cooling rates calculated based on those values and can be measured using a spare sample of a steel plate in which a thermocouple is embedded and performing water cooling simulating actual water cooling.
Below, other preferable conditions in the method of production will be explained. Before the rolling, the steel slab or cast slab is heated. If the heating temperature is less than 900° C., part of the structure from before the heating will remain untransformed, so the material will become nonhomogeneous, while if the heating temperature exceeds 1350° C., the austenite will become coarser, the final structure will also become coarser, the matrix toughness will greatly fall, and also the formation of ferrite will be suppressed and the machineability will fall, so the heating temperature is preferably made 900° C. to 1350° C. Further, the water cooled, then air cooled steel plate may be heat treated if necessary. For example, it may be tempered from the viewpoint of improving the matrix toughness.
Next, the second method of production (methods of production 5, 5′) among the methods of production of steel plate described in (2) and (3) of the present invention, that is, the method of heating again when the temperature of the steel plate falls after rolling, then water cooling, and the methods of production described in (11) and (16) of the present invention will be defined. The heating, rough rolling, and finish rolling may be performed under any conditions. After the end of the heating, rough rolling, and finish rolling, the steel plate is cooled to 500° C. or less by any means, then is again heated to 900° C. to 1050° C. After the heating, it is water cooled by a cooling rate of 1° C./s to 100° C./s. The end temperature of the water cooling is made 500° C. or less. After the water cooling, air cooling is performed.
Due to the reheating after rolling, fine austenite is obtained and ferrite can be stably formed. If the reheating temperature is less than 900° C., austenite is formed with a high concentration of C. This becomes martensite after transformation and causes the matrix toughness to greatly fall. Further, if the reheating temperature is over 1050° C., ferrite is not stably produced, and the machineability falls. Therefore, the reheating temperature is defined as 900° C. to 1050° C. If the cooling rate after reheating is less than 1° C./s, the residual austenite after formation of ferrite will not transform at a low temperature and the strength will fall, while if the cooling rate exceeds 100° C./s, ferrite will not be stably formed, so the cooling rate after reheating is defined as 1° C./s to 100° C./s. If the end temperature of the water cooling exceeds 500° C., the residual austenite after formation of ferrite will not transform at a low temperature and the strength will fall, so the end temperature of the water cooling is defined as 500° C. or less. Further, the water cooled, then air cooled steel plate may be heat treated if necessary. For example, it may be tempered from the viewpoint of improving the matrix toughness.
Here, the “end temperature of water cooling” means the maximum value of the steel plate surface temperature measured speedily after heat recovery after water cooling. The cooling rate of the water cooling is made the average cooling rate calculated based on the temperatures measured at the steel plate t/4 part and can be estimated by using a spare sample of a steel plate in which a thermocouple is embedded and performing water cooling simulating actual water cooling.
Next, the third method of production (methods of production 6, 6′) among the methods of production of steel plate described in (2) and (3) of the present invention, that is, the method of air cooling until the start of formation of ferrite after rolling, then water cooling, and the methods of production described in (12) and (17) of the present invention will be defined. The heating is the same as said first method of production (method of production 4). In the rough rolling, if the total reduction rate is less than 30%, the toughness falls, while if over 95%, the productivity greatly falls, so the total reduction rate of the rough rolling is defined as 30% to 95%. The finish rolling is not defined as to temperature like with the first method and may be performed under any conditions. If the total reduction rate of the finish rolling is less than 30%, the toughness falls, while if over 95%, the productivity greatly falls, so the total reduction rate of the finish rolling is defined as 30% to 95%. After the heating, rough rolling, and finish rolling end, air cooling is performed. During the air cooling, ferrite starts to form, then water cooling is performed. When the temperature for starting the water cooling exceeds the Ar3 point, the ferrite is not stably formed and the machineability falls, while if lower than a temperature lower than the Ar3 point by 150° C., the strength falls, so the water cooling start temperature is defined as the Ar3 point to a temperature lower than the Ar3 point by 150° C. or higher. Here, the “water cooling start temperature” means the steel plate surface temperature measured before water cooling. The Ar3 point can be estimated by thermo-mechanical treatment simulating actual production conditions while measuring the expansion curve. If the flow rate at the time of water cooling falls below 0.2 m3/m2·min, the strength will fall, while if over 5.0 m3/m2·min, the productivity will fall, so the flow rate at the time of water cooling is defined as 0.2 m3/m2·min to 5.0 m3/m2·min. If the end temperature of the water cooling exceeds 500° C., the residual austenite after formation of ferrite will not transform at a low temperature and the strength will fall, so the end temperature of the water cooling is made 500° C. or less. Here, the “end temperature of water cooling” means the maximum value of the steel plate surface temperature measured speedily after heat recovery after water cooling. After the water cooling, air cooling is performed. Further, the water cooled, then air cooled steel plate may be heat treated if necessary. For example, it may be tempered from the viewpoint of improving the matrix toughness.
Next, the fourth method of production (methods of production 7, 7′) among the methods of production of steel plate described in (2) and (3) of the present invention, that is, the method of heating again to the two-phase region when the temperature of the steel plate falls after rolling, and the methods of production described in (13) and (18) of the present invention will be defined. The heating, rough rolling, and finish rolling are similar to said third method of production (methods of production 6, 6′). After the heating, rough rolling, and finish rolling end, the steel plate is cooled to 500° C. or less by any means, then is again heated. If the reheating temperature is less than 730° C., the machineability falls, while if 900° C. or more, the strength falls, so the reheating temperature is defined as 730° C. to less than 900° C. After reheating, the plate may be water cooled by any method. If the end temperature of water cooling is over 500° C., the residual austenite after formation of ferrite will not transform at a low temperature and the strength will fall, so the end temperature of water cooling is made 500° C. or less. After the water cooling, air cooling is performed. Further, the cooled steel plate may be heat treated if necessary. For example, it may be tempered from the viewpoint of improving the matrix toughness.
Examples of the steel plate described in (1) of the present invention will be explained next.
Test steel materials of various chemical ingredients were used to produce steel plates of plate thicknesses of 6, 18, 40, and 100 mm under various production conditions. Their properties were evaluated. The evaluated items were, as strength, the yield stress and tensile strength, as toughness, the Charpy impact absorption energy, as the weld heat affected zone toughness in weldability, the Charpy impact absorption energy of the weld joint, and as the machineability, the drilling property. The chemical ingredients, plate thickness, X1, and ferrite fraction measured at various locations of each of the steel plates are shown in Table 1 to Table 6, the production conditions (methods of production 1 to 3) are shown in Table 7 to Table 9, and the results of evaluation of properties are shown in Table 10 to Table 12.
The yield stress and the tensile strength were measured by the metal material tensile test method described in JIS Z 2241. The test piece was a metal material test piece described in JIS Z 2201. No. 5 test pieces taken from steel plates having plate thicknesses of 6 mm and 18 mm, while No. 10 test pieces taken from the t/4 parts of steel plates having plate thicknesses of 40 mm and 100 mm were used. The test pieces were taken so that their longitudinal directions became vertical to the rolling direction. The yield stress was made 0.2% of the yield strength calculated by the lower yield stress or offset method. Two tests were conducted at ordinary temperature and the average values were used.
The matrix toughness was measured by the metal material impact test method described in JIS Z 2242. The test pieces used were the metal material impact test pieces described in JIS Z 2202. For steel plates having thicknesses of 6 mm, subsize test pieces of widths of 5 mm were taken from the plate thickness center parts, for steel plates having thicknesses of 18 mm, test pieces of widths of 10 mm were taken from the plate thickness center parts, while for steel plates having thicknesses of 40 mm and 100 mm, test pieces of widths of 10 mm were taken from the t/4 parts. The shapes were all made V-notch test pieces. The test pieces were taken so that the lines formed by the notch bottoms became parallel to the plate thickness direction and so that the longitudinal directions of the test piece became perpendicular to the rolling direction. The test temperature was made −5° C. The average value of three tests was employed.
For the weld heat affected zone toughness, Charpy test pieces were taken from weld joints prepared by CO2 gas shield arc welding and submerge arc welding and measured for absorption energy at −5° C. The welding heat input was made 2 to 3 kJ/mm in the case of CO2 gas shield arc welding and was made 3 kJ/mm for plate thickness 6 mm materials, 5 kJ/mm for plate thickness 18 mm materials, and 7 kJ/mm for plate thickness 40 mm materials and 100 mm materials in the case of submerge arc welding. The test nieces were taken so that the locations 0.5 mm from the weld bond part corresponded to the Charpy test piece notch positions. The average value of three impact absorption energies was used.
The machineability was evaluated by a drilling test using a drilling machine and a high speed drill. The drilling distance was 42 mm in the case of plate thickness 6 mm steel plates piled up in seven layers, 36 mm in the case of plate thickness 18 mm steel plates piled up in two layers, 40 mm in the case of one plate thickness 40 mm steel plate, and 100 mm in the case of one plate thickness 100 mm steel plate for the test. The drill was used to make a through hole using a 6 mmφ diameter high speed drill SKH51. The rotational speed was 1610 rpm, the feed speed was 190 mm/min, and the machining oil was a water soluble machining oil. Under the above conditions, drilling was performed until drilling was no longer possible. The number of holes bored until the limit was reached was measured.
Invention Examples 1-1 to 1-11 are steel plates produced by first method of production (method of production 1), that is, the method of speedily water cooling after rolling (the method of production described in (5) of the present invention). Along with these, Comparative Examples 1-1 to 1-11 are also shown.
Invention Example 1-1 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling and water cooling conditions, etc. to produce plate thickness 6 mm steel plate. Ferrite is stably formed in the entire plate thickness range, in particular near the steel plate surface, so this has a tensile strength of 570 MPa or more and is excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 1-1 is similar to Invention Example 1-1 in ingredients and method of production, but the amount of C and X1 are outside the range of the present invention, so the matrix toughness and weld heat affected zone toughness are inferior.
Invention Example 1-2 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling and water cooling conditions, etc. to produce plate thickness 18 ace steel plate. Ferrite is stably formed in the entire plate thickness range, in particular near the steel plate surface, so this has a tensile strength of 570 MPa or more and is excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 1-2 is similar to Invention Example 1-2 in ingredients and method of production, but the Si is outside the range of the present invention, so the matrix toughness and weld heat affected zone toughness are inferior.
Invention Example 1-3 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling and water cooling conditions, etc. to produce plate thickness 40 mm steel plate. Ferrite is stably formed in the entire plate thickness range, in particular near the steel plate surface, so this has a tensile strength of 570 MPa or more and is excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 1-3 is similar to Invention Example 1-3 in ingredients and method of production, but the total reduction rate of the rough rolling and ferrite fraction are outside the range of the present invention, so the machineability is inferior.
Invention Example 1-4 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling and water cooling conditions, etc. to produce plate thickness 100 mm steel plate. Ferrite is stably formed in the entire plate thickness range, in particular near the steel plate surface, so this has a tensile strength of 570 MPa or more and is excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 1-4 is similar to Invention Example 1-4 in ingredients and method of production, but the finish rolling first pass bite temperature and ferrite fraction are outside the range of the present invention, so the machineability is inferior.
Invention Example 1-5 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling and water cooling conditions, etc. to produce plate thickness 6 mm steel plate. Ferrite is stably formed in the entire plate thickness range, in particular near the steel plate surface, so this has a tensile strength of 570 MPa or more and is excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 1-5 is similar to Invention Example 1-5 in ingredients and method of production, but the amount of P is outside the range of the present invention, so the matrix toughness and weld heat affected zone toughness are inferior.
Invention Example 1-6 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling and water cooling conditions, etc. to produce plate thickness 18 mm steel plate. Ferrite is stably formed in the entire plate thickness range, in particular near the steel plate surface, so this has a tensile strength of 570 MPa or more and is excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 1-6 is similar to Invention Example 1-6 in ingredients and method of production, but the amount of Mo and the finish rolling first pass bite temperature are outside the range of the present invention, so the matrix toughness, weld heat affected zone toughness, and machineability are inferior.
Invention Example 1-7 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling and water cooling conditions, etc. to produce plate thickness 40 mm steel plate. Ferrite is stably formed in the entire plate thickness range, in particular near the steel plate surface, so this has a tensile strength of 570 MPa or more and is excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 1-7 is similar to Invention Example 1-7 in ingredients and method of production, but the total reduction rate of the finish rolling and ferrite fraction are outside the range of the present invention, so the machineability is inferior.
Invention Example 1-8 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling and water cooling conditions, etc. to produce plate thickness 100 mm steel plate. Ferrite is stably formed in the entire plate thickness range, in particular near the steel plate surface, so this has a tensile strength of 570 MPa or more and is excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 1-8 is similar to Invention Example 1-8 in ingredients and method of production, but the amount of Mn and X1 are outside the range of the present invention, so the weld heat affected zone toughness is inferior.
Invention Example 1-9 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling and water cooling conditions, etc. to produce plate thickness 6 mm steel plate. Ferrite is stably formed in the entire plate thickness range, in particular near the steel plate surface, so this has a tensile strength of 570 MPa or more and is excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 1-9 is similar to Invention Example 1-9 in ingredients and method of production, but the amount of N and water cooling end temperature are outside the range of the present invention, so the strength, toughness, and weld heat affected zone toughness are inferior.
Invention Example 1-10 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling and water cooling conditions, etc. to produce plate thickness 18 mm steel plate. Ferrite is stably formed in the entire plate thickness range, in particular near the steel plate surface, so this has a tensile strength of 570 MPa or more and is excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 1-10 is similar to Invention Example 1-10 in ingredients and method of production, but the amount of S and flow rate are outside the range of the present invention, so the strength, toughness, and weld heat affected zone toughness are inferior.
Invention Example 1-11 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling and water cooling conditions, etc. to produce plate thickness 40 mm steel plate. Ferrite is stably formed in the entire plate thickness range, in particular near the steel plate surface, so this has a tensile strength of 570 MPa or more and is excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 1-11 is similar to Invention Example 1-11 in ingredients and method of production, but the amount of Al, the amount of Cr, ferrite fraction, and flow rate are outside the range of the present invention, so the toughness, weld heat affected zone toughness, and machineability are inferior.
Invention Examples 1-12 to 1-17 are steel plates produced by the second method of production (method of production 2), that is, the method of air cooling until starting to form ferrite after rolling, then water cooling, and the method of production described in (7) of the present invention. Along with these, Comparative Examples 1-12 to 1-17 are also shown.
Invention Example 1-12 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling and water cooling conditions, etc. to produce plate thickness 6 mm steel plate. Ferrite is stably formed in the entire plate thickness range, in particular near the steel plate surface, so this has a tensile strength of 570 MPa or more and is excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 1-12 is similar to Invention Example 1-12 in ingredients and method of production, but the amount of V and the water cooling start temperature are outside the range of the present invention, so the strength, toughness, weld heat affected zone toughness, and machineability are inferior.
Invention Example 1-13 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling and water cooling conditions, etc. to produce plate thickness 18 mm steel plate. Ferrite is stably formed in the entire plate thickness range, in particular near the steel plate surface, so this has a tensile strength of 570 MPa or more and is excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 1-13 is similar to Invention Example 1-13 in ingredients and method of production, but the water cooling end temperature and the amount of S are outside the range of the present invention, so the strength, toughness, and weld heat affected zone toughness are inferior.
Invention Example 1-14 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling and water cooling conditions, etc. to produce plate thickness 40 mm steel plate. Ferrite is stably formed in the entire plate thickness range, in particular near the steel plate surface, so this has a tensile strength of 570 MPa or more and is excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 1-14 is similar to Invention Example 1-14 in ingredients and method of production, but the water cooling start temperature and ferrite fraction are outside the range of the present invention, so the machineability is inferior.
Invention Example 1-15 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling and water cooling conditions, etc. to produce plate thickness 100 mm steel plate. Ferrite is stably formed in the entire plate thickness range, in particular near the steel plate surface, so this has a tensile strength of 570 MPa or more and is excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 1-15 is similar to Invention Example 1-15 in ingredients and method of production, but the amount of Nb and the total reduction rate of the finish rolling are outside the range of the present invention, so the toughness, weld heat affected zone toughness, and machineability are inferior.
Invention Example 1-16 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling and water cooling conditions, etc. to produce plate thickness 6 mm steel plate. Ferrite is stably formed in the entire plate thickness range, in particular near the steel plate surface, so this has a tensile strength of 570 MPa or more and is excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 1-16 is similar to Invention Example 1-16 in ingredients and method of production, but the amount of Mg and the flow rate are outside the range of the present invention, so the strength and toughness are inferior.
Invention Example 1-17 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling and water cooling conditions, etc. to produce plate thickness 18 mm steel plate. Ferrite is stably formed in the entire plate thickness range, in particular near the steel plate surface, so this has a tensile strength of 570 MPa or more and is excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 1-17 is similar to Invention Example 1-17 in ingredients and method of production, but the amount of Ti and the total reduction rate of the rough rolling are outside the range of the present invention, so the toughness, weld heat affected zone toughness, and machineability are inferior.
Invention Examples 1-18 to 1-21 are steel plates produced by the third method of production (method of production 3), that is, the method of heating until the two-phase region again after the temperature the steel plate falls after rolling (the method of production described in (8) of the present invention). Along with these, Comparative Example 1-18 to 1-21 are also shown.
Invention Example 1-18 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling and water cooling conditions, etc. to produce plate thickness 18 mm steel plate. Ferrite is stably formed in the entire plate thickness range, in particular near the steel plate surface, so this has a tensile strength of 570 MPa or more and is excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 1-18 is similar to Invention Example 1-18 in ingredients and method of production, but the amount of Cu, X1, and reheating temperature are outside the range of the present invention, so the toughness and weld heat affected zone toughness are inferior.
Invention Example 1-19 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling and water cooling conditions, etc. to produce plate thickness 40 mm steel plate. Ferrite is stably formed in the entire plate thickness range, in particular near the steel plate surface, so this has a tensile strength of 570 MPa or more and is excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 1-19 is similar to Invention Example 1-19 in ingredients and method of production, but the amount of Ca and reheating temperature are outside the range of the present invention, so the toughness is inferior.
Invention Example 1-20 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling and water cooling conditions, etc. to produce plate thickness 6 mm steel plate. Ferrite is stably formed in the entire plate thickness range, in particular near the steel plate surface, so this has a tensile strength of 570 MPa or more and is excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 1-20 is similar to Invention Example 1-20 in ingredients and method of production, but the X1 and the total reduction rate of the finish rolling are outside the range of the present invention, so the toughness and weld heat affected zone toughness are inferior.
Invention Example 1-21 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling and water cooling conditions, etc. to produce plate thickness 100 mm steel plate. Ferrite is stably formed in the entire plate thickness range, in particular near the steel plate surface, so this has a tensile strength of 570 MPa or more and is excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 1-21 is similar to Invention Example 1-21 in ingredients and method of production, but the amount of Ni, total reduction rate of the rough rolling, and water cooling end temperature are outside the range of the present invention, so the strength, toughness, and weld heat affected zone toughness are inferior.
From the above examples, it is clear that the steel materials produced by the present invention, that is, the steel plates of Invention Examples 1-1 to 1-21, are steel materials having tensile strengths of 570 to 720 MPa or so, high in matrix toughness, high in weld heat affected zone toughness, and excellent in machineability.
Examples of the steel plate described in (2) of the present invention will be explained next.
Test steel materials of various chemical ingredients were used to produce steel plates of plate thicknesses of 6, 20, 40, and 100 mm under various production conditions. These were evaluated for, as strength, the yield stress and tensile strength of the matrix, as toughness, the Charpy impact absorption energy of the matrix, as the weld heat affected zone toughness in weldability, the Charpy impact absorption energy of the weld joint, and as the machineability, the drilling property. The chemical ingredients, plate thickness, X1, X2, ferrite fraction, ratio of micro Vickers hardness in a specific range, and Vlckers hardness of each of the steel plates are shown in Table 13 to Table 22, the production conditions (methods of production 4 to 7) are shown in Table 23 to Table 27, and the results of evaluation of properties are shown in Table 28 to Table 32.
The yield stress and the tensile strength were measured by the metal material tensile test method described in JIS Z 2241. The test piece was a metal material test piece described in JIS Z 2201. No. 5 test pieces taken from steel plates having plate thicknesses of 6 mm and 20 mm, while No. 10 test pieces taken from the t/4 parts of steel plates having thicknesses of 40 mm and 100 mm were used. The test pieces were taken so that their longitudinal directions became vertical to the rolling direction. The yield stress was made 0.2% of the yield strength calculated by the lower yield stress or offset method. Two tests were conducted at ordinary temperature and the average values were used.
The matrix toughness was measured by the metal material impact test method described in JIS Z 2242. The test pieces used were the metal material impact test pieces described in JIS Z 2202. For steel plates having thicknesses of 6 mm, subsize test pieces of widths of 5 mm were taken from the plate thickness center parts, for steel plates having thicknesses of 18 mm, test pieces of widths of 10 mm were taken from the plate thickness center parts, while for steel plates having thicknesses of 40 mm and 100 mm, test pieces of widths of 10 mm were taken from the t/4 parts. The shapes were all made V-notch test pieces. The test pieces were taken so that the lines formed by the notch bottoms became parallel to the plate thickness direction and so that the longitudinal directions of the test piece became perpendicular to the rolling direction. The test temperature was made −5° C. The average value of three tests was employed.
For the weld heat affected zone toughness, Charpy test pieces were taken from weld joints prepared by CO2 gas shield arc welding and submerge arc welding and measured for absorption energy at −5° C. The welding heat input was made 2 to 3 kJ/mm in the case of CO2 gas shield arc welding and was made 3 kJ/mm for plate thickness 6 mm materials, 5 kJ/mm for plate thickness 20 mm materials, and 7 kJ/mm for plate thickness 40 mm materials and 100 mm materials in the case of submerge arc welding. The test pieces were taken so that the locations 0.5 mm from the weld bond part corresponded to the Charpy test piece notch positions. The average value of three impact absorption energies was used.
The machineability was evaluated by a drilling test using a drilling machine and a high speed drill. The drilling distance was 42 mm in the case of plate thickness 6 mm steel plates piled up in seven layers, 40 mm in the case of plate thickness 20 mm steel plates piled up in two layers, 40 mm in the case of one plate thickness 40 mm steel plate, and 100 mm in the case of one plate thickness 100 mm steel plate for the test. The drill was used to make a through hole using a 6 mmφ diameter high speed drill SKH51. The rotational speed was 1610 rpm, the feed speed was 190 mm/min, and the machining oil was a water soluble machining oil. Under the above conditions, drilling was performed until drilling was no longer possible. The number of holes bored until the limit was reached was measured.
Invention Examples 2-1 to 2-19 are steel plates produced by first method of production of steel plates described in (2) of the present invention (method of production 4), that is, the method of speedily water cooling after rolling, and the method of production described in (9) of the present invention. Along with these, Comparative Examples 2-1 to 2-19 are also shown.
Invention Example 2-1 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling conditions, water cooling conditions, etc. to produce plate thickness 6 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-1 is similar to Invention Example 2-1 in ingredients and method of production, but X2 is outside the range of the present invention, so the machineability is inferior.
Invention Example 2-2 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling conditions, water cooling conditions, etc. to produce plate thickness 6 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-2 is similar to Invention Example 2-2 in ingredients and method of production, but the amount of Si is outside the range of the present invention, so the toughness and weld heat affected zone toughness are inferior.
Invention Example 2-3 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling conditions, water cooling conditions, etc. to produce plate thickness 6 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-3 is similar to Invention Example 2-3 in ingredients and method of production, the amount of Mn is outside the range of the present invention, so the weld heat affected zone toughness is inferior.
Invention Example 2-4 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling conditions, water cooling conditions, etc. to produce plate thickness 6 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-4 is similar to Invention Example 2-4 in ingredients and method of production, but the finish rolling first pass bite temperature and ferrite fraction are outside the range of the present invention, so the machineability is inferior.
Invention Example 2-5 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling conditions, water cooling conditions, etc. to produce plate thickness 20 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-5 is similar to Invention Example 2-5 in ingredients and method of production, but the amount of A1 and X2 are outside the range of the present invention, so the toughness is inferior.
Invention Example 2-6 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling conditions, water cooling conditions, etc. to produce plate thickness 20 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-6 is similar to Invention Example 2-6 in ingredients and method of production, but the amount of S is outside the range of the present invention, so the toughness and weld heat affected zone toughness are inferior.
Invention Example 2-7 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling conditions, water cooling conditions, etc. to produce plate thickness 20 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-7 is similar to Invention Example 2-7 in ingredients and method of production, but the amount of P is outside the range of the present invention, so the toughness and weld heat affected zone toughness are inferior.
Invention Example 2-8 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling conditions, water cooling conditions, etc. to produce plate thickness 20 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-8 is similar to Invention Example 2-8 in ingredients and method of production, but the finish rolling first pass bite temperature and the micro Vickers hardness are outside the range of the present invention, so the machineability is inferior.
Invention Example 2-9 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling conditions, water cooling conditions, etc. to produce plate thickness 40 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-9 is similar to Invention Example 2-9 in ingredients and method of production, but the amount of Mo and the X1 are outside the range of the present invention, so the weld heat affected zone toughness is inferior.
Invention Example 2-10 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling conditions, water cooling conditions, etc. to produce plate thickness 40 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-10 is similar to Invention Example 2-10 in ingredients and method of production, but the amount of Cr is outside the range of the present invention, so the weld heat affected zone toughness is inferior.
Invention Example 2-11 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling conditions, water cooling conditions, etc. to produce plate thickness 40 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-11 is similar to Invention Example 2-11 in ingredients and method of production, but the Vickers hardness and flow rate are outside the range of the present invention, so the strength is inferior.
Invention Example 2-12 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling conditions, water cooling conditions, etc. to produce plate thickness 40 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-12 is similar to Invention Example 2-12 in ingredients and method of production, but the amount of N is outside the range of the present invention, so the toughness and weld heat affected zone toughness are inferior.
Invention Example 2-13 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling conditions, water cooling conditions, etc. to produce plate thickness 100 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-13 is similar to Invention Example 2-13 in ingredients and method of production, but the total reduction rate of the finish rolling and the micro Vickers hardness are outside the range of the present invention, so the machineability is inferior.
Invention Example 2-14 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling conditions, water cooling conditions, etc. to produce plate thickness 100 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-14 is similar to Invention Example 2-14 in ingredients and method of production, but the amount of B and the X1 are outside the range of the present invention, so the toughness is inferior.
Invention Example 2-15 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling conditions, water cooling conditions, etc. to produce plate thickness 100 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-15 is similar to Invention Example 2-15 in ingredients and method of production, but the ferrite fraction and the total reduction rate of the rough rolling are outside the range of the present invention, so the machineability is inferior.
Invention Example 2-16 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling conditions, water cooling conditions, etc. to produce plate thickness 100 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-16 is similar to Invention Example 2-16 in ingredients and method of production, but the X2 is outside the range of the present invention, so the weld heat affected zone toughness is inferior.
Invention Example 2-17 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling conditions, water cooling conditions, etc. to produce plate thickness 20 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-17 is similar to Invention Example 2-17 in ingredients and method of production, but the water cooling end temperature and the Vickers hardness are outside the range of the present invention, so the strength is inferior.
Invention Example 2-18 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling conditions, water cooling conditions, etc. to produce plate thickness 20 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-18 is similar to Invention Example 2-18 in ingredients and method of production, but the flow rate and ferrite fraction are outside the range of the present invention, so the machineability is inferior.
Invention Example 2-19 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling conditions, water cooling conditions, etc. to produce plate thickness 20 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-19 is similar to Invention Example 2-19 in ingredients and method of production, but the amount of C and the X1 are outside the range of the present invention, so the toughness and the weld heat affected zone toughness are inferior.
Invention Examples 2-20 to 2-23 are steel plates produced by the second method of production of steel plates described in (2) of the present invention (method of production 5), that is, the method of reheating after the temperature of the steel plate falls after rolling, then water cooling, and the method of production described in (11) of the present invention. Along with these, Comparative Examples 2-20 to 2-23 are also shown.
Invention Example 2-20 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, water cooling conditions, etc. to produce plate thickness 6 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-20 is similar to Invention Example 2-20 in ingredients and method of production, but the reheating temperature and ferrite fraction are outside the range of the present invention, so the machineability is inferior.
Invention Example 2-21 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, water cooling conditions, etc. to produce plate thickness 20 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-21 is similar to Invention Example 2-21 in ingredients and method of production, but the water cooling end temperature after reheating and the Vickers hardness are outside the range of the present invention, so the strength is inferior.
Invention Example 2-22 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, water cooling conditions, etc. to produce plate thickness 40 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-22 is similar to Invention Example 2-22 in ingredients and method of production, but the amount of Nb and the reheating temperature are outside the range of the present invention, so the toughness, weld heat affected zone toughness, and machineability are inferior.
Invention Example 2-23 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, water cooling conditions, etc. to produce plate thickness 100 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-23 is similar to Invention Example 2-23 in ingredients and method of production, but the amount of Ti, cooling rate after reheating, and Vickers hardness are outside the range of the present invention, so the strength, toughness, weld heat affected zone toughness, and machineability are inferior.
Invention Examples 2-24 to 2-29 are steel plates produced by the third method of production of steel plates described in (2) of the present invention (method of production 6), that is, the method of air cooling until starting to form ferrite after rolling, then water cooling, and the method of production described in (12) of the present invention. Along with these, Comparative Examples 2-24 to 2-29 are also shown.
Invention Example 2-24 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling conditions, water cooling conditions, etc. to produce plate thickness 6 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-24 is similar to Invention Example 2-24 in ingredients and method of production, but the water cooling start temperature and the micro Vickers hardness are outside the range of the present invention, so the machineability is inferior.
Invention Example 2-25 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling conditions, water cooling conditions, etc. to produce plate thickness 20 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-25 is similar to Invention Example 2-25 in ingredients and method of production, but the amount of Zr, water cooling end temperature, and Vickers hardness are outside the range of the present invention, so the strength and toughness are inferior.
Invention Example 2-26 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability arc achieved by a method of production controlling the balance of alloying ingredients, rolling conditions, water cooling conditions, etc. to produce plate thickness 40 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-26 is similar to Invention Example 2-26 in ingredients and method of production, but the amount of V, water cooling start temperature, and Vickers hardness are outside the range of the present invention, so the strength, toughness, weld heat affected zone toughness, and machineability are inferior.
Invention Example 2-27 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling conditions, water cooling conditions, etc. to produce plate thickness 100 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-27 is similar to Invention Example 2-27 in ingredients and method of production, but the amount of Ni, X1, and the total reduction rate of the finish rolling are outside the range of the present invention, so the toughness and weld heat affected zone toughness are inferior.
Invention Example 2-28 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling conditions, water cooling conditions, etc. to produce plate thickness 20 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-28 is similar to Invention Example 2-28 in ingredients and method of production, but the flow rate and Vickers hardness are outside the range of the present invention, so the strength is inferior.
Invention Example 2-29 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling conditions, water cooling conditions, etc. to produce plate thickness 100 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-29 is similar to Invention Example 2-29 in ingredients and method of production, but the amount of Cu and the total reduction rate of the rough rolling are outside the range of the present invention, so the toughness and weld heat affected zone toughness are inferior.
Invention Examples 2-30 to 2-34 are steel plates produced by the fourth method of production of steel plates described in (2) of the present invention (method of production 7), that is, the method of heating until the two-phase region again after the temperature the steel plate falls after rolling and the method of production described in (13) of the present invention. Along with these, Comparative Examples 2-30 to 2-34 are also shown.
Invention Example 2-30 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling conditions, water cooling conditions, etc. to produce plate thickness 6 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-30 is similar to Invention Example 2-30 in ingredients and method of production, but the amount of RPM and reheating temperature are outside the range of the present invention, so the toughness and the machineability are inferior.
Invention Example 2-31 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling conditions, water cooling conditions, etc. to produce plate thickness 20 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-31 is similar to Invention Example 2-31 in ingredients and method of production, but the amount of Ca, the water cooling end temperature after reheating, and the Vickers hardness are outside the range of the present invention, so the strength and toughness are inferior.
Invention Example 2-32 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling conditions, water cooling conditions, etc. to produce plate thickness 40 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-32 is similar to Invention Example 2-32 in ingredients and method of production, but the amount of Mg, reheating temperature, and Vickers hardness are outside the range of the present invention, so the strength and toughness are inferior.
Invention Example 2-33 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling conditions, water cooling conditions, etc. to produce plate thickness 100 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-33 is similar to Invention Example 2-33 in ingredients and method of production, but the amount of V and the total reduction rate of the finish rolling are outside the range of the present invention, so the toughness, weld heat affected zone toughness, and machineability are inferior.
Invention Example 2-34 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling conditions, water cooling conditions, etc. to produce plate thickness 100 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-34 is similar to Invention Example 2-34 in ingredients and method of production, but the amount of Nb and the total reduction rate of the rough rolling are outside the range of the present invention, so the toughness, weld heat affected zone toughness, and machineability are inferior.
From the above examples, it is clear that the steel materials produced by the present invention, that is, the steel plates of Invention Examples 2-1 to 2-34, are steel materials having tensile strengths of 570 to 720 MPa or so and excellent in all of toughness, weldability, and machineability.
Examples of the steel plate described in (3) of the present invention will be explained next.
Test steel materials of various chemical ingredients were used to produce steel plates of plate thicknesses of 6, 18, 40, and 100 mm under various production conditions. These were evaluated for, as strength, the yield stress and tensile strength of the matrix, as toughness, the Charpy impact absorption energy of the matrix, and as the machineability, the drilling property. The chemical ingredients, plate thickness, X1, X2, ferrite fraction, Vickers hardness, and ratio of micro Vickers hardness in a specific range of each of the steel plates are shown in Table 33 to Table 40, the production conditions (methods of production 4′ to 7′) are shown in Table 41 to Table 44, and the results of evaluation of properties are shown in Table 45 to Table 48.
The yield stress and the tensile strength were measured by the metal material tensile test method described in JIS Z 2241. The test piece was a metal material test piece described in JIS Z 2201. No. 5 test pieces taken from steel plates having thicknesses of 6 mm and 18 mm, while No. 10 test pieces taken from the t/4 parts of steel plates having thicknesses of 40 mm and 100 mm were used. The test pieces were taken so that their longitudinal directions became vertical to the rolling direction. The yield stress was made 0.2% of the yield strength calculated by the lower yield stress or offset method. Two tests were conducted at ordinary temperature and the average values were used.
The matrix toughness was measured by the metal material impact test method described in JIS Z 2242. The test pieces used were the metal material impact test pieces described in JIS Z 2202. For steel plates having thicknesses of 6 mm, subsize test pieces of widths of 5 mm were taken from the plate thickness center parts, for steel plates having thicknesses of 18 mm, test pieces of widths of 10 mm were taken from the plate thickness center parts, while for steel plates having thicknesses of 40 mm and 100 mm, test pieces of widths of 10 mm were taken from the t/4 parts. The shapes were all made V-notch test pieces. The test pieces were taken so that the lines formed by the notch bottoms became parallel to the plate thickness direction and so that the longitudinal directions of the test piece became perpendicular to the rolling direction. The test temperature was made −5° C. The average value of three tests was employed.
The machineability was evaluated by a drilling test using a drilling machine and a high speed drill. The drilling distance was 42 mm in the case of plate thickness 6 mm steel plates piled up in seven layers, 36 mm in the case of plate thickness 18 mm steel plates piled up in two layers, 40 mm in the case of one plate thickness 40 mm steel plate, and 100 mm in the case of one plate thickness 100 mm steel plate for the test. The drill was used to make a through hole using a 6 mmφ diameter high speed drill SKH51. The rotational speed was 1610 rpm, the feed speed was 190 mm/min, and the machining oil was a water soluble machining oil. Under the above conditions, drilling was performed until drilling was no longer possible. The number of holes bored until the limit was reached was measured.
Invention Examples 3-1 to 3-14 are steel plates produced by the first method of production of steel plate described in (3) of the present invention (method of production 4′), that is, the method of water cooling speedily after rolling, and the method of production described in (14) of the present invention. Comparative Example 3-1 to 3-14 are also shown.
Invention Example 3-1 is plate thickness 18 mm steel plate produced by the first method of production (method of production 4′). It satisfies all of the requirements defined in the present invention, so has a 570 MPa or higher tensile strength and simultaneously exhibits an excellent machineability and good toughness. On the other hand, Comparative Example 3-1 is similar to Invention Example 3-1 in ingredients and method of production, but the amount of C and X1 are outside the range of the present invention, so the toughness is extremely low.
Invention Example 3-2 is plate thickness 18 mm steel plate produced by the first method of production (method of production 4′). It satisfies all of the requirements defined in the present invention, so has a 570 MPa or higher tensile strength and simultaneously exhibits an excellent machineability and good toughness. On the other hand, Comparative Example 3-2 is similar to Invention Example 3-2 in ingredients and method of production, but the finish rolling first pass bite temperature and the ferrite fraction are outside the range of the present invention, so the machineability is extremely low.
Invention Example 3-3 is plate thickness 18 mm steel plate produced by the first method of production (method of production 4′). It satisfies all of the requirements defined in the present invention, so has a 570 MPa or higher tensile strength and simultaneously exhibits an excellent machineability and good toughness. On the other hand, Comparative Example 3-3 is similar to Invention Example 3-3 in ingredients and method of production, but the X2 is outside the range of the present invention, so the machineability is extremely low.
Invention Example 3-4 is a plate thickness 6 mm steel plate produced by the first method of production (method of production 4′). It satisfies all of the requirements defined in the present invention, so has a 570 MPa or higher tensile strength and simultaneously exhibits an excellent machineability and good toughness. On the other hand, Comparative Example 3-4 is similar in ingredients and method of production with Invention Example 3-4, but the amount of Si is outside the range of the present invention, so the toughness is extremely low.
Invention Example 3-5 is a plate thickness 6 mm steel plate produced by the first method of production (method of production 4′). It satisfies all of the requirements defined in the present invention, so has a 570 MPa or higher tensile strength and simultaneously exhibits an excellent machineability and good toughness. On the other hand, Comparative Example 3-5 is similar in ingredients and method of production with Invention Example 3-5, but the finish rolling first pass bite temperature and the micro Vickers hardness are outside the range of the present invention, so the toughness and machineability are extremely low.
Invention Example 3-6 is a plate thickness 6 mm steel plate produced by the first method of production (method of production 4′). It satisfies all of the requirements defined in the present invention, so has a 570 MPa or higher tensile strength and simultaneously exhibits an excellent machineability and good toughness. On the other hand, Comparative Example 3-6 is similar to Invention Example 3-6 in ingredients and method of production, but the water cooling end temperature is outside the range of the present invention, so the strength is extremely low.
Invention Example 3-7 is plate thickness 40 mm steel plate produced by the first method of production (method of production 4′). It satisfies all of the requirements defined in the present invention, so has a 570 MPa or higher tensile strength and simultaneously exhibits an excellent machineability and good toughness. On the other hand, Comparative Example 3-7 is similar to Invention Example 3-7 in ingredients and method of production, but the amount of Mn and X2 are outside the range of the present invention, so the machineability is extremely low.
Invention Example 3-8 is plate thickness 40 mm steel plate produced by the first method of production (method of production 4′). It satisfies all of the requirements defined in the present invention, so has a 570 MPa or higher tensile strength and simultaneously exhibits an excellent machineability and good toughness. On the other hand, Comparative Example 3-8 is similar to Invention Example 3-8 in ingredients and method of production, but the amount of Mo is outside the range of the present invention, so the toughness is extremely low.
Invention Example 3-9 is plate thickness 40 mm steel plate produced by the first method of production (method of production 4′). It satisfies all of the requirements defined in the present invention, so has a 570 MPa or higher tensile strength and simultaneously exhibits an excellent machineability and good toughness. On the other hand, Comparative Example 3-9 is similar to Invention Example 3-9 in ingredients and method of production, but the amount of P is outside the range of the present invention, so the toughness is extremely low.
Invention Example 3-10 is plate thickness 100 mm steel plate produced by the first method of production (method of production 4′). It satisfies all of the requirements defined in the present invention, so has a 570 MPa or higher tensile strength and simultaneously exhibits an excellent machineability and good toughness. On the other hand, Comparative Example 3-10 is similar to Invention Example 3-10 in ingredients and method of production, but the total reduction rate of the finish rolling and the ferrite fraction are outside the range of the present invention, so the machineability is extremely low.
Invention Example 3-11 is plate thickness 100 mm steel plate produced by the first method of production (method of production 4′). It satisfies all of the requirements defined in the present invention, so has a 570 MPa or higher tensile strength and simultaneously exhibits an excellent machineability and good toughness. On the other hand, Comparative Example 3-11 is similar to Invention Example 3-11 in ingredients and method of production, but the amount of S is outside the range of the present invention, so the toughness is extremely low.
Invention Example 3-12 is plate thickness 100 mm steel plate produced by the first method of production (method of production 4′). It satisfies all of the requirements defined in the present invention, so has a 570 MPa or higher tensile strength and simultaneously exhibits an excellent machineability and good toughness. On the other hand, Comparative Example 3-12 is similar to Invention Example 3-12 in ingredients and method of production, but the amount of Al, total reduction rate of the rough rolling, and ferrite fraction are outside the range of the present invention, so the machineability and toughness are extremely low.
Invention Example 3-13 is a plate thickness 6 mm steel plate produced by the first method of production (method of production 4′). It satisfies all of the requirements defined in the present invention, so has a 570 MPa or higher tensile strength and simultaneously exhibits an excellent machineability and good toughness. On the other hand, Comparative Example 3-13 is similar to Invention Example 3-13 in ingredients and method of production, but the flow rate and Vickers hardness are outside the range of the present invention, so the strength is extremely low.
Invention Example 3-14 is a plate thickness 6 mm steel plate produced by the first method of production (method of production 4′). It satisfies all of the requirements defined in the present invention, so has a 570 MPa or higher tensile strength and simultaneously exhibits an excellent machineability and good toughness. On the other hand, Comparative Example 3-14 is similar to Invention Example 3-14 in ingredients and method of production, but the flow rate and ferrite fraction are outside the range of the present invention, so the machineability is extremely low.
Invention Examples 3-15 to 3-19 are steel plates produced by the second method of production of steel plate described in (3) of the present invention (method of production 5′), that is, the method of reheating when the temperature of the steel plate falls after rolling, then water cooling, and the method of production described in (16) of the present invention. Comparative Examples 3-15 to 3-19 are also shown.
Invention Example 3-15 is plate thickness 6 mm steel plate produced by the second method of production (method of production 5′). It satisfies all of the requirements defined in the present invention, so has a 570 MPa or higher tensile strength and simultaneously exhibits an excellent machineability and good toughness. On the other hand, Comparative Example 3-15 is similar to Invention Example 3-15 in ingredients and method of production, but the amount of N, the cooling rate of water cooling after reheating, and the micro Vickers hardness are outside the range of the present invention, so the machineability and toughness are extremely low.
Invention Example 3-16 is plate thickness 18 mm steel plate produced by the second method of production (method of production 5′). It satisfies all of the requirements defined in the present invention, so has a 570 MPa or higher tensile strength and simultaneously exhibits an excellent machineability and good toughness. On the other hand, Comparative Example 3-16 is similar to Invention Example 3-16 in ingredients and method of production, but the amount of C, cooling rate of water cooling after reheating, and Vickers hardness are outside the range of the present invention, so the strength is extremely low.
Invention Example 3-17 is plate thickness 40 mm steel plate produced by the second method of production (method of production 5′). It satisfies all of the requirements defined in the present invention, so has a 570 MPa or higher tensile strength and simultaneously exhibits an excellent machineability and good toughness. On the other hand, Comparative Example 3-17 is similar to Invention Example 3-17 in ingredients and method of production, but the amount of Cr, the end temperature of water cooling after reheating, and the Vickers hardness are outside the range of the present invention, so the strength and toughness are extremely low.
Invention Example 3-18 is plate thickness 100 mm steel plate produced by the second method of production (method of production 5′). It satisfies all of the requirements defined in the present invention, so has a 570 MPa or higher tensile strength and simultaneously exhibits an excellent machineability and good toughness. On the other hand, Comparative Example 3-18 is similar to Invention Example 3-18 in ingredients and method of production, but the reheating temperature and ferrite fraction are outside the range of the present invention, so the machineability is extremely low.
Invention Example 3-19 is plate thickness 18 mm steel plate produced by the second method of production (method of production 5′). It satisfies all of the requirements defined in the present invention, so has a 570 MPa or higher tensile strength and simultaneously exhibits an excellent machineability and good toughness. On the other hand, Comparative Example 3-19 is similar to Invention Example 3-19 in ingredients and method of production, but the amount of S and reheating temperature are outside the range of the present invention, so the toughness is extremely low.
Invention Examples 3-20 to 3-24 are steel plate produced by the third method of production of steel plate described in (3) of the present invention (method of production 6′), that is, the method of air cooling until the start of formation of ferrite after rolling, then water cooling, and the method of production described in (17) of the present invention. Comparative Examples 3-20 to 3-24 are also shown.
Invention Example 3-20 is plate thickness 6 mm steel plate produced by the third method of production (method of production 6′). It satisfies all of the requirements defined in the present invention, so has a 570 MPa or higher tensile strength and simultaneously exhibits an excellent machineability and good toughness. On the other hand, Comparative Example 3-20 is similar to Invention Example 3-20 in ingredients and method of production, but the water cooling start temperature and Vickers hardness are outside the range of the present invention, so the strength is extremely low.
Invention Example 3-21 is plate thickness 18 mm steel plate produced by the third method of production (method of production 6′). It satisfies all of the requirements defined in the present invention, so has a 570 MPa or higher tensile strength and simultaneously exhibits an excellent machineability and good toughness. On the other hand, Comparative Example 3-21 is similar to Invention Example 3-21 in ingredients and method of production, but the flow rate and Vickers hardness are outside the range of the present invention, so the strength is extremely low.
Invention Example 3-22 is plate thickness 40 mm steel plate produced by the third method of production (method of production 6′) It satisfies all of the requirements defined in the present invention, so has a 570 MPa or higher tensile strength and simultaneously exhibits an excellent machineability and good toughness. On the other hand, Comparative Example 3-22 is similar to Invention Example 3-22 in ingredients and method of production, but the amount of V and the total reduction rate in the finish rolling are outside the range of the present invention, so the toughness and machineability are extremely low.
Invention Example 3-23 is plate thickness 100 mm steel plate produced by the third method of production (method of production 6′). It satisfies all of the requirements defined in the present invention, so has a 570 MPa or higher tensile strength and simultaneously exhibits an excellent machineability and good toughness. On the other hand, Comparative Example 3-23 is similar to Invention Example 3-23 in ingredients and method of production, but the total reduction rate in the rough rolling, the water cooling end temperature, and the Vickers hardness are outside the range of the present invention, so the strength and toughness are extremely low.
Invention Example 3-24 is plate thickness 18 mm steel plate produced by the third method of production (method of production 6′). It satisfies all of the requirements defined in the present invention, so has a 570 MPa or higher tensile strength and simultaneously exhibits an excellent machineability and good toughness. On the other hand, Comparative Example 3-24 is similar to Invention Example 3-24 in ingredients and method of production, but the water cooling start temperature and ferrite fraction are outside the range of the present invention, so the machineability is extremely low.
Invention Examples 3-25 to 3-29 are steel plates produced by the fourth method of production of steel plate described in (3) of the present invention (method of production 7′), that is, the method of heating again to the two-phase region after the temperature of the steel plate falls after rolling, and the method of production described in (18) of the present invention. Comparative Examples 3-25 to 3-29 are also shown.
Invention Example 3-25 is plate thickness 6 mm steel plate produced by the fourth method of production (method of production 7′). It satisfies all of the requirements defined in the present invention, so has a 570 MPa or higher tensile strength and simultaneously exhibits an excellent machineability and good toughness. On the other hand, Comparative Example 3-25 is similar to Invention Example 3-25 in ingredients and method of production, but the X2, reheating temperature, and ferrite fraction are outside the range of the present invention, so the machineability is extremely low.
Invention Example 3-26 is plate thickness 18 mm steel plate produced by the fourth method of production (method of production 7′). It satisfies all of the requirements defined in the present invention, so has a 570 MPa or higher tensile strength and simultaneously exhibits an excellent machineability and good toughness. On the other hand, Comparative Example 3-26 is similar to Invention Example 3-26 in ingredients and method of production, but the Vickers hardness and reheating temperature are outside the range of the present invention, so the strength is extremely low.
Invention Example 3-27 is plate thickness 40 mm steel plate produced by the fourth method of production (method of production 7′). It satisfies all of the requirements defined in the present invention, so has a 570 MPa or higher tensile strength and simultaneously exhibits an excellent machineability and good toughness. On the other hand, Comparative Example 3-27 is similar to Invention Example 3-27 in ingredients and method of production, but the water cooling end temperature and the Vickers hardness are outside the range of the present invention, so the strength is extremely low.
Invention Example 3-28 is plate thickness 100 mm steel plate produced by the fourth method of production (method of production 7′). It satisfies all of the requirements defined in the present invention, so has 570 MPa or higher tensile strength and simultaneously exhibits an excellent machineability and good toughness. On the other hand, Comparative Example 3-28 is similar to Invention Example 3-28 in ingredients and method of production, but the amount of V and the total reduction rate of the finish rolling are outside the range of the present invention, so the toughness and machineability are extremely low.
Invention Example 3-29 is plate thickness 100 mm steel plate produced by the fourth method of production (method of production 7′). It satisfies all of the requirements defined in the present invention, so has a 570 MPa or higher tensile strength and simultaneously exhibits an excellent machineability and good toughness. On the other hand, Comparative Example 3-29 is similar to Invention Example 3-29 in ingredients and method of production, but the amount of B and the total reduction rate of the rough rolling are outside the range of the present invention, so the toughness is extremely low.
From the above examples, it is clear that the steel materials produced by the present invention, that is, the steel plates of Invention Examples 3-1 to 3-29, are steel materials having a tensile strength of 570 to 720 MPa or so, a high matrix toughness, and an excellent machineability.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2004-069407 | Mar 2004 | JP | national |
| 2004-069408 | Mar 2004 | JP | national |
| 2004-069406 | Mar 2004 | JP | national |
| 2005-059314 | Mar 2005 | JP | national |
| 2005-059315 | Mar 2005 | JP | national |
| 2005-059313 | Mar 2005 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP05/04849 | 3/11/2005 | WO | 9/7/2006 |
