Patent Application
20190211413
The present invention relates to a high-strength coated steel sheet which is used mainly as a material for automobile parts and a method for manufacturing the steel sheet. More specifically, the present invention relates to a high-strength coated steel sheet having high strength represented by yield strength of 550 MPa or more and excellent weldability.
Nowadays, for example, in the automobile industry, improving the fuel efficiency of automobiles to decrease the amount of carbon dioxide gas (CO2) emission continues to be an important issue to be addressed from the viewpoint of global environment conservation. Although decreasing the weight of automobile bodies is effective for improving the fuel efficiency of automobiles, it is necessary to decrease the weight of automobile bodies while maintaining satisfactory strength of the automobile bodies. It is possible to achieve weight reduction in the case where an automobile structure can be simplified to decrease the number of parts and the thickness of the material can be decreased by increasing the strength of a steel sheet which is used as a material for automobile parts.
However, in the case of a high-strength steel sheet having yield strength of 550 MPa or more where large amounts of alloy elements, which are necessary to increase strength, are typically added, there is a decrease in the toughness of a weld zone, in particular, the toughness of a heat-affected zone in the vicinity of a melt-solidified zone, which is called a nugget, when resistance spot welding is performed, often resulting in a fracture occurring in the weld zone at the time of an automobile collision, and, as a result, it is not possible to maintain satisfactory collision strength of the whole automobile body. Although various techniques have been proposed to date, none are directly intended to improve the strength of such a welded joint.
For example, Patent Literature 1 discloses a high-strength hot-dip coated steel sheet having a TS of 980 MPa or more which is excellent in terms of formability and impact resistance and a method for manufacturing the steel sheet. In addition, Patent Literature 2 discloses a high-strength hot-dip coated steel sheet having a TS: 590 MPa or more and excellent workability and a method for manufacturing the steel sheet. In addition, Patent Literature 3 discloses a high-strength hot-dip coated steel sheet having a TS of 780 MPa or more and excellent formability and a method for manufacturing the steel sheet. In addition, Patent Literature 4 discloses a high-strength cold-rolled steel sheet having excellent forming workability and weldability and a method for manufacturing the steel sheet. In addition, Patent Literature 5 discloses a high-strength thin steel sheet having a TS of 800 MPa or more which is excellent in terms of hydrogen embrittlement resistance, weldability, hole expansion formability, and ductility and a method for manufacturing the steel sheet.
In the case of the high-strength hot-dip coated steel sheet according to Patent Literature 1, it is difficult to achieve a high strength represented by yield strength of 550 MPa or more, and there is a decrease in the toughness of a heat-affected zone. Therefore, there is room for improvement in the torsional strength of a resistance spot weld zone under a condition of high-speed deformation.
In the case of the high-strength hot-dip coated steel sheet according to Patent Literature 2, since the steel has a microstructure including, in terms of area fraction, 30% or more and 90% or less of a ferrite phase, 3% or more and 30% or less of a bainite phase, and 5% or more and 40% or less of a martensite phase, it is difficult to achieve a high strength represented by yield strength of 550 MPa or more, and there is a decrease in the toughness of a heat-affected zone. Therefore, there is room for improvement in the torsional strength of a resistance spot weld zone under a condition of high-speed deformation.
In the case of the high-strength hot-dip coated steel sheet according to Patent Literature 3, it is difficult to achieve a high strength represented by yield strength of 550 MPa or more, and there is a decrease in the toughness of a heat-affected zone and the toughness of the heat-affected zone is deteriorated. Therefore, there is room for improvement in the torsional strength of a resistance spot weld zone under a condition of high-speed deformation.
In the case of a high-strength hot-dip coated steel sheet according to Patent Literature 4, Patent Literature 4 states that it is possible to obtain a steel sheet having excellent weldability by controlling a Ceq value to be 0.25 or less. However, although such a technique is effective in relation to conventional static tensile shear and peeling strength, it may be said that there is insufficient toughness in consideration of a configuration factor regarding a ferrite phase. Therefore, there is room for improvement in the torsional strength of a resistance spot weld zone under a condition of high-speed deformation.
In the case of a microstructure proposed in Patent Literature 5, since bainite and/or bainitic ferrite are included in a total amount of 34% to 97% in terms of area fraction, there is room for improvement in the torsional strength of a resistance spot weld zone under a condition of high-speed deformation.
As described above, in the case of all the conventional techniques, since there is a problem to be solved regarding the torsional strength of a resistance spot weld zone under the condition of high-speed deformation, and since, for example, there is a case where fracture is practically prevented by using stiffening members, it may now be said that there is an insufficient effect of weight reduction.
Aspects of the present invention are intended to advantageously solve the problems of the conventional techniques described above, and an object according to aspects of the present invention is to provide a high-strength coated steel sheet which has high strength represented by yield strength of 550 MPa or more and with which it is possible to form a resistance spot weld zone having high torsional strength under the condition of high-speed deformation and a method for manufacturing the steel sheet. Here, in accordance with aspects of the present invention, the term “excellent weldability” refers to high torsional strength under the condition of high-speed deformation.
To achieve the object described above, the present inventors eagerly conducted investigations regarding torsional strength of a resistance spot weld zone under the condition of high-speed deformation and, as a result, obtained the following knowledge by changing a microstructure, which has yet to be subjected to welding heat, to increase the toughness of a heat-affected zone.
(1) In the case where a torsion test is performed under the condition of high-speed deformation, a crack in a heat-affected zone is generated in a direction (in the thickness direction) perpendicular to the rolling direction.
(2) It is possible to inhibit a crack from being generated in such a direction by controlling a microstructure in a cross section in a thickness direction perpendicular to a rolling direction, as observed the cross section in the thickness direction perpendicular to the rolling direction, to be a microstructure including a martensite phase and a ferrite phase, in which a volume fraction of the martensite phase is 50% to 80%, in which a volume fraction of tempered martensite with respect to the whole martensite phase is 50% or more and 85% or less, in which an average grain diameter of the ferrite phase is 13 μm or less, and in which a volume fraction of ferrite grains having an aspect ratio of 2.0 or less with respect to the whole ferrite phase is 70% or more.
(3) In the case where a large number of ferrite grains elongated in a width direction exist in a parent phase of a heat-affected zone, since stress is concentrated at tips of the grains elongated in the width direction, voids tend to be generated when the tips of the grains are located adjacent to hard martensite or the like. Then, as a result of voids combining with each other, a crack is easily generated in a vicinity of a nugget. As a result, since a crack is generated in a direction (in the thickness direction) perpendicular to the rolling direction in a nugget in a torsion test under a condition of high-speed deformation, there is a decrease in strength. By forming the microstructure according to aspects of the present invention, since tempered martensite decreases a difference in hardness between hard martensite and soft ferrite, a void is less likely to be generated, which results in an increase in strength.
Aspects of the present invention have been completed on the basis of the knowledge described above, and, more specifically, aspects of the present invention provide the following.
[1] A high-strength coated steel sheet including a base steel sheet and a coating layer formed on a surface of the base steel sheet, the base steel sheet including a chemical composition containing, by mass %, C: 0.05% to 0.15%, Si: 0.01% to 1.80%, Mn: 1.8% to 3.2%, P: 0.05% or less, S: 0.02% or less, Al: 0.01% to 2.0%, one or more of B: 0.0001% to 0.005%, Ti: 0.005% to 0.04%, and Mo: 0.03% to 0.50%, and the balance being Fe and inevitable impurities, and a microstructure, as observed a cross section in a thickness direction perpendicular to a rolling direction, including a martensite phase and a ferrite phase, in which a volume fraction of the martensite phase is 50% to 80%, in which a volume fraction of tempered martensite with respect to the whole martensite phase is 50% or more and 85% or less, in which an average grain diameter of the ferrite phase is 13 μm or less, and in which a volume fraction of ferrite grains having an aspect ratio of 2.0 or less with respect to the whole ferrite phase is 70% or more, in which yield strength (YP) of the high-strength coated steel sheet is 550 MPa or more.
[2] The high-strength coated steel sheet according to item [1], in which the chemical composition further contains, by mass %, Cr: 1.0% or less.
[3] The high-strength coated steel sheet according to item [1] or [2], in which the chemical composition further contains, by mass %, one or more of Cu, Ni, Sn, As, Sb, Ca, Mg, Pb, Co, Ta, W, REM, Zn, Nb, V, Cs, and Hf in a total amount of 1% or less.
[4] A method for manufacturing a high-strength coated steel sheet, the method including a hot rolling process in which a steel slab having the chemical composition according to any one of items [1] to [3] is hot-rolled, in which the hot-rolled steel sheet is cooled at an average cooling rate of 10° C./s to 30° C./s, and in which the cooled steel sheet is coiled at a coiling temperature of 470° C. to 700° C., a cold rolling process in which the hot-rolled steel sheet obtained in the hot rolling process is cold-rolled, an annealing process in which the cold-rolled steel sheet obtained in the cold rolling process is heated to an annealing temperature range of 750° C. to 900° C., in which the heated steel sheet is held at the annealing temperature range for 30 seconds to 200 seconds, in which the steel sheet is subjected to reverse bending through rolls having a radius of 200 mm or more eight times or more in total during the holding, and in which the held steel sheet is cooled to a cooling stop temperature of 400° C. to 600° C. at an average cooling rate of 10° C./s or more, and a coating process in which the annealed steel sheet is subjected to a coating treatment and in which the coated steel sheet is cooled at an average cooling rate of 10° C./s to 25° C./s.
The high-strength coated steel sheet according to aspects of the present invention has yield strength of 550 MPa or more and is excellent in terms of high-speed torsional strength in a joint formed by performing resistance spot welding.
Hereafter, an embodiment of the present invention will be described. Here, the present invention is not limited to the embodiment described below.
The high-strength coated steel sheet according to aspects of the present invention has a base steel sheet and a coating layer formed on the surface of the base steel sheet.
The base steel sheet of the high-strength coated steel sheet according to aspects of the present invention has a chemical composition containing, by mass %, C: 0.05% to 0.15%, Si: 0.01% to 1.80%, Mn: 1.8% to 3.2%, P: 0.05% or less, S: 0.02% or less, Al: 0.01% to 2.0%, one or more of B: 0.0001% to 0.005%, Ti: 0.005% to 0.04%, and Mo: 0.03% to 0.50%, and the balance being Fe and inevitable impurities.
In addition, the chemical composition described above may further contain, by mass %, Cr: 1.0% or less.
In addition, the chemical composition described above may further contain, by mass %, one or more of Cu, Ni, Sn, As, Sb, Ca, Mg, Pb, Co, Ta, W, REM, Zn, Nb, V, Cs, and Hf in a total amount of 1% or less.
Hereafter, the constituents of the chemical composition described above will be described. “%” representing the contents of the constituents refers to “mass %”.
C: 0.05% to 0.15%
C is an element which is necessary to increase strength by forming martensite. In the case where the C content is less than 0.05%, since the effect of increasing strength caused by martensite is insufficient, it is not possible to achieve yield strength of 550 MPa or more. On the other hand, in the case where the C content is more than 0.15%, since a large amount of cementite is formed in a heat-affected zone, there is a decrease in toughness in a portion of the heat-affected zone where martensite is formed, which results in a decrease in strength in a torsion test under the condition of high-speed deformation. Therefore, the C content is set to be 0.05% to 0.15%. It is preferable that the lower limit of the C content be 0.06% or more, more preferably 0.07% or more, or even more preferably 0.08% or more. It is preferable that the upper limit of the C content be 0.14% or less, more preferably 0.12% or less, or even more preferably 0.10% or less.
Si: 0.01% to 1.80%
Si is an element which has a function of increasing the strength of a steel sheet through solid-solution strengthening. It is necessary that the Si content be 0.01% or more to stably achieve satisfactory yield strength. On the other hand, in the case where the Si content is more than 1.80%, since cementite is finely precipitated in martensite, there is a decrease in torsional strength under the condition of high-speed deformation. In addition, the upper limit of the Si content is set to be 1.80% to inhibit a crack from being generated in a heat-affected zone. It is preferable that the lower limit of the Si content be 0.50% or more, more preferably 0.60% or more, or even more preferably 0.90% or more. It is preferable that the upper limit of the Si content be 1.70% or less, more preferably 1.60% or less, or even more preferably 1.55% or less.
Mn: 1.8% to 3.2%
Mn is an element which has a function of increasing the strength of a steel sheet through solid-solution strengthening. Mn is an element which increases the strength of a material by forming martensite as a result of inhibiting, for example, ferrite transformation and bainite transformation. It is necessary that the Mn content be 1.8% or more to stably achieve satisfactory yield strength. On the other hand, in the case where the Mn content is large, cementite is formed when tempering is performed, and there is a decrease in toughness in a heat-affected zone, which results in a decrease in torsional strength under the condition of high-speed deformation. Therefore, the Mn content is set to be 3.2% or less. It is preferable that the upper limit of the Mn content be 2.8% or less.
P: 0.05% or Less
P decreases toughness as a result of being segregated at grain boundaries. Therefore, the P content is set to be 0.05% or less, preferably 0.03% or less, or more preferably 0.02% or less. Although it is preferable that the P content be as small as possible, it is preferable that the P content be 0.0001% or more in consideration of costs incurred to decrease the P content.
S: 0.02% or Less
S decreases toughness by combining with Mn to form coarse MnS. Therefore, it is preferable that the S content be decreased. In accordance with aspects of the present invention, the S content should be 0.02% or less, preferably 0.01% or less, or more preferably 0.002% or less. Although it is preferable that the S content be as small as possible, it is preferable that the S content be 0.0001% or more in consideration of costs incurred to decrease the S content.
Al: 0.01% to 2.0%
Since there is a decrease in toughness in the case where large amounts of oxides exist in steel, deoxidation is important. In addition, Al is effective for inhibiting the precipitation of cementite, and it is necessary that the Al content be 0.01% or more to realize such an effect. On the other hand, in the case where the Al content is more than 2.0%, since oxides and nitrides coagulate and are coarsened, there is a decrease in toughness. Therefore, the Al content is set to be 2.0% or less. It is preferable that the lower limit of the Al content be 0.03% or more, more preferably 0.04% or more, or even more preferably 0.05% or more. It is preferable that the upper limit of the Al content be 0.10% or less, more preferably 0.08% or less, or even more preferably 0.06% or less.
As described above, the chemical composition described above contains one or more of B: 0.0001% to 0.005%, Ti: 0.005% to 0.04%, and Mo: 0.03% to 0.50%.
B: 0.0001% to 0.005%
B is an element which is necessary to increase toughness by strengthening grain boundaries. It is necessary that the B content be 0.0001% or more to realize such an effect. On the other hand, in the case where the B content is more than 0.005%, B decreases toughness by forming Fe23(CB)6. Therefore, the B content is limited to be in the range of 0.0001% to 0.005%. It is preferable that the lower limit of the B content be 0.0005% or more, more preferably 0.0010% or more, or even more preferably 0.0015% or more. It is preferable that the upper limit of the B content be 0.004% or less or more preferably 0.003% or less.
Ti: 0.005% to 0.04%
Ti brings out the effect of B by inhibiting the formation of BN as a result of combining with N to form nitrides, and Ti increases toughness by decreasing the diameter of crystal grains as a result of forming TiN. It is necessary that the Ti content be 0.005% or more to realize such effects. On the other hand, in the case where the Ti content is more than 0.04%, such effects become saturated, and it is difficult to stably manufacture a steel sheet due to an increase in rolling load. Therefore, the Ti content is limited to be in a range of 0.005% to 0.04%. It is preferable that the lower limit of the Ti content be 0.010% or more, or more preferably 0.020% or more. It is preferable that the upper limit of the Ti content be 0.03% or less.
Mo: 0.03% to 0.50%
Mo is an element which further increases the effect according to aspects of the present invention. Mo increases the toughness of a heat-affected zone by preventing the formation of cementite and coarsening of crystal grains in the heat-affected zone. It is necessary that the Mo content be 0.03% or more. On the other hand, in the case where the Mo content is more than 0.50%, since Mo carbides are precipitated, there is conversely a decrease in toughness. Therefore, the Mo content is limited to be in a range of 0.03% to 0.50%. In addition, by controlling the Mo content to be within the range described above, since it is also possible to inhibit lowering of the liquid-metal embrittlement of a welded joint, it is possible to increase the strength of the joint. It is preferable that the lower limit of the Mo content be 0.08% or more, more preferably 0.09% or more, or even more preferably 0.10% or more. It is preferable that the upper limit of the Mo content be 0.40% or less, more preferably 0.35% or less, or even more preferably 0.30% or less.
As described above, the chemical composition according to aspects of the present invention may contain the elements below as optional constituents.
Cr: 1.0% or Less
Cr is an element which is effective for inhibiting temper embrittlement. Therefore, the addition of Cr further increases the effects according to aspects of the present invention. It is preferable that the Cr content be 0.01% or more to realize such an effect. However, in the case where the Cr content is more than 1.0%, since Cr carbides are formed, there is a decrease in the toughness of a heat-affected zone. Therefore, it is preferable that the Cr content be 1.0% or less, more preferably 0.5% or less, or even more preferably 0.1% or less.
In addition, one or more of Cu, Ni, Sn, As, Sb, Ca, Mg, Pb, Co, Ta, W, REM, Zn, Nb, V, Cs, and Hf may be added in a total amount of 1% or less, preferably 0.1% or less, or even more preferably 0.03% or less. In addition, the constituents other than those described above are Fe and inevitable impurities.
The remainder is Fe and inevitable impurities. When the B content is less than 0.0001%, the Ti content is less than 0.005%, or the Mo content is less than 0.03% in the case where at least one of the B content, the Ti content, and the Mo content is within a range according to aspects of the present invention, the elements having contents less than their lower limits are regarded as being contained as inevitable impurities.
Although the chemical composition is described above, controlling only the chemical composition to be within the range described above is not sufficient for realizing the intended effects according to aspects of the present invention, that is, controlling the microstructure of steel (microstructure) is also important. The conditions applied for controlling the microstructure will be described hereafter. Here, the configuration of the microstructure described below is that which is viewed in a cross section in the thickness direction perpendicular to the rolling direction. In addition, volume fraction, average grain diameter, and aspect ratio are determined by using the methods described in EXAMPLES below.
Volume Fraction of Martensite Phase: 50% to 80%
A martensite phase is a hard phase and has a function of increasing the strength of a steel sheet through transformation microstructure strengthening. In addition, it is necessary that the volume fraction of a martensite phase be 50% or more, preferably 53% or more, or more preferably 56% or more to achieve yield strength of 550 MPa or more. On the other hand, in the case where the volume fraction is more than 80%, since voids generated at the interface between a martensite phase and other phases are locally concentrated, there is a decrease in the toughness of a heat-affected zone. Therefore, the volume fraction is set to be 80% or less, preferably 79% or less, more preferably 75% or less, or even more preferably 70% or less.
Area Fraction of Tempered Martensite with Respect to Whole Martensite Phase: 50% or More and 85% or Less
Tempered martensite, whose hardness is lower than that of as-quenched martensite, is capable of decreasing the difference in hardness between hard as-quenched martensite and soft ferrite. In the case where the volume fraction of tempered martensite is within the range described above, since a void is less likely to be generated in a torsion test under the condition of high-speed deformation, there is an increase in strength. Therefore, the volume fraction of tempered martensite in martensite is set to be 50% or more, preferably 53% or more, or more preferably 56% or more. In addition, in the case where the volume fraction of tempered martensite in martensite is excessively large, there is a decrease in yield strength. Therefore, the volume fraction of tempered martensite in martensite is set to be 85% or less, preferably 75% or less, or more preferably 65% or less.
The steel microstructure according to aspects of the present invention includes a ferrite phase in addition to a martensite phase. It is preferable that the volume fraction of a ferrite phase be 30% or more, more preferably 32% or more, or even more preferably 34% or more to increase the toughness of a heat-affected zone by inhibiting voids from being locally concentrated in the vicinity of martensite. In addition, it is preferable that the volume fraction be 50% or less, more preferably 45% or less, or even more preferably 40% or less to achieve satisfactory yield strength.
In addition, other phases such as cementite, pearlite, a bainite phase, and a retained austenite phase may be included in addition to a martensite phase and a ferrite phase. The total volume fraction of such other phases should be 8% or less.
Average Grain Diameter of Ferrite Phase: 13 μm or Less
In the case where the average grain diameter of a ferrite phase is more than 13 μm, there is a decrease in the strength of a steel sheet, and there is a decrease in toughness due to low-toughness ferrite which has been subjected to aging caused by a thermal influence. In addition, there is a decrease in the strength of a weld zone due to grain growth in a heat-affected zone (HAZ). Therefore, the average grain diameter of a ferrite phase is set to be 13 μm or less. It is preferable that the lower limit of the average grain diameter be 3 μm or more, more preferably 5 μm or more, or even more preferably 7 μm or more. It is preferable that the upper limit of the average grain diameter be 12 μm or less, more preferably 11 μm or less, or even more preferably 10 μm or less.
Here, the above-described average grain diameter of a ferrite phase is determined by etching a portion located at ¼ of the thickness in a cross section (C-cross section) in the thickness direction perpendicular to the rolling direction with a 1-vol % nital solution to expose the microstructure, by taking photographs in 10 fields of view by using a scanning electron microscope (SEM) at a magnification of 1000 times, and by using a cutting method in accordance with ASTM E 112-10.
Volume Fraction of Ferrite Grains Having an Aspect Ratio of 2.0 or Less with Respect to Whole Ferrite Phase: 70% or More
In, the case where the aspect ratios of a large number of ferrite grains are more than 2.0, because the grain growth in the thickness direction is stopped by the pinning effect of precipitates, the grains become flattened in shape through thermal influence, which results in a decrease in toughness. Here, the lower limit of the aspect ratio of ferrite grains formed in accordance with aspects of the present invention is substantially 0.8. In accordance with aspects of the present invention, the volume fraction of ferrite grains having an aspect ratio of 2.0 or less with respect to the whole ferrite phase is set to be 70% or more to increase toughness.
The aspect ratios of ferrite grains are determined by etching a portion located at ¼ of the thickness in a cross section (C-cross section) in the thickness direction perpendicular to the rolling direction with a 1-vol % nital solution to expose the microstructure, by taking photographs in 10 fields of view by using a scanning electron microscope (SEM) at a magnification of 1000 times, and by calculating the ratio of the length in the width direction (C-direction) to the length in the thickness direction as an aspect ratio.
The base steel sheet having the chemical composition and the microstructure described above has a coating layer on a surface thereof. It is preferable that the coating layer be a zinc coating layer, or more preferably a galvanizing layer or a galvannealed layer. Here, the coating layer may be composed of a metal other than zinc.
The high-strength coated steel sheet according to aspects of the present invention has yield strength of 550 MPa or more or preferably 600 MPa or more. Although there is no particular limitation on the upper limit of the yield strength, the upper limit is 800 MPa or less in many cases.
The high-strength coated steel sheet according to aspects of the present invention is excellent in terms of weldability. Specifically, in the case of such a steel sheet, the crack length, which is determined by using the method described in EXAMPLES below, is 50 μm or less (including a case where no crack is generated).
It is preferable that the tensile strength of the high-strength coated steel sheet according to aspects of the present invention be 950 MPa or more, or more preferably 1000 MPa or more, although this is not indispensable for achieving the object according to aspects of the present invention. The upper limit of the tensile strength is 1200 MPa or less in many cases.
It is preferable that the elongation of the high-strength coated steel sheet according to aspects of the present invention be 14.0% or more, or more preferably 16.0% or more, although this is not indispensable for achieving the object according to aspects of the present invention. The upper limit of the elongation is 22.0% or less in many cases.
Hereafter, the method for manufacturing the high-strength coated steel sheet according to aspects of the present invention will be described. The method for manufacturing the high-strength coated steel sheet according to aspects of the present invention includes a hot rolling process, a cold rolling process, an annealing process, and a coating process. Hereafter, these processes will be described.
The hot rolling process is a process in which a steel slab having the chemical composition is hot-rolled, in which the hot-rolled steel sheet is cooled at an average cooling rate of 10° C./s to 30° C./s, and in which the cooled steel sheet is coiled at a coiling temperature of 470° C. to 700° C.
In accordance with aspects of the present invention, there is no particular limitation on the method used for preparing molten steel for a steel raw material (steel slab), and a known method such as one which utilizes a converter or an electric furnace may be used. In addition, after having prepared molten steel, although it is preferable that a steel slab be manufactured by using a continuous casting method from a viewpoint of problems such as segregation, a slab may be manufactured by using a known casting method such as an ingot casting-slabbing method or a thin-slab continuous casting method. Here, when hot rolling is performed on the cast slab, rolling may be performed after the slab has been reheated in a heating furnace, or hot direct rolling may be performed without heating the slab in the case where the slab has a temperature equal to or higher than a predetermined temperature.
The steel raw material obtained as described above is subjected to hot rolling which includes rough rolling and finish rolling. In accordance with aspects of the present invention, it is preferable that carbides in the steel raw material be dissolved before rough rolling is performed. In the case where the slab is heated, it is preferable that the slab be heated to a temperature of 1100° C. or higher to dissolve carbides and to prevent an increase in rolling load. In addition, it is preferable that the slab heating temperature be 1300° C. or lower to prevent an increase in the amount of scale loss. In addition, as described above, in the case where the steel raw material which has yet to be subjected to rough rolling has a temperature equal to or higher than a predetermined temperature and where carbides in the steel raw material are dissolved, a process in which the steel raw material which has yet to be subjected to rough rolling is heated may be omitted. Here, it is not necessary to put a particular limitation on the conditions applied for rough rolling and finish rolling.
Average Cooling Rate of Cooling after Hot Rolling: 10° C./s to 30° C./s
After hot rolling has been performed, in the case where the average cooling rate to a coiling temperature is less than 10° C./s, since ferrite grains do not grow, the aspect ratio tends to be more than 2.0 so that there is a decrease in “the volume fraction of ferrite grains having an aspect ratio of 2.0 or less with respect to the whole ferrite phase” described above, which results in a decrease in the toughness of a heat-affected zone. On the other hand, in the case where the average cooling rate is more than 30° C./s, since ferrite grains grow excessively, there is a decrease in strength. Therefore, the average cooling rate is set to be 10° C./s to 30° C./s. It is preferable that the lower limit of the above-described average cooling rate be 15° C./s or more. It is preferable that the upper limit of the above-described average cooling rate be 25° C./s or less. Here, it is preferable that a cooling start temperature, that is, a finishing delivery temperature, be 850° C. to 980° C., because this results in ferrite grains in the hot-rolled steel sheet growing uniformly and having the desired aspect ratio.
Coiling Temperature: 470° C. to 700° C.
In the case where the coiling temperature is lower than 470° C., since low-temperature-transformation phases such as bainite are formed, softening occurs in a heat-affected zone. On the other hand, in the case where the coiling temperature is higher than 700° C., since there is an excessive coarsening in ferrite grain diameter, there is a decrease in the toughness of a heat-affected zone. Therefore, the coiling temperature is set to be 470° C. to 700° C. It is preferable that the lower limit of the coiling temperature be 500° C. or higher. It is preferable that the upper limit of the coiling temperature be 600° C. or lower.
In the cold rolling process, cold rolling is performed on the hot-rolled steel sheet obtained in the hot rolling process described above. Although there is no particular limitation on the rolling reduction ratio of cold rolling, the rolling reduction ratio is usually 30% to 60%. Here, cold rolling may be performed after pickling has been performed, and, in this case, there is no particular limitation on the conditions applied for pickling.
An annealing process is performed on the cold-rolled steel sheet obtained in the cold rolling process described above. Specific conditions applied for the annealing process are as follows.
Annealing Condition: Holding at an Annealing Temperature of 750° C. to 900° C. for 30 Seconds to 200 Seconds
It is necessary that annealing be performed by holding the cold-rolled steel sheet at an annealing temperature of 750° C. to 900° C. for 30 seconds to 200 seconds to form a microstructure in which the average grain diameter of the ferrite phase is 13 μm or less and in which the volume fraction of ferrite grains having an aspect ratio of 2.0 or less with respect to the whole ferrite phase is 70% or more. In the case where the annealing temperature is lower than 750° C. or the holding time is less than 30 seconds, since the progress of recovery is delayed, it is not possible to achieve the desired aspect ratio. On the other hand, in the case where the annealing temperature is higher than 900° C., since there is an increase in the volume fraction of martensite, there is a decrease in the toughness of a heat-affected zone. In addition, in the case where the annealing time is more than 200 seconds, there may be a decrease in the ductility due to a large amount of iron carbides being precipitated. Therefore, the annealing temperature is set to be 750° C. to 900° C. or preferably 800° C. to 900° C., and the holding time is set to be 3.0 seconds to 200 seconds or preferably 50 seconds to 150 seconds. Here, there is no particular limitation on the conditions applied for heating to the annealing temperature range described above.
Reverse Bending Through Rolls Having a Radius of 200 mm or More During the Holding Described Above: Eight Times or More in Total
In the case where a large number of ferrite grains have an aspect ratio of more than 2.0 such that “the volume fraction of ferrite grains having an aspect ratio of 2.0 or less with respect to the whole ferrite phase” described above is out of the desired range, there is a decrease in toughness. To control “the volume fraction of ferrite grains having an aspect ratio of 2.0 or less with respect to the whole ferrite phase” described above to be within the desired range, it is necessary to grow the grains during annealing. For this purpose, in the holding in the annealing temperature range described above, it is necessary to perform reverse bending through rolls having a radius of 200 mm or more eight times or more in total. It is considered that, in the case where rolls having a radius of less than 200 mm are used, since there is an increase in the amount of bending strain, there is an increase in the amount of elongation of a steel sheet, which results in a tendency for ferrite grains to have an aspect ratio of more than 2.0. Therefore, the radius of the rolls is set to be 200 mm or more. In addition, in the case where the number of times of reverse bending is less than 8, ferrite grains tend to have an aspect ratio of more than 2.0. Therefore, the number of times of reverse bending is set to be 8 or more, or preferably 9 or more. Here, in the case where there is an increase in the amount of bending strain, there is a decrease in the toughness of a heat-affected zone. Therefore, it is preferable that the number of times of reverse bending be 15 or less. Here, the expression “the number of times of reverse bending is 8 or more in total” refers to a case where the sum of the number of times of bending and the number of times of unbending is 8 or more. Now, the term “reverse bending” means “bending in one direction, and bending in the opposite direction repeatedly”.
Average Cooling Rate of Cooling Performed after Holding in the Annealing Temperature Range: 10° C./s or More
In the case where the average cooling rate is less than 10° C./s, since ferrite grains are coarsened, there is a decrease in strength and the toughness of a heat-affected zone. Therefore, the average cooling rate is set to be 10° C./s or more. In the case where the cooling rate is excessively increased, it is not possible to achieve the desired aspect ratio. Therefore, it is preferable that the average cooling rate be 30° C./s or less.
Cooling Stop Temperature of Cooling after Holding in the Annealing Temperature Range: 400° C. to 600° C.
In the case where the cooling stop temperature is lower than 400° C., since it is not possible to achieve the desired volume fraction of a martensite phase, there is a decrease in strength. On the other hand, in the case where the cooling stop temperature is higher than 600° C., since ferrite grains grow, there is a decrease in strength and the toughness of a heat-affected zone. Therefore, the cooling stop temperature described above is set to be 400° C. to 600° C.
A coating process in which a coating treatment described below is performed is performed after the annealing process described above has been performed. There is no particular limitation on the kind of the coating treatment, and an electroplating treatment or a hot-dip plating treatment may be performed. An alloying treatment may be performed after a hot-dip plating treatment has been performed. It is preferable that a galvanizing treatment or a galvannealing treatment, in which an alloying treatment is performed after a galvanizing treatment has been performed, be performed.
Average Cooling Rate after the Coating Treatment: 10° C./s to 25° C./s
Controlling the average cooling rate after the coating treatment has been performed is important for forming tempered martensite. In the case where the average cooling rate is less than 10° C./s, since a large amount of tempered martensite is formed, it is not possible to achieve the desired yield strength. On the other hand, in the case where the average cooling rate is more than 25° C./s, since the volume fraction of tempered martensite formed is 50% or less, there is a decrease in the toughness of a heat-affected zone. Therefore, the average cooling rate is set to be 10° C./s to 25° C./s.
High-strength coated steel sheets were manufactured by performing a hot rolling process, a cold rolling process, an, annealing process, and a coating process on slabs having the chemical compositions given in Table 1 under the conditions given in Table 2. In addition, the methods used for microstructure observation and property evaluation were as follows.
E
0.034
F
0.185
J
2.02
K
M
1.51
N
3.64
35
20
240
11
430
12
750
13
14
17
720
18
950
20
21
30
22
23
25
350
26
650
27
28
(1) Microstructure Observation
A cross-section in the thickness direction perpendicular to the rolling direction of the obtained steel sheet was polished and etched with a 1-vol % nital solution to expose a microstructure. By using a scanning electron microscope at a magnification of 1000 times, images were obtained in 10 fields of view in a region from the surface to a ¼t position. “t” denotes the thickness of a steel sheet, that is, a steel sheet thickness. The area fraction of each of the constituent phases was determined by using the images obtained as described above, and the determined area fraction was defined as the volume fraction of the constituent phase. A ferrite phase is a microstructure having a grain in which an etching mark or an iron-based carbide is not observed. As-quenched martensite phase is a microstructure having a grain in which no carbide is observed and which is observed to be white. A tempered martensite phase is a microstructure having a grain in which a large number of fine iron-based carbides and corrosion marks are observed. The area fraction of a martensite phase described above was defined as the volume fraction of a martensite phase. Here, as other phases, a bainite phase, a pearlite phase, and retained austenite phase were observed.
The average grain diameter of a ferrite phase was determined by using the above-described sample used for determining the volume fraction, by using a scanning electron microscope (SEM) at a magnification of 1000 times to obtain images in 10 fields of view, and by using a cutting method in accordance with ASTM E 112-10. The calculated average grain diameter of a ferrite phase is given in Table 3.
The aspect ratio of ferrite grains was determined by using the above-described sample used for determining the volume fraction, by using a scanning electron microscope (SEM) at a magnification of 1000 times to obtain images of the exposed microstructure which was prepared by performing etching using a 1-vol % nital solution in 10 fields of view, and by defining the ratio of the length in the width direction (C-direction) to the length in the thickness direction as an aspect ratio. The volume fraction of ferrite grains having an aspect ratio of 2.0 with respect to the whole ferrite phase was calculated by calculating the total volume fraction of ferrite grains having an aspect ratio of 2.0 and by using the volume fraction of a ferrite phase determined as described above.
(2) Tensile Property
By performing a tensile test five times in accordance with JIS Z 2241 on a JIS No. 5 tensile test piece in accordance with JIS Z 2201 whose longitudinal direction (tensile direction) was a direction perpendicular to the rolling direction, average yield strength (YP), tensile strength (TS), and butt elongation (EL) were determined. The results are given in Table 3.
(3) Torsion Test Under Condition of High-Speed Deformation
A test piece was prepared by overlapping two steel sheets, across the full width thereof as illustrated in
45
68
45
43
17
50
14
55
68
14
50
11
48
45
14
60
12
45
17
55
13
40
16
50
510
14
55
17
50
18
88
60
20
90
540
21
30
22
15
62
23
45
14
60
520
25
44
48
68
530
26
48
16
56
27
45
14
60
540
28
68
| Number | Date | Country | Kind |
|---|---|---|---|
| 2016-193564 | Sep 2016 | JP | national |
This is the U.S. National Phase application of PCT/JP2017/035100, filed Sep. 28, 2017, which claims priority to Japanese Patent Application No. 2016-193564, filed Sep. 30, 2016, the disclosures of these applications being incorporated herein by reference in their entireties for all purposes.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2017/035100 | 9/28/2017 | WO | 00 |
