Patent Application
20200230681
The present invention relates to a hot stamped body used for structural members or reinforcing members of automobiles or structures where strength is required, in particular a hot stamped body excellent in strength, impact resistance, ductility, and hydrogen embrittlement resistance after hot stamping and small in scattering in hardness.
In recent years, from the viewpoints of environmental protection and resource saving, lighter weight of automobile bodies is being sought. For this reason, application of high strength steel sheet to automobile members has been accelerating. However, along with the increase in strength of steel sheets, the formability deteriorates, and therefore in high strength steel sheets, formability into members with complicated shapes is a problem.
To solve this problem, hot stamping, where the steel sheet is heated to a high temperature of the austenite region, then is press-formed, is increasingly being applied. Since hot stamping performs press-forming and simultaneously quenching in the die, it is possible to obtain a strength corresponding to the C amount of the steel sheet. This is being taken note of as a technique achieving both formation of a material into an automobile member and securing strength.
However, since in conventional hot pressed parts which were produced by press hardening, the entire sheet thickness is formed by hard structures (mainly martensite), if bending deformation occurs at the time of collision of the automobile, the largest strain will be applied to the bent portion of the part, cracks will advance starting from the vicinity of the surface layer of the steel sheet, and finally fracture will easily be caused.
For example, in a conventional hat-shaped member or other hot stamped body produced by press hardening, if bending deformation occurs at the time of collision of an automobile, the hat-shaped member will buckle and thereby deformation will become localized and the load resistance of the member will fall. That is, the maximum load of a member of a hot stamped body is affected not only by the strength of the member, but also the ease of buckling. If the ductility of the steel sheet is high, in the state of a member formed into a certain shape, it becomes harder for localization of the deformation region to occur. That is, the member becomes resistant to buckling.
Further, in a hot stamped body, the way of contact with the die is not necessarily uniform. For example, at the vertical wall parts of a hat-shaped member etc., the cooling rate easily falls. For this reason, steel sheet is sometimes locally formed with regions with low hardnesses. Deformation concentrates in a local soft part at the time of collision and becomes a cause of cracking, so a small scattering in hardness of the body, that is, securing stable strength, is important in securing impact resistance.
Therefore, in a hot stamped part as well, ductility is important, but in general the ductility of martensite is low. Further, the density of lattice defects of the surface layer of the steel sheet is high, so there is the problem that penetration by hydrogen is promoted and the part becomes poor in hydrogen embrittlement resistance. Due to such reasons, hot stamped parts produced by press hardening have been limited in locations of use in auto parts.
To deal with this problem, art has been proposed for raising the deformability of hot pressed parts to suppress cracking. PTL 1 discloses making the hardness of the middle in sheet thickness of a hot pressed part 400 Hv or more and forming a softened layer with a thickness of 20 μm to 200 μm and a hardness of 300 Hv or less on a surface layer so as to secure a strength of a tensile strength of 1300 MPa or more while suppressing cracking at the time of automobile collision. PTL 2 discloses controlling the concentration of carbon at a surface layer in sheet thickness to ⅕ or less of the concentration of carbon of the middle part in sheet thickness so as to reduce the density of lattice defects of the surface layer and improve the hydrogen embrittlement resistance. PTL 3 discloses to make the middle part in sheet thickness a dual phase structure of ferrite and martensite and raise the structural fraction of ferrite of a surface layer portion so as to ease the stress even if the surface layer part receives severe bending deformation.
However, in the members described in PTL 1 and PTL 2, by making a surface layer portion in sheet thickness by soft structures and making a middle part in sheet thickness by hard structures, a sharp gradient in hardness ends up being formed in the sheet thickness direction. For this reason, when subjected to bending deformation, there is the issue that cracking easily occurs near the boundary between the soft structures and hard structures where this sharp gradient of hardness occurs. Further, in PTL 3, a surface layer portion in sheet thickness is made by soft structures and the middle part in sheet thickness is made by a dual phase structure of hard structures and soft structures so as to reduce the sharp gradient in hardness in the sheet thickness direction. However, since making the middle part in sheet thickness a dual phase structure, the upper limit of tensile strength ends up becoming 1300 MPa or so. It is difficult to secure the tensile strength of 1500 MPa or more sought for hot pressed parts.
[PTL 1] Japanese Unexamined Patent Publication No. 2015-30890
[PTL 2] Japanese Unexamined Patent Publication No. 2006-104546
[PTL 3] WO 2015/097882
The present invention, in consideration of the technical issues in the prior art, has as its object to provide a hot stamped body achieving both a high bendability and high ductility for realizing impact resistance and hydrogen embrittlement resistance and keeping down the scattering in hardness.
The inventors engaged in an in-depth study of a method for solving the above technical issues. As a result, to improve the hydrogen embrittlement resistance, it is effective to reduce the density of lattice defects at the surface layer in sheet thickness. For this reason, it is necessary to form soft structures at the surface layer. On the other hand, to secure a 1500 MPa or more tensile strength, it is necessary to form the middle part in sheet thickness by only hard structures. In this way, the inventors thought that if forming the surface layer in sheet thickness by soft structures and forming the middle part in sheet thickness by hard structures, if it were possible to reduce the sharp gradient of hardness in the sheet thickness direction occurring near the boundary of the hard structures and soft structures, a strength of a tensile strength of 1500 MPa or more and excellent hydrogen embrittlement resistance could be secured while excellent bendability could be obtained.
Therefore, the inventors investigated and engaged in intensive studies on metal structures of steel sheets where good bendability was obtained by controlling the structures of a surface layer of soft structures. As a result, it was discovered that the metal structures forming the surface layer should be comprised of crystal grains with a maximum crystal orientation difference inside the crystal grains of 1° or less and crystal grains with a maximum crystal orientation difference inside the crystal grains of 8° or more and less than 15° when a region surrounded by grain boundaries having an orientation difference of 15° or more in the sheet thickness cross-section is defined as a “crystal grain”. These measurements were performed in the region from a position of a depth of 20 μm below the surface of the surface layer to a position of a depth of ½ of the thickness of the surface layer (center of surface layer). It was discovered that the effects of the surface properties of the hot stamped body and the effects of the transitional part from the middle part in sheet thickness to the surface layer can be eliminated by such metal structures.
Further, by controlling the amounts of addition of Mn and Si at the middle part in sheet thickness, the inventors raised the ductility and raised the hardenability to stably secure high strength. As a result, it is possible to keep down the occurrence of cracking at the time of bending deformation. The inventors succeeded in securing a 1500 MPa or more tensile strength and good hydrogen embrittlement resistance while realizing excellent bendability, ductility, and stability of strength and were able to obtain a hot stamped body excellent in impact resistance and hydrogen embrittlement resistance.
The present invention was completed based on the above discovery and has as its gist the following:
(1) A hot stamped body comprising a middle part in sheet thickness and a softened layer arranged at both sides or one side of the middle part in sheet thickness, wherein
the middle part in sheet thickness comprises, by mass %,
C: 0.20% or more and less than 0.70%,
Si: less than 3.00%,
Mn: 0.20% or more and less than 3.00%,
P: 0.10% or less,
S: 0.10% or less,
sol. Al: 0.0002% or more and 3.0000% or less,
N: 0.01% or less, and
a balance of Fe and unavoidable impurities, and has a hardness of 500 Hv or more and 800 Hv or less,
in the metal structures from a depth of 20 μm below the surface of the softened layer to a depth of ½ of the thickness of the softened layer, when defining a region surrounded by grain boundaries having a 15° or higher orientation difference in a cross-section parallel to the sheet thickness direction as a “crystal grain”, the area rate of the total of crystal grains with a maximum crystal orientation difference inside the crystal grains of 1° or less and crystal grains with a maximum crystal orientation difference inside the crystal grains of 8° or more and less than 15° is 50% or more and less than 85%,
the tensile strength is 1500 MPa or more.
(2) The hot stamped body according to (1), wherein the Si content is 0.50% or less and the Mn content is 0.20% or more and less than 1.50%.
(3) The hot stamped body according to (1), wherein the Si content is 0.50% or less and the Mn content is 1.50% or more and less than 3.00%.
(4) The hot stamped body according to (1), wherein the Si content is more than 0.50% and less than 3.00%, the Mn content is 0.20% or more and less than 1.50%, and the middle part in sheet thickness comprises, by area percent, 1.0% or more and less than 5.0% of residual austenite.
(5) The hot stamped body according to (1), wherein the Si content is more than 0.50% and less than 3.00%, the Mn content is 1.50% or more and less than 3.00%, and the middle part in sheet thickness comprises, by area percent, 1.0% or more and less than 5.0% of residual austenite.
(6) The hot stamped body according to any one of (1) to (5), where the middle part in sheet thickness further comprises, by mass %, Ni: 0.01% or more and 3.00% or less.
(7) The hot stamped body according to any one of (1) to (6), where the middle part in sheet thickness further comprises, by mass %, one or more of Nb: 0.010% or more and 0.150% or less, Ti: 0.010% or more and 0.150% or less, Mo: 0.005% or more and 1.000% or less, and B: 0.0005% or more and 0.0100% or less.
(8) The hot stamped body according to any one of (1) to (7), where a plated layer is formed on the softened layer.
According to the present invention, it is possible to provide a hot stamped body excellent in bendability, ductility, impact resistance, and hydrogen embrittlement resistance and with small scattering in hardness.
The hot stamped body according to the present invention is a structure with a softened layer arranged on the surface at both sides or one side. The softened layer has a region having a hardness 10 Hv or more lower than the hardness of the middle part in sheet thickness.
The middle part in sheet thickness of the hot stamped body according to the present invention must have a hardness of 500 Hv to 800 Hv. The reasons for limiting the composition of constituents at the middle part in sheet thickness to make the hardness of the middle part in sheet thickness the above-mentioned range are explained below. Below, the % relating to the component of constituents means mass %.
(C: 0.20% or More and Less than 0.70%))
C is an important element for obtaining a 500 Hv to 800 Hv hardness at the middle part in sheet thickness. With less than 0.20%, it is difficult to secure 500 Hv or more at the middle part in sheet thickness, and therefore C is 0.20% or more. Preferably it is 0.30% or more. On the other hand, with more than 0.70%, the hardness of the middle part in sheet thickness exceeds 800 Hv and the bendability falls, and therefore C is 0.70% or less. Preferably, it is 0.50% or less.
(Si: Less than 3.00%)
Si is an element contributing to improvement of strength by solution strengthening. The amount of addition of Si for obtaining the effect of improvement of strength of the steel sheet by formation of a solid solution of Si in the metal structures is preferably 0.30% or more, but even if adding more than 0.5% of Si, the effect becomes saturated.
Si also has the effect of causing the formation of residual austenite and raising the ductility. To obtain this effect, addition of more than 0.50% is at least necessary. On the other hand, even if adding more than 3.00%, the effect becomes saturated, and therefore the amount of addition of Si is one with an upper limit of less than 3.00%. Preferably, the amount is less than 2.0%.
(Mn: 0.20% or More and Less than 3.00%)
Mn is an element contributing to improvement of strength by solution strengthening. The effect of improving the strength of the steel sheet by solid solution of Mn in the metal structures cannot be obtained with an amount of addition of less than 0.20%, so 0.20% or more is added. Preferably the content is 0.70% or more. On the other hand, even if adding 1.50% or more, the effect becomes saturated.
Mn, further, has the effect of raising the hardenability. By adding 1.50% or more, it is possible to raise the hardenability and stably obtain high strength. The preferable amount of addition for obtaining the effect of raising the hardenability is 1.70% or more. Even if adding 3.00% or more, the effect becomes saturated, and therefore the upper limit of the amount of addition of Mn is 3.00%. Preferably, the content is less than 2.00%.
P is an element segregating at the grain boundaries and impairing the strength of the grain boundaries. If more than 0.10%, the strength of the grain boundaries remarkably falls and the hydrogen embrittlement resistance and bendability fall, and therefore P is 0.10% or less. Preferably, it is 0.05% or less. The lower limit is not particularly prescribed, but if reducing this to less than 0.0001%, the dephosphorizing cost greatly rises and the result becomes economically disadvantageous, so in practical steel sheet, 0.0001% is the substantive lower limit.
S is an element forming inclusions. If more than 0.10%, inclusions are formed and the hydrogen embrittlement resistance and bendability fall, and therefore S is 0.10% or less. Preferably, it is 0.0025% or less. The lower limit is not particularly prescribed, but if reducing this to less than 0.0015%, the desulfurizing cost greatly rises and the result becomes economically disadvantageous, so in practical steel sheet, 0.0001% is the substantive lower limit.
(Sol. Al: 0.0002% or More and 3.0000% or Less)
Al is an element acting to deoxidize the molten steel and make the steel sounder. In the present invention, to obtain the deoxidizing action, the range of content of not all of the Al contained in the steel, but the content of so-called “acid soluble aluminum” (sol. Al) is prescribed. With a sol. Al content of less than 0.0002%, the deoxidizing is insufficient, and therefore sol. Al is 0.0002% or more. Preferably the content is 0.0010% or more. On the other hand, even if adding more than 3.0%, the effect becomes saturated, and therefore the content is 3.0000% or less.
N is an impurity element and is an element which forms nitrides and impairs bendability. If more than 0.01%, coarse nitrides are formed and the bendability remarkably falls, and therefore N is 0.01% or less. Preferably the content is 0.0075% or less. The lower limit is not particularly prescribed, but if reducing this to less than 0.0001%, the denitriding cost greatly rises and the result becomes economically disadvantageous, so in practical steel sheet, 0.0001% is the substantive lower limit.
Ni is an element contributing to improvement of strength by solution strengthening, so may be added as needed. With less than 0.010%, the effect is not obtained, so 0.010% or more is added. Preferably, the content is 0.5% or more. On the other hand, even if added in more than 3.00%, the effect becomes saturated, and therefore the content is 3.00% or less. Preferably, the content is 2.50% or less.
Nb is an element contributing to improvement of strength by solution strengthening, so may be added as needed. With less than 0.010%, the effect is not obtained, so 0.010% or more is added. Preferably, the content is 0.035% or more. On the other hand, even if added in more than 0.150%, the effect becomes saturated, and therefore the content is 0.150% or less. Preferably, the content is 0.120% or less.
Ti is an element contributing to improvement of strength by solution strengthening, so may be added as needed. With less than 0.010%, the effect is not obtained, and therefore the content is 0.010% or more. Preferably, the content is 0.020%. On the other hand, even if added in more than 0.150%, the effect becomes saturated, and therefore the content is 0.150% or less. Preferably, the content is 0.120% or less.
Mo is an element contributing to improvement of strength by solution strengthening, so may be added as needed. With less than 0.005%, the effect is not obtained, and therefore the content is 0.005% or more. Preferably, the content is 0.0100% or more. On the other hand, even if added in more than 1.000%, the effect becomes saturated, and therefore the content is 1.000% or less. Preferably, the content is 0.800% or less.
B is an element segregating at the grain boundaries and improving the strength of the grain boundaries, so may be added as needed. With less than 0.0005%, the effect of addition is not sufficiently obtained, so 0.0005% or more is added. Preferably, the content is 0.0010% or more. On the other hand, even if added in more than 0.0100%, the effect becomes saturated, and therefore the content is 0.0100% or less. Preferably, the content is 0.0075% or less.
The balance of the composition of constituents of the middle part in sheet thickness consists of Fe and unavoidable impurities. The unavoidable impurities are elements which unavoidably enter from the steel raw materials and/or in the steelmaking process and are allowed in ranges not impairing the characteristics of the hot stamped body of the present invention.
If the hardness of the middle part in sheet thickness is 500 Hv or more, as the tensile strength of the hot stamped body of the present invention, 1500 MPa or more can be secured. Preferably, it is 600 Hv or more. On the other hand, if the hardness of the middle part in sheet thickness is more than 800 Hv, since the difference in hardness with the softened layer becomes too large and deterioration of the bendability is invited, 800 Hv is the upper limit. Preferably the hardness is 720 Hv or less.
The method of measurement of the hardness of the middle part in sheet thickness is as follows: A cross-section vertical to the sheet surface of the hot stamped body is taken to prepare a sample of the measurement surface. This is supplied to a hardness test. The method of preparing the measurement surface may be based on JIS Z 2244. For example, #600 to #1500 silicon carbide paper may be used to polish the measurement surface, then a solution of particle size 1 μm to 6 μm diamond powder dispersed in alcohol or another diluent or pure water may be used to finish the sample to a mirror surface. The hardness test may be performed by the method described in JIS Z 2244. A micro-Vickers hardness tester is used to measure 10 points at the ½ position of thickness of the hot stamped body by a load of 1 kgf and intervals of 3 times or more of the dents. The average value was defined as the hardness of the middle part in sheet thickness.
The middle part in sheet thickness can be improved in ductility by including residual austenite in an area percent of 1% or more. The area percent of residual austenite at the middle part in sheet thickness is preferably 2% or more. However, if making the area percent of the residual austenite 5% or more, since deterioration of the bendability is invited, the upper limit is less than 5.0%. Preferably, the fraction is less than 4.5%.
The area percent of the residual austenite can be measured by the following method. A sample is taken from a hot stamped member and ground down at its surface to a depth of ½ of the sheet thickness from the normal direction of the rolling surface. The ground down surface is used for X-ray diffraction measurement. From the image obtained by the X-ray diffraction method using Kα rays of Mo, the area rate Vγ of residual austenite can be determined using the following formula:
Vγ=(⅔){100/(0.7×α(211)/γ(220)+1)}+(⅓){100/(0.78×α(211)/γ(311)+1)}
Here, α(211) is the X-ray diffraction intensity at the (211) face of ferrite, γ(220) is the X-ray diffraction intensity at the (220) face of austenite, and γ(311) is the X-ray diffraction intensity at the (311) face of austenite.
As explained above, in the present invention, the “softened layer” is the region in the sheet thickness direction of the cross-section of sheet thickness of the hot stamped body from the position where the hardness falls by 10 Hv or more from hardness of the middle part in sheet thickness (hardness at position of ½ of sheet thickness) to the surface of the stamped body.
The inventors investigated the metal structures of steel sheets where good bendability was obtained and as a result discovered that the metal structures forming the softened layer should be comprised of crystal grains with a maximum crystal orientation difference inside the crystal grains of 1° or less and crystal grains with a maximum crystal orientation difference inside the crystal grains of 8° or more and less than 15° when defining a region surrounded by grain boundaries having a 15° or higher orientation difference in a cross-section of sheet thickness as a “crystal grain”. These measurements were performed in the region from a position of a depth of 20 μm below the surface of the softened layer to a position of a depth of ½ of the thickness of the softened layer (center of softened layer). The inventors engaged in intensive studies and as a result discovered that from the viewpoint of the bendability and other effects, the fractions of structures from a position of 20 μm from the surface of the softened layer to a position of a depth of ½ of the thickness of the softened layer are important. The effects of the surface properties of the hot stamped body and the effects of the transitional part from the middle part in sheet thickness to the softened layer can be eliminated by such metal structures.
In the above-mentioned metal structures of the softened layer, the area rate of the total of crystal grains with a maximum crystal orientation difference inside the crystal grains of 1° or less and crystal grains with a maximum crystal orientation difference inside the crystal grains of 8° or more and less than 15° should be 50% or more, more preferably 55% or more. On the other hand, with an area rate of the total of the metal structures of the softened layer of 85% or more, the difference in hardness of the softened layer and the middle part in sheet thickness becomes too great and the effect of reduction of the sharp gradient of hardness in the sheet thickness direction occurring at the time of bending deformation cannot be obtained, and therefore the area rate is less than 85%. More preferably, it is 80% or less.
Between the position of a depth of ½ of the thickness of the softened layer (center of softened layer) to the middle part in sheet thickness, if the hardness at the sheet thickness middle part side of the softened layer (boundary with middle part in sheet thickness) is HvA and the hardness of the center of the softened layer is HvB, they are in the relationship of HvA−HvB≥10 Hv.
The method of determining the region from 20 μm below the surface of the softened layer to a position of ½ of the thickness of the softened layer will be explained below. A cross-section vertical to the surface of the hot stamped body being measured (cross-section of sheet thickness) is taken to prepare a sample of the measurement surface. This is used for a hardness test. The method of preparing the measurement surface may be based on JIS Z 2244. For example, #600 to #1500 silicon carbide paper may be used to polish the measurement surface, then a solution of particle size 1 μm to 6 μm diamond powder dispersed in alcohol or another diluent or pure water may be used to finish the sample to a mirror surface. The sample with the prepared measurement surface is measured two times based on the method described in JIS Z 2244 using a micro Vickers hardness tester. The first time measures the hardness from the region within 20 μm from the surface of the hot stamped body in the sheet thickness direction to the middle part in sheet thickness (position of ½ of sheet thickness) in the direction vertical to the surface (sheet thickness direction) by a load of 0.3 kgf at intervals of 3 times or more the dents. However, if there is a plated layer, this is measured from the region within 20 μm right under the plating or coating or the alloy layer of the plating or coating and material of the softened layer. The position where the hardness starts to drop by 10 Hv or more from the hardness of the middle part in sheet thickness (hardness at position of ½ of sheet thickness) is determined and the layer from that sheet thickness position to the surface of the hot stamped body is defined as the “softened layer”. If the softened layer is present at both surfaces, the second measurement is performed at the surface at the opposite side to the first one (back surface) by a similar method to determine the position where the hardness starts to drop by 10 Hv or more from the hardness of the middle part in sheet thickness.
Next, the method of calculating the area rates of metal structures of the softened layer will be explained. A sample is cut out from a hot stamped body to enable examination of a cross-section vertical to its surface (sheet thickness direction). The length of the sample depends on the measuring device, but may be about 50 μm. The region in the sheet thickness direction of the sample from the surface of the softened layer to the position of ½ of the thickness of the softened layer (center of softened layer) is analyzed at 0.2 μm measurement intervals by EBSD to obtain information on the crystal orientation. Here, this EBSD analysis is performed using an apparatus comprised of a thermal field emission type scan electron microscope (JSM-7001F made by JEOL) and EBSD detector (DVCS type detector made by TSL) at an analysis speed of 200 to 300 points/second.
Next, based on the obtained crystal orientation information, a region surrounded by grain boundaries having an orientation difference of 15° or more is defined as one crystal grain and a crystal orientation map in the sheet surface direction is prepared. The obtained crystal orientation map is used to find the crossing points of the long axis of one crystal grain and the crystal grain boundaries. Among the two crossing points, one is designated as the starting point and the other is designated as the end point and the difference in orientation among all measurement points contained on the long axis of the crystal grain is calculated. The maximum value of the orientation difference obtained was defined as the maximum crystal orientation difference at that crystal grain. The above analysis was performed for all crystal grains included in the measurement region, then the average of these values was defined as the maximum crystal orientation difference inside a region surrounded by grain boundaries of 15° or more.
The above-defined maximum crystal orientation difference can be simply calculated, for example, if using the “Inverse Pole Figure Map” and “Profile Vector” functions included in the software (OIM Analysis®) attached to the EBSD analysis system. With the “Inverse Pole Figure Map” function, it is possible to draw grain boundaries having slants of 15° or more as large angle grain boundaries and further possible to prepare a crystal orientation map in the sheet surface direction. With the “Profile Vector” function, it is possible to calculate the misorientation angle (difference in crystal orientations) between all measurement points included on any line. All crystal grains contained in the measurement region (crystal grains at end parts of measurement region not included) are analyzed as explained above and the area rate of the total of the crystal grains with a maximum crystal orientation difference inside the regions surrounded by grain boundaries of 15° or more of 1° or less and the crystal grains with a crystal orientation difference of 8° or more and less than 15° is calculated. If the softened layer is formed on both surfaces, the above procedure is performed at the back surface side of the hot stamped body as well and the average value of the area rates obtained from the front surface side and the back surface side is employed.
The composition of the softened layer is not particularly limited other than regarding the unavoidable impurity elements of P, S, and N impairing the strength and/or bendability, but the layer is preferably the following composition so as to secure the strength of the hot stamped body and steel exhibiting excellent bendability.
In the composition of the softened layer, one or more of the C content, Si content, and Mn content are preferably respectively 0.6 time the corresponding contents of elements of the middle part in sheet thickness. The preferable ranges of the constituents in this case are as follows:
(C: 0.05% or More and Less than 0.42%)
C may be added in 0.05% or more so as to raise the strength. From the viewpoint of raising the load resistance as a member and improving the impact characteristics, preferably the content is 0.10% or more. To make the hardness of the softened layer lower than the hardness of the middle part in sheet thickness, it is preferable to make the content smaller than the middle part in sheet thickness. For this reason, the preferable C content of the softened layer is less than 0.42%. Preferably the content is 0.35% or less.
(Si: Less than 2.00%)
Si is an element contributing to improvement of strength by solution strengthening, so is added for raising the strength. However, to make the hardness of the softened layer lower than the hardness of the middle part in sheet thickness, it is preferable to make this smaller in content than the middle part in sheet thickness.
If the Si content of the middle part in sheet thickness is 0.50% or less, the preferable Si content of the softened layer is 0.30% or less, more preferably 0.20% or less. Further, if the Si content of the middle part in sheet thickness is more than 0.50% and less than 3.00%, the preferable Si content of the softened layer is less than 2.00%, more preferably 1.50% or less.
(Mn: 0.12% or More and Less than 1.80%)
Mn is an element contributing to improvement of strength by solution strengthening, so may be added in 0.12% or more for raising the strength. However, to make the hardness of the softened layer lower than the hardness of the middle part in sheet thickness, it is preferably smaller in content than the middle part in sheet thickness.
If the Mn content at the middle part in sheet thickness is 0.20% to less than 1.50%, the preferable Mn content of the softened layer is less than 0.90%, more preferably is 0.70% or less. Further, if the Mn content of the middle part in sheet thickness is 1.50% to less than 3.00%, the preferable Mn content of the softened layer is less than 1.80%, preferably 1.40% or less.
P is an element segregating at the grain boundaries and impairing the strength of the grain boundaries. If more than 0.10%, the strength of the grain boundaries remarkably falls and the hydrogen embrittlement resistance and bendability fall, and therefore P is 0.1% or less. Preferably, it is 0.05% or less. The lower limit is not particularly prescribed, but if reducing this to less than 0.0001%, the dephosphorizing cost greatly rises and the result becomes economically disadvantageous, so in practical steel sheet, 0.0001% is the substantive lower limit.
S is an element forming inclusions. If more than 0.10%, inclusions are formed and the hydrogen embrittlement resistance and bendability fall, and therefore S is 0.10% or less. Preferably, it is 0.0025% or less. The lower limit is not particularly prescribed, but if reducing this to less than 0.0015%, the desulfurizing cost greatly rises and the result becomes economically disadvantageous, so in practical steel sheet, 0.0001% is the substantive lower limit.
(Sol. Al: 0.0002% or More and 3.0000% or Less)
Al is an element acting to deoxidize the molten steel and make the steel sounder. In the present invention, to obtain this deoxidizing action, the range of content of not all of the Al contained in the steel, but the so-called “acid soluble aluminum” (sol. Al) is prescribed. With a sol. Al content of less than 0.0002%, the deoxidizing is insufficient, and therefore the sol. Al is preferably 0.0002% or more. More preferably the content is 0.0010% or more. On the other hand, even if adding more than 3.0000%, the effect becomes saturated, and therefore the content is 3.0000% or less.
N is an impurity element and is an element which forms nitrides and impairs bendability. If more than 0.01%, coarse nitrides are formed and the bendability remarkably falls, and therefore N is 0.01% or less. Preferably the content is 0.0075% or less. The lower limit is not particularly prescribed, but if reducing this to less than 0.0001%, the denitriding cost greatly rises and the result becomes economically disadvantageous, so in practical steel sheet, 0.0001% is the substantive lower limit.
Regarding the constituents of the softened layer, one or more of the C content, Si content, and Mn content are preferably respectively 0.6 time or less the C content, Si content, and Mn content of the middle part in sheet thickness. Other than the upper limits of the unavoidable impurity elements of P, S, and N impairing the strength and/or bendability being prescribed, the other constituents are not particularly limited. In general, the softened layer may optionally and selectively include one or more of the following constituents besides C, Si, and Mn.
Ni is an element contributing to improvement of strength by solution strengthening, so may be added as needed. With less than 0.01%, the effect is not obtained, and therefore preferably 0.01% or more is added. More preferably, the content is 0.50% or more. On the other hand, even if added in more than 3.00%, the effect becomes saturated, and therefore the content is 3.00% or less. Preferably, the content is 2.50% or less.
Nb is an element contributing to improvement of strength by solution strengthening, so may be added as needed. With less than 0.010%, the effect is not obtained, so preferably 0.010% or more is added. More preferably, the content is 0.035% or more. On the other hand, even if added in more than 0.150%, the effect becomes saturated, and therefore the content is 0.150% or less. Preferably, the content is 0.120% or less.
Ti is an element contributing to improvement of strength by solution strengthening, so may be added as needed. With less than 0.010%, the effect is not obtained, and therefore preferably the content is 0.010% or more. More preferably, the content is 0.020%. On the other hand, even if added in more than 0.150%, the effect becomes saturated, and therefore the content is 0.150% or less. Preferably, the content is 0.120% or less.
Mo is an element contributing to improvement of strength by solution strengthening, so may be added as needed. With less than 0.005%, the effect is not obtained, and therefore preferably the content is 0.005% or more. More preferably, the content is 0.010% or more. On the other hand, even if added in more than 1.000%, the effect becomes saturated, and therefore the content is 1.000% or less. Preferably, the content is 0.800% or less.
B is an element segregating at the grain boundaries and improving the strength of the grain boundaries, so may be added as needed. With less than 0.0005%, the effect of addition is not sufficiently obtained, and therefore preferably 0.0005% or more is added. More preferably, the content is 0.0010% or more. On the other hand, even if added in more than 0.0100%, since the effect becomes saturated, the content is 0.0100% or less. Preferably, the content is 0.0075% or less.
At the cross-section vertical to the surface of the hot stamped body, the distribution of hardness at the middle part in sheet thickness is preferably uniform with no scattering. In a hat-shaped structure, at the vertical wall parts, contact with the die is difficult and the cooling rate becomes low, so sometimes the hardness falls. If there is a region where the hardness falls by 100 Hv or more from the average hardness of the cross-section vertical to the longitudinal direction of the hat-shaped member, at the time of impact, the deformation will concentrate at the softened part and the part will fracture early, so a high impact resistance cannot be obtained. For this reason, there must not be a point with a hardness more than 100 HV below the average value of the distribution of hardness in the cross-section vertical to the surface of the hot stamped body (below, referred to as the “average hardness of cross-section”). The distribution of hardness at the cross-section and the average hardness of the cross-section are obtained by obtaining a cross-section vertical to the longitudinal direction of a long hot stamped body at any position in the longitudinal direction and measuring the Vickers hardness between the end parts of the cross-section at equal intervals of 1 mm pitch or less at the middle position of sheet thickness of the entire cross-sectional region including the vertical walls using a Vickers hardness tester (load of 1 kgf).
The surface of the softened layer may be formed with a plated layer for the purpose of improving the corrosion resistance. The plated layer may be either an electroplated layer or a hot dip coated layer. An electroplated layer includes, for example, an electrogalvanized layer, electro Zn—Ni alloy plated layer, etc. As a hot dip coated layer, a hot dip galvanized layer, a hot dip galvannealed layer, a hot dip aluminum coated layer, a hot dip Zn—Al alloy coated layer, a hot dip Zn—Al—Mg alloy coated layer, a hot dip Zn—Al—Mg—Si alloy coated layer, etc., may be mentioned. The amount of deposition of the layer is not particularly limited and may be a general amount of deposition.
Next, the method of production for obtaining the hot stamped body according to the present invention will be explained, but the present invention is not limited to the form of the double layer steel sheet explained below.
As one embodiment of the method of production of the present invention, first, a steel sheet satisfying the requirements of the composition of constituents of the middle part in sheet thickness explained above is ground down at its front surface and/or back surface to remove surface oxides, then a steel sheet for softened layer formation use (below, referred to as a “steel sheet for surface layer”) is superposed on each ground down surface side. The method of joining the steel sheet for surface layer and the steel sheet for sheet thickness middle part is not particularly limited, but they may be joined by arc welding. A steel sheet for surface layer wherein one or more of the C content, Si content, and Mn content are 0.6 time or less the content of the corresponding element of the steel sheet for sheet thickness middle part is preferably superposed.
Further, by controlling the casting rate to ton/min or more in the continuous casting process of the steel sheet for surface layer, it is possible to keep down microsegregation of Mn in the steel sheet for surface layer and possible to make the distribution of concentration of Mn at the steel sheet for surface layer uniform. Mn raises the yield strength of austenite to thereby affect the behavior in formation of grain boundaries in the transformed structures, so when defining a region surrounded with grain boundaries having orientation differences of 15° or more as a “crystal grain”, it has the effect of promoting the formation of crystal grains with a maximum crystal orientation difference inside the crystal grains of 8° or more and less than 15°. For this reason, it is also possible to control the casting rate to 6 ton/min or more in the continuous casting process of steel sheet for surface layer for the purpose of promoting the formation of the above microstructures.
Further, a double layer steel sheet fabricated by the above method is preferably held at 1100° C. or more and 1350° C. or less in temperature for 20 minutes to less than 60 minutes. The held sheet is preferably used as the steel sheet for hot stamped body according to the present invention. The inventors studied this and as a result learned that by performing heat treatment holding the steel sheet at 1100° C. or more and 1350° C. or less for 20 minutes to less than 60 minutes, in the metal structures in the region from a position of a depth of 20 μm below the surface of the softened layer to the center of the softened layer, the area rate of the total of crystal grains with a maximum crystal orientation difference inside the crystal grains of 1° or less and crystal grains with a maximum crystal orientation difference inside the crystal grains of 8° or more and less than 15° becomes 50% to less than 85% when a region surrounded by grain boundaries having an orientation difference of 15° or more is defined as a “crystal grain” and that excellent bendability and hydrogen embrittlement resistance can be obtained.
The multilayer member produced by the above method of production (double layer steel sheet) can be treated by hot rolling, cold rolling, hot stamping, continuous hot dip coating, etc., to obtain the hot stamped body according to the present invention.
The hot rolling may be hot rolling performed under usual conditions. For example, the finishing temperature may also be in the temperature range of 810° C. or more. The subsequent following cooling conditions also do not have to be particularly prescribed. The steel sheet is coiled in the temperature region of 750° C. or less. Further, it may be reheated for the purpose of softening the double layer steel sheet after hot rolling.
Further, to promote more the formation of the middle part in sheet thickness, the hot rolling after the above heat treatment of the double layer steel sheet preferably includes rough rolling and finish rolling with the rough rolling being performed twice under conditions of a temperature of 1100° C. or more, a sheet thickness reduction rate per pass of 5% or more and less than 50%, and a time between passes of 3 seconds or more.
Specifically, to promote more the formation of the middle part in sheet thickness in the present invention, the concentrations of alloy elements, in particular C atoms, have to be controlled to become more moderately distributed. The distribution of concentration of C is obtained by diffusion of C atoms. The diffusion frequency of C atoms increases the higher the temperature. Therefore, to control the C concentration, control in the rough rolling from the hot rolling heating becomes important. In hot rolling heating, to promote the diffusion of C atoms, the heating temperature has to be high. Preferably, it is 1100° C. or more and 1350° C. or less, more preferably more than 1150° C. and 1350° C. or less. With hot rolled heating, the changes of (i) and (ii) shown in
After adjusting the hot rolling heating temperature to obtain the preferable distribution of concentration of C, to obtain a further optimum distribution of concentration of C, pass control in rough rolling becomes extremely important. Rough rolling is performed two times or more under conditions of a rough rolling temperature of 1100° C. or more, a sheet thickness reduction rate per pass of 5% or more and less than 50%, and a time between passes of 3 seconds or more. This is so as to promote the diffusion of C atoms of (i) in
The cold rolling may be cold rolling performed by a usual rolling reduction, for example, 30 to 90%. The hot rolled steel sheet and the cold rolled steel sheet include steel sheets as hot rolled and cold rolled and also steel sheets obtained by recrystallization annealing hot rolled steel sheet or cold rolled steel sheet under usual conditions and steel sheets obtained by skin pass rolling under usual conditions.
The heating, shaping, and cooling steps at the time of hot stamping may also be performed under usual conditions. For example, hot rolled steel sheet obtained by uncoiling hot rolled steel sheet coiled in the hot rolling step, cold rolled steel sheet obtained by uncoiling and cold rolling coiled hot rolled steel sheet, or steel sheet obtained by plating or coating cold rolled steel sheet, heating this by a 0.1° C./s to 200° C./s heating rate up to 810° C. or more and 1000° C. or less in temperature, and holding it at this temperature is formed into the required shape by the usual hot stamping.
The holding time may be set according to the mode of forming, so is not particularly limited. For example, if 30 seconds or more and 600 seconds or less, a good hot stamped body is cooled to room temperature.
The cooling rate may also be set to a usual condition. For example, the average cooling rate in the temperature region from the heating temperature to more than 400° C. may be 50° C./s or more. In the case of steel sheet with an Si content at the middle part in sheet thickness of more than 0.50% and less than 3.00% and an Mn content at the middle part in sheet thickness of 0.20% or more and less than 1.50% and steel sheet with an Si content at the middle part in sheet thickness of more than 0.50% and less than 3.00% and an Mn content at the middle part in sheet thickness of 1.50% or more and less than 3.00%, for the purpose of increasing the amount of formation of residual austenite to improve the ductility, it is preferable to control the average cooling rate at the cooling after heating and holding at the 200° C. to 400° C. temperature region to less than 50° C./s.
Further, for the purpose of adjusting the strength etc., it is possible to temper the body cooled down to room temperature in the range of 150° C. to 600° C.
In the method of production of the hot stamped body of the above-mentioned embodiment, the middle part in sheet thickness and the softened layer were configured by separate steel sheets. However, the hot stamped body of the present invention is not limited to double layer steel sheet comprised of two of the above-mentioned steel sheets superposed. The middle part in sheet thickness and the softened layer may be formed inside a single material steel sheet. For example, it is possible to treat a single layer steel sheet to decarburize it and soften the surface layer part to thereby produce high strength steel sheet comprised of a softened layer and a middle part in sheet thickness.
Next, examples of the present invention will be explained, but the conditions in the examples are just illustrations of conditions employed for confirming the workability and advantageous effects of the present invention. The present invention is not limited to the illustration of conditions. The present invention can employ various conditions so long as not departing from the gist of the present invention and achieving the object of the present invention.
The Nos. 1 to 18 steel sheets for sheet thickness middle part having the chemical compositions shown in Table A-1-1 (in the table, “Steel Nos. 1 to 18”) were ground down at their surfaces to remove the surface oxides. After that, the respective steel sheets for sheet thickness middle part were welded with steel sheets for surface layer having the chemical compositions shown in Table A-1-2 at both surfaces or single surfaces by arc welding to fabricate the Nos. 1 to 43 multilayer steel sheets for hot stamped body. The total of the sheet thicknesses of the steel sheet for surface layer and the steel sheet for sheet thickness middle part after arc welding is 200 mm to 300 mm and the thickness of the steel sheet for surface layer is ⅓ or so of the thickness of the steel sheet for sheet thickness middle part (¼ or so in case of single side). The No. 37 multilayer steel sheet is steel with the steel sheet for surface layer welded to only one surface. In the Nos. 1 to 43 multilayer steel sheets of Table A-1-1 to Table A-1-2, ones with a steel sheet for sheet thickness middle part not satisfying the requirement for composition of the middle part in sheet thickness of the hot stamped body according to the present invention are indicated as “comparative steel” in the remarks column.
The Nos. 1 to 43 multilayer steel sheets were respectively treated under the conditions of the Nos. 1 to 43 manufacturing conditions shown in Table A-2-1 to Table A-2-2 by heat treatment before hot rolling, rough rolling, hot rolling, and cold rolling to obtain steel sheets. Next, the steel sheets were heat treated as shown in Table A-2-1 and Table A-2-2 (in the tables, “heat treatment of hot stamped body”) for hot stamping to manufacture the Nos. 1A to 43A hot stamped bodies (“stamped bodies” of Table A-3). Further, the Nos. 35A and 36A hot stamped bodies were coated on a hot dip coating line at the surfaces with 120 to 160 g/m2 amounts of aluminum.
In the tables, the item “sheet thickness reduction rate” of the “rough rolling” means the sheet thickness reduction rate per pass of the rough rolling. The item “number of rolling operations” means the number of rolling operations under the conditions of a time between passes of 3 seconds or more. Further, the item in the tables of “heating rate (° C./s)” means the rate of temperature rise until reaching the heating temperature of the “heat treatment at the time of hot stamping” after the cold rolling process. Further, in the tables, the item “heating temperature (° C.)” of the “heat treatment at the time of hot stamping” is the temperature at the time of hot stamping, the “average cooling rate (° C./s) (more than 400° C.)” means the average cooling rate (° C./s) in the temperature region from the heating temperature to more than 400° C., and the “average cooling rate (° C./s) (400° C. or less)” means the average cooling rate (° C./s) in the temperature region from 200° C. to 400° C. Further, in the tables, the fields with the notations “-” indicate no corresponding treatment performed.
Table A-3 shows the metal structures and characteristics of the Nos. 1A to 43A hot stamped bodies. The constituents obtained by analyzing the positions of ½ of the sheet thicknesses of the samples taken from the hot stamped bodies and positions of 20 μm from the surfaces of the softened layers were equivalent to the constituents of the steel sheets for sheet thickness middle part and steel sheets for surface layer of the Nos. 1 to 43 multilayer steel sheets of Table A-1-1 to Table A-1-2.
The metal structures of the hot stamped steel sheets were measured by the above-mentioned method. The hardness of the steel sheet for sheet thickness middle part forming the middle part in sheet thickness and the area rate of the total of the crystal grains with a maximum crystal orientation difference inside the regions surrounded by grain boundaries of 15° or more of 1° or less and the crystal grains with a crystal orientation difference of 8° or more and less than 15° in the metal structures from the surface of the steel sheet for surface layer forming the softened layer to ½ of the thickness were calculated. The calculated values of the area rate are shown in the item “area rate (%) of total of crystal grains with maximum crystal orientation difference inside large angle grain boundaries of 1° or less and crystal grains with maximum crystal orientation difference of 8° or more and less than 15°” of Table A-3.
Further, a tensile test of the hot stamped body was performed. The results are shown in Table A-3. The tensile test was performed by preparing a No. 5 test piece described in JIS Z 2201 and following the test method described in JIS Z 2241.
The hydrogen embrittlement resistance of the hot stamped body was evaluated using a test piece cut out from the stamped body. In general, a hot stamped body is joined with other parts using spot welding or another joining method. Depending upon the precision of the shape of the part, the hot stamped body will be subjected to twisting and stress will be applied. The stress differs depending on the position of the part. Accurately calculating this is difficult, but if there is no delayed fracture at the yield stress, it is believed there is no problem in practical use. Therefore, a sheet thickness 1.2 mm×width 6 mm×length 68 mm test piece was cut out from the stamped body, a strain corresponding to the yield stress was imparted in a four-point bending test, then the body was immersed in pH3 hydrochloric acid for 100 hours. The presence of any cracking was used to evaluate the hydrogen embrittlement resistance. A case of no cracking was indicated as passing (“good”) and a case with cracking was indicated as failing (“poor”).
For the purpose of evaluating the impact resistance of the hot stamped body, the body was evaluated based on the VDA standard (VDA238-100) prescribed by the German Association of the Automotive Industry under the following measurement conditions. In the present invention, the displacement at the time of maximum load obtained in the bending test was converted to angle by the VDA standard to find maximum bending angle and thereby evaluate the impact resistance of the hot stamped body.
Test piece dimensions: 60 mm (rolling direction)×60 mm (direction vertical to rolling) or 30 mm (rolling direction)×60 mm (direction vertical to rolling)
Bending ridgeline: direction perpendicular to rolling
Test method: roll support, punch pressing
Roll diameter: φ30 mm
Punch shape: tip R=0.4 mm
Distance between rolls: 2.0×sheet thickness (mm)+0.5 mm
Indentation rate: 20 mm/min
Tester: SHIMAZU AUTOGRAPH 20 kN
If the tensile strength is 1500 MPa or more, the maximum bending angle (°) was 70(°) or more, and the hydrogen embrittlement resistance was a passing level, it was judged that the impact resistance and hydrogen embrittlement resistance were excellent and the case was indicated as an “invention example”. If even one of the three aspects of performance is not satisfied, the case was indicated as a “comparative example”.
In each hot stamped body of the invention examples, the area rate of the total of the crystal grains with a maximum crystal orientation difference inside the regions surrounded by grain boundaries of 15° or more of 1° or less and the crystal grains with a crystal orientation difference of 8° or more and less than 15° in the metal structures from the surface of the steel sheet for surface layer to ½ of the thickness was 50% to less than 85%. Further, each hot stamped body of the invention examples was excellent in tensile strength, bendability, and hydrogen embrittlement resistance.
As opposed to this, the No. 5A hot stamped body was low in carbon content of the steel sheet for sheet thickness middle part, so the hardness of the middle part in sheet thickness became insufficient and the tensile strength became insufficient. The No. 9A hot stamped body was excessive in carbon content of the steel sheet for sheet thickness middle part, so the hardness of the middle part in sheet thickness became excessive and the targeted bendability could not be obtained. Further, the No. 11A hot stamped body was low in Mn content at the steel sheet for sheet thickness middle part, so the hardness of the middle part in sheet thickness became insufficient and the tensile strength became insufficient.
The Nos. 30A to 32A hot stamped bodies are comparative examples produced using the multilayer steel sheets for hot stamped body to which the desirable heat treatment had not been applied before the hot stamping process. The No. 30A hot stamped body was low in heat treatment temperature before the hot stamping process, while the No. 31A hot stamped body was short in heat treatment time before the hot stamping process, so in the metal structures from the surface of the softened layer to ½ of the thickness, the soft structures and metal structures with intermediate hardnesses insufficiently grew and the target bendability could not be obtained. Further, the No. 32A hot stamped body was excessively high in heat treatment temperature before the hot stamping process, so the effect of reduction of the sharp gradient in hardness in the sheet thickness direction occurring at the time of bending deformation could not be obtained.
The No. 40A hot stamped body was low in rolling temperature of the rough rolling. Further, the No. 41A hot stamped body was low in sheet thickness reduction rate of the rough rolling. Further, the No. 42A hot stamped body was low in number of rolling operations under conditions of a time between passes of 3 seconds or more. These hot stamped bodies were not manufactured under the suitable rough rolling conditions, so the soft structures and metal structures with intermediate hardnesses insufficiently grew, it was not possible to ease the strain occurring due to bending deformation, and the targeted bendability could not be obtained.
The No. 43A hot stamped body is a steel sheet controlled in casting rate to 6 ton/min or more in the continuous casting process of steel sheet for surface layer. It can raise the area rate (%) of the total of the crystal grains with a maximum crystal orientation difference inside the regions surrounded by grain boundaries of 15° or more of 1° or less and the crystal grains with a crystal orientation difference of 8° or more and less than 15° in the metal structures from the surface of the steel sheet for surface layer to ½ of the thickness and is excellent in bendability.
The Nos. 1 to 18 steel sheets for sheet thickness middle part having the chemical compositions shown in Table B-1-1 (“Steel Nos. 1 to 18” in Table B-1-1) were ground down at their surfaces to remove the surface oxides. After that, the respective steel sheets for sheet thickness middle part were welded with steel sheets for surface layer having the chemical compositions shown in Table B-1-2 at both surfaces or single surfaces by arc welding to fabricate the Nos. 1 to 41 multilayer steel sheets for hot stamped body. The sheet thickness of the total of the steel sheet for surface layer and the steel sheet for sheet thickness middle part after arc welding was 200 mm to 300 mm and the thickness of the steel sheet for surface layer was ⅓ or so of the thickness of the steel sheet for sheet thickness middle part (in case of single side, ¼ or so). The No. 37 multilayer steel sheet was steel with steel sheet for surface layer welded to only one side. The multilayer steel sheets other than No. 37 respectively had steel sheets for surface layer welded to both sides of the steel sheet for sheet thickness middle part. Among the Nos. 1 to 41 multilayer steel sheets of Table B-1-3, ones where the steel sheet for sheet thickness middle part did not satisfy the requirements of composition of the middle part in sheet thickness of the hot stamped body according to the present invention are indicated as “comparative steels” in the remarks columns.
The Nos. 1 to 41 multilayer steel sheets were respectively treated under the conditions of the Nos. 1 to 41 manufacturing conditions shown in Table B-2-1 to Table B-2-2 by heat treatment before hot rolling, rough rolling, hot rolling, and cold rolling to obtain steel sheets. Next, the steel sheets were heat treated as shown in Table B-2-1 and Table B-2-2 (in the tables, “heat treatment of hot stamped body”) for hot stamping to manufacture the Nos. 1B to 41B hot stamped bodies (“stamped bodies” of Table B-3-1 and Table B-3-2). Further, the Nos. 35B and 36B hot stamped bodies were coated on a hot dip coating line at their surfaces with 120 to 160 g/m2 amounts of aluminum. Further, the items in Table B-2-1 to Table B-2-2 correspond to the items in Table A-2-1 to Table A-2-2. Further, in the tables, the fields with the notations “-” indicate no corresponding treatment performed.
Table B-3-1 and Table B-3-2 show the metal structures and characteristics of the Nos. 1B to 41B hot stamped bodies. The constituents obtained by analyzing the positions of ½ of the sheet thicknesses of the samples taken from the hot stamped bodies (middle parts in sheet thickness) and positions of 20 μm from the surfaces of the softened layers were equivalent to the constituents of the steel sheets for sheet thickness middle part and steel sheets for surface layer of the Nos. 1 to 41 multilayer steel sheets of Table B-1-1 to Table B-1-3.
The metal structures of the hot stamped steel sheets were measured by the above-mentioned method. The hardness of the steel sheet for sheet thickness middle part forming the middle part in sheet thickness and the area rate (%) of the total of the crystal grains with a maximum crystal orientation difference inside the regions surrounded by grain boundaries of 15° or more of 1° or less and the crystal grains with a crystal orientation difference of 8° or more and less than 15° in the metal structures from the surface of the steel sheet for surface layer forming the softened layer to ½ of the thickness of that softened layer were calculated. The calculated values of the area rate are shown in the items “area rate (%) of total of crystal grains with maximum crystal orientation difference inside large angle grain boundaries of 1° or less and crystal grains with maximum crystal orientation difference of 8° or more and less than 15°” of Tables B-3-1 to Table B-3-2.
Further, the Nos. 1B to 41B hot stamped bodies were respectively measured for average hardness (HV) and minimum hardness (HV) at the middle part in sheet thickness (position of ½ of sheet thickness) by the above method. The measurement results are shown in Table B-3-1 to Table B-3-2. The Nos. 1B to 41B hot stamped bodies had differences of the average hardness (HV) and minimum hardness (HV) shown in the “scattering in cross-sectional hardness” of Table B-3-1 to Table B-3-2. Further, cases with a scattering in cross-sectional hardness of 100 HV or more were indicated as failing.
The hot stamped bodies were subjected to tensile tests. The results are shown in Table B-3-1 to Table B-3-2. The tensile tests were performed by fabricating No. 5 test pieces described in JIS Z 2201 and testing them by the method described in JIS Z 2241.
The hydrogen embrittlement resistance of the hot stamped body, in the same way as Manufacturing Example A, was evaluated using a test piece cut out from the stamped body. That is, a test piece of a sheet thickness of 1.2 mm×width 6 mm×length 68 mm was cut out from the stamped body, given a strain corresponding to the yield stress in a four-point bending test, then immersed in pH3 hydrochloric acid for 100 hours and evaluated for hydrogen embrittlement resistance by the presence of any cracks. The case of no cracks was indicated as passing (“Good”) and the case of cracks was evaluated as failing (“Poor”).
For the purpose of evaluating the impact resistance of the hot stamped body, the body was evaluated based on the VDA standard (VDA238-100) prescribed by the German Association of the Automotive Industry under the same measurement conditions as Manufacturing Example A. In the present invention, the displacement at the time of maximum load obtained in the bending test was converted to angle by the VDA standard to find maximum bending angle and thereby evaluate the impact resistance of the hot stamped body.
If the tensile strength is 1500 MPa or more, the maximum bending angle (°) was 70(°) or more, and the hydrogen embrittlement resistance was a passing level, it was judged that the impact resistance and hydrogen embrittlement resistance were excellent and the case was indicated as an “invention example”. If even one of the three aspects of performance is not satisfied, the case was indicated as a “comparative example”.
In each hot stamped body of the invention examples, the area rate (%) of the total of the crystal grains with a maximum crystal orientation difference inside the regions surrounded by grain boundaries of 15° or more of 1° or less and the crystal grains with a crystal orientation difference of 8° or more and less than 15° in the metal structures from the surface of the steel sheet for surface layer to ½ of the thickness was 50% to less than 85%. Further, each hot stamped body of the invention examples was excellent in tensile strength, bendability, and hydrogen embrittlement resistance.
As opposed to this, the No. 5B hot stamped body was low in carbon content at the steel sheet for sheet thickness middle part, so the hardness of the middle part in sheet thickness became insufficient and the tensile strength became insufficient. The No. 9B hot stamped body was excessive in carbon content of steel sheet for sheet thickness middle part, so the hardness of the middle part in sheet thickness also became excessive and the targeted bendability could not be obtained. Further, the No. 11B hot stamped body was sparse in Mn content at the steel sheet for sheet thickness middle part, so the hardness of the middle part in sheet thickness became insufficient and the tensile strength became insufficient.
The Nos. 30B to 32B hot stamped bodies are comparative examples produced using the multilayer steel sheets for hot stamped body to which the desirable heat treatment had not been applied before the hot stamping process. The No. 30B hot stamped body was low in heat treatment temperature before the hot stamping process, while the No. 31B hot stamped body was short in heat treatment time before the hot stamping process, so in the metal structures of the softened layer from the surface of the softened layer to ½ of the thickness, the soft structures and metal structures with intermediate hardnesses insufficiently grew and the target bendability could not be obtained. Further, the No. 32B hot stamped body was excessively high in heat treatment temperature before the hot stamping process, so the effect of reduction of the sharp gradient in hardness in the sheet thickness direction occurring at the time of bending deformation could not be obtained.
The No. 38B hot stamped body was low in rolling temperature of the rough rolling. Further, the No. 39B hot stamped body was low in sheet thickness reduction rate of the rough rolling. Further, the No. 40B hot stamped body was low in number of rolling operations under conditions of a time between passes of 3 seconds or more. These hot stamped bodies were not manufactured under the suitable rough rolling conditions, so the soft structures and metal structures with intermediate hardnesses insufficiently grew, it was not possible to ease the strain occurring due to bending deformation, and the targeted bendability could not be obtained.
The No. 41B hot stamped body is a steel sheet controlled in casting rate to 6 ton/min or more in the continuous casting process of steel sheet for surface layer. It can raise the area rate of the total of the crystal grains with a maximum crystal orientation difference inside the regions surrounded by grain boundaries of 15° or more of 1° or less and the crystal grains with a crystal orientation difference of 8° or more and less than 15° in the metal structures from the surface of the steel sheet for surface layer to ½ of the thickness and is excellent in bendability.
Steel sheets for sheet thickness middle part having the chemical compositions shown in Table C-1-1 to Table C-1-2 were ground down at their surfaces to remove the surface oxides. After that, the respective steel sheets for sheet thickness middle part were welded with steel sheets for surface layer having the chemical compositions shown in Table C-1-3 to Table C-1-4 at both surfaces or single surfaces by arc welding to fabricate the Nos. 1 to 49 multilayer steel sheets for hot stamped body. The sheet thickness of the total of the steel sheet for surface layer and the steel sheet for sheet thickness middle part after arc welding was 200 mm to 300 mm and the thickness of the steel sheet for surface layer was ⅓ or so of the thickness of the steel sheet for sheet thickness middle part (in case of single side, ¼ or so). The No. 31 multilayer steel sheet was steel with steel sheet for surface layer welded to only one side. Among the Nos. 1 to 53 multilayer steel sheets of Table C-1-1 to Table C-1-4, ones where the steel sheet for sheet thickness middle part did not satisfy the requirements of composition of the middle part in sheet thickness of the hot stamped body according to the present invention are indicated as “comparative steels” in the remarks columns.
The “ratio of C, Si, and Mn contents of steel sheet for surface layer to steel sheet for sheet thickness middle part” of Table C-1-3 to Table C-1-4 show the ratios of C, Si, and Mn contents of steel sheet for surface layer to the C, Si, and Mn contents of steel sheet for sheet thickness middle part in the Nos. 1 to 53 multilayer steel sheets for hot stamped body.
The Nos. 1 to 53 multilayer steel sheets were respectively treated under the conditions of the Nos. 1 to 53 manufacturing conditions shown in Table C-2-1 to Table C-2-2 by heat treatment before hot rolling, rough rolling, hot rolling, and cold rolling to obtain steel sheets. Next, the steel sheets were heat treated as shown in Table C-2-1 to Table C-2-2 (in the tables, “heat treatment of hot stamped body”) for hot stamping to manufacture the Nos. 1C to 53C hot stamped bodies (“stamped bodies” of Table C-3-1 to Table C-3-2). Further, the No. 30C hot stamped body was coated on a hot dip coating line at the surface with a 120 to 160 g/m2 amount of aluminum. Further, the items in Table C-2-1 to Table C-2-2 correspond to the items in Table A-2-1 to Table A-2-2. Further, in the tables, the fields with the notations “-” indicate no corresponding treatment performed.
Table C-3-1 to Table C-3-2 show the metal structures and characteristics of the Nos. 1C to 53C hot stamped bodies. The constituents obtained by analyzing the positions of ½ of the sheet thicknesses of the samples taken from the hot stamped bodies (middle parts in sheet thickness) and positions of 20 μm from the surfaces of the softened layers were equivalent to the constituents of the steel sheets for sheet thickness middle part and the steel sheets for surface layer of the Nos. 1 to 53 multilayer steel sheets of Table C-1-1 to Table C-1-4.
The metal structures of the hot stamped steel sheets were measured by the above-mentioned method. The hardness of the steel sheet for sheet thickness middle part forming the middle part in sheet thickness and the area rate of the total of the crystal grains with a maximum crystal orientation difference inside the regions surrounded by grain boundaries of 15° or more of 1° or less and the crystal grains with a crystal orientation difference of 8° or more and less than 15° in the metal structures from the surface of the steel sheet for surface layer forming the softened layer to ½ of the thickness of that softened layer were calculated. The calculated values of the area rate are shown in the items “area rate (%) of total of crystal grains with maximum crystal orientation difference inside large angle grain boundaries of 1° or less and crystal grains with maximum crystal orientation difference of 8° or more and less than 15°” of Tables C-3-1 to C-3-2.
The hot stamped bodies were subjected to tensile tests. The results are shown in Table C-3-1 to Table C-3-2. The tensile tests were performed by fabricating No. 5 test pieces described in JIS Z 2201 and testing them by the method described in JIS Z 2241.
The hydrogen embrittlement resistance of the hot stamped body, in the same way as Manufacturing Example A, was evaluated using a test piece cut out from the stamped body. That is, a test piece of a sheet thickness of 1.2 mm×width 6 mm×length 68 mm was cut out from the stamped body, given a strain corresponding to the yield stress in a four-point bending test, then immersed in pH3 hydrochloric acid for 100 hours and evaluated for hydrogen embrittlement resistance by the presence of any cracks. The case of no cracks was indicated as passing (“Good”) and the case of cracks was evaluated as failing (“Poor”).
For the purpose of evaluating the impact resistance of the hot stamped body, the body was evaluated based on the VDA standard (VDA238-100) prescribed by the German Association of the Automotive Industry under the same measurement conditions as Manufacturing Example A. In the present invention, the displacement at the time of maximum load obtained in the bending test was converted to angle by the VDA standard to find maximum bending angle and thereby evaluate the impact resistance of the hot stamped body.
If the tensile strength is 1500 MPa or more, the maximum bending angle (°) was 70(°) or more, the uniform elongation was 5% or more, and the hydrogen embrittlement resistance was a passing level, it was judged that the impact resistance, hydrogen embrittlement resistance, and ductility were excellent and the case was indicated as an “invention example”. If even one of the three aspects of performance is not satisfied, the case was indicated as a “comparative example”.
In each hot stamped body of the invention examples, the area rate of the total of the crystal grains with a maximum crystal orientation difference inside the regions surrounded by grain boundaries of 15° or more of 1° or less and the crystal grains with a crystal orientation difference of 8° or more and less than 15° in the metal structures from the surface of the steel sheet for surface layer to ½ of the thickness of the steel sheet for surface layer was 50% to less than 85%. Further, each hot stamped body of the invention examples was excellent in tensile strength, bendability, and hydrogen embrittlement resistance.
As opposed to this, the No. 5C hot stamped body was low in carbon content of the steel sheet for sheet thickness middle part, so became insufficient in hardness of the middle part in sheet thickness and became insufficient in tensile strength. The No. 9C hot stamped body was excessive in carbon content of the steel sheet for sheet thickness middle part, so also became excessive in hardness of the middle part in sheet thickness and could not be given the targeted bendability. Further, the No. 11C hot stamped body was low in Si content of the steel sheet for sheet thickness middle part, so the area percent of the residual austenite of the metal structures at the middle part in sheet thickness was less than 1.0% and the uniform elongation was low.
The Nos. 25C to 27C and 49C hot stamped bodies are comparative examples manufactured using the multilayer steel sheets for hot stamped body to which the preferable heat treatment is not applied before the hot stamping process. The No. 25C hot stamped body is too low in heat treatment temperature before the hot stamping process, so the soft structures and metal structures with intermediate hardnesses insufficiently grew, the effect of surface properties of the hot stamped body and effect of the transitional part from the middle part in sheet thickness to the softened layer could not be eliminated, and excellent bendability could not be obtained.
Further, the No. 26C hot stamped body was excessively high in heat treatment time before the hot stamping process, so the soft structures and metal structures with intermediate hardnesses excessively grew, the difference in hardness between the softened layer and the middle part in sheet thickness became too large, and the effect of reducing the sharp gradient of hardness in the sheet thickness direction occurring at the time of bending deformation could not be obtained. For this reason, the No. 26C hot stamped body could not be given the targeted bendability.
Further, the Nos. 27C and 49C hot stamped bodies were too long in heat treatment time before the hot stamping process, the difference in hardness between the softened layer and the middle part in sheet thickness become too great. Further, the heat treatment temperature was excessively high, so the effect of reducing the sharp gradient of hardness in the sheet thickness direction occurring at the time of bending deformation could not be obtained. For this reason, the Nos. 27C and 49C hot stamped bodies could not be given excellent bendability.
The No. 50C hot stamped body was low in rolling temperature of the rough rolling. Further, the No. 51C hot stamped body was low in sheet thickness reduction rate of the rough rolling. Further, the No. 52C hot stamped body was low in number of rolling operations under conditions of a time between passes of 3 seconds or more. These hot stamped bodies were not manufactured under the suitable rough rolling conditions, so the soft structures and metal structures with intermediate hardnesses insufficiently grew, it was not possible to ease the strain occurring due to bending deformation, and the targeted bendability could not be obtained.
The No. 53C hot stamped body is a steel sheet controlled in casting rate to 6 ton/min or more in the continuous casting process of steel sheet for surface layer. It can raise the area rate of the total of the crystal grains with a maximum crystal orientation difference inside the regions surrounded by grain boundaries of 15° or more of 1° or less and the crystal grains with a crystal orientation difference of 8° or more and less than 15° in the metal structures from the surface of the steel sheet for surface layer to ½ of the thickness and is excellent in bendability.
1070
1020
1370
15
1013
4
1
402
1116
Comp. ex.
823
39.4
Comp. ex.
4.2
0.8
Comp. ex.
15
54.3
Poor
Comp. ex.
92
54.2
Comp. ex.
13
62
Poor
Comp. ex.
9
59.1
Poor
Comp. ex.
10
59.8
Poor
Comp. ex.
13
60.4
Poor
Comp. ex.
Steel sheets for sheet thickness middle part having the Nos. 1 to 37 chemical compositions shown in Table D-1-1 to Table D-1-2 (in the tables, “Steel Nos. 1 to 37”) were ground down at their surfaces to remove the surface oxides. After that, the respective steel sheets for sheet thickness middle part were welded with steel sheets for surface layer having the chemical compositions shown in Table D-1-3 to Table D-1-4 at both surfaces or single surfaces by arc welding to fabricate the Nos. 1 to 60 multilayer steel sheets for hot stamped body. The sheet thickness of the total of the steel sheet for surface layer and the steel sheet for sheet thickness middle part after arc welding was 200 mm to 300 mm and the thickness of the steel sheet for surface layer was ⅓ or so of the thickness of the steel sheet for sheet thickness middle part (in case of single side, ¼ or so). The No. 37 multilayer steel sheet is steel with the steel sheet for surface layer welded to only one surface. The multilayer steel sheets other than No. 37 have steel sheets for surface layer welded to both surfaces of the steel sheet for sheet thickness middle part. In the Nos. 1 to 60 multilayer steel sheets of Table D-1-1 to Table D-1-4, cases where the steel sheet for sheet thickness middle part does not satisfy the requirement of the composition of the middle part in sheet thickness of the hot stamped body according to the present invention are indicated as “comparative steels” in the remarks column.
The Nos. 1 to 60 multilayer steel sheets were treated under the conditions of the Nos. 1 to 60 manufacturing conditions shown in Table D-2-1 to Table D-2-3 by heat treatment before hot rolling, rough rolling, hot rolling, and cold rolling to obtain steel sheets. Next, the steel sheets were heat treated as shown in Table D-2-1 to Table D-2-3 (in the tables, “heat treatment of hot stamped bodies”) for hot stamping to produce the Nos. 1D to 60D hot stamped bodies (“stamped bodies” of Tables D-3-1 to D-3-3). Further, the Nos. 38D and 39D hot stamped bodies were coated on a hot dip coating line at the surfaces with 120 to 160 g/m2 amounts of aluminum. Further, the items of Table D-2-1 to Table D-2-3 correspond to the items of Table A-2-1 to Table A-2-2. Further, in the tables, the fields with the notations “-” indicate no corresponding treatment performed.
Tables D-3-1 to D-3-3 show the metal structures and characteristics of the Nos. 1D to 60D hot stamped bodies. The constituents obtained by analyzing the positions of ½ of the sheet thicknesses of the samples taken from hot stamped bodies (middle parts in sheet thickness) and positions of 20 μm from the surfaces of the softened layers were equivalent to the constituents of the steel sheets for sheet thickness middle part and the steel sheets for surface layer of the Nos. 1 to 60 multilayer steel sheets of Table D-1-1 to Table D-1-3.
The metal structures of the hot stamped steel sheets were measured by the above-mentioned method. The hardness of the steel sheet for sheet thickness middle part forming the middle part in sheet thickness and the area rate of the total of the crystal grains with a maximum crystal orientation difference inside the regions surrounded by grain boundaries of 15° or more of 1° or less and the crystal grains with a crystal orientation difference of 8° or more and less than 15° in the metal structures from the surface of the steel sheet for surface layer forming the softened layer to ½ of the thickness of that softened layer were calculated. The calculated values of the area rate are shown in the items “area rate (%) of total of crystal grains with maximum crystal orientation difference inside large angle grain boundaries of 1° or less and crystal grains with maximum crystal orientation difference of 8° or more and less than 15°” of Tables D-3-1 to D-3-3.
The Nos. 1D to 60D hot stamped bodies were subjected to tensile tests. The results are shown in Tables D-3-1 to D-3-3. The tensile tests were performed by fabricating No. 5 test pieces described in JIS Z 2201 and testing them by the method described in JIS Z 2241.
The hot stamped bodies were evaluated for hydrogen embrittlement resistance in the same way as Manufacturing Example A using test pieces cut out from the stamped bodies. That is, test pieces of a sheet thickness of 1.2 mm×width 6 mm×length 68 mm were cut out from the stamped bodies, given strain corresponding to the yield stress in four-point bending tests, then immersed in pH3 hydrochloric acid for 100 hours and evaluated for hydrogen embrittlement resistance by the presence of any cracks. Cases of no fracture were evaluated as passing (“good”) and cases of fracture were evaluated as failing (“Poor”).
For the purpose of evaluating the impact resistance of the hot stamped body, the body was evaluated based on the VDA standard (VDA238-100) prescribed by the German Association of the Automotive Industry under the same measurement conditions as Manufacturing Example A. In the present invention, the displacement at the time of maximum load obtained in the bending test was converted to angle by the VDA standard to find maximum bending angle and thereby evaluate the impact resistance of the hot stamped body.
The hot stamped bodies were also evaluated for impact resistance from the viewpoint of ductility. Specifically, the hot stamped steel sheets were subjected to tensile tests to find the uniform elongations of the steel sheet to evaluate the impact resistance. The tensile tests were performed by fabricating No. 5 test pieces described in JIS Z 2201 and testing them by the method described in JIS Z 2241. The elongations where the maximum tensile loads were obtained were defined as the uniform elongations.
Deformation concentrates at a local softened part at the time of collision and becomes a cause of cracking, so a small scattering in hardness at the stamped body, that is, securing stable strength, is important in securing impact resistance. Therefore, the impact resistance of a hot stamped body was also evaluated from the viewpoint of the scattering in hardness. A cross-section vertical to the longitudinal direction of a long hot stamped body was taken at any position in that longitudinal direction and measured for hardness at the middle position in sheet thickness at the entire cross-sectional region including the vertical walls. For the measurement, use was made of a Vickers hardness tester. The measurement load was 1 kgf, 10 points were measured, and the measurement interval was 1 mm. The difference between the average cross-sectional hardness and the minimum hardness is shown in Table D-3-1 to Table D-3-3. Cases with no measurement points of below 100 Hv from the average value of all measurement points were evaluated as being small in scattering in hardness, that is, excellent in stability of strength and, as a result, were evaluated as excellent in impact resistance and therefore passing, while cases with measurement points below 100 Hv were evaluated as failing.
Cases where the tensile strength was 1500 MPa or more, the uniform elongation was 5% or more, the scattering in hardness was a passing level, the maximum bending angle (°) was 70.0(°) or more, and the hydrogen embrittlement resistance was passing were evaluated as hot stamped bodies excellent in impact resistance and hydrogen embrittlement resistance (“invention examples” in Table D-3-1 to Table D-3-3). On the other hand, cases where even one of the above five aspects of performance was not satisfied are indicated as “comparative examples”.
In each of the hot stamped bodies of the invention examples, the area rate of the total of crystal grains with a maximum crystal orientation difference inside regions surrounded by grain boundaries of 15° or higher of 1° or less and crystal grains with a crystal orientation difference of 8° or more and less than 15° in the metal structures from the surface of the steel sheet for surface layer to ½ of the thickness was 50% to less than 85%. Further, in each of the hot stamped bodies of the invention examples, the tensile strength, bendability, and hydrogen embrittlement resistance were excellent.
As opposed to this, the No. 5D hot stamped body was low in carbon content of the steel sheet for sheet thickness middle part, so became insufficient in hardness of the middle part in sheet thickness and became insufficient in tensile strength. The No. 9D hot stamped body was excessive in carbon content of the steel sheet for sheet thickness middle part, so became excessive in hardness of the middle part in sheet thickness as well and could not be given the targeted bendability. Further, the Nos. 10D and 11D hot stamped bodies were sparse in Si content of the steel sheet for sheet thickness middle part, so were insufficient in uniform elongation. Further, the No. 12D hot stamped body was insufficient in Mn content, so became insufficient in hardness of the middle part in sheet thickness and were insufficient in tensile strength. The No. 14D and the No. 15D hot stamped bodies were sparse in Si content and Mn content, so had an area percent of residual austenite of less than 1.0% and an insufficient uniform elongation. Further, the No. 12D to No. 15D hot stamped bodies were large in scattering in hardness and deemed failing.
The Nos. 33D to 35D hot stamped bodies are comparative examples produced using multilayer steel sheets for hot stamped body which were not subjected to the desirable heat treatment before the hot stamping process. The No. 33D hot stamped body was low in heat treatment temperature before the hot stamping process, so became insufficient in growth of soft structures and metal structures of intermediate hardnesses in the metal structures of the softened layer from the surface of the softened layer to ½ of the thickness and was not able to be given the targeted bendability. The No. 34D hot stamped body was excessively high in heat treatment temperature before the hot stamping process, so the fraction of structures from a position of 20 μm from the surface of the softened layer to a position of a depth of ½ of the thickness of the softened layer exceeded 85%. For this reason, in the No. 34D hot stamped body, the difference in hardness between the softened layer and the middle part in sheet thickness became too large, and the effect of reduction of the sharp gradient in hardness in the sheet thickness direction occurring at the time of bending deformation could not be obtained. Further, the No. 35D hot stamped body was short in heat treatment time before the hot stamping process, so in the metal structures from the surface of the softened layer to ½ of the thickness, the soft structures and metal structures with intermediate hardnesses insufficiently grew and the target bendability could not be obtained.
The No. 40D hot stamped body was excessive in Si content, so residual austenite was excessively produced exceeding an area percent of 5%. For this reason, the No. 40D hot stamped body was inferior in bendability. The No. 41D hot stamped body was excessive in Mn content, so became the greatest in tensile strength among the Nos. 1D to 56D hot stamped bodies and was inferior in bendability. The No. 42D hot stamped body was poor in content of acid soluble aluminum, so inclusions containing oxygen were excessively produced and bendability was inferior. Further, the No. 45D hot stamped body included excessive aluminum, so oxides mainly comprised of aluminum were excessively produced and bendability was inferior.
The No. 57D hot stamped body was low in rolling temperature of the rough rolling. Further, the No. 58D hot stamped body was low in sheet thickness reduction rate of the rough rolling. Further, the No. 59D hot stamped body was low in number of rolling operations under conditions of a time between passes of 3 seconds or more. These hot stamped bodies were not produced under optimal rough rolling conditions, so were insufficient in growth of soft structures and metal structures of intermediate hardnesses, were not able to be eased in strain caused by bending deformation, and were not able to be given the targeted bendability.
The No. 60D hot stamped body is steel sheet with a casting rate controlled to 6 ton/min or more in a continuous casting process of steel sheet for surface layer. It can raise the area rate of the total of crystal grains with a maximum crystal orientation difference inside regions surrounded by grain boundaries of 15° or higher of 1° or less and crystal grains with a crystal orientation difference of 8° or more and less than 15° in the metal structures from the surface of the steel sheet for surface layer to ½ of the thickness and is excellent in bendability.
0.08
Comp. steel
0.83
Comp. steel
0.13
Comp. steel
0.41
Comp. steel
0.19
Comp. steel
0.90
Comp. steel
0.48
1.34
Comp. steel
0.27
1.18
Comp. steel
5.30
Comp. steel
4.90
Comp. steel
0.0001
Comp. steel
4.200
Comp. steel
Comp. steel
Comp. steel
Comp. steel
Comp. steel
Comp. steel
Comp. steel
Comp. steel
Comp. steel
Comp. steel
Comp. steel
Comp. steel
Comp. steel
1084
1052
1372
14
1019
420
1218
Comp. ex.
1014
61.2
Comp. ex.
0.2
2.6
Comp. ex.
0.5
4.3
Comp. ex.
443
1285
167
Comp. ex.
492
1427
155
Comp. ex.
0.7
4.8
115
Comp. ex.
0.3
3.7
127
Comp. ex.
14
66.8
Poor
Comp. ex.
95
67.4
Comp. ex.
17
61.9
Poor
Comp. ex.
12.5
61.8
Comp. ex.
Comp. ex.
64.1
Comp. ex.
56.1
Comp. ex.
10
59.2
Poor
Comp. ex.
11
61.4
Poor
Comp. ex.
12
61.9
Poor
Comp. ex.
The hot stamped body of the present invention is excellent in strength, ductility, bendability, impact resistance, and hydrogen embrittlement resistance and is small in scattering in hardness, so can be suitably used for structural members or reinforcing members for automobiles or structures requiring strength.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2017-029318 | Feb 2017 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2018/006086 | 2/20/2018 | WO | 00 |
