HOT STAMPED BODY

FIELD

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, ductility, impact resistance, and hydrogen embrittlement resistance after hot stamping.

BACKGROUND

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.

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, when the hardenability of the steel sheet is low etc., 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. Furthermore, if bending deformation occurs at the time of collision of an automobile, a 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 is affected not only by the strength of the member, but also the ease of buckling. In the state of a member, if the ductility of the steel sheet is high, it becomes harder for localization of the deformation region to occur. That is, the sheet becomes resistant to buckling.

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 member 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.

CITATION LIST
Patent Literature
[PTL 1] Japanese Unexamined Patent Publication No. 2015-30890
[PTL 2] Japanese Unexamined Patent Publication No. 2006-104546
[PTL 3] WO 2015/097882
SUMMARY
Technical Problem

The present invention, in consideration of the technical issues in the prior art, has as its technical problem to obtain a strength of a tensile strength of 1500 MPa or more and achieve both a high bendability for realizing impact resistance and hydrogen embrittlement resistance and keep down the scattering in hardness and has as its object the provision of a hot stamped body solving this technical problem. Further, the present invention has as its object the provision of a hot stamped body achieving both high ductility and high hydrogen embrittlement resistance.

Solution to Problem

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 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 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”. Further, it was discovered that these measurements should be 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 this.

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 and ductility and keeping down the scattering in hardness 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 20% or more and less than 50%,

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% to 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 3.0%, 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.

Advantageous Effects of Invention

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.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic view for explaining the diffusion of C atoms when producing a hot stamped body of the present invention.

FIG. 2 is a graph showing the change in dislocation density after a rolling pass relating to rough rolling used in the method for producing the hot stamped body of the present invention.

DESCRIPTION OF EMBODIMENTS
(Structure of Hot Stamped Body According to Present Invention)

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.

(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, so may be added up to 0.50% as an upper limit from the viewpoint of improvement of strength. On the other hand, even if added in more than 0.50%, the effect of improvement of strength becomes saturated, and therefore 0.50% is the upper limit. Preferably it is 0.30% or less.

Si further is an element having the effect of raising the ductility without impairing the hydrogen embrittlement resistance and bendability manifested by control of the structures of the surface layer. In particular, if bending deformation occurs at the time of collision of an automobile, buckling of a hat-shaped member causes the deformation to become localized and the load resistance of the member to drop. That is, the maximum load of the member is affected by not only the strength of the member, but also the ease of buckling. In the state of the member, if the ductility of the steel sheet is high, it becomes harder for localization of the deformation region to occur. That is, the sheet becomes resistant to buckling.

In a hot stamped member as well, while the ductility is important, in general the ductility of martensite is low. By adding Si in more than 0.50%, it is possible to secure residual austenite in an area percent of 1.0% or more and thereby improve the ductility. From such a viewpoint, Si is preferably added in more than 0.50%. More preferably, the content is 1.00% or more. On the other hand, if adding 3.00% or more, the residual austenite becomes present in an area percent of 5.0% or more and deterioration of the bendability is invited, and therefore the upper limit is less than 3.00%. Preferably, the content is less than 2.00%.

(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: 0.10% 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 0.10% 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: 0.10% or Less)

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 3.0000%, the effect becomes saturated, and therefore the content is 3.0000% or less.

(N: 0.01% 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: 0.01% or More and 3.00% or Less)

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, so 0.01% or more is added. 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: 0.010% or More and 0.150% 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: 0.010% or More and 0.150% 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: 0.005% or More and 1.000% 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: 0.0005% or More and 0.0100% 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 0.01% 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.

(Hardness of Middle Part in Sheet Thickness is 500 Hv or More and 800 Hv or Less)

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.

(Metal Structures at 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 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 at the middle part in sheet thickness 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.

(Softened Layer)

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. Below, the metal structures and composition etc., of the softened layer will be explained.

(Metal Structures of Softened Layer)

The inventors engaged in intensive studies and as a result discovered, as a result of investigation of the metal structures of steel sheets where good bendability was obtained, 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”. It was discovered that these measurements should be 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 learned 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. 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 softened layer can be eliminated by this.

In the above-mentioned metal structures of the softened layer, if 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 less than 20%, this effect is not sufficiently obtained, and therefore the lower limit is 20%. Preferably, the area rate is 20% or more, more preferably it may be 25% or more. On the other hand, with an area rate of the total of the metal structures of the softened layer of 50% or more, the difference in hardness of the softened layer and the middle part in sheet thickness becomes greater 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 sufficiently obtained, and therefore the area rate is less than 50%. More preferably, it may be 45% 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-HvB10 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 perpendicular 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 (DVC5 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.

(Composition of Softened Layer)

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 0.6 time or less 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, 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: 0.01% or More and 1.80% or Less)

Mn is an element contributing to improvement of strength by solution strengthening, so is added for raising the strength. To make the hardness of the surface 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. For this reason, the preferable Mn content of the surface layer is less than 1.80%, preferably 1.40% or less, more preferably less than 0.90%, still more preferably 0.70% or less.

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, the preferable Mn content of the softened layer is 0.12% to less than 0.90%, preferably 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 1.80% or less.

(P: 0.10% 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: 0.10% or Less)

(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 3.0%, the effect becomes saturated, and therefore the content is 3.0% or less.

(N: 0.01% or Less)

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: 0.01% or More and 3.00% or Less)

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 the content is 0.01% or more. 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: 0.010% or More and 0.150% 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 the content is made 0.010% or 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: 0.010% or More and 0.150% or Less)

(Mo: 0.005% or More and 1.000% 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 0.010% 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: 0.0005% or More and 0.01% 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 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%, since the effect becomes saturated, the content is 0.0100% or less. Preferably, the content is 0.0075% or less.

(Cross-Sectional Distribution of Hardness of Hot Stamped Body)

At the cross-section vertical to the surface of the hot stamped body, the distribution of hardness is preferably uniform. 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 using a Vickers hardness tester (load of 1 kgf).

(Formation of Plated Layer)

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.

(Method of Production of Hot Stamped Body According to Present Invention)

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 is superposed on each ground down surface side. The method of joining the steel sheet for softened 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 softened 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 6 ton/min or more in the continuous casting process of the steel sheet for softened layer, it is possible to keep down microsegregation of Mn in the steel sheet for softened layer and possible to make the distribution of concentration of Mn at the steel sheet for softened 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/h or more in the continuous casting process of steel sheet for softened layer for the purpose of promoting the formation of the above microstructures.

Further, a double layer steel sheet fabricated by the above method and further held at 1100° C. or more and 1350° C. or less in temperature for 60 minutes or more 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 60 minutes or more, 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 20% to less than 50% 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 upper limit is not particularly limited, but if holding the sheet for more than 300 minutes, the heating cost greatly rises and the result becomes economically disadvantageous, so in actual operation, 300 minutes is the substantive upper limit.

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 FIG. 1 occur. (i) shows the diffusion of C atoms from the middle part in sheet thickness to the surface layer, while (ii) shows the decarburization reaction of C being desorbed from the surface layer to the outside. A distribution occurs in the concentration of C due to the balance between this diffusion of C atoms and the desorption reaction of (i) and (ii). With less than 1100° C., the reaction of (i) is insufficient, so the preferable distribution of the concentration of C cannot be obtained. On the other hand, with more than 1350° C., the reaction of (ii) excessively occurs, so similarly a preferable distribution of concentration cannot be obtained.

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 FIG. 1 by the strain introduced in the rough rolling. Even if using an ordinary method to rough roll and finish roll a slab controlled in concentration of C to a preferable state by hot rolling heating, the sheet thickness will be reduced without the C atoms sufficiently diffusing in the surface layer. Therefore, if manufacturing hot rolled steel sheet of a thickness of several mm from a slab having a thickness more than 200 mm through a general hot rolling process, the result will be a steel sheet changing rapidly in concentration of C at the surface layer. A moderate hardness change will no longer be able to be obtained. The method discovered to solve this is the above pass control of the rough rolling. The diffusion of C atoms is greatly affected by not only the temperature, but also the strain (dislocation density). In particular, compared with lattice diffusion, with dislocation diffusion, the diffusion frequency becomes 10 times or more higher, so steps have to be taken to leave the dislocation density while rolling to reduce the sheet thickness. Curve 1 of FIG. 2 shows the change in the dislocation density after a rolling pass in the case where the sheet thickness reduction rate per pass in the rough rolling is small. It will be understood that strain remains over a long time period. By causing strain to remain at the surface layer over a long time period in this way, C atoms sufficiently diffuse in the surface layer and the optimum distribution of concentration of C can be obtained. On the other hand, curve 2 shows the change in dislocation density in the case where the sheet thickness reduction rate per pass of rough rolling is large. If the amount of strain introduced by the rolling rises, recovery is easily promoted and the dislocation density rapidly falls. For this reason, to obtain the optimal distribution of concentration of C, it is necessary to prevent the occurrence of a change in dislocation density like the curve 2. From such a viewpoint, the upper limit of the sheet thickness reduction rate per pass becomes less than 50%. To promote the diffusion of C atoms at the surface layer, certain amounts of dislocation density and holding time have to be secured, so the lower limit of the sheet thickness reduction rate becomes 5%. As the time between passes, 3 seconds or more has to be secured.

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.

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.

EXAMPLES

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.

Manufacturing Example A

The Nos. 1 to 19 steel sheets for sheet thickness middle part having the chemical compositions shown in Table A-1-1 to Table A-1-2 (in the tables, “Steel Nos. 1 to 19”) 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 softened layer forming use having the chemical compositions shown in Table A-1-1 to Table A-1-2 (below, referred to as the “steel sheets for surface layer”) at both surface or single surfaces by arc welding to fabricate the Nos. 1 to 44 multilayer steel sheets for hot stamped body. In the tables, fields in which the constituents are indicated as 0 show that the corresponding constituents are not intentionally added.

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. 38 multilayer steel sheet is steel with the steel sheet for surface layer welded to only one surface. In the Nos. 1 to 44 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 44 multilayer steel sheets were respectively treated under the conditions of the Nos. 1 to 44 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 44A hot stamped bodies (“shaped bodies” of Table A-3). Further, the Nos. 36A and 37A hot stamped bodies were coated on a hot dip coating line at the surfaces of the matrix steel sheets with 120 to 160 g/m²amounts 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 44A 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 44 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 90(°) 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 20% to less than 50%. 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 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. 31A hot stamped body was excessively high in heat treatment temperature 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 reduction of the sharp gradient in hardness in the sheet thickness direction occurring at the time of bending deformation could not be obtained. For this reason, the No. 31A hot stamped body could not be given excellent bendability. The No. 32A hot stamped body was too 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. 41A hot stamped body was low in rolling temperature of the rough rolling. Further, the No. 42A hot stamped body was low in sheet thickness reduction rate of the rough rolling. Further, the No. 43A 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. 44A 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.

TABLE A-1-1

Multilayer
Chemical constituents of steel sheet for sheet thickness middle part (mass %)

steel
Steel

sol.

sheet no.
no.
C
Si
Mn
P
S
Al
N
Ni
Nb
Ti
Mo
B
Remarks

1
1
0.21
0.13
1.31
0.016
0.0026
0.0370
0.003
0
0
0
0
0

2
2
0.35
0.08
1.35
0.013
0.0011
0.0370
0.003
0
0
0
0
0

3
3
0.29
0.1
1.25
0.016
0.0008
0.0390
0.0042
0
0
0
0
0

4
4
0.53
0.18
1.37
0.009
0.0007
0.0440
0.0023
0
0
0
0
0

5
5
0.17
0.13
1.26
0.01
0.0005
0.0470
0.0027
0
0
0
0
0
Comp.

steel

6
6
0.21
0.11
1.35
0.003
0.001
0.0400
0.0022
0
0
0
0
0

7
7
0.23
0.14
1.37
0.009
0.0004
0.0510
0.0037
0
0
0
0
0

8
8
0.26
0.15
1.22
0.005
0.0012
0.0520
0.0031
0
0
0
0
0

9
9
0.86
0.11
1.34
0.002
0.0011
0.0380
0.0024
0
0
0
0
0
Comp.

steel

10
10
0.23
0.33
1.26
0.006
0.0002
0.0410
0.0026
0
0
0
0
0

11
11
0.36
0.23
0.08
0.005
0.0005
0.0460
0.0022
0
0
0
0
0
Comp.

steel

12
12
0.33
0.22
0.74
0.002
0.001
0.0500
0.0027
0
0
0
0
0

13
13
0.25
0.15
1.23
0.012
0.0004
0.0480
0.0038
0.07
0
0
0
0

14
14
0.26
0.08
1.31
0.005
0.0004
0.0430
0.0024
0
0.032
0
0
0

15
15
0.28
0.12
1.36
0.011
0.0006
0.0340
0.0032
0
0
0.026
0
0

16
16
0.22
0.18
1.31
0.014
0.0008
0.0430
0.0028
0
0
0
0.04
0

17
17
0.27
0.12
1.37
0.012
0.001
0.0500
0.0028
0
0
0
0
0.0015

18
1
0.24
0.13
1.31
0.016
0.0026
0.0370
0.003
0
0
0
0
0

19
1
0.23
0.13
1.31
0.016
0.0026
0.0370
0.003
0
0
0
0
0

20
1
0.25
0.13
1.31
0.016
0.0026
0.0370
0.003
0
0
0
0
0

21
2
0.35
0.08
1.35
0.013
0.0011
0.0370
0.003
0
0
0
0
0

22
2
0.35
0.08
1.35
0.013
0.0011
0.0370
0.003
0
0
0
0
0

23
2
0.35
0.08
1.35
0.013
0.0011
0.0370
0.003
0
0
0
0
0

24
3
0.29
0.1
1.25
0.016
0.0008
0.0390
0.0042
0
0
0
0
0

25
3
0.29
0.1
1.25
0.016
0.0008
0.0390
0.0042
0
0
0
0
0

26
3
0.29
0.1
1.25
0.016
0.0008
0.0390
0.0042
0
0
0
0
0

27
4
0.53
0.18
1.37
0.009
0.0007
0.0440
0.0023
0
0
0
0
0

28
4
0.53
0.18
1.37
0.009
0.0007
0.0440
0.0023
0
0
0
0
0

29
4
0.53
0.18
1.37
0.009
0.0007
0.0440
0.0023
0
0
0
0
0

30
2
0.35
0.08
1.35
0.013
0.0011
0.0370
0.003
0
0
0
0
0

31
2
0.35
0.08
1.35
0.013
0.0011
0.0370
0.003
0
0
0
0
0

32
2
0.35
0.08
1.35
0.013
0.0011
0.0370
0.003
0
0
0
0
0

33
2
0.35
0.08
1.35
0.013
0.0011
0.0370
0.003
0
0
0
0
0

34
2
0.35
0.08
1.35
0.013
0.0011
0.0370
0.003
0
0
0
0
0

35
18
0.61
0.14
1.32
0.003
0.0004
0.0520
0.0037
0
0
0
0
0

36
18
0.61
0.14
1.32
0.003
0.0004
0.0520
0.0037
0
0
0
0
0

37
2
0.35
0.08
1.35
0.013
0.0011
0.0370
0.003
0
0
0
0
0

38
2
0.35
0.08
1.35
0.013
0.0011
0.0370
0.003
0
0
0
0
0

39
19
0.45
0.17
1.35
0.009
0.0001
0.0400
0.0028
0
0
0
0
0

40
19
0.45
0.17
1.35
0.009
0.0001
0.0400
0.0028
0
0
0
0
0

41
2
0.35
0.08
1.35
0.013
0.0011
0.0370
0.003
0
0
0
0
0

42
2
0.35
0.08
1.35
0.013
0.0011
0.0370
0.003
0
0
0
0
0

43
2
0.35
0.08
1.35
0.013
0.0011
0.0370
0.003
0
0
0
0
0

44
2
0.35
0.08
1.35
0.013
0.0011
0.0370
0.003
0
0
0
0
0

TABLE A-1-2

Multilayer
Composition of constituents of steel sheet for surface layer (mass %)

steel sheet no.
C
Si
Mn
P
S
sol.Al
N
Ni
Nb
Ti
Mo
B
Remarks

1
0.080
0.072
0.642
0.015
0.0028
0.0390
0.0029
0
0
0
0
0

2
0.196
0.038
0.608
0.012
0.0011
0.0380
0.0029
0
0
0
0
0

3
0.107
0.044
0.638
0.016
0.0010
0.0400
0.0043
0
0
0
0
0

4
0.260
0.083
0.630
0.011
0.0008
0.0460
0.0023
0
0
0
0
0

5
0.092
0.068
0.554
0.009
0.0006
0.0450
0.0027
0
0
0
0
0

Comp. steel

6
0.116
0.053
0.621
0.004
0.0012
0.0380
0.0022
0
0
0
0
0

7
0.138
0.069
0.575
0.007
0.0006
0.0520
0.0037
0
0
0
0
0

8
0.114
0.069
0.464
0.005
0.0013
0.0510
0.0032
0
0
0
0
0

9
0.404
0.046
0.643
0.003
0.0009
0.0360
0.0024
0
0
0
0
0

Comp. steel

10
0.104
0.185
0.693
0.006
0.0002
0.0400
0.0026
0
0
0
0
0

11
0.212
0.113
0.035
0.006
0.0003
0.0450
0.0023
0
0
0
0
0

Comp. steel

12
0.165
0.103
0.281
0.003
0.0011
0.0480
0.0026
0
0
0
0
0

13
0.123
0.072
0.677
0.013
0.0003
0.0500
0.0039
0.02
0
0
0
0

14
0.125
0.033
0.537
0.003
0.0002
0.0450
0.0023
0
0.031
0
0
0

15
0.123
0.053
0.653
0.010
0.0008
0.0330
0.0032
0
0
0.023
0
0

16
0.130
0.083
0.59
0.014
0.0007
0.0410
0.0027
0
0
0
0.04
0

17
0.113
0.055
0.699
0.014
0.0012
0.0520
0.0028
0
0
0
0
0.0017

18
0.103
0.116
1.245
0.017
0.0027
0.0380
0.0029
0
0
0
0
0

19
0.101
0.118
0.655
0.018
0.0026
0.0390
0.0030
0
0
0
0
0

20
0.145
0.057
1.061
0.015
0.0024
0.0360
0.0031
0
0
0
0
0

21
0.312
0.038
0.662
0.014
0.0012
0.0390
0.0030
0
0
0
0
0

22
0.308
0.044
1.134
0.014
0.0009
0.0360
0.0030
0
0
0
0
0

23
0.301
0.070
0.594
0.012
0.0012
0.0350
0.0029
0
0
0
0
0

24
0.220
0.043
0.563
0.017
0.0008
0.0370
0.0043
0
0
0
0
0

25
0.133
0.091
0.638
0.014
0.0008
0.0400
0.0041
0
0
0
0
0

26
0.128
0.045
0.975
0.016
0.0007
0.0370
0.0041
0
0
0
0
0

27
0.429
0.092
0.685
0.008
0.0009
0.0430
0.0023
0
0
0
0
0

28
0.217
0.160
0.617
0.007
0.0008
0.0450
0.0022
0
0
0
0
0

29
0.233
0.074
1.206
0.007
0.0005
0.0440
0.0023
0
0
0
0
0

30
0.151
0.04
0.554
0.011
0.0013
0.0360
0.0031
0
0
0
0
0

31
0.165
0.044
0.486
0.011
0.0009
0.0380
0.0031
0
0
0
0
0

32
0.147
0.047
0.486
0.012
0.0010
0.0370
0.0031
0
0
0
0
0

33
0.179
0.037
0.581
0.011
0.0012
0.0390
0.0031
0
0
0
0
0

34
0.182
0.046
0.621
0.011
0.0010
0.0360
0.0029
0
0
0
0
0

35
0.348
0.081
0.488
0.002
0.0002
0.0500
0.0037
0
0
0
0
0

36
0.299
0.066
0.581
0.005
0.0005
0.0520
0.0036
0
0
0
0
0

37
0.154
0.042
0.648
0.013
0.0010
0.0370
0.0029
0
0
0
0
0

38
0.196
0.038
0.608
0.012
0.0011
0.0360
0.0030
0
0
0
0
0

39
0.221
0.092
0.689
0.011
0.0004
0.0380
0.0027
0
0
0
0
0

40
0.410
0.148
1.094
0.007
0.0007
0.0410
0.0028
0
0
0
0
0

41
0.196
0.038
0.608
0.012
0.0011
0.0380
0.0029
0
0
0
0
0

42
0.196
0.038
0.608
0.012
0.0011
0.0380
0.0029
0
0
0
0
0

43
0.196
0.038
0.608
0.012
0.0011
0.0380
0.0029
0
0
0
0
0

44
0.196
0.038
0.608
0.012
0.0011
0.0380
0.0029
0
0
0
0
0

TABLE A-2-1

Rough rolling

Heat treatment at hot stamping

Heat treatment

Rate of

Cold

Average

before hot

reduction

roll-

cooling
Average

Multi-

rolling

of
No. of
Hot rolling
ing

rate
cooling

layer
Manu-
Heat-
Hold-
Roll-
sheet
rolling
Finish
Coil-
Roll-
Heat-
Heat-
(° C./s)
rate
Temper-

Sheet

steel
facturing
ing
ing
ing
thick-
oper-
rolling
ing
ing
ing
ing
(more
(° C./s)
ing

thick-

sheet
condition
temp.
time
temp.
ness
ations
temp.
temp.
rate
rate
temp.
than
(400° C.
temp.
Plat-
ness

no.
no.
(° C.)
(min)
(° C.)
(%)
(times)
(° C.)
(° C.)
(%)
(° C./s)
(° C.)
400° C.)
or less)
(° C.)
ing
(mm)

1
1
1317
115
1159
39
3
892
721
58
37
847
65
58
—
None
1.2

2
2
1256
96
1161
31
3
848
699
61
39
848
102
96
—
None
1.1

3
3
1301
86
1135
24
3
892
674
45
51
882
78
71
—
None
1.5

4
4
1279
112
1151
39
3
910
651
53
57
916
95
88
—
None
1.3

5
5
1276
118
1140
35
3
882
569
55
56
849
94
89
—
None
1.3

6
6
1307
128
1158
35
3
879
675
61
71
891
68
59
—
None
1.1

7
7
1329
102
1194
43
3
889
688
48
63
822
84
78
—
None
1.5

8
8
1315
122
1129
44
3
904
698
40
50
838
84
79
—
None
1.7

9
9
1294
96
1165
25
3
901
705
63
26
872
72
63
—
None
1.0

10
10
1319
109
1135
35
3
838
574
48
44
836
75
68
—
None
1.5

11
11
1327
109
1125
30
3
885
693
54
58
903
100
93
—
None
1.3

12
12
1251
106
1181
38
3
849
527
48
62
873
79
73
—
None
1.5

13
13
1284
86
1186
28
3
870
659
44
25
898
99
90
—
None
1.6

14
14
1262
83
1134
42
3
918
632
57
39
826
71
63
—
None
1.2

15
15
1295
96
1163
39
3
848
694
41
26
873
85
75
—
None
1.7

16
16
1252
125
1145
37
3
835
693
52
32
883
102
93
—
None
1.3

17
17
1337
122
1135
45
3
835
730
39
68
869
115
110
—
None
1.7

18
18
1318
118
1146
37
3
843
672
38
48
925
91
85
—
None
1.7

19
19
1344
115
1163
44
3
862
557
56
22
904
70
61
—
None
1.2

20
20
1336
96
1129
44
3
919
648
45
21
850
101
91
—
None
1.5

21
21
1279
70
1153
46
3
840
702
58
19
826
100
92
—
None
1.2

22
22
1275
118
1164
36
3
849
630
55
25
900
97
87
—
None
1.3

23
23
1286
83
1136
42
3
904
594
47
66
917
93
85
—
None
1.5

24
24
1262
102
1166
33
3
909
626
49
68
889
76
70
—
None
1.4

25
25
1274
102
1142
39
3
896
645
52
60
934
95
87
—
None
1.3

TABLE A-2-2

Rough rolling

Heat treatment at hot stamping

Heat treatment

Rate of

Cold

Average

before hot

reduction

roll-

cooling
Average

Multi-

rolling

of
No. of
Hot rolling
ing

rate
cooling

layer
Manu-
Heat-
Hold-
Roll-
sheet
rolling
Finish
Coil-
Roll-
Heat-
Heat-
(° C./s)
rate
Temper-

Sheet

steel
facturing
ing
ing
ing
thick-
oper-
rolling
ing
ing
ing
ing
(more
(° C./s)
ing

thick-

sheet
condition
temp.
time
temp.
ness
ations
temp.
temp.
rate
rate
temp.
than
(400° C.
temp.
Plat-
ness

no.
no.
(° C.)
(min)
(° C.)
(%)
(times)
(° C.)
(° C.)
(%)
(° C./s)
(° C.)
400° C.)
or less)
(° C.)
ing
(mm)

26
26
1308
128
1135
21
3
907
713
41
20
881
89
82
—
None
1.7

27
27
1315
106
1181
26
3
843
696
42
66
886
93
88
—
None
1.6

28
28
1255
77
1189
23
3
842
647
51
27
896
109
101
—
None
1.4

29
29
1291
90
1141
38
3
888
686
53
40
855
84
74
—
None
1.3

30
30
992
118
980
34
3
896
682
47
48
892
84
78
—
None
1.5

31
31

1378

90
1146
46
3
854
661
55
69
907
67
60
—
None
1.3

32
32
1132
16
1122
34
3
876
615
41
70
903
81
72
—
None
1.7

33
33
1123
73
1113
22
3
834
550
46
68
912
79
71
—
None
1.5

34
34
1329
96
1128
27
3
879
675
0
46
914
74
68
—
None
2.8

35
35
1317
122
1141
46
3
844
545
58
53
919
93
87
267
None
1.2

36
36
1288
74
1172
34
3
875
533
47
49
926
98
93
274
Yes
1.5

37
37
1292
80
1129
39
3
849
559
45
28
847
80
71
—
Yes
1.5

38
38
1249
92
1120
40
3
840
678
61
32
852
101
91
—
None
1.1

39
39
1245
91
1169
36
3
883
671
47
64
848
86
76
—
None
1.5

40
40
1249
62
1145
20
3
881
703
59
30
868
115
110
—
None
1.1

41
41
1337
81

1007

41
3
840
557
58
73
917
109
101
—
None
1.4

42
42
1336
77
1151
3
2
843
594
52
31
934
77
72
—
None
1.7

43
43
1275
79
1147
35

1

896
696
51
68
903
86
78
—
None
1.6

44
44
1308
62
1121
37
3
843
702
49
65
892
100
94
—
None
1.6

TABLE A-3

Metal structures

Area rate (%) of total of crystal

grains with maximum difference

Hardness
of crystal orientation inside large

of middle
angle grain boundaries of 1° or
Mechanical properties

part in
less and crystal grains with

Max.

Multilayer

sheet
maximum difference of crystal
Tensile
bending
Hydrogen

Stamped
steel sheet
Manufacturing
thickness
orientation of 8° or more and
strength
angle
embrittlement

body no.
no.
condition no.
(Hv)
less than 15°
(MPa)
(°)
resistance
Remarks

1A
1
1
518
48
1533
105.4
Good
Inv. ex.

2A
2
2
639
35
1927
109
Good
Inv. ex.

3A
3
3
717
26
2138
110.1
Good
Inv. ex.

4A
4
4
795
27
2330
95.9
Good
Inv. ex.

5A
5
5

391

32

1163

109.9
Good

Comp. ex.

6A
6
6
583
43
1718
105.4
Good
Inv. ex.

7A
7
7
603
42
1839
110
Good
Inv. ex.

8A
8
8
684
31
2026
106.4
Good
Inv. ex.

9A
9
9

893

21
2676
55.5
Good

Comp. ex.

10A
10
10
636
36
1918
101.5
Good
Inv. ex.

11A
11
11

441

35

1455

108.8
Good

Comp. ex.

12A
12
12
647
31
1925
108.3
Good
Inv. ex.

13A
13
13
649
33
1905
101.5
Good
Inv. ex.

14A
14
14
635
38
1912
111.3
Good
Inv. ex.

15A
15
15
645
34
1925
98.7
Good
Inv. ex.

16A
16
16
653
36
1924
96.4
Good
Inv. ex.

17A
17
17
654
31
1935
100.7
Good
Inv. ex.

18A
18
18
507
47
1537
104.1
Good
Inv. ex.

19A
19
19
525
48
1522
103.9
Good
Inv. ex.

20A
20
20
504
46
1551
105.3
Good
Inv. ex.

21A
21
21
639
32
1928
110.4
Good
Inv. ex.

22A
22
22
642
34
1934
109.4
Good
Inv. ex.

23A
23
23
654
35
1916
111.2
Good
Inv. ex.

24A
24
24
721
25
2121
109.8
Good
Inv. ex.

25A
25
25
723
27
2147
110.2
Good
Inv. ex.

26A
26
26
718
24
2139
108.7
Good
Inv. ex.

27A
27
27
782
29
2586
94.8
Good
Inv. ex.

28A
28
28
788
31
2580
95.2
Good
Inv. ex.

29A
29
29
770
27
2577
96.1
Good
Inv. ex.

30A
30
30
649

13

1930
61.5

Poor

Comp. ex.

31A
31
31
655

85

1909
66.6
Good

Comp. ex.

32A
32
32
651

12

1929
68.2

Poor

Comp. ex.

33A
33
33
636
35
1932
95.1
Good
Inv. ex.

34A
34
34
639
32
1908
95.2
Good
Inv. ex.

35A
35
35
726
29
2167
106
Good
Inv. ex.

36A
36
36
729
25
2142
103
Good
Inv. ex.

37A
37
37
640
34
1863
122.7
Good
Inv. ex.

38A
38
38
649
34
2142
98.1
Good
Inv. ex.

39A
39
39
722
25
2139
109.1
Good
Inv. ex.

40A
40
40
782
36
2181
90.1
Good
Inv. ex.

41A
41
41
632

11

2086
63.2

Poor

Comp. ex.

42A
42
42
640

12

2112
59.6

Poor

Comp. ex.

43A
43
43
637

13

2102
57.9

Poor

Comp. ex.

44A
44
44
628
45
2072
108.4
Good
Inv. ex.

Manufacturing Example B

Steel sheets for sheet thickness middle part having the chemical compositions shown in Table B-1-1 to Table B-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 B-1-3 to Table B-1-4 at both surfaces or single surfaces by arc welding to fabricate the Nos. 1 to 52 multilayer steel sheets for hot stamped body. In the tables, fields in which the constituents are indicated as 0 show that the corresponding constituents are not intentionally added.

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. 32 multilayer steel sheet was steel with steel sheet for surface layer welded to only one side. Among the Nos. 1 to 52 multilayer steel sheets of Table B-1-1 to 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 52 multilayer steel sheets were respectively treated under the conditions of the Nos. 1 to 52 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 52B hot stamped bodies (“stamped bodies” of Table B-3-1 and Table B-3-2). Further, the Nos. 30B and 31B hot stamped bodies were coated on a hot dip coating line at the surfaces of the matrix steel sheets with 120 to 160 g/m²amounts 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 52B 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 52 multilayer steel sheets of Table B-1-1 to Table B-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 B-3-1 to Table B-3-2.

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.

The scattering in hardness of the stamped bodies was evaluated by the results of measurement of the hardness at the cross-section vertical to the longitudinal direction of the stamped bodies. On a line passing through the middle of sheet thickness of a total cross-sectional region and parallel to the surface of the stamped body, the Vickers hardness was measured using a Vickers hardness tester by a load of 1 kgf and 1 mm pitches. For the Nos. 1B to 52B hot stamped bodies, the average values of the hardnesses measured and the minimum hardnesses are shown in Table B-3-1 and Table B-3-2 in the items “average cross-sectional hardness” and “minimum hardness”. The “average cross-sectional hardness-minimum hardness” is the difference between the average cross-sectional hardness and minimum hardness. Further, for the Nos. 1B to 52B hot stamped bodies, cases with no regions with hardnesses falling more than 100 HV from the average values were indicated as “passing”.

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 became large in scattering in hardness of the cross-section of the stamped body.

The Nos. 25B to 27B 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. 25B hot stamped body was 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. 26B hot stamped body was excessively high in heat treatment temperature 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. 26B hot stamped body could not be given excellent bendability.

The No. 27B hot stamped body was too 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 targeted bendability could not be obtained.

The No. 49B hot stamped body was low in rolling temperature of the rough rolling. Further, the No. 50B hot stamped body was low in sheet thickness reduction rate of the rough rolling. Further, the No. 51B 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. 52B 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.

TABLE B-1-1

Multilayer

steel sheet
Chemical constituents of steel sheet for sheet thickness middle part (mass %)

no.
C
Si
Mn
P
S
sol.Al
N
Ni
Nb
Ti
Mo
B
Remarks

1
0.19
0.35
2.93
0.0162
0.0032
0.0230
0.0052
0
0
0
0
0

2
0.36
0.11
2.27
0.0096
0.0037
0.0550
0.005
0
0
0
0
0

3
0.29
0.43
2.55
0.0141
0.0026
0.0550
0.0068
0
0
0
0
0

4
0.5
0.07
2.64
0.0137
0.0006
0.0460
0.0019
0
0
0
0
0

5

0.15

0.40
1.98
0.0163
0.0032
0.0360
0.0059
0
0
0
0
0

Comp. steel

6
0.21
0.23
2.94
0.0124
0.0048
0.0260
0.0052
0
0
0
0
0

7
0.22
0.14
1.56
0.0075
0.0048
0.0540
0.0058
0
0
0
0
0

8
0.25
0.24
2.45
0.0141
0.005
0.0210
0.0069
0
0
0
0
0

9

0.77

0.25
2.51
0.0093
0.0054
0.0240
0.0022
0
0
0
0
0

Comp. steel

10
0.23
0.15
2.83
0.0115
0.002
0.0340
0.0026
0
0
0
0
0

11
0.34
0.32

1.42

0.0138
0.0056
0.0340
0.0023
0
0
0
0
0

Comp. steel

12
0.36
0.36
2.37
0.0107
0.0022
0.0460
0.0034
0
0
0
0
0

13
0.26
0.40
1.99
0.0174
0.0054
0.0310
0.0027
0.10
0
0
0
0

14
0.27
0.33
2.96
0.0144
0.002
0.0270
0.0066
0
0
0
0
0.0020

15
0.26
0.08
2.01
0.015
0.0056
0.0520
0.0028
0
0.040
0.020
0
0.0015

16
0.23
0.32
1.53
0.0176
0.0054
0.0280
0.0057
0
0
0
0
0

17
0.23
0.32
2.07
0.0048
0.0057
0.0480
0.0063
0
0
0
0
0

18
0.39
0.12
2.32
0.0158
0.0055
0.0520
0.005
0
0
0
0
0

19
0.33
0.40
2.20
0.0123
0.0046
0.0520
0.0047
0
0
0
0
0

20
0.37
0.44
2.49
0.0148
0.0053
0.0380
0.0051
0
0
0
0
0

21
0.30
0.45
1.70
0.0072
0.0051
0.0290
0.0046
0
0
0
0
0

22
0.58
0.18
2.32
0.0109
0.0059
0.0420
0.0042
0
0.020
0.020
0
0.0020

23
0.56
0.2
2.80
0.0154
0.0038
0.0440
0.0048
0
0
0
0
0

24
0.53
0.11
1.96
0.0103
0.005
0.0230
0.0015
0
0
0
0
0

25
0.38
0.27
1.98
0.0045
0.0042
0.0500
0.0031
0
0
0
0
0

26
0.34
0.34
2.62
0.0098
0.0035
0.0340
0.003
0
0
0
0
0

27
0.35
0.34
1.76
0.0069
0.0056
0.0240
0.006
0
0.050
0.030
0
0.0015

28
0.34
0.14
2.34
0.008
0.0011
0.0350
0.0035
0
0
0
0
0

29
0.63
0.22
2.45
0.0135
0.0058
0.0240
0.006
0
0
0
0
0

30
0.66
0.11
2.93
0.0151
0.004
0.0240
0.0065
0
0
0
0
0

TABLE B-1-2

Multilayer

steel sheet
Chemical constituents of steel sheet for sheet thickness middle part (mass %)

no.
C
Si
Mn
P
S
sol.Al
N
Ni
Nb
Ti
Mo
B
Remarks

31
0.37
0.37
2.12
0.0165
0.0031
0.0590
0.0036
0
0
0
0
0

32
0.33
0.13
1.99
0.0071
0.0028
0.0560
0.0025
0
0
0
0
0

33
0.31
0.22
2.23
0.093
0.006
2.4180
0.0069
0
0
0
0
0

34
0.28
0.3
2.83
0.084
0.004
0.0540
0.0024
2.40
0
0
0
0

35
0.25
0.43
2.45
0.099
0.003
0.0540
0.0041
0.06
0
0
0
0

36
0.34
0.39
1.73
0.071
0.002
0.0410
0.0049
0
0.120
0
0
0

37
0.4
0.42
1.87
0.127
0.002
0.0640
0.0045
0
0
0.150
0
0

38
0.28
0.21
2.19
0.076
0.003
0.0490
0.0022
0
0
0
0.500
0

39
0.36
0.37
2.74
0.068
0.005
0.0630
0.0048
0
0
0
0.200
0

40
0.38
0.48
2.45
0.082
0.006
0.0550
0.0071
0
0
0
0
0.0080

41
0.31
0.20
2.00
0.107
0.002
0.0420
0.0066
0
0
0
0
0

42
0.36
0.33
2.16
0.057
0.004
0.0350
0.0032
0
0
0
0
0

43
0.32
0.30
2.52
0.119
0.005
0.0590
0.004
0
0
0
0
0

44
0.31
0.28
2.76
0.076
0.004
0.0520
0.0022
0
0
0
0
0

45
0.27
0.19
2.18
0.107
0.004
0.0470
0.0026
0
0
0
0
0

46
0.37
0.48
2.88
0.082
0.006
0.0520
0.0029
0
0
0
0
0

47
0.25
0.25
2.7
0.087
0.003
0.0580
0.0066
0
0
0
0
0

48
0.34
0.18
2.31
0.097
0.004
0.0340
0.0065
0
0
0
0
0

49
0.36
0.11
2.27
0.0096
0.0037
0.0550
0.005
0
0
0
0
0

50
0.36
0.11
2.27
0.0096
0.0037
0.0550
0.005
0
0
0
0
0

51
0.36
0.11
2.27
0.0096
0.0037
0.0550
0.005
0
0
0
0
0

52
0.36
0.11
2.27
0.0096
0.0037
0.0550
0.005
0
0
0
0
0

TABLE B-1-3

Thickness of

Multilayer

steel sheet

steel sheet
Composition of constituents of steel sheet for surface layer (mass %)
for surface

no.
C
Si
Mn
P
S
sol.Al
N
Ni
Nb
Ti
Mo
B
layer (mm)
Remarks

1
0.072
0.193
1.940
0.062
0.0040
0.0200
0.0049
0
0
0
0
0
%

2
0.202
0.052
1.710
0.12
0.0005
0.030
0.0057
0
0
0
0
0
91

3
0.108
0.189
1.060
0.2
0.0005
0.0390
0.0022
0
0
0
0
0
95

4
0.247
0.032
1.750
0.145
0.0040
0.0330
0.0024
0
0
0
0
0
96

5
0.084
0.208
1.850
0.043
0.0026
0.0210
0.0049
0
0
0
0
0
78

Comp. steel

6
0.114
0.11
1.820
0.112
0.0030
0.0350
0.0051
0
0
0
0
0
82

7
0.132
0.069
1.870
0.052
0.0022
0.0340
0.0026
0
0
0
0
0
84

8
0.112
0.11
1.650
0.096
0.0017
0.0360
0.006
0
0
0
0
0
106

9
0.364
0.105
2.360
0.110
0.0037
0.0430
0.0036
0
0
0
0
0
85

Comp. steel

10
0.101
0.084
1.370
0.119
0.0008
0.0360
0.0063
0
0
0
0
0
85

11
0.202
0.157
1.290
0.065
0.0026
0.0300
0.0024
0
0
0
0
0
103

Comp. steel

12
0.18
0.169
2.170
0.162
0.0025
0.0490
0.0055
0
0
0
0
0
75

13
0.127
0.192
2.060
0.177
0.0013
0.0430
0.0058
0.02
0
0
0
0
94

14
0.115
0.152
1.030
0.176
0.0006
0.0310
0.0034
0
0
0
0
0.0017
89

15
0.112
0.071
2.450
0.054
0.0005
0.0460
0.0037
0
0
0
0
0
83

16
0.102
0.291
1.970
0.081
0.0034
0.0260
0.0061
0
0
0
0
0
87

17
0.135
0.141
1.810
0.174
0.0009
0.0460
0.0065
0
0
0
0
0
86

18
0.343
0.056
1.530
0.163
0.0006
0.0240
0.0026
0
0
0
0
0
90

19
0.286
0.22
1.760
0.089
0.0030
0.0320
0.0034
0
0
0
0
0
102

20
0.322
0.383
2.300
0.166
0.0034
0.0480
0.0023
0
0
0
0
0
101

21
0.225
0.194
1.370
0.119
0.0008
0.0430
0.0026
0
0
0
0
0
105

22
0.468
0.092
1.290
0.178
0.0009
0.0200
0.0042
0
0
0
0
0
84

23
0.228
0.178
1.500
0.122
0.0028
0.0260
0.0039
0
0
0
0
0
102

24
0.233
0.045
2.170
0.152
0.0018
0.0390
0.0055
0
0
0
0
0
88

25
0.163
0.135
2.370
0.142
0.0035
0.0200
0.0052
0
0
0
0
0
85

TABLE B-1-4

Thickness of

Multilayer

steel sheet

steel sheet
Composition of constituents of steel sheet for surface layer (mass %)
for surface

no.
C
Si
Mn
P
S
sol.Al
N
Ni
Nb
Ti
Mo
B
layer (mm)
Remarks

26
0.161
0.187
2.200
0.073
0.0040
0.038
0.0064
0
0
0
0
0
89

27
0.147
0.201
1.680
0.124
0.0029
0.041
0.0050
0
0
0
0
0
91

28
0.178
0.081
2.430
0.131
0.0022
0.037
0.0032
0
0
0
0
0
101

29
0.362
0.128
1.350
0.153
0.0030
0.022
0.0039
0
0
0
0
0
86

30
0.323
0.052
2.090
0.097
0.0029
0.033
0.0061
0
0
0
0
0
81

31
0.165
0.196
1.800
0.094
0.0014
0.042
0.0022
0
0
0
0
0
84

32
0.184
0.061
1.850
0.103
0.0027
0.030
0.0048
0
0
0
0
0
162

33
0.146
0.101
1.127
0.006
0.004
0.040
0.0045
0
0
0
0
0
103

34
0.118
0.162
1.120
0.007
0.005
0.039
0.0088
0
0
0
0
0
88

35
0.13
0.254
1.128
0.010
0.003
0.033
0.0077
0
0
0
0
0
107

36
0.136
0.234
0.893
0.012
0.002
0.021
0.0073
0
0
0
0
0
81

37
0.2
0.189
0.680
0.012
0.006
0.043
0.0051
0
0
0
0
0
102

38
0.12
0.126
0.966
0.012
0.005
0.023
0.0056
0
0
0
0
0
96

39
0.18
0.148
1.680
0.007
0.002
0.050
0.0099
0
0
0
0
0
100

40
0.163
0.264
1.250
0.007
0.003
0.036
0.0053
0
0
0
0
0
92

41
0.177
0.118
0.800
0.009
0.003
0.047
0.0097
2.10
0
0
0
0
91

42
0.209
0.182
1.081
0.009
0.006
0.042
0.0041
0.04
0
0
0
0
86

43
0.157
0.177
1.368
0.011
0.004
0.023
0.0067
0
0.150
0
0
0
103

44
0.183
0.112
1.566
0.012
0.004
0.046
0.0064
0
0
0.140
0
0
89

45
0.13
0.103
1.260
0.010
0.002
0.048
0.0065
0
0
0
0.700
0
92

46
0.215
0.25
1.120
0.013
0.005
0.045
0.0052
0
0
0
0.150
0
109

47
0.13
0.11
1.653
0.009
0.005
0.036
0.0059
0
0
0
0
0.0090
82

48
0.16
0.085
1.276
0.007
0.005
0.028
0.0049
0
0
0
0
0.0015
88

49
0.202
0.052
1.710
0.120
0.0005
0.030
0.0057
0
0
0
0
0
91

50
0.202
0.052
1.710
0.120
0.0005
0.030
0.0057
0
0
0
0
0
91

51
0.202
0.052
1.710
0.120
0.0005
0.030
0.0057
0
0
0
0
0
91

52
0.202
0.052
1.710
0.120
0.0005
0.030
0.0057
0
0
0
0
0
91

TABLE B-2-1

Rough rolling

Heat treatment at hot stamping

Heat treatment

Rate of

Cold

Average

before hot

reduction

roll-

cooling
Average

rolling

of
No. of
Hot rolling
ing

rate
cooling

Manu-
Heat-
Hold-
Roll-
sheet
rolling
Finish
Coil-
Roll-
Heat-
Heat-
(° C./s)
rate
Temper-

facturing
ing
ing
ing
thick-
oper-
rolling
ing
ing
ing
ing
(more
(° C./s)
ing

condition
temp.
time
temp.
ness
ations
temp.
temp.
rate
rate
temp.
than
(400° C.
temp.
Plat-

no.
(° C.)
(min)
(° C.)
(%)
(times)
(° C.)
(° C.)
(%)
(° C./s)
(° C.)
400° C.)
or less)
(° C.)
ing

1
1189
100
1161
34
3
912
726
75
38
847
63
58
—
None

2
1301
64
1152
32
3
904
732
70
34
848
104
94
—
None

3
1327
103
1133
29
3
874
589
59
49
882
76
67
—
None

4
1109
99
1107
37
3
915
524
61
52
916
96
89
—
None

5
1219
105
1145
30
3
948
690
50
56
849
99
89
—
None

6
1209
105
1154
35
3
865
645
62
76
891
72
63
—
None

7
1348
74
1202
39
3
851
571
69
65
822
88
82
—
None

8
1164
117
1129
41
3
862
746
70
51
838
79
70
—
None

9
1251
108
1166
27
3
879
551
47
29
872
68
63
—
None

10
1184
94
1142
36
3
877
563
78
43
836
72
62
—
None

11
1260
114
1126
30
3
941
546
72
60
903
105
97
—
None

12
1101
65
1100
35
3
948
684
41
61
873
78
73
—
None

13
1322
104
1183
29
3
865
550
54
23
898
96
86
—
None

14
1318
100
1134
43
3
912
517
44
43
869
68
59
—
None

15
1332
84
1167
36
3
876
680
50
31
925
87
81
—
None

16
1306
72
1150
36
3
877
629
49
29
904
98
91
—
None

17
1175
67
1144
40
3
869
567
60
72
850
120
113
—
None

18
1154
71
1145
38
3
944
511
40
48
826
96
89
—
None

19
1178
113
1168
46
3
852
605
51
25
900
72
63
—
None

20
1288
98
1139
47
3
854
624
50
18
917
98
90
—
None

21
1324
68
1151
44
3
936
502
40
23
889
98
88
—
None

22
1170
76
1160
31
3
937
506
65
22
886
95
89
—
None

23
1155
65
1138
38
3
890
572
77
70
896
92
85
—
None

24
1326
104
1162
36
3
901
611
58
69
855
72
67
—
None

25

1081

104
1051
43
3
922
547
67
63
892
100
90
—
None

TABLE B-2-2

Rough rolling

Heat treatment at hot stamping

Heat treatment

Rate of

Cold

Average

before hot

reduction

roll-

cooling
Average

rolling

of
No. of
Hot rolling
ing

rate
cooling

Manu-
Heat-
Hold-
Roll-
sheet
rolling
Finish
Coil-
Roll-
Heat-
Heat-
(° C./s)
rate
Temper-

facturing
ing
ing
ing
thick-
oper-
rolling
ing
ing
ing
ing
(more
(° C./s)
ing

condition
temp.
time
temp.
ness
ations
temp.
temp.
rate
rate
temp.
than
(400° C.
temp.
Plat-

no.
(° C.)
(min)
(° C.)
(%)
(times)
(° C.)
(° C.)
(%)
(° C./s)
(° C.)
400° C.)
or less)
(° C.)
ing

26

1366

117
1140
25
3
890
572
68
17
907
86
78
—
None

27
1132
15
1122
25
3
949
681
68
63
903
88
83
—
None

28
1114
80
1107
19
3
913
738
65
24
914
107
97
—
None

29
1239
92
1134
40
3
909
592
41
44
919
82
75
290
None

30
1341
72
1142
36
3
897
649
80
45
926
83
73
244
Yes

31
1292
63
1143
45
3
894
682
74
65
847
62
55
—
Yes

32
1244
76
1153
29
3
863
652
78
66
852
80
75
—
None

33
1186
87
1136
27
3
889
619
48
68
854
76
66
—
None

34
1177
110
1133
22
3
858
593
44
44
935
76
66
—
None

35
1222
70
1135
47
3
895
645
40
48
886
97
89
—
None

36
1158
104
1145
32
3
904
580
57
48
897
99
89
—
None

37
1192
111
1135
44
3
850
610
55
23
858
79
72
—
None

38
1230
72
1116
39
3
912
600
50
33
872
106
98
—
None

39
1153
85
1143
40
3
907
550
45
63
859
83
76
—
None

40
1152
109
1122
19
3
897
618
57
26
899
118
110
—
None

41
1217
89
1159
33
3
910
637
54
57
877
110
100
—
None

42
1194
108
1135
35
3
850
571
52
31
883
99
94
—
None

43
1233
76
1138
35
3
950
638
55
46
949
90
82
—
None

44
1193
99
1138
39
3
950
553
46
56
923
91
83
—
None

45
1174
119
1169
32
3
940
639
44
31
874
88
78
—
None

46
1218
102
1138
35
3
947
605
53
34
936
63
55
—
None

47
1245
101
1136
33
3
940
648
40
21
893
92
86
—
None

48
1217
106
1124
36
3
907
590
49
21
895
108
100
—
None

49
1337
99

1005

41
3
840
557
58
68
917
113
108
—
None

50
1336
78
1158
4
2
843
594
52
26
934
80
74
—
None

51
1275
88
1147
39

1

896
696
51
73
903
86
78
—
None

52
1308
63
1126
42
3
843
702
49
65
892
97
87
—
None

TABLE B-3-1

Metal structures

Area rate (%) of total

of crystal grains with

maximum difference of
Mechanical properties

Hardness
crystal orientation inside

Average

of
large angle grain

cross-

Multi-

middle
boundaries of 1° or less

Maxi-

Average

sectional

layer
Manu-
part in
and crystal grains with

mum
Hydrogen
cross-
Mini-
hardness-

steel
facturing
sheet
maximum difference of
Tensile
bending
embrittle-
sectional
mum
minimum

Stamped
sheet
condition
thickness
crystal orientation of 8°
strength
angle
ment
hardness
hardness
hardness

body no.
no.
no.
(Hv)
or more and less than 15°
(MPa)
(°)
resistance
(Hv)
(Hv)
(Hv)
Remarks

1B
1
1
534
46
1564
105.4
Good
496
460
36
Inv. ex.

2B
2
2
684
33
2004
112.3
Good
663
657
6
Inv. ex.

3B
3
3
703
25
2245
111.2
Good
668
666
2
Inv. ex.

4B
4
4
755
28
2260
94.9
Good
702
678
24
Inv. ex.

5B
5
5

414

31

1175

106.6
Good
385
339
46

Comp. ex.

6B
6
6
589
43
1718
103.3
Good
565
520
45
Inv. ex.

7B
7
7
579
43
1747
104.5
Good
562
493
69
Inv. ex.

8B
8
8
643
32
1985
103.2
Good
611
602
9
Inv. ex.

9B
9
9

839

21
2596
56.6
Good
772
757
15

Comp. ex.

10B
10
10
642
36
1860
97.4
Good
617
600
17
Inv. ex.

11B
11
11
604
36
2022
113.2
Good
602
489

113

Comp. ex.

12B
12
12
699
30
1983
105.1
Good
671
653
18
Inv. ex.

13B
13
13
610
33
1962
105.6
Good
567
553
14
Inv. ex.

14B
14
14
680
31
2012
100.7
Good
619
617
2
Inv. ex.

15B
15
15
502
45

1460

103.1
Good
457
429
28

Comp. ex.

16B
16
16
546
49
1583
103.9
Good
524
515
9
Inv. ex.

17B
17
17
509
48
1535
108.5
Good
468
416
52
Inv. ex.

18B
18
18
697
33
1947
104.9
Good
634
627
7
Inv. ex.

19B
19
19
648
33
1837
103.9
Good
597
534
63
Inv. ex.

30B
30
30
621
36
1935
110.1
Good
596
586
10
Inv. ex.

21B
21
21
692
26
2163
106.5
Good
637
576
61
Inv. ex.

22B
22
22
704
30
2612
91
Good
676
616
60
Inv. ex.

23B
23
23
780
33
2477
97.1
Good
710
647
63
Inv. ex.

24B
24
24
847
27
2551
96.1
Good
762
729
33
Inv. ex.

25B
25
25
714

13

1988
65.1

Poor

657
646
11

Comp. ex.

TABLE B-3-2

Metal structures

Area rate (%) of total

of crystal grains with

maximum difference of
Mechanical properties

Hardness
crystal orientation inside

Average

of
large angle grain

cross-

Multi-

middle
boundaries of 1° or less

Maxi-

Average

sectional

layer
Manu-
part in
and crystal grains with

mum
Hydrogen
cross-
Mini-
hardness-

steel
facturing
sheet
maximum difference of
Tensile
bending
embrittle-
sectional
mum
minimum

Stamped
sheet
condition
thickness
crystal orientation of 8°
strength
angle
ment
hardness
hardness
hardness

body no.
no.
no.
(Hv)
or more and less than 15°
(MPa)
(°)
resistance
(Hv)
(Hv)
(Hv)
Remarks

26B
26
26
668

92

1909
68.1
Good
628
599
29

Comp. ex.

27B
27
27
592

12

1852
67.4

Poor

581
511
70

Comp. ex.

28B
28
28
645
33
1965
113.3
Good
617
572
45
Inv. ex.

29B
29
29
799
28
2189
124.9
Good
780
733
47
Inv. ex.

30B
30
30
671
26
2228
117.9
Good
642
620
22
Inv. ex.

31B
31
31
659
34
1770
148.8
Good
645
642
3
Inv. ex.

32B
32
32
668
35
2227
118.1
Good
653
600
53
Inv. ex.

33B
33
33
635
37
2109
94
Good
614
606
8
Inv. ex.

34B
34
34
700
54
2017
110
Good
684
664
20
Inv. ex.

35B
35
35
632
66
2065
93
Good
626
593
33
Inv. ex.

36B
36
36
610
49
2278
102
Good
593
548
45
Inv. ex.

37B
37
37
613
57
2105
106
Good
589
563
26
Inv. ex.

38B
38
38
653
54
2167
95
Good
630
611
19
Inv. ex.

39B
39
39
697
52
2071
107
Good
668
646
22
Inv. ex.

40B
40
40
613
29
2043
95
Good
587
562
25
Inv. ex.

41B
41
41
647
44
2189
103
Good
633
631
2
Inv. ex.

42B
42
42
615
60
2020
110
Good
609
588
21
Inv. ex.

43B
43
43
605
62
2287
103
Good
585
562
23
Inv. ex.

44B
44
44
611
44
2165
95
Good
604
549
55
Inv. ex.

45B
45
45
622
32
2141
108
Good
609
579
30
Inv. ex.

46B
46
46
604
56
2275
98
Good
581
531
50
Inv. ex.

47B
47
47
610
63
2010
110
Good
582
557
25
Inv. ex.

48B
48
48
631
47
2109
110
Good
613
584
29
Inv. ex.

49B
49
49
629

10

2076
59.1

Poor

629
622
33

Comp. ex.

50B
50
50
644

12

2125
63.2

Poor

644
627
35

Comp. ex.

51B
51
51
638

12

2105
60.1

Poor

638
612
29

Comp. ex.

52B
52
52
633
44
2089
102.1
Good
633
603
30
Inv. ex.

Manufacturing Example C

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 59 multilayer steel sheets for hot stamped body. In the tables, fields in which the constituents are indicated as 0 show that the corresponding constituents are not intentionally added.

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. 38 multilayer steel sheet was steel with steel sheet for surface layer welded to only one side. The multilayer steel sheets of other than No. 38 had steel sheets for surface layer welded to both surfaces of the steel sheets for sheet thickness middle part. Among the Nos. 1 to 59 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 Nos. 1 to 59 multilayer steel sheets were respectively treated under the conditions of the Nos. 1 to 59 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 and Table C-2-2 (in the tables, “heat treatment of hot stamped body”) for hot stamping to manufacture the Nos. 1C to 59C hot stamped bodies (“stamped bodies” of Table C-3-1 and Table C-3-2). Further, the Nos. 36C and 37C hot stamped bodies were coated on a hot dip coating line at the surfaces of the matrix steel sheets with 120 to 160 g/m²amounts 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 and Table C-3-2 show the metal structures and characteristics of the Nos. 1C to 59C 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 59 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. 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.

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.

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 sparse in Si content of the steel sheet for sheet thickness middle part, so the area percent of the residual austenite became less than 1% and uniform elongation became insufficient.

The “ratios of constituents of the steel sheet for sheet thickness middle part and the steel sheet for surface layer” in Table C-1-3 and Table C-1-4 are the ratios of the C content, Si content, and Mn content at the steel sheet for surface layer with respect to the contents at the steel sheet for sheet thickness middle part. The Nos. 30C and 37C hot stamped bodies had each of the C content, Si content, and Mn content at more than 0.6 time the content of the corresponding element of the middle part in sheet thickness.

The Nos. 30C to 32C 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. 30C hot stamped body is too low in heat treatment temperature 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 targeted bendability could not be obtained. Further, the No. 31C hot stamped body was excessively high in heat treatment temperature 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. 31C hot stamped body could not be given excellent bendability. The No. 32C hot stamped body was too 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 targeted bendability could not be obtained.

The No. 56C hot stamped body was low in rolling temperature of the rough rolling. Further, the No. 57C hot stamped body was low in sheet thickness reduction rate of the rough rolling. Further, the No. 58C 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. 59C 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.

TABLE C-1-1

Multilayer

steel sheet
Chemical constituents of steel sheet for sheet thickness middle part (mass %)

no.
C
Si
Mn
P
S
sol.Al
N
Ni
Kb
Ti
Mo
B
Remarks

1
0.24
2.94
0.89
0.0130
0.0049
0.056
0.0059
0
0
0
0
0

2
0.34
2.56
0.89
0.0120
0.0080
0.053
0.0064
0
0
0
0
0

3
0.29
2.06
1.49
0.0120
0.0019
0.034
0.0061
0
0
0
0
0

4
0.48
2.18
0.97
0.0090
0.0037
0.023
0.005
0
0
0
0
0

5

0.18

2.99
0.55
0.0080
0.0013
0.055
0.0069
0
0
0
0
0

Comp. steel

6
0.22
2.84
1.28
0.0110
0.0068
0.025
0.0038
0
0
0
0
0

7
0.21
1.69
1.10
0.0130
0.0007
0.036
0.0033
0
0
0
0
0

8
0.25
1.18
1.43
0.0060
0.0041
0.048
0.0052
0
0
0
0
0

9

0.75

2.52
1.27
0.0080
0.0014
0.042
0.0038
0
0
0
0
0

Comp. steel

10
0.22
1.71
1.36
0.0100
0.0045
0.027
0.0032
0
0
0
0
0

11
0.35

0.45

0.76
0.0100
0.008
0.026
0.0052
0
0
0
0
0

Comp. steel

12
0.33
1.18
0.56
0.0090
0.0035
0.0.31
0.0038
0
0
0
0
0

13
0.23
2.71
1.19
0.0140
0.0026
0.041
0.0068
0.10
0
0
0
0

17
0.29
1.57
1.42
0.0070
0.0045
0.044
0.0061
0
0
0
0
0.0015

18
0.25
1.82
0.98
0.0120
0.0057
0.060
0.0035
0
0.045
0.025
0
0.0020

19
0.21
2.77
1.26
0.0050
0.0014
0.045
0.0064
0
0
0
0
0

20
0.22
1.51
1.37
0.0070
0.0075
0.034
0.0034
0
0
0
0
0

21
0.35
1.97
0.94
0.0120
0.0043
0.0.30
0.0062
0
0
0
0
0

22
0.33
1.81
0.90
0.0040
0.0063
0.051
0.0043
0
0
0
0
0

23
0.39
1.22
1.01
0.0050
0.0064
0.053
0.0036
0
0
0
0
0

24
0.28
2.32
0.74
0.0060
0.0079
0.048
0.006
0
0
0
0
0

27
0.56
2.30
0.82
0.0050
0.0035
0.058
0.0062
0
0.020
0.025
0
0.0015

28
0.55
1.19
0.85
0.0110
0.0011
0.026
0.0035
0
0
0
0
0

29
0.49
1.59
0.79
0.0120
0.0058
0.051
0.0076
0
0
0
0
0

30
0.38
2.47
0.58
0.0110
0.0064
0.042
0.0038
0
0
0
0
0

TABLE C-1-2

Multilayer

steel sheet
Chemical constituents of steel sheet for sheet thickness middle part (mass %)

no.
C
Si
Mn
P
S
sol.Al
N
Ni
Nb
Ti
Mo
B
Remarks

31
0.32
1.86
0.87
0.005
0.0058
0.034
0.0057
0
0
0
0
0

32
0.36
2.24
0.69
0.010
0.0005
0.048
0.0039
0
0.06
0.032
0
0.0022

34
0.30
2.35
0.64
0.014
0.0061
0.046
0.0034
0
0
0
0
0

35
0.65
1.88
0.58
0.010
0.0021
0.019
0.0044
0
0
0
0
0

36
0.6
1.02
0.94
0.008
0.0073
0.037
0.0037
0
0
0
0
0

37
0.33
1.74
0.98
0.005
0.0015
0.031
0.0045
0
0
0
0
0

38
0.36
2.27
1.19
0.011
0.0058
0.051
0.0066
0
0
0
0
0

39
0.33
2.89

0.15

0.006
0.003
0.051
0.0039
0
0
0
0
0

Comp. steel

40
0.33
2.72
0.65
0.100
0.005
2.660
0.0066
0
0
0
0
0

41
0.27
1.74
0.93
0.091
0.005
0.051
0.0025
2.592
0
0
0
0

42
0.24
2.95
0.77
0.092
0.003
0.056
0.0037
0.0612
0
0
0
0

43
0.36
2.05
0.68
0.070
0.002
0.044
0.0051
0
0.1164
0
0
0

44
0.43
2.68
0.91
0.119
0.002
0.068
0.005
0
0
0.150
0
0

45
0.29
2.64
0.65
0.074
0.003
0.046
0.0022
0
0
0
0.520
0

46
0.35
1.69
0.90
0.072
0.005
0.069
0.0044
0
0
0
0.214
0

47
0.34
1.98
0.76
0.088
0.006
0.055
0.0072
0
0
0
0
0.0076

48
0.32
1.90
0.73
0.108
0.002
0.045
0.0069
0
0
0
0
0

49
0.36
2.50
1.00
0.063
0.004
0.035
0.0034
0
0
0
0
0

50
0.29
1.99
0.98
0.107
0.005
0.061
0.0042
0
0
0
0
0

51
0.33
2.07
0.85
0.073
0.005
0.050
0.0022
0
0
0
0
0

52
0.26
1.88
0.79
0.107
0.004
0.047
0.0028
0
0
0
0
0

53
0.34
1.51
0.8
0.079
0.005
0.052
0.0028
0
0
0
0
0

54
0.24
2.85
0.7
0.091
0.003
0.053
0.006
0
0
0
0
0

55
0.34
2.42
0.84
0.103
0.003
0.033
0.006
0
0
0
0
0

56
0.34
2.56
0.89
0.012
0.008
0.053
0.0064
0
0
0
0
0

57
0.34
2.56
0.89
0.012
0.008
0.053
0.0064
0
0
0
0
0

58
0.34
2.56
0.89
0.012
0.008
0.053
0.0064
0
0
0
0
0

59
0.34
2.56
0.89
0.012
0.008
0.053
0.0064
0
0
0
0
0

TABLE C-1-3

Thickness

of steel

Multilayer

sheet for

steel sheet
Composition of constituents of steel sheet for surface layer (mass %)
surface

no.
C
Si
Mn
P
S
sol.Al
N
Ni
Nb
Ti
Mo
B
layer (mm)
Remarks

1
0.09
1.76
0.5
0.011
0.0048
0.043
0.0053
0
0
0
0
0
82

2
0.16
0.90
0.34
0.007
0.0068
0.038
0.0056
0
0
0
0
0
78

3
0.10
1.01
0.86
0.009
0.0068
0.035
0.0056
0
0
0
0
0
120

4
0.23
0.65
0.3
0.013
0.0053
0.036
0.0076
0
0
0
0
0
72

5
0.06
1.41
0.25
0.011
0.0046
0.034
0.0063
0
0
0
0
0
88

Comp. steel

6
0.10
0.97
0.47
0.008
0.0049
0.043
0.0041
0
0
0
0
0
70

7
0.10
0.81
0.61
0.012
0.0079
0.043
0.007
0
0
0
0
0
106

8
0.08
0.57
0.76
0.014
0.0076
0.029
0.008
0
0
0
0
0
110

9
0.23
1.49
0.75
0.014
0.008
0.046
0.0056
0
0
0
0
0
89

Comp. steel

10
0.11
0.74
0.76
0.01
0.0076
0.03
0.0067
0
0
0
0
0
92

11
0.11
0.17
0.33
0.01
0.0056
0.036
0.0075
0
0
0
0
0
101

Comp. steel

12
0.15
0.60
0.32
0.01
0.0064
0.039
0.0057
0
0
0
0
0
120

13
0.12
1.36
0.56
0.008
0.007
0.049
0.0062
0
0
0
0
0
88

17
0.10
0.58
0.51
0.006
0.0047
0.042
0.0041
0
0
0
0
0.0190
109

18
0.12
1.62
0.60
0.014
0.0046
0.026
0.0055
0
0.050
0.024
0
0.0160
93

19
0.07
2.60
0.38
0.010
0.006
0.049
0.0072
0
0
0
0
0
104

20
0.07
0.82
0.90
0.014
0.0068
0.048
0.0074
0
0
0
0
0
93

21
0.31
0.71
0.39
0.011
0.0049
0.038
0.0058
0
0
0
0
0
112

22
0.27
1.03
0.83
0.009
0.0065
0.047
0.0056
0
0
0
0
0
114

23
0.36
1.02
0.51
0.013
0.0073
0.046
0.0049
0
0
0
0
0
78

24
0.26
1.16
0.35
0.008
0.0042
0.036
0.0044
0
0
0
0
0
106

27
0.34
0.78
0.37
0.012
0.0053
0.036
0.0043
0
0
0
0
0
102

28
0.21
0.73
0.45
0.012
0.0053
0.045
0.0066
0
0
0
0
0
118

29
0.17
0.70
0.64
0.012
0.0046
0.034
0.0079
0.15
0
0
0
0
89

30
0.26
1.65
0.57
0.014
0.0067
0.042
0.004
0
0
0
0
0
70

TABLE C-1-4

Multi-

layer

steel

sheet
Composition of constituents of steel sheet for surface layer (mass %)

no.
C
Si
Mn
P
S
sol.Al
N
Ni
Nb

31
0.16
0.71
0.27
0.009
0.0048
0.031
0.0047
0
0

32
0.11
0.9
0.25
0.012
0.0069
0.043
0.0057
0
0

34
0.16
0.87
0.31
0.010
0.0045
0.045
0.0067
0
0

35
0.32
0.83
0.21
0.008
0.0056
0.032
0.0077
0
0

36
0.30
0.42
0.36
0.007
0.0066
0.044
0.0063
0
0

37
0.24
1.17
0.65
0.013
0.0068
0.043
0.0057
0
0

38
0.12
0.91
0.51
0.011
0.0061
0.033
0.0063
0
0

39
0.165
1.0982
0.048
0.008
0.0048
0.047
0.0056
0
0

40
0.138
0.107
1.059
0.006
0.004
0.038
0.0045
0
0

41
0.108
0.159
1.198
0.008
0.005
0.037
0.0094
0
0

42
0.126
0.271
1.015
0.009
0.003
0.034
0.0083
0
0

43
0.141
0.246
0.911
0.012
0.002
0.020
0.0066
0
0

44
0.214
0.206
0.673
0.012
0.006
0.047
0.0056
0
0

45
0.11
0.135
0.927
0.012
0.004
0.022
0.0055
0
0

46
0.169
0.158
1.697
0.007
0.002
0.047
0.0101
0
0

47
0.178
0.288
1.200
0.007
0.002
0.038
0.0056
0
0

48
0.182
0.12
0.816
0.009
0.003
0.048
0.0097
2.30
0

49
0.228
0.182
1.092
0.009
0.006
0.045
0.004

0

0

50
0.141
0.188
1.505
0.010
0.005
0.024
0.007
0
0.150

51
0.188
0.102
1.644
0.013
0.004
0.049
0.007
0
0

52
0.131
0.111
1.147
0.010
0.002
0.044
0.0066
0
0

53
0.225
0.252
1.086
0.013
0.005
0.049
0.0047
0
0

54
0.127
0.101
1.587
0.010
0.005
0.037
0.0055
0
0

55
0.149
0.092
1.161
0.007
0.005
0.031
0.005
0
0

56
0.16
0.9
0.34
0.007
0.0068
0.038
0.0056
0
0

57
0.16
0.9
0.34
0.007
0.0068
0.038
0.0056
0
0

58
0.16
0.9
0.34
0.007
0.0068
0.038
0.0056
0
0

59
0.16
0.9
0.34
0.007
0.0068
0.038
0.0056
0
0

Thick-

ness

Multi-

of steel

layer

sheet for

steel

layer

sheet

surface
Re-

no.
Ti
Mo
B
(mm)
marks

31
0
0
0
117

32
0
0
0
99

34
0
0
0
73

35
0
0
0
73

36
0
0
0
115

37
0
0
0
79

38
0
0
0
88

39
0
0
0
70

Comp.

steel

40
0
0
0
103

41
0
0
0
88

42
0
0
0
107

43
0
0
0
81

44
0
0
0
102

45
0
0
0
96

46
0
0
0
100

47
0
0
0
92

48
0
0
0
91

49
0
0
0
86

50
0
0
0
103

51
0.150
0
0
89

52
0
0.700
0
92

53
0
0.160
0
109

54
0
0
0.0081
82

55
0
0
0.0015
88

56
0
0
0
78

57
0
0
0
78

58
0
0
0
78

59
0
0
0
78

TABLE C-2-1

Rough rolling

Heat

Heat treatment

Rate of

Hot rolling
Cold
treatment at

Manu-
before hot rolling

reduction
No. of
Finish

rolling
hot stamping

Multilayer
facturing
Heating
Holding
Rolling
of sheet
rolling
rolling
Coiling
Rolling
Heating

steel
condition
temp.
time
temp.
thickness
operations
temp.
temp.
rate
rate

sheet no.
no.
(° C.)
(min)
(° C.)
(%)
(times)
(° C.)
(° C.)
(%)
(° C./s)

1
1
1250
62
1161
38
3
856
704
52
40

2
2
1174
87
1158
32
3
920
716
46
31

3
3
1125
89
1110
28
3
856
523
62
43

4
4
1160
62
1148
37
3
945
619
62
64

5
5
1149
114
1142
29
3
872
577
59
44

6
6
1131
127
1121
34
3
888
563
61
60

7
7
1316
127
1191
36
3
881
606
60
65

8
8
1294
125
1131
46
3
941
548
57
53

9
9
1317
62
1150
24
3
867
702
60
30

10
10
1123
71
1117
31
3
872
587
45
42

11
11
1105
147
1101
31
3
908
615
63
56

12
12
1239
120
1187
41
3
912
657
49
61

13
13
1254
142
1176
26
3
889
627
51
24

17
17
1350
121
1154
44
3
859
638
46
44

18
18
1179
66
1174
34
3
894
626
59
28

19
19
1115
117
1105
34
3
925
650
60
36

20
20
1294
119
1127
46
3
853
580
48
71

21
21
1301
146
1140
43
3
865
518
62
47

22
22
1330
137
1153
35
3
932
706
55
22

23
23
1184
70
1127
45
3
913
699
46
22

24
24
1260
94
1164
47
3
912
637
45
17

27
27
1105
109
1101
38
3
899
589
61
15

28
28
1251
70
1135
36
3
895
719
50
64

29
29
1331
129
1163
35
3
933
739
46
66

30
30

1085

81
1075
40
3
939
562
63
55

Heat treatment at hot stamping

Average
Average

cooling rate
cooling rate

Multilayer
Heating
(° C./s)
(° C./s)
Tempering

Sheet

steel
temp.
(more than
(400° C.
temp.

thickness

sheet no.
(° C.)
400° C.)
or less)
(° C.)
Plating
(mm)

1
976
85
34
—
None
1.6

2
945
111
12
—
None
1.4

3
845
83
21
—
None
1.4

4
923
108
33
—
None
1.2

5
965
75
19
—
None
1.6

6
971
63
32
—
None
1.6

7
845
74
32
—
None
1.6

8
910
75
16
—
None
1.4

9
890
86
13
—
None
1.0

10
983
65
13
—
None
1.4

11
851
83
13
—
None
1.2

12
904
77
13
—
None
1.2

13
918
70
13
—
None
1.2

17
894
89
13
—
None
1.4

18
899
78
13
—
None
1.6

19
884
85
13
—
None
1.6

20
866
100
13
—
None
1.6

21
940
83
13
—
None
1.2

22
971
85
13
—
None
1.4

23
986
93
13
—
None
1.4

24
847
98
13
—
None
1.2

27
960
92
13
—
None
1.0

28
927
118
13
—
None
1.2

29
908
92
13
—
None
1.0

30
957
108
13
—
None
1.4

TABLE C-2-2

Rough rolling

Heat

Heat treatment

Rate of

Hot rolling
Cold
treatment at

Manu-
before hot rolling

reduction
No. of
Finish

rolling
hot stamping

Multilayer
facturing
Heating
Holding
Rolling
of sheet
rolling
rolling
Coiling
Rolling
Heating

steel
condition
temp.
time
temp.
thickness
operations
temp.
temp.
rate
rate

sheet no.
no.
(° C.)
(min)
(° C.)
(%)
(times)
(° C.)
(° C.)
(%)
(° C./s)

31
31

1380

112
1132
25
3
879
738
62
33

32
32
1231
10
1192
36
3
928
652
59
76

34
34
1341
100
1165
25
3
906
641
60
28

35
35
1166
131
1133
38
3
853
660
60
40

36
36
1191
130
1140
40
3
876
636
45
47

37
37
1276
139
1142
46
3
901
560
60
69

38
38
1281
141
1139
36
3
902
549
60
61

39
39
1237
71
1121
27
3
915
633
52
62

40
40
1216
82
1125
33
3
863
577
60
54

41
41
1236
82
1145
43
3
894
608
64
60

42
42
1163
85
1151
31
3
862
561
46
54

43
43
1240
61
1121
42
3
862
625
54
26

44
44
1247
113
1168
39
3
920
561
55
30

45
45
1175
104
1130
31
3
910
648
61
63

46
46
1172
113
1167
28
3
876
557
47
24

47
47
1228
103
1173
29
3
925
613
58
51

48
48
1212
65
1137
27
3
927
604
51
37

49
49
1214
69
1106
42
3
859
609
64
45

50
50
1164
77
1115
32
3
867
648
59
18

51
51
1152
63
1144
32
3
891
553
65
67

52
52
1159
83
1152
35
3
865
595
50
49

53
53
1201
83
1132
31
3
871
615
48
68

54
54
1232
65
1117
31
3
917
592
63
45

55
55
1248
75
1111
33
3
856
639
49
72

56
56
1276
86

1005

38
3
879
699
48
60

57
57
1236
77
1155
4
2
901
739
45
67

58
58
1247
91
1149
44

1

863
636
63
59

59
59
1228
64
1132
20
3
862
561
60
28

Heat treatment at hot stamping

Multi-

Average
Average

layer

cooling rate
cooling rate

Sheet

steel
Heating
(° C./s)
(° C./s)
Tempering

thick-

sheet
temp.
(more than
(400° C.
temp.

ness

no.
(° C.)
400° C.)
or less)
(° C.)
Plating
(mm)

31
909
88
13
—
None
1.4

32
963
79
13
—
None
1.2

34
858
116
13
—
None
1.4

35
902
75
13
310
None
1.0

36
900
95
13
420
Yes
1.2

37
990
79
13
—
Yes
1.4

38
922
66
13
—
None
1.2

39
852
62
13
—
None
1.7

40
886
70
13
—
None
1.7

41
855
75
13
—
None
1.2

42
930
87
13
—
None
1.4

43
880
93
13
—
None
1.8

44
931
84
13
—
None
1.6

45
858
73
13
—
None
1.7

46
934
98
13
—
None
1.8

47
851
70
13
—
None
1.6

48
853
106
13
—
None
1.7

49
889
78
13
—
None
1.7

50
887
105
13
—
None
1.2

51
863
81
13
—
None
1.4

52
872
50
13
—
None
1.4

53
864
74
13
—
None
1.4

54
901
86
13
—
None
1.4

55
922
80
13
—
None
1.4

56
855
89
13
—
None
1.7

57
931
67
13
—
None
1.4

58
851
67
13
—
None
1.7

59
853
61
13
—
None
1.7

TABLE C-3-1

Metal structures

Area rate (%) of

total of crystal grains

Hard-
with maximum difference

ness
of crystal orientation

of
inside large angle grain

middle
boundaries of 1° or
Mechanical properties

Multi-

part
less and crystal grains

Max-
Hydrogen
Residual

layer
Manu-
in sheet
with maximum difference

imum
embrit-
γ

steel
facturing
thick-
of crystal orientation
Tensile
Uniform
bending
tlement
area

Stamped
sheet
condition
ness
of 8° or more
strength
elongation
angle
re-
rate
Re-

body no.
no.
no.
(Hv)
and less than 15°
(MPa)
(%)
(°)
sistance
(%)
marks

1C
1
1
576
32
1516
6
103
Good
4.8
Inv. ex.

2C
2
2
738
31
2083
7.7
107
Good
2.5
Inv. ex.

3C
3
3
639
31
2027
8.3
114
Good
3.4
Inv. ex.

4C
4
4
831
30
2256
8.2
91
Good
1.4
Inv. ex.

5C
5
5

402

25

1426

5.4
106
Good
3.9

Comp.

ex.

6C
6
6
554
35
1628
6.4
105
Good
1.9
Inv. ex.

7C
7
7
567
47
1594
5.9
99
Good
4.6
Inv. ex.

8C
8
8
592
29
1845
8.5
117
Good
3.1
Inv. ex.

9C
9
9

823

40
2344
8.1
78
Good
4.3

Comp.

ex.

10C
10
10
694
29
1895
5.3
97
Good
3.9
Inv. ex.

11C
11
11
664
47
1931

3.8

90
Good

0.7

Comp.

ex.

12C
12
12
727
30
2041
8.8
97
Good
4.3
Inv. ex.

13C
13
13
622
40
1996
5
98
Good
5.0
Inv. ex.

17C
17
17
667
39
1851
6.8
95
Good
2.7
Inv. ex.

18C
18
18
542
42
1550
7
94
Good
1.3
Inv. ex.

19C
19
19
590
38
1615
5.7
118
Good
1.4
Inv. ex.

20C
20
20
473
42
1588
5.6
115
Good
3.0
Inv. ex.

21C
21
21
759
25
2004
6
112
Good
4.6
Inv. ex.

22C
22
22
603
31
1826
5.3
118
Good
4.0
Inv. ex.

23C
23
23
578
38
1762
6.9
96
Good
2.1
Inv. ex.

24C
24
24
713
32
2035
5.5
100
Good
3.8
Inv. ex.

27C
27
27
683
46
2556
7.7
101
Good
4.1
Inv. ex.

28C
28
28
726
31
2277
7.8
115
Good
2.4
Inv. ex.

29C
29
29
771
44
2303
6.6
117
Good
4.3
Inv. ex.

30C
30
30
700

18

1768
8.2
69

Poor

4.0

Comp.

ex.

TABLE C-3-2

Metal structures

Area rate (%) of

total of crystal grains

Hard-
with maximum difference

ness
of crystal orientation

of
inside large angle grain

middle
boundaries of 1° or
Mechanical properties

Multi-

part
less and crystal grains

Max-
Hydrogen
Residual

layer
Manu-
in sheet
with maximum difference

imum
embrit-
γ

steel
facturing
thick-
of crystal orientation
Tensile
Uniform
bending
tlement
area

Stamped
sheet
condition
ness
of 8° or more
strength
elongation
angle
re-
rate

body no.
no.
no.
(Hv)
and less than 15°
(MPa)
(%)
(°)
sistance
(%)
Remarks

31C
31
31
728

92

1795
7
65
Good
3.3

Comp. ex.

32C
32
32
592

15

1940
6.8
66

Poor

3.9

Comp. ex.

34C
34
34
639
78
1860
6
109
Good
3.8
Inv. ex.

35C
35
35
815
51
2312
6.9
96
Good
5.0
Inv. ex.

36C
36
36
698
70
2155
6.3
118
Good
1.6
Inv. ex.

37C
37
37
600
52
1802
8.1
96
Good
4.0
Inv. ex.

38C
38
38
695
56
2246
6.6
107
Good
2.7
Inv. ex.

39C
39
39

480

45

1435

9.3
110
Good
3.2

Comp. ex.

40C
40
40
635
37
2109
8.2
94
Good
3.7
Inv. ex.

41C
41
41
700
54
2017
5.8
110
Good
4.1
Inv. ex.

42C
42
42
632
66
2065
7.4
93
Good
2.9
Inv. ex.

43C
43
43
610
49
2278
6.3
102
Good
3.0
Inv. ex.

44C
44
44
613
57
2105
9
106
Good
2.0
Inv. ex.

45C
45
45
653
54
2167
8
95
Good
4.4
Inv. ex.

46C
46
46
697
52
2071
5.5
107
Good
3.9
Inv. ex.

47C
47
47
613
29
2043
7
95
Good
4.2
Inv. ex.

48C
48
48
647
44
2189
6
103
Good
4.6
Inv. ex.

49C
49
49
615
60
2020
6.5
110
Good
2.1
Inv. ex.

50C
50
50
605
62
2287
9
103
Good
3.7
Inv. ex.

51C
51
51
611
44
2165
6.4
95
Good
4.3
Inv. ex.

52C
52
52
622
32
2141
7.7
108
Good
3.3
Inv. ex.

53C
53
53
604
56
2275
6.6
98
Good
3.1
Inv. ex.

54C
54
54
610
63
2010
7.2
110
Good
4.4
Inv. ex.

55C
55
55
631
47
2109
8.8
110
Good
2.5
Inv. ex.

56C
56
56
642

12

2119
6.1
63.2

Poor

2.8

Comp. ex.

57C
57
57
638

11

2105
6.6
59.6

Poor

2.9

Comp. ex.

58C
58
58
633

13

2089
6.4
57.9

Poor

3.2

Comp. ex.

59C
59
59
629
46
2076
6.9
109
Good
2.9
Inv. ex.

Manufacturing Example D

Steel sheets for sheet thickness middle part having the Nos. 1 to 38 chemical compositions shown in Table D-1-1 to Table D-1-2 (in the tables, “Steel Nos. 1 to 38”) 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. 38 multilayer steel sheet is steel with the steel sheet for surface layer welded to only one surface. The multilayer steel sheets other than No. 38 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-1-3, 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. 38 and 39 hot stamped bodies were coated on a hot dip coating line at the surfaces of the matrix steel sheets with 120 to 160 g/m²amounts 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-1-4.

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 D-3-1 to D-3-3.

The 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 and the maximum bending angle (°) was 90(°) or more and further the hydrogen embrittlement resistance was passing were evaluated as excellent in impact resistance and hydrogen embrittlement resistance and indicated as “invention examples”. Cases where even one of the above three 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 20% to less than 50%. 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 had an area percent of residual austenite of less than 1% and were insufficient in uniform elongation. Further, the Nos. 12D and 13D hot stamped bodies were 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.

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 too 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. Further, the No. 34D hot stamped body was excessively high in heat treatment temperature before the hot stamping process, so became excessive in growth of soft structures and metal structures of intermediate hardnesses, became excessively large in difference of hardnesses between the softened layer and middle part in sheet thickness, and was not able to obtain the effect of reduction of the sharp gradient of hardness in the sheet thickness direction formed at the time of bending deformation. For this reason, the No. 34D hot stamped body could not be given excellent bendability. The No. 35D hot stamped body was too short in heat treatment time 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. 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 was inferior in bendability. The No. 42D hot stamped body was poor in content of acid soluble aluminum, so was inferior in bendability. Further, the No. 45D hot stamped body included an excessive content of acid soluble aluminum, so was inferior in bendability.

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.

TABLE D-1-1

Multilayer
Chemical constituents of steel sheet for sheet thickness middle part (mass %)

steel
Steel

sheet no.
no.
C
Si
Mn
P
S
sol.Al
N
Ni
Nb
Ti
Mo
B
Remarks

1
1
0.23
1.43
1.74
0.023
0.0029
0.061
0.0029
0
0
0
0
0

2
2
0.28
1.31
1.97
0.007
0.0024
0.039
0.0023
0
0
0
0
0

3
3
0.34
1.53
1.65
0.017
0.0009
0.025
0.0029
0
0
0
0
0

4
4
0.40
1.38
2.05
0.016
0.0014
0.030
0.0041
0
0
0
0
0

5
5

0.13

1.36
1.86
0.015
0.0018
0.038
0.0044
0
0
0
0
0

Comp. steel

6
6
0.28
1.47
1.90
0.004
0.0024
0.043
0.0048
0
0
0
0
0

7
7
0.36
1.86
1.86
0.010
0.0029
0.046
0.0036
0
0
0
0
0

8
8
0.40
1.78
2.03
0.003
0.0003
0.060
0.0034
0
0
0
0
0

9
9

0.80

1.73
1.86
0.008
0.0032
0.043
0.0027
0
0
0
0
0

Comp. steel

10
10
0.29

0.22

1.91
0.008
0.0024
0.043
0.0016
0
0
0
0
0

Comp. steel

11
11
0.26

0.32

1.85
0.014
0.0006
0.049
0.0028
0
0
0
0
0

Comp. steel

12
12
0.36
1.27

0.18

0.009
0.0031
0.062
0.0035
0
0
0
0
0

Comp. steel

15
15
0.23

0.38

0.73

0.012
0.0006
0.064
0.0028
0
0
0
0
0

Comp. steel

16
16
0.40
1.61
1.79
0.007
0.0034
0.042
0.0033
1.71
0
0
0
0

17
17
0.39
1.07
1.66
0.011
0.003
0.047
0.0020
0
0.082
0
0
0

18
18
0.38
1.55
1.98
0.018
0.0035
0.058
0.0026
0
0
0.032
0
0

19
19
0.28
1.23
1.94
0.013
0.0009
0.061
0.0028
0
0
0
0.04
0

20
20
0.28
1.4
1.81
0.015
0.0011
0.028
0.0037
0
0
0
0
0.0019

21
1
0.23
1.43
1.74
0.023
0.0029
0.061
0.0029
0
0
0
0
0

22
1
0.23
1.43
1.74
0.023
0.0029
0.061
0.0029
0
0
0
0
0

23
1
0.23
1.43
1.74
0.023
0.0029
0.061
0.0029
0
0
0
0
0

24
2
0.28
1.31
1.97
0.007
0.0024
0.039
0.0023
0
0
0
0
0

25
2
0.28
1.31
1.97
0.007
0.0024
0.039
0.0023
0
0
0
0
0

26
2
0.28
1.31
1.97
0.007
0.0024
0.039
0.0023
0
0
0
0
0

27
3
0.34
1.53
1.65
0.017
0.0009
0.025
0.0029
0
0
0
0
0

28
3
0.34
1.53
1.65
0.017
0.0009
0.025
0.0029
0
0
0
0
0

29
3
0.34
1.53
1.65
0.017
0.0009
0.025
0.0029
0
0
0
0
0

30
4
0.40
1.38
2.05
0.016
0.0014
0.03
0.0041
0
0
0
0
0

TABLE D-1-2

Multilayer
Chemical constituents of steel sheet for sheet thickness middle part (mass %)

steel
Steel

sheet no.
no.
C
Si
Mn
P
S
sol.Al
N
Ni
Nb
Ti
Mo
B
Remarks

31
4
0.4
1.38
2.05
0.016
0.0014
0.030
0.0041
0
0
0
0
0

32
4
0.4
1.38
2.05
0.016
0.0014
0.030
0.0041
0
0
0
0
0

33
2
0.28
1.31
1.97
0.007
0.0024
0.039
0.0023
0
0
0
0
0

34
2
0.28
1.31
1.97
0.007
0.0024
0.039
0.0023
0
0
0
0
0

35
2
0.28
1.31
1.97
0.007
0.0024
0.039
0.0023
0
0
0
0
0

36
2
0.28
1.31
1.97
0.007
0.0024
0.039
0.0023
0
0
0
0
0

37
21
0.67
1.26
1.82
0.013
0.0033
0.027
0.0023
0
0
0
0
0

38
21
0.67
1.26
1.82
0.013
0.0033
0.027
0.0023
0
0
0
0
0

39
2
0.28
1.31
1.97
0.007
0.0024
0.039
0.0023
0
0
0
0
0

40
22
0.38

4.9

1.87
0.009
0.0022
0.058
0.003
0
0
0
0
0

Comp. steel

41
23
0.25
1.21

4.5

0.012
0.0009
0.046
0.0023
0
0
0
0
0

Comp. steel

42
24
0.26
1.32
1.82
0.013
0.0028

0.0001

0.0036
0
0
0
0
0

Comp. steel

43
25
0.26
1.32
1.82
0.013
0.0028
0.002
0.0036
0
0
0
0
0

44
26
0.26
1.32
1.82
0.013
0.0028
2.500
0.0036
0
0
0
0
0

45
27
0.26
1.32
1.82
0.013
0.0028

4.100

0.0036
0
0
0
0
0

Comp. steel

46
28
0.30
1.59
1.75
0.004
0.0012
0.052
0.003
0.04
0
0
0
0

47
29
0.30
1.59
1.75
0.004
0.0012
0.052
0.003
2.60
0
0
0
0

48
30
0.36
1.00
1.78
0.022
0.0007
0.045
0.0032
0
0.030
0
0
0

49
31
0.36
1.00
1.78
0.022
0.0007
0.045
0.0032
0
0.120
0
0
0

50
32
0.27
1.63
1.97
0.016
0.0012
0.051
0.0029
0
0
0.030
0
0

51
33
0.27
1.63
1.97
0.016
0.0012
0.051
0.0029
0
0
0.100
0
0

52
34
0.29
1.27
2.01
0.013
0.0013
0.057
0.003
0
0
0
0.010
0

53
35
0.29
1.27
2.01
0.013
0.0013
0.057
0.003
0
0
0
0.800
0

54
36
0.3
1.45
1.72
0.014
0.0016
0.043
0.0032
0
0
0
0
0.0009

55
37
0.3
1.45
1.72
0.014
0.0016
0.043
0.0032
0
0
0
0
0.0060

56
38
0.4
1.38
2.05
0.016
0.0014
0.03
0.0041
0
0
0
0
0

57
2
0.28
1.31
1.97
0.007
0.0024
0.039
0.0023
0
0
0
0
0

58
2
0.28
1.31
1.97
0.007
0.0024
0.039
0.0023
0
0
0
0
0

59
2
0.28
1.31
1.97
0.007
0.0024
0.039
0.0023
0
0
0
0
0

60
2
0.28
1.31
1.97
0.007
0.0024
0.039
0.0023
0
0
0
0
0

TABLE D-1-3

Multilayer

steel sheet
Chemical constituents of steel sheet for surface layer (mass %)

no.
C
Si
Mn
P
S
sol.Al
N
Ni
Nb
Ti
Mo
B
Remarks

1
0.11
0.80
0.90
0.020
0.0025
0.054
0.0027
0
0
0
0
0

2
0.13
0.73
1.08
0.005
0.0021
0.038
0.0018
0
0
0
0
0

3
0.14
0.64
0.68
0.014
0.0008
0.020
0.0028
0
0
0
0
0

4
0.2
0.70
0.94
0.014
0.0013
0.025
0.0039
0
0
0
0
0

5
0.05
0.65
1.00
0.012
0.0016
0.034
0.0039
0
0
0
0
0

Comp. steel

6
0.16
0.78
1.06
0.001
0.0023
0.036
0.0045
0
0
0
0
0

7
0.18
0.84
1.00
0.009
0.0026
0.041
0.0032
0
0
0
0
0

8
0.18
0.75
1.12
0.001
0.0026
0.053
0.0032
0
0
0
0
0

9
0.33
0.80
0.86
0.005
0.0031
0.036
0.0025
0
0
0
0
0

Comp. steel

10
0.15
0.11
0.80
0.007
0.0021
0.040
0.0015
0
0
0
0
0

Comp. steel

11
0.11
0.13
0.80
0.011
0.0005
0.042
0.0027
0
0
0
0
0

Comp. steel

12
0.16
0.57
0.08
0.007
0.0027
0.055
0.0033
0
0
0
0
0

Comp. steel

15
0.13
0.19
0.34
0.009
0.0003
0.061
0.0025
0
0
0
0
0

Comp. steel

16
0.22
0.69
0.79
0.004
0.003
0.041
0.0029
1.51
0
0
0
0

17
0.17
0.56
0.83
0.009
0.0027
0.045
0.0018
0
0.065
0
0
0

18
0.19
0.84
0.83
0.016
0.0034
0.052
0.0022
0
0
0.028
0
0

19
0.13
0.60
0.93
0.010
0.0008
0.059
0.0023
0
0
0
0.030
0

20
0.15
0.60
0.74
0.013
0.0009
0.021
0.0033
0
0
0
0
0.0016

21
0.16
0.66
0.77
0.021
0.0025
0.057
0.0027
0
0
0
0
0

22
0.09
0.94
0.77
0.020
0.0026
0.054
0.0026
0
0
0
0
0

23
0.1
0.76
1.18
0.022
0.0025
0.055
0.0025
0
0
0
0
0

24
0.22
0.64
1.08
0.004
0.0022
0.033
0.0019
0
0
0
0
0

25
0.16
1.02
0.95
0.004
0.0020
0.032
0.002
0
0
0
0
0

26
0.12
0.54
1.28
0.004
0.0023
0.034
0.002
0
0
0
0
0

27
0.29
0.7
0.71
0.016
0.0008
0.018
0.0025
0
0
0
0
0

28
0.17
0.98
0.86
0.014
0.0006
0.021
0.0025
0
0
0
0
0

29
0.19
0.8
1.17
0.015
0.0006
0.022
0.0026
0
0
0
0
0

30
0.32
0.63
1.15
0.014
0.0012
0.026
0.0037
0
0
0
0
0

TABLE D-1-4

Multilayer

steel sheet
Chemical constituents of steel sheet for surface layer (mass %)

no.
C
Si
Mn
P
S
sol.Al
N
Ni
Nb
Ti
Mo
B
Remarks

31
0.18
1.12
1.05
0.014
0.0011
0.022
0.0039
0
0
0
0
0

32
0.17
0.63
1.39
0.013
0.001
0.024
0.0037
0
0
0
0
0

33
0.12
0.59
1.06
0.006
0.0022
0.032
0.0018
0
0
0
0
0

34
0.15
0.52
0.97
0.006
0.0022
0.031
0.0022
0
0
0
0
0

35
0.14
0.55
0.85
0.004
0.0023
0.037
0.002
0
0
0
0
0

36
0.13
0.59
0.99
0.006
0.0021
0.034
0.0021
0
0
0
0
0

37
0.27
0.66
0.80
0.01
0.0031
0.02
0.0019
0
0
0
0
0

38
0.24
0.69
0.80
0.01
0.0029
0.02
0.0022
0
0
0
0
0

39
0.12
0.72
0.85
0.005
0.0021
0.032
0.002
0
0
0
0
0

40
0.21
0.06
0.94
0.006
0.0018
0.055
0.0025
0
0
0
0
0

Comp. steel

41
0.14
0.64
0.11
0.011
0.0008
0.039
0.0021
0
0
0
0
0

Comp. steel

42
0.15
0.66
0.91
0.011
0.0026
0.039
0.0032
0
0
0
0
0

Comp. steel

43
0.15
0.66
0.91
0.01
0.0027
0.031
0.0033
0
0
0
0
0

44
0.15
0.66
0.91
0.011
0.0024
2.497
0.0033
0
0
0
0
0

45
0.15
0.66
0.91
0.011
0.0026
2.78
0.0035
0
0
0
0
0

Comp. steel

46
0.15
0.78
0.95
0.001
0.0008
0.047
0.0028
0.03
0
0
0
0

47
0.15
0.78
0.95
0.003
0.0009
0.048
0.0029
2.40
0
0
0
0

48
0.17
0.55
0.91
0.019
0.0004
0.043
0.0031
0
0.020
0
0
0

49
0.17
0.55
0.91
0.021
0.0003
0.037
0.0027
0
0.100
0
0
0

50
0.15
0.85
1.04
0.013
0.001
0.045
0.0028
0
0
0.040
0
0

51
0.15
0.85
1.04
0.013
0.0009
0.043
0.0024
0
0
0.090
0
0

52
0.15
0.74
1.13
0.011
0.0011
0.05
0.0025
0
0
0
0.020
0

53
0.15
0.74
1.13
0.011
0.001
0.053
0.0027
0
0
0
0.700
0

54
0.17
0.78
0.81
0.012
0.0015
0.038
0.003
0
0
0
0
0.0080

55
0.17
0.78
0.81
0.012
0.0015
0.04
0.0028
0
0
0
0
0.0050

56
0.17
0.91
1.37
0.013
0.001
0.024
0.0037
0
0
0
0
0

57
0.13
0.73
1.08
0.005
0.0021
0.038
0.0018
0
0
0
0
0

58
0.13
0.73
1.08
0.005
0.0021
0.038
0.0018
0
0
0
0
0

59
0.13
0.73
1.08
0.005
0.0021
0.038
0.0018
0
0
0
0
0

60
0.13
0.73
1.08
0.005
0.0021
0.038
0.0018
0
0
0
0
0

TABLE D-2-1

Rough rolling

Heat

Heat treatment

Rate of

Hot rolling
Cold
treatment at

before hot rolling

reduction
No. of
Finish

rolling
hot stamping

Multilayer

Heating
Holding
Rolling
of sheet
rolling
rolling
Coiling
Rolling
Heating

steel
Manufacturing
temp.
time
temp.
thickness
operations
temp
temp.
rate
rate

sheet no.
condition no.
(° C.)
(min)
(° C.)
(%)
(times)
(° C.)
(° C.)
(%)
(° C./s)

1
1
1281
129
1153
42
3
855
663
47
40

2
2
1125
108
1105
27
3
848
669
54
31

3
3
1120
128
1110
24
3
830
633
53
44

4
4
1279
81
1158
36
3
859
590
49
64

5
5
1194
114
1152
27
3
908
683
45
44

6
6
1269
132
1143
39
3
907
672
48
65

7
7
1299
98
1201
35
3
906
561
53
60

8
8
1148
87
1123
43
3
855
627
44
56

9
9
1125
135
1115
23
3
892
615
50
29

10
10
1187
135
1153
28
3
850
703
45
46

11
11
1210
144
1160
36
3
865
565
46
58

12
12
1225
78
1194
38
3
879
586
55
66

15
15
1305
136
1164
31
3
892
657
55
31

16
16
1248
85
1143
30
3
867
570
55
39

17
17
1183
81
1133
49
3
846
566
49
68

18
18
1277
124
1133
48
3
868
652
44
50

19
19
1210
81
1143
35
3
832
666
53
24

20
20
1195
144
1124
48
3
851
608
51
27

Heat treatment at hot stamping

Average
Average

cooling rate
cooling rate

Heating
(° C./s)
(° C./s)
Tempering

Sheet

temp.
(more than
(400° C.
temp.

thickness

(° C.)
400° C.)
or less)
(° C.)
Plating
(mm)

904
88
32
None
None
1.5

900
113
11
None
None
1.3

872
82
26
None
None
1.3

892
104
35
None
None
1.4

895
83
23
None
None
1.5

869
70
33
None
None
1.5

872
74
28
None
None
1.3

850
80
15
None
None
1.6

878
79
14
None
None
1.4

865
68
9
None
None
1.5

898
91
8
None
None
1.5

900
72
12
None
None
1.3

861
68
10
None
None
1.3

858
87
16
None
None
1.3

906
102
15
None
None
1.4

877
80
12
None
None
1.6

856
87
13
None
None
1.3

903
88
12
None
None
1.4

TABLE D-2-2

Rough rolling

Heat

Heat treatment

Rate of

Hot rolling
Cold
treatment at

before hot rolling

reduction
No. of
Finish

rolling
hot stamping

Multilayer

Heating
Holding
Rolling
of sheet
rolling
rolling
Coiling
Rolling
Heating

steel
Manufacturing
temp.
time
temp.
thickness
operations
temp
temp.
rate
rate

sheet no.
condition no.
(° C.)
(min)
(° C.)
(%)
(times)
(° C.)
(° C.)
(%)
(° C./s)

21
21
1180
94
1160
45
3
888
584
45
21

22
22
1164
140
1156
34
3
891
690
49
15

23
23
1189
89
1143
38
3
883
594
48
64

24
24
1297
136
1169
35
3
876
696
46
68

25
25
1153
149
1129
35
3
880
575
50
57

26
26
1163
139
1123
27
3
863
626
55
37

27
27
1303
129
1190
31
3
899
578
51
75

28
28
1219
147
1161
22
3
880
641
46
31

29
29
1216
78
1127
41
3
838
663
48
38

30
30
1170
107
1144
43
3
869
650
48
49

31
31
1280
130
1149
48
3
837
585
50
64

32
32
1237
100
1133
35
3
893
685
46
64

33
33

1072

125
1022
24
3
890
720
46
64

34
34

1368

122
1131
28
3
878
657
44
51

35
35
1130
12
1113
46
3
896
623
50
64

36
36
1244
131
1147
29
3
883
710
0
58

37
37
1121
118
1102
44
3
861
622
47
21

38
38
1165
80
1110
40
3
903
602
48
32

39
39
1144
137
1131
34
3
877
644
47
60

40
40
1239
115
1164
24
3
879
624
53
28

Heat treatment at hot stamping

Average
Average

cooling rate
cooling rate

Heating
(° C./s)
(° C./s)
Tempering

Sheet

temp.
(more than
(400° C.
temp.

thickness

(° C.)
400° C.)
or less)
(° C.)
Plating
(mm)

891
107
9
None
None
1.5

901
93
9
None
None
1.4

895
126
11
None
None
1.5

877
101
9
None
None
1.5

885
103
8
None
None
1.4

851
81
16
None
None
1.3

904
87
17
None
None
1.4

866
111
15
None
None
1.5

855
83
11
None
None
1.5

870
86
12
None
None
1.5

889
72
9
None
None
1.4

882
70
13
None
None
1.5

859
58
14
None
None
1.5

902
71
10
None
None
1.6

903
84
13
None
None
1.4

896
96
13
None
None
2.8

893
92
13
267
None
1.5

859
84
11
279
Yes
1.5

858
74
18
None
Yes
1.5

898
88
11
None
None
1.3

TABLE D-2-3

Rough rolling

Heat

Heat treatment

Rate of

Hot rolling
Cold
treatment at

before hot rolling

reduction
No. of
Finish

rolling
hot stamping

Multilayer

Heating
Holding
Rolling
of sheet
rolling
rolling
Coiling
Rolling
Heating

steel
Manufacturing
temp.
time
temp.
thickness
operations
temp
temp.
rate
rate

sheet no.
condition no.
(° C.)
(min)
(° C.)
(%)
(times)
(° C.)
(° C.)
(%)
(° C./s)

41
41
1266
123
1182
31
3
839
578
54
49

42
42
1280
101
1128
31
3
858
568
54
33

43
43
1268
119
1104
45
3
860
648
54
48

44
44
1270
109
1107
32
3
844
592
54
19

45
45
1251
113
1145
28
3
883
607
54
63

46
46
1241
99
1146
39
3
872
573
55
53

47
47
1235
96
1132
34
3
851
615
55
63

48
48
1272
95
1110
36
3
871
629
49
49

49
49
1272
115
1106
35
3
887
567
49
67

50
50
1267
99
1106
36
3
843
631
44
65

51
51
1257
82
1145
28
2
845
662
44
69

52
52
1241
104
1159
43
3
858
638
53
62

53
53
1242
127
1139
23
3
886
645
53
24

54
54
1244
90
1172
27
3
863
617
51
46

55
55
1261
89
1130
26
3
888
606
51
23

56
56
1231
99
1132
35
3
895
681
46
72

57
57
1276
98

1004

38
3
879
699
48
59

58
58
1236
88
1164
7
2
901
739
45
64

59
59
1247
78
1148
39

1

863
636
63
58

60
60
1228
62
1133
24
3
862
561
60
26

Heat treatment at hot stamping

Average
Average

cooling rate
cooling rate

Heating
(° C./s)
(° C./s)
Tempering

Sheet

temp.
(more than
(400° C.
temp.

thickness

(° C.)
400° C.)
or less)
(° C.)
Plating
(mm)

876
67
14
None
None
1.3

884
107
10
None
None
1.3

895
79
17
None
None
1.3

865
102
17
None
None
1.3

878
91
12
None
None
1.3

894
49
15
None
None
1.3

870
64
11
None
None
1.3

891
82
14
None
None
1.4

882
75
11
None
None
1.4

893
95
11
None
None
1.6

879
70
17
None
None
1.6

869
69
17
None
None
1.3

868
51
15
None
None
1.3

887
101
9
None
None
1.4

877
83
8
None
None
1.4

889
75
15
None
None
1.5

855
83
12
—
None
1.7

931
76
18
—
None
1.4

851
61
8
—
None
1.7

853
55
14
—
None
1.7

TABLE D-3-1

Metal structures

Area rate (%) of

total of crystal grains

Hard-
with maximum difference

ness
of crystal orientation

of
inside large angle grain

Mechanical properties

middle
boundaries of 1° or

Average

Multi-

part
less and crystal grains
Residual

cross-
Max-

layer
Manu-
in sheet
with maximum difference
γ

sectional
imum

steel
facturing
thick-
of crystal orientation
area
Tensile
Uniform
hardness-
bending
Hydrogen

Stamped
sheet
condition
ness
of 8° or more
rate
strength
elongation
minimum
angle
embrittlement

body no.
no.
no.
(Hv)
and less than 15°
(%)
(MPa)
(%)
hardness
(°)
resistance
Remarks

1D
1
1
610
38
3.3
1825
6.3
26
103.9
Good
Inv. ex.

2D
2
2
725
29
3
2169
5.7
75
102.9
Good
Inv. ex.

3D
3
3
797
24
4.1
2383
6.9
62
101
Good
Inv. ex.

4D
4
4
798
24
3.5
2509
6.9
50
96.8
Good
Inv. ex.

5D
5
5

476

48
3.5

1423

6.5
66
101.6
Good

Comp. ex.

6D
6
6
785
25
3.4
2346
6.3
66
101.5
Good
Inv. ex.

7D
7
7
772
26
4.6
2307
6.8
31
101.8
Good
Inv. ex.

8D
8
8
788
25
4.2
2356
6.7
37
97.1
Good
Inv. ex.

9D
9
9

1467
47
4.6
4386
6.9
61
58.1
Good

Comp. ex.

10D
10
10
699
31

0.4

2090

0.9

41
102.5
Good

Comp. ex.

11D
11
11
792
24

0.5

2369

1.4

33
99.2
Good

Comp. ex.

12D
12
12

459

49
2.9

1372

5.8

177
104
Good

Comp. ex.

15D
15
15
720
30

0.8

2154

2.7

161
103
Good

Comp. ex.

16D
16
16
789
25
3.9
2359
5.7
34
99.9
Good
Inv. ex.

17D
17
17
780
25
2.5
2331
5.1
53
100.7
Good
Inv. ex.

18D
18
18
781
24
4
2389
5.6
26
100.4
Good
Inv. ex.

19D
19
19
721
30
2.9
2156
5.4
25
101
Good
Inv. ex.

20D
20
20
716
30
3.5
2141
6.8
41
105
Good
Inv. ex.

TABLE D-3-2

Metal structures

Area rate (%) of

total of crystal grains

Hard-
with maximum difference

ness
of crystal orientation

of
inside large angle grain

Mechanical properties

middle
boundaries of 1° or

Average

Multi-

part
less and crystal grains

cross-
Max-

layer
Manu-
in sheet
with maximum difference
Residual

sectional
imum

steel
facturing
thick-
of crystal orientation
γ
Tensile
Uniform
hardness-
bending
Hydrogen

Stamped
sheet
condition
ness
of 8° or more
area rate
strength
elongation
minimum
angle
embrittlement

body no.
no.
no.
(Hv)
and less than 15°
(%)
(MPa)
(%)
hardness
(°)
resistance
Remarks

21D
21
21
631
35
3.6
1888
6.9
61
97.3
Good
Inv. ex.

22D
22
22
624
27
3.4
1867
6.7
52
99.4
Good
Inv. ex.

23D
23
23
619
35
3.6
1852
6.8
36
99.1
Good
Inv. ex.

24D
24
24
734
31
3.2
2196
6.3
71
96
Good
Inv. ex.

25D
25
25
732
40
3.5
2190
6.9
58
97.2
Good
Inv. ex.

26D
26
26
731
41
3.5
2187
6.9
56
98.1
Good
Inv. ex.

27D
27
27
784
23
3.9
2344
5.6
28
99.2
Good
Inv. ex.

28D
28
28
788
34
3.8
2356
5.7
57
96.5
Good
Inv. ex.

29D
29
29
780
43
3.6
2332
6.9
64
98.8
Good
Inv. ex.

30D
30
30
781
33
3.5
2510
6.7
75
96
Good
Inv. ex.

31D
31
31
792
28
3.7
2504
5.8
30
97.1
Good
Inv. ex.

32D
32
32
794
36
3.3
2501
6.3
29
96.7
Good
Inv. ex.

33D
33
33
733

16

3.5
2193
6.8
50

67.1

Poor

Comp. ex.

34D
34
34
731

87

3.3
2187
6.2
58

64.9

Good

Comp. ex.

35D
35
35
741

15

3.4
2217
6.5
52

65.8

Poor

Comp. ex.

36D
36
36
733
29
3.1
2193
5.8
72
103.9
Good
Inv. steel

37D
37
37
788
25
3.3
2356
6.1
63
99.2
Good
Inv. steel

38D
38
38
799
24
3.1
2389
5.9
68
99.2
Good
Inv. steel

39D
39
39
743
28
3
2223
6
41
103.3
Good
Inv. steel

40D
40
40
772
26

10.3

2307
6.8
53

61.8

Good

Comp. ex.

TABLE D-3-3

Metal structures

Area rate (%) of

total of crystal grains

Hard-
with maximum difference

ness
of crystal orientation

of
inside large angle grain

Mechanical properties

middle
boundaries of 1° or

Average

Multi-

part
less and crystal grains

cross-

layer
Manu-
in sheet
with maximum difference
Residual

sectional

steel
facturing
thick-
of crystal orientation
γ
Tensile
Uniform
hardness-
Maximum
Hydrogen

Stamped
sheet
condition
ness
of 8° or more
area rate
strength
elongation
minimum
bending
embrittlement

body no.
no.
no.
(Hv)
and less than 15°
(%)
(MPa)
(%)
hardness
angle (°)
resistance
Remarks

41D
41
41
781
29
3
2577
5.7
33

51.9

Good

Comp. ex.

42D
42
42
733
31
2.7
2419
5.7
53

67.1

Good

Comp. ex.

43D
43
43
734
34
3.2
2422
5.8
56
96.1
Good
Inv. ex.

44D
44
44
717
28
3.3
2366
5.7
55
100.6
Good
Inv. ex.

45D
45
45
731
34
2.9
2412
5.6
51

64.3

Good

Comp. ex.

46D
46
46
761
24
3.5
2511
5.2
71
92.3
Good
Inv. ex.

47D
47
47
799
26
3.9
2637
5.5
36
91.7
Good
Inv. ex.

48D
48
48
741
23
2.5
2445
4.9
28
93.1
Good
Inv. ex.

49D
49
49
793
29
2.6
2617
5.2
48
98.1
Good
Inv. ex.

50D
50
50
738
22
3.1
2435
5.5
29
93.1
Good
Inv. ex.

51D
51
51
788
24
3.5
2600
5.6
56
94.7
Good
Inv. ex.

52D
52
52
651
29
2.8
2148
5.1
25
92.1
Good
Inv. ex.

53D
53
53
731
31
2.9
2412
5.4
33
94.4
Good
Inv. ex.

54D
54
54
655
28
3.2
2162
6.3
46
93.1
Good
Inv. ex.

55D
55
55
725
29
3.5
2393
6.8
50
96.7
Good
Inv. ex.

56D
56
56
799
32
3.1
2636
6.1
30
95.7
Good
Inv. ex.

57D
57
57
710

13

2.7
2343
6.2
31

60.2

Poor

Comp. ex.

58D
58
58
708

10

2.9
2336
6.6
33

59.1

Poor

Comp. ex.

59D
59
59
701
12
2.9
2313
6.4
28

55.1

Poor

Comp. ex.

60D
60
60
698
45
3.1
2303
6.9
29
111
Good
Inv. ex.

INDUSTRIAL APPLICABILITY

The hot stamped body of the present invention is excellent in bendability, ductility, 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.

HOT STAMPED BODY

Information

Publication Number

Date Filed

Date Published

Inventors

Original Assignees

CPC

International Classifications

Abstract

Description

Claims

Priority Claims (1)

PCT Information