Steel sheets for hot stamping and hot stamped products

Abstract

This steel sheet for hot stamping has a specific chemical composition, while having a metal structure that contains, in area%, 75% to 95% of ferrite, 5% to 25% of martensite, and a total of 0% to 5% of pearlite, bainite and cementite; the proportion of ferrite having a GAM value of 0.5 or less among the ferrite is 70% or more in percentage; the ferrite has an average grain size of 1.0 µm to 7.0 µm; the martensite has an average grain size of 0.5 µm to 3.0 µm; and the solid solution Nb concentration is 25 ppm or more.

Description

[Technical Field]

[0001] The present invention relates to a steel sheet for hot stamping and a hot stamped steel. This application claims priority based on Japanese Patent Application No. 2022-001752, filed on January 7, 2022, the contents of which are incorporated herein by reference. [Background technology]

[0002] In recent years, there has been a demand for lighter automobile bodies from the perspectives of environmental protection and resource conservation, and the application of high-strength steel sheets to autobody parts has accelerated. Autobody parts are manufactured by press forming. As the strength of the steel sheets that make up autobody parts increases, not only does the forming load during press forming increase, but formability also decreases. Therefore, when press forming high-strength steel sheets, formability into components with complex shapes becomes an issue.

[0003] To solve the above-mentioned problems, the application of hot stamping technology, in which steel sheets are heated to a high temperature in the austenite region where they soften, and then press-formed, is being promoted. Hot stamping is attracting attention as a technology that achieves both forming into vehicle body parts and ensuring strength by performing quenching treatment in a die simultaneously with press-forming.

[0004] Among autobody parts, members used for impact absorption and frame deformation control are required to be less susceptible to fracture due to deformation during a collision. To prevent fracture due to deformation during a collision, autobody parts must have excellent bendability. Furthermore, they must have small anisotropy in bendability so that fracture can be prevented even when deformed in various deformation modes during a collision.

[0005] The bendability of a material is correlated with its tensile strength, and reducing the tensile strength improves the bendability. The main phase of the metal structure of a hot-stamped body is martensite, and the tensile strength of martensite is greatly affected by the carbon in the chemical composition.

[0006] For example, Patent Document 1 discloses a steel sheet having an area ratio of martensite of 95% or more in the entire structure, a solid solution C of the martensite of 0.05 mass% or less, and a density of carbides having a major axis of 200 nm or more of 50 particles / μm 3 The document discloses a high-strength steel plate characterized by having a tensile strength of 1270 MPa or more and a thickness of 1.0 mm or less.

[0007] Patent Document 2 discloses a non-heat treated high tensile steel plate having a yield strength of 885 MPa or more, characterized in that the microstructure is a mixed structure of martensite and lower bainite, and the total area ratio of both structures is 95% or more.

[0008] Patent Document 3 discloses a high-strength steel with excellent delayed fracture resistance, characterized in that 70% by volume or more of the metal structure is a martensite phase or a tempered martensite phase, and 50% by volume or more of the martensite phase or the tempered martensite phase is a martensite phase or a tempered martensite phase formed from an unrecrystallized austenite phase. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Japanese Patent Publication No. 2018-109222 [Patent Document 2] Japanese Patent Application Publication No. 2011-12315 [Patent Document 3] Japanese Patent Publication No. 11-229075 Summary of the Invention [Problem to be solved by the invention]

[0010] The techniques of Patent Documents 1 to 3 mentioned above improve various properties by specifying the martensite fraction and controlling the metal structure, but do not mention anything about improving bendability or reducing the anisotropy of bendability.

[0011] In view of the above problems, an object of the present invention is to provide a hot-stamped steel sheet having high strength, excellent bendability, and small anisotropy of bendability, and a steel sheet for hot stamping from which the hot-stamped steel sheet can be produced. [Means for solving the problem]

[0012] The gist of the present invention, which was made based on the above findings, is as follows. [1] A steel sheet for hot stamping according to one aspect of the present invention has a chemical composition, in mass%, C: 0.050% or more, less than 0.150% Si: 0.010 to 1.000%, Mn: 1.00-2.00%, P: 0.100% or less, S: 0.0100% or less, Al: 0.001 to 0.500%, N: 0.0001 to 0.0100%, O: 0.1000% or less, Nb: 0.015 to 0.100%, Ti: 0.005 to 0.100%, B: 0.0005~0.0050%, Cr: 0 to 0.500%, Mo: 0 to 0.500%, Ni: 0 to 3.000% Cu: 0~3.000%, Co: 0 to 0.50% W: 0~3.00%, Sn: 0 to 0.500% V: 0 to 0.100%, Zr: 0 to 0.100%, Ca: 0 to 0.0050%, Mg: 0 to 0.0050%, REM: 0~0.0050%, Sb: 0 to 0.0200%, and As: 0 to 1.0000% the balance being Fe and impurities, The metal structure is, in area%, Ferrite: 75-95%, Martensite: 5-25% The sum of pearlite, bainite and cementite: 0 to 5% Among the ferrites, The average misorientation within a grain surrounded by a grain boundary with a misorientation of 5° or more. GAM value is 0.5 ° The percentage of ferrite is 70% or more, The average particle size of the ferrite is 1.0 to 7.0 μm, The average grain size of the martensite is 0.5 to 3.0 μm, The concentration of solute Nb is 25 ppm or more. [2] The steel sheet for hot stamping according to the above item [1], wherein the chemical composition is, in mass%, Cr: 0.100~0.500%, Mo: 0.050~0.500%, Ni: 0.050~3.000%, Cu: 0.050~3.000%, Co: 0.05 to 0.50%, W: 0.05~3.00%, Sn: 0.005 to 0.500%, V: 0.005~0.100%, Zr: 0.005 to 0.100%, Ca: 0.0005 to 0.0050%, Mg: 0.0005 to 0.0050%, REM: 0.0005~0.0050%, Sb: 0.0005 to 0.0200%, and As: 0.0005 to 1.0000% The compound may contain one or more of the group consisting of: [3] The steel sheet for hot stamping according to the above [1] or [2] may have a plating layer on the surface. [4] A hot-stamped product according to another embodiment of the present invention has a chemical composition, in mass%, C: 0.050% or more, less than 0.150% Si: 0.010 to 1.000%, Mn: 1.00-2.00%, P: 0.100% or less, S: 0.0100% or less, Al: 0.001 to 0.500%, N: 0.0001 to 0.0100%, O: 0.1000% or less, Nb: 0.015 to 0.100%, Ti: 0.005 to 0.100%, B: 0.0005~0.0050%, Cr: 0 to 0.500%, Mo: 0 to 0.500%, Ni: 0 to 3.000% Cu: 0~3.000%, Co: 0 to 0.50% W: 0~3.00%, Sn: 0 to 0.500% V: 0 to 0.100%, Zr: 0 to 0.100%, Ca: 0 to 0.0050%, Mg: 0 to 0.0050%, REM: 0~0.0050%, Sb: 0 to 0.0200%, and As: 0 to 1.0000% the balance being Fe and impurities, The metal structure is, in area%, Martensite: 90-100% The sum of ferrite, pearlite, bainite, cementite and retained austenite: 0 to 10% The average grain size of the prior austenite grains is 1.5 to 7.0 μm. [5] The hot-stamped steel according to the above item [4], wherein the chemical composition is, in mass%, Cr: 0.100~0.500%, Mo: 0.050~0.500%, Ni: 0.050~3.000%, Cu: 0.050~3.000%, Co: 0.05 to 0.50%, W: 0.05~3.00%, Sn: 0.005 to 0.500%, V: 0.005~0.100%, Zr: 0.005 to 0.100%, Ca: 0.0005 to 0.0050%, Mg: 0.0005 to 0.0050%, REM: 0.0005~0.0050%, Sb: 0.0005 to 0.0200%, and As: 0.0005 to 1.0000% The compound may contain one or more of the group consisting of: [6] The hot stamped compact according to the above [4] or [5], wherein the standard deviation σ of the nanoindentation hardness of the metallographic structure Hn may satisfy the following formula (1). σ Hn ≦0.235×Hn …(1) Here, Hn in the above formula (1) is the average value of the nanoindentation hardness of the metal structure. [7] The hot-stamped product according to any one of the above items [4] to [6] may have a plating layer on the surface. [Effects of the Invention]

[0013] According to the above aspects of the present invention, it is possible to provide a hot-stamped steel sheet having high strength, excellent bendability, and small anisotropy of bendability, as well as a steel sheet for hot stamping from which the hot-stamped steel sheet can be manufactured. DETAILED DESCRIPTION OF THE INVENTION

[0014] Hereinafter, the steel sheet for hot stamping and the hot-stamped steel according to the present embodiment will be described in detail. First, the reasons for limiting the chemical composition of the steel sheet for hot stamping according to the present embodiment will be described.

[0015] In the following, the numerical ranges indicated by "to" include the lower and upper limits. Numerical values ​​indicated as "less than" and "greater than" are not included in the numerical range. All % in chemical compositions indicate mass %.

[0016] The chemical composition of the steel sheet for hot stamping according to this embodiment is, in mass%, C: 0.050% or more but less than 0.150%, Si: 0.010 to 1.000%, Mn: 1.00 to 2.00%, P: 0.100% or less, S: 0.0100% or less, Al: 0.001 to 0.500%, N: 0.0001 to 0.0100%, O: 0.1000% or less, Nb: 0.015 to 0.100%, Ti: 0.005 to 0.100%, B: 0.0005 to 0.0050%, and the balance being Fe and impurities. Each element will be described in detail below.

[0017] C: 0.050% or more, less than 0.150% C is an element that greatly affects the strength of a hot-stamped steel sheet. If the C content is less than 0.050%, the strength of the hot-stamped steel sheet will be low. Therefore, the C content is set to 0.050% or more, preferably 0.070% or more, or 0.090% or more. On the other hand, if the C content is 0.150% or more, the strength of the hot stamped steel sheet becomes too high, which may result in poor bendability and increased anisotropy of bendability. Therefore, the C content is set to less than 0.150%, preferably 0.130% or less, or 0.120% or less.

[0018] Si: 0.010 to 1.000% Si has temper softening resistance and is effective in suppressing a decrease in strength due to auto-tempering during hot stamp quenching. If the Si content is less than 0.010%, the above effect cannot be obtained, and strength may decrease or bendability may deteriorate. Therefore, the Si content is set to 0.010% or more. Preferably, it is 0.020% or more, 0.030% or more, 0.150% or more, or 0.200% or more. On the other hand, if the Si content exceeds 1.000%, problems with surface scale occur. That is, after the scale formed during hot rolling is pickled, a pattern due to surface irregularities occurs, resulting in poor surface appearance. Furthermore, when plating is performed on the surface of a steel sheet for hot stamping, plating ability deteriorates. Therefore, the Si content is set to 1.000% or less. Preferably, it is 0.700% or less, 0.500% or less, 0.450% or less, or 0.400% or less.

[0019] Mn: 1.00 to 2.00% Mn is an element that improves the strength of a hot-stamped body and the hardenability of steel. If the Mn content is less than 1.00%, the strength of the hot-stamped body decreases. Therefore, the Mn content is set to 1.00% or more, preferably 1.20% or more, or 1.40% or more. On the other hand, if the Mn content exceeds 2.00%, the above effects saturate, the bendability of the hot-stamped steel sheet decreases, and the anisotropy of the bendability increases. Therefore, the Mn content is set to 2.00% or less. Preferably, the Mn content is less than 2.00%, 1.80% or less, or 1.60% or less.

[0020] P:0.100% or less P is an element that segregates at grain boundaries and reduces the strength of the grain boundaries. If the P content exceeds 0.100%, the strength of the grain boundaries is significantly reduced, resulting in reduced toughness and bendability of the hot-stamped steel. Therefore, the P content is set to 0.100% or less, preferably 0.080% or less, or 0.050% or less. Although there is no particular lower limit for the P content, if the P content is reduced too much, the refining cost increases, so the P content may be set to 0.001% or more.

[0021] S: 0.0100% or less S is an element that affects non-metallic inclusions in steel and deteriorates the bendability of hot-stamped steel. Therefore, the S content is set to 0.0100% or less, preferably 0.0080% or less, or 0.0050% or less. Although there is no particular lower limit for the S content, if the S content is reduced too much, the manufacturing cost of the desulfurization step increases, so the S content may be set to 0.0001% or more.

[0022] Al: 0.001 to 0.500% Al is an element used as a deoxidizer for molten steel. If deoxidation is insufficient, the bendability of the hot-stamped steel will be reduced due to the excess oxides produced. To ensure sufficient deoxidation of the molten steel, the Al content is set to 0.001% or more. Preferably, the Al content is 0.010% or more, or 0.030% or more. On the other hand, if the Al content exceeds 0.500%, many nonmetallic inclusions are formed, making the hot stamped steel more susceptible to surface defects. Therefore, the Al content is set to 0.500% or less, preferably 0.300% or less, 0.200% or less, or 0.100% or less.

[0023] N: 0.0001 to 0.0100% If the N content exceeds 0.0100%, coarse nitrides are formed in the steel, significantly reducing the bendability of the hot-stamped steel. Therefore, the N content is set to 0.0100% or less. The N content is preferably 0.0080% or less, and more preferably 0.0060% or less. If the N content is reduced to less than 0.0001%, the cost of denitrification will increase significantly, which is not economically preferable. Therefore, the N content is set to 0.0001% or more. The N content may also be set to 0.0005% or more.

[0024] O: 0.1000% or less If the O content in steel is too high, it forms coarse oxides that become the starting points for fracture. As a result, the bendability of the hot-stamped steel deteriorates. Therefore, the O content is set to 0.1000% or less, and preferably 0.0080% or less or 0.0050% or less. In order to disperse a large number of fine oxides during deoxidation of molten steel, the O content may be set to 0.0005% or more, or 0.0010% or more.

[0025] Nb: 0.015 to 0.100% Nb acts as a solid solution element to refine the grain structure, thereby improving the bendability of the hot-stamped steel and reducing bending anisotropy. If the Nb content is less than 0.015%, the above effects cannot be obtained, and the bendability of the hot-stamped steel deteriorates. If the Nb content is less than 0.015%, the anisotropy of the bendability of the hot-stamped steel increases. Therefore, the Nb content is set to 0.015% or more. Preferably, it is 0.020% or more, 0.030% or more, or 0.040% or more. On the other hand, if the Nb content exceeds 0.100%, a large amount of carbonitrides is formed, which reduces the bendability of the hot-stamped steel and increases the anisotropy of the bendability. Therefore, the Nb content is set to 0.100% or less, preferably 0.080% or less or 0.070% or less.

[0026] Ti: 0.005 to 0.100% Ti forms carbonitrides in steel and has the effect of improving the strength of hot-stamped steel through precipitation strengthening. Furthermore, it fixes N as nitrides, suppresses BN formation, and exerts the effect of B in improving hardenability. If the Ti content is less than 0.005%, the above effect cannot be obtained, and the strength of the hot-stamped steel decreases. Therefore, the Ti content is set to 0.005% or more. It is preferably 0.010% or more, 0.020% or more, or 0.030% or more. On the other hand, if the Ti content exceeds 0.100%, a large amount of carbonitrides is formed, resulting in a decrease in the bendability of the hot-stamped steel sheet. Therefore, the Ti content is set to 0.100% or less, preferably 0.080% or less or 0.070% or less.

[0027] B: 0.0005 to 0.0050% B has the effect of improving the hardenability during hot stamping or during cooling after hot stamping, thereby improving the strength of the hot stamped body. If the B content is less than 0.0005%, the above effect cannot be obtained, and the strength of the hot stamped body decreases. Therefore, the B content is set to 0.0005% or more, preferably 0.0007% or more, or 0.0010% or more. On the other hand, if the B content exceeds 0.0050%, the above effects will saturate, cracks may occur during hot rolling, and borides may reduce the bendability of the hot-stamped steel. Therefore, the B content is set to 0.0050% or less, preferably 0.0040% or less or 0.0030% or less.

[0028] The balance of the chemical composition of the steel sheet for hot stamping according to the present embodiment may be Fe and impurities. Examples of impurities include elements that are inevitably mixed in from steel raw materials or scrap and / or during the steelmaking process and are permissible within a range that does not impair the properties of the hot stamped steel according to the present embodiment.

[0029] The steel sheet for hot stamping according to this embodiment may contain the following elements as optional elements in place of a portion of Fe. When the following optional elements are not contained, the content is 0%.

[0030] Cr: 0.100~0.500% Mo: 0.050 to 0.500% Ni: 0.050 to 3.000% Cu: 0.050 to 3.000% Cr, Mo, Ni, and Cu are elements that improve the hardenability of steel and have the effect of improving the strength of hot-stamped steel. These elements also have the effect of improving the corrosion resistance of hot-stamped steel. Therefore, one or more of these elements may be contained as needed. To ensure the above effects, it is preferable that the Cr content be 0.100% or more, or that the content of at least one of Mo, Ni, and Cu be 0.050% or more. On the other hand, if the Cr content or Mo content exceeds 0.500%, or the Ni content or Cu content exceeds 3.000%, the carbides present after hot rolling, cold rolling, or annealing (including after plating) may be stabilized, delaying the dissolution of the carbides during heating during hot stamping and reducing hardenability. As a result, the strength of the hot-stamped compact may decrease. Therefore, the Cr and Mo contents are each limited to 0.500% or less, and the Ni and Cu contents are each limited to 3.000% or less.

[0031] Co: 0.05 to 0.50% Co is an element that has the effect of raising the Ms point and improving the bendability of the hot-stamped steel. Therefore, Co may be added as needed. To ensure the above effect, the Co content is preferably 0.05% or more. On the other hand, if the Co content exceeds 0.50%, the hardenability of the steel decreases, so the Co content is set to 0.50% or less.

[0032] W: 0.05 to 3.00% W is an element that suppresses phase transformation at high temperatures and contributes to improving the strength of the hot-stamped steel. W also has the effect of improving the corrosion resistance of the hot-stamped steel. Therefore, W may be added as needed. To ensure that the above effects are exerted, the W content is preferably 0.05% or more. On the other hand, if the W content exceeds 3.00%, the hot workability may deteriorate, resulting in a decrease in productivity, or the strength of the hot stamped steel may decrease. Therefore, the W content is set to 3.00% or less.

[0033] Sn: 0.005 to 0.500% Sn has the effect of improving the corrosion resistance of the hot stamped steel. Therefore, Sn may be added as needed. To ensure this effect, the Sn content is preferably 0.005% or more. On the other hand, even if the Sn content exceeds 0.500%, the above effect saturates, so the Sn content is set to 0.500% or less.

[0034] V: 0.005 to 0.100% V forms carbonitrides in steel and has the effect of improving the strength of hot-stamped steel through precipitation strengthening. Furthermore, as a solid solution element, it also has the effect of refining the grain structure, thereby improving the strength and bendability of the hot-stamped steel. Therefore, V may be added as needed. To ensure the above effects, the V content is preferably 0.005% or more. On the other hand, if the V content exceeds 0.100%, a large amount of carbonitrides is formed, which deteriorates the bendability of the hot-stamped steel sheet, so the V content is set to 0.100% or less.

[0035] Zr: 0.005 to 0.100% Zr has the effect of forming carbonitrides in steel and improving the strength of hot-stamped steel through precipitation strengthening. Furthermore, it has the effect of immobilizing N as nitrides, suppressing the formation of BN, and enhancing the hardenability-improving effect of B. Therefore, Zr may be added as needed. To ensure the above effects, the Zr content is preferably 0.005% or more. On the other hand, if the Zr content exceeds 0.100%, a large amount of carbonitrides is formed, which reduces the bendability of the hot-stamped steel sheet, so the Zr content is set to 0.100% or less.

[0036] Ca: 0.0005 to 0.0050% Mg: 0.0005 to 0.0050% REM: 0.0005 to 0.0050% Ca, Mg, and REM have the effect of refining inclusions in steel and preventing cracks caused by the inclusions during hot stamping. Therefore, one or more of these elements may be added as needed. To ensure the above effect, it is preferable that the content of at least one of Ca, Mg, and REM is 0.0005% or more. On the other hand, if the content of Ca, Mg, or REM exceeds 0.0050%, the effect of refining inclusions in the steel saturates and the alloy cost increases. Therefore, the content of Ca, Mg, and REM is each set to 0.0050% or less. In this embodiment, REM refers to a total of 17 elements consisting of Sc, Y and lanthanoids, and the content of REM refers to the total content of these elements.

[0037] Sb: 0.0005 to 0.0200% Sb may be added as needed to suppress decarburization during hot rolling. By adding Sb, decarburization can be suppressed during hot rolling and hot stamping when using an unplated cold-rolled steel sheet. To ensure this effect, the Sn content is preferably 0.0005% or more. On the other hand, if the Sb content exceeds 0.0200%, the above effects saturate, so the Sb content is set to 0.0200% or less.

[0038] As: 0.0005 to 1.0000% As has the effect of inhibiting grain boundary migration and suppressing grain growth. This reduces the ferrite grain size after annealing and the prior austenite grain size after hot stamping, thereby improving the bendability of the hot stamped steel. Therefore, As may be added as needed. To ensure the above effects, the As content is preferably 0.0005% or more. On the other hand, if the As content exceeds 1.0000%, the hot ductility decreases and cracks occur during casting and hot rolling, so the As content is set to 1.0000% or less.

[0039] The chemical composition of the above-mentioned steel sheet for hot stamping may be measured by a general analytical method. For example, it may be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). C and S may be measured using a combustion-infrared absorption method, N may be measured using an inert gas fusion-thermal conductivity method, and O may be measured using an inert gas fusion-non-dispersive infrared absorption method. When the steel sheet for hot stamping has a plating layer on the surface, the plating layer on the surface may be removed by mechanical grinding before analyzing the chemical composition.

[0040] Next, the metal structure of the steel sheet for hot stamping according to this embodiment will be described. The metal structure of the steel sheet for hot stamping according to this embodiment is, in area %, ferrite: 75 to 95%, martensite: 5 to 25%, and a total of pearlite, bainite, and cementite: 0 to 5%. Of the ferrite, the proportion of ferrite having a GAM value of 0.5 or less is 70% or more, the average grain size of the ferrite is 1.0 to 7.0 μm, the average grain size of the martensite is 0.5 to 3.0 μm, and the concentration of solute Nb is 25 ppm or more.

[0041] In this embodiment, the metallographic structure at the 1 / 4 position of the sheet thickness of the steel sheet for hot stamping (the region from the surface to the depth of 1 / 8 of the sheet thickness to the depth of 3 / 8 of the sheet thickness) is specified because the metallographic structure at this position shows a typical metallographic structure of the steel sheet for hot stamping.

[0042] Ferrite area ratio: 75 to 95% If the area ratio of ferrite is less than 75%, the desired metal structure cannot be obtained in the hot stamped steel. Therefore, the area ratio of ferrite is set to 75% or more, preferably 80% or more or 85% or more. On the other hand, if the area ratio of ferrite exceeds 95%, the desired metal structure cannot be obtained in the hot stamped steel. Therefore, the area ratio of ferrite is set to 95% or less, and preferably 90% or less.

[0043] Martensite area ratio: 5 to 25% If the area ratio of martensite is less than 5%, the desired metal structure cannot be obtained in the hot stamped steel. Therefore, the area ratio of martensite is set to 5% or more, preferably 10% or more. On the other hand, if the area ratio of martensite exceeds 25%, the desired metal structure cannot be obtained in the hot stamped steel. Therefore, the area ratio of martensite is set to 25% or less, and preferably 20% or less.

[0044] Total area ratio of pearlite, bainite and cementite: 0-5% The steel sheet for hot stamping according to this embodiment may contain one or more of pearlite, bainite, and cementite as the remaining structure. These remaining structures may not be contained, and therefore the area ratio may be 0%. If the area ratio of the remaining structure exceeds 5%, the desired metal structure cannot be obtained in the hot stamped steel. Therefore, the area ratio of the remaining structure is set to 5% or less, preferably 3% or less, 2% or less, or 1% or less. The cementite in this embodiment does not include plate-like cementite in pearlite, and refers to granular cementite that is not contained in pearlite.

[0045] The area ratio of each structure is measured by the following method. A sample is cut out from the hot stamping steel plate so that a cross section perpendicular to the surface can be observed at 1 / 4 of the plate thickness (the area from 1 / 8 of the plate thickness depth from the surface to 3 / 8 of the plate thickness depth from the surface). After polishing this cross section of the sample using #600 to #1500 silicon carbide paper, it is polished to a mirror finish using a liquid in which diamond powder with a particle size of 1 to 6 μm is dispersed in a diluted solution such as alcohol or pure water, and then nital etching is performed.

[0046] Next, photographs of at least five fields of view are taken at any position in the longitudinal direction of the sample cross section using a thermal field emission scanning electron microscope (JEOL JSM-7001F). An evenly spaced grid is drawn on the photograph, and the structure at the grid points is identified. The number of grid points corresponding to each structure is determined and divided by the total number of grid points to obtain the area ratio of each structure. The greater the total number of grid points, the more accurately the area ratio can be determined. In this embodiment, the grid spacing is 2 μm × 2 μm, and the total number of grid points is 1,500.

[0047] In the image analysis of the photographs, each tissue is identified using the following method. The region where cementite is precipitated in lamellar form within the grains is determined to be pearlite. Granular regions with high brightness and a particle size (circle equivalent diameter) of 2 μm or less are judged to be cementite. Areas with low brightness and no visible underlying structure are judged to be ferrite. Areas with high brightness and where the underlying structure has not been revealed by etching are judged to be martensite. Areas that do not fall into any of the above categories are determined to be bainite.

[0048] The percentage of ferrite with a GAM value of 0.5 or less: 70% or more In this embodiment, the area ratio of ferrite generated by recrystallization (recrystallized ferrite) among the above-mentioned ferrite is increased. Ferrite with a GAM value of 0.5 or less can be determined to be recrystallized ferrite. In a steel sheet for hot stamping, a ratio of recrystallized ferrite to ferrite of less than 70% indicates that a large amount of unrecrystallized ferrite remains. Martensite is generated in rows along the rolling direction around the unrecrystallized ferrite. C-enriched portions are formed in the row-shaped martensite portions by heating during hot stamping, and the hardness of these C-enriched portions increases after hot stamping. As a result, the bendability of the hot-stamped steel in the direction along the rolling direction deteriorates, resulting in increased anisotropy of bendability.

[0049] For the above reasons, the proportion of ferrite having a GAM value of 0.5 or less is set to 70% or more, preferably 75% or more, 80% or more, or 85% or more. There is no particular upper limit, but it may be set to 100%.

[0050] The proportion of ferrite with a GAM value of 0.5 or less is measured by the following method. The same area as that where the above-mentioned structure area ratio was measured was polished for 8 minutes at room temperature using colloidal silica that does not contain an alkaline solution to remove the strain introduced into the surface layer of the sample.

[0051] Crystal orientation information is obtained by measuring at 0.1 μm measurement intervals using electron backscatter diffraction at a position 1 / 4 of the plate thickness from the surface of the sample cross section (the region from 1 / 8 of the plate thickness depth from the surface to 3 / 8 of the plate thickness depth from the surface). For the measurements, an EBSD analyzer consisting of a thermal field emission scanning electron microscope (JEOL JSM-7001F) and an EBSD detector (TSL DVC5 detector) is used. The degree of vacuum inside the EBSD analyzer is 9.6 x 10 -5 Pa or less, acceleration voltage is 15 kV, probe current level is 13, and electron beam irradiation level is 62.

[0052] From the obtained crystal orientation information, the "Phase Map" function installed in the software "OIM Analysis (registered trademark)" attached to the EBSD analyzer is used to identify regions with an fcc crystal structure and regions with a bcc crystal structure. In the regions with a bcc crystal structure, crystal grains surrounded by grain boundaries with a misorientation of 5° or more are identified. For each identified crystal grain, a region with an intragrain misorientation (GAM value: Grain Average Misorientation value) of 0.5 or less is identified, and this region is considered to be ferrite with a GAM value of 0.5 or less. The above operation is performed for at least five regions, and the average area fraction of ferrite with a GAM value of 0.5 or less is calculated. This gives the area fraction of ferrite with a GAM value of 0.5 or less. The area ratio of ferrite with a GAM value of 0.5 or less obtained is divided by the area ratio of ferrite obtained by image analysis of the photographs taken as described above, and multiplied by 100 to obtain the percentage of ferrite with a GAM value of 0.5 or less ((area ratio of ferrite with a GAM value of 0.5 or less / area ratio of ferrite) × 100).

[0053] Average grain size of ferrite: 1.0 to 7.0 μm Since martensite forms at the ferrite grain boundaries, a small average ferrite grain size allows for a uniform carbon distribution during heating in hot stamping. As a result, the anisotropy of bendability of the hot-stamped steel can be reduced. If the average ferrite grain size exceeds 7.0 μm, the prior austenite grains of the hot-stamped steel become coarse, resulting in a deterioration in bendability and an increased anisotropy of bendability. Therefore, the average ferrite grain size is set to 7.0 μm or less. Preferably, it is 6.0 μm or less, 5.0 μm or less, or 4.0 μm or less. On the other hand, if the average grain size of ferrite is less than 1.0 μm, the desired metal structure cannot be obtained in the hot stamped steel, and the bendability cannot be improved or the anisotropy of the bendability cannot be reduced. Therefore, the average grain size of ferrite is set to 1.0 μm or more. Preferably, it is 1.5 μm or more or 2.0 μm or more.

[0054] Average grain size of martensite: 0.5 to 3.0 μm Martensite has a higher carbon concentration than ferrite and serves as a carbon supply source during hot stamping heating. If the average grain size of martensite is small, carbon is supplied uniformly during hot stamping heating, resulting in a uniform carbon distribution. As a result, the anisotropy of bendability of the hot stamped body can be reduced. If the average grain size of martensite exceeds 3.0 μm, the anisotropy of the hot stamped body increases. Therefore, the average grain size of martensite is set to 3.0 μm or less. Preferably, it is 2.5 μm or less, 2.0 μm or less, or 1.5 μm or less. On the other hand, if the average grain size of martensite is less than 0.5 μm, the desired metal structure cannot be obtained in the hot stamped steel, the bendability deteriorates, and the anisotropy of the bendability cannot be reduced. Therefore, the average grain size of martensite is set to 0.5 μm or more, preferably 1.0 μm or more.

[0055] The average grain sizes of ferrite and martensite are measured by the following method. The photographs taken when measuring the above-mentioned structure area ratio are used to calculate the circle-equivalent diameters of ferrite and martensite. The average circle-equivalent diameters of ferrite and martensite obtained are calculated to obtain the average grain size of ferrite and the average grain size of martensite. The average grain size of ferrite and martensite is measured based on the counting method described in Appendix A of JIS G 0551:2020.

[0056] Solid solution Nb concentration: 25ppm or more Increasing the solute Nb concentration can suppress the growth of austenite grains during heating in hot stamping. This can refine the prior austenite grains in the hot-stamped steel. As a result, the bendability of the hot-stamped steel can be improved and the anisotropy of bendability can be reduced. If the solute Nb concentration is less than 25 ppm, the above effects cannot be obtained. Therefore, the solute Nb concentration is set to 25 ppm or more. Preferably, it is 30 ppm or more, 35 ppm or more, or 40 ppm or more. Although there is no particular upper limit for the solute Nb concentration, a concentration exceeding 200 ppm would require special heat treatment different from the normal iron-making process, which would entail excessive costs, so the upper limit may be set to 200 ppm or less, or may be set to 100 ppm or less.

[0057] The concentration of solute Nb is measured by the following method. A test piece is taken from the hot stamping steel sheet and immersed in 5 L of an electrolyte containing 200 g of tetramethylammonium guanide, 1 L of ethylene glycol, 1000 g of maleic acid, and the remainder methanol. 2 1 g of the powder is electrolyzed at a current range of 1000 kJ / s. The residue is obtained by suction filtration and then dissolved in sodium hydrogen sulfate. The resulting molten solution is analyzed by ICP (inductively coupled plasma) to obtain the Nb concentration (mass%). The solid solution Nb concentration (mass%) is obtained by subtracting this Nb concentration (mass%) from the Nb content (mass%) obtained by measuring the chemical composition as described above. When the steel sheet for hot stamping has a plating layer on the surface, the plating layer on the surface may be removed by mechanical grinding before the above-mentioned analysis is carried out.

[0058] plating layer The steel sheet for hot stamping according to this embodiment may have a plating layer on its surface for the purpose of further improving corrosion resistance. Examples of the plating layer include an Al-based plating layer such as a hot-dip aluminum plating layer or an aluminum-galvanized layer, and a Zn-based plating layer such as a hot-dip galvanized layer, a galvannealed hot-dip galvanized layer, an electrogalvanized layer, and a zinc-nickel plating layer.

[0059] The plating layer may be formed on either one surface or both surfaces of the steel sheet for hot stamping. The coating amount is not particularly limited, but the Al-based plating layer: 15 to 120 g / m per surface 2 , hot-dip galvanized layer: 30 to 120 g / m per side 2 , Galvannealed layer: 30 to 120 g / m per side 2 , electrogalvanized layer and zinc-nickel plated layer: 5 to 100 g / m per side 2It is preferable that:

[0060] In this embodiment, the Al-based plating layer refers to a plating layer containing 50% or more by mass of Al. Elements other than Al may be contained, such as 0.1 to 20% by mass of Si, 0.1 to 10% by mass of Fe, 0.1 to 45% by mass of Zn, and the balance (Cu, Na, K, Co, Ni, Mg, etc.): less than 0.5% by mass.

[0061] In this embodiment, the Zn-based plating layer refers to a plating layer containing 50% by mass or more of Zn. Elements other than Zn may be contained, such as 0.01 to 20% by mass of Si, 0.1 to 10% by mass of Fe, 0.01 to 45% by mass of Al, and the balance (Cu, Na, K, Co, Ni, Mg, etc.): less than 0.5% by mass.

[0062] The component analysis of the plating layer is carried out by the following method. A sample is cut out from any position at least 50 mm away from the end face of the steel sheet for hot stamping (if it is not possible to take a sample from this position, a position that avoids the end) so that the cross section of the sheet thickness can be observed. The size of the sample depends on the measuring device, but it should be large enough to allow observation of about 10 mm in the rolling direction.

[0063] The sample is embedded in resin and polished, after which the layer structure of the cross-section of the plate thickness is observed using a scanning electron microscope (SEM). Specifically, the SEM is used at a magnification that allows the steel plate and plating layer to fit within the observation field. For example, by observing using a backscattered electron composition image (COMPO image), it is possible to infer how many layers the cross-sectional structure is made up of.

[0064] Next, an electron probe microanalyzer (EPMA) is used to map and analyze a range of 50 μm in the sheet surface direction and 30 μm in the sheet thickness direction. If the coating layer is an Al-based coating layer, the average values ​​of the Fe concentration and Al concentration in the sheet surface direction are determined. Next, the relationship between the sheet thickness position and the Al concentration, and the relationship between the sheet thickness position and the Fe concentration are determined. The sheet thickness position where the Al concentration and Fe concentration become the same as the Al concentration and Fe concentration of the steel sheet can be determined to be the interface between the steel sheet and the Al-based coating layer. The Al concentration and Fe concentration of the steel sheet referred to here are obtained by measurement using an EPMA.

[0065] Furthermore, if the coating layer is a Zn-based coating layer, the average values ​​of the Fe concentration and Zn concentration in the sheet surface direction are determined. Next, the relationship between the sheet thickness position and the Zn concentration, and the relationship between the sheet thickness position and the Fe concentration are determined. The sheet thickness position where the Zn concentration and Fe concentration become the same as the Zn concentration and Fe concentration of the steel sheet can be determined to be the interface between the steel sheet and the Zn-based coating layer. The Zn concentration and Fe concentration of the steel sheet referred to here are obtained by measurement using an EPMA.

[0066] Plate thickness The thickness of the steel sheet for hot stamping according to this embodiment is not particularly specified, but may be 0.5 to 3.5 mm from the viewpoint of reducing the weight of the car body.

[0067] Next, a hot-stamped steel obtained by hot stamping the above-mentioned steel sheet for hot stamping will be described. The hot-stamped steel according to this embodiment has the same chemical composition as the above-described steel sheet for hot stamping, and therefore, a description of the chemical composition will be omitted.

[0068] The hot stamped steel according to this embodiment has a metal structure, in area %, of martensite: 90 to 100%, and a total of ferrite, pearlite, bainite, cementite, and retained austenite: 0 to 10%, and the average grain size of prior austenite grains is 1.5 to 7.0 μm.

[0069] In this embodiment, the metallographic structure at the 1 / 4 position in the sheet thickness of the hot stamped steel (the region from the surface to the depth of 1 / 8 of the sheet thickness to the depth of 3 / 8 of the sheet thickness) is specified because the metallographic structure at this position represents the typical metallographic structure of the hot stamped steel.

[0070] Martensite area ratio: 90-100% Martensite is a structure with high strength. Furthermore, the higher the area fraction of martensite, the more homogeneous the metal structure becomes, and the more the anisotropy of bendability can be reduced. If the area fraction of martensite is less than 90%, the strength may deteriorate and / or the anisotropy of bendability may increase. Therefore, the area fraction of martensite is set to 90% or more. It is preferably 92% or more, 95% or more, or 98% or more. The higher the area fraction of martensite, the better, and therefore 100% is more preferable.

[0071] Total area ratio of ferrite, pearlite, bainite, cementite and retained austenite: 0 to 10% The hot stamped steel according to this embodiment may contain one or more of ferrite, pearlite, bainite, cementite, and retained austenite as a residual structure. These residual structures may not be present, and therefore the area ratio may be 0%. If the area ratio of the remaining structure exceeds 10%, the area ratio of martensite decreases, which may result in a decrease in the strength of the hot stamped body and / or anisotropy in bendability. Therefore, the area ratio of the remaining structure is set to 10% or less, preferably 8% or less, 5% or less, or 2% or less.

[0072] The area ratio of each structure is measured by the following method. A sample is cut out from the hot-stamped product so that a cross section of the plate thickness perpendicular to the surface can be observed at a position 1 / 4 of the plate thickness (a region from 1 / 8 depth of the plate thickness from the surface to 3 / 8 depth of the plate thickness from the surface). The area ratio of each structure is measured for this sample using the same method as for the measurement of the steel plate for hot stamping described above. The area fraction of retained austenite is measured by X-ray diffraction. In a hot-stamped steel, the integrated intensity of six peaks, α(110), α(200), α(211), γ(111), γ(200), and γ(220), is determined using Co-Kα radiation at a position 1 / 4 of the way down from the surface in a thickness cross section parallel to the rolling direction (a region from 1 / 8 of the thickness down from the surface to 3 / 8 of the thickness down from the surface), and the volume fraction of retained austenite is calculated using the intensity averaging method. This volume fraction of retained austenite is considered to be the area fraction of retained austenite.

[0073] Average grain size of prior austenite grains: 1.5 to 7.0 μm The finer the prior austenite grains, the easier it is to form fine-grained martensite, resulting in a more homogeneous metal structure. As a result, the anisotropy of the bendability of the hot-stamped steel can be reduced. If the average grain size of the prior austenite grains exceeds 7.0 μm, the anisotropy of the bendability of the hot-stamped steel increases. Therefore, the average grain size of the prior austenite grains is set to 7.0 μm or less. Preferably, it is 6.5 μm or less or 6.0 μm or less. If the average grain size of the prior austenite grains is less than 1.5 μm, the hardenability is significantly reduced. As a result, ferrite transformation, pearlite transformation, and bainite transformation are likely to occur, and the strength of the hot-stamped steel sheet is reduced. Therefore, the average grain size of the prior austenite grains is set to 1.5 μm or more. Preferably, it is 2.5 μm or more, 3.5 μm or more, or 4.0 μm or more.

[0074] The average grain size of prior austenite grains is measured by the following method. A test piece is taken from the hot-stamped product in a thickness cross section parallel to the rolling direction so that the metal structure can be observed at a position 1 / 4 of the thickness from the surface (the region from 1 / 8 of the thickness depth from the surface to 3 / 8 of the thickness depth from the surface). The observation surface of the test piece is corroded with a saturated aqueous solution of picric acid to reveal the prior austenite grain boundaries.

[0075] Enlarged photographs of the corroded observation surface at a position 1 / 4 of the plate thickness from the surface (the region from 1 / 8 of the plate thickness depth from the surface to 3 / 8 of the plate thickness depth from the surface) are taken using an optical microscope at a magnification of 1000x, covering at least five fields of view. Based on "A.2 Intersection Method" in Appendix A of JIS G 0551:2020, the average line segment lengths of at least 20 prior austenite grains contained in each photograph are determined, and the average of these is calculated. This gives the average grain size of the prior austenite grains.

[0076] σ Hn ≦0.235×Hn …(1) In the hot stamped steel according to this embodiment, the standard deviation σ of the nanoindentation hardness of the metal structure Hn preferably satisfies the above formula (1), where Hn in the above formula (1) is the average value of the nanoindentation hardness of the metal structure. Satisfying the above formula (1) indicates that the hardness is more uniform in the metal structure. By satisfying the above formula (1), the bendability of the hot-stamped steel can be further improved and the anisotropy of bendability can be further reduced.

[0077] The nanoindentation hardness of the metal structure of the hot stamped body is measured by the following method. A sample is cut out from the hot stamped body so that it can be measured at a thickness cross section perpendicular to the surface at a position 1 / 4 of the thickness (the region from 1 / 8 of the thickness from the surface to 3 / 8 of the thickness from the surface). Hardness is measured by nanoindentation at a position 1 / 4 of the thickness of the obtained sample (the region from 1 / 8 of the thickness from the surface to 3 / 8 of the thickness from the surface). The nanoindentation hardness is measured at at least 50 points and the average value is calculated to obtain the average nanoindentation hardness value Hn. In addition, the standard deviation of the nanoindentation hardness at all measurement points is calculated to obtain the standard deviation of the nanoindentation hardness σ Hn get. The measurement is carried out using a TriboScope / TriboIndenter manufactured by Hysitron, and the measurement load is set to 1 mN.

[0078] plating layer The hot-stamped steel according to this embodiment may have a plating layer similar to that of the steel sheet for hot stamping described above, in order to further improve corrosion resistance. The type, definition, measurement method, etc. of the plating layer are the same as those of the steel sheet for hot stamping, and therefore description thereof will be omitted.

[0079] Plate thickness The thickness of the hot stamped steel according to this embodiment is not particularly limited, but may be 0.5 to 3.5 mm from the viewpoint of reducing the weight of the vehicle body.

[0080] Tensile strength In order to enhance the effect of reducing the vehicle body weight, the hot-stamped steel according to this embodiment preferably has a tensile strength of 980 MPa or more. More preferably, it is 1000 MPa or more, 1050 MPa or more, or 1100 MPa or more. If the tensile strength is too high, bendability decreases, so the tensile strength may be 1380 MPa or less. The tensile strength is determined by preparing a No. 5 test piece as specified in JIS Z 2241: 2011 and following the test method as specified in JIS Z 2241: 2011. The tensile test piece is taken from the center position in the sheet width direction, with the direction perpendicular to the rolling direction as the longitudinal direction.

[0081] Next, a preferred method for manufacturing the steel sheet for hot stamping according to this embodiment will be described. The steel sheet for hot stamping according to this embodiment can be stably manufactured by a manufacturing method including the following steps. The temperatures described below refer to the surface temperatures of the slab or steel plate.

[0082] A suitable method for manufacturing a steel sheet for hot stamping according to this embodiment includes the following steps. (1) A slab having the above-mentioned chemical composition is heated to a temperature range of 1200°C or higher. (2) Rough rolling is performed so that the cumulative reduction in the temperature range of 1050°C or less is 88% or less, and the residence time from 1050°C to the finish rolling start temperature is 210 seconds or less. (3) Finish rolling is performed so that the finish rolling completion temperature is in a temperature range of Ar3 (°C) or higher. Ar3 (°C) is expressed by the following formula (A): Each element symbol in the following formula (A) indicates the content of each element in mass %. Ar3=901-325×C+33×Si―92×Mn+287×P+40×Al…(A) (4) Coiling is carried out in a temperature range of 650°C or less. (5) Heat to a temperature range of Ta (°C) or higher, hold for 5 seconds or more, and then perform annealing by cooling from the holding temperature to 700°C at an average cooling rate of 3 to 30°C / s and so that the residence time in the temperature range of 650°C to Tb (°C) is 250 seconds or less. Ta (°C) is expressed by the following formula (B), and Tb (°C) is expressed by the following formula (C). Each element symbol in the following formula (B) indicates the content of each element in mass%, and 0 is substituted when the element is not contained. Ta=885-1500×(Nb+0.65×Ti+0.45×V+0.45×Zr)+20×Sn+20×Sb+20×As+6×Cu+2000×B+1000×O …(B) Tb=Ta-40...(C) Each step will be described below.

[0083] Slab heating The slab is not particularly limited as long as it has the above-mentioned chemical composition. The slab may be produced by a conventional method, for example, a slab produced by a general method such as a continuous casting slab or a thin slab caster. By setting the heating temperature to 1200°C or higher, the alloy carbide can be sufficiently dissolved. Therefore, the heating temperature is preferably set to 1200°C or higher. The holding time in the temperature range of 1200°C or higher may be 20 minutes or more. The upper limit of the heating temperature is not particularly limited, but may be set to 1400° C. or less from the viewpoint of productivity.

[0084] Rough rolling In rough rolling, by setting the cumulative reduction rate in the temperature range of 1050°C or less to 88% or less, it is possible to impart sufficient cumulative strain and suppress the precipitation of Nb-based compounds. As a result, the solute Nb concentration in the steel sheet for hot stamping can be increased. Therefore, it is preferable to set the cumulative reduction rate in the temperature range of 1050°C or less to 88% or less. In addition, in the rough rolling, from the viewpoint of improving productivity, the cumulative rolling reduction in the temperature range of 1050°C or less may be 10% or more, and is preferably 20% or more.

[0085] In this embodiment, the cumulative rolling reduction can be expressed as {1-(t1 / t0)}×100(%), where t0 is the entry thickness of the first stage and t1 is the delivery thickness of the final stage.

[0086] In rough rolling, by setting the residence time from 1050°C to the finish rolling start temperature to 210 seconds or less, the precipitation of Nb-based compounds can be suppressed. As a result, the solid solution Nb concentration of the steel sheet for hot stamping can be increased. Therefore, it is preferable that the residence time from 1050°C to the finish rolling start temperature be 210 seconds or less. The residence time mentioned above refers to the time elapsed from when the steel sheet temperature reaches 1050°C until it reaches the finish rolling start temperature.

[0087] Finishing rolling In finish rolling, setting the finish rolling completion temperature to a temperature range of Ar3 (°C) or higher can achieve a uniform structure in the sheet thickness direction and suppress the occurrence of ridging (irregularities on the steel sheet surface). If only the surface layer of the steel sheet is below Ar3 (°C), a structure with larger grain sizes than the center will occur in the surface layer, resulting in a non-uniform structure in the sheet thickness direction. This structure will continue even after cold rolling, annealing, and hot stamping, making the structure of the hot-stamped product non-uniform in the sheet thickness direction. As a result, the strength and bendability of the hot-stamped part may deteriorate. Furthermore, if not only the surface layer but the entire steel sheet is below Ar3 (°C), texture will develop, making ridging more likely to occur during cold rolling. If ridging occurs, the thickness of the steel sheet will become non-uniform, resulting in non-uniform contact between the die and the steel sheet during die quenching in hot stamping. As a result, the strength and bendability of the hot-stamped product will deteriorate. Therefore, it is preferable to set the finish rolling completion temperature to a temperature range of Ar3 (°C) or higher.

[0088] Winding By setting the coiling temperature in the temperature range of 650°C or less, it is possible to suppress the precipitation of Nb-based compounds. As a result, it is possible to increase the solute Nb concentration in the steel sheet for hot stamping. In addition, it is possible to preferably control the average grain size of ferrite. Therefore, it is preferable that the coiling temperature be in the temperature range of 650°C or less. It is more preferable that the coiling temperature be 600°C or less. If the coiling temperature is too low, the steel sheet may harden and the cold rolling properties may decrease, so the coiling temperature may be 400°C or more. After coiling, the steel sheet is preferably pickled and then cold rolled. The cumulative reduction during cold rolling may be within a range that does not impair productivity, for example, 30 to 80%.

[0089] annealing After cold rolling, annealing is preferably performed. By setting the holding temperature during annealing to Ta (°C) or higher and holding at this holding temperature for 5 seconds or longer, the solute Nb concentration of the steel sheet for hot stamping can be increased. Therefore, it is preferable that the holding temperature during annealing is Ta (°C) or higher and the holding time is 5 seconds or longer. The holding temperature during annealing may be 950°C or lower from the viewpoint of sheet passing properties in continuous annealing, and the holding time may be 600 seconds or shorter from the viewpoint of productivity in continuous annealing.

[0090] After holding in the above-mentioned temperature range, the solute Nb concentration of the steel sheet for hot stamping can be increased by cooling from the holding temperature to 700°C at an average cooling rate of 3 to 30°C / s and with a residence time in the temperature range of 650°C to Tb (°C) of 250 seconds or less. Therefore, it is preferable to cool from the holding temperature to 700°C at an average cooling rate of 3 to 30°C / s and with a residence time in the temperature range of 650°C to Tb (°C) of 250 seconds or less.

[0091] The residence time refers to the time elapsed from the start of cooling from the holding temperature until the temperature reaches 700°C. Furthermore, when only continuous annealing is performed without hot-dip galvanizing, the average cooling rate in the temperature range of 600 to 350°C is preferably 5 to 50°C / s, from the viewpoint of productivity, and when hot-dip galvanizing is performed, the average cooling rate in the temperature range from after passing through the galvanizing bath to 350°C is preferably 5 to 50°C / s. When only continuous annealing is performed, overaging may be performed, but from the viewpoint of productivity, the heating temperature is preferably 350°C or less. In this embodiment, the average cooling rate is the temperature difference between the start point and the end point of the set range divided by the elapsed time from the start point to the end point.

[0092] The steel sheet for hot stamping according to this embodiment is obtained by the manufacturing method described above.

[0093] The above-mentioned plating may be formed on one or both surfaces of the steel sheet for hot stamping. The plating may be applied under ordinary plating conditions. For aluminum plating, the Si concentration in the bath is preferably 5 to 12 mass%, the Fe concentration in the bath is preferably 0 to 5 mass%, and the balance is aluminum and less than 0.5 mass% of impurities. For aluminum-zinc plating, the Zn concentration in the bath is preferably 40 to 50 mass%, and the balance is aluminum and less than 0.5 mass% of impurities. There is no particular problem if Mg or Zn is mixed in the aluminum plating, or if Mg is mixed in the aluminum-zinc plating.

[0094] The atmosphere during plating may be normal plating conditions whether the continuous plating equipment has a non-oxidizing furnace or does not have a non-oxidizing furnace. In the case of zinc plating, the Al concentration in the bath is preferably 0.05 to 0.5 mass %, with the balance being zinc and less than 0.5 mass % of impurities. For galvanizing, methods such as hot dip galvanizing, electrogalvanizing, and galvannealed hot dip galvanizing may be employed.

[0095] Before plating, the steel sheet surface may be pre-plated with a metal. Examples of pre-plated metals include Ni pre-plating, Fe pre-plating, and other pre-plated metals that improve plating properties. There is no particular problem with applying a different type of metal plating or a coating of an inorganic or organic compound to the surface of the plated layer.

[0096] Next, a preferred method for producing the hot-stamped steel according to this embodiment will be described. The hot-stamped steel according to this embodiment can be stably produced by applying the following hot stamping conditions to the steel sheet for hot stamping obtained by the above-mentioned method.

[0097] The steel sheet for hot stamping is heated to a temperature range of Ac3 (°C) or higher, quickly transferred onto a die, and hot stamped at a temperature range of Ar3 (°C) or higher. After that, the steel sheet is cooled in the die at an average cooling rate of 30°C / s or higher by heat transfer between the steel sheet and the die.

[0098] Ac3 (°C) can be calculated using the following formula. Ac3=exp(X)+31.5×Mo-28 X=6.8165-0.47132×C-0.057321×Mn+0.0660261×Si-0.050211×Cr+0.10593×Ti+2.0272×N+1.0536×S-0.12024×Si×C+0.11629×Cr×C+0.29225×C 2 +0.01566×Mn 2 +0.017315×Cr 2 The element symbols in the above formulas represent the content of the element in mass %, and 0 is substituted when the element is not contained.

[0099] By heating to a temperature range of Ac3 (°C) or higher, the metal structure can be sufficiently austenitized. By sufficiently austenitizing the metal structure, a desired amount of martensite can be obtained by cooling, which will be described later. Therefore, it is preferable that the heating temperature before hot stamping be in a temperature range of Ac3 (°C) or higher. The retention time in the temperature range of Ac3 (°C) or higher may be 0.1 to 30.0 minutes.

[0100] Hot stamping in a temperature range of Ar3 (°C) or higher allows cooling before the onset of ferrite transformation, pearlite transformation, and bainite transformation, thereby enabling martensitic transformation to occur. Therefore, it is preferable that the hot stamping start temperature (forming start temperature) be in a temperature range of Ar3 (°C) or higher.

[0101] A desired amount of martensite can be obtained by cooling at an average cooling rate of 30°C / s or more after hot stamping. Therefore, it is preferable to cool at an average cooling rate of 30°C / s or more after hot stamping. [Example]

[0102] Next, examples of the present invention will be described, but the conditions in the examples are merely examples adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to these examples. Various conditions can be adopted in the present invention as long as they do not deviate from the gist of the present invention and the object of the present invention is achieved.

[0103] Using slabs having the chemical compositions shown in Tables 1A to 2C, hot stamping steel sheets (cold-rolled steel sheets and plated steel sheets) shown in Tables 4A to 4C were produced under the conditions shown in Tables 3A to 3C.

[0104] The obtained steel sheets for hot stamping were subjected to metallographic observation and measurement of the solute Nb concentration by the methods described above.

[0105] The "plating types" listed in Tables 4A to 4C and Tables 6A to 6C are as follows: CR: No plating layer GI: Hot-dip galvanized layer (target coating weight 60g / m on one side) 2 , double-sided plating) GA: Galvannealed layer (target coating weight 45g / m on one side) 2 , double-sided plating) Al: Al-based plating layer (target coating weight 80g / m on one side) 2 , double-sided plating) EG: Electrogalvanized layer (target coating weight 20g / m on one side) 2 , double-sided plating)

[0106] The produced cold-rolled steel sheets and plated steel sheets were subjected to hot stamping under the conditions shown in Tables 5A to 5C. Hot stamping was performed by clamping the flat steel sheets for hot stamping between water-cooled dies and applying a pressure of 20 MPa for 10 seconds to facilitate the preparation of test pieces for tensile tests and metallographic observation. This resulted in the hot-stamped products shown in Tables 6A to 6C.

[0107] The metal structure of the hot stamped steel sheet was observed and the tensile strength was measured by the methods described above.

[0108] If the obtained tensile strength was 980 MPa or more, the hot-stamped body was judged to have high strength and to have passed the test. On the other hand, if the obtained tensile strength was less than 980 MPa, the hot-stamped body was judged to have low strength and to have failed the test. Furthermore, if the obtained tensile strength was more than 1380 MPa, the hot-stamped body was judged to have too high strength and to have failed the test.

[0109] The bendability was evaluated by the following method based on the VDA standard (VDA238-100) specified by the German Association of the Automotive Industry. In this example, the displacement at the maximum load obtained in the bending test was converted to an angle according to the VDA standard to determine the bend angle α (°).

[0110] The conditions for the bending test were as follows: Test piece dimensions: 60 mm (rolling direction) x 30 mm (direction parallel to the plate width direction) Bending ridge: parallel to the rolling direction, 45° direction, perpendicular direction Test method: Roll support, punch indentation Roll diameter: φ30mm Punch shape: Tip R = 0.4 mm Distance between rolls: 2.0 x plate thickness (mm) + 0.5 mm Push-in speed: 20mm / min Testing machine: SHIMADZU AUTOGRAPH 20kN

[0111] From the bending test in three directions, the average bending angle α m The bending anisotropy Δα (%) was calculated as follows. Δα=(α m -α min ) / α m ×100 α m =(α L +2×α D +α C ) / 4 α min :α L、 α D、 α C The smallest value of α L: α when bent along an axis parallel to the rolling direction α D : α when bent at an axis of 45° to the rolling direction α C : α when bent on an axis perpendicular to the rolling direction

[0112] If the rolling direction is unknown, the test is carried out in five directions at intervals of 22.5°, and the direction in which the minimum bending angle α is obtained is regarded as the rolling direction. L , α D and α C If the rolling direction is unknown even with this method, tests are conducted in 10 directions at intervals of 11.25°, and the direction in which the minimum bending angle α is obtained is regarded as the rolling direction, and α L , α D and α C You may ask for:

[0113] α m is affected by plate thickness and tensile strength. m The α value is also affected by the plating layer of the hot stamped body. When aluminum plating is used, a hard Fe-Al alloy layer is formed on the surface by the heat of hot stamping, so the α value is higher than that of GA, GI, EG or CR (which are shot blasted after hot stamping). m is low.

[0114] In this embodiment, α m When α was equal to or greater than the lower limit shown in Tables 7 and 8, the specimen was judged to have excellent bendability and to have passed the test. m When the value was less than the lower limit shown in Tables 7 and 8, the sample was judged to have poor bendability and to have failed. Furthermore, when Δα was 15% or less, the anisotropy of bending property was deemed small and the sample was judged as passing, whereas when Δα was more than 15%, the anisotropy of bending property was deemed large and the sample was judged as failing.

[0115] The corrosion resistance after painting was evaluated according to the method specified in JASO M609-91 established by the Society of Automotive Engineers of Japan. Specifically, it was evaluated according to the following method.

[0116] Samples were taken from the hot-stamped compacts, and a 70 mm-long scratch was made with a cutter on the flat surface of the sample, which had a 15 μm-thick electrocoated coating. The samples were then subjected to cyclic corrosion testing. After 120 cycles, the samples were removed and immersed in a commercially available paint remover for 30 minutes, after which the coating was removed with a brush. The samples were then immersed in a 5% by volume ammonium citrate solution containing a steel sheet inhibitor, and the rust that had formed in the corroded areas was removed with a brush. Using a Keyence VHX-7000 digital microscope, the maximum thickness loss from the reference plane was measured every 35 mm along the 70 mm scratch, with the center of the scratch as the boundary. The reference plane was the uncorroded surface after the coating was removed, regardless of whether the coating was plated or not. The average of the two maximum thickness losses was calculated.

[0117] The average value of the maximum thickness reduction obtained was evaluated according to the following criteria: When the evaluation was E, the hot stamped steel sheet was judged to have particularly excellent corrosion resistance. E (Excellent): Less than 0.05 mm V (Very Good): 0.05mm or more, less than 0.10mm G (Good): 0.10mm or more, less than 0.15mm B(Bad): 0.15mm or more

[0118] Tables 6A to 6C show that the examples of the present invention produced hot-stamped steel sheets with high strength, excellent bendability, and small anisotropy in bendability, while the comparative examples did not satisfy the pass criteria for one or more of the above properties.

[0119] [Table 1A]

[0120] [Table 1B]

[0121] [Table 1C]

[0122] Table 2A

[0123] Table 2B

[0124] Table 2C

[0125] Table 3A

[0126] Table 3B

[0127]

Table 3C

[0128] Table 4A

[0129] Table 4B

[0130] Table 4C

[0131] Table 5A

[0132] [Table 5B]

[0133] [Table 5C]

[0134] [Table 6A]

[0135] [Table 6B]

[0136] [Table 6C]

[0137] [Table 7]

[0138] [Table 8] [Industrial Applicability]

[0139] According to the above aspects of the present invention, it is possible to provide a hot-stamped steel sheet having high strength, excellent bendability, and small anisotropy of bendability, as well as a steel sheet for hot stamping from which the hot-stamped steel sheet can be manufactured.

Claims

1. The chemical composition, in mass%, is C: 0.050% or more and less than 0.150%; Si: 0.010 to 1.000%, Mn: 1.00-2.00%, P: 0.100% or less, S: 0.0100% or less, Al: 0.001-0.500%, N: 0.0001 to 0.0100%, O: 0.1000% or less, Nb: 0.015-0.100%, Ti: 0.005-0.100%, B: 0.0005 to 0.0050%, Cr: 0-0.500%, Mo: 0-0.500%, Ni: 0-3.000%, Cu: 0-3.000%, Co: 0 to 0.50%, W: 0-3.00%, Sn: 0-0.500%, V: 0 to 0.100%, Zr: 0 to 0.100%, Ca: 0-0.0050%, Mg: 0 to 0.0050%, REM: 0 to 0.0050%, Sb: 0 to 0.0200%, and As: 0 to 1.0000%; the balance being Fe and impurities; The metal structure is, in area%, Ferrite: 75 to 95%, Martensite: 5 to 25%, The sum of pearlite, bainite and cementite: 0 to 5%; The proportion of ferrite having a GAM value, which is an average misorientation within crystal grains surrounded by crystal grain boundaries having a misorientation of 5° or more, of 0.5° or less is 70% or more, The average particle size of the ferrite is 1.0 to 7.0 μm, The average grain size of the martensite is 0.5 to 3.0 μm, A steel sheet for hot stamping, characterized in that the concentration of solute Nb is 25 ppm or more.

2. The chemical composition is, in mass %, Cr: 0.100-0.500%, Mo: 0.050-0.500%, Ni: 0.050-3.000%, Cu: 0.050-3.000%, Co: 0.05-0.50%, W: 0.05-3.00%, Sn: 0.005-0.500%, V: 0.005-0.100%, Zr: 0.005-0.100%, Ca: 0.0005-0.0050%, Mg: 0.0005-0.0050%, REM: 0.0005-0.0050%, Sb: 0.0005 to 0.0200%, and As: 0.0005-1.0000% The steel sheet for hot stamping according to claim 1, characterized in that it contains one or more of the group consisting of

3. 3. The steel sheet for hot stamping according to claim 1, further comprising a plating layer on the surface thereof.

4. The chemical composition, in mass%, is C: 0.050% or more and less than 0.150%; Si: 0.010 to 1.000%, Mn: 1.00-2.00%, P: 0.100% or less, S: 0.0100% or less, Al: 0.001-0.500%, N: 0.0001 to 0.0100%, O: 0.1000% or less, Nb: 0.015-0.100%, Ti: 0.005-0.100%, B: 0.0005 to 0.0050%, Cr: 0-0.500%, Mo: 0-0.500%, Ni: 0-3.000%, Cu: 0-3.000%, Co: 0 to 0.50%, W: 0-3.00%, Sn: 0-0.500%, V: 0 to 0.100%, Zr: 0 to 0.100%, Ca: 0-0.0050%, Mg: 0 to 0.0050%, REM: 0 to 0.0050%, Sb: 0 to 0.0200%, and As: 0 to 1.0000%; the balance being Fe and impurities; The metal structure is, in area%, Martensite: 90-100%, The total of ferrite, pearlite, bainite, cementite and retained austenite: 0 to 10%; A hot stamped body characterized in that the average grain size of prior austenite grains is 1.5 to 7.0 μm.

5. The chemical composition is, in mass %, Cr: 0.100-0.500%, Mo: 0.050-0.500%, Ni: 0.050-3.000%, Cu: 0.050-3.000%, Co: 0.05-0.50%, W: 0.05-3.00%, Sn: 0.005-0.500%, V: 0.005-0.100%, Zr: 0.005-0.100%, Ca: 0.0005-0.0050%, Mg: 0.0005-0.0050%, REM: 0.0005-0.0050%, Sb: 0.0005 to 0.0200%, and As: 0.0005-1.0000% The hot-stamped product according to claim 4, characterized in that it contains one or more members selected from the group consisting of

6. The standard deviation σ of the nanoindentation hardness of the metal structure Ｈｎ The hot-stamped steel according to claim 4 or 5, characterized in that: s Ｈｎ ≦0.235×Hn …(1) Here, Hn in the above formula (1) is the average value of the nanoindentation hardness of the metal structure.

7. The hot-stamped product according to any one of claims 4 to 6, characterized in that it has a plating layer on its surface.

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