Hot stamped compact

Abstract

This hot-stamp formed article has a prescribed chemical composition, and at a location 1 / 4 of the sheet thickness from the surface thereof and in a prior-austenite texture, the maximum pole density of an orientation group, expressed by Euler angle Φ = 60-90°, φ1 = 60-90° and φ2 = 45°, is at least 3.0, and the average block particle size of martensite, tempered martensite, and bainite is not more than 1.20 μm.

Description

[Technical Field]

[0001] The present invention relates to a hot stamped product. This application claims priority based on Japanese Patent Application No. 2022-090847, filed on June 3, 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 high-strength steel sheets are being used in automobile components. Automotive components are manufactured by press forming, but as the strength of steel sheets increases, not only do the forming loads increase but also formability decreases. Therefore, the formability of high-strength steel sheets 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 formability into automotive parts and strength by performing quenching treatment in a die simultaneously with press working.

[0004] For example, US Pat. No. 5,629,999 discloses a hardenable steel that has excellent cold formability properties and can be reheated and hardened to yield a steel with excellent impact strength and hardness. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japan Special Publication No. 2020-508393 [Non-patent literature]

[0006] [Non-Patent Document 1] Acta Materialia, 58(2010), 6393-6403 Summary of the Invention [Problem to be solved by the invention]

[0007] Using a hot-stamped steel sheet with improved tensile strength as an automotive component can achieve a greater effect in reducing the weight of the vehicle body. However, because it is an automotive component, it may be subject to bending deformation due to a collision or the like, and therefore the hot-stamped steel sheet must have high bendability. However, Patent Document 1 does not take bendability into consideration.

[0008] The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide a hot-stamped steel sheet having high strength and excellent bendability. [Means for solving the problem]

[0009] The gist of the present invention is as follows. (1) A hot-stamped steel according to one aspect of the present invention has a chemical composition, in mass%, of C: 0.40~0.70%, Si: 0.010 to 3.000%, Mn: 0.10% or more, less than 0.60% P: 0.100% or less, S: 0.0100% or less, N: 0.0100% or less, O: 0.0200% or less, Al: 0.0010~0.5000%, Nb: 0.0010~0.1000%, Ti: 0.010 to 0.100%, Cr: 0.010~1.000%, Mo: 0.050 to 1.000%, B: 0.0005~0.0100%, Co: 0-3.00%, Ni: 0-3.00% Cu: 0-3.00% V: 0~3.00%, W: 0~3.00%, Ca: 0 to 0.1000%, Mg: 0 to 1.0000%, REM: 0 to 1.0000%, Sb: 0 to 1.000%, Sn: 0 to 1.000%, Zr: 0 to 1.000%, As: 0~0.100%, The balance is Fe and impurities. At 1 / 4 of the plate thickness from the surface, In the texture of prior austenite, the maximum value of the pole density of the orientation group expressed by Euler angles Φ=60-90°, φ1=60-90°, and φ2=45° is 3.0 or more, The average block grain size of martensite, tempered martensite, and bainite is 1.20 μm or less. (2) The hot stamped steel according to (1) above, wherein the chemical composition is, in mass%, Co: 0.01 to 3.00%, Ni: 0.01 to 3.00% Cu: 0.01 to 3.00%, V: 0.01~3.00%, W: 0.01 to 3.00%, Ca: 0.0001 to 0.1000%, Mg: 0.0001 to 1.0000%, REM: 0.0001 to 1.0000%, Sb: 0.001 to 1.000%, Sn: 0.001 to 1.000%, Zr: 0.001 to 1.000%, and As: 0.001 to 0.100% The compound may contain one or more selected from the group consisting of: [Effects of the Invention]

[0010] According to the above aspects of the present invention, it is possible to provide a hot-stamped steel sheet having high strength and excellent bendability. DETAILED DESCRIPTION OF THE INVENTION

[0011] The present inventors have found that the bendability of a hot-stamped steel sheet can be improved by controlling the texture of prior austenite and the average block grain sizes of martensite, tempered martensite, and bainite at a position from the surface of the hot-stamped steel sheet to 1 / 4 of the thickness. In particular, the present inventors have found that the bendability of a hot-stamped steel sheet can be improved by controlling, within a specific range, not the textures of martensite, tempered martensite, bainite, etc., which are the metallographic structures of the hot-stamped steel sheet, but the texture of prior austenite before it transforms into martensite, bainite, etc. (i.e., the austenite state at high temperatures of Ar3 or higher).

[0012] The present inventors have also found that in order to obtain a hot stamped steel sheet having the above characteristics, it is particularly effective to strictly control the finish rolling conditions during hot rolling.

[0013] The hot stamped steel according to this embodiment will be described in detail below. First, the reasons for limiting the chemical composition of the hot stamped steel according to this embodiment will be described.

[0014] 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 %.

[0015] The hot stamped steel according to this embodiment has a chemical composition, in mass%, of C: 0.40 to 0.70%, Si: 0.010 to 3.000%, Mn: 0.10% or more but less than 0.60%, P: 0.100% or less, S: 0.0100% or less, N: 0.0100% or less, O: 0.0200% or less, Al: 0.0010 to 0.5000%, Nb: 0.0010 to 0.1000%, Ti: 0.010 to 0.100%, Cr: 0.010 to 1.000%, Mo: 0.050 to 1.000%, B: 0.0005 to 0.0100%, and the balance: Fe and impurities. Each element will be explained below.

[0016] C: 0.40 to 0.70% C is an element that improves the strength of a hot-stamped steel sheet. If the C content is less than 0.40%, the desired strength cannot be obtained in the hot-stamped steel sheet. Therefore, the C content is set to 0.40% or more. The C content is preferably more than 0.40%, 0.42% or more, or 0.44% or more. On the other hand, if the C content exceeds 0.70%, the strength becomes too high and the bendability of the hot-stamped steel deteriorates. Therefore, the C content is set to 0.70% or less. Preferably, the C content is 0.65% or less or 0.60% or less.

[0017] Si: 0.010 to 3.000% Silicon is an element that improves the strength of hot-stamped steel sheets through solid solution strengthening. If the Si content is less than 0.010%, the desired strength cannot be obtained in the hot-stamped steel sheet. Therefore, the Si content is set to 0.010% or more. The Si content is preferably 0.100% or more, 0.300% or more, or 0.500% or more. On the other hand, if the Si content exceeds 3.000%, the amount of ferrite increases, making it impossible to obtain the desired strength in the hot stamped steel. Therefore, the Si content is set to 3.000% or less. The Si content is preferably 2.000% or less, 1.000% or less, or 0.800% or less.

[0018] Mn: 0.10% or more, less than 0.60% Mn is an element that improves the hardenability of steel and increases the strength of hot-stamped steel. If the Mn content is less than 0.10%, the desired strength cannot be obtained in the hot-stamped steel. Therefore, the Mn content is set to 0.10% or more. The Mn content is preferably 0.20% or more or 0.35% or more. On the other hand, if the Mn content is 0.60% or more, the desired texture cannot be obtained in the prior austenite. Therefore, the Mn content is set to less than 0.60%. Preferably, the Mn content is 0.55% or less or 0.50% or less.

[0019] P:0.100% or less P segregates at grain boundaries, reducing grain boundary strength. This deteriorates the bendability of the hot-stamped steel. If the P content exceeds 0.100%, the bendability of the hot-stamped steel will deteriorate significantly. Therefore, the P content is set to 0.100% or less. The P content is preferably 0.050% or less or 0.010% or less. The P content may be 0%. However, if the P content is reduced to less than 0.0001%, the dephosphorization cost increases significantly, which is economically undesirable. Therefore, the P content may be set to 0.0001% or more.

[0020] S: 0.0100% or less S forms inclusions in steel. If the S content exceeds 0.0100%, the bendability of the hot-stamped steel sheet deteriorates significantly. Therefore, the S content is set to 0.0100% or less. The S content is preferably 0.0080% or less, 0.0050% or less, or 0.0030% or less. The S content may be 0%. However, if the S content is reduced to less than 0.0001%, the cost of desulfurization increases significantly, which is not economically preferable. Therefore, the S content may be set to 0.0001% or more.

[0021] N: 0.0100% or less N forms nitrides in steel. If the N content exceeds 0.0100%, the bendability of the hot-stamped steel sheet deteriorates significantly. Therefore, the N content is set to 0.0100% or less. The N content is preferably 0.0080% or less, 0.0060% or less, or 0.0040% or less. The N content may be 0%. However, if the N content is reduced to less than 0.0001%, the cost of denitrification increases significantly, which is not economically preferable. Therefore, the N content may be set to 0.0001% or more.

[0022] O: 0.0200% or less When O is contained in a large amount in steel, it forms coarse oxides. If the O content exceeds 0.0200%, the bendability of the hot-stamped steel sheet deteriorates significantly. Therefore, the O content is set to 0.0200% or less. The O content is preferably 0.0100% or less, 0.0070% or less, 0.0040% or less, or 0.0030% or less. The O content may be 0%, but in order to disperse a large number of fine oxides during deoxidation of molten steel, the O content may be 0.0005% or more.

[0023] Al: 0.0010 to 0.5000% Al is an element that deoxidizes molten steel to improve its soundness (suppressing the occurrence of defects such as blowholes in the steel). If the Al content is less than 0.0010%, deoxidation is insufficient and coarse oxides are formed. This deteriorates the bendability of the hot-stamped steel. Therefore, the Al content is set to 0.0010% or more. The Al content is preferably 0.0050% or more, 0.0100% or more, or 0.0300% or more. On the other hand, if the Al content exceeds 0.5000%, coarse oxides are formed in the steel, which deteriorates the bendability of the hot stamped steel. Therefore, the Al content is set to 0.5000% or less. The Al content is preferably 0.4000% or less, 0.3000% or less, 0.2000% or less, or 0.1000% or less.

[0024] Nb: 0.0010 to 0.1000% Nb is an element that forms carbonitrides in steel and improves the strength of hot-stamped steel through precipitation strengthening. If the Nb content is less than 0.0010%, the desired strength cannot be obtained in the hot-stamped steel. Therefore, the Nb content is set to 0.0010% or more. The Nb content is preferably 0.0050% or more, 0.0100% or more, or 0.0200% or more. On the other hand, if the Nb content exceeds 0.1000%, a large amount of carbonitrides is formed in the steel, deteriorating the bendability of the hot-stamped steel. Therefore, the Nb content is set to 0.1000% or less. The Nb content is preferably 0.0800% or less or 0.0600% or less.

[0025] Ti: 0.010 to 0.100% Ti is an element that forms carbonitrides in steel and improves the strength of hot-stamped steel through precipitation strengthening. If the Ti content is less than 0.010%, the desired strength cannot be obtained in the hot-stamped steel. Therefore, the Ti content is set to 0.010% or more. The Ti content is preferably 0.020% or more or 0.025% or more. On the other hand, if the Ti content exceeds 0.100%, a large amount of coarse carbonitrides is formed in the steel, deteriorating the bendability of the hot-stamped steel. Therefore, the Ti content is set to 0.100% or less. The Ti content is preferably 0.080% or less, 0.060% or less, or 0.050% or less.

[0026] Cr: 0.010 to 1.000% Cr is an element that dissolves in prior austenite grains during heating before hot stamping, thereby increasing the strength of the hot stamped steel. If the Cr content is less than 0.010%, the desired strength cannot be obtained in the hot stamped steel. Therefore, the Cr content is set to 0.010% or more. The Cr content is preferably 0.100% or more, 0.150% or more, or 0.200% or more. On the other hand, if the Cr content exceeds 1.000%, the desired texture cannot be obtained in the prior austenite. Therefore, the Cr content is set to 1.000% or less. The Cr content is preferably 0.700% or less, 0.500% or less, or 0.400% or less.

[0027] Mo: 0.050 to 1.000% Mo is an element that dissolves in prior austenite grains during heating before hot stamping, thereby increasing the strength of the hot stamped steel. If the Mo content is less than 0.050%, the desired strength cannot be obtained in the hot stamped steel. Therefore, the Mo content is set to 0.050% or more. The Mo content is preferably 0.100% or more or 0.150% or more. On the other hand, if the Mo content exceeds 1.000%, the desired texture cannot be obtained in the prior austenite. Therefore, the Mo content is set to 1.000% or less. The Mo content is preferably 0.800% or less, 0.600% or less, or 0.400% or less.

[0028] B: 0.0005 to 0.0100% B is an element that improves the hardenability of steel. If the B content is less than 0.0005%, the desired strength cannot be obtained in the hot stamped steel. Therefore, the B content is set to 0.0005% or more. The B content is preferably 0.0020% or more or 0.0030% or more. On the other hand, if the B content exceeds 0.0100%, coarse intermetallic compounds are formed in the hot-stamped steel sheet. This deteriorates the bendability of the hot-stamped steel sheet. Therefore, the B content is set to 0.0100% or less. The B content is preferably 0.0080% or less, 0.0060% or less, or 0.0040% or less.

[0029] The chemical composition of the hot stamped steel may contain the following elements as optional elements in place of part of Fe. When the following optional elements are not contained, the content is 0%.

[0030] Co: 0.01 to 3.00% Co is an element that improves the strength of the hot stamped steel by solid solution strengthening. To reliably obtain this effect, the Co content is preferably 0.01% or more, and more preferably 0.05% or more. On the other hand, since the above effects are saturated even if Co is added in a large amount, the Co content is set to 3.00% or less. If necessary, the Co content may be limited to 2.00% or less, 1.50% or less, 1.00% or less, or 0.50% or less.

[0031] Ni: 0.01 to 3.00% Ni has the effect of increasing the strength of the hot stamped body by dissolving in prior austenite grains during heating before hot stamping. To ensure this effect, the Ni content is preferably 0.01% or more. On the other hand, since the above effect saturates even if Ni is added in a large amount, the Ni content is set to 3.00% or less. If necessary, the Ni content may be limited to 2.00% or less, 1.50% or less, 1.00% or less, or 0.50% or less.

[0032] Cu: 0.01 to 3.00% Cu has the effect of increasing the strength of the hot stamped steel by dissolving in prior austenite grains during heating before hot stamping. To ensure this effect, the Cu content is preferably 0.01% or more, and more preferably 0.05% or more. On the other hand, since the above effect saturates even if Cu is added in a large amount, the Cu content is set to 3.00% or less. If necessary, the Cu content may be limited to 2.00% or less, 1.50% or less, 1.00% or less, or 0.50% or less.

[0033] V: 0.01 to 3.00% V forms carbonitrides in steel and has the effect of improving the strength of the hot-stamped steel by precipitation strengthening. To reliably obtain this effect, the V content is preferably 0.01% or more, and more preferably 0.05% or more. On the other hand, if the V content exceeds 3.00%, a large amount of coarse carbonitrides is formed in the steel, which deteriorates the bendability of the hot stamped steel. Therefore, the V content is set to 3.00% or less. If necessary, the V content may be limited to 2.00% or less, 1.50% or less, 1.00% or less, or 0.50% or less.

[0034] W: 0.01 to 3.00% W has the effect of improving the strength of the hot stamped steel. To reliably obtain this effect, the W content is preferably 0.01% or more, and more preferably 0.05% or more. On the other hand, since the above effect saturates even if W is added in a large amount, the W content is set to 3.00% or less. If necessary, the W content may be limited to 2.00% or less, 1.50% or less, 1.00% or less, or 0.50% or less.

[0035] Ca: 0.0001 to 0.1000% Ca is an element that suppresses the generation of oxides that serve as fracture initiation sites and contributes to improving the bendability of the hot-stamped steel. To reliably obtain this effect, the Ca content is preferably 0.0001% or more, and more preferably 0.0010% or more. On the other hand, since the above effect saturates even if Ca is added in a large amount, the Ca content is set to 0.1000% or less. If necessary, the Ca content may be limited to 0.0500% or less, 0.0200% or less, 0.0100% or less, or 0.0060% or less.

[0036] Mg: 0.0001 to 1.0000% Mg forms oxides and sulfides in molten steel, suppressing the formation of coarse MnS and dispersing many fine oxides, thereby refining the metal structure. This contributes to improving the bendability of the hot-stamped steel. To ensure these effects, the Mg content is preferably 0.0001% or more, and more preferably 0.0010% or more. On the other hand, since the above effect saturates even if Mg is added in a large amount, the Mg content is set to 1.0000% or less. If necessary, the Mg content may be limited to 0.0500% or less, 0.0200% or less, 0.0100% or less, or 0.0060% or less.

[0037] REM: 0.0001 to 1.0000% REM suppresses the formation of coarse oxides, thereby contributing to improving the bendability of the hot-stamped steel. To reliably obtain this effect, the REM content is preferably 0.0001% or more, and more preferably 0.0010% or more. On the other hand, since the above effect saturates even if the REM content is contained in a large amount, the REM content is set to 1.0000% or less. If necessary, the REM content may be limited to 0.0500% or less, 0.0200% or less, 0.0100% or less, or 0.0060% 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.

[0038] Sb: 0.001 to 1.000% Sb suppresses the formation of coarse oxides, thereby contributing to improving the bendability of the hot-stamped steel. To reliably obtain this effect, the Sb content is preferably 0.001% or more. On the other hand, since the above effect saturates even if Sb is added in a large amount, the Sb content is set to 1.000% or less. If necessary, the Sb content may be limited to 0.500% or less, 0.200% or less, 0.100% or less, or 0.050% or less.

[0039] Sn: 0.001 to 1.000% Sn suppresses the formation of coarse oxides, thereby contributing to improving the bendability of the hot-stamped steel. To reliably obtain this effect, the Sn content is preferably 0.001% or more. On the other hand, even if Sn is added in a large amount, the above effect saturates, so the Sn content is set to 1.000% or less. If necessary, the Sn content may be limited to 0.500% or less, 0.200% or less, 0.100% or less, or 0.050% or less.

[0040] Zr: 0.001 to 1.000% Zr suppresses the formation of coarse oxides, thereby contributing to improving the bendability of the hot-stamped steel. To reliably obtain this effect, the Zr content is preferably 0.001% or more. On the other hand, since the above effect saturates even if Zr is added in a large amount, the Zr content is set to 1.000% or less. If necessary, the Zr content may be limited to 0.500% or less, 0.200% or less, 0.100% or less, or 0.050% or less.

[0041] As: 0.001 to 0.100% As reduces the austenite single-phase temperature, thereby refining prior austenite grains. This contributes to improving the bendability of the hot-stamped steel. To reliably obtain this effect, the As content is preferably 0.001% or more. On the other hand, since the above effect saturates even if As is contained in a large amount, the As content is set to 0.100% or less. If necessary, the As content may be limited to 0.500% or less, 0.200% or less, 0.100% or less, or 0.050% or less.

[0042] The remainder of the chemical composition of the hot stamped steel 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 this embodiment.

[0043] The chemical composition of the hot-stamped body can be measured by a common analytical method, such as inductively coupled plasma-atomic emission spectrometry (ICP-AES). C and S can be measured using a combustion-infrared absorption method, N can be measured using an inert gas fusion-thermal conductivity method, and O can be measured using an inert gas fusion-non-dispersive infrared absorption method. When the surface of the hot-stamped body is provided with a plating layer, a paint film, or the like, the plating layer, the paint film, or the like may be removed by mechanical grinding before analyzing the chemical composition.

[0044] Next, the metal structure of the hot stamped steel according to this embodiment will be described. In the hot stamped steel according to this embodiment, at a position from the surface to 1 / 4 of the plate thickness, in the prior austenite texture, the maximum value of the pole density of the orientation group expressed by Euler angles Φ = 60 to 90°, φ1 = 60 to 90°, and φ2 = 45° is 3.0 or more, and the average value of the block grain size of martensite, tempered martensite, and bainite is 1.20 μm or less.

[0045] In this embodiment, the metallographic structure at a position 1 / 4 of the plate thickness from the surface of the hot stamped steel (a region from 1 / 8 depth of the plate thickness to 3 / 8 depth of the plate thickness from the surface) is specified because the metallographic structure at this position represents a typical metallographic structure of the hot stamped steel. In addition, when the hot stamped body has a plating layer, a paint film, or the like on the surface, the "surface" here refers to the interface between the plating layer, the paint film, or the like and the base steel sheet.

[0046] In the texture of prior austenite, the maximum value of the pole density of the orientation group expressed by Euler angles Φ=60-90°, φ1=60-90°, φ2=45°: 3.0 or more The present inventors have made the following findings regarding the texture of prior austenite. By developing the texture of prior austenite, it is possible to alleviate the strain concentration introduced by bending deformation, thereby reducing the load increase at the initial stage of bending deformation and improving the bendability of the hot-stamped steel.

[0047] In the prior austenite texture, if the maximum value of the pole density of an orientation group expressed by Euler angles Φ=60 to 90°, φ1=60 to 90°, and φ2=45° (hereinafter sometimes referred to as the pole density of the prior austenite texture) is less than 3.0, the desired bendability cannot be obtained in the hot-stamped steel. Therefore, the maximum value of the pole density of the prior austenite texture of the orientation group is set to 3.0 or more, preferably 5.0 or more. Although the upper limit is not particularly specified, the maximum value of the pole density of the texture of the prior austenite in the orientation group may be 50.0 or less, 20.0 or less, 15.0 or less, or 10.0 or less.

[0048] The pole density of the texture of prior austenite is measured by the following method. The pole density of the prior austenite texture is measured using an EBSD analyzer consisting of a thermal field emission scanning electron microscope and an EBSD detector, and the software "OIM Analysis (registered trademark)" that comes with the EBSD analyzer. The pole density of the prior austenite texture can be determined by using the orientation data measured by the EBSD (Electron Back Scattering Diffraction) method and the crystal orientation distribution function (ODF), which displays the three-dimensional texture calculated using spherical harmonic functions.

[0049] The specimen used for EBSD analysis is mechanically polished on a cross section parallel to the rolling direction and perpendicular to the plate surface, and strain is removed by chemical polishing or electrolytic polishing. EBSD measurements are performed on this specimen at a position 1 / 4 of the plate thickness from the surface (the region from 1 / 8 of the plate thickness depth to 3 / 8 of the plate thickness depth from the surface), with a measurement range of 150 μm in length and 50 μm in the plate thickness direction, at a measurement interval of 0.2 μm. An EBSD analyzer consisting of a thermal field emission scanning electron microscope and an EBSD detector can be used for the measurement; for example, an EBSD analyzer consisting of a JEOL JSM-7001F and a TSL DVC5 detector can be used. The vacuum level inside the EBSD analyzer is 9.6 × 10 -5 The acceleration voltage may be 15 kV and the probe current level may be 13. The crystal orientation of the prior austenite is calculated using the following method. The crystal orientation of the prior austenite is calculated using the method described in Non-Patent Document 1, and the crystal orientation of the prior austenite at each coordinate in the region measured by EBSD is identified. Next, a crystal orientation map of the prior austenite is created using the "Inverse Pole Figure" function included in the software "OIM Analysis (registered trademark)" attached to the EBSD analyzer. Based on this crystal orientation map, the maximum pole density of the orientation group within the ranges of Φ = 60 to 90° and Φ = 60 to 90° in the Φ = 45° cross section is calculated. This obtains the maximum pole density of the orientation group expressed by Euler angles Φ = 60 to 90°, Φ = 60 to 90°, and Φ = 45° in the texture of the prior austenite. Texture analysis using Euler angles (φ1, Φ, φ2) is widely used. For example, the definitions of Euler angles (φ1, Φ, φ2) are described in Hiroshi Inoue's "Lecture (Easy Materials Analysis Techniques) - Three-Dimensional Texture Orientation Analysis," Light Metals, Vol. 41, No. 6 (1992), 358. By performing analysis using the aforementioned software, even those who do not fully understand the definitions of Euler angles (φ1, Φ, φ2) can easily calculate the maximum pole density of the orientation group within the ranges of Φ = 60 to 90° and φ1 = 60 to 90° on a φ2 = 45° cross section.

[0050] Average block grain size of martensite, tempered martensite, and bainite: 1.20 μm or less If the average block grain size of martensite, tempered martensite, and bainite exceeds 1.20 μm, the desired bendability cannot be obtained in the hot stamped steel. Therefore, the average block grain size of martensite, tempered martensite, and bainite is set to 1.20 μm or less, preferably 1.00 μm or less, and more preferably 0.90 μm or less. The lower limit is not particularly specified, but may be 0.30 μm or more, 0.40 μm or more, or 0.50 μm or more.

[0051] The average block grain sizes of martensite, tempered martensite, and bainite are measured by the following method. A sample is cut out from any position at least 50 mm away from the end face of the hot-stamped product (if a sample cannot be taken from this position, a position that avoids the end as much as possible) so that a thickness cross section parallel to the rolling direction can be observed. The size of the sample depends on the measuring device, but it should be large enough to allow observation of at least about 10 mm in the rolling direction.

[0052] The cross section of the sample was polished using #600 to #1500 silicon carbide paper, then polished to a mirror finish using a diluted solution such as alcohol or a liquid dispersion of diamond powder with a grain size of 1 to 6 μm dispersed in pure water. The observation surface was then polished by electrolytic polishing. Using this sample, a measurement range of 150 μm in length and 50 μm in the thickness direction was measured at a depth of 1 / 4 of the plate thickness from the surface (from 1 / 8 of the plate thickness depth to 3 / 8 of the plate thickness depth from the surface), with a measurement interval of 0.2 μm, using electron backscatter diffraction to obtain crystal orientation information. Measurements can be performed using an EBSD analyzer consisting of a thermal field emission scanning electron microscope and an EBSD detector, such as a JEOL JSM-7001F and a TSL DVC5 detector. The vacuum level inside the EBSD analyzer was 9.6 × 10 -5 The acceleration voltage may be 15 kV and the probe current level may be 13.

[0053] Using the obtained crystal orientation information and the "Phase Map" function of the "OIM Analysis®" software provided with the EBSD analyzer, regions with a bcc crystal structure were extracted. For these regions, the "Grain Average Misorientation" function of the "OIM Analysis®" software provided with the EBSD analyzer was used to extract regions with a "Grain Average Misorientation" of more than 0.5° as martensite, tempered martensite, and bainite, under the condition that grain boundaries with a crystal misorientation of 5° or more are considered as grain boundaries. For the obtained regions, the "Grain Properties" function was used to obtain values ​​calculated by the Number method using the "Grain Size (diameter)" function, under the condition that grain boundaries with a crystal misorientation of 15° or more are considered as grain boundaries, to obtain the average block grain sizes of martensite, tempered martensite, and bainite.

[0054] The rolling direction of the hot stamped steel is determined by the following method. First, a test piece is taken so that the thickness cross section of the hot-stamped body can be observed. The thickness cross section of the taken test piece is mirror-polished and then observed using an optical microscope. The observation range is a 500 μm width × full thickness range, and areas with dark brightness are determined to be inclusions. Next, using the thickness cross section initially observed by the above method as a reference, a plane parallel to a plane rotated in 5° increments in the range of 0° to 180° around the thickness direction is observed. The average value of the length of the major axes of the inclusions in each obtained cross section is calculated for each cross section, and the direction parallel to the major axis of the inclusions in the cross section where the average value of the length of the major axes of the inclusions is the largest is determined to be the rolling direction. When the rolling direction of the hot-stamped steel is known in advance, the rolling direction of the hot-stamped steel may be determined without using the above-described determination method.

[0055] The metal structure of the hot stamped steel sheet is not particularly limited as long as it can provide the desired strength and bendability, and may, for example, be composed of a total of 90% or more of martensite, bainite, and tempered martensite, and 10% or less of ferrite and retained austenite, in terms of area percentage.

[0056] The area ratio of each structure is measured by the following method. A sample is cut out from any position at least 50 mm away from the end face of the hot-stamped product (if a sample cannot be taken from this position, a position that avoids the end as much as possible) so that a thickness cross section parallel to the rolling direction can be observed. The size of the sample depends on the measuring device, but it should be large enough to allow observation of at least about 10 mm in the rolling direction.

[0057] The cross section of the sample is polished using #600 to #1500 silicon carbide paper, then polished to a mirror finish using a liquid in which diamond powder with a grain size of 1 to 6 μm is dispersed in a diluted solution such as alcohol or pure water. The observation surface is then polished by electrolytic polishing. At any position along the longitudinal direction of the sample cross section, a 50 μm long region extending from 1 / 8 of the plate thickness to 3 / 8 of the plate thickness is measured at 0.1 μm measurement intervals using electron backscatter diffraction to obtain crystal orientation information. Measurements can be performed using an EBSD analyzer consisting of a thermal field emission scanning electron microscope and an EBSD detector, such as a JEOL JSM-7001F and a TSL DVC5 detector. The vacuum level inside the EBSD analyzer is 9.6 × 10 -5 The acceleration voltage may be 15 kV and the probe current level may be 13.

[0058] The obtained crystal orientation information is used with the "Phase Map" function of the "OIM Analysis®" software provided with the EBSD analyzer to determine whether the crystal structure is fcc or retained austenite. The area fraction of this retained austenite is calculated to obtain the area fraction of retained austenite. Next, regions with bcc crystal structures are determined to be bainite, tempered martensite, martensite, and ferrite. From these regions, the "Grain Average Misorientation" function of the "OIM Analysis®" software provided with the EBSD analyzer is used to extract regions with a "Grain Average Misorientation" of 0.5° or less as ferrite, under the condition that grain boundaries with a crystal orientation misorientation of 5° or more are considered to be grain boundaries. The area fraction of the extracted ferrite is then calculated to obtain the area fraction of ferrite.

[0059] Next, the remaining region (the region where "Grain Average Misorientation" exceeds 0.5°) is taken as the total area ratio of martensite, tempered martensite, and bainite.

[0060] The hot-stamped product according to this embodiment may have a plating layer, a paint film, or the like on its surface. By having a plating layer, a paint film, or the like on its surface, corrosion resistance can be improved after hot stamping. Examples of the plating layer include an aluminum plating layer, an aluminum-zinc plating layer, an aluminum-silicon plating layer, a hot-dip galvanized layer, an electrogalvanized layer, a galvannealed hot-dip galvanized layer, a zinc-nickel plating layer, and an aluminum-magnesium-zinc based plating layer.

[0061] The thickness of the hot stamped steel according to this embodiment is not particularly limited, but is preferably 0.5 to 3.5 mm from the viewpoint of reducing the weight of the vehicle body. The shape of the hot stamped body is not particularly limited, and may be, for example, a flat shape, a curved shape, or a three-dimensional shape such as a hat shape.

[0062] The hot-stamped steel according to this embodiment preferably has a tensile strength of 2300 MPa or more, more preferably 2400 MPa or more, and even more preferably 2500 MPa or more. There is no need to specify an upper limit for the tensile strength, but the tensile strength may be set to 3000 MPa or less or 2800 MPa or less, as necessary. The tensile strength is measured by preparing a No. 5 test piece as specified in JIS Z 2241: 2011 from a flat position on the hot-stamped body and following the test method as specified in JIS Z 2241: 2011. The crosshead speed is 1 mm / min.

[0063] When the hot-stamped steel according to this embodiment has a flat plate shape (having no curved portions, etc.), the load at 1 / 2 of the stroke at the maximum load is preferably 8050 N or more, more preferably 8100 N or more, and even more preferably 8150 N or more. However, these standards are based on the case where the thickness of the hot-stamped steel is 1.6 mm. The load at 1 / 2 stroke is obtained by conducting a bending test under the following conditions based on the VDA standard (VDA238-100:2017-04) specified by the German Association of the Automotive Industry, and determining the load at 1 / 2 stroke of the stroke at maximum load. When the thickness of the hot stamped steel sheet exceeds 1.6 mm, the thickness is reduced to 1.6 mm before the bending test is carried out. When the thickness of the hot stamped body is less than 1.6 mm, the load at 1 / 2 of the stroke at the maximum load is preferably 8050×t / 1.6 (N) or more, where t is the thickness of the hot stamped body. The load at 1 / 2 stroke of the stroke at the maximum load (however, when the thickness of the hot-stamped steel sheet is less than 1.6 mm, the value obtained by multiplying the load at 1 / 2 stroke by 1.6 / t (t is the sheet thickness in mm)) rarely exceeds 8500 N, 8300 N, or 8200 N.

[0064] Test piece dimensions: 60 mm (rolling direction) x 30 mm (direction parallel to the plate width direction) Bending ridge: parallel to the plate width 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: For example, SHIMADZU AUTOGRAPH 20kN

[0065] Next, a steel sheet for hot stamping for obtaining the hot stamped steel according to this embodiment will be described. The steel sheet for hot stamping has the above-mentioned chemical composition. The metal structure of the steel sheet for hot stamping is not particularly limited as long as it can obtain the desired strength and bendability after hot stamping. For example, it may be composed of, by area %, ferrite: 0 to 90%, bainite and martensite: 0 to 100%, pearlite: 0 to 80%, and retained austenite: 0 to 5%.

[0066] Furthermore, the steel sheet for hot stamping may have a plating layer, a paint film, or the like on its surface. By having a plating layer, a paint film, or the like on its surface, corrosion resistance can be improved after hot stamping. Examples of the plating layer include an aluminum plating layer, an aluminum-zinc plating layer, an aluminum-silicon plating layer, a hot-dip galvanized layer, an electrogalvanized layer, a galvannealed hot-dip galvanized layer, a zinc-nickel plating layer, and an aluminum-magnesium-zinc based plating layer.

[0067] Manufacturing method of steel sheet for hot stamping Hereinafter, a method for producing a steel sheet for hot stamping for obtaining the hot-stamped steel according to this embodiment will be described. In order to obtain the hot-stamped steel, it is particularly effective to control the finish rolling conditions during hot rolling in the method for producing a steel sheet for hot stamping.

[0068] Finishing rolling In the finish rolling, it is preferable that the rolling one stage before the final stage and the final stage rolling are each performed at a reduction ratio of 50% or more. By performing rolling at a reduction ratio of 50% or more in the rolling one stage before the final stage and the final stage rolling, it is possible to control the prior austenite to have a predetermined texture. The reduction ratio here can be expressed as (1-t1 / t0) x 100 (%), where t0 is the entry thickness of each stage and t1 is the exit thickness.

[0069] After the completion of finish rolling (after the final stage of rolling), it is preferable to start cooling after 5.0 seconds or more have elapsed. By allowing 5.0 seconds or more to elapse before starting cooling, granular austenite grains can be generated. As a result, flat austenite grains are reduced, and a sufficient amount of granular austenite grains can be secured. The cooling here does not include air cooling (cooling at an average cooling rate of less than 10°C / s), but includes, for example, water cooling at an average cooling rate of 10°C / s or more. The cooling stop temperature is preferably 550 to 650°C.

[0070] Cooling after finish rolling transforms austenite into ferrite and pearlite. At this time, pearlite transformation progresses from the grain boundaries of prior austenite grains. Pearlite with a specified texture is produced by transformation from austenite grains with a specified texture. Furthermore, in order to soften the hot-rolled steel sheet, the coil after coiling may be subjected to a softening heat treatment. The method of the softening heat treatment is not particularly limited, and general conditions may be used. The total reduction during cold rolling is preferably 50% or less. The total reduction here can be expressed as (1-t3 / t2) x 100 (%), where t3 is the thickness after cold rolling and t2 is the thickness before cold rolling.

[0071] Hot stamping The hot stamped steel sheet manufactured by the above-mentioned method is hot stamped to obtain a hot stamped product according to this embodiment. As a condition for hot stamping, for example, it is preferable to heat the steel sheet to a temperature range of 800°C to 1000°C and hold it at this temperature range for 60 to 1200 seconds.

[0072] Heating during hot stamping causes a reverse transformation from pearlite to austenite. Because pearlite has a specific texture, the texture of the austenite generated by the reverse transformation develops. Cooling after hot stamping causes a transformation from austenite to martensite. If the final structure becomes martensite, the austenite texture is preserved. Therefore, the texture of the prior austenite remains developed in the structure after hot stamping.

[0073] If the heating temperature is less than 800°C or the holding time is less than 60 seconds, austenitization will be insufficient, and the bendability of the hot-stamped steel sheet may deteriorate or the desired strength may not be obtained. On the other hand, if the heating temperature is more than 1000°C or the holding time is more than 1200 seconds, the prior austenite grains will grow excessively, and the bendability of the hot-stamped steel sheet may deteriorate or the desired strength may not be obtained.

[0074] The heating atmosphere may be, for example, air, a gas combustion atmosphere in which the ratio of air to fuel is controlled, or a nitrogen atmosphere, and the dew point of these gases may be controlled. After holding the material in this temperature range, it is hot stamped. After hot stamping, it is sufficient to cool the material to a temperature range of 250°C or lower at an average cooling rate of 20°C / s or more.

[0075] Examples of the heating method before hot stamping include heating with an electric furnace or gas furnace, flame heating, electrical heating, high frequency heating, and induction heating.

[0076] The hot-stamped product according to this embodiment is obtained by the above method. To soften the product, tempering at 130 to 600°C may be performed after hot stamping, or baking hardening may be performed after painting. Furthermore, a portion of the hot-stamped product may be tempered by laser irradiation or the like to provide a partially softened region. [Example]

[0077] 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.

[0078] Molten steel having the chemical compositions shown in Tables 1A to 1F was cast to produce billets, which were then held at a temperature of 1200°C or higher for 20 minutes or longer, followed by hot rolling, coiling, and cold rolling. The finish rolling conditions were as shown in Tables 2A to 2E. The cooling that began 5.0 seconds or more after the completion of finish rolling was carried out at an average cooling rate of 10° C. / s or more, and the cooling stop temperature was 550 to 650° C. The total reduction rate of the cold rolling was 50% or less.

[0079] The obtained steel sheets for hot stamping were hot stamped under the conditions shown in Tables 2A to 2E, and then cooled to a temperature range of 250° C. or lower at an average cooling rate of 20° C. / s or higher, thereby obtaining hot-stamped products shown in Tables 3A to 3G. However, in some examples, plating was applied or heat treatment for softening was carried out as shown in the table.

[0080] The underlines in the table indicate that the product is outside the range of the present invention, that the production conditions are not within the preferred range, or that the characteristic values ​​are not preferred. The metal structure of the hot stamped steel according to the present invention was composed of a total of 90% or more of martensite, bainite, and tempered martensite, and 10% or less of ferrite and retained austenite, in terms of area percentage. The thickness of the hot stamped steel according to the present invention was 0.5 to 3.5 mm.

[0081] The metallographic structure and mechanical properties of the hot stamped bodies were measured by the above-mentioned measuring methods. Although the bending test according to the aforementioned VDA standard (VDA238-100:2017-04) is widely performed on automotive parts and other products, it only tests flat plates. Therefore, this VDA standard cannot evaluate the bendability of hot-stamped steel sheets with shapes other than flat. On the other hand, if the hot-stamped steel sheet has bent portions, the bendability is affected by factors such as the curvature of the bent portions. Therefore, the inventors considered it appropriate to evaluate the bendability of flat hot-stamped steel sheets using this VDA standard. Therefore, the bending test was performed on flat hot-stamped steel sheets obtained by hot stamping without bending (using a die capable of obtaining a hot-stamped steel sheet without bent portions). Furthermore, because the rolling direction of the hot-stamped steel sheets was known in advance, the rolling direction of the hot-stamped steel sheets was determined without determining the rolling direction using the above-mentioned determination method. A SHIMADZU AUTOGRAPH 20kN testing machine was used for the bending test.

[0082] A tensile strength TS of 2300 MPa or more was judged to have high strength and pass, while a tensile strength of less than 2300 MPa was judged to not have high strength and fail.

[0083] When the load at 1 / 2 stroke of the stroke at the maximum load was 8050 N or more, the specimen was judged to have excellent bendability and to have passed the test. On the other hand, when the load at 1 / 2 stroke was less than 8050 N, the specimen was judged to have poor bendability and to have failed the test. However, when the thickness of the hot-stamped steel sheet was less than 1.6 mm, the specimen was judged to have excellent bendability and to have passed the test if the load at 1 / 2 stroke of the stroke at the maximum load was 8050 × t / 1.6 (N) or more, where t is the thickness of the hot-stamped steel sheet. On the other hand, when the load at 1 / 2 stroke was less than 8050 × t / 1.6 (N), the specimen was judged to have poor bendability and to have failed the test. Note that when the thickness of the hot-stamped steel sheet was less than 1.6 mm, the value obtained by multiplying the load at 1 / 2 stroke by 1.6 / t (where t is the thickness in mm) was entered in the "Load at 1 / 2 stroke" column of Tables 3A to 3G.

[0084] [Table 1A]

[0085] [Table 1B]

[0086] [Table 1C]

[0087] [Table 1D]

[0088] [Table 1E]

[0089] [Table 1F]

[0090] Table 2A

[0091] Table 2B

[0092] Table 2C

[0093]

Table 2D

[0094] Table 2E

[0095] Table 3A

[0096] Table 3B

[0097]

Table 3C

[0098]

Table 3D

[0099] Table 3E

[0100] [Table 3F]

[0101] [Table 3G]

[0102] It can be seen from Tables 3A to 3G that the hot stamped steel sheets according to the present invention have high strength and excellent bendability. On the other hand, it is clear that the hot stamped steel sheets of the comparative examples are inferior in one or more properties. [Industrial Applicability]

[0103] According to the above aspects of the present invention, it is possible to provide a hot-stamped steel sheet having high strength and excellent bendability.

Claims

1. The chemical composition, in mass%, is C: 0.40-0.70%, Si: 0.010-3.000%, Mn: 0.10% or more and less than 0.60%; P: 0.100% or less, S: 0.0100% or less, N: 0.0100% or less, O: 0.0200% or less, Al: 0.0010-0.5000%, Nb: 0.0010-0.1000%, Ti: 0.010 to 0.100%, Cr: 0.010-1.000%, Mo: 0.050-1.000%, B: 0.0005-0.0100%, Co: 0-3.00%, Ni: 0-3.00%, Cu: 0-3.00%, V: 0 to 3.00%, W: 0-3.00%, Ca: 0-0.1000%, Mg: 0 to 1.0000%, REM: 0-1.0000%, Sb: 0 to 1.000%, Sn: 0-1.000%, Zr: 0 to 1.000%, As: 0 to 0.100%, The balance is Fe and impurities. At 1 / 4 of the plate thickness from the surface, In the texture of prior austenite, the maximum value of the pole density of the orientation group expressed by Euler angles Φ = 60 to 90°, φ1 = 60 to 90°, and φ2 = 45° is 3.0 or more, A hot stamped steel, characterized in that the average block grain size of martensite, tempered martensite and bainite is 1.20 μm or less.

2. The chemical composition is, in mass %, Co: 0.01 to 3.00%, Ni: 0.01 to 3.00%, Cu: 0.01-3.00%, V: 0.01 to 3.00%, W: 0.01-3.00%, Ca: 0.0001-0.1000%, Mg: 0.0001 to 1.0000%, REM: 0.0001-1.0000%, Sb: 0.001 to 1.000%, Sn: 0.001 to 1.000%, Zr: 0.001 to 1.000%, and As: 0.001-0.100% The hot-stamped product according to claim 1, characterized in that it contains one or more selected from the group consisting of

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