Hot press-formed body

Abstract

The hot press-formed body has a prescribed chemical composition, a metal structure, in terms of area ratio, of 90 to 100% of martensite, 0 to 10% of remaining structure, a proportion of martensite having a GAIQ value of 40,000 or less in all of the martensite being less than 5.0%, an average grain size of the prior austenite grains being 6.0 μm or less, and a standard deviation of the grain size of the above prior austenite grains being 2.6 μm or less.

Description

Technical Field

[0001] This invention relates to hot-stamped formed articles.

[0002] This application claims priority based on Japanese Patent Application No. 2021-175240 filed on October 27, 2021, the contents of which are incorporated herein by reference. Background Technology

[0003] In recent years, from the perspectives of environmental protection and resource conservation, there has been a demand for lightweight automobile bodies, and high-strength steel sheets are being used in automotive components. Automotive components are manufactured through pressing, but with the increasing strength of the steel sheets, not only does the forming load increase, but formability also decreases. Therefore, the formability of high-strength steel sheets for complex-shaped components has become a key issue.

[0004] To address the aforementioned issues, the application of hot stamping technology, which involves heating the steel sheet to a high temperature in the austenitic region to soften it before pressing, is being promoted. Hot stamping is attracting attention as a technology that balances the formability and strength of automotive components by simultaneously performing quenching treatment within a die during the pressing process.

[0005] For example, Patent Document 1 discloses a high-yield-ratio, high-strength zinc-based electroplated steel sheet with excellent bending properties, wherein the diffusible hydrogen content in the steel is less than 0.20 ppm by mass.

[0006] Patent document 2 discloses a hot-stamped formed body, characterized by having a steel structure in which the area fraction of primary martensite and tempered martensite is 80% or more, the grain size of primary austenite is 20 μm or less, and the average grain size of carbides is 0.5 μm or less.

[0007] Patent document 3 discloses a hot-stamped formed body in which the average grain size of the original austenite grains in the microstructure is less than 5.0 μm and the average Mn concentration of the grain boundaries of the original austenite grains is less than 1.0% by mass.

[0008] Existing technical documents

[0009] Patent documents

[0010] Patent Document 1: International Publication No. 2020 / 079925

[0011] Patent Document 2: International Publication No. 2018 / 134874

[0012] Patent Document 3: International Publication No. 2020 / 189767 Summary of the Invention

[0013] The problem that the invention aims to solve

[0014] To further reduce the weight of automobile bodies, increasing the strength of the steel sheets is an effective approach. One method for increasing the strength of the steel sheets is to increase the amount of martensite in the metal structure. However, increasing the amount of martensite increases the number of hydrogen trapping sites, making it easier for hydrogen to penetrate and causing hydrogen embrittlement cracking in hot-stamped parts.

[0015] Hydrogen embrittlement cracking is the phenomenon of sudden fracture in steel components subjected to high stress under service conditions due to hydrogen intrusion from the external environment. Depending on the fracture morphology, it is also known as delayed fracture. Generally, it is known that the higher the tensile strength of a steel plate, the more prone it is to hydrogen embrittlement cracking. This is believed to be because higher tensile strength results in greater residual stress in the steel plate after component forming. The sensitivity to this hydrogen embrittlement cracking (delayed fracture) is termed hydrogen embrittlement resistance.

[0016] Patent document 1 considers flexibility, but does not consider resistance to hydrogen embrittlement.

[0017] In patent documents 2 and 3, there is room for further improvement regarding the hydrogen embrittlement resistance.

[0018] The present invention was made in view of the above-mentioned problems. The object of the present invention is to provide a hot-stamped formed article having high strength and excellent resistance to hydrogen embrittlement.

[0019] Methods for solving problems

[0020] The main points of this invention are as follows.

[0021] (1) The chemical composition of the hot-stamped formed article according to one embodiment of the present invention, expressed in mass percent, is as follows:

[0022] C: 0.42–0.70%

[0023] Si: 0.010~1.300%

[0024] Mn: 0.100~3.000%

[0025] P: below 0.100%

[0026] S: below 0.0100%

[0027] N: below 0.0200%

[0028] O: Below 0.0200%

[0029] Al: 0.001~0.500%

[0030] Cr: 0.010~0.800%

[0031] Ti: 0.010~0.100%

[0032] Nb: 0.0010~0.1000%

[0033] B: 0.0005~0.0200%

[0034] Mo: 0–1.000%

[0035] Co: 0-4.00%

[0036] Ni: 0-3.00%

[0037] Cu: 0–3.00%

[0038] V: 0~1.00%

[0039] W: 0~1.00%

[0040] Ca: 0–1.0000%

[0041] Mg: 0~1.0000%

[0042] REM: 0~1.0000%

[0043] Sb: 0~1.00%

[0044] Zr: 0~1.00%

[0045] Sn: 0~1.00%

[0046] As: 0~1.0000%

[0047] Remaining components: Fe and impurities.

[0048] The metal structure, expressed as an area ratio, is as follows:

[0049] Martensite: 90-100%

[0050] Remaining tissue: 0-10%,

[0051] The proportion of martensite with a GAIQ value below 40,000 in all martensite is less than 5.0%.

[0052] The average grain size of the original austenite grains is less than 6.0 μm.

[0053] The standard deviation of the crystal grain size of the aforementioned original austenite grains is less than 2.6 μm.

[0054] (2) The hot-stamped formed body according to (1) above, wherein the above chemical composition may also contain one or more elements selected from the group consisting of the following elements, expressed in mass %:

[0055] Mo: 0.001~1.000%

[0056] Co: 0.01~4.00%

[0057] Ni: 0.01~3.00%

[0058] Cu: 0.01–3.00%

[0059] V: 0.01~1.00%

[0060] W: 0.01~1.00%

[0061] Ca: 0.0001~1.0000%

[0062] Mg: 0.0001~1.0000%

[0063] REM: 0.0001~1.0000%

[0064] Sb: 0.001~1.00%

[0065] Zr: 0.001~1.00%

[0066] Sn: 0.001–1.00%, and

[0067] As: 0.0001~1.0000%.

[0068] (3) The hot stamping formed body according to (1) or (2) above, wherein the average crystal grain size of the original austenite grains may also be greater than 3.0 μm.

[0069] Invention Effects

[0070] According to the above-described solution of the present invention, a hot-stamped formed article with high strength and excellent resistance to hydrogen embrittlement can be provided. Attached Figure Description

[0071] Figure 1 This is a diagram showing the shape of the test piece used in the evaluation of hydrogen embrittlement resistance. Detailed Implementation

[0072] The present invention demonstrates that by reducing the average grain size and standard deviation of the original austenite grains, and by reducing the amount of martensite in regions with high dislocation density, the hydrogen embrittlement resistance of hot-stamped articles can be improved.

[0073] The inventors have realized that, in order to obtain a hot-stamped article with the above-mentioned characteristics, it is effective to perform multiple heat treatments under desired conditions during the heating process before hot stamping.

[0074] The hot-stamped formed article of this embodiment will now be described in detail. First, the reasons for limiting the chemical composition of the hot-stamped formed article of this embodiment will be explained.

[0075] It should be noted that for the numerical ranges specified below, indicated by “~”, both the lower and upper limits are included within that range. Values ​​expressed as “below” or “above” are not included in the numerical range. All percentages related to chemical composition are expressed as mass%.

[0076] The chemical composition of the hot-stamped formed body of this embodiment, by mass%, contains C: 0.42-0.70%, Si: 0.010-1.300%, Mn: 0.100-3.000%, P: less than 0.100%, S: less than 0.0100%, N: less than 0.0200%, O: less than 0.0200%, Al: 0.0010-0.5000%, Cr: 0.010-0.800%, Nb: 0.0010-0.1000%, Ti: 0.010-0.100%, B: 0.0005-0.0200%, and the remainder: Fe and impurities.

[0077] The following is an explanation of each element.

[0078] C: 0.42–0.70%

[0079] Carbon (C) is an element that improves the strength of hot-stamped formed articles. When the C content is below 0.42%, the desired strength cannot be obtained in the hot-stamped formed article. Therefore, the C content is set to 0.42% or more. The C content is preferably 0.44% or more, 0.45% or more, or 0.50% or more.

[0080] On the other hand, excellent resistance to hydrogen embrittlement cannot be obtained when the carbon content exceeds 0.70%. Therefore, the carbon content is set to 0.70% or less. Preferably, the carbon content is 0.65% or less, 0.60% or less, or 0.55% or less.

[0081] Si: 0.010~1.300%

[0082] Si is an element that enhances the strength of hot-stamped parts through solid solution strengthening. When the Si content is below 0.010%, the desired strength cannot be obtained. Therefore, the Si content is set to 0.010% or more. Preferably, the Si content is 0.050% or more, 0.100% or more, 0.200% or more, 0.300% or more, 0.400% or more, or 0.500% or more.

[0083] On the other hand, when the Si content exceeds 1.300%, the ferrite content increases, making it impossible to obtain the desired metallic structure. Therefore, the Si content is set to 1.300% or less. The Si content is preferably 1.100% or less, 0.900% or less, 0.700% or less, or 0.600% or less.

[0084] Mn: 0.100~3.000%

[0085] Mn is an element that improves the hardenability of steel. To improve hardenability and obtain the desired strength of the hot-stamped part, the Mn content is set to 0.100% or more. The preferred Mn content is 0.200% or more, 0.250% or more, 0.300% or more, 0.350% or more, or 0.400% or more.

[0086] On the other hand, if the Mn content exceeds 3.000%, cracking due to Mn segregation becomes more likely, making it impossible to obtain excellent resistance to hydrogen embrittlement. Therefore, the Mn content is set to 3.000% or less. Preferably, the Mn content is 2.700% or less, 2.500% or less, 2.300% or less, 2.000% or less, 1.600% or less, 1.200% or less, 0.900% or less, or 0.600% or less.

[0087] P: below 0.100%

[0088] P is an impurity element that segregates at grain boundaries and becomes the starting point for fracture. Therefore, the P content is set to be 0.100% or less. The P content is preferably 0.050%, 0.030%, or 0.020% or less.

[0089] There is no need to specify a lower limit for the phosphorus (P) content, but it is 0%. However, if the P content is reduced to below 0.0001%, the cost of P removal increases significantly, which is not economically preferable. Therefore, the P content can also be set at 0.0001% or higher, 0.001% or higher, 0.003% or higher, or 0.005% or higher.

[0090] S: below 0.0100%

[0091] Sulfur (S) is an impurity element that forms inclusions in steel. These inclusions can become the starting point for fracture, therefore the S content is set to be 0.0100% or less. The S content is preferably 0.0080%, 0.0050%, 0.0030%, or 0.0020% or less.

[0092] There is no need to specify a lower limit for sulfur content, but it is 0%. However, if the sulfur content is reduced to below 0.0001%, the cost of sulfur removal increases significantly, which is not economically preferable. Therefore, the sulfur content can also be set at 0.0001% or higher, 0.0002% or higher, or 0.0003% or higher.

[0093] N: below 0.0200%

[0094] Nitrogen (N) is an impurity element that forms nitrides in steel. These nitrides can become the initiation point for fracture, therefore the N content is set to be 0.0200% or less. The preferred N content is 0.0100%, 0.0080%, or 0.0050% or less.

[0095] There is no need to specify a lower limit for nitrogen (N) content, but it is 0%. However, if the N content is reduced to below 0.0001%, the cost of nitrogen removal increases significantly, which is not economically preferable. Therefore, the N content can also be set at 0.0001% or higher, 0.0004% or higher, 0.0008% or higher, or 0.0012% or higher.

[0096] O: Below 0.0200%

[0097] If oxygen (O) is present in large quantities in steel, it forms coarse oxides that become the starting point for fracture, thus deteriorating the hydrogen embrittlement resistance of the hot-stamped part. Therefore, the O content is set to 0.0200% or less. Preferably, the O content is set to 0.0080%, 0.0050%, or 0.0030% or less.

[0098] There is no need to specify a lower limit for the O content, but it is 0%. In order to disperse a large amount of fine oxides during the deoxidation of molten steel, the O content can also be set to 0.0005% or more or 0.0010% or more.

[0099] Al: 0.001~0.500%

[0100] Al is an element that deoxidizes molten steel, thus improving its quality (suppressing defects such as porosity). When the Al content is below 0.001%, deoxidation is insufficient, resulting in coarse oxides and failing to achieve the desired effect. Therefore, the Al content is set to 0.001% or more. Preferably, the Al content is 0.005% or more, 0.010% or more, 0.015% or more, 0.020% or more, or 0.025% or more.

[0101] On the other hand, if the Al content exceeds 0.500%, coarse oxides are formed in the steel, reducing the hydrogen embrittlement resistance of the hot-stamped part. Therefore, the Al content is set to 0.500% or less. The Al content is preferably 0.400%, 0.300%, 0.200%, 0.150%, 0.100%, or 0.075% or less.

[0102] It should be noted that in this embodiment, the Al content refers to the total Al content.

[0103] Cr: 0.010~0.800%

[0104] Cr is an element that increases the strength of hot-stamped parts by dissolving in the original austenite grains during heating before hot stamping. If the Cr content is less than 0.010%, the desired strength cannot be obtained. Therefore, the Cr content is set to 0.010% or more. The Cr content is preferably set to 0.100% or more or 0.200% or more.

[0105] On the other hand, if the Cr content exceeds 0.800%, the hydrogen embrittlement resistance of the hot-stamped part deteriorates. Therefore, the Cr content is set to 0.800% or less. The Cr content is preferably 0.700%, 0.650%, 0.600%, or 0.550% or less.

[0106] Ti: 0.010~0.100%

[0107] Ti is an element that forms carbonitrides in steel, thereby increasing the strength of hot-stamped parts through precipitation strengthening. If the Ti content is less than 0.010%, the desired strength cannot be obtained. The Ti content is preferably 0.020% or more, or 0.025% or more.

[0108] On the other hand, if the Ti content exceeds 0.100%, a large amount of carbonitrides will be generated in the steel, and the hydrogen embrittlement resistance of the hot-stamped form will deteriorate. Therefore, the Ti content is set to 0.100% or less. The Ti content is preferably 0.080% or less, 0.060% or less, 0.045% or less, or 0.035% or less.

[0109] Nb: 0.0010~0.1000%

[0110] Nitrogen (Nb) is an element that forms carbonitrides in steel, thereby increasing the strength of hot-stamped parts through precipitation strengthening. If the Nb content is below 0.0010%, the desired strength cannot be obtained. Therefore, the Nb content is set to 0.0010% or more. Preferably, the Nb content is 0.0050% or more, 0.0090% or more, or 0.0150% or more.

[0111] On the other hand, if the Nb content exceeds 0.1000%, a large amount of carbonitrides will be generated in the steel, and the hydrogen embrittlement resistance of the hot-stamped part will deteriorate. Therefore, the Nb content is set to 0.1000% or less. The Nb content is preferably 0.0800%, 0.0600%, or 0.0500% or less.

[0112] B: 0.0005~0.0200%

[0113] Boron (B) is an element that improves the hardenability of steel. If the B content is below 0.0005%, the desired strength cannot be obtained. Therefore, the B content is set to 0.0005% or more. Preferably, the B content is set to 0.0010% or more or 0.0015% or more.

[0114] On the other hand, if the boron content exceeds 0.0200%, the hydrogen embrittlement resistance of the hot-stamped part deteriorates. Therefore, the boron content is set to 0.0200% or less. The boron content is preferably 0.0080%, 0.0060%, 0.0040%, or 0.0030% or less.

[0115] The remaining portion of the chemical composition of the hot-stamped formed article may also be Fe and impurities. Examples of impurities include elements that are unavoidably mixed in from steel raw materials or scrap iron and / or during the steelmaking process, and are permissible within a range that does not impair the characteristics of the hot-stamped formed article of this embodiment.

[0116] The chemical composition of hot-stamped parts may also contain the following elements as optional elements to replace a portion of Fe. The content of any of the following optional elements is 0%.

[0117] Mo: 0.001~1.000%

[0118] Mo is an element that increases the strength of hot-stamped parts by dissolving in the original austenite grains during heating before hot stamping. To reliably achieve this effect, the Mo content is preferably set to 0.001% or more.

[0119] On the other hand, if the Mo content exceeds 1.000%, the hydrogen embrittlement resistance of the hot-stamped part deteriorates. Therefore, the Mo content is set to 1.000% or less. The Mo content is preferably 0.800% or less or 0.600% or less.

[0120] Co: 0.01~4.00%

[0121] Co is an element that increases the strength of hot-stamped parts through solid solution strengthening. To reliably achieve this effect, the Co content is preferably set to 0.01% or more.

[0122] On the other hand, even with a large amount, the above effects are saturated, so the Co content is set to 4.00% or less.

[0123] Ni: 0.01~3.00%

[0124] Ni has the effect of increasing the strength of hot-stamped articles by dissolving in the original austenite grains during heating before hot stamping. To reliably obtain this effect, the Ni content is preferably set to 0.01% or more.

[0125] On the other hand, even with a large amount, the above-mentioned effects are saturated, so the Ni content is set to 3.00% or less. The Ni content is preferably 2.00% or less, 1.00% or less, 0.60% or less, or 0.30% or less.

[0126] Cu: 0.01–3.00%

[0127] Cu has the effect of increasing the strength of hot-stamped parts by dissolving in the original austenite grains during heating before hot stamping. To reliably obtain this effect, it is preferable to set the Cu content to 0.01% or more.

[0128] On the other hand, even if it contains a large amount, the above-mentioned effect will be saturated, so the Cu content is set to 3.00% or less. The Cu content is preferably 2.00% or less, 1.00% or less, 0.60% or less, or 0.30% or less.

[0129] V: 0.01~1.00%

[0130] V has the effect of increasing the strength of hot-stamped parts by forming carbonitrides in steel and thus strengthening them through precipitation. To reliably obtain this effect, the V content is set to 0.01% or more.

[0131] On the other hand, when the V content is set to more than 1.00%, a large amount of carbonitrides are generated in the steel, and the hydrogen embrittlement resistance of the hot-stamped part deteriorates. Therefore, the V content is set to 1.00% or less. The V content is preferably 0.80% or less, 0.60% or less, or 0.30% or less.

[0132] W: 0.01~1.00%

[0133] W has the effect of improving the strength of hot-stamped parts. To reliably obtain this effect, it is preferable to set the W content to 0.01% or more.

[0134] On the other hand, even with a large amount, the above-mentioned effects are saturated, so the W content is set to 1.00% or less. The W content is preferably 0.80%, 0.60%, or 0.30% or less.

[0135] Ca: 0.0001~1.0000%

[0136] Ca is an element that inhibits the formation of oxides that become the starting point for fracture. To reliably achieve this effect, it is preferable to set the Ca content to 0.0001% or more.

[0137] On the other hand, even if it contains a large amount, the above-mentioned effect is saturated, so the Ca content is set to 1.0000% or less. The Ca content is preferably 0.4000%, 0.1000%, 0.0500%, 0.0200%, 0.0100%, or 0.070% or less.

[0138] Mg: 0.0001~1.0000%

[0139] Mg has the effect of inhibiting the formation of coarse MnS by forming oxides or sulfides in molten steel, dispersing fine oxides in large quantities, and refining the metal structure. To reliably obtain these effects, it is preferable to set the Mg content to 0.0001% or more.

[0140] On the other hand, if the Mg content exceeds 1.0000%, the oxide content in the steel increases, which adversely affects the toughness of the hot-stamped part. Therefore, the Mg content is set to 1.0000% or less. The Mg content is preferably 0.4000%, 0.1000%, 0.0500%, 0.0200%, 0.0100%, or 0.070% or less.

[0141] REM: 0.0001~1.0000%

[0142] REM is an element that inhibits the formation of oxides that become the starting point for fracture. To reliably achieve this effect, it is preferable to set the REM content to 0.0001% or more.

[0143] On the other hand, even with a large amount, the above-mentioned effects are saturated, so the REM content is set to 1.0000% or less. The REM content is preferably 0.4000%, 0.1000%, 0.0500%, 0.0200%, 0.0100%, or 0.070% or less.

[0144] It should be noted that in this embodiment, REM refers to a total of 17 elements including Sc, Y and lanthanides, and the content of REM refers to the total content of these elements.

[0145] Sb: 0.001~1.00%

[0146] Sb improves the deformability of hot-stamped articles by suppressing the formation of oxides that become the initiation point of fracture. To reliably achieve this effect, the Sb content is preferably set to 0.001% or more.

[0147] On the other hand, even with a large amount, the above-mentioned effects are saturated, so the Sb content is set to 1.00% or less. The Sb content is preferably 0.4000%, 0.1000%, 0.0500%, 0.0200%, 0.0100%, or 0.070% or less.

[0148] Zr: 0.001~1.00%

[0149] Zr is an element that helps control inclusions, especially the fine dispersion of inclusions, and improves the toughness of hot-stamped articles. To reliably achieve this effect, it is preferable to set the Zr content to 0.001% or more.

[0150] On the other hand, if the Zr content is high, the deterioration of surface properties may become more pronounced. Therefore, the Zr content is set to 1.00% or less. The Zr content is preferably 0.4000%, 0.1000%, 0.0500%, 0.0200%, 0.0100%, or 0.070% or less.

[0151] Sn: 0.001~1.00%

[0152] Sn inhibits the formation of oxides that become the starting point for fracture, thus contributing to improved resistance to hydrogen embrittlement. To reliably achieve this effect, the Sn content is preferably set to 0.001% or higher.

[0153] On the other hand, even if it contains a large amount, the above-mentioned effect is saturated, so the Sn content is set to 1.00% or less. The Sn content is preferably 0.4000%, 0.1000%, 0.0500%, 0.0200%, 0.0100%, or 0.070% or less.

[0154] As: 0.0001~1.0000%

[0155] As helps improve resistance to hydrogen embrittlement by lowering the austenite single-phase formation temperature and refining the original austenite grains. To reliably achieve this effect, it is preferable to set the As content to 0.0001% or higher.

[0156] On the other hand, even if it contains a large amount, the above-mentioned effect is saturated, so the As content is set to 1.0000% or less. The As content is preferably 0.4000%, 0.1000%, 0.0500%, 0.0200%, 0.0100%, or 0.070% or less.

[0157] The chemical composition of the hot-stamped formed articles described above can be determined using general analytical methods. For example, it can be determined using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). It should be noted that C and S are determined using the combustion-infrared absorption method, N using the inert gas melting-thermal conductivity method, and O using the inert gas melting-non-dispersive infrared absorption method.

[0158] When a hot-stamped part has a coating on its surface, the chemical composition can be analyzed after the coating is removed by mechanical grinding.

[0159] Next, the metal structure of the hot-stamped formed article of this embodiment will be described.

[0160] The microstructure of the hot-stamped formed body of this embodiment, in terms of area ratio, is: martensite: 90-100%, residual microstructure: 0-10%. In all the martensite, the proportion of martensite with a GAIQ value of 40,000 or less is less than 5.0%, the average grain size of the original austenite grains is 6.0 μm or less, and the standard deviation of the grain size of the original austenite grains is 2.6 μm or less.

[0161] In this embodiment, the metal structure at a location 1 / 4 of the plate thickness from the surface (a region ranging from 1 / 8 to 3 / 8 of the plate thickness from the surface) is specified. This is because the metal structure at this location represents the representative metal structure of the steel plate.

[0162] Martensite area ratio: over 90%

[0163] If the martensite area ratio is less than 90%, the desired strength cannot be obtained in the hot-stamped formed body. Therefore, the martensite area ratio is set to 90% or more. Preferably, it is 93% or more, 95% or more, 97% or more, or 99% or more. The martensite area ratio can also be set to 100%.

[0164] There is no specific upper limit, but it is 100%.

[0165] The microstructure of a hot-stamped formed body may also include bainite, ferrite, and retained austenite as residual microstructure. The area ratio of the residual microstructure may also be set to less than 10%, less than 7%, less than 5%, less than 3%, or less than 1%. The area ratio of the residual microstructure may also be set to 0%.

[0166] The microstructure of hot-stamped parts is determined by the following methods.

[0167] A sample is cut from any position more than 50 mm from the end face of the hot-stamped body (avoiding the end position if the sample cannot be collected from that position) in a way that allows observation of the sheet thickness section parallel to the rolling direction. Although the size of the sample varies depending on the measuring device, it is set to be approximately 10 mm in size that can be observed in the rolling direction.

[0168] The cross-section of the above sample was ground using #600 to #1500 silicon carbide paper, and then refined into a mirror finish using a liquid obtained by dispersing diamond powder with a particle size of 1 to 6 μm in a diluent such as alcohol or pure water. Next, it was ground for 8 minutes at room temperature using colloidal silica with a particle size of 0.25 μm without an alkaline solution to remove the strain introduced into the sample surface. At any location along the length of the sample cross-section, regions with a length of 50 μm and a depth from 1 / 8 to 3 / 8 of the plate thickness from the surface were measured at 0.1 μm intervals using electron backscatter diffraction to obtain crystal orientation information. For the measurements, an EBSD analysis apparatus consisting of a thermal field emission scanning electron microscope (JEOL JSM-7001F) and an EBSD detector (TSL DVC5 detector) was used. The vacuum level within the EBSD analysis apparatus was set to 9.6 × 10⁻⁶. -5 Below Pa, the accelerating voltage is set to 15kV, the irradiation current level is set to 13, and the electron beam irradiation level is set to 62.

[0169] Using the obtained crystal orientation information, the "Phase Map" function of the "OIM Analysis" software (registered trademark) included in the EBSD analysis device was used to identify regions with an fcc crystal structure as retained austenite. The area ratio of this retained austenite was calculated to obtain the area ratio of the retained austenite. Next, regions with a bcc crystal structure were identified as bainite, martensite, and ferrite. For these regions, the "Grain Orientation Spread" function of the "OIM Analysis" software (registered trademark) included in the EBSD analysis device was used to extract regions with a "Grain Orientation Spread" of less than 1° as ferrite, under the condition that 15° grain boundaries are considered as crystal boundaries. The area ratio of the extracted ferrite was calculated to obtain the area ratio of the ferrite.

[0170] Next, in the remaining region (the region where the "Grain Orientation Spread" exceeds 1°), under the condition that the 5° grain boundary is considered a crystal boundary, when the maximum value of the "Grain Average IQ" of the ferrite region is set to Iα, the region that becomes more than Iα / 2 is extracted as bainite, and the region that becomes less than Iα / 2 is extracted as martensite. The area ratio of bainite is obtained by calculating the area ratio of the extracted bainite. Similarly, the area ratio of martensite is obtained by calculating the area ratio of the extracted martensite.

[0171] If no ferrite is extracted from the field of view, the GAM "Grain Average Misorientation" function is used to extract regions with a "Grain Average Misorientation" greater than 0.50° and less than 0.75° as bainite, and regions with a "Grain Average Misorientation" greater than 0.75° as martensite and tempered martensite, assuming 5° grain boundaries are considered as solid grain boundaries. The area ratios of these extracted regions are then calculated to obtain the area ratio of bainite and the combined area ratio of martensite and tempered martensite.

[0172] The proportion of martensite with a GAIQ value below 40,000 in all martensite is less than 5.0%.

[0173] A higher GAIQ value indicates a lower dislocation density, while a lower GAIQ value indicates a higher dislocation density. Therefore, the GAIQ value is a parameter that reflects the dislocation density of a grain.

[0174] If the proportion of martensite with a GAIQ value of 40,000 or less in the total martensite is 5.0% or more, the hydrogen embrittlement resistance of the hot-stamped formed body deteriorates. Therefore, the proportion of martensite with a GAIQ value of 40,000 or less in the total martensite is set to be less than 5.0%. Preferably, it is 4.0% or less, 3.0% or less, or 2.0% or less, and it can also be 0.0%.

[0175] The proportion of martensite with a GAIQ value below 40,000 in all martensite was obtained using the following method.

[0176] The sample is cut from a position at least 50 mm away from the end face of the hot-stamped body (avoiding the end face if it cannot be sampled from this position), with the thickness section observable. The thickness section of the sample is then ground using #600 to #1500 silicon carbide paper, followed by a mirror finish using a liquid obtained by dispersing diamond powder with a particle size of 1–6 μm in a diluent such as alcohol or pure water. Next, the sample is ground for 8 minutes at room temperature using colloidal silica with a particle size of 0.25 μm and without an alkaline solution to remove strain introduced into the surface layer.

[0177] At arbitrary locations along the length of the sample's cross-section, measuring a 50 μm length and a quarter-thickness region (a depth from 1 / 8 to 3 / 8 of the thickness from the surface) using electron backscatter diffraction at 0.1 μm intervals, crystal orientation information was obtained. For the measurements, an EBSD analysis apparatus consisting of a thermal field emission scanning electron microscope (JEOL JSM-7001F) and an EBSD detector (TSL DVC5 type detector) was used. The vacuum level within the EBSD analysis apparatus was set to 9.6 × 10⁻⁶. -5 Below Pa, the accelerating voltage is set to 15kV, the working distance is set to 15mm, the irradiation current level is set to 13, and the electron beam irradiation level is set to 62.

[0178] The obtained crystal orientation information was used to generate a Grain Average Image Quality (GAIQ) map using the "OIM DataCollection" function in the EBSD analysis device's accompanying software and the "Grain Average Misorientation" function in "OIM Analysis (registered trademark)". In the OIM DataCollection, the camera's EXPOSURE TIME was set to 3.65, and the Gain was set to 0.39. Furthermore, during the detection of bands in the EBSD pattern, the Hough transform's Max Peak Count was set to 9. In the obtained GAIQ map, regions with a crystal orientation difference of 5° or more were defined as grains, and the area ratio of martensite with a GAIQ value of 40,000 or less was calculated. For a total of 10 observation areas, the area ratio of martensite with a GAIQ value of 40,000 or less was calculated. The average of these area ratios was then used to obtain the area ratio of martensite with a GAIQ value of 40,000 or less. By dividing the obtained area ratio by the martensite area ratio obtained through the above method, the proportion of martensite with a GAIQ value of 40,000 or less in the total martensite is obtained. It should be noted that in regions with a GAIQ value of 40,000 or less, bainite may sometimes be present in addition to martensite. Therefore, martensite is identified using the above method, and for the identified martensite, the area ratio of martensite with a GAIQ value of 40,000 or less is determined.

[0179] Average grain size of the original austenite grains: below 6.0 μm

[0180] By reducing the average grain size of the original austenite grains, the grain boundary area increases, and the amount of hydrogen per unit grain boundary area decreases. This improves the hydrogen embrittlement resistance of the hot-stamped form. If the average grain size of the original austenite grains exceeds 6.0 μm, the hydrogen embrittlement resistance of the hot-stamped form deteriorates. Therefore, the average grain size of the original austenite grains is set to 6.0 μm or less. Preferably, it is 5.5 μm or less, or 5.0 μm or less.

[0181] The lower limit is not specifically specified, but it can also be set to 2.0 μm or more. The average grain size of the original austenite grains is preferably more than 3.0 μm. More preferably, the average grain size of the original austenite grains is 3.3 μm or more, 3.6 μm or more, 3.9 μm or more, 4.2 μm or more, 4.5 μm or more, or 4.7 μm or more.

[0182] Standard deviation of the original austenite grain size: below 2.6 μm

[0183] By reducing the non-uniformity of the original austenite grain size, i.e., reducing the standard deviation, the increase of local residual stress can be suppressed. As a result, the hydrogen embrittlement resistance of the hot-stamped part can be improved. If the standard deviation of the original austenite grain size exceeds 2.6 μm, the hydrogen embrittlement resistance deteriorates. Therefore, the standard deviation of the original austenite grain size is set to 2.6 μm or less. Preferably, it is 2.4 μm or less, 2.2 μm or less, or 2.0 μm or less.

[0184] The lower limit of the standard deviation of the original austenite grain size does not need to be specifically limited, but it can be set to 1.0 μm.

[0185] The average grain size and standard deviation of the original austenite grains were obtained by the following method.

[0186] A sample is cut from any position more than 50 mm from the end face of the hot-stamped body (avoiding the end position if the sample cannot be collected from that position) in a way that allows observation of the sheet thickness section parallel to the rolling direction. Although the size of the sample varies depending on the measuring device, it is set to be approximately 10 mm in size that can be observed in the rolling direction.

[0187] Next, for the thick section of the sample, the microstructure was revealed using an etching solution containing sodium dodecylbenzenesulfonate in a saturated aqueous solution of picric acid. At any point along the length of the sample, at a length of 50 μm and a distance of 1 / 4 of the plate thickness from the surface (the region from 1 / 8 to 3 / 8 of the plate thickness from the surface), a scanning electron microscope was used to capture micrographs at 500x magnification. Using these micrographs, the equivalent circle diameter of the original austenite grains was determined.

[0188] It should be noted that the scanning electron microscope was configured to be equipped with two electron detectors. Tissue photographs were taken at 9.6 × 10⁻⁶. -5 In a vacuum below Pa, the sample was irradiated with electron beams at an accelerating voltage of 15 kV and an irradiation current level of 13, and a secondary electron image was captured. The number of fields of view was set to 10 or more. In the captured secondary electron image, the proto-austenite grain boundaries were captured as bright contrast. For each proto-austenite grain included in the field of view, the equivalent circle diameter was calculated. The same operation was performed on all proto-austenite grains included in the field of view, except for those grains at the ends of the field of view that were not entirely included, and the equivalent circle diameter of all proto-austenite grains in the field of view was calculated. The average grain size of the proto-austenite grains was obtained by calculating the average of the obtained equivalent circle diameters of the proto-austenite grains. Furthermore, the standard deviation of the grain size of the proto-austenite grains was obtained by calculating the standard deviation from the obtained equivalent circle diameters of the proto-austenite grains.

[0189] The hot-stamped formed body of this embodiment may also have a coating on its surface. By having a coating on the surface, corrosion resistance can be improved after hot stamping. Examples of coatings include aluminum coatings, aluminum-zinc coatings, aluminum-silicon coatings, hot-dip galvanized coatings, electroplated zinc coatings, and alloyed hot-dip galvanized coatings.

[0190] Next, the hot stamping steel sheet used to obtain the hot stamping formed body of this embodiment will be described.

[0191] The hot-stamping steel sheet has the above-mentioned chemical composition. The microstructure of the hot-stamping steel sheet is not particularly limited as long as the desired strength and resistance to hydrogen embrittlement can be obtained after hot stamping. However, for example, it may contain, in terms of area ratio, ferrite: 0-90%, bainite and martensite: 0-100%, pearlite: 0-80%, and retained austenite: 0-5%.

[0192] Furthermore, hot-stamped steel sheets can also have a coating on their surface. By having a coating on the surface, corrosion resistance can be improved after hot stamping. Examples of coatings include aluminum coatings, aluminum-zinc coatings, aluminum-silicon coatings, hot-dip galvanized coatings, electro-galvanized coatings, and alloyed hot-dip galvanized coatings.

[0193] Manufacturing method of steel sheet for hot stamping

[0194] The following describes a method for manufacturing a hot-stamping steel sheet used to obtain the hot-stamped formed body of this embodiment. There are no particular limitations on the manufacturing conditions of the hot-stamping steel sheet; it can be manufactured under normal conditions.

[0195] The hot-stamped formed article of this embodiment is obtained by hot-stamping a hot-stamping steel sheet. In order to obtain the hot-stamped formed article of this embodiment, it is effective to perform heat treatment on the hot-stamping steel sheet three or more times (including the final hot stamping).

[0196] It should be noted that all temperatures mentioned below refer to the surface temperature of the steel plate.

[0197] First heat treatment

[0198] In the first heat treatment, the hot stamping steel plate is heated to the temperature range of Ac3 point to "Ac3 point + 200°C", held in this temperature range, and then cooled to the temperature range of 250 to 350°C.

[0199] It should be noted that point Ac3 is represented by the following formula.

[0200] Ac3 (°C) = 910 - 203 × C 0.5 +66×Si-25×Mn+700×P-11×Cr+109×Al+400×Ti-15.2×Ni+104×V+31.5×Mo (1)

[0201] The element symbols in the above formula represent the content of each element in terms of mass %; if the element is not present, substitute 0.

[0202] If the heating temperature is below point Ac3 or exceeds "Ac3 point + 200°C", the carbides cannot be fully melted, resulting in the inability to optimally control the average grain size and standard deviation of the original austenite grains. Therefore, the heating temperature is set in the temperature range of point Ac3 to "Ac3 point + 200°C".

[0203] The average heating rate to the aforementioned temperature range is set to 2°C / s or higher. If the average heating rate is lower than 2°C / s, the original austenite grains will coarsen during the heating process, and even the second heat treatment described later will not be able to refine the original austenite grains of the hot-stamped body.

[0204] There are no particular limitations on the heating method; examples include atmospheric heating, electric heating, and infrared heating.

[0205] The holding time in the aforementioned temperature range is set to be 1 second or more. If the holding time is less than 1 second, the carbide does not melt sufficiently. Furthermore, if the holding time exceeds 600 seconds, the effect saturates, productivity decreases, and costs increase; therefore, the holding time is set to be less than 600 seconds.

[0206] After holding in the aforementioned temperature range, the temperature is cooled to a range of 250–350°C at an average cooling rate of 10°C / s or higher. If the average cooling rate is less than 10°C / s, pearlite containing coarse, plate-like carbides is formed, and the carbides do not fully melt in subsequent heat treatments. Furthermore, if the cooling stop temperature exceeds 350°C, coarse granular and plate-like carbides are formed, and the carbides do not fully melt in subsequent heat treatments, failing to achieve the desired strength. If the cooling stop temperature is less than 250°C, the carbides in the martensite become excessively fine, and Ostwald ripening of the original austenite grains occurs in subsequent heat treatments. Consequently, it is sometimes impossible to preferably control the average grain size and standard deviation of the original austenite grains.

[0207] Cooling methods with an average cooling rate of 10°C / s or higher include mold cooling, gas cooling, and water cooling.

[0208] After cooling to the temperature range of 250–350°C, air cooling can be performed. It should be noted that air cooling here refers to cooling with an average cooling rate of less than 10°C / s.

[0209] Second heat treatment

[0210] In the second heat treatment, the heat treatment is carried out under the same conditions as the first heat treatment.

[0211] In either the first or second heat treatment, the cooling stop temperature is set to 260°C or higher. If the cooling stop temperature is not 260°C or higher in either the first or second heat treatment, it is not possible to optimally control the average grain size and standard deviation of the original austenite grains.

[0212] 3rd heat treatment

[0213] In the third heat treatment, the temperature is heated to the range of Ac3 point to "Ac3 point + 200°C", held in this temperature range, and then cooled to a temperature range below 250°C at an average cooling rate of 10°C / s or more. Except for the cooling to a temperature range below 250°C, the process is the same as the first and second heat treatments, and therefore, the explanation is omitted.

[0214] By performing a third heat treatment under the conditions described above, carbides can be finely dispersed in martensite. This reduces the average grain size and standard deviation of the original austenite grains.

[0215] It should be noted that in the third heat treatment, the temperature can also be heated to the range of Ac3 point to "Ac3 point + 200℃", and hot stamping can be performed after maintaining this temperature range. In this case, as long as the average cooling rate to the temperature range below 250℃ is above 10℃ / s through contact with the mold, it is sufficient.

[0216] Furthermore, if hot stamping is not performed in the third heat treatment, multiple heat treatments can be performed under the same conditions as the third heat treatment after the third heat treatment. The more times the heat treatment is performed, the more effectively the average grain size and standard deviation of the original austenite grains can be reduced.

[0217] In this case, as long as the final heat treatment involves heating to the temperature range of Ac3 point to "Ac3 point + 200°C" and maintaining this temperature range before hot stamping, it is sufficient. At this point, the average cooling rate to the temperature range below 250°C, achieved through contact with the mold, should be at least 10°C / s.

[0218] The hot-stamped formed article of this embodiment is obtained by the above method. It should be noted that tempering treatment at 150 to 600°C can also be performed after hot stamping. In addition, a portion of the hot-stamped formed article can be tempered by laser irradiation or the like to partially form a softened area.

[0219] Example

[0220] Next, embodiments of the present invention will be described. However, the conditions in the embodiments are merely examples of conditions adopted to confirm the feasibility and effectiveness of the present invention, and the present invention is not limited to these specific examples. Various conditions may be employed in the present invention as long as the objectives of the invention are achieved, without departing from the spirit of the invention.

[0221] Hot stamping steel plates are obtained by hot rolling and cold rolling slabs manufactured by casting molten steel with the chemical compositions shown in Tables 1A to 1C.

[0222] By heat-treating the obtained hot-stamping steel sheet under the conditions shown in Tables 2A to 2D, hot-stamped formed bodies as shown in Tables 3A to 3D are obtained. It should be noted that in all heat treatments, the average heating rate to the heating temperature is set to be 2°C / s or higher, the holding time at the heating temperature is set to be 1–600 seconds, the average cooling rate from the heating temperature to the cooling stop temperature is set to be 10°C / s or higher, and air cooling is performed after cooling stops (average cooling rate less than 10°C / s).

[0223] It should be noted that the underlined parts in the table indicate that the invention is outside the scope of the invention, deviates from the preferred manufacturing conditions, or has a non-preferred characteristic value.

[0224] The measurement of the metallographic structure of the hot stamping formed body is carried out by the above-mentioned measurement method. In addition, the mechanical properties of the hot stamping formed body are evaluated by the following method.

[0225] Tensile strength

[0226] The tensile strength TS of the hot stamping formed body is obtained by conducting a tensile test on a No. 5 test piece made from an arbitrary position of the hot stamping formed body in accordance with JIS Z 2241:2011. It should be noted that the crosshead speed is set to 3 mm / min. When the tensile strength TS is 2300 MPa or more, it is determined to have high strength and is judged as qualified, and when it is less than 2300 MPa, it is determined not to have high strength and is judged as unqualified.

[0227] Hydrogen embrittlement resistance property

[0228] Figure 1 The shape of the test piece used in the evaluation of the hydrogen embrittlement resistance property is shown. The test piece with a V-notch is Figure 1 immersed in an aqueous solution obtained by dissolving 5 g / l of ammonium thiocyanate in 3 vol% saline at room temperature, and the hydrogen embrittlement resistance property is judged by the presence or absence of fracture after 12 hours, 18 hours, and 24 hours. It should be noted that for the V-notch of the test piece, a load of 40% of the tensile strength obtained in the tensile test is pre-applied. When there is no fracture even after immersion for 12 hours or more, it is judged as qualified. Specifically, when there is no fracture after 12 hours and there is fracture after 18 hours, it is recorded as "Fair" in the table, when there is no fracture after 18 hours and there is fracture after 24 hours, it is recorded as "Good" in the table, when there is no fracture after 24 hours, it is recorded as "Very Good" in the table, and when there is fracture after 12 hours, it is judged as unqualified and recorded as "Bad" in the table.

[0229]

[0230]

[0231]

[0232]

[0233]

[0234]

[0235]

[0236]

[0237]

[0238]

[0239]

[0240] It has a high degree of resistance and excellent resistance to hydrogen embrittlement.

[0241] On the other hand, it was found that more than one characteristic of the hot-stamped formed body used as a comparative example was inferior.

[0242] Industrial availability

[0243] According to the above-described solution of the present invention, a hot-stamped formed article with high strength and excellent resistance to hydrogen embrittlement can be provided.

Claims

1. A hot-stamped formed body, characterized in that, The chemical composition, expressed as a percentage by mass, is as follows: C：0.42～0.70%、 Si: 0.010~1.300% Mn: 0.100~3.000% P: Below 0.100% S: Below 0.0100% N: below 0.0200% O: Below 0.0200% Al：0.001～0.500%、 Cr：0.010～0.800%、 Ti: 0.010~0.100% Nb: 0.0010~0.1000% B：0.0005～0.0200%、 Mo: 0~1.000%, Co: 0-4.00%, Ni: 0~3.00%, Cu: 0–3.00% V：0～1.00%、 W：0～1.00%、 Ca: 0~1.0000% Mg: 0~1.0000%, REM: 0~1.0000% Sb: 0~1.00%, Zr：0～1.00%、 Sn: 0~1.00% As: 0~1.0000% Remaining components: Fe and impurities. The metal structure, expressed as an area ratio, is as follows: Martensite: 90-100% Remaining tissue: 0-10%, The proportion of martensite with a GAIQ value below 40,000 in all martensite is less than 5.0%. GAIQ, or Grain Average Image Quality, refers to the average image quality of the grains. This value is calculated by defining regions with an orientation difference of 5° or more in the obtained GAIQ image as grains. The average grain size of the original austenite grains is less than 6.0 μm. The standard deviation of the original austenite grain size is below 2.6 μm.

2. The hot-stamped formed body according to claim 1, characterized in that, The chemical composition, expressed as a percentage by mass, contains one or more elements selected from the group consisting of: Mo: 0.001~1.000% Co: 0.01~4.00%, Ni: 0.01~3.00% Cu: 0.01~3.00% V：0.01～1.00%、 W：0.01～1.00%、 Ca: 0.0001~1.0000% Mg: 0.0001~1.0000%, REM: 0.0001~1.0000% Sb: 0.001~1.00% Zr：0.001～1.00%、 Sn: 0.001~1.00%, and As: 0.0001~1.0000%.

3. The hot-stamped formed body according to claim 1 or 2, characterized in that, The average grain size of the original austenite grains exceeds 3.0 μm.

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