Hot stamping steel sheet and hot stamping formed body

Abstract

The hot stamped body has a predetermined chemical composition and a metal structure in which the average grain size of the prior austenite grains is 5 to 25 μm and the standard deviation of the grain size of the prior austenite grains is 0.1 to 2.0 μm. In addition, the hot stamping steel sheet has a predetermined chemical composition and a metal structure in which the ferrite is composed of {100} <011> ~{223} <110> The average value of the pole density of the formed orientation group is less than 10.0; the proportion of the number of the above-mentioned ferrite containing carbides with an equivalent circle diameter of 0.2 μm or more in the grains of the entire ferrite is more than 20%; in terms of area ratio, pearlite is 10 to 90%, and ferrite is 10 to 90%.

Description

Technical Field

[0001] The present invention relates to a steel sheet for hot stamping and a hot stamped body.

[0002] This application claims priority based on Japanese Patent Application No. 2021-081621 filed in Japan on May 13, 2021, the contents of which are incorporated herein by reference. Background Art

[0003] In the past, from the perspective of global environmental issues and collision safety performance, there has been a demand for thinner and stronger automotive components. In order to meet these requirements, the number of automotive components made of high-strength steel plates as raw materials is increasing. In addition, as a forming method for high-strength steel plates, a method called hot stamping is known. In hot stamping, high-strength steel plates are press-formed in a high-temperature region of 700°C or above and quenched inside or outside the pressing die. According to hot stamping, since forming is carried out in a high-temperature region where the strength of the steel plate is reduced, forming defects such as those caused by cold pressing can be suppressed. In addition, since a structure with martensite as the main phase can be obtained by quenching after forming, high strength can be obtained. Therefore, hot stamped parts with a tensile strength of about 1500 MPa are widely used in the world.

[0004] In order to achieve greater weight reduction in automotive components formed from high-strength steel sheets by hot stamping, high-strength and excellent collision resistance are required. To improve the collision resistance of automotive components, excellent bendability is particularly required.

[0005] Patent Document 1 discloses a steel plate and a method for producing the same, which has improved hardenability and material formability and is particularly suitable for forming into parts such as gears by cold forging such as wall expansion.

[0006] The inventors of the present invention have discovered that in order to achieve a higher vehicle body weight reduction effect in automobile components with further improved tensile strength, it is necessary to further improve the bendability.

[0007] Prior art literature

[0008] Patent Literature

[0009] Patent Document 1: International Publication No. 2016 / 190396

[0010] Non-patent literature

[0011] Non-patent document 1: Acta Materialia, 58 (2010), 6393-6403 Summary of the Invention

[0012] Problems to be solved by the invention

[0013] The present invention has been made in view of the above-mentioned problems and has an object to provide a hot stamped body having high strength and excellent bendability and a hot stamping steel sheet capable of producing the hot stamped body.

[0014] Means for solving problems

[0015] The gist of the present invention is as follows.

[0016] [1] The chemical composition of the hot stamping steel sheet according to one embodiment of the present invention contains, in mass %, the following:

[0017] C: more than 0.40% and less than 0.70%,

[0018] Si: 0.010-1.30%,

[0019] Mn: 0.10~0.60%,

[0020] P: 0.100% or less,

[0021] S: 0.0100% or less,

[0022] N: 0.0140% or less,

[0023] O: 0.0200% or less,

[0024] Al: 0.0010~0.500%,

[0025] Cr: 0.010~0.80%,

[0026] Nb: 0~0.100%,

[0027] Ti: 0~0.100%,

[0028] B: 0~0.0100%,

[0029] Mo: 0-1.00%,

[0030] Co: 0-2.00%,

[0031] Ni: 0% or more and less than 3.00%,

[0032] Cu: 0-1.00%,

[0033] V: 0~1.00%

[0034] W: 0~1.000%,

[0035] Ca: 0~0.010%,

[0036] Mg: 0-1.000%,

[0037] REM: 0~1.000%

[0038] Sb: 0~1.000%,

[0039] Zr: 0~1.000%,

[0040] Sn: 0-1.000%, and

[0041] As: 0~0.100%,

[0042] The rest contains Fe and impurities.

[0043] The hot stamping steel sheet has the following metal structure:

[0044] Ferrite is made of {100} <011> ~{223} <110> The average value of the pole density of the formed orientation group is less than 10.0;

[0045] The proportion of the number of ferrite containing carbides having an equivalent circle diameter of 0.2 μm or more in the grains of the ferrite in the entire ferrite is 20% or more;

[0046] In terms of area ratio, pearlite accounts for 10 to 90%, and ferrite accounts for 10 to 90%.

[0047] [2] The hot stamping steel sheet according to [1], wherein the chemical composition may contain, in mass %, one or more elements selected from the group consisting of:

[0048] Nb: 0.001~0.100%,

[0049] Ti: 0.010~0.100%,

[0050] B: 0.0015~0.0100%,

[0051] Mo: 0.05-1.00%,

[0052] Co: 0.05-2.00%,

[0053] Ni: 0.01% or more and less than 3.00%,

[0054] Cu: 0.01-1.00%,

[0055] V: 0.01~1.00%,

[0056] W: 0.001~1.000%,

[0057] Ca: 0.001~0.010%,

[0058] Mg: 0.001~1.000%,

[0059] REM: 0.001~1.000%,

[0060] Sb: 0.005~1.000%,

[0061] Zr: 0.001~1.000%,

[0062] Sn: 0.001~1.000%, and

[0063] As: 0.001~0.100%.

[0064] [3] The chemical composition of the hot stamped product according to another embodiment of the present invention contains, in mass %, the following:

[0065] C: more than 0.40% and less than 0.70%,

[0066] Si: 0.010-1.30%,

[0067] Mn: 0.10~0.60%,

[0068] P: 0.100% or less,

[0069] S: 0.0100% or less,

[0070] N: 0.0140% or less,

[0071] O: 0.0200% or less,

[0072] Al: 0.0010~0.500%,

[0073] Cr: 0.010~0.80%,

[0074] Nb: 0~0.100%,

[0075] Ti: 0~0.100%,

[0076] B: 0~0.0100%,

[0077] Mo: 0-1.00%,

[0078] Co: 0-2.00%,

[0079] Ni: 0% or more and less than 3.00%,

[0080] Cu: 0-1.00%,

[0081] V: 0~1.00%

[0082] W: 0~1.000%,

[0083] Ca: 0~0.010%,

[0084] Mg: 0-1.000%,

[0085] REM: 0~1.000%

[0086] Sb: 0~1.000%,

[0087] Zr: 0~1.000%,

[0088] Sn: 0-1.000%, and

[0089] As: 0~0.100%,

[0090] The rest contains Fe and impurities.

[0091] The hot stamped body has the following metal structure: the average grain size of the prior austenite grains is 5 to 25 μm, the standard deviation of the grain size of the prior austenite grains is 0.1 to 2.0 μm,

[0092] The hot stamping formed body has a tensile strength of 2200 MPa or more.

[0093] [4] The hot stamped product according to [3], wherein the chemical composition may contain, in mass %, one or more elements selected from the group consisting of:

[0094] Nb: 0.001~0.100%,

[0095] Ti: 0.010~0.100%,

[0096] B: 0.0015~0.0100%,

[0097] Mo: 0.05-1.00%,

[0098] Co: 0.05-2.00%,

[0099] Ni: 0.01% or more and less than 3.00%,

[0100] Cu: 0.01-1.00%,

[0101] V: 0.01~1.00%,

[0102] W: 0.001~1.000%,

[0103] Ca: 0.001~0.010%,

[0104] Mg: 0.001~1.000%,

[0105] REM: 0.001~1.000%,

[0106] Sb: 0.005~1.000%,

[0107] Zr: 0.001~1.000%,

[0108] Sn: 0.001~1.000%, and

[0109] As: 0.001~0.100%.

[0110] [5] The hot stamped body according to [3] or [4], wherein the area ratio of the prior austenite grains having an average grain size of 0.5 to 3.0 μm may be 60% or less.

[0111] Effects of the Invention

[0112] According to the above aspects of the present invention, it is possible to provide a hot stamped product having high strength and excellent bendability, and a steel sheet for hot stamping that can produce the hot stamped product. DETAILED DESCRIPTION

[0113] The inventors of the present invention have studied the bendability of hot stamped parts. As a result, the inventors have recognized that the bendability of hot stamped parts deteriorates if a large number of fine prior austenite grains are present in the metal structure of the hot stamped parts. Furthermore, the inventors have recognized that the bendability of the hot stamped parts can be further improved by setting the prior austenite grains to a desired size in the metal structure of the hot stamped parts and suppressing the size variation of the prior austenite grains, that is, by granulating (sizing) the prior austenite grains.

[0114] Next, the inventors of the present invention studied the method for obtaining the hot stamped steel. As a result, the inventors of the present invention found that by setting the Mn content in the chemical composition of the hot stamping steel sheet to 0.60% or less, the ferrite in the metal structure is reduced from {100} <011> ~{223} <110> The extreme density of the formed orientation group is increased, and the number ratio of ferrite containing carbides in the grains is increased, so that the above-mentioned hot stamped body can be obtained.

[0115] Hereinafter, the hot stamping steel sheet and the hot stamped product of the present embodiment obtained based on the above findings will be described. First, the reasons for limiting the chemical composition of the hot stamping steel sheet of the present embodiment will be described.

[0116] In the following numerical ranges, the lower and upper limits are included in the ranges. Numerical values ​​expressed as "below" or "exceed" are not included in the numerical ranges. "%" in chemical compositions all indicate "mass %."

[0117] The chemical composition of the hot stamping steel sheet of this embodiment includes, by mass%, C: more than 0.40% and less than 0.70%, Si: 0.010% to 1.30%, Mn: 0.10% to 0.60%, P: less than 0.100%, S: less than 0.0100%, N: less than 0.0140%, O: less than 0.0200%, Al: 0.0010% to 0.500%, Cr: 0.010% to 0.80%, and the balance Fe and impurities. Each element is described below.

[0118] C: more than 0.40% and less than 0.70%

[0119] A high C content contributes to the improvement of the strength of hot stamped parts. When the C content is 0.40% or less, it becomes difficult to obtain sufficient strength in the hot stamped parts. Therefore, the C content is set to more than 0.40%. It is preferably 0.42% or more, more preferably 0.45% or more, and even more preferably 0.47% or more.

[0120] On the other hand, if the C content exceeds 0.70%, coarse carbides are formed, deteriorating the bendability of the hot stamped body. Therefore, the C content is set to 0.70% or less, preferably 0.65% or less, and more preferably 0.60% or less.

[0121] Si: 0.010~1.30%

[0122] Si is an element that improves the deformability of hot stamped parts by combining with oxygen to suppress the formation of oxides that can become the starting point of fracture. If the Si content is less than 0.010%, coarse oxides will form in the hot stamped parts, making it impossible to achieve the desired bendability. Therefore, the Si content is set to 0.010% or more. It is preferably 0.05% or more, and more preferably 0.10% or more.

[0123] On the other hand, if the Si content exceeds 1.30%, coarse oxides are generated, deteriorating the bendability of the hot stamped part. Therefore, the Si content is set to 1.30% or less, preferably less than 1.00%, and more preferably 0.50% or less.

[0124] Mn: 0.10~0.60%

[0125] Mn stabilizes austenite and improves the hardenability of the steel sheet. If the Mn content is less than 0.10%, sufficient hardenability cannot be achieved. Therefore, the Mn content is set to 0.10% or more, preferably 0.20% or more, and more preferably 0.30% or more.

[0126] On the other hand, if the Mn content exceeds 0.60%, cracking due to Mn segregation becomes more likely to occur if the manufacturing process is not properly controlled, and excellent bendability cannot be achieved in the hot stamped part. Therefore, the Mn content is set to 0.60% or less. It is preferably 0.55% or less, and more preferably 0.50% or less.

[0127] P: 0.100% or less

[0128] P segregates at the grain boundaries of the steel sheet, degrading the bendability of the hot stamped product. Therefore, a lower P content is preferred. In particular, if the P content exceeds 0.100%, the workability of the steel sheet and the bendability of the hot stamped product significantly deteriorate. Therefore, the P content is set to 0.100% or less. It is preferably 0.080% or less, and more preferably 0.020% or less.

[0129] The lower limit of the P content is not particularly limited, but may be 0%. However, if the P content is reduced to less than 0.0001%, the cost of P removal increases significantly, which is not economically desirable. Therefore, the P content may be set to 0.0001% or more.

[0130] S: 0.0100% or less

[0131] S forms coarse inclusions, which can degrade the bendability of hot stamped parts. Therefore, a lower S content is preferred. In particular, if the S content exceeds 0.0100%, the formability of the steel sheet and the bendability of the hot stamped parts significantly deteriorate. Therefore, the S content is set to 0.0100% or less. It is preferably 0.0050% or less, and more preferably 0.0010% or less.

[0132] The lower limit of the S content is not particularly limited, but may be 0%. However, if the S content is reduced to less than 0.0001%, the cost of removing the S increases significantly, which is not economically desirable. Therefore, the S content may be set to 0.0001% or more.

[0133] N: 0.0140% or less

[0134] Nitrogen forms coarse nitrides, which can degrade the bendability of hot stamped parts. Therefore, a lower N content is preferred. In particular, if the N content exceeds 0.0140%, the formability of the steel sheet significantly deteriorates. Therefore, the N content is set to 0.0140% or less. It is preferably 0.0100% or less or 0.0070% or less, and more preferably 0.0040% or less.

[0135] The lower limit of the N content is not particularly limited, but may be 0%. However, if the N content is reduced to less than 0.0001%, the cost of N removal increases significantly, which is not economically preferable. Therefore, the N content may be set to 0.0001% or more.

[0136] O: 0.0200% or less

[0137] O forms coarse oxides in steel, degrading the bendability of hot-stamped parts. Therefore, a lower O content is preferred. In particular, if the O content exceeds 0.0200%, the bendability of the hot-stamped parts significantly deteriorates. Therefore, the O content is set to 0.0200% or less. It is preferably 0.0150% or less, more preferably 0.0100% or less, and even more preferably 0.0060% or less.

[0138] The lower limit of the O content is not particularly limited, but may be 0%. However, if the O content is reduced to less than 0.0001%, the manufacturing cost will increase significantly, which is not economically preferable. Therefore, the O content may be set to 0.0001% or more.

[0139] Al: 0.0010~0.500%

[0140] Al is an element that deoxidizes molten steel, suppressing the formation of oxides that can cause fracture, thereby improving deformability and the bendability of hot-stamped parts. If the Al content is less than 0.0010%, deoxidation will not be sufficient, resulting in the formation of coarse oxides and the failure to achieve the aforementioned effects. Therefore, the Al content is set to 0.0010% or more. It is preferably 0.010% or more, and more preferably 0.030% or more.

[0141] On the other hand, if the Al content exceeds 0.500%, coarse oxides will form in the steel, reducing the bendability of the hot stamped body. Therefore, the Al content is set to 0.500% or less, preferably 0.450% or less, and more preferably 0.350% or less.

[0142] Cr: 0.010~0.80%

[0143] Cr improves the strength of hot stamped parts by dissolving into the prior austenite grains during hot stamping. This effect is not achieved when the Cr content is less than 0.010%. Therefore, the Cr content is set to 0.010% or more. It is preferably 0.10% or more, and more preferably 0.20% or more.

[0144] On the other hand, if the Cr content exceeds 0.80%, coarse carbides are formed, deteriorating the bendability of the hot stamped body. Therefore, the Cr content is set to 0.80% or less, preferably 0.60% or less, and more preferably 0.40% or less.

[0145] The remainder of the chemical composition of the hot stamping steel sheet of this embodiment may be Fe and impurities. Examples of impurities include elements that are inevitably mixed in from steel raw materials or scrap and / or during the steelmaking process, or elements that are allowed within a range that does not hinder the characteristics of the hot stamped body of this embodiment.

[0146] The hot stamping steel sheet of the present embodiment may contain the following elements as optional elements in place of a portion of Fe. When the following optional elements are not contained, the content is 0%.

[0147] Nb: 0~0.100%

[0148] Nb forms carbonitrides in steel and improves the strength of hot stamped parts through precipitation strengthening. To achieve this effect, the Nb content is preferably set to 0.001% or more.

[0149] On the other hand, if the Nb content exceeds 0.100%, a large amount of carbonitrides are generated in the steel, and the bendability of the hot stamped body is reduced. Therefore, the Nb content is set to 0.100% or less.

[0150] Ti: 0~0.100%

[0151] Like Nb, Ti forms carbonitrides in steel, thereby increasing the strength of the hot stamped product through precipitation strengthening. To achieve this effect, the Ti content is preferably set to 0.010% or more.

[0152] On the other hand, if the Ti content exceeds 0.100%, a large amount of carbonitrides are generated in the steel, and the bendability of the hot stamped body is reduced. Therefore, the Ti content is set to 0.100% or less.

[0153] B: 0~0.0100%

[0154] B improves the hardenability of steel and thus increases the strength of hot stamped products. To achieve this effect, the B content is preferably set to 0.0015% or more.

[0155] On the other hand, if the B content exceeds 0.0100%, coarse carbides are generated, and the bendability of the hot stamped body deteriorates. Therefore, the B content is set to 0.0100% or less.

[0156] Mo: 0~1.00%

[0157] Mo improves the hardenability of the steel sheet and thus increases the strength of the hot stamped body. To achieve this effect, the Mo content is preferably set to 0.05% or more.

[0158] On the other hand, if the Mo content exceeds 1.00%, coarse carbides are generated, and the bendability of the hot stamped body deteriorates. Therefore, the Mo content is set to 1.00% or less.

[0159] Co: 0-2.00%

[0160] Co improves the hardenability of the steel sheet and thus increases the strength of the hot stamped body. To reliably exhibit this effect, the Co content is preferably set to 0.05% or more.

[0161] On the other hand, if the Co content exceeds 2.00%, coarse carbides are generated, and the bendability of the hot stamped body deteriorates. Therefore, the Co content is set to 2.00% or less.

[0162] Ni: 0% or more and less than 3.00%

[0163] Ni improves the hardenability of the steel sheet and thus increases the strength of the hot stamped product. To achieve this effect, the Ni content is preferably set to 0.01% or more.

[0164] On the other hand, if the Ni content is 3.00% or more, segregation is promoted and the bendability of the hot stamped body is deteriorated. Therefore, the Ni content is set to less than 3.00%.

[0165] Cu: 0-1.00%

[0166] Cu, like Ni, improves the hardenability of the steel sheet and thus increases the strength of the hot stamped product. To achieve this effect, the Cu content is preferably set to 0.01% or more.

[0167] On the other hand, if the Cu content exceeds 1.00%, segregation is promoted and the bendability of the hot stamped body is deteriorated. Therefore, the Cu content is set to 1.00% or less.

[0168] V: 0~1.00%

[0169] V improves the hardenability of the steel sheet and thus increases the strength of the hot stamped product. To achieve this effect, the V content is preferably set to 0.01% or more.

[0170] On the other hand, if the V content exceeds 1.00%, carbonitrides precipitate in large quantities, deteriorating the bendability of the hot stamped body. Therefore, the V content is set to 1.00% or less.

[0171] W: 0~1.000%

[0172] W improves the hardenability of the steel sheet and thus increases the strength of the hot stamped product. To achieve this effect, the W content is preferably set to 0.001% or more.

[0173] On the other hand, if the W content exceeds 1.000%, segregation is promoted and the bendability of the hot stamped body is deteriorated. Therefore, the W content is set to 1.000% or less.

[0174] Ca: 0~0.010%

[0175] Ca improves deformability by suppressing the formation of oxides that become the starting point of fracture, thereby improving the bendability of the hot stamped body. To achieve this effect, the Ca content is preferably set to 0.001% or more.

[0176] On the other hand, if the Ca content exceeds 0.010%, coarse oxides are generated, and the bendability of the hot stamped body deteriorates. Therefore, the Ca content is set to 0.010% or less.

[0177] Mg: 0~1.000%

[0178] Mg improves deformability by suppressing the formation of oxides that become the starting point of fracture, thereby improving the bendability of the hot stamped body. To achieve this effect, the Mg content is preferably set to 0.001% or more.

[0179] On the other hand, if the Mg content exceeds 1.000%, coarse oxides are generated, and the bendability of the hot stamped body deteriorates. Therefore, the Mg content is set to 1.000% or less.

[0180] REM: 0~1.000%

[0181] REM improves deformability and bendability of hot stamped parts by suppressing the formation of oxides that become the starting point of fracture. To achieve this effect, the REM content is preferably set to 0.001% or more.

[0182] On the other hand, if the REM content exceeds 1.000%, coarse oxides are generated, and the bendability of the hot stamped body deteriorates. Therefore, the REM content is set to 1.000% or less.

[0183] In addition, in this embodiment, REM refers to a total of 17 elements including Sc, Y, and lanthanoid elements, and the content of REM refers to the total content of these elements.

[0184] Sb: 0~1.000%

[0185] Sb suppresses the formation of oxides that become the starting point of fracture, thereby improving the deformability and the bendability of the hot stamped body. To achieve this effect, the Sb content is preferably set to 0.005% or more.

[0186] On the other hand, if the Sb content exceeds 1.000%, coarse oxides are generated, and the bendability of the hot stamped body deteriorates. Therefore, the Sb content is set to 1.000% or less.

[0187] Zr: 0~1.000%

[0188] Zr suppresses the formation of oxides that become the starting point of fracture, thereby improving the deformability and the bendability of the hot stamped body. To achieve this effect, the Zr content is preferably set to 0.001% or more.

[0189] On the other hand, if the Zr content exceeds 1.000%, coarse oxides are generated, and the bendability of the hot stamped body deteriorates. Therefore, the Zr content is set to 1.000% or less.

[0190] Sn: 0~1.000%

[0191] Sn improves deformability and bendability of hot stamped parts by suppressing the formation of oxides that become fracture starting points. To reliably achieve this effect, the Sn content is preferably set to 0.001% or more.

[0192] On the other hand, even if Sn is contained in a large amount, the above-mentioned effect is saturated, so the Sn content is made 1.000% or less.

[0193] As: 0~0.100%

[0194] As lowers the austenite single-phase transformation temperature, thereby refining the prior austenite grains and improving the bendability of the hot stamped body. To reliably achieve this effect, the As content is preferably set to 0.001% or more.

[0195] On the other hand, even if a large amount of As is contained, the above-mentioned effect is saturated, so the As content is made 0.100% or less.

[0196] The chemical composition of the hot stamping steel sheet can be measured using standard analytical methods. For example, ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry) can be used. Furthermore, C and S can be measured using the combustion-infrared absorption method, N using the inert gas fusion-thermal conductivity method, and O using the inert gas fusion-non-dispersive infrared absorption method. If the hot stamping steel sheet has a coating on its surface, the coating can be removed by mechanical grinding before chemical composition analysis.

[0197] Next, the metal structure of the hot stamping steel sheet according to this embodiment will be described.

[0198] The hot stamping steel sheet of the present embodiment has the following metal structure: ferrite composed of {100} <011> ~{223} <110> The average value of the pole density of the formed orientation group is 10.0 or less, the number ratio of ferrite containing carbides with an equivalent circle diameter of 0.2 μm or more in the grains of the total ferrite is 20% or more, and the area ratio of pearlite is 10 to 90%, and that of ferrite is 10 to 90%. Each of the provisions is explained below.

[0199] It should be noted that in this embodiment, the metal structure at a depth 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) in a plate thickness cross section parallel to the rolling direction is defined. This is because the metal structure at this position represents the representative metal structure of the steel plate.

[0200] "Ferrite is made of {100} <011> ~{223} <110> The average value of the pole density of the formed orientation group is less than 10.0"

[0201] If the ferrite is composed of {100} <011> ~{223} <110> If the average value of the pole density of the formed orientation group exceeds 10.0, the average grain size of the prior austenite in the hot stamped body cannot be controlled to a predetermined value, and a hot stamped body with excellent bendability cannot be obtained. <011> ~{223} <110> The average value of the pole density of the formed orientation group is preferably 9.0 or less, more preferably 7.0 or less, further preferably 6.0 or less, and further preferably 5.0 or less. <011> ~{223} <110> The lower limit of the pole density of the orientation group to be formed is not particularly limited, but may be set to 0.1 or more.

[0202] It should be noted that in {100} <011> ~{223} <110> The orientation group formed includes {100} <011> 、{116} <110> 、{114} <110> 、{112} <110> 、{223} <110> crystal orientation.

[0203] Method for determining extreme density

[0204] Ferrite is made of {100} <011> ~{223} <110> The extreme density of the formed orientation group can be obtained as follows: using a device composed of a scanning electron microscope and an EBSD analyzer and OIM Analysis (registered trademark) manufactured by TSL, the orientation data is measured by the EBSD (Electron Back Scattering Diffraction) method. The orientation data obtained is calculated using spherical harmonic functions to calculate the crystal orientation distribution function (ODF) representing the three-dimensional texture, and the orientation distribution function is obtained from this crystal orientation distribution function. In order to observe the position at a depth of 1 / 4 of the plate thickness from the surface, the measurement area is set to the area from 1 / 8 of the plate thickness to 3 / 8 of the plate thickness from the surface. The measurement pitch is set to 5μm / step.

[0205] It should be noted that {hkl} represents the crystal plane parallel to the rolling surface. <uvw>represents the crystal direction parallel to the rolling direction. That is, {hkl} <uvw>Indicates that {hkl} is toward the normal direction of the plate surface, <uvw>Crystals oriented in the rolling direction.

[0206] "The proportion of ferrite containing carbides with an equivalent circle diameter of 0.2 μm or more in its grains is 20% or more of the total ferrite"

[0207] If the number ratio of ferrite containing carbides with an equivalent circle diameter of 0.2 μm or more within the grains of the total ferrite is less than 20%, the prior austenite grains cannot be granulated in the hot stamped body, resulting in a hot stamped body with excellent bendability. By setting the number ratio of ferrite containing carbides with an equivalent circle diameter of 0.2 μm or more within the grains of the total ferrite to 20% or more, the carbides within the grains ideally function as the starting points of prior austenite grains during heating before hot stamping. It is presumed that as a result, the prior austenite grains are uniformly dispersed and granulated in the metal structure of the hot stamped body. The number ratio of ferrite containing carbides with an equivalent circle diameter of 0.2 μm or more within the grains of the total ferrite is preferably 40% or more, preferably 50% or more, and even more preferably 60% or more. The upper limit of the number ratio of ferrite containing carbides having an equivalent circle diameter of 0.2 μm or more in grains in the entire ferrite is not particularly specified, but may be set to 90% or less.

[0208] Method for measuring the number ratio of ferrite containing carbides

[0209] From any position of the hot stamping steel plate that is more than 50 mm away from the end face (if the sample cannot be collected from this position, avoid the position of the end), collect the sample in such a way that the plate thickness section parallel to the rolling direction becomes the observation surface. Then, the observation surface is fine-machined by electric field grinding. After that, in order to be able to observe the position at a depth of 1 / 4 of the plate thickness from the surface, the area at a depth of 1 / 8 of the plate thickness from the surface to 3 / 8 of the plate thickness from the surface is observed for more than 10 fields of view at a magnification of 20,000 times. For the grains identified as ferrite by the metal structure measurement method described later, the equivalent circle diameter of each carbide is calculated from the area of ​​each carbide observed in the ferrite grains by image analysis. Calculate the number of ferrite grains containing carbides with an equivalent circle diameter of 0.2 μm or more in all the observed ferrite grains. The obtained value is divided by the number of all ferrite grains and multiplied by 100 to obtain the ratio of the number of ferrite grains containing carbides having an equivalent circle diameter of 0.2 μm or more.

[0210] In addition, in this embodiment, particles having an equivalent circle diameter of 0.2 to 30 μm are regarded as carbides.

[0211] "Pearlite is 10 to 90 area %"

[0212] "Ferrite is 10 to 90 area %"

[0213] If the area fraction of ferrite is less than 10% and the area fraction of pearlite exceeds 90%, pearlite will preferentially become the starting point of prior austenite during the hot stamping process, and the granulation effect of the prior austenite grains will not be achieved. Therefore, the area fraction of ferrite is set to 10% or more, and the area fraction of pearlite is set to 90% or less. The area fraction of ferrite is preferably 20% or more, more preferably 40% or more. The area fraction of pearlite is preferably 80% or less, more preferably 60% or less.

[0214] On the other hand, if the area fraction of ferrite exceeds 90% and the area fraction of pearlite is less than 10%, carbon becomes too concentrated in the pearlite, and the temperature for transformation to austenite decreases. As a result, the transformation begins at a low temperature during the hot stamping process, and the prior austenite grains tend to coarsen, making it impossible to achieve the granulation effect of the prior austenite grains. Therefore, the area fraction of ferrite is set to 90% or less, and the area fraction of pearlite is set to 10% or more. The area fraction of ferrite is preferably 70% or less, and more preferably 60% or less. The area fraction of pearlite is preferably 30% or more, and more preferably 40% or more.

[0215] In the metal structure of the hot stamping steel sheet of this embodiment, the remainder of the structure is one or more of martensite, lower bainite, retained austenite, and tempered martensite. The area ratio of the remainder of the structure may be set to 20% or less.

[0216] Method for measuring the metal structure of steel sheets for hot stamping

[0217] Samples are cut from any position of the hot stamping steel sheet at least 50 mm from the end face (if sampling from this position is not possible, avoid the end face) so that a cross section parallel to the rolling direction can be observed. The sample size varies depending on the measuring device, but is set to a size that allows observation of approximately 10 mm in the rolling direction.

[0218] After the cross section of the above sample was ground using silicon carbide paper of #600 to #1500, it was finished into a mirror surface using a liquid obtained by dispersing diamond powder having a particle size of 1 to 6 μm in a diluent such as alcohol or pure water, and fine grinding was performed by electrolytic grinding. Next, in order to be able to observe a depth position of 1 / 4 of the thickness of the plate from the surface, an area with a length of 50 μm and a depth of 1 / 8 of the thickness of the plate from the surface to a depth of 3 / 8 of the thickness of the plate from the surface at any position in the longitudinal direction of the sample cross section was observed using a device consisting of a thermal field emission scanning electron microscope (JSM-7001F manufactured by JEOL) and an EBSD detector (DVC5 type detector manufactured by TSL). The scanning electron microscope used is set to a microscope equipped with two electron detectors. At 9.6×10 -5 The sample was irradiated with electron beams at an acceleration voltage of 15 kV and an irradiation current level of 13 in a vacuum of less than Pa, and electron images were taken twice using a scanning electron microscope.

[0219] In the obtained photographs, the area where cementite precipitates in the form of lamellae within the grains is judged to be pearlite. The area ratio of pearlite is obtained by calculating the area ratio of the area judged to be pearlite. The lath-shaped grains are judged to be lower bainite, martensite and tempered martensite. Next, an EBSD analysis device is used for this field of view at an analysis speed of 200 to 300 points per second. The area ratio of ferrite is calculated using the "Grain Average Misorientation" function in the software "OIM Analysis (registered trademark)" attached to the EBSD analysis device. With this function, for grains with a body-centered structure, after calculating the orientation difference between adjacent measurement points, the average value of all measurement points within the grain can be obtained. For the crystal orientation information obtained by EBSD analysis, the area surrounded by grain boundaries with an average crystal orientation difference of 5° or more is defined as a grain, and a distribution map is drawn using the "Grain Average Misorientation" function. Excluding the regions identified as pearlite, lower bainite, martensite, and tempered martensite from the distribution diagram, regions with an average intra-grain crystal orientation difference of less than 5.0° were identified as ferrite. The area ratio of the regions identified as ferrite was calculated to obtain the area ratio of ferrite.

[0220] The hot stamping steel sheet of this embodiment may also be coated on the surface for the purpose of improving corrosion resistance after hot stamping. The coating may be any of an electroplated layer and a hot-dip coated layer. The electroplated layer may include, for example, an electrogalvanized layer, an electroplated Zn-Ni alloy layer, etc. The hot-dip coated layer may include, for example, a hot-dip galvanized layer, an alloyed hot-dip galvanized layer, a hot-dip aluminum layer, a hot-dip Zn-Al alloy layer, a hot-dip Zn-Al-Mg alloy layer, a hot-dip Zn-Al-Mg-Si alloy layer, etc. The coating weight is not particularly limited and may be a general weight.

[0221] The plate thickness of the hot stamping steel sheet of the present embodiment is not particularly limited, but is preferably set to 0.5 to 3.5 mm from the viewpoint of reducing the weight of a vehicle body.

[0222] Next, the hot stamped body of the present embodiment obtained by hot stamping the above-mentioned hot stamping steel sheet will be described. The hot stamped body of the present embodiment has the same chemical composition as the above-mentioned hot stamping steel sheet. The method for measuring the chemical composition may be the same method as that for the hot stamping steel sheet. In addition, the hot stamped body of the present embodiment has the original austenite grains granulated in the metal structure. That is, the hot stamped body of the present embodiment has the following metal structure: the average grain size of the original austenite grains is 5 to 25 μm, and the standard deviation of the grain size of the above-mentioned original austenite grains is 0.1 to 2.0 μm.

[0223] It should be noted that in this embodiment, the metal structure at a depth 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) of a cross section perpendicular to the plate surface is defined. This is because the metal structure at this location represents a representative metal structure of a hot stamped part. The metal structure is described below.

[0224] "The average grain size of the original austenite grains is 5 to 25 μm"

[0225] "The standard deviation of the grain size of the prior austenite grains is 0.1 to 2.0 μm."

[0226] In the metal structure of the hot stamped body, the bendability of the hot stamped body can be improved by setting the average grain size of the prior austenite grains to 5 to 25 μm and the standard deviation of the grain size of the prior austenite grains to 0.1 to 2.0 μm. If the average grain size of the prior austenite grains or the standard deviation of the grain size of the prior austenite grains are outside the above range, excellent bendability cannot be achieved in the hot stamped body.

[0227] The average grain size of the prior austenite grains is preferably set to 10 μm or more, more preferably 15 μm or more, and preferably 20 μm or less.

[0228] By setting the standard deviation of the prior austenite grain size to 2.0 μm or less, excellent bendability can be achieved in the hot stamped body. Therefore, the standard deviation of the prior austenite grain size is set to 2.0 μm or less. It is more preferably 1.2 μm or less, even more preferably 1.1 μm or less, and even more preferably 0.4 μm or less.

[0229] In practice, it is difficult to set the standard deviation of the grain size of prior austenite grains to less than 0.1 μm, so the actual lower limit is 0.1 μm or more.

[0230] If the area ratio of prior austenite grains with an average grain size of 0.5 to 3.0 μm is 60% or less, better bendability can be achieved in the hot stamped body. Therefore, the area ratio of prior austenite grains with an average grain size of 0.5 to 3.0 μm can be set to 60% or less. It is more preferably 50% or less, and even more preferably 40% or less.

[0231] Method for determining the average grain size and standard deviation of prior austenite grains

[0232] Next, the method for measuring the average crystal grain size of the original austenite grains is described. From any position of the hot stamped body that is more than 50 mm away from the end face (if the sample cannot be collected from this position, avoid the position of the end), cut the sample in such a way that the plate thickness section parallel to the rolling direction can be observed. Although the size of the sample varies depending on the measuring device, it is set to a size of about 10 mm that can be observed in the rolling direction. After the cross section of the above sample is ground using silicon carbide paper of #600 to #1500, it is finished into a mirror surface 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, and fine grinding is performed using electrolytic grinding.

[0233] Next, in order to observe the position at a depth of 1 / 4 of the plate thickness from the surface, an apparatus consisting of a thermal field emission scanning electron microscope (JEOL JSM-7001F) and an EBSD detector (TSL DVC5 detector) was used to observe the region of 100 μm in length and 100 μm in the plate thickness direction at a depth of 1 / 8 to 3 / 8 of the plate thickness from the surface at any position in the longitudinal direction of the sample cross section. -5 In a vacuum of less than 1 Pa, the sample was irradiated with electron beams at an accelerating voltage of 15 kV and an irradiation current level of 13, with EBSD analysis performed at an analysis rate of 200 to 300 points / second. The resulting crystal orientation information was used to calculate the crystal orientation of the prior austenite grains based on the crystal orientation relationship between normal prior austenite grains and the transformed body-centered structured grains. This information was then used to calculate the average grain size of the prior austenite grains.

[0234] The method for calculating the crystal orientation of the original austenite grains is not particularly limited, but for example, it can be calculated by the following method. First, the crystal orientation of the original austenite grains is calculated by the method described in non-patent literature 1, and the crystal orientation of the original austenite in each coordinate of the region measured by EBSD is determined. Then, the "Inverse Pole Figure" function carried in the software "OIM Analysis (registered trademark)" attached to the EBSD analysis device is used to make a crystal orientation distribution diagram of the original austenite grains. For one original austenite grain contained in the observation field, the average value of the shortest diameter and the longest diameter is calculated, and its average value is used as the particle size of the original austenite grain. Except for the original austenite grains that are not included in the shooting field as a whole, the above operation is performed on all original austenite grains to obtain the particle size of all original austenite grains in the shooting field. The average grain size of the prior austenite grains in the captured field of view is calculated by dividing the sum of the obtained prior austenite grain sizes by the total number of prior austenite grains whose grain sizes were measured. This operation is repeated for each captured field of view, and the average grain size of the prior austenite grains in all captured fields of view is calculated to obtain the average grain size of the prior austenite grains.

[0235] The standard deviation of the prior austenite grain size is calculated by calculating the standard deviation from the prior austenite grain size. In order to eliminate the influence of locally generated fine grains and coarse grains, the standard deviation is calculated by excluding the minimum and maximum values ​​of the prior austenite grain size.

[0236] The area ratio of prior austenite grains having an average grain size of 0.5 to 3.0 μm was obtained by dividing the area of ​​prior austenite grains having an average grain size of 0.5 to 3.0 μm by the area of ​​the entire measurement field.

[0237] The metal structure of a hot stamped body is not particularly limited as long as the desired strength and bendability are achieved after hot stamping. However, for example, it may include, by area%, ferrite: 0-50%, bainite and martensite: 0-100%, pearlite: 0-30%, and retained austenite: 0-5%. The metal structure of a hot stamped body can be measured by the following method.

[0238] Method for measuring the metal structure of hot stamped parts

[0239] From any position of the hot stamped body at a distance of 50 mm or more from the end face (if the sample cannot be collected from this position, avoid the position of the end), cut the sample in such a way that a cross section perpendicular to the plate surface can be observed. The cross section of the sample is polished using silicon carbide paper of #600 to #1500, and then finished into a mirror surface 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, and nitric alcohol etching is performed. In order to observe the position at a depth of 1 / 4 of the plate thickness from the surface, a thermal field emission scanning electron microscope (JEOL JSM-7001F) is used to take photographs of multiple fields of view at any position in the longitudinal direction of the sample cross section, with a length of 100 μm and a depth of 1 / 8 of the plate thickness from the surface to a depth of 3 / 8 of the plate thickness from the surface. A grid of equal intervals is drawn on the photograph, and the structure at the grid point is identified. The area ratio of each structure is obtained by calculating the number of grid points corresponding to each structure and dividing it by the total number of grid points. The greater the total number of grid points, the more accurately the area ratio can be determined. In this embodiment, the grid pitch is set to 2 μm×2 μm, and the total number of grid points is set to 1500 points.

[0240] Regions where cementite precipitated in lamellar form within the grains were judged to be pearlite. Regions with low brightness and no underlying structure were judged to be ferrite. Regions with high brightness and no underlying structure revealed by etching were judged to be martensite and retained austenite. Regions that did not meet any of the above criteria were judged to be bainite.

[0241] The area ratio of martensite is obtained by subtracting the area ratio of retained austenite determined by the EBSD analysis described later from the area ratios of martensite and retained austenite determined from the above-mentioned photographs.

[0242] The area ratio of retained austenite is measured by backscattered electron diffraction (EBSD). Regarding the analysis using EBSD, a sample collected at the same sample collection position as the measurement using the above-mentioned photographs is used, and the area from the surface to the surface is 1 / 8 of the plate thickness to 3 / 8 of the plate thickness. The sample is set as the following sample: after being polished using silicon carbide paper of #600 to #1500, it is finished into a mirror surface 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, and then finished by electrolytic polishing in order to fully remove the strain of the measured cross section. It should be noted that, with respect to electrolytic polishing, in order to remove the mechanical polishing strain of the observation surface, a minimum of 20 μm polishing is sufficient, and a maximum of 50 μm polishing is sufficient. If the collapse of the end is considered, it is preferably less than 30 μm.

[0243] Regarding the measurement using EBSD, the acceleration voltage is set to 15-25 kV, and the measurement is performed at intervals of at least 0.25 μm or less, obtaining crystal orientation information for each measurement point within a range of 150 μm or more in the plate thickness direction and 250 μm or more in the rolling direction. Among the obtained crystal structures, the "Phase Map" function in the "OIMAnalysis (registered trademark)" software attached to the EBSD analysis device is used to determine that the crystal structure is fcc as retained austenite. By calculating the ratio of the measurement points determined to be retained austenite, the area ratio of retained austenite is obtained. Here, the more measurement points, the better, so it is better to have a narrow measurement interval and a wide measurement range. However, when the measurement interval is less than 0.01 μm, adjacent points interfere with each other in the diffusion width of the electron beam. Therefore, the measurement interval is set to 0.01 μm or more. In addition, the maximum measurement range is set to 200 μm in the plate thickness direction and 400 μm in the plate width direction. In addition, an EBSD device consisting of a thermal field emission scanning electron microscope (JEOL JSM-7001F) and an EBSD detector (TSL DVC5 detector) was used for the measurement. The vacuum degree in the device was set to 9.6×10 -5 Pa or less, the irradiation current level is set to 13, and the electron beam irradiation level is set to 62.

[0244] The hot stamping formed body of this embodiment may also be coated on the surface for the purpose of improving corrosion resistance after hot stamping. The coating may be any one of an electroplating layer and a hot-dip coating. The electroplating layer includes, for example, an electrogalvanized layer, an electroplated Zn-Ni alloy layer, etc. The hot-dip coating includes, for example, a hot-dip galvanized layer, an alloyed hot-dip galvanized layer, a hot-dip aluminum layer, a hot-dip Zn-Al alloy layer, a hot-dip Zn-Al-Mg alloy layer, a hot-dip Zn-Al-Mg-Si alloy layer, etc. The coating weight is not particularly limited and may be a general weight.

[0245] The plate thickness of the hot stamped body of the present embodiment is not particularly limited, but is preferably set to 0.5 to 3.5 mm from the viewpoint of reducing the weight of the vehicle body.

[0246] The tensile (maximum) strength of the hot stamped part of this embodiment is 2200 MPa or greater. It is preferably 2400 MPa or greater, and more preferably 2550 MPa or greater. The tensile strength is determined using the test method described in JIS Z 2241:2011 using a No. 5 test piece prepared from a position as flat as possible on the hot stamped part.

[0247] Furthermore, the hot stamped body of the present embodiment preferably has a maximum bending angle of 20° or greater obtained by a bending test based on the VDA standard (VDA 238-100) defined by the German Association of the Automotive Industry. The bending test conditions are set as follows.

[0248] Test piece size: 60 mm (rolling direction) x 30 mm (parallel to the plate width direction)

[0249] Test piece thickness: 1.6mm

[0250] Bending ridge: parallel to the width of the plate

[0251] Test method: Roller support, punch press

[0252] Roller diameter: φ30mm

[0253] Punch shape: Front end R = 0.4mm

[0254] Distance between rollers: 2.0×plate thickness (mm)+0.5mm

[0255] Pressing speed: 20mm / min

[0256] Testing machine: SHIMADZU AUTOGRAPH 20kN

[0257] Next, a method for producing a hot stamping steel sheet according to the present embodiment will be described.

[0258] In the method for producing a hot stamping steel sheet according to the present embodiment, in order to obtain a hot stamping steel sheet having the above-described metal structure, the rolling reduction rate in the first pass of the final pass of finish rolling in hot rolling is set high.

[0259] The steel slab (steel material) to be subjected to hot rolling may be a steel slab produced by a conventional method, for example, a steel slab produced by a common method such as continuous casting of a slab or a thin slab continuous caster.

[0260] In hot rolling, rough rolling and finish rolling are performed. In finish rolling, the slab after rough rolling is rolled by a plurality of finish rolling mills. In the present embodiment, the first pass of the final pass of finish rolling is performed in a temperature range of 900 to 1050°C at a reduction rate of 10 to 25%. After this rolling, the final pass is performed in a temperature range of 850°C to less than 1000°C at a reduction rate (final reduction rate) of 6% or more. When the plate thickness before the first pass of the final pass is set to t0 and the plate thickness after the first pass of the final pass is set to t1, the reduction rate of the first pass of the final pass can be expressed by {(t0-t1) / t0}×100(%). When the plate thickness before the final pass of finish rolling is t1 and the plate thickness after the final pass of finish rolling is t2, the final reduction ratio can be expressed by {(t1-t2) / t1}×100(%).

[0261] By setting the reduction ratio of the rolling pass before the final pass to 10-25%, the dislocations in the austenite are reduced, and by setting the reduction ratio of the subsequent final pass (final reduction ratio) to 6% or more, a small amount of dislocations can be introduced into the austenite grains. It is speculated that since the dislocations introduced into the austenite grains function as the precipitation starting points of carbides, as a result, the desired amount of ferrite containing carbides in the grains can be formed. It is speculated that the dislocations in the austenite before the final rolling merge with the dislocations introduced in the final pass and disappear. Therefore, if the reduction ratio of the rolling pass before the final pass is not controlled within the above range, the precipitation starting points of carbides are reduced.

[0262] Typically, in finish rolling, the reduction ratio is gradually reduced with each pass. However, in this embodiment, in the first pass before the final pass of finish rolling, the reduction ratio is increased compared to the previous passes (two passes before the final pass), and rolling is performed at the above reduction ratio. This allows the desired metal structure to be obtained.

[0263] If the rolling reduction in the first pass before the final pass is less than 10% or exceeds 25%, the recrystallization of austenite in the final pass is suppressed, and the desired texture cannot be obtained. The rolling reduction in the first pass before the final pass is preferably 13% or more, more preferably 16% or more, and even more preferably 18% or more.

[0264] If the rolling temperature before the final pass is lower than 900°C, the recrystallization of austenite in the final pass is suppressed, and the desired texture cannot be obtained. The rolling temperature before the final pass is preferably 910°C or higher, more preferably 930°C or higher.

[0265] On the other hand, if the rolling temperature in the first pass before the final pass exceeds 1050°C, the austenite grains coarsen, which inhibits the ferrite transformation and prevents the required amount of ferrite from being obtained in the hot stamping steel sheet. The rolling temperature in the first pass before the final pass is preferably 1040°C or lower, and more preferably 1020°C or lower.

[0266] If the final reduction (final reduction) is less than 6%, the introduced dislocations decrease, making it impossible to control the number ratio of ferrite containing carbides with an equivalent circle diameter of 0.2 μm or greater within the grains to the specified level. The final reduction is preferably 8% or greater, more preferably 10% or greater, and even more preferably 12% or greater. The upper limit of the final reduction is not particularly specified, but may be set to less than 40%.

[0267] If the final rolling temperature is lower than 850°C, the austenite grains become too fine, the ferrite transformation is excessively accelerated, and the required amount of pearlite cannot be obtained in the hot stamped steel sheet. The final rolling temperature is preferably 860°C or higher, more preferably 870°C or higher.

[0268] On the other hand, if the final rolling temperature is 1000°C or higher, the austenite grains coarsen, the ferrite transformation is inhibited, and the required amount of ferrite cannot be obtained in the hot stamped steel sheet. The final rolling temperature is preferably 980°C or lower, more preferably 960°C or lower.

[0269] The heating temperature and holding time of the steel slab before hot rolling are not particularly limited, but are preferably held in a temperature range of 1200° C. or higher for 20 minutes or longer.

[0270] After finish rolling, coiling is preferably performed in the temperature range of 400-750°C. If the coiling temperature is lower than 400°C, the area ratio of pearlite in the hot stamping steel sheet exceeds 90% and the area ratio of ferrite falls below 10%. The coiling temperature is preferably 450°C or higher, and more preferably 530°C or higher.

[0271] On the other hand, if the coiling temperature exceeds 750°C, the area ratio of pearlite in the hot stamping steel sheet becomes less than 10% and the area ratio of ferrite becomes more than 90%. The coiling temperature is preferably 700°C or lower, more preferably 660°C or lower.

[0272] After coiling, cold rolling may be performed as needed. Furthermore, the above-mentioned coating may be formed after finish rolling or cold rolling. Furthermore, pickling may be performed between hot rolling and cold rolling. During cold rolling, a typical cumulative reduction ratio, such as 30 to 90%, may be employed. Tempering rolling may also be performed under typical conditions. Furthermore, to soften the hot-rolled steel sheet, hot-rolled sheet annealing may be performed by heating the hot-rolled steel sheet to a temperature range below 730°C.

[0273] The hot stamping steel sheet of this embodiment can be manufactured by the above method. Next, a method for manufacturing a hot stamped formed body of this embodiment that can be manufactured using the above hot stamping steel sheet will be described. The method for manufacturing the hot stamped formed body of this embodiment is not particularly limited, but for example, the following manufacturing method can be used.

[0274] First, the hot stamping steel sheet is heated to a temperature of 800°C or higher. If the heating temperature is lower than 800°C, coarse carbides may remain during heating, which may reduce the bendability of the hot stamped product. The heating temperature is preferably 820°C or higher, and more preferably 860°C or higher.

[0275] The upper limit of the heating temperature is not particularly limited, but if the heating temperature is too high, decarburization will be promoted in the surface layer of the steel sheet, and the strength of the hot stamped product will be reduced. Therefore, the heating temperature is preferably 1000°C or lower, more preferably 960°C or lower, and even more preferably 930°C or lower.

[0276] The holding time at the above heating temperature is preferably set to 1.0 to 10.0 minutes. If the holding time is less than 1.0 minute, coarse carbides may remain, thereby reducing the bendability of the hot stamped part. On the other hand, if the holding time exceeds 10.0 minutes, decarburization may be promoted in the surface layer of the steel sheet, thereby reducing the strength of the hot stamped part.

[0277] Furthermore, the average heating rate up to the aforementioned heating temperature is preferably set to 1.0°C / second or higher. If the average heating rate is lower than 1.0°C / second, decarburization will be promoted in the surface layer of the steel sheet, reducing the strength of the hot stamped product. While there is no specific upper limit, it is difficult to set it above 1000°C / second in practice, so 1000°C / second or lower is the practical upper limit.

[0278] After the above heating and holding, hot stamping is performed. After hot stamping, cooling is preferably performed to a temperature range of 300°C or less, for example, at an average cooling rate of 10°C / second or more. An average cooling rate lower than 10°C / second may result in insufficient strength. There is no specific upper limit, but in practice, it is difficult to set it above 1000°C / second, so 1000°C / second or less is the practical upper limit.

[0279] It should be noted that preheating, i.e., two-stage heating, is not preferred during hot stamping. This is because the carbon segregation regions at the grain boundaries created during the hot stamping steel sheet stage are eliminated, making it impossible to form uniformly dispersed prior austenite grains. As a result, the standard deviation of the prior austenite grains cannot be controlled within the desired range.

[0280] The hot stamping part of the present embodiment can be obtained by the preferred manufacturing method described above. It should be noted that tempering treatment can also be performed at 150 to 600°C after hot stamping. In addition, a part of the hot stamping part can also be tempered by laser irradiation or the like to partially set a softening area. The weldability is improved in the softening area. For example, if the end of the hot stamping part is softened and then spot welded, the strength difference between the softened end and the spot weld in the end can be reduced, so that damage from the interface between the two can be suppressed. In addition, for example, in the case where the hot stamping part is applied to a high-strength component of an automobile, by providing a softening area in a part of the high-strength component, the damage and deformation mode of the high-strength component during a collision can be controlled.

[0281] Example

[0282] Next, the embodiments of the present invention will be described. However, the conditions in the embodiments are merely examples of conditions employed to confirm the feasibility and effectiveness of the present invention, and the present invention is not limited to this single example. Various conditions may be employed in the present invention as long as they do not deviate from the spirit of the present invention and achieve the purpose of the present invention.

[0283] Steel slabs produced by casting molten steel having the chemical compositions shown in Tables 1A to 1D were heated, held in a temperature range of 1200°C or higher for 20 minutes or longer, and then hot-rolled and coiled under the conditions shown in Tables 2A to 2G. Cold rolling, hot-rolled sheet annealing, pickling, and plating were performed as needed. Thus, the hot stamping steel sheets shown in Tables 2A to 2G were obtained. It should be noted that, in the finish rolling, except for No. 195 marked with an "*," the reduction ratio in the first pass before the final pass was increased compared to the previous passes (two passes before the final pass).

[0284] Furthermore, Steel Plate No. 149 was subjected to hot-rolled plate annealing in which the steel plate was heated to a temperature range of 730° C. or lower and held therein.

[0285] Steel plate No. 150 was not cold-rolled.

[0286] Steel sheet No. 151 has an electroplated zinc layer formed on its surface.

[0287] Steel sheet No. 152 has a Zn—Ni alloy electroplating layer formed on the surface.

[0288] Steel sheet No. 153 has a hot-dip galvanized layer formed on its surface.

[0289] Steel sheet No. 154 has an alloyed hot-dip galvannealed layer formed on its surface.

[0290] Steel sheet No. 155 has a hot-dip aluminum coating layer formed on the surface.

[0291] Steel sheet No. 156 has a hot-dip Zn—Al alloy layer formed on the surface.

[0292] Steel sheet No. 157 has a hot-dip Zn-Al-Mg alloy layer formed on the surface.

[0293] Steel sheet No. 158 has a hot-dip Zn-Al-Mg-Si alloy layer formed on the surface.

[0294] Steel Plate No. 195 was rolled with the reduction ratio gradually reduced in each pass during the finish rolling.

[0295] In addition, the "extreme density" in Table 2A to Table 2G indicates "the density of ferrite from {100} <011> ~{223} <110> The “average value of the pole density of the formed orientation group” and the “number ratio of ferrite containing carbides” represent “the number ratio of ferrite containing carbides with an equivalent circle diameter of 0.2 μm or more in grains in the entire ferrite”.

[0296] The obtained steel sheets for hot stamping were hot stamped under the conditions described in Tables 3A to 3G to obtain hot stamped bodies shown in Tables 3A to 3G.

[0297] Production No. 186 was tempered at 150 to 600° C. after hot stamping.

[0298] In production No. 187, a portion of the hot stamped body was subjected to laser irradiation for tempering, thereby forming a partially softened region.

[0299] Production No. 188 was heated to the heating temperature listed in Table 3G, cooled to a temperature range of 250° C. or lower, then heated to 900° C. and hot stamped, and then cooled at the average cooling rate listed in Table 3G.

[0300] In the examples of the present invention in Tables 3A to 3G, the metal structures include, by area%, ferrite: 0-50%, bainite and martensite: 0-100%, pearlite: 0-30%, and retained austenite: 0-5%.

[0301] In addition, the method for measuring the metal structure of the hot stamping steel sheet and the method for measuring the metal structure and mechanical properties of the hot stamped formed body are set as described above. If the tensile strength of the hot stamped formed body is 2200 MPa or more, it is judged as having high strength and qualified. If the tensile strength is less than 2200 MPa, it is judged as not having high strength and unqualified. In addition, if the maximum bend angle is 20° or more, it is judged as having excellent bendability and qualified. If the maximum bend angle is less than 20°, it is judged as not having excellent bendability and unqualified.

[0302] [Table 1A]

[0303]

[0304] [Table 1B]

[0305]

[0306] [Table 1C]

[0307]

[0308] [Table 1D]

[0309]

[0310] [Table 2A]

[0311]

[0312] [Table 2B]

[0313]

[0314] [Table 2C]

[0315]

[0316] [Table 2D]

[0317]

[0318] [Table 2E]

[0319]

[0320] [Table 2F]

[0321]

[0322] [Table 2G]

[0323]

[0324] [Table 3A]

[0325]

[0326] [Table 3B]

[0327]

[0328] [Table 3C]

[0329]

[0330] [Table 3D]

[0331]

[0332] [Table 3E]

[0333]

[0334] [Table 3F]

[0335]

[0336] [Table 3G]

[0337]

[0338] From Tables 3A to 3G, it can be seen that the hot stamped bodies of the examples of the present invention have high strength and excellent bendability. On the other hand, it can be seen that the hot stamped bodies of the comparative examples have deteriorated in one of their characteristics.

[0339] Industrial applicability

[0340] According to the above aspects of the present invention, it is possible to provide a hot stamped product having high strength and excellent bendability, and a steel sheet for hot stamping that can produce the hot stamped product.< / uvw> < / uvw> < / uvw>

Claims

1. A steel sheet for hot stamping, characterized in that: The chemical composition contains in mass %: C: more than 0.40% and less than 0.70%, Si: 0.010-1.30%, Mn: 0.10~0.60%, P: 0.100% or less, S: 0.0100% or less, N: 0.0140% or less, O: 0.0200% or less, Al：0.0010～0.500％、 Cr：0.010～0.80％、 Nb: 0~0.100%, Ti: 0~0.100%, B：0～0.0100％、 Mo: 0-1.00%, Co: 0-2.00%, Ni: 0% or more and less than 3.00%, Cu: 0-1.00%, V：0～1.00％、 W：0～1.000％、 Ca: 0~0.010%, Mg: 0-1.000%, REM: 0~1.000% Sb: 0~1.000%, Zr：0～1.000％、 Sn: 0-1.000%, and As: 0~0.100%, The rest is Fe and impurities. The hot stamping steel sheet has the following metal structure: Ferrite is made of {100} <011> ~{223} <110> The average value of the pole density of the formed orientation group is less than 10.0; The number ratio of the ferrite containing carbides having an equivalent circle diameter of 0.2 μm or more in the grains of the entire ferrite is 20% or more; In terms of area ratio, pearlite accounts for 10 to 90%, and ferrite accounts for 10 to 90%.

2. The hot stamping steel sheet according to claim 1, wherein: The chemical composition contains, in mass %, one or more elements selected from the following: Nb: 0.001~0.100%, Ti: 0.010~0.100%, B：0.0015～0.0100％、 Mo: 0.05-1.00%, Co: 0.05-2.00%, Ni: 0.01% or more and less than 3.00%, Cu: 0.01-1.00%, V：0.01～1.00％、 W：0.001～1.000％、 Ca: 0.001~0.010%, Mg: 0.001~1.000%, REM: 0.001~1.000%, Sb: 0.005~1.000%, Zr：0.001～1.000％、 Sn: 0.001~1.000%, and As: 0.001~0.100%.

3. The hot stamping steel sheet according to claim 1, wherein: The chemical composition contains in mass %: C: more than 0.40% and less than 0.70%, Si: 0.010-1.30%, Mn: 0.10~0.60%, P: 0.100% or less, S: 0.0100% or less, N: 0.0100% or less, O: 0.0200% or less, Al：0.0010～0.500％、 Cr：0.010～0.80％、 Nb: 0~0.100%, Ti: 0~0.100%, B：0～0.0100％、 Mo: 0-1.00%, Co: 0-2.00%, Ni: 0% or more and less than 3.00%, Cu: 0-1.00%, V：0～1.00％、 W：0～1.000％、 Ca: 0~0.010%, Mg: 0-1.000%, REM: 0~1.000% Sb: 0-1.000%, and Zr：0～1.000％， The remainder is Fe and impurities.

4. A hot stamping formed body, characterized in that The chemical composition contains in mass %: C: more than 0.40% and less than 0.70%, Si: 0.010-1.30%, Mn: 0.10~0.60%, P: 0.100% or less, S: 0.0100% or less, N: 0.0140% or less, O: 0.0200% or less, Al：0.0010～0.500％、 Cr：0.010～0.80％、 Nb: 0~0.100%, Ti: 0~0.100%, B：0～0.0100％、 Mo: 0-1.00%, Co: 0-2.00%, Ni: 0% or more and less than 3.00%, Cu: 0-1.00%, V：0～1.00％、 W：0～1.000％、 Ca: 0~0.010%, Mg: 0-1.000%, REM: 0~1.000% Sb: 0~1.000%, Zr：0～1.000％、 Sn: 0-1.000%, and As: 0~0.100%, The rest is Fe and impurities. The hot stamped body has the following metal structure: the average grain size of the prior austenite grains is 5 to 25 μm, the standard deviation of the grain size of the prior austenite grains is 0.1 to 2.0 μm, The hot stamping formed body has a tensile strength of 2200 MPa or more.

5. The hot stamped body according to claim 4, characterized in that The chemical composition contains, in mass %, one or more elements selected from the following: Nb: 0.001~0.100%, Ti: 0.010~0.100%, B：0.0015～0.0100％、 Mo: 0.05-1.00%, Co: 0.05-2.00%, Ni: 0.01% or more and less than 3.00%, Cu: 0.01-1.00%, V：0.01～1.00％、 W：0.001～1.000％、 Ca: 0.001~0.010%, Mg: 0.001~1.000%, REM: 0.001~1.000%, Sb: 0.005~1.000%, Zr：0.001～1.000％、 Sn: 0.001~1.000%, and As: 0.001~0.100%.

6. The hot stamped body according to claim 4, wherein: The chemical composition contains in mass %: C: more than 0.40% and less than 0.70%, Si: 0.010-1.30%, Mn: 0.10~0.60%, P: 0.100% or less, S: 0.0100% or less, N: 0.0100% or less, O: 0.0200% or less, Al：0.0010～0.500％、 Cr：0.010～0.80％、 Nb: 0~0.100%, Ti: 0~0.100%, B：0～0.0100％、 Mo: 0-1.00%, Co: 0-2.00%, Ni: 0% or more and less than 3.00%, Cu: 0-1.00%, V：0～1.00％、 W：0～1.000％、 Ca: 0~0.010%, Mg: 0-1.000%, REM: 0~1.000% Sb: 0-1.000%, and Zr：0～1.000％， The remainder is Fe and impurities.

7. The hot stamped body according to any one of claims 4 to 6, characterized in that The area ratio of the prior austenite grains having an average grain size of 0.5 to 3.0 μm is 60% or less.

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