Steel sheet and component including same

Abstract

The present disclosure provides, through a novel configuration, a high-strength steel sheet having exceptional hydrogen embrittlement resistance characteristics, and a component including the steel sheet.　A steel sheet and a component including the same according to the present disclosure have a prescribed chemical composition and microstructure. The average particle diameter in terms of equivalent circles is 10 μm or less. In relation to the signal intensity of BO2 − measured through time-of-flight secondary ion mass spectrometry, the average value Ib of the signal intensity of BO2 − within the range from a first reference signal intensity I1 to a second reference signal intensity I2 is at least 2.0 times an average value Ia of the signal intensity of BO2 −, where the first reference signal intensity I1 is a signal intensity 1.2 times the average value Ia, and the second reference signal intensity I2 is a signal intensity 7.5 times the aforementioned average value. The tensile strength of the steel sheet is at least 1760 MPa.

Description

Steel plates and parts containing them

[0001] This disclosure relates to steel plates and parts containing them.

[0002] In recent years, there has been a trend towards increasing the strength of steel sheets used in various fields such as automobiles, home appliances, and building materials. For example, in the automotive sector, the use of thin, high-strength steel sheets is increasing in order to lighten vehicle bodies and components and improve fuel efficiency.

[0003] In high-strength steel plates like these, hydrogen embrittlement cracking can be a problem. Hydrogen embrittlement cracking is a phenomenon in which a steel component subjected to high stress under operating conditions suddenly fractures due to hydrogen entering the steel from the surrounding environment. This phenomenon is also called delayed fracture due to the manner in which the fracture occurs. Generally, it is known that hydrogen embrittlement cracking in steel plates is more likely to occur as the tensile strength of the steel plate increases. This is thought to be because the higher the tensile strength of the steel plate, the greater the stress remaining in the steel plate after the part is formed.

[0004] Various proposals have been made to improve the hydrogen embrittlement resistance of steel sheets to address this type of hydrogen embrittlement cracking.

[0005] For example, Patent Document 1 discloses a hot-dip galvanized steel sheet having a hot-dip galvanized layer on at least one surface of a base steel sheet, wherein the base steel sheet has a predetermined chemical composition, and the microstructure in the range of 1 / 8 to 3 / 8 thickness centered at a position 1 / 4 thickness from the surface of the base steel sheet contains, by volume fraction, ferrite: 0% to 50%, retained austenite: 0% to 30%, tempered martensite: 5% or more, fresh martensite: 0% to 10%, and the sum of pearlite and cementite: 0% to 5%, and if a remaining microstructure exists, the remaining microstructure consists of bainite, the concentration of B atoms at the prior austenite grain boundaries is 2.0 atm% or more, and the average effective grain size is 7.0 μm or less.

[0006] Furthermore, Patent Document 2 describes a material having a predetermined chemical composition and an area ratio of: polygonal ferrite: 10% or less, upper bainite: 20% or less, retained austenite: 5% or less, martensite: 70% or more, 1 × 10６ / mm ２ A hot-dip galvanized steel sheet is disclosed having a structure represented by martensite with Fe carbides at the above number density: 50% or more of the total martensite, and an average effective grain size of 5.0 μm or less.

[0007] Furthermore, Patent Document 3 describes a material having a predetermined chemical composition, satisfying the relationship where the product of the amount of solid-solution B in the steel (solB [mass%]) and the prior austenite particle size Dγ [μm] is 0.0010 or more, and furthermore, in terms of area percentage, polygonal ferrite is 10% or less, bainite is 30% or less, retained austenite is 6% or less, and tempered martensite is 60% or more, and the number density of Fe carbides in the tempered martensite is 1 × 10⁻⁶ ６ / mm ２ Therefore, the average dislocation density of the entire steel is 1.0 × 10⁻⁶. １５ / m ２ The above is 2.0 x 10 １６ / m ２ The following discloses an ultra-high-strength cold-rolled steel sheet having a steel structure with an effective grain size of 7.0 μm or less, a tensile strength of 1300 MPa or more, and excellent resistance to hydrogen embrittlement.

[0008] International Publication No. 2020 / 162561, International Publication No. 2018 / 011978, Japanese Patent Publication No. 2016-50343

[0009] The steel sheets disclosed in Patent Documents 1 to 3 are all described as high-strength steel sheets with excellent hydrogen embrittlement resistance.

[0010] Therefore, the present disclosure aims to provide a high-strength steel plate and a component containing the same that have excellent hydrogen embrittlement resistance properties through a novel configuration.

[0011] This disclosure includes the following aspects:

[0012] (Aspect 1) In terms of chemical composition by mass%, C: 0.27 to 0.40%, Si: 0.01 to 2.50%, Mn: 1.00 to 4.00%, Al: 0.001 to 1.500%, Ti: 0.001 to 0.100%, B: 0.0005 to 0.0100%, P: 0.050% or less, S: 0.0100% or less, N: 0.0200% or less, O: 0.0100% or less, Cr: 0 to 1.00%, Cu: 0 to 1.00%, Mo: 0 to 1.00%, Ni: 0 to 1.00%, Co: 0 to 3.00%, W: 0 to 1.00%, Sn: 0 to 1.00%, Sb: 0 to 0.50%, Nb: 0 to 0.200%, V: 0 to 1.00%, As: 0 to 0.10%, Zn: 0 to 1.00%, Mg: 0 to 0.0500%, Ca: 0 to 0.0500%, Zr: 0 to 0.5000%, Ce: 0 to 0.0300%, La: 0 to 0.0150%, Hf: 0 to 0.0100%, Bi: 0 to 0.0100%, REM other than Ce and La: 0 to 0.1000%, and the balance: Fe and impurities, and the contents of C and B satisfy the following formula (1), and the microstructure at a depth of 1 / 4 of the plate thickness from the steel plate surface is, in terms of area%, the total of ferrite and bainite: 5% or less, retained austenite: 10% or less, martensite: 85% or more, and the balance: ２ － Regarding the signal intensity of BO ２ － The average value I ａ Of the signal intensity 1.2 times that of BO １ As the first reference signal intensity I ａ And the signal intensity 7.5 times that of the average value I ２ As the second reference signal intensity I １ When the above first reference signal intensity I ２ Is above and within the range of the above second reference signal intensity I ２ － The average value I ｂ Of the signal intensity of BOａ A steel plate characterized in that the strength is 2.0 times or more, and the tensile strength of the steel plate is 1760 MPa or more. However, the first reference signal intensity I １ If pixels showing a signal strength exceeding 0 are adjacent to each other, the BO from adjacent pixels ２ － Among the signal strengths, the highest BO ２ － Other than the signal strength of the BO ２ － The signal intensity of the average value I ｂ Exclude from calculation. T ｐ <1000 ... (1) However, T ｐ = 555 + 179 × [C] + 8083 × √[B] [C]: Content of C (mass%) [B]: Content of B (mass%)

[0013] (Aspect 2) The above chemical composition is as follows, in mass%, Cr: 0.001 to 1.00%, Cu: 0.001 to 1.00%, Mo: 0.01 to 1.00%, Ni: 0.01 to 1.00%, Co: 0.01 to 3.00%, W: 0.001 to 1.00%, Sn: 0.001 to 1.00%, Sb: 0.001 to 0.50%, Nb: 0.001 to 0.200%, V: 0.001 to 1.00%, As: 0.001 to 0.10%, Zn: 0.001 to 1.00%, Mg: 0.0001 to 0.0500%, Ca: 0.0001 to 0.0500%, The steel sheet according to embodiment 1, characterized by containing one or more of the following: Zr: 0.0001 to 0.5000%, Ce: 0.0001 to 0.0300%, La: 0.0001 to 0.0150%, Hf: 0.0001 to 0.0100%, Bi: 0.0001 to 0.0100%, and REM other than Ce and La: 0.0001 to 0.1000%.

[0014] (Aspect 3) The steel sheet according to aspect 1, characterized in that the above chemical composition contains, by mass%, N: 0.0100% or less, Co: 0 to 1.00%, B: 0.0005 to 0.0030%, Mg: 0 to 0.0100%, Ca: 0 to 0.0100%, Zr: 0 to 0.0100%, Ce: 0 to 0.0150%, and REM other than Ce and La: 0 to 0.0100%.

[0015] (Aspect 4) A component characterized by including the steel plate described in any of the above aspects 1 to 3.

[0016] According to this disclosure, it is possible to provide high-strength steel plates having excellent hydrogen embrittlement resistance and parts including the same.

[0017] Figure 1 is a schematic diagram illustrating the binning process performed on SIMS measurement data. Figure 2 is "BO ２ － This is an example of SIMS measurement data to explain the method for calculating the "signal intensity". Figure 3 is a schematic diagram to explain the method for evaluating the hydrogen embrittlement resistance of steel plates using a U-bending test.

[0018] This disclosure may define characteristics of specific locations in the thickness direction of a steel plate. Such characteristics of specific locations in the thickness direction of a steel plate are characteristics that use the position of the steel plate in the thickness direction with respect to the surface of the steel plate. Therefore, we will first explain such specific locations in the thickness direction of a steel plate.

[0019] First, since the "thickness direction" and "depth direction" of a steel plate are synonymous, in this specification, the position of the steel plate in the thickness direction relative to the steel plate surface may be referred to as the "depth position." In this regard, in this specification, the "x / y depth position of the plate thickness (where x and y are natural numbers satisfying x < y)" means the position moved from the surface of the steel plate in the thickness direction, i.e., the steel plate surface, toward the center of the steel plate by a distance (depth) of x / y of the plate thickness. For example, if the thickness of the steel plate is t mm, the "1 / 8 depth position of the plate thickness" means the position at a depth of 1 × t / 8 mm from the steel plate surface in the thickness direction. Note that the "steel plate" covered by this disclosure may be a "base steel plate" of a steel plate having some kind of coating on its surface, such as a plated steel plate. In such cases, the "steel plate surface" that serves as the reference for the depth position of the steel plate is the steel plate surface of the base steel plate. For example, in the time-of-flight secondary ion mass spectrometry method described later, Fe ＋ The shallowest position (i.e., the position closest to the steel plate surface) where the ion signal intensity is 0.7 times that at a depth of 1 / 4 of the plate thickness is defined as the surface of the base steel plate, i.e., the 0 μm position.

[0020] Preferred embodiments of the steel sheet, the component containing the same, and the method for manufacturing the steel sheet described herein will be explained in detail below. In this specification, unless otherwise specified, the "~" indicating a numerical range includes the numbers before and after it as the lower and upper limits. However, if "greater than" or "less than" is appended to a number, that number is not included.

[0021] Hydrogen embrittlement cracking occurs when cracks propagate along grain boundaries. Therefore, stabilizing grain boundaries is effective in improving hydrogen embrittlement resistance. The disclosers focused on boron (B), which is known to stabilize grain boundaries by segregating at them, and diligently investigated methods to increase the segregation concentration of B. As a result, the disclosers refined the microstructure of the steel sheet so that the average grain size of the crystal grains at a depth of 1 / 4 of the sheet thickness from the steel sheet surface was 10 μm or less, and measured the BO by time-of-flight secondary ion mass spectrometry. ２ － Regarding the signal strength, BO２ － The average value of the signal intensity across the entire field of view I ａ A signal strength 1.2 times that of the first reference signal strength I １ And the above average value I ａ A signal strength 7.5 times that of the second reference signal strength I ２ In this case, the first reference signal intensity I １ The above and the second reference signal intensity I ２ BO within the following range ２ － The average value of the signal intensity I ｂ However, the above average value I ａ 2.0 times or more (provided that the first reference signal intensity I １ If pixels showing a signal strength exceeding a certain level are adjacent to each other, then the BO from the adjacent pixels ２ － Among the signal strengths, the highest BO ２ － BO other than signal strength ２ － The signal intensity of the average value I ｂ We found that by segregating the solid-solution B in such a way that it is excluded from the calculation, hydrogen embrittlement cracking can be significantly suppressed.

[0022] Furthermore, the Disclosers have found that, in addition to the above configuration, by appropriately controlling the chemical composition and microstructure of the steel sheet, excellent hydrogen embrittlement resistance can be obtained even for high-strength steel sheets that are prone to hydrogen embrittlement cracking, more specifically, high-strength steel sheets with a tensile strength (TS) of 1760 MPa or higher.

[0023] This disclosure is completed based on these findings and includes aspects of each of the following embodiments.

[0024] First, preferred embodiments of the steel sheet of this disclosure will be described in detail.

[0025] <Steel Plate> A steel plate according to one embodiment of the present disclosure has the following unique characteristic configuration. First, the chemical composition of the steel sheet in this embodiment is as follows (in mass%): C: 0.27-0.40%, Si: 0.01-2.50%, Mn: 1.00-4.00%, Al: 0.001-1.500%, Ti: 0.001-0.100%, B: 0.0005-0.0100%, P: 0.050% or less, S: 0.0100% or less, N: 0.0200% or less, O: 0.0100% or less, Cr: 0-1.00%, Cu: 0-1.00%, Mo: 0-1.00%, Ni: 0-1.00%, Co: 0-3.00%, W: 0-1.00%, Sn: 0-1.00%. Sb: 0-0.50%, Nb: 0-0.200%, V: 0-1.00%, As: 0-0.10%, Zn: 0-1.00%, Mg: 0-0.0500%, Ca: 0-0.0500%, Zr: 0-0.5000%, Ce: 0-0.0300%, La: 0-0.0150%, Hf: 0-0.0100%, Bi: 0-0.0100%, REM other than Ce and La: 0-0.1000%, and the remainder: Fe and impurities.

[0026] Furthermore, the steel sheet of this embodiment satisfies the following formula (1) in terms of the content of C and B. ｐ <1000 ... (1) However, T ｐ = 555 + 179 × [C] + 8083 × √[B] [C]: Content of C (mass%) [B]: Content of B (mass%)

[0027] Furthermore, in this embodiment, the microstructure of the steel sheet at a depth of 1 / 4 of the sheet thickness from the surface of the steel sheet is as follows in area percentage: total of ferrite and bainite: 5% or less, retained austenite: 10% or less, martensite: 85% or more, and the remainder: 5% or less.

[0028] Furthermore, the steel plate of this embodiment has an average grain size of 10 μm or less at a depth of 1 / 4 of the plate thickness from the surface of the steel plate, and the BO is measured by time-of-flight secondary ion mass spectrometry. ２ － Regarding the signal strength, the above BO２ － The average value of the signal intensity I ａ A signal strength 1.2 times that of the first reference signal strength I １ The above average value I ａ A signal strength 7.5 times that of the second reference signal strength I ２ In this case, the first reference signal intensity I １ The above and the second reference signal intensity I ２ The above BO is within the following range ２ － The average value of the signal intensity I ｂ However, the above total average I ａ It is 2.0 times or more. However, the first reference signal intensity I mentioned above １ If pixels showing a signal strength exceeding a certain level are adjacent to each other, then the BO from the adjacent pixels ２ － Among the signal strengths, the highest BO ２ － BO other than signal strength ２ － The signal strength of the above average value I ｂ Exclude it from the calculation.

[0029] Furthermore, the steel plate of this embodiment has a tensile strength of 1760 MPa or more.

[0030] As described above, the steel sheet of this embodiment has a refined microstructure such that the average grain size of the crystal grains is 10 μm or less at a depth of 1 / 4 of the sheet thickness from the surface of the steel sheet. Furthermore, the steel sheet of this embodiment has the first reference signal intensity I １ The above and the second reference signal intensity I ２ The above BO is within the following range ２ － The average value of the signal intensity I ｂ However, the above average value I ａ B is segregated to a ratio of 2.0 times or more. As a result, the steel sheet of this embodiment can significantly suppress hydrogen embrittlement cracking, that is, it can exhibit excellent hydrogen embrittlement resistance.

[0031] Furthermore, in addition to the microstructure being refined and B being segregated in the steel sheet of the present embodiment, the chemical composition and microstructure of the steel sheet are controlled within the appropriate ranges as described above. Therefore, even in a high-strength steel sheet with a high tendency to generate hydrogen embrittlement cracks, more specifically, a high-strength steel sheet with a strength of 1760 MPa or more, excellent hydrogen embrittlement resistance characteristics can be fully exhibited.

[0032] Hereinafter, each characteristic configuration in the steel sheet of the present embodiment will be described. Here, in the present disclosure, "the average value I of the signal intensity of BO within the range of not less than the first reference signal intensity I" is defined to suppress hydrogen embrittlement cracks in which cracks progress at grain boundaries. Generally, dissolved B tends to segregate at grain boundaries, and it is known that the grain boundary strength is improved by the segregation of dissolved B at grain boundaries. Among grain boundaries, cracks tend to progress at unstable grain boundaries such as large-angle grain boundaries. Therefore, the hydrogen embrittlement resistance characteristics can be effectively improved by increasing the segregation amount of dissolved B at such grain boundaries. Also, at unstable grain boundaries, the segregation of B tends to progress due to its instability. Therefore, the location where the "signal intensity of BO within the range of not less than the first reference signal intensity I and not more than the second reference signal intensity I" is observed is a region where dissolved B is particularly concentrated and is presumed to be an "unstable grain boundary". That is, by increasing the segregation amount of B at the location where the "signal intensity of BO within the range of not less than the first reference signal intensity I and not more than the second reference signal intensity I" is observed, unstable grain boundaries where cracks tend to progress can be effectively strengthened, and the hydrogen embrittlement resistance characteristics can be improved. Specifically, the "average value I of the signal intensity of BO within the range of not less than the first reference signal intensity I and not more than the second reference signal intensity I" is such that all BO １ not less than and the second reference signal intensity I ２ within the following range of BO ２ － is defined to suppress hydrogen embrittlement cracks in which cracks progress at grain boundaries. Generally, dissolved B tends to segregate at grain boundaries, and it is known that the grain boundary strength is improved by the segregation of dissolved B at grain boundaries. Among grain boundaries, cracks tend to progress at unstable grain boundaries such as large-angle grain boundaries. Therefore, the hydrogen embrittlement resistance characteristics can be effectively improved by increasing the segregation amount of dissolved B at such grain boundaries. Also, at unstable grain boundaries, the segregation of B tends to progress due to its instability. Therefore, the location where the "signal intensity of BO within the range of not less than the first reference signal intensity I and not more than the second reference signal intensity I" is observed is a region where dissolved B is particularly concentrated and is presumed to be an "unstable grain boundary". That is, by increasing the segregation amount of B at the location where the "signal intensity of BO within the range of not less than the first reference signal intensity I and not more than the second reference signal intensity I" is observed, unstable grain boundaries where cracks tend to progress can be effectively strengthened, and the hydrogen embrittlement resistance characteristics can be improved. Specifically, the "average value I of the signal intensity of BO within the range of not less than the first reference signal intensity I and not more than the second reference signal intensity I" is such that all BO ｂ １ not less than and the second reference signal intensity I ２ within the following range of BO ２ － is defined to suppress hydrogen embrittlement cracks in which cracks progress at grain boundaries. Generally, dissolved B tends to segregate at grain boundaries, and it is known that the grain boundary strength is improved by the segregation of dissolved B at grain boundaries. Among grain boundaries, cracks tend to progress at unstable grain boundaries such as large-angle grain boundaries. Therefore, the hydrogen embrittlement resistance characteristics can be effectively improved by increasing the segregation amount of dissolved B at such grain boundaries. Also, at unstable grain boundaries, the segregation of B tends to progress due to its instability. Therefore, the location where the "signal intensity of BO within the range of not less than the first reference signal intensity I and not more than the second reference signal intensity I" is observed is a region where dissolved B is particularly concentrated and is presumed to be an "unstable grain boundary". That is, by increasing the segregation amount of B at the location where the "signal intensity of BO within the range of not less than the first reference signal intensity I and not more than the second reference signal intensity I" is observed, unstable grain boundaries where cracks tend to progress can be effectively strengthened, and the hydrogen embrittlement resistance characteristics can be improved. Specifically, the "average value I of the signal intensity of BO within the range of not less than the first reference signal intensity I and not more than the second reference signal intensity I" is such that all BO １ not less than and the second reference signal intensity I ２ within the following range of BO ２ － is defined to suppress hydrogen embrittlement cracks in which cracks progress at grain boundaries. Generally, dissolved B tends to segregate at grain boundaries, and it is known that the grain boundary strength is improved by the segregation of dissolved B at grain boundaries. Among grain boundaries, cracks tend to progress at unstable grain boundaries such as large-angle grain boundaries. Therefore, the hydrogen embrittlement resistance characteristics can be effectively improved by increasing the segregation amount of dissolved B at such grain boundaries. Also, at unstable grain boundaries, the segregation of B tends to progress due to its instability. Therefore, the location where the "signal intensity of BO within the range of not less than the first reference signal intensity I and not more than the second reference signal intensity I" is observed is a region where dissolved B is particularly concentrated and is presumed to be an "unstable grain boundary". That is, by increasing the segregation amount of B at the location where the "signal intensity of BO within the range of not less than the first reference signal intensity I and not more than the second reference signal intensity I" is observed, unstable grain boundaries where cracks tend to progress can be effectively strengthened, and the hydrogen embrittlement resistance characteristics can be improved. Specifically, the "average value I of the signal intensity of BO within the range of not less than the first reference signal intensity I and not more than the second reference signal intensity I" is such that all BO １ not less than and the second reference signal intensity I ２ [[ID=3o]]within the following range of BO ２ － is defined to suppress hydrogen embrittlement cracks in which cracks progress at grain boundaries. Generally, dissolved B tends to segregate at grain boundaries, and it is known that the grain boundary strength is improved by the segregation of dissolved B at grain boundaries. Among grain boundaries, cracks tend to progress at unstable grain boundaries such as large-angle grain boundaries. Therefore, the hydrogen embrittlement resistance characteristics can be effectively improved by increasing the segregation amount of dissolved B at such grain boundaries. Also, at unstable grain boundaries, the segregation of B tends to progress due to its instability. Therefore, the location where the "signal intensity of BO within the range of not less than the first reference signal intensity I and not more than the second reference signal intensity I" is observed is a region where dissolved B is particularly concentrated and is presumed to be an "unstable grain boundary". That is, by increasing the segregation amount of B at the location where the "signal intensity of BO within the range of not less than the first reference signal intensity I and not more than the second reference signal intensity I" is observed, unstable grain boundaries where cracks tend to progress can be effectively strengthened, and the hydrogen embrittlement resistance characteristics can be improved. Specifically, the "average value I of the signal intensity of BO within the range of not less than the first reference signal intensity I and not more than the second reference signal intensity I" is such that all BO ｂ is such that all BO​２ － The average value of the signal intensity I ａ The inventors discovered that exceeding 2.0 times this value can improve the hydrogen embrittlement resistance of steel plates.

[0033] First, the chemical composition of the steel sheet in this embodiment will be described. In the following description, "%", which is the unit for the content of each element, means "mass%" unless otherwise specified.

[0034] [C: 0.27-0.40%] Carbon (C) is an element necessary for improving the strength of steel sheets. C is also an effective element for stabilizing austenite. If the C content is less than 0.27%, these effects cannot be obtained sufficiently, so the C content should be 0.27% or more. The C content may be 0.30% or more, 0.32% or more, or 0.34% or more. On the other hand, if the C content exceeds 0.40%, the strength may increase and weldability and hydrogen embrittlement resistance may decrease, so the C content should be 0.40% or less. The C content may be 0.38% or less, or 0.36% or less.

[0035] [Si: 0.01-2.50%] Silicon (Si) is an element that suppresses the precipitation of iron-based carbides and contributes to improved strength and moldability. In addition, Si is Fe ２３ (C, B) ６ It is an element that contributes to suppressing coarsening. However, if the Si content is less than 0.01%, the above effect may not be sufficiently obtained. Therefore, the Si content should be 0.01% or more. The Si content may be 0.10% or more, 0.50% or more, or 1.00% or more. On the other hand, if the Si content is excessive, the press formability may deteriorate, the chemical treatment properties may decrease, or the hydrogen embrittlement resistance may decrease. Therefore, the Si content should be 2.50% or less. The Si content may be 1.80% or less, 1.60% or less, or 1.50% or less.

[0036] [Mn: 1.00–4.00%] Manganese (Mn) is an element that enhances hardenability and contributes to improving the strength of steel sheets. Mn is also an effective element for stabilizing austenite. However, if the Mn content is less than 1.00%, these effects may not be fully obtained. Therefore, the Mn content should be 1.00% or more. The Mn content may also be 1.20% or more, 1.50% or more, or 1.70% or more. On the other hand, if the Mn content is excessive, the grain boundaries may become brittle, and the hydrogen embrittlement resistance may decrease. Therefore, the Mn content should be 4.00% or less. The Mn content may also be 3.60% or less, 3.40% or less, 3.20% or less, 3.00% or less, 2.50% or less, or 2.00% or less.

[0037] [Al: 0.001 to 1.500%] Aluminum (Al) is a deoxidizing element. However, if the Al content is less than 0.001%, the deoxidizing effect may not be sufficient. Therefore, the Al content should be 0.001% or more. The Al content may also be 0.005% or more, 0.008% or more, or 0.010% or more. On the other hand, if the Al content exceeds 1.500%, the ferrite transformation in the steel sheet may be accelerated, and sufficient strength may not be obtained. Therefore, the Al content should be 1.500% or less. The Al content may also be 1.000% or less, 0.800% or less, 0.500% or less, 0.300% or less, 0.100% or less, or 0.050% or less.

[0038] [Ti: 0.001 to 0.100%] Titanium (Ti) is an element that contributes to improving the strength of steel sheets through precipitation strengthening, fine grain strengthening by suppressing grain growth, and dislocation strengthening through suppression of recrystallization. Furthermore, when manufacturing the steel sheet of this embodiment, Ti also has the effect of suppressing the formation of boron nitride (BN) by combining with solid-solution nitrogen in the steel by forming titanium nitride and consuming the solid-solution nitrogen in the steel. However, if the Ti content is less than 0.001%, these effects may not be sufficiently obtained. Therefore, the Ti content should be 0.001% or more. The Ti content may also be 0.005% or more, 0.008% or more, or 0.010% or more. On the other hand, if the Ti content exceeds 0.100%, coarse Ti oxide or titanium nitride may be formed, which may reduce the formability and hydrogen embrittlement resistance of the steel sheet. Therefore, the Ti content should be 0.100% or less. The Ti content may be 0.080% or less, 0.060% or less, or 0.050% or less.

[0039] [B: 0.0005 to 0.0100%] Boron (B) is an element that segregates at grain boundaries and contributes to improving hydrogen embrittlement resistance. However, if the B content is less than 0.0005%, B may not be sufficiently segregated at the grain boundaries, and the effect of improving hydrogen embrittlement resistance may not be fully obtained. Therefore, the B content should be 0.0005% or more. The B content may also be 0.0008% or more, 0.0010% or more, or 0.0015% or more. On the other hand, if the B content exceeds 0.0100%, coarse carbides and / or nitrides are more likely to be formed, making it difficult to fully obtain the above effect, and the steel sheet may become more prone to cracking and its ductility may decrease. Therefore, the B content should be 0.0100% or less. The B content may be 0.0080% or less, 0.0060% or less, 0.0040% or less, 0.0030% or less, 0.0028% or less, 0.0025% or less, or 0.0020% or less.

[0040] [P: 0.050% or less] Phosphorus (P) is an element that causes embrittlement of steel sheets and degrades the plating properties. The P content may be 0%, but reducing the P content to less than 0.0001% requires more time for refining, which may lead to a decrease in productivity. Therefore, the P content may be 0.0001% or more. The P content may be 0.0005% or more, 0.0010% or more, or 0.0020% or more. On the other hand, if the P content exceeds 0.050%, the steel sheet may become embrittle due to P segregation at the grain boundaries, which may reduce its resistance to hydrogen embrittlement. Therefore, the P content should be 0.050% or less. The P content may be 0.045% or less, 0.040% or less, or 0.035% or less.

[0041] [S: 0.0100% or less] Sulfur (S) is an element that causes hot embrittlement and also inhibits weldability and corrosion resistance. The S content may be 0%, but reducing the S content to less than 0.0001% requires more time for refining, which may lead to a decrease in productivity. Therefore, the S content may be 0.0001% or more. The S content may be 0.0005% or more, 0.0010% or more, or 0.0020% or more. On the other hand, if the S content exceeds 0.0100%, it may lead to embrittlement of the steel plate and a decrease in hydrogen embrittlement resistance. Therefore, the S content should be 0.0100% or less. The S content may be 0.0080% or less, 0.0060% or less, or 0.0050% or less.

[0042] [N: 0.0200% or less] Nitrogen (N) is an element that forms nitrides, inhibiting ductility and flange properties, and also causes blowholes during welding. The N content may be 0%, but reducing the N content to less than 0.0001% requires time for refining, which may lead to a decrease in productivity. Therefore, the N content may be 0.0001% or more. The N content may be 0.0005% or more, 0.0010% or more, or 0.0020% or more. On the other hand, if the N content exceeds 0.0200%, ductility and flange properties may decrease significantly, and blowholes may easily occur during welding. In addition, if the N content exceeds 0.0200%, boron nitride (BN) may be generated, which may result in a decrease in hydrogen embrittlement resistance. Therefore, the N content should be 0.0200% or less. The N content may be 0.0180% or less, 0.0160% or less, 0.0140% or less, 0.0120% or less, 0.0100% or less, 0.0080% or less, 0.0060% or less, or 0.0050% or less.

[0043] [O: 0.0100% or less] Oxygen (O) is an element that forms oxides, inhibiting ductility and flange properties, and is also a cause of blowhole formation during welding. The O content may be 0%, but reducing the O content to less than 0.0001% requires time for refining, which may lead to a decrease in productivity. Therefore, the O content may be 0.0001% or more. The O content may be 0.0005% or more, 0.0010% or more, or 0.0015% or more. On the other hand, if the O content exceeds 0.0100%, ductility and flange properties may be significantly reduced, and blowholes are more likely to occur during welding. Therefore, the O content should be 0.0100% or less. The O content may be 0.0080% or less, 0.0060% or less, or 0.0050% or less.

[0044] The basic component composition of the steel sheet of this embodiment is as described above. Furthermore, the steel sheet of this embodiment may contain one or more of the following optional elements in place of a portion of the remaining Fe, if necessary.

[0045] [Cr: 0-1.00%] Chromium (Cr) is an element that enhances the hardenability of steel and contributes to improving the strength of steel sheets. The Cr content may be 0%, but to obtain these effects fully, it is preferable that the Cr content be 0.001% or more. The Cr content may be 0.005% or more, 0.010% or more, or 0.020% or more. On the other hand, if the Cr content exceeds 1.00%, pickling properties, weldability, and hot workability may decrease. Therefore, the Cr content should be 1.00% or less. The Cr content may be 0.80% or less, 0.60% or less, 0.50% or less, 0.60% or less, or 0.50% or less.

[0046] [Cu: 0-1.00%] Copper (Cu) is an element that contributes to improving the strength of steel sheets and also to improving hydrogen embrittlement resistance. The Cu content may be 0%, but in order to fully obtain these effects, it is preferable that the Cu content be 0.001% or more. The Cu content may be 0.005% or more, 0.010% or more, or 0.015% or more. On the other hand, if the Cu content exceeds 1.00%, it may lead to embrittlement of the steel sheet and a decrease in ductility. Therefore, the Cu content should be 1.00% or less. The Cu content may be 0.80% or less, 0.60% or less, or 0.50% or less.

[0047] [Mo: 0-1.00%] Molybdenum (Mo) is an element that enhances the hardenability of steel, contributes to improving the strength of steel sheets, and also contributes to improving hydrogen embrittlement resistance. The Mo content may be 0%, but to fully obtain these effects, it is preferable that the Mo content be 0.01% or more. The Mo content may be 0.02% or more, 0.03% or more, or 0.04% or more. On the other hand, if the Mo content exceeds 1.00%, the strength of the hot-rolled sheet may increase and the cold-rollability may decrease. Therefore, the Mo content should be 1.00% or less. The Mo content may be 0.90% or less, 0.80% or less, 0.70% or less, 0.60% or less, 0.50% or less, 0.40% or less, 0.30% or less, 0.28% or less, or 0.15% or less.

[0048] [Ni: 0-1.00%] Nickel (Ni) is an element that contributes to improving strength and also to improving hydrogen embrittlement resistance. The Ni content may be 0%, but in order to fully obtain these effects, it is preferable that the Ni content be 0.01% or more. The Ni content may be 0.02% or more, 0.03% or more, or 0.04% or more. On the other hand, if the Ni content exceeds 1.00%, the strength of the hot-rolled sheet may increase and the cold-rollability may decrease. Therefore, the Ni content should be 1.00% or less. The Ni content may be 0.90% or less, 0.80% or less, 0.70% or less, or 0.50% or less.

[0049] [Co: 0-3.00%] Cobalt (Co) is an effective element for increasing the strength of steel sheets. The Co content may be 0%, but to obtain these effects fully, it is preferable that the Co content be 0.01% or more. The Co content may be 0.02% or more, 0.03% or more, or 0.04% or more. On the other hand, if the Co content exceeds 3.00%, ferrite transformation and / or pearlite transformation in the steel sheet may be promoted, and sufficient strength may not be obtained. Therefore, the Co content should be 3.00% or less. The Co content may be 2.50% or less, 2.00% or less, 1.50% or less, 1.00% or less, 0.80% or less, 0.60% or less, 0.50% or less, 0.30% or less, or 0.10% or less.

[0050] [W: 0-1.00%] Tungsten (W) is an element that is effective in increasing the strength of steel and also contributes to improving hydrogen embrittlement resistance. The W content may be 0%, but in order to fully obtain these effects, it is preferable that the W content be 0.001% or more. The W content may be 0.005% or more, 0.007% or more, or 0.010% or more. On the other hand, if the W content exceeds 1.00%, the strength of the hot-rolled sheet may increase and the cold-rollability may decrease. Therefore, the W content should be 1.00% or less. The W content may be 0.80% or less, 0.60% or less, 0.50% or less, 0.30% or less, 0.10% or less, or 0.05% or less.

[0051] [Sn: 0-1.00%] Tin (Sn) is an element contained in steel when scrap is used as a raw material, and a lower amount is preferable. The Sn content may be 0%, but reducing the Sn content to less than 0.001% requires more time for refining, which may lead to a decrease in productivity. Therefore, the Sn content may be 0.001% or more. The Sn content may be 0.002% or more, 0.003% or more, or 0.005% or more. On the other hand, if the Sn content exceeds 1.00%, it can lead to embrittlement of the steel sheet, and cracks may occur during hot rolling. Therefore, the Sn content should be 1.00% or less. The Sn content may be 0.80% or less, 0.60% or less, 0.50% or less, 0.30% or less, 0.10% or less, or 0.05% or less.

[0052] [Sb: 0-0.50%] Antimony (Sb), like Sn, is an element present when scrap is used as a raw material for steel, and a lower amount is preferable. The Sb content may be 0%, but reducing the Sb content to less than 0.001% requires more time for refining, which may lead to a decrease in productivity. Therefore, the Sb content may be 0.001% or more. The Sb content may be 0.002% or more, 0.003% or more, or 0.005% or more. On the other hand, if the Sb content exceeds 0.50%, it may lead to embrittlement of the steel sheet and a decrease in ductility. Therefore, the Sb content should be 0.50% or less. The Sb content may be 0.45% or less, 0.40% or less, 0.35% or less, 0.25% or less, 0.10% or less, or 0.05% or less.

[0053] [Nb: 0-0.200%] Niobium (Nb) is an element that contributes to improving steel sheet strength through precipitation strengthening, fine grain strengthening by suppressing grain growth, and dislocation strengthening through suppression of recrystallization. In addition, Nb has the effect of suppressing the precipitation of boron. The Nb content may be 0%, but in order to obtain these effects to the fullest extent, it is preferable that the Nb content be 0.001% or more. The Nb content may also be 0.005% or more, 0.008% or more, or 0.010% or more. On the other hand, if the Nb content exceeds 0.200%, coarse carbides precipitate and the amount of solid-solution carbon decreases, which can lower the martensite fraction and prevent the steel sheet from obtaining sufficient strength. Therefore, the Nb content should be 0.200% or less. The Nb content may be 0.150% or less, 0.120% or less, 0.100% or less, 0.080% or less, 0.060% or less, or 0.040% or less.

[0054] [V: 0-1.00%] Vanadium (V) is an element that contributes to improving the strength of steel sheets through precipitation strengthening, fine grain strengthening by suppressing grain growth, and dislocation strengthening through suppression of recrystallization. In addition, V is an element that contributes to improving hydrogen embrittlement resistance. The V content may be 0%, but in order to obtain these effects sufficiently, it is preferable that the V content be 0.001% or more. The V content may be 0.005% or more, 0.008% or more, or 0.010% or more. On the other hand, if the V content exceeds 1.00%, coarse carbides may form and become the starting point for cracks, which may lead to a decrease in hydrogen embrittlement resistance. Therefore, the V content should be 1.00% or less. The V content may be 0.80% or less, 0.60% or less, 0.50% or less, 0.40% or less, or 0.30% or less.

[0055] [As: 0-0.10%] Arsenic (As), like Sn and Sb, is an element present when scrap is used as a raw material for steel, and it is also an element that strongly segregates at grain boundaries. A lower As content is preferable. The As content may be 0%, but reducing the As content to less than 0.001% requires more time for refining, which may lead to a decrease in productivity. Therefore, the As content may be 0.001% or more. The As content may be 0.002% or more, 0.003% or more, or 0.005% or more. On the other hand, if the As content exceeds 0.10%, it may lead to embrittlement of the steel sheet and a decrease in ductility. Therefore, the As content should be 0.10% or less. The As content may be 0.08% or less, 0.06% or less, or 0.05% or less.

[0056] [Zn: 0-1.00%] Zinc (Zn) is an effective element for controlling the morphology of sulfides and improving local ductility and elongation flange properties. The Zn content may be 0%, but to obtain these effects fully, it is preferable that the Zn content be 0.001% or more. The Zn content may be 0.002% or more, 0.003% or more, or 0.005% or more. On the other hand, if the Zn content exceeds 1.00%, the number of inclusions increases, which may cause defects on the surface and inside the steel sheet. Therefore, the Zn content should be 1.00% or less. The Zn content may be 0.80% or less, 0.60% or less, 0.40% or less, 0.25% or less, or 0.10% or less.

[0057] [Mg: 0-0.0500%] Magnesium (Mg) is an element whose sulfide morphology can be controlled by adding trace amounts. The Mg content may be 0%, but to fully obtain these effects, it is preferable that the Mg content be 0.0001% or more. The Mg content may also be 0.0005% or more, 0.0010% or more, or 0.0015% or more. On the other hand, if the Mg content exceeds 0.0500%, it may lead to embrittlement of the steel sheet and a decrease in ductility. Therefore, the Mg content should be 0.0500% or less. The Mg content may be 0.0450% or less, 0.0400% or less, 0.0350% or less, 0.0300% or less, 0.0250% or less, 0.0200% or less, 0.0150% or less, 0.0100% or less, 0.0080% or less, 0.0060% or less, or 0.0050% or less.

[0058] [Ca: 0-0.0500%, Zr: 0-0.5000%, Ce: 0-0.0300%, La: 0-0.0150%, Hf: 0-0.0100%, Bi: 0-0.0100%, REM other than Ce and La: 0-0.1000%] Calcium (Ca), zirconia (Zr), cerium (Ce), lanthanum (La), hafnium (Hf), and REM (rare earth elements) other than Ce and La are all elements that can control the form of sulfides. In addition, Bi (bismuth) is an element that reduces the microsegregation of substitutional alloy elements such as Mn and Si in steel. For this reason, one or more of these elements may be included as needed. The content of each of these elements may be 0%, but in order to fully obtain the above effects, it is preferable that the content of Ca, Zr, Ce, La, Hf, Bi, and REM other than Ce and La be 0.0001% or more. The content of Ca, Zr, Ce, La, Hf, Bi, and REM other than Ce and La may be 0.0002% or more, 0.0003% or more, 0.005% or more, or 0.0010% or more, respectively. On the other hand, if the Ca content exceeds 0.0500%, it may lead to embrittlement of the steel sheet and a decrease in ductility. Therefore, the Ca content should be 0.0500% or less. Similarly, if the Ce content exceeds 0.0300%, it may lead to embrittlement of the steel sheet and a decrease in ductility. Therefore, the Ce content should be 0.0300% or less. Similarly, if the Zr content exceeds 0.5000%, it may lead to embrittlement of the steel sheet and a decrease in ductility. Therefore, the Zr content should be 0.5000% or less. Similarly, if the Hf and Bi content each exceeds 0.0100%, or if the La content exceeds 0.0150%, it may lead to embrittlement of the steel sheet and a decrease in ductility. Therefore, the Hf and Bi content each should be 0.0100% or less, and the La content should be 0.0150% or less. In addition, if the content of REM other than Ce and La exceeds 0.1000%, it may lead to embrittlement of the steel sheet and a decrease in ductility. Therefore, the content of REM other than Ce and La should be 0.1000% or less.The Ca content may be 0.0400% or less, 0.0300% or less, 0.0200% or less, 0.0100% or less, 0.0080% or less, 0.0060% or less, or 0.0050% or less. The Ce content may be 0.0250% or less, 0.0200% or less, 0.0150% or less, 0.0100% or less, 0.0080% or less, 0.0060% or less, or 0.0050% or less. The Zr content may be 0.4000% or less, 0.3000% or less, 0.2000% or less, 0.1000% or less, 0.0750% or less, 0.0500% or less, 0.0300% or less, or 0.0100% or less. Furthermore, the content of Hf and Bi may be 0.0080% or less, 0.0060% or less, or 0.0050% or less, respectively. Also, the content of La may be 0.0120% or less, 0.0110% or less, or 0.0100% or less. And the content of REM other than Ce and La may be 0.0800% or less, 0.0600% or less, 0.0400% or less, 0.0200% or less, 0.0100% or less, 0.0080% or less, 0.0060% or less, or 0.0050% or less.

[0059] In this specification, REM refers to the collective term for 17 elements, including scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and the lanthanides lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71. The REM content is the total content of these elements.

[0060] In this embodiment, the chemical composition of the steel sheet is as follows, in mass%, for the above optional elements: Cr: 0.001 to 1.00%, Cu: 0.001 to 1.00%, Mo: 0.01 to 1.00%, Ni: 0.01 to 1.00%, Co: 0.01 to 3.00%, W: 0.001 to 1.00%, Sn: 0.001 to 1.00%, Sb: 0.001 to 0.50%, Nb: 0.001 to 0.200%, V: 0.001 to 1.00%, As: 0.001 to 0.10%, Zn: 0.001 to 1.00%, Mg: 0.0001 to 0.0500%. It may also contain one or more of the following: Ca: 0.0001 to 0.0500%, Zr: 0.0001 to 0.5000%, Ce: 0.0001 to 0.0300%, La: 0.0001 to 0.0150%, Hf: 0.0001 to 0.0100%, Bi: 0.0001 to 0.0100%, and REM other than Ce and La: 0.0001 to 0.1000%. Furthermore, the above chemical composition may contain, in mass percent, N: 0.0100% or less, Co: 0 to 1.00%, B: 0.0005 to 0.0030%, Mg: 0 to 0.0100%, Ca: 0 to 0.0100%, Zr: 0 to 0.0100%, Ce: 0 to 0.0150%, and REM other than Ce and La: 0 to 0.0100%.

[0061] In the steel sheet of this embodiment, the remainder of the elements other than those mentioned above consists of Fe and impurities. Here, impurities are components that are mixed in during the industrial production of steel sheets due to various factors in the manufacturing process, including raw materials such as ore and scrap. It should be noted that impurities include components that were not intentionally added to the steel sheet of this embodiment (so-called unavoidable impurities). Furthermore, impurities also include elements other than those mentioned above that are present in the steel sheet to such an extent that their specific effects do not affect the properties of the steel sheet of this embodiment.

[0062] [Content of C and B: Formula (1)] In the steel sheet of this embodiment, the content (mass%) of C and B satisfies the following formula (1). T calculated by the following formula (1) ｐThis refers to the precipitation start temperature (°C) of B, and T ｐ The temperature should be 1000°C or lower. ｐ <1000 ... (1) However, T ｐ = 555 + 179 × [C] + 8083 × √[B] [C]: Content of C (mass%) [B]: Content of B (mass%)

[0063] In the above formula (1), T ｐ B is Fe ２３ (C, B) ６ This refers to the precipitation initiation temperature when precipitation occurs, and in this specification, it is conveniently referred to as the "precipitation initiation temperature of B". Fe ２３ (C, B) ６ When precipitation occurs, B cannot be sufficiently segregated at the grain boundaries, and the effect of improving hydrogen embrittlement resistance may not be fully obtained. Therefore, T ｐ The temperature should be less than 1000°C. ｐ The temperature may be less than 950°C, less than 900°C, less than 850°C, or less than 800°C. Note that the above formula (1) is an approximate formula derived from calculated values ​​obtained using the integrated thermodynamic calculation software "Thermo-Calc" (registered trademark). Also, the above T ｐ In the expression representing T ｐ The unit is "℃", but the unit on the right side also includes the term √B. However, if we calculate [C] and [B] in the formula using "mass%", the unit of the calculation result will be "℃".

[0064] The chemical composition of steel sheets can be measured using general analytical methods. For example, the chemical composition of steel sheets can be measured using inductively coupled plasma atomic emission spectroscopy (ICP-AES). Specifically, the front and back surfaces of the steel sheet are ground to a depth of 200 μm from each surface to take test pieces, and the chemical composition of the steel sheet can be determined by measuring it using a measuring device such as the Shimadzu ICPS-8100 under conditions based on a pre-established calibration curve. C and S, which cannot be measured by ICP-AES, can be measured using combustion-infrared absorption spectroscopy, N can be measured using inert gas fusion-thermal conductivity spectroscopy, and O can be measured using inert gas fusion-nondispersive infrared absorption spectroscopy. Furthermore, if analysis values ​​for molten steel, slabs, or other steel sheets manufactured from the same molten steel are available, the analysis of test pieces taken from the steel sheet may be omitted, and those analysis values ​​may be considered as the chemical composition of the steel sheet.

[0065] Next, the microstructure of the steel sheet of this embodiment will be described. As described above, the microstructure of the steel sheet of this embodiment at a depth of 1 / 4 of the sheet thickness from the surface of the steel sheet is as follows in area percentage: total of ferrite and bainite: 5% or less, retained austenite: 10% or less, martensite: 85% or more, and the remainder: 5% or less.

[0066] The following describes each of these organizations.

[0067] [Total content of ferrite and bainite: 5% or less] Ferrite is a soft structure with excellent ductility. On the other hand, bainite is a structure that is effective in ensuring the ductility and strength of steel sheets. These structures are included in order to improve the elongation of the steel sheet, depending on the required strength or ductility. From the viewpoint of ensuring the ductility and strength of the steel sheet, the total content of ferrite and bainite shall be 5% or less by area percentage. The total content of ferrite and bainite may be 4% or less, 3% or less, or 2% or less by area percentage. The total content of ferrite and bainite may be 0% by area percentage, but from the viewpoint of improving the ductility of the steel sheet, it may be 1% or more or 2% or more.

[0068] [Retained Austenite: 10% or less] Retained austenite is a structure that improves the ductility of steel sheets through the TRIP effect, which is a transformation into martensite during deformation of the steel sheet (i.e., a work-induced transformation). The retained austenite content may be 0% by area percentage, or it may be 1% or more, 3% or more, or 5% or more. On the other hand, excessive retained austenite content may reduce toughness. Therefore, the retained austenite content should be 10% or less by area percentage, and may be 8% or less, 6% or less, 4% or less, or 2% or less.

[0069] [Martensite: 85% or more] Martensite is a high-strength structure that increases the tensile strength of steel sheets. Therefore, it is an important structure for ensuring the desired steel sheet strength. The martensite referred to here includes fresh martensite and tempered martensite. Fine cementite present within tempered martensite is considered part of the tempered martensite. To ensure sufficient strength, the martensite content shall be 85% or more by area percentage. The martensite content may be 90% or more, 92% or more, or 95% or more. Alternatively, the martensite content may be 100% or less, 99% or less, or 98% or less.

[0070] [Remainder: 5% or less] Considering the chemical composition of the steel sheet of this embodiment and the preferred manufacturing conditions, cementite and other precipitates are measured as the area percentage of the microstructure in which they exist. These microstructures are common in high-strength steel sheets used in fields such as automobiles, home appliances, and building materials, which are the subject of this application. In this disclosure, the effect on hydrogen embrittlement resistance due to martensite, which makes up most of the microstructure, and the ferrite, bainite, and retained austenite present with it is significant, and even if cementite and alloy carbides of at most 5% are formed, it is not necessary to consider the effect on hydrogen embrittlement resistance. Furthermore, if the manufacturing method conforms to the preferred manufacturing conditions of this disclosure, it is easy to keep the area percentage of the remaining region to 5% or less. The area percentage of the remaining region is more preferably 2% or less, and even more preferably 1% or less or 0%. The effect of the remaining region on hydrogen embrittlement resistance is negligible. The remaining region may contain pearlite. However, since there is almost no pearlite, the remaining region does not need to contain pearlite.

[0071] (Identification of Microstructure and Calculation of Area Ratio) For the above-mentioned microstructure, the area ratio of retained austenite can be calculated by X-ray diffraction. In addition, the total of ferrite and bainite, as well as martensite, can be identified and their area ratios calculated by observing and measuring the cross-section of the steel sheet using a scanning electron microscope.

[0072] Specifically, the area ratio of retained austenite is calculated by X-ray diffraction. First, the area from the surface of the steel plate to a depth of 1 / 4 of the plate thickness is removed by mechanical and chemical polishing. Next, MoKα rays are used as characteristic X-rays on the surface of the polished sample to obtain the diffraction peaks of (200) and (211) for the bcc phase, and (200), (220), and (311) for the fcc phase. At this time, the total irradiated area is 1 mm². ２ This concludes the explanation. The integral intensity ratio of each diffraction peak is used to calculate the structural fraction of retained austenite, and this is taken as the area fraction of retained austenite.

[0073] The total area ratio of ferrite and bainite is measured by the following method. First, a sample is taken from the thickness cross section of the steel plate parallel to the rolling direction, and the observation surface is mechanically polished to a mirror finish, followed by electropolishing. Next, in one or more observation fields centered in the thickness direction at a depth of 1 / 4 of the steel plate thickness on the observation surface, crystal structure analysis and orientation analysis are performed for Iron-Alpha and Iron-Gamma by electron backscatter diffraction using a scanning electron microscope. At this time, each observation field has a length of 200 μm in the thickness direction and a length of 200 μm in the direction parallel to the plate surface, and the distance between evaluation points (step) is 0.20 μm. For the analysis of the data obtained by electron backscatter diffraction, "OIM Analysis® 6.0" or "OIM Analysis® 7.0" manufactured by TSL Corporation is used. From the measurement data, the average grain orientation difference (GAM) for Iron-Alpha is calculated. Then, the area fraction of the region where the GAM value is less than 0.1° is multiplied by the area fraction of the bcc phase obtained from the aforementioned X-ray diffraction, and this value is taken as the total area fraction of ferrite and bainite. Here, "the value obtained by multiplying the area fraction of the region where the GAM value is less than 0.1° by the area fraction of the bcc phase" is "the value of (area of ​​the region where the GAM value is less than 0.1° / area of ​​Iron-Alpha) × (1 - area fraction of retained austenite)". Furthermore, the average grain orientation difference is the value obtained by calculating the orientation difference between adjacent measurement points in the region surrounded by grain boundaries where the crystal orientation difference is 5° or more, and averaging this value over all measurement points within the crystal grain.

[0074] The total area percentage of martensite is measured by the following method. First, nital etching is performed on the same observation surface of the sample that was observed using the electron backscatter diffraction method described above. Then, secondary electron imaging is performed using a scanning electron microscope on the same field of view as observed using electron backscatter diffraction on the etched observation surface. This observation should be performed at a magnification of 1,000 to 50,000. To observe the same field of view as observed using electron backscatter diffraction, it is advisable to mark the surface beforehand with Vickers indentations or other markers. In the field of view of the obtained secondary electron image, the area percentage of tissue that is relatively unetched, has a lath-like understructure, and contains fine cementite or retained austenite less than 1 μm in size is measured by point counting, and the area percentage of martensite is calculated by subtracting the area percentage of the retained austenite. The finer the lattice spacing when performing point counting, the more accurate the value will be. For example, a lattice spacing of 2 μm is appropriate.

[0075] If the total area ratio of retained austenite, ferrite, bainite, and martensite obtained by the above method is less than 100%, the difference from 100% will be the area ratio of the remaining material. If the total area ratio of retained austenite, ferrite, bainite, and martensite obtained by the above method exceeds 100%, the area ratio of each tissue will be calculated by multiplying the area ratio of each tissue by 100 / (total area ratio of each tissue).

[0076] [Average grain size of crystal grains is 10 μm or less at a depth of 1 / 4 of the plate thickness from the surface of the steel plate] As described above, the microstructure of the steel plate in this embodiment is refined such that the average grain size of crystal grains is 10 μm or less at a depth of 1 / 4 of the plate thickness from the surface of the steel plate.

[0077] The average grain size of the crystal grains in this disclosure is determined as follows. First, a test specimen is taken from a steel plate so that the cross section in the L direction (rolling direction) at a depth of 1 / 4 of the plate thickness is the observation position. The L-direction cross section of the test specimen is mirror-polished with diamond paste, and then finished polished with colloidal silica. Subsequently, with a step distance (step) of 0.2 μm in a hexagonal grid, measurements are performed on Iron-Alpha and Iron-Gamma using electron backscatter diffraction for a field of view of 500 points in the rolling direction and 500 points in the plate thickness direction. For the analysis of the data obtained by electron backscatter diffraction, "OIM Analysis® 6.0" or "OIM Analysis® 7.0" manufactured by TSL Corporation is used. Based on the observation results, for Iron-Alpha, the region enclosed by grain boundaries with an orientation difference of 15° or more between measurement points is defined as the "region enclosed by large-angle grain boundaries." In this case, if Iron-Gamma is included in the measurement results, it is excluded, and the boundary between Iron-Alpha and Iron-Gamma is considered to be a "grain boundary with an orientation difference of 15° or more." Furthermore, within the "region enclosed by large-angle grain boundaries," the region containing five or more measurement points is defined as a crystal grain. Then, the area of ​​each crystal grain is converted to a circle of equal area to determine the equivalent diameter of the circle. The average value of the equivalent diameter of the crystal grains within the field of view is calculated and taken as the average grain size of the crystal grain.

[0078] Furthermore, a smaller average grain size is preferable, for example, it may be 8 μm or less, 5 μm or less, or 3 μm or less. Also, there is no particular lower limit to the average grain size, but it is, for example, 2 μm.

[0079] [First reference signal intensity I １ The above and the second reference signal intensity I ２ BO within the following range ２ － The average value of the signal intensity I ｂ However, the average value I ａ [More than 2.0 times] Furthermore, the steel plate of this embodiment is measured by time-of-flight secondary ion mass spectrometry. ２ － Regarding the signal strength, the above BO ２ － The average value of the signal intensity Iａ A signal strength 1.2 times that of the first reference signal strength I １ The above average value I ａ A signal strength 7.5 times that of the second reference signal strength I ２ In this case, the first reference signal intensity I １ The above and the second reference signal intensity I ２ The above BO is within the following range ２ － The average value of the signal intensity I ｂ However, the above average value I ａ B is segregated to be 2.0 times or more of the above. However, the first reference signal intensity I １ If pixels showing a signal strength exceeding a certain level are adjacent to each other, then the BO from the adjacent pixels ２ － Among the signal strengths, the highest BO ２ － BO other than signal strength ２ － The signal strength of the above average value I ｂ Exclude it from the calculation.

[0080] BO at grain boundaries ２ － The signal intensity is measured by the following method. First, a sample is taken from the cross-section of the steel plate parallel to the rolling direction, and the observation surface is mirror-polished with diamond paste. Then, the signal intensity is measured by time-of-flight secondary ion mass spectrometry (TOF-SIMS). ２ ― Measure the signal strength.

[0081] In TOF-SIMS analysis, commercially available analytical instruments can be used. In this embodiment, the TOF-SIMS5 manufactured by ION-TOF is used as the analytical instrument. The measuring ion gun is Bi １ ＋ (30kV) will be used. The measurement pitch will be 0.05μm, with 2000 points in the thickness direction and 2000 points horizontally to the plate surface, for a total of 4,000,000 points. ２ － The signal strength (hereafter referred to as "BO") ２ －The signal strength of the measurement is sometimes simply referred to as "signal strength." The measurement is taken at a depth of 1 / 4 of the thickness of the steel plate on the measurement surface, which is taken as the center position in the thickness direction of the measurement range. The obtained data is binned to convert it into 250 x 250 point data. Here, Figure 1 is a schematic diagram to explain the binning process. The binning process is an operation that adds adjacent 8 x 8 point data to convert it into a single point data. Coordinates are assigned to the measurement points before conversion as shown in Figure 1(a), and the signal strength at that position is measured as I ｎ （ｉ，ｊ） In this case, the signal intensity I at the transformed coordinate (i,j) shown in Figure 1(b) （ｉ，ｊ） It can be calculated using the following formula. Furthermore, binning the 4,000,000 measurement data points reduces the number of data points to 1 / 64th. First, the arithmetic mean of the 250 x 250 signal intensities after binning is calculated as the "average signal intensity I". ａ The average value of the signal intensity is I. ａ The value obtained by multiplying this by 1.2 is called "First Reference Signal Intensity I" １ " and the above average value I ａ A signal strength 7.5 times greater than that is defined as "Second Reference Signal Strength I" ２ "

[0082] Next, the binned 250x250 signal intensity data consists of 250 interconnected surface analysis profiles for each region with one point in the thickness direction and 250 points in the horizontal direction of the plate surface. Here, the signal intensity profile for the region with one point in the thickness direction and 250 points in the horizontal direction of the plate surface will be referred to as the "horizontal line analysis profile of the plate surface," and the nth one counting from the steel plate surface will be referred to as the "nth horizontal line analysis profile of the plate surface." Similarly, the signal intensity profile for the region with 250 points in the thickness direction and one point in the horizontal direction of the plate surface will be referred to as the "line analysis profile in the thickness direction," and the nth one counting from the left when the steel plate surface is facing upwards will be referred to as the "nth line analysis profile in the thickness direction." Here, n is an integer from 1 to 250. Figure 2 shows an example of a portion of the horizontal line analysis profile of the plate surface.

[0083] Next, within the signal intensity profile in the horizontal direction of the board surface, the following is determined. In Figure 2, the dashed line shows "Average Value I ａ This is shown by the dashed line, and the first reference signal intensity I １ This is shown by the dashed line, and the second reference signal intensity I ２ Next, within the signal intensity profile in the horizontal direction of the board surface, the signal intensity is the first reference signal intensity I １ The above points are identified as regions where one or more points are consecutive. In Figure 2, (a1) to (a2), (b), and (c1) to (c3) are regions identified by this procedure. As in (b), there may be only one signal intensity data point included in a single B-enriched region.

[0084] Then, for each identified region, the maximum signal intensity within each region is the second reference signal intensity I ２ If the following conditions are met, the maximum value will be considered the "signal intensity in the B-enriched region". For example, in Figure 2, (a1) and (b) are considered the signal intensity in the B-enriched region, but (c2) is not considered the signal intensity in the B-enriched region, and (c1) to (c3) are not used in the calculation of the signal intensity in the B-enriched region. In other words, the "first reference signal intensity I １ Pixels showing a signal intensity less than "" are excluded from the "calculation of signal intensity in the B-enhancement region", however, if the signal intensities of adjacent pixels in the horizontal direction of the board surface are consecutively "first reference signal intensity I １ If it exceeds "the first reference signal intensity I", then the "first reference signal intensity I" will continue to be exceeded. １ Among the pixels that exceed "", only the pixel showing the maximum signal intensity is included in the calculation of the signal intensity in the B-enriched region, and pixels that do not show the maximum signal intensity are excluded from the calculation of the signal intensity in the B-enriched region. Furthermore, the signal intensity of the pixel showing the maximum signal intensity is the "second reference signal intensity I ２ Pixels exceeding this value are also excluded from the calculation of signal intensity in the B-enriched region. ２ The reason for establishing the second reference signal intensity I ２This is because regions with higher signal intensity are judged to be B precipitates (inclusions containing B), rather than regions where solid solution B is concentrated. On the other hand, the first reference signal intensity I １ The reason for establishing the first reference signal intensity I １ Regions with lower signal intensity are regions with relatively low solid-solution B concentration and are judged not to be B-enriched regions where solid-solution B has segregated (accumulated). In this disclosure, the hydrogen embrittlement resistance is improved by making the B concentration (solid-solution B concentration) in the B-enriched region (excluding B precipitates) relatively high. The reason for adopting this measurement method is that, since solid-solution B segregates along grain boundaries, it was thought that the B concentration (solid-solution B concentration) at the grain boundary could be determined by extracting the signal intensity at the intersection of the "one-dimensional line analysis line in the horizontal direction of the plate surface" and the "solid-solution B segregation line (at the grain boundary)". In other words, in the horizontal direction of the plate surface, the "first reference signal intensity I １ Only consecutive (adjacent) pixels beyond the specified interval are considered "adjacent to each other." Therefore, in this embodiment, pixels adjacent vertically or diagonally are excluded from the calculation of signal intensity in the B-enhancement region, regardless of the signal intensity of those pixels.

[0085] Using the same procedure as described above, the signal intensity in the enriched region for 250 line analysis profiles in the horizontal direction of the board surface is calculated. Then, the average value of all signal intensities designated as "the target for calculating the signal intensity in the enriched region B" among the 250 x 250 binned signal intensities is used as the "first reference signal intensity I" as defined in this disclosure. １ The above and the second reference signal intensity I ２ BO within the following range ２ － The average value of the signal intensity I ｂ As stated above, first, the first reference signal intensity I １ BO less than ２ － The signal strength of the above average value I ｂ It is excluded from the calculation. After this exclusion, the first reference signal intensity I １ If pixels showing a signal strength exceeding a certain level are adjacent to each other in the horizontal direction of the board, then the BO from adjacent pixels in the horizontal direction of the board ２－ Among the signal strengths, the highest BO ２ － BO other than signal strength ２ － The signal strength of the above average value I ｂ It is excluded from the calculation. After this exclusion, the remaining BO ２ － Among the signal strengths, the second reference signal strength I ２ BO that surpassed ２ － The signal strength is also the above average value I ｂ Exclude from the calculation. Then, all remaining BO ２ － The average value of the signal intensity is the above average value I ｂ This disclosure refers to "the first reference signal intensity I １ The above and the second reference signal intensity I ２ BO within the following range ２ － The average value of the signal intensity I ｂ " is the above average value I ａ It is more than 2.0 times that amount.

[0086] Note that "First reference signal strength I １ The above and the second reference signal intensity I ２ BO within the following range ２ － The average value of the signal intensity I ｂ " is the above average value I ａ It may be 2.2 times or more, 2.4 times or more, 2.6 times or more, 2.8 times or more, 3.0 times or more, 3.2 times or more, 3.4 times or more, 3.6 times or more, 3.8 times or more, or 4.0 times or more. Also, "First reference signal intensity I １ The above and the second reference signal intensity I ２ BO within the following range ２ － The average value of the signal intensity I ｂ " is the above average value I ａ It may be 7.5 times or less, 7.4 times or less, 7.2 times or less, 7.0 times or less, 6.8 times or less, 6.6 times or less, 6.4 times or less, 6.2 times or less, or 6.0 times or less. In this embodiment, the first reference signal intensity I １ Lower BO ２－ Locations with a signal intensity of I (hereinafter also simply referred to as "signal intensity") indicate that the solid solution B is not segregated (concentrated), meaning they are either inside a crystal grain or, if they are grain boundaries, stable grain boundaries. In this embodiment, the intention is to improve hydrogen embrittlement resistance by increasing the solid solution B concentration at unstable grain boundaries, therefore the first reference signal intensity I １ Locations with lower signal intensity are given an average value of I ｂ It is excluded from the calculation of the second reference signal intensity I ２ Locations with higher signal intensity were determined to be B precipitates (inclusions containing B, hereinafter the same) rather than unstable grain boundaries. To exclude signal intensity from locations containing B precipitates, a second reference signal intensity I was used. ２ Higher signal intensity, average value I ｂ This was excluded from the calculation. In this embodiment, the hydrogen embrittlement resistance is improved by making the B concentration (concentration of solid-solution B) in the B-enriched region (excluding B precipitates) relatively high.

[0087] By possessing characteristic structures related to the refinement of these structures and characteristic structures related to the segregation of B, the steel sheet of this embodiment can strengthen grain boundaries where grain boundary fracture is likely to occur, and significantly suppress hydrogen embrittlement cracking. In other words, the steel sheet of this embodiment can exhibit excellent resistance to hydrogen embrittlement.

[0088] The means for realizing such characteristic structures related to the refinement of the microstructure and the segregation of B, that is, the means for refining the microstructure and segregating B, will be explained later in the description of the steel sheet manufacturing method.

[0089] [Tensile strength: 1760 MPa or higher] As described above, the steel sheet of this embodiment has the characteristic structure related to the refinement of the microstructure and the characteristic structure related to the segregation of B, and in addition, the chemical composition and microstructure of the steel sheet are appropriately controlled. Therefore, even at high strength, which is prone to hydrogen embrittlement cracking, and more specifically at high strength with a tensile strength (TS) of 1760 MPa or higher, it can fully exhibit excellent resistance to hydrogen embrittlement. In other words, the steel sheet of this embodiment can achieve high strength with a tensile strength of 1760 MPa or higher while having excellent resistance to hydrogen embrittlement.

[0090] The tensile strength of the steel plate may be 1780 MPa or higher, 1800 MPa or higher, or 2000 MPa or higher. The upper limit of the tensile strength is not particularly limited, but from the viewpoint of workability, for example, it may be 3000 MPa or lower, or 2800 MPa or lower, 2100 MPa or lower, or 1950 MPa or lower.

[0091] The tensile strength (TS) of a steel sheet can be measured by taking a No. 5 tensile test specimen from the steel sheet, with its longitudinal direction perpendicular to the rolling direction and thickness direction, and performing a tensile test in accordance with JIS Z2241:2022. If a No. 5 tensile test specimen cannot be taken from the steel sheet being measured (for example, if the size of the sample is small), a tensile test specimen of any size with its longitudinal direction perpendicular to the rolling direction and thickness direction may be used instead of the No. 5 tensile test specimen.

[0092] If the rolling direction cannot be determined by the electron beam backscatter diffraction analysis, TOF-SIMS measurement, and tensile test described above, a sample with an observation surface in any direction perpendicular to the thickness direction or a tensile test specimen with the longitudinal direction may be used.

[0093] (Plating layer) The steel sheet of this embodiment may also have a zinc-containing plating layer (hereinafter sometimes referred to as the "zinc-containing plating layer") on at least one surface. The plating layer may be a zinc-containing plating layer having any known composition, and may also contain additive elements other than zinc, such as aluminum or magnesium. Furthermore, this zinc-containing plating layer may or may not be subjected to alloying treatment. In addition, the amount of zinc-containing plating layer that adheres is not particularly limited and may be a general amount.

[0094] (Application Examples) As described above, the steel sheet of this embodiment is a high-strength steel sheet with excellent hydrogen embrittlement resistance, and can therefore be applied to various structural components such as automobiles and other transportation machinery, industrial machinery, and buildings where excellent strength and durability are required. In particular, this disclosure can be used in the manufacture of automobile bodies and parts.

[0095] <Parts> In one preferred embodiment of the present disclosure, an automotive part is provided which includes a steel sheet according to the embodiment of the present disclosure described above. Examples of automotive parts include frame parts, bumpers, and other structural and reinforcing parts that require strength. Further examples of automotive parts include exterior parts such as roofs, hoods, fenders, and doors that require high aesthetic appeal. These parts only need to include a steel sheet according to the embodiment of the present disclosure in at least a portion of them. Therefore, these parts satisfy the characteristics of the steel sheet of the embodiment described above in at least a portion of them. In forming processes such as press forming, the characteristics of the steel sheet do not particularly change before and after forming in areas of the steel sheet that do not come into direct contact with the mold or, even if they come into direct contact with the mold, are processed to a relatively low degree. For example, in a part including a steel sheet according to the embodiment of the present disclosure, the portion of the sample collection site described later (i.e., the portion avoiding the locations described in (i) to (iv) below) can be recognized as an unprocessed portion, and this portion retains the characteristics of the steel sheet of the embodiment described above before and after forming as a part.

[0096] The steel sheet according to the embodiments of this disclosure can be used as various automotive parts as described above, for example, after a chemical conversion coating or paint film is optionally formed on its surface. Whether or not an automotive part having a paint film or chemical conversion coating includes the steel sheet according to the embodiments of this disclosure can be determined (without removing the paint film or chemical conversion coating) by performing TOF-SIMS analysis or the like on a sample taken from the automotive part using the method described above. However, the paint film, chemical conversion coating, or plating may be removed. The sampling locations are as follows.

[0097] When taking samples from automotive parts for various measurements and analyses, the following locations (i) to (iv) should be avoided: (i) Welded areas: within 20 mm of the toe of spot welds, and within 20 mm of the toe of arc / laser welds. (ii) Machined areas: machined areas with a radius of curvature of less than 15 mm, and within 5 mm of the said machined areas. (iii) Ends: ends within 5 mm of the cut end face of the part. (iv) Red rust: within 5 mm of areas where red rust is visible.

[0098] Methods for removing coatings, chemical conversion coatings, or plating include known chemical methods (for example, a method for removing coatings using a coating remover (specifically, Neoriver® #160, manufactured by Sansai Chemical Co., Ltd.), a method for removing chemical conversion coatings in accordance with JIS K 3151:1996, or a method for dissolving and removing the plating layer with an acidic aqueous solution containing an inhibitor (for example, an acidic aqueous solution at room temperature with 10% hydrochloric acid and 0.04% by mass of Ibit 710K (manufactured by Asahi Chemical Industry Co., Ltd.)), known mechanical methods (removal using a grinder or sandpaper, etc.), or methods combining these. Since there is a possibility of accidentally removing a portion of the base steel sheet, it is preferable to limit the removal to a portion and leave the portion intact before subjecting it to analysis and testing using the methods described above. If these materials are completely removed, it is preferable to confirm by some means that a portion of the base steel sheet has not been removed, or that the analysis position is at the plate thickness position specified in each analysis method (for example, 1 / 4 of the plate thickness from the surface of the steel sheet).

[0099] <Method for Manufacturing Steel Sheets> Next, a method for manufacturing steel sheets according to one embodiment of the present disclosure will be described. The following description is intended to illustrate characteristic methods for manufacturing steel sheets according to the embodiment of the present disclosure, and is not intended to limit the steel sheets to those manufactured by the manufacturing method described below.

[0100] A method for manufacturing a steel sheet according to one embodiment of this disclosure is a manufacturing method having the following steps (A) to (D). These steps will be described below.

[0101] [Process (A)] In this embodiment, process (A) is a process of hot rolling a slab having the specific chemical composition described above with respect to a steel sheet. Process (A) is a process in which finish rolling is performed in three or more passes, the hot-rolled steel sheet is cooled after the completion of finish rolling and wound up to form a hot-rolled coil, in which the reduction ratio of each of the last three passes of finish rolling is 14% or more, and the inter-pass time of the last three passes is 1.0 second or less. Furthermore, in process (A), the above T ｐ The cumulative reduction ratio at the following temperatures is 30% or less, and the finish rolling completion temperature is 800°C or higher. Furthermore, in process (A), the temperature of the hot-rolled steel sheet during the cooling process after the completion of the finish rolling is 800°C or higher. ｐ The thermal history from reaching the temperature to the start of winding satisfies equation (2) below, and the thermal history from the start of winding to cooling to 100°C satisfies equation (3) below. x1 calculated by equation (3) below is when the temperature of the hot-rolled steel sheet is T ｐThis refers to the cumulative average diffusion distance (m) of B from the time the temperature reaches T until the start of winding, and x1 is set to 0.0060 m or less in order to suppress nucleation of B between finish rolling and winding. x2, calculated by the following formula (4), refers to the cumulative average diffusion distance (m) of B from the start of winding until the steel sheet temperature reaches 100°C, and x2 is set to 3.7 m or less in order to suppress the growth and coarsening of precipitates after winding. Here, the cooling process refers to the process in which heat is removed from the steel sheet via cooling water and air after finish rolling. In cases where the heat generation associated with phase transformation etc. is large, it is conceivable that the steel sheet temperature may rise, but the section in which the steel sheet temperature rises due to the heat generated by the steel sheet itself is also included in the cooling process. Note that if the steel sheet temperature rises during the cooling process due to the heat generation mentioned above, ｐ If the temperature exceeds T ｐ The point at which the temperature of the hot-rolled steel sheet reaches the T temperature during the cooling process is defined as "the temperature of the hot-rolled steel sheet during the cooling process is T ｐ It is considered that the temperature has reached that point.

[0102] t: The temperature of the hot-rolled steel sheet is T ｐ The elapsed time (in seconds) from the point when the temperature reached t ＣＴ : The temperature of the hot-rolled steel sheet is T ｐ Time (seconds) from reaching the specified temperature until winding begins. T(t): Temperature of the steel plate at time t (°C). However, the integration step is 1 second. If the measurement interval is not 1 second, the temperature at each second is calculated by linear interpolation of the temperatures between measurement points. For example, if a certain measurement point is the time origin (t=0) and the measurement interval is every 3 seconds, data points t=0, 3, 6, ... are accumulated. In this case, when determining the temperatures at t=1 and t=2, the temperature gradient (dT / dt) between t=0 and t=3 is calculated, and T(1) = T(0) + dT / dt and T(2) = T(0) + 2 × dT / dt are calculated.

[0103] Furthermore, in equation (2), the unit of x1 is "m", but the unit of the right-hand side is t, where T(t) in the equation is in "℃" from t=0 seconds. ＣＴ When calculations are performed down to the second, the unit of the calculation result becomes "m".

[0104] t: Elapsed time (seconds) since winding began １００℃ : Time elapsed (seconds) from the start of winding until the temperature of the hot-rolled steel sheet reaches 100°C T(t): Temperature of the steel sheet at time t (°C) However, the integration step is 1 second. If the measurement interval is not 1 second, the temperature every second is calculated by linearly interpolating the temperature between measurement points in the same way as in equation (2) above.

[0105] Furthermore, in equation (3), the unit of x² is "m", but the unit of the right-hand side is t, where T(t) in the equation is in "℃" from t=0 seconds. １００℃ When calculations are performed down to the second, the unit of the calculation result becomes "m".

[0106] In this specification, "final three passes" refers to the three passes in the finish rolling process that are counted from the final pass: the first pass, the second pass, and the third pass.

[0107] In process (A), by performing finish rolling for three or more passes, setting the reduction ratio of each of the last three passes in the finish rolling to 14% or more, setting the interval between passes in the last three passes to 1.0 second or less, and setting the finish rolling completion temperature to 800°C or higher, it is possible to generate bainite and martensite while suppressing the formation of ferrite and pearlite after strain accumulation in austenite. This results in a uniform and fine hot-rolled microstructure and refines the austenite structure during heat treatment (annealing).

[0108] In process (A), the above T is applied during finish rolling. ｐ The cumulative reduction ratio at the following temperatures shall be 30% or less, and the temperature of the hot-rolled steel sheet during the cooling process after the completion of finish rolling shall be the above T ｐ By ensuring that the thermal history from the time the temperature is reached until winding begins satisfies equation (2) above, Fe ２３ (C, B) ６ This suppresses nucleation, thereby inhibiting the formation of B precipitates after winding, and contributing to the promotion of B segregation in the final product.

[0109] In equation (2) above, x1 is less than 0.0060m, but may be 0.0050m or less, or 0.0030m or less. The lower limit of x1 is not particularly limited, but <x> may be, for example, 0.0001m or more, 0.0003m or more, or 0.0006m or more.

[0110] Furthermore, by ensuring that the thermal history in process (A), from the start of winding to cooling to 100°C, satisfies the above equation (3), the formation of B precipitates after winding can be suppressed, contributing to the refinement of B precipitates in the final product.

[0111] In equation (3) above, x² is less than 3.7m, but may be 3.6m or less, 3.5m or less, or 3.4m or less. The lower limit of x² is not particularly limited, but x² may be, for example, 1.0m or more, 1.5m or more, 2.0m or more, or 2.5m or more.

[0112] In process (A), the reduction ratio, inter-pass time, and T are determined for each of the final three passes in the finish rolling process. ｐ The following are the cumulative reduction ratio at the following temperatures, the temperature at which finish rolling is completed, and the temperature of the hot-rolled steel sheet during the cooling process after finish rolling is completed. ｐ Other conditions are not particularly limited, except for the thermal history from the time the temperature is reached until the start of winding, and the thermal history from the start of winding of the hot-rolled steel sheet until it cools to 100°C. For example, the slab may be reheated and hot-rolled immediately after casting or after it has cooled once. When reheating, the heating temperature of the slab is, for example, 1150°C or higher, preferably 1200°C or higher. There is no particular upper limit to the heating temperature of the slab, but it may be, for example, 1300°C or lower.

[0113] The reduction ratio of each of the three final passes in finish rolling is 14% or more, but may be 16% or more, 17% or more, or 18% or more. There is no particular upper limit to the reduction ratio of each of the three final passes, but for example it is 50%. The reduction ratio of each of the three final passes may be 45% or less, or 40% or less.

[0114] The interval between each of the final three passes in finish rolling is 1.0 second or less, but may be 0.8 seconds or less. There is no particular lower limit to the interval between each of the final three passes, but for example, it is 0.3 seconds.

[0115] T in finish rolling ｐ The cumulative reduction ratio at the following temperatures is 30% or less, but may be 29% or less, 28% or less, 25% or less, or 20% or less. ｐ The lower limit of the cumulative reduction rate at the following temperatures is not particularly limited and may be 0%. ｐ The cumulative reduction rate at the following temperatures may be 1% or more, 5% or more, or 10% or more.

[0116] In process (A), for example, the cast slab may be subjected to rough rolling before finish rolling, for purposes such as adjusting the plate thickness. Such rough rolling is only required to ensure the desired sheet bar dimensions are achieved, and the conditions are not particularly limited.

[0117] Slabs that have undergone rough rolling or slabs that have not undergone rough rolling are subjected to the three or more passes of finish rolling described above. The completion temperature in finish rolling shall be 800°C or higher. The completion temperature in finish rolling may be controlled within the range of 800 to 940°C. If the completion temperature of finish rolling is 940°C or lower, the ferrite content in the hot-rolled steel sheet increases, making it less prone to embrittlement. Therefore, cracks that occur during cold rolling are less likely to occur. The completion temperature of finish rolling may be 930°C or lower or 910°C or lower.

[0118] In process (A), finish rolling is performed on a slab that has undergone rough rolling or a slab that has not undergone rough rolling. After the finish rolling is completed, the hot-rolled steel sheet is cooled and wound up to form a hot-rolled coil. At this time, the temperature of the hot-rolled steel sheet is T ｐ The thermal history from the time the temperature is reached until the start of winding must satisfy equation (2) above, and the thermal history from the start of winding until cooling to 100°C must satisfy equation (3) above.

[0119] In process (A), for example, during the cooling process from the completion of finish rolling to the start of coiling, the hot-rolled steel sheet may be air-cooled, water-cooled, or intermediate air-cooling may be performed in the range of 600 to 750°C during water cooling. If the intermediate air-cooling temperature is 750°C or higher, Ti and Nb carbides precipitate inside the ferrite contained in the hot-rolled steel sheet, increasing the yield stress of the hot-rolled steel sheet. This makes it easier to obtain sufficient friction on the surface of the steel sheet during cold rolling, as described later. The intermediate air-cooling temperature may be 730°C or lower, or 700°C or lower. On the other hand, if the intermediate air-cooling temperature is 600°C or higher, the ferrite content in the hot-rolled steel sheet increases, making it less prone to embrittlement, and thus less likely to crack during cold rolling. The intermediate air-cooling temperature may be 620°C or higher, or 650°C or higher.

[0120] In process (A), for example, the winding temperature when winding the hot-rolled steel sheet may be 400 to 700°C. If the winding temperature is 700°C or lower, the ferrite grain size in the hot-rolled steel sheet does not become too large, and sufficient strength can be easily obtained after annealing of the cold-rolled steel sheet. The winding temperature may be 680°C or lower, or 650°C or lower. On the other hand, if the winding temperature is 400°C or higher, the hot-rolled steel sheet does not become too hard and is less prone to embrittlement, so cracks are less likely to occur on the surface of the steel sheet in subsequent processes, and it is easier to ensure the ductility of the cold-rolled steel sheet after annealing. The winding temperature may be 420°C or higher, or 450°C or higher.

[0121] [Process (B)] In this embodiment, process (B) is a process in which the hot-rolled steel sheet after process (A) described above is unwound and pickled. By performing such pickling, the oxide scale on the surface of the hot-rolled steel sheet can be removed, thereby improving the chemical conversion treatment properties and plating properties of the cold-rolled steel sheet. The pickling conditions are not particularly limited, and general pickling conditions can be used. In addition, pickling may be performed in one step or in multiple steps.

[0122] [Process (C)] In this embodiment, process (C) is a process of cold rolling the hot-rolled steel sheet after pickling at a reduction ratio of 30% to 75%. By setting the cold rolling reduction ratio to 30% or more, the shape of the cold-rolled steel sheet can be kept flat, and a decrease in ductility in the final product can be suppressed. The cold rolling reduction ratio may be 35% or more or 40% or more. On the other hand, by setting the cold rolling reduction ratio to 75% or less, it is possible to prevent the rolling load from becoming excessive and making rolling difficult. The cold rolling reduction ratio may be 70% or less, 65% or less, or 60% or less. The number of rolling passes and the reduction ratio for each pass are not particularly limited and should be set appropriately so that the overall cold rolling reduction ratio falls within the above range.

[0123] In process (C), as long as cold rolling is performed at the specified reduction ratio described above, other conditions are not particularly limited, and any known cold rolling conditions may be adopted.

[0124] [Process (D)] In this embodiment, process (D) is a process of heat-treating the cold-rolled steel sheet obtained in process (C) described above, and is a process of heat-treating under the following conditions (D1) to (D5).

[0125] (D1): In the process of heating the cold-rolled steel sheet to the maximum heating temperature of Ac3 + 20°C to Ac3 + 75°C, the heating is performed at an average heating rate of 0.5°C / sec to 500°C / sec from 650°C until the maximum heating temperature is reached. The upper limit of the maximum heating temperature is set to Ac3 + 75°C in order to refine the particle size. However, if Ac3 is less than 780°C, the heating is performed to 800°C to 855°C.

[0126] (D2): Hold at Ac3 + 20°C to Ac3 + 75°C for 1 second to 1000 seconds. However, if Ac3 is less than 780°C, hold at 800°C to 855°C for 1 second to 1000 seconds.

[0127] (D3): The steel plate, maintained at Ac3 + 20°C to Ac3 + 75°C, is cooled to a temperature of Ms - 100°C or lower, with an average cooling rate of 10°C / second or more from 700°C to 500°C. However, if Ac3 is less than 780°C, the steel plate, maintained at 800°C to 855°C, is cooled to a temperature of Ms - 100°C or lower, with an average cooling rate of 10°C / second or more from 700°C to 500°C.

[0128] (D4): After cooling, the steel plate is held at 200°C to 350°C for 1 to 600 seconds.

[0129] (D5): In (D3) above, the thermal history from when the steel plate temperature reaches 800°C until it first reaches 550°C satisfies the following equation (4). x3 calculated by the following equation (3) represents the cumulative average diffusion distance (m) of B from when the steel plate temperature reaches 800°C until it reaches 550°C, and x3 shall be greater than 0.013m and less than 0.042m. t: Time elapsed (seconds) since the steel plate reached a temperature of 800°C. ５５０℃ : The time (seconds) from when the steel plate temperature starts at 800°C until it reaches 550°C. T(t): The temperature of the steel plate at time t (°C). However, the integration step is 1 second. If the measurement interval is not 1 second, the temperature every second is calculated by linearly interpolating the temperature between measurement points in the same way as in equation (2) above.

[0130] Note that in equation (4), the unit of x3 is "m", but the unit of the right-hand side is t, where T(t) in the equation is in "℃" from t=0 seconds. ５５０℃ When calculations are performed down to the second, the unit of the calculation result becomes "m".

[0131] In process (D), the Ac3 point (°C) is determined according to the following formula: Ac3 = 912 - 230.5 [C] - 20.4 [Mn] + 31.6 [Si] - 14.8 [Cr] - 18.1 [Ni] + 16.8 [Mo] + 100 [Al] - 39.8 [Cu] In the above formula, [C], [Mn], [Si], [Cr], [Ni], [Mo], [Al], and [Cu] represent the content (mass %) of each element.

[0132] Furthermore, in (D3) above, the Ms point (°C) is determined according to the following formula: Ms = 561 - 474 [C] - 33 [Mn] - 7.5 [Si] - 17 [Cr] - 17 [Ni] - 21 [Mo] + 10 [Co] In the above formula, [C], [Mn], [Si], [Cr], [Ni], [Mo], and [Co] represent the content (mass %) of each element.

[0133] In process (D), by heat-treating the cold-rolled steel sheet under the conditions (D1) to (D5) described above, the segregation of B at the grain boundaries during heat treatment (annealing) can be promoted.

[0134] In (D1) above, the average heating rate to the maximum heating temperature is set to 0.5 to 500°C / second from the viewpoint of promoting ferrite recrystallization and suppressing austenite coarsening. The average heating rate to the maximum heating temperature may be 1.0°C / second or more, or 2.0°C / second or more. Alternatively, the average heating rate may be 400°C / second or less, 300°C / second or less, 200°C / second or less, or 100°C / second or less. Here, "average heating rate" refers to the value obtained by dividing the difference between 650°C and the maximum heating temperature by the elapsed time from 650°C to the maximum heating temperature.

[0135] In (D1) to (D2) above, the maximum heating temperature shall be between Ac3 + 20°C and Ac3 + 75°C, from the viewpoint of promoting austenitization and suppressing the coarsening of the austenite diameter. The holding time between Ac3 + 20°C and Ac3 + 75°C shall be between 1 and 1000 seconds, from the viewpoint of promoting austenitization and productivity. The holding time at the maximum heating temperature may be 800 seconds or less, 600 seconds or less, 400 seconds or less, or 200 seconds or less. Also, the holding time between Ac3 + 20°C and Ac3 + 75°C may be 10 seconds or more, 20 seconds or more, 40 seconds or more, 60 seconds or more, or 80 seconds or more. During holding, the steel plate does not necessarily need to be held at a constant temperature and may fluctuate within the above maximum temperature range.

[0136] In (D3) above, in order to obtain the desired structure, after holding the steel plate at Ac3 + 20°C to Ac3 + 75°C, the steel plate is cooled to a temperature of Ms point - 100°C or lower, with an average cooling rate of 10°C / second or more from 700°C to 500°C. However, if Ac3 is less than 780°C, the steel plate held at 800°C to 855°C is cooled to a temperature of Ms - 100°C or lower, with an average cooling rate of 10°C / second or more from 700°C to 500°C. The average cooling rate from 700°C to 500°C may be 20°C / second or more, 30°C / second or more, or 50°C / second or more. In addition, when process (D) and the hot-dip galvanizing process described later are carried out as separate processes, it is conceivable that the "cooling process from 700°C to 500°C" may be repeated multiple times. In this case, the "average cooling rate from 700°C to 500°C" explained as (D3) is calculated for the steel plate held at Ac3 + 20°C to Ac3 + 75°C, or, if Ac3 is less than 780°C, for the steel plate held at 800°C to 855°C, from the point when the steel plate temperature first reaches 700°C to the point when it first reaches 500°C.

[0137] In (D4) above, in order to obtain the desired structure, the steel plate is cooled to a temperature below the Ms point -100°C and then held at 200 to 350°C for 1 to 600 seconds. The holding temperature of the steel plate after cooling may be 300°C or lower, or 250°C or lower. The holding time of the steel plate after cooling may be 500 seconds or lower, or 400 seconds or lower. The holding time of the steel plate after cooling may be 50 seconds or more, 100 seconds or more, or 150 seconds or more. During holding, the steel plate does not necessarily need to be held at a constant temperature and may fluctuate within the range of 200 to 350°C.

[0138] The holding time in the 200-350°C temperature range is calculated by summing the residence times in the 200-350°C temperature range for the thermal history after the steel plate temperature reaches Ms point - 100°C due to the cooling in (D3) above. The steel plate may also be reheated after the temperature falls below 200°C and held in the 200-350°C temperature range. In addition to the case of reheating, it is also conceivable that the "holding process in the 200-350°C range" may be repeated multiple times, such as when process (D) and the hot-dip galvanizing process described later are carried out as separate processes. In this case as well, the "holding time in the 200-350°C range" explained as (D4) is calculated by summing the residence times in the 200-350°C temperature range for the thermal history after the steel plate temperature reaches Ms point - 100°C due to the cooling in (D3) above.

[0139] In (D5) above, the thermal history from when the steel plate temperature in (D3) first reaches 550°C from 800°C satisfies the above equation (4). In equation (4), x3 is preferably 0.040 or less from the viewpoint of suppressing the precipitation of B. x3 may be 0.036 or less or 0.032 or less. Also, x3 may be 0.014 or more from the viewpoint of promoting the segregation of B to the grain boundaries. x3 may be 0.018 or more or 0.022 or more. In addition, when process (D) and the hot-dip galvanizing process described later are carried out as separate processes, it is conceivable that the "thermal history from 800°C to reaching 550°C for the first time" may be repeated multiple times. In this case, the "thermal history from 800°C to reaching 550°C for the first time," as explained as (D5), is calculated for the period from when the steel plate temperature first reaches 800°C to when it first reaches 500°C during the cooling process of a steel plate held at Ac3 + 20°C to Ac3 + 75°C, or, if Ac3 is less than 780°C, when the steel plate temperature is held at 800°C to 855°C.

[0140] In this step (D), as long as the cold-rolled steel sheet is heat-treated under the conditions (D1) to (D5) described above, the other conditions are not particularly limited, and any known heat treatment (annealing) conditions may be adopted.

[0141] (Plating Treatment) In this embodiment, a zinc-containing plating layer may be formed on at least one surface of the steel sheet. The means for forming such a plating layer is not particularly limited, and any known method may be used. For example, during or after step (D3) of process (D), the sheet may be heated or cooled to (zinc plating bath temperature - 40)°C to (zinc plating bath temperature + 50)°C as needed, and hot-dip galvanizing may be performed. The hot-dip galvanizing process forms a hot-dip galvanized layer on at least one surface of the cold-rolled steel sheet. When such a hot-dip galvanized layer is formed, the corrosion resistance of the cold-rolled steel sheet can be improved.

[0142] (Post-treatment) In addition, the surface of the hot-dip galvanized steel sheet and alloyed hot-dip galvanized steel sheet may be subjected to a top-layer plating treatment or other various treatments, such as chromate treatment, phosphate treatment, lubricity improvement treatment, and weldability improvement treatment, for the purpose of improving paintability, weldability, etc.

[0143] (Skin Pass Rolling) Furthermore, skin pass rolling may be applied to the steel sheet for the purpose of correcting the shape of the steel sheet and introducing movable dislocations to improve ductility. The reduction ratio of skin pass rolling after heat treatment may be in the range of 0.1 to 1.5%. If the reduction ratio is 0.1% or more, the desired effect can be sufficiently obtained and it is easy to control. Also, if the reduction ratio is 1.5% or less, productivity can be easily ensured.

[0144] The steel plates, parts containing them, and methods for manufacturing steel plates described herein are not limited to the embodiments described above or the examples described later, and can be appropriately combined, substituted, or modified without departing from the purpose and spirit of this disclosure.

[0145] The present disclosure will be described in more detail below with reference to examples, but the present disclosure is not limited in any way to these examples. The present disclosure may adopt various conditions, insofar as they do not depart from the gist of the present disclosure and achieve the objectives of the present disclosure.

[0146] (Preparation of steel plates) First, slabs were produced by casting steel with various chemical compositions. These slabs were then hot-rolled to produce hot-rolled steel plates. Furthermore, these hot-rolled steel plates were sequentially subjected to grinding, cold-rolling, and heat treatment to produce cold-rolled steel plates.

[0147] A portion of the obtained cold-rolled steel sheets was subjected to plating.

[0148] Table 1 below shows the molten steel analysis values ​​for the various steel sheets obtained as described above. The remainder other than the components shown in Table 1 consists of Fe and impurities. Underlines next to the chemical compositions in Table 1 indicate that they are outside the scope of this disclosure.

[0149]

[0150] Hot rolling was carried out under the conditions shown in Table 2 below. Next, the hot-rolled steel sheets were pickled. Furthermore, cold rolling was performed on the pickled steel sheets at the reduction ratios shown in Table 3 below.

[0151] Subsequently, the cold-rolled steel sheets were subjected to heat treatment under the conditions described in Table 3 below. Here, R1, R2, and R3 are the reduction ratios for the 3rd pass, 2nd pass, and 1st pass (final pass), respectively, counting from the final pass during the hot-rolling finish rolling process. Also, t1 and t2 are the inter-pass times between the 3rd and 2nd passes, and between the 2nd and 1st passes (final pass), respectively, counting from the final pass during the hot-rolling finish rolling process. The heat treatment involved heating the steel sheets to the maximum heating temperature at a predetermined average heating rate, holding them at that maximum heating temperature for a predetermined time, and then cooling them at a predetermined cooling rate. Furthermore, the steel sheets were cooled to a temperature below the Ms point - 100°C and then held at 200-350°C. In Table 3, Ms is the martensitic transformation point (°C) of the steel used.

[0152]

[0153]

[0154] Subsequently, some of the steel sheets underwent continuous hot-dip galvanizing, and some of those underwent alloying treatment. The plating conditions were not special, but rather known and general conditions. In Table 2, GA refers to alloyed hot-dip galvanized steel sheets. GI refers to hot-dip galvanized steel sheets that have not undergone alloying treatment. CR refers to cold-rolled steel sheets that have not been plated.

[0155] The various steel sheets obtained as described above were subjected to microstructural identification and time-of-flight secondary ion mass spectrometry (TIR) ​​measurements according to the various measurement methods described above. The results of these measurements are shown in Table 4 below. Note that the hot-dip galvanized steel sheets were used in the test without removing the hot-dip galvanizing.

[0156] In Table 4, "α + B" represents ferrite and bainite, "γ" represents retained austenite, and "M" represents martensite.

[0157] In addition, the underlines next to various numerical values ​​in Tables 2, 3, and 4 indicate that they are outside the scope of this disclosure, that the manufacturing conditions are such that the steel sheet described herein cannot be obtained, or that the various properties of the steel sheet are undesirable.

[0158] (Evaluation of Hydrogen Embrittlement Resistance) The hydrogen embrittlement resistance of the steel sheet was evaluated by the following U-bending test. Here, Figure 3 is a schematic diagram illustrating the method for evaluating the hydrogen embrittlement resistance of the steel sheet using the U-bending test.

[0159] First, a strip-shaped test piece 11 measuring 30 mm x 120 mm was taken from a steel plate. Holes for bolt fastening were drilled at both ends of the longitudinal direction of this test piece 11. Next, as shown in Figure 3, the test piece 11 was bent 180° using a punch 12 with a radius of 10 mm and a die (support roll) 13. At this time, the clearance between the punch 12 and the die (support roll) 13 was set to the thickness of the test piece 11 + 1.0 mm. Next, stress was applied to the springback U-bent test piece 14 by fastening it with bolts 15 and nuts 16 as shown in Figure 1. A strain gauge 17 with a GL of 3 mm was attached to the top of the U-bent test piece 14, and stress was applied by controlling the amount of strain. The applied stress was equivalent to 1050 MPa and 1350 MPa. At this time, the strain was converted to stress from the stress-strain curve obtained in advance in a tensile test.

[0160] Next, each U-bending test specimen 14 was immersed for 48 hours in 1000 mL of pH 3.0 hydrochloric acid aqueous solution for 48 hours, with stress applied by bolts 15 and nuts 16. The end faces of the U-bending test specimens 14 were milled. After the test was completed, the bending apex of the U-bending test specimen 14 was observed with a microscope or optical microscope, and the length of the largest crack was measured. Cracks exceeding 3 mm were judged to be "cracks". Then, specimens that cracked under a load stress of 1000 MPa were evaluated as "C", specimens that did not crack under a load stress of 1000 MPa but cracked under a load stress of 1200 MPa were evaluated as "B", and specimens that did not crack under a load stress of 1200 MPa were evaluated as "A".

[0161] Table 4 shows the evaluation results for hydrogen embrittlement resistance.

[0162]

[0163] As shown in Table 4, the above-mentioned specific chemical composition and microstructure, the average grain size of the crystal grains being 10 μm or less, and the first reference signal intensity I １ The above and the second reference signal intensity I ２ BO within the following range ２ － The average value of the signal intensity I ｂ The average value is I ａThe steel plates No. 1-8 and 12-18, which are embodiments of this disclosure and are more than 2.0 times the average value, were found to all have excellent hydrogen embrittlement resistance and tensile strength. In particular, the average value I ｂ The average value is I ａ Steel plates No. 3, 17, and 18, which are more than 3.2 times stronger than the others, were found to all possess extremely excellent hydrogen embrittlement resistance.

[0164] On the other hand, the steel plates No. 9 to 11, which serve as comparative examples, do not meet the above specific range of manufacturing conditions, therefore the average value I ｂ It was found that the value decreased, resulting in inferior hydrogen embrittlement resistance. Furthermore, it was found that the comparative steel sheets No. 19, 21, and 22 had inferior hydrogen embrittlement resistance because their chemical composition was not within the specified range. In addition, the average value I of steel sheets No. 24 and 25 was found to be inferior because their chemical composition or the precipitation initiation temperature of B was not within the specified range. ｂ It was found that the amount of hydrogen embrittlement was reduced, resulting in inferior hydrogen embrittlement resistance. Furthermore, although comparative example No. 20 steel sheet had excellent hydrogen embrittlement resistance, its chemical composition (specifically, the C content) was not within the specified range, resulting in low tensile strength and inferior overall strength. Similarly, although comparative example No. 23 steel sheet had excellent hydrogen embrittlement resistance, its chemical composition (specifically, the Mn content) was not within the specified range, resulting in excessive formation of ferrite and bainite, low tensile strength, and inferior overall strength.

[0165] 11 Strip-shaped test piece 12 Punch 13 Die 14 U-bend test piece 15 Bolt 16 Nut 17 Strain gauge

Claims

1. The chemical composition is in mass %, C: 0.27 to 0.40%, Si: 0.01 to 2.50%, Mn: 1.00 to 4.00%, Al: 0.001 to 1.500%, Ti: 0.001 to 0.100%, B: 0.0005 to 0.0100%, P: 0.050% or less, S: 0.0100% or less, N: 0.0200% or less, O: 0.0100% or less, Cr: 0 to 1.00%, Cu: 0 to 1.00%, Mo: 0 to 1.00%, Ni: 0 to 1.00%, Co: 0 to 3.00%, W: 0 to 1.00%, Sn: 0 to 1.00%, Sb: 0 to 0.50%, Nb: 0 to 0.200%, V: 0 to 1.00%, As: 0 to 0.10%, Zn: 0 to 1.00%, Mg: 0 to 0.0500%, Ca: 0 to 0.0500%, Zr: 0 to 0.5000%, Ce: 0 to 0.0300%, La: 0 to 0.0150%, Hf: 0 to 0.0100%, Bi: 0 to 0.0100%, REM other than Ce and La: 0 to 0.1000%, and the balance: Fe and impurities, and the contents of C and B satisfy the following formula (1), and the microstructure at a depth of 1 / 4 of the plate thickness from the steel plate surface is in area %, the total of ferrite and bainite: 5% or less, retained austenite: 10% or less, martensite: 85% or more, and the balance: 5% or less, at the position of 1 / 4 of the plate thickness from the steel plate surface, the average grain size of the crystal grains is 10 μm or less, and BO － , ｂ , ２ , ２ － Regarding the signal intensity of, the BO ２ － The average value I of the signal intensity ａ The signal intensity 1.2 times that of is taken as the first reference signal intensity I １ And the average value I ａ The signal intensity 7.5 times that of is taken as the second reference signal intensity I ２ When, the first reference signal intensity I １ Is greater than or equal to and the second reference signal intensity I ２ Within the range of less than or equal to, the average value I of the signal intensity of the BO ２ － Of is the average value I ｂ Of the average value I ａ A steel plate characterized in that the first reference signal intensity I is 2.0 times or more, and the tensile strength of the steel plate is 1760 MPa or more. １ If pixels showing a signal strength exceeding 0 are adjacent to each other, the BO from adjacent pixels ２ － Among the signal strengths, the highest BO ２ － Other than the signal strength of the BO ２ － The signal intensity of the average value I ｂ Exclude from calculation. T ｐ <1000 ... (1) However, T ｐ = 555 + 179 × [C] + 8083 × √[B] [C]: Content of C (mass%) [B]: Content of B (mass%) 2. The chemical composition is as follows, in mass%, Cr: 0.001-1.00%, Cu: 0.001-1.00%, Mo: 0.01-1.00%, Ni: 0.01-1.00%, Co: 0.01-3.00%, W: 0.001-1.00%, Sn: 0.001-1.00%, Sb: 0.001-0.50%, Nb: 0.001-0.200%, V: 0.001-1.00%, As: 0.001-0.10%, Zn: 0.001-1.00%, Mg: 0.0001-0.0500%, Ca: 0.0001-0.0500%, The steel sheet according to claim 1, characterized by containing one or more of the following: Zr: 0.0001 to 0.5000%, Ce: 0.0001 to 0.0300%, La: 0.0001 to 0.0150%, Hf: 0.0001 to 0.0100%, Bi: 0.0001 to 0.0100%, and REM other than Ce and La: 0.0001 to 0.1000%.

3. The steel sheet according to claim 1, characterized in that the chemical composition contains, by mass%, N: 0.0100% or less, Co: 0 to 1.00%, B: 0.0005 to 0.0030%, Mg: 0 to 0.0100%, Ca: 0 to 0.0100%, Zr: 0 to 0.0100%, Ce: 0 to 0.0150%, and REM other than Ce and La: 0 to 0.0100%.

4. A component characterized by comprising a steel plate as described in any one of claims 1 to 3.

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