Steel sheet, component including same, and method for producing steel sheet

Abstract

By means of a novel configuration, the present invention provides: a steel sheet which has high strength and excellent hydrogen embrittlement resistance; a component comprising the steel sheet; and a method for producing the steel sheet. Provided are a steel sheet , a component comprising same, and a method for producing a steel sheet, the steel sheet being characterized in that: the steel sheet has a predetermined chemical composition; at a position at 1 / 4 of the sheet thickness from a surface of the steel sheet, the steel sheet contains, in terms of area%, ferrite and bainite in a total amount of 5% or less, and 10% or less of residual austenite, with the balance being martensite; and the steel sheet has a steel structure in which when time of flight secondary ion mass spectrometry is performed at a position at 1 / 4 of the sheet thickness from the surface of the steel sheet, an average value of BO2 - detection signal intensity is 8.0 or less at a measurement point at which BO2 - detection signal intensity is 4 times the mode or less.

Description

Steel plate, parts containing the same, and method for manufacturing steel plate

[0001] This invention relates to steel plates, parts containing the same, and methods for manufacturing steel plates.

[0002] In recent years, there has been a growing demand for improved fuel efficiency in automobiles from the perspective of regulating greenhouse gas emissions as a measure against global warming. As a result, the application of high-strength steel plates is expanding to reduce vehicle weight and ensure collision safety.

[0003] In high-strength steel sheets, hydrogen embrittlement cracking (also known as delayed fracture) 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 environment. Generally, it is known that hydrogen embrittlement cracking in steel sheets is more likely to occur as the strength of the steel sheet increases. This is thought to be because the higher the tensile strength of the steel sheet, the greater the stress remaining in the steel sheet after the part is formed. This susceptibility to hydrogen embrittlement cracking is called hydrogen embrittlement resistance.

[0004] Various attempts have been made to improve the hydrogen embrittlement resistance of steel sheets.

[0005] For example, Patent Document 1 describes a precipitate having a predetermined chemical composition, a structure in which the area ratio of the martensite to the entire structure is 95% or more and 100% or less, an average particle size of prior austenite grains of less than 11.2 μm, and an equivalent circle diameter of 500 nm or more, with a number density A of A (particles / mm²). ２ ) ≤ 8.5 × 10 ５ ×A steel sheet satisfying [B] is disclosed. Here, [B] represents the content (mass%) of B. Patent Document 1 teaches that the above configuration provides a steel sheet that is high in strength and has excellent resistance to delayed fracture.

[0006] International Publication No. 2023 / 008003

[0007] As mentioned above, hydrogen embrittlement cracking is known to occur more easily as the strength of the steel plate increases.

[0008] Therefore, the present invention aims to provide a steel sheet with high strength and excellent resistance to hydrogen embrittlement, a component containing the same, and a method for manufacturing the steel sheet, through a novel configuration.

[0009] To achieve the above objective, the inventors focused particularly on the microstructure of the steel sheet. Specifically, the inventors first found that by optimizing the chemical composition of the steel sheet, in particular by optimizing the amount of B contained, and by constructing the microstructure of the steel sheet with a structure mainly composed of martensite, it is possible to increase the strength of the steel sheet, for example, to a tensile strength of 1700 MPa or more, more preferably 1760 MPa or more, while improving the hydrogen embrittlement resistance of the steel sheet. In addition, the inventors found that by lowering the amount of solid-solution B in the prior austenite grains, more specifically, as will be explained in more detail later, when a flight-type secondary ion mass spectrometry is performed, BO ２ － At measurement points where the detected signal intensity is 4 times or less the mode, BO ２ － By controlling the average value of the detected signal intensity to 8.0 or less, B can be sufficiently segregated at the prior austenite grain boundaries. As a result, we have found that even at high strengths of 1700 MPa or more, more preferably 1760 MPa or more, the occurrence of hydrogen embrittlement cracking can be significantly suppressed, thus completing the present invention.

[0010] The present invention that can achieve the above object is as follows. (1) 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 0.0030%, P: 0.050% or less, S: 0.0100% or less, N: 0.0150% 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 1.00%, W: 0 to 1.00%, Sn: 0 to 1.000%, Sb: 0 to 0.500%, Nb: 0 to 0.200%, V: 0 to 1.00%, As: 0 to 0.100%, Zn: 0 to 1.000%, Mg: 0 to 0.0100%, Ca: 0 to 0.0100%, Zr: 0 to 0.0100%, Ce: 0 to 0.0150%, La: 0 to 0.0150%, Hf: 0 to 0.0100%, Bi: 0 to 0.0100%, REM other than Ce and La: 0 to 0.0100%, and the balance: consisting of Fe and impurities, at a position 1 / 4 of the plate thickness from the surface, in area %, the total of ferrite and bainite: 5% or less, retained austenite: 10% or less, and martensite: 85% or more, when performing flight-type secondary ion mass spectrometry at a position 1 / 4 of the plate thickness from the surface, BO ２ － At the measurement point where the detection signal intensity is 4 times or less the most frequent value, BO ２ －A steel plate characterized by having a steel structure in which the average value of the detected signal intensity is 8.0 or less. (2) The above chemical composition is as follows in mass%: Cr: 0.001-1.00%, Cu: 0.001-1.00%, Mo: 0.001-1.00%, Ni: 0.001-1.00%, Co: 0.001-1.00%, W: 0.001-1.00%, Sn: 0.001-1.000%, Sb: 0.001-0.500%, Nb: 0.001-0.200%, V: 0.001-1.00%, As: 0.001-0.100%, Zn: 0.001-1.000%, Mg: 0.0001-0.0100%, Ca: 0.0001-0.0100% The steel sheet as described in (1) above, characterized in that it contains at least one of the following: Zr: 0.0001 to 0.0100%, Ce: 0.0001 to 0.0150%, 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 0.0100%. (3) When the above-mentioned flight-type secondary ion mass spectrometry is performed, BO ２ －(1) or (2) above, characterized in that the sample skewness of the detected signal intensity distribution is 20.0 or less. (4) A steel sheet according to any one of (1) to (3) above, characterized in that it has a tensile strength of 1700 MPa or more. (5) A steel sheet according to any one of (1) to (4) above, characterized in that it has a hot-dip galvanized layer or an alloyed hot-dip galvanized layer on at least one surface. (6) A component characterized in that it includes a steel sheet according to any one of (1) to (5) above. (7) (A) A hot rolling process comprising hot rolling a slab having the chemical composition described in (1) or (2) above, then winding the resulting hot-rolled steel sheet and cooling, wherein the following conditions (A1) to (A3) are met: (A1) Three or more rolling passes are performed in the temperature range of 950 to 1050°C with a reduction ratio of 0.2 or more, and the cumulative reduction ratio is 0.60 or more; (A2) The cumulative reduction ratio is 0.50 or less in the temperature range of 900 to 950°C; (A3) For each rolling pass, the temperature difference between the rolling entry plate temperature between the nth pass and the (n+1)th pass satisfies the following formula (1): Here, n: number of rolling passes T ｎ : Rolling entry plate temperature (°C) for the nth pass t ｎ : Rolling entry side thickness (mm) of the nth pass (B) A cold rolling step in which the hot-rolled steel sheet is pickled and cold-rolled with a reduction ratio of 0.30 to 0.75 (C) A heat treatment step in which the obtained cold-rolled steel sheet is heated to a maximum heating temperature of Ac3 to 950°C, held for 1 to 1000 seconds, and then cooled to room temperature while the thermal history from when it reaches 600°C or below until it first reaches (Ms-100)°C or below satisfies the following formula (2). Here, t: Elapsed time (seconds) from when the steel plate temperature reaches 600°C or below until it first reaches (Ms-100)°C or below. T: Average temperature (°C) from when the steel plate temperature reaches 600°C or below until it first reaches (Ms-100)°C or below. Here, the average temperature (°C) is calculated by measuring the temperature every 1 second during the above elapsed time t, calculating the accumulated temperature, and dividing the obtained accumulated temperature by the above elapsed time t. C ∞: Grain boundary segregation concentration (mass%) of boron at equilibrium at temperature T C ｔ A method for manufacturing a steel sheet, characterized by comprising: [B]: the concentration of boron grain boundary segregation when held at temperature T for time t (mass%); [B]: the B content in the chemical composition of the steel sheet (mass%). (8) The manufacturing method described in (7) above, characterized in that the heat treatment step further satisfies the following conditions (C1) to (C3): (C1) Cooling the cold-rolled steel sheet in a temperature range of 600 to 750°C at an average cooling rate of 10°C / second or more; (C2) Allowing it to remain in a temperature range of (Ms-100) to 600°C for 1 to 1000 seconds and cooling it to (Ms-100)°C or below; (C3) Reheating the cold-rolled steel sheet cooled to (Ms-100)°C or below to a temperature range of 200°C or higher and allowing it to remain there for 1 to 1000 seconds, then cooling it to 100°C or below, and the thermal history from when it reaches (Ms-100)°C or below until it first reaches 100°C or below satisfies the following formula (4). Here, r ｆ / r ０ : Particle size ratio of precipitate B before and after reheating treatment t: Elapsed time (integer seconds) from the start of reheating treatment t ｆ : Reheating completion time (integer seconds) T(t): Process temperature at time t (°C) However, the integration step is 1 second. If the measurement interval is less than 1 second, the temperature every second is calculated by linearly correcting the temperature between measurement points. [B]: B content in the chemical composition of the steel plate (mass %)

[0011] According to the present invention, it is possible to provide a steel sheet with high strength and excellent resistance to hydrogen embrittlement, a component containing the same, and a method for manufacturing the steel sheet.

[0012] Figures 1(A) and 1(B) are schematic diagrams illustrating a method for evaluating the hydrogen embrittlement resistance of steel plates using a four-point bending test.

[0013] <Steel Sheet> The steel sheet according to the embodiment of the present invention has the following composition 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: 0.0005-0.0030%, P: 0.050% or less, S: 0.0100% or less, N: 0.0150% or less, O: 0.0100% or less, Cr: 0-1.00%, Cu: 0-1.00%, Mo: 0-1.00%, Ni: 0-1.00%, Co: 0-1.00%, W: 0-1.00%, Sn: 0-1.00% The chemical composition consists of Sb: 0-0.500%, Nb: 0-0.200%, V: 0-1.00%, As: 0-0.100%, Zn: 0-1.000%, Mg: 0-0.0100%, Ca: 0-0.0100%, Zr: 0-0.0100%, Ce: 0-0.0150%, La: 0-0.0150%, Hf: 0-0.0100%, Bi: 0-0.0100%, REM other than Ce and La: 0-0.0100%, and the remainder being Fe and impurities. At a position 1 / 4 of the plate thickness from the surface, the area percentage contains: total of ferrite and bainite: 5% or less, retained austenite: 10% or less, and martensite: 85% or more. When a flight-type secondary ion mass spectrometry was performed at a position 1 / 4 of the plate thickness from the surface, BO ２ － At measurement points where the detected signal intensity is 4 times or less the mode, BO ２ － It is characterized by having a steel structure in which the average value of the detected signal intensity is 8.0 or less.

[0014] As mentioned earlier, hydrogen embrittlement cracking is known to occur more easily as the strength of the steel sheet increases. In particular, in steel sheets with very high strength, such as those with a tensile strength of 1700 MPa or more, more preferably 1760 MPa or more, the steel structure of the steel sheet generally contains martensite as the main component in order to ensure high strength. On the other hand, in the case of such high-strength steel sheets mainly composed of martensite, hydrogen embrittlement cracking may occur when hydrogen that has penetrated the steel accumulates at the prior austenite grain boundaries in the martensite structure, reducing the interatomic bonding force.

[0015] Therefore, in addition to optimizing the chemical composition of the steel sheet, the inventors focused particularly on the microstructure of the steel sheet. More specifically, the inventors found that by configuring the microstructure of a steel sheet having an optimized chemical composition, particularly one containing B: 0.0005 to 0.0030% by mass%, with a structure mainly composed of martensite, and more specifically, by configuring it to contain 85% or more martensite by area%, it is possible to improve the hydrogen embrittlement resistance of the steel sheet while achieving the desired high strength, for example, a tensile strength of 1700 MPa or more, more preferably 1760 MPa or more.

[0016] B is known to be an effective element for improving hydrogen embrittlement resistance by strengthening prior austenite grain boundaries through segregation. However, simply adding a predetermined amount of B sometimes did not yield sufficient hydrogen embrittlement resistance. In other words, in order to improve the hydrogen embrittlement resistance of steel sheets by adding B, it is necessary to sufficiently segregate B at the prior austenite grain boundaries.

[0017] Therefore, the inventors focused on other characteristics of the steel microstructure and conducted investigations. As a result, the inventors found that reducing the amount of solid-solution B in the prior austenite grains, more specifically when performing flight-type secondary ion mass spectrometry, ２ － At measurement points where the detected signal intensity is 4 times or less the mode, BO ２ －We found that by controlling the average value of the detected signal intensity to 8.0 or less, B can be sufficiently segregated at the prior austenite grain boundaries, thereby strengthening these prior austenite grain boundaries. As a result, even at high strengths of 1700 MPa or more, more preferably 1760 MPa or more, the occurrence of hydrogen embrittlement cracking can be significantly suppressed.

[0018] First, we will explain the relationship between flight-type secondary ion mass spectrometry and the characteristics of steel microstructure. In flight-type secondary ion mass spectrometry, BO ２ － The detected signal intensity depends on the concentration of B at the measurement point and provides information to determine whether B is in a solid solution state or exists as a precipitate, etc. For example, at a measurement point where B is concentrated, such as as a precipitate, BO ２ － While the detection signal intensity increases, at measurement points where B is diluted, such as in a solid solution state, BO ２ － The detection signal intensity will be lower. Also, in the microstructure recognized as prior austenite grains in cross-sectional observation of the steel plate, the area recognized as the inside of the prior austenite grain (matrix phase) is larger than the area recognized as the prior austenite grain boundary. Therefore, the number of measurement points in the measurement field of the flying secondary ion mass spectrometer will be greater for the matrix phase than for the grain boundaries of the prior austenite grains. Therefore, in cross-sectional observation of the steel plate, ２ － The most frequent value of the detected signal intensity will originate from the matrix phase B. Here, B in the matrix phase is mainly in a solid solution state, and therefore BO ２ － Measurement points where the detected signal intensity is four times or less the mode mainly originate from the solid solution state of B within the prior austenite grains. Thus, when flight-type secondary ion mass spectrometry is performed on the cross section of a steel plate, BO ２ － At measurement points where the detected signal intensity is 4 times or less the mode, BO ２ － Controlling the average value of the detected signal intensity to 8.0 or less will result in a lower amount of solid-solution B within the existing austenite grains.

[0019] Furthermore, finely controlling the particle size of the prior austenite grains is effective in reducing the amount of dissolved B within the prior austenite grains. When the particle size of the prior austenite grains is small, the proportion of grain boundaries relative to the matrix phase of the prior austenite grains becomes larger. As a result, the number of sites where B can segregate increases, and the distance required for diffusion to the grain boundaries also decreases, making it easier for B in the matrix phase to move to the grain boundaries. Consequently, the amount of dissolved B within the prior austenite grains, corresponding to the amount of BO, ２ － BO at measurement points where the detected signal intensity is 4 times or less the mode. ２ － The average value of the detected signal intensity becomes lower. As a result, B can be sufficiently segregated at the prior austenite grain boundaries, making it possible to significantly suppress the occurrence of hydrogen embrittlement cracking.

[0020] Therefore, according to the steel sheet according to the embodiment of the present invention, high strength, more specifically 1700 MPa or more, more preferably 1760 MPa or more, can be achieved by a steel structure mainly composed of martensite, while at the same time, hydrogen embrittlement resistance can also be significantly improved. For this reason, the steel sheet according to the embodiment of the present invention is particularly useful for use in the automotive sector, where a high level of both high strength and hydrogen embrittlement resistance is required.

[0021] The steel sheets according to embodiments of the present invention will be described in more detail below. In the following description, "%", which is the unit for the content of each element, means "mass%" unless otherwise specified. In this specification, "~", which indicates a numerical range, is used to mean that the numbers written before and after it are included as the lower limit and upper limit, respectively, unless otherwise specified.

[0022] [C: 0.27-0.40%] Carbon (C) is an essential element for ensuring the strength of steel plates. To obtain this effect sufficiently, the C content should be 0.27% or more. The C content may be 0.29% or more, 0.31% or more, or 0.33% or more. On the other hand, if the C content is excessive, the hydrogen embrittlement resistance may decrease due to an excessive increase in strength. For this reason, the C content should be 0.40% or less. The C content may be 0.38% or less, 0.36% or less, or 0.34% or less.

[0023] [Si: 0.01-2.50%] Si is an element that suppresses the formation of iron-based carbides and contributes to improved strength and formability. To fully obtain these effects, the Si content should be 0.01% or more. The Si content may be 0.50% or more, 0.70% or more, 0.90% or more, or 1.10% or more. On the other hand, if the Si content is excessive, local ductility may decrease and hydrogen embrittlement resistance may decrease. Therefore, the Si content should be 2.50% or less. The Si content may be 2.00% or less, 1.80% or less, 1.60% or less, or 1.40% or less.

[0024] [Mn: 1.00–4.00%] Mn is a strong austenite-stabilizing element and is effective in increasing the strength of steel sheets. To obtain this effect fully, the Mn content should be 1.00% or more. The Mn content may be 1.80% or more, 2.00% or more, 2.20% or more, or 2.40% or more. On the other hand, excessive Mn content can lead to a decrease in grain boundary bonding strength due to Mn segregation to prior austenite grain boundaries, which may reduce hydrogen embrittlement resistance. Therefore, the Mn content should be 4.00% or less. The Mn content may be 3.00% or less, 2.80% or less, or 2.60% or less.

[0025] [Al: 0.001-1.500%] Al is an element that acts as a deoxidizing agent. To obtain this effect sufficiently, the Al content should be 0.001% or more. The Al content may be 0.010% or more, 0.020% or more, 0.030% or more, 0.040% or more, or 0.050% or more. On the other hand, if Al is included in excess, the effect will saturate, and including more Al in the steel plate than necessary will lead to an increase in manufacturing costs. Therefore, the Al content should be 1.500% or less. The Al content may be 1.000% or less, 0.500% or less, 0.200% or less, or 0.100% or less.

[0026] [Ti: 0.001 to 0.100%] Ti is an effective element for fixing N, which is present as an impurity in steel, as TiN, and for suppressing the precipitation of B as a nitride. To obtain these effects sufficiently, the Ti content should be 0.001% or more. The Ti content may be 0.005% or more, 0.010% or more, 0.015% or more, or 0.020% or more. On the other hand, if Ti is included in excess, the effect will saturate, and including more Ti in the steel sheet than necessary will lead to an increase in manufacturing costs. Therefore, the Ti content should be 0.100% or less. The Ti content may be 0.080% or less, 0.060% or less, 0.040% or less, or 0.020% or less.

[0027] [B: 0.0005 to 0.0030%] B is an element that enhances hardenability and contributes to improved strength. B is also an effective element for strengthening prior austenite grain boundaries by segregating at them, thereby improving hydrogen embrittlement resistance. To obtain these effects fully, the B content should be 0.0005% or more. The B content may be 0.0008% or more, 0.0010% or more, or 0.0015% or more. On the other hand, if B is included in excess, excessive borides may be generated in the steel, which may reduce the hardenability of the steel sheet or reduce its hydrogen embrittlement resistance. Therefore, the B content should be 0.0030% or less. The B content may be 0.0028% or less, 0.0026% or less, 0.0024% or less, 0.0022% or less, or 0.0020% or less.

[0028] [P: 0.050% or less] P is a solid solution strengthening element and is effective in increasing the strength of steel plates, but excessive addition may degrade weldability and toughness. Therefore, the P content should be 0.050% or less. Preferably, the P content is 0.045% or less, 0.035% or less, or 0.020% or less. The P content may be 0%, but reducing the P content to an extreme degree will increase the cost of removing P. For this reason, from an economic standpoint, the P content may be 0.0001% or more, 0.0005% or more, or 0.001% or more.

[0029] [S: 0.0100% or less] S is an element contained as an impurity and can form MnS in steel, degrading toughness and hole-expanding properties. Therefore, the S content should be 0.0100% or less. Preferably, the S content is 0.0050% or less, 0.0040% or less, or 0.0030% or less. The S content may be 0%, but extremely low S content increases desulfurization costs. For this reason, from an economic standpoint, the S content may be 0.0001% or more, 0.0003% or more, or 0.0005% or more.

[0030] [N: 0.0150% or less] N is an element contained as an impurity, and if the N content is high, coarse nitrides may form in the steel, reducing its bendability and hole-expanding properties, or B may precipitate as nitrides. Therefore, the N content should be 0.0150% or less. Preferably, the N content is 0.0100% or less, 0.0080% or less, 0.0060% or less, or 0.0040% or less. The N content may be 0%, but reducing the N content to an extreme degree will increase the cost of removing N. For this reason, from an economic standpoint, the N content may be 0.0001% or more, 0.0003% or more, or 0.0005% or more.

[0031] [O: 0.0100% or less] O is an element contained as an impurity, and if the O content is high, coarse oxides may form in the steel, reducing its bendability and hole-expanding properties. Therefore, the O content should be 0.0100% or less. Preferably, the O content is 0.0080% or less, 0.0060% or less, or 0.0040% or less. The O content may be 0%, but extremely low O content increases manufacturing costs. For this reason, from the viewpoint of manufacturing costs, the O content may be 0.0001% or more, 0.0003% or more, or 0.0005% or more.

[0032] The basic chemical composition of the steel sheet according to the embodiment of the present invention is as described above. Furthermore, the steel sheet may, if necessary, contain at least one of the following optional elements in place of a portion of the remaining Fe.

[0033] [Cr: 0-1.00%] 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, 0.01% or more, or 0.05% or more. On the other hand, if the Cr content is excessive, the 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, 0.40% or less, or 0.20% or less.

[0034] [Cu: 0-1.00%] 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, 0.01% or more, or 0.05% or more. On the other hand, if the Cu content is excessive, 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, 0.40% or less, or 0.20% or less.

[0035] [Mo: 0-1.00%] Mo is an element that enhances the hardenability of steel and contributes to improving the strength of steel sheets, as well as improving hydrogen embrittlement resistance. The Mo content may be 0%, but to obtain these effects fully, it is preferable that the Mo content be 0.001% or more. The Mo content may be 0.005% or more, 0.01% or more, or 0.05% or more. On the other hand, if the Mo content is excessive, 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 0.80% or less, 0.60% or less, 0.40% or less, or 0.20% or less.

[0036] [Ni: 0-1.00%] 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 0.001% or more. The Ni content may be 0.005% or more, 0.01% or more, or 0.05% or more. On the other hand, if the Ni content is excessive, 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 0.80% or less, 0.60% or less, 0.40% or less, or 0.20% or less.

[0037] [Co: 0-1.00%] 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 0.001% or more. The Co content may be 0.005% or more, 0.01% or more, or 0.05% or more. On the other hand, if the Co content is excessive, the 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 1.00% or less. The Co content may be 0.80% or less, 0.60% or less, 0.40% or less, or 0.20% or less.

[0038] [W: 0-1.00%] W is an element effective in increasing the strength of steel and also contributes to improving hydrogen embrittlement resistance. The W content may be 0%, but to obtain these effects fully, it is preferable that the W content be 0.001% or more. The W content may be 0.005% or more, 0.01% or more, or 0.05% or more. On the other hand, if W is included in excess, 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, 0.40% or less, or 0.20% or less.

[0039] [Sn: 0-1.000%] 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, 0.005% or more, or 0.010% or more. On the other hand, excessive Sn content can lead to embrittlement of the steel sheet, which may cause cracking during hot rolling. Therefore, the Sn content should be 1.000% or less. The Sn content may be 0.800% or less, 0.600% or less, 0.400% or less, 0.200% or less, 0.100% or less, or 0.080% or less.

[0040] [Sb: 0-0.500%] 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. On the other hand, excessive Sb content may lead to embrittlement of the steel sheet and a decrease in ductility. Therefore, the Sb content should be 0.500% or less. The Sb content may be 0.400% or less, 0.300% or less, 0.200% or less, 0.100% or less, 0.050% or less, 0.020% or less, or 0.010% or less.

[0041] [Nb: 0-0.200%] Nb is an element that contributes to improving steel sheet strength through precipitation strengthening, fine grain strengthening by suppressing grain growth, and dislocation strengthening by suppressing recrystallization. The Nb content may be 0%, but to obtain these effects sufficiently, it is preferable that the Nb content be 0.001% or more. The Nb content may be 0.005% or more, 0.008% or more, or 0.010% or more. On the other hand, if the Nb content is excessive, coarse carbides will precipitate and the amount of solid-solution carbon will decrease, which may lower the martensite fraction and prevent sufficient steel sheet strength from being obtained. Therefore, the Nb content should be 0.200% or less. The Nb content may be 0.150% or less, 0.100% or less, 0.080% or less, or 0.050% or less.

[0042] [V: 0-1.00%] 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. V also contributes to improving hydrogen embrittlement resistance. The V content may be 0%, but to obtain these effects sufficiently, it is preferable that the V content be 0.001% or more. The V content may be 0.002% or more, 0.005% or more, or 0.01% or more. On the other hand, if V is included in excess, the effect will saturate, and including more V in the steel sheet than necessary will lead to an increase in manufacturing costs. Therefore, the V content should be 1.00% or less. The V content may be 0.80% or less, 0.60% or less, 0.40% or less, or 0.20% or less.

[0043] [As: 0-0.100%] As is an element effective in improving corrosion resistance. The As content may be 0%, but to obtain such an effect, it is preferable that the As content be 0.001% or more. The As content may also be 0.002% or more or 0.005%. On the other hand, if the As content is excessive, the effect will saturate, and including more As than necessary in the steel plate will lead to an increase in manufacturing costs. Therefore, the As content should be 0.100% or less, and may also be 0.050% or less, 0.030% or less, 0.020% or less, or 0.010% or less.

[0044] [Zn: 0-1.000%] Zn is an effective element for controlling the morphology of sulfides and improving local ductility and elongation flangeability. 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, 0.005% or more, or 0.010% or more. On the other hand, if the Zn content is excessive, the number of inclusions will increase, which may cause defects on the surface and inside the steel sheet. Therefore, the Zn content should be 1.000% or less. The Zn content may be 0.800% or less, 0.600% or less, 0.400% or less, 0.200% or less, 0.100% or less, 0.080% or less, or 0.060% or less.

[0045] [Mg: 0-0.0100%] Mg is an element whose addition in trace amounts can control the form of sulfides. The Mg content may be 0%, but to obtain these effects fully, it is preferable that the Mg content be 0.0001% or more. The Mg content may be 0.0002% or more, 0.0003% or more, 0.0005% or more, or 0.0007% or more. On the other hand, if the Mg content is excessive, it may lead to embrittlement of the steel sheet and a decrease in ductility. Therefore, the Mg content should be 0.0100% or less. The Mg content may be 0.0080% or less, 0.0060% or less, 0.0040% or less, 0.0020% or less, 0.0015% or less, or 0.0010% or less.

[0046] [Ca: 0-0.0100%, Zr: 0-0.0100%, Ce: 0-0.0150%, La: 0-0.0150%, Hf: 0-0.0100%, Bi: 0-0.0100%, REM other than Ce and La: 0-0.0100%] Ca, Zr, Ce, La, Hf, and REM other than Ce and La are all elements that can control the form of sulfides. In addition, Bi 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, 0.0005% or more, or 0.0007% or more, respectively. On the other hand, excessive content of Ca, Zr, Ce, La, Hf, Bi, and REM other than Ce and La may lead to embrittlement of the steel sheet and a decrease in ductility. Therefore, the content of Ca, Zr, Hf, Bi, and REM other than Ce and La should be 0.0100% or less, and the content of Ce and La should be 0.0150% or less, respectively. The content of Ca, Zr, Hf, Bi, and REM other than Ce and La may be 0.0080% or less, 0.0060% or less, 0.0040% or less, 0.0020% or less, or 0.0015% or less, respectively. In addition, the content of Ce and La may be 0.0120% or less, 0.0100% or less, 0.0080% or less, 0.0050% or less, 0.0020% or less, or 0.0010% or less, respectively.

[0047] 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 other than Ce and La refers to the total content of these elements excluding Ce and La.

[0048] In the steel sheet according to the embodiment of the present invention, the remainder other than the above-mentioned elements consists of Fe and impurities. Impurities are components that are mixed in during the industrial manufacture of steel sheets due to various factors in the manufacturing process, including raw materials such as ore and scrap, and components that are included in a range that does not affect the effects of the present invention.

[0049] The chemical composition of the steel sheet according to the embodiment of the present invention can be measured by general analytical methods. For example, the chemical composition of the steel sheet can be measured using inductively coupled plasma atomic emission spectroscopy (ICP-AES). C and S can be measured using the combustion-infrared absorption method, N can be measured using the inert gas fusion-thermal conductivity method, and O can be measured using the inert gas fusion-nondispersive infrared absorption method.

[0050] [Steel Structure] [Total of Ferrite and Bainite: 5% or less] The metallic structure of the steel sheet according to the embodiment of the present invention, more specifically the metallic structure in the region from the surface of the steel sheet to a position 1 / 4 of the sheet thickness, includes, in area %, a total of 5% or less of ferrite and bainite. Of ferrite and bainite, ferrite in particular has excellent ductility and is a structure that contributes to improved elongation. However, if the total area ratio of ferrite and bainite becomes too high, the area ratio of martensite decreases, and therefore the desired strength may not be achieved. Therefore, the total area ratio of ferrite and bainite is set to 5% or less. The total area ratio of ferrite and bainite may be 4% or less or 3% or less. The lower limit is not particularly limited, and therefore the total area ratio of ferrite and bainite may be 0%. From the viewpoint of improving the ductility of the steel sheet, the total area ratio of ferrite and bainite may be 1% or more or 2% or more.

[0051] [Retained Austenite: 10% or less] The metallic structure of the steel sheet according to the embodiment of the present invention, more specifically the metallic structure in the region from the surface of the steel sheet to a position 1 / 4 of the sheet thickness, contains retained austenite: 10% or less in area %. Retained austenite is a structure that improves the ductility of the steel sheet by the TRIP effect, which is transformed into martensite (i.e., work-induced transformation) during the deformation of the steel sheet. However, if retained austenite is present in excess, the toughness may decrease, or the area ratio of martensite may decrease, and therefore the desired strength may not be achieved. For this reason, the area ratio of retained austenite is set to 10% or less, and may be 8% or less or 6% or less. The lower limit is not particularly limited, and therefore the area ratio of retained austenite may be 0%, or for example, 1% or more, 3% or more or 5% or more.

[0052] [Martensite: 85% or more] The metallic structure of the steel sheet according to the embodiment of the present invention, more specifically the metallic structure in the region from the surface of the steel sheet to a position 1 / 4 of the sheet thickness, contains 85% or more martensite in area percent. The martensite referred to herein includes fresh martensite and tempered martensite. Fine cementite present within the tempered martensite is considered as part of the tempered martensite. Martensite is a high-strength structure and is a structure that increases the tensile strength of the steel sheet. Therefore, it is an important structure for ensuring the desired tensile strength. The area ratio of martensite is 85% or more, and may be 90% or more or 95% or more. There is no particular upper limit, but for example, the area ratio of martensite may be 100% or less, 99% or less or 98% or less.

[0053] [Residual structure: 0-10% in total] Residual structures other than ferrite, bainite, retained austenite, and martensite may account for 0% of the area. If residual structures are present, they may be pearlite or the like, although this is not particularly limited. From the viewpoint of ensuring the above-mentioned effects based on ferrite, bainite, retained austenite, and martensite, the area ratio of the residual structures is preferably 10% or less in total, and may be, for example, 8% or less, 6% or less, 5% or less, 4% or less, 3% or less, 2% or less, or 1.5% or less. On the other hand, the area ratio of the residual structures may be 0.5% or more, or 1% or more.

[0054] [Identification of Steel Microstructure and Calculation of Area Ratio] For the steel microstructure described above, the identification of ferrite, bainite, retained austenite, and martensite, and the calculation of their area ratios, are performed using secondary electron images captured with a thermal field emission scanning electron microscope (FE-SEM) and X-ray diffraction. First, a sample is taken from the steel plate so that the thickness cross section perpendicular to the plate surface becomes the observation surface. Although it is preferable that the thickness cross section is parallel to the rolling direction, it is not necessary for the thickness cross section to be parallel to the rolling direction, for example, when the rolling direction of the steel plate cannot be determined. Next, the cross section of the sample is polished using silicon carbide paper from #600 to #1500, then finished to a mirror surface using a liquid in which diamond powder with a particle size of 1 to 6 μm is dispersed in a diluent such as alcohol or pure water, and then nital etching is performed. Next, a secondary electron image is observed using a thermal field emission scanning electron microscope (JEOL JSM-7001F) in a 100 × 100 μm area centered at a point 1 / 4 of the plate thickness from the surface of the steel plate on the observation surface. From the obtained secondary electron image, the total area ratio of martensite and retained austenite, and the total area ratio of ferrite and bainite are measured. First, areas with high brightness and where the underlying structure is not revealed by etching are identified as fresh martensite and retained austenite. Next, areas with a underlying structure and where multiple cementite deposits with different elongation directions are precipitated are identified as tempered martensite. The remaining areas other than the above structures are identified as ferrite and bainite. For reference, areas with low brightness and no underlying structure can be identified as ferrite, and areas that do not fall into any of the above categories can be identified as bainite. The area ratio of each structure identified in this way is calculated by the point method.

[0055] Since martensite is not sufficiently etched by nital etching, it can be distinguished from other structures that are etched. However, retained austenite is also not sufficiently etched, similar to martensite. Therefore, the area ratio of martensite is determined by subtracting the area ratio of retained austenite, obtained by the X-ray diffraction method described below, from the total area ratio of tempered martensite, fresh martensite, and retained austenite.

[0056] The area fraction of retained austenite is calculated by X-ray diffraction. First, the region from the surface of the steel plate to the 1 / 4 position of the plate thickness is removed by mechanical and chemical polishing. Next, the surface of the polished sample is treated with MoKα rays as characteristic X-rays to obtain diffraction peaks at (200) and (211) for the bcc phase, and at (200), (220), and (311) for the fcc phase. The structural fraction of retained austenite is calculated from the integral intensity ratio of these diffraction peaks, and this is taken as the area fraction of retained austenite.

[0057] [BO ２ － Average value of detected signal intensity: 8.0 or less] When a flight-type secondary ion mass spectrometry is performed on a steel plate according to an embodiment of the present invention, more specifically at a position 1 / 4 of the plate thickness from the surface of the steel plate, BO ２ － BO at measurement points where the detected signal intensity is 4 times or less the mode. ２ － The average value of the detected signal intensity is 8.0 or less. As explained above, BO ２ － Measurement points where the detected signal intensity is four times or less the mode primarily originate from the solid solution state B within the prior austenite grains. ２ －By controlling the average value of the detected signal intensity to 8.0 or less, it is possible not only to control the amount of solid-solution B in the prior austenite grains to a low level, but also to finely control the particle size of the prior austenite grains. By controlling in this way, B is sufficiently segregated at the prior austenite grain boundaries, strengthening these prior austenite grain boundaries, thereby significantly suppressing the occurrence of hydrogen embrittlement cracks. From the viewpoint of improving hydrogen embrittlement resistance, BO ２ － A smaller average value of the detected signal intensity is preferable. ２ － The average value of the detected signal intensity may be 7.0 or less, 6.0 or less, or 5.0 or less. The lower limit is not particularly limited, but for example, BO ２ － The average value of the detected signal intensity may be 0.5 or higher, and may also be 1.0 or higher, 2.0 or higher, 3.0 or higher, or 4.0 or higher.

[0058] [Flying secondary ion mass spectrometry: BO] ２ － BO at measurement points where the detected signal intensity is 4 times or less the mode. ２ － [Measurement of the average value of the detected signal intensity] BO ２ － BO at measurement points where the detected signal intensity is 4 times or less the mode. ２ － The average value of the detected signal intensity is measured by flight-type secondary ion mass spectrometry using 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 mirror-polished with diamond paste. Although it is preferable that the thickness cross section be parallel to the rolling direction, it is not necessary to be parallel to the rolling direction if the rolling direction of the steel plate cannot be determined. Subsequently, by time-of-flight secondary ion mass spectrometry (TOF-SIMS), the BO (Body Occlusion) in a 100 μm × 100 μm region centered at the 1 / 4 thickness position from the surface of the steel plate is measured. ２ － Measure the intensity of the detected signal.

[0059] In TOF-SIMS analysis, a commercially available analyzer can be used. In this embodiment, TOF-SIMS5 manufactured by ION-TOF is used as the analyzer. The measurement ion gun is Bi １ ＋ (30 kV) is used. The measurement pitch is 0.05 μm, and the measurement range is 100 μm × 100 μm (2048 × 2048 pixel).

[0060] In the measurement region of TOF-SIMS analysis, by performing binning processing on the obtained data, it is converted into data of 512 points × 512 points. The binning processing is to repeat the operation of adding the signal intensities of adjacent 4 points × 4 points of BO ２ － and converting them into 1 point of data 512 × 512 times. Next, the most frequent value of the BO ２ － detection signal intensity in the data of 512 points × 512 points is obtained. Next, 4 times the most frequent value of the BO ２ － detection signal intensity obtained above is used as the threshold value, and for the data below the threshold value among the data of 512 points × 512 points, the average value of the BO ２ － detection signal intensity is obtained. The average value is used as the average value of the BO ２ － detection signal intensity at the measurement points where the BO ２ － detection signal intensity is 4 times or less the most frequent value.

[0061] [Sample skewness of BO ２ － detection signal intensity distribution: 20.0 or less] In a preferred embodiment of the present invention, when performing time-of-flight secondary ion mass spectrometry at a position 1 / 4 of the plate thickness from the surface of the steel plate, the sample skewness of the BO ２ － detection signal intensity distribution is 20.0 or less. BO ２ [[ID=3By controlling the sample skewness of the detected signal intensity to 20.0 or less, it becomes possible to more remarkably improve the hydrogen embrittlement resistance characteristics of the steel sheet. More specifically, as described above, B is known to be an element effective in improving the hydrogen embrittlement resistance characteristics by segregating at the prior austenite grain boundaries and strengthening the prior austenite grain boundaries thereby. On the other hand, B may form precipitates on the prior austenite grain boundaries during the heat treatment in the manufacturing process, and hydrogen embrittlement cracks may be induced starting from these precipitates. Therefore, from the viewpoint of further improving the hydrogen embrittlement resistance characteristics, it is preferable to suppress the formation of such B precipitates. As explained above, BO ２ － The mode value of the detected signal intensity mainly originates from B in the solid solution state present in the matrix phase. Therefore, BO ２ － In the histogram of the detected signal intensity and frequency, BO derived from B in the solid solution state ２ － has the highest frequency. Here, when B at the measurement point is segregated at the prior austenite grain boundaries or forms B precipitates, since B is concentrated, a strong BO ２ － detected signal intensity is detected. Especially when a large number of B precipitates are formed and / or coarser B precipitates are formed, since B is particularly concentrated, a very strong BO ２ － detected signal intensity is detected. Therefore, when such B precipitates are formed, BO ２ － since very strong detected signal intensities have a certain frequency, BO ２ － distortion occurs in the histogram of the detected signal intensity and frequency. Therefore, from the viewpoint of suppressing the formation of B precipitates and thus further improving the hydrogen embrittlement resistance characteristics of the steel sheet, BO ２ － the smaller the sample skewness of the detected signal intensity distribution, the more preferable, and BO ２ － the sample skewness of the detected signal intensity distribution may be 19.0 or less, 18.0 or less, or 17.0 or less. The lower limit is not particularly limited, for example, BO ２－ The sampling skewness of the detected signal intensity distribution may be 1.0 or greater, 3.0 or greater, 5.0 or greater, or 10.0 or greater.

[0062] [Flying secondary ion mass spectrometry: BO] ２ － [Calculation of sampling skewness of detected signal intensity distribution] BO ２ － The sample skewness of the detected signal intensity distribution is determined by flight-type secondary ion mass spectrometry using 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 mirror-polished with diamond paste. Although it is preferable that the thickness cross section be parallel to the rolling direction, it is not necessary to be parallel to the rolling direction if the rolling direction of the steel plate cannot be determined. Subsequently, TOF-SIMS analysis is performed to determine the BO in a 100 μm × 100 μm region centered at the 1 / 4 thickness position from the surface of the steel plate. ２ － Measure the intensity of the detected signal.

[0063] 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, and the measurement range will be 100μm × 100μm (2048 × 2048 pixels).

[0064] TOF-SIMS analysis revealed that BO ２ － The detection signal is measured, and the obtained data is converted into 512-point x 512-point data by performing binning. The binning process is performed by dividing adjacent 4x4 points into BOs. ２ － The signal strengths are added together and converted into a single data point, and this operation is repeated 512 x 512 times. Next, the BO in the 512 x 512 data points ２ － The detection signal intensity distribution is obtained, and then the sampling skewness, which is an indicator of the asymmetry of that distribution, is calculated from the following formula (5).

[0065] [Plate Thickness] The steel plate according to the embodiment of the present invention is not particularly limited, but generally has a plate thickness of 0.6 to 6.0 mm. For example, the plate thickness may be 1.0 mm or more, 1.2 mm or more, or 1.4 mm or more, and / or 5.0 mm or less, 4.0 mm or less, 3.0 mm or less, or 2.5 mm or less.

[0066] [Plating Layer] The steel sheet according to the embodiment of the present invention may be a plated steel sheet having a plating layer on at least one surface, preferably both surfaces. The plating layer is not particularly limited, but may be, for example, a hot-dip galvanized layer (GI), an alloyed hot-dip galvanized layer (GA), or an electroplated galvanized layer (EG). Among these, it is preferable to have a hot-dip galvanized layer (GI) or an alloyed hot-dip galvanized layer (GA) on the surface. These zinc plating layers may have any composition known to those skilled in the art, and may contain additive elements such as Al and Mg in addition to Zn. Furthermore, the amount of the plating layer is not particularly limited and may be a general amount.

[0067] As described above, the steel sheet according to the embodiment of the present invention achieves high strength, more specifically 1700 MPa or more, more preferably 1760 MPa or more, due to its martensite-based steel structure, while also achieving significantly improved hydrogen embrittlement resistance. Therefore, the steel sheet according to the embodiment of the present invention is particularly useful for use in parts in technical fields that require a high level of both high strength and hydrogen embrittlement resistance, and is especially useful for use in automotive parts. In a preferred embodiment, an automotive part including the steel sheet according to the embodiment of the present invention is provided. Examples of automotive parts include structural parts such as front pillars, center pillars, side sills, and cross members, as well as bumpers, and other structural and reinforcing parts that require strength. These parts only need to include the steel sheet according to the embodiment of the present invention in at least a portion of them, and therefore at least a portion of these parts will satisfy the characteristics of the steel sheet described above. In forming processes such as press forming, the characteristics of the steel sheet do not particularly change before and after forming in parts of the steel sheet that do not directly contact the mold or, even if they do, are processed to a relatively low degree.

[0068] [Mechanical Strength] [Tensile Strength (TS)] According to the steel sheet having the above chemical composition and steel structure, a high tensile strength, specifically a tensile strength (TS) of 1700 MPa or more, more preferably 1760 MPa or more, can be achieved. The tensile strength is preferably 1800 MPa or more, 1850 MPa or more, or 1900 MPa or more. According to the steel sheet according to the embodiment of the present invention, despite having such a very high tensile strength, the occurrence of hydrogen embrittlement cracking can be significantly suppressed by the specific combination of chemical composition and steel structure described above. The upper limit of the tensile strength is not particularly limited, but for example, the tensile strength of the steel sheet may be 3000 MPa or less, 2500 MPa or less, or 2000 MPa or less. The tensile strength is determined by taking a JIS No. 5 test piece from a direction (C direction) where the longitudinal direction of the test piece is preferably parallel to the direction perpendicular to the rolling direction of the steel sheet, and performing a tensile test in accordance with JIS Z 2241:2022. If the rolling direction of the steel plate cannot be determined, a JIS No. 5 test specimen may be taken from any direction within the surface of the steel plate. If it is difficult to take a JIS No. 5 test specimen, a JIS No. 13B test specimen may be used, or a small test specimen with a similar shape to a JIS No. 13B test specimen may be used.

[0069] <Method for Manufacturing Steel Sheets> Next, preferred methods for manufacturing steel sheets according to embodiments of the present invention will be described. The following description is intended to illustrate characteristic methods for manufacturing steel sheets according to embodiments of the present invention, and is not intended to limit the steel sheets to those manufactured by the manufacturing methods described below.

[0070] A method for manufacturing a steel sheet according to an embodiment of the present invention includes (A) a hot rolling step that includes hot rolling a slab having the chemical composition described above, then winding the obtained hot-rolled steel sheet and cooling, and satisfying the following conditions (A1) to (A3): (A1) Three or more rolling passes are performed in the temperature range of 950 to 1050°C with a reduction ratio of 0.20 or more, and the cumulative reduction ratio is 0.60 or more; (A2) The cumulative reduction ratio is 0.50 or less in the temperature range of 900 to 950°C; (A3) For each rolling pass, the temperature difference between the rolling entry plate temperature between the nth pass and the (n+1)th pass satisfies the following formula (1). Here, n: number of rolling passes T ｎ : Rolling entry plate temperature (°C) for the nth pass t ｎ : Rolling entry side thickness (mm) of the nth pass (B) Cold rolling process in which the hot-rolled steel sheet is pickled and cold-rolled with a reduction ratio of 0.30 to 0.75 (C) The obtained cold-rolled steel sheet is heated to a maximum heating temperature of Ac3 to 950°C, held for 1 to 1000 seconds, and the thermal history from when it reaches 600°C or below until it first reaches (Ms-100)°C or below satisfies the following formula (2), Here, t: Elapsed time (seconds) from when the steel plate temperature reaches 600°C or below until it first reaches (Ms-100)°C or below. T: Average temperature (°C) from when the steel plate temperature reaches 600°C or below until it first reaches (Ms-100)°C or below. Here, the average temperature (°C) is calculated by measuring the temperature at each 1-second accumulation step over the above elapsed time t, calculating the accumulated temperature, and dividing the obtained accumulated temperature by the above elapsed time t. C ∞ : Grain boundary segregation concentration (mass%) of boron at equilibrium at temperature T C ｔ : Grain boundary segregation concentration of boron (mass%) when held at temperature T for time t [B]: B content (mass%) in the chemical composition of the steel sheet. The following describes each process in detail.

[0071] [(A) Hot Rolling Process] [Heating of Slab] First, a slab having the chemical composition described above in relation to the steel plate is heated. From the viewpoint of productivity, the slab to be used is preferably cast by the continuous casting method, but it may also be manufactured by the ingot casting method or the thin slab casting method. The slab to be used contains a relatively large amount of alloying elements in order to obtain a high-strength steel plate. For this reason, it is necessary to heat the slab before subjecting it to hot rolling to solid dissolve the alloying elements in the slab. If the heating temperature is low, the alloying elements will not sufficiently solid dissolve in the slab, leaving coarse alloy carbides, which may cause brittle cracking during hot rolling. For this reason, the heating temperature is preferably 1100°C or higher, and more preferably 1200°C or higher. The upper limit of the heating temperature is not particularly limited, but from the viewpoint of the capacity of the heating equipment and productivity, it is preferably 1300°C or lower.

[0072] [Rough Rolling] In this manufacturing method, for example, rough rolling may be performed on a heated slab before finish rolling to adjust the plate thickness. The conditions for rough rolling are not particularly limited, as long as the desired sheet bar dimensions are secured.

[0073] [Finish Rolling] [(A1) Three or more rolling passes with a reduction ratio of 0.20 or more are performed in a temperature range of 950 to 1050°C, and the cumulative reduction ratio is 0.60 or more.] The heated slab, or a slab that has been roughly rolled in addition as necessary, is then subjected to finish rolling. In this manufacturing method, finish rolling is performed using a tandem rolling mill consisting of three or more rolling stands, more specifically four to eight rolling stands. In this manufacturing method, by performing three or more rolling passes with a reduction ratio of 0.20 or more in a recrystallization temperature range such as 950 to 1050°C, and by the cumulative reduction ratio being 0.60 or more, recrystallization is promoted to the center of the plate thickness, thereby refining the austenite grains throughout the entire plate thickness. In the temperature range of 950 to 1050°C, if there are two or fewer rolling passes with a reduction ratio of 0.20 or more, and / or if the cumulative reduction ratio is less than 0.60, recrystallization cannot be sufficiently promoted across the entire thickness of the plate. As a result, austenite grains cannot be refined across the entire thickness of the plate, and consequently, the diffusion of B into the grain boundaries cannot be promoted, and when flight-type secondary ion mass spectrometry is performed on the resulting steel microstructure, BO ２ － At measurement points where the detected signal intensity is 4 times or less the mode, BO ２ － The average value of the detected signal intensity may exceed 8.0. Therefore, in the temperature range of 950 to 1050°C, three or more rolling passes with a reduction ratio of 0.20 or more are performed, and the cumulative reduction ratio is 0.60 or more. The upper limit of the reduction ratio in this temperature range is not particularly limited, but may be 0.50 or less. Also, the upper limit of the number of rolling passes with a reduction ratio of 0.20 or more in this temperature range is not particularly limited, and the number of such rolling passes may be, for example, 8 or less or 7 or less. The upper limit of the cumulative reduction ratio in this temperature range is not particularly limited, but may be 0.80 or less. The cumulative reduction ratio in the temperature range of 950 to 1050°C refers to the reduction ratio determined by the thickness of the sheet at 950°C relative to the thickness of the sheet at 1050°C, and is calculated using the following formula. Cumulative reduction ratio in the temperature range of 950°C to 1050°C = (plate thickness at 1050°C (mm) - plate thickness at 950°C (mm)) / plate thickness at 1050°C (mm)

[0074] [(A2) Cumulative reduction ratio in the temperature range of 900 to 950°C: 0.50 or less] In this manufacturing method, the cumulative reduction ratio is 0.50 or less in the temperature range of 900 to 950°C. If the cumulative reduction ratio exceeds 0.50 in the non-recrystallized temperature range such as 900 to 950°C, the precipitation of B is promoted by processing, a large amount of ferrite is produced, and the ferrite and bainite in the final steel structure may exceed 5%. The lower limit of the cumulative reduction ratio in this temperature range is not particularly limited, but may be 0 or more, or 0.01 or more, or 0.05 or more. The cumulative reduction ratio in the temperature range of 900 to 950°C refers to the reduction ratio obtained by the plate thickness at 900°C relative to the plate thickness at 950°C, and is calculated by the following formula. Cumulative reduction ratio in the temperature range of 900°C to 950°C = (plate thickness at 950°C (mm) - plate thickness at 900°C (mm)) / plate thickness at 950°C (mm)

[0075] [(A3) For each rolling pass, the temperature difference between the nth pass and the (n+1)th pass of the rolling entry plate satisfies the following equation (1)] In the hot rolling process, for each rolling pass, the temperature difference between the nth pass and the (n+1)th pass satisfies the following equation (1). Here, n: number of rolling passes T n : Rolling entry plate temperature (°C) for the nth pass t n : Rolling entry side thickness (mm) of the nth pass

[0076] Equation (1) specifies the temperature difference between the nth pass and the (n+1)th pass of the rolling entry plate. It can be understood that this means the amount of temperature drop in the steel plate in each rolling pass (corresponding to the left side of equation (1)) should be smaller than a predetermined value calculated from the number of rolling passes and the thickness of the rolling entry plate in the nth pass (corresponding to the right side of equation (1)). For example, if the amount of temperature drop in the steel plate in each rolling pass is large, or more specifically, if equation (1) is not satisfied, then, for example, only the surface portion of the steel plate may be excessively cooled, causing temperature unevenness in the thickness direction of the steel plate, and it may not be possible to obtain fine and uniform prior austenite grains at the 1 / 4 thickness position. As a result, when a flight-type secondary ion mass spectrometry is performed on the final steel structure, BO２ － At measurement points where the detected signal intensity is 4 times or less the mode, BO ２ － The average value of the detected signal intensity may exceed 8.0. Therefore, when performing flight-type secondary ion mass spectrometry on the final steel microstructure, BO ２ － At measurement points where the detected signal intensity is 4 times or less the mode, BO ２ － In order to control the average value of the detected signal intensity to be 8.0 or less, in addition to refining the austenite grains by satisfying the conditions (A1) above, it is necessary to obtain austenite grains with a uniform distribution and fineness by satisfying the above formula (1).

[0077] [Cooling and Winding] Next, the finish-rolled steel sheet is cooled to 580°C or lower at an average cooling rate of preferably 20°C / second or more, and then wound up. If the average cooling rate is less than 20°C / second or the winding temperature is above 500°C, segregation of P may occur during the hot rolling process, causing the hot-rolled steel sheet to become brittle. For example, the average cooling rate is preferably 25°C / second or higher, and the winding temperature is preferably 550°C or lower. The lower limit of the winding temperature is not particularly limited, but for example, the winding temperature may be 450°C or higher.

[0078] [(B) Cold Rolling Process] [Pickling] Next, the obtained hot-rolled steel sheet is pickled to remove the oxide scale formed on the surface of the hot-rolled steel sheet. Pickling can be carried out under conditions suitable for removing the oxide scale, and may be done once or in multiple steps to ensure complete removal of the oxide scale.

[0079] [Cold Rolling] Pickled hot-rolled steel sheets are cold-rolled with a reduction ratio of 0.30 to 0.75. By setting the cold-rolling reduction ratio to 0.30 or higher, the shape of the cold-rolled steel sheet can be kept flat, and a decrease in ductility in the final product can be suppressed. On the other hand, by setting the cold-rolling reduction ratio to 0.75 or lower, it is possible to prevent the rolling load from becoming excessive and making rolling difficult. 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.

[0080] [(C) Heat Treatment Process] Next, the obtained cold-rolled steel sheet is heated to a maximum heating temperature of Ac3 to 950°C in the heat treatment process and held for 1 to 1000 seconds. It is necessary to heat the cold-rolled steel sheet to a maximum heating temperature of Ac3 to 950°C in order to sufficiently advance austenitization and obtain the desired steel structure in the subsequent cooling process. If austenitization is insufficient, a large amount of ferrite may be formed in the final steel structure, and the desired martensite area ratio may not be achieved. On the other hand, if the maximum heating temperature exceeds 950°C, the austenite will grow excessively, and when flight-type secondary ion mass spectrometry is performed on the final steel structure, BO ２ － At measurement points where the detected signal intensity is 4 times or less the mode, BO ２ － The average value of the detected signal intensity may exceed 8.0. During holding, the cold-rolled steel sheet does not necessarily need to be kept at a constant temperature and may fluctuate within the range of the maximum heating temperature mentioned above. Here, Ac3 (°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.

[0081] [Thermal history satisfying the following formula (2) from below 600°C to below (Ms-100)°C] Next, the cold-rolled steel sheet, which has been heated to the maximum heating temperature and held for a predetermined time, is then cooled to below (Ms-100)°C. At this time, the thermal history from when it reaches below 600°C until it first reaches below (Ms-100)°C satisfies the following formula (2). Here, Ms (°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. Here, t: Elapsed time (seconds) from when the steel plate temperature reaches 600°C or below until it first reaches (Ms-100)°C or below. T: Average temperature (°C) from when the steel plate temperature reaches 600°C or below until it first reaches (Ms-100)°C or below. Here, the average temperature (°C) is calculated by measuring the temperature at each 1-second accumulation step over the above elapsed time t, calculating the accumulated temperature, and dividing the obtained accumulated temperature by the above elapsed time t. C ∞ : Grain boundary segregation concentration (mass%) of boron at equilibrium at temperature T C ｔ : Grain boundary segregation concentration of boron when held at temperature T for time t (mass%) [B]: B content in the chemical composition of the steel sheet (mass%)

[0082] The middle side of equation (2) above represents the grain boundary segregation concentration (mass%) of boron (B) when the steel plate is held for a predetermined time t (seconds) at the average temperature T (°C) from when the steel plate temperature reaches 600°C or below until it first reaches (Ms-100)°C or below. Equation (3) above represents the grain boundary segregation concentration (mass%) of boron in equilibrium at temperature T. In the thermal history from when the temperature reaches 600°C or below until it first reaches (Ms-100)°C or below, by controlling the middle side to be greater than 2000 times the B content (the right side of equation (2)), B can be sufficiently segregated at the prior austenite grain boundaries. On the other hand, if the middle side of equation (2) is less than 2000 times the B content, B cannot be sufficiently segregated at the prior austenite grain boundaries, and when flight-type secondary ion mass spectrometry is performed on the final steel structure, BO ２ － At measurement points where the detected signal intensity is 4 times or less the mode, BO ２ － The average value of the detected signal intensity may exceed 8.0. Therefore, in order to sufficiently segregate B at the prior austenite grain boundaries and thereby strengthen those prior austenite grain boundaries to improve hydrogen embrittlement resistance, it is important to control the heat treatment process so that the thermal history from when the temperature reaches 600°C or below until it first reaches (Ms-100)°C or below satisfies the above equation (2), that is, to control it so that Ct / 2000[B] > 1.0.

[0083] [(C1) Average cooling rate in the temperature range of 600 to 750°C: 10°C / second or more] In the method for manufacturing steel sheets according to a preferred embodiment of the present invention, the cold-rolled steel sheet held at the highest heating temperature is cooled in the temperature range of 600 to 750°C at an average cooling rate of 10°C / second or more. Since B precipitates are easily formed in the temperature range of 600 to 750°C, if the cold-rolled steel sheet is cooled relatively slowly in this temperature range, more specifically if the average cooling rate in this temperature range is less than 10°C / second, a relatively large amount of B precipitates will be formed, and as a result, BO ２ －The sample skewness of the detected signal intensity distribution may not satisfy the requirement of 20.0 or less. Therefore, from the viewpoint of further improving hydrogen embrittlement resistance, it is preferable to suppress the formation of such B precipitates. Accordingly, the average cooling rate in the temperature range of 600 to 750°C is 10°C / second or more, preferably 13°C / second or more. The upper limit of the average cooling rate is not particularly limited and may be 100°C / second or less or 50°C / second or less.

[0084] [(C2) Residence time in the temperature range of (Ms-100) to 600°C: 1 to 1000 seconds] Next, in the method for manufacturing steel sheets according to a preferred embodiment of the present invention, the cold-rolled steel sheet resides for 1 to 1000 seconds in the temperature range of (Ms-100) to 600°C. If the residence time in the temperature range of (Ms-100) to 600°C is too long, specifically if the residence time in the temperature range of (Ms-100) to 600°C exceeds 1000 seconds, B segregated at the austenite grain boundaries may form precipitates on the grain boundaries. As a result, BO ２ － The sample skewness of the detected signal intensity distribution may not satisfy the requirement of 20.0 or less. Therefore, from the viewpoint of further improving hydrogen embrittlement resistance, it is preferable to suppress the formation of such B precipitates, and it is preferable that the residence time in the temperature range of (Ms-100) to 600°C is 1000 seconds or less.

[0085] [(C3) Residence time in a temperature range of 200°C or higher: 1 to 1000 seconds, and thermal history from (Ms-100)°C or lower to 100°C or lower that satisfies the following formula (4)] In a preferred embodiment of the present invention, the heat treatment step further includes reheating a cold-rolled steel sheet cooled to (Ms-100)°C or lower to a temperature range of 200°C or higher and leaving it there for 1 to 1000 seconds, and then cooling it to 100°C or lower. By appropriately reheating (tempering) a cold-rolled steel sheet that has undergone martensitic transformation by cooling to (Ms-100)°C or lower in such a temperature range for 1 to 1000 seconds, the toughness of the steel sheet can be increased, thereby further improving the hydrogen embrittlement resistance of the steel sheet. At this time, it is important to control the thermal history from when it reaches (Ms-100)°C or lower until it first reaches 100°C or lower that it satisfies the following formula (4). Here, r ｆ / r ０ : Particle size ratio of precipitate B before and after reheating treatment t: Elapsed time (seconds) from the start of reheating treatment t ｆ : Reheating completion time (seconds) T(t): Process temperature at time t (°C) However, the integration step is 1 second. If the measurement interval is less than 1 second, the temperature every second is calculated by linearly correcting the temperature between measurement points. [B]: B content in the chemical composition of the steel plate (mass %)

[0086] To explain in detail, the middle term of the above equation (4) represents the particle size ratio of B precipitates before and after reheating treatment, and the thermal history is controlled from when the temperature reaches (Ms-100)°C or below until it first reaches 100°C or below, so that this middle term does not exceed 1.5. By controlling it in this way, the growth of B precipitates at the prior austenite grain boundaries is suppressed by the tempering treatment in which the temperature range is maintained at 200°C or above for 1 to 1000 seconds, and when a flight-type secondary ion mass spectrometry is performed on the final steel sheet, BO ２ － At measurement points where the detected signal intensity is 4 times or less the mode, BO ２ － Not only can it be controlled so that the average value of the detected signal intensity is 8.0 or less, but BO ２ － It becomes possible to control the sampling skewness of the detected signal intensity distribution to satisfy 20.0 or less. As a result, it becomes possible to more significantly improve the hydrogen embrittlement resistance of the steel sheet due to appropriate tempering treatment. The lower limit of the temperature when cooling the cold-rolled steel sheet to (Ms - 100°C) or below is not particularly limited, but may be 100°C or higher, for example. Also, the upper limit of the reheating temperature is not particularly limited, but may be 400°C or lower, for example. Also, the lower limit of the temperature when cooling to 100°C or below is not particularly limited, but may be 20°C or higher, for example.

[0087] [Plating] When manufacturing plated steel sheets, for example, after the cold-rolled steel sheet reaches the maximum heating temperature in the (C) heat treatment process, cooling should be stopped near the plating bath temperature (approximately 460°C in the case of a Zn bath), and the sheet should be immersed in the plating bath. When manufacturing steel sheets having an alloyed hot-dip galvanized layer, the sheet should be reheated after immersion in the plating bath and subjected to alloying treatment. The alloying treatment temperature should be in the range of 460 to 600°C. Alternatively, a steel sheet having a hot-dip galvanized layer without alloying treatment may be used. However, the conditions described in the (C) heat treatment process must be met, including the plating time and alloying treatment time. When manufacturing steel sheets having an electro-galvanized layer, the treatment should be performed after the (C) heat treatment process has cooled to room temperature. These zinc plating processes may be carried out according to any suitable method known to those skilled in the art. Similarly, these zinc plating processes may have any composition known to those skilled in the art, and may contain additive elements such as Al and Mg in addition to Zn. Furthermore, the amount of zinc plating applied is not particularly limited and may be a general amount.

[0088] According to the steel sheet manufactured by the above manufacturing method, by optimizing the chemical composition, particularly by configuring the steel sheet having a chemical composition containing B: 0.0005 to 0.0030% by mass%, with a structure mainly composed of martensite, and more specifically, with martensite accounting for 85% or more by area%, it is possible to improve the hydrogen embrittlement resistance of the steel sheet while achieving the desired high strength, for example, a tensile strength of 1700 MPa or more, more preferably 1760 MPa or more. Furthermore, when flight-type secondary ion mass spectrometry is performed, BO ２ － At measurement points where the detected signal intensity is 4 times or less the mode, BO ２ － By controlling the average value of the detected signal intensity to 8.0 or less, it becomes possible to significantly suppress the occurrence of hydrogen embrittlement cracking even at high strengths of 1700 MPa or more, more preferably 1760 MPa or more. Therefore, steel sheets manufactured by the above manufacturing method are particularly useful in the automotive sector, where a high level of both high strength and hydrogen embrittlement resistance is required.

[0089] The steel sheet according to the embodiment of the present invention can be used, for example, as various automobile parts as described above. The sample collection locations in this case are as follows.

[0090] [Sampling Locations] When sampling from a wound coil, the outermost edge of the coil may have a changed surface condition. Therefore, samples should be taken from the outermost edge (1st turn) of the coil, starting from the 3rd turn and beyond, avoiding the area within 100 mm of the widthwise end. On the other hand, when sampling from automotive parts, the following locations (i) to (iv) should be avoided: (i) within 20 mm of the toe of a spot weld, and within 20 mm of the bead toe of an arc / laser weld (ii) processed areas with a radius of curvature less than 15 mm, and within 5 mm of such processed areas (iii) the end within 5 mm of the cut end face of the part (iv) within 5 mm of any area where red rust is visible

[0091] The present invention will be described in more detail below with reference to examples, but the present invention is not limited in any way to these examples.

[0092] First, molten steel was cast using a continuous casting method to form slabs with various chemical compositions as shown in Table 1. These slabs were then heated to the temperatures shown in Table 2 and hot-rolled. Hot rolling was carried out by rough rolling and finish rolling. More specifically, the conditions for rough rolling were the same for all examples and comparative examples, while the conditions for finish rolling and winding were as shown in Table 2. The obtained hot-rolled steel sheets were pickled, and then cold-rolled under the conditions shown in Table 2 to obtain cold-rolled steel sheets with a thickness of 1.4 mm. Next, the obtained cold-rolled steel sheets were heated to the maximum heating temperature shown in Table 2 in a heat treatment process, and heat treatment was carried out under the conditions shown in Table 2 to obtain cold-rolled steel sheets (CR), steel sheets with an alloyed hot-dip galvanized layer (GA), or steel sheets with a hot-dip galvanized layer (GI). Subsequently, in Examples 19 and 22, electro-galvanizing was performed to obtain steel sheets with an electro-galvanized layer (EG).

[0093]

[0094]

[0095] The properties of the obtained steel plates were measured and evaluated by the following method.

[0096] [Tensile Strength (TS)] Tensile strength (TS) was measured by taking a JIS No. 5 test specimen, 200 mm in length and 2.5 mm in thickness, from the direction (C direction) where the longitudinal direction of the test specimen is parallel to the direction perpendicular to the rolling direction of the steel plate, and performing a tensile test in accordance with JIS Z 2241:2022. More specifically, the test was performed at room temperature in the range of 10 to 35°C, and a tensile test force was applied to the test specimen, allowing strain to be introduced until fracture occurred.

[0097] [Hydrogen Embrittlement Resistance] The hydrogen embrittlement resistance of the steel sheet was evaluated by the following four-point bending test. Here, Figures 1(A) and 1(B) are schematic diagrams illustrating the evaluation method for the hydrogen embrittlement resistance of the steel sheet using the four-point bending test.

[0098] First, a strip-shaped test piece 11 measuring 10 mm x 68 mm was taken from a steel plate. Next, as shown in Figures 1(A) and 1(B), a ceramic pin 12 with a diameter of 3.8 mm, a push-in block 13, and a push-in bolt 14 were attached to a holder 15, and stress was applied by tightening the push-in bolt. The bolt fastening force at this time was determined based on the relationship between the bolt fastening force and the strain at the top of the four-point bending test piece, which had been obtained in advance by attaching a GL 3 mm strain gauge 17 to the strain at the top of the four-point bending test piece 16, so that the desired stress was applied to the top of the four-point bending test piece. The applied stress was set to be equivalent to 0.65 times the tensile strength (TS) or more. At this time, the strain was converted to stress from the stress-strain curve obtained in advance in a tensile test.

[0099] Next, the four-point bending test specimens 16, each subjected to a stress of 0.65 times or more of the tensile strength (TS) applied by a push-in bolt 14, were immersed for 48 hours in 1000 mL of 100 g / L ammonium thiocyanate solution per four-point bending test specimen 11. The end faces of the four-point bending test specimens 11 were milled. After the test was completed, the presence or absence of fracture was checked, and the hydrogen embrittlement resistance was evaluated according to the following criteria: AA: No fracture occurred after 48 hours with a stress of 0.70 times the TS applied. A: No fracture occurred after 48 hours with a stress of 0.65 times the TS applied. B: Fracture occurred before 48 hours had elapsed with a stress of 0.65 times the TS applied. C: Test not performed

[0100] Steel plates with a tensile strength (TS) of 1700 MPa or higher and hydrogen embrittlement resistance of AA or A were evaluated as high-strength steel plates with excellent hydrogen embrittlement resistance. The evaluation results are shown in Table 3.

[0101]

[0102] In Table 1, "REM" refers to "REM other than Ce and La." In Table 2, "GA," "GI," "EG," and "CR" refer to "steel sheet with alloyed hot-dip galvanized layer," "steel sheet with hot-dip galvanized layer," "steel sheet with electro-galvanized layer," and "cold-rolled steel sheet," respectively. In Table 3, "M," "α+B," and "Residual γ" refer to the area percentages of "martensite," "ferrite and bainite," and "residual austenite," respectively.

[0103] Referring to Tables 1-3, Comparative Example 11 showed a decrease in TS due to its low C content. On the other hand, Comparative Example 12 showed a decrease in hydrogen embrittlement resistance due to an excessive increase in TS caused by its high C content.

[0104] Comparative Example 13 had a high Si content, which resulted in excessive formation of retained austenite. The presence of retained austenite is thought to have increased hydrogen storage in the steel. Consequently, its resistance to hydrogen embrittlement decreased.

[0105] Comparative Example 14 showed a decrease in TS due to its low Mn content. On the other hand, Comparative Example 15's high Mn content likely led to a decrease in grain boundary bonding strength due to Mn segregation at the prior austenite grain boundaries. As a result, its hydrogen embrittlement resistance decreased.

[0106] In Comparative Example 16, the low B content likely resulted in insufficient segregation at the prior austenite grain boundaries, thus failing to adequately strengthen them. As a result, the hydrogen embrittlement resistance decreased. On the other hand, in Comparative Example 17, the high B content likely led to excessive boride formation in the steel. Consequently, the hydrogen embrittlement resistance also decreased.

[0107] In Comparative Example 25, the cumulative reduction ratio at 950-1050°C was small, which prevented sufficient recrystallization across the entire thickness of the plate. Consequently, the austenite grains could not be refined across the entire thickness of the plate, and relatedly, the diffusion of B into the grain boundaries could not be promoted, resulting in the final steel plate containing BO ２ － It is thought that the average value of the detected intensity increased. As a result, the hydrogen embrittlement resistance decreased. In Comparative Example 26, because the cumulative reduction ratio at 900-950°C was large, it is thought that the precipitation of B was promoted by processing, and a large amount of ferrite was formed in the final steel structure. As a result, TS decreased. In Comparative Example 27, because equation (1) was not satisfied, only the surface portion of the steel sheet was excessively cooled, causing temperature unevenness in the thickness direction of the steel sheet, and it was not possible to obtain fine and uniform prior austenite grains throughout the entire thickness of the sheet, and in the final steel sheet, BO ２ － It is believed that the average detection intensity increased. As a result, the hydrogen embrittlement resistance decreased.

[0108] In Comparative Example 30, the maximum heating temperature was lower than that of Ac3, resulting in insufficient austenitization. This led to the formation of a large amount of ferrite in the resulting steel structure, which is thought to have resulted in a lower area ratio of martensite. As a result, the TS decreased. In Comparative Example 31, the maximum heating temperature was higher than 950°C, causing excessive grain growth of austenite, resulting in a lower BO in the resulting steel sheet. ２ － It is thought that the average detected intensity increased. As a result, the hydrogen embrittlement resistance decreased. In Comparative Example 34, because Ct / 2000 [B] was 1.0 or less, B could not be sufficiently segregated at the prior austenite grain boundaries, and in the steel sheet that was finally obtained, BO ２ － It is believed that the average detection intensity increased. As a result, the hydrogen embrittlement resistance decreased.

[0109] In contrast, in all the steel sheets relating to the invention, having a predetermined chemical composition, and by appropriately controlling each condition in the manufacturing method, at a position 1 / 4 of the thickness from the surface, the total area percentage of ferrite and bainite is 5% or less, retained austenite is 10% or less, and the remainder is martensite, and when flight-type secondary ion mass spectrometry is performed at a position 1 / 4 of the thickness from the surface, BO ２ － At measurement points where the detected signal intensity is 4 times or less the mode, BO ２ － We were able to obtain a steel plate having a steel structure in which the average value of the detected signal intensity was 8.0 or less. As a result, it had high strength and excellent hydrogen embrittlement resistance. In particular, in addition to the above configuration, BO ２ － In Examples 1-10 and 18-20, which were controlled to satisfy the requirement that the sample skewness of the detected signal intensity distribution be 20.0 or less, the evaluation result for hydrogen embrittlement resistance was AA, indicating superior hydrogen embrittlement resistance.

[0110] 11 Test specimen 12 Ceramic pin 13 Indentation block 14 Indentation bolt 15 Holder 16 Top of test specimen 17 Strain gauge

Claims

1. In mass percent, 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: 0.0005-0.0030%, P: 0.050% or less, S: 0.0100% or less, N: 0.0150% or less, O: 0.0100% or less, Cr: 0-1.00%, Cu: 0-1.00%, Mo: 0-1.00%, Ni: 0-1.00%, Co: 0-1.00%, W: 0-1.00%, Sn: 0-1.000%, Sb: 0-0.500% The chemical composition consists of Nb: 0-0.200%, V: 0-1.00%, As: 0-0.100%, Zn: 0-1.000%, Mg: 0-0.0100%, Ca: 0-0.0100%, Zr: 0-0.0100%, Ce: 0-0.0150%, La: 0-0.0150%, Hf: 0-0.0100%, Bi: 0-0.0100%, REM other than Ce and La: 0-0.0100%, and the remainder being Fe and impurities. At a position 1 / 4 of the plate thickness from the surface, the area percentage consists of: total ferrite and bainite: 5% or less, retained austenite: 10% or less, and martensite: 85% or more. When a flight-type secondary ion mass spectrometry was performed at a position 1 / 4 of the plate thickness from the surface, BO ２ － At measurement points where the detected signal intensity is 4 times or less the mode, BO ２ － A steel plate characterized by having a steel structure in which the average value of the detected signal intensity is 8.0 or less.

2. The chemical composition is as follows, in mass percent: Cr: 0.001-1.00%, Cu: 0.001-1.00%, Mo: 0.001-1.00%, Ni: 0.001-1.00%, Co: 0.001-1.00%, W: 0.001-1.00%, Sn: 0.001-1.000%, Sb: 0.001-0.500%, Nb: 0.001-0.200%, V: 0.001-1.00%, As: 0.001-0.100%, Zn: 0.001-1.000%, Mg: 0.0001-0.0100%, Ca: 0.0001-0.0100% The steel sheet according to claim 1, characterized in that it contains at least one of the following: Zr: 0.0001 to 0.0100%, Ce: 0.0001 to 0.0150%, 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 0.0100%.

3. When the aforementioned flight-type secondary ion mass spectrometry is performed, BO ２ － The steel plate according to claim 1 or 2, characterized in that the sampling skewness of the detected signal intensity distribution is 20.0 or less.

4. A steel plate according to any one of claims 1 to 3, characterized in that it has a tensile strength of 1700 MPa or more.

5. The steel sheet according to any one of claims 1 to 4, characterized in that it has a hot-dip galvanized layer or an alloyed hot-dip galvanized layer on at least one surface.

6. A component characterized by comprising a steel plate as described in any one of claims 1 to 5.

7. (A) A hot rolling process including hot rolling a slab having the chemical composition according to claim 1 or 2, then winding up the obtained hot rolled steel sheet, and cooling, satisfying the following conditions (A1) to (A3): (A1) In the temperature range of 950 to 1050 °C, applying a rolling pass with a reduction rate of 0.20 or more three or more times, and the cumulative reduction rate is 0.60 or more; (A2) In the temperature range of 900 to 950 °C, the cumulative reduction rate is 0.50 or less; (A3) For each rolling pass, the temperature difference between the inlet side plate temperature of the n-th pass and the (n + 1)-th pass satisfies the following formula (1): Here, n: Number of rolling passes T ｎ : Inlet side plate temperature of the n-th pass (°C) t ｎ : Inlet side plate thickness of the n-th pass (mm) (B) A cold rolling process of pickling the hot rolled steel sheet and cold rolling it with a reduction rate of 0.30 to 0.75; (C) A heat treatment process including heating the obtained cold rolled steel sheet to a maximum heating temperature of Ac3 to 950 °C, holding it for 1 to 1000 seconds, and then cooling to room temperature, where the heat history from when it first reaches 600 °C or lower until it first reaches (Ms - 100) °C or lower satisfies the following formula (2): Here,[ t: Elapsed time (seconds) from when the steel sheet temperature first reaches 600 °C or lower until it first reaches (Ms - 100) °C or lower T: Average temperature (°C) from when the steel sheet temperature first reaches 600 °C or lower until it first reaches (Ms - 100) °C or lower Here, the average temperature (°C) is calculated by measuring the temperature every 1 second for the integration step over the elapsed time t to calculate the integrated temperature, and then dividing the obtained integrated temperature by the elapsed time t. C ∞ : Equilibrium grain boundary segregation concentration of boron at temperature T (mass%) C ｔ : Grain boundary segregation concentration of boron when held at temperature T for time t (mass%) [B]: B content in the chemical composition of the steel sheet (mass%) A method for manufacturing a steel sheet, characterized by including the above.

8. The manufacturing method according to claim 7, characterized in that the following conditions (C1) to (C3) are further satisfied in the heat treatment step: (C1) The cold-rolled steel sheet is cooled in a temperature range of 600 to 750°C at an average cooling rate of 10°C / second or more; (C2) The sheet is left in a temperature range of (Ms-100) to 600°C for 1 to 1000 seconds and cooled to (Ms-100)°C or below; (C3) The cold-rolled steel sheet cooled to (Ms-100)°C or below is reheated to a temperature range of 200°C or higher and left for 1 to 1000 seconds, then cooled to 100°C or below, and the thermal history from when it reaches (Ms-100)°C or below until it first reaches 100°C or below satisfies the following formula (4). Here, r ｆ / r ０ : Particle size ratio of precipitate B before and after reheating treatment t: Elapsed time (integer seconds) from the start of reheating treatment t ｆ : Reheating completion time (integer seconds) T(t): Process temperature at time t (°C) However, the integration step is 1 second. If the measurement interval is less than 1 second, the temperature every second is calculated by linearly correcting the temperature between measurement points. [B]: B content in the chemical composition of the steel plate (mass %)

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