WO2023181643A1

WO2023181643A1 - High-strength steel sheet and manufacturing method therefor

Info

Publication number: WO2023181643A1
Application number: PCT/JP2023/002917
Authority: WO
Inventors: 潤也戸畑; 勇樹田路; 秀和南
Original assignee: Ｊｆｅスチール株式会社
Priority date: 2022-03-25
Filing date: 2023-01-30
Publication date: 2023-09-28

Abstract

The purpose of the present invention is to provide: a high-strength steel sheet having a TS of 980 MPa or more, an El of 10% or more, and excellent toughness, flatness in the sheet width direction, and resistance to machining embrittlement; and a manufacturing method therefor.　This high-strength steel sheet has a prescribed component composition. The amount of martensite is 60% or more in area fraction, and the amount of retained austenite is 3% to 15%, inclusive, in volume fraction at a position at 1/4 of the plate thickness. The sum of the amount of ferrite and the amount of bainitic ferrite is more than 10% in area fraction, and the average value of the occupancy of packets having the maximum occupancy in the prior austenite grains is 70% or less in area fraction.

Description

High strength steel plate and its manufacturing method

The present invention relates to a high-strength steel plate that is excellent in tensile strength, El, toughness, flatness in the width direction, and resistance to work embrittlement, and a method for manufacturing the same. The high-strength steel sheet of the present invention can be suitably used as a structural member for automobile parts and the like.

With the aim of both reducing _CO2 emissions by reducing the weight of vehicles and improving collision resistance by reducing the weight of the car body, the strength of thin steel sheets for automobiles is progressing, and new laws and regulations are being introduced one after another. . Therefore, in order to increase the strength of the car body, high-strength steel plates with a tensile strength (TS) of 980 MPa or higher are increasingly being used in the main structural parts forming automobiles.

High-strength steel sheets used in automobiles require excellent press forming. For example, for frame parts such as automobile bumpers, it is preferable to use high-strength steel plates with high El. In addition, from the viewpoint of collision safety, excellent toughness and resistance to work embrittlement are required.

Furthermore, high-strength steel sheets used in automobiles are required to have excellent flatness. Patent Document 1 describes that warpage of a steel plate adversely affects operational troubles in a forming line and dimensional accuracy of products. As a result of extensive studies, the present inventors found that the dimensional accuracy of a product is affected not only by the warpage of the steel plate but also by the flatness in the width direction of the plate, which is evaluated using steepness. For example, in order to achieve excellent dimensional accuracy, the steepness in the width direction is preferably 0.02 or less.

In response to these demands, for example, Patent Document 2 provides a high-strength steel plate having a tensile strength of 1100 MPa or more and excellent YR, surface texture, and weldability, and a method for manufacturing the same. However, the technique described in Patent Document 2 does not take into consideration El, toughness, flatness in the sheet width direction, and resistance to work embrittlement.

Patent Document 3 provides a hot-dip galvanized steel sheet with excellent press formability and low-temperature toughness and a tensile strength of 980 MPa or more, and a method for manufacturing the same. However, although the technique described in Patent Document 3 can improve the embrittlement of the steel plate due to a temperature drop, it does not take into account the embrittlement of the steel plate due to processing. Flatness in the width direction of the plate is also not considered.

Patent No. 4947176 Patent No. 6525114 Patent No. 6777272

The present invention was developed in view of the above circumstances, and includes a high-strength steel plate having a TS of 980 MPa or more, an El of 10% or more, and excellent toughness, flatness in the width direction, and resistance to work embrittlement, and a method for manufacturing the same. The purpose is to provide

In order to achieve the above-mentioned problems, the present inventors have made the following findings as a result of extensive studies.
(1) By setting the amount of martensite to 60% or more in terms of area fraction, a TS of 980 MPa or more can be achieved.
(2) El of 10% or more can be achieved by setting the amount of retained austenite to 3% or more in volume fraction and the total amount of ferrite and bainitic ferrite to more than 10% in area fraction.
(3) Excellent toughness can be achieved by setting the amount of retained austenite to 3% or more in volume fraction.
(4) Excellent work embrittlement resistance can be achieved by setting the average value of the occupancy of packets having the maximum occupancy in prior austenite grains to 70% or less in terms of area fraction.
(5) Excellent durability can be achieved by setting the amount of retained austenite to 15% or less in terms of volume fraction and the average value of the occupancy of the packets having the maximum occupancy in the prior austenite grains to 70% or less in terms of area fraction. Process embrittlement properties can be achieved.

The present invention has been made based on the above findings. That is, the gist of the present invention is as follows.
[1] In mass %, C: 0.030% or more and 0.500% or less, Si: 0.50% or more and 2.50% or less, Mn: 1.00% or more and 5.00% or less, P: 0. Contains 100% or less, S: 0.0200% or less, Al: 1.000% or less, N: 0.0100% or less, and O: 0.0100% or less, with the remainder consisting of Fe and inevitable impurities. Regarding the component composition, at the 1/4 position of the plate thickness, the amount of martensite is 60% or more in area fraction, the amount of retained austenite is 3% or more and 15% or less in volume fraction, and the amount of ferrite and bainitic ferrite is A high-strength steel plate, wherein the total area fraction is more than 10%, and the average value of the occupancy of packets having the maximum occupancy in prior austenite grains is 70% or less in area fraction.
[2] The component composition further includes, in mass %, Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less, Ta: 0.10% or less, W: 0. 10% or less, B: 0.0100% or less, Cr: 1.00% or less, Mo: 1.00% or less, Co: 0.010% or less, Ni: 1.00% or less, Cu: 1.00% Below, Sn: 0.200% or less, Sb: 0.200% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less, Zr: 0.100% or less, The high-strength steel plate according to [1], containing at least one element selected from Te: 0.100% or less, Hf: 0.10% or less, Bi: 0.200% or less.
[3] The high-strength steel plate according to [1] or [2], which has a plating layer on the surface of the steel plate.
[4] The method for producing a high-strength steel plate according to [1] or [2], wherein a cold-rolled plate produced by subjecting steel having the above-mentioned composition to hot rolling, pickling, and cold rolling, Heating and annealing is performed under conditions where the annealing temperature Ta is 700°C or more and 900°C or less, and the holding time at the annealing temperature Ta is 10 seconds or more and 1000 seconds or less, and during the annealing, bending and unbending are performed using a roll with a radius of 800 mm or less. Processed 1 to 15 times, and the average cooling rate in the temperature range of 700°C to 600°C is 20°C/s or more, and the average cooling rate in the temperature range of 499°C to Ms is less than 20°C/s. The average cooling rate in the temperature range from Ms to the cooling stop temperature Tb is performed by bending and unbending a total of 1 to 15 times using a roll with a radius of 800 mm or less in the temperature range of 499 ° C to Ms. is cooled at 150°C/s or less, the tension applied to the steel plate in the temperature range from Ms to cooling stop temperature Tb is 5 MPa or more and 100 MPa or less, and the cooling stop temperature Tb is 100°C or more (Ms - 80°C). In addition, Ms is the martensitic transformation start temperature (°C) defined by formula (1), the tempering temperature is Tb or more and 450°C or less, and the holding time at the tempering temperature is 10 seconds or more and 1000 A method for producing high-strength steel sheets that can be tempered in seconds or less.
Ms=519-474×[%C]-30.4×[%Mn]-12.1×[%Cr]-7.5×[%Mo]-17.7×[%Ni]-Ta/80 ...(1)
Here, [%C], [%Mn], [%Cr], [%Mo], and [%Ni] represent the respective contents (mass%) of C, Mn, Cr, Mo, and Ni, and do not include In that case, it is set to 0.
[5] The method for producing a high-strength steel plate according to [4], which comprises performing a plating treatment.

According to the present invention, it is possible to obtain a high-strength steel plate having a TS of 980 MPa or more, an El of 10% or more, and excellent toughness, flatness in the width direction, and resistance to work embrittlement. Furthermore, by applying the high-strength steel plate of the present invention to, for example, automobile structural members, it is possible to improve fuel efficiency by reducing the weight of the vehicle body. Therefore, the industrial value is extremely large.

FIG. 1 is a diagram showing the structure of a packet having the maximum occupancy in prior austenite grains and a method for calculating the occupancy of the packet according to the present invention. FIG. 2 is a diagram illustrating the concept of the steepness λ in the width direction of a steel plate and the calculation method thereof according to the present invention.

Hereinafter, embodiments of the present invention will be described.

First, the appropriate range of the composition of high-strength steel sheets and the reason for its limitation will be explained. In the following description, "%" representing the content of component elements of steel means "mass %" unless otherwise specified.

[C: 0.030% or more and 0.500% or less]
C is one of the important basic components of steel, and particularly in the present invention, it is an important element that affects the amount of martensite. If the C content is less than 0.030%, the amount of martensite decreases, making it difficult to achieve a TS of 980 MPa or more. On the other hand, when the C content exceeds 0.500%, martensite becomes brittle and the toughness and resistance to work embrittlement deteriorate. Therefore, the content of C is 0.030% or more and 0.500% or less. The lower limit of the C content is preferably 0.050% or more. The upper limit of the C content is preferably 0.400% or less. The lower limit of the C content is more preferably 0.100% or more. The upper limit of the C content is more preferably 0.350% or less.

[Si: 0.50% or more and 2.50% or less]
Si is one of the important basic components of steel and is an important element that affects TS and the amount of retained austenite. If the Si content is less than 0.50%, the strength of martensite decreases, making it difficult to achieve a TS of 980 MPa or more. On the other hand, when the Si content exceeds 2.50%, retained austenite increases excessively, and toughness and resistance to work embrittlement deteriorate. Therefore, the Si content is set to 0.50% or more and 2.50% or less. The lower limit of the Si content is preferably 0.55% or more. The upper limit of the Si content is preferably 2.00% or less. The lower limit of the Si content is more preferably 0.60% or more. The upper limit of the Si content is more preferably 1.80% or less.

[Mn: 1.00% or more and 5.00% or less]
Mn is one of the important basic components of steel and is an important element that affects the amount of martensite. If the Mn content is less than 1.00%, the amount of martensite decreases, making it difficult to achieve a TS of 980 MPa or more. On the other hand, when the Mn content exceeds 5.00%, martensite becomes brittle and the toughness and resistance to work embrittlement deteriorate. Therefore, the Mn content is 1.00% or more and 5.00% or less. The lower limit of the Mn content is preferably 1.50% or more. The upper limit of the Mn content is preferably 4.50% or less. The lower limit of the Mn content is more preferably 2.00% or more. The upper limit of the Mn content is more preferably 4.00% or less.

[P: 0.100% or less]
Since P segregates at prior austenite grain boundaries and embrittles the grain boundaries, it lowers the ultimate deformability of the steel sheet, resulting in a decrease in toughness and work embrittlement resistance. Therefore, the content of P needs to be 0.100% or less. Although the lower limit of the P content is not particularly defined, it is preferably 0.001% or more since P is a solid solution strengthening element and can increase the strength of the steel sheet. Therefore, the content of P is 0.100% or less. The lower limit of the P content is preferably 0.001% or more. The upper limit of the P content is preferably 0.070% or less.

[S: 0.0200% or less]
S exists as a sulfide and reduces the ultimate deformability of the steel sheet, resulting in a reduction in toughness and resistance to work embrittlement. Therefore, the S content needs to be 0.0200% or less. Although the lower limit of the S content is not particularly specified, it is preferably 0.0001% or more due to production technology constraints. Therefore, the S content is set to 0.0200% or less. The lower limit of the S content is preferably 0.0001% or more. The upper limit of the S content is preferably 0.0050% or less.

[Al: 1.000% or less]
Al exists as an oxide and reduces the ultimate deformability of the steel sheet, thereby reducing the toughness and resistance to work embrittlement. Therefore, the Al content needs to be 1.000% or less. Although the lower limit of the Al content is not particularly defined, the Al content is preferably 0.001% or more because it suppresses the formation of carbides during continuous annealing and promotes the formation of retained austenite. Therefore, the Al content is set to 1.000% or less. The lower limit of the Al content is preferably 0.001% or more. The upper limit of the Al content is preferably 0.500% or less.

[N: 0.0100% or less]
N exists as a nitride and reduces the ultimate deformability of the steel sheet, resulting in a reduction in toughness and resistance to work embrittlement. Therefore, the N content needs to be 0.0100% or less. Although the lower limit of the N content is not particularly specified, it is preferable that the N content is 0.0001% or more due to constraints on production technology. Therefore, the N content is set to 0.0100% or less. The lower limit of the N content is preferably 0.0001% or more. The upper limit of the N content is preferably 0.0050% or less.

[O: 0.0100% or less]
O exists as an oxide and reduces the ultimate deformability of the steel sheet, thereby reducing the toughness and resistance to work embrittlement. Therefore, the content of O needs to be 0.0100% or less. Although the lower limit of the O content is not particularly defined, it is preferable that the O content is 0.0001% or more due to production technology constraints. Therefore, the O content is set to 0.0100% or less. The lower limit of the O content is preferably 0.0001% or more. The upper limit of the O content is preferably 0.0050% or less.

A high-strength steel plate according to an embodiment of the present invention has a composition containing the above-mentioned components, with the remainder containing Fe and inevitable impurities. Here, unavoidable impurities include Zn, Pb, As, Ge, Sr, and Cs. A total content of these impurities of 0.100% or less is allowed.

In addition to the above-mentioned composition, the high-strength steel plate of the present invention further includes, in mass%,
Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less, Ta: 0.10% or less, W: 0.10% or less, B: 0.0100% or less, Cr: 1.00% or less, Mo: 1.00% or less, Ni: 1.00% or less, Co: 0.010% or less, Cu: 1.00% or less, Sn: 0.200% or less, Sb: 0. 200% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less, Zr: 0.100% or less, Te: 0.100% or less, Hf: 0.10% At least one element selected from the following and Bi: 0.200% or less may be contained alone or in combination.

If Ti, Nb, and V are each 0.200% or less, large amounts of coarse precipitates and inclusions will not be generated and the ultimate deformability of the steel plate will not be reduced, resulting in a decrease in toughness and resistance to work embrittlement. do not. Therefore, it is preferable that the contents of Ti, Nb, and V are each 0.200% or less. Note that the lower limits of the content of Ti, Nb, and V are not particularly specified, but the strength of the steel sheet can be increased by forming fine carbides, nitrides, or carbonitrides during hot rolling or continuous annealing. Therefore, it is more preferable that the contents of Ti, Nb, and V are each 0.001% or more. Therefore, when Ti, Nb, and V are contained, their contents are each 0.200% or less. The lower limit in the case of containing Ti, Nb and V is more preferably 0.001% or more. When Ti, Nb and V are contained, the upper limit is more preferably 0.100% or less.

If Ta and W are each 0.10% or less, large amounts of coarse precipitates and inclusions will not be generated and the ultimate deformability of the steel plate will not be reduced, so the toughness and work embrittlement resistance will not deteriorate. Therefore, it is preferable that the contents of Ta and W are each 0.10% or less. Note that there is no particular lower limit to the content of Ta and W, but the strength of the steel sheet is increased by forming fine carbides, nitrides, or carbonitrides during hot rolling or continuous annealing. It is more preferable that the contents of Ta and W are each 0.01% or more. Therefore, when Ta and W are contained, their contents are each 0.10% or less. The lower limit in the case of containing Ta and W is more preferably 0.01% or more. The upper limit when Ta and W are contained is more preferably 0.08% or less.

If B is 0.0100% or less, it will not generate cracks inside the steel plate during casting or hot rolling, and will not reduce the ultimate deformability of the steel plate, so the toughness and work embrittlement resistance will not deteriorate. Therefore, the content of B is preferably 0.0100% or less. Note that the lower limit of the B content is not particularly specified, but since it is an element that segregates at austenite grain boundaries during annealing and improves hardenability, it is preferable that the B content is 0.0003% or more. preferable. Therefore, when B is contained, its content should be 0.0100% or less. The lower limit in the case of containing B is more preferably 0.0003% or more. The upper limit when B is contained is more preferably 0.0080% or less.

If each of Cr, Mo, and Ni is 1.00% or less, coarse precipitates and inclusions do not increase and the ultimate deformability of the steel sheet does not decrease, so the toughness and work embrittlement resistance do not decrease. Therefore, it is preferable that the contents of Cr, Mo, and Ni are each 1.00% or less. Note that the lower limit of the content of Cr, Mo, and Ni is not particularly specified, but since they are elements that improve hardenability, it is more preferable that the content of Cr, Mo, and Ni is each 0.01% or more. . Therefore, when Cr, Mo, and Ni are contained, their contents are each 1.00% or less. The lower limit in the case of containing Cr, Mo and Ni is more preferably 0.01% or more. The upper limit when Cr, Mo and Ni are contained is more preferably 0.80% or less.

If Co is 0.010% or less, coarse precipitates and inclusions do not increase and the ultimate deformability of the steel sheet does not decrease, so the toughness and work embrittlement resistance do not decrease. Therefore, the Co content is preferably 0.010% or less. Although the lower limit of the Co content is not particularly specified, since it is an element that improves hardenability, the Co content is more preferably 0.001% or more. Therefore, when Co is contained, the content should be 0.010% or less. The lower limit in the case of containing Co is more preferably 0.001% or more. The upper limit when Co is contained is more preferably 0.008% or less.

If Cu is 1.00% or less, coarse precipitates and inclusions do not increase and the ultimate deformability of the steel sheet does not decrease, so the toughness and work embrittlement resistance do not deteriorate. Therefore, the Cu content is preferably 1.00% or less. Although the lower limit of the Cu content is not particularly specified, since it is an element that improves hardenability, the Cu content is preferably 0.01% or more. Therefore, if Cu is contained, the content should be 1.00% or less. The lower limit in the case of containing Cu is more preferably 0.01% or more. The upper limit when Cu is contained is more preferably 0.80% or less.

If Sn is 0.200% or less, it will not generate cracks inside the steel sheet during casting or hot rolling, and will not reduce the ultimate deformability of the steel sheet, so the toughness and resistance to work embrittlement will not deteriorate. Therefore, the content of Sn is preferably 0.200% or less. Note that the lower limit of the Sn content is not particularly specified, but since Sn is an element that improves hardenability (generally an element that improves corrosion resistance), the Sn content should be 0.001% or more. It is more preferable. Therefore, if Sn is contained, the content should be 0.200% or less. The lower limit in the case of containing Sn is more preferably 0.001% or more. The upper limit when Sn is contained is more preferably 0.100% or less.

If Sb is 0.200% or less, coarse precipitates and inclusions will not increase and the ultimate deformability of the steel plate will not be reduced, so the toughness and work embrittlement resistance will not deteriorate. Therefore, the content of Sb is preferably 0.200% or less. Although the lower limit of the Sb content is not particularly defined, it is more preferable that the Sb content is 0.001% or more since it is an element that controls the softening thickness of the surface layer and enables strength adjustment. Therefore, if Sb is contained, the content should be 0.200% or less. The lower limit in the case of containing Sb is more preferably 0.001% or more. The upper limit when Sb is contained is more preferably 0.100% or less.

If each of Ca, Mg and REM is 0.0100% or less, coarse precipitates and inclusions will not increase and the ultimate deformability of the steel plate will not be reduced, so the toughness and work embrittlement resistance will not deteriorate. Therefore, the content of Ca, Mg and REM is preferably 0.0100% or less. Note that the lower limits of the contents of Ca, Mg, and REM are not particularly stipulated, but since they are elements that spheroidize the shape of nitrides and sulfides and improve the ultimate deformability of steel sheets, the contents of Ca, Mg, and REM are More preferably, each amount is 0.0005% or more. Therefore, when Ca, Mg and REM are contained, their contents are each 0.0100% or less. The lower limit in the case of containing Ca, Mg and REM is more preferably 0.0005% or more. The upper limit when Ca, Mg and REM are contained is more preferably 0.0050% or less.

If each of Zr and Te is 0.100% or less, coarse precipitates and inclusions will not increase and the ultimate deformability of the steel plate will not be reduced, so the toughness and work embrittlement resistance will not deteriorate. Therefore, the contents of Zr and Te are preferably 0.100% or less. Note that the lower limits of the contents of Zr and Te are not particularly specified, but since they are elements that spheroidize the shape of nitrides and sulfides and improve the ultimate deformability of the steel sheet, the contents of Zr and Te are respectively 0. More preferably, the content is .001% or more. Therefore, when Zr and Te are contained, their contents are each 0.100% or less. The lower limit in the case of containing Zr and Te is more preferably 0.001% or more. The upper limit when Zr and Te are contained is more preferably 0.080% or less.

If Hf is 0.10% or less, coarse precipitates and inclusions will not increase and the ultimate deformability of the steel plate will not be reduced, so the toughness and work embrittlement resistance will not deteriorate. Therefore, the Hf content is preferably 0.10% or less. Note that there is no particular lower limit to the Hf content, but since it is an element that spheroidizes the shape of nitrides and sulfides and improves the ultimate deformability of steel sheets, the Hf content should be 0.01% or more. It is more preferable to do so. Therefore, if Hf is contained, the content should be 0.10% or less. The lower limit in the case of containing Hf is more preferably 0.01% or more. The upper limit when containing Hf is more preferably 0.08% or less.

If Bi is 0.200% or less, coarse precipitates and inclusions will not increase and the ultimate deformability of the steel plate will not be reduced, so the toughness and work embrittlement resistance will not deteriorate. Therefore, the Bi content is preferably 0.200% or less. Although the lower limit of the Bi content is not particularly defined, since it is an element that reduces segregation, the Bi content is more preferably 0.001% or more. Therefore, when Bi is contained, the content should be 0.200% or less. The lower limit in the case of containing Bi is more preferably 0.001% or more. The upper limit in the case of containing Bi is more preferably 0.100% or less.

In addition, each content of Ti, Nb, V, Ta, W, B, Cr, Mo, Ni, Co, Cu, Sn, Sb, Ca, Mg, REM, Zr, Te, Hf and Bi is preferable. If it is less than the lower limit, the effect of the present invention will not be impaired, and therefore it is included as an unavoidable impurity.

Next, the steel structure of the high-strength steel plate of the present invention will be explained.

[Amount of martensite is 60% or more in terms of area fraction]
This is an extremely important feature of the invention. By setting the amount of martensite to 60% or more in terms of area fraction, it becomes possible to realize a TS of 980 MPa or more. Therefore, the area fraction of martensite is 60% or more. Preferably it is 62% or more. More preferably, it is 64% or more.

[Retained austenite amount is 3% or more and 15% or less in volume fraction]
This is an extremely important feature of the invention. If the amount of retained austenite is less than 3% in volume fraction, it becomes difficult to achieve an El of 10% or more, and the toughness improvement effect of retained austenite cannot be obtained, making it difficult to achieve excellent toughness. become. Additionally, if the amount of retained austenite exceeds 15%, the retained austenite will excessively transform into hard martensite during processing, resulting in a decrease in the ultimate deformability of the steel sheet, making it difficult to obtain excellent work embrittlement resistance. It becomes difficult. Therefore, the retained austenite is set to 3% or more and 15% or less. The lower limit of the amount of retained austenite is preferably 5% or more. The upper limit of the amount of retained austenite is preferably 14% or less. The lower limit of the amount of retained austenite is more preferably 7% or more. The upper limit of the amount of retained austenite is more preferably 13% or less.

Here, the method for measuring retained austenite is as follows. Retained austenite was determined by polishing the steel plate from 1/4 part of the plate thickness to a surface of 0.1 mm, and then chemically polishing the surface to a further 0.1 mm using an X-ray diffractometer using CoKα rays. }, {220}, {311} planes and the diffraction peaks of {200}, {211}, {220} planes of BCC iron were measured, and the nine integrated intensity ratios obtained were averaged. Convert and seek.

[The total amount of ferrite and bainitic ferrite exceeds 10% in area fraction]
This is an extremely important feature of the invention. If the total amount of ferrite and bainitic ferrite is 10% or less, it becomes difficult to achieve El of 10% or more. Therefore, the total amount of ferrite and bainitic ferrite is more than 10%. Preferably it is 12% or more. More preferably, it is 13% or more. Note that the upper limit of the total amount of ferrite and bainitic ferrite is not particularly limited.

Here, the method for measuring the total amount of ferrite and bainitic ferrite is as follows. After polishing the L cross section of the steel plate, 3vol. % nital, and 1/4 part of the plate thickness (a position corresponding to 1/4 of the plate thickness in the depth direction from the steel plate surface) is observed in 10 fields at a magnification of 2000 times using an SEM. In the above structure image, ferrite and bainitic ferrite have concave portions and a flat structure inside, and have no carbide inside. From the average value of those values, the total amount of ferrite and bainitic ferrite can be determined.

The method for measuring the amount of martensite is as follows. The amount of martensite can be determined by measuring the amount of retained austenite, the amount of ferrite, and the amount of bainitic ferrite based on the method described above, and subtracting the total from 100%. Therefore, the amount of martensite in the present invention is an amount that includes both quenched martensite and tempered martensite. In addition, in subtraction, since the volume ratio of the retained austenite amount is approximately the area ratio, it is subtracted from 100% together with the ferrite amount and the bainitic ferrite amount expressed by the area ratio.

[The average value of the occupancy of the packets with the maximum occupancy in the prior austenite grains is 70% or less in terms of area fraction]
This is an extremely important feature of the invention. The occupancy of the packets having the maximum occupancy within the prior austenite grains influences the flatness in the width direction and the resistance to work embrittlement. The packet with the maximum occupancy rate in the prior austenite grains means that, as shown in Figure 1, there are up to four regions in the prior austenite grains, called packets, that have the same crystal habit plane during transformation. indicates the packet with the largest occupancy rate.

The occupancy of one packet within the prior austenite grains is determined by dividing the area of the specified packet by the entire area within the prior austenite grains.

As a result of extensive studies, the present inventors found that by reducing the occupancy of the packets with the maximum occupancy within the prior austenite grains, the strain between the packets was alleviated and the flatness in the sheet width direction was improved. I discovered that. They also found that by reducing the occupancy of packets, which have the highest occupancy within prior austenite grains, the structure becomes finer and crack propagation can be suppressed, thereby improving the work embrittlement resistance of the steel sheet. . Therefore, the average value of the occupancy of the packets having the maximum occupancy in the prior austenite grains is set to be 70% or less. Preferably it is 60% or less. Note that the lower limit of the average value of the occupancy of packets having the maximum occupancy in prior austenite grains is not particularly limited. There are a maximum of four types of packets, and when the four packets are evenly distributed, the occupancy rate of the packet having the maximum occupancy rate in the prior austenite grains is 25%. Therefore, although the lower limit of the average value of the occupancy of packets having the maximum occupancy in prior austenite grains is preferably 25% or more, it is not necessary to be limited to this.

Here, the method for measuring the average value of the occupancy of the packets having the maximum occupancy in the prior austenite grains is as follows. First, a test piece for microstructural observation is taken from a cold-rolled steel sheet. Next, the sampled test piece is polished by colloidal silica vibration polishing so that the cross section in the rolling direction (L cross section) becomes the observation surface. The observation surface shall be a mirror surface. Next, electron beam backscatter diffraction (EBSD) measurement is performed on a 1/4 part of the plate thickness (a position corresponding to 1/4 of the plate thickness in the depth direction from the steel plate surface) to obtain local crystal orientation data. At this time, the SEM magnification is 1000 times, the step size is 0.2 μm, the measurement area is 80 μm square, and the WD is 15 mm. The obtained local orientation data is analyzed using OIM Analysis 7 (OIM), and a color-coded diagram (CP map) for each Close-packed Plane group (CP group) is created using the method described in Non-Patent Document 1. . In the present invention, packets are defined as areas to which the same CP group belongs. The area of the packet with the largest occupancy is determined from the obtained CP map and divided by the total area within the prior austenite grains, thereby obtaining the occupancy of the packet with the maximum occupancy within the prior austenite grains. This analysis is performed on ten or more adjacent prior austenite grains, and the average value is taken as the average value of the occupancy of the packets having the maximum occupancy within the prior austenite grain.

Next, the manufacturing method of the present invention will be explained.

In the present invention, the method of melting the steel material (steel slab) is not particularly limited, and any known melting method such as a converter or an electric furnace is suitable. The steel slab (slab) is preferably manufactured by a continuous casting method in order to prevent macro segregation.

In the present invention, the slab heating temperature, slab soaking time and coiling temperature in hot rolling are not particularly limited. Methods for hot rolling steel slabs include rolling the slab after heating, directly rolling the slab after continuous casting without heating it, and rolling after subjecting the slab after continuous casting to a short heat treatment. Examples include. Although the slab heating temperature, slab soaking time, finish rolling temperature, and coiling temperature in hot rolling are not particularly limited, the lower limit of the slab heating temperature is preferably 1100° C. or higher. The upper limit of the slab heating temperature is preferably 1300°C or less. The lower limit of the slab soaking time is preferably 30 min or more. The upper limit of the slab soaking time is preferably 250 min or less. The lower limit of the finish rolling temperature is preferably equal to or higher than the Ar ₃ transformation point. Further, the lower limit of the winding temperature is preferably 350°C or higher. Moreover, the upper limit of the winding temperature is preferably 650° C. or less.

The hot-rolled steel sheet produced in this way is pickled. Since pickling can remove oxides on the surface of the steel sheet, it is important for ensuring good chemical conversion treatability and plating quality in the final high-strength steel sheet. Further, the pickling may be carried out once or may be carried out in multiple steps. Moreover, cold rolling may be performed on the pickled plate after hot rolling, or cold rolling may be performed after heat treatment.

The rolling reduction in cold rolling and the plate thickness after rolling are not particularly limited, but the lower limit of the rolling reduction is preferably 30% or more. Further, the upper limit of the rolling reduction ratio is preferably 80% or less. Note that the effects of the present invention can be obtained without any particular limitations on the number of rolling passes and the rolling reduction rate of each pass.

The cold rolled steel sheet obtained as described above is annealed. The annealing conditions are as follows.

[Annealing temperature Ta is 700°C or higher and 900°C or lower]
When the annealing temperature Ta is less than 700°C, the amount of martensite decreases, making it difficult to achieve a TS of 980 MPa or more. On the other hand, when the annealing temperature exceeds 900° C., the total amount of ferrite and bainitic ferrite decreases, making it difficult to achieve El of 10% or more. Therefore, the annealing temperature is set to 700°C or more and 900°C or less. The lower limit of the annealing temperature is preferably 750°C or higher. The upper limit of the annealing temperature is preferably 870°C or lower.

[Annealing with holding time at annealing temperature Ta of 10 seconds or more and 1000 seconds or less]
If the holding time at the annealing temperature Ta is less than 10 seconds, the amount of martensite decreases, making it difficult to achieve a TS of 980 MPa or more. On the other hand, if the holding time at the annealing temperature Ta exceeds 1000 seconds, the total amount of ferrite and bainitic ferrite decreases, making it difficult to achieve El of 10% or more. Therefore, the holding time at the annealing temperature Ta is 10 seconds or more and 1000 seconds or less. The lower limit of the holding time at the annealing temperature Ta is preferably 50 seconds or more. The upper limit of the holding time at the annealing temperature Ta is preferably 500 seconds or less.

[Bending and unbending a total of 1 to 15 times with rolls with a radius of 800 mm or less during annealing]
As a result of extensive studies, the present inventors have found that applying unbending to a steel sheet during annealing affects the occupancy of packets having the maximum occupancy within prior austenite grains. If bending and unbending is not performed using rolls with a radius of 800 mm or less during annealing, the number of nucleation sites for martensitic transformation is reduced. Therefore, the average value of the occupancy of the packets having the maximum occupancy in the prior austenite grains exceeds 70%, the flatness in the sheet width direction deteriorates, and the work embrittlement resistance deteriorates. On the other hand, when bending and unbending is performed 16 times or more using rolls with a radius of 800 mm or less during annealing, the ultimate deformability of the steel sheet decreases, and the work embrittlement resistance decreases. Therefore, during annealing, bending and unbending are performed at least once and at most 15 times in total using rolls with a radius of 800 mm or less. Preferably, the radius of the roll diameter is 600 mm or less. Preferably, the lower limit of the number of times of bending and unbending is 3 or more times in total. Preferably, the upper limit of the number of times of bending and unbending is 10 times or less in total. Although the lower limit of the radius of the roll diameter does not need to be particularly limited, it is preferably 50 mm or more.

Incidentally, the term "bending and unbending" refers to a process of bending a material in one direction with rolls and then bending it back by the amount of bending in the opposite direction using a known method. The number of times of bending and unbending is counted as one bending and one unbending, rather than one bending and unbending.

[Average cooling rate of 20°C/s or more in the temperature range of 700°C to 600°C]
As a result of intensive studies, the present inventors found that the average cooling rate in the temperature range of 700° C. to 600° C. affects the occupancy rate of packets having the maximum occupancy rate in prior austenite grains. When the average cooling rate in the temperature range of 700° C. to 600° C. is less than 20° C./s, the influence of bending and unbending the steel sheet during annealing is reduced, and the number of nucleation sites for martensitic transformation is reduced. Therefore, the average value of the occupancy of the packets having the maximum occupancy in the prior austenite grains exceeds 70%, the flatness in the sheet width direction deteriorates, and the work embrittlement resistance deteriorates. Therefore, the average cooling rate from 750°C to 600°C is set to 20°C/s or more. Preferably it is 30°C/s or more. The upper limit does not need to be particularly limited, but is preferably 100° C./s or less.

[Average cooling rate in the temperature range of 499°C to Ms is less than 20°C/s]
The average cooling rate in the temperature range of 499° C. to Ms affects the total area fraction of the amount of ferrite and bainitic ferrite. When the average cooling rate in the temperature range of 499° C. to Ms is 20° C./s or more, the total amount of ferrite and bainitic ferrite decreases, making it difficult to achieve El of 10% or more. Therefore, the average cooling rate in the temperature range of 499°C to Ms is less than 20°C/s. Preferably it is 18°C/s or less. The lower limit does not need to be particularly limited, but is preferably 5° C./s or more.

Note that the martensitic transformation start temperature Ms (° C.) is defined by the following equation (1).
Ms=519-474×[%C]-30.4×[%Mn]-12.1×[%Cr]-7.5×[%Mo]-17.7×[%Ni]-Ta/80 ...(1)
Here, [%C], [%Mn], [%Cr], [%Mo], and [%Ni] represent the respective contents (mass%) of C, Mn, Cr, Mo, and Ni, and do not include In that case, it is set to 0.

[Bending and unbending a total of 1 to 15 times with a roll with a radius of 800 mm or less in a temperature range of 499°C to Ms]
As a result of extensive studies, the present inventors found that applying bending and unbending to a steel sheet in the temperature range of 499°C to Ms affects the occupancy rate of packets having the maximum occupancy rate in prior austenite grains. Ta. If bending and unbending is not performed in the temperature range of 499° C. to Ms with a roll having a radius of 800 mm or less, the number of martensite nucleation sites is reduced. Therefore, the average value of the occupancy of the packets having the maximum occupancy in the prior austenite grains exceeds 70%, the flatness in the sheet width direction deteriorates, and the work embrittlement resistance deteriorates. On the other hand, when bending and unbending is performed more than 16 times in the temperature range of 499° C. to Ms with a roll having a radius of 800 mm or less, the ultimate deformability of the steel sheet decreases and the work embrittlement resistance decreases. Therefore, bending and unbending is performed a total of 1 to 15 times in a temperature range of 499° C. to Ms with a roll having a radius of 800 mm or less. Preferably, the radius of the roll diameter is 600 mm or less. Preferably, the lower limit of the number of times of bending and unbending is 3 or more times in total. Preferably, the lower limit of the number of times of bending and unbending is 10 times or less in total. Although the lower limit of the radius of the roll diameter does not need to be particularly limited, it is preferably 50 mm or more.

[Average cooling rate in the temperature range from Ms to cooling stop temperature Tb is 150°C/s or less]
As a result of extensive studies, the present inventors found that the average cooling rate in the temperature range from Ms to the cooling stop temperature Tb affects the occupancy of packets having the maximum occupancy in prior austenite grains. When the average cooling rate in the temperature range from Ms to the cooling stop temperature Tb exceeds 150°C/s, one packet tends to grow due to the fast martensitic transformation rate, so the maximum occupancy within the prior austenite grains increases. The average value of the occupancy rate of packets having a ratio exceeds 70%, the flatness in the sheet width direction deteriorates, and the work embrittlement resistance deteriorates. Therefore, the average cooling rate in the temperature range from Ms to the cooling stop temperature Tb is set to 150° C./s or less. Preferably it is 120°C/s or less. The lower limit does not need to be particularly limited, but is preferably 5° C./s or more.

[Tension applied to the steel plate in the temperature range from Ms to cooling stop temperature Tb is 5 MPa or more and 100 MPa or less]
As a result of extensive studies, the present inventors found that applying tension to the steel sheet in the temperature range from Ms to cooling stop temperature Tb affects the occupancy rate of packets having the maximum occupancy rate in prior austenite grains. . When the tension applied to the steel plate in the temperature range from Ms to the cooling stop temperature Tb is less than 5 MPa, the number of martensite nucleation sites is reduced. Therefore, the average value of the occupancy of the packets having the maximum occupancy in the prior austenite grains exceeds 70%, the flatness in the sheet width direction deteriorates, and the work embrittlement resistance deteriorates. On the other hand, if the tension applied to the steel sheet in the temperature range from Ms to cooling stop temperature Tb exceeds 100 MPa, the total amount of ferrite and bainitic ferrite increases excessively, so the amount of martensite decreases, and the tension exceeds 980 MPa. It becomes difficult to realize the TS of Therefore, the tension applied to the steel plate in the temperature range from Ms to the cooling stop temperature Tb is set to be 5 MPa or more and 100 MPa or less. The lower limit of the tension applied to the steel plate in the temperature range from Ms to cooling stop temperature Tb is preferably 6 MPa or more. The upper limit of the tension applied to the steel plate in the temperature range from Ms to cooling stop temperature Tb is preferably 50 MPa or less. Note that the tension is applied by a known method. As an example, tension may be applied by controlling the speed of the rolls in the furnace.

Note that while the bending and unbending process increases the number of nucleation sites that are martensitic transformation initiation sites, the tension application process has a different effect in that it promotes martensitic transformation itself.

[Cooling stop temperature Tb is 100°C or higher (Ms-80°C) or lower]
When the cooling stop temperature Tb is less than 100°C, the amount of retained austenite decreases and bendability decreases. On the other hand, when the cooling stop temperature Tb exceeds (Ms-80°C), the amount of retained austenite increases excessively, the prior austenite grain size increases excessively, and the work embrittlement resistance deteriorates. Therefore, the cooling stop temperature Tb is set to 100°C or more (Ms-80°C) or less. The lower limit of the cooling stop temperature Tb is preferably 120°C or higher. The upper limit of the cooling stop temperature Tb is preferably (Ms-100°C) or less.

[Tempering temperature is Tb or higher and 450°C or lower]
After cooling is stopped at the cooling stop temperature Tb, the remaining austenite is stabilized by holding at that temperature or by reheating and holding at a temperature of 450° C. or lower. When the tempering temperature is lower than Tb, a predetermined level of retained austenite cannot be obtained, so that El decreases and it becomes difficult to obtain excellent toughness. If the tempering temperature exceeds 450° C., the martensite will be excessively tempered, making it difficult to achieve a TS of 980 MPa or more. Therefore, the tempering temperature is set to be higher than Tb and lower than 450°C. The lower limit of the tempering temperature is preferably (Tb+10°C) or higher. The upper limit of the tempering temperature is preferably 420°C or less.

[Holding time at tempering temperature is 10 seconds or more and 1000 seconds or less]
If the holding time at the tempering temperature is less than 10 seconds, the stabilization of austenite will be insufficient and the desired residual austenite will not be obtained, resulting in a decrease in El and difficulty in obtaining excellent toughness. . If the holding time at the tempering temperature exceeds 1000 seconds, the martensite will be excessively tempered, making it difficult to achieve a TS of 980 MPa or more. Therefore, the holding time at the tempering temperature is 10 seconds or more and 1000 seconds or less. The lower limit of the holding time at the tempering temperature is preferably 50 seconds or more. The upper limit of the holding time at the tempering temperature is preferably 800 seconds or less.

Cooling after tempering does not need to be particularly specified, and may be cooled to a desired temperature by any method. Note that the desired temperature is preferably about room temperature.

Furthermore, the above-mentioned high-strength steel plate may be processed under conditions that result in an equivalent plastic strain amount of 0.10% or more and 5.00% or less. Further, after processing, reheating may be performed again under the conditions of 100° C. or more and 400° C. or less.

Note that when high-strength steel sheets are traded, they are usually traded after being cooled to room temperature.

The high-strength steel plate may be subjected to plating treatment during or after annealing.

Examples of plating treatment during annealing include hot-dip galvanizing treatment during or after cooling at a temperature of 700°C to 600°C after annealing at an average cooling rate of 20°C/s or more, and alloying after hot-dip galvanizing. can. Further, as the plating treatment after annealing, for example, Zn-Ni electroplating treatment or pure Zn electroplating treatment can be exemplified after tempering. The plating layer may be formed by electroplating, or hot-dip zinc-aluminum-magnesium alloy plating may be applied. In addition, although the above-mentioned plating treatment was mainly explained in the case of zinc plating, the type of plating metal such as Zn plating and Al plating is not particularly limited. Conditions for other manufacturing methods are not particularly limited, but from the viewpoint of productivity, a series of treatments such as annealing, hot-dip galvanizing, and galvanizing alloying are performed on a continuous galvanizing line (CGL), which is a hot-dip galvanizing line. It is preferable to carry out the process using Line). After hot-dip galvanizing, wiping can be performed to adjust the coating weight. Note that the conditions for plating and the like other than the above-mentioned conditions can be based on a conventional method for hot-dip galvanizing.

After the plating treatment after annealing, processing may be performed again under conditions that result in an equivalent plastic strain amount of 0.10% or more and 5.00 or less. Further, after processing, reheating may be performed again under the conditions of 100° C. or more and 400° C. or less.

A steel having the composition shown in Tables 1 and 2, with the remainder consisting of Fe and unavoidable impurities, was melted in a converter and made into a slab by continuous casting. Next, the obtained slab was heated, hot rolled, pickled, cold rolled, and annealed as shown in Tables 3 and 4 to give a plate thickness of 0.6 to 2.2 mm. A high-strength cold-rolled steel sheet was obtained. For bending and unbending during annealing, a roll with a radius of 300 mm was used, and for bending and unbending in the temperature range of 499° C. to Ms, a roll with a radius of 300 mm was used. Note that some steel plates are manufactured by performing plating treatment during or after annealing.

Using the high-strength cold-rolled steel sheet obtained as described above as a test steel, tensile properties, flatness in the sheet width direction, toughness, and resistance to work embrittlement were evaluated according to the following test methods.

(organizational observation)
According to the method described above, the total amount of martensite, retained austenite, ferrite, and bainitic ferrite was determined.

(Occupancy of packets with maximum occupancy in prior austenite grains)
According to the method described above, the average value of the occupancy of the packets having the maximum occupancy within the prior austenite grains was determined.

(Tensile test)
For the tensile test, a JIS No. 5 test piece (gauge length 50 mm, parallel part width 25 mm) was taken so that the longitudinal direction of the test piece was perpendicular to the rolling direction, and tested in accordance with JIS Z 2241. A tensile test was conducted at a crosshead speed of 1.67×10 ⁻¹ mm/sec, and TS and El were measured. In addition, in the present invention, TS of 980 MPa or more was judged to be acceptable, and El of 10% or more was judged to be acceptable.

(toughness)
Toughness was evaluated by Charpy test. For the Charpy test piece, a plurality of steel plates were stacked together and fastened together with bolts, and after confirming that there were no gaps between the steel plates, a test piece with a V-notch having a depth of 2 mm was prepared. The number of steel plates to be stacked was set so that the thickness of the test piece after stacking was closest to 10 mm. For example, if the plate thickness is 1.2 mm, 8 plates are laminated, resulting in a test piece thickness of 9.6 mm. The laminated Charpy test piece was judged to have "excellent toughness" if it had a strength of 40 J/cm ² or more. Note that conditions other than the above were in accordance with JIS Z 2242:2018.

(Flatness in board width direction)
The flatness in the width direction of the various cold-rolled steel sheets obtained as described above was measured by the method shown in FIG. 2. Specifically, a plate with a length of 500 mm in the rolling direction (coil width x 500 mm L x plate thickness) is cut out from the coil, placed on a surface plate so that the warp of the end face faces upward, and the stylus moves over the object to be measured. The height of the steel plate was continuously measured over the entire width direction using a moving contact displacement meter. Based on the results, the steepness, which is an index indicating the flatness of the steel plate shape, was measured according to the method shown in FIG. If the steepness exceeds 0.02, it is evaluated as "×", if the steepness exceeds 0.01 and is less than 0.02, it is evaluated as "○", and if the steepness is less than 0.01, it is evaluated as "◎". Those with a degree of 0.02 or less were judged to have "excellent flatness in the board width direction".

(Processing embrittlement resistance)
The resistance to work embrittlement was evaluated by Charpy test. For the Charpy test piece, a plurality of steel plates were stacked together and fastened together with bolts, and after confirming that there were no gaps between the steel plates, a test piece with a V-notch having a depth of 2 mm was prepared. The number of steel plates to be stacked was set so that the thickness of the test piece after stacking was closest to 10 mm. For example, if the plate thickness is 1.2 mm, 8 plates are laminated, resulting in a test piece thickness of 9.6 mm. The laminated Charpy test piece was taken with the width direction of the plate as the longitudinal direction. As an index indicating resistance to work embrittlement, the ratio vE _0% /vE _10% of impact absorption energy at room temperature between an as-produced (unprocessed) steel plate and a 10% rolled steel plate was measured. "x" indicates that vE _0% /vE _10% is less than 0.6; "○" indicates that vE _0% /vE ₁₀ _% is _0.6 or more and less than 0.7; Those with vE _0% /vE _10% of 0.6 or more were evaluated as "excellent in work embrittlement resistance." Note that conditions other than the above were in accordance with JIS Z 2242:2018.

The results are summarized in Tables 5 to 7. As shown in Tables 5 to 7, the examples of the present invention have a TS of 980 MPa or more, an El of 10% or more, and are excellent in toughness, flatness in the width direction, and resistance to work embrittlement. On the other hand, the comparative examples are inferior in any one or more of TS, El, toughness, flatness in the width direction, or resistance to work embrittlement.

Claims

In mass%,
C: 0.030% or more and 0.500% or less,
Si: 0.50% or more and 2.50% or less,
Mn: 1.00% or more and 5.00% or less,
P: 0.100% or less,
S: 0.0200% or less,
Al: 1.000% or less,
N: 0.0100% or less, and
A component composition containing O: 0.0100% or less, with the remainder consisting of Fe and inevitable impurities;
At the plate thickness 1/4 position,
The amount of martensite is 60% or more in terms of area fraction,
The amount of retained austenite is 3% or more and 15% or less in volume fraction,
The total amount of ferrite and bainitic ferrite is more than 10% in area fraction,
A high-strength steel sheet in which the average value of the occupancy of packets having the maximum occupancy in prior austenite grains is 70% or less in terms of area fraction.
The component composition further includes, in mass%,
Ti: 0.200% or less, Nb: 0.200% or less,
V: 0.200% or less, Ta: 0.10% or less,
W: 0.10% or less, B: 0.0100% or less,
Cr: 1.00% or less, Mo: 1.00% or less,
Co: 0.010% or less, Ni: 1.00% or less,
Cu: 1.00% or less, Sn: 0.200% or less,
Sb: 0.200% or less, Ca: 0.0100% or less,
Mg: 0.0100% or less, REM: 0.0100% or less,
Zr: 0.100% or less, Te: 0.100% or less,
Hf: 0.10% or less, Bi: 0.200% or less,
The high-strength steel plate according to claim 1, containing at least one element selected from the following.
The high-strength steel plate according to claim 1 or 2, which has a plating layer on the surface of the steel plate.
A method for manufacturing a high-strength steel plate according to claim 1 or 2, comprising:
A cold-rolled plate produced by subjecting steel having the above-mentioned composition to hot rolling, pickling and cold rolling,
The annealing temperature Ta is 700°C or more and 900°C or less,
Heating and annealing under conditions where the holding time at the annealing temperature Ta is 10 seconds or more and 1000 seconds or less,
During the annealing, bending and unbending is performed a total of 1 to 15 times with a roll having a radius of 800 mm or less,
The average cooling rate in the temperature range of 700°C to 600°C is 20°C/s or more,
Cooling at an average cooling rate of less than 20°C/s in a temperature range of 499°C to Ms,
Bending and unbending a total of 1 to 15 times using rolls with a radius of 800 mm or less in the temperature range of 499 ° C to Ms,
Cooling at an average cooling rate of 150 ° C / s or less in the temperature range from Ms to cooling stop temperature Tb,
The tension applied to the steel plate in the temperature range from Ms to cooling stop temperature Tb is 5 MPa or more and 100 MPa or less,
The cooling stop temperature Tb is 100°C or more (Ms - 80°C) or less, where Ms is the martensitic transformation start temperature (°C) defined by formula (1),
Tempering temperature is Tb or higher and 450°C or lower,
A method for producing a high-strength steel plate, wherein the holding time at the tempering temperature is 10 seconds or more and 1000 seconds or less.
Ms=519-474×[%C]-30.4×[%Mn]-12.1×[%Cr]-7.5×[%Mo]-17.7×[%Ni]-Ta/80 ...(1)
Here, [%C], [%Mn], [%Cr], [%Mo], and [%Ni] represent the respective contents (mass%) of C, Mn, Cr, Mo, and Ni, and do not include In that case, it is set to 0.
The method for manufacturing a high-strength steel plate according to claim 4, which comprises performing a plating treatment.