KR20240152339A - High-strength steel plate and its manufacturing method - Google Patents

Abstract

The purpose of this invention is to provide a high-strength steel plate having a TS of 980 MPa or higher and an El of 10% or higher, and also having excellent toughness, flatness in the width direction, and embrittlement resistance when processed, and a method for manufacturing the same.
A high-strength steel plate having a predetermined composition, wherein, at a position of 1/4 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, the sum of the amount of ferrite and the amount of bainitic ferrite exceeds 10% in area fraction, and the average value of the occupancy of packets having the maximum occupancy within 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 sheet having excellent tensile strength, El, toughness, flatness in the width direction, and embrittlement resistance during processing, and a method for manufacturing the same. The high-strength steel sheet of the present invention can be suitably used as a structural member of automobile parts and the like.

In order to achieve both reductions in _CO2 emissions through vehicle weight reduction and improved crashworthiness through body weight reduction, the strength of automotive sheet steel is being improved, and new laws and regulations are being introduced one after another. Therefore, in order to increase body strength, the application of high-strength steel sheets with a tensile strength (TS) of 980 MPa or higher is increasing in major structural parts that form automobiles.

High-strength steel sheets used in automobiles require excellent press forming. For example, in the case of skeletal parts such as automobile bumpers, it is appropriate to apply high-strength steel sheets with high El. In addition, from the viewpoint of collision safety, excellent toughness and embrittlement resistance during processing are required.

In addition, high-strength steel plates used in automobiles are also required to have excellent flatness of the steel plates. Patent Document 1 describes that warpage of steel plates adversely affects operational troubles in forming lines and the dimensional accuracy of products. As a result of repeated examinations, the inventors of the present invention have found that the dimensional accuracy of products is affected not only by the warpage of the steel plates but also by the flatness in the width direction evaluated using the steepness. For example, in order to realize excellent dimensional accuracy, the steepness in the width direction is suitably 0.02 or less.

For these requirements, for example, Patent Document 2 provides a high-strength steel plate having a tensile strength of 1100 MPa or more and excellent YR, surface quality and weldability, and a method for manufacturing the same. However, the technology described in Patent Document 2 does not take into consideration El, toughness, flatness in the plate width direction, and embrittlement resistance during processing.

Patent Document 3 provides a hot-dip galvanized steel sheet having a tensile strength of 980 MPa or more, excellent in press formability and low-temperature toughness, and a method for manufacturing the same. However, the technology described in Patent Document 3 can improve embrittlement of the steel sheet due to a decrease in temperature, but does not take into account embrittlement of the steel sheet due to processing. It also does not take into account flatness in the width direction of the sheet.

Japanese Patent Publication No. 4947176 Japanese Patent Publication No. 6525114 Japanese Patent Publication No. 6777272

Journal of Smart Process Society 2013, Vol. 2, No. 3, p.110-118

The present invention has been developed in consideration of these circumstances, and has as its object the provision of a high-strength steel plate having a TS of 980 MPa or more and an El of 10% or more, and also having excellent toughness, flatness in the width direction, and embrittlement resistance when processed, and a method for manufacturing the same.

The inventors of the present invention have conducted repeated examinations to achieve the above-mentioned task and have discovered the following facts.

(1) By increasing the martensite content to 60% or more in area fraction, a TS of 980 MPa or more can be achieved.

(2) By making the amount of retained austenite more than 3% in volume fraction and the sum of the amount of ferrite and bainitic ferrite more than 10% in area fraction, an El of 10% or more can be realized.

(3) By increasing the volume fraction of retained austenite to 3% or more, excellent toughness can be achieved.

(4) By setting the average area fraction of the packet having the maximum area fraction within the old austenite grain to 70% or less, excellent processing embrittlement characteristics can be realized.

(5) By setting the amount of retained austenite to 15% or less in volume fraction and, furthermore, setting the average occupancy of packets having the maximum occupancy within the old austenite grains to 70% or less in area fraction, it is possible to realize excellent embrittlement resistance during processing.

The present invention has been made based on the above knowledge. That is, the gist of the present invention is as follows.

[1] A composition containing, 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 O: 0.0100% or less, with the remainder being Fe and inevitable impurities, and at a position of 1/4 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 sum of the amount of ferrite and the amount of bainitic ferrite exceeds 10% in area fraction, and the average value of the occupancy of packets having the maximum occupancy within the prior austenite grains is High-strength steel plate with a fraction of 70% or less.

[2] The above component composition is, 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% A high-strength steel plate as described in [1], containing at least one element selected from the following: Bi: 0.200% or less.

[3] A high-strength steel plate as described in [1] or [2], having a plating layer on the surface of the steel plate.

[4] A method for manufacturing a high-strength steel plate described in [1] or [2], wherein a cold-rolled plate is manufactured by hot-rolling, pickling and cold-rolling a steel having the above-mentioned component composition, and annealing is performed by heating under the conditions of an annealing temperature Ta of 700°C or more and 900°C or less and a holding time at the annealing temperature Ta of 10 seconds or more and 1000 seconds or less, and during the annealing, processing is performed such that bending and unbending are performed a total of 1 time or more and 15 times or less with a roll having a radius of 800 mm or less, and cooling is performed such that an average cooling rate in a temperature range of 700°C to 600°C is 20°C/s or more and an average cooling rate in a temperature range of 499°C to Ms is less than 20°C/s, and bending and unbending are performed a total of 1 time or more and 15 times with a roll having a radius of 800 mm or less in the temperature range of 499°C to Ms. A method for manufacturing a high-strength steel sheet, comprising: performing the processing described below, cooling at an average cooling rate of 150°C/s or less in a temperature range from Ms to a cooling stop temperature Tb, setting the tension imparted to the steel sheet in the temperature range from Ms to the cooling stop temperature Tb to 5 MPa or more and 100 MPa or less, and wherein the cooling stop temperature Tb is 100°C or more (Ms-80°C) or less, and further, Ms is a martensitic transformation initiation temperature (°C) defined by Equation (1), and tempering is performed at a tempering temperature of Tb or more and 450°C or less, and a holding time at the tempering temperature is 10 seconds or more and 1000 seconds or less.

Here, [% C], [% Mn], [% Cr], [% Mo], and [% Ni] represent the contents (mass%) of C, Mn, Cr, Mo, and Ni, respectively, and are set to 0 if not included.

[5] A method for manufacturing a high-strength steel plate as described in [4], wherein a plating treatment is performed.

According to the present invention, a high-strength steel sheet having a TS of 980 MPa or more and an El of 10% or more, and also having excellent toughness, flatness in the width direction, and embrittlement resistance during processing can be obtained. In addition, by applying the high-strength steel sheet of the present invention to, for example, automobile structural members, it is possible to improve fuel efficiency by reducing the weight of the automobile body. Therefore, the industrial utility value is very large.

FIG. 1 is a drawing showing the structure of a packet having the maximum occupancy rate within austenite grain according to the present invention and a method for calculating the occupancy rate of the packet.
Figure 2 is a drawing showing the concept of steepness λ in the width direction of a steel plate related to the present invention and a method for calculating the same.

Hereinafter, embodiments of the present invention will be described.

First, the appropriate range of the composition of high-strength steel plates and the reasons for their limitations are explained. In addition, in the following explanation, "%" indicating the content of the steel component elements 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 is an important element affecting the martensite amount, particularly in the present invention. When the C content is less than 0.030%, the martensite amount decreases, making it difficult to achieve a TS of 980 MPa or more. On the other hand, when the C content exceeds 0.500%, the martensite becomes embrittled, and the toughness and the embrittlement resistance during processing deteriorate. Therefore, the C content is set to 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 affecting TS and the amount of retained austenite. When 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%, the retained austenite increases excessively, deteriorating the toughness and the embrittlement resistance during processing. 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 affecting the martensite amount. When the Mn content is less than 1.00%, the martensite amount decreases, making it difficult to achieve a TS of 980 MPa or more. On the other hand, when the Mn content exceeds 5.00%, the martensite becomes brittle, and the toughness and the embrittlement resistance during processing deteriorate. Therefore, the Mn content is set to 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 is segregated in the old austenite grain boundary and embrittles the grain boundary, it lowers the ultimate deformation capacity of the steel sheet, thereby lowering the toughness and embrittlement resistance during processing. Therefore, the P content needs to be 0.100% or less. In addition, although the lower limit of the P content is not specifically stipulated, it is preferable that it be 0.001% or more because P is a solid solution strengthening element that can increase the strength of the steel sheet. Therefore, the P content is set to 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 lowers the ultimate deformation capacity of the steel sheet, thereby lowering the toughness and embrittlement resistance during processing. Therefore, the S content must be 0.0200% or less. In addition, although the lower limit of the S content is not specifically stipulated, it is preferably 0.0001% or more due to constraints in production technology. 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 deformation capacity of the steel sheet, thereby lowering the toughness and embrittlement resistance during processing. Therefore, the Al content must be 1.000% or less. In addition, although the lower limit of the Al content is not specifically stipulated, it is preferable that the Al content be 0.001% or more in order to suppress the formation of carbides during continuous annealing and promote 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 lowers the ultimate deformation capacity of the steel sheet, thereby lowering the toughness and embrittlement resistance during processing. Therefore, the N content must be 0.0100% or less. In addition, although the lower limit of the N content is not specifically stipulated, due to constraints in production technology, it is preferable that the N content be 0.0001% or more. Therefore, the N content is 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 lowers the ultimate deformation capacity of the steel sheet, thereby lowering the toughness and embrittlement resistance during processing. Therefore, the O content must be 0.0100% or less. In addition, although the lower limit of the O content is not specifically stipulated, due to constraints in production technology, it is preferable that the O content be 0.0001% or more. 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 one embodiment of the present invention has a composition containing the above components, with the remainder comprising Fe and inevitable impurities. Here, the inevitable impurities include Zn, Pb, As, Ge, Sr, and Cs. It is permissible for these impurities to be contained in a total amount of 0.100% or less.

The high-strength steel plate of the present invention, in addition to the above-mentioned component composition, further comprises, 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% or less, and Bi: 0.200% or less may be contained alone or in combination.

When Ti, Nb and V are each 0.200% or less, coarse precipitates or inclusions are not formed in large quantities, so the ultimate deformation capacity of the steel sheet is not reduced, and the toughness and embrittlement resistance to processing are not reduced. Therefore, the contents of Ti, Nb and V are each preferably 0.200% or less. In addition, although the lower limit of the contents of Ti, Nb and V is not specifically stipulated, from the viewpoint of increasing the strength of the steel sheet by forming fine carbides, nitrides or carbonitrides during hot rolling or continuous annealing, 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, the contents are each set to 0.200% or less. The lower limit when Ti, Nb and V are contained is more preferably 0.001% or more. The upper limit when containing Ti, Nb and V is more preferably 0.100% or less.

Since Ta and W do not form a large amount of coarse precipitates or inclusions when they are each 0.10% or less, the ultimate deformation capacity of the steel sheet is not reduced, and thus the toughness and embrittlement resistance against processing are not reduced. Therefore, it is preferable that the contents of Ta and W are each 0.10% or less. In addition, although the lower limit of the contents of Ta and W is not specifically stipulated, from the viewpoint of increasing the strength of the steel sheet 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, the contents are each set to 0.10% or less. The lower limit when Ta and W are contained is more preferably 0.01% or more. The upper limit when Ta and W are contained is more preferably 0.08% or less.

When B is 0.0100% or less, cracks do not form inside the steel sheet during casting or hot rolling, and the ultimate deformation capacity of the steel sheet is not reduced, so the toughness and embrittlement resistance against processing are not reduced. Therefore, the B content is preferably 0.0100% or less. In addition, although the lower limit of the B content is not specifically stipulated, since it is an element that segregates in the austenite grain boundary during annealing and improves the quenching property, it is more preferably 0.0003% or more. Therefore, when B is contained, the content is set to 0.0100% or less. The lower limit when B is contained is more preferably 0.0003% or more. The upper limit when B is contained is more preferably 0.0080% or less.

When Cr, Mo and Ni are each 1.00% or less, coarse precipitates or inclusions do not increase, and the ultimate deformation capacity of the steel sheet is not lowered, so the toughness and embrittlement resistance are not lowered. Therefore, the contents of Cr, Mo and Ni are each preferably 1.00% or less. In addition, although the lower limit of the contents of Cr, Mo and Ni is not specifically stipulated, since they are elements that improve quenching property, the contents of Cr, Mo and Ni are each preferably 0.01% or more. Therefore, when Cr, Mo and Ni are contained, the contents are each set to 1.00% or less. The lower limit when Cr, Mo and Ni are contained is more preferably 0.01% or more. The upper limit when Cr, Mo and Ni are contained is more preferably 0.80% or less.

When Co is 0.010% or less, coarse precipitates or inclusions do not increase, and the ultimate deformation capacity of the steel plate is not lowered, so the toughness and embrittlement resistance during processing are not lowered. Therefore, the Co content is preferably 0.010% or less. In addition, although the lower limit of the Co content is not specifically stipulated, since it is an element that improves quenching property, the Co content is preferably 0.001% or more. Therefore, when Co is contained, the content is set to 0.010% or less. The lower limit when Co is contained is more preferably 0.001% or more. The upper limit when Co is contained is further preferably 0.008% or less.

When Cu is 1.00% or less, coarse precipitates or inclusions do not increase, and the ultimate deformation capacity of the steel sheet is not lowered, so the toughness and embrittlement resistance during processing are not lowered. Therefore, the Cu content is preferably 1.00% or less. In addition, although the lower limit of the Cu content is not specifically stipulated, since it is an element that improves quenching property, the Cu content is preferably 0.01% or more. Therefore, when Cu is contained, the content is set to 1.00% or less. The lower limit when Cu is contained is more preferably 0.01% or more. The upper limit when Cu is contained is more preferably 0.80% or less.

When Sn is 0.200% or less, cracks do not form inside the steel sheet during casting or hot rolling, and the ultimate deformation capacity of the steel sheet is not reduced, so the toughness and embrittlement resistance during processing are not reduced. Therefore, the Sn content is preferably 0.200% or less. In addition, although the lower limit of the Sn content is not specifically stipulated, since Sn is an element that improves quenching properties (generally an element that improves corrosion resistance), it is more preferably that the Sn content is 0.001% or more. Therefore, when Sn is contained, the content is set to 0.200% or less. The lower limit when Sn is contained is more preferably 0.001% or more. The upper limit when Sn is contained is further preferably 0.100% or less.

When Sb is 0.200% or less, coarse precipitates or inclusions do not increase, and the ultimate deformation capacity of the steel sheet is not lowered, so the toughness and embrittlement resistance during processing are not lowered. Therefore, the Sb content is preferably 0.200% or less. In addition, although the lower limit of the Sb content is not specifically stipulated, since it is an element that controls the surface softening thickness and enables strength adjustment, the Sb content is preferably 0.001% or more. Therefore, when Sb is contained, the content is set to 0.200% or less. The lower limit when Sb is contained is more preferably 0.001% or more. The upper limit when Sb is contained is more preferably 0.100% or less.

When Ca, Mg, and REM are each 0.0100% or less, coarse precipitates or inclusions do not increase, and the ultimate deformation capacity of the steel sheet is not lowered, so the toughness and embrittlement resistance are not lowered. Therefore, the content of Ca, Mg, and REM is preferably 0.0100% or less. In addition, although the lower limit of the content of Ca, Mg, and REM is not specifically stipulated, since they are elements that spheroidize the shape of nitrides or sulfides and improve the ultimate deformation capacity of the steel sheet, the content of Ca, Mg, and REM is preferably 0.0005% or more. Therefore, when Ca, Mg, and REM are contained, the contents are each set to 0.0100% or less. The lower limit when Ca, Mg, and REM are contained is more preferably 0.0005% or more. The upper limit when containing Ca, Mg and REM is more preferably 0.0050% or less.

Zr and Te, when each is 0.100% or less, do not increase coarse precipitates or inclusions, and do not lower the ultimate deformation capacity of the steel sheet, so the toughness and embrittlement resistance are not lowered. Therefore, the content of Zr and Te is preferably 0.100% or less. In addition, although the lower limit of the content of Zr and Te is not specifically stipulated, since they are elements that spheroidize the shape of nitrides or sulfides and improve the ultimate deformation capacity of the steel sheet, the contents of Zr and Te are preferably 0.001% or more each. Therefore, when Zr and Te are contained, the contents are each set to 0.100% or less. The lower limit when Zr and Te are contained is more preferably 0.001% or more. The upper limit when Zr and Te are contained is more preferably 0.080% or less.

When Hf is 0.10% or less, coarse precipitates or inclusions do not increase, and the ultimate deformation capacity of the steel sheet is not lowered, so the toughness and embrittlement resistance to processing are not lowered. Therefore, the Hf content is preferably 0.10% or less. In addition, although the lower limit of the Hf content is not specifically stipulated, since it is an element that spheroidizes the shape of nitrides or sulfides and improves the ultimate deformation capacity of the steel sheet, it is more preferably 0.01% or more. Therefore, when Hf is contained, the content is set to 0.10% or less. The lower limit when Hf is contained is more preferably 0.01% or more. The upper limit when Hf is contained is further preferably 0.08% or less.

When Bi is 0.200% or less, coarse precipitates or inclusions do not increase, and the ultimate deformation capacity of the steel plate is not reduced, so the toughness and embrittlement resistance during processing are not reduced. Therefore, the Bi content is preferably 0.200% or less. In addition, although the lower limit of the Bi content is not specifically stipulated, since it is an element that reduces segregation, the Bi content is preferably 0.001% or more. Therefore, when Bi is contained, the content is set to 0.200% or less. The lower limit when Bi is contained is more preferably 0.001% or more. The upper limit when Bi is contained is further preferably 0.100% or less.

In addition, for the above-mentioned Ti, Nb, V, Ta, W, B, Cr, Mo, Ni, Co, Cu, Sn, Sb, Ca, Mg, REM, Zr, Te, Hf and Bi, if the content is below the preferred lower limit, it does not hinder the effect of the present invention, so it is included as an unavoidable impurity.

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

[Martensite content is 60% or more in area fraction]

In the present invention, it is a very important invention component requirement. By making the martensite amount 60% or more in terms of area fraction, it becomes possible to realize a TS of 980 MPa or more. Therefore, the martensite amount is 60% or more in terms of area fraction. It is preferably 62% or more. It is more preferably 64% or more.

[The amount of residual austenite is 3% or more and 15% or less in volume fraction]

In the present invention, it is a very important invention configuration requirement. When the amount of retained austenite is less than 3% in volume fraction, it becomes difficult to realize El of 10% or more, and furthermore, the effect of improving toughness by the retained austenite is not obtained, making it difficult to realize excellent toughness. Furthermore, when the amount of retained austenite exceeds 15%, since the retained austenite is excessively transformed into hard martensite when working is applied, the ultimate deformability of the steel sheet decreases, making it difficult to obtain excellent embrittlement resistance during working. Therefore, the retained austenite is set to be 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 is obtained by measuring the integrated intensity ratios of the diffraction peaks of the {200}, {220}, {311} planes of fcc iron and the {200}, {211}, {220} planes of bcc iron using CoKα lines on a surface where a steel plate is polished from a 1/4 section of the plate thickness to a surface of 0.1 mm, and then further polished by 0.1 mm by chemical polishing, and averaging the nine obtained integrated intensity ratios.

[The sum of the amount of ferrite and the amount of bainitic ferrite exceeds 10% in area fraction]

In the present invention, it is a very important invention configuration requirement. When the sum of the ferrite amount and the bainitic ferrite amount is 10% or less, it becomes difficult to realize El of 10% or more. Therefore, the sum of the ferrite amount and the bainitic ferrite amount is set to exceed 10%. Preferably, it is set to 12% or more. More preferably, it is set to 13% or more. In addition, the upper limit of the sum of the ferrite amount and the bainitic ferrite amount is not particularly limited.

Here, the method for measuring the sum of the ferrite amount and the bainitic ferrite amount is as follows. After polishing the L cross-section of the steel plate, it is corroded with 3 vol.% nital, and a 1/4 section 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 with a 10-field view at a magnification of 2000 times using a SEM. In addition, in the above-mentioned tissue image, ferrite and bainitic ferrite are tissues in which the inside of the tissue in the concave part is flat, and further, it is a tissue that does not have carbides inside. From the average of their values, the sum of the ferrite amount and the bainitic ferrite amount can be obtained.

The method for measuring the above martensite amount is as follows. The martensite amount can be obtained by measuring the retained austenite amount, the ferrite amount, and the bainitic ferrite amount based on the above-described method, and subtracting their total from 100%. Therefore, the martensite amount of the present invention is an amount including both ??chilled martensite and tempered martensite. In addition, in the subtraction, since the volume ratio of the retained austenite amount ≒ the area ratio, the ferrite amount and the bainitic ferrite amount expressed by the area ratio are subtracted from 100% together.

[The average area fraction of the packets with the maximum area fraction within the old austenite grain is 70% or less]

In the present invention, it is a very important invention configuration requirement. The occupancy of the packet having the largest occupancy in the old austenite grain affects the flatness in the plate width direction and the embrittlement characteristic during processing. The packet having the largest occupancy in the old austenite grain refers to a packet having up to four regions in the old austenite grain in which the crystal wall surfaces at the time of transformation are the same, as shown in Fig. 1.

The occupancy rate of one packet within the old austenite grain is obtained by dividing the area of the specified packet by the total area within the old austenite grain.

The present inventors have found, through repeated examination, that by reducing the occupancy of packets having the maximum occupancy within the old austenite grains, deformation between packets is alleviated, and flatness in the plate width direction is improved. Furthermore, they have found that by reducing the occupancy of packets having the maximum occupancy within the old austenite grains, the structure is refined, and crack propagation can be suppressed, thereby improving the process embrittlement characteristics of the steel plate. Therefore, the average occupancy of packets having the maximum occupancy within the old austenite grains is set to 70% or less. Preferably, it is set to 60% or less. Meanwhile, the lower limit of the average occupancy of packets having the maximum occupancy within the old austenite grains is not particularly limited. There are at most four types of packets, and when four packets exist evenly, the occupancy of packets having the maximum occupancy within the old austenite grains becomes 25%. Therefore, the lower limit of the average occupancy of packets having the maximum occupancy within the old austenite grain is preferably 25% or more, but it is not necessary to be limited to this.

Here, the method for measuring the average occupancy of the packet having the maximum occupancy in the old austenite grain is as follows. First, a test piece for structural observation is collected from a cold-rolled steel plate. Next, the collected test piece is polished by colloidal silica vibration polishing so that the rolling direction cross-section (L cross-section) becomes the observation surface. The observation surface is made into a mirror surface. Next, electron backscatter diffraction (EBSD) measurement is performed on a 1/4 portion 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 ㎛, the measurement area is 80 ㎛ square, and the WD is 15 mm. The obtained local orientation data is analyzed using OIM Analysis 7 (OIM), and a color-coded drawing (CP map) is created for each Close-packed Plane group (CP group) using the method described in Non-Patent Document 1. In the present invention, a packet is defined as an area to which the same CP group belongs. The area of the packet with the largest occupancy is obtained from the obtained CP map, and the occupancy rate of the packet with the largest occupancy rate within the old austenite grain is obtained by dividing it by the total area within the old austenite grain. This analysis is performed on 10 or more adjacent old austenite grains, and the average value is taken as the average value of the occupancy rates of the packets with the largest occupancy rate within the old austenite grain.

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

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

In the present invention, the slab heating temperature, slab crack holding time, and coiling temperature in hot rolling are not particularly limited. As a method for hot rolling a steel slab, examples thereof include a method of heating a slab and then rolling it, a method of directly rolling a slab after continuous casting without heating it, and a method of performing a short-term heat treatment on a slab after continuous casting and then rolling it. The slab heating temperature, slab crack holding time, finish rolling temperature, and coiling temperature in hot rolling are not particularly limited, but 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 lower. The lower limit of the slab crack holding time is preferably 30 min or higher. The upper limit of the slab crack holding time is preferably 250 min or lower. The lower limit of the finish rolling temperature is preferably the Ar ₃ transformation point or higher. In addition, the lower limit of the coiling temperature is preferably 350°C or higher. Additionally, the upper limit of the coiling temperature is preferably 650°C or lower.

The hot-rolled steel plate manufactured in this manner is pickled. Pickling is important for securing good chemical treatment properties and plating quality in the high-strength steel plate, which is the final product, because it can remove oxides on the surface of the steel plate. In addition, the pickling may be performed once or divided into multiple times. In addition, cold rolling may be performed while the pickled plate is hot-rolled, or cold rolling may be performed after heat treatment.

The reduction ratio in cold rolling and the thickness of the plate after rolling are not particularly limited, but the lower limit of the reduction ratio is preferably 30% or more. In addition, the upper limit of the reduction ratio is preferably 80% or less. In addition, the number of rolling passes and the reduction ratio of each pass are not particularly limited, and the effects of the present invention can be obtained.

Annealing is performed on the cold rolled steel sheet obtained as described above. The annealing conditions are as follows.

[Annealing temperature Ta is 700 ℃ or more and 900 ℃ or less]

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 sum of the amount of ferrite and the amount of bainitic ferrite decreases, making it difficult to achieve an 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 more. The upper limit of the annealing temperature is preferably 870°C or less.

[Annealing at annealing temperature Ta for a holding time of 10 seconds or more and 1000 seconds or less]

When the holding time at the annealing temperature Ta is less than 10 seconds, the martensite amount decreases, making it difficult to realize a TS of 980 MPa or more. On the other hand, when the holding time at the annealing temperature Ta exceeds 1000 seconds, the sum of the ferrite amount and the bainitic ferrite amount decreases, making it difficult to realize an El of 10% or more. Therefore, the holding time at the annealing temperature Ta is set to 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.

[During annealing, bending and bending-unbending with a roll of radius 800 mm or less a total of 1 to 15 times]

The inventors of the present invention have found, through repeated examination, that the application of bending and unbending to a steel sheet during annealing affects the occupancy of packets having the maximum occupancy within the old austenite grains. If bending and unbending are not performed with a roll having a radius of 800 mm or less during annealing, nucleation sites of martensitic transformation are reduced. Therefore, the average occupancy of packets having the maximum occupancy within the old austenite grains exceeds 70%, the flatness in the sheet width direction deteriorates, and the process embrittlement resistance deteriorates. On the other hand, if bending and unbending are performed 16 or more times with a roll having a radius of 800 mm or less during annealing, the ultimate deformability of the steel sheet deteriorates, and the process embrittlement resistance deteriorates. Therefore, during annealing, bending and unbending are performed a total of 1 to 15 times 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 bending and bending-unbending is a total of 3 times or more. Preferably, the upper limit of the number of bending and bending-unbending is a total of 10 times or less. The lower limit of the radius of the roll diameter does not need to be specifically limited, but is preferably 50 mm or more.

In addition, bending and bend-unbending refer to a process of bending in one direction with a roll by a known method, and then bending back in the opposite direction by the amount of bending. The number of bending and bend-unbending is counted as 1 time for bending and 1 time for bend-unbending, not 1 time for bend-bend-unbend.

[Average cooling rate of 20 ℃/s or more in the temperature range of 700 ℃ to 600 ℃]

The present inventors have repeatedly studied the above and 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 within the 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 imparting bending and bending unbending to the steel sheet during annealing decreases, and the nucleation sites of martensite transformation are reduced. Therefore, the average occupancy rate of packets having the maximum occupancy rate within the prior austenite grains exceeds 70%, so that the flatness in the sheet width direction deteriorates, and further, the process embrittlement resistance deteriorates. Therefore, the average cooling rate of 750°C to 600°C is 20°C/s or more. It is preferably 30°C/s or more. The upper limit need not be particularly limited, but is preferably 100°C/s or less.

[Average cooling rate less than 20 ℃/s in the temperature range of 499 ℃ ∼ Ms]

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

In addition, the martensite transformation initiation temperature Ms (℃) is defined by the following equation (1).

Here, [% C], [% Mn], [% Cr], [% Mo], and [% Ni] represent the contents (mass%) of C, Mn, Cr, Mo, and Ni, respectively, and are set to 0 if not included.

[Bending and bending-unbending a total of 1 to 15 times with a roll of radius 800 mm or less in the temperature range of 499 ℃ to Ms]

The inventors of the present invention have found, as a result of repeated examinations, that the application of bending and bend-unbending to a steel sheet in a temperature range of 499°C to Ms affects the occupancy rate of packets having the maximum occupancy rate within the old austenite grains. If bending and bend-unbending are not performed with a roll having a radius of 800 mm or less in the temperature range of 499°C to Ms, the nucleation sites of martensite are reduced. Therefore, the average occupancy rate of packets having the maximum occupancy rate within the old austenite grains exceeds 70%, the flatness in the sheet width direction deteriorates, and further, the processing embrittlement resistance deteriorates. On the other hand, if bending and bend-unbending are performed 16 times or more with a roll having a radius of 800 mm or less in the temperature range of 499°C to Ms, the ultimate deformability of the steel sheet deteriorates, and the processing embrittlement resistance deteriorates. Therefore, the bending and bend-unbending are performed at a total of 1 to 15 times in a temperature range of 499 ℃ 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 bending and bend-unbending is a total of 3 times or more. Preferably, the lower limit of the number of bending and bend-unbending is a total of 10 times or less. There is no need to specifically limit the lower limit of the radius of the roll diameter, but it is preferably 50 mm or more.

[Average cooling rate of 150 ℃/s or less in the temperature range from Ms to cooling stop temperature Tb]

The present inventors have repeatedly studied the above and found that the average cooling rate in the temperature range from Ms to the cooling stop temperature Tb affects the occupancy rate of the packet having the maximum occupancy rate within the prior austenite grains. When the average cooling rate in the temperature range from Ms to the cooling stop temperature Tb exceeds 150°C/s, since the martensite transformation rate is high and one packet is likely to grow, the average occupancy rate of the packet having the maximum occupancy rate within the prior austenite grains exceeds 70%, the flatness in the sheet width direction deteriorates, and further, the processing embrittlement characteristic 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. It is preferably 120°C/s or less. The lower limit need not be particularly limited, but it is preferably 5°C/s or more.

[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]

As a result of repeated examinations, the present inventors have found that the application of tension to a steel sheet in the temperature range from Ms to the cooling stop temperature Tb affects the occupancy rate of packets having the maximum occupancy rate within prior austenite grains. When the tensile strength applied to the steel sheet in the temperature range from Ms to the cooling stop temperature Tb is less than 5 MPa, the number of nucleation sites of martensite is reduced. Therefore, the average occupancy rate of packets having the maximum occupancy rate within prior austenite grains exceeds 70%, the flatness in the sheet width direction deteriorates, and further, the process embrittlement resistance deteriorates. On the other hand, when the tensile strength applied to the steel sheet in the temperature range from Ms to the cooling stop temperature Tb exceeds 100 MPa, the sum of the amount of ferrite and the amount of bainitic ferrite excessively increases, so that the amount of martensite decreases, making it difficult to realize a TS of 980 MPa or more. Therefore, the tension applied to the steel sheet in the temperature range from Ms to the cooling stop temperature Tb is set to 5 MPa or more and 100 MPa or less. The lower limit of the tension applied to the steel sheet in the temperature range from Ms to the cooling stop temperature Tb is preferably 6 MPa or more. The upper limit of the tension applied to the steel sheet in the temperature range from Ms to the cooling stop temperature Tb is preferably 50 MPa or less. In addition, the tension is applied by a known method. As an example, a method of applying the tension by controlling the roll speed of the rolls in the furnace may be implemented.

In addition, the above bending and bend-unbending processes have different effects in that they increase the number of nucleation sites, which are the initiation points of martensite transformation, while the above tension-applying process promotes the martensite transformation itself.

[Cooling stop temperature Tb is 100 ℃ or higher (Ms－80 ℃) or lower]

When the cooling stop temperature Tb is less than 100°C, the amount of retained austenite decreases, thereby deteriorating the bendability. 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 excessively increases, and the process embrittlement property 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 more. The upper limit of the cooling stop temperature Tb is preferably (Ms-100°C) or less.

[Tempering temperature Tb or higher and 450 ℃ or lower]

After cooling is stopped at the above cooling stop temperature Tb, the temperature is maintained as is, or reheated and maintained at a temperature of 450°C or lower to stabilize the retained austenite. If the tempering temperature is lower than Tb, the desired retained austenite is not obtained, so El decreases and it becomes difficult to obtain excellent toughness. If the tempering temperature exceeds 450°C, tempering of martensite progresses excessively, making it difficult to achieve a TS of 980 MPa or higher. Therefore, the tempering temperature is set to Tb or higher and 450°C or lower. 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 lower.

[Holding time at tempering temperature is 10 seconds or more and 1000 seconds or less]

When the holding time at the tempering temperature is less than 10 seconds, stabilization of austenite becomes insufficient, so that the desired retained austenite is not obtained, El decreases, and it becomes difficult to obtain excellent toughness. When the holding time at the tempering temperature exceeds 1000 seconds, tempering of martensite progresses excessively, making it difficult to realize a TS of 980 MPa or more. Therefore, the holding time at the tempering temperature is set to 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 specifically specified, and may be cooled to a desired temperature by any method. In addition, the desired temperature is preferably about room temperature.

In addition, processing may be performed on the above high-strength steel plate under the condition that the corresponding plastic deformation amount is 0.10% or more and 5.00% or less. In addition, after processing, reheating may be performed under the condition that the temperature is 100°C or more and 400°C or less.

In addition, when high-strength steel plates are traded, they are usually traded after they have cooled to room temperature.

High-strength steel sheets may be plated during or after annealing.

As a plating treatment during annealing, for example, a treatment in which hot-dip galvanizing is performed during or after cooling under the condition of an average cooling rate of 20°C/s or more at 700°C to 600°C after annealing, or an alloying treatment after hot-dip galvanizing can be exemplified. In addition, as a plating treatment after annealing, for example, a Zn-Ni electroalloy plating treatment or a pure Zn electroplating treatment can be exemplified after tempering. A plating layer may be formed by electroplating, or hot-dip galvanizing-aluminum-magnesium alloy plating may be performed. In addition, in the above plating treatment, the description was made mainly with reference to the case of zinc plating, but the type of plating metal such as Zn plating or Al plating is not particularly limited. The conditions of other manufacturing methods are not particularly limited, but from the viewpoint of productivity, it is preferable that the series of treatments such as the above annealing, hot-dip galvanizing, and alloying of the zinc plating be performed by a CGL (Continuous Galvanizing Line), which is a hot-dip galvanizing line. After hot-dip galvanizing, wiping is possible to adjust the apparent weight of the plating. In addition, conditions for plating, etc. other than the above conditions can be based on the conventional method of hot-dip galvanizing.

After plating treatment after annealing, processing may be performed again under the condition that the equivalent plastic strain is 0.10% or more and 5.00 or less. In addition, after processing, reheating may be performed again under the condition that the temperature is 100°C or more and 400°C or less.

Example

Steel having the component composition shown in Tables 1 and 2, the remainder being Fe and unavoidable impurities, was melted in a converter and made into a steel slab by a continuous casting method. Next, the obtained slab was heated, hot-rolled, pickled, and then cold-rolled, and annealed as shown in Tables 3 and 4, to obtain a high-strength cold-rolled steel sheet having a thickness of 0.6 to 2.2 mm. For bending and unbending during annealing, a roll having a radius of 300 mm was used, and for bending and unbending in the temperature range of 499°C to Ms, a roll having a radius of 300 mm was used. In addition, some of the steel sheets were manufactured by performing a plating treatment during or after annealing.

The high-strength cold-rolled steel plate obtained as described above was used as a test steel, and the tensile properties, flatness in the width direction of the plate, toughness, and embrittlement resistance against processing were evaluated according to the following test methods.

(organizational observation)

According to the method described above, the sum of the amount of martensite, the amount of retained austenite, the amount of ferrite, and the amount of bainitic ferrite was obtained.

(The share of the packet with the largest share within the austenitic grain)

According to the method described above, the average occupancy of the packet having the maximum occupancy within the old austenite grain was obtained.

(Tensile test)

The tensile test was conducted in accordance with JIS Z 2241 by collecting JIS No. 5 test pieces (gauge length 50 mm, parallel section width 25 mm) such that the length of the test piece was perpendicular to the rolling direction. The tensile test was conducted under the condition of a crosshead speed of 1.67×10 ^-1 mm/sec, and TS and El were measured. In the present invention, TS was judged to be acceptable when it was 980 MPa or more, and El was judged to be acceptable when it was 10% or more.

(tenacity)

The toughness was evaluated by the Charpy test. For the Charpy test piece, multiple steel plates were overlapped and fastened with bolts, and after confirming that there was no gap between the steel plates, a test piece with a V-notch having a depth of 2 mm was produced. The number of overlapping steel plates was set so that the thickness of the test piece after lamination was closest to 10 mm. For example, when the plate thickness is 1.2 mm, 8 sheets were laminated to give a test piece thickness of 9.6 mm. The laminated Charpy test piece was judged to have “excellent toughness” when it had 40 J/cm ² or more. In addition, conditions other than the above were in accordance with JIS Z 2242: 2018.

(Flatness in the direction of the board width)

For each type of cold rolled steel sheet obtained as described above, the flatness in the width direction of the sheet was measured by the method described in Fig. 2. Specifically, a sheet (coil width × 500 mm L × sheet thickness) 500 mm long in the rolling direction was cut from the coil, installed on a platen so that the cross-section was warped upward, and a contact-type displacement meter whose stylus moved on the measuring object was used to continuously measure the height of the sheet over the entire width direction. Based on the results, the steepness, which is an index indicating the flatness of the shape of the sheet, was measured by the method shown in Fig. 2. A sheet having a steepness of more than 0.02 was evaluated as “×”, a sheet having a steepness of more than 0.01 and less than or equal to 0.02 was evaluated as “○”, a sheet having a steepness of 0.01 or less was evaluated as “◎”, and a sheet having a steepness of 0.02 or less was judged to have “excellent flatness in the width direction”.

(Processing embrittlement characteristics)

The processing-resistant embrittlement characteristic was evaluated by a Charpy test. For a Charpy test piece, multiple steel plates were overlapped and fastened with bolts, and after confirming that there was no gap between the steel plates, a test piece in which a V-notch with a depth of 2 mm was formed was produced. The number of overlapping steel plates was set so that the thickness of the test piece after lamination was as close as possible to 10 mm. For example, when the plate thickness is 1.2 mm, 8 sheets were laminated, and the test piece thickness became 9.6 mm. The laminated Charpy test piece was collected with the plate width direction as the length. As an index representing the processing-resistant embrittlement characteristic, the ratio of the shock absorption energy at room temperature, vE _0% /vE _10% , was measured between the steel plate in the manufactured state (unprocessed) and the steel plate subjected to 10% rolling. When vE _0％ /vE _10％ is less than 0.6, it is evaluated as 「×」, when vE _{0 ％} /vE _10％ is 0.6 or more but less than 0.7, it is evaluated as 「○」, when vE 0％ /vE _10％ is 0.7 or more, it is evaluated as 「◎」, and when vE _0％ /vE _10％ is 0.6 or more, it is judged as “excellent in machining embrittlement characteristics”. In addition, conditions other than the above follow JIS Z 2242: 2018.

The results are summarized in Tables 5 to 7. As shown in Tables 5 to 7, in the examples of the present invention, TS is 980 MPa or more, El is 10% or more, and further, toughness, flatness in the plate width direction, and embrittlement resistance during processing are excellent. On the other hand, in the comparative examples, at least one of TS, El, toughness, flatness in the plate width direction, or embrittlement resistance during processing is inferior.

Claims (5)

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,
O: Composition containing 0.0100% or less, with the remainder being Fe and unavoidable impurities,
At the 1/4 position of the plate thickness,
The martensite content is 60% or more in area fraction,
The amount of retained austenite is 3% or more and 15% or less in volume fraction,
The sum of the amount of ferrite and the amount of bainitic ferrite exceeds 10% in area fraction,
A high-strength steel plate, wherein the average area fraction of packets having the maximum area fraction within the old austenitic grains is 70% or less. In paragraph 1,
The above composition of ingredients is further 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,
High-strength steel plate containing at least one element selected from Hf: 0.10% or less and Bi: 0.200% or less. In claim 1 or 2,
High-strength steel plate having a plating layer on the surface of the steel plate. A method for manufacturing a high-strength steel plate as described in claim 1 or claim 2,
Cold rolled sheet manufactured by hot rolling, pickling and cold rolling of steel having the above composition,
Annealing temperature Ta is 700 ℃ or more and 900 ℃ or less,
Annealing is performed by heating under the condition of holding time at the above annealing temperature Ta for 10 seconds or more and 1000 seconds or less,
During the above annealing, the process of bending and bending-unbending is performed at least once and no more than 15 times in total with a roll having a radius of 800 mm or less,
The average cooling rate in the temperature range of 700 ℃ to 600 ℃ is 20 ℃/s or more,
Cooling at an average cooling rate of less than 20 ℃/s in the temperature range of 499 ℃ ∼ Ms,
Processing is performed by bending and bending-unbending a total of 1 to 15 times with a roll having a radius of 800 mm or less in the temperature range of 499 ℃ to Ms.
Cooling at an average cooling rate of 150 ℃/s or less in the temperature range from Ms to cooling stop temperature Tb,
The tension applied to the steel sheet in the temperature range of Ms to cooling stop temperature Tb is set to 5 MPa or more and 100 MPa or less,
The above cooling stop temperature Tb is 100 ℃ or higher (Ms－80 ℃) or lower, and further, Ms is the martensite transformation initiation temperature (℃) specified in equation (1),
Tempering temperature is Tb or higher and 450 ℃ or lower,
A method for manufacturing a high-strength steel plate, wherein tempering is performed at the above tempering temperature for a holding time of 10 seconds or more and 1000 seconds or less.

Here, [% C], [% Mn], [% Cr], [% Mo], and [% Ni] represent the contents (mass%) of C, Mn, Cr, Mo, and Ni, respectively, and are set to 0 if not included. In paragraph 4,
A method for manufacturing a high-strength steel plate by performing a plating treatment.

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