JP7845583B2 - Steel plates, components, and methods for manufacturing them. - Google Patents

Description

This invention relates to steel sheets, components, and methods for manufacturing them, which are excellent in tensile strength, yield ratio, ductility, stress corrosion cracking resistance, and material stability. The steel sheets of this invention can be suitably used as structural components for automobile parts and the like.

With the aim of simultaneously reducing _CO2 emissions through vehicle weight reduction and improving collision resistance through vehicle weight reduction, the strengthening of thin steel sheets for automobiles is progressing, and new legal regulations are being introduced one after another. As a result, in order to increase the strength of the vehicle body, the application of high-strength steel sheets with a tensile strength (TS) of 1180 MPa or higher is increasing in major structural components that make up automobiles.

High-strength steel sheets used in automobiles are required to have excellent yield ratio, excellent ductility, and excellent stress corrosion cracking resistance. For example, in structural components such as floor cross members of automobiles, it is preferable to use steel sheets with excellent ductility from the viewpoint of formability. Furthermore, from the viewpoint of component performance, excellent yield ratio and excellent stress corrosion cracking resistance are required. In addition, a challenge when using high-strength steel sheets as components is the significant decrease in shape retention during press forming due to the increased springback of the steel sheet as its strength increases. Therefore, in the field of press technology, in order to ensure shape retention, it is common practice to predict the amount of shape change after demolding during press forming and design the press die shape in anticipation of this change. However, when the TS fluctuation of the steel sheet, which is the press material, increases, the springback fluctuation also increases, making it difficult to obtain parts of the same shape even when press forming with the same die. This necessitates rework such as sheet metal processing of each part after press forming, which significantly reduces mass production efficiency. Therefore, it is required to minimize the amount of TS fluctuation, that is, to improve material stability.

Patent Document 1 discloses a high-strength steel sheet with a yield ratio of 1180 MPa or higher, excellent in flatness in the width direction and resistance to work-induced embrittlement, and a method for manufacturing the same. However, the technology described in Patent Document 1 does not consider high-strength steel sheets with excellent ductility, stress corrosion cracking resistance, and material stability.

Patent Document 2 discloses a high-strength steel sheet with a strength of 1180 MPa or higher, which has excellent ductility and resistance to bending cracks at the shear end face, and a high yield ratio, as well as a method for manufacturing the same. However, the technology described in Patent Document 2 does not consider high-strength steel sheets that have excellent stress corrosion cracking resistance and material stability.

Patent Document 3 discloses a method for manufacturing high-strength steel sheets with a strength of 980 MPa or higher, which exhibits excellent ductility, elongation flangeability, and mechanical stability. However, the technology described in Patent Document 3 does not consider high-strength steel sheets with excellent yield ratio and stress corrosion cracking resistance.

Patent No. 7323093 Patent No. 7298647 Patent No. 5333298

The present invention was developed in view of these circumstances, and aims to provide steel plates, components, and methods for manufacturing them, which have a tensile strength TS of 1180 MPa or more, a yield ratio YR of 65% or more, and excellent ductility, stress corrosion cracking resistance, and material stability.

Here, the tensile strength TS (hereinafter also simply referred to as TS) and the yield ratio YR (hereinafter also simply referred to as YR) can be measured according to JIS Z 2241 (2022).
Excellent ductility means that the TS × El value, determined by the measurement method according to JIS Z 2241 (2022), is 14,000 MPa・% or higher.
Excellent stress corrosion cracking resistance means that, when a test specimen obtained from a steel plate is subjected to four-point bending according to ASTM (G39-99), stresses equivalent to YS and TS are applied to the bending apex of the test specimen, and the stressed test specimen is immersed in 1% sulfuric acid at 25°C for 100 hours, and no cracks are observed in the test specimen subjected to stress equivalent to YS.
Excellent material stability means that the total stress point (TS) is measured at 10 or more locations at a distance of 50 mm or more in the width direction of the plate, and the difference between the maximum TS and the minimum TS among the measured TS is calculated as TS fluctuation (ΔTS = maximum TS - minimum TS), and ΔTS is less than 60 MPa.

In order to achieve the above-mentioned objectives, the inventors conducted extensive research and, as a result, discovered the following:
(1) By setting the tempered martensite content to 83% or more, a total pressure (TS) of 1180 MPa or higher can be achieved.
(2) By making the total amount of ferrite and bainitic ferrite 5% or more, excellent ductility can be achieved.
(3) By keeping the amount of retained austenite to less than 3% and the total amount of ferrite and bainitic ferrite to less than 15%, a YR of 65% or more can be achieved.
(4) In tempered martensite, within grains surrounded by grain boundaries of 15° or more, 5% to 50% of the martensite contains 5 or more carbides with a particle size of 0.1 μm to 1.0 μm. This allows for the realization of excellent stress corrosion cracking resistance and material stability.

This invention is based on the above findings. Specifically, the gist of this invention is as follows:
[1] In mass%,
C: 0.030% or more and 0.450% or less,
Si: 0.010% or more and 2.500% or less,
Mn: 0.10% 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,
The composition has the following characteristics: O: contains 0.0100% or less, with the remainder being Fe and unavoidable impurities.
At the point where the plate thickness is 1/4,
Area fraction of tempered martensite: 83% or more,
Volume fraction of retained austenite: less than 3%,
The total area fraction of ferrite and bainitic ferrite is 5% or more and less than 15%.
A steel sheet having a structure in which, among the tempered martensite, the area fraction of tempered martensite containing five or more carbides with a particle size of 0.1 μm or more and 1.0 μm or less within grains surrounded by grain boundaries of 15° or more is 5% or more and 50% or less.
[2] The above component composition 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
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,
Bi: 0.200% or less,
The steel sheet according to [1], which contains at least one element selected from among the following.
[3] The steel sheet according to [1] or [2], wherein the steel sheet surface has a plating layer.
[4] A member made of a steel plate as described in any of [1] to [3] above.
[5] A cold-rolled sheet made by hot-rolling, pickling and cold-rolling a steel having the component composition described in [1] or [2] above,
Annealing temperature T1: 750°C or higher and 850°C or lower.
The material is heated under the condition that the holding time t1 at the annealing temperature T1 is between 10 seconds and 1000 seconds.
Cooling with an average cooling rate CR1 of less than 5°C/s between 700°C and 500°C,
A cooling process is performed that includes water quenching, where the average cooling rate CR2 from the rapid cooling start temperature T2 (Ms - 80°C) to 80°C is 300°C/s or more.
Tempering temperature T3: 100°C or higher and less than 250°C.
The process includes an annealing step in which the heating is performed under the condition that the holding time t3 at the tempering temperature T3 is between 10 seconds and 10,000 seconds.
In the cooling process in the annealing step, the time t2 during which the steel plate is held in a temperature range of 250°C or higher and Ms or lower is set to 1.0 seconds or more and 10.0 seconds or less.
A method for manufacturing a steel sheet, wherein, during the cooling of the water quenching in the annealing process, pressure is applied to the front and back surfaces of the steel sheet using two rolls placed on either side of the steel sheet, and this pressure is applied under the conditions that the distance between the two rolls in the steel sheet conveying direction is 20 mm or more and 250 mm or less, and the applied pressure is 196 N or more.
[6] A method for manufacturing a steel sheet as described in [5], wherein the sheet is plated.
[7] A method for manufacturing a member, comprising the step of forming and joining a steel plate according to any one of [1] to [3] above to make a member.

According to the present invention, a steel sheet can be obtained that has a TS of 1180 MPa or higher, a YR of 65% or higher, and excellent ductility, stress corrosion cracking resistance, and material stability. Furthermore, by applying the steel sheet of the present invention to, for example, automotive structural members, it is possible to improve fuel efficiency by reducing the weight of the vehicle body. Therefore, its industrial utility is extremely large.

Figure 1 is a schematic diagram illustrating the pressurization method during water cooling in the steel sheet manufacturing method of the present invention.

[Steel plate]
Embodiments of the present invention will be described below.
The steel sheet of the present invention has a composition in mass% of C: 0.030% to 0.450%, Si: 0.010% to 2.500%, Mn: 0.10% to 5.00%, 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 unavoidable impurities.
This steel sheet has a structure that satisfies the following conditions: at the 1/4 thickness position, the area fraction of tempered martensite is 83% or more, the volume fraction of retained austenite is less than 3%, the total area fraction of ferrite and bainitic ferrite is 5% or more and less than 15%, and within the tempered martensite, within grains surrounded by grain boundaries of 15° or more, the area fraction of tempered martensite containing 5 or more carbides with a particle size of 0.1 μm or more and 1.0 μm or less is 5% or more and 50% or less.

First, we will explain the appropriate range of the steel sheet's composition and the reasons for its limitations. In the following explanation, the "%" representing the content of the steel's constituent elements refers to "mass percent" unless otherwise specified.

C: 0.030% or more and 0.450% or less. Carbon (C) is one of the important basic components of steel, and in particular in this invention, it is an important element that affects the area fraction (hereinafter also referred to as fraction) and stress corrosion cracking resistance properties of tempered martensite. If the C content is less than 0.030%, the fraction of tempered martensite decreases, and it may become difficult to achieve a TS of 1180 MPa or more.
On the other hand, if the C content exceeds 0.450%, the tempered martensite becomes brittle, making it difficult to achieve excellent stress corrosion cracking resistance. Therefore, the C content should be between 0.030% and 0.450%. Preferably, the C content should be 0.050% or more. More preferably, the C content should be 0.100% or more. Preferably, the C content should be 0.400% or less. More preferably, the C content should be 0.350% or less.

Si: 0.010% to 2.500% Si is one of the important basic components of steel, and in this invention in particular, it is an important element that affects TS and retained austenite content because it suppresses carbide formation during continuous annealing and promotes the formation of retained austenite. If the Si content is less than 0.010%, it becomes difficult to achieve a TS of 1180 MPa or higher.
On the other hand, if the Si content exceeds 2,500%, the amount of retained austenite increases excessively, making it difficult to achieve YR ≥ 65%. Therefore, the Si content should be between 0.010% and 2,500%. Preferably, the Si content should be 0.050% or more. More preferably, the Si content should be 0.100% or more. Preferably, the Si content should be 2,000% or less. More preferably, the Si content should be 1,200% or less. More preferably, the Si content should be 0.500% or less, and even more preferably 0.300% or less.

Mn: 0.10% to 5.00%. Mn is one of the important basic components of steel, and in this invention in particular, it is an important element that affects the fraction of tempered martensite and the stress corrosion cracking resistance properties. If the Mn content is less than 0.10%, the fraction of tempered martensite decreases, making it difficult to achieve a TS of 1180 MPa or higher.
On the other hand, if the Mn content exceeds 5.00%, corrosion at the grain boundaries of the steel plate is promoted, making it difficult to achieve excellent stress corrosion cracking resistance. Therefore, the Mn content should be between 0.10% and 5.00%. Preferably, the Mn content should be 0.50% or more. More preferably, the Mn content should be 0.80% or more. Preferably, the Mn content should be 4.50% or less. More preferably, the Mn content should be 4.00% or less.

P: 0.100% or less. Since P segregates at the prior austenite grain boundaries and embrittles them, it becomes difficult to achieve excellent stress corrosion cracking resistance if the P content exceeds 0.100%. Therefore, the P content must be 0.100% or less. Preferably, the P content is 0.070% or less. More preferably, the P content is 0.050% or less, and even more preferably 0.020% or less.
Although there is no specific lower limit for the P content, since P is a solid solution strengthening element and can increase the strength of the steel sheet, it is preferable that the P content be 0.001% or more. More preferably, the P content is 0.003% or more, and even more preferably 0.005% or more.

S: 0.0200% or less. Since sulfur exists as a sulfide and acts as an initiation point for stress corrosion cracking, it becomes difficult to achieve excellent stress corrosion cracking resistance if the content exceeds 0.0200%. Therefore, the sulfur content must be 0.0200% or less. Preferably, the sulfur content is 0.0050% or less. More preferably, the sulfur content is 0.0030% or less, and even more preferably 0.0020% or less.
Although there is no specific lower limit for the sulfur content, it is preferable to have a content of 0.0001% or more due to production technology constraints. More preferably, the sulfur content should be 0.0002% or more.

Al: 1.000% or less. Since Al exists as an oxide and acts as an initiation point for stress corrosion cracking, it becomes difficult to achieve excellent stress corrosion cracking resistance if the content exceeds 1.000%. Therefore, the Al content must be 1.000% or less. Preferably, the Al content is 0.500% or less. More preferably, the Al content is 0.200% or less, and even more preferably, 0.100% or less.
Although there is no specific lower limit for the Al content, it is preferable to have an Al content of 0.001% or more due to production technology constraints. More preferably, the Al content is 0.002% or more. More preferably, the Al content is 0.005% or more, and even more preferably 0.010% or more.

N: 0.0100% or less. Since N exists as a nitride and acts as an initiation point for stress corrosion cracking, it becomes difficult to achieve excellent stress corrosion cracking resistance if the content exceeds 0.0100%. Therefore, the N content must be 0.0100% or less. Preferably, the N content is 0.0050% or less.
Although there is no specific lower limit for the N content, due to production technology constraints, it is preferable that the N content be 0.0001% or more. More preferably, the N content is 0.0002% or more. More preferably, the N content is 0.0010% or more, and even more preferably, 0.0020% or more.

O: 0.0100% or less. Since oxygen exists as an oxide and acts as an initiation point for stress corrosion cracking, it becomes difficult to achieve excellent stress corrosion cracking resistance if the content exceeds 0.0100%. Therefore, the oxygen content must be 0.0100% or less. Preferably, the oxygen content is 0.0050% or less.
Although there is no specific lower limit for the O content, due to production technology constraints, it is preferable that the O content be 0.0001% or higher.

A steel sheet according to one embodiment of the present invention has a component composition containing the above-mentioned components, with the remainder being Fe and unavoidable impurities. In the steel sheet of the present invention, it is preferable that the component composition consists of Fe and unavoidable impurities. Examples of unavoidable impurities include Zn, Pb, As, Ge, Sr, and Cs. The total amount of these impurities is acceptable to be 0.100% or less.

In addition to the above component composition, the steel sheet of the present invention further comprises, by 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. The material may contain at least one element selected from 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, either alone or in combination.

Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less. If Ti, Nb, and V are each at 0.200% or less, large amounts of coarse precipitates and inclusions will not be generated, and they will not become the starting points for stress corrosion cracking, thus the stress corrosion cracking resistance will not decrease. For this reason, when at least one of Ti, Nb, and V is included, the content of Ti, Nb, and V should each be 0.200% or less. Preferably, the content of each should be 0.100% or less.
While there is no specific lower limit for the content of Ti, Nb, and V, it is preferable that the content of Ti, Nb, and V be 0.001% or more, since the strength of the steel sheet is increased by forming fine carbides, nitrides, or carbonitrides during hot rolling or continuous annealing. More preferably, the content of each is 0.002% or more, and even more preferably 0.003% or more.

Ta: 0.10% or less, W: 0.10% or less. If Ta and W are each present in amounts of 0.10% or less, large amounts of coarse precipitates and inclusions will not be generated, and they will not become the starting points for stress corrosion cracking, thus the stress corrosion cracking resistance will not decrease. For this reason, when at least one of Ta and W is included, the content of Ta and W should be 0.10% or less each. Preferably, the content of each should be 0.08% or less.
Although there is no specific lower limit for the content of Ta and W, it is preferable that the content of Ta and W be 0.01% or more, since the strength of the steel sheet is increased by forming fine carbides, nitrides, or carbonitrides during hot rolling or continuous annealing. More preferably, the content of each is 0.02% or more, and even more preferably 0.03% or more.

B: 0.0100% or less. If B is present at 0.0100% or less, it will not cause cracks to form inside the steel sheet during casting or hot rolling, and the stress corrosion cracking resistance will not decrease. Therefore, when B is included, the B content should be 0.0100% or less. Preferably, the B content should be 0.0080% or less.
Although there is no specific lower limit for the content of B, it is preferable that the content of B be 0.0003% or more, as it is an element that segregates at austenite grain boundaries during annealing and improves hardenability.

Cr: 1.00% or less, Mo: 1.00% or less, Ni: 1.00% or less. If Cr, Mo, and Ni are each at 1.00% or less, the amount of coarse precipitates and inclusions will not increase, and they will not become the starting point for stress corrosion cracking, thus the stress corrosion cracking resistance will not decrease. For this reason, when at least one of Cr, Mo, and Ni is included, the content of Cr, Mo, and Ni should each be 1.00% or less. Preferably, the content of each should be 0.90% or less. Preferably, the content of each should be 0.80% or less.
While there is no specific lower limit for the content of Cr, Mo, and Ni, it is more preferable that the content of each of these elements be 0.01% or more, as they are elements that improve hardenability. More preferably, the content of each of these elements is 0.02% or more, and even more preferably 0.03% or more.

Co: 0.010% or less. If the Co content is 0.010% or less, the amount of coarse precipitates and inclusions will not increase, and it will not become the starting point for stress corrosion cracking, thus the stress corrosion cracking resistance will not decrease. For this reason, when Co is included, it is preferable to keep the Co content at 0.010% or less. Preferably, the Co content should be 0.008% or less.
Although there is no specific lower limit for the Co content, it is more preferable that the Co content be 0.001% or more, as it is an element that improves hardenability. More preferably, the Co content should be 0.002% or more.

Cu: 1.00% or less. If the Cu content is 1.00% or less, the amount of coarse precipitates and inclusions will not increase, and it will not become the starting point for stress corrosion cracking, thus the stress corrosion cracking resistance will not decrease. For this reason, when Cu is included, the Cu content should be 1.00% or less. Preferably, the Cu content should be 0.80% or less.
Although there is no specific lower limit for the Cu content, it is preferable that the Cu content be 0.01% or more, as Cu is an element that improves hardenability.

Sn: 0.200% or less. If Sn is present at a concentration of 0.200% or less, it will not cause cracks to form inside the steel sheet during casting or hot rolling, and will not become the starting point for stress corrosion cracking, thus preventing a decrease in stress corrosion cracking resistance. Therefore, when Sn is included, the Sn content should be 0.200% or less. Preferably, the Sn content should be 0.190% or less.
Although there is no specific lower limit for the Sn content, since Sn is an element that improves hardenability (generally an element that improves corrosion resistance), it is more preferable that the Sn content be 0.001% or more. More preferably, the Sn content is 0.002% or more, and even more preferably 0.003% or more.

Sb: 0.200% or less. If the Sb content is 0.200% or less, the amount of coarse precipitates and inclusions will not increase, and it will not become the starting point for stress corrosion cracking, thus the stress corrosion cracking resistance will not decrease. For this reason, when Sb is included, the Sb content should be 0.200% or less. Preferably, the Sb content should be 0.190% or less.
Although there is no specific lower limit for the Sb content, it is more preferable that the Sb content be 0.001% or more, as it is an element that allows control of the surface softening thickness and strength adjustment. More preferably, each of these content levels is 0.002% or more, and even more preferably 0.003% or more.

Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less. If Ca, Mg, and REM are each at 0.0100% or less, the amount of coarse precipitates and inclusions will not increase, and they will not become the starting point for stress corrosion cracking, thus the stress corrosion cracking resistance will not decrease. For this reason, when at least one of Ca, Mg, and REM is included, the content of Ca, Mg, and REM should be 0.0100% or less. Preferably, the content of each is 0.0070% or less. More preferably, the content of each is 0.0050% or less.
While there is no specific lower limit for the content of Ca, Mg, and REM, it is preferable that the content of Ca, Mg, and REM be 0.0005% or more, since these elements spheroidize the shape of nitrides and sulfides and reduce the number of locations that can initiate stress corrosion cracking. More preferably, the content of each is 0.0006% or more, and even more preferably 0.0007% or more.

Zr: 0.100% or less, Te: 0.100% or less. If the content of Zr and Te is 0.100% or less, the amount of coarse precipitates and inclusions will not increase, and they will not become the starting point for stress corrosion cracking, so the stress corrosion cracking resistance will not decrease. For this reason, when at least one of Zr and Te is included, it is preferable that the content of Zr and Te be 0.100% or less. Preferably, the content of each is 0.080% or less.
While there is no specific lower limit for the content of Zr and Te, it is more preferable that the content of Zr and Te be 0.001% or more, since these elements spheroidize the shape of nitrides and sulfides and reduce the number of locations that can initiate stress corrosion cracking. More preferably, the content of each is 0.002% or more, and even more preferably 0.003% or more.

Hf: 0.10% or less. If Hf is 0.10% or less, the amount of coarse precipitates and inclusions does not increase, and it does not become the starting point for stress corrosion cracking, so the stress corrosion cracking resistance does not decrease. For this reason, when Hf is included, the Hf content should be 0.10% or less. Preferably, the Hf content should be 0.08% or less.
Although there is no specific lower limit for the Hf content, it is preferable that the Hf content be 0.001% or more, and more preferably 0.010% or more, since Hf is an element that spheroidizes the shape of nitrides and sulfides and reduces the number of sites that can become the starting point for stress corrosion cracking.

Bi: 0.200% or less. If Bi is present at 0.200% or less, the amount of coarse precipitates and inclusions does not increase, and it does not become an initiation point for stress corrosion cracking, thus the stress corrosion cracking resistance does not decrease. For this reason, when Bi is included, the Bi content should be 0.200% or less. Preferably, the Bi content should be 0.100% or less.
Although there is no specific lower limit for the Bi content, it is more preferable that the Bi content be 0.001% or more, as Bi is an element that reduces segregation. More preferably, the Bi content is 0.002% or more, and even more preferably 0.003% or more.

Furthermore, regarding 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 their respective contents are below the preferred lower limit, they will not impair the effects of the present invention and will therefore be included as unavoidable impurities.

Next, the structural framework of the steel sheet of the present invention will be described.
Tempered martensite: Area fraction of 83% or more. This is one of the important constituent elements of the present invention. By using tempered martensite as the main phase, it is possible to achieve a TS of 1180 MPa or more. In order to obtain this effect, it is necessary for the area fraction of tempered martensite to be 83% or more. Therefore, the area fraction of tempered martensite is set to be 83% or more. The area fraction of tempered martensite is preferably 84% or more. There is no particular upper limit to the area fraction of tempered martensite, but the area fraction of tempered martensite is preferably 95% or less.

The method for measuring the area fraction of tempered martensite is as follows: After polishing the L-section of the steel plate, it is etched with 1 vol. % nital, and 1/4 of the plate thickness (a position corresponding to 1/4 of the plate thickness in the depth direction from the surface of the steel plate) is observed using a SEM at a magnification of 2000x with a field of view of 30 μm × 30 μm for 10 fields.
In the above microstructure image, tempered martensite is a structure with fine irregularities within its interior and contains carbides. The area fraction of tempered martensite can be determined from the average of these values.

Retained austenite: less than 3% by volume fraction. This is one of the important constituent elements of the present invention. If the retained austenite is 3% or more by volume fraction, it becomes difficult to achieve YR ≥ 65%. The reason why it becomes difficult to achieve YR ≥ 65% is that the retained austenite transforms into martensite during the tensile test, causing a decrease in YS. Therefore, the retained austenite should be less than 3%. Preferably, the volume fraction of retained austenite should be 1% or less. There is no particular lower limit to the retained austenite. The volume fraction of retained austenite may be 0%.

The method for measuring the volume fraction of retained austenite is as follows: After polishing the steel plate to a surface that is 1/4 of its thickness, the surface was further polished by chemical polishing to a depth of 0.1 mm. Using an X-ray diffractometer with CoKα rays, the integral intensity ratios of the diffraction peaks of the {200}, {220}, and {311} planes of fcc iron and the {200}, {211}, and {220} planes of bcc iron were measured, and the nine resulting integral intensity ratios were averaged to determine the volume fraction of retained austenite.

The total area fraction of ferrite and bainitic ferrite is 5% or more and less than 15%. This is one of the important constituent elements of the present invention. If the total area fraction of ferrite and bainitic ferrite is 15% or more, it becomes difficult to achieve YR ≥ 65%.
On the other hand, if the total area fraction of ferrite and bainitic ferrite falls below 5%, it becomes difficult to achieve excellent ductility. Therefore, the total area fraction of ferrite and bainitic ferrite should be 5% or more and less than 15%. Preferably, the total area fraction of these materials should be 6% or more. Preferably, the total area fraction of these materials should be 13% or less.

The method for measuring the total area fraction of ferrite and bainitic ferrite is as follows: After polishing the L-section of the steel plate, it is etched with 1 vol. % nital, and 1/4 of the plate thickness (a position corresponding to 1/4 of the plate thickness in the depth direction from the surface of the steel plate) is observed using a SEM at 2000x magnification with a field of view of 30 μm × 30 μm for 10 fields. In the above microstructure image, ferrite and bainitic ferrite are structures with a flat interior in the recesses and do not contain carbides. The total of ferrite and bainitic ferrite can be determined from the average value of these values.

Other tissues besides those listed above may include perlite, fresh martensite, etc. These remaining tissues do not affect the properties as long as their area fraction is 5% or less, so their inclusion is acceptable.

In the present invention, the area fraction of tempered martensite containing five or more carbides with a particle size of 0.1 μm to 1.0 μm within grain boundaries of 15° or more is 5% to 50%. This is one of the important constituent elements of the present invention. The area fraction of tempered martensite containing five or more carbides with a particle size of 0.1 μm to 1.0 μm within grain boundaries of 15° or more affects stress corrosion cracking resistance and material stability. If the area fraction of tempered martensite containing five or more carbides with a particle size of 0.1 μm to 1.0 μm within grain boundaries of 15° or more exceeds 50%, it becomes the starting point for stress corrosion cracking. Therefore, it becomes difficult to achieve excellent stress corrosion cracking resistance.
As a result of diligent research, the inventors have found that by increasing the area fraction of tempered martensite containing five or more carbides with a particle size of 0.1 μm to 1.0 μm, it is possible to suppress the excessive rise in TS that accompanies the increase in the area fraction of tempered martensite. Therefore, it is possible to suppress the change in TS when the fraction of tempered martensite changes in the plate width direction, thereby improving material stability. Accordingly, the area fraction of tempered martensite containing five or more carbides with a particle size of 0.1 μm to 1.0 μm within grains surrounded by grain boundaries of 15° or more is set to 5% to 50%. This area fraction is preferably 7% or more. Furthermore, this area fraction is preferably 30% or less.

The method for measuring the fraction of tempered martensite containing five or more carbides with a particle size of 0.1 μm to 1.0 μm is as follows: Using a TEM (transmission electron microscope), the steel sheet structure is observed at a position 1/4 of the sheet thickness from the surface of the steel sheet at 20,000x magnification with a field of view of 4 μm × 4 μm, and the particle size and number of carbides present in all tempered martensite grains within the field of view are calculated. The particle size of the carbides is determined by importing data, in which the carbides have been previously identified, into MediaCybernetics' Image-Pro and calculating the equivalent diameter of the circle. Within grains surrounded by grain boundaries of 15° or more, the total area fraction of tempered martensite containing five or more carbides with a particle size of 0.1 μm to 1.0 μm is calculated. The total area fraction of all tempered martensite is also calculated. The area fraction of tempered martensite containing five or more carbides with a particle size of 0.1 μm to 1.0 μm is divided by the total area fraction of all tempered martensite to calculate the area fraction of tempered martensite containing five or more carbides with a particle size of 0.1 μm to 1.0 μm.
Furthermore, grains in tempered martensite surrounded by grain boundaries of 15° or greater can be identified, for example, as follows: Using specimens that have undergone TEM observation, transmission electron backscatter diffraction (transmission EBSD) measurements are performed to obtain local crystal orientation data. Then, the obtained local crystal orientation data is analyzed using the analysis software: OIM Analysis 7. Prior to the analysis of the local crystal orientation data, a cleanup process is performed once using the Grain Dilation function of the analysis software (Grain Tolerance Angle: 5, Minimum Grain Size: 2, Single Iteration: ON). Then, by displaying the grain boundaries of 15° or greater in the tempered martensite, grains in tempered martensite surrounded by grain boundaries of 15° or greater can be identified.
Furthermore, for each carbide, a single region is measured as a whole in the microscope image, where the outer edge is surrounded by material other than the carbide and the region is formed as a continuous whole without interruption.

The steel sheet of the present invention may have a plating layer on its surface. Examples of plating layers include zinc plating layers such as a hot-dip galvanized layer, an alloyed hot-dip galvanized layer, and an electroplated zinc layer. Examples of plating layers other than zinc plating layers include aluminum plating layers and alloy plating layers. Examples of alloy plating layers include a hot-dip zinc-aluminum-magnesium alloy plating layer and a Zn-Ni electroalloy plating layer.

The thickness of the steel plate of the present invention is preferably 0.5 mm or more. Furthermore, the thickness is preferably 3.0 mm or less.
The board width is preferably 600 mm or more. Furthermore, the board width is preferably 1500 mm or less.

[Method of manufacturing steel plates]
Next, the method for manufacturing steel sheets according to the present invention will be described.
The present invention provides a method for manufacturing steel sheets, in which a cold-rolled sheet is produced by hot-rolling, pickling, and cold-rolling a steel having a specific composition, and the sheet is heated under the conditions of an annealing temperature T1 of 750°C to 850°C and a holding time t1 at the annealing temperature T1 of 10 seconds to 1000 seconds.
The cooling process includes cooling with an average cooling rate CR1 of less than 5°C/s from 700 to 500°C, and water quenching with an average cooling rate CR2 of 300°C/s or more from a rapid cooling start temperature T2 (Ms - 80°C) to 80°C.
The process includes an annealing step in which the heating is performed under the conditions of tempering temperature T3: 100°C or higher and less than 250°C, and holding time t3 at tempering temperature T3: 10 seconds or higher and 10,000 seconds or lower.
In the cooling process during the annealing stage, the time t2 during which the steel plate is held in a temperature range of 250°C or higher and Ms or lower is set to 1.0 seconds or more and 10.0 seconds or less.
During the cooling process of water quenching in the annealing process, pressure is applied to the front and back surfaces of the steel plate using two rolls placed on either side of the steel plate. This pressure is applied under the following conditions: the distance between the two rolls in the steel plate conveying direction is 20 mm to 250 mm, and the applied pressure is 196 N or more.

In this invention, the method for melting the steel material (steel slab) is not particularly limited, and any known melting method, such as a converter or electric furnace, is suitable. The steel slab (slab) is preferably manufactured by a continuous casting method to prevent macrosegregation.

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 a steel slab include a method of heating the slab before rolling, a method of directly rolling a slab after continuous casting without heating it, and a method of subjecting a slab after continuous casting to a short-time heat treatment before rolling.
The slab heating temperature, slab soaking time, finish rolling temperature, and coiling temperature in hot rolling are not particularly limited, but the slab heating temperature is preferably 1100°C or higher. The slab heating temperature is preferably 1300°C or lower.
A slab heating and holding time of 30 minutes or more is preferable. A slab heating and holding time of 250 minutes or less is preferable.
The finishing rolling temperature is preferably above the Ar ₃ transformation point.
Furthermore, a winding temperature of 350°C or higher is preferable. Also, a winding temperature of 650°C or lower is preferable.

The hot-rolled steel sheet manufactured in this manner is then pickled. Pickling is important for ensuring good chemical conversion treatment properties and plating quality in the final high-strength steel sheet, as it removes oxides from the surface of the steel sheet. Pickling may be performed once or in multiple stages. Furthermore, the sheet may be cold-rolled immediately after hot-rolling and pickling, or it may be cold-rolled after heat treatment.

The reduction ratio (cumulative reduction ratio) in cold rolling and the thickness of the sheet after rolling are not particularly limited, but a reduction ratio of 30% or more is preferred. Furthermore, a reduction ratio of 80% or less is preferred. The effects of the present invention can be obtained without any particular limitations on the number of rolling passes or the reduction ratio of each pass.

The cold-rolled sheet obtained as described above is then annealed. The annealing conditions are as follows:

Annealing temperature T1: 750°C or higher and 850°C or lower. If the annealing temperature T1 is less than 750°C, the total area fraction of ferrite and bainitic ferrite will be 15% or more, making it difficult to achieve YR ≥ 65%.
On the other hand, if the annealing temperature T1 exceeds 850°C, the total area fraction of ferrite and bainitic ferrite will be less than 5%, making it difficult to achieve excellent ductility. Therefore, the annealing temperature T1 should be between 750°C and 850°C.
The annealing temperature T1 is preferably 760°C or higher, and more preferably 770°C or higher. Furthermore, the annealing temperature T1 is preferably 840°C or lower, and more preferably 830°C or lower.
The annealing temperature referred to here is the holding temperature during the annealing process. The annealing temperature may remain constant during the holding period. Furthermore, the annealing temperature does not need to be constantly constant during the holding period, as long as it is within the temperature range of 750°C to 850°C and the temperature fluctuation is within ±10°C of the set temperature.

Holding time t1 at annealing temperature T1: 10 seconds or more and 1000 seconds or less. If the holding time t1 at annealing temperature T1 is less than 10 seconds, the total area fraction of ferrite and bainitic ferrite will be 15% or more, making it difficult to achieve YR ≥ 65%.
On the other hand, if the holding time t1 at annealing temperature T1 exceeds 1000 seconds, the total area fraction of ferrite and bainitic ferrite becomes less than 5%, making it difficult to achieve excellent ductility. Therefore, the holding time t1 at annealing temperature T1 should be between 10 seconds and 1000 seconds. Preferably, the holding time t1 at annealing temperature T1 is 30 seconds or more. Preferably, the holding time at annealing temperature T1 is 800 seconds or less.

Average cooling rate CR1 at 700-500°C: Less than 5°C/s If the average cooling rate CR1 at 700-500°C is 5°C/s or higher, the total area fraction of ferrite and bainitic ferrite will be less than 5%, making it difficult to achieve excellent ductility. Therefore, the average cooling rate CR1 at 700-500°C should be less than 5°C/s. The average cooling rate CR1 is preferably 3°C/s or less. There is no particular need to limit the lower limit, but an average cooling rate CR1 of 1°C/s or higher is preferable.
Here, the average cooling rate CR1 is calculated as (cooling start temperature (700°C) - cooling stop temperature (500°C)) / cooling time (s) from the cooling start temperature (700°C) to the cooling stop temperature.
Cooling at an average cooling rate of CR1 can be achieved through methods such as water cooling or mist cooling.

Rapid cooling start temperature T2: (Ms-80°C) or higher and Ms or lower This is one of the important constituent elements of the present invention. By setting the rapid cooling start temperature T2 to (Ms-80°C) or higher and Ms or lower, it is possible to obtain a structure in which, within the grains surrounded by grain boundaries of 15° or higher, the area fraction of tempered martensite containing 5 or more carbides with a particle size of 0.1 μm or more and 1.0 μm or less is 5% or higher and 50% or lower. If the rapid cooling start temperature T2 is less than (Ms-80°C), the area fraction of tempered martensite containing 5 or more carbides with a particle size of 0.1 μm or more and 1.0 μm or less exceeds 50%, making it difficult to achieve excellent stress corrosion cracking resistance.
On the other hand, if the quenching start temperature T2 exceeds Ms, the area fraction of tempered martensite containing five or more carbides with a particle size of 0.1 μm to 1.0 μm becomes less than 5%, making it difficult to achieve excellent material stability. Therefore, the quenching start temperature T2 should be between (Ms - 80°C) and Ms. The quenching start temperature T2 is preferably above (Ms - 40°C). The quenching start temperature T2 is preferably below (Ms - 10°C). Hereinafter, Ms is the martensitic transformation start temperature (°C), and 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]...(1)
Here, [%C], [%Mn], [%Cr], [%Mo], and [%Ni] represent the mass percentage content of C, Mn, Cr, Mo, and Ni in steel (steel sheet), and 0 is used if none of these elements are present.

The average cooling rate CR2 from the rapid cooling start temperature T2 to 80°C is 300°C/s or more. If the average cooling rate CR2 from the rapid cooling start temperature T2 to 80°C is less than 300°C/s, the retained austenite will be 3% or more by volume fraction, making it difficult to achieve a YR of 65% or more. Therefore, the average cooling rate CR2 from the rapid cooling start temperature T2 to 80°C should be 300°C/s or more. The average cooling rate CR2 is preferably 800°C/s or more. There is no particular upper limit, but the average cooling rate CR2 is preferably 3000°C/s or less.

In the cooling process, the time t2 for which the steel plate is held in the temperature range of 250°C or higher and Ms or lower is 1.0 seconds or more and 10.0 seconds or less. This is one of the important constituent elements of the present invention. By setting the time for which the steel plate is held in the temperature range of 250°C or higher and Ms or lower in the cooling process to 1.0 seconds or more and 10.0 seconds or less, it is possible to obtain a structure in which, within the grains surrounded by grain boundaries of 15° or higher, the area fraction of tempered martensite containing 5 or more carbides with a particle size of 0.1 μm or more and 1.0 μm or less is 5% or more and 50% or less. If the time for which the steel plate is held in the temperature range of 250°C or higher and Ms or lower in the cooling process exceeds 10.0 seconds, the fraction of tempered martensite containing 5 or more carbides with a particle size of 0.1 μm or more and 1.0 μm or less within the grains surrounded by grain boundaries of 15° or higher exceeds 50%, making it difficult to achieve excellent stress corrosion cracking resistance.
On the other hand, if the steel sheet is held in the temperature range of 250°C or higher and Ms or lower for less than 1.0 second during the cooling process, the area fraction of tempered martensite containing 5 or more carbides with a particle size of 0.1 μm or more and 1.0 μm or less within grains surrounded by grain boundaries of 15° or higher will be less than 5%, making it difficult to achieve excellent material stability. Therefore, the time the steel sheet is held in the temperature range of 250°C or higher and Ms or lower during the cooling process should be 1.0 second or more and 10.0 seconds or less. Preferably, the time the steel sheet is held in the temperature range of 250°C or higher and Ms or lower during the cooling process is 1.5 seconds or more. Preferably, the time the steel sheet is held in the temperature range of 250°C or higher and Ms or lower during the cooling process is 5.0 seconds or less.

Tempering temperature T3: 100°C or higher and less than 250°C. This is one of the important constituent elements of the present invention. In the present invention, tempered martensite refers to a structure in which martensite at 80°C or lower has been subjected to a heat treatment at a tempering temperature of 100°C or higher and a holding time of 10 seconds or more. Therefore, if the tempering temperature T3 is less than 100°C, the martensite is not sufficiently tempered, resulting in a structure mainly composed of as-quenched martensite, and as-quenched martensite deteriorates in stress corrosion cracking resistance.
On the other hand, if the tempering temperature T3 is 250°C or higher, the tempering of the martensite proceeds excessively, and within the tempered martensite, the area fraction of tempered martensite containing 5 or more carbides with a particle size of 0.1 μm to 1.0 μm within grains surrounded by grain boundaries of 15° or more exceeds 50%, making it difficult to achieve excellent stress corrosion cracking resistance. Therefore, the tempering temperature T3 should be 100°C or higher and less than 250°C. Preferably, the tempering temperature T3 should be 150°C or higher. Preferably, the tempering temperature T3 should be 220°C or lower.

Holding time t3 at tempering temperature T3: 10 seconds or more and 10,000 seconds or less. In the present invention, tempered martensite refers to a structure in which martensite below 80°C has been subjected to a heat treatment at a tempering temperature of 100°C or higher and a holding time of 10 seconds or more. Therefore, if the holding time t3 at tempering temperature T3 is less than 10 seconds, the martensite is not sufficiently tempered, resulting in a structure mainly composed of as-quenched martensite, and as-quenched martensite deteriorates the stress corrosion cracking resistance properties.
On the other hand, if the holding time t3 exceeds 10,000 seconds, the tempering of the martensite proceeds excessively, and the fraction of tempered martensite containing 5 or more carbides with a particle size of 0.1 μm to 1.0 μm within grain boundaries of 15° or more exceeds 50%. Therefore, it becomes difficult to achieve excellent stress corrosion cracking resistance. Accordingly, the holding time t3 at tempering temperature T3 should be between 10 seconds and 10,000 seconds. Preferably, the holding time t3 at tempering temperature T3 should be 50 seconds or more. Preferably, the holding time t3 at tempering temperature T3 should be 5,000 seconds or less.

Cooling after tempering does not need to be specifically defined and may be cooled to the desired temperature by any method. The desired temperature is preferably around room temperature.

Furthermore, the above-mentioned steel plate (high-strength steel plate) may be processed under conditions that result in an equivalent plastic strain of 0.05% to 5.00%. Additionally, after processing, it may be reheated again under conditions of 100°C to 400°C.

Furthermore, when high-strength steel plates are traded, they are usually cooled to room temperature before being traded.

During the cooling process of water quenching, pressure is applied to the front and back surfaces of the steel plate by two rolls placed on either side of the steel plate. The distance between the two rolls in the steel plate transport direction during this application of pressure is 20 mm to 250 mm, and the applied pressure is 196 N or more. During the cooling (water cooling) process of water quenching in the annealing process, pressure is applied to the front and back surfaces of the steel plate by two rolls placed on either side of the steel plate. At this time, the distance between the two rolls in the steel plate transport direction (hereinafter also simply referred to as the distance between the rolls) is 20 mm to 250 mm, and the applied pressure is 196 N or more. In this invention, "distance between the two rolls" refers to the distance between the contact points between one roll and the steel plate, and between the contact points between the other roll and the steel plate, as shown in Figure 1.
If no pressure is applied during the water cooling process of the water quenching described above, the fraction of tempered martensite containing five or more carbides with a particle size of 0.1 μm to 1.0 μm within grain boundaries of 15° or more will be less than 5%, making it difficult to achieve excellent material stability. After diligent research by the inventors, it was discovered that applying pressure during the cooling process of water quenching affects the progress of martensitic transformation. By applying this pressure under the conditions of a roll-to-roll distance of 20 mm to 250 mm and a pressure of 196 N or more, nucleation of martensitic transformation is promoted, increasing the area fraction of martensite that transforms at 250°C or higher. As a result, the area fraction of tempered martensite containing five or more carbides with a particle size of 0.1 μm to 1.0 μm becomes 5% or more, thus achieving excellent material stability. To obtain this effect, the roll-to-roll distance (see Figure 1) must be 20 mm or more.
On the other hand, if the distance between the rolls exceeds 250 mm, the pressure will weaken, so the distance between the rolls must be 250 mm or less. Pressure is applied by sandwiching the material between rolls that are far apart from each other, and the pressure required to achieve the above effect is 196 N or more. This pressure corresponds to the load of one roll. In other words, a pressure of 196 N or more means that each of the two rolls applies pressure of 196 N or more.
For pressurization conditions, the preferred distance between rolls is 30 mm or more. Alternatively, the preferred distance between rolls is 220 mm or less.
Furthermore, a preferred pressing force is 294 N or higher. A preferred pressing force is 4900 N or lower.
Furthermore, while the applied pressure varies depending on the steel plate strength and tension, it can be adjusted by the tension and the amount of indentation. The pressure being within the above range can be confirmed using a load cell attached to the roll, and the amount of indentation can be calculated from the roll diameter and roll position.
Based on the above, in the present invention, during the cooling (water cooling) of water quenching, pressure is applied to the front and back surfaces of the steel plate by two rolls placed on either side of the steel plate, and this pressure is applied under conditions where the distance between the two rolls is 20 mm or more and 250 mm or less, and the pressure is 196 N or more.

Furthermore, the steel plate may optionally be plated. The plating treatment is not particularly limited. Examples of plating treatments include zinc plating treatments such as hot-dip galvanizing, alloyed hot-dip galvanizing, and electroplating. Examples of plating treatments other than zinc plating include aluminum plating and alloy plating. Examples of alloy plating treatments include hot-dip zinc-aluminum-magnesium alloy plating and Zn-Ni electroalloy plating. The treatment conditions in all cases should follow conventional methods. As mentioned above, the plating treatment is preferably performed during the cooling from the annealing temperature T1 to the rapid cooling start temperature T2, or after the tempering process. For example, hot-dip galvanizing and alloyed hot-dip galvanizing are preferably performed during the cooling from the annealing temperature T1 to the rapid cooling start temperature T2. Also, electroplating and Zn-Ni electroalloy plating are preferably performed after the tempering process.
Furthermore, in the case of hot-dip galvanizing and alloyed hot-dip galvanizing, from the viewpoint of productivity, it is preferable to perform the series of processes, such as the heating process, annealing process, and plating process, on a CGL (Continuous Galvanizing Line), which is a hot-dip galvanizing line. After hot-dip galvanizing, wiping is possible to adjust the plating thickness.

Other than the conditions mentioned above, there are no particular limitations, and conventional methods may be followed. According to the steel sheet manufacturing method of the embodiment described above, a steel sheet can be obtained having a TS of 1180 MPa or more, a YR of 65% or more, and excellent ductility, stress corrosion cracking resistance, and material stability. The obtained steel sheet can be suitably used, for example, as a material for automobile parts.

After annealing and plating, the material may be processed again under conditions that result in an equivalent plastic strain of 0.05% to 5.00%. Alternatively, after processing, the material may be reheated again under conditions of 100°C to less than 250°C.

[Components and methods for manufacturing components]
Next, the component of the present invention and its manufacturing method will be described.

The component of the present invention is obtained by subjecting a steel sheet of the present invention to at least one of forming and joining processes. Furthermore, the method for manufacturing the component of the present invention includes the step of subjecting a steel sheet of the present invention to at least one of forming and joining processes to form the component.

The steel sheet of the present invention has a tensile strength of 1180 MPa or more, a YR of 65% or more, and excellent ductility, stress corrosion cracking resistance, and material stability. Therefore, members obtained using the steel sheet of the present invention also have a tensile strength of 1180 MPa or more, a YR of 65% or more, and excellent ductility, stress corrosion cracking resistance, and material stability. By using the members of the present invention, weight reduction is possible. Therefore, the members of the present invention can be suitably used, for example, in automotive structural members.

Forming processes can utilize general processing methods such as press working without restriction. Joining processes can utilize general welding methods such as spot welding and arc welding, as well as riveting and crimping without restriction.

Steel having the component composition shown in Table 1 (Table 1-1, Table 1-2) (the remainder being Fe and unavoidable impurities) was melted in a converter and formed into steel slabs by continuous casting. Next, the steel slabs were heated. Next, the steel slabs were hot-rolled to form hot-rolled steel sheets. Next, the hot-rolled steel sheets were pickled. Next, the hot-rolled steel sheets were cold-rolled to form cold-rolled steel sheets. In this way, the raw material steel sheets were prepared. Next, the prepared raw material steel sheets were annealed under the conditions shown in Table 2 (Table 2-1, Table 2-2) to obtain the final product steel sheets (thickness: 1.4 mm, width: 1000 mm).
Furthermore, some steel plates (those with the type column GI, GA, and EG in Table 2) were plated. Of these, those with the type column GI and GA in Table 2 were plated during the cooling process from the annealing temperature T1 to the rapid cooling start temperature T2. Those with the type column EG in Table 2 were plated after the tempering process. Conditions not specified were governed by conventional methods.

The steel sheets (high-strength cold-rolled steel sheets) obtained as described above were used as test materials, and their tensile properties, ductility, stress corrosion cracking resistance, and material stability were evaluated according to the following test methods.

(Tissue observation)
Following the method described above, the amount of tempered martensite (area fraction), the amount of retained austenite (volume fraction), the sum of the amount of ferrite and bainitic ferrite (area fraction), and the fraction (area fraction) of tempered martensite containing five or more carbides with a particle size of 0.1 μm or more and 1.0 μm or less were determined.

(Tensile test)
For the tensile test, a JIS No. 5 test specimen (gauge length 50 mm, parallel section width 25 mm) was taken so that the longitudinal direction of the specimen was perpendicular to the rolling direction, and the test was conducted according to JIS Z 2241 (2022). The tensile test was performed under the condition that the crosshead speed was 1.67 × ^10⁻¹ mm/sec, and the tensile strength TS was measured. In this invention, a TS of 1180 MPa or higher was considered acceptable. A yield ratio YR of 65% or higher was also considered acceptable. YR is calculated using the following formula (2).
YR=100×YS/TS...(2)
Ductility was evaluated using TS × El, and a score of TS × El = 14000 MPa·% or higher was considered acceptable.

(Stress corrosion cracking resistance properties)
The obtained steel plate was sheared to a size of 20 mm x 75 mm with the longitudinal side perpendicular to the rolling direction. 2 mm was removed from both sides of the longitudinal end face by mechanical grinding to produce 16 mm x 75 mm test specimens. Four-point bending was performed on the obtained test specimens according to ASTM (G39-99), and stresses equivalent to YS and TS were applied to the bending apex of the specimens. The stressed test specimens were immersed in 1% sulfuric acid at 25°C for 100 hours. After the test, the presence or absence of cracks was visually checked for each test specimen. The stress corrosion cracking resistance characteristics were then evaluated according to the following criteria.
◎ (Pass, Excellent): No cracks in samples subjected to stress equivalent to YS and TS ○ (Pass, Excellent): No cracks in samples subjected to stress equivalent to YS × (Fail): Cracks present in both samples subjected to stress equivalent to YS and TS

(Material stability)
Material stability can be evaluated using coils and cut plates. The total surface temperature (TS) was measured at 10 or more locations within the coil at distances of 50 mm or more in the plate width direction. The difference between the maximum and minimum TS values among those measured was calculated as the TS variation (ΔTS = maximum TS - minimum TS). The material stability was then evaluated according to the following criteria.
◎ (Pass, Excellent): ΔTS less than 40 MPa ○ (Pass, Excellent): ΔTS less than 60 MPa × (Fail): ΔTS 60 MPa or more

The examples of the present invention shown in Table 3 (Tables 3-1 and 3-2) have a tensile strength TS of 1180 MPa or higher, a yield ratio YR of 65% or higher, and excellent ductility, stress corrosion cracking resistance, and material stability, whereas the comparative examples were inferior in at least one of these aspects.

Furthermore, it was found that members obtained by forming and joining using the steel plate of the present invention example exhibited the same properties as the steel plate of the present invention: a tensile strength TS of 1180 MPa or higher, a yield ratio YR of 65% or higher, and excellent ductility, stress corrosion cracking resistance, and material stability.

Claims (8)

In mass percent,
C: 0.030% or more and 0.450% or less,
Si: 0.010% or more and 2.500% or less,
Mn: 0.10% 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,
The composition has the following characteristics: O: contains 0.0100% or less, with the remainder being Fe and unavoidable impurities.
At the point where the plate thickness is 1/4,
Area fraction of tempered martensite: 83% or more,
Volume fraction of retained austenite: less than 3%,
The total area fraction of ferrite and bainitic ferrite is 5% or more and less than 15%.
The tempered martensite has a structure that satisfies the condition that within grains surrounded by grain boundaries of 15° or more, the area fraction of tempered martensite containing 5 or more carbides with a particle size of 0.1 μm or more and 1.0 μm or less is 5% or more and 50% or less .
Steel plates with a width of 600 to 1500 mm . The aforementioned component composition is further expressed 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,
Bi: 0.200% or less,
The steel sheet according to claim 1, comprising at least one element selected from among the following. The steel sheet according to claim 1, having a plating layer on its surface. The steel sheet according to claim 2, having a plating layer on its surface. A member made using the steel plate described in any one of claims 1 to 4. A cold-rolled sheet is made by hot-rolling, pickling, and cold-rolling a steel having the component composition described in claim 1 or 2.
Annealing temperature T1: 750°C or higher and 850°C or lower.
The material is heated under the condition that the holding time t1 at the annealing temperature T1 is between 10 seconds and 1000 seconds.
Cooling with an average cooling rate CR1 of less than 5°C/s between 700°C and 500°C,
A cooling process is performed that includes water quenching, where the average cooling rate CR2 from the rapid cooling start temperature T2 (Ms - 80°C) to 80°C is 300°C/s or more.
Tempering temperature T3: 100°C or higher and less than 250°C.
The process includes an annealing step in which the heating is performed under the condition that the holding time t3 at the tempering temperature T3 is between 10 seconds and 10,000 seconds.
In the cooling process in the annealing step, the time t2 during which the steel plate is held in a temperature range of 250°C or higher and Ms or lower is set to 1.0 seconds or more and 10.0 seconds or less.
During the cooling of the water quenching in the annealing process, pressure is applied to the front and back surfaces of the steel plate by two rolls placed on either side of the steel plate, and this pressure is applied under the following conditions: the distance between the two rolls in the steel plate conveying direction is 20 mm or more and 250 mm or less, and the applied pressure is 196 N or more.
At the point where the plate thickness is 1/4,
Area fraction of tempered martensite: 83% or more,
Volume fraction of retained austenite: less than 3%,
The total area fraction of ferrite and bainitic ferrite is 5% or more and less than 15%.
The tempered martensite has a structure that satisfies the condition that within grains surrounded by grain boundaries of 15° or more, the area fraction of tempered martensite containing 5 or more carbides with a particle size of 0.1 μm or more and 1.0 μm or less is 5% or more and 50% or less.
A method for manufacturing steel plates with a width of 600 to 1500 mm . A method for manufacturing a steel sheet according to claim 6, comprising applying a plating treatment. A method for manufacturing a component, comprising the step of subjecting a steel plate according to any one of claims 1 to 4 to a forming process and a joining process to form the component.