JP7597270B1 - Steel plates and members, and their manufacturing methods - Google Patents

Abstract

The present invention provides a steel sheet having a TS of 1,320 MPa or more, excellent stretch flangeability, and excellent toughness and impact properties after paint baking. The steel sheet has a specified chemical composition, and has a structure in which the area fraction of tempered martensite is 95% or more, the area fraction of retained austenite is less than 3%, the total area fraction of ferrite and bainitic ferrite is less than 5%, the density of grain boundaries of 20° or more in the tempered martensite is 1.0 μm/ ^μm2 or more, and the following formula (1) is satisfied:
KAM(S)/KAM(C) > 1.00 (1)

Description

The present invention relates to a steel plate, a component made from the steel plate, and a method for manufacturing the same.

In order to achieve both reduction in _CO2 emissions by reducing the weight of automobile bodies and improvement in crashworthiness, efforts are being made to increase the strength of steel sheets, which are the raw material for automobile parts. In addition, new legal regulations are being introduced one after another. As a result, the number of cases in which steel sheets with a tensile strength (hereinafter also referred to as TS) of 1320 MPa or more are used in the main structural parts of automobiles is increasing.

Steel sheets used as raw materials for automobile parts are often required to have excellent stretch flangeability. For example, automobile parts such as crash boxes have punched end faces. Therefore, from the standpoint of formability, steel sheets used as raw materials for such automobile parts are required to have excellent stretch flangeability.

As an example of a steel sheet used as a material for automobile parts, Patent Document 1 describes:
"In mass percent,
C: 0.12% or more and 0.40% or less,
Si: 0.01% or more and 1.5% or less,
Mn: more than 1.7% and not more than 3.5%;
P: 0.05% or less,
S: 0.010% or less,
sol. Al: 1.00% or less,
N: 0.010% or less,
B: 0.0002% or more and 0.0050% or less, and one or two of Nb and Ti in total of 0.010% or more and 0.080% or less, with the balance being Fe and unavoidable impurities;
and a steel structure having an area ratio of martensite of 70% or more, an area ratio of bainite of 30% or less, and a total area ratio of ferrite and retained austenite of 10% or less,
The number density of carbides having a major axis of 0.5 μm or more at a 1/4 position of the sheet thickness of the steel sheet is 60,000 pieces/mm2 ^or less,
the number density of inclusion particles having a circle equivalent diameter of 4.0 μm or more in a range of 1/4 to 3/4 of the sheet thickness of the steel sheet is 10 particles/mm2 or ^more and 30 particles/mm2 ^or less,
the number density of inclusion particles having a circular equivalent diameter of 4.0 μm or more in the range from the surface of the steel plate to 1/4 of the plate thickness is 27 particles/ ^mm2 or less,
A steel plate with a tensile strength of 1,310 MPa or more.
has been disclosed.

In Patent Document 2,
"In mass percent,
C: 0.05% to 0.40%,
Si: 0.05% to 3.0%,
Mn: 1.5% to 3.5%,
Al: 1.5% or less,
N: 0.010% or less,
P: 0.10% or less,
S: 0.005% or less,
Cr, Cu, Ni, Sn and Mo: 0.0% to 1.0% in total,
B: 0.000% to 0.005%,
Ca: 0.000% to 0.005%,
Ce: 0.000% to 0.005%, and La: 0.000% to 0.005%,
and further comprising
Nb: 0.0002% to 0.04%,
Ti: 0.0002% to 0.08%, and V and Ta: 0.01% to 0.3% in total;
Contains one or more selected from the group consisting of
The balance: Fe and impurities,
The chemical composition is represented by
In area %,
A first martensite having two or more iron carbides having an equivalent circle diameter of 2 nm to 500 nm in the lath: 20% to 95%,
Ferrite: 15% or less,
The steel has a steel structure represented by: retained austenite: 15% or less; and the balance: bainite or a second martensite having less than two iron carbides with a circle equivalent diameter of 2 nm to 500 nm in the lath, or both of these;
The total area fraction of ND//<111> oriented grains and ND//<100> oriented grains is 40% or less;
The amount of solute C is 0.44 ppm or more,
The ND//<111> oriented grains are crystal grains whose crystal orientation parallel to the normal direction of the plate surface is deviated from the <111> direction by 10° or less;
The ND//<100> orientation grains are crystal grains whose crystal orientation parallel to the normal direction of the sheet surface is deviated from the <100> direction by 10° or less.
has been disclosed.

In Patent Document 3,
"In mass percent,
C: 0.09% or more and 0.37% or less,
Si: more than 0.70% and less than 2.00%,
Mn: 2.60% or more and 3.60% or less,
P: 0.001% or more and 0.100% or less,
S: 0.0200% or less,
A composition comprising Al: 0.010% or more and 1.000% or less, N: 0.0100% or less, and the balance being Fe and unavoidable impurities;
The area ratio of martensite having a carbon concentration greater than 0.7×[%C] and smaller than 1.5×[%C] is 55% or more,
The area ratio of tempered martensite having a carbon concentration of 0.7×[% C] or less is 5% or more and 40% or less,
a ratio of a carbon concentration in the retained austenite to a volume fraction of the retained austenite is 0.05 or more and 0.40 or less;
The martensite and the tempered martensite have an average grain size of 5.3 μm or less,
The steel structure further has a surface softened thickness of 10 μm or more and 100 μm or less,
A high-strength steel plate having a tensile strength of 1180 MPa or more.
In addition, [%C] indicates the content (mass%) of the component element C in the steel.
has been disclosed.

Patent No. 7001197 Patent No. 6497443 Patent No. 6747612

Meanwhile, paint baking is often performed on automotive parts. Here, the toughness and impact properties of the steel plate can change significantly before and after the paint baking. For this reason, in recent years, the steel plate used as the material for automotive parts is also required to have excellent toughness and impact properties after paint baking in order to further improve the safety of automobiles.

However, none of the steel sheets disclosed in Patent Documents 1 to 3 take into consideration the toughness and impact properties after paint baking. Therefore, there is currently a demand for the development of a steel sheet with a TS of 1,320 MPa or more, which is excellent in stretch flangeability, and in toughness and impact properties after paint baking.

The present invention has been developed in view of the above-mentioned current situation, and has an object to provide a steel sheet having a TS of 1320 MPa or more and excellent stretch flangeability, as well as toughness and impact properties after paint baking, together with an advantageous manufacturing method thereof.
Another object of the present invention is to provide a member made of the above-mentioned steel plate and a method for manufacturing the same.

Here, TS is measured by a tensile test in accordance with JIS Z 2241:2022.
Excellent stretch flangeability means that the limit hole expansion ratio λ is 30% or more. The limit hole expansion ratio λ is measured by a hole expansion test in accordance with JIS Z 2256:2020.
"Excellent toughness after paint baking" means that the brittle-ductile transition temperature after aging treatment is -40°C or lower. Here, the aging treatment conditions are a treatment temperature of 170°C and a treatment time of 20 minutes. The brittle-ductile transition temperature is measured by a Charpy impact test in accordance with JIS Z 2242:2018.
"Excellent impact properties after baking paint" means that the YR after aging treatment is 0.85 or more and the fracture stress ratio after aging treatment is 0.90 or less. Here, the aging treatment conditions are a treatment temperature of 170°C and a treatment time of 20 minutes. The YR after aging treatment and the fracture stress ratio after aging treatment are calculated from the TS, YS (yield stress) and fracture stress after aging treatment measured by a tensile test in accordance with JIS Z 2241:2022, respectively, according to the following formulas.
[YR after aging treatment] = [YS after aging treatment] / [TS after aging treatment]
[Fracture stress ratio after aging treatment] = [Fracture stress after aging treatment] / [TS after aging treatment]
Details of the measurement methods are as described in the Examples below.

In order to achieve the above object, the inventors have conducted extensive research and have obtained the following findings.
(A) In order to achieve TS: 1320 MPa or more, it is important to make the area fraction of tempered martensite 95% or more. This makes it possible to obtain TS: 1320 MPa or more while ensuring the specified required properties.
(B) In order to obtain excellent stretch flangeability, it is important that the total area fraction of ferrite and bainitic ferrite is less than 5%. This makes it possible to obtain excellent stretch flangeability while ensuring the required properties.
(C) In order to obtain excellent toughness after baking paint, it is important to make the area fraction of retained austenite less than 3% and the density of grain boundaries of 20° or more in tempered martensite 1.0 μm/μm2 ^{or more} . This makes it possible to obtain excellent toughness after baking paint while ensuring the specified required properties.
(D) In order to obtain excellent impact properties after baking paint, it is important that the grain boundary density of 20° or more in the tempered martensite is 1.0 μm/μm2 or ^more and that the following formula (1) is satisfied. This makes it possible to obtain excellent impact properties after baking paint while ensuring the specified required properties.
KAM(S)/KAM(C) > 1.00 (1)
In the formula,
KAM(S): KAM value at a depth of 100 μm from the steel sheet surface,
KAM (C): The KAM value at the center position of the thickness of the steel plate.
The present invention was completed based on the above findings and through further investigation.

That is, the gist and configuration of the present invention are as follows.
1. In mass percent,
C: 0.030% or more and 0.500% 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,
N: 0.0100% or less,
A composition comprising O: 0.0100% or less and Al: 1.000% or less, with the balance being Fe and unavoidable impurities;
Area fraction of tempered martensite: 95% or more,
Area fraction of retained austenite: less than 3%;
Total area fraction of ferrite and bainitic ferrite: less than 5%;
The tempered martensite has a grain boundary density of 20° or more of 1.0 μm/μm2 ^{or more} ;
A steel sheet having a structure that satisfies the following formula (1):
KAM(S)/KAM(C) > 1.00 (1)
In the formula,
KAM (S): average KAM value at a depth of 100 μm from the steel sheet surface,
KAM (C): Average KAM value at the center position of the sheet thickness of the steel sheet.

2. The 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,
2. The steel plate according to 1 above, containing at least one selected from Hf: 0.10% or less and Bi: 0.200% or less.

3. The steel plate described in 1 or 2 above, having a plating layer on the surface.

4. A component made using a steel plate described in any one of 1 to 3 above.

5. A method for producing the steel plate according to any one of 1 to 3 above,
A preparation step of preparing a base steel sheet having the component composition according to 1 or 2;
Next, the base steel sheet is
A heating step of heating to an annealing temperature T1 under the condition of an average heating rate in a temperature range of 700 ° C. to 750 ° C.: 5.0 ° C./s or less;
Next, the base steel sheet is
The annealing temperature T1: 800° C. or higher;
An annealing step of annealing under the condition of an annealing time t1: 10 seconds or more;
Next, the base steel plate is
A bending process in which bending is performed at least once using a roll with a radius of 800 mm or less in a temperature range of the annealing temperature T1 to 700 ° C.;
Next, the base steel sheet is
A first cooling step of cooling to a first cooling end temperature under the condition of an average cooling rate of 10°C/s or more in a temperature range of 700°C to 550°C;
Next, the base steel sheet is
Average cooling rate in the temperature range of 300°C to 100°C: 300°C/s or more;
A second cooling step of cooling the base steel sheet to a second cooling end temperature under a condition of a tension of 5 MPa or more applied to the base steel sheet in a temperature range of 300° C. to 100° C.;
Next, the base steel sheet is
Tempering temperature T2: 100°C or more and 400°C or less,
A tempering process in which tempering is performed under the condition of a tempering time t2 of 10 seconds or more and 10,000 seconds or less;
Next, the base steel plate is
Straightening start temperature: 100℃ or less,
Inlet intermesh push-in amount: 4.0 mm or more and 10.0 mm or less,
Amount of intermesh pushed in at the outlet: 1.0 mm or more and 10.0 mm or less,
Inlet tension: 20 MPa or more and 500 MPa or less,
A straightening process in which straightening is performed by leveling under the condition of an outlet tension of 25 MPa or more and 550 MPa or less;
A manufacturing method for steel plates that provides the following:

6. The method for manufacturing steel plate described in 5, further comprising a plating process for applying a plating treatment to the base steel plate between the first cooling process and the second cooling process, or between the tempering process and the straightening process.

7. A method for manufacturing a component, comprising the step of subjecting a steel plate described in any one of 1 to 3 above to at least one of forming and joining processes to form a component.

According to the present invention, a steel sheet having a TS of 1320 MPa or more and excellent stretch flangeability, toughness after paint baking, and crashworthiness can be obtained. By applying the steel sheet of the present invention to, for example, a material for automobile parts, it is possible to improve fuel efficiency by reducing the weight of the automobile body, which can greatly contribute to reducing _CO2 emissions. Therefore, the industrial utility value is extremely high.

FIG. 2 is a schematic diagram for explaining the definitions of an inlet intermesh push-in amount and an outlet intermesh push-in amount.

The present invention will be described based on the following embodiments.
[1] Steel Plate First, the composition of the steel plate according to one embodiment of the present invention will be described. Note that the unit of the composition is "mass%", but hereinafter, unless otherwise specified, it will be simply shown as "%".

[C: 0.030% or more and 0.500% or less]
C is one of the important basic components of steel. In particular, in a steel plate according to an embodiment of the present invention, C is an important component that affects the area fraction of tempered martensite and the impact properties after paint baking. If the C content is less than 0.030%, the area fraction of tempered martensite decreases, making it difficult to achieve a TS of 1320 MPa or more. On the other hand, if the C content exceeds 0.500%, the tempered martensite becomes embrittled, making it difficult to achieve excellent toughness after baking paint. The C content is set to 0.030% or more and 0.500% or less. The C content is preferably set to 0.050% or more, and more preferably set to 0.100% or more. , preferably 0.400% or less, and more preferably 0.350% or less.

[Si: 0.010% or more and 2.500% or less]
Si is one of the important basic components of steel. In particular, in a steel sheet according to an embodiment of the present invention, Si suppresses the formation of carbides during annealing and promotes the formation of retained austenite. That is, Si Si is an important element that affects the area fraction of retained austenite. If the Si content is less than 0.010%, it becomes difficult to achieve a TS of 1320 MPa or more. If the Si content exceeds 500%, the amount of retained austenite increases excessively, making it difficult to achieve excellent toughness after baking paint. Therefore, the Si content is set to 0.010% or more and 2.500% or less. The Si content is preferably 0.050% or more, more preferably 0.100% or more. The Si content is preferably 2.000% or less, more preferably 1.200% or less. do.

[Mn: 0.10% or more and 5.00% or less]
Mn is one of the important basic components of steel. In particular, in a steel plate according to an embodiment of the present invention, Mn is an important element that affects the area fraction of tempered martensite and the toughness after paint baking. When the Mn content is less than 0.10%, the area fraction of tempered martensite decreases, making it difficult to achieve a TS of 1320 MPa or more. %, the tempered martensite becomes embrittled, making it difficult to achieve excellent toughness after paint baking. Therefore, the Mn content is set to 0.10% or more and 5.00% or less. The Mn content is preferably 0.50% or more, and more preferably 0.80% or more.The Mn content is preferably 4.50% or less, and more preferably 4.00% or less.

[P: 0.100% or less]
P segregates at prior austenite grain boundaries, embrittling the grain boundaries and reducing the ultimate deformability of steel sheets. Therefore, if the P content is excessive, it is difficult to achieve excellent toughness after paint baking. Therefore, the P content is set to 0.100% or less. The P content is preferably set to 0.070% or less. There is no particular lower limit for the P content. P is a solid solution strengthening element that can increase the strength of the steel sheet, and therefore the P content is preferably 0.001% or more.

[S: 0.0200% or less]
S exists as sulfide and reduces the ultimate deformability of the steel sheet. Therefore, if the S content is excessive, it becomes difficult to achieve excellent toughness after baking paint. The amount of S is 0.0200% or less. The S content is preferably 0.0050% or less. There is no particular lower limit for the S content. However, due to restrictions in production technology, the S content is The content is preferably 0.0001% or more.

[N: 0.0100% or less]
N exists as a nitride and reduces the ultimate deformability of the steel sheet. Therefore, if the N content is excessive, it becomes difficult to achieve excellent toughness after baking paint. The N content is set to 0.0100% or less. The N content is preferably set to 0.0050% or less. There is no particular lower limit for the N content. However, due to production technology constraints, the N content is The content is preferably 0.0001% or more.

[O: 0.0100% or less]
O exists as an oxide and reduces the ultimate deformability of the steel sheet. Therefore, if the O content is excessive, it becomes difficult to achieve excellent toughness after baking paint. The amount of O is 0.0100% or less. The content of O is preferably 0.0050% or less. There is no particular lower limit for the content of O. However, due to restrictions in production technology, the amount of O is The content is preferably 0.0001% or more.

[Al: 1.000% or less]
Al exists as an oxide and reduces the ultimate deformability of the steel sheet. Therefore, if the Al content is excessive, it becomes difficult to achieve excellent toughness after baking paint. Therefore, the content of Al is The content of Al is set to 1.000% or less. The content of Al is preferably set to 0.500% or less. There is no particular lower limit for the content of Al. However, due to the constraints of production technology, the content of Al is set to 1.000% or less. The content is preferably 0.001% or more.

The basic composition of the steel sheet according to one embodiment of the present invention has been described above, but the steel sheet according to one embodiment of the present invention has a composition containing the above basic components, with the balance other than the above basic components including Fe (iron) and unavoidable impurities. Here, the steel sheet according to one embodiment of the present invention preferably has a composition containing the above basic components, with the balance consisting of Fe and unavoidable impurities. In addition to the above basic components, the steel sheet according to one embodiment of the present invention may contain at least one element selected from the following, either alone or in combination, as an optional added element.
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. The above-mentioned optional elements are not particularly limited in terms of lower limit because the effects of the present invention can be obtained as long as they are contained in amounts not exceeding the upper limits. In addition, when the above-mentioned optional elements are contained in amounts less than the preferred lower limits described below, the elements are considered to be contained as unavoidable impurities.

[Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less]
If the Ti, Nb and V contents are each 0.200% or less, large amounts of coarse precipitates and inclusions are not generated, and the ultimate deformability of the steel sheet is not reduced. Therefore, the toughness after baking the paint is not reduced. Therefore, when Ti, Nb and V are contained, the content of each is preferably 0.200% or less. The content of each of Ti, Nb and V is more preferably 0.100% or less. The lower limits of the contents of Ti, Nb and V are not particularly specified. However, Ti, Nb and V form fine carbides, nitrides or carbonitrides during hot rolling or annealing, and thus improve the hardness of the steel sheet. Therefore, the contents of Ti, Nb and V are each preferably 0.001% or more.

[Ta: 0.10% or less, W: 0.10% or less]
If the Ta and W contents are each 0.10% or less, large coarse precipitates and inclusions are not generated in large quantities, and the ultimate deformability of the steel sheet is not reduced, so that the toughness after paint baking is not reduced. Therefore, when Ta and W are contained, the content is preferably 0.10% or less, and more preferably 0.08% or less. There is no particular lower limit. However, Ta and W increase the strength of the steel sheet by forming fine carbides, nitrides, or carbonitrides during hot rolling or annealing. Therefore, the content of Ta and W is preferably within the range of 100 to 1500 μm. The content of each of these elements is preferably 0.01% or more.

[B: 0.0100% or less]
If B is 0.0100% or less, cracks will not form inside the steel sheet during casting or hot rolling, and the ultimate deformability of the steel sheet will not decrease. Therefore, there will be no decrease in toughness after paint baking. When B is contained, the content is preferably 0.0100% or less. The content of B is more preferably 0.0080% or less. There is no particular lower limit for the content of B. However, since B is an element that segregates at austenite grain boundaries during annealing and improves hardenability, the B content is preferably 0.0003% or more.

[Cr: 1.00% or less, Mo: 1.00% or less, Ni: 1.00% or less]
If the content of Cr, Mo and Ni is 1.00% or less, a large amount of coarse precipitates and inclusions are not generated, and the ultimate deformability of the steel plate is not reduced. Therefore, the toughness after baking the paint is not reduced. Therefore, when Cr, Mo and Ni are contained, the content of each is preferably 1.00% or less. The content of each of Cr, Mo and Ni is more preferably 0.80% or less. The lower limits of the contents of Cr, Mo and Ni are not particularly specified. However, Cr, Mo and Ni are elements that improve hardenability. Therefore, the contents of Cr, Mo and Ni are each 0.01 % or more is preferable.

[Co: 0.010% or less]
If the Co content is 0.010% or less, large precipitates and inclusions are not generated in large quantities, and the ultimate deformability of the steel sheet is not reduced. Therefore, the toughness after baking is not reduced. When Co is contained, its content is preferably 0.010% or less. The Co content is more preferably 0.008% or less. There is no particular lower limit for the Co content. However, Co is an element that improves hardenability. Therefore, the Co content is preferably 0.001% or more.

[Cu: 1.00% or less]
If the Cu content is 1.00% or less, large precipitates and inclusions are not generated in large quantities, and the ultimate deformability of the steel sheet is not reduced. Therefore, the toughness after baking is not reduced. When Cu is contained, its content is preferably 1.00% or less. The Cu content is more preferably 0.80% or less. There is no particular lower limit for the Cu content. However, Cu is an element that improves hardenability. Therefore, the Cu content is preferably 0.01% or more.

[Sn: 0.200% or less]
If the Sn content is 0.200% or less, cracks will not form inside the steel sheet during casting or hot rolling, and the ultimate deformability of the steel sheet will not decrease. Therefore, there will be no decrease in toughness after paint baking. When Sn is contained, the content is preferably 0.200% or less. The Sn content is more preferably 0.100% or less. There is no particular lower limit for the Sn content. However, Sn is an element that improves hardenability and generally improves corrosion resistance, so the Sn content is preferably 0.001% or more.

[Sb: 0.200% or less]
If the Sb content is 0.200% or less, large precipitates and inclusions are not generated in large quantities, and the ultimate deformability of the steel sheet is not reduced. Therefore, the toughness after baking is not reduced. When Sb is contained, its content is preferably 0.200% or less. The Sb content is more preferably 0.100% or less. There is no particular lower limit for the Sb content. However, Sb Sb is an element that controls the thickness of the softened surface layer and enables strength adjustment. Therefore, the Sb content is preferably 0.001% or more.

[Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less]
If the Ca, Mg and REM contents are each 0.0100% or less, large amounts of coarse precipitates and inclusions are not generated, and the ultimate deformability of the steel sheet is not reduced. Therefore, when Ca, Mg and REM are contained, the content of each is preferably 0.0100% or less. The content of each of Ca, Mg and REM is more preferably 0.0050% or less. The lower limits of the contents of Ca, Mg and REM are not particularly specified. However, Ca, Mg and REM are elements that make the shape of nitrides and sulfides spheroidal and improve the ultimate deformability of the steel sheet. The Ca, Mg and REM contents are each preferably 0.0005% or more.

[Zr: 0.100% or less, Te: 0.100% or less]
If the content of Zr and Te is 0.100% or less, large coarse precipitates and inclusions are not generated in large quantities, and the ultimate deformability of the steel sheet is not reduced, so that the toughness after baking the paint is not reduced. Therefore, when Zr and Te are contained, the content is preferably 0.100% or less, and more preferably 0.080% or less. There is no particular lower limit for the amount. However, Zr and Te are elements that make the shape of nitrides and sulfides spheroidal and improve the ultimate deformability of the steel sheet. Therefore, the content of Zr and Te should not exceed 0. It is preferable that the content is 0.01% or more.

[Hf: 0.10% or less]
If the Hf content is 0.10% or less, large precipitates and inclusions are not generated in large quantities, and the ultimate deformability of the steel sheet is not reduced. Therefore, the toughness after baking is not reduced. When Hf is contained, its content is preferably 0.10% or less. The Hf content is more preferably 0.08% or less. There is no particular lower limit for the Hf content. Hf is an element that makes the shape of nitrides and sulfides spheroidal and improves the ultimate deformability of the steel sheet, so the Hf content is preferably 0.01% or more.

[Bi: 0.200% or less]
If the Bi content is 0.200% or less, a large amount of coarse precipitates or inclusions are not generated, and the ultimate deformability of the steel sheet is not reduced. Therefore, the toughness after baking is not reduced. When Bi is contained, its content is preferably 0.200% or less. The Bi content is more preferably 0.100% or less. There is no particular lower limit for the Bi content. Bi is an element that reduces segregation, so the Bi content is preferably 0.001% or more.

The elements other than those mentioned above are Fe and unavoidable impurities. Examples of unavoidable impurities include Zn, Pb, As, Ge, Sr and Cs. These unavoidable impurities are permitted to be contained if their total amount is 0.100% or less.

Next, the structure of a steel sheet according to one embodiment of the present invention will be described.
The structure of the steel plate according to one embodiment of the present invention is
Area fraction of tempered martensite: 95% or more,
Area fraction of retained austenite: less than 3%;
Total area fraction of ferrite and bainitic ferrite: less than 5%;
The density of grain boundaries at angles of 20° or more in the tempered martensite is 1.0 μm/μm2 or ^more ;
An organization that satisfies the above formula (1).
The reasons for each of the limitations will be explained below. The area fraction of each phase is the area ratio that each phase occupies with respect to the entire structure.

[Area fraction of tempered martensite: 95% or more]
In the steel sheet according to one embodiment of the present invention, it is extremely important that the area fraction of tempered martensite is 95% or more. That is, by making tempered martensite the main phase, and in particular by making its area fraction 95% or more, it is possible to realize TS: 1320 MPa or more. Therefore, the area fraction of tempered martensite is 95% or more. The area fraction of tempered martensite is preferably 96% or more, more preferably 97% or more. There is no particular upper limit for the area fraction of tempered martensite. The area fraction of tempered martensite may be 100%.

[Area fraction of retained austenite: less than 3%]
In the steel sheet according to one embodiment of the present invention, it is extremely important that the area fraction of the retained austenite is less than 3%. That is, if the area fraction of the retained austenite is 3% or more, it becomes difficult to achieve excellent toughness after baking paint. One of the causes of the decrease in toughness after baking paint is that the retained austenite is transformed into high-hardness martensite during processing, which becomes the starting point of fracture. Therefore, the area fraction of the retained austenite is less than 3%. The area fraction of the retained austenite is preferably 1% or less. The lower limit of the area fraction of the retained austenite is not particularly specified. The area fraction of the retained austenite may be 0%.

[Total area fraction of ferrite and bainitic ferrite: less than 5%]
In the steel sheet according to one embodiment of the present invention, it is extremely important that the total area fraction of ferrite and bainitic ferrite is less than 5%. That is, when the total area fraction of ferrite and bainitic ferrite is 5% or more, it becomes difficult to realize a TS of 1320 MPa or more. It also becomes difficult to realize excellent stretch flangeability. Therefore, the total area fraction of ferrite and bainitic ferrite is less than 5%. The total area fraction of ferrite and bainitic ferrite is preferably 3% or less, more preferably 2% or less. The lower limit of the total area fraction of ferrite and bainitic ferrite is not particularly specified. The total area fraction of ferrite and bainitic ferrite may be 0%. Ferrite and bainitic ferrite may be contained alone, or both of them may be contained.

It is preferable that the area fraction of the remaining structure other than the above is 5% or less. Examples of the remaining structure include pearlite, fresh martensite, and acicular ferrite. These remaining structures may be included as long as they are 5% or less and do not affect the properties. The area fraction of the remaining structure may be 0%.

Here, the area fraction of tempered martensite and the total area fraction of ferrite and bainitic ferrite are measured, for example, as follows.
That is, a sample is cut out from the steel plate so that the plate thickness cross section (L cross section) parallel to the rolling direction of the steel plate becomes the observation surface. Next, the observation surface of the sample is polished. Next, the observation surface of the sample is corroded with 3 vol. % nital to reveal the structure. Next, the plate thickness 1/4 position of the steel plate (a position corresponding to 1/4 of the plate thickness in the depth direction from the surface of the steel plate) is observed by SEM at a magnification of 2000 times in 10 fields of view. In addition, in the observation image, the tempered martensite has fine irregularities inside the structure and has carbides inside the structure. In addition, the ferrite and bainitic ferrite are structures in which the inside of the structure is flat at the recesses and does not have carbides inside. Then, the areas of the regions occupied by the tempered martensite, ferrite, and bainitic ferrite are obtained for each field of view. Next, the areas of the regions occupied by tempered martensite, and ferrite and bainitic ferrite are each divided by the area of the entire observation field, and multiplied by 100. Then, the average values are determined as the area fraction of tempered martensite and the total area fraction of ferrite and bainitic ferrite, respectively.
In addition, the structure of a steel sheet is generally symmetrical in the thickness direction, so any one of the surfaces (front surface and back surface) of the steel sheet may be set as the starting point of the thickness position (0 thickness position), such as a 1/4 thickness position or a position at a depth of 100 μm from the steel sheet surface.

The area fraction of retained austenite is measured, for example, as follows.
That is, the steel plate is mechanically ground to a depth of 1/4-0.1 mm of the plate thickness so that the observation position is the 1/4 position of the plate thickness of the steel plate, and then further polished by 0.1 mm by chemical polishing. The polished surface is used as the observation surface, and the ratio of the integrated intensity of the diffraction peaks of the {200}, {211}, and {220} planes of fcc iron (austenite) to the integrated intensity of the diffraction peaks of the {200}, {220}, and {311} planes of bcc iron is obtained using CoKα radiation by an X-ray diffractometer. Next, the volume fraction of the retained austenite is calculated from the ratio of the integrated intensities of the respective planes. Then, the retained austenite is considered to be three-dimensionally homogeneous, and the volume fraction of the retained austenite is taken as the area fraction of the retained austenite.

Furthermore, the area fraction of the remaining structure is determined by subtracting the area fraction of tempered martensite, the total area fraction of ferrite and bainitic ferrite, and the area fraction of retained austenite determined as described above from 100%.
[Area fraction of remaining structure (%)] = 100 - [Area fraction of tempered martensite (%)] - [Total area fraction of ferrite and bainitic ferrite (%)] - [Area fraction of retained austenite (%)]

[Grain boundary density of 20° or more in tempered martensite: 1.0 μm/μm2 or ^more ]
In the steel sheet according to the embodiment of the present invention, it is extremely important that the grain boundary density of 20° or more in the tempered martensite is 1.0 μm/μm2 ^{or more} . The high-angle grain boundaries in the tempered martensite, particularly the grain boundaries of 20° or more in the tempered martensite, become carbon segregation sites during paint baking and suppress the destruction of the steel sheet. As a result, the fracture stress during tensile deformation is reduced. Therefore, if the grain boundary density of 20° or more in the tempered martensite (hereinafter also referred to as the high-angle grain boundary density of the tempered martensite) is less than 1.0 μm/ ^μm2 , it becomes difficult to realize excellent impact characteristics after paint baking. Therefore, the high-angle grain boundary density of the tempered martensite is 1.0 μm/μm2 or ^more . The high-angle grain boundary density of the tempered martensite is preferably 1.2 μm/μm2 or ^more , more preferably 1.3 μm/μm2 or ^more . The upper limit of the high-angle grain boundary density of the tempered martensite is not particularly specified. For example, the high angle grain boundary density of tempered martensite is preferably 3.0 μm/μm2 or ^less .

Here, the high angle grain boundary density of the tempered martensite is determined, for example, as follows.
A test piece for observing the structure is taken from the steel sheet. The taken test piece is then polished by colloidal silica vibration polishing so that the rolling direction cross section (L cross section) becomes the observation surface. The observation surface is mirror finished. Next, electron backscatter diffraction (EBSD) measurement is performed at a 1/4 position of the steel sheet thickness (a position corresponding to 1/4 of the sheet thickness in the depth direction from the steel sheet surface) to obtain local crystal orientation data. In the EBSD measurement, the step size is 0.10 μm, and the measurement area is 50 μm square (50 μm × 50 μm). Next, the obtained local crystal orientation data is analyzed using analysis software: OIM Analysis 7. The analysis of the local crystal orientation data is performed for each of 10 fields of view at the 1/4 position of the steel sheet thickness, and the average value is used. In addition, prior to the analysis of the local crystal orientation data, a cleanup process is performed once each using the grain dilation function (grain tolerance angle: 5, minimum grain size: 2, single iteration: ON) of the analysis software. Next, the grain boundaries of 20° or more in the tempered martensite are displayed, and the total length of the grain boundaries of 20° or more in the tempered martensite is obtained. Then, the total length of the grain boundaries of 20° or more in the tempered martensite is divided by the area of the measurement region to obtain the high angle grain boundary density of the tempered martensite.

[KAM(S)/KAM(C)>1.00]
In the steel sheet according to the embodiment of the present invention, it is extremely important that KAM(S)/KAM(C) exceeds 1.00. Here, KAM(S) is the value at a depth of 100 μm from the surface of the steel sheet. The KAM value of the steel plate is the KAM value at the center of the thickness of the steel plate, and KAM(C) is the KAM value at the center of the thickness of the steel plate. It has been found that it is effective to change the dislocation distribution state toward the inside. In particular, by making KAM(S)/KAM(C) exceed 1.00, excellent impact properties after baking paint can be obtained. Therefore, KAM(S)/KAM(C) is set to be more than 1.00. KAM(S)/KAM(C) is preferably 1.03 or more. There is no particular upper limit for (C). For example, KAM(S)/KAM(C) is preferably 1.110 or less.

Here, KAM(S) and KAM(C) are calculated, for example, as follows.
EBSD measurement is performed in the same manner as the measurement of the high angle boundary density of tempered martensite, and the obtained local crystal orientation data is analyzed. The analysis of the local crystal orientation data is performed for 10 fields of view at a depth of 100 μm from the steel sheet surface and at the center of the steel sheet thickness, and the average value is used. Then, from the analysis results of the local crystal orientation data at a depth of 100 μm from the steel sheet surface and at the center of the steel sheet thickness, a chart of the KAM value of the bcc phase is created for each position, and the average values are designated as KAM (S) and KAM (C), respectively.

The mechanical properties of the steel plate according to one embodiment of the present invention are as described above.

The steel sheet according to one embodiment of the present invention may have a plating layer on the surface. The plating layer may be provided on only one surface of the steel sheet, or on both surfaces. The plating layer is not particularly limited. An example of the plating layer is a zinc plating layer mainly composed of Zn (Zn content is 50.0 mass% or more). Examples of the zinc plating layer include a hot-dip galvanized layer, an alloyed hot-dip galvanized layer, and an electrogalvanized layer. The steel sheet having a zinc plating layer can also be called a zinc-plated steel sheet. The above-mentioned steel sheets having the hot-dip galvanized layer, alloyed hot-dip galvanized layer, and electrogalvanized layer can also be called hot-dip galvanized steel sheet (GI), alloyed hot-dip galvanized steel sheet (GA), and electrogalvanized steel sheet (EG), respectively.

Examples of plating layers other than zinc plating layers include aluminum plating layers and alloy plating layers. Examples of alloy plating layers include hot-dip zinc-aluminum-magnesium alloy plating layers and Zn-Ni electric alloy plating layers.

In addition, the plating weight per side of the plating layer is not particularly limited, but is preferably 20 g/m ² or more and 80 g/m ² or less.

The thickness of the steel plate according to one embodiment of the present invention is not particularly limited, but is preferably 0.50 mm or more and 2.50 mm or less.

[2] Members Next, members according to one embodiment of the present invention will be described.
A member according to an embodiment of the present invention is a member made using the above-mentioned steel plate (as a raw material). For example, the raw material steel plate is subjected to at least one of forming and joining to form a member.
Here, the above steel sheet has a TS of 1320 MPa or more, and is excellent in stretch flangeability, toughness after paint baking, and impact properties. Therefore, the member according to one embodiment of the present invention is particularly suitable for use as a material for automobile parts, for example. This allows for the improvement of fuel efficiency by reducing the weight of the automobile body, which contributes greatly to the reduction of _CO2 emissions.

[3] Manufacturing Method of Steel Sheet Next, a manufacturing method of a steel sheet according to one embodiment of the present invention will be described.

A method for producing a steel sheet according to one embodiment of the present invention includes the steps of:
A preparation step of preparing a base steel sheet having the above-mentioned composition;
Next, the base steel sheet is
A heating step of heating to an annealing temperature T1 under the condition of an average heating rate in a temperature range of 700 ° C. to 750 ° C.: 5.0 ° C./s or less;
Next, the base steel sheet is
The annealing temperature T1: 800° C. or higher;
An annealing step of annealing under the condition of an annealing time t1: 10 seconds or more;
Next, the base steel plate is
A bending process in which bending is performed at least once using a roll with a radius of 800 mm or less in a temperature range of the annealing temperature T1 to 700 ° C.;
Next, the base steel sheet is
A first cooling step of cooling to a first cooling end temperature under the condition of an average cooling rate of 10°C/s or more in a temperature range of 700°C to 550°C;
Next, the base steel sheet is
Average cooling rate in the temperature range of 300°C to 100°C: 300°C/s or more;
A second cooling step of cooling the base steel sheet to a second cooling end temperature under a condition of a tension of 5 MPa or more applied to the base steel sheet in a temperature range of 300° C. to 100° C.;
Next, the base steel sheet is
Tempering temperature T2: 100°C or more and 400°C or less,
A tempering process in which tempering is performed under the condition of a tempering time t2 of 10 seconds or more and 10,000 seconds or less;
Next, the base steel plate is
Straightening start temperature: 100℃ or less,
Inlet intermesh push-in amount: 4.0 mm or more and 10.0 mm or less,
Amount of intermesh pushed in at the outlet: 1.0 mm or more and 10.0 mm or less,
Inlet tension: 20 MPa or more and 500 MPa or less,
A straightening process in which straightening is performed by leveling under the condition of an outlet tension of 25 MPa or more and 550 MPa or less;
It has the following characteristics.
Unless otherwise specified, the above temperatures are the surface temperatures of the steel sheet. Furthermore, unless otherwise specified, the average heating rate and the average cooling rate are also based on the surface temperatures of the steel sheet.

Preparation step: First, a base steel sheet having the above-mentioned composition is prepared. For example, the base steel sheet can be prepared by hot rolling a steel slab to obtain a hot-rolled steel sheet, optionally pickling and heat treating the hot-rolled steel sheet, and then cold rolling the hot-rolled steel sheet to obtain a cold-rolled steel sheet. The conditions for these steps are not particularly limited, and may be in accordance with conventional methods.

For example, any known melting method such as converter or electric furnace is suitable for melting steel slabs (steel material). In order to prevent macrosegregation, it is preferable to melt steel slabs using a continuous casting method.

Examples of hot rolling include a method of rolling a steel slab after heating, a method of directly rolling a steel slab after continuous casting without heating, and a method of rolling a steel slab after continuous casting by subjecting it to a short-term heat treatment. In addition, the slab heating temperature, slab soaking holding time, and coiling temperature in hot rolling are not particularly limited. The slab heating temperature is preferably 1100°C or higher. The slab heating temperature is preferably 1300°C or lower. The slab soaking holding time is preferably 30 min or more. The slab soaking holding time is preferably 250 min or less. The finish rolling temperature is preferably the _Ar3 transformation point or higher. The coiling temperature is preferably 350°C or higher. The coiling temperature is preferably 650°C or lower. The _Ar3 transformation point is calculated by the following formula.
Ar ₃ transformation point (°C) = 868-396×[%C]+24.6×[%Si]-68.1×[%Mn]-36.1×[%Ni]-20.7×[%Cu]-24.8×[%Cr]
In the above formula, the symbol [% element symbol] represents the content (mass %) of the corresponding element in the above composition.

Pickling can remove oxides from the surface of the hot-rolled steel sheet, and is preferably performed to ensure good chemical conversion treatability and plating quality in the final steel sheet product. Pickling may be performed once or multiple times. The hot-rolled steel sheet after pickling may be subjected to heat treatment.

The total reduction ratio in cold rolling is preferably 30% or more. The total reduction ratio in cold rolling is preferably 80% or less. The number of rolling passes and the reduction ratio of each pass are not particularly limited, and the desired effect can be obtained.

Heating Step Next, the base steel sheet prepared in the preparation step is heated to an annealing temperature T1 under the condition of an average heating rate of 5.0°C/s or less in a temperature range of 700°C to 750°C. The annealing temperature T1 will be described in the annealing step described later.

[Average heating rate in the temperature range of 700°C to 750°C: 5.0°C/s or less]
The inventors have conducted extensive research and found that the average heating rate in the temperature range of 700°C to 750°C (hereinafter, simply referred to as the average heating rate) affects the high-angle grain boundary density of tempered martensite. That is, by setting the average heating rate to 5.0°C/s or less, the dissolution of carbides is promoted. As a result, the prior austenite grain boundaries are refined, and the grain boundaries of 20° or more increase after martensite transformation. Therefore, the high-angle grain boundary density of tempered martensite in the final steel sheet product also increases, improving the impact properties after paint baking. Therefore, the average heating rate is set to 5.0°C/s or less. The average heating rate is preferably 3.0°C/s or less. The upper limit of the average heating rate is not particularly specified. For example, the average heating rate is preferably 0.1°C/s or more.

Annealing Step Next, the base steel sheet is annealed under the conditions of an annealing temperature T1: 800° C. or higher and an annealing time t1: 10 seconds or longer.

[Annealing temperature T1: 800° C. or higher]
When the annealing temperature T1 is less than 800°C, the total area fraction of ferrite and bainitic ferrite is 5% or more, and it is difficult to achieve a TS of 1320 MPa or more. It is also difficult to achieve excellent stretch flangeability. Therefore, the annealing temperature T1 is set to 800°C or more. The annealing temperature T1 is preferably 820°C or more. The upper limit of the annealing temperature T1 is not particularly specified. For example, the annealing temperature T1 is preferably 1000°C or less. The annealing temperature here is the holding temperature in the annealing process. The annealing temperature may be constant during holding. In addition, the annealing temperature does not have to be constant during holding as long as it is in a temperature range of 800°C or more and the temperature fluctuation is within the set temperature ±10°C.

[Annealing time t1: 10 seconds or more]
If the annealing time t1 is less than 10 seconds, the total area fraction of ferrite and bainitic ferrite will be 5% or more, making it difficult to achieve a TS of 1320 MPa or more. It will also be difficult to achieve excellent stretch flangeability. Therefore, the annealing time t1 is set to 10 seconds or more. The annealing time t1 is preferably 30 seconds or more. There is no particular upper limit for the annealing time t1. For example, the annealing time t1 is preferably 1000 seconds or less. The annealing time t1 here refers to the holding time at the annealing temperature T1.

Bending Step Next, the base steel sheet is bent at least once using rolls with a radius of 800 mm or less in a temperature range of annealing temperatures T1 to 700°C.

[Number of times of bending with a roll of radius 800 mm or less in the temperature range of annealing temperature T1 to 700 ° C.: 1 time or more]
As a result of intensive research, the inventors have found that bending in the temperature range from annealing temperature T1 to 700°C (hereinafter also referred to as bending temperature range) affects the high angle grain boundary density of tempered martensite. In particular, when bending is performed using a roll with a radius of 800 mm or less in the bending temperature range, nucleation of martensitic transformation is promoted. This makes martensite finer, and grain boundaries with angles of 20° or more increase after martensitic transformation. Therefore, the high angle grain boundary density of tempered martensite in the final steel sheet product also increases, improving the impact properties after paint baking. Therefore, the number of times bending using a roll with a radius of 800 mm or less in the bending temperature range (hereinafter also referred to as bending number) is set to one or more.

The radius of the roll used in the bending process is preferably 600 mm or less. There is no particular lower limit to the radius of the roll used in the bending process. For example, the radius of the roll used in the bending process is preferably 100 mm or more.

The number of times of bending may be one or more. The number of times of bending is preferably two or more. There is no particular upper limit on the number of times of bending. For example, the number of times of bending is preferably 15 or less. The bending process may be performed by bending in one direction with a roll, and then bending back the same amount as the amount of bending in the opposite direction. In this case, the number of times of bending is counted as two (one bending and one bending back). The bending angle is preferably 80 to 110°. This further enhances the effect of promoting the nucleation of martensitic transformation. The bending angle is the angle (acute angle) between the passing direction of the steel plate on the roll entry side and the passing direction of the steel plate on the roll exit side.

In addition, if bending is performed at least once using a roll with a radius of 800 mm or less in the bending temperature range, further bending that does not satisfy the above condition may be performed.

First Cooling Step Next, the base steel plate is cooled to a first cooling end temperature under conditions of an average cooling rate of 10°C/s or more in a temperature range of 700°C to 550°C.

[Average cooling rate in the temperature range of 700°C to 550°C: 10°C/s or more]
When the average cooling rate in the temperature range of 700°C to 550°C (hereinafter also referred to as the first average cooling rate) is less than 10°C/s, the total area fraction of ferrite and bainitic ferrite becomes 5% or more, and it becomes difficult to realize a TS of 1320 MPa or more. It also becomes difficult to realize excellent stretch flangeability. Therefore, the first average cooling rate is set to 10°C/s or more. The first average cooling rate is preferably 30°C/s or more. The upper limit of the first average cooling rate is not particularly specified. For example, the first average cooling rate is preferably 2000°C/s or less.

The first cooling end temperature may be, for example, 550°C to 300°C. In addition, between the first cooling process and the second cooling process described below, the base steel sheet may be subjected to a plating process. The plating process is described below.

Second Cooling Step Next, the base steel plate is cooled to a second cooling end temperature under the conditions of an average cooling rate of 300°C/s or more in the temperature range of 300°C to 100°C and a tension applied to the base steel plate in the temperature range of 300°C to 100°C: 5 MPa or more.

[Average cooling rate in the temperature range of 300°C to 100°C: 300°C/s or more]
If the average cooling rate in the temperature range of 300°C to 100°C (hereinafter also referred to as the second average cooling rate) is less than 300°C/s, the area fraction of retained austenite will be 3% or more, making it difficult to achieve excellent toughness after paint baking. Therefore, the second average cooling rate is set to 300°C/s or more. The second average cooling rate is preferably 800°C/s or more. There is no particular upper limit for the second average cooling rate. For example, the second average cooling rate is preferably 2000°C/s or less.

[Tension applied to base steel sheet in temperature range of 300°C to 100°C: 5 MPa or more]
As a result of intensive research by the inventors, it was found that the tension applied to the material steel sheet during cooling in the temperature range of 300°C to 100°C affects the high angle grain boundary density of tempered martensite. In particular, when the tension applied to the material steel sheet in the temperature range of 300°C to 100°C (hereinafter also simply referred to as the applied tension) is 5 MPa or more, martensitic transformation is promoted. As a result, martensite becomes finer, and the grain boundaries of 20° or more increase after martensitic transformation. Therefore, the high angle grain boundary density of tempered martensite in the final product steel sheet also increases, and the impact properties after paint baking are improved. Therefore, the applied tension is 5 MPa or more. The applied tension is preferably 10 MPa or more. The upper limit of the applied tension is not particularly specified. For example, the applied tension is preferably 100 MPa or less.

The second cooling end temperature may be, for example, less than 100°C. The second cooling end temperature may be, for example, about room temperature.

The bending process in the bending step described above increases the number of nucleation sites, which are the locations where martensitic transformation begins. On the other hand, the application of tension in the second cooling step promotes the martensitic transformation itself. In other words, the effects obtained by the two methods are different.

Tempering Step Next, the base steel sheet is tempered under the conditions of a tempering temperature T2: 100° C. or more and 400° C. or less, and a tempering time t2: 10 seconds or more and 10,000 seconds or less.

[Tempering temperature T2: 100°C or more and 400°C or less]
Tempered martensite is produced by tempering martensite through a tempering process. If the tempering temperature T2 is less than 100° C., the martensite is not sufficiently tempered, and the as-quenched martensite is the main component. In such a structure mainly composed of martensite as quenched, excellent toughness after paint baking cannot be obtained. On the other hand, when the tempering temperature T2 exceeds 400°C, the tempering of martensite proceeds excessively. Therefore, the tempering temperature T2 is set to 100° C. or higher and 400° C. or lower. The tempering temperature T2 is preferably set to 150° C. or higher. The tempering temperature T2 is set to The tempering temperature is preferably 350° C. or less. The tempering temperature here is the holding temperature in the tempering process. The tempering temperature may be constant during holding. The tempering temperature is preferably 100° C. or less. As long as the temperature is in the range of from 400° C. to 400° C. and the temperature fluctuation is within ±10° C. of the set temperature, the temperature does not have to be constant during the holding period.

[Tempering time t2: 10 seconds or more and 10,000 seconds or less]
As described above, tempered martensite is produced by tempering martensite through a tempering process. If the tempering time t2 is less than 10 seconds, the martensite is not sufficiently tempered, and the quenching In the case of such a martensite-based structure as quenched, excellent toughness after paint baking cannot be obtained. On the other hand, when the tempering time t2 exceeds 10,000 seconds, the tempering of martensite The tempering process proceeds excessively, making it difficult to achieve a TS of 1320 MPa or more. Therefore, the tempering time t2 is set to 10 seconds or more and 10,000 seconds or less. The tempering time t2 is preferably set to 50 seconds or more. The tempering time t2 is preferably 5000 seconds or less. Note that the tempering time t2 here refers to the holding time at the tempering temperature T2.

There are no particular regulations regarding the cooling after tempering. For example, cooling may be performed by any method according to conventional methods. The end temperature of cooling after tempering may be, for example, about room temperature. In addition, the base steel sheet may be plated between the tempering process and the straightening process described below. The plating process is described below.

Straightening Step Next, the raw steel sheet is straightened by levelling. At this time, it is extremely important in the manufacturing method of the steel sheet according to the embodiment of the present invention that the following conditions are satisfied.
Straightening start temperature: 100℃ or less,
Inlet intermesh push-in amount: 4.0 mm or more and 10.0 mm or less,
Amount of intermesh pushed in at the outlet: 1.0 mm or more and 10.0 mm or less,
Inlet tension: 20 MPa or more and 500 MPa or less,
Output tension: 25MPa or more and 550MPa or less

[Straightening start temperature: 100℃ or less]
When the straightening start temperature exceeds 100°C, the steel sheet becomes soft. This changes the amount of strain introduced into the surface layer and center of the steel sheet by the leveling process, and the ratio KAM(S)/KAM(C) becomes 1. Therefore, the impact properties after painting and baking are deteriorated. Therefore, the straightening start temperature is set to 100°C or less. The straightening start temperature is preferably set to 80°C or less. The lower limit of the straightening start temperature is particularly Not specified. For example, the straightening start temperature is preferably -10°C or higher.

[Inlet intermesh push-in amount: 4.0 mm or more and 10.0 mm or less]
If the inlet intermesh push-in amount is less than 4.0 mm, the processing amount will be insufficient. As a result, KAM(S)/KAM(C) will be 1.00 or less, and the impact properties after paint baking will deteriorate. The upper limit of the inlet intermesh push-in amount is set to 10.0 mm or less due to production technology constraints. Therefore, the inlet intermesh push-in amount is set to 4.0 mm or more and 10.0 mm or less. The inlet intermesh push-in amount is preferably 5.0 mm or more.

[Outlet intermesh push-in amount: 1.0 mm or more and 10.0 mm or less]
If the amount of push-in of the exit intermesh is less than 1.0 mm, the amount of processing will be insufficient. As a result, KAM(S)/KAM(C) will be 1.00 or less, and the impact characteristics after baking will deteriorate. Note that the upper limit of the amount of push-in of the exit intermesh is set to 10.0 mm or less due to production technology constraints. Therefore, the amount of push-in of the exit intermesh is set to 1.0 mm or more and 10.0 mm or less.

Here, the inlet intermesh push-in amount is the push-in amount of the second roll from the inlet side in the leveller process (roll 2 in Fig. 1) against the steel sheet plane (the steel sheet surface on the side where roll 2 is located), as shown in Fig. 1. The outlet intermesh push-in amount is the push-in amount of the second roll from the outlet side in the leveller process (roll 8 in Fig. 1) against the steel sheet plane (the steel sheet surface on the side where roll 8 is located). The number of rolls in the leveller process (leveller) is not particularly limited, but for example, the number of rolls is preferably 5 or more.

[Inlet tension: 20 MPa or more and 500 MPa or less]
If the entry tension is less than 20 MPa, the processing amount will be insufficient. As a result, KAM(S)/KAM(C) will be 1.00 or less, and the impact properties after painting and baking will deteriorate. Due to restrictions on production technology, the entry side tension is set to 500 MPa or less. Therefore, the entry side tension is set to 20 MPa or more and 500 MPa or less. The entry side tension is preferably 100 MPa or more.

[Output tension: 25 MPa or more and 550 MPa or less]
Due to equipment restrictions in the leveller process, the tension on the exit side is higher than the tension on the entry side. If the intermesh push-in amount on the exit side is less than 25 MPa, the processing amount is insufficient. Therefore, KAM(S) The KAM(C) is 1.00 or less, and the crashworthiness after painting and baking deteriorates. The upper limit of the tension on the exit side is 550 MPa due to the constraints of production technology. Therefore, the tension on the exit side is 25 MPa or more. The exit tension is preferably 100 MPa or more.

- Plating process The steel sheet may be optionally plated. The plating process is not particularly limited. Examples of the plating process include galvanizing processes such as hot-dip galvanizing, alloyed hot-dip galvanizing, and electric galvanizing. Examples of plating processes other than galvanizing include aluminum plating and alloy plating. Examples of alloy plating processes include hot-dip zinc-aluminum-magnesium alloy plating and Zn-Ni electric alloy plating. All of the treatment conditions may be in accordance with conventional methods. As described above, the plating process is preferably performed between the first cooling step and the second cooling step, or between the tempering step and the straightening step. For example, the hot-dip galvanizing process and the alloyed hot-dip galvanizing process are preferably performed between the first cooling step and the second cooling step. Furthermore, the electric galvanizing process and the Zn-Ni electric alloy plating process are preferably performed between the tempering step and the straightening step.

In the case of hot-dip galvanizing and alloyed hot-dip galvanizing, from the viewpoint of productivity, it is preferable to carry out a series of processes such as the above-mentioned heating process, annealing process, and plating process in a continuous galvanizing line (CGL). After hot-dip galvanizing, wiping is possible to adjust the coating weight.

The conditions other than those mentioned above are not particularly limited and may be the same as usual. According to the method for producing steel sheet according to one embodiment of the present invention described above, a steel sheet having a TS of 1320 MPa or more and excellent stretch flangeability, toughness after paint baking, and impact properties can be obtained. The obtained steel sheet can be suitably used, for example, as a material for automobile parts.

[4] Manufacturing Method of Member Next, a manufacturing method of a member according to one embodiment of the present invention will be described.
A method for manufacturing a component according to one embodiment of the present invention includes a step of subjecting the above-mentioned steel plate to at least one of forming and joining to form a component.
Here, the molding method is not particularly limited, and for example, a general processing method such as press molding can be used. The joining method is also not particularly limited, and for example, general welding such as spot welding, laser welding, and arc welding, rivet joining, crimp joining, etc. The molding conditions and joining conditions are not particularly limited, and may be in accordance with ordinary methods.

Steel having the composition shown in Table 1 (the balance being Fe and unavoidable impurities) was melted in a converter and made into a steel slab by continuous casting. The steel slab was then heated. The steel slab was then hot-rolled to obtain a hot-rolled steel sheet. The hot-rolled steel sheet was then pickled. The hot-rolled steel sheet was then cold-rolled to obtain a cold-rolled steel sheet. In this manner, a base steel sheet was prepared. The prepared base steel sheet was then subjected to a heating process, an annealing process, a bending process, a first cooling process, a second cooling process, a tempering process, and a straightening process under the conditions shown in Table 2 to obtain a steel sheet (thickness: 0.6 to 2.2 mm) as a final product. The first cooling end temperature was 550°C to 300°C in all cases. The second cooling end temperature and the cooling end temperature after tempering were both room temperature. The bending angle in the bending process was 80 to 110° in all cases. In addition, some of the steel sheets (those in the type column of Table 2 as GI, GA, and EG) were subjected to a plating treatment. Among these, those in the type column of Table 2 as GI and GA were subjected to a plating treatment between the first cooling step and the second cooling step. In addition, those in the type column of Table 2 as EG were subjected to a plating treatment between the tempering step and the straightening step. Conditions not specified were in accordance with conventional methods.

Using the steel sheets thus obtained, the area fraction of tempered martensite, the area fraction of retained austenite, the total area fraction of ferrite and bainitic ferrite, the high angle grain boundary density of tempered martensite, and KAM(S)/KAM(C) were determined in the manner described above. The results are shown in Table 3.

In addition, each evaluation was performed according to the following procedures. The evaluation results are shown in Table 3.

(TS Evaluation)
From the obtained steel plate, a JIS No. 5 test piece (gauge length: 50 mm, parallel part width: 25 mm) was taken so that the direction perpendicular to the rolling direction of the steel plate was the longitudinal direction of the test piece. Next, a tensile test according to JIS Z 2241:2022 was performed using the taken test piece, and TS was measured. The crosshead speed was set to 1.67×10 ⁻¹ mm/sec. Then, evaluation was performed according to the following criteria.
Good (passed, excellent): TS is 1320 MPa or more. Poor (failed): TS is less than 1320 MPa.

(Evaluation of stretch flangeability)
The evaluation of the stretch flangeability was performed by a hole expansion test conforming to JIS Z 2256:2020. That is, the obtained steel sheet was sheared to 100 mm x 100 mm, and a test piece was taken. Next, a hole having a diameter of 10 mm was punched in the test piece with a clearance of 12.5%. Next, the test piece was held down with a wrinkle holding force of 9 ton (88.26 kN) using a die with an inner diameter of 75 mm. Next, in that state, a conical punch with an apex angle of 60° was pressed into the hole of the test piece, and the diameter of the hole of the test piece at the crack generation limit (when a crack occurred) was measured. Then, the limit hole expansion ratio λ was calculated by the following formula.
λ (%) = {(D _f - D ₀ )/D ₀ }×100
Where:
_Df : Diameter (mm) of the hole in the test piece at the crack initiation limit (when a crack occurs)
D ₀ : Initial diameter of the hole of the test piece (mm)
It is.
The stretch flangeability was evaluated according to the following criteria.
Good (pass, excellent): λ is 30% or more. Poor (fail): λ is less than 30%.

(Evaluation of toughness after painting and baking)
The obtained steel plates were stacked and fastened with bolts. Then, after confirming that there was no gap between the steel plates, a V-notch with a depth of 2 mm was given to the stacked steel plates to prepare a laminated Charpy test piece (hereinafter, also simply referred to as a test piece). The number of steel plates to be stacked was set to the number that would make the thickness of the test piece closest to 10 mm (if there were two numbers that would make the thickness of the test piece closest to 10 mm, the smaller number would be used). For example, when the thickness of the steel plate was 1.2 mm, eight steel plates were stacked. That is, the thickness of the test piece was 9.6 mm. The test piece was prepared so that the width direction of the steel plate was the longitudinal direction of the test piece. Then, the prepared test piece was subjected to aging treatment at a treatment temperature of 170°C and a treatment time of 20 minutes. Then, a Charpy impact test was performed using the aging-treated test piece in a test temperature range of -120°C to +120°C. A transition curve was obtained from the brittle fracture surface ratio, and the temperature at which the brittle fracture surface ratio was 50% was determined as the brittle-ductile transition temperature. The toughness after paint baking was evaluated according to the following criteria. The other conditions were in accordance with JIS Z 2242:2018.
Excellent (pass, especially excellent): Brittle-ductile transition temperature after aging treatment is -60°C or less. Good (pass, excellent): Brittle-ductile transition temperature after aging treatment is -40°C or less (excluding excellent).
Bad (failed): Brittle-ductile transition temperature after aging is over -40°C

(Impact characteristics after paint baking)
The obtained steel plate was subjected to aging treatment at a treatment temperature of 170°C for a treatment time of 20 minutes. Then, a JIS No. 5 test piece (gauge length: 50 mm, parallel part width: 25 mm) was taken from the aging-treated steel plate so that the direction perpendicular to the rolling direction of the steel plate was the longitudinal direction of the test piece. Then, using the taken test piece, a tensile test according to JIS Z 2241:2022 was performed in the same manner as the above-mentioned evaluation of TS, and TS, YS (yield stress) and fracture stress after aging treatment were measured. Then, the impact properties after baking the paint were evaluated according to the following criteria.
Good (pass, excellent): YR after aging treatment is 0.85 or more and the fracture stress ratio after aging treatment is 0.90 or less. Poor (fail): Does not satisfy at least one of the following: YR after aging treatment is 0.85 or more and the fracture stress ratio after aging treatment is 0.90 or less. The YR and fracture stress ratio after aging treatment are calculated by the following formulas.
[YR after aging treatment] = [YS after aging treatment] / [TS after aging treatment]
[Fracture stress ratio after aging treatment] = [Fracture stress after aging treatment] / [TS after aging treatment]
The breaking stress is the stress at the breaking point in the above tensile test (the stress applied when the test piece breaks).

As shown in Table 3, all of the inventive examples passed the TS, stretch flangeability, and toughness and crash properties after paint baking. In addition, all of the members obtained by forming or joining using the steel plates of the inventive examples had the desired shape without cracking, and all of the TS, stretch flangeability, toughness and crash properties after paint baking met the pass criteria.
On the other hand, the comparative examples failed in at least one of TS, stretch flangeability, and toughness and impact properties after paint baking.

Claims (8)

In mass percent,
C: 0.030% or more and 0.500% 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,
N: 0.0100% or less,
A composition comprising O: 0.0100% or less and Al: 1.000% or less, with the balance being Fe and unavoidable impurities;
Area fraction of tempered martensite: 95% or more,
Area fraction of retained austenite: less than 3%;
Total area fraction of ferrite and bainitic ferrite: less than 5%;
The tempered martensite has a grain boundary density of 20° or more of 1.0 μm/μm2 ^{or more} ;
A structure that satisfies the following formula (1) ,
The density of grain boundaries of 20° or more in the tempered martensite is obtained by dividing the total length of grain boundaries of 20° or more in the tempered martensite by the area of the structure .
KAM(S)/KAM(C) > 1.00 (1)
In the formula,
KAM (S): average KAM value at a depth of 100 μm from the steel sheet surface,
KAM (C): Average KAM value at the center position of the sheet thickness of the steel sheet. The 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,
The steel sheet according to claim 1, containing at least one selected from Hf: 0.10% or less and Bi: 0.200% or less. 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 according to any one of claims 1 to 4. A method for producing a steel sheet according to any one of claims 1 to 4, comprising the steps of:
A preparation step of preparing a base steel sheet having the component composition according to claim 1 or 2;
Next, the base steel sheet is
A heating step of heating to an annealing temperature T1 under the condition of an average heating rate in a temperature range of 700 ° C. to 750 ° C.: 5.0 ° C./s or less;
Next, the base steel sheet is
The annealing temperature T1: 800° C. or higher;
An annealing step of annealing under the condition of an annealing time t1: 10 seconds or more;
Next, the base steel plate is
A bending process in which bending is performed at least once using a roll with a radius of 800 mm or less in a temperature range of the annealing temperature T1 to 700 ° C.;
Next, the base steel sheet is
A first cooling step of cooling to a first cooling end temperature under the condition of an average cooling rate of 10°C/s or more in a temperature range of 700°C to 550°C;
Next, the base steel sheet is
Average cooling rate in the temperature range of 300°C to 100°C: 300°C/s or more;
A second cooling step of cooling the base steel sheet to a second cooling end temperature under a condition of a tension of 5 MPa or more applied to the base steel sheet in a temperature range of 300° C. to 100° C.;
Next, the base steel sheet is
Tempering temperature T2: 100°C or more and 400°C or less,
A tempering process in which tempering is performed under the condition of a tempering time t2 of 10 seconds or more and 10,000 seconds or less;
Next, the base steel plate is
Straightening start temperature: 100℃ or less,
Inlet intermesh push-in amount: 4.0 mm or more and 10.0 mm or less,
Amount of intermesh pushed in at the outlet: 1.0 mm or more and 10.0 mm or less,
Inlet tension: 20 MPa or more and 500 MPa or less,
A straightening process in which straightening is performed by leveling under the condition of an outlet tension of 25 MPa or more and 550 MPa or less;
A manufacturing method for steel plates that provides the following: The method for manufacturing steel sheet according to claim 6, further comprising a plating process for plating the base steel sheet between the first cooling process and the second cooling process, or between the tempering process and the straightening process. A method for manufacturing a component, comprising the steps of subjecting the steel plate according to any one of claims 1 to 4 to at least one of forming and joining processes to produce a component.

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