EP4656756A1

EP4656756A1 - Steel sheet, member, and methods for producing same

Info

Publication number: EP4656756A1
Application number: EP23930853.9A
Authority: EP
Inventors: Junya TOBATA; Hideyuki Kimura
Original assignee: JFE Steel Corp
Current assignee: JFE Steel Corp
Priority date: 2023-03-27
Filing date: 2023-11-21
Publication date: 2025-12-03
Also published as: CN121079445A; MX2025009603A; WO2024202227A1; JP7597273B1; JPWO2024202227A1; EP4656756A4; KR20250157552A

Abstract

Provided is a steel sheet having a TS of 1180 MPa or more, and also excellent dimensional accuracy and shear angle range. The steel sheet has a defined chemical composition and a microstructure where an area fraction of tempered martensite is 83 % or more, an area fraction of retained austenite is less than 3 %, a total area fraction of ferrite and bainitic ferrite is 5 % or more and less than 15 %, and a prior y grain boundary occupancy rate of 20 % or more.

Description

TECHNICAL FIELD

The present disclosure relates to a steel sheet, a member using the steel sheet as material, and methods of producing same.

BACKGROUND

Higher strength steel sheets as material for automotive parts are being developed to both decrease CO₂ emissions by decreasing automobile body weight and improve crashworthiness. Further, new laws and regulations are being introduced one after another. As a result, in the main structural parts of automobiles (hereinafter also referred to as automotive frame parts), the application of steel sheets that have a tensile strength (hereinafter also referred to as TS) of 1180 MPa or more is increasing.
Steel sheets used as material for automotive parts are also required to have excellent dimensional accuracy when formed into parts (hereinafter also referred to simply as dimensional accuracy). For example, in automotive frame parts such as bumpers, it is possible to suppress springback and improve dimensional accuracy by controlling the yield ratio (hereinafter also referred to as YR) of the steel to a certain range.
As a steel sheet used as material for an automotive part, for example, Patent Literature (PTL) 1 describes:

"a steel sheet comprising: a chemical composition containing, in mass%,
C: 0.09 % or more and 0.37 % or less,
Si: more than 0.70 % and 2.00 % or less,
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,
Al: 0.010 % or more and 1.000 % or less, and
N: 0.0100 % or less, with the balance being Fe and inevitable impurity; and
a steel microstructure that has an area fraction of martensite with a carbon concentration greater than 0.7 × [%C] and less than 1.5 × [%C] of 55 % or more,
an area fraction of tempered martensite with a carbon concentration of 0.7 × [%C] or less of 5 % or more and 40 % or less,
a ratio of carbon concentration in retained austenite to a volume fraction of retained austenite of 0.05 or more and 0.40 or less, and
an average grain size of the martensite and the tempered martensite of 5.3 µm or less, wherein
the steel microstructure further has a surface layer softening thickness of 10 µm or more and 100 µm or less, and
tensile strength is 1180 MPa or more,
where [%C] represents content in mass% of the component element C in the steel."

PTL 2 describes:

"a cold-rolled steel sheet comprising: a chemical composition containing, in mass%, C: 0.15 % or more and 0.40 % or less, Si: 1.5 % or less, Mn: 0.9 % to 1.7 %, P: 0.03 % or less, S: less than 0.0020 %, sol.Al: 0.2 % or less, N: less than 0.0055 %, and O: 0.0025 % or less, satisfying the relationship of the following expression (1), with the balance being Fe and inevitable impurity; and
a microstructure wherein an area fraction of tempered martensite and bainite relative to the entire microstructure of 95 % or more and 100 % or less in total,
inclusions are composed of one or more inclusion particles having a major axis of 0.3 µm or more, extended and/or distributed in a dotted row pattern in the rolling direction, and when an inclusion is composed of two or more particles, the distance between the inclusion particles is 30 µm or less, where the number of inclusions having a total length of more than 120 µm in the rolling direction is 0.8/mm² or less,
a number of carbides mainly composed of Fe that have an aspect ratio of 2.5 or less with a major axis of 0.20 µm or more and 2 µm or less is 3500/mm² or less,
a number of carbides having a diameter of 10 nm to 50 nm distributed in the tempered martensite and/or the bainite is 0.7 × 107/mm² or more, and
average grain size of prior y grains is 18 µm or less, wherein
the cold-rolled steel sheet has a thickness of 0.5 mm to 2.6 mm, a tensile strength of 1320 MPa or more, and excellent delayed fracture resistance. $5 [% S] + [% N] < 0.0115$

Here, [%S] and [%N] indicate S and N content, in mass%, in the steel, respectively.
Here, excellent delayed fracture resistance means that a time to delayed fracture when cold press forming involving shearing and punching is:

more than 200 h when tensile strength is 1320 MPa or more and less than 1530 MPa,
24 h or more when tensile strength is 1530 MPa or more and less than 1550 MPa,
12 h or more when tensile strength is 1550 MPa or more and less than 1570 MPa,
9 h or more when tensile strength is 1570 MPa or more and less than 1610 MPa,
1.0 h or more when tensile strength is 1610 MPa or more and less than 1960 MPa, and
0.2 h or more when tensile strength is 1960 MPa or more.

The time to delayed fracture is the time from the start of immersion to the beginning of the formation of microcracks when immersed in hydrochloric acid (hydrogen chloride aqueous solution) with a pH of 1 at an aqueous solution temperature of 20 °C."
PTL 3 describes:

"a steel sheet comprising: a chemical composition containing, in mass%,
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 3.5 % or less,
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 both of Nb and Ti for a total of 0.010 % or more and 0.080 % or less, with the balance being Fe and inevitable impurity; and
a steel microstructure that has a martensite area fraction of 70 % or more, a bainite area fraction of 30 % or less, and a total area fraction of ferrite and retained austenite of 10 % or less, wherein
a number density of carbides that have a major axis length of 0.5 µm or more at a 1/4 sheet thickness position of the steel sheet is 60,000/mm² or less,
a number density of inclusion particles that have a circle equivalent diameter of 4.0 µm or more in a range from 1/4 to 3/4 sheet thickness of the steel sheet is 10/mm² or more and 30/mm² or less,
a number density of inclusion particles that have a circle equivalent diameter of 4.0 µm or more in a range from a surface to 1/4 sheet thickness of the steel sheet is 27/mm² or less, and
tensile strength is 1310 MPa or more."

CITATION LIST

Patent Literature

PTL 1: JP 6747612 B2
PTL 2: JP 6112261 B2
PTL 3: JP 7001197 B2

SUMMARY

(Technical Problem)

Automotive parts, particularly automotive frame parts, have many end faces formed by shearing (hereinafter also referred to as sheared end faces). Therefore, steel sheets used as material for automotive parts are also required to have excellent delayed fracture resistance after shearing. Delayed fracture is a phenomenon that leads to failure as follows. When a part is subjected to high stress due to forming or the like and is placed in a hydrogen entry environment, hydrogen enters the part. Hydrogen that enters into a part causes a decrease in interatomic bonding strength and causes localized deformation. This causes microcracks to form in the part, which eventually leads to failure when the microcracks propagate.
Delayed fracture resistance is affected by the morphology of sheared end faces. Further, the shape of a sheared end face is affected by a shear angle during shearing (hereinafter also referred to simply as the shear angle). That is, the delayed fracture resistance is affected by the shear angle. For example, even when parts are made from the same steel sheet, when the shear angle is outside an appropriate range, delayed fracture resistance will decrease. The shear angle is an angle between upper and lower blades used in shearing (blade angle).
Steel sheets used as material for automotive parts are sheared at various shear angles depending on the required dimensional accuracy, productivity, equipment constraints, and other factors. For this reason, it is also required that an appropriate range of the shear angle versus delayed fracture (that is, the range of the shear angle at which excellent delayed fracture resistance of the steel sheet after shearing is obtainable, hereinafter also referred to as shear angle range) is wide, that is, that the shear angle range of the steel sheet is excellent.
However, regarding the steel sheets described in PTL 1 to 3, no consideration is given to shear angle range. Accordingly, there is currently a demand for the development of a steel sheet having a TS of 1180 MPa or more that has excellent dimensional accuracy and shear angle range.
In view of the above circumstances, it would be helpful to provide a steel sheet that has a TS of 1180 MPa or more and excellent dimensional accuracy and shear angle range, together with an advantageous method of producing the steel sheet.
Further, it would be helpful to provide a member using the steel sheet as a material and a method of producing the member.
Here, TS is measured by a tensile test in accordance with JIS Z 2241:2022.
Excellent dimensional accuracy means that the YR is 65 % or more and 85 % or less. Here, YR is calculated by the following expression. $YR = 100 \times YS / TS$
In this expression, YS is yield stress, which, like TS, is measured by a tensile test in accordance with JIS Z 2241:2022.
Excellent shear angle range means that an appropriate range of the shear angle at which delayed fracture does not occur when load stress is 1000 MPa is 0° to 0.5° or more.
Details of measurement methods are described in the EXAMPLES section below.

(Solution to Problem)

The inventors conducted intensive studies to achieve the above, and made the following discoveries:

(A) To obtain a TS of 1180 MPa or more, it is important that the area fraction of tempered martensite is 83 % or more and the total area fraction of ferrite and bainitic ferrite is less than 15 %. This allows for obtaining a TS of 1180 MPa or more while securing defined required properties.
(B) To obtain excellent dimensional accuracy, it is important that the total area fraction of ferrite and bainitic ferrite is 5 % or more. This allows for obtaining excellent dimensional accuracy while securing defined required properties.
(C) To obtain an excellent shear angle range, it is important that an area fraction of retained austenite is less than 3 % and an occupancy rate of prior austenite grain boundaries by ferrite and bainitic ferrite is 20 % or more. This allows for obtaining excellent shear angle range while securing defined required properties.

The present disclosure is based on these discoveries and further studies.
Primary features of the present disclosure are as follows.

1. A steel sheet comprising: a chemical composition containing (consisting of), in mass%,
- 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,
- O: 0.0100 % or less, and
- Al: 1.000 % or less,
- with the balance being Fe and inevitable impurity; and a steel microstructure wherein
- area fraction of tempered martensite is 83 % or more,
- area fraction of retained austenite is less than 3 %,
- total area fraction of ferrite and bainitic ferrite is 5 % or more and less than 15 %, and
- occupancy rate of prior austenite grain boundaries by the ferrite and the bainitic ferrite is 20 % or more.
2. The steel sheet according to 1, above, wherein the chemical composition further contains, in mass%, at least one selected from the group consisting of
- 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.
3. The steel sheet according to 1 or 2, above, further comprising a coated or plated layer on a surface.
4. A member formed using the steel sheet according to any one of 1 to 3, above.
5. A method of producing the steel sheet according to any one of 1 to 3, above, the method comprising:
- a preparation process of preparing a blank sheet having the chemical composition according to 1 or 2, above;
- a heating process of heating the blank sheet under a set of conditions including an average heating rate in a temperature range from 700 °C to 750 °C of 5.0 °C/s or less, and
- a maximum arrival temperature T1 of 800 °C or more and 900 °C or less;
- a first cooling process of cooling the blank sheet under a set of conditions including an average cooling rate in a temperature range from the maximum arrival temperature T1 to an intermediate holding temperature T2 of 0.10 °C/s or more and 5.00 °C/s or less;
- an intermediate holding process of holding the blank sheet under a set of conditions including the intermediate holding temperature T2 being 600 °C or more and 750 °C or less,
- an intermediate holding time t2 of 1.0 s or longer and 2000.0 s or shorter, and
- a tension applied to the blank sheet of 5 MPa or more;
- a second cooling process of cooling the blank sheet under a set of conditions including an average cooling rate in a temperature range from 300 °C to 100 °C of 300 °C/s or more,
- to a second cooling end temperature; and
- a tempering process of tempering the blank sheet under a set of conditions including a tempering temperature T3 of 100 °C or more and 400 °C or less, and
- a tempering time t3 of 10 s or longer and 10,000 s or shorter.
6. The method of producing a steel sheet according to 5, above, further comprising a coating or plating process of applying a coating or plating treatment to the blank sheet between the intermediate holding process and the second cooling process, or after the tempering process.
7. A method of producing a member, wherein the steel sheet according to any one of 1 to 3, above, is subjected to at least one of a forming process or a joining process to produce the member.

(Advantageous Effect)

According to the present disclosure, a steel sheet having a TS of 1180 MPa or more and excellent dimensional accuracy and shear angle range is obtainable. Further, the steel sheet of the present disclosure can be applied to a wider range of automotive part materials, which can further improve fuel efficiency by decreasing automotive body weight, thereby greatly contributing to decreasing CO₂ emissions. Therefore, the industrial utility value is extremely high.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:
FIG. 1 is a schematic diagram for explaining the definition of prior y grain boundary occupancy rate.

DETAILED DESCRIPTION

The following describes embodiments of the present disclosure.

[1] Steel sheet

First, the chemical composition of a steel sheet according to an embodiment of the present disclosure is described. Hereinafter, although the unit in all chemical compositions is "mass%", this may be indicated simply as "%", unless otherwise specified.

[C: 0.030 % or more and 0.500 % or less]

C is an important basic component of steel. In particular, in the steel sheet according to an embodiment of the present disclosure, C is an important element that affects the area fraction of tempered martensite. When C content is less than 0.030 %, the area fraction of tempered martensite decreases, and achieving a TS of 1180 MPa or more becomes difficult. On the other hand, when the C content exceeds 0.500 %, tempered martensite becomes brittle, and achieving excellent shear angle range becomes difficult. The C content is therefore 0.030 % or more and 0.500 % or less. The C content is preferably 0.050 % or more. The C content is more preferably 0.100 % or more. The C content is preferably 0.400 % or less. The C content is more preferably 0.350 % or less.

[Si: 0.010 % or more and 2.500 % or less]

Si is an important basic component of steel. In particular, in the steel sheet according to an embodiment of the present disclosure, Si suppresses carbide formation during annealing and promotes formation of retained austenite. That is, Si is an important element that affects the area fraction of retained austenite. When Si content is less than 0.010 %, achieving a TS of 1180 MPa or more becomes difficult. On the other hand, when the Si content exceeds 2.500 %, retained austenite increases excessively, and achieving excellent shear angle range becomes difficult. The Si content is therefore 0.010 % or more and 2.500 % or less. The Si content is preferably 0.050 % or more. The Si content is more preferably 0.100 % or more. The Si content is preferably 2.000 % or less. The Si content is more preferably 1.200 % or less.

[Mn: 0.10 % or more and 5.00 % or less]

Mn is an important basic component of steel. In particular, in the steel sheet according to an embodiment of the present disclosure, Mn is an important element that affects the area fraction of tempered martensite and shear angle range. When Mn content is less than 0.10 %, the area fraction of tempered martensite decreases, and achieving a TS of 1180 MPa or more becomes difficult. On the other hand, when the Mn content exceeds 5.00 %, tempered martensite becomes embrittled, and achieving excellent shear angle range becomes difficult. The Mn content is therefore 0.10 % or more and 5.00 % or less. The Mn content is preferably 0.50 % or more. The Mn content is more preferably 0.80 % or more. The Mn content is preferably 4.50 % or less. The Mn content is more preferably 4.00 % or less.

[P: 0.100 % or less]

P segregates at prior austenite grain boundaries, embrittling grain boundaries and becoming the initiation point of delayed fracture. Therefore, when P content is excessive, achieving an excellent shear angle range becomes difficult. The P content is therefore 0.100 % or less. The P content is preferably 0.070 % or less. A lower limit of the P content is not particularly specified. However, P is a solid-solution-strengthening element and can increase steel sheet strength. The P content is therefore preferably 0.001 % or more.

[S: 0.0200 % or less]

S exists as sulfides and becomes initiation points of delayed fracture. Therefore, when S content is excessive, achieving an excellent shear angle range becomes difficult. The S content is therefore 0.0200 % or less. The S content is preferably 0.0050 % or less. A lower limit of the S content is not particularly specified. However, in view of production technology constraints, the S content is preferably 0.0001 % or more.

[N: 0.0100 % or less]

N exists as nitrides and becomes initiation points of delayed fracture. Therefore, when N content is excessive, achieving an excellent shear angle range becomes difficult. The N content is therefore 0.0100 % or less. The N content is preferably 0.0050 % or less. A lower limit of the N content is not particularly specified. However, in view of production technology constraints, the N content is preferably 0.0001 % or more.

[O: 0.0100 % or less]

O exists as oxides and becomes initiation points of delayed fracture. Therefore, when O content is excessive, achieving an excellent shear angle range becomes difficult. The O content is therefore 0.0100 % or less. The O content is preferably 0.0050 % or less. A lower limit of the O content is not particularly specified. However, in view of production technology constraints, the O content is preferably 0.0001 % or more.

[Al: 1.000 % or less]

Al exists as oxides and becomes initiation points of delayed fracture. Therefore, when Al content is excessive, achieving an excellent shear angle range becomes difficult. The Al content is therefore 1.000 % or less. The Al content is preferably 0.500 % or less. A lower limit of the Al content is not particularly specified. However, in view of production technology constraints, the Al content is preferably 0.001 % or more.
Basic chemical composition of the steel sheet according to an embodiment of the present disclosure is described above. The steel sheet according to an embodiment of the present disclosure has a chemical composition including the basic composition above, with the balance being Fe (iron) and inevitable impurity. Here, the steel sheet according to an embodiment of the present disclosure preferably has a chemical composition consisting of the basic composition above, with the balance being Fe and inevitable impurity. In addition to the basic components described above, the steel sheet according to an embodiment of the present disclosure may contain one or more elements selected from the following as optional additive elements, either alone or in combination.

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 effects of the present disclosure are obtainable whenever content is equal to or less than the upper limit indicated above, and therefore there is no particular lower limit for the above optional additive elements. Further, when any of the above optional additional elements are included below a preferred lower limit described below, such an element is included as an inevitable impurity.

[Ti: 0.200 % or less, Nb: 0.200 % or less, V: 0.200 % or less]

When each of Ti, Nb, and V are 0.200 % or less, these elements do not cause large amounts of coarse precipitates or inclusions to form or become initiation points of delayed fracture. Therefore, this does not lead to a decrease in shear angle range. Therefore, when Ti, Nb, and V are included, the content of each is preferably 0.200 % or less. The content of each of Ti, Nb, and V is respectively more preferably 0.100 % or less. A lower limit of the content of each of Ti, Nb, and V is not particularly specified. However, Ti, Nb, and V increase the strength of steel sheets by forming fine carbides, nitrides, or carbonitrides during hot rolling or annealing. Therefore, the content of each of Ti, Nb, and V is respectively preferably 0.001 % or more.

[Ta: 0.10 % or less, W: 0.10 % or less]

When each of Ta and W are 0.10 % or less, these elements do not cause large amounts of coarse precipitates or inclusions to form or become initiation points of delayed fracture. Therefore, this does not lead to a decrease in shear angle range. Therefore, when Ta and W are included, the content of each is preferably 0.10 % or less. The content of each of Ta and W is respectively more preferably 0.08 % or less. A lower limit of the content of each of Ta and W is not particularly specified. However, Ta and W increase the strength of steel sheets by forming fine carbides, nitrides or carbonitrides during hot rolling or annealing. Therefore, the content of each of Ta and W is respectively preferably 0.01 % or more.

[B: 0.0100 % or less]

When B content is 0.0100 % or less, this element does not cause cracks inside the steel sheet during casting or hot rolling or become an initiation point of delayed fracture. Therefore, this does not lead to a decrease in shear angle range. Therefore, when B is included, the B content is preferably 0.0100 % or less. The B content is more preferably 0.0080 % or less. A lower limit of the B content is not particularly specified. However, B is an element that segregates at an austenite grain boundary during annealing and improves hardenability. The B content is therefore preferably 0.0003 % or more.

[Cr: 1.00 % or less, Mo: 1.00 % or less, Ni: 1.00 % or less]

When each of Cr, Mo, and Ni are 1.00 % or less, these elements do not cause large amounts of coarse precipitates or inclusions to form or become initiation points of delayed fracture. Therefore, this does not lead to a decrease in shear angle range. Therefore, when Cr, Mo, and Ni are included, the content of each is preferably 1.00 % or less. The content of each of Cr, Mo, and Ni is respectively more preferably 0.80 % or less. A lower limit of the content of each of Cr, Mo, and Ni is not particularly specified. However, Cr, Mo, and Ni are elements that improve hardenability. Therefore, the content of each of Cr, Mo, and Ni is respectively preferably 0.01 % or more.

[Co: 0.010 % or less]

When Co is 0.010 % or less, this element does not cause large amounts of coarse precipitates or inclusions to form or become an initiation point of delayed fracture. Therefore, this does not lead to a decrease in shear angle range. Therefore, when Co is included, the Co content is preferably 0.010 % or less. The Co content is more preferably 0.008 % or less. A lower limit of the Co content is not particularly specified. However, Co is an element that improves hardenability. The Co content is therefore preferably 0.001 % or more.

[Cu: 1.00 % or less]

When Cu is 1.00 % or less, this element does not cause large amounts of coarse precipitates or inclusions to form or become an initiation point of delayed fracture. Therefore, this does not lead to a decrease in shear angle range. Therefore, when Cu is included, the Cu content is preferably 1.00 % or less. The Cu content is more preferably 0.80 % or less. A lower limit of the Cu content is not particularly specified. However, Cu is an element that improves hardenability. The Cu content is therefore preferably 0.01 % or more.

[Sn: 0.200 % or less]

When Sn content is 0.200 % or less, this element does not cause cracks inside the steel sheet during casting or hot rolling or become an initiation point of delayed fracture. Therefore, this does not lead to a decrease in shear angle range. Therefore, when Sn is included, the Sn content is preferably 0.200 % or less. The Sn content is more preferably 0.100 % or less. A lower limit of the Sn content is not particularly specified. However, Sn is an element that improves hardenability and is generally also an element that improves corrosion resistance. The Sn content is therefore preferably 0.001 % or more.

[Sb: 0.200 % or less]

When Sb is 0.200 % or less, this element does not cause large amounts of coarse precipitates or inclusions to form or become an initiation point of delayed fracture. Therefore, this does not lead to a decrease in shear angle range. Therefore, when Sb is included, the Sb content is preferably 0.200 % or less. The Sb content is more preferably 0.100 % or less. A lower limit of the Sb content is not particularly specified. However, Sb is an element that controls surface layer softening thickness and allows strength adjustment. The Sb content is therefore preferably 0.001 % or more.

[Ca: 0.0100 % or less, Mg: 0.0100 % or less, REM: 0.0100 % or less]

When each of Ca, Mg, and REM are 0.0100 % or less, these elements do not cause large amounts of coarse precipitates or inclusions to form or become initiation points of delayed fracture. Therefore, this does not lead to a decrease in shear angle range. Therefore, when Ca, Mg, and REM are included, the content of each is preferably 0.0100 % or less. The content of each of Ca, Mg, and REM is respectively more preferably 0.0050 % or less. A lower limit of the content of each of Ca, Mg, and REM is not particularly specified. However, Ca, Mg, and REM are elements that spheroidize the shape of nitrides and sulfides and improve steel sheet ultimate deformability. Therefore, the content of each of Ca, Mg, and REM is respectively preferably 0.0005 % or more.

[Zr: 0.100 % or less, Te: 0.100 % or less]

When each of Zr and Te are 0.100 % or less, these elements do not cause large amounts of coarse precipitates or inclusions to form or become initiation points of delayed fracture. Therefore, this does not lead to a decrease in shear angle range. Therefore, when Zr and Te are included, the content of each is preferably 0.100 % or less. The content of each of Zr and Te is respectively more preferably 0.080 % or less. A lower limit of the content of each of Zr and Te is not particularly specified. However, Zr and Te are elements that spheroidize the shape of nitrides and sulfides and improve steel sheet ultimate deformability. Therefore, the content of each of Zr and Te is respectively preferably 0.001 % or more.

[Hf: 0.10 % or less]

When Hf is 0.10 % or less, this element does not cause large amounts of coarse precipitates or inclusions to form or become an initiation point of delayed fracture. Therefore, this does not lead to a decrease in shear angle range. Therefore, when Hf is included, the content is 0.10 % or less. The Hf content is more preferably 0.08 % or less. A lower limit of the Hf content is not particularly specified. However, Hf is an element that spheroidizes the shape of nitrides and sulfides and improves steel sheet ultimate deformability. The Hf content is therefore preferably 0.01 % or more.

[Bi: 0.200 % or less]

When Bi is 0.200 % or less, this element does not cause large amounts of coarse precipitates or inclusions to form or become an initiation point of delayed fracture. Therefore, this does not lead to a decrease in shear angle range. Therefore, when Bi is included, the content is 0.200 % or less. The Bi content is more preferably 0.100 % or less. A lower limit of the Bi content is not particularly specified. However, Bi is an element that reduces segregation. The Bi content is therefore preferably 0.001 % or more.
Elements other than those described above are Fe and inevitable impurity. Examples of inevitable impurity include Zn, Pb, As, Ge, Sr, and Cs. Such inevitable impurity is allowed to be included as long as a total amount is 0.100 % or less.
The following describes the microstructure of the steel sheet according to an embodiment of the present disclosure.
The microstructure of the steel sheet according to an embodiment of the present disclosure satisfies the following conditions:

area fraction of tempered martensite is 83 % or more,
area fraction of retained austenite is less than 3 %,
total area fraction of ferrite and bainitic ferrite is 5 % or more and less than 15 %, and

This is a microstructure in which the occupancy rate of prior austenite grain boundaries by ferrite and bainitic ferrite is 20 % or more.
The reasons for each of these limitations are described below. The area fraction of each phase is the area ratio occupied by each phase relative to the entire microstructure.

[Area fraction of tempered martensite: 83 % or more]

In the steel sheet according to an embodiment of the present disclosure, it is extremely important that the area fraction of tempered martensite is 83 % or more. That is, by making tempered martensite the main phase, in particular by making the area fraction 83 % or more, a TS of 1180 MPa or more is possible to achieve. The area fraction of tempered martensite is therefore 83 % or more. The area fraction of tempered martensite is preferably 85 % or more. The area fraction of tempered martensite is more preferably 87 % or more. An upper limit of the area fraction of tempered martensite is not specifically defined. The area fraction of tempered martensite is, for example, preferably less than 95 %. The area fraction of tempered martensite is more preferably 94 % or less. The area fraction of tempered martensite is even more preferably 93 % or less.

[Area fraction of retained austenite: less than 3 %]

In the steel sheet according to an embodiment of the present disclosure, it is extremely important that the area fraction of retained austenite is less than 3 %. That is, when the area fraction of retained austenite is 3 % or more, achieving excellent shear angle range becomes difficult. One of the causes of decreased shear angle range is that retained austenite transforms into deformation-induced martensite during shearing, resulting in high-hardness martensite, which becomes an initiation point of a fracture. The area fraction of retained austenite is therefore less than 3 %. The area fraction of retained austenite is preferably 1 % or less. A lower limit of the area fraction of retained austenite is not specifically defined. The area fraction of retained austenite may be 0 %.

[Total area fraction of ferrite and bainitic ferrite: 5 % or more and less than 15 %]

In the steel sheet according to an embodiment of the present disclosure, it is extremely important that the total area fraction of ferrite and bainitic ferrite is more than 5 % and less than 15 %. That is, when the total area fraction of ferrite and bainitic ferrite is 15 % or more, achieving a TS of 1180 MPa or more becomes difficult. On the other hand, when the total area fraction of ferrite and bainitic ferrite is less than 5 %, achieving excellent dimensional accuracy becomes difficult. Therefore, the total area fraction of ferrite and bainitic ferrite is 5 % or more and less than 15 %. The total area fraction of ferrite and bainitic ferrite is preferably 6 % or more. The total area fraction of ferrite and bainitic ferrite is more preferably 7 % or more. The total area fraction of ferrite and bainitic ferrite is preferably 14 % or less. The total area fraction of ferrite and bainitic ferrite is more preferably 13 % or less. Ferrite and bainitic ferrite may be included individually, or both may be included.
The area fraction of residual microstructure other than described above is preferably 5 % or less. Examples of residual microstructure include pearlite, fresh martensite, and acicular ferrite. These residual microstructures may be included as long as the content is 5 % or less, as they do not affect the properties. The area fraction of the residual microstructure may be 0 %.
Here, the area fraction of tempered martensite, as well as the total area fraction of ferrite and bainitic ferrite, is measured, for example, as follows.
A sample is cut from the steel sheet such that a thickness cross-section parallel to the rolling direction of the steel sheet (L-section) becomes an observation plane. The observation plane of the sample is then polished. The observation plane of the sample is then corroded with 1 vol% nital to reveal the microstructure. Then, a 1/4 sheet thickness position of the steel sheet (a position corresponding to 1/4 of the sheet thickness in the depth direction from a steel sheet surface) is observed at 2000× magnification by SEM for ten fields of view. In the observation images, tempered martensite has fine irregularities in the microstructure and contains carbides in the microstructure. Further, ferrite and bainitic ferrite have a flat microstructure in recessed portions and do not contain carbides. Then, for each field of view, the areas occupied by tempered martensite, as well as ferrite and bainitic ferrite, are determined. Next, for each field of view, the area occupied by tempered martensite, and the area occupied by ferrite and bainitic ferrite, are each divided by the total area of the observed field of view and multiplied by 100. Then, the average values of these are taken as the area fraction of tempered martensite and the total area fraction of ferrite and bainitic ferrite, respectively.
The microstructure of steel sheets is normally approximately vertically symmetrical in the thickness direction. Therefore, any one surface of the steel sheet (front or back) can be set as an initiation point of a thickness position (sheet thickness 0 position), such as the 1/4 sheet thickness position or a depth of 100 µm from the steel sheet surface.
Further, the area fraction of retained austenite is measured as follows.
That is, the steel sheet is mechanically ground to a depth of 1/4 - 0.1 mm so that the 1/4 sheet thickness position of the steel sheet becomes the observation position, and then further polished by 0.1 mm by chemical polishing. Using the polished surface as the observation plane, an integrated intensity of the diffraction peaks of bcc iron {200}, {211}, and {220} is compared to that of fcc iron (austenite) {200}, {220}, and {311} using Co Kα radiation with an X-ray diffractometer. A volume fraction of retained austenite is then calculated from the ratio of the integrated intensity of each plane. Then, assuming that the retained austenite is uniform in three dimensions, the volume fraction of the retained austenite is taken as the area fraction of retained austenite.
Further, the area fraction of the residual microstructure 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 from 100 %. [Area fraction of residual microstructure (%)] = 100 - [area fraction of tempered martensite (%)] - [total area fraction of ferrite and bainitic ferrite (%)] - [area fraction of retained austenite (%)]

[Occupancy rate of prior austenite grain boundaries by ferrite and bainitic ferrite (hereinafter also referred to as prior y grain boundary occupancy rate): 20 % or more]

In the steel sheet according to an embodiment of the present disclosure, it is extremely important to have a prior y grain boundary occupancy rate of 20 % or more in order to realize an excellent shear angle range. Prior austenite grain (hereinafter also referred to as prior y grain) boundaries become initiation points of delayed fracture. Here, it is important that the prior y grain boundaries are occupied by soft ferrite and bainitic ferrite, and in particular that the prior y grain boundary occupancy rate is 20 % or more. This makes it possible to minimize the effect of the shear angle during shearing and suppress the occurrence of delayed fracture, thereby achieving an excellent shear angle range. The prior y grain boundary occupancy rate is therefore 20 % or more. The prior y grain boundary occupancy rate is preferably 22 % or more. The prior y grain boundary occupancy rate is more preferably 24 % or more. There is no particular upper limit to the prior y grain boundary occupancy rate. The prior y grain boundary occupancy rate may be 100 %.
Here, the prior y grain boundary occupancy rate is determined, for example, as follows (see FIG. 1).
One prior y grain confirmed in an observation image in measurement of the area fraction of tempered martensite and the total area fraction of ferrite and bainitic ferrite is hereinafter also referred to as the prior y grain. The total circumferential length of the prior y grain (the circumference of the prior y grain, which is the sum of the solid line (prior austenite grain boundary not occupied by ferrite or bainitic ferrite) and the dotted line in FIG. 1, hereinafter also referred to as L_T) is measured. The length of the interface between the prior y grain and the ferrite and bainitic ferrite in contact with the prior y grain (the sum of the prior y grain boundary length of the dotted lines in FIG. 1, hereinafter also referred to as L_F) is measured. Then, the prior y grain boundary occupancy rate of the prior y grain is calculated using the following expression. Prior y grain boundary occupancy rate of the prior y grain (%) = (L_F/L_T) × 100
This measurement is performed on 30 prior y grains in order from the closest to the prior y grain, and the average of the prior y grain boundary occupancy rates measured for each prior y grain is regarded as the prior y grain boundary occupancy rate of the steel sheet being measured.
Further, L_T and L_F are measured, for example, as follows.
A sample is cut from the steel sheet such that a thickness cross-section parallel to the rolling direction of the steel sheet (L-section) becomes an observation plane. The observation plane of the sample is then polished. The observation plane of the sample is then corroded with 1 vol% nital to reveal the microstructure. Then, a 1/4 sheet thickness position of the steel sheet (a position corresponding to 1/4 of the sheet thickness in the depth direction from a steel sheet surface) is observed at 2000× magnification by SEM. From the obtained microstructure image, L_T and L_F are measured using an object function of Adobe Illustrator.
Mechanical properties of the steel sheet according to an embodiment of the present disclosure are as described above.
Further, the steel sheet according to an embodiment of the present disclosure may include a coated or plated layer on a surface. The coated or plated layer may be on only one surface of the steel sheet or may be on both surfaces. The coated or plated layer is not particularly limited. As a coated or plated layer, a galvanized layer with Zn as the main component (Zn content of 50.0 mass% or more) is an example. Further, examples of galvanized layers include hot-dip galvanized layers, galvannealed layers, and electrogalvanized layers. A steel sheet that has a galvanized layer may also be referred to as a galvanized steel sheet. Further, a steel sheet that has a hot-dip galvanized layer, a galvannealed layer, or an electrogalvanized layer may also be referred to as a hot-dip galvanized steel sheet (GI), a galvannealed steel sheet (GA), or an electrogalvanized steel sheet (EG), respectively.
Coated or plated layers other than galvanized layers may include aluminum coated or plated layers and alloy coated or plated layers. As alloy coated or plated layers, examples include hot-dip zinc-aluminum-magnesium alloy coated layers and Zn-Ni electroplated alloy layers.
Further, coating weight per side of the coated or plated layer is not particularly limited. The coating weight per side of the coated or plated layer is preferably 20 g/m² or more. The coating weight per side is preferably 80 g/m² or less.
The thickness of the steel sheet according to an embodiment of the present disclosure is not particularly limited. The thickness of the steel sheet is preferably 0.50 mm or more. The thickness of the steel sheet is preferably 2.50 mm or less.

[2] Member

A member according to an embodiment of the present disclosure is described below.
The member according to an embodiment of the present disclosure is a member formed using the steel sheet described above as a material. For example, the material, the steel sheet, is subjected to at least one of a forming process or a joining process to make the member.
Here, the steel sheet has a TS of 1180 MPa or more, and also has excellent dimensional accuracy and shear angle range. Therefore, the member according to an embodiment of the present disclosure is particularly suitable for application as a material for automotive parts. This allows for improved fuel efficiency due to an automotive body weight decrease, which can greatly contribute to a decrease in CO₂ emissions.

[3] Method of producing steel sheet

The following describes a method of producing a steel sheet according to an embodiment of the present disclosure.
The method of producing a steel sheet according to an embodiment of the present disclosure includes:

a preparation process of preparing a blank sheet having the chemical composition described above;
a heating process of heating the blank sheet under a set of conditions including an average heating rate in a temperature range from 700 °C to 750 °C of 5.0 °C/s or less, and
a maximum arrival temperature T1 of 800 °C or more and 900 °C or less;
a first cooling process of cooling the blank sheet under a set of conditions including an average cooling rate in a temperature range from the maximum arrival temperature T1 to an intermediate holding temperature T2 of 0.10 °C/s or more and 5.00 °C/s or less;
an intermediate holding process of holding the blank sheet under a set of conditions including the intermediate holding temperature T2 being 600 °C or more and 750 °C or less,
an intermediate holding time t2 of 1.0 s or longer and 2000.0 s or shorter, and
a tension applied to the blank sheet of 5 MPa or more;
a second cooling process of cooling the blank sheet under a set of conditions including an average cooling rate in a temperature range from 300 °C to 100 °C of 300 °C/s or more,
to a second cooling end temperature; and
a tempering process of tempering the blank sheet under a set of conditions including a tempering temperature T3 of 100 °C or more and 400 °C or less, and
a tempering time t3 of 10 s or longer and 10,000 s or shorter.

Unless otherwise specified, each of the temperatures above refers to a surface temperature of the steel sheet. Further, the average heating rate and the average cooling rate are based on the surface temperature of the steel sheet, unless otherwise specified.

• Preparation process

First, a blank sheet having the chemical composition described above is prepared. For example, a blank sheet can be prepared by hot rolling a steel slab into a hot-rolled steel sheet, then subjecting the hot-rolled steel sheet to optional pickling and heat treatment, and then cold rolling to obtain a cold-rolled steel sheet. The conditions of these processes are not particularly limited and may follow a conventional method.
For example, a method of smelting the steel slab (steel material) may be any known method, such as by use of a converter, an electric furnace, or the like. The steel slab is preferably smelted by continuous casting to help prevent macro-segregation.
Examples of hot rolling include methods such as rolling the steel slab after heating, direct rolling the steel slab after continuous casting without heating, and rolling the steel slab after applying a short heating treatment following continuous casting. Further, slab heating temperature, slab soaking duration, and coiling temperature in hot rolling are not particularly limited. The slab heating temperature is preferably 1100 °C or more. The slab heating temperature is preferably 1300 °C or less. The slab soaking duration is preferably 30 min or more. The slab soaking duration is preferably 250 min or less. The rolling finish temperature is preferably the Ar₃ transformation temperature or more. The coiling temperature is preferably 350 °C or more. The coiling temperature is preferably 650 °C or less. The Ar₃ transformation temperature is determined by the following expression. Ar₃ transformation temperature (°C) = 868 - 396 × [%C] + 24.6 × [%Si] - 68.1 × [%Mn] - 36.1 × [%Ni] - 20.7 × [%Cu] - 24.8 × [%Cr]
Here, [%element symbol] in the above expression represents the content in mass% of the element in the chemical composition described above.
Pickling is capable of removing oxides from the surface of the hot-rolled steel sheet, and is preferably carried out to secure good chemical convertibility and coating quality in the final steel sheet product. Pickling may be carried out in one or more batches. Further, the hot-rolled steel sheet after pickling may be subjected to heat treatment.
The total rolling reduction in the cold rolling is preferably 30 % or more. The total rolling reduction in the cold rolling is preferably 80 % or less. The defined effect can be obtained without limiting the number of rolling passes or the rolling reduction for each pass.

• Heating process

Next, the blank sheet prepared in the preparation process is heated to the maximum arrival temperature T1 under a set of conditions including an average heating rate of 5.0 °C/s or less in the temperature range of 700 °C to 750 °C.

[Average heating rate in temperature range of 700 °C to 750 °C: 5.0 °C/s or less]

The inventors have carried out intensive studies and found that the average heating rate in the temperature range of 700 °C to 750 °C (hereinafter also referred to simply as average heating rate) affects the prior y grain boundary occupancy rate. That is, by setting the average heating rate to 5.0 °C/s or less, the dissolution of carbides is promoted. This refines the prior y grains, contributing to an increase in the prior y grain boundary occupancy rate. As a result, the shear angle range is improved. Accordingly, the average heating rate is 5.0 °C/s or less. The average heating rate is preferably 3.0 °C/s or less. A lower limit of the average heating rate is not specifically defined. For example, the average heating rate is preferably 0.1 °C/s or more.

[Maximum arrival temperature T1: 800 °C or more and 900 °C or less]

When the maximum arrival temperature T1 is less than 800 °C, the total area fraction of ferrite and bainitic ferrite becomes 15 % or more, and achieving a TS of 1180 MPa or more becomes difficult. On the other hand, when the maximum arrival temperature T1 exceeds 900 °C, the total area fraction of ferrite and bainitic ferrite becomes less than 5 %, making achieving excellent dimensional accuracy of parts difficult. The maximum arrival temperature T1 is therefore 800 °C or more and 900 °C or less. The maximum arrival temperature T1 is preferably 810 °C or more. The maximum arrival temperature T1 is preferably 890 °C or less.
After the maximum arrival temperature T1 is reached, processing may immediately proceed to the cooling process described later, or the maximum arrival temperature T1 may be held for a certain period of time, for example, 1.0 s to 5.0 s, before proceeding to the cooling process.

• First cooling process

The blank sheet is then cooled under a set of conditions including an average cooling rate in a temperature range from the maximum arrival temperature T1 to the intermediate holding temperature T2 of 0.10 °C/s or more and 5.00 °C/s or less.

[Average cooling rate in temperature range from maximum arrival temperature T1 to intermediate holding temperature T2 (hereinafter also referred to as first average cooling rate): 0.10 °C/s or more and 5.00 °C/s or less]

As a result of intensive studies, the inventors found that the first average cooling rate affects the prior y grain boundary occupancy rate. That is, by setting the first average cooling rate to 5.00 °C/s or less, the nucleation of ferrite from prior y grain boundaries is promoted, which contributes to an increase in the prior y grain boundary occupancy rate. As a result, the shear angle range is improved. On the other hand, when the first average cooling rate is less than 0.10 °C/s, the total area fraction of ferrite and bainitic ferrite becomes 15 % or more, and achieving a TS of 1180 MPa or more becomes difficult. The first average cooling rate is therefore 0.10 °C/s or more and 5.00 °C/s or less. The first average cooling rate is preferably 0.20 °C/s or more. The first average cooling rate is preferably 3.00 °C/s or less.
The first cooling end temperature may be 600 °C or more and 750 °C or less. For example, the first cooling end temperature may be the intermediate holding temperature T2.

• Intermediate holding process

The blank sheet is then held under a set of conditions including the intermediate holding temperature T2 being 600 °C or more and 750 °C or less,

an intermediate holding time t2 of 1.0 s or longer and 2000.0 s or shorter, and
a tension applied to the blank sheet of 5 MPa or more.

[Intermediate holding temperature T2: 600 °C or more and 750 °C or less]

When the intermediate holding temperature T2 is less than 600 °C, transformation of ferrite and bainitic ferrite from places other than prior y grain boundaries may be promoted. Therefore, it becomes difficult to make the prior y grain boundary occupancy rate 20 % or more, and it also becomes difficult to realize an excellent shear angle range. On the other hand, when the intermediate holding temperature T2 exceeds 750 °C, the total area fraction of ferrite and bainitic ferrite becomes less than 5 %, making it difficult to achieve excellent dimensional accuracy of parts. The intermediate holding temperature T2 is therefore 600 °C or more and 750 °C or less. The intermediate holding temperature T2 is preferably 610 °C or more. The intermediate holding temperature T2 is preferably 740 °C or less. The intermediate holding temperature here refers to the holding temperature in the intermediate holding process. The intermediate holding temperature may be constant during holding. Further, the intermediate holding temperature is the temperature range of 600 °C or more and 750 °C or less, and when temperature fluctuation is within ±10 °C of the set temperature, the annealing temperature does not have to be constant during holding.

[Intermediate holding time t2: 1.0 s or longer and 2000.0 s or shorter]

When the intermediate holding time t2 is less than 1.0 s (including a case where no intermediate holding is carried out), the prior y grain boundary occupancy rate becomes less than 20 %, making it impossible to realize an excellent shear angle range. When the intermediate holding time t2 exceeds 2000.0 s, the total area fraction of ferrite and bainitic ferrite becomes 15 % or more, and achieving a TS of 1180 MPa or more becomes difficult. The intermediate holding time t2 is therefore 1.0 s or longer and 2000.0 s or shorter. The intermediate holding time t2 is preferably 10.0 s or longer. The intermediate holding time t2 is preferably 1500.0 s or shorter. The intermediate holding time t2 is a holding time at the intermediate holding temperature T2.

[Tension applied to blank sheet: 5 MPa or more]

As a result of intensive studies, the inventors have found that applying tension to the blank sheet during intermediate holding affects the prior y grain boundary occupancy rate. In this case, by applying a tension to the blank sheet (hereinafter also referred to simply as applied tension) of 5 MPa or more, ferrite nucleation from prior y grain boundaries is promoted, which contributes to an increase in the prior y grain boundary occupancy rate. This makes it possible to make the prior y grain boundary occupancy rate 20 % or more, and to realize an excellent shear angle range. Accordingly, the applied tension is 5 MPa or more. The applied tension is preferably 10 MPa or more. An upper limit of the applied tension is not specifically defined. The applied tension is, for example, preferably 100 MPa or less.
Further, a coating or plating treatment may be applied to the blank sheet between the intermediate holding process and the second cooling process described below. Details about the coating or plating treatment are described later.

• Second cooling process

The blank sheet is cooled under a set of conditions including an average cooling rate in a temperature range from 300 °C to 100 °C of 300 °C/s or more,
to the second cooling end temperature.

[Average cooling rate in temperature range of 300 °C to 100 °C (hereinafter also referred to as second average cooling rate): 300 °C/s or more]

When the second average cooling rate is less than 300 °C/s, the area fraction of retained austenite becomes 3 % or more, making it difficult to achieve an excellent shear angle range. The second average cooling rate is therefore 300 °C/s or more. The second average cooling rate is preferably 800 °C/s or more. An upper limit of the second average cooling rate is not specifically defined. For example, the second average cooling rate is preferably 2000 °C/s or less.
The second cooling end temperature may be, for example, less than 100 °C. Further, the second cooling end temperature may be, for example, around room temperature.

• Tempering process

Next, the blank sheet is tempered under a set of conditions including the tempering temperature T3 being 100 °C or more and 400 °C or less, and the tempering time t3 being 10 s or longer and 10,000 s or shorter.

[Tempering temperature T3: 100 °C or more and 400 °C or less]

Tempered martensite is formed by tempering treatment, where martensite is tempered. Here, when the tempering temperature T3 is less than 100 °C, martensite is not sufficiently tempered, resulting in a microstructure mainly composed of quenched martensite. In such a microstructure mainly composed of quenched martensite, excellent shear angle range cannot be obtained. On the other hand, when the tempering temperature T3 exceeds 400 °C, tempering of martensite progresses excessively, and achieving a TS of 1180 MPa or more becomes difficult. The tempering temperature T3 is therefore 100 °C or more and 400 °C or less. The tempering temperature T3 is preferably 150 °C or more. The tempering temperature T3 is preferably 350 °C or less. The tempering temperature referred to here is the holding temperature during the tempering process. The tempering temperature may be constant during holding. Further, the tempering temperature is in the range from 100 °C or more to 400 °C or less, and when temperature fluctuation is within ±10 °C of the set temperature, the tempering temperature does not have to be constant during holding.

[Tempering time t3: 10 s or longer and 10,000 s or shorter]

As mentioned above, tempered martensite is formed by tempering treatment, where martensite is tempered. Here, when the tempering time t3 is shorter than 10 s, martensite is not sufficiently tempered, resulting in a microstructure mainly composed of quenched martensite. In such a microstructure mainly composed of quenched martensite, excellent shear angle range cannot be obtained. On the other hand, when the tempering time t3 exceeds 10,000 s, tempering of martensite progresses excessively, and achieving a TS of 1180 MPa or more becomes difficult. Accordingly, the tempering time t3 is 10 s or longer and 10,000 s or shorter. The tempering time t3 is preferably 50 s or longer. The tempering time t3 is preferably 5000 s or shorter. Here, the tempering time t3 refers to the holding time at the tempering temperature T3.
The cooling after tempering is not specifically defined. For example, it is sufficient to cool by any method according to a conventional method. The cooling end temperature after tempering may be, for example, around room temperature.
Further, after the tempering process, the blank sheet may be worked under conditions that result in an equivalent plastic strain of 0.10 % or more and 5.00 % or less. Further, after the working, the blank sheet may be reheated to a temperature of 100 °C or more and 400 °C or less.
Further, after the tempering process, the blank sheet may be subjected to a coating or plating treatment. Details about the coating or plating treatment are described below.

• Coating or plating process

Further, optionally, the blank sheet may be subjected to coating or plating treatment. Coating or plating treatment is not particularly limited. Examples of coating or plating treatment include galvanizing treatment such as hot-dip galvanizing treatment, galvannealing treatment, and electrogalvanization treatment. Other than galvanizing treatment, examples of coating or plating treatment include aluminum coating or plating treatment and alloy coating or plating treatment. Examples of alloy coating or plating treatment include hot-dip zinc-aluminum-magnesium alloy coating treatment and Zn-Ni electro-alloy plating treatment. Treatment conditions may follow conventional methods. As mentioned above, the coating or plating treatment is preferably carried out between the intermediate holding process and the second cooling process, or after the tempering process. For example, hot-dip galvanizing treatment or galvannealing treatment is preferably carried out between the intermediate holding process and the second cooling process. Further, electrogalvanization treatment or Zn-Ni electro-alloy plating treatment is preferably carried out after the tempering process.
In the case of hot-dip galvanizing treatment and galvannealing treatment, from the perspective of productivity, the series of treatments including the heating process and the coating or plating treatment process is preferably carried out on a continuous galvanizing line (CGL). After the hot-dip galvanizing, wiping may be carried out for adjusting the coating amount.
Further, after the coating or plating treatment process, the blank sheet may be worked under conditions that result in an equivalent plastic strain of 0.10 % or more and 5.00 % or less. Further, after the working, the blank sheet (coated or plated steel sheet) may be reheated to a temperature of 100 °C or more and 400 °C or less.
Conditions other than those described above are not particularly limited, and a conventional method may be used. According to the method of producing the steel sheet according to an embodiment of the present disclosure described above, a steel sheet is obtainable that has a TS of 1180 MPa or more and excellent dimensional accuracy and shear angle range. The obtained steel sheet may be suitably used as a material for automotive parts, for example. When the steel sheet is to be traded, the steel sheet is typically cooled to room temperature before being traded.

[4] Method of producing member

The following describes a method of producing a member according to an embodiment of the present disclosure.
The method of producing a member according to an embodiment of the present disclosure includes process of at least one of forming or joining the steel sheet described above to make the member.
Here, a forming method is not particularly limited, and a typical processing method such as press forming may be used, for example. Further, a joining method is also not particularly limited, and for example, typical welding such as spot welding, laser welding, arc welding, and the like, rivet joining, swaging joining, and the like may be used. Forming and joining conditions are not particularly limited and may follow a conventional method.

EXAMPLES

Steel having the chemical compositions listed in Table 1 (the balance being Fe and inevitable impurity) was melted in a converter and made into steel slabs by a continuous casting method. The steel slabs were then heated. The steel slabs were then hot rolled to produce hot-rolled steel sheets. Pickling treatment was then carried out on the hot-rolled steel sheets. The hot-rolled steel sheets were then subjected to cold rolling to obtain cold-rolled steel sheets. In this way, blank sheets were prepared. The prepared blank sheets then underwent the heating process, the first cooling process, the intermediate holding process, the second cooling process, and the tempering process, under conditions including the conditions listed in Table 2 to obtain final product steel sheets (thickness: 0.6 mm to 2.2 mm). Further, some of the steel sheets (those listed as GI, GA, and EG in Table 2) were subjected to coating or plating treatment. Among these, for those listed as GI and GA in Table 2, coating treatment was carried out between the intermediate holding process and the second cooling process. For those listed as EG in Table 2, plating treatment was carried out after the tempering process. Conditions not specified were followed according to 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, and the prior y grain boundary occupancy rate were determined as described above. Results are listed in Table 3.
Further, each evaluation was carried out according to the following procedure. The evaluation results are listed in Table 3.

(Evaluation of TS)

From each obtained steel sheet, a JIS No. 5 test piece (gauge length: 50 mm, parallel portion width: 25 mm) was taken so that the direction perpendicular to the rolling direction of the steel sheet was the longitudinal direction of the test piece. Next, a tensile test was conducted according to JIS Z 2241:2022 using the test piece, and TS and YS were measured. The crosshead speed was set to 1.67 × 10^-1 mm/s. The TS was evaluated according to the following criteria:

Good (pass, very good): TS was 1180 MPa or more
Poor (fail): TS was less than 1180 MPa

(Evaluation of dimensional accuracy)

From the TS and YS measured in the above evaluation of TS, YR was calculated according to the following expression. $YR = 100 \times YS / TS$
The dimensional accuracy was evaluated according to the following criteria:

Good (pass, very good): YR was 65 % or more and 85 % or less
Poor (fail): YR was less than 65 % or YR was more than 85 %

(Evaluation of shear angle range)

The obtained steel sheets were each sheared into a size of 16 mm × 75 mm with the longitudinal direction perpendicular to the rolling direction to prepare test pieces. The clearance during shearing was set to 15 % in each case. Further, the shear angle during shearing was changed in increments of 0.25° within a range of 0° to 2.0°. Next, a four-point bending test was carried out in accordance with ASTM (G39-99), and a stress of 1000 MPa was applied to the tip of the bend test piece. Next, while the stress was applied, the test piece was immersed in hydrochloric acid of pH 3 at 25 °C for 100 h. After immersion, each test piece was visually inspected for the presence or absence of cracks. The shear angle range was evaluated according to the following criteria.

Excellent (pass, particularly good): appropriate range of shear angle versus delayed fracture was 0° to 1.0° or more
Good (pass, very good): appropriate range of shear angle versus delayed fracture was 0° to 0.5° or more and less than 1.0°
Poor (fail): appropriate range of shear angle versus delayed fracture was 0° to less than 0.5°.

The appropriate range of shear angle versus delayed fracture was the range of shear angles at which no cracks were observed in the test pieces in the test described above. For example, when no cracks were observed in any of the test pieces prepared with shear angles of 0° to 0.75° during shearing, but cracks were observed in the test pieces prepared with a shear angle of 1.00° or more during shearing, then the appropriate range of shear angles versus delayed fracture was "0° to 0.75°", and the test piece was evaluated as "Good (pass, very good)". Further, when no cracks were observed in any of the test pieces prepared with shear angles of 0° to 0.25° during shearing, but cracks were observed in the test pieces prepared with a shear angle of 0.50° or more during shearing, the appropriate range of shear angles versus delayed fracture becomes "0° to 0.25°", and the test piece was evaluated as "Poor (fail)".

[Table 1]

Table 1

Steel sample ID	Chemical composition (mass%)													Remarks
Steel sample ID	C	Si	Mn	P	S	N	O	Al	TI	Nb	B	Cu	Other	Remarks
A	0.231	0.249	1.55	0.014	0.0013	0.006	0.006	0.041						Conforming steel
B	0.221	0.116	1.67	0.013	0.0014	0.006	0.006	0.038						Conforming steel
C	0.220	0.259	1.75	0.012	0.0008	0.006	0.003	0.058						Conforming steel
D	0.205	0.242	1.55	0.011	0.0007	0.005	0.003	0.014						Conforming steel
E	0.203	0.155	1.46	0.014	0.0010	0.001	0.006	0.029						Conforming steel
F	0.040	0.112	1.69	0.013	0.0013	0.003	0.002	0.023						Conforming steel
G	0.024	0.286	1.61	0.011	0.0006	0.006	0.003	0.019						Comparative steel
H	0.477	0.224	1.88	0.010	0.0014	0.005	0.007	0.035						Conforming steel
I	0.505	0.243	1.33	0.013	0.0013	0.006	0.007	0.048						Comparative steel
J	0.205	0.025	1.39	0.006	0.0009	0.001	0.006	0.034						Conforming steel
K	0.247	0.005	1.50	0.010	0.0006	0.005	0.006	0.044						Comparative steel
L	0.237	2.481	1.26	0.010	0.0009	0.007	0.007	0.038						Conforming steel
M	0.244	2.580	1.16	0.007	0.0011	0.005	0.003	0.047						Comparative steel
N	0.218	0.176	0.11	0.008	0.0014	0.004	0.006	0.049						Conforming steel
O	0.243	0.283	0.07	0.009	0.0012	0.004	0.005	0.017						Comparative steel
P	0.214	0.105	4.77	0.013	0.0011	0.003	0.002	0.029						Conforming steel
Q	0.219	0.194	5.16	0.006	0.0007	0.005	0.003	0.013						Comparative steel
R	0.211	0.184	1.60	0.097	0.0013	0.001	0.007	0.036						Conforming steel
S	0.205	0.246	1.37	0.109	0.0006	0.002	0.003	0.020						Comparative steel
T	0.249	0.252	1.40	0.007	0.0192	0.006	0.005	0.039						Conforming steel
U	0.218	0.172	1.43	0.006	0.0203	0.004	0.002	0.019						Comparative steel
V	0.232	0.248	1.56	0.006	0.0006	0.006	0.006	0.924						Conforming steel
W	0.247	0.169	1.17	0.014	0.0012	0.007	0.003	1.049						Comparative steel
X	0.229	0.194	1.72	0.005	0.0015	0.0094	0.004	0.019						Conforming steel
Y	0.211	0.277	1.76	0.008	0.0009	0.0111	0.004	0.036						Comparative steel
Z	0.202	0.291	1.63	0.014	0.0010	0.002	0.0088	0.028						Conforming steel
AA	0.201	0.254	1.05	0.014	0.0008	0.004	0.0112	0.025						Comparative steel
AB	0.217	0.200	1.21	0.008	0.0009	0.005	0.006	0.036						Conforming steel
AC	0.233	0.199	1.29	0.006	0.0010	0.002	0.001	0.031	0.001					Conforming steel
AD	0.248	0.241	1.98	0.006	0.0007	0.003	0.006	0.055	0.195					Conforming steel
AE	0.237	0.279	1.08	0.011	0.0008	0.004	0.002	0.021	0.214					Comparative steel
AF	0.208	0.138	1.50	0.006	0.0014	0.007	0.002	0.037			0.0004			Conforming steel
AG	0.239	0.137	1.23	0.011	0.0009	0.003	0.001	0.043			0.0084			Conforming steel
AH	0.247	0.220	1.90	0.006	0.0012	0.006	0.006	0.028			0.0120			Comparative steel
AI	0.245	0.160	1.72	0.006	0.0011	0.005	0.005	0.045		0.003				Conforming steel
AJ	0.242	0.267	1.41	0.010	0.0008	0.001	0.002	0.013		0.187				Conforming steel
AK	0.247	0.260	1.40	0.009	0.0013	0.006	0.006	0.054		0.209				Comparative steel
AL	0.208	0.284	1.03	0.011	0.0009	0.002	0.004	0.022				0.03		Conforming steel
AM	0.242	0.240	1.59	0.010	0.0007	0.004	0.004	0.028				0.95		Conforming steel
AN	0.226	0.245	1.29	0.013	0.0007	0.006	0.006	0.052				1.15		Comparative steel
AO	0.223	0.134	1.86	0.005	0.0013	0.006	0.006	0.052					V:0.140	Conforming steel
AP	0.246	0.147	1.10	0.005	0.0009	0.005	0.005	0.037					Ta:0.09	Conforming steel
AQ	0.242	0.136	1.53	0.007	0.0015	0.006	0.001	0.058					W:0.05	Conforming steel
AR	0.219	0.272	1.53	0.007	0.0010	0.001	0.003	0.031					Cr:0.32	Conforming steel
AS	0.236	0.142	1.25	0.005	0.0015	0.002	0.001	0.020					Mo:0.53	Conforming steel
AT	0.223	0.123	1.15	0.012	0.0006	0.005	0.005	0.024					Co:0.008	Conforming steel
AU	0.235	0.254	1.87	0.006	0.0014	0.006	0.001	0.026					Ni:0.06	Conforming steel
AV	0.236	0.278	1.91	0.012	0.0008	0.002	0.004	0.029					Sn:0.126	Conforming steel
AW	0.237	0.260	1.42	0.012	0.0013	0.004	0.007	0.054					Sb:0.104	Conforming steel
AX	0.235	0.279	1.76	0.007	0.0006	0.002	0.006	0.059					Ca:0.0040	Conforming steel
AY	0.219	0.268	1.59	0.008	0.0011	0.002	0.006	0.021					Mg:0.0028	Conforming steel
AZ	0.202	0.257	1.44	0.009	0.0011	0.002	0.004	0.043					Zr:0.061	Conforming steel
BA	0.223	0.243	1.56	0.005	0.0013	0.007	0.005	0.014					Te:0.046	Conforming steel
BB	0.208	0.230	1.35	0.007	0.0005	0.006	0.003	0.022					Hf:0.08	Conforming steel
BC	0.239	0.240	1.74	0.013	0.0010	0.004	0.002	0.049					REM:0.0058	Conforming steel
BD	0.244	0.273	1.57	0.006	0.0010	0.001	0.006	0.016					Bi:0.029	Conforming steel
BE	0.218	0.203	1.55	0.007	0.0008	0.007	0.006	0.058					Zn:0.030	Conforming steel
BF	0.215	0.274	1.22	0.014	0.0009	0.002	0.001	0.050					Pb:0.023	Conforming steel
BG	0.249	0.172	1.64	0.012	0.0015	0.001	0.003	0.058					As:0.033	Conforming steel
BH	0.235	0.253	1.02	0.008	0.0012	0.002	0.004	0.013					Ge:0.054	Conforming steel
BI	0.218	0.234	1.26	0.007	0.0012	0.005	0.005	0.026					Sr:0.036	Conforming steel
BJ	0.223	0.230	1.56	0.013	0.0012	0.004	0.007	0.028					Cs:0.063	Conforming steel
BK	0.223	0.179	1.20	0.008	0.0006	0.006	0.003	0.012						Conforming steel
BL	0.215	0.117	1.91	0.015	0.0011	0.003	0.004	0.026						Conforming steel
BM	0.236	0.268	1.26	0.011	0.0005	0.005	0.006	0.059						Conforming steel
BN	0.240	0.134	1.75	0.012	0.0009	0.006	0.001	0.018						Conforming steel
BO	0.229	0.169	1.87	0.011	0.0010	0.007	0.003	0.055						Conforming steel

Underlined: indicates value outside scope of present disclosure.

[Table 2]

Table 2

No.	Steel sample ID	Heating process		First cooling process	Intermediate holding process			Second cooling process	Tempering process		Type*	Remarks
No.	Steel sample ID	Average heating rate (°C/s)	Maximum arrival temp. T1 (°C)	First average cooling rate (°C/s)	Intermediate holding temp. T2 (°C)	Intermediate holding time t2 (s)	Applied tension (MPa)	Second average cooling rate (°C/s)	Tempering temp. T3 (°C)	Tempering time t3 (s)	Type*	Remarks
1	A	0.4	843	0.41	717	50.3	14	867	174	707	CR	Example
2	B	0.9	836	0.32	691	42.9	20	904	173	694	CR	Example
3	B	0.1	820	0.62	727	61.8	11	958	178	594	CR	Example
4	B	0.3	833	0.37	725	938.9	18	961	197	878	CR	Example
5	B	4.7	834	0.40	703	88.0	11	918	177	866	CR	Example
6	B	5.3	833	0.90	712	95.2	19	941	193	911	CR	Comparative Example
7	B	0.4	808	1.22	722	58.6	11	964	176	848	CR	Example
8	B	0.6	795	1.19	724	41.8	12	829	173	928	CR	Comparative Example
9	B	0.5	894	1.23	715	70.9	19	929	179	949	CR	Example
10	B	0.5	911	0.33	714	48.0	15	976	186	545	CR	Comparative Example
11	B	0.2	850	0.10	725	31.3	18	925	210	609	CR	Example
12	B	0.5	834	0.08	726	63.9	11	875	202	564	CR	Comparative Example
13	B	0.6	840	4.90	692	99.8	17	886	198	652	CR	Example
14	B	0.3	827	5.11	717	93.3	18	815	175	964	CR	Comparative Example
15	B	0.4	836	0.34	612	35.0	10	911	178	637	CR	Example
16	B	0.5	854	0.74	590	88.2	10	864	204	712	CR	Comparative Example
17	B	0.4	844	0.93	748	74.2	18	872	186	573	CR	Example
18	B	0.4	841	0.92	755	77.7	17	895	183	552	CR	Comparative Example
19	B	0.6	856	0.43	724	1.7	12	825	208	756	CR	Example
20	B	0.6	839	1.12	715	0.8	13	958	198	730	CR	Comparative Example
21	B	0.3	850	1.08	694	1987.5	11	991	192	844	CR	Example
22	B	0.6	823	0.70	701	2024.9	19	850	198	594	CR	Comparative Example
23	B	0.4	821	1.00	Not held	Not held	17	916	203	752	CR	Comparative Example
24	B	0.8	836	0.95	Not held	Not held	18	995	205	777	CR	Comparative Example
25	B	0.5	835	0.47	705	94.8	7	973	179	772	CR	Example
26	B	0.6	849	0.69	705	88.8	4	968	196	865	CR	Comparative Example
27	B	0.9	847	0.36	708	40.1	48	988	201	676	CR	Example
28	B	0.2	844	0.87	708	1065.2	16	893	177	900	CR	Example
29	B	0.7	842	1.28	713	76.2	16	314	192	599	CR	Example
30	B	0.9	826	0.39	722	81.8	11	284	190	715	CR	Comparative Example
31	B	0.9	856	1.09	726	62.5	11	1974	179	814	CR	Example
32	B	0.7	840	1.13	698	37.0	11	839	185	542	CR	Example
33	B	0.9	846	0.99	695	62.5	13	998	124	928	CR	Example
34	B	0.3	844	1.09	706	98.5	15	852	109	714	CR	Example
35	B	0.7	838	1.08	696	35.5	12	822	389	983	CR	Example
36	B	0.5	833	0.84	690	64.7	19	907	394	877	CR	Example
37	B	0.5	826	0.72	721	82.4	14	935	198	14	CR	Example
38	B	0.7	835	0.96	703	94.2	15	883	174	10	CR	Example
39	B	0.8	837	0.51	722	33.4	19	896	180	9981	CR	Example
40	B	0.9	833	0.60	701	33.7	11	944	200	9998	CR	Example
41	C	0.6	834	0.90	714	1088.1	15	974	187	842	CR	Example
42	D	0.6	848	0.64	708	1009.9	14	990	206	751	CR	Example
43	E	0.8	858	1.06	694	924.5	18	838	177	795	CR	Example
44	F	0.5	824	0.93	730	36.1	15	867	195	554	CR	Example
45	G	0.9	846	0.89	720	54.3	12	843	198	589	CR	Comparative Example
46	H	0.9	835	1.23	715	81.0	18	932	206	990	CR	Example
47	I	0.3	839	1.01	714	54.1	10	991	206	855	CR	Comparative Example
48	J	0.3	838	0.78	701	62.5	19	846	199	824	CR	Example
49	K	0.3	850	1.25	710	44.8	13	904	187	602	CR	Comparative Example
50	L	0.9	856	1.03	701	47.8	18	854	204	884	CR	Example
51	M	0.4	826	0.89	711	91.0	12	815	199	836	CR	Comparative Example
52	N	0.9	833	1.04	728	36.3	18	824	206	520	CR	Example
53	O	0.9	849	1.13	696	48.8	19	842	198	761	CR	Comparative Example
54	P	0.5	832	0.67	726	80.4	14	897	191	674	CR	Example
55	Q	0.7	833	0.48	729	87.2	13	807	174	936	CR	Comparative Example
56	R	0.3	839	0.95	702	79.1	14	927	205	946	CR	Example
57	S	0.9	856	0.50	711	63.1	18	923	195	513	CR	Comparative Example
58	T	0.4	849	0.67	712	31.0	17	987	187	759	CR	Example
59	U	0.9	840	0.55	693	59.2	15	828	195	925	GA	Comparative Example
60	V	1.0	839	0.69	705	51.4	14	911	195	838	GA	Example
61	W	0.3	826	0.97	701	57.9	13	837	182	862	GA	Comparative Example
62	X	0.7	845	0.51	713	47.5	19	876	172	697	GA	Example
63	Y	0.4	849	0.78	723	87.7	17	988	190	637	GA	Comparative Example
64	Z	0.3	841	1.12	715	72.7	17	984	200	986	CR	Example
65	AA	1.0	837	0.35	718	53.9	12	862	177	617	CR	Comparative Example
66	AB	1.0	821	0.69	721	1040.2	18	878	179	587	GA	Example
67	AC	0.3	821	0.61	721	69.0	12	831	195	894	GA	Example
68	AD	1.0	847	0.66	726	70.9	11	886	209	694	GI	Example
69	AE	0.9	842	0.30	692	73.7	19	859	173	674	GA	Comparative Example
70	AF	0.9	859	0.45	713	75.5	19	824	171	514	GA	Example
71	AG	0.9	860	0.83	695	87.0	13	867	173	973	GA	Example
72	AH	0.8	823	0.58	716	75.5	15	872	202	706	GA	Comparative Example
73	AI	0.7	821	1.28	700	43.5	20	953	186	883	GI	Example
74	AJ	0.6	825	1.17	700	51.5	11	938	183	927	GA	Example
75	AK	0.7	844	0.89	728	59.6	11	839	207	689	GA	Comparative Example
76	AL	0.8	855	0.63	726	36.5	12	941	195	583	GA	Example
77	AM	0.7	821	0.97	728	60.4	13	978	196	866	GA	Example
78	AN	0.4	841	0.96	715	48.2	10	859	188	957	GA	Comparative Example
79	AO	0.2	820	0.65	712	41.0	11	955	171	767	CR	Example
80	AP	4.8	849	0.92	723	84.5	18	822	205	584	CR	Example
81	AQ	0.6	805	1.14	704	62.0	13	983	203	799	CR	Example
82	AR	0.4	894	0.83	694	44.6	12	977	175	842	CR	Example
83	AS	0.9	860	0.17	720	58.2	10	911	202	721	CR	Example
84	AT	0.7	824	4.87	699	63.0	11	841	186	530	GA	Example
85	AU	1.0	849	0.30	611	31.2	11	843	171	969	GA	Example
86	AV	0.5	827	0.32	748	70.6	10	843	187	653	CR	Example
87	AW	0.3	822	0.64	717	1.6	15	848	178	991	CR	Example
88	AX	1.0	859	1.18	725	1995.7	14	874	184	568	CR	Example
89	AY	0.4	837	0.84	725	38.4	6	997	209	819	EG	Example
90	AZ	0.4	830	0.43	707	84.4	49	998	179	529	GI	Example
91	BA	0.7	828	1.12	727	81.7	15	311	176	952	EG	Example
92	BB	0.3	830	1.14	710	84.5	14	1986	178	745	GI	Example
93	BC	0.7	823	0.82	693	58.1	20	936	104	697	CR	Example
94	BD	0.5	859	0.57	729	86.8	12	988	397	830	CR	Example
95	BE	0.7	826	0.51	698	53.3	16	863	187	14	CR	Example
96	BF	0.4	822	0.73	721	81.0	14	906	208	9987	CR	Example
97	BG	0.9	845	0.98	705	46.5	11	840	198	849	CR	Example
98	BH	0.4	837	0.85	701	87.1	12	887	190	843	CR	Example
99	BI	0.9	850	1.17	703	97.7	16	811	201	536	CR	Example
100	BJ	0.7	842	0.60	722	95.0	14	936	179	913	CR	Example
101	BK	0.9	830	1.07	697	48.0	16	821	183	827	EG	Example
102	BL	0.6	857	1.26	693	84.0	17	922	183	921	GI	Example
103	BM	0.5	832	0.87	725	82.0	12	849	196	659	EG	Example
104	BN	0.9	832	0.69	728	62.8	16	888	207	765	GI	Example
105	BO	0.8	857	1.03	717	69.9	11	840	185	733	GA	Example

Underlined: indicates value outside scope of present disclosure.
(*) CR: cold-rolled steel sheet (uncoated), GI: hot-dip galvanized steel sheet (without alloying treatment), GA: galvannealed steel sheet, EG: electrogalvanized steel sheet

[Table 3]

Table 3

No.	Steel sample ID	Microstructure				Evaluation result						Remarks
		Area fraction			Prior γ grain boundary occupancy (%)	TS		Dimensional accuracy			Shear angle range
		Tempered M (%)	Retained γ (%)	F+B (%)	Prior γ grain boundary occupancy (%)	TS (MPa)	Evaluation	YS (MPa)	YR (%)	Evaluation	Evaluation
1	A	89	0	11	30	1479	Good	1183	80	Good	Excellent	Example
2	B	89	0	11	25	1458	Good	1181	81	Good	Excellent	Example
3	B	90	0	10	97	1476	Good	1181	80	Good	Excellent	Example
4	B	92	0	8	31	1502	Good	1232	82	Good	Excellent	Example
5	B	89	1	10	20	1480	Good	1169	79	Good	Good	Example
6	B	92	1	7	14	1538	Good	1200	78	Good	Poor	Comparative Example
7	B	86	0	14	27	1215	Good	996	82	Good	Excellent	Example
8	B	78	0	22	27	1135	Poor	908	80	Good	Excellent	Comparative Example
9	B	93	1	6	30	1590	Good	1336	84	Good	Excellent	Example
10	B	97	1	2	15	1704	Good	1517	89	Poor	Poor	Comparative Example
11	B	85	1	14	97	1222	Good	990	81	Good	Excellent	Example
12	B	83	1	16	98	1048	Poor	838	80	Good	Excellent	Comparative Example
13	B	93	0	7	22	1529	Good	1193	78	Good	Good	Example
14	B	88	0	12	8	1419	Good	1149	81	Good	Poor	Comparative Example
15	B	88	0	12	22	1411	Good	1157	82	Good	Good	Example
16	B	91	1	8	7	1487	Good	1204	81	Good	Poor	Comparative Example
17	B	95	0	5	31	1607	Good	1366	85	Good	Excellent	Example
18	B	96	1	3	4	1681	Good	1513	90	Poor	Poor	Comparative Example
19	B	91	1	8	22	1473	Good	1193	81	Good	Good	Example
20	B	90	0	10	11	1441	Good	1167	81	Good	Poor	Comparative Example
21	B	85	1	14	97	1226	Good	993	81	Good	Excellent	Example
22	B	83	1	16	99	1137	Poor	921	81	Good	Excellent	Comparative Example
23	B	88	1	11	6	1399	Good	1147	82	Good	Poor	Comparative Example
24	B	91	1	8	15	1482	Good	1200	81	Good	Poor	Comparative Example
25	B	90	1	9	22	1509	Good	1252	83	Good	Good	Example
26	B	93	0	7	14	1535	Good	1213	79	Good	Poor	Comparative Example
27	B	88	0	12	29	1369	Good	1109	81	Good	Excellent	Example
28	B	88	0	12	32	1417	Good	1148	81	Good	Excellent	Example
29	B	88	2	10	31	1448	Good	1187	82	Good	Good	Example
30	B	87	6	7	32	1547	Good	1284	83	Good	Poor	Comparative Example
31	B	92	1	7	27	1567	Good	1222	78	Good	Excellent	Example
32	B	91	0	9	26	1493	Good	1194	80	Good	Excellent	Example
33	B	93	0	7	28	1619	Good	1295	80	Good	Good	Example
34	B	92	1	7	35	1631	Good	1321	81	Good	Good	Example
35	B	91	0	9	25	1225	Good	956	78	Good	Excellent	Example
36	B	90	1	9	30	1199	Good	959	80	Good	Excellent	Example
37	B	88	0	12	25	1632	Good	1338	82	Good	Good	Example
38	B	92	0	8	31	1638	Good	1294	79	Good	Good	Example
39	B	89	1	10	32	1223	Good	1003	82	Good	Excellent	Example
40	B	91	1	8	26	1216	Good	997	82	Good	Excellent	Example
41	C	90	1	9	32	1498	Good	1213	81	Good	Excellent	Example
42	D	91	0	9	34	1396	Good	1131	81	Good	Excellent	Example
43	E	90	1	9	33	1440	Good	1152	80	Good	Excellent	Example
44	F	86	2	12	31	1205	Good	964	80	Good	Excellent	Example
45	G	78	2	20	29	1031	Poor	845	82	Good	Excellent	Comparative Example
46	H	91	1	8	35	1647	Good	1334	81	Good	Good	Example
47	I	91	0	9	33	1770	Good	1398	79	Good	Poor	Comparative Example
48	J	90	0	10	26	1210	Good	968	80	Good	Excellent	Example
49	K	88	1	11	31	1056	Poor	845	80	Good	Excellent	Comparative Example
50	L	86	2	12	31	1545	Good	1221	79	Good	Good	Example
51	M	83	5	12	27	1574	Good	1275	81	Good	Poor	Comparative Example
52	N	88	0	12	35	1223	Good	1003	82	Good	Excellent	Example
53	O	77	1	22	28	1129	Poor	903	80	Good	Excellent	Comparative Example
54	P	87	1	12	31	1586	Good	1237	78	Good	Good	Example
55	Q	90	1	9	28	1757	Good	1423	81	Good	Poor	Comparative Example
56	R	87	1	12	33	1328	Good	1076	81	Good	Good	Example
57	S	90	0	10	25	1384	Good	1135	82	Good	Poor	Comparative Example
58	T	93	0	7	29	1619	Good	1311	81	Good	Good	Example
59	U	91	0	9	29	1453	Good	1191	82	Good	Poor	Comparative Example
60	V	90	1	9	32	1503	Good	1172	78	Good	Good	Example
61	W	89	0	11	28	1483	Good	1216	82	Good	Poor	Comparative Example
62	X	89	0	11	30	1495	Good	1166	78	Good	Good	Example
63	Y	87	1	12	26	1382	Good	1133	82	Good	Poor	Comparative Example
64	Z	92	1	7	26	1474	Good	1194	81	Good	Good	Example
65	AA	90	1	9	26	1412	Good	1101	78	Good	Poor	Comparative Example
66	AB	90	0	10	27	1432	Good	1174	82	Good	Excellent	Example
67	AC	89	0	11	32	1421	Good	1151	81	Good	Excellent	Example
68	AD	87	1	12	33	1609	Good	1319	82	Good	Good	Example
69	AE	89	1	10	29	1817	Good	1508	83	Good	Poor	Comparative Example
70	AF	90	0	10	34	1437	Good	1178	82	Good	Excellent	Example
71	AG	91	0	9	29	1638	Good	1360	83	Good	Good	Example
72	AH	90	0	10	30	1800	Good	1476	82	Good	Poor	Comparative Example
73	AI	92	0	8	26	1595	Good	1260	79	Good	Excellent	Example
74	AJ	90	1	9	27	1604	Good	1283	80	Good	Good	Example
75	AK	89	1	10	33	1734	Good	1439	83	Good	Poor	Comparative Example
76	AL	92	1	7	26	1458	Good	1166	80	Good	Excellent	Example
77	AM	89	1	10	31	1635	Good	1357	83	Good	Good	Example
78	AN	88	1	11	29	1834	Good	1486	81	Good	Poor	Comparative Example
79	AO	93	0	7	28	1603	Good	1330	83	Good	Excellent	Example
80	AP	90	1	9	22	1493	Good	1179	79	Good	Excellent	Example
81	AQ	86	0	14	31	1219	Good	1012	83	Good	Excellent	Example
82	AR	95	0	5	32	1622	Good	1362	84	Good	Excellent	Example
83	AS	85	1	14	29	1210	Good	992	82	Good	Excellent	Example
84	AT	93	0	7	23	1522	Good	1233	81	Good	Good	Example
85	AU	90	0	10	23	1551	Good	1272	82	Good	Good	Example
86	AV	93	1	6	35	1654	Good	1406	85	Good	Excellent	Example
87	AW	89	1	10	22	1512	Good	1210	80	Good	Good	Example
88	AX	86	0	14	25	1216	Good	948	78	Good	Excellent	Example
89	AY	92	1	7	20	1502	Good	1172	78	Good	Good	Example
90	AZ	90	0	10	26	1413	Good	1145	81	Good	Excellent	Example
91	BA	87	2	11	34	1455	Good	1149	79	Good	Good	Example
92	BB	93	0	7	27	1510	Good	1208	80	Good	Excellent	Example
93	BC	91	1	8	26	1650	Good	1304	79	Good	Good	Example
94	BD	92	0	8	26	1211	Good	981	81	Good	Excellent	Example
95	BE	87	1	12	27	1632	Good	1289	79	Good	Good	Example
96	BF	88	0	12	35	1201	Good	973	81	Good	Excellent	Example
97	BG	88	1	11	27	1490	Good	1222	82	Good	Excellent	Example
98	BH	89	1	10	27	1457	Good	1180	81	Good	Excellent	Example
99	BI	91	1	8	27	1461	Good	1169	80	Good	Excellent	Example
100	BJ	92	0	8	33	1539	Good	1200	78	Good	Excellent	Example
101	BK	93	0	7	27	1536	Good	1275	83	Good	Excellent	Example
102	BL	90	0	10	27	1469	Good	1161	79	Good	Excellent	Example
103	BM	92	0	8	30	1530	Good	1255	82	Good	Excellent	Example
104	BN	89	1	10	31	1477	Good	1182	80	Good	Excellent	Example
105	BO	89	0	11	28	1474	Good	1223	83	Good	Excellent	Example

Underlined: indicates value outside scope of present disclosure.

As indicated in Table 3, all of the Examples passed testing in terms of TS, dimensional accuracy, and shear angle range. Further, all of the members obtained by forming or joining using the steel sheets of the Examples had the desired shapes without any cracks. Further, in such members, delayed fracture did not occur even when the shear angle of the shearing was changed. The dimensional accuracy was also good.
In contrast, for the Comparative Examples, at least one of TS, dimensional accuracy, and shear angle range did not pass testing.

Claims

A steel sheet comprising: a chemical composition containing, in mass%,
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,

O: 0.0100 % or less, and

Al: 1.000 % or less,

with the balance being Fe and inevitable impurity; and a steel microstructure wherein

area fraction of tempered martensite is 83 % or more,

area fraction of retained austenite is less than 3 %,

total area fraction of ferrite and bainitic ferrite is 5 % or more and less than 15 %, and

occupancy rate of prior austenite grain boundaries by the ferrite and the bainitic ferrite is 20 % or more.
The steel sheet according to claim 1, wherein the chemical composition further contains, in mass%, at least one selected from the group consisting of
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 steel sheet according to claim 1, further comprising a coated or plated layer on a surface.
The steel sheet according to claim 2, further comprising a coated or plated layer on a surface.
A member made using the steel sheet according to any one of claims 1 to 4.
A method of producing the steel sheet according to any one of claims 1 to 4, the method comprising:
a preparation process of preparing a blank sheet having the chemical composition according to claim 1 or 2;

a heating process of heating the blank sheet under a set of conditions including an average heating rate in a temperature range from 700 °C to 750 °C of 5.0 °C/s or less, and

a maximum arrival temperature T1 of 800 °C or more and 900 °C or less;

a first cooling process of cooling the blank sheet under a set of conditions including an average cooling rate in a temperature range from the maximum arrival temperature T1 to an intermediate holding temperature T2 of 0.10 °C/s or more and 5.00 °C/s or less;

an intermediate holding process of holding the blank sheet under a set of conditions including the intermediate holding temperature T2 being 600 °C or more and 750 °C or less,

an intermediate holding time t2 of 1.0 s or longer and 2000.0 s or shorter, and

a tension applied to the blank sheet of 5 MPa or more;

a second cooling process of cooling the blank sheet under a set of conditions including an average cooling rate in a temperature range from 300 °C to 100 °C of 300 °C/s or more,

to a second cooling end temperature; and

a tempering process of tempering the blank sheet under a set of conditions including a tempering temperature T3 of 100 °C or more and 400 °C or less, and

a tempering time t3 of 10 s or longer and 10,000 s or shorter.
The method of producing a steel sheet according to claim 6, further comprising a coating or plating process of applying a coating or plating treatment to the blank sheet between the intermediate holding process and the second cooling process, or after the tempering process.
A method of producing a member, wherein the steel sheet according to any one of claims 1 to 4 is subjected to at least one of a forming process or a joining process to produce the member.