EP4621090A1

EP4621090A1 - Steel sheet and member, and method for producing said steel sheet and method for producing said member

Info

Publication number: EP4621090A1
Application number: EP23918506.9A
Authority: EP
Inventors: Katsuya Hata; Shotaro TERASHIMA; Tatsuya Nakagaito; Seisuke Tsuda
Original assignee: JFE Steel Corp
Current assignee: JFE Steel Corp
Priority date: 2023-01-26
Filing date: 2023-10-19
Publication date: 2025-09-24
Also published as: CN120584208A; WO2024157551A1; JPWO2024157551A1; KR20250107865A; EP4621090A4; JP7541653B1; MX2025008614A

Abstract

Provided is a steel sheet having high strength, high YS, excellent ductility, and excellent bendability. A steel sheet comprises a specified chemical composition and steel microstructure. In particular, a grain number density of retained austenite, a ratio, average C concentration, average intergrain distance, and maximum intergrain distance of grains with an aspect ratio of 3 or less, and [Mn]c/[Mn] in the steel microstructure are appropriately controlled.

Description

TECHNICAL FIELD

The present disclosure relates to a steel sheet, a member made from the steel sheet, and production methods therefor.

BACKGROUND

In recent years, the automobile industry has been attempting to reduce exhaust gases such as CO₂ for global environment protection. Specifically, efforts have been made to reduce the amount of exhaust gas by strengthening and reducing the thickness of steel sheets as material for automotive members to thus make the automotive body more lightweight and improve fuel efficiency.
As such steel sheets used as material for automotive members, for example, Patent Literature (PTL) 1 discloses "A high-strength thin steel sheet with excellent elongation and uniform paint bake hardening performance, comprising: a steel composition containing, in mass%, C: 0.05 % or more and 0.35 % or less, Si: 0.05 % or more and 2.5 % or less, Mn: 0.6 % or more and 3.0 % or less, P: 0.001 % or more and 0.1 % or less, S: 0.0002 % or more and 0.05 % or less, N: 0.0010 % or more and 0.020 % or less, and Al: 0.001 % or more and 2.0 % or less with the balance consisting of iron and inevitable impurities; and a metallic microstructure containing one or more of ferrite, bainite, and tempered martensite and containing 3 % or more of retained austenite, wherein an average grain size of the austenite is 1 µm or more and 8 µm or less, and at interfaces where the austenite grains contact the ferrite, bainite, and tempered martensite, 50 % or more of austenite grains have an austenite grain central concentration Cgc and an austenite grain boundary concentration Cgb that satisfy (Formula 1): $Cgb / Cgc \geq 1.1$
PTL 2 discloses "A galvannealed steel sheet with excellent ductility and corrosion resistance, containing, in mass%, C: 0.10 % to 0.50 %, Mn: 1.0 % to 3.0 %, Si: 0.005 % to 2.5 %, Al: 0.005 % to 2.5 %, P: 0.05 % or less, S: 0.02 % or less, and N: 0.006 % or less where Si + Al ≥ 0.8 %, and comprising a microstructure that contains, in area ratio, ferrite: 10 % to 75 %, and retained austenite: 2 % to 30 %, wherein C content in the retained austenite is 0.8 % to 1.0 %."
PTL 3 discloses "A high-strength steel sheet with excellent elongation and press forming stability, comprising: a steel composition containing, in mass%, C: 0.05 % or more and 0.35 % or less, Si: 0.05 % or more and 2.0 % or less, Mn: 0.8 % or more and 3.0 % or less, P: 0.0010 % or more and 0.1 % or less, S: 0.0005 % or more and 0.05 % or less, N: 0.0010 % or more and 0.010 % or less, and Al: 0.01 % or more and 2.0 % or less with the balance consisting of iron and inevitable impurities; and a microstructure containing, in area ratio, ferrite phase and bainite phase: 10 % or more and 93 % or less in total, retained austenite phase: 5 % or more and 30 % or less, and martensite phase: 5 % or more and 20 % or less, wherein the retained austenite phase is in a lath-like and island-like form, and an area ratio yi of the island-like retained austenite phase and an area ratio y of the total retained austenite phase satisfy the following Formula (1): $0.7 \geq γi / γ \geq 0.3$

CITATION LIST

Patent Literature

PTL 1: JP 2012-031505 A
PTL 2: JP 2011-168816 A
PTL 3: JP 2012-041573 A

SUMMARY

(Technical Problem)

Typically, increasing the strength of a steel sheet causes a decrease in ductility. Steel sheets as material for automotive members are, however, required to have both high strength and excellent ductility, specifically, improved total elongation (hereinafter also simply referred to as El) in tensile tests.
In particular, steel sheets used in automotive framework structural members and the like among automotive members are required to have high member strength when press-formed. For example, an effective way of improving the strength of automotive members is to enhance the yield stress (hereinafter also simply referred to as YS) of steel sheets.
Steel sheets used in automotive framework structural members and the like are also required to have excellent formability, especially excellent bendability, because they are formed into complex shapes.
The steel sheets disclosed in PTL 1 to PTL 3 do not satisfy all of these required properties. Moreover, with the technique described in PTL 2, it is necessary to hold the steel sheet for a long time after annealing in order to stabilize retained austenite. This requires a large annealing line, and is likely to cause an increase in plant costs.
It could therefore be helpful to provide a steel sheet having high strength, high YS, excellent ductility, and excellent bendability, together with an advantageous production method therefor.
It could also be helpful to provide a member made from the steel sheet and a production method therefor.
Here, "high strength" and "high YS" mean that the tensile strength (hereinafter also referred to as TS) and YS measured in a tensile test conforming to JIS Z 2241 satisfy the following respective formulas:

TS $780 MPa \leq TS$
YS $420 MPa \leq YS when 780 MPa \leq TS < 980 MPa$ $550 MPa \leq YS when 980 MPa \leq TS .$

"Excellent ductility" means that the total elongation (El) measured in a tensile test conforming to JIS Z 2241 satisfies the following formula: $19 % \leq El when 780 MPa \leq TS < 980 MPa$ $10 % \leq El when 980 MPa \leq TS .$
"Excellent bendability" means that R/t measured in a V-bend test conforming to JIS Z 2248 satisfies the following formula: $2.0 \geq R / t when 780 MPa \leq TS < 980 MPa$ $4.0 \geq R / t when 980 MPa \leq TS,$ where R is the limit bending radius (mm), and t is the sheet thickness (mm) of the steel sheet.

(Solution to Problem)

Upon careful examination, we discovered the following.

(1) By adjusting the chemical composition to a specified range and limiting each of the total area ratio of ferrite and bainite and the area ratio of martensite to 10 % or more and 87 % or less, it is possible to achieve both high strength and excellent ductility.
(2) By limiting the area ratio of retained austenite to 3 % or more and the number density of grains constituting retained austenite to 0.05 grains/µm² or more, it is possible to improve both ductility and bendability.
(3) By causing grains with an aspect ratio of 3 or less among the grains constituting retained austenite to satisfy the following (A), (B), (C), and (D), it is possible to improve the stability of retained austenite. Consequently, deformation-induced transformation from retained austenite to hard fresh martensite is suppressed. Here, deformation-induced transformation occurs during primary processing such as press forming. As a result of controlling the distribution form of retained austenite, stress concentration can be suppressed even if hard fresh martensite forms. This further improves both ductility and bendability.
1. (A) The ratio of grains with an aspect ratio of 3 or less to all grains constituting retained austenite is 60 % or more in area ratio.
2. (B) The average C concentration of grains with an aspect ratio of 3 or less is 0.3 mass% or more.
3. (C) The average value of the shortest distance between grains with an aspect ratio of 3 or less is 5 µm or less.
4. (D) The maximum value of the shortest distance between grains with an aspect ratio of 3 or less is 15 µm or less.
(4) By appropriately controlling the concentration distribution of Mn in the steel, specifically, by limiting [Mn]c/[Mn] to 1.05 or more and 2.00 or less, it is possible to further improve both ductility and bendability. Here, [Mn]c is the average Mn concentration (mass%) in the Mn-enriched regions in the steel, and [Mn] is the average Mn concentration (mass%) in the steel.

The present disclosure is based on these discoveries and further studies.
We thus provide:

1. A steel sheet comprising: a chemical composition containing (consisting of), in mass%, C: 0.05 % or more and 0.20 % or less, Si: 0.1 % or more and 1.8 % or less, Mn: 1.5 % or more and 3.0 % or less, P: 0.001 % or more and 0.100 % or less, S: 0.0500 % or less, Al: 0.010 % or more and 1.000 % or less, N: 0.0100 % or less, and one or both of Nb and Ti: 0.005 % or more and 0.200 % or less in total, with a balance consisting of Fe and inevitable impurities; a steel microstructure in which an area ratio of one or both of ferrite and bainite is 10 % or more and 87 % or less in total, an area ratio of martensite is 10 % or more and 87 % or less, an area ratio of retained austenite is 3 % or more, a number density of grains constituting the retained austenite is 0.05 grains/µm² or more, grains with an aspect ratio of 3 or less among the grains constituting the retained austenite satisfy the following (A), (B), (C), and (D), and [Mn]c/[Mn] is 1.05 or more and 2.00 or less, where [Mn]c is an average Mn concentration in mass% in a Mn-enriched region in steel and [Mn] is an average Mn concentration in mass% in the steel; and a tensile strength of 780 MPa or more,
1. (A) a ratio of the grains with an aspect ratio of 3 or less to all of the grains constituting the retained austenite is 60 % or more in area ratio,
2. (B) an average C concentration of the grains with an aspect ratio of 3 or less is 0.3 mass% or more,
3. (C) an average value of a shortest distance between the grains with an aspect ratio of 3 or less is 5 µm or less,
4. (D) a maximum value of the shortest distance between the grains with an aspect ratio of 3 or less is 15 µm or less.
2. The steel sheet according to 1., wherein the chemical composition further contains, in mass%, at least one selected from V: 0.45 % or less, B: 0.010 % or less, Cr: 1.0 % or less, Ni: 1.0 % or less, Mo: 1.0 % or less, Sb: 0.1 % or less, Sn: 0.1 % or less, Cu: 1.0 % or less, Ta: 0.1 % or less, W: 0.2 % or less, Mg: 0.01 % or less, Zn: 0.02 % or less, Co: 0.02 % or less, Zr: 0.2 % or less, Ca: 0.02 % or less, Se: 0.02 % or less, Te: 0.02 % or less, Ge: 0.02 % or less, As: 0.05 % or less, Sr: 0.02 % or less, Cs: 0.02 % or less, Hf: 0.02 % or less, Pb: 0.02 % or less, Bi: 0.02 % or less, and REM: 0.02 % or less.
3. The steel sheet according to 1. or 2., comprising a soft layer with a thickness of 1 µm or more and 50 µm or less, the soft layer being a region whose hardness is 65 % or less of hardness at a position of 1/4 of a sheet thickness of the steel sheet.
4. The steel sheet according to any one of 1. to 3., comprising a galvanized layer on a surface thereof.
5. The steel sheet according to 4., wherein the galvanized layer is a hot-dip galvanized layer or a galvannealed layer.
6. A member produced from the steel sheet according to any one of 1. to 5.
7. A production method for a steel sheet, the production method comprising: heating a steel slab having the chemical composition according to 1. or 2. under a condition of a slab heating temperature of 1220 °C or more and a slab heating time of 1.0 hour or more; thereafter hot rolling the steel slab under a condition of a rolling finish temperature of 840 °C or more and 1000 °C or less, an average cooling rate in a temperature range from the rolling finish temperature to 700 °C of 10 °C/sec or more, and a coiling temperature of 620 °C or less, to obtain a hot-rolled steel sheet; thereafter cold rolling the hot-rolled steel sheet under a condition of a rolling reduction ratio of 20 % or more and 80 % or less to obtain a cold-rolled steel sheet; thereafter heating the cold-rolled steel sheet under a condition of an average heating rate in a temperature range from 600 °C to 750 °C of 1 °C/sec or more and 15 °C/sec or less; thereafter annealing the cold-rolled steel sheet under a condition of an annealing temperature of 750 °C or more and 920 °C or less and an annealing time of 1 second or more and 30 seconds or less; thereafter cooling the cold-rolled steel sheet under a condition of an average cooling rate in a temperature range from the annealing temperature - 30 °C to 600 °C of 5 °C/sec or more and 100 °C/sec or less and a cooling stop temperature of 400 °C or more and 600 °C or less; and thereafter retaining the cold-rolled steel sheet under a condition of a retention time in a temperature range of 400 °C or more and 600 °C or less of 1 second or more and 90 seconds or less.
8. The production method for a steel sheet according to 7., wherein a dew point of an atmosphere in each of the heating of the cold-rolled steel sheet and the annealing is -35 °C or more, and a dew point of an atmosphere in the cooling is -35 °C or less.
9. The production method for a steel sheet according to 7. or 8., further comprising subjecting the cold-rolled steel sheet to galvanizing treatment after the retaining.
10. The production method for a steel sheet according to 9., wherein the galvanizing treatment is hot-dip galvanizing treatment or galvannealing treatment.
11. A production method for a member, the production method comprising subjecting the steel sheet according to any one of 1. to 5. to at least one of forming and joining to obtain a member.

(Advantageous Effect)

It is thus possible to obtain a steel sheet having high strength, high YS, excellent ductility, and excellent bendability. Such a steel sheet having high strength, high YS, excellent ductility, and excellent bendability can be used advantageously as material for automotive framework structural members having complex shapes, etc.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:
FIG. 1 illustrates an example of an observation image used to measure the grain number density of retained austenite, etc.

DETAILED DESCRIPTION

The presently disclosed technology will be described by way of embodiments below.

[1] Steel sheet

First, the chemical composition of a steel sheet according to one embodiment of the present disclosure will be described. The unit of the chemical composition is "mass%", which is hereinafter simply indicated as "%" unless otherwise noted.

C: 0.05 % or more and 0.20 % or less

C has the effect of enhancing the strength of martensite and bainite and forming the appropriate amount of these phases. This effect makes it possible to ensure the specified strength. If the C content is less than 0.05 %, the strength of martensite decreases. In addition, the area ratio of ferrite increases excessively, making it difficult to obtain the specified strength. If the C content is more than 0.20 %, TS increases excessively and El decreases. Moreover, the strength of martensite increases excessively, causing a decrease in bendability. The C content is therefore 0.05 % or more and 0.20 % or less. The C content is preferably 0.07 % or more, and more preferably 0.09 % or more. The C content is preferably 0.18 % or less, and more preferably 0.17 % or less.

Si: 0.1 % or more and 1.8 % or less

Si is an element that improves the strength of the steel sheet by solid solution strengthening. Si also increases the strength of ferrite, thus improving ductility while suppressing a decrease in strength. Si also promotes ferrite transformation in annealing and subsequent cooling. That is, Si influences the area ratio of ferrite. If the Si content is less than 0.1 %, the area ratio of ferrite decreases and ductility decreases. If the Si content is excessively high, particularly if the Si content is more than 1.8 %, the rolling load during hot rolling and cold rolling increases significantly, and also toughness decreases. The Si content is therefore 0.1 % or more and 1.8 % or less. The Si content is preferably 0.3 % or more, and more preferably 0.5 % or more. The Si content is preferably 1.5 % or less, and more preferably 1.0 % or less.

Mn: 1.5 % or more and 3.0 % or less

Mn is added to improve the hardenability of the steel and ensure the specified area ratios of martensite and bainite. If the Mn content is less than 1.5 %, hardenability is insufficient and ferrite forms excessively. This makes it difficult to achieve a TS of 780 MPa or more. If the Mn content is excessively high, bainite transformation is delayed and it is difficult to obtain the specified amount of retained austenite. This causes a decrease in ductility. The Mn content is therefore 1.5 % or more and 3.0 % or less. The Mn content is preferably 1.65 % or more, and more preferably 1.8 % or more. The Mn content is preferably 2.85 % or less, and more preferably 2.7 % or less.

P: 0.001 % or more and 0.100 % or less

P is an element that has the effect of solid solution strengthening and increases the TS of the steel sheet. To achieve this effect, the P content is 0.001 % or more. If the P content is more than 0.100 %, P segregates at and embrittles the prior austenite grain boundaries. In such a case, when bending stress is applied to the steel sheet, voids form and cracks propagate along the prior austenite grain boundaries, making it impossible to achieve the desired bendability. The P content is therefore 0.100 % or less. The P content is preferably 0.002 % or more due to production technology constraints. The P content is preferably 0.050 % or less, and more preferably 0.030 % or less.

S: 0.0500 % or less

S forms MnS and the like, causing a decrease in ductility. Moreover, if Ti is contained together with S, TiS, Ti(C, S), etc. may form, causing a decrease in bendability. The S content is therefore 0.0500 % or less. The S content is preferably 0.0100 % or less, more preferably 0.0080 % or less, and further preferably 0.0050 % or less. No lower limit is placed on the S content, but the S content is preferably 0.0001 % or more, and more preferably 0.0005 % or more.

Al: 0.010 % or more and 1.000 % or less

Al is an element that promotes ferrite transformation in annealing and subsequent cooling. That is, Al influences the area ratio of ferrite. If the Al content is less than 0.010 %, the area ratio of ferrite decreases and ductility decreases. If the Al content is more than 1.000 %, the area ratio of ferrite increases excessively, making it difficult to achieve a TS of 780 MPa or more. The Al content is therefore 0.010 % or more and 1.000 % or less. The Al content is preferably 0.015 % or more, and more preferably 0.030 % or more. The Al content is preferably 0.500 % or less, and more preferably 0.100 % or less.

N: 0.0100 % or less

N is an element that forms nitride-based precipitates such as AlN that pin crystal grain boundaries, and can be added to improve elongation. If the N content is more than 0.0100 %, nitride-based precipitates such as AlN coarsen, so that elongation decreases. The N content is therefore 0.0100 % or less. The N content is preferably 0.0070 % or less, and more preferably 0.0050 % or less. No lower limit is placed on the N content, but the N content is preferably 0.0006 % or more due to production technology constraints.

One or both of Nb and Ti: 0.005 % or more and 0.200 % or less in total

Nb and Ti are elements that contribute to improving TS, ductility, and bendability through, for example, the refinement of prior austenite grains. That is, Nb and Ti contribute to increasing TS through the refinement of the internal structure of martensite and bainite by the refinement of prior austenite grains. Nb and Ti also contribute to increasing TS through the formation of fine precipitates, such as carbides, nitrides, and carbonitrides, in hot rolling and annealing. In addition, Nb and Ti increase the nucleation sites of ferrite and bainite in cooling and retention by the foregoing refinement of prior austenite grains and formation of fine precipitates, and promote ferrite transformation and bainite transformation. With the increase of the nucleation sites of ferrite and bainite, the distribution state of the grains constituting retained austenite can be appropriately controlled. In particular, grains with an aspect ratio of 3 or less can be appropriately controlled to satisfy the foregoing (C) and (D). Thus, Nb and Ti also contribute to improving ductility and bendability. To achieve these effects, the total content of Nb and Ti is 0.005 % or more. If the total content of Nb and Ti is excessively high, a large amount of coarse precipitates and inclusions form, causing decreases in ductility and bendability. In particular, if Nb and Ti are added in combination, the precipitates tend to stabilize and remain as coarse inclusions. The total content of Nb and Ti is therefore 0.200 % or less. The total content of Nb and Ti is preferably 0.008 % or more, more preferably 0.010 % or more, further preferably 0.011 % or more, and even more preferably 0.015 % or more. The total content of Nb and Ti is preferably 0.150 % or less, and more preferably 0.080 % or less.
The respective contents of Nb and Ti are not limited as long as the total content of Nb and Ti is 0.005 % or more and 0.200 % or less,
For example, the Nb content is preferably 0.002 % or more, more preferably 0.005 % or more, and further preferably 0.010 % or more. The Nb content is preferably 0.200 % or less, more preferably 0.150 % or less, and further preferably 0.080 % or less.
The Ti content is preferably 0.002 % or more, more preferably 0.005 % or more, and further preferably 0.010 % or more. The Ti content is preferably 0.200 % or less, more preferably 0.150 % or less, and further preferably 0.080 % or less.
The basic chemical composition of the steel sheet according to one embodiment of the present disclosure has been described above. The steel sheet according to one embodiment of the present disclosure has a chemical composition containing the foregoing basic components with the balance containing Fe (iron) and inevitable impurities. It is preferable that the steel sheet according to one embodiment of the present disclosure has a chemical composition containing the foregoing basic components with the balance consisting of Fe and inevitable impurities. The steel sheet according to one embodiment of the present disclosure may contain, in addition to the foregoing basic components, at least one selected from the following as an optionally added element:

V: 0.45 % or less,
B: 0.010 % or less,
Cr: 1.0 % or less,
Ni: 1.0 % or less,
Mo: 1.0 % or less,
Sb: 0.1 % or less,
Sn: 0.1 % or less,
Cu: 1.0 % or less,
Ta: 0.1 % or less,
W: 0.2 % or less,
Mg: 0.01 % or less,
Zn: 0.02 % or less,
Co: 0.02 % or less,
Zr: 0.2 % or less,
Ca: 0.02 % or less,
Se: 0.02 % or less,
Te: 0.02 % or less,
Ge: 0.02 % or less,
As: 0.05 % or less,
Sr: 0.02 % or less,
Cs: 0.02 % or less,
Hf: 0.02 % or less,
Pb: 0.02 % or less,
Bi: 0.02 % or less, and
REM: 0.02 % or less.

No lower limits are placed on these optionally added elements because the effects according to the present disclosure are achieved as long as the contents of these elements are not more than the respective upper limits. If the content of any of these optionally added elements is less than the below-described preferred lower limit, the element is contained as an inevitable impurity.

V: 0.45 % or less

V increases TS by forming fine precipitates, such as carbides, nitrides, and carbonitrides, in hot rolling and annealing, as with Nb and Ti. To achieve this effect, the V content is preferably 0.001 % or more. The V content is more preferably 0.005 % or more. If the V content is more than 0.45 %, a large amount of coarse precipitates and inclusions may form, causing a decrease in ductility. Accordingly, in the case of adding V, the V content is preferably 0.45 % or less. The V content is more preferably 0.060 % or less.

B: 0.010 % or less

B is an element that enhances hardenability by segregating at the austenite grain boundaries. B also controls the formation and grain growth of ferrite in cooling after annealing. To achieve these effects, the B content is preferably 0.0001 % or more. The B content is more preferably 0.0002 % or more. If the B content is more than 0.010 %, the amount of nitride-based precipitates such as BN is excessive, which may cause a decrease in ductility. Accordingly, in the case of adding B, the B content is preferably 0.010 % or less. The B content is more preferably 0.0050 % or less, and further preferably 0.0030 % or less.

Cr: 1.0 % or less

Cr is an element that enhances hardenability and promotes the formation of martensite, thereby increasing TS. To achieve this effect, the Cr content is preferably 0.0005 % or more. The Cr content is more preferably 0.010 % or more. If the Cr content is more than 1.0 %, the area ratio of martensite may increase, causing a decrease in ductility. Accordingly, in the case of adding Cr, the Cr content is preferably 1.0 % or less. The Cr content is more preferably 0.60 % or less, and further preferably 0.30 % or less.

Ni: 1.0 % or less

Ni is an element that enhances hardenability and promotes the formation of martensite, thereby increasing TS. To achieve this effect, the Ni content is preferably 0.005 % or more. The Ni content is more preferably 0.020 % or more. If the Ni content is more than 1.0 %, the area ratio of martensite may increase, causing a decrease in ductility. Accordingly, in the case of adding Ni, the Ni content is preferably 1.0 % or less. The Ni content is more preferably 0.5 % or less.

Mo: 1.0 % or less

Mo is an element that enhances hardenability and promotes the formation of martensite, thereby increasing TS. To achieve this effect, the Mo content is preferably 0.010 % or more. The Mo content is more preferably 0.030 % or more. If the Mo content is more than 1.0 %, the area ratio of martensite may increase, making it impossible to achieve the desired ductility. Accordingly, in the case of adding Mo, the Mo content is preferably 1.0 % or less. The Mo content is more preferably 0.5 % or less, and further preferably 0.3 % or less.

Sb: 0.1 % or less

Sb is an element effective in suppressing the diffusion of C near the steel sheet surface during annealing and controlling the formation of a soft layer near the steel sheet surface. If the soft layer increases excessively near the steel sheet surface, it may be difficult to achieve a TS of 780 MPa or more. The Sb content is therefore preferably 0.002 % or more. The Sb content is more preferably 0.005 % or more. If the Sb content is more than 0.1 %, castability decreases. Accordingly, in the case of adding Sb, the Sb content is preferably 0.1 % or less. The Sb content is more preferably 0.06 % or less, and further preferably 0.04 % or less.

Sn: 0.1 % or less

Sn suppresses oxidation and nitridation near the steel sheet surface to thus suppress the resulting decrease in the C and B contents near the steel sheet surface. This suppresses excessive ferrite formation near the steel sheet surface, and contributes to achieving a TS of 780 MPa or more. From this viewpoint, the Sn content is preferably 0.002 % or more. If the Sn content is more than 0.1 %, castability decreases. Accordingly, in the case of adding Sn, the Sn content is preferably 0.1 % or less. The Sn content is more preferably 0.04 % or less, and further preferably 0.02 % or less.

Cu: 1.0 % or less

Cu is an element that enhances hardenability and promotes the formation of martensite, thereby increasing TS. To achieve this effect, the Cu content is preferably 0.005 % or more. The Cu content is more preferably 0.020 % or more. If the Cu content is more than 1.0 %, the area ratio of martensite may increase excessively, causing a decrease in ductility. In addition, a large amount of coarse precipitates and inclusions may form, causing a decrease in ductility. Accordingly, in the case of adding Cu, the Cu content is preferably 1.0 % or less. The Cu content is more preferably 0.2 % or less.

Ta: 0.1 % or less

Ta increases TS by forming fine precipitates, such as carbides, nitrides, and carbonitrides, in hot rolling and annealing, as with Ti, Nb, and V. Ta also partially dissolves in Nb carbides and Nb carbonitrides to form composite precipitates such as (Nb,Ta)(C,N). This suppresses the coarsening of precipitates and stabilizes strengthening by precipitation, thereby further improving TS. To achieve these effects, the Ta content is preferably 0.001 % or more. If the Ta content is more than 0.1 %, a large amount of coarse precipitates and inclusions may form. Such coarse precipitates and inclusions may reduce ductility and bendability. Accordingly, in the case of adding Ta, the Ta content is preferably 0.1 % or less. The Ta content is more preferably 0.05 % or less.

W: 0.2 % or less

W increases TS by forming fine precipitates, such as carbides, nitrides, and carbonitrides, in hot rolling and annealing, as with Ti, Nb, and V. To achieve this effect, the W content is preferably 0.001 % or more. The W content is more preferably 0.005 % or more. If the W content is more than 0.2 %, a large amount of coarse precipitates and inclusions form, causing a decrease in ductility. Accordingly, in the case of adding W, the W content is preferably 0.2 % or less. The W content is more preferably 0.060 % or less.

Mg: 0.01 % or less

Mg is an effective element for spheroidizing inclusions such as sulfides and oxides to improve the hole expansion formability and bendability of the steel sheet. To achieve this effect, the Mg content is preferably 0.0001 % or more. If the Mg content is more than 0.01 %, surface quality decreases. Besides, bendability may decrease. Accordingly, in the case of adding Mg, the Mg content is preferably 0.01 % or less. The Mg content is more preferably 0.005 % or less, and further preferably 0.001 % or less.

Zn: 0.02 % or less

Zn is an effective element for spheroidizing inclusions to improve the bendability of the steel sheet. To achieve this effect, the Zn content is preferably 0.001 % or more. If the Zn content is more than 0.02 %, a large amount of coarse precipitates and inclusions may form, causing a decrease in bendability. Accordingly, in the case of adding Zn, the Zn content is preferably 0.02 % or less.

Co: 0.02 % or less

Co is an effective element for spheroidizing inclusions to improve the bendability of the steel sheet, as with Zn. To achieve this effect, the Co content is preferably 0.001 % or more. If the Co content is more than 0.02 %, a large amount of coarse precipitates and inclusions may form, causing a decrease in bendability. Accordingly, in the case of adding Co, the Co content is preferably 0.02 % or less.

Zr: 0.2 % or less

Zr contributes to higher strength by refining prior austenite grains. Zr also contributes to higher strength by reducing, for example, the block size and Bain unit size which are the internal structural units of martensite and bainite, through the refinement of prior austenite grains. In addition, Zr improves castability. To achieve these effects, the Zr content is preferably 0.001 % or more. If the Zr content is high, the amount of coarse ZrN-based and ZrS-based precipitates remaining undissolved in slab heating increases, and ductility decreases. Accordingly, in the case of adding Zr, the Zr content is preferably 0.2 % or less. The Zr content is more preferably 0.05 % or less, and further preferably 0.01 % or less.

Ca: 0.02 % or less

Ca exists as inclusions in steel. If the Ca content is more than 0.02 %, a large amount of coarse inclusions may form, causing ductility and bendability to decrease. Surface quality degrades, too. Accordingly, in the case of adding Ca, the Ca content is preferably 0.02 % or less. No lower limit is placed on the Ca content, but the Ca content is preferably 0.0005 % or more, for example. The Ca content is more preferably 0.0010 % or more due to production technology constraints.
Se: 0.02 % or less, Te: 0.02 % or less, Ge: 0.02 % or less, As: 0.05 % or less, Sr: 0.02 % or less, Cs: 0.02 % or less, Hf: 0.02 % or less, Pb: 0.02 % or less, Bi: 0.02 % or less, and REM: 0.02 % or less
Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi, and REM are each an element effective in improving the bendability of the steel sheet. To achieve this effect, the Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi, and REM contents are each preferably 0.0001 % or more. If the Se, Te, Ge, Sr, Cs, Hf, Pb, Bi, and REM contents are each more than 0.02 % or the As content is more than 0.05 %, a large amount of coarse precipitates and inclusions may form, causing a decrease in bendability. Accordingly, in the case of adding these elements, the Se, Te, Ge, Sr, Cs, Hf, Pb, Bi, and REM contents are each preferably 0.02 % or less, and the As content is preferably 0.05 % or less. Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi, and REM may be added alone or in combination.
The elements other than those described above are Fe and inevitable impurities.
Next, the steel microstructure of the steel sheet according to one embodiment of the present disclosure will be described.
The steel microstructure of the steel sheet according to one embodiment of the present disclosure is a steel microstructure in which the area ratio of one or both of ferrite and bainite is 10 % or more and 87 % or less in total, the area ratio of martensite is 10 % or more and 87 % or less, the area ratio of retained austenite is 3 % or more, the number density of grains constituting retained austenite is 0.05 grains/µm² or more, grains with an aspect ratio of 3 or less among the grains constituting retained austenite satisfy the foregoing (A), (B), (C), and (D), and [Mn]c/[Mn] is 1.05 or more and 2.00 or less, where [Mn]c is the average Mn concentration (mass%) in the Mn-enriched regions in the steel and [Mn] is the average Mn concentration (mass%) in the steel.
The reasons for these limitations will be given below. The area ratio of each phase is the ratio of the area occupied by the phase to the area of the entire steel microstructure.
Total area ratio of one or both of ferrite and bainite (hereinafter also referred to as "total area ratio of ferrite and bainite"): 10 % or more and 87 % or less
Ferrite and bainite are soft, and thus are effective in obtaining excellent ductility. To achieve the desired ductility, the total area ratio of ferrite and bainite is 10 % or more. If the area ratio of ferrite and bainite is excessively high, it is difficult to achieve a TS of 780 MPa or more. The total area ratio of ferrite and bainite is therefore 87 % or less. The total area ratio of ferrite and bainite is preferably 20 % or more, and more preferably 30 % or more. The total area ratio of ferrite and bainite is preferably 75 % or less, and more preferably 65 % or less. One of ferrite and bainite may be contained. Both ferrite and bainite may be contained.

Area ratio of martensite: 10 % or more and 87 % or less

Martensite is a microstructure that is hard and is necessary to increase the strength of the steel sheet. If the area ratio of martensite is less than 10 %, the desired TS cannot be achieved. If the area ratio of martensite is excessively high, ductility decreases. The area ratio of martensite is therefore 10 % or more and 87 % or less. The area ratio of martensite is preferably 20 % or more, and more preferably 30 % or more. The area ratio of martensite is preferably 75 % or less, and more preferably 65 % or less.
Martensite is a hard microstructure formed as a result of transformation from austenite at the martensite transformation point (also simply referred to as "Ms point") or less, and includes both fresh martensite as quenched and tempered martensite obtained by tempering fresh martensite.

Area ratio of retained austenite: 3 % or more

Retained austenite is a microstructure necessary to achieve both strength and ductility. If the area ratio of retained austenite is less than 3 %, it is impossible to achieve both strength and ductility. The area ratio of retained austenite is therefore 3 % or more. The area ratio of retained austenite is preferably 5 % or more, and more preferably 7 % or more. No upper limit is placed on the area ratio of retained austenite. If retained austenite is excessive, however, for example when the steel sheet is formed into a part, retained austenite transforms into martensite and bending crack initiation points increase. The area ratio of retained austenite is therefore preferably 20 % or less, and more preferably 15 % or less.
Retained austenite is austenite remaining as a result of austenite not transforming into ferrite, martensite, bainite, or any other metallic phase. Retained austenite forms, for example, when elements such as C are concentrated in austenite and as a result the martensite transformation point fall to room temperature or below (austenite remains without transforming).
The area ratio of the residual microstructures other than the above is preferably 15 % or less. The area ratio of the residual microstructures is more preferably 10 % or less, and further preferably 5 % or less. The area ratio of the residual microstructures may be 0 %.
The residual microstructures are not limited, and examples thereof include carbides such as cementite and pearlite. The types of the residual microstructures can be determined, for example, by observation with a scanning electron microscope (SEM). Pearlite is a microstructure that forms from austenite at a relatively high temperature and consists of layered ferrite and cementite.
The total area ratio of ferrite and bainite and the area ratio of martensite are measured at a position of 1/4 of the sheet thickness of the steel sheet as follows.
A sample is cut out from the steel sheet so that a sheet thickness section (L-section) parallel to the rolling direction of the steel sheet will be the observation plane. The observation plane of the sample is then polished using diamond paste, and then finish-polished using alumina. Following this, the observation plane of the sample is etched with nital to reveal the microstructure. The observation plane of the sample is then observed using a scanning electron microscope (SEM) at a magnification of 1500 times for five observation fields. Next, from the obtained microstructure images, the following regions are color-coded (defined) using Adobe Photoshop available from Adobe Systems Co., Ltd. The total area ratio of ferrite and bainite and the area ratio of martensite are then calculated by the point counting method. Specifically, 16 × 15 lattice points are set at intervals of 4.8 µm in a region of actual length: 82 µm × 57 µm of each SEM image. Next, the number of lattice points on ferrite and bainite and the number of lattice points on martensite are counted. The number of lattice points on ferrite and bainite and the number of lattice points on martensite are then each divided by the total number of lattice points and multiplied by 100, thus calculating the total area ratio of ferrite and bainite and the area ratio of martensite.
Ferrite: a massive black region. Ferrite is a microstructure made of crystal grains of a bcc lattice. Ferrite is formed as a result of transformation from austenite at relatively high temperatures.
Bainite: a black to dark gray region of a massive form, an irregular form, or the like. Bainite is a hard microstructure in which fine carbides are dispersed in acicular or platelike ferrite, as mentioned above. Bainite is formed from austenite at relatively low temperatures (higher than or equal to the Ms point). Bainite contains a relatively small amount of carbides.
Martensite: a white to light gray region. Martensite is a hard microstructure formed as a result of transformation from austenite at the Ms point or less, as mentioned above. Martensite includes both fresh martensite as quenched and tempered martensite obtained by tempering fresh martensite.
The area ratio of retained austenite is measured at a position of 1/4 of the sheet thickness of the steel sheet as follows.
The steel sheet is mechanically ground to the position of 1/4 of the sheet thickness in the sheet thickness direction (depth direction), and then chemically polished with oxalic acid to form an observation plane. The observation plane is then observed by X-ray diffractometry. CoKα rays are used for incident X-rays to determine the ratio of the diffraction intensity of each of (200), (220), and (311) planes of fcc iron (austenite) to the diffraction intensity of each of (200), (211), and (220) planes of bcc iron. The volume fraction of retained austenite is then calculated from the ratio of the diffraction intensity of each plane. Assuming that retained austenite is three-dimensionally homogeneous, the volume fraction of retained austenite is taken as the area ratio of retained austenite.
The area ratio of residual microstructures is calculated by subtracting the total area ratio of ferrite and bainite, the area ratio of martensite, and the area ratio of retained austenite calculated as described above from 100 %. [Area ratio of residual microstructures (%)] = 100 - [Total area ratio of ferrite and bainite (%)] - [Area ratio of martensite (%)] - [Area ratio of retained austenite (%)].
Number density of grains constituting retained austenite (hereinafter also referred to as "grain number density of retained austenite"): 0.05 grains/µm² or more
In the steel sheet according to one embodiment of the present disclosure, it is important that the grain number density of retained austenite is 0.05 grains/µm² or more. Hard martensite (hereinafter also referred to as "deformation-induced martensite") formed as a result of deformation-induced transformation from retained austenite during processing to form the steel sheet into a part (hereinafter also referred to simply as "processing") is a microstructure that promotes void formation and crack propagation during processing. If the grain number density of retained austenite is 0.05 grains/µm² or more, the stress on deformation-induced martensite is dispersed and stress concentration is suppressed, with it being possible to suppress void formation and crack propagation. The grain number density of retained austenite is therefore 0.05 grains/µm² or more. The grain number density of retained austenite is preferably 0.15 grains/µm² or more, and more preferably 0.25 grains/µm² or more. No upper limit is placed on the grain number density of retained austenite. If the grain number density of retained austenite is excessively high, however, the number of bending crack initiation points increases. The grain number density of retained austenite is therefore preferably 100 grains/µm² or less, and more preferably 10 grains/µm² or less.

(A) Ratio of grains with aspect ratio of 3 or less to all grains constituting retained austenite (hereinafter also referred to as "ratio of grains with an aspect ratio of 3 or less"): 60 % or more in area ratio
If the aspect ratio of retained austenite is high, stress concentration is likely to occur around deformation-induced martensite formed due to processing. This promotes void formation and crack propagation and reduces ductility and bendability. The ratio of the grains with an aspect ratio of 3 or less is therefore 60 % or more in area ratio. The ratio of the grains with an aspect ratio of 3 or less is preferably 65 % or more and more preferably 70 % or more in area ratio. No upper limit is placed on the ratio of the grains with an aspect ratio of 3 or less, and the ratio of the grains with an aspect ratio of 3 or less may be 100 % in area ratio.
(B) Average C concentration of grains with aspect ratio of 3 or less: 0.3 mass% or more
In the steel sheet according to one embodiment of the present disclosure, it is important that the average C concentration of the grains with an aspect ratio of 3 or less is 0.3 mass% or more. When the average C concentration of the grains with an aspect ratio of 3 or less is higher, the stability of retained austenite is higher and the balance between strength and ductility is better. If the average C concentration of the grains with an aspect ratio of 3 or less is less than 0.3 mass%, a good balance between strength and ductility cannot be achieved. Moreover, since the stability of retained austenite is low, for example, the amount of retained austenite transformed into deformation-induced martensite due to processing increases, and bendability decreases. The average C concentration of the grains with an aspect ratio of 3 or less is therefore 0.3 mass% or more. The average C concentration of the grains with an aspect ratio of 3 or less is preferably 0.5 mass% or more, and more preferably 0.7 mass% or more. No upper limit is placed on the average C concentration of the grains with an aspect ratio of 3 or less. If the C concentration of the grains with an aspect ratio of 3 or less is excessively high, however, the progress of deformation-induced transformation from retained austenite to martensite may be excessively suppressed, making it impossible to achieve sufficient strain hardenability. The average C concentration of the grains with an aspect ratio of 3 or less is therefore preferably 2.0 mass% or less.
(C) Average value of shortest distance between grains with aspect ratio of 3 or less (hereinafter also referred to as "average intergrain distance"): 5 µm or less
In the steel sheet according to one embodiment of the present disclosure, it is important to uniformly disperse the grains of retained austenite. By uniformly dispersing the grains of retained austenite, particularly grains with aspect ratio of 3 or less, stress concentration on the deformation-induced transformed martensite caused by processing is suppressed, and ductility and bendability are improved. For this reason, it is important to set the average intergrain distance to 5 µm or less. The average intergrain distance is preferably 4 µm or less, and more preferably 3 µm or less. Although no lower limit is placed on the average intergrain distance, the average intergrain distance is preferably 0.8 µm or more from the viewpoint of cost and productivity.
(D) Maximum value of shortest distance between grains with aspect ratio of 3 or less (hereinafter also referred to as "maximum intergrain distance"): 15 µm or less

In the steel sheet according to one embodiment of the present disclosure, it is important to uniformly disperse retained austenite. By uniformly dispersing the grains of retained austenite, particularly the grains with aspect ratio of 3 or less, stress concentration on the deformation-induced transformed martensite caused by processing is suppressed, and ductility and bendability are improved. If the maximum intergrain distance is large, particularly if the maximum intergrain distance is more than 15 µm, the grains of retained austenite exist locally, and ductility and bendability decrease. The maximum intergrain distance is therefore 15 µm or less. The maximum intergrain distance is preferably 10 µm or less, and more preferably 7 µm or less. Although no lower limit is placed on the maximum intergrain distance, for example, the maximum intergrain distance is preferably 1 µm or more.
The grain number density of retained austenite, the ratio of the grains with an aspect ratio of 3 or less, the average C concentration of the grains with an aspect ratio of 3 or less, the average intergrain distance, and the maximum intergrain distance are each determined using electron backscatter diffraction (EBSD) attached to FE-SEM.
A sample is cut out from the steel sheet so that a sheet thickness section (L-section) parallel to the rolling direction of the steel sheet will be the observation plane. The observation plane of the sample is then polished using diamond paste, and then finish-polished using alumina. Next, a 50 µm × 50 µm region is observed by EBSD, with the observation position being a position of 1/4 of the sheet thickness of the steel sheet. Image analysis is then performed by ImageJ using phase map, and the aspect ratio and barycentric position of each grain of retained austenite observed are obtained by the grain analysis function. For reference, FIG. 1 illustrates an example of an observation image. In the observation image, the white regions are grains of retained austenite. The grains constituting retained austenite are assumed to have an equivalent circular diameter of 0.8 µm or more. The number of grains constituting retained austenite is counted, and the number of grains is divided by the area of the observation region to yield the grain number density of retained austenite. Moreover, the area occupied by the grains with an aspect ratio of 3 or less among all of the counted grains constituting retained austenite is divided by the area occupied by all of the grains constituting retained austenite, and the result is multiplied by 100 to determine the ratio of the grains with an aspect ratio of 3 or less. Furthermore, for each grain with an aspect ratio of 3 or less, the distance to the nearest grain with an aspect ratio of 3 or less (the distance between the centroids of grains) is determined. The average value and maximum value of the calculated distances are then taken as the average intergrain distance and the maximum intergrain distance, respectively.
The average C concentration of the grains with an aspect ratio of 3 or less is determined as follows.
For the sample used in the above EBSD observation, the C concentration is measured in a lattice pattern in a 23 µm square region with a measurement interval of 0.1 µm at a position of 1/4 of the sheet thickness of the steel sheet as the observation position as in EBSD. A region of grains with an aspect ratio of 3 or less is extracted from the EBSD phase map, and the average value of the C concentration at each measurement point in the region is taken as the average C concentration of the grains with an aspect ratio of 3 or less.

[Mn]c/[Mn]: 1.05 or more and 2.00 or less

A large value of [Mn]c/[Mn], which is the ratio of the average Mn concentration in the Mn-enriched regions in the steel to the average Mn concentration in the steel, means that the concentration of Mn in austenite has progressed in annealing. The Mn concentration of austenite in the steel sheet immediately after annealing is one of the factors that determine whether the phase transformed from austenite in cooling and retention after annealing is ferrite and bainite, or martensite. Excessive concentration of Mn in austenite in annealing causes a delay in ferrite transformation and bainite transformation in cooling. This may make it impossible to obtain the desired area ratio of ferrite and bainite, and reduce ductility and bendability. In addition, the delay in ferrite transformation and bainite transformation suppresses concentration of C in untransformed austenite. This causes an insufficient amount of retained austenite that contributes to improving ductility. [Mn]c/[Mn] is therefore 2.00 or less. [Mn]c/[Mn] is preferably 1.80 or less, and more preferably 1.60 or less. If [Mn]c/[Mn] is 1.05 or more, that is, if the distribution of Mn concentration is moderately nonuniform, ferrite transformation and bainite transformation are promoted in austenite with a low Mn concentration. Accordingly, concentration of C in untransformed austenite progresses. As a result, good ductility and bendability are obtained. [Mn]c/[Mn] is therefore 1.05 or more. [Mn]c/[Mn] is preferably 1.10 or more, and more preferably 1.15 or more.

[Mn]c/[Mn] is determined as follows.

A sample is cut out from the steel sheet so that a sheet thickness section (L-section) parallel to the rolling direction of the steel sheet will be the observation plane. The observation plane of the sample is then polished using diamond paste, and then finish-polished using alumina. Next, the Mn concentration is measured in a lattice pattern in a 23 µm square region with a measurement interval of 0.1 µm by EPMA, at a position of 1/4 of the sheet thickness of the steel sheet as the observation position. The average value of the Mn concentration at all measurement points is taken as the average Mn concentration [Mn] (mass%) in the steel. In addition, the top 10 % of all measurement points with the highest Mn concentration are taken as Mn-enriched regions. The average value of the Mn concentration measured in the Mn-enriched regions is then taken as the average Mn concentration [Mn]c (mass%) in the Mn-enriched regions in the steel. Dividing [Mn]c by [Mn] yields [Mn]c/[Mn].
The steel sheet according to one embodiment of the present disclosure preferably includes a soft layer with a thickness of 1 µm or more and 50 µm or less. In particular, as a result of the steel sheet including a soft layer with a thickness of 1 µm or more and 50 µm or less from the steel sheet surface in the sheet thickness direction, better bendability can be obtained. Hence, it is preferable that the steel sheet includes a soft layer from the steel sheet surface in the sheet thickness direction and the thickness of the soft layer is 1 µm or more. If the soft layer is formed excessively, it is difficult to obtain the desired TS. Accordingly, in the case of providing a soft layer, the thickness of the soft layer is preferably 50 µm or less, and more preferably 40 µm or less.
Herein, a soft layer is a region whose hardness is 65 % or less of the hardness at the position of 1/4 of the sheet thickness of the steel sheet. The thickness of the soft layer is measured as follows.
A sheet thickness section (L-section) parallel to the rolling direction of the steel sheet is subjected to surface smoothing by wet polishing. Next, using a Vickers hardness meter, hardness measurement is performed at 1 µm intervals in the sheet thickness (depth) direction from a position 1 µm deep to a position 100 µm deep from the steel sheet surface under a load of 10 gf. Hardness measurement is also performed at 20 µm intervals in the sheet thickness (depth) direction from a position 100 µm deep from the steel sheet surface to the sheet thickness center position under the same conditions. Using the hardness measured at the position of 1/4 of the sheet thickness of the steel sheet as reference hardness, a depth position at which the hardness is 65 % or less of the reference hardness is identified on the surface side relative to the position of 1/4 of the sheet thickness of the steel sheet. The distance from the steel sheet surface to the deepest depth position at which the hardness is 65 % or less of the reference hardness (hereinafter also referred to as "depth of the region whose hardness is 65 % or less of the reference hardness") is then measured. This measurement is performed at five locations spaced at least 3 mm apart in the rolling direction, and the average depth of the regions where the measured hardness is 65 % or less of the reference hardness is taken as the thickness of the soft layer.
Since the steel microstructure of a steel sheet is approximately symmetrical in the sheet thickness direction, any one of the (front and back) surfaces of the steel sheet is used as a representative in measuring the thickness of the soft layer. For example, any one of the (front and back) surfaces of the steel sheet may be used as the starting point (0 sheet thickness position) of the sheet thickness position such as the position of 1/4 of the sheet thickness. If the soft layer is present on only one side of the steel sheet, the surface on which the soft layer is present is taken as the starting point (0 sheet thickness position) of the sheet thickness position. The thickness of the soft layer is the thickness per side.
A smaller fluctuation range of the thickness of the soft layer in the longitudinal direction (rolling direction) of the steel sheet is better. In particular, if the fluctuation range of the thickness of the soft layer is more than 20 µm, bendability may vary in the longitudinal direction of the steel sheet and decrease locally. The fluctuation range of the thickness of the soft layer is therefore preferably 20 µm or less. The fluctuation range of the thickness of the soft layer is preferably 15 µm or less, and more preferably 10 µm or less. No lower limit is placed on the fluctuation range of the thickness of the soft layer, and the fluctuation range of the thickness of the soft layer may be 0 µm.
Here, the fluctuation range of the thickness of the soft layer is the difference between the maximum and minimum values (maximum value - minimum value) of the depth of the region having a hardness of 65 % or less of the reference hardness measured in the above-described soft layer thickness measurement.
Next, the mechanical properties of the steel sheet according to one embodiment of the present disclosure will be described.

Tensile strength (TS): 780 MPa or more

The tensile strength of the steel sheet according to one embodiment of the present disclosure is 780 MPa or more. Although no upper limit is placed on the tensile strength of the steel sheet according to one embodiment of the present disclosure, for example, the tensile strength of the steel sheet according to one embodiment of the present disclosure is preferably less than 1180 MPa.
The yield stress (YS), total elongation (El), and R/t of the steel sheet according to one embodiment of the present disclosure are as described above. The tensile strength (TS), yield stress (YS), total elongation (El), and R/t are measured as described in the EXAMPLES section below.
The steel sheet according to one embodiment of the present disclosure may have a galvanized layer on its surface. The galvanized layer may be provided on only one side or both sides of the steel sheet. The term "galvanized layer" refers to a coated or plated layer containing Zn as a main component (Zn content: 50.0 mass% or more). Examples of the galvanized layer include a hot-dip galvanized layer and a galvannealed layer. A steel sheet including a galvanized layer is also referred to as a galvanized steel sheet. A steel sheet including a hot-dip galvanized layer and a steel sheet including a galvannealed layer are also referred to as a hot-dip galvanized steel sheet (GI) and a galvannealed steel sheet (GA), respectively.
For example, the hot-dip galvanized layer is preferably composed of Zn, 20.0 mass% or less of Fe, and 0.001 mass% or more and 1.0 mass% or less of Al. The hot-dip galvanized layer may optionally contain one or more elements selected from the group consisting of Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM in a total amount of 0.0 mass% or more and 3.5 mass% or less. The Fe content in the hot-dip galvanized layer is more preferably less than 7.0 mass%. The balance other than the foregoing elements consists of inevitable impurities.
For example, the galvannealed layer is preferably composed of Zn, 20 mass% or less of Fe, and 0.001 mass% or more and 1.0 mass% or less of Al. The galvannealed layer may optionally contain one or more elements selected from the group consisting of Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM in a total amount of 0.0 mass% or more and 3.5 mass% or less. The Fe content in the galvannealed layer is more preferably 7.0 mass% or more, and further preferably 8.0 mass% or more. The Fe content in the galvannealed layer is more preferably 15.0 mass% or less, and further preferably 12.0 mass% or less. The balance other than the foregoing elements consists of inevitable impurities.
The coating weight of the galvanized layer per side is not limited, but is preferably 20 g/m² or more and 80 g/m² or less.
The coating weight of the galvanized layer is measured as follows.
A treatment solution is prepared by adding 0.6 g of a corrosion inhibitor for Fe (IBIT^® 700BK (IBIT is a registered trademark in Japan, other countries, or both) produced by Asahi Chemical Co., Ltd.) to 1 L of a 10 mass% hydrochloric acid aqueous solution. A sample of the steel sheet is then immersed in the treatment solution to dissolve the galvanized layer. The mass loss of the sample before and after dissolution is measured, and the value is divided by the surface area of the steel sheet (the surface area of the coated portion) to calculate the coating weight (g/m²).
The sheet thickness of the steel sheet according to one embodiment of the present disclosure is not limited, but is preferably 0.5 mm or more and 3.5 mm or less.

[2] Member

Next, a member according to one embodiment of the present disclosure will be described.
The member according to one embodiment of the present disclosure is a member produced from (using) the above-described steel sheet (as material). For example, the steel sheet as material is subjected to at least one of forming and joining to produce the member.
Here, the steel sheet has a TS of 780 MPa or more, high YS, excellent ductility, and excellent bendability. Hence, the member according to one embodiment of the present disclosure has high strength and is particularly suitable for use in complex shaped members used in the automotive field.

[3] Production method for steel sheet

Next, a production method for a steel sheet according to one embodiment of the present disclosure will be described.
The production method for a steel sheet according to one embodiment of the present disclosure comprises: heating (slab heating) a steel slab having the above-described chemical composition under the condition of a slab heating temperature of 1220 °C or more and a slab heating time of 1.0 hour or more; thereafter hot rolling the steel slab under the condition of a rolling finish temperature of 840 °C or more and 1000 °C or less, an average cooling rate in the temperature range from the rolling finish temperature to 700 °C of 10 °C/sec or more, and a coiling temperature of 620 °C or less, to obtain a hot-rolled steel sheet; thereafter cold rolling the hot-rolled steel sheet under the condition of a rolling reduction ratio of 20 % or more and 80 % or less to obtain a cold-rolled steel sheet; thereafter heating the cold-rolled steel sheet under the condition of an average heating rate in the temperature range from 600 °C to 750 °C of 1 °C/sec or more and 15 °C/sec or less; thereafter annealing the cold-rolled steel sheet under the condition of an annealing temperature of 750 °C or more and 920 °C or less and an annealing time of 1 second or more and 30 seconds or less; thereafter cooling the cold-rolled steel sheet under the condition of an average cooling rate in the temperature range from (the annealing temperature - 30 °C) to 600 °C of 5 °C/sec or more and 100 °C/sec or less and a cooling stop temperature of 400 °C or more and 600 °C or less; and thereafter retaining the cold-rolled steel sheet under the condition of a retention time in the temperature range of 400 °C or more and 600 °C or less of 1 second or more and 90 seconds or less.
Here, each temperature is the surface temperature of the steel slab or steel sheet unless otherwise noted.
First, a steel slab having the above-described chemical composition is prepared. For example, steel material is subjected to smelting to obtain molten steel having the above-described chemical composition. The smelting method is not limited, and any known smelting method such as converter or electric furnace may be used. The obtained molten steel is then solidified into a steel slab. The method of obtaining the steel slab from the molten steel is not limited, and continuous casting, ingot casting, or thin slab casting may be used, for example. Continuous casting is preferable from the viewpoint of preventing macrosegregation. After producing the steel slab, a conventional method of cooling the steel slab to room temperature and then reheating the steel slab may be used. Alternatively, an energy-saving process such as hot direct rolling (a method in which the steel slab as a warm slab is, without being cooled to room temperature, charged into a heating furnace and hot rolled) or direct rolling (a method in which the steel slab is subjected to a short period of heat retention and then immediately rolled) may be used.

[Slab heating]

The steel slab is then heated. In the production method for a steel sheet according to one embodiment of the present disclosure, it is important to satisfy the following conditions.

Slab heating temperature: 1220 °C or more

If the slab heating temperature is 1220 °C or more, Nb- and Ti-based coarse precipitates formed during casting dissolve sufficiently. Thus, coarse precipitates can be reduced. It is also possible to uniformly disperse the grains of retained austenite, and in particular to limit the average intergrain distance and maximum intergrain distance to the specified ranges. This improves the ductility and bendability of the steel sheet as a finished product. The slab heating temperature is therefore 1220 °C or more. The slab heating temperature is preferably 1230 °C or more, and more preferably 1240 °C or more. Although no upper limit is placed on the slab heating temperature, the slab heating temperature is preferably 1400 °C or less, for example. The slab heating temperature is the maximum arrival temperature of the steel slab in slab heating.

Slab heating time: 1.0 hour or more

If the slab heating temperature is 1220 °C or more and the slab heating time is 1.0 hour or more, Nb- and Ti-based coarse precipitates formed during casting dissolve sufficiently. Thus, coarse precipitates can be reduced. It is also possible to uniformly disperse the grains of retained austenite, and in particular to limit the average intergrain distance and maximum intergrain distance to the specified ranges. This improves the ductility and bendability of the steel sheet as a finished product. The slab heating time is therefore 1.0 hour or more. The slab heating time is preferably 1.1 hours or more, and more preferably 1.2 hours or more. Although no upper limit is placed on the slab heating time, the slab heating time is preferably 3.0 hours or less, for example. The slab heating time is the holding time in the temperature range of 1220 °C or more.

[Hot rolling]

The steel slab is then hot rolled to obtain a hot-rolled steel sheet. In hot rolling, it is important to satisfy the following conditions.

Rolling finish temperature: 840 °C or more and 1000 °C or less

It is important to suppress concentration of Mn (variation in Mn concentration) in the microstructure of the steel sheet before annealing from the viewpoint of suppressing concentration of Mn in the steel microstructure of the steel sheet as a finished product. If the rolling finish temperature is less than 840 °C, the formation of ferrite is promoted and ferrite forms excessively before the hot-rolled steel sheet is coiled. This causes concentration of C in untransformed austenite. Excessive concentration of C in untransformed austenite promotes pearlite transformation. In other words, pearlite forms excessively in the steel microstructure of the hot-rolled steel sheet obtained after hot rolling. Pearlite is a layered microstructure of ferrite and cementite, and Mn is concentrated in cementite. This results in variation in Mn concentration. The rolling finish temperature is therefore 840 °C or more. The rolling finish temperature is preferably 850 °C or more. If the rolling finish temperature is excessively high, it may be difficult to cool the steel sheet to the below-described coiling temperature. The rolling finish temperature is therefore 1000 °C or less. The rolling finish temperature is preferably 950 °C or less, and more preferably 920 °C or less.
Average cooling rate in temperature range from rolling finish temperature to 700 °C (hereinafter also referred to as "first cooling rate"): 10 °C/sec or more
It is important to suppress concentration of Mn (variation in Mn concentration) in the microstructure of the steel sheet before annealing from the viewpoint of suppressing concentration of Mn in the steel microstructure of the finished product, as mentioned above. If the first cooling rate is low, an excessive amount of ferrite forms during cooling, causing concentration of C in untransformed austenite. Excessive concentration of C in untransformed austenite promotes pearlite transformation. In other words, pearlite forms excessively in the steel microstructure of the hot-rolled steel sheet obtained after hot rolling. As mentioned above, pearlite is a layered microstructure of ferrite and cementite, and Mn is concentrated in cementite. This results in variation in Mn concentration. Moreover, if the first cooling rate is low, precipitates such as carbides and nitrides of Nb and Ti coarsen. This makes it impossible to achieve the effect of increasing TS and improving ductility by prior austenite grain refinement and fine precipitates. The first cooling rate is therefore 10 °C/sec or more. The first cooling rate is preferably 15 °C/sec or more. Although no upper limit is placed on the first cooling rate, the first cooling rate is preferably 1000 °C/sec or less from the viewpoint of energy saving of the cooling line.

Coiling temperature: 620 °C or less

If the coiling temperature is more than 620 °C, pearlite increases excessively during coiling, and concentration of Mn is promoted. When the coiling temperature is lower, the amount of pearlite formed is smaller. Accordingly, a lower coiling temperature is preferable. A lower coiling temperature is also preferable from the viewpoint of finely precipitating carbides and nitrides of Nb and Ti. The coiling temperature is therefore 620 °C or less. The coiling temperature is preferably 600 °C or less, and more preferably 580 °C or less. If the coiling temperature is less than 400 °C, the steel sheet may harden excessively and fracture during cold rolling. The coiling temperature is therefore preferably 400 °C or more. The coiling temperature is more preferably 450 °C or more.
Descaling may be optionally performed in order to remove primary and secondary scale formed on the surface of the hot-rolled steel sheet. It is preferable to thoroughly pickle the hot-rolled steel sheet to reduce the amount of remaining scale before cold rolling the hot-rolled steel sheet. The hot-rolled steel sheet may be optionally subjected to hot-rolled sheet annealing from the viewpoint of reducing the load during cold rolling.

[Cold rolling]

The hot-rolled steel sheet is then cold rolled to obtain a cold-rolled steel sheet.

Rolling reduction ratio: 20 % or more and 80 % or less

The rolling reduction ratio in cold rolling is 20 % or more. If the rolling reduction ratio is less than 20 %, the steel microstructure tends to become coarse and non-uniform in annealing, causing a decrease in the TS and bendability of the finished product. The rolling reduction ratio is therefore 20 % or more. If the rolling reduction ratio is more than 80 %, the shape of the steel sheet is likely to be defective. There is also a possibility of non-uniform steel microstructure due to temperature unevenness in annealing and non-uniform galvanized coating weight. The rolling reduction ratio is therefore 80 % or less. The rolling reduction ratio is preferably 30 % or more. The rolling reduction ratio is preferably 70 % or less.

[Heating]

The cold-rolled steel sheet is then heated to the annealing temperature. Here, it is important to appropriately control the average heating rate in the temperature range from 600 °C to 750 °C
Average heating rate in temperature range from 600 °C to 750 °C (hereinafter also referred to as "heating rate"): 1 °C/sec or more and 15 °C/sec or less
If the time during which the cold-rolled steel sheet is retained in the temperature range from 600 °C to 750 °C (hereinafter also referred to as "heating temperature range") in the heating decreases, Mn diffuses and concentration of Mn in austenite is suppressed. In other words, when the retention time in the heating temperature range is longer, concentration of Mn in austenite is promoted more. Hence, it is effective to shorten the retention time in the heating temperature range, that is, to increase the heating rate. Moreover, by shortening the retention time in the heating temperature range, coarsening of prior austenite grains is suppressed. This makes it possible to secure the specified ratio of the grains with an aspect ratio of 3 or less. A higher heating rate is better also from the viewpoint of promoting ferrite transformation and bainite transformation and uniformly dispersing retained austenite. The heating rate is therefore 1 °C/sec or more. The heating rate is preferably 2 °C/sec or more, and more preferably 3 °C/sec or more. If the heating rate is more than 15 °C/sec, concentration of Mn in austenite in the heating is excessively suppressed. The heating rate is therefore 15 °C/sec or less. The heating rate is preferably 12 °C/sec or less, and more preferably 9 °C/sec or less.

Dew point of atmosphere: -35 °C or more

The dew point of the atmosphere in the heating is preferably -35 °C or more from the viewpoint of forming a soft layer of the desired thickness from the steel sheet surface in the sheet thickness direction and obtaining excellent bendability. If the dew point of the atmosphere is less than -35 °C, it is difficult to form a soft layer of the desired thickness. The dew point of the atmosphere in the heating is therefore preferably -35 °C or more. The dew point of the atmosphere in the heating is more preferably -20 °C or more, and further preferably -10 °C or more. Although no upper limit is placed on the dew point of the atmosphere in the heating, the dew point of the atmosphere in the heating is preferably 15 °C or less and more preferably 5 °C or less in order to limit the TS to the preferred range.

[Annealing]

The cold-rolled steel sheet is then annealed under the condition of an annealing temperature of 750 °C or more and 920 °C or less and an annealing time of 1 second or more and 30 seconds or less.

Annealing temperature: 750 °C or more and 920 °C or less

If the annealing temperature is less than 750 °C, the proportion of austenite formed during heating in the ferrite-austenite dual phase region is insufficient. Consequently, the area ratio of ferrite increases excessively after annealing, and the desired TS cannot be obtained. If the annealing temperature is more than 920 °C, the desired area ratio of ferrite and bainite cannot be obtained and ductility decreases. The annealing temperature is therefore 750 °C or more and 920 °C or less. The annealing temperature is preferably 880 °C or less. The annealing temperature herein is the maximum arrival temperature in annealing.

Annealing time: 1 second or more and 30 seconds or less

In the production method for a steel sheet according to one embodiment of the present disclosure, the annealing time is important for controlling the aspect ratio of grains constituting retained austenite. In detail, a shorter annealing time is better from the following viewpoints:

Suppressing grain growth during annealing and suppressing concentration of Mn in austenite.
Promoting ferrite transformation and bainite transformation and reducing the aspect ratio of grains constituting retained austenite.
Promoting concentration of C in grains with an aspect ratio of 3 or less.
Suppressing coarsening of austenite (grains) during annealing and uniformly dispersing retained austenite.

The annealing time is therefore 30 seconds or less. The annealing time is preferably 25 seconds or less, and more preferably 20 seconds or less.
If the annealing time is less than 1 second, coarse Fe-based precipitates do not melt, so that elongation decreases. The annealing time is therefore 1 second or more. The annealing time is preferably 3 seconds or more, and more preferably 5 seconds or more. The annealing time herein is the holding time at the annealing temperature.

Dew point of atmosphere: -35 °C or more

In the annealing following the foregoing heating, too, the dew point of the atmosphere is preferably -35 °C or more from the viewpoint of forming a soft layer of the desired thickness from the steel sheet surface in the sheet thickness direction and obtaining excellent bendability. If the dew point of the atmosphere is less than -35 °C, it is difficult to form a soft layer of the desired thickness. The dew point of the atmosphere in the annealing is therefore preferably -35 °C or more. The dew point of the atmosphere in the annealing is more preferably -20 °C or more, and further preferably -10 °C or more. Although no upper limit is placed on the dew point of the atmosphere in the annealing, the dew point of the atmosphere in the annealing is preferably 15 °C or less and more preferably 5 °C or less in order to limit the TS to the preferred range.

[Cooling]

The cold-rolled steel sheet annealed as described above is then cooled under the following conditions.

Average cooling rate in temperature range from (annealing temperature - 30 °C) to 600 °C: 5 °C/sec or more and 100 °C/sec or less

In cooling, it is necessary to appropriately control the cooling rate, particularly the average cooling rate in the temperature range from (annealing temperature - 30 °C) to 600 °C (hereinafter also referred to as "second cooling rate") in order to form ferrite and bainite. If the second cooling rate is low, ferrite forms excessively. In addition, pearlite forms excessively, too, and TS decreases. Moreover, an appropriate amount of retained austenite cannot be obtained. The second cooling rate is therefore 5 °C/sec or more. The second cooling rate is preferably 9 °C/sec or more, and more preferably 12 °C/sec or more. If the second cooling rate is more than 100 °C/sec, ferrite transformation and bainite transformation may be suppressed excessively, causing a decrease in ductility. The second cooling rate is therefore 100 °C/sec or less. The second cooling rate is preferably 75 °C/sec or less, and more preferably 50 °C/sec or less.

Cooling stop temperature: 400 °C or more and 600 °C or less

If the cooling stop temperature is less than 400 °C, the aspect ratio of bainite increases, as a result of which the number of grains with an aspect ratio of more than 3 among all grains constituting retained austenite increases. The cooling stop temperature is therefore 400 °C or more. The cooling stop temperature is preferably 430 °C or more, and more preferably 460 °C or more. If the cooling stop temperature is more than 600 °C, pearlite may form excessively, making it impossible to obtain the desired TS. The cooling stop temperature is therefore 600 °C or less. The cooling stop temperature is preferably 570 °C or less, and more preferably 540 °C or less.

Dew point of atmosphere: -35 °C or less

The dew point of the atmosphere in the cooling is preferably -35 °C or less from the viewpoint of homogenizing the soft layer formed from the steel sheet surface in the sheet thickness direction. If the dew point of the atmosphere during cooling is more than -35 °C, the soft layer may not be homogenized and unevenness may occur. The dew point of the atmosphere in the cooling is therefore preferably -35 °C or less. The dew point of the atmosphere in the cooling is more preferably -40 °C or less. Although no lower limit is placed on the dew point of the atmosphere in the cooling, the dew point of the atmosphere in the cooling is preferably -60 °C or more and more preferably -55 °C or more from the viewpoint of controllability.

[Retention]

The cold-rolled steel sheet cooled as described above is then retained in the temperature range of 400 °C or more and 600 °C or less for 1 second or more and 90 seconds or less.

Retention temperature range: 400 °C or more and 600 °C or less

The retention temperature range is 400 °C or more and 600 °C or less from the viewpoint of ensuring the appropriate amount of bainite and retained austenite. If the retention temperature range is less than 400 °C, the number of grains with an aspect ratio of more than 3 among all grains constituting retained austenite increases. If the retention temperature range is more than 600 °C, ferrite and bainite may form excessively, making it impossible to achieve the desired TS. The retention temperature range is therefore 400 °C or more and 600 °C or less. The retention temperature range is preferably 420 °C or more, and more preferably 440 °C or more. The retention temperature range is preferably 560 °C or less, and more preferably 520 °C or less. In the case of performing the below-described galvanizing treatment, particularly hot-dip galvanizing treatment or galvannealing treatment, it is preferable to reheat the cold-rolled steel sheet immediately before galvanizing treatment so that the temperature of the sheet entering the galvanizing bath will be higher than the temperature of the galvanizing bath.

Retention time: 1 second or more and 90 seconds or less

It is necessary to appropriately control the retention time in the retention temperature range (hereinafter also simply referred to as "retention time") in order to ensure the appropriate amount of retained austenite. When the retention time is longer, the amount of retained austenite is larger. The retention time is therefore 1 second or more. The retention time is preferably 7 seconds or more, and more preferably 15 seconds or more. If the retention time is excessively long, the amount of bainite is excessive, and martensite necessary to ensure strength cannot be obtained. The retention time is therefore 90 seconds or less. The retention time is preferably 80 seconds or less, and more preferably 70 seconds or less. Here, the retention time does not include the retention time in the temperature range of 400 °C or more and 600 °C or less in the cooling (before cooling is stopped).
After the retention, the cold-rolled steel sheet may be further subjected to surface treatment such as chemical conversion treatment or organic coating treatment.

[Galvanizing treatment]

The cold-rolled steel sheet may then be optionally subjected to galvanizing treatment. Examples of the galvanizing treatment include hot-dip galvanizing treatment and galvannealing treatment. The treatment conditions may be in accordance with conventional methods.
For example, in the case of hot-dip galvanizing treatment, it is preferable to immerse the cold-rolled steel sheet in a galvanizing bath at 440 °C or more and 500 °C or less and then adjust the coating weight by gas wiping or the like. The galvanizing bath is not limited as long as the composition of the galvanized layer described above is obtained. For example, a galvanizing bath having an Al content of 0.10 mass% or more and 0.23 mass% or less with the balance consisting of Zn and inevitable impurities is preferably used. In the case of performing hot-dip galvanizing treatment or galvannealing treatment (described below), it is preferable to reheat the cold-rolled steel sheet immediately before the galvanizing treatment so that the temperature of the sheet entering the galvanizing bath will be higher than the temperature of the galvanizing bath.
In the case of galvannealing treatment, it is preferable to perform hot-dip galvanizing treatment in the above-described manner and then perform alloying treatment in the temperature range of 450 °C or more and 600 °C or less. If the alloying temperature is less than 450 °C, the Zn-Fe alloying rate is excessively slow and alloying may be hindered. If the alloying temperature is more than 600 °C, untransformed austenite may transform into pearlite, causing decreases in TS and ductility. The alloying temperature in the alloying treatment is therefore preferably 450 °C or more and 600 °C or less. The alloying temperature in the alloying treatment is more preferably 460 °C or more, and further preferably 470 °C or more. The alloying temperature in the alloying treatment is more preferably 580 °C or less, and further preferably 560 °C or less.
The coating weight is preferably 20 g/m² or more and 80 g/m² or less per side. The coating weight can be adjusted by gas wiping or the like.
The steel sheet obtained as described above may be further subjected to temper rolling. If the elongation rate of temper rolling is more than 2.00 %, yield stress may increase, causing a decrease in dimensional accuracy when the steel sheet is formed into a member. The elongation rate of temper rolling is therefore preferably 2.00 % or less. Although no lower limit is placed on the elongation rate of temper rolling, the elongation rate of temper rolling is preferably 0.05 % or more from the viewpoint of productivity. Temper rolling may be performed by a device continuous with the annealing device for performing each process (i.e. online). Temper rolling may be performed by a device not continuous with the annealing device for performing each process (i.e. offline). The number of times temper rolling is performed may be one, or two or more. Rolling by a leveler or the like may be performed as long as the same elongation rate as in temper rolling can be achieved.
It is preferable to perform a series of treatments such as the above-described annealing and galvanizing treatment in a continuous annealing line (CAL) or a continuous galvanizing line (CGL) which is a hot-dip galvanizing line, from the viewpoint of productivity. After hot-dip galvanizing treatment, wiping may be performed to adjust the coating weight.
Conditions other than those described above are not limited and may be in accordance with conventional methods. The above-described production method for a steel sheet according to one embodiment of the present disclosure enables obtaining a steel sheet having high strength, high YS, excellent ductility, and excellent bendability, which is suitable for use in automotive members and the like.

[4] Production method for member

Next, a production method for a member according to one embodiment of the present disclosure will be described.
The production method for a member according to one embodiment of the present disclosure comprises subjecting the above-described steel sheet to at least one of forming and joining to obtain a member.
The forming method is not limited and, for example, a typical processing method such as press forming may be used. The joining method is not limited and, for example, typical welding such as spot welding, laser welding, or arc welding, riveting, or caulking may be used. Forming conditions and joining conditions are not limited, and may be in accordance with conventional methods.

EXAMPLES

Steel materials having the chemical compositions shown in Table 1 (the balance consisting of Fe and inevitable impurities) were each smelted in a converter and continuously cast to obtain a steel slab. Following this, the steel slab was heated under the conditions shown in Table 2, and then subjected to hot rolling consisting of rough rolling and finish rolling to obtain a hot-rolled steel sheet. The slab heating time of No. 13 is the holding time at the slab heating temperature. The obtained hot-rolled steel sheet was then pickled and subjected to cold rolling under the conditions shown in Table 2 to obtain a cold-rolled steel sheet. Next, the obtained cold-rolled steel sheet was subjected to heating, annealing, and cooling under the conditions shown in Table 2. Some of the cold-rolled steel sheets were then subjected to galvanizing treatment under the conditions shown in Table 2. This yielded a steel sheet as a finished product. Conditions not specified were in accordance with conventional methods.
In the galvanizing treatment, hot-dip galvanizing treatment or galvannealing treatment was performed to obtain a hot-dip galvanized steel sheet (hereinafter also referred to as "GI") or a galvannealed steel sheet (hereinafter also referred to as "GA"). In Table 2, "GI" and "GA" each indicate the type of galvanizing treatment, and "CR" as the type of galvanizing treatment means that the steel sheet was as cold-rolled with no galvanizing treatment performed.
In hot-dip galvanizing treatment, a galvanizing bath having a composition containing Al: 0.20 mass% with the balance consisting of Zn and inevitable impurities was used. The galvanizing bath temperature was 470 °C. The coating weight was about 45 g/m² to 72 g/m² per side (double-sided coating). The composition of the galvanized layer of the finally obtained GI contained Fe: 0.1 mass% to 1.0 mass% and Al: 0.2 mass% to 1.0 mass% with the balance consisting of Zn and inevitable impurities.
In galvannealing treatment, a galvanizing bath having a composition containing Al: 0.14 mass% with the balance consisting of Zn and inevitable impurities was used. The galvanizing bath temperature was 470 °C. The coating weight was about 45 g/m² per side (double-sided coating). The alloying temperature was 520 °C. The composition of the galvanized layer of the finally obtained GA contained Fe: 7 mass% to 15 mass% and Al: 0.1 mass% to 1.0 mass% with the balance consisting of Zn and inevitable impurities.
The steel sheets thus obtained were each used to identify the steel microstructure of the steel sheet and to measure the grain number density of retained austenite, (A) ratio of grains with an aspect ratio of 3 or less (area%), (B) average C concentration with an aspect ratio of 3 or less (mass%), (C) average intergrain distance (µm), (D) maximum intergrain distance (µm), [Mn]c/[Mn], and soft layer thickness according to the above-described procedures. The measurement results are shown in Table 3. For each steel sheet having a soft layer, the soft layer was formed on both sides of the steel sheet with the same thickness. In No. 25, no soft layer was found (the thickness of the soft layer was less than 1 µm), and accordingly the "Soft layer thickness" column in Table 2 is "0".
Moreover, a tensile test and a V-bend test were conducted according to the following procedures, and the tensile strength (TS), yield stress (YS), total elongation (El), and R/t were evaluated based on the following criteria.

TS
- Pass: 780 MPa ≤ TS
- Fail: TS < 780 MPa
YS
- Pass: 420 MPa ≤ YS when 780 MPa ≤ TS < 980 MPa
  550 MPa ≤ YS when 980 MPa ≤ TS
- Fail: YS < 420 MPa when 780 MPa ≤ TS < 980 MPa
  YS < 550 MPa when 980 MPa ≤ TS
El
- Pass: 19 % ≤ El when 780 MPa ≤ TS < 980 MPa
  10 % ≤ El when 980 MPa ≤ TS
- Fail: El < 19 % when 780 MPa ≤ TS < 980 MPa
  El < 10 % when 980 MPa ≤ TS
R/t
- Pass: 2.0 ≥ R/t when 780 MPa ≤ TS < 980 MPa
  4.0 ≥ R/t when 980 MPa ≤ TS
- Fail: R/t > 2.0 when 780 MPa ≤ TS < 980 MPa
  R/t > 4.0 when 980 MPa ≤ TS

The tensile test was conducted in accordance with JIS Z 2241. In detail, a JIS No. 5 test piece was collected from the obtained steel sheet so that the longitudinal direction would be orthogonal to the rolling direction of the steel sheet. The tensile test was conducted using the collected test piece at a crosshead speed of 10 mm/min to measure TS, YS, and El. The results are shown in Table 3.
The V (90°)-bend test was conducted in accordance with JIS Z 2248. In detail, a 100 mm × 35 mm test piece was collected from the steel sheet by shearing and end grinding, with the 100 mm side parallel to the direction (C) (transverse direction) orthogonal to the rolling direction. The V(90°)-bend test was conducted using the collected test piece under the following conditions:

Bending radius R: change with 0.5 mm pitch
Test method: die support, punch press
Forming load: 10 tons
Test speed: 30 mm/min
Holding time: 5 seconds
Bending direction: direction (C) orthogonal to rolling direction.

The test was conducted three times, and the minimum bending radius at which no cracks occurred in any of the three tests was determined as R. R was divided by the sheet thickness t to calculate R/t. The test piece was observed using a stereoscopic microscope produced by Leica at a magnification of 25 times. If a crack of 200 µm or more in length was found, it was determined that a crack had occurred. The results are shown in Table 3.
As shown in Table 3, all Examples were determined as pass in all of TS, YS, El, and R/t. In addition, members obtained by forming or joining the steel sheets of Examples all had the desired shape without cracking and were determined as pass in all of TS, YS, El, and R/t.
Meanwhile, Comparative Examples were determined as fail in at least one of TS, YS, El, and R/t.

INDUSTRIAL APPLICABILITY

It is thus possible to obtain a steel sheet having high strength, high YS, excellent ductility, and excellent bendability. Such a steel sheet can be used advantageously as material for automotive framework structural members having complex shapes, etc. This can make the automotive body more lightweight and thus improve fuel efficiency. The presently disclosed technology therefore has high industrial applicability.

Claims

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

C: 0.05 % or more and 0.20 % or less,

Si: 0.1 % or more and 1.8 % or less,

Mn: 1.5 % or more and 3.0 % or less,

P: 0.001 % or more and 0.100 % or less,

S: 0.0500 % or less,

Al: 0.010 % or more and 1.000 % or less,

N: 0.0100 % or less, and

one or both of Nb and Ti: 0.005 % or more and 0.200 % or less in total,

with a balance consisting of Fe and inevitable impurities;

a steel microstructure in which an area ratio of one or both of ferrite and bainite is 10 % or more and 87 % or less in total, an area ratio of martensite is 10 % or more and 87 % or less, an area ratio of retained austenite is 3 % or more, a number density of grains constituting the retained austenite is 0.05 grains/µm² or more, grains with an aspect ratio of 3 or less among the grains constituting the retained austenite satisfy the following (A), (B), (C), and (D), and [Mn]c/[Mn] is 1.05 or more and 2.00 or less, where [Mn]c is an average Mn concentration in mass% in a Mn-enriched region in steel and [Mn] is an average Mn concentration in mass% in the steel; and

a tensile strength of 780 MPa or more,
(A) a ratio of the grains with an aspect ratio of 3 or less to all of the grains constituting the retained austenite is 60 % or more in area ratio,

(B) an average C concentration of the grains with an aspect ratio of 3 or less is 0.3 mass% or more,

(C) an average value of a shortest distance between the grains with an aspect ratio of 3 or less is 5 µm or less,

(D) a maximum value of the shortest distance between the grains with an aspect ratio of 3 or less is 15 µm or less.
The steel sheet according to claim 1, wherein the chemical composition further contains, in mass%, at least one selected from
V: 0.45 % or less,

B: 0.010 % or less,

Cr: 1.0 % or less,

Ni: 1.0 % or less,

Mo: 1.0 % or less,

Sb: 0.1 % or less,

Sn: 0.1 % or less,

Cu: 1.0 % or less,

Ta: 0.1 % or less,

W: 0.2 % or less,

Mg: 0.01 % or less,

Zn: 0.02 % or less,

Co: 0.02 % or less,

Zr: 0.2 % or less,

Ca: 0.02 % or less,

Se: 0.02 % or less,

Te: 0.02 % or less,

Ge: 0.02 % or less,

As: 0.05 % or less,

Sr: 0.02 % or less,

Cs: 0.02 % or less,

Hf: 0.02 % or less,

Pb: 0.02 % or less,

Bi: 0.02 % or less, and

REM: 0.02 % or less.
The steel sheet according to claim 1, comprising a soft layer with a thickness of 1 µm or more and 50 µm or less, the soft layer being a region whose hardness is 65 % or less of hardness at a position of 1/4 of a sheet thickness of the steel sheet.
The steel sheet according to claim 2, comprising a soft layer with a thickness of 1 µm or more and 50 µm or less, the soft layer being a region whose hardness is 65 % or less of hardness at a position of 1/4 of a sheet thickness of the steel sheet.
The steel sheet according to any one of claims 1 to 4, comprising a galvanized layer on a surface thereof.
The steel sheet according to claim 5, wherein the galvanized layer is a hot-dip galvanized layer or a galvannealed layer.
A member produced from the steel sheet according to any one of claims 1 to 4.
A member produced from the steel sheet according to claim 5.
A member produced from the steel sheet according to claim 6.
A production method for a steel sheet, the production method comprising:
heating a steel slab having the chemical composition according to claim 1 or 2 under a condition of a slab heating temperature of 1220 °C or more and a slab heating time of 1.0 hour or more;

thereafter hot rolling the steel slab under a condition of a rolling finish temperature of 840 °C or more and 1000 °C or less, an average cooling rate in a temperature range from the rolling finish temperature to 700 °C of 10 °C/sec or more, and a coiling temperature of 620 °C or less, to obtain a hot-rolled steel sheet;

thereafter cold rolling the hot-rolled steel sheet under a condition of a rolling reduction ratio of 20 % or more and 80 % or less to obtain a cold-rolled steel sheet;

thereafter heating the cold-rolled steel sheet under a condition of an average heating rate in a temperature range from 600 °C to 750 °C of 1 °C/sec or more and 15 °C/sec or less;

thereafter annealing the cold-rolled steel sheet under a condition of an annealing temperature of 750 °C or more and 920 °C or less and an annealing time of 1 second or more and 30 seconds or less;

thereafter cooling the cold-rolled steel sheet under a condition of an average cooling rate in a temperature range from the annealing temperature - 30 °C to 600 °C of 5 °C/sec or more and 100 °C/sec or less and a cooling stop temperature of 400 °C or more and 600 °C or less; and

thereafter retaining the cold-rolled steel sheet under a condition of a retention time in a temperature range of 400 °C or more and 600 °C or less of 1 second or more and 90 seconds or less.
The production method for a steel sheet according to claim 10, wherein a dew point of an atmosphere in each of the heating of the cold-rolled steel sheet and the annealing is -35 °C or more, and
a dew point of an atmosphere in the cooling is -35 °C or less.
The production method for a steel sheet according to claim 10, further comprising subjecting the cold-rolled steel sheet to galvanizing treatment after the retaining.
The production method for a steel sheet according to claim 11, further comprising subjecting the cold-rolled steel sheet to galvanizing treatment after the retaining.
The production method for a steel sheet according to claim 12, wherein the galvanizing treatment is hot-dip galvanizing treatment or galvannealing treatment.
The production method for a steel sheet according to claim 13, wherein the galvanizing treatment is hot-dip galvanizing treatment or galvannealing treatment.
A production method for a member, the production method comprising subjecting the steel sheet according to any one of claims 1 to 4 to at least one of forming and joining to obtain a member.
A production method for a member, the production method comprising subjecting the steel sheet according to claim 5 to at least one of forming and joining to obtain a member.
A production method for a member, the production method comprising subjecting the steel sheet according to claim 6 to at least one of forming and joining to obtain a member.