EP4790162A1

EP4790162A1 - Steel sheet and component

Info

Publication number: EP4790162A1
Application number: EP24874434.4A
Authority: EP
Inventors: Mutsumi Yoshitake; Eisaku Sakurada; Jun Ando
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 2023-10-02
Filing date: 2024-09-10
Publication date: 2026-08-12
Also published as: KR20260028819A; JPWO2025074822A1; CN121464234A; MX2026000869A; JP7759017B2; WO2025074822A1

Abstract

The steel sheet has a desired chemical composition, and in a microstructure at a position at a 1/4 sheet thickness from a surface, by area%, prior austenite grains having an aspect ratio less than 2.0 account for less than 20%, prior austenite grains having an aspect ratio of 2.0 or more and less than 6.0 account for 40% or more, prior austenite grains having an aspect ratio of 6.0 or more account for less than 30%, bainite accounts for 70% to 95%, and martensite accounts for 5% to 30%.

Description

TECHNICAL FIELD

The present disclosure relates to a steel sheet and a component. Specifically, the present disclosure relates to a steel sheet having high strength, excellent ductility, and excellent hole expansibility, in which occurrence of unevenness on an inside of a bend is suppressed, and a component manufactured using the steel sheet.
Priority is claimed on Japanese Patent Application No. 2023-171497, filed on October 2, 2023 , the content of which is incorporated herein by reference.

BACKGROUND ART

In recent years, a reduction in weight of vehicles and mechanical components has proceeded. The reduction in weight of vehicles and mechanical components can be achieved by designing a component shape into an optimum shape and securing stiffness. Furthermore, a reduction in weight of a blank formed component such as a press-formed component can be achieved by reducing a sheet thickness of a component material.
However, in a case of securing strength characteristics of a component such as static fracture strength and yield strength while reducing the sheet thickness, it is necessary to use a high strength material. In particular, for vehicle suspension components such as lower control arms, trailing links, and knuckles, studies have begun about the application of steel sheets having higher strength. These vehicle suspension components are manufactured by performing burring, stretch flanging, bending forming, or the like on steel sheets. Therefore, the steel sheet applied to these vehicle suspension components is required to have excellent formability, particularly, ductility and hole expansibility.
In addition, in a case where the steel sheet subjected to high-strengthening treatment is used in manufacturing the automotive suspension component as described above, fine cracks (unevenness) are likely to occur on an inside of a bend during forming. In a case where fine cracks occur on the inside of the bend, collision resistance characteristics of the component deteriorate.
For example, Patent Document 1 discloses a high strength steel sheet containing a martensite phase having an area ratio of 95% or more, having a structure with an average aspect ratio of the prior austenite grains of 3.0 or more, in which a stress value after 5-min relaxation upon application of 400 MPa in a stress relaxation test is 20 MPa or less, and tensile strength is 1,180 MPa or more. Patent Document 1 discloses that, with the above-described configuration, a high strength steel sheet having a tensile strength TS of 1,180 MPa or more can be manufactured, which has significantly improved delayed fracture resistance, is suitable as a material for a component for a vehicle, and has excellent delayed fracture resistance.
Patent Document 2 discloses a manufacturing method of a high strength steel sheet, in which an average aspect ratio of the prior austenite grains is 1.3 or more and 5.0 or less, and an area ratio of a bainite phase is 80% or more. Patent Document 2 discloses that, with the above-described configuration, a high strength steel sheet having excellent hole expansibility can be obtained.
Patent Document 3 discloses a steel sheet having excellent fatigue crack propagation resistance, in which acicular ferrite is contained at an area fraction of 1% to 60%, and a number proportion of the acicular ferrite having a major axis in a range of 5 to 100 µm is 80% or more.

Citation List

Patent Documents

Patent Document 1: PCT International Publication No. WO2021/193310
Patent Document 2: Japanese Unexamined Patent Application, First Publication No. 2021-116476
Patent Document 3: Japanese Unexamined Patent Application, First Publication No. 2007-321220

SUMMARY OF INVENTION

Technical Problem

However, in Patent Documents 1 to 3, occurrence of unevenness on the inside of the bend is not considered.
The present disclosure has been made in view of the above-described problems, and an object thereof is to provide a steel sheet having high strength, excellent ductility, and excellent hole expansibility, in which occurrence of unevenness on the inside of the bend during forming is suppressed, and a component manufactured using the steel sheet. Solution to Problem
The present inventors have found that, by controlling the distribution of aspect ratios of the prior austenite grains, occurrence of unevenness on the inside of the bend during forming can be suppressed while ensuring high strength, excellent ductility, and excellent hole expansibility. In addition, the present inventors have found that, by suppressing occurrence of unevenness on the inside of the bend during forming, collision resistance characteristics of the steel sheet can be improved.
The gist of the present disclosure made based on the above findings is as follows.

(1) A steel sheet containing, as a chemical composition, by mass%:
- C: 0.045% to 0.120%;
- Si: 0% to 3.00%;
- Mn: 1.20% to 3.00%;
- sol. Al: 0.001% to 0.500%;
- P: 0% to 0.0800%;
- S: 0% to 0.0100%;
- N: 0% to 0.0050%;
- O: 0% to 0.0100%;
- Ti: 0% to 0.180%;
- Nb: 0% to 0.100%;
- V: 0% to 1.000%;
- Cu: 0% to 1.000%;
- Cr: 0% to 2.000%;
- Mo: 0% to 3.000%;
- Ni: 0% to 0.500%;
- B: 0% to 0.0100%;
- Ca: 0% to 0.0500%;
- Mg: 0% to 0.050%;
- REM: 0% to 0.100%;
- Bi: 0% to 0.100%;
- Ta: 0% to 0.100%;
- Zr: 0% to 0.500%;
- Co: 0% to 3.000%;
- Zn: 0% to 0.200%;
- W: 0% to 0.200%;
- Sb: 0% to 0.500%;
- As: 0% to 0.050%;
- Sn: 0% to 0.050%, and
- a remainder including Fe and impurities,
- in which, in a microstructure at a position at a 1/4 sheet thickness from a surface, by area%,
- prior austenite grains having an aspect ratio of 1.0 or more and less than 2.0 account for less than 20%,
- prior austenite grains having an aspect ratio of 2.0 or more and less than 6.0 account for 40% or more,
- prior austenite grains having an aspect ratio of 6.0 or more account for less than 30%,
- bainite accounts for 70% to 95%, and
- martensite accounts for 5% to 30%.
(2) The steel sheet according to (1),
in which the chemical composition contains, by mass%, one or more selected from the group consisting of,
- Ti: 0.001% to 0.180%,
- Nb: 0.001% to 0.100%,
- V: 0.001% to 1.000%,
- Cu: 0.001% to 1.000%,
- Cr: 0.001% to 2.000%,
- Mo: 0.001% to 3.000%,
- Ni: 0.001% to 0.500%,
- B: 0.0001% to 0.0100%,
- Ca: 0.0001% to 0.0500%,
- Mg: 0.001% to 0.050%,
- REM: 0.001% to 0.100%,
- Bi: 0.001% to 0.100%,
- Ta: 0.001% to 0.100%,
- Zr: 0.001% to 0.500%,
- Co: 0.001% to 3.000%,
- Zn: 0.001% to 0.200%,
- W: 0.001% to 0.200%,
- Sb: 0.001% to 0.500%,
- As: 0.001% to 0.050%, and
- Sn: 0.001% to 0.050%.
(3) The steel sheet according to (1) or (2),
- in which in the microstructure at the position at the 1/4 sheet thickness from the surface, by area%,
- prior austenite grains having an aspect ratio of 2.0 or more and less than 4.0 account for 30% or more, and
- prior austenite grains having an aspect ratio of 4.0 or more and less than 6.0 account for 10% or more.
(4) The steel sheet according to any one of (1) to (3),
in which, in the microstructure at the position at the 1/4 sheet thickness from the surface, a maximum value of a major axis length of the prior austenite grains is 200 µm or less.
(5) A component including the steel sheet according to any one of (1) to (4). Advantageous Effects of Invention

According to the aspect of the present disclosure, it is possible to provide a steel sheet having high strength, excellent ductility, and excellent hole expansibility, in which occurrence of unevenness on the inside of the bend during forming is suppressed, and a component manufactured using the steel sheet.

DESCRIPTION OF EMBODIMENTS

A steel sheet and component according to an embodiment of the present disclosure (hereinafter, may be referred to as the steel sheet and component according to the present embodiment) will be described. However, the present disclosure is not limited to the configuration disclosed in the present embodiment, and various modifications can be made without departing from the gist of the present disclosure.
Hereinafter, individual configuration requirements of the present disclosure will be described in detail. First, reasons for limiting the chemical composition of the steel sheet according to the present embodiment will be described.
The numerical limit range described below with "to" includes the lower limit and the upper limit. Numerical values indicated as "less than" or "more than" do not fall within the numerical range. In the following description, % regarding the chemical composition is mass% unless otherwise specified.
A steel sheet according to the present embodiment contains, as a chemical composition, by mass%, C: 0.045% to 0.120%, Si: 0% to 3.00%, Mn: 1.20% to 3.00%, sol. Al: 0.001% to 0.500%, P: 0% to 0.0800%, S: 0% to 0.0100%, N: 0% to 0.0050%, O: 0% to 0.0100%, and a remainder: Fe and impurities. Each element will be described in detail below.

C: 0.045% to 0.120%

C is an important element for improving the strength of the steel sheet. To obtain a desired strength, the C content is set to 0.045% or more. The C content is preferably 0.050% or more, 0.55% or more, or 0.60% or more.
On the other hand, when the C content is more than 0.120%, the hole expansibility of the steel sheet deteriorates. For this reason, the C content is set to 0.120% or less. The C content is preferably 0.110% or less, and 0.100% or less.

Si: 0% to 3.00%

Si is an element that has an effect of suppressing the generation of carbide during ferritic transformation and improving the toughness of the steel sheet. Since Si does not have to be contained, the Si content may be 0%. To reliably obtain the effect, the Si content is preferably set to 0.10% or more. The Si content is more preferably 0.20% or more or 0.50% or more.
On the other hand, when the Si content is more than 3.00%, crack susceptibility of a slab increases, and it becomes difficult to handle the slab. For this reason, the Si content is set to 3.00% or less. The Si content is preferably 2.50% or less, 2.00% or less, or 1.50% or less.

Mn: 1.20% to 3.00%

Mn is an element effective in improving hardenability and improving the strength of the steel sheet using solid solution strengthening. To obtain a desired strength, the Mn content is set to 1.20% or more. The Mn content is preferably 1.30% or more, 1.50% or more, or 1.70% or more.
On the other hand, when the Mn content is more than 3.00%, the bainitic transformation is delayed, and it is difficult to obtain a desired amount of bainite. Therefore, the Mn content is set to 3.00% or less. The Mn content is preferably 2.70% or less, 2.50% or less, or 2.30% or less.

sol. Al: 0.001% to 0.500%

Al has an action of achieving soundness of steel by deoxidation and also has an action of controlling ferritic transformation. To obtain these effects, the sol.Al content is set to 0.001% or more. The sol. Al content is preferably 0.010% or more or 0.020% or more.
On the other hand, when the Al content is more than 0.500%, ferrite is likely to be excessively generated, and the strength of the hot-rolled steel sheet decreases. For that reason, the Al content is set to 0.500% or less. The Al content is preferably 0.400% or less, 0.300% or less, or 0.200% or less.

P: 0% to 0.0800%

P is an element that affects the weldability of the steel sheet. In particular, when the P content is more than 0.0800%, the weldability of the steel sheet significantly deteriorates. Furthermore, the crack susceptibility of the slab increases, and it becomes difficult to handle the slab. Therefore, the P content is set to 0.0800% or less. The P content is preferably 0.0600% or less or 0.0400% or less.
The P content may be 0%. From the viewpoint of the refining cost, the P content may be set to 0.0010% or more.

S: 0% to 0.0100%

S is an element that affects the ductility and the hole expansibility of the steel sheet. In particular, when the S content is more than 0.0100%, a large amount of inclusions such as MnS harmful to the ductility and the hole expansibility of the steel sheet, are generated. For this reason, the S content is set to 0.0100% or less. The S content is preferably 0.0080% or less or 0.0060% or less.
The S content may be 0%. From the viewpoint of the refining cost, the P content may be set to 0.0001 % or more.

N: 0% to 0.0050%

N is an element that bonds with Ti to form Ti nitride. In particular, when the N content is more than 0.0050%, crack susceptibility of a slab increases, and it becomes difficult to handle the slab. For this reason, the N content is set to 0.0050% or less. The N content is preferably 0.0040% or less or 0.0030% or less.
The N content may be 0%. From the viewpoint of the refining cost, the N content may be set to 0.0001 % or more.

O: 0% to 0.0100%

O is an element that, when contained in steel in a large amount, forms a coarse oxide that becomes an origin of fracture, causing brittle fractures and hydrogen-induced cracks. When an O content is more than 0.0100%, brittle fractures and hydrogen-induced cracks are likely to be initiated. Therefore, the O content is set to 0.0100% or less. The O content is preferably 0.0080% or less, 0.0060% or less, or 0.0040% or less.
Since O may not be contained, the O content may be 0%. The O content may be set to 0.0005% or more or 0.0010% or more to disperse a large number of fine oxides when deoxidation of molten steel.
The steel sheet according to the present embodiment may contain the above-mentioned chemical composition and a remainder may consist of Fe and impurities. In the present embodiment, the impurities mean substances that are incorporated from ores, scrap, or a manufacturing environment as a raw material, and/or substances permitted insofar as they do not adversely affect the characteristics of the steel sheet according to the present embodiment, for example, the characteristics related to the problems to be solved by the present disclosure.
Although not essential to impart desired characteristics, the following optional elements may be contained to reduce manufacturing variations or to further improve the strength of the steel sheet. However, since it is not essential that these elements are contained, the lower limits of the content of these elements are 0%.

Ti: 0.001% to 0.180%

Ti is precipitated in steel as a carbide or a nitride and has an action of refining the microstructure by an austenite pinning effect and increasing the strength and yield ratio of the steel sheet by precipitation hardening. In the case of reliably obtaining these effects, the content of Ti is preferably set to 0.001% or more. The Ti content is more preferably 0.005% or more or 0.010% or more.
On the other hand, when the Ti content is more 0.180%, the ductility and the hole expansibility of the steel sheet deteriorate because of excessive precipitation of TiC. Therefore, the Ti content is set to 0.180% or less. The Ti content is preferably 0.160% or less, 0.150% or less, and 0.130% or less.

Nb: 0.001% to 0.100%

Nb has an effect of increasing the strength of the steel sheet by refining the crystal grain size of the steel sheet and precipitation hardening of NbC. In a case where this effect is to be reliably obtained, it is preferable that the Nb content is set to 0.001% or more. The Nb content is more preferably 0.005% or more or 0.010% or more.
On the other hand, in a case where the Nb content exceeds 0.100%, the above-mentioned effect is saturated. For this reason, even in a case where Nb is to be contained, the Nb content is set to 0.100% or less. The Nb content is preferably 0.080% or less or 0.060% or less.

V: 0.001% to 1.000%

V has an effect of increasing the strength of the steel sheet via strengthening caused by precipitates, grain refinement strengthening caused by the suppression of the growth of ferrite grains, and dislocation strengthening caused by the suppression of recrystallization. In a case where this effect is to be reliably obtained, it is preferable that the V content is set to 0.001% or more. The V content is more preferably 0.005% or more or 0.010% or more.
On the other hand, in a case where the V content is excessively high, a large amount of carbonitride is precipitated, so that the ductility and the hole expansibility of the steel sheet deteriorate. Therefore, the V content is set to 1.000% or less. The V content is preferably 0.800% or less or 0.600% or less.

Cu: 0.001% to 1.000%

Cu is present in steel in the form of fine grains and has an effect of increasing the strength of the steel sheet. In the case of reliably obtaining this effect, a Cu content is preferably set to 0.001% or more. The Cu content is more preferably 0.005% or more or 0.010% or more.
On the other hands, in a case where the Cu content is excessively high, the weldability of the steel sheet deteriorates. Therefore, the Cu content is set to 1.000% or less. The Cu content is preferably 0.800% or less or 0.600% or less.

Cr: 0.001% to 2.000%

Cr is an element effective in improving the strength of the steel sheet. In a case where this effect is to be reliably obtained, it is preferable that the Cr content is set to 0.001% or more. The Cr content is more preferably 0.005% or more or 0.010% or more.
On the other hand, when the Cr content is excessively high, the ductility and the hole expansibility of the steel sheet deteriorates. For that reason, the Cr content is set to 2.000% or less. The Cr content is preferably 1.500% or less, 1.200% or less, and 1.000% or less.

Mo: 0.001% to 3.000%

Mo is an element effective for precipitation hardening of ferrite. In a case where this effect is to be reliably obtained, it is preferable that the Mo content is set to 0.001% or more. The Mo content is more preferably 0.005% or more or 0.010% or more.
On the other hand, in a case where the Mo content is excessively high, the crack susceptibility of a slab is increased, so that it is difficult to handle the slab. Therefore, the Mo content is set to be 3.000% or less. The Mo content is preferably 2.500% or less, 2.000% or less, or 1.500% or less.

Ni: 0.001% to 0.500%

Ni has an effect of suppressing a phase transformation at a high temperature and increasing the strength of the steel sheet. In a case where this effect is to be reliably obtained, it is preferable that the Ni content is set to 0.001% or more. The Ni content is more preferably 0.005% or more or 0.010% or more.
On the other hands, in a case where the Ni content is excessively high, the weldability of the steel sheet deteriorates. Therefore, the Ni content is set to 0.500% or less. The Ni content is preferably 0.300% or less or 0.150% or less.

B: 0.0001% to 0.0100%

B has an effect of suppressing a phase transformation at a high temperature and increasing the strength of the steel sheet. In a case where this effect is to be reliably obtained, it is preferable that the B content is set to 0.0001% or more. The B content is more preferably 0.0005% or more or 0.0010% or more.
On the other hand, in a case where the B content is excessively high, a B precipitate is generated, so that the strength of the steel sheet is reduced. Therefore, the B content is set to 0.0100% or less. The B content is preferably 0.0080% or less or 0.0050% or less.

Ca: 0.0001% to 0.0500%

Ca has an effect of refining the structure of the steel sheet by causing a number of fine oxides to be dispersed during the deoxidation of molten steel. Furthermore, Ca has an effect of increasing the hole expansibility of the steel sheet by fixing S in the steel as spherical CaS and suppressing the generation of elongated inclusions, such as MnS. In a case where these effects are to be reliably obtained, it is preferable that the Ca content is set to 0.0001% or more. The Ca content is more preferably 0.0005% or more or 0.0010% or more.
On the other hand, even though the Ca content exceeds 0.0500%, the above-mentioned effects are saturated. For this reason, the Ca content is set to 0.0500% or less. The Ca content is preferably 0.0300% or less or 0.0200% or less.

Mg: 0.001% to 0.050%

Mg has an action of increasing the yield ratio of the steel sheet by adjusting a shape of an inclusion in steel to a preferable shape. To reliably obtain this effect, a Mg content is preferably set to 0.001% or more. The Mg content is more preferably 0.005% or more or 0.010% or more.
On the other hand, when the Mg content is more than 0.050%, an excessive amount of the inclusion is generated in steel, and the yield ratio of the steel sheet decreases. Therefore, the Mg content is set to 0.050% or less. The Mg content is preferably 0.040% or less or 0.030% or less.

REM: 0.001% to 0.100%

REM has an effect of increasing the yield ratio of the steel sheet by adjusting a shape of an inclusion in steel to a preferable shape. To reliably obtain this effect, a REM content is preferably set to 0.001% or more. The REM content is more preferably 0.005% or more or 0.010% or more.
On the other hand, when the REM content is more than 0.100%, an excessive amount of the inclusion is generated in steel, and the yield ratio of the steel sheet decreases. Therefore, the REM content is set to 0.100% or less. The REM content is preferably 0.080% or less or 0.060% or less.
Here, REM refers to a total of 17 elements consisting of Sc, Y, and lanthanoids, and the REM content refers to the total amount of these elements. Lanthanoids are added in the form of mischmetal in industry.

Bi: 0.001% to 0.100%

Bi has an effect of increasing the yield ratio of the steel sheet by refining a solidification structure. To reliably obtain this effect, the Bi content is preferably set to 0.001% or more. The Bi content is more preferably 0.005% or more or 0.010% or more.
On the other hand, when the Bi content is more than 0.100%, the effect by the action is saturated, which is not economically preferable. Therefore, the Bi content is set to 0.100% or less. The Bi content is preferably 0.080% or less, 0.060% or less, and 0.040% or less.

Ta: 0.001% to 0.100%

Ta has an effect of increasing the strength of the steel sheet by forming a fine carbide in steel, as in V. To reliably obtain this effect, the Ta content is preferably set to 0.001% or more. The Ta content is more preferably 0.005% or more or 0.010% or more.
On the other hand, when the Ta content is more than 0.100%, the ductility and the hole expansibility of the steel sheet deteriorate. Therefore, the Ta content is set to 0.100% or less. The Ta content is preferably 0.080% or less or 0.050% or less.

Zr: 0.001% to 0.500%

Zr has an effect of increasing the strength of the steel sheet by solid solution strengthening. To reliably obtain this effect, the Zr content is preferably set to 0.001% or more. The Zr content is more preferably 0.005% or more or 0.010% or more.
On the other hand, when the Zr content is more than 0.500%, the ductility and the hole expansibility of the steel sheet deteriorate. Therefore, the Zr content is set to 0.500% or less. The Zr content is preferably 0.300% or less or 0.100% or less.

Co: 0.001% to 3.000%

Co has an effect of increasing the strength of the steel sheet by solid solution strengthening. To reliably obtain this effect, the Co content is preferably set to 0.001% or more. The Co content is more preferably 0.005% or more or 0.010% or more.
On the other hand, when the Co content is more than 3.000%, the ductility and the hole expansibility of the steel sheet deteriorate. For this reason, the Co content is set to 3.000% or less. The Co content is preferably 1.000% or less or 0.500% or less.

Zn: 0.001% to 0.200%

Zn has an effect of increasing the strength of the steel sheet by solid solution strengthening. To reliably obtain this effect, the Zn content is preferably set to 0.001% or more. The Zn content is more preferably 0.005% or more or 0.010% or more.
On the other hand, when the Zn content is more than 0.200%, the ductility and the hole expansibility of the steel sheet deteriorate. Therefore, the Zn content is set to 0.200% or less. The Zn content is preferably 0.150% or less or 0.100% or less.

W: 0.001% to 0.200%

W has an effect of increasing the strength of the steel sheet by solid solution strengthening. To reliably obtain this effect, the W content is preferably set to 0.001% or more. The W content is more preferably 0.005% or more or 0.010% or more.
On the other hand, when the W content is more than 0.200%, the ductility and the hole expansibility of the steel sheet deteriorate. Therefore, the W content is set to 0.200% or less. The W content is preferably 0.150% or less or 0.100% or less.

Sb: 0.001% to 0.500%

Sb has an effect of increasing the ductility and the hole expansibility of the steel sheet by suppressing the generation of oxides that becomes an origin of fracture. To reliably obtain this effect, the Sb content is preferably set to 0.001% or more. The Sb content is more preferably 0.005% or more or 0.010% or more.
On the other hand, the above effect is saturated even when a large amount of Sb is contained. Therefore, the Sb content is set to 0.500% or less. The Sb content is preferably 0.300% or less or 0.100% or less.

As: 0.001% to 0.050%

As has an effect of refining the prior austenite grains by lowering an austenite single-phase formation temperature and increasing the hole expansibility of the steel sheet. In the case of reliably obtaining the effects, the As content is preferably set to 0.001% or more. The As content is more preferably 0.005% or more or 0.010% or more.
On the other hand, since the above effects are saturated even in a case where a large amount of As is contained, the As content is set to 0.050% or less. The As content is preferably 0.040% or less or 0.030% or less.

Sn: 0.001% to 0.050%

Sn has an effect of increasing the ductility and the hole expansibility of the steel sheet by suppressing the generation of oxides that becomes an origin of fracture. In a case of reliably obtaining the effects, a Sn content is preferably set to 0.001% or more. The Sn content is more preferably 0.005% or more or 0.010% or more.
On the other hand, the above effect is saturated even when a large amount of Sn is contained. Therefore, the Sn content is set to 0.050% or less. The Sn content is preferably 0.040% or less or 0.030% or less.
The above-described chemical composition of the steel sheet may be analyzed using a spark discharge emission spectrophotometer or the like. For C and S, values identified by burning the steel sheet in an oxygen stream using a gas component analyzer or the like and measuring C and S by an infrared absorption method are adopted. In addition, for O and N, a value identified by melting a test piece collected from the steel sheet in a helium stream and measuring N by a thermal conductivity method is adopted.
In a case where the steel sheet is provided with a plating layer, a coating film, or the like on the surface, the chemical composition is analyzed after removing the plating layer, the coating film, or the like by mechanical grinding or the like, as necessary.
Next, a microstructure of the steel sheet according to the present embodiment will be described.
In the steel sheet according to the present embodiment, in a microstructure at a position at a 1/4 sheet thickness from a surface, by area%, prior austenite grains having an aspect ratio of 1.0 or more and less than 2.0 account for less than 20%, prior austenite grains having an aspect ratio of 2.0 or more and less than 6.0 account for 40% or more, prior austenite grains having an aspect ratio of 6.0 or more account for less than 30%, bainite accounts for 70% to 95%, and martensite accounts for 5% to 30%.
In the present embodiment, the position at a 1/4 sheet thickness from the surface indicates a region from a depth corresponding to 1/8 of the sheet thickness from the surface to a depth corresponding to 3/8 of the sheet thickness from the surface. In other words, it refers to a region from a depth corresponding to 1/8 of the sheet thickness from the surface as a start point to a depth corresponding to 3/8 of the sheet thickness from the surface as an end point. The reason for specifying the microstructure at this position is that the microstructure at this position indicates a typical microstructure of the steel sheet.
In a case where the steel sheet includes a plating layer, a coating film, or the like on the surface, the surface referred to here means an interface between the steel sheet and the plating layer or the coating film.

Prior austenite grains having an aspect ratio of 1.0 or more and less than 2.0: less than 20 area%
Prior austenite grains having an aspect ratio of 2.0 or more and less than 6.0: 40 area% or more
Prior austenite grains having an aspect ratio of 6.0 or more: less than 30 area%

In a case where the area ratio of the prior austenite grains having a predetermined aspect ratio is outside the above-described range, unevenness generated on the inside of the bend during forming cannot be suppressed. Therefore, the prior austenite grains having an aspect ratio of 1.0 or more and less than 2.0 account for less than 20 area%, the prior austenite grains having an aspect ratio of 2.0 or more and less than 6.0 account for 40 area% or more, and the prior austenite grains having an aspect ratio of 6.0 or more account for less than 30 area%.
Since the prior austenite grains having an aspect ratio of 2.0 or more and less than 6.0 is more preferable in a larger amount, it may account for 100 area%. The prior austenite grains having an aspect ratio of 2.0 or more and less than 6.0 may account for 95 area% or less or 90 area% or less.
The prior austenite grains having an aspect ratio of 1.0 or more and less than 2.0 account for preferably 15 area% or less and more preferably 10 area% or less. Since the prior austenite grains having an aspect ratio of 1.0 or more and less than 2.0 is more preferable in a smaller amount, it may account for 0 area%.
In addition, the prior austenite grains having an aspect ratio of 6.0 or more account for preferably 25 area% or less or 20 area% or less and more preferably 15 area% or less or 10 area% or less. Since the prior austenite grains having an aspect ratio of 6.0 or more is more preferable in a smaller amount, it may account for 0 area%.

Prior austenite grains having an aspect ratio of 2.0 or more and less than 4.0: 30 area% or more
Prior austenite grains having an aspect ratio of 4.0 or more and less than 6.0: 10 area% or more

It is preferable that the prior austenite grains having an aspect ratio of 2.0 or more and less than 6.0 account for 40 area% or more, the prior austenite grains having an aspect ratio of 2.0 or more and less than 4.0 account for 30 area% or more, and the prior austenite grains having an aspect ratio of 4.0 or more and less than 6.0 account for 10 area% or more. By setting the prior austenite grains having an aspect ratio of 2.0 or more and less than 4.0 to 30 area% or more and the prior austenite grains having an aspect ratio of 4.0 or more and less than 6.0 to 10 area% or more, occurrence of unevenness on the inside of the bend can be further suppressed.
An aspect ratio of the prior austenite grain is a value obtained by dividing a major axis of the prior austenite grain by a minor axis and takes a value of 1.0 or more.
An aspect ratio of prior austenite grains is measured by the following method.
A sample is collected such that the microstructure of a cross section (cross section in a sheet thickness direction × a rolling direction) with the sheet width direction as a normal direction can be observed at a position of 1/4 from an end surface of the steel sheet in the sheet width direction. The size of the sample also depends on the measurement device, but may be, for example, a rectangular parallelepiped having a total thickness in the sheet thickness direction, 15 mm in the rolling direction, and 10 mm in the sheet width direction. Next, the observation surface is mirror-finished, and then the surface is etched by a Bechet-Beaujard method in accordance with JIS G 0551: 2020 using a saturated aqueous solution of picric acid. The grains appearing black due to the etching are specified as the prior austenite grains. The observation surface on which the prior austenite grains are exposed is observed with an optical microscope, and at a position at a 1/4 sheet thickness from the surface (region from a depth corresponding to 1/8 of the sheet thickness from the surface to a depth corresponding to 3/8 of the sheet thickness from the surface), 8 visual fields, each having an area of 200 µm in the sheet thickness direction × 600 µm in the rolling direction, were photographed at a magnification of 500 times. From the photographed structure image, the aspect ratio of each prior austenite grains is obtained by calculating a value of major axis/minor axis for each prior austenite grain. The area ratio of each of the prior austenite grains having an aspect ratio of 1.0 or more and less than 2.0, the prior austenite grains having an aspect ratio of 2.0 or more and less than 4.0, the prior austenite grains having an aspect ratio of 4.0 or more and less than 6.0, and the prior austenite grains having an aspect ratio of 6.0 or more is calculated to obtain the area ratio of the prior austenite grains having each aspect ratio.
In a case where the prior austenite grains cannot be sufficiently exposed by the above-described method, the prior austenite grains are specified by a reconstruction method described in "Study for Improving the Accuracy of the Reconstruction Method of Austenite Structure in Steel" (Kengo Hata, Masayuki Wakita, Tomoya Fujiwara, and Kaori Kawano, Nippon Steel & Sumitomo Metal Technical Report, No. 404 (2016), pp. 24 to 30), and the aspect ratio of the prior austenite grains is obtained.
The EBSD measurement data used in the reconstruction method is obtained by the following method.
For the above-described observation visual field (visual field of 200 µm in the sheet thickness direction × 600 µm in the rolling direction), crystal orientation information is obtained by an electron backscatter diffraction method at a measurement interval of 0.1 µm after colloidal polishing or electrolytic polishing.
For the measurement, an EBSD analysis device including a thermal field emission scanning electron microscope (JSM-7200F manufactured by JEOL Ltd.) and an EBSD detector (EDAX Velocity (registered trademark), an ultra-high-speed EBSD detector) is used. In this case, the degree of vacuum in the device is 9.6 × 10^-5 Pa or less, the acceleration voltage is 15 kV, and the irradiation current is 36 nA. It is noted that after the accelerating voltage is determined, the irradiation current may be adjusted to obtain a clearer pattern.
In displaying the EBSD map, version 7 or later of OIM Analysis (registered trademark) manufactured by EDAX/TSL solution is used for the obtained crystal orientation information.
In a case where prior austenite grains having a circle equivalent diameter of less than 2 µm are included, the prior austenite grains are excluded and the above-described measurement is performed. The reason for this is that the prior austenite grains having a circle equivalent diameter of less than 2 µm does not adversely affect the characteristics of the steel sheet according to the present embodiment.
In addition, in a case where only a part of the prior austenite grains is included in the structure image at an end portion of the structure image, the measurement is performed by excluding the part.
These exclusion conditions are also applied in the measurement of the major axis length of the prior austenite grains described below.
In addition, the rolling direction of the steel sheet is discriminated by the following method.
A test piece is collected so that a sheet thickness cross section of the steel sheet can be observed. The test piece is collected in a total of 12 pieces by rotating the test piece in a direction perpendicular to the sheet surface as a Z direction by 30° around the Z direction. The sheet thickness cross section of the collected test piece is polished, the prior austenite grains boundary is exposed by the above-described etchant, and the average value of the aspect ratio of the prior austenite grains is calculated by an intercept method. The test piece having the largest average value of the aspect ratio of the prior austenite grains is specified, and the direction in which the test piece is collected is discriminated as the rolling direction of the steel sheet. That is, a direction parallel to the sheet thickness cross section of the test piece and perpendicular to the sheet thickness direction is discriminated as the rolling direction of the steel sheet.
Maximum value of major axis length of prior austenite grains: 200 µm or less
In the microstructure at a position at a 1/4 sheet thickness from the surface, the maximum value of the major axis length of the prior austenite grains is preferably 200 µm or less. By setting the maximum value of the major axis length of the prior austenite grains to 200 µm or less, occurrence of unevenness on the inside of the bend during forming can be further suppressed.
The maximum value of the major axis length of the prior austenite grains is obtained by measuring the major axis length of each prior austenite grain in the structure image photographed by the above-described method and obtaining the maximum value thereof.

Area ratio of bainite: 70% to 95%

Bainite is a structure including fine crystal grains and a carbide. When an area ratio of bainite is less than 70%, a desired ductility cannot be obtained in the steel sheet. Therefore, the area ratio of the bainite is set to 70% or more. The area ratio of the bainite is preferably 75% or more or 80% or more.
On the other hand, when an area ratio of bainite is more than 95%, a desired strength cannot be obtained in the steel sheet. Therefore, the area ratio of the bainite is set to 95% or less. The area ratio of the bainite is preferably 93% or less, 90% or less, or 87% or less.

Area ratio of martensite: 5% to 30%

Martensite is a structure that increases the strength of the steel sheet. When an area ratio of martensite is less than 5%, a desired strength cannot be obtained. For this reason, the area ratio of the martensite is set to 5% or more. The area ratio of the martensite is preferably more than 5%, 6% or more, 7% or more, 10% or more, or 13% or more.
On the other hand, when the area ratio of martensite is more than 30%, a desired ductility cannot be obtained. For this reason, the area ratio of the martensite is set to 30% or less. The area ratio of the martensite is preferably 25% or less or 20% or less.

Remainder in microstructure: 0% to 5%

In the microstructure of the internal region of the steel sheet according to the present embodiment, in addition to the bainite and the martensite, ferrite and pearlite may be contained as the remainder in microstructure. The area ratio of the remainder in microstructure may be set to 0% to 5%.
The area ratios of the bainite, the martensite, and the remainder in microstructure are measured by the following method.
A sample is collected such that the microstructure of a cross section (cross section in a sheet thickness direction × a rolling direction) with the sheet width direction as a normal direction can be observed at a position of 1/4 from an end surface of the steel sheet in the sheet width direction. The size of the sample also depends on the measurement device, but may be, for example, a rectangular parallelepiped having a total thickness in the sheet thickness direction, 15 mm in the rolling direction, and 10 mm in the sheet width direction. Next, the observation surface is mirror-finished, and then subjected to nital etching. For the observation surface after the etching, using an optical microscope and a scanning electron microscope (SEM), at a position at a 1/4 sheet thickness from the surface (region from a depth corresponding to 1/8 of the sheet thickness from the surface to a depth corresponding to 3/8 of the sheet thickness from the surface), 8 visual fields, each having an area of 200 µm in the sheet thickness direction × 600 µm in the rolling direction, were photographed at a magnification of 1,000 times or more. The area ratio of each of the ferrite, the pearlite, and the bainite are obtained by performing image analysis on the obtained structure image.
Thereafter, for the same observation position, only the etched layer is removed by polishing, followed by mirror finishing, and LePera etching, the observation of the structure is then performed using an optical microscope and a scanning electron microscope, and the area ratio of the martensite is obtained by performing image analysis on the obtained structure image.
In the above structure observation, each structure is identified by the following method.
Since the martensite is a structure having a high dislocation density and having a substructure called a block or a packet in the grain, it is possible to distinguish the fresh martensite from other microstructures in an electron channeling contrast image for which a scanning electron microscope is used.
Among structures that are each an aggregate of lath-shaped crystal grains and do not contain any Fe-based carbides having a major axis of 20 nm or more in the structure, a structure that is not martensite or a structure in which Fe-based carbides having a major axis of 20 nm or more are contained in the structure and the Fe-based carbides have a single variant, that is, the Fe-based carbides extend in the same direction, is regarded as the bainite.
Here, the Fe-based carbides extending in the same direction refer to Fe-based carbides for which a difference in extending direction between the Fe-based carbides is 5° or less.
A structure that is lumpy crystal grains and does not contain any substructures such as a lath in the structure is regarded as the ferrite.
A structure in which sheet-shaped ferrite and an Fe-based carbide are overlapped in layers is regarded as the pearlite.

Tensile strength (TS): 980 MPa or more

The tensile strength of the steel sheet according to the present embodiment may be 980 MPa or more. The tensile strength is more preferably 1,000 MPa or more. When the tensile strength is set to 980 MPa or more, it is possible to make the hot rolled steel sheet significantly contribute to the weight reduction of vehicle bodies without limiting applicable parts.
An upper limit of the tensile strength does not need to be particularly limited, and may be set to 1500 MPa or less or 1300 MPa or less from the viewpoint of suppressing the wear of a die.

Total elongation (El): 10.0% or more
Hole expansion ratio (λ): 50% or more

The total elongation (total elongation at maximum test force) of the steel sheet according to the present embodiment may be 10.0% or more, and the hole expansion ratio may be 50% or more. The total elongation is preferably 12.0% or more or 14.0% or more. The hole expansion ratio is preferably 55.0% or more or 60.0% or more.
A tensile test is performed according to JIS Z 2241:2022 to evaluate a tensile strength and total elongation. As a test piece, a No. 5 test piece of JIS Z 2241: 2022 is used. As a position where a tensile test piece is collected, a 1/4 portion extending from the end portion in the sheet width direction is set, and a direction perpendicular to the rolling direction is set as a longitudinal direction.
In a case where the No. 5 test piece cannot be collected since the steel sheet or the component has a small size or a complicated shape, a tensile strength and a total elongation may be obtained by collecting a strip-shaped small piece having a parallel portion with any width and performing a tensile test using the small piece. Regarding the direction of the strip-shaped small piece, a direction perpendicular to the rolling direction is the longitudinal direction.
The hole expansion ratio is measured by performing a hole expansion test in accordance with JIS Z 2256:2020.

Evaluation of unevenness on the inside of the bend

In the steel sheet according to the present embodiment, in a case where the bending test described later is performed and the surface roughness on the inside of the bend is measured, a maximum valley depth (Rv) of the roughness profile may be 20 µm or less. In a case where the maximum valley depth of the roughness profile is 20 µm or less in the bending after the bending test described later, it can be determined that the occurrence of cracks in the bending is suppressed and the collision resistance characteristics are excellent.
The bending test is performed by the following method.
A strip-shaped test piece of 100 mm × 30 mm is cut out from a 1/2 position in a width direction of the steel sheet. For bending (L-axis bending) in which a bending ridge is parallel to the rolling direction (L direction), a bending test is performed in accordance with the V-block method of JIS Z 2248:2022 (bending angle θ is 90°).
Next, the test piece is cut out in a direction parallel to the bending ridge such that the apex portion of the bending can be observed, and the roughness is measured by a contact-type surface roughness tester. In the bending of the apex portion of the bending, the roughness of the surface is measured using a contact-type surface roughness tester along a direction perpendicular to the ridge, at two points separated by 2.0 mm. A roughness profile is obtained by applying a contour curve filter with cut-off values of λc to a measured cross-sectional curve obtained by the measurement. Specifically, a component having a wavelength λc of 0.8 mm or less is removed from the obtained measurement result to obtain a roughness profile. The maximum valley depth Rv of the roughness profile is calculated in accordance with JIS B 0601: 2013 based on the obtained roughness profile.
It is noted that the bend test is considered to be equivalent in effect as long as the value obtained by dividing the bend radius R by the sheet thickness t is the same, thereby performing the test using R/t of 1.3. As an example, when the sheet thickness t is 3.0 mm, R is 3.9 mm.
A sheet thickness of the steel sheet according to the present embodiment is not particularly limited and may be set to 1.2 to 8.0 mm. When the sheet thickness of the steel sheet is less than 1.2 mm, it may be difficult to secure the rolling completion temperature and the rolling force may become excessive, making hot rolling difficult. Therefore, the sheet thickness of the steel sheet according to the present invention may be 1.2 mm or more. The sheet thickness of the steel sheet according to the present embodiment is preferably 1.4 mm or more.
On the other hand, in a case where the sheet thickness exceeds 8.0 mm, it may be difficult to obtain the above-described microstructure after hot rolling. Therefore, the sheet thickness may be set to 8.0 mm or less. The sheet thickness is preferably 6.0 mm or less.
The steel sheet according to the present embodiment having the chemical composition and the microstructure described above may be provided with a plating layer on the surface thereof for the purpose of improving corrosion resistance and the like to be made into a surface-treated steel sheet. The plating layer may be an electro plating layer or a hot-dip plating layer. Examples of the electro plating layer include electrogalvanizing, and electro Zn-Ni alloy plating. Examples of the hot-dip plating layer include hot-dip galvanizing, hot-dip galvannealing, hot-dip aluminum plating, hot-dip Zn-Al alloy plating, hot-dip Zn-Al-Mg alloy plating, and hot-dip Zn-Al-Mg-Si alloy plating. A plating adhesion amount is not particularly limited and may be the same as in the related art.
In addition, it is also possible to further enhance the corrosion resistance by performing an appropriate chemical conversion treatment (for example, application and drying of a silicate-based chromium-free chemical conversion treatment liquid) after plating.
It is considered that the steel sheet according to the present embodiment has high strength, excellent ductility, and excellent hole expansibility, and has excellent collision resistance characteristics because occurrence of unevenness on the inside of the bend during forming is suppressed, whereby the steel sheet can be suitably used for a component, particularly a vehicle component. Among the vehicle components, the steel sheet can be suitably used for a vehicle suspension component such as a lower control arm, a trailing link, and a knuckle. These vehicle components may be formed of only the steel sheet according to the present embodiment, or may be formed by bonding the steel sheet according to the present embodiment and another steel sheet.
The component manufactured using the steel sheet according to the present embodiment has the same chemical composition as that of the above-described steel sheet. In addition, in the component, a portion subjected to processing and a portion not subjected to processing may be mixed. The portion not subjected to processing has the same microstructure as the above-described steel sheet. The portion subjected to processing basically has the same microstructure as the above-described steel sheet, but may not have the above-described microstructure in a case of subjecting to severe processing or an end portion of the component. Therefore, in a case of measuring the microstructure in the component, the measurement is performed on the portion not subjected to processing, avoiding the end portion. In a case where there is no portion not subjected to processing, the measurement is performed on the portion not subjected to severe processing. The portion not subjected to processing or to severe processing refers, for example, to a flat portion of the component, as well as a portion outside areas subjected to punching, hole-expanding, bending, or the like. As an example, in the case of the above-described component, a portion that is flat and has the largest area is applied, a test piece is collected near the centroid portion thereof and investigated. However, the surface roughness on the inner peripheral side of the bending processing portion that is considered to have a large influence on the collision resistance characteristics is measured at a portion subjected to processing with the smallest bend radius in the component.
Next, a preferable manufacturing method of the steel sheet according to the present embodiment will be described. According to the manufacturing method described below, the steel sheet according to the present embodiment can be stably manufactured. A temperature of a slab and a temperature of a steel sheet in the present embodiment refer to a surface temperature of the slab and a surface temperature of the steel sheet.
A preferred manufacturing method of the steel sheet according to the present embodiment is as follows.

(1) Rough rolling is performed with a total rolling reduction of 50% or more and a rough rolling completion temperature of 1,100°C or higher. Provided that the rough rolling conditions are more preferable.
(2) Finish rolling is performed under the following conditions.

A finish rolling start temperature is set to 1,020°C to 1,100°C.
A rolling reduction of the first stage is set to 40% or more.
A total rolling reduction of the final two stages is set to less than 40%.
A finish rolling completion temperature is set to 900°C or higher and lower than 980°C.
In the finish rolling, it is more preferable to perform rolling two or more times at a rolling reduction of 30% or more in a temperature range of 980°C or higher and lower than 1,020°C.
(3) After the finish rolling is completed, cooling is started within 1 second and is performed at an average cooling rate of 30°C/s or faster until a temperature reaches 200°C.
Hereinafter, the steps will be described.

(1) Rough rolling

In the rough rolling, it is more preferable to set the total rolling reduction to 50% or more and the rough rolling completion temperature to 1,100°C or higher. By performing the rough rolling under this condition, the maximum value of the major axis length of the prior austenite grains can be preferably controlled.
The slab is not particularly limited except that the slab has the above-described chemical composition. For example, a slab manufactured by melting molten steel having the above chemical composition using a converter or an electric furnace and performing a continuous casting method thereon can be used. Instead of the continuous casting method, an ingot-making method, a thin slab casting method, or the like may also be adopted. In the slab heating before the rough rolling, the heating temperature may be held in a temperature range of 1,100°C or higher for 3,000 seconds or longer.

(2) Finish rolling

In the finish rolling, it is preferable to set the finish rolling start temperature (temperature on the inlet side of the first stage) to 1,020°C to 1,100°C, set the rolling reduction of the first stage to 40% or more, set the total rolling reduction of the final two stages to less than 40%, and set the finish rolling completion temperature (temperature on the outlet side of the final stage) to 900°C or higher and lower than 980°C. By performing the finish rolling under these conditions, the distribution of the aspect ratios of the prior austenite grains can be preferably controlled.
In addition, in the finish rolling, it is more preferable to perform rolling two or more times at a rolling reduction of 30% or more in a temperature range of 980°C or higher and lower than 1,020°C. By satisfying this condition as well, the area ratio of the prior austenite grains having an aspect ratio of 2.0 or more and less than 4.0 and the area ratio of the prior austenite grains having an aspect ratio of 4.0 or more and less than 6.0 can be preferably controlled.
The rolling reduction referred to here can be represented by (1 - t₁/t₀) × 100 (%) in a case where a sheet thickness before the rolling is t₀ and a sheet thickness after the rolling is t₁. In addition, the total rolling reduction of the final two stages can be represented by (1 - t₃/t₂) × 100 (%) in a case where a sheet thickness on the entry-side thickness of the stage before the final stage is t₂ and a sheet thickness on the exit-side thickness of the final stage is t₃.

(3) Cooling

After the finish rolling is completed, it is preferable to start cooling within 1 second and to perform cooling at an average cooling rate of 30°C/s or faster until a temperature reaches 200°C. By cooling under this condition, a desired amount of bainite can be obtained.
After cooling to 200°C, the steel sheet may be subjected to coiling.
The average cooling rate mentioned in the present embodiment is a value obtained by dividing a temperature difference between a start point and an end point in a set range by an elapsed time from the start point to the end point.

Examples

Next, effects of one aspect of the present disclosure will be more specifically described using examples, but conditions in the examples are simply examples of the conditions adopted to confirm the feasibility and effects of the present disclosure, and the present disclosure is not limited to these examples of the conditions. The present disclosure may adopt various conditions as long as the object of the present disclosure is achieved without departing from the gist of the present disclosure.
Steels having a chemical composition shown in Tables 1 and 2 were melted and continuously cast to manufacture slabs having a thickness of 240 to 300 mm. The obtained slabs were used to obtain steel sheets shown in Table 4A and Table 4B under the manufacturing conditions shown in Table 3A and Table 3B. The sheet thickness of the obtained steel sheet was 1.2 to 8.0 mm.
It is noted that cooling was started within 1 second after the finish rolling was completed.
For the obtained steel sheet, the area ratio of the prior austenite grains having an aspect ratio of 1.0 or more and less than 2.0, the area ratio of the prior austenite grains having an aspect ratio of 2.0 or more and less than 4.0, the area ratio of the prior austenite grains having an aspect ratio of 4.0 or more and less than 6.0, the area ratio of the prior austenite grains having an aspect ratio of 6.0 or more, the maximum value of the major axis length of the prior austenite grains, and the area ratios of the bainite and the martensite at the position at a 1/4 sheet thickness from the surface were measured by the above-described method. In addition, by the above-described method, the tensile strength, the total elongation, and the hole expansion ratio were measured, and the evaluation of the unevenness on the inside of the bend was performed.
There was also an example in which the remainder in microstructure included ferrite and pearlite in addition to the bainite and the martensite, in the microstructure at the position of 1/4 from the surface of the steel sheet.
The obtained measurement results are shown in Table 5.

Method of evaluating characteristics of steel sheet

In a case where the tensile strength was 980 MPa or more, the steel sheet was considered to be a steel sheet having high strength and determined to be acceptable. On the other hand, in a case where the tensile strength was less than 980 MPa, the steel sheet was not regarded as a steel sheet having high strength and determined to be unacceptable.
In a case where the total elongation was 10.0% or more, the steel sheet was considered to be a steel sheet having excellent ductility and determined to be acceptable. On the other hand, in a case where the total elongation was less than 10.0%, the steel sheet was considered not to be a steel sheet having excellent ductility and determined to be not acceptable.
In a case where the hole expansion ratio was 50% or more, the steel sheet was considered to be a steel sheet having excellent hole expansibility and determined to be acceptable. In a case where the hole expansion ratio was less than 50%, the steel sheet was considered not to be a steel sheet having excellent hole expansibility and determined to be not acceptable.
After the bending test, the maximum valley depth of the roughness profile in the bending was evaluated according to the following standard. In a case where the evaluation was A or higher, the steel sheet was considered to be a steel sheet in which occurrence of unevenness on the inside of the bend during forming was suppressed, and determined to be acceptable. On the other hand, in a case where the evaluation was B, the steel sheet was considered not to be a steel sheet in which occurrence of unevenness on the inside of the bend during forming was suppressed, and determined to be not acceptable.
In addition, in a case where the evaluation was AA, the steel sheet was determined to be a steel sheet in which occurrence of unevenness on the inside of the bend during forming was further suppressed.

AA: The maximum valley depth of the roughness profile was 15 µm or less.
A: The maximum valley depth of the roughness profile was more than 15 µm and 20 µm or less.
B: The maximum valley depth of the roughness profile was more than 20 µm.

[Table 1]

Kind of steel	Chemical composition (by mass%), remainder including Fe and impurities
Kind of steel	C	Si	Mn	sol. Al	P	S	N	O	Ti	Nb	V	Cu	Cr	Mo
A	0.075	0.79	2.19	0.032	0.0074	0.0042	0.0031	0.0032	0.115	0.025			0.320
B	0.062	0.95	2.21	0.027	0.0045	0.0039	0.0037	0.0014	0.124	0.020
C	0.118	0.92	1.23	0.310	0.0035	0.0080	0.0037	0.0020	0.112	0.018			0.050
D	0.046	0.32	2.97	0.022	0.0057	0.0062	0.0034	0.0034	0.151	0.022			0.199
E	0.099	0.65	1.85	0.044	0.0054	0.0061	0.0021	0.0018	0.112	0.019			0.270
F	0.052	0.78	2.35	0.070	0.0060	0.0054	0.0027	0.0024	0.124	0.007
G	0.098	2.97	2.52	0.031	0.0080	0.0036	0.0029	0.0028	0.112	0.023
H	0.087	0.02	2.31	0.250	0.0089	0.0030	0.0015	0.0022	0.115	0.014	0.120			0.280
I	0.071	0.74	2.01	0.033	0.0057	0.0055	0.0024	0.0026	0.158				0.180
J	0.068	0.68	2.31	0.045	0.0051	0.0050	0.0023	0.0034		0.082
K	0.081	0.82	2.09	0.029	0.0034	0.0069	0.0016	0.0027
L	0.082	0.42	1.48	0.350	0.0064	0.0045	0.0023	0.0013				0.021	1.960
M	0.057	0.81	1.75	0.206	0.0084	0.0055	0.0035	0.0031					0.520
N	0.068	1.01	2.15	0.149	0.0041	0.0031	0.0022	0.0024	0.001	0.098
O	0.059	0.72	1.99	0.220	0.0083	0.0047	0.0026	0.0038	0.177	0.002
P	0.082	0.12	2.10	0.392	0.0085	0.0075	0.0015	0.0030	0.133	0.017
Q	0.075	0.68	1.40	0.032	0.0065	0.0070	0.0019	0.0023	0.134				1.520
R	0.064	0.52	1.75	0.420	0.0000	0.0065	0.0032	0.0010
S	0.043	0.72	2.85	0.030	0.0065	0.0034	0.0025	0.0030	0.153	0.023			0.220
T	0.125	0.68	1.38	0.027	0.0075	0.0046	0.0029	0.0039
U	0.072	3.05	2.11	0.024	0.0048	0.0055	0.0039	0.0018	0.112				0.210
V	0.091	0.55	1.17	0.045	0.0049	0.0034	0.0032	0.0036		0.035			0.420
W	0.078	0.62	3.12	0.025	0.0059	0.0071	0.0016	0.0024	0.102	0.023
X	0.069	0.72	2.21	0.520	0.0059	0.0062	0.0026	0.0015	0.112	0.018

The underline indicates that it is outside the scope of the present disclosure.

[Table 2]

Kind of steel	Chemical composition (by mass%), remainder including Fe and impurities
Kind of steel	Ni	B	Ca	Mg	REM	Bi	Ta	Zr	Co	Zn	W	Sb	As	Sn
A
B
C													0.002
D								0.003	0.002
E			0.0025	0.003
F
G												0.120
H							0.002
I
J										0.160
K											0.020
L	0.080
M		0.0035
N														0.030
O
P					0.001	0.002
Q
R
S
T
U
V
W
X

[Table 5]

Production No.	Kind of steel	Tensile strength TS	Total elongation EL	Hole expansion ratio λ	Maximum valley depth	Notes
Production No.	Kind of steel	MPa	%	%	Maximum valley depth	Notes
1	A	1092	11.8	74	AA	Invention Example
2	A	1066	12.9	70	AA	Invention Example
3	A	1032	14.8	65	AA	Invention Example
4	A	1058	13.6	55	A	Invention Example
5	A	1031	11.7	55	B	Comparative Example
6	A	1063	13.1	56	B	Comparative Example
7	A	1050	13.0	61	A	Invention Example
8	A	1098	13.0	65	A	Invention Example
9	A	1038	14.5	65	A	Invention Example
10	A	1015	13.9	58	B	Comparative Example
11	A	1087	11.7	63	B	Comparative Example
12	A	1042	13.0	51	B	Comparative Example
13	A	978	13.1	75	AA	Comparative Example
14	B	987	13.0	68	AA	Invention Example
15	C	1126	11.5	58	AA	Invention Example
16	D	991	12.7	75	AA	Invention Example
17	E	1077	10.2	53	AA	Invention Example
18	F	985	13.5	78	AA	Invention Example
19	G	1201	10.4	77	AA	Invention Example
20	H	1120	13.5	57	AA	Invention Example
21	I	1013	13.7	70	AA	Invention Example
22	J	987	12.8	60	AA	Invention Example
23	K	992	10.1	71	AA	Invention Example
24	L	1033	14.5	65	AA	Invention Example
25	M	989	12.1	75	AA	Invention Example
26	N	997	12.7	70	AA	Invention Example
27	O	985	12.8	69	AA	Invention Example
28	P	986	12.5	70	AA	Invention Example
29	Q	1095	11.2	64	AA	Invention Example
30	R	983	14.2	52	AA	Invention Example
31	S	972	12.9	51	AA	Comparative Example
32	T	1005	13.7	45	A	Comparative Example
33	U	Slab cracking				Comparative Example
34	V	830	15.4	53	AA	Comparative Example
35	W	1194	9.8	55	A	Comparative Example
36	X	955	14.2	45	A	Comparative Example

The underline indicates that it is outside the scope of the present disclosure, or that the characteristic value is not preferable.

From Tables 1 to 5, it can be seen that the steel sheet according to the example of the present invention has high strength, excellent ductility, and excellent hole expansibility, in which occurrence of unevenness on the inside of the bend is suppressed.
On the other hand, it can be seen that any one or more of the above-described characteristics of the steel sheet according to the comparative example is inferior.
In addition, the lower control arm (component) was manufactured by press working for all examples. The flat portion of the lower control arm was evaluated in the same manner as the above-described method. The measurement results and the evaluation results were the same as the results shown in Tables 4A, 4B, and 5.

INDUSTRIAL APPLICABILITY

According to the aspect of the present disclosure, it is possible to provide a steel sheet having high strength, excellent ductility, and excellent hole expansibility, in which occurrence of unevenness on the inside of the bend is suppressed, and a component manufactured using the steel sheet.

Claims

A steel sheet comprising, as a chemical composition, by mass%:
C: 0.045% to 0.120%;

Si: 0% to 3.00%;

Mn: 1.20% to 3.00%;

sol. Al: 0.001% to 0.500%;

P: 0% to 0.0800%;

S: 0% to 0.0100%;

N: 0% to 0.0050%;

O: 0% to 0.0100%;

Ti: 0% to 0.180%;

Nb: 0% to 0.100%;

V: 0% to 1.000%;

Cu: 0% to 1.000%;

Cr: 0% to 2.000%;

Mo: 0% to 3.000%;

Ni: 0% to 0.500%;

B: 0% to 0.0100%;

Ca: 0% to 0.0500%;

Mg: 0% to 0.050%;

REM: 0% to 0.100%;

Bi: 0% to 0.100%;

Ta: 0% to 0.100%;

Zr: 0% to 0.500%;

Co: 0% to 3.000%;

Zn: 0% to 0.200%;

W: 0% to 0.200%;

Sb: 0% to 0.500%;

As: 0% to 0.050%;

Sn: 0% to 0.050%, and

a remainder including Fe and impurities,

wherein, in a microstructure at a position at a 1/4 sheet thickness from a surface, by area%,

prior austenite grains having an aspect ratio of 1.0 or more and less than 2.0 account for less than 20%,

prior austenite grains having an aspect ratio of 2.0 or more and less than 6.0 account for 40% or more,

prior austenite grains having an aspect ratio of 6.0 or more account for less than 30%,

bainite accounts for 70% to 95%, and

martensite accounts for 5% to 30%.
The steel sheet according to Claim 1,
wherein the chemical composition comprises, by mass%, one or more selected from the group consisting of,
Ti: 0.001% to 0.180%,

Nb: 0.001% to 0.100%,

V: 0.001% to 1.000%,

Cu: 0.001% to 1.000%,

Cr: 0.001% to 2.000%,

Mo: 0.001% to 3.000%,

Ni: 0.001% to 0.500%,

B: 0.0001% to 0.0100%,

Ca: 0.0001% to 0.0500%,

Mg: 0.001% to 0.050%,

REM: 0.001% to 0.100%,

Bi: 0.001% to 0.100%,

Ta: 0.001% to 0.100%,

Zr: 0.001% to 0.500%,

Co: 0.001% to 3.000%,

Zn: 0.001% to 0.200%,

W: 0.001% to 0.200%,

Sb: 0.001% to 0.500%,

As: 0.001% to 0.050%, and

Sn: 0.001% to 0.050%.
The steel sheet according to Claim 1 or 2,
wherein in the microstructure at the position at the 1/4 sheet thickness from the surface, by area%,

prior austenite grains having an aspect ratio of 2.0 or more and less than 4.0 account for 30% or more, and

prior austenite grains having an aspect ratio of 4.0 or more and less than 6.0 account for 10% or more.
The steel sheet according to Claim 1 or 2,
wherein, in the microstructure at the position at the 1/4 sheet thickness from the surface, a maximum value of a major axis length of the prior austenite grains is 200 µm or less.
The steel sheet according to Claim 3,
wherein, in the microstructure at the position at the 1/4 sheet thickness from the surface, a maximum value of a major axis length of the prior austenite grains is 200 µm or less.
A component comprising:
the steel sheet according to Claim 1 or 2.
A component comprising:
the steel sheet according to Claim 3.
A component comprising:
the steel sheet according to Claim 4.
A component comprising:
the steel sheet according to Claim 5.