TECHNICAL FIELD

This disclosure relates to a high-strength cold-rolled steel sheet, and in particular, a high-strength cold-rolled steel sheet excellent in ductility and stretch flangeability. This disclosure also relates to a method for manufacturing the same.

BACKGROUND

In recent years, demand for improvement of crashworthiness and fuel efficiency of automobiles has increased, and high strength steel has been increasingly applied. Since thin steel sheets for automobiles are formed into a desired shape on a cold-pressing process, high ductility is required. However, commonly, there is a trade-off between the strength and the ductility of a steel sheet, and thus, the improvement of ductility of a high strength steel sheet has been variously examined. In the process, using the transformation induced plasticity (TRIP) effect of retained austenite, a low alloy TRIP steel sheet has been developed and widely applied. However, since martensite formed by transformation of retained austenite of the TRIP steel sheet during working is excessively hard, and thus, easily acts as starting points of crack during formation of stretch flange, the TRIP steel sheet has low stretch flangeability.

To improve the low stretch flangeability, various studies have been conducted. For example, JP 2006-104532 A (PTL 1) describes a technique related to a steel sheet having ferrite as a matrix phase microstructure and tempered martensite, retained austenite, and bainite as a hard phase microstructure, excellent ductility and stretch flangeability, and a tensile strength of 528 MPa to 1445 MPa.

Further, WO 2013/051238 A (PTL 2) describes a technique related to a steel sheet containing tempered martensite, retained austenite, and bainite as a dominant, hard phase microstructure and a predetermined polygonal ferrite, and having excellent ductility and stretch flangeability and a tensile strength of 813 MPa to 1393 MPa.

CITATION LIST

Patent Literatures

PTL 1: JP 2006-104532 A

PTL 2: WO 2013/051238 A

SUMMARY

Technical Problem

Though even in the conventional steel sheets as described in PTL 1 and PTL 2, ductility and stretch flangeability are improved in some degree, there has been a demand for the development of a high-strength cold-rolled steel sheet having higher levels of ductility and stretch flangeability.

It could thus be helpful to provide a high-strength steel sheet having a tensile strength (TS) of 750 MPa or more and excellent in ductility and stretch flangeability. Further, it could also be helpful to provide a method for manufacturing the high-strength steel sheet. In this disclosure, “excellent in ductility” means that the product of TS and total elongation (El) (TS·El) is 20000 (MPa×%) or more. Further, in this disclosure, “excellent in stretch flangeability” means that the product of TS and hole expansion ratio (λ) (TS·λ) is 30000 (MPa×%) or more.

Solution to Problem

The inventors made intensive studies to obtain a high-strength steel sheet having a tensile strength of 750 MPa or more and excellent in ductility and stretch frangeability, and as a result made the following discoveries.

(1) The circularity index of quenched martensite can be controlled by controlling the steel slab heating temperature, the finisher delivery temperature, the coiling temperature, the cold rolling reduction ratio, and the heating rate to a first soaking temperature.

(2) The area ratio of ferrite in the microstructure after annealing can be controlled by controlling the first soaking temperature for obtaining a ferrite-austenite dual phase and the average cooling rate from the first soaking temperature to 500° C.

(3) The area ratios of tempered martensite, quenched martensite, and retained austenite in the microstructure after annealing can be controlled by controlling the cooling stop temperature, the cooling rate, and the soaking temperature during a process in which cooling is performed to a martensite transformation start temperature or lower on the cooling process and subsequently the temperature is raised to a temperature range in which upper bainite is formed to perform soaking treatment.

(4) The area ratio of quenched martensite to the total area ratio of quenched martensite and tempered martensite can be controlled by controlling the cooling rate to 200° C. in a final cooling process to room temperature after the soaking treatment at the temperature range in which upper bainite is formed.

(5) A steel sheet having TS of 750 MPa or more and excellent in ductility and stretch flangeability can be obtained by controlling manufacturing conditions on each process in the specific ranges.

This disclosure is based on the findings described above and has the following primary features.

1. A high-strength cold-rolled steel sheet comprising: a chemical composition containing (consisting of), in mass %,

C: 0.060% to 0.250%,

Si: 0.70% to 1.80%,

Mn: 1.00% to 2.80%,

P: 0.100% or less,

S: 0.0100% or less,

Al: 0.010% to 0.100%, and

N: 0.0100% or less,

with the balance being Fe and inevitable impurities, wherein the high-strength cold-rolled steel sheet has a microstructure containing, in area ratio,

ferrite: 50% to 90%,

quenched martensite: 1% to 8%,

tempered martensite: 3% to 40%, and

retained austenite: 6% to 15%,

the quenched martensite has an average grain size of 2.5 μm or less, the quenched martensite has an average circularity index of 0.50 or more, the circularity index being defined as 4 πM/D2, where D is a perimeter of the quenched martensite and M is an area of the quenched martensite, and

the high-strength cold-rolled steel sheet has a ratio of an area ratio of the quenched martensite fMto a total area ratio of the quenched martensite and the tempered martensite fM+TM, fM/fM+TM, of 50% or less.

2. The high-strength cold-rolled steel sheet according to 1., wherein the chemical composition further contains, in mass %, at least one selected from the group consisting of:

Mo: 0.50% or less,

Ti: 0.100% or less,

Nb: 0.050% or less,

V: 0.100% or less,

B: 0.0100% or less,

Cr: 0.50% or less,

Cu: 1.00% or less,

Ni: 0.50% or less,

As: 0.500% or less,

Sb: 0.100% or less,

Sn: 0.100% or less,

Ta: 0.100% or less,

Ca: 0.0200% or less,

Mg: 0.0200% or less,

Zn: 0.020% or less,

Co: 0.020% or less,

Zr: 0.020% or less, and

REM: 0.0200% or less.

3. A method for manufacturing a high-strength cold-rolled steel sheet, comprising:

heating a steel slab having the chemical composition according to 1. or 2. to a steel slab heating temperature of 1100° C. to 1300° C. to obtain a heated steel slab,

hot rolling the heated steel slab with a finisher delivery temperature of 800° C. to 950° C., a coiling temperature of 300° C. to 700° C. to obtain a hot-rolled steel sheet,

cold rolling the hot-rolled steel sheet with a rolling reduction of 30% or more to obtain a cold-rolled steel sheet,

subjecting the cold-rolled steel sheet to a first soaking treatment, whereby the cold-rolled steel sheet is heated under conditions of a first soaking temperature of a T1 temperature or higher and a T2 temperature or lower and an average heating rate of less than 5.0° C./s within a temperature range of 500° C. to an Ac1transformation temperature and subsequently cooled to a cooling stop temperature of 100° C. to 250° C. with an average cooling rate of 10° C./s or more in a temperature range down to 500° C., and

subjecting the cold-rolled steel sheet after the first soaking treatment to a second soaking treatment, whereby the cold-rolled steel sheet is re-heated to a second soaking temperature of 350° C. to 500° C., held at the second soaking temperature for 10 seconds or more, subsequently cooled to 200° C. with an average cooling rate of 50° C./s or less, and then cooled to a room temperature,

the T1 temperature being defined by the following formula (1):
T1 temperature (° C.)=751−27×[% C]+18×[% Si]−12×[% Mn]−169×[% Al]−6×[% Ti]+24×[% Cr]−895×[% B]  (1)

the T2 temperature being defined by the following formula (2):
T2 temperature (° C.)=937−477×[% C]+56×[% Si]−20×[% Mn]+198×[% Al]+136×[% Ti]−5×[% Cr]+3315×[% B]  (2)
where brackets of the formula (1) and formula (2) indicate content by mass % of an element of the chemical composition enclosed in the brackets.

Advantageous Effect

According to this disclosure, it is possible to obtain a high-strength steel sheet having a tensile strength (TS) of 750 MPa or more and excellent in ductility and stretch flangeability. Further, high-strength steel sheets according to this disclosure are highly beneficial in industrial terms because, for example, they can improve fuel efficiency through a reduction in the weight of automotive bodies when applied to automobile structural parts.

