TECHNICAL FIELD

The present invention relates to a cold-rolled steel sheet and a hot-dip galvanized cold-rolled steel sheet.

BACKGROUND ART

In recent years, from the viewpoint of complying with greenhouse gas emissions regulations that accompany measures to tackle global warming, improvements in the fuel consumption of automobiles are being sought. In order to reduce the weight of automobile bodies and ensure collision safety, the application of high-strength steel sheets is increasing more and more. Recently, there is an increasing need for ultra-high strength steel sheets having a tensile strength of 980 MPa or more. There is a demand for an ultra-high strength hot-dip galvanized steel sheet whose surface has undergone hot-dip galvanization for use at regions in an automobile body that require rust preventing properties.

Steel sheets that are provided for use in components for automobile use are required to have not only high strength, but also various working properties required when forming components, such as press-formability and weldability. Specifically, from the viewpoint of press-formability, a steel sheet is often required to be excellent in elongation (total elongation in a tensile test: El) and stretch flangeability (hole expansion ratio: λ).

Although it is generally difficult to secure both a high level of total elongation (El) and a high hole expansion ratio (λ) accompanying enhancement of the strength of a steel sheet, a TRIP (transformation induced plasticity) steel sheet is known in which both enhanced strength and workability are achieved by utilizing transformation induced plasticity of retained austenite.

On the other hand, when the application of high-strength steel sheets to automobiles to be used in cold regions is considered, the high-strength steel sheets are required to have properties such that brittle fractures do not occur under a low-temperature environment. In particular, when the application thereof to components for automobile use is considered, low-temperature toughness after plastic strain is introduced by press working is required. However, it is commonly known that a TRIP steel sheet is inferior in low-temperature toughness.

Patent Documents 1 to 3 disclose a technique that relates to high-strength TRIP steel sheets in which constituent fractions of the microstructure are controlled to be within a predetermined range to thereby improve elongation and the hole expansion ratio.

Patent Document 4 and Patent Document 5 disclose a technique that relates to high-strength TRIP steel sheets in which the low-temperature toughness is improved by controlling constituent fractions of the microstructure to be within a predetermined range, and furthermore controlling the distribution of IQ (image quality) values of grains determined by EBSD analysis to be within a predetermined range.

Patent Document 6 discloses a technique that relates to a high-strength TRIP steel in which the microstructure is principally composed of tempered martensite containing retained austenite and MA, and the hole expandability is improved by increasing the proportion of the MA and retained austenite that comes in contact with tempered martensite or that exists within grains of tempered martensite.

Patent Document 7 discloses a technique that improves the toughness of a DP (dual-phase) steel sheet. Patent Document 8 and Patent Document 9 disclose a technique that relates to a high-strength steel sheet in which the low-temperature toughness is improved by controlling constituent fractions of the microstructure to be within a predetermined range, and furthermore controlling the stacking fault density of the retained austenite so as to fall within a predetermined range.

LIST OF PRIOR ART DOCUMENTS

Patent Document

Patent Document 1: WO 2013/151238

Patent Document 2: JP2006-104532A

Patent Document 3: JP2007-262494A

Patent Document 4: JP2015-086468A

Patent Document 5: JP2015-200006A

Patent Document 6: JP2014-34716A

Patent Document 7: JP2011-132602A

Patent Document 8: JP2015-025208A

Patent Document 9: JP2014-133944A

SUMMARY OF INVENTION

Technical Problem

In the techniques disclosed in Patent Documents 1 to 3, consideration is not given to low-temperature toughness. In the technique disclosed in Patent Document 4, because the structural fraction of ferrite is 50% or more, it is difficult to secure a strength of 980 MPa-class or higher. In the technique disclosed in Patent Document 5, consideration is not given to low-temperature toughness after working that is necessary as a steel sheet for automobile use. In the technique disclosed in Patent Document 6, consideration is not given to low-temperature toughness. In the steel sheet disclosed in Patent Document 7, ductility is insufficient since the steel sheet contains almost no retained austenite. In the technique disclosed in Patent Document 8 and Patent Document 9, consideration is not given to hole expandability that is important with regard to the workability of a high-strength steel sheet.

In view of the current state of the prior art, a problem to be solved by the present invention is to increase workability and low-temperature toughness, especially low-temperature toughness after the introduction of plastic strain, in a high-strength cold-rolled steel sheet and a high-strength hot-dip galvanized cold-rolled steel sheet, and an objective of the present invention is to provide a high-strength cold-rolled steel sheet and a high-strength hot-dip galvanized cold-rolled steel sheet that solve the aforementioned problem.

Solution to Problem

In considering approaches for solving the aforementioned problem, the present inventors conducted intensive studies regarding a microstructure with which, in addition to high strength, workability and low-temperature toughness can be secured.

As a result, the present inventors discovered that in order to secure the target strength, elongation, hole expansion ratio and low-temperature toughness, it is necessary for the microstructure to simultaneously satisfy the following conditions (i) to (v).

(i) Ferrite: 1 to 29 area %

(ii) Retained austenite: 5 to 20 area %

(iii) Martensite: less than 10 area %

(iv) Pearlite: less than 5 area %

(v) Bainite and/or tempered martensite: balance

In addition, the present inventors ascertained that the phase boundary between ferrite that has the softest structure in the microstructure and martensite or retained austenite that has the hardest structure becomes a starting point for fractures, and discovered that the low-temperature toughness after working can be further improved if the length of phase boundaries at which both microstructures come in contact is not more than a predetermined value, specifically, if the length satisfies the condition described in (vi) hereunder.

(vi) The total sum of the lengths of phase boundaries at which ferrite comes in contact with martensite or retained austenite having a circle-equivalent radius of 1 μm or more is 100 μm or less per 1000 μm2.

FIG. 1shows results obtained by measuring vTrs when a pre-strain of 5% was applied to steel sheets having various σMA, and thereafter a Charpy impact test was performed. Note that, in the present specification, a total sum of the lengths of phase boundaries at which ferrite comes in contact with martensite or retained austenite having a circle-equivalent radius of 1 μm or more is referred to as “σMA”.

As illustrated inFIG. 1, there is a tendency for vTrs after application of 5% pre-strain to decrease as σMA decreases, and in particular vTrs decreases significantly when σMA is 100 μm or less. With regard to the mechanism whereby σMA affects the low-temperature toughness after working, it is considered that when a steel sheet is subjected to working, strain concentrates at phase boundaries between ferrite which is the softest structure in the microstructure and martensite or retained austenite which is the hardest structure in the microstructure, and minute phase boundary separation or cracking occurs. Such phase boundary separation or cracking becomes the starting points for brittle fracture. Hence, it is considered that the smaller the phase boundary is, in other words, the smaller σMA is, the more excellent the low-temperature toughness after working will be.

The present invention has been made based on the aforementioned findings, and the gist of the present invention is as described hereunder.

(1) A cold-rolled steel sheet having a tensile strength of 980 MPa or more, including a chemical composition consisting of, in mass %:

C: 0.10 to 0.30%,

Si: 0.50 to 2.50%,

Mn: 1.50 to 3.50%,

Al: 0.001 to 1.00%,

P: 0.05% or less,

S: 0.01% or less,

N: 0.01% or less,

O: 0.01% or less,

Cr: 0 to 1.00%,

Mo: 0 to 1.00%,

Sn: 0 to 1.00%,

Cu: 0 to 1.00%,

Ni: 0 to 1.00%,

B: 0 to 0.005%,

Ti: 0 to 0.30%,

V: 0 to 0.50%,

Nb: 0 to 0.10%,

W: 0 to 0.50%,

Ca: 0 to 0.010%,

Mg: 0 to 0.010%,

Sb: 0 to 0.200%,

Zr: 0 to 0.010%,

Bi: 0 to 0.010%,

REM: 0 to 0.100%, and

the balance: Fe and impurities,

wherein:

a microstructure consists of, in area %:

ferrite: 1 to 29%,

retained austenite: 5 to 20%,

martensite: less than 10%,

pearlite: less than 5%, and

the balance: bainite and/or tempered martensite; and

a total sum of lengths of phase boundaries where ferrite comes in contact with martensite or retained austenite having a circle-equivalent radius of 1 μm or more is 100 μm or less per 1000 μm2.

(2) The cold-rolled steel sheet according to the above (1), wherein a thickness of the steel sheet is in a range of 0.5 to 3.2 mm.

(3) A hot-dip galvanized cold-rolled steel sheet, including: a hot-dip galvanized layer on a surface of the cold-rolled steel sheet according to the above (1) or (2).

(4) A hot-dip galvanized cold-rolled steel sheet, including: a galvannealed layer on a surface of the cold-rolled steel sheet according to the above (1) or (2).

Advantageous Effects of Invention

According to the present invention, a high-strength cold-rolled steel sheet and a high-strength hot-dip galvanized cold-rolled steel sheet can be provided which are excellent in workability and low-temperature toughness, and in particular, are excellent in low-temperature toughness after plastic strain introduction.

