High-strength steel sheet and method for producing same

1. A high-strength steel sheet comprising:

a chemical composition consisting of, by mass %,

C: 0.030% or more and 0.250% or less,

Si: 0.01% or more and 3.00% or less,

Mn: 2.60% or more and 4.20% or less,

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

S: 0.0001% or more and 0.0200% or less,

N: 0.0005% or more and 0.0100% or less, and

Ti: 0.003% or more and 0.200% or less, and

optionally at least one selected from

Al: 0.01% or more and 2.00% or less,

Nb: 0.005% or more and 0.200% or less,

B: 0.0003% or more and 0.0050% or less,

Ni: 0.005% or more and 1.000% or less,

Cr: 0.005% or more and 1.000% or less,

V: 0.005% or more and 0.500% or less,

Mo: 0.005% or more and 1.000% or less,

Cu: 0.005% or more and 1.000% or less,

Sn: 0.002% or more and 0.200% or less,

Sb: 0.002% or more and 0.200% or less,

Ta: 0.001% or more and 0.010% or less,

Ca: 0.0005% or more and 0.0050% or less,

Mg: 0.0005% or more and 0.0050% or less, or

REM: 0.0005% or more and 0.0050% or less, and

the balance being Fe and inevitable impurities, and

a microstructure where

ferrite is 35% or more and 80% or less in area ratio,

martensite is 5% or more and 25% or less in area ratio, and

retained austenite is 8% or more in volume fraction, wherein

an average grain size of the ferrite is 6.0 μm or less,

an average grain size of the martensite is 3.0 μm or less,

an average grain size of the retained austenite is 3.0 μm or less,

an average aspect ratio of crystal grain of each of the ferrite, the martensite and the retained austenite is more than 2.2 and 15.0 or less,

a value obtained by dividing a Mn content (mass %) in the retained austenite by a Mn content (mass %) in the ferrite is 2.0 or more, and

the high-strength steel sheet has a tensile strength of 590 MPa or more and a yield ratio of less than 68%.

2. The high-strength steel sheet according to claim 1 , comprising a hot-dip galvanized layer on a surface.

3. The high-strength steel sheet according to claim 1 , comprising a hot-dip aluminum-coated layer on a surface.

4. The high-strength steel sheet according to claim 1 , comprising an electrogalvanized layer on a surface.

5. A method for producing the high-strength steel sheet according to claim 1 , comprising:

(i) subjecting a steel slab to hot rolling, in which the steel slab is heated to 1100° C. or higher and 1300° C. or lower, hot rolled with a finisher delivery temperature of 750° C. or higher and 1000° C. or lower, and coiled at an average coiling temperature of 300° C. or higher and 750° C. or lower to obtain a hot-rolled sheet;

(ii) subjecting the hot-rolled sheet to pickling, in which scales are removed;

(iii) subjecting the hot-rolled sheet to hot band annealing, in which the hot-rolled sheet is held in a temperature range of (Ac 1 transformation point+20° C.) or higher and (Ac 1 transformation point+120° C.) or lower for 600 seconds or more and 21600 seconds or less;

(iv) subjecting the hot-rolled sheet to cold rolling, in which the hot-rolled sheet is cold rolled with a rolling reduction of 3% or more and less than 30% to obtain a cold-rolled sheet; and

(v) subjecting the cold-rolled sheet to cold-rolled sheet annealing, in which the cold-rolled sheet is held in a temperature range of (Ac 1 transformation point+10° C.) or higher and (Ac 1 transformation point+100° C.) or lower for more than 900 seconds and 21600 seconds or less and then cooled,

wherein the steel slab has a chemical composition consisting of, by mass %,

C: 0.030% or more and 0.250% or less,

Si: 0.01% or more and 3.00% or less,

Mn: 2.60% or more and 4.20% or less,

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

S: 0.0001% or more and 0.0200% or less,

N: 0.0005% or more and 0.0100% or less, and

Ti: 0.003% or more and 0.200% or less, and

optionally at least one selected from

Al: 0.01% or more and 2.00% or less,

Nb: 0.005% or more and 0.200% or less,

B: 0.0003% or more and 0.0050% or less,

Ni: 0.005% or more and 1.000% or less,

Cr: 0.005% or more and 1.000% or less,

V: 0.005% or more and 0.500% or less,

Mo: 0.005% or more and 1.000% or less,

Cu: 0.005% or more and 1.000% or less,

Sn: 0.002% or more and 0.200% or less,

Sb: 0.002% or more and 0.200% or less,

Ta: 0.001% or more and 0.010% or less,

Ca: 0.0005% or more and 0.0050% or less,

Mg: 0.0005% or more and 0.0050% or less, or

REM: 0.0005% or more and 0.0050% or less, and

the balance being Fe and inevitable impurities.

6. A method for producing the high-strength steel sheet according to claim 2 , comprising:

(i) subjecting a steel slab to hot rolling, in which the steel slab is heated to 1100° C. or higher and 1300° C. or lower, hot rolled with a finisher delivery temperature of 750° C. or higher and 1000° C. or lower, and coiled at an average coiling temperature of 300° C. or higher and 750° C. or lower to obtain a hot-rolled sheet;

(ii) subjecting the hot-rolled sheet to pickling, in which scales are removed;

(iii) subjecting the hot-rolled sheet to hot band annealing, in which the hot-rolled sheet is held in a temperature range of (Ac 1 transformation point+20° C.) or higher and (Ac 1 transformation point+120° C.) or lower for 600 seconds or more and 21600 seconds or less;

(iv) subjecting the hot-rolled sheet to cold rolling, in which the hot-rolled sheet is cold rolled with a rolling reduction of 3% or more and less than 30% to obtain a cold-rolled sheet; and

(v) subjecting the cold-rolled sheet to cold-rolled sheet annealing, in which the cold-rolled sheet is held in a temperature range of (Ac 1 transformation point+10° C.) or higher and (Ac 1 transformation point+100° C.) or lower for more than 900 seconds and 21600 seconds or less and then cooled,

after (v) the cold-rolled sheet annealing, the cold-rolled sheet is further subjected to hot-dip galvanizing treatment, or

after (v) the cold-rolled sheet annealing, the cold-rolled sheet is further subjected to hot-dip galvanizing treatment, and then to alloying treatment in a temperature range of 450° C. or higher and 600° C. or lower,

wherein the steel slab has a chemical composition consisting of, by mass %,

C: 0.030% or more and 0.250% or less,

Si: 0.01% or more and 3.00% or less,

Mn: 2.60% or more and 4.20% or less,

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

S: 0.0001% or more and 0.0200% or less,

N: 0.0005% or more and 0.0100% or less, and

Ti: 0.003% or more and 0.200% or less, and

optionally at least one selected from

Al: 0.01% or more and 2.00% or less,

Nb: 0.005% or more and 0.200% or less,

B: 0.0003% or more and 0.0050% or less,

Ni: 0.005% or more and 1.000% or less,

Cr: 0.005% or more and 1.000% or less,

V: 0.005% or more and 0.500% or less,

Mo: 0.005% or more and 1.000% or less,

Cu: 0.005% or more and 1.000% or less,

Sn: 0.002% or more and 0.200% or less,

Sb: 0.002% or more and 0.200% or less,

Ta: 0.001% or more and 0.010% or less,

Ca: 0.0005% or more and 0.0050% or less,

Mg: 0.0005% or more and 0.0050% or less, or

REM: 0.0005% or more and 0.0050% or less, and

the balance being Fe and inevitable impurities.

7. A method for producing the high-strength steel sheet according to claim 3 , comprising:

(i) subjecting a steel slab to hot rolling, in which the steel slab is heated to 1100° C. or higher and 1300° C. or lower, hot rolled with a finisher delivery temperature of 750° C. or higher and 1000° C. or lower, and coiled at an average coiling temperature of 300° C. or higher and 750° C. or lower to obtain a hot-rolled sheet;

(ii) subjecting the hot-rolled sheet to pickling, in which scales are removed;

(iii) subjecting the hot-rolled sheet to hot band annealing, in which the hot-rolled sheet is held in a temperature range of (Ac 1 transformation point+20° C.) or higher and (Ac 1 transformation point+120° C.) or lower for 600 seconds or more and 21600 seconds or less;

(iv) subjecting the hot-rolled sheet to cold rolling, in which the hot-rolled sheet is cold rolled with a rolling reduction of 3% or more and less than 30% to obtain a cold-rolled sheet; and

(v) subjecting the cold-rolled sheet to cold-rolled sheet annealing, in which the cold-rolled sheet is held in a temperature range of (Ac 1 transformation point+10° C.) or higher and (Ac 1 transformation point+100° C.) or lower for more than 900 seconds and 21600 seconds or less and then cooled,

after (v) the cold-rolled sheet annealing, the cold-rolled sheet is further subjected to hot-dip aluminum-coating treatment,

wherein the steel slab has a chemical composition consisting of, by mass %,

C: 0.030% or more and 0.250% or less,

Si: 0.01% or more and 3.00% or less,

Mn: 2.60% or more and 4.20% or less,

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

S: 0.0001% or more and 0.0200% or less,

N: 0.0005% or more and 0.0100% or less, and

Ti: 0.003% or more and 0.200% or less, and

optionally at least one selected from

Al: 0.01% or more and 2.00% or less,

Nb: 0.005% or more and 0.200% or less,

B: 0.0003% or more and 0.0050% or less,

Ni: 0.005% or more and 1.000% or less,

Cr: 0.005% or more and 1.000% or less,

V: 0.005% or more and 0.500% or less,

Mo: 0.005% or more and 1.000% or less,

Cu: 0.005% or more and 1.000% or less,

Sn: 0.002% or more and 0.200% or less,

Sb: 0.002% or more and 0.200% or less,

Ta: 0.001% or more and 0.010% or less,

Ca: 0.0005% or more and 0.0050% or less,

Mg: 0.0005% or more and 0.0050% or less, or

REM: 0.0005% or more and 0.0050% or less, and

the balance being Fe and inevitable impurities.

8. A method for producing the high-strength steel sheet according to claim 4 , comprising

(i) subjecting a steel slab to hot rolling, in which the steel slab is heated to 1100° C. or higher and 1300° C. or lower, hot rolled with a finisher delivery temperature of 750° C. or higher and 1000° C. or lower, and coiled at an average coiling temperature of 300° C. or higher and 750° C. or lower to obtain a hot-rolled sheet;

(ii) subjecting the hot-rolled sheet to pickling, in which scales are removed;

(iii) subjecting the hot-rolled sheet to hot band annealing, in which the hot-rolled sheet is held in a temperature range of (Ac 1 transformation point+20° C.) or higher and (Ac 1 transformation point+120° C.) or lower for 600 seconds or more and 21600 seconds or less;

(iv) subjecting the hot-rolled sheet to cold rolling, in which the hot-rolled sheet is cold rolled with a rolling reduction of 3% or more and less than 30% to obtain a cold-rolled sheet; and

(v) subjecting the cold-rolled sheet to cold-rolled sheet annealing, in which the cold-rolled sheet is held in a temperature range of (Ac 1 transformation point+10° C.) or higher and (Ac 1 transformation point+100° C.) or lower for more than 900 seconds and 21600 seconds or less and then cooled,

after (v) the cold-rolled sheet annealing, the cold-rolled sheet is further subjected to electrogalvanizing treatment,

wherein the steel slab has a chemical composition consisting of, by mass %,

C: 0.030% or more and 0.250% or less,

Si: 0.01% or more and 3.00% or less,

Mn: 2.60% or more and 4.20% or less,

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

S: 0.0001% or more and 0.0200% or less,

N: 0.0005% or more and 0.0100% or less, and

Ti: 0.003% or more and 0.200% or less, and

optionally at least one selected from

Al: 0.01% or more and 2.00% or less,

Nb: 0.005% or more and 0.200% or less,

B: 0.0003% or more and 0.0050% or less,

Ni: 0.005% or more and 1.000% or less,

Cr: 0.005% or more and 1.000% or less,

V: 0.005% or more and 0.500% or less,

Mo: 0.005% or more and 1.000% or less,

Cu: 0.005% or more and 1.000% or less,

Sn: 0.002% or more and 0.200% or less,

Sb: 0.002% or more and 0.200% or less,

Ta: 0.001% or more and 0.010% or less,

Ca: 0.0005% or more and 0.0050% or less,

Mg: 0.0005% or more and 0.0050% or less, or

REM: 0.0005% or more and 0.0050% or less, and

the balance being Fe and inevitable impurities.

9. The high-strength steel sheet according to claim 1 , wherein the average aspect ratio of crystal grain of each of the ferrite, the martensite and the retained austenite is more than 2.4 and 15.0 or less.