High-strength steel sheet, high-strength hot-dip galvanized steel sheet, high-strength hot-dip aluminum-coated steel sheet, and high-strength electrogalvanized steel sheet, and methods for manufacturing same

1. A high-strength steel sheet, having a tensile strength of 980 MPa or more, comprising:

a chemical composition containing, in mass %, C: 0.030% or more and 0.250% or less, Si: 0.01% or more and 3.00% or less, Mn: more than 4.20% and 6.00% 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: 0.005% or more and 0.200% or less, and the balance being Fe and incidental impurities; and

a steel microstructure that contains, in area ratio, 15% or more and 55% or less of polygonal ferrite, 8% or more of non-recrystallized ferrite, and 15% or more and 30% or less of martensite, and that contains, in volume fraction, 12% or more of retained austenite,

wherein the polygonal ferrite has a mean grain size of 4 μm or less, the martensite has a mean grain size of 2 μm or less, the retained austenite has a mean grain size of 2 μm or less, and a value obtained by dividing an Mn content in the retained austenite in mass % by an Mn content in the polygonal ferrite in mass % equals 2.0 or more.

2. The high-strength steel sheet according to claim 1 , wherein the chemical composition further contains, in mass %, at least one selected from the group consisting of 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, and REM: 0.0005% or more and 0.0050% or less.

3. The high-strength steel sheet according to claim 2 , wherein the steel microstructure further contains, in area ratio, 2% or more of c phase with an hcp structure.

4. The high-strength steel sheet according to claim 2 , wherein the retained austenite has a C content that satisfies the following formula in relation to the Mn content in the retained austenite:

0.04*[Mn content]+0.056−0.180[C content]≤0.04*[Mn content]+0.056+0.180

where

[C content] is the C content in the retained austenite in mass %, and

[Mn content] is the Mn content in the retained austenite in mass %.

5. The high-strength steel sheet according to claim 2 , wherein when the steel sheet is subjected to tensile working with an elongation value of 10%, a value obtained by dividing a volume fraction of the retained austenite after the tensile working by a volume fraction of the retained austenite before the tensile working equals 0.3 or more.

6. The high-strength steel sheet according to claim 2 , wherein the high-strength steel sheet is a high-strength hot-dip galvanized steel sheet comprising a hot-dip galvanized layer, a high-strength hot-dip aluminum-coated steel sheet comprising a hot-dip aluminum-coated layer or a high-strength electrogalvanized steel sheet comprising an electrogalvanized layer.

7. The high-strength steel sheet according to claim 1 , wherein the steel microstructure further contains, in area ratio, 2% or more of c phase with an hcp structure.

8. The high-strength steel sheet according to claim 1 , wherein the retained austenite has a C content that satisfies the following formula in relation to the Mn content in the retained austenite:

0.04*[Mn content]+0.056−0.180[C content]≤0.04*[Mn content]+0.056+0.180

where

[C content] is the C content in the retained austenite in mass %, and

[Mn content] is the Mn content in the retained austenite in mass %.

9. The high-strength steel sheet according to claim 1 , wherein when the steel sheet is subjected to tensile working with an elongation value of 10%, a value obtained by dividing a volume fraction of the retained austenite after the tensile working by a volume fraction of the retained austenite before the tensile working equals 0.3 or more.

10. The high-strength steel sheet according to claim 1 , wherein the high-strength steel sheet is a high-strength hot-dip galvanized steel sheet comprising a hot-dip galvanized layer, a high-strength hot-dip aluminum-coated steel sheet comprising a hot-dip aluminum-coated layer or a high-strength electrogalvanized steel sheet comprising an electrogalvanized layer.

11. A method for manufacturing the high-strength steel sheet as recited in claim 1 , the method comprising:

heating a steel slab having the chemical composition containing, in mass %, C: 0.030% or more and 0.250% or less, Si: 0.01% or more and 3.00% or less, Mn: more than 4.20% and 6.00% 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: 0.005% or more and 0.200% or less, and the balance being Fe and incidental impurities, to 1100° C. or higher and 1300° C. or lower;

hot rolling the steel slab with a finisher delivery temperature of 750° C. or higher and 1000° C. or lower to obtain a steel sheet;

coiling the steel sheet at 300° C. or higher and 750° C. or lower;

then subjecting the steel sheet to pickling to remove scales;

retaining the steel sheet in a temperature range of Ac 1 transformation temperature+20° C. to Ac 1 transformation temperature+120° C. for 600 s to 21,600 s;

cold rolling the steel sheet at a rolling reduction of 30% or more; and

then retaining the steel sheet in a temperature range of Ac 1 transformation temperature to Ac 1 transformation temperature+100° C. for 20 s to 900 s, and subsequently cooling the steel sheet.

12. A method for manufacturing the high-strength steel sheet as recited in claim 10 , the method comprising:

heating a steel slab having the chemical composition containing, in mass %, C: 0.030% or more and 0.250% or less, Si: 0.01% or more and 3.00% or less, Mn: more than 4.20% and 6.00% 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: 0.005% or more and 0.200% or less, and the balance being Fe and incidental impurities, to 1100° C. or higher and 1300° C. or lower;

hot rolling the steel slab with a finisher delivery temperature of 750° C. or higher and 1000° C. or lower to obtain a steel sheet;

coiling the steel sheet at 300° C. or higher and 750° C. or lower;

then subjecting the steel sheet to pickling to remove scales;

retaining the steel sheet in a temperature range of Ac 1 transformation temperature+20° C. to Ac 1 transformation temperature+120° C. for 600 s to 21,600 s;

cold rolling the steel sheet at a rolling reduction of 30% or more;

then retaining the steel sheet in a temperature range of Ac 1 transformation temperature to Ac 1 transformation temperature+100° C. for 20 s to 900 s, and subsequently cooling the steel sheet; and

then subjecting the steel sheet to any one of the following:

galvanizing treatment, either alone or followed by alloying treatment at 450° C. or higher and 600° C. or lower,

hot-dip aluminum-coating treatment, or

electrogalvanizing treatment.

13. A method for manufacturing the high-strength steel sheet as recited in claim 2 , the method comprising:

heating a steel slab having the chemical composition containing, in mass %, C: 0.030% or more and 0.250% or less, Si: 0.01% or more and 3.00% or less, Mn: more than 4.20% and 6.00% 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: 0.005% or more and 0.200% or less, and at least one selected from the group consisting of 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, and REM: 0.0005% or more and 0.0050% or less, and the balance being Fe and incidental impurities, to 1100° C. or higher and 1300° C. or lower;

hot rolling the steel slab with a finisher delivery temperature of 750° C. or higher and 1000° C. or lower to obtain a steel sheet;

coiling the steel sheet at 300° C. or higher and 750° C. or lower;

then subjecting the steel sheet to pickling to remove scales;

retaining the steel sheet in a temperature range of Ac 1 transformation temperature+20° C. to Ac 1 transformation temperature+120° C. for 600 s to 21,600 s;

cold rolling the steel sheet at a rolling reduction of 30% or more; and

then retaining the steel sheet in a temperature range of Ac 1 transformation temperature to Ac 1 transformation temperature+100° C. for 20 s to 900 s, and subsequently cooling the steel sheet.

14. A method for manufacturing the high-strength steel sheet as recited in claim 6 , the method comprising:

heating a steel slab having the chemical composition containing, in mass %, C: 0.030% or more and 0.250% or less, Si: 0.01% or more and 3.00% or less, Mn: more than 4.20% and 6.00% 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: 0.005% or more and 0.200% or less, and at least one selected from the group consisting of 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, and REM: 0.0005% or more and 0.0050% or less, and the balance being Fe and incidental impurities, to 1100° C. or higher and 1300° C. or lower;

hot rolling the steel slab with a finisher delivery temperature of 750° C. or higher and 1000° C. or lower to obtain a steel sheet;

coiling the steel sheet at 300° C. or higher and 750° C. or lower;

then subjecting the steel sheet to pickling to remove scales;

retaining the steel sheet in a temperature range of Ac 1 transformation temperature+20° C. to Ac 1 transformation temperature+120° C. for 600 s to 21,600 s;

cold rolling the steel sheet at a rolling reduction of 30% or more;

then retaining the steel sheet in a temperature range of Ac 1 transformation temperature to Ac 1 transformation temperature+100° C. for 20 s to 900 s, and subsequently cooling the steel sheet; and

then subjecting the steel sheet to any one of the following:

galvanizing treatment, either alone or followed by alloying treatment at 450° C. or higher and 600° C. or lower,

hot-dip aluminum-coating treatment, or

electrogalvanizing treatment.