High-strength steel sheet and method for producing the same

1. A high-strength steel sheet comprising a composition containing, by mass, C: 0.05% to 0.15%, Si: 0.01% to 1.00%, Mn: 1.5% to 4.0%, P: 0.100% or less, S: 0.02% or less, Al: 0.01% to 0.50%, Cr: 0.010% to 2.000%, Nb: 0.005% to 0.100%, Ti: 0.005% to 0.100%, B: 0.0005% to 0.0050%, and the balance being Fe and inevitable impurities, wherein K represented by Formula (I) below is 3.0 or more; and a microstructure in which, when the microstructure is observed in a cross section of the steel sheet which is perpendicular to the surface of the steel sheet at a depth of ¼ thickness below the surface of the steel sheet, the area ratio of ferrite is 8% to 45%, the area ratio of martensite is 55% to 85%, the proportion of martensite grains adjacent to only ferrite grains in the entire microstructure is 15% or less, the average crystal grain sizes of ferrite and martensite are each 10 μm or less, and the area ratio of ferrite grains having a size of 10 μm or more to all ferrite grains included in a portion of the steel sheet which extends from 20 to 100 μm below the surface of the steel sheet is less than 5%,

K=−0.4×[Si]+1.0×[Mn]+1.3×[Cr]+200×[B]  Formula (I),

where [Si] represents the Si content [mass %], [Mn] represents the Mn content [mass %], [Cr] represents the Cr content [mass %], and [B] represents the B content [mass], and

wherein the steel sheet has a yield strength (YS) of 780 MPa or more and a tensile strength (TS) of 1180 MPa or more.

2. The high-strength steel sheet according to claim 1 , further containing, by mass, at least one element selected from Mo: 0.005% to 2.000%, V: 0.005% to 2.000%, Ni: 0.005% to 2.000%, and Cu: 0.005% to 2.000%.

3. The high-strength steel sheet according to claim 1 , further containing, by mass, at least one element selected from Ca: 0.001% to 0.005% and REM: 0.001% to 0.005%.

4. The high-strength steel sheet according to claim 1 , being a high-strength cold-rolled steel sheet.

5. The high-strength steel sheet according to claim 1 , provided with a hot-dip galvanizing film deposited on a surface of the steel sheet.

6. The high-strength steel sheet according to claim 5 , wherein the hot-dip galvanizing film is a hot-dip galvannealing film.

7. A method for producing a high-strength steel sheet having a yield strength (YS) of 780 MPa or more and a tensile strength (TS) of 1180 MPa or more, the method comprising a hot-rolling step in which, when a slab having the composition according to claim 1 is hot-rolled, a finish-rolled steel sheet is cooled such that the steel sheet is retained at 600° C. to 700° C. for 1-10 seconds in total after being finish-rolled, and the cooled steel sheet is coiled at 400° C. or more and less than 600° C.; a cold-rolling step in which, subsequent to the hot-rolling step, the hot-rolled steel sheet is cold-rolled at a rolling reduction ratio of more than 20%; an annealing step in which, subsequent to the cold-rolling step, the cold-rolled steel sheet is heated to a target heating temperature of 680° C. or more at an average heating rate of 5° C./sec. or more, further heated to an annealing temperature of 720° C. to 820° C. within 500 seconds, and subsequently held at the annealing temperature for 10 to 1000 seconds; and a cooling step in which the annealed cold-rolled steel sheet is cooled to a cooling-stop temperature of 450° C. to 550° C. at an average cooling rate of 3° C./sec. or more and subsequently held at the cooling-stop temperature for 1000 seconds or less.

8. The method for producing a high-strength steel sheet according to claim 7 , the method further comprising a galvanizing step in which a hot-dip galvanizing treatment is performed subsequent to the cooling step after annealing the cold-rolled steel sheet.

9. The method for producing a high-strength steel sheet according to claim 8 , the method further comprising an alloying step in which an alloying treatment is performed subsequent to the galvanizing step.