| 摘要 |
Provided are: a high-strength steel sheet having excellent stretchability and stretch flangeability, and a low yield ratio; and a method for manufacturing the high-strength steel sheet. A low-yield-ratio high-strength cold-rolled steel sheet having a chemical composition containing, by mass, 0.05 to 0.10% of C, 0.6 to 1.3% of Si, 1.4 to 2.2% of Mn, 0.08% or less of P, 0.010% or less of S, 0.01 to 0.08% of Al, and 0.010% or less of N, with the remainder made up by Fe and unavoidable impurities. The low-yield-ratio high-strength cold-rolled steel sheet has a microstructure in which the average crystal grain size of ferrite is equal to or smaller than 15 μm, the volume fraction of ferrite is equal to or greater than 70%, the volume fraction of bainite is equal to or greater than 3%, the volume fraction of residual austenite is 4 to 7%, the average crystal grain size of martensite is equal to or smaller than 5 μm, and the volume fraction of martensite is 1 to 6%. The average C concentration (mass%) in the residual austenite is 0.30 to 0.70%. The steel sheet characteristics are such that the yield ratio is equal to or less than 64% and the tensile strength is equal to or greater than 590 MPa. |
