| 摘要 |
<p>Disclosed is a high-strength steel sheet in which ferrite crystal grains are finely divided to achieve static strength of 750 MPa or more and which has excellent dynamic deformation properties. Specifically disclosed is a steel sheet having a chemical composition comprising 0.05 to 0.20% of C, 0.02 to 3.0% of Si, 0.5 to 3.0% of Mn, 0.5% or less of P, 0.05% or less of S, 0.05 to 1.0% of Cr, 0.01 to 1.0% of sol.Al, one or more elements selected from the group consisting of Ti, Nb, Mo, V and W in the total amount of 0.002 to 0.03%, and the remainder made up by Fe and impurities. The steel sheet has such a metallic structure that ferrite has an average crystal grain diameter of 3.0 µm or less in a region lying between the surface of the steel sheet and the depth of at least 100 to 200 µm in the thicknesswise direction of the steel sheet, the region is made up by ferrite at an areal ratio of 30 to 80% and the remainder structure, and the average spacing of the remainder structure in the thicknesswise direction is 3.0 µm or less. The steel sheet has such mechanical properties that the tensile strength is 750 MP or more, the product of the tensile strength and the breaking elongation is 13000 MPa·% or more, and the difference between the dynamic tensile strength at a tensile strain rate of 103/sec and static tensile strength at a tensile strain rate of 0.01/sec is 80 MPa or more.</p> |
