To provide a high-yield-ratio high-strength cold-rolled steel sheet of excellent elongation and stretch-flangeability, the steel sheet having a tensile strength of 590 MPa or greater.
A steel sheet has the following chemical composition comprising, by mass, 0.06-0.13% of C, 1.2-2.3% of Si, 0.6-1.6% of Mn, 0.10% or less of P, 0.010% or less of S, 0.01-0.10% of Al, 0.010% or less of N, and the remainder comprising Fe with inevitable impurities. The microstructure of the steel sheet includes, in volume fraction: 90% or more of ferrite having a mean grain size of less than 20 μm; and 1.0-10% of perlite having a mean grain size less than 5 μm. A high-yield-ratio high-strength cold-rolled steel sheet of excellent elongation and stretch-flangeability where the ferrite has: a mean Vickers hardness of 130 or higher; a yield ratio of 65% or higher; and a tensile strength of 590 MPa or greater.
TAJI YUUKI
HASEGAWA KOHEI
Kazuhiro Mori