| 摘要 |
<P>PROBLEM TO BE SOLVED: To provide a hot-rolled steel sheet suitable for use in a cold-rolled steel sheet or a hot-dip galvanized steel sheet, excellent in material uniformity and having a tensile strength of 590 MPa or higher. <P>SOLUTION: The chemical composition of the steel sheet includes, in mass%, 0.060-0.150% of C, 0.15-0.70% of Si, 1.00-1.90% 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 0.010-0.100% of Nb, with the remainder including Fe and inevitable impurities. The microstructure of the steel sheet has a composite structure that includes 75% or more, by volume fraction, of ferrite having an average crystal grain size of 18 μm or smaller, and 5% or more, by volume fraction, of pearlite having an average crystal grain size of 2 μm or larger, with the remainder including a phase formed at low temperatures. The mean free path of the pearlite is 5.0 μm or greater. <P>COPYRIGHT: (C)2013,JPO&INPIT |
