| 摘要 |
A hot-rolled steel sheet has a chemical composition containing, by mass %, C: 0.030% or more and 0.120% or less, Si: 0.05% or more and 0.50% or less, Mn: 1.00% or more and 2.20% or less, P: 0.025% or less, S: 0.0050% or less, N: 0.0060% or less, Al: 0.005% or more and 0.100% or less, Nb: 0.020% or more and 0.100% or less, Mo: 0.05% or more and 0.50% or less, Ti: 0.001% or more and 0.100% or less, Cr: 0.05% or more and 0.50% or less, Ca: 0.0005% or more and 0.0050% or less, and the balance being Fe and inevitable impurities, and has a microstructure including bainitic ferrite as a main phase and martensite and retained austenite as second phases, wherein a volume fraction of the main phase is 90% or more, an average grain diameter of the main phase is 10 μm or less, a volume fraction of the martensite is 0.5% or more and 9.5% or less, and a volume fraction of the retained austenite is 0.5% or more and 9.5% or less, wherein the steel sheet has a yield ratio of 90% or less, a yield strength of 555 MPa or more, and a tensile strength of 650 MPa or more. |
