| 摘要 |
Provided are: a hot-rolled steel sheet which is suitable as a material for an API X80 grade steel pipe, has high strength and a low yield ratio, can keep the yield ratio thereof at a low level even after being worked regardless of the amount of working strain, and has excellent post-working property stability; and a method for manufacturing the hot-rolled steel sheet. A high-strength hot-rolled steel sheet having excellent post-working property stability and a low yield ratio, which has a chemical composition comprising, in % by mass, 0.030 to 0.120% inclusive of C, 0.05% to 0.50% inclusive of Si, 1.00 to 2.20% inclusive of Mn, 0.025% or less of P, 0.0050% or less of S, 0.0060% or less of N, 0.005 to 0.100% inclusive of Al, 0.020 to 0.100% inclusive of Nb, 0.05 to 0.50% inclusive of Mo, 0.001 to 0.100% inclusive of Ti, 0.05 to 0.50% inclusive of Cr, 0.0005% to 0.0050% inclusive of Ca and a remainder made up by Fe and unavoidable impurities, and has such a structure that bainitic ferrite is contained as the main phase and martensite and retained austenite are contained as the second phase, wherein the fraction of the main phase is 90% by volume or more, the average crystal grain diameter of the main phase is 10 μm or less, the fraction of martensite is 0.5 to 9.5% by volume inclusive and the fraction of retained austenite is 0.5 to 9.5% by volume inclusive in the structure. |
