| 摘要 |
PROBLEM TO BE SOLVED: To provide a hot rolled steel sheet having high strength and a low yield ratio, and suitable as a raw material for an API standard X80 class steel tube, capable of maintaining a low yield ratio regardless of an amount of residual strain by working even after worked and excellent in post-working characteristic stability, and to provided a production method of the hot rolled steel sheet.SOLUTION: The hot rolled steel sheet having high strength and a low yield ratio and excellent in post-working characteristic stability is provided which has a composition containing, by mass%, C:0.030% to 0.120%, Si:0.05% to 0.50%, Mn:1.00% to 2.20%, P:0.025% or less, S:0.0050% or less, N:0.0060% or less, Al:0.005% to 0.100%, Nb:0.020% to 0.100%, Mo:0.05% to 0.50%, Ti:0.001% to 0.100%, Cr:0.05% to 0.50% and Ca:0.0005% to 0.0050% and the balance Fe with inevitable impurities. The hot rolled steel sheet has structure in which a bainitic ferrite is included as a main phase, martensite and retained austenite are included as a second phase and a volume fraction of the main phases is 90% or more, an average crystal grain diameter of the main phases is 10 μm or less, the volume fraction of the martensite is 0.5% to 9.5% and the volume fraction of the retained austenite is 0.5% to 9.5%. |
