| 摘要 |
Provided are a hot-rolled steel sheet which can be preferably used as a material for an X80 grade steel pipe specified in the API standards having a high strength, a low-yield-ratio, and excellent stability of properties after pipe forming with which it is possible to maintain a low-yield ratio independently of the amount of forming strain applied during pipe forming and a method for manufacturing the steel sheet. A high-strength hot-rolled steel sheet having a low yield ratio excellent in terms of stability of properties after processing has been performed is obtained, by providing 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 by forming a microstructure including bainitic ferrite as a main phase and martensite and retained austenite as second phases, in which the volume fraction of the main phase is 90% or more, in which the average grain diameter of the main phase is 10 µm or less, in which the volume fraction of the martensite is 0.5% or more and 9.5% or less, and in which the volume fraction of the retained austenite is 0.5% or more and 9.5% or less. |
