HIGH STRENGTH HOT ROLLED STEEL SHEET HAVING EXCELLENT LOW CYCLE FATIGUE PROPERTY AND CORROSION RESISTANCE AFTER COATING, AND METHOD FOR PRODUCING THE SAME

摘要

PROBLEM TO BE SOLVED: To provide a high strength hot rolled steel sheet which is excellent in both of low cycle fatigue properties and corrosion resistance after coating.;SOLUTION: The high strength hot rolled steel sheet having excellent low cycle fatigue properties and corrosion resistance after coating has a composition containing, by mass, 0.06 to 0.12% C, 0.6 to 1.25% Si, 1.60 to 2.20% Mn and 0.20 to 0.65% Al so as to satisfy [Si]≥[Mn]/4+[Al]/1.2, and in which the contents of P, S and N are limited, and the balance Fe with inevitable impurities, and has a metallic structure composed of the two phases of a ferrite phase and a martensite phase, or the three phases of a ferrite phase, a martensite and a bainite phase, and in which the area ratio of the ferrite phase is 60%, the area ratio of the martensite phase is 10% and the area ratio of the bainite phase is 5%. Further, in the method for producing a high strength hot rolled steel sheet, a slab is reheated to ≤1,300°C, is hot-rolled, is subsequently subjected to cooling of three stages, is cooled to ≤250°C and is coiled.;COPYRIGHT: (C)2010,JPO&INPIT