Patent ID: 9523134
Filing Date: 2016-12-20
CPC Classification: C21D,C22C,C23C

Claim Text:
1. A method for producing a high strength hot-rolled steel sheet that is superior in an acid pickling property, a chemical conversion processability, a fatigue property, a hole expandability, and a resistance to surface deterioration during forming, and that has isotropic strength and isotropic ductility, the method comprising: a process of heating a slab at a heating temperature in a range of T1 or less and subjecting the slab to rough rolling under conditions in which a rolling reduction ratio is in a range of 80% or more and a final temperature is in a range of T2 or less to produce a rough rolled material; a process of subjecting the rough rolled material to descaling and subsequent finish rolling under a condition in which a finish temperature is set to be in a range of 700 to 950° C. to produce a rolled sheet; a process of cooling the rolled sheet to a temperature in a range of 550 to 750° C. at an average cooling rate of 5 to 90° C./s, further cooling the rolled sheet to a temperature in a range of 450 to 700° C. at an average cooling rate of 15° C./s or less, and further cooling the rolled sheet to a temperature in a range of 250° C. or less at an average cooling rate of 30° C./s or more to produce a hot-rolled steel sheet; and a process of coiling the hot-rolled steel sheet, wherein [Si] and [Al] represent a Si content (mass %) in the slab, and an Al content (mass %) in the slab, respectively.