Method for producing a high strength hot-rolled steel plate exhibiting excellent acid pickling property, chemical conversion processability, fatigue property, stretch flangeability, and resistance to surface deterioration during molding, and having isotropic strength and ductility
View Patent ↗This high strength hot-rolled steel sheet includes: in terms of percent by mass, C: 0.05 to 0.12%; Si: 0.8 to 1.2%; Mn: 1.6 to 2.2%; Al: 0.30 to 0.6%; P: 0.05% or less; S: 0.005% or less; and N: 0.01% or less, with the remainder being Fe and unavoidable impurities, wherein a microstructure includes specific ranges (in area %) of ferrite phases as well as martensite phases, and a maximum concentration of Al detected by a glow discharge emission spectroscopic analysis is in a range of 0.75 mass % or less in a region from a surface of the steel sheet to a thickness of 500 nm after being acid-pickled.
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
T 1=1215+35×[Si]−70×[Al],
T 2=1070+35×[Si]−70×[Al], and
[Si] and [Al] represent a Si content (mass %) in the slab, and an Al content (mass %) in the slab, respectively.
2. The method for producing of 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 according to claim 1 ,
wherein in the process of subjecting the slab to the rough rolling, the heating temperature of the slab is set to be in a range of less than 1200° C., and the final temperature of the rough rolling is set to be in a range of 960° C. or less, and
in the process of subjecting the rough rolled material to the finish rolling, the finish temperature is set to be in a range of 700 to 900° C.