| 摘要 |
PURPOSE:To produce a high-ductility high-strength cold rolled steel sheet of the composite structure having stable base material strength by subjecting a specifically composed steel to hot and cold rolling under specific conditions and further subjecting the steel successively to recrystallization annealing and overaging treatment under specific conditions. CONSTITUTION:The steel consisting, per weight %, 0.05-0.30% C, 1.2-2.5% Si, 1.2-2.5% Mn, having 1.0-1.5 weight ratio of Mn/Si and consisting of the balance iron and inevitable impurities is hot rolled to a hot rolled sheet of the composite structure consisting of ferrite and <=70vol% low temp. transformation product. The hot rolled sheet is then picked and is cold rolled at >=30% draft. The cold rolled sheet is subjected to the recrystallization annealing in the temp. region of 128C+1.0<=gammav%<=128C+55 to control the volumetric rate gammav of the austenite phase in the recrystallization heating stage by the content of C at the heating temp. in the two-phase co-existence region of ferrite and austenite having Ac1-Ac2. The steel sheet is further forcibly air-cooled and is quickly cooled at a cooling rate of >=100 deg.C/sec from the temp. at which the content of C in the austenite is made >=0.5%. Such steel is subjected to overaging treatment at 200-400 deg.C.
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