PURPOSE: To manufacture a cold rolled high strength steel sheet excellent in local deformability by specifying hot rolling, continuous annealing after cold rolling, and reheating after water quenching, respectively, at the time of manufacturing a cold rolled steel sheet from a steel having a specific composition.
CONSTITUTION: At the time of hot-rolling a steel having a composition which consists of, by weight, 0.10-0.25% C, ≤0.4% Si, 1.0-2.5% Mn, ≤0.010% S, 0.01-0.05% Al, and the balance iron with inevitable impurities and in which Ac3 defined by an equation I is regulated to ≤850°C and the relations in an inequality II are satisfied, finishing temp., average cooling rate from the finishing temp. to coiling, and coiling temp. are regulated to ≥800°C, 15-60°C/sec, and ≤600°C, respectively, by which the structure of the hot rolled plate is formed into bainite-ferrite structure. Subsequently, after pickling and cold rolling, the resulting cold rolled steel sheet is subjected, in continuous annealing, to heating up to a temp. in an austenite range at ≤850°C and then to slow cooling until a temp. at which polygonal ferrite in 5-15% area ratio is formed is reached. Further, after water quenching, reheating is applied to the above steel sheet at 300-500°C for 30-300sec.
TANAKA FUKUTERU
MIYOSHI TETSUJI
Next Patent: GOLD ALLOY CAPABLE OF BLACKENING AND BLACKENING TREATMENT