PURPOSE: To obtain inexpensively a high-strength thin steel sheet having good ductility and spot weldability by coiling a hot rolled steel strip contg. a specific ratio of C, Mn, P, Al and N at a specific temp., subjecting the strip to pickling and cold rolling then heating and holding the same under specific conditions and cooling the heated steel sheet.
CONSTITUTION: A steel contg., by weight, 0.02W0.15% C, 0.8W3.5% Mn, 0.02W 0.15% P, ≤0.10% Al and 0.005W0.025% N and consisting of the balance Fe and unavoidable impurities is melted and is hot rolled. The hot rolled steel strip is coiled at ≤550°C and is cold rolled after pickling. The cold rolled steel strip is heated and held for 10secW10min in a temp. range of the Ac1 transformation pointW950°C and thereafter the strip is cooled in such a way that the average cooling rate between 600W300°C attains the range of the critical cooling rate CR(°C/sec) determined by the equation or above and up to 200°C/sec. The high- tension thin steel sheet having about ≥50kg.f/mm2 tensile strength is thus obtd. at a low cost.
KATOU TOSHIYUKI
NISHIDA MINORU
| JPS5822332A | 1983-02-09 |