Nippon Steel Corporation

Ikushi Yoshida

Munetsugu Matsuo

Mitsunobu Abe

Hiroshi Hayakawa

A process for producing a cold rolled steel strip having excellent workability by a short-time continuous annealing, which process is constituted by hot rolling a low carbon steel slab of steel having a carbon content of from about 0.003 to 0.08% into a steel strip; cold rolling the hot rolled steel strip; rapidly heating the cold rolled steel strip to a recrystallization temperature range at a heating rate not less than 40.degree. C./second; slowly heating the thus rapidly heated steel strip to an annealing temperature at a heating rate ranging from 5.degree. to 30.degree. C./second; annealing the thus slowly heated steel strip at an annealing temperature T which is no lower than 700.degree. C. and no higher than the A.sub.3 transformation temperature and for an annealing time t no shorter than [8-0.03 (T-680)] seconds and no longer than [80-0.15 (T-680)] seconds, initially slowly cooling the thus annealed steel strip at a cooling rate less than 50.degree. C./second, and then rapidly cooling the strip at a cooling rate not less than 50.degree. C./second from a temperature T.sub.Q ranging from higher than 600.degree. C. to no higher than [T-0.027 (T-680)(.sqroot.t+23.7)].degree. C. to an over-ageing temperature range; and subjecting the thus cooled steel strip to an over-ageing treatment at a temperature ranging from 300.degree. to 500.degree. C. for a period of time ranging from 10 seconds to 5 minutes.

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Yokohama, Japan

Ikushi Yoshida