Nippon Kokan Kabushiki Kaisha

Hisatoshi Tagawa

Makoto Yamada

Itaru Watanabe

Kazuaki Matsumoto

Masataka Suga

Seishi Tsuyama

Toru Izawa

Nobuhiro Iwasaki

Kazuhide Takahashi

Yoshitaka Yamasaki

Kazuyuki Matsui

A steel which is excellent in mechanical properties after a warm working process is produced by specifying a chemical composition as claimed, subjecting the steel to a controlled rolling under conditions at temperatures of not more than 900.degree. C. and accumulated reduction of more than 30%, leaving it, after the controlled rolling, as it is in the air or performing it to an accelerated cooling wherein the steel is cooled at rate between the air cooling and 100.degree. C./sec until temperatures where a transformation finishes, subsequently reheating the steel to ranges between 400.degree. C. and 750.degree. C., and carrying out the warm working thereon instantaneously or after the air cooling at the temperatures between 250.degree. C. and 750.degree. C.

Method for producing high strength steel excellent in properties after

The steel plate having a high tensile strength is manufactured from a steel consisting essentially of 0.04-0.16% by weight of C, 0.02-0.50% by weight of Si, 0.4-1.2% by weight of Mn, 0.2-5.0% by weight of Ni, 0.2-1.5% by weight of Cr, 0.2-1.0% by weight of Mo, 0.01-0.10% by weight of acid soluble Al, 0.03-0.15% by weight of one or more of V, Ti and Nb, 0.015% or less by weight of P, 0.006% or less by weight of S and the balance of iron and inherent impurities. The steel is heated to a temperature above a temperature at which carbo-nitrides of V and Nb and carbides of Ti become complete solid solution state, rolled with total reduction of 40% or more below 950.degree. C., quenched by simultaneous cooling immediately after completion of the rolling from a temperature above (A.sub.3 -50).degree.C. and tempered at a temperature lower than Ac.sub.1 temperature. The density of cooling water (W) for the quenching is detemined by the following equation (I) or (II) in accordance with the plate thickness (t):

Method of manufacturing high tensile strength steel plates

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Kinosho, Japan

Hisatoshi Tagawa