Nippon Steel Corporation

Hiroki Miyawaki

Masanao Yamamoto

Takashi Fujita

Atsuhiko Yoshie

Masahiro Doki

Masaaki Fujioka

Katsushige Matsuzaki

Yuji Nomiyama

Sumitada Kakimoto

Harutoshi Ogai

Kiyoshi Tanehashi

Hiroshi Uekaji

Masato Sanazawa

A steel containing predetermined components is rolled in a recrystallization temperature region or a non-recrystallization temperature region of an austenite and is subsequently subjected to repeated hot bending. Alternatively, a surface layer portion is cooled during rolling of the steel described above to an .alpha. single phase or a .gamma./.alpha. dual phase temperature region, rolling is then effected and is finished at the point of time when the surface temperature of the plate rises above an Ac.sub.3 point due to recuperative heat, and repeated bending is carried out. Still alternatively, the steel described above is rolled to a cumulative reduction ratio of at least 20% in the non-recrystallization temperature region and is then subjected to repeated bending. Further alternatively, the surface layer portion is cooled during hot rolling of the steel described above to an .alpha. single or .gamma./.alpha. dual phase temperature region, rolling is then continued at a cumulative reduction ratio of at least 20% and is finished at the point when the surface temperature of the steel plate rises less than (Ac.sub.3 point--200.degree. C.) due to recuperative heat, and subsequently, repeated bending is carried out.

Production method of strong and tough thick steel plate

A method of water cooling a hot-rolled steel plate is disclosed. In this method, while a hot-rolled steel plate is being advanced lengthwise, a plurality of nozzles spray cooling water to the steel plate, and thus the plate is cooled to a predetermined temperature at a predetermined cooling rate. Before, while and after the plurality of nozzles carry out water-cooling, temperatures are consistently measured at predetermined temperature measurement points in cross-sectional areas of temperature measurement positions which are arranged lengthwise at predetermined locations of the steel plate. Each time such measurement is performed, degree of deformation of the plate is calculated on the basis of temperature differences between the temperature measurement points. When the calculated degree of deformation is not within an allowable range, corrections are made on the distribution of the water supplied by the plurality of nozzles.

Method of cooling hot-rolled steel plate

Hot steel plate as hot-rolled is passed forward longitudinally through pairs of top and bottom rollers disposed in the direction of the plate travel. The steel plate is cooled with water supplied to its top and bottom surfaces from the nozzles on a plurality of cooling units disposed in the same longitudinal direction. Each cooling unit is disposed between adjoining pairs of the top and bottom rollers. Before cooling is started, the temperature distribution in the steel plate is determined and the desired mean cooling rate is set. The distance from the plate edge over which the supply of the cooling water at least to the bottom side of the plate is to be cut off is determined for each cooling unit on the basis of the determined temperature distribution and preset mean cooling rate. By so doing, the temperature of the inner edge portion of the steel plate is kept above the temperature of the middle portion, thereby allowing the Ar.sub.3 transformation in the inner edge portion to occur simultaneously with or after the Ar.sub.3 transformation in the middle portion. The cooling water directly supplied to the edge portion of the steel plate is cut off over the distance thus determined.

Method of cooling hot steel plates

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Kitakyushu, Japan

Hiroki Miyawaki