Nkk Corporation

Other

Hiroyasu Kikuchi

Tadashi Inoue

Sadanori Imada

Yoichi Motoyashiki

Toru Inazumi

Yoshimasa Funakawa

Hiroshi Nakata

Yasuhide Ishiguro

Akira Hiura

Takayuki Odake

Takeshi Nakahara

Satoshi Ishijima

A method for manufacturing a steel sheet comprising continuously casting a steel containing 0.04 to 0.2 wt. % C, 0.25 to 2 wt. % Si, 0.5 to 2.5 wt. % Mn, and 0.1 wt. % or less Al to form a slab; hot-rolling by rough-rolling the slab to form a sheet bar and finish-rolling the sheet bar with a reduction in thickness at the final stand of less than 30%, the finish-rolling being completed at a temperature from the Ar transformation point to the Ar transformation point &plus;60&deg; C.; primary-cooling the hot-rolled steel sheet, the primary cooling being started within 1 second after the completion of hot-rolling and conducting the cooling at a cooling speed of higher than 200&deg; C./sec down to a temperature of Ar &minus;30&deg; C. to the Ar transformation point; slow cooling or air-cooling the primary-cooled steel sheet at a temperature of the Ar transformation point to the Ar transformation point at 10&deg; C./sec or less for 2 seconds or more; secondary-cooling the steel sheet after the slow cooling or the air-cooling at 30&deg; C./sec or more; and coiling the secondary-cooled steel sheet at 300&deg; C. or less.

Method for manufacturing a steel sheet

The present invention relates to a high strength steel sheet consisting essentially of 0.04 to 0.1% C, 0.5% or less Si, 0.5 to 2% Mn, 0.05% or less P, 0.005% or less 0, 0.005% or less S, by weight, having 10 &mgr;m or less of average ferritic grain size, and 20 mm/mm or less of generation frequency A, which generation frequency A is defined as the total length of a banded secondary phase structure observed per 1 mm of steel sheet cross section along the rolling direction thereof. The steel sheet is manufactured by, for example, a method comprising the steps of: hot-rolling a continuously cast slab having the composition described above at temperatures of Ar transformation point or above directly or after reheating thereof; and cooling the hot-rolled steel sheet within 2 seconds down to the temperatures of from 600 to 750&deg; C. at cooling speeds of from 100 to 2,000&deg; C./sec, followed by coiling the cooled steel sheet at temperatures of from 450 to 650&deg; C. The present invention provides a high strength steel sheet having strengths of 340 MPa or more and having excellent stretch flanging performance, ductility, and shock resistance, providing a sufficient coil shape with good surface properties, even when hot dip zinc-coating is applied.

High strength steel sheet and method for manufacturing the same

The method for manufacturing steel sheet comprises the steps of: rough-rolling to form a sheet bar; finish-rolling the sheet bar to form a steel strip; applying primary cooling and secondary cooling to the finish-rolled steel strip; and coiling the secondary-cooled steel strip. The primary cooling is conducted at cooling speeds of 120&deg; C./sec or more down to the temperatures of from 500 to 800&deg; C. The secondary cooling is conducted at cooling speeds of less than 60&deg; C./sec.

Steel sheet and method for manufacturing the same

High tensile strength hot-rolled steel sheet and method for manufacturing the same

The method for manufacturing steel sheet comprises the steps of: forming a sheet bar; forming a steel strip; primary-cooling; air-cooling; secondary-cooling; and coiling. The sheet bar is finish-rolled at finish temperatures of (Ar transformation point &minus;20&deg; C.) or more. The primary cooling cools the finish-rolled steel strip at cooling speeds of more than 120&deg; C./sec down to the temperatures ranging from 500 to 800&deg; C.

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Fukuyama, Japan

Hiroyasu Kikuchi