Nkk Corporation

Jfe Steel Corporation

Hideyuki Tsurumaru

Kohei Hasegawa

Toshiaki Urabe

Hideyuki Takahashi

Takeshi Fujita

Nobuyuki Nakamura

Hiroki Nakamaru

Junichiro Hirasawa

Naoto Yoshimi

Masato Sasaki

Kenichi Mitsuzuka

Katsutoshi Ito

Keita Yonetsu

Akihide Yoshitake

A method for the manufacturing of high strength cold rolled steel sheets includes continuously annealing a cold rolled steel sheet that has a composition containing C: 0.05-0.3% mass, Si: 0.6-3.0% mass, Mn: 1.0-3.0% mass, P: &#x2266;0.1% mass, S: &#x2266;0.02% mass, Al: 0.01-1% mass, N: &#x2266;0.01% mass, and Fe and inevitable impurities: balance, in a manner such that the cold rolled steel sheet is heated in a furnace using an oxidizing burner to a steel sheet temperature of &#x2267;700&#xb0; C., then soak-annealed in a reducing atmosphere furnace at 750-900&#xb0; C., then cooled so the average cooling rate between 500&#xb0; C. and 100&#xb0; C. is &#x2267;50&#xb0; C./s. High-Si cold rolled steel sheets with high strength and good phosphatability while containing Si&#x2267;0.6% are obtained without controlling conditions so as to increase the dew point in the reducing atmosphere in the soaking furnace or to increase the vapor hydrogen partial pressure ratio.

Method for manufacturing of high strength cold rolled steel sheet of excellent phosphatability

The present invention relates to a steel sheet that consists essentially of 0.10 to 0.37% C, 1% or less Si, 2.5% or less Mn, 0.1% or less P, 0.03% or less S, 0.01 to 0.1% sol.Al, 0.0005 to 0.0050% N, 0.0003 to 0.0050% B, by mass, and balance of Fe, &lsqb;14B/(10.8N)&rsqb; being 0.5 or more, average particle size of precipitate BN being 0.1 &mgr;m or more, and grain size of prior austenite after the hardening treatment being 2 to 25 &mgr;m. The steel sheet according to the present invention allows an inexpensive hardening treatment method to be applied to improve the strength to a level required for the structural components and the functional components of automobiles, while providing excellent toughness after the hardening treatment.

Steel sheet and method for manufacturing the same

An ultra-high strength cold rolled steel sheet having a tensile strength of 880 to 1170 MPa, that consists essentially of 0.01 to 0.07% C, 0.3% or less Si, 0.1% or less P, 0.01% or less S, 0.01 to 0.1% sol.Al, 0.0050% or less N, 1.6 to 2.5% of the sum of at least one element selected from the group consisting of Mn, Cr, and Mo, by mass, and a balance of Fe, and that has an inner zone deeper than 10 &mgr;m from the surface of the steel sheet being substantially a martensitic single phase structure. Since the steel sheet has a hole expansion ratio of 75% or more, specified by the Standard of Japan Iron and Steel Federation, JFST1001-1996, the steel sheet has a tensile strength in a range of from 880 to 1170 MPa, and an excellent mechanically joining property, and is suitable for fabricating automobile seat frames.

Ultra-high strength cold rolled steel sheet and method for manufacturing the same

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Fukuyama, Japan

Hideyuki Tsurumaru