Nippon Steel Corporation

Junichi Wakita

Osamu Kawano

Manabu Takahashi

Kazuyoshi Esaka

Riki Okamoto

Akihiro Uenishi

Yukihisa Kuriyama

Yasuharu Sakuma

Yuzo Takahashi

Hidesato Mabuchi

Hiroshi Abe

Norio Ikenaga

The object of the present invention is to provide high-strength steel sheets exhibiting high impact energy absorption properties, as steel sheets to be used for shaping and working into such parts to front side members which absorb impact energy upon collision, as well as a method for their production. The high-strength steel sheets of the invention which exhibit high impact energy absorption properties are press formable high-strength steel sheets with high flow stress during dynamic deformation characterized in that the microstructure of the steel sheets in their final form is a composite microstructure of a mixture of ferrite and/or bainite, either of which is the dominant phase, and a third phase including retained austenite at a volume fraction between 3% and 50%, wherein the difference between the static tensile strength as when deformed in a strain rate range of 5&times;10 &tilde;5&times;10 (l/s) after pre-deformation at an equivalent strain of greater than 0% and less than or equal to 10%, and the dynamic tensile strength &sgr;d when deformed at a strain rate of 5&times;10 &tilde;5&times;10 (l/sec) after the pre-deformation, i.e. &sgr;d&minus;&sgr;s, is at least 60 MPa, and the work hardening coefficient between 5% and 10% of a strain is at least 0.130.

High-strength steel sheet highly resistant to dynamic deformation and excellent in workability and process for the production thereof

The object of the present invention is to provide high-strength steel sheets exhibiting high impact energy absorption properties, as steel materials, to be used for shaping and working into such parts as front side members of automobiles which absorb impact energy upon collision, as well as a method for their production. The high-strength steel sheets of the invention which exhibit high impact energy absorption properties are high-strength steel sheets with high flow stress during dynamic deformation characterized in that the microstructure of the steel sheets in their final form is a composite microstructure of a mixture of ferrite and/or bainite, either of which is the dominant phase, and a third phase including retained austenite at a volume fraction between 3% and 50%, wherein the average value &sgr;dyn (MPa) of the flow stress in the range of 3&tilde;10% of equivalent strain when deformed in a strain rate range of 5&times;10,&tilde;5&times;10,(1/sec) after pre-deformation of greater than 0% and less than or equal to 10% of equivalent strain, satisfies the inequality: &sgr;dyn&gE;0.766&times;TS&plus;250 as expressed in terms of the maximum stress TS (MPa) in the static tensile test as measured in a strain rate range of 5&times;10,&tilde;5&times;10,(1/s) without deformation, and the work hardening coefficient between 1% and 5% of a strain is at least 0.080.

High-strength steel plate having high dynamic deformation resistance and method of manufacturing the same

High yield ratio-type, hot rolled high strength steel sheet excellent in

A hot rolled steel sheet with a high strength and a distinguished formability, and a process for producing the same are disclosed. The steel sheet comprises 0.15 to 0.4% by weight of C, 0.5 to 2.0% by weight of Si, and 0.5 to 2.0% by weight of Mn, the balance being iron and inevitable impurities, and has a microstructure composed of ferrite, bainite and retained austenite phases with the ferrite phase being a ratio (V.sub.PF /d.sub.PF) of polygonal ferrite volume fraction V.sub.PF (%) to polygonal ferrite average grain size d.sub.PF (.mu.m) of 7 or more and the retained austenite phase being contained in an amount of 5% by volume or more on the basis of the total phases. The steel sheet can be produced with a high productivity and without requiring special alloy elements.

Process for producing a hot rolled steel sheet with high strength and

A hot rolled steel sheet with a high strength and a distinguished formability, and a process for producing the same are disclosed. The steel sheet comprises 0.15 to 0.4% by weight of C, 0.5 to 2.0% by weight of Si, and 0.5 to 2.0% by weight of Mn, the balance being iron and inevitable impurities, and has a microstructure composed of ferrite, bainite and retained austenite phases with the ferrite phase being in a ratio (V.sub.PF /d.sub.PF) of polygonal ferrite volume fraction V.sub.PF (%) to polygonal ferrite average grain size d.sub.PF (.mu.m) of 7 or more and the retained austenite phase being contained in an amount of 5% by volume or more on the basis of the total phases. The steel sheet can be produced with a high productiviity and without requiring special alloy elements.

Hot rolled steel sheet with high strength and distinguished formability

Growing community of inventors

Oita, Japan

Junichi Wakita