Jfe Steel Corporation

Daigo Itoh

Takeshi Yokota

Reiko Sugihara

Hidetaka Kawabe

Kazunori Tahara

A cold-rolled steel sheet having a steel composition comprising, by mass %, 0.12% to 0.22% C, 0.8% to 1.8% Si, 1.8% to 2.8% Mn, 0.020% or less P, 0.0040% or less S, 0.005% to 0.08% Al, 0.008% or less N, 0.001% to 0.040% Ti, 0.0001% to 0.0020% B, 0.0001% to 0.0020% Ca, and Fe and incidental impurities. The steel sheet includes a microstructure in which ferrite and bainite phases are 50% to 70% of the total area, the average grain size of the ferrite and bainite phase is 1 to 3 &#x3bc;m, a tempered martensite phase is 25% to 45% of the total area, the average grain size of the tempered martensite phase is 1 to 3 &#x3bc;m, and a retained austenite phase is 2% to 10% of the total area.

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Chiba, Japan

Daigo Itoh