Nippon Steel Corporation

Yuji Fukumoto

Takashi Aramaki

Junichi Yasui

Norimitsu Harada

A steel sheet of the present invention has a steel structure obtained by performing a soaking at a dual phase region temperature of Ac1 temperature or higher and lower than Ac3 temperature for a soaking time of 15 seconds or longer and 35 seconds or shorter, next, performing a primary cooling to a temperature range of 250&#xb0; C. or higher and 380&#xb0; C. or lower within 3 seconds at a cooling rate of 0.5&#xb0; C./s or more and 30&#xb0; C./s or less, and performing a retention in a temperature range of 260&#xb0; C. or higher and 370&#xb0; C. or lower for 180 seconds or longer and 540 seconds or shorter, in which a yield ratio is 65% or less and tensile strength is 590 MPa or more after the primary cooling.

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Tokyo, Japan

Yuji Fukumoto