| 摘要 |
According to the present invention, a predetermined steel component is included; a steel plate composition is made to have 5-70% of a martensite, 5-40% of a residual austenite, and 5% or more of a bainitic ferrite in an upper bainite, by area in relation to the entire steel plate composition; the total area of the martensite, the residual austenite, and the bainitic ferrite is made to be 40% or more; 25% or more of the martensite is made to be tempered martensite; the area of a polygonal ferrite in relation to the entire steel plate composition is more than 10% and less than 50%, and the average grain size thereof is made to be 8 mum or less; and when a group of ferrite grains comprising adjacent polygonal ferrite grains is made to be a polygonal ferrite grain group, the average diameter thereof is 15 mum or less, the average amount of C in the residual austenite is made to be 0.70 mass% or more, and the tensile strength is made to be 780 MPa or more; whereby it is possible to obtain a high-strength press member having excellent ductility and stretch flangeability, as well as a tensile strength of 780-1400 MPa. |
