A cold-rolled steel sheet having a tensile strength of 780 MPa or more an excellent local formability and a suppressed increase in weld hardness

Abstract

The present invention provides a high-strength cold-rolled steel sheet and a high-strength surface treated steel sheet 780 MPa or more in tensile strength, said steel sheets having excellent local formability and suppressed weld hardness increase and being characterized by: said steel sheets containing, in weight, C: 0.05 to 0.09%, Si: 0.4 to 1.3%, Mn: 2.5 to 3.2%, P: 0.001 to 0.05%, N: 0.0005 to 0.006%, Al: 0.005 to 0.1%, Ti: 0.001 to 0.045%, and S in the range stipulated by the following expression (A), with the balance consisting of Fe and unavoidable impurities; the microstructures of said steel sheets being composed of bainite of 7% or more in terms of area percentage and the balance consisting of one or more of ferrite, martensite, tempered martensite and retained austenite; and said components in said steel sheets satisfying the following expressions (C) and (D) when Mneq. is defined by the following expression (B); S≦̸0.08×(Ti(%)−3.43×N(%)+0.004 . . . (A), where, when a value of the member Ti(%)−3.43×N(%) of said expression (A) is negative, the value is regarded as zero. Mneq.=Mn(%)−0.29×Si(%)+6.24×C(%) . . . (B), 950≦̸(Mneq. / (C(%)−(Si(%) / 75)))×bainite area percentage (%) . . . (C), C(%)+(Si(%) / 20)+(Mn(%) / 18)50.30 . . . (D).

Description

technical field [0001] The present invention relates to a high-strength cold-rolled steel sheet having a tensile strength of 780 MPa or more and a high-strength surface-treated steel sheet having excellent local formability and suppressed increase in weld hardness. Background technique [0002] To date, thin steel sheets with a tensile strength standard of 590MPa or less have generally been used for most components, including car or motorcycle bodies. [0003] In recent years, research has been conducted on maximizing the strength of materials, and attempts have been made to apply further reinforced high-strength thin steel sheets in order to reduce the weight of the vehicle body, thereby improving fuel efficiency and improving crash safety. [0004] Most of the high-strength steels produced for the above purposes are used in body frame members and reinforcements, seat frame parts, etc. of automobiles or motorcycles, and there is an urgent need for base steels with a tensile...

AI Technical Summary

Problems solved by technology

However, as the C content increases, the local formability may deteriorate, and at the same time, the hardness of the weld increases significantly

In any case, with regard to the above-mentioned problems of high-strength thin steel sheets with a base steel tensile strength of 780 MPa or more, no proposal emphasizing local formability improvement and weld hardening suppression has been found

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