High-strength cold-rolled steel plate and method for producing same

Abstract

Provided are: a high-strength cold-rolled steel plate having excellent elongation (EL), an excellent hole expansion ratio (lambda), and a low yield ratio (YR); and a method for producing same. The high-strength cold-rolled steel plate has a component composition comprising, in mass%, 0.15-0.25% of C, 1.0-2.0% of Si, 1.8-2.5% of Mn, 0.10% or less of P, 0.010% or less of S, 0.10% or less of Al, 0.010% or less of N, and a remainder of Fe and unavoidable impurities. The high-strength cold-rolled steel plate has a composite structure comprising 30-55% by volume fraction of ferrite having an average crystal grain size of 5 [mu]m or less, 5-15% by volume fraction of residual austenite having an average crystal grain size of 2 [mu]m or less, and 30-60% by volume fraction of tempered martensite having an average crystal grain size of 2 [mu]m or less. The high-strength cold-rolled steel plate is configured so that 10 or more pieces of the residual austenite are present in 1000 [mu]m2 thereof.

Description

technical field [0001] The present invention relates to a high-strength cold-rolled steel sheet having a high elongation (EL) and a high hole expansion rate (λ) and a low yield ratio (YR) and a method for producing the same, and in particular to a member suitable as a structural part of an automobile or the like High-strength cold-rolled steel sheet. Background technique [0002] In the automotive field, improvement in fuel consumption due to weight reduction of the vehicle body has become an important issue. In this process, the application of high-strength steel sheets to automotive parts is being promoted to reduce wall thickness, and the application of tensile strength is being promoted. (TS) is a steel plate of 980 MPa or more. For high-strength steel sheets used in structural and reinforcing members of automobiles, when forming parts with complex shapes, not only elongation (EL) and tensile flangeability (hereinafter also referred to as It is required to be excellent...

AI Technical Summary

Problems solved by technology

However, although the DP steel sheet has a high elongation (EL), it has the following disadvantages: since the stress concentrates on the interface between the ferrite and the martensite, it is easy to generate cracks, so the bendability and hole expandability are poor.

However, this TRIP steel sheet has the following disadvantages: since retained austenite transforms into martensite during punching, cracks occur at the interface with ferrite, resulting in poor hole expandability.

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