High-strength cold-rolled steel sheet and method for manufacturing the same

Abstract

A high-strength cold-rolled steel sheet excellent in terms of elongation (EL) and hole expansion ratio (A) having a low yield ratio (YR), as well as a method for manufacturing the steel sheet. The steel sheet has a chemical composition containing, by mass %, C: 0.15% to 0.25%, Si: 1.0% to 2.0%, Mn: 1.8% to 2.5%, P: 0.10% or less, S: 0.010% or less, Al: 0.10% or less, N: 0.010% or less, and the balance being Fe and inevitable impurities, and a multi-phase microstructure including ferrite having an average crystal grain diameter of 5 μm or less in an amount of 30% to 55% in terms of volume fraction, retained austenite having an average crystal grain diameter of 2 μm or less in an amount of 5% to 15% in terms of volume fraction, and tempered martensite having an average crystal grain diameter of 2 μm or less in an amount of 30% to 60% in terms of volume fraction, in which the number of grains of the retained austenite existing in an area of 1000 μm2 is 10 or more.

Description

CROSS REFERENCE TO RELATED APPLICATIONS[0001]This is the U.S. National Phase application of PCT / JP2015 / 006347, filed Dec. 21, 2015, which claims priority to Japanese Patent Application No. 2015-038506, filed Feb. 27, 2015, the disclosures of these applications being incorporated herein by reference in their entireties for all purposes.TECHNICAL FIELD OF THE INVENTION[0002]The present invention relates to a high-strength cold-rolled steel sheet having a high elongation (EL), a high hole expansion ratio (λ), and a low yield ratio (YR) and a method for manufacturing the steel sheet, and in particular, to a high-strength cold-rolled steel sheet which can be used for structural parts of, for example, an automobile.BACKGROUND OF THE INVENTION[0003]In a situation where an increase in fuel efficiency through the weight reduction of an automobile body is an important issue to be addressed in the automobile field, since there has been progress in reducing the thickness of automobile parts by ...

AI Technical Summary

Benefits of technology

The patent is about a method for manufacturing a high-strength cold-rolled steel sheet with excellent elongation and hole expansion ratio while maintaining low yield ratio. The method involves controlling the crystal grain diameters and volume fractions of steel sheet microstructures, such as ferrite, retained austenite, and tempered martensite. By forming a microstructure composed of bainite and martensite and allowing bainite transformation to occur through cooling, the resulting steel sheet has fine grained structures that facilitate elongation and hole expansion. The method allows for the stable production of a high-strength cold-rolled steel sheet with high elongation, hole expansion ratio, and low yield ratio.

Problems solved by technology

However, since the steel sheet disclosed in Patent Literature 1 has an insufficient elongation (EL) in the case where it has a tensile strength (TS) of 980 MPa or more, it cannot be said that sufficient formability is achieved.

However, since such martensite is hard, voids are formed at its interface, in particular, its interface with soft ferrite when punching work is performed in a hole expansion process, the voids then combine with each other when the punched hole is expanded, and a crack occurs as the combination of the voids progresses.

As described above, it is conventionally difficult to improve the balance among elongation (EL), hole expansion ratio (λ), and yield ratio (YR).