High-strength cold-rolled steel sheet having excellent stretch flange properties, and process for production thereof

Abstract

The present invention provides: a high-strength cold-rolled steel sheet which has excellent stretch flange properties, and which has a chemical composition comprising, in mass%, 0.050-0.090% of C:, 0.05% or less of Si, 1.5-2.0% of Mn, 0.030% or less of P, 0.0050% or less of S, 0.005-0.1% of Al, 0.01% or less of N, 0.005-0.050% of Ti, 0.020-0.080% of Nb and a remainder made up by Fe and unavoidable impurities, has a tissue composed of, in vol%, 50-77% of a ferrite phase, 20-50% of a bainite phase, 2-10% of a martensite phase and 1-5% of a residual austenite phase, and has such high strength that the tensile strength (TS) is 590 MPa or more, a strength-elongation balance (TSEl) of 16000 MPa% or more and a strength-(hole expanding ratio) balance (TS[lambda]) of 40000 MPa% or more; and a process for producing the high-strength cold-rolled steel sheet.

Description

technical field [0001] The present invention relates to a high-strength cold-rolled steel sheet suitable for use in press-forming automobile parts and the like into complex shapes, and particularly to improvement of stretch-flangeability. The "high-strength steel sheet" as used herein refers to a high-strength steel sheet having a tensile strength of 590 MPa or higher. In addition, the "steel plate" mentioned here includes a steel plate and a steel strip. Background technique [0002] In recent years, from the viewpoint of protecting the global environment, the fuel efficiency of automobiles has been required to be improved, and the weight of automobile bodies has been reduced. In addition, from the viewpoint of securing the safety of crew members, it is required to improve the collision safety of automobiles. In view of such requirements, the application of high-strength steel sheets in automobile bodies is expanding. [0003] However, as the strength of the steel sheet ...

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Problems solved by technology

[0024] However, in the technology described in Patent Document 4, it is required to use a steel slab whose composition is different from the surface layer and the interior other than the surface layer. In order to manufacture such a steel slab, it is necessary to use a special cladding technique, etc., and there is a problem that the production cost increases.

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