High-strength cold-rolled steel sheet having excellent bendability

Abstract

This high-strength rolled steel sheet has a component composition containing, by mass, 0.15 to 0.30% of C, 1.0 to 3.0% of Si, 0.1 to 5.0% of Mn, 0.1% or less of P (not including 0%), 0.010% or less of S (not including 0%), 0.01% or less of N (not including 0%), and 0.001 to 0.10% of Al, with the remainder being made up by iron and unavoidable impurities, martensite being contained at 95% or above by area ratio, the total area ratio of residual austenite and ferrite being 5% or less (including 0%), the average size of carbides being no greater than 60 nm in terms of equivalent circle diameter, and the number density of carbides having an equivalent circle diameter of at least 25 nm being no greater than 5.0 × 105 per 1 mm2. The high-strength rolled steel sheet has a yield strength of at least 1180 MPa and a tensile strength of at least 1470 MPa.

Description

technical field [0001] The present invention relates to a high-strength cold-rolled steel sheet excellent in workability for use in automobile parts, and more specifically, to a high-strength cold-rolled steel sheet having a yield strength of 1,180 MPa or more and a tensile strength of 1,470 MPa or more, and is particularly excellent in bendability among workability Cold rolled steel. Background technique [0002] In general, for steel sheets used in frame parts of automobiles, high strength is required for the purpose of collision safety, fuel consumption reduction due to weight reduction of the vehicle body, etc., and excellent stamping is required for processing frame parts with complex shapes Formability. Furthermore, conventionally, material design based only on tensile strength (TS) has been carried out, but material design based on yield strength (YP) is required in consideration of crash safety, so it is required to have an excellent tensile strength (TS) A high-st...

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Problems solved by technology

However, since it contains 25% or more of ferrite which is a soft phase, it is estimated that, as shown in this example, the tensile strength of 1470 MPa or more is not satisfied, and the yield strength is of course insufficient at this level of tensile strength. 1180MPa

However, it is difficult for these steel sheets to obtain the bendability that satisfies the above-mentioned requirements in the more stringent evaluation method, that is, the V-shaped block method, and the workability is improved by the precipitation of a large amount of carbides, and the amount of solid solution of C is small, and the soft iron The area ratio of the element body is large, so the yield strength (YP: expressed as YS in this document) is high, but even so, it is only at the level of 1100MPa or less, and it does not meet the level of 1180MPa or more

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