Ultra-high strength steel plate having excellent formability and hole-expandability, and method for manufacturing same

Abstract

The present invention relates to an ultra-high strength steel plate for an automobile and, more specifically, to an ultra-high strength steel plate having excellent formability and hole-expandability,and a method for manufacturing the same.

Description

technical field [0001] The present invention relates to an ultra-high-strength steel sheet used in automobiles, and more particularly, to an ultra-high-strength steel sheet excellent in formability and hole expandability, and a method for manufacturing the same. Background technique [0002] In order to reduce the weight of steel sheets for automobiles, the thickness of the steel sheets should be reduced. However, in order to ensure crash stability, the thickness of the steel sheets should be increased or the strength should be increased. Therefore, there are conflicting issues. [0003] In order to solve the above-mentioned problems, formability should be improved while increasing the strength of the base material. It is well known that a suitable steel plate includes dual-phase steel (Dual Phase Steel, DP steel) called Advanced High Strength Steel (AHSS). ), transformation induced plasticity steel (Transformation Induced Plasticity, TRIP steel), multi-phase steel (Complex ...

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Problems solved by technology

[0013] In addition, in Patent Document 3, a method of increasing the temperature of Ac1 by adding Al to steel containing 3 to 7% of Mn to enable continuous annealing is proposed, although this method has a product of tensile strength and elongation High advantage, but the hole expandability is not improved, so the press formability cannot be clearly ensured

[0014] Furthermore, in Patent Document 4 and Patent Document 5, there is provided a method of producing a high-strength steel sheet having a tensile strength of 980 MPa or more and a product of tensile strength and elongation of 24000 MPa% or more using steel containing 3.5 to 10% of Mn. method, however, when heat-treating coils coiled after hot rolling, since the coiling is performed below the Ac1 transformation point, the increase of austenite and the annealed martensite due to the preferential distribution (partitioning) of Mn are suppressed. Formation, so that the cold deformability cannot be ensured effectively, and, as final annealing and intermediate annealing etc. are performed only in the two-phase domain, in the final structure, the hardness difference between ferrite and other phases (phase) may be very large large, which is likely to result in poorer yield strength and hole expandability of the final product

Not only that, the literature does not try to improve the yield strength and hole expandability, but simply evaluates the bendability. Although it is applicable to the molding of simple parts, it cannot give a suitable solution for complex stamping

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