High Strength Cold Rolled Steel Sheet Having Excellent Shape Freezability, and Method for Manufacturing the Same

Abstract

A high-strength isotropic cold-rolled steel sheet having excellent shape-fixability, and a method for manufacturing the same are disclosed. The steel sheet is made using a low-carbon steel comprising a little amounts of Ti to have an r of 1.3 or less, r approaching 1, and a low in-plane anisotropy index (Dr) of 0.15 or less, ensuring excellent shape-fixability so as to be suitable for automotive outer panels which are essentially subjected to deformation in a stretching mode.

Description

TECHNICAL FIELD [0001] The present invention relates to a high-strength cold-rolled steel sheet suitable for automotive outer panels. More particularly, the present invention relates to a high-strength cold-rolled steel sheet, which has an r90 of 1.3 or less, an average plastic strain ratio rm near 1, a low in-plane anisotropy index □r of 0.15 or less, thereby providing excellent shape-fixability so as to allow isotropic plastic deformation of the steel sheet during press forming, and a method for manufacturing the same. BACKGROUND ART [0002] As for steel plates for automobile bodies, cold-rolled steel sheets of excellent formability have been required to perform press forming without work defects and to efficiently manufacture automotive components of a desired shape after the press forming. In particular, for automotive outer panels, it is necessary to have dent resistance and shape-fixability. [0003] In view of dent resistance, bake hardened steel sheets have been generally appli...

AI Technical Summary

Benefits of technology

[0036] As apparent from the above description, the present invention can provide a high-strength isotropic cold-rolled steel sheet, which has an r90 of 1.3 or less, an rm approaching 1, and a low □r of 0.15 or less, ensuring excellent shape-fixability so as to be suitable for steel for automotive outer panels which are essentially subjected to deformation in a stretching mode, and the method for manufacturing the same. With the steel sheet of the invention, it is possible to easily process complicated automotive components in the stretching mode when forming the automotive components.

Problems solved by technology

Thus, high elongation of a steel sheet permits large deformation of the steel sheet.

A high plastic strain ratio results in high biaxial yield strength, and makes it difficult to work the outer panels of the complicated shape. FIG. 1 shows theoretical results based on Taylor polycrystal modeling as to influence of plastic strain ratio on a locus of the yield strength of steel which comprises two different major textures.

However, the conventional techniques have a problem in that, since the Ti and / or Nb-added ultra-low carbon steel has relatively high rm and □r, it exhibits severe in-plane anisotropy and high biaxial yield strength for the deformation in the stretching mode, and thus is disadvantageous in terms of shape fixability even though it exhibits excellent formability in a deep drawing mode.

In addition, according to the conventional techniques, since only 0.005% or less of carbon is generally added for enhancing deep drawability, it is difficult to obtain high strength.

However, the conventional techniques have problems in that these techniques are performed using a batch annealing apparatus, and require a long period of time for the process, thereby lowering productivity per unit time.

In addition, since the conventional steel sheet has □r in the range of 0.15˜0.28, it is undesirable in view of isotropy.

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