Improvement of surface roughening resistance of ultra-low-carbon steel sheet by reducing {001} oriented crystal grains

Abstract

Because of the increasing demand for automobile outer panels with sharper streamlines, it is important to improve the surface qualities of outer panel products. In the present study, the effects of microstructural properties - particularly the deformation-mode dependence of heterogeneous deformation at the grain scale and the fraction of {001} oriented grains - on surface roughening during deformation were investigated experimentally. The results showed that surface roughening decreased as the average grain size and the fraction of {001} oriented grains decreased. These trends were more pronounced under equibiaxial tension than under plane-strain tension. Surface roughening decreased with the fraction of {001} oriented grains, presumably because the heterogeneity of deformation resistance decreased at the grain scale. Moreover, if the texture and average grain size were similar, the development of surface roughening in the low-strain range (equivalent plastic strain of less than 0.3) was independent of the work-hardening coefficient n. On the basis of the abovementioned results, the optimum annealing conditions were examined to realize crystal grain refinement and reduction of {001} orientation. As a result, we obtained a high-strength steel with excellent surface roughening resistance.