Effect of Microstructure on Surface Roughening in Stretch Forming of Steel Sheets

Surface quality is an important requirement in outer automobile panels, and to obtain products with an excellent surface quality the development of surface roughening during press forming should be prevented. However, the influence of microstructures on surface roughening is not yet sufficiently understood, and thus it is difficult to control surface roughening during press forming. In this study, the mutual effect of the development of texture and the surface profile variation is examined in detail using two types of tests: a Marciniak type test used for surface roughening observation, and a microscopic biaxial tensile test in a vacuum chamber of scanning electron microscope. The latter test enables in-situ continuous observations of microstructure evolution under biaxial deformation, which is a typical deformation mode in press forming. Interstitial free (IF) steel sheets are used in this study because these sheets are mainly used for outer panels. Results show that surface roughening becomes large for sheets which have a larger number of crystal grains with orientations in the vicinity of ND(001) due to inhomogeneous deformation derived from their lower deformation resistance. It is therefore suggested that surface quality after press forming may be further improved by reducing the number of grains with crystal orientation of ND(001) in IF steel sheets.