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

Abstract

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.

Influence of Strain Ratio on Surface Roughening in Biaxial Stretching of IF Steel Sheets

“…The equivalent plastic strain was calculated by von Mises yield criterion, though the materials had some anisotropy ( Table 1). As in our previous study, 20) the development of surface roughening was observed after stopping the test at certain punch strokes, until necking occurred. After stopping the test at intervals, 25 mm square specimens were cut from the center part, where the most significant surface roughening could be observed.…”

“…13 depending on strain ratio is derived from the difference in deformation resistance between the neighboring grains. This result implies that the inhomogeneous deformations of polycrystalline grains are effected not only by the initial crystal orientations 20) but also by their deformation mode. Figure 15 shows a comparison between the experimental and calculated changes of the parameter P a during biaxial stretch forming.…”

“…To maintain surface unevenness as much as possible the polishing of specimens for EBSD was limited to a range of 2 μm depth. 20,29)…”

“…Studies to overcome surface roughening in press forming have been conducted on steel sheets and aluminum alloy for a long time. [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] In our previous paper, 20) the authors surveyed the history of these researches. The research not only dealt with the relationship between surface roughening and forming conditions, [6][7][8][9]14) such as amount of strain and strain path, but also with material factors, 4,5,[10][11][12][13][14][15][16][17][18] such as crystal grain size, crystal structure, crystal orientation and texture.…”

“…In our previous study, 20) however, only equi-biaxial tensile deformation was examined. There are various deformation modes in biaxial stretching, and as pointed out by Osakada et al, 9) the behavior of surface roughening may also be affected by the strain ratio.…”