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

Abstract

Due to an increasing demand for automobile outer panels with sharper streamlines, surface roughening during press forming is recognized as an important problem to be solved. However, although sheets are subjected to various deformation modes during press forming, the influence of deformation mode on surface roughening is not yet understood. Moreover, surface roughening behavior in Interstitial Free (IF) steels, which are now commonly utilized for outer panels, has not been studied. In this study the effect of deformation mode on surface roughening behavior in IF steels was examined, focusing in detail on the effect of texture development. Differences in surface roughness development and changes of microstructure were examined under equi-biaxial and plane-strain tension, using a macroscopic Marciniak test and microscopic in-situ observations. In addition, the influence of the distribution of crystal orientations on surface roughness development was numerically examined using a crystal plasticity finite-element analysis. The results showed that surface roughening was larger for plane-strain tension than for equi-biaxial tension regardless of the IF steels tested, due to the larger difference in deformation resistance among crystal grains depending on crystal orientation. It is therefore suggested that surface quality after press forming could be improved by reducing the difference in deformation resistance among the grains.

Effect of strain mode and blank orientation on waviness for paint appearance of automotive outer panels

“…Some research work indicated that the surface roughening of IF steels depends upon the strain level as well as the strain ratio. Increased strain causes an increase in roughness, with strain paths close to plane strain exhibiting the highest rate of roughening [12,13]. Most outer panels, such as the fender and hood, are pressed at a typical strain of approximately 5%.…”

Study on Size Effect of Surface Roughness Based on the 3D Voronoi Model and Establishment of Roughness Prediction Model in Micro-Metal Forming

“…Many studies have reported that under uniaxial tensile deformation, the uneven plastic deformation between grain boundaries leads to surface roughness evolution [13,14]. Kubo et al [14] reported the difference in the crystal orientations because of the stress state on the surface roughness of a low-carbon steel sheet.…”

“…Many studies have reported that under uniaxial tensile deformation, the uneven plastic deformation between grain boundaries leads to surface roughness evolution [13,14]. Kubo et al [14] reported the difference in the crystal orientations because of the stress state on the surface roughness of a low-carbon steel sheet. 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 interstitial-free (IF) steel sheets.…”

Constitutive Model of the Surface Roughening Behavior of Austenitic Stainless Steel

“…To understand the mechanisms behind surface roughening in austenitic stainless steel, many studies have been conducted on stainless steel sheets [2,4,[11][12][13][14]. It has been generally reported that surface roughening linearly increases with plastic strain [14][15][16][17] and is affected not only by plastic deformation conditions such as the strain ratio or strain path but also by material factors such as the grain size crystal structure, crystal orientation and texture [15,[17][18][19]. When the austenitic stainless steel is subjected to plastic deformation, martensitic phase transformation (MPT) occurs [15,[20][21][22].…”