Evolution of appearance profile of hot-dip galvanized car-body steel sheet during deformation process

In recent years, the waterborne free intermediate coating process has been widely used in the automotive industry. Because the baking times and coating thickness are decreased, the surface covering capability of the painting process is reduced,which directly affects the appearance quality（ long-and short-wave values） of the body paint. Thus, there are correspondingly higher requirements for the white body surface profile prior to painting. The surface profile of the white body is mainly affected by the plate material, the surface profile, and the deformation process. So, the change rule for the surface profile during deformation of the steel plate is a key factor in coating appearance optimization. In this paper,we first analyze the typical deformation of the outer cover of a car body.Then,we examine the change tendency of the surface profile of steel plates with respect to different deformation rates, specifically for a steel plate comprising a hot-dip galvanized bake-hardened steel sheet. Based on our analysis of the influence of the deformation on the coating appearance,we selected 3%,5%, and 8% deformation rates in this research. We found the roughness（Ra） value in the typical deformation range（3%-8%） of the car body to exhibit a decreasing trend at first and then an increasing trend. The Ra value of the 8% deformation is not more than the original plate test value. When the P;value of the original plate is in the lower range（about 60）, it exhibits a slight increasing trend in the deformation process（3%-8%）. And when the P;value of the original plate is in the higher range（about 120）, it exhibits no increasing trend in the deformation process（3%-8%）. In contrast, the waviness(W;) value in the car body’s typical deformation range（3%-8%） shows a significant growth trend.