Improved Scale Adhesion of Uncoated Hot Stamped Steel

The yearly production of hot stamped automotive parts is increasing due to several advantages over the conventional cold forming processes, such as allowing the manufacture of components of much higher strength and of more complex geometries. Hot stamping of uncoated boron steel results in the formation of a loose oxide on the steel surface. This oxide has to be subsequently removed by shot blasting for satisfactory painting. In this paper, a method to avoid shot blasting is presented. The method is based on a chemical pre-treatment allowing anchoring of the formed oxide to the steel surface during hot stamping. Characterization by Scanning Electron Microscopy (SEM) as well as results from adhesion tests and a cyclic corrosion test of the produced material indicate that the scale adhesion is unaffected by subsequent process steps (cleaning, phosphating, and painting). The results also indicate that the corrosion properties of the hot stamped material are directly affected by scale adhesion; improved adhesion results in improved corrosion resistance. Furthermore, the chemical pre-treatment allows more rapid heating than non-modified material.