Effects of Surface Oxides on the Phosphatability of the High Strength Cold Rolled Steels

Abstract

It is well known that Si and Mn, the basic alloying elements for steel materials, are susceptible to oxidation and form oxides on the steel surface easily, which has a great influence upon the surface performance of steel sheets such as phosphatability. In recent years, on the other hand, the cold rolled steel sheets for automobiles are required to have increasingly higher strength in order to combine improved crashworthiness and weight saving of automobiles. To increase the strength of steel sheets, the addition of these alloying elements is very effective. In this work, the kinds and the state of distribution of oxides formed on the surface of steel sheets alloyed with Si and Mn were analyzed to investigate the effects of Si and Mn upon phosphatability from the standpoint of surface oxides. It was found that phosphatability is deteriorated by the addition of Si because Si oxide films are formed on the surface of steel sheets and serve as the barriers against the reactions in phosphating treatment. It was also found that the formation of oxides is also affected by the concentration of Mn except Si and that the steel sheets excellent in phosphatability can be produced by controlling the concentration of Mn even when the concentration of Si is comparatively high.