Surface and Coating Analysis of Press-Hardened Hot-Dip Galvanized Steel Sheet
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The use of continuous galvanized steel sheet as feedstock material for press hardening leads to components of very high strength levels with classical cathodic corrosion protection. The present work provides an insight into this technology with special focus on surface oxidation and intermetallic phase formation during the austenisation process. For that reason hot dipped galvanized 22MnB5 steel sheets, with a blank thickness of 1.5?mm and an average coating weight of 70?g?m?2, were annealed in a temperature range of 400?900°C in steps of 50°C without soaking before quenching in water. Surface and cross-cuts were analyzed by SEM, EDX, and XRD to illustrate the phase formation and the surface changes during thermal treatment. Corrosion behavior was also studied based on electrochemical investigations and an accelerated, cyclic, automotive corrosion tests called VDA, which is a mixture of salt spray test and alternating climate test. It was found that austenisation of galvanized steel sheet causes a significant change of the coating. The generated coating still offers cathodic protection for the steel substrate and has higher corrosion resistance than standard galvanized steel sheet. Surface oxidation also occurs during the press hardening process leading to a surface covered with successively arranged layers of Al2O3 and ZnO, containing also further oxidized alloying elements.