Metallurgical Behaviour of Uncoated and Galvannealed Boron Steels in Hot Stamping Process

Understanding metallurgical behaviour of steel and surface reaction in hot stamping process is important for product quality and productivity. However they are often explained empirically. This paper is intended to describe them quantitatively with focusing on austenitization, martensite transformation, iron oxide scale formation and zinc coating reaction. Heating affects not only austenitization but also hardenability when short time heating is employed. Iron oxide scale is as less as 5 gm in thickness, if the furnace atmosphere is appropriate. Zinc coating of galvannealed steel interdiffuses with the substrate. After complete development of Fe-Zn solid solution, LME (Liquid- Metal-Embrittlement) does not occur. Cooling rate below M-s temperature affects hardness. Slow cooling below Ms temperature results in decreasing hardness due to auto-tempering of mart-ensite, even if any secondary phase is prevented.