Development of deep drawable galvannealed Ti-IF HSS containing niobium in solution for automotive body weight reduction

It is important to improve (1) the Lankford value and (2) the wetting characteristics by molten Zn for the development of deep drawable galvannealed Ti-IF HSS containing Nb in solution having a tensile strength of more than 390 MPa corresponding to automobile industry’s demand for weight reduction and high corrosion resistance. Concerning item (1), the effect of the coiling temperature under hot-rolling conditions on the Lankford value and the recrystallization texture of cold-rolled and annealed steel sheets were investigated together with basic research on hotrolled microstructures and textures. In addition the metallurgical role of Nb in solution related to the improvement of the recrystallization texture and the Lankford value was discussed. Concerning item (2), the effect of reduction heating conditions before hot dipping on the Zn wetability was examined. Here the relationship between the formation of a Mn-related oxide layer and the Zn wetability was considered. Furthermore, Ti-IF mild steels containing Nb in solution are described as a base metal of deep drawable galvannealed Ti-IF HSS and various properties including the mechanical ones of industrial products developed by Nisshin Steel Co., LTD are introduced together with their automotive application. (AU)