Hot rolled stretch flange steel (SFHS-600): experience at Tata Steel

Fuel economy together with safety considerations is the driving forces for the steadily increasing usages of higher strength steels in automotive industry. Dual phase steel based on ferrite and martensite structure possesses a potential to reduce the weight of the automobile components as they have good combination of strength and elongations, and a low yield ratio. However, their use is not fully established for wheel rim application due to softening in HAZ leading to localized necking during stretching of the welded hoop. This paper presents the development work on high strength steel for wheel rim and disc applications. The steel chemistry selected has low carbon, high manganese and approximately 0.04%Nb. Thermodynamic calculations were made to predict various phases formed during run out table (ROT) cooling. The steel was commercially processed through the basic oxygen process (BOF)-ladle furnace (LF)-slab caster route. Transformation behavior of the steel and ROT simulation was carried out using a thermo mechanical simulator. The microstructure of the steel consisted of ferrite and 15-20% bainite. The material showed yield strength of the order of 520 MPa while ultimate tensile strength and percentage elongation were found to be 610 MPa and 29 percent respectively. The hot rolled strip was commercially processed into wheel rim and disc successfully. The weld joint of the rim was examined in the as welded condition to assess its soundness. (AU)