Design and Development of Galvannealed Dual-Phase Steel: Microstructure, Mechanical Properties and Weldability

It was attempted to produce galvannealed dual-phase (DP) steel through a commercial continuous galvanizing line (CGL). The main challenge was to form the DP microstructure even with the slow cooling rate after annealing in the CGL. A two-pronged approach was adopted for this, viz. design of the steel chemistry with calculations and optimization of the CGL parameters with simulation using Gleeble-3800 thermomechanical simulator. The steel thus produced in CGL revealed ferrite and 18 +/- 2% martensite microstructure, with tensile strength exceeding 600MPa and 24% total elongation along with good formability. Weldability study using resistance spot welding indicated the possibility of achieving sound joint with a 4-kA width in weld time-current lobe despite the high carbon equivalence.