The Effects of Cold-rolling Strain on Microstructure and Formability of Al Bearing TRIP-aided Cold-rolled Steel Sheets with Annealed Bainitic Lath Structure Matrix

The effects of cold-rolling strain on microstructure and formability of 0.2%C-0.5-1.5%Si-1.5%Mn-0.04-1.0%Al, in mass%, TRIP-aided cold-rolled sheet steels with annealed bainitic lath structure matrix (TAB steels) were investigated. Cold rolling straining changed the matrix structure after intercritical annealing front annealed bainitic lath structure to granular structure with a change of retained austenite morphology, if the structure before cold-rolling was changed to bainitic lath structure. Both the largest elongation and the best stretch-flangeability were achieved in 1.0% Al bearing TAB steel subjected to cold-rolling strain of 40%, followed by intercritical annealing and austempering. This was caused by the increased retained austenite stability and the remained bainitic lath structure due to Al addition.