Abstract:
The purpose of this paper is to present research whose goal was to improve the substrate in galvanized or galvannealed sheet steel,such that high strength can be obtained while maintaining good global formability(tensile ductility),local formability(sheared-edge ductility),and good spot weldability.It is well-known that the strength of DP steels is controlled by several factors,including the amount of fresh(untempered) martensite found in the final microstructure.However,the factors that control the amount of martensite in the final microstructure are not all together clear,and their identification represents a large portion of the research conducted in this program.The amount of fresh martensite found in the final microstructure can be considered equal to the amount of austenite formed in the intercritical anneal less the amount of austenite lost to various transformation products as the steel undergoes cooling from the intercritical annealing temperature(IAT) to 460℃,isothermal holding at 460℃,final cooling to RT and any remaining austenite retained at RT.Recent research has revealed that the amount of austenite formed during intercritical annealing of a given steel can be strongly influenced by the annealing temperature and the pre-annealing conditions of the hot band(coiling temperature) and cold band(% cold reduction).Current experiments have explored the combination of pre-annealing conditions and four annealing practices to help define the best thermal path needed to optimize the strength-formability balance in these higher strength DP steels.The steels used in these experiments contained(i) low carbon content for good spot weldability,(ii) the hardenability additions Mo and Cr for strength,and(iii) V for grain refinement,precipitation hardening and temper resistance.When processed correctly,these steels exhibited UTS levels up to 1 000 MPa,total elongation to 25%,reduction in area to 45%,and Hole Expansion ratios to 50%.The results of this program will be presented and discussed.
