Microstructure And Mechanical Properties Of Nb Cold Rolled Galvanized Steel Containing Duplex

Energy-saving, environmental protection and safety have become the major concerns in the automobile industry because of the exacerbating of energy crisis, the rising oil price, and the enhancing of greenhouse effect. Auto lightweight technology is just an important way to solve these problems. Therefore, automotive steel should be oriented to high strength and low cost. Dual-phase steel is widely used in the automotive industry owing to high tensile strength,low yield strength, high initial work hardening rate and excellent strength-toughness match. Moreover, hot dipped galvanized dual-phase steel have been widely concerned with the improvement of surface quality requirement of modern automobile materials.Nb can effective refine grain. In order to improve the hardenability of the austenite and obtain ferrite+martensite microstrucutres, Nb+Cr has been used to take the place of expensive Mo in hot dipped galvanized dual-phase steel. The effect of Nb on micro structures and mechanical properties of cold rolled hot dip galvanized dual phase steel in different intercritical annealing temperature, intercritical holding time, slowing cooling temperature and cooling rate were analyzed by using of Gleeble-3500thermo-mechanical simulation machine.Nb can refine grain obviously and improve distribution of the second phase in hot dip galvanized dual-phase steel. Martensite with irregular shape in Nb-free steel tend to aggregated distribution when martensite with the shape of sphericity or spheroidicity in Nb-added steel is fine and disperses in micrstrucutre, and martenite in Nb-free steel is larger than that in Nb-added steel. Microstructure of tested steel is deeply influenced by critical annealing temperature. In these experiments, holding time of heating, slow cooling temperature, cooling rate have little effect on the mechanical properties of the tested steel, but yield strength, tensile strength, yield ratio, elongation and n value of Nb-add steel are higher than that of Nb-free steel because annealed steel strip possesses the higher strength, high ductility and high work hardening ability owing to Nb addition. The optimized process of DP780Nb-add cold rolled hot dip galvanized steel is that the annealing temperature is820℃-840℃, holding time of heating is120s, cooling temperature is700℃-720℃. The actual DP590Nb-add cold rolled hot galvanized steel fully meet the mechanical performance requirements of DP590dual phase steel. While strength meets the requirements, the ductility and strain hardening ability are more excellent.