Microstructure and Mechanical Properties of Direct Quenched and Tempered N610E Steel for Oil Storage Tank

N610E grade steel (12MnNiVR) for oil storage tank is normally produced by reheating quenching and tempering process. Effects of tempering temperature on the microstructure and mechanical properties of the direct-quenched N610E steels were investigated by OM, TEM and other experimental methods. The results indicate that: after direct quenched and tempered at 655℃, the tested steel obtains the optimum comprehensive mechanical properties with tensile strength of 640MPa, yield strength of 570MPa, elongation of 22% and impact energy of 273J at -20℃, brittle transition temperature of N610E steel tempered at different temperature are lower than -60℃. The recovery of lath bainite becomes gradually obvious with increasing tempering temperature through motion, merging and reorganization of dislocations, which cause lath wider. Mechanical properties variation with tempering temperature shows non-monotonic nature, which attributes to the comprehensive mechanism including recovery and softening process of substructure, and the strengthening mechanism due to carbon dissolution and the second phase precipitation in bainite during tempering.