Metallurgy of continuously annealed high strength TRIP steel sheet

The effects of heat-treatment conditions on mechanical properties are comprehensively investigated to optimise the industrial process of the 590 MPa grade TRIP steel sheet with the metallurgical understanding. The substantial effect of the thermal conditions are first clarified by laboratory investigation, which includes the effects of annealing conditions, cooling conditions from intercritical temperature to austempering temperature and austempering conditions. The results indicate that the optimum annealing temperature is between 800 and 850 °C and the mechanical properties are hardly influenced by the annealing time between 30 and 120 s at an annealing temperature of 825 °C. It is also suggested that the optimum quenching rate is 45 °C/s to obtain the stable properties of the products and the optimum austempering conditions are 425 °C with over 300 s in case of a constant temperature austempering. Based on the laboratory investigation, mill trial is performed using the NKK No.4-CAL in Fukuyama works. The heat treatment conditions are intentionally varied to examine minutely the stability of the production. The mechanical properties are sensitive to the austempering start temperature, when the austempering temperature is gradually decreased during austempering in the industrial conditions for the stable operation without meanders. Excellent mechanical properties can be obtained by controlling the austempering start temperature between 445 and 460 °C. On the contrary, the properties deteriorate in case of the austempering start temperature over 470 °C although the amount of retained austenite is the same or slightly larger than the material which exhibits excellent properties. This is because the retained austenite is less stable in the high-temperature austempered material caused by less bainite transformation.