Continuous Annealing Process and Microstructure Property of 1200mpa High Strength Steel by Ultrafast Cooling

Abstract

The different dilatometric curves of continuous cooling transformation have been determined by DIL805 thermal mechanical simulate, through metallographic and hardness method to study the effect of different cooling rate on the microstructure of transition. The critical point Ac1 and Ac3 of the tested steel are 709°C and 865°C. With the increase of cooling rate, the hardness of the steel and the content of martensite increase. In the laboratory conditions, the steel in this experiment was heated to 780°C, 800°C, 820°C, 840°C and 860°C, for 80s, then slowly cooled to 680°C, and water quenched to room temperature finally. The aging temperature was 240°C for 300s, and the last the sample was air cooled to room temperature. The results show that the microstructure of the annealed experimental steel belongs to martensite and ferrite. With the increase of annealing temperature, the content of martensite, the tensile strength and yield strength of the experimental steel increase, and the elongation decreases continuously. The sample was annealed at 800°C for 80s, then slowly cooled to 680°C and finally water quenched to room temperature. After overaging at 240°C, the samples were obtained with high mechanical properties. The tensile strength, yield strength and elongation are 1223MPa, 605MPa and 9.2%, respectively.