Effect of intercritical annealing temperature and holding time on microstructure and mechanical properties of dual phase low carbon steel

Dual phase steels are an important advanced high strength steel, which have been widely used in the automotive industry for vehicle components requiring light weight and safety. In this study, the formation of dual phase structure with various volume fraction of martensite in a low carbon steel SS400 during intercritical annealing were investigated. It was found that intercritical annealing temperature and holding time affected the microstructure and mechanical properties of dual phase low carbon steel. The specimens were heated at intercritical annealing temperature of 750 degrees C, 775 degrees C, 800 degrees C and 825 degrees C, for holding periods of 6-18 minutes, followed by water quenching in order to get a dual phase ferrite and martensite. After quenching, it was obtained the optimal annealing conditions at 800 degrees C with a holding periods of 10 minutes. In this condition, the tensile strength was increased up to 621 N/mm(2) or 39.24% higher than the initial condition, while the elongation decreased up to 13.8%. The hardness of specimens increased from 127.7 to 235.83 HVN or up to 84.67% higher than the initial condition. Meanwhile the volume fraction of martensite was 24.08%. The higher the temperature of the heating value of grain growth rate constant (K) increases. In addition, at the optimal poin, the value of K (grain growth rate constant) and n (Avrami's exponent) were 0.263 and 0.318, respectively, with activation energy (Q) of 3.98 J/mol.