The effects of starting microstructure on the results of double intercritical annealing process of 0.107%C-2.39%Mn-0.453%Si dual phase steel

This study investigates and compares the effects of the starting microstructure on the double intercritical annealing (DIA) process on the microstructure and mechanical properties of 0.11%C-2.39%Mn-0.45%Si dual phase steel. The specimens were austenitized, followed by water quenching (WQ) or air cooling (AC), then DIA at 730°C. For the WQ method, high cooling rate results in the formation of a high strength martensitic structure. Conversely, AC method yields a moderate cooling rate, forming a bainitic structure. After single intercritical annealing (SIA), the microstructure transformed into ferrite and martensite. WQ method specimens contain high stress concentration, leading to earlier recrystallization compared to the AC method resulting in a higher amount of martensite and a larger ferrite grain area. In contrast, the AC method results in more homogeneous structure, thus it shows lower strength with higher ductility compared to the WQ method. After DIA, the WQ method has a higher strength than the AC method, with similar ductility. This is because the morphology of the martensite in WQ730SIA is thin and sharp needle dispersed irregularly in matrix which has a higher stress concentration than AC730SIA which has short and rounded martensite dispersed regularly in ferrite matrix. However, regardless of slightly higher mechanical properties, the WQ method causes the risk of the workpiece breaking during first water quenching, therefore, the AC method provides a better heat treatment option.