Dilatometric study of phase transformations in advanced high-strength bainitic steel

Abstract

The work deals with dilatometric studies of a new-developed advanced high-strength bainitic 3Mn-1.5Al steel. Ferritic, bainitic and martensitic phase transformations are investigated in detail in respect of their temperature range forming and microstructures produced under various conditions of both continuous and isothermal cooling. The equilibrium temperatures of A e1 and A e3 and phase composition of the investigated steel were initially calculated whereas critical temperatures of A c1 and A c3 as well as the decomposition of retained austenite were determined upon heating. The major tests consisted of controlled cooling of undeformed or plastically deformed austenite using the dilatometer within the cooling rate range of 2-0.5 °C s-1. The effects of the cooling rate and deformation at temperatures of 900 and 1,050°C on the phase transformation behaviour and microstructure were explained. The final experiment was carried out using a thermo-mechanical simulator under conditions of multi-step deformation and isothermal holding of the steel at 400 °C. Microstructural features were revealed using light microscopy and scanning electron microscopy techniques.