Influence of hot plastic deformation and cooling rate on martensite and bainite start temperatures in 22MnB5 steel

During hot stamping process, hot forming, cooling and phase transformations are performed in a single step. As a matter of fact, multifunctional phenomena happen and affect each other. Among these phenomena, martensitic and bainitic transformations have the greatest importance. In the current research, the start temperatures of martensite and bainite of 22MnB5 boron steel have been measured in undeformed and 40% deformed conditions, and in various cooling rates from 0.4 °C/s to 100 °C/s by means of deformation dilatometer. It is concluded that, reduction of cooling rate, could bring about an increase or decrease in Ms and Mf, depending on other phases formation before martensite. Also, hot plastic deformation, hindered the martensitic transformation and decreased Mf and Ms especially at lower cooling rates, while Bs increased. Furthermore, the critical cooling rate, increased about 40 °C/s by applying 40% hot plastic deformation.