Effect of Plastic Deformation on CCT-Diagram of Multi-Phase Forging Steel

The paper presents the results of research on the influence of plastic deformation and cooling conditions on microstructure, hardness and a shape of CCT-diagram (Continuous Cooling Transformations) of newly developed multi-phase steel assigned for die forgings, combining high strength, crack resistance and fatigue strength. The diagrams of undeformed and plastically deformed supercooled austenite transformations of steel, containing 0.175% C, 1.87% Mn, 1.0% Si, 0.22% Mo as well as Ti and V microadditions in concentration of 0.031% and 0.022%, respectively, were determined. Dilatometric tests were performed using a Bahr 805 A/D dilatometer. Specimens were austenitized at the temperature of 1000°C for 300 s and successively cooled to ambient temperature at a rate ranging from 60°C/s to 0.1°C/s. In order to determine the influence of plastic deformation on the shape of CCT-diagram, samples were deformed at the temperature of 1000°C, using a 50% degree of deformation, and then cooled in the same rate range as the samples which were not plastically deformed. The tests showed the following temperature results: Ac3 = 960°C, Ac1 = 832°C and a relatively low MS temperature equal 330°C. Plastic deformation of steel at the temperature of 1000°C, prior to the beginning of phase transformations, leads to significant increase in the ferritic transformation range, shifting the temperature of the beginning of this transformation to higher temperature in the entire range of cooling rates. It was also revealed that the specimens, plastically deformed at the austenitizing temperature, exhibit higher hardness compared to the specimens which were not plastically deformed, cooled with the same cooling rate. The elaborated CCT-diagrams of supercooled austenite transformations constitute the basis for correct development of the conditions of thermo-mechanical treatment of forgings from the tested steel