ELABORATION OF THERMOMECHANICAL TREATMENT CONDITIONS OF Ti-V AND Ti-Nb-V MICROALLOYED FORGING STEELS

The goal of the work was to describe the forging conditions of thermomechanical treatment for Ti-V and Ti-Nb-V microalloyed steels. Conditions of hot-working allowing to obtain both the desired microstructure and mechanical properties of forgings were selected taking into consideration: precipitation analysis of MX-type (M-Nb, Ti, V; X-N, C) interstitial phases in austenite; research on the influence of the austenitizing temperature on the gamma-phase grain size; investigation of the continuous compression of specimens; and examination of the kinetics of recrystallization of plastically deformed austenite. The precipitation analysis of MX-type interstitial phases in austenite was conducted on the basis of a simplified thermodynamic model for equilibrium conditions as proposed by Adrian, assuming that individual MX phases are soluble in austenite. The effect of the austenitizing temperature in a range from 900 to 1200 degrees C on the prior austenite grain size was investigated to verify the precipitation analysis of MX-type phases. The work also presents the results of the effect of Nb, Ti and V microadditions on flow stress, recrystallization kinetics and microstructure. Plastometric tests were carried out using the Gleeble 3800 thermomechanical test simulator. The studies provide the basis for a proper design of the manufacturing process for thermomechanical treatment of forged machine parts obtained from high-strength microalloyed steels.