On the nature of hot work hardening phenomenon in die steel with regulated austenitic transformation during exploitation

Die steels with regulated austenitic transformation during exploitation (RATE steels) are a new type of tool steels. These steels are the only ones for which the phenomenon of ‘hot work hardened state’ enhancement has been clearly demonstrated. But the nature of hot work hardening of such steels remained unexplored. A systematic study of the mechanical behavior and changes in the structure during hot deformation of RATE steel in the temperature range of 450–750°С was carried out in this work. A new medium carbon (0.46% C) RATE die steel of the Si–Cr–Ni–Mn–Mo–V–Ti–Nb alloying system was used. The level of strain hardening for steel obtained after austenitization at a temperature of 1150 °C with subsequent cyclic deformation at 450 °C increases with further cyclic deformation at a temperature of 750 °C. Austenitization at a temperature of 1150°С, holding and preliminary deformation at 450°С, followed by cooling to room temperature and rapid heating to a deformation temperature of 750°С enhances the hot work hardened state and promotes maximum strength of steel at a temperature of 750°С. On the basis of the results obtained, the mechanisms and structural factors of hot work hardening of RATE steel have been identified, namely strain hardening (increase in dislocation density at high temperatures); dispersion and nanophase hardening; hardening from polymorphic transformations and hardening upon deformation in a two-phase (γ + α)-region. Their separate contribution to the complex hardening of steel is quantified. The most important factor affecting the presence/absence of the hot work hardened state enhancement in the RATE steel is the austenitizing temperature as well as high stability of supercooled austenite and the lower α↔γ-transformation temperatures.