Hot Deformation Behaviour of Mn-Cr-Mo Low-Alloy Steel in Various Phase Regions

The deformation behaviour of a coarse-grained as-cast medium-carbon steel, alloyed with 1.2% Mn, 0.8% Cr and 0.2% Mo, was studied by uniaxial compression tests for the strain rates of 0.02 s(-1)-20 s(-1)in the unusually wide range of temperatures (650-1280 degrees C), i.e., in various phase regions including the region with predominant bainite content (up to the temperature of 757 degrees C). At temperatures above 820 degrees C, the structure was fully austenitic. The hot deformation activation energies of 648 kJ center dot mol(-1)and 364 kJ center dot mol(-1)have been calculated for the temperatures <= 770 degrees C and >= 770 degrees C, respectively. This corresponds to the significant increase of flow stress in the low-temperature bainitic region. Unique information on the hot deformation behaviour of bainite was obtained. The shape of the stress-strain curves was influenced by the dynamic recrystallization of ferrite or austenite. Dynamically recrystallized austenitic grains were strongly coarsened with decreasing strain rate and growing temperature. For the austenitic region, the relationship between the peak strain and the Zener-Hollomon parameter has been derived, and the phenomenological constitutive model describing the flow stress depending on temperature, true strain rate and true strain was developed. The model can be used to predict the forming forces in the seamless tubes production of the given steel.