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

Abstract

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 °C), i.e., in various phase regions including the region with predominant bainite content (up to the temperature of 757 °C). At temperatures above 820 °C, the structure was fully austenitic. The hot deformation activation energies of 648 kJ·mol−1 and 364 kJ·mol−1 have been calculated for the temperatures ≤770 °C and ≥770 °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.

Effects of Austenitization Temperature and Pre-Deformation on CCT Diagrams of 23MnNiCrMo5-3 Steel

“…This occurrence of martensite was so small that it could not be manifested in the analysis of the dilatation curves. The minor local occurrence of martensite in microsegregations (probably of manganese and chromium [ 57 ]) was documented in the photographs (see Figure 9 a for an example) but was not included in Table 2 . In case of the cooling rate minimization and quasi-equilibrium conditions’ achievement in the dilatometry of the investigated steel, the fraction of ferrite and perlite would further increase at the expense of the bainite and martensite content.…”

“…This occurrence of martensite was so small that it could not be manifested in the analysis of the dilatation curves. The minor local occurrence of martensite in microsegregations (probably of manganese and chromium [57]) was documented in the photographs (see Figure 9a for an example) but was not included in Table 2. In case of the cooling It should be noted that martensite was found in all samples, at least in trace amounts, even after cooling at the lowest rate.…”