Constitutive Constants for Hot Working of Steels: The Critical Strain for Dynamic Recrystallisation in C-Mn Steels

The primary focus of this work was to determine the relationship between chemical composition and the Zener-Hollomon exponent q in the peak strain equation for hot working of C-Mn steels with the ultimate aim of improving the modeling of mean flow stresses for hot strip mills. Therefore, the hot deformation behavior of C-Mn steels was examined in which the C and Mn contents were increased systematically between the levels 0.035 up to 0.52%C and 0.22 to 1.58%Mn, respectively. In addition, data from other workers were also analyzed to complement the results from this investigation. As opposed to the observations from some other workers that the apparent activation energy for hot working QHW appears to decrease with an increase in the carbon content, it was found that, despite a weak relationship, QHW increases with an increase in carbon content i.e., from 300 to 355 kJ/mol as was also found by others. Consequently, the Zener-Hollomon exponent q in the peak strain equation for hot working and hence also the critical strain for dynamic recrystallisation, εc = 0.65AD0mZq, was found to decrease with an increase in carbon content as follows, q = 0.21 − 14[%C] for C ≤ 0.8%C. The possible role played by manganese-carbon complexes in the austenite during hot working and the consequences of the variation in q for compact strip production rolling are discussed.