The softening processes that take place after hot deformation in two high Mn steels, one of them microalloyed with 0.1%Nb, have been investigated. Double hit torsion tests were carried out in order to determine the mechanical softening at temperatures ranging from 900 to 1100°C. In addition, the applicability of different parameters obtained by means of the Electron Back Scattering Diffraction (EBSD) technique to estimate the recrystallized fraction was investigated. It has been found that the Grain Orientation Spread (GOS) is the parameter that best allows quantifying the recrystallized fraction under the conditions investigated. The correlation between the mechanical softening and the recrystallized fraction measured by EBSD depends on the material and deformation conditions. A good correlation between both values is observed for the base steel at all the temperatures. However, for the Nb microalloyed steel, although at high temperatures a good correlation is also observed, at low temperatures the mechanical softening fraction tends to be larger than the recrystallized fraction denoting that recovery has an important contribution.

This work has focused on the study of hot working behavior of Ti-Nb microalloyed steels with high Ti contents (> 0.05%). The role of Nb during the hot deformation of low carbon steels is well known: it mainly retards austenite recrystallization, leading to pancaked austenite microstructures before phase transformation and to refined room temperature microstructures. However, to design rolling schedules that result in properly conditioned austenite microstructures, it is necessary to develop models that take into account the effect of high Ti concentrations on the microstructural evolution of austenite. To that end, in this work torsion tests were performed to investigate the microstructural evolution during hot deformation of steels microalloyed with 0.03% Nb and different high Ti concentrations (0.05%, 0.1%, 0.15%). It was observed that the 0.1% and 0.15% Ti additions resulted in retarded softening kinetics at all the temperatures. This retardation can be mainly attributed to the solute drag effect exerted by Ti in solid solution. The precipitation state of the steels after reheating and after deformation was characterized and the applicability of existing microstructural evolution models was also evaluated. Determined recrystallization kinetics and recrystallized grain sizes reasonably agree with those predicted by equations previously developed for Nb-Ti microalloyed steels with lower Ti concentrations (<0.05%).

In this work, multipass torsion tests followed by coiling simulations under different conditions have been performed with a reference Nb (0.03 pct) and a high Ti (0.1 pct)–Nb-microalloyed (0.03 pct) steel. In the case of the high Ti steel, estimated yield strengths close to or over 700 MPa were obtained for some of the conditions researched. However, a very significant effect of previous austenite grain size and strain accumulation on precipitation strengthening has also been observed. As a result, depending on deformation sequence and final cooling conditions, the coiling simulation temperatures that lead to the highest mechanical strength varied from 600 °C to 500 °C. The effect of increasing strain accumulation was mainly related to higher phase transformation temperatures, which led to a lower driving force for precipitation and higher microalloying element diffusivity, resulting in the formation of less and coarser precipitates.