Characterization of the microstructure and transformation behaviour of strained and nonstrained austenite in Nb–V-alloyed C–Mn steel

In order to optimise the hot rolling schedule of a Nb–V-alloyed C–Mn steel, a precise determination of the continuous cooling transformation (CCT) diagram is required. To achieve this goal, a series of dilatometer tests were carried out in addition to hardness measurements and a metallographic analysis. These data allowed delineation of the various regions of the CCT diagram under the condition that the parent austenite phase was not deformed prior to transformation. In the hot rolling process, however, the austenite is subjected to a rolling strain, which may accumulate into a deformed microstructure at the onset of transformation. The processing conditions of hot rolling were simulated in a series of hot torsion experiments. The critical transformation temperatures thus obtained gave rise to a modified CCT diagram for a strained parent phase. The volume fractions of the various constituents in the steel were determined both by conventional optical metallography methods and by employing orientation imaging microscopy data. The data produced by these two methods were in excellent agreement.