A new type of thermomechanical processing with rough rolling below the recrystallisation-stop temperature (T5%), followed by a rapid reheat to 1200 °C for 10 s, and then finish rolling at the same temperature as the rough rolling, was investigated to better understand the kinetics of strain-induced precipitates (SIP). In addition, the deformation-induced ferrite transformation (DIFT) during the finishing passes was investigated as an efficient method to achieve the ultrafine ferrite grains. For comparative purposes, two experimental 0.06 wt% C steels were studied; one with 0.03 wt% Nb (Nb steel), and another one with both 0.03 wt% Nb and 0.02 wt% Ti (Nb-Ti steel).
To understand SIP behaviour and its effect on the recrystallisation, six different rough rolling temperatures were used in the range of 850-1100°C. The full recrystallisation temperature (Tl) and recrystallisation stop temperature (Ts) contributions of the precipitates start temperature were determined using the Dutta and Sellars’s model approach on the basis of optical microscopy (OM) and transmission electron microscopy (TEM) characterization. It was found that there were mixed structures after rolling at 950 °C in Nb and Nb-Ti steels, which is consistent with the Tl (960 °C in Nb steel) and Ts (950 °C) by Dutta and Sellars’s model. The number density and volume fraction of SIP increased as the rolling temperature was decreased, consistent with the prediction that the nose of the precipitates-time-temperature diagrams (PTT) is at 840 °C in both steels. The orientation relationship (OR) between precipitates and ferrite matrix has been identified by analysis of selection area electron diffraction patterns. While obeying OR, the nanoscale particles precipitated in the austenite region during deformation.
After the roughing schedule, with the final roughing pass taking place at 850 °C, the steels were rapidly reheated at a rate of 10 °C/sec to a temperature of 1200 °C, held at temperature for various times, and water quenched to room temperature. Then, both the precipitate dissolution kinetics, together with the austenite grain coarsening kinetics was established as a function of the holding time at 1200 °C. The importance to subsequent finish rolling was then used for the final part of the project.
Finally, processing these steels consisted of a simulated roughing rolling schedule with the final roughing pass taking place at 850 °C, followed by rapidly reheating at a rate of 10°C/sec to a temperature of 1200 °C, held at temperature for 10 s and 100 s, and then air cooled to the final rolling temperature of 850 °C followed by deformation using different parameters. The effect of different rolling and reheating parameters on the microstructures and the SIP was fully analysed using various techniques. This thermomechanical process route resulted in DIFT, with ferrite nucleated primarily on the prior-austenite grain boundaries. The ferrite/martensite phase transformation temperature was increased by the heat treatment at 1200 °C. The volume fraction of SIP after finish rolling was influenced by the supersaturation of microalloyed elements in solution during heat treatment. The variation of micro-hardness correlated well with the change of volume fraction of SIP and DIFT, as well as the refinement of prior-austenite grain size.
