Study of ferrite grain growth and transformation mechanisms, at different stages during steel processing, aided by the use of modern OIM techniques

When in process like phase transformation, recrystallisation and grain growth, the selective dissolution, nucleation and/or preferential growth of certain orientations are involved, the conventional observation techniques(Transmission Electron Microscopy and X-ray texture analysis) reveal insufficient to some extent. In the present work, OIM (Orientation Imaging Microscopy) techniques have been used to investigate the effect of the local orientations and mesotexture on the microstructure and texture evolution during transformation, recrystallisation and grain growth in steels. The following topics are treated: Recrystallisation and grain growth after cold rolling Grain evolution during the austenite to ferrite transformation Selective transformation with special reference to TRIP and Dual-phase steel. TKS has concentrated on the study of the cold rolled and partly recrystallised materials concerning their global texture (X-ray diffraction) and the local one (EBSP technique). The results indicated that the cold rolling texture component in these samples dominated and that the intensity of the γ fibre increases with cold reduction. In partially recrystallised samples, a small degree of recovery seems to be active and the microstructure into the non recrystallised regions appear very similar to the one in the cold rolled sample. Local texture analysis of the recrystallised grains showed, that many of the freshly recrystallised grains had a gamma fibre orientation. CEIT has concentrated on the Study of ferrite grain coarsening during continuous annealing. Parameters as grain size and number of grains by selected orientations were calculated. A number advantage of the {111} grains is observed. However, the size advantage of them is not clear. The application of EBSD to the study of the transformation from both recrystallised and deformed austenite shows that the main contribution to the ferrite grain coarsening taking place behind the transfromation front is due to normal grain growth. The investigation of the deviation from an ideal Kurdjumov-Sachs orientation relationship between ferrite and the parent austenite indicated that a variant selection seems to be operative during the transformation from recrystallised austenite, leading to a high density of low angle ferrite-ferrite boundaries. When transformation takes place from a deformed austenite, this effect cannot clearly be appreciated due to the misorientations produced into the parent austenite by the effect of the deformation. CRM and RUG concentrated on the selective transformation. CRM completed the analysis of the influence of the annealing temperature on the texture and r-value evolution on three CMn steel grades. An optimal annealing temperature is found for the three steels, corresponding with an optimal fraction α → γ → a transformation. For quenched steels a big systematic difference in measured and calculated r-value was observed. Two-step annealing treatments were performed in order to strengthen the steel by introducing a hard second phase into the microstructure, while optimising the texture. RUG continued with the work done previously on TRIP steels. At conventional heating rates, the recrystallization finishes before the start of α-γ phase transformation and the austenitic phase emerges from the completely recrystallized ferritic phase. At heating rates higher than 3000 °C/s the phase transformation starts before the completion of the recrystallization.