Papers by Author: D. Jorge-Badiola

Abstract: New trends focused on achieving higher performance steels has led to a so-called 3^rd Generation Advanced High Strength Steels (AHSS), in which the typical polygonal ferrite found in TRIP steels as a matrix phase is replaced by harder phases as Carbide-Free Bainite (CFB) and/or (tempered) martensite. Besides, large volume fractions of retained austenite (R.A.) with adequate stability are aimed for to improve the formability of the steels. Si containing steels are regarded as the most suitable to retard cementite formation and consequently reach high volume fractions of RA. In this work, CFB annealing schedules were applied to dilatometer samples of Fe-0.22C-2.0Mn-1.3Si. The overaging temperature T_B was varied between 390 oC and 480 oC, and other processing variables investigated were the austenitizing temperature T_aus, and the overaging holding time t_B. The annealed samples analyzed with LOM, FEG-SEM, EBSD and X-ray diffraction techniques show that markedly different complex microstructures made up of bainite, ferrite, MA phase and retained austenite (R.A) are accomplished depending on the specific thermal cycle. These results are described in detail and discussed in relation to the dilatometry measurements.

Abstract: A reversion of the strain produces a modification of the static recrystallization kinetics. Initially, the reversion increases the recrystallization time, that reaches a maximum at a certain strain, and decreases again for increasing reverse strains. This transient on recrystallization kinetics develops over a strain interval similar to that of the microstructural and stress-strain transients. At strains beyond the transient, the reversion can be regarded as a shift on the strain axis. However, at the authors knowledge there is no formulation able to describe the material behaviour during the transient. The present work introduces an equivalent strain concept based on the substructural dissolution/build-up processes taking place as a result of the strain reversal. This formulation allows including the effect of the strain path on recrystallization models.

Abstract: A combination of monotonic and reverse tests has been carried out in order to assess the strain path effects on an austenitic stainless steel hot deformed by torsion. Microstructural results have been obtained by EBSD. The misorientation average parameter measured at different step size scans, the Kernel parameter and the orientation spread average parameter, provide a picture of the in-grain curvature developed during the different strain paths. The results show that these parameters are sensitive to the strain path.

Abstract: It has been demonstrated recently that when a reversion of the strain is applied during the hot working of a Nb-microalloyed steel, the subsequent static recrystallization kinetics is significantly affected. However, depending on the magnitude of the reversion, the static softening kinetics can be accelerated or delayed. This relates to the substructure dissolution taking place by the effect of the reversal. In the present work, new microstructural results obtained by EBSD on an austenitic stainless steel hot deformed by torsion is used to explain better the observed effect of the reversion of the strain.