The bainitic ferrite plate thickness is the main parameter that controls the strength of this type of microstructures. Such thickness has been proved to mainly depend on the austenite yield strength, the driving force for the transformation and the transformation temperature. However, no research has focused on how these parameters evolve throughout the transformation and how this evolution can affect the outcome. In this study, thermal and thermomechanical treatments have been performed in three selected steels. The treatments have been designed in such a way that all the mentioned parameters are comparable, aiming to obtain similar microstructures in terms of bainitic ferrite plate thickness. However, significant differences have been found among the microstructures, with variations in plate thickness larger than 100 nm. These results indicate that there might be other factors that take part in the scale of bainitic microstructures. To explain these differences and based on the kinetics of the transformation and on the carbon content of austenite at the end of the transformation, a possible explanation has been proposed.@en

The mechanisms driving variant selection phenomena in ausformed bainitic microstructures are not fully controlled yet. Previous results are not congruent among them, as they cannot be explained by the same rule. In this work, we aimed to understand the mechanisms governing variant selection in a medium carbon-high silicon steel subjected to ausforming treatments. To do so, the microstructural characterization in different regions of barreled samples has been combined with the simulation of the stress and strain state at the macro and micro level, by finite elements simulations and crystal plasticity simulations, respectively. The main conclusion is that the variant selection shown in these microstructures can be explained by understanding the distribution of the plastic strains at the macro and micro level. The selection of crystallographic variants is more pronounced in the regions which have been more deformed. Also, the deformation distributes along deformation microbands, where the most promoted variants seem to grow, in good agreement with previous results in ausformed lath martensite.@en

The novel use of double austempering treatments in a multiphase steel to refine and homogenize the final microstructure and thus improve the material strength have been studied in the 3.3Mn-0.17C-1.6Al-0.23Mo-0.22Si alloy. The microstructural features developed after conventional isothermal austempering treatments at 450 °C and 400 °C were compared with those obtained after two-step heat treatments. These treatments consisted of a first isothermal holding at a temperature slightly above the initial Ms, that was interrupted at 25 and 50% of transformation, followed by a second stage treatment at a lower temperature to complete the bainitic transformation. One- and two-step treatments were performed in a high-resolution dilatometer, and the critical transformation temperatures and phase transformation kinetics were determined from the longitudinal changes recorded during these tests. It was shown that blocky-type austenite was almost completely eliminated after the two-step treatments, which in turn positively reduced the amount of fresh martensite from ∼6 to «1%. It was possible to keep the volume fraction of retained austenite above 10%, while reducing both the thickness of the bainitic plates and the film-like retained austenite by 20% and more than 40%, respectively. These microstructural characteristics made it possible to increase the hardness of the alloy by approximately 50 HV and yield strength by 180 MPa.@en

The development of superior mechanical properties in medium-Mn requires the optimization of microstructural parameters such as retained austenite (RA) stability, volume fraction, and morphology. The present work explores the possibility of using a continuous annealing approach instead of conventional batch annealing to perform an intercritical annealing (IA) treatment in a hot-rolled strip of an Al-alloyed 5Mn steel. Dilatometric studies were performed at a temperature of 680 ºC with soaking times ranging from 1 to 300 min to follow the microstructural changes as a function of time. The microstructures thus obtained were thoroughly characterized by means of X-ray diffraction, SEM and TEM, TEM-EDS microanalysis and EBSD phase and orientation maps. It was observed that with increasing soaking times, the volume fraction of retained austenite gradually increases, albeit at the cost of its stability. The comparison of martensite start temperatures (Ms) based on the chemical composition of austenite at 680 ºC with that experimentally obtained at higher process temperature revealed the effect of the grain size on the reduction of RA stability for longer process times. Accordingly, mechanical tests results showed that the yield stress, tensile strength and hardness decrease with an increase in the IA soaking time.@en