Estudio de la precipitación inducida por deformación y su interacción con los procesos de ablandamiento en aceros mecroaleados con alto contenido en

Resumen

Nowadays, due to the demand of the automotive industry and the competence with lighter materials, new grades of steels are under investigation. Among others, high-Mn steels (15-30%), also known as TWIP (TWinning Induced Plasticity) steels, are highlighted due to their excellent strength-ductility combination. As a result, a significant thickness reduction of some of the automotive components can be achieved. These steels present also a high ductility, which helps to preserve vehicle structural stability in case of impact.However, the main disadvantage found in the production of these steels is their low yield strength. Accordingly, several strategies are under investigation to improve this property. Adding microalloying elements (Nb, V and/or Ti) has been proposed as an ideal hardening mechanism for TWIP steels. These elements can precipitate in the form of carbides and/or nitrides during the different stages of the steel production process. Depending on the volume fraction, coherency and precipitate size, they can contribute to increase steel yield strength. Although several works have shown encouraging results on the hardening potential of microalloying elements in TWIP steels, the influence of high Mn contents on the precipitation kinetics has not been investigated in detail. The information concerning the effect of these elements on other processing stages, such as hot deformation, is also scarce. It has to be taken into account that during hot working, microalloying elements in solid solution or in the form of strain-induced precipitates can result in a notable increase in the rolling loads, making even more difficult the already challenging processing conditions of these steels. In addition, if strain-induced precipitation takes place during hot rolling, the amount of microalloying element available for precipitation at later stages is reduced, which can suppose a loss of microalloying efficiency.Taking this into account, this thesis has focused on the characterisation of the softening and strain-induced precipitation kinetics during hot working of several steels with high Mn levels (20 and 30%) microalloyed with Nb or V. In order to do so, the specimens have been deformed in the torsion machine using two types of tests; double-hit torsion tests, that allow the determination of the softening kinetics, and single-hit tests followed by quenching, in order to analyse the microstructure and the precipitation state of the specimens in the required conditions of temperature and holding time. The presence of strain-induced precipitation has been characterised through the analysis of the carbon extraction replicas in TEM and the results have been related to the softening kinetics determined from the double-hit torsion tests. In addition, thin foils have been examined in order to perform a more detailed analysis of the precipitation for several of the Nb microalloyed steels. Similarly, in some cases the precipitate volume fraction has been quantified through the EFTEM technique. Different samples have been also analysed via ESBD for determining the recrystallised fraction and compare it with the fractional softening obtained from the mechanical tests. Finally, a semi-empiric model for the prediction of the recrystallisation kinetics valid for those cases where strain-induced precipitation does not occur has been developed. In addition, the applicability of a physical model that takes into account the interaction between recovery, recrystallisation and strain-induced precipitation has been investigated. The experimental results obtained during this work have been used to determine the unknown physical parameters required for model implementation. Finally, the modelling results have been validated through the comparison of the precipitate volume fraction predictions with the experimental results obtained from the thin foils analysis.