Towards the modelling of recrystallization phenomena in multi-pass conditions: application to 304L steel

Recrystallization, which can occur dynamically or statically, is an importantphenomenon causing microstructure changes in deformed metals and therefore affecting theproperties of the material. Even though extensive investigations have been carried out on thenumerical modeling of recrystallization, the literature lacks accurate recrystallization modelswhich are able to predict microstructure evolution under multi-pass conditions. Although someefforts have been reported in this direction, most of them either lack experimental validation oronly provide qualitative agreement in selected deformation conditions. The connectionsbetween static recrystallization (SRX), dynamic recrystallization (DRX), post-dynamicrecrystallization (PDRX) and grain growth (GG) are usually oversimplified. Furthermore, most ofthem are not designed for variable thermal and/or mechanical conditions and are thereforedifficult to use for industrial applications.In this work a physically-based two-site mean field model has been developed to describethe microstructural evolution of 304L austenitic stainless steels. The originality of the model liesin: (a) the interaction of each representative grain with two homogeneous equivalent media, withhigh and low dislocation density, respectively; (b) the relative weight of the two media isfunctionally related to their volume fractions; (c) nucleation and disappearance of grains makethe data structure variable in time; (d) the model parameters vary with temperature and strainrate but do not depend on grain size in DRX conditions, and become only temperaturedependent in static conditions (SRX/PDRX/GG); (e) quantitative agreement with experimentalresults is obtained in terms of (i) recrystallization kinetics, (ii) stress-strain curves, (iii)recrystallized grain size, and (f) it is designed to be used in multi-pass conditions, with variabletemperature and strain rate. To verify and validate the model, torsion tests were conducted over a wide range ofconditions for investigation of DRX. Subsequent annealing after termination of deformation,which led to SRX or PDRX depending on the applied strain, was also carried out. The modelparameters were first estimated from experimental and literature data, and were further tunedby inverse analysis. It is found that all identified model parameters evolve with temperature andstrain rate in a physically consistent way. The application of this proposed model to DRX, SRX/PDRX/GG is then analyzed, taking intoaccount the effects of deformation temperature, strain rate, applied strain, as well as initial grainsize. Good quantitative agreements with measured data are obtained in the differentrecrystallization regimes, which opens the possibility of modeling multi-pass operationscompatible with industrial applications. A few in situ heating experiments were carried out to provide a better understanding of theSRX/PDRX/GG mechanisms. The role of annealing twins is tentatively discussed: it seems topromote both nucleation and grain boundary migration.