Design of thermo mechanical processing and transformation behaviour of bulk Si-Mn TRIP steel

In the last decade, a lot of effort has been paid to optimising the thermomechanical processing of TRIP steels that stands for transformation induced plasticity. The precise characterization of the resulting multiphase microstructure of low alloyed TRIP steels is of great importance for the interpretation and optimisation of their mechanical properties. The results obtained in situ neutron diffraction laboratory experiment concerning the austenite to ferrite transformation in Si-Mn bulk TRIP steel specimens, displaying the transformation induced plasticity (TRIP), are presented. The advancement of ferrite formation during transformation in conditioned austenite is investigated at different transformation temperatures and has been monitored using neutron diffraction method. The relevant information on transformation proceeding is extracted from neutron diffraction spectra. The integrated intensities of austenite and ferrite neutron diffraction profiles developed during the transformation are then assumed as a measure of the phase volume fractions of both phases in dependence on transformation temperature and austenite conditioning. According to the yielding information on ferrite volume fractions from isothermal transformation kinetics data the thermo mechanical processing of bulk specimen was designed in order to support austenite stabilization through bainitic transformation. The volume fractions of retained austenite resulting at alternating transformation conditions were measured by neutron and X-ray diffraction respectively. The stability of retained austenite in bulk specimens during room temperature mechanical testing was characterized by in situ neutron diffraction experiments as well. Termomehanicka obrada i ponasanje pri transformaciji Si-Mn TRIP celika. Tijekom zadnjeg desetljeca uložen je veliki napor u optimiziranje termomehanickog procesa TRIP celika, tj. plasticnosti celika inducirane transformacijom. Precizno objasnjavanje svojstava dobivene visefazne strukture niskolegiranih TRIP celika je od velike važnosti za intervenciju i optimizaciju njihovih mehanickih svojstava. Prikazani su rezultati dobiveni in situ laboratorijskog eksperimenta difrakcijom neutrona a u vezi transformacije austenita u ferit u uzorcima Si-Mn TRIP celika koji pri tome pokazuju plasticnost induciranu transformacijom (TRIP). Razvijanje feritne formacije tijekom transformiranja u kondicionirani austenit istraživalo se pri razlicitim temperaturama transformacije i praceno je uporabom metode difrakcije neutrona. Relevantna informacija o razvoju transformacije dobivena je iz spektra difrakcije neutrona. Integrirani intenziteti profila difrakcije neutrona austenita i ferita razvili su se tijekom transformacije i onda su uzeti kao mjerilo za volumski udjel obadvije faze u ovisnosti o temperaturi transformacije i kondicioniranje austenita. Prema informacijama o popustanju volumskih udjela ferita dobiveni iz izotermnih podataka o kinetickoj transformaciji projektirana je termomehanicka obrada krupno uzorka radi podrske stabilizaciji austenita kroz transformaciju bainita. Difrakcijom neutrona, odnosno x-zraka, izmjereni su volumski udjeli zaostalog austenita pri promjenjljivim uvjetima transformacije. Svojstva stabilnosti zaostalog austenita u krupnim uzorcima tijekom mehanickog testiranja na sonoj temperaturi obilježena su i in situ eksperimentima s difrakcijom neutrona.