Microstructure evolution of advanced high-strength TRIP-aided bainitic steel

Abstract

The present work is focused on monitoring the microstructure evolution of the thermomechanically rolled medium-C steel containing Si and Al to prevent the precipitation of carbides. The initial microstructure consists of carbide-free bainite, polygonal ferrite and blocky-type and interlath retained austenite. To monitor the transformation behaviour of the retained austenite transforming into strain-induced martensite, tensile tests were interrupted at particular strains (5 %, 10 %, 15 %, 18.4 %). The amount of the retained austenite determined with XRD and EBSD techniques decreased from 17 % to 6.8 % upon straining, which was related to the transformation-induced plasticity effect. The identification of the retained austenite and strain-induced martensite was carried out using an electron scanning microscope equipped with EBSD (electron backscatter diffraction). It was found that sheet samples are strongly strengthened by the transformation of the blocky retained austenite, whereas the layers and films of the g phase are over-stabilized and they do not transform into martensite. Keywords: AHSS, bainitic steel, retained austenite, thermomechanical treatment, TRIP effect, strain-induced martensite Delo obravnava pregled razvoja mikrostrukture pri termomehanskem valjanju srednje oglji~nega jekla s Si in Al, ki prepre~ita izlo~anje karbidov. Za~etna mikrostruktura je bila iz bainita brez karbidov, poligonalnega ferita ter zaostalega avstenita kockaste oblike in zaostalega avstenita med lamelami. Za spremljanje vedenja zaostalega avstenita pri pretvorbi v deformacijsko induciran martenzit so bili natezni preizkusi prekinjeni pri posameznih raztezkih (5 %, 10 %, 15 %, 18,4 %). Dele`zaostalega avstenita, dolo~en z XRD in EBSD, se je zmanj{al po natezni deformaciji iz 17 % na 6,8 %, kar je posledica vpliva z deformacijo povzro~ene pretvorbe na plasti~nost. Identifikacija zaostalega avstenita in napetostno induciranega martenzita je bila izvr{ena z vrsti~nim elektronskim mikroskopom, opremljenim z EBSD (difrakcija odbitih elektronov). Ugotovljeno je, da se vzorci plo~evine bolj utrjujejo s pretvorbo kockastega zaostalega avstenita, medtem ko so plasti in sloji g-faze bolj stabilizirani in se ne pretvorijo v martenzit.

Improving the high-temperature properties of a new generation of Fe-Al-O oxide-precipitation-hardened steels

“…A model for coarsening in multicomponent systems shows that the coarsening kinetics is proportional to the product of the solubility and the diffusion coefficient of oxygen in the matrix. [16][17][18] As the solubility of oxygen in the matrix is determined by the chemical stability of the oxide, a significant resistance of chemically reported very stable yttrium particles against coarsening. [19][20][21][22] The high-temperature applications of ODS steels are thus rather limited by the existence of grain boundaries allowing diffusional creep and intergranular damage.…”

Using thermomechanical treatments to improve the grain growth of new-generation ODS alloys

“…The processing temperatures are critical during the consolidation step in order to retain the nanocrystalline structure generated during the mechanical alloying and to impede particle coarsening and grain growth. [13][14][15][16][17][18][19] In the present study, the ODS alloys consist of a ferritic Fe-Al matrix strengthened with about 7 % volume fraction of Al 2 O 3 particles. In order to investigate these new groups of materials with a more detailed analysis, this paper concentrates on the mechanical properties of ODS alloys, the microstructure and the phenomenon of recrystallization, leading to an evaluation of the grain growth.…”

Influence of thermomechanical treatment on the grain-growth behaviour of new Fe-Al based alloys with fine Al2O3 precipitates

“…In the present study, the ODS alloys consist of a ferritic Fe-Al matrix strengthened with about 4 % volume fraction of Al 2 O 3 particles. [10][11][12][13][14][15] In order to obtain a more detailed insight into these new groups of materials, this paper will concentrate on both the general microstructure and mechanical properties of ODS steels, the phenomenon of recrystallization and the related microstructural evolutions.…”