Modeling for onset strain of deformation-induced martensite transformation in Q&P steel by a mean-field Eshelby approach

Based on Eshelby model, a multi-domain incremental mean-field method was revised to investigate the microstructure effect on the onset strain of deformation-induced martensite transformation (DIMT) in Q&P980 steel, which is designed to show good energy absorbability when the car meets with an accident. Inputting elastoplastic numerical constitutive relationships of both particulate retained austenite (RA) and primary martensite innovatively, this model predicts the overall steel stress-strain curve successfully so that it can calculate stress evolution in any constituent phase in the steel during straining. The model is used to eventually examine the onset strain of DIMT determined by a stress criterion of martensite transformation as a function of far-field strain and a numerical relationship between transformation fraction of newborn martensite from DIMT and the applied strain is fitted out to quantify the amount of DIMT in the steel. The effects of volume fraction and aspect ratio of the original RA and primary martensite on the onset strain of DIMT are quantitatively distinguished. The microstructure effect on the rest fraction of the RA during straining is also examined. These results are to supply guidance by varying microstructure of the Q&P steel to achieve high onset strain and low speed in DIMT to protect the RA during automobile manufacture.