Effect of Microstructure on the Onset Strain and Rate per Strain of Deformation-Induced Martensite Transformation in Q&P Steel by Modeling.

The effect of microstructure on the onset strain and rate of deformation-induced martensitic transformation (DIMT) in Q&P steel is studied by a mean-field micromechanics model, in which the residual austenite ( RA ) and primary martensite ( M ) phases are treated as elastoplastic particles embedded into the ferrite ( F ) matrix. The results show that when the volume fraction of the RA increases with a constant fraction of the M , the onset strain of DIMT increases and transformation rate decreases, in contrast to the case of the RA fraction effect with a fixed F fraction. Increasing the volume fraction of the M postpones the DIMT, regardless of the corresponding change from the RA or F fraction, which is similar to the effect of the RA fraction with the constant M but to a higher degree. Conversely, when increasing the fraction of the matrix F , the onset strain of DIMT increases and the rate decreases, and the effect is greater when the corresponding fraction change comes from the M rather than from the RA . Moreover, when the aspect ratio of the RA increases, the onset strain of DIMT decreases with a gradual increase in transformation rate, in agreement with the experimental observation that the equiaxial austenite is more stable in Q&P steels. However, the aspect ratio effect of the M is opposite to that of the RA , indicating that the lath-shaped primary martensite could protect the austenite from DIMT.