Quantitative Relation of Strengthening in SUS 304 Steel due to Deformation-Induced Transformation

Volume fractions of α' and ε' martensite phases induced in tensile deformation in SUS 304 steel at temperature ranges from 20°C to -196°C are investigated by X-ray using the multi-diffraction peak method with an Mo-target. The former is presented well with the relation by the shear-band intersection mechanism and the latter with an exponential function of total plastic strain. The experimental plastic strain and flow stress of a dual phase steel at those temperatures are expressed by the variable volume model equations proposed here. The equation of total plastic strain comprises both components of the austenite and martensite phases and the transformation strain which is larger the larger the total plastic strain and the volume fraction of transformation. The equation of total plastic stress comprises both components of the austenite and martensite phases and a small additive component of the ε' martensite rate and a small stress relaxation due to transformation strain in the austenite phase. Experimental data of these strains obtained from the X-ray residual stress measurement at 20°C have shown that the predicted relations agree excellently with the data.