EFFECT OF RETENTION AND MECHANICAL STABILITY OF RETAINED AUSTENITE ON TENSILE PROPERTIES IN LOW CARBON-LOW ALLOY TRIPHASE STEEL

A low C-low alloy triphase ferrite-martensite-austenite steel containing 7-18% retained austenite was obtained by composition design and intercritical annealing. The effect of retention and the mechanical stability of the retained austenite on the tensile properties and strain induced martensite transformation in the ratained austenite during tensile deformation were studied by X-ray diffraction, tensile testing, and transmission electron microscopy. The experimental results show that the amount of retained austenite is sensitive to the intercritical annealing regime. The retained austenite of isolated type and is mechanically unstable; the strain induced martensite transformation occurs within the retained austenite, the sequence of transformation being retained austenite (fcc)-->stacking fault (hcp)-->martensite (bcc). The effect of the retained austenite on the tensile properties of the triphase steel is very marked: the ultimate tensile strength, initial and overall work hardening rates, and uniform elongation are enhanced; the yield strength, ratio of yield strength to ultimate strength, total elongation, and section compression are reduced; and the resistance to necking is increased.