EFFECT OF RETAINED AUSTENITE ON DUCTILITY AND TOUGHNESS OF A LOW ALLOYED MULTI-PHASE STEEL

High performance steels require not only high strength, but also the combination of high ductility, high toughness and good weldability. Retained austenite in multi-phase steels has been widely reported to be helpful for obtaining high strength, high ductility and high toughness. In this work, steel with multi-phase microstructure consisting of intercritical ferrite, tempered martensite/bainite and different volume fractions retained austenite was obtained by intercritical annealing and tempering at 600 similar to 680 degrees C. The volume fractions of retained austenite were 2%, 5%, 10% for samples tempered at 600, 650 and 680 degrees C, respectively. The effect of retained austenite on ductility and toughness was studied in detail. Results showed that there was no obvious change in strength by varying the volume fraction of retained austenite, yield strength of the steel was 540 similar to 590 MPa, tensile strength was 720 similar to 780 MPa. Retained austenite could largely improve both the ductility and toughness of the steel. With increasing the volume fraction of retained austenite from 2% to 10%, the uniform elongation and total elongation were enhanced from 10.3% and 23.8% to 20.4% and 33.8%, respectively. The underlying reason for the improvement of ductility was attributed to the transformation induced plasticity of retained austenite by providing sustainable high work hardening rate. The improvement of toughness by retained austenite became more obvious when testing temperature was lower. When impact test temperature was higher than 60 degrees C, the Charpy impact energy of samples with 2%similar to 10% retained austenite were larger than 120 J. When test temperature was 80 degrees C, Charpy impact energy of sample with 2% retained austenite decreased to 14 J, while that of sample with 10% retained austenite remained as 60 similar to 80 J when test temperature was as low as 80 and 100 degrees C. Results from instrument impact test indicated that retained austenite was helpful for enhancing plasticity before crack initiation at low temperature, leading to improvement of crack initiation energy, resulting in excellent low temperature toughness.