Microstructure Analysis and Mechanical Properties of Low Alloyed Steel with Retained Austenite Obtained by Heat Treatment

Abstract

Four different heating temperatures in the range of 770 °C - 950 °C were used for laboratory heat treatment of low carbon low alloyed steel. Chemical composition of the steel was based on the most common TRIP steel concept, only the silicon content was lowered to 0.6% and it was partially replaced by 1.4 % of aluminium. The steel was further micro-alloyed by niobium. Two different ways of cooling were applied to the samples. The first set was cooled to 425 °C in a salt bath with the temperature of 200 °C, the second set was cooled to 425 °C in a salt bath heated to the temperature of 400 °C. In this way, two distinctive cooling rates were achieved for every soaking temperature. Once the samples reached 4250 °C, they were in all cases removed to the furnace for 20 minute hold at the temperature of 425 °C. The final cooling was carried out in air. Resulting microstructures were analysed by scanning electron microscopy and consisted of various amounts of ferrite, bainite and retained austenite. Tensile strength in the range of 750 - 908 MPa was obtained with total elongation of 33-42%.