Heat treatment of the TRIP steel with partial substitution of silicon by aluminium
Low alloyed high strength steel utilizing TRIP (transformation induced plasticity) effect achieve interesting combinations of relatively good strengths around 800 MPa and high elongation above 30%. This is the reason why these steels are developed mainly for an automotive industry. Good mechanical properties of TRIP steels are obtained due to the multiphase microstructure of these steels consisting of ferrite, carbide-free bainite and metastable retained austenite. This microstructure can be prepared by two step annealing with the second hold in temperature region of bainite transformation. This heat treatment was carried out in a laboratory furnace in this work using 0.2C-1.5Mn-0.6S-1.5Al steel. Heating temperatures of 770-950 °C were used with subsequent cooling either in water, or in salt bath heated to 400 °C to the temperature of the second hold. The temperature of the second hold varied from 410 °C to 460 °C. Final microstructures were characterised by scanning electron microscopy and mechanical properties were measured by tensile test. Resulting microstructures achieved the strengths of 750 – 908 MPa and total elongation of 33-42%.