Comparison of thermo-mechanical treatment of C-Mn-Si-Nb and C-Mn-Si-Al-Nb TRIP steels

Abstract

Two low alloyed steels with 0.2% C, 1.5% Mn, 0.06% Nb and various silicon and aluminium contents were used in this work. The first steel possessed only 1.8 %Si without aluminium, in the second one the silicon content was lowered to 0.5% and 1.3% aluminium was added. The same methods of thermo-mechanical processing were carried out on a thermo-mechanical simulator and consisted of soaking at the temperatures either in the one-phase austenite region or in the intercritical region, with one compressive deformation applied at the end of the soaking hold and the second one at 720 °C. Two cooling rates, 14 °C/s and 30 °C/s were used to cool the samples to a bainitic hold carried out at 425 °C for 600 s. The final microstructures consisted of various volume fractions of ferrite, bainite, retained austenite and finally martensite and they were analysed using scanning electron microscopy. Good combinations of high tensile strength above 800 MPa accompanied by elongation above 40% were obtained for both steels. Higher amounts of austenite were retained in the aluminium alloyed steel than in the aluminium free steel for the same processing conditions and the final microstructures achieved slightly higher elongation at comparable ultimate tensile strengths.