Bainite Formation at Low Temperatures in High C-Si Steel and its Mechanical Behavior

Abstract

A significant amout of stabilized austenite can be obtained in high carbon steel containing high amounts of manganese and silicon (1.5-2 %). At relatively low temperatures the bainite plates formed are extremely thin, making the material very strong. In this study, the influence of the thermal cycle of austempering on the mechanical behavior of a spring steel 0.56C-1.43Si-0.58Mn-0.47Cr (wt. %), with TRIP effect was investigated. The thermal cycle consisted of heating three groups of hot-rolled wire steel at austenite field of 900°C for 300 s, and quickly transferring those to a metallic bath maintained at 200, 220 or 270°C, respectively, for different heat treatment times. The samples were then tested in tension and their microstructures were examined by scanning and transmission electron microscopy. The samples treated at 220°C showed higher elongation, yield strength and tensile strength than those maintained at 200 or 270°C. The high level of strength and ductility is due to a mixture of martensite and very fine bainitic ferrite with interlath film of retained austenite. The temperature has shown a strong influence on bainite formation kinetics. The fracture behavior of the steel was also evaluated using SEM fractography.