Structure and properties of advanced fine grained steels produced using novel thermal treatments

Abstract: Fine grained advanced steels exhibit favourable mechanical properties for applications requiring high strength, ductility and impact toughness. These properties result from a microstructure containing a fine distribution of several phases including ferrite, austenite, martensite and bainite. The bainite phase is in the form of fine lamellas of ferrite and carbon-enriched austenite which due to proper control of the chemical composition is lacking the nanometre scaled carbides associated with traditional bainite. The mechanisms of bainite phase transformations in steels have been debated since the naming of bainite in 1934, and range from diffusion-controlled (reconstructive) to diffusionless (displacive). Interest in the manufacture and application of fine grained advanced multiphase steels can be dated back to the 1970s, and it has been intensified after the turn of the century with the industrial production and application of such steels. The structure and mechanical properties of fine grained advanced steels produced using novel thermal treatments are described. The results of in-situ x-ray diffraction studies of the austenite to bainite transformation process provide information about the effect of carbon redistribution on the formation of transformation products. Transformation microstructures created by various thermal treatments are characterised using optical and scanning electron microscopy, which reveal the presence of martensite, bainite (in various forms), ferrite and retained austenite. Microstructural control is found to be possible by quenching the steels from the austenite phase to temperatures below the start temperature of martensite formation (determined by steel composition). The quenching is followed by isothermal treatment for varying times at temperatures related to the formation of transformation products (bainite in particular). This combined heat treatment also increases the rate of phase transformation in comparison with isothermal treatments. Thus multiphase microstructures are produced, which are found to possess favourable mechanical properties, in particular tensile strength, toughness and wear resistance. In press hardening of 0.26wt-carbon steels, by using the combined heat treatment described, the yield strength was found to be comparable to existing materials and the ductility was found to be higher. Welding tests of medium carbon steel with control of the post weld time temperature cycle in accordance with the combined heat treatment process minimized the risk of brittle phase formation in the weld as well as the heat affected zone. In addition the wear resistance in sliding as well as in rolling-sliding was shown to be better for austempered medium carbon high-silicon steels with fine grained ferritic austenitic microstructure in comparison with conventional steels. The presented work can be regarded as a contribution to the current world-wide effort to develop the 3rd generation advanced high-strength steels, which are expected to be a substantial part of the structural steel market before the end of the present decade. The switch to such steels in a large number of applications in manufacturing of transport vehicles and other products, will be comparable with the switch from plain carbon steels to HSLA steels in the seventies and eighties.