Abstract

A combination of physical simulation and laboratory rolling experiments, including thermomechanical rolling and low-temperature ausforming, was conducted for designing a suitable processing route to enable phase transformation from austenite to ultrafine bainite in a medium-carbon steel. Following low-temperature ausforming at 500–550 °C, two different cooling and holding paths were tried in the study: (1) water cooling close to martensite start temperature (300 °C), followed by isothermal holding (route A), and (2) air cooling to 350 °C followed by isothermal holding (route B). For reference, a third sample was directly water-cooled to 300 °C after hot rolling without ausforming treatment, followed by isothermal holding (route C). Field emission scanning electron microscopy and electron backscatter diffraction, as well as x-ray diffraction, were employed for microstructural analysis and correlations with the mechanical properties evaluated in respect of hardness and tensile properties. The low-temperature ausforming and subsequent cooling schedules resulted in the decomposition of austenite into ultrafine bainite and some martensite, while stabilizing a fraction of finely divided, carbon-enriched interlath austenite. Results suggested the development of a novel, multiphase bainite-martensite-austenite microstructure, achieved via low-temperature ausforming and subsequent air-cooling (route B), was beneficial in respect of mechanical properties. Most of the bainitic plates were in the range of 50–200 nm with the occasional presence of coalesced plates as wide as 2000 nm. Despite the differences in the microstructure, the mechanical behavior of non-ausformed samples was not significantly different. The technique paves way for developing medium-carbon nanostructured/ultrafine bainitic steels with high mechanical properties achieved via innovative modification of processing routes including low-temperature ausforming.