In order to develop tough ductile ultrahigh strength (TS > 1500 MPa) nanostructured bainite in medium carbon steels, a novel concept based on low temperature ausforming of suitably designed steels followed by bainitic transformation at relatively low temperatures has been considered. Enhanced austenite strength in respect of increased dislocation density by means of low temperature ausforming can prominently provide new nucleation sites, such as deformation shear bands, dislocation substructure, etc. to enable extreme refinement of bainitic ferrite plates. In this study, an attempt has been made to carry out low temperature deformation either in the bay between ferrite and bainite C-curves (500°C) or at still lower temperatures between the bainite (Bs) and martensite start (Ms) temperatures (300-400°C) in order to impart high dislocation densities in the microstructures prior to isothermal holding to enable bainitic transformation. A limiting factor in low temperature ausforming, however, is the substantial increase in flow stress, which may cause increase in rolling loads and hence required evolution of austenite strength at low temperatures.
To evaluate the constitutive flow behaviour of austenite essentially below the no-recrystallization temperature (Tnr), flow stress measurements were conducted as single-hit compression tests in a Gleeble 3800 simulator. The compression tests were performed in the temperature range 300-900°C (in steps of 100°C) to a strain of 0.6 using three different true strain rates: 0.1, 1 and 10 s-1. For given experimental steels, deformation at 0.1 s-1 resulted in a higher flow stress compared to that at the highest strain rate of 10 s-1 at temperatures in the bainite transformation range, obviously as a result of strain induced bainite transformation, marked by noticeable increase in temperature (50-80°C). The low temperature ausformed samples were held at different transformation temperatures for one hour to complete the bainitic transformation, with the desired results mainly achieved at low temperatures in the vicinity of Ms. The paper presents the first account of flow stress and transformation behaviors with details of retained austenite and hardness variation as a function of hold temperature.