2021 年 61 巻 2 号 p. 582-590
Bainite transformation and the resultant tensile properties of medium-carbon Si-bearing steels having upper bainite with retained austenite were clarified focusing on the effect of prior austenite grain size. Two different conditions (850°C and 1050°C) for austenitization were adopted to obtain the different prior-austenite grain sizes. The bainite structure was obtained by austempering, and the condition of the isothermal holding was decided according to the Time-Temperature-Transformation diagrams. The refinement of prior-austenite grains brought significant shortening of the incubation time of bainite transformation and the morphology of retained austenite grains occasionally appeared more blocky. Meanwhile, the mean grain size of the retained austenite measured by electron back scattering diffraction (EBSD) analysis did not change largely (~3 µm). The samples that were held isothermally at 400°C for 30 min showed fully bainite structure with much amount of retained austenite whose area fraction was ~40%, and these tensile tests of the both samples revealed high strength (1.4 GPa) with adequate ductility (more than 25%). Significant amount of retained austenite disappeared in the breaking samples and this indicates that the transformation induced plasticity (TRIP) occurs. The dependence of prior-austenite grain size on the deformation-induced martensitic transformation could not be found, while the orientation dependence was clearly detected in both the samples. According to these experimental results, the benefit of the refinement of the prior austenite grains is difficult to be discovered with the improvement of the tensile properties. However, it can be found in the shortening of the heat treatment process.
