Characteristics of Phase Transformations in Medium- Carbon Nanobainitic Steel of High Hardenability

The article presents the results of phase transformations taking place in steels having a carbon content of 0.55 % and characterised by high hardenability (NBH), intended for the industrial fabrication of sheets. Phase transformations were examined using the dilatometric method under continuous cooling conditions (CCT) and during isothermal holding (TTT), applying results of microstructural observations and those of hardness measurements of dilatometric specimens. The research work involved developing the diagram of phase transformations under continuous cooling conditions, using the cooling rate range of 0.025 °C/s to 50 °C/s and a fragment of a diagram of isothermal phase transformations within the temperature range of 200 °C to 275 °C. The Authors proposed a new type of
isothermal diagram of phase transformations developed for a specific cooling rate from austenitisation temperature equal to v (TTTv), enabling its application in the precise design of industrial  isothermal specimens. The analysis of dilatograms as well as that of microstructural observations and dilatometric specimen hardness test results enabled the identification of experimental parameters applied in the heat treatment of specimens made of the NBH steel used in tensile tests. The research-related tests confirmed information contained in scientific reference publications stating that the presence of retained austenite of blocky morphology did not necessarily result in the low plasticity of nanobainitic steels. After the isothermal transformation into nanobainite at a temperature of 225 °C and that of 235 °C, the steel (NBH) contained approximately 20 % of retained austenite, including blocky austenite transformed into martensite during deformation and characterised by favourable plasticity restricted within the range of 13.6 % to 15.5 % of ultimate elongation in relation to a strength of 1.9 GPa.