The γ–α Transformation Kinetics of Low Carbon Steels under Ultra-fast Cooling Conditions

Abstract

Current evolutions in the processing techniques of hot rolled steel sheet, like the development of an ultra fast cooling unit which cools the material after the final rolling step, stretch the domain of the material models used to control these processes. When employing a so-called ultra fast cooling unit an accurate description of the transformation kinetics is needed at cooling rates that are significantly higher than the range in which most austenite to ferrite transformation models have ever been tested. In this paper a physically based transformation model is applied to a set of dilatometer experiments involving four different commercial steel grades and cooling rates from 20 K/s to 600 K/s. The model describes the transformation by means of moving γ–α interfaces in a single austenite grain. Parameters that were varied in the modelling procedure are the degree of undercooling below the A3-temperature, ΔT, at which the transformation is assumed to start, and the intrinsic interface mobility pre-factor, M₀. By analysing the errors in the fit of the calculated transformation behaviour to the experimental data, ranges of optimum fit in the ΔT–M₀ space were determined. In order to determine a unique combination of ΔT and M₀, a physically justified value of ΔT was determined using the classical nucleation theory. The resulting values of M₀ increase with increasing cooling rate and decrease with increasing carbon content.