On the formation of duplex precipitate phases in an ultra-low carbon microalloyed steel

Abstract

The formation of duplex carbonitride precipitates in an ultra-low carbon Ti-Nb steel, either isothermally annealed or controlled rolled after a high temperature solutionizing treatment, has been studied experimentally and the relative phase stabilities of the precipitate and matrix phases, over the temperature range of the experiments, have been calculated. The austenite and ferrite phases were modelled using the Redlich-Kister-Muggianu interpolation formula and the (Ti XNb 1−X)(C 1−YN Y) phases, which were assumed to be stoichiometric, using the Hillert-Staffansson formalism. Two possible ways in which the duplex precipitates could have been formed were explored: Equilibrium phase separation within a (Ti XNb 1−X(C 1−YN Y) phase or two-step precipitation, the low-temperature precipitate being nucleated on a precipitate which had been formed earlier at a higher temperature. Comparison of the experimental and theoretical results clearly indicates that both the duplex precipitates formed by precipitation in austenite in specimens double-annealed at 1250C and 900C and those formed by controlled rolling specimens to 700C from a solution temperature of 1050C were formed by two-step precipitation processes.