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Abstract
To retain a large fraction of austenite in 5 wt-%Mn alloyed transformation induced plasticity (TRIP) steel after short intercritical annealing (IA), the influence of the carbide that precipitates during various heating processes on austenitisation kinetics and amount of retained austenite was investigated. It was found that a considerable amount of Mn rich cementite particles precipitated when heating was interrupted at 550°C for 5 min; they remained undissolved during IA at 650°C, impairing the austenitisation kinetics and decreasing the fraction of retained austenite. In contrast, relatively fine para-carbide particles precipitated extensively, without Mn enrichment, when heating was interrupted at 250°C for 20 min. Both, experimental investigations and numerical simulations, indicate that these para-carbide particles were quickly dissolved during IA, thereby accelerating the austenitisation kinetics and increasing the fraction of retained austenite. In addition, the influence of the heating rate was investigated and is discussed. It is concluded that the carbide particles that precipitate before IA can either positively or negatively influence the fraction of retained austenite in the TRIP steel, depending on whether they dissolve during IA. Moreover, their dissolution kinetics strongly depends on the extent of Mn enrichment of the carbide compared to that of austenite, which can be predicted by the presented numerical simulations on dissolution kinetics of cementite.
Acknowledgements
The authors are grateful to the CISRI-TCS United Open Laboratory, Central Iron & Steel Research Institute in Beijing for permission to use Thermo-Calc and DICTRA software. This research is supported by National Basic Research Program of China 973 (no. 2010CB630800).
