Papers by Author: Sang Hwan Lee

Abstract: It is generally accepted that Si promotes kinetics of polygonal ferrite due to thermodynamic factors such as Ae3 and maximum amount of ferrite formed. However, in this study, it was found that the difference between the measured rates of ferrite formation in C-Mn steel and Si added steel was much larger than that expected considering only thermodynamic factors. The classical nucleation theory with pillbox model was adopted to figure out what is the most controlling factor in formation of ferrite. The volume free energy change was calculated by use of the dilute solution model. The diffusivity of carbon (DC) was formulated as functions of C, Mn and Si by using experimental data. It was found that the volume free energy change was still predominant but the kinetic factors such as interfacial energy and the diffusivity of carbon by addition of Si were not negligible at lower undercooling. However, with increasing undercooling, the diffusivity of C was the most effective on the ferrite kinetics, though the ambiguity of treating interfacial energy was not yet clear.

Abstract: The effect of Si on phase transformation was well known in dual phase steels. Si promoted the ferrite transformation and the enriched C in untransformed austenite prohibited the transformation at intermediate temperature range resulting in the formation of lower bainite and martensite at low temperature range. In addition, during continuous cooling with fast cooling rate, it was very hard to differentiate one phase from the others. In order to clarify the effects of Si on the austenite-to-ferrite transformation quantitatively, the start temperatures of bainite(BS) and martensite(MS) as well as ferrite(Ae3) and pearlite(Ae1) were calculated by thermodynamic analysis. LVDT measured by dilatometer and 1st differentiation peaks of LVDT were examined with microstructures, which gives a possibility of the phase separation. In CCT diagrams, it was also found that large austenite grain size(AGS) widened the gap between the transformation start(Ts) and end(Tf) when Si was added.

Abstract: The dilute solution model is quite widely used because the chemical potential is more easily defined than that in the sub-lattice model. In the present study, the thermodynamic model for the Fe-Mn-Si-Nb-Ti-V-C system was conducted by evaluating Wagner interaction parameters. The data used in this work was collected and modified by means of TCFE 2000-database in Thermo-Calc and up-to date references. The relationship of interaction parameters(L) in the sub-lattice model and Wagner interaction parameters in the dilute solution model was derived. The composition dependency of reference state and the higher order interaction parameters of the excess Gibbs energy were considered to evaluate Wagner interaction parameters. The equilibrium compositions of austenite and fractions of phases and the dissolution temperature of precipitates(NbC, VC, and TiC) were evaluated by the dilute solution model and compared with the results by the sub-lattice model.

Abstract: The quantitative analysis of precipitates in ferrite was investigated using the fact that the formation of precipitates in Nb, V and Ti added steels accelerated austenite/ferrite transformation. The major factors on austenite/ferrite transformation were cooling rate, austenite grain size, and deformation. The slower cooling rate, the smaller austenite grain size and the higher deformation accelerated austenite/ferrite transformation. In 0.34wt%V micro-alloyed steel, the influence of cooling rate and austenite grain size on austenite/ferrite transformation was investigated without deformation. In addition, the experimental method was set up to measure the amount of precipitates in ferrite by the transformation dilatometer. The amount of precipitates was controlled by the holding temperature and time in ferrite. Then, the specimen was inversely transformed without the formation or dissolution of precipitates. Volume fractions transformed were measured by dilatometer during cooling. Iso-precipitation kinetics was determined by comparing the mean transformation temperatures at various conditions, respectively. The result was compared with the calculated.