The precipitation behaviors of sulfides and carbonitrides in V(Ti) bearing C-Mn steels have been calculated using the thermodynamic and kinetic models. The results have been used to study the role of carbonitrides and sulfides as the nucleation sites for intragranular ferrite formation.
The variation of solution behavior of MnS has been thermodynamically calculated in V-bearing C-Mn steels by substituting Ti for V. The use of two different set of solubility data for $Ti_4C_2S_2$ and TiS reported in literatures leads to a completely reversed solution behavior of MnS. The result of experimental observations is in qualitative agreement with the calculated result using the solubility data of Liu and Jonas. The calculated result showed that the solution behavior of MnS becomes retrograde in steels bearing Ti-only and that this is due to the formation of $Ti_4C_2S_2$. Controlled cooling experiments with interrupted quenching technique showed that MnS directly acts as the nucleation site for ferrite and that its nucleation potency depends on the solution behavior of MnS. Namely, MnS rarely acts a nucleation site for ferrite in steels showing a retrograde solution behavior. The addition of Ti has an effect to refine the dispersion of MnS particles and this was primary due to the formation of TiN particles in liquid state which act as the nucleation site for MnS particles during solidification. The MnS particles formed during solid state cooling mostly nucleate along prior austenite grain boundary.
The precipitation kinetics of V-carbonitride$(VC_yN_{1-y})$ in V-bearing C-Mn steel was calculated by taking into account the diffusivity of V along dislocation and the elastic strain energy of dislocation. The calculated result showed that the precipitation rate and mole fraction of $VC_yN_{1-y}$ is the largest at around 900℃. This was in a good agreement with the experimental observation. In the steels subjected to a prior heat treatment for $VC_yN_{1-y}$ precipitation, ferrites were formed both in the matrix of the austenite grain and on the austenite grain boundary, and the ferrite grain size was sensitively depend on the $VC_yN_{1-y}$ precipitation rate. This revealed that $VC_yN_{1-y}$ particles act as potential nucleation sites for intragranular ferrite. A very fine ferrite grain size of 2-3㎛ was obtained in a sample solutionized, water-quenched and precipitation heat treated at 900℃. There are two primary causes for this extensive refinement, the extensive refinement of austenite grain size during reverting i.e. martensite → austenite and the $VC_y_N{1-y}$ particles acting as the potential nucleation sites for intragranular ferrite.
The influence of Nb addition and C content on the precipitation behavior of carbonitride on the austenite grain growth in Ti-microalloyed steels during reheating to the austenite region has been studied using optical microscopy and transmission electron microscopy.
In Nb added Ti-microalloyed steels, carbonitride particles during reheating were coarser than those in Ti only-microalloyed steel because of a faster coarsening rate of carbonitride in Nb added Ti-microalloyed steel. This is believed to be due to the fact that solute solubility in Nb added Ti-microalloyed steel is higher than in Ti steel. So the addition of small amount of Nb to the Ti-microalloyed steel did not significantly changed the GCT of Ti-microalloyed steel. But the GCT of high-Nb containing Ti-microalloyed steel is observed to be higher than that of the Ti steel. The higher volume fraction of carbonitride precipitates is responsible for the high GCT of high-Nb containing Ti-microalloyed steel. Calculated GCT using Gladman`s Model was in good agreement with the measured GCT.
The austenite grain coarsening temperature (GCT) at the center of slab decreases as the alloy carbon content increases from 0.09wt.% to near peritectic composition 0.16wt.%. This is believed to arise from the effect of steel carbon content on the segregation behavior of the interdendritic region. The degree of segregation during continuous cooling is distinctively small in low carbon steel because of two simultaneous effects, namely the faster diffusion rate and fine dendrite size: The faster diffusion rate is because the dendrite in low carbon steel is primarily δ-phase. However the reason for the finer dendrite size is not clearly understood at the present time. The austenite grain size during reheating was observed to be distinctively small in the case of low carbon steel. This is due to the fact that the upper temperature for the austenite single phase region is comparatively low in the case of low carbon steel.
