The effect of V-Ti composite addtion on the 2nd phase(manganese sulfides, vanadium carbonitrides and titanium carbonitride) precipitation and ferrite grain growth behavior have been studied by applying controlled cooling rate experiment.
The composition of specimen used experiment was designed in accordance with equilibrium calculation between 2nd phases, and the basic composition was 0.15C-1.5Mn. Also, V/Ti contents were varied.
The ferrite grain was fined with addition of Ti and grain size of the alloy added both V and Ti was finer than that of alloy added only Ti. This is relative to refinement of MnS with addtion of Ti.
In the alloy added Ti (both only and composite), a number of MnS particles were complex precipitates with TiN. It seems that TiN plays role of heterogeneous nucleation site of MnS, therofore controls growth of MnS particles. The mole fraction of Mns is decreased with only addition of Ti because of precipitation of T$i_4$C_2$S_2$.
In the alloy added both V and Ti, when rapid cooling rate, ferrite grain size was finer. It seems that MnS particles didn't grow because of the lack of time to do. Therefore the finer MnS was, the finer ferrite grain size was.
Interrupted Quenching experiment during cooling show the temperature of formation of precipitates. TiN was formed in high temperature but V-carbonitrides were low temperature relatively. V-carbonitrides exist in complex precipitates with MnS likewise TiN. In the alloy cadded both V and Ti, they exist in MnS-TiN-VCN complex precipitates.
In also case of holding experiment at 900℃ for 50min. in order to promote precipitation of VCN, ferrite grain size of alloy added both V and Ti was the finest, and that of V free alloy was finer than that of the alloy added only V. In addition, ferrite grain of held specimen at 900℃ was coarser than that of non-held specimen at 900℃.
In all case, the alloy added both V and Ti have the finest ferrite grain, and the faster cooling rate is, the finer ferrite grain is. Also MnS particles size is the finest according to addition of both V and Ti.
