The effect of vanadium on the austenite grain size.

Vanadium in both low and high nitrogen C-Mn steels was found to refine the recrystallized austenite grain size observed immediately after rolling, Fig. 57. This refinement of the recrystallized grain size was also observed in steels containing niobium, and is due to an increased nucleation rate of the recrystallized grains coupled with slow growth of these grains. The slow growth of recrystallized grains was due to i.V\€- pinning effect by strain induced Nb(C,N) precipitates. Whether a similar concept can be applied to vanadium steels to explain the refinement of recrystallized grains by vanadium must be considered,

During rolling at 1250°C, all the V C and VN was in solution in both

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low and high nitrogen steels. Therefore, there was no strain induced precipitation of V(C,N) at 1250°C. This indicates that the growth of recrystallized grains was not inhibited by strain induced precipitates of V(C,N). However, very little time was available after rolHng at

1250°C for the growth of recrystallized grains to occur, indicating less importance of strain induced precipitates in refining the recrystallized grain size, when the grain size was measured immediately after rolling. Another factor by which vanadium can refine the recrystallized grains is by increasing their nucleation rate. In niobium steels, it was shown that new grains nucleated intragranularly and at interfaces of recrystallized grains with the unrecrystallized matrix, which in turn increased the number of nucleation sites, and produced a fine recrysta­ llized grain size. In vanadium steels, on the other hand, nucleation at such sites'may not occur due to the fact that there were no precipitates to inhibit the growth of recrystallized grains into the unrecrystallized matrix, which is a requirement for such nucleation to occur. Vanadium in solution however, does not retard the nucleation as much as does

niobium and therefore in vanadium steels the nucleation rate can increase with an increase in the number of nucleatiop sites or stored energy.

to•retardation of recovery, and thereby, will increase the rate of nucleation, leading to the refinement of the recrystallized grain size. An increase in vanadium in both the low and high nitrogen steels,

refined the recrystallized grain size. This may be due to the increased stored energy at a given deformation condition with increasing vanadium contents.

The presence of high nitrogen in vanadium steels, tended to increase the refinement of the recrystallized austenite grain size, for a given vanadium content and thermo-mechanical treatment, Fig. 57* All the VN was in solution at 1250°C and therefore enhanced refinement of the grain size due to increased nitrogen, cannot be attributed to the increased volume fraction of precipitates. A possible reason for the enhanced nucleation of recrystallized grains in the presence of high nitrogen may be a further increase in stored energy when high nitrogen is present* Such a further increase in stored energy when nitrogen is present in vanadium steels, can be due to:-

(i) V-N interactions around dislocations: .This can retard the movement of dislocations and hence will increase the stored energy. V-N interaction is likely to be stronger than V-C interaction due to higher affinity of vanadium for nitrogen than for carbon.

(ii) Segregation of nitrogen atoms to dislocations: This is a well known phenomena, in which nitrogen locks the dislocations and therefore as mentioned above would be expected to increase the stored energy, but is not likely to be operative at the high temperature involved.

The time dependent growth of recrystallized grains at 1250°C and 950°C in vanadium steels is shown in Figs. 58 and 59* and it can be seen that grain growth occurred at 1250°C but was inhibited at 950°C, At 1250°C all the V.C„ and V_{4 3} N was in solution and hence there was no inhibition

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of grain growth, but at 950 C sufficient precipitation in austenite occurred to inhibit the grain growth,

13.2.4 • Comparison of the effect of. niobium and vanadium.

can retard recrystallization. The basic processes which retard recrysta­ llization are:-

(i) The retardation of nucleation of recrystallized grains; and

(ii) The retardation of the growth of nuclei into the unrecrystallized matrix.

The retardation of nucleation has been shown to occur when the movement of the dislocations is inhibited either by solute-vacancy interaction, by segregation of solute atoms at dislocations or by precipitate forma­ tion at dislocations. Both, the solute-vacancy interaction and segrega­ tion of solute atoms are likely to be more pronounced when the solute atom has a greater atomic size difference with the iron. It is thus possible to explain the effect of niobium and vanadium as solutes on the nucleation process for recrystallization, i.e. niobium which is the larger atom is more likely to retard nucleation than vanadium. This is in agreement with the observations. With regard to precipitation on the dislocations, the Nb(C,N) is much less soluble in austenite than V(C,N) and again it will be the Nb(C,N) which will precipitate in preference to V(C,N), Therefore, niobium is most likely to retard nucleation at much higher temperature than will vanadium.

After nucleation, recrystallization proceeds by the growth of the

recrystallized grains into unrecrystallized matrix. Niobium in solution does not retard this process. Calculations on the solute drag effects exerted by niobium in solution on the moving grain b o u n d a r y , also suggest that niobium in solution cannot retard the growth of recrysta­ llized grains, nor will vanadium.

On the other hand, strain induced precipitates of Nb(C,N) or VN are very fine and hence can retard the growth of recrystallized grains into unreciystallized matrix. Nb(C,N) will precipitate at higher temperature than V(C,N) and therefore niobium additions will retard the growth of recrystallized grains at much higher temperature than vanadium. To inhibit the growth of recrystallized grains into the unrecrystallized matrix over a longer time, it would be necessary to keep the size of the precipitates small. However, the precipitates tends to coarsen during holding, which intum decreases their number and increases their size,

thus giving less effective pinning of grain, boundaries, and leading to an increased rate of recrystallization. At a given temperature, the

coarsening rate of V(C,N) is much faster than that of Nb(C,N), suggesting that vanadium will not be able to retard recrystallization so effectively as niobium.

13.3 Effect of Thermo-Mechanical Treatment on the Transformation