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is not well understood, see section 8.6. In the present investigation an attempt has been made to study whether niobium in solution or as precipitates retard the recrystallization. Discussion of the effect of niobium content on the recrystallization of austenite, section 13.2.1.2,

could offer no conclusive evidence for the mechanism involved, but did show that niobium in solution retarded the recrystallization.

From Fig, 42 it is evident that > 0.46% Nb gave maximum retardation of recrystallization, i.e. when ^0.27% Nb was undissolved, iv 0.05% Nb was strain induced as Nb(C,N) and 0.14% Nb was in solution. In this steel the undissolved precipitates were mainly in the size range 0.10-0.30/cm, Fig* 119(a)t and as discussed earlier this sizes of precipitates cannot nucleate recrystallized grains. However, it can retard the growth of

recrystallized grains and therby retard the recrystallization rate. Strain induced precipitates will coarsen rapidly at 1250°C and the volume fraction will be too low to be effective in retarding recrysta­ llization, Niobium in solution can retard recrystallization by either retarding nucleation or the growth of recrystallized grains.

The effect of niobium in solution on the recrystallization has been investigated by using a 0,29% Nb steel containing less than 0.008/6

total (C+N). In this steel all the Nb(C,N) was in solution above 1100°C. Rolling 50% reduction at 1250°C after reheating at 1300°C gave 0/5CP/o

recrystallization of austenite, Fig, 120 , compared with 10056 ^cry­ stallization in the base steel. This provides positive evidence that niobium in solution retards recrystallization of austenite. To identify the individual effects of niobium in solution on the nucleation and the growth of reciystallized grains, the steel was held after rolling at

1250°C for 100s and 1000s. After 100s holding the austenite was comple- tly recrystallized and further increase in holding time simply coarsened the austenite grain size. This suggest that niobium in solution did not retard the growth of recrystallized grains, and therefore it can be concluded that niobium in solution retarded the recrystallization by retarding' the nucleation of recrystallized grains. This was further confirmed by press forging experiments.

Niobium in solution retards the nucleation of recrystallized grains by retarding the movement of dislocations by;-

(i) Solute-vacancy interaction: This lowers the tendency for dislocation climb and therefore the formation of the sub-grains necessary for nucleation by either sub-grain growth or sub-grain coalescence mechanisms.

(ii) Segregation of niobium and carbon atoms around dislocations: Such segregation of niobium and carbon atoms is known to retard the movement of dislocations and thereby retards the nucleation of

recrystallized grains by the above mechanisms. It should be noted . that the segregation of niobium and carbon atoms at dislocations is a first step towards precipitation on dislocations.

The present work provides unequivocal evidence for the retardation of nucleation by niobium in solution. Although a similar effect of niobium

in solution on nucleation has been reported by Roberts' during dynamic recrystallization and by Coldas et a l ^ ^ during static recrystallization ,these author's did not provide direct evidence*

The above discussion stressed the effect of niobium in solution on the retardation of nucleation, but there exists another mechanism by which niobium can retard nucleation, and this is by strain induced precipita-

/ -i O/C i 7«| \

tion of lvb(C,N) on dislocations 9 . These precipitates are very fine and hence can pin the dislocations. Strain induced precipitation of Nb(C,N) on sub-grain boundaries in the 0,11% Nb steel are shown in Pig. 121.

Also the effect of holding temperature on recrystallization after 50% reduction at 1250°C was investigated to ascertain whether the influence of niobium in solution on reciystallization is dependent on temperature. The holding temperatures used were 1250°C, 1150°C,

1050°C and 950°C for 100s and 1000s, There was a^5 0 % recrystallization

after rolling at 1250°C, but complete recrystallization occurred during holding at 1250°C, 1150°C and 1050°C, On the other hand, holding at

950°C did not lead to complete recrystallization of austenite. This evidence suggests that niobium in solution did not retard growth of the recrystallized grains into unrecrystallized grain, nor would it be expected to do so even at lower temperatures than 1050°Ce The reason for retardation of recrystallization during holding at 950°C is mainly associated with the inhibition of growth of recrystallized grains, by freshly precipitated Nb(C,N). It should be noted that such precipita­ tion in austenite can occur even at this low interstitial content below a/ 1 1 0 0 ° C , The amount of Nb(C,N) precipitated at 1050°C was not

sufficient however to retard the recrystallization.

It can therefore be concluded that although niobium in solution retards recrystallization of austenite by decreasing the nucleation rate, the presence of a fine, uniform distribution of Nb(C,N) is needed to

enharice this effect.

15.2.1,4 The effect of niobium on the austenite grain size. The presence of niobium up to ru0.16% refined the recrystallized

austenite grain size, Pig. 49» immediately after rolling. Steels contai­ ning >0.16% Nb contained un'recrystallized austenite immediately after rolling, but the extent of recrystallization increased during holding at 1250°C. After 1000s hold, it was possible to determine the recrysta­ llized grain size in all the steels, Pig. 49* and niobium up to/vO.73% refined the recrystallized grain size but higher niobium contents caused the recrystallized austenite grain size to coarsen. The effect of niobium in refining recrystallized austenite grain size has also been observed by other worker , and secondary recrystallization in

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niobium treated steels y has also been reported.

At a given starting austenite grain size, fine recrystallized grains can form only when there isjhigh nucleation rate and a low growth rate of the recrystallized grains. An increase in deformation and decreasing deformation temperature increases the nucleation rate and can refine the‘austenite grain size. However, in niobium steels increasing defoli­ ation increases the incubation time for nucleation of recrystallized grains, and hence this decreases the nucleation rate, so that it may be argued that niobium should not refine austenite grain size$. On the other hand, it has also been shown that strain induced precipitates of Nb(C,N) retard recrystallized grain growth, and therefore promote intragranular and interfacial nucleation for recrystallized grains. Thus it is not surprising to have finer recrystallized austenite grain size in niobium treated steels than in niobium free steels.

An increase in niobium increases the volume fraction of strain induced precipitate which,by retarding the growth of recrystallized grains, increases the refinement of recrystallized grains as shown in Pig, 49- At the highest niobium content of 1.03% the recrystallized grains coarsened due to the fact that in this steel the amount of strain - induced Nb(C,N) was very low and also the undissolved precipitates were too coarse to be effective in pinning the grain boundaries. It can be concluded that an optimum amount of niobium would be that which gives maximum amount of strain induced Nb(C,N) and minium undissolved Nb(C,N).

transformed ferrite grain size is also conditional on the inhibition of austenite grain growth. The effect of holding time at 1250°C and 950°C on the grain growth of austenite is shown in Pigs. 50(a) and (b). It can be seen that at 1250°C there was no significant grain growth in steels containing >>0*07% Nb for holding up to 100s, but grain growth occurred in the base and the 0,07% Nb steel. The growth of recrystallized grains was slightly slower in the niobium steel than in base steel due to Nb(C,N) precipitates pinning the grain boun­ daries. At 1250°C in the 0.07% Nb steel,the amount of Nb(C,N) precipi­ tates was small and grain growth was not retarded effectively. On the other hand in the 0.16% Nb steel,where the amount of Nb(C,N) precipi­ tates was large, there was no grain coarsening at 1250°C up to 1000s holding time.

The time dependent growth of recrystallized grains in the base steel was different after 50% and .20% rolling reduction, Pig. 50, there being faster growth after 20% reduction than after 50% reduction. Such

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