Doctor of Philosophy
Department of Materials Engineering
Abdollah-Zadeh, Amir, The investigation of deformation, recovery, recrystallization and precipitation in austenitic HSLA steel analogue alloys, Doctor of Philosophy thesis, Department of Materials Engineering, University of Wollongong, 1996. http://ro.uow.edu.au/theses/1530
In high strength low alloy steels, the deformation, restoration and precipitation effects which occur during finish rolling in the austenitic condition are difficult to study because of transformation to ferrite and/or martensite on cooling. In the current research program, a series of Fe-31Ni-0.15C analogue alloys (with and without 0.02%Nb), which remain austenitic on cooling to room temperature, were prepared to allow a more detailed study of phenomena occurring during simulated finish rolling.
The kinetics of static softening and precipitation were studied using uniaxial compression testing, hardness testing, optical microscopy, and scanning and transmission electron microscopy. Specimens were deformed in uniaxial compression at a constant strain rate of 0.7sec-1 to strains of 0.25, 0.5 and 0.9 in the temperature range of 850-1000°C, followed by holding at the deformation temperature before quenching.
Retardation of static recrystallization was produced by either niobium in solution and/or strain-induced precipitates of NbC . Static recrystallization in the niobium steel was retarded when niobium was in solution, due to solute drag, by approximately an order of magnitude compared to the niobium-free steel. However, much stronger retardation in the niobium steel was observed when strain-induced precipitates were formed, due to the interaction between dislocations and strain-induced precipitates.
An increase of strain and/or temperature accelerated the rate of recrystallization. Deformation bands and distorted recrystallization twin boundaries were observed in as-deformed structures for all strains. Although grain boundaries were the most potent sites for recrystallization, nucleation at twin boundaries was also observed to occur at a lower rate, after deformation to strains of 0.5 and 0.9. During the early stages of recrystallization, nuclei formed preferentially at serrated boundaries produced by thermally induced boundary migration. The experimental evidence indicated that serrations reached stable sizes and did not progress into recrystallization by strain-induced grain boundaries motion. Rather, recrystallization appeared to proceed from heavily deformed pocket between the serrations by subgrain coalescence.
The formation of bands of recrystallized grains in zones around boundaries at high strains indicated the operation of subgrain growth and/or subgrain coalescence mechanisms. Nucleated grains tended to grow into only one side of the two deformed grains. A heavily twinned structure was observed in growing recrystallized grains and after recrystallization, consistent with the low stacking fault energy of these steels.
Recrystallized grain size decreased markedly with increasing strain and only slightly with decreasing temperature. The recrystallized grain size data were found to be closely predicted by the equations proposed for statically recrystallized austenite in C-Mn and Nb-microalloyed steels. In addition, existing equations for predicting the recrystallization times were found to be consistent with the experimental kinetics data obtained in the present work.
It was observed that the dislocations, subgrain boundaries, and grain boundaries were the preferential sites for strain-induced NbC precipitation. Very small precipitates (
By using an analogue alloy, this work provides the first direct observations of NbC precipitation behaviour in deformed austenite under simulated hot working conditions and confirms, the interactions occurring during finish rolling of low carbon niobium steels, which have been previously inferred using indirect experimental techniques.