On the origin of equiaxed austenite grains that result from the hot rolling of steel
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AN observation
frequently made during hot rolling experiments of low alloy steel is that when the deformation takes place at rather high temperatures, i.e. > - 1 0 0 0 ~ (1832 ~ the prior-austenite grains that are observed upon quenching immediately after rolling exhibit an equiaxed morphology. It is also frequently observed that the equiaxed prior-austenite grains that are found after rolling are somewhat smaller than those present prior to rolling, i.e. the hot rolling had caused grain refinement of the austenite? -3 It is generally assumed that this new set of equiaxed prioraustenite grains has resulted from the recrystallization of the original grains, and that this recrystallization is a consequence of the hot rolling treatment. The principal goal of the work reported herein was to investigate the nature of the recrystallization which attends high temperature hot rolling and which is responsible for the observed grain refinement of the equiaxed prior-austenite. There are four mechanisms that, individually, could cause an original set of equiaxed grains to be replaced by a second set during a thermomechanical process. These are: a) static recrystallization, b) dynamic recrystallization, c) metadynamic recrystallization, and d) continuous recrystallization. Static recrystallization occurs when both the nucleation and growth of the new grains take place after the deformation. 4 Dynamic recrystallization results when both the nucleation and growth take place during the deformation. 5,6 Metadynamic recrystallization obtains when the nucleation of the new grains occurs during deformation but the growth takes place after deformation. 7,8 Continuous recrystallization is somewhat different from the others in that it is not strictly a recrystallization process but rather an adE. L. BROWN, formerly a Graduate Student, University of Pittsburgh, is now with National Bureau of Standards, Dept. of Commerce, Deformation and Fracture Div., Boulder, CO 80303. A. J. DEARDO is Associate Professor, Metallurgical and Materials Engineering, University of Pittsburgh, 848 Benedum Hall, Pittsburgh, PA 15261. Manuscript submitted July 12, 1979. METALLURGICAL TRANSACTIONS A
vanced form of recovery in which there is no gross movement of high angle grain boundaries. 9,1~Since there are several ways that hot rolling can lead to a final, equiaxed, prior-austenite grain morphology, it would appear quite tenuous to try to assign a mechanism to this process merely on the basis of observations of grain morphology made before and after rolling. The problem becomes even more complicated when considering the rolling behavior of low alloy steel austenite, since, in this case, observations must be made on the transformation products, ferrite or martensite, rather than on the phase of interest, the parent austenite. In an attempt to address the question of which recrystallization mechanism is operative in association with hot rolling, it was decided to investigate the microstructural changes that attend high temperature rolling, and, in particular,
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