Grain Refinement of Nb Steels by Control of Recrystallization during Hot Rolling
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I.
INTRODUCTION
THE object of modern hot-rolling technology (aside from shape change) is to produce uniform fine-grained structures in the as-rolled product to meet stringent strength and toughness specifications. Maximum grain refinement requires an understanding of the importance of a number of process and compositional variables on austenite recrystallization and grain growth. For example, Tanaka e t al. 1 (Figure 1) and Kozasu e t al. 2 have demonstrated the importance of initial grain size and deformation temperature on the critical strain (reduction) necessary for static recrystallization of austenite during hot rolling. The reduction per pass, total reduction, and interpass time also have to be controlled to optimize austenite grain refinement by recrystallization.~ 5 The concentrations of microalloying elements also have marked effects on austenite recrystallization and growth because they form alloy carbides and nitrides during hot rolling that pin grain boundaries. 6 For example, higher Nb levels should promote precipitation of NbCN and thereby inhibit recrystallization at higher temperatures, whereas the removal of soluble nitrogen by Ti additions should prevent the high temperature formation of NbCN and thereby promote recrystallization to lower temperatures. Although these two compositional changes have opposite effects on the recrystallization temperature, they both are potentially useful in producing fine-grained product. Raising the tem-
perature at which recrystallization stops allows control rolling at higher temperatures and therefore lower mill loads. Lowering the recrystallization temperature produces finer recrystallized austenite grains that transform to finer ferrite. To study the effects of these parameters in detail, a laboratory simulation of the hot rolling of slabs to plates was required which duplicated the time-temperature-deformation program to which the plates are subjected, and which allowed the extraction and quenching of the sample at any point in the processing to follow the development of the austenite grain structure. To this end, a technique was developed to simulate plate rolling by modification of a small REDUCTION, 10 2 0
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L.J. CUDDY is Associate Research Consultant, Heavy Products Division, United States Steel Corporation Research Laboratory, Monroeville, PA 15146. Manuscript submitted May 10, 1983.
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