Austenite recrystallization and carbonitride precipitation in niobium microalloyed steels

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I. I N T R O D U C T I O N T H E technological significance of niobium microalloying additions to low-carbon steels cannot be overstated, because of the substantial improvements in toughness and strength which have been made possible through the development of high-strength, low-alloy (HSLA) steels. Niobium carbonitrides which are present at high temperature can improve the resistance of a steel to graincoarsening during austenitizing treatments, and the fine precipitates which form during later processing can also provide precipitation strengthening. However, of most importance is the well-known ability of niobium additions to provide substantial grain refinement during specialized thermomechanical treatments (e.g., controlledrolling of steel plates). Basically, niobium additions retard austenite recrystallization, thereby causing substantial flattening of the austenite grains in the later stages of commercial rolling. The resulting high ratio of (austenite) grain boundary surface area to grain volume enhances the ferrite nucleation kinetics, and a fine ferrite grain size is obtained after completion of the austeniteto-ferrite transformation during cooling. The role of niobium in retarding the austenite recrystallization reaction in HSLA steels has been the subject of considerable interest and controversy during the past 15 years. [1-41 Previous research suggests that the retardation of austenite recrystallization in Nb-steels results from the pinning of austenite grain boundaries and subboundaries by either: (1) niobium carbonitride precipiJ.G. SPEER, Research Engineer, and S.S. HANSEN, Supervisor, are with Homer Research Laboratories, Bethlehem Steel Corporation, Bethlehem, PA 18016. Manuscript submitted May 9, 1988. METALLURGICAL TRANSACTIONS A

tates; (2) niobium atoms in solution in the austenite. Most investigators agree that the dramatic retardation of austenite recrystallization which occurs during commercially controlled rolling results from the strain-induced precipitation of carbonitrides on the austenite substructure. Hansen et al. have suggested that there is a critical precipitate supersaturation which is required for significant retardation to occur, tS] Nevertheless, other investigators argue strongly for the solute-drag effect (e.g., Coladas et al.[61), or for a combination of the two effects. [7,81 The exact mechanism of recrystallization inhibition is of considerable importance in the design of microalloyed steels. For example, if the recrystallization kinetics are controlled by solute-drag, then it should only be necessary to consider the concentration of microalloying elements dissolved in the austenite matrix. On the other hand, if carbonitride precipitation controls the recrystallization kinetics, then the levels of carbon and nitrogen in solution should also be important. These elements influence the thermodynamics and kinetics of the precipitation reaction, and, therefore, would be expected to affect the recrystallization behavior. The purpose of this paper is to contribute to the understa

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