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I.

INTRODUCTION

THE role

of microalloying elements in improving the strength and toughness of high-strength, low-alloy (HSLA) steels has been the subject of numerous investigations during the past 15 years (e.g., References 1 through 3). The alloying elements of interest are typically niobium, vanadium, and titanium, either individually or in combination. These elements precipitate in austenite and ferrite as fine carbides, nitrides, or carbonitrides, and contribute to mechanical properties via grain refinement and precipitation hardening. Carbonitride precipitates can greatly improve the resistance of a steel to grain coarsening during hightemperature austenitizing treatments. In addition, carbonitride precipitation in austenite during controlled rolling is used to retard austenite recrystallization, which results in substantial flattening of the austenite grains in the later stages of commercial rolling. The resulting high ratio of grain boundary surface area to grain volume enhances the ferrite nucleation kinetics, and a fine ferrite grain size is obtained after completion of the 3 / ~ a transformation during cooling. Previous research in the area of austenite recrystallization behavior suggests that the retardation of austenite recrystallization kinetics results from the pinning of austenite grain boundaries by either (1) alloy carbonitride precipitates, or (2) microalloying elements in solution in the austenite. Although the exact mechanism of recrystallization

J.G. SPEER and J.R. MICHAEL, Research Engineers, and S.S. HANSEN, Supervisor, are with the Research Department, Bethlehem Steel Corporation, Bethlehem, PA 18016. Manuscript submitted March 17, 1986. METALLURGICALTRANSACTIONS A

retardation is controversial, most investigators agree that the dramatic retardation of austenite recrystallization which occurs during commercial thermomechanical processing results from the precipitation of alloy carbonitrides. 4-8 Obviously a thorough understanding of carbonitride precipitation behavior is necessary in order to control the response of microalloyed steels to thermomechanical processing. The solubility of carbonitrides is fairly well understood in commercial steels containing a single microalloying addition. Experimental determinations of carbonitride compositions (viz., the C/N ratio in the precipitates) are generally lacking, but theoretical models have been developed to describe these systems. 9-~3In systems containing mixed carbonitrides, very few observations have been made. Crooks et al. first showed that the carbonitrides which form in Nb/V steels contain both Nb and V. 14 Mehta et al. measured the Nb and V concentrations in HSLA steels containing relatively high aluminum levels, and found that the niobium concentration in the precipitates increased with time during isothermal annealing. 15 Michael has suggested recently that the vanadium level in these precipitates decreased as nitrogen was tied up with aluminum during the (relatively sluggish) precipitation of A1N. 16Modeling of the thermodynamics o