Ferrite grain refinement by controlled rolling of lovv-carbon and microalloyed steel

Abstract

A C–Mn steel and two microalloyed steels, one containing niobium and the other vanadium, were subjected to controlled hot rolling followed by cooling in air or vermiculite. The experiments were designed to allow differentiation between cases in which transformation started after the end of rolling and those in which it started in the roll gap during deformation. When transformation began after the end of rolling, austenite grains of the C–Mn steel were refined by recrystallization, while in the microalloyed steels austenite grains were elongated due to the absence of recrystallization. In all three steels ferrite grain refinement originated wholly from changes in size or shape of austenite grains and constancy of the ratio of austenite to ferrite grain sizes. Significant change in the transformation mechanism need not be invoked in order to explain the observed degrees of ferrite grain refinement. On the other hand, when transformation began during rolling, more efficient grain refinement was obtained, and an increase in the ratio of austenite to ferrite grain sizes indicated a significant increase in ferrite nucleation rate, relative to growth rate, in all three steels. For the microalloyed steels cooled in vermiculite the extra efficiency of grain refinement was quantitatively explained by additional nucleation on deformation bands when nucleation was concomitant with deformation, and this was probably true for the C–Mn steel. When the steels were cooled in air, however, the additional nucleation sites made a lesser contribution to grain-refining efficiency, probably because they contributed at the expense of intragranular nucleation events normally utilized during transformation at the faster cooling rate. Because of the similar behaviours of the steels when transformation began during rolling, the microalloying constituents appeared not to play any direct role in controlling nucleation on deformation substructure. The nature of the results suggests a strategy whereby the total reduction available in controlled rolling may be used to maximum benefit.