Precipitation and fine structure in medium-carbon vanadium and vanadium/niobium microalloyed steels
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I.
INTRODUCTION
M I C R O A L L O Y I N G and controlled thermomechanical treatment are effectively used to increase the strength and toughness of low-carbon plate and strip steels, m:,3] The improvements in properties are largely due to very fine ferrite grain sizes produced by the transformation of austenite in which recrystallization has been suppressed by niobium carbonitride, Nb(C, N), precipitation during low-temperature finishing passes in plate or strip hot rolling. I41 Microalloying is now also being applied to medium-carbon bar and forging steels to produce strengths comparable to those of quenched and hightemperature-tempered alloy steels, tSm However, because bar rolling and forging are usually completed at high finishing temperatures, t5"71 the strength enhancement of medium-carbon steels by microalloying is not accomplished by grain size refinement. Rather, strength is due to large volume fractions of pearlite and the precipitation of carbonitrides of microalloying elements, primarily vanadium carbonitrides, V(C, N), as first demonstrated by Gladman et al.IS] The fine microalloy precipitate dispersions in mediumcarbon steels have received limited attention in the literature. In vanadium-containing steels, V(C, N) particles have been found in interphase or row precipitate arrays, not only in ferrite-precipitate colonies having an equiaxed ferrite morphology, but also in the ferrite of pearlite colonies. [9,1~ Niobium, which is known to precipitate on cooling or dissolve on heating in austenite at higher temperatures than vanadium, is sometimes added to medium-carbon steels to limit austenite grain growth S.W. T H O M P S O N , Assistant Professor, and G. K R A U S S , A M A X Foundation Professor and Director, Advanced Steel Processing and Products Research Center, are with the Department of Metallurgical and Materials Engineering, Colorado School of Mines, Golden, CO 80401. Manuscript submitted November 22, 1988. METALLURGICAL TRANSACTIONS A
during high-temperature forging.rSJ Thermodynamic modeling of Nb, V, C, and N solubility in austenite t~21 shows that carbonitride precipitates which form in austenite should be richer in Nb with increasing temperature, a prediction verified in several low-carbon steels by scanning transmission electron microscopy (STEM) analyses. [12] The purpose of the investigation described in this paper was to extend the characterization of the fine structures produced in microalloyed medium-carbon steels. Steels with V and Nb were selected to evaluate the possibility of different precipitate arrays due to the two elements, and the effect of carbon was evaluated by selecting steels with 0.2 and 0.4 wt pct C. II.
EXPERIMENTAL PROCEDURE
The chemical compositions of the alloys are listed in Table I. The alloys are designated 2V, 4V, 2V + Nb, and 4V + Nb. The number indicates the nominal carbon content, and the V and Nb notation indicates the presence of deliberate microalloying additions. All elements other than C and Nb are at approximately constant levels. The alloys were
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