Intragranular ferrite nucleation in medium-carbon vanadium steels
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I.
INTRODUCTION
IT has been recognized for many years that proeutectoid ferrite can nucleate at inclusions within austenite grains and that as a result, a fine ferrite-pearlite microstructure can be obtained in steels with a coarse austenite grain size. Recently, this phenomenon has been :receiving great attention for the following reasons. First, the need to improve the toughness of the heataffected zone in welded structural members has increased. Second, in manufacturing structural parts and components of automobiles, the heat treatment after the hot-forging process needs to be eliminated for cost reduction. In both cases, sufficient toughness cannot be obtained only by the conventional austenite grain refinement techniques. Some studies show that intragranular ferrite plates or idiomorphs nucleate at compound precipitates composed of titanium oxide (Ti203), manganese sulfide (MnS), and vanadium nitride (VN). ll,2] In these studies, the precipitates at which intragranular ferrite nucleation actually occurs were identified but the reasons for which such precipitates act as viable sites for intragranular ferrite nucleation were not clarified. According to classical nucleation theory, the following are the key factors controlling ferrite nucleation: (1) the existence of the driving free energy for ferrite nucleation, (2) the large diffusivity of carbon atoms in austenite, and (3) the reduction of the nucleus-matrix interracial energy. I3,4] Especially, the significance of condition (3) is elucidated by the fact that ferrite crystals nucleated at austenite grain boundaries have KurdjumovSachs (K-S) orientation relationships with respect to at FUSAO ISHIKAWA, Researcher, and TOSHIHIKO TAKAHASHI, General Manager, are with Bar, Shape & Wire Rod, Steel Research Laboratories, Technical Development Bureau, Nippon Steel Corporation, Futtsu, Chiba 229-12, Japan. TATSUROU OCHI, Senior Researcher, is with Muroran R&D Laboratory Nippon Steel Corporation, Muroran, Hokkaido 050, Japan. Manuscript submitted July 1, 1993. METALLURGICALAND MATERIALS TRANSACTIONSA
least one austenite grain so as to decrease the interracial energy, especially at lower undercooling, t51 The purpose of this article is to clarify experimentally the conditions under which intragranular ferrite nucleation is enhanced in medium-carbon vanadium steels and to identify the governing factor of intragranular ferrite nucleation in terms of classical nucleation theory. II.
EXPERIMENTAL PROCEDURE
A. Sample Preparation Vacuum-melted, medium-carbon steels containing 0.25 wt pct carbon were used in this study. The chemical compositions of the steels are given in Table I. They were prepared to investigate the effect of the amount of vanadium and nitrogen on intragranular ferrite nucleation. They were hot-rolled into 30-mm-diameter bars. B. H e a t Treatment
Samples of 14-mm width and 60-mm length were austenitized at 1523 K for 180 seconds and subsequently cooled to room temperature at a rate of 0.1 or 0.3 K/s. Some of them were quenched from various temperatures
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