Comparative Study of VC, NbC, and TiC Interphase Precipitation in Microalloyed Low-carbon Steels
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UCTION
MICROALLOYED ferritic steels strengthened by B1-structured nano-sized alloy carbide are widely used in industry for its high strength accompanied by reasonable formability. Generally, densely dispersed nano-sized alloy carbide can be obtained by two different processes: precipitation by tempering of supersaturated bainite/martensite, or interphase precipitation during ferrite transformation. The latter is formed by periodic nucleation at the migrating ferrite (a)/austenite (c) interface, featuring its characteristic sheet-like dispersion.[1] Since this phenomenon can occur even by continuous cooling after hot rolling of steel sheet, it has already been commercialized for high-strength sheet steels for automotive application,[2] and attracted increasing research interests in recent years. In the past decades, many studies have been carried out to understand the nature of interphase precipitation in various alloy systems. Honeycombe and his coworkers studied the steels with the addition of strong carbide-forming elements (M), i.e., V,[3–5] Nb[6,7] and Ti,[8] respectively. Based on the variations in inter-sheet spacing and size of precipitates with various alloy compositions and heat treatment conditions, as was
YONGJIE ZHANG, GORO MIYAMOTO, KUNIO SHINBO and TADASHI FURUHARA are with the Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai, Miyagi 980-8577, Japan. Contact e-mail: [email protected] Manuscript submitted June 4, 2020.
METALLURGICAL AND MATERIALS TRANSACTIONS A
quantified through transmission electron microscopy (TEM) observations, they reported that the dispersion of interphase precipitation becomes finer at lower transformation temperature, or with larger amount of microalloying addition. These results are also supported by the works in these years by different groups.[9–11] However, to the best of our knowledge, almost no comparison of interphase precipitation behavior in different types of alloy carbide has been presented in the literature. Our group recently investigated the effects of a/c crystallography,[12] transformation temperature,[13] and alloy composition[14,15] on the dispersion of interphase precipitation in a series of V-added low-carbon steels through the quantification mainly by three-dimensional atom probe (3DAP). It was found that the deviation of a/c orientation relationship from the exact Kurdjumov–Sachs orientation relationship (K-S OR: (111)c// (011)a, 101 c// 111 a) is necessary for interphase precipitation to occur.[12] In addition, the dispersion of interphase precipitation is dominantly determined by its driving force,[13,14] but only slightly influenced by the a growth rate.[15] Compared with the case of V, microalloying additions of Nb and Ti are more frequently applied in industry, mainly due to their relatively lower solubility than V both in a and c, as a result of which larger precipitation strengthening is expected.[1] Therefore, in this study, interphase precipitation behavior of alloy carbide (MC) in low-carbon steels
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