Duplex Heterogeneous Nucleation Behavior of Precipitates in C-Mn Steel Containing Sn
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he main modes of alloy solidification nucleation are homogeneous and heterogeneous nucleation. Heterogeneous nucleation starts with the formation of the absorbed layer of the nucleated phase on the substrate and the cavities in the substrate that act as nucleation sites. The interfacial free energy at the nucleating interface is the controlling factor in heterogeneous nucleation behavior.[1–5] Alloy solidification is mainly nucleated by heterogeneous nucleation because, more or less, the actual liquid alloy always contains some solid particles. The addition of small precipitates with high melting points to a supersaturated liquid alloy can greatly increase the rate at which crystals heterogeneously nucleate and thus lead to improvement of organization and performance. Extensive studies on heterogeneous nucleation have mainly involved the substrate nucleation, such as intragranular ferrite on inclusions or precipitates. For example, the concept of oxide metallurgy was first proposed by Takamura and Mizoguchi.[6] Based on this concept, non-metallic inclusions such as oxides are regarded as nucleation sites for intragranular acicular ferrite.[7,8] Various studies[9–11] have shown that B1-type carbonitride and carbide nucleate at the MnS inclusions and oxides of Ti and rare earth elements. However, the nucleation behavior
GUILIN SUN and SUFEN TAO are with the School of Metallurgical Engineering, Anhui University of Technology, Maanshan 243002, China. Contact e-mail: [email protected] Manuscript submitted October 9, 2017. METALLURGICAL AND MATERIALS TRANSACTIONS B
of precipitates at high melting point inclusions or precipitates that nucleated at precipitates and inclusions with a higher melting point, has not been investigated. Once the appropriate higher melting point inclusions are formed, they would act as effective nuclei for low melting point inclusions, so that hazards caused by low melting point metals such as tramp element can be prevented. Tramp elements, such as Sn, cannot be removed by the currently used techniques,[12–14] and segregation of Sn along the proeutectoid ferrite–austenite interface or the austenite grain boundaries may cause deterioration of hot ductility. Various studies[15–17] have shown that Sn could nucleate at some suitable particles, such as MnS and Fe4N; however, the heterogeneous nucleation rate of Sn was low. If appropriate second-phase particles or precipitates are observed, the heterogeneous nucleation of Sn would increase, while the hot ductility of steel would decrease. In this study, we successfully developed a C-Mn steel with the Sn phase effectively precipitated on the MnS inclusions that precipitated on the Al2O3 inclusions. The morphologies and element distribution of the precipitates were characterized by transmission electron microscopy (TEM). The continuous double heterogeneous nucleation behavior of Sn is discussed below. The test steel doped with Sn was smelted in a Mo wire resistance furnace. The chemical position of the prepared steel is as follows (wt pct): 0.14 C, 0.33 Si, 1.2
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