Abstract
Refinement of as-cast structures is one of the most effective approaches to improve mechanical properties, formability, and surface quality of steel castings and ingots. In the past few decades, addition of rare earths (REs), lanthanum and cerium in particular, has been considered as a practical and effective method to refine the as-cast steels. However, previous reports contained inconsistent, sometime even contradictory, results. This review summaries the major published results on investigation of the roles of lanthanum or/and cerium in various steels, provides reviews on the similarity and difference of previous studies, and clarifies the inconsistent results. The proposed mechanisms of grain refinement by the addition of lanthanum or/and cerium are also reviewed. It is concluded that the grain refinement of steels by RE additions is attributed to either heterogeneous nucleation on the in-situ formed RE inclusions, a solute effect, or the combined effect of both. The models/theories for evaluation of heterogeneous nucleation potency and for solute effect on grain refinement of cast metals are also briefly summarized.
Highlights
Fine-grained microstructure leads to improved formability and to higher strength and toughness of steels
The type of rare earths (REs) sulfides is related to the ratio of RE content to S content, i.e., the i.e., the value of [RE]/[S], in the molten steel
The addition of lanthanum or/and cerium in steel leads to the increase in the proportion of equiaxed crystal zone or the reduction in the average grain size or both
Summary
Fine-grained microstructure leads to improved formability and to higher strength and toughness of steels. Grain refinement is promoted through heterogeneous nucleation by the addition of a master alloy containing inoculant particles This is commonly referred to as inoculation treatment and is considered to be the most cost-effective and practical technique to produce small equiaxed grains in a metal casting process [14,15]. When carbon content is between 0.09 wt% and 0.17 wt%, primary δ-ferrite directly forms from the liquid followed by a peritectic reaction (δ + liquid→γ), and δ-ferrite transforms to γ-austenite. In these two cases, the nucleation and growth of δ-ferrite define the size of austenite. The development of effective techniques for grain refinement of steels should use δ-ferrite and γ-austenite as two different targets
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