A fine-grained microstructure yields the optimum combination of strength and toughness of steel. Moreover refinement of the as-cast structure can reduce the tendency for hot-cracking during forging and rolling. This paper describes how small inclusions can be used to control the microstructure of steels. These small inclusions (dispersoids) are oxides, sulfides, nitrides and carbides which are in the 1 mm size range and capable of promoting grain refinement during solidification by a process of epitaxial nucleation or in the solid state through intragranular nucleation of ferrite. Such particles are sufficiently small to be harmless from a toughness point of view, but at the same time large enough to act as potent nucleation sites during phase transformation. The dispersoids can either be created by balanced additions of strong oxide and sulfide forming elements to an impure steel melt or be added directly into the liquid steel through a specially designed master alloy containing the nucleating particles. In both cases it is possible to manipulate the steel microstructure in a positive direction, but the latter method, involving the use of a master alloy, has probably a wider industrial application.KEY WORDS: steels; solidification structure; oxides, sulfides; inclusions; nucleation.Therefore, and for the sake on inclusion size control, a number of special "grain refiner alloys" for steels is now under development. First Generation Grain Refiner AlloysIn order to utilize the inclusions for grain refinement purposes, it is essential to have control over their size distribution. An average size of about 1 mm is desirable. This is a compromise between two conflicting requirements. On the one hand, a submicron particle size implies that the dispersoids start to lose their potency because a curved interface increases the associated energy barrier against nucleation. On the other hand, if the particle size is significantly larger than 1 mm the dispersoids may become detrimental to toughness. At the same time the particle number density drops rapidly, which makes grain nucleation at such sites less likely. 1)So far, the new steel developments have been hampered by the fact that the nucleating dispersoids used to control the microstructure evolution must be created within the system as a result of deoxidation or desulfurization reactions. The problem is the uncontrolled coarsening of the inclusions, with subsequent loss of toughness. Elkem has developed 23) a Ce containing ferroalloy that can be added as a cored wire at a late stage during steelmaking reducing the time for coarsening. The advantages of using such a ferroalloy to add Ce contra the addition of pure Ce metal are 1) a higher Ce yield and 2) no oxidation of such a ferroalloy in air. This is a so-called first generation grain refiner. Application of the First Generation Grain Refinerto Austenitic Steels Grain refinement of highly alloyed austenitic steels is important because these steels maintain their solidification microstructure during cooling due to the ...