Microstructure control of steels through dispersoid metallurgy using novel grain refining alloys

This paper describes how small inclusions (designated dispersoids) can be used to control the microstructure of steels. The term "dispersoids" refers to oxides, sulphides, nitrides and carbides which are in the 1 mu m 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 sulphide 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. The direction of the research now being undertaken at NTNU/SINTEF to make the grain refining alloys commercially available is briefly described towards the end of the paper.