Effect of cerium sulphide particle dispersions on acicular ferrite microstructure development in steels

Abstract

Of the phases found in wrought steels, cerium sulphide particles are notable in that they can be both stable at liquid metal temperatures and exhibit good lattice coherency with α iron. An investigation has been carried out to determine the effectiveness of cerium sulphide particle dispersions in nucleating intragranular acicular ferrite microstructures in steels. Vacuum melts of 50 kg have been manufactured of appropriate base steel compositions with varying additions of cerium (0·04–0·18%) and sulphur (0·01–0·04%). The work has shown that 0·02–0·12% cerium and 70–340 ppm sulphur may be retained in steels after deoxidation and desulphurisation reactions while oxygen can be reduced to <20 ppm. Resulting inclusions are largely spheroidal in shape and consist of several crystalline compounds, notably CeS, Ce3S4 and Ce2O2S. The inclusion numbers are of the order of 0·68–6·12 × 106 mm−3 with mean diameters of 0·63–1·70 μm. The densities of these inclusion dispersions are approaching those in weld metals where acicular ferrite is the dominant microstructure constituent. Significant volume fractions of acicular ferrite (up to 65%) have been obtained in steels after thermally cycling in a dilatometer and cooling at rates simulating transformation conditions ranging from high heat input welding to air cooling of forgings and water cooling of plate. A potential beneficial effect of acicular ferrite on mechanical properties in high heat input welding (heat affected zone grain refinement) and in thermo-mechanically processed steels (relaxed schedules) has been highlighted. A pilot plant billet cast of steel has shown the feasibility of achieving the required particle dispersions and acicular ferrite microstructure in tonnage steelmaking.