This master thesis has the objective to improve the mechanical properties of Super Duplex steel by adding Elkem Grain Refiner (EGR). EGR is commercial grain refiner where the active element is cerium. Cerium is a strong oxide and sulphide former whose inclusions are to act as sites for heterogeneous nucleation during casting. The cerium inclusions will form at low undercooling making it possible to grow equiaxed grains ahead of the solidifying front, reducing the size of the columnar zone normally seen in an ingot. By reducing the columnar zone the steel will have a larger zone of equiaxed, and smaller, grains improving the mechanical properties of the steel and reducing the segregation throughout the ingot. The experiment was performed with S4501 Super Duplex steel provided by Scana Steel Stavanger. The casting was done at Frekhaug stål, Bergen by adding 0.05, 0.075 and 0.1 wt% cerium to 120 kg casts. There were a total of 3 parallels, where 1 was cast at 1525ºC and 2 series at 1540ºC. The as cast structure was significantly refined, the columnar zone was reduced from 22 mm to being absent in the cast with most cerium added. The mechanical results show a linear increase in both yield and ultimate tensile strength with increasing amount of cerium. The elongation increased somewhat, but the impact toughness decreased linearly with increasing cerium content. The casting temperature did not seem to have an effect on the grain refining.