During the 1930s the plant was further developed, turning from raw iron production to steel production using open hearth furnaces; again "Freyr, Brassert & Co." was chosen to supply the plant's design. A pipe foundry was opened in 1936, and in 1938 construction of a steel conversion plant using the Siemens-Martin (Open Hearth) process was begun. The first 60-ton capacity open hearth furnace opened 19 March 1939, additional furnaces were added during the 1940s and early 1950s, and the capacity of the furnaces increased - by 1956 the plant had six furnaces, each of 190-ton capacity. Construction of the plant's first rolling mill ("Walserij West") was begun in the late 1930s. During the World War II occupation the mill was confiscated and shipped to Watenstedt (Salzgitter), Germany where it was installed in the Reichswerke Hermann Göring steel plant. After the war the mill was returned to the Netherlands, and remained in operation until decommissioned in 1992.