Oxygen Removal and Deoxidation Products of an Aluminium Killed and Calcium Treated Steel with Low Sulfur Contents for Premium Quality

摘要

The production of cold heading steels with 0.34% C for the automotive industry requires an outstanding cleanness in the finished product. To meet these tough specifications the oxygen removal in the ladle after tapping has to be done in such a way that the oxygen content is in order of magnitude of 10 to 20 ppm at the end of the ladle treatment and in the wire rod of the RH degassed heats. In each production stage the measured oxygen activity is reflecting the course of the total oxygen in the metallic system Fe-Al-O depending on the equilibria of the different temperatures. The ladle metallurgy for these steels includes the Ca treatment for the modification of Al_2O_3 into low melting Ca aluminates assuring a good castability. The evolution of the inclusions with regard to size and composition follows these productions stages characterized by Al_2O_3 after tapping and small Ca aluminates during casting in the tundish and in the finished product. In addition the inclusions contain small quantities of MgO and CaS. The cleanness of the semi-finished products was evaluated by ultrasonic inspections acc. to the German SEP 1927 and blue brittle tests indicating an excellent cleanness. The metallographic results of the wire rod acc. to DIN 50602 confirm those of the ultrasonic inspection.