Development of Low Aluminum and Nitrogen Continuous Cast Steels for Bar and Wire Rod

Continuously cast low Al and N steels have been developed as mild steels for bar and wire rod applications. Al content is controlled, using R-H degassing process, from 0.003 to 0.01% not only to minimize Al2O3 inclusions but also to prevent the subsurface blow holes in the bloom section. N content is also reduced under 0.003 % by the hard-blowing of LD process and shrouding during casting.Compared with rimmed steel wire rod, this new steel shows better drawability and higher ductility of wire at the high speed continuous drawing. On the cold forming test the surf face and center of this steel has shown the higher ductility. At higher annealing temperature the, ferrite grain of this Al-killed steel tends to grow suddenly. B addition is effective to the prevention of the abnormal grain growth and brittle behavior of annealed products. Because of the high cleanliness and this segregation of continuous cast bloom, the size and amount of inclusions are smaller and hence the machinability of this steel is poor. Higher S content is desirable for improving it. In addition, the depth of carburization is relatively small. However B addition increases the austenite grain size and hardenability of carburized products.