Method of Production of Ultralow Carbon Cast Slab

Abstract

A method of production of an ultralow carbon cast slab characterized by adding Ti to molten steel decarburized to a carbon concentration of 0.01 mass % or less, further adding at least one of Ce, La, and Nd, and using an immersion nozzle to inject the above molten steel from a tundish to a casting mold for continuous casting while maintaining a flow rate of Ar gas blown from any location in a range from a tundish upper nozzle to discharge ports of said immersion nozzle at 3 Nl (normal liter)/min or less.

1 . A method of production of an ultralow carbon cast slab characterized by adding Ti to molten steel decarburized to a carbon concentration of 0.01 mass % or less, further adding at least one of Ce, La, and Nd, and using an immersion nozzle to inject the above molten steel from a tundish to a casting mold for continuous casting while maintaining a flow rate of Ar gas blown from any location in a range from a tundish upper nozzle to discharge ports of said immersion nozzle at 3N1 (normal liter)/min or less. 
   
   
       2 . A method of production of an ultralow carbon cast slab as set forth in  claim 1 , characterized in that said immersion nozzle has an orifice in its inner bore. 
   
   
       3 . A method of production of an ultralow carbon cast slab as set forth in  claim 2 , characterized in that
 said inner bore has a circular cross-sectional shape and   (i) a relation 3≦R−r≦30 stands between a radius R of a top end of the inner bore [mm] and a smallest radius r of the orifice [mm] and   (ii) a length L from a top end to a bottom end of the orifice [mm] is 50≦L≦150.   
   
   
       4 . A method of production of an ultralow carbon cast slab as set forth in  claim 2 , characterized in that
 said inner bore has an elliptical cross-sectional shape and   (i) a relation 3≦A−a≦30 stands between a radius A of a long-axis direction of a top end of the inner bore [mm] and a smallest radius a of the orifice [mm] and   (ii) a length L from a top end to a bottom end of the orifice [mm] is 50≦L≦150.   
   
   
       5 . A method of production of an ultralow carbon cast slab as set forth in  claim 1 , characterized in that
 said immersion nozzle has a closed bottom cylindrical shape and   (i) two discharge ports are arranged at axially symmetric positions to a cylinder at a bottom part of side walls of the cylindrical shape and   (ii) a slit is provided connecting a cylinder bottom part and bottom parts of the two discharge ports and opening to the outside.   
   
   
       6 . A method of production of an ultralow carbon cast slab as set forth in  claim 5 , characterized in that, in said immersion nozzle,
 (i) portions contiguous with the slit of the cylinder bottom part are inclined upward toward the cylinder side walls   (ii) portions contiguous with the slit of discharge port bottom parts are inclined upward toward discharge port side walls, and   (iii) there is substantially no step difference between the surface forming the cylinder bottom part and the surfaces forming the discharge port bottom parts.   
   
   
       7 . A method of production of an ultralow carbon cast slab as set forth in  claim 6 , characterized in that, in said immersion nozzle, an inclination angle by which “portions contiguous with the slit” of the cylinder bottom part head toward the cylinder side walls and an inclination angle by which “portions contiguous with the slit” of the discharge port bottom parts head toward discharge port side walls are both at least 30° upward. 
   
   
       8 . A method of production of an ultralow carbon cast slab as set forth in  claim 5 , characterized in that, in said immersion nozzle, portions of the top parts of the discharge ports contiguous with the side walls of the cylinder are formed by curved surfaces smoothly contiguous with the side walls of the cylinder. 
   
   
       9 . A method of production of an ultralow carbon cast slab as set forth in  claim 5 , characterized in that said immersion nozzle is provided with a rib connecting the two side surfaces of the slit. 
   
   
       10 . A method of production of an ultralow carbon cast slab as set forth in  claim 5 , characterized in that, in said immersion nozzle, the slit has an opening width of 0.15 to 0.40 of a square root of the cross-sectional area of the discharge port openings. 
   
   
       11 . A method of production of an ultralow carbon cast slab as set forth in  claim 1 , characterized in that, in said immersion nozzle, the side surfaces and bottom part of the cylinder and part or all of the surfaces of the discharge ports and slit contiguous with the melt are formed by a material of any of carbon-less spinel, low-carbon spinel, magnesia graphite, zirconia graphite, and silica-less alumina graphite.