Method for producing ultra low carbon steel slab

Abstract

An ultra-low carbon steel slab having a carbon content of about 0.01 mass percent or less is produced by casting at a casting speed of more than about 2.0 m/min using a mold provided with a casting space having a short side length D of about 150 to about 240 mm and an immersion nozzle provided with discharge spouts each having a lateral width d, the ratio D/d being in the range of from about 1.5 to about 3.0. Accordingly, a ultra-low carbon steel slab can be obtained having superior surface quality without performing slab conditioning such as scarfing.

1 - 24 . (canceled)  
   
   
       25 . A method for producing an ultra-low carbon steel slab comprising: 
 providing a continuous casting apparatus comprising a mold having a casting space with a short side length D of about 150 to about 240 mm and an immersion nozzle having at least one discharge spout with a lateral width d, in which a ratio D/d is in the range of from about 1.5 to about 3.0;    introducing molten steel into the mold through the immersion nozzle;    casting the molten steel at a casting speed of more than about 2.0 mm/min with the continuous casting apparatus to produce a ultra-low carbon steel slab having a carbon content of about 0.01 mass percent or less; and    applying a brake using an electromagnetic force to the flow of molten steel by superimposingly applying a static magnetic field and an AC magnetic field to the mold in a direction intersecting the mold thickness with a magnetic field application device provided at an upper portion of the mold including a surface level of the molten steel in the mold,    wherein the immersion nozzle is disposed at a lower side of the magnetic field application device and has an immersion depth of about 200 to about 350 mm.    
   
   
       26 . The method according to  claim 25 , further comprising applying a static magnetic field to the mold in a direction intersecting the mold thickness using a lower magnetic field application device, the lower magnetic field application device being provided at a lower side of the upper magnetic field application device.  
   
   
       27 . The method according to  claim 25 , further comprising oscillating the mold at a frequency of about 185 cycles/min or less.  
   
   
       28 . The method according to  claim 25 , wherein the casting speed is about 2.4 m/min or more.  
   
   
       29 . The method according to  claim 25 , wherein the immersion nozzle is a two-spout nozzle.  
   
   
       30 . The method according to  claim 25 , wherein the ratio D/d is about 2.1 to about 2.9.  
   
   
       31 . The method according to  claim 25 , wherein the ultra-low carbon steel slab is a starting material for a cold-rolled steel sheet for forming outer plates of automobiles.  
   
   
       32 . The method according to  claim 25 , wherein applying a brake using an electromagnetic force to the flow of molten steel is performed by superimposingly applying a static magnetic field and an AC magnetic field to the entire mold in the direction intersecting the mold thickness using an upper magnetic field application device and by applying a static magnetic field to the mold in a direction intersecting the mold thickness using a lower magnetic field application device, 
 the upper magnetic field application device is provided at an upper portion of the mold including a surface level of molten steel in the mold, and the lower magnetic field application device is provided at a lower side of the upper magnetic field application device, and    the immersion nozzle is disposed between the upper and the lower magnetic application devices and has an immersion depth of about 200 to about 350 mm.    
   
   
       33 . The method according to  claim 25 , wherein the molten steel comprises about 0.01 mass percent or less of C; about 0.01 to about 0.04 mass percent of Si, about 0.08 to about 0.20 mass percent of Mn, about 0.008 to about 0.020 mass percent of P, about 0.003 to about 0.008 mass percent of S, about 0.015 to about 0.060 mass percent of Al, about 0.03 to about 0.080 mass percent of Ti, about 0.002 to about 0.017 mass percent of Nb, and 0 to about 0.0007 mass percent of B; and the balance Fe and inevitable impurities.  
   
   
       34 . The method according to  claim 33 , wherein the molten steel comprises 0.0005 to 0.0090 mass percent of C.  
   
   
       35 . A method of producing an ultra-low carbon steel slab comprising: 
 introducing molten steel into a mold having a casting space with a short side length D of about 150 to about 240 mm through an immersion nozzle having at least one discharge spout with a lateral width d, in which a ratio D/d is in the range of from about 1.5 to about 3.0;    casting the molten steel at a casting speed of more than about 2.0 mm/min with the continuous casting apparatus to produce an ultra-low carbon steel slab having a carbon content of about 0.01 mass percent or less; and    applying a brake using an electromagnetic force to the flow of molten steel by superimposingly applying a static magnetic field and an AC magnetic field to the mold in a direction intersecting the mold thickness with a magnetic field application device provided at an upper portion of the mold including a surface level of the molten steel in the mold,    wherein the immersion nozzle is disposed at a lower side of the magnetic field application device and has an immersion depth of about 200 to about 350 mm.    
   
   
       36 . The method according to  claim 35 , further comprising applying a static magnetic field to the mold in a direction intersecting the mold thickness using a lower magnetic field application device, the lower magnetic field application device being provided at a lower side of the upper magnetic field application device.  
   
   
       37 . The method according to  claim 35 , further comprising oscillating the mold at a frequency of about 185 cycles/min or less.  
   
   
       38 . The method according to  claim 35 , wherein the casting speed is about 2.4 m/min or more.  
   
   
       39 . The method according to  claim 35 , wherein the immersion nozzle is a two-spout nozzle.  
   
   
       40 . The method according to  claim 35 , wherein the ratio D/d is about 2.1 to about 2.9.  
   
   
       41 . The method according to  claim 35 , wherein the ultra-low carbon steel slab is a starting material for a cold-rolled steel sheet for forming outer plates of automobiles.  
   
   
       42 . The method according to  claim 35 , wherein applying a brake using an electromagnetic force to the flow of molten steel is performed by superimposingly applying a static magnetic field and an AC magnetic field to the entire mold in the direction intersecting the mold thickness using an upper magnetic field application device and by applying a static magnetic field to the mold in a direction intersecting the mold thickness using a lower magnetic field application device, 
 the upper magnetic field application device is provided at an upper portion of the mold including a surface level of molten steel in the mold, and the lower magnetic field application device is provided at a lower side of the upper magnetic field application device, and    the immersion nozzle is disposed between the upper and the lower magnetic application devices and has an immersion depth of about 200 to about 350 mm.    
   
   
       43 . The method according to  claim 35 , wherein the molten steel comprises about 0.01 mass percent or less of C; about 0.01 to about 0.04 mass percent of Si, about 0.08 to about 0.20 mass percent of Mn, about 0.008 to about 0.020 mass percent of P, about 0.003 to about 0.008 mass percent of S, about 0.015 to about 0.060 mass percent of Al, about 0.03 to about 0.080 mass percent of Ti, about 0.002 to about 0.017 mass percent of Nb, and 0 to about 0.0007 mass percent of B; and the balance Fe and inevitable impurities.  
   
   
       44 . The method according to  claim 43 , wherein the molten steel comprises 0.0005 to 0.0090 mass percent of C.