Mold flux for continuously casting steel and method of continuously casting steel using the same

Abstract

Mold flux for continuously casting steel which contains SiO 2 , T.CaO, Al 2 O 3 and MgO as main components, one or two or more of alkali metal oxides, and further the constituent F, while the ratio of the T.CaO content to the SiO 2 content being from 0.7 to 2.0, the Al 2 O 3 content being 35% or less, the MgO content being 20% or less, the total content of alkali metal oxides being 8% or less, the F content being 7% or less, characterized in that each of the contents expressed by formulae (b) to (e) below satisfies the formula (a): 0.63+2.51×Y Al2O3 ≦Y CaO /Y SiO2 ≦1.23+2.51×Y Al2O3 . . . (a), Y SiO2 =X SiO2 /(X SiO2 +X CaO +X Al2O3 +X MgO ) . . . (b), Y CaO =X CaO /(X SiO2 +X CaO +X Al2O3 +X MgO ) . . . (c), Y Al2O3 =X Al2O3 /(X SiO2 +X CaO +X Al2O3 +X MgO ) . . . (d), and Y MgO =X MgO /(X SiO2 +X CaO +X Al2O3 +X MgO ) . . . (e). Continuous casting of a round billet using this mold flux can effectively prevent the generation of pin-hole defects and longitudinal cracking.

1 . Mold flux for continuously casting steel which contains SiO 2 , T.CaO, Al 2 O 3  and MgO as main components, one or two or more of alkali metal oxides, and further constituent F, while, by mass %, the ratio “T.CaO/SiO 2 ” of the T.CaO content to the SiO 2  content being from 0.7 to 2.0, the Al 2 O 3  content being 35% or less, the MgO content being 20% or less, the total content of alkali metal oxides being 8% or less, and the F content being 7% or less, wherein each of the contents expressed by formulae (b) to (e) below satisfies the formula (a):
   0.63+2.51 Y Al2O3 ≦Y CaO /Y SiO2 ≦1.23+2.51 Y Al2O3    (a)
 
   where, 
   Y SiO2 =X SiO2 /(X SiO2 +X CaO +X Al2O3 +X MgO )   (b)
 
   Y CaO   =X   CaO /(X SiO2 +X CaO +X Al2O3 +X MgO )   (c)
 
   Y Al2O3 =X Al2O3 /(X SiO2 +X CaO +X Al2O3 +X MgO )   (d) and
 
   Y MgO =X MgO /(X SiO2 +X CaO +X Al2O3 +X MgO )   (e).
 
 
       In addition,
   X CaF2 =(W F −1.27 W Li2O −0.613 W Na2O −0.403 W K2O )2.05   (f),
 
   X CaO =W T.CaO −X CaF2  0.718   (g),
 
   X SiO2 =W SiO2    (h),
 
   X Al2O3 =W Al2O3    (i) and
 
   X MgO =W MgO    (j),
 
 
       given that W T.CaO  denotes the T.CaO content, likewise, W F  as the F content, W Li2O , W Na2O  and W K2O , respectively, as the contents of Li 2 O, Na 2 O and K 2 O that are alkali metal oxides, W SiO2  as the SiO 2  content, W Al2O3  as the Al 2 O 3  content, and W MgO  as the MgO content, in terms of mass %. 
     
     
         2 . The mold flux for continuously casting steel according to  claim 1 , wherein the mold flux further contains one or more of TiO 2  and ZrO 2  and the total content thereof is not more than 8 mass %. 
     
     
         3 . A method of continuously casting steel, wherein a round billet is cast using the mold flux for continuous casting according to  claim 1 . 
     
     
         4 . The method of continuously casting steel according to  claim 3 , wherein when secondary cooling is applied to cast steel by using cooling water, the specific amount of the cooling water is set in the range of 0.2 to 1.6 L/kg. 
     
     
         5 . A method of continuously casting steel, wherein a round billet is cast using the mold flux for continuous casting according to  claim 2 .