Cast piece and sheet of ferritic stainless steel, and method for production thereof

Abstract

The present invention provides a ferritic stainless steel casting and a sheet thereof excellent in deep drawability, punch stretchability and ridging resistance and a method for producing the casting and the sheet. In the present invention, a chemical composition is controlled so that the amounts of C, N, Si, Mn, P and Ti may be reduced to the utmost for securing high workability and, on the basis of the chemical composition, the roping and ridging of a steel sheet product is reduced by adding Mg, thus dispersing Mg containing oxides that accelerate the formation of nuclei for solidification and, resultantly, suppressing the development of coarse columnar crystals in a casting. The present invention is characterized in that the average composition of the Mg containing oxides dispersing in a casting satisfies the following expressions <2> and <3>, 17.4(Al 2 O 3 )+3.9(MgO)+0.3(MgAl 2 O 4 )+18.7(CaO)≦500  <2>, (Al 2 O 3 )+(MgO)+(MgAl 2 O 4 )+(CaO)≧95  <3>.

1 . A ferritic stainless steel casting characterized in that: said casting contains, in mass, 0.001 to 0.010% C, 0.01 to 0.30% Si, 0.01 to 0.30% Mn, 0.01 to 0.04% P, 0.0010 to 0.0100% S, 10 to 20% Cr, 0.001 to 0.020% N, 0.05 to 0.30% Ti, and 0.0002 to 0.0050% Mg, with the balance consisting of Fe and unavoidable impurities, and the value of Σ defined by the expression <1> is 0.70 or less,  
       Σ=0.9Si+8.6P+2Ti+0.5Mn−0.5  <1>;  
       and the average composition of the Mg containing oxides dispersing in said casting satisfies the expressions <2> and <3>,  
         17 . 4 (Al 2 O 3 )+3.9(MgO)+0.3(MgAl 2 O 4 )+18.7(CaO)≦500  <2>, (Al 2 O 3 )+(MgO)+(MgAl 2 O 4 )+(CaO)≧95  <3>,  
       where the chemical components in the parentheses mean mol % of the relevant chemical components, respectively.  
     
     
         2 . A ferritic stainless steel casting according to  claim 1 , characterized in that said casting further contains, in mass, 0.0003 to 0.0050% B and/or 0.005 to 0.1% Al.  
     
     
         3 . A ferritic stainless steel casting according to  claim 1  or  2 , characterized in that said casting further contains, in mass, one or more of 0.1 to 2.0% Mo, 0.1 to 2.0% Ni, and 0.1 to 2.0% Cu.  
     
     
         4 . A ferritic stainless steel casting according to any one of  claims 1  to  3 , characterized in that said casting further contains, in mass, one or more of 0.01 to 0.5% Nb, 0.1 to 3.0% V, and 0.01 to 0.5% Zr.  
     
     
         5 . A ferritic stainless steel casting according to any one of  claims 1  to  4 , characterized in that the average width of the columnar crystals is 4 mm or less at a portion in the depth of one fourth of the thickness of said casting.  
     
     
         6 . A ferritic stainless steel sheet characterized by being produced from a casting according to any one of  claims 1  to  5 .  
     
     
         7 . A method for producing a ferritic stainless steel sheet, characterized by using a casting according to any one of  claims 1  to  5 .  
     
     
         8 . A method for producing a ferritic stainless steel sheet according to  claim 7 , characterized by charging MgO and/or metallic Mg in molten steel at not less than 0.30 kg per molten steel ton in terms of Mg equivalent.  
     
     
         9 . A method for producing a ferritic stainless steel sheet according to  claim 7  or  8 , characterized in that: when a casting is hot rolled, the reheating temperature T1 of said casting is controlled to within the range defined by the expression <4>, said heated casting is subjected to rough rolling of plural passes, thereafter finish rolling of plural passes is finished at a temperature of 850° C. or lower, and subsequently said hot-rolled steel sheet is coiled at a temperature of 700° C. or lower; and thereafter said hot-rolled steel sheet is annealed at a heating temperature T2 in the range defined by the expression <5> and cold rolled, and subsequently said cold-rolled steel sheet is annealed at a heating temperature T3 in the range defined by the expression <6>,  
       1,000 ≦T 1(°  C .)≦−8,714/(log([Ti]×[C] 0.5 ×[S] 0.5 )−3.4)  <4>, −5,457/(log([Ti]×[C])−2.6)≦ T 2(°  C .)≦1,000  <5>, −100−5,457/(log([Ti]×[C])−2.6)≦ T 3(°  C .)≦−5,457/(log([Ti]×[C])−2.6)  <6>.  
     
     
         10 . A method for producing a ferritic stainless steel sheet according to  claim 7  or  8 , characterized in that: a casting is hot rolled; thereafter said hot-rolled steel sheet, without subjected to hot band annealing, is cold rolled at a reduction ratio of 30% or more in a rolling mill equipped with rolls 300 mm or larger in diameter; and, thereafter, said cold-rolled steel sheet is subjected to intermediate annealing at a heating temperature T4 in the range defined by the expression <7>, cold rolled again to a prescribed thickness and, thereafter, subjected to final annealing at a heating temperature T5 in the range defined by the expression <8>,  
       700 ≦T 4(°  C .)≦−50−5,457/(log([Ti]×[C])−2.6)  <7>, −100−5,457/(log([Ti]×[C])−2.6)≦ T 5(°  C .)≦−5,457/(log([Ti]×[C])−2.6)  <8>.