Mold flux for continuous-casting Ti-containing hypo-peritectic steel and method therefor
A mold flux is used in continuous casting of Ti-containing hypo-peritectic steel so as to prevent longitudinal cracks from forming on a surface of a slab. The mold flux contains CaO, SiO 2 , an alkali metal oxide and a fluorine compound as major components. f(1), f(2) and f(3), which are calculated from the initial chemical composition, are (1.1−0.5×T) to (1.9−0.5×T), 0.05 to 0.40 and 0 to 0.40, respectively, if the Ti content of the molten steel (mass %) is T. The TiO 2 content in the melting state during the casting is no more than 20 mass % and the ratio of the first peak height of perovskite to the first peak height of cuspidine in the mold flux film is no more than 1.0.
1. A method for continuous-casting Ti-containing hypo-peritectic steel, the method comprising: continuous-casting hypo-peritectic steel containing 0.1 to 1 mass % of Ti, using a mold flux,
wherein
the mold flux contains CaO, SiO 2 , an alkali metal oxide and a fluorine compound as major components,
a chemical composition of the mold flux before the mold flux is put into a mold satisfies the formulas (1), (2) and (3),
a TiO 2 content of the mold flux in a melting state during the casting is no more than 20 mass %, and
a strength ratio of a film of the mold flux in a solidifying state after the casting is no more than 1.0:
1.1−0.5× T≤f (1)≤1.9−0.5× T (1)
0.05≤ f (2)≤0.40 (2)
0≤ f (3)≤0.40 (3),
wherein in the formulas (1) to (3),
f (1)=( CaO ) h /( SiO 2 ) h (A)
f (2)=( CaF 2 ) h /{( CaO ) h +( SiO 2 ) h +( CaF 2 ) h } (B)
f (3)={(alkali metal fluoride) h }/{( CaO ) h +( SiO 2 ) h +(alkali metal fluoride) h )} (C),
wherein in the formulas (A) to (C),
( CaO ) h =W CaO −( CaF 2 ) h ×0.718 (D)
( SiO 2 ) h =W SiO2 (E)
( CaF 2 ) h =( W F −W Li2O ×1.27− W Na2O ×0.613− W K2O ×0.403)×2.05 (F)
(alkali metal fluoride) h =W Li2O ×1.74+ W Na2O ×1.35+ W K2O ×1.23 (G)
wherein T is a Ti content of molten steel (mass %), W CaO is a CaO content of the mold flux (mass %), W SiO2 is a SiO 2 content of the mold flux (mass %), W F is a F content of the mold flux (mass %), and W Li2O , W Na2O and W K2O are contents of Li 2 O, Na 2 O and K 2 O respectively, which are alkali metal oxides, of the mold flux (mass %), and
wherein the strength ratio of the film means a ratio of a first peak height of perovskite to a first peak height of cuspidine, the ratio being obtained from X-ray diffraction analysis of powder obtained by pulverizing the film of the mold flux.