Abstract: Particulate casting material is applied onto the upper surface of molten steel in a vertical continuous casting mold where it rapidly melts to form a protective flux-lubricant molten glass covering which also flows down between the solidifying cast steel and the mold wall. Casting material is a substantially chemically homogeneous, pre-reacted, but substantially unfused mixture of multiple crystalline phases and preferably in a thorough blend of the mixture with up to 10% by weight finely divided graphite. The mixture: (a) analytically consists essentially of, by weight thereof, 20-45% SiO.sub.2, 10-40% CaO and/or SrO and/or BaO, 0-10% Al.sub.2 O.sub.3, 0-11% Fe.sub.2 O.sub.3, 5-25% Na.sub.2 O and/or K.sub.2 O, 0-15% MgO and 3-15% F ion; (b) exhibits not more than 2% weight loss upon heating thereof at 900.degree.C. for 1 hour; and (c) uniformly melts at temperatures of molten steel to form a fluid glass having the following viscosity characteristics: 1-80 poises at 1100.degree.C., 0.1-3 poises at 1500.