An alloy steel having particular utility for roll caster shells used in the direct casting of molten aluminum to sheet, consists of, in weight %, 0.25-0.45% C, 1.75-3.75% Cr, 0.75-2.5% Mo, 0.35 to 0.8% V, 0.3-1% Mn, greater than 0.4-1% Ni, 0.02% max P, 0.02% max S, up to 0.35% Si and balance essentially Fe. Roll caster shells fabricated from the steel have excellent toughness and ductility at room temperature to enable a greater shrink-fit with the core. The shells also have good resistance to heat checking. The excellent properties are provided by a controlled amount of Mo 2 C and VC as well as limiting the amount of C in solution.