PURPOSE: To suppress formation of coarse metal carbides and to stably produce castings of a high-carbon alloy steel having excellent mechanical properties by forming molten metal into a half solidified slurry form of solid-liquid mixed phases and casting such molten metal at the time of casting the melt of the high-carbon alloy steel.

CONSTITUTION: While the melt of the high-carbon alloy steel, such as high-speed steel, contg. 0.4 to 1.5% C and ≥1% in total of one or ≥2 kinds of alloy elements, such as Cr, Mo, W and V, is previously kept stirred at a low temp. prior to casting, the melt is made into the half-solidified slurry in which the non-dendrite primary crystal grains are uniformly dispersed and suspended wherein the solid and liquid of a solid phase rate 0.15 to 0.6 coexist and, thereafter, such molten metal is cast at the time of producing steel products by casting the melt of the high-carbon alloy steel. The solidification progresses in the uniformly dispersed state of the primary crystal grains and, therefore, the metal carbides formed at the grain boundaries are made uniform and fine and the formation of the coarse metal carbides does not arise. The castings of the high-carbon alloy steel having the excellent mechanical properties are consequently obtd.