Abstract: The cellular eutectic carbide is eliminated, the matrix fined and the segregation of elements W and Mo reduced in M2 cast high speed steel after it is metamorphism-treated with modificator RE-Al-N. Also its toughness is greatly increased without increasing its hardness. Thanks to the said treatment the M2 cast high speed steel possesses excellent heat fatigue resistance and high temperature wear resistance, which leads to satis-factory effectiveness of the plug made thereof for seamless steel tube reeling mill.