PURPOSE: To provide a screw and screw parts excellent in resistance to seizing properties in a correlation between a cylinder and parts by constituting so the manufacturing method of the screw parts that secondary carbide is manufactured in a large quantity not less than the specified amount at an area rate in comparison with the quantity of secondary carbide of material wherein the material having the same chemical components as the heat-treated article of the secondary carbide is hardened at the temperature for obtaining the uppermost as-quenched hardness.

CONSTITUTION: A screw 4 and a check ring 2 are a hardening and tempering heat-treated article and used as screw parts excellent in toughness and resistance to sizing properties for molding plastic. The manufacturing method of the screw parts is so constituted that secondary carbide of ≥0.2μm is manufactured in a large quantity not less than 20% at an area rate in comparison with the amount of secondary carbide of ≥0.2μm of material wherein the material having the same chemical components as the heat-treated article of the secondary carbide is hardened at the temperature for obtaining the uppermost as-quenched hardeness. In such a way, the quantity of primary carbide is regulated to the necessary limit of required wear resistance. Secondary carbide small in adverse effect for toughness is increased from a point of size and a roundish shape. Resistance to seizing properties is enhanced without drastically deteriorating toughness.