Composite steel part and manufacturing method for the same

A manufacturing method for a composite steel part formed by welding a plurality of steel parts to each other, comprising:

• manufacturing a first steel part, which includes a cylindrical portion formed in a cylindrical shape and a flange portion provided to extend radially outward from one end of the cylindrical portion, the cylindrical portion being a carburized quenched portion which has been subjected to a carburizing quenching hardening process and the flange portion including a welding expected portion to be welded to a second steel part, by
• preparing an intermediate product in which an extra portion, which has a thickness equal to or more than that of a carburized layer to be formed in a subsequent carburizing step, has been added to the welding expected portion, and
• performing
• the carburizing step in which the intermediate product is heated to an austenitizing temperature or more in a carburizing atmosphere to form the carburized layer on a surface of the intermediate product,
• a cooling step, subsequent to the carburizing step, in which the intermediate product is cooled at a cooling rate less than a cooling rate at which martensitic transformation is caused and in which the intermediate product is cooled to a temperature equal to or less than a temperature at which structure transformation due to the cooling is completed,
• a quenching step in which a desired portion of the cylindrical portion of the intermediate product is heated to an austenitizing range by high-density energy and thereafter cooled at a cooling rate equal to or more than the cooling rate at which martensitic transformation is caused to form the carburized quenched portion in the desired portion, and
• a cutting step in which the extra portion of the intermediate product is cut;
• and
• then performing a welding step in which the second steel part is brought into abutment with the welding expected portion of the flange portion of the obtained first steel part to weld the first steel part and the second steel part to each other.