Gear and method for manufacturing the same

A gear that is formed of material steel molded to include a disc portion and a plurality of tooth portions circumferentially discretely formed on the disc portion and have a shape in which tooth root portions are formed between the tooth portions, and is subjected to vacuum carburizing treatment and subsequent quenching treatment with high-density energy heating after being molded, wherein

• the material steel has a chemical composition of:
• C:
• 0.10% by mass to 0.30% by mass;
• Si:
• 0.50% by mass to 3.00% by mass;
• Mn:
• 0.30% by mass to 3.00% by mass;
• P:
• 0.030% by mass or less;
• S:
• 0.030% by mass or less;
• Cu:
• 0.01% by mass to 1.00% by mass;
• Ni:
• 0.01% by mass to 3.00% by mass;
• Cr:
• 0.20% by mass to 1.00% by mass;
• Mo:
• 0.10% by mass or less;
• N:
• 0.05% by mass or less;
• and
• Fe and unavoidable impurities:
• a residual portion, where
• Si(% by mass)+Ni(% by mass)+Cu(% by mass)−Cr(% by mass)>0.5 is satisfied,
• a partially tempered region is provided in a surface layer of at least a part of a portion including an edge part at an end in an axial direction in the tooth portions and the tooth root portions,
• the partially tempered region has hardness lower than hardness of a martensitic structure generated in the surface layer of the part of the portion by the quenching treatment, and
• a surface layer of a portion other than the partially tempered region in the tooth portions and the tooth root portions is formed by the martensitic structure generated by the quenching treatment.

A method for manufacturing a gear that is formed of material steel molded to include a disc portion and a plurality of tooth portions circumferentially discretely formed on the disc portion and have a shape in which tooth root portions are formed between the tooth portions, wherein

• the material steel has a chemical composition of:
• C:
• 0.10% by mass to 0.30% by mass,
• Si:
• 0.50% by mass to 3.00% by mass,
• Mn:
• 0.30% by mass to 3.00% by mass,
• P:
• 0.030% by mass or less,
• S:
• 0.030% by mass or less,
• Cu:
• 0.01% by mass to 1.00% by mass,
• Ni:
• 0.01% by mass to 3.00% by mass,
• Cr:
• 0.20% by mass to 1.00% by mass,
• Mo:
• 0.10% by mass or less,
• N:
• 0.05% by mass or less, and
• Fe and unavoidable impurities:
• a residual portion, where
• Si(% by mass)+Ni(% by mass)+Cu(% by mass)−Cr(% by mass)>0.5 is satisfied is used,
• the method for manufacturing a gear comprising:
• a vacuum carburizing step of heating the gear formed of the molded material steel to a temperature at or higher than an austenitizing temperature of the material steel in a carburizing atmosphere at a pressure lower than the atmospheric pressure to form a carburized layer on a surface of the gear;
• a cooling step of cooling the gear, after the vacuum carburizing step, to a temperature lower than a temperature at which structure transformation due to the cooling is completed, at a cooling rate lower than a cooling rate at which the material steel is transformed into martensite;
• a quenching step of heating the gear after the cooing step with high-density energy heating to increase the temperature of the gear to a temperature at or higher than the austenitizing temperature of the material steel and, from that state, cooling the gear at a cooling rate at or higher than the cooling rate at which the material steel is transformed into martensite to form a martensitic structure at least in a portion of the carburized layer;
• and
• a partial tempering step of heating at least a part of a portion including an edge part at an end in an axial direction in the tooth portions and the tooth root portions of the gear with the high-density energy heating after the quenching step to increase the temperature of at least the part of the portion including the edge part to a temperature of 180° C. or higher at which austenitization of the material steel does not occur and, from that state, cooling the gear to reduce a concentration of carbon dissolved in a solid state in the martensitic structure in the portion of the carburized layer in at least the part of the portion including the edge part.