A steel for a carburizing and a carburized steel component having a steel portion and a carburized layer with a thickness of more than 0.4 mm to less than 2 mm which is formed on an outside of the steel portion. A chemical composition of the steel for the carburizing and the steel portion of the carburized steel component satisfies simultaneously equations of a hardness parameter, a hardenability parameter, and a TiC precipitation parameter.

A steel for a carburizing and a carburized steel component having a steel portion and a carburized layer with a thickness of more than 0.4 mm to less than 2 mm which is formed on an outside of the steel portion. A chemical composition of the steel for the carburizing and the steel portion of the carburized steel component satisfies simultaneously equations of a hardness parameter, a hardenability parameter, and a TiC precipitation parameter.

A nitrided component has a chemical composition consisting of, by mass percent, C: 0.07-0.14%, Si: 0.10-0.30%, Mn: 0.4-1.0%, S: 0.005-0.030%, Cr: 1.0-1.5%, Mo: ≦0.05% (including 0%), Al: 0.010% or more to less than 0.10%, V: 0.10-0.25%, optionally at least one element selected from Cu: ≦0.30% and Ni: ≦0.25%, [0.61Mn+1.11Cr+0.35Mo+0.47V≦2.30], and the balance of Fe and impurities. P, N, Ti and O among the impurities are P: ≦0.030%, N: ≦0.008%, Ti: ≦0.005%, and O: ≦0.0030%. The nitrided composition is suitable for use as an automobile ring gear. The nitrided component has a surface hardness of 650-900 HV, core hardness being ≧150 HV, effective case depth of ≧0.15 mm, has excellent bending fatigue strength and surface fatigue strength although the content of Mo is as low as ≦0.05% and has a small amount of expansion caused by nitriding.

A nitrided component has a chemical composition consisting of, by mass percent, C: 0.07-0.14%, Si: 0.10-0.30%, Mn: 0.4-1.0%, S: 0.005-0.030%, Cr: 1.0-1.5%, Mo: ≦0.05% (including 0%), Al: 0.010% or more to less than 0.10%, V: 0.10-0.25%, optionally at least one element selected from Cu: ≦0.30% and Ni: ≦0.25%, [0.61Mn+1.11Cr+0.35Mo+0.47V≦2.30], and the balance of Fe and impurities. P, N, Ti and O among the impurities are P: ≦0.030%, N: ≦0.008%, Ti: ≦0.005%, and O: ≦0.0030%. The nitrided composition is suitable for use as an automobile ring gear. The nitrided component has a surface hardness of 650-900 HV, core hardness being ≧150 HV, effective case depth of ≧0.15 mm, has excellent bending fatigue strength and surface fatigue strength although the content of Mo is as low as ≦0.05% and has a small amount of expansion caused by nitriding.

A steel for a carburizing and a carburized steel component having a steel portion and a carburized layer with a thickness of more than 0.4 mm to less than 2 mm which is formed on an outside of the steel portion. A chemical composition of the steel for the carburizing and the steel portion of the carburized steel component satisfies simultaneously equations of a hardness parameter, a hardenability parameter, and an AlN precipitation parameter.

A steel for a carburizing and a carburized steel component having a steel portion and a carburized layer with a thickness of more than 0.4 mm to less than 2 mm which is formed on an outside of the steel portion. A chemical composition of the steel for the carburizing and the steel portion of the carburized steel component satisfies simultaneously equations of a hardness parameter, a hardenability parameter, and an AlN precipitation parameter.

A novel combination of heat treatment steps includes the steps of carburizing a component fabricated of a medium carbon alloy steel at an elevated temperature for between three and six hours, subjecting the component to an austempering bath and holding it there for between fifteen and two hundred forty minutes and finally cooling the component to room temperature to allow martensitic transformation. These steps may be followed with cryogenic treatment to reduce retained austenite if needed. The process produces components with low distortion, high surface hardness, from HRC 56 to 62, and high surface compressive residual stress.

A novel combination of heat treatment steps includes the steps of carburizing a component fabricated of a medium carbon alloy steel at an elevated temperature for between three and six hours, subjecting the component to an austempering bath and holding it there for between fifteen and two hundred forty minutes and finally cooling the component to room temperature to allow martensitic transformation. These steps may be followed with cryogenic treatment to reduce retained austenite if needed. The process produces components with low distortion, high surface hardness, from HRC 56 to 62, and high surface compressive residual stress.

The present invention is steel for surface hardening for machine structural use which contains, by mass %, C: 0.3 to 0.6%, Si: 0.02 to 2.0%, Mn: 0.35 to less than 1.5%, and Al: 0.01 to 0.5%, is restricted to B: less than 0.0003%, S: 0.0001 to 0.021%, N: 0.003 to 0.0055%, P: 0.0001 to 0.03%, and O: 0.0001 to 0.0050%, has a ratio Mn/S of Mn and S satisfying 70 to 30,000, has a balance of Fe and unavoidable impurities, and, when nitrided, then induction hardened, has a surface hardenability of a Vicker's hardness when tempered at 300° C. of 650 or more.

A continuous nitriding treatment furnace includes a nitriding chamber, a heater, a first nitriding zone, and a second nitriding zone lower in atmosphere gas temperature than the first nitriding zone by 25° C. to 150° C., the continuous nitriding treatment furnace being configured such that an atmosphere gas in the first nitriding zone flows into the second nitriding zone and being configured to execute a nitriding treatment that forms an iron nitride compound layer composed of an ε phase or of the ε phase and a γ′ phase on a surface of the steel member in the first nitriding zone and precipitates the γ′ phase in the iron nitride compound layer in the second nitriding zone.