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.

Provided is a steel for nitrocarburizing that can ensure hardened case depth by suppressing precipitation of Cr, V, and Nb in a part of the surface layer very close to the surface. The provided steel comprises: a specific chemical composition satisfying 9.5≤([Cr]/52+[V]/50.9+[Nb]/92.9+M)×10.sup.3≤18.5, with the balance being Fe and inevitable impurities; and a steel microstructure in which an area ratio of bainite phase with respect to the entire microstructure is more than 50%.

A separator plate for use as a clutch plate for a multiplate wet clutch is formed of a steel plate. The steel plate has a chemical composition containing, on a basis of percent by mass, C from 0.03 to 0.08%, Si from 0 to 1.0%, Mn from 0.2 to 0.8%, P at 0.03% or less, S at 0.01% or less, and Al at 0.05% or less, so as to satisfy a formula, 5*C %−Si %+Mn %−1.5*Al %<1. In addition, the steel plate has the chemical component containing at least one of Nb from 0.03 to 0.4%, V from 0.01 to 0.3%, and Ti from 0.01 to 0.3%, so as to satisfy a formula, 0.04<(Nb %/1.4)+(V %/1.1)+Ti %<0.3. Then, an average diameter of particles of a carbide as a precipitate is controlled to be from 20 to 100 nm. The plate is formed by heating, hot rolling, winding and forming.

A separator plate for use as a clutch plate for a multiplate wet clutch is formed of a steel plate. The steel plate has a chemical composition containing, on a basis of percent by mass, C from 0.03 to 0.08%, Si from 0 to 1.0%, Mn from 0.2 to 0.8%, P at 0.03% or less, S at 0.01% or less, and Al at 0.05% or less, so as to satisfy a formula, 5*C %−Si %+Mn %−1.5*Al %<1. In addition, the steel plate has the chemical component containing at least one of Nb from 0.03 to 0.4%, V from 0.01 to 0.3%, and Ti from 0.01 to 0.3%, so as to satisfy a formula, 0.04<(Nb %/1.4)+(V %/1.1)+Ti %<0.3. Then, an average diameter of particles of a carbide as a precipitate is controlled to be from 20 to 100 nm. The plate is formed by heating, hot rolling, winding and forming.

To reduce the roughening of a surface of a workpiece whose hardness is increased by heat treatment. In a workpiece such as a guide plate (11), not a surface (11a1) of a groove (11a) that is a desired part for hardening (hardening target position) but a surface (11a2) opposite the surface (11a1) is heat-treated as a surface to be heated that is to be directly heated by an induction heating device. This can inhibit the surface (11a1) of the hardening target position from being roughened by the heat treatment and becoming lower in surface precision than before the heat treatment. Further, according to the present invention, the use of the induction heating device makes it possible to heat-treat only a necessary part.

To reduce the roughening of a surface of a workpiece whose hardness is increased by heat treatment. In a workpiece such as a guide plate (11), not a surface (11a1) of a groove (11a) that is a desired part for hardening (hardening target position) but a surface (11a2) opposite the surface (11a1) is heat-treated as a surface to be heated that is to be directly heated by an induction heating device. This can inhibit the surface (11a1) of the hardening target position from being roughened by the heat treatment and becoming lower in surface precision than before the heat treatment. Further, according to the present invention, the use of the induction heating device makes it possible to heat-treat only a necessary part.

A sprocket wheel, which is a machine component configured to slide relative to a bushing while being in contact with the bushing in an outer peripheral surface, includes a base made of a first metal, and an overlay that covers the base so as to constitute the outer peripheral surface. The surface of the overlay constituting the outer peripheral surface has been smoothed. Such a smoothed surface of the overlay makes the sprocket wheel less damaging to the bushing.

A steel material for nitriding has a composition comprising, by mass percent, C: more than 0.15% and not more than 0.35%, Si≦0.20%, Mn: 0.10 to 2.0%, P≦0.030%, S≦0.050%, Cr: 0.80 to 2.0%, V: 0.10 to 0.50%, Al: 0.01 to 0.06%, N≦0.0080%, O≦0.0030%, and optionally one or more elements of Mo, Cu, Ni, Ti, Nb, Zr, Pb, Ca, Bi, Te, Se and Sb, the balance being Fe and impurities. The composition satisfies the conditions of [20≦(669.3×log.sub.eC−1959.6×log.sub.eN−6983.3)×(0.067×Mo+0.147×V)≦80] and [140×Cr+125×Al+235×V≧160]. The microstructure is a ferritic-pearlitic structure, a ferritic-bainitic structure, or a ferritic-pearlitic-bainitic structure. The area fraction of ferrite is 20% or more and the precipitate content of V is 0.10% or less.

A steel material for nitriding has a composition comprising, by mass percent, C: more than 0.15% and not more than 0.35%, Si≦0.20%, Mn: 0.10 to 2.0%, P≦0.030%, S≦0.050%, Cr: 0.80 to 2.0%, V: 0.10 to 0.50%, Al: 0.01 to 0.06%, N≦0.0080%, O≦0.0030%, and optionally one or more elements of Mo, Cu, Ni, Ti, Nb, Zr, Pb, Ca, Bi, Te, Se and Sb, the balance being Fe and impurities. The composition satisfies the conditions of [20≦(669.3×log.sub.eC−1959.6×log.sub.eN−6983.3)×(0.067×Mo+0.147×V)≦80] and [140×Cr+125×Al+235×V≧160]. The microstructure is a ferritic-pearlitic structure, a ferritic-bainitic structure, or a ferritic-pearlitic-bainitic structure. The area fraction of ferrite is 20% or more and the precipitate content of V is 0.10% or less.

A method for producing a metallic component, The method includes the method steps of first machining (103) the component and thereafter cooling (105) the component from a first temperature down to a lower second temperature. The cooling (105) occurs after the machining (103) of the component.