A steel for forging mechanical parts including the following elements, expressed in percentage by weight 0.2%≤C≤0.5%; 0.8%≤Mn≤1.5% ; 0.4%≤Si≤1%; 0.15%≤V≤0.6%; 0.01%≤Nb≤0.15%; 0.01%≤Cr≤0.5%; 0.01%≤P≤0.05%; 0.04%≤S≤0.09%; 0.01%≤N≤0.025%; and can contain one or more of the following optional elements 0%≤Al≤0.05%; 0%≤Mo≤0.5%; 0.01%≤Ni≤0.5%; 0%≤Ti≤0.2%; 0%≤B≤0.008%; 0%≤Cu≤0.5%; the remainder composition being composed of iron and unavoidable impurities caused by processing, the microstructure of said steel comprising 50% to 90% of Pearlite, 10% to 40% of Ferrite, with an optional presence of acicular ferrite between 0% and 2%, a niobium equivalent of 80% or more.

A crankshaft with improved seizure resistance is provided. A crankshaft having journals 11 and pins 12 includes a compound layer containing iron and nitrogen on its surface, wherein, in the compound layer, for both the journals 11 and pins 12, the porosity area ratio of the thinner one of a region from the surface to a depth of 3.0 μm and a region across the total thickness of the compound layer is not higher than 10.0%, and both the journals 11 and pins 12 have such a surface geometry that the arithmetical mean deviation of the primary profile, Pa, is not larger than 0.090 μm.

An automotive shaft includes a journal having a crest-to-crest contact surface area defined by and between undercut regions of the shaft, an entirety of the crest-to-crest contact surface area being laser hardened to a depth no greater than 1 mm to form a layer that does not contain unhardened portions.

An automotive shaft includes a journal having a crest-to-crest contact surface area defined by and between undercut regions of the shaft, an entirety of the crest-to-crest contact surface area being laser hardened to a depth no greater than 1 mm to form a layer that does not contain unhardened portions.

A method of manufacturing a cam piece for a continuously variable valve duration and a cam piece manufactured therefrom, and more particularly, to material and heat treatment conditions of a cam piece, may include manufacturing a cam piece by casting; heating the cam piece; maintaining a heating temperature; and salt-bathing the cam piece, in which the cam piece includes 3.2 to 4.2 wt % of carbon (C), 2.2 to 3.4 wt % of silicon (Si), and the balance iron (Fe), and may have a carbon equivalent value of 4.4 to 4.6.

A method of manufacturing a cam piece for a continuously variable valve duration and a cam piece manufactured therefrom, and more particularly, to material and heat treatment conditions of a cam piece, may include manufacturing a cam piece by casting; heating the cam piece; maintaining a heating temperature; and salt-bathing the cam piece, in which the cam piece includes 3.2 to 4.2 wt % of carbon (C), 2.2 to 3.4 wt % of silicon (Si), and the balance iron (Fe), and may have a carbon equivalent value of 4.4 to 4.6.

Disclosed is a steel composition including specified ranges of Ni; Mo; Co; Mo+Co+Si+Mn+Cu+W+V+Nb+Zr+Ta+Cr+C; Co+Mo; Ni+Co+Mo; and traces of Al; Ti; N; Si; Mn; C; S; P; B; H; O; Cr; Cu; W; Zr; Ca; Mg; Nb; V; and Ta in specified ranges; the remainder being iron and impurities. The inclusion population, as observed by image analysis over a polished surface measuring 650 mm.sup.2 if hot-formed or hot-rolled; and measuring 800 mm.sup.2 if cold-rolled, does not contain non-metallic inclusions of diameter >10 μm, and, in the case of a hot-rolled sheet, does not contain more than four non-metallic inclusions of diameter 5-10 μm over 100 mm.sup.2, the observation being performed by image analysis over a polished surface measuring 650 mm.sup.2.

Disclosed is a steel composition including specified ranges of Ni; Mo; Co; Mo+Co+Si+Mn+Cu+W+V+Nb+Zr+Ta+Cr+C; Co+Mo; Ni+Co+Mo; and traces of Al; Ti; N; Si; Mn; C; S; P; B; H; O; Cr; Cu; W; Zr; Ca; Mg; Nb; V; and Ta in specified ranges; the remainder being iron and impurities. The inclusion population, as observed by image analysis over a polished surface measuring 650 mm.sup.2 if hot-formed or hot-rolled; and measuring 800 mm.sup.2 if cold-rolled, does not contain non-metallic inclusions of diameter >10 μm, and, in the case of a hot-rolled sheet, does not contain more than four non-metallic inclusions of diameter 5-10 μm over 100 mm.sup.2, the observation being performed by image analysis over a polished surface measuring 650 mm.sup.2.

The present invention provides a steel material which is excellent in both of the strength (particularly, fatigue strength) and the manufacturability (particularly, bending straightening properties), and thus can be used as an automobile component such as a crankshaft by being formed into a product shape, being subjected to a high strength treatment such as a nitrocarburizing treatment, and then being subjected to the bending straightening.

The present invention provides a steel material which is excellent in both of the strength (particularly, fatigue strength) and the manufacturability (particularly, bending straightening properties), and thus can be used as an automobile component such as a crankshaft by being formed into a product shape, being subjected to a high strength treatment such as a nitrocarburizing treatment, and then being subjected to the bending straightening.