1. Field of the Invention
The present invention relates to a nitrocarburized crankshaft member wherein at least a portion of the surface is nitrocarburized by a nitrocarburizing process, and a steel for nitrocarburized crankshafts that can be used therefor.
2. Description of the Background Art
In a reciprocating engine of an automobile or the like, a crankshaft is used to extract an output of rotating motion from a piston that linearly reciprocates. This crankshaft, as shown in the drawings of JP2007-197812A, for example, comprises a journal portion located around a shaft that is the same as the rotating shaft of an output shaft, a pin portion located around a shaft that moves the rotating shaft in parallel by a predetermined distance, and an arm portion provided in a plurality at a predetermined interval along the rotating shaft, connecting the journal portion and pin portion.
In the manufacturing method of this bent shape crankshaft, for example, a steel rod is hot-forged into the required shape, and this roughly formed shape is then machined into an integrated crankshaft member. Subsequently, as required, a normalizing process is carried out to remove residual stress, and a surface hardening process such as nitrocarburizing or nitriding is carried out to improve fatigue strength. During the forging or the surface hardening process, the crankshaft member is likely to bend or warp. Therefore, subsequently, a bending correction is carried out to correct bending, warping, and the like.
Any bends or warps of the crankshaft member is corrected by a bending correction bending the crankshaft member in the direction opposite the direction of the bend. At this time, cracks readily occur on the hard surface layer formed by a surface hardening process, such as nitrocarburizing or nitriding, and these cracks may cause damage to the crankshaft. Given this factor, a crankshaft member made of a steel that is superior in bending correctability, which makes it possible to perform a bending correction process without imparting excessive strain, has been in demand.
In response to such a demand, there have been developed steels for crankshafts that are made of a medium-carbon steel wherein the carbon content is suppressed to about 0.3 wt % to achieve a reduction in average hardness. For example, JP2002-226939A discloses a steel for nitrocarburized crankshafts that includes C in the amount of 0.2 to 0.6 wt %, Si in the amount of 0.05 to 1.0 wt %, Mn in the amount of 0.25 to 1.0 wt %, S in the amount of 0.03 to 0.2 wt %, Cr in the amount of 0.2 wt % or less, s-M in the amount of 0.045 wt % or less, Ti in the amount of 0.002 to 0.010 wt %, N in the amount of 0.005 to 0.025 wt %, and O in the amount of 0.001 to 0.005 wt %, and satisfies the condition 0.12×Ti wt %<0 wt %<2.5×Ti wt % and 0.04×N wt %<O wt %<0.7×N wt %.
Now, from the viewpoints of cost and ease of manufacture, a solution strategy for the demand described above that proactively uses Cu, which may exist as a trapped element in scraps of raw material, is highly preferred. For example, the technical journal “DENKI-SEIKOU”, Volume 77, No. 1 (February 2006), discloses that adding Cu to medium-carbon steel can increase ferrite hardness, improving yield strength, and form a harder yet thinner compound layer, which is formed by nitrocarburizing. That is, by adding Cu to a medium-carbon steel, the achievement of a nitrocarburized crankshaft member that is superior in bending correctability and superior in fatigue strength is expected.
Steels for nitrocarburized crankshafts made of a medium-carbon steel that include Cu are disclosed in JP2002-226939A, for example. This steel for nitrocarburized crankshafts includes C in an amount greater than or equal to 0.30 wt % and less than or equal to 0.50 wt %, Si in an amount greater than or equal to 0.05 wt % and less than or equal to 0.30 wt %, Mn in an amount greater than or equal to 0.50 wt % and less than or equal to 1.00 wt %, S in an amount greater than or equal to 0.03 wt % and less than or equal to 0.20 wt %, Cu in an amount greater than or equal to 0.05 wt % and less than or equal to 0.60 wt %, Ni in an amount greater than or equal to 0.02 wt % and less than or equal to 1.00 wt %, and Cr in an amount greater than or equal to 0.05 wt % and less than or equal to 0.30 wt %, and satisfies the condition that composition parameter F1>20 and F2>0 given F1=185 WCr+50 WCu, and F2=8+4 WNi+1.5 WCu−44 WCr, where WCu, WNi, and WCr are the content percentages (unit: wt %) of Cu, Ni, and Cr, respectively.
It is now expected that a crankshaft member that increases the performance requirements of crankshafts, excels in cost and ease of manufacture, and achieves both high fatigue strength and superior bending correctability will be developed.
The present invention was made in view of such circumstances, and it is therefore an object of the present invention to obtain a nitrocarburized crankshaft member having at least a portion of its surface subjected to nitrocarburizing, excels in cost and ease of manufacture, and achieves both high fatigue strength and superior bending correctability compared to prior art crankshaft members, and a steel for nitrocarburized crankshafts that can be used therefor.