The steel for machine structural use such as a gear, shaft, pulley, constant velocity joint and the like utilized for a variety of gear transmission devices, to begin with a transmission for an automobile and a differential gear, as well as a crank shaft, con'rod and the like, is generally finished into a final shape by performing the work of forging and the like and thereafter performing cutting work. Because the cost required for the cutting work occupies a major portion in the manufacturing cost, steel material constituting the steel for machine structural use is required to be excellent in machinability. Therefore, technologies for improving machinability have been disclosed from the past.
The typical examples of such technologies are to add Pb and to form MnS by adding S. However, because Pb is hazardous for a human body, its use has come to be restricted. Also, with respect to the parts in which deterioration of the mechanical property caused by sulfide becomes a problem, there is a limit in using S. Further, in cutting work of a gear and the like particularly, gear cutting with a hob is generally performed, however, the cutting in this case differs from the continuous cutting such as what is called lathe turning but is in a manner called as the intermittent cutting. At present, steel material improving the machinability in hobbing has been scarcely materialized. The tool raw material used for a hob is a high-speed steel, and is generally performed with coating of TiAlN and the like. In this case, it is known that the tool surface wears while being oxidized by repeating cutting and idle rotation in working under a comparative low speed.
As a method for improving the intermittent cutting machinability, in the patent document 1, steel material is described which is excellent in the intermittent cutting machinability (tool life) under a high speed (cutting speed: 200 m/min or more) by containing Al: 0.04-0.20% and O: 0.0030% or less.
In the patent document 2, a steel for machine structural use is described which contains C: 0.05-1.2%, Si: 0.03-2%, Mn: 0.2-1.8%, P: 0.03% or less, S: 0.03% or less, Cr: 0.1-3%, Al: 0.06-0.5%, N: 0.004-0.025%, O: 0.003% or less respectively, contains Ca: 0.0005-0.02% and Mg: 0.0001-0.005% with solid solution N: 0.002% or more in steel, the remainder being iron and unavoidable impurities, and satisfies
(0.1×[Cr]+[Al])/[O]≦150.
Also, in the patent document 3, a steel for machine structural use is described which contains C: 0.1-0.85%, Si: 0.01-1.0%, Mn: 0.05-2.0%, P: 0.005-0.2%, total Al: exceeding 0.1% and 0.3% or less, total N: 0.0035-0.020%, with solid solution N being limited to 0.0020% or less.