1. Field of the Invention
The present invention relates to steel particularly suited for both general use and the manufacture of a component for bearings, especially a race for ball bearings, needle bearings or roller bearings.
2. Background of the Invention
Components for bearings, such as races, balls, needles or rollers, are generally manufactured from steel of the 100Cr6 or 100CrMn6 type containing from 0.6 to 1.5% of carbon, from 1.3 to 1.6% of chromium, from 0.3 to 1% of manganese and less than 0.4% of silicon and having a very high degree of cleanliness in terms of inclusions. The steel is used in the form of rolled bar, seamless tube or wire, from which are cut blanks or slugs which are formed by cold or hot plastic deformation and then hardened by tempering and annealing, before being machined. The components thus obtained have a high hardness and the toughness required for them to be able to withstand the rolling fatigue well, at least under the normal conditions of use, especially for in-service temperatures below 150.degree. C. However, the components thus formed have an insufficient rolling fatigue resistance for more severe service conditions, which are tending to become common. These more severe service conditions are characterized, in particular, by a service temperature above 150.degree. C. and possibly as high as 350.degree. C., and/or by the presence of the phenomenon of bearing surface deterioration by indentation. This phenomenon consists of the initiation of cracks on the surface, caused by the indentations, i.e. deformations generated by hard particles present in the lubricant.
In order to limit the effect of the indentation, it has been proposed to use materials having a very high hardness such as ceramics or deposits of hard materials. However, this technique has the drawback of being not very reliable because of the excessively high brittleness of these materials, which brittleness makes them very sensitive to the slightest defect.
It has also been proposed, for example in U.S. Pat. No. 5,030,017, to use a steel containing, in particular, from 0.3% to 0.6% of carbon, from 3% to 14% of chromium, from 0.4% to 2% of molybdenum, from 0.3% to 1% of vanadium and less than 2% of manganese. The components are carburized or carbonitrided in the region of the bearing surface, so as to obtain a sum of the carbon and nitrogen contents of between 0.03% and 1%, and are then hardened so that their micrographic structure comprises from 20% to 50% (in % by volume) of residual austenite in a surface layer representing from 10% to 25% of the volume of the component. This technique has the double drawback of requiring the use of a steel which is highly loaded with alloying elements, and is hence expensive, and the execution of a carburizing or carbonitriding treatment, this treatment being lengthy and expensive.
It has also been proposed, in German Patent Application DE 195 24 957, to use a steel containing from 0.9% to 1.3% of carbon, from 0.6% to 1.2% of silicon, from 1.1% to 1.6% of manganese and from 1.3% to 1.7% of chromium, the balance being iron and impurities resulting from smelting, the structure of this steel containing from 7% to 25% of residual austenite. However, this steel, because of its chemical composition, provides no guarantee of castability, of cold deformability and of residual austenite content and stability. The specified residual austenite content necessary for improving the resistance to indentation fatigue moreover requires, in the case of this steel, subjecting the bearings to a not very convenient heat treatment comprising a step of holding them at approximately 100.degree. C. for more than 10 hours between tempering and annealing without returning to ambient temperature after tempering or before annealing. Moreover, in the presence of multidirectional stresses below the cyclic yield stress, its austenite is stable for more than 2000 hours only for thermal stresses below 120.degree. C., which is too low for some applications.