1. Field of the Invention
The present invention relates to a railway wheel having excellent wear resistance and heat-crack resistance and to a method of manufacturing such a railway wheel. More particularly, the invention relates to a solid railway wheel having wear resistance and heat-crack resistance which can cope with high speed of an engine, a passenger car, a freight car, and the like and to a method of manufacturing such a railway wheel.
2. Description of Related Art
There are railway wheels (hereinafter, simply referred to wheels) for an engine, a passenger car, a freight car, and the like having different functions, shapes, and so on. Therefore, wheels of different dimensions, shapes, and steels are used for respective purposes. Since the life of each wheel is usually determined by the degree of wear of the tread and the flange face, it is requested to use a steel which basically has high wear resistance for the wheel.
A solid wheel which is not produced by a shrinkage fit or the like is produced by the following method. First, a molten steel prepared to have predetermined chemical composition is cast in steel ingots having a circular shape of a diameter of about 360 to 450 mm in cross section by ingot making process or continuous casting processes. Each steel ingot is cut so as to have the thickness of about 300 to 500 mm and is formed in an almost wheel shape of the product by processes of forging, rolling, piercing, and the like. After that, heat treatment such as quenching and tempering is performed to the rim. Steps of machining and the like are further performed, thereby finishing it as a product.
In the quenching step, as shown in FIG. 7, cooling water is injected from nozzles 3 of a quenching ring 2 arranged so as to surround a rim portion 1 of a circular wheel toward a tread 4 and a region from the tread surface 4 to a depth of few tens mm is quenched. By such a quenching, the metallographic structure of an ordinary steel for a wheel becomes pearlitic structure. The wheel having a pearlitic structured region including the tread is wear-resistant and consequently has sufficient properties as a railway wheel ordinarily used.
However, since the speed of the railway vehicle is being increased in recent years, a change such as an increase in a heat generating amount when the brakes are applied occurs. In association with the change, heat-crack of the wheel, which is not conventionally a big issue, becomes an issue. In the increase in speed of the railway vehicle, the heat-crack resistance of the wheel is an important factor in determining the life of the wheel. The heat-crack of the wheel denotes a phenomenon such that tensile stress occurs on the tread and the flange face of the wheel due to thermal stress caused by heat generated by application of the brakes and cracking takes place by repetitive occurrence of the tensile stress. When the heat-crack occurs, the tread and the flange face of the wheel have to be machined to eliminate a portion in which the cracking occurs. The life of the wheel is consequently shortened. Further, when the heat-crack reaches the limit in length, a serious situation such as fracture of the wheel is caused.
The wheel for the recent high-speed railway vehicle is strongly requested to have both of the following properties at the same time:
A. strong wear resistance PA1 B. strong heat-crack resistance
The wear resistance and the heat-crack resistance are contrary properties of steels of the wheels and are generally incompatible properties. The reasons are as follows. In order to enhance the wear resistance, hardness has to be enhanced as much as possible while keeping the pearlitic metallographic structure of the tread portion as it is. However, when an alloying element for improving hardenability is added to increase the hardness, martensitic structure or bainitic structure is accordingly apt to be formed upon quenching or during use of the wheel. The heat-crack is apt to occur during use, so the heat-crack resistance cannot be satisfied. On the other hand, in order to obtain the heat-crack resistance, it is necessary to change the metallographic structure to the tough pearlitic structure and is necessary to reduce the hardness. In this case, however, sufficient wear resistance cannot be obtained. To satisfy both of the wear resistance and the heat-crack resistance at the same time is basically a difficult subject to be achieved.
Hitherto, when greater importance is attached to the wear resistance, some heat-crack resistance is sacrificed and a medium carbon steel (C: 0.65 to 0.75% by weight) is used. The steel is hard and highly wear-resistant and has a relatively high carbon content. When greater importance is attached to the heat-crack resistance, proof stress and wear resistance are ignored to a certain extent and a tough medium carbon steel (C: 0.45 to 0.55%) having a low carbon content and excellent heat-crack resistance is used.
Meanwhile, the applicant of the present invention have already proposed a steel for a wheel which simultaneously satisfies the two properties, that is, the wear resistance and the heat-crack resistance by choosing a proper chemical composition (Japanese Patent Application Laid-Open No. 57-143465). The chemical composition consists of 0.55 to 0.80% by weight C, 0.40 to 1.20% by weight Si, 0.60 to 1.20% by weight Mn, 0.20 to 0.70% by weight Cr, and the rest Fe and other incidental impurities. It is characterized in that the metallographic structure is mainly the pearlitic structure. The Si content of the steel of the wheel lies in a range from 0.4 to 1.2% and the Cr content lies in a range from 0.2 to 0.7% and the contents are high (in case of an ordinary wheel, the Si content is approximately 0.25% and the Cr content is approximately 0.10%). Thermal sensitivity (hardenability) is consequently rather high. Therefore, when heat is much generated when the brakes are applied or the like in a recent high-speed railway vehicle, there is a tendency such that a small quantity of the martensitic structure is formed on the surface layer of the wheel being used. When such a change in metallographic structure occurs, the heat-crack or spalling on the tread occurs or the martensitic structure peels off. Therefore, the life of the wheel is not always satisfactorily long.
When the deformation by wear or heat-crack occurs in the tread portion or the flange portion, the surface layer is removed by machining and the wheel is repeatedly used. In this case, it is desirable from the viewpoint of the life that the metallographic structure and hardness are uniform in the region from the new surface of the condemning limit. In the conventional wheel having a low Si content, since the hardenability is suppressed to obtain the high wear-resistant pearlitic structure, the region in which predetermined hardness is obtained is limited to a region from the tread surface to a relatively shallow point. Consequently, the life of the wheel is not satisfactorily long.
The invention is provided to solve the problems and it is an object of the invention to provide a long-life solid railway wheel and a method of manufacturing such a wheel which simultaneously satisfies both of wear resistance and heat-crack resistance good for use in a high-speed railway vehicle.