Rolling element for rolling bearing and rolling bearing made using the same

Abstract

To provide a rolling bearing component, namely a rolling element, which can achieve a remarkable improvement in pressure indentation resistance required for rolling bearings, and also to provide a rolling bearing made using the rolling element.SOLUTION: A rolling element for a rolling bearing, composed of an alloy steel containing, in mass%, C: 0.40 to 0.95%, Si: 0.70 to 2.50%, Mn: 0.10 to 1.00%, Cr: 1.00 to 4.00%, W+2Mo: 0.40 to 3.00%, and V: 0.05 to 0.80%, the remainder consisting of Fe and inevitable impurities, the rolling element having on its surface a carburized layer with a thickness of 500 μm or greater, the retained austenite level of the carburized layer being controlled within a range of 15 vol.% or more to 30 vol.% or less.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a rolling bearing component (rolling element for rolling bearing) to be built into automobiles, industrial machinery, robots, etc., and to a rolling bearing using the same. [Background technology]

[0002] Rolling bearings used in fields such as automobiles and industrial machinery require various properties, including heat resistance and wear resistance, and in particular, static strength (resistance to indentations) is required in addition to dynamic strength of the inner and outer rings and rolling elements. Furthermore, as machines become smaller, lighter, and less expensive, the environments in which bearings are used are becoming even more severe than before, and rolling bearings used in such harsh environments often encounter foreign matter such as burrs and wear debris in the lubricating oil, so they also require resistance to foreign matter.

[0003] For example, a technology has been disclosed in which a high-hardness material such as high-speed tool steel is used as the material for the raceway rings and rolling elements, and carbides of several μm or less are precipitated in the structure at a predetermined depth on the raceway surface of the raceway rings, followed by the formation of a carbonitrided layer (see Patent Documents 1 and 2). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 4-9449 [Patent Document 2] Japanese Patent Application Publication No. 8-49057 Summary of the Invention [Problem to be solved by the invention]

[0005] However, when a high alloy steel is used as the base material (substrate), the carbon content becomes relatively high, which makes it easy for coarse carbides to precipitate in the matrix structure, significantly reducing machinability and polishability when processed into the final shape.On the other hand, when low-alloy steel is processed into steel balls and high hardness is imparted by work hardening, there is a problem in that the rolling fatigue life of the steel balls is shortened.

[0006] Therefore, an object of the present invention is to provide a rolling bearing component (rolling element) that maintains the processing characteristics (machinability and polishability) required when manufacturing a rolling bearing, while at the same time being expected to improve the indentation resistance and rolling fatigue life required when using the rolling bearing, and a rolling bearing using the same. [Means for solving the problem]

[0007] The rolling element for a rolling bearing of the present invention is made of an alloy steel containing, by mass, 0.40-0.95% C, 0.70-2.50% Si, 0.10-1.00% Mn, 1.00-4.00% Cr, 0.40-3.00% W+2Mo, 0.05-0.80% V, and the remainder being iron and unavoidable impurities, and the rolling element for a rolling bearing has a carburized layer on its surface that is 500 μm thick and has a retained austenite content of 15-30% by volume. Preferably, the carburized layer has a Vickers hardness of 800 HV or more, and the alloy steel directly below the carburized layer has a Vickers hardness of 760 HV or more. [Effects of the Invention]

[0008] The rolling element for a rolling bearing of the present invention is made of an alloy steel containing, by mass, 0.40-0.95% C, 0.70-2.50% Si, 0.10-1.00% Mn, 1.00-4.00% Cr, 0.40-3.00% W+2Mo, 0.05-0.80% V, with the balance being iron and unavoidable impurities, and has a carburized layer formed on its surface to a thickness of 500 μm or more. The amount of retained austenite on the rolling element surface is set to 15-30% by volume, thereby improving the indentation resistance required of a rolling bearing. Furthermore, by setting the Vickers hardness of the alloy steel directly below the carburized layer to 760 HV or more, the alloy steel substrate itself can maintain its indentation resistance even if the carburized layer wears away after long-term use. DETAILED DESCRIPTION OF THE INVENTION

[0009] The contents of the main chemical components of the rolling element for a rolling bearing according to one embodiment of the present invention will now be described. First, the C (carbon) content in the rolling element for a rolling bearing (alloy steel) of the present invention is 0.40 to 0.95% by weight. Carbon ensures hardness in the steel after quenching and tempering, and plays a role in ensuring a high level of fatigue life when used as a rolling element for a rolling bearing. If the C content in the alloy steel is less than 0.40%, the required surface and internal hardness cannot be obtained, and if it exceeds 0.95%, the amount of retained austenite (γ amount) increases, deteriorating the fatigue life of mechanical components.

[0010] The Si (silicon) content is 0.70 to 2.50% by weight. Silicon plays a role in increasing the temper softening resistance of alloy steel. If the Si content in alloy steel is less than 0.70%, the required temper softening resistance cannot be obtained, and if it exceeds 2.50%, hot forgeability deteriorates significantly.

[0011] The Mn (manganese) content is 0.10 to 1.00% by weight. Manganese improves the hardenability of alloy steel, and is effective in improving fatigue life when used as a rolling element for a rolling bearing. If the Mn content in the steel is less than 0.10%, the hardenability of the alloy steel deteriorates, and if it exceeds 1.00%, hot forgeability deteriorates significantly.

[0012] The Cr (chromium) content is 1.00 to 4.00% by weight. Chromium increases the hardenability of the alloy steel and also thermally stabilizes cementite, preventing the cementite from dissolving in the matrix at high temperatures. If the Cr content in the alloy steel is less than 1.00%, the hardenability of the alloy steel will deteriorate, and if it exceeds 4.00%, coarse carbides will form in the alloy steel.

[0013] W (tungsten) can be included in the range of 0.40 to 3.00% by weight as the W equivalent (W + 2Mo). In this case, tungsten, like molybdenum, forms carbides in the steel, contributing to ensuring hardness. If the W equivalent in the steel is less than 0.40%, the required temper hardness and softening resistance cannot be obtained. On the other hand, if the W equivalent exceeds 3.00%, coarse carbides will form.

[0014] The V (vanadium) content is 0.05 to 0.80% by weight. V has the role of increasing temper softening resistance when added in combination with silicon in the alloy steel. Furthermore, if the V content in the alloy steel is less than 0.10%, the required temper softening resistance cannot be obtained, and if it exceeds 0.80%, coarse carbides are generated. Preferably, V is 0.05 to 0.40%.

[0015] Mo (molybdenum) is contained as the W equivalent (W + 2Mo), but when Mo is contained alone, the content is set to 0.20 to 1.50% by weight. Molybdenum forms carbides in the alloy steel, contributing to ensuring hardness. If the Mo content in the alloy steel is less than 0.20%, the required temper hardness and softening resistance cannot be obtained. If the Mo content exceeds 1.50%, coarse carbides are generated. Preferably, Mo is 0.20 to 0.70%.

[0016] Next, we will explain the carburized layer formed on the surface of the rolling element for a rolling bearing. The carburized layer is formed for the purpose of further increasing the hardness of the base alloy steel itself, and its thickness (thickness of the carburized layer) is at least 500 μm (0.5 mm) from the surface of the rolling element, and preferably 1.0 mm or more.

[0017] The amount of retained austenite (γ amount) in the carburized layer is set to a range of 15% to 30% by volume, and considering that the material will be machined (ground) into a predetermined shape after carburizing, the range is preferably 15% to 25% by volume. By setting the amount of retained austenite within this range, spheroidized carbides with an average particle size of less than 10 μm are evenly dispersed throughout the base structure, resulting in a stable hardness value (hardness) directly below the carburized layer regardless of the depth (thickness) direction. The amount of retained austenite in the structure is a value calculated by converting it into a volume ratio based on X-ray diffraction measurement.

[0018] Furthermore, the hardness of the carburized layer must be at least 800 HV on the Vickers hardness scale (equivalent to 64 HRC on the Rockwell C scale). In particular, if the Vickers hardness of the base structure directly below the carburized layer is less than 760 HV, the indentation resistance required of rolling elements for rolling bearings will be significantly reduced. For this reason, the hardness of the base structure directly below the carburized layer must be 760 HV on the Vickers hardness scale. Note that "directly below the carburized layer" is defined as a position at a depth of 1 mm (1000 μm) or more from the surface of the rolling element.

Claims

1. A rolling element for a rolling bearing made of alloy steel containing, in mass %, C: 0.40 to 0.95%, Si: 0.70 to 2.50%, Mn: 0.10 to 1.00%, Cr: 1.00 to 4.00%, W+2Mo: 0.40 to 3.00%, V: 0.05 to 0.80%, and the balance being iron and inevitable impurities, wherein the surface of the rolling element for a rolling bearing has a carburized layer having a thickness of 500 μm or more, and the amount of retained austenite on the surface of the rolling element is 15 to 30 volume %.

2. 2. The rolling element for a rolling bearing according to claim 1, wherein the hardness of the carburized layer is 800 HV or more in Vickers hardness, and the hardness of the alloy steel directly below the carburized layer is 760 HV in Vickers hardness.

3. A rolling bearing comprising the rolling element for a rolling bearing according to claim 1 or 2.