Rolling slide member, rolling bearing using same, and method for manufacturing rolling slide member

Abstract

A rolling-sliding member that is high in hardness and continues to have a passivation film reliably even after being subjected to a process that does not require any processing for removal of scale, etc., as well as a rolling bearing using the same and a method for manufacturing the rolling-sliding member.

Claims

1. A method for manufacturing a rolling-sliding member, wherein the rolling-sliding member is an outer ring of a rolling bearing, the outer ring having an outer circumferential surface, an inner surface and side surfaces on each side of the outer ring between the outer circumferential surface and the inner surface, the method comprising: performing vacuum heat treatment, to attain hardness of 55 HRC or higher, on the outer ring of the rolling bearing, the outer ring being comprised of: 0.15 mass % or larger and smaller than 0.70 mass % of C; 0.05 to 1.00 mass % of Si; 0.05 to 1.00 mass % of Mn; 0.03 mass % or smaller of P; 0.03 mass % or smaller of S; 0.001 to 0.500 mass % of Cu; 0.05 to 0.50 mass % of Ni; 11 to 18.0 mass % of Cr; 0.05 to 2.00 mass % of Mo; 0.01 to 0.50 mass % of W; 0.01 to 0.50 mass % of V; 0.05 to 0.40 mass % of N; 0.02 mass % or smaller of O; 0.080 mass % or smaller of Al; 0.0005 to 0.0050 mass % of B; provided that the total content of C and N is larger than 0.4 mass % and smaller than 0.7 mass %, and a content ratio C/N is 0.75 or larger; and the remainder being Fe and unavoidable impurities; wherein the vacuum heat treatment forms a Cr-deficient layer in a surface layer part of the outer ring of the rolling bearing; and thereafter performing processing of removing the surface layer part of the outer circumferential surface and the side surfaces of the outer ring of the rolling bearing such that no Cr-deficient layer exists on the outer circumferential surface and the side surfaces of the outer ring of the rolling bearing, but a Cr-deficient layer exists on the inner surface of the outer ring of the rolling bearing.

2. The method for manufacturing a rolling-sliding member according to claim 1, wherein a region of performing the processing of removing the surface layer part is a region having a depth of at least 40 nm from the outer circumferential surface and the side surfaces of the outer ring of the rolling bearing.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a partially cutaway perspective view showing the structure of a common rolling bearing.

(2) FIG. 2 is a sectional view of a rolling bearing that has a sealing member for sealing drive surfaces.

(3) FIG. 3 is a graph showing to what extent Cr exists in a surface layer part.

MODE FOR CARRYING OUT THE INVENTION

(4) First, a description will be made of a fundamental composition of a martensite steel material that is a base of a rolling-sliding member. This rolling-sliding member is the same in fundamental composition as the rolling-sliding member disclosed in Patent document 1, and has DSR-40N as a product on the market. More specifically, the fundamental composition includes 0.15 mass % or larger and smaller than 0.70 mass % of C; 0.05 to 1.00 mass % of Si; 0.05 to 1.00 mass % of Mn; 0.03 mass % or smaller of P; 0.03 mass % or smaller of S; 0.001 to 0.500 mass % of Cu; 0.05 to 0.50 mass % of Ni; 11.0 to 18.0 mass % of Cr; 0.05 to 2.00 mass % of Mo; 0.01 to 0.50 mass % of W; 0.01 to 0.50 mass % of V; 0.05 to 0.40 mass % of N; 0.02 mass % or smaller of O; 0.080 mass % or smaller of Al; 0.0005 to 0.0050 mass % of B; provided that the total content of C and N is larger than 0.4 mass % and smaller than 0.7 mass %, and a content ratio C/N is 0.75 or larger; and the remainder being Fe and unavoidable impurities.

(5) Carbon (C) serves to secure necessary strength and corrosion resistance, and forms carbides by combining with carbide forming elements such as Cr, Mo, W, V, and Nb. Carbon also serves to secure necessary hardness by forming a solid solution in the matrix at the time of quenching, thereby causing the matrix to have a martensite structure.

(6) Being an interstitial element, N increases both of corrosion resistance and hardness. The corrosion resistance is increased when N is added to the matrix in the form of a solid solution. The elements Ni, Cr, and V increase the amount of dissolved nitrogen.

(7) Chromium (Cr), which is the main component of a surface layer passivation film, increases the corrosion resistance. Copper (Cu) also increases the corrosion resistance, and is particularly effective at suppressing erosion of hydrochloric acid. Molybdenum (Mo) also increases the corrosion resistance.

(8) Silicon (Si) is added mainly as a deoxidizer or for addition of nitrogen. Manganese (Mn) increases the quenching performance, and has an effect of preventing reduction in toughness by fixing S that is contained unavoidably. Tungsten (W) increases the quenching performance. Aluminum (Al) is added as a deoxidizer. Boron (B) strengthens grain boundaries and thereby lowers the probability of occurrence of braking at the time of quenching or sub-zero treatment. Phosphorus (P), S, and O are contained in steel unavoidably.

(9) The rolling-sliding member may contain the following elements in the following content ranges in addition to the above essential elements:

(10) 0.001 mass %≤Co≤0.500 mass %

(11) 0.001 mass %≤Se≤0.300 mass %

(12) 0.001 mass %≤Te≤0.300 mass %

(13) 0.0002 mass %≤Ca≤0.1000 mass %

(14) 0.001 mass %≤Pb≤0.200 mass %

(15) 0.001 mass %≤Nb≤0.300 mass %

(16) 0.001 mass %≤Ta≤0.300 mass %

(17) Ti≤0.200 mass %

(18) 0.001 mass %≤Zr≤0.300 mass %.

(19) In the case of manufacturing a rolling-sliding member using the above martensite steel material, the material is formed into a predetermined shape by casting or forging and then subjected to heat treatment such as quenching, sub-zero treatment, and tempering to obtain certain hardness and for other purposes. In the case where high dimensional accuracy is required, it is preferable to cut a member formed by casting or forging so as to have a rough shape, into predetermined dimensions.

(20) The quenching treatment is performed by heating a steel material to a quenching temperature or a solution treatment temperature of 1,020° C. to 1,150° C. and then cooling it quickly at a predetermined cooling rate by oil quenching, gas cooling, or the like. In the sub-zero treatment, the steel material is again cooled quickly using a freezing mixture or refrigerant of 0° C. or lower soon after the quenching. For example, dry ice, dry ice plus alcohol (−80° C.), carbon dioxide gas (−130° C.), or liquid nitrogen (−196° C.) can be used as the freezing mixture or refrigerant. In the tempering treatment, the steel material as subjected to the sub-zero treatment is heated to 150° C. to 450° C. As a result of the above heat treatment, the steel material is given Rockwell hardness that is at least larger than or equal to 55 HRC, which is equivalent to the value of SUS440C.

(21) The heat treatment described above is performed in a vacuum atmosphere. It is also preferable to perform the heat treatment in an inert gas atmosphere by introducing a nitrogen gas or the like, rather than in a vacuum atmosphere. Since the above heat treatment is performed in an oxygen-absent atmosphere, no scale, nitrogen absorption region, or decarburization region is formed in the surface of the rolling-sliding member and hence it is not necessary to perform any processing for removal of scale etc. after the heat treatment.

(22) However, we found that a Cr-deficient layer where the content of Cr is clearly small is formed within a region of a depth (thickness) of about 40 nm from the surface. Thus, if the steel material is used without being subjected to any processing after the heat treatment, no effective passivation film is formed due to the shortage of Cr and an iron oxide coating is formed instead as a result of a phenomenon that iron in the steel material combines with oxygen in the air, resulting in large reduction in corrosion resistance. The term “Cr-deficient layer” as used herein means a layer where the content of Cr is clearly smaller than an intrinsic range 11.0 to 18.0 mass % of the rolling-sliding member.

(23) In view of the above, it is necessary to remove a very thin surface layer part after the heat treatment. The thickness (depth) of a layer to be removed is at least larger than or equal to 40 nm, preferably larger than or equal to 100 nm, from the surface. A clear Cr-deficient layer can be removed properly if a layer whose thickness is larger than or equal to 40 nm from the surface is removed. A Cr-deficient layer can be removed fully if a layer whose thickness is larger than or equal to 100 nm from the surface is removed. On the other hand, although there are no particular limitations on the upper limit of the thickness of a layer to be removed, it is preferable that the upper limit be at least smaller than a thickness (0.7 mm) of the conventional processing for removal of scale etc. This is because it is desirable to reduce the amount of removed material to a minimum necessary level if the material cost and the productivity (shortening of the processing time) are taken into consideration. Thus, it is much preferable that the thickness of a layer to be removed be smaller than or equal to 100 μm from the surface and it is far preferable that it be smaller than or equal to 150 nm from the surface. There are no particular limitations on the processing method for removing a surface layer part; any method capable of removing a surface layer part can be employed, such as polishing, barrel polishing, cutting, and chemical processing.

(24) A rolling-sliding member produced in the above-described manner is high in corrosion resistance because an effective passivation film is formed reliably in the surface thereof. In terms of pitting potential, this rolling-sliding member is equivalent to SUS630 which is said to be high in corrosion resistance. As such, this rolling-sliding member is suitable as a member for use in an exposed-to-water environment, and can be used as a rolling bearing used for, for example, a slide door that is installed in an automobile, an industrial machine, a warehouse, or the like so as to be exposed directly to an external environment (ambient atmosphere). This rolling-sliding member is particularly suitable for use as the outer ring of a rolling bearing among such uses.

(25) As shown in FIG. 1, a common rolling bearing 1 of this type includes an outer ring 2, an inner ring 3, a plurality of rolling bodies 4 which are disposed rollably between the outer ring 2 and the inner ring 3, and a holding device 5 for holding the rolling bodies 4 so that the rolling bodies 4 are arranged at predetermined intervals in the circumferential direction. Since in this manner the outer ring 2 is the outermost component of the rolling bearing 1, it is most prone to rust when the rolling bearing 1 is used in an exposed-to-water environment. If the outer ring 2 rusts, resulting rust powder sticks to a slide rail and possibly impair its appearance. It is therefore preferable to use the rolling-sliding member according to the present invention as the outer ring 2.

(26) In this case, basically, it is necessary to perform processing of removing a Cr-deficient layer on all of the outer circumferential surface, the side surfaces, and the inner circumferential surface (the surface opposed to the inner ring 3) of the outer ring 2. However, among rolling bearings are ones in which, as in a rolling bearing 10 for a slide door shown in FIG. 2, the space between an outer ring 12 and an inner ring 13 is sealed by sealing members 16. In the rolling bearing 10 having such a structure, it may suffice to perform processing of removing a Cr-deficient layer on only the outer circumferential surface and the side surfaces of the outer ring 2. In FIG. 2, symbol 14 denotes a rolling body and symbol 15 denotes a holding device for holding the rolling bodies 14 at predetermined intervals in the circumferential direction.

(27) In this rolling bearing 10 for a slide door, in many cases, the inner circumferential surface, including the raceway surface, of the outer ring 12 is not subjected to finishing such as polishing after being subjected to cutting and vacuum heat treatment as in the conventional case. More specifically, in the common rolling bearing 1, it is found frequently that finishing is performed on only the raceway surface (inner circumferential surface) which is a curved surface but is not performed on the side surfaces and the outer circumferential surface. On the other hand, in the present invention, finishing is performed on only the side surfaces or the outer circumferential surface but not performed on the raceway surface; the present invention is right opposite to the conventional case in this respect. Thus, in such a rolling bearing 10 for a slide door, the time and labor and the cost of processing can be reduced.

(28) In the case where rolling bearings are used in an exposed-to-water environment, the inner ring may also rust. It is therefore preferable to use a rolling-sliding member according to the present invention also as the inner ring. As in the case of the outer ring, basically, it is preferable to perform processing of removing a Cr-deficient layer on all of the outer circumferential surface (the surface opposed to the outer ring), the side surfaces, and the inner circumferential surface. However, in the case shown in FIG. 2 in which the space between the outer ring and the inner ring is sealed by the sealing members, the processing of removing a Cr-deficient layer may be performed on only the inner circumferential surface and the side surfaces.

(29) The rolling bodies and the holding device may be made of the same materials as those of known, common rolling bearings. For example, the rolling bodies may be made of a carbon steel or SUJ2. For example, the holding device may be made of a resin-based material such as reinforced polyamide.

EXAMPLES

(30) Hardness (Rockwell hardness) of the outer rings of rolling bearings that were treated under various sets of heat treatment conditions shown in Table 1 was measured and evaluated. Results are also shown in Table 1. As for the dimensions of the outer ring, the outer diameter was 24 mm, the inner diameter was 18 mm, and the width was 8 mm. DSR-40N was used as a martensite steel material. The outer ring was produced by cutting a rod material without subjecting a resulting material to processing of removing a surface layer such as polishing. Heat treatment was performed using a vacuum furnace. Quick cooling that was performed after quenching was carried out by introducing a nitrogen gas into the furnace.

(31) TABLE-US-00001 TABLE 1 Manufacture Manufacture Manufacture Manufacture Manufacture Manufacture Example 1 Example 2 Example 3 Example 4 Example 5 Example 6 Quenching Temperature (° C.) 1,062 1,038 1,050 1,050 1,050 1,050 Time (min) 70 50 60 60 60 60 Sub-zero treatment Temperature (° C.) −60 −60 −72 −48 −60 −60 Time (min) 30 30 40 20 30 30 Tempering Temperature (° C.) 180 180 180 180 192 168 Time (min) 120 120 120 120 130 110 Hardness (HRC) 58.2 57.3 57.9 57.3 57.1 58.4

(32) It was confirmed from the results of Table 1 that the Rockwell hardness can be made larger than or equal to 55 HRC by performing vacuum heat treatment on DSR-40N under the above sets of heat treatment conditions.

(33) Subsequently, influence of execution/non-execution of the surface layer part removal processing on the corrosion resistance was measured and evaluated using the outer ring of Manufacture Example 1. The outer ring of Manufacture Example 1 whose outer circumferential surface was subjected to the surface layer part removal processing by polishing away a surface layer part having a depth of 100 μm from the surface was employed as Working Example. On the other hand, the outer ring of Manufacture Example 1 itself that was not subjected to the surface layer part removal processing was employed as Comparative Example. As for corrosion resistance, pitting potential was measured according to JIS G 0577: 2014. The contents of JIS G 0577: 2014 are incorporated herein by reference.

(34) A measured pitting potential of Working Example (with surface layer part removal) was 116 mV, which is equivalent to the value of SUS630 which is generally said to be high in corrosion resistance. In contrast, a measured pitting potential of Comparative Example was −12 mV. Thus, whereas in Comparative Example (without surface layer part removal) proper corrosion resistance cannot be obtained though it is high in hardness, it was confirmed that in Example (with surface layer part removal) both of high hardness and high corrosion resistance are secured.

(35) Subsequently, why execution/non-execution of the surface layer part removal causes a difference in corrosion resistance was investigated. In the investigation, an element composition profile in the depth direction was analyzed using an Auger electron spectroscopic analyzer by repeating, starting from the outermost surface layer part, an element composition analysis and nm-order removal of a material layer by ion sputtering. The depth was obtained by converting the ion sputtering time. Results are shown in FIG. 3.

(36) It is seen from the results shown in FIG. 3 that in Comparative Example (without surface layer part removal), the Cr ratio clearly is smaller in a region having a depth of about 40 nm from the surface (i.e., Cr-deficient region) than a reference (constant) Cr ratio in a region where the depth is around 100 nm. Thus, it was found that the reason of the corrosion resistance reduction is that no effective passivation film is formed adjacent to the surface. It is seen from the above measurement result that it suffices that the thickness (depth) of a layer to be removed be at least larger than or equal to 40 nm from the surface in the surface layer part removal processing. That is, it is expected that a state that the Cr ratio is constant from the surface part, that is, no Cr-deficient layer exists, would be established by removing a surface layer part having a depth of 40 nm or more from the surface.

(37) In contrast, in Working Example (with surface layer part removal), it is seen that no Cr-deficient layer exists within a region having a depth of 10 nm or more from the surface. Furthermore, a sufficiently large Cr ratio was detected even at a depth of 2 nm, which means that no Cr-deficient layer exists even substantially at the surface. That is, it can be said that the Cr ratio is kept almost constant as the position goes deeper from the surface. It has thus been found that in Working Example almost no Cr-deficient layer occurs and high corrosion resistance can be secured because Cr exists adjacent to the surface.

(38) The present application is based on Japanese Patent Application No. 2016-030498 filed on Feb. 19, 2016, the contents of which are incorporated herein by reference.

DESCRIPTION OF REFERENCE NUMERALS AND SIGNS

(39) 1, 10: Rolling bearing 2, 12: Outer ring 3, 13: Inner ring 4, 14: Rolling body 5, 15: Holding device 16: Sealing member