GB2345296A

GB2345296A - Rolling member and production

Info

Publication number: GB2345296A
Application number: GB9927194A
Authority: GB
Inventors: Kenji Yamamura; Manabu Ohori; Kazuo Miyaguchi
Original assignee: NSK Ltd
Current assignee: NSK Ltd
Priority date: 1998-11-19
Filing date: 1999-11-17
Publication date: 2000-07-05
Anticipated expiration: 2019-11-17
Also published as: DE19955565A1; DE19955565B4; JP2000212721A; GB2345296B; GB9927194D0

Abstract

At least one of the rolling members of a rolling bearing or a ball screw apparatus is formed with an alloy steel containing C: 0.1 to 0.7 wt%, Si: 0.1 to 1.5 wt%, Mn: 0.1 to 1.5 wt%, Cr: 0.5 to 3.0 wt%, V: 0.6 to 2.0 wt%, Mo: 3.0 wt% or less, Ni: 2.0 wt% or less and the balance being Fe and unavoidable impurities, and is subjected to a carbonitriding at temperature of 920{C or higher, so that a carbon density in the surface of a completed product is set to be 0.7 to 1.3 wt% and a nitrogen density therein is set to be 0.15 to 0.3 wt%, whereby carbide, nitride and carbonitride of grain diameter being 0.1 žm or less are precipitated at least 400 pieces/100 žm<SP>2</SP> or more in the completed surface.

Description

2345296 ROLLING MEMBER AND PRODUCTION The invention relates to a rolling

member of a rolling bearing or a ball screw bearing to be used in iron and steel equipment, agricultural machinery, vehicles, building machinery and other industrial machinery, and in particularly to a rolling member excellent in abrasion resistance to be suitably served in applications especially requiring the abrasion resistance. In addition, the invention relates to a method of producing the rolling member.

The rolling bearing is severely practiced as repeatedly receiving shearing stresses under bearing high pressure, and so it is necessary to secure rolling fatigue lives (called briefly as "life" or lives" hereafter) by standing up against the shearing stress. For this purpose, in the related art, a high carbon chromium (SUJ2) is employed forbearing raceways (inne,r and outer races) being the rolling members of the rolling bearing or blank materials for fabricating the rolling elements, and is subjected to hardening and tempering to a Rockwell hardness of HRC58 to 64 for attempting to secure lives.

There is an example to improve lives by using a case -I- hardening steel as a substitution for SUJ2. In this case, it is necessary to determine a hardness curve following distribution of internal shearing stress caused by a contacting pressure. Therefore, SCR420H, SCM420H, SAE8620H or SAE4320 of low carbon case hardening steels having a good hardening property is employed and subjected to the carbonization or the carbonitriding treatment, hardening and tempering, so that -surface hardness of the bearing part is made HRC30 to 48 for securing a required life.

However, recently, high load and high speed of machinery using the rolling bearing go forward, and the using conditions of the bearings have remarkably been severer. Accompanying therewith, the following problems appear.

Namely, the problems are that temperature is heightened by the high load and high speed, and the hardness of the rolling member of the bearing is thereby deteriorated, resulting in inviting rolling fatigue characteristic and abrasion characteristic. In particular, the abrasion resistance is remarkably deteriorated and the life is shortened by increasing slippage due to the high speed and by a shortage of lubricating capacity due to lowering viscosity of a lubricant ef f ected by heightening of the temperature.

on the other hand, with respect to the bearing used at ultra low speed as a continuous casting equipment, abrasions have been problems because oil films are formed insufficiently. As measures thereto, JP-A-8-049057 discloses that a steel containing V: 0. 8 to 2. 0 wt% is treated with a carbonizing or carbonitriding for satisfying relations that a carbon density in the surface of the rolling member of the rolling bearing is 0. 8 to 1.5 wt% and the ratio of a densi ty of V/C in the surface is 1 to 2. 5, thereby to precipitate VC carbide in the surface of the product. But considerations are not given to the density of nitrogen influencing the abrasion resistance.

JP-A-8-311603 shows that the abrasion resistance is by far improved if the nitrogen density in the completed surface of the rolling member is 0. 3 wt% or more, but there remain problems unsolved that since a diffusion coefficient of nitrogen is small, it takes very long time in the heat treatment in big scaled products which take large grinding margins after the heating treatments, and the grinding takes more time than conventionally.

The problems involved in the abrasions accompanied with the severe using conditions may be applicable to ball screw shafts, ball nuts or balls as the rolling members of the ball screw apparatus. Namely, the ball screw apparatus has recently been used under heavy loading conditions instead of hydraulic cylinders of injection molding machine or pressing machines, and in such cases, ball screw apparatus is once stopped as a maximum load is added, and turned over. But, before and after stopping, a lubricant is difficult to be lead in between a screw groove and a ball, and an oil f ilm is hardly formed so as to cause a metal-to-metal contact, resulting in easily generating the abrasion.

Accordingly, the invention is to solve the unsolved 10 problems in the related art in regard to the rolling members constituting the rolling bearings or ball screw apparatus, paying attentions to that if adding V: 0.6 wt% or more and carbonitriding at 920 'C or higher, fine carbide or carbonitrides of 0. 1 lim or less are precipitated in the product surface, and it is an object of the invention to offer the rolling member excellent in the abrasion resistance by securing the precipitation amount per unit area of the fine carbide and carbonitride.

For accomplishing the object, there is provided at least one of the rolling members of a rolling bearing or a ball screw apparatus formed with an alloy steel containing C: 0.1 to 0.7 wt%, Si: 0.1 to 1.5 wt%, Mn: 0.1 to 1.5 wt%, Cr: 0.5 to 3.0 wt%, V: 0.6 to 2.0 wt%, Mo: 3.0 wt% or less, Ni: 2.0 wt% or less and the balance being Fe and unavoidable impurities, and is subjected to a carbonitriding at temperature of 9200C or higher, so that a carbon density in the surface of a completed product is set to be 0.7 to 1.3 wt% and a nitrogen density therein is set to be 0.15 to 0.3 wt%, whereby carbide, nitride and carbonitride of grain diameter being 0.1 11m or less are precipitated at least 400 pieces/100 A, m 2 or more in the completed surface.

A particular embodiment in accordance with this invention will now be described with reference to the accompanying drawings; in which:- Figs. 1A and 1B are schematic views of the abrasion testing apparatus of two cylinder type, Fig. IA is a front view thereof and Fig. 1B is a side view of the same; Fig. 2 is a graph showing the relation between the V density (wt%) and the depth where the nitrogen density is 0.25 wt% (a distance from the surface: mm), classified by the heat treatments; Fig. 3 is a graph showing distributions of grain diameter of carbide, nitride and carbonitride in the surface of the completed products of the rolling bearing of the embodiment; and Fig. 4 is a graph showing distributions of grain diameter of carbide, nitride and carbonitride in the surface of the completed products of the rolling bearing of the comparative example.

Inventors of this.patent application repeatedly made studies on the relation between the abrasion resistance of the rolling members constituting the rolling bearing and the ball screw apparatus as well as the nitrogen density of the product surface and further chemical compositions of materials, and consequently found that if carbonitriding a steel material added with V: 0.6 wt% or more at high temperatures as 920T or higher, the diffusion coefficient of nitrogen is made high, and concurrently very fine carbide, nitride and carbonitrides of 0. 1 Xim or less are precipitated.

This phenomenon is thought as follows. Carbide, nitride and carbonitride of V all have structures of NaCl type. In case the carbonitriding treatment is carried out at high temperatures so as to form carbide, nitride and carbonitride of V not including alloying elements other than V (e.g., Cr or Fe), the diffusion coefficient of nitrogen comes near to the diffusion coefficient of carbon, and the carbide, nitride and carbonitride of V are very slow in growing speed.

The inventors further found that if these fine carbide, nitride and carbonitride are much precipitated, an excellent abrasion resistance may be obtained without heightening the nitrogen density more than a necessary amount, and particularly, the carbide, nitride and carbonitride of grain diameter being 0.1 gm or less are prominently useful to the improvement of the abrasion resistance of the rolling member, and they established this invention.

Specifically, taking up the rolling bearing, forexample, at least one of an inner race, an outer race and a rolling member which are the rolling members thereof, or taking up the ball screw apparatus, for example, at least one of a ball screw shaft, a ball nut and a ball which are the rolling members thereof is formed with an alloy steel containing in weight percent C: 0. 1 to 0. 7%, Si: 0. 1 to 1. 5%, Mn: 0. 1 to 1. 5%, Cr:

0. 5 to 3. 0%, V: 0. 6 to 2. 0%, Mo: 3. 0 or less, Ni: 2. 0 or less and the balance being Fe and unavoidable impurities, the alloy steel is performed with the carbonitriding at 9200C or higher, so that the carbon density in the surface of a completed product is set to be 0. 7 to 1. 3 wt% and the nitrogen density therein is set to be 0. 15 to 0. 3 wt%, whereby carbide, nitride and carbonitride of grain diameter being 0. 1 gm or less are precipitated at least 400 pieces/100 lim 2 or more in the surface of the completed surface.

In this manner, as a result of V accelerating the diffusion of the nitrogen, the excellent abrasion resistance is available though not heightening the nitrogen density more than necessary, and for example, the heat treating time may be shortened in big scaled products having the large grinding margin after the heat treatment, and it is possible to cheaply offer the rolling members having the excellent abrasion resistance.

A further reference will be made to critical significances of the chemical composition of the alloy steel for fabricating the rolling member of the invention having the excellent abrasion resistance.

[C content: 0.1 to 0.7 wt%l C is a necessary element for improving the hardness after the hardening and the tempering by making a matrix martensite.

The reason for determining the content to be 0. 1 wt% or more is for securing the necessary strength as the rolling member. The reason for determining the upper limit to be 0.7 wt% is because exceeding this range, carbide is already precipitated during being still a blank material, processability in plastic working or machining for forming products prior to the heating treatment is deteriorated.

[Si content: 0.1 to 1.5 wt%l Si is a necessary element as a deoxidizing agent in making steel, and is effective for heightening temper- so f tening resistance and improving rolling fatigue live. Therefore, it is contained 0.1 wt% or more, but the upper limit is set to be 1.5 wt%, since it arrests invasions of carbon or nitrogen from the surface when carbonitriding and lowers a heat treating 5 productivity.

[Mn: 0.1 to 1.5 wt%l Mn is a necessary element as a deoxidizing agent and a desulfurizing agent, and is effective for improving hardenability. Therefore, it is contained 0.1 wt% or more.

However, the upper limit is set to be 1.5 wt%, since much addition lowers the machinability.

[V: 0.6 to 2.0 wt%j V increases the temper-softening resistance and forms very fine carbide, nitride and carbonitride of high hardness effective for improving the abrasion resistance. For providing the nitrogen density growing in a deep part when carrying out the carbonitriding at high temperature, it is preferable to add V 0.6 wt% or more, and so the lower limit is set to be 0.6 wt%. on the other hand, if much adding, an ef f ect of the addition cannot be absorbed, the processability is lowered. Further, this element is expensive d.isadvantageously in cost, and so the upper limit is set to be 2.0 wt%.

[Cr: 0.5 to 3.0 wt% -9" In addition to the improvement of the hardenability and the solid solution strengthening of the matrix, Cr is useful_ for precipitating carbides, nitrides and carbonitrides in the surf ace layers of the rolling members by use of carbonitriding, thereby improving the rolling fatigue life and the abrasion resistance. As a preferable lower limit of the Cr content, it is determined to be 0.5 wt% because being lower than that, the addition effect is low. In contrast, much addition forms Cr oxides in the product surface to prevent invasion of carbon or nitrogen f rom the surf ace when carbonitriding, resulting in lowering the heat treating productivity. Therefore, the upper limit is set to be 3. 0 wt%.

[Mo: 3.0 wt% or less] Mo is effective for increasing the temper-softening resistance and precipitating, as well as Cr, carbides, nitrides and carbonitrides in the surface layers of the rolling members when carbonitriding and improving the rolling fatigue life and the abrasion resistance. The upper limit is determined to be 3.0 wt% because much addition makes the plastic workability bad and is expensive. [Ni: 2.0 wt% or less) Ni is an effective element for improving toughness by being solid in the matrix. However, too much addition makes residual austenite amount in the surface layer too increase, resulting in lowering the hardness, and therefore, the upper limit is set to be 2.0 wt%.

other these alloying elements, Pt--:0.02%, S-"0.05%, Cu <0.10%, 05-'15 ppmmaybe contained as unavoidable impurities, and a limitation to 0:510 ppm is desirable for keeping non metallic inclusions harmful to the rolling fatigue life characteristic as low as possible.

A still further reference will be made to critical signif icances of the carbon density, the nitrogen density, the fine carbide, nitride and carbonitride in the surface of the completed product being the rolling member excellent in abrasion resistance.

[The carbon density in the surface: 0.7 to 1.3 wt%l For providing the necessary surface hardness as the rolling member, 0.8 wt% or more is ordinarily required, but in the invention, as the nitrogen is included by the carbonitriding, the lower limit is set to be 0. 7 wt%. However, if it is excessive together with the nitrogen, the residual austenite amount in the surface grows overmuch to lower the hardness or pro-eutectoid cementite is precipitated, so that the rolling fatigue life may be probably lowered, and therefore the upper limit is determined to be 1.3 wt%.

[The nitrogen density in the surface: 0.15 to 0.3 wt%l The nitrogen is a very effective element for improving the abrasion resistance, and is added in the surface layer by the carbonitriding treatment, and the content of less than 0. 15 wt% does not bring about a suf f icient ef f ect. However, a too much content worsens the grindability. In the production of big scaled products, the high nitrogen density in the deep part is required. Then, the heating treatment takes a long time, the cost is increased, and so the upper limit is set to be 0.3 wt%.

[Carbide, nitride and carbonitride of grain diameter being 0. 1 10 11m or less are 400 pieces/100 =2 or more] Fine carbide, nitride and carbonitride have high ef fects for improving the abrasion resistance, in particular those of 0.1 = or less are highly effective, and the effects thereof are made remarkable by precipitating at the density of 400 15 pieces/100 lim or more.

(Examples]

Explanation will be made to comparative tests between examples of the invention and comparative examples.

Firstly, a description will be given of tests made on 20 the rolling members of the rolling bearing.

Table 1 shows main chemical compositions of various kinds of alloyed steel materials used in the tests.

Abrasion tests of a two cylinder type as shown in Fig. 1 were carried out to investigate influences to the abrasion Table 1 (Weight %) Steel kinds c si Mn Cr v MO Ni A 0.10 0.40 0.75 1.45 0.98 B 0.25 0.10 0.80 1.50 0.96 c 0.42 0.38 0.82 1.51 1.00 - D 0.41 0.35 0.81 1.51 1.05 0.05 E 0.39 0.37 0.10 1.48 0.95 - F 0.35 0.36 0.78 0.50 0.99 G 0.45 0.37 0.80 1.55 0.60 H 0.40 0.35 0.82 1.49 1.49 1 0.40 0.35 0.80 1.50 2.00 j 0.70 0.25 0.30 1.51 1.00 K 0.39 0.88 0.80 1.50 1.01 L 0.38 1.50 0.82 1.51 1.02 m 0.41 0.36 1.50 1.50 1.01 N 0.40 0,37 0.81 3.00 1.00 - 0 0.25 0.37 0.81 1.51 1.01 0.98 p 0.18 0.25 0.42 1.50 1.01 3.00 Q 0.25 0.28 0,40 1.52 1.02 1.50 R 0.25 0.28 0.39 1.50 1.00 S 0.42 0.36 0.81 1.51 T 0.20 0.32 0.75 1.08 resistance caused by these alloying components and the heating treatments. The tests were carried out in that a pair of axes vertically opposed were attached with test pieces S respectively, and the axes were rotated at low speed in opposite directions while a load P was ef f ected thereon under a condition that the axes were contacted, so as to obtain an average value of abrasion rates (g/m) of both test pieces S. In particular, a lubricant of low viscosity in which oil film is easily run out was poured during rotation for testing the abrasion characteristic in inferior lubrication condition.

The abrasion testing conditions are as follows.

Load: 200 kgf(2.0 kN) Rotation number: 10 rpm Slip percentage: 20% Lubricant: Spindle oil oil temperature: 8CC The test pieces were heat-treated as follows to watch influences of the heat treatment to the abrasion resistance.

[Heat treatment A] The carbonitriding was performed at 920 to 9SO'C for 6 to 8 hours while an enriched gas and ammonia gas were added to an endothermic gas atmosphere, and then the test pieces were air-cooled or slowly cooled to a room temperature. Then, the test pieces were subjected to a secondary hardening at 820 to 8800C, and tempered at 160 to 1800C for 2 to 3 hours.

[Heat treatment B] The carbonitriding was performed at 870 to 90CC for 6 to 8 hours while the enriched gas and ammonia gas were added to the endothermic gas atmosphere, and then the test pieces were air-cooled or slowly cooled to a room temperature. Then, the test pieces were subjected to a secondary hardening at 820 to 8800C, and tempered at 160 to 1800C for 2 to 3 hours.

[Heat treatment Cl An ordinary carbonizing treatment was performed at 920 to 9500C for 6 to 8 hours, and then the test pieces were left in an air to the room temperature. Then, the test pieces were subjected to the secondary hardening at 820 to 88CC, and tempered at 160 to ACC for 2 to 3 hours. Herein, the relation between the heat treating conditions and the V density will be explained. 5 Fig. 2 shows the relation between the V density (wt%) and the depth where the nitrogen density is 0. 25 wt% (a distance from the surface: mm) when the heat treatments A and B are.performed. As seen from this figure, when the carbonitriding is performed at high temperature as the heat treatment A, if the V addition amount is wanted, since a nitrogen potential in the surface is lowered, the high nitrogen density till the deep part is not provided. However, when the V density is high, the high nitrogen density till the deep part is provided. This effect is remarkable at the V addition amount of 0.6 wt% or more.

However, at the low treating temperature below 90CC as the heat treatment B, the effect of the V addition is' small. To obtain the high nitrogen density till the deep part, a heat treating time is necessarily much taken.

Successively, a description will be given of results of the abrasion test. The used test pieces were grind-finished in testing faces after the heat treatment.

Table 2 shows the abrasion tests classified by the combinations of steel kinds and the heat treatments.

Table 2

No. Steel Heat Carbon Nitrogen N Abrasion Remarks kinds treatments density C1o) density (%) (Piece/100 rate (x 10' in surface in surface____ grn gim) 1 A A 0.83 0.23 542 1.15 Example 2 B A 0.93 0.25 534 1.08 Example 3 c A 0.91 0.23 592 1.02 Example 4 D A 0.86 0.22 624 0.96 Example E A 0.95 0.21 526 1.08 Example 6 F A 0.78 0.23 466 1.19 Example 7 G A 0.80 0.15 408 1.34 Example 8 H A 1.21 0.29 734 0.87 Example 9 1 A 1.27 0.26 650 0.92 Example j A 0.92 0.20 482 1.18 Example 11 K A 0,95 0.20 444 1.22 Example 12 L A 0.83 0.19 522 1.09 Example 13 m A 0.88 0.22 476 1.23 Example 14 N A 0.71 0.19 428 1.31 Example 0 A 0.93 0.24 528 1.11 Example 16 p A 0.89 0.23 474 1.14 Example 17 Q A 0.87 0.21 510 1.18 Example 18 R A 0.82 0.23 548 1.09 Example 19 S A 0.97 0.11 182 6.13 Comparative e le T A 1.02 0.01 156 7.02 Comparative example

21 c B 0.87 0.24 266 4.58 Comparative example

22 c c 0.91 242 4.27 Comparative example

T c 0.87 64 9.38 Conventional example

Table 2 shows the carbon density, nitrogen density, piece number N of carbide, nitride and carbonitride of grain diameter being 0.1 lim or less in the surface layers of the test pieces together.

Nos. 1 to 18 are Examples of the invention provided with the excellent abrasion resistance. Comparative Examples Nos. 19 and 20 with the lower V addition amount, and due to the insufficient V addition the necessary nitrogen density is not available, so that an enough abrasion resistance is not provided.

Comparative Example No. 21 was carried out with the carbonitriding treatment at the low temperature, and so there are small piece numbers of carbide, nitride and carbonitride of grain diameter being 0.1 m or less, and the required abrasion resistance is not available.

Comparative Example No. 22 is of the carbonizing treatment, and because of absence of the nitrogen, the required abrasion resistance is not available.

Comparative Example No. 23 is a conventional one of no V addition, the carbonization and a largest abrasion degree, Finally, a description will be given of influences of distributions of grain diameters of the carbide, nitride and carbonitride in surfaces of test pieces (surfaces of completed products) giving to decrease in abrasion rate.

Fig. 3, with respect to Example No. 3, shows and Fig. 4, with respect to Comparative Example No. 21, shows measured results of distributions of grain diameters in the surfaces of the test pieces, respectively.

Comparing both, it is apparent that the distributions of the grain diameters over 0.1 gm are not much different, and those of 0.1 pm or less prominently different. Seeing the 5 abrasion rates of both in Table 2, Example No. 3 is 1.02 x 10-6 g/M, while Comparative Example No. 21 is four times as 4.58 x 10-6 g/m. It is seen from this fact that the number of grains of diameter being 0.1 lim or less is largely relative with the decrease of the abrasion rate. Seeing counted results of the number N of grains of diameter being 0.1 pm or less existing in the area of 100 m' or less shown in Table 2, Examples Nos. 1 to 18 are all over 400 pieces. Thus, the invention determines the number N to be 400 pieces/100 M2 or more.

Seeing the counted results of the number N of grains of diameter being 0. 1 pm or less in Table 2, Example No. 8 is most as 734 pieces/100 pm, but the invention is not limited to it. But when too many, grains are combined one another and grow, and as a result that the grain diameter is large, the abrasion resistance might be probably spoiled. Therefore, the piece number N of the grain diameter of 0. 1 11m or less of the invention may be increased to an extent that grains do not grow by the combination one another.

In addition, in case that the grains of diameter being 0. 1 11m or less exists in the area of 100 m' in theory, assuming - is- that: 100pm' = 10 gm x 10 11m, the maximum of the number N of grains is computed as follows.

(10lim / 0.1 pm) x (10 pm / 0.1 pm) = 10000pieces/100pm2 However, as described the above, it is preferable that the piece number N of the grain diameter of 0.1 lim or less is increased to an extent that grains do not grow by the combination one another. Therefore, the upper limit of the number N of the.grain diameter of 0.1 pm or less is preferably set to be 2500pieces/ilm'.

Next, a description will be given of tests performed by making self-aligning roll bearings.

The self-aligning roll bearing is relatively large in a contacting ellipse between the bearing racewayand the rolling element and accordingly differential slipping and spin slipping are large. In particular, when the self-aligning roll bearing is served to a continuous casting equipment, an abrasion of an outer race to be used as a f ixed race is a problem.

Then, the self-aligning roll bearing of Type No. 22210CD (outer diameter: 90 mm, inner diameter: 50 mm, width: 23 mm) was made as follows for carrying out an endurance test. The steel kinds A to T in Table 1 were employed as blank materials for the outer races of the rolling members to be tested, and SUJ2 of JIS steel kinds was employed for the inner races and the rolling elements. The heat treating conditions of the outer race were the same as those of Table 2. Further, the carbon density in the surface of the outer race, the nitrogen density in the surface, the piece number of the carbide, nitride and carbonitride of grain diameter being 0.1 pm or less were 5 also all the same as Table 2.

The thus produced self-aligning roll bearings as test pieces were tested under the following conditions.

Load: 25 N Rotation number: 10 rpm Testing temperature: 800C Lubricant: Mineral oil based grease Testing time: 300 hr.

The tests were practiced while supplying an ion exchanging water 0.1 cc/hr for easily running out of an oil film.

After the tests, shapes of bus lines of the bearing faces of the outer race in a maximum loading position were measured for reading out the depth of a maximum abrasion. Values of the depths of the maximum abrasions are shown in Table 3 together with the steel kinds, heating treatments, carbon and nitrogen densities in the surfaces, piece number N of carbide, nitride and carbonitride of grain diameter being 0. 1 pm or less, the abrasion rate and the like.

As apparently from Table 3, also in case of the self- aligning roll bearings, it may be said that Examples of the invention Nos. 1 to 18 have the excellent abrasion resistance in comparison with Comparative Examples and Conventional Examples of Nos. 19 to 23.

Table 3

No. Steel Heat Carbon Nitrogen N Abrasion Maximum Remarks kinds treatments density density (%) (Piece/100 rate (x 101 abrasion (%) in surface PM g/m) depth in surface 1 A A 0.83 0.23 542 1.15 5 Example 2 B A 0.93 0.25 534 1.08 4 Example 3 C A 0.91 0.23 592 1.02 4 Example 4 D A 0.86 0.22 624 0.96 4 Example E A 0.95 0.21 526 1.08 4 Example 6 F A 0.78 0.23 466 1.19 5 Example 7 G A 0.80 0.15 408 1.34 5 Example 8 H A 1.21 0.29 734 0.87 3 Example 9 1 A 1.27 0.26 650 0.92 3 Example j A 0.92 0.20 482 1.18 4 Example 11 K A 0.95 0.20 444 1.22 4 Example 12 L A 0.83 0.19 522 1.09 4 Example 13 m A 0.88 0.22 476 1.23 4 Example 14 N A 0.71 0.19 428 1.31 4 Example 0 A 0.93 0.24 528 1.11 4 Example 16 p A 0.89 0.23 474 1.14 3 Example 17 Q A 0.87 0.21 510 1.18 4 Example 18 R A 0.82 0.23 548 1.09 4 Example 19 S A 0.97 0.11 182 6.13 24 Comparative example

T A 1.02 0.01 156 7.02 27 Comparative examWe 21 C B 0.87 0.24 266 4.58 19 Comparative exa le 22 c c 0.91 - 242 4.27 20 Comparative example

23 T c 0.87 64 9.38 35 Conventional example

Successively, a description will be given of the endurance tests performed by making the ball screw apparatus.

The ball screw apparatus of axial diameter being 80 mm and lead being 20 mm was made for testing. As used materials, any of the steel kind D shown in Table 1 or SCM420H of JIS steel kind were used for the ball screw axes and the ball nuts, and any of the steel kind D or SUJ2 of JIS steel kind were used for balls.

These steel kinds were variously combined for making ball screw axes, ball nuts and ball as the rolling members, and setting up the ball screw apparatus so as to prepare test pieces as shown with I to IV in Table 4.

TablP 4 Marks Axes Nuts Balls Abrasion loss Remarks of axis D D D 2 Inventive example

II D SCM420H D 2 Rm Inventive example

III D SCM420H SUJ2 4 jAm Inventive example

IV SCM420H SCM420H SUJ2 12 tAm Conventional example

The heat treating conditions were set as follows per steel kind of the blank material.

With respect to the rolling members made of the steel kind D, the carbonitriding was performed at 920 to 95CC for 1 to 24 hours while the enriched gas and ammonia gas were added to an endothermic gas atmosphere, and then the rolling members were air-cooled or slowly cooled to a room temperature. Then, the rolling members were subjected to a secondary hardening at 820 to 880"C, and tempered at 160 to 180'C for 2 to 3 hours.

With respect to the rolling members made of SCM420H of JIS steel kind, the ordinary carbonizing treatment was performed at 920 to 9500C for 6 to 24 hours, left in an air to the room temperature, subjected to the secondary hardening at 820 to 880"C, and tempered at 160 to 1800C for 2 to 3 hours. The balls made of SUJ2 of JIS steel kind was subjected to the hardening at

820 to 8600C and tempered at 180 to 220C.

The ball screw axes, ball nuts and balls to be tested were subjected to the grinding process after the heating treatment, and the balls were further subjected to a lapping finish.

With respect to the thus produced ball screw apparatus to be tested, the endurance tests were practiced under the following conditions.

Load: Maximum 280 N Rotation number: Maximum 150 rpm (reciprocation) Lubricant: Mineral oil based grease Shot number (frequency of reciprocation) of the endurance tests were million shots. The shapes of the bearing grooves in the ball screw axes before and after tests were measured to obtain maximum depths of abrasion for valuation. Valued results are shown in Table 4. As apparently from this, according to the invention, it is possible to reduce abrasion of the ball screw axis to be served under heavy loading conditions to 1/3 to 1/6 of the abrasion according to the conventional art.

As explained, according to the first aspect of the invention, the alloyed steel added with V: 0. 6 wt% or more is subjected to the carbonitriding at 920C or more, carbon, nitride and carbonitride of grain diameter being 0. 1 pm or less are precipitated at least 400 piecess/100 pm2 in the surface of at least one of the inner race, outer race and rolling element of the rolling bearing as the completed product. Thus, the invention takes the effect capable of offering the rolling member very excellent in the abrasion resistance.

Claims

1 A rolling member, wherein at least one of the rolling members of a rolling bearing or a ball screw apparatus is formed with an alloy steel containing C: 0.1 to 0.7 wt%, Si: 0.1 to 1.5 wt%, Mn: 0.1 to 1.5 wt%, Cr: 0.5 to 3.0 wt%, V: 0.6 to 2.0 wt%, Mo: 3.0 wt% or less, Ni: 2.0 wt% or less and the balance being Fe and unavoidable impurities, a carbon density in the surface of a completed product is set to be 0.7 to 1.3 wt% and a nitrogen density therein is set to be 0.15 to 0.3 wt%, and the number of at least one kind of carbide, nitride and carbonitride of grain diameter being 0. 1 11 m or less precipitated in the completed surface is at least 400 pieces/100 /Am 2 or more.

2. The rolling member as set forth in claim 1, wherein the rolling bearing is a self-aligning roll bearing.

3. The rolling member as set forth in claim 1 or 2, wherein the number of at least one kind of carbide, nitride and carbonitride of grain diameter being 0.1 Im or less precipitated in the completed surface is 400 - 2500 pieces/10Ogm'.

CZ,

4. A method of producing a rolling member, comprising the steps of:

preparing at least one of the rolling members of a rolling bearing or a rolling member of a ball screw apparatus is formed with an alloy steel containing C: 0.1 to 0.7 wt%, Si: 0.1 to 1. 5 wt%, Mn: 0. 1 to 1. 5 wt%, Cr: 0. 5 to 3. 0 wt%, V: 0. 6 to 2. 0 wt%, Mo: 3. 0 wt% or less, Ni: 2. 0 wt% or less and the balance being Fe and linavoidable impurities; and cabonitriding the rolling member at temperature of 920T or higher, so that at least one kind of fine carbide, nitride and carbonitride is precipitated in the completed surf ace of the rolling member.

5. The method as set forth in claim 4, further comprising the step of:

cooling the rolling member to a room temperature, after the carbonitriding; and reheating the rolling member to temperature of 820 to HTC, to thereby performing the hardening of the rolling member.

6. The method as set forth in claim 4 or 5, wherein the carbonitriding is performed at temperature of 9200C to 9501,C.

7. A rolling member substantially as described.

8. A method of producing a rolling member substantially as described.