Method of producing roughly shaped material for rolling bearing

Abstract

A roughly shaped material for a rolling bearing of the present invention is produced by forging a steel composed of a high-carbon chrome bearing steel containing 0.7 mass % to 1.2 mass % of a carbon, and 0.8 mass % to 1.8 mass % of a chromium to a predetermined shape while heating the steel to a forging temperature in a range of (Ae.sub.1 point+25 C.) to (Ae.sub.1 point+105 C.), cooling a forged article to a temperature of Ae.sub.1 point or lower, and performing an annealing in which the forged article that is obtained is heated to a soaking temperature in a range of (Ae.sub.1 point+25 C.) to (Ae.sub.1 point+85 C.), the forged article is retained for 0.5 hours or longer, and the forged article is cooled down to 700 C. or lower at a cooling rate of 0.30 C./s or slower.

Claims

1. A method of producing a roughly shaped material for a rolling bearing by forging a steel composed of a high-carbon chrome bearing steel of which a Vickers hardness is Hv240 or less containing 0.7 mass % to 1.2 mass % of a carbon, and 0.8 mass % to 1.8 mass % of a chromium, the method comprising: (A) forging the steel to a predetermined shape while heating the steel to a forging temperature in a range of (Ae.sub.1 point+25 C.) to (Ae.sub.1 point+105 C.), and cooling a forged article to a temperature of Ae.sub.1 point or lower; and (B) performing an annealing in which the forged article obtained in (A) is heated to a soaking temperature in a range of (Ae.sub.1 point+25 C.) to (Ae.sub.1 point+85 C.), the forged article is retained for 0.5 hours or longer and 1 hour or shorter, and the forged article is cooled down to 700 C. or lower at a cooling rate of 0.020 C./s or faster and 0.30 C./s or slower.

2. The method of producing a roughly shaped material for a rolling bearing according to claim 1, wherein the soaking temperature is set to 760 C. to 820 C.

3. The method of producing a roughly shaped material for a rolling bearing according to claim 1, wherein the cooling rate is set to 0.27 C./s or slower.

4. The method of producing a roughly shaped material for a rolling bearing according to claim 2, wherein the cooling rate is set to 0.27 C./s or slower.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a cross-sectional view of a rolling bearing including bearing rings which are formed by a roughly shaped material for bearing rings which is produced by a production method according to an embodiment of the invention.

(2) FIG. 2 is a process diagram showing a sequence and heat treatment conditions of the production method according to the embodiment of the invention.

(3) FIG. 3 is a process diagram showing a sequence of a warm forging process in the production method according to the embodiment of the invention.

(4) FIG. 4 is a drawing-substituting photograph showing results obtained by observing a structure state in each process of the production method according to the embodiment of the invention, (A) is a drawing-substituting photograph of a steel structure before the warm forging process, (B) is a drawing-substituting photograph of a structure of a forged article at the time of terminating the warm forging process, and (C) is a drawing-substituting photograph of a structure of a roughly shaped material for bearing rings which is obtained after an annealing process.

(5) FIG. 5 is a view showing results obtained by investigating a relationship between a soaking temperature and hardness, and results obtained by observing a structure of a roughly shaped material for bearing rings which is obtained after an annealing process in Test Example 3. (a) is a graph showing the results obtained by investigating the relationship between the soaking temperature and the hardness, (b) is a drawing-substituting photograph of the structure of the roughly shaped material for bearing rings when the soaking temperature is set to 740 C., (c) is a drawing-substituting photograph of the structure of the roughly shaped material for bearing rings when the soaking temperature is set to 760 C., (d) is a drawing-substituting photograph of the structure of the roughly shaped material for bearing rings when the soaking temperature is set to 820 C., and (e) is a drawing-substituting photograph of the structure of the roughly shaped material for bearing rings when the soaking temperature is set to 840 C.

(6) FIG. 6 is a view showing results obtained by investigating a relationship between a cooling rate and hardness, and results obtained by observing a structure of a roughly shaped material for bearing rings which is obtained after an annealing process in Test Example 5. (a) is a graph showing the results obtained by investigating the relationship between the cooling rate and the hardness, (b) is a drawing-substituting photograph of the structure of the roughly shaped material for bearing rings when the cooling rate is set to 7 C./min (0.12 C./s), (c) is a drawing-substituting photograph of the structure of the roughly shaped material for bearing rings when the cooling rate is set to 16 C./min (0.27 C./s), (d) is a drawing-substituting photograph of the structure of the roughly shaped material for bearing rings when the cooling rate is set to 26 C./min (0.43 C./s), and (e) is a drawing-substituting photograph of the structure of the roughly shaped material for bearing rings when the cooling rate is set to 93 C./min (1.55 C./s).

(7) FIG. 7 is a view showing results obtained by investigating a relationship between a cooling temperature and hardness, and results obtained by observing a structure of a roughly shaped material for bearing rings which is obtained after an annealing process in Test Example 6. (a) is a graph showing the results obtained by investigating the relationship between the cooling temperature and the hardness, (b) is a drawing-substituting photograph of the structure of the roughly shaped material for bearing rings when the cooling temperature is set to 650 C., (c) is a drawing-substituting photograph of the structure of the roughly shaped material for bearing rings when the cooling temperature is set to 700 C., and (d) is a drawing-substituting photograph of the structure of the roughly shaped material for bearing rings when the cooling temperature is set to 730 C.

(8) FIG. 8 is a diagram showing heat treatment conditions in Example 1.

(9) FIG. 9 is a diagram showing heat treatment conditions in Example 2.

(10) FIG. 10 is a drawing-substituting photograph showing results obtained by observing a structure of a forged article that is obtained after an annealing process in Test Example 7. (a) is a drawing-substituting photograph showing a result obtained by observing a structure of a forged article in Example 1, and (b) is a drawing-substituting photograph showing a result obtained by observing a structure of a forged article in Example 2.

(11) FIG. 11 is a view showing heat treatment conditions in a method of producing a roughly shaped material for bearing rings in the related art in which hot forging is performed, and results obtained by observing a state of a structure in each process.

EMBODIMENTS OF THE INVENTION

(12) Hereinafter, a method (hereinafter, also simply referred to as production method) of producing a roughly shaped material for a rolling bearing of the invention will be described in detail with reference to the accompanying drawings.

(13) FIG. 1 is a cross-sectional view of a rolling bearing including bearing rings which are formed by a roughly shaped material for bearing rings which is produced by a production method according to an embodiment of the invention. Furthermore, in this embodiment, description will be given of a conical roller bearing as an example of the rolling bearing, but the invention is not limited to the example. In addition, in this embodiment, description will be given of a roughly shaped material for formation of bearing rings (an inner ring and an outer ring) of the rolling bearing as an example of the roughly shaped material. However, the invention is not limited to the example, and is also applicable to a roughly shaped material for formation of a rolling body (roller, ball).

(14) A rolling bearing 1 shown in FIG. 1 includes an inner ring 11, an outer ring 12, a plurality of rollers (rolling bodies) 13 which are arranged between the inner and outer rings 11 and 12, and a retainer 14 that retains the plurality of rollers 13. The inner ring 11 and the outer ring 12 of the rolling bearing 1 are formed from a roughly shaped material for bearing rings (a roughly shaped material for an inner ring and a roughly shaped material for an outer ring) which is produced by the production method of the invention.

(15) FIG. 2 is a process diagram showing a sequence and heat treatment conditions of the production method according to the embodiment of the invention, FIG. 3 is a process diagram showing a sequence of a warm forging process in the production method according to the embodiment of the invention, and FIG. 4 is a drawing-substituting photograph showing results obtained by observing a structure in each process of the production method according to the embodiment of the invention. In FIG. 4, a scale bar represents 10 m.

(16) The production method according to the embodiment of the invention includes (A) forging a steel composed of a high-carbon chrome bearing steel containing 0.7 mass % to 1.2 mass % of a carbon, and 0.8 mass % to 1.8 mass % of a chromium to a predetermined shape while heating the steel to a forging temperature in a range of (Ae.sub.1 point+25 C.) to (Ae.sub.1 point+105 C.), and cooling a forged article to a temperature of Ae.sub.1 point or lower (refer to (a) warm forging process in FIG. 2), and (B) performing an annealing in which the forged article obtained in the process (A) is heated to a soaking temperature in a range of (Ae.sub.1 point+25 C.) to (Ae.sub.1 point+85 C.), and the forged article is retained for 0.5 hours or longer, and cooling the forged article to 700 C. or lower at a cooling rate of 0.30 C./s or slower (refer to (b) annealing process in FIG. 2). Furthermore, in this specification, Ae.sub.1 point represents an equilibrium temperature (austenitizing temperature) that defines the lower limit presence of austenite. Hereinafter, respective processes will be described.

(17) (Warm Forging Process)

(18) In the warm forging process, first, steel composed of high-carbon chrome bearing steel containing 0.7 mass % to 1.2 mass % of carbon, and 0.8 mass % to 1.8 mass % of chromium is heated until a temperature of the central portion of the steel reaches a forging temperature in a range of (Ae.sub.1 point+25 C.) to (Ae.sub.1 point+105 C.) (refer to (a) in FIG. 2).

(19) Examples of the steel composed of high-carbon chrome bearing steel include steel composed of high-carbon chrome bearing steel containing 0.7 mass % to 1.2 mass % of carbon, 0.8 mass % to 1.8 mass % of chromium, 0.15 mass % to 0.35 mass % of silicon, 0.25 mass % to 0.50 mass % of manganese, 0.025 mass % or less of phosphorous, 0.008 mass % or less of sulfur, and a remainder including iron and unavoidable impurities.

(20) The amount of carbon contained in the steel is 0.7 mass % or greater from the viewpoint of securing fatigue life sufficient for usage as a bearing ring by increasing the amount of cementite in the roughly shaped material for bearing rings, and is preferably 0.98 mass % or greater. In addition, the amount of carbon is 1.2 mass % or less from the viewpoint of securing satisfactory machinability during manufacturing of the bearing rings, and preferably 1.05 mass % or less.

(21) The amount of chromium contained in the steel is 0.8 mass % or greater from the viewpoint of securing hardenability of steel and thermal stability of cementite, and preferably 1.4 mass % or greater. In addition, the amount of chromium is 1.8 mass % or less from the viewpoints of suppressing quenching cracks during quenching, and preventing a decrease in mechanical properties, and preferably 1.6 mass % or less.

(22) The amount of silicon contained in the steel is preferably 0.15 mass % or greater from the viewpoint of securing rolling fatigue life sufficient for usage as the bearing ring, and is preferably 0.35 mass % or less from the viewpoint of improving heat treatment efficiency during manufacturing of the bearing ring.

(23) The amount of manganese contained in the steel is preferably 0.25 mass % or greater from the viewpoint of securing hardenability of steel.

(24) Phosphorous in the steel is an impurity. According to this, the amount of phosphorous contained in the steel is preferably 0.025 mass % or less from the viewpoint of securing toughness or from the viewpoint of rolling fatigue life, and is more preferably 0.015 mass % or less. The smaller the amount of phosphorous contained in the steel is, the more preferable it is. Accordingly, the lower limit of the amount of phosphorous contained in the steel may be 0 mass %. However, it is not technically easy to reduce the amount of phosphorous to 0 mass %. In addition, the steel-making cost increases for stable reducing of the amount of phosphorous to less than 0.001 mass %. Accordingly, the lower limit of the amount of phosphorus contained in the steel may be set to 0.001 mass %.

(25) Sulfur in the steel is an impurity. According to this, the amount of sulfur contained in the steel is preferably 0.008 mass % or less from the viewpoint of improving rolling fatigue life, and more preferably 0.005 mass % or less. The smaller the amount of sulfur contained in the steel is, the more preferable. However, it is not easy to reduce the amount of sulfur to 0 mass % from the viewpoint of a steel-making technology, and the steel-making cost increases for stable reducing to less than 0.001 mass %. Accordingly, the lower limit of the amount of sulfur contained in the steel may be set to 0.001 mass %.

(26) Examples of unavoidable impurities other than phosphorous and sulfur include nickel, copper, aluminum, nitrogen, oxygen, and the like.

(27) The amount of nickel contained in the steel is 0.25 mass % or less. Furthermore, it is preferable that the lower limit of the amount of nickel contained in the steel is a certain degree of amount capable of being regarded as an unavoidable impurity. Typically, the lower limit is preferably 0 mass %.

(28) Furthermore, the amount of iron contained in the steel is the remainder excluding the respective amounts of carbon, silicon, manganese, chromium, phosphorous, sulfur, and the unavoidable impurities from the entire composition of the steel, and is typically 96 mass % to 99 mass %.

(29) The forging temperature is (Ae.sub.1 point+25 C.) or higher from the viewpoints of suppressing formation of sheet-shaped or layered cementite by promoting decomposition of cementite, and promoting formation of fine spheroidal cementite, and preferably (Ae.sub.1 point+35 C.) or higher. At the forging temperature, plastic working is performed to apply at least 0.5 equivalent strain. On the other hand, the forging temperature is (Ae.sub.1 point+105 C.) or lower from the viewpoint of suppressing dissolution of a carbide and from the viewpoint of promoting processing-induced precipitation of the spheroidal cementite in accordance with the plastic working, and preferably (Ae.sub.1 point+95 C.) or lower. Specifically, the forging temperature is 760 C. or higher from the viewpoint of promoting decomposition of layered cementite (lamella cementite) in steel before heating, and promoting formation of fine spheroidal cementite, and preferably 770 C. or higher. On the other hand, the forging temperature is 840 C. or lower from the viewpoints of suppressing dissolution of the spheroidal cementite, and promoting processing-induced precipitation of the spheroidal cementite, and preferably 830 C. or lower. Furthermore, in this specification, the forging temperature represents a forging temperature at the central portion of steel. In addition, in this specification, the equivalent strain represents an average value of an equivalent plastic strain that is applied to steel. The equivalent plastic strain can be obtained by converting a strain in a cross-section that is deformed through forging into a plastic strain of uniaxial stretching in accordance with a method described in page 113 of Revised industrial plastic mechanics by MASUDA et al., (Feb. 20, 1995, Fifteenth edition, published by yokendo co. ltd.). Furthermore, an average value of the equivalent plastic strain that is applied to steel can be obtained by a finite element method.

(30) A temperature rising rate when heating steel to the forging temperature may be in a range capable of forming fine spheroidal cementite, and can be appropriately determined in accordance with a composition of steel, a micro structure state, and the like.

(31) When a temperature at the central portion of steel reaches the forging temperature, forging is initiated. Forging is performed under the forging temperature condition (refer to (a) in FIG. 2). As described above, when forging is performed at a forging temperature in a range of (Ae.sub.1 point+25 C.) to (Ae.sub.1 point+105 C.), dislocation, which is introduced to the inside of a crystal of a steel structure is less likely to disappear, and the dislocation is integrated to the vicinity of a grain boundary of austenite or the vicinity of residual cementite. Accordingly, precipitation of spheroidal cementite, that is, processing-induced precipitation is promoted. For occurrence of the processing-induced precipitation, plastic working for application of at least 0.5 of equivalent strain is necessary. For example, forging can be performed by a process including an upsetting process (refer to (a) in FIG. 3), a forming process (refer to (b) in FIG. 3), and a separation and punching process (refer to (c) in FIG. 3).

(32) In the upsetting process, cylindrical steel (not shown) composed of the high-carbon chrome bearing steel is set on an upsetting device (not shown), and compression is performed from both sides of the cylindrical steel in an axial direction to perform upsetting forming, thereby obtaining a disk member B1 (refer to (a) in FIG. 3).

(33) Next, in the forming process, the disk member B1 is subjected to plastic deformation by using a die including a male tool and a female tool (not shown) to obtain a formed blank B2 having an outer tubular portion 50, an inner tubular portion 60, and a bottom portion 60a that is formed at a lower side of the inner tubular portion 60 (refer to (b) in FIG. 3).

(34) Then, in the separation and punching process, the inner tubular portion 60 of the formed blank B2 is punched by using a punch (not shown), and the bottom portion 60a of the inner tubular portion 60 is punched by using the punch, thereby separating a forged article 51 for an outer ring and a forged article 61 for an inner ring from each other, and separating the forged article 61 for an inner ring and the bottom portion 61b from each other to form a prepared hole 61a in the forged article 61 for an inner ring (refer to (c) in FIG. 3).

(35) After termination of forging, a forged article that is obtained is cooled down to a temperature (cooling temperature) of Ae.sub.1 point or lower (refer to (a) in FIG. 2). As described above, when the forged article that is obtained is cooled down to the cooling temperature, it is possible to efficiently form fine spheroidal cementite. The cooling temperature is Ae.sub.1 point or lower, and is preferably 400 C. or lower, from the viewpoint of completely completing transformation of an austenite structure after termination of forging so as to suppress formation of a pearlite structure and formation of a martensite structure or a bainite structure which is an overcooled structure. In addition, the cooling temperature is preferably room temperature (25 C.) or higher from the viewpoints of reducing a load on cooling facilities, improving production efficiency, and reducing production cost.

(36) A cooling rate when steel is cooled down to the cooling temperature can be appropriately determined in accordance with a composition of steel, a forging shape, and the like. Furthermore, in this specification, the cooling temperature represents a cooling temperature at the central portion of steel.

(37) Through the warm forging process, it is possible to form a structure (refer to step (B) in FIG. 2, and FIG. 4(B)) in which fine spheroidal cementite is dispersed after the warm forging process (step (B) in FIG. 2) from a structure (refer to FIG. 4(A)) that includes pearlite before the warm forging process (step (A) in FIG. 2).

(38) (Annealing Process)

(39) In the annealing process, first, the forged article, which is obtained in the warm forging process, is heated until the temperature of the central portion of the forged article reaches a soaking temperature in a range of (Ae.sub.1 point+25 C.) to (Ae.sub.1 point+85 C.) (refer to (b) in FIG. 2).

(40) The soaking temperature is (Ae.sub.1 point+25 C.) or higher, and is preferably (Ae.sub.1 point+35 C.) or higher, from the viewpoint of obtaining a roughly shaped material capable of stably securing satisfactory machinability. In addition, the soaking temperature is (Ae.sub.1 point+85 C.) or lower, and is preferably (Ae.sub.1 point+75 C.) or lower, from the viewpoint of suppressing formation of sheet-shaped or layered cementite. Specifically, the soaking temperature is 760 C. or higher, and is preferably 770 C. or higher, from the viewpoint of obtaining a roughly shaped material capable of securing satisfactory machinability. The soaking temperature is 820 C. or lower, and is preferably 810 C. or lower, from the viewpoint of suppressing formation of sheet-shaped or layered cementite. Furthermore, in this specification, the soaking temperature represents a soaking temperature at the central portion of steel.

(41) A temperature rising rate when heating steel to the soaking temperature can be appropriately determined in accordance with a composition of steel, a forging shape, and the like.

(42) When a temperature at the central portion of the forged article reaches a soaking temperature in a range of (Ae.sub.1 point+25 C.) to (Ae.sub.1 point+85 C.), the forged article is retained at the soaking temperature for 0.5 hours or longer (refer to (b) in FIG. 2).

(43) The time (soaking time) for retention at the soaking temperature is preferably 1.0 hour or longer from the viewpoint of obtaining a roughly shaped material capable of stably securing satisfactory machinability. Furthermore, even when the soaking time is lengthened to be longer than 10 hours, an additional improvement in characteristics of the roughly shaped material, which corresponds to a period of the soaking time, is not exhibited. Therefore, it is preferable that the soaking time is as shorter as possible from the viewpoint of shortening the production time. The upper limit of the soaking time is typically 5.0 hours or shorter.

(44) After the forged article is retained at the soaking time for 0.5 hours or longer, the forged article is cooled down to a cooling temperature of 700 C. or lower at a cooling rate of 0.30 C./s or slower (refer to (b) in FIG. 2).

(45) The cooling rate is preferably 0.007 C./s or faster, and is more preferably 0.020 C./s or faster, from the viewpoint of improving productivity (shortening of production time). In addition, the cooling rate is 0.30 C./s or slower, is preferably 0.27 C./s or slower, and is more preferably 0.25 C./s or slower, from the viewpoint of suppressing formation of sheet-shaped or layered cementite.

(46) The cooling temperature is 700 C. or lower, and is preferably 650 C. or lower, from the viewpoint of suppressing formation of sheet-shaped or layered cementite. In addition, the cooling temperature is preferably room temperature (25 C.) or higher from the viewpoints of reducing a load on cooling facilities, improving production efficiency, and of reducing the production cost.

(47) Through the annealing process, fine spheroidal cementite (refer to FIG. 4(B)) is allowed to efficiently grow in a structure after the warm forging process (step (B) in FIG. 2). Accordingly, it is possible to form a structure (refer to FIG. 4(C)) in which the spheroidal cementite is dispersed after the annealing process (step (C) in FIG. 2).

(48) In this manner, it is possible to obtain a roughly shaped material for bearing rings of a rolling bearing.

EXAMPLES

(49) Next, the effect of a rolling member and a method of manufacturing the rolling member according to the embodiment of the invention will be verified with reference to examples and the like.

Experiment Example 1

(50) Steel (Ae.sub.1=735 C.) composed of high-carbon chrome bearing steel A (composition: 0.98 mass % of carbon, 1.48 mass % of chromium, 0.25 mass % of silicon, 0.45 mass % of manganese, 0.012 mass % of phosphorous, 0.006 mass % of sulfur, and a remainder including iron and unavoidable impurities) was forged and annealed under conditions shown in Table 1, thereby obtaining test pieces (model number: 6208) of a roughly shaped material for bearing rings in Experiment Nos. 1 to 17.

(51) [Table 1]

Test Example 1

(52) Vickers hardness at 55 sites among the test pieces of Experiment Nos. 1 to 17 was measured in accordance with JIS Z 2244, and an average value of the Vickers hardness measured at the 55 sites was obtained. In addition, 5 mass % of picral etchant was brought into contact with the central portion of each of the test pieces of Experiment Nos. 1 to 17 for 10 seconds to corrode the central portion, and then a corroded surface was observed with a scanning electron microscope (product name: EPMA-1600, manufactured by Shimadzu Corporation). Next, an evaluation of hardness and structure spheroidizing, and an overall evaluation were performed for the respective test pieces. The results thereof are shown in Table 2. Furthermore, the evaluation standards for the hardness and the structure spheroidizing, and for the overall evaluation of each of the respective test pieces are as follows.

(53) (Evaluation Standards for Hardness)

(54) Good: Vickers hardness is Hv240 or less.

(55) Poor: Vickers hardness is greater than Hv240.

(56) (Evaluation Standards for Structure Spheroidizing)

(57) Good: Sheet-shaped or layered cementite is not shown, and spheroidal cementite is uniformly dispersed in a structure.

(58) Poor: Sheet-shaped or layered cementite is shown.

(59) (Evaluation Standards for Overall Evaluation)

(60) Good: Evaluation of the hardness and the evaluation of the structure spheroidizing are good.

(61) Poor: At least one of the evaluation of the hardness and of the evaluation of the structure spheroidizing is poor.

(62) [Table 2]

(63) From results shown in Table 2, in a case where the following conditions were satisfied (Experiment Nos. 1 to 3, 6, 10, 11, and 13 to 15), that is, in a case where the forging temperature was in a range of (Ae.sub.1 point+25 C.) to (Ae.sub.1 point+105 C.), the cooling temperature after forging was Ae.sub.1 point or lower, the soaking temperature during annealing was in a range of (Ae.sub.1 point+25 C.) to (Ae.sub.1 point+85 C.), the soaking time was 0.5 hours or longer, the cooling rate after soaking was 0.30 C./s or slower, and the cooling temperature was 700 C. or lower, it could be seen that all of the test pieces which were obtained had Vickers hardness of Hv240 or less (hardness capable of stably obtaining satisfactory machinability), and had a structure in which sheet-shaped or layered cementite was not shown and spheroidal cementite was uniformly dispersed, and thus hardness and structure spheroidizing were suitable for the roughly shaped material for bearing rings. In contrast, in a case where the above-described conditions were not satisfied, that is, in a case where the soaking temperature was Ae.sub.1 point+5 C. (Experiment No. 12), Ae.sub.1 point+105 C. (Experiment No. 16), or Ae.sub.1 point+135 C. (Experiment No. 17), the soaking time was 0 hour (Experiment No. 9), the cooling temperature after soaking was 730 C. (Experiment No. 4) or 780 C. (Experiment No. 5), the cooling rate after soaking was 0.50 C./s (Experiment No. 7), and the cooling rate after soaking was 1.50 C./s (Experiment No. 8), it could be seen that at least one of hardness and structure spheroidizing of the test pieces was not suitable for the roughly shaped material for bearing rings.

(64) Even in a case using steel composed of high-carbon chrome bearing steel B (Ae.sub.1=735 C.) containing 1.0 mass % of carbon, 1.5 mass % of chromium, 0.3 mass % of silicon, 0.45 mass % of manganese, 0.01 mass % of phosphorous, 0.005 mass % of sulfur, and a remainder including iron and unavoidable impurities, high-carbon chrome bearing steel C (Ae.sub.1=735 C.) containing 0.8 mass % of carbon, 1.4 mass % of chromium, 0.2 mass % of silicon, 0.4 mass % of manganese, 0.005 mass % of phosphorous, 0.004 mass % of sulfur, and a remainder including iron and unavoidable impurities, or high-carbon chrome baring steel D (Ae.sub.1=745 C.) containing 0.9 mass % of carbon, 1.7 mass % of chromium, 0.2 mass % of silicon, 0.45 mass % of manganese, 0.005 mass % of phosphorous, 0.005 mass % of sulfur, and a remainder including iron and unavoidable impurities instead of steel composed of the high-carbon chrome bearing steel A, it could be seen that the same tendency as in steel composed of high-carbon chrome bearing steel A was shown.

Experiment Example 2

(65) Steel (Ae.sub.1=735 C.) composed of high-carbon chrome bearing steel (composition: 0.98 mass % of carbon, 1.48 mass % of chromium, 0.25 mass % of silicon, 0.45 mass % of manganese, 0.012 mass % of phosphorous, 0.006 mass % of sulfur, and a remainder including iron and unavoidable impurities) was forged and annealed under conditions shown in Table 3, thereby obtaining test pieces (model number: 6208) of a roughly shaped material for bearing rings in Experiment Nos. 18 to 41.

(66) [Table 3]

Test Example 2

(67) The Vickers hardness at 55 sites among the test pieces of Experiment Nos. 18 to 19 was measured in accordance with JIS Z 2244, and the average value of the Vickers hardness measured at the 55 sites was obtained. In addition, 5 mass % of picral etchant was brought into contact with the central portion of each of the test pieces of Experiment Nos. 18 and 19 for 10 seconds to corrode the central portion, and then a corroded surface was observed with a scanning electron microscope (product name: EPMA-1600, manufactured by Shimadzu Corporation). Furthermore, the test pieces of Experiment Nos. 18 and 19 were produced under the same conditions except that the temperature rising rates in the annealing process were different from each other.

(68) As a result, it could be seen that all of the test pieces in Experiment Nos. 18 and 19 had a Vickers hardness of Hv240 or less (a hardness capable of obtaining satisfactory machinability), and had a structure in which sheet-shaped or layered cementite was not shown and spheroidal cementite was uniformly dispersed, and thus hardness and structure spheroidizing were suitable for the roughly shaped material for bearing rings (not shown). Accordingly, it is implied that an effect of the temperature rising rate in the annealing process on hardness and a structure state of the roughly shaped material for bearing rings is small.

Test Example 3

(69) Vickers hardness at 55 sites among the test pieces of Experiment Nos. 20 to 27 was measured in accordance with JIS Z 2244, and an average value of the Vickers hardness measured at the 55 sites was obtained. In addition, 5 mass % of picral etchant was brought into contact with the central portion of each of the test pieces of Experiment Nos. 20 to 27 for 10 seconds to corrode the central portion, and then a corroded surface was observed with a scanning electron microscope (product name: EPMA-1600, manufactured by Shimadzu Corporation). Furthermore, the test pieces of Experiment Nos. 20 and 27 were produced under the same conditions except that soaking temperatures in the annealing process were different from each other. In Test Example 3, results obtained by investigating a relationship between the soaking temperature and the hardness, and results obtained by observing the structure of the roughly shaped material for bearing rings which was obtained after the annealing process are shown in FIG. 5. In FIG. 5, (a) is a graph showing the results obtained by investigating the relationship between the soaking temperature and the hardness, (b) is a drawing-substituting photograph of the structure of the roughly shaped material for bearing rings when the soaking temperature is set to 740 C., (c) is a drawing-substituting photograph of the structure of the roughly shaped material for bearing rings when the soaking temperature is set to 760 C., (d) is a drawing-substituting photograph of the structure of the roughly shaped material for bearing rings when the soaking temperature is set to 820 C., and (e) is a drawing-substituting photograph of the structure of the roughly shaped material for bearing rings when the soaking temperature is set to 840 C. In the drawing, the scale bar represents 5 m.

(70) As shown in FIG. 5, in a case where the soaking temperature is 760 C. to 820 C. ((Ae.sub.1 point+25 C.) to (Ae.sub.1 point+85 C.)) (Experiment Nos. 22 to 25), it could be seen that all of the test pieces which were obtained had a Vickers hardness of Hv240 or less (a hardness capable of obtaining satisfactory machinability), and tended to have a structure in which sheet-shaped or layered cementite was not shown and spheroidal cementite was uniformly dispersed, and thus all of the hardness and structure spheroidizing were suitable for the roughly shaped material for bearing rings. In contrast, in a case where the soaking temperature was 740 C. (Experiment No. 20), 750 C. (Experiment No. 21), 840 C. (Experiment No. 26), 850 C. (Experiment No. 27), and 870 C. (Experiment No. 28), Vickers hardness of the test pieces was greater than Hv240 (the hardness at which machinability deteriorates), or the sheet-shaped or layered cementite was shown. Accordingly, from the results, it is implied that the soaking temperature is preferably (Ae.sub.1 point+25 C.) to (Ae.sub.1 point+85 C.).

Test Example 4

(71) Vickers hardness at 55 sites among the test pieces of Experiment Nos. 29 to 33 was measured in accordance with JIS Z 2244, and an average value of the Vickers hardness measured at the 55 sites was obtained. In addition, 5 mass % of picral etchant was brought into contact with the central portion of each of the test pieces of Experiment Nos. 29 and 33 for 10 seconds to corrode the central portion, and then a corroded surface was observed with a scanning electron microscope (product name: EPMA-1600, manufactured by Shimadzu Corporation). Furthermore, the test pieces of Experiment Nos. 29 to 33 were produced under the same conditions except that the soaking time in the annealing process is different from each other.

(72) As a result, in a case where the soaking time was 0.5 hours (Experiment No. 30), 1 hour (Experiment No. 31), 1.5 hours (Experiment No. 32), and 2 hours (Experiment No. 33), it could be seen that all of the test pieces which were obtained had a Vickers hardness of Hv240 or less (a hardness capable of obtaining satisfactory machinability), and had a structure in which sheet-shaped or layered cementite was not shown and spheroidal cementite was uniformly dispersed, and thus hardness and structure spheroidizing were suitable for the roughly shaped material for bearing rings (not shown). In contrast, in a case where the soaking time was 0 hour (Experiment No. 29), Vickers hardness of the test piece was greater than Hv240 (hardness at which machinability deteriorates) (not shown). Accordingly, from the results, it is implied that the soaking time is preferably 0.5 hours or longer.

Test Example 5

(73) Vickers hardness at 55 sites among the test pieces of Experiment Nos. 34 to 37 was measured in accordance with JIS Z 2244, and the average value of the Vickers hardness measured at the 55 sites was obtained. In addition, 5 mass % of picral etchant was brought into contact with the central portion of each of the test pieces of Experiment Nos. 34 to 37 for 10 seconds to corrode the central portion, and then a corroded surface was observed with a scanning electron microscope (product name: EPMA-1600, manufactured by Shimadzu Corporation). Furthermore, the test pieces of Experiment Nos. 34 and 37 were produced under the same conditions except that the cooling rates in the annealing process were different from each other. In Test Example 5, results obtained by investigating a relationship between the cooling rate and the hardness, and results obtained by observing the structure of the roughly shaped material for bearing rings which was obtained after the annealing process are shown in FIG. 6. In the drawing, (a) is a graph showing the results obtained by investigating the relationship between the cooling rate and the hardness, (b) is a drawing-substituting photograph of the structure of the roughly shaped material for bearing rings when the cooling rate is set to 7 C./min (0.12 C./s), (c) is a drawing-substituting photograph of the structure of the roughly shaped material for bearing rings when the cooling rate is set to 16 C./min (0.27 C./s), (d) is a drawing-substituting photograph of the structure of the roughly shaped material for bearing rings when the cooling rate is set to 26 C./min (0.43 C./s), and (e) is a drawing-substituting photograph of the structure of the roughly shaped material for bearing rings when the cooling rate is set to 93 C./min (1.55 C./s). In the drawing, the scale bar represents 5 m.

(74) From the results shown in FIG. 6, in a case where the cooling rate was 0.12 C./s (Experiment No. 34) and 0.27 C./s (Experiment No. 35), it could be seen that all of the test pieces which were obtained had Vickers hardness of Hv240 or less (hardness capable of stably obtaining satisfactory machinability), and had a structure in which sheet-shaped or layered cementite was not shown and spheroidal cementite was uniformly dispersed, and thus hardness and structure spheroidizing were suitable for the roughly shaped material for bearing rings. In contrast, in a case where the cooling rate was 1.55 C./s (Experiment No. 37), it could be seen that the Vickers hardness of the test piece was greater than Hv240 (the hardness at which machinability deteriorates), and the sheet-shaped or layered cementite was seen. Accordingly, the results implies that the cooling rate in the annealing process is preferably 0.30 C./s or lower, and is more preferably 0.27 C./s or lower.

Test Example 6

(75) Vickers hardness at 55 sites among the test pieces of Experiment Nos. 38 to 43 was measured in accordance with JIS Z 2244, and an average value of the Vickers hardness measured at the 55 sites was obtained. In addition, 5 mass % of picral etchant was brought into contact with the central portion of each of the test pieces of Experiment Nos. 38 to 43 for 10 seconds to corrode the central portion, and then a corroded surface was observed with a scanning electron microscope (product name: EPMA-1600, manufactured by Shimadzu Corporation). Furthermore, the test pieces of Experiment Nos. 38 and 43 were produced under the same conditions except that the cooling temperature in the annealing process was different from each other. In Test Example 6, results obtained by investigating a relationship between the cooling temperature and the hardness, and results obtained by observing the structure of the roughly shaped material for bearing rings which is obtained after the annealing process are shown in FIG. 7. In the drawing, (a) is a graph showing the results obtained by investigating the relationship between the cooling temperature and the hardness, (b) is a drawing-substituting photograph of the structure of the roughly shaped material for bearing rings when the cooling temperature is set to 650 C., (c) is a drawing-substituting photograph of the structure of the roughly shaped material for bearing rings when the cooling temperature is set to 700 C., and (d) is a drawing-substituting photograph of the structure of the roughly shaped material for bearing rings when the cooling temperature is set to 730 C. In the drawing, a scale bar represents 5 m.

(76) From the results shown in FIG. 7, in a case where the cooling temperature was 600 C. (Experiment No. 38), 650 C. (Experiment No. 39), and 700 C. (Experiment No. 40), it could be seen that all of the test pieces which were obtained had Vickers hardness of Hv240 or less (hardness capable of obtaining satisfactory machinability), and tended to have a structure in which sheet-shaped or layered cementite was not shown and spheroidal cementite was uniformly dispersed, and thus the hardness and the structure spheroidizing were suitable for the roughly shaped material for bearing rings. In contrast, in a case where the cooling temperature was 750 C. (Experiment No. 42) and 780 C. (Experiment No. 43), it could be seen that there is a tendency where the Vickers hardness of the test pieces was greater than HIV240 (hardness at which machinability deteriorates), and the sheet-shaped or layered cementite was shown. Accordingly, from the results, it is implied that the cooling temperature in the annealing process is preferably 700 C. or lower.

Example 1 and Example 2

(77) Steel (Ae.sub.1=735 C.) Composed of high-carbon chrome bearing steel (composition: 0.98 mass % of carbon, 1.48 mass % of chromium, 0.25 mass % of silicon, 0.45 mass % of manganese, 0.012 mass % of phosphorous, 0.006 mass % of sulfur, and a remainder including iron and unavoidable impurities) was forged and annealed under conditions shown in Table 4. Then, the forged articles which were obtained were subjected to a quenching treatment and a tempering treatment under conditions shown in FIG. 8 (Example 1) and under conditions shown in FIG. 9 (Example 2), thereby obtaining test pieces (model number: 6208) of a roughly shaped material for bearing rings.

(78) [Table 4]

Test Example 7

(79) 5 mass % of picral etchant was brought into contact with the central portion of each of forged articles after the annealing process in Example 1 and Example 2 for 10 seconds to corrode the central portion, and then a corroded surface was observed with a scanning electron microscope (product name: EPMA-1600, manufactured by Shimadzu Corporation). In addition, the Vickers hardness at 55 sites among the test pieces of Example 1 and Example 2 was measured in accordance with JIS Z 2244, and the average value of the Vickers hardness measured at the 55 sites was obtained. Next, evaluation of hardness of the respective test pieces and structure spheroidizing of the forged articles after the annealing process was performed. In addition, the X-ray diffraction intensity was measured at a surface layer portion and the center portion of the test pieces of the roughly shaped material for bearing rings in Example 1 and Example 2 by an X-ray diffraction method. The X-ray diffraction intensity I of fcc that is a crystal structure of residual austenite, and the X-ray diffraction intensity I of bcc that is a crystal structure of a tempered martensite were obtained. From the X-ray diffraction intensity ratio which was obtained, the amount of residual austenite at the surface layer portion and the center portion of the test pieces of the roughly shaped material for bearing rings in Example 1 and 2 was obtained by using the following theoretical Expression (I):
1/V=1+(R/R)(I/I)(I)

(80) (In Expression, V represents the amount of residual austenite, R and R represent coefficients depending on crystal orientations that is measured, I and I represent X-ray diffraction intensities).

(81) In test Example 7, results, which are obtained by observing a structure of the forged article obtained after the annealing process, are shown in FIG. 10. In the drawing, (a) is a drawing-substituting photograph showing a result obtained by observing the structure of the forged article in Example 1, and (b) is a drawing-substituting photograph showing a result obtained by observing the structure of the forged article in Example 2. Furthermore, a scale bar in the drawing represents 10 m. The amount of residual austenite at the surface layer portion and the center portion of the test pieces of the roughly shaped material for bearing rings in Example 1 and Example 2, and results obtained by performing evaluation of the hardness of the respective test pieces and the structure spheroidizing of the forged articles after the annealing process are shown in Table 5. In addition, the evaluation standards for the evaluation of the hardness of the respective test pieces, and the structure spheroidizing of the forged articles obtained after the annealing process are the same as the evaluation standards used in Test Example 1. In addition, a case where the amount of residual austenite was an amount (15 vol % or less), which is suitable for compatibility between securement of rolling fatigue life and securement of dimensional stability in bearing rings of a rolling bearing is regarded as Good.

(82) [Table 5]

(83) From results shown in FIG. 10 and Table 5, in Example 1 and Example 2 which satisfies conditions in which the forging temperature is in a range of (Ae.sub.1 point+25 C.) to (Ae.sub.1 point+105 C.), the cooling temperature after forging was Ae.sub.1 point or lower, the soaking temperature during annealing was in a range of (Ae.sub.1 point+25 C.) to (Ae.sub.1 point+85 C.) and the soaking temperature during annealing was in a range of preferably (Ae.sub.1 point+35 C.) to (Ae.sub.1 point+75 C.), the soaking time was 0.5 hours or longer, the cooling rate after soaking was 0.30 C./s or slower, and the cooling temperature was 700 C. or lower, it can be seen that all of the amount of residual austenite, the hardness, and the structure spheroidizing are in ranges which are suitable for manufacturing of bearing rings.

BRIEF DESCRIPTION OF THE REFERENCE SYMBOLS

(84) 1: CONICAL ROLLER BEARING (ROLLING BEARING)

(85) 11: INNER RING (BEARING RING)

(86) 12: OUTER RING (BEARING RING)

(87) TABLE-US-00001 TABLE 1 WARM FORGING PROCESS ANNEALING PROCESS FORGING COOLING TEMPERATURE SOAKING SOAKING COOLING COOLING EXPERIMENT Ae.sub.1 TEMPERATURE TEMPERATURE RISING RATE TEMPERATURE TIME RATE TEMPERATURE No. ( C.) ( C.) ( C.) ( C./s) ( C.) (h) ( C./s) ( C.) 1 735 820(Ae.sub.1 + 95) AIR COOLING 0.08 780(Ae.sub.1 + 45) 1.0 0.10 600 2 735 830(Ae.sub.1 + 95) AIR COOLING 0.40 780(Ae.sub.1 + 45) 1.0 0.10 600 3 735 830(Ae.sub.1 + 95) AIR COOLING 0.40 780(Ae.sub.1 + 45) 1.0 0.10 700 4 735 830(Ae.sub.1 + 95) AIR COOLING 0.40 780(Ae.sub.1 + 45) 1.0 0.10 730 5 735 830(Ae.sub.1 + 95) AIR COOLING 0.40 780(Ae.sub.1 + 45) 1.0 0.10 780 6 735 830(Ae.sub.1 + 95) AIR COOLING 0.40 780(Ae.sub.1 + 45) 1.0 0.25 600 7 735 830(Ae.sub.1 + 95) AIR COOLING 0.40 780(Ae.sub.1 + 45) 1.0 0.50 600 8 735 830(Ae.sub.1 + 95) AIR COOLING 0.40 780(Ae.sub.1 + 45) 1.0 1.50 600 9 735 830(Ae.sub.1 + 95) AIR COOLING 0.40 780(Ae.sub.1 + 45) 0.0 0.10 700 10 735 830(Ae.sub.1 + 95) AIR COOLING 0.40 780(Ae.sub.1 + 45) 0.5 0.10 700 11 735 830(Ae.sub.1 + 95) AIR COOLING 0.40 780(Ae.sub.1 + 45) 1.5 0.10 700 12 735 830(Ae.sub.1 + 95) AIR COOLING 0.40 740(Ae.sub.1 + 5) 1.0 0.10 700 13 735 830(Ae.sub.1 + 95) AIR COOLING 0.40 760(Ae.sub.1 + 25) 1.0 0.10 700 14 735 830(Ae.sub.1 + 95) AIR COOLING 0.40 800(Ae.sub.1 + 65) 1.0 0.10 700 15 735 830(Ae.sub.1 + 95) AIR COOLING 0.40 820(Ae.sub.1 + 85) 1.0 0.10 700 16 735 830(Ae.sub.1 + 95) AIR COOLING 0.40 840(Ae.sub.1 + 105) 1.0 0.10 700 17 735 830(Ae.sub.1 + 95) AIR COOLING 0.40 870(Ae.sub.1 + 135) 1.0 0.10 700

(88) TABLE-US-00002 TABLE 2 EVALUATION EXPERIMENT HARDNESS STRUCTURE No. (Hv240 or less) SPHEROIDIZING OVERALL 1 GOOD GOOD GOOD 2 GOOD GOOD GOOD 3 GOOD GOOD GOOD 4 GOOD POOR POOR 5 POOR POOR POOR 6 GOOD GOOD GOOD 7 POOR POOR POOR 8 POOR POOR POOR 9 POOR GOOD POOR 10 GOOD GOOD GOOD 11 GOOD GOOD GOOD 12 POOR GOOD POOR 13 GOOD GOOD GOOD 14 GOOD GOOD GOOD 15 GOOD GOOD GOOD 16 GOOD POOR POOR 17 POOR POOR POOR

(89) TABLE-US-00003 TABLE 3 WARM FORGING PROCESS ANNEALING PROCESS FORGING COOLING TEMPERATURE SOAKING SOAKING COOLING COOLING EXPERIMENT Ae.sub.1 TEMPERATURE TEMPERATURE RISING RATE TEMPERATURE TIME RATE TEMPERATURE No. ( C.) ( C.) ( C.) ( C./s) ( C.) (h) ( C./s) ( C.) 18 735 830(Ae.sub.1 + 95) AIR COOLING 0.08 780(Ae.sub.1 + 45) 1.0 0.10 600 19 735 830(Ae.sub.1 + 95) AIR COOLING 0.40 780(Ae.sub.1 + 45) 1.0 0.10 600 20 735 830(Ae.sub.1 + 95) AIR COOLING 0.40 740(Ae.sub.1 + 5) 1.0 0.10 700 21 735 830(Ae.sub.1 + 95) AIR COOLING 0.40 750(Ae.sub.1 + 15) 1.0 0.10 700 22 735 830(Ae.sub.1 + 95) AIR COOLING 0.40 760(Ae.sub.1 + 25) 1.0 0.10 700 23 735 830(Ae.sub.1 + 95) AIR COOLING 0.40 780(Ae.sub.1 + 45) 1.0 0.10 700 24 735 830(Ae.sub.1 + 95) AIR COOLING 0.40 800(Ae.sub.1 + 65) 1.0 0.10 700 25 735 830(Ae.sub.1 + 95) AIR COOLING 0.40 820(Ae.sub.1 + 85) 1.0 0.10 700 26 735 830(Ae.sub.1 + 95) AIR COOLING 0.40 840(Ae.sub.1 + 105) 1.0 0.10 700 27 735 830(Ae.sub.1 + 95) AIR COOLING 0.40 850(Ae.sub.1 + 115) 1.0 0.10 700 28 735 830(Ae.sub.1 + 95) AIR COOLING 0.40 870(Ae.sub.1 + 135) 1.0 0.10 700 29 735 830(Ae.sub.1 + 95) AIR COOLING 0.40 780(Ae.sub.1 + 45) 0.0 0.10 700 30 735 830(Ae.sub.1 + 95) AIR COOLING 0.40 780(Ae.sub.1 + 45) 0.5 0.10 700 31 735 830(Ae.sub.1 + 95) AIR COOLING 0.40 780(Ae.sub.1 + 45) 1.0 0.10 700 32 735 830(Ae.sub.1 + 95) AIR COOLING 0.40 780(Ae.sub.1 + 45) 1.5 0.10 700 33 735 830(Ae.sub.1 + 95) AIR COOLING 0.40 780(Ae.sub.1 + 45) 2.0 0.10 700 34 735 830(Ae.sub.1 + 95) AIR COOLING 0.40 780(Ae.sub.1 + 45) 1.0 0.12 600 35 735 830(Ae.sub.1 + 95) AIR COOLING 0.40 780(Ae.sub.1 + 45) 1.0 0.27 600 36 735 830(Ae.sub.1 + 95) AIR COOLING 0.40 780(Ae.sub.1 + 45) 1.0 0.43 600 37 735 830(Ae.sub.1 + 95) AIR COOLING 0.40 780(Ae.sub.1 + 45) 1.0 1.55 600 38 735 830(Ae.sub.1 + 95) AIR COOLING 0.40 780(Ae.sub.1 + 45) 1.0 0.10 600 39 735 830(Ae.sub.1 + 95) AIR COOLING 0.40 780(Ae.sub.1 + 45) 1.0 0.10 650 40 735 830(Ae.sub.1 + 95) AIR COOLING 0.40 780(Ae.sub.1 + 45) 1.0 0.10 700 41 735 830(Ae.sub.1 + 95) AIR COOLING 0.40 780(Ae.sub.1 + 45) 1.0 0.10 730 42 735 830(Ae.sub.1 + 95) AIR COOLING 0.40 780(Ae.sub.1 + 45) 1.0 0.10 750 43 735 830(Ae.sub.1 + 95) AIR COOLING 0.40 780(Ae.sub.1 + 45) 1.0 0.10 780

(90) TABLE-US-00004 TABLE 4 WARM FORGING PROCESS ANNEALING PROCESS FORGING COOLING TEMPERATURE SOAKING SOAKING COOLING COOLING Ae.sub.1 TEMPERATURE TEMPERATURE RISING RATE TEMPERATURE TIME RATE TEMPERATURE EXAMPLE ( C.) ( C.) ( C.) ( C./s) ( C.) (h) ( C./s) ( C.) 1 735 770 (Ae.sub.1 + 35) AIR 0.40 770 (Ae.sub.1 + 35) 0.5 0.27 700 COOLING 2 735 830 (Ae.sub.1 + 95) AIR 0.40 810 (Ae.sub.1 + 75) 2.5 0.27 700 COOLING

(91) TABLE-US-00005 TABLE 5 EVALUATION RESIDUAL AMOUNT OF RESID- AUSTENITE UAL AUSTENITE HARDNESS STRUCTURE (Vol %) (15 vol % or less) (Hv240 or less) SPHEROIDIZING EXAMPLE 1 SURFACE 13.1 SURFACE GOOD GOOD GOOD LAYER LAYER CENTER 10.3 CENTER GOOD EXAMPLE 2 SURFACE 12.1 SURFACE GOOD GOOD GOOD LAYER LAYER CENTER 9.1 CENTER GOOD