JP5202040B2 - Steel material for bearings with excellent wear resistance - Google Patents

Description

  The present invention relates to a steel material for producing a bearing that exhibits excellent wear resistance when used as a bearing component.

  From the viewpoint of protecting the global environment, bearing components (hereinafter sometimes simply referred to as “bearings”) tend to be lighter and smaller, and the use environment of bearings has become increasingly severe. In a severe usage environment, the bearing pressure on the bearing increases, and as a result, the wear of the bearing during rolling becomes remarkable. When such wear becomes prominent, the accuracy of the bearing is lowered, so that the function as a bearing is not fully exhibited, and it is necessary to perform maintenance such as replacement in a short period of time, which is a problem.

  As steel materials (steel materials for bearings) used as bearing materials, conventionally, high carbon chromium bearing steels such as SUJ2 defined in JIS G 4805 (1999) have been used in various fields such as automobiles and various industrial machines. Yes. However, even if such a bearing steel material is used to form a bearing by an ordinary method, it is not possible to meet the recently required wear resistance, and further improvements are required.

  In response to this problem, a technique for improving the wear resistance of the bearing by improving the steel material (steel material for the bearing) itself has been proposed. For example, Patent Document 1 discloses that a large amount of carbide is generated by adding a carbide forming element such as Mo, W, V or the like to the steel material, thereby improving the wear resistance of the steel material for bearings. . However, the use of such a large amount of alloy elements is likely to cause problems in cost, workability, and manufacturability, and is unlikely to be as versatile as SUJ2.

  Further, in Patent Document 2, the case-hardened steel is used as a raw material, and by performing induction hardening after carburizing and carbonitriding, carbide is finely dispersed, and by defining the carbon content and hardness of the surface layer portion of the raceway surface, Techniques for improving wear resistance have been proposed. Unlike SUJ2, which can manufacture bearings only by quenching and tempering processes, the carburizing process and induction hardening process are used to manufacture bearings. It will be forced to change the manufacturing process of this, which will not only raise the cost, but it seems difficult to put into practical use.

  On the other hand, on the premise that versatile SUJ2 steel is used, a method for improving the characteristics has also been proposed. From this point of view, Patent Document 3 uses SUJ2 steel as a raw steel material, and by performing warm rolling on the steel, a carbide having an average particle size of 0.5 μm or less with respect to all carbides It is shown that the wear amount of the bearing can be reduced, and the life can be improved and the acoustic characteristics can be improved by making the carbide having an average particle size of 1 μm or more 2 area% or less, and 50 area% or more. .

  However, even such techniques may be insufficient in terms of improving wear resistance. In other words, there is no concept of the distance between carbides simply by defining the area ratio and size of carbides, and in normal manufacturing methods, carbides may be generated unevenly, and wear occurs from places where the amount of carbide generation is small. It is likely to occur and good wear resistance may not be exhibited.

In addition, when large plate-like cementite was generated in the rolled structure before warm rolling, not only the refinement of carbides was insufficient with warm rolling alone, but also large plate-like cementite was present. In the vicinity, since the carbides are present in a biased state, good wear resistance may not be obtained. Moreover, since this technique performs further hot rolling after hot rolling, it not only increases the cost required for capital investment, but also the versatility remains unresolved.
JP 63-143239 A JP 2002-194438 A Japanese Patent Laid-Open No. 10-259451

  The present invention has been made in view of such circumstances, and the purpose thereof is general-purpose, which is excellent in wear resistance at the time of rolling when applied to a bearing and can be manufactured without causing an increase in cost. An object of the present invention is to provide a steel material for bearings.

The steel for bearings of the present invention that has solved the above problems is SUJ2-equivalent steel specified by JIS G 4805 (1999), that is, C: 0.95 to 1.10% (meaning of mass%, chemical composition) Are the same), Si: 0.15 to 0.35%, Mn: 0.50% or less (not including 0%), P: 0.025% or less (not including 0%), S: 0.0. 025% or less (excluding 0%), Cr: 1.30 to 1.60%, respectively, the balance is made of iron and inevitable impurities, and the average area ratio of carbide in any cross section after quenching and tempering is In addition to being 10 to 16%, the average number of carbides is 0.35 to 0.50 / μm ² . In addition, after quenching and tempering, the steel material is hot-rolled and then spheroidized, and subsequently formed into a predetermined shape and then quenched and tempered.

  The bearing steel of the present invention is based on the premise that SUJ2 steel is used as a raw material, and by making the manufacturing conditions appropriate using existing equipment, it is possible to make hard carbides in an ideal dispersed state, and cost It is possible to produce a steel material that can produce a bearing with excellent wear resistance that can reduce the wear rate to ½ or less compared to conventional materials without incurring an increase in the wear rate. It will be.

  The present inventors have studied from various angles in order to realize a steel material (bearing steel material) for manufacturing a bearing having excellent wear resistance. As a result, in order to make the bearings excellent in wear resistance, it is important to disperse hard carbide (cementite) in an ideal state and to reduce the locally worn parts as much as possible. Knowledge was obtained. For example, even if the carbide area ratio in the steel material is large, if the dispersion state is poor (the number per unit area is small) and the area where no carbide is present increases, good wear resistance as a whole bearing component can be obtained. It cannot be secured.

  On the other hand, even if the dispersion state of carbide is good (the number per unit area is large), if the area ratio of carbide is small, the portion where carbide is not present increases, and in this case as well, the wear resistance of the entire bearing Sexuality gets worse. That is, in order to improve the wear resistance during rolling of the bearing, it is necessary to increase the area ratio of carbide in the bearing and increase the number of carbide per unit area (arbitrary cross section).

  The present inventors used SUJ2 equivalent steel as a raw material steel, and examined conditions for increasing the area ratio of carbide in the bearing and increasing the number of carbide per unit area (arbitrary cross section). In addition, since the carbide dispersion state of the bearing greatly depends on the structure before quenching and tempering, it is necessary to disperse the area ratio and number of carbides to an ideal state in the structure before quenching and tempering. In other words, if the area ratio and number of carbides are dispersed in an ideal state in the structure of the steel material before quenching and tempering, the steel material is molded into a predetermined shape and then quenched and tempered. Even after being made into bearing parts, the area ratio and number of carbides remain dispersed in an ideal state.

  As a result, if hot rolling is performed at a relatively low temperature in the austenite region and the conditions for the subsequent spheroidizing treatment (spheroidizing annealing treatment) are made appropriate, even if SUJ2-equivalent steel is used, the carbide area ratio is a predetermined amount. The steel material (bearing steel material) for obtaining a bearing capable of securing an ideal dispersion state of carbide and exhibiting excellent wear resistance could be realized.

  The bearing steel of the present invention was obtained by subjecting a slab satisfying the above component composition (for example, a slab slab obtained by continuous casting) to normal segment rolling and soaking (soaking furnace soaking). Thereafter, when performing hot rolling and spheroidizing treatment (spheroidizing annealing treatment), these conditions are obtained by appropriately controlling, and the specific conditions at this time are as follows.

In order to secure carbides in the spheroidized structure, it is effective to perform hot rolling at the lowest possible temperature in the austenite region. That is, if hot rolling is performed at a low temperature, the austenite crystal grains become finer, thereby increasing the number of grain boundaries that serve as starting points for carbide formation nuclei, and further, it takes time to dissolve in the matrix during spheroidization. Since the pro-eutectoid cementite is also dispersed and formed, the number of carbides in the steel material can be finally increased. Further, along with the refinement of crystal grains, the hardenability is lowered and the lamellar spacing of the pearlite structure of the rolled material is increased, so that it takes time to dissolve the carbide into the matrix during spheroidization. As a result, since the carbide is likely to remain, the carbide can be further increased. Therefore, in order to as much as possible austenite grains fine, A ₁ transformation point or more, it is effective to perform hot rolling at 850 ° C. or less.

  After the above hot rolling, spheroidizing treatment (spheroidizing annealing treatment) is performed. The conditions at this time are 680 to 680 ° C./hour after heating at 780 to 810 ° C. for 4 to 6 hours. It is effective to cool to below ℃. In the cooling process in the spheroidization process (cooling process after heating), carbides in the steel material grow. If the cooling rate at this time is reduced, the amount of carbon diffusion increases, so that the area ratio of carbides can be increased. However, if the cooling rate is too slow, further carbon diffusion proceeds, so that the carbides are coarsely aggregated and the number of carbides decreases. On the other hand, if the cooling rate is increased, carbon is not sufficiently diffused, so that the carbide does not grow and the carbide area ratio cannot be secured. For this reason, in order to ensure a predetermined amount of carbide area ratio and number in the present invention, the cooling rate during spheroidization (hereinafter sometimes referred to as “sphering rate during spheroidizing treatment”) is 12 to 18 ° C. Need to be controlled.

  The cooling stop temperature during the spheroidizing treatment needs to be 680 ° C. or less, which is a temperature at which the spheroidizing hardly proceeds. As general spheroidizing treatment conditions, heating at 780 to 810 ° C. for 4 to 6 hours, gradually cooling at a cooling rate of 10 ° C./hour or less, stopping cooling at 600 ° C., and allowing to cool. Although it has been carried out (for example, “Steel Heat Treatment Revision 5th Edition” edited by the Iron and Steel Institute of Japan, August 20, 1981, p. 427), the number of carbides cannot be secured by such spheroidizing treatment.

By using a steel material that has been subjected to hot rolling and spheroidizing treatment under the manufacturing conditions as described above, quenching and tempering are performed to obtain an average area ratio of carbides (hereinafter, sometimes simply referred to as “area ratio”). It is a steel material that can secure 10% or more and can secure an average number of carbides per unit area (hereinafter, simply referred to as “number”) of 0.35 / μm ² or more. A bearing having good properties can be manufactured. As the area ratio of the carbide increases, the wear resistance improves, but the upper limit is about 16% due to the chemical components of the steel material, production conditions, and the like. The same applies to the number of carbides per unit area. The larger the number of carbides, the better the wear resistance. However, from the relationship of the chemical composition and production conditions of the steel, 0.50 The upper limit is about pieces / μm ² .

  The steel material for bearings of the present invention is processed into a predetermined part shape (drawn if necessary and then processed into a part shape), and then quenched and tempered to form a bearing (the bearing surface is polished if necessary). ) Although the steel material for bearings of the present invention is basically obtained as a linear or rod-like steel material that can easily produce a bearing, the size can be appropriately determined according to the final product. That is, the steel material for bearings of the present invention is intended to include those that have been drawn as needed before being formed into products (after rolling).

  For example, the quenching process is heated to 800 to 840 ° C., held at that temperature for a predetermined time (for example, 30 to 60 minutes), then oil-quenched, and then tempered. The tempering conditions at this time vary depending on the size and shape of the steel material (for example, spheres, rollers, rings), but are usually about 120 to 180 ° C. × 60 to 250 minutes. That is, in the steel for bearings of the present invention, the area ratio and the number of carbides in the steel can be secured under the above-described quenching and tempering conditions.

  However, with regard to the quenching conditions, if the heating time is too long, carbides may excessively dissolve in the matrix phase, and the area ratio and number of carbides cannot be ensured, and wear resistance may not be obtained. For these reasons, the heating time is preferably up to about 50 minutes (Test No. 25 described later).

  The steel material of the present invention is SUJ2-equivalent steel specified by JIS G 4805 (1999), C: 0.95 to 1.10%, Si: 0.15 to 0.35%, Mn: 0.50% or less (Not including 0%), P: not exceeding 0.025% (not including 0%), S: not exceeding 0.025% (not including 0%), Cr: satisfying 1.30 to 1.60% Is.

  The basic chemical composition of the steel material of the present invention is as described above, with the balance being iron and inevitable impurities. Examples of the inevitable impurities include Ni, Mo, Cu, and the like, as defined in JIS, Ni: less than 0.25%, Mo: less than 0.08%, and Cu: less than 0.25%.

  Hereinafter, the present invention will be described in more detail by way of examples.However, the present invention is not limited by the following examples as a matter of course, and may be implemented with modifications within a range that can meet the gist of the preceding and following descriptions. Of course, they are all possible and are included in the technical scope of the present invention.

C: 1.01%, Si: 0.25%, Mn: 0.36%, P: 0.0014%, S: 0.0005%, Cr: 1.45%, balance: iron and inevitable impurities Steel (SUJ2-equivalent steel) was melted by continuous casting, rolled into 155 mm square billets, and then heated in a soaking furnace (soaking furnace) (heating temperature: 1200 ° C.). Thereafter, hot rolling was performed at the rolling temperature shown in Table 1 below to obtain a rolled material (round bar) having a diameter of 70 mm. Subsequently, after heating to 795 ° C. (heating time: 6 hours), spheronization treatment (spheroidizing annealing treatment) was performed at the cooling rate (cooling rate during spheroidizing treatment) shown in Table 1 below (cooling stop temperature) : 680 ° C. and then allowed to cool). Note for the steel, A ₁ transformation point obtained based on the following equation (1) is 751 ° C.. At this time, samples (steel materials) were put into a furnace whose atmosphere was controlled by a thermocouple at an upper part of the furnace body (height: about 4200 mm, width: about 2000 mm) at intervals of 2500 mm.

A ₁ transformation point = 723-10.7 × [Mn] −16.9 × [Ni] + 29.1 × [Si]
+ 16.9 × [Cr] + 290 × [As] + 6.83 × [W] (1)
However, [Mn], [Ni], [Si], [Cr], [As] and [W] indicate the contents (mass%) of Mn, Ni, Si, Cr, As and W, respectively.

  The spheroidized material thus obtained was heated to 840 ° C. for oil quenching and tempered at 160 ° C. for 120 minutes. In the quenching, the heating time was basically 30 minutes, and some were 60 minutes (test No. 25 described later).

  About the obtained steel materials (quenched / tempered materials), a thrust test piece having a diameter of 60 mm and a thickness of 6 mm was prepared, and the area ratio, number, and surface hardness of carbides were measured by the following methods, and abrasion resistance was obtained. Evaluated. The external shape of the produced thrust test piece is shown in FIG. In FIG. 1, 1 is a thrust test piece, 2 is a load ball (diameter: 9.525 mm), 3 is a raceway, diameter D is 60 mm, and raceway width W is 38.5 mm. These results are collectively shown in Table 2 below.

(Measurement method of area ratio and number of carbides)
The area ratio of the carbide was measured at 4000 magnifications using a scanning electron microscope (“JSM-T220” manufactured by JEOL Ltd.) near the raceway 3 of the test surface of the thrust test piece. .3.0 "(manufactured by Sumitomo Metal Technology Co., Ltd.), the area ratio was determined, and the average value of three fields of view was determined. As for the number of carbides, an average value of values obtained by dividing the number of carbides obtained by the above-mentioned carbide area ratio measurement by the measured field area (356 μm ² ) was obtained. In addition, since the carbide | carbonized_material becomes difficult to judge the circumference | surroundings of carbide | carbonized_material when it becomes small too much, a circle equivalent diameter (diameter of a circle | round | yen equivalent when the area is the same) is 0.1 micrometer or more (on a 4000 times photograph, 0.4 mm). The above was measured.

(Hardness measurement method)
1 at four positions (indicated by a to d in FIG. 1) at 90 degrees on the raceway ring 3 of the thrust specimen shown in FIG. 1 (untested: spare sample not to be tested). Measurement was performed at 98N, and the average value was obtained.

(Abrasion resistance evaluation method)
The thrust test piece was evaluated using a rolling fatigue tester (“FJ-5T” manufactured by Fuji Testing Machine Co., Ltd.). At this time, the number of tests for each steel material was 16 times (n = 16). At this time, the contact pressure was 5.24 GPa and the load speed was 1500 cpm. “Krysef Oil F8” (manufactured by Nippon Oil Co., Ltd.) is used as the lubricant, and after the wear rate reaches the end of its life (determination is made when the raceway is peeled off), the wear depth of the raceway is loaded. The average value (n = 16) was used by dividing by the number of times. About wear depth, the said 4 places were measured using roughness measuring instrument "SE3300" (made by Kosaka Laboratory Ltd.), and the average value was calculated | required. If the wear rate was 10 (× 10 ⁻⁸ ) μm / time or less, it was judged that the wear resistance was excellent.

  These results can be considered as follows. That is, no. 1 to 9 (meaning of test No., the same applies hereinafter) are examples that satisfy the requirements (area ratio of carbide, number of carbides) defined in the present invention, and all exhibit excellent wear resistance. I understand that.

  In contrast, no. Nos. 10 to 25 deviate from the requirements (the area ratio of carbides and the number of carbides) defined in the present invention, and wear resistance is poor.

  Specifically, no. 10 and 11 are cases where the rolling temperature is higher than 850 ° C. and the cooling rate during the spheroidizing process is slow. Although these have a low austenite refining effect, since the carbide is sufficiently diffused, the area ratio of the carbide is increased. However, since the cooling rate during the spheroidizing treatment is slow, the agglomeration of carbides proceeds, the number of carbides is insufficient, and the wear resistance is inferior.

  No. Nos. 12 to 14 have a rolling temperature higher than 850 ° C., and even when the cooling rate at the time of spheronization is appropriate, the austenite refining effect is low, so the number of carbides cannot be ensured and wear resistance. Lack of sex.

  No. 15 and 16 are cases where the rolling temperature is higher than 850 ° C. and the cooling rate during the spheroidizing treatment is too fast. Since austenite has a low refining effect and carbides do not grow, sufficient carbide area ratio and number cannot be ensured, and wear resistance is poor.

  No. In Nos. 17 to 20, since the rolling temperature is too high and the effect of refining austenite cannot be obtained, a sufficient carbide area ratio and number can be ensured regardless of whether the cooling rate during spheroidization is fast or slow. The wear resistance is inferior.

  No. 21 and 22 are cases where the cooling rate during the spheroidizing treatment is slow after performing appropriate low-temperature rolling, and a sufficient carbide area ratio is ensured. Cannot be secured, and wear resistance is poor.

  No. Nos. 23 and 24 are cases where the cooling rate during the spheroidizing treatment is high after performing appropriate low-temperature rolling, and the agglomeration of carbides does not proceed, so that the number of carbides can be secured, but the diffusion of carbon required for growth Is insufficient, the carbide area ratio cannot be secured, and the wear resistance is inferior.

  No. No. 25 is a case where the heating time at the time of quenching is long, and the area ratio and the number of carbides are not secured, and the wear resistance is inferior.

It is explanatory drawing which shows the external appearance shape of a thrust test piece.

Explanation of symbols

1 Thrust test piece 2 Load ball 3 Race ring

Claims (1)

C: 0.95 to 1.10% (meaning of mass%, chemical components are the same hereinafter), Si: 0.15 to 0.35%, Mn: 0.50% or less (excluding 0%), P: 0.025% or less (not including 0%), S: 0.025% or less (not including 0%), Cr: 1.30 to 1.60%, respectively, the balance being iron and inevitable impurities The carbide has an average area ratio of 10 to 16% in an arbitrary cross-section after quenching and tempering, and an average number of carbides of 0.41 to 0.50 / μm ^2. Steel material for bearings with excellent wear resistance.

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