JP2007169673A - Heat-treatment method for steel, method for producing rolling member in rolling-support device and rolling-support device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To hardly occur the inner part starting point type breakage without developing a large deformation, even in the case of producing a rolling member in the large bearing, such as the bearing for backup roll. <P>SOLUTION: At least one of rolling member among an inner ring 1, an outer ring 2 and a roller 3, is made by applying a carbonizing or a carbonitriding after processing a blank made of a steel into a prescribed shape and further, by applying a cooling at 15°C/min-300°C/min cooling rate, and a quenching and a tempering in this order, an austenite crystal grain diameter in this core part is made to ≤19.2μm of the average value and ≤5.5 of the standard deviation. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a steel heat treatment method, a rolling support device manufacturing method (rolling bearing, ball screw, linear guide, etc.), and a rolling support device.

In the hot rolling process of a steel plate, a pair of rolling rolls arranged above and below the steel plate is required to withstand a high load with a smaller diameter from the viewpoint of productivity. For this reason, the backup roll with a large diameter is arrange | positioned at the up-down direction outer side of the steel plate, and the bending of the rolling roll by the load and impact which are applied at the time of a rolling process is suppressed.
A backup roll bearing that rotatably supports the backup roll is required to have excellent load resistance and impact resistance in addition to a long life. For this reason, as a rolling member (inner ring, outer ring, rolling element) constituting a backup roll bearing, after carving or carburizing after processing a material made of a steel type having good hardenability (for example, carburized steel) into a predetermined shape. By performing nitriding and quenching and tempering, the hardness of the surface layer part forming the rolling surface is set to HRC60 (Hv697) or more, and the hardness of the core part is set to HRC30 to 48 (Hv302 to 484). Yes.

  However, in the rolling member obtained by the above-described conventional manufacturing method, coarse particles are generated in the austenite crystal of the core during heat treatment and become a starting point of breakage, or the core and the surface layer portion subjected to carburizing or carbonitriding When the tensile stress is generated between the two, the material strength of the core portion is lower than the material strength required in the use environment, and the internal origin type damage may occur in the core portion.

  For this reason, in Patent Document 1, in order to reduce the tensile stress generated between the core portion and the surface layer portion, and to suppress damage of the internal origin type that occurs in the core portion, carburization or at least one of the inner ring and the outer ring It has been proposed to perform surface hardening treatment of carbonitriding and to set the hardness of the surface layer portion of at least one of the inner ring and the outer ring to Hv550 or higher and Hv690 or lower.

Further, in Patent Document 2, in order to suppress internal origin type breakage that occurs in the core portion, the inner ring of the backup roll bearing is made of oxide inclusions having an average particle diameter of 3 μm or more and 30 μm or less in unit area (160 mm ² ). It has been proposed to form a bearing steel having a structure ratio of oxide inclusions of not more than 80 per particle, of which the average particle diameter is 10 μm or more and less than 2%.

By the way, as the usage environment of the backup roll bearing becomes more severe, the techniques described in Patent Document 1 and Patent Document 2 described above make it difficult to cause the above-described internal origin type breakage, and in addition to a long life, excellent It has become difficult to obtain high load resistance and impact resistance.
For this reason, in recent years, a technique for increasing material strength by refining the austenite grain size of steel has attracted attention.
Since the austenite crystal grain size depends on the structure before quenching, the finer the structure before quenching, the more austenite nucleation sites increase after quenching, and the austenite crystal grain size can be refined. . Here, it is said that it is effective to increase the cooling rate after carburizing or carbonitriding in order to refine the structure before quenching.

JP 2000-314427 A JP 2004-84869 A

However, when the above-described crystal grain refinement technique is applied to a rolling member of a large bearing (for example, a bearing having an inner diameter of 120 m or more) such as a backup roll bearing, the following problems occur. . That is, in a large-sized bearing such as a backup roll bearing, since the steel material having good hardenability is used as the material of the rolling member, the entire structure is not uniform when rapidly cooled by quenching after carburizing or carbonitriding. Since it transforms into martensite by cooling, deformation due to burning strain becomes large and productivity is not good.
Accordingly, the present invention is intended to make it difficult to cause internal origin type damage without causing large deformation even when a rolling member of a large-sized bearing such as a backup roll bearing is manufactured. It is said.

In order to solve such problems, the present invention is characterized in that quenching and tempering are performed after cooling after carburizing or carbonitriding at a cooling rate of 15 ° C./min to 300 ° C./min. A method for heat treating steel is provided.
According to the heat treatment method for steel according to the present invention, steel having good hardenability is used as a material by performing cooling after carburizing or carbonitriding at a cooling rate of 15 ° C./min to 300 ° C./min. Even in this case, since the entire structure is transformed into martensite by uniform cooling, the internal origin type damage caused by the austenite grain size of the steel core and residual tensile stress occurs without causing large deformation. It can be difficult.

  The present invention also includes a first member and a second member having raceway surfaces arranged opposite to each other, and a rollable surface disposed between the first member and the second member, the rolling surface with respect to the raceway surface. The rolling member of the rolling support device in which one of the first member and the second member moves relative to the other when the rolling member rolls. In this method, after a steel material is processed into a predetermined shape, carburization or carbonitriding, cooling at a cooling rate of 15 ° C./min to 300 ° C./min, quenching and tempering are performed in this order. Accordingly, the present invention provides a method for producing a rolling member of a rolling support device, characterized in that the austenite grain size of the core is 19.2 μm or less and the standard deviation is 5.5 or less.

  Here, when the average value of the austenite crystal grain size in the core is larger than 19.2 μm or the standard deviation is larger than 5.5, the effect of suppressing the internal origin type peeling due to the refinement of crystal grains is obtained. Disappear. In order to reliably prevent internal origin type damage without causing large deformation, cooling after carburizing or carbonitriding is performed at a cooling rate of 30 ° C./min to 300 ° C./min, It is preferable that the average value of the austenite grain size of the core is 17.8 μm or less and the standard deviation thereof is 4.8 or less.

  According to the present invention, using the steel heat treatment method according to the present invention described above, the average austenite grain size of the core of the rolling member is 19.2 μm or less and the standard deviation is 5.5 or less. As a result, even when a steel having good hardenability is used as a material for forming the rolling member, austenite crystal grains in the core of the rolling member without causing large deformation in the rolling member. It is possible to make it difficult to cause breakage of the internal origin type due to the diameter and residual tensile stress.

Therefore, by using the rolling member obtained by the manufacturing method according to the present invention, in a large-sized rolling support device such as a backup roller bearing, in addition to long life, excellent load resistance and impact resistance Can be obtained.
Further, the steel structure forming the rolling member of the rolling support device is generally mixed and affected by composition fluctuation (segregation) and forging. For this reason, when observing with a wide field of view, it is difficult to accurately grasp the grain size. On the other hand, when observing with a narrow field of view using the crystal grain size microscopic test method defined in JIS G 0551, observation of one field of view It is difficult to grasp the granularity of the entire organization.

Therefore, as a result of intensive studies on a method for grasping the prior austenite crystal grain size of the entire core portion of the rolling member without being affected by the mixed grains, the inventor has obtained an area per field of view of 30000 μm ² or more. It was found that the austenite grain size of the core part of the rolling member can be accurately grasped by randomly observing 30 fields or more at 500,000 μm ² or less.
That is, in the method for producing a rolling member of a rolling support device according to the present invention, the austenite crystal grain size is preferably a value obtained by observing 30 or more fields at 30000 μm ² or more and 500000 μm ² or less per field of view. .

Hereinafter, the heat treatment method for steel according to the present invention will be described in detail.
First, “carburization” is performed by heating and holding in a furnace in which a mixed gas (for example, RX gas + enriched gas) is introduced, or in a furnace in which a mixed gas (for example, RX gas + enriched gas + ammonia gas) is introduced. "Carbonitriding" is carried out by heating and holding.
Next, cooling in which the cooling rate is 15 ° C./min or more and 300 ° C./min or less is performed by air cooling, furnace cooling, gas cooling, or the like. Here, if the cooling rate is slower than 15 ° C./min, the austenite crystal grain size in the core cannot be sufficiently refined, and the internal origin type breakage cannot be sufficiently suppressed. On the other hand, when the cooling rate is higher than 300 ° C./min, the entire structure is transformed into martensite by non-uniform cooling, and deformation due to burning strain increases.

In order to reliably prevent internal origin type damage without causing large deformation, cooling after carburizing or carbonitriding should be performed at a cooling rate of 30 ° C./min to 300 ° C./min. preferable.
Next, after heating again to a temperature equal to or higher than the A1 transformation point, quenching is performed by quenching with water cooling or oil cooling, and then tempering is performed. This tempering is preferably performed at a low temperature (for example, about 150 to 220 ° C.) in order to reduce the austenite crystal grain size in the core of the steel without causing significant deformation of the steel.

  The rolling support device according to the present invention refers to, for example, a rolling bearing, a ball screw, and a linear guide. Here, when the rolling support device is a rolling bearing, the first member and the second member refer to the inner ring and the outer ring. Similarly, when the rolling support device is a ball screw, the first member and the second member are When the rolling support device is a linear guide, the first member and the second member indicate a guide rail and a slider, respectively.

Moreover, the core part which concerns on this invention points out the part (part deeper than a carburizing layer or a carbonitriding layer) which is not influenced by carburizing or carbonitriding.
Furthermore, as the steel used in the present invention, for example, manganese steel such as SMn420 and SMn420C, chromium steel such as SCr415 and SCr420, chromium molybdenum steel such as SCM415 and SCM420, nickel chromium steel such as SNC415, SNCM420, A carburized steel of nickel chrome molybdenum steel such as SNCM815 can be suitably used.

According to the heat treatment method for steel according to the present invention, the steel is made by quenching and tempering after cooling after carburizing or carbonitriding at a cooling rate of 15 ° C./min to 300 ° C./min. It is possible to make it difficult to cause internal origin type breakage without causing large deformation in the material. Moreover, according to the manufacturing method of the rolling member of the rolling support device according to the present invention, the average value of the austenite grain size of the core part of the rolling member is 19. By setting the standard deviation to be 2 μm or less and having a standard deviation of 5.5 or less, it is possible to make it difficult to cause internal origin type damage at the core of the rolling member without causing significant deformation of the rolling member.
Therefore, by using the rolling member obtained by the manufacturing method according to the present invention, in addition to a long life, excellent load resistance and impact resistance can be obtained even in a large-sized rolling support device such as a backup roll bearing. Can be obtained.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
In the present embodiment, a cylindrical roller bearing having a designation number NU228 (inner diameter: 140 mm, outer diameter: 250 mm, width: 42 mm) manufactured by NSK Ltd. shown in FIG. 1 was produced by the following procedure. As shown in FIG. 1, the cylindrical roller bearing includes an inner ring (first member) 1 and an outer ring (second member) 2 having raceway surfaces 1 a and 2 a arranged to face each other, and an inner ring 1 and an outer ring 2. And a roller (rolling element) 3 having a surface (rolling surface) with respect to the raceway surface, and a cage 4.

The inner ring 1, the outer ring 2, and the roller 3 were produced by the following procedure.
First, after processing a material made of nickel chrome molybdenum steel (SNCM815) into a predetermined shape, as shown in FIG. 2, carburization that is heated and held for a predetermined time in a furnace in which a mixed gas (RX gas + enriched gas) is introduced, Heat treatment in which cooling (air cooling) performed at a cooling rate in the range of 1 to 900 ° C./min, quenching (oil cooling) for rapid cooling after heating for a predetermined time in a furnace introduced with RX gas, and tempering in this order Went.

Using the sample for observation and inspection of the inner ring 1 obtained in this way, the average value of the austenite crystal grain size (γ crystal grain size) of the core part (the part where the carbon content and nitrogen content are the same as the material) and The standard deviation was measured by the following procedure.
As for the average value of the austenite crystal grain size, first, the inner ring 1 was cut, the cut surface was polished, and then etched with a saturated picric acid solution to reveal the austenite grain boundary in the core. Next, photographs of 50 visual fields (30000 to 500,000 μm ² per visual field) were taken using an optical microscope (200 to 1000 times) at the core where the austenite grain boundaries appeared. Next, the austenite crystal grain size was measured using the obtained photograph, and the average value of 50 visual fields was calculated. Moreover, the standard deviation of the austenite crystal grain diameter was calculated using the obtained photograph. These results are also shown in Table 1.

Further, the roundness of the inner ring 1 before and after heat treatment was measured using the obtained sample for observation and inspection of the inner ring 1, and the dimensional difference was calculated. This result is shown in No. Table 1 also shows the ratio when the dimensional difference of 11 is 1.
In addition, since the obtained outer ring 2 and roller 3 were subjected to the same heat treatment as that of the inner ring 1, the average value and standard deviation of the austenite crystal grain size in the surface layer portion were the same as those of the inner ring 1 described above.
Furthermore, after assembling a roller bearing using the obtained inner ring 1, outer ring 2, and roller 3 life test samples and a brass cage holder 4, a life test was performed under the following conditions. . <Life test conditions>
Radial load: P / C = 0.6
Rotational speed: 1000min ^-1
Lubricating oil: Ro68

As shown in Table 1, the cooling rate after carburizing was 15 to 300 ° C./min. 1-No. In No. 6, no. 7-No. Compared with 11, a long life was obtained and the amount of deformation was small.
On the other hand, the cooling rate after carburizing was higher than 300 ° C / min. 7, no. In No. 8, a long life was obtained, but the amount of deformation was large.
Moreover, No. which made the cooling rate after carburization slower than 15 degrees C / min. 9-No. In No. 11, the amount of deformation was small, but the life was short.
Based on the result shown in Table 1, the graph of FIG. 3 which shows the relationship between the cooling rate after carburizing and a lifetime was created. From the graph of FIG. 3, by setting the cooling rate to 15 ° C./min or more, No. It can be seen that a life of 1.5 times longer than 11 is obtained.

Moreover, based on the results shown in Table 1, the graph of FIG. 4 showing the relationship between the cooling rate after carburizing and the deformation amount was created. From the graph of FIG. 4, the deformation amount is set to No. by setting the cooling rate to 300 ° C./min or less. It can be seen that it can be 3.2 times or less of 11.
Furthermore, based on the results shown in Table 1, the graph of FIG. 5 showing the relationship between the cooling rate after carburizing and the average value of the austenite crystal grain size of the core was prepared. From the graph of FIG. 5, it can be seen that the austenite crystal grain size of the core is 19.2 μm or less in the range where the cooling rate is 15 to 300 ° C./min.
Furthermore, based on the results shown in Table 1, the graph of FIG. 6 showing the relationship between the cooling rate after carburizing and the standard deviation of the austenite crystal grain size in the core was prepared. From the graph of FIG. 6, it can be seen that the standard deviation of the austenite crystal grain size in the core is 5.5 or less in the range where the cooling rate is 15 to 300 ° C./min.

  From the above results, after processing the material made of carburized steel into a predetermined shape for the inner ring 1, outer ring 2 and roller 3 of the roller bearing, carburizing, cooling at a cooling rate of 15 to 300 ° C./min, quenching and It is produced by performing tempering in this order, and in the core portion, the average value of the austenite crystal grain size is set to 19.2 μm or less, and the standard deviation of the austenite crystal grain size is set to 5.5 or less, so that large deformation It was confirmed that the life could be extended without causing

It is sectional drawing which shows a roller bearing as an example of the rolling support apparatus which concerns on this invention. It is a figure which shows an example of the heat processing of the steel which concerns on this invention. It is a figure which shows the relationship between the cooling rate after carburizing, and lifetime. It is a figure which shows the relationship between the cooling rate after carburizing, and a deformation amount. It is a figure which shows the relationship between the cooling rate after carburizing, and the average value of the austenite crystal grain diameter of a core part. It is a figure which shows the relationship between the cooling rate after carburizing, and the standard deviation of the austenite crystal grain diameter of a core part.

Explanation of symbols

1 Inner ring (first member)
1a Raceway surface 2 Outer ring (second member)
2a Raceway surface 3 Rolling element

Claims (4)

  A steel heat treatment method characterized by performing quenching and tempering after cooling after carburizing or carbonitriding at a cooling rate of 15 ° C / min to 300 ° C / min. A first member and a second member having raceway surfaces arranged to face each other; a rolling element which is disposed between the first member and the second member so as to freely roll and has a rolling surface with respect to the raceway surface; In the method of manufacturing the rolling member of the rolling support device in which one of the first member and the second member moves relative to the other by rolling the rolling element. ,
After processing the material made of steel into a predetermined shape, carburizing or carbonitriding, cooling at a cooling rate of 15 ° C./min to 300 ° C./min, quenching and tempering in this order,
A method for producing a rolling member of a rolling support device, characterized in that an average austenite grain size of the core is 19.2 μm or less and a standard deviation is 5.5 or less. 3. The method for producing a rolling member of a rolling support device according to claim 2, wherein the austenite crystal grain size is a value obtained by observing 30 or more fields of view at 30000 μm ² or more and 500000 μm ² or less per field of view. . A first member and a second member having raceway surfaces arranged to face each other; a rolling element which is disposed between the first member and the second member so as to freely roll and has a rolling surface with respect to the raceway surface; In a rolling support device in which one of the first member and the second member moves relative to the other by rolling the rolling element.
The rolling support according to claim 2, wherein at least one of the first member, the second member, and the rolling element is a rolling member obtained by the manufacturing method according to claim 2. apparatus.

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