JPH10204534A - Track bushing and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a track bushing increased in productivity, reduced in costs, and excellent in wear resistance and strength, as compared with that obtained by a carbonizing method or an induction hardening method, and its production. SOLUTION: By controlling cooling capacity so that the temp. gradient in the inner part of wall thickness during cooling from hardening temp. becomes gradual, residual tensile stress due to martensitic transformation is controlled and residual compressive stress is formed in respective surface of inside diameter and outside diameter, and simultaneously, the central part of the inside and outside diameters is regulated so that it has a hardness not lower than 50% martensite hardness of the steel material used and also becomes a practically through hardened state, and further, the internal structure is regulated so that it is composed essentially of lath martensite.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a track bush used for a construction machine such as a bulldozer and a method for manufacturing the same.

[0002]

2. Description of the Related Art As shown in FIG. 11, a crawler belt bush 52 used for a crawler belt 51 of a construction machine or the like has a function of meshing with a sprocket tooth for transmitting rotational motion from a final reduction gear to rotate the crawler belt 51. Therefore, wear resistance is required on the inner and outer diameter surfaces, and at the same time, severe strength and toughness are required. In order to satisfy these required characteristics, conventionally, the following method has been employed in manufacturing this crawler belt bush. Case hardening steel is carburized to form high-hardness martensite on the inner and outer surfaces to ensure abrasion resistance and strength (for example, see Japanese Patent Publication No. 52-34806). Using medium carbon steel, induction hardening the inner and outer diameter parts respectively to form high hardness martensite on the inner and outer surface layers,
Those designed to ensure abrasion resistance and strength.
9-77979). Using medium carbon steel with very fine control of hardenability while finely adjusting the chemical composition of the steel material, rapidly cooling the bush heated in the furnace to 800 ° C or more and adjusting the hardening depth of the inner and outer diameter surface , Designed to ensure wear resistance and strength. FIG. 12 shows a schematic diagram of a typical hardening pattern of a bush produced by the conventional method and a hardness distribution diagram of a cross section, respectively.

[0003]

However, in the carburizing method, the carburizing time is long,
Cost problems such as the large use of carburizing gas are significant. For example, as the wall thickness increases, the required hardened layer depth increases from the viewpoint of strength and abrasion resistance. Need to be improved. Further, since the inner and outer surface layers are made of a martensite layer containing a high concentration of carbon, there is also a problem that the impact fatigue resistance tends to deteriorate.

In the above-mentioned induction hardening method,
Although it is easier to improve the cost compared to the carburizing method, since the inner and outer diameters cannot be heated at the same time,
It is necessary to harden the inner and outer diameters twice, and unevenness and detachment of the quenched part at the end face of the bush cannot be avoided. There is a problem in ensuring abrasion (especially important in oil filling). Further, FIG.
As shown in Fig. 3, a very large tensile residual stress is likely to occur near the center of the wall thickness. In particular, when the depth of the hardened layer is increased in order to improve the wear resistance, fatigue fracture occurs during use. There are also problems such as easy occurrence.

Next, the above-mentioned quenching method can solve the above-mentioned cost problems, but it is necessary to accurately grasp the relationship between the thickness of the bush to be used and the cooling rate before using it. It is necessary to control the hardenability (DI value) of steel materials accurately and narrowly, and there is a problem in the availability of steel materials as a production method. Also, as the bush thickness becomes thinner, through hardening occurs in the entire thickness area, and a martensitic transformation type residual stress due to a sharp temperature gradient inside and outside the thickness occurs as a large tensile residual stress on the inner and outer surfaces, Since the occurrence of quenching cracks during quenching and the remarkable deterioration of fatigue strength are caused, the DI value is inevitably reduced. Is liable to occur.

An object of the present invention is to eliminate such problems by increasing the productivity, lowering the cost, and improving the wear resistance and strength of the crawler belt bush as compared with the carburizing method or the induction hardening method. And a method for manufacturing the same.

[0007]

In order to achieve the above-mentioned object, a crawler belt bush according to the present invention has a compression residual stress formed on each surface of an inner diameter and an outer diameter, and has a hard portion at the center of the inner and outer diameters. The hardness of the steel is 50% or more of the martensite hardness of the steel material used, and is substantially adjusted to a through-hardened state, and the internal structure is adjusted to be substantially lath martensite. .

Further, the method for manufacturing a crawler belt bush according to the present invention for manufacturing the crawler belt bush includes controlling the cooling capacity so that the temperature gradient inside the wall thickness during cooling from the quenching temperature becomes gentle. Suppresses the tensile residual stress caused by the martensitic transformation to form compressive residual stress on each of the inner and outer diameter surfaces, and the center of the inner and outer diameters is substantially more than 50% martensite hardness of the used steel material and is substantially centerless. It is characterized in that it is adjusted so as to be in a quenched state and is adjusted so that the internal structure is substantially composed of lath martensite.

Further, in order to prevent the occurrence of incompletely quenched layers due to grain debound oxidation and decarburized layers on the inner diameter surface which significantly deteriorates the fatigue strength of the crawler bush, the inner and outer diameter surfaces are stable after quenching. Ammonia was added to the heating atmosphere at the quenching temperature in order to generate compressed residual stress and to form the residual austenite in an amount of 20% by volume or more in order to prevent the hardness of the outermost surface layer from becoming too high. It is characterized by a carburizing / nitriding atmosphere in which nitrogen is permeated within 0.3 mm from the surface.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION A steel material which can be used for manufacturing a track bush according to the present invention has a carbon content of 0.35 to 0.60.
Weight percent, and is adjusted by adding alloying elements such as Cr, Mn, Si, B, and Al so that the DI value is 0.7 in ^-1 or more, and the quenching hardness is Rockwell hardness. It is preferably from 45 to 60. Here, the reason why the DI value is set to 0.7 in ⁻¹ or more is mainly that it is difficult to uniformly cool the inner diameter portion of the bush at the time of rapid cooling. This is because it becomes difficult to form uniform martensite, which causes a considerable increase in the cost of cooling equipment. Further, even in ordinary steel, a material having a DI value of less than 0.7 in ^-1 requires special composition adjustment and causes an increase in cost.
It is desirable that the value be 0.7 in ^-1 or more. The use of the boron-treated steel is preferable for reducing the low-temperature brittleness property by setting the tempering or quenching and pulling temperature high. When the carbon content of the steel material used is increased to 0.60% or more, the impact fatigue becomes worse in spite of the small contribution of hardness, and the carbon content is reduced to 0.10%.
If it is less than 3%, sufficient hardness cannot be obtained,
The carbon content of the present invention was set to 0.35 to 0.60%.

Since the crawler belt bush of the present invention is manufactured based on a heat treatment operation for heating and cooling the entire bush almost uniformly, adjustment of the hardening depth is performed by adjusting the high-frequency heating depth as in the conventional induction hardening method. Since there is no need to perform the control and the internal and external diameters are not separately heated and quenched, high productivity can be realized. In addition, the depth of the inner and outer diameter hardened layers is controlled so as to significantly reduce the cooling capacity during the cooling from the relationship between the temperature distribution inside the wall thickness during the cooling and the DI value specific to the steel material. There is no need to finely control the hardenability of the steel while corresponding to the thickness of the bush, and there is no particular problem in the availability of steel, making it possible to solve the problems of the prior art.

Further, in the present invention, a tough lath martensite structure can be mainly formed in the vicinity of the center of the thickness, and the hardness in the vicinity of the center of the thickness can be set high. No substantial distribution of hardness with the part. This is because the hardness distribution of the hardened layer of the conventional induction hardened bush enters the unhardened layer, and at the same time, suddenly softens into a U-shape and the tensile residual stress rises sharply in this part, which is a strength constraint. Further, as a means for increasing the hardness of the thick inner diameter portion, it is an effective solution to the problem such as the strength limitation due to the local softening phenomenon appearing in the bush subjected to the induction hardening after the tempering treatment.

[0013] In the present invention, after substantially uniform heating of the entire bushing, since it mainly heat treatment to slowly cool the vicinity of M _s point, the carbon content of the steel to be used as the bush Even if it is set high, since it is different from non-uniform quenching work such as induction hardening, the use of boron (B) -treated steel further increases the DI value without lowering the M _s point, thereby reducing the occurrence of quenching cracks. By eliminating the danger, it is possible to achieve higher hardness of the inner and outer diameter surface layers of the crawler belt bush, improve the abrasion resistance, and remove the hardened layer at the end face of the bush, which occurs with the conventional induction hardening method. For example, it can greatly contribute to the improvement of the life of crawler tracks of construction machines.

In the present invention, after heating to a quenching temperature of 770 to 950 ° C. in a carburizing, carburizing, or nitriding atmosphere, the oil is quenched using oil of 100 ° C. or more as a cooling medium. After quenching, the quenching temperature is adjusted to 100 to 250 ° C., or the quenching temperature is almost uniformly heated to 770 to 950 ° C. in a carburizing, carburizing and / or nitriding atmosphere, and then the cooling medium When the temperature near the center of the wall thickness is cooled to a range of 250 to 400 ° C. in the first stage of cooling, the cooling is stopped to eliminate the temperature gradient inside the wall thickness by using water or soluble. It is preferable to return the temperature to ~ 250 ° C. By doing so, the residual stress on the inner and outer diameter portions of the bush is always made to be a compressive stress, and the surface hardness is 45 to 60 in terms of Rockwell hardness. A large lath martensite phase can be formed, the above-described wear resistance, strength and toughness can be exhibited, and a great advantage can be obtained in terms of cost.

In particular, when carbon and / or nitrogen is diffused and infiltrated into the surface by carburizing or nitriding, controlling the depth of penetration from the surface to 0.3 mm or less causes a problem even under an impact load condition. -3
Fatigue strength can be improved by stably generating a remarkable compressive residual stress exceeding 0 kg / mm ² .

[0016]

Next, specific examples of the crawler belt bush and the method of manufacturing the same according to the present invention will be described.

Example 1 Table 1 shows the components of steel materials used in this example. As shown in Table 1, in this example, S4 was used as the boron (B) treated steel.
The types 0BC and SCr440B were selected. In addition,
The Jominy curves of these boron treated steels are shown in FIG. Further, as shown in FIG. 2, the bush shape used in this embodiment has an outer diameter of 59 mm, an inner diameter of 38 mm, and a height of 138 mm. As the heat treatment method, FIG.
As shown in FIG. 3 (b), the quenching method was set on a metal grid of a stirred water tank, pulled out of the water tank after a predetermined time, and allowed to cool. Note that, for comparison, the test was also performed on a completely cooled level without raising after a predetermined cooling time. Further, for reference, an induction hardened bush using the same SCr440B material was also implemented.

[0018]

[Table 1]

The quenching time in the water tank is 3, 4, 5,
After the quenching, it was allowed to cool to the atmosphere. FIG. 4 shows a hardness distribution in a thick cross section of a steel type having a low DI value which is left to cool after 5 seconds of water cooling. As apparent from FIG. 4, it is possible to also conveniently evenly quenching process at an inner diameter portion, in order to obtain the H _R C45 or more through hard bushing inside wall thickness, the DI value when the steels selected It turns out that a steel material of 0.7 in ^-1 or more is necessary (however, when the wall thickness is 10.5 mm).
As a matter of course, when the bush becomes thicker, a steel material having higher hardenability is required to satisfy the above-mentioned condition (through hard). This is the thickness of 14.5mm
FIG. 5 shows the cross-sectional hardness distribution of the bush that was allowed to cool after 7 seconds. FIG. 5 shows that the required DI value is 0.95 in ⁻¹ or more.

FIG. 6 shows the relationship between the residual stress of the bush and the quench cracking when directly cooled to the water temperature. From this figure, it can be seen that when the through-hard bush is directly cooled to the water temperature, a tensile residual stress is generated on the inner and outer diameter surfaces of the bush, and consequently burning cracks are significantly generated.

FIG. 7 shows the relationship between the water tank cooling time and the surface residual stress for a 10.5 mm thick bush (see FIG. 2) using S40BC and SCr440B shown in Table 1. As is clear from FIG.
There is an optimum value for the cooling time, and it is clear that the bush having a thickness of 10.5 mm used in the present embodiment generates a compressive residual stress in the bush of 5 to 6 seconds. In addition, the cooling temperature of 3 seconds raises the pulling temperature to about 350 ° C., which is too low in hardness, and the cooling time of 7 seconds or more is almost equal to the hardness of martensite expected from the Jominy curve in FIG. It is clear that the hardness at the center of the wall thickness is also hardened to almost the same level, but it can be seen that the residual stress on the surface becomes the tensile stress.

The reheating temperature of the surface is about 170 ° C. after cooling for 7 seconds. In this case, the residual stress on the surface is almost 0 kg / mm ² , The time cooling causes tensile residual stress on the surface. In addition, despite the fact that the steel materials used differ greatly in hardenability, the obtained surface residual stress shows almost the same result, and the residual stress control by thermal control, which is the gist of the present invention, has a wide hardenability. This shows that the technology can be applied to different steel materials.

Example 2 An oil quenching method was used as a cooling means. In this case, the quenching oil is usually 50 to 80
It was carried out using a material heated to 120 ° C. higher than 100 ° C. The main point was that the cooling time was not controlled as described in Example 1 and that the oil was sufficiently quenched in the quenched oil until the temperature reached the oil temperature. This is for cooling. From the results of the preliminary experiments, it is necessary to ensure that the steel material to be subjected to the experiment has a hardenability of 1.8 in ^-1 or more in terms of DI value. Therefore, in this example, the boron-treated steel shown in Table 1 was examined.

In this embodiment, a bush having the shape shown in FIG.
The sample was kept at a constant temperature for 60 minutes in a protective atmosphere of gas and RX gas: NH ₃ = 5: 1 and in a carburizing / nitriding atmosphere, and then immersed in quenching oil and cooled. The carburizing / nitriding depth at that time is about 0.15 mm. After cooling, a tempering treatment was performed for 3 hours at each temperature from 150 to 570 ° C., and the hardness, residual stress, and half-value width of martensite at that time were measured by X-ray. FIG. 8 shows the measurement results of the sample quenched in the RX gas protective atmosphere. 180 for residual stress
It was found that the compression residual stress was stably generated by tempering at a temperature of at least 250C, and the effect was particularly remarkable at around 250C. It was also found that the compressive residual stress was generated stably even at 400 ° C. or higher, but it is considered that it cannot be used as a crawler bush from the viewpoint of wear resistance due to a large decrease in hardness. Therefore, preferably 180 to 3
A tempering range of 00 ° C. can be selected. When the pulling temperature was adjusted to 182 ° C., 231 ° C., and 275 ° C., almost the same result as the tempering level was obtained in terms of hardness and residual stress, and when the pulling temperature was adjusted, a new tempering process was performed. Turned out to be unnecessary.

In a carburizing / nitriding atmosphere, the carburizing / nitriding depth
Of Table 1 which was heat-treated so as to be 0.15 mm.
5, No. All 11 composition crawler bushes are -30kg
/ Mm ^TwoThe above compressive residual stress is generated,
Compressed residue larger than any of those heat treated in a protective atmosphere
It was confirmed that it was a stress. Also, the surface under this condition
The carbon concentration of the layer is 0.8% by weight, the nitrogen concentration is 0.7% by weight
The amount of retained austenite in the surface layer was 55% by volume.
Yes, the outermost surface hardness is H_V= 500-600. Sa
In addition, the surface layer has fine C particles having an average particle diameter of about 0.3 μm.
that the carbonitride or nitride of r, Al
confirmed.

Example 3 The impact fatigue strength of the crawler belt bushes treated in Examples 1 and 2 was investigated. FIG. 9 shows a test method used for the test. The track bush has a thickness of 10.5 mm shown in FIG.
The load conditions were such that the hammer was dropped on the bush attached to the crawler belt link as shown in FIG. 9, and the stress generated on the inner diameter of the bush was adjusted so as to correspond to 2, 3, and 4 times the body load (20 ton). A comparative study was conducted by comparing the number of repetitions up to. The test results are as shown in FIG. Using the boron-treated steel shown in Table 1 as a comparative material, an impact fatigue test was performed on both a crawler track bush whose inner and outer diameter surfaces were induction hardened and a crawler track bush whose SCM420 was carburized and hardened.

Based on the test results, the tracked bush according to the present invention has significantly improved impact fatigue characteristics compared to the carburized tracked bush for bulldozers requiring extremely abrasion resistance, and has a resistance almost equal to or higher than that of the high-frequency tracked bush. Impact fatigue properties were obtained. In particular, when the present invention is applied to a crawler bush for a bulldozer that requires wear resistance, the material is hardened in the entire thickness range, so that the wear life can be improved. The excellent impact fatigue resistance of the crawler bush heat-treated under the conditions of carburizing and nitriding also means that the amount of retained austenite in the surface layer is remarkably large, and the carbonitride and nitride precipitates themselves This indicates that the reduction in impact strength is prevented by making the particles extremely fine. Therefore, an appropriate amount of retained austenite is preferably about 20% by volume or more produced by a conventional carburizing method, and is preferably 80% by volume or less at which the surface hardness does not become H _RC = 45 or less. . Further, the average particle diameter of the precipitated carbonitride and nitride can be discussed in relation to the hardness of the martensitic matrix in relation to the notch effect inside the structure, but it is preferable that the average particle diameter be approximately 1 μm or less. Conceivable.

Example 4 Using the same condition as the crawler bush used in Example 3, the sliding tester shown in FIG. 14 was used to investigate the heat generation characteristics of the bush using an oil-filled crawler track assembly. did. The crawler belt pins used were S53C induction hardened materials.

FIG. 15 shows the result of the investigation. The heat generation characteristics are excellent in the order of IQT, crawler bush, and carburized carbonized QT (product of the present invention). It shows good sliding characteristics and is suitable for measures against overheating of crawler belts when running at high speeds such as bulldozers. It is considered that this is due to the effect of improving the conformability by the large amount of retained austenite generated by carbonitriding and the effect of improving the seizure by hard particles such as carbonitride and / or nitride which are finely dispersed.

According to the present invention, a high quality bush can be provided at a lower cost than an induction hardened bush or a carburized bush.
A manufacturing method that makes it easier to select the hardenability of bushing steel materials that are likely to become through hard by ordinary quenching, a hardened layer with excellent abrasion resistance is stably applied to the inside, and the surface layer is By providing a bush having excellent sliding characteristics, it is possible to improve the life of crawler tracks in construction machines and the like.

[Brief description of the drawings]

FIG. 1 is a graph showing a Jominy curve of boron-treated steel.

FIG. 2 is a cross-sectional view showing a bush shape in the embodiment.

FIG. 3 is a graph (a) showing a quenching pattern;
FIG. 4B is a diagram illustrating a quenching method.

FIG. 4 is a graph showing a relationship between a hardness distribution and a DI value.

FIG. 5 is a graph showing a relationship between a hardness distribution and a DI value.

FIG. 6 is a graph showing a relationship between residual stress and quenching crackability.

FIG. 7 is a graph showing a relationship between a cooling time and a residual stress.

FIG. 8 is a graph showing a relationship between residual stress and hardness in oil quenching.

FIG. 9 is an explanatory view of a crawler belt bush impact fatigue test method.

FIG. 10 is a graph showing the results of an impact fatigue test.

FIG. 11 is a perspective view showing the structure of a crawler belt.

FIG. 12 is a view showing hardening patterns (a) to (c) and a cross-sectional strength distribution of a bush according to a conventional method.

FIG. 13 is a graph showing an analysis result of a residual stress distribution by an induction hardening method.

FIG. 14 is a schematic diagram of a sliding tester and a diagram showing test conditions.

FIG. 15 is a graph showing a result of an investigation on the heat generation characteristics of the crawler belt bush.

Claims (12)

[Claims] 1. A compressive residual stress is formed on each surface of an inner diameter and an outer diameter, and the hardness of the center of the inner and outer diameters is not less than 50% martensite hardness of the used steel material, and the material is substantially in a centerless hardened state. A tracked bush characterized by being adjusted and being adjusted so that the internal structure is substantially composed of lath martensite. 2. A means for forming a compressive residual stress on each of the surfaces, the cooling rate is extremely increased when the temperature at the substantially central portion of the thickness of the crawler belt bush reaches 250 to 400 ° C. during cooling from the quenching temperature. The crawler belt bush according to claim 1, wherein the quenching and pulling temperature is adjusted to 100 to 250C by slowing down. 3. A method for controlling the cooling temperature, wherein in oil quenching, the quenching oil temperature is 100 to 180.
3. The crawler belt bush according to claim 2, wherein the cooling temperature is adjusted so that the cooling temperature becomes slower after the temperature of the crawler bush becomes substantially equal to 250 to 400C. 4. As means for controlling the cooling temperature, in water or soluble quenching, cooling is stopped or switched to air cooling when the temperature at the substantially central portion of the thickness of the crawler bush reaches 250 to 400 ° C. The crawler bush according to claim 2, wherein 5. As a means for forming a compressive residual stress on each of the surfaces, the heating atmosphere at the quenching temperature is adjusted to a carburizing / nitriding atmosphere so that the carbon content in the surface layer region within 0.3 mm from the surface is reduced. After satisfying either or both of the conditions of 0.4 to 0.8% by weight and the amount of nitrogen of 0.4 to 1.0% by weight, a quenching treatment is performed to generate a compressive residual stress. The crawler belt bush according to any one of claims 1 to 4, wherein an incompletely quenched layer in the surface layer is prevented from being generated, and at least 20% by volume of a retained austenite phase is contained. 6. The steel material used has a carbon content of 0.35.
This is a boron-treated steel pretreated with Al and Ti at ~ 0.60% by weight, and is adjusted by adding alloy elements such as Cr, Mn, and Si so that the DI value becomes 0.7 in- ¹ or more. And quenching hardness of 45 in Rockwell hardness
The crawler bush according to any one of claims 1 to 5, wherein 7. At the center of the wall thickness during cooling from the quenching temperature, the cooling rate at 250 to 400 ° C. or later is controlled so that the temperature gradient inside the wall thickness becomes gentle, and the tensile force caused by martensitic transformation is obtained. Controlling residual stress, forming compressive residual stress on each surface of inner and outer diameter,
It is characterized in that the thickness is adjusted so that the center of the thickness is at least 50% martensite hardness of the used steel material and is substantially in a state of hardened hardening, and the internal structure is substantially made of lath martensite. Crawler bush manufacturing method. 8. As a means for forming a compressive residual stress on each of the surfaces, after substantially uniformly heating at a quenching temperature of 770 to 950 ° C., and substantially in the middle of the thickness of the crawler bush during cooling using a cooling medium. The method for manufacturing a crawler belt bush according to claim 7, wherein the quenching and pulling-up temperature is adjusted to 100 to 250 ° C by extremely decreasing the cooling rate when the temperature of the section reaches 250 to 400 ° C. . 9. A method for controlling the cooling temperature, wherein in oil quenching, the quenching oil temperature is 100 to 180.
9. The manufacturing of the crawler belt bush according to claim 8, wherein the cooling temperature is adjusted so that the cooling temperature becomes slower after the temperature of the crawler bush becomes approximately 250 to 400 [deg.] C. by increasing the temperature to not less than 250C. Method. 10. As means for controlling the cooling temperature,
The method for manufacturing a crawler bush according to claim 8, wherein in water or soluble quenching, cooling is stopped or switched to air cooling when the temperature at the substantially central portion of the crawler bush thickness becomes 250 to 400C. 11. As a means for forming a compressive residual stress on each of the surfaces, heating is performed almost uniformly at a quenching temperature of 770 to 950 ° C., and the heating atmosphere is adjusted to a carburizing / nitriding atmosphere to reduce the surface temperature to 0%. 0.4 to 0.8% by weight of carbon and 0.4% of nitrogen in the surface layer region within 0.3 mm
After satisfying either or both of the conditions of 1.0 to 1.0% by weight, a quenching treatment is performed to generate a compressive residual stress, and furthermore, it is possible to prevent the occurrence of an incompletely quenched layer in the surface layer, 2. The composition according to claim 1, further comprising a residual austenite phase in an amount of at least volume%.
0. The method for manufacturing a crawler belt bush according to any one of 0. 12. The steel material used has a carbon content of 0.3.
5 to 0.60% by weight, pre-treated with Al and Ti
Boron-treated steel with DI value of 0.7in ^-1Over
As described above, by adding alloying elements such as Cr, Mn, and Si,
And quenching hardness is 4 in Rockwell hardness
It is 5 to 60, characterized by the above-mentioned.
The method for manufacturing a crawler bush according to any one of the above.