JP2008255470A - Carburized component superior in low cycle fatigue characteristic - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve both of tooth flank strength and low cycle fatigue strength of the root of tooth, which are required for deferential gear (a carburized component). <P>SOLUTION: The carburized component comprises 0.10 to less than 0.30% C, 0.10% or less Si, 0.20-0.60% Mn, 0.015% or less P, 0.035% or less S, 0.50-1.00% Cr, 0.50-1.00% Mo, 0.0005-0.0030% B, 0.010-0.100% Ti, 0.010-0.100% Nb and balance Fe with inevitable impurities; has a surface layer C concentration after gas carburization treatment of 0.40-0.60%, an effective hardened layer depth of 0.6-1.2 mm defined by limit hardness of 513 HV, and a surface hardness after shot peening treatment of 700 HV or higher. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to carburized parts that are required to have high low cycle bending fatigue strength, and more particularly to drive system parts such as automobiles.

  Carburized parts are applied to power transmission parts such as automobiles, for example, differential gears. As one of the carburizing methods for carburized parts, gas carburizing is widely known. Gas carburizing has the advantages that the carbon potential of the gas can be freely controlled and that continuous operation from carburizing to quenching is easy. In this gas carburizing, as described in Patent Document 1 and Non-Patent Document 1 below, the C concentration of the surface layer of the carburized component is generally set to about 0.8%. That is, if the C concentration of the surface layer of the carburized component is set lower than 0.8%, the hardness of the surface layer is lowered, and it becomes difficult to ensure the strength of the carburized component (for example, the tooth surface strength of the differential gear). . Further, the surface of the carburized component is oxidized during carburization (formation of a grain boundary oxide layer), and it is difficult to ensure fatigue strength due to the presence of the incompletely quenched layer due to this surface oxidation.

  Moreover, in the following Patent Document 2, although the carburizing method is not particularly defined, the C concentration of the surface layer of the carburized component is set to 0.5 to 1.0% (invention example is 0.62 to 0.83%). Techniques to do this are disclosed. In Patent Document 2, toughness is ensured by preventing the formation of coarse cementite at the austenite grain boundaries of the surface layer while ensuring the hardness of the surface layer of the carburized component.

  On the other hand, in Patent Document 3 below, the C concentration of the surface layer of the carburized part is set to 0.7 to 0.9% by vacuum carburizing treatment, and the second stage is larger than the first stage shot particle size on the surface. A technique for performing a double shot peening process with a small shot particle size is disclosed. In Patent Document 3, fatigue strength is ensured by reducing the amount of retained austenite by double shot peening while preventing formation of a grain boundary oxide layer by vacuum carburization.

Japanese Patent Laid-Open No. 10-8199 JP-A-8-92690 JP 2002-30344 A Nikkan Kogyo Shimbun, “Case carburizing and quenching second edition”, February 26, 1999, first two editions, one print, p. 101

  By the way, the fatigue strength against low cycle fatigue at the root of a carburized part, for example, a differential gear (fatigue failure occurring at a repetition rate of, for example, 10,000 cycles or less when a large repetitive load causing plastic deformation is applied) is improved. In order to achieve this, it is effective to set the C concentration of the gear surface layer low. However, in this case, it is not possible to ensure the above-described gear surface strength of the gear. For this reason, there has been a problem that the characteristics of the tooth surface strength required for the gear and the characteristics of the low cycle fatigue strength of the tooth root cannot be made compatible. In this case, if vacuum carburizing is employed instead of gas carburizing, the above-described problem of fatigue strength reduction due to surface oxidation can be solved. However, in this vacuum carburization, excessive carburization occurs at the edge portion of the carburized component, and thus the low cycle fatigue strength cannot be improved as in the case of the gas carburizing.

  The present invention has been made in order to cope with the above-described problems, and its purpose is to improve both the tooth surface strength required for, for example, a differential gear as a carburized part and the low cycle fatigue strength of the tooth base. An object of the present invention is to provide a carburized part excellent in low cycle fatigue characteristics that can be made to occur.

  As a result of intensive studies to solve the above problems, the present inventors have obtained the following knowledge. (A) It is effective to suppress segregation of C present at the surface grain boundary of the carburized steel in order to suppress the surface grain boundary fracture serving as a fracture starting point. For that purpose, it is effective to lower the control value of the surface layer C concentrated by carburizing treatment to 0.40 to 0.60%. (B) When the control value of the surface layer C is lowered, the surface hardness is naturally reduced. In order to ensure surface hardness, it is effective to perform shot peening after carburizing. According to the shot peening treatment, surface layer hardness (700 HV or more) can be ensured by the effects of work hardening and compressive residual stress. Further, according to the shot peening process, the progress of fatigue cracks can be suppressed. (C) Even if the control value of the surface layer C is lowered and only the shot peening process is performed, for example, the tooth surface strength of the differential gear can be improved. The low cycle fatigue strength is not improved sufficiently. For this, it is effective to set the effective hardened layer depth at which the limit hardness is 513 HV as the management value.

  That is, the carburized parts excellent in low cycle fatigue characteristics of the present invention are in mass%, C: 0.10 to less than 0.30%, Si: 0.10% or less, Mn: 0.20 to 0.60% , P: 0.015% or less, S: 0.035% or less, Cr: 0.50 to 1.00%, Mo: 0.50 to 1.00%, B: 0.0005 to 0.0030%, It contains Ti: 0.010-0.100%, Nb: 0.010-0.100%, the balance consists of Fe and inevitable impurities, and the surface layer C concentration after gas carburizing treatment is 0.40-0.60. %, The effective hardened layer depth with a limit hardness of 513 HV is 0.6 to 1.2 mm, and the surface hardness after the shot peening treatment is 700 HV or higher.

  In this case, after the shot peening treatment, the maximum value of the compressive residual stress is 800 MPa or more, and the depth position (peak depth) at which the compressive residual stress is maximum is preferably within 100 μm from the surface layer. Preferably, the maximum value of compressive residual stress is 1000 MPa or more, and the peak depth is within 50 μm from the surface layer. The carburized component may be a differential gear, for example.

  Hereinafter, the reasons for limiting the composition of each element and the limiting conditions will be described.

(1) C: 0.10 to less than 0.30% C is an element for ensuring the strength of the carburized component (strength of the core). In order to obtain this effect, a content of 0.10% or more is necessary. On the other hand, if it is contained excessively, the toughness and the low cycle fatigue strength decrease, so the upper limit is made less than 0.30%. Preferably, it is 0.25% or less.

(2) Si: 0.10% or less Si is added as a deoxidizer during melting. This Si is an element that promotes grain boundary oxidation at the time of carburizing, and causes a decrease in low cycle fatigue strength. Therefore, it is necessary to limit its content as much as possible. Specifically, the content is 0.10% or less. Preferably it is 0.05% or less. In addition, since Si is effective in improving the hardenability of steel, in order to acquire this effect, 0.01% or more can be contained.

(3) Mn: 0.20 to 0.60%
Mn is an element that promotes grain boundary oxidation during carburizing, and lowers the low cycle fatigue strength. Therefore, its content must be limited as much as possible. Specifically, the content is 0.60% or less. On the other hand, Mn is an element effective for enhancing the hardenability of steel, and moderate austenite remains after carburizing in order to improve toughness. In order to obtain these effects, a content of 0.20% or more is necessary. Preferably it is 0.40% or more.

(4) P: 0.015% or less P is an element that deteriorates the toughness of the carburized layer. In particular, when the content exceeds 0.015%, the low cycle fatigue strength is significantly reduced. Further, since P is an impurity element, the content is preferably as close to 0% as possible.

(5) S: 0.035% or less S is an element that deteriorates the toughness of the carburized layer. If the content exceeds 0.035% as in the case of P, the low cycle fatigue strength is remarkably reduced. However, if machinability is particularly required, it may be contained at 0.010 to 0.020%.

(6) Cr: 0.50 to 1.00%
Cr, like Mn, is an element that promotes grain boundary oxidation during carburizing, and lowers the low cycle fatigue strength. Therefore, its content must be limited as much as possible. Specifically, the content is limited to 1.00% or less. Preferably it is 0.80% or less. On the other hand, Cr is an element effective for enhancing the hardenability of steel, so to obtain this effect, it is necessary to contain 0.50% or more.

(7) Mo: 0.50 to 1.00%
Mo is an element effective for enhancing the hardenability of steel, and is added to supplement the hardenability of steel which is insufficient due to the limited Cr content. It is also an element effective for improving the toughness of the carburized surface layer. In order to obtain these effects, a content of 0.50% or more is necessary. On the other hand, excessive content causes the hardness after carburizing and quenching to be too high, and the manufacturability deteriorates, so the content is 1.00% or less. Preferably it is 0.80% or less.

(8) B (Solubility B): 0.0005 to 0.0030%
B is an element effective for improving the hardenability of the core of the carburized steel. That is, the addition of B prevents the strength from being lowered by incomplete quenching, and the effect of increasing the effective hardened layer depth described later can be obtained. B is also an element effective for preferential segregation at the grain boundaries of the carburized layer and strengthening the grain boundaries of the carburized layer. In order to obtain this effect, the content is preferably 0.0005% or more. On the other hand, an excessive content not only saturates the effect of improving the hardenability but also decreases the workability in hot and cold, so the content is 0.0030% or less.

(9) Ti: 0.010 to 0.100%
Ti combines with N in carburized steel to form a nitride, and prevents N from combining with B, thereby securing solid solution B and maintaining the effect of improving the hardenability of B. It is an effective element. However, if it is less than 0.010%, it is not sufficient to fix N in the carburized steel, and if it exceeds 0.100%, the workability in the cold state decreases due to the enlargement of TiN, so 0.100% or less It is assumed to contain.

(10) Nb: 0.010 to 0.100%
Nb is an element effective to react with C and N in carburized steel to form carbonitrides and prevent coarsening of austenite crystal grains during carburizing. However, if it is less than 0.010%, it is difficult to obtain the effect of preventing the coarsening of the austenite crystal grains, and if it exceeds 0.100%, the effect is saturated.

(11) Surface layer C concentration: 0.40 to 0.60%
The carburized component of the present invention is subjected to gas carburizing treatment. According to the gas carburizing process, the surface layer C concentration can be easily set to a predetermined set value. In this case, as described above, in the normal carburizing process, the surface layer C concentration of the carburized component is set to about 0.8%. However, in the present invention, from the viewpoint of improving the ductility of the carburized layer in the carburized component and improving the crack initiation strength (lifetime until cracks are generated by a fatigue test), 0 .40 to 0.60% or less.

(12) Effective hardened layer depth (depth position from the surface where the limit hardness is 513HV. In Tables 3 and 5, it is expressed as ECD): 0.6 to 1.2 mm
ECD needs to be 0.6 mm or more from the viewpoint of ensuring plastic deformation resistance. Preferably, it is 0.7 mm or more. On the other hand, carburizing for a long time is required to make the thickness 1.2 mm or more, and the cost is high. In addition, grain boundary oxidation and generation of an incompletely hardened layer become remarkable, and the strength is lowered. preferable.

(13) Gas carburizing treatment The gas carburizing treatment of carburized parts generally includes a carburizing and quenching process of a carburizing period in which the surface layer has a high C concentration and a diffusion period in which the C concentration in the surface layer is diffused. In this gas carburizing process, when the carbon potential of the carburizing period and the diffusion period (hereinafter also referred to as CP) is set high, the effective hardened layer depth should be increased while shortening the carburizing period and the diffusion period. Can do. However, if the CP in the diffusion period is increased, the final surface C concentration is increased, and the crack initiation strength may be reduced. On the other hand, when both the carburizing period and the diffusion period CP are set low, the effective hardened layer depth can be increased by lengthening the carburizing period and the diffusion period. However, if the carburizing period and the diffusion period are lengthened, the cost of the carburizing process may increase. Therefore, the CP of the carburizing period is set higher and the CP of the diffusion period is set lower, that is, the carbon potential in the carburizing period is preferably set higher by 0.30% or more than the carbon potential in the diffusion period. According to this, the effective hardened layer depth can be increased in a short processing time compared to the case where both the carburizing period and the diffusion period CP are set low, and cracks are generated while suppressing the cost of the carburizing process. It is possible to improve the strength.

(14) Shot peening If the surface C concentration is lowered, the surface hardness decreases and it becomes difficult to secure the surface strength of the carburized component (for example, gear). By performing shot peening treatment on carburized parts after gas carburizing treatment, it is possible to remove the surface grain boundary oxide layer, which is the starting point of cracks, and to ensure good surface hardness by applying compressive residual stress be able to.

(15) Surface layer hardness: 700 HV or more From the viewpoint of securing the surface strength of the carburized component, the surface layer needs to have a hardness of at least 700 HV. Here, the surface hardness means the hardness at a depth position of 0.05 mm from the surface.

(16) Maximum value of compressive residual stress: 800 MPa or more From the viewpoint of suppressing the occurrence of cracks due to bending fatigue, a compressive residual stress of at least 800 MPa is required. Preferably, the pressure is set to 1000 MPa or more based on the test results described later. A higher absolute value is preferable, but the cost of shot peening increases, and if the yield strength of the projection target member is exceeded, cracks occur in the surface layer and the bending fatigue strength is reduced.

(17) Peak depth: within 100 μm This is to ensure the stress value of the surface layer of the carburized component. Preferably, it is within 50 μm. The purpose of defining the peak depth of the present invention is to form a portion having high residual stress in the shallow portion of the surface layer, and after making the portion of 100 μm from the surface layer high residual stress as in the present invention. Also, those having a peak at 100 μm or more are also targets (combined by shot peening).

  The present invention is suitable for use in a differential gear of an automobile in which low cycle fatigue strength (cracking strength) is particularly required. Hereinafter, an embodiment in which a carburized component according to the present invention is applied to a differential gear will be described with reference to the drawings.

  FIG. 1 shows a differential unit 10 having the differential gear. In the differential unit 10, a pair of left and right side gears 12 as differential gears are provided in a differential case 11 to which a ring gear (not shown) is fixed. 12 and differential pinions 13 and 13 engaged with both side gears 12 are accommodated. Each side gear 12 is connected to an axle shaft (not shown), and each differential pinion 13 is rotatably attached to a pinion shaft 14 provided in the differential case 11.

  When the vehicle travels, the differential unit 10 transmits the rotational torque from the engine to the differential case 11, the pinion shaft 14, the differential pinion 13, and the side gears 12 in this order via a drive pinion (not shown) and a ring gear. Rotate the axle shaft.

  As mentioned above, although one Embodiment which applied the carburized component by this invention to the differential gear was described, the application range of a carburized component is not restricted to this, It can apply widely to the mechanical structure components by which low cycle fatigue strength is requested | required. .

a. First Example First, steel A having chemical components shown in Table 1 was melted using an electric furnace. This steel A was hot-rolled into a round bar having a diameter of 22 mm, held at 920 ° C. for 1 hour and air-cooled, and then each test piece having the shape shown in FIG. 2 was produced. This test piece has a 1.5R circular groove-shaped notch, and simulates the tooth root (strength) of the gear. And what changed the surface layer C density | concentration according to any carburizing condition (carburizing quenching tempering process) which shows each test piece to FIG. 3, FIG. 4 (A), 4 (B), Examples 1-7, Comparative example 1-9.

  The carburizing condition a in FIG. 3 is that the carburizing period CP in the carburizing and quenching process is set to a high C concentration (0.9%), the diffusion period CP is set to a low C concentration (0.55%), The diffusion period is set to 90 minutes. The carburizing condition b is a condition that is often employed in a normal carburizing and quenching process. Both the carburizing period and the diffusion period CP have a high C concentration (the carburizing period: 0.8%, the diffusion period: 0.7%). ), The carburizing period is set to 240 minutes, and the diffusion period is set to 180 minutes. Carburizing condition c is set to low C concentration (carburizing period: 0.65%, diffusion period: 0.55%) in both carburizing period and diffusion period, and carburizing period is set to 90 minutes and diffusion period is set to 90 minutes. It is a thing.

  The carburizing conditions a1 to a5 of FIGS. 4A and 4B are obtained by appropriately changing individual settings based on the carburizing condition a of FIG. Specifically, the carburizing conditions a1 and a2 are obtained by extending both the carburizing period and the diffusion period in the carburizing condition a. In the carburizing condition a3, the CP of the diffusion period in the carburizing condition a is lowered and the diffusion period is extended within a range where the lower limit value of the surface layer C concentration is 0.40% or more. In the carburizing condition a4, the CP of the diffusion period in the carburizing condition a is lowered and the diffusion period is extended so that the lower limit value of the surface layer C concentration is less than 0.40%. In the carburizing condition a5, the CP of the diffusion period in the carburizing condition a is increased and the diffusion period is extended so that the lower limit value of the surface layer C concentration exceeds 0.60%. In addition, about any of carburizing conditions ac, a1-a5, the holding temperature in a carburizing and quenching process was 930 degreeC, and after oil diffusion, it hold | maintained at 850 degreeC for 30 minutes. Further, in the subsequent carburizing and tempering process, the steel was kept at 180 ° C. for 120 minutes and then cooled by air. The amount of surface layer C and the effective hardened layer depth (ECD) at this time are shown in Table 3 described later.

  Further, in Examples 1 to 7, shot peening (hereinafter also referred to as SP) was performed. In addition, about Comparative Examples 1-9, SP was given only about Comparative Examples 7-9. Table 2 shows the SP conditions. In Table 2, A and B perform SP twice while changing the SP condition, and C and D perform SP once. The shot particle size “large” indicates that the shot particle diameter is about 0.8 mm, “medium” indicates that the diameter is about 0.15 mm to 0.3 mm, and “small” indicates 0.05 mm to 0.00 mm. The thing of about 15 mm is shown. Further, the arc height “large” indicates a warpage amount of about 1.0 mm (A), and the “small” indicates a warpage amount of about 0.2 mm (A). Refer to JIS B 2711 (2005) for details of the measurement method.

  Next, evaluation methods and tests performed to confirm the effects of the present invention will be described.

(1) Surface layer C density | concentration The surface layer C density | concentration of each test piece was measured by the emission spectroscopic analysis based on JISG1253. Here, a calibration curve was prepared so that the C content could be measured up to 1% (error is ± 0.01%). Further, the surface layer C concentration distribution of each test piece was measured by line analysis using an X-ray microanalyzer (EPMA). Here, the surface layer C concentration is 0.40 to 0.60%.

(2) Effective hardened layer depth and surface hardness The effective hardened layer depth and the surface hardness of each test piece were measured using a Vickers hardness meter with a load of 300 g in accordance with ISO2639 (1982). However, the critical hardness of the effective hardened layer depth was 513 HV, and the hardness at a depth position of 0.05 mm from the surface was measured as the surface layer hardness. Here, the effective hardened layer depth is 0.6 to 1.2 mm, and the surface layer hardness is 700 HV or higher.

(3) Compressive residual stress For Examples 1 to 7 and Comparative Examples 1 to 9, the surface was polished by electrolytic polishing, and based on the X-ray diffraction profile by the well-known diffractometer method, the half width of the peak and the peak center position Compressive residual stress (maximum residual stress) was measured from the relationship. Further, the stress distribution from the surface layer was obtained, and the peak depth was measured. Here, a material having a compressive residual stress of 800 MPa or more is considered good.

(4) Bending fatigue test Each specimen was subjected to a bending fatigue test (4-point bending test) according to the test method shown in FIG. Then, for example, the number of repeated loads (cracking life) until the strain gauge attached to the notch of the test piece shown in FIG. Here, a load having a number of repetitions of 1100 times or more is considered good. Table 3 shows the above SP conditions, compressive residual stress, surface hardness, and load repetition number.

  According to Table 3, it can be seen that when the surface layer C concentration is 0.40 to 0.60%, the crack generation life is prolonged regardless of the presence or absence of SP. In addition, those with SP have a longer crack generation life than those without SP.

  In Examples 1 to 6, excellent surface layer hardness (700 HV or more) and crack generation life (1100 times or more) are obtained. From this, it can be seen that when the maximum value of the compressive residual stress is 1000 MPa or more, the crack generation life becomes long. In Example 7, the maximum value of the compressive residual stress is 806 MPa and does not reach 1000 MPa, but by applying SP (D condition), the surface hardness (762 HV) is good as low cycle fatigue characteristics. And a crack generation life (1104 times).

  From Comparative Examples 1 and 7, it can be seen that when the surface layer C concentration is low (0.35%), sufficient surface layer hardness cannot be obtained even when SP is applied (559 HV, 639 HV). From Comparative Example 5, it can be seen that when the surface layer C concentration is high (0.65%), a sufficient crack initiation life cannot be obtained (585 times). From Comparative Examples 2 to 4, it can be seen that even if the surface layer C concentration is in the range of 0.40 to 0.60%, sufficient surface layer hardness cannot be obtained unless SP is applied (614HV, 662HV, 698HV). ). From Comparative Examples 8 and 9, the surface layer C concentration is in the range of 0.40 to 0.60%, and even when SP is applied, the ECD is not in the range of 0.6 to 1.2 mm. It can be seen that a sufficient crack initiation life cannot be obtained (838 times, 805 times).

b. Second Example Next, in order to judge the influence of the alloy composition, the alloy composition was changed as shown in Table 4 so that the concentration of the surface layer C was 0.40 to 0.60%. Carburizing conditions a were employed, and SP was applied under SP conditions (A conditions) to produce Examples 8 to 13 and Comparative Examples 10 to 15. And also about these steel, the same evaluation method and test as the said 1st Example were done. The results are shown in Table 5.

  According to Table 5, when the composition of C was changed within the range of 0.10 to 0.30% as in Examples 8 and 9, the compositions of Cr and Mo were as in Examples 10, 12, and 13. Even when each is changed within the range of 0.50 to 1.00%, excellent surface hardness (700 HV or more) and crack generation life (1100 times or more) are obtained.

  It can be seen from Comparative Example 10 that when the content of C is low, sufficient ECD cannot be obtained even when SP is applied (0.55 mm). Moreover, since the core hardness cannot be obtained sufficiently, it can be seen that a sufficient crack initiation life cannot be obtained (963 times). From Comparative Examples 12 and 14, it can be seen that when the Cr and Mo contents are low, sufficient surface hardness cannot be obtained even when SP is applied (506HV, 583HV). From Comparative Examples 11 and 13, it can be seen that when the contents of C and Cr are high, sufficient crack initiation life cannot be obtained (973 times and 1007 times).

  As a result, in the present invention, it was confirmed that both the surface layer hardness and the crack generation life were excellent. Therefore, by applying the present invention to a differential gear, it is possible to improve both the tooth surface strength and the low cycle fatigue strength of the tooth root.

The cross-sectional schematic diagram which shows the differential unit provided with the differential gear which concerns on one Embodiment of the carburized component by this invention. (A) is a front view which shows a test piece. (B) is explanatory drawing which shows the outline | summary of a bending fatigue test (4-point bending test). Explanatory drawing which shows carburizing conditions. (A), (B) is explanatory drawing which shows the carburizing condition which changed each setting suitably based on the carburizing condition a of FIG.

Explanation of symbols

  12 Side gear (Differential gear)

Claims (4)

In mass%, C: 0.10 to less than 0.30%, Si: 0.10% or less, Mn: 0.20 to 0.60%, P: 0.015% or less, S: 0.035% or less , Cr: 0.50 to 1.00%, Mo: 0.50 to 1.00%, B: 0.0005 to 0.0030%, Ti: 0.010 to 0.100%, Nb: 0.010 Containing ~ 0.100%, the balance consisting of Fe and inevitable impurities,
Surface layer C concentration after gas carburizing treatment is 0.40 to 0.60%, effective hardened layer depth with limit hardness being 513HV is 0.6 to 1.2 mm, and surface layer after shot peening treatment A carburized part excellent in low cycle fatigue characteristics characterized by having a hardness of 700 HV or more.   The low value according to claim 1, wherein after the shot peening treatment, the maximum value of the compressive residual stress is 800 MPa or more, and the depth position where the compressive residual stress is maximum is within 100 μm from the surface layer. Carburized parts with excellent cycle fatigue characteristics.   The carburized part having excellent low cycle fatigue characteristics according to claim 1 or 2, wherein the carburized part is a differential gear.   The carburizing and quenching step in the carburized component according to any one of claims 1 to 3, including a carburizing period in which a surface layer has a high C concentration and a diffusion period in which the C concentration in the surface layer is diffused, A method for producing a carburized part excellent in low cycle fatigue characteristics, characterized in that a carbon potential in a period is set to be 0.30% or more higher than a carbon potential in the diffusion period.

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