JP4490874B2 - Steel parts having splines and methods for improving their fatigue properties - Google Patents

Description

  The present invention relates to a steel part having a spline with excellent fatigue strength in which a spline end portion that is a starting point of fatigue fracture of a machine part is strengthened and a method for improving the fatigue strength thereof. More specifically, the present invention relates to a steel mechanical part constituting a power transmission system of a shaft having a spline portion and a method for improving fatigue characteristics thereof.

  Drive shafts and gear shafts used in engines such as automobiles are required to have high strength because a large fluctuating load acts as the engine rotates. In particular, the spline end provided in the power transmission system part is a stress concentration part, and is the weakest part.

  Regardless of the spline end, various proposals have been made regarding the method of strengthening the stress concentration portion. For example, in Patent Document 1, the strength around the oil hole, which is the stress concentration portion, is increased by induction hardening, And the invention regarding the method of introduce | transducing the residual stress of a compression to an oil hole surface layer is disclosed. Further, in Patent Document 1, when heating is performed with a half-release type high-frequency heating coil while rotating the crankshaft, the rotation is slowed down when the oil hole is located on the opposite side of the heating coil, A method is disclosed in which the heating layer in the vicinity is thickened to deepen the hardened hardened layer and strengthen the periphery of the oil hole.

  However, at the so-called “hardened boundary” where quenching occurs, tensile residual stress is generated, so that cracks are easily generated and fatigue cracks are generated from the “hardened boundary”. There was a problem that it was difficult to improve significantly.

  Further, as a technique related to a shaft having a spline portion, for example, Patent Document 2 discloses a quenching depth between a spline processing part for assembling a universal joint at the tip of a power transmission shaft and an intermediate part of a shaft which is a non-processing part. A method for making a difference between the two is disclosed.

  This is because, in a power transmission shaft, the intermediate part of the shaft has a lower static strength than the splined part that is the tip of the shaft, and the shaft intermediate part and the spline processed part are reduced due to the reduced diameter associated with the weight reduction of the shaft. The difference in static strength from the above is taken into account, and the static strength is reinforced only at the middle part of the shaft. Note that the spline portion, which is the tip portion of the shaft, is subjected to shot peening for imparting residual stress in order to improve its fatigue strength.

  Similarly, in Patent Document 3, the entire shaft is heat-treated, thereby increasing the strength, and by applying shot peening to the end of the spline that is a stress concentration portion, compressive residual stress is applied, and the fatigue strength of the part is increased. Is described.

  However, in the residual stress applying method using shot peening as described in Patent Documents 2 and 3, it is difficult to apply sufficient residual stress to prevent fatigue failure, and sufficient residual stress is not applied. Even if it can be applied, the treatment takes time and the treatment surface becomes rough, which is not practical.

  Patent Document 4 discloses a method for reducing stress concentration by adjusting the shape of the spline end.

  However, in the method described in Patent Document 4, numerical conditions for the optimum shape are not shown, and a large number of prototypes are required to obtain a sufficient effect. Further, since the shape is complicated, there is a problem that the number of processes is increased and the cost cannot be denied.

JP 2002-038220 A JP 2000-240669 A JP 2003-307111 A JP 2004-125000 A

  Therefore, the present invention has been made in view of such problems, and its purpose is to provide a new and improved fatigue resistance property capable of applying a sufficient compressive residual stress to the spline end. An object of the present invention is to provide a steel part having a spline and a method for improving the fatigue strength thereof.

  The present invention has been made as a result of intensive studies in order to solve the above-mentioned problems. By applying ultrasonic hitting treatment to the end of the spline and strengthening it, sufficient compressive residual stress is applied to the end of the spline. The present invention provides a steel part having a spline with excellent fatigue resistance, and a simple method for improving fatigue strength, the gist of which is as follows.

(1) By mass%, C: 0.1 to 0.8%, Si: 0.05 to 2.5%, Mn: 0.2 to 3%, Al: 0.005 to 0.1%, N A spline containing 0.001 to 0.02%, the balance being Fe and inevitable impurities, and the compressive residual stress in the surface layer of the spline end satisfying the following formula (A): Has steel parts.
−2.4 <(residual stress [MPa]) / (surface Vickers hardness Hv.) ≦ −1.8
... (A)
(2) By mass%, Cr: 0.1-2%, Ni: 0.1-2%, Mo: 0.1-2%, Cu: 0.1-2%, Ti: 0.003-0 The spline according to (1), further comprising one or more of 2%, V: 0.05 to 0.5%, Nb: 0.01 to 0.1% Has steel parts.
(3) A method for improving the fatigue characteristics of a steel part having a spline as described in (1) or (2) above, wherein the spline end is vibrated at a frequency of 10 to 60 kHz and an amplitude of 0.5 to 50 μm. A method for improving fatigue characteristics of steel parts having splines, characterized by striking with vibration terminals.

  According to the present invention, it is possible to provide a steel part having a spline with excellent fatigue resistance, which can apply a sufficient compressive residual stress to the spline end, and a simple method for improving fatigue strength. As a result, in the steel product having the spline according to the present invention, the spline end portion is not broken and the reliability of the part is increased, and the weight of the part corresponding to the reinforcement can be reduced, which contributes to the improvement of fuel consumption and cost. It has significant industrially useful effects.

  Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. In the present specification and drawings, components having substantially the same functional configuration are denoted by the same reference numerals, and redundant description is omitted.

  The best mode for carrying out the present invention will be described in detail with reference to FIGS.

  First, the technical idea forming the basis of the present invention is as follows. That is, the spline end portion provided on the shaft has a stress concentration shape, and further has a shape in which the cross-sectional area decreases. In other words, the fatigue strength of the spline end determines the fatigue strength of the entire shaft.

  In order to improve the fatigue strength around the spline end, two measures are conceivable: increasing the strength around the spline end or introducing compressive residual stress.

  The present inventors have found that it is possible to satisfy both of the above two measures by hitting a steel material with a vibration terminal that vibrates ultrasonically.

  In other words, a large compressive residual stress is applied to the surface of the spline end, and the surface is plastic-worked in the same way as shot peening, so that the fatigue strength of the shaft with the spline end can be greatly improved. I found out.

  FIG. 1 shows an example of a steel part having a spline and a shaft targeted by the present invention. FIG. 1A is a view showing an appearance of a spline portion in the shaft, and FIG. 1B is an enlarged side view of the spline end portion 1 of the spline and its peripheral region. As shown in FIG. 1B, the tooth groove of the spline end portion 1 has a shape in which the groove depth continuously decreases toward the smooth portion, and the groove depth is 0 mm. That is, the portion where the groove is eliminated shifts to the smooth portion.

  2 and 3 are views showing an embodiment in which the spline end portion of the shaft according to the present invention is hit. 2A shows a cross section perpendicular to the axis near the end of the spline, and FIG. 2B shows an axial cross section of the spline tooth groove. In the figure, 2 is a spline tooth, and 3 is an ultrasonic vibration terminal, where the tooth groove at the end of the spline becomes gradually shallower (from the end of the tooth groove, more specifically from the tooth groove to the smooth part). The processing method in (region) is shown.

  At this time, as shown in FIG. 2 (a), the ultrasonic vibration terminal 3 vibrates in the length direction, but the ultrasonic vibration terminal 3 is used to efficiently transfer energy in the shaft radial direction. Oscillates at an angle of ± 5 ° or less with respect to the perpendicular from the tooth root (the deepest part of the tooth gap), the compressive residual stress can be effectively applied.

  Further, when the radius of curvature R of the tip of the ultrasonic vibration terminal 3 is greater than or equal to the radius of curvature of the tooth root, the tip of the ultrasonic vibration terminal 3 cannot reach the tooth root, that is, the deepest part of the tooth gap. Therefore, it is impossible to apply energy by ultrasonic vibration. Therefore, the curvature radius R of the tip portion of the ultrasonic vibration terminal 3 needs to be equal to or less than the curvature radius of the tooth root. Also, at a radius of curvature less than half of the radius of curvature of the tooth root, when the ultrasonic vibration terminal 3 is pressed, the plastic deformation becomes too large, and on the contrary, the fatigue life is reduced, so the minimum radius of curvature is set to the radius of curvature of the tooth root. The radius of curvature is more than half of the above.

Therefore, the radius of curvature R at the tip of the ultrasonic vibration terminal 3 is preferably in the range of the following formula (B).
(Curvature radius of tooth root) ≧ R ≧ (curvature radius of tooth root) / 2 (B)

  In FIG. 3, the processing method of the smooth part without the tooth | gear 2 of a spline edge part is shown. 2A shows an axial cross section near the end of the spline, as in FIG. 2B, and FIG. 2B shows a splined shaft that is sandwiched between lathes and rotated to provide an ultrasonic transducer. It shows a method of efficiently performing surface treatment by making contact and scanning in the axial direction. In FIG. 3, the shaft without the change in the diameter of the smooth portion is illustrated, but it is also effective when the diameter of the smooth portion changes.

  In the present invention, the part where the hitting process is performed is limited to the spline end of the shaft because the fatigue failure is the main problem in the shaft because of the end of the shaft. The spline end mentioned here refers to a region where the spline tooth groove gradually becomes shallow and a smooth portion near the spline end. The width of the smooth part near the spline end (length in the axial direction) is not specified, but the area corresponding to the axial radius of the smooth part near the spline end (the width of the smooth part near the spline end is It is desirable to perform ultrasonic treatment on a region having the same length as the shaft radius. In addition, when the position where the fatigue crack is generated in the vicinity of the spline end is known in advance, the portion may be processed intensively. By performing the ultrasonic treatment, an impact mark is generated by the ultrasonic vibration terminal. When the depth of the impact mark is 10 μm or less, the effect of improving the fatigue strength is not sufficient, and when it exceeds 50 μm, work hardening is saturated, so the range of 10 to 50 μm is preferable.

  Here, it is preferable that the residual stress on the surface of the spline end portion satisfies the following formula (A).

−2.4 <(residual stress [MPa]) / (surface Vickers hardness Hv.) ≦ −1.8
... (A)

  Here, in this embodiment, the measurement of the surface Vickers hardness is performed by cutting the processed part perpendicularly to the axial direction and applying a force of 300 gf from the surface in the cross section to a part having a depth of 50 μm.

  In order to improve the fatigue characteristics, the higher the surface Vickers hardness, that is, the higher the strength, the better. However, the present invention further requires that the compressive residual stress be increased in proportion to the surface Vickers hardness.

In the ultrasonic striking treatment used in the present invention, it is difficult to set (residual stress [MPa]) / (surface Vickers hardness Hv.) To −2.4 or less, so the lower limit is set to −2.4. . On the other hand, when (residual stress [MPa]) / (surface Vickers hardness Hv.) Is −1.5 or more, the fatigue strength cannot be sufficiently improved, so the upper limit is set to −1.5. More preferably, as shown in the examples described later, (residual stress [MPa]) / (surface Vickers hardness Hv.) Is −1.8 or less.

  When processing the spline end, the shape of the tip of the ultrasonic vibration terminal 3 may be a hemispherical shape, a bowl shape, a bowl shape, or the like, but is not particularly limited. However, when processing a smooth part without the spline end teeth 2, the hemispherical or bowl-shaped tip shape may cause convexity and convexity to come together, which may result in unstable processing. is there. The most preferable one is a bowl shape that combines convex and concave, but there is a possibility that the manufacturing cost of the ultrasonic vibration terminal 3 is increased.

  The reason why the frequency of the ultrasonic vibrator used in the present invention is limited to 10 to 60 kHz is that the compressive residual stress applied to the steel material increases in this region. Similarly, the amplitude of the tip of the vibration terminal that vibrates ultrasonically is limited to 0.5 to 50 μm, because sufficient compressive residual stress cannot be applied to the steel material with an amplitude of less than 0.5 μm. Although the residual stress increases as the amplitude increases, the plastic deformation becomes too large at more than 50 μm, and the dimensional accuracy of the parts decreases and the fatigue strength also decreases. Therefore, the upper limit of the amplitude is limited to 50 μm.

Next, the reasons for limiting the components of the steel material constituting the shaft of the present invention will be described below.
C is an element effective for strengthening steel, but if it is less than 0.1%, sufficient strength cannot be obtained. On the other hand, if added excessively, the toughness decreases, so the upper limit of the amount added is 0.8%.
Si is effective as a steel strengthening element, but less than 0.05% has no effect. On the other hand, if added in excess, the toughness and machinability deteriorate, so the upper limit of the amount added is 2.5%.
Mn is an element effective for strengthening steel, but if it is less than 0.2%, a sufficient effect cannot be obtained. On the other hand, if added in excess, the toughness and machinability decrease, so the upper limit of the amount added is 3%.
Al is an element effective for deoxidation of steel and refinement of crystal grains, but if it is less than 0.005%, there is no effect. On the other hand, since the machinability deteriorates if added excessively, the upper limit of the addition amount is set to 0.1%.
N forms V carbonitride and Nb carbonitride and is an element necessary for precipitation strengthening, but if it is less than 0.001%, sufficient effects cannot be obtained. On the other hand, if added excessively, the toughness deteriorates, so the upper limit of the amount added is 0.02%.
Cr, Ni, Mo, and Cu are all elements that increase strength without impairing toughness when added in appropriate amounts. Cr, Ni, Mo, and Cu are not effective when the content is less than 0.1%, and the toughness is greatly deteriorated when the content exceeds 2%. And
Ti is an effective element because it produces nitrides and carbides and increases the strength by precipitation strengthening. Furthermore, since the Ti nitride remains without dissolving at high temperatures, it is effective in preventing austenite coarsening during heating. If the content is less than 0.003%, these effects do not appear. If the content exceeds 0.2%, the toughness deteriorates, so the lower limit of the amount added is 0.003% and the upper limit is 0.2%.
V is also an effective element because it produces nitrides and carbides as in the case of Ti and increases in strength due to precipitation strengthening, but 0.05% or more of addition is necessary for the effect to occur. On the other hand, if added excessively, toughness deteriorates, so the upper limit of the amount added is 0.5%.
Nb is also an effective element because it produces nitrides and carbides as well as Ti, and the strength is increased by precipitation strengthening. However, if it is less than 0.01%, a sufficient effect cannot be obtained for its effect. . On the other hand, if added excessively, the toughness deteriorates, so the upper limit of the amount added is 0.1%.

  In addition to these elements, for example, Pb, S, Bi or the like may be added as an element for improving machinability, and such a case is also included in the present invention.

  In addition to the above elements, unavoidable impurities such as P are included, and such cases are also included in the present invention.

  Ono-type rotating bending test pieces provided with grooves simulating spline ends and grooves simulating smooth portions were cut out from the steels having the components shown in Table 1 below. Moreover, about the steel of the component of the steel number S shown in Table 1, after cutting out to the Ono type | formula rotation bending test piece of the same shape as the above, it carburized on the conditions of FIG.

  FIG. 4 shows details of the test piece. JIS Z 2274, No. 1 test piece, a test piece conforming to symbol 1-6 was prepared, and an axial groove and a smooth part simulating a spline as shown in FIG. Circumferential grooves were machined. FIG. 4B shows an axial section of the test piece. The length of the axial groove imitating the spline was 15 mm. FIG. 4C shows an A-A ′ cross section of FIG. Grooves simulating splines were 8 in 45 ° increments, the groove radius of curvature was 0.8R, and the groove depth was 0.5 mm. FIG. 4D shows an enlarged view of an axial cross section in the vicinity of the groove simulating a smooth portion. The radius of curvature of the groove imitating the smooth portion was 0.5R, and the depth was 0.5 mm.

  An example in which the ultrasonic treatment of the present invention was applied to this test piece and a comparative example in which no treatment or a treatment outside the range of numerical values of the present invention was applied were prepared. Asked. The results are shown in Table 2 below.

  The ultrasonic treatment was performed using a vibrator having a hemispherical tip with a radius of curvature of 0.5 mm. For the groove simulating a spline, a smooth part was simulated so that the vibrator reciprocated once in each groove. The groove was subjected to ultrasonic treatment so that the vibrator made two rounds of the groove.

  The value of surface Vickers hardness shown in Table 1 is the average of Vickers hardness measured at three points under a load of 300 gf from the surface of the cross section cut perpendicular to the axial direction after machining the part at a depth of 50 μm. Value is adopted.

  The measured values of the residual stress shown in Table 2 are obtained by separately preparing a test piece that has not been subjected to a fatigue test and measuring the residual stress of the joint surface layer. The residual stress is measured using X-rays, and the intensity of diffracted X-rays is measured and obtained from the half-value width of the peak intensity.

  Samples without ultrasonic striking treatment have only low fatigue strength compared to specimens with ultrasonic striking treatment. This is considered to be caused by the fact that the spline end portion reduces the fatigue strength. On the other hand, the fatigue strength can be improved by increasing the strength of the spline end by introducing an appropriate ultrasonic striking treatment and introducing compressive residual stress.

  As a result, the Example using the method for improving fatigue strength of the present invention showed a significant improvement in fatigue strength compared to the comparative example.

  As mentioned above, although preferred embodiment of this invention was described referring an accompanying drawing, it cannot be overemphasized that this invention is not limited to this example. It will be apparent to those skilled in the art that various changes and modifications can be made within the scope of the claims, and these are naturally within the technical scope of the present invention. Understood.

  INDUSTRIAL APPLICABILITY The present invention can be applied to a steel part having a spline with excellent fatigue strength in which a spline end portion that is a starting point for fatigue failure of a mechanical part is strengthened and a method for improving the fatigue strength thereof. In particular, the present invention can be applied to steel mechanical parts constituting a power transmission system of a shaft having a spline portion and methods for improving the fatigue characteristics thereof.

It is a figure which illustrates the form of the shaft which has a spline. It is a figure which illustrates embodiment which strikes the spline part at the time of striking the spline edge part of the shaft which has a spline in this invention. It is a figure which illustrates embodiment which strikes the smooth part at the time of hit | damaging the spline edge part of the shaft which has a spline in this invention. It is a figure which shows the test piece used for the Example of this invention. It is a figure which shows the carburizing quenching conditions used for the Example of this invention.

Explanation of symbols

1 Spline end 2 Spline teeth 3 Ultrasonic vibration terminal

Claims (5)

% By mass
C: 0.1 to 0.8%,
Si: 0.05 to 2.5%,
Mn: 0.2 to 3%,
Al: 0.005 to 0.1%,
N: 0.001 to 0.02%
And the balance consists of Fe and inevitable impurities,
A steel part having a spline, wherein the compressive residual stress in the surface layer at the end of the spline satisfies the following formula (A).
-2.4 <(residual stress [MPa]) / (surface Vickers hardness Hv.) < -1.8
... (A) % By mass
Cr: 0.1 to 2%,
Ni: 0.1 to 2%,
Mo: 0.1 to 2%,
Cu: 0.1 to 2%,
Ti: 0.003 to 0.2%,
V: 0.05-0.5%,
Nb: 0.01 to 0.1%
The steel part having a spline according to claim 1, further comprising one or more of the following. A method for improving fatigue characteristics of a steel part having a spline according to claim 1 or 2,
A method for improving fatigue characteristics of a steel part having a spline, wherein the spline end portion is hit with a vibration terminal that vibrates at a frequency of 10 to 60 kHz and an amplitude of 0.5 to 50 μm.   4. The fatigue property improvement of a steel part having a spline according to claim 3, wherein the spline end portion is a region where the tooth groove of the spline gradually becomes shallow and a smooth portion near the region. Method.   The length of the smooth portion in the axial direction is the same length as the axial radius of the smooth portion,
  The method for improving fatigue characteristics of a steel part having splines according to claim 3 or 4, wherein the vibration terminal is an ultrasonic vibration terminal.

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