JP2005321086A - Crank shaft fatigue characteristics improving method and crank shaft - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method of improving the fatigue characteristics of a crank shaft by strengthening a fillet portion, and to provide the crank shaft. <P>SOLUTION: Supersonic blowing treatment is given to the crank shaft on condition that a supersonic vibrator has a frequency of 10-60kHz, a supersonic output of 500-5000W and a thrust of 10-400N. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a method for strengthening a shaft having a fillet portion, and more particularly to a method for improving fatigue characteristics of a crankshaft of an internal combustion engine such as an automobile, and the crankshaft.

  As shown in FIG. 1, in a crankshaft 1 of an internal combustion engine, joints between an arm 2 and a pin 3 or a journal 4 are fillet portions 5 and 6 having a rounded cross section. The fillet portions 5 and 6 are locations where torsional stress and bending stress tend to concentrate during crankshaft rotation. For this reason, it is practiced to reinforce the fillet part, and examples thereof include a roll processing method and an induction hardening method.

The roll processing method is a technique for improving the strength by cold working the boundary between the pin / flange and the journal / flange with a fillet roll. As a conventional technique, for example, Patent Document 1 discloses a technique for reducing the stress concentration by increasing the radius of curvature of a portion where tensile stress is applied, and Patent Document 2 suppresses a decrease in the shaft diameter of the fret portion, resulting in stress. Techniques to ease concentration are introduced.
On the other hand, the induction hardening method is a technique for increasing the strength of the pin, the journal portion, the pin / flange, and the fillet portion at the boundary between the journal / flange and martensite by induction hardening. As a prior art, for example, Patent Document 3 introduces an induction hardening method and apparatus that hardly cause quenching cracks.
JP 2002-122126 A JP 2002-224920 A JP 2002-173711 A

However, the invention described in Patent Document 1 increases the cost because fillet forming control is difficult, and the invention described in Patent Document 2 has a problem in improving fatigue strength because sufficient work strengthening cannot be performed. It was.
In addition, in the induction hardening method represented by the invention described in Patent Document 3, it is impossible to avoid the problem that the strength of the burning boundary decreases due to the tensile residual stress generated in the burning boundary, and to strengthen the fillet portion. The deterioration of other parts was incurred.
Therefore, in order to overcome these problems, an object of the present invention is to provide a method for improving fatigue characteristics of a crankshaft by strengthening a fillet portion and a crankshaft.

The present invention provides a crankshaft fatigue characteristics improving method and a crankshaft by solving the problems of the prior art as described above and strengthening the fillet portion by applying an ultrasonic hitting process. The following is the contents as described in the claims.
(1) Ultrasonic impact treatment is performed under the conditions of the ultrasonic vibrator having a frequency of 10 to 60 kHz, an ultrasonic output of 500 to 5000 W, and a pressing force of the ultrasonic vibrator to the crankshaft of 10 to 400 N. A method for improving fatigue characteristics of a crankshaft.
(2) The crank according to claim 1, wherein the fillet portion is hit by a single or a plurality of pins whose curvature radius R of the tip of the ultrasonic transducer is defined by the following formula (A). A method for improving the fatigue characteristics of a shaft.
(Curvature of fillet portion × 0.5) ≦ R ≦ (curvature of fillet portion × 1.05)
... (A)
(3) The fatigue characteristics of the crankshaft according to claim 1, wherein the fillet portion is hit with a roll having a radius of curvature R of the tip portion of the ultrasonic transducer defined by the following equation (A). How to improve.
(Curvature of fillet portion × 0.5) ≦ R ≦ (curvature of fillet portion × 1.05)
... (A)
(4) When performing the ultrasonic hitting treatment on the fillet portion, when the Vickers hardness of the tip of the ultrasonic transducer is H1 and the Vickers hardness of the crankshaft is H2, the hardness defined by the following formula (B) The method for improving fatigue characteristics of a crankshaft according to any one of (1) to (3), wherein the processing is performed by an ultrasonic transducer having the crankshaft.
1.2 ≦ (H1 / H2) ≦ 5 (B)
(5) By mass%, C: 0.1 to 0.8%, Si: 0.05 to 2.5%,
Mn: 0.2 to 2%, Al: 0.005 to 0.1%, N: 0.001 to 0.02%, the balance is made of Fe and inevitable impurities, and the tensile strength is 800 MPa or more A crankshaft having excellent fatigue resistance, characterized in that the compressive residual stress in the surface layer of the fillet portion of the crankshaft is 50% to 90% of the tensile strength of the steel material. shaft.
(6) The steel material is mass%, and Cr: 0.1 to 2%, Ni: 0.1 to 2%, Mo: 0.1 to 2%, Cu: 0.1 to 2%, Ti (5), characterized by containing one or more of 0.003-0.05%, V: 0.05-0.5%, Nb: 0.01-0.1% Crankshaft with excellent fatigue strength as described.

  According to the present invention, it is possible to provide a method for improving fatigue characteristics of a crankshaft and a crankshaft by strengthening the fillet portion by applying an ultrasonic hitting process. Therefore, it is possible to reduce the weight of the crankshaft and thus improve the fuel efficiency. It has a significant industrially useful effect.

Hereinafter, the present invention will be described in detail. The technical idea forming the basis of the present invention is as follows.
In order to improve the fatigue strength of a component, the strength of the material is improved, the design of the component is changed so as to reduce the stress concentration, or a compressive residual stress is applied to the surface of the part where the fatigue failure of the component occurs. There are three possible points to introduce.
The inventors of the present invention have found that a large compressive residual stress is applied to the steel material surface by hitting the steel material with a terminal that vibrates ultrasonically. Furthermore, it has been found that the fatigue strength can be improved by applying this ultrasonic striking treatment to a portion where fatigue fracture of a part occurs.

The reasons for limiting the present invention will be described below.
The reason why the ultrasonic hitting process is limited to the fillet portion is that the fatigue failure is a major problem in the crankshaft because it is the fillet portion. It is effective to perform ultrasonic striking treatment in a narrow place where stress concentration occurs. The fatigue crack from the fillet portion is usually from the portion 5 in FIG. 1, and if only the portion 5 is subjected to ultrasonic treatment, the fatigue resistance is improved. Ultrasonic treatment is also effective in other places where fatigue failure is a problem, such as oil holes.

The reason why the frequency of the ultrasonic vibrator is limited to 10 to 60 kHz is that the compressive residual stress applied to the steel material increases in this region. Similarly, the reason why the ultrasonic output is limited to 500 to 5000 W is that the compressive residual stress applied to the steel material increases in this region. Further, the pressing force is limited to 10 to 400N because sufficient residual stress does not enter when the pressing force is less than 10N, and when the pressing force exceeds 400N, plastic deformation increases, the dimensional accuracy of the parts decreases and fatigue occurs. This is because the strength also decreases.
The ultrasonic vibration terminal used in the present invention is preferably one or a plurality of pins or rolls, and the shape of the tip of the ultrasonic hitting pin may be a hemispherical shape, a saddle shape, a saddle shape or the like, but is not particularly limited. However, in the case of a hemispherical tip shape, when processing the fillet portion, the projections and projections are brought together, so the processing may become unstable. The best is a saddle shape that combines convex and concave, but there is a possibility that the manufacturing cost of the ultrasonic hitting pin becomes high.

<Pin tip and roll curvature>
As shown in the upper part of FIG. 4, when the curvature of the pin tip is smaller than the curvature of the fillet, the tip of the pin reaches the bottom of the groove from the start of processing, and the entire groove is processed by the axial shake caused by ultrasonic vibration.
The smaller the curvature of the pin tip, the smaller the contact area, so that the energy density of the ultrasonic wave increases, and the compressive stress introduced into the fret portion or the processing portion of the journal portion also increases. However, since the curvature of the processing part becomes equal to the curvature radius of the pin due to deformation due to processing, when the curvature radius of the pin is less than half of the curvature radius of the processing part, the stress concentration factor increases and the fatigue strength increases so much. Disappear.

On the other hand, as shown in the middle of FIG. 4, when the curvature of the tip of the pin is larger than the curvature of the processing section and less than 1.05 times the curvature of the processing section, the processing section is deformed and the processing reaches the groove bottom. However, if the curvature of the tip of the pin is 5% or more larger than the curvature of the processing portion as shown in the lower part of FIG. 4, the processing does not reach the bottom of the groove. It is preferable that the fillet portion is hit with a single or a plurality of pins whose curvature radius R is defined by the following formula (A).
(Curvature of fillet portion × 0.5) ≦ R ≦ (curvature of fillet portion × 1.05)
... (A)
More preferably, it is desirable that the radius of curvature of the processing portion and the radius of curvature of the pin tip are equal.

The ultrasonic hitting process in the present invention is performed along the fillet journal portion. Processing may be performed over the entire circumference of the fillet portion and the journal portion, but since the fillet portion 5 in FIG. 1 is a crack generation point, it is efficient to perform processing only on the fillet 5.
As an example, when the hitting process is performed with a single pin, as shown in FIG. 5, a sufficient effect can be exhibited if the process is performed in a range of about ± 30 ° around the fret crack initiation point (fillet 5).
In addition, when there are a plurality of processing pins, it is desirable to arrange all the pins along the fillet or journal so that they are in contact with the portion to be processed.
As shown in FIG. 6, by arranging a plurality of pins radially or in parallel, processing can be performed in a short time.
In the case of a roll, the curvature defined by the above formula (A) is preferable for the same reason as the pin.

<Hardness of tip>
The tip is required to have hardness. If the tip tip has a Vickers hardness of 1.2 times or less than that of the material to be treated, the tip tip is deformed by the treatment and not enough compressive residual stress is applied to the material to be treated, and the life of the tip is significantly reduced. In order to increase the processing cost, the tip needs to have a Vickers hardness of 1.2 times or more that of the material to be processed. Functionally, the harder the tip, the better, but the harder the manufacturing cost of the tip, the higher the hardness ratio is preferably 5.
Accordingly, when the Vickers hardness of the tip of the ultrasonic transducer is H1 and the Vickers hardness of the crankshaft is H2, it is preferable to perform processing with an ultrasonic transducer having a hardness defined by the following formula (B).
1.2 ≦ (H1 / H2) ≦ 5 (B)
The material for the tip is not particularly defined, but alloys such as tool steel, sintered materials such as silicon carbide and tungsten carbide are conceivable.

<Reason for limiting steel components>
In the crankshaft of the present invention, by applying the ultrasonic hitting treatment as described above to the fillet portion, the compressive residual stress in the surface layer of the fillet portion of the crankshaft is 50% to the tensile strength of the steel material constituting the crankshaft. It is characterized by 90%.
The reasons for limiting the components of the steel material constituting the crankshaft of the present invention are shown 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, 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 deteriorate, so the upper limit of the amount added is 2%.
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, if added in excess, the machinability decreases, so the upper limit of the amount added is 0.1%.
N is an element necessary for precipitation strengthening by generating V carbonitride and Nb carbonitride, 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%. Therefore, the lower limit of addition amount is 0.1% and the upper limit is 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 0.05%, the toughness deteriorates, so the lower limit of the amount added is 0.003% and the upper limit is 0.05%.
V is also an effective element because it produces nitrides and carbides as well as Ti, and the strength is increased by precipitation strengthening. However, in order to enjoy the effect, addition of 0.05% or more is necessary. On the other hand, if added excessively, the 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%, sufficient effects cannot be obtained in order to enjoy the effect. On the other hand, if added excessively, toughness deteriorates, so the upper limit of the amount added is 0.1%.
In addition to these elements, Pb, S, Bi and the like may be added as elements for improving machinability, and such cases are also included in the present invention.
In addition to the above elements, unavoidable impurities such as P and S are included, but such cases are also included in the present invention.

<Tensile strength>
The reasons for limiting the tensile strength of the steel material constituting the crankshaft of the present invention are shown below.
In a steel material having a tensile strength of 800 MPa or less, a sufficient fatigue strength improvement effect cannot be obtained at 50%, which is the lower limit of the compressive residual stress of the fillet portion, so the lower limit value was set to 800 MPa.
<Residual stress>
As shown in FIG. 2, when the base material strength is 810 MPa, the effect of improving the fatigue strength is remarkable when the residual stress is −400 MPa or less, and when the base material strength is 1005 MPa as shown in FIG. In the case of 500 MPa or less, the effect of improving fatigue strength is remarkable.
Therefore, in a steel material having a strength of 800 MPa or more, which is the subject of the present invention, a sufficient fatigue strength improvement cannot be observed with a compressive residual stress of 50% or less of the tensile strength, and a compression of 90% or more of the tensile strength. Since it is difficult to apply the residual stress in the present invention, the upper limit is made 90%.
The residual stress is measured using X-rays, and the intensity of diffracted X-rays is measured and obtained from the half width of the peak intensity.

An Ono rotary bending test piece with an annular semicircular groove simulating a crankshaft fillet portion was cut out from the steel components shown in Table 1. The axial diameter D of the test piece was 6 mm, the curvature ρ of the annular semicircular groove was 0.5 mm, and the axial diameter d of the groove portion was 5 mm. Using this test piece, the test was conducted under the conditions shown in Table 2, and the test results shown in Table 2 were obtained. The ultrasonic hitting treatment of the test piece was performed only on the groove.
Moreover, although this test piece is a thing imitating a crankshaft, since the stress conditions are the same throughout the circumference unlike the crankshaft, the ultrasonic impact treatment was applied to the entire groove of the test piece. The shape of the ultrasonic striking pin used at this time was bowl-shaped, and the curvature of the tip was the same as that of the test piece groove.
As a result, the present invention example using the fatigue strength improving method of the present invention showed a significant improvement in fatigue strength compared to the comparative example.
In addition, Table 3 shows the results of the experiment conducted by changing the hardness of the tip of the ultrasonic vibrator.
In the above-described examples of the present invention in which the hardness ratio H1 / H2 between the Vickers hardness H1 of the tip of the ultrasonic transducer and the Vickers hardness H2 of the crankshaft is in the range of 1.2 to 5, a remarkable improvement effect of fatigue strength has been confirmed. It was done.
In addition, the tool tip (SUJ2) was mainly used for the front-end | tip tip in a present Example.
Further, for the investigation on the influence of the hardness of the tip, tempered tool steel and steel type A were used.

It is a figure which shows typically the crankshaft which this invention makes object. It is a figure which shows the relationship between residual stress and fatigue strength in case base material strength is 810 Mpa. It is a figure which shows the relationship between residual stress and fatigue strength in case base material strength is 1005 Mpa. It is a figure explaining the deformation | transformation condition of the hit | damage part according to the curvature radius of a pin front-end | tip. It is a figure which illustrates arrangement | positioning of an ultrasonic transducer | vibrator. It is a figure which illustrates arrangement | positioning of an ultrasonic transducer | vibrator.

Explanation of symbols

1 Crankshaft 2 Arm 3 Pin 4 Journal 5 Fillet

Claims (6)

  The ultrasonic striking process is performed under the conditions of an ultrasonic vibrator having a frequency of 10 to 60 kHz, an ultrasonic output of 500 to 5000 W, and a pressing force of the ultrasonic vibrator to a crankshaft of 10 to 400 N. Crankshaft fatigue characteristics improvement method. 2. The crankshaft fatigue according to claim 1, wherein the fillet portion is hit with a single or a plurality of pins whose curvature radius R of the tip portion of the ultrasonic transducer is defined by the following formula (A). Characteristic improvement method.
(Curvature of fillet portion × 0.5) ≦ R ≦ (curvature of fillet portion × 1.05)
... (A) 2. The method for improving fatigue characteristics of a crankshaft according to claim 1, wherein the fillet portion is hit with a roll having a curvature radius R of the tip portion of the ultrasonic transducer defined by the following expression (A).
(Curvature of fillet portion × 0.5) ≦ R ≦ (curvature of fillet portion × 1.05)
... (A) When performing ultrasonic striking treatment on the fillet portion, an ultrasonic wave having a hardness defined by the following equation (B), where V1 hardness of the tip of the ultrasonic transducer is H1 and Vickers hardness of the crankshaft is H2 The method for improving fatigue characteristics of a crankshaft according to any one of claims 1 to 3, wherein the treatment is performed by a vibrator.
1.2 ≦ (H1 / H2) ≦ 5 (B) % By mass
C: 0.1-0.8%
Si: 0.05 to 2.5%,
Mn: 0.2-2%
Al: 0.005 to 0.1%,
N: 0.001 to 0.02%
A crankshaft made of a steel material having a balance of Fe and inevitable impurities and a tensile strength of 800 MPa or more, wherein the compressive residual stress in the surface layer of the fillet portion of the crankshaft is the tensile strength of the steel material Crankshaft with excellent fatigue resistance, characterized by 50% to 90% The steel material is in mass%, and
Cr: 0.1 to 2%,
Ni: 0.1 to 2%,
Mo: 0.1 to 2%,
Cu: 0.1 to 2%,
Ti: 0.003 to 0.05%,
V: 0.05-0.5%
Nb: 0.01 to 0.1%
The crankshaft excellent in fatigue strength according to claim 5, comprising one or more of the following.

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