JP2006104552A - Automotive undercarriage parts with excellent fatigue characteristic, and method for improving fatigue characteristic thereof - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide automotive undercarriage parts having excellent fatigue characteristics and also to provide a method for improving fatigue strength thereof. <P>SOLUTION: The automotive undercarriage parts with excellent fatigue characteristics are composed of hot forged non-heat treated steel which has a composition consisting of, by mass, 0.1 to 0.8% C, 0.05 to 2.5% Si, 0.2 to 3% Mn, 0.005 to 0.1% Al, 0.001 to 0.02% N and the balance Fe with inevitable impurities and has ≥600 MPa tensile strength and a ferrite-pearlite structure. The residual compressive stress in the neck part surface layer of the undercarriage parts is 50 to 80% of the tensile strength of the hot forged non-heat treated steel. The method for improving the fatigue strength of the parts is also provided. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

The present invention relates to an automobile undercarriage component having excellent fatigue characteristics using hot forged non-heat treated steel and a method for improving the fatigue characteristics thereof.
More specifically, the present invention relates to an undercarriage component that strengthens the neck of an automobile undercarriage component that becomes the starting point of fatigue failure, has excellent fatigue strength, and is comfortable to ride, and a method for improving the fatigue strength thereof.

Heavy loads are repeatedly applied to the undercarriage parts for automobiles, and the necks of undercarriage parts such as tie rods and lower arms are the most vulnerable to fatigue.
In addition, undercarriage parts for automobiles are required to reduce the rigidity of the parts in order to make riding comfort mild, and the cross-sectional area is intentionally reduced or the shape is curved in an Ω shape or an arc shape. Efforts were made to lower the rigidity, and this part had the problem that it was likely to become the starting point of fatigue failure.
Therefore, conventionally, methods for reinforcing automobile undercarriage parts as described below have been proposed.

<Improvement of fatigue strength by strength improvement>
For example, Patent Document 1 relates to a method for increasing the overall strength by adjusting the structure using a specific component and increasing the fatigue strength by increasing the strength, and by reducing the pearlite particle size, the pearlite colony particle size, and the lamellar interval. An invention of a method for improving fatigue life has been proposed.
However, the invention described in Patent Document 1 has a problem in that machinability is deteriorated in order to increase the strength of the entire base material.

<Improvement of fatigue strength by compressive residual stress>
For example, Patent Document 2 discloses an invention relating to a method for improving fatigue strength by introducing compressive residual stress using so-called hard shot, which is a strong shot peening process, and grinding in combination.
However, since the surface of the present invention may be roughened only by hard shots and may reduce the fatigue life, it is essential to use it together with grinding. It was not applicable.
JP-A-9-143610 JP-A-2-301313

The present invention solves the problems of the prior art as described above, and provides an automobile undercarriage part having excellent fatigue characteristics and a method for improving the fatigue strength thereof. Specifically, the following three problems are to be solved. And
1) A large compressive residual stress can be applied.
2) Less rough skin.
3) A strengthening method that does not reduce the machinability.

The present invention has been made as a result of intensive studies to solve the above-mentioned problems, by subjecting the neck of an automobile undercarriage part to ultrasonic striking treatment and introducing compressive residual stress into the neck surface layer, It is an object of the present invention to provide an automobile undercarriage component having excellent fatigue characteristics and a method for improving the fatigue strength thereof. The gist of the present invention is the following contents as described in the claims.
(1) By mass%, C: 0.1-0.8%, Si: 0.05-2.5%, Mn: 0.2-3%, Al: 0.005-0.1%, N : 0.001 to 0.02% content, balance of Fe and unavoidable impurities, tensile strength of 600 MPa or more, and suspension for automobiles using heat-forged non-tempered steel having ferrite / pearlite structure An automobile undercarriage having excellent fatigue characteristics, characterized in that the compressive residual stress in the neck surface layer of the undercarriage part is 50% to 80% of the tensile strength of the heat forged non-heat treated steel parts.
(2) The automobile undercarriage part having excellent fatigue characteristics according to (1), wherein an average surface roughness of the neck part of the undercarriage part is 0.5 μm or less.
(3) A method for improving the fatigue characteristics of an undercarriage part for an automobile, with an ultrasonic vibration terminal that vibrates at a frequency of 10 to 60 kHz and an amplitude of 0.3 to 50 μm, while striking the surface of the neck part of the undercarriage part A method for improving fatigue characteristics of undercarriage parts, characterized by scanning.

By applying ultrasonic impact treatment to the neck part of automobile undercarriage parts such as tie rods and lower arms according to the present invention, and introducing compressive residual stress into the neck surface layer, it has excellent fatigue characteristics, less skin roughness, and machinability It is possible to provide an undercarriage part for automobiles and a method for improving fatigue strength that does not deteriorate, and the reliability of the part increases without being broken from the neck.
In addition, the weight of the parts corresponding to the strengthened parts can be reduced, which contributes to improved fuel efficiency and cost reduction. Furthermore, since weight reduction and thickness reduction bring about a reduction in rigidity, it is possible to improve riding comfort, and there are significant industrially useful effects.

The best mode for carrying out the present invention will be described in detail with reference to FIGS.
FIG. 1 and FIG. 3 are diagrams illustrating an automobile undercarriage component that is an object of the present invention, FIG. 1 shows a lower arm, and FIG. 3 shows a tie rod.
1 and 3, 1 indicates a neck.
The neck 1 of the undercarriage parts for automobiles such as the lower arm and tie rod shown in FIGS. 1 and 3 has a reduced cross-sectional area and a shape in which stress concentration is likely to occur. ing.
That is, the fatigue strength of the neck 1 determines the fatigue strength of the entire underbody part.
In order to improve the fatigue strength of the neck 1, two measures are conceivable: increasing the strength of the neck 1 or introducing a compressive residual stress.
The present inventors have found that by hitting the surface of the neck 1 with the ultrasonic vibration terminal 2 as shown in FIGS. 2 and 4, it is possible to satisfy both of the above-mentioned two countermeasures. .

The undercarriage parts for automobiles of the present invention are in mass%, C: 0.1 to 0.8%, Si: 0.05 to 2.5%, Mn: 0.2 to 3%, Al: 0.005. -0.1%, N: 0.001-0.02%, the balance is Fe and inevitable impurities, the tensile strength is 600MPa or more, and a heat-forged non-tempered steel having a ferrite pearlite structure A compressive residual stress in the neck surface layer of the undercarriage part is 50% to 80% of the tensile strength of the heat forged non-heat treated steel.
Suspension parts Applying ultrasonic striking treatment to the surface of the neck 1 gives a large compressive residual stress, and the work is hardened by plastic working the surface like shot peening. The fatigue strength of the neck can be greatly improved.
In addition, in the undercarriage parts for automobiles of the present invention, the steel material structure and the treatment process are made of hot-forged non-tempered steel with a ferrite-pearlite structure from the viewpoint of reducing manufacturing costs.

The reasons for limiting the present invention will be described below.
<Reason for limiting steel components>
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 required for precipitation strengthening by generating V carbonitride or Nb carbonitride, but if it is less than 0.001%, a sufficient effect cannot be obtained. On the other hand, if added excessively, the toughness deteriorates, so the upper limit of the amount added is 0.02%.
In addition to these elements, Cr, Ni, Mo, and Cu can be expected as solid solution strengthening, Ti, V, and Nb can be expected as precipitation strengthening, and Pb, S as elements that improve machinability. , Bi, etc. are conceivable, but not particularly limited.

<Reason for limitation of strength and compressive residual stress>
In steel materials of 600 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 present invention, so the lower limit value was set to 600 MPa.
In a steel material having a strength of 600 MPa or more, a sufficient fatigue strength improvement cannot be observed with a compressive residual stress of 50% or less of the tensile strength, and a compressive residual stress of 80% or more of the tensile strength is imparted. Since this is difficult in the present invention, the upper limit is made 80%.
<Reason for limiting surface roughness>
In the present invention, the surface roughness of the undercar parts for automobiles is not limited, but the average surface roughness of the rim portion of the neck is preferably 0.5 μm or less.
By setting the average surface roughness of the rim portion of the neck portion to 0.5 μm or less, there is little skin roughness and the fatigue strength can be remarkably improved.

2 and 4 are views illustrating an embodiment of the method for improving the fatigue strength of an undercarriage part for automobiles according to the present invention. FIG. 2 is a view taken along an arrow A in FIG. FIG.
2 and 4, reference numeral 1 denotes a neck portion, and 2 denotes an ultrasonic vibration terminal.
The undercarriage parts for automobiles are subject to repeated loads, and particularly the neck 1 is most susceptible to fatigue cracks.
Therefore, the method for improving the fatigue characteristics of the undercarriage part for automobiles according to the present invention hits the surface of the neck 1 of the undercarriage part for automobiles with an ultrasonic vibration terminal that vibrates at a frequency of 10 to 60 kHz and an amplitude of 0.3 to 50 μm. Scanning while scanning.

The reason why the ultrasonic hitting process is limited to the neck 1 is that the neck 1 is the main problem of fatigue failure in the suspension parts.
In addition, when the position where the fatigue crack enters is known in advance, the portion may be processed intensively.
In the present invention, the shape of the ultrasonic vibration terminal is not limited. However, in order to make the surface roughness of the hitting treatment portion 0.5 μm or less as described above, the radius of the tip portion of the ultrasonic vibration terminal 2 is 5 mm. It is preferable that the ultrasonic vibration terminal 2 is reciprocated in a direction perpendicular to the direction in which the fatigue crack enters to perform scanning.
This is because the compressive residual stress introduced by the processing by the ultrasonic vibration terminal 2 is anisotropic and a stronger compressive residual stress is applied in the scanning direction.

The tip shape of the ultrasonic vibration terminal 2 may be a hemispherical shape, a bowl shape, a bowl shape, or the like, but is not particularly limited. However, in the case of a hemispherical or bowl-shaped tip, the projections and projections are brought together, so the processing may become unstable. The best is a bowl shape that combines convex and concave, but there is a possibility that the manufacturing cost of the ultrasonic vibration terminal 2 is increased.
The reason why the frequency of the ultrasonic vibration terminal 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 pin that vibrates ultrasonically is limited to 0.3 μm or more because sufficient compressive residual stress cannot be applied to the steel material with an amplitude below this. Although the residual stress increases as the amplitude increases, the plastic deformation becomes too large at 50 μm or more, 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.

From the steels having the components shown in Table 1, Ono-type rotating bending test pieces (JIS Z-2274, No. 1 test piece, symbol 1-6) having a parallel part diameter d = 6 mm as shown in FIG. 5 were cut out.
The test piece was subjected to the ultrasonic treatment of the present invention, and a comparative material subjected to no treatment or treatment outside the range was prepared, and an Ono-type rotary bending fatigue test was performed to determine the fatigue strength. The results are shown in Table 2. In addition, since this test piece is a simple bending fatigue test, a fatigue crack is generated in the vicinity of ½ L of the sample and breaks on a plane perpendicular to the axial direction. Therefore, the ultrasonic treatment was performed by scanning the entire parallel portion in the axial direction as shown in FIG.
The residual stress measurement values in Table 2 are obtained by separately preparing a test piece that has not been subjected to a fatigue test and measuring the axial residual stress of the 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.
In the comparative example in which the ultrasonic hitting treatment was not performed and the frequency / amplitude was outside the range of the present invention, only a fatigue strength less than 1/2 of the tensile strength was obtained.
On the other hand, it was found that in the invention example in which an appropriate ultrasonic striking treatment was performed, the compressive residual stress of 50 to 80% of the tensile strength was introduced and the fatigue strength was improved.
In addition, when the average surface roughness of the hit | damage process part was measured, although it was 0.5 micrometer or less in the invention example, it exceeded 0.5 micrometer in the comparative example.
From the above, according to the present invention, a significant improvement in fatigue strength was recognized as compared with the comparative example, and it was confirmed that it was effective.

It is a figure which illustrates the lower arm which is the suspension part for motor vehicles which is the object of this invention. It is a figure which illustrates embodiment of the fatigue strength improvement method of the lower arm which is a suspension part for motor vehicles of this invention. It is a figure which illustrates the tie rod which is the suspension part for motor vehicles which is the object of this invention. It is a figure which illustrates embodiment of the fatigue strength improvement method of the tie rod which is an underbody part for motor vehicles of this invention. It is a figure which shows the test piece used for the Example of this invention.

Explanation of symbols

1 Neck 2 Ultrasonic vibration terminal

Claims (3)

% By mass
C: 0.1-0.8%
Si: 0.05 to 2.5%,
Mn: 0.2-3%,
Al: 0.005 to 0.1%,
N: 0.001 to 0.02%
A suspension part for automobiles using a heat-forged non-heat treated steel having a ferrite and pearlite structure, the balance being Fe and inevitable impurities, and having a tensile strength of 600 MPa or more, The undercarriage part for automobiles having excellent fatigue characteristics, characterized in that the compressive residual stress in the surface layer of the neck is 50% to 80% of the tensile strength of the heat forged non-heat treated steel.   2. The automobile undercarriage part having excellent fatigue characteristics according to claim 1, wherein an average surface roughness of a neck part of the undercarriage part is 0.5 μm or less. A method for improving fatigue characteristics of undercarriage parts for automobiles, wherein an ultrasonic vibration terminal that vibrates at a frequency of 10 to 60 kHz and an amplitude of 0.3 to 50 μm is scanned while striking the surface of the neck part of the undercarriage part. A method for improving fatigue characteristics of undercarriage parts.

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