JPH0657325A - Method for improving fatigue strength of high strength plate having sheared part - Google Patents

Abstract

PURPOSE:To improve the fatigue strength of a high strength plate having a sheared part by subjecting the sheared burr part of the high strength plate having a sheared part to coining in such a manner that the degree of the hardness to be increased and the amt. to be worked are controlled. CONSTITUTION:At the time of subjecting the plate having 490N/mm<2> or higher tensile strength to shearing, the sheared burr part is subjected to coining. At this time, the degree of the hardness to be increased directly below the coining is controlled to 50 or lower in Vickers hardness (Hv) and the amt. to be worked in controlled to 3 to 30% in a local volume ratio. By the light coining to the sheared part of this high strength material, the fatigue strength of the sheared part can easily be improved.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for improving the fatigue strength of a high-strength plate material having a sheared portion such as an automobile and a construction member.

[0002]

2. Description of the Related Art From the viewpoint of reducing the weight of automobiles, various members have been made to have high strength and thin wall, and high strength plate materials having excellent press workability have been developed. As a result, the fatigue strength of the notch has been greatly reduced, and various improvement techniques are currently being sought.

That is, in the case of a smooth material having no notch, as shown in FIG.
As shown in, the fatigue strength increases as the material strength increases, but for materials with notches due to shearing, the fatigue strength cannot be expected to increase in spite of the increase in material strength.
High strength thinning of the member is a problem because the fatigue strength is substantially reduced.

Punching (piercing), shear cutting, and other shearing processes are frequently used in the field of press working of plate materials, and there is a strong demand for improvement in the fatigue strength of the notches formed by these processes. The current situation.

[0005]

As a conventional technique for improving the fatigue strength of a notched material, a method of subjecting the notched portion to shot peening to release local residual stress and further imparting compressive residual stress, notch processing Although methods such as drilling, lasering, etc. have been proposed, these methods are difficult to use at the press working site in the mass production process.

Further, in the molding of some automobile wheel discs, in the processing of the nut seat, etc., the plate material is pierced and then coined, and in this case, the entire thickness of the plate is coined to make contact with the nut. However, this method cannot be applied to high strength materials. because,
When severe coining is performed on high-strength materials, minute cracks often occur due to insufficient workability of the material, which reduces the fatigue strength of the member. This is because it has the drawback of shrinking.

The present invention has been made in order to solve the above-mentioned problems, and the shearing by performing coining is performed on the shearing flash part by controlling the amount of hardness increase and the amount of machining by coining. An object of the present invention is to provide a method for improving fatigue strength of a high-strength plate having a portion.

[0008]

Means for Solving the Problems The gist of the present invention is that a plate shearing beam portion having a tensile strength of 490 N / mm ² or more has a Vickers hardness (HV) of 50 or less and an increase in hardness immediately below the coining, and This is a method for improving the fatigue strength of a high-strength plate material having a shearing portion subjected to coining with a processing volume of 3 to 30% in a local volume ratio.

[0009]

The inventors of the present invention have examined the method of relatively easily applying the fatigue strength of the high strength notched material having the sheared portion at the press working site, and as a result, unlike the conventional low strength material, The present invention has been completed by finding that in high-strength materials, a slight coining process on a sheared beam portion is effective in improving fatigue strength.

The results of the study by the inventors will be described below. The inventors performed a double-sided plane bending fatigue test on a hot rolled high-strength material having a plate thickness of 2.0 to 2.6 mm using an hourglass-shaped fatigue test piece. The test piece is a smooth material and a punched punch with a diameter of 8 mm in the center of the smooth material. Furthermore, for the punched punches, coining was applied to the burrs so that the amount of processing was 8% of the local volume.

The coining amount is as shown in FIG.
It is shown by the ratio (%) of the volume of the deformed portion by coining to the volume consisting of the coining area and the plate thickness. That is, in FIG. 2, the volume consisting of the coining area and the plate thickness is A ₀ + B ₀ , the volume of the deformed portion by coining is B ₀ , and the machining amount (local volume ratio) is [B ₀ / ( A ₀ + B ₀ )] × 100 (%).

FIG. 1 shows the results of a double swing plane bending fatigue test using the above test piece. As described above, the fatigue strength of the smooth material increases as the material strength increases, but in the notched material as punched, the fatigue strength cannot be expected to increase in spite of the increase of the material strength. However, it can be seen that the fatigue strength is significantly improved by lightly coining the notched material. This improvement is more remarkable as the material strength is higher, and can be utilized as a measure to reduce the fatigue strength due to the high strength thinning. Table 1 shows a quantitative summary of the increase in fatigue strength due to coining.

[0013] As shown in Table 1, the rate of increase in fatigue strength due to coining is respect notched material remains punch punching, 120% in 590N / mm ² class, at 780N / mm ² class 129
%, 169% at 980 N / mm ² grade.

The reason why the fatigue strength of the high-strength material can be improved by lightly coining the flash portion of the sheared portion is not clear yet, but the flashing site which is a fatigue crack initiation site due to stress concentration by the coining is not clear. It is considered that the resistance to fatigue fracture is increased by the fact that the microvoids that are frequently generated are pressure-bonded to the fracture region near the flash, and that the work-hardening of that portion is also performed. The great effect of light coining can be explained by the fact that the higher the strength of the material, the higher the notch sensitivity. Therefore, as described above, excessive coining is not preferable for high strength materials.

[0015]

[Table 1]

Next, the reasons for limitation of the present invention will be described. Regarding the strength of the plate material, as shown in Fig. 1, the decrease in fatigue strength due to the notch of the plate material is not a problem for low strength materials, and the significant improvement in fatigue strength due to light coining is the tensile strength 490N. / mm ² because it is recognized by the above materials, the tensile strength of the applied material of the present invention is 490 N / mm ²
Limited to the above.

Regarding the amount of increase in hardness just below the coining,
Qualitatively, the greater the hardness increase in the material notch due to coining, the greater the fatigue strength of the material.However, excessive coining promotes the generation of microcracks in high-strength materials with relatively low ductility. On the contrary, the fatigue strength decreases. Therefore, the amount of increase in hardness just below the coining is the Vickers hardness.
(HV) limited to 50 or less.

The coining amount is limited to the range of 3 to 30% in terms of the local volume ratio shown in FIG. The reason is that if the machining amount is less than 3%, the fatigue strength improving effect is too small, and if it exceeds 30%, the fatigue strength of the high-strength material may rather decrease. This is because the amount of deformation of the entire notched portion becomes large, which causes problems such as the appearance of the product being damaged.

The present invention is basically limited to the above limiting conditions. Therefore, although the shape of the coining is not particularly specified, it is considered that a simple method such as pushing with a conical punch of the piercing part is performed when the coining is performed in a series of pressing processes at the press working site.

Although the results shown in FIG. 1 are the results of the double swing flat bending fatigue test, the method of the present invention has the same effect as the flat bending also on the axial load. Further, the method of the present invention can be expected for a cut hole portion such as a rivet joint, and can be applied to both a steel plate and an aluminum plate as a material, and a plate material subjected to a surface treatment such as galvanizing.

[0021]

EXAMPLES Examples of the present invention will be described below.
The test materials were three kinds of 2.9 mm thick hot rolled steel sheets with different strength levels, and hourglass-shaped fatigue test pieces were taken from these steel sheets. There are two types of test pieces, a punched piece with a diameter of 8 mm at the center and a coined piece after punching. The fatigue test was a double-sided plane bending test, and the endurance stress at 10 ⁷ cycles was obtained. Table 2 shows the strength level of the test material, the local volume ratio during coining, the increase in hardness immediately below the coining, and the increase rate of each endurance stress and fatigue strength.

As shown in Table 2, it can be seen that those within the limited range of the present invention are greatly improved in durability stress by coining. Comparative example No. 1 is the result for mild steel, and no significant improvement in durability stress due to coining is observed.

[0023]

[Table 2]

In Comparative Example No. 4, since the coining process is excessive, the hardness increase immediately below the coining exceeds HV50.
A decrease in endurance stress is observed. In addition, since No. 6 of the comparative example also has excessive coining, the improvement in durability stress is small,
On the other hand, there is a problem that tool wear becomes severe.

[0025]

EFFECTS OF THE INVENTION The present invention has a tensile strength of 490 N / mm ² or more in a plate shearing beam portion, a hardness increase immediately below the coining is Vickers hardness (HV) of 50 or less, and the processing amount is a local volume ratio. 3
A method of improving fatigue strength of a high-strength plate material having a shear-processed portion to which a coining process of up to 30% is applied. According to the method of the present invention, the fatigue strength of the shear-processed portion of the high-strength plate material can be improved.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between strength and endurance stress.

FIG. 2 is a diagram illustrating a local volume ratio.

Claims (1)

[Claims] 1. The shearing beam portion of a plate material having a tensile strength of 490 N / mm ² or more has a Vickers hardness (HV) of 50 or less in the hardness increase immediately below the coining, and the processing amount is 3 to 30% of the local volume ratio. A method for improving fatigue strength of a high-strength plate material having a sheared portion, characterized by performing coining.