JPH07144232A - Bending method for obtaining steel pipe with low residual stress - Google Patents

Abstract

(57) [Summary] [Purpose] For example, in various mechanical parts such as link parts and support parts, which are used while bending the steel pipe, residual stress is generated in parts where tensile force or bending force acts on the parts. Bending method for obtaining less steel pipe. A method for bending a steel pipe having a small residual stress, which comprises bending the steel pipe smaller than a predetermined bending radius by a maximum of 3% and then bending the steel pipe reversely to the predetermined bending radius. [Effects] According to the present invention, residual stress can be easily reduced with simple equipment, which is economical, as compared with conventional methods such as strain relief annealing and shot peening. Further, if the bending machine is modified so that the same machine can perform bending and reverse bending, further effects can be expected.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to mechanical parts such as various link parts and supporting parts that are used while bending a steel pipe and in which a tensile force or a bending force acts on the parts. The present invention relates to a bending method for obtaining a steel pipe with low stress.

[0002]

2. Description of the Related Art Thorough weight reduction and functional enhancement of parts for automobiles and industrial machines have been studied, and a steel pipe of a steel bar and a steel pipe having a high strength have been made thin.

For example, automobile link parts are made into steel pipes,
Further thinning is being planned. However, it was clarified that when there is a bent part in these parts, the residual stress in the tensile direction generated by bending reduces the fatigue strength. For the bending method, see Iron and Steel Handbook III- (2),
Although there is a method shown in P179 to P180, in any case, the residual stress in the tensile direction and the convex portion in the concave portion of the steel pipe bent at the bending radius R (radius at the pipe center, the same applies hereinafter) shown in FIG. Residual stress in the compressive direction is generated. As methods for reducing or removing this residual stress, strain relief annealing, shot peening and the like are known. However, these methods have a drawback that special equipment is required and the manufacturing cost becomes high.

[0004]

DISCLOSURE OF INVENTION Problems to be Solved by the Invention
Shot peening and the like require special equipment and have a drawback of high manufacturing cost. The present invention provides a method to remedy such shortcomings of conventional methods.

[0005]

The subject matter of the present invention is as follows.

(1) A method of bending a steel pipe having a small residual stress, which comprises bending the steel pipe to a smaller bend radius than a predetermined bend radius and then bending the steel pipe back to the predetermined bend radius.

(2) The steel pipe has a bend radius of 0.2 or more.
A method for bending a steel pipe having a small residual stress, which comprises bending a small amount by 3% and then performing a reverse bending to a predetermined bending radius.

The present invention will be described in detail below.

In the method of the present invention, first, as shown in FIG. 1, the steel pipe 1 is bent to have a bending radius R1 slightly smaller than a predetermined bending radius R. Here, R1 is a bending radius after springback, and naturally, the radius to be actually bent needs to be further reduced in consideration of the springback after bending. This amount depends on the bending method and is empirically determined.
When the bending shown in FIG. 1 is performed, a tensile residual stress is generated in the concave portion 2 and a compressive residual stress is generated in the convex portion 3. The residual tensile stress generated in the recess 2 becomes more than half of the yield stress of the steel pipe, and when it is used as it is, it becomes the same as the conventional method, and the fatigue strength is significantly lowered. However, in the present invention, further, as shown in FIG. 2, the reverse bending is performed to the bending radius R2 in which the bending radius after the springback is equal to the predetermined bending radius R. As shown in FIG. 3, the method of reverse bending is to place the steel pipe 1 bent in FIG. 1 on a support die 5 having a predetermined distance L, and perform reverse bending with a punch 4. This method is one of the methods for straightening steel pipes and has a proven track record as press straightening. The amount of reverse bending is empirically determined in consideration of the springback amount. By setting an appropriate amount of reverse bending with such simple reverse bending, the tensile residual stress in the recess can be halved.
The direction can also be reversed. Therefore, no special residual stress reduction treatment is required, and the fatigue strength is hardly affected even if it is used as it is. As for the method of reverse bending, reverse bending can be performed using the same equipment depending on the bending method. It is more effective to make the bending radius R2 after the reverse bending smaller than the initial bending radius R1 by 0.2 to 3.0% in order to substantially reduce the residual stress by half.

[0010]

Example No. 1 shown in Table 1 Using the steel tube of No. 1, the radius after bending was set to 100 mm by press bending, and the residual stress was measured. To measure residual stress, attach a strain gauge,
It was calculated from the difference in strain before and after cutting. As a result, as shown in FIG. 4, the residual stress in the recess was a tensile stress of 400 N / mm ² . Generally, the residual stress after bending often reaches more than half of the yield stress as in this example. afterwards,
According to the method of the present invention, the amount of reverse bending (R2-R1) is calculated by the method of FIG.
/ R2 × 100 (%) was changed, and the residual stress was similarly measured. The results are shown in FIG. In this result, the residual stress value is halved when the reverse bending amount is 0.2 to 0.3%, and the residual stress is compressed in the opposite direction when the reverse bending amount is 0.4% or more. Of these, the amount of reverse bending is 1% and the residual stress is 11
As a result of carrying out a fatigue test of compression by pulling the material compressed to 0 N / mm ^{2 in} the axial direction of the tube, the fatigue life of the number of repetitions of 5 × 10 ⁵ was three times or more that in the case without reverse bending.
This result showed no difference from the case of performing conventional strain relief annealing (600 ° C.). The fatigue test was carried out by welding the spectacles to both ends of the bent steel pipe, fixing one spectacle to the fixed part with a pin, and connecting the other spectacle to the moving part with a pin, and moving the moving part in the axial direction of the pipe. It was carried out by moving with a predetermined tension and compression load. Table 1 shows examples of other steel pipes having different bending radii. It can be seen that the amount of reverse bending that halves the residual stress value should be 0.2 to 3.0%.

[0011]

[Table 1]

[0012]

According to the present invention, the conventional strain relief annealing,
Compared to shot peening, etc., residual stress can be easily reduced with simple equipment, which is economical. Further, if the bending machine is modified so that the same machine can perform bending and reverse bending, further effects can be expected.

[Brief description of drawings]

FIG. 1 is a view showing a tubular shape after bending according to the present invention.

FIG. 2 is a view showing a tubular shape after bending according to the present invention.

FIG. 3 is a diagram showing an example of a method for converting the bending pipe of FIG. 1 into the bending pipe of FIG.

FIG. 4 is a diagram showing a residual stress reducing effect of the present invention.

[Explanation of symbols]

 1 ... Steel pipe 2 ... Recessed portion 3 ... Convex portion 4 ... Punch 5 ... Support die

Claims (2)

[Claims] 1. A method for bending a steel pipe having a small residual stress, which comprises bending the steel pipe to a smaller bend radius than a predetermined bend radius and then bending the steel pipe reversely to a predetermined bend radius. 2. A steel pipe having a bend radius of 0.2-3.
% A method for bending a steel pipe having a small residual stress, which comprises bending a small amount and then performing a reverse bending to a predetermined bending radius.