JPH08174080A - Manufacture of small diameter resistance welded tube with superior fatigue characteristics in bent part - Google Patents

Abstract

PURPOSE: To manufacture a small diameter resistance welded tube which is used for a structural part in an automobile and machine and which is superior in fatigue characteristics in a bent part. CONSTITUTION: In a small diameter resistance welded tube with a bent part worked in a circular-arc shape from a part or the entirety of a straight tube, after being bent smaller by 5-10% than a prescribed bending radius, the tube is bent in the reverse direction larger by 5-10% than a specific bending radius. Consequently, without quenching or tempering in the intermediary process or without stress relieving annealing, a small diameter resistance welded tube with superior fatigue characteristics in the bent part is manufactured by regulating a bending method, without causing the problems in industrial productivity and economy.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a small diameter electric resistance welded steel pipe which is excellent in fatigue characteristics of a bent portion used in automobiles, machine structural parts and the like.

[0002]

2. Description of the Related Art Thorough weight reduction of structural members such as automobiles and mechanical structural parts is under consideration for improving fuel efficiency and environmental measures. For that reason, the whole process is designed to be compact and the processing itself is becoming more and more difficult. But,
For this reason, in a pipe or the like that is bent, the fatigue characteristics deteriorate as the working becomes more severe.

Conventionally, as a method for preventing the fatigue characteristics of a pipe from being deteriorated even when the bending process becomes severe, there is a method disclosed in Japanese Patent Laid-Open No.
A method of increasing the strength of a hot-rolled sheet which is a raw material by a method as described in Japanese Patent No. 289122 and the like, and a method of performing stress relief annealing after bending have been performed.

[0004]

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention
There are two types as described in the background section.
One is a method described in Japanese Patent Application Laid-Open No. 4-289122 or the like, which is a method of increasing the strength of a hot rolled sheet as a raw material. In this method, the working such as bending becomes strict, and as a result, the residual stress becomes large and the fatigue property deteriorates, but by strengthening the material strength of it, it is attempted to prevent the deterioration of the fatigue property and measure the increase. To do. However, the manufacturing process as in the present invention, that is, hot rolling,
After cold rolling, it is necessary to quench and temper by continuous annealing, the process is long, quenching and tempering equipment is required, the cost is high, and it is not industrially established as a method for producing electric resistance welded steel pipe. . There is also a method of increasing the strength by adding an additional element without performing such quenching and tempering, but it goes without saying that the cost increases by the amount of the additional element.

Next, there is a method of performing stress relief annealing after bending. In this method, the fatigue strength is reduced due to the residual stress caused by the bending work, and the fatigue is reduced by strain relief annealing to remove the residual stress. It is intended to prevent the strength from decreasing. However, the number of steps for strain relief annealing increases, and the cost rises.

The present invention solves these problems when the bending work becomes severe, the residual stress increases and the fatigue strength decreases, and a small diameter electric resistance welded steel pipe having excellent bending fatigue properties is manufactured. The purpose is to do.

[0007]

The subject matter of the present invention is as follows.

According to the present invention, in a small-diameter electric resistance welded steel pipe having a bent portion obtained by processing a part or the whole of a straight pipe into an arc shape, after bending it by 5 to 10% smaller than the predetermined bending radius, Also, it is a method for producing a small diameter electric resistance welded steel pipe excellent in fatigue property of a bent portion, which is characterized by performing reverse bending largely by 5 to 10%.

The present invention will be described in detail below. Conventionally, as mentioned above, even if the bending process becomes severe, as a method to prevent the fatigue property of the pipe from being lowered, a method of increasing the strength of the hot-rolled sheet that is the material, and a strain relief annealing after the bending process Has been done. In any case, it is necessary to perform quenching, quenching and tempering, or strain relief annealing in the middle of the process, which is inevitably costly in terms of capital investment and productivity.

Therefore, in the present invention, it is intended to manufacture a small-diameter electric resistance welded steel pipe excellent in fatigue characteristics of a bent portion by defining a bending method without performing quenching / tempering treatment or strain relief annealing in an intermediate step. To do.

FIG. 1 shows a manufacturing process according to the present invention. There are various methods of bending the pipe, but in the first step, whichever method is used, the bending radius is 5 to 10% smaller than the final bending radius required when bending from the straight pipe. In the second step, by removing the bending jig, the bent pipe springs back and returns to its original shape to some extent. However, at this point, the effect of bending by 5 to 10% smaller than the springback amount is greater, and is still smaller than the required final bending radius. Next, reverse bending is performed so as to be 5 to 10% larger than the final bending radius required in the third step. In the final fourth step, the tube that has been bent backward by removing the bending jig and has a large bending radius is spring-backed to a predetermined final bending radius.

Next, the reason for such bending will be described with reference to FIG. FIG. 2 shows the stress distribution in the pipe from the outside to the inside when the pipe is bent. By bending in the first step, tensile stress is generated outside the bend and compressive stress is generated inside the bend. In the second step, spring-back causes a residual stress distribution as shown in FIG. 2, with compressive stress on the outside of the bending and tensile stress on the inside of the bending. If left as it is, the fatigue strength will decrease due to these residual stresses. In the third step, since reverse bending is performed, compressive stress is generated outside the bend and tensile stress is generated inside the bend. In the fourth step, due to springback, the stress in the third step becomes tensile on the outside of the bending and becomes compressive residual stress on the inside of the bending.
However, although it will be combined with the residual stress of the second step,
Since they are opposite to each other, the residual stress inside and outside the bend is reduced. Therefore, it is possible to prevent the fatigue strength from lowering and further increase it.

Here, in the first bending, that is, in the first step, the bending radius is 5 to 10% smaller than the final bending radius required when bending from a straight pipe, but this is 5
If it is less than%, the reverse bending must be small and the effect is not good,
On the other hand, if it exceeds 10%, the amount of reverse bending becomes large this time, and the processing becomes difficult, and further processing is not necessary to obtain the effect.

In the reverse bending, that is, the reverse bending in the third step, the reverse bending is performed so as to be 5 to 10% larger than the required final bending radius. However, if the bending amount in the first step is less than 5%, the final bending radius will not reach the final bending radius and the effect will not be obtained, and it is not necessary to increase the bending amount to more than 10%. Is.

Although the present invention has been described above, the reason why a small-diameter electric resistance welded steel pipe is used in the claims is that a component specified by such bending work has a so-called small diameter with an outer diameter of 114.3 mm or less, and Since there are many electric resistance welded steel pipes, they are limited, but it goes without saying that other pipes that can be bent can be applied.

[0016]

EXAMPLES Table 1 shows the results of pipe-making of an electric resistance welded steel pipe having a size of φ34.0 × t2.1 mm by the conventional method as a comparative example with the method of the present invention.

As shown in Table 1, according to the present invention, it is possible to significantly reduce the residual stress on the outside and inside of the bend, and to manufacture a small diameter electric resistance welded steel pipe having excellent fatigue characteristics at the bend. .

[0018]

[Table 1]

[0019]

As described above, as described above, as a method of preventing the fatigue property of the pipe from being deteriorated even when the bending work becomes severe, a method of increasing the strength of the hot-rolled sheet which is a raw material and a method after the bending work A method of performing stress relief annealing has been used. In any case, it is necessary to perform quenching, quenching and tempering, or strain relief annealing in the middle of the process, which is inevitably costly in terms of capital investment and productivity.

Therefore, according to the present invention, the bending method is defined without quenching and tempering treatment or strain relief annealing in the middle step, so that the bending portion can be solved without such problems in industrial productivity and economical efficiency. Since it becomes possible to manufacture a small diameter electric resistance welded steel pipe having excellent fatigue characteristics, the industrial contribution is extremely large.

[Brief description of drawings]

FIG. 1 is a diagram showing a manufacturing process of a method of the present invention.

FIG. 2 is a diagram showing a stress distribution in a pipe in each process.

FIG. 3 is a diagram showing manufacturing steps of a conventional method.

Claims (1)

[Claims] 1. A small-diameter electric resistance welded steel pipe having a bent portion obtained by processing a part or the whole of a straight pipe into an arc shape. A method for producing a small diameter electric resistance welded steel pipe having excellent fatigue characteristics in a bent portion, characterized by performing reverse bending by 10%