JPH08238526A - Production of small diameter electric resistance welded steel tube having excellent fatigue characteristic on bending part - Google Patents

Abstract

PURPOSE: To provide a small electric resistance welded steel tube having the excellent fatigue characteristics on the bending part by bending it to a prescribed bending radius of curvature while loading a stress less than the yield point on the cross-section face of the tube in the tube axial direction without quenching, tempering or annealing of straightening in the process. CONSTITUTION: At first, as a 1st stage, both ends of a tube are gripped by a some method, stretched in the axial direction of the tube and a tension is loaded. The tension is loaded so that the stress less than the yield point is applied on the cross-section of the tube in the axial direction of the tube. This is a reason because when the stress more than the yield point is loaded on the tube cross-section face, the tube is deformed and, in some cases, the outer diameter is reduced by drawing. Next as a 2nd stage, in the stage of loading the tension, it is bend while holding it with a press jig tool set in smaller than a prescribed bending radius. As a 3rd stage, the press jig tool is detached while relieving the load. In that time, the bending radius is made larger caused on the spring back, but because the tension is loaded at it is, so it is made further larger than the prescribed bending. As a 4th stage, the tension is removed.

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 this reason, the processing itself is gradually becoming stricter because the whole is designed to be compact. However, because of this, in a pipe or the like that is bent, the fatigue characteristics deteriorate as the working becomes 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.
This is a method for increasing the strength of the hot-rolled sheet which is a raw material by the method described in JP-A-4-289122.
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, it is necessary to carry out quenching and tempering by continuous annealing after the manufacturing process as in the patent, that is, hot rolling, cold rolling,
The process is long, quenching and tempering equipment is required, the cost is high, and it is not industrially viable as a method for manufacturing ERW 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.

In a small diameter electric resistance welded steel pipe having a bent portion in which a part or the whole of a straight pipe is processed into an arc shape, it is bent to a predetermined bending radius while applying a stress below the yield point to the pipe cross section in the pipe axial direction. This is a method for producing a small diameter electric resistance welded steel pipe having excellent fatigue properties at the bent portion.

The present invention will be described in detail below.

Conventionally, as described above, as a method of preventing the fatigue characteristics of the pipe from being deteriorated even if the bending process becomes severe, a method of increasing the strength of the hot-rolled sheet as a raw material and a strain relief after the bending process A method of performing annealing has been used. In any case, quenching or quenching and tempering in the middle process,
It is necessary to perform stress relief annealing, which is inevitably high cost 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 having excellent fatigue characteristics at the bent portion by defining the bending method without performing quenching and tempering treatment or strain relief annealing in the intermediate step. To do.

FIG. 1 shows a manufacturing process according to the present invention.

There are various methods for bending the tube, and the pressing method shown in FIG. 2 and the winding method shown in FIG. 3 are considered as typical ones. The pressing method will be described in the manufacturing process according to the present invention in FIG. . First, as the first step, both ends of the tube are grasped by some method, and the tube is pulled in the axial direction (longitudinal direction) to apply tension. The tension is applied so that stress below the yield point is applied to the cross section of the pipe in the axial direction of the pipe.
This is because when a stress exceeding the yield point is applied to the cross section of the pipe, the pipe is deformed and, in some cases, the outer diameter is reduced by being narrowed. Next, as a second step, bending is performed while holding tension with a pressing jig set smaller than a predetermined bending radius. The bend radius is preferably 5-10% smaller than the given bend radius, but the tube size,
Since it depends on the material, an appropriate bending radius is determined. As the third step, the press jig is removed while unloading. At that time, the bending radius increases due to spring back,
Since the tension is still applied, the bending radius becomes larger than the predetermined bending radius. The tension is removed as the fourth step. This results in a predetermined bending radius.

Next, the reason for such bending will be described with reference to FIG.

FIG. 4 shows the stress distribution in the pipe from the outside of the bend to the inside of the bend when the pipe is bent. In the first step, tensile stress is applied uniformly in the thickness direction to apply tension. By bending in the second step, a tensile stress is generated on the outside of the bending and a compressive stress is generated on the inside of the bending. In the third step, by spring packing, the residual stress distribution becomes as shown in the figure, and the compressive stress on the outside of the bending and the tensile residual stress on the inside of the bending. In the fourth step, the tension is unloaded, but the spring pack balances the tensile residual stress and the compressive residual stress. As a result, both the compressive residual stress on the outside of the bend and the tensile residual stress on the inside of the bend are reduced. Although the residual stress distribution in the case of bending without applying tension is shown in the figure, the residual stress of the method of the present invention is significantly reduced in comparison with this. As a result, it is possible to prevent the fatigue strength from decreasing and further increase it.

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

[0017]

Example: Size φ34.0 × t2.1 mm, tensile strength 80 kgf / mm ² , yield point strength 55 kgf / m
Table 1 shows an example in which an electric resistance welded steel pipe of m ² was produced by the conventional method as a comparative example with the method of the present invention. In the embodiment, as the method of applying tension in the first step, as shown in FIG. 5, after a tube was gripped by a chuck, tension was applied in a fixed direction by a universal joint regardless of the degree of bending. The bending method in the second stage was the pressing method shown in FIG. or,
The unloading in the 3rd and 4th steps was simply done.

As shown in Table 1, according to the present invention, the residual stress on the outside and inside of the bend can be significantly reduced,
It is possible to manufacture a small-diameter electric resistance welded steel pipe having excellent fatigue properties in the bent portion.

[0019]

[Table 1]

[0020]

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 the quenching and tempering treatment or the strain relief annealing in the intermediate step, so that there is no problem 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 bending method with a press.

FIG. 3 is a diagram showing a bending method in a winding method.

FIG. 4 is a diagram showing a residual stress distribution.

FIG. 5 is a diagram showing a method of applying tension in the tube axis direction.

[Explanation of symbols]

 1 ... Tube 2 ... Chuck 3 ... Universal joint

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, while applying a stress equal to or lower than the yield point to the pipe cross section in the pipe axial direction, to a predetermined bending radius. A method for producing a small diameter electric resistance welded steel pipe having excellent fatigue properties in a bent portion, which is characterized by bending.