JPH05149478A - Lightweight seam welded steel pipe excellent in three point bending strength - Google Patents

Abstract

PURPOSE:To provide a seam welded steel pipe for mechanical structure, which is excellent in three point bending strength, and is used as a reinforcing member and the like for automobile doors. CONSTITUTION:In a seam welded steel pipe in a round shape in cross section to be used as a member subjected to three point bending moment, a lightweight seam welded steel pipe excellent in three point bending characteristics is provided wherein the thickness (t1) of the pipe in a range where the maximum bending moment is incurred, is thickened over an appropriate length (S). As raw material, a seam welded steel pipe in a round shape in cross section is used, which can be manufactured with ease and also is economical. Since the steel pipe excellent in three point bending strength can be obtained by thickening a part of the steel pipe only by an appropriate length, the aforesaid steel pipe is advantageous in weight saving and is also economical.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electric resistance welded steel pipe having excellent three-point bending strength which is used, for example, as a reinforcing material for automobile doors.

[0002]

2. Description of the Related Art Steel pipes used as conventional members that are required to have bending strength are mostly round or square and partly
Various shapes were manufactured and used by drawing or the like. For example, a method for manufacturing a high-strength steel pipe for machine structure has the same circular cross-section in the axial direction of the pipe, as shown in Japanese Patent Laid-Open No. 122219/1993. A method for producing a square tube from a round tube by drawing is disclosed in JP-A-63-1577.
There is a manufacturing method of a rectangular tube disclosed in Japanese Patent Laid-Open No. This square tube manufacturing method is a method of manufacturing a square tube by pulling out a round tube using a die and a plug. According to this method, the entire length of the round tube is finished into a square tube uniformly.

[0003]

The conventional steel pipe described above has a uniform shape in the axial direction of the pipe due to the manufacturing method. Therefore, in order to increase the bending strength, it is necessary to increase the material strength of the entire steel pipe or change the shape of the steel pipe to increase the section modulus. For example, in order to obtain a high-strength steel pipe of 100 kgf / mm ² or more, a seamless steel pipe is easy but the manufacturing cost is high, and it is difficult to form and weld the electric resistance welded steel pipe. Further, the square tube has an advantage of having a larger section modulus than the round tube, but it is more difficult to manufacture the high strength square tube than the round tube.

As shown in FIG. 1, the bending moment at the time of three-point bending is not uniform in the pipe axis direction, and the maximum bending moment is generated at the load point where the load between the fulcrums acts. With a uniform shape and material in the pipe axis direction as in the past, the bending strength is
It is determined by the strength of this maximum bending moment portion, and this portion buckles like the buckling portion 2 of the steel pipe 1 in FIG. Therefore, conventionally, in order to increase the bending strength of the steel pipe, it is necessary to increase the strength of the entire steel pipe, which is difficult to manufacture and the manufacturing cost increases. On the contrary, the strength of the portion other than the maximum bending moment portion of the steel pipe is excessive, which is disadvantageous in terms of weight. An object of the present invention is to provide a light-weight electric resistance welded steel pipe having a three-point bending strength superior to the conventional one.

[0005]

SUMMARY OF THE INVENTION The gist of the present invention is that in a steel pipe having a round cross section which is used as a member to which a three-point bending moment is applied, the portion where the maximum bending moment occurs is locally formed into a large section modulus. It is an electric resistance welded steel pipe having excellent three-point bending strength, which is characterized by being formed.

[0006]

The present invention uses an electric resistance welded steel pipe, which is a round steel pipe that can be easily manufactured, as a material, and makes the wall thickness of the portion where the maximum bending moment of three-point bending occurs larger than that of the other portions.
Molding is performed in a cold state to locally increase the section modulus.

There are various processes for changing the wall thickness in the axial direction of the pipe, but the cold drawing method is generally used. That is, as shown in FIG. 3, when the core tube 3 is cored to reduce the outer diameter and the wall thickness by drawing between the die 6 and the plug 7, the wall thickness of the finish tube 4 becomes t ₂ . Where the plug 7
Are fixed by a supporting mandrel 8 so as not to move in the tube axis direction. Next, as shown in FIG. 4 during pulling out, when the plug 7 is pulled out to the raw pipe 3 side by the support mandrel 8, only the outer diameter of the raw pipe 3 is reduced and the wall thickness is not reduced.
A so-called sinking, the wall thickness t ₁ of the finishing tube 5 becomes substantially close to base pipe wall thickness t _0, steel pipe wall thickness of different t ₁ and t ₂ is the is the outer diameter is the same tube axis direction You can easily.

FIG. 5 shows a sectional view of the steel pipe of the present invention. The thickness of the portion where the maximum bending moment is generated at the central portion in the pipe axis direction is thick, t ₁ , and the other portions are appropriately thin, t ₂ . The length S of the thick-walled portion is experimentally determined to be an appropriate length based on the thickness reduction rate R and the total length L. Here, the thinning rate R = (1
−t / t ₀ ) × 100 (%). Examples will be shown below.

[0009]

【Example】

<Manufacturing process> ERW pipe making → Heat treatment of base pipe → Cold drawing <Heat treatment of base pipe> Annealing or normalization <Unit pipe> Pipe size; Outer diameter φ25.4 to 50.8 mm, Wall thickness 2.0 to 3. 0 mm strength; tensile strength 100 to 150 kgf / mm ² <Cold drawing conditions> (1) Finishing dimensions Outer diameter: φ25.4 to 31.8 mm Wall thickness; Thick part t ₁ 2.0 to 3.2 mm Thin part t ₂ 1.4 to 2.9 mm Length of thick part S Various changes according to the thickness of thin part (2) Drawing speed 15 to 30 m / min (3) Lubricating oil commercially available product <Bending conditions> In Fig. 1, span (A + b) is 600
mm, the fulcrum is free only in the pipe axis direction. The load point is a = 200 to 300 mm, and the displacement of the load point is up to 200 mm.

The thick wall portion of the steel pipe according to the present invention shown in FIG.
It was placed in the center of the bending span and a bending test was performed under the above conditions. As a result, when the wall thickness is changed in the pipe axis direction, the condition that the total length has the same bending strength as that of the steel pipe having the same thickness as that of the thick wall portion is as shown in FIG. It was found that the ratio can be obtained by appropriately taking the ratio of the total length L according to the thickness reduction rate R. That is, S / L ≧ (11.1 + 1.8R−0.018R ² ) / 1
It takes a range of 00. It should be noted that it is advantageous to reduce the weight if the upper limit of S is made as small as possible in terms of molding precision.

According to the present invention, when the total length and the same thickness steel pipe have the same bending strength, the wall thickness of the steel pipe can be reduced because the partial thickness in the pipe axial direction can be reduced. FIG. 7 shows the relationship between the thickness reduction rate R and the steel pipe weight reduction rate J. Here, the weight reduction ratio J = (1-W /
W ₀ ) × 100 (%), W is the weight of the same length thick steel pipe, W
₀ indicates the weight of a different thickness steel pipe having the same strength as the entire length same thickness steel pipe. It is possible to reduce the weight by 15% with a thickness reduction rate of about 30%.

In the above-mentioned embodiments, the case of use as cold drawing is described. However, when deterioration of ductility and toughness due to cold working becomes a problem, finishing such as annealing and normalizing after cold drawing. Heat treatment may be performed.

[0013]

According to the present invention described above, by using a circular electric resistance welded steel pipe that is easy to manufacture and is economical as a material, and by changing the wall thickness in the pipe axial direction of the steel pipe by cold drawing,
Since the wall thickness near the load point where the maximum bending moment is generated can be increased, compared to a steel pipe with the same thickness as the full length thick wall portion, a member with high bending strength can be obtained with the same steel pipe weight,
Further, when the bending strength is the same, the weight can be reduced and it is economical.

[Brief description of drawings]

FIG. 1 is a diagram showing an outline of three-point bending and a bending moment generated during bending.

FIG. 2 is a view showing a buckling state of a conventional steel pipe when it is bent at three points.

FIG. 3 is a cross-sectional view of a steel pipe showing a method for obtaining the steel pipe of the present invention.

FIG. 4 is a cross-sectional view of a steel pipe showing a method for obtaining the steel pipe of the present invention.

5 is a longitudinal sectional view of the tube of FIG.

FIG. 6 is a diagram showing a condition range of the present invention.

FIG. 7 is a diagram showing an example of effects of the present invention.

[Explanation of symbols]

 1 Round steel pipe 2 Buckling part 3 Element pipe 4 Finishing pipe 5 Finishing pipe 6 Dice 7 Plug 8 Support mandrel

Claims (2)

[Claims] 1. A steel pipe having a round cross section, which is used as a member to which a three-point bending moment is applied, is characterized in that the portion where the maximum bending moment occurs is thicker than the other portions and the section modulus is increased. A lightweight ERW steel pipe with excellent 3-point bending strength. 2. In a steel pipe having a round cross section used as a member to which a three-point bending moment is applied, the wall thickness of the portion where the maximum bending moment occurs is made thicker than the other portion, and the length S of the thickened portion is the total length. S / L ≧ (11.
^{1 + 1.8R-0.018R 2) /} 100 ( However, t
₀ : thickness of thick portion, t: thickness of thin portion, R = (1-t /
t ₀ ) × 100), which is a lightweight electric resistance welded steel pipe having excellent three-point bending strength.