JPH06106978A - Ultra high tensile deformed ERW steel pipe with excellent bending properties - Google Patents

Abstract

(57) [Summary] [Object] The present invention provides an ultra-high-strength deformed electric resistance welded steel pipe having a tensile strength of 150 kgf / mm ² or more and excellent bending properties, which is used for a vehicle door reinforcement. [Structure] Oval shape with a cross-sectional shape in which the upper and lower shapes are convex outward and the horizontal portion is straight, or a gourd-shaped shape in which the upper and lower shapes are convex outward and the horizontal portion is convex inward , Or an oval or gourd type, an ultra-high-strength ERW steel pipe with excellent bending properties, characterized in that the lower side has a radius of curvature close to a straight line. According to the present invention, the bending characteristics can be improved and the weight can be reduced as compared with the round pipe. Moreover, since buckling can be prevented, the strain generated during bending can be reduced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an ultra-high-strength electric resistance welded steel pipe used for structural members of automobiles, etc., in particular, an ultra-high-strength electric resistance welded pipe having a tensile strength of 150 kgf / mm ² or more for door reinforcement and having excellent bending characteristics. Regarding steel pipes.

[0002]

2. Description of the Related Art Regarding structural members such as automobiles, thorough weight reduction is being considered to improve fuel efficiency and environmental measures, and 150 kgf / mm for some members is one of the measures to achieve compatibility with safety. More than ² ultra high strength steel pipes are being adopted.

As a steel pipe material for reinforcing automobile doors, a method of tempering after electric resistance welding, that is, quenching or quenching and tempering, as described in JP-A-3-122219, etc. Although a method of normalizing a predetermined low alloy steel such as Japanese Patent Laid-Open No. 3-140441 is generally used, the cross-sectional shape of each is a round steel pipe.

[0004]

FIG. 1 is a diagram showing the results of an examination of the displacement and load characteristics of a punch at the time of three-point bending of a conventional round steel pipe having an outer diameter of 31.8 mm and a wall thickness of 2.6 mm by the method of FIG. 2 shows. As described above, in the conventional round steel pipe, under bending conditions where the span L is short, buckling (local depression) of the steel pipe occurs immediately below the punch as shown in FIG. 3, and the load decreases. Therefore, the bending energy E represented by the shaded area is smaller than that in the case where the buckling does not occur, which is disadvantageous. Moreover, since the buckling causes local deformation, the maximum strain generated is large. Ultra high-strength steel pipes of 150 kgf / mm ² or more generally have low elongation and deformability of drawing, so cracking during bending,
Breakage easily occurs.

In view of the above situation, the present invention is less likely to cause buckling than a round steel pipe, has a large bending energy E, and has a small cross-sectional area of 150 kgf / mm ² or more in order to reduce the weight. It is intended to provide a high-strength electric resistance welded steel pipe.

[0006]

The subject matter of the present invention is as follows. That is, (1) the cross-sectional shape is such that the upper and lower shapes are convex outward,
An ultra-high-strength ERW steel pipe with excellent bending characteristics, characterized by an oval shape with a straight horizontal portion. (2) As for the cross-sectional shape, the upper and lower shapes are convex outward,
Ultra-high-strength deformed electric resistance welded steel pipe with excellent bending properties, characterized by being a gourd type with the shape of the lateral part convex inward. And (3) An ultra-high-strength ERW steel pipe with excellent bending properties, characterized in that the oval-shaped steel pipe of the preceding clause 1 or the gourd-shaped steel pipe of the preceding clause 2 has a lower side with a radius of curvature close to a straight line. .

The present invention will be described in detail below. First, the reason for limitation described in claim 1 of the present invention will be described. As with a round steel pipe, the punch load when buckling does not occur is almost proportional to the second moment of area. When the conventional round steel pipe shown in FIG. 4 is bent, the cross-sectional shape is made elliptical and then buckled, the upper part A on the punch side is dented inward and the lateral part B is bent outwardly as shown in FIG. Therefore, the lateral portion B of the round steel pipe has a small contribution to the bending strength and is convex outward, which causes buckling. Therefore, as shown in FIG. 6, when the horizontal portion is straight, the second moment of area can be made larger and the buckling is less likely to occur when the cross sectional area is the same as that of the round tube, so that the bending absorbed energy can be increased. The oval cross-sectional shape of FIG. 6 is claim 1. In this case, the upper and lower radii of curvature Ru and Rb are made larger than V / 2.

Further, as shown in FIG. 7, by making the lateral portion convex inward, buckling is less likely to occur. However, if the amount S of the concave portion inward is excessively large, buckling is likely to occur. The cross-sectional shape of the gourd type shown in FIG. 7 is claim 2. In this case, the upper and lower radii of curvature Ru and Rb are made larger than V / 2.

Next, claim 3 will be described. The lower part C of the round steel pipe shown in FIG. 4 has a smaller influence on buckling occurrence than the upper part A, and may not necessarily be convex outward.
Rather, the second moment of area is greater on a straight line or closer to a straight line, and therefore it is desirable to be closer to a straight line in terms of bending strength.
An example of this shape is shown in FIG. Although FIG. 8 shows the lower part of FIG. 6 as a straight line, the same effect can be obtained by making the lower part of FIG. 7 a straight line. This cross-sectional shape is claim 3. The smaller the corner radius R of the lower part is, the more difficult it is to buckle. However, if R of 1.5t or less is used, cracks are likely to occur during cold working into a square tube.
Is preferably 1.5 to 5.0R.

[0010]

EXAMPLE The composition of the components is C by weight; 0.24%, Si;
0.25%, Mn; 2.4%, Cr; 0.5%, Mo;
0.7%, Ti: 0.03%, B: 20 ppm, balance F
It manufactured with the manufacturing process of FIG. 9 using e and the hot rolled steel strip which is an unavoidable element.

The ERW pipe had a diameter of 38.1 mm in outer diameter, and the heat treatment for the raw pipe was carried out by air-cooling at 700 ° C. in order to soften the work-hardened portion by molding and the quench-hardened portion during welding almost uniformly. The cold drawing was performed by cold drawing using a die.

Table 1 shows the shapes of the examples of the present invention. The dimensions of the examples of the present invention and the following comparative materials are selected so as to have substantially the same cross-sectional area. As a comparative material, a round steel pipe having the same height and the same cross-sectional area as the example was manufactured by the same manufacturing process. In the finish heat treatment, normalization was performed at 850 ° C. in order to make the microstructure of the welded portion and the base metal portion uniform. As a result of the tensile test after the finish heat treatment, TS = 170 kgf / mm ² , El =
It was 10%. The bending test results are shown in Table 1.

[0013]

[Table 1]

In each of the present inventions, the maximum load is larger than that of the round pipe, and the displacement causing buckling is large. Therefore, the load is superior to the round pipe in almost all the displacements. As a result, the present invention was able to increase the bending energy by 20 to 30% compared with the round tube. Further, since the deformed pipe of the present invention is less likely to buckle, as shown in FIG. 10, the maximum equivalent plastic strain of the lower surface of the pipe immediately below the punch can be reduced by about 10 to 20% as compared with the round pipe, and elongation, drawing, etc. This is very advantageous because it reduces the required material properties. Although cold-rolled pipes were used in this example, similar deformed pipe shapes can be obtained by drawing by roll forming.

[0015]

According to the present invention, the bending characteristics can be improved and the weight can be reduced as compared with the round tube. Moreover, since it is possible to prevent buckling, it is possible to reduce strain generated during bending. Therefore, the manufacturing is easy, and the contribution to the industry such as the use of the ultra-high tensile ERW steel pipe of 150 kgf / mm ² or more can be greatly contributed to the industry.

[Brief description of drawings]

FIG. 1 is a front view showing a bending test method.

[Fig. 2] Punch displacement in bending test of a conventional round tube-
The figure which shows a punch load characteristic.

FIG. 3 is a view showing a shape of a steel pipe immediately below a punch after buckling.

FIG. 4 is a sectional view of a round pipe before bending.

FIG. 5 is a cross-sectional view immediately below the punch after bending the round tube.

FIG. 6 is a sectional view of the deformed pipe of the present invention (claim 1).

FIG. 7 is a sectional view of the deformed pipe of the present invention (claim 2).

FIG. 8 is a sectional view of the deformed pipe of the present invention (claim 3).

FIG. 9 is a diagram showing a manufacturing process of the deformed pipe of the present invention.

FIG. 10 is a diagram comparing the maximum equivalent plastic strain of the lower part of the pipe that occurs during the bending test of the deformed pipe of the present invention.

Claims (3)

[Claims] 1. An ultra-high-strength deformed electric resistance welded steel pipe having excellent bending characteristics, characterized in that the cross-sectional shape is an oval type in which the shape of the upper and lower parts is convex outward and the lateral part is straight. 2. An ultra-high tensile deformed electric sewing machine having excellent bending characteristics, characterized in that the cross-sectional shape is a gourd type in which the upper and lower shapes are convex outward and the lateral shape is convex inward. Steel pipe. 3. The oval steel pipe according to claim 1 or the gourd steel pipe according to claim 2, wherein the lower side has a radius of curvature close to a straight line, which is an ultra-high-strength deformed electric resistance welded steel pipe having excellent bending characteristics. .