JPH0938722A - Production of square steel tube - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce residual stress of a square steel tube at low cost and to restrain sectional deformation developed with the residual stress, in a producing method of the square steel tube with a roll forming system. SOLUTION: In the producing method of the square steel tube 2 by squarely forming a round steel tube with a roll to produce the square steel tube 2, after executing forming so that the side parts of the square steel tube 2 projects toward the inner surface side of the tube 2 in the square forming process, bending work is executed to the sides of the square steel tube 2 from the inner surface side so as to develop tension strain at the outer surface side of the tube and compression strain at the inner surface side of the tube in the longitudinal direction of the tube.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a method for manufacturing a square steel pipe by a roll forming system.

[0002]

2. Description of the Related Art A roll forming method, which is one of the methods for manufacturing square steel pipes, forms a steel strip as a raw material into an open pipe shape by passing it through a plurality of round-shaped rolls, and A round steel pipe is obtained by electric resistance welding, and the round steel pipe is passed through a plurality of stages of corner forming rolls (reshaping stands R1 to R4 in FIG. 3) to form a square steel pipe.

However, when the square steel pipe manufactured by the roll forming method is cut, as shown in FIG. 1, a phenomenon of opening deformation occurs in which the side portion of the cut edge bulges outward along the forming direction.

Such a cut deformation deteriorates the dimensional accuracy of the square steel pipe, and when the square steel pipe is used as a building member, the dimensional accuracy of the square steel pipe is originally assumed in the process of joining and welding. It is impossible to perform automatic welding, and manual welding and welding repair accompanying it are unavoidable.Because the dimensions of the brackets do not match, cutting repair of some bracket welded parts is required. descend. Furthermore, when connecting two square steel pipes and providing a backing metal on the inner surface of the abutting portion of both square steel pipes, the above-mentioned cut deformation causes an excessive gap between the inner surface of the pipe and the backing metal, resulting in a welding defect. This results in deterioration of connection strength and occurrence of welding repair. In addition, since the side of the square steel pipe bulges outward due to the above-mentioned cut deformation, a high residual tensile stress is generated on the inner surface of the pipe corner, cracks due to welding processing performed at low temperatures in winter, during hot dip galvanizing There is a case where cracks occur, and it is difficult to secure the performance as a building member.

Therefore, as a conventional method for reducing residual stress, which is considered to be the cause of cut end deformation of a square steel tube, for example, Japanese Patent Laid-Open No.
As described in JP-A-5-23738, a part or all of the corner forming process is hot-formed, or as described in JP-A-5146821, the entire surface of the tube after corner forming is heated, and SR ( There is something that causes stress relief.

[0006]

However, in the prior art, the pipe is hot-formed or heat-treated in order to reduce the residual stress that is considered to be the cause of the cut deformation of the square steel pipe, and fuel such as heavy oil or gas is used. Or even if it uses electricity,
Heating energy is required, resulting in high cost. Also, SR
On the other hand, when an off-line heating furnace is used, the productivity is lowered and the cost is further increased.

It is an object of the present invention to reduce residual stress of a square steel pipe at a low cost in a method of manufacturing a square steel pipe by a roll forming system and suppress a cut deformation caused thereby.

[0008]

SUMMARY OF THE INVENTION The present invention is a method for manufacturing a square steel pipe in which a round steel pipe is formed into a square shape by rolls to produce a square steel tube. In the corner forming step, the sides of the square steel tube are directed toward the inner surface of the pipe. After being formed so as to be convex, the side of the square steel pipe is bent from the pipe inner surface side so that the pipe outer surface side has tensile strain and the pipe inner surface side has compressive strain in the pipe longitudinal direction. is there.

The present inventors investigated the cause of the cut deformation of a square steel pipe and obtained the following findings. The cause of the deformation of the cut end of the square steel pipe is as shown in FIG. First, the reason why both ends of the square steel pipe are deformed by opening will be described. That is, the bending strain in the longitudinal direction of the pipe (tensile on the inner surface of the plate thickness and compressed on the outer surface) caused by winding of the material to be rolled around the roll causes the material thickness to return to the linear shape after the completion of roll forming. Bending residual stress is compressive on the inner surface and tensile on the outer surface. After that, when the residual stress is released by cutting the pipe, the cut end has the same shape as the winding on the roll, that is, the side portion bends downwardly in the longitudinal direction, and as shown in FIG. Both cut ends Deform in the direction of opening to the outside. Also,
Shear stress acts in the plate thickness direction in the corner forming type. Then, a shear residual stress in the plate thickness direction is generated in the square steel pipe after the corner forming, whereby the cutting end cut end of the square steel pipe is deformed in the direction in which the upstream cutting end cut end is closed as shown in FIG. Side cut end The cut end deforms in the direction of opening to the outside. Cut deformation due to the above-mentioned longitudinal bending residual stress and plate thickness direction shear residual stress is overlapped, and there is a tendency that the opening amount is larger at the downstream cutting end cutting and smaller at the upstream cutting end cutting. is there. The larger the product side length D is,
The smaller the product plate thickness t and the higher the yield strength of the product, the larger the tendency tends to be.

Therefore, in the present invention, in the corner forming step, after the side of the square steel pipe is formed to be convex toward the inner surface of the pipe, the bending strain and the shear strain in the pipe longitudinal direction at the time of roll forming described above. Deformation opposite to the above is applied to the side portions of the square steel pipe at a stage after the corner forming step so that each side portion has a flat shape. As a result, the bending residual stress and the shear residual stress in the longitudinal direction of the product remaining in the product square steel pipe become extremely small, and as a result, the occurrence of cut end deformation of the pipe end is suppressed. As a result, it is possible to manufacture a square steel pipe with high dimensional accuracy in which both the uncut portion (body portion) of the square steel pipe and the sides of the cut end are flat.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a schematic view showing a cut deformation of a square steel pipe, FIG. 2 is a schematic view showing a cause of a cut deformation of a square steel pipe, and FIG.
Is a schematic diagram showing a round steel pipe forming process in one embodiment of the present invention, FIG. 4 is a schematic diagram showing a square steel pipe forming process in one embodiment of the present invention, and FIG. 5 is a square steel pipe forming process in one embodiment of the present invention. FIG. 6 is a schematic view showing a subsequent tube straightening step, FIG. 6 is an enlarged view of a main part of FIG. 5, FIG. 7 is a schematic view showing a modified example of the lining roll arrangement,
FIG. 8 is a schematic diagram showing the effect of the present invention, and FIG. 9 is an explanatory diagram of the cut deformation amount after cutting the square steel pipe.

FIG. 3 shows a round steel pipe forming process, in which an open pipe 1A which is formed into an open pipe shape by a round forming roll group is passed through a fin pass roll 11 which is the final roll of the round forming roll group. After that, open pipe 1
Two contact tips 12 are brought into contact with both edge portions of A and a high-frequency current is flown, and the heated edge is pressure welded by a squeeze roll 13 (electric resistance welding).
Then, a round electric resistance welded steel pipe (round steel pipe 1B) is obtained. Then, the weld beads of the round steel pipe 1B are cut and removed by the outer bead cutting tool 14. The inner bead may be cut and removed by the inner bead cutting tool depending on the specifications.

Then, the round steel pipe 1B is continuously passed through the angle forming roll group shown in FIG. In FIG. 4, 16 is a sizing roll for the round steel pipe 1B, and 17A to 17D are reshaping rolls for forming the square steel pipe 2 into a square shape.

However, in this embodiment, in the corner forming step of FIG. 4, for example, the final reshaping roll 17D is formed so that the four sides of the square steel pipe 2 are convex toward the inner surface of the pipe ( FIG. 5A, convex deformation amount ΔH ₁ ).

In the present embodiment, the square steel pipe 2 which has undergone the corner forming process of FIG. 4 is cut off from the round steel pipe 1B at an off-line stage, and as shown in FIG. Bending and shearing are applied from the inner surface side of the pipe by the four lining rolls 21A to 21D (FIG. 5 (B), lining bending amount ΔH2), and each side portion is subjected to the lining bending process as shown in FIG.
The flat shape as shown in FIG.
Each of the lining rolls 21 </ b> A to 21 </ b> D is subjected to a bending process and a shearing process such that the pipe outer surface side has a tensile strain and the pipe inner surface side has a compression strain in the longitudinal direction of the square steel pipe 2.

The inventors of the present invention have found that the uncut portion (body portion) of the square steel pipe and the four sides of both the upstream and downstream cut ends after the lining and bending work from the inner surface side of the pipe are flat. In order to have a shape, the amount of convex deformation (ΔH ₁ ) to the inner surface side applied to those four sides in the corner forming step and the amount of lining bending (ΔH ₂ ) to be applied in the subsequent step from the pipe inner surface side It is understood by a series of experiments that there is an appropriate value between the sides depending on the side length (L), plate thickness (t), and strength (s) of the tube. The relationship can be expressed by the following functional expression, for example. ΔH ₁ = f (L, t, s) ΔH ₂ = g (L, t, s, ΔH ₁ )

Here, the lining rolls 21A to 21D are rotatably pivotally attached to the tip end portion of the holder bar 15 extending inside the square steel pipe 2, and the roll positions can be adjusted in the roll radial direction. It has a structure, and the amount of lining can be controlled by adjusting the roll position. Moreover, when inserting the holder bar 15 incorporating the lining rolls 21A to 21D into the square steel pipe 2, the lining rolls 21A to
21D is retracted, the side of the square steel pipe 2 and the rolls 21A to 2A
The positions of the lining rolls 21A to 21D are adjusted so as not to come into contact with 1D. The square steel pipe 2 is held by the stopper 22, and the holder bar 15 inserted inside the square steel pipe 2
By pulling out, the lining bending by the lining rolls 21A to 21D is performed from the pipe inner surface side. At this time, the lining rolls 21A to 21D may be arranged on the same plane in the pipe longitudinal direction (FIG. 6), and the arrangement surface of the upper and lower lining rolls 21A and 21C and the arrangement of the left and right lining rolls 21B and 21D. The planes may be displaced from each other in the pipe longitudinal direction (FIG. 7), or they may be arranged one by one at different positions in the pipe longitudinal direction.

Further, the roll diameter D of the lining rolls 21A to 21D for bending the square steel pipe 2 and the amount of projection are determined according to the side length, plate thickness, strength, cut deformation amount, etc. of the square steel pipe 2, Can be adjusted.

The above-mentioned inner rolls 21A to 21D are
It can be replaced with a shoe. At this time, the shoe does not rotate, and the lubricant is supplied to the contact portion with the inner surface of the pipe.

The operation of this embodiment will be described below.
Each of the forming rolls (the sizing roll 16 and the reshaping roll 17A) forming the corner forming process is formed on the square steel pipe 2.
~ 17D), the bending strain (tensile on the inner surface of the plate thickness, compression on the outer surface) and shear strain in the pipe longitudinal direction are imparted by wrapping around the molding roll, and when returning to a straight line after the roll molding. Bending residual stress and tensile residual stress of compression remain on the inner surface of the plate thickness and tensile on the outer surface. However, in this embodiment,
(a) In the corner forming process, for example, the reshaping roll 17
D imparts a convex deformation to the side of the square steel pipe 2 toward the pipe inner surface side, and (b) the inner-lining rolls 21A to 21D are bent from the pipe inner surface side of the square steel pipe 2 after corner forming. Then, the side of the square steel pipe 2 is given a tensile strain on the outer side of the pipe in the longitudinal direction of the pipe and a compressive strain on the inner side of the pipe, and a shear strain opposite to the shear strain acting at the time of corner forming.

That is, the square steel pipe 2 after the corner forming process is the round pipe 1.
In the off-line stage of long cutting from B, bending strain (tensile on the inner surface, compression on the outer surface) and shear strain in the longitudinal direction of the pipe at the time of roll forming in the corner forming process, the deformation opposite to the square steel pipe 2
Bending residual stress and shear residual stress in the longitudinal direction of the square steel tube 2 are extremely small by applying the bending residual stress and the shear residual stress to the sides of the square steel tube 2. Therefore, even if the square steel pipe 2 is cut to the product length and its residual stress is released, the occurrence of the cut end deformation at the pipe end is minimized.

[0022]

EXAMPLES The concrete results of this example will be described below. In an electric resistance welded steel pipe manufacturing facility in which a round forming roll group and a square forming roll group are continuously arranged, an inner surface side convex provided on four sides of the square steel pipe after the final reshaping roll 17D of the square forming roll group. Deformation amount (ΔH ₁ ) is 4 mm
And the amount of lining bending (ΔH ₂ ) given by the inner lining bending by the lining rolls 21A to 21D thereafter is 2.5 m
By setting m, 550mm square, 16mm plate thickness, STKC49
0 square steel pipe was manufactured. The uncut portion (body portion) of this square steel pipe and the amount of cut deformation of both upstream and downstream cut ends were investigated, and FIG. 8 and Table 1 were obtained. The cut deformation amount is
It is represented by the difference in flatness of each side before and after cutting. When the flatness of the side before cutting is Δd ₁ = d −d ₁ and the flatness of the side after cutting is Δd ₂ = d −d ₂ , the cut deformation amount is represented by Δd ₂ −Δd ₁ (FIG. 9).

According to FIG. 8 and Table 1, by carrying out the method of the present invention, the amount of deformation of the cut can be markedly reduced as compared with the conventional method, and both the uncut portion (body portion) and the cut end portion of the square steel pipe can be reduced. It is recognized that it is possible to manufacture a square steel pipe having a flat side shape and high dimensional accuracy.

[0024]

[Table 1]

The embodiment of the present invention has been described in detail above with reference to the drawings. However, the specific configuration of the present invention is not limited to this embodiment, and changes in design within the scope not departing from the gist of the present invention can be made. Even if it exists, it is included in the present invention. For example, the present invention is not limited to a square steel pipe, but can be applied to any square steel pipe such as a triangle or a pentagon.

[0026]

As described above, according to the present invention, in the method of manufacturing a square steel pipe by the roll forming method, it is possible to reduce the residual stress of the square steel pipe at a low cost and suppress the cut deformation caused thereby.

[Brief description of drawings]

FIG. 1 is a schematic view showing a cut end deformation of a square steel pipe.

FIG. 2 is a schematic diagram showing a cause of deformation of a cut end of a square steel pipe.

FIG. 3 is a schematic view showing a round steel pipe forming process in one embodiment of the present invention.

FIG. 4 is a schematic diagram showing a process of forming a square steel pipe in one embodiment of the present invention.

FIG. 5 is a schematic view showing a pipe straightening process after a square steel pipe forming process in one embodiment of the present invention.

FIG. 6 is an enlarged view of a main part of FIG. 5;

FIG. 7 is a schematic diagram showing a modified example of the arrangement of the lining rolls.

FIG. 8 is a schematic diagram showing the effect of the present invention.

FIG. 9 is an explanatory diagram of the amount of cut deformation after cutting a square steel pipe.

[Explanation of symbols]

 1B Round steel pipe 2 Square steel pipe 11 Fin pass roll (roll) 13 Squeeze roll 17A-17D Reshaping roll 21A-21D Liner roll

Claims (1)

[Claims] 1. A method of manufacturing a square steel pipe, wherein a round steel pipe is formed into a square shape by rolls to produce a square steel tube. In the corner forming step, the side portion of the square steel tube is formed to be convex toward the inner surface of the pipe. After that, the side of the square steel pipe is bent from the pipe inner surface side so that the pipe outer surface side has a tensile strain and the pipe inner surface side has a compressive strain in the pipe longitudinal direction.