JP3248393B2 - Steel pipe with excellent bending workability - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel pipe which is used by bending a construction machine, an automobile part, a piping system of various plants, and the like.

[0002]

2. Description of the Related Art A large number of bent steel pipes are used for hydraulic pipes for construction machines, parts for automobiles, intake and exhaust pipes for automobile engines, plumbing pipes for buildings, and piping systems for various plants. I have.

Generally, in the bending of a steel pipe, it becomes more difficult to ensure the circularity of the cross section and the uniformity of the wall thickness as the bending radius becomes smaller. This tendency is particularly remarkable when the bending radius is as small as twice or less the outer diameter of the raw tube. When the cross section is flattened by bending, stress concentration occurs, and the pressure resistance decreases. For this reason, for example, in the case of high-pressure piping, it is desired that the flatness after bending is 5% or less.

Of course, even in fields other than applications requiring strength, such as steel pipes for mechanical structures and steel pipes for pressure pipes, flattening or uneven wall thickness is required in order to secure the flow rate and shape in the pipes. There is a need for a bending method that does not cause excessive bending.

[0005] Limitations and problems in bending of a pipe are listed as follows: 1) Since the pipe is hollow, the magnitude and direction of the generated stress differs depending on the position of the pipe when the bending is performed. Non-uniform thickness occurs. Poor shape often leads to a decrease in cross-sectional area, leading to a non-uniform or increased flow velocity. In addition, it leads to a decrease in pressure resistance.

2) Due to the hollow cross section, when bending is performed, the magnitude and direction of the generated stress differs depending on the position of the pipe, and the yield strength, tensile strength and ductility of the pipe become non-uniform. Therefore, heat treatment may be required after bending.

3) When bending is performed, the yield strength and tensile strength of the steel pipe increase, and the ductility decreases. Therefore, it may be necessary to perform heat treatment to obtain the required characteristics.

4) In order to easily perform bending, it is necessary that the elongation of the material is secured to a certain value or more. Of course, in many cases, the strength must be ensured according to the application.

5) In the state of a raw tube, the strength is too high or the ductility is too low to bend easily.
Heat treatment may be required. And so on.

[0010] Although there are problems as described above, a general countermeasure is to first obtain a bent tube without flattening, uneven thickness, thinning, and rough skin. Non-uniformity and the like are dealt with by heat treatment.

However, for the reason described in 1) above, it is not always easy to obtain a bent pipe having an excellent shape, and various techniques aimed at this purpose have been developed. One of them is core metal optimization,
Further, a method of passing a ball using a mold after bending is performed.

In other words, in order to prevent the cross section from becoming flat or uneven at the time of bending, a foam core or a ball core corresponding to the bending radius of the pipe is used. Is required to push through a super hard ball or the like using a mold.

However, the use of an optimum core metal is to use a metal core that matches the bending radius. To cope with bending work of various curvatures, naturally a large number of metal cores are required, and in terms of efficiency, And economic disadvantages are inevitable. In addition, the method of passing a ball using a mold after bending is limited to a material having a thin and low-strength material in a target tube, and does not cover the recovery of uneven thickness.

Japanese Patent Application Laid-Open No. 63-235026 discloses a method in which a flexible tube is inserted into a tube and a bending process is performed with an incompressible fluid being pressed into the tube. However, it is clear that there are efficiency and cost issues.

Although the above is a mechanical method, there is another method of processing by giving a temperature difference to a position on the circumference of a pipe. For example, Japanese Unexamined Patent Publication (Kokai) No. 62-161428 discloses a method for producing a bent tube with less wrinkles by heating and bending the contact side of a mold of a seamless steel tube for a boiler to below the A _c1 transformation point. Also, JP-A-63-171220
Japanese Patent Application Laid-Open Publication No. H11-133873 also discloses a method of performing bending while maintaining the inside of the bend at a higher temperature than the outside. Needless to say, the above two techniques can be used together.

The technique described above is basically applicable regardless of the type and composition of the material.
Japanese Patent Application Laid-Open No. 2-166028 discloses a technique in which liquid nitrogen is introduced into an Al-Mg alloy pipe and the pipe is cooled to an extremely low temperature to perform bending work with less unevenness and flatness.

In the bending of steel pipes, a method of heating and bending the entire pipe is widely used. However, the purpose of this method is to reduce the external force required for the processing and increase the ductility. Not meat prevention. For example, JP-A-3-35
No. 823 discloses a method of heating or cooling a zirconium alloy tube or the like to bend the tube, but also in this case, the main purpose is to increase ductility.

When a product is manufactured by bending a steel pipe or when a plant is constructed using the same, the selection of the steel type of the steel pipe to be used often depends on characteristics such as strength, ductility, weldability, and corrosion resistance. It is done with consideration.

[0019]

For example, a steel pipe for a boiler is assembled into a boiler panel after being subjected to a complicated bending process. In this case, selection of a steel type is made with a primary purpose of ensuring high-temperature strength. And its components are usually limited to a narrower range within the specified component range. In the bending of these steel pipes, there is a problem how to bend the selected steel pipe skillfully.

However, since the steel material has relatively high strength and is inexpensive as compared with other materials, the type and composition of the steel tube used as the material does not matter so much depending on the application. Etc. may be a problem exclusively.
Also, even when the steel type and the component range are limited, when the selection of the component range of the steel is not possible at all, the number is unexpectedly small.

The present invention has been made in view of the above points, and an object of the present invention is to provide a steel pipe having good bending workability by adjusting the composition of steel.

[0022]

The steel pipe according to the present invention comprises:
% By weight, C: 0.05 to 0.2%, Si: 0.01
% Or less (including 0), Mn: 0.4% or less (including 0), Al: 0.005 to 0.03%, Al + Ti + N
The sum of b + Ta + Zr + V + B: 0.05% or less, Ni
+ Co + Cr + Cu + Mo + W; a steel pipe containing 1% or less (including 0), N: 0.004% or less (including 0), and the balance being substantially Fe and having excellent bending workability.

[0023]

The present inventors have found that a steel pipe with reduced N content and nitride has less wall thickness and flattening during bending, and to what extent the N content and nitride should be reduced. The present invention has been completed in pursuit of an effect. N is, of course, an important element that strengthens steel. However, almost all elements such as C, Mn, and Ni have a certain degree of difference in terms of strengthening action, and are not unique to N.

It is not clearly understood why, among many reinforcing elements, only N affects the shape at the time of bending, but unlike many other added elements, it is an interstitial element. And the formation of nitrides in the steel.

N, which is an interstitial impurity element, enters the crystal lattice of iron. Also, in combination with Al which is a deoxidizing element, A
It becomes 1N and precipitates. In this way, N hinders the movement of dislocations and prevents plastic deformation. By limiting N,
It is considered that the distortion of the crystal lattice is reduced, and the amount of nitride of Al or Fe, which causes an increase in strength or hardness, is reduced, leading to a reduction in the wall thickness unevenness and the flatness during bending.

It has been found that it is sensitive to the amount of Si, but the reason is not clear. Si is added as a deoxidizer to form an oxide. This may be detrimental to bending workability.

FIG. 1 shows the effect of N on the flatness ratio during bending, and FIG. 2 shows the effect of N on the wall thickness ratio during bending. The composition of the steel pipe is N: 18-67 ppm,
C: 0.07 to 0.08%, Si: 0.003 to 0.0
07%, Mn: 0.12-0.15%, Al: 0.01
Rolled in the range of 1 to 0.013% by a Mannesmann mandrel mill method to produce a hot-finished steel pipe of 25A schedule 40. This steel pipe is bent at a bending radius of 2D.

The bending is performed by the rotary pull bending method shown in FIG. Reference numeral 1 in the figure denotes a steel pipe to be bent. 2
Is a disk-shaped rotatable guide for determining a bending radius at the time of bending, and has a depression corresponding to the outer peripheral surface of the steel pipe on the circumferential surface. Reference numeral 3 denotes a clamp for fixing the steel pipe to the guide 2, and presses and fixes the steel pipe 1 that has moved from the left direction at the position 3a to the guide 2. As in the case of the guide 2, a depression corresponding to the outer peripheral surface of the steel pipe is also provided on the surface of the clamp in contact with the steel pipe.

After fixing the steel pipe 1 with the guide 2 and the clamp 3 at the position 3a, the guide 2 and the clamp 3 are rotated in the direction of the arrow while holding the steel pipe 1 to bend the steel pipe. As shown in 2a, the guide 2 is provided with a convex portion for fixing a steel pipe extending tangentially from one point on the circumference, and this convex portion also has a depression corresponding to the outer peripheral surface of the steel pipe. It is attached.

Reference numeral 4 denotes a cored bar. The metal core is inserted into the inside of the steel pipe from the left direction in the figure to restrain the inside diameter of the steel pipe and prevent the steel pipe from becoming flat.

FIG. 4 shows a cross section taken along the line AA in FIG. The flatness ratio and the wall thickness reduction ratio were determined by the following equations (1) and (2). Flatness = [(maximum outer diameter-minimum outer diameter) / outer diameter before processing] x 100% ... (1) Unevenness ratio = [(maximum thickness-minimum thickness) / thickness before processing] x 100% (2) As shown in FIG. 1, the flatness after bending increases with an increase in the amount of N. However, in bending using a cored bar,
In order to satisfy a flattening rate of 5% or less, which is a rough guide,
It is understood that the amount of N may be set to 40 ppm or less. Further, in order to make the content 2.5% or less, the content may be made 20 ppm or less. In addition, when there is no core metal, the content of N is 20 pp.
It can be seen that the flattening rate becomes 10% or less by setting m or less.

FIG. 2 shows the effect on the uneven thickness ratio. The effect of the amount of N is not clearly recognized as much as the flattening rate, but is also improved by reducing the N content.
In general, it is known that it is difficult to simultaneously reduce the thickness unevenness and the flatness in a mechanical improvement method. For example, in rotation drawing and bending, the flatness can be reduced by using a metal core, but in this case, the unevenness tends to increase. On the other hand, the improvement from the viewpoint of the material, that is, the reduction of the amount of N is a very noticeable method that can reduce both of them.

The present invention analyzes the effects of various components on the bending workability, and reduces the flatness and the wall thickness by limiting the N content to 40 ppm or less, preferably 20 ppm or less. However, in order to make the effect of reducing the amount of N work effectively, it is necessary to limit other components.

The ranges of other components and the reasons for restricting them are shown below. C is set to 0.20% or less. If the content is more than this, the strength becomes too high and the ductility also decreases, so that it is not practical. The lower limit of C need not be particularly set from the viewpoint of bending workability, but is required to be 0.05% or more in order to secure the strength as a steel material.

The content of Si is limited to 0.01% or less. Si is an important deoxidizing element, but if it exceeds 0.01%, the flatness and the wall thickness increase. For unknown reasons, S
The i-type inclusions may be affecting.

Mn is set to 0.4% or less. Mn is an element necessary for deoxidation and desulfurization, but if it exceeds 0.4%, the strength becomes too high, the ductility is reduced, and the flatness and the wall thickness are increased. Although there is no particular need to set a lower limit, about 0.05% is necessary for deoxidation and desulfurization, and 0.3% or more is preferable when strength is desired.

Al is used as a deoxidizing material in an amount of 0.005 to 0.03.
%Added. If the content is less than 0.005%, the effect of deoxidation is not sufficient, and if it exceeds 0.03%, the inclusions of oxides and nitrides increase, and the flatness and the thickness unevenness increase.

There are many elements other than Al that combine with N to form a nitride. For example, Ti, Nb, Ta, Z
r, V, B, etc. are often added to steel or mixed as impurities from scrap. Naturally, nitrides of these elements have the same effect as Al nitride,
It is necessary to limit the total amount of addition including Al to 0.05% or less.

The additional elements that do not or hardly form nitrides do not need to be so limited, but most of them increase the strength, so that Ni, Co, Cr, Cu, M
The total addition amount of o and W is 1% or less.

Incidentally, P and S, which are impurities usually contained in steel, are set to 0.02% or less and 0.01% or less, respectively, but are not particularly limited to these values.

[0041]

EXAMPLES Table 1 shows the chemical components of Examples of the seamless steel pipe of the present invention and the chemical components of Comparative Examples. Comparative Examples A and B contain N at or above the upper limit. C in Comparative Example contains Si at the upper limit or more. D in Comparative Example has Al equal to or more than the upper limit, and the total content of Al, Ti, Nb, Ta, Zr, V, and B also exceeds the upper limit. E in Comparative Example is Ti,
The total content of Nb, V, Al and the like exceeds the upper limit, and the total content of Cr and Mo also exceeds the upper limit.

[0042]

[Table 1]

The steel pipe was manufactured by a Mannesmann-mandrel mill method to produce a hot-finished steel pipe as rolled and a cold-finished steel pipe further subjected to cold drawing and annealing. Each size is a 25A schedule 40. Bending is 2D.

The method of bending and the method of obtaining the flatness ratio and the wall thickness unevenness are the same as those shown in FIGS. 3 and 4 and the equations (1) and (2). Table 2 shows the flatness and wall thickness after bending of these steel pipes. The results of the examples of the present invention show that both the hot-finished steel pipe and the cold-finished steel pipe are good, but the steel pipe of the comparative example has a large flatness and a thin wall thickness. Incidentally, the cold-finished steel pipe has a small thickness unevenness and a small flattening rate as compared with the hot finished steel pipe.

[0045]

[Table 2]

Table 3 shows the chemical components of the examples of the electric resistance welded steel pipe of the present invention and the chemical components of the comparative example. Comparative Example F
And G contain N more than the upper limit. H in Comparative Example contains Si at an upper limit or more. In Comparative Example I, Al was equal to or more than the upper limit, and Al, Ti, Nb, Ta, Zr, V,
The total content of B also exceeds the upper limit. In J of the comparative example, the total content of Ti, Nb, V, Al and the like exceeds the upper limit, and the total content of Cr, Mo and the like also exceeds the upper limit.

[0047]

[Table 3]

All sizes are 50A schedule 40
It is. Bending is 2D. The method of bending and the method of obtaining the flatness ratio and the wall thickness unevenness are the same as those shown in FIGS. 3 and 4 and the equations (1) and (2) above.

Table 4 shows the flatness and wall thickness after bending of these steel pipes. It can be seen that the steel pipes of the examples of the present invention are low in both flatness and wall thickness and are good, but the flatness and wall thickness of the steel pipes of the comparative examples are large. In addition, it can be seen that the ERW welded steel pipe has a small thickness unevenness and flatness, although slightly, as compared with the seamless steel pipe.

[0050]

[Table 4]

When bending the steel pipe of the present invention, various methods as described above, that is, heating the inside of the steel pipe to be bent, using a foam-type core metal, or Of course, it is also possible to apply a method in which a fluid is put into the inside of a pipe and bending is performed.

[0052]

According to the present invention, it is possible to obtain a bent pipe having a small flatness ratio and a small wall thickness ratio by using no core metal or using only a simple core metal. The invention of a steel pipe with controlled components, but other properties of the steel pipe, namely strength, ductility,
There are no strict restrictions that have a significant effect on toughness, weldability, manufacturability, corrosion resistance, etc. Therefore, it can be said that the applicable range of the present invention is extremely wide. Specifically, it is applicable to various piping materials such as high-pressure hydraulic piping, various heat exchanger materials typified by steel tubes for boilers, and various mechanical component materials, and has a large economic effect.

[Brief description of the drawings]

FIG. 1 is a diagram showing the relationship between the amount of N and the flattening rate.

FIG. 2 is a diagram showing the relationship between the amount of N and the uneven thickness ratio.

FIG. 3 is a view schematically showing a rotary drawing method for bending a steel pipe according to the present invention.

FIG. 4 is a view schematically showing a steel pipe after bending.

[Explanation of symbols]

 Reference Signs List 1 steel pipe 2 bending guide 2a steel pipe fixing part of bending guide 3 clamp 3a position where clamp fixes steel pipe 4 cored bar

──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Kenichi Toyama 1-1-2 Marunouchi, Chiyoda-ku, Tokyo Inside Nippon Kokan Co., Ltd. (72) Inventor Takashi Suzuki 1-1-2 Marunouchi, Chiyoda-ku, Tokyo Nippon Kokan (56) References JP-A-56-23254 (JP, A) JP-A-56-119756 (JP, A) JP-A-5-287379 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C22C 38/00-38/60

Claims (1)

(57) [Claims] (1) C: 0.05 to 0.2% by weight,
Si: 0.01% or less (including 0), Mn: 0.4% or less (including 0), Al: 0.005 to 0.03%, Al
+ Ti + Nb + Ta + Zr + V + B total: 0.05%
Hereinafter, the total of Ni + Co + Cr + Cu + Mo + W; 1%
A steel pipe having excellent bending workability, comprising the following (including 0), N: 0.004% or less (including 0), and the balance substantially consisting of Fe.