JPH1180900A - Steel tube excellent in earthquake resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent the occurrence of local buckling against the compression and bending stress acting in the axial direction of a tubular body at the time of occurrence of a big earthquake by regulating the slope of the curve, in the strain region from the yield point to 5% amount of on-load strain on the nominal stress/nominal strain curve obtained by the tensile test in the axial direction of a tubular body, to positive. SOLUTION: In this steel tube where the slope of nominal stress/nominal strain is positive in the strain region from the yield point to 5% amount of on-load strain on the nominal stress/nominal strain curve, critical outside diameter/tube thickness ratio causing local buckling is remarakbly high and accordingly local buckling is hardly brought about. Such a characteristic can be provided by controlling a chemical composition of steel tube, rolling conditions for steel plate, etc., and applying heat treatment to steel tube. As to this chemical composition, it is preferable to incorporate 0.03-0.25 wt.% C and 0.5-2.0 wt.% Mn as essential elements and also to incorporate at least one element among Cu, Ni, Cr, Mo, Nb, V, and Ti.

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 as a gas pipeline, a water supply pipe or the like and has excellent local buckling resistance, that is, earthquake resistance during an earthquake.

[0002]

2. Description of the Related Art Carbon steel pipes such as UOE steel pipes, spiral steel pipes, seamless steel pipes, electric resistance welded steel pipes, and press-bend steel pipes or low alloy steel pipes can be manufactured in large quantities and stably. It is widely used as a pipe for transporting fluids such as gas pipelines and water pipes, or as a steel pipe for construction and civil engineering columns, in combination with economy and welding workability.

[0003] However, when a large earthquake occurs, the outer diameter of the steel pipe is increased because tensile force and compression in the longitudinal direction of the tubular body and large lateral force in the axial direction of the tubular body are repeatedly applied to these steel pipes. In the case of a steel pipe having a large pipe thickness ratio, local buckling occurs, and in some cases, a crack is generated in the circumferential direction, leading to breakage.

Conventionally, as steel pipes for construction, for example, Japanese Patent Application Laid-Open Nos. Hei 3-173719, Hei 5-65535, Hei 5
-117746, JP-A-5-117747, JP-A-5-17747
156357, JP-A-6-49540, JP-A-6-49
541, JP-A-6-128864, JP-A-6-2641
43, JP-A-6-264144, etc.
A method for manufacturing a steel pipe to reduce the yield ratio, which is the ratio between the yield stress and the tensile strength, is disclosed.

However, the above prior arts all relate to the ability of a column to absorb plastic deformation against bending stress, and prevent the occurrence of brittle cracks due to local buckling against the axial force of compression and tension after local buckling occurs. No consideration has been given to doing so.

The effect of reducing the yield ratio of steel on the local buckling is described in Toyoda et al., "Effect of Steel Deformation Characteristics on Deformability of Steel Welded Structure" (Journal of the Japan Welding Society). vol.8. No.1 p112. 1990).

[0007] With respect to line pipes for transporting fluids such as gas, resistance to internal pressure acting in the circumferential direction such as ductile fracture or brittle fracture has been studied, but laying is required for external force in the pipe axis direction. Almost no consideration is given other than bending deformation at the time.

Various studies have been made on the buckling behavior of steel pipes with respect to the compressive force in the pipe axis direction.
Suzuki, Kiba, "Inelastic buckling strength of buried steel pipes against cyclic deformation in the axial direction" (JSCE Structural Engineering Transactions vol.35
A. p1351-1358. March 1989)
In the case of 2.6 and 35.0, the buckling strain was 3% or more, whereas when the ratio of outer diameter / tube thickness increased to 55.4, 1
It is described that buckling occurs even with an extremely small strain amount of less than%.

[0009]

However, no buckling behavior has been studied from a viewpoint other than the yield ratio of the steel pipe. Particularly, when the outer diameter / pipe thickness ratio is large, the buckling strain amount is 1%. % Or more, a steel pipe having excellent buckling resistance, which is excellent in buckling resistance and hardly causes local buckling even during bending deformation, has not yet been developed.

[0010] Accordingly, an object of the present invention is to solve the above-mentioned problems, and to prevent local buckling of compression or bending stress acting in the axial direction of a pipe even in the event of a large earthquake. An object of the present invention is to provide a steel pipe excellent in earthquake resistance suitable for piping and the like.

[0011]

Means for Solving the Problems The present inventors have evaluated the buckling resistance of a tube against the compressive force acting in the axial direction.
Using various steel pipes with different material and outer diameter / pipe thickness ratio,
The relationship between the material properties of the pipe body and the local buckling behavior was investigated by performing an actual pipe compression test and various material examination tests, and the bending deformation of the steel pipe was also investigated.

FIG. 1 is a schematic front view of an actual pipe compression tester. As shown in FIG. 1, a test steel pipe 1 vertically arranged on a base 2 via a load cell 5 is connected to a hydraulic mechanism. 3 was pressed from the upper surface, and the displacement meter 4 was used to measure the amount of strain generated in the tube. FIG. 2 is a schematic front view of the test steel pipe 1, and the length a of the test portion of the steel pipe 1 is 500 mm.

As a result of the examination by the above-mentioned test, the presence or absence of local buckling in the compression test with a nominal strain of 1% and the occurrence of buckling in the bending buckling test at a bending angle of 3 ° are determined by the axial tensile characteristics of the tube and the It was found that there was a correlation as described below.

That is, a tensile test was carried out using a tensile test piece sampled with the longitudinal direction of the test piece coinciding with the axial direction of the tube, and the on-road strain was calculated from the yield point in the obtained nominal stress-nominal strain curve. In a strain range up to 5%, a steel pipe having a positive nominal stress / nominal strain gradient has a critical outer diameter / tube thickness ratio that causes local buckling as compared with a steel pipe having the above-mentioned gradient of zero or negative. Is extremely large, so that local buckling is unlikely to occur.

The present invention has been made on the basis of the above findings, and the invention according to claim 1 is based on the fact that a nominal stress / nominal strain curve obtained by an axial tensile test on a tubular body is obtained from a yield point. It is characterized in that the slope of the nominal stress / nominal strain curve is positive in the strain region where the on-load strain amount is up to 5%.

According to a second aspect of the present invention, the chemical composition of the steel pipe according to the first aspect is C: 0.03 to 0.25 wt.
%, Mn: 0.5 to 2.0 wt.%, And the balance: Fe and unavoidable impurities. The invention according to claim 3, wherein the chemical composition of the steel pipe is: C: 0.03-0.25 wt.%, Mn: 0.5-2.
0 wt.%, And if necessary, Cu: 0.05-0.5
0 wt.%, Ni: 0.05 to 0.50 wt.%, Cr: 0.0
5 to 0.50 wt.%, Mo: 0.05 to 0.50 wt.%, N
b: 0.005 to 0.10 wt.%, V: 0.005 to 0.
It is characterized by containing at least one element of 10 wt.% And Ti: 0.005 to 0.10 wt.%.

[0017]

Next, the present invention will be described in detail. The method for producing the steel pipe of the present invention is not limited.
For any of steel pipes, spiral steel pipes, seamless steel pipes, ERW steel pipes, press-bend steel pipes, etc., the above characteristics, namely, the amount of on-load strain from the yield point to the nominal stress / nominal strain curve obtained by a tensile test in the pipe axis direction. Satisfies that the slope of the nominal stress / nominal strain curve is positive in the strain range up to 5%.

Such properties may be imparted by controlling the chemical composition of the steel pipe or the rolling conditions of the steel sheet, or may be imparted by subjecting the steel pipe to heat treatment. There is no particular limitation.

The chemical composition of the steel pipe is as follows: C: 0.0
3 to 0.25 wt.%, Mn: 0.5 to 2.0 wt.% As essential elements, and Cu: 0.05 to 0.5 if necessary.
0 wt.%, Ni: 0.05 to 0.50 wt.%, Cr: 0.0
5 to 0.50 wt.%, Mo: 0.05 to 0.50 wt.%, N
b: 0.005 to 0.10 wt.%, V: 0.005 to 0.
Preferably, the steel contains at least one of 10 wt.% And Ti: 0.005 to 0.10 wt.% As a selective element. The reason will be described below.

C: The welding of steel with a C content of less than 0.03 wt.% Or more than 0.25 wt.% Increases the possibility of weld cracking. Therefore, the C content is 0.03-0.25 wt.%.
Is preferably within the range.

Mn: In order to obtain sufficient strength and toughness as structural steel, it is necessary that Mn is contained in an amount of 0.5 wt.% Or more. However, the Mn content is 2.0 wt.%
Exceeding this causes deterioration in toughness and weldability of the base material and the welded portion. Therefore, the Mn content is preferably in the range of 0.5 to 2.0 wt.%.

Cu, Ni, Cr, Mo, Nb, V, Ti
At least one of the following: Cu, Ni, Cr, Mo is
All are effective elements for improving the strength, and their preferable contents are 0.05 to 0.50 wt.%. If the content is less than 0.05 wt.%, The effect is weak, while
If it exceeds 0 wt.%, The toughness and weldability of the base metal welded portion of the steel pipe tend to deteriorate.

Nb is an element effective for improving the strength and toughness of a steel pipe.
It is within the range of 0.10 wt.%. Content is 0.005wt.%
If it is less than 0.10 wt.%, The effect will be weak, and if it exceeds 0.10 wt.%, The toughness of the welded portion tends to deteriorate.

V is an element effective for improving the strength of the steel sheet, and its preferable content is 0.005 to 0.10 wt.%.
Is within the range. If the content is less than 0.005 wt.%, The effect is small, while if it exceeds 0.10 wt.%, The toughness of the weld tends to deteriorate.

[0025] Ti is an element effective for improving the toughness of the steel sheet and preventing flaws in the slab at the time of casting, and its preferable content is in the range of 0.005 to 0.10 wt.%. If the content is less than 0.005 wt.%, The effect is thin,
On the other hand, if it exceeds 0.10 wt.%, The toughness of the weld tends to deteriorate.

The provision of the characteristic that the slope of the nominal stress / nominal strain curve is positive to the steel pipe having the chemical composition described above is performed by controlling the rolling conditions of the steel sheet as the material of the steel pipe, or
It can be performed by heat treatment of the steel pipe during or after pipe formation.

For example, during the cooling process at the time of rolling or during hot pipe forming, after air cooling to a temperature not higher than the Ar ₃ transformation temperature,
Accelerated cooling from the two-phase temperature range above the Ar ₁ transformation temperature, or heating and cooling to a two-phase temperature range between Ac ₁ and Ac ₃ after forming the pipe in the cold state to reduce the metal structure to ferrite and bainite or ferrite The above characteristics can be imparted by forming a two-phase structure of and martensite.

[0028]

Next, the present invention will be described with reference to embodiments. Steel Nos. 1 to 3 having the chemical component compositions shown in Table 1 were hot-rolled into steel sheets, which were formed into a tube, and both ends in the width direction were welded to prepare a welded steel pipe. At that time, the rolling was completed at a temperature of 750 ° C., air-cooled, and then accelerated cooling was performed at a cooling rate of 10 ° C./sec. For steel Nos. 1 and 2, rolling was completed at 750.degree.

[0029]

[Table 1]

Test specimens were taken from the steel pipe thus produced, and for each test specimen, the nominal gradient of the nominal stress / nominal strain up to 5% by a tensile test in the pipe axis direction, the outer diameter / pipe thickness ratio, The presence / absence of buckling due to 1% compression and the presence / absence of buckling during 3 ° bending deformation were investigated and are shown in Table 2.

[0031]

[Table 2]

In Table 2, Nos. 1 to 8 are steel pipes of the present invention, and Nos. 9 to 12 are comparative steel pipes outside the scope of the present invention.
The mark * indicates the minimum value of the gradient from the proportional limit or the upper yield point to the 5% on-load strain in the nominal stress / nominal strain curve. E indicates a longitudinal elastic modulus, and ** indicates a steel pipe in which an upper yield point and a lower yield point are recognized.

As shown in Table 2, in the case of the invention steel pipes Nos. 1 to 8 in which the minimum gradient of the nominal stress / nominal strain up to 5% strain is positive, the outer diameter / pipe thickness ratio is 60. Neither compression buckling nor bending buckling occurred. On the other hand, in the case of Comparative Steel Pipe Nos. 9 to 12 in which the minimum gradient was negative or 0, even when the outer diameter / pipe thickness ratio was 35, compression buckling and bending buckling occurred.

[0034]

As described above, according to the present invention,
Even in the event of a large earthquake, it is possible to obtain a steel pipe with excellent seismic resistance that does not easily cause local buckling against compression or bending deformation.Therefore, by using the steel pipe of the present invention as a gas pipeline or a water pipe, Prevents damage to pipes and outflow of internal fluids, and can be used as a bridge pier on an expressway to prevent the occurrence of disasters such as collapse due to its breakage, and has an industrially useful effect. .

[Brief description of the drawings]

FIG. 1 is a schematic front view of an actual pipe compression tester.

FIG. 2 is a schematic front view of a test steel pipe used in an actual pipe compression tester.

[Explanation of symbols]

 Reference Signs List 1 steel pipe 2 base 3 hydraulic mechanism 4 displacement gauge 5 load cell

Claims (3)

[Claims] In a nominal stress / nominal strain curve obtained by an axial tensile test on a tubular body, the slope of the nominal stress / nominal strain curve is positive in a strain range from the yield point to an on-load strain amount of 5%. A steel pipe with excellent earthquake resistance. 2. The steel pipe having excellent earthquake resistance according to claim 1, wherein the steel pipe has the following chemical composition. C: 0.03 to 0.25 wt.%, Mn: 0.5 to 2.0 wt.%, And the balance: Fe and unavoidable impurities. 3. The steel pipe according to claim 1, wherein the steel pipe has the following chemical composition. C: 0.03 to 0.25 wt.%, Mn: 0.5 to 2.0 wt.%, At least one element selected from the group consisting of: Cu: 0.05 to 0.50 wt.%, Ni: 0 0.05 to 0.50 wt.%, Cr: 0.05 to 0.50 wt.%, Mo: 0.05 to 0.50 wt.%, Nb: 0.005 to 0.10 wt.%, V: 0.005 ０．１0.10 wt.%, Ti: 0.005 to 0.10 wt.%, And balance: Fe and unavoidable impurities.