JP2002194503A - Steel tube with superior buckling resistance, and production method and evaluation system for the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel tube with superior buckling resistance together with the production method and evaluation system for the same. SOLUTION: This steel tube having superior buckling resistance is characterized in that the stress ratio (σr-C) which is a ratio of the yield strength and the stress (σ1.5) at the nominal distortion of 1.5% in the steel tube tensile test in the tube's circumferential direction is 1.05 or greater and/or that the stress ratio (σr-L) which is a ratio of the yield strength and the stress (σ1.5) at the nominal distortion of 1.5% in the steel tube tensile test in the tube's axial direction is 1.05 or greater. Furthermore, the steel tube may contain at mass% C: 0.03-0.1%, Si: 0.05-0.5%, Mn: 1-2% and the remainder substantially consisting of Fe, with PCM value shown by the formula, PCM=C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/15+V/10+5×B, satisfying the range of 0.10-0.20.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel pipe suitable as a gas pipeline, a water supply pipe, a pillar for construction and civil engineering, a method for producing the same, and a method for evaluating the same.

[0002]

2. Description of the Related Art Carbon steel pipes such as UOE steel pipes, electric resistance welded steel pipes, spiral steel pipes, and press-bend steel pipes and low alloy steel pipes can be manufactured in large quantities and in a stable manner. It is widely used as a pipe for transporting fluids such as gas pipelines and water pipes.

However, when a large earthquake occurs or when a submarine pipe is laid, a large force such as tension, compression, or bending is applied in the longitudinal direction of the steel pipe, causing local buckling and, in some cases, circumferential cracks. May result in breakage or breakage.

To cope with such measures against local buckling, Japanese Patent Application Laid-Open Nos. 3-173719, 5-65535, 5-117746, and 5-11
7747, JP-A-5-156357, JP-A-6-49540, JP-A-6-49541, JP-A-6-128641, JP-A-6-264
No. 143 and JP-A-6-264144 disclose a method of manufacturing a steel pipe in which the yield ratio, which is the ratio of yield stress to tensile strength, is reduced to improve buckling resistance.

Further, Japanese Patent Application Laid-Open Nos. 9-196243, 9-196244 and 9-316599.
In the publication, buckling during compression or bending deformation of a line pipe is controlled by controlling a gradient of a stress-strain curve, a work hardening index (n value), and a yield ratio (YR) obtained by dividing a yield strength by a tensile strength. Methods for improving performance are disclosed.

[0006]

However, JP-A-3-173719, JP-A-5-65535, JP-A-5-117746, and JP-A-5-117.
747, JP-A-5-156357, JP-A-6-49540, JP-A-6-49541,
JP-A-6-128541, JP-A-6-26414
No. 3 and Japanese Patent Application Laid-Open No. 6-264144 each relate to the ability of a column to absorb plastic deformation against bending stress, and studies are being conducted to prevent the occurrence of local buckling due to the axial force of compression. Not done.

Further, Japanese Patent Application Laid-Open Nos. 9-196243, 9-196244 and 9-316599.
In any of the techniques disclosed in Japanese Patent Application Laid-Open Publication No. H11-107, the strain range in the stress-strain curve is not clear.

As described above, the prior art does not show a method of controlling the stress-strain relationship by focusing on a low strain region where the line pipe buckles. Furthermore, there is no disclosure of a technique for controlling characteristics in the circumferential direction and the longitudinal direction of the steel pipe to improve buckling resistance.

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems, and is not liable to cause local buckling against a compressive or bending stress acting in an axial direction at the time of a large earthquake or laying a submarine pipe. An object of the present invention is to provide a steel pipe excellent in buckling resistance suitable for a water pipe, a pillar for construction and civil engineering, and the like, a manufacturing method thereof, and an evaluation method thereof.

[0010]

Means for Solving the Problems In order to achieve the above object, the present inventors have intensively studied the buckling performance. As a result, prevention of local buckling due to compression is of utmost importance in improving buckling resistance, and local buckling due to this compression involves yield strength and nominal strain in the axial direction of the steel pipe:
Stress ratio (σ _rL ), which is the ratio to the stress at 1.5% (σ _1.5 ), or the ratio of the yield strength in the circumferential direction of the steel pipe to the nominal strain: stress at 1.5% (σ _1.5 ). It has been found that controlling the stress ratio (σ _rC ) to an appropriate value is effective.

The present invention has been made based on such findings, and has the following configuration.

[1] In a tensile test in the axial direction of a steel pipe, a stress ratio (σ _rL ) which is a ratio of a yield strength to a stress (σ _1.5 ) at 1.5% of nominal strain is 1.05 or more. A steel pipe with excellent buckling resistance characterized by the following.

[2] In the tensile test in the circumferential direction of the steel pipe, the stress ratio (σ _rC ), which is the ratio between the yield strength and the stress (σ _1.5 ) at 1.5% of nominal strain, and the tensile test in the axial direction of the steel pipe A steel pipe excellent in buckling resistance, wherein a stress ratio (σ _rL ) which is a ratio of a yield strength and a stress at 1.5% (σ _1.5 ) at each time is 1.05 or more.

[3] In the above [1] or [2], mas
s%, C: 0.03-0.1%, Si: 0.05-
0.5%, Mn: 1-2%, the balance being substantially F
consist e, and P _CM represented by the following formula (1) is 0.1
A steel pipe excellent in buckling resistance, having a component composition satisfying 0 to 0.20. P _CM = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5 × B (1) [4] In the above [1] to [3], mass%
, Cu: 0.05-0.5%, Ni: 0.05-0.
5%, Cr: 0.05 to 0.5%, and Mo: 0.05
A steel pipe excellent in buckling resistance, comprising one or more selected from the group of ~ 0.5%.

[5] In the above [1] to [4], further, in mass%, Nb: 0.005 to 0.1%, V:
0.005 to 0.1%, and Ti: 0.005 to 0.0
A steel pipe excellent in buckling resistance, comprising one or more selected from the group of 8%.

[6] In the above items [1] to [5], Ca: 0.0005 to 0.003
%, Mg: one or two selected from the group of 0.0005 to 0.003%.

[7] In mass%, C: 0.03 to 0.1
% Si: 0.05 to 0.5% Mn: it contains 1-2%, the balance being substantially Fe, and the P _CM represented by the following formula (1) from 0.10 to 0. After hot rolling a steel satisfying No. 20, a cooling rate to an arbitrary temperature of 500 ° C. or less:
The steel sheet obtained by accelerated cooling at 5 ° C./sec or more is formed into a tube by cold forming, whereby the yield strength in the tube axis direction and the nominal strain: stress at 1.5% (σ _1.5 )
A method for producing a steel pipe having excellent buckling resistance, characterized by obtaining a steel pipe having a stress ratio (σ _rL ) that is 1.05 or more. P _CM = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5 × B (1) [8] Mass%, C: 0.03-0.1%, Si:
0.05 to 0.5%, Mn: 1 to 2%, the balance substantially consisting of Fe, and P represented by the following formula (1)
After hot rolling a steel satisfying _CM of 0.10 to 0.20, the cooling rate to an arbitrary temperature of 500 ° C. or less: 5 ° C./s
The steel sheet obtained by accelerated cooling at ec or more is formed by cold forming at a pipe expansion ratio of 1.4% or less, whereby the yield strength and nominal strain in the steel pipe circumferential direction and pipe axis direction are 1.5%. A method for producing a steel pipe having excellent buckling resistance, characterized by obtaining a steel pipe having a stress ratio (σ _rC , σ _rL ) which is a ratio with _{respect to} a stress at the time (σ _1.5 ) of 1.05 or more. P _CM = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5 × B (1) [9] In the above [7] or [8], mass%
, Cu: 0.05-0.5%, Ni: 0.05-0.
5%, Cr: 0.05 to 0.5%, and Mo: 0.05
A method for producing a steel pipe excellent in buckling resistance, characterized by using steel containing one or more selected from the group of 0.5% to 0.5%.

[10] In the above-mentioned [7] to [9], Nb: 0.005 to 0.1%, V:
0.005 to 0.1%, and Ti: 0.005 to 0.0
A method for producing a steel pipe having excellent buckling resistance, characterized by using a steel containing one or more selected from the group of 8%.

[11] In the above items [7] to [10], further, by mass%, Ca: 0.0005 to 0.003
%, Mg: 0.0005 to 0.003%. A method for producing a steel pipe having excellent buckling resistance, characterized by using steel containing one or two kinds selected from the group of 0.0005 to 0.003%.

[12] Using a stress ratio (σ _rL ) which is a ratio of a yield strength and a nominal strain: stress (σ _1.5 ) at 1.5% in a tensile test in the axial direction of the steel pipe, the seat resistance of the steel pipe is determined. A method for evaluating buckling resistance of a steel pipe, characterized by evaluating buckling property.

[13] Stress ratio (σ _rC ) which is a ratio of yield strength and nominal strain: stress (σ _1.5 ) at 1.5% in a tensile test in the circumferential direction of the steel pipe, and a tensile test in the axial direction of the steel pipe. Strength and nominal strain at 1.5% stress at 1.5% (σ _1.5 )
A method for evaluating buckling resistance of a steel pipe, wherein the buckling resistance of the steel pipe is evaluated using a stress ratio (σ _rL ) which is a ratio of the buckling resistance to the steel pipe.

[14] In the above _item [12] or [13], the stress ratio in the steel pipe axial direction (σ _{r -L} ) or the stress ratio in the steel pipe circumferential direction (σ _rC ) is 1.05 or more. Characteristic method for evaluating buckling resistance of steel pipes.

In these means, "substantially Fe" means, unless the effects of the present invention are lost.
It means that those containing other trace elements including unavoidable impurities can be included in the scope of the present invention.

Further, in this specification, all percentages indicating components of steel are mass%.

[0025]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The details of the present invention and the reasons for limiting the same will be described below.

First, the reason for limiting the stress ratio (σ _r ) will be described.

In the present invention, the stress ratio (σ_r) And descend
Yield strength and nominal strain: stress at 1.5% (σ_1.5) And ratio
And the stress ratio (σ_r) Is greater than 1.05
Or is the buckling resistance at the time of applying bending strain improved?
From the stress ratio (σ_r) Is 1.05 or more, which is the present invention.
Is the most important requirement. Further, the stress ratio (σ
_r) Is the stress ratio in the pipe axis direction (σ_rL) Only 1.05 or more
Ratio and stress ratio in steel pipe circumferential direction and pipe axis direction
(Σ_rC, Σ_rL) Is 1.05 or more.
Stipulate.

By defining the stress ratio (σ _rL ) in the tube axis direction, local buckling due to compression can be prevented.
Furthermore, by defining the stress ratio (σ _rC ) in the circumferential direction of the steel pipe, it is possible to impart bending strain and prevent local buckling at the time of external pressure. Therefore, for example, when used for land-based seismic line pipes and the like, when the steel pipe of the present invention that defines only the stress ratio (σ _rL ) in the pipe axis direction is used for submarine pipes that are subjected to bending and external pressure during laying, the steel pipe circumference is used. It is preferable to appropriately use the steel pipe according to the situation at the time of use, such as using the steel pipe of the present invention in which the stress ratio in the direction and the pipe axis direction (σ _rC , σ _rL ) is specified.

Next, the reasons for limiting the component composition range will be described.

C: When C is added in less than 0.03% or more than 0.1%, a stress ratio (σ _r ) of 1.05 or more cannot be obtained stably, and the weldability also deteriorates. I do. Therefore, in the present invention, the C content is 0.03% to 0.1%.
1%.

Si: In order to obtain sufficient strength and toughness as structural steel, it is necessary to add 0.05% or more. On the other hand, if added over 0.5%, the weldability will be degraded. Therefore, in the present invention, the amount of Si is 0.05% to
0.5%.

Mn: In order to obtain sufficient strength and toughness as a structural steel, 1% or more must be added. Meanwhile, 2
%, The weldability is degraded. Therefore, in the present invention, the amount of Mn is set to 1 to 2%.

Cu, Ni, Cr, Mo: an element effective for increasing the strength, one or more selected from the group consisting of Cu, Ni, Cr, and Mo can be added. However, in order to increase the strength, each of the elements has a content of 0.1.
Addition of not less than 0.5% is required, while addition of more than 0.5% deteriorates the toughness and weldability of the base material and the welded portion of the steel pipe. Therefore, in the present invention, Cu, Ni, C
When one or two or more selected from the group of r and Mo are added, it is preferable to add each component at 0.05 to 0.5%.

Nb, V, Ti: elements that are effective for improving the toughness and strength of the steel sheet or for preventing damage to the slab during casting, and one or two elements selected from the group consisting of Nb, V, and Ti.
More than one species can be added. However, in order to improve the toughness and strength of the steel sheet, Nb: 0.005% or more,
V: 0.005% or more must be added. To improve the toughness of the steel sheet and to prevent slab damage during casting, Ti: 0.0
More than 05% is required. On the other hand, Nb: exceeds 0.1%,
Addition of V: more than 0.1% and Ti: more than 0.08% deteriorates weldability and toughness of a welded portion. Therefore, in the present invention, when one or more kinds selected from the group of Nb, V, and Ti are added, each component is Nb: 0.005 to 0.005.
0.1%, V: 0.005 to 0.1%, Ti: 0.00
It is preferable to add 5 to 0.08%.

Ca, Mg: Elements indispensable for controlling the form of sulfide inclusions, and one or two selected from the group consisting of Ca and Mg can be added. However, each element is 0.0005 in order to control the morphology of sulfide inclusions.
%, On the other hand, if it exceeds 0.003%, the effect is saturated, and conversely, the cleanliness is reduced and the buckling resistance is deteriorated. Therefore, in the present invention, when one or two kinds selected from the group of Ca and Mg are added, it is preferable to add each component at 0.0005 to 0.003%.

[0036] P _CM: In the present invention, the P _CM C + Si
/ 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr /
20 + Mo / 15 + V / 10 + 5 × B, which is an index having a good correlation with the strength and toughness of steel. In order to obtain sufficient strength as a structure and to obtain good buckling resistance, 0.10 or more is required. On the other hand, when it exceeds 0.20, buckling resistance and weldability are deteriorated. Therefore, in the present invention, P _CM is set to 0.10 to 0.20.

Next, the reasons for limiting the steel sheet manufacturing process will be described.

The steel sheet used for the steel pipe of the present invention is obtained by heating steel having the above-mentioned components, for example, at a heating temperature of 1050 ° C. to 12 ° C.
It is obtained by hot rolling at 50 ° C. and then accelerated cooling to an arbitrary temperature of 500 ° C. or less at a cooling rate of 5 ° C./sec or more. If the cooling stop temperature is higher than 500 ° C. or the cooling rate is lower than 5 ° C./sec, the stress ratio (σ _r ) of 1.05 or more cannot be stably obtained in any case, and the buckling resistance Excellent steel pipe cannot be obtained.

Next, the reasons for limiting the steel pipe manufacturing process will be described.

The steel pipe of the present invention can be obtained by cold forming the steel sheet obtained as described above at a pipe expansion ratio of 1.4% or less. If the pipe expansion ratio exceeds 1.4%, the stress ratio in the circumferential direction of the steel pipe decreases, and good buckling resistance cannot be obtained. Further, the cold forming method is not particularly limited, for example, UOE,
It is formed into a steel pipe by cold working with a bending roll, press bend, or the like.

As described above, it is possible to obtain a steel pipe which is less likely to cause local buckling and has excellent buckling resistance. The occurrence of local buckling due to the stress acting in the axial direction of the steel pipe and the brittle cracks and Breakage can be prevented, bending strain can be applied, and local buckling at the time of external pressure can be prevented.

[0042]

(Embodiment 1) The ratio (D / t) of the outer diameter to the tube thickness is 3
The stress ratio (σ _rL ) in the pipe axis direction and the buckling strain of the steel pipe were determined for a steel pipe _having a pipe _outer diameter of 600 mm changed from 5 to 60. The tensile test in the tube axis direction for _{obtaining the} stress ratio (σ _rL ) in the tube axis direction was performed using a full-thickness test piece having a parallel portion width of 38 mm and a gauge length of 50 mm. The buckling strain of the steel pipe was a value obtained by dividing a buckling strain obtained by a compression test of an actual steel pipe by a value obtained by an equation: εb (buckling strain) = 35 · t / D. Table 1 shows the performance evaluation results. Here, the evaluation of the buckling strain was 1.2 or better.

[0043]

[Table 1]

From Table 1, it can be seen that the stress ratio (σ
_rL ) falls within the scope of the present invention. In the cases of 1, 2, and 3, the buckling strain is 1.2 times or more, and it can be seen that a steel pipe excellent in buckling resistance is obtained.

On the other hand, the stress ratio in the pipe axis direction (σ _rL )
Is a steel pipe No. out of the scope of the present invention. In No. 4, the buckling strain was less than 1.2, and the buckling resistance was poor.

(Example 2) The stress ratio (σ) in the steel pipe circumferential direction and the pipe axis direction was applied to a steel pipe having a pipe outer diameter of 600 mm in which the ratio of the outer diameter to the pipe thickness (D / t) was changed from 35 to 60.
_rC , σ _rL ) and the buckling strain of the steel pipe were determined. The tensile test in the circumferential direction of the pipe in determining the stress ratio ( _{σr - C} , _σrL ) in the circumferential direction of the steel pipe and in the axial direction of the pipe was performed using a straight pipe having a diameter of 6 mm taken from the circumferential direction of the uncorrected steel pipe.
Using a round bar test piece with φ, gauge length of 25 mm, the tensile test in the tube axis direction was parallel part width 38 mm, gauge length 50 mm
Was carried out by using a full thickness test piece. The buckling strain of the steel pipe was defined as a value obtained by dividing a buckling strain obtained by a compression test (with external pressure) of an actual steel pipe by a value obtained by an equation: εb (buckling strain) = 35 · t / D. . Table 2 shows the performance evaluation results.
Here, the evaluation of the buckling strain was 1.2 or better.

[0047]

[Table 2]

From Table 2, it can be _seen that the stress ratio (σ _rC , σ _rL ) in the circumferential direction and the axial direction of the steel pipe is within the range of the present invention. 5, 6, and 7, it can be seen that a buckling strain of 1.2 times or more and a steel pipe excellent in buckling resistance were obtained.

On the other hand, the steel pipe No. _whose stress ratio (σ _rC , σ _rL ) in the circumferential direction or the axial direction of the steel pipe is out of the range of the present invention. In Examples 8 and 9, the buckling strain was as low as less than 1.2, and the buckling resistance was poor.

(Example 3) A steel pipe having a pipe outer diameter of 600 mm having the components shown in Table 3 and having an outer diameter to pipe thickness ratio (D / t) of 35 to 60 was used. The stress ratio (σ _rL ) and the buckling strain of the steel pipe were determined. In determining the stress ratio (σ _rL ) in the tube axis direction, a tensile test in the tube axis direction was performed using a full-thickness test piece having a parallel portion width of 38 mm and a gauge length of 50 mm. The buckling strain of the steel pipe is
The buckling strain obtained by the compression test of an actual steel pipe is expressed by the following equation:
(Buckling strain) = A value obtained by dividing by a value obtained at 35 · t / D. Table 4 shows the performance evaluation results. Here, the evaluation of the buckling strain was 1.2 or better. The weldability was checked by checking for the presence of weld cracks in the vertical seam weld after forming the steel pipe.

[0051]

[Table 3]

[0052]

[Table 4]

From Table 4, it can be seen that the stress ratio (σ
_rL ) falls within the scope of the present invention. 10, 14, 1
5 and 16, it can be seen that a steel pipe having a buckling strain of 1.2 times or more, excellent weldability, and excellent buckling resistance was obtained.

On the other hand, the stress ratio (σ _rL ) in the pipe axis direction
Is a steel pipe No. out of the scope of the present invention. In 11, 12, and 13, the buckling strain was as low as less than 1.2, and the buckling resistance was poor. In particular, steel pipe No. In Nos. 12 and 13, the weldability is also poor.

Example 4 Steel having the components shown in Table 5 was heated.
Hot rolling, then accelerated cooling under the conditions shown in Table 5,
The obtained steel sheet was subjected to cold working to obtain a steel pipe having a pipe outer diameter of 600 mm. At this time, the ratio (D / t) between the outer diameter and the tube thickness was changed from 35 to 60.

[0056]

[Table 5]

With respect to the obtained steel pipe, the stress ratio (σ _rL ) in the pipe axis direction and the buckling strain of the steel pipe were determined. In _{obtaining the} stress ratio (σ _rL ) in the pipe axis direction, a tensile test in the pipe axis direction was performed with a parallel part width of 38 mm and a gauge length of 50 mm.
Was carried out by using a full thickness test piece. The buckling strain of a steel pipe is calculated by using the buckling strain obtained by a compression test of an actual steel pipe as follows:
εb (buckling strain) = 35 · t / D divided by the value obtained. Table 6 shows the performance evaluation results together with the manufacturing conditions. Here, the evaluation of the buckling strain was 1.2 or better. The weldability was checked by checking for the presence of weld cracks in the vertical seam weld after forming the steel pipe.
Was determined to have weld cracks.

[0058]

[Table 6]

From Table 6, it can be seen that the steel pipe No. having the accelerated cooling stop temperature and the cooling rate falling within the range of the present invention. In 17, 20, and 21, a steel pipe having a stress ratio (σ _rL ) in the pipe axis direction of 1.05 or more, a buckling strain of 1.2 times or more, excellent weldability, and excellent buckling resistance was obtained. You can see that it is done.

On the other hand, in the case of steel pipe No. whose accelerated cooling stop temperature and cooling rate are outside the range of the present invention. In Nos. 18 and 19, the stress ratio (σ _rL ) in the tube axis direction was less than 1.05, the buckling strain was as low as less than 1.2, and the buckling resistance was poor.

(Example 5) Steel having the components shown in Table 6 was heated,
Hot rolling, then accelerated cooling under the conditions shown in Table 7,
The obtained steel plate was subjected to cold working at a pipe expansion ratio shown in Table 8 to obtain a steel pipe having a pipe outer diameter of 600 mm. At this time, the ratio (D / t) between the outer diameter and the tube thickness was changed from 35 to 60.

[0062]

[Table 7]

With respect to the obtained steel pipe, the stress ratio (σ _rC , σ _rL ) in the circumferential direction of the steel pipe and in the pipe axis direction, and the buckling strain of the steel pipe were determined. The tensile test in the circumferential direction of the pipe for determining the stress ratio (σ _rC , σ _rL ) in the circumferential direction of the steel pipe and in the axial direction of the pipe was performed using a straight section having a diameter of 6 mmφ and a gauge point taken from the circumferential direction of the uncorrected steel pipe. Using a round bar test piece with a distance of 25 mm, the tensile test in the tube axis direction was 38 m in parallel part width.
m, a test piece with a total thickness of 50 mm between gauge points.
The buckling strain of the steel pipe was defined as a value obtained by dividing a buckling strain obtained by a compression test (with external pressure) of an actual steel pipe by a value obtained by an equation: εb (buckling strain) = 35 · t / D. . Table 8 shows the performance evaluation results together with the manufacturing conditions. here,
The evaluation of buckling strain was 1.2 or better. The weldability was checked by checking for the presence of weld cracks in the vertical seam weld after forming the steel pipe.

[0064]

[Table 8]

From Table 8, it can be seen that the steel pipe No. having the accelerated cooling stop temperature, the cooling rate and the pipe expansion ratio within the range of the present invention. 22, 27,
In _Nos . 28 and 29, the stress ratio (σ _rC , σ _rL ) in the steel pipe circumferential direction and pipe axis direction is 1.05 or more, the buckling strain is 1.2 times or more, and the weldability is excellent and the buckling resistance is excellent. It can be seen that an excellent steel pipe has been obtained.

On the other hand, in the case of the steel pipe No. having the accelerated cooling stop temperature, the cooling rate and the expansion ratio outside the range of the present invention. 23, 24, 2
In _{Nos. 5} and ₂₆ , either the stress ratio (σ _rC , σ _rL ) in the circumferential direction of the steel pipe or in the pipe axis direction is less than 1.05, the buckling strain is less than 1.2, and the buckling resistance is low. Inferior.

[0067]

As described above, according to the present invention, a steel pipe excellent in buckling resistance can be obtained. Since the steel pipe obtained by the present invention has excellent buckling resistance, it is most suitable as a steel pipe used for gas pipelines, water pipes, columns for construction and civil engineering, and the like.

Further, the steel pipe of the present invention is resistant to compressive and bending stresses acting in the axial direction when a large earthquake or submarine pipe is laid.
Since local buckling is unlikely to occur, it is possible to prevent disasters such as damage to gas pipelines and water pipes and the outflow of internal fluid when a large earthquake occurs, or collapse due to breakage of pier pillars on a highway.

 ──────────────────────────────────────────────────続 き Continued on front page (72) Inventor Nobuhisa Suzuki 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nihon Kokan Co., Ltd. (72) Inventor Takeshi Kondo 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Sun F-term (reference) in this steel pipe Co., Ltd.

Claims (14)

[Claims] In a tensile test in the axial direction of a steel tube, a stress ratio (σ _rL ) which is a ratio of a yield strength to a stress (σ _1.5 ) at 1.5% of nominal strain is 1.05 or more. A characteristic steel pipe with excellent buckling resistance. Wherein yield strength in a tensile test of the steel pipe circumferential direction and a nominal strain 1.5% when the stress (sigma _1.5) and which is the ratio stress ratio (sigma _rC) and the yield in the tensile test of the steel pipe tube axis direction A steel pipe having excellent buckling _{resistance, wherein} stress ratios (σ _rL ), which are ratios between strength and nominal strain: stress (σ _1.5 ) at 1.5%, are each 1.05 or more. 3. Mass%, C: 0.03 to 0.1
% Si: 0.05 to 0.5% Mn: it contains 1-2%, the balance being substantially Fe, and the P _CM represented by the following formula (1) from 0.10 to 0. The steel pipe having excellent buckling resistance according to claim 1 or 2, wherein the steel pipe has a component composition satisfying 20. P _CM = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5 × B (1) 4. Further, in mass%, Cu: 0.05
0.5%, Ni: 0.05 to 0.5%, Cr: 0.0
The buckling resistance according to any one of claims 1 to 3, wherein the buckling resistance is one or more selected from the group of 5 to 0.5% and Mo: 0.05 to 0.5%. Excellent steel pipe. 5. The method according to claim 1, wherein the mass% is Nb: 0.005%.
5 to 0.1%, V: 0.005 to 0.1%, and Ti:
One or two selected from the group of 0.005 to 0.08%
The steel pipe excellent in buckling resistance according to claim 1, wherein the steel pipe contains at least one kind. 6. Further, in mass%, Ca: 0.00
0.05 to 0.003%, Mg: 0.0005 to 0.003
The steel pipe excellent in buckling resistance according to claim 1, wherein the steel pipe contains one or two selected from the group of%. 7. Mass%, C: 0.03 to 0.1
% Si: 0.05 to 0.5% Mn: it contains 1-2%, the balance being substantially Fe, and the P _CM represented by the following formula (1) from 0.10 to 0. After hot rolling a steel satisfying No. 20, a cooling rate to an arbitrary temperature of 500 ° C. or less:
The steel sheet obtained by accelerated cooling at 5 ° C./sec or more is formed into a tube by cold forming, whereby the yield strength in the tube axis direction and the nominal strain: stress at 1.5% (σ _1.5 )
A method for producing a steel pipe having excellent buckling resistance, characterized by obtaining a steel pipe having a stress ratio (σ _rL ) that is 1.05 or more. P _CM = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5 × B (1) 8. Mass%, C: 0.03 to 0.1
% Si: 0.05 to 0.5% Mn: it contains 1-2%, the balance being substantially Fe, and the P _CM represented by the following formula (1) from 0.10 to 0. After hot rolling a steel satisfying No. 20, a cooling rate to an arbitrary temperature of 500 ° C. or less:
A steel sheet obtained by accelerated cooling at 5 ° C./sec or more is subjected to cold forming at a pipe expansion ratio of 1.4% or less to yield yield strength and nominal distortion in the circumferential direction of the steel pipe and in the pipe axis direction: A method for producing a steel pipe having excellent buckling resistance, wherein a steel pipe having a stress ratio (σ _rC , σ _rL ) which is a ratio to a stress (σ _1.5 ) at _0.5% is 1.05 or more. . P _CM = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5 × B (1) 9. Further, in mass%, Cu: 0.05
0.5%, Ni: 0.05 to 0.5%, Cr: 0.0
The steel according to claim 7 or 8, wherein a steel containing one or more selected from the group of 5 to 0.5% and Mo: 0.05 to 0.5% is used. A method for manufacturing steel pipes with excellent buckling properties. 10. Further, in mass%, Nb: 0.0
05-0.1%, V: 0.005-0.1%, and T
10. The steel pipe having excellent buckling resistance according to claim 7, wherein a steel containing one or more kinds selected from the group of i: 0.005 to 0.08% is used. Production method. 11. Further, in mass%, Ca: 0.0
005 to 0.003%, Mg: 0.0005 to 0.00
11. The method for producing a steel pipe having excellent buckling resistance according to claim 7, wherein steel containing one or two selected from the group of 3% is used. 12. The buckling resistance of a steel pipe is determined by using a stress ratio (σ _rL ) which is a ratio of a yield strength and a nominal strain: stress (σ _1.5 ) at 1.5% in a tensile test in an axial direction of the pipe. A method for evaluating buckling resistance of a steel pipe, characterized by evaluating the buckling resistance. 13. A stress ratio (σ _rC ) which is a ratio between a yield strength and a nominal strain: stress at 1.5% (σ _1.5 ) in a tensile test in the circumferential direction of a steel pipe, and a tensile test in an axial direction of the steel pipe. The buckling resistance of a steel pipe is evaluated using a stress ratio (σ _rL ) which is a ratio of a yield strength and a nominal strain: stress (σ _1.5 ) at 1.5%. Buckling evaluation method. 14. A stress ratio (σ _rL ) in the axial direction of the steel pipe or a stress ratio (σ _rL ) in the circumferential direction of the steel pipe.
_{2. The method according} to claim 1, wherein _rC ) is 1.05 or more.
14. The method for evaluating buckling resistance of a steel pipe according to 2 or 13.