JP2002206140A - Steel tube and production method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel tube which has an excellent buckling resisting performance and a high internal pressure fracture performance, and a production method therefor. SOLUTION: In the steel tube, provided that the yield strength and tensile strength in the circumferential direction of the tube are respectively defined as YSC and TSC, and the yield strength and tensile strength in the axial direction of the tube respectively as YSL and TSL, YSL/YSC<=0.9 and the yield ratio in the axial direction of the tube: YSL/TSL<=85% are satisfied. The steel tube is obtained by subjecting a steel containing, by mass, 0.03 to 0.15% C, 0.01 to 1% Si and 0.5 to 2.0% Mn to hot rolling, thereafter cooling the steel at the cooling starting temperature of (Ar3+40) to (Ar3-80) deg.C at the cooling rate of >=2 deg.C/sec to obtain a steel sheet, and subjecting the steel sheet to cold forming at the tube expansion rate of >=0.4% in the final stage of tube making.

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 for a fluid transport pipe such as a gas pipeline or a water pipe, a buried pipe for storage, and the like.

[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, many steel pipes used for the above applications have a large outer diameter / pipe thickness ratio, and when a large earthquake occurs, large tensile and compressive forces are repeatedly applied in the longitudinal direction of the steel pipe, resulting in local buckling. Raising and cracking due to buckling may result in fracture.

Conventionally, measures against local buckling due to the axial force of compression have been proposed in, for example, Japanese Patent Application Laid-Open No. 3-173719.
JP, JP-A-5-65535, JP-A-5-11
7746, JP-A-5-117747, JP-A-5-156357, JP-A-6-49540, JP-A-6-49541, JP-A-6-2641
No. 43, JP-A-6-264144 and the like propose a low yield ratio steel pipe as an earthquake-resistant steel pipe, which lowers the yield ratio, which is the ratio of yield stress to tensile strength, and utilizes energy absorption by plastic deformation. However, the above-mentioned earthquake-resistant steel pipe was inferior in buckling resistance.

For the purpose of solving the above problems, Japanese Patent Application Laid-Open Nos. 9-196244, 10-52713 and 11-6032 disclose a stress-strain curve obtained by a tensile test in the tube axis direction. A steel pipe having excellent buckling resistance and excellent earthquake resistance by increasing the work hardening index (n value) of the steel pipe is disclosed.

[0005]

However, the techniques described in JP-A-9-196244, JP-A-10-52713, and JP-A-11-6032 take into consideration the strength in the circumferential direction of the tube. Therefore, it cannot be said that the internal pressure breaking resistance is sufficient.

In recent years, in line pipes for transporting fluids such as gas, there has been an increasing demand for higher transport efficiency due to a higher pressure, a reduction in construction cost or an improvement in storage efficiency, etc. The development of high-strength steel pipes with internal pressure fracture performance is being actively pursued. But,
Such high-strength steel pipes generally have a high yield ratio and little consideration is given to buckling resistance properties. Therefore, there is a concern that buckling may occur due to bending stress during embedding.

As described above, a steel pipe having both high buckling resistance and excellent internal pressure fracture resistance has not yet been proposed.

The present invention has been made in view of the above-mentioned problems, and it is hard to cause local buckling against a compressive stress or a bending stress generated at the time of laying a pipeline due to ground deformation or the like due to a large earthquake. An object of the present invention is to provide a steel pipe having high performance and high internal pressure fracture performance, and a method for manufacturing the same.

[0009]

Means for Solving the Problems In order to achieve the above object, the present inventors have intensively studied the buckling resistance of a steel pipe against a compressive axial force. As a result, it was found that by controlling the tensile properties in the axial direction and the circumferential direction of the steel pipe, respectively, the buckling resistance and the internal pressure fracture resistance are greatly improved. In other words, the yield strength in the circumferential direction of the pipe is higher than that in the pipe axis direction, and the yield ratio in the pipe axis direction is suppressed to a certain value or less. Was found to improve.

[0010] The present invention has been made based on such knowledge, and has the following configuration.

[0011] [1] tube circumferential direction of the yield strength and tensile strength, respectively YS _C and TS _C, in the tube axis direction yield strength and tensile strength when formed into a YS _L and TS _L respectively, YS _L / Y
S _C ≦ 0.9 and the yield ratio in the tube axis direction: YS _L / TS _L ≦
A steel pipe characterized by satisfying 85%.

[2] In the above [1], mass%
C: 0.03 to 0.15%, Si: 0.01 to 1%, M
n: a steel pipe containing 0.5 to 2.0%.

[3] In the above [2], mas is further added.
s%, Cu: 0.05-0.5%, Ni: 0.05-
0.5%, Cr: 0.05 to 0.5%, Mo: 0.05
0.5%, Nb: 0.005 to 0.1%, V: 0.0
A steel pipe containing one or two or more selected from the group of 0.05 to 0.1% and Ti: 0.005 to 0.1%.

[4] In mass%, C: 0.03 to 0.1
5%, Si: 0.01-1%, Mn: 0.5-2.0%
After hot-rolling a steel containing, the cooling start temperature: (Ar
3 + 40) to (Ar3-80) ° C, cooling rate: 2 ° C / s
The steel sheet obtained by cooling at a temperature of at least ec is cold-formed at a pipe expansion ratio of 0.4% or more in the final pipe-forming step to form a pipe, so that YS _L / YS _C ≦ 0.9 and the pipe axis. A method for producing a steel pipe, comprising obtaining a steel pipe satisfying a yield ratio in the direction: YS _L / TS _L ≦ 85%. Here, YS _C : yield strength in the circumferential direction of the steel pipe, TS _C : tensile strength in the circumferential direction of the steel pipe, YS _L : yield strength in the axial direction of the pipe, TS _L : tensile strength in the axial direction of the pipe.

[5] Mass%, C: 0.03 to 0.1
5%, Si: 0.01-1%, Mn: 0.5-2.0%
 And Cu: 0.05-0.5%, Ni:
0.05-0.5%, Cr: 0.05-0.5%, M
o: 0.05-0.5%, Nb: 0.005-0.1
%, V: 0.005 to 0.1%, Ti: 0.005 to
Hot-pressing steel containing 0.1% or more of one or more
After extending, the cooling start temperature: (Ar3 + 40) to (Ar3 + 40)
-80) ° C, cooling rate: obtained by cooling at 2 ° C / sec or more
The expanded steel sheet in the final process of pipe making: 0.4%
By cold forming and pipe forming, YS_L/ YS_C
≦ 0.9 and the yield ratio in the tube axis direction: YS_L/ TS_L≦ 85
% Of steel pipes characterized by obtaining steel pipes satisfying%
Law. However, YS _C: Yield strength of steel pipe in circumferential direction, T
S_C: Tensile strength of steel pipe in the circumferential direction, YS_L: Downward in the pipe axis direction
Yield strength, TS_L: Tensile strength in the pipe axis direction.

In this specification, all the percentages indicating the components of steel are mass%.

[0017]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention provides anisotropy of mechanical properties in the pipe circumferential direction and the pipe axial direction, that is, increases the yield strength in the pipe circumferential direction as compared with the pipe axial direction, The essence of the invention is to significantly improve the buckling resistance against compression axial force and the internal pressure fracture resistance of the steel pipe by suppressing the yield ratio to a certain value or less. Hereinafter, details of the circumstances leading to the present invention and the reasons for the limitation will be described.

Local buckling (plastic buckling) of a steel pipe is caused by local concentration of plastic strain distribution in the pipe axis direction when an external force is applied. At this time, at the same time as the compressive deformation progresses in the tube axis direction, tensile deformation occurs in the tube circumferential direction. Here, if the pipe has a high yield strength in the circumferential direction of the pipe, the tensile deformation in the circumferential direction of the pipe is restricted, and thereby local concentration of compressive deformation in the axial direction of the pipe can be suppressed. The lower yield ratio in the tube axis direction facilitates the axial propagation of the plasticized region and further suppresses the local concentration of strain. I do.

In order to obtain such anisotropy of mechanical properties in the pipe circumferential direction and the pipe axial direction, hot rolling is performed using steel having a specific component, and the transformation determined by the steel component is performed. The metal structure can be obtained by performing accelerated cooling from a temperature near or below the temperature to make the metal structure a structure containing bainite or martensite. Furthermore, after forming the steel pipe by cold forming, the pipe is expanded at a specific expansion rate or more, thereby reducing the strength in the pipe axis direction due to the Bauschinger effect, and balancing the optimum strength characteristics in the pipe axis direction and pipe circumferential direction. Can be obtained.

On the other hand, it is important to increase the strength in the pipe axis direction against the internal pressure of the steel pipe. As described above, by increasing the yield strength in the pipe circumferential direction with respect to the pipe axis direction, it is possible to simultaneously improve buckling resistance and internal pressure fracture resistance,
Further, by specifying the composition of the steel and the method of manufacturing the steel sheet, a high Charpy absorbed energy value can be provided, and a steel pipe having a high resistance to unstable fracture can be obtained.

As described above, the steel pipe of the present invention has a YS _L / YS _C
≦ 0.9 and the yield ratio in the tube axis direction: YS _L / TS _L ≦ 85
%, Which are the most important requirements in the present invention. Hereinafter, this will be described in detail. _{Here, YS L, YS C, TS} L and TS _C are defined as follows.

YS_L: Yield strength in pipe axis direction, YS_C： Yield strength in pipe circumferential direction TS_L: Tensile strength in the pipe axis direction, TS_C: Tensile strength in pipe circumferential direction In the present invention, yield strength in pipe axis direction and yield strength in pipe circumferential direction
YS indicating the ratio of intensity _L/ YS_CIs set to 0.9 or less. Book
The feature of the invention is that by suppressing plastic deformation in the circumferential direction of the pipe,
Facilitates the propagation of the plastic zone in the direction and suppresses local deformation
It's in the matter, YS _L/ YS_CIs greater than 0.9,
Local compression change in the tube axis direction due to insufficient effect
Concentration of shapes tends to occur, and buckling strain is reduced.

In the present invention, the yield ratio in the tube axis direction: YS
_L / TS _L shall be 85% or less. Yield ratio in tube axis direction: YS
Promote the propagation in the axial direction of the plasticized region in the tube axis direction by reducing the _L / TS _L, by preventing local concentration of compressive deformation of the tube axis direction, is improved buckling resistance ability . However, if the yield ratio in the tube axis direction: YS _L / TS _L exceeds 85%, the effect cannot be obtained sufficiently, so that local strain is likely to be concentrated in the tube axis direction, and buckling strain is reduced. I do.

The ratio (TS _L / TS _C ) of the tensile strength in the pipe axis direction and the pipe circumferential direction is 0.9 ≦ TS _L / T
It is desirable to satisfy S _C ≦ 1.1. When a compressive or bending load is applied, the buckling initiation strain of a steel pipe is at most several percent, but if it undergoes a large deformation that exceeds the buckling initiation strain, the tensile strength in the pipe axis direction and in the pipe circumferential direction decreases. If the difference is large,
The anisotropy of the plastic flow is accelerated, cracks are easily generated in a specific direction, and there is a danger of brittle fracture when receiving a load in the opposite direction due to ground vibration. The ratio of the tensile strength in the pipe axis direction to the tensile strength in the pipe circumferential direction is 0.9 ≦ TS _L /
If TS _C ≦ 1.1, the above problem can be solved.

In the tensile test in the circumferential direction of the pipe, generally, a full-thickness test piece having a curvature cut out of a steel pipe is made into a flat plate by press straightening. Accurate circumferential material properties cannot be determined. Therefore, the tensile properties in the circumferential direction of the pipe used in the present invention are to be evaluated by using a round bar specimen cut out from an uncorrected steel pipe. In addition, since correction is not normally performed in the tube axis direction, either a full thickness test piece or a round bar test piece may be used.

Next, the reasons for limiting the steel composition of the steel sheet will be described.

C is an element necessary for ensuring the strength of the steel sheet and promoting the formation of bainite. If it is less than 0.03%, the strength is insufficient and bainite transformation hardly occurs. Also,
If it is added in excess of 0.15%, not only the yield ratio in the tube axis direction: YS _L / TS _{L of} 85% or less is not obtained, but also the weldability is reduced and the Charpy absorbed energy is reduced. Therefore, in the present invention, C is preferably set to 0.03 to 0.15%.

Si is used as a deoxidizing agent in the steel making process.
And it is added to increase the strength. If it is less than 0.01%, the effect is not sufficient. On the other hand, if added in excess of 1%, the toughness of the weld is degraded. Therefore, in the present invention, S
i is preferably set to 0.01 to 1%.

Mn is added to increase the strength.
If it is less than 0.5%, the strength will be insufficient, and if it exceeds 2.0%, the toughness and weldability of the base material and the weld will deteriorate. Therefore, in the present invention, Mn is preferably set to 0.5 to 2.0%.

Cu, Ni, Cr and Mo are effective elements for improving the hardenability. Therefore, when increasing the strength of steel, one or more of these can be added. If each steel component is less than 0.05%, there is no effect.
If it exceeds 5%, the weldability deteriorates. Therefore, in the present invention, when one or more kinds selected from the group consisting of Cu, Ni, Cr, and Mo are contained, each steel component is set to 0.0
It is preferable to add 5 to 0.5%.

Nb, V, and Ti are effective elements for precipitation strengthening, and therefore, when increasing the toughness and strength of steel,
One or more of these can be added. Less than 0.005% for each steel component has no effect, 0.1%
If it exceeds, the toughness of the welded portion will deteriorate. Therefore, in the present invention, when one or two or more selected from the group of Nb, V, and Ti are contained, each steel component is 0.005 to 0.005.
It is preferable to add at 0.1%.

Al added as a deoxidizing element is
May be included as needed. In addition, it is better that impurities such as P, S, O, and N are small. Elements such as Ca and REM for controlling inclusions may be included because they do not particularly impair the object of the present invention. Also,
B may be contained in a range not exceeding 0.001%.

Next, a method of manufacturing a steel pipe according to the present invention will be described.

In the steel pipe of the present invention, steel having the above-mentioned components is formed into a steel sheet by hot rolling, and then formed into a steel pipe by cold forming. At this time, in order to obtain a steel pipe having excellent buckling resistance and high internal pressure fracture performance, the cooling start temperature after hot rolling, the cooling rate, and the expansion ratio in the final pipe forming process are as follows. It is prescribed as follows.

In the present invention, the cooling start temperature after hot rolling is (Ar3 + 40) to (Ar3-80) ° C. Here, the Ar3 temperature is determined by the following equation: Ar3 (° C.) = 910−
310C (%)-80Mn (%)-20Cu (%)-15Cr
(%)-55Ni (%)-80Mo (%). If the cooling start temperature exceeds (Ar3 + 40) ° C., a yield ratio in the tube axis direction: YS _L / TS _{L of} 85% or less cannot be obtained. On the other hand, (Ar
If the temperature is lower than 3-80) ° C., not only the Charpy absorbed energy is reduced, but also the production efficiency is significantly reduced.

In the present invention, the cooling rate after hot rolling is 2 ° C.
/ Sec or more. Here, the cooling rate indicates an average cooling rate from the start of cooling to 500 ° C. Cooling rate 2 ℃ /
If it is less than sec, the transformation is insufficient and high strength cannot be obtained, and further, a yield ratio in the tube axis direction: YS _L / TS _{L of} 85% or less cannot be obtained. Since the metal structure after cooling may be a structure containing bainite or martensite, the upper limit of the cooling rate does not need to be particularly defined.

In the method of the present invention, pipe expansion is performed as a final step of pipe formation, and the expansion rate at this time is set to 0.4% or more. If the expansion ratio in the final pipe forming step is less than 0.4%, a yield ratio in the pipe axis direction of YS _L / TS _L of 85% or less cannot be obtained, and the difference in yield strength between the pipe axis direction and the pipe circumferential direction is not good. It is enough. Although there is no particular upper limit on the pipe expansion ratio, it is not preferable to give excessive deformation to the seam welded portion of the steel pipe, and the yield strength in the pipe axis direction is too low due to the Bauschinger effect.
% Is preferable.

[0038]

Examples of the present invention will be described below. A steel having the composition shown in Table 1 was heated to 1100 ° C., and 50% in a non-recrystallization temperature range.
After applying the above reduction, accelerated cooling was performed under the conditions shown in Table 2 to obtain a steel plate having a thickness of 15 mm. Then, after a steel pipe having an outer diameter of 610 mm was formed by the UOE process, the pipe was expanded under the conditions shown in Table 2.

[0039]

[Table 1]

[0040]

[Table 2]

The tensile properties in the circumferential direction of the pipe were measured using a round bar test piece having a parallel part diameter of 6 mmφ and a distance between gauge points of 25 mm taken from the circumferential direction of the uncorrected steel pipe, or a tensile property in the axial direction of the pipe. The evaluation was performed using a full-thickness test piece having a parallel portion width of 38 mm and a distance between gauge points of 50 mm. The Charpy absorbed energy at 0 ° C. was measured using a V-notch Charpy test piece taken from the circumferential direction of the steel pipe.

In order to evaluate the buckling resistance, a buckling test was performed by compressing the axial force. In the buckling test, after a steel plate is welded to both ends of a steel pipe having a length of 1800 mm, a compression test is performed by a large-sized press test device, and a strain at which load reduction starts due to buckling (a reduction amount / total length) is defined as a buckling strain. evaluated. Table 2 shows the tensile properties, Charpy absorbed energy, and buckling strain of the steel pipe in the pipe circumferential direction and the pipe axis direction, together with the production conditions.

As shown in Table 2, Examples 1 to 18 of the present invention include:
In any case, since YS _L / YS _C is 0.9 or less and the yield ratio in the tube axis direction: YS _L / TS _L is 85% or less, the buckling strain is 1.0% or more and high. It has tube circumferential strength and Charpy absorbed energy, and it is clear that it has excellent buckling resistance and internal pressure fracture resistance.

On the other hand, No. Comparative examples 19 to 23 are YS
_{Since L} / YS _C or the yield ratio in the tube axis direction: YS _L / TS _L is out of the range of the present invention, the buckling resistance is inferior.

[0045]

According to the present invention, it is possible to provide a steel pipe which is excellent in buckling resistance against a large compressive load and has high internal pressure fracture resistance. Since the steel pipe obtained by the present invention has excellent buckling resistance and internal pressure fracture resistance, it is most suitable as a steel pipe used for fluid transportation or storage such as gas pipelines and water pipes.

Further, the steel pipe of the present invention has excellent buckling resistance performance against compressive stress or bending stress generated when laying a submarine line pipe, especially when a submarine line pipe is laid, such as during a large earthquake, and has a high internal pressure. Has destructive performance.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI theme coat ゛ (Reference) C22C 38/58 C22C 38/58

Claims (5)

[Claims] Yield strength of 1. A pipe circumferential direction and tensile strength, respectively YS _C and TS _C, in the tube axis direction yield strength and tensile strength when formed into a YS _L and TS _L respectively, YS _L / YS _C
≦ 0.9 and the yield ratio in the tube axis direction: YS _L / TS _L ≦ 85
% Steel pipe. 2. Mass%, C: 0.03 to 0.15
%, Si: 0.01 to 1%, and Mn: 0.5 to 2.0%. 3. The method according to claim 1, further comprising:
0.5%, Ni: 0.05 to 0.5%, Cr: 0.0
5 to 0.5%, Mo: 0.05 to 0.5%, Nb: 0.
005 to 0.1%, V: 0.005 to 0.1%, and T
The steel pipe according to claim 2, wherein the steel pipe contains one or more kinds selected from the group of i: 0.005 to 0.1%. 4. Mass%, C: 0.03 to 0.15
%, Si: 0.01 to 1%, and Mn: 0.5 to 2.0%, after hot rolling, a cooling start temperature: (Ar3
+40) to (Ar3-80) ° C, cooling rate: 2 ° C / sec
By subjecting the steel sheet obtained by cooling at a temperature of c or more to cold forming at a pipe expansion ratio of 0.4% or more in the final pipe-forming step and forming a pipe, YS _L / YS _C ≦ 0.9 and the pipe axis Yield ratio in direction:
A method for producing a steel pipe, wherein a steel pipe satisfying YS _L / TS _L ≦ 85% is obtained. Here, YS _C : yield strength in the circumferential direction of the steel pipe, TS _C : tensile strength in the circumferential direction of the steel pipe, Y
S _L : Yield strength in the pipe axis direction, TS _L : Tensile strength in the pipe axis direction. 5. Mass%, C: 0.03 to 0.15
%, Si: 0.01 to 1%, Mn: 0.5 to 2.0%
, And Cu: 0.05-0.5%, Ni: 0.
05 to 0.5%, Cr: 0.05 to 0.5%, Mo:
0.05-0.5%, Nb: 0.005-0.1%,
V: 0.005 to 0.1%, Ti: 0.005 to 0.1
% After hot-rolling a steel containing one or more of the following types: (Ar3 + 40) to (Ar3-8)
0) The cooling rate is 2 ° C./sec or more, and the steel sheet obtained is cooled at a pipe expansion ratio of 0.4% or more in the final pipe-forming step to form YS _L / YS _C ≦
0.9 and the yield ratio in the tube axis direction: YS _L / TS _L ≦ 85%
A method for producing a steel pipe, characterized by obtaining a steel pipe satisfying the following. Here, YS _C : yield strength in the circumferential direction of the steel pipe, T
S _C : tensile strength in the circumferential direction of the steel pipe, YS _L : yield strength in the axial direction of the pipe, TS _L : tensile strength in the axial direction of the pipe.