JP5292869B2 - High strength steel pipe excellent in internal pressure fracture resistance and method for producing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high strength steel pipe in which occurrence of fracture from a weld zone can be suppressed in a using form where high internal pressure is applied thereto, and burst fracture and unstable ductile fracture can be prevented. <P>SOLUTION: Disclosed is a steel pipe produced by an UOE pipe making process and having a base material tensile strength of &ge;600 MPa, wherein, the ratio between the tensile strength (TSw) of a seam weld metal and the base material tensile strength (TSb), [TSw/TSb] is &ge;0.95, the yield ratio in the peripheral direction of the pipe in the base material is &le;92%, and the difference between the hardness (HVs) of the surface layer part in the base material and the average hardness (HVm) of the central part in the base material, [HVs-HVm] is &le;30HV. By reducing the yield ratio of the base material and narrowing the difference in the hardness between the surface layer part and the inside in the base material, concentration of strain on a weld heat affected zone is suppressed, and the occurrence of fracture from the weld zone caused by internal pressure can be prevented. <P>COPYRIGHT: (C)2010,JPO&amp;INPIT

Description

  The present invention relates to a high-strength steel pipe suitable for fluid transportation pipes such as gas pipelines and water pipes, steel pipes for gas storage, and the like, and particularly to a high-strength steel pipe having high resistance to breakage due to internal pressure. .

UOE steel pipes are widely used as fluid transportation pipes such as gas pipelines and water pipes. Recently, in order to reduce the cost of transporting fluids using such transportation pipes, the demand for high pressure pipelines has increased. ing. When a ductile fracture that causes a ductile crack occurs from a weld defect of a line pipe, a scratch caused by an external factor, or a thinned part due to corrosion, a burst fracture or long-distance crack propagation ( (Unstable ductile fracture) may occur. In particular, when the pressure of the pipeline is increased, it is expected that the risk that the ductile fracture will progress to burst fracture or unstable ductile fracture and the steel pipe will be destroyed increases.
For the purpose of preventing unstable ductile fracture, Patent Document 1 discloses a method for producing a steel sheet for line pipes in which the absorbed energy is increased by making the metal structure a single bainite structure. Discloses a steel material that has improved the stopping performance of unstable ductile fracture by making the surface layer portion of the steel sheet into an ultrafine structure.

In addition, seam welded parts of UOE steel pipes are inferior in toughness or softened parts occur in the weld heat affected zone, so the seam welded heat affected zone can become a location where fracture occurs or a crack propagation path, possibly causing significant damage. is there. In order to prevent the occurrence and propagation of such a fracture in the welded part, Patent Document 3 discloses a technique for making the weld metal stronger than the base metal and further making the weld bead height below a certain value. Further, in Patent Document 4, the peaking amount of the welded portion and the welding heat affected zone of the welded heat affected zone are satisfied so as to satisfy the relational expression defined by the peaking amount of the welded portion, the hardness of the welded heat affected zone and the hardness of the base material. A technique for reducing the hardness is disclosed.
Japanese Patent Laid-Open No. 62-4826 Japanese Patent Laid-Open No. 10-17986 JP 2002-309336 A JP 2002-346629 A

  However, even if the techniques such as Patent Documents 1 and 2 are applied, the propagation of a crack once generated may not be stopped depending on the operating conditions of the pipeline, and the phase transformation behavior (gas → gas In the case of natural gas or the like indicating (+ mist), the decompression of the gas is inhibited by the phase transformation, so that it may be more difficult to stop the crack. Therefore, in order to prevent the unstable ductile fracture of the line pipe, it is necessary to suppress the generation of cracks from a weld defect, a flaw caused by external factors, or a thinned portion due to corrosion. In order to suppress such breakage from the welded portion, the techniques of Patent Documents 3 and 4 have been proposed. The technology of Patent Document 3 makes it possible to reduce the pressure concentration on the toe of the welded part by reducing the weld bead height by making the weld metal stronger than the base metal and suppressing the fracture at the welded part. It is effective to do. However, since the weld metal strength and the base metal strength vary in the longitudinal direction of the pipe, it is difficult to maintain the relationship between the weld metal strength and the base metal strength over the entire length of the steel pipe, and due to variations in welding conditions such as current and voltage. It is also difficult to control the height of the weld bead below a certain value over the entire length of the steel pipe. Therefore, there is a possibility that a fragile portion that does not satisfy the predetermined condition in the steel pipe may occur. Similarly, in the technique of Patent Document 4, it is difficult to control the height of peaking over the entire length of the steel pipe, and there may be a portion in the steel pipe that does not satisfy a predetermined condition. Furthermore, none of the technologies has examined the material of the base material except for the relationship with the strength of the weld metal. If the material of the base material is likely to cause stress concentration on the welded part, how to weld it? Even if the shape of the part is controlled, the occurrence of breakage from the welded part may not be prevented.

  Therefore, the object of the present invention is to solve such problems of the prior art, suppress the occurrence of fracture from the welded portion in a usage form subject to high internal pressure, and prevent high-strength steel pipes that can prevent burst fracture and unstable ductile fracture, and its It is to provide a manufacturing method.

The gist of the present invention for solving the above problems is as follows.
[1] A steel pipe having a base metal tensile strength of 600 MPa or more manufactured by a UOE pipe manufacturing process, wherein the base material is C: 0.03 to 0.08 mass%, Si: 0.01 to 0.5 mass %, Mn: 1.5 to 2.0 mass%, P: 0.02 mass% or less, S: 0.002 mass% or less, Al: 0.08 mass% or less, Nb: 0.01 to 0.05 % By mass, Ti: 0.005 to 0.02% by mass, comprising the balance Fe and inevitable impurities, and the ratio of the tensile strength (TSw) and the base material tensile strength (TSb) of the seam weld metal [TSw / TSb ] Is 0.95 or more, the pipe circumferential yield ratio of the base metal is 92% or less, and the difference between the hardness of the base metal surface layer (HVs) and the average hardness of the base metal center (HVm) [HVs−HVm ] Is an HV30 or less, a high-strength steel pipe excellent in internal pressure fracture resistance .

[2] In the steel pipe of the above [1] , the base material is further Cu: 0.5 mass% or less, Ni: 0.5 mass% or less, Cr: 0.5 mass% or less, Mo: 0.5 mass % High-strength steel pipe excellent in internal pressure fracture resistance, characterized by containing one or more selected from below.
[3] In the steel pipe of the above [1] or [2] , the base material is further V: 0.1 mass% or less, Ca: 0.0005 to 0.0030 mass%, B: 0.005 mass% or less. A high-strength steel pipe excellent in internal pressure fracture resistance, characterized by containing one or more selected from the above.
[4] In the method for producing a high-strength steel pipe according to any one of [1] to [3 ] above,
This is a method of manufacturing a steel pipe by a UOE pipe manufacturing process using a steel sheet having a yield ratio in the vertical direction of rolling of 88% or less, and the pipe expansion ratio in the pipe expansion process applied after seam welding is 0.6 to 1.4%. A method for producing a high-strength steel pipe excellent in internal pressure fracture resistance.

  The high-strength steel pipe of the present invention has a high tensile strength of 600 MPa or more and excellent internal pressure fracture resistance that suppresses the occurrence of fracture from the weld due to internal pressure. For this reason, it is a high-strength steel pipe useful as a line pipe etc. in which especially high destruction safety is required. Moreover, the production method of the present invention can stably produce a high-strength steel pipe having such excellent internal pressure fracture resistance.

  In order to suppress the occurrence of fracture from the weld due to the internal pressure, the present inventors have diligently studied the fracture behavior of the UOE steel pipe due to the internal pressure, and reduced the yield ratio of the base metal in the pipe circumferential direction. It is effective to reduce the difference between the hardness of the surface layer and the inside, and this makes it possible to prevent the concentration of strain on the weld heat affected zone and to suppress the occurrence of fracture from the weld due to internal pressure. I found out. And if it is a base material which has such a material, if the ratio [TSw / TSb] of the tensile strength (TSw) of a seam weld metal and the tensile strength (TSb) of a base material is 0.95 or more, a welded part It is possible to suppress the occurrence of fracture from the steel, and it can be greatly deformed by the burst of the steel pipe due to internal pressure.

The results of a water pressure burst test performed by the method shown in FIG. 1 (schematic diagram) are shown below. A UOE steel pipe having a tensile strength of 630 to 750 MPa was used, end caps were welded to both ends of the steel pipe, and the steel pipe was filled with water. And the steel pipe was destroyed by inject | pouring water further into a steel pipe with a high pressure pump. FIG. 2 (schematic diagram) shows an example of a fractured form of a steel pipe burst. When the steel pipe breaks at the base material part away from the seam welded part, the deformation of the whole steel pipe can be absorbed by the deformation of the base material, so that the fracture can be suppressed to a high pressure. On the other hand, when strain concentrates on the seam welded portion, particularly the weld heat affected zone (HAZ), the portion is destroyed at an early stage, so that bursting occurs at a low pressure, that is, a small water injection amount. The results of the water pressure burst test were evaluated by evaluating the ratio of the amount of water injected before the burst (ΔV) to the initial internal volume (V ₀ ) of the steel pipe as the fracture volume strain and organizing it in relation to the pipe pipe circumferential yield ratio. Is shown in FIG. According to this, it can be seen that the lower the yield ratio of the base material, the larger the fracture volume strain, and the better the internal pressure fracture resistance. Conversely, when the yield ratio is equal to or greater than a certain value, fracture occurs from the weld heat affected zone (HAZ), and it can be seen that the fracture volume strain is greatly reduced.
As described above, the greatest feature of the present invention is that the yield ratio of the base metal in the pipe circumferential direction is reduced, and the difference between the hardness of the surface layer portion and the inside of the base material is reduced, so that the weld heat affected zone is reduced. This is to suppress strain concentration. As a result, the occurrence of fracture from the weld due to the internal pressure can be suppressed, and the internal pressure fracture resistance can be improved.

Hereinafter, the reasons for limitation of the high-strength steel pipe of the present invention will be described.
-Base material tensile strength: 600 MPa or more The object of the present invention is to prevent fracture from the weld due to internal pressure, which is a problem with high-strength UOE steel pipes, but it is not particularly problematic for steel pipes with a tensile strength of less than 600 MPa. Therefore, the strength of the target steel pipe is set to 600 MPa or more.
・ The ratio of the tensile strength (TSw) of the seam weld metal to the base material tensile strength (TSb) [TSw / TSb]: 0.95 or more Depending on the internal pressure if the tensile strength of the seam weld metal is significantly lower than the tensile strength of the base metal When stress in the pipe circumferential direction is applied, the welded part is preferentially deformed, resulting in destruction from the welded part. However, if the base material characteristics as described later are present, welding is performed if the ratio [TSw / TSb] of the tensile strength (TSw) of the seam weld metal to the base material tensile strength (TSb) is 0.95 or more. Since the destruction from the part can be suppressed, the lower limit is defined as 0.95. Also, the greater the ratio of the seam weld metal to the base metal, the lower the risk of fracture from the weld. Therefore, in order to obtain a more stable internal pressure fracture resistance, the tensile strength of the seam weld metal ( The ratio [TSw / TSb] of TSw) to base material tensile strength (TSb) is preferably 1.0 or more.
It should be noted that seam welding may be performed by applying submerged arc welding or the like used in the manufacture of ordinary UOE steel pipes, and by appropriately selecting a welding material used for welding and adjusting the strength of the weld metal.

・ Pipe circumferential yield ratio of base metal: 92% or less When the pipe peripheral yield ratio of the base metal is high, strain concentration tends to occur in the welded part due to deformation in the pipe circumferential direction due to internal pressure. Cause. However, if the pipe circumferential direction yield ratio of the base material is 92% or less, the strain concentration on the welded portion is reduced due to the deformation of the base material, and the breakage from the welded portion can be suppressed. For this reason, the yield ratio of the base material in the pipe circumferential direction is specified to be 92% or less. In the tensile test in the pipe circumferential direction of the base material, a full-thickness test piece obtained by straightening a steel pipe may be used. It is evaluated by a round bar tensile specimen taken from the circumferential direction of a straightened steel pipe.

・ Difference between the hardness of the base metal surface layer (HVs) and the average hardness of the base metal center (HVm) [HVs-HVm]: HV30 or less When the hardness of the base metal surface layer is significantly higher than the hardness of the central part Even if the yield ratio of the base metal in the pipe circumferential direction is low, strain concentration tends to occur at the weld. However, if the difference [HVs-HVm] between the hardness of the base metal surface layer (HVs) and the average hardness of the base metal center (HVm) [HVs−HVm] is HV30 or less in Vickers hardness, large strain concentration will not occur. The upper limit of the difference is defined as HV30. In addition, the hardness (HVs) of the base material surface layer part is measured at a depth of 1 mm from the front and back surfaces of the steel pipe, and is the maximum value. The average hardness (HVm) at the center of the base material is between the position of the tube thickness 1/4 on the surface side and the position of the tube thickness 1/4 on the back side when the hardness is measured in the tube thickness direction. The average value of the hardness.
Note that a steel sheet having a small difference between the hardness of the base metal surface layer (HVs) and the average hardness of the base metal center (HVm) conforms to the chemical composition and production conditions described below, and in particular, the cooling stop temperature of accelerated cooling is 500. It can be obtained by setting the reheating temperature after accelerated cooling to 570 ° C. or higher.

The high-strength steel pipe of the present invention as described above can be manufactured by the following manufacturing method.
When a steel pipe is manufactured by a UOE pipe manufacturing process, distortion is imparted to the steel sheet by cold forming, and in particular, in the pipe expansion process after seam welding, the steel sheet is subjected to tensile deformation in the pipe circumferential direction, so that the yield ratio increases. Therefore, in order to obtain a low yield ratio in the pipe circumferential direction, it is necessary to sufficiently reduce the yield ratio in the state of the steel sheet. Furthermore, it becomes possible to obtain a predetermined pipe circumferential direction yield ratio by controlling the tube expansion rate in the tube expansion step within a certain range.

Hereinafter, the reasons for limitation of the production method of the present invention will be described.
-Yield ratio in the vertical direction of rolling of the steel sheet: 88% or less Since UOE steel pipes are manufactured by cold forming, tensile properties vary greatly between the steel sheet used and the steel pipe after forming. In order to obtain a steel pipe having a low yield ratio, it is necessary to lower the yield ratio of the steel sheet used therefor, and in order to obtain a steel pipe having a yield ratio in the pipe circumferential direction of 92% or less, the yield ratio of the steel sheet in the vertical direction of rolling. Must be 88% or less. For this reason, the upper limit of the yield ratio in the rolling vertical direction of the steel sheet used for manufacturing the steel pipe is defined as 88%.

-Tube expansion rate in the tube expansion process after seam welding: 0.6-1.4%
In the manufacturing process of a UOE steel pipe, the process that most affects the yield ratio in the pipe circumferential direction is the pipe expansion process, and the yield ratio is greatly increased by the application of strain in the pipe circumferential direction as the pipe expansion ratio increases. However, if the tube expansion ratio is 1.4% or less, the yield ratio in the pipe circumferential direction can be suppressed to 92% or less, so the upper limit of the tube expansion ratio is 1.4%. Further, if the expansion ratio is too low, the roundness of the steel pipe is lowered and causes strain concentration on the welded portion, so the lower limit is made 0.6%.

In the present invention, excellent internal pressure fracture resistance can be obtained as long as the above-described conditions are satisfied. Therefore, the components and manufacturing method of the steel material used therefor may be arbitrary conditions. However, in order to stably obtain a steel pipe having the above-described performance, it is desirable that the chemical composition and production conditions of the steel plate are as follows.
First, chemical components of the steel plate will be described. In addition, all "%" of content of each element means the mass%.
・ C: 0.03-0.08%
C is an element necessary for securing the strength of the steel material, promoting the formation of bainite or martensite, and obtaining a multiphase structure advantageous for a low yield ratio. However, if it is less than 0.03%, sufficient strength cannot be obtained. On the other hand, if it exceeds 0.08%, weldability deteriorates, so the C content may be 0.03 to 0.08%. preferable.

・ Si: 0.01-0.5%
Si is added for deoxidation, but if it is less than 0.01%, the deoxidation effect is not sufficient. On the other hand, if it exceeds 0.5%, the toughness and weldability are deteriorated. It is preferable to set it to 0.5%.
Mn: 1.5 to 2.0%
Mn is added to ensure strength and toughness, but if it is less than 1.5%, the effect is not sufficient, while if it exceeds 2.0%, the weldability deteriorates, so the Mn content is 1.5 to It is preferable to set it to 2.0%.
-P: 0.02% or less P is contained as an inevitable impurity, but in order to deteriorate toughness and weldability, the P content is preferably 0.02% or less.
-S: 0.002% or less S is contained as an inevitable impurity, but in general, in steel, it becomes a MnS inclusion and becomes a starting point of voids, reducing the Charpy absorbed energy. It is necessary to regulate. However, since there is no problem if it is 0.002% or less, the S content is preferably 0.002% or less.

-Al: 0.08% or less Al is added as a deoxidizer, but if it exceeds 0.08%, the cleanliness of the steel decreases and the toughness deteriorates, so the Al content is 0.08% or less. It is preferable to do. More preferably, it is 0.01 to 0.08%.
・ Nb: 0.01-0.05%
Nb is an element necessary to refine the structure by controlled rolling and increase the toughness and strength. However, if it is less than 0.01%, its effect is small. Since the toughness of (HAZ) is deteriorated, the addition amount is preferably 0.01 to 0.05%.
Ti: 0.005-0.02%
Ti precipitates as carbonitride and is an effective element for suppressing grain coarsening during slab heating and for miniaturizing the weld heat affected zone. However, if the amount is less than 0.005%, the effect cannot be obtained. On the other hand, if the amount exceeds 0.02%, the precipitate becomes coarse and deteriorates the toughness of the welded portion. % Is preferable.

The steel sheet can further contain one or more of the following elements selectively.
Cu, Ni, Cr, Mo is one or more selected from Cu: 0.5% or less, Ni: 0.5% or less, Cr: 0.5% or less, Mo: 0.5% or less Although these elements are effective elements for increasing the strength, the weldability deteriorates if they are added in excess of 0.5%. Therefore, when they are added, the content is preferably 0.5% or less.
V: 0.1% or less V is an element effective for improving the strength by precipitating as carbonitride. However, if added over 0.1%, the toughness of the weld heat-affected zone deteriorates. Therefore, when added, the content is preferably made 0.1% or less.

・ Ca: 0.0005 to 0.0030%
Ca can be added for the control of inclusions. If it is less than 0.0005%, there is no effect. On the other hand, if it exceeds 0.0030%, the amount of inclusions increases and the toughness deteriorates. Therefore, when added, the content is preferably 0.0005 to 0.0030%.
B: 0.005% or less B is an element that contributes to strength increase and HAZ toughness improvement. In order to acquire the effect, it is preferable to add 0.0005% or more, but if added over 0.005%, the weldability is deteriorated. .
The balance other than the above consists essentially of Fe. That the balance is substantially made of Fe means that an element containing other trace elements including inevitable impurities can be included in the scope of the present invention unless the effects of the present invention are lost. For example, one or more of Mg: 0.02% or less and REM: 0.02% or less may be added.

Moreover, as preferable manufacturing conditions of a steel plate, using steel having the above component composition, after performing hot rolling at a heating temperature of 1000 to 1200 ° C. and a rolling end temperature of Ar ₃ points or more, 5 ° C./s or more. Accelerated cooling to 500 to 620 ° C. at a cooling rate of 0.5 ° C./s, followed by reheating to a temperature higher than the accelerated cooling stop temperature and a temperature of 570 to 700 ° C. By carrying out, a metal structure is made into the multiphase structure of a bainite and an island-like martensite (MA).
Here, the heating temperature, the rolling end temperature, the accelerated cooling stop temperature (cooling end temperature), the reheating temperature, and the like described above are the average thickness direction temperature of the steel sheet. This average thickness direction temperature is obtained by calculation from the surface temperature of the slab or steel plate in consideration of parameters such as plate thickness and thermal conductivity. The cooling rate is an average cooling rate obtained by dividing the temperature difference required for cooling to the accelerated cooling stop temperature by the time required for cooling after completion of hot rolling. The temperature increase rate is an average temperature increase rate obtained by dividing the temperature difference necessary for reheating after cooling by the time required for reheating.

Hereinafter, the reasons for limiting the manufacturing conditions will be described.
The heating temperature in the hot rolling is 1000 to 1200 ° C. If the heating temperature is less than 1000 ° C., the solid solution of carbide is insufficient and the required strength and yield ratio cannot be obtained, while if it exceeds 1200 ° C., the base material toughness deteriorates.
The rolling end temperature of hot rolling is Ar ₃ points or more. If the rolling end temperature is less than Ar ₃ , the subsequent ferrite transformation rate is lowered, so that sufficient precipitation of fine precipitates cannot be obtained during ferrite transformation by reheating, and the strength is lowered. Moreover, the concentration of C into untransformed austenite during reheating becomes insufficient, and island martensite is not generated. Here, the Ar ₃ point is a temperature at which the ferrite transformation starts, and can be determined by the following equation (1), for example.
Ar ₃ (° C.) = 910-310C-80Mn-20Cu-15Cr-55Ni-80Mo (1)
However, the element symbol shown in the formula (1) represents mass% of each element.

Immediately after the completion of rolling, cooling (accelerated cooling) is performed at a cooling rate of 5 ° C./s or more. When the cooling rate is less than 5 ° C./s, pearlite is generated at the time of cooling, so that island-like martensite is not generated and strengthening by bainite cannot be obtained, so that sufficient strength cannot be obtained. In addition, when the cooling start temperature is less than Ar ₃ point and ferrite is generated, dispersion precipitation of fine precipitates is not obtained during reheating, resulting in insufficient strength, and generation of island martensite does not occur. Is Ar ₃ or more. About the cooling method at this time, it is possible to use arbitrary cooling equipment by a manufacturing process.

  The cooling stop temperature of the accelerated cooling is 500 to 620 ° C. This process is an important manufacturing condition in the present invention. In the present invention, C-concentrated untransformed austenite present after reheating is transformed into island martensite during subsequent air cooling. That is, it is necessary to stop the cooling in a temperature range where untransformed austenite during the bainite transformation exists. If the cooling stop temperature is less than 500 ° C., the bainite transformation is completed, so that island martensite is not generated during air cooling, and a low yield ratio cannot be achieved. In addition, when the cooling stop temperature is lowered, the surface layer of the steel sheet is further cooled to a low temperature, and a hardened structure such as upper bainite and martensite is formed, so that the surface hardness increases remarkably. The lower limit of the stop temperature is 500 ° C. On the other hand, when the cooling stop temperature exceeds 620 ° C., pearlite is precipitated during cooling, so that fine carbides are not sufficiently precipitated, and sufficient strength cannot be obtained. Further, C is consumed in the pearlite and island martensite is formed. Do not generate.

  Immediately after the accelerated cooling is stopped, reheating is performed to a temperature higher than the accelerated cooling stop temperature and a temperature of 570 to 750 ° C. at a temperature rising rate of 0.5 ° C./s or more. This process is also an important production condition in the present invention. Precipitates of fine composite carbides that contribute to strengthening precipitate during reheating. Furthermore, bainite transformation from untransformed austenite at the time of reheating, and discharge of C into the untransformed austenite accompanying the transformation, untransformed austenite enriched with C during air cooling after reheating transforms into island martensite. . In order to obtain such fine composite carbide precipitates and island martensite, it is necessary to reheat to a temperature higher than the accelerated cooling stop temperature and to a temperature range of 570 to 750 ° C. If the heating rate is less than 0.5 ° C./s, it takes a long time to reach the target reheating temperature, so that the production efficiency deteriorates and pearlite transformation occurs. The site cannot be obtained, and sufficient strength and low yield ratio cannot be obtained. When the reheating temperature is lower than 570 ° C., sufficient precipitation driving force cannot be obtained, and since the amount of fine composite carbide is small, sufficient precipitation strengthening cannot be obtained. In addition, by reheating after accelerated cooling, the hardened surface layer part at the time of accelerated cooling is tempered and the hardness is reduced, and the difference in hardness between the steel plate center part and the surface layer part is reduced, but when the reheating temperature is low, Since the hardness of the surface layer portion is not sufficiently reduced, the lower limit of the reheating temperature is set to 570 ° C. from this point. On the other hand, when the reheating temperature exceeds 700 ° C., the precipitate becomes coarse and sufficient strength cannot be obtained.

If it is said manufacturing conditions, even if it cools immediately after reheating, high intensity | strength will be obtained since sufficient fine composite carbide is obtained, but in order to ensure sufficient fine composite carbide, in the state reheated. It is also possible to hold the temperature within 30 minutes. If the temperature is maintained for more than 30 minutes, the composite carbide may become coarse and the strength may decrease. In the cooling process after reheating, the fine composite carbide is not coarsened regardless of the cooling rate. Therefore, it is preferable that the cooling rate after reheating is basically air cooling.
By using a steel plate manufactured under the above conditions and manufacturing a steel pipe by a UOE pipe manufacturing process, it is possible to stably obtain a steel pipe with more excellent internal pressure fracture resistance.

Steels (steel types A to G) having chemical components shown in Table 1 were made into slabs by a continuous casting method, and thick steel plates (Nos. 1 to 17) having various plate thicknesses were produced using the slabs.
The heated slab is hot-rolled, and the hot-rolled steel sheet is immediately cooled using water-cooled accelerated cooling equipment, and then reheated using an induction heating furnace installed on the same line as the accelerated cooling equipment. . The cooling rate during accelerated cooling was 40 to 55 ° C./s, and the heating rate during reheating was 20 to 30 ° C./s. In addition, some steel plates were not reheated for comparison. Using these steel plates, steel pipes were manufactured by the UOE pipe manufacturing process.
A water pressure burst test was performed on each manufactured steel pipe by the method shown in FIG. End caps were welded to both ends of the test steel pipe, and the interior was filled with water. Thereafter, water was further injected with a high-pressure pump, and the ratio (percentage) of the amount of water (ΔV) injected until the steel pipe burst to the initial internal volume (V ₀ ) of the steel pipe was evaluated as fracture volume strain. Table 2 shows the manufacturing conditions and characteristics of the steel plates used, and Table 3 shows the manufacturing conditions and characteristics of the steel pipes.

In Table 2 and Table 3, No. Each of the steel pipes 1 to 10 has a yield ratio of the base metal in the pipe circumferential direction of 92% or less, and the ratio [TSw / TSb] of the tensile strength (TSw) to the base metal tensile strength (TSb) of the seam weld metal is 0. Since the difference [HVs−HVm] between the hardness of the base metal surface layer (HVs) and the average hardness (HVm) of the base metal center is not more than HV30, the fracture volume strain in the hydraulic burst test is large, and All cracking positions are the base material.
On the other hand, No. which is a comparative example. Nos. 11 to 17 have a high pressure burst test because the pipe yield ratio is high or the ratio of the seam weld metal tensile strength (TSw) to base metal tensile strength (TSb) is small [TSw / TSb]. The fracture volume strain at is small and all fracture from the HAZ part.

Explanatory drawing showing the test method of water pressure burst test Explanatory diagram showing crack initiation position after hydraulic pressure burst test A graph showing the relationship between the pipe circumferential yield ratio of the base metal and the fracture volume strain measured by the hydraulic burst test Graph showing the relationship between the tensile strength of the base metal and the fracture volume strain measured by the hydraulic burst test

Claims (4)

It is a steel pipe having a base metal tensile strength of 600 MPa or more manufactured by the UOE pipe manufacturing process, and the base material is C: 0.03 to 0.08 mass%, Si: 0.01 to 0.5 mass%, Mn : 1.5 to 2.0 mass%, P: 0.02 mass% or less, S: 0.002 mass% or less, Al: 0.08 mass% or less, Nb: 0.01 to 0.05 mass%, Ti: 0.005 to 0.02% by mass, consisting of remainder Fe and inevitable impurities, and the ratio [TSw / TSb] of the tensile strength (TSw) and base material tensile strength (TSb) of the seam weld metal is 0 .95 or more, the pipe circumferential yield ratio of the base metal is 92% or less, and the difference [HVs−HVm] between the hardness of the base metal surface layer (HVs) and the average hardness of the base metal center (HVm) is HV30 A high-strength steel pipe with excellent internal pressure fracture resistance characterized by: The base material further includes at least one selected from Cu: 0.5% by mass or less, Ni: 0.5% by mass or less, Cr: 0.5% by mass or less, Mo: 0.5% by mass or less. The high-strength steel pipe having excellent internal pressure fracture resistance according to claim 1, which is contained. The base material further contains one or more selected from V: 0.1% by mass or less, Ca: 0.0005-0.0030% by mass, and B: 0.005% by mass or less. A high-strength steel pipe excellent in internal pressure fracture resistance according to claim 1 or 2 . In the manufacturing method of the high strength steel pipe according to any one of claims 1 to 3 ,
This is a method of manufacturing a steel pipe by a UOE pipe manufacturing process using a steel sheet having a yield ratio in the vertical direction of rolling of 88% or less, and the pipe expansion ratio in the pipe expansion process applied after seam welding is 0.6 to 1.4%. A method for producing a high-strength steel pipe excellent in internal pressure fracture resistance.

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