JP2012241270A - High strength sour-resistant linepipe superior in collapse resistance and method for producing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high strength linepipe which is inexpensively produced with high productivity without degrading collapse resistance and sour resistance, and to provide a method for producing the high strength linepipe.SOLUTION: A high strength sour-resistant linepipe is a welded steel pipe which is obtained by forming a base material composed of a thick steel sheet into a tubular shape and joining the butted parts by welding the butted parts in two or more layers, and which contains, by mass, 0.02-0.08% of C, 0.01-0.50% of Si, 0.5-1.5% of Mn, a certain content of each of P, S, Al, Nb, Ca and O, and a certain amount of at least one selected from Cu, Ni, Cr and Mo, wherein Ceq is 0.30 or more, PHIC is 0.10 or less, ACR is 1.00-6.00, and the remainder comprises Fe and inevitable impurities. In the high strength sour-resistant linepipe, the metal structure and hardness of the surface layer part and the center part of pipe thickness are specified. The method for producing the linepipe is also provided.

Description

  The present invention relates to a welded steel pipe for line pipes used for the transportation of oil and natural gas and a method for producing the same, and in particular, a high-strength sour-resistant line having excellent crushing strength produced by cold-forming and welding thick steel plates. The present invention relates to a pipe and a manufacturing method thereof.

  Generally, a line pipe laid on the seabed may be crushed by receiving a high external pressure from the outside during laying. Therefore, the line pipe laid on the seabed is required to have a high pressure crushing property. The crush resistance is governed by the shape of the line pipe and the compressive yield stress. Generally, it is known that the more the shape of the line pipe is a perfect circle and the greater the compressive yield stress, the better the crush resistance. For this reason, it is desirable that the line pipe laid on the seabed has a sufficient compressive yield stress in the piped state, but it is piped by cold-working a thick steel plate like a UOE steel pipe and then expanding it. In the case of steel pipes, a large tensile load is applied in the final process of pipe expansion. Therefore, the compressive yield stress of the steel pipe is lower than the tensile yield stress of the steel pipe due to the back stress generated during the tensile load.

  Therefore, in order to ensure the crushing resistance of the steel pipe, it is necessary to design the steel sheet with a high design strength or to increase the pipe thickness. However, to increase the strength or increase the tube thickness, both increase the alloy cost and promote toughness deterioration of the base metal and the weld heat affected zone. Therefore, it is required to establish a method for manufacturing a welded steel pipe that can ensure the property.

  Moreover, when using as a line pipe, hydrogen sulfide may be contained in transport gas, In that case, in addition to the characteristic mentioned above, it is calculated | required to ensure the outstanding sour-proof performance.

  In response to such a request, in Patent Document 1 and Patent Document 2, the O-press compression ratio and pipe expansion ratio during pipe making are used as parameters, and the compression ratio / pipe expansion ratio is reduced to the optimum range, so that A method for suppressing a decrease in compressive yield stress of a steel pipe is disclosed. For example, Patent Document 2 discloses a technique in which the ratio of the upset rate (that is, the compression rate) α during O molding to the tube expansion rate β during tube expansion is α / β ≧ 0.35. Patent Document 2 also discloses a method for suppressing a reduction in the compressive yield stress of a steel pipe after pipe making by increasing the pipe expansion rate extremely. Patent Document 3 discloses a method for improving the crushing strength of a steel pipe by external pressure by optimizing the order and degree of contraction and expansion.

  Patent Documents 4 to 7 disclose a method for suppressing a decrease in compressive yield stress of a steel pipe by reducing a back stress applied to the steel pipe in a pipe making process by low-temperature strain aging by heat treatment or coating heating after pipe forming. Is disclosed.

  Moreover, in patent document 8 and patent document 9, compression by the Bauschinger effect is performed by decomposing island martensite (MA) contained in the structure of the thick steel plate and further reducing the hardness of bainite of ferrite + bainite. A method is disclosed in which the yield stress is improved and, at the same time, the collapse resistance is improved by improving the roundness by uniformizing the hardness in the tube thickness direction.

JP 2002-102931 A JP 2003-340518 A Japanese Patent Laid-Open No. 9-1233 JP-A-9-3545 JP 2002-295736 A JP 2003-342639 A JP 2004-35925 A JP 2009-275261 A JP 2010-84171 A

  However, in order to set the pipe making conditions to the optimum compression ratio / pipe expansion ratio as shown in Patent Document 1 and Patent Document 2, it is necessary to make the O-press compression ratio much larger than usual. Increasing the compression rate of the O press requires an increase in the press capability of the O press machine, and increases the cost due to the introduction of new equipment and the repair of equipment.

  Furthermore, line pipes for submarine pipelines, where securing compressive strength is a problem, are often designed with a thick wall from the viewpoint of securing buckling resistance, which increases the compressibility of the O-press. . Moreover, although it can also be set to an optimal range by reducing a pipe expansion rate, the roundness of a steel pipe will be reduced and a crushing property will deteriorate. In the present invention, the term “crush resistance” is used in the meaning of resistance to buckling failure due to external pressure when laying a steel pipe.

  Further, as described in Patent Documents 2 and 3, increasing the tube expansion ratio or performing tube contraction and tube expansion is caused by an increase in surface hardness due to excessive work hardening, or by tube expansion and tube contraction dies. May remain on the surface of the steel pipe.

  Moreover, as described in Patent Documents 4 to 7, performing the low-temperature strain aging treatment by optimizing the coating heating conditions after pipe forming is a great effect from the viewpoint of suppressing the decrease in compressive yield stress. However, the tensile stress-strain curve of the steel pipe is changed from the round house type after pipe making to the Luders type, and the deformability of the steel pipe such as bending buckling performance is lowered.

  Furthermore, the coating heating conditions vary depending on the coating material used, and may not necessarily match the target coating heating conditions. When performing low-temperature strain aging treatment by heat treatment instead of coating heating, As a result, the productivity will be significantly impaired.

  Moreover, as described in Patent Document 8 and Patent Document 9, by applying rapid heating immediately after accelerated cooling, the crush resistance can be improved from both sides of the steel material characteristics and the steel pipe shape. However, the difference in hardness in the pipe thickness direction can be reduced, but the difference in hardness in the pipe diameter direction cannot be reduced sufficiently. Cracks (hereinafter referred to as “SSC”) may occur.

  As described above, it is difficult to produce a sour-resistant line pipe with excellent crush resistance without causing deterioration in appearance, weldability, productivity, or sour-resistant performance with the conventional technology. Met.

  Therefore, an object of the present invention is to provide a high-strength line pipe that can be manufactured at high productivity and low cost without reducing the crush resistance and sour resistance performance, and a method for manufacturing the same.

  In order to solve the above-mentioned problems, the present inventors diligently studied the microstructure of the steel sheet and the method for achieving the microstructure, in particular, the manufacturing process of steel material components, controlled rolling, and accelerated cooling, and obtained the following knowledge. Obtained.

  First, it was found that in order to secure excellent sour resistance performance, it is necessary to add an appropriate amount of Ca in order to spheroidize MnS generated in the central segregation. At that time, by setting the ACR of the formula (3) described later to 1.0 or more, the needle-like MnS is changed to spherical CaOS, and it becomes a starting point of fracture occurrence in the hydrogen-induced cracking (hereinafter referred to as “HIC”) test. It turned out that it can suppress becoming. Furthermore, it has been found that by providing an upper limit to the contents of Ca, S, and O, it is possible to suppress the occurrence of a fracture starting point in the HIC test due to the aggregation and coarsening of Ca-based inclusions. Further, even when acicular MnS is not generated in the central segregation, HIC may be generated starting from NbTi-CN, Ca-Al inclusions, bubbles, etc., but PHIC of formula (2) described later is 1 0.0 or less prevents alloy elements with high hardenability from concentrating in the central segregation, and by making the structure at the center of the tube thickness into a bainite single phase, C to austenite during two-phase transformation. It has been found that the concentration of NO can be suppressed and the generation of HIC can be suppressed.

  On the other hand, for the occurrence of SSC from the surface layer, it has been conventionally known that the occurrence of SSC can be suppressed by suppressing the surface layer hardness to 248 or less, but after the end of rolling to ensure HIC resistance, It has been pointed out before that the surface layer is significantly hardened by accelerated cooling before the surface layer is sufficiently transformed.

  As a result of detailed investigation of the surface hardness distribution in the plate width direction in order to investigate the cause, the present inventors have found that the portion where the surface layer is significantly hardened is locally present in the tube width direction, and its position is It was found that the scale could not be peeled off sufficiently by descaling after rolling, and that a thick scale was generated during accelerated cooling from the end of rolling. Therefore, the present inventors perform high-pressure descaling immediately before performing accelerated cooling, thereby uniformly peeling the scale on the surface of the steel sheet and minimizing scale growth from descaling to accelerated cooling. Thus, it is clear that the hardened portion that existed locally in the plate width direction can be eliminated, and the hardness of the steel pipe surface can be suppressed to 248 or less over the entire length, and as a result, SSC can be suppressed. did.

  In addition, it is advantageous from the viewpoint of securing the roundness of the steel pipe by performing descaling and flattening the hardness distribution in the plate width direction before accelerated cooling, ensuring roundness without excessively increasing the pipe expansion rate. can do. Decreasing the expansion ratio leads to an increase in the compressive yield stress of the steel pipe, making it possible to improve both the roundness and the compressive yield stress, and to improve the collapse resistance. I made it.

The present invention is a further study based on the above knowledge,
[1] A welded steel pipe in which a base material made of a thick steel plate is formed into a tubular shape, and a seam portion thereof is joined by welding of two or more layers,
% By mass
C: 0.02 to 0.08%
Si: 0.01 to 0.50%
Mn: 0.5 to 1.5%
P: 0.010% or less S: 0.001% or less Al: 0.06% or less Nb: 0.002 to 0.100%
Ca: 0.0005 to 0.0040%
O: contains 0.0030% or less,
Cu: 1.0% or less Ni: 1.0% or less Cr: 1.00% or less Mo: containing one or more selected from 0.5% or less,
Furthermore, Ceq defined by the formula (1) is 0.30 or more,
PHIC defined by the formula (2) is 0.10 or less,
ACR defined by equation (3) is 1.00 to 6.00,
The balance Fe and inevitable impurities,
The base metal surface layer metallographic structure is upper bainite or ferrite and upper bainite,
The metal structure of the base metal tube thickness center is the upper bainite single phase,
The volume fraction of island martensite (MA) is less than 4% throughout the tube thickness,
And the maximum value of the hardness difference in the pipe thickness direction at the same position in the pipe circumferential direction is 50 or less,
The maximum value of the hardness difference in the pipe circumferential direction at the same position in the pipe thickness direction is 50 or less,
A high-strength sour-line pipe excellent in crushing resistance, characterized in that the maximum surface hardness is 248 or less.
Ceq = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5 Formula (1)
PHIC = 4.46C + 2.37Mn / 6 + (1.18Cr + 1.95Mo + 1.74V) / 5 + (1.74Cu + 1.7Ni) /15+22.36P Formula (2)
ACR = (Ca− (0.18 + 130Ca) O) /1.25S Formula (3)
Here, the element symbol on the right side of each formula represents the content (% by mass), and is 0 when not contained.
[2] Furthermore, in mass%,
V: 0.005 to 0.100%
Ti: 0.005 to 0.050%
Mg: 0.0005 to 0.0040%
A high-strength sour-line pipe excellent in crush resistance according to [1], comprising one or more selected from the group consisting of:
[3] The high-strength sour line pipe excellent in crush resistance according to [1] or [2], wherein the roundness satisfies the following formula (4) or (5):
When D / t ^0.6 ≦ 85 Dmax−Dmin ≦ 3.0 Formula (4)
When D / t ^0.6 > 85 Dmax−Dmin ≦ 0.04 D / t ^0.6 −0.4 Formula (5)
Here, D: nominal outer diameter (mm), t: tube thickness (mm), Dmax-Dmin: roundness (mm), Dmax: maximum measured outer diameter (mm), Dmin: minimum measured outer diameter (mm) It is.
[4] After heating the steel material to 900 to 1200 ° C., after performing hot rolling with a cumulative reduction rate of 900 ° C. or less at 30 to 90% and a rolling end temperature of (Ar ₃ −10 ° C.) or more,
Immediately before accelerated cooling, descaling with a jet collision pressure on the steel sheet surface of 1 MPa or more,
Immediately _(Ar 3 -30 ℃) cooling rate of the surface layer of the cooling rate from the temperature of 200 ° C. / s or less and an average or perform accelerated cooling stop temperature is 300 ° C. to 600 ° C. cooled at 10 ° C. / s or higher,
Thereafter, the steel plate obtained by cooling to room temperature is formed into a tubular shape in the cold, and the butt portion is welded to form a steel pipe, and further, the pipe is expanded at a pipe expansion ratio of 0.5 to 1.1%. The method for producing a high-strength sour-line pipe excellent in crush resistance according to any one of [1] to [3], wherein the method is produced.

  According to the present invention, high toughness produced by cold forming and welding thick steel plates suitable for thick and high-strength line pipes with excellent crush resistance for use in oil and natural gas transportation, especially in submarine pipelines. Welded steel pipes can be manufactured, which is extremely effective in industry.

  The form for implementing this invention is demonstrated below. First, the reasons for limiting the constituent requirements of the present invention will be described. The thick steel plate to be used in the present invention refers to a steel plate having a thickness of 20 mm or more manufactured by hot rolling.

1. The reasons for limiting the component composition will be described below. In addition, the unit which shows a component composition shall be mass% altogether.

C: 0.02 to 0.08%
C is an element that increases the hardenability and is important for securing the strength, but if it is less than 0.02%, sufficient strength cannot be secured. Moreover, when it adds exceeding 0.08%, the production | generation of a hard 2nd phase will become remarkable and it will become difficult to ensure sour resistance. In addition, an increase in the hard second phase reduces the compressive strength of the steel pipe and also reduces the crushing resistance. Therefore, the C content is in the range of 0.02 to 0.08%. More preferably, it is 0.03 to 0.06%.

Si: 0.01 to 0.50%
Si is added for deoxidation, but if it is less than 0.01%, the deoxidation effect is not sufficient, and if it exceeds 0.50%, sour resistance, crush resistance, toughness and welding due to an increase in the martensite volume fraction The Si content is in the range of 0.01 to 0.50%. More preferably, it is 0.01 to 0.30% of range.

Mn: 0.5 to 1.5%
Mn is an element effective for improving strength and toughness, but if it is less than 0.5%, the effect is not sufficient, and if it exceeds 1.5%, it segregates in the center, and due to the increase in center segregation hardness and generation of MnS. The sour resistance performance is significantly degraded. Therefore, the Mn content is in the range of 0.5 to 1.5%. More preferably, it is 1.0 to 1.5%.

P: 0.010% or less P is an element concentrated in segregation, and even if contained in a small amount, the hardness of central segregation is remarkably increased and sour resistance is deteriorated. However, up to 0.010% is acceptable.

S: 0.001% or less S combines with Mn to generate MnS. Further, since S is an element that is easily segregated in the center like Mn, if the amount of S is large, a large number of central segregations of MnS are generated, and sour resistance is remarkably deteriorated. Therefore, it is desirable to reduce S as much as possible, but it is acceptable up to 0.001%.

Al: 0.06% or less Al is added as a deoxidizer, but if it exceeds 0.06%, the cleanliness of the steel is lowered, and sour resistance is deteriorated due to the formation of Al inclusions. The Al content is 0.06% or less. More preferably, it is 0.01 to 0.05% of range.

Nb: 0.002 to 0.100%
Nb is an element that enhances the effect of controlled rolling and improves strength and toughness by refining the structure. However, if it is less than 0.002%, there is no effect, and if it exceeds 0.100%, the toughness of the weld heat-affected zone is remarkably deteriorated, so the Nb content is in the range of 0.002 to 0.100%. More preferably, it is 0.005 to 0.060%.

Ca: 0.0005 to 0.0040%
Ca improves the anti-HIC performance by making the shape of acicular MnS formed in the central segregation spherical. In order to obtain the effect, it is preferable to add 0.0005% or more. However, if it exceeds 0.0040%, CaOS clusters are formed, and the HIC resistance performance is rather deteriorated. Is 0.0005 to 0.0040%. More preferably, it is 0.0015 to 0.0040%.

O: 0.0030% or less O is an element inevitably contained in steel, and usually exists as an oxide combined with Al or Ca. If the content of these Al and Ca-based oxides in the steel is too large, a cluster is formed and the HIC resistance is deteriorated, so the content of O is made 0.0030% or less.

  Furthermore, in order to improve the intensity | strength of a steel plate, 1 or more types chosen from Cu, Ni, Cr, and Mo shown below are added.

Cu: 1.0% or less Cu is an element effective for improving toughness and increasing strength. However, if added over 1.0%, the base metal due to weldability degradation or precipitation embrittlement, the toughness degradation of HAZ, and further the decrease in compressive strength due to an increase in the MA fraction becomes a problem. When adding, the upper limit is made 1.0%. More preferably, it is 0.05 to 0.45%.

Ni: 1.0% or less Ni is an element effective for improving toughness and increasing strength. However, if added over 1.0%, the slab cracks during continuous casting, requiring surface care, resulting in a significant decrease in productivity and a decrease in compressive strength due to an increase in the MA fraction. Therefore, when Ni is added, the upper limit is made 1.0%. More preferably, it is 0.05 to 0.45% Cr: 1.00% or less Cr is an element effective for obtaining sufficient strength even at low C as in the case of Mn. However, if added over 1.0%, the weldability deteriorates and the compressive strength decreases due to an increase in the MA fraction. Therefore, when Cr is added, its content is made 1.00% or less. More preferably, it is 0.10 to 0.40%.

Mo: 0.50% or less Mo is an element that improves hardenability and greatly contributes to an increase in strength. However, addition exceeding 0.50% leads to a decrease in compressive strength and deterioration of weld heat affected zone toughness due to an increase in the MA fraction. Therefore, when Mo is added, its content is 0.50% or less. And More preferably, it is 0.05 to 0.30%.

Ceq: 0.30 or more Ceq defined by the following formula (1) is originally an index indicating the HAZ (welding heat affected zone) maximum hardness at the time of welding, but at the same time, it shows a good correlation with the base material strength. Are known. When Ceq is less than 0.30, the desired base material strength cannot be obtained, so the lower limit of Ceq is set to 0.30.

Ceq = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5 Formula (1)
Here, the element symbol on the right side of each formula represents the content (% by mass), and is 0 when not contained.

PHIC: 1.00 or less The PHIC defined by the following formula (2) is a concentration obtained by determining the degree of concentration at the central segregation part by thermodynamic calculation for the alloy element and P used in the general carbon equivalent formula. The hardness of the final solidified part of the central segregation part can be indirectly displayed. When this PHIC exceeds 1.00, even if coarse MnS is not generated in the central segregation, HIC cracks are generated starting from NbTi-CN or the like, so the upper limit is set to 1.00.
PHIC = 4.46C + 2.37Mn / 6 + (1.18Cr + 1.95Mo + 1.74V) / 5 + (1.74Cu + 1.7Ni) /15+22.36P Formula (2)
Here, the element symbol on the right side of each formula represents the content (% by mass), and is 0 when not contained.

ACR: 1.00 to 6.00
The ACR defined by the following formula (3) is an index for evaluating whether MnS generated in the central segregation can be spheroidized by Ca. When the MCR is less than 1.00, coarse MnS remains in the central segregation and is resistant to HIC. Degrading performance. In the case of 1.00 or more, CaOS is generated and coarse MnS is not generated. However, if it exceeds 6.00, CaOS generates a cluster and deteriorates the HIC resistance, so the ACR range is set to 1.00. The range is ˜6.00.
ACR = (Ca− (0.18 + 130Ca) O) /1.25S Formula (3)
Here, the element symbol on the right side of each formula represents the content (% by mass).

  Furthermore, in order to improve the strength, base metal toughness, and HAZ toughness of the steel sheet, one or more selected from V, Ti, and Mg shown below can be added.

V: 0.005-0.100
V can improve the strength of the base material mainly by improving the hardenability. The effect does not appear when the content is less than 0.005%. On the other hand, the addition exceeding 0.100% causes precipitation embrittlement and deteriorates the base metal toughness and the HAZ toughness. Therefore, when V is added, the range is 0. 0.005 to 0.100% is preferable. More preferably, it is 0.005 to 0.050%.

Ti: 0.005 to 0.050%
Ti is an effective element for suppressing the austenite coarsening during heating due to the pinning effect of TiN and improving the toughness of the base metal and the weld heat affected zone. However, if it is less than 0.005%, there is no effect, and if it exceeds 0.050%, TiN becomes coarse and conversely deteriorates the toughness of the weld heat affected zone. Therefore, when Ti is added, its content is 0. It is preferable to make it into the range of 0.005 to 0.050%. Furthermore, when the Ti content is 0.005 to 0.030%, more excellent toughness is exhibited.

Mg: 0.0005 to 0.0040%
Mg is an element that contributes to improvement of the base material and HAZ toughness by finely dispersing alumina clusters (Al ₂ O ₃ ) as an Al—Mg-based oxide. In order to obtain the effect, it is preferable to add 0.0005% or more, but when adding over 0.0040%, MgCaOS clusters are formed and the HIC resistance is deteriorated. The addition amount is preferably 0.0005 to 0.0040%.

Other than the above elements, Fe and unavoidable impurities are not added intentionally. More preferably, the upper limit of the content of inevitable impurities is 0.0060% or less for N and 0.0005% or less for B.
2. Metal structure (micro structure)
In the present invention, the shape and volume fraction of the metal structure of the base material are defined. Here, the volume fraction is regarded as the volume fraction by measuring the area ratio of each metal structure.

Surface structure It is necessary to prevent the surface structure from becoming an excessively baked structure in order to secure SSC resistance. In the present invention, by performing descaling immediately before accelerated cooling, the generation of a hardened structure in the surface layer due to the formation of a thick scale is suppressed, and the surface layer structure is made to be upper bainite, or ferrite + upper bainite (ferrite And the upper bainite are expressed in this manner) to prevent excessive increase in surface hardness. In addition to these main structures, there are martensite, MA, lower bainite, pearlite, and cementite, all of which are harder than ferrite and upper bainite, and therefore less is preferable. The volume fraction of the main tissue is not particularly specified, but is more preferably 85% or more. At this time, cementite generated between the laths of the upper bainite is measured as a part of the upper bainite. The surface layer is a region from the outermost layer to the tube thickness direction of 2 mm.

Central structure of tube thickness The structure of the center of the tube thickness is an important factor in securing the base material strength and the HIC performance. From the viewpoint of ensuring HIC performance and compressive strength, it is desirable that the structure be as uniform as possible. From the viewpoint of ensuring strength, the ferrite single-phase structure is not suitable, and the lower bainite or martensite single-phase structure has high hardness. Since it becomes too much and cracks occur from the center segregation during the HIC test, it is necessary to have an upper bainite single phase structure in order to achieve both the strength of the base material and the HIC resistance.

  In addition to the upper bainite structure, there are martensite, MA, lower bainite, pearlite, cementite and ferrite, all of which cause a hardness difference from the upper bainite which is the main structure as the second phase, Less is better because it degrades the compressive strength. In addition, it is preferable that the volume fraction of the upper bainite which is a main body structure is 85% or more. At this time, cementite generated between the laths of the upper bainite is measured as a part of the upper bainite. The tube thickness center is a region at a position ± 2 mm from the tube thickness center and excluding the center segregation portion.

The M-A fraction M-A is a structure having the largest hardness among the hard second phases described above, and it is preferable that the M-A fraction M-A is as small as possible because it significantly deteriorates the HIC resistance and compressive strength. When manufacturing a steel pipe material by accelerated cooling, MA is unavoidably present, but up to 4% can be allowed. The whole pipe thickness is the whole steel pipe base material excluding the central segregation part.

3. Hardness In the present invention, the hardness distribution in the pipe circumferential direction and the pipe thickness direction and the maximum value of the hardness are defined. The hardness is measured with a load of 10 kgf (98 N) using a Vickers hardness tester.

Maximum value of the hardness difference in the pipe circumferential direction: 50 or less The hardness difference in the pipe circumferential direction is mainly caused by the surface properties during accelerated cooling. A portion where the scale is thick is excessively cooled and the surface layer is markedly cured. On the other hand, in a portion where the scale thickness is thin, the surface layer is not hardened so much, resulting in a large difference in surface hardness. When the hardness difference in the pipe circumferential direction is large, the shape is disturbed in the CU-O molding at the time of UOE pipe forming, and as a result, a larger pipe expansion rate is required to obtain a desired roundness. When the tube expansion rate is increased, the compressive strength is reduced due to the Bauschinger effect, so that the crushing resistance is reduced. On the other hand, if the hardness difference in the pipe circumferential direction is small, high roundness can be obtained even if the pipe expansion rate is small, and the crush resistance is improved in terms of both suppressing the decrease in compressive strength and ensuring the roundness. be able to. The effect is not seen if the hardness difference in the pipe circumferential direction exceeds 50, so the upper limit is set to 50. More preferably, it is 40 or less. In addition, the hardness difference in the pipe circumferential direction is to be compared for those measured at a pitch of 1 mm in the pipe thickness direction at the same position in the pipe length direction, and as described above, the hardness difference in the pipe circumferential direction mainly occurs in the surface layer portion. If two locations of 1 mm from the surface layer and 1 mm from the back layer are measured, the hardness difference in the circumferential direction of the steel pipe can be regarded as a representative.

Maximum value of hardness difference in the tube thickness direction: 50 or less The difference in hardness in the tube thickness direction is mainly caused by the surface layer structure before accelerated cooling, the cooling rate of accelerated cooling, and the surface properties during accelerated cooling. Similar to the circumferential hardness difference, the shape is disturbed in the CU-O molding at the time of UOE pipe forming, and as a result, a larger pipe expansion rate is required to obtain a desired roundness. When the tube expansion rate is increased, the compressive strength is reduced due to the Bauschinger effect, so that the crushing resistance is reduced. On the other hand, if the hardness difference in the tube thickness direction is small, a high roundness can be obtained even if the tube expansion rate is small, and the crush resistance is improved in terms of both suppressing the decrease in compressive strength and ensuring the roundness. be able to. The effect is not seen if the hardness difference in the pipe circumferential direction exceeds 50, so the upper limit is set to 50. More preferably, it is 40 or less. In addition, the hardness difference in the pipe thickness direction shall be compared for those measured at the same position in the pipe circumferential direction and the pipe length direction.

Maximum surface layer hardness: 248 or less Since the surface layer hardness exceeds 248, cracks occur in SSC, so the maximum surface layer hardness is 248 or less. In addition, the measurement position of the surface layer hardness is 1 mm from the surface layer and 1 mm from the back layer. As described above, the local hardened portion is caused by the unevenness of the scale of the surface layer due to the application of the descaling water. It is possible to use the maximum value of the pipe circumferential direction positions measured at a pitch of 20 mm at the maximum at twice as long as the larger one of the nozzle intervals of the mill descaling device and the descaling device before accelerated cooling.

4). The higher the roundness of the steel pipe, the higher the crush resistance. Here, the roundness is defined as Dmax−Dmin. Dmax is the maximum measured outer diameter (mm), and Dmin is the minimum measured outer diameter (mm). Here, the roundness is determined by measuring the outer diameter at the opposite position by dividing the pipe circumference into 12 or 24 equal parts at any pipe length position of the manufactured steel pipe, and the maximum value and the minimum value among them. Can be determined as Dmax and Dmin.

The crushing resistance becomes higher as the circle becomes more round (that is, the roundness is closer to 0). However, UOE pipe-making cannot achieve a perfect circle, and the outer diameter is large. The roundness worsens as the thickness decreases. In the present invention, the roundness shown in the following formulas (4) and (5) can be obtained by making the hardness distribution in the pipe circumferential direction uniform.
When D / t ^0.6 ≦ 85 Dmax−Dmin ≦ 3.0 Formula (4)
When D / t ^0.6 > 85 Dmax−Dmin ≦ 0.04 D / t ^0.6 −0.4 Formula (5)
Here, D: nominal outer diameter (mm), t: tube thickness (mm).

By setting the roundness to 3.0 or less, the crushing resistance can be remarkably improved. Therefore, when particularly high crushing performance is required, the steel pipe dimension D / t ^0.6 is 85. In the following cases, it is preferable that Dmax−Dmin has an upper limit of 3.0. When D / t ^0.6 exceeds 85, Dmax-Dmin is preferably not more than 0.04 D / t ^0.6 -0.4.

  In the present invention, the high-strength steel sheet refers to a steel sheet having a tensile strength of 520 MPa or more corresponding to the SAWL415 grade of DNV-OS-F101.

5. Manufacturing Method In the present invention, a steel pipe raw material and a manufacturing method of the steel pipe for obtaining the above-mentioned base material microstructure and hardness distribution and desired performance are defined.

Slab heating temperature: 900-1200 ° C
The minimum temperature is 900 ° C. in order to obtain a minimum amount of Nb solid solution while austenizing the slab. On the other hand, when the slab is heated to a temperature exceeding 1200 ° C., the pinning effect by NbC and TiN is weakened, austenite grains grow significantly, and the base material toughness deteriorates. For this reason, slab heating temperature shall be the range of 900-1200 degreeC.

Cumulative rolling reduction below 900 ° C: 30-90%
The steel according to the present invention is in the austenite non-recrystallization temperature region at 900 ° C. or less by adding Nb. Austenite grains are extended by rolling under a large cumulative pressure below this temperature range, and in particular, the base material toughness is improved by making fine grains in the plate thickness direction. When the cumulative rolling reduction is less than 30%, fine graining is not sufficient and the toughness deteriorates, so the cumulative rolling reduction in the temperature range of 900 ° C. or lower is set to 30% or more. The larger the cumulative rolling reduction, the more problems are the warpage of the steel sheet during rolling and the reduction of rolling efficiency, and even if the rolling reduction exceeding 90% is secured, there is no significant change in material properties, so the upper limit is 90%. To do. Preferably it is 50 to 90%.

Rolling end temperature: (Ar ₃ −10 ° C.) or higher The rolling end temperature is lower when the base metal toughness is better, but when it is lower than (Ar ₃ −10 ° C.), the processed ferrite is formed in the base metal structure near the center of the tube thickness. Is generated and the HIC resistance is deteriorated, the lower limit is set to (Ar ₃ -10 ° C). More preferably, it is (Ar ₃ −10 ° C.) or more and 830 ° C. or less. In addition, the measurement of temperature shall measure a steel plate surface temperature with a radiation thermometer immediately after completion | finish of rolling. Although it is desirable to measure the transformation start temperature after processing by a thermal expansion test of steel having chemical components that can be regarded as substantially the same for the _three Ar points, the following equation (6) may be used instead.
Ar ₃ (° C.) = 910-310C-80Mn-20Cu-55Ni-15Cr-80Mo
Formula (6)
Here, each element symbol is content (mass%), and is 0 when not included.

Descaling before accelerated cooling Further, in addition to the manufacturing process described above, descaling is performed using a jet stream of high collision pressure immediately before accelerated cooling. In order to make a high-strength steel sheet with excellent material uniformity within the steel sheet, it is necessary to reduce the variation in hardness within the steel sheet, and in particular, while suppressing the hardness of the surface layer while maintaining the strength inside the steel sheet It is important to. In a steel sheet after rolling, unevenness in the thickness of the scale may occur in the width direction due to descaling or the like before rolling and during rolling. Further, when the scale thickness is large, the scale may be partially peeled off. If the scale thickness varies during accelerated cooling after rolling, the cooling speed of the steel sheet surface changes according to the thickness, and the hardness of the steel sheet surface also changes according to the cooling speed. . To increase the strength of steel sheets, it is effective to increase the cooling rate during accelerated cooling, but the effect of scale thickness on the surface layer hardness becomes significant when cooling at a high cooling rate. If there is unevenness, the variation in hardness increases and the material uniformity in the steel sheet deteriorates. As a countermeasure, the scale thickness can be uniformly reduced to such an extent that a large difference in the cooling rate does not occur by descaling the high collision pressure.

  In the present invention, descaling is performed such that the impinging pressure of the jet flow on the steel sheet surface is 1 MPa or more immediately before accelerated cooling. If the collision pressure of the jet flow on the surface of the steel sheet is less than 1 MPa, the descaling may be insufficient and unevenness in scale may occur, resulting in variations in surface hardness. Therefore, the collision pressure of the jet flow is set to 1 MPa or more. Descaling is performed using high-pressure water, but other jet streams may be used as long as the collision pressure of the jet stream on the steel sheet surface is 1 MPa or more. In addition, it is desirable to perform accelerated cooling within 5 seconds after descaling. This is because when the accelerated cooling is performed for more than 5 seconds after descaling, the scale grows, so that the cooling rate of the surface layer portion increases and the variation in hardness may increase.

When the accelerated cooling start temperature accelerated cooling start temperature is lower, mid-thickness of the steel sheet, i.e., ferrite is generated in the pipe wall thickness center, since the HIC resistance is degraded, and (Ar 3 _-30 ℃) or higher. More preferably, it is in the range of (Ar ₃ −30 ° C.) to (Ar ₃ + 10 ° C.). Note that the temperature is measured by measuring the surface temperature with a radiation thermometer immediately before descaling before accelerated cooling. Ar ₃ may substitute formula (6).

Accelerated cooling rate of the surface layer The accelerated cooling rate of the surface layer is an important factor that determines the structure and hardness of the surface layer. When the cooling rate of the surface layer exceeds 200 ° C / s, many martensites and lower bainite are generated in the surface layer structure. Since the surface hardness increases, the upper limit is set to 200 ° C./s. Preferably, it is not less than the cooling rate at the center of the plate thickness and not more than 150 ° C./s.

Average cooling rate of steel sheet The average cooling rate of steel sheet is an important factor that determines the thickness and thickness of the steel sheet, that is, the structure and hardness at the center of the pipe thickness. If it is less than 10 ° C./s, ferrite is generated in the center of the tube thickness and the HIC resistance is deteriorated, so the lower limit is made 10 ° C./s. More preferably, it is 10-100 degreeC / s.

  Note that the average temperature and cooling rate in the plate thickness direction of the steel sheet cannot be physically measured directly, but can be obtained in real time by performing a simulation calculation based on the temperature change of the steel sheet surface. .

Accelerated cooling stop temperature After completion of rolling, the bainite phase is generated by accelerated cooling to 300 to 600 ° C., which is a temperature range of bainite transformation. When the cooling stop temperature is lower than 300 ° C., a large amount of MA is generated from austenite that has not been transformed at the time of accelerated cooling, and the HIC resistance and compressive strength are lowered. On the other hand, when the temperature exceeds 600 ° C., not only the desired strength cannot be obtained, but also a part of austenite that has not been transformed at the time of accelerated cooling undergoes ferrite transformation, and the HIC resistance and compressive strength are lowered. ℃. The temperature is measured by measuring the surface with a radiation thermometer when the temperature difference between the surface layer and the center of the plate thickness is reduced by recuperation. More preferably, the measurement is performed within a time range of 10 to 120 seconds after the water cooling of the entire steel plate is completed.

Cooling to room temperature;
In the present invention, the influence of the cooling process after stopping the accelerated cooling on the material and shape of the steel sheet is not great. Therefore, the cooling means for cooling the steel plate after accelerated cooling to room temperature is air cooling, for example. Here, air cooling may be allowed to cool, or may be positively cooled by blowing air to the steel plate. Other than this, water cooling or other means may be used.

  After the above accelerated cooling, the thick steel plate obtained by cooling to room temperature is formed into a cold tube.

  The present invention forms a steel pipe using the thick steel plate manufactured by the above-described method, and the steel pipe is formed into a steel pipe shape by cold forming such as UOE process or press bend. Thereafter, seam welding is performed, and any welding method can be used as long as sufficient joint strength and joint toughness can be obtained, but it is preferable to use submerged arc welding in terms of excellent welding quality and production efficiency. . The present invention is directed to a welded steel pipe in which the butt portion is joined by welding of two or more layers and expanded. By welding two or more butt portions, it is easy to prevent deterioration of toughness due to an excessive increase in welding heat input, and a good appearance shape can be stably obtained with respect to the weld bead. After welding the butt, pipe expansion is performed to remove residual welding stress and improve the roundness of the steel pipe. Hereinafter, the tube expansion rate will be described.

Tube expansion rate: 0.5-1.1%
In general, a thick-walled and high-strength UOE steel pipe is formed at a pipe expansion rate in the range of about 0.9 to 1.2%. The tube expansion rate is an important factor in securing the crushing resistance. The lower the tube expansion rate, the higher the compressive strength, but the lower the roundness. On the other hand, the higher the tube expansion rate, the higher the roundness, but the compressive strength decreases, and the steel pipe is damaged by a die. Even if the tube expansion rate is less than 0.5%, the compression strength increase effect cannot be expected so much, so the lower limit is made 0.5%. On the other hand, in the present invention, since the formability is remarkably improved by making the tube thickness and the hardness in the tube circumferential direction uniform, a desired roundness can be obtained even if the tube expansion rate is low. When the tube expansion rate exceeds 1.1, the roundness increases after that because the effect of improving the circularity by increasing the tube expansion rate is saturated, so the upper limit is set to 1.1%. If the pipe expansion rate is in the range of 0.5 to 1.1% as defined above, excellent crushing performance can be obtained. Moreover, More preferably, it is 0.5 to 1.0%.

  Steel with the chemical composition shown in Table 1 is made into a slab by continuous casting, and the heated slab is rolled by hot rolling, followed by high impact pressure descaling just before the accelerating cooling device and water-cooled cooling equipment within 5 seconds Accelerated cooling was used to produce a thick steel plate. The manufactured steel plate was piped by UOE forming. The welding of the butt portion was performed by submerged arc welding of one layer on each of the inner surface and the outer surface. In addition, the compression rate of O press was 0.3%. Table 2 shows the details of the manufacturing method of the welded steel pipe manufactured by the above method. The heating temperature was the average temperature of the entire steel sheet, the rolling end temperature and the cooling start temperature were the steel sheet surface temperature, and the cooling stop temperature was the steel sheet surface temperature after reheating.

  The roundness was calculated by measuring the outer diameter of the steel pipe at six or more locations in the pipe circumferential direction and obtaining the maximum value / minimum value. The target value of roundness (Dmax−Dmin) is set in the range described in claim 3.

  The fraction of the microstructure of the steel pipe is obtained by averaging the total area fraction of the ferrite phase and the bainite phase from image analysis of 10 optical micrographs observed at 400 times the position of 1 mm from the surface layer and the tube thickness center position. Asked. The central segregation part was excluded for the tube thickness center. The M-A fraction was determined from the image analysis of five SEM (scanning electron microscope) photographs of the structure observed at a position 1 mm from the surface layer, 1/4 position from the inner surface of the tube thickness, and 2000 times the center of the tube thickness. The rate was averaged and the area fraction value was considered equal to the volume fraction value, assuming that the second phase was uniformly dispersed in the steel pipe.

  Hardness distribution in the pipe circumferential direction is measured at a pitch of 1 mm from the front surface at 40 to 320 ° and a 1 mm position from the back surface at a 20 mm pitch, starting from the seam welded portion of the steel pipe. The difference in values was determined. The hardness distribution in the tube thickness direction was measured at a 1 mm pitch from the surface layer to 1 mm from the surface and 1 mm from the back surface at a 90 ° position of the steel pipe, and the difference between the minimum value and the maximum value was obtained. The maximum value of the hardness result measured above was used for the maximum hardness of the steel pipe. The hardness was measured with a load of 10 kgf (98 N) using a Vickers hardness tester.

  The tensile yield stress and tensile strength of the steel pipe were obtained by collecting a full thickness tensile test piece from the circumferential direction of the steel pipe at the 90 ° position. The target value of tensile strength is 520 MPa or more. For the compressive yield stress, a cylindrical test piece having a diameter of 20 mm and a length of 60 mm in accordance with ASTM E9 was taken from a position 1 mm from the inner surface of the steel pipe 180 ° position, and the stress at 0.5% was obtained. The target value of compressive yield stress was 80% or more of the tensile yield stress. For the DWTT (Drop Weight Tear Test) test, a 19 mm DWTT specimen taken from the circumferential direction of the steel pipe was used at -17 ° C. to determine the fracture surface ratio. Two tests were carried out each, and the average value was set to 85% or more.

Crush resistance was evaluated crush resistance _{p c} with DNV-OS-F101 defined in external pressure of crushing resistance by the following formula (7) to (11).

Here, α _U = 0.96, α _fab = 1.00, E = 206000 MPa, ν = 0.3, t = tube thickness (mm), D = outer diameter (mm), Dmax−Dmin = roundness (Mm), YS = compressive yield stress at normal temperature (MPa), fy, temp = difference between compressive yield stress at room temperature and the maximum use temperature (in the present invention, assuming use at 50 ° C. or lower and 0 MPa) is there. The target value of the crush resistance is 1.05 times or more with reference to a value calculated by substituting the tensile yield stress, f ₀ = 0.0050, α _fab = 0.85 into the equations (7) to (11) Decided to become.

Based on NACE Standard TM0284-2003, the HIC characteristics were determined by taking three samples each and placing 96 specimens in a 5% NaCl + 0.5% CH ₃ COOH aqueous solution saturated with hydrogen sulfide having a pH of about 3. After immersion for a period of time, the presence or absence of cracks on the entire surface of the test piece was investigated by ultrasonic flaw detection, and evaluated by a crack area ratio (CAR). Here, the maximum value of each steel plate was regarded as the CAR of the steel plate, and CAR ≦ 5% was regarded as acceptable. Based on NACE Standard TM0177-2005, the SSC characteristics were obtained by applying a load stress of 90% of the yield stress of the base material to a 3-point bendable sample with a thickness of 5 mm taken from the inner surface side, and having a pH of about 3 It was evaluated whether or not the test piece was immersed in a NaCl + 0.5% CH ₃ COOH aqueous solution saturated with hydrogen for 720 hours to break. The test was performed three by three, and when all three were not broken, No crack was evaluated, and when even one was broken, it was evaluated as Crack. It can be evaluated that the sour resistance is excellent when all three pieces are not broken in this test.

  Table 3 shows the microstructure and mechanical properties of the welded steel pipe obtained by the manufacturing method shown in Table 2. It can be seen that all of the welded steel pipes within the scope of the claims of the present invention have both excellent crushing resistance and sour resistance while satisfying the strength and DWTT performance required for a line pipe. On the other hand, the welded steel pipe outside the scope of the present invention does not satisfy any of these characteristics.

Claims (4)

A welded steel pipe in which a base material made of a thick steel plate is formed into a tubular shape, and a butt portion thereof is joined by welding of two or more layers,
% By mass
C: 0.02 to 0.08%
Si: 0.01 to 0.50%
Mn: 0.5 to 1.5%
P: 0.010% or less S: 0.001% or less Al: 0.06% or less Nb: 0.002 to 0.100%
Ca: 0.0005 to 0.0040%
O: contains 0.0030% or less,
Cu: 1.0% or less Ni: 1.0% or less Cr: 1.00% or less Mo: 0.5% or less
Furthermore, Ceq defined by the formula (1) is 0.30 or more,
PHIC defined by the formula (2) is 1.00 or less,
ACR defined by equation (3) is 1.00 to 6.00,
The balance Fe and inevitable impurities,
The base metal surface layer metallographic structure is upper bainite or ferrite and upper bainite,
The metal structure of the base metal tube thickness center is the upper bainite single phase,
The volume fraction of island martensite (MA) is less than 4% throughout the tube thickness,
And the maximum value of the hardness difference in the pipe thickness direction at the same position in the pipe circumferential direction is 50 or less,
The maximum value of the hardness difference in the pipe circumferential direction at the same position in the pipe thickness direction is 50 or less,
A high-strength sour-line pipe excellent in crushing resistance, characterized in that the maximum surface hardness is 248 or less.
Ceq = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5 Formula (1)
PHIC = 4.46C + 2.37Mn / 6 + (1.18Cr + 1.95Mo + 1.74V) / 5 + (1.74Cu + 1.7Ni) /15+22.36P Formula (2)
ACR = (Ca− (0.18 + 130Ca) O) /1.25S Formula (3)
Here, the element symbol on the right side of each formula represents the content (% by mass), and is 0 when not contained. Furthermore, in mass%,
V: 0.005 to 0.100%
Ti: 0.005 to 0.050%
Mg: 0.0005 to 0.0040%
The high-strength sour line pipe excellent in crush resistance according to claim 1, comprising at least one selected from the group consisting of: 3. A high-strength sour line pipe excellent in crush resistance according to claim 1, wherein the roundness satisfies the following formula (4) or (5).
When D / t ^0.6 ≦ 85 Dmax−Dmin ≦ 3.0 Formula (4)
When D / t ^0.6 > 85 Dmax−Dmin ≦ 0.04 D / t ^0.6 −0.4 Formula (5)
Here, D: nominal outer diameter (mm), t: tube thickness (mm), Dmax-Dmin: roundness (mm), Dmax: maximum measured outer diameter (mm), Dmin: minimum measured outer diameter (mm) It is. After heating the steel material to 900 to 1200 ° C., after performing hot rolling with a cumulative reduction rate of 900 ° C. or less being 30 to 90% and a rolling end temperature being (Ar ₃ −10 ° C.) or more,
Immediately before accelerated cooling, descaling with a jet collision pressure on the steel sheet surface of 1 MPa or more,
Immediately _(Ar 3 -30 ℃) cooling rate of the surface layer of the cooling rate from the temperature of 200 ° C. / s or less and an average or perform accelerated cooling stop temperature is 300 ° C. to 600 ° C. cooled at 10 ° C. / s or higher,
After that, the steel plate obtained by cooling to room temperature is formed into a tubular shape in the cold,
After welding the butt to make a steel pipe,
Furthermore, it manufactures by performing pipe expansion with a pipe expansion rate of 0.5 to 1.1%, The high-strength sour-proof line excellent in crushing resistance of any one of Claims 1-3 characterized by the above-mentioned. Pipe manufacturing method.