JP2012241273A - High strength linepipe superior in collapse resistance and sour-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: The high strength linepipe having excellent collapse resistance and sour resistance is a welded steel pipe, 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 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 further contains a certain amount of at least one selected from Cu, Ni, Cr and Mo, wherein Ceq is 0.30 or more, PHIC is 1.00 or less, ACR is 1.0-6.0, and the remainder comprises Fe and inevitable impurities. In the high strength linepipe, the volume fraction of island martensite (M-A) is 1% or less in the whole area of pipe thickness, and the metal structure and hardness of the surface layer part of the base material and the center part of pipe thickness of the base material 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.

  In addition, hydrogen sulfide may be contained in oil and natural gas transported by the line pipe. In that case, in addition to the above-described characteristics, it is required to ensure excellent sour resistance performance. Corrosion cracking due to hydrogen sulfide includes hydrogen induced cracking (HIC) and hydrogen sulfide stress corrosion cracking (SSC). When steel pipe materials that ensure strength by accelerated cooling, such as line pipes, are used, the SSC of the surface layer Occurrence becomes a problem. In order to suppress the occurrence of SSC on the surface layer, it is generally known that it is effective to suppress the surface layer hardness to Vickers hardness (HV) 248 or less, but recently, as a stricter standard, There is an increasing demand for consumers to suppress the surface hardness to Vickers hardness (HV) 230 or less.

  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 8 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 9 and patent document 10, by decomposing the island martensite (MA) contained in the structure of a thick steel plate, and further reducing the hardness of bainite in a mixed structure of ferrite and bainite, A method is disclosed in which the compressive yield stress is improved by improving the roundness by improving the compressive yield stress due to the effect and at the same time making the hardness in the tube thickness direction uniform.

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 2010-235993 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 8, 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 a 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 9 and Patent Document 10, 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. When the scale is thick, the surface layer is rapidly cooled and hardened only in the part where the scale exists, so the surface hardness distribution in the plate width direction does not become flat, and the SSC caused by the local hardening region Problems remain, such as the occurrence of defects and the irregular shape during pipe making. Furthermore, in order to suppress the surface layer hardness to 230 or less while ensuring the desired strength of the steel pipe, it is assumed that the scale is sufficiently peeled off during accelerated cooling only by performing rapid heating treatment after normal accelerated cooling. Is insufficient.

  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 ensure excellent sour resistance performance, it is necessary to add an appropriate amount of Ca in order to spheroidize MnS generated in the central segregation part. At that time, by changing the ACR of the formula (3) described later to 1.00 to 6.00, the needle-like MnS is changed to spherical CaOS, and the hydrogen-induced cracking (hereinafter referred to as “HIC”) fracture during the test It turned out that it can suppress becoming an origin. 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. Even when needle-like MnS in the central segregation part is not generated, HIC may be generated starting from NbTi—CN, Ca—Al inclusions, bubbles, etc., but PHIC of formula (2) is 1. By making it 0 or less, alloy elements with high hardenability are prevented from concentrating in the central segregation, and by making the structure at the center of the tube thickness into a bainite single phase, C of austenite during the two-phase region transformation It was found that concentration can be suppressed and generation of HIC can be suppressed.

  On the other hand, for the occurrence of SSC from the surface layer, it is conventionally known that the occurrence of cracks can be suppressed by suppressing the surface layer hardness to 230 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. As a result, the hardened portion that existed locally in the plate width direction could be eliminated.

  On the other hand, when the desired tube thickness was large or the target strength was large, a portion where the surface hardness exceeded 230 was partially observed even in the portion where the scale was sufficiently peeled off by descaling. Therefore, as a result of examining the accelerated cooling conditions, when accelerated cooling is performed such that the cooling rate of the surface layer exceeds 100 ° C./s, depending on the chemical composition of the steel material, even if the scale is not located, Has been found to be over 230. Therefore, in the present invention, accelerated cooling is divided into two stages, and until the surface layer completes transformation, cooling conditions with a low cooling rate of 100 ° C./s or less are applied to the surface layer, and after the surface layer completes transformation, We examined in detail the pattern of rapid cooling. As a result, by setting the average temperature of the steel sheet after the first stage cooling to 630 ° C. or higher, the transformation due to the first stage cooling inside the steel sheet can be suppressed, and by performing the low temperature transformation by the second stage strong cooling, the steel sheet It was found that the inside can obtain almost the same strength as normal cooling. As described above, by applying descaling before accelerated cooling and two-stage accelerated cooling controlled to an optimal pattern, the surface of the steel pipe is not affected by much strength compared to normal accelerated cooling. It was found that the hardness of can be suppressed to 230 or less.

  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. Lowering the tube expansion rate in this way 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 crush resistance. Clarified what can be done.

  In the present invention, the heat treatment conditions after accelerated cooling are also examined, and by setting the reheating conditions to the cooling stop temperature or higher and 400 ° C. or higher, the MA contained in the steel sheet is decomposed into cementite, and the Bausinger effect is improved. Generation | occurrence | production can be reduced and the compressive strength of a steel pipe can be improved, As a result, it turned out that the outstanding crushing property can be ensured. Moreover, since reheating and tempering the steel sheet has the effect of flattening the hardness distribution of the steel sheet, it is advantageous from the viewpoint of ensuring the roundness.

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 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: 1.0% or less Mo: containing at least one 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 1.00 or less,
ACR defined by equation (3) is 1.00 to 6.00,
The balance Fe and inevitable impurities,
The metal structure of the base material surface layer portion 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 1.0% or less 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 30 or less,
The maximum value of the hardness difference in the pipe circumferential direction at the same position in the pipe thickness direction is 30 or less,
A high-strength line pipe excellent in crushing resistance and sour resistance, characterized in that the maximum surface hardness is 230 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 line pipe excellent in crushing resistance and sour resistance according to [1], comprising at least one selected from the group consisting of:
[3] The high-strength line pipe excellent in crush resistance and sour resistance according to [1] or [2], wherein the roundness satisfies the following expression (4) or (5):
In the case of D / t ^0.6 ≦ 135 Dmax−Dmin ≦ 3.0 Formula (4)
When D / t ^0.6 > 135 Dmax−Dmin ≦ 0.04 D / t ^0.6 −1.4 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 900-1200 ° C,
After performing hot rolling with a cumulative rolling reduction of 900 ° C. or lower at 30 to 90% and a rolling end temperature of (Ar ₃ −10 ° C.) or higher,
Immediately before accelerated cooling, descaling with a jet collision pressure on the steel sheet surface of 1 MPa or more,
Thereafter, the steel sheet surface temperature _(Ar 3 -80 ℃) above temperature range, the steel sheet surface layer cooling rate is 10 ° C. / s or higher 100 ° C. / s or less,
The steel plate surface layer temperature is 300 ° C. or higher and 600 ° C. or lower, and the steel plate average temperature is 630 ° C. or higher.
Subsequently, the second stage accelerated cooling is performed until the steel sheet average cooling rate is 10 ° C./s or higher and the steel plate average temperature is 300 ° C. or higher and 600 ° C. or lower, and then the steel plate average temperature is set to the cooling stop temperature or higher and 400 to 600. Reheated to a temperature range of ℃, cooled to room temperature, formed into a tubular shape in the cold,
After welding the butt to make a steel pipe,
Furthermore, it excels in the crushing resistance and the sour resistance according to any one of [1] to [3], which is produced by expanding the tube at a tube expansion rate of 0.5 to 1.1%. Manufacturing method of high strength line pipe.
(700-T) / V ≧ 3 Formula (6)
Here, T: steel plate surface layer cooling stop temperature (° C.) for the first stage accelerated cooling, and V: steel sheet surface layer cooling rate (° C./s) for the first stage accelerated cooling.

  According to the present invention, a thick steel plate suitable for thick-walled high-strength line pipes used for transportation of oil and natural gas, particularly for submarine pipelines, which has excellent crushing resistance and sour resistance, is produced by cold forming and welding. This makes it possible to produce high toughness welded steel pipes and is extremely effective in the industry.

2 is a diagram illustrating one embodiment of an arrangement of devices.

  The component composition and microstructure of the welded steel pipe for high-strength line pipe excellent in crush resistance and sour resistance according to the present invention are defined. 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. Component composition Reasons for limitation of 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 the strength and toughness, but if it is less than 0.5%, the effect is not sufficient, and if it exceeds 1.5%, it concentrates in the central segregation part, and increases the hardness of the central segregation part and The generation of MnS significantly degrades sour resistance performance. 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 that is easily segregated and is concentrated in the center, and even if contained in a small amount, the hardness of the center segregation is significantly increased and sour resistance is deteriorated. Moderate. However, 0.010% is allowed.

S: 0.001% or less S binds Mn to generate MnS. In addition, since S is an element that is also likely to concentrate Mn in central segregation, a large amount of S causes a large amount of central segregation of MnS to be generated, and sour resistance is significantly 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 HIC resistance performance by making the shape of acicular MnS formed in the central segregation part 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 O is contained excessively, the content of these Al and Ca-based oxides in the steel becomes too large, forming clusters and degrading the HIC resistance, 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 contained. That is, it is necessary to contain one or more selected from Cu: 1.0% or less, Ni: 1.0% or less, Cr: 1.0% or less, and Mo: 0.5% or less. Hereinafter, the reasons for limitation will be described for each element.

Cu: 1.0% or less Cu is an element effective for improving toughness and increasing strength. However, if added over 1.0%, the base material due to weldability deterioration or precipitation embrittlement, HAZ toughness deterioration, and further the decrease in compressive strength due to an increase in the MA volume fraction becomes a problem. Is added, 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 is cracked 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 volume fraction. Since it becomes a problem, when adding Ni, the upper limit is made 1.0%. More preferably, it is 0.05 to 0.45%. Cr: 1.0% or less Cr is an element effective for obtaining sufficient strength even at low C like Mn. However, if added over 1.0%, the weldability deteriorates and the compressive strength decreases due to an increase in the MA volume fraction. Therefore, when Cr is added, its content is 1.0% or less. And More preferably, it is 0.10 to 0.40%.

Mo: 0.5% or less Mo is an element that improves hardenability and greatly contributes to an increase in strength. However, addition exceeding 0.5% leads to a decrease in compressive strength due to an increase in the MA volume fraction and a deterioration in weld heat affected zone toughness. Therefore, when Mo is added, the content is 0.5%. The following. 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 maximum hardness at the time of welding, but is also known to show a good correlation with the base material strength at the same time. 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 the formula represents the respective content (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 the formula represents the respective content (% by mass). 0 if not contained.

ACR: 1.00 to 6.00
ACR defined by the following formula (3) is an index for evaluating whether or not MnS generated in the central segregation part can be spheroidized by Ca. When it is less than 1.00, coarse MnS remains in the central segregation and resistance Degrading HIC performance. In the case of 1.00 or more, CaOS is generated and coarse MnS is not generated. However, when it exceeds 6.00, CaOS generates a cluster and deteriorates the HIC resistance. Therefore, 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 the formula represents the respective 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 the following V, Ti, and Mg can be selectively contained.
V: 0.005-0.100
V can improve the strength of the base material mainly by improving the hardenability. The effect does not appear at less than 0.005%. On the other hand, the addition of more than 0.100% causes precipitation embrittlement and deteriorates the base metal toughness and 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, the weld heat affected zone toughness deteriorates. Therefore, when adding Ti, its content is A range of 0.005 to 0.050% is preferable. 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 content of inevitable impurities is N: 0.0060% or less, B: 0.0005% or less.

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 upper bainite or a mixed structure of ferrite and upper bainite. By making the main component (hereinafter also referred to as “ferrite + upper bainite”), an excessive increase in surface hardness is prevented. Other than these main structures, there are martensite, MA, lower bainite, pearlite, and cementite, and since all of them are harder than ferrite and upper bainite, the smaller one is more preferable. The volume fraction of the main tissue is not particularly defined, but is 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 central segregation part 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. Here, the upper bainite single phase means that the bainite structure is 85% or more.

  Other than the structure mainly composed of upper bainite, there are martensite, MA, lower bainite, pearlite, cementite, and ferrite, all of which cause a hardness difference from the main structure as the second phase, and have HIC resistance and compressive strength. It is better to use less. The volume fraction of the main tissue is more preferably 95% 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 position that is ± 2 mm from the tube thickness center and that excludes the central segregation part.

The M-A volume fraction M-A is a structure having the highest hardness among the hard second phases described above, and it is preferable that the M-A volume fraction M-A is as small as possible because it significantly deteriorates the HIC resistance and compressive strength. In the present invention, MA can be decomposed by reheating after accelerated cooling and reduced to a level that does not substantially contain it. In addition, although MA may remain a little by reheating, the volume fraction of MA can be tolerated up to 1.0%. More preferably, it is 0.5% or less. The entire pipe thickness is the entire 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 at the same position in the pipe thickness direction: 30 or less The difference in hardness in the pipe circumferential direction at the same position in the pipe thickness direction (sometimes simply referred to as “hardness difference in the pipe circumferential direction”) is mainly used. Occurs due to 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 making, 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. Since the effect is conspicuous when the hardness difference in the pipe circumferential direction is made Vickers hardness (HV) 30 or less, the upper limit is set to 30. More preferably, it is 20 or less.

  In addition, the hardness difference in the pipe circumferential direction is to be compared for those measured at the same position in the pipe thickness direction and the pipe length direction, and as described above, the hardness difference in the pipe circumferential direction mainly occurs in the surface layer portion, so that 1 mm from the surface layer. And if two places of 1 mm from the back layer are measured, the difference in hardness in the pipe circumferential direction of the steel pipe can be regarded as a representative.

Maximum hardness difference in the pipe thickness direction at the same position in the pipe circumferential direction: 30 or less The difference in hardness in the pipe thickness direction at the same position in the pipe circumferential direction (sometimes simply referred to as “hardness difference in the pipe thickness direction”) is mainly. This is caused by the surface layer structure before accelerated cooling, the cooling rate of accelerated cooling, and the surface properties during accelerated cooling, as well as the hardness difference in the circumferential direction of the pipe, and the shape in CU-O forming during UOE pipe forming As a result, a larger tube 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 appears remarkably when the difference in hardness in the pipe circumferential direction is made Vickers hardness (HV) 30 or less, so the upper limit is set to 30. More preferably, it is 20 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 value of surface hardness: 230 or less As described above, recently, in order to ensure SSC resistance, it is often required that the surface layer hardness is Vickers hardness (HV) 230 or less. Since this invention is related to the steel pipe which can respond to this specification, the maximum value of surface hardness is set to 230. 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 assumed that the maximum value of the pipe circumference direction position twice as long as the larger nozzle spacing of the mill descaling device and the pre-acceleration cooling descaling device measured at a pitch of 20 mm is used.

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). The roundness is measured by measuring the outer diameter at the opposite position by dividing the pipe circumference into 12 or 24 parts at an arbitrary pipe length position of the manufactured steel pipe, and the maximum value and the minimum value among them are respectively set to Dmax. , 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, roundness shown in the following formulas (4) and (5) can be obtained by making the hardness distribution in the pipe circumferential direction uniform.
In the case of D / t ^0.6 ≦ 135 Dmax−Dmin ≦ 3.0 Formula (4)
When D / t ^0.6 > 135 Dmax−Dmin ≦ 0.04 D / t ^0.6 −1.4 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.

When the roundness is 3.0 or less, the crushing property can be remarkably improved. Therefore, when particularly high crushing performance is required, the steel pipe dimension D / t ^0.6 is 135. In the following cases, the upper limit of Dmax−Dmin is preferably 3.0. When D / t ^0.6 exceeds 135, it is preferable that Dmax−Dmin is not more than 0.04 D / t ^0.6 −0.4.

5. Manufacturing Method In the present invention, a steel pipe material and a manufacturing method of the steel pipe are specified in order to obtain the above-mentioned base material microstructure and hardness distribution and desired performance.

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 steel material 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%
In the steel according to the present invention, 900 ° C. or lower is the austenite non-recrystallization temperature region due to the addition of Nb. Below this temperature range, hot rolling under cumulative large pressure is performed to extend the austenite grains, and in particular to make them fine in the plate thickness direction to improve the base metal toughness. 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 in the range of 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. It is preferable 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, but the following formula (7) may be used instead.
Ar ₃ (° C.) = 910-310C-80Mn-20Cu-55Ni-15Cr-80Mo formula (7)
Here, each element symbol is content (mass%), and is 0 when not included.

Descaling immediately before accelerated cooling Further, in addition to the above-described manufacturing process, descaling is performed using an injection flow having a 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. Moreover, it is necessary 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.

Cooling start temperature The steel sheet surface temperature at the start of cooling is (Ar ₃ -80 ° C) or higher. If the steel plate surface temperature at the start of cooling is lower than this, the amount of ferrite produced inside the steel pipe material before accelerated cooling will increase and the strength will decrease and the HIC resistance will deteriorate. Therefore, the steel sheet surface temperature at the start of cooling is set to (Ar ₃ -80 ° C.) or higher. Subsequent accelerated cooling (sometimes simply referred to as cooling) employs a two-stage accelerated cooling method. That is, in the first stage accelerated cooling, the cooling rate of the surface layer is 10 ° C./s or more and 100 ° C./s or less, the steel sheet surface layer temperature is 300 ° C. or more and 600 ° C. or less, and the steel plate average temperature is 630 ° C. or more. , Which satisfies the condition (6).
(700-T) / V ≧ 3 Formula (6)
Here, T: Steel sheet surface layer cooling stop temperature (° C.) for the first stage cooling, and V: Steel sheet surface layer cooling rate (° C./s) for the first stage cooling.
The second stage of accelerated cooling is characterized in that the steel plate average cooling rate is 10 ° C./s or more and the steel plate average temperature is 300 ° C. or more and 600 ° C. or less. This will be described below.

First-stage accelerated cooling In the first-stage accelerated cooling, it is necessary to select a cooling pattern in which transformation near the surface layer is completed at a low cooling rate and transformation does not occur as much as possible inside the steel sheet.
First-stage surface layer cooling rate In order to reduce the hardness of the steel sheet surface layer to 230 or less within the range of strength and tube thickness desired by the present invention, the cooling rate of the steel sheet surface layer is set to 100 ° C. in a state where the scale is uniformly peeled. / S or less. On the other hand, under the cooling conditions in which the cooling rate of the surface layer is less than 10 ° C./s, a difference in cooling rate from the inside of the steel sheet cannot be established and the inside of the steel sheet is transformed, so the lower limit is set to 10 ° C./s.
First-stage surface layer cooling stop temperature When the surface layer temperature of the steel sheet at the time of cooling stop exceeds 600 ° C., the surface layer has not completely transformed and is rapidly cooled and hardened by the second-stage accelerated cooling. On the other hand, when the temperature is less than 300 ° C., recuperation does not occur sufficiently, a low temperature transformation structure is generated, and the surface layer portion is excessively hardened. The first-stage surface cooling stop temperature range for setting the surface hardness to 230 or less is set to 300 ° C. or more and 600 ° C. or less. More preferably, it is 350 degreeC or more and 600 degrees C or less.

Furthermore, in order to make the surface layer hardness 230 or less, it is necessary to set the surface layer cooling rate and the surface layer cooling stop temperature in the first stage to a range satisfying the formula (6). What is meant by the left side of the above equation (6) is the cooling time of the first stage cooling. That is, the first stage cooling needs to be continued for 3 seconds or more. This is because a time of 3 seconds or more is required for the ferrite and bainite phases to be sufficiently generated so that the surface layer structure is not hard. When the formula (6) is not satisfied, martensite and island-like martensite (MA) are generated in the steel sheet surface layer, the hardness of the surface layer increases significantly, and the variation in hardness increases. It is necessary to satisfy the expression (6) as the eye constraint.
Average cooling stop temperature of the first steel plate Average cooling stop temperature of the steel plate is an index for controlling the strength inside the steel plate, and the cooling rate of the first stage accelerated cooling is slow, so the inside of the steel plate is not transformed as much as possible. Is good. When the average cooling stop temperature in the thickness direction of the steel sheet is less than 630 ° C., the transformation inside the steel sheet progresses and the strength decreases in the first stage accelerated cooling and in the air cooling state between the first and second stages. Is 630 ° C. Therefore, the first-stage accelerated cooling is performed under the condition that satisfies the formula (6) until the steel sheet average temperature reaches 630 ° C. or higher. More preferably, the first stage accelerated cooling is performed under the condition that satisfies the formula (6) up to the temperature range where the average temperature of the steel sheet is 680 ° C. or higher.

The average temperature and cooling rate in the steel sheet thickness direction cannot be directly measured physically, but can be obtained in real time by performing a simulation calculation based on the temperature change of the steel sheet surface.
Average cooling rate of the second stage steel plate The second stage accelerated cooling is performed after the transformation of the surface layer is complete, so even if the cooling rate is increased, the surface layer will not be excessively hardened, and the cooling is as fast as possible to ensure the strength of the base metal. Speed is better. In addition, this manufacturing method is an effective process for reducing the hardness of the surface layer of a thick material. For example, a cooling rate of 10 ° C./s or more is necessary to obtain sufficient strength at 60 mm. Is 10 ° C./s. More preferably, it is 20 ° C./s or more.
Second stage steel plate average cooling stop temperature If the second stage steel plate average cooling stop temperature exceeds 600 ° C, not only the desired strength is not obtained, but also the transformation is not completely completed, so some of the untransformed austenite It becomes M-A and reduces HIC resistance and crush resistance. On the other hand, even when the temperature is lower than 300 ° C., MA is generated during the transformation, and the HIC resistance and the crush resistance are lowered. Therefore, the second stage accelerated cooling is performed to a temperature range where the average temperature of the steel sheet is 300 ° C. or more and 600 ° C. or less.

Although the time between the first stage and the second stage of accelerated cooling is not particularly specified, the second stage of cooling is performed as quickly as possible in order to suppress the transformation inside the steel sheet due to air cooling after the first stage of cooling. The start of the second stage accelerated cooling is preferably within 20 seconds after the first stage cooling.
Reheating In this invention, in order to decompose | disassemble MA contained in a steel plate structure | tissue, reheating heat processing is performed to the steel plate after an accelerated cooling stop. If the heat treatment temperature is not higher than the cooling stop temperature and 400 ° C. or higher, the MA is not sufficiently decomposed, so the lower limit is set to the cooling stop temperature or higher and 400 ° C. or higher. On the other hand, when the steel plate average temperature is reheated to a temperature range exceeding 600 ° C., the DWTT performance deteriorates due to cementite agglomeration and precipitation of carbides such as Nb, so the upper limit is set to 600 ° C. Therefore, the steel sheet average temperature is reheated to a temperature range of 400 to 600 ° C. above the cooling stop temperature. Furthermore, it is preferable to reheat the steel sheet average temperature to a temperature range of 400 to 550 ° C. above the cooling stop temperature. The temperature at the center of the tube thickness (that is, the center of the thickness of the steel plate as the base material) cannot be physically measured directly, but is calculated in real time by performing a simulation calculation based on the temperature change of the steel plate surface. be able to.

  From the viewpoint of reducing the manufacturing efficiency and the fuel cost required for heat treatment, the above reheating of the steel plate after the accelerated cooling stop is preferably started immediately after the accelerated cooling stop. For example, the reheating is performed within 120 seconds after the cooled stop. It is desirable to start heating.

FIG. 1 shows an example of equipment suitable for manufacturing a steel sheet according to the present invention. A hot rolling mill 2, a high collision pressure descaling device 3, an acceleration cooling device 4, and an induction heating device 7 are arranged in the steel plate conveyance line 1 from the upstream side toward the downstream side. Moreover, the hot straightening machine 5 can also be installed in front of the descaling device 3 (upstream side). By improving the shape of the steel sheet with a hot straightening machine, the collision pressure of the jet flow can be increased, so that descaling can be performed more efficiently at low cost. On the other hand, the hot straightening device 6 can also be installed in front of the induction heating device 7 (upstream side). By improving the shape of the steel sheet with a hot straightening machine, for example, it is possible not only to avoid an accident where a steel sheet having a large warp collides with the induction heating device, but also improves the temperature uniformity during induction heating. As a result, the safety of the equipment and the material uniformity can be improved.
Cooling to room temperature;
The reheating temperature of the present invention does not affect the material and shape of the steel sheet by the subsequent cooling process. Therefore, the cooling means for cooling the steel plate after reheating to room temperature may be air cooling, for example. It may be allowed to cool, or may be actively air-cooled by blowing air onto the steel plate. Other than this, water cooling or other means may be used.

  After the above reheating heat treatment, the thick steel plate obtained by cooling to room temperature by air cooling is formed into a tubular shape in the cold.

  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. The upper limit of the roundness is 1.1% because the roundness improvement effect due to the increase in the pipe expansion rate exceeding 1.1 is saturated. As described above, excellent pipe crushing performance can be obtained if the pipe is formed in the range of the specified expansion ratio of 0.5 to 1.1%. 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 accelerated cooling device, and water-cooled cooling equipment within 5 seconds Was used for accelerated cooling. Accelerated cooling is performed using the equipment shown in FIG. 1, and after the first stage of accelerated cooling is completed, the second stage of accelerated cooling is started within 20 seconds, and immediately after the completion of accelerated cooling, rapid heating is performed by induction heating. Then, a thick steel plate was produced by air cooling. 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 is the average temperature of the entire steel sheet, the rolling end temperature and cooling start temperature, the surface layer cooling stop temperature is the steel sheet surface temperature, the surface heating temperature by induction heating is the steel sheet surface temperature, the plate average temperature and the steel sheet average cooling rate. Was calculated from the heating pattern and the actual surface heating temperature by heat conduction calculation.

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.
For the measurement position in the longitudinal direction, a total of 7 points were measured including 5 points of both tube ends and 2000 mm inside the tube (5 mm inside the tube because the tube length was 12000 mm), and the largest value was used for evaluation. The measurement was performed with a caliper at the end of the tube and with a contact-type profile meter while rotating the steel tube inside the tube.

  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 1 mm position and the tube thickness center position from the surface layer. Asked. The central segregation part was excluded for the tube thickness center. The M-A volume fraction was determined from the image analysis of five SEM (scanning electron microscope) photographs of the structure observed at a magnification of 2000 times about 1 mm from the surface layer, 1/4 position from the inner surface of the tube thickness, and 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. The target form of the microstructure is within the scope of the claims of the present invention.

  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. In addition, all the hardness was measured with the load of 10 kgf (98N) with the 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. DWTT (Drop Weight Tear Test) characteristics were tested at −17 ° C. using a 19 mm DWTT specimen taken from the circumferential direction of the steel pipe, and the fracture surface ratio (SA (%)) was obtained. Two tests were carried out each, and the target was an average value of 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 (8) to (12).

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 (MPa) at room temperature. The target value of the crush resistance is 1.13 times or more with a value calculated by substituting the tensile yield stress, f ₀ = 0.0050, α _fab = 0.85 into the equations (8) to (12). Decided to become.

In DNV-OS-F101, since a roundness of 5% or less cannot normally be ensured over the entire length, it is not used in the evaluation according to the equations (8) to (12), and is evaluated as f ₀ = 0.05. However, in the present invention, since the roundness is guaranteed also inside the pipe, the expressions (8) to (12) are established. In addition, it has been confirmed separately that the equations (8) to (12) hold even when f ₀ <0.05 by numerical calculation by FEM.

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.

DESCRIPTION OF SYMBOLS 1 Steel plate conveyance line 2 Hot rolling mill 3 High collision pressure descaling device 4 Accelerated cooling device and 5 Hot straightening device 6 Induction heating device 7 Hot straightening device

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: 1.0% or less Mo: containing at least one 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 1.00 or less,
ACR defined by equation (3) is 1.00 to 6.00,
The balance Fe and inevitable impurities,
The metal structure of the base material surface layer portion 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 1.0% or less throughout the tube thickness,
And the maximum value of the hardness difference in the pipe circumferential direction at the same position in the pipe thickness direction is 30 or less,
The maximum value of the hardness difference in the pipe thickness direction at the same position in the pipe circumferential direction is 30 or less,
A high-strength line pipe excellent in crushing resistance and sour resistance, characterized in that the maximum surface hardness is 230 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 line pipe excellent in crushing resistance and sour resistance according to claim 1, comprising at least one selected from the group consisting of: The high-strength line pipe with excellent crushing resistance and sour resistance according to claim 1 or 2, wherein the roundness satisfies the following formula (4) or (5).
In the case of D / t ^0.6 ≦ 135 Dmax−Dmin ≦ 3.0 Formula (4)
When D / t ^0.6 > 135 Dmax−Dmin ≦ 0.04 D / t ^0.6 −1.4 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 900-1200 ° C,
After performing hot rolling with a cumulative rolling reduction of 900 ° C. or lower at 30 to 90% and a rolling end temperature of (Ar ₃ −10 ° C.) or higher,
Immediately before accelerated cooling, descaling with a jet collision pressure on the steel sheet surface of 1 MPa or more,
Thereafter, the steel sheet surface temperature _(Ar 3 -80 ℃) above temperature range, the steel sheet surface layer cooling rate is 10 ° C. / s or higher 100 ° C. / s or less,
The steel plate surface layer temperature is 300 ° C. or higher and 600 ° C. or lower, and the steel plate average temperature is 630 ° C. or higher.
Subsequently, the second stage accelerated cooling is performed until the steel sheet average cooling rate is 10 ° C./s or higher and the steel plate average temperature is 300 ° C. or higher and 600 ° C. or lower, and then the steel plate average temperature is set to the cooling stop temperature or higher and 400 to 600. Reheated to a temperature range of ℃, cooled to room temperature, formed into a tubular shape in the cold,
After welding the butt to make a steel pipe,
Furthermore, it manufactures by performing a pipe expansion with a pipe expansion rate of 0.5 to 1.1%, The high excellent in crushing resistance and sour resistance of any one of Claims 1-3 characterized by the above-mentioned. Strength line pipe manufacturing method.
(700-T) / V ≧ 3 Formula (6)
Here, T: steel plate surface layer cooling stop temperature (° C.) for the first stage accelerated cooling, and V: steel sheet surface layer cooling rate (° C./s) for the first stage accelerated cooling.

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