CN115210396A - Steel pipe and steel plate - Google Patents
Steel pipe and steel plate Download PDFInfo
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- CN115210396A CN115210396A CN202080097912.9A CN202080097912A CN115210396A CN 115210396 A CN115210396 A CN 115210396A CN 202080097912 A CN202080097912 A CN 202080097912A CN 115210396 A CN115210396 A CN 115210396A
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- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
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- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/10—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of tubular bodies
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- C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
- C21D1/56—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering characterised by the quenching agents
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Abstract
A steel pipe having a base material portion and a welded portion, wherein the base material portion has a predetermined chemical composition, the metallic structure of a surface layer portion of the base material portion, which is in a range from the surface to a depth of 1mm, is composed of 1 or more kinds selected from polygonal ferrite, granular bainite, acicular ferrite, and bainite, the maximum hardness of the surface layer portion of the base material portion is 250HV or less, the yield stress is 415 to 630MPa, and the limit of proportion in a stress-strain curve is 90% or more of the yield stress.
Description
Technical Field
The present invention relates to a steel pipe and a steel plate. The present invention particularly relates to a welded steel pipe for a line pipe and a steel sheet suitable as a raw material thereof.
Background
A system that is installed above the ground, on the sea floor, or the like and transports oil and gas is called a line pipe. The steel pipe for pipeline constituting such a pipeline is called a pipeline pipe. Straight-seam arc welded steel pipes (hereinafter referred to as arc-welded steel pipes, or steel pipes) are widely used for large-diameter line pipes having a pipe diameter of 508mm or more, which constitute long-distance lines. Here, the straight arc welded steel pipe is a steel pipe produced by forming a thick steel plate into a cylindrical open pipe and welding a butt portion (seam portion) by an arc welding method such as submerged arc welding. Depending on the forming method, it may be called UOE steel pipe or JCOE steel pipe.
In recent years, pipeline construction has been expanding to severe environmental regions such as cold regions and acid environments. Here, the acid atmosphere means that the corrosive gas H is contained 2 The oxidized wet hydrogen sulfide environment of S. It is known that Hydrogen Induced Cracking (HIC) sometimes occurs if the line pipe is exposed to an acid environment. On the other hand, in an oil country tubular good having a strength higher than that of a line pipe, sulfide Stress Cracking (SSC) may occur. However, SSC may occur even in line pipes when the partial pressure of hydrogen sulfide becomes high or the stress becomes high. As described above, line pipes used in severe acid environments (acid-resistant line pipes) are required to have SSC resistance in addition to HIC resistance.
Further, patent document 2 proposes a high-strength steel sheet for an acid-resistant line pipe, in which a CP value (= 4.46[% C ] +2.37[% Mn ]/6+ (1.74 [% Cu ] +1.7[% Ni ])/15 + (1.18 [% Cr ] +1.95[% Mo ] +1.74[% V ])/5 +22.36[% P ]), which is an index representing the hardness of a center segregation portion, is 1.0 or less in mass%, a steel structure is a bainite structure, a deviation Δ HV of the hardness in a sheet thickness direction is 30 or less, and a deviation Δ HV of the hardness in a sheet width direction is 30 or less.
The steel sheets of patent documents 1 to 4 and non-patent document 2 satisfy acid resistance in an environment where the hydrogen sulfide partial pressure is 0.1MPa (1 bar) or less and the load stress is 90% or less of the yield stress. However, the recent oil country tubular goods and line pipes are used in more severe environments, and the level of acid resistance required for welded steel pipes for line pipes is becoming higher.
Conventionally, acid resistance under an environment with a hydrogen sulfide partial pressure of 0.1MPa (1 bar) or less has been required, but recently, a material design capable of withstanding a high-pressure hydrogen sulfide environment exceeding 0.1MPa has been required. Further, conventionally, the load stress is 90% or less of the yield stress, but recently, a material design capable of withstanding a high-pressure hydrogen sulfide environment under a load stress exceeding 90% of the yield stress has been demanded.
According to the study of the present inventors, the steel sheets of patent documents 1 to 4 and the steel sheet of non-patent document 2 have insufficient acid resistance in an environment where the partial pressure of hydrogen sulfide exceeds 0.1MPa (1 bar) and exceeds 90% of the yield stress.
In order to solve such problems, patent document 5 discloses a steel pipe having HIC resistance equal to or higher than that of conventional steels, having a yield strength of 350MPa or higher, and having excellent SSC resistance in which cracks do not occur even under a stress of 90% or more of the yield strength under load in an environment of 30 ℃ or lower containing hydrogen sulfide having a partial pressure of hydrogen sulfide exceeding 0.1 MPa.
However, patent document 5 shows that the SSC resistance is excellent in which the load stress in the sulfide stress corrosion cracking test is 90% of the yield stress, but does not show that the load stress exceeds 90% of the yield stress.
Prior art documents
Patent document
Patent document 1: japanese patent application laid-open No. 2011-017048
Patent document 2: japanese unexamined patent publication No. 2012-077331
Patent document 3: japanese patent application laid-open No. 2013-139630
Patent document 4: japanese unexamined patent publication No. 2014-218707
Patent document 5: japanese patent No. 6369658 and its applications
Non-patent document
Non-patent document 1: the Xinri iron Tou jin technical report No. 397 (2013), p.17-22
Non-patent document 2: JFE technical report No.9 (8 months in 2005), p.19-24
Disclosure of Invention
As described above, the recent line pipes are used in more severe environments, and the level of the requirement for acid resistance of the welded steel pipes for line pipes is further increased. Accordingly, an object of the present invention is to provide a welded steel pipe, particularly a straight arc welded steel pipe, which has excellent acid resistance and can be used in a severe high-pressure hydrogen sulfide environment, and a steel plate (particularly a thick steel plate) as a raw material thereof.
More specifically, the object is to provide a steel pipe excellent in SSC resistance which has HIC resistance equal to or higher than that of conventional steel, has a yield stress of 350MPa or higher, and does not crack even when a stress exceeding 90% of the yield stress, specifically a stress exceeding 95% of the yield stress is applied in an environment of 30 ℃ or lower containing hydrogen sulfide exceeding 0.1MPa, and a steel sheet to be a raw material thereof.
The present invention has been made to solve the above problems, and the gist of the present invention is steel pipes and steel plates described below.
(1) A steel pipe according to an aspect of the present invention is a steel pipe having a matrix portion and a welded portion, the matrix portion having a chemical composition containing, in mass%, C:0.030 to 0.100%, si:0.50% or less, mn:0.80 to 1.60%, P:0.020% or less, S:0.0030% or less, al:0.060% or less, ti:0.001 to 0.030%, nb:0.006 to 0.100%, N:0.0010 to 0.0080%, ca:0.0005 to 0.0050%, O:0.0050% or less, cr:0 to 1.00%, mo:0 to 0.50%, ni:0 to 1.00%, cu:0 to 1.00%, V:0 to 0.10%, mg:0 to 0.0100%, REM:0 to 0.0100%, the balance being Fe and impurities, wherein ESSP represented by the following formula (i) is 1.5 to 3.0, ceq represented by the following formula (ii) is 0.20 to 0.50, the microstructure of the surface layer portion, which is in the range from the surface to the depth of 1mm, of the matrix portion is composed of 1 or more kinds selected from polygonal ferrite, granular bainite, acicular ferrite, and bainite, the maximum hardness in the surface layer portion of the matrix portion is 250HV or less, the yield stress is 415 to 630MPa, and the proportional limit in the stress-strain curve is 90% or more of the yield stress.
ESSP=Ca×(1-124×O)/(1.25×S)...(i)
Ceq=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5...(ii)
In the formula, each element symbol represents the content (mass%) of each element contained in the steel, and is zero when not contained.
(2) The steel pipe according to item (1) above, wherein a total area ratio of the granular bainite, the acicular ferrite, and the bainite may exceed 80% in the microstructure of the surface layer portion of the matrix portion.
(3) The steel sheet according to the above (1) or (2), wherein the chemical composition of the matrix portion may contain, in mass%, a chemical component selected from the group consisting of Cr:0.10 to 1.00%, mo:0.03 to 0.50%, ni:0.10 to 1.00%, cu:0.10 to 1.00%, V:0.005 to 0.10%, mg:0.001 to 0.0100% and REM: 0.001-0.0100% of more than 1.
(4) The steel pipe of any one of (1) to (3) above, wherein the chemical composition of the matrix section contains, in mass%: 0.01 to 0.04%, wherein the welded portion includes a weld heat affected zone and a weld metal portion, the surface layer portion in the weld heat affected zone has a metal structure including 1 or more selected from bainite and acicular ferrite, the surface layer portion in the weld heat affected zone has a maximum hardness of 250HV or less, and the angle of the weld toe portion on the inner side of the steel pipe is in the range of 130 to 180 °.
(5) The steel pipe according to any one of (1) to (4) above, wherein: the thickness of the parent material part is 10-40 mm, and the pipe diameter is more than 508 mm.
(6) A steel sheet according to another aspect of the present invention is used for the parent metal portion of the steel pipe according to any one of (1) to (5).
According to the above aspect of the present invention, it is possible to provide a steel pipe having excellent SSC resistance and a steel sheet usable as a raw material thereof, in which cracks do not occur even when a stress exceeding 90% of the yield stress is applied in an environment of 30 ℃ or lower containing hydrogen sulfide exceeding 0.1 MPa.
Further, according to a preferred aspect of the present invention, it is possible to provide a steel pipe having a welded portion excellent in acid resistance, which can be used in a severe high-pressure hydrogen sulfide environment.
Drawings
Fig. 1 is a schematic view for explaining the angle of the weld toe portion of the steel pipe according to the present embodiment.
Fig. 2 is a schematic view showing a portion where a sample is cut out from the steel pipe according to the present embodiment.
Detailed Description
In order to investigate a method for solving the above problems, the present inventors conducted a high-pressure hydrogen sulfide atmosphere (for example, H containing 5% of common salt and acetic acid) of more than 0.1MPa 2 S in a saturated solution), and the load stress exceeds 90%, the cross section, the structure, and the like of the parent metal portion and the welded portion of the steel pipe that cracked in the test were observed. The stress-strain curve of the steel pipe was also examined. As a result, the following findings were obtained.
(a) In order to improve acid resistance in a high-pressure hydrogen sulfide environment exceeding 0.1MPa or more, it is necessary to control not only HIC resistance but also SSC resistance. HIC occurs at a center segregation portion, which exists in the vicinity of the center portion in the thickness direction of the steel pipe. On the other hand, SSC depends on the structure and hardness of a steel pipe in a range of 1mm from the surface (surface layer portion), which have not been considered in the past.
(b) When the metal structure of the surface layer portion is made mainly of 1 or more kinds selected from polygonal ferrite, granular bainite, acicular ferrite, and bainite, and the maximum hardness is made to be 250HV or less, the acid resistance is improved. Further, if the total area ratio of 1 or more selected from the group consisting of granular bainite, acicular ferrite, and bainite exceeds 80%, the SSC property is further improved.
(c) In the case where the structure of the layer portion is controlled as described above, it is important to strictly control the cooling mode while controlling the carbon equivalent Ceq to 0.20 to 0.50.
(d) When the method for manufacturing a hot-rolled steel sheet on the premise of coiling is applied, the cooling rate after the stop of accelerated cooling is slower than the cooling rate of cooling by cooling. In this case, although the variation in hardness is small, the above-described structure and/or hardness of the surface layer portion cannot be obtained. Therefore, in order to obtain the structure and hardness of the surface layer portion, it is necessary to manufacture the surface layer portion through a thick plate process.
(e) By appropriately controlling the hardness of the weld heat-affected zone and the shape of the weld toe portion (see fig. 1), the stress concentration in the weld toe portion is relaxed, and the SSC resistance of the weld portion is improved.
The present invention has been completed based on the above findings.
A steel pipe according to an embodiment of the present invention (steel pipe according to the present embodiment) and a steel sheet for the steel pipe (steel sheet according to the present embodiment) will be described below.
The steel pipe according to the present embodiment is a welded steel pipe having a base material portion and a welded portion. The base material portion is cylindrical, and the welded portion extends in a direction parallel to the axial direction of the steel pipe. The welded portion includes a bead metal portion that is a metal portion that melts and solidifies at the time of welding, and a welding heat-affected zone that is a region that, although not melted at the time of welding, has a change in structure or the like due to heat input and subsequent cooling of the welding.
The steel sheet according to the present embodiment is used for the base material portion of the steel pipe. That is, as described later, the steel pipe can be obtained by forming the steel plate into a cylindrical shape, and butt-welding both end portions of the steel plate. Therefore, the chemical composition, the metal structure, and the mechanical properties of the steel sheet are the same as those of the base material portion of the steel pipe. Therefore, the following description of the base material portion of the steel pipe according to the present embodiment is also applicable to the steel sheet according to the present embodiment.
1. Chemical composition
The reasons for limiting the elements are as follows. In the following description, "%" as to the content means "% by mass".
1-1. Chemical composition of mother Material portion (Steel plate) of Steel pipe
The chemical composition of the base material portion of the steel pipe according to the present embodiment (steel plate according to the present embodiment) will be described.
C:0.030~0.100%
C is an element for improving the strength of the steel. When the C content is less than 0.030%, the strength-improving effect cannot be sufficiently obtained. Therefore, the C content is set to 0.030% or more. Preferably 0.035% or more.
On the other hand, if the C content exceeds 0.100%, the hardness of the surface layer portion becomes high, and SSC is easily generated. In addition, carbide is formed, and HIC is likely to occur. Therefore, the C content is set to 0.100% or less. In the case where more excellent SSC resistance and HIC resistance are to be secured and deterioration of weldability and toughness is to be suppressed, the C content is preferably 0.070% or less, more preferably 0.060% or less.
Si: less than 0.50%
If the Si content exceeds 0.50%, the toughness of the weld portion is reduced. Therefore, the Si content is set to 0.50% or less. Preferably 0.35% or less, more preferably 0.30% or less. The lower limit of the Si content includes 0%.
On the other hand, since Si is inevitably mixed from the steel raw material and/or during the steel making process, 0.01% is a substantial lower limit of the Si content in the practical steel. In addition, si may be added for deoxidation, and in this case, the lower limit of the Si content may be set to 0.10%.
Mn:0.80~1.60%
Mn is an element that improves the strength and toughness of steel. When the Mn content is less than 0.80%, these effects cannot be sufficiently obtained. Therefore, the Mn content is 0.80% or more. The Mn content is preferably 0.90% or more, and more preferably 1.00% or more.
On the other hand, if the Mn content exceeds 1.60%, the acid resistance is lowered. Therefore, the Mn content is 1.60% or less. Preferably 1.50% or less.
P:0.020% or less
P is an element inevitably contained as an impurity. If the P content exceeds 0.020%, HIC resistance is lowered, and toughness of the weld portion is lowered. Therefore, the P content is 0.020% or less. Preferably 0.015% or less, more preferably 0.010% or less. Preferably, the P content is small, and the lower limit includes 0%. However, since the production cost is greatly increased when the P content is reduced to less than 0.001%, 0.001% is a substantial lower limit of the P content in practical steel.
S:0.0030% or less
S is an element inevitably contained as an impurity. S is an element that forms MnS extending in the rolling direction during hot rolling and reduces HIC resistance. If the S content exceeds 0.0030%, the HIC resistance is significantly reduced, so the S content is set to 0.0030% or less. Preferably 0.0020% or less, more preferably 0.0010% or less. The lower limit includes 0%, but if the S content is reduced to less than 0.0001%, the manufacturing cost is greatly increased, so 0.0001% is a substantial lower limit in practical steel sheets.
Al: less than 0.060%
If the Al content exceeds 0.060%, clusters of Al oxides are formed, and the HIC resistance is lowered. Therefore, the Al content is set to 0.060% or less. Preferably 0.050% or less, more preferably 0.035% or less, and still more preferably 0.030% or less. Preferably, the Al content is small, and the lower limit of the Al content includes 0%.
On the other hand, since Al is inevitably mixed from the steel material and/or during the steel making process, 0.001% is a substantial lower limit of the Al content in practical steel. In addition, al may be added for deoxidation, and in this case, the lower limit of the Al content may be set to 0.010%.
Ti:0.001~0.030%
Ti is an element that forms carbonitrides to contribute to grain refinement of crystal grains. If the Ti content is less than 0.001%, the effect cannot be sufficiently obtained. Therefore, the Ti content is 0.001% or more. Preferably 0.008% or more, and more preferably 0.010% or more.
On the other hand, if the Ti content exceeds 0.030%, carbonitride is excessively generated, and HIC resistance and toughness are lowered. Therefore, the Ti content is set to 0.030% or less. Preferably 0.025% or less, more preferably 0.020% or less.
Nb:0.006~0.100%
Nb is an element that forms carbide and/or nitride and contributes to strength improvement. When the Nb content is less than 0.006%, these effects cannot be sufficiently obtained. Therefore, the Nb content is set to 0.006% or more. Preferably 0.008% or more, and more preferably 0.010% or more. Particularly, when the hardness of the weld heat affected zone is to be secured, the Nb content is preferably 0.010% or more, more preferably 0.015% or more, and still more preferably 0.017% or more.
On the other hand, if the Nb content exceeds 0.100%, carbonitrides of Nb accumulate in the center segregation portion, and HIC resistance is lowered. Therefore, the Nb content is 0.100% or less. Preferably 0.080% or less, more preferably 0.060% or less.
In order to improve the toughness of the weld (weld heat affected zone and weld metal part), the Nb content is preferably 0.040% or less, more preferably 0.035% or less, and still more preferably 0.033% or less.
N:0.0010~0.0080%
N is an element that forms a nitride by bonding with Ti and/or Nb and contributes to refinement of the austenite grain size during heating. When the N content is less than 0.0010%, the above-mentioned effects cannot be sufficiently obtained. Therefore, the N content is set to 0.0010% or more. Preferably 0.0020% or more.
On the other hand, if the N content exceeds 0.0080%, nitrides of Ti and/or Nb are accumulated, and the HIC resistance is lowered. Therefore, the N content is set to 0.0080% or less. Preferably 0.0060% or less, more preferably 0.0050% or less.
Ca:0.0005~0.0050%
Ca is an element that suppresses the formation of MnS that elongates in the rolling direction by forming CaS in the steel, and as a result, contributes to the improvement of HIC resistance. If the Ca content is less than 0.0005%, the above effects cannot be sufficiently obtained. Therefore, the Ca content is set to 0.0005% or more. Preferably 0.0010% or more, more preferably 0.0015% or more.
On the other hand, if the Ca content exceeds 0.0050%, oxides accumulate and the HIC resistance is lowered. Therefore, the Ca content is set to 0.0050% or less. Preferably 0.0045% or less, and more preferably 0.0040% or less.
O:0.0050% or less
O is an element that inevitably remains. When the O content exceeds 0.0050%, oxides are generated, and HIC resistance is lowered. Therefore, the O content is set to 0.0050% or less. From the viewpoint of ensuring the toughness of the steel sheet and the toughness of the welded portion, it is preferably 0.0040% or less, and more preferably 0.0030% or less. The smaller the O content is, the more preferable it may be 0%. However, if O is reduced to less than 0.0001%, the production cost is greatly increased. Therefore, the O content may be set to 0.0001% or more. From the viewpoint of production cost, 0.0005% or more is preferable.
Cr:0~1.00%
Mo:0~0.50%
Ni:0~1.00%
Cu:0~1.00%
V:0~0.10%
Cr, mo, ni, cu and V are elements that improve the hardenability of steel. Therefore, 1 or more selected from these elements may be contained as necessary.
In order to obtain the above effects, it is preferable to contain a compound selected from Cr:0.10% or more, mo:0.03% or more, ni:0.10% or more, cu:0.10% or more and V:0.005% or more of 1 or more.
On the other hand, if the contents of Cr, ni, and Cu exceed 1.00%, the content of Mo exceeds 0.50%, or the content of V exceeds 0.10%, respectively, the hardness increases and the acid resistance decreases. Therefore, the contents of Cr, ni, and Cu are all 1.00% or less, the Mo content is 0.50% or less, and the V content is 0.10% or less. Preferably, cr:0.50% or less, mo:0.40% or less, ni:0.50% or less, cu:0.50% or less, V: less than 0.06 percent.
Mg:0~0.0100%
REM:0~0.0100%
Mg and REM are elements that control sulfide morphology. In order to obtain the above effects, it is preferable to contain a compound selected from Mg:0.001% or more and REM:0.001% or more of 1 or 2.
On the other hand, when the contents of Mg and REM exceed 0.0100%, the sulfide coarsens and the effect thereof cannot be exerted. Therefore, the contents of Mg and REM are both 0.0100% or less. Preferably 0.0050% or less.
Here, REM is a rare earth element and is a general term for 16 elements in total of Sc and lanthanoid, and REM content means a total content of these elements.
In the above chemical composition, the balance is Fe and impurities. Here, the "impurities" are components mixed in due to raw materials such as ores and scraps and various factors of a manufacturing process when industrially manufacturing steel, and refer to components that are allowed within a range not adversely affecting the present invention.
When Sb, sn, co, as, pb, bi, H, W, zr, ta, B, nd, Y, hf and Re are included As impurities, the content of each is preferably controlled to be in the range described later.
Sb: less than 0.10%
Sn: less than 0.10%
Co: less than 0.10%
As: less than 0.10%
Pb: less than 0.005%
Bi: less than 0.005%
H: less than 0.0005%
Sb, sn, co, as, pb, bi, and H may be mixed As impurities or inevitable mixing elements from the steel material, but if the contents are within the above ranges, the properties of the steel pipe according to the present embodiment are not impaired. Therefore, it is preferable that these elements are limited to the above-mentioned ranges.
W, zr, ta, B, nd, Y, hf, and Re: 0.10% or less in total
These elements may be mixed as impurities or inevitable mixing elements from the steel raw material, but if the contents are within the above ranges, the properties of the steel pipe according to the present embodiment are not impaired. Therefore, the total content of these elements is limited to 0.10% or less.
The chemical composition of the matrix portion is required to satisfy a predetermined condition in terms of the values of ESSP and Ceq calculated from the contents of the components, as shown below, except that the contents of the respective elements are within the above-described ranges.
ESSP:1.5~3.0
ESSP is a value which is an index of the amount of effective Ca representing the presence or absence of an amount corresponding to the S content on the premise that the remaining Ca (effective Ca) obtained by subtracting the Ca bonded to oxygen is bonded to S in an atomic weight ratio, and is represented by the following formula (i). In the steel pipe according to the present embodiment, in order to ensure HIC resistance equal to or higher than that of conventional steel, it is necessary to set the value of the ESSP within a range of 1.5 to 3.0.
ESSP=Ca×(1-124×O)/(1.25×S)...(i)
In the formula, each element symbol represents the content (mass%) of each element contained in the steel, and is zero when not contained.
In order to ensure the HIC resistance, it is effective to suppress the generation of MnS extending in the rolling direction. In addition, in order to suppress the formation of MnS extending in the rolling direction, it is effective to reduce the S content and add Ca to form CaS and fix S. On the other hand, since Ca has a stronger oxygen affinity than S, it is necessary to reduce the O content in order to form a necessary amount of CaS.
If ESSP is less than 1.5, the content of Ca is insufficient for the O content and the S content to produce MnS. MnS extended during rolling becomes a cause of deteriorating HIC resistance, so the ESSP is 1.5 or more. Preferably 1.6 or more, and more preferably 1.7 or more.
On the other hand, if the Ca content becomes excessive, a large amount of cluster-like inclusions are generated, and the morphological control of MnS may be inhibited. When the O content and the S content are reduced, the generation of cluster inclusions can be suppressed, but when the ESSP exceeds 3.0, the production cost for reducing the O content and the S content significantly increases. Therefore, the ESSP is set to 3.0 or less. Preferably 2.8 or less, more preferably 2.6 or less.
If the ESSP value is in the range of 1.5 to 3.0, the effective Ca amount is adjusted to be not less than the minimum amount required for controlling the form of MnS and not more than the critical amount at which cluster inclusions are not generated, and therefore, excellent HIC resistance can be obtained.
Ceq:0.20~0.50
Ceq is a value indicating the carbon equivalent, which is an index of hardenability, and is represented by the following formula (ii). In the base material portion of the steel pipe according to the present embodiment, as described later, it is necessary to appropriately control the hardenability of the steel in order to obtain a structure composed of 1 or more kinds selected from polygonal ferrite, granular bainite, acicular ferrite, and bainite, preferably a metal structure containing 1 or more kinds selected from granular bainite, acicular ferrite, and bainite in a total amount exceeding 80% in the surface layer portion. Therefore, the value of Ceq needs to be 0.20 to 0.50.
Ceq=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5...(ii)
In the formula, each element symbol represents the content (mass%) of each element contained in the steel, and is zero when not contained.
When Ceq is less than 0.20, a tensile strength of 530MPa or more cannot be obtained. Therefore, ceq is 0.20 or more. Preferably 0.25 or more. On the other hand, when Ceq exceeds 0.50, the surface hardness of the welded portion becomes high, and the acid resistance is lowered. Therefore, ceq is set to 0.50 or less. Preferably 0.45 or less.
1-2 chemical composition of weld
The welding heat affected zone is a portion where the base material portion is not melted even by welding. Therefore, the chemical composition is the same as that of the base metal portion, and the reason for limitation is the same.
On the other hand, the chemical composition of the weld metal portion in the welded portion is not particularly limited. However, in order to increase the strength of the weld metal portion to a level equal to or higher than that of the base metal portion, the chemical composition of the weld metal portion is preferably set to the following range.
That is, the chemical composition of the bead metal part in the welded part is preferably C:0.02 to 0.20%, si:0.01 to 1.00%, mn:0.1 to 2.0%, P:0.015% or less, S:0.0050% or less, cu:1.0% or less, ni:1.0% or less, mo:1.0% or less, cr:0.1% or less, nb:0.5% or less, V:0.3% or less, ti:0.05% or less, al:0.005 to 0.100%, O: 0.010-0.070%, cr:0 to 1.00%, ni:0 to 1.00%, cu:0 to 1.00%, mo:0 to 0.50%, V:0 to 0.10%, mg:0 to 0.01%, REM:0 to 0.01%, and the balance: fe and impurities.
The chemical composition of the weld metal portion is determined by the inflow ratio of the base metal and the welding material at the time of welding. As the solder material, any commercially available material may be used, and for example, Y-D, Y-DM, Y-DMH wire, and NF5000B or NF2000 solder may be used. In order to control the composition range of the weld metal portion, it is preferable to adjust the welding conditions to a range described later.
2. Metallic structure
2-1. Metallic structure of parent material part
Next, the metal structure of the base material portion (steel plate) of the steel pipe will be described.
The metal structure in the surface layer portion of the matrix portion is a structure composed of 1 or more kinds selected from polygonal ferrite, granular bainite, acicular ferrite, and bainite. In the present embodiment, the surface layer means a range of 1.0mm from the surface of the base material portion.
In the steel pipe according to the present embodiment, in order to suppress the maximum hardness of the surface layer portion of the matrix portion to 250HV or less and ensure the required strength and excellent acid resistance, the metal structure in the surface layer portion is a structure composed of 1 or more kinds selected from polygonal ferrite, granular bainite, acicular ferrite, and bainite. Preferably: the total area ratio of 1 or more selected from granular bainite, acicular ferrite and bainite exceeds 80%. When the total area ratio exceeds 80%, the strength and acid resistance are further improved. More preferably 85% or more.
The area ratio of each structure was measured by observing a metal structure which was corroded by a mixed solution of 3% nitric acid and 97% ethanol or the like with a Scanning Electron Microscope (SEM). The structure of the surface layer portion may be measured at a position 0.5mm from the surface of the steel sheet.
The metal structure of the surface layer portion of the base material portion means a metal structure of the base material portion that is not affected by welding. The steel pipe according to the present embodiment refers to a metal structure or the like of a surface layer portion at positions of 90 °, 180 °, and 270 ° in the circumferential direction of the steel pipe from a butted portion (a seam portion, which corresponds to an end in the width direction of the steel plate). The above-mentioned positions correspond to the metal structures of the surface layer portions at positions 1/4, 1/2, and 3/4 in the width direction of the steel sheet.
In the present embodiment, polygonal ferrite is a structure observed as a massive structure containing no coarse cementite, coarse precipitates such as MA, and the like in grains, and acicular ferrite is a structure in which the original austenite grain boundaries are unknown and acicular ferrite (no carbide, austenite-martensite component) is generated in random (random) crystal orientation in grains.
On the other hand, the processed ferrite is processed ferrite, and grains flattened in the rolling direction are observed in an optical microscope and SEM observation. The flattening means that the aspect ratio (the ratio of the ferrite length in the rolling direction to the ferrite length in the plate thickness direction) is 2.0 or more. Pearlite is a structure in which ferrite and cementite are layered, and a structure in which cementite forming a layer breaks in the middle of pearlite is a quasi-pearlite (suspected pearlite: 12597125125401251251251251251252112412412412488.
Regarding the retained austenite, a structure that appears white using the modified Lepera liquid is determined as the retained austenite.
The granular bainite is formed at an intermediate transformation temperature between acicular ferrite and bainite, and has an intermediate structure characteristic. The microstructure is a microstructure in which the prior austenite grain boundaries are partially observed and a coarse lath structure is present in grains, fine carbides and austenite-martensite components are dispersed in and between laths, and a needle-like or amorphous ferrite portion in which the prior austenite grain boundaries are not known is present.
Bainite and martensite are structures in which the prior austenite crystal is clear and the intragranular fine lath structure is developed. However, in the present embodiment, a structure in which prior austenite crystals are defined, fine lath structures are developed in grains, and the hardness is 250Hv or more is regarded as martensite, and a structure in which prior austenite crystals are defined, fine lath structures are developed in grains, and the hardness is less than 250Hv is regarded as bainite. Whether the hardness is 250Hv or more or less than 250Hv is determined by measuring the target tissue at 10 points using a micro vickers hardness tester with a load of 100gf and determining whether the maximum value is 250Hv or less than 250Hv. All structures were tempered during reheating and during heat treatment of steel pipes, but they were not particularly distinguished by the presence or absence of tempering.
In the steel pipe according to the present embodiment, the structure other than the surface layer portion is not particularly limited. However, when the structure of the layer portion is controlled as described above by the manufacturing method described later, it is preferable that: the structure other than the surface layer portion, for example, the structure of the thick central portion (the thick central portion of the steel sheet) is a structure mainly composed of acicular ferrite, pearlite (including quasi-pearlite), martensite, and does not contain processed ferrite, and the maximum hardness is 250Hv or less.
2-2 welding the metal structure of the heat affected zone
In the steel pipe according to the present embodiment, in order to form a similar microstructure in the entire steel pipe, the microstructure of the surface layer portion in the weld heat affected zone preferably includes 1 or more kinds selected from bainite and acicular ferrite. The metal structure of the surface layer portion in the welding heat affected zone is preferably a uniform structure, that is, a structure composed of bainite and/or acicular ferrite.
The weld metal portion preferably has a structure made of acicular ferrite.
In order to make the welding heat affected zone the above-mentioned metal structure, the following conditions are desirable as welding conditions. For example, as the solder material, Y-D, Y-DM, Y-DMH wire, and NF5000B or NF2000 solder are preferably used. Further, inner surface welding and outer surface welding are preferably performed, and submerged arc welding is preferably performed using an inner surface 3 electrode and an outer surface 4 electrode. The heat input during welding is preferably in the range of 2.0kJ/mm to 10kJ/mm depending on the plate thickness.
As for the metal structure of the weld heat affected zone, a sample including a weld metal portion was cut out from the welded portion of the steel pipe, and a sample for microstructure observation was prepared. Then, observation was performed in the same manner as the base material portion.
3. Mechanical characteristics
Next, the mechanical properties of the steel pipe will be described.
3-1 mechanical Properties of the parent Material portion
Maximum hardness of surface layer portion: below 250HV
SSC occurs due to minute flaws or minute cracks on the surface of a steel sheet, and therefore the microstructure and hardness of the surface layer portion that is the source of the minute flaws or minute cracks are important.
In the steel pipe according to the present embodiment, in order to ensure excellent SSC resistance, the microstructure of the surface layer portion of the matrix portion is controlled as described above, and the maximum hardness of the surface layer portion of the matrix portion is set to 250HV or less. The maximum hardness of the surface layer portion is preferably 245HV or less, and more preferably 240HV or less.
The maximum hardness of the surface layer portion was measured by the following method. First, samples having axial lengths of 20mm and circumferential lengths of 20mm were prepared by mechanical cutting from positions 90 °, 180 °, and 270 ° from the welded portion in the circumferential direction of the steel pipe. In the case of the steel sheet, samples having a length of 20mm and a width of 20mm were prepared from positions 1/4, 1/2, and 3/4 of the ends of the steel sheet in the width direction.
Next, the sample was polished by mechanical polishing. The total of 100 points of the polished sample was measured by using a Vickers hardness tester (test force: 100 gf), measuring 10 points at 0.1mm intervals in the thickness direction from the surface as a starting point, and measuring 10 points at 1mm intervals in the width direction at the same depth.
As a result of the above measurement, if the measurement points exceeding 250HV do not continuously appear at 2 points or more in the plate thickness direction, it is determined that the maximum hardness of the surface layer portion is 250HV or less.
In the base material portion of the steel pipe, a high value (abnormal value) may occur locally due to inclusions and the like. However, since the inclusion does not cause cracking, the SSC resistance can be secured even if such an abnormal value occurs. On the other hand, when 2 or more measurement points exceeding 250HV are continuously present in the thickness direction, they are not allowed because they are not caused by inclusions and their SSC resistance is lowered.
Therefore, in the present invention, even if there are 1 point and more than 250Hv measurement points, if 2 or more points do not appear continuously in the plate thickness direction, the points are regarded as abnormal points and are not used, and the next highest value is regarded as the highest hardness. On the other hand, when 2 or more points exceeding 250Hv are continuously present in the thickness direction, the hardness is set as the highest hardness.
And (4) limiting the proportion: yield stress of 90% or more
The present inventors have studied the SSC resistance under a severer environment. The results thereof revealed that: if the proportional limit in the stress-strain curve is 90% or more of the yield stress, SSC does not occur even when the load stress exceeds 90% (for example, 95%) of the yield stress.
When the proportional limit is less than 90% of the yield stress, plastic deformation occurs in the case of an actual yield stress in which the load stress in the sulfide stress corrosion cracking test is 90%, and thus dislocations proliferate. As a result, hydrogen entering during the sulfide stress corrosion test is trapped by the grown dislocations, and the amount of hydrogen increases, thereby causing cracks. On the other hand, if the limit of the proportion is 90% or more of the yield stress, plastic deformation does not occur even if the yield stress exceeds 90%. Therefore, the number of dislocations to be propagated does not increase, and hydrogen does not accumulate there. Also, as a result, cracking can be prevented.
As described above, when the limit ratio is 90% or more of the yield stress, sulfide stress cracking does not occur even if the base material portion of the steel pipe according to the present embodiment (the steel sheet according to the present embodiment) is subjected to a stress exceeding 90% of the yield stress in a solution environment containing 5% of salt and acetic acid at 30 ℃. The proportional limit is more preferably 95% or more of the yield stress.
In the present embodiment, the ratio limit is measured by the following procedure.
First, according to API5L, a round bar tensile sample was prepared in a direction (C direction) perpendicular to the longitudinal direction of the steel pipe, and a tensile test was performed. The tensile test was conducted under the control of the stroke (tensile speed: 1 mm/min), the test force and displacement were measured at intervals of 0.05 second, and the stress and strain were determined for each measurement time based on them. Then, the Yield Stress (YS) was determined from the obtained stress-strain curve. When the yield point is not clearly confirmed as YS, the conditional yield strength σ is adopted 0.2 。
Then, smoothing (smoothing) processing of the values of stress and strain is performed in consideration of the measurement error. Specifically, the average value of the measurement time ± 2.50 seconds was calculated for each measurement time, and the value was used as the result for each measurement time. For example, as the values of stress and strain at 2.50 seconds, an average value of 101 measured values over a period of 0 to 5.00 seconds was used.
Next, the slope of the linear portion of the stress-strain curve after the smoothing process was obtained. The slope of the straight line portion was calculated by the least square method using the value of the time period during which the stress changed from 0.2YS to 0.4YS as a representative value.
Next, the slope of the stress-strain curve at each measurement time was calculated. Specifically, the slope is calculated by the least square method from the value of the period of ± 0.50 seconds of the measurement time for each measurement time. For example, the slope of the stress-strain curve at 60.00 seconds is calculated by the least square method using 21 measurement values over a period of 59.50 to 60.50 seconds.
Then, the value of the former stress in which the slope of the stress-strain curve is continuously lower than 0.95 times the slope of the straight line portion is set as a proportional limit. Even if the slope of the stress-strain curve is once less than 0.95 times the slope of the straight portion due to the influence of the measurement error, the slope does not take on this value when it exceeds 0.95 times the slope of the straight portion again.
Yield stress: 415MPa or more
Tensile strength: over 530MPa
In order to ensure a required strength in the steel pipe according to the present embodiment, the yield stress of the parent metal portion of the steel pipe according to the present embodiment is 415MPa or more. Preferably 430MPa or more. The upper limit of the yield stress is a substantial upper limit in processability, namely, the 630MPa level defined by X70 of API 5L. In terms of workability, the yield stress is preferably 600MPa or less.
In order to ensure the required strength in the steel pipe according to the present embodiment, the tensile strength of the parent metal portion of the steel pipe according to the present embodiment is preferably 530MPa or more. More preferably 550MPa or more. The upper limit of the tensile stress is not particularly limited, but 690MPa defined by X70 of API5L is a substantial upper limit in view of processability. From the viewpoint of workability, 650MPa or less is preferred.
3-2 mechanical characteristics of the weld
Highest hardness of the surface layer portion in the welding heat affected zone: 250Hv or less
In the steel pipe according to the present embodiment, in order to ensure good SSC resistance, the maximum hardness of the surface layer portion in the weld heat affected zone is preferably 250HV or less. The maximum hardness of the surface layer portion is more preferably 245HV or less, and still more preferably 240HV or less.
On the other hand, in order to obtain a strength of X60 or more of the API standard, the maximum hardness of the surface layer portion in the weld heat affected zone is preferably 150HV or more. The maximum hardness of the surface layer portion is more preferably 160HV or more, and still more preferably 170HV or more.
The highest hardness of the surface layer portion in the welding heat affected zone means the highest hardness measured in the region from the surface to the depth position of 0.9mm in the thickness direction. The highest hardness of the surface layer portion in the weld heat affected zone was measured by cutting out a sample as shown in fig. 2, measuring 40 points at a 0.5mm pitch from the toe portion (the boundary between the weld metal portion and the base material portion) toward the base material portion side at positions 0.3mm, 0.6mm, and 0.9mm from the surface, and measuring 120 points in total.
As a result of the above measurement, if the measured point less than 150HV or more than 250HV does not continuously appear at 2 points or more in the wall thickness direction, it is determined that the maximum hardness of the surface layer portion in the weld heat affected zone is 150 to 250HV. The reason why the hardness is measured in this way is the same as the highest hardness of the surface layer portion of the base material portion.
4. Size of
The thickness of the plate is 10-40 mm
Pipe diameter: 508mm (20 inches) or more
In the case of a steel pipe for drilling or a steel pipe for line pipe for oil, natural gas, or the like, the plate thickness is preferably 10 to 40mm, and the pipe diameter (outer diameter) is preferably 508mm or more. The upper limit of the pipe diameter is not particularly limited, but 1422.4mm (56 inches) or less is a substantial upper limit.
5. Angle of the toe portion
In the steel pipe according to the present embodiment, it is preferable to control the angle of the toe portion of the seam-welded portion in order to improve the SSC resistance of the welded portion. In the present embodiment, the angle of the weld toe portion is an angle as shown in fig. 1. That is, the angle of the toe portion is the angle of the excess tip portion of the weld metal portion, that is, the angle formed by the tangential direction of the weld metal and the surface of the base material portion. Also called the so-called flank angle.
In order to suppress SSC, the angle of the weld toe portion on the inner side of the steel pipe is preferably in the range of 130 ° to 180 °. When the angle of the weld toe portion is less than 130 ° and is a sharper angle, strain is accumulated in the weld heat affected zone, and penetration of hydrogen is promoted, so that cracks are likely to occur. In fig. 1, only the angle at the lower left is described as being measured, but in the present embodiment, the angle at the lower left is measured and the angle at the lower right is defined as the angle of the weld toe portion (toe angle).
5. Manufacturing method
A preferred method for producing the steel pipe according to the present embodiment and the steel sheet used as the raw material thereof will be described.
The steel pipe according to the present embodiment can obtain the effects as long as it has the above-described configuration regardless of the production method, but is preferably obtained stably if the following production method is employed, for example.
The steel sheet according to the present embodiment can be obtained by a manufacturing method including the following steps.
(A) A hot rolling step of heating the slab having the predetermined chemical composition to 1000 to 1250 ℃ to perform hot rolling, and then performing Ar hot rolling 3 Finishing hot rolling at the temperature above the point;
(B) A 1 st cooling step of performing accelerated cooling in multiple stages, wherein the steel sheet after the hot rolling step is subjected to Ar 3 Starting water cooling at a temperature not lower than the above point 3 times or more, the stop temperature of water cooling being not higher than 500 ℃, and the maximum reaching temperature by reheating after stopping water cooling exceeding 500 ℃; and
(C) And a 2 nd cooling step of cooling the steel sheet to a temperature of 500 ℃ or lower at an average cooling rate of 0.2 ℃/s or higher.
The steel pipe according to the present embodiment can be obtained by performing the following steps in addition to the steps (a) to (C).
(D) A forming step of forming the steel sheet into a cylindrical shape;
(E) A welding step of welding the two end portions of the cylindrical steel plate in a butt joint manner; and
(F) And a heat treatment step of heat-treating the welded steel pipe at a temperature in the range of 100 to 300 ℃ for a holding time of 1 minute or more.
Preferred conditions are described for each step.
(Hot Rolling Process)
A steel slab produced by casting molten steel having the same chemical composition as that of the parent metal portion of the steel pipe according to the present embodiment is heated to 1000 to 1250 ℃. Casting of molten steel and production of steel slabs before hot rolling may be performed according to conventional methods.
When the heating temperature is less than 1000 ℃ during the rolling of the billet, the heating temperature is 1000 ℃ or higher because the deformation resistance is not reduced and the load on the rolling mill is increased. Preferably 1100 ℃ or higher. On the other hand, when the heating temperature exceeds 1250 ℃, the crystal grains of the billet become coarse, and the strength and toughness deteriorate, so the heating temperature is 1250 ℃ or less. Preferably 1210 ℃ or lower.
Heating the billet in Ar 3 Hot rolling at a temperature not lower than the above-mentioned temperature to produce a steel sheet, and Ar 3 The hot rolling is finished at a temperature above the above point. If the hot rolling temperature is lower than Ar 3 As a result, work ferrite is generated in the steel sheet structure, and the strength is reduced. Therefore, the hot rolling temperature is Ar 3 The point is above.
(the 1 st Cooling step)
For the hot rolled steel sheet, from Ar 3 Temperatures above this point begin to accelerate cooling. In this case, the multistage accelerated cooling is performed in which water cooling is performed with a surface thermometer so that the water cooling stop temperature is 500 ℃ or lower for 2 or more times and the maximum reaching temperature by reheating after the water cooling is stopped exceeds 500 ℃. Preferably, the reaction is carried out 3 or more times.
In order to make the maximum reached temperature due to reheat more than 500 ℃, it is important to increase the temperature difference between the surface and the inside. The temperature difference between the surface and the inside can be adjusted by changing the water flow density and the collision pressure during water cooling.
If the maximum reaching temperature by reheating is 500 ℃ or lower, the hardness of the steel sheet, particularly the maximum hardness of the surface layer portion from the surface to the depth of 1mm, cannot be made 250HV or lower. Further, when the number of reheating at more than 500 ℃ is less than 2, the maximum hardness of the surface layer portion cannot be 250HV or less. Therefore, accelerated cooling is performed so that the reheating at the maximum reached temperature exceeding 500 ℃ is 3 times or more.
For the reason that the hard phase is not generated, it is preferable that each water-cooling stop temperature in the multistage cooling is a temperature exceeding the Ms point.
Further, since a predetermined structure cannot be obtained if the water cooling stop temperature before reheating exceeds 500 ℃, the water cooling stop temperature is set to 500 ℃ or lower. The water cooling stop temperature is preferably 500 ℃ or lower.
By reheating 3 times or more, the maximum hardness HVmax of the surface layer portion of the steel sheet from the surface to a depth of 1mm is reduced to 250HV or less. The number of reheating times is the number of times until the maximum hardness HVmax of the surface layer portion reaches 250HV or less, and therefore, the upper limit of the number of reheating times does not need to be defined.
(cooling step 2)
In the 1 st cooling step, after the completion of water cooling and reheating for 3 or more times, the steel sheet is cooled to a temperature of 500 ℃ or lower at an average cooling rate of 0.2 ℃/s or higher. When the cooling is completed at a temperature exceeding 500 ℃ or the cooling rate is reduced by winding or the like, and the average cooling rate up to 500 ℃ or less is less than 0.2 ℃/s, the variation in hardness becomes small, but the structure and/or hardness of the surface layer portion cannot be obtained.
(Molding step and welding step)
The forming of the steel sheet into the steel pipe according to the present embodiment is not limited to a specific forming method. For example, warm working can be used, but cold working is preferable from the viewpoint of dimensional accuracy.
After the steel sheet is formed into a cylindrical shape, both ends of the steel sheet are butted and arc welded (seam welding). The arc welding is not limited to a specific welding, but submerged arc welding is preferable. The welding conditions may be performed under known conditions. For example, it is preferable to perform welding with 3 electrodes or 4 electrodes in the range of on-line energy of 2.0 to 10kJ/mm depending on the plate thickness. In order to make the weld heat affected zone into the above-mentioned metal structure, for example, as the welding material, Y-D, Y-DM, Y-DMH wire, and NF5000B or NF2000 flux are preferably used. Further, inner surface welding and outer surface welding are preferably performed, and submerged arc welding is preferably performed using an inner surface 3 electrode and an outer surface 4 electrode.
(Heat treatment Process)
Thereafter (after pipe production), the steel pipe is heat-treated at a temperature in the range of 100 to 300 ℃ for a holding time of 1 minute or more. The upper limit is not particularly limited, and is, for example, 60 minutes or less.
(other steps)
Further, the welded portion may be heated to Ac so that a structure harmful to acid resistance (ferrite-pearlite exceeding 20% in area ratio) is not generated in the welded portion 1 And (4) performing seam heat treatment for tempering below a point. The heat treatment may be performed just after seam welding.
The base material portion of the steel pipe according to the present embodiment is not subjected to a treatment exceeding Ac 1 The metal structure of the base material portion is the same as that of the steel sheet according to the present embodiment because of the heat treatment at the point temperature. Therefore, the steel pipe according to the present embodiment has excellent SSC resistance in both the parent metal portion and the welded portion, in addition to HIC resistance equal to or higher than that of conventional steel.
The present invention will be described more specifically with reference to the following examples, but the present invention is not limited to these examples.
Examples
Steel slabs 240mm thick were produced by continuously casting molten steel having the chemical compositions shown in tables 1-1 and 1-2, and steel sheets were produced under the production conditions shown in tables 2-1 to 2-3 (heating temperature, finish rolling temperature, maximum reaching temperature by reheating after the 1 st water cooling stop in the multistage cooling, and the number of reheating exceeding 500 ℃. In tables 2-1 to 2-3, in the column of the water cooling stop temperature, OK indicates an example in which the water cooling stop temperature is 500 ℃ or less after each water cooling of the multistage accelerated cooling, and NG indicates an example in which the cooling stop temperature exceeds 500 ℃.
Round bar tensile specimens were prepared from the obtained steel sheets according to API5L, and the tensile strength was measured. Further, the highest hardness of the surface layer portion from the surface to the depth of 1mm was measured, and the metal structure was observed by SEM. For reference, the structure at a position 5mm from the surface and the structure at a position 1/2 of the thickness of the sheet from the surface (1/2 part) were also observed.
The highest hardness of the surface layer portion was obtained by first cutting a 300mm square steel sheet from the widthwise ends of the steel sheet at positions 1/4, 1/2 and 3/4 of the widthwise direction of the steel sheet by gas cutting, mechanically cutting the cut steel sheet from the center to prepare a block sample having a length of 20mm and a width of 20mm, and grinding the block sample by mechanical grinding. The block sample was measured at 10 points at 0.1mm intervals in the plate thickness direction and at 20 points at 1mm intervals in the width direction at the same depth using a vickers hardness tester (load 100 g) from a position at a depth of 0.1mm from the surface of the steel plate, and the total of 200 points was measured to obtain the highest hardness. At this time, even if there are measurement points having 1 point exceeding 250HV, if 2 or more points do not appear continuously in the plate thickness direction, this point is regarded as an abnormal point and is not adopted, and the next highest value is regarded as the highest hardness. On the other hand, when 2 or more measurement points exceeding 250HV are continuously present in the thickness direction, the highest value is the highest hardness.
The metal structure was observed by immersing a sample prepared so that the position of 0.5mm (surface layer portion) from the surface, 5mm from the surface, and 1/2 of the plate thickness from the surface was observed in a mixed solution of 3% nitric acid and 97% ethanol for several seconds to several tens of seconds to corrode the sample, and the bainite and martensite were classified by the micro vickers hardness. The results are shown in tables 3-1 to 3-3. For the observation of the metal structure, a modified Lepera solution was also used as needed.
Thereafter, each steel sheet was cold-worked into a cylindrical shape, both end portions of the cylindrical steel sheet were butted, and Submerged Arc Welding (SAW) was performed using 3 electrodes or 4 electrodes under conditions in which the in-line energy was in the range of 2.0kJ/mm to 10kJ/mm depending on the sheet thickness, to manufacture a steel pipe.
As the solder material, a flux of Y-D, Y-DM, Y-D wire and NF-5000B was used on the inner surface side, a flux of Y-DM, Y-DMH, Y-DM wire and NF-5000 was used on the outer surface side. The welding conditions are as follows: the inner surface 3 electrode and the outer surface 4 electrode were adjusted in the linear energy at the time of welding in the range of 2.0kJ/mm to 10kJ/mm in terms of the plate thickness.
With respect to the obtained steel pipes, with respect to some of the steel sheets, the base material portions were heat-treated under the conditions shown in tables 2-1 to 2-3. In addition, some of the steel pipes (test No. 58) were heated to 400 ℃ to Ac in the welded portion 1 And (4) carrying out heat treatment on the spots.
For each of the obtained steel pipes, samples having an axial length of 20mm and a circumferential length of 20mm were prepared by mechanical cutting from positions 90 °, 180 °, and 270 ° from the welded portion in the circumferential direction of the steel pipe. Then, using this sample, the highest hardness of the surface layer portion of the steel pipe was determined by the same method as described above. Since the microstructure after the pipe is formed into a steel pipe is considered to be the same as that of a steel sheet, the above measurement results are used as they are.
Further, as evaluation of SSC resistance, round bar samples were prepared from the obtained steel pipes in accordance with API5L, and yield stress and tensile strength were measured.
Further, 4-point bending samples having a width of 15mm, a length of 115mm and a thickness of 5mm were prepared from the inner surface of the base material portion of the steel pipe as a residual inner surface, and the occurrence of cracks was examined under various solution environments having a hydrogen sulfide partial pressure and a pH of 3.5 according to NACE TM 0316-2016. The load stress in the 4-point bending test was set to 90% and 95% of the actual yield stress.
Further, as an evaluation of HIC resistance, a hydrogen induced cracking test (hereinafter referred to as "HIC test") was performed. HIC testing was performed on a NACE TM 0284 2016 basis. Specifically, a sample prepared from the base material portion and having a length of 100mm and a width of 20mm along the curvature of the inner surface was saturated with 100% H in Solution A (Solution A) (5 mass% NaCl +0.5 mass% glacial acetic acid aqueous Solution) 2 The test solution of S gas was immersed for 96 hours. Thereafter, the surface portion and the central portion were measured for the area ratio (CAR) at which cracks occurred. If CAR is 5% or less, HIC resistance is judged to be excellent.
Further, based on the results of the round bar tensile test, the proportional limit of each steel sheet was calculated by the above-described method. The results are shown in tables 4-1 to 4-3.
The test Nos. 1 to 22 and 60 to 65 (inventive steel pipes) have HIC resistance equal to or higher than that of conventional steel pipes and are excellent in SSC resistance.
The chemical composition of the weld metal portion was determined from the steel pipe No. 1. As a result, the weld metal had a chemical composition of C:0.07%, si:0.41%, mn:1.45%, P:0.010%, S:0.0030%, cu:0.04%, ni:0.12%, cr:0.16%, mo:0.24%, nb:0.02, ti:0.02%, al:0.02%, O:0.045%, and the balance of Fe and impurities.
(shape of weld toe)
The angle of the excess height tip portion of the weld metal portion, i.e., the angle formed by the tangential direction of the weld metal on both sides and the surface of the base metal portion, was determined for the resulting steel pipe, and the smaller angle was defined as the angle of the weld toe portion.
(SSC resistance)
In addition, as an evaluation of SSC resistance, 4-point bent samples having a width of 15mm, a length of 115mm and a thickness of 5mm were prepared from the inner surface of the steel pipe so as to leave the inner surface and so that the toe portion was disposed at the center portion in the longitudinal direction of the sample, and the occurrence of cracks was examined under various hydrogen sulfide partial pressures and solution environments of ph3.5 in accordance with NACE TM 0316-2016. The load stress in the 4-point bending test was set to 90% and 95% of the actual yield stress.
(highest hardness of surface layer part of welding heat-affected zone)
The hardness of the surface layer portion in the welding heat affected zone was measured. The hardness was measured in the surface layer portion from the surface to a depth of 1.0mm or 0.9mm from the center in the circumferential direction and the longitudinal direction of the steel pipe. The method of cutting out the test piece for the hardness test of the welding heat affected zone was as described above.
Specifically, for the hardness measurement of the weld heat affected zone, 40 points were measured at a 0.5mm pitch from the weld toe (the boundary between the weld metal portion and the base material portion) toward the base material portion side at positions 0.3mm, 0.6mm, and 0.9mm from the surface, and the total of 120 points was measured to calculate the maximum hardness.
The metal structure in the surface layer portion of the weld heat affected zone was also observed, and the area ratio was also measured. The metal structure of the surface layer portion was a metal structure at a position 0.5mm deep from the surface in the thickness direction. The results are shown in Table 5.
TABLE 5
B bainite
The tests nos. 2, 2', 11, and 11' also have excellent SSC resistance including the welded portions. On the other hand, in test Nos. 2 "and 11", SSC occurred from the toe.
Industrial applicability
According to the present invention, it is possible to provide a steel pipe having excellent SSC resistance in which the yield stress is 350MPa or more and cracks are not generated even when a stress exceeding 90% of the yield stress is applied in an environment of 30 ℃ or less containing hydrogen sulfide exceeding 0.1MPa, and a steel sheet usable as a raw material thereof. Specifically, the steel pipe according to the present invention is suitably used for a steel pipe used in a high-pressure hydrogen sulfide environment, such as a steel pipe for drilling or a steel pipe for transportation of oil, natural gas, or the like.
Description of the reference numerals
1. Weld metal part
2. Mother material part
3. Angle of the toe portion
4. Weld heat affected zone
5. Sample cutting part
Claims (6)
1. A steel pipe having a base material portion and a welded portion,
the chemical composition of the matrix part comprises in mass%
C:0.030~0.100%、
Si: less than 0.50 percent,
Mn:0.80~1.60%、
P: less than 0.020%,
S: less than 0.0030%,
Al: less than 0.060%,
Ti:0.001~0.030%、
Nb:0.006~0.100%、
N:0.0010~0.0080%、
Ca:0.0005~0.0050%、
O: less than 0.0050 wt%,
Cr:0~1.00%、
Mo:0~0.50%、
Ni:0~1.00%、
Cu:0~1.00%、
V:0~0.10%、
Mg:0~0.0100%、
REM:0~0.0100%,
The balance of Fe and impurities,
(ii) an ESSP represented by the following formula (i) is 1.5 to 3.0,
ceq represented by the following formula (ii) is 0.20 to 0.50,
the surface layer part of the parent metal part ranging from the surface to the depth of 1mm has a metal structure composed of 1 or more kinds selected from polygonal ferrite, granular bainite, acicular ferrite and bainite,
the surface layer portion of the base material portion has a maximum hardness of 250HV or less, a yield stress of 415 to 630MPa, and a proportional limit in a stress-strain curve of 90% or more of the yield stress,
ESSP=Ca×(1-124×O)/(1.25×S) …(i)
Ceq=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5 …(ii)
in the formula, each element symbol represents the content of each element in mass% contained in the steel, and is zero when not contained.
2. The steel pipe according to claim 1, wherein,
in the metal structure of the surface layer portion of the base material portion, the total area ratio of granular bainite, acicular ferrite, and bainite exceeds 80%.
3. The steel pipe according to claim 1 or 2,
the chemical composition of the matrix part contains, in mass%, a chemical component selected from the group consisting of
Cr:0.10~1.00%、
Mo:0.03~0.50%、
Ni:0.10~1.00%、
Cu:0.10~1.00%、
V:0.005~0.10%、
Mg:0.001 to 0.0100%, and
REM:0.001~0.0100%
1 or more of them.
4. The steel pipe according to any one of claims 1 to 3,
the chemical composition of the matrix section contains, in mass%: 0.01 to 0.04 percent,
the weld comprises a weld heat affected zone and a weld metal portion,
the metal structure of the surface layer portion in the welding heat affected zone includes 1 or more kinds selected from bainite and acicular ferrite,
the surface layer portion in the welding heat affected zone has a maximum hardness of 250HV or less,
the angle of the weld toe portion on the inner side of the steel pipe is in the range of 130-180 degrees.
5. The steel pipe according to any one of claims 1 to 4,
the thickness of the parent material part is 10-40 mm, and the pipe diameter is more than 508 mm.
6. A steel sheet used for the parent material portion of the steel pipe according to any one of claims 1 to 5.
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KR20220131992A (en) | 2022-09-29 |
WO2021176590A1 (en) | 2021-09-10 |
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JP7360075B2 (en) | 2023-10-12 |
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BR112022013767A2 (en) | 2022-10-11 |
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