EP4116453A1

EP4116453A1 - Steel pipe and steel sheet

Info

Publication number: EP4116453A1
Application number: EP20923002.8A
Authority: EP
Inventors: Takuya Hara; Yasuhiro Shinohara; Taishi Fujishiro; Kiyoshi Ebihara; Eiji Tsuru
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 2020-03-04
Filing date: 2020-03-04
Publication date: 2023-01-11
Also published as: KR20220131992A; BR112022013767A2; JP7360075B2; EP4116453A4; WO2021176590A1; KR102792300B1; JPWO2021176590A1; CN115210396A

Abstract

What is provided is a steel pipe including a base material portion and a welded portion, in which the base material portion has a predetermined chemical composition, a metallographic structure of a surface layer area in a range of 1 mm of a depth from a surface of the base material portion consists of one or more selected from polygonal ferrite, granular bainite, acicular ferrite, and bainite, a maximum hardness of the base material portion in the surface layer area is 250 HV or less, a yield stress is 415 to 630 MPa, and a proportional limit in a stress-strain curve is 90% or more of the yield stress.

Description

[Technical Field of the Invention]

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 plate suitable as a material thereof.

[Background Art]

A system that is installed on the ground or on the seabed and transfers oil or gas is referred to as a pipeline. A steel pipe for a pipeline that constitutes such a pipeline is referred to as a line pipe. A straight seam arc welded steel pipe (hereinafter, referred to as an arc welded steel pipe, a welded steel pipe, or a steel pipe) is widely used for a large-diameter line pipe having a pipe diameter of 508 mm or more, which constitutes a long-range pipeline. Here, the straight seam arc welded steel pipe is a steel pipe manufactured by forming a thick steel plate into a tubular open pipe and welding a butt portion (seam portion) by an arc welding method such as a submerged arc welding method. Depending on the forming method, the pipe may be called a UOE steel pipe or a JCOE steel pipe.
In recent years, pipeline construction has expanded to harsh environmental regions such as cold regions and sour environments. Here, the sour environment means an acidified wet hydrogen sulfide environment containing H₂S which is a corrosive gas. It is known that in a case where the line pipe is exposed to a sour environment, hydrogen-induced crack (HIC) may occur. On the other hand, sulfide stress crack (SSC) may occur in an oil country tubular goods having higher strength than a line pipe. However, even in a line pipe, SSC may occur in a case where a hydrogen sulfide partial pressure becomes high or the stress becomes high. As described above, the line pipe (sour resistant line pipe) used in a harsh sour environment is required to have SSC resistant properties in addition to HIC resistant properties.
Patent Document 1 and Non-Patent Document 2 suggest a welded steel pipe or a steel plate for a steel pipe having excellent sour resistant properties, in which the hardness of a base material portion and a welded portion is defined as 220 Hv or less based on the finding that the hardness affects the sour resistant properties.
In addition, Patent Document 2 suggests a high-strength steel plate for a sour resistant line pipe in which in a case where 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 indicating the hardness of a center segregation portion, by mass%, is a1.0 or less, the steel structure is a bainite structure, the hardness unevenness ΔHV in a plate thickness direction is 30 or less, and the hardness unevenness ΔHV in a plate width direction is 30 or less.
Patent Document 3 suggests a high-strength steel plate for a sour resistant line pipe having excellent material uniformity in a steel plate, in which the metallographic structure is a bainite structure, the hardness unevenness in a plate thickness direction is ΔHv₁₀25 or less, the hardness unevenness in a plate width direction is ΔHv₁₀25 or less, and the maximum hardness of a surface layer area of the steel plate is Hv₁₀220 or less.
In addition, Patent Document 4 suggests a heat treated steel plate having excellent hydrogen-induced crack resistance, in which a metallographic structure in a range of 1 mm from a steel plate surface in a plate thickness direction consists of one or both of tempered martensite and tempered bainite, in the metallographic structure in a range of ± 1 mm from a plate thickness center portion in the plate thickness direction, a primary phase consisting of one or both of tempered martensite and tempered bainite is 80% or more in area ratio, the remainder other than the primary phase consists of one or more selected from ferrite, pearlite, cementite, and residual austenite, and the hardness at a position of 1 mm from a steel plate surface in the plate thickness direction is 250 HV or less in Vickers hardness, and the hardness difference between the position of 1 mm from the steel plate surface and the plate thickness center portion is 60 HV or less in Vickers hardness.
In the steel plates of Patent Documents 1 to 4 and Non-Patent Document 2, sour resistant properties are satisfied in an environment where the hydrogen sulfide partial pressure is 0.1 MPa (1 bar) or less and a load stress is 90% or less of the yield stress. However, a usage environment of the oil country tubular goods or the line pipe has become more severe recently, and the required level for sour resistant properties of the welded steel pipe for a line pipe has become higher.
In the related art, sour resistant properties were required in an environment of the hydrogen sulfide partial pressure of 0.1 MPa (1 bar) or less, but recently, a material design capable of withstanding a high-pressure hydrogen sulfide environment of more than 0.1 MPa has been required. In addition, in the related art, the load stress was 90% or less of the yield stress, but recently, a material design capable of withstanding a high-pressure hydrogen sulfide environment of the load stress of more than 90% of the yield stress.
According to the studies by the present inventors, the steel plates of Patent Documents 1 to 4 and the steel plates of Non-Patent Document 2 did not have sufficient sour resistant properties in an environment in which the hydrogen sulfide partial pressure exceeds 0.1 MPa (1 bar) and exceeds 90% of the yield stress.
In response to such problems, Patent Document 5 discloses a steel pipe having excellent SSC resistant properties, having HIC resistant properties equal to or higher than those of the steel in the related art, and having a yield strength of 350 MPa or more, in which crack does not occur even in a case where a stress of 90% or more of the yield strength is loaded in an environment of 30°C or lower containing hydrogen sulfide of a hydrogen sulfide partial pressure of more than 0.1 MPa.
However, Patent Document 5 shows that SSC resistant properties are excellent in the sulfide stress corrosion crack test in a case where a load stress is 90% of the yield stress, but does not show those in a case where the load stress is more than 90% of the yield stress.

[Prior Art Document]

[Patent Documents]

[Patent Document 1] Japanese Unexamined Patent Application, First Publication No. 2011-017048
[Patent Document 2] Japanese Unexamined Patent Application, First Publication No. 2012-077331
[Patent Document 3] Japanese Unexamined Patent Application, First Publication No. 2013-139630
[Patent Document 4] Japanese Unexamined Patent Application, First Publication No. 2014-218707
[Patent Document 5] Japanese Patent No. 6369658

[Non-Patent Documents]

[Non-Patent Document 1] Nippon Steel & Sumitomo Metal Technical Report No. 397 (2013), p. 17-22
[Non-Patent Document 2] JFE Technical Report No. 9 (August 2005), p. 19-24

[Disclosure of the Invention]

[Problems to be Solved by the Invention]

As mentioned above, the use environment of a line pipe has become more severe these days, and the required level of sour resistant properties of a welded steel pipe for a line pipe has become more sophisticated. Therefore, an object of the present invention is to provide a welded steel pipe capable of being used in a harsh high-pressure hydrogen sulfide environment and having excellent sour resistant properties, particularly a straight seam arc welded steel pipe, and a steel plate (particularly a thick steel plate) as a material thereof.
More specifically, the object of the present invention is to provide a steel pipe having a HIC resistant properties equal to or higher than those of steel in the related art, having a yield stress of 350 MPa or more, and having excellent SSC resistant properties, in which crack does not occur even in a case where a stress of more than 90% of the yield stress, specifically a stress of 95% of the yield stress, is loaded in an environment of 30°C or lower containing hydrogen sulfide of more than 0.1 MPa, and a steel plate as a material thereof.

[Means for Solving the Problem]

The present invention has been made to achieve the above objects, and the following steel pipe and steel plate are the gist of the present invention.

(1) A steel pipe according to an aspect of the present invention is a steel pipe including a base material portion and a welded portion, in which a chemical composition of the base material portion includes, by 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%, and a remainder: Fe and impurities, 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, a metallographic structure of a surface layer area in a range of 1 mm of a depth from a surface of the base material portion is one or more selected from polygonal ferrite, granular bainite, acicular ferrite, and bainite, the maximum hardness of the base material portion in the surface layer area is 250 HV or less, the yield stress is 415 to 630 MPa, and the proportional limit in a stress-strain curve is 90% or more of the yield stress. $ESSP = Ca \times (1 - 124 \times O) / (1.25 \times S)$
$Ceq = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5$
Here, each element symbol in the formula represents the amount (mass%) of one element included in steel, and is zero in a case where no element is included.
(2) In the steel pipe according to (1), a total area ratio of the granular bainite, the acicular ferrite, and the bainite may be more than 80% in the metallographic structure of the surface layer area of the base material portion.
(3) In the steel plate according to (1) or (2), the chemical composition of the base material portion may include one or more selected from, by mass%, 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% to 0.0100%.
(4) In the steel pipe according to any one of (1) to (3), the chemical composition of the base material portion includes Nb: 0.01% to 0.04%, by mass%, the welded portion includes a welded heat-affected zone and a weld metal portion, the metallographic structure of the surface layer area in the welded heat-affected zone includes one or both of bainite and acicular ferrite, the maximum hardness of the surface layer area in the welded heat-affected zone is 250 HV or less, and the angle of a weld toe portion on an inner side of the steel pipe is in a range of 130° to 180°.
(5) In the steel pipe according to any one of (1) to (4), the thickness of the base material portion may be 10 to 40 mm, and the pipe diameter may be 508 mm or more.
(6) A steel plate according to another aspect of the present invention is used for the base material portion of the steel pipe according to any one of (1) to (5).

[Effects of the Invention]

According to the aspect of the present invention, it is possible to provide a steel pipe having excellent SSC resistant properties, in which crack does not occur even in a case where a stress of more than 90% of a yield stress is loaded in an environment of 30°C or lower containing hydrogen sulfide of more than 0.1 MPa, and a steel plate capable of being used as a material thereof.
In addition, according to a preferable aspect of the present invention, it is possible to provide a steel pipe including a welded portion having excellent sour resistant properties, which can be used in a harsh high-pressure hydrogen sulfide environment.

[Brief Description of the Drawings]

Fig. 1 is a schematic view showing an angle of a weld toe portion of a steel pipe according to the present embodiment.
Fig. 2 is a schematic view showing a portion where a test piece is cut out from the steel pipe according to the present embodiment.

[Embodiments of the Invention]

In order to study a method for achieving the object, the present inventors observed fracture surfaces of a base material portion and a welded portion, structures, and the like of the steel pipe cracked in a high-pressure hydrogen sulfide environment of more than 0.1 MPa (for example, in an H₂S saturated solution containing 5% salt and acetic acid) in a test in which the load stress is more than 90%. In addition, the stress-strain curve of the steel pipe was also examined. As a result, the following findings were obtained.

(a) In order to improve the sour resistant properties in a high-pressure hydrogen sulfide environment of more than 0.1 MPa, it is required to control not only the HIC resistant properties but also the SSC resistant properties. HIC occurs in the center segregation portion existing in the vicinity of the center portion in the thickness direction of the steel pipe. On the other hand, the SSC depends on the structure and hardness in a range of 1 mm (surface layer area) from the surface of the steel pipe, which was not considered in the related art.
(b) In a case where the metallographic structure of the surface layer area is a structure consisting of one or more selected from polygonal ferrite, granular bainite, acicular ferrite, and bainite, and the maximum hardness is 250 HV or less, the sour resistant properties are improved. In addition, in a case where the total area ratio of one or more selected from granular bainite, acicular ferrite, and bainite is more than 80%, the SSC properties are further improved.
(c) In a case of controlling the structure of the surface layer area as described above, it is important to control the carbon equivalent Ceq to 0.20 to 0.50 and then strictly control a cooling pattern.
(d) In a case of applying a method for manufacturing a hot-rolled steel sheet on the premise of coiling, a cooling rate after the accelerated cooling is stopped is slower than the cooling rate of air cooling. In this case, the unevenness in hardness becomes small, but the above-mentioned structure and/or hardness of the surface layer area cannot be obtained. Therefore, in order to obtain the above-mentioned structure and hardness of the surface layer area, it is required to perform manufacturing by a plate manufacturing process.
(e) By appropriately controlling the hardness of the welded heat-affected zone and the shape of the weld toe portion (refer to Fig. 1), the stress concentration of the toe portion is relaxed and the SSC resistant properties of the welded portion are improved.

The present invention has been made based on the above findings.
Hereinafter, a steel pipe according to an embodiment of the present invention (steel pipe according to the present embodiment) and a steel plate for the steel pipe (steel plate according to the present embodiment) will be described.
The steel pipe according to the present embodiment is a welded steel pipe including a base material portion and a welded portion. The base material portion is cylindrical, and the welded portion extends in a direction parallel to an axial direction of the steel pipe. The welded portion includes a weld metal portion, which is a metal portion that melts and solidifies during welding, and a welded heat-affected zone, which is a region that did not melt during welding but caused changes in the structure and the like due to heat input by welding and subsequent cooling.
In addition, the steel plate according to the present embodiment is used for the base material portion of the steel pipe. That is, as will be described later, the steel pipe is obtained by forming the steel plate into a tubular shape and butt welding both end portions of the steel plate. Therefore, the chemical composition, metallographic structure, and mechanical properties of the steel plate are the same as those of the base material portion of the steel pipe. Therefore, hereinafter, the description of the base material portion of the steel pipe according to the present embodiment is also applied to the steel plate according to the present embodiment.

1. Chemical composition

Reasons for limiting each element are as follows. In the following description, "%" for the amount means "mass%".

1-1. Chemical composition of base material portion (steel plate) of steel pipe

The chemical composition of the base material portion (steel plate according to the present embodiment) of the steel pipe according to the present embodiment will be described.

C: 0.030% to 0.100%

C is an element that increases the strength of steel. In a case where the C content is less than 0.030%, the strength increase effect cannot be sufficiently obtained. Therefore, the C content is 0.030% or more. It is preferably 0.035% or more.
On the other hand, in a case where the C content is more than 0.100%, the hardness of the surface layer area becomes high and SSC easily occurs. In addition, carbides are generated, and HIC easily occurs. Therefore, the C content is set to 0.100% or less. In a case of ensuring more excellent SSC resistant properties and HIC resistant properties, and suppressing lowering of weldability and toughness, the C content is preferably 0.070% or less, and more preferably 0.060% or less.

Si: 0.50% or less

In a case where the Si content is more than 0.50%, the toughness of the welded portion is lowered. Therefore, the Si content is set to 0.50% or less. It is preferably 0.35% or less, and 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 a steel raw material and/or in a steelmaking process, 0.01% is a substantial lower limit of the Si content in practical steel. In addition, Si may be added for deoxidation, and in this case, the lower limit of the Si content may be 0.10%.

Mn: 0.80% to 1.60%

Mn is an element that improves the strength and toughness of steel. In a case where the Mn content is less than 0.80%, these effects cannot be sufficiently obtained. Therefore, the Mn content is set to 0.80% or more. The Mn content is preferably 0.90% or more, and more preferably 1.00% or more.
On the other hand, in a case where the Mn content is more than 1.60%, the sour resistant properties are lowered. Therefore, the Mn content is set to 1.60% or less. It is preferably 1.50% or less.

P: 0.020% or less

P is an element that is inevitably included as impurities. In a case where the P content is more than 0.020%, the HIC resistant properties are lowered and the toughness of the welded portion is lowered. Therefore, the P content is 0.020% or less. It is preferably 0.015% or less, and more preferably 0.010% or less. The P content is preferably low, and the lower limit includes 0%. However, in a case where the P content is lowered to less than 0.001%, a manufacturing cost significantly increases, and thus 0.001% is the substantial lower limit of the P content in practical steel.

S: 0.0030% or less

S is an element that is inevitably included as impurities. In addition, S is an element that forms MnS that stretches in a rolling direction during hot rolling and lowers the HIC resistant properties. In a case where the S content is more than 0.0030%, the HIC resistant properties are significantly lowered, and the S content is set to 0.0030% or less. It is preferably 0.0020% or less, and more preferably 0.0010% or less. The lower limit includes 0%, but in a case where the S content is reduced to less than 0.0001%, the manufacturing cost increases significantly, and thus 0.0001% is a substantial lower limit on a practical steel plate.

Al: 0.060% or less

In a case where the Al content is more than 0.060%, clusters in which Al oxides are aggregated are generated, and the HIC resistant properties are lowered. Therefore, the Al content is 0.060% or less. It is preferably 0.050% or less, more preferably 0.035% or less, and further more preferably 0.030% or less. It is preferable that the Al content is low, and the lower limit of the Al content includes 0%.
On the other hand, since Al is inevitably mixed from the steel raw material and/or in the steelmaking 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 0.010%.

Ti: 0.001% to 0.030%

Ti is an element that forms carbonitrides and contributes to the refinement of crystal grains. In a case where the Ti content is less than 0.001%, this effect cannot be sufficiently obtained. Therefore, the Ti content is set to 0.001% or more. It is preferably 0.008% or more, and more preferably 0.010% or more.
On the other hand, in a case where the Ti content is more than 0.030%, carbonitride is excessively generated, and HIC resistant properties and toughness are lowered. Therefore, the Ti content is 0.030% or less. It is preferably 0.025% or less, and more preferably 0.020% or less.

Nb: 0.006% to 0.100%

Nb is an element that forms carbides and/or nitrides and contributes to the increase of strength. In a case where 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. It is preferably 0.008% or more, and more preferably 0.010% or more. In particular, in a case of ensuring the hardness of the welded heat-affected zone, the Nb content is preferably 0.010% or more, more preferably 0.015% or more, and further more preferably 0.017% or more.
On the other hand, in a case where the Nb content is more than 0.100%, the carbonitride of Nb is aggregated in the center segregation portion, and the HIC resistant properties are lowered. Therefore, the Nb content is set to 0.100% or less. It is preferably 0.080% or less, and more preferably 0.060% or less.
In addition, in a case of improving the toughness of the welded portion (welded heat-affected zone and weld metal portion), the Nb content is preferably 0.040% or less, more preferably 0.035% or less, and further more preferably 0.033% or less.

N: 0.0010% to 0.0080%

N is an element that bonds with Ti and/or Nb to form a nitride and contributes to the refinement of the austenite grain size during heating. In a case where the N content is less than 0.0010%, the effects cannot be sufficiently obtained. Therefore, the N content is set to 0.0010% or more. It is preferably 0.0020% or more.
On the other hand, in a case where the N content is more than 0.0080%, nitrides of Ti and/or Nb are aggregated and the HIC resistant properties are lowered. Therefore, the N content is set to 0.0080% or less. It is preferably 0.0060% or less, and more preferably 0.0050% or less.

Ca: 0.0005% to 0.0050%

Ca is an element that suppresses the formation of MnS that extends in the rolling direction by forming CaS in steel, and as a result, contributes to the improvement of HIC resistant properties. In a case where the Ca content is less than 0.0005%, the effects cannot be sufficiently obtained. Therefore, the Ca content is set to 0.0005% or more. It is preferably 0.0010% or more, and more preferably 0.0015% or more.
On the other hand, in a case where the Ca content is more than 0.0050%, oxides are aggregated and the HIC resistant properties are lowered. Therefore, the Ca content is set to 0.0050% or less. It is preferably 0.0045% or less, and more preferably 0.0040% or less.

O: 0.0050% or less

O is an element that inevitably remains. In a case where the O content is more than 0.0050%, oxides are generated and the HIC resistant properties are lowered. Therefore, the O content is set to 0.0050% or less. From a viewpoint of ensuring the toughness of the steel plate and the toughness of the welded portion, 0.0040% or less is preferable, and 0.0030% or less is more preferable. The O content is preferably low, and may be 0%. However, in a case where O is reduced to less than 0.0001%, the manufacturing cost will increase significantly. Therefore, the O content may be 0.0001% or more. From a viewpoint of manufacturing cost, 0.0005% or more is preferable.

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%
Cr, Mo, Ni, Cu, and V are elements that enhance the hardenability of steel. Therefore, one or more selected from these elements may be included, if necessary.

In order to obtain the effects, it is preferable to include one or more 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.
On the other hand, in a case where the amounts of Cr, Ni, and Cu each are more than 1.00%, or the Mo content is more than 0.50%, or the V content is more than 0.10%, the hardness increases and the sour resistant properties are lowered. 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: 0.06% or less.

Mg: 0% to 0.0100%
REM: 0% to 0.0100%
Mg and REM are elements that control the morphology of sulfides. In order to obtain the effects, it is preferable to include one or both of Mg: 0.001% or more and REM: 0.001% or more.

On the other hand, in a case where the amounts of Mg and REM each are more than 0.0100%, the sulfide becomes coarse and the effects cannot be exhibited. Therefore, the amounts of Mg and REM are both 0.0100% or less. It is preferably 0.0050% or less.
Here, REM is a rare earth element and is a collective term for 16 elements of Sc and lanthanoid, and the REM content means the total amount of these elements.
In the chemical composition, the remainder is Fe and impurities. Here, the "impurities" means a component mixed due to raw materials such as ore and scrap and various factors in the manufacturing step when steel is industrially manufactured, and is acceptable in a range not imparting an adverse effect to the present invention.
In a case where Sb, Sn, Co, As, Pb, Bi, H, W, Zr, Ta, B, Nd, Y, Hf and Re are included as impurities, the amount of each is preferably controlled within the range described later.

Sb: 0.10% or less
Sn: 0.10% or less
Co: 0.10% or less
As: 0.10% or less
Pb: 0.005% or less
Bi: 0.005% or less
H: 0.0005% or less
Sb, Sn, Co, As, Pb, Bi, and H may be mixed from the steel raw material as impurities or inevitable mixed elements, but within the above range, the properties of the steel pipe according to the present embodiment are not impaired. Therefore, it is preferable to limit these elements to within the range.

W, Zr, Ta, B, Nd, Y, Hf, and Re: Total 0.10% or less

These elements may be mixed from the steel raw material as impurities or inevitable mixing elements, but within the range, the properties of the steel pipe according to the present embodiment are not impaired. Therefore, the total amount of these elements is limited to 0.10% or less.
In the chemical composition of the base material portion, in addition to the amount of each element being within the range, the values of ESSP and Ceq calculated from the amount of the components are required to satisfy predetermined conditions, as shown below.

ESSP: 1.5 to 3.0

ESSP is a value that is an index showing whether or not there is an amount of effective Ca commensurate with the S content, assuming that the residual Ca (effective Ca) obtained by subtracting Ca bound to oxygen is bound to S in an atomic weight ratio, and is represented by the following Formula (i). In the steel pipe according to the present embodiment, the value of ESSP is required to be in a range of 1.5 to 3.0 in order to ensure the HIC resistant properties equal to or higher than those of steel in the related art. $ESSP = Ca \times (1 - 124 \times O) / (1.25 \times S)$
Here, each element symbol in the formula represents the amount (mass%) of each element included in steel, and is zero in a case where no element is included.
In order to ensure the HIC resistant properties, it is effective to suppress generation of MnS stretched in the rolling direction. In addition, in order to suppress the generation of MnS stretched in the rolling direction, it is an effective method to reduce the S content, add Ca, and fix S by forming CaS. On the other hand, since Ca has a stronger oxygen affinity than S, it is required to reduce the O content in order to form the required amount of CaS.
In a case where the ESSP is less than 1.5, the Ca content is insufficient with respect to the O content and the S content, and MnS is generated. Since MnS stretched by rolling causes deterioration of HIC resistant properties, the ESSP is set to 1.5 or more. It is preferably 1.6 or more, and more preferably 1.7 or more.
On the other hand, in a case where the Ca content becomes excessive, a large amount of cluster-like inclusions are generated, and there is a concern that the morphological control of MnS is inhibited. In a case where the O content and the S content are reduced, the generation of cluster-like inclusions can be suppressed, but in a case where the ESSP is more than 3.0, the manufacturing cost for reducing the O content and the S content is significantly increased. Therefore, the ESSP is set to 3.0 or less. It is preferably 2.8 or less, and more preferably 2.6 or less.
In a case where the value of ESSP is in a range of 1.5 to 3.0, the amount of effective Ca is equal to or more than the minimum required amount for controlling the morphology of MnS, and is adjusted to equal to or less than the critical amount at which cluster-like inclusions are not generated, and thus excellent HIC resistant properties can be obtained.

Ceq: 0.20 to 0.50

Ceq is a value that is an index of hardenability, which means carbon equivalent, and is represented by the following Formula (ii). In the base material portion of the steel pipe according to the present embodiment, as will be described later, in order to obtain a structure consisting of one or more selected from polygonal ferrite, granular bainite, acicular ferrite, and bainite, and preferably a metallographic structure consisting of one or more selected from granular bainite, acicular ferrite, and bainite, in total of more than 80% in the surface layer area, it is required to appropriately control the hardenability of steel. Therefore, it is required to set the value of Ceq to 0.20 to 0.50. $Ceq = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5$
Here, each element symbol in the formula represents the amount (mass%) of one element included in steel, and is zero in a case where no element is included.
In a case where Ceq is less than 0.20, a tensile strength of 530 MPa or more cannot be obtained. Therefore, Ceq is set to 0.20 or more. It is preferably 0.25 or more. On the other hand, in a case where Ceq is more than 0.50, the surface hardness of the welded portion becomes high and the sour resistant properties are lowered. Therefore, Ceq is set to 0.50 or less. It is preferably 0.45 or less.

1-2. Chemical composition of welded portion

The welded heat-affected zone is a portion where the base material portion is not melted by welding. Therefore, the chemical composition is the same as that of the base material portion, and the reason for limitation is also 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 the same level as or higher than the strength of the base material portion, the chemical composition of the weld metal portion is preferably in the following range.
That is, the chemical composition of the weld metal portion in the welded portion is, by mass%, 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% to 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%, a remainder: Fe and impurities.
The chemical composition of the weld metal portion is determined by an inflow ratio of the base material and the welding material during welding. As the welding material, a commercially available material may be used, and for example, Y-D, Y-DM, Y-DMH wire, and a flux of NF5000B or NF2000 can be used. In addition, in order to control the composition range of the weld metal portion, it is desirable to adjust welding conditions to a range described later.

2. Metallographic structure

2-1. Metallographic structure of base material portion

Next, the metallographic structure of the base material portion (steel plate) of the steel pipe will be described.
The metallographic structure in the surface layer area of the base material portion is a structure consisting of one or more selected from polygonal ferrite, granular bainite, acicular ferrite, and bainite. In the present embodiment, the surface layer area means a range up to 1.0 mm 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 area of the base material portion to 250 HV or less and to ensure the required strength and excellent sour resistant properties, the metallographic structure in the surface layer area is a structure consisting of one or more selected from polygonal ferrite, granular bainite, acicular ferrite, and bainite. Preferably, a total area ratio of one or more selected from granular bainite, acicular ferrite, and bainite is more than 80%. In a case where the total area ratio is more than 80%, the strength and sour resistant properties are further improved. More preferably, it is 85% or more.
The measurement of the area ratio of each structure is performed by observing the metallographic structure etched with a mixed solution of 3% nitric acid and 97% ethanol with a scanning electron microscope (SEM). The structure of the surface layer area may be measured at a position of 0.5 mm from the surface of the steel plate as a representative.
The metallographic structure of the surface layer area in the base material portion refers to the metallographic structure of the base material portion that is not affected by welding. The steel pipe according to the present embodiment refers to the metallographic structure of the surface layer area at positions of 90°, 180°, and 270° in a circumferential direction of the steel pipe from a butt portion (corresponding to the seam portion and the end portion of the steel plate in a width direction). The position corresponds to the metallographic structure of the surface layer area at positions of 1/4, 1/2, and 3/4 of the steel plate in a width direction in the steel plate.
In the present embodiment, the polygonal ferrite is a structure observed as a massive structure containing no coarse cementite or coarse precipitates such as MA inside the grain, and acicular ferrite is a structure in which prior austenite grain boundary is unclear, and inside the grain, needle-like ferrite (carbide and austenite/martensite mixture do not exist) is generated in random crystal orientation.
On the other hand, the worked ferrite is a ferrite subjected to working, and grains flattened in the rolling direction are observed by an optical microscope or SEM observation. Flattening means that an aspect ratio (ferrite length in the rolling direction with respect to the ferrite length in the plate thickness direction) is 2.0 or more. In addition, pearlite is a structure in which ferrite and cementite are layered, and among pearlite, a structure in which the cementite forming a layer is cut off in the middle is pseudo-pearlite.
As for the residual austenite, the one appeared in white by a modified LePera solution is determined as residual austenite.
Granular bainite is generated at an intermediate transformation temperature between that of acicular ferrite and bainite and has intermediate structural properties. Granular bainite is a structure in which prior austenite grain boundaries are partially visible, coarse lath structures are present in the grains, and a portion in which fine carbides and austenite-martensite constituent are scattered in and between the laths, and a portion of needle-like or amorphous ferrite in which the prior austenite grain boundaries are unclear are mixedly present.
Bainite and martensite are structures in which the prior austenite grain boundaries are clear and fine lath structures are developed in the grains. Bainite and martensite cannot be easily distinguished by SEM observation, but in the present embodiment, are structures in which the prior austenite grain boundaries are clear, and the inside the grain, a fine lath structure is developed, a structure having a hardness of 250 Hv or more is martensite, and a structure in which prior austenite grain boundaries are clear, a fine lath structure is developed inside the grain, and the hardness is less than 250 Hv is bainite. Whether the hardness is 250 Hv or more or less than 250 Hv is determined by measuring 10 points of the target structure with Micro Vickers with a load of 100 gf and determining whether the maximum value is 250 Hv or less than 250 Hv. All structures are tempered during double heating and heat treatment with steel pipes, but there is no particular distinction between the presence or absence of tempering.
In the steel pipe according to the present embodiment, the structure other than the surface layer area is not particularly limited. However, in a case where the structure of the surface layer area is controlled as described above by the manufacturing method described later, the structure other than the surface layer area, for example, the structure of a wall thickness center portion (thickness middle portion of the steel plate) does not contain worked ferrite, or pearlite (containing pseudo-pearlite), and martensite, mainly contains acicular ferrite and bainite, and preferably has a maximum hardness of 250 Hv or less.

2-2. Metallographic structure of welded heat-affected zone

In the steel pipe according to the present embodiment, in order to obtain similar metallographic structure over the whole steel pipe, the metallographic structure of the surface layer area in the welded heat-affected zone preferably contains one or both of bainite and acicular ferrite. In addition, the metallographic structure of the surface layer area in the welded heat-affected zone is preferably a uniform structure, that is, a structure consisting of bainite and/or acicular ferrite.
The weld metal portion is preferably a structure consisting of acicular ferrite.
The following conditions are desirable as the welding conditions in order to control the welded heat-affected zone to the above metallographic structure. For example, it is preferable to use Y-D, Y-DM, Y-DMH wire, and a flux of NF5000B or NF2000 as the welding material. In addition, it is preferable to carry out inner surface welding and outer surface welding, and it is preferable to carry out submerged arc welding on the inner surface 3 electrodes and the outer surface 4 electrodes. The heat input during welding is preferably in a range of 2.0 kJ/ mm to 10 kJ/mm depending on the plate thickness.
For the metallographic structure of the welded heat-affected zone, a test piece including the weld metal portion is cut out from the welded portion of the steel pipe to prepare a sample for microstructure observation. Then, observation is performed in the same method as the base material portion.

3. Mechanical properties

Next, the mechanical properties of the steel pipe will be described.

3-1. Mechanical properties of the base material portion

Maximum hardness of surface layer area: 250 HV or less

Since SSC is caused by micro defects or micro cracks on the surface of the steel plate, the metallographic structure and hardness of the surface layer area that is a source of the micro defects and micro cracks are important.
In the steel pipe according to the present embodiment, in order to ensure excellent SSC resistant properties, the metallographic structure of the surface layer area of the base material portion is controlled as described above, and the maximum hardness of the surface layer area of the base material portion is 250 HV or less. The maximum hardness of the surface layer area is preferably 245 HV or less, and more preferably 240 HV or less.
The maximum hardness of the surface layer area is measured by the following method. First, a test piece having an axial length of 20 mm and a circumferential length of 20 mm is collected by mechanical cutting from positions 90°, 180°, and 270° away from the welded portion in the circumferential direction of the steel pipe. In a case of the steel plate, a test piece having a length of 20 mm and a width of 20 mm is collected from positions of 1/4, 1/2, and 3/4 of the steel plate in a width direction from an end portion in the width direction.
Subsequently, the test piece is polished by mechanical polishing. For the test piece after polishing, using a Vickers hardness tester (test force: 100 gf), with a position of 0.1 mm from the surface as a starting point, 10 points at 0.1 mm intervals in the plate thickness direction, 10 points at 1 mm intervals in the width direction for the same depth, and a total of 100 points are measured.
Then, as a result of the measurement, in a case where two or more measurement points of more than 250 HV do not appear continuously in the plate thickness direction, it is determined that the maximum hardness of the surface layer area is 250 HV or less.
In the base material portion of the steel pipe, a high value (abnormal value) may appear locally due to inclusions or the like. However, since inclusions do not cause crack, SSC resistant properties can be ensured even if such an abnormal value appears. On the other hand, in a case where two or more measurement points of more than 250 HV are continuously present in the plate thickness direction, it is not acceptable since it is not caused by inclusions and the SSC resistant properties are lowered.
Therefore, in the present invention, even if there is one measurement point of more than 250 HV, in a case where two or more points do not appear continuously in the plate thickness direction, the point is not adopted as an abnormal point, and the next highest value is denoted as the maximum hardness. On the other hand, in a case where two or more measurement points of more than 250 HV are continuously present in the plate thickness direction, the hardness is denoted as the maximum hardness.

Proportional limit: 90% or more of yield stress

The present inventors performed examination on SSC resistant properties in a harsher environment. As a result, it was found that in a case where the proportional limit in the stress-strain curve is 90% or more of the yield stress, SSC does not occur even in a case where the load stress is more than 90% of the yield stress (for example, 95%).
In a case where the proportional limit is less than 90% of the yield stress, when the load stress in the sulfide stress corrosion crack test is 90% of actual yield stress, dislocations proliferate due to plastic deformation. As a result, the hydrogen that intruded during the sulfide stress corrosion test is trapped by the proliferated dislocations, and the amount of hydrogen increases, and crack occurs. In contrast, in a case where the proportional limit is 90% or more of the yield stress, plastic deformation does not occur even if the yield stress is more than 90%. Therefore, the proliferated dislocations do not increase, and hydrogen is not concentrated there. As a result, it becomes possible to prevent cracking.
As described above, when the proportional limit is 90% or more of the yield stress, in the base material portion of the steel pipe according to the present embodiment (steel plate according to the present embodiment), sulfide stress crack does not occur even if a stress of more than 90% of the yield stress is loaded in a solution environment of 30°C or lower containing 5% salt and acetic acid. The proportional limit is more preferably 95% or more of the yield stress.
In the present embodiment, the proportional limit is measured by the following procedure.
First, according to API 5L, a round bar tensile test piece is collected at a right angle (C direction) to the longitudinal direction of the steel pipe, and a tensile test is performed. The tensile test is performed under stroke control (tensile speed: 1 mm/min), the test force and displacement are measured at intervals of 0.05 s, and the stress and strain for each measurement time are obtained based thereon. Then, the yield stress (YS) is obtained from the obtained stress-strain curve. In a case where the yield point is not clearly recognized, 0.20% proof stress is adopted as YS.
After that, the stress and strain values are subjected to smoothing treatment in consideration of the measurement error. Specifically, an average value of the measurement time ± 2.50 s is calculated for each measurement time, and the value is used as the result at each measurement time. For example, as the stress and strain values at 2.50 s, the average value of 101 measurement values between 0 and 5.00 s is adopted.
Next, an inclination of the stress-strain curve after the smoothing treatment in a straight line portion is obtained. The inclination of the straight line portion is calculated by the least squares method using a value between 0.2 YS and 0.4 YS as a representative value.
Subsequently, the inclination of the stress-strain curve at each measurement time is calculated. Specifically, for each measurement time, the inclination is calculated by the least squares method from the value between the measurement time ± 0.50 s. For example, the inclination of the stress-strain curve at 60.00 s is calculated by the least squares method using 21 measurement values between 59.50 and 60.50 s.
Then, a value of one before the stress in which the inclination of the stress-strain curve continues to be less than 0.95 times the inclination of the straight line portion is set as the proportional limit. Even if the inclination of the stress-strain curve falls below 0.95 times the inclination of the straight line portion due to the influence of measurement error, in a case where the inclination of the stress-strain curve is more than 0.95 times the inclination of the straight line portion again, the value will not be adopted.

Yield stress: 415 MPa or more
Tensile strength: 530 MPa or more

The yield stress of the base material portion of the steel pipe according to the present embodiment is 415 MPa or more in order to ensure the required strength in the steel pipe according to the present embodiment. It is preferably 430 MPa or more. As for an upper limit of the yield stress, about 630 MPa defined in X70 of API 5L is a substantial upper limit, from a viewpoint of workability. From the viewpoint of workability, the yield stress is preferably 600 MPa or less.
In addition, the tensile strength of the base material portion of the steel pipe according to the present embodiment is preferably 530 MPa or more in order to ensure the required strength in the steel pipe according to the present embodiment. More preferably, it is 550 MPa or more. An upper limit of the tensile stress is not particularly limited, but from the viewpoint of workability, 690 MPa defined in X70 of API 5L is a substantial upper limit. From the viewpoint of workability, 650 MPa or less is preferable.

3-2. Mechanical properties of welded portion

Maximum hardness of surface layer area in welded heat-affected zone: 250Hv or less

In the steel pipe according to the present embodiment, in order to ensure good SSC resistant properties, it is preferable that the maximum hardness of the surface layer area in the welded heat-affected zone is 250 HV or less. The maximum hardness of the surface layer area is more preferably 245 HV or less, and further more preferably 240 HV or less.
On the other hand, in order to obtain the strength of X60 or more of the API standard, it is preferable that the maximum hardness of the surface layer area in the welded heat-affected zone is 150 HV or more. The maximum hardness of the surface layer area is more preferably 160 HV or more, and further preferably 170 HV or more.
The maximum hardness of the surface layer area in the welded heat-affected zone is a maximum hardness measured in a region from the surface to a depth position of 0.9 mm in the wall thickness direction. As for the maximum hardness of the surface layer area in the welded heat-affected zone, 40 points at 0.5 mm pitch at positions of 0.3 mm, 0.6 mm, and 0.9 mm from the surface, a total of 120 points, on a base material portion side from the welded toe (the boundary between the weld metal portion and the base material portion) by cutting out the sample as shown in Fig. 2, are measured to measure the maximum hardness.
As a result of the measurement, in a case where two or more measurement points of less than 150 HV or more than 250 HV do not appear continuously in the wall thickness direction, it is determined that the maximum hardness of the surface layer area in the welded heat-affected zone is 150 to 250 HV. The reason for measuring the hardness in this way is the same as the reason for measuring the maximum hardness of the surface layer area in the above-mentioned base material portion.

4. Dimension

Plate thickness: 10 to 40 mm
Pipe diameter: 508 mm (20 inches) or more

In a case where a steel pipe for excavation of petroleum and natural gas or a steel pipe for a line pipe is used, it is preferable that the plate thickness is 10 to 40 mm, and the pipe diameter (outer diameter) is 508 mm or more. An upper limit of the pipe diameter is not particularly limited, but 1,422.4 mm (56 inches) or less is a substantial upper limit.

5. Angle of weld toe portion

In the steel pipe according to the present embodiment, it is preferable to control the angle of the weld toe portion of the seam welded portion in order to improve the SSC resistant properties 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 weld toe portion is an angle of an excess weld tip end portion of the weld metal portion, that is, an angle formed by a tangential direction of the weld metal and the surface of the base material portion. It can also be referred to as a so-called flank angle.
In order to suppress SSC, the angle of the weld toe portion on an inner side of the steel pipe is preferably in a range of 130° to 180°. In a case where the angle of the weld toe portion is less than 130° and the angle is sharper, strain is piled up in the welded heat-affected zone, hydrogen intrusion is promoted, and crack easily occurs. In Fig. 1, it is described that only a lower left angle is measured, but in the present embodiment, the left and right angles are measured, and the smaller angle is the angle of the weld toe portion (toe angle).

5. Manufacturing method

A preferable manufacturing method for manufacturing a steel pipe according to the present embodiment and a steel plate as a material thereof will be described.
Regardless of the manufacturing method, the steel pipe according to the present embodiment can obtain the effect as long as the steel pipe has the above-mentioned configuration, but the steel pipe is preferable since it can be stably obtained by the following manufacturing method, for example.
The steel plate according to the present embodiment is obtained by a manufacturing method including:

(A) a hot rolling step in which a steel piece having the above-mentioned predetermined chemical composition is heated to 1,000°C to 1,250°C and subjected to hot rolling, and hot rolling is completed at a temperature of Ar₃ points or higher,
(B) a first cooling step in which a steel plate after the hot rolling step is subjected to multi-stage accelerated cooling from a temperature of Ar₃ points or higher, in which water cooling is performed three times of more such that a water-cooling stop temperature is 500°C or lower, and a maximum attainment temperature by recure-heating after stopping the water cooling is higher than 500°C.
(C) then, a second cooling step of cooling to a temperature of 500°C or lower at an average cooling rate of 0.2°C/s or higher.

The steel pipe according to the present embodiment is obtained by further performing, in addition to steps of (A) to (C):

(D) a forming step for forming the steel plate into a tubular shape,
(E) a welding step of butting and welding both end portions of the tubular steel plate, and
(F) a heat treatment step of heat-treating the steel pipe obtained by welding under conditions in which a temperature range is 100°C to 300°C and a retention time is 1 minute or more.

Preferable conditions will be described for each step.

(Hot rolling step)

Steel piece manufactured by casting molten steel having the same chemical composition as that of the base material portion of the steel pipe according to the present embodiment is heated to 1,000°C to 1,250°C for hot rolling. Casting of molten steel and manufacturing of a steel piece prior to hot rolling may be performed according to a general method.
In a case of rolling a steel piece, in a case where the heating temperature is lower than 1,000°C, the deformation resistance does not decrease, the load on a rolling mill increases, and the heating temperature is set to 1,000°C or higher. It is preferably 1,100°C or higher. On the other hand, in a case where the heating temperature is higher than 1,250°C, the crystal grains of the steel piece become coarse and the strength and toughness are lowered, and thus the heating temperature is set to 1,250°C or lower. It is preferably 1,210°C or lower.
The heated steel piece is hot-rolled in a temperature range of Ar₃ points or higher to form a steel plate, and hot rolling is completed at Ar₃ points or higher. In a case where the hot rolling finish temperature is less than Ar3 points, worked ferrite is generated in the steel plate structure and the strength is lowered. Therefore, the hot rolling finish temperature is set to Ar₃ points or higher.

(First cooling step)

Accelerated cooling is started from the temperature of Ar₃ points or higher on the steel plate for which hot rolling has been completed. At this time, multi-stage accelerated cooling is performed, in which water cooling is performed twice or more such that the water-cooling stop temperature is 500°C or lower and the maximum attainment temperature due to recure-heating after stopping water cooling is higher than 500°C, at the surface temperature. It is preferably performed 3 times or more.
In order for the maximum attainment temperature by recure-heating to be higher than 500°C, it is important to increase the temperature difference between the surface and the inner side. The temperature difference between the surface and the inner side can be adjusted by changing the sprayed water density, collision pressure, and the like in water cooling.
In a case where the maximum attainment temperature by recure-heating is 500°C or lower, the hardness of the steel plate, particularly the maximum hardness of the surface layer area from the surface to a depth of 1 mm cannot be reduced to 250 HV or less. In addition, even if the number of times of recure-heating of higher than 500°C is less than 2, the maximum hardness of the surface layer area cannot be reduced to 250 HV or less. Therefore, accelerated cooling is performed such that the recure-heating by which the maximum attainment temperature becomes higher than 500°C is performed three times or more.
Each water-cooling stop temperature in the multi-stage cooling is preferably a temperature higher than the Ms point since the temperature does not generate a hard phase.
In addition, in a case where the water-cooling stop temperature before recure-heating is higher than 500°C, a predetermined structure cannot be obtained, and the water-cooling stop temperature is set to 500°C or lower. The water-cooling stop temperature is preferably 500°C or lower.
By performing recure-heating three times or more, the maximum hardness HVmax of the surface layer area from the surface of the steel plate to the depth of 1 mm is lowered to 250 HV or less. Since the number of recure-heating is the number of times until the maximum hardness HVmax of the surface layer area reaches 250 HV or less, it is not required to define the upper limit of the number of recure-heating.

(Second cooling step)

In the first cooling step, after the completion of water cooling and recure-heating three times or more, cooling is performed at an average cooling rate of 0.2°C/s or more to a temperature of 500°C or lower. In a case where cooling is completed at a temperature of higher than 500°C, the cooling rate becomes slow by performing coiling or the like, and thereby the average cooling rate up to 500°C is less than 0.2°C/s, the hardness unevenness becomes small but the structure and/or hardness of above-mentioned surface layer area cannot be obtained.

(Forming step and welding step)

The forming of the steel plate according to the present embodiment into a steel pipe is not limited to a specific forming method. For example, warm working also can be used, but cold working is preferable from a viewpoint of dimensional accuracy.
After forming the steel plate into a tubular shape, both end portions of the steel plate are butted and arc welded (seam welding). Arc welding is not limited to specific welding, but submerged arc welding is preferable. In addition, the welding conditions may be known conditions. For example, it is preferable to perform welding with 3 electrodes or 4 electrodes in a heat input range of 2.0 to 10 kJ/mm depending on the plate thickness. In order to obtain the welded heat-affected zone by using the above-mentioned metallographic structure, for example, it is preferable to use Y-D, Y-DM, Y-DMH wire, and a flux of NF5000B or NF2000 as the welding material. In addition, it is preferable to carry out inner surface welding and outer surface welding, and it is preferable to carry out submerged arc welding on the inner surface 3 electrodes and the outer surface 4 electrodes.

(Heat treatment step)

After that (after pipe formation), the steel pipe is heat-treated under the conditions that the temperature range is 100°C to 300°C and the retention time is 1 minute or more. The upper limit is not particularly limited, but is, for example, 60 minutes or less.

(Other steps)

In addition, seam heat treatment may be performed by heating the welded portion to Ac₁ point or lower and tempering thereof such that a structure harmful to sour resistant properties (ferrite-pearlite of more than 20% in area ratio) is not generated on the welded portion. This heat treatment may be performed immediately after seam welding.
Since the base material portion of the steel pipe according to the present embodiment is not heat-treated at a temperature of more than the Ac₁ point, the metallographic structure of the base material portion is the same as the metallographic structure of the steel plate according to the present embodiment. Therefore, the steel pipe according to the present embodiment has excellent SSC resistant properties in addition to HIC resistant properties equal to or higher than that of the steel in the related art in both the base material portion and the welded portion.
Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited thereto.

[Examples]

The molten steel having the chemical composition shown in Tables 1-1 and 1-2 was continuously cast to manufacture a steel slab having a thickness of 240 mm, and the steel plate was manufactured under the manufacturing conditions (heating temperature, finish rolling temperature, maximum attainment temperature by recure-heating after the first water-cooling stop in the multi-stage cooling, and number of times of recure-heating of higher than 500°C) shown in Tables 2-1 to 2-3. In Tables 2-1 to 2-3, in the column of water-cooling stop temperature, OK means an example in which the water-cooling stop temperature was 500°C or lower after each water cooling of multi-stage accelerated cooling, and NG means an example in which there is a case where the cooling stop temperature is higher than 500°C.

[Table 2-1]

Test No.	Steel No.	Heating temperature (°C)	Finish rolling temperature (°C)	Ar₃ point (°C)	Cooling start temperature (°C)	Water-cooling stop temperature (°C)	Maximum attainment temperature after first water-cooling stop (°C)	Number of recure-heating of higher than 500°C (times)	Second coolingcooling rate (°C/s)	Cooling stop temperature (°C)	Steel pipe heat treatment temperature (°C)	Steel pipe heat treatment time (min)
1	1	1230	822	772	782	OK	570	3	100	420	250	5
2	2	1200	824	774	784	OK	600	3	150	450	220	5
3	3	1210	826	776	786	OK	590	3	100	440	180	10
4	4	1200	809	759	769	OK	600	3	70	450	200	5
5	5	1200	817	767	777	OK	560	3	50	410	260	3
6	6	1180	826	776	786	OK	680	4	100	490	280	1
7	7	1250	827	777	787	OK	550	3	150	400	230	5
8	8	1200	821	771	781	OK	570	3	100	420	240	5
9	9	1240	820	770	780	OK	590	3	50	440	150	60
10	10	1210	816	766	776	OK	580	3	150	430	230	5
11	11	1230	819	769	779	OK	565	3	100	415	190	5
12	12	1200	826	776	786	OK	560	3	200	410	200	5
13	13	1230	818	768	778	OK	570	3	150	420	260	3
14	14	1210	813	763	773	OK	560	3	50	410	280	1
15	15	1200	821	771	781	OK	550	3	100	400	230	5
16	16	1200	821	771	781	OK	550	3	50	400	240	5
17	17	1210	844	794	804	OK	560	3	200	410	200	5
18	18	1180	827	777	787	OK	570	3	80	420	260	3
19	19	1250	821	771	781	OK	550	3	100	400	280	1
20	20	1200	816	766	776	OK	550	3	150	400	230	5
21	21	1210	816	766	776	OK	570	3	200	420	240	5
22	22	1200	827	777	787	OK	570	3	160	420	180	10

[Table 2-2]

Test No.	Steel No.	Heating temperature (°C)	Finish rolling temperature (°C)	Ar₃ point (°C)	Cooling start temperature (°C)	Water-cooling stop temperature (°C)	Maximum attainment temperature after first water-cooling stop (°C)	Number of recure-heating of higher than 500°C (times)	Second coolingcooling rate (°C/s)	Cooling stop temperature (°C)	Steel pipe heat treatment temperature (°C)	Steel pipe heat treatment time (min)
23	23	1200	900	698	708	OK	550	3	200	400	250	5
24	24	1000	800	798	800	OK	580	3	70	500	250	5
25	25	1170	750	697	707	OK	590	3	50	440	250	5
26	26	1200	818	768	778	OK	570	3	70	420
26'	26	1200	818	768	778	OK	570	3	70	420	280	1
27	27	1240	823	773	783	OK	575	3	50	425	250	5
28	28	1200	823	773	783	OK	600	3	130	450	250	5
29	29	1240	816	766	776	OK	550	3	200	400	250	5
30	30	1200	815	765	775	OK	570	3	230	420	250	5
31	31	1180	814	752	762	OK	580	3	50	430	250	5
32	32	1240	818	768	778	OK	550	3	140	400	250	5
33	33	1200	826	776	786	OK	550	3	60	400	250	5
34	34	1230	825	775	785	OK	560	3	120	410	250	5
35	35	1200	780	778	780	OK	570	3	300	500	250	5
36	36	1210	800	767	768	OK	550	3	100	400	250	5
37	37	1200	809	759	769	OK	550	3	300	25	250	5
38	38	1215	815	765	775	OK	700	3	50	600	250	5
39	39	1200	817	767	777	OK	500	3	150	400	250	5
40	40	1200	810	745	755	OK	570	3	110	420	250	5
41	41	1180	880	854	864	OK	580	3	140	430	250	5
42	42	1190	750	699	709	OK	590	3	200	440	250	5
43	43	1210	780	752	762	OK	550	3	100	400	250	5
44	44	1230	830	804	814	OK	570	3	300	420	250	5

[Table 2-3]

Test No.	Steel No.	Heating temperature (°C)	Finish rolling temperature (°C)	Ar₃ point (°C)	Cooling start temperature (°C)	Water-cooling stop temperature (°C)	Maximum attainment temperature after first water-cooling stop (°C)	Number of recure-heating of higher than 500°C (times)	Second cooling cooling rate (°C/s)	Cooling stop temperature (°C)	Steel pipe heat treatment temperature (°C)	Steel pipe heat treatment time (min)
45	45	1240	770	754	764	OK	550	3	50	400	250	5
46	46	1150	780	766	776	OK	560	3	140	400	250	5
47	2	1300	790	774	784	OK	600	3	180	450	250	5
48	2	900	780	774	784	OK	590	3	30	440	250	5
49	2	1200	710	774	784	OK	570	3	30	420	250	5
50	2	1190	800	774	784	NG	560	3	50	410	250	5
51	2	1180	810	774	784	OK	500	1	100	400	250	5
52	2	1150	820	774	784	OK	540	2	50	400	250	5
53	2	1160	800	774	784	OK	570	3	0.1	420	250	5
54	2	1180	820	774	794	OK	600	3	10	430
55	2	1190	830	774	800	OK	650	4	500	460
56	2	1200	840	774	810	OK	620	3	200	490
57	2	1230	822	774	808	OK	570	3	100	420	80	5
58	2	1200	824	774	808	OK	600	3	150	450	500	5
59	2	1210	826	774	808	OK	590	3	100	440
60	2	1200	820	774	800	OK	590	3	100	450	240	5
61	2	1120	800	774	780	OK	700	3	150	490	230	10
62	2	1150	830	774	785	OK	650	3	100	480	220	15
63	2	1200	816	774	790	OK	580	3	150	440	250	1
64	2	1200	835	774	795	OK	640	3	100	450	250	1
65	2	1200	838	774	800	OK	630	3	150	420	240	5

A round bar tensile test piece was collected from the obtained steel plate according to API 5L, and the tensile strength was measured. In addition, the maximum hardness of the surface layer area from the surface to a depth of 1 mm was measured, and the metallographic structure was observed by SEM. In addition, as a reference, the structure at a position 5 mm away from the surface and the structure at a position of 1/2 (1/2 portion) of the plate thickness from the surface were also observed.
For the maximum hardness of the surface layer area, first, a 300 mm square steel plate was cut out by gas cutting from positions of 1/4, 1/2, and 3/4 of the steel plate in a width direction from an end portion of the steel plate in the width direction, a block test piece having a length of 20 mm and a width of 20 mm was collected from a center of the cut-out steel plate by mechanical cutting, and polished by mechanical polishing. For the block test piece, 10 points at 0.1 mm intervals in a plate thickness direction, and 20 points at 1 mm intervals in a width direction for the same depth, a total of 200 points were measured, with a position of 0.1 mm depth from a steel plate surface as a starting point, using a Vickers hardness tester (load 100 g) to obtain the maximum hardness. At this time, even if there was one measurement point of more than 250 HV, in a case where two or more points did not appear continuously in the plate thickness direction, the point was regarded as an abnormal point and was not adopted, and the next highest value was set to the maximum hardness. On the other hand, in a case where there were two or more measurement points of more than 250 HV continuously in the plate thickness direction, the highest value was set to the maximum hardness.

Regarding the metallographic structure, a test piece obtained by polishing a sample collected such that positions of 0.5 mm from the surface (surface layer area), 5 mm from the surface, and 1/2 of the plate thickness from the surface can be observed was immersed in a mixed solution of 3% nitric acid and 97% ethanol for several seconds to several tens of seconds and etched, the metallographic structure was exposed and observed by SEM, and bainite and martensite were classified by micro Vickers hardness. The results are shown in Tables 3-1 to 3-3. A modified LePera solution was also used depending on the necessity to observe the metallographic structure.

[Table 3-1]

Test No.	Metallographic structure^# of surface layer area of steel plate	Total area ratio^## of GB + AF + B (%)	Area ratio other than GB + AF + B (%)	Metallographic structure at position of 5 mm from surface	Metallographic structure of 1/2 portion	Maximum hardness of surface layer area (HV)	Tensile strength (MPa)	Plate thickness (mm)
1	AF + B	100	0	AF + B	GB + B	207	570	14
2	AF + B + PF	95	5	AF + B + PF	GB + B	221	612	19
3	AF + B + PF	98	2	AF + B + PF	GB + B	229	643	15
4	AF + B + PF	97	3	AF + B + PF	GB + B	223	623	12
5	AF + B + PF	96	4	AF + B + PF	GB + B	223	622	16
6	GB + AF + B + PF	81	19	GB + AF + B + PF	GB + B	215	580	13
7	AF + B + PF	95	5	AF + B + PF	GB + B	222	615	17
8	AF + B + PF	97	3	AF + B + PF	GB + B	222	615	18
9	AF + B + PF	97	3	AF + B + PF	GB + B	221	613	14
10	AF + B + PF	97	3	AF + B + PF	GB + B	209	575	16
11	AF + B + PF	94	6	AF + B + PF	GB + B	225	625	15
12	AF + B + PF	92	8	AF + B + PF	GB + B	213	587	17
13	GB + AF + B + PF	85	15	GB + AF + B + PF	GB + B	218	605	13
14	AF + B + PF	96	4	AF + B + PF	GB + B	210	576	16
15	AF + B	100	0	AF+B	GB + B	223	620	14
16	AF + B + PF	94	6	AF + B + PF	GB + B	225	625	13
17	AF + B	100	0	AF+B	GB + B	217	600	19
18	AF + B + PF	94	6	AF + B + PF	GB + B	223	619	16
19	AF + B + PF	96	4	AF+B+PF	GB + B	223	620	13
20	AF + B + PF	97	3	AF + B + PF	GB + B	227	631	11
21	AF + B + PF	95	5	AF + B + PF	GB + B	228	621	15
22	AF + B + PF	97	3	AF + B + PF	GB + B	213	589	14

# PF: polygonal ferrite, AF: aciclular ferrite, GB: granular bainite, B: bainite, M: martensite, DF: worked ferrite, P: pearlite, γ: residual austenite ## means total area ratio of granular bainite, acicular ferrite, and bainite.

[Table 3-2]

Test No.	Metallographic structure^# of surface layer area of steel plate	Total area ratio^## of GB + AF + B (%)	Area ratio other than GB + AF + B (%)	Metallographic structure at position of 5 mm from surface	Metallographic structure of 1/2 portion	Maximum hardness of surface layer area (HV)	Tensile strength (MPa)	Plate thickness (mm)
23	AF + B + M	79	21	AF + B + M	GB + B	262	744	16
24	PF	5	95	PF	GB + B	177	472	30
25	B+M	70	30	B+M	B+M	402	1377	10
26	AF + B + PF	93	7	AF + B + PF	GB + B	270	613	14
26'	AF + B + PF	93	7	AF + B + PF	GB + B	270	613	14
27	AF + B + PF	93	7	AF + B + PF	GB + B	218	603	19
28	AF + B + PF	92	8	AF + B + PF	GB + B	214	591	13
29	AF + B + PF	92	8	AF + B + PF	GB + B	260	614	14
30	AF + B + PF	93	7	AF + B + PF	GB + B	255	631	13
31	AF + B + PF	94	6	AF + B + PF	GB + B	271	642	16
32	AF + B + PF	92	8	AF+B+PF	GB + B	218	603	19
33	AF + B + PF	90	10	AF + B + PF	GB + B	202	552	17
34	AF + B + PF	93	7	AF+B+PF	GB + B	213	587	18
35	AF + B + PF	95	5	AF + B + PF	GB + B	216	620	30
36	DF + AF + B	77	23	DF+AF+B	GB + B	223	612	32
37	AF+B+M	78	22	AF+B+M	B + M	255	674	16
38	PF + AF + B	79	21	PF + AF + B	GB + B	206	510	13
39	AF+B+M	79	21	AF + B + M	GB + B + M	245	652	10
40	AF+B+M	75	25	AF+B+M	GB + B + M	267	759	16
41	AF + B + M + γ	79	21	AF + B + M + γ	B + M	251	697	20
42	B+M	78	22	B+M	B + M	366	1074	12
43	B+M	78	22	B+M	B+M	357	1045	14
44	PF+AF+P	15	85	PF+AF+P	GB + B	154	398	16

# PF: polygonal ferrite, AF: acicular ferrite, GB: granular bainite, B: bainite, M: martensite, DF: worked ferrite, P: peartile, γ: residual austenite
## means total area ratio of granular bainite, acicular ferrite, and bainite.

[Table 3-3]

Test No.	Metallographic structure^# of surface layer area of steel plate	Total area ratio^## of GB + AF + B (%)	Area ratio other than GB+AF+B (%)	Metallographic structure at position of 5 mm from surface	Metallographic structure of 1/2 portion	Maximum hardness of surface layer area (HV)	Tensile strength (MPa)	Plate thickness (mm)
45	B + M	78	22	B + M	B + M	378	1115	20
46	AF + B + PF	95	5	AF + B + PF	GB + B	226	626	15
47	AF + B + M	79	21	AF + B + M	B + M	255	670	13
48	PF + P + B	20	80	PF + P + B	GB + B	176	500	20
49	DF + AF + B	77	23	DF + AF + B	GB + B	225	624	15
50	B + M	60	40	B + M	B + M	252	710	18
51	B + M	70	30	B + M	B + M	260	740	15
52	B + M	76	24	B + M	B + M	255	730	17
53	PF + AF + B	77	23	PF + AF + B	GB + B	186	500	30
54	GB + B + AF + PF	95	5	GB + B + AF + PF	GB + B	218	600	15
55	M + B	96	4	M + B	M + B	225	625	20
56	AF + B + PF	94	6	AF + B + PF	GB + B	208	570	35
57	AF + B	100	0	AF + B	GB + B	213	587	14
58	AF + B + PF	95	5	AF + B + PF	GB + B	221	612	19
59	AF + B + PF	98	2	AF + B + PF	GB + B	238	631	15
60	B + PF	85	15	B + PF	B	230	633	25
61	PF + B	85	15	PF + B	GB + B	200	550	38
62	AF + PF	90	10	AF + PF	GB + B	205	564	35
63	GB + PF	84	16	GB + PF	GB + B	225	619	25
64	AF + B + PF	80	20	AF + B + PF	GB + B	220	605	29
65	GB + AF + B + PF	80	20	GB + AF + B + PF	GB + B	223	613	30

# PF: polygonal ferrite, AF: acicular ferrite, GB: granular bainite, B: bainite, M: martensite, DF: worked ferrite, P: pearlite, γ: residual austenite
## means total area ratio of granular bainite, acicular ferrite, and bainite.

After that, each steel plate was cold-worked into a tubular shape, both end portions of the tubular steel plate were butted against each other, and a steel pipe was manufactured by submerged arc welding (SAW) in which heat input was under a condition in a range of 2.0 kJ/mm to 10 kJ/mm depending on the plate thickness with 3 electrodes or 4 electrodes.
As the welding material, Y-D, Y-DM, Y-D wire and a flux of NF-5000B were used on the inner surface side, and Y-DM, Y-DMH, Y-DM, Y-DM and a flux of NF-5000 were used on the outer surface side. As for welding conditions, 3 electrodes were used on the inner surface and 4 electrodes were used on the outer surface, and the heat input during welding was adjusted in a range of 2.0 kJ/mm to 10 kJ/mm depending on the plate thickness.
Heat treatment was performed on the obtained steel pipe, and on the base material portion for some of the steel plates, under the conditions as shown Tables 2-1 to 2-3. In addition, heat treatment of heating to 400°C to Ac₁ point was performed on the welded portion regarding some of the steel pipes (Test No. 58).
Regarding each of the obtained steel pipes, test pieces having an axial length of 20 mm and a circumferential length of 20 mm were collected by mechanical cutting from positions 90°, 180°, and 270° away from the welded portion in the circumferential direction of the steel pipe. Then, using the test pieces, the maximum hardness of the surface layer area of the steel pipe was obtained by the same method as described above. Since it is considered that the metallographic structure after the pipe was made into a steel pipe is the same as the metallographic structure of the steel plate, the measurement results were used as they were.
In addition, as an evaluation of SSC resistant properties, a round bar test piece was collected from the obtained steel pipe according to API 5L, and the yield stress and tensile strength were measured.
In addition, a 4-point bending test piece having a width of 15 mm, a length of 115 mm, and a thickness of 5 mm was collected from an inner surface of the base material portion of the steel pipe so as to remain the inner surface, and the presence or absence of crack in a solution environment of pH 3.5 with various hydrogen sulfide partial pressures was examined in accordance with NACE TM 0316-2016. The load stress during the 4-point bending test was 90% and 95% of the actual yield stress.
Then, as an evaluation of HIC resistant properties, a hydrogen-induced crack test (hereinafter, referred to as "HIC test") was carried out. The HIC test was carried out in accordance with NACE TM0284 2016. Specifically, a test piece having a length of 100 mm and a width of 20 mm with a curvature along an inner surface, collected from the base material portion, was immersed in a test solution obtained by saturating 100% H₂S gas in Solution A (5 mass% NaCl + 0.5 mass% glacial acetic acid aqueous solution) for 96 hours. After that, the area ratio (CAR) at which crack occurred was measured for the surface layer area and the center portion. In a case where the CAR is 5% or less, it was determined that the HIC resistant properties were excellent.

In addition, a proportional limit of each steel plate was calculated by the above-mentioned method based on the result of the round bar tensile test. The results are summarized in Tables 4-1 to 4-3.

[Table 4-1]

Test No.	Metallographic structure^# of surface layer area of steel pipe	Total area ratio^## (%)	Maximum hardness of surface layer area (HV)	Round bar tensile test					SSC resistant properties		HIC resistant properties		Pipe diameter (mm)	Plate thickness (mm)
Test No.		Total area ratio^## (%)	Maximum hardness of surface layer area (HV)	Yield stress (MPa)	Tensile strength (MPa)	Proportional limit/yield stress (%)	Yield ratio (%)	Hydrogen sulfide partial pressure (MPa)	A result of 4-point bending test in 90% of actual vield stress	A result of 4-point bending test in 95% of actual yield stress	Immersion test center portion CAR (%)	Immersion test 1 mm from surface layer CAR (%)	Pipe diameter (mm)	Plate thickness (mm)
1	AF + B	100	217	580	598	96	97	0.2	No Crack	No Crack	2	0	560	14
2	AF + B + PF	95	225	611	624	96	98	0.3	No Crack	No Crack	1	0	760	19
3	AF + B + PF	98	243	621	643	96	97	0.4	No Crack	No Crack	0	0	600	15
4	AF + B + PF	97	234	627	640	97	98	0.5	No Crack	No Crack	1	0	343	12
5	AF + B + PF	96	234	623	633	97	98	1.2	No Crack	No Crack	1	0	640	16
6	GB + AF + B + PF	81	220	588	600	97	98	0.5	No Crack	No Crack	2	0	520	13
7	AF + B + PF	95	226	617	628	97	98	1.7	No Crack	No Crack	1 '	0	680	17
8	AF + B + PF	97	226	608	627	96	97	2.0	No Crack	No Crack	3	0	720	18
9	AF + B + PF	97	225	612	625	97	98	1.0	No Crack	No Crack	2	0	560	14
10	AF + B + PF	97	213	569	587	96	97	0.5	No Crack	No Crack	1	0	640	16
11	AF + B + PF	94	229	618	637	96	97	0.6	No Crack	No Crack	2	0	600	15
12	AF + B + PF	92	217	586	598	97	98	0.2	No Crack	No Crack	3	0	680	17
13	GB + AF + B + PF	85	222	598	610	97	98	0.3	No Crack	No Crack	1	0	520	13
14	AF + B + PF	96	214	579	588	96	98	0.4	No Crack	No Crack	0	0	640	16
15	AF + B	100	228	620	633	97	98	0.5	No Crack	No Crack	0	0	560	14
16	AF + B + PF	94	229	624	637	97	98	0.7	No Crack	No Crack	1	0	520	13
17	AF + B	100	221	600	612	97	98	0.9	No Crack	No Crack	2	0	760	19
18	AF + B + PF	94	227	621	631	97	98	1.2	No Crack	No Crack	1	0	640	16
19	AF + B + PF	96	228	623	633	96	98	1.0	No Crack	No Crack	1	0	520	13
20	AF + B + PF	97	231	624	644	96	97	0.5	No Crack	No Crack	1	0	440	11
21	AF + B + PF	95	239	610	620	97	98	0.2	No Crack	No Crack	1	0	600	15
22	AF + B + PF	97	218	588	601	96	98	0.3	No Crack	No Crack	2	0	560	14

[Table 4-2]

Test No.	Metallographic structure^# of surface layer area of steel pipe	Total area ratio" (%)	Maximum hardness of surface layer area (HV)	Round bar tensile test				Hydrogen sulfide partial pressure (MPa)	SSC properties		HIC properties		Pipe diameter (mm)	Plate thickness (mm)
Test No.		Total area ratio" (%)	Maximum hardness of surface layer area (HV)	Yield stress (MPa)	Tensile strength (MPa)	Proportional limit/yield stress (%)	Yield ratio (%)	Hydrogen sulfide partial pressure (MPa)	resistant A result of 4-point bending test in 90% of actual yield stress	A result of 4-point bending test in 95% of actual yield stress	resistant Immersion test center portion CAR (%)	Immersion test 1 mm from surface layer CAR (%)	Pipe diameter (mm)	Plate thickness (mm)
23	AF + B + M	79	267	652	759	94	95	0.4	Crack	Crack	10	10	640	16
24	PF	5	182	430	481	92	93	0.5	No Crack	Crack	2	0	1200	30
25	B + M	70	407	1234	1404	95	97	1.4	Crack	Crack	12	10	400	10
26	AF + B + PF	93	275	551	625	84	88	1.2	Crack	Crack	15	10	560	14
26'	AF + B + PF	93	272	612	625	97	98	1.2	Crack	Crack	15	5	560	14
27	AF + B + PF	93	223	561	615	96	97	1.3	No Crack	No Crack	30	0	760	19
28	AF + B + PF	92	219	551	603	97	98	1.2	No Crack	No Crack	10	0	520	13
29	AF + B + PF	92	265	561	626	96	97	0.8	Crack	Crack	14	10	560	14
30	AF + B + PF	93	260	561	643	96	97	0.2	Crack	Crack	20	10	520	13
31	AF + B + PF	94	276	575	655	96	97	0.4	Crack	Crack	25	15	640	16
32	AF + B + PF	92	223	541	615	97	98	0.5	No Crack	No Crack	10	0	760	19
33	AF + B + PF	90	207	505	563	96	97	0.2	No Crack	No Crack	13	0	680	17
34	AF + B + PF	93	218	526	598	96	97	0.3	No Crack	No Crack	15	0	720	18
35	AF + B + PF	95	221	613	625	97	98	0.4	No Crack	No Crack	10	5	1200	30
36	DF + AF + B	77	255	505	624	96	97	0.5	No Crack	No Crack	30	10	1280	32
37	AF + B + M	78	260	604	688	94	95	0.2	Crack	Crack	9	10	640	16
38	PF + AF + B	79	211	414	520	91	92	0.4	No Crack	No Crack	1	0	520	13
39	AF + B + M	79	260	584	665	96	98	0.2	No Crack	No Crack	2	10	250	10
40	AF + B + M	75	272	667	774	93	95	1.6	Crack	Crack	2	10	640	16
41	AF + B + M + γ	79	256	657	711	92	94	1.6	Crack	Crack	15	10	800	20
42	B + M	78	371	1000	1095	96	98	1.6	Crack	Crack	1	15	480	12
43	B + M	78	362	889	1066	96	97	1.6	Crack	Crack	2	19	560	14
44	PF + AF + P	15	159	344	406	90	91	1.6	No Crack	No Crack	17	0	640	16

# PF: polygonal ferrite, AF: acicular ferrite, GB: granular bainite, B: bainite, M: martensite, DF: worked ferrite, P: pearlite, y: residual austenite ## means total area ratio of granular bainite, acicular ferrite, and bainite.

[Table 4-3]

Test No.	Metallographic structure^# of surface layer area of steel pipe	Total area ratio^## (%)	Maximum hardness of surface layer area (HV)	Round bar tensile test				Hydrogen sulfide partial pressure (MPa)	SSC resistant properties		HIC resistant properties		Pipe diameter (mm)	Plate thickness (mm)
Test No.		Total area ratio^## (%)	Maximum hardness of surface layer area (HV)	Yield stress (MPa)	Tensile strength (MPa)	Proportional limit/yield stress (%)	Yield ratio (%)	Hydrogen sulfide partial pressure (MPa)	A result of 4-point bending test in 90% of actual yield stress	A result of 4-point bending test in 95% of actual yield stress	Immersion test center portion CAR (%)	Immersion test 1 mm from surface layer CAR (%)	Pipe diameter (mm)	Plate thickness (mm)
45	B + M	78	383	1010	1137	94	95	1.6	Crack	Crack	1	15	800	20
46	AF + B + PF	95	231	562	639	96	97	1.6	No Crack	Crack	17	0	600	15
47	AF + B + M	79	260	606	683	94	95	1.6	Crack	Crack	1	10	520	13
48	PF + P + B	20	181	414	480	91	93	1.6	No Crack	Crack	0	0	800	20
49	DF+AF+B	77	230	556	636	93	95	1.6	No Crack	Crack	18	0	600	15
50	B + M	60	257	667	724	94	95	1.6	Crack	Crack	8	10	720	18
51	B + M	70	265	657	755	93	95	1.6	Crack	Crack	1	10	600	15
52	B + M	76	260	697	745	93	95	1.6	Crack	Crack	1	10	680	17
53	PF + AF + B	77	191	465	510	93	95	1.6	No Crack	Crack	1	0	1200	30
54	GB + B + AF + PF	95	223	545	612	85	89	1.6	No Crack	Crack	0	0	600	15
55	M + B	96	230	562	638	84	88	1.6	No Crack	Crack	0	0	800	20
56	AF + B + PF	94	213	525	581	87	90	1.6	No Crack	Crack	0	0	1400	35
57	AF + B	100	217	518	598	83	87	0.2	No Crack	Crack	2	0	560	14
58	AF + B + PF	95	225	546	624	84	88	0.3	No Crack	Crack	1	0	760	19
59	AF + B + PF	98	243	595	681	84	87	0.4	No Crack	Crack	0	0	600	15
60	B + PF	85	239	623	640	96	97	0.4	No Crack	No Crack	0	0	660	25
61	PF + B	85	208	539	561	94	96	0.2	No Crack	No Crack	0	0	559	38
62	AF + PF	90	213	564	575	96	98	0.3	No Crack	No Crack	0	0	610	35
63	GB + PF	84	234	618	631	96	98	0.4	No Crack	No Crack	0	0	660	25
64	AF + B + PF	80	229	605	617	96	98	0.5	No Crack	No Crack	0	0	559	29
65	GB + AF + B + PF	80	232	613	626	96	98	0.3	No Crack	No Crack	0	0	610	30

Test Nos. 1 to 22 and 60 to 65 (steel pipe of the present invention) had HIC resistant properties equal to or higher than that of the steel pipe in the related art, and were excellent in SSC resistant properties.
The chemical composition of the weld metal portion was obtained from Steel pipe No. 1. As a result, the chemical composition of the weld metal was 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 a remainder of Fe and impurities.

(Shape of weld toe portion)

For the obtained steel pipe, an angle of the excess weld tip end portion of the weld metal portion, that is, an angle between the tangential direction of the weld metal and the surface of the base material portion on both sides, and use the smaller angle as the angle of the weld toe portion.

(SSC resistant properties)

In addition, as an evaluation of SSC resistant properties, a 4-point bending test piece having a width of 15 mm, a length of 115 mm, and a thickness of 5 mm was collected from an inner surface of the steel pipe so as to remain the inner surface such that the weld toe portion is disposed in a center portion of the test piece in a longitudinal direction, and the presence or absence of crack in a solution environment of pH 3.5 with various hydrogen sulfide partial pressures was examined in accordance with NACE TM 0316-2016. The load stress during the 4-point bending test was 90% and 95% of the actual yield stress.

(Maximum hardness of surface layer area of welded heat-affected zone)

The hardness of the surface layer area in the welded heat-affected zone was measured. The hardness was measured in the surface layer area from the center portion in the circumferential direction and the longitudinal direction of the steel pipe to a depth position of 1.0 mm or 0.9 mm from the surface. A method of cutting out the test piece for the hardness test of the welded heat-affected zone is as described above.
Specifically, regarding the hardness measurement of the welded heat-affected zone, 40 points in each, at positions of 0.3 mm, 0.6 mm, and 0.9 mm from the surface at 0.5 mm pitch, on a base material portion side from a welded toe (boundary between weld metal portion and base material portion), a total of 120 points, were measured to calculate the maximum hardness.

In addition, the metallographic structure in the surface layer area of the welded heat-affected zone was observed, and the area ratio was also measured. The metallographic structure of the surface layer area is a metallographic structure at a depth position of 0.5 mm in the wall thickness direction from the surface. The results are summarized in Table 5.

[Table 5]

Test No.	Test No. of Table 2	Welded heat-affected zone		Weld toe portion	Hydrogen sulfide partial pressure (MPa)	SSC resistant properties
Test No.	Test No. of Table 2	Metallo graphic structure of surface layer area	Maximum hardness of surface layer area (HV)	Angle of weld toe portion (°)	Hydrogen sulfide partial pressure (MPa)	90% of actual yield stress as a result of 4-point bending test	95% of actual yield stress as a result of 4-point bending test
2	2	B	225	160	0.3	No Crack	No Crack
2'	2	B	229	130	0.3	No Crack	No Crack
2"	2	B	227	110	0.3	Crack	Crack
11	11	B	229	150	0.6	No Crack	No Crack
11'	11	B	233	140	0.6	No Crack	No Crack
11"	11	B	231	100	0.6	Crack	Crack

Test Nos. 2, 2', 11, and 11' were excellent in SSC resistant properties including the welded portion. On the other hand, in Test Nos. 2" and 11", SSC occurred from the weld toe portion.

[Industrial Applicability]

According to the present invention, it is possible to provide a steel pipe having a yield stress of 350 MPa or more and having excellent SSC resistant properties, in which crack does not occur even if a load of more than 90% of the yield stress is applied in an environment of 30°C or lower containing hydrogen sulfide of more than 0.1 MPa, and a steel plate capable of being used as the material thereof. Specifically, the steel pipe according to the present invention is suitable for a steel pipe used in a highpressure hydrogen sulfide environment such as a steel pipe for excavation of petroleum or natural gas or a steel pipe for transportation.

[Brief Description of the Reference Symbols]

1 Weld metal portion
2 Base material portion
3 Angle of weld toe portion
4 Welded heat-affected zone
5 Sample cut-out portion

Claims

A steel pipe comprising:
a base material portion; and

a welded portion,

wherein a chemical composition of the base material portion includes, by 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%, and

a remainder: Fe and impurities,

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,

a metallographic structure of a surface layer area in a range of 1 mm of a depth from a surface of the base material portion is one or more selected from polygonal ferrite, granular bainite, acicular ferrite, and bainite,

a maximum hardness of the base material portion in the surface layer area is 250 HV or less,

a yield stress is 415 to 630 MPa, and

a proportional limit in a stress-strain curve is 90% or more of the yield stress, $ESSP = Ca \times (1 - 124 \times O) / (1.25 \times S)$
$Ceq = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5$

here, each element symbol in the formula represents an amount (mass%) of each element included in steel, and is zero in a case where no element is included.
The steel pipe according to claim 1, wherein a total area ratio of the granular bainite, the acicular ferrite, and the bainite is more than 80% in the metallographic structure of the surface layer area of the base material portion.
The steel pipe according to claim 1 or 2,
wherein the chemical composition of the base material portion includes one or more selected from, by mass%,

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% to 0.0100%.
The steel pipe according to any one of claims 1 to 3,
wherein the chemical composition of the base material portion includes, by mass%, Nb: 0.01% to 0.04%,

the welded portion includes a welded heat-affected zone and a weld metal portion,

the metallographic structure of the surface layer area in the welded heat-affected zone includes one or both of bainite and acicular ferrite,

a maximum hardness of the surface layer area in the welded heat-affected zone is 250 HV or less, and

an angle of a weld toe portion on an inner side of the steel pipe is in a range of 130° to 180°.
The steel pipe according to any one of claims 1 to 4,
wherein a thickness of the base material portion is 10 to 40 mm, and a pipe diameter is 508 mm or more.
A steel plate used for the base material portion of the steel pipe according to any one of claims 1 to 5.