JP2013119657A - HIGH STRENGTH WELDED STEEL PIPE EXCELLENT IN SULFIDE STRESS CORROSION CRACKING RESISTANCE AND HAVING TENSILE STRENGTH OF 600 MPa OR MORE - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high strength welded steel pipe excellent in SSC (Sulfide Stress corrosion Cracking) resistance and having a tensile strength of 600 MPa or more.SOLUTION: The high strength welded steel pipe excellent in sulfide stress corrosion cracking resistance and having a tensile strength of 600 MPa or more is a welded steel pipe, in which a chemical component of a mother material satisfies a CP value of ≤0.95 calculated by formula (1), a Py value of ≤0.160 calculated by formula (2), and a chemical composition of a welded metal satisfies a Py value of 0.140 to 0.160 calculated by formula (2), wherein formula (1): CP=4.46C(%)+2.37 Mn(%)/6+{1.18Cr(%)+1.95Mo(%)+1.74V(%)}/5+{1.74Cu(%)+1.7Ni(%)}/15+22.36P(%), and formula (2): Py=C(%)+Si(%)/30+Mn(%)/20+{Cu(%)+Cr(%)}/20+Ni(%)/60+Mo(%)/7+V(%)/10+5×B(%).

Description

  The present invention includes a hydrogen gas containing a hydrogen sulfide gas having a partial pressure of 0.1 MPa or less, and a natural gas or crude oil transportation pipeline having a severe corrosion environment of pH 4 or less, not only in the steel pipe itself, but also in a circumferential weld between steel pipes. The present invention relates to a high-strength welded steel pipe excellent in sulfide stress corrosion cracking resistance and having a tensile strength of 600 MPa or more.

Welded pipes used for transportation of crude oil and natural gas containing hydrogen sulfide, strength, toughness, in addition to weldability, resistance to hydrogen-induced cracking resistance (hydrogen induced cracking: H ydrogen I nduced C racking; hereinafter, abbreviated as HIC) and, resistance to sulfide stress corrosion cracking (sulfide stress corrosion cracking: S ulfide S tress corrosion C racking ; hereinafter, abbreviated as SSC) so-called sour resistance is required, such as.

  In HIC, hydrogen ions from the corrosion reaction are adsorbed on the steel surface, penetrate into the steel as atomic hydrogen, diffuse and accumulate around non-metallic inclusions such as MnS in the steel, and cracks are generated by the internal pressure. It is supposed to be. As a technique for preventing HIC, by adding an appropriate amount of Ca or Ce to the amount of S, generation of MnS in a form with a large stress concentration (for example, needle-like or plate-like) is suppressed, and fine dispersion with a small stress concentration is achieved. A method of suppressing the occurrence of cracks by changing the shape to the spherical inclusions (for example, Patent Document 1), reduction of elements with high tendency to segregation (C, Mn, P, etc.), and further specifying the upper limit of hardness of the segregation part Methods (for example, Patent Document 2 and Patent Document 3) are known.

  On the other hand, in the SSC, when the internal pressure is applied to the steel pipe, hydrogen ions from the corrosion reaction are adsorbed on the surface of the steel pipe facing the corrosive environment, like the HIC, and enter the steel as atomic hydrogen. It is thought that embrittlement occurred. SSC sensitivity has a strong correlation with the hardness of steel, and ISO 15156 defines an upper limit of hardness for preventing SSC in each hydrogen sulfide corrosive environment. For this reason, it is necessary to reduce at least the hardness of the surface portion of the steel pipe, and the higher the required strength level of the steel pipe, the more difficult it is to achieve both. In order to solve this problem, a method of heating and tempering only the steel sheet surface layer portion by induction heating immediately after performing accelerated cooling to increase the strength in the manufacturing process of the steel pipe base material (for example, Patent Document 4) It has been known.

However, because natural gas or crude oil transportation pipelines are thick and large in diameter, welded steel pipes that are manufactured by welding thick steel plates into tubes and then welded are common, and in order to prevent SSC also at the welds, Although it is necessary to achieve both high strength and low hardness, none of the above-described techniques disclose an improvement in the SSC resistance of the welded portion of the welded steel pipe. In addition, in the circumferential welded portion that connects the welded steel pipes shown in FIG. 1, HAZ is formed by circumferential welding of the steel pipe base material, and HAZ is produced by circumferential welding of the steel pipe welded metal part, particularly formed in the steel pipe welded metal part. It is known that the hardening of the HAZ is remarkable, and it is extremely difficult to prevent SSC not only in the steel pipe body but also in the circumferential welded portion. Note that the HAZ, is substantially of the heat-affected zone by welding (H eat A ffected Z one) .

Japanese Patent Laid-Open No. 54-110119 JP-A-52-111815 JP 2009-133005 A JP 2002-327212 A

  The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a high-strength steel pipe having a tensile strength of 600 MPa or more excellent in SSC resistance not only in a welded steel pipe but also in a circumferential welded portion between steel pipes. To do.

First, the present inventors consider a steel pipe base material (hereinafter sometimes simply referred to as “base material” or “base material part”) and a steel pipe weld metal (which may be considered to affect the SSC resistance of the circumferential welded part). Hereinafter, it may be simply referred to as “welded metal” or “welded metal part.”) The behavior of each HAZ hardness was investigated using an existing welded steel pipe (hereinafter sometimes simply referred to as “steel pipe”). . Samples of the base metal part and the welded part are taken from welded steel pipes with different chemical constituents of the steel pipe base metal and the chemical constituents of the steel pipe weld metal. the inner surface weld metal, weld carbonate gas arc weld bead simulating the circumferential welding on the surface of the sample (B ead O n P late; . hereinafter, abbreviated as "BOP"), and perpendicular to the bead in the bead longitudinal central portion After the sample is cut, the cut surface is mirror-polished and then corroded so that the welded portion macrostructure can be seen, and the maximum hardness of the weld heat affected zone (HAZ) is determined according to JIS Z3101 by the Vickers hardness test method. It measured by (JIS Z2244).

  The hardness of the BOP HAZ simulating the circumferential welding thus obtained was subjected to multiple regression arrangement with the chemical components of the steel. The formula (2): Py = C + Si / 30 + Mn / 20 + (Cu + Cr) / The Py value calculated by 20 + Ni / 60 + Mo / 7 + V / 10 + 5 × B was found to accurately arrange the HAZ hardness of the base metal part and the HAZ hardness of the inner surface weld metal part by circumferential welding. Here, each element symbol is mass%, and is 0 when not contained.

  FIG. 2 (c) shows a relationship diagram between Py and circumferential welding simulated BOP part hardness Hv. Here, the HAZ of the base metal part (“base metal HAZ” in the figure) is the heat-affected part of the base material located under the BOP reheated by the carbon dioxide arc welding bead (BOP) of FIG. The weld metal part HAZ ("weld metal part HAZ" in the figure) is a steel pipe inner surface weld metal located under the BOP reheated by the carbon dioxide arc weld bead (BOP) in FIG. 2 (a). Refers to the heat affected zone.

  Next, the inventors have a tensile strength of 610 MPa, the main chemical component is 0.05% C-0.30% Si-1.30% Mn-0.045% Nb-0.01% by mass%. Using a steel plate with a thickness of 20 mm of Ti-0.002% Ca-0.19% Mo as a base material, a welded joint simulating inner and outer surface single-layer submerged arc welding of a welded steel pipe was prepared, and a joint tensile test was performed. The effect of weld metal chemical composition on steel pipe joint strength was investigated. In addition, the submerged arc welding was adjusted so that the welding heat input per 1 mm of the tube thickness was 0.21 kJ / mm on both the inner and outer surfaces and the welding amount was the same.

Here, the welding heat input per 1 mm of tube thickness is defined by the following formula (3).
Weld heat input per 1 mm of pipe thickness (kJ / mm) = 60 × welding current (A) × welding voltage (V) ÷ welding speed (mm / min) ÷ tube thickness (mm) Equation (3)
In the present invention, unless otherwise specified, the term welding heat input or welding heat input means the welding heat input per 1 mm of pipe thickness or plate thickness. The amount of welding heat input per 1 mm of pipe thickness is determined because welding heat input increases with increasing pipe thickness in the case of inner / outer surface single layer submerged arc welding generally used for welded steel pipes. This is because the amount of welding heat input per unit pipe thickness (sheet thickness) should be used as an index as a guide for welding conditions when the size of the steel pipe varies.

  As a result, when the same weld material is used for the inner and outer surfaces and the same weld metal chemical composition is used, the Py value of the formula (2) calculated by the chemical composition of the weld metal in order to make the joint strength 600 MPa or more. When the welded portion of the welded steel pipe is designed with such a Py value, the HAZ hardness exceeds 248 Hv in the Vickers hardness at the circumferential welded portion, and the classification of ISO 15156 It is predicted that the SSC resistance cannot be satisfied in a severe sour environment where the hydrogen sulfide partial pressure is 0.1 MPa at pH 4.0 or lower.

  Here, the inventors changed the way of thinking and thought that if the cooling rate after welding was increased even with the same weld metal composition, the hardenability could be improved and the joint strength could be improved, and the tube thickness of submerged arc welding was 1 mm. The welding heat input per contact was reduced to 0.19, 0.17, 0.15, and 0.13 kJ / mm, and the joint strength was evaluated. As a result, as shown in FIG. 3, it was confirmed that the strength of the welded joint increased as the welding heat input per 1 mm of pipe thickness was reduced even with the Py value of the same weld metal. As a result, the weld metal upper limit Py value for preventing SSC in the Reason 3 environment (a severe corrosion environment including a hydrogen sulfide gas having a partial pressure of 0.1 MPa or less and a pH of 4 or less) in the ISO 15156 classification is less than 0.160. However, it was found that if the welding heat input per 1 mm of plate thickness is reduced to 0.15 kJ / mm or less, the joint strength can be 600 MPa or more.

The present invention is a further study based on the above knowledge,
[1] A base material of a thick steel plate;
A welded steel pipe having a weld metal formed by submerged arc welding with a butt portion on each inner and outer surface layer,
The base material is mass%,
C: 0.02 to 0.06%,
Si: 0.05 to 0.5%,
Mn: 0.75 to 1.75%,
Al: 0.01 to 0.08%,
Nb: 0.005 to 0.06%,
Ti: 0.005 to 0.025%,
Ca: 0.0010 to 0.0035% is contained,
P: 0.01% or less,
S: 0.001% or less,
B: 0.004% or less,
N: 0.008% or less
Cu: 0.30% or less,
Ni: 0.50% or less,
Cr: 0.50% or less,
Mo: 0.20% or less,
V: contains one or more selected from 0.05% or less,
The CP value represented by the following formula (1) is 0.95 or less,
The Py value represented by the following formula (2) is 0.160 or less,
The balance consists of Fe and inevitable impurities,
And the said weld metal is the mass%,
C: 0.04 to 0.08%
Si: 0.05-0.5%
Mn: 1.0 to 1.6%
Nb: 0.01 to 0.05%
Ti: 0.01-0.04%
B: 0.001 to 0.003%
O: 0.020 to 0.035%
Al: containing 0.02% or less,
Further, Cu: 0.20% or less Ni: 0.30% or less Cr: 0.30% or less Mo: 0.20% or less V: 0.05% or less
It is composed of the remaining Fe and inevitable impurities, and the Py value of the formula (2) calculated by the chemical composition of the weld metal satisfies 0.140 to 0.160. High strength welded steel pipe with excellent tensile strength of 600 MPa or more.
CP = 4.46C (%) + 2.37Mn (%) / 6+ {1.18Cr (%) + 1.95Mo (%) + 1.74V (%)} / 5+ {1.74Cu (%) + 1.7Ni (% )} / 15 + 22.36P (%) (1)
Py = C (%) + Si (%) / 30 + Mn (%) / 20+ {Cu (%) + Cr (%)} / 20 + Ni (%) / 60 + Mo (%) / 7 + V (%) / 10 + 5 × B (%) · ..Formula (2)
Here, the element symbol on the right side of each formula represents the content (% by mass), and is 0 when not contained.
[2] The metal structure of the surface layer portion of the base material has a volume fraction of island martensite of 2% or less, and the balance is bainite or a mixed structure of bainite and ferrite, [1] A high-strength welded steel pipe having a tensile strength of 600 MPa or more and excellent in sulfide stress corrosion cracking resistance described in 1.
[3] The submerged arc welding has a resistance to sulfide stress corrosion cracking as described in [1] or [2] above, wherein a welding heat input per 1 mm of tube thickness is 0.15 kJ / mm or less. High strength welded steel pipe with excellent tensile strength of 600 MPa or more.

  According to the present invention, not only the HIC resistance and SSC resistance of a steel pipe base material in a natural gas or crude oil transportation pipeline where a severe hydrogen sulfide corrosion environment with a pH of 4 or less is assumed, but also a circumferential weld that connects steel pipes together It is possible to provide a high-strength steel pipe having excellent SSC resistance, and to improve the efficiency of transportation of natural gas or crude oil by high-pressure operation.

It is a figure explaining the circumferential weld part of welded steel pipes. It is a figure explaining the HAZ hardness of the base material and a weld metal by the welding test which simulated circumferential welding. It is a figure explaining the influence of the welding heat input per 1 mm of plate thickness and Py value of a weld metal which gives to the welded joint strength of a welded steel pipe. It is a figure explaining the hardness measurement position of a welded steel pipe base material, a welded part, and a circumferential welded part. It is a figure explaining the stress provision method to the test piece in a 4-point bending SSC test.

  Hereinafter, the reasons for limitation of each component of the present invention will be described separately.

1. The chemical component of the steel pipe base material First, the reasons for limiting the chemical composition of the steel pipe base material will be described. In addition, the unit of chemical components is all mass%.

C: 0.02 to 0.06%
C is the most effective element for increasing the strength of steel. However, if it is less than 0.02%, sufficient strength cannot be ensured, and if it exceeds 0.06%, the hardenability is increased, and the sour resistance is deteriorated due to the increased hardness of the segregated portion and the steel surface portion. Therefore, the C content is in the range of 0.02 to 0.06%. More preferably, it is 0.03 to 0.05%.

Si: 0.05-0.5%
Since Si exhibits the effect of solid solution strengthening of steel, it is effective for increasing the strength by containing 0.05% or more. However, if the content exceeds 0.5%, the toughness is remarkably lowered, so the Si content is in the range of 0.05 to 0.5%.

Mn: 0.75 to 1.75%
Mn is added to increase the strength of the steel. However, if it is less than 0.75%, the effect is not sufficient, and if it exceeds 1.75%, the hardness of the segregated part is particularly increased and the HIC resistance is deteriorated. . Therefore, the Mn content is in the range of 0.75 to 1.75%. Preferably, it is 1.2 to 1.4%.

Al: 0.01 to 0.08%
Al acts as a deoxidizing element. Sufficient deoxidation effect can be obtained with addition of 0.01% or more, but addition over 0.08% lowers the cleanliness in the steel and lowers the HIC resistance of the base metal part as the starting point of HIC. Therefore, the Al content is in the range of 0.01 to 0.08%. More preferably, it is 0.02 to 0.05% of range.

Nb: 0.005 to 0.06%
Nb is an element for improving the hardenability of steel and is added to increase the strength. However, if it is less than 0.005%, there is no effect, and if it exceeds 0.06%, coarse Nb carbonitride in the segregation part. Therefore, the Nb content is in the range of 0.005 to 0.06% in order to reduce the HIC resistance of the base metal part as a starting point of HIC. More preferably, it is 0.02 to 0.05% of range.

Ti: 0.005-0.025%
Ti is added for the purpose of suppressing formation of coarse Nb carbonitride that forms a fine carbonitride in steel in advance of Nb and serves as a starting point of HIC in the segregation part. However, if it is less than 0.005%, there is no effect, and if it exceeds 0.025%, Ti carbonitride itself is coarsened and becomes the starting point of HIC, which decreases the HIC resistance of the base metal part. Is in the range of 0.005 to 0.025%. More preferably, it is 0.007 to 0.020% of range.

Ca: 0.0010 to 0.0035%
Ca is an element necessary for controlling the form of sulfide inclusions that are the starting point of HIC, and particularly for preventing the generation of HIC by MnS. However, if it is less than 0.0010%, there is no effect, and 0.0035 Even if added in excess of%, the effect is saturated, and rather coarse CaO · CaS inclusions are formed, which becomes the starting point of HIC, and rather deteriorates the HIC resistance. Therefore, the Ca content is 0.0010 to 0.0035%. More preferably, it is 0.0015 to 0.0030% of range.

P: 0.01% or less P is an unavoidable impurity and significantly increases the hardness of the segregated portion due to center segregation, thereby degrading the HIC resistance. This tendency becomes remarkable when it exceeds 0.01%. Therefore, it is desirable to reduce P as much as possible, but it is acceptable up to 0.01%. More preferably, it is 0.006% or less.

S: 0.001% or less S is generally a MnS-based inclusion in steel, but the form is controlled from MnS-based to CaS-based inclusion by addition of Ca. However, when the content of S is large, the amount of CaS inclusions increases, and a high-strength steel sheet can be a starting point of cracking. This tendency becomes remarkable when the S amount exceeds 0.001%. Therefore, it is desirable to reduce S as much as possible, but it is acceptable up to 0.001%. More preferably, it is 0.0006% or less.

B: 0.0004% or less B is an element for improving hardenability and is effective in increasing the strength of steel, but at the same time, the effect of increasing the hardness of HAZ is remarkable, and the SSC resistance at the circumferential welds between steel tubes is improved. In order to make it deteriorate, it is necessary to reduce as much as possible with a steel pipe preform, and the upper limit is made 0.0004%.

N: 0.008% or less N is an unavoidable impurity element, but as described above, it forms coarse carbonitrides of Nb and Ti, and lowers the HIC resistance of the base metal part as the starting point of HIC. 0.008%.

  In this invention, in order to improve the intensity | strength of a steel pipe base material, 1 or more types chosen from Cu, Ni, Cr, Mo, and V shown below are added.

Cu: 0.30% or less Cu is an element effective for increasing the strength, and forms a dense corrosion product when the steel pipe base material is exposed to a mild hydrogen sulfide corrosion environment of pH 4-5. However, even if added in excess of 0.30%, the effect is saturated, and due to dilution into the steel pipe weld metal part described later, the weld metal part Causes hot cracking. Therefore, when adding Cu, the upper limit is made 0.30%.

Ni: 0.50% or less Ni is an element effective for improving toughness and increasing strength. However, when added over 0.50%, hair cracking occurs on the surface of the steel pipe base material exposed to the hydrogen sulfide corrosion environment. Therefore, when Ni is added, the upper limit is made 0.50%.

Cr: 0.50% or less Cr is an effective element for obtaining strength by improving hardenability. However, if added over 0.50%, the weldability deteriorates. Therefore, when adding Cr, it is 0.50% or less.

Mo: 0.20% or less Mo is an element that improves hardenability and greatly contributes to an increase in strength. However, the effect of increasing the hardness of the HAZ is remarkable, and if it exceeds 0.20%, the SSC resistance at the circumferential welded portion between the steel pipes is deteriorated. Therefore, when adding Mo, it is made 0.20% or less.

V: 0.05% or less V is an element that increases the strength. However, if added over 0.05%, the effect of increasing the hardness of HAZ is remarkably the same as that of Mo, and the SSC resistance at the circumferential welded portion between the steel pipes is degraded. Therefore, when V is added, the content is made 0.05% or less.

CP value calculated by Formula (1) is 0.95 or less Formula (1): CP = 4.46C (%) + 2.37Mn (%) / 6+ {1.18Cr (%) + 1.95Mo (%) + 1 .74 V (%)} / 5+ {1.74 Cu (%) + 1.7 Ni (%)} / 15 + 22.36 P (%) Here, the element symbol on the right side of each formula represents the content (mass%) of each. , 0 if not contained.

  The CP value is an expression devised to estimate the material of the central segregation part from the content of each alloy element. The higher the CP value, the higher the concentration of the central segregation part, and the hardness of the central segregation part. There is a technical significance of rising. As a result of intensive studies, the inventors clarified the limit hardness at which HIC occurs in the central segregation part for each hydrogen sulfide environment, and attempted to arrange the CP value as an index for not exceeding the hardness. As a result, it was found that HIC can be suppressed by setting this CP value to 0.95 or less in the Region 3 environment of ISO 15156 classification to be solved by the present invention. Therefore, the CP value is set to 0.95 or less.

Py value calculated by formula (2) is 0.160 or less Py = C (%) + Si (%) / 30 + Mn (%) / 20+ {Cu (%) + Cr (%)} / 20 + Ni (%) / 60 + Mo ( %) / 7 + V (%) / 10 + 5 × B (%) (2)
Here, the element symbol on the right side of each formula represents the content (% by mass), and is 0 when not contained.
As shown in FIG. 2 (c), the Py value indicates the HAZ hardness of the steel pipe base material and the weld metal part when the steel pipes are circumferentially welded ("base metal HAZ" and "welded metal HAZ" in the figure, respectively). There is a good correlation with the description. Furthermore, in order to prevent the occurrence of SSC in the Region 3 environment of the ISO 15156 classification to be solved by the present invention, it is necessary to make the Vickers hardness of the steel 248 or less, so the HAZ hardness in the circumferential weld is reduced. In order to make it 248 or less, the Py value is made 0.160 or less.

  In the present invention, other than the above elements are Fe and inevitable impurities. Although it does not need to add intentionally, elements other than the above and unavoidable impurities can be contained as long as the effects of the present invention are not impaired.

  In addition, about the manufacturing method of a steel pipe base material, the steel melted by the converter method was made into the slab by the continuous casting method from the point of a material and manufacturing efficiency, and after applying thick plate rolling, accelerated cooling is applied and 600 MPa or more It is desirable to obtain high strength.

2. Weld metal 2.1 Chemical component of weld metal Next, the reason for limiting the chemical component of the weld metal part of the steel pipe will be described. In addition, the unit of chemical components is all mass%. In the description of the present invention, unless otherwise specified, the weld metal refers to a weld metal by butt welding at the time of manufacturing a steel pipe, and does not target a weld metal at the time of circumferential welding.

C: 0.04 to 0.08%
C, like the base material, is the most effective element for increasing the strength of the weld metal. In particular, 0.04% or more is necessary to obtain high strength in a weld metal that is a solidified structure. On the other hand, if it exceeds 0.08%, the HAZ hardness of the weld metal is remarkably increased during circumferential welding, and the SSC resistance of the circumferential weld is deteriorated, so the upper limit is made 0.08%. In addition, More preferably, it is 0.04 to 0.06%.

Si: 0.05-0.5%
Si acts as a deoxidizing element in the weld metal and is an element necessary for controlling the amount of oxygen in the weld metal. If the Si content in the weld metal is less than 0.05%, deoxidation is insufficient and the amount of oxygen in the weld metal increases, resulting in a decrease in strength. On the other hand, the effect is saturated even if the addition exceeds 0.5%. Therefore, the Si content is in the range of 0.05 to 0.5%.

Mn: 1.0 to 1.6%
Mn also acts as a hardenability improving element in the weld metal. In order to increase the strength of the weld metal, Mn of at least 1.0% is required. However, if it exceeds 1.6%, the HAZ hardness of the weld metal is significantly increased during circumferential welding, and circumferential welding is performed. In order to degrade the SSC resistance of the part, the upper limit is made 1.6%. Therefore, the Mn content is in the range of 1.0 to 1.6%.

Nb: 0.01 to 0.05%
Nb is required to be at least 0.01% or more in order to cause solute N in the weld metal to form nitrides prior to B so as to exist as solute B at the austenite grain boundaries. On the other hand, if it exceeds 0.05%, carbides are formed, the weld metal is precipitated and hardened, and the toughness is reduced, so the upper limit is made 0.05%. More preferably, it is 0.01 to 0.03% of range.

Ti: 0.01-0.04%
Ti reacts with oxygen in the weld metal to form TiO or TiO ₂ and functions as an acicular ferrite transformation nucleus from within the weld metal austenite grains. In order to obtain the effect of increasing the strength by refining the acicular ferrite structure, it is necessary to generate a large number of TiO or TiO ₂ , and Ti must be at least 0.01% or more. On the other hand, if it exceeds 0.04%, TiO or TiO ₂ in the weld metal is agglomerated and coarsened to cause a decrease in toughness, so the upper limit is made 0.04%. More preferably, it is 0.02 to 0.04% of range.

B: 0.001 to 0.003%
B suppresses the formation of polygonal ferrite from the austenite grain boundary of the weld metal, and has the effect of forming a metal structure mainly composed of acicular ferrite, contributing to high strength. In order to completely suppress the formation of polygonal ferrite from the grain boundary, at least 0.001% or more is necessary, but even if it exceeds 0.003%, the effect is saturated, so the upper limit is made 0.003%. More preferably, it is 0.002 to 0.003% of range.

O: 0.020 to 0.035%
O reacts with the above-mentioned Ti to form TiO or TiO ₂ and functions as an acicular ferrite transformation nucleus from within the weld metal austenite grains. In order to obtain an acicular ferrite structure which is a fine metal structure, it is necessary to generate a large number of TiO or TiO ₂ , and O is required to be at least 0.020% or more. On the other hand, if it exceeds 0.035%, a part of the grain boundary ferrite is generated and causes a decrease in the strength of the weld metal, so the upper limit is made 0.035%. More preferably, it is 0.025 to 0.035%.

Al: 0.02% or less Al is present in the weld metal as an unavoidable impurity due to dilution from the base metal part. However, if it exceeds 0.02%, the above-described formation of TiO is inhibited, and the acicular ferrite structure of the weld metal Since the effect of increasing the strength due to the refinement of the metal structure cannot be obtained, the upper limit is made 0.02%.

  In the present invention, in order to further improve the strength of the weld metal part, at least one selected from the following Cu, Ni, Mo, Cr, and V is added. In addition, when adding, it is preferable to add the same element as the element added to the base material.

Cu: 0.20% or less Cu acts as a hardenability improving element and can be used as a substitute for Mn. However, if it exceeds 0.20%, Cu liquefaction cracks may cause welding defects significantly. Therefore, when adding Cu, the upper limit is made 0.20%.

Ni: 0.30% or less Ni acts as a hardenability improving element and can be substituted for Mn. However, it is an expensive element and if it exceeds 0.30%, the effect of increasing the strength is saturated. Therefore, when Ni is added, the upper limit is made 0.30%.

Cr: 0.30% or less Cr also acts as a hardenability improving element and can substitute for Mn. However, even if added over 0.30%, the effect of increasing the strength is saturated. Therefore, when adding Cr, the upper limit is made 0.30%.

Mo: 0.20% or less Mo also acts as a hardenability improving element and can be used as an alternative to Mn addition. However, as with the steel pipe base material, when the weld metal is affected by the welding heat during the circumferential welding of the steel pipes, the effect of increasing the hardness of the HAZ is remarkable. SSC property may be deteriorated. Therefore, when adding Mo, the upper limit is made 0.20%.

V: 0.05% or less V, like Mo, acts as a hardenability improving element and can be substituted for Mn. However, as with the steel pipe base material, when the weld metal is affected by welding heat during the circumferential welding of steel pipes, the HAZ hardness increase effect is significant. SSC property may be deteriorated. Therefore, when V is added, the upper limit is made 0.05%.

Py value (Py) of weld metal: 0.140 to 0.160
Furthermore, in the present invention, in order to achieve both high joint strength of the steel pipe welded portion and reduced hardness of the HAZ formed in the weld metal during steel pipe circumferential welding, both the inner and outer weld metals of the steel pipe The Py value calculated by equation (2) is in the range of 0.140 to 0.160.
Py = C (%) + Si (%) / 30 + Mn (%) / 20+ {Cu (%) + Cr (%)} / 20 + Ni (%) / 60 + Mo (%) / 7 + V (%) / 10 + 5 × B (%) · ..Formula (2)
Here, the element symbol on the right side of each formula represents the content (% by mass), and is 0 when not contained.

  As a result of various studies by the present inventors, the hardness of the HAZ formed when the inner surface weld metal is circumferentially welded also correlates with the Py value, and from the viewpoint of preventing the occurrence of SSC in the Region 3 environment defined in ISO 15156, In order to set the upper limit of hardness to Vickers hardness Hv248 or less, the upper limit is set to 0.160. On the other hand, in order to satisfy the joint strength of 600 MPa or more like the base metal with the low Py weld metal composition, it is necessary to increase the cooling rate after welding by reducing the welding heat input per 1 mm of pipe thickness described later. However, if the welding heat input is lower than necessary, it causes welding defects such as undercuts in the welded part. Therefore, when the lower limit of the Py value at which the joint strength can be achieved at the lowest possible welding heat input is obtained, 0. Since it was 140, the lower limit is set to 0.140.

  Other than the above elements, Fe and unavoidable impurities are not added intentionally. However, components other than those described above can be contained as long as the effects of the present invention are not impaired.

  In addition, the steel pipe weld is usually divided into an inner surface weld metal and an outer surface weld metal, but both weld metals do not necessarily have the same composition. From the viewpoint of increasing joint strength, the outer surface weld metal composition is slightly higher Py. It is preferable to use a value.

  In order to control the chemical component in the weld metal within the above range, it is preferable to appropriately select a welding material (welding wire and flux) used for welding according to the chemical component of the base metal and the welding conditions. For example, for each element, a welding wire having a component obtained by dividing the target component of the component element in the weld metal by the base material dilution rate is produced and welded using this.

2.2 Welding method As a welding method for butt welding at the time of manufacturing a steel pipe, submerged arc welding is used from the viewpoint of excellent welding quality and manufacturing efficiency. In this case, it is preferable that the welding heat input per 1 mm of pipe thickness of submerged arc welding is 0.15 kJ / mm or less.

  As shown in FIG. 3, the strength of the welded joint is increased even if the Py value is the same by reducing the amount of heat input during welding of the steel pipe. The welding heat input changes depending on the pipe thickness, and a good correlation was obtained by arranging the welding heat input per 1 mm of the pipe thickness. From FIG. 3, a joint strength of 600 MPa or more is satisfied with a Py value of 0.160 or less. Therefore, it can be seen that the welding heat input per 1 mm of the tube thickness should be 0.15 kJ / mm or less.

  In addition, since the welding amount of a weld metal falls by lowering the welding heat input, it is preferable to use a welding wire having a higher melting efficiency and a smaller diameter. Further, in order to suppress defects such as undercuts in the welded portion, it is more preferable that the amount of welding heat input per 1 mm of the tube thickness is 0.12 kJ / mm or more.

3. In the present invention, from the viewpoint of preventing the occurrence of SSC in the steel pipe base material, the metal structure of the steel pipe surface layer part is specified as follows. Here, the notation of the volume fraction (%) of the metal structure is applied by measuring the area ratio (%) of each metal structure by image analysis and regarding the volume fraction (%).

The volume fraction of island martensite in the steel pipe surface layer: 2% or less Island martensite (hereinafter sometimes simply referred to as “MA”) is a structure generated by accelerated cooling, and is generated by MA. The hardness increases greatly. As described above, in order to prevent the occurrence of SSC in the Region 3 environment of ISO 15156 classification, it is necessary to make the Vickers hardness of the steel pipe surface layer part, which is the base material part, 248 or less, and at least directly exposed to a hydrogen sulfide corrosive environment. It is important to reduce the hardness of the steel pipe surface layer. As a result of intensive studies, the inventors focused on MA in the metal structure of the steel pipe base metal. As the volume fraction of MA increased, the hardness of the steel pipe surface layer increased, and at an MA volume fraction of at least 2%. It has been found that the Vickers hardness exceeds 248 Hv and the SSC resistance may deteriorate. Therefore, the volume fraction of island martensite in the steel pipe surface layer is preferably 2% or less.

  In order to obtain a base material portion tensile strength of 600 MPa or more, at least the metal structure of the base material portion needs to be mainly bainite, and the bainite structure is preferably 70% or more in terms of volume fraction. However, even if the hardness is reduced by generating soft ferrite only in the surface layer portion, since the influence on the tensile strength of the base material portion is small, the ferrite containing a ferrite structure having a volume fraction of 20% or less and It can be a mixed structure of bainite.

  Steel of the chemical composition shown in Table 1 (steel types A to H) was made into a slab by a continuous casting method, and a steel pipe material having a plate thickness of 19 to 25 mm was manufactured by a thick plate rolling / accelerated cooling process.

  The steel pipe material was subjected to submerged arc welding of one inner surface and one outer surface using various welding wires under the welding heat input conditions shown in Table 2 for each pipe thickness of 1 mm to obtain steel pipe welds. A chemical component analysis sample was taken from the inner surface weld metal of the weld and analyzed for each chemical component. Table 2 shows the chemical component analysis results.

  Full thickness tensile test pieces were sampled from the base metal part and the steel pipe welded part according to the API standard, and the tensile strength was measured. The tensile strength of 600 MPa or more was determined as the strength required for the present invention. Further, in order to evaluate the HIC performance of the base metal part and the steel pipe welded part, in accordance with NACE TM0284, HIC test specimens were collected from the base metal part and the welded part, and mixed with an acetic acid aqueous solution and a sodium chloride aqueous solution to adjust the pH to 3.0. The immersion liquid adjusted to 100% was saturated with 100% hydrogen sulfide gas and immersed for 96 hours. After the immersion, the HIC test piece was cut into three sections at equal intervals, each mirror-polished, and then each section was observed with an optical microscope at a magnification of 100 times, and found in the manner described in Section 7 of NACE TM0284-2003 The length of the crack in the sample width direction was recorded, and the crack length ratio CLR (%) was calculated. In the present invention, the CLR of 15% or less is assumed to satisfy the HIC resistance performance.

  Next, as shown in FIG. 4, circumferential welding simulated multi-layer welding in which the steel pipe base material and the steel pipe welded portion were brought together was performed by a carbon dioxide arc welding method. The average value of welding heat input was about 1.0 kJ / mm. Then, one section of the produced circumferential weld joint was cut and mirror-polished, and as shown in FIG. 4, the base metal part surface, the inner surface weld metal surface of the steel pipe welded part, the base metal side HAZ of the circumferential weld simulated part, Vickers hardness was measured at five points for each of four locations on the inner surface weld metal side HAZ of the circumferential weld simulation portion, and the average value was calculated. Further, a specimen for observing the metal structure was taken from the side of the Vickers hardness measurement location on the surface of the base material part, first subjected to nital etching, and the type of microstructure was examined with a 400 × optical microscope. Next, two-stage etching was performed on the test piece to reveal MA, and then five random fields of view were taken with a 1500x scanning electron microscope, and the area ratio of MA in the photograph was measured and calculated by image analysis. did.

  Next, rectangular test pieces having a thickness of 5 mm, a width of 15 mm, and a length of 115 mm were sampled from three locations of the base metal part surface, the steel pipe welded part inner surface, and the circumferential weld simulation part inner surface, and are shown in FIG. After applying a stress corresponding to 90% of the yield strength to the center of the specimen by four-point bending using a jig, the pH was adjusted to 3.0 by mixing an acetic acid aqueous solution and a sodium chloride aqueous solution as in the HIC test. The immersion liquid was saturated with 100% hydrogen sulfide gas and immersed for 720 hours. After the immersion, the test piece was removed from the jig, washed with water, and the presence or absence of SSC generation on the surface of the test piece was confirmed at a magnification of 100 times.

  Table 3 summarizes the results of microstructural investigation on the surface of the base metal part, tensile test results of the base metal part and the welded part, hardness measurement results, HIC test results, and SSC test results.

  In Table 3, Nos. 1 to 5 as examples of the present invention all have the chemical composition of the base metal, the inner surface weld metal, and the outer surface weld metal and the microstructure of the surface of the base material within the scope of the present invention. -Both welds have high strength of tensile strength of 600 MPa and higher, and HIC CLR is 0%. Furthermore, no cracks occurred in the four-point bending SSC test of all the base metal parts, weld parts, and circumferential weld simulation parts. .

  On the other hand, Comparative Example No. 6 in which the amount of C of the base material exceeded the upper limit of the present invention was such that the MA fraction exceeded the upper limit of the present invention in the microstructure inspection of the surface portion of the base material, and the nearby hardness was 271. As a result of being extremely hard, cracks occurred in the 4-point bending SSC test of the base material. Also, in the HIC test of the base material part, many cracks occurred on the surface side, and the CLR exceeded 15%. In Comparative Example No. 7 in which the CP value of the base material exceeded the upper limit of the present invention, many cracks occurred in the central segregation part in the HIC test of the base material part, and the CLR exceeded 15%. In Comparative Example No. 8 in which the Py value of the base material exceeded the upper limit, the base metal side HAZ hardness of the circumferential weld simulation part was very hard and cracked in the 4-point bending SSC test of the circumferential weld simulation part. There has occurred.

  In Comparative Example No. 9, in which Py of the inner surface weld metal was less than the lower limit of the present invention, the tensile strength of the welded portion was less than 600 MPa. On the other hand, in Comparative Example No. 10, in which Py exceeded the upper limit of the present invention, the inner surface weld metal side HAZ hardness of the circumferential welding simulated portion was very hard, and the four-point bending SSC of the circumferential welding simulated portion. Cracks occurred in the test. In Comparative Example No. 10, in which the welding heat input per 1 mm of the pipe thickness exceeded the range of 0.15 kJ / mm in the present application range, the tensile strength of the welded portion was less than 600 MPa.

  According to the present invention, not only the HIC resistance and SSC resistance of the steel pipe base material in the natural gas and crude oil transportation pipeline assumed to be severe hydrogen sulfide corrosive environment of pH 4 or less, but also in the circumferential welded part connecting the steel pipes. In addition, it is possible to provide a high-strength steel pipe having excellent SSC resistance, and it is possible to improve the efficiency of transportation of natural gas or crude oil by high-pressure operation.

Claims (3)

A thick steel base material,
A welded steel pipe having a weld metal formed by submerged arc welding with a butt portion on each inner and outer surface layer,
The base material is mass%,
C: 0.02 to 0.06%,
Si: 0.05 to 0.5%,
Mn: 0.75 to 1.75%,
Al: 0.01 to 0.08%,
Nb: 0.005 to 0.06%,
Ti: 0.005 to 0.025%,
Ca: 0.0010 to 0.0035% is contained,
P: 0.01% or less,
S: 0.001% or less,
B: 0.004% or less,
N: 0.008% or less
Cu: 0.30% or less,
Ni: 0.50% or less,
Cr: 0.50% or less,
Mo: 0.20% or less,
V: contains one or more selected from 0.05% or less,
The CP value represented by the following formula (1) is 0.95 or less,
The Py value represented by the following formula (2) is 0.160 or less,
The balance consists of Fe and inevitable impurities,
And the said weld metal is the mass%,
C: 0.04 to 0.08%
Si: 0.05-0.5%
Mn: 1.0 to 1.6%
Nb: 0.01 to 0.05%
Ti: 0.01-0.04%
B: 0.001 to 0.003%
O: 0.020 to 0.035%
Al: containing 0.02% or less,
Further, Cu: 0.20% or less Ni: 0.30% or less Cr: 0.30% or less Mo: 0.20% or less V: 0.05% or less
It is composed of the remaining Fe and inevitable impurities, and the Py value of the formula (2) calculated by the chemical composition of the weld metal satisfies 0.140 to 0.160. High strength welded steel pipe with excellent tensile strength of 600 MPa or more.
CP = 4.46C (%) + 2.37Mn (%) / 6+ {1.18Cr (%) + 1.95Mo (%) + 1.74V (%)} / 5+ {1.74Cu (%) + 1.7Ni (% )} / 15 + 22.36P (%) (1)
Py = C (%) + Si (%) / 30 + Mn (%) / 20+ {Cu (%) + Cr (%)} / 20 + Ni (%) / 60 + Mo (%) / 7 + V (%) / 10 + 5 × B (%) · ..Formula (2)
Here, the element symbol on the right side of each formula represents the content (% by mass), and is 0 when not contained.   The metal structure of the surface layer portion of the base material has a volume fraction of island martensite of 2% or less, and the balance is bainite or a mixed structure of bainite and ferrite. A high-strength welded steel pipe with a tensile strength of 600 MPa or more with excellent sulfide stress corrosion cracking properties.   The submerged arc welding has a heat input per pipe thickness of 1 mm of 0.15 kJ / mm or less, and has a tensile strength of 600 MPa or more excellent in resistance to sulfide stress corrosion cracking according to claim 1 or 2. High strength welded steel pipe.