JP2013119658A - 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 HIC (Hydrogen Induced Cracking) resistance and SSC (Sulfide Stress corrosion Cracking) resistance, particularly, SSC resistance not only in steel pipe but also in circumferential welded part of steel pipes with each other.SOLUTION: The high strength welded steel pipe excellent in sulfide stress corrosion cracking resistance and having a tensile strength of 600 MPa or more satisfies: a CP value, obtained from a specified formula which is a function of a chemical composition of a mother material, of ≤1.1; a Py value, obtained by another formula, of ≤0.175; and a Py value, calculated by a chemical composition of a welded metal, of ≤0.175, wherein the area of an inner surface welded metal portion is 2.5 times or more the area of an outer surface welded metal portion.

Description

  The present invention is a natural gas or crude oil transportation pipeline that is in a Region 2 environment (including hydrogen sulfide gas having a partial pressure of 0.01 MPa or less and a relatively mild corrosive environment having a pH of about 4 to 5) in the ISO 15156 classification. The present invention relates to a high-strength welded steel pipe having a tensile strength of 600 MPa or more that is excellent in resistance to sulfide stress corrosion cracking not only in the steel pipe itself but also in a circumferential welded portion between the steel pipes.

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 excellent in SIC resistance not only in HIC resistance and SSC resistance, particularly in a circumferential welded portion between steel pipes.

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 collected from the welded steel pipes having different chemical components of the steel pipe base metal and the chemical composition of the steel pipe weld metal, and in the manner shown in FIGS. 2 (a) and 2 (b), the base metal part and 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 HAZ of the weld metal part (“weld metal part HAZ” in the figure) is the steel pipe inner surface weld metal located under the BOP reheated by the carbon dioxide arc weld bead (BOP) in FIG. Refers to the heat affected zone.

  Next, the inventors have a tensile strength of 610 MPa, and the main chemical component is 0.05% C-0.30% Si-1.30% Mn-0.045% Nb-0.01% in 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 the submerged arc welding, the welding heat input amount was set to 4.2 kJ / mm on both the inner and outer surfaces, and the welding amount was adjusted to be substantially the same as shown in the macro photograph of the welded portion cross section shown in FIG.

Here, the welding heat input is defined by the following equation (3).
Weld heat input (kJ / mm) = 60 × welding current (A) × welding voltage (V) ÷ welding speed (mm / min) (3)
In the present invention, unless otherwise specified, the term "welding heat input" or "welding heat input" means welding input heat according to the formula defined here.

  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 280 Hv in the Vickers hardness at the circumferential welded portion, and the classification of ISO 15156 It is predicted that the SSC resistance in the Region 2 environment cannot be satisfied.

Here, the inventors conducted extensive investigations on the welded joints used in the experiments, and in particular, as a result of measuring the pipe thickness direction hardness distribution of the weld metal part, as shown in FIG. It was found to show a higher hardness value. From this, it is thought that if the proportion of the outer surface welded portion, which is considered to be lower in strength, occupies the cross section of the joint, the joint strength will be relatively increased, and the welding groove shape, welding current, etc. The weld joints in which the ratio of the inner surface weld metal part area to the outer surface weld metal part area in the weld joint cross section was systematically changed were prepared, and the joint tensile test was performed again. Here, the calculation of the area of the inner surface weld metal and the outer surface weld metal in the joint section cross section, as shown in FIG. 5 (a), with a line connecting the meeting portion and a line drawn so as to exclude the excess portion near the surface. Approximate to the trapezoid surrounded. That is,
External weld metal area: So = (w + wo) × ho × 1/2
Internal weld metal area: Si = (w + wi) × hi × 1/2
w: the length of the line connecting the meeting part wi: the length of the surface side when the inner surface weld metal surplus is removed hi: the distance between w and wi wo: the length of the surface side when the outer surface weld metal surplus is removed The distance between ho and w and the ratio PSw (also referred to as “welding area ratio”) of the inner surface weld metal part area and the outer surface weld metal part area in the weld cross section excluding the surplus part from the calculated Si and So. ) Is PSw = Si / So = {(w + wi) × hi} / {(w + wo) × ho}
Is calculated as As a result, as shown in FIG. 5B, it was confirmed that the joint strength increased as PSw increased. And in the classification of ISO15156, the inner surface weld metal upper limit Py value 0.175 for preventing SSC in the Region 2 environment (relatively mild corrosive environment having a pH of about 4 to 5 including hydrogen sulfide gas having a partial pressure of 0.01 MPa or less). It was found that the joint strength can be increased to 600 MPa or more by setting PSw to 2.5 or more even if it is less than 2.5.

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: 0.8 to 1.7%,
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: 0.30% or less,
V: contains one or more selected from 0.1% or less,
The CP value represented by the following formula (1) is 1.1 or less,
The Py value represented by the following formula (2) is 0.175 or less,
The balance consists of Fe and inevitable impurities,
And the said weld metal is the mass%,
C: 0.04 to 0.10%
Si: 0.05-0.5%
Mn: 1.0 to 1.8%
Nb: 0.01 to 0.05%
Ti: 0.01-0.04%
B: 0.001 to 0.004%
O: 0.025 to 0.045%
Al: containing 0.02% or less,
Furthermore, Cu: 0.30% or less Ni: 0.50% or less Cr: 0.50% or less Mo: 0.30% or less V: containing at least one selected from 0.1% or less,
The Py value of the formula (2) consisting of the balance Fe and inevitable impurities and calculated by the chemical composition of the weld metal is 0.175 or less,
In the welded section, the inner surface weld metal part area excluding the surplus part is 2.5 times or more the outer surface weld metal part area, and has a tensile strength of 600 MPa or more excellent in resistance to sulfide stress corrosion cracking. High strength welded steel pipe.
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 of the base material has a volume fraction of island martensite of 5% or less, and the balance is bainite or a mixed structure of bainite and ferrite, [1 The high strength welded steel pipe having a tensile strength of 600 MPa or more and excellent in sulfide stress corrosion cracking resistance.

  According to the present invention, only HIC resistance and SSC resistance of a pipe base material in a natural gas or crude oil transportation pipeline assuming a relatively mild hydrogen sulfide corrosion environment defined as a Region 2 environment in the classification of ISO 15156. In addition, it is possible to provide a high-strength steel pipe having excellent SSC resistance even in a circumferential welded portion that connects pipes, and it is possible to improve the efficiency of natural gas / crude oil transportation 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 Py value of the weld metal which gives to the weld joint strength of a welded steel pipe. It is a figure explaining the hardness distribution of the weld metal of a welded steel pipe. It is a figure explaining the influence which it has on the joint strength of the area ratio of the inner surface weld metal and outer surface weld metal in the welding part of a welded steel pipe, and the area ratio. It is a figure explaining the hardness measurement position of a welded steel pipe base material part, 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.

  In the following, the invention will be described with respect to reasons for limiting the respective constituent requirements of the present invention.

1. Steel Pipe Base Material 1.1 Chemical Components of Steel Pipe Base Material First, the reasons for limiting the chemical components 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: 0.8 to 1.7%
Mn is added to increase the strength of the steel, but if it is less than 0.8%, the effect is not sufficient, and if it exceeds 1.7%, the hardness of the segregated part is particularly increased and the HIC resistance is deteriorated. Therefore, the amount of Mn is made 0.8 to 1.7%. Preferably, it is 1.0 to 1.5%.

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 when the steel pipe base material is exposed to a mild hydrogen sulfide corrosive environment having a pH of about 4.0 to 5.0, dense corrosion is caused. It suppresses the accumulation of hydrogen at the origin of the HIC by forming a product, but the effect is saturated even if added over 0.3%, and diluted to a steel pipe weld metal part described later to be welded metal part Cause hot cracking. Therefore, when adding Cu, the upper limit is made 0.3%.

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: 0.30% 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 HAZ is remarkable, and if added over 0.30%, the SSC resistance in the circumferential welded portion between the steel pipes is deteriorated. Therefore, when adding Mo, it is 0.30% or less.

V: 0.1% or less V is an element that increases the strength. However, if added over 0.1%, 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 deteriorated. Therefore, when V is added, the content is made 0.1% or less.

CP value calculated by Formula (1) is 1.1 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 from the ISO 15156 that is to be solved by the present invention that the HIC can be suppressed by setting the CP value to 1.1 or less in the Region 2 environment. Therefore, the CP value is 1.1 or less.

Py value calculated by formula (2) is 0.175 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 2 environment of ISO 15156 classification to be solved by the present invention, it is necessary to make the Vickers hardness of steel 280 or less, so the HAZ hardness at the circumferential weld is reduced. In order to make it 280 or less, the Py value is made 0.175 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.
1.2 Metal Structure of Steel Pipe Base Material Part In the present invention, from the viewpoint of preventing the occurrence of SSC in the steel pipe base material part, it is particularly preferable to define the metal structure of the steel pipe surface layer part 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: 5% or less Island martensite (hereinafter sometimes simply referred to as “MA”) is a structure generated by accelerated cooling. Hardness increases greatly. As described above, in order to prevent SSC generation in the Region 2 environment from ISO 15156, it is necessary to reduce the Vickers hardness of the steel to 280 or less, and it is important to reduce the hardness of the steel pipe surface layer directly exposed to at least a hydrogen sulfide corrosion environment. is there. As a result of diligent study, the inventors focused on island martensite (MA) in the metal structure of the steel pipe base metal part, and the hardness of the steel pipe surface layer increased with an increase in the volume fraction of MA, and the MA exceeded at least 5%. It was found that the Vickers hardness was over 280 in volume fraction and the SSC resistance deteriorated. Therefore, it is preferable that the volume fraction of island martensite in the steel pipe surface layer portion is 5% or less.

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

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 present invention, unless otherwise specified, the weld metal means both circumferential welding and weld metal (steel pipe weld metal) by butt welding at the time of steel pipe production.

C: 0.04 to 0.10%
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 0.10%, the HAZ hardness of the weld metal is remarkably increased during circumferential welding, and the SSC resistance of the circumferential welded portion is deteriorated, so the upper limit is made 0.10%. In addition, More preferably, it is 0.05 to 0.09%.

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 1.8%
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 set to 1.8%. Therefore, the Mn content is in the range of 1.0 to 1.8%. Preferably it is 1.3 to 1.7%.

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 0.004%
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 0.004%, the effect is saturated, so the upper limit is made 0.004%. More preferably, it is 0.002 to 0.003% of range.

O: 0.025 to 0.045%
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 0.025% or more. On the other hand, if it exceeds 0.045%, 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 0.045%. More preferably, it is 0.030 to 0.040%.

Al: 0.02% or less Al is present in the weld metal as an inevitable impurity due to dilution from the base metal part. However, if it exceeds 0.02%, the above-described formation of TiO or TiO ₂ is inhibited, and the weld metal is removed. Since the effect of increasing the strength due to the refinement of the metal structure of the cura ferrite 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: 0.30% or less Cu acts as a hardenability improving element and can substitute for Mn. However, if it exceeds 0.30%, Cu liquefaction cracks may significantly cause welding defects. Therefore, when adding Cu, the upper limit is made 0.30%.

Ni: 0.50% or less Ni acts as a hardenability improving element and can substitute for Mn. However, it is an expensive element and if it exceeds 0.50%, the effect of increasing the strength is saturated. Therefore, when Ni is added, the upper limit is made 0.50%.

Cr: 0.50% or less Cr also acts as a hardenability improving element and can substitute for Mn. However, even if added over 0.50%, the effect of increasing the strength is saturated. Therefore, when adding Cr, the upper limit is made 0.50%.

Mo: 0.30% 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 Mo is added, the upper limit is made 0.30%.

V: 0.1% 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 0.1%.

In the steel pipe weld metal of the present invention, components other than those described above are Fe and inevitable impurities. However, components other than those described above can be contained as long as the effects of the present invention are not impaired.
2.2 Py value of weld metal and ratio of inner surface weld metal part area to outer surface weld metal part area Furthermore, in the present invention, the strength of the joint of the steel pipe welded part is increased, and it is formed on the weld metal during circumferential welding of the steel pipe In order to achieve both HAZ hardness reduction, the range of the Py value calculated by equation (2) for the weld metal of the steel pipe, and the ratio of the inner surface weld metal part area to the outer surface weld metal part area in the weld joint cross section Stipulate.

2.2.1 Py value (Py) of weld metal: 0.175
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 described above, the weld metal of the steel pipe is hardened under the influence of circumferential welding, and its value is determined by the Py value as shown in FIG. In the present application, the upper limit is set to 0.175 in order to make the upper limit of hardness 280 or less for preventing the occurrence of SSC in the Region 2 environment. In addition, although the minimum of Py value is not prescribed | regulated in particular, 0.165 or more is preferable from a viewpoint of joint strength.
2.3 Ratio of inner weld metal area to outer weld metal area in cross section of welded joint: 2.5 times or more SSC, which is a problem in circumferential welds between steel pipes and steel pipes, is sulfided by the natural gas / crude oil being transported Occurs in a corrosive environment containing hydrogen. That is, it is important to improve the SSC resistance on the inner surface side of the steel pipe and the circumferential welded portion of the steel pipe. Therefore, as described above, the weld metal of the steel pipe welded portion needs to lower Py in order to prevent SSC. On the other hand, the joint strength needs to be equal to or higher than that of the base material, and the inner surface weld with high strength at the same Py value. The joint strength is secured by increasing the proportion of the metal cross-section.

  FIG. 5A schematically shows a method for obtaining the ratio of the inner surface weld metal part area to the outer surface weld metal part area (deposition area ratio) in the weld joint cross section. As described above, the outer surface weld metal weld area S0 and the inner surface weld metal weld area S1 are obtained by approximation as trapezoidal shapes as shown in FIG. w is a length obtained by connecting the left and right meeting points of the inner surface and the outer surface weld metal with a straight line. The weld joint cross section to be measured is obtained in advance for each welding condition, and when the welding condition is the same, the cross section shape of the same weld is calculated. The reason why the surplus is excluded from the measurement object is that the welding area ratio can be sufficiently evaluated by the above approximate expression. It is also possible to obtain and calculate the cross-sectional area of the weld metal on the inner and outer surfaces including the surplus by macro observation of the cross section.

  As a result of the inventors' extensive studies, in order to make the joint tensile strength 600 MPa or more from FIG. 5B, the ratio of the inner surface weld metal part area to the outer surface weld metal part area in the weld joint cross section is 2.5 times or more. And In addition, since submerged arc welding of the steel pipe inner surface has restrictions on welding conditions, the ratio of the inner surface weld metal part area to the outer surface weld metal part area is preferably 3.5 times or less.

2.4 Welding method The steel pipe welding method uses submerged arc welding in terms of excellent welding quality and manufacturing efficiency. The welding of the butt portion when manufacturing the welded steel pipe is performed by submerged arc welding for each of the inner and outer surfaces. The present invention was developed on the premise of this method.
In this case, the welding heat input of the submerged arc welding is preferably 8.0 kJ / mm or less for inner surface welding and 6.0 kJ / mm to 3.0 kJ / mm for outer surface welding. It is preferable from the viewpoint of welding work efficiency that the welding is performed in the order of inner surface welding and then outer surface welding.

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

  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 16 to 31 mm was produced 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 to form a steel pipe welded portion. Chemical component analysis samples were collected from the inner surface weld metal and the outer surface weld metal of the weld, and each chemical component was analyzed. Further, a sample for observing the joint cross section is taken from the welded portion, mirror-polished, etched using a nitrate alcoholic corrosive solution, a macro photograph of the joint is taken, and each outer surface weld metal area S (o) as shown in FIG. The ratio PSw between the inner surface weld metal area and the outer surface weld metal area was calculated from the inner surface weld metal area S (i). Table 2 shows the analysis result of the weld metal component and the value of PSw.

  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 acetate aqueous solution to adjust the pH to 4.0. The mixed gas adjusted to 10% hydrogen sulfide and 90% nitrogen in the pressure ratio was saturated in the immersion liquid adjusted to 1, and immersed for 96 hours. After the immersion, the HIC test piece was cut into three sections at equal intervals, each mirror-polished, and each section was observed with an optical microscope at a magnification of 100 times, and found as described in Section 7 of NACE TM0284-2003. The sample width direction length of the crack was recorded, and the crack length ratio CLR (%) was calculated. Note that when the CLR was 15% or less, the HIC resistance performance in the present invention was considered good.

  Next, as shown in FIG. 4, circumferential welding simulated multilayer welding in which the base metal part and the steel pipe welded part were brought together was carried out by a carbon dioxide arc welding method. The average value of welding heat input was about 10 kJ / cm. Then, after cutting one section of the produced circumferential welded simulated joint, it is 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 welded 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. In addition, a specimen for metallographic observation was taken from the side of the Vickers hardness measurement point 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 simulated part inner surface side, and are shown in FIG. After applying a stress corresponding to 90% of the yield strength to the center of the test piece by 4-point bending using a jig, the pH was adjusted to 4.0 by mixing an acetic acid aqueous solution and an aqueous sodium acetate solution as in the HIC test. A mixed gas adjusted to 10% hydrogen sulfide and 90% nitrogen was saturated in the immersion liquid and immersed in 720 hr. 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 the microstructure investigation on the surface of the base metal part, the tensile test results of the base metal part and the welded part, the hardness measurement result, the HIC test result, and the SSC test result. In the SSC test, ◯ indicates that no cracking occurred, and x indicates the result of cracking.

  In Table 3, No. which is an example of the present invention. 1 to 5 all have the chemical composition of the base metal, the inner surface weld metal, the outer surface weld metal, and the microstructure of the surface of the base material within the scope of the present invention, and both the base material and the weld have high strength of 600 MPa or higher. In addition, the CIC of the HIC was 0.0%, and further, no cracks were generated in the four-point bending SSC test at all the base metal part, the welded part, and the circumferential welded simulated part.

  On the other hand, Comparative Example No. in which the C content of the base material exceeded the upper limit of the present invention. No. 6 shows that the volume fraction of MA exceeds 5% in the microstructure inspection of the surface of the base material, and the hardness of the vicinity becomes very hard at 288. As a result, the base material is cracked in the 4-point bending SSC test. There has occurred. Also, in the HIC test of the base material part, many cracks occurred on the surface side, and the CLR exceeded 15%. Comparative Example No. in which the CP value of the base material exceeded the upper limit of the present invention. In No. 7, many cracks occurred in the central segregation part in the HIC test of the base material part and the weld part, and the CLR exceeded 15%. Comparative example No. in which the Py value of the base material exceeded the upper limit No. 8 shows that the base metal side HAZ hardness of the circumferential welding simulated portion is very hard and cracked in the 4-point bending SSC test of the circumferential welding simulated portion (shown as “circumferential welded portion” in Table 3). There has occurred.

  Comparative example No. in which the C amount of the weld metal exceeded the upper limit of the present invention. In No. 9, the weld metal side HAZ hardness of the SAW welded and circumferential weld simulation part was very hard, and cracks occurred in the 4-point bending SSC test of both the pipe weld part and the circumferential weld simulation part. Further, the comparative example No. in which the Mn amount of the weld metal exceeded the upper limit of the present invention. In No. 10, the weld metal side HAZ hardness of the circumferential weld simulation portion was very hard, and cracks occurred in the 4-point bending SSC test of the circumferential weld simulation portion.

  Similarly, comparative example No. in which the Py value of the weld metal exceeded the upper limit of the present invention. No. 11, the weld metal side HAZ hardness of the circumferential weld simulation part was very hard, and cracks occurred in the 4-point bending SSC test of the circumferential weld simulation part.

  Comparative Example No. in which the ratio of the inner surface weld metal area and the outer surface weld metal area in the joint cross section was below the lower limit of the present invention. No. 12 had a joint strength of less than 600 MPa.

  According to the present invention, not only in the HIC resistance and SSC resistance of the pipe base material in the natural gas and crude oil transportation pipeline where a relatively mild hydrogen sulfide corrosion environment is assumed, but also in the circumferential welded portion connecting the pipes. In addition, it is possible to provide a high-strength steel pipe having excellent SSC resistance and to improve the efficiency of natural gas / crude oil transportation by high-pressure operation.

Claims (2)

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: 0.8 to 1.7%,
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: 0.30% or less,
V: contains one or more selected from 0.1% or less,
The CP value represented by the following formula (1) is 1.1 or less,
The Py value represented by the following formula (2) is 0.175 or less,
The balance consists of Fe and inevitable impurities,
And the said weld metal is the mass%,
C: 0.04 to 0.10%
Si: 0.05-0.5%
Mn: 1.0 to 1.8%
Nb: 0.01 to 0.05%
Ti: 0.01-0.04%
B: 0.001 to 0.004%
O: 0.025 to 0.045%
Al: containing 0.02% or less,
Furthermore, Cu: 0.30% or less Ni: 0.50% or less Cr: 0.50% or less Mo: 0.30% or less V: containing at least one selected from 0.1% or less,
The Py value of the formula (2) consisting of the balance Fe and inevitable impurities and calculated by the chemical composition of the weld metal is 0.175 or less,
In the welded section, the inner surface weld metal part area excluding the surplus part is 2.5 times or more the outer surface weld metal part area, and has a tensile strength of 600 MPa or more excellent in resistance to sulfide stress corrosion cracking. High strength welded steel pipe.
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 5% 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.