WO2013161089A1

WO2013161089A1 - Cr-CONTAINING STEEL PIPE FOR LINEPIPE EXCELLENT IN INTERGRANULAR STRESS CORROSION CRACKING RESISTANCE OF WELDED HEAT AFFECTED ZONE

Info

Publication number: WO2013161089A1
Application number: PCT/JP2012/061699
Authority: WO
Inventors: 宮田　由紀夫; 木村　光男
Original assignee: Ｊｆｅスチール株式会社
Priority date: 2012-04-26
Filing date: 2012-04-26
Publication date: 2013-10-31
Also published as: BR112014025818B1; EP2843068A1; EP2843068B1; CN104254625A; EP2843068A4

Abstract

A Cr-containing steel pipe for a linepipe, exhibiting a high strength of X65 to X80 grade, excellent toughness and corrosion resistance and excellent resistance to sulfide stress corrosion cracking and ensuring excellent resistance of a welded heat affected zone to intergranular stress corrosion cracking. Specifically, this steel pipe has a composition which contains, in mass%, 0.001 to 0.015% of C, 0.05 to 0.50% of Si, 0.10 to 2.0% of Mn, 0.001 to 0.10% of Al, 13 to less than 15% of Cr, 2.0 to 5.0% of Ni, 1.5 to 3.5% of Mo, 0.001 to 0.20% of V, and up to 0.015% of N so as to satisfy the relationships: P₁ = 11.5 to 13.3 and P₂ = (0.5Cr+5.0) - P₁ ≥ 0. When the steel pipe is subjected to a welding process which comprises heating to a ferrite single-phase temperature region of 1300°C or higher and then cooling, the resulting welded heat affected zone has a structure such that: martensite accounts for at least 50% of the prior ferrite grain boundary in terms of the ratio to the overall length of the grain boundary; and the formation of Cr carbide depleted zones is minimized. Therefore, the welded heat affected zone exhibits remarkably improved resistance to intergranular stress corrosion cracking. The present invention can dispense with post weld heat treatment and thus has an effect of remarkably shortening the construction period of a welded steel pipe structure.

Description

Cr-containing steel pipe for line pipes with excellent intergranular stress corrosion cracking resistance in weld heat affected zone

The present invention is suitable as a steel pipe for line pipe used in a pipeline for transporting crude oil or natural gas produced in an oil well or a gas well. It is related to Cr-containing steel pipes (Cr-containing steel pipes), and in particular, it is related to the resistance to intergranular stress corrosion cracking cracking cracking resistance of the welded heat affected zone (welded heat affected zone).

In recent years, deep oil fields with a deep depth that has not been excluded in the past from the viewpoint of oil crude oil price increases and oil resources expected in the near future. Development of deep layer oil wells) and gas fields, or oil fields and gas fields that have been abandoned, are highly corrosive. Such oil fields and gas fields are generally deep and the atmosphere is high in temperature, including carbon dioxide gas (CO ₂ ), chlorine ion (Cl ⁻ ), ^{and the} like, resulting in a severe corrosive environment. Yes. In addition, the development of oil fields and gas fields with severe drilling environments such as the bottom of the ocean is also active. Pipelines for transporting crude oil and natural gas produced in such oil and gas fields have high strength, high toughness, excellent corrosion resistance, and laying of pipelines From the viewpoint of reducing the cost of laying, it is required to use a steel pipe that also has excellent weldability.

In response to such a request, for example, Patent Document 1 discloses an intergranular stress corrosion crack (intergranular stress cracking) that occurs in a weld heat affected zone without performing post-weld heat treatment suitable for a line pipe. There is described a martensitic stainless steel pipe that can prevent corrosion cracking (abbreviated as IGSCC) and is excellent in intergranular stress corrosion cracking resistance of a weld heat-affected zone. The martensitic stainless steel pipe described in Patent Document 1 is mass%, C: less than 0.0100%, N: less than 0.0100%, Cr: 10-14%, Ni: 3-8%, Si: 0.05 to 1.0%, Mn: 0.1 to 2.0%, P: 0.03% or less, S: 0.010% or less, Al: 0.001 to 0.10%, and One or more selected from Cu: 4% or less, Co: 4% or less, Mo: 4% or less, W: 4% or less, and Ti: 0.15% or less, Nb: 0.10 % Or less, V: 0.10% or less, Zr: 0.10% or less, Hf: 0.20% or less, Ta: 0.20% or less. It has a composition containing so as to satisfy less than 0.0050%. In the technique described in Patent Document 1, Csol, which is an effective content of dissolved carbon (Csol) that effectively acts on the formation of Cr carbide, is less than 0.0050%. Preventing formation of Cr carbide in prior austenite grain boundaries, preventing formation of Cr-depleted layers (Cr depleted zones) that are the cause of intergranular stress corrosion cracking in the weld heat affected zone, It is said that intergranular stress corrosion cracking that occurs in the weld heat affected zone can be suppressed without performing post-weld heat treatment.
Patent Document 3 contains Cr for line pipes with high strength of X65 to X80, excellent toughness, corrosion resistance, and sulfide stress corrosion cracking resistance, and excellent resistance to intergranular stress corrosion cracking in the weld heat affected zone. Steel pipes are described. The Cr-containing steel pipe for line pipe described in Patent Document 3 is mass%, C: 0.001 to 0.015%, Si: 0.05 to 0.50%, Mn: 0.10 to 2.0. %, Al: 0.001 to 0.10%, Cr: 15.0 to 18.0%, Ni: 2.0 to 6.0%, Mo: 1.5 to 3.5%, V: 0.00. 001 to 0.20%, N: 0.015% or less so that Cr + Mo + 0.4W + 0.3Si-43.5C-0.4Mn-Ni-0.3Cu-9N: 11.5 to 13.3 It is set as the composition containing. As a result, the welding heat-affected zone heated to 1300 ° C. or more in the ferrite single-phase temperature range at the time of welding and cooled is a ratio to the total length, and 50% or more of the old ferrite grain boundaries are martensite phase and / or austenite phase. It becomes an occupied structure, the formation of a Cr carbide-deficient layer is suppressed, and a steel pipe with markedly improved intergranular stress corrosion cracking resistance of the weld heat affected zone is obtained. There is no need to perform heat treatment after welding, and the construction period of the welded steel pipe structure can be greatly shortened.

Patent Document 2 describes a high-strength stainless steel pipe for line pipes having excellent corrosion resistance. The high-strength stainless steel pipe described in Patent Document 2 is mass%, C: 0.001 to 0.015%, N: 0.001 to 0.015%, Cr: 15 to 18%, Ni: 0.00. 5% or more and less than 5.5%, Mo: 0.5 to 3.5%, V: 0.02 to 0.2%, Si: 0.01 to 0.5%, Mn: 0.1 to 1 0.8%, P: 0.03% or less, S: 0.005% or less, N: 0.001 to 0.015%, O: 0.006% or less, Cr + 0.65Ni + 0.6Mo + 0.55Cu-20C ≧ 18.5, and Cr + Mo + 0.3Si-43.5C-0.4Mn-Ni-0.3Cu-9N ≧ 11.5 and C + N ≦ 0.025 so as to satisfy simultaneously. In the technique described in Patent Document 2, an appropriate amount of ferrite phase is included, and while maintaining a ferrite-martensite two-phase structure (dual phase structure), the Cr content is adjusted to be as high as 15 to 18%. Therefore, it is excellent in hot workability and low temperature toughness, has sufficient strength for line pipes, and even in a corrosive environment at a high temperature of 200 ° C. containing carbon dioxide and chlorine ions. The steel pipe has excellent corrosion resistance.

JP 2005-336601 A (WO 2005/073419 A1) JP 2005-336599 A Unexamined-Japanese-Patent No. 2011-241477 (WO2011-132765 A1)

However, under severe corrosive environment, there is still a problem that the intergranular stress corrosion cracking generated in the weld heat-affected zone cannot be completely suppressed even by the technique described in Patent Document 1, and heat treatment after welding is performed. In the present situation, the intergranular stress corrosion cracking generated in the weld heat affected zone is prevented. In addition, although the technique described in Patent Document 1 was previously developed by the present inventors, the structure of the steel pipe of Patent Document 1 is a martensitic stainless steel pipe that does not include a ferrite phase in the structure. .
Moreover, the steel pipe manufactured by the technique described in Patent Document 2 is not considered at all with respect to intergranular stress corrosion cracking resistance, and despite the increase in Cr content, intergranular stress corrosion cracking resistance. From this point of view, there is a problem that the intergranular stress corrosion cracking generated in the weld heat-affected zone is not completely suppressed, rather than the steel pipe described in Patent Document 1 having a low Cr content. It was.
Moreover, the steel pipe manufactured by the technique described in Patent Document 3 has a problem that the alloy addition amount is relatively large and the material cost is increased.

The present invention solves the problems of the prior art, and has the desired high strength, toughness, corrosion resistance, resistance to sulfide stress corrosion cracking, and intergranular stress resistance of the weld heat affected zone. It aims at providing the Cr containing steel pipe for line pipes excellent in corrosion cracking property. The steel pipe targeted by the present invention is an X65 to X80 grade steel pipe (steel pipe having a yield strength (YS) of 448-651 MPa). Here, “excellent toughness” means that the absorbed energy E ₋₄₀ (J) at −40 ° C. in the Charpy impact test is 50 J or more. And The term “excellent in corrosion resistance” as used herein means that the corrosion rate (mm / y) in a 200 g / liter NaCl aqueous solution at 150 ° C. saturated with 3.0 MPa of carbon dioxide gas is 0.10 mm / y or less. It shall mean a certain case. Here, the “steel pipe” includes a seamless steel pipe and a welded steel pipe.

In order to achieve the above-mentioned object, the inventors of the present invention have provided a ferrite-martensitic stainless steel pipe with intergranular stress corrosion cracking of a weld heat-affected zone in a high-temperature corrosive environment containing carbon dioxide and chlorine ions. We intensively studied various factors affecting sex.
As a result, in such a ferrite-martensitic stainless steel, intergranular stress corrosion cracking is caused by the formation of coarse ferrite grains during the heating cycle during welding, followed by a cooling cycle. It has been found that Cr carbide precipitates at the grain boundaries of the coarse ferrite grains and a Cr-depleted layer is formed at the grain boundaries. And in this type of steel, the present inventors have found that at least the ferrite (α) → austenite (γ) from the grain boundary before Cr carbide precipitates at the grain boundary of coarse ferrite grains. If transformation can be caused and most grain boundaries can be occupied by austenite, precipitation of Cr carbides at the grain boundaries can be prevented, formation of Cr-deficient layers can be suppressed, and the occurrence of intergranular stress corrosion cracking can be prevented. I came up with it.

As a result of further research, before Cr carbide precipitates at the grain boundaries, the α to γ transformation is caused from the grain boundaries, and in order to prevent the occurrence of intergranular stress corrosion cracking, the composition range is as follows ( 1) _{P 1 = Cr + Mo + 0.4W} + 0.3Si-43.5C-0.4Mn-Ni-0.3Cu-9N ‥‥ (1)
P ₁ defined by the following formula satisfies 11.5 or more and 13.3 or less, and the following formula (2): P ₂ = (0.5Cr + 5.0) −P ₁ (2)
In it found that it is necessary to P _2, which is defined to optimize the composition range so as to satisfy 0 or more.

According to the studies of the present inventors, it is P ₁ is 13.3 or less, and, by a composition such as P ₂ becomes 0 or more, difficult to precipitate carbides (Cr carbides) is the grain boundary, thus It has been newly found that formation of a Cr-deficient layer is difficult to occur and it is possible to prevent intergranular stress corrosion cracking.
Because, and _{P 1} is 13.3 or less as described above, when the proportion of ferrite forming elements is low composition, upon circumferential weld, such as during laying of the pipeline (girth welding), at the time of heating, melting point A coarse ferrite phase single phase structure is formed in a region exposed to a high temperature exceeding 1200 ° C. in the vicinity of (melting point), but during cooling, α → γ transformation occurs, and the γ phase occurs from the grain boundary or within the grain. Occurs. In such a case, since the solubility product of the carbide is larger in the γ phase than in the α phase, the carbide (Cr carbide) is less likely to precipitate at the grain boundary, and therefore the formation of a Cr-deficient layer is less likely to occur. It is possible to prevent field stress corrosion cracking. Needless to say, most or all of the γ phase is transformed into a martensite phase by subsequent cooling.

On the other hand, when the composition of the ferrite-forming element is high such that P ₁ exceeds 13.3, the formed coarse ferrite phase single phase structure undergoes α → γ transformation during subsequent cooling. Since it does not occur and reaches room temperature as it is, Cr carbide precipitates at the grain boundary, a Cr-deficient layer is formed, and intergranular stress corrosion cracking is likely to occur.
If the composition can be adjusted so that P ₁ becomes 13.3 or less and P ₂ becomes 0 or more even if Cr and further Ni are reduced by further study, the above-mentioned is described. We obtained the knowledge that it was possible to ensure the structural change and prevent the intergranular stress corrosion cracking of the weld heat affected zone.

The present invention has been completed based on such findings and further studies. That is, the gist of the present invention is as follows.
(1) In mass%, C: 0.001 to 0.015%, Si: 0.05 to 0.50%, Mn: 0.10 to 2.0%, P: 0.020% or less, S: 0.010% or less, Al: 0.001 to 0.10%, Cr: 13% or more and less than 15%, Ni: 2.0 to 5.0%, Mo: 1.5 to 3.5%, V: 0.001 to 0.20%, N: 0.015% or less, the following formula (1),
P ₁ = Cr + Mo + 0.4W + 0.3Si-43.5C-0.4Mn-Ni-0.3Cu-9N (1)
(Here, Cr, Mo, W, Si, C, Mn, Ni, Cu, N: content of each element (mass%))
P ₁ defined by 11.5 or more and 13.3 or less, and the following equation (2):
P ₂ = (0.5Cr + 5.0) −P ₁ (2)
(Here, Cr: Cr content (mass%))
In comprising as P ₂ defined satisfies 0 or more, having a composition the balance being Fe and unavoidable impurities, it is heated in the ferrite single phase temperature range of not lower than 1300 ° C. during welding, the cooled weld heat affected In the intergranular stress corrosion cracking resistance of the weld heat-affected zone characterized in that the ratio of the former ferrite grain boundary to the total length of the old ferrite grain boundary is a structure in which 50% or more of the old ferrite grain boundary is occupied by the martensite phase. Excellent Cr-containing steel pipe for line pipe.

(2) In (1), in addition to the above-mentioned composition, it is further one of mass%, selected from Cu: 0.01 to 3.5%, W: 0.01 to 3.5%, or 2 A Cr-containing steel pipe for line pipes, characterized by comprising a seed-containing composition.
(3) In (1) or (2), in addition to the above composition, further, in mass%, Ti: 0.01 to 0.20%, Nb: 0.01 to 0.20%, Zr: 0.01 A Cr-containing steel pipe for line pipes, characterized in that the composition contains one or more selected from 0.20%.

(4) In any one of (1) to (3), in addition to the above-mentioned composition, further, in mass%, Ca: 0.0005 to 0.0100%, REM: 0.0005 to 0.0100% A Cr-containing steel pipe for line pipes, characterized in that the composition contains one or two selected ones.

According to the present invention, a Cr-containing steel pipe for a line pipe excellent in resistance to intergranular stress corrosion cracking in a weld heat-affected zone can be produced at low cost without performing post-weld heat treatment, and a remarkable industrial effect can be achieved. Moreover, according to this invention, steel pipe structures, such as a pipeline, can be constructed without performing post-weld heat treatment, and there is also an effect that construction costs can be significantly reduced, such as shortening the construction period.

It is explanatory drawing which shows typically the welding reproduction thermal cycle (simulated welded thermal cycle) used in the Example. It is explanatory drawing which shows typically the bending condition of the test piece for U bending stress corrosion cracking tests (test special for U-bend test) used in the Example.

First, the reasons for limiting the composition of the steel pipe of the present invention will be described. Hereinafter, unless otherwise specified, mass% is simply expressed as%.
C: 0.001 to 0.015%
C is an element that contributes to an increase in strength. In the present invention, it is necessary to contain 0.001% or more.
On the other hand, if the content exceeds 0.015%, the toughness of the weld heat affected zone is deteriorated. If it is contained in a large amount, it becomes difficult to prevent intergranular stress corrosion cracking in the weld heat affected zone. For this reason, C is limited to the range of 0.001 to 0.015%. Preferably, the content is 0.002 to 0.010%.

Si: 0.05 to 0.50%
Si is an element that acts as a deoxidizing agent and increases the strength by solid solution. In the present invention, it is necessary to contain 0.05% or more. However, a large content exceeding 0.50% lowers the toughness of the base metal and the weld heat affected zone. For this reason, Si was limited to the range of 0.05 to 0.50%. Note that the content is preferably 0.10 to 0.40%.

Mn: 0.10 to 2.0%
Mn is a solid solution that contributes to increasing the strength of the steel and is an austenite-generating element that suppresses the formation of ferrite and improves the toughness of the base material and the weld heat affected zone. Such an effect requires the content of 0.10% or more, but even if the content exceeds 2.0%, the effect is saturated and an effect commensurate with the content cannot be expected. For this reason, Mn was limited to the range of 0.10 to 2.0%. The content is preferably 0.20 to 1.5%.

P: 0.020% or less P is an element that deteriorates corrosion resistance such as carbon dioxide corrosion resistance (CO ₂ corrosion resistance) and sulfide stress corrosion cracking resistance, and can be reduced as much as possible in the present invention. Although desirable, extreme reduction results in increased manufacturing costs. P is limited to 0.020% or less as a range that can be industrially implemented at a relatively low cost and does not deteriorate the corrosion resistance. In addition, Preferably it is 0.015% or less.

S: 0.010% or less S is an element that significantly deteriorates the hot workability in the pipe manufacturing process, and is preferably as small as possible, but if it is reduced to 0.010% or less, the pipe in the normal process Since production is possible, S is limited to 0.010% or less. In addition, Preferably it is 0.004% or less.
Al: 0.001 to 0.10%
Al is an element having a strong deoxidizing action, and in order to obtain such an effect, it is necessary to contain 0.001% or more. However, if it exceeds 0.10%, the toughness is adversely affected. Effect. For this reason, Al was limited to 0.10% or less. In addition, Preferably it is 0.05% or less.

Cr: 13% or more and less than 15% Cr is an element that forms a protective surface film and improves corrosion resistance such as carbon dioxide corrosion resistance and sulfide stress corrosion cracking resistance. In the present invention, the content of 13% or more is required for the purpose of improving the corrosion resistance in a severe corrosive environment. On the other hand, when the content is excessively 15% or more, in order to adjust the P ₁ value to a predetermined range, it is necessary to contain a large amount of other alloy elements such as Ni, resulting in an increase in material cost. For this reason, Cr was limited to the range of 13% or more and less than 15%. More preferably, it is more than 14% and less than 15%.

Ni: 2.0-5.0%
Ni is an element that has a function of strengthening the protective coating, enhances corrosion resistance such as carbon dioxide corrosion resistance and sulfide stress corrosion cracking resistance, and contributes to an increase in strength. In order to obtain such an effect, the content of 2.0% or more is required. However, if the content exceeds 5.0%, the hot workability tends to decrease and the material cost increases. . For this reason, Ni was limited to the range of 2.0 to 5.0%. Preferably, the content is 2.5 to 5.0%.

Mo: 1.5-3.5%
Mo is an element that has an effect of increasing resistance to pitting corrosion caused by Cl ⁻ (chlorine ions) and effectively acts to improve corrosion resistance. In order to acquire such an effect, it is necessary to contain 1.5% or more. On the other hand, if the content exceeds 3.5%, the hot workability is lowered and the production cost is increased. For this reason, Mo was limited to the range of 1.5 to 3.5%. Note that the content is preferably 1.8 to 3.0%.

V: 0.001 to 0.20%
V is an element that contributes to an increase in strength and has an effect of improving stress corrosion cracking resistance. Such an effect becomes remarkable when the content is 0.001% or more, but the content exceeding 0.20% lowers the toughness. For this reason, V is limited to a range of 0.001 to 0.20%. Note that the content is preferably 0.010 to 0.10%.

N: 0.015% or less N is an element that has an effect of improving pitting corrosion resistance but significantly reduces weldability. In the present invention, N can be reduced as much as possible. Although desirable, extreme reduction results in increased manufacturing costs. The upper limit was set to 0.015% as a range that can be industrially implemented at a relatively low cost and does not deteriorate the weldability.

The above-mentioned components are basic components, but in addition to the basic composition, Cu: 0.01 to 3.5%, W: 0.01 to 3.5% 1 is selected as a selective element. One or two species and / or one selected from Ti: 0.01 to 0.20%, Nb: 0.01 to 0.20%, Zr: 0.01 to 0.20% Two or more kinds and / or one or two kinds selected from Ca: 0.0005 to 0.0100% and REM: 0.0005 to 0.0100% can be selected and contained as necessary. .

One or two selected from Cu: 0.01 to 3.5%, W: 0.01 to 3.5% Cu and W are both elements that improve the corrosion resistance of carbon dioxide gas. , And can be selected and contained as necessary.
Cu is an element that further contributes to an increase in strength. In order to obtain such an effect, it is desirable to contain 0.01% or more, but even if it exceeds 3.5%, the effect is saturated, and an effect commensurate with the content cannot be expected, and it is economical. Disadvantageous. For this reason, when it contains, it is preferable to limit Cu to 0.01 to 3.5% of range. More preferably, it is 0.30 to 2.0%.

W is an element that further improves stress corrosion cracking resistance, sulfide stress corrosion cracking resistance, and pitting corrosion resistance. In order to obtain such an effect, it is desirable to contain 0.01% or more, but even if it exceeds 3.5%, the effect is saturated, and an effect commensurate with the content cannot be expected, and it is economical. Disadvantageous. For this reason, when contained, W is preferably limited to a range of 0.01 to 3.5%. More preferably, it is 0.30 to 2.0%.

One or more selected from Ti: 0.01-0.20%, Nb: 0.01-0.20%, Zr: 0.01-0.20% Ti, Nb, Zr are Both are elements that have a strong tendency to form carbides compared to Cr, have the effect of suppressing the precipitation of Cr carbides at grain boundaries during cooling, and can be selected as needed to contain one or more. . In order to obtain such an effect, it is desirable to contain Ti: 0.01% or more, Nb: 0.01% or more, Zr: 0.01% or more, respectively, Ti: 0.20%, If Nb: 0.20% and Zr: 0.20% are contained in excess, weldability and toughness are deteriorated. For this reason, when it contains, it is respectively limited to the range of Ti: 0.01-0.20%, Nb: 0.01-0.20%, Zr: 0.01-0.20%. preferable. More preferably, Ti is 0.020 to 0.10%, Nb is 0.020 to 0.10%, and Zr is 0.020 to 0.10%.

One or two types selected from Ca: 0.0005 to 0.0100%, REM: 0.0005 to 0.0100% Ca and REM are both hot through the control of the form of inclusions. It is an element that improves processability and production stability during continuous casting, and can be selected and contained as necessary. In order to obtain such an effect, it is desirable to contain Ca: 0.0005% or more and REM: 0.0005% or more, respectively, but Ca: 0.0100% and REM: more than 0.0100%, respectively. Inclusion increases the amount of inclusions and lowers the cleanliness of the steel. For this reason, when it contains, it is preferable to limit to Ca: 0.0005-0.0100% and REM: 0.0005-0.0100%, respectively. More preferably, Ca: 0.0010 to 0.0050%, REM: 0.0010 to 0.0050%.

In the present invention, within the range of the components described above, the following formula (1) P ₁ = Cr + Mo + 0.4W + 0.3Si-43.5C-0.4Mn-Ni-0.3Cu-9N (1)
(Here, Cr, Mo, W, Si, C, Mn, Ni, Cu, N: content of each element (mass%))
P ₁ defined by the following formula is 11.5 or more and 13.3 or less, and the following formula (2): P ₂ = (0.5Cr + 5.0) −P ₁ (2)
(Here, Cr: Cr content (mass%))
In such P ₂ defined satisfies 0 or more, to adjust the content of each component.

P ₁ is an index for evaluating hot workability, and further resistance to intergranular stress corrosion cracking, and in the present invention, each element of P ₁ satisfies the range of 11.5 to 13.3. The content is adjusted within the above range. P ₁ is less than 11.5, the hot workability is insufficient, can not secure the necessary sufficient hot workability in the production of seamless steel pipe, it is difficult to manufacture a seamless steel pipe. On the other hand, when P ₁ exceeds 13.3, as described above, the intergranular stress corrosion cracking resistance decreases. Similarly, P ₂ is intergranular stress corrosion cracking resistance is lowered below the 0. For this reason, the content of each element is adjusted so as to satisfy the above-described range and satisfy P ₁ : 11.5 to 13.3 and P ₂ : 0 or more.

The balance other than the components described above consists of Fe and inevitable impurities. As an inevitable impurity, O: 0.010% or less is acceptable.
The steel pipe of the present invention has a structure having the above-described composition, further comprising a martensite phase as a base phase, a ferrite phase having a volume ratio of 10 to 35%, and an austenite phase having a volume ratio of 30% or less. Have. Note that the martensite phase includes a tempered martensite phase (tempered martensite phase). The martensite phase is preferably contained in a volume ratio of 40% or more in order to secure a desired strength. Further, the ferrite phase is a soft structure that improves workability, and is preferably contained in a volume ratio of 10% or more from the viewpoint of improving workability. On the other hand, if it exceeds 35%, the desired high strength (X65) cannot be secured. The austenite phase is a structure that improves toughness. From the viewpoint of securing toughness, 15% or more is preferable. However, when the austenite phase exceeds 30%, it is difficult to ensure the strength.

Note that the austenite phase does not completely transform into a martensite phase during the quenching process, and a part of the austenite phase remains, and a part of the martensite phase and the ferrite phase during the tempering process reversely transforms and stabilizes. After cooling, it may remain as an austenite phase.
In the steel pipe of the present invention having the above-described composition and the above-described structure, when the welded portion is formed, the weld heat-affected zone that is heated and cooled to a ferrite single-phase temperature range of 1300 ° C. or higher at the time of welding, The ratio of the former ferrite grain boundaries to the total length of the prior ferrite grain boundaries is a structure in which 50% or more of the old ferrite grain boundaries are occupied by the martensite phase. Thereby, precipitation of Cr carbide can be avoided at the grain boundaries of coarse old ferrite grains, the occurrence of intergranular stress corrosion cracking is suppressed, and the intergranular stress corrosion cracking resistance of the weld heat affected zone is improved.

Next, a preferred method for producing the steel pipe of the present invention will be described using a seamless steel pipe as an example.
First, molten steel having the above composition is melted by a conventional melting method such as a converter, an electric furnace, a vacuum melting furnace, and the like, and a continuous casting method. It is preferable to use a steel material such as a billet by a conventional method such as (continuous casting method) or a slabbing-mill method for rolling an ingot. Subsequently, these steel materials are heated and hot-rolled using a normal Mannesmann-plug mill method or Mannesmann-mandrel mill method manufacturing process. The pipe is made into a seamless steel pipe having a desired size. The seamless steel pipe after pipe forming is subjected to accelerated cooling (air-cooling rate) or higher, preferably cooled to room temperature at an average cooling rate of 0.5 ° C./s or higher at 800 to 500 ° C. It is preferable to apply. Thereby, if it is a steel pipe which has a composition within the composition range of this invention, it can be set as the structure | tissue based on a martensite phase as mentioned above. When the cooling rate is less than 0.5 ° C./s, it becomes impossible to obtain a structure based on the martensite phase as described above. Here, the structure based on the martensite phase is a structure in which the martensite phase has the largest volume ratio or has a volume ratio substantially equal to the volume ratio of another structure having the largest volume ratio. Means that

In addition, it replaces with the above-mentioned accelerated cooling after rolling, reheating, quenching (tempering) and tempering (tempering) may be performed. As the quenching treatment, it is reheated to 800 ° C. or higher, held at that temperature for 10 minutes or more, and then cooled to 100 ° C. or lower at a cooling rate of 0.5 ° C./s or higher on average by air cooling or 800 to 500 ° C. It is preferable to set it as a treatment. When the reheating temperature is less than 800 ° C., it becomes impossible to secure a structure based on a desired martensite phase.

As the tempering treatment, after the quenching treatment, heating to a temperature of 500 ° C. or more and 700 ° C. or less, preferably 500 ° C. or more and 680 ° C. or less, holding for a predetermined time, and air cooling is preferable. Thereby, desired high strength, desired high toughness, and desired excellent corrosion resistance can be combined.
So far, the seamless steel pipe has been described as an example, but the present invention is not limited to this. By using a steel pipe material (steel plate) having the above-described composition, an electric-welded steel pipe and a UOE steel pipe can be manufactured by a normal process to obtain a steel pipe for a line pipe. In addition, it is preferable to perform the above-mentioned quenching-tempering process also about an electric resistance steel pipe and a UOE steel pipe, and to make it the steel pipe which has the above-mentioned structure.

Also, the above-described steel pipe of the present invention can be welded to form a welded structure (steel pipe structure). In addition, the welding joining of this invention steel pipe shall include the case where welding joining of this invention steel pipe and another kind of steel pipe is included. In these welded structures formed by welding the steel pipe of the present invention, the welded heat-affected zone heated to a ferrite single-phase temperature range of 1300 ° C. or higher and preferably cooled at the time of welding is compared with the total length of the old ferrite grain boundary. In terms of the ratio, 50% or more of the prior ferrite grain boundaries have welds that have a structure occupied by the martensite phase and / or austenite phase. Thereby, intergranular stress corrosion cracking is suppressed, and the intergranular stress corrosion cracking resistance of the weld heat affected zone is improved without performing post-weld heat treatment.

Hereinafter, the present invention will be further described based on examples.

Molten steel having the composition shown in Table 1 was melted and degassed in a vacuum melting furnace, cast into a 100 kgf steel ingot, and made into a steel pipe material of a predetermined size by hot forging. These steel pipe materials are heated and then piped by hot working using a model seamless mill (small seamless mill for experiments) to produce a seamless steel pipe (outer diameter 72 mmφ x wall thickness 5.5 mm). ).
About the obtained seamless steel pipe, the presence or absence of the crack generation | occurrence | production of the inner and outer surface was examined visually with the cooling after pipe forming, and hot workability was evaluated. In addition, when the crack of length 5mm or more was recognized in the pipe | tube longitudinal direction end surface, it was set as "with a crack: x" and other than it was set as "without a crack: (circle)".

Next, a test material (steel pipe) was collected from the obtained seamless steel pipe, and the test material (steel pipe) was quenched and tempered under the conditions shown in Table 2.
A test piece is taken from a test material (steel pipe) that has been subjected to quenching treatment and tempering treatment, and the structure is observed (microstructure observation), tensile test (impact test), impact test (corrosion test), and corrosion test (corrosion test). A sulfide stress corrosion cracking test and a U bending stress corrosion cracking test were conducted. The test method was as follows.
(1) Structure observation A test piece for structure observation was collected from the obtained test material (steel pipe). After polishing and corroding the specimen for tissue observation, it was observed using an optical microscope (magnification ratio: 1000 times), imaged, the tissue was identified, and an image analyzer was used. Utilizing this, the structure fraction of each phase in the base metal was determined. In addition, the amount of γ was measured using an X-ray diffraction method (X-ray diffraction method).
(2) Tensile test From the obtained test material (steel pipe), an API arc-shaped tensile test piece in ten API standards is taken so that the pipe axis direction is the tensile direction. Tensile tests were conducted to determine tensile properties (yield strength YS, tensile strength TS) and to evaluate the base material strength.
(3) Impact test A V-notch test piece (5.0 mm thick) was sampled from the obtained test material (steel pipe) in accordance with JIS Z 2242, and a Charpy impact test was conducted. The absorption energy vE- ₄₀ (J / cm < ² >) at _-40 degreeC was calculated | required, and the base material toughness was evaluated.
(4) Corrosion test From the obtained test material (steel pipe), a corrosion test piece with a thickness of 3 mm x width 25 mm x length 50 mm was sampled by machining, and the corrosion test was carried out. And pitting corrosion resistance). In the corrosion test, a 200 g / liter NaCl aqueous solution at 150 ° C. saturated with 3.0 MPa of carbon dioxide gas was held in an autoclave, and a corrosion test piece was immersed in the aqueous solution and held for 30 days. After completion of the corrosion test, the weight of the test piece was measured, the corrosion rate was calculated from the weight change (weight loss) before and after the corrosion test, and the CO ₂ corrosion resistance was evaluated. Moreover, after the corrosion test, the presence or absence of pitting corrosion on the surface of the test piece was observed using a 10-fold loupe. When pitting corrosion occurred, it was evaluated as x, and when it did not occur, it was evaluated as o.
(5) Sulfide stress corrosion cracking (SSC) test From the obtained test material (steel pipe), a four-point bending test piece (size: thickness 4 mm x width 15 mm x length 115 mm) EFC (European Federation of Corrosion) No. A four-point bending test based on No. 17 was conducted to evaluate sulfide stress corrosion cracking resistance (SSC resistance). The test solution used was 50 g / liter NaCl + NaHCO ₃ solution (pH: 4.5), and the test was conducted while flowing a 1 vol% H ₂ S + 99 vol% CO ₂ mixed solution to investigate the presence or absence of breakage. The applied stress was YS (yield strength) of the base material, and the test period was 720 hour (hereinafter abbreviated as h). The broken piece was evaluated as x, and the broken piece was evaluated as o.
(6) U bending stress corrosion cracking test From the obtained test material (steel pipe), a specimen material of size: thickness 4 mm x width 15 mm x length 115 mm was sampled, and the conditions shown in FIG. The welding heat cycle was applied. In addition, the test piece for structure | tissue observation was extract | collected from the test piece after the welding heat cycle provision of the conditions shown in FIG. 1, and it grind | polished and corroded and observed the structure | tissue after welding heat cycle provision. Investigate the presence of transformation products (martensite phase and / or austenite phase) from the former α grain boundary, and the former α grains in which the old α grain boundary is occupied by the transformation product (martensite phase and / or austenite phase) The length of the boundary was measured, and the occupation ratio with respect to the total length of the old α grain boundary was calculated.

Further, a test piece having a thickness of 2 mm, a width of 15 mm, and a length of 75 mm was cut out from the center portion of the obtained test piece material having been subjected to welding heat cycle, and U-bending stress corrosion cracking was performed using the jig shown in FIG. The test was conducted. The U bending stress corrosion cracking test was a test in which a test piece was bent into a U shape with an inner radius of 8.0 mm and immersed in a corrosive solution using the jig shown in FIG. The following two types of corrosive liquids were used.
(1) 50 g / liter NaCl solution having a liquid temperature of 100 ° C., a CO ₂ pressure of 0.1 MPa, and a pH of 2.0.
(2) Liquid temperature: 150 ° C., CO ₂ pressure: 0.1 MPa, pH: 2.0 200 g / liter NaCl solution.
The test period was 168h.

After the test, the cross section of the test piece was observed with a 100 × optical microscope, the presence or absence of cracking was investigated, and the intergranular stress corrosion cracking resistance (welding heat affected zone IGSCC resistance) of the weld heat affected zone was evaluated. The case where there was a crack was rated as x, and the case where there was no crack was marked as ○.
The obtained results are shown in Table 3.

Each of the inventive examples (tube Nos. 1 to 19) is excellent in hot workability, high strength of YS: 450 or more, high toughness of vE- ₄₀ : 50 J / cm ² or more, and corrosion rate; High corrosion resistance of 10 mm / y or less, no occurrence of sulfide stress corrosion cracking, no occurrence of intergranular stress corrosion cracking in the welding heat affected zone heated to 1300 ° C. or higher, The steel pipe has excellent intergranular stress corrosion cracking resistance.
Comparative examples (tube Nos. 20 to 30) outside the scope of the present invention have reduced hot workability, reduced toughness, reduced corrosion resistance, or reduced resistance to sulfide stress cracking. Or the IGSCC resistance of the weld heat affected zone is reduced.
Specifically, tube No. In Nos. 20 to 23, P2 is out of the range of the present invention, so the intergranular stress corrosion cracking resistance of the weld heat affected zone is lowered.
Tube No. In Nos. 24 and 25, P1 is out of the scope of the present invention, so the hot workability is reduced.
Tube No. In No. 26, since the range of C exceeds the upper limit of the present invention, the toughness is lowered.
Tube No. 28 to 30 correspond to F steel, K steel, and M steel in the examples of Patent Document 1, respectively, but the Cr range is less than the lower limit of the present invention, and the Ni range is the upper limit of the present invention. In addition, since P1 is less than the lower limit of the present invention, the ferrite phase has a structure fraction of 0%, and the intergranular stress corrosion cracking resistance of the weld heat-affected zone has a more severe corrosion solution (2). In this case, the intergranular stress corrosion cracking resistance of the weld heat affected zone is lowered.

Claims

mass%,
C: 0.001 to 0.015%, Si: 0.05 to 0.50%,
Mn: 0.10 to 2.0%, P: 0.020% or less,
S: 0.010% or less, Al: 0.001 to 0.10%,
Cr: 13% or more and less than 15%, Ni: 2.0 to 5.0%,
Mo: 1.5 to 3.5%, V: 0.001 to 0.20%,
N: include 0.015% or less, as described below (1) P 1 is 11.5-defined by the equation 13.3, P 2 is defined by the following equation (2) satisfies 0 or more, The composition comprising the remaining Fe and inevitable impurities, heated to a ferrite single-phase temperature range of 1300 ° C. or higher during welding, and the cooled weld heat-affected zone is the ratio of the old ferrite grains to the total length of the old ferrite grain boundaries. Cr-containing steel pipe for line pipes in which 50% or more of the boundary is a structure occupied by the martensite phase.
P 1 = Cr + Mo + 0.4W + 0.3Si-43.5C-0.4Mn-Ni-0.3Cu-9N (1)
P 2 = (0.5Cr + 5.0) −P 1 (2)
Here, Cr, Mo, W, Si, C, Mn, Ni, Cu, N: Content of each element (mass%)
In addition to the above composition, the composition further comprises one or two selected by mass% from Cu: 0.01 to 3.5% and W: 0.01 to 3.5%. Item 2. A Cr-containing steel pipe for line pipes according to Item 1.
In addition to the above-mentioned composition, the mass is 1% selected from Ti: 0.01 to 0.20%, Nb: 0.01 to 0.20%, and Zr: 0.01 to 0.20%. The Cr-containing steel pipe for line pipes according to claim 1 or 2, wherein the composition contains seeds or two or more kinds.
In addition to the above composition, the composition further comprises one or two kinds selected from Ca: 0.0005 to 0.0100% and REM: 0.0005 to 0.0100% in mass%. Item 4. A Cr-containing steel pipe for a line pipe according to any one of Items 1 to 3.