JP2558403B2 - Line pipe with excellent corrosion resistance and weldability - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a line pipe having excellent weldability and corrosion resistance.
As a line pipe used for the transportation of natural gas, it has excellent corrosion resistance in an environment containing wet carbon dioxide and a trace amount of wet hydrogen sulfide, and also has excellent weldability such as the circumferential welds at the site where the line pipe is laid. Regarding steel pipes.

[0002]

2. Description of the Related Art In recent years, petroleum and natural gas produced have increased containing moist carbon dioxide and hydrogen sulfide. It is a well-known fact that carbon steel and low alloy steel are significantly corroded in such an environment. Therefore, when transporting such corrosive oil and natural gas, addition of a corrosion inhibitor has conventionally been common as a measure for preventing corrosion of line pipes.

[0003] However, the use of corrosion inhibitors in marine line pipes requires enormous costs for adding and recovering the corrosion inhibitors, and the use of such inhibitors is becoming difficult due to the problem of marine pollution. Therefore, the need for corrosion resistant materials that do not require the addition of corrosion inhibitors has increased recently.

As a corrosion-resistant material for petroleum and natural gas containing a large amount of carbon dioxide, the use of stainless steel has been studied first. J. Klein, Corrosion
Osion) '84, paper number 211, AISI4 as a high strength and relatively low cost steel
About 0.2% C and 12-1 as represented by 20 steel
Martensitic stainless steel containing about 3% Cr is widely used. However, a relatively large amount of carbon is added to these steels to obtain strength.

In the case of line pipes, pipes are usually connected by welding when laying on-site, but when such martensitic stainless steel containing a relatively large amount of carbon is welded by a normal welding method, The hardness of the heat affected zone is significantly increased and the impact toughness is deteriorated.
Further, when hydrogen sulfide is contained in the transport fluid, the increase in hardness of the weld heat affected zone increases the risk of sulfide stress cracking, resulting in a decrease in the safety of the line pipe.

Although it is possible to reduce the hardness of the heat affected zone and improve the toughness to some extent by performing post-weld heat treatment of heating to, for example, 600 ° C. or more after welding, such heat treatment is performed at the line pipe laying site. Doing so is substantially difficult in terms of temperature control and quality assurance, and requires enormous costs. For this reason, there is a demand for a line pipe which has a small increase in hardness of the weld heat affected zone even when welded by a normal welding method and has excellent low temperature impact toughness including the weld heat affected zone.

On the other hand, if the carbon content of the martensitic stainless steel is reduced, the increase in hardness of the weld heat affected zone can be reduced, but the microstructure of the weld heat affected zone becomes coarse ferrite. Impact toughness is significantly reduced. On the other hand, the present inventors have already proposed line pipe steel excellent in weldability in Japanese Patent Application No. 3-28960, but the suppression of the hardness of the welded portion is not always sufficient.

[0008]

SUMMARY OF THE INVENTION In view of these circumstances, it is an object of the present invention to provide a line pipe having excellent corrosion resistance and weldability.

[0009]

As a result of various studies to achieve the above object, the present inventors have found that at least the surface in contact with a corrosive transport fluid is made of low-carbon high-Cr steel having excellent corrosion resistance, and the inner layer is made of It has been found that the above object can be achieved by using a low alloy steel having excellent strength and toughness.

That is, high Cr steel with a reduced carbon content is disposed at least on the inner surface of the line pipe directly in contact with the transport fluid in order to reduce the increase in hardness due to the influence of welding heat, and the base material not in contact with the transport fluid has a strength. It has been found that a line pipe excellent in both corrosion resistance and weldability can be obtained by using low alloy steel having excellent toughness.

The present inventors have further studied and, as a basic composition of the high Cr steel giving the above-mentioned properties, contain 7.5 to 16% of Cr, 0.03% or less of C and 0 to 0% of N. .
It was found that the reduction to 02% or less is very effective in improving the corrosion resistance and decreasing the hardness of the weld heat affected zone.

Further, Ni, Cu, Mo are added to the high Cr steel.
Alternatively, it is possible to further improve the corrosion resistance in a wet carbon dioxide environment by adding any one or more of W, and it is effective to improve the corrosion resistance by reducing P to 0.02% or less and S to 0.005% or less. I found out that there is something.

On the other hand, the present inventor has also studied a base material which is responsible for strength and toughness, and has C: 0.02 to 0.25.
%, Si: 0.01 to 1.0%, Mn: 0.05 to 2.
0%, P is 0.015% or less, S is 0.005%
% Steel is suitable, Nb:
0.01 to 0.15%, V: 0.01 to 0.1%, T
i: 0.005 to 0.1%, Mo: 0.05 to 0.7%
It has been found that the strength and toughness can be further improved by adding one or more of these.

Further, according to the study by the present inventor, even in the case of high Cr steel in which the toughness of the heat affected zone is not necessarily good, the ratio of the thickness of the high Cr steel to the total thickness is 25% or less. For example, it was found that by using steel having excellent toughness in the weld heat affected zone as the base material, good toughness in the weld heat affected zone can be secured for the entire line pipe.

The present invention has been made mainly on the basis of the above findings. The gist of the first invention is that at least the inner surface of the line pipe, which is in direct contact with the transport fluid, has a weight ratio of Cr: 7. 5 to 16%, Si: 0.01 to 1.0
%, Mn: 0.01 to 3.0%, C: 0.03%
% Or less, N is reduced to 0.02% or less, and a high Cr steel consisting of the balance unavoidable impurities and Fe is used, and the base material that does not come into direct contact with the transport fluid is C: 0.02 to 0.25%, Si: 0. 0
1 to 1.0%, Mn: 0.05 to 2.0%, P as an impurity is reduced to 0.02% or less, S is reduced to 0.005% or less, and the balance is inevitable impurities. A line pipe having excellent corrosion resistance and weldability, characterized in that the ratio of the thickness of the high Cr steel portion to the total wall thickness is 25% or less.

The gist of the second invention is that the high Cr steel of the line pipe targeted by the first invention further contains Ni: 4% or less, Cu: 4.5% or less, Mo:
2.5% or less, W: 5% or less, any one or two
A line pipe using steel containing more than one kind and having excellent corrosion resistance and weldability.

The gist of the third invention is that, as impurities in the high Cr steel of the line pipe targeted by the first invention and the second invention, P is 0.02% or less and S is 0.00
A line pipe that uses steel reduced to 5% or less and has excellent corrosion resistance and weldability. The gist of the fourth invention is that the steel of the base material that does not come into direct contact with the transport fluid of the line pipe targeted by the first, second, and third inventions has Nb: 0.01 to 0.15%, V:
0.01-0.1%, Ti: 0.005-0.1%, M
o: A line pipe containing at least one of 0.05 to 0.7% and having excellent corrosion resistance and weldability.

[0018]

First, in the present invention, at least the inner surface side of the line pipe which is in direct contact with the transport fluid is made of high Cr steel, which, of course, imparts corrosion resistance to corrosive internal fluid. Therefore, the inner surface must be made of high Cr steel. On the other hand, since corrosive fluid does not flow on the outer surface side, it is not always necessary to use high Cr steel, but for the purpose of ensuring corrosion resistance of the outer surface or for reasons of the manufacturing process, the outer surface side is also made of high Cr steel. There is no problem at all, and a line pipe made of high Cr steel on the outer surface side is also an object of the present invention. In short, it is important that the inner surface or the inner and outer surfaces be made of high Cr steel having excellent corrosion resistance, and that steel having excellent weldability be disposed in a portion not in direct contact with the transport fluid.

Next, the reason for limiting the components of the high Cr steel in the present invention will be described. Cr: Cr is an element necessary for imparting corrosion resistance in a wet carbon dioxide environment, and if the content is less than 7.5%, the corrosion resistance is not sufficient, while if it exceeds 16%, its effect is saturated. Not only that, the decrease in toughness after welding is more remarkable, so the upper limit content should be 16%.

Si: Si is a useful element for deoxidation and 0.01% or more is required for deoxidation.
If it exceeds 0%, the toughness of the high Cr steel is significantly reduced, so the upper limit content is set to 1.0%.

Mn: Mn needs to be added in an amount of 0.01% or more for deoxidation and toughness of a high Cr steel, but 3.0%.
Even if added in excess, the effect will no longer be saturated, and manufacturing problems such as the generation of Mn vapor during melting will occur, so the upper limit content is made 3.0%.

C: C is an element that remarkably increases the hardness of the high Cr steel in the weld heat affected zone and lowers the corrosion resistance. In order to suppress the hardness in the weld heat affected zone to be low and to secure the corrosion resistance, The upper limit content of C is 0.03%
And must be.

N: Similar to C, N is also an element that significantly increases the hardness of the high Cr steel in the heat affected zone. To suppress the hardness in the heat affected zone to a low level, the upper limit content of N is set to 0. 0.02%.

The above are the basic components of the high Cr steel used on at least the inner surface side of the line pipe targeted by the present invention, but in the present invention, the following elements may be further added or reduced if necessary. It also targets those with further improved characteristics.

Ni: Ni is effective in further improving the corrosion resistance of high Cr steel, but even if it is added in excess of 4%, the effect is not only saturated, but also the cost is unnecessarily increased and the welding heat is increased. The upper limit content is 4% because it only unnecessarily increases the hardness of the affected zone.

Cu: Cu, like Ni, is effective in further improving the corrosion resistance of high Cr steel, but even if added in an amount of more than 4.5%, the effect is not only saturated but also in the heat affected zone of welding. Therefore, the upper limit of the content is set to 4.5%.

Mo: Mo is also effective in further improving the corrosion resistance, especially pitting corrosion resistance, of high Cr steel, but if it is added in an amount of more than 2.5%, the effect is not only saturated but also the cost is unnecessarily increased. Is increased and the toughness of the weld heat affected zone is excessively reduced, so the upper limit content is made 2.5%.

W: W is the corrosion resistance of high Cr steel, similar to Mo,
Particularly effective in further improving pitting corrosion resistance, but 5%
If the addition exceeds 3, the effect is not only saturated, but also the cost is unnecessarily increased and the toughness of the heat affected zone is excessively reduced. Therefore, the upper limit content is set to 5%.

P: If a large amount of P is present, the corrosion resistance of high Cr steel is impaired. Therefore, if more excellent corrosion resistance is required, it is effective to set P to 0.02% or less.

S: If a large amount of S is present, it impairs the corrosion resistance, especially pitting corrosion resistance, of the high Cr steel. Therefore, when further excellent corrosion resistance is required, S is 0.005% or less, which is very effective.

In addition, addition of Al as a deoxidizing agent as a component of high Cr steel is not contrary to the gist of the present invention. In addition, it is possible to add Ca, B, or a rare earth element to the high Cr steel, if necessary, and these are of course not against the gist of the present invention.

Next, the reason for limiting the components of the steel used as the base material in the present invention will be described. C: C is an element necessary for the base material to secure the strength as a line pipe, and if the content is less than 0.02%, sufficient strength cannot be obtained, while if it exceeds 0.25%. The upper limit content is 0.25% because the impact toughness of the heat-affected zone of welding is reduced.

Si: It is necessary to add 0.01% or more of Si to deoxidize and improve the strength. If it exceeds 1.0%, the ductility is reduced.
%.

Mn: Mn must also be added in an amount of 0.05% or more for strengthening, but if it exceeds 2.0%, its effect is saturated, so the upper limit content is made 2.0%.

P: Since P impairs weldability and resistance to sulfide stress cracking in a wet hydrogen sulfide environment, it should be limited to 0.015% or less.

S: Since S also impairs weldability and resistance to sulfide stress cracking in a wet hydrogen sulfide environment, S should be limited to 0.005% or less.

Al: Al is an element required as a deoxidizing element and needs to be added in an amount of 0.005% or more.
If it exceeds 0%, coarse inclusions are formed and the impact toughness is reduced, so the content should be in the range of 0.005 to 0.10%.

The above are the basic components of the steel used as the base material of the line pipe which is the subject of the present invention in the position where it does not come into direct contact with the transport fluid, but in the present invention, the following elements are further added as necessary. It is also intended to improve the characteristics by adding them. Nb: Nb is an element useful for refining the structure and therefore improving impact toughness, and it is necessary to add 0.01% or more to obtain a sufficient effect. On the other hand, even if added in excess of 0.15%, the effect is saturated, but only the cost is unnecessarily increased, so the upper limit content is made 0.15%.

V: V also has the effect of refining the structure and is also effective for improving the strength, and in order to obtain a sufficient effect, addition of 0.01% or more is necessary. On the other hand, if the addition exceeds 0.1%, the effect is saturated but the cost is merely increased unnecessarily, so the upper limit content is set to 0.1%.

Ti: Ti is also an element useful for refining the structure and thus improving the impact toughness. To obtain a sufficient effect, 0.005% or more must be added. On the other hand, 0.
Even if it is added in excess of 1%, the effect is saturated but the cost is simply increased unnecessarily, so the upper limit content is set to 0.1%.

Mo: Mo is an element useful for improving the strength of the substrate, and it is necessary to add 0.05% or more to obtain a sufficient effect. On the other hand, even if added in excess of 0.7%, the effect is saturated, but only the cost is unnecessarily increased, so the upper limit content is made 0.7%.

In addition, the addition of Cr, Ni, Cu, B, and N is not an essential component of the present invention. However, since the addition of these elements is effective in increasing the strength, it is not necessary to selectively add them. It is not contrary to the gist of. Further, addition of Zr for the purpose of deoxidation and addition of Ca or a rare earth element for the purpose of controlling the form of nonmetallic inclusions do not contradict the gist of the present invention.

In the present invention, the ratio of the thickness of the high Cr steel portion to the total thickness is limited to 25% or less. The reason is that the thickness of the high Cr steel portion to the total thickness is This is because if the ratio exceeds 25%, it will be difficult to secure the toughness in the weld heat affected zone of the line pipe, no matter how high the toughness steel is used as the base material. This is because it is necessary to secure a ratio of an excellent base material of 75% or more.

In the present invention, at least the inner surface side directly in contact with the transport fluid is a high Cr steel, the base material not in direct contact with the transport fluid is a steel satisfying the above-mentioned components, and the high Cr steel portion occupying the entire wall thickness is The manufacturing method is not particularly limited as long as the thickness ratio is 25% or less.

For example, a molten high Cr steel and ordinary steel are simultaneously cast to produce a multi-layered slab having an outer surface made of high Cr steel and an interior made of ordinary steel. A method of using a welded steel pipe as the UOE steel pipe or the electric resistance welded steel pipe can be used. Or molten high Cr
Steel and ordinary steel can be simultaneously cast to produce a composite bloom made of ordinary steel on the outer surface and high Cr steel inside, and this can be hot rolled or hot extruded to form a seamless steel pipe. Of course it is good.

[0046]

EXAMPLE A composite slab or a composite bloom having the components shown in Table 1 was melted to obtain a steel pipe having a wall thickness of 10 to 20 mm as shown in the table. The ratio of the thickness of the high Cr steel portion to the total wall thickness of each steel pipe is as shown in Table 1.

Next, these steel pipes were circumferentially welded by hand welding to form a joint corresponding to the field welded portion of the line pipe. The welding heat input was 17 kJ / cm. A JIS No. 4 impact test piece (full size) was sampled from the base material and the weld heat affected zone of the circumferential weld zone to perform an impact test. Also,
The maximum hardness of the heat affected zone was determined by micro Vickers measurement with a load of 100 g.

A test piece was sampled from the high Cr steel portion of the base material, and was placed in an autoclave at a test temperature of 120 ° C.
The sample was immersed in a 3% aqueous NaCl solution at 0 atm for 30 days to perform a corrosion test in a wet carbon dioxide gas environment, and the corrosion rate was calculated from the weight change before and after the test. The unit of corrosion rate is expressed in mm / y. Generally, if the corrosion rate of a material in an environment is 0.1 mm / y or less, it is considered that the material is sufficiently corrosion resistant in that environment and can be used. To be

The test results are also shown in Table 1. In the impact test results of Table 1, ◯ ○ has a fracture surface transition temperature of −40 ° C. or less, and ◯ has a fracture surface transition temperature of more than −40 ° C. and −20 ° C. or less,
X indicates that the fracture surface transition temperature was more than -20 ° C and 0 ° C or less, and XX indicates that the fracture surface transition temperature was more than 0 ° C. In the welding heat affected zone maximum hardness, XX represents the maximum hardness. Saga 2
Less than 50, ○ has a maximum hardness of more than 250 and less than 280, × has a maximum hardness of more than 280 and less than 400, and XX has a maximum hardness of 400.
In the corrosion test results, ○○ indicates a corrosion rate of less than 0.05 mm / y, ○ indicates a corrosion rate of 0.05 mm / y or more and less than 0.1 mm / y, and × indicates a corrosion rate. Is 0.1 mm / y or more and less than 0.5 mm / y, and XX indicates that the corrosion rate is 0.5 mm / y or more.

In Table 1, the comparative steel No. 9 is AI
SI420 steel, No. No. 10 is 9Cr-1Mo steel, and all are conventional steels that have been conventionally used in a wet carbon dioxide environment. No. For both 9 and 10, a single pipe was used as a test material for comparison.

[Table 1]

[Table 2] As is clear from Table 1, the line pipe No. Nos. 1 to 8 have remarkably excellent impact toughness of the base metal and the heat affected zone of welding, the hardness of the heat affected zone is sufficiently low, and are practical even at a very high temperature of 120 ° C in a wet carbon dioxide gas environment as a line pipe. It can be seen that the corrosion rate is lower than 0.1 mm / y, which is a corrosion rate that can be used in general, and that it has excellent corrosion resistance and weldability.

On the other hand, No. 9 and N
o. No. 10 has weld cracks at the weld and has a very high maximum hardness. In Comparative Example No. In Nos. 11 to 13, since the requirements of the present invention are not satisfied, the hardness or impact toughness of the heat affected zone is poor.

[0052]

INDUSTRIAL APPLICABILITY The present invention makes it possible to provide a line pipe having excellent corrosion resistance in a wet carbon dioxide gas environment and excellent weldability, and is extremely significant in contributing to the development of industry.

Continuation of front page (51) Int.Cl. ⁶ Identification code Office reference number FI Technical display location C22C 38/18 C22C 38/18 F16L 9/02 F16L 9/02

Claims (4)

(57) [Claims] 1. At least the inner surface of the line pipe that is in direct contact with the transport fluid, in weight%, Cr: 7.5-16%, Si: 0.01-1.0%, Mn: 0.01-3.0. %, C is reduced to 0.03% or less, N is reduced to 0.02% or less, and a high Cr content consisting of balance unavoidable impurities and Fe
A base material that is steel and is not in direct contact with the transport fluid is C: 0.02 to 0.25%, Si: 0.01 to 1.0%, Mn: 0.05 to 2.0%, Al: 0.005. To 0.1% of P, 0.015% or less of P and 0.005% or less of S as impurities, and a balance of unavoidable impurities to make the steel a high-Cr steel portion with respect to the total thickness. A line pipe with excellent corrosion resistance and weldability, characterized in that the ratio of the thickness is 25% or less. 2. The high Cr steel further comprises one or more of Ni: 4% or less, Cu: 4.5% or less, Mo: 2.5% or less, and W: 5% or less as additional components. The line pipe having excellent corrosion resistance and weldability according to claim 1, wherein the line pipe contains at least one kind. 3. The corrosion resistance and weldability according to claim 1 or 2, wherein as impurities in the high Cr steel, P is reduced to 0.02% or less and S is reduced to 0.005% or less. Excellent line pipe. 4. The steel of the base material which is not in direct contact with the transport fluid further contains Nb: 0.01 to 0.15%, V: 0.01 to 0.1%, Ti: 0.005 to 0.5%. The line pipe having excellent corrosion resistance and weldability according to claim 1, 2 or 3, wherein the line pipe contains one or more of Mo: 0.05 to 0.7%. .