JP2000288775A - Welded joint of martensitic stainless steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a molded joint having high strength and high corrosion resistance, made of a martensitic stainless steel. SOLUTION: This welded joint is composed of a base material of the martensitic stainless steel contained of <=0.1% C and 8-15% Cr, and having >=80% area ratio of the ratio of martensitic phase and >=550 MPa yield stress and a welded metal mainly contained of <=0.1% C, <=0.35% N, 0.01-1% Si, 0.01-3% Mn, 15-28% Cr, 5-10% Ni, 0.01-3% Cu, 0.005-1% Al and 1-5% Mo or/and 0.1-5% W and having cross sectional shape of 30-60 deg. angle θD/2.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a martensitic stainless steel welded joint.

[0002]

2. Description of the Related Art Oil and natural gas produced in recent years often contain moist carbon dioxide and hydrogen sulfide. If carbon steel or low alloy steel is used in such an environment, corrosion of the steel is remarkable. Therefore, a corrosion inhibitor is added as a countermeasure against the corrosion. However, the effect is lost at high temperatures, and it takes enormous costs to add and recover such as in submarine pipelines.

Therefore, recently, a corrosion resistant material which does not require the addition of a corrosion inhibitor, specifically, 12 to 13% by weight of Cr represented by 420 steel specified by the AISI standard.
In recent years, martensitic stainless steels containing iron have been widely used.

However, conventional martensitic stainless steels typified by the above-mentioned 420 steels contain a relatively large amount (0.16-0.22% by weight) of C in order to obtain high strength. I have. Therefore, there were the following problems.

[0005] That is, the pipeline as described above,
The line pipes, which are the constituent materials, are butt-butted to each other and welded along the circumference. At this time, when a martensitic stainless steel line pipe containing a relatively large amount of C, such as 420 steel, is butt-welded by a normal welding method, the hardness of the weld heat-affected zone increases, and the impact characteristics are increased. Is significantly reduced. Further, when the hardness increases, the susceptibility to sulfide stress corrosion cracking becomes remarkable. As a method of solving the above-mentioned problem due to the increase in hardness, a method of performing a post-heat treatment on the welded portion,
A method for reducing the C content of the base material (for example,
243740 and 5-287455),
Method using welding material made of duplex stainless steel (for example,
JP-A-8-57683).

However, the above method is very expensive. In addition, the above method has a drawback that a welded joint exhibiting good corrosion resistance and hot crack resistance cannot be obtained after securing 850 MPa or more in required strength (tensile strength (TS)). I have.

[0007]

SUMMARY OF THE INVENTION An object of the present invention is to provide a marten having not only the required strength but also the required corrosion resistance and good hot cracking resistance even when it is welded. An object of the present invention is to provide a welded joint made of a site-based stainless steel.

[0008]

The gist of the present invention resides in the following martensitic stainless steel welded joint.

In weight%, C: 0.1% or less, Cr: 8 to
An arc-welded joint made of a martensitic stainless steel containing 15% as a base material, wherein the weld metal of the joint is 0.1% or less by weight, C: 0.1% or less, and N: 0.
35% or less, Si: 0.01 to 1%, Mn: 0.01 to
3%, P: 0.03% or less, S: 0.01% or less, C
r: 15 to 28%, Ni: 5 to 10%, Cu: 0.01
３3%, Al: 0.005 to 1%, O (oxygen): 0.0
35% or less, and Mo: 1 to 5% and W: 0.1
-5%, the balance being substantially Fe, having a chemical composition that satisfies the following formula and the following formula, wherein the cross-sectional shape of the weld metal is An angle θ _D formed by a line L connecting an intersection A with the center line Lt of the material thickness and an intersection B between the center line Lw of the weld metal and an extension of the base metal surface Sm and an extension of the base metal surface Sm. _{/ 2} is 30-60
Welded joint of martensitic stainless steel with a degree shape.

C + N ≧ 0.2 0 ≦ 0.6 × Cr equivalent−Ni equivalent ≦ 10−100 × C where Cr equivalent = Cr + Mo + 1.5 × Si + 0.5 × Nb Ni equivalent = Ni + 30 × (C + N) + 0.5 × Mn Here, the symbol of each element in each of the above formulas means the content (% by weight) of each element contained in the weld metal.

[0011] The welded joint according to the present invention is characterized in that the weld metal contains one or more selected from Hf, Nb, V, Ti, Zr, Ta and Co in addition to the above components in a total of 1%. It is preferable that the content is not more than% by weight.

The present invention has been completed based on the following findings. That is, the present inventors have studied in detail the effects of the chemical composition and shape of the weld metal on the strength and corrosion resistance of the welded joint, as well as on the hot cracking susceptibility.
As a result, the following was found.

In order to secure the required joint strength described above, it is most convenient to increase the contents of C and N in the weld metal. If the total amount of both is 0.2% by weight or more, The desired strength is ensured.

However, if the content of C exceeds 0.1% by weight, the required corrosion resistance cannot be secured due to precipitation of Cr-based carbides, and if the content of N exceeds 0.35% by weight,
Welding defects such as blowholes occur during welding, and a healthy welded joint cannot be obtained.

In addition, it is necessary to add the above amounts of each component in addition to C and N. However, the value obtained by subtracting the Ni equivalent from the 0.6 times the Cr equivalent obtained by the above equations is 0 (zero). )
As described above, unless the components are adjusted so as to be not more than the value of the expression (10-100 × C), the desired strength and corrosion resistance cannot be secured.

Furthermore, the O content in the weld metal is 0.03
More than 5% by weight or the cross-sectional shape is not appropriate,
Specifically, the angle θ _{D / 2} shown in FIG.
An intersection A between f and the center line Lt of the base material thickness, and a line L connecting the center line Lw of the weld metal and an intersection B of the extension line of the base material surface Sm and an extension line of the base material surface Sm are formed. Angle θ _{D / 2} is 30 ~
If the temperature is not within the range of 60 degrees, high-temperature cracking occurs and a sound welded joint cannot be obtained.

[0017]

BEST MODE FOR CARRYING OUT THE INVENTION The reason why the welded joint of the present invention is determined as described above will be described in detail below. In the following, “%” means “% by weight”.

<< Base material of martensitic stainless steel >>
As the martensitic stainless steel of the base material, it is necessary to use a martensitic stainless steel having a C content of 0.1% or less and a Cr content of 8 to 15%. The reason is as follows.

If the content of C is more than 0.1%, the hardness of the heat affected zone after pipe welding or on-site circumferential welding becomes too high, and the toughness and stress corrosion cracking resistance decrease. I do. Therefore, the C content is set to 0.1% or less. Note that the lower limit is not particularly defined, but is preferably 0.001% or more from the viewpoint of securing strength.

[0020] Cr is the most basic element constituting martensitic stainless steel, but if its content is less than 8%, corrosion resistance in a carbon dioxide gas environment cannot be ensured. Conversely, if the content exceeds 15%, the strength cannot be adjusted. Therefore, the Cr content was set to 8 to 15%.

The martensitic stainless steel of the base material used in the present invention may be any martensitic stainless steel as long as the above conditions are satisfied.
For example, the effects of the present invention described below do not change at all even if the following components are included in addition to the above two components.

The martensitic stainless steel as a base material preferably has a martensite phase in the structure of not less than 80 area%. This is because if the proportion of the martensite phase is too small, local corrosion is likely to occur, and the desired local corrosion resistance may not be secured.

Si: Si is usually added as a deoxidizing element, but if it exceeds 1%, the toughness is reduced.
Therefore, the content of Si when added is preferably 1% or less.

Mn: Mn is added as a deoxidizing element or as a strengthening element as in the case of the above Si.
When the content exceeds 2%, the stress corrosion cracking resistance is reduced. Therefore, when added, the Mn content is preferably 2% or less.

Al: Like Al, Al is usually added as a deoxidizing element. However, if it is contained in excess of 0.1%, the cleanliness of steel is reduced. Therefore, the content of Al when added is preferably 0.1% or less.

N: N is an impurity element inevitably contained in steel, but if present excessively, it lowers the toughness of the base material. Therefore, the N content is preferably set to 0.02% or less.

P: P is an impurity element inevitably contained in steel, as in the case of N described above. If P is present in excess, P reduces hot workability and increases hot cracking susceptibility during welding. , It is desirable to use as little as possible, but 0.02%
There is no particular problem if it is below.

S: 0.005% or less S is an impurity element unavoidably contained in steel, like N and P described above, and when present in excess, has the same hot workability as P described above. It is desirable to reduce as much as possible, as it lowers or increases the susceptibility to hot cracking during welding.
There is no particular problem if it is 0.005% or less.

Ni, Cu, Co: These elements, when added, all improve the toughness of the base material and the corrosion resistance in a wet carbon dioxide gas environment. The effect of each element is 0.1%.
It becomes remarkable at 5% or more. However, the effect is saturated when Ni is 10% and Cu and Co are 5%, and further addition only causes an increase in cost. Therefore, the content of these elements when added is preferably 0.5 to 10% for Ni and 0.5 to 5% for Cu and Co. These elements may be added alone or in combination of two or more.

Mo, W: If these elements are added,
In each case, the corrosion resistance and the stress corrosion cracking resistance of the base material in a wet carbon dioxide gas environment are improved. The effect becomes remarkable at 0.2% or more for each element. However, the effect is saturated when Mo is 3% and W is 6%, and further addition only causes an increase in cost. Therefore, when these elements are added, the content of Mo is 0.2 to 3%, and the content of W is 0.2 to 6%.
It is good to do. One of these elements may be added alone or both may be added in combination.

Ti, Nb, V, Zr, Ta, Hf: These elements, when added, all enhance the strength of the base material. The effect is not less than 0.003% for Ti and N
b, V, Zr, Ta and Hf are all 0.01
It becomes remarkable at% or more. However, when the content exceeds 0.2% in the case of Ti, and exceeds 0.1% in the case of Nb, V, Zr, Ta and Hf, the toughness of the weld heat affected zone and the weld metal decreases. Invite. Therefore, when these elements are added, the content of Ti is 0.003 to 0.2%,
Nb, V, Zr, Ta and Hf are all 0.01 to
It is good to make it 0.1%. These elements may be added alone or in combination of two or more.

<< Weld Metal >> The weld metal constituting the weld joint of the present invention must have the following chemical composition. The reason is as follows.

C, N: C is 0.1% or less, N is 0.35
% And C + N ≧ 0.2% C and N are the most important elements for securing the strength of the weld metal. If the total content is less than 0.2%, the desired strength cannot be secured. . However, as described above, when the content of C exceeds 0.1%, a large amount of Cr-based carbides precipitate, making it impossible to secure desired corrosion resistance. Further, the content of N is 0.35%
If it exceeds, welding defects such as blow holes occur during welding, and a sound welded joint cannot be obtained. Therefore, C content is 0.1% or less, N content is 0.35% or less, and C +
N was set to 0.2% or more. Note that a preferable upper limit of the C content is 0.09% and a preferable upper limit of the N content is 0.3%.

Si: 0.01 to 1% Si is necessary as a deoxidizing agent for molten metal during welding and as a strengthening element for weld metal, and its content is 0.01%.
If less than that, the effect is not sufficient. However, if the content exceeds 1%, the effect is saturated and the toughness is rather deteriorated. Therefore, the Si content is set to 0.01 to 1%.

Mn: 0.01 to 3% Like Mn, Mn is necessary as a deoxidizer for molten metal at the time of welding and a strengthening element for weld metal. If less than that, the effect is not sufficient.
However, if the content exceeds 3%, the effect is saturated,
Instead, the stress corrosion cracking resistance is reduced. Therefore,
The Mn content was set to 0.01 to 3%.

P: 0.03% or less P is an impurity element and is inevitably mixed during welding.
When the content exceeds 0.03%, not only does the toughness of the weld metal decrease, but also the susceptibility to hot cracking increases. Therefore, the P content is set to 0.03% or less. The lower the P content, the better.

S: 0.01% or less S is an impurity element inevitably mixed at the time of welding, similar to the above P, and if the content exceeds 0.01%, the ductility and corrosion resistance of the weld metal are reduced. Not only decreases, but also the hot cracking sensitivity increases. Therefore, the S content is 0.01%
It was as follows. The lower the S content, the better.

Cr: 15 to 28% Cr is an important element that is indispensable for securing the corrosion resistance and strength of the weld metal. However, if the content is less than 15%, the effect is not sufficient. On the other hand, 28
%, The effect is saturated, not only increasing the cost but also making it difficult to adjust the strength. Therefore, the Cr content was set to 15 to 28%.

Ni: 5 to 10% Ni is an important element that is indispensable for securing the toughness and corrosion resistance of the weld metal. However, if the content is less than 5%, the corrosion resistance and toughness are not sufficient. Meanwhile, 1
If the content exceeds 0%, the effect is saturated, not only increasing the cost, but also increasing the austenite phase and making it impossible to secure a desired strength. Therefore, the Ni content was set to 5 to 10%.

Mo: 1 to 5% or / and W: 0.1
５5% These elements are important elements indispensable for securing the corrosion resistance and strength of the weld metal, and one or both of them are added. However, if the content is less than 1% for Mo and less than 0.1% for W, the above effect is not sufficient. On the other hand, when any of the elements is contained in excess of 5%, the effect is saturated, not only increasing the cost but also making it difficult to adjust the strength. Therefore, the Mo content is 1 to 5%, and the W content is 0.1 to 5%.
%.

Cu: 0.01 to 3% Cu, like Mo and W described above, is an indispensable element for securing the strength and corrosion resistance of the weld metal. However, if the content is less than 0.01%, the effect is not sufficient. On the other hand, if the content exceeds 3%, the effect is saturated and the hot cracking sensitivity is increased. Therefore, the Cu content is set to 0.01 to 3%.

Al: 0.005 to 1% Al is necessary as a deoxidizing agent for molten metal at the time of welding, like Si and Mn described above.
If it is less than 05%, the effect is not sufficient. However, when the content exceeds 1%, coarse oxide-based inclusions are formed, and the corrosion resistance and toughness of the weld metal deteriorate. Therefore,
The Al content was 0.005 to 1%.

O (oxygen): 0.035% or less O is an impurity element, but its content is 0.035%.
If it exceeds 300, hot cracking will occur and a healthy welded joint will not be obtained. Therefore, the O content is set to 0.035% or less. The lower the O content, the better.

Relationship between Cr equivalent and Ni equivalent: Even when the content of each component is within the above range, when the relationship between Cr equivalent and Ni equivalent determined by the following equations does not satisfy the following equation. However, as described above, desired strength and corrosion resistance cannot be secured. This is because, if the value obtained by subtracting the Ni equivalent from the 0.6 equivalent of the Cr equivalent is less than 0, the ratio of the austenite phase in the structure becomes too large and the strength is reduced. On the other hand, if the value exceeds the value of the formula (10-100 × C), C does not sufficiently form a solid solution, and a large amount of Cr-based carbides precipitate, thereby deteriorating the corrosion resistance.

0 ≦ 0.6 × Cr equivalent−Ni equivalent ≦ 10−100 × C Cr equivalent = Cr + Mo + 1.5 × Si + 0.5 × Nb Ni equivalent = Ni + 30 × (C + N) + 0.5 × Mn The element symbol in each of the above formulas means the content (% by weight) of each element contained in the weld metal.

The weld metal constituting the weld joint of the present invention is:
If the chemical composition described above is satisfied, desired strength and corrosion resistance are provided. However, if the corrosion resistance and toughness are desired to be further improved, the following elements may be added.

Hf, Nb, V, Ti, Zr, Ta, C
o: These elements, when added, all have the effect of improving the toughness and corrosion resistance of the weld metal. Therefore, when this effect is desired, any one of these elements can be added alone or in combination of two or more, and the effect is 0.01% or more for each element. Become noticeable. However, the total content of these elements is 1%
If it exceeds, the effect is saturated, and the toughness is rather reduced. Therefore, when these elements are added, the content is preferably 0.01% or more, and the total content is preferably 1% or less.

Cross-sectional shape: As shown in FIG. 1, the cross-sectional shape of the weld metal is represented by the intersection A between the melting line Lf and the center line Lt of the base metal thickness, the center line Lw of the weld metal and the base metal surface. It is necessary that the angle θ _{D / 2} between the line L connecting the intersection B with the extension line of Sm and the extension line of the base metal surface Sm is in the range of 30 to 60 degrees. The reason is as follows.

That is, when the angle θ _{D / 2} is less than 30 degrees, as shown in FIG. 2, the weld metal has a shallow penetration depth and a wide width, and stress concentrates on the portion indicated by the arrow in the figure. However, a hot crack is generated in the portion, and a sound welded joint cannot be obtained. Conversely, when the above angle θ _{D / 2} exceeds 60 degrees, as shown in FIG. 3, the weld metal has a deeper penetration depth and a narrower weld metal, and the alloy elements are markedly marked by the arrows shown in the figure. This is due to the occurrence of severe segregation and the occurrence of hot cracks in the form of cracks at the portions, making it impossible to obtain a sound welded joint.

The welding method for obtaining the above-described welded joint of the present invention may be any method as long as it is an arc welding method.
Method, TIG method, SAW method and the like. The welding conditions at that time may be appropriately selected so as to obtain a weld metal having the above-described chemical composition and cross-sectional shape, and are not particularly limited.

[0051]

EXAMPLES The chemical compositions shown in Table 1 were used, and in each case, the area ratio of the martensite phase in the structure was 80% or more.
And a tensile strength of 620 Mpa or more and a yield strength of 550 M
Two kinds of plate thickness 20mm adjusted to be more than pa
Was prepared.

The prepared two types of base steel sheets were compared with each other and various welding methods and welding conditions (heat input) shown in Table 3 were used.
As shown in Tables 2 and 3, various welded joints having different chemical compositions and cross-sectional shapes (angle θ _{D / 2} ) of the weld metal were obtained.

[0053]

[Table 1]

[0054]

[Table 2]

[0055]

[Table 3]

From the obtained welded portion of each welded joint, the weld metal is located at the center of the parallel portion with the longitudinal direction perpendicular to the welding progress direction, and the weld heat affected zone and the base metal portion are located on both sides thereof. No. 5 test pieces specified in JIS Z2201 in which are sequentially located were sampled and subjected to a tensile test at room temperature to examine the tensile strength (TS).

Similarly to the above, from a position 5 mm in the thickness direction from the surface of the base material of the welded portion, a width 22 mm, a thickness 3 mm and a length 1 where the weld metal is located at the center in the longitudinal direction.
A 00 mm corrosion test piece was sampled, the test surface was polished with No. 600 emery paper, degreased and dried, and then subjected to a carbon dioxide gas corrosion test under the following conditions to examine its carbon dioxide gas corrosion resistance.

Carbon dioxide gas corrosion test:
Liquid temperature of 125 ° C. saturated with carbon dioxide at 30 bar 5
In this test, a% salt solution was continuously flowed at a flow rate of 2.5 m / sec for 720 hours, and the surface of the test piece after the test was visually inspected for the occurrence of local corrosion.

The results of these tests are also shown in Table 3.

As can be seen from the results shown in Table 3, the welded joints of Test Examples 1 to 6 of the present invention have no high-temperature cracking, have a high TS of 891 MPa or more, and have good resistance to carbon dioxide gas corrosion. Met.

On the other hand, among the comparative examples, the welded joints of test numbers 7 and 8 had a low TS of 606 MPa or less and a desired strength because the total content of C and N in the weld metal was too small. Could not be obtained. Since the value obtained by subtracting the Ni equivalent from the 0.6 times the Cr equivalent in the weld metal was larger than the equation (10-100 × C), the weld joint of Test No. 9 was poor in carbon dioxide gas corrosion resistance. . Since the value obtained by subtracting the Ni equivalent from the 0.6 times the Cr equivalent in the weld metal was smaller than 0, the welded joint of Test No. 10 had a low TS of 605 MPa,
Did not have the desired strength. The welded joint of test number 11 had poor carbon dioxide gas corrosion resistance because the C content in the weld metal was too large. The welded joint of Test No. 12 had a high temperature crack because the angle θ _{D / 2} was too small and was a wide weld metal, and a sound joint was not obtained. The welded joint of test number 13 was a weld metal having a too large angle θ _{D / 2} and a deep penetration depth and a narrow width, so that hot cracking occurred and a sound joint was not obtained. The No. 14 welded joint has a high-temperature crack because the O content in the weld metal is too large,
A healthy joint was not obtained. The No. 15 joint had poor carbon dioxide corrosion resistance because the Cr content in the weld metal was too low. In the welded joint of Test No. 16, since the N content in the weld metal was too large, a blowhole was generated during welding, and a sound joint was not obtained.

[0062]

The martensitic stainless steel welded joint of the present invention has high strength as it is welded, and also has excellent corrosion resistance. Therefore, the present invention can be applied to a pipeline composed of a martensitic stainless steel pipe, and the facility cost can be reduced.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating a cross-sectional shape of a weld metal constituting a weld joint of the present invention.

FIG. 2 is a diagram illustrating a cross-sectional shape of a weld metal and a site where a hot crack occurs when an angle θ _{D / 2} is too small.

FIG. 3 is a view showing a cross-sectional shape of a weld metal and a site where a hot crack occurs when an angle θ _{D / 2} is too large.

[Explanation of symbols]

 Lf: melting line, Lt: center line of base material thickness, Lw: center line of weld metal, Sm: base material surface.

Claims (2)

[Claims] (1) In terms of% by weight, C: 0.1% or less, Cr: 8 to
An arc-welded joint made of a martensitic stainless steel containing 15% as a base material, wherein the weld metal of the joint is 0.1% or less by weight, C: 0.1% or less, and N: 0.
35% or less, Si: 0.01 to 1%, Mn: 0.01 to
3%, P: 0.03% or less, S: 0.01% or less, C
r: 15 to 28%, Ni: 5 to 10%, Cu: 0.01
３3%, Al: 0.005 to 1%, O (oxygen): 0.0
35% or less, and Mo: 1 to 5% and W: 0.1
-5%, and the balance is substantially Fe, and has the following chemical formula and the chemical composition satisfying the following formula, and the cross-sectional shape of the weld metal is the melting line Lf and the base metal. The intersection A between the center line Lt of the thickness and the center line Lw of the weld metal
A martensite system characterized in that an angle θ _{D / 2} formed by a line L connecting the point L and an extension line of the base material surface Sm and an extension line of the base material surface Sm is 30 to 60 degrees. Stainless steel welded joint. C + N ≧ 0.2 0 ≦ 0.6 × Cr equivalent−Ni equivalent ≦ 10−100 × C where Cr equivalent = Cr + Mo + 1.5 × Si + 0.5 × Nb Ni equivalent = Ni + 30 × (C + N) + 0.5 × Mn Here, the symbol of each element in each of the above formulas means the content (% by weight) of each element contained in the weld metal. 2. The welding metal according to claim 1, further comprising Hf,
The martensitic stainless steel welding according to claim 1, wherein one or more kinds selected from Nb, V, Ti, Zr, Ta and Co are contained in a total of 1% by weight or less. Fittings.