JP2001107200A - Martensitic stainless steel welded joint excellent in toughness and strength - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a martensitic stainless steel welded joint excellent in toughness and strength, applicable with the general melting and welding a process and also stably producible even in a wide welding condition range. SOLUTION: In this welded joint, both the base material part and the weld metal part are composed of martensitic stainless steel containing <=0.04% C, 0.01 to 1% Si, 0.1 to 1.5% Mn, <=0.02% P, <=0.005% S, 7 to 17% Cr, 0.5 to 9% Ni and 0 to 4% Mo, and the weld metal satisfies the inequality of Cr+ Mo+1.8×Ni<=30 and the inequality of -1<=Cr+Mo-1.7×Ni<=8.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a welded portion of a welded structure or a welded pipe obtained by a fusion welding method such as an arc welding method or a plasma welding method, and a butt circumferential welded portion of a welded pipe or a seamless pipe. The present invention relates to a martensitic stainless steel welded joint having excellent toughness and strength.

[0002]

2. Description of the Related Art Martensitic stainless steels have recently been widely used as corrosion-resistant materials having excellent economic efficiency. However, when a welded structure is presupposed, the formation of a hard martensite structure accompanying heating and cooling during welding tends to cause problems such as a decrease in toughness of a welded portion and generation of a weld crack. In addition, if austenitic weld metal is used as a countermeasure for low-temperature cracking, the strength of the weld metal will decrease,
This is so-called undermatching.

To solve these problems, Japanese Patent Laid-Open No. 10-1
As disclosed in Japanese Patent No. 46691, in the gas shielded arc welding method, the stability of the ferrite phase and the stability of the austenite phase are controlled by controlling the chemical composition to change the structure of the weld metal to the ferrite phase-austenite phase-martenite. There is a method to make a three-phase structure of the site phase. Specifically, the stability of the ferrite phase of the welding material is represented by Cr equivalents, and the stability of the austenitic phase is represented by Ni equivalents, and by defining these equivalents in a predetermined range, the structure of the weld metal is defined as above. This is a method for making a three-phase structure.

However, in the case of a martensitic structure, the stability of the austenite phase is not governed by the Ni equivalent, but rather becomes more stable as the Cr or Mo content on the Cr equivalent side increases. In addition, the above method
Even welding methods such as a covered arc welding method and a submerged arc welding method, and even a gas shielded arc welding method, have a disadvantage that they are not sufficient for a wide range of welding conditions. That is, when the welding method and welding conditions change, the dilution ratio with the base material and the cooling rate change, and as a result, stable toughness and strength are not always obtained.

Japanese Patent Application Laid-Open No. H11-80881 discloses that the base material has the formula “(Cr + 1.3 × Mo—Ni) ≦ 14”.
Steel and weld metal satisfying the formula “15 ≦ ΔNi + 30 × C +
0.5 × Mn + 0.8 × (Cr + Mo + 1.5 × Si +
0.5 × Nb)｝ ≦ 25 ”and the formula“ −5.8 ≦ ｛Ni + 3
0 × C + 0.5 × Mn−0.72 × (Cr + Mo + 1.
5 × Si + 0.5 × Nb)}, and shows a welded steel pipe excellent in toughness and corrosion resistance of a welded portion.
However, the welded steel pipe shown therein has a base material having a Cr content of only 14% by mass or less. This is because, as described in the publication, the Cr content of the base material is 14% by mass.
Is exceeded, the above equation “(Cr + 1.3 ×
Mo−Ni) ≦ 14 ”, because the amount of ferrite increases in the heat affected zone of welding and toughness cannot be ensured.

[0006]

The object of the present invention is not limited to the above-described covered arc welding method and submerged arc welding method, but can be applied to all fusion welding methods such as TIG welding, MIG welding and plasma welding. Another object of the present invention is to provide a martensitic stainless steel welded joint which is stable and can be manufactured stably in a wide range of welding conditions and has excellent toughness and strength.

[0007]

Means for Solving the Problems The inventors have made various studies in order to achieve the above object. As a result, in a wide range of welding conditions, martensitic stainless steel welding metal ensures stable toughness and strength as it is, and also prevents welding hot cracking during the cooling process in the welding heat cycle. It has been found that the time of martensite formation and the amount of ferrite immediately after solidification may be controlled.

That is, the strength is ensured by delaying the generation time of martensite in the welding heat cycle process to disperse and generate a fine austenite phase. Specifically, the generation time of martensite in the welding heat cycle process largely depends on the value obtained by the equation “Cr + Mo + 1.8 × Ni”, and when the value is set to 30 or less, the generation time of martensite is delayed. As a result, a fine austenite phase is dispersed and generated, and the strength is secured. In addition, when the expression “Cr + Mo + 1.8 × Ni ≦ 30” is satisfied, the effect does not change even if the welding conditions change and the cooling rate changes.

On the other hand, the formation of a ferrite phase is desirable to prevent welding hot cracking, but in a structure where martensite is dominant, the ferrite phase significantly increases toughness. Therefore, to achieve both high-temperature welding crack prevention and ensuring toughness, it is necessary to have a ferrite phase during solidification, reduce the ferrite phase as solidification progresses, and eliminate the ferrite phase in the subsequent heat cycle process. The value obtained by the expression “Cr + Mo−1.7 × Ni” is −1 to
With the setting of 8, the ferrite phase is present in the solidification process, and the ferrite phase disappears in the later stage of the solidification and in the subsequent heat cycle process, thereby preventing hot cracking at the time of welding and ensuring toughness. In addition, the formula “-1 ≦ Cr + Mo-1.7 × Ni
When ≦ 8 is satisfied, the effect does not change even if the welding conditions change and the cooling rate changes.

The gist of the present invention based on the above findings is the following martensitic stainless steel welded joint having excellent toughness and strength.

The base material portion is, in mass%, C: 0.04% or less, Si: 0.01 to 1%, Mn: 0.1 to 1.5%,
P: 0.02% or less, S: 0.005% or less, Cr: 7
-17%, Ni: 0.5-9%, Mo: 0-4%, T
i: 0 to 0.1%, Zr: 0 to 0.1%, V: 0 to 0.
1%, Al: 0 to 0.05%, N: 0 to 0.04%, martensitic stainless steel with the balance being substantially Fe, the weld metal having the same chemistry as the base metal It has a composition, and the relationship between Cr, Ni and Mo is expressed by the following equation (1).
A martensitic stainless steel welded joint with excellent toughness and strength, which is a martensitic stainless steel that satisfies both equations (2).

Cr + Mo + 1.8 × Ni ≦ 30 (1) −1 ≦ Cr + Mo−1.7 × Ni ≦ 8 (2) Here, the above equation (1) and (2) The symbol of the element in the formula means the content (% by mass) of each element contained in the weld metal.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The reasons why the chemical compositions of the base metal and the weld metal constituting the martensitic stainless steel welded joint of the present invention are determined as described above will be described in detail below. Here, the martensitic stainless steel referred to in the present invention refers to a martensitic stainless steel having a metal structure in which the ratio of martensite is 50% or more by volume%.

In the following, "%" means "% by mass".
Means The purpose of addition of each element and the reason for limiting the content are the same for both the base metal and the weld metal unless otherwise specified.

C: not more than 0.04% If the C content exceeds 0.04%, the hardness of the weld heat affected zone (hereinafter simply referred to as “HAZ”) as it is welded and the hardness of the weld metal become too high. Weld cold cracking occurs. For this reason, the C content is set to 0.04% or less. A preferred upper limit is 0.035%, and a more preferred upper limit is 0.030%. The lower the C content, the better, and in this case, the toughness of the as-welded HAZ is improved.

Si: 0.01 to 1.0% Si is an element necessary as a deoxidizing component, and
The above is necessary. However, when the content exceeds 1.0%, hot workability deteriorates. For this reason, the Si content is set to 0.01 to 1.0%. The preferred range is 0.1 to
0.6%, and a more preferable range is 0.2 to 0.5%.

Mn: 0.1 to 1.5% Mn is an element necessary as a deoxidizing component, as in the case of Si described above, and requires 0.1% or more. However, when the content exceeds 1.5%, hot workability deteriorates. For this reason, the Mn content is set to 0.1 to 1.5%. A preferred range is 0.3 to 1.0%, and a more preferred range is 0.3 to 1.0%.
0.8%.

P: 0.02% or less P is an impurity element inevitably contained in steel, and if its content exceeds 0.02%, the susceptibility to welding hot cracking increases and weld cracking occurs. It will be easier. Therefore, the P content is set to 0.02% or less. Preferred upper limit is 0.01
5%, and a more preferred upper limit is 0.010%. In addition,
The lower the P content, the better.

S: 0.005% or less S is an impurity element inevitably contained in steel, as in the case of the above P, and if its content exceeds 0.005%,
Hot cracking susceptibility is increased, and welding cracks are likely to occur. For this reason, the S content is set to 0.005% or less. A preferred upper limit is 0.003%, and a more preferred upper limit is 0.03%.
02%. The lower the S content, the better.

Cr: 7 to 17% Cr is an element that improves the carbon dioxide gas corrosion resistance. However, if the content is less than 7%, sufficient carbon dioxide corrosion resistance cannot be ensured. Conversely, if the content exceeds 17%, it becomes difficult to secure toughness in HAZ. For this reason, Cr
The content was 7 to 17%. The preferred range is 10-17
%, And a more preferable range is 11 to 16.5%.

Ni: 0.5 to 9% Ni, as-quenched, has a martensite ratio of 50%.
In order to obtain a structure of not less than 0.5% by volume, if the content is less than 0.5%, a structure having a martensite ratio of not less than 50% by volume cannot be obtained. Conversely, its content is 9%
If it exceeds, economic efficiency is impaired. For this reason, the Ni content is set to 0.5 to 9%. A preferred range is 2 to 9%, and a more preferred range is 4 to 8%.

Mo: 0 to 4% Mo may not be added, but if added, is an element having an effect of preventing local corrosion in a carbon dioxide gas environment in the presence of Cr. Therefore, if it is desired to obtain this effect, it can be added, and the effect becomes significant at 0.7% or more. However, even if the content exceeds 4%, the effect is saturated, and conversely, a large amount of expensive Ni must be added to secure toughness, which impairs economic efficiency. Therefore, the Mo content when added is preferably 0.7 to 4%.

Ti: 0 to 0.1% Ti does not need to be added, but if added, it is an element having the effect of fixing C and improving the toughness and strength after quenching and tempering. Therefore, if this effect is desired, it can be added, and the effect becomes significant at 0.01% or more. However, if it is contained in excess of 0.1%, the susceptibility to welding hot cracking is increased, and welding cracks are likely to occur. Therefore, the Ti content when added is 0.01
It is preferable to set it to 0.1%.

Zr: 0 to 0.1% Zr may not be added, but if added, the above-mentioned Ti
Similarly to the above, it is an element having an effect of fixing C and improving the toughness and strength after quenching and tempering. Therefore, if it is desired to obtain this effect, it can be added.
It becomes significant at 005% or more. However, if it is contained in excess of 0.1%, the susceptibility to welding hot cracking is increased, and welding cracks are likely to occur. Therefore, when Zr is added,
The content is preferably set to 0.005 to 0.1%.

V: 0 to 0.1% V may not be added, but if added, is an element having the effect of forming a precipitate with C and improving the strength.
Therefore, if it is desired to obtain this effect, it can be added, and the effect becomes significant at 0.03% or more. But,
If the content exceeds 0.1%, the toughness is impaired. Therefore, when added, the V content is preferably set to 0.03 to 0.1%.

Al: 0 to 0.05% Al does not have to be added, but if added, it is an element having a strong deoxidizing effect, so that the above-mentioned addition amounts of Si and Mn can be reduced. Therefore, if this effect is desired, it can be added, and the effect becomes significant at 0.01% or more. However, when the content exceeds 0.05%, a coarse oxide is generated, and the toughness is reduced. Therefore, the content of Al when added is preferably 0.01 to 0.05%.

N: 0 to 0.04% N may not be added, but if added, it is an element that contributes to solid solution strengthening and has the effect of improving strength. Therefore, if you want to get this effect, you can add
The effect becomes significant at 0.001% or more. But,
If the content exceeds 0.04%, embrittlement is caused. Therefore, the N content when added is 0.001 to 0.04.
%.

However, as for the weld metal constituting the welded joint of the present invention, the Cr, N
It is necessary that the contents of i and Mo satisfy both the following expressions (1) and (2) for the following reasons.

Cr + Mo + 1.8 × Ni ≦ 30 (1) −1 ≦ Cr + Mo−1.7 × Ni ≦ 8 (2) Value obtained by equation “Cr + Mo + 1.8 × Ni” Is 30
If it exceeds, the generation time of the martensite phase in the welding heat cycle process is too early, so that the fine austenite phase is not dispersed and generated, and the desired toughness cannot be ensured by welding.

If the value obtained by the formula "Cr + Mo-1.7 × Ni" is less than -1, no ferrite phase is formed during solidification, the susceptibility to hot cracking is increased, and hot cracking occurs. Conversely, the formula “Cr + Mo−1.7 × Ni”
If the value determined in step (1) exceeds 8, many ferrite phases remain even after cooling, and the toughness is reduced.

Therefore, in the present invention, Cr of the weld metal,
When the content of Ni and Mo is within the above range,
It is determined that the quantity satisfies both the equations (1) and (2).

The preferred upper limit of the above formula (1) is 29,
A more preferred upper limit is 28. The preferred lower limit of the above formula (2) is 0 (zero), the more preferred lower limit is 2, the preferred upper limit is 7, and the more preferred upper limit is 6.

The base material constituting the weld joint is melted and mixed into the weld metal during welding. Therefore, in producing the austenitic alloy welded joint of the present invention, the chemical composition of the base metal constituting the joint and the dilution ratio of the base metal determined by the welding method and the welding conditions are taken into consideration, and the chemical composition of the weld metal is determined by the following formula. It goes without saying that it is necessary to select and use a welding material having a chemical composition falling within the above range specified in the invention.

[0034]

EXAMPLES Six types of sheet thicknesses 1 having the chemical compositions shown in Table 1
A 7 mm base steel sheet and 10 types of welding materials were prepared.

[0035]

[Table 1]

The prepared base steel sheet was subjected to groove processing for forming a V groove having a groove angle of 60 ° and a root face thickness of 2 mm, and butted in the combinations shown in Table 2. Melt welding was performed with the welding material and the welding method to produce a welded joint.

[0037]

[Table 2]

[0038]

[Table 3]

Each symbol shown in the welding method column of Table 2 means the following welding method, respectively.

MAG: MAG welding, PAW: Plasma welding, TIG: TIG welding, SAW: Submerged arc welding, SMAW: Covered arc welding.

The welding materials prepared were SAW and SM.
For AW, 4mm outer diameter, for MAG, PAW and TIG, it is processed into a wire rod with 1.6mm outer diameter. Was used.

Further, MAG is a shielding gas containing 10% by volume of CO ₂ , the balance being Ar mixed gas, PAW and TIG.
Is performed using pure Ar gas as a shielding gas, and the flux of SAW is a product No. LP3 manufactured by Sumitomo Welding Industry Co., Ltd.
Was used.

Further, the welding conditions of each welding method are such that the heat input (kJ / cm) shown in Table 2 at which a good bead shape is obtained.
= [Welding current x welding voltage / welding speed].

Then, while the chemical composition of the weld metal of each of the obtained welded joints was examined, it was subjected to the following tests to evaluate its performance.

Tensile test: A test piece having a parallel part diameter of 6 mm and a length of 30 mm was taken from the weld metal part and subjected to a tensile test at room temperature. , Those less than 800 MPa were rejected "x"
And

Charpy impact test: 10 mm in width and 10 mm in thickness provided with a V notch having a depth of 2 mm at the center of the weld metal.
A test piece for evaluating the toughness of a weld metal having a length of 50 mm, and a HA similar to the above having a V notch having a depth of 2 mm at the fusion boundary.
A test specimen for evaluating the toughness of Z was taken, and the test temperature was -30 ° C.
Tested in impact value passed 50 J / cm ² or more of "○", those of less than 50 J / cm ² was unacceptable "×".

Side bending test: thickness 17 mm, width 10 mm, length 200 mm where the weld metal part is located at the center in the longitudinal direction
And a 180 ° bending test was performed at a radius of curvature of 20 mm. A test piece with no occurrence of cracks in the weld metal was evaluated as “OK”, and a test piece with “X” was rejected.

The above results are shown in Tables 2 and 3.

As can be seen from the results shown in Tables 2 and 3,
As can be seen from the test marks A1 to A14 satisfying the requirements specified in the present invention, crack resistance is obtained by various welding methods and welding conditions.
A welded joint satisfying both strength and toughness has been obtained.

On the other hand, test signs B1, B2 and B
When the value of the expression “Cr + Mo−1.7 × Ni” is excessive, as in the case of No. 6, the toughness deteriorates, and the test codes B3, B5 and B
As in 6, when the value of the expression “Cr + Mo + 1.8 × Ni” was excessive, the strength was reduced. When the value of the expression “Cr + Mo−1.7 × Ni” is too small, as in the test code B4, welding cracks occurred.

[0051]

The martensitic stainless steel welded joint of the present invention is excellent in toughness, strength and crack resistance, so that, for example, the safety of welded structures and welded pipes is improved. In addition, since this welded joint can be obtained in a wide range of welding conditions, welding workability is improved.

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Tomohiko Omura 4-33, Kitahama, Chuo-ku, Osaka City, Osaka Prefecture Inside Sumitomo Metal Industries, Ltd. (72) Kunio Kondo 4-chome, Kitahama, Chuo-ku, Osaka City, Osaka Prefecture No. 5-33 Sumitomo Metal Industries, Ltd. (72) Inventor Masahiko Hamada 4-33 Kitahama, Chuo-ku, Osaka City, Osaka Prefecture F-term in Sumitomo Metal Industries, Ltd. 4E001 AA03 BB05 BB07 BB08 BB11 CA03 4E028 CA13

Claims (1)

[Claims] 1. The base material portion is, in mass%, C: 0.04% or less, Si: 0.01 to 1%, Mn: 0.1 to 1.5%,
P: 0.02% or less, S: 0.005% or less, Cr: 7
-17%, Ni: 0.5-9%, Mo: 0-4%, T
i: 0 to 0.1%, Zr: 0 to 0.1%, V: 0 to 0.
1%, Al: 0 to 0.05%, N: 0 to 0.04%, martensitic stainless steel with the balance being substantially Fe, the weld metal having the same chemistry as the base metal It has a composition, and the relationship between Cr, Ni and Mo is expressed by the following equation (1).
A martensitic stainless steel welded joint having excellent toughness and strength, characterized in that it is a martensitic stainless steel satisfying both equations (2). Cr + Mo + 1.8 × Ni ≦ 30 (1) −1 ≦ Cr + Mo−1.7 × Ni ≦ 8 (2) Here, in the above equations (1) and (2), The element symbol means the content (% by mass) of each element contained in the weld metal.