JP2020104141A - Ferritic heat-resistant steel weld metal and weld joint including the same - Google Patents

Abstract

To provide a ferritic heat-resistant steel weld metal having excellent freezing cracking resistance in addition to high creep strength and toughness and a weld joint including the same.SOLUTION: A ferritic heat-resistant steel weld metal has a chemical composition containing, in mass%, C: 0.06-0.10%, Si: 0.10-0.40%, Mn: 0.30-0.70%, P: 0.010% or less, S: 0.003% or less, Cr: 8.5-9.5%, Mo: 0.01-1.0%, Co: 2.6-3.4%, Ni: 0.01-1.10%, W: 2.5-3.5%, Nb: 0.02-0.08%, Ta: 0.02-0.08%, V: 0.1-0.3%, B: 0.007-0.015%, Al: 0.030% or less, N: 0.005-0.017%, O: 0.020% or less, Cu: 0-1.0%, Ti: 0-0.30%, Ca: 0-0.050%, Mg: 0-0.050%, REM: 0-0.10%, with the balance being Fe and impurities, and satisfies [sol.B≥0.005], [insol.B≤0.0045] and [(20(C+N)+Cr+4Mo+2W+10(Nb+Ta+V)+500 sol.B)/(Mn+Co+Ni) ≥4.5].SELECTED DRAWING: None

Description

The present invention relates to a ferritic heat-resistant steel weld metal and a welded joint provided with the same.

In recent years, in thermal power generation, high temperature and high pressure steam conditions have been promoted in order to improve thermal efficiency. In the future, operation under ultra-supercritical pressure conditions of 650°C and 350 atm is planned. The ferritic heat resistant steel is less expensive than the austenitic heat resistant steel and the Ni-based heat resistant steel. The ferritic heat-resistant steel further has an advantage as a heat-resistant steel that the coefficient of thermal expansion is small. Therefore, ferritic heat resistant steel is widely used in high temperature and high pressure environments.

Further, the ferritic heat-resistant steel may be welded and used as a welded joint in a structure. In this case, the creep strength of the weld heat affected zone (hereinafter referred to as "HAZ") of the welded joint may be reduced. Therefore, Patent Documents 1 to 3 propose ferritic heat-resisting steels that suppress the decrease in creep strength in HAZ.

The ferritic heat-resistant steel disclosed in Patent Document 1 contains 0.003 to 0.03% by mass of B to suppress grain refinement in the HAZ. This suppresses the decrease in creep strength in HAZ. The ferritic heat-resistant steels disclosed in Patent Documents 2 and 3 contain a large amount of B, and the C content is adjusted according to the welding heat input or the B content. This suppresses the strength reduction in the HAZ and suppresses liquefaction cracking during welding.

When welding a ferritic heat resistant steel containing a large amount of B, a welding material is generally used. A weld metal formed using a commercially available welding material for Ni-based heat-resistant alloys (for example, JIS Z 3334 (2011) SNi6082) has high creep strength and toughness. However, during welding, B flows into the weld metal from the base metal, especially in the first layer weld where the base metal is highly diluted.

In this case, solidification cracking may occur. Therefore, the welding material used for welding the ferritic heat-resistant steel containing B is required not only to have high creep strength and toughness in the weld metal, but also to suppress solidification cracking during welding.

Therefore, Patent Document 4 discloses a welding material for ferritic heat resistant steel and a welded joint for ferritic heat resistant steel, which can form a weld metal having high creep strength and toughness when welding ferritic heat resistant steel containing B. Has been done.

JP, 2004-300532, A JP, 2010-7094, A International Publication No. 2008/149703 Japanese Patent No. 6338028

When the ferritic heat-resistant steel containing B is welded using the welding material for ferritic heat-resistant steel described in Patent Document 4, a weld metal having high creep strength and toughness can be formed. However, as a result of further studies by the present inventors, it was found that there is room for further improvement in terms of solidification crack resistance.

It is an object of the present invention to solve the above problems, and to provide a ferritic heat-resistant steel weld metal having high creep strength and toughness as well as excellent solidification cracking resistance, and a welded joint including the same.

The present invention has been made in order to solve the above problems, and has as its gist the following ferritic heat resistant steel weld metal and ferritic heat resistant steel welded joint.

(1) The chemical composition is mass%,
C: 0.06 to 0.10%,
Si: 0.10 to 0.40%,
Mn: 0.30 to 0.70%,
P: 0.010% or less,
S: 0.003% or less,
Cr: 8.5-9.5%,
Mo: 0.01 to 1.0%,
Co: 2.6-3.4%,
Ni: 0.01-1.10%,
W: 2.5-3.5%,
Nb: 0.02 to 0.08%,
Ta: 0.02 to 0.08%,
V: 0.1 to 0.3%,
B: 0.007 to 0.015%,
Al: 0.030% or less,
N: 0.005-0.017%,
O: 0.020% or less,
Cu: 0 to 1.0%,
Ti: 0 to 0.30%,
Ca: 0 to 0.050%,
Mg: 0 to 0.050%,
REM: 0 to 0.10%,
Balance: Fe and impurities,
Satisfies the following expressions (i) to (iii),
Ferritic heat resistant steel weld metal.
sol. B≧0.005 (i)
insol. B≦0.0045 (ii)
(20(C+N)+Cr+4Mo+2W+10(Nb+Ta+V)+500sol.B)/(Mn+Co+Ni)≧4.5 (iii)
However, each element symbol in the formula represents the content (mass %) of each element contained in the weld metal, and the sol. B is the content (% by mass) of B dissolved in the weld metal, insol. B is the B content (mass %) present as a precipitate in the weld metal.

(2) The chemical composition is mass%,
Cu: 0.05-1.0%,
Ti: 0.02 to 0.30%,
Ca: 0.001 to 0.050%,
Mg: 0.001 to 0.050%, and
REM: 0.001 to 0.10%,
Containing one or more selected from,
The ferritic heat-resistant steel weld metal according to (1) above.

(3) Includes the ferritic heat-resistant steel weld metal according to (1) or (2) above, and a base material made of ferritic heat-resistant steel.
Ferritic heat resistant steel welded joint.

(4) The chemical composition of the base material is% by mass,
C: 0.04 to 0.12%,
Si: 0.05-0.60%,
Mn: 0.10 to 0.80%,
P: 0.020% or less,
S: 0.010% or less,
Cr: 8.0 to 10.0%,
Co: 2.0-4.0%,
Ni: 0 to 0.4%,
W: 2.0 to 4.0%,
Nb+Ta: 0.02 to 0.18% in total,
V: 0.05 to 0.40%,
B: 0.005-0.020%,
Al: 0.030% or less,
N: 0.002-0.025%,
O: 0.020% or less,
Nd: 0.01 to 0.06%,
Remainder: Fe and impurities,
The ferritic heat resistant steel welded joint according to (3) above.

According to the present invention, it becomes possible to obtain a ferritic heat-resistant steel weld metal having excellent creep resistance and solidification cracking resistance in addition to high creep strength and toughness, and a welded joint provided with the same.

Based on the technique described in Patent Document 4, the present inventors have made extensive studies on a method for further improving solidification crack resistance, and as a result, have obtained the following findings.

B is an element indispensable for improving the creep strength, but on the other hand, since it deteriorates the solidification crack resistance, its content needs to be controlled within an appropriate range.

Further, B may exist as a solid solution or as a precipitate in the weld metal, but B existing as a precipitate hardly contributes to the improvement of creep strength and causes deterioration of solidification crack resistance. Only. Therefore, in order to achieve both the creep strength and the solidification cracking resistance, it is necessary to suppress the precipitation of B as much as possible and to secure the solid solution amount.

In order to suppress the precipitation of B, it is important to control the welding conditions, and it is necessary to keep the temperature in a temperature range where B is likely to precipitate as much as possible.

The present invention has been made based on the above findings. Hereinafter, each requirement of the present invention will be described in detail.

1. Weld Metal The ferritic heat-resistant steel weld metal according to the present invention has the chemical composition shown below. The reasons for limiting each element are as follows. In the following description, “%” regarding the content means “mass %”.

C: 0.06 to 0.10%
Carbon (C) suppresses the formation of δ ferrite in the weld metal, and makes the main structure of the weld metal a martensite structure. C further produces fine carbides (M ₂₃ C ₆ carbides) when used at high temperatures, increasing the creep strength. If the C content is too low, these effects cannot be obtained. On the other hand, if the C content is too high, a large amount of coarse carbide is deposited, and the toughness of the weld metal is reduced. Therefore, the C content is 0.06 to 0.10%. The C content is preferably 0.07% or more, and preferably 0.09% or less.

Si: 0.10 to 0.40%
Silicon (Si) has the effect of deoxidizing steel. Si further enhances the steam oxidation resistance properties of the weld metal. If the Si content is too low, these effects cannot be obtained. On the other hand, if the Si content is too high, the formation of δ ferrite is promoted, the toughness of the weld metal is lowered, and the creep ductility is also lowered. Therefore, the Si content is 0.10 to 0.40%. The Si content is preferably 0.25% or more, and preferably 0.35% or less.

Mn: 0.30 to 0.70%
Manganese (Mn) has an effect of deoxidizing steel similarly to Si. Mn further promotes martensitic transformation of the weld metal structure. If the Mn content is too low, these effects cannot be obtained. On the other hand, if the Mn content is too high, creep embrittlement easily occurs in the weld metal. Therefore, the Mn content is 0.30 to 0.70%. The Mn content is preferably 0.40% or more, and preferably 0.60% or less.

P: 0.010% or less Phosphorus (P) is an element contained in steel as an impurity. P reduces the toughness of the weld metal. Therefore, the P content is 0.010% or less. The P content is preferably 0.008% or less, and is preferably as low as possible. However, from the viewpoint of material cost, the P content is preferably 0.0005% or more.

S: 0.003% or less Sulfur (S) is an element contained in steel as an impurity. S segregates at the prior austenite grain boundaries and lath interfaces in the weld metal containing B, and reduces the adhesion strength at the grain boundaries and lath interfaces, and as a result, reduces the toughness of the weld metal. Therefore, the S content is 0.003% or less. The S content is preferably less than 0.002%, more preferably less than 0.0015%. It is preferable that the S content is as low as possible. However, the S content is preferably 0.0002% or more from the viewpoint of the effect and the material cost.

Cr: 8.5-9.5%
Chromium (Cr) enhances the steam oxidation resistance and corrosion resistance of the weld metal. Cr also precipitates as carbides during use at high temperatures, increasing creep strength. If the Cr content is too low, these effects cannot be obtained. On the other hand, if the Cr content is too high, the stability of the carbide is reduced and the creep strength is reduced. If the Cr content is too high, the formation of δ ferrite is further promoted, and the toughness is reduced. Therefore, the Cr content is 8.5 to 9.5%. The Cr content is preferably 8.7% or more, and preferably 9.3% or less.

Mo: 0.01-1.0%
Molybdenum (Mo) forms a solid solution in the matrix and enhances the creep strength of the weld metal. If the Mo content is too low, this effect cannot be obtained. On the other hand, when the Mo content is too high, Mo is solidified and segregated, and the long-term stability of the W-containing intermetallic compound and carbide described later is reduced. Further, it precipitates as an intermetallic compound, and the toughness of the weld metal decreases. Therefore, the Mo content is 0.01 to 1.0%. The Mo content is preferably 0.1% or more, and preferably 0.5% or less.

Co: 2.6-3.4%
Cobalt (Co) suppresses the formation of δ ferrite and is effective in obtaining a martensite structure. Unlike the base metal, since the weld metal is not heat treated, the lower limit of the Co content is 2.6% in order to sufficiently obtain the above effect. On the other hand, if the Co content is too high, the creep strength is rather reduced and the creep ductility is also reduced. Further, since Co is an expensive element, the material cost is high. Therefore, the Co content is 2.6 to 3.4%. The Co content is preferably 2.8% or more, and preferably 3.3% or less.

Ni: 0.01-1.10%
Nickel (Ni) is effective in suppressing the formation of δ ferrite and obtaining a martensite structure. Ni further enhances the toughness of the weld metal. If the Ni content is too low, these effects cannot be obtained. On the other hand, if the Ni content is too high, the creep ductility decreases. Further, since Ni is an expensive element, the material cost is high. Therefore, the Ni content is 0.01 to 1.10%. The Ni content is preferably 0.04% or more, and preferably 1.00% or less.

W: 2.5-3.5%
Tungsten (W) is solid-solved in the matrix or is precipitated as an intermetallic compound during long-term use, and enhances the creep strength of the weld metal at high temperatures. If the W content is too low, this effect cannot be obtained. On the other hand, if the W content is too high, a large amount of precipitates are formed. Furthermore, the formation of δ ferrite is promoted, and the toughness of the weld metal is reduced. Therefore, the W content is 2.5 to 3.5%. The W content is preferably 2.7% or more, and preferably 3.3% or less.

Nb: 0.02-0.08%
Niobium (Nb) precipitates in the grains as fine carbonitrides during use at high temperatures, increasing the creep strength of the weld metal. If the Nb content is too low, this effect cannot be obtained. On the other hand, if the Nb content is too high, a large amount of coarse carbonitride precipitates, and the creep strength and creep ductility decrease. Furthermore, the formation of δ ferrite is promoted, and the toughness of the weld metal is reduced. Therefore, the Nb content is 0.02 to 0.08%. The Nb content is preferably 0.03% or more, and preferably 0.07% or less.

Ta: 0.02 to 0.08%
Tantalum (Ta), like Nb, precipitates in the grains as fine carbonitrides during use at high temperatures and enhances the creep strength of the weld metal. If the Ta content is too low, this effect cannot be obtained. On the other hand, if the Ta content is too high, a large amount of coarse carbonitride precipitates, and the creep strength and creep ductility decrease. Therefore, the Ta content is 0.02 to 0.08%. The Ta content is preferably 0.03% or more, and preferably 0.07% or less.

V: 0.1-0.3%
Vanadium (V), like Nb and Ta, precipitates in the grains as fine carbonitrides during use at high temperatures and enhances the creep strength of the weld metal. If the V content is too low, this effect cannot be obtained. On the other hand, if the V content is too high, a large amount of coarse carbonitride precipitates, and the creep strength and creep ductility decrease. Furthermore, the formation of δ ferrite is promoted, and the toughness of the weld metal is reduced. Therefore, the V content is 0.1 to 0.3%. The V content is preferably 0.15% or more, and preferably 0.25% or less.

B: 0.007 to 0.015%
Boron (B) enhances the hardenability and is effective in obtaining a martensitic structure in the weld metal. B further finely disperses the carbides at the prior austenite boundaries and martensite lath boundaries during use at high temperatures, suppressing the recovery of the structure and increasing the creep strength. If the B content is too low, these effects cannot be obtained. On the other hand, if the B content is too high, the martensite lath abruptly extends during the martensitic transformation, and the fracture unit increases. Furthermore, the formation of δ ferrite is promoted. Therefore, the toughness of the weld metal is extremely reduced. Therefore, the B content is 0.007 to 0.015%. The B content is preferably 0.009% or more, and preferably 0.012% or less.

Here, B exists as a solid solution or as a precipitate in the weld metal. As described above, B exhibits the effect of increasing the creep strength when it is contained as a solid solution, and when it exists as a precipitate, it has little effect and only causes deterioration of solidification crack resistance. Is. Therefore, it is necessary to control the contents of B existing as a solid solution or as a precipitate so as to satisfy the following expressions (i) and (ii), respectively.
sol. B≧0.005 (i)
insol. B≦0.0045 (ii)
However, sol. B is the content (mass %) of B dissolved in the weld metal. B is the B content (mass %) present as a precipitate in the weld metal.

In addition, sol. B and insol. B is obtained by the following procedure. About 0.4 g of the weld metal is electrolyzed with 10% acetylacetone-1% tetramethylammonium chloride/methanol at a current value of 20 mA/cm ² . After that, the solution containing the electrolyzed weld metal is filtered through a 0.2 μm filter, and the residue is acid-decomposed with a mixed acid of sulfuric acid+phosphoric acid+nitric acid+perchloric acid. Then, the amount of residue of B is obtained by an ICP emission spectroscopic analyzer, and B. In addition, from the B content in the molten steel, insol. By subtracting B, the sol. B.

Al: 0.030% or less Aluminum (Al) has an effect of deoxidizing steel. However, if the Al content is too high, the cleanability is deteriorated and the toughness and creep strength are deteriorated. Therefore, the Al content is 0.030% or less. The Al content is preferably 0.010% or less. Considering the manufacturing cost, the Al content is preferably 0.001% or more. In the present specification, the Al content is sol. It means Al (acid-soluble Al).

N: 0.005-0.017%
Nitrogen (N) finely precipitates in the grains as fine nitrides during use at high temperature, and increases the creep strength. N further suppresses the formation of δ ferrite. If the N content is too low, these effects cannot be obtained. On the other hand, if the N content is too high, coarse nitrides crystallize out during solidification of the weld metal, and the toughness of the weld metal deteriorates. Therefore, the N content is 0.005 to 0.017%. The N content is preferably 0.008% or more, and preferably 0.015% or less.

O: 0.020% or less Oxygen (O) is an element contained in steel as an impurity. If the O content is too high, the toughness decreases. Therefore, the content of O is 0.020% or less. The O content is preferably 0.010% or less. Considering the effect and the manufacturing cost, the O content is preferably 0.001% or more.

Cu: 0 to 1.0%
Copper (Cu) is effective in generating a martensite structure, and thus may be contained if necessary. However, if the Cu content is too high, the creep ductility of the weld metal deteriorates. Therefore, the Cu content is 0 to 1.0%. The Cu content is preferably 0.8% or less. To obtain the above effect, the Cu content is preferably 0.05% or more, more preferably 0.2% or more.

Ti: 0 to 0.30%
Titanium (Ti), like Nb, Ta, and V, precipitates in the grains as fine carbonitrides during use at high temperatures and increases the creep strength of the weld metal. Good. However, if the Ti content is too high, it crystallizes as a coarse nitride during welding, or precipitates in large amounts as a coarse nitride during use at high temperatures, which lowers the toughness of the weld metal. Therefore, the Ti content is 0 to 0.30%. In order to obtain the above effect, the Ti content is preferably 0.02% or more, more preferably 0.05% or more.

Ca: 0 to 0.050%
Mg: 0 to 0.050%
REM: 0 to 0.10%
Calcium (Ca), magnesium (Mg), and rare earth element (REM) are used to enhance hot workability during the production of welding materials, and thus may be contained in the metal material as necessary. However, if the content of these elements is too high, these elements combine with oxygen and deteriorate the cleanliness of the weld metal. In this case, the hot workability of the weld metal is reduced. Therefore, the Ca content is 0 to 0.050%, the Mg content is 0 to 0.050%, and the REM content is 0 to 0.10%.

Each of the Ca and Mg contents is preferably 0.020% or less, and the REM content is preferably 0.06% or less. In order to obtain the above effect, it is preferable to contain one or more selected from Ca: 0.001 or more, Mg: 0.001 or more, and REM: 0.001 or more.

Here, REM refers to a total of 17 elements of Sc, Y and lanthanoid, and the content of the REM means the total content of these elements.

Further, the chemical composition of the weld metal according to the present invention needs to satisfy the following formula (iii). The value on the left side of the equation (iii) is an index of creep strength. By satisfying the expression (iii), sufficient creep strength can be secured. The value on the left side of the formula (iii) is preferably 5.0 or more, and more preferably 5.5 or more.
(20(C+N)+Cr+4Mo+2W+10(Nb+Ta+V)+500sol.B)/(Mn+Co+Ni)≧4.5 (iii)
However, each element symbol in the formula represents the content (mass %) of each element contained in the weld metal.

In the chemical composition of the weld metal of the present invention, the balance is Fe and impurities. Here, the "impurity" is a component that is mixed in by raw materials such as ore and scrap when manufacturing steel industrially, and various factors in the manufacturing process, and is allowed within a range that does not adversely affect the present invention. Means something.

2. Welded Joint The ferritic heat resistant steel welded joint according to the present invention includes the above-described weld metal and a base material made of ferritic heat resistant steel. The chemical composition of the base material is not particularly limited, but preferably has the chemical composition shown below.

C: 0.04 to 0.12%
Carbon (C) is an element effective for obtaining a martensite structure. C further produces fine carbides when used at high temperatures, increasing the creep strength of the base material. If the C content is too low, these effects cannot be obtained. However, unlike the weld metal, the base metal is suppressed in solidification segregation and is used after being heat treated. Therefore, even if the C content is lower than that of the weld metal, the above effect can be obtained. On the other hand, if the C content is too high, the effect of improving creep strength is saturated. Therefore, the C content is 0.04 to 0.12%. The C content is preferably 0.06% or more, and preferably 0.10% or less.

Si: 0.05-0.60%
Silicon (Si) has the effect of deoxidizing steel. Si further enhances the steam oxidation resistance of the base material. If the Si content is too low, these effects cannot be obtained. However, unlike the weld metal, the base metal is suppressed in solidification segregation and is used after being heat treated. Therefore, even if the Si content is lower than that in the case of weld metal, the above effect can be obtained. On the other hand, if the Si content is too high, the creep ductility and toughness of the base material deteriorate. Therefore, the Si content is 0.05 to 0.60%. The Si content is preferably 0.10% or more, and more preferably 0.40% or less.

Mn: 0.10 to 0.80%
Manganese (Mn) has an effect of deoxidizing steel similarly to Si. Mn further promotes martensite formation of the base metal structure. If the Mn content is too low, these effects cannot be obtained. However, unlike the weld metal, the base metal is suppressed in solidification segregation and is used after being heat treated. Therefore, even if the Mn content is lower than that in the case of weld metal, the above effect can be obtained. On the other hand, if the Mn content is too high, creep embrittlement easily occurs. Therefore, the Mn content is 0.10 to 0.80%. The Mn content is preferably 0.20% or more, and preferably 0.70% or less.

P: 0.020% or less Phosphorus (P) is an element contained in steel as an impurity. If the P content is too high, the creep ductility decreases. Therefore, the P content is 0.020% or less. The P content is preferably 0.018% or less, and is preferably as low as possible. However, from the viewpoint of material cost, the P content is preferably 0.0005% or more.

S: 0.010% or less Sulfur (S) is an element contained in steel as an impurity. If the S content is too high, the creep ductility decreases. Therefore, the S content is 0.010% or less. The S content is preferably 0.005% or less, and is preferably as low as possible. However, from the viewpoint of material cost, the S content is preferably 0.0002% or more.

Cr: 8.0-10.0%
Chromium (Cr) enhances steam oxidation resistance and corrosion resistance of the base material at high temperatures. Cr also precipitates as carbides during use at high temperatures, increasing the creep strength of the base material. If the Cr content is too low, these effects cannot be obtained. However, unlike the weld metal, the base metal is suppressed in solidification segregation and is used after being heat treated. Therefore, even if the Cr content is lower than that in the case of weld metal, the above effect can be obtained. On the other hand, if the Cr content is too high, the stability of the carbides decreases and the creep strength of the base material decreases. Therefore, the Cr content is 8.0 to 10.0%. The Cr content is preferably 8.5% or more, and more preferably 9.5% or less.

Co: 2.0-4.0%
Cobalt (Co) is effective in increasing the creep strength by making the structure of the base material a martensite structure. If the Co content is too low, this effect cannot be obtained. However, unlike the weld metal, the base metal is suppressed in solidification segregation and is used after being heat treated. Therefore, even if the Co content is lower than that of the weld metal, the above effect can be obtained. On the other hand, if the Co content is too high, the creep strength and the creep ductility of the base material deteriorate. Further, since Co is an expensive element, the material cost is high. Therefore, the Co content is 2.0 to 4.0%. The Co content is preferably 2.5% or more, and preferably 3.5% or less.

Ni: 0 to 0.4%
Nickel (Ni) is effective for obtaining a martensitic structure, and thus may be contained if necessary. However, if the Ni content is too high, the above effect is saturated. Therefore, the Ni content is 0 to 0.4%. The Ni content is preferably 0.2% or less. In order to obtain the above effects, the Ni content is preferably 0.05% or more, more preferably 0.1% or more.

W: 2.0 to 4.0%
Tungsten (W) is solid-dissolved in the matrix or precipitates as an intermetallic compound during long-term use and enhances creep strength at high temperatures. If the W content is too low, this effect cannot be obtained. However, unlike the weld metal, the base metal is suppressed in solidification segregation and is used after being heat treated. Therefore, even if the W content is lower than that of the weld metal, the above effect can be obtained. On the other hand, if the W content is too high, the above effect is saturated. Therefore, the W content is W: 2.0 to 4.0%. The W content is preferably 2.5% or more, and preferably 3.5% or less.

Nb+Ta: 0.02 to 0.18% in total
Niobium (Nb) and tantalum (Ta) precipitate in the grains as fine carbonitrides during use at high temperatures, increasing creep strength. If the content of Nb and/or Ta is too low, this effect cannot be obtained. However, unlike the weld metal, the base metal is suppressed in solidification segregation and is used after being heat treated. Therefore, even if the contents of these elements are lower than in the case of weld metal, the above effects can be obtained. On the other hand, when the content of Nb and/or Ta is too high, a large amount of coarse carbonitride precipitates, and the creep strength and creep ductility decrease. Therefore, the total content of Nb and Ta is 0.02 to 0.18%. The total content of Nb and Ta is preferably 0.05% or more, and preferably 0.12% or less.

V: 0.05 to 0.40%
Vanadium (V), like Nb and Ta, precipitates in the grains as fine carbonitrides during use at high temperatures and enhances creep strength. If the V content is too low, this effect cannot be obtained. However, unlike the weld metal, the base metal is suppressed in solidification segregation and is used after being heat treated. Therefore, even if the V content is lower than that of the weld metal, the above effect can be obtained. On the other hand, if the V content is too high, a large amount of coarse carbonitride precipitates, and the creep strength and creep ductility decrease. Therefore, the V content is 0.05 to 0.40%. The V content is preferably 0.10% or more, and preferably 0.30% or less.

B: 0.005-0.020%
Boron (B) is effective in enhancing the hardenability and obtaining a martensite structure. Further, B finely disperses carbides at the former austenite boundary and the martensite lath boundary during use at a high temperature to suppress the recovery of the structure and enhance the creep strength. If the B content is too low, this effect cannot be obtained. However, unlike the weld metal, the base metal is suppressed in solidification segregation and is used after being heat treated. Therefore, even if the B content is lower than in the case of weld metal, the above effect can be obtained. On the other hand, if the B content is too high, the toughness decreases. Therefore, the B content is 0.005 to 0.020%. The B content is preferably 0.007% or more, and more preferably 0.015% or less.

Al: 0.030% or less Aluminum (Al) has an effect of deoxidizing steel. However, if the Al content is too high, the cleanability of the base material is lowered and the workability is lowered. If the Al content is too high, the creep strength further decreases. Therefore, the Al content is 0.030% or less. The Al content is preferably 0.010% or less. Considering the manufacturing cost, the Al content is preferably 0.001% or more. In the present specification, the Al content is sol. It means Al (acid-soluble Al).

N: 0.002-0.025%
Nitrogen (N) finely precipitates in the grains as fine nitrides during use at high temperature, and increases the creep strength. If the N content is too low, this effect cannot be obtained. However, unlike the weld metal, the base metal is suppressed in solidification segregation and is used after being heat treated. Therefore, even if the N content is lower than that of the weld metal, the above effect can be obtained. On the other hand, if the N content is too high, the nitride is coarsened and the creep ductility is reduced. Therefore, the N content is 0.002 to 0.025%. The N content is preferably 0.005% or more, and preferably 0.015% or less.

O: 0.020% or less Oxygen (O) is an element contained in steel as an impurity. If the O content is too high, the workability of the base material is reduced. Therefore, the O content is 0.020% or less. The O content is preferably 0.010% or less. Considering the manufacturing cost, the O content is preferably 0.001% or more.

Nd: 0.01 to 0.06%
Neodymium (Nd) improves the creep ductility of the base material. If the Nd content is too low, this effect cannot be obtained. The above effect of Nd can be effectively utilized in the base material in which there is no fear of reduction as slag during welding. On the other hand, if the Nd content is too high, the hot workability deteriorates. Therefore, the Nd content is 0.01 to 0.06%. The Nd content is preferably 0.02% or more, and preferably 0.05% or less.

In the chemical composition of the base material of the present invention, the balance is Fe and impurities. Here, the "impurity" is a component that is mixed in by raw materials such as ore and scrap when manufacturing steel industrially, and various factors in the manufacturing process, and is allowed within a range that does not adversely affect the present invention. Means something.

3. Welded joint manufacturing method In the above-mentioned welded joint manufacturing method, the base material is welded using a welding material for ferritic heat-resistant steel (welding step), and the heat treatment is performed on the weld metal after welding. And a step (heat treatment step). Hereinafter, each step will be described in detail.

<Welding process>
Welding is performed on the base material using a welding material to form a weld metal. The chemical composition of the welding material used for welding is not particularly limited and may be selected so that the chemical composition of the weld metal is the above-mentioned composition. When the ferritic heat resistant steel having the above chemical composition is used as the base material, the chemical composition of the welding material may be within the range of the content of each element described in the chemical composition of the weld metal. The shape of the base material is not particularly limited. The base material may be a steel plate or a steel pipe.

Gas tungsten arc welding is preferably used as the welding method. This is because gas tungsten arc welding has a small amount of oxygen mixed in during welding and suppresses deterioration of the cleanliness of the weld metal. The welding conditions for gas tungsten arc welding are as follows.

Welding heat input range: 6 to 20 kJ/cm
In the gas tungsten arc welding, if the welding heat input is too low, defective fusion tends to occur depending on the size and shape of the base material. If the welding heat input is too low, the cooling rate becomes too high, and the growth of martensite lath is promoted. In this case, the fracture unit becomes large and the toughness of the weld metal deteriorates.

On the other hand, when the welding heat input is too high, the cooling rate is low and B is likely to precipitate, so that the solidification cracking resistance decreases. Therefore, the welding heat input is set to 6 to 20 kJ/cm. The welding heat input is preferably 8 kJ/cm or more, and preferably 18 kJ/cm or less. If the welding heat input range satisfies this condition, excellent toughness and solidification crack resistance are likely to be obtained.

Further, in order to suppress the precipitation of B, it is important not to maintain the temperature range where B is likely to precipitate. If the time between welding passes is not sufficiently secured, the temperature of the welded metal part is kept high, and the amount of B existing as a precipitate increases. Therefore, it is preferable that the temperature of the welded joint between the passes is 300° C. or less and the welded joint is sufficiently cooled.

<Heat treatment process>
After forming the weld metal, heat treatment is performed on the weld metal. The heat treatment reduces the hardness of the weld metal and increases the toughness. For example, a heat treatment apparatus such as a band heater and an induction heater is arranged at a welded portion including a weld metal portion to perform heat treatment. Alternatively, the entire welded structure is heated in the heating furnace. The heat treatment temperature in the heat treatment and the holding time (heat treatment time) at the heat treatment temperature are as follows.

Heat treatment temperature: 740-780°C
Heat treatment time: 0.5 to 4.0 hours per 25.4 mm thickness of the base metal The unit thickness of the base metal is 25.4 mm (1 inch), which is often specified by welding construction standards and the like. .. If the heat treatment temperature is too low, or if the heat treatment time per unit thickness of the base material is too short, tempering of the martensite structure is insufficient and sufficient toughness cannot be obtained.

On the other hand, if the heat treatment temperature is too high, a part of the weld metal exceeds the austenite transformation temperature, and the toughness decreases. Further, if the heat treatment time per unit thickness of the base material is too long, tempering becomes excessive and the creep strength decreases. Therefore, the heat treatment temperature is 740 to 780° C., and the heat treatment time is 0.5 to 4.0 hours per 25.4 mm of the base material thickness.

Here, the thickness of the base material is the plate thickness when the base material is a steel plate, and the wall thickness when the base material is a steel pipe. The preferable lower limit of the heat treatment time is 1.0 hour per 25.4 mm of the thickness of the base material, and the preferable upper limit is 3.0 hours. When the heat treatment temperature and the heat treatment time satisfy this condition, for example, the creep rupture time of the weld metal can be set to 3000 hours or more, and excellent toughness is easily obtained.

Hereinafter, the present invention will be described more specifically by way of examples, but the present invention is not limited to these examples.

An ingot was manufactured using molten steel having the chemical composition shown in Table 1, and then hot forging and hot rolling were performed to manufacture a steel sheet. By quenching and tempering the steel plate, a base steel plate having a plate thickness of 12 mm, a width of 50 mm and a length of 200 mm (hereinafter simply referred to as a base material) was manufactured. In quenching, the steel sheet was held at 1100° C. for 1 hour and then air-cooled (air-cooled quenching). In the tempering, the steel sheet after quenching was held at 770° C. for 1.5 hours.

Next, an ingot was manufactured using molten steel having the chemical composition shown in Table 2, and then hot forging, hot rolling and machining were performed to manufacture a filler wire having a diameter of 2.4 mm. The manufactured filler wire was used as the welding material.

A V groove having an angle of 30° and a root thickness of 1 mm was processed in the longitudinal direction of the base material. V-grooves of a pair of base materials were butted against each other, and welding was performed using the above-mentioned welding material. Specifically, the welding material was laminated and welded in the groove to form weld metal by gas tungsten arc welding using Ar as a shielding gas, and each welded joint was manufactured (Test Nos. 1 to 15).

At this time, the welding heat input was 6 kJ/cm for the first layer welding and 15 kJ/cm for the laminated welding. In addition, the test No. With the exception of 11, the weld joint temperature between passes was controlled to be 300°C or lower. On the other hand, the test No. For No. 11, the weld joint temperature between passes was in the range of 300 to 400°C. The welded joint after welding was held at 740° C. for 0.5 hour and then air-cooled.

The chemical composition of the weld metal of the obtained welded joint was measured. The method for measuring the chemical composition of the weld metal was as follows. Chips were collected so that the base metal did not mix from the weld metal. The collected chips were analyzed by inductively coupled plasma optical emission spectroscopy and high frequency combustion method. Table 3 shows the measurement results of the chemical composition of the weld metal.

Separately from the above-mentioned welded joint, multi-layer welding was performed on the surface of the base material shown in Table 1 by using the welding material shown in Table 2 and gas tungsten arc welding using Ar as a shielding gas until the thickness became 12 mm. .. This produced the weld metal. The welding conditions at this time are the same as the above-mentioned conditions.

Then, 6 mm square and 40 mm long test pieces for electrolytic extraction were collected from the above-mentioned weld metal. Then, using the above test piece, sol. B and insol. B was measured. First, about 0.4 g of the above test piece was electrolyzed with 10% acetylacetone-1% tetramethylammonium chloride/methanol at a current value of 20 mA/cm ² . Then, the electrolyzed solution was filtered through a 0.2 μm filter, and the residue was acid-decomposed with a mixed acid of sulfuric acid+phosphoric acid+nitric acid+perchloric acid. Then, the amount of residue of B is obtained by an ICP emission spectroscopic analyzer, and It was set to B. In addition, from the B content in the molten steel, insol. By subtracting B, the sol. It was set to B.

Next, a round bar creep rupture test piece (referred to as a joint test piece) was sampled from the above-mentioned welded joint so that the weld metal was located at the center of the parallel portion. Further, a round bar creep rupture test piece (referred to as a molten metal test piece) was also taken from the weld metal formed on the surface of the base material. A creep rupture test was performed on each test piece under the test conditions of 650° C. and 147 MPa at which the target creep rupture time of the base material was about 3000 hours.

From the test results, the creep strength was judged by the following evaluations. When the joint test piece broke in the base material (HAZ) and the creep rupture time of the molten metal test piece was 6000 hours or more, it was defined as “good”. The case where the joint test piece broke in the base material (HAZ) and the creep rupture time of the molten metal test piece was 3000 hours or more and less than 6000 hours was evaluated as "OK". In the joint test piece, the test piece that broke at the weld metal portion or the creep rupture time of the molten metal test piece of less than 3000 hours was regarded as "fail".

Next, test specimens for observation were sampled from three locations of the above-mentioned welded joint so that the observation surface was the cross-section of the joint (a section perpendicular to the weld bead). The collected test piece for observation was mirror-polished and corroded with Virella reagent (1 g of picric acid, 5 ml of hydrochloric acid, 100 ml of ethanol) for about 30 seconds to 2 minutes, and then observed with an optical microscope to check for cracks in the heat affected zone. investigated. Weld joints in which cracks were not observed in all three samples were judged as “good”, and weld joints in which cracks were observed were judged as “fail”.

Further, three full-size V-notch Charpy impact test pieces (notch depth 2 mm) obtained by processing a notch in the weld metal were taken from the above-mentioned welded joint. A Charpy impact test according to JIS Z2242 (2005) was performed on each test piece at 0°C. Based on the test results, toughness was judged as follows.

"Good" when all three absorbed energy values in the Charpy impact test exceed 27J, and when at least one of the three absorbed energy values is less than 27J, the average value satisfies 27J. “Fair”, and those in which the average value of the three pieces was less than 27 J were defined as “fail”.

The results are summarized in Table 4.

With reference to Tables 3 and 4, the test No. The weld metals 1 to 7 all satisfy the requirements of the present invention. Therefore, it can be seen that these weld metals have excellent creep strength, solidification crack resistance and toughness. The obtained welded joint also showed sufficient creep strength and toughness.

On the other hand, the test No. In the weld metal of No. 8, Mo content, B content and sol. Since the B content was below the lower limit, the creep rupture time was less than 3000 hours and the creep strength was unacceptable. Test No. In the weld metal of No. 9, since the Mo content exceeds the upper limit, the Charpy impact value was below 27 J, and the toughness was unacceptable.

Test No. In the weld metals 10 and 13, since the N content exceeds the upper limit, the B content (insol.B) contained as a precipitate was high. Therefore, the Charpy impact value was less than 27 J, and the toughness was unacceptable. Test No. Although the weld metal of No. 11 has an appropriate chemical composition, it has a high inter-pass temperature and a high B content (insol.B) existing as a precipitate, so the Charpy impact value is below 27 J, The toughness was rejected.

Test No. The weld metal No. 12 did not satisfy the formula (iii), although the content of each element was within the specified range. Therefore, the creep rupture time was shorter than 3000 hours, and the creep strength failed. Test No. In the weld metal of No. 14, since the B content was less than the lower limit value, the B content (sol.B) in solid solution was low, the creep rupture time was less than 3000 hours, and the creep strength was unacceptable. ..

Test No. In the weld metal of No. 15, since the B content exceeded the upper limit, the creep strength was good, but the toughness failed and solidification cracking occurred.

According to the present invention, it becomes possible to obtain a ferritic heat-resistant steel weld metal having excellent creep resistance and solidification cracking resistance in addition to high creep strength and toughness, and a welded joint provided with the same.

Claims (4)

The chemical composition is% by mass,
C: 0.06 to 0.10%,
Si: 0.10 to 0.40%,
Mn: 0.30 to 0.70%,
P: 0.010% or less,
S: 0.003% or less,
Cr: 8.5-9.5%,
Mo: 0.01 to 1.0%,
Co: 2.6-3.4%,
Ni: 0.01-1.10%,
W: 2.5-3.5%,
Nb: 0.02 to 0.08%,
Ta: 0.02 to 0.08%,
V: 0.1 to 0.3%,
B: 0.007 to 0.015%,
Al: 0.030% or less,
N: 0.005-0.017%,
O: 0.020% or less,
Cu: 0 to 1.0%,
Ti: 0 to 0.30%,
Ca: 0 to 0.050%,
Mg: 0 to 0.050%,
REM: 0 to 0.10%,
Balance: Fe and impurities,
Satisfies the following expressions (i) to (iii),
Ferritic heat resistant steel weld metal.
sol. B≧0.005 (i)
insol. B≦0.0045 (ii)
(20(C+N)+Cr+4Mo+2W+10(Nb+Ta+V)+500sol.B)/(Mn+Co+Ni)≧4.5 (iii)
However, each element symbol in the formula represents the content (mass %) of each element contained in the weld metal, and the sol. B is the content (% by mass) of B dissolved in the weld metal, insol. B is the B content (mass %) present as a precipitate in the weld metal. The chemical composition is% by mass,
Cu: 0.05-1.0%,
Ti: 0.02 to 0.30%,
Ca: 0.001 to 0.050%,
Mg: 0.001 to 0.050%, and
REM: 0.001 to 0.10%,
Containing one or more selected from,
The ferritic heat resistant steel weld metal according to claim 1. The ferritic heat-resistant steel weld metal according to claim 1 or 2, and a base material made of ferritic heat-resistant steel,
Ferritic heat resistant steel welded joint. The chemical composition of the base material is% by mass,
C: 0.04 to 0.12%,
Si: 0.05-0.60%,
Mn: 0.10 to 0.80%,
P: 0.020% or less,
S: 0.010% or less,
Cr: 8.0 to 10.0%,
Co: 2.0-4.0%,
Ni: 0 to 0.4%,
W: 2.0 to 4.0%,
Nb+Ta: 0.02 to 0.18% in total,
V: 0.05 to 0.40%,
B: 0.005-0.020%,
Al: 0.030% or less,
N: 0.002-0.025%,
O: 0.020% or less,
Nd: 0.01 to 0.06%,
Remainder: Fe and impurities,
The ferritic heat resistant steel welded joint according to claim 3.