JP2017202494A - Austenitic heat-resistant steel weld metal and weld joint having the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an austenitic heat-resistant steel weld metal that is prevented from decreasing in toughness and has excellent creep strength and a weld joint having the same.SOLUTION: An austenitic heat-resistant steel weld metal contains, in mass%, C: 0.06%-0.14%, Si: 0.1%-0.8%, Mn: 0.1%-1.8%, P: 0.025% or less, Ni: 25%-35%, Cr: 20%-24%, W: 2%-4.5%, Nb: 0.05%-0.4%, V: 0.05%-0.4%, N: 0.1%-0.35%, Al: 0.08% or less, and O: 0.08% or less, with the balance being Fe and impurities, with S, Sn, Pb and Zn as impurities satisfying the following formula (1): [%S]+0.5×{[%Sn]+[%Pb]+[%Zn]}≤0.0030% ([%S], [%Sn], [%Pb], and [%Zn] represent contents of S, Sn, Pb, and Zn as impurities (mass%)).SELECTED DRAWING: None

Description

  The present invention relates to an austenitic heat-resistant steel weld metal and a welded joint having the same.

In recent years, high-temperature and high-pressure operating conditions have been promoted on a global scale for power generation boilers and the like from the viewpoint of reducing environmental impact. Materials used for superheater tubes and reheater tubes are required to have more excellent properties such as high-temperature strength and corrosion resistance.
As materials that satisfy such requirements, various austenitic heat resistant steels containing a large amount of nitrogen and a large amount of nickel (hereinafter, austenitic heat resistant steels containing a large amount of nitrogen and nickel are referred to as “high nitrogen and high nickel”. Also referred to as “containing austenitic heat resistant steel” has been developed.

For example, Patent Document 1 discloses that N is 0.02% to 0.3%, Ni is 17% to 50%, and Cr is 18% to 25%, and Nb is 0.05% to 0.6%. , Austenitic heat-resisting steels having excellent high-temperature strength, containing 0.03% to 0.3% Ti and 0.3% to 5% Mo have been proposed.
In Patent Document 2, N is added to 0.1% to 0.30%, Ni is added to 22.5% to 32%, and Cr is added to 20% to 27%. An austenitic heat-resistant steel excellent in high-temperature strength containing ˜4% and Nb of 0.20% to 0.60% has been proposed.
Further, Patent Document 3 includes N over 0.05% to 0.3%, Ni over 15% to 55%, Cr over 20% to less than 28%, and Nb over 0.1% to 0%. An austenitic heat resistant steel excellent in creep characteristics and hot workability, containing 0.8%, V in a range of 0.02% to 1.5%, and W in a range of 0.05% to 10% has been proposed.
These austenitic heat resistant steels are generally used as a welded structure having a weld metal. Therefore, various weld metals that can utilize the performance of these austenitic heat resistant steels as welded structures have been proposed.

  For example, Patent Document 4 contains 0.3% to 3.5% Nb, 0.1% to 0.35% N, 0.2% to 1.8% Mo, and Ni. It is disclosed that the austenitic heat-resistant steel weld metal containing 35% to 45% has high-temperature strength, corrosion resistance, and weld crack resistance during welding.

  Patent Document 5 contains Nb of 0.5% to 4%, N of 0.1% to 0.35%, and Mo of 0.2% to 1.8%, and Ni of 30% to 50%. In addition, austenitic heat-resistant steel weld metal with a C content adjusted to an appropriate range according to the amount of Nb can improve weld crack resistance during welding, and achieve both high-temperature strength and corrosion resistance. It is disclosed.

  Patent Document 6 discloses austenite containing 0.1% to 1.5% Nb, 0.5% to 3% W, 0.1% to 0.35% N, and 15% to 25% Ni. It is disclosed that a heat resistant steel weld metal is excellent in high temperature strength.

JP 59-173249 A Special table 2002-537486 gazette Japanese Patent No. 3838216 Japanese Patent No. 3329262 Japanese Patent No. 3918670 Japanese Patent No. 3329261

  By the way, since the austenitic heat-resistant steel weld metal disclosed in these Patent Documents 4 to 6 mainly uses Nb as a precipitation strengthening element, it certainly satisfies excellent properties such as high strength and corrosion resistance. However, since the austenitic heat-resistant steel weld metal disclosed in Patent Documents 4 to 6 has a very high strengthening ability, a significant decrease in toughness may occur at the beginning of use at high temperatures. Therefore, development of a weld metal that can fully utilize the performance of a high nitrogen high nickel content austenitic heat resistant steel and a welded joint having the weld metal has been awaited.

  The present invention has been made in view of the above situation, and when the high nitrogen high nickel content austenitic heat resistant steel is used as a welded structure, the performance of the high nitrogen high nickel content austenitic heat resistant steel is fully utilized. An object of the present invention is to provide an austenitic heat-resistant steel weld metal that is excellent in creep strength and suppresses a decrease in toughness, and a welded joint having the same.

As a result of examining whether or not there is a problem during use in the high nitrogen high nickel content austenitic heat resistant steel weld metal containing Nb, the present inventors have found the following matters.
(A) The toughness of the weld metal decreases in the initial use at high temperatures. The tendency becomes conspicuous with an increase in the amount of Nb in the weld metal and an increase in the amount of impurities of one or more of S, Sn, Pb, and Zn.
(B) The fracture surface of the weld metal after the impact test is a columnar crystal boundary.
(C) A large amount of fine carbonitride is precipitated in the weld metal.

  From the above findings (a) to (c), the present inventors have reached the following conclusion. That is, carbon, nitrogen, and Nb are combined in the weld metal due to use at a high temperature, and a large amount of carbonitrides such as Nb (C, N) and NbCrN are precipitated in the grains from the beginning of use. The deformation resistance inside increases. Further, in this process, S, Sn, Pb, and Zn contained as impurities are segregated at the columnar crystal boundaries. And when it received the impact from the outside, it was thought that the toughness of a weld metal fell as a result of the deformation | transformation concentrating on the grain boundary embrittled by the segregation of the impurity.

Therefore, the present inventors examined measures for preventing a decrease in toughness of the weld metal. As a result, by replacing Nb in the weld metal with V, which has a weak affinity for carbon and nitrogen compared to Nb and has a low tendency to form a compound (carbonitride), Nb charcoal at the initial stage of use at high temperatures It was found that the amount of nitride precipitation was reduced. At the same time, it has been clarified that by gradually precipitating V carbide on the long time side, it is possible to reduce rapid toughness deterioration at the initial stage of use and to secure the creep strength during long time use. In addition, it has also been clarified that it is necessary to strictly control the impurity contents of S, Sn, Pb, and Zn in the weld metal to reduce the toughness reduction of the weld metal. Specifically, the amounts of S, Sn, Pb, and Zn are determined according to a predetermined relational expression (formula (1) [% S] + 0.5 × {[% Sn] + [% Pb] + [% Zn]. } ≦ 0.0030%), it was revealed that the above-described deterioration of the toughness of the weld metal can be prevented.
The present invention has been completed by repeating the above studies.
That is, the gist of the present invention is as follows.

<1> By mass%
C: 0.06% to 0.14%,
Si: 0.1% to 0.8%,
Mn: 0.1% to 1.8%
P: 0.025% or less,
Ni: 25% to 35%,
Cr: 20% to 24%,
W: 2% to 4.5%,
Nb: 0.05% to 0.4%
V: 0.05% to 0.4%
N: 0.1% to 0.35%
Al: 0.08% or less,
O: 0.08% or less is included, the balance consists of Fe and impurities,
An austenitic heat-resistant steel weld metal in which S, Sn, Pb and Zn as impurities satisfy the following formula (1).
Formula (1) [% S] + 0.5 × {[% Sn] + [% Pb] + [% Zn]} ≦ 0.0030%
(In formula (1), [% S], [% Sn], [% Pb], and [% Zn] represent the contents (mass%) of S, Sn, Pb, and Zn as impurities. )

<2> The austenitic heat-resistant steel weld metal according to <1>, which contains one or more of the following elements in mass% instead of Fe as an alloy component.
Ti: 0% to 0.25%,
Cu: 0% to 4%,
Co: 0% to 2%
Mo: 0% to 2%,
B: 0% to 0.005%
Ca: 0% to 0.02%,
Mg: 0% to 0.02%,
REM: 0% to 0.06%

<3> A welded joint comprising the austenitic heat resistant steel weld metal according to <1> or <2> and a base material of the austenitic heat resistant steel.

<4> The base material is mass%,
C: 0.04% to 0.14%,
Si: 0.05% to 1%
Mn: 0.5% to 2.5%
P: 0.03% or less,
S: 0.002% or less,
Ni: 23% to 32%,
Cr: 20% to 25%,
W: 1% to 5%
Nb: 0.1% to 0.6%,
V: 0.1% to 0.6%,
N: 0.1% to 0.3%
B: 0% to 0.01%
REM: 0% to 0.06%
Al: 0.03% or less,
O: 0.02% or less,
The weld joint according to <3>, further comprising Fe and impurities.

  According to the present invention, when a high nitrogen high nickel content austenitic heat resistant steel is used as a welded structure, the performance of the high nitrogen high nickel content austenitic heat resistant steel can be fully utilized, and a reduction in toughness can be suppressed. An austenitic heat-resistant steel weld metal having excellent creep strength and a welded joint having the same are provided.

Hereinafter, an embodiment of the austenitic heat-resistant steel weld metal of the present invention and a welded joint having the same will be described.
In the present specification, a numerical range expressed using “to” means a range including numerical values described before and after “to” as a lower limit and an upper limit unless otherwise specified. However, when there is a notice such as “exceeding” and “less than”, it means that numerical values described before and after “to” are not included as at least one of the lower limit value and the upper limit value.

<Welded metal>
The austenitic heat-resistant steel weld metal of the present invention is, in mass%, C: 0.06% to 0.14%, Si: 0.1% to 0.8%, Mn: 0.1% to 1.8%. , P: 0.025% or less, Ni: 25% to 35%, Cr: 20% to 24%, W: 2% to 4.5%, Nb: 0.05% to 0.4%, V: 0 0.05% to 0.4%, N: 0.1% to 0.35%, Al: 0.08% or less, O: 0.08% or less, with the balance being Fe and impurities.
And S, Sn, Pb and Zn as impurities satisfy the following formula (1).
Formula (1) [% S] + 0.5 × {[% Sn] + [% Pb] + [% Zn]} ≦ 0.0030%
(In formula (1), [% S], [% Sn], [% Pb], and [% Zn] represent the contents (mass%) of S, Sn, Pb, and Zn as impurities. )

  In this specification, “impurities” are components that are mixed from ore, scrap, or the production environment as raw materials when industrially producing austenitic heat-resistant alloys, and are intentionally included. Refers to ingredients that are not.

In the present invention, the reason for limiting the chemical composition of the austenitic heat-resistant steel weld metal is as follows.
In the following description, “%” display of the content of each element means “mass%”.

(C: 0.06% to 0.14%)
C (carbon) stabilizes the austenite structure, forms fine carbides, and improves the creep strength during high temperature use. In order to sufficiently obtain these effects, C needs to be contained by 0.06% or more. However, when the C content is excessive, a large amount of carbide is present in the weld metal, so that ductility and toughness are lowered. Therefore, the upper limit of the C content is 0.14% or less. A desirable range for the C content is 0.07% to 0.13%, and a more desirable range is 0.08% to 0.12%.

(Si: 0.1% to 0.8%)
Si (silicon) is an element that has a deoxidizing action and is effective in improving corrosion resistance and oxidation resistance at high temperatures. In order to acquire the effect, it is necessary to contain Si 0.1% or more. However, when Si is excessively contained, the stability of the structure is lowered, and the toughness and the creep strength are lowered. Therefore, the upper limit of Si content is 0.8% or less. A desirable range for the Si content is 0.12% to 0.78%, and a more desirable range is 0.15% to 0.75%.

(Mn: 0.1% to 1.8%)
Mn (manganese) has a deoxidizing action like Si. Mn stabilizes the austenite structure and contributes to the improvement of creep strength. In order to acquire these effects, it is necessary to contain Mn 0.1% or more. However, when the Mn content is excessive, embrittlement is caused, and further, creep ductility is reduced. Therefore, the upper limit of the Mn content is 1.8% or less. A desirable range of the Mn content is 0.15% to 1.6%, and a more desirable range is 0.2% to 1.4%.

(P: 0.025% or less)
P (phosphorus) is an element contained in the weld metal as an impurity and reduces creep ductility. Therefore, an upper limit is set for the P content to 0.025% or less. The upper limit of the P content is desirably 0.023% or less, and more desirably 0.020% or less. In addition, although it is desirable to reduce P content as much as possible, extreme reduction leads to the increase in manufacturing cost. Therefore, a desirable lower limit of the P content is 0.0005% or more, and a more desirable lower limit is 0.0008% or more.

(Ni: 25% to 35%)
Ni (nickel) increases the stability of the austenite structure when used for a long time and contributes to the improvement of the creep strength. In order to obtain the effect sufficiently, Ni needs to be contained by 25% or more. However, Ni is an expensive element, and a large amount causes an increase in cost. Therefore, the Ni content is set to 35% or less with an upper limit. A desirable range of the Ni content is 25.5% to 34.5%, and a more desirable range is 26% to 34%.

(Cr: 20% to 24%)
Cr (chromium) is an essential element for securing oxidation resistance and corrosion resistance at high temperatures. It also contributes to securing the creep strength by forming fine carbides. In order to obtain the effect sufficiently, the Cr content needs to be 20% or more. However, if the Cr content exceeds 24%, the stability of the austenite structure at high temperatures deteriorates, causing a significant decrease in creep strength. Therefore, the Cr content is 20% to 24%. A desirable range for the Cr content is 20.5% to 23.5%, and a more desirable range is 21% to 23%.

(W: 2% to 4.5%)
W (tungsten) is an element that makes a solid solution in the matrix and greatly contributes to improvement in creep strength and tensile strength at high temperatures. In order to fully exhibit the effect, W needs to contain at least 2% or more. However, since W is an expensive element, excessive inclusion of W causes an increase in cost. Even if W is contained excessively, the effect is saturated. Therefore, the upper limit of the W content is 4.5% or less. A desirable range of W content is 2.2% to 4.3%, and a more desirable range is 2.5% to 4%.

(Nb: 0.05% to 0.4%)
Nb (niobium) has a strong affinity for carbon and nitrogen, precipitates in the grains as fine carbonitrides, and contributes to improvement of the creep strength and tensile strength of the weld metal at high temperatures. In order to obtain the effect sufficiently, the Nb content needs to be 0.05% or more. However, when the content of Nb is excessive, the amount of precipitation at the initial stage of use at high temperatures increases, leading to a decrease in toughness. Therefore, the upper limit of Nb content needs to be 0.4% or less. A desirable range of Nb content is 0.08% to 0.38%, and a more desirable range is 0.1% to 0.35%.

(V: 0.05% to 0.4% or less)
V (vanadium), like Nb, forms fine carbonitrides, but has a weaker affinity for carbon and nitrogen than Nb. Therefore, V contributes to the improvement of the creep strength of the weld metal without affecting the toughness at the initial use as much as Nb. In order to acquire this effect, V needs to contain 0.05% or more. However, when V is contained excessively, it precipitates in a large amount, and the coarsening of the precipitate becomes remarkable, resulting in a decrease in creep strength and ductility. Therefore, the upper limit of V content needs to be 0.4% or less. A desirable range for the V content is 0.08% to 0.38%, and a more desirable range is 0.1% to 0.35%.

(N: 0.1% to 0.35%)
N (nitrogen) stabilizes the austenite structure and precipitates as a solid solution or nitride, contributing to the improvement of the high temperature strength. In order to obtain the effects not a little, N needs to be contained by 0.1% or more. However, if N is contained in excess of 0.35%, a large amount of nitride precipitates, resulting in a decrease in toughness. Therefore, the N content is 0.1% to 0.35%. A desirable range of N content is 0.12% to 0.32%, and a more desirable range is 0.15% to 0.3%.

(Al: 0.08% or less)
Al (aluminum) is contained as a deoxidizer during the production of the base material, and is also contained as a deoxidizer during the production of the welding material. As a result, the weld metal contains Al. However, when a large amount of Al is contained, ductility is lowered. For this reason, the upper limit of Al content needs to be 0.08% or less. The upper limit of the Al content is desirably 0.06% or less, and more desirably 0.04% or less. In addition, although it is not necessary to provide the minimum of Al content in particular, the extreme reduction of Al content leads to the increase in manufacturing cost. Therefore, a desirable lower limit of the Al content is 0.0005% or more, and a desirable lower limit is 0.001% or more.

(O: 0.08% or less)
O (oxygen) is contained in the weld metal as an impurity. However, when the content of O (oxygen) is excessive, toughness and ductility are deteriorated. For this reason, the upper limit of O (oxygen) content needs to be 0.08% or less. The upper limit of the O (oxygen) content is desirably 0.06% or less, and more desirably 0.04% or less. Although there is no particular need to set a lower limit for the O (oxygen) content, an extreme reduction leads to an increase in manufacturing cost. Therefore, the desirable lower limit of the O (oxygen) content is 0.0005% or more, and the more desirable lower limit is 0.0008% or more.

(S, Sn, Pb, and Zn: [% S] + 0.5 × {[% Sn] + [% Pb] + [% Zn]} ≦ 0.0030%)
These elements are contained as impurities in the weld metal, and segregate at the columnar crystal boundaries at the initial stage of use at high temperatures, thereby reducing toughness. As a result of repeating various experiments, the present inventors have found that since S has a large segregation energy, it is easy to segregate at the grain boundaries, and the degree of influence that causes ductile drop cracking of S is twice as high as that of Sn, Pb, and Zn. I found out that there was.
Therefore, in order to stably reduce the toughness deterioration of the weld metal, it is obtained by the formula (1) ([% S] + 0.5 × {[% Sn] + [% Pb] + [% Zn]}). It was found that the value needs to be 0.0030% or less. The upper limit of the value of Formula (1) is desirably 0.0025% or less, and more desirably 0.0020% or less. Although it is desirable to reduce these impurities (S, Sn, Pb, and Zn) as much as possible, extreme reduction causes an increase in manufacturing cost. Therefore, the desirable lower limit of the value obtained by the formula (1) is 0.0001% or more, and the more desirable lower limit is 0.0002% or more.

  The austenitic heat-resistant steel weld metal of the present invention has a chemical composition containing the above-mentioned elements, with the balance being Fe and impurities.

  The austenitic heat-resisting steel weld metal of the present invention is replaced by Fe as an alloy component in mass%, Ti: 0% to 0.25%, Cu: 0% to 4%, Co: 0% to 2%, Mo: 0% to 2%, B: 0% to 0.005%, Ca: 0% to 0.02%, Mg: 0% to 0.02%, REM: 0% to 0.06% Or you may contain 2 or more types of elements. Below, each component is demonstrated.

(Ti: 0% to 0.25%)
Ti (titanium), like Nb and V, forms fine carbonitrides and contributes to the improvement of creep strength and tensile strength at high temperatures. Therefore, you may contain as needed. However, when the Ti content is excessive, a large amount is precipitated in the initial stage of use like Nb, resulting in a decrease in toughness. Therefore, when it contains Ti, the upper limit of Ti content shall be 0.25% or less. The upper limit of the Ti content is desirably 0.23% or less, and more desirably 0.2% or less. When Ti is contained, the desirable lower limit of the Ti content is 0.01% or more, and the more desirable lower limit is 0.03% or more.

(Cu: 0% to 4%)
Cu (copper) enhances the stability of the austenite structure and precipitates finely during use, contributing to the improvement of creep strength. However, when Cu is contained excessively, ductility is reduced. Therefore, when it contains Cu, the upper limit of Cu content shall be 4% or less. The upper limit of the Cu content is desirably 3.8% or less, and more desirably 3.5% or less. When Cu is contained, a desirable lower limit of the Cu content is 0.01% or more, and a more desirable lower limit is 0.03% or more.

(Co: 0% to 2%)
Co (cobalt), like Ni and Cu, is an austenite-generating element and contributes to the improvement of creep strength by increasing the stability of the austenite structure. However, since Co is an extremely expensive element, excessive content of Co causes a significant cost increase. Therefore, when it contains Co, the upper limit of Co content shall be 2% or less. The upper limit of the Co content is desirably 1.8% or less, and more desirably 1.5% or less. When Co is contained, a desirable lower limit of the Co content is 0.01% or more, and a more desirable lower limit is 0.03% or more.

(Mo: 0% to 2%)
Mo (molybdenum), like W, is an element that contributes to the improvement of creep strength and tensile strength at high temperatures by dissolving in a matrix. However, when Mo is excessively contained, the structural stability is lowered, and on the contrary, the creep strength may be lowered. Furthermore, since Mo is an expensive element, excessive inclusion causes an increase in cost. Therefore, when it contains Mo, the upper limit of Mo content shall be 2% or less. The upper limit of the Mo content is desirably 1.5% or less, and more desirably 1.2% or less. When Mo is contained, the desirable lower limit of the Mo content is 0.01% or more, and the more desirable lower limit is 0.03% or more.

(B: 0% to 0.005%)
B (boron) is an element that improves the creep strength of the weld metal by finely dispersing carbides, and strengthens the grain boundaries and contributes to the improvement of toughness. However, when B is contained excessively, the solidification cracking susceptibility during welding is increased. Therefore, when it contains B, the upper limit of B content shall be 0.005% or less. The upper limit of the B content is desirably 0.003% or less, and more desirably 0.002% or less. When B is contained, the desirable lower limit of the B content is 0.0003% or more, and the more desirable lower limit is 0.0005% or more.

(Ca: 0% to 0.02%)
Since Ca (calcium) has an effect of improving hot deformability, it may be contained as necessary. However, the excessive content of Ca combines with oxygen, significantly lowering the cleanliness and deteriorating hot deformability. Therefore, when Ca is contained, the upper limit of the Ca content is 0.02% or less. The upper limit of the Ca content is desirably 0.015% or less, and more desirably 0.01% or less. When Ca is contained, the desirable lower limit of the Ca content is 0.0005% or more, and the more desirable lower limit is 0.001% or more.

(Mg: 0% to 0.02%)
Since Mg (magnesium) has the effect of improving the hot deformability like Ca, it may be contained as necessary. However, an excessive content of Mg combines with oxygen, significantly lowering the cleanliness and deteriorating hot deformability. Therefore, when it contains Mg, the upper limit of Mg content shall be 0.02% or less. The upper limit of the Mg content is desirably 0.015% or less, and more desirably 0.01% or less. When Mg is contained, the desirable lower limit of the Mg content is 0.0005% or more, and the more desirable lower limit is 0.001% or more.

(REM: 0% to 0.06%)
Since REM (rare earth element) has an effect of improving hot deformability like Ca and Mg, it may be contained as necessary. However, an excessive content of REM combines with oxygen, significantly lowering cleanliness and degrading hot workability. Therefore, when it contains REM, the upper limit of REM content shall be 0.06% or less. The upper limit of the REM content is desirably 0.04% or less, and more desirably 0.03% or less. When REM is contained, the desirable lower limit of the REM content is 0.0005% or more, and the more desirable lower limit is 0.001% or more.

  “REM” is a generic name for a total of 17 elements of Sc, Y, and lanthanoid, and the content of REM refers to the total content of one or more elements of REM. Further, REM is generally contained in misch metal. For this reason, for example, REM may be contained in the form of misch metal so that the content of REM is in the above range.

<Welded joint>
With the weld metal of the present invention and the base material of the austenitic heat-resisting steel, a welded joint having excellent weld hot cracking resistance can be obtained. Specifically, the welded joint has a weld metal of the joint part and two base materials made of austenitic heat-resistant steel sandwiching the weld metal.
The specific shape of the welded joint and the specific mode of welding (welding posture) for obtaining the welded joint are not particularly limited. For example, in the case of butt welding after groove processing on a steel pipe, groove processing is performed on a thick plate. For example, it may be applied to the case where butt welding is performed.
Hereinafter, the base material constituting the welded joint will be described.

<Base material>
As described above, the chemical composition of the austenitic heat-resistant steel weld metal according to the present invention has been described in detail. When obtaining a welded joint having the austenitic heat-resistant steel weld metal according to the present invention, the base material has the following chemical composition. It is desirable.

Desirable chemical composition of the base material is mass%, C: 0.04% to 0.14%, Si: 0.05% to 1%, Mn: 0.5% to 2.5%, P: 0.00. 03% or less, S: 0.002% or less, Ni: 23% to 32%, Cr: 20% to 25%, W: 1% to 5%, Nb: 0.1% to 0.6%, V: 0.1% to 0.6%, N: 0.1% to 0.3%, B: 0% to 0.01%, REM: 0 to 0.06%, Al: 0.03% or less, O : Containing 0.02% or less, with the balance being Fe and impurities.
The reason for this is described below. In the following description, “%” display of the content of each element means “mass%”.

(C: 0.04% to 0.14%)
C stabilizes the austenite structure, forms fine carbides, and improves the creep strength during high temperature use. If the C content is 0.04% or more, the above effect can be obtained effectively. However, when C is contained excessively, a large amount of carbide precipitates and creep ductility and toughness are lowered. Therefore, the upper limit of the C content is preferably 0.14% or less. A desirable range for the C content is 0.05% to 0.13%, and a more desirable range is 0.06% to 0.12%.

(Si: 0.05% to 1%)
Si is an element that has a deoxidizing action and is effective in improving corrosion resistance and oxidation resistance at high temperatures. If the Si content is 0.05% or more, the above effect can be obtained effectively. However, when Si is excessively contained, the stability of the structure is lowered, and the toughness and the creep strength are lowered. Therefore, the upper limit of the C content is preferably 1% or less. A desirable range of the Si content is 0.08% to 0.8%, and a more desirable range is 0.1% to 0.5%.

(Mn: 0.5% to 2.5%)
Mn has a deoxidizing action like Si. Further, Mn contributes to stabilization of the austenite structure. If the Mn content is 0.5% or more, the above effect can be obtained effectively. However, when the Mn content is excessive, embrittlement is caused, and further, creep ductility is reduced. Therefore, the upper limit of the Mn content is preferably 2.5% or less. A desirable range for the Mn content is 0.6% to 2%, and a more desirable range is 0.8% to 1.5%.

(P: 0.03% or less)
P is an element which is contained in the alloy as an impurity and segregates at the grain boundary of the weld heat affected zone during welding to increase the liquefaction cracking sensitivity. Furthermore, the creep ductility after long-time use is also reduced. Therefore, the upper limit of the P content is preferably 0.03% or less. The upper limit of the P content is desirably 0.028% or less, and more desirably 0.025% or less. Although it is desirable to reduce the P content as much as possible, extreme reduction leads to an increase in manufacturing cost. Therefore, a desirable lower limit of the P content is 0.0005% or more, and a more desirable lower limit is 0.0008% or more.

(S: 0.002% or less)
S is an element which is contained in the alloy as an impurity as in the case of P and segregates at the grain boundary in the weld heat-affected zone during welding to increase liquefaction cracking sensitivity. Therefore, the S content can be prevented by setting the upper limit to 0.002% or less. The upper limit of the S content is desirably 0.0018% or less, and more desirably 0.0015% or less. In addition, although it is desirable to reduce S content as much as possible, extreme reduction leads to the increase in manufacturing cost. Therefore, the desirable lower limit of the S content is 0.0001% or more, and the more desirable lower limit is 0.0002% or more.

(Ni: 23% to 32%)
Ni is an element for ensuring the stability of the austenite structure during long-time use and ensuring the creep strength. If Ni content is 23% or more, the said effect will be acquired effectively. However, Ni is an expensive element, and a large amount causes an increase in cost. Therefore, the upper limit of the Ni content is preferably 32% or less. A desirable range for the Ni content is 24% to 31.5%, and a more desirable range is 25% to 31%.

(Cr: 20% to 25%)
Cr is an element for ensuring oxidation resistance and corrosion resistance at high temperatures. It also contributes to securing the creep strength by forming fine carbides. If the Cr content is 20% or more, the above effect can be obtained effectively. However, if the Cr content exceeds 25%, the stability of the austenite structure at high temperatures deteriorates, leading to a decrease in creep strength. Therefore, the Cr content is preferably 20% to 25%. A desirable range for the Cr content is 20.5% to 24.5%, and a more desirable range is 21% to 24%.

(W: 1% to 5%)
W is an element that contributes greatly to the improvement in creep strength and tensile strength at high temperatures by dissolving in the matrix. If the W content is 1% or more, the above effect can be obtained effectively. However, even if W is excessively contained, the effect is saturated or the creep strength is lowered in some cases. Furthermore, since W is an expensive element, excessive inclusion of W causes an increase in cost. Therefore, the upper limit of the W content is preferably 5% or less. A desirable range of W content is 1.2% to 4.8%, and a more desirable range is 1.5% to 4.5%.

(Nb: 0.1% to 0.6%)
Nb precipitates in the grains as fine carbonitrides and contributes to the improvement of creep strength and tensile strength at high temperatures. If the Nb content is 0.1% or more, the above effect can be obtained effectively. However, if the content of Nb is excessive, a large amount of carbonitride precipitates, causing a decrease in creep ductility and toughness. For this reason, the upper limit of Nb content is preferably 0.6% or less. A desirable range of Nb content is 0.12% to 0.55%, and a more desirable range is 0.15% to 0.5%.

(V: 0.1% to 0.6%)
V, like Nb, forms fine carbonitrides and contributes to the improvement of creep strength and tensile strength at high temperatures. If the V content is 0.1% or more, the above effect can be obtained effectively. However, when the V content is excessive, it precipitates in a large amount, resulting in a decrease in creep ductility and toughness. For this reason, the upper limit of V content is preferably 0.6% or less. A desirable range for the V content is 0.12% to 0.55%, and a more desirable range is 0.15% to 0.5%.

(N: 0.1% to 0.3%)
N stabilizes the austenite structure and precipitates as a solid solution or a nitride, contributing to an improvement in high temperature strength. If the N content is 0.1% or more, the above effect can be obtained effectively. However, when N is excessively contained, a large amount of fine nitride precipitates in the grains during long-time use, which causes deterioration of creep ductility and toughness. Therefore, the upper limit of the N content is preferably 0.3% or less. A desirable range of N content is 0.12% to 0.28%, and a more desirable range is 0.15% to 0.25%.

(B: 0% to 0.01%)
B is an element effective for improving the creep strength by finely dispersing the grain boundary carbides and segregating at the grain boundaries to strengthen the grain boundaries. Therefore, B may be contained as necessary. However, when the B content is excessive, the liquefaction cracking sensitivity of the heat-affected zone during welding is increased. Therefore, the upper limit of the B content is preferably 0.01% or less. The upper limit of the B content is desirably 0.008% or less, and more desirably 0.006% or less. When B is contained, a desirable lower limit of the B content is 0.0005% or more, and a more desirable lower limit is 0.001% or more.

(REM: 0% to 0.06%)
Since REM has the effect of improving the hot deformability at the time of manufacture, it may be contained as necessary. However, excessive content of REM binds to oxygen, significantly reduces cleanliness, and adversely affects hot deformability. Therefore, the upper limit of the REM content is preferably 0.06% or less. The upper limit of the REM content is desirably 0.04% or less, and more desirably 0.03% or less. When REM is contained, the desirable lower limit of the REM content is 0.0005% or more, and the more desirable lower limit is 0.001% or more.

(Al: 0.03% or less)
Al is contained as a deoxidizer during the production of the base material. However, when a large amount of Al is contained, the cleanliness of the steel deteriorates and the hot workability decreases. Therefore, the upper limit of the Al content is preferably 0.03% or less. The upper limit of the Al content is desirably 0.025% or less, and more desirably 0.02% or less. In addition, although it is not necessary to provide the minimum of Al content in particular, extreme reduction invites the increase in manufacturing cost. Therefore, the desirable lower limit of the Al content is 0.0005% or more, and the more desirable lower limit is 0.001% or more.

(O: 0.02% or less)
O (oxygen) is contained as an impurity in the alloy, and when it is contained excessively, hot workability is lowered, and toughness and ductility are deteriorated. For this reason, the O (oxygen) content is preferably 0.02% or less. The upper limit of the O (oxygen) content is desirably 0.018% or less, and more desirably 0.015% or less. In addition, although it is not necessary to set a minimum in particular about O content, an extreme reduction causes the raise of manufacturing cost. Therefore, the desirable lower limit of the O (oxygen) content is 0.0005% or more, and the more desirable lower limit is 0.0008% or more.

  The main elements of the base material have been described above. However, the base material may further include Ti: 0.01% to 0.5%, Co: 0.01% to 2%, Cu: 0.0. It may contain 01% to 4%, Mo: 0.01% to 4%, Ca: 0.0005% to 0.02%, Mg: 0.0005% to 0.02%.

  EXAMPLES Hereinafter, although an Example demonstrates this invention more concretely, this invention is not limited to these Examples. It is obvious for those skilled in the art that various modifications or modifications can be conceived within the scope of the idea described in the claims, and these naturally belong to the technical scope of the present invention. It is understood.

From an ingot in which a material having the chemical composition shown in Table 1 (the balance is Fe and impurities) was melted and cast in a laboratory, the thickness was obtained by hot forging, hot rolling, cold rolling, heat treatment and machining. A plate material (plate material (1)) of 12 mm, a width of 50 mm, and a length of 120 mm was produced. The plate material (1) was a welding base material.
Further, from an ingot in which a material having a chemical composition shown in Table 2 was melted and cast in a laboratory, a plate material (thickness 4 mm, width 200 mm, length 500 mm) was obtained by hot forging, hot rolling, heat treatment and machining. A plate material (2)) was produced. A cut filler of 2 mm square and 500 mm length was produced from the plate material (2) by machining.

[Charpy impact test]
In the longitudinal direction of the plate material (1), after processing a V groove having an angle of 30 ° and a root surface (route face) of 1 mm, a commercially available steel plate (JIS G 3160 (2008) specified in SM400B, thickness 25 mm, Four rounds were restrained and welded using a coated arc welding rod (“DNiCrFe-3” defined in JIS Z3224 (1999)) on a width of 150 mm and a length of 200 mm.
Then, using the produced cut filler, lamination welding was performed in the groove by manual TIG welding in which the shielding gas was Ar, and a welded joint was produced. In the welding, the heat input was set to 9 kJ / cm to 15 kJ / cm.
Chips were collected from the weld metal of the obtained welded joint and subjected to chemical analysis. Table 3 shows the results.

Further, the remaining welded joint was cut to a length of 50 mm in the longitudinal direction and subjected to aging heat treatment at 700 ° C. for 300 hours. Thereafter, three 2 mm V-notch full-size Charpy impact test pieces obtained by processing a notch in the weld metal were collected and subjected to an impact test at 20 ° C.
Then, all the individual values of the absorbed energy of the three test pieces were 27 J or more were “passed”, and even one of the three samples was determined to be “failed” if the absorbed energy was less than 27 J.

[Creep rupture test]
As for the welded joint in which the result of the Charpy impact test was “accepted”, a round bar creep rupture test piece with the weld metal at the center of the parallel part was collected from the remaining welded joint. Then, a creep rupture test is performed under the conditions of 700 ° C. and 177 MPa, and when the rupture time exceeds about 500 hours, which is approximately 80% of the target rupture time of the base material, the “pass” is obtained, and for those that are 500 hours or less. “Fail”.
Table 4 also shows the results of the above tests.

From Table 4, the welded joints having the weld metals of reference signs A1 to A3 and B1 to B3 whose chemical compositions are in the range defined by the present invention have the required toughness even after aging heat treatment at high temperature, and creep rupture It is clear that the time satisfies 80% or more of the target fracture time of the base material.
On the other hand, in the welded joint having the weld metals of the symbols A4, A6, B4 and B6, [% S] + 0.5 × {[% Sn] + [% Pb] + [% Zn]} of the weld metal is Since it exceeded 0.0030%, the toughness of the weld metal after the aging heat treatment did not satisfy the required performance. Further, in the welded joint having the weld metals of reference signs A5 and B5, the Nb content of the weld metal exceeded 0.4%, which is the upper limit of the range of the present invention, so the toughness of the weld metal after aging heat treatment was poor. Met.
Thus, the weld metal that satisfies the requirements of the present invention is excellent in toughness after aging heat treatment and also satisfies the creep strength necessary as a welded structure. It can be seen that the performance can be fully utilized.

  By utilizing the present invention, when using a high nitrogen high nickel content austenitic heat resistant steel as a welded structure, the austenitic heat resistant steel that can sufficiently utilize its performance, suppresses a decrease in toughness, and is excellent in creep strength. A weld metal and a weld joint having the weld metal can be provided. Therefore, the weld metal of the present invention and the weld joint having the weld metal and the weld metal constituting the weld structure applied to equipment used at a high temperature such as a power generation boiler, a high-nitrogen high-nickel-containing austenitic heat-resistant steel, It is useful as a welded joint having it.

Claims (4)

% By mass
C: 0.06% to 0.14%,
Si: 0.1% to 0.8%,
Mn: 0.1% to 1.8%
P: 0.025% or less,
Ni: 25% to 35%,
Cr: 20% to 24%,
W: 2% to 4.5%,
Nb: 0.05% to 0.4%
V: 0.05% to 0.4%
N: 0.1% to 0.35%
Al: 0.08% or less,
O: 0.08% or less is included, the balance consists of Fe and impurities,
An austenitic heat-resistant steel weld metal in which S, Sn, Pb and Zn as impurities satisfy the following formula (1).
Formula (1) [% S] + 0.5 × {[% Sn] + [% Pb] + [% Zn]} ≦ 0.0030%
(In formula (1), [% S], [% Sn], [% Pb], and [% Zn] represent the contents (mass%) of S, Sn, Pb, and Zn as impurities. ) The austenitic heat-resistant steel weld metal according to claim 1, which contains one or more of the following elements in mass% instead of Fe as an alloy component.
Ti: 0% to 0.25%,
Cu: 0% to 4%,
Co: 0% to 2%
Mo: 0% to 2%,
B: 0% to 0.005%
Ca: 0% to 0.02%,
Mg: 0% to 0.02%,
REM: 0% to 0.06%   A welded joint comprising the austenitic heat resistant steel weld metal according to claim 1 and a base material of the austenitic heat resistant steel. The base material is mass%,
C: 0.04% to 0.14%,
Si: 0.05% to 1%
Mn: 0.5% to 2.5%
P: 0.03% or less,
S: 0.002% or less,
Ni: 23% to 32%,
Cr: 20% to 25%,
W: 1% to 5%
Nb: 0.1% to 0.6%,
V: 0.1% to 0.6%,
N: 0.1% to 0.3%
B: 0% to 0.01%
REM: 0% to 0.06%
Al: 0.03% or less,
O: 0.02% or less,
The weld joint according to claim 3, wherein the balance is made of Fe and impurities.