JP2017202492A - Austenitic heat-resistant steel weld metal and weld joint having the same - Google Patents

Abstract

An austenitic heat-resistant steel weld metal having excellent resistance to welding hot cracking and a welded joint having the same.
SOLUTION: C: 0.06% to 0.14%, Si: 0.1% to 0.8%, Mn: 0.1% to 1.8%, P: 0.025% or less, Cu: 2% -4. %, Ni: 12% to 16%, Cr: 16.5% to 19.5%, W: 2% to 4.5%, Ti: 0.05% to 0.35%, Nb: 0.05% -0.5%, N: 0.001% -0.015%, Al: 0.08% or less, O: 0.08% or less, and S, Sn, Pb and Zn as impurities are represented by the following formula ( Austenitic heat-resistant steel weld metal satisfying 1).
Formula (1) [% S] + 0.5 × {[% Sn] + [% Pb] + [% Zn]} ≦ 0.0030% ([% S], [% Sn], [% Pb], and [% Zn] represents the content (mass%) of S, Sn, Pb, and Zn as impurities.)
[Selection figure] 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 satisfying such requirements, various austenitic heat resistant steels containing a large amount of nitrogen (hereinafter, austenitic heat resistant steels containing a large amount of nitrogen are also referred to as “high nitrogen containing austenitic heat resistant steels”). ) Has been developed.

For example, in Patent Document 1, N is 0.1% to 0.35% and Cr is more than 22% to less than 30%, and a high nitrogen content austenite having a specific metal structure and excellent in high temperature strength and corrosion resistance. A heat resistant steel has been proposed.
Patent Document 2 also describes that N is 0.1% to 0.35% and Cr is more than 22% to less than 30%, and high nitrogen content is excellent in high temperature strength and corrosion resistance in which impurities are specified under specific conditions. Austenitic heat-resistant steel has been proposed.
These high nitrogen content austenitic heat resistant steels are generally used as welded structures having weld metals. Therefore, various weld metals that can utilize the performance of these high nitrogen-containing austenitic heat resistant steels have been proposed as welded structures.

  For example, Patent Document 3 includes a high Nb content of 0.5% to 3.5% and N content of 0.1% to 0.35% and Mo content of 0.2% to 1.8%. It is disclosed that a nitrogen-containing austenitic heat-resistant steel weld metal satisfies high-temperature strength, corrosion resistance, and weld crack resistance during welding.

  Patent Document 4 discloses a high-nitrogen-containing austenitic heat-resistant steel weld metal containing 0.8% to 4.5% Nb and 0.1% to 0.35% N and adjusting the amount of C by the amount of Nb. Discloses that it is possible to improve the weld crack resistance during welding and to achieve both high-temperature strength and corrosion resistance.

  Patent Document 5 discloses a high nitrogen content austenitic heat-resistant steel weld metal containing 0.15% to 1.5% Nb, 0.5% to 3% W, and 0.1% to 0.35% N. Is disclosed to be excellent in high-temperature strength.

Japanese Patent No. 4424471 Japanese Patent No. 4946758 Japanese Patent No. 3329262 Japanese Patent No. 3918670 Japanese Patent No. 3329261

  By the way, these high nitrogen-containing austenitic heat resistant steels certainly satisfy excellent properties such as high strength and corrosion resistance. However, high nitrogen-containing austenitic heat-resistant steel precipitates a large amount of nitride, and therefore has poor toughness after aging. In addition, nitride grows during long-term use, and the creep strength may be lowered depending on the use conditions. Therefore, even in the case of an austenitic heat-resistant steel having a low nitrogen content (hereinafter also referred to as “low nitrogen-containing austenitic heat-resistant steel”), development of austenitic heat-resistant steel capable of achieving high strength has been promoted.

These low nitrogen-containing austenitic heat resistant steels are also used as welded structures, and at that time, it is possible to weld as welded structures having a weld metal for Ni-based alloys. However, although Ni-based alloys are excellent in creep strength, they are not preferable from the viewpoint of economy because they are expensive. In addition, when the component of the Ni-based alloy is significantly different from the component of the base material, sufficient weld hot cracking resistance (specifically, solidification cracking resistance) may not be obtained.
Therefore, a weld metal that can utilize the performance of the low nitrogen content austenitic heat resistant steel is also required in the same manner as the base metal. Therefore, when low nitrogen content austenitic heat resistant steel is used as a welded structure, development of a weld metal capable of fully utilizing the performance of the low nitrogen content austenitic heat resistant steel and a welded joint having the same has been awaited.

  The present invention has been made in view of the above situation, and when using low-nitrogen-containing austenitic heat-resistant steel as a welded structure, it is possible to fully utilize the performance of low-nitrogen-containing austenitic heat-resistant steel, which has high resistance to welding. An object of the present invention is to provide an austenitic heat-resistant steel weld metal having excellent crackability (specifically, ductile drop cracking) and a welded joint having the same.

In order to examine the presence or absence of a problem when the nitrogen content of the weld metal is reduced, the present inventors, in mass%, C: 0.06% to 0.14%, Si: 0.1% to 0.8 %, Mn: 0.1% to 1.8%, Cu: 2% to 4%, Ni: 12% to 16%, Cr: 16.5% to 19.5%, W: 2% to 4.5 %, Ti: 0.05% to 0.35%, and Nb: 0.05% to 0.5% were produced, and various studies were made. As a result, it was found that although necessary strength was obtained, there were the following problems.
(A) When welding which simulates repair after welding is performed on the weld metal, when the amount of nitrogen in the weld metal is reduced, the susceptibility to cracks generated in the original weld metal is increased during welding.
(B) The crack is a so-called ductile deterioration crack that occurs at a position slightly away from the melting boundary due to the influence of the welding heat cycle of the repair.
(C) The structure of the weld metal has a tendency that the shape of the columnar crystal boundary becomes smooth due to the reduction in nitrogen, and the amount of eutectic carbonitride existing at the columnar crystal boundary decreases.
(D) As a result of the detailed investigation of the cracked fracture surface of the weld metal, at least one enrichment of S, Sn, Pb and Zn was detected on the fracture surface.

From the above findings (a) to (d), the inventors have reached the following conclusion.
That is, when the content of nitrogen in the weld metal is high, with the solidification of the weld metal, alloy elements having strong affinity with C and N, such as Ti and Nb, are discharged and concentrated in the liquid phase. The eutectic reaction between the carbonitride and the substrate that occurs is generated from a high temperature. Therefore, the columnar crystal boundary of the final layer becomes complicated.

On the other hand, when nitrogen in the weld metal is reduced, the eutectic reaction does not occur to a low temperature. Therefore, the columnar crystal boundary becomes smooth and the boundary area per unit volume decreases.
As a result, when nitrogen in the weld metal is reduced, segregation to the columnar crystal boundaries of S, Sn, Pb, and Zn, which lowers the grain boundary fixing force, increases. Furthermore, when thermal stress due to a thermal cycle simulating repair welding is applied to the weld metal, it is considered that grain boundary slip is likely to occur in shape, so that the ductility lowering cracking sensitivity is increased.

Therefore, the present inventors examined the above-mentioned crack prevention measures. As a result, the amounts of S, Sn, Pb, and Zn contained as impurities in the weld metal are determined according to a predetermined relational expression (formula (1) [% S] + 0.5 × {[% Sn] + [% Pb] + It was revealed that the above cracks can be prevented by managing so as to satisfy [% Zn]} ≦ 0.0030%).
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,
Cu: 2% to 4%,
Ni: 12% to 16%,
Cr: 16.5% to 19.5%,
W: 2% to 4.5%,
Ti: 0.05% to 0.35%,
Nb: 0.05% to 0.5%,
N: 0.001% to 0.015%,
Al: 0.08% or less,
O: 0.08% or less,
And 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.
V: 0% to 0.35%,
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 0.15%,
Si: 0.05% to 1%
Mn: 0.3% to 2.5%
P: 0.04% or less,
S: 0.002% or less,
Cu: 2% to 4%,
Ni: 11% to 16%,
Cr: 16% to 20%,
W: 2% -5%
Nb: 0.1% to 0.8%
Ti: 0.05% to 0.35%,
N: 0.001% to 0.015%,
B: 0% to 0.01%
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 using a low nitrogen content austenitic heat resistant steel as a welded structure, it is possible to fully utilize the performance of the low nitrogen content austenitic heat resistant steel. Provided are an austenitic heat-resistant steel weld metal excellent in reduced cracking) and a welded joint having the same.

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, Cu: 2% to 4%, Ni: 12% to 16%, Cr: 16.5% to 19.5%, W: 2% to 4.5%, Ti: 0 0.05% to 0.35%, Nb: 0.05% to 0.5%, N: 0.001% to 0.015%, Al: 0.08% or less, O: 0.08% or less The balance is 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% (where [% S], [% Sn], [% Pb] And [% Zn] represents 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. Further, C combines with Ti, Nb, and the like in the solidification process of the weld metal to generate eutectic carbide. Eutectic carbides such as Ti and Nb also contribute to a reduction in ductility reduction cracking susceptibility during repair welding. In order to sufficiently obtain these effects, C needs to be contained by 0.06% or more. However, when the C content is excessive, the ductility is lowered because a large amount of carbide is present in the weld metal. 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 the Si content is excessive, the stability of the structure decreases, leading to a decrease in toughness and creep strength. Therefore, the upper limit of Si content is 0.8% or less. A desirable range for the Si content is 0.12% to 0.7%, and a more desirable range is 0.15% to 0.6%.

(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 1.2%, and a more desirable range is 0.2% to 1.0%.

(P: 0.025% or less)
P (phosphorus) is an element contained in the weld metal as an impurity and reduces creep ductility. For this reason, the upper limit of P content needs to be 0.025% or less. The upper limit of the P content is desirably 0.023% or less, and more desirably 0.02% 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.

(Cu: 2% to 4%)
Cu (copper) enhances the stability of the austenite structure and precipitates finely during use, contributing to the improvement of creep strength. In order to acquire the effect, Cu needs to contain 2% or more. However, when the Cu content is excessive, the ductility is lowered, so the upper limit of the Cu content is 4% or less. A desirable range for the Cu content is 2.3% to 3.8%, and a more desirable range is 2.5% to 3.5%.

(Ni: 12% to 16%)
Ni (nickel) increases the stability of the austenite structure when used for a long time and contributes to the creep strength. In order to obtain the effect sufficiently, Ni needs to be contained by 12% or more. However, Ni is an expensive element, and a large amount causes an increase in cost. Therefore, the Ni content is 16% or less with an upper limit. A desirable range of the Ni content is 12.5% to 15.5%, and a more desirable range is 13% to 15%.

(Cr: 16.5% to 19.5%)
Cr (chromium) is an essential element for securing oxidation resistance and corrosion resistance at high temperatures. Further, Cr contributes to ensuring creep strength by forming fine carbides. In order to obtain the effect sufficiently, it is necessary to contain 16.5% or more of Cr. However, if the Cr content exceeds 19.5%, the stability of the austenite structure at a high temperature deteriorates and the creep strength is reduced. Therefore, the Cr content is 16.5% to 19.5%. A desirable range for the Cr content is 17% to 19%, and a more desirable range is 17.5% to 18.5%.

(W: 2% to 4.5%)
W (tungsten) is an element that contributes greatly to the improvement of creep strength and tensile strength at high temperatures by forming a solid solution or a fine intermetallic compound phase in the matrix. In order to fully exhibit the effect, W needs to be contained 2% or more. However, since W is an expensive element, excessive content 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%.

(Ti: 0.05% to 0.35%)
Ti (titanium) forms fine carbonitrides and contributes to improvement of creep strength and tensile strength at high temperatures. In order to acquire the effect, it is necessary to contain Ti 0.05% or more. However, when the Ti content is excessive, it precipitates in a large amount, resulting in a decrease in creep ductility and toughness. Therefore, the upper limit of the Ti content needs to be 0.35% or less. A desirable range of Ti content is 0.08% to 0.32%, and a more desirable range is 0.12% to 0.3%.

(Nb: 0.05% to 0.5%)
Nb (niobium), like Ti, precipitates in the grains as fine carbonitride, and is effective in improving the creep strength and tensile strength of the weld metal at high temperatures. Furthermore, Nb also contributes to a reduction in ductility lowering cracking sensitivity during repair welding. In order to obtain the effect sufficiently, Nb needs to be contained by 0.05% or more. However, if the Nb content is excessive, a large amount of carbonitride precipitates, resulting in a decrease in creep ductility and toughness. Therefore, the upper limit of Nb content is 0.5% or less. A desirable range of Nb content is 0.08% to 0.45%, and a more desirable range is 0.12% to 0.4%.

(N: 0.001% to 0.015%)
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 0.001% or more. However, when N is contained in excess of 0.015%, a large amount of nitride precipitates, resulting in a decrease in toughness. Therefore, the N content is set to 0.001% to 0.015%. A desirable range of the N content is 0.002% to 0.012%, and a more desirable range is 0.003% to 0.01%.

(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 limited to 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 0.0008% or more.

(O: 0.08% or less)
O (oxygen) is contained in the weld metal as an impurity. However, when the O (oxygen) content is excessive, toughness and ductility are deteriorated. For this reason, the upper limit of O (oxygen) content needs to be limited to 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 in the weld metal as impurities, and are elements that increase the susceptibility to ductile deterioration cracks during repair welding. As a result of various experiments in the range of the nitrogen content described above, the present inventors have found that S has a large segregation energy, so that it is easy to segregate at the grain boundary, and the degree of influence causing ductile drop cracking is Sn, Pb, and Zn. It was found that there are twice as much.
Therefore, in order to stably prevent the ductile drop cracking, the value obtained by the formula (1) ([% S] + 0.5 × {[% Sn] + [% Pb] + [% Zn]}) is used. It is necessary to make it 0.0030% or less. 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 mass% in place of Fe as an alloy component, V: 0% to 0.35%, Co: 0% to 2%, Mo: 0% to 2%, Contains one or more elements of B: 0% to 0.005%, Ca: 0% to 0.02%, Mg: 0% to 0.02%, REM: 0% to 0.06% May be. Below, each component is demonstrated.

(V: 0% to 0.35% or less)
V (vanadium), like Nb and Ti, forms fine carbonitrides and contributes to the improvement of creep strength, and may be contained as necessary. However, when V is contained excessively, it precipitates in a large amount and causes a decrease in creep ductility. Therefore, when V is included, the upper limit of V content is 0.35% or less. The upper limit of the V content is desirably 0.32% or less, and more desirably 0.3% or less. When V is contained, the desirable lower limit of the V content is 0.01% or more, and the more desirable lower limit is 0.03% or more.

(Co: 0% to 2%)
Co (cobalt) is an austenite-forming element like Ni and Cu, and contributes to the improvement of creep strength by increasing the stability of the austenite structure, so it may be contained if necessary. However, since Co is an extremely expensive element, if it is excessively contained, the cost of the material is greatly increased. Therefore, when Co is contained, the upper limit of the Co content is 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 dissolves in the matrix and contributes to the improvement of the creep strength and tensile strength at high temperatures, and may be contained as necessary. However, when Mo is contained excessively, the structure 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 Mo is included, the upper limit of the Mo content is 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 by finely dispersing carbides, strengthens the grain boundary, and contributes to a reduction in cracking susceptibility and toughness during repair welding. However, when B is contained excessively, the liquefaction cracking sensitivity at the time of repair welding is increased. Therefore, when B is included, the upper limit of the B content is 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, a desirable lower limit is 0.0003% or more, and a 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, an excessive content of Ca combines with oxygen, significantly lowers cleanliness, and deteriorates hot deformability. Therefore, the upper limit of the Ca content is 0.02% or less. When Ca is contained, the upper limit of the Ca content is desirably 0.015% or less, and more desirably 0.01% or less. When Ca is contained, a desirable lower limit is 0.0005% or more, and a more desirable lower limit is 0.001% or more.

(Mg: 0% to 0.02%)
Since Mg (magnesium) has the effect of improving hot deformability like Ca, it may be contained as necessary. However, the excessive content of Mg combines with oxygen, significantly lowers cleanliness, and deteriorates hot deformability. Therefore, the upper limit of the Mg content is 0.02% or less. When Mg is contained, the upper limit of the Mg content is desirably 0.015% or less, and more desirably 0.01% or less. When Mg is contained, a desirable lower limit is 0.0005% or more, and a 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 binds to oxygen, significantly reduces cleanliness, and deteriorates hot deformability. 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 is 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 0.15%, Si: 0.05% to 1%, Mn: 0.3% to 2.5%, P: 0.00. 04% or less, S: 0.002% or less, Cu: 2% to 4%, Ni: 11% to 16%, Cr: 16% to 20%, W: 2% to 5%, Nb: 0.1% -0.8%, Ti: 0.05% -0.35%, N: 0.001% -0.015%, B: 0% -0.01%, Al: 0.03% or less, O: It contains 0.02% or less, and the balance consists of 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 0.15%)
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 0.15% 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 Si 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: 0.3% to 2.5%)
Mn has a deoxidizing action like Si. Further, Mn contributes to stabilization of the austenite structure. If the Mn content is 0.3% 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 0.4% to 2%, and a more desirable range is 0.5% to 1.5%.

(P: 0.04% or less)
P is an element contained in the alloy as an impurity and segregates at the grain boundary of the heat affected zone during welding to increase the liquefaction cracking sensitivity. Furthermore, the creep ductility after long-time use is also reduced. Therefore, the P content is preferably limited to 0.04% or less. The upper limit of the P content is desirably 0.035% or less, and more desirably 0.03% 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.

(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, if the S content is limited to 0.002% or less, these can be prevented. 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.

(Cu: 2% to 4%)
Cu enhances the stability of the austenite structure and precipitates finely during use, contributing to the improvement of the creep strength of the base material. If the Cu content is 2% or more, the above effect can be obtained effectively. However, when Cu is excessively contained, the hot workability is lowered, so the upper limit of the Cu content is preferably 4% or less. A desirable range for the Cu content is 2.3% to 3.8%, and a more desirable range is 2.5% to 3.5%.

(Ni: 11% to 16%)
Ni is an element for ensuring the stability of the austenite structure during long-time use and ensuring the creep strength. If the Ni content is 11% or more, the above effect can be obtained 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 16% or less. A desirable range for the Ni content is 12% to 15.5%, and a more desirable range is 13% to 15%.

(Cr: 16% to 20%)
Cr is an element for ensuring oxidation resistance and corrosion resistance at high temperatures. Further, Cr contributes to ensuring creep strength by forming fine carbides. If the Cr content is 16% or more, the above effect can be obtained effectively. However, when the Cr content is excessive, the stability of the austenite structure at high temperatures deteriorates, leading to a decrease in creep strength. Therefore, the Cr content is preferably 16% to 20%. A desirable range for the Cr content is 16.5% to 19.5%, and a more desirable range is 17% to 19%.

(W: 2% to 5%)
W is an element that greatly contributes to the improvement of creep strength and tensile strength at high temperatures by forming a solid solution or a fine intermetallic compound phase in the matrix. If the W content is 2% 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 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 2.2% to 4.8%, and a more desirable range is 2.5% to 4.5%.

(Nb: 0.1% to 0.8%)
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 Nb content is excessive, a large amount of carbonitride precipitates, resulting in a decrease in creep ductility and toughness. For this reason, the upper limit of Nb content is preferably 0.8% or less. A desirable range of Nb content is 0.12% to 0.7%, and a more desirable range is 0.15% to 0.6%.

(Ti: 0.05% to 0.35%)
Ti, like Nb, forms fine carbonitrides and contributes to the improvement of creep strength and tensile strength at high temperatures. If the Ti content is 0.05% or more, the above effect can be obtained effectively. However, when the Ti content is excessive, it precipitates in a large amount and causes a decrease in creep ductility and toughness. For this reason, the upper limit of the Ti content is preferably 0.35% or less. A desirable range of Ti content is 0.08% to 0.32%, and a more desirable range is 0.12% to 0.3%.

(N: 0.001% to 0.015%)
N stabilizes the austenite structure and precipitates as a solid solution or a nitride, contributing to an improvement in high temperature strength. If N content is 0.001% or more, the said effect will be acquired effectively. However, when N is excessively contained, a large amount of fine nitride precipitates in the grains during long-time use, resulting in a decrease in creep ductility and toughness. Therefore, the upper limit of the N content is preferably 0.015% or less. A desirable range of the N content is 0.002% to 0.012%, and a more desirable range is 0.004% to 0.01%.

(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.

(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 Al content is preferably limited to 0.03% or less. The upper limit of the Al content is desirably 0.025% or less, and more desirably 0.02% or less. Note that the lower limit of the Al content is not particularly required, but since an extreme reduction leads to an increase in manufacturing cost, the desirable lower limit of the Al content is 0.0005% or more, and the more desirable lower limit is 0.001% or more. is there.

(O: 0.02% or less)
O (oxygen) is an element contained in the alloy as an impurity, and when it is excessively contained, the hot workability is lowered and the toughness and ductility are deteriorated. For this reason, the O (oxygen) content is preferably limited to 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. Although it is not necessary 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.

  Although the main elements of the base material have been described above, the base material may further include Mo: 0.01% to 2.0%, V: 0.01% to 0.4%, Nd as necessary. : 0.001% to 0.10% may be included.

  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.

A material having the chemical composition shown in Table 1 (the balance is Fe and impurities) was melted and cast in a laboratory, and then hot forging, hot rolling, heat treatment and machining were performed to obtain a plate thickness of 12 mm and a width of 50 mm. A plate material (plate material (1)) having a length of 100 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.

[Weld crack resistance test]
After processing a V groove having an angle of 30 ° and a root thickness of 1 mm in the longitudinal direction of the plate material (1), the groove is formed by manual TIG welding using Ar as the shielding gas using the cut filler shown in Table 2. A welded joint was produced by laminating the inside. In the welding, the heat input was set to 9 kJ / cm to 15 kJ / cm.
After processing a groove having an angle of 60 ° and a depth of 4 mm in a range of 30 mm in length on the weld line of this welded joint, a commercially available steel plate (SM400B defined in JIS G 3160 (2008), thickness 25 mm, width 150 mm, On the length of 200 mm), four rounds were restrained and welded using a covered arc welding rod (“DNiCrFe-3” defined in JIS Z 3224 (1999)).
And the welding which simulated repair welding was performed using the same cut filler as what produced the original weld metal in the process groove | channel. In the welding, the heat input was 15 kJ / cm.
Chips were collected from a portion (original weld metal) of the finally obtained welded joint that was not subjected to simulated repair welding and subjected to chemical analysis. Table 3 shows the results.

  Also, the cross section of the sample taken from the three places where simulated repair welding was performed was mirror polished, corroded, and examined with an optical microscope, and there was no crack in the original weld metal near the melting boundary of simulated repair welding investigated. As a result of microscopic examination, the weld metal in which no crack was recognized was “accepted”, and among the three test pieces, one in which crack was observed was regarded as “fail”.

[Creep rupture test]
Round bar creep rupture test piece so that the weld metal is in the center of the parallel part from the part of the welded joint where the crack was "passed" in the evaluation of the weld crack resistance test that was not subjected to simulated repair welding. Were collected. Then, a creep rupture test is performed under the conditions of 700 ° C. and 196 MPa, and when the rupture time exceeds about 500%, which is about 80% of the target rupture time of the base material, the “pass” is obtained, and when the rupture time is 500 hours or less, “Fail”.
Table 4 also shows the evaluation results of the above tests.

From Table 4, the welded joints having the weld metal of reference signs A1 to A3 and B1 to B3 whose chemical composition is in the range specified in the present invention does not generate cracks in the weld metal affected by the heat of repair welding, It is apparent that 80% or more of the target fracture time of the base material is satisfied.
On the other hand, in the welded joint having the weld metals of the symbols A4, A5, B4, and B5, the value of [% S] + 0.5 × {[% Sn] + [% Pb] + [% Zn]} is 0. Since it exceeded .0030%, ductile drop cracking occurred in the weld metal affected by the heat of repair welding. As described above, the weld metal that satisfies the requirements of the present invention is excellent in crack resistance and also satisfies the creep strength necessary as a welded structure, so that the performance of the low nitrogen-containing austenitic heat-resistant steel can be fully utilized. I know you get.

  By utilizing the present invention, when using a low nitrogen content austenitic heat resistant steel as a welded structure, a weld metal that can fully utilize its performance, and a welded joint that has excellent weld crack resistance and use performance including the weld metal Can provide. Therefore, the weld metal of the present invention and the welded joint having the same have a weld metal constituting a welded structure applied to equipment used at a high temperature such as a power generation boiler using a low nitrogen content austenitic heat-resistant steel and the weld metal. It is useful as a welded joint.

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,
Cu: 2% to 4%,
Ni: 12% to 16%,
Cr: 16.5% to 19.5%,
W: 2% to 4.5%,
Ti: 0.05% to 0.35%,
Nb: 0.05% to 0.5%,
N: 0.001% to 0.015%,
Al: 0.08% or less,
O: 0.08% or less, the balance is 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.
V: 0% to 0.35%,
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 0.15%,
Si: 0.05% to 1%
Mn: 0.3% to 2.5%
P: 0.04% or less,
S: 0.002% or less,
Cu: 2% to 4%,
Ni: 11% to 16%,
Cr: 16% to 20%,
W: 2% -5%
Nb: 0.1% to 0.8%
Ti: 0.05% to 0.35%,
N: 0.001% to 0.015%,
B: 0% to 0.01%
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.