JP2021080510A - Austenitic heat-resistant steel welded joint - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an austenitic heat-resistant steel welded joint having low crack sensitivity of a weld metal in the first layer and excellent creep strength. SOLUTION: The austenite is provided with a base material made of austenitic heat-resistant steel having a predetermined chemical composition and satisfying the relationship of the formula (1), and a weld metal satisfying the relations of the formulas (2) and (3). Austenitic steel welded joint. 30C + 3W + 2Cu + 10Nb + 700B + 40N ≧ 30 ... (1) [% BWM1] ≦ 0.0025 ... (2) [% NiWM3] ≧ 30 ... (3) However, each element symbol in the above formula is for each element. The content (% by mass) and [% BWM1] are the content of B contained in the weld metal of the first layer (% by mass), and [% NiWM3] is the content of Ni contained in the weld metal of the third layer (% by mass). Mass%) respectively. [Selection diagram] None

Description

The present invention relates to an austenitic heat resistant steel welded joint.

In recent years, new ultra-supercritical boilers with increased steam temperature and pressure have been installed all over the world to improve efficiency. The operating conditions of the equipment in such a high temperature environment have become extremely harsh, and the required performance for the materials used has become stricter accordingly. For example, in the field of boilers for thermal power generation, since the steam temperature is 650 ° C. or higher, high creep strength is required.

Further, in order to use the material as a structural member or the like, welding work is indispensable, and high creep strength is also required in the welded portion. Therefore, an austenitic stainless steel having improved creep strength by containing various alloying elements in an optimum amount has been invented (Patent Document 1 and the like).

Japanese Unexamined Patent Publication No. 2000-239807

In Patent Document 1, an element such as B is contained in order to improve the creep rupture strength.

However, there is a problem that solidification cracks occur in the first layer weld metal when multi-layer welding is performed using austenitic heat-resistant steel containing B as a base material and a welding material containing a large amount of Ni.

An object of the present invention is to solve the above problems and to provide an austenitic heat-resistant steel welded joint having low crack sensitivity of the weld metal in the first layer and excellent creep strength.

The present inventors conducted studies to solve the above problems and obtained the following findings.

(1) In order to improve the creep rupture strength of the welded joint, it is important that the base metal contains a large amount of elements such as B. However, if a large amount of B is present in the weld metal in such a welded joint using steel as a base material, it becomes a major cause of solidification cracking. Therefore, if a welding material having the same chemical composition as the base material is used, solidification cracks occur.

(2) Here, the weld metal is derived from the base metal and the welding material, and its chemical composition is determined by the dilution ratio of the base metal into the welding material. Dilution of the base material, in a cross section perpendicular to the welding direction (bead direction), the area of the melted base metal and [S _BM], when an area of the weld metal _{[S WM], [S BM} ] / It is a calculated value of [ _SWM]. The dilution ratio of the base metal is affected by the welding method and the like, but is generally the highest in the welding metal of the first layer, decreases in the welding metal of the second and subsequent layers, and hardly changes in the welding metal of the third and subsequent layers.

(3) Therefore, if a welding material having a low B content is selected, the B content of the third and subsequent layers can be reduced, which contributes to the suppression of solidification cracking. However, simply reducing the B content in the welding material reduces the creep rupture strength of the weld metal. Therefore, it is important to reduce the B content in the weld metal while supplementing the creep strength of the weld metal with an element different from the base metal.

(4) On the other hand, since the B content of the first layer is affected by the dilution rate of the base metal, simply selecting a welding material with a low B content completely prevents solidification cracking of the welding metal in the first layer. Does not reach. Therefore, it is important to keep the cracking susceptibility of the weld metal in the initial layer low by adjusting the dilution ratio of the base metal and keeping the B content of the initial layer within a predetermined range.

The present invention has been made based on such findings, and the following austenitic heat-resistant steel welded joints are the gist of the present invention.

An austenitic heat-resistant steel welded joint including a base material made of austenitic heat-resistant steel and a weld metal.
The chemical composition of the base material is mass%.
C: 0.04 to 0.10%,
Si: 1.0% or less,
Mn: 2.0% or less,
P: 0.03% or less,
S: 0.015% or less,
Ni: 22.5 to 32.0%,
Cr: 20.0 to 27.0%,
W: 2.0-4.0%,
Co: 0.5-3.0%,
Cu: 2.0-3.5%,
Nb: 0.25 to 0.60%,
B: 0.0015 to 0.008%,
Al: 0.003 to 0.05%,
N: 0.10 to 0.30%,
O: 0.02% or less,
Mo: 0-0.5%,
Ca: 0-0.02%,
Mg: 0-0.02%,
REM: 0-0.06%,
The balance is Fe and impurities, satisfying the following equation (1).
The weld metal satisfies the following equations (2) and (3).
Austenitic heat resistant steel welded joint.
30C + 3W + 2Cu + 10Nb + 700B + 40N ≧ 30 ... (1)
[% B _WM1 ] ≤0.0025 ... (2)
[% Ni _WM3 ] ≧ 30 ... (3)
However, each element symbol in the above formula is the content (mass%) of each element, [% B _WM1 ] is the content of B contained in the welding metal of the first layer (mass%), and [% Ni _WM3]. ] Means the content (mass%) of Ni contained in the weld metal of the third layer.

According to the present invention, it is possible to obtain an austenitic heat-resistant steel suitable for producing a welded joint having low crack sensitivity of the first layer weld metal and excellent creep strength.

It is a schematic diagram which shows the shape of the test piece for restraint weld cracking test.

Hereinafter, each requirement of the present invention will be described in detail. The austenitic heat-resistant steel welded joint according to the present invention is composed of a base material made of austenitic heat-resistant steel and a weld metal.

[Chemical composition of austenitic heat-resistant steel (base material)]
The reasons for limiting each element are as follows. In the following description, "%" for the content means "mass%".

C: 0.04 to 0.10%
C is an element that has the effect of stabilizing the austenite phase and, together with N, forms fine intragranular carbonitrides and contributes to the improvement of high temperature strength. However, if the C content is excessive, coarse carbides are generated during use at high temperatures, resulting in a decrease in creep strength and a decrease in corrosion resistance. Therefore, the C content is set to 0.04 to 0.10%. The C content is preferably 0.05% or more, and preferably 0.085% or less.

Si: 1.0% or less Si is an element that has a deoxidizing effect and is effective for corrosion resistance and oxidation resistance at high temperatures. However, if the content is excessive, the stability of the austenite phase is reduced, resulting in a decrease in creep strength and toughness. Therefore, the Si content is set to 1.0% or less. The Si content is preferably 0.8% or less, more preferably 0.5% or less.

It is not necessary to set a lower limit for the Si content, but if it is extremely lowered, the deoxidizing effect cannot be sufficiently obtained, the cleanliness of the steel is deteriorated, and the manufacturing cost is increased. Therefore, the Si content is preferably 0.02% or more.

Mn: 2.0% or less Mn has a deoxidizing action like Si. Mn also contributes to the stabilization of the austenite phase. However, if its content is excessive, it leads to embrittlement and a decrease in creep ductility and toughness. Therefore, the Mn content is set to 2.0% or less. The Mn content is preferably 1.5% or less, more preferably 0.6% or less.

It is not necessary to set a lower limit for the Mn content, but if it is extremely lowered, the deoxidizing effect cannot be sufficiently obtained, the cleanliness of the steel is deteriorated, and the manufacturing cost is increased. Therefore, the Mn content is preferably 0.02% or more.

P: 0.03% or less S: 0.015% or less P and S are elements contained as impurities in the alloy. All of these elements lower the melting point of the final solidified part during solidification of the weld metal, significantly increase the solidification cracking sensitivity, and cause intergranular embrittlement during high-temperature use, resulting in a decrease in stress relaxation cracking resistance. It is an element. Therefore, the content is limited to P: 0.03% or less and S: 0.015% or less, respectively.

Ni: 22.5 to 32.0%
Ni is an element effective for obtaining an austenite structure, and is an essential element for ensuring tissue stability during long-term use and obtaining a desired creep strength. On the other hand, since Ni is an expensive element, excessive content causes an increase in cost. Therefore, the Ni content is set to 22.5 to 32.0%. The Ni content is preferably 24.0% or more, and preferably 28.0% or less.

Cr: 20.0 to 27.0%
Cr is an essential element for ensuring oxidation resistance and corrosion resistance at high temperatures. However, excessive content reduces the stability of the austenite phase at high temperatures, leading to a decrease in creep strength. Therefore, the Cr content is set to 20.0 to 27.0%. The Cr content is preferably 21.5% or more, and preferably 23.5% or less.

W: 2.0-4.0%
W is an element that dissolves in the matrix and contributes to the improvement of high temperature strength, especially the creep strength at high temperature. However, if the W content becomes excessive, the stability of the austenite phase is lowered, and thus the creep strength is rather lowered. In addition, there is a risk of increasing the crack sensitivity of the weld metal. Therefore, the W content is set to 2.0 to 4.0%.
The W content is preferably 2.3% or more, and preferably 3.6% or less.

Co: 0.5-3.0%
Like Ni and Cu, Co is an austenite-forming element, which enhances the stability of the austenite structure and contributes to the improvement of creep strength. However, since Co is an extremely expensive element, excessive content causes a significant cost increase. Therefore, the Co content is set to 0.5 to 3.0%. The Co content is preferably 1.0% or more, and preferably 2.0% or less.

Cu: 2.0-3.5%
Cu is an element effective for improving creep rupture strength. However, if it is contained in an excessive amount, the creep ductility is rather lowered. It is an element that causes embrittlement when it is contained in excess. Therefore, the Cu content is therefore set to 2.0-3.5%. The Cu content is preferably 2.3% or more, and preferably 3.2% or less.

Nb: 0.25 to 0.60%
Nb is an element that is finely precipitated in the grain as a carbonitride and contributes to the improvement of creep strength at high temperature. However, if the Nb content is excessive, the carbonitride will rapidly coarsen during use at high temperatures, resulting in an extreme decrease in creep strength and toughness. In addition, there is a risk of increasing the crack sensitivity of the weld metal. Therefore, the Nb content is set to 0.20 to 0.60%. The Nb content is preferably 0.30% or more, and preferably 0.55% or less.

B: 0.0015 to 0.008%
B is an element that contributes to strengthening the grain boundaries by segregating at the grain boundaries and finely dispersing the carbides at the grain boundaries. However, when the B content becomes excessive, the melting point of the final solidified portion is lowered during solidification of the weld metal, and the solidification crack sensitivity is significantly increased. Therefore, the B content is set to 0.0015 to 0.008%. The B content is preferably 0.0020% or more, and preferably 0.0055% or less.

Al: 0.003 to 0.05%
Al has a deoxidizing effect, but the addition of a large amount significantly impairs cleanliness and deteriorates processability and ductility. Therefore, the Al content is Al: 0.003 to 0.05%. The Al content is preferably 0.01% or more, and preferably 0.03% or less.

N: 0.10 to 0.30%
N is an austenite stabilizing element, which dissolves in a matrix and forms fine intragranular carbonitrides like C, which contributes to ensuring creep strength at high temperatures. It is also an element that is effective in improving corrosion resistance. However, when the N content becomes excessive, a large amount of nitride is precipitated, which not only lowers the creep ductility but also lowers the hot workability, which causes surface defects of the base metal. Therefore, the N content is set to 0.10 to 0.30%. The N content is preferably 0.15% or more, and preferably 0.25% or less.

O: 0.02% or less O is contained in the alloy as an impurity, and if it is excessively contained, the hot workability is lowered and the toughness and ductility are deteriorated. Therefore, the O content is set to 0.02% or less. The O content is preferably 0.018% or less.

Mo: 0-0.5%
Mo is an element that dissolves in the matrix and contributes to the improvement of high temperature strength, especially the creep strength at high temperature. However, if the Mo content is excessive, the stability of the austenite phase is lowered, and thus the creep strength is rather lowered. In addition, there is a risk of increasing the crack sensitivity of the weld metal. Therefore, the Mo content is set to 0.5% or less. The above effect is exhibited even in a small amount, but is particularly remarkable when it is contained in an amount of 0.01% or more. The Mo content is preferably 0.1% or less.

Ca: 0-0.02%
Since Ca has an effect of improving the hot deformability, it may be contained if necessary. However, the excessively contained Ca combines with oxygen to significantly reduce the cleanliness, and thus deteriorates the hot deformability. Therefore, the Ca content is preferably 0.02% or less. The Ca content is more preferably 0.015% or less. In order to obtain the above effects, the Ca content is preferably 0.001% or more, and more preferably 0.002% or more.

Mg: 0-0.02%
Like Ca, Mg has an effect of improving the hot deformability, and therefore may be contained if necessary. However, the excessively contained Mg combines with oxygen to significantly reduce the cleanliness, and thus deteriorates the hot deformability. Therefore, the Mg content is preferably 0.02% or less. The Mg content is more preferably 0.015% or less. In order to obtain the above effects, the Mg content is preferably 0.001% or more, and more preferably 0.002% or more.

REM: 0-0.06%
REM contributes to the improvement of hot deformability during manufacturing. However, the excessively contained REM combines with oxygen to significantly reduce the cleanliness, and thus deteriorates the hot deformability. Therefore, the REM content is preferably 0.06% or less. The REM content is more preferably 0.04% or less, and even more preferably 0.03% or less. In order to obtain the above effects, the REM content is preferably 0.001% or more, and more preferably 0.01% or more.

In the chemical composition of the austenitic heat-resistant steel of the present invention, the balance is Fe and impurities. Here, the "impurity" is a component mixed with raw materials such as ore and scrap, and various factors in the manufacturing process when an alloy is industrially manufactured, and is allowed as long as it does not adversely affect the present invention. Means something. Further, the chemical composition of the austenitic heat-resistant steel of the present invention needs to satisfy the following formula (1).
30C + 3W + 2Cu + 10Nb + 700B + 40N ≧ 30 ... (1)
However, each element symbol in the above formula represents the content (mass%) of each element.
The above equation (1) is an equation in which the degree of influence of each element is arranged for the elements that improve the creep rupture strength. Then, when the value calculated on the left side of the above equation (1) is less than the value (30) on the right side, the creep rupture strength decreases. The rvalue in the above equation (1) is preferably 31 and more preferably 33.

[Chemical composition of weld metal]
(First layer weld metal)
The weld metal is derived from the base metal and the welding material, and its chemical composition is determined by the dilution ratio of the base metal into the welding material. Although the dilution ratio of the base metal is affected by the welding method, etc., it is generally the highest in the welding metal of the first layer of the welding metal, decreases in the welding metal of the second and subsequent layers, and is almost the same in the welding metal of the third and subsequent layers. It does not change. Therefore, if a welding material having a low B content is selected, the B content of the third and subsequent layers can be reduced, which contributes to the suppression of solidification cracking. However, since the B content of the first layer is affected by the dilution rate of the base metal, it cannot be adjusted simply by selecting a welding material with a low B content, and solidification cracking of the weld metal in the first layer is caused. It cannot be completely prevented. Then, if the dilution ratio of the base metal can be adjusted and the B content of the initial layer can be kept within a predetermined range, the crack sensitivity of the weld metal of the initial layer can be suppressed to be low. _{Specifically, the B content [% B WM1} ] (mass%) contained in the weld metal of the first layer must satisfy the relationship of the following equation (2).
[% B _WM1 ] ≤0.0025 ... (2)
The rvalue in the above equation (2) is preferably 0.0020, more preferably 0.0016.

(Third layer weld metal)
In the present invention, it is important to select a welding material having a low B content and to contain a large amount of Ni in order to suppress solidification cracking in the welding metal of the first layer while not lowering the creep rupture strength of the welding metal. Is. That is, the Ni content [% Ni _WM3 ] (mass%) contained in the weld metal of the third layer must satisfy the relationship of the following equation (3).
[% Ni _WM3 ] ≧ 30 ... (3)
The rvalue in the above equation (3) is preferably 40, more preferably 50.
More specifically, the weld metal of the third layer preferably has the following chemical composition.

C: 0.005 to 0.180%
C is an austenite-forming element and is an element effective for enhancing the stability of the austenite structure when used at a high temperature. However, if the C content is excessive, ductility and toughness are reduced due to the large amount of carbides present in the weld metal. Therefore, the C content is preferably 0.005 to 0.180%. The C content is preferably 0.008% or more, and preferably 0.150% or less.

Si: 0 to 1.20%
Si is an element that has a deoxidizing effect and is effective in corrosion resistance and oxidation resistance at high temperatures. However, if the content is excessive, the stability of the austenite phase is reduced, resulting in a decrease in creep strength and toughness. Therefore, the Si content is preferably 1.20% or less. The Si content is preferably 1.0% or less, more preferably 0.8% or less. In order to obtain the above effects, the Si content is preferably 0.10% or more, and more preferably 0.20% or more.

Mn: 0.02 to 4.00%
Mn has a deoxidizing action like Si. In addition, Mn stabilizes the austenite structure and contributes to the improvement of creep strength. However, an excessive Mn content causes embrittlement and further reduces creep ductility. Therefore, the Mn content is preferably 0.02 to 4.00%. The Mn content is preferably 0.05% or more, and preferably 3.00% or less.

P: 0.030% or less P is contained as an impurity, which increases the solidification crack sensitivity during solidification of the weld metal and causes a decrease in creep ductility. Therefore, the P content is preferably 0.030% or less. The P content is more preferably 0.020% or less, and further preferably 0.015% or less.

S: 0.010% or less S is contained as an impurity like P, and increases the solidification cracking sensitivity at the time of solidification of the weld metal and causes a decrease in creep ductility. Therefore, the S content is preferably 0.010% or less. The S content is more preferably 0.008% or less, and further preferably 0.005% or less.

Cu: 0-3.5%
Like Co, Cu may be contained because it is an element effective for stabilizing the structure of the weld metal at high temperature and improving the creep strength. However, if it is contained in an excessive amount, the creep ductility is rather lowered. Therefore, the Cu content is preferably 3.5% or less. The Cu content is more preferably 3.0% or less, and even more preferably 2.5% or less. In order to obtain the above effects, the Cu content is preferably 0.01% or more, and more preferably 0.05% or more.

Co: 0 to 15.0%
Co may be contained because it is an element effective for stabilizing the structure of the weld metal at high temperature and improving the creep strength. However, if it is contained in an excessive amount, the creep strength and creep ductility are rather lowered. In addition, it is a very expensive element, which increases material costs. Therefore, the Co content is preferably 15.0% or less. The Co content is more preferably 14.5% or less, and even more preferably 14.0% or less. In order to obtain the above effects, the Co content is preferably 0.50% or more, and more preferably 1.00% or more.

Cr: 20.0 to 27.0%
Cr is an element effective for water vapor oxidation resistance and corrosion resistance at high temperature of the weld metal. In addition, it precipitates as carbide during use at high temperature, which also contributes to the improvement of creep strength. In order to obtain these effects, it is preferable to contain 20.0% or more. However, if it is excessively contained, the tissue stability at high temperature may be lowered and the creep strength may be lowered. Therefore, the Cr content is preferably 27.0% or less. The Cr content is preferably 20.5% or more, and more preferably 21.0% or more. The Cr content is preferably 26.5% or less, more preferably 26.0% or less.

Ni: 30.0 to 70.0%
Ni is an element effective for obtaining an austenite structure, and is an essential element for ensuring tissue stability during long-term use and obtaining a desired creep strength. On the other hand, since Ni is an expensive element, excessive content causes an increase in cost. Therefore, the Ni content is preferably 30.0% to 70.0%. The Ni content is preferably 35.0% or more, and preferably 40.0% or more. The Ni content is preferably 68.0% or less, more preferably 65.0% or less, and even more preferably 60.0% or less.

Mo: 0.01 to 12.0%
Mo may be contained because it is an element that dissolves in the matrix and contributes to ensuring the creep strength of the weld metal at a high temperature. However, if it is contained in an excessive amount, the tissue stability at high temperature is lowered, and the creep strength is rather lowered. Therefore, the Mo content is preferably 12.0% or less. The Mo content is more preferably 11.5% or less, and even more preferably 11.0% or less. In order to obtain the above effects, the Mo content is preferably 0.01% or more, and more preferably 0.05% or more.

Ti: 0.01-0.50%
Like Nb, Ti may be contained as a fine carbonitride during use at a high temperature because it precipitates in the grains and contributes to the improvement of the creep strength of the weld metal. However, if the content is excessive, a large amount and coarse precipitation may occur, which may lead to a decrease in creep strength and creep ductility. Therefore, the Ti content is preferably 0.50% or less. The Ti content is more preferably 0.48% or less, and further preferably 0.45% or less. In order to obtain the above effects, the Ti content is preferably 0.01% or more, and more preferably 0.05% or more.

W: 0-4.0%
Like Mo, W may be contained because it is an element that dissolves in the matrix and contributes to the improvement of creep strength at high temperatures. However, if the W content becomes excessive, the stability of the austenite phase is lowered, and thus the creep strength is rather lowered. In addition, there is a risk of increasing the crack sensitivity of the weld metal. Therefore, the W content is preferably 4.0% or less. The W content is more preferably 3.8% or less, and even more preferably 3.6% or less. In order to obtain the above effects, the W content is preferably 0.01% or more, and more preferably 0.05% or more.

Nb: 0 to 1.00%
Nb may be contained in one or both of the Nb because it is precipitated in the grains as fine carbonitride during use at a high temperature and contributes to the improvement of the creep strength of the weld metal. However, if the content is excessive, a large amount and coarse precipitation may occur, which may lead to a decrease in creep strength and creep ductility. Therefore, the Nb content is preferably 1.00% or less. The Nb content is more preferably 0.90% or less, and further preferably 0.80% or less. In order to obtain the above effects, the Nb content is preferably 0.01% or more, and more preferably 0.05% or more.

N: 0-0.30%
Since N is an element effective for enhancing the stability of the weld metal structure at high temperatures, it may be contained. However, if it is contained in an excessive amount, a large amount of nitride is precipitated during use at a high temperature, and the toughness and ductility are lowered. Therefore, the content is preferably 0.30% or less. The N content is more preferably 0.28% or less, and further preferably 0.25% or less. In order to obtain the above effects, the N content is more preferably 0.01% or more.

B: 0 to 0.008%
B may be contained because it is an element that contributes to the creep strength of the weld metal by segregating at the grain boundaries and finely dispersing the carbides at the grain boundaries. However, when the B content becomes excessive, the toughness of the weld metal is lowered. Therefore, the B content is set to 0.008% or less. The B content is more preferably 0.006% or less, and further preferably 0.004% or less. In order to obtain the above effects, the content of B is preferably 0.0001% or more, and more preferably 0.0002% or more.

Al: 0.01-2.00%
Al may be contained because it binds to Ni and is finely precipitated in the grain as an intermetallic compound and contributes to the improvement of the creep strength of the weld metal. On the other hand, excessive content causes excessive precipitation of the intermetallic compound phase and lowers toughness. Therefore, the Al content is preferably 2.00% or less. The Al content is more preferably 1.80% or less, and further preferably 1.60% or less. In order to obtain the above effects, the Al content is preferably 0.01% or more, and more preferably 0.05% or more.

O: 0.020% or less O is contained as an impurity, but when it is contained in a large amount, the ductility of the weld metal is lowered. Therefore, the O content is preferably 0.020% or less. The O content is more preferably 0.018% or less, and further preferably 0.015% or less.

Ca: 0-0.02%
Since Ca has an effect of improving the hot deformability, it may be contained if necessary. However, the excessively contained Ca combines with oxygen to significantly reduce the cleanliness, and thus deteriorates the hot deformability. Therefore, the Ca content is preferably 0.02% or less. The Ca content is more preferably 0.015% or less. In order to obtain the above effects, the Ca content is preferably 0.001% or more, and more preferably 0.002% or more.

Mg: 0-0.02%
Like Ca, Mg has an effect of improving the hot deformability, and therefore may be contained if necessary. However, the excessively contained Mg combines with oxygen to significantly reduce the cleanliness, and thus deteriorates the hot deformability. Therefore, the Mg content is preferably 0.02% or less. The Mg content is more preferably 0.015% or less. In order to obtain the above effects, the Mg content is preferably 0.001% or more, and more preferably 0.002% or more.

REM: 0-0.06%
REM contributes to the improvement of hot deformability during manufacturing. However, the excessively contained REM combines with oxygen to significantly reduce the cleanliness, and thus deteriorates the hot deformability. Therefore, the REM content is preferably 0.06% or less. The REM content is more preferably 0.04% or less, and even more preferably 0.03% or less. In order to obtain the above effects, the REM content is preferably 0.001% or more, and more preferably 0.01% or more.

In the chemical composition of the weld metal of the third layer, the balance is Fe and impurities. Here, the "impurity" is a component mixed by various factors of raw materials such as ore and scrap, and various factors in the manufacturing process when an alloy is industrially manufactured, and is allowed as long as it does not adversely affect the present invention. Means something.

[Chemical composition of welding material]
The composition of the welding material used when welding the base metal is not particularly limited, but it is preferable to have the chemical composition shown below.

C: 0.005 to 0.180%
C is an austenite-forming element and is an element effective for enhancing the stability of the austenite structure when used at a high temperature. Further, C enhances high temperature crack resistance during welding. Specifically, C mainly combines with Cr in the solidification process during welding to form eutectic carbides. This accelerates the disappearance of the liquid phase and makes the structure of the final solidified part _{a lamellar structure of (Cr, M) 23} C ₆ and austenite. As a result, the residual morphology of the liquid phase changes from a planar shape to a dot shape, stress concentration on a specific surface is suppressed, and solidification cracking is suppressed. Further, since C increases the final solidification boundary area which is a segregation site of impurities, it also contributes to the prevention of ductile reduction cracks during welding and the reduction of the sensitivity of stress relaxation cracks during high temperature use.

In order to obtain the above effect sufficiently within the range of Cr content described later, it is necessary to set the C content to 0.005% or more. However, when C is excessively contained, excess C, which does not become carbide during solidification, is finely precipitated as carbide during high-temperature use, which rather increases the stress relaxation cracking sensitivity. Therefore, the content of C is set to 0.005 to 0.180%. The C content is preferably 0.02% or more, and more preferably 0.06% or more. The C content is preferably 0.15% or less.

Si: 1.5% or less Si is contained as an antacid, but segregates at the columnar grain boundaries during solidification of the weld metal, lowers the melting point of the liquid phase, and increases the solidification cracking sensitivity. Therefore, the Si content needs to be 1.5% or less. It is not necessary to set a lower limit for the Si content, but if it is extremely lowered, the deoxidizing effect cannot be sufficiently obtained, the cleanliness of the steel is deteriorated, and the manufacturing cost is increased. Therefore, the Si content is preferably 0.02% or more.

Mn: 2.0% or less Mn is contained as a deoxidizer like Si. Mn suppresses the scattering of N from the arc atmosphere by lowering the activity of N in the weld metal, and also contributes to ensuring the strength. However, if Mn is excessively contained, embrittlement will occur, so the Mn content needs to be 2.0% or less. The Mn content is preferably 1.5% or less.

It is not necessary to set a lower limit for the Mn content, but if it is extremely lowered, the deoxidizing effect cannot be sufficiently obtained, the cleanliness of the steel is deteriorated, and the manufacturing cost is increased. Therefore, the Mn content is preferably 0.02% or more.

P: 0.020% or less S: 0.030% or less P and S are contained as impurities, lower the melting point of the final solidified portion during solidification of the weld metal, and significantly increase the solidification crack sensitivity. Therefore, the P content needs to be 0.020% or less, and the S content needs to be 0.030% or less. The P content is preferably 0.015% or less, and the S content is preferably 0.020% or less.

Cu: 0.15% or less Cu is an element that causes embrittlement when it is contained in excess. Therefore, it is desirable to reduce the Cu content as much as possible, and the Cu content should be 0.15% or less. The Cu content is preferably 0.10% or less.

Cr: 20.0 to 25.0%
Cr is an essential element for ensuring oxidation resistance and corrosion resistance at high temperatures. Cr combines with C in the solidification process to generate eutectic carbides, prevents solidification cracks and ductility-reducing cracks during welding, and also has the effect of reducing stress relaxation crack susceptibility during high-temperature use. In order to obtain these effects, the Cr content needs to be 20.0% or more. However, if the Cr content becomes excessive and exceeds 25.0%, the stability of the structure at high temperature deteriorates, resulting in a decrease in creep strength. Therefore, the Cr content is set to 20.0 to 25.0%. The Cr content is preferably 20.5% or more, and preferably 24.5% or less.

Mo: 0-10.0%
Mo is an element that dissolves in the matrix and contributes to the improvement of high temperature strength, especially the creep strength at high temperature. However, when the Mo content becomes excessive, the stability of the austenite phase decreases and local corrosion at high temperature increases. Therefore, the Mo content is set to 10.0% or less. The Mo content is preferably 9.5% or less. The lower limit does not need to be specified and is 0%. However, when the above effect is desired, the Mo content is preferably 0.5% or more, and more preferably the Mo content or more in the base material.

W: 0-4.0%
Like Mo, W may be contained because it is an element that dissolves in the matrix and contributes to the improvement of creep strength at high temperatures. However, if the W content becomes excessive, the stability of the austenite phase is lowered, and thus the creep strength is rather lowered. In addition, there is a risk of increasing the crack sensitivity of the weld metal. Therefore, the W content is preferably 4.0% or less. The W content is more preferably 3.8% or less, and even more preferably 3.6% or less. In order to obtain the above effects, the W content is preferably 0.01% or more, and more preferably 0.05% or more.

Nb: 0-1.0%
Nb is an element that is finely precipitated in the grain as a carbonitride and contributes to the improvement of creep strength at high temperature. However, if the Nb content is excessive, the carbonitride will rapidly coarsen during use at high temperatures, resulting in an extreme decrease in creep strength and toughness. In addition, there is a risk of increasing the crack sensitivity of the weld metal. Therefore, the Nb content is set to 1.0% or less. The Nb content is preferably 0.6% or less. The lower limit does not need to be specified and is 0%. However, when the above effect is desired, the Nb content is preferably 0.1% or more, and more preferably 0.4% or more.

Ti: 0 to 0.50%
Ti is an element that is finely precipitated as a carbonitride in the grain and contributes to the improvement of creep strength at high temperature. However, if the content of Ti is excessive, the carbonitride rapidly becomes coarse during use at high temperature. This not only results in an extreme decrease in creep strength and toughness, but also a significant increase in liquefaction cracking susceptibility during welding. Therefore, the Ti content is preferably reduced to 0.50% or less. The lower limit does not need to be specified and is 0%. However, when the above effect is desired, the Ti content is preferably 0.1% or more, and more preferably 0.3% or more.

Co: 15.0% or less Co, like Ni and Cu, is an austenite-forming element, which enhances the stability of the austenite structure and contributes to the improvement of creep strength. However, since Co is an extremely expensive element, excessive content causes a significant cost increase. Therefore, the Co content is set to 15.0% or less. The Co content is preferably 14.0% or less. The lower limit does not need to be specified and is 0%. However, when the above effect is desired, the Co content is preferably 0.5% or more.

Al: 0-2.0%
Al is an element having a deoxidizing action. However, the addition of large amounts significantly impairs cleanliness and deteriorates processability and ductility. Therefore, the Al content is set to 2.0% or less. The lower limit does not need to be specified and is 0%. However, when the above effect is desired, the Al content is preferably 0.5% or more.

N: 0-0.30%
Since N is an element effective for enhancing the stability of the weld metal structure at high temperatures, it may be contained. However, when it is contained in an excessive amount, a large amount of nitride is precipitated during use at a high temperature, and the toughness and ductility are lowered. Therefore, the content is preferably 0.30% or less. The N content is more preferably 0.28% or less, and further preferably 0.25% or less. In order to obtain the above effects, the N content is more preferably 0.01% or more.

B: 0 to 0.005%
B is an element effective for improving creep strength by segregating at grain boundaries during use at high temperature, strengthening the grain boundaries, and finely dispersing carbides at the grain boundaries. Therefore, B may be contained in order to obtain this effect. However, the excessive content of B increases the susceptibility to solidification cracking during gas shielded arc welding. Therefore, the B content is set to 0.005% or less. The B content is preferably 0.0045% or less. The lower limit does not need to be specified and is 0%. However, when the above effect is desired, the B content is preferably 0.002% or more.

Ni: 30.0 to 70.0%
Ni is an element effective for obtaining an austenite structure, and is an essential element for ensuring tissue stability during long-term use and obtaining a desired creep strength. On the other hand, since Ni is an expensive element, excessive content causes an increase in cost. Therefore, the Ni content is set to 30.0% to 70.0%. The Ni content is preferably 35.0% or more, and preferably 40.0% or more. The Ni content is preferably 65.0% or less, and preferably 60.0% or less.

O: 0.020% or less O is contained as an impurity, but when it is contained in a large amount, the ductility of the weld metal is lowered. Therefore, the O content is preferably 0.020% or less. The O content is more preferably 0.018% or less, and further preferably 0.015% or less.

In the chemical composition of the welding material described above, the balance is Fe and impurities. Here, the "impurity" is a component mixed by various factors of raw materials such as ore and scrap, and various factors in the manufacturing process when an alloy is industrially manufactured, and is allowed as long as it does not adversely affect the present invention. Means something.

[Manufacturing method of welded joint]
The austenitic heat-resistant steel welded joint according to the present invention is manufactured by welding base materials made of austenitic heat-resistant steel with each other using a predetermined welding material to form a weld metal.

(Manufacturing method of austenitic heat resistant steel (base material))
As the austenitic heat-resistant steel (base material), those manufactured by a usual method are used. That is, a heat-resistant steel having a predetermined chemical composition is melted, and a heat-resistant steel material having a predetermined shape is produced by hot forging, hot rolling, cold rolling, and solution heat treatment.

(Manufacturing method of welded joint)
For welding, for example, GTAW welding (Gas Tungsten Arc Welding) can be used. By changing the supply rate of the welding material, the dilution rate of the base metal in the initial layer can be changed. Specifically, a welded joint was produced in which the supply rate of the welding material was changed between 45 and 800 mm / min and the dilution rate of the base material was variously changed.

Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited to these Examples.

The heat-resistant steel having the chemical composition shown in Table 1 was melted, hot forged, hot-rolled and cold-rolled, and subjected to solution heat treatment at 1230 ° C. After that, in JIS Z 3001-1 (2013) No. 14349, U groove processing was performed so that the root radius r = 0.5 mm, the root surface b = 1.2 mm, and the groove angle θ = 40 °, and the thickness was 15 mm. , A test piece for restraint weld cracking test having a width of 120 mm and a length of 200 mm was prepared (see FIG. 1).

Using each restraint weld crack test test piece obtained as described above, using ENi6182 specified in JIS Z 3224 (2010) as a shielded metal arc welding rod, JIS having a thickness of 25 mm, a width of 200 mm, and a length of 300 mm is used. Four circumferences were restrained welded on a commercially available steel plate of SM400C specified in G 3106 (2008).

Then, the first layer TIG welding was performed using a spool welding material having a diameter of 1.2 mm having the chemical composition shown in Table 2 inside the groove. The heat input was 6 to 10 kJ / cm, and the feeding speed of the welding material was variously changed in the range of 45 to 800 mm / min. Then, about half of the first layer weld metal was left, and the rest was laminated welded under the condition of heat input of 9 to 12 kJ / cm. At that time, the temperature between passes was controlled to 150 ° C. or lower. Then, the composition of the weld metal of the first layer and the third layer was measured by quantifying the central portion of the weld metal by EPMA analysis. The results are shown in Table 3.

After the above welding, two test pieces for observing the cross-sectional microstructure of the joint were collected from the welded portion only in the first layer for each test piece. Then, after the cross section was mirror-polished, chromic acid electrolytically corroded, and the presence or absence of cracks was observed using an optical microscope at a magnification of 500 times. The results are shown in Table 3.

Next, a stepped round bar creep test piece was cut out so that the weld metal was located at the center of a parallel portion having a diameter of 6 mm and a length of 10 mm, and a creep break test was performed. Then, assuming an actual use environment, a case where the breaking time is 1,000 hours or more under a stress load of 200 MPa at 650 ° C. is regarded as "pass", and a case where the breaking time is less than 1,000 hours is regarded as "fail". The results are shown in Table 3.

As shown in Table 3, when a base metal having a predetermined chemical composition is used and appropriate welding is performed to form a weld metal having a predetermined chemical composition, solidification cracking of the initial layer does not occur. It had good creep strength. On the other hand, Test No. in which the chemical composition of the base material is outside the range specified in the present invention. In 9 to 16, the creep rupture strength was deteriorated. Test No. in which the B content of the weld metal in the first layer was too high. In 18 to 20, solidification cracking of the first layer occurred. Test No. 2 in which the Ni content of the weld metal was too low. At 21 and 22, the creep rupture strength was deteriorated.

According to the present invention, it is possible to obtain an austenitic heat-resistant steel suitable for producing a welded joint having low crack sensitivity of the first layer weld metal and excellent creep strength. Therefore, the austenitic heat-resistant steel of the present invention can be suitably used as a material for equipment such as boilers used in a high temperature environment.

Claims (3)

An austenitic heat-resistant steel welded joint including a base material made of austenitic heat-resistant steel and a weld metal.
The chemical composition of the base material is mass%.
C: 0.04 to 0.10%,
Si: 1.0% or less,
Mn: 2.0% or less,
P: 0.03% or less,
S: 0.015% or less,
Ni: 22.5 to 32.0%,
Cr: 20.0 to 27.0%,
W: 2.0-4.0%,
Co: 0.5-3.0%,
Cu: 2.0-3.5%,
Nb: 0.25 to 0.60%,
B: 0.0015 to 0.008%,
Al: 0.003 to 0.05%,
N: 0.10 to 0.30%,
O: 0.02% or less,
Mo: 0-0.5%,
Ca: 0-0.02%,
Mg: 0-0.02%,
REM: 0-0.06%,
The balance is Fe and impurities, satisfying the following equation (1).
The weld metal satisfies the following equations (2) and (3).
Austenitic heat resistant steel welded joint.
30C + 3W + 2Cu + 10Nb + 700B + 40N ≧ 30 ... (1)
[% B _WM1 ] ≤0.0025 ... (2)
[% Ni _WM3 ] ≧ 30 ... (3)
However, each element symbol in the above formula is the content (mass%) of each element, [% B _WM1 ] is the content of B contained in the welding metal of the first layer (mass%), and [% Ni _WM3]. ] Means the content (mass%) of Ni contained in the weld metal of the third layer. The chemical composition of the base material is mass%.
Mo: 0.01-0.5%,
Ca: 0.0005% -0.02%
Mg: 0.0005% to 0.02% and REM: 0.0003% to 0.06%
Contains one or more selected from,
The austenitic heat-resistant steel welded joint according to claim 1. The chemical composition of the weld metal of the third layer is mass%.
C: 0.005 to 0.180%,
Si: 0 to 1.20%,
Mn: 0.02 to 4.00%,
P: 0.030% or less,
S: 0.010% or less,
Cu: 0-3.5%,
Co: 0 to 15.0%,
Cr: 20.0 to 27.0%,
Ni: 30.0 to 70.0%,
Mo: 0.01 to 12.0%,
Ti: 0.01 to 0.50%,
W: 0-4.0%,
Nb: 0-1.00%,
N: 0-0.30%,
B: 0-0.008%,
Al: 0.01-2.00%,
O: 0.020% or less,
Ca: 0-0.02%,
Mg: 0-0.02%,
REM: 0-0.06%,
The balance is Fe and impurities,
The austenitic heat-resistant steel welded joint according to claim 1 or 2.

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