JP6225598B2 - Austenitic stainless steel welding material - Google Patents

Description

  The present invention relates to an austenitic stainless steel welding material. Specifically, it is used to obtain welded joints with excellent resistance to polythionic acid stress corrosion cracking and high-temperature strength, which are characteristics required for heating furnace tubes of petroleum refining and petrochemical plants, and has excellent weldability. The present invention relates to an austenitic stainless steel welding material. In the following description, “stress corrosion cracking” is referred to as “SCC”.

  Equipment such as heating furnace tubes of petroleum refining and petrochemical plants operate in a high temperature environment, and iron sulfide is generated due to sulfur (S) contained in the raw material. When the iron sulfide comes into contact with water or air, it produces polythionic acid and induces SCC. Therefore, the material used for these facilities is required to have excellent resistance to polythionic acid SCC.

  Further, depending on the raw material, a catalytic reaction at a high temperature exceeding 600 ° C. is achieved, and not only the polythionic acid SCC resistance but also excellent high temperature strength and aging characteristics are required.

  Therefore, not only the steel material as a base material but also welded joints, particularly weld metals, have excellent polythionic acid SCC resistance, high-temperature strength and aging characteristics, and a welding material capable of obtaining them is indispensable.

  In view of this, the present inventors disclosed in Patent Document 1 an austenitic stainless steel that is excellent in resistance to polythionate SCC and high-temperature strength, and also has excellent resistance to weld cracking such as liquefaction cracking and embrittlement cracking, that is, excellent weld cracking resistance. Proposed.

  In addition, various austenitic welding materials have been proposed in consideration of the low C region necessary for securing grain boundary sensitization characteristics such as polythionic acid SCC.

  For example, Patent Document 2 proposes a welding material having a C content of 0.01 to 0.15% by mass and excellent in reheat cracking resistance and creep characteristics.

  Patent Document 3 and Patent Document 4 each have a C content of 0.01 to 0.05% by mass, and Patent Document 5 has a C content of 0.01 to 0.07% by mass. TIG welding materials excellent in high temperature strength have been proposed.

  Patent Document 6 and Patent Document 7 propose a welding material having a low C content of 0.030% by mass or less, containing Nb as required, and further containing a specific amount of ferrite in the structure. Has been.

  In addition, Patent Document 8 and Patent Document 9 each have a low C content of 0.01% by mass or less, a welding material excellent in stress corrosion cracking resistance, and a welding material excellent in hot cracking resistance. In both Patent Document 10 and Patent Document 11, a welding material in which the C content is as low as 0.030% by mass or less and the ferrite content of the weld metal part is in a specific range. Has been proposed.

  Further, Patent Document 12 also proposes a low-C welding material having a C content of 0.01% by mass or less, and Patent Document 13 discloses that the ratio of the content of C and Nb falls within a specific range. Some welding materials have been proposed.

International Publication No. 2009/044802 JP 2000-102891 A Japanese Patent Laid-Open No. 11-285889 Japanese Patent Laid-Open No. 10-225792 JP 2001-300763 A JP-A-5-50287 JP-A-5-50288 JP 2010-142843 A JP-A-4-237592 Japanese Patent Laid-Open No. 2-99295 JP-A-3-169497 International Publication No. 2010/071146 JP 2004-58062 A

  The austenitic stainless steel proposed by the present inventors in Patent Document 1 is indeed excellent in weld crack resistance, and has good polythionic acid SCC resistance and high-temperature strength. However, in order to use this stainless steel as a welding material, there are still points to be improved.

  The welding material proposed in Patent Document 2 is inferior in aging characteristics because the Ni content is as low as 10% or less with respect to the Cr content and the structure stability is not sufficient.

  Although the welding materials proposed in Patent Literature 3, Patent Literature 4 and Patent Literature 5 are excellent in high temperature strength, since the high temperature strength is increased by containing W, which is an expensive element, as an essential element, There is a problem in terms of economy. Furthermore, the examination regarding the anti-polythionic acid SCC property remains as a subject.

  Although the welding materials proposed in Patent Document 6 and Patent Document 7 are expected to have good grain boundary sensitization characteristics, the Ni content is low and the structure contains a specific amount of ferrite, so that the aging characteristics And creep strength is insufficient.

  The welding materials proposed in Patent Documents 8 to 11 do not contain an element for fixing C such as Nb, have insufficient polythionic acid SCC resistance, and further contain elements that cause precipitation strengthening. The strength at high temperatures is inferior. Furthermore, the welding materials proposed in Patent Document 10 and Patent Document 11 are inferior in aging characteristics because of the structure in which ferrite exists in the weld metal part.

  Although the welding material proposed in Patent Document 12 is expected to have excellent grain boundary sensitization properties, it has poor aging properties because of its high Cr content and insufficient structure stability. Furthermore, since no creep strengthening element is contained, the high temperature strength is insufficient.

  The welding material proposed in Patent Document 13 has a high Cr content. For this reason, since Ni content must be raised from a viewpoint of structure | tissue stability, it is inferior to economical efficiency.

  The present invention has been made in view of the above situation, and provides an austenitic stainless steel welding material having excellent polythionic acid SCC resistance and high-temperature strength, as well as excellent aging characteristics, weld crack resistance and economy. The purpose is to do.

  The present inventors have repeatedly studied to solve the above-described problems. As a result, the following findings (a) to (e) were obtained.

  (A) Polythionic acid SCC is stress corrosion cracking due to grain boundary sensitization. Therefore, in order to suppress the generation of polythionic acid SCC, it is necessary to suppress the precipitation of Cr-based carbides that causes grain boundary sensitization.

  (B) By containing a specific amount of Nb while reducing the C content as much as possible, if the activity is reduced by fixing C and Cr as carbides, Cr-based nitrides, etc., excellent polythione resistance Acid SCC property can be expressed.

  (C) The decrease in high-temperature strength due to low C can be compensated by an increase in the N content, which is excellent in economic efficiency. Further, if Cu, which is excellent in economic efficiency, is added, precipitation strengthening due to the Cu phase is achieved. Further, higher strength can be realized economically.

  (D) From the viewpoint of aging characteristics, it is best to have a complete austenite structure, and for this purpose, it is necessary to ensure a Ni content commensurate with the Cr content. However, if a complete austenite structure is formed, it is difficult to avoid solidification cracking due to a decrease in the amount of ferrite.

  (E) If the MX-based carbonitride and the Z phase are eutectic crystallized at the end of solidification to accelerate the progress of solidification, it is possible to reduce the solidification cracking susceptibility while ensuring the polythionic acid SCC resistance, For this purpose, an amount of Nb commensurate with the N content may be contained.

  Therefore, the present inventors further examined the effects of the contents of various alloy elements such as C, Nb, N, Cu, Cr on the polythionic acid SCC resistance, high-temperature strength, and aging characteristics, and as a result, the following (f ).

(F) The content of the alloy element is in a specific range, and the element symbol in the formula is the content in mass% of the element, and Fn1 represented by the following formula (1) is [40 ≧ Fn1 ≧ −2] and when Fn2 represented by the following formula (2) is [Fn2 ≧ 1.3], the austenitic stainless steel welding material has excellent aging characteristics and excellent In addition, it has SCC resistance and high temperature strength.
Fn1 = Ni + 0.31Mn + 22C + 14.2N + Cu-Cr-1.5Si-2Nb (1),
Fn2 = Cu-10C- (Nb / 10) (2).

  This invention is completed based on said knowledge, The summary exists in the austenitic stainless steel welding material shown below.

(1) By mass%, C: 0.02% or less, Si: 2% or less, Mn: 2.98 % or less, Ni: 10-40%, Cr: 15-25%, Cu: 1.5-5 %, Al: 0.05% or less, Nb: 0.2 to 1.0% and N: more than 0.1% and 0.45% or less,
The balance consists of Fe and impurities,
Fn1 and Fn2 represented by the following formulas (1) and (2) are 40 ≧ Fn1 ≧ −2 and Fn2 ≧ 1.3, respectively.
Austenitic stainless steel characterized in that P, S and O as impurities have chemical compositions of P: 0.03% or less, S: 0.02% or less, and O: 0.02% or less, respectively. Welding material.
Fn1 = Ni + 0.31Mn + 22C + 14.2N + Cu-Cr-1.5Si-2Nb (1)
Fn2 = Cu-10C- (Nb / 10) (2)
However, the element symbol in the formulas (1) and (2) represents the content in mass% of the element.

(2) Instead of a part of Fe of the austenitic welding material described in (1) above, it contains one or more elements selected from the following [1] to [4], Austenitic stainless steel welding material.
[1]: Mo: 5% or less, Co: 5% or less and W: 1.5% or less [2]: V: 0.3% or less, Ti: 0.3% or less and Ta: 1% or less [3] ]: B: 0.01% or less [4]: Ca: 0.01% or less, Mg: 0.01% or less, and REM: 0.1% or less

  According to the present invention, it is possible to obtain an austenitic stainless steel welding material having excellent polythionic acid SCC resistance and high-temperature strength, as well as excellent aging characteristics, weld crack resistance and economy.

  Hereinafter, each requirement of the present invention will be described in detail. In the following description, “%” of the content of each element means “mass%”.

C: 0.02% or less C has an effect of stabilizing the austenite structure. However, C increases the weld solidification cracking susceptibility, and in the weld metal, it combines with Cr and precipitates as Cr carbide mainly at the grain boundary due to the thermal cycle of subsequent welding, resulting in grain boundary sensitization. When the content of the polythionic acid SCC is more than 0.02%, the polythionic acid SCC resistance is significantly lowered. Therefore, the C content needs to be 0.02% or less. Further reduction of the C content is more preferable from the viewpoint of polythionic acid SCC resistance, but from the viewpoint of securing strength, the lower limit of the C content is preferably 0.005%.

Si: 2% or less Si is an element that has a deoxidizing action and is also effective in improving corrosion resistance. However, excessive Si content lowers the stability of the austenite structure, leading to a decrease in structure stability. In weld metal, segregation occurs at the columnar crystal boundary during solidification, lowering the melting point of the liquid phase, and susceptibility to solidification cracking. To increase. Therefore, the Si content is 2% or less. The Si content is desirably 1.5% or less, and more desirably 1.0% or less. In addition, it is not necessary to set a lower limit in particular for the content of Si, but an extreme decrease does not provide a sufficient deoxidation effect, and the cleanliness of the steel increases and the cleanliness deteriorates. Invite rise. Therefore, the lower limit of the Si content is desirably 0.01%, and more desirably 0.05%.

Mn: 10% or less Mn is an element that has a deoxidizing action and is also effective in stabilizing the austenite structure. Further, Mn contributes to an increase in high temperature strength by increasing the solubility of N in the molten metal. However, excessive inclusion of Mn causes a decrease in ductility. Therefore, the Mn content is 10% or less. The Mn content is desirably 8% or less, and more desirably 6% or less. In addition, although it is not necessary to provide a minimum in particular in content of Mn, the extreme reduction cannot fully obtain the effect of deoxidation effect and austenite structure stabilization. Therefore, the lower limit of the Mn content is desirably 0.1%, and more desirably 0.5%. The lower limit of the Mn content is extremely desirable if it is 0.8%.

Ni: 10-40%
Ni is an essential element for obtaining a stable austenite structure, and a content of 10% or more is necessary to sufficiently obtain the effect. However, since Ni is an expensive element, inclusion of a large amount causes an increase in cost and reduces the solubility of N in the molten metal. Therefore, the upper limit of Ni content is 40%. The lower limit of the Ni content is desirably 14%, and more desirably 15%. Further, the upper limit of the Ni content is desirably 30%, and more desirably 25%.

Cr: 15-25%
Cr is an essential element for ensuring the corrosion resistance under the use environment, and a content of 15% or more is necessary to sufficiently obtain the effect. However, if the Cr content is too high, the stability of the austenite structure decreases and the high temperature strength decreases. Therefore, the upper limit of Cr content is 25%. The lower limit of the Cr content is desirably 16%, and more desirably 17%. Further, the upper limit of the Cr content is desirably 22%, and more desirably 20%.

Cu: 1.5 to 5%
Cu is an element effective for obtaining a stable austenite structure, and at the same time has an effect of increasing the high temperature strength (creep strength) by being finely precipitated in the steel. In order to obtain these effects, it is necessary to contain 1.5% or more of Cu. However, since Cu is easily segregated at the grain boundary, excessive inclusion of Cu causes a decrease in hot workability and weldability. Therefore, the Cu content is 1.5 to 5%. The lower limit of the Cu content is desirably 2%, and more desirably 2.5%. Further, the upper limit of the Cu content is desirably 4.5%, and more desirably 4%.

Al: Less than 0.05% Al, like Si and Mn, has a deoxidizing action. However, when Al is contained excessively, a large amount of nitride is formed and ductility is reduced. Therefore, the Al content is less than 0.05%. The Al content is desirably 0.04% or less, and more desirably 0.03% or less. In addition, although it is not necessary to provide a lower limit in particular for the content of Al, an extreme decrease does not provide a sufficient deoxidation effect, and the cleanliness of the steel increases and the cleanliness deteriorates. Invite rise. Therefore, the lower limit of the Al content is desirably 0.001%, and more desirably 0.002%.

Nb: 0.2-1.0%
Nb is an important element in the present invention, and precipitates as MX-based carbonitride or Z phase, and has an effect of exhibiting excellent polythionic acid resistance SCC property. In addition, the above-described precipitate is also effective in improving high temperature strength (creep strength). Furthermore, if Nb is contained together with an appropriate amount of N, MX carbonitride and Z phase eutectic crystallize at the end of solidification and the progress of solidification is accelerated. Cracking susceptibility can also be reduced. In order to obtain these effects, an Nb content of 0.2% or more is necessary. However, if the Nb content is excessive, the weld metal segregates at the boundary between columnar crystals during solidification, lowering the melting point of the liquid phase, and increasing the susceptibility to solidification cracking. Therefore, the content of Nb is set to 0.2 to 1.0% in order to ensure SCC resistance and high-temperature strength and prevent solidification cracking. The lower limit of the Nb content is desirably 0.25%, and more desirably 0.3%. Moreover, the upper limit of Nb content is desirably 0.9%, and more desirably 0.8%.

N: more than 0.1% but not more than 0.45% N is essential for obtaining high strength by forming a solid solution in the substrate and forming a fine Cr-based nitride of Z phase with Nb Elements. Furthermore, if N is contained together with an appropriate amount of Nb, the MX-based carbonitride and the Z phase are eutectic crystallized at the end of solidification, and the progress of solidification is accelerated. Cracking susceptibility can also be reduced. In order to obtain such effects sufficiently, an N content exceeding 0.1% is necessary. However, when N is contained excessively, precipitation of Cr-based nitrides at the grain boundaries is induced, leading to a decrease in resistance to polythionic acid SCC, and coarse Z-phase eutectic nitrides are crystallized during solidification. This increases the susceptibility to solidification cracking, and also induces the formation of blowholes. Therefore, the upper limit of N content is 0.45%. The lower limit of the N content is desirably 0.18%, and more desirably 0.20%. Further, the upper limit of the N content is desirably 0.42%, and more desirably 0.40%.

  The austenitic stainless steel welding material according to the present invention is composed of the above-described elements, the balance being Fe and impurities, further satisfying the conditions for Fn1 and Fn2 described later, and the contents of P, S and O in the impurities The chemical composition is limited to the range described below.

  In addition, “impurities” in “Fe and impurities” as the balance refer to those mixed from ore, scrap, or production environment as raw materials when industrially producing steel materials.

Fn1: −2 or more and 40 or less Even when the content of each element from C to N is in the above-described range, when Fn1 represented by the above formula (1) is less than −2, Since the structure stability of the weld metal obtained using such austenitic stainless steel welding material is lowered, sufficient aging characteristics cannot be obtained. Moreover, since the increase of Fn1 exceeding 40 deteriorates economical efficiency, it was decided that 40 ≧ Fn1 ≧ −2 for Fn1. Fn1 is desirably −1 or more and 30 or less.

Fn2: 1.3 or more In addition, even if the content of each element from C to N is in the above-described range, if Fn2 represented by the above formula (2) is less than 1.3, The austenitic stainless steel welding material according to the invention cannot obtain excellent polythionic acid SCC resistance and high temperature strength at the same time. Therefore, for Fn2, Fn2 ≧ 1.3. Fn2 is desirably 1.5 or more and 4.0 or less.

  Furthermore, in the present invention, the contents of P, S, and O in impurities must be limited to P: 0.03% or less, S: 0.02% or less, and O: 0.02% or less, respectively. There is.

  This will be described below.

P: 0.03% or less P is contained as an impurity, impedes hot workability during production, and lowers the melting point of the liquid phase during solidification in a weld metal, thereby increasing the susceptibility to solidification cracking. Therefore, it is preferable to reduce the P content as much as possible. However, since extreme reduction leads to an increase in manufacturing cost, it is set to 0.03% or less. The upper limit of the P content is desirably 0.025%, and more desirably 0.02%.

S: 0.02% or less S, which is contained as an impurity like P, inhibits hot workability during production, and lowers the melting point of the liquid phase during solidification in weld metal, increasing the susceptibility to solidification cracking. Let Therefore, it is preferable to reduce the S content as much as possible as in the case of P. However, since the extreme reduction leads to an increase in manufacturing cost, the upper limit is made 0.02%. The upper limit of the S content is desirably 0.015%, more desirably 0.01%.

O: 0.02% or less O is present as an impurity, but when it is contained in a large amount, it causes a decrease in hot workability at the time of production, and further a deterioration in toughness and ductility. Therefore, the content of O needs to be 0.02% or less. The upper limit of the O content is desirably 0.015%, more desirably 0.01%.

  The austenitic stainless steel welding material according to the present invention may further contain the following elements as necessary.

Mo: 5% or less Mo has the effect of increasing the high-temperature strength. That is, Mo dissolves in the substrate to increase the high temperature strength, especially the creep strength. Mo also has the effect of suppressing the precipitation of Cr-based carbides at the grain boundaries and increasing the resistance to polythionic acid SCC. For this reason, in order to acquire these effects, you may contain Mo. However, the excessive content of Mo causes a decrease in the stability of the structure and is an expensive element, which also causes a decrease in economic efficiency. Therefore, the upper limit of the amount of Mo in the case of containing is 5%. The upper limit of the Mo content is desirably 4%, and more desirably 3%.

  On the other hand, in order to stably obtain the effect of Mo described above, the Mo content is desirably 0.05% or more, and more desirably 0.1% or more.

Co: 5% or less Co has the effect of increasing the high-temperature strength. That is, Co, like Ni, stabilizes the austenite structure and increases the high temperature strength of the steel. For this reason, in order to acquire this effect, you may contain Co. However, since Co is an expensive element, excessive inclusion causes a decrease in economic efficiency. Therefore, the upper limit of the amount of Co when contained is 5%. The upper limit of the Co content is desirably 3%, and more desirably 1%.

  On the other hand, in order to stably obtain the above Co effect, the Co content is preferably 0.03% or more, and more preferably 0.1% or more.

W: 1.5% or less W has the effect of increasing the high-temperature strength. That is, W, like Mo, dissolves in the substrate and increases the high temperature strength, especially the creep strength. For this reason, in order to acquire this effect, you may contain W. However, since W is an expensive element, excessive inclusion causes a decrease in economic efficiency, and unlike Mo, it causes a decrease in high-temperature strength, and further, the effect of improving the SCC resistance of polythionic acid is small. Therefore, the upper limit of the amount of W when contained is 1.5%. The upper limit of the W content is desirably 1.2%, and more desirably 1.0%.

  On the other hand, in order to stably obtain the effect of W described above, the W content is preferably 0.1% or more, and more preferably 0.2% or more.

V: 0.3% or less V has an effect of increasing high temperature strength. That is, V precipitates as a solid solution or Z phase in the substrate and improves the high temperature strength. For this reason, in order to acquire this effect, you may contain V. However, an excessive content of V causes a large amount of carbonitride to precipitate, resulting in a decrease in ductility. Therefore, the upper limit of the V amount in the case of inclusion is set to 0.3%. The upper limit of the V content is desirably 0.25%, and more desirably 0.2%.

  On the other hand, in order to stably obtain the effect of V described above, the V content is preferably 0.05% or more, and more preferably 0.1% or more.

Ti: 0.3% or less Ti has an effect of increasing the high-temperature strength. That is, Ti precipitates as a solid solution or MX carbonitride in the substrate and improves the high temperature strength. For this reason, in order to acquire this effect, you may contain Ti. However, an excessive content of Ti generates coarse nitrides and causes a reduction in ductility. Therefore, the upper limit of the Ti amount in the case of inclusion is set to 0.3%. The upper limit of the Ti content is desirably 0.25%, and more desirably 0.2%.

  On the other hand, in order to stably obtain the effect of Ti described above, the Ti content is desirably 0.05% or more, and more desirably 0.1% or more.

Ta: 1% or less Ta has an effect of increasing high-temperature strength. That is, Ta precipitates as a solid solution or Z phase in the substrate, and improves the high temperature strength. For this reason, in order to acquire this effect, you may contain Ta. However, excessive inclusion of Ta causes a decrease in economic efficiency. Therefore, the upper limit of the Ta amount in the case of inclusion is 1%. The upper limit of the Ta content is desirably 0.6%, and more desirably 0.5%.

  On the other hand, in order to stably obtain the effect of Ta described above, the Ta content is desirably 0.1% or more, and more desirably 0.2% or more.

B: 0.01% or less B has the effect of increasing the high temperature strength (creep strength) by segregating at the grain boundaries and finely dispersing the grain boundary carbides. For this reason, in order to acquire this effect, you may contain B. However, an increase in the B content promotes the precipitation of Cr-based carbides at the grain boundaries, leading to a reduction in various corrosion resistances. In a weld metal, segregation at the grain boundaries during solidification also increases the susceptibility to solidification cracking. Therefore, the upper limit of the amount of B in the case of containing is 0.01%. The upper limit of the B content is desirably 0.005%, and more desirably 0.003%.

  On the other hand, in order to stably obtain the above-described effect of B, the B content is preferably 0.001% or more, and more preferably 0.0015% or more.

  Since Ca, Mg, and REM all have high affinity with S, they have the effect of improving hot workability, and further have the effect of reducing the susceptibility to solidification cracking due to S grain boundary segregation. For this reason, in order to ensure better hot workability and prevent solidification cracking, it may be contained in the following range.

Ca: 0.01% or less Ca has an effect of improving hot workability and reducing susceptibility to solidification cracking in a weld metal. For this reason, in order to acquire these effects, you may contain Ca. However, when the content of Ca is excessive, it combines with O to remarkably reduce cleanliness, and on the contrary, deteriorate hot workability. Therefore, the upper limit of the Ca content in the case of containing is 0.01%. The upper limit of the Ca content is desirably 0.007%, and more desirably 0.005%.

  On the other hand, in order to stably obtain the above Ca effect, the Ca content is preferably 0.0001% or more, and more preferably 0.0003% or more.

Mg: 0.01% or less Mg, like Ca, has an effect of improving hot workability and reducing the susceptibility to solidification cracking in a weld metal. For this reason, in order to acquire these effects, you may contain Mg. However, when the Mg content is excessive, it combines with O to significantly reduce cleanliness, and on the contrary, deteriorate hot workability. Therefore, the upper limit of the amount of Mg when contained is 0.01%. The upper limit of the Mg content is desirably 0.007%, and more desirably 0.005%.

  On the other hand, in order to stably obtain the above-described effect of Mg, the content of Mg is preferably 0.0001% or more, and more preferably 0.0003% or more.

REM: 0.1% or less REM also has an effect of improving hot workability and reducing susceptibility to solidification cracking in weld metal. For this reason, in order to acquire these effects, you may contain REM. However, when the content of REM becomes excessive, it combines with O to remarkably reduce cleanliness, and on the contrary, deteriorate hot workability. Therefore, the upper limit of the amount of REM when contained is 0.1%. The upper limit of the REM content is desirably 0.06%, and more desirably 0.05%.

  On the other hand, in order to stably obtain the above-described effect of REM, the REM content is desirably 0.001% or more, and more desirably 0.005% 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, it may be added in the form of misch metal and contained so that the amount of REM falls within the above range.

  Said Ca, Mg, and REM can be contained only in one of them, or 2 or more types of composites.

  EXAMPLES Hereinafter, although an Example demonstrates this invention more concretely, this invention is not limited to these Examples.

  Austenitic stainless steels A to N having the chemical compositions shown in Table 1 were melted in a 30 kg high-frequency vacuum melting furnace and then ingoted to produce ingots.

  Steels A to I in Table 1 are steels whose chemical compositions are within the range defined by the present invention. On the other hand, steels J to N are steels of comparative examples whose chemical compositions deviate from the conditions specified in the present invention.

  A part of each ingot was subjected to hot forging and hot rolling, and then subjected to a solution heat treatment at 1100 ° C. for 30 minutes, followed by water cooling. Thereafter, a test piece for a transbalance test having a thickness of 4 mm, a width of 100 mm, and a length of 100 mm was collected.

  Moreover, after the hot forging was carried out by the usual method using the remainder of each ingot, it was cold worked while repeating the solution heat treatment in the middle to produce a welding material made of a solid wire having a diameter of 2.0 mm.

  First, in order to evaluate the solidification cracking susceptibility of each welding material in more detail, welding current: 100 A, welding voltage: 15 V, welding speed: 15 cm / min, additional strain: 2 using the above-described transbalance test specimen. % Was conducted under the condition of% and the maximum crack length was measured.

  In addition, it is known that the maximum crack length evaluated by the transbarrain train test of the weld metal of SUS310S which is an austenitic stainless steel which solidifies completely austenite is 1 mm or less. Therefore, the austenitic welding material having a maximum crack length of 1 mm or less evaluated by the transbalance train test is considered to have excellent solidification cracking resistance, and this was the target.

  Next, in order to evaluate the aging characteristics, high-temperature strength (creep strength), and polythionic acid SCC resistance of each welding material, a fully welded metal specimen was produced using the welding material produced as described above.

And the raw material which can extract | collect the subsize Charpy impact test piece with a V notch of 5 mm of width described in JISZ2242 (2005) from each weld metal test body was extract | collected. The specimen was cut out after aging at 700 ° C. for 300 hours, and a Charpy impact test was performed at 0 ° C. to evaluate the aging characteristics. In addition, as an evaluation standard of aging characteristics, the target was a Charpy impact value of 50 J / cm ² or more.

  Further, a creep rupture test piece having a parallel part diameter of 6 mm was taken from each weld metal specimen, and the creep rupture life was evaluated under the condition of 700 ° C. × 150 MPa. In addition, as an evaluation standard of creep rupture strength, the target was that the creep rupture life was 300 hours or longer.

  Furthermore, in order to quantitatively evaluate the SCC resistance of polythionic acid, after sensitizing the remainder of each weld metal specimen by aging at 650 ° C. × 1000 h, a test in which the test area size becomes 10 mm × 10 mm from this aging material Pieces were collected and subjected to an electrochemical reactivation rate test (hereinafter referred to as “EPR test”) by the method defined in JIS G 0580 (2003). In addition, it is known that the criterion for determining good SCC resistance of polythionic acid in the EPR test is “reactivation rate ≦ 5%”. For this reason, it aimed at the reactivation rate being 5% or less.

Table 2 summarizes the results of each of the above investigations. In the “aging characteristics” column, “◯” and “×” indicate that the target was reached when the Charpy impact value at 0 ° C. was 50 J / cm ² or more, and the Charpy impact value at 0 ° C. was 50 J / cm ^{2, respectively.} It means that the target was not achieved at less than. Further, “−” in the “creep rupture life” orchid of test number 10 and test number 13 indicates that the creep rupture test was not performed. Furthermore, “−” in the reactivation rate column of test number 13 indicates that the EPR test was not performed.

  From Table 2, in the case of test numbers 1 to 9 using welding materials made of steels A to I that satisfy the conditions specified by the present invention in the chemical composition, all have low solidification cracking susceptibility and excellent aging characteristics. The creep rupture life under the condition of 700 ° C. × 150 MPa achieves the target 300 h or more and is excellent in high-temperature strength, and the reactivation rate evaluated by the EPR test is significantly less than 5% and is polythione resistant. It is clear that the acid SCC property is also excellent.

  On the other hand, in the case of test numbers 10 to 14 using a welding material composed of steels J to N whose chemical composition deviates from the conditions specified in the present invention, at least solidification cracking resistance, aging characteristics, high temperature strength, Any of the polythionic acid SCC resistance has not reached its target.

  In the case of test No. 10, the C content of steel J is 0.040%, and exceeds the range specified in the present invention. Therefore, the maximum crack length in the transbalance train test is 1.44 mm and the solidification cracking sensitivity. In addition, the reactivation rate in the EPR test is very high at 10.4%, and the resistance to polythionic acid SCC is extremely inferior.

  In the case of test number 11, since the N content of steel K is 0.095%, which is lower than the range specified in the present invention and is smaller than the Nb content, solidification due to crystallization of MX-based carbonitride and Z phase The effect of reducing crack sensitivity is insufficient and the weld crack resistance is poor.

  In the case of test number 12, the W content of steel L is 1.73%, which exceeds the range specified in the present invention. Therefore, the creep rupture life under conditions of 700 ° C. × 150 MPa is as short as 282 h, which is the target. Not reached. Thus, it is clear that excessive inclusion of the expensive element W should be avoided.

  In the case of test number 13, the Ni content of steel M is 8.73%, and Fn1 is −7.26, both of which are below the range defined in the present invention, so that the structural stability is lowered and targeted. Aging characteristics are not obtained.

  In the case of test number 14, since the Fn2 of steel N is 1.29, which is below the range specified in the present invention, the creep rupture life under the condition of 700 ° C. × 150 MPa has not reached the target, and It is clear that the activation rate has not reached the target.

  According to the present invention, it is possible to obtain an austenitic stainless steel welding material having excellent polythionic acid SCC resistance and high-temperature strength, as well as excellent aging characteristics, weld crack resistance and economy.

Claims (2)

In mass%, C: 0.02% or less, Si: 2% or less, Mn: 2.98 % or less, Ni: 10-40%, Cr: 15-25%, Cu: 1.5-5%, Al : 0.05% or less, Nb: 0.2 to 1.0% and N: more than 0.1% and 0.45% or less,
The balance consists of Fe and impurities,
Fn1 and Fn2 represented by the following formulas (1) and (2) are 40 ≧ Fn1 ≧ −2 and Fn2 ≧ 1.3, respectively.
Austenitic stainless steel characterized in that P, S and O as impurities have chemical compositions of P: 0.03% or less, S: 0.02% or less, and O: 0.02% or less, respectively. Welding material.
Fn1 = Ni + 0.31Mn + 22C + 14.2N + Cu-Cr-1.5Si-2Nb (1 )
Fn2 = Cu-10C- (Nb / 10) (2 )
However, the element symbol in the formulas (1) and (2) represents the content in mass% of the element. An austenitic stainless steel comprising one or more elements selected from the following [1] to [4] instead of a part of Fe of the austenitic welding material according to claim 1. Welding material.
[1]: Mo: 5% or less, Co: 5% or less and W: 1.5% or less [2]: V: 0.3% or less, Ti: 0.3% or less and Ta: 1% or less [3] ]: B: 0.01% or less [4]: Ca: 0.01% or less, Mg: 0.01% or less, and REM: 0.1% or less