JP2014148702A - Ni-GROUP HEAT-RESISTANT ALLOY MEMBER - Google Patents

Abstract

The present invention provides a Ni-base heat-resistant alloy member having a thickness exceeding 30 mm, which is excellent in creep strength and crack resistance at the time of multi-layer welding.
The surface of a member subjected to solution heat treatment is subjected to strong processing such as cutting, polishing or roll, and then recrystallized by heat treatment [from the member surface to a depth of 0.2 mm. Mean grain size (μm) ≦ −5 × {2 × Al + Ti + (Mo / 10) −5 × B} +60] and [−4 × {10 × B + 2 × Al + Ti + (Mo / 10)} + 85 ≦ member Ni-base heat-resistant alloy member satisfying the average crystal grain size (μm) ≤ -30 x {10 x B + 2 x Al + Ti + (Mo / 10)} + 650] in the region excluding each surface side that is 25% of the thickness .
[Selection figure] None

Description

  The present invention relates to a Ni-base heat-resistant alloy member. Specifically, the present invention relates to a Ni-based heat-resistant alloy member that is excellent in creep strength and crack resistance during welding and can be suitably used as a thick high-temperature member such as a main steam pipe of a power generation boiler.

  In recent years, high-temperature and high-pressure operating conditions have been promoted on a global scale in power generation boilers and the like from the viewpoint of reducing environmental impact, and austenitic heat-resistant alloys used as materials for superheater tubes and reheater tubes Therefore, it is required to have superior high-temperature strength and corrosion resistance.

  Furthermore, the application of austenitic heat-resistant alloys is also being studied in various members such as thick-walled members such as main steam pipes and high-temperature reheat steam pipes that conventionally used ferritic heat-resistant steel.

Under such technical background, for example, Patent Documents 1 to 4 contain Mo and / or W to enhance solid solution strengthening, and Al and Ti are contained to form an γ ′ phase that is an intermetallic compound. Specifically, a Ni-based alloy that utilizes precipitation strengthening of Ni ₃ (Al, Ti) is disclosed.

  On the other hand, Patent Document 5 proposes a high Ni austenitic heat-resistant alloy in which the creep strength is improved by adjusting the addition range of Al and Ti and precipitating the γ 'phase.

  Patent Documents 6 and 7 also disclose Ni-based alloys containing Co in addition to Cr and Mo for the purpose of further strengthening.

  Further, in Patent Document 8, the contents of Cr, B and P and the contents of Al, Ti and Nb are controlled within predetermined ranges, respectively, and Nd is contained so that creep strength and liquefaction cracking resistance at the time of welding are included. An austenitic heat-resistant alloy with improved properties has been proposed.

JP-A-51-84726 Japanese Patent Laid-Open No. 51-84727 Japanese Unexamined Patent Publication No. 7-150277 JP 2002-518599 A JP-A-9-157779 JP 60-110856 A Japanese Patent Application Laid-Open No. 2-1077736 International Publication No. 2011/071054 JP 2000-265249 A

  When the above-mentioned austenitic heat-resistant alloy or Ni-based alloy is used as a structure, it is generally assembled by welding. At this time, it is known that various cracks are likely to occur in the welded portion mainly due to metallurgical factors.

  However, in the case of the various alloys disclosed in the above-mentioned Patent Documents 1 to 7, sufficient consideration is not made from the viewpoint of “weldability” when assembling as a structural member.

  On the other hand, the present inventors changed the austenitic heat-resistant alloy (Ni-based heat-resistant alloy) disclosed in Patent Document 8 to a thick member such as a main steam pipe, particularly a member having a thickness exceeding 30 mm. As a result of assembling and investigating for application, it was confirmed that liquefaction cracking during welding could be stably prevented.

  However, from a further detailed investigation conducted by the present inventors after that, simply managing the chemical composition of the austenitic heat-resistant alloy, a site different from the crack generation site targeted by Patent Document 8, specifically, It has been clarified that fine cracks may occur in the HAZ in the vicinity of the welds on the surface of the first layer and the last layer during multilayer welding.

  In addition, the austenitic heat-resistant alloy (Ni-based heat-resistant alloy) disclosed in Patent Document 8 can surely obtain sufficient creep strength in a member having a thickness of less than 30 mm, but when the thickness exceeds 30 mm, It has also been found that it may be difficult to stably obtain sufficient creep strength.

  The present invention has been made in view of the above-described situation. Creep strength and crack resistance during welding, in particular, weld crack resistance in the vicinity of the surface, specifically, a multilayer member as a thick member having a thickness exceeding 30 mm. Ni is excellent in crack resistance in the HAZ near the welded portion on the surface of the first layer and the last layer when welded, and can be suitably used as a thick high-temperature member such as a main steam pipe of a power generation boiler. An object is to provide a base heat-resistant alloy member.

  In order to solve the above-described problems, the present inventors first conducted a detailed investigation on minute cracks generated in the HAZ near the welded portion on the surface of the multi-layer welded member. As a result, the following items [i] to [iii] were clarified.

  [i] Cracks occur at the HAZ crystal grain boundary several hundred μm away from the weld on the first layer side member surface, and at the HAZ crystal grain boundary about 500 to 1000 μm away from the weld part on the final layer side member surface. did.

  [ii] No cracks were observed on the fracture surface, and the fracture surface was poor in ductility.

  [iii] The portion where cracks occurred was a portion where strain was introduced during surface processing of the member, and a clear increase in hardness was observed. Further, cracks may occur on the final layer side even when the degree of increase in hardness is small compared to the initial layer side.

  From the above [i] to [iii], the present inventors indicate that the fine cracks generated in the HAZ near the welded portion on the surface of the multi-layer welded member are generated by the following mechanisms (a) to (c). Estimated.

  (A) Thermal stress due to welding occurs in the HAZ near the weld. In particular, when thick-walled members are welded in multiple layers, the surrounding members of the first layer are thicker than the thickness of the weld metal part, so thermal deformation during welding is caused by the HAZ near the weld part including the weld metal and the surface part. The tensile thermal stress acts. On the other hand, since the weld metal portion is almost equal to the base metal thickness on the surface on the final layer side, the restraint becomes large, and a large tensile thermal stress acts on the HAZ away from the weld portion. And any thermal stress becomes especially large in the surface part to about 0.2 mm depth.

  (B) Since the surface of the member is strained and increased in hardness by processing, the deformation resistance in the grains increases, and the thermal stress concentrates on the crystal grain boundaries. As a result, when the thermal stress exceeds the strength of the grain boundary, the grain boundary opens and becomes a fine crack.

  (C) The reason for the difference in crack occurrence position between the first layer side and the last layer side is that the site where the tensile stress acts differs depending on the above (a) and (b). In addition, even when the degree of increase in hardness is small compared to the first layer side, cracks occur on the final layer side because cracks occur at sites exposed to lower temperatures. This is because it is more affected.

  In addition, the fine crack generated in the HAZ near the welded portion on the surface of the multilayer welded member is a crack that has been confirmed so far, for example, the crack described in Patent Document 8 described above is also the position of occurrence. Needless to say, the mechanism is completely different.

  Based on the above estimation, the present inventors repeated intensive studies in order to prevent the fine cracks described above. As a result, the following item (d) became clear.

(D) In order to prevent the fine cracks described above, the average crystal grain size (μm) of the region from the member surface to a depth of 0.2 mm (hereinafter, the region is defined as “surface layer”, and the average of the surface layer) The crystal grain size is sometimes referred to as “G _S ”.) Must be controlled to a specific value or less depending on the contents of Al, Ti, Mo, and B, more specifically, the initial value at the time of welding. The average crystal grain size of the region from the member surface that becomes the layer side to a depth of 0.2 mm and the region from the member surface that becomes the final layer side to the depth of 0.2 mm at the time of welding satisfy the following equation [1]. Need to be managed.
G _S ≦ −5 × P1 + 60 [1]
However,
P1 = 2 × Al + Ti + (Mo / 10) −5 × B
Thus, Al, Ti, Mo and B in the parameter P1 mean the content (mass%) of the element.

  The above formula [1] has a large content of Al, Ti, and Mo that strengthens the inside of the grains and promotes stress concentration at the grain boundaries, and conversely, when the content of B that strengthens the grain boundaries is small, that is, When P1 is large, in order to suppress stress concentration at a specific grain boundary, the upper limit of the average crystal grain size of the surface layer needs to be reduced, whereas the contents of Al, Ti and Mo are small, When the content of B is large, it means that the upper limit of the average crystal grain size of the surface layer may be increased.

  The above formula [1] is achieved by subjecting the surface of the member subjected to solution heat treatment to, for example, strong processing such as cutting, polishing, or roll, followed by heat treatment and recrystallization. Can do.

  Further, it has also been clarified that the following layer (e) is preferably satisfied on the final layer side during welding that is easily affected by the hardness of the surface portion of the member in order to prevent fine cracks. .

(E) Maximum hardness HV0.01 (hereinafter sometimes referred to as HV0.01 (max) for simplicity) in the region from the member surface on the final layer side during welding to a depth of 0.2 mm is 380. The following.
In addition, said "HV0.01" means the "hardness symbol" at the time of implementing a micro Vickers hardness test by making test force 0.098N (10gf) (refer JISZ2244 (2009)).

  Next, the inventors investigated the reason why it may be difficult to stably obtain a sufficient creep strength when the thickness exceeds 30 mm in the Ni-base heat-resistant alloy. As a result, the following items [iv] and [v] were confirmed.

  [iv] When the member thickness exceeds 30 mm, the average crystal grain size tends to vary, and the creep strength becomes unstable.

  [v] When the member thickness exceeds 30 mm, the content of B, Al, Ti, and Mo, particularly the content of B, Al, and Ti, which has an effect on creep strength but also affects weld cracking, is small. In some cases, instability of the creep strength tends to be manifested.

  From the above [iv] and [v], the present inventors estimated that the instability of creep strength occurs due to the following reasons (f) and (g).

  (F) When the thickness of the member exceeds 30 mm, the average crystal grain size tends to vary due to the temperature distribution during the production of the Ni-base heat-resistant alloy material, particularly the temperature non-uniformity during the solution heat treatment.

  (G) When the Ni-base heat-resistant alloy sufficiently contains B, Al, Ti and Mo, stable creep strength can be obtained. However, when the above-described element content is controlled to be low in order to prevent weld cracking, the influence of the average crystal grain size becomes obvious, and the creep strength becomes unstable.

  Under the above estimation, the present inventors have repeated intensive studies in order to stably obtain a sufficient creep strength in a member having a thickness exceeding 30 mm. As a result, the following items (h) and (i) were clarified.

(H) Average crystal grain size (μm) of the region excluding each surface side that is 25% of the member thickness (hereinafter, the region is defined as “member thickness central part”, and the member thickness center part The average crystal grain size of “G _B ” is sometimes referred to as “GB”.) More specifically, the depth from the surface on the first layer side during welding exceeds 25% of the member thickness, and the final By controlling the average crystal grain size in a region where the depth from the surface on the layer side exceeds 25% of the member thickness to a specific value or more according to the contents of B, Al, Ti and Mo, the creep strength Is stabilized.

  (I) However, if the average crystal grain size in the central portion of the member is too large, the susceptibility to liquefaction cracking during welding increases. For this reason, it is necessary to manage the upper limit of the average crystal grain size at the central portion of the member according to the contents of B, Al, Ti and Mo.

  Therefore, as a result of further detailed studies by the present inventors, the following important item (j) has been clarified.

The average crystal grain size of the thickness of the central portion of the (j) member (μm) G _B is satisfies the following formula [2], sufficient creep strength even member of more than 30mm thick can be stably obtained.
−4 × P2 + 85 ≦ G _B ≦ −30 × P2 + 650 (2)
However,
P2 = 10 × B + 2 × Al + Ti + (Mo / 10)
In the parameter P2, B, Al, Ti and W mean the content (% by mass) of the element.

  In the above formula [2], since B, Al, Ti and Mo are effective elements for improving the creep strength, when the content is large, the thickness center of the member necessary for stabilizing the creep strength is obtained. The lower limit of the average crystal grain size of the part may be small, but as the content decreases, it is necessary to increase the lower limit of the average crystal grain size of the central part of the thickness of the member, and these elements Since it also has the effect of increasing the susceptibility to liquefaction cracking during welding, it means that it is necessary to reduce the upper limit of the average crystal grain size at the center of the thickness of the member as the content of these elements increases.

  Note that the average crystal grain size in the central portion of the thickness of the member can be managed by selecting the temperature and time during the solution heat treatment of the member. For this reason, the formula [2] can be satisfied by performing an appropriate solution heat treatment.

  The present invention has been completed based on the above findings, and the gist of the present invention is the Ni-base heat-resistant alloy member shown below.

(1) Ni-base heat-resistant alloy member having a thickness exceeding 30 mm, and in mass%, C: 0.03 to 0.15%, Si: 1% or less, Mn: 2% or less, P: 0.03% Hereinafter, S: 0.01% or less, Ni: 48-58%, Co: 8-16%, Cr: 18-25%, Mo: 6-12%, Ti: 0.05-0.8%, Al : 0.05-1.6%, B: 0.0001-0.01%, REM: 0.001-0.1%, N: 0.02% or less and O: 0.01% or less, A Ni-base heat-resistant alloy member characterized in that the balance has a chemical composition composed of Fe and impurities and satisfies the following formulas [1] and [2].
G _S ≦ −5 × P1 + 60 [1]
−4 × P2 + 85 ≦ G _B ≦ −30 × P2 + 650 (2)
However,
P1 = 2 × Al + Ti + (Mo / 10) −5 × B
P2 = 10 × B + 2 × Al + Ti + (Mo / 10)
In the parameters P1 and P2, Al, Ti, Mo, and B mean the content (mass%) of the element.
Each G _S and G _B, refers to a surface layer having an average grain size ([mu] m) and the average crystal grain size of the thickness of the central portion of the member ([mu] m).

(2) The Ni-base heat-resistant alloy member as described in (1) above, which contains at least one element selected from the group shown below in mass% instead of part of Fe.
First group: Ca: 0.05% or less and Mg: 0.05% or less,
Second group: Cu: 1% or less, W: 1% or less, V: 0.5% or less, and Nb: 2% or less.

  (3) The maximum hardness HV0.01 in the region from the member surface on the final layer side during welding to a depth of 0.2 mm is 380 or less, as described in (1) or (2) above Ni-base heat-resistant alloy member.

  Although the Ni-based heat-resistant alloy member of the present invention is a thick member having a thickness of more than 30 mm, the crack resistance during welding, in particular, the weld crack resistance near the surface during multilayer welding, more specifically, In addition to being excellent in crack resistance in the HAZ near the welded portion on the surface of the first and last layer members, it has a stable and excellent creep strength. For this reason, the Ni-base heat-resistant alloy member of the present invention can be suitably used as a thick high-temperature member such as a main steam pipe of a power generation boiler.

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

(A) Chemical composition:
C: 0.03-0.15%
C stabilizes austenite, forms fine carbides at grain boundaries, and improves creep strength at high temperatures. In order to sufficiently obtain this effect, a C content of 0.03% or more is necessary. However, when C is contained excessively, the carbide becomes coarse and precipitates in a large amount, so that the ductility of the grain boundary is lowered, and further, the toughness and the creep strength are also lowered. Therefore, an upper limit is provided and the C content is 0.03 to 0.15%. A desirable lower limit of the C content is 0.04%, and a more desirable lower limit is 0.05%. The desirable upper limit of the C content is 0.12%, and the more desirable upper limit is 0.10%.

Si: 1% or less Si is an element that has a deoxidizing action and is effective for improving corrosion resistance and oxidation resistance at high temperatures. However, when Si is excessively contained, toughness is reduced. Therefore, an upper limit is set for the Si content to 1% or less. The Si content is desirably 0.8% or less, and more desirably 0.6% or less.

  Although there is no need to set a lower limit in particular for the Si content, an extreme reduction does not provide a sufficient deoxidation effect, which increases the cleanliness of the alloy and deteriorates the cleanliness, as well as corrosion resistance and acid resistance at high temperatures. It is difficult to obtain the effect of improving the chemical property, and the manufacturing cost increases greatly. Therefore, the desirable lower limit of the Si content is 0.01%, and the more desirable lower limit is 0.03%.

Mn: 2% or less Mn, like Si, has a deoxidizing action. Mn also contributes to stabilization of austenite. However, when the Mn content is excessive, embrittlement is caused, and the toughness and creep ductility are also reduced. Therefore, an upper limit is set for the Mn content to 2% or less. The Mn content is desirably 1.8% or less, and more desirably 1.5% or less.

  In addition, it is not necessary to provide a lower limit for the Mn content. However, an extreme reduction does not provide a sufficient deoxidation effect, deteriorates the cleanliness of the alloy, and makes it difficult to obtain an austenite stabilizing effect. Costs also rise significantly. Therefore, the desirable lower limit of the Mn content is 0.01%, and the more desirable lower limit is 0.03%.

P: 0.03% or less P is an element which is contained in the alloy as an impurity and segregates at the grain boundary of HAZ during welding to increase the liquefaction cracking sensitivity. Furthermore, P also reduces creep ductility after long-term use. Therefore, an upper limit is set for the P content to 0.03% or less. The P content is desirably 0.025% or less, and more desirably 0.02% or less.

  Although the P content is preferably reduced as much as possible, the extreme reduction leads to an increase in manufacturing cost. Therefore, the desirable lower limit of the P content is 0.0005%, and the more desirable lower limit is 0.0008%.

S: 0.01% 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 boundaries of HAZ during welding to increase the liquefaction cracking sensitivity. Furthermore, S is an element that adversely affects creep ductility and toughness after long-term use. Therefore, an upper limit is set for the S content to 0.01% or less. The S content is desirably 0.008% or less, and more desirably 0.005% or less.

  Although the S content is preferably reduced as much as possible, the extreme reduction leads to an increase in manufacturing cost. Therefore, the desirable lower limit of the S content is 0.0001%, and the more desirable lower limit is 0.0002%.

Ni: 48-58%
Ni is an effective element for obtaining austenite, and is an essential element for ensuring the structural stability when used for a long time. Furthermore, Ni combines with Al and Ti to form a fine intermetallic compound phase and has an effect of increasing the creep strength. In order to obtain the above-described effects of Ni sufficiently within the range of the Cr content of the present invention of 18 to 25% described later, a Ni content of 48% or more is necessary. However, Ni is an expensive element, and a large amount causes an increase in cost. Therefore, an upper limit is provided so that the Ni content is 48 to 58%. A desirable lower limit of the Ni content is 49%, and a more desirable lower limit is 50%. The desirable upper limit of the Ni content is 56%, and the more desirable upper limit is 55%.

Co: 8-16%
Co, like Ni, is an austenite-forming element and contributes to the improvement of creep strength by increasing the stability of austenite. In order to sufficiently obtain this effect, a Co content of 8% or more is necessary. However, since Co is an extremely expensive element, a large amount causes an increase in cost. Therefore, an upper limit is set so that the Co content is 8 to 16%. A desirable lower limit of the Co content is 8.5%, and a more desirable lower limit is 9%. The desirable upper limit of the Co content is 15.5%, and the more desirable upper limit is 15%.

Cr: 18-25%
Cr is an essential element for securing oxidation resistance and corrosion resistance at high temperatures. In order to obtain the effect of Cr described above within the range of the Ni content of the present invention of 48 to 58%, a Cr content of 18% or more is necessary. However, if the Cr content exceeds 25%, the stability of austenite at high temperatures deteriorates and the creep strength decreases. Therefore, the Cr content is 18 to 25%. A desirable lower limit of the Cr content is 18.5%, and a more desirable lower limit is 19%. The desirable upper limit of the Cr content is 24.5%, and the more desirable upper limit is 24%.

Mo: 6-12%
Mo is an element that makes a solid solution in the matrix and greatly contributes to the improvement of the creep strength and tensile strength at high temperatures. In order to fully exhibit the effect, a Mo content of 6% or more is necessary. However, even if Mo is excessively contained, the above effect is saturated, and on the contrary, a coarse precipitate phase is generated, and the creep strength may be lowered. Furthermore, since Mo is an expensive element, excessive inclusion causes an increase in cost. Therefore, an upper limit is provided so that the Mo content is 6 to 12%.

  Mo is an element that has the effect of increasing the strength, while increasing the deformation resistance in the grains, and increasing the cracking susceptibility during multilayer welding of thick members. Therefore, in particular, in order to prevent fine cracks generated in the HAZ in the vicinity of the welded portion on the surface of the member and to ensure stable creep strength, the Mo content needs to satisfy the above formulas [1] and [2]. is there.

  A desirable lower limit of the Mo content is 6.5%, and a more desirable lower limit is 7%. The desirable upper limit of the Mo content is 11.5%, and the more desirable upper limit is 11%.

Ti: 0.05 to 0.8%
Ti combines with Ni and precipitates in the grains as a fine intermetallic compound, which contributes to the improvement of creep strength and tensile strength at high temperatures. In order to obtain the effect, a Ti content of 0.05% or more is necessary. However, when the Ti content is excessive, a large amount of intermetallic compounds are precipitated, resulting in a decrease in creep ductility and toughness. For this reason, an upper limit is set so that the Ti content is 0.05 to 0.8%.

  Ti is an element that has the effect of increasing the strength, while increasing the deformation resistance within the grains, and increasing the cracking susceptibility during multilayer welding of thick members. Therefore, in particular, in order to prevent fine cracks generated in the HAZ near the welded portion on the surface of the member and to ensure stable creep strength, the Ti content needs to satisfy the above formulas [1] and [2]. is there.

  A desirable lower limit of the Ti content is 0.07%, and a more desirable lower limit is 0.1%. The desirable upper limit of the Ti content is 0.7%, and the more desirable upper limit is 0.6%.

Al: 0.05 to 1.6%
Al, like Ti, binds to Ni and precipitates in the grains as a fine intermetallic compound, contributing to the improvement of creep strength and tensile strength at high temperatures. Furthermore, Al is an element having a deoxidizing action. However, when the Al content is excessive, a large amount of intermetallic compounds are precipitated, leading to a decrease in creep ductility and toughness, and the cleanliness of the alloy is significantly deteriorated, resulting in a decrease in hot workability and ductility. Therefore, an upper limit is set for the Al content to be 0.05 to 1.6%.

  Al is an element that has the effect of increasing the strength, while increasing the deformation resistance within the grains, and increasing the cracking susceptibility during multilayer welding of thick members. Therefore, in particular, in order to prevent fine cracks generated in the HAZ near the welded portion on the surface of the member and to ensure stable creep strength, the Al content needs to satisfy the above formulas [1] and [2]. is there.

  A desirable lower limit of the Al content is 0.1%, and a more desirable lower limit is 0.3%. A desirable upper limit of the Al content is 1.5%, and a more desirable upper limit is 1.4%.

B: 0.0001 to 0.01%
B is an element effective for improving the creep strength by finely dispersing grain boundary carbides and segregating at the grain boundaries to strengthen the grain boundaries. In order to obtain this effect, a B content of 0.0001% or more is necessary. However, if the B content is excessive, B is segregated in a large amount in the high-temperature HAZ in the vicinity of the weld due to the welding thermal cycle during welding, lowering the melting point of the grain boundary, and increasing the HAZ liquefaction cracking susceptibility. Therefore, an upper limit is provided so that the B content is 0.0001 to 0.01%.

  In addition, since B has the above-mentioned action, in order to prevent fine cracks generated in the HAZ near the welded portion on the surface of the member and to secure stable creep strength, the content of B is the above-mentioned formula [1]. And [2] must also be satisfied.

  A desirable lower limit of the B content is 0.0005%, and a more desirable lower limit is 0.001%. The desirable upper limit of the B content is 0.008%, and the more desirable upper limit is 0.006%.

REM: 0.001 to 0.1%
REM is an element that has a strong affinity for S, has an effect of improving hot workability, and is effective in improving creep ductility during use at high temperatures. In order to obtain this effect, it is necessary to contain 0.001% or more of REM. However, when the content of REM becomes excessive, it combines with O to remarkably reduce cleanliness, and on the contrary, deteriorate hot workability and also reduce ductility. For this reason, an upper limit is set so that the content of REM is 0.001 to 0.1%. A desirable lower limit of the REM content is 0.005%, and a more desirable lower limit is 0.008%. The desirable upper limit of the REM content is 0.09%, and the more desirable upper limit is 0.08%.

  “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.

N: 0.02% or less N is an element effective for stabilizing austenite. However, if it is excessively contained, a large amount of fine nitride precipitates in the grains during use at high temperatures and creeps. It causes a reduction in ductility and toughness. Therefore, an upper limit is set for the N content to 0.02% or less. The N content is desirably 0.018% or less, and more desirably 0.015% or less.

  In addition, although there is no particular need to set a lower limit for the N content, an extreme reduction makes it difficult to obtain the effect of stabilizing austenite, and the manufacturing cost also increases greatly. Therefore, the desirable lower limit of the N content is 0.0005%, and the more desirable lower limit is 0.0008%.

O: 0.01% or less O (oxygen) is contained as an impurity in the alloy, and when its content is excessive, hot workability is lowered, and further, toughness and ductility are deteriorated. For this reason, an upper limit is set for the O content to 0.01% or less. The O content is desirably 0.008% or less, and more desirably 0.005% or less.

  Although there is no particular need to set a lower limit for the O content, an extreme reduction leads to an increase in manufacturing cost. Therefore, the desirable lower limit of the O content is 0.0005%, and the more desirable lower limit is 0.0008%.

  One of the Ni-base heat-resistant alloy members of the present invention has a chemical composition containing each of the elements described above, with the balance being Fe and impurities.

  The “impurity” refers to a material mixed from ore, scrap, or a manufacturing environment as a raw material when an Ni-base heat-resistant alloy member is manufactured industrially.

The Ni-base heat-resistant alloy member of the present invention may contain one or more elements selected from Ca, Mg, Cu, W, V, and Nb instead of a part of the above-described Fe.
First group: Ca: 0.05% or less and Mg: 0.05% or less,
Second group: Cu: 1% or less, W: 1% or less, V: 0.5% or less, and Nb: 2% or less.

  Hereinafter, the effect of these arbitrary elements and the reason for limiting the content will be described.

  Ca and Mg, which are elements of the first group, all have an effect of improving hot workability. For this reason, you may contain these elements.

Ca: 0.05% or less Ca has an effect of improving hot workability. For this reason, Ca may be contained. However, when the content of Ca is excessive, it combines with O to remarkably reduce cleanliness, and on the contrary, deteriorate hot workability. For this reason, when making it contain, the upper limit is provided in the quantity of Ca, and it is 0.05% or less. The upper limit of the Ca content is desirably 0.03%.

  On the other hand, in order to stably obtain the effect of Ca, the Ca content is preferably 0.0001% or more. A more desirable lower limit of the Ca content is 0.0005%.

Mg: 0.05% or less Mg, like Ca, has an effect of improving hot workability. For this reason, 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. For this reason, the upper limit is set to the amount of Mg in the case of containing 0.05% or less. The upper limit of the Mg content is desirably 0.03%.

  On the other hand, in order to stably obtain the effect of Mg, the Mg content is preferably 0.0001% or more. A more desirable lower limit of the Mg content is 0.0005%.

  Said Ca and Mg can be contained only in any 1 type or 2 types of composites. The total amount when these elements are contained in combination may be 0.1%.

  Cu, W, V, and Nb that are elements of the second group all have an effect of improving the creep strength. For this reason, you may contain these elements.

Cu: 1% or less Cu has an effect of improving creep strength. That is, Cu is an austenite-forming element like Ni and Co, and contributes to improvement of creep strength by increasing phase stability. Therefore, Cu may be contained. However, when Cu is contained excessively, the hot workability is lowered. For this reason, when making it contain, the upper limit is provided in the quantity of Cu, and it is 1% or less. The upper limit of the Cu content is desirably 0.8%.

  On the other hand, in order to stably obtain the effect of Cu, the Cu content is preferably 0.01% or more. A more desirable lower limit of the Cu content is 0.03%.

W: 1% or less W has an effect of improving creep strength. That is, W has a function of improving the creep strength at a high temperature by dissolving in the matrix. Therefore, W may be contained. However, when W is contained excessively, the stability of austenite is lowered, and on the contrary, the creep strength is lowered. Therefore, an upper limit is set for the amount of W in the case of inclusion to 1% or less. The upper limit of the W content is desirably 0.8%.

  On the other hand, in order to stably obtain the effect of W, the W content is preferably 0.01% or more. A more desirable lower limit of the W content is 0.03%.

V: 0.5% or less V has an effect of improving creep strength. That is, V combines with C or N to form fine carbides or carbonitrides, and has the effect of improving creep strength. Therefore, V may be contained. However, when V is contained excessively, it precipitates in a large amount as a carbide or carbonitride, resulting in a decrease in creep ductility. Therefore, an upper limit is set for the amount of V in the case of inclusion to 0.5% or less. The upper limit of the V content is desirably 0.4%.

  On the other hand, in order to stably obtain the effect of V, the V content is preferably 0.01% or more. A more desirable lower limit of the V content is 0.02%.

Nb: 2% or less Nb combines with C and N in the same manner as V and precipitates in the grains as fine carbides and carbonitrides, and also binds with Ni to form an intermetallic compound, and creep at high temperature. Contributes to strength. Therefore, you may contain Nb. However, when the Nb content is excessive, a large amount of carbides, carbonitrides and intermetallic compounds are precipitated, resulting in a decrease in creep ductility and toughness. Therefore, an upper limit is set for the amount of Nb in the case of inclusion, and it is 2% or less. The upper limit of the Nb content is desirably 1.8%.

  On the other hand, in order to stably obtain the effect of Nb, the Nb content is preferably 0.01% or more. A more desirable lower limit of the Nb content is 0.05%.

  Said Cu, W, V, and Nb can be contained in only 1 type in them, or 2 or more types of composites. The total amount when these elements are contained in combination may be 4.5%.

(B) Average crystal grain size (G _S ) of the surface layer:
When the Ni-base heat-resistant alloy member having the chemical composition described in the above item (A) is used as a member having a thickness exceeding 30 mm, the HAZ in the vicinity of the welded portion on the surface of the member of the first layer and the last layer in multilayer welding is used. In addition, fine cracks may occur. According to the estimation of the present inventors, the above-mentioned fine cracks are generated because the thermal stress during welding is concentrated on the crystal grain boundary and exceeds the strength of the grain boundary.

  In order to prevent the fine cracks described above, it is effective to reduce the average crystal grain size in the region where the thermal stress of the member surface portion increases. It should be noted that the content of B, which strengthens the grain boundaries, in other words, increases the deformation resistance within the grains, promotes stress concentration at the grain boundaries, and conversely strengthens the grain boundaries. Depending on the content, the upper limit of the average crystal grain size for preventing the cracking varies.

However, G _S (μm), which is the average crystal grain size of the surface layer, is the above formula [1], that is,
G _S ≦ −5 × P1 + 60 [1]
If the above is satisfied, the above-described fine cracks can be stably prevented.

As already mentioned, the “surface layer” refers to a region from the member surface to a depth of 0.2 mm,
P1 = 2 × Al + Ti + (Mo / 10) −5 × B
Thus, Al, Ti, Mo and B in the parameter P1 mean the content (mass%) of the element.

The average grain size G _S of the surface layer is obtained by subjecting the surface of the solution heat-treated member to cutting or polishing with a tool, shot peening using laser blasting or sand blasting, cold rolling with a roll or a hydraulic press, It is possible to control by subjecting the surface portion to mechanical processing and then recrystallizing it by heat treatment.

  As the condition of the “heat treatment”, a condition of holding for 0.1 to 1.5 h in a temperature range of 900 to 1150 ° C. is preferable, and a condition of holding for 0.5 to 1.5 h in a temperature range of 950 to 1075 ° C. More desirable.

  Patent Document 9 discloses an austenitic alloy structure in which recrystallized grains are formed as a surface layer of a structure to enhance intergranular corrosion resistance. The structure and the Ni base according to the present invention are disclosed. The chemical composition is completely different from that of the heat-resistant alloy member, and the problem to be solved is completely different. For this reason, it is obvious that the Ni-base heat-resistant alloy member according to the present invention cannot be obtained from the technique disclosed in Patent Document 9.

(C) an average crystal grain size of the thickness of the central portion of the member (G _B):
When the Ni-base heat-resistant alloy member having the chemical composition described in the above item (A) is used as a member having a thickness exceeding 30 mm, it may be difficult to stably obtain a sufficient creep strength. According to the estimation of the present inventors, when the thickness of the member exceeds 30 mm, the average crystal grain size is likely to vary, particularly due to temperature non-uniformity during solution heat treatment, and further contributes to the creep strength. The creep strength becomes unstable due to variations in the contents of B, Al, Ti and Mo, which are the elements to be used.

  In order to solve the above-described instability of the creep strength, it is effective to increase the average crystal grain size of the member, but the contents of B, Al, Ti and Mo effective for improving the creep strength Accordingly, the lower limit of the average crystal grain size necessary for ensuring the creep strength varies. On the other hand, since B, Al, Ti, and Mo also have an effect of increasing the susceptibility to liquefaction cracking during welding, the upper limit of the average crystal grain size varies depending on the content of these elements.

However, G _B (μm), which is the average crystal grain size in the central part of the thickness of the member, is the formula [2], that is,
−4 × P2 + 85 ≦ G _B ≦ −30 × P2 + 650 (2)
If the above condition is satisfied, sufficient creep strength can be stably obtained even in a member having a thickness exceeding 30 mm.

As already described, the “member thickness central portion” refers to a region excluding each surface side that is 25% of the member thickness,
P2 = 10 × B + 2 × Al + Ti + (Mo / 10)
Thus, B, Al, Ti and Mo in the parameter P2 mean the content (% by mass) of the element.

The average crystal grain size G _B thick central portion of the member can be controlled by the selection of temperature and time during the solution heat treatment of the member.

  As conditions for the above-mentioned “solution heat treatment”, conditions for holding for 0.1 to 3 hours in a temperature range of 900 to 1240 ° C. are preferable, and conditions for holding for 0.2 to 1.5 hours in a temperature range of 1050 to 1210 ° C. More desirable.

(D) HV0.01 (max):
The Ni-base heat-resistant alloy member that satisfies the requirements described in the above items (A) to (C) has a stable and excellent creep strength regardless of its thickness even when used as a member having a thickness exceeding 30 mm. In addition, when multi-layer welding is performed, crack resistance to such an extent that it does not cause a practical problem, specifically, crack resistance at the HAZ near the welded portion of the member surface of the first layer and the last layer, and in the member meat It has the liquefaction cracking resistance.

  However, since cracks near the welded part on the surface of the member occur at a position exposed to a low temperature on the final layer side compared to the initial layer side, the surface of the surface is more strongly affected by the hardness of the member surface part. It is preferable to suppress the hardness of the part. Specifically, HV0.01 (max), that is, the maximum hardness HV0.01 in the region from the member surface on the final layer side during welding to a depth of 0.2 mm is desirably 380 or less. HV0.01 (max) is more preferably 350 or less.

  As described above, the above “HV0.01” means a “hardness symbol” when a micro Vickers hardness test is performed with a test force of 0.098 N (10 gf).

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

  An ingot was prepared by melting a Ni-base heat-resistant alloy having the chemical composition shown in Table 1 in a laboratory.

  Next, using the above ingot, forming by hot forging, solution heat treatment and partly surface grinding with a cutting tool or cold surface treatment with a rolling roll, and then heat treatment under different conditions For each Ni-base heat-resistant alloy, a plurality of alloy plates having a thickness of 35 mm, a width of 100 mm, and a length of 250 mm were produced.

  From each alloy plate obtained as described above, a test piece was cut out and mirror-polished so that the cross section was the test surface.

After that, for each test piece, the mirror-polished surface was corroded with aqua regia, and the surface of both surfaces that became the “surface layer” was observed from the two surfaces to 0.2 mm in the depth direction using an optical microscope. Then, the average grain slice length for each region is measured by a cutting method, the average grain slice length for each region is further arithmetically averaged, multiplied by 1.128, and the average crystal grain size of the surface layer G _S (μm) was determined. Similarly, a region where the depth from the surface processed surface that becomes the “member thickness central portion” exceeds 8.75 mm is randomly observed with three optical microscopes, and an average grain section length is obtained by a cutting method. Subsequently, the average grain section length was multiplied by 1.128 to determine the average crystal grain size G _B (μm) at the center of the thickness of the member.

  Further, for each of the above test pieces, the micro Vickers hardness in the region from the surface to a depth of 0.2 mm is defined as a surface that is arbitrarily selected from the both surfaces subjected to the surface processing and becomes the final layer side during welding. Was measured. Specifically, the micro Vickers hardness up to the position where the depth from the surface is 0.2 mm is randomly measured at a test force of 0.098 N (10 gf) at 20 points, and the largest value is “HV0.01 (max ) ”.

  In addition, for each alloy plate, a round bar creep rupture test piece having a diameter of 6 mm and a gauge distance of 30 mm was sampled from “the thickness central portion of the member”, and the target rupture time of the base plate was 700 ° C. A creep rupture test was conducted under the condition of 176 MPa. In addition, the thing whose creep rupture time becomes 1000 h or more which is the target rupture time of a base material board material was set as "pass", and the thing less than 1000 h was set as "fail".

  In addition, for each alloy plate, using the remaining part of each of the test pieces described above, in the longitudinal direction of the alloy plate, after processing a V groove having an angle of 30 ° and a root thickness of 1 mm, a thickness of 60 mm, Using “E Ni 6182” defined in JIS Z 3224 (2010) as a coated arc welding rod on a commercially available SM400C steel plate defined in JIS G 3106 (2008) having a width of 500 mm and a length of 500 mm, four rounds were performed. Restraint welded. Thereafter, multilayer welding was performed in the groove at a heat input of 9 to 15 kJ / cm by TIG welding using a welding wire (AWS A5.14 ER NiCrCoMo-1).

  After removing the welded joint obtained in this way from the restraint plate, a penetration inspection test on the surface of the first layer side and the surface of the final layer side is performed according to JIS Z 2343-1 (2001), and the presence or absence of the indication pattern is confirmed. In the case where there is no indication pattern, “Pass” is given, and in the case where there is no instruction pattern, “Fail” is given.

  Further, five test pieces were cut out and mirror-polished so that the cross section of the weld became the test surface.

  For each of the five mirror-polished specimens, first, the surface vicinity of the first layer and the last layer was observed with an optical microscope. And although it was not detected by the penetrant flaw test, it is “OK” when a minute microcrack is observed near the surface with an optical microscope. In addition to the fact that micro cracks were observed near the surface, it was classified as “impossible”.

  Subsequently, portions other than the vicinity of the surface were also observed with an optical microscope to investigate the presence of so-called “mesh cracks”. In addition, as for the crack in the meat, the thing in which the crack was not observed in all the five cross sections examined with the optical microscope was set as "pass", and the thing in which the crack was observed also in one cross section was set as "fail".

  Tables 2 and 3 collectively show the results of the above tests, together with the conditions of solution heat treatment, surface treatment, and heat treatment applied to each alloy plate.

  “◎” in the “Evaluation” column in Tables 2 and 3 indicates that “Creep Strength”, “Penetration Flaw Test”, and “Crack in Meat” are all “Pass”, and “Micro Crack near the Surface”. The column is “good”, indicating that the creep strength is excellent and the crack resistance during welding is extremely excellent. “○” indicates that “creep strength”, “penetration test”, and “crack in meat” are all “pass”, and “micro crack near surface” is “possible”. In addition to excellent resistance to cracking during welding. “×” indicates that the “creep strength” column is “fail” and the creep strength is inferior, or the “penetration test” column is “fail” or the “penetration test” column is “pass”. The “crack” column is “fail”, indicating that the crack resistance during welding is poor.

  From Tables 2 and 3, in the case of specimens A2, A3, B2 to B7, C2, C3, D2, D3, E2, and E3 using alloy plates that satisfy the conditions specified in the present invention, the surface was measured by the penetrant flaw test. It is apparent that the welded joint is practically sound and has no indication pattern, and no weld cracks occur in any part other than the surface. Further, it is clear that the creep rupture time also has excellent creep strength by clearing the target rupture time of the base plate. In addition, when HV0.01 (max) was less than 380, minute microcracks were not observed near the surface in cross-sectional observation with an optical microscope.

  In contrast, the specimens A1, A4, A5, B1, B8, B9, C1, C4, C5, D1, D4, D5, E1, E4, and E5 using alloy plates that deviate from the conditions specified in the present invention. In this case, the creep strength is poor or the crack resistance during welding is poor.

Specifically, specimens A4, B8, C4, D4 and E4, because the G _S alloy plate used (average crystal grain size of the surface layer) did not satisfy the above equation [1], in penetrant An indication pattern was observed on the surface, and cracks were also observed on the surface in cross-sectional observation, and the crack resistance during welding was poor.

For specimens A1, B1, C1, D1 and E1, since the G _B alloy plate used (average crystal grain size of the thickness of the central portion of the member) is below the lower limit of the above formula [2], sufficient creep Strength was not obtained.

On the other hand, in the case of specimens A5, B9, C5, D5 and E5, G _B (average crystal grain size at the center of the thickness of the member) of the alloy plate used was too large, and the upper limit of the above formula [2] was exceeded. Since it exceeded, the crack judged to be a liquefaction crack generate | occur | produced in the meat in the cross-sectional observation with an optical microscope.

  Although the Ni-based heat-resistant alloy member of the present invention is a thick member having a thickness of more than 30 mm, the crack resistance during welding, in particular, the weld crack resistance near the surface during multilayer welding, more specifically, In addition to being excellent in crack resistance in the HAZ near the welded portion on the surface of the first and last layer members, it has a stable and excellent creep strength. For this reason, the Ni-base heat-resistant alloy member of the present invention can be suitably used as a thick high-temperature member such as a main steam pipe of a power generation boiler.

Claims (3)

Ni-based heat-resistant alloy member having a thickness exceeding 30 mm, and in mass%, C: 0.03 to 0.15%, Si: 1% or less, Mn: 2% or less, P: 0.03% or less, S : 0.01% or less, Ni: 48-58%, Co: 8-16%, Cr: 18-25%, Mo: 6-12%, Ti: 0.05-0.8%, Al: 0. 05-1.6%, B: 0.0001-0.01%, REM: 0.001-0.1%, N: 0.02% or less and O: 0.01% or less, with the balance being Fe And a Ni-based heat-resistant alloy member having a chemical composition comprising impurities and satisfying the following formulas [1] and [2]:
G _S ≦ −5 × P1 + 60 [1]
−4 × P2 + 85 ≦ G _B ≦ −30 × P2 + 650 (2)
However,
P1 = 2 × Al + Ti + (Mo / 10) −5 × B
P2 = 10 × B + 2 × Al + Ti + (Mo / 10)
In the parameters P1 and P2, Al, Ti, Mo, and B mean the content (mass%) of the element.
Each G _S and G _B, refers to a surface layer having an average grain size ([mu] m) and the average crystal grain size of the thickness of the central portion of the member ([mu] m). The Ni-base heat-resistant alloy member according to claim 1, wherein the Ni-base heat-resistant alloy member contains at least one element selected from the group shown below in mass% instead of a part of Fe.
First group: Ca: 0.05% or less and Mg: 0.05% or less Second group: Cu: 1% or less, W: 1% or less, V: 0.5% or less and Nb: 2% or less   The Ni-base heat-resistant alloy member according to claim 1 or 2, wherein a maximum hardness HV0.01 in a region from the member surface on the final layer side during welding to a depth of 0.2 mm is 380 or less.