JP2014034725A - Austenitic heat resistant alloy member - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an austenitic heat resistant alloy member excellent in crack resistance during a hot bending method, capable of being used preferably as a thick high temperature member having large diameter.SOLUTION: An austenitic heat resistant alloy member contains a chemical composition containing C:0.03 to 0.15%, Si≤1%, Mn≤2%, P≤0.03%, S≤0.01%, Ni:40 to 55%, Cr:20 to 35%, W:3 to 10%, Ti:0.01 to 1.2%, Al≤0.3%, B:0.0001 to 0.01%, N≤0.02% and O≤0.01% and the balance Fe with impurities, wherein an average crystal particle diameter d (μm) and the maximum hardness at the area from an external surface of the member to 5 mm depth HV0.1(max) satisfy [HV0.1(max)≤(-1/6)×d+300]. The alloy member may contain specific amounts of one or more Ca, REM, Co, Cu, Mo, V, Nb, Zr in place of a portion of Fe.

Description

  The present invention relates to an austenitic heat-resistant alloy member. Specifically, the present invention relates to an austenitic heat-resistant alloy member excellent in hot workability, which can be suitably used as a thick-walled, large-diameter high-temperature member such as a main steam pipe and a reheat steam pipe of a power generation boiler.

  The above-mentioned “excellent in hot workability” means that the surface of the member is cracked particularly when bending is performed while locally heating a part of the member with a heating device such as high-frequency induction heating. It means that it can be processed without occurring.

  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, application of austenitic heat-resistant alloys is also being studied for large-diameter and thick-walled members such as main steam pipes and reheat steam pipes, which conventionally used ferritic heat-resistant steels.

  Based on such a technical background, technologies relating to various austenitic heat-resistant alloys have been proposed.

  Specifically, for example, in Patent Document 1, after surface processing is performed to form a plastic work hardened layer that is 330 HV or higher on the surface, sufficient recrystallization is generated on the hardened surface portion. Austenitic alloy structure with improved intergranular corrosion resistance and stress corrosion cracking resistance by local heat treatment for dispersing and precipitating Cr carbide in recrystallized grains or grain boundaries and its production The law is disclosed.

  Patent Document 2 discloses a high-Ni, high-Cr stainless steel that has improved hot workability by minimizing crystal grains and suppressing S precipitated at the crystal grain boundaries. .

  Further, Patent Document 3 proposes a Ni-based alloy product. This Ni-based alloy product uses W to increase the high-temperature strength and manage the amount of effective B, thereby improving hot workability and preventing weld cracking. It is an alloy product.

  Patent Document 4 proposes an austenitic heat resistant alloy, a heat resistant pressure resistant member made of the alloy, and a manufacturing method thereof, in which the creep strength is increased by using an α-Cr phase as a strengthening phase by utilizing Cr, Ti and Zr.

  Patent Document 5 proposes a Ni-base heat-resistant alloy containing a large amount of W and using Al and Ti to enhance the strength by solid solution strengthening and precipitation strengthening of the γ 'phase.

JP 2000-265249 A JP 2002-80942 A JP 2011-63838 A International Publication No. 2009/154161 International Publication No. 2010/038826

  In order to use an austenitic heat-resistant alloy as a structure, welding or hot bending is generally performed.

  In addition, when welding is performed, it is known that various cracks mainly caused by metallurgical factors are likely to occur in the welded portion.

  However, Patent Document 1 is only a technique for improving intergranular corrosion resistance and stress corrosion cracking resistance, and is not developed in consideration of occurrence of cracks in a welded portion. Furthermore, no consideration has been given to hot bending workability.

  Similarly, Patent Document 2 is not developed considering the occurrence of cracks in the welded portion. In addition, no specific study has been conducted on hot bending workability.

  On the other hand, since all the austenitic heat-resistant alloys disclosed in Patent Documents 3 to 5 are sufficiently low in weld cracking sensitivity, they can be suitably used as materials for structures to be welded.

  However, from the detailed investigation conducted by the present inventors, even when using the austenitic heat-resistant alloy disclosed in Patent Documents 3 to 5, in a member bent hot, particularly a thick member, It has been clarified that fine cracks that have not been confirmed before may occur in the surface and in the vicinity of the surface.

  Therefore, it became clear that the prevention of minute cracks occurring on the outer surface of the member is a new issue, especially when a member made of an austenitic heat-resistant alloy with a thickness of about 5 mm or more is bent hot. did.

  The present invention has been made in view of the above-mentioned present situation, and is resistant to cracking at the time of hot bending, particularly cracking resistance at the outer surface and in the vicinity of the surface when hot bending is performed as a thick member. An object of the present invention is to provide an austenitic heat-resistant alloy member that is excellent and suitable for hot working on a thick, large-diameter high-temperature member such as a main steam pipe or a reheat steam pipe of a power generation boiler.

  In order to solve the above-described problems, the present inventors have conducted a detailed investigation on minute cracks that have occurred on the outer surface of a member that has been bent hot and the vicinity of the surface. As a result, the following items (i) to (iv) were clarified.

  (I) The larger the crystal grain size of the member, the larger the size of cracks generated in the member bent hot, and the higher the frequency of cracks.

  (Ii) The higher the hardness in the vicinity of the outer surface of the member, the larger the size of cracks generated in the hot-bent member and the higher the frequency of cracks.

  (Iii) Cracks occurred at crystal grain boundaries (hereinafter simply referred to as “grain boundaries”) in a range of about 5 mm depth from the outer surface of the member.

  (Iv) Both cracks including the outer surface of the member and those existing in the vicinity of the surface were observed in the crack.

  From the above (i) to (iv), the present inventors generate fine cracks in the outer surface of the member bent in the hot state and in the vicinity of the surface by the following mechanisms (a) to (c). Estimated.

  (A) A large tensile stress is generated on the outer surface of the member during hot bending, and the deformability of the member decreases as the crystal grain size of the member increases. Therefore, the larger the crystal grain, the more stress is applied to the grain boundary. It becomes easier to concentrate.

  (B) Dislocations are introduced into the outer surface of the member by processing when the member is manufactured, and the hardness is increased. For this reason, deformation resistance within crystal grains (hereinafter simply referred to as “intra-grain”) increases. Furthermore, dislocations are also introduced into the grains during bending, and as a result, dynamic precipitation occurs within the grains.

  (C) Grain boundaries are weakened relatively in the grains that are remarkably strengthened by dynamic precipitation. For this reason, stress concentrates at the grain boundary. As a result, the grain boundaries are opened and become fine cracks.

  Under the above estimation, the present inventors conducted a detailed study in order to prevent fine cracks generated on the outer surface of the member bent hot and inside the vicinity of the surface.

  As a result, in order to prevent the above-described cracking, the hardness of the outer surface portion of the member resulting from the processing of the surface at the time of manufacturing the member is controlled within a specific range according to the crystal grain size of the member. It became clear that the stress concentration at the grain boundary, which is relatively generated by the intragranular strengthening associated with mechanical precipitation, can be reduced, specifically, the following measure (d) may be taken.

(D) The maximum hardness HV0.1 (max) in the region from the outer surface of the member to a depth of 5 mm and the average crystal grain size d (μm) of the member are managed so as to satisfy the following formula.
HV0.1 (max) ≦ (−1/6) × d + 300
In addition, said "HV0.1" means the "hardness symbol" at the time of implementing a micro Vickers hardness test by setting test force as 0.9807N (100gf) (refer JISZ2244 (2009)).

  The present invention has been completed based on the above findings, and the gist thereof is the following austenitic heat-resistant alloy member.

(1) By 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: 40 to 55%, Cr: 20-35%, W: 3-10%, Ti: 0.01-1.2%, Al: 0.3% or less, B: 0.0001-0.01%, N: 0 0.02% or less and O: 0.01% or less, the balance being a chemical composition consisting of Fe and impurities, the average crystal grain size d (μm) of the member, and the region from the outer surface of the member to a depth of 5 mm An austenitic heat-resistant alloy member characterized in that the maximum hardness HV0.1 (max) in the above satisfies the following formula [1].
HV0.1 (max) ≦ (−1/6) × d + 300 (1).

(2) The austenitic system according to the above (1), which contains one or more elements selected from the following first group or second group in mass% instead of part of Fe Heat-resistant alloy member.
Group 1: Ca: 0.05% or less and REM: 0.1% or less Group 2: Co: 1% or less, Cu: 1% or less, Mo: 1% or less, V: 0.5% or less, Nb : 0.5% or less and Zr: 0.5% or less.

(3) The austenite according to (1) or (2), wherein the processing temperature T (° C.) and the strain amount ε (%) are used for hot bending processing satisfying the following formula [2]: Heat-resistant alloy members.
T ≧ 200 × log ₁₀ ε + 900 (2).

  The austenitic heat-resistant alloy member of the present invention is excellent in crack resistance during hot bending, particularly crack resistance on the outer surface and in the vicinity of the surface when hot bent as a thick member. For this reason, the austenitic heat-resistant alloy member of the present invention is suitable for hot working on thick, large-diameter high-temperature members such as main steam pipes and reheat steam pipes of power generation boilers.

  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 contained excessively, the stability of austenite is lowered, leading to a decrease in toughness and creep strength. 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 0.02%, and the more desirable lower limit is 0.05%.

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, a desirable lower limit of the Mn content is 0.02%, and a more desirable lower limit is 0.05%.

P: 0.03% or less P is contained in the alloy as an impurity. When P is contained in a large amount, the hot workability and weldability are remarkably lowered, and the creep ductility after long-time use is also lowered. . 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 contained in the alloy as an impurity in the same manner as P, and when it is contained in a large amount, the hot workability and weldability are remarkably deteriorated, and further, the creep after long-time use. It also reduces the ductility. 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: 40-55%
Ni is an effective element for obtaining austenite, and is an essential element for ensuring the structural stability when used for a long time. In order to sufficiently obtain the effect of Ni described above within the range of the Cr content of the present invention of 20 to 35% described later, a Ni content of 40% 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 40 to 55%. A desirable lower limit of the Ni content is 41%, and a more desirable lower limit is 42%. The desirable upper limit of the Ni content is 54%, and the more desirable upper limit is 53%.

Cr: 20 to 35%
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 40 to 55%, a Cr content of 20% or more is necessary. However, if the Cr content exceeds 35%, the stability of austenite at a high temperature deteriorates and the creep strength decreases. Therefore, the content of Cr is set to 20 to 35%. A desirable lower limit of the Cr content is 20.5%, and a more desirable lower limit is 21%. The desirable upper limit of the Cr content is 34.5%, and the more desirable upper limit is 34%.

W: 3-10%
W is an element that makes a solid solution in the matrix and greatly contributes to the improvement of the creep strength at high temperatures. In order to fully exhibit the effect, W content of at least 3% or more is necessary. However, even if W is excessively contained, the effect is saturated and the creep strength may be lowered. Furthermore, since W is an expensive element, excessive W content causes an increase in cost. Therefore, an upper limit is provided and the W content is 3 to 10%. A desirable lower limit of the W content is 3.5%, and a more desirable lower limit is 4%. The desirable upper limit of the W content is 9.5%, and the more desirable upper limit is 9%.

Ti: 0.01 to 1.2%
Ti precipitates in the grains as fine carbonitrides and contributes to the creep strength at high temperatures. In order to obtain the effect, a Ti content of 0.01% or more is necessary. However, if the Ti content is excessive, a large amount of carbonitride precipitates, causing a decrease in creep ductility and toughness. For this reason, an upper limit is provided and the Ti content is set to 0.01 to 1.2%. A desirable lower limit of the Ti content is 0.03%, and a more desirable lower limit is 0.05%. The desirable upper limit of the Ti content is 1.0%, and the more desirable upper limit is 0.8%.

Al: 0.3% or less Al is an element having a deoxidizing action. However, when the Al content is excessive, the cleanliness of the alloy is remarkably deteriorated and the hot workability and ductility are lowered. Therefore, an upper limit is set for the Al content to 0.3% or less. The Al content is desirably 0.2% or less, and more desirably 0.1% or less.

  In addition, although it is not necessary to set a minimum in particular about content of Al, extreme reduction will not obtain a sufficient deoxidation effect, but will deteriorate the cleanliness of an alloy conversely, and will raise the manufacturing cost. Therefore, the desirable lower limit of the Al content is 0.0005%. In order to stably obtain the deoxidation effect of Al and to ensure good cleanability of the alloy, the lower limit of the Al content is more preferably 0.001%.

B: 0.0001 to 0.01%
B is an element necessary for improving the creep strength by segregating at the grain boundary during use at a high temperature to strengthen the grain boundary and finely dispersing the grain boundary carbide. In order to obtain this effect, a B content of 0.0001% or more is necessary. However, when the content of B becomes excessive, the hot workability deteriorates in addition to the weldability deterioration. Therefore, an upper limit is provided so that the B content is 0.0001 to 0.01%. 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%.

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%.

  The austenitic heat-resistant alloy member 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 an impurity mixed from ore, scrap, or a manufacturing environment as a raw material when an austenitic heat-resistant alloy member is industrially manufactured.

  The austenitic heat-resistant alloy member of the present invention may contain one or more elements selected from Ca, REM, Co, Cu, Mo, V, Nb and Zr instead of a part of the above-mentioned Fe. Good.

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 it contains Ca, the content shall be 0.05% or less. The upper limit of the Ca content is desirably 0.04%.

  On the other hand, the effect of Ca described above can be stably obtained when the Ca content is 0.0001% or more.

REM: 0.1% or less REM has an effect of improving hot workability. That is, REM has a strong affinity with S and contributes to improvement of hot workability. For this reason, you may contain REM. However, when the content of REM becomes excessive, it combines with O to significantly reduce cleanliness and, on the contrary, deteriorate hot workability. For this reason, when it contains REM, the content shall be 0.1% or less. The upper limit of the REM content is desirably 0.08%.

  On the other hand, the above-described REM effect can be stably obtained when the REM content is 0.001% or more.

  “REM” is a generic name for a total of 17 elements of Sc, Y, and lanthanoid, and the content of REM refers to the total content of one or more elements of REM. Further, REM is generally contained in misch metal. For this reason, for example, 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 and REM 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 0.15%.

Co: 1% or less Co has the effect of improving the creep strength. That is, Co is an austenite generating element like Ni, and contributes to the improvement of creep strength by increasing phase stability. Therefore, Co may be contained. However, since Co is an extremely expensive element, excessive content of Co causes a significant cost increase. For this reason, when it contains Co, the content shall be 1% or less. The upper limit of the Co content is desirably 0.8%.

  On the other hand, the effect of Co described above can be stably obtained when the Co content is 0.01% or more.

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 it contains Cu, the content shall be 1% or less. The upper limit of the Cu content is desirably 0.8%.

  On the other hand, the effect of Cu described above can be stably obtained when the Cu content is 0.01% or more.

Mo: 1% or less Mo has an effect of improving creep strength. That is, Mo has a function of improving the creep strength at a high temperature by dissolving in the matrix. Therefore, you may contain Mo. However, when Mo is excessively contained, the stability of austenite is lowered, and instead the creep strength is lowered. Therefore, when it contains Mo, the content shall be 1% or less. The upper limit of the Mo content is desirably 0.8%.

  On the other hand, the effect of Mo described above is stably obtained when the Mo content is 0.01% or more.

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, when V is contained, the content is set to 0.5% or less. The upper limit of the V content is desirably 0.4%.

  On the other hand, the effect of V described above can be stably obtained when the V content is 0.01% or more.

Nb: 0.5% or less Nb combines with C and N in the same manner as V and precipitates in the grains as fine carbides and carbonitrides, contributing to the improvement of creep strength at high temperatures. Therefore, you may contain Nb. However, when the Nb content is excessive, a large amount of carbides and carbonitrides are precipitated, resulting in a decrease in creep ductility and toughness. Therefore, when Nb is contained, the content is set to 0.5% or less. The upper limit of the Nb content is desirably 0.4%.

  On the other hand, the effect of Nb described above can be stably obtained when the Nb content is 0.01% or more.

Zr: 0.5% or less Zr has an effect of improving creep strength. That is, Zr is a grain boundary strengthening element and contributes to the improvement of creep strength at high temperatures, and further contributes to the improvement of creep ductility. Therefore, Zr may be contained. However, when the Zr content exceeds 0.5%, the hot workability may decrease. Therefore, when Zr is contained, the content is set to 0.5% or less. The upper limit of the Zr content is desirably 0.4%.

  On the other hand, the effect of Zr described above can be stably obtained when the Zr content is 0.01% or more.

  Said Co, Cu, Mo, V, Nb, and Zr can be made to contain only any 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 of member and hardness of outer surface portion of member:
The austenitic heat-resistant alloy member of the present invention having the chemical composition described in the above item (A) has an average crystal grain size d (μm) of the member and a maximum hardness HV0 in a region from the outer surface of the member to a depth of 5 mm. .1 (max) is
HV0.1 (max) ≦ (−1/6) × d + 300 [1]
Must satisfy the expression.

  Since the surface of the austenitic heat-resistant alloy member of the present invention is cut with a tool or polished with a grinder or belter for the purpose of removing surface flaws and oxide films during production, The dislocation is introduced by the above processing and the hardness is increased. For this reason, the deformation resistance in the grains is increased, and further, dislocations are introduced during the bending process, so that dynamic precipitation occurs in the grains. The grain boundary is weakened relatively in the grains remarkably strengthened by the dynamic precipitation. In addition, a large tensile stress is generated on the outer surface of the member during hot bending, and the deformability of the member decreases as the crystal grain size of the member increases. For this reason, as the crystal grain size of the member increases, the stress tends to concentrate on the grain boundary, and as a result, the grain boundary opens and becomes a fine crack.

  The above “HV0.1” means a “hardness symbol” when a micro Vickers hardness test is performed with a test force of 0.9807 N (100 gf).

  The condition that the average crystal grain size d (μm) of the member and the maximum hardness HV0.1 (max) in the region from the outer surface of the member to a depth of 5 mm satisfy the above-mentioned formula [1] is as follows: For example, using the austenitic heat-resistant alloy of the present invention having the chemical composition described in the item (A) as a raw material, after manufacturing by melting, forging, hot pipe forming, solution heat treatment, the following <A> or < By performing the process of B>, it can be stably achieved.

  <A> After cutting the surface of the member with a cutting tool, the surface is polished once or more with a grindstone having a particle size smaller than 60.

  <B> After cutting the surface of the member with a cutting tool, heat treatment is performed in a temperature range of 900 to 1200 ° C. for 0.1 to 5 hours.

  As described above, as the average crystal grain size d (μm) of the member increases, stress is more likely to concentrate at the grain boundaries, and fine cracks are likely to occur. For this reason, the average crystal grain size d of the member is desirably 520 μm or less.

  On the other hand, when a desired member is manufactured by the above-described method, the average crystal grain size d of the member is usually about 50 μm because the creep strength decreases when the member becomes finer.

The processing temperature T (° C.) and the strain amount ε in the case of hot bending using the austenitic heat-resistant alloy member of the present invention having the chemical composition described in the item (A) and satisfying the formula [1] (%) Is the following
T ≧ 200 × log ₁₀ ε + 900 (2)
It is preferable that the formula is satisfied.

  Even if it is a case where it bends hot using the austenitic heat-resistant alloy member of the present invention which has the chemical composition described in the item (A) and satisfies the formula [1], the processing temperature T (° C.) Is low and the strain amount ε (%) is large, the amount of dynamically precipitated carbides increases and precipitates quickly. For this reason, the deformability within the grains decreases, and as a result, the grain boundaries weaken relatively with respect to the remarkably strengthened grains, and cracks tend to occur.

  However, the processing temperature T (° C.) and the strain amount ε (%) in the case of hot bending using the austenitic heat-resistant alloy member of the present invention that satisfies the formula [1] are the above formula [2]. If satisfied, it is possible to stably suppress the occurrence of fine cracks in the outer surface of the bent member and in the vicinity of the surface.

  The lower limit of the processing temperature T (° C.) in the formula [2] is preferably 950 ° C., and the upper limit thereof is preferably 1250 ° C.

  The upper limit of the strain amount ε (%) in the formula [2] may be about 56% when the processing temperature T (° C.) is 1250 ° C., which is the preferable upper limit, but is preferably 40%. On the other hand, the lower limit of the strain amount ε (%) may be about 1.8% when the processing temperature T (° C.) is 950 ° C. which is the preferable lower limit.

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

  An austenitic heat-resistant alloy having the chemical composition shown in Table 1 was melted in the laboratory to produce an ingot.

  Using the above ingot, hot forging and rolling forming, solution heat treatment and surface cutting with a cutting tool are performed, and for each austenitic heat-resistant alloy, an alloy plate having a thickness of 15 mm, a width of 100 mm, and a length of 500 mm is obtained. Several sheets were produced.

  Then, about a part of alloy plate, it grind | polished 1 to 4 times with the grindstone of the particle size 40th or 60th. Further, a part of the alloy plate was subjected to a heat treatment in which air was cooled after heating at 950 ° C. for 3 h, or a heat treatment in which water was cooled after being heated at 1200 ° C. for 0.5 h.

  A test piece having a diameter of 10 mm and a length of 130 mm, which is parallel to the longitudinal direction from the center in the thickness direction of each alloy plate obtained as described above, and further includes an outer surface of the plate of about 3 mm in width. Multiple pieces were produced by machining.

  Using the test piece having a diameter of 10 mm and a length of 130 mm, first, the average crystal grain size d (μm) of the test piece and the region from the outer surface to the center (ie, 5 mm depth) of the test piece. The maximum hardness HV0.1 (max) was investigated.

  Specifically, the average crystal grain size d (μm) of the test piece is determined by cutting the test piece so that the cross section becomes the test surface, mirror-polishing, then corroding with aqua regia, and a magnification of 100 A three-field optical microscope was observed at a magnification of 2, and the average grain section length was measured by a cutting method. Then, the average grain diameter d (μm) was obtained by multiplying the average grain section length by 1.128.

  The maximum hardness HV0.1 (max) in the above region is the micro Vickers hardness from the outer surface to the center after the test piece is cut so that the cross section becomes the test surface and mirror-polished, About 50 points were measured randomly at a test force of 0.9807 N (100 gf), and the highest value was designated as the highest hardness.

  Next, using a test piece having a diameter of 10 mm and a length of 130 mm, a reproduction test for hot bending of an actual boiler pipe was performed.

Specifically, using a greeble tester, the above test piece was subjected to a tensile test at an extremely low strain rate of 10 ⁻⁴ s ⁻¹ to 10 ⁻³ s ⁻¹ with various processing temperatures T (° C.). Then, the tensile test was interrupted in the middle so that the strain amount ε (%) changed variously, and the test piece after the interruption of the tensile test was examined for cracks on the surface and inside of the test piece.

The greeble test is a tensile test conducted while energizing and heating the central part of the test piece having a diameter of 10 mm and a length of 130 mm as described above. The processing temperature T (° C.) is a thermocouple welded to the central part of the greeble test piece. And measured. Further, the strain amount ε (%) was obtained by measuring the drawing Ra at the center of the test piece after interruption of the tensile test and converting it using the following formula.
ε = {Ra / (1-Ra)} × 100.

  The presence or absence of cracks on the surface of the test piece after the interruption of the tensile test was investigated by a penetration flaw test defined in JIS Z 2343-1 (2001). Further, the presence or absence of cracks in the test piece was investigated by cutting the center part of the test piece after interruption of the tensile test so that the cross section was the test surface, mirror polishing, and then observing with an optical microscope at a magnification of 100 times. .

  As a result of the above investigation, "No good" when no cracks were observed on both the test piece surface and the inside of the test piece. In this case, the crack resistance at the time of hot bending was evaluated as “No” if no crack was found on the surface of the test piece, and “No” if the crack was found on both the test piece surface and inside the test piece. did. Although there are no cracks on the surface, the reason for determining “OK” despite the presence of microcracks inside is that the cracks are so small that it is unlikely to lead to a serious accident in the actual plant. Because it is.

  Table 2 shows the results of the above tests together with the polishing and heat treatment conditions applied to each alloy plate.

  From Table 2, test numbers 2 to 6 were used to perform a greeble test, which is a reproduction test of hot bending of an actual boiler pipe, using test pieces taken from an alloy plate that satisfies the conditions specified in the present invention. In the case of 8 to 11, 13, 14, 16 to 19, 22 and 24 to 26, it is clear that no cracks are generated on the surface of the test piece and the crack resistance during hot bending is excellent.

Among the above test numbers, the processing temperature T (° C.) and strain amount ε (%) of the greeble test are as follows.
T ≧ 200 × log ₁₀ ε + 900 (2)
In the case of test numbers 9, 19, and 26 that satisfy the equation, it is clear that cracks are not generated not only on the surface of the test piece but also on the inside, and the crack resistance during hot bending is extremely excellent.

  On the other hand, in the case of test numbers 1, 7, 12, 15, 20, 21, and 23 in which a greeble test was performed using a test piece collected from an alloy plate that does not satisfy the conditions specified in the present invention, the surface of the test piece It is clear that cracks are observed both inside the test piece and at least inferior in hot workability.

  The austenitic heat-resistant alloy member of the present invention is excellent in crack resistance during hot bending, particularly crack resistance on the outer surface and in the vicinity of the surface when hot bent as a thick member. For this reason, the austenitic heat-resistant alloy member of the present invention is suitable for hot working on thick, large-diameter high-temperature members such as main steam pipes and reheat steam pipes of power generation boilers.

Claims (3)

In mass%, C: 0.03-0.15%, Si: 1% or less, Mn: 2% or less, P: 0.03% or less, S: 0.01% or less, Ni: 40-55%, Cr: 20-35%, W: 3-10%, Ti: 0.01-1.2%, Al: 0.3% or less, B: 0.0001-0.01%, N: 0.02% And O: 0.01% or less, the balance being a chemical composition consisting of Fe and impurities, the average crystal grain size d (μm) of the member, and the highest hardness in the region from the outer surface of the member to a depth of 5 mm An austenitic heat-resistant alloy member characterized in that the HV0.1 (max) satisfies the following formula [1].
HV0.1 (max) ≦ (−1/6) × d + 300 [1] The austenitic heat-resistant alloy member according to claim 1, wherein the austenitic heat-resistant alloy member contains at least one element selected from the following first group or second group in mass% instead of a part of Fe.
Group 1: Ca: 0.05% or less and REM: 0.1% or less Group 2: Co: 1% or less, Cu: 1% or less, Mo: 1% or less, V: 0.5% or less, Nb : 0.5% or less and Zr: 0.5% or less The austenitic heat-resistant alloy member according to claim 1 or 2, wherein the processing temperature T (° C) and the strain amount ε (%) are used for hot bending processing that satisfies the following formula [2].
T ≧ 200 × log ₁₀ ε + 900 (2)