JP6085989B2 - Ni-base heat-resistant alloy member and Ni-base heat-resistant alloy material - Google Patents

Description

  The present invention relates to a Ni-base heat-resistant alloy member and a Ni-base heat-resistant alloy material, and in particular, can prevent cracks on the surface during hot working (hereinafter sometimes referred to as “surface defects”), and is a power generation boiler. The present invention relates to a Ni-base heat-resistant alloy member that can be suitably used as a thick-walled, large-diameter high-temperature member, such as a main steam pipe or a reheat steam pipe, and a Ni-base heat-resistant alloy material used in manufacturing the Ni-base heat-resistant alloy member.

  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 Ni-based 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.

  In addition, the application of Ni-based 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 steel.

  Under such technical background, for example, Patent Documents 1 to 4 include Mo and / or W to enhance solid solution strengthening, and Al and Ti are included to form an intermetallic compound γ ′. Ni-base heat-resistant alloys that utilize phase precipitation strengthening have been disclosed. Patent Document 5 proposes a Ni-base heat-resistant alloy having improved creep strength by adjusting the composition of Al and Ti and precipitating a γ 'phase. Furthermore, Patent Documents 6 to 9 disclose Ni-based heat-resistant alloys containing Co for the purpose of further increasing strength in addition to Cr and Mo.

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 JP-A-63-76840 JP 2001-107196 A

  From the detailed investigation conducted by the present inventors, even when using the Ni-based heat-resistant alloy disclosed in Patent Documents 1 to 9, when hot working, particularly in a thick member having a thickness of 20 mm or more, It has become clear that surface defects that have not been confirmed before may occur.

  The present invention has been made in view of the above situation, and prevents surface defects during hot working, for example, during hot bending or hot forging, even when the thickness is 20 mm or more. An object of the present invention is to provide a Ni-base heat-resistant alloy member suitable for hot working on a thick, large-diameter high-temperature member such as a main steam pipe and a reheat steam pipe of a power generation boiler. .

  As a result of intensive studies to solve the above-mentioned problems, the present inventors have obtained the following knowledge.

  (A) The occurrence of surface defects during hot working is related to the metal structure of the member surface before hot working. For example, if hot working with the processed layer remaining on the surface of the member, carbide precipitates on the dislocations in the processed layer during the hot working, and as a result, the grains are strengthened and the grain boundaries are relatively weakened. Therefore, grain boundary cracks occur near the surface. In addition, the coarser the crystal grain size of the member surface, the more difficult the member is to be deformed, and the coarse grain structure also promotes grain boundary weakening.

  (B) In order to prevent surface defects during hot working, heat treatment is performed with the processed layer remaining on the surface of the member, and recrystallization in the vicinity of the surface suppresses precipitation on dislocations. It was newly found that lowering the deformation resistance in the vicinity of the surface by the fine grain effect is effective in reducing the grain boundary weakening.

  (C) When a surface defect exists, it becomes a starting point of a creep crack. Therefore, it is necessary to prevent surface defects in order to maintain excellent creep characteristics.

  (D) In order to prevent surface defects at the time of hot working, the metallographic structure in the region where the depth from the member surface is up to 50 μm (hereinafter, the region is sometimes referred to as “surface layer”) is average crystal It is necessary to use recrystallized grains having a grain size of ASTM grain size number 7 or more.

  (E) Moreover, in order to maintain the excellent creep characteristics, the surface metal structure is used as the above-mentioned recrystallized grains to prevent surface defects, and regions excluding each surface side that is 25% of the member thickness ( Hereinafter, the metal structure in the region may be referred to as “the thickness central portion of the member.”) It is necessary to make the coarse grain having an average grain size of ASTM grain size number 4 or less.

  The present invention has been made on the basis of the above findings, and the gist thereof is the following Ni-base heat-resistant alloy member.

(1) A Ni-based heat-resistant alloy member having a thickness of 20 mm or more,
In mass%, C: 0.01 to 0.15%, Si: 1% or less, Mn: 2% or less, P: 0.03% or less, S: 0.01% or less, Ni: 48 to 58%, Co: 8-16%, Cr: 18-25%, Mo: 6-12%, Ti: 0.05-0.8%, Al: 0.05-1.6%, B: 0.0001-0 0.01%, N: not more than 0.02% and O: not more than 0.01%, the balance having a chemical composition consisting of Fe and impurities,
The surface layer has a metal structure composed of recrystallized grains having an ASTM grain size number of 7 or more,
A Ni-based heat-resistant alloy member, characterized in that the central portion of the member has a metal structure composed of crystal grains having ASTM grain size number 4 or less.

(2) The Ni-base heat-resistant alloy member as described in (1) above, wherein the Ni-based heat-resistant alloy member 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, Mg: 0.05% or less, REM: 0.1% or less Second group: Cu: 1% or less, W: 1% or less, V: 0.5% or less , Nb: 2% or less

(3) A tensile test was performed on the test piece including the surface layer at a strain rate of 0.0001 s ⁻¹ at 1100 ° C., and the crack depth inside the test piece when the elongation reached 10% was 20 μm or less (0 The Ni-base heat-resistant alloy member as described in (1) or (2) above,

(4) Table layer has a metallic structure consisting of ASTM grain size number # 7 or more recrystallized grains,
The thickness of the member central portion an Ni-base heat-resistant alloy material which need use in preparing the Ni-base heat-resistant alloy member having a metallic structure consisting of ASTM grain size number # 4 following grain,
The thickness is 20 mm or more,
In mass%, C: 0.01 to 0.15%, Si: 1% or less, Mn: 2% or less, P: 0.03% or less, S: 0.01% or less, Ni: 48 to 58%, Co: 8-16%, Cr: 18-25%, Mo: 6-12%, Ti: 0.05-0.8%, Al: 0.05-1.6%, B: 0.0001-0 0.01%, N: not more than 0.02% and O: not more than 0.01%, the balance having a chemical composition consisting of Fe and impurities,
Ni-base heat-resistant alloy material hardness of the surface layer of the material, characterized in that 300 or more in HV0.1.

(5) The Ni-base heat-resistant alloy material as described in (4) above, which contains, in mass%, one or more elements selected from the group shown below instead of part of Fe.
First group: Ca: 0.05% or less, Mg: 0.05% or less, REM: 0.1% or less Second group: Cu: 1% or less, W: 1% or less, V: 0.5% or less , Nb: 2% or less

  According to the present invention, even when a thick alloy member having a thickness of 20 mm or more is used, surface defects during hot working can be prevented. Therefore, the Ni-base 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.

1. Chemical composition C: 0.01 to 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 0.01% 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 set to 0.01 to 0.15%. A desirable lower limit of the C content is 0.03%, a more desirable lower limit 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 for the Si content, an extreme reduction is not enough to obtain a deoxidizing effect, and the cleanliness of the alloy is deteriorated, and the effect of improving the corrosion resistance and oxidation resistance at high temperatures is obtained. It becomes difficult to obtain, 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.

  Although there is no particular need to set a lower limit for the Mn content, the extreme reduction does not provide a sufficient deoxidation effect, which 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 0.02%, and the 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 content of P 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. Note that the S content is preferably reduced as much as possible.

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 or Ti to form a fine intermetallic compound phase, and has the effect of increasing the creep strength. In order to obtain a sufficient effect within the range of the Cr content of the present invention, a Ni content of 48% or more is necessary. However, Ni is an expensive element, and if it is contained in a large amount, the cost increases. 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 phase stability. In order to sufficiently obtain this effect, a Co content of 8% or more is necessary. However, since Co is an extremely expensive element, excessive content of Co causes a significant cost increase. 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 above effect within the range of the Ni content of the present invention, 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 sufficiently exhibit this effect, a Mo content of 6% or more is necessary. However, even if Mo is contained excessively, the effect is saturated, and on the contrary, a coarse precipitate phase is generated, and the creep strength is lowered. Furthermore, since Mo is an effective element, if it is excessively contained, the cost increases. Therefore, an upper limit is provided so that the Mo content is 6 to 12%. 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, contributing 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 compound phases are precipitated, leading to a decrease in creep ductility and toughness. Therefore, an upper limit is provided so that the Ti content is 0.05 to 0.8%. A desirable lower limit of the Ti content is 0.07%, and a more desirable lower limit is 0.1%. Moreover, the upper limit with preferable Ti content is 0.7%, and a more preferable 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. Al is an element having a deoxidizing action. In order to obtain the effect, an Al content of 0.05% or more is necessary. However, if the Al content is excessive, a large amount of intermetallic compound phases 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 provided so that the Al content is 0.05 to 1.6%. 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, the B content needs to be 0.0001% or more. 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, more desirably 0.015% or less.

  Although it is not necessary to set a lower limit in particular for the N content, if it is extremely reduced, it becomes difficult to obtain the effect of stabilizing austenite, and the manufacturing cost also greatly increases. 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 Ni-base heat-resistant alloy member of the present invention has a chemical composition containing the above-mentioned elements, 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.

In addition to the above elements, the Ni-base heat-resistant alloy member of the present invention may further contain one or more elements selected from the following first group and second group.
First group: Ca: 0.05% or less, Mg: 0.05% or less, REM: 0.1% or less Second group: Cu: 1% or less, W: 1% or less, V: 0.5% or less , Nb: 2% or less

  Below, the effect of said element and the reason for limitation of content are demonstrated.

  The first group of Ca, Mg, and REM are all elements that have the effect of improving hot workability. Therefore, these elements may be included.

Ca: 0.05% or less Ca has an effect of improving hot workability. Specifically, Ca is an element having an effect of improving hot workability by generating CaS and suppressing grain boundary segregation of S. For this reason, you may contain Ca. However, when the content of Ca is excessive, it combines with O to remarkably reduce cleanliness, and on the contrary, deteriorate hot workability. For this reason, when it contains Ca, the content shall be 0.05% or less. The upper limit of the Ca content is desirably 0.03%.

  On the other hand, the effect of Ca described above can be stably obtained when the Ca content is 0.0001% or more. When Ca is contained, the amount of Ca is more preferably 0.0005% or more.

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, when it contains Mg, the content shall be 0.05% or less. The upper limit of the Mg content is desirably 0.03%.

  On the other hand, the effect of Mg described above can be stably obtained when the Mg content is 0.0001% or more. When Mg is contained, the amount of Mg is more preferably 0.0005% or more.

REM: 0.1% or less 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. For this reason, you may contain REM. However, when the content of REM becomes excessive, it combines with O to remarkably reduce cleanliness, and on the contrary, deteriorate hot workability. For this reason, when it contains REM, the content shall be 0.1% or less. The upper limit of the REM content is preferably 0.09%, more preferably 0.08%.

  On the other hand, the effect of the REM described above can be stably obtained when the REM content is 0.001% or more. When it is contained, the amount of REM is more preferably 0.005% or more, and further preferably 0.008% or more.

  “REM” is a generic name for a total of 17 elements of Sc, Y, and lanthanoid, and the content of REM refers to the total content of one or more elements of REM. Further, REM is generally contained in misch metal. For this reason, for example, it may be added in the form of misch metal and contained so that the amount of REM falls within the above range.

  Said Ca, Mg, and REM can be contained only in one of them, or 2 or more types of composites. The total amount when these elements are contained in combination may be 0.2%.

  Cu, W, V, and Nb in the second group are all elements that have the effect of improving the creep strength. Therefore, these elements may be included.

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. When Cu is contained, the amount of Cu is more preferably 0.03% or more.

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 included. However, when W is contained excessively, the stability of austenite is lowered, and on the contrary, the creep strength may be lowered. Furthermore, since W is an expensive element, if it is excessively contained, the cost increases. For this reason, when it contains W, the content shall be 1% or less. The upper limit of the W content is desirably 0.8%.

  On the other hand, the effect of W described above is stably obtained when the W content is 0.01% or more. When W is contained, the amount of W is more preferably 0.03% 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. When V is contained, the amount of V is more preferably 0.02% or more.

Nb: 2% or less Nb combines with C or N in the same manner as V and precipitates in the grains as fine carbides or carbonitrides, contributing to the improvement of creep strength at high temperatures. Therefore, Nb may be included. 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 made 2% or less. The upper limit of the Nb content is desirably 1.8%.

  On the other hand, the effect of Nb described above can be stably obtained when the Nb content is 0.01% or more. When Nb is contained, the amount of Nb is more preferably 0.05% or more.

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

2. Average grain size of member The Ni-based heat-resistant alloy member of the present invention has a metal structure composed of recrystallized grains having an average grain size of ASTM grain size number 7 or more in the surface layer, and the central part of the member has an average grain size. It has a metal structure consisting of coarse crystal grains with ASTM grain size number 4 or less.

  As described above, in the present invention, the “surface layer” refers to a region having a depth of up to 50 μm from the member surface. Moreover, the average crystal grain size in the central part of the thickness of the member is the average crystal grain size in a region excluding each surface side that is 25% of the member thickness.

  In order to prevent surface defects during hot working, the surface layer has a finer metal structure, and the upper limit of the average grain size is not particularly defined. However, it is not only technically difficult to obtain an excessively fine grain structure, but also causes an increase in manufacturing cost.

  In order to obtain excellent creep characteristics, the average crystal grain size at the central portion of the member is preferably ASTM grain size number 3 or less, and more preferably 2 or less. On the other hand, if the metal structure in the central portion of the member is excessively coarse, the creep ductility may be deteriorated and the impact value may be reduced. Therefore, the average grain size in the central portion of the member is determined by the ASTM grain size number. It is preferably -2 or more, more preferably -1 or more.

  The average grain size of the surface layer and the central part of the thickness of the member can be determined by the following procedure. After cutting so that the cross-section of the member becomes the test surface, mirror polishing, corroded with aqua regia, observed with a three-field optical microscope at a magnification of 100 times, and measured the average grain slice length by the cutting method The average grain size is obtained by multiplying the average grain section length by 1.128. Furthermore, it converts into a crystal grain size by JIS G 0551-02 (2009).

  Note that the average crystal grain size at the central portion of the thickness of the member can be adjusted by managing the temperature and time during the solution heat treatment of the member. As conditions for solution heat treatment, it is preferable to hold for 0.1 to 5 hours in a temperature range of 1000 to 1280 ° C. The temperature range of the heat treatment is more preferably 1100 to 1250 ° C., and the holding time is more preferably 0.2 to 1.5 h.

  In addition, the average grain size of the surface layer is the surface of the member subjected to the above solution heat treatment, cutting with a tool, polishing, shot peening using laser or sand blasting, cold rolling with a roll or a hydraulic press, or cold It is possible to control by subjecting the surface part to mechanical processing and then recrystallizing it by heat treatment. As conditions for heat treatment for recrystallization, it is preferable to hold for 0.1 to 3 hours in a temperature range of 980 to 1180 ° C. The temperature range of the heat treatment is more preferably 1000 to 1150 ° C., and the holding time is more preferably 0.5 to 1.5 h.

  Regarding the hardness of the surface layer, it is preferable that HV0.1 in the surface layer of the material after performing the above-described strong processing and before the recrystallization heat treatment is 300 or more, and more preferably 350 or more. This is because the average crystal grain size of the member surface layer after the recrystallization heat treatment does not become the ASTM grain size number 7 or more when the HV0.1 of the material surface layer after strong processing is less than 300. And as for the hardness of the surface layer of the member after recrystallization heat processing, it is preferable that HV0.1 is 270 or less, and it is more preferable that it is 220 or less. This is because if the hardness of the surface layer of the member after the recrystallization heat treatment exceeds 270, the deformation resistance in the grains is high and the grain boundary fracture is promoted.

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

In the hardness of the surface layer of the Ni-base heat-resistant alloy material according to the present invention, HV0.1 of 300 or more corresponds to a dislocation density of the surface layer of 2.0 × 10 ¹⁴ / m ² or more. That is, it is preferable that the dislocation density in the surface layer of the raw material after the strong processing and before the recrystallization heat treatment is 2.0 × 10 ¹⁴ / m ² or more.

  The “dislocation density” is the Lorentz function of {111}, {200}, {220} and {311} planes from the X-ray diffraction data obtained by measuring the sample surface by XRD using a Co tube. The angle, half width, and diffraction intensity of the diffraction peak are obtained by approximation, and can be calculated from the modified Williamson-Hall equation and the modified Warren-Averbach equation.

3. Crack Evaluation Method The Ni-base heat-resistant alloy member of the present invention can prevent surface defects during hot working. On the other hand, it goes without saying that it is most desirable not to cause a micro crack inside the member, but even if it occurs, if the crack depth is small, it is unlikely to lead to a serious accident in the actual plant. It is not a big problem.

Therefore, the Ni-base heat-resistant alloy member of the present invention was subjected to a tensile test at 1100 ° C. at a strain rate of 0.0001 s ⁻¹ on a test piece including a surface layer of a member made of recrystallized grains, and the elongation was 10%. It is desirable that the crack depth inside the test piece is 20 μm or less.

  The test piece used in the above test includes the surface layer of at least one surface of the member, and may include both surfaces. Moreover, about a shape, the plate-shaped test piece or rod-shaped test piece which a cross section becomes a rectangle or a square prescribed | regulated to JISZ2241 (2011) can be used. At this time, it is desirable to produce the test piece so that the member surface is included in at least one surface of the test piece.

A tensile test at a low strain rate simulating hot working is performed using the above test piece. Specifically, using a greeble tester, the above test piece was subjected to a tensile test at a processing temperature of 1100 ° C. and a low strain rate of 0.0001 s ⁻¹ , and the elongation (strain amount) became 10%. At that time, the tensile test is interrupted, and the surface of the test piece and internal cracks are confirmed using the test piece after the tensile test is interrupted.

  The greeble test is a tensile test performed while energizing and heating the center part of the test piece. The processing temperature is measured by welding a thermocouple to the center of the greeble test piece.

  The presence or absence of cracks on the surface of the test piece after the suspension of the tensile test is determined by the penetrant flaw test defined in JIS Z 2343-1 (2001). Also, the presence or absence of cracks inside the test piece is confirmed by observing the center of the test piece after interruption of the tensile test so that the cross section becomes the test surface, mirror-polishing, and observing with an optical microscope at a magnification of 100 times I will do it.

  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. Using the above ingot, hot forging and rolling, and solution heat treatment under the conditions shown in Table 2 were performed to produce a plurality of alloy plates having a thickness of 25 mm, a width of 100 mm, and a length of 500 mm. Thereafter, the alloy plate was subjected to surface cutting with a cutting bite and further subjected to heat treatment under the conditions shown in Table 2 to form a recrystallized layer on the surface portion of the alloy plate.

  From each alloy plate obtained as described above, a test piece for determining the average crystal grain size was cut out and mirror-polished so that the cross section was the test surface. After that, for each test piece, the average grain size of each of the regions from the surface of the alloy plate subjected to surface processing as a surface layer to a depth of 50 μm and the region excluding each surface side that becomes 25% of the thickness of the alloy plate Was determined by the following method.

  The above specimen is mirror-polished and corroded with aqua regia, and then observed with an optical microscope at a magnification of 100 times for each of the three visual fields of the surface layer and the central part of the member, and the average grain slice length is determined by a cutting method. The average grain size was determined by multiplying the average grain section length by 1.128. Furthermore, it converted into the crystal grain size according to JIS G 0551-02 (2009).

  Furthermore, about each said alloy plate, the test piece for using for a tensile test was cut out. The test piece for the tensile test is machined by a plurality of square bar-like test pieces each having a length of 10 mm and a length of 130 mm parallel to the longitudinal direction of the alloy plate so that the surface of the alloy plate subjected to surface processing is included. Produced.

Using the above test piece, a tensile test was performed at a low strain rate of 1100 ° C. and a strain rate of 0.0001 s ⁻¹ . Then, when the elongation (strain amount) reached 10%, the tensile test was interrupted, and the test piece surface portion and internal cracks were examined using the test piece after the tensile test was interrupted.

  As described above, the presence or absence of cracks on the surface of the test piece after interruption of the tensile test is conducted by the penetrant flaw test specified in JIS Z 2343-1 (2001). The sample was cut so that the cross section was the test surface, mirror-polished, and then examined by observation with an optical microscope at a magnification of 100 times.

  In addition, a round bar creep rupture test piece having a diameter of 6 mm and a gauge distance of 30 mm was sampled from the center of the thickness of each alloy plate and subjected to a creep rupture test at 700 ° C. and 150 MPa. In addition, the thing whose creep rupture time becomes 1000 h or more was set as the pass, and the thing below 1000 h was set as the rejection.

  The average crystal grain size in the surface layer and the central part of the thickness of the member and the test results are shown in Table 2. As for surface cracks, “O” indicates that no cracks occurred, and “X” indicates that cracks occurred. As for the internal crack, “◯” indicates that no crack occurred, and “Δ” indicates that the crack was generated but the depth was 20 μm or less, and a crack having a depth exceeding 20 μm occurred. The thing was made into "x".

  In the comprehensive evaluation of Table 2, the case where the creep rupture time was long and no cracks were observed on both the surface and the inside of the tensile test piece, which was an evaluation of crack resistance during hot working, was determined as “good”. Moreover, although the creep rupture strength was long and no crack was observed on the surface of the test piece in the penetrant flaw detection test, the case where a micro crack with a depth of 20 μm or less was found inside the test piece was determined as “OK”. And when the creep rupture time was short, the case where the crack was recognized on the surface of the tensile test piece, or the case where the crack of the depth exceeding 20 micrometers generate | occur | produced inside the tensile test piece was made "impossible".

  As shown in Table 2, test numbers 2 to 4, 8 to 10, 14 to 16, and 20 to 22 in which the average crystal grain size in the surface layer and the central portion of the member is within the range defined by the present invention are comprehensive evaluations. Was “good” or “possible”.

  On the other hand, Test Nos. 1, 7, 13 and 19 had a large average crystal grain size at the center of the thickness of the member, so that the creep rupture time was short and the creep strength was inferior. Test numbers 5, 11, 17 and 23 resulted in surface defects during hot working because the average grain size of the surface layer was small. Furthermore, test Nos. 6, 12, 18 and 24 show that the hardness of the material surface layer after strong processing is low, so the average grain size of the member surface layer after recrystallization heat treatment is small, and surface defects occur during hot working. became.

  According to the present invention, even when a thick alloy member having a thickness of 20 mm or more is used, surface defects during hot working can be prevented. Therefore, the Ni-base 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 (5)

A Ni-based heat-resistant alloy member having a thickness of 20 mm or more,
In mass%, C: 0.01 to 0.15%, Si: 1% or less, Mn: 2% or less, P: 0.03% or less, S: 0.01% or less, Ni: 48 to 58%, Co: 8-16%, Cr: 18-25%, Mo: 6-12%, Ti: 0.05-0.8%, Al: 0.05-1.6%, B: 0.0001-0 0.01%, N: not more than 0.02% and O: not more than 0.01%, the balance having a chemical composition consisting of Fe and impurities,
The surface layer has a metal structure composed of recrystallized grains having an ASTM grain size number of 7 or more,
A Ni-based heat-resistant alloy member, characterized in that the central portion of the member has a metal structure composed of crystal grains having ASTM grain size number 4 or less. The Ni-base heat-resistant alloy member according to claim 1, further comprising at least one element selected from the group shown below in mass% instead of part of Fe.
First group: Ca: 0.05% or less, Mg: 0.05% or less, REM: 0.1% or less Second group: Cu: 1% or less, W: 1% or less, V: 0.5% or less , Nb: 2% or less The test piece including the surface layer is subjected to a tensile test at a strain rate of 0.0001 s ⁻¹ at 1100 ° C., and the crack depth inside the test piece when the elongation becomes 10% is 20 μm or less (including 0). The Ni-base heat-resistant alloy member according to claim 1 or 2, characterized in that Table layer has a metallic structure consisting of ASTM grain size number # 7 or more recrystallized grains,
The thickness of the member central portion an Ni-base heat-resistant alloy material which need use in preparing the Ni-base heat-resistant alloy member having a metallic structure consisting of ASTM grain size number # 4 following grain,
The thickness is 20 mm or more,
In mass%, C: 0.01 to 0.15%, Si: 1% or less, Mn: 2% or less, P: 0.03% or less, S: 0.01% or less, Ni: 48 to 58%, Co: 8-16%, Cr: 18-25%, Mo: 6-12%, Ti: 0.05-0.8%, Al: 0.05-1.6%, B: 0.0001-0 0.01%, N: not more than 0.02% and O: not more than 0.01%, the balance having a chemical composition consisting of Fe and impurities,
Ni-base heat-resistant alloy material hardness of the surface layer of the material, characterized in that 300 or more in HV0.1. The Ni-base heat-resistant alloy material according to claim 4, wherein the Ni-base heat-resistant alloy material further contains at least one element selected from the group shown below by mass% instead of a part of Fe.
First group: Ca: 0.05% or less, Mg: 0.05% or less, REM: 0.1% or less Second group: Cu: 1% or less, W: 1% or less, V: 0.5% or less , Nb: 2% or less