WO2018151222A1

WO2018151222A1 - Ni-BASED HEAT-RESISTANT ALLOY AND METHOD FOR MANUFACTURING SAME

Info

Publication number: WO2018151222A1
Application number: PCT/JP2018/005298
Authority: WO
Inventors: 仙波　潤之; 友彰浜口; 伸之佑栗原
Original assignee: 新日鐵住金株式会社
Priority date: 2017-02-15
Filing date: 2018-02-15
Publication date: 2018-08-23
Also published as: JP6819700B2; JPWO2018151222A1; CN110291216A; US20200010931A1; EP3584335A1; EP3584335A4; CA3053741A1; KR20190117605A

Abstract

This Ni-based heat-resistant alloy contains predetermined amounts of C, Si, Mn, P, S, N, O, Ni, Co, Cr, Mo, W, B, Al, Ti, Nb, REM, Mg, and Ca, the remainder being Fe and impurities, the expressions [0.1 ≤ Mo + W ≤ 12.0], [1.0 ≤ 4 × Al + 2 × Ti + Nb ≤ 12.0], and [P + 0.2 × Cr × B < 0.035] being satisfied, the shortest distance from a center part to an outer surface part in a cross-section perpendicular to the longitudinal direction of an alloy member being 40 mm or greater, the austenite crystal grain size in the outer surface part being -2.0 to 4.0, the total content of Al, Ti, and Nb present as precipitates obtained by extraction residue analysis satisfying the expression [(Al + Ti + Nb)PB/(Al + Ti + Nb)PS ≤ 10.0], and the expressions [YSS/YSB ≤ 1.5] and [TSS/TSB ≤ 1.2] being satisfied at normal temperature.

Description

Ni-base heat-resistant alloy and method for producing the same

The present invention relates to a Ni-base heat-resistant alloy and a method for producing the same.

In recent years, in order to increase efficiency, new super-critical pressure boilers with increased steam temperature and pressure are being developed all over the world. These ultra-supercritical boilers are also planned to raise the steam temperature, which was conventionally around 600 ° C., to 650 ° C. or more, and further to 700 ° C. or more, and technical development is underway in Japan and overseas.

This is based on the fact that energy conservation, effective utilization of resources, and reduction of CO ₂ gas emissions for environmental conservation are one of the challenges for solving energy problems and are important industrial policies. In the case of a power generation boiler for burning fossil fuel, a reaction furnace for chemical industry, and the like, a highly efficient ultra super critical pressure boiler and reaction furnace are advantageous.

Steam high-temperature high-pressure increases the temperature of boiler superheater tubes and chemical reactor reaction furnace tubes, and thick plates and forgings as heat and pressure resistant members during actual operation to 700 ° C or higher. Therefore, alloys that are used for a long time in such harsh environments are required to have not only high-temperature strength and high-temperature corrosion resistance, but also good long-term microstructure stability, creep rupture ductility, and creep fatigue resistance. The

For the above strict requirements, Fe-based alloys such as austenitic stainless steel have insufficient creep rupture strength. For this reason, it is essential to use a Ni-based alloy utilizing precipitation such as γ ′ phase. Furthermore, since it is unavoidable that the steel pipe for a boiler / chemical industry is welded, it is required to have excellent weldability.

In response to the above strict requirements, for example, Patent Document 1 discloses an austenitic heat-resistant alloy that is excellent in both weld crack resistance and toughness of HAZ and also excellent in creep strength at high temperatures.

Japanese Patent No. 4697357

By the way, large structural members such as materials for equipment such as boilers and chemical plants are used after being subjected to final heat treatment without hot working after hot rolling or hot forging. Is relatively large. Therefore, there is a problem that the 0.2% proof stress and the tensile strength at normal temperature, which are normally specified as the material specifications, are lower than those subjected to the final heat treatment after cold working.

In addition, in large structural members, the cooling rate during heat treatment varies greatly depending on the site, so the amount of solid solution elements that contribute to strengthening as precipitates when used at high temperatures varies depending on the site. As a result, there is also a problem that variation in creep rupture strength occurs. Therefore, it is difficult to apply the alloy described in Patent Document 1 to a large structural member.

The present invention solves the above problems, and provides a Ni-base heat-resistant alloy that exhibits 0.2% proof stress and tensile strength at room temperature sufficient as a large structural member, and creep rupture strength at high temperature, and a method for producing the same. The purpose is to provide.

The present invention has been made in order to solve the above-mentioned problems, and provides the following Ni-base heat-resistant alloy and its manufacturing method.

(1) The chemical composition of the alloy is mass%,
C: 0.005 to 0.15%,
Si: 2.0% or less,
Mn: 3.0% or less,
P: 0.030% or less,
S: 0.010% or less,
N: 0.030% or less,
O: 0.030% or less,
Ni: 40.0-60.0%,
Co: 0.01-25.0%,
Cr: 15.0% or more and less than 28.0%,
Mo: 12.0% or less,
W: less than 4.0%,
B: 0.0005 to 0.006%,
Al: 0 to 3.0%,
Ti: 0 to 3.0%,
Nb: 0 to 3.0%,
REM: 0 to 0.1%,
Mg: 0 to 0.02%,
Ca: 0 to 0.02%,
Balance: Fe and impurities,
Satisfying the following formulas (i) to (iii):
In the cross section perpendicular to the longitudinal direction of the alloy, the shortest distance from the center portion to the outer surface portion is 40 mm or more,
The austenite grain size number in the outer surface portion is -2.0 to 4.0,
The total content of Al, Ti and Nb present as precipitates obtained by extraction residue analysis satisfies the following formula (iv):
Mechanical properties at room temperature satisfy the following formulas (v) and (vi):
Ni-base heat-resistant alloy.
0.1 ≦ Mo + W ≦ 12.0 (i)
1.0 ≦ 4 × Al + 2 × Ti + Nb ≦ 12.0 (ii)
P + 0.2 × Cr × B <0.035 (iii)
(Al + Ti + Nb) _PB / (Al + Ti + Nb) _PS ≦ 10.0 (iv)
YS _S / YS _B ≦ 1.5 (v)
TS _S / TS _B ≦ 1.2 (vi)
However, the element symbols in the above formulas (i) to (iii) represent the content (% by mass) of each element, and the meaning of each symbol in the above formulas (iv) to (vi) is as follows.
(Al + Ti + Nb) _PB : Total content of Al, Ti and Nb present as precipitates obtained by extraction residue analysis in the central portion (Al + Ti + Nb) _PS : Al, Ti present as precipitates obtained by extraction residue analysis in the outer surface portion And Nb total content YS _B : 0.2% yield strength at the center YS _S : 0.2% yield strength at the outer surface TS _B : Tensile strength at the center TS _S : Tensile strength at the outer surface

(2) The chemical composition is mass%,
Mg: 0.0001 to 0.02%, and
Ca: 0.0001 to 0.02%,
Containing one or two selected from
The Ni-base heat-resistant alloy as described in (1) above.

(3) The 10,000-hour creep rupture strength at 700 ° C. in the longitudinal direction in the central portion is 150 MPa or more.
The Ni-base heat-resistant alloy as described in (1) or (2) above.

(4) Hot-working the steel ingot or slab having the chemical composition described in (1) or (2) above;
Thereafter, heating to a heat treatment temperature T (° C.) in the range of 1070 to 1220 ° C., holding 1150 D / T to 1500 D / T (min), and then performing a heat treatment of cooling with water,
Manufacturing method of Ni-base heat-resistant alloy.
However, D is the maximum value (mm) of the linear distance between an arbitrary point on the outer edge of the cross section and another arbitrary point on the outer edge in a cross section perpendicular to the longitudinal direction of the alloy.

(5) In the step of performing the hot working, the hot working is performed at least once in a direction substantially perpendicular to the longitudinal direction of the hot working.
The manufacturing method of the Ni-base heat-resistant alloy as described in said (4).

The Ni-based heat-resistant alloy of the present invention has little variation in mechanical properties depending on the part, and is excellent in creep rupture strength at high temperatures.

Hereinafter, each requirement of the present invention will be described in detail.

1. Chemical composition The reasons for limiting each element are as follows. In the following description, “%” for the content means “% by mass”.

C: 0.005 to 0.15%
C stabilizes the austenite structure, forms fine carbides at the grain boundaries, and improves the creep strength at high temperatures. Therefore, the C content needs to be 0.005% or more. However, when the content becomes excessive, the carbide becomes coarse and precipitates in a large amount, thereby lowering the ductility of the grain boundary, leading to a reduction in toughness and creep strength. Therefore, the C content is 0.15% or less. The C content is preferably 0.01% or more. Further, the C content is preferably 0.12% or less, and more preferably 0.10% or less.

Si: 2.0% or less Si is contained as a deoxidizing element. Si is an element effective for improving the corrosion resistance and oxidation resistance at high temperatures. However, if the Si content exceeds 2.0%, the stability of the austenite phase decreases, leading to a decrease in toughness and creep strength. Therefore, the Si content is 2.0% or less. The Si content is preferably 1.5% or less, and more preferably 1.0% or less. In addition, although it is not necessary to provide a minimum in particular about Si content, extreme reduction will not obtain a sufficient deoxidation effect, will deteriorate the cleanliness of an alloy, and will raise the manufacturing cost. Therefore, the Si content is preferably 0.02% or more, and more preferably 0.10% or more.

Mn: 3.0% or less Mn is an element that has a deoxidizing action like Si and contributes to stabilization of austenite. However, if the Mn content exceeds 3.0%, embrittlement is caused and the toughness and creep ductility are lowered. Therefore, the Mn content is 3.0% or less. The Mn content is preferably 2.5% or less, more preferably 2.0% or less, and even more preferably 1.5% or less. Although it is not necessary to provide a lower limit for the Mn content, an extreme decrease results in an insufficient deoxidation effect and deteriorates the cleanliness of the alloy, and increases the manufacturing cost. Therefore, the Mn content is preferably 0.02% or more, more preferably 0.10% or more, and further preferably 0.15% or more.

P: 0.030% or less P is an element contained in the alloy as an impurity, but segregates at the grain boundaries of HAZ during welding to increase liquefaction cracking sensitivity and adversely affect toughness after long-term use. is there. Therefore, although it is preferable to reduce as much as possible, extreme reduction leads to an increase in steelmaking cost. Therefore, the P content is 0.030% or less, and preferably 0.020% or less.

S: 0.010% or less S is an element contained in the alloy as an impurity, but segregates at the grain boundaries of HAZ during welding to increase liquefaction cracking sensitivity and adversely affect toughness after long-term use. is there. Therefore, although it is preferable to reduce as much as possible, extreme reduction leads to an increase in steelmaking cost. Therefore, the S content is 0.010% or less, and preferably 0.005% or less.

N: 0.030% or less N is an element effective for stabilizing the austenite phase. However, in the range of the Cr content of the present invention, a large amount of fine nitriding is required during use at high temperatures if contained in excess. The substance is precipitated in the grains, resulting in a decrease in creep ductility or toughness. Therefore, the N content is 0.030% or less, preferably 0.020% or less, and more preferably 0.015% or less. In addition, although there is no need to provide a lower limit in particular for the N content, an extreme reduction causes an increase in manufacturing cost. Therefore, the N content is preferably 0.0005% or more, more preferably 0.001% or more, and further preferably 0.005% or more.

O: 0.030% or less O is contained in the alloy as an impurity, but if it is excessively contained, it causes a decrease in hot workability, toughness and ductility. Therefore, the O content is 0.030% or less, preferably 0.020% or less, more preferably 0.010% or less, and still more preferably 0.005% or less. In addition, although it is not necessary to set a minimum in particular about content of O, an extreme fall invites the raise of manufacturing cost. Therefore, the O content is preferably 0.001% or more.

Ni: 40.0-60.0%
Ni is an effective element for obtaining an austenite structure, and is an essential element for ensuring the structural stability after long-term use. Further, Ni combines with Al, Ti, and Nb to form a fine intermetallic compound phase, and also has an effect of increasing creep strength. In order to sufficiently obtain the above Ni effect within the Cr content range of the present invention, the Ni content needs to be 40.0% or more. However, since Ni is an expensive element, if its content exceeds 60.0%, the cost increases. Therefore, the Ni content is 40.0 to 60.0%. The Ni content is preferably 42.0% or more, more preferably 45.0% or more, further preferably 48.0% or more, and preferably 58.0% or less.

Co: 0.01-25.0%
Co, like Ni, is an austenite-forming element and contributes to the improvement of creep strength by increasing the stability of the austenite phase. In order to obtain this effect, the Co content needs to be 0.01% or more. However, since Co is an extremely expensive element, if its content exceeds 25.0%, a significant cost increase is caused. Therefore, the Co content is 0.01-25.0%. The Co content is preferably 0.1% or more, more preferably 2.0% or more, and even more preferably 8.0% or more. Further, the Co content is preferably 23.0% or less, more preferably 21.0% or less.

Cr: 15.0% or more and less than 28.0% Cr is an essential element for ensuring oxidation resistance and corrosion resistance at high temperatures. In order to obtain the above Cr effect within the Ni content range of the present invention, the Cr content needs to be 15.0% or more. However, when the Cr content is 28.0% or more, the stability of the austenite phase at a high temperature is deteriorated, and the creep strength is lowered. Therefore, the Cr content is 15.0% or more and less than 28.0%. The Cr content is preferably 17.0% or more, and more preferably 19.0% or more. Moreover, it is preferable that Cr content is 26.0% or less, and it is more preferable that it is 24.0% or less.

Mo: 12.0% or less W: less than 4.0% Both Mo and W are elements that contribute to improvement in creep strength at high temperatures by dissolving in the austenite structure as a matrix. In order to obtain this effect, it is necessary to contain one or both of Mo and W. However, when the content of these elements is excessive, the stability of the austenite phase is decreased, and the creep strength is decreased. Therefore, the Mo content is 12.0% or less. The Mo content is preferably 10.0% or less.

In addition, since W has a larger atomic weight than Mo, it needs to be contained in a larger amount in order to obtain the same effect as Mo, which is disadvantageous from the viewpoint of ensuring cost and phase stability. For this reason, the W content is less than 4.0%. It is not necessary to contain Mo and W in combination. When Mo or W is contained alone, the content is preferably 0.1% or more.

B: 0.0005 to 0.006%
B is an element necessary for improving the creep strength by segregating at the grain boundary in use to strengthen the grain boundary and finely dispersing the grain boundary carbide. In addition, it has the effect of segregating at the grain boundaries to improve the fixing force and contribute to toughness improvement. In order to obtain these effects, the B content needs to be 0.0005% or more. However, when the B content increases and exceeds 0.006% in particular, a large amount of segregation occurs in the high-temperature HAZ near the melting boundary due to the welding heat cycle during welding, and overlaps with P to lower the melting point of the grain boundary. Enhances liquefaction cracking sensitivity of HAZ. Therefore, the B content is 0.0005 to 0.006%. The B content is preferably 0.001% or more, and preferably 0.005% or less.

Al: 0 to 3.0%
Ti: 0 to 3.0%
Nb: 0 to 3.0%
Al, Ti, and Nb are all elements that improve creep strength at high temperatures by binding to Ni and finely precipitating in the grains as intermetallic compounds. However, if the content is too large and exceeds 3.0% for any of the elements, the above effects are saturated and creep ductility and toughness after prolonged heating are reduced. Therefore, the content of each of Al, Ti, and Nb is set to 3.0% or less. The content of these elements is preferably 2.8% or less, and more preferably 2.5% or less.

REM: 0 to 0.1%
Rare earth elements (REM) have a strong affinity with P, have a high melting point, and form a compound with P that is stable up to high temperatures, thereby fixing P and removing the adverse effects of P on liquefaction cracking and toughness of HAZ. . Moreover, it is an element which precipitates as a carbide | carbonized_material and contributes also to the improvement of high temperature strength. Therefore, you may make it contain as needed. However, if the content of REM becomes excessive and exceeds 0.1%, the effect of reducing the adverse effect of P is saturated, and in addition, a large amount of carbide precipitates, leading to a decrease in toughness. Therefore, the REM content is 0.1% or less. The REM content is preferably 0.08% or less, and more preferably 0.06% or less. In order to obtain the above effect, the REM content is preferably 0.001% or more, more preferably 0.005% or more, and further preferably 0.01% or more.

REM refers to a total of 17 elements of Sc, Y and lanthanoid, and the content of REM means the total content of these elements.

Mg: 0 to 0.02%
Mg has a strong affinity with S and has an effect of increasing hot workability, and also has an effect of reducing both the occurrence of liquefaction cracking of HAZ and a decrease in toughness due to S. Therefore, you may make it contain as needed. However, excessive addition of Mg causes a decrease in cleanliness due to bonding with oxygen. In particular, when the content exceeds 0.02%, the cleanliness decreases significantly, and the hot workability is deteriorated. Therefore, the Mg content is 0.02% or less. The Mg content is preferably 0.01% or less. On the other hand, in order to obtain the above effect, the Mg content is preferably 0.0001% or more, more preferably 0.0005% or more, and further preferably 0.001% or more.

Ca: 0 to 0.02%
Ca has a strong affinity with S and has an effect of improving hot workability, and also has an effect of reducing both the occurrence of liquefaction cracking of HAZ and a decrease in toughness due to S. Therefore, you may make it contain as needed. However, excessive addition of Ca leads to a decrease in cleanliness due to bonding with oxygen. In particular, when the content exceeds 0.02%, the cleanliness decreases remarkably, and hot workability is deteriorated. Therefore, the Ca content is 0.02% or less. The Ca content is preferably 0.01% or less. On the other hand, in order to obtain the above effect, the Ca content is preferably 0.0001% or more, more preferably 0.0005% or more, and further preferably 0.001% or more.

The alloy according to the present invention must satisfy the following formulas (i) to (iii) in addition to the content of each element being in the above range. The element symbols in the following formulas (i) to (iii) represent the content (% by mass) of each element.

0.1 ≦ Mo + W ≦ 12.0 (i)
As described above, both Mo and W are elements that contribute to the improvement of creep strength at high temperatures by dissolving in the austenite structure that is a matrix. On the other hand, when the content of these elements is excessive, On the other hand, the stability of the austenite phase is lowered and the creep strength is lowered. Therefore, the total content of Mo and W needs to satisfy the above formula (i). The middle value of the formula (i) is preferably 1.0 or more, and preferably 10.0 or less.

1.0 ≦ 4 × Al + 2 × Ti + Nb ≦ 12.0 (ii)
In order to ensure good creep strength at high temperatures and toughness after prolonged heating by finely precipitating intermetallic compounds bonded to Ni, one kind selected from Al, Ti and Nb While containing the above, the content needs to satisfy said (ii) Formula. The middle value of the formula (ii) is preferably 3.0 or more, and preferably 11.0 or less.

P + 0.2 × Cr × B <0.035 (iii)
P and B are elements that segregate at the grain boundaries of the HAZ in the vicinity of the melting boundary during the heat cycle during welding, thereby lowering the melting point and increasing the susceptibility to liquefaction cracking of the HAZ. On the other hand, during a long period of use, P segregated at the grain boundaries decreases the fixing force of the grain boundaries, whereas B conversely strengthens the grain boundaries, so P adversely affects toughness and B is reversed. To reduce toughness. Further, Cr is an element that affects the grain boundary segregation behavior of P and B, and indirectly affects their performance. That is, the degree of influence of B on liquefaction cracking of HAZ becomes more prominent as the Cr content increases. Moreover, about the toughness of HAZ after long-time use, although the bad influence of P is large, when it contains substantially the same amount of P and B, there exists a tendency for the fall of toughness to become large, so that there is little Cr content.

In order to control the grain boundary segregation of P and B in HAZ and to reduce the resistance to liquefaction cracking and toughness reduction after long-time heating, it is necessary to satisfy the above formula (iii). The value on the left side of the formula (iii) is preferably 0.030 or less. The lower limit of the left side value of the formula (iii) is not particularly limited, but the content of P as an impurity is extremely low, and is close to 0.0015 when Cr is 15.0% and B is 0.0005%. It may be a value.

In the chemical composition of the Ni-base heat-resistant alloy of the present invention, the balance is Fe and impurities. Here, “impurities” are components mixed in due to various factors of raw materials such as ores and scraps and manufacturing processes when the alloy is industrially manufactured, and are allowed within a range that does not adversely affect the present invention. Means something.

2. Grain size Austenite grain size number in the outer surface: -2.0 to 4.0
If the austenite grain size in the outer surface portion is too coarse, the 0.2% proof stress and tensile strength at room temperature will be low, while if too fine, it will not be possible to maintain high creep rupture strength at high temperatures. Therefore, the austenite grain size number in the outer surface portion is set to -2.0 to 4.0.

In the present invention, the crystal grain size number is determined by the intersecting line segment (grain size) defined in JIS G 0551 (2013). In addition, in the manufacturing process of the Ni-base alloy, the crystal grain size number of the outer surface portion after the final heat treatment can be set to the above range by appropriately adjusting the heat treatment temperature and holding time after the hot working and the cooling method. .

3. Dimensions Shortest distance from the center to the outer surface: 40 mm or more As described above, in a large structural member, in addition to the 0.2% proof stress and tensile strength at room temperature, the creep rupture strength varies depending on the part. There is also a problem that occurs. However, the Ni-base heat-resistant alloy according to the present invention exhibits 0.2% proof stress and tensile strength at room temperature sufficient for a large structural member, and creep rupture strength at high temperature. That is, the effect of the present invention is remarkably exhibited for a thick member.

Therefore, in the Ni-base heat-resistant alloy of the present invention, the shortest distance from the center portion to the outer surface portion is set to 40 mm or more in the cross section perpendicular to the longitudinal direction. In order to obtain the effects of the present invention more remarkably, the shortest distance from the center portion to the outer surface portion is preferably 80 mm or more, and more preferably 100 mm or more. Here, the shortest distance from the center portion to the outer surface portion is, for example, a radius of the cross section (mm) when the alloy is cylindrical, and a length (mm) that is half the short side of the cross section when the alloy is a quadrangular prism. It becomes.

The heat-resistant alloy according to the present invention is obtained, for example, by subjecting a steel ingot or a cast piece obtained by continuous casting to hot working such as hot forging or hot rolling, as will be described later. . The longitudinal direction of the heat-resistant alloy is generally the direction connecting the top and bottom portions of the steel ingot when using a steel ingot, and the length direction when using a slab.

4). Precipitation amount of γ ′ phase obtained by extraction residue analysis (Al + Ti + Nb) _PB / (Al + Ti + Nb) _PS ≦ 10.0 (iv)
However, the meaning of each symbol in the formula (iv) is as follows.
(Al + Ti + Nb) _PB : Total content of Al, Ti and Nb present as precipitates obtained by extraction residue analysis in the central portion (Al + Ti + Nb) _PS : Al, Ti present as precipitates obtained by extraction residue analysis in the outer surface portion And Nb total content

In the alloy manufacturing process, an insoluble γ ′ phase (Ni ₃ (Al, Ti, Nb)) is generated mainly in the grains after the heat treatment after hot working. In particular, since the cooling rate is slower at the center of the alloy than at the outer surface, the amount of undissolved γ ′ phase tends to increase. Therefore, the precipitation amount of Al, Ti, and Nb precipitated as γ 'at the central portion with respect to the outer surface portion of the alloy increases, and when the value of (Al + Ti + Nb) _PB / (Al + Ti + Nb) _PS exceeds 10.0, High creep rupture strength cannot be maintained. On the other hand, the lower limit value of (Al + Ti + Nb) _PB / (Al + Ti + Nb) _PS is not required, but is preferably 1.0 or more because the central portion tends to increase the amount of precipitates more than the outer surface portion.

The precipitate obtained by the extraction residue analysis is an insoluble γ ′ phase contained in the alloy. The extraction residue analysis is performed according to the following procedure. First, a test piece for measuring the γ ′ phase is collected from the center portion and the outer surface portion in a cross section perpendicular to the longitudinal direction of the alloy sample. After obtaining the surface area of the above test piece, only the base material of the heat-resistant alloy is completely electrolyzed in an electrolysis condition of 20 mA / cm ^{2 in} a 1% tartaric acid-1% ammonium sulfate aqueous solution. And the solution after electrolysis is filtered with a 0.2 micrometer filter, and deposits are extracted as a residue. Then, the content (mass%) of Al, Ti and Nb contained as an undissolved γ ′ phase is measured by ICP-AES measurement after acid decomposition of the extraction residue, and based on the measured value (Al + Ti + Nb ) _PB / (Al + Ti + Nb) _Determine the value of _PS .

5). Mechanical properties YS _S / YS _B ≦ 1.5 (v)
_{_{TS S / TS B ≦ 1.2 ···}} (vi)
However, the meaning of each symbol in the above formula is as follows.
YS _B : 0.2% yield strength at the center portion YS _S : 0.2% yield strength at the outer surface portion TS _B : Tensile strength at the center portion TS _S : Tensile strength at the outer surface portion

Large structural members tend to have large variations in the mechanical properties of each part due to the fact that the cooling rate during heat treatment varies from part to part. In a large-sized structural member, when the 0.2% proof stress and tensile strength at normal temperature are greatly different between the central portion and the outer surface portion, there is a problem that the specification is not satisfied depending on the part. Therefore, the Ni-base heat-resistant alloy according to the present invention satisfies the above formulas (v) and (vi) in mechanical properties at room temperature. In addition, although it is not necessary to set a lower limit value for each, since the mechanical properties of the central portion tend to be inferior to the mechanical properties of the outer surface portion, both formulas (v) and (vi) are set to 1.0 or more. It is preferable.

The 0.2% proof stress and tensile strength were determined by cutting a round bar tensile test piece with a parallel part length of 40 mm from the center part and outer surface part of the alloy in parallel with the longitudinal direction, and conducting a tensile test at room temperature. It asks by carrying out. In addition, the tensile test is performed according to JIS Z 2241 (2011).

6). Creep rupture strength Since the Ni-base heat-resistant alloy of the present invention is used in a high-temperature environment, a high high-temperature strength, particularly a high creep rupture strength is required. Therefore, the alloy of the present invention needs to have a 10,000 hour creep rupture strength at 700 ° C. in the longitudinal direction of 150 MPa or more at the center.

Creep rupture strength is obtained by the following method. First, a round bar creep rupture test piece having a diameter of 6 mm and a gauge distance of 30 mm described in JIS Z 2241 (2011) is cut out by machining from the center of the alloy in parallel with the longitudinal direction. Then, a creep rupture test is performed in the atmosphere at 700 ° C., 750 ° C., and 800 ° C., and the creep rupture strength at 700 ° C. for 10,000 hours is obtained using the Larson-Miller parameter method. The creep rupture test is performed in accordance with JIS Z 2271 (2010).

7). Manufacturing Method The Ni-base heat-resistant alloy of the present invention is manufactured by subjecting a steel ingot or slab having the above-described chemical composition to hot working. In the hot working step described above, the treatment is performed so that the longitudinal direction of the final shape of the alloy coincides with the longitudinal direction of the steel ingot or slab as the raw material. Although the hot working may be performed only in the longitudinal direction, the hot working is performed once or more in the direction substantially perpendicular to the longitudinal direction in order to provide a higher degree of working and a more homogeneous structure. You may give it. Moreover, you may further give hot processing of different methods, such as hot extrusion, as needed after the said hot processing.

In producing the Ni-base heat-resistant alloy of the present invention, after the above-described steps, the final heat treatment described below is performed in order to suppress the variation in the metal structure and mechanical properties of each part and maintain high creep rupture strength. Apply.

First, the hot-worked alloy is heated to a heat treatment temperature T (° C.) in the range of 1070 to 1220 ° C., and within that range, 1150 D / T to 1500 D / T (min) is maintained. Here, D is, for example, a diameter (mm) of the alloy when the alloy is cylindrical, and a diagonal distance (mm) when the alloy is square. That is, D is the maximum value (mm) of the linear distance between an arbitrary point on the outer edge of the cross section and another arbitrary point on the outer edge in a cross section perpendicular to the longitudinal direction of the alloy.

When the heat treatment temperature is lower than 1070 ° C., the insoluble γ ′ phase increases and the creep rupture strength decreases. On the other hand, when it exceeds 1220 ° C., the ductility is lowered due to melting of the grain boundary or markedly coarsening of the crystal grains. The heat treatment temperature is more preferably 1100 ° C. or higher, and more preferably 1200 ° C. or lower. When the holding time is less than 1150 D / T (min), the γ ′ phase in the central portion increases, and (Al + Ti + Nb) _PB / (Al + Ti + Nb) _PS is outside the range defined in the present invention. On the other hand, if it exceeds 1500 D / T (min), the crystal grains in the outer surface portion become coarse, and the austenite grain size number falls outside the range specified in the present invention.

・ After heating and holding, immediately cool the alloy with water. This is because when the cooling rate is slow, a large amount of undissolved γ ′ phase is generated mainly in the grains, particularly in the center of the alloy, and the above formula (iv) may not be satisfied.

Hereinafter, the present invention will be described more specifically by way of examples. However, the present invention is not limited to these examples.

An alloy having the chemical composition shown in Table 1 was melted in a high-frequency vacuum melting furnace to form a steel ingot having an outer diameter of 550 mm and a weight of 3 t.

The obtained steel ingot was processed into a cylindrical shape having an outer diameter of 200 to 480 mm by hot forging and subjected to final heat treatment under the conditions shown in Table 2 to obtain an alloy member sample. For alloys 1, 2, 3 and 5, after forging in the longitudinal direction and before final heat treatment, forging is performed in a direction substantially perpendicular to the longitudinal direction, and then the final hot forging is further performed in the longitudinal direction. went.

For each sample, a specimen for observing the structure was collected from the outer surface, and the longitudinal section was polished with emery paper and buff, then corroded with mixed acid and observed with an optical microscope. The crystal grain size number on the observation surface was determined according to the determination method based on the intersection line segment (grain size) defined in JIS G 0551 (2013).

Next, a test piece for measuring the γ ′ phase was collected from the center portion and the outer surface portion in the cross section perpendicular to the longitudinal direction of each sample. After determining the surface area of the above test piece, only the base material of the heat-resistant alloy was completely electrolyzed in an electrolysis condition of 20 mA / cm ^{2 in} a 1% tartaric acid-1% ammonium sulfate aqueous solution. And the solution after electrolysis was filtered with a 0.2 micrometer filter, and the deposit was extracted as a residue. Then, the content (mass%) of Al, Ti and Nb contained as an undissolved γ ′ phase is measured by ICP-AES measurement after acid decomposition of the extraction residue, and based on the measured value (Al + Ti + Nb ) _PB / (Al + Ti + Nb) The value of _PS was determined.

In addition, a tensile test piece having a parallel part length of 40 mm was cut out by machining from the center part and the outer surface part of each sample, and a tensile test was performed at room temperature. I asked for strength. Furthermore, a round bar creep rupture test piece having a diameter of 6 mm and a gauge distance of 30 mm described in JIS Z 2241 (2011) was cut out from the center of each sample in parallel with the longitudinal direction by machining. And the creep rupture test was implemented in 700 degreeC, 750 degreeC, and 800 degreeC air | atmosphere, The 700 degreeC creep rupture strength was calculated | required using the Larson-Miller parameter method. The results are summarized in Table 3.

The results are summarized in Table 3.

Alloys 1 to 8 are examples of the present invention, and the alloy composition, grain size number, (Al + Ti + Nb) _PB / (Al + Ti + Nb) _PS , YS _S / YS _B , TS _S / TS _B , and creep rupture strength are defined by the present invention. The variation in mechanical properties was small, and the creep rupture strength was good.

On the other hand, alloys A and B have substantially the same chemical composition as alloy 1 and have the same final shape by hot forging. However, the holding time at the time of heat treatment is outside the range of manufacturing conditions defined in the present invention. As a result, the grain size number of the outer surface portion of alloy A is outside the specified range of the present invention, and the values of YS _S / YS _B and TS _S / TS _B are out of the specified range of the present invention. As a result, the variation in mechanical properties increased depending on the part. Further, regarding the alloy B, the value of (Al + Ti + Nb) _PB / (Al + Ti + Nb) _PS was outside the specified range of the present invention, and the creep rupture strength was significantly lower than that of the alloy 1.

Alloys C, D, and E have substantially the same chemical composition as alloy 2 and have the same final shape by hot forging. Since the heat treatment temperature of Alloy C is lower than the specified range of the present invention, the value of (Al + Ti + Nb) _PB / (Al + Ti + Nb) _PS and the crystal grain number of the outer surface part are outside the range specified by the present invention. Compared to 2, the creep rupture strength was extremely low. Since the heat treatment temperature of the alloy D is higher than the specified range of the present invention, the crystal grain size number of the outer surface portion and the values of YS _S / YS _B and TS _S / TS _B are outside the specified range of the present invention. Compared to Alloy 2, the creep rupture strength was remarkably low. In addition, the cooling method at the time of final heat treatment of alloy E is not water cooling but air cooling, and the cooling rate is extremely slow, so that the value of (Al + Ti + Nb) _PB / (Al + Ti + Nb) _PS falls outside the specified range of the present invention. As a result, the creep rupture strength was significantly lower than that of Alloy 3.

Alloys F, G, and H are comparative examples whose chemical compositions deviate from the provisions of the present invention. Specifically, Alloy F is an example in which the W content is high, Alloy G has a high median value in equation (i), and Alloy H has a low median value in equation (ii). Therefore, in these examples, the creep rupture strength was reduced.

The Ni-base heat-resistant alloy according to the present invention has little variation in mechanical properties depending on the part, and is excellent in creep rupture strength at high temperatures. Therefore, the Ni heat-resistant alloy of the present invention can be suitably used as large structural members such as boilers and chemical plants used in high temperature environments.

Claims

The chemical composition of the alloy is mass%,
C: 0.005 to 0.15%,
Si: 2.0% or less,
Mn: 3.0% or less,
P: 0.030% or less,
S: 0.010% or less,
N: 0.030% or less,
O: 0.030% or less,
Ni: 40.0-60.0%,
Co: 0.01-25.0%,
Cr: 15.0% or more and less than 28.0%,
Mo: 12.0% or less,
W: less than 4.0%,
B: 0.0005 to 0.006%,
Al: 0 to 3.0%,
Ti: 0 to 3.0%,
Nb: 0 to 3.0%,
REM: 0 to 0.1%,
Mg: 0 to 0.02%,
Ca: 0 to 0.02%,
Balance: Fe and impurities,
Satisfying the following formulas (i) to (iii):
In the cross section perpendicular to the longitudinal direction of the alloy, the shortest distance from the center portion to the outer surface portion is 40 mm or more,
The austenite grain size number in the outer surface portion is -2.0 to 4.0,
The total content of Al, Ti and Nb present as precipitates obtained by extraction residue analysis satisfies the following formula (iv):
Mechanical properties at room temperature satisfy the following formulas (v) and (vi):
Ni-base heat-resistant alloy.
0.1 ≦ Mo + W ≦ 12.0 (i)
1.0 ≦ 4 × Al + 2 × Ti + Nb ≦ 12.0 (ii)
P + 0.2 × Cr × B <0.035 (iii)
(Al + Ti + Nb) PB / (Al + Ti + Nb) PS ≦ 10.0 (iv)
YS S / YS B ≦ 1.5 (v)
TS S / TS B ≦ 1.2 (vi)
However, the element symbols in the above formulas (i) to (iii) represent the content (% by mass) of each element, and the meaning of each symbol in the above formulas (iv) to (vi) is as follows.
(Al + Ti + Nb) PB : Total content of Al, Ti and Nb present as precipitates obtained by extraction residue analysis in the central portion (Al + Ti + Nb) PS : Al, Ti present as precipitates obtained by extraction residue analysis in the outer surface portion And Nb total content YS B : 0.2% yield strength at the center YS S : 0.2% yield strength at the outer surface TS B : Tensile strength at the center TS S : Tensile strength at the outer surface
The chemical composition is mass%,
Mg: 0.0001 to 0.02%, and
Ca: 0.0001 to 0.02%,
Containing one or two selected from
The Ni-base heat resistant alloy according to claim 1.
10,000 hours creep rupture strength at 700 ° C. in the longitudinal direction in the central portion is 150 MPa or more,
The Ni-base heat-resistant alloy according to claim 1 or 2.
A step of hot-working the steel ingot or slab having the chemical composition according to claim 1 or 2,
Thereafter, heating to a heat treatment temperature T (° C.) in the range of 1070 to 1220 ° C., holding 1150 D / T to 1500 D / T (min), and then performing a heat treatment of cooling with water,
Manufacturing method of Ni-base heat-resistant alloy.
However, D is the maximum value (mm) of the linear distance between an arbitrary point on the outer edge of the cross section and another arbitrary point on the outer edge in a cross section perpendicular to the longitudinal direction of the alloy.
In the step of performing the hot working, the hot working is performed at least once in a direction substantially perpendicular to the longitudinal direction of the hot working.
The manufacturing method of the Ni-base heat-resistant alloy of Claim 4.