WO2018146783A1

WO2018146783A1 - Austenitic heat-resistant alloy and method for producing same

Info

Publication number: WO2018146783A1
Application number: PCT/JP2017/004824
Authority: WO
Inventors: 仙波　潤之; 岡田　浩一; 吉澤　満; 敏秀小野
Original assignee: 新日鐵住金株式会社
Priority date: 2017-02-09
Filing date: 2017-02-09
Publication date: 2018-08-16
Also published as: CA3052547A1; JPWO2018146783A1; EP3581669A1; CA3052547C; EP3581669A4; US20200232081A1; CN110268079A; JP6816779B2; KR20190117598A

Abstract

Provided is an austenitic heat resistant alloy containing, in mass%, 0.02-0.12% of C, at most 2.0% of Si, at most 3.0% of Mn, at most 0.030% of P, at most 0.015% of S, at least 20.0% and less than 28.0% of Cr, more than 35.0% and at most 55.0% of Ni, 0-20% of Co, 4.0-10.0% of W, 0.01-0.50% of Ti, 0.01-1.0% of Nb, less than 0.50% of Mo, less than 0.50% of Cu, at most 0.30% of Al, less than 0.10% of N, 0-0.05% of Mg, 0-0.05% of Ca, 0-0.50% of REM, 0-1.5% of V, 0-0.1% of B, 0-0.10% of Zr, 0-1.0% of Hf, 0-8.0% of Ta, and 0-8.0% of Re, with the remainder comprising Fe and inevitable impurities, wherein, in a cross section perpendicular to the longitudinal direction of the alloy, the shortest distance from the center to the outer surface portion is 40 mm or more, the austenite grain size number of the outer surface portion is -2.0 to 4.0, the amount of Cr present as a precipitate satisfies [CrPB/CrP S≤10.0], and [YSS/YSB≤1.5] and [TSS/TSB≤1.2] are satisfied at room temperature.

Description

Austenitic heat-resistant alloy and method for producing the same

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

Conventionally, 18-8 austenitic stainless steels such as SUS304H, SUS316H, SUS321H, and SUS347H have been used as equipment materials in thermal power generation boilers and chemical plants used in high temperature environments.

However, in recent years, new super-supercritical boilers with higher steam temperatures and pressures have been developed all over the world for higher efficiency. The use conditions of the apparatus in such a high temperature environment have become extremely severe, and accordingly, the required performance for the materials used has become severe. The conventionally used 18-8 austenitic stainless steel is in a state where the high temperature strength, particularly the creep rupture strength, is remarkably insufficient in addition to the corrosion resistance.

* Various studies have been conducted so far to solve the above problems. For example, Patent Documents 1 to 4 disclose highly corrosion-resistant austenitic steels having good high-temperature strength. Patent Document 5 discloses an austenitic stainless steel excellent in high temperature strength and corrosion resistance. According to Patent Documents 1 to 5, the high temperature strength is improved by increasing the Cr content to 20% or more and containing W and / or Mo.

JP-A 61-179833 Japanese Patent Application Laid-Open No. Sho 61-179834 JP-A 61-179835 Japanese Patent Laid-Open No. 61-179836 JP 2004-3000 A

By the way, since large structural members such as boilers for thermal power generation and materials for devices such as chemical plants are used after hot rolling or hot forging, without performing cold processing, and are used, The crystal grain size 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 steels described in Patent Documents 1 to 5 to large structural members.

The present invention solves the above problems, and provides an austenitic 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 to solve the above-mentioned problems, and the gist thereof is the following austenitic heat-resistant alloy and a method for producing the same.

(1) The chemical composition of the alloy is mass%,
C: 0.02 to 0.12%,
Si: 2.0% or less,
Mn: 3.0% or less,
P: 0.030% or less,
S: 0.015% or less,
Cr: 20.0% or more and less than 28.0%,
Ni: more than 35.0% and 55.0% or less,
Co: 0-20.0%,
W: 4.0-10.0%,
Ti: 0.01 to 0.50%,
Nb: 0.01 to 1.0%,
Mo: less than 0.50%,
Cu: less than 0.50%,
Al: 0.30% or less,
N: less than 0.10%,
Mg: 0 to 0.05%,
Ca: 0 to 0.05%,
REM: 0 to 0.50%,
V: 0 to 1.5%
B: 0 to 0.01%
Zr: 0 to 0.10%,
Hf: 0 to 1.0%
Ta: 0 to 8.0%,
Re: 0 to 8.0%,
Balance: Fe and impurities,
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 amount of Cr present as a precipitate obtained by extraction residue analysis satisfies the following formula (i):
Mechanical properties at room temperature satisfy the following formulas (ii) and (iii):
Austenitic heat-resistant alloy.
Cr _PB / Cr _PS ≦ 10.0 (i)
YS _S / YS _B ≦ 1.5 (ii)
TS _S / TS _B ≦ 1.2 (iii)
However, the meaning of each symbol in the above formula is as follows.
Cr _PB : Cr amount existing as precipitates obtained by extraction residue analysis in the central portion Cr _PS : Cr amount existing as precipitates obtained by extraction residue analysis in the outer surface portion YS _B : 0.2% proof stress in the central portion YS _S : 0.2% proof stress at the outer surface portion TS _B : Tensile strength at the central portion TS _S : Tensile strength at the outer surface portion

(2) The chemical composition is mass%,
Mg: 0.0005 to 0.05%,
Ca: 0.0005 to 0.05%,
REM: 0.0005 to 0.50%,
V: 0.02 to 1.5%,
B: 0.0005 to 0.01%,
Zr: 0.005 to 0.10%,
Hf: 0.005 to 1.0%,
Ta: 0.01-8.0%, and
Re: 0.01 to 8.0%,
Containing one or more selected from
The austenitic heat-resistant alloy according to (1) above.

(3) The 10,000-hour creep rupture strength at 700 ° C. in the longitudinal direction in the central portion is 100 MPa or more.
The austenitic heat-resistant alloy according to (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 1100 to 1250 ° C., holding 1000 D / T to 1400 D / T (min), and then performing a heat treatment of cooling with water,
A method for producing an austenitic 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 working is performed once or more in a direction substantially perpendicular to the longitudinal direction.
The manufacturing method of the austenitic heat-resistant alloy as described in said (4).

The austenitic 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: 0.02 to 0.12%
C is an element essential for forming carbides and maintaining high-temperature tensile strength and creep rupture strength necessary for an austenitic heat-resistant alloy. Therefore, the C content needs to be 0.02% or more. However, if its content exceeds 0.12%, not only undissolved carbides are produced, but also Cr carbides increase, which deteriorates mechanical properties such as ductility and toughness and weldability. Therefore, the C content is 0.02 to 0.12%. The C content is preferably 0.05% or more, and preferably 0.10% or less.

Si: 2.0% or less Si is contained as a deoxidizing element. Further, Si is an element effective for enhancing oxidation resistance, steam oxidation resistance, and the like. It is also an element that improves the hot water flow in the cast material. However, when the Si content exceeds 2.0%, the formation of intermetallic compounds such as the σ phase is promoted, so that the stability of the structure at high temperatures deteriorates and the toughness and ductility decrease. Furthermore, the weldability is also reduced. Therefore, the Si content is 2.0% or less. When the structural stability is important, the Si content is preferably 1.0% or less. In addition, when the deoxidation effect | action is fully ensured with another element, it is not necessary to provide a minimum especially about Si content. However, when importance is attached to deoxidation, oxidation resistance, steam oxidation resistance, etc., the Si content is preferably 0.05% or more, and more preferably 0.10% or more.

Mn: 3.0% or less Mn has a deoxidizing effect similar to Si, and has an effect of improving ductility at high temperatures by fixing S unavoidably contained in the alloy as a sulfide. However, if the Mn content exceeds 3.0%, precipitation of intermetallic compounds such as the σ phase is promoted, so that mechanical properties such as structure stability and high temperature strength deteriorate. Therefore, the Mn content is 3.0% or less. The Mn content is preferably 2.0% or less, and more preferably 1.5% or less. In addition, although it is not necessary to provide a lower limit for the Mn content, when importance is placed on the ductility improving effect at high temperature, the Mn content is preferably 0.10% or more, and is preferably 0.20% or more. More preferred.

P: 0.030% or less P is inevitably mixed into the alloy as an impurity, and significantly reduces weldability and ductility at high temperatures. Therefore, the P content is 0.030% or less. The P content is preferably as low as possible, preferably 0.020% or less, and more preferably 0.015% or less.

S: 0.015% or less S, like P, is inevitably mixed into the alloy as an impurity, and significantly reduces weldability and ductility at high temperatures. Therefore, the S content is 0.015% or less. When emphasizing hot workability, the S content is preferably 0.010% or less, more preferably 0.005% or less, and even more preferably 0.003% or less.

Cr: 20.0% or more and less than 28.0% Cr is an important element that exhibits an excellent action in improving corrosion resistance such as oxidation resistance, steam oxidation resistance, and high temperature corrosion resistance. However, if the content is less than 20.0%, these effects cannot be obtained. On the other hand, if the Cr content is increased, particularly 28.0% or more, the structure becomes unstable due to precipitation of the σ phase and the weldability is also deteriorated. Therefore, the Cr content is 20.0% or more and less than 28.0%. The Cr content is preferably 21.0% or more, and more preferably 22.0% or more. Moreover, it is preferable that Cr content is 26.0% or less, and it is more preferable that it is 25.0% or less.

Ni: more than 35.0% and not more than 55.0% Ni is an element that stabilizes the austenite structure, and is also an important element for ensuring corrosion resistance. From the balance with the above Cr content, Ni needs to be contained exceeding 35.0%. On the other hand, if the Ni content is excessive, the cost is increased. The Ni content is preferably 40.0% or more, and more preferably 42.0% or more. Further, the Ni content is preferably 50.0% or less, and more preferably 48.0% or less.

Co: 0-20.0%
Co is not necessarily contained, but it may be contained in place of a part of Ni in order to stabilize the austenite structure and contribute to the improvement of creep rupture strength as in the case of Ni. However, if the content exceeds 20.0%, the effect is saturated and the economic efficiency is lowered. Therefore, the Co content is 0 to 20.0%. The Co content is preferably 15.0% or less. In addition, when obtaining said effect, it is preferable that Co content shall be 0.5% or more.

W: 4.0-10.0%
W not only contributes to the improvement of creep rupture strength as a solid solution strengthening element by dissolving in the matrix, but also precipitates as a Fe ₂ W type Laves phase or Fe ₇ W ₆ type μ phase, and increases the creep rupture strength. It is an important element that greatly improves. However, if the W content is less than 4.0%, the above-described effects cannot be obtained. On the other hand, even if W is contained in an amount exceeding 10.0%, the effect of improving the strength is saturated and the structure stability and ductility at high temperature deteriorate. Therefore, the W content is 4.0 to 10.0%. The W content is preferably 5.0% or more, and more preferably 5.5% or more. Moreover, it is preferable that W content is 9.0% or less, and it is more preferable that it is 8.5% or less.

Ti: 0.01 to 0.50%
Ti is an element that has the effect of forming carbonitrides and improving creep rupture strength. However, if the Ti content is less than 0.01%, a sufficient effect cannot be obtained. On the other hand, if it exceeds 0.50%, the ductility at high temperatures decreases. Therefore, the Ti content is set to 0.01 to 0.50%. The Ti content is preferably 0.05 or more, and more preferably 0.10% or more. Further, the Ti content is preferably 0.40% or less, and more preferably 0.35% or less.

Nb: 0.01 to 1.0%
Nb has the effect of forming a carbonitride to improve the creep rupture strength. However, if the Nb content is less than 0.01%, a sufficient effect cannot be obtained. On the other hand, if the Nb content exceeds 1.0%, the ductility at high temperatures decreases. Therefore, the Nb content is set to 0.01 to 1.0%. The Nb content is preferably 0.10% or more. Moreover, it is preferable that Nb content is 0.90% or less, and it is more preferable that it is 0.70% or less.

Mo: Less than 0.50% Conventionally, Mo has been considered to be an element having a function equivalent to that of W as an element contributing to an improvement in creep rupture strength as a solid solution strengthening element by dissolving in a matrix. However, as a result of studies by the present inventors, when Mo is contained in a composite containing the above-mentioned amounts of W and Cr, a σ phase may precipitate when used for a long time, For this reason, it has been found that creep rupture strength, ductility and toughness may be reduced. For this reason, it is desirable to make Mo content as low as possible, and to be less than 0.50%. Note that the Mo content is preferably limited to less than 0.20%.

Cu: Less than 0.50% In the present invention, Cu lowers the melting point and decreases hot workability and weldability. Therefore, it is desirable to make Cu content as low as possible, and to be less than 0.50%. Note that the Cu content is preferably limited to less than 0.20%.

Al: 0.30% or less Al is an element to be contained as a deoxidizer for molten steel. However, when the Al content exceeds 0.30%, ductility at high temperatures deteriorates. Therefore, the Al content is set to 0.30% or less. The Al content is preferably 0.25% or less, and more preferably 0.20% or less. In addition, when obtaining said effect, it is preferable that Al content shall be 0.01% or more, and it is more preferable to set it as 0.02% or more.

N: Less than 0.10% N is an element having an effect of stabilizing the austenite structure, and is an element inevitably included in a normal melting method. However, in the present invention in which the inclusion of Ti is essential, it is better to reduce as much as possible to avoid the consumption of Ti due to TiN formation. However, since it is difficult to reduce extremely in the case of dissolution in the atmosphere, the N content is set to less than 0.10%.

In the chemical composition of the austenitic heat-resistant alloy of the present invention, the balance is Fe and impurities. Fe is preferably contained in an amount of 0.1 to 40.0%. In addition, “impurities” as used herein are components that are mixed due to various factors of raw materials such as ores and scraps and manufacturing processes when the alloy is industrially manufactured, and do not adversely affect the present invention. It means what is allowed.

The austenitic heat-resistant alloy of the present invention may further contain one or more selected from Mg, Ca, REM, V, B, Zr, Hf, Ta and Re.

Mg, Ca, and REM all have the effect of fixing S as a sulfide to improve high temperature ductility. For this reason, when it is desired to obtain better high-temperature ductility, one or more of these elements may be positively contained in the following range.

Mg: 0.05% or less Mg has the action of fixing S, which inhibits ductility at high temperatures, as a sulfide to improve high-temperature ductility. Therefore, Mg may be included to obtain this effect. However, when the Mg content exceeds 0.05%, the cleanliness is lowered, and the high temperature ductility is impaired. Therefore, the Mg content in the case of inclusion is 0.05% or less. The Mg content is more preferably 0.02% or less, and still more preferably 0.01% or less. On the other hand, in order to reliably obtain the above effect, the Mg content is preferably 0.0005% or more, and more preferably 0.001% or more.

Ca: 0.05% or less Ca has an action of fixing S as a sulfide to inhibit high temperature ductility to improve high temperature ductility. Therefore, Ca may be contained to obtain this effect. However, when the Ca content exceeds 0.05%, the cleanliness is deteriorated and the high temperature ductility is impaired. Therefore, the Ca content when contained is 0.05% or less. The Ca content is more preferably 0.02% or less, and still more preferably 0.01% or less. On the other hand, in order to surely obtain the above effect, the Ca content is preferably 0.0005% or more, and more preferably 0.001% or more.

REM: 0.50% or less REM has an action of fixing S as sulfide to improve high temperature ductility. REM also improves the adhesion of the Cr ₂ O ₃ protective coating on the steel surface, in particular, improves the oxidation resistance during repeated oxidation, and further contributes to the strengthening of grain boundaries. It also has the effect of improving creep rupture ductility. However, when the REM content exceeds 0.50%, inclusions such as oxides increase and workability and weldability are impaired. Therefore, the amount of REM when contained is 0.50% or less. The REM content is more preferably 0.30% or less, and further preferably 0.15% or less. On the other hand, in order to reliably obtain the above effect, the REM content is preferably 0.0005% or more, more preferably 0.001% or more, and further preferably 0.002% or more. preferable.

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.

The total content of Mg, Ca and REM may be 0.6% or less, more preferably 0.4% or less, and still more preferably 0.2% or less.

V, B, Zr and Hf all have the effect of improving high temperature strength and creep rupture strength. For this reason, when it is desired to obtain a higher high-temperature strength and creep rupture strength, one or more of these elements may be positively contained in the following range.

V: 1.5% or less V has an action of forming a carbonitride to improve high temperature strength and creep rupture strength. For this reason, in order to acquire these effects, you may contain V. However, when the V content exceeds 1.5%, the high temperature corrosion resistance is lowered, and further ductility and toughness are deteriorated due to precipitation of the embrittled phase. Therefore, when V is included, the amount of V is 1.5% or less. The V content is more preferably 1.0% or less. On the other hand, in order to surely obtain the above effect, the V content is preferably 0.02% or more, and more preferably 0.04% or more.

B: 0.01% or less B exists in the carbide or in the matrix, and not only promotes refinement of the precipitated carbide, but also has an effect of improving the creep rupture strength by strengthening the grain boundary. However, if the B content exceeds 0.01%, the ductility at high temperatures is lowered and the melting point is also lowered. Therefore, the amount of B when contained is 0.01% or less. The B content is more preferably 0.008% or less, and further preferably 0.006% or less. On the other hand, in order to reliably obtain the above effect, the B content is preferably 0.0005% or more, more preferably 0.001% or more, and further preferably 0.0015% or more. preferable.

Zr: 0.10% or less Zr is an element that promotes refinement of carbonitride and improves creep rupture strength as a grain boundary strengthening element. However, when the Zr content exceeds 0.10%, the ductility at high temperatures is lowered. Therefore, the amount of Zr in the case of containing is 0.10% or less. The Zr content is more preferably 0.06% or less, and even more preferably 0.05% or less. On the other hand, in order to reliably obtain the above effect, the Zr content is preferably 0.005% or more, and more preferably 0.01% or more.

Hf: 1.0% or less Hf contributes to precipitation strengthening as a carbonitride and has an effect of improving creep rupture strength. Therefore, Hf may be contained in order to obtain these effects. However, if the Hf content exceeds 1.0%, workability and weldability are impaired. Therefore, the amount of Hf when contained is 1.0% or less. The Hf content is more preferably 0.8% or less, and further preferably 0.5% or less. On the other hand, in order to reliably obtain the above effect, the Hf content is preferably 0.005% or more, more preferably 0.01% or more, and further preferably 0.02% or more. preferable.

The total content of V, B, Zr and Hf described above is preferably 2.6% or less, and more preferably 1.8% or less.

Ｔａ Both Ta and Re have a solid solution strengthening action by dissolving in austenite as a matrix. For this reason, when it is desired to obtain higher high-temperature strength and creep rupture strength by the solid solution strengthening action, one or both of these elements may be positively contained in the following range.

Ta: 8.0% or less Ta has the effect of forming carbonitride and improving the high temperature strength and creep rupture strength as a solid solution strengthening element. For this reason, in order to acquire these effects, you may contain Ta. However, if the Ta content exceeds 8.0%, workability and mechanical properties are impaired. Therefore, when Ta is included, the amount of Ta is set to 8.0% or less. The Ta content is more preferably 7.0% or less, and even more preferably 6.0% or less. On the other hand, in order to surely obtain the above effect, the Ta content is preferably 0.01% or more, more preferably 0.1% or more, and further preferably 0.5% or more. preferable.

Re: 8.0% or less Re has a function of improving high-temperature strength and creep rupture strength mainly as a solid solution strengthening element. Therefore, Re may be contained in order to obtain these effects. However, if the Re content exceeds 8.0%, workability and mechanical properties are impaired. Therefore, the amount of Re when contained is 8.0% or less. The Re content is more preferably 7.0% or less, and even more preferably 6.0%. On the other hand, in order to reliably obtain the above effect, the Re content is preferably 0.01% or more, more preferably 0.1% or more, and further preferably 0.5% or more. preferable.

The total content of Ta and Re is preferably 14.0% or less, and more preferably 12.0% or less.

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 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 austenitic 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 austenitic 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). Cr amount present as precipitate obtained by extraction residue analysis Cr _PB / Cr _PS ≦ 10.0 (i)
However, the meaning of each symbol in the formula (i) is as follows.
Cr _PB : Cr amount present as precipitates obtained by extraction residue analysis in the center portion Cr _PS : Cr amount present as precipitates obtained by extraction residue analysis in the outer surface portion In the manufacturing process of the alloy, heat treatment after hot working An insoluble Cr precipitate (mainly carbide) is formed in the crystal grain boundary or in the grain after the treatment. In particular, since the cooling rate is slower at the center of the alloy than at the outer surface, the amount of Cr precipitates tends to increase. Therefore, if the value of Cr _PB / Cr _PS exceeds 10.0, it becomes impossible to maintain high creep rupture strength at high temperatures. On the other hand, the lower limit of Cr _PB / Cr _PS does not need to be determined, but is preferably set to 1.0 or more because the central portion tends to increase the amount of precipitates more than the outer surface portion.

The extraction residue analysis is performed according to the following procedure. First, a test piece for measuring Cr precipitates 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 test piece, only the base material of the alloy sample is completely electrolyzed in an electrolysis condition of 20 mA / cm ^{2 in} a 10% acetylacetone-1% tetramethylammonium chloride-methanol solution. And the solution after electrolysis is filtered with a 0.2 micrometer filter, and deposits are extracted as a residue. After that, the extraction residue is acid-decomposed, and then analyzed using an inductively coupled plasma optical emission spectrometer (ICP-AES) to determine the Cr content (mass%) contained as undissolved Cr precipitates. Measure and obtain the value of Cr _PB / Cr _PS based on the measured value.

5). Mechanical properties YS _S / YS _B ≦ 1.5 (ii)
TS _S / TS _B ≦ 1.2 (iii)
However, the meaning of each symbol in the above formula is as follows.
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 Cooling during heat treatment for large structural members Due to the fact that the speed varies from site to site, the mechanical properties of each site tend to vary greatly. 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, it is assumed that the austenitic heat-resistant alloy according to the present invention satisfies the above formulas (ii) and (iii) in mechanical properties at room temperature. In addition, although it is not necessary to set a lower limit 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 (ii) and (iii) 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 austenitic heat-resistant alloy of the present invention is used in a high temperature environment, it requires high high temperature strength, particularly high creep rupture strength. Therefore, the heat-resistant alloy of the present invention preferably has a 10,000-hour creep rupture strength at 700 ° C. in the longitudinal direction of 100 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). Production Method The austenitic heat-resistant alloy of the present invention can be produced 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 austenitic heat-resistant alloy of the present invention, after the above 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 1100 to 1250 ° C., and within that range, 1000 D / T to 1400 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 1100 ° C., undissolved chromium carbide and the like increase, and the creep rupture strength decreases. On the other hand, when the temperature exceeds 1250 ° C., the ductility decreases due to melting of the grain boundary or markedly coarsening of the crystal grains. The heat treatment temperature is more preferably 1150 ° C. or higher, and more preferably 1230 ° C. or lower. Further, the holding time is less than 1000D / T (min), undissolved chromium carbide increases _Cr PB / _{Cr PS} central portion is out of the range defined in the present invention. On the other hand, if it exceeds 1400 D / T (min), the crystal grains in the outer surface portion become coarse, and the austenite grain size number falls outside the range defined 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 Cr precipitates are generated at the grain boundaries or in the grains, particularly at the center of the alloy, and the above formula (i) 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 120 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 and 4, forging in the direction substantially perpendicular to the longitudinal direction was performed after hot forging in the longitudinal direction and before the final heat treatment, and then final hot forging was further performed in the longitudinal direction. .

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 Cr precipitates 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 alloy sample was completely electrolyzed in an electrolytic condition of 20 mA / cm ^{2 in} a 10% acetylacetone-1% tetramethylammonium chloride-methanol solution. And the solution after electrolysis was filtered with a 0.2 micrometer filter, and the deposit was extracted as a residue. After that, the extraction residue is subjected to acid decomposition, and then ICP-AES measurement is performed to measure the Cr content (mass%) contained as an undissolved Cr precipitate. Based on the measured value, Cr _PB / Cr _{The PS} value 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. Further, a creep rupture test piece having a parallel part length of 30 mm was cut out from the center part 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.

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, with respect to Alloy B, the creep rupture strength was outside the specified range of the present invention, and the result was significantly lower than that of 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 grain size number of the outer surface portion and the value of Cr _PB / Cr _PS are outside the range specified by the present invention. 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 the final heat treatment of the alloy E is not water cooling but air cooling, and because the cooling rate is extremely slow, the value of Cr _PB / Cr _PS is outside the specified range of the present invention. Compared to Alloy 2, the creep rupture strength was significantly lower. On the other hand, Alloys 1 to 9 satisfying all the provisions of the present invention had small variations in mechanical properties and good creep rupture strength.

The austenitic 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. Therefore, the austenitic heat-resistant alloy of the present invention can be suitably used as a large structural member such as a thermal power generation boiler and a chemical plant used in a high temperature environment.

Claims

The chemical composition of the alloy is mass%,
C: 0.02 to 0.12%,
Si: 2.0% or less,
Mn: 3.0% or less,
P: 0.030% or less,
S: 0.015% or less,
Cr: 20.0% or more and less than 28.0%,
Ni: more than 35.0% and 55.0% or less,
Co: 0-20.0%,
W: 4.0-10.0%,
Ti: 0.01 to 0.50%,
Nb: 0.01 to 1.0%,
Mo: less than 0.50%,
Cu: less than 0.50%,
Al: 0.30% or less,
N: less than 0.10%,
Mg: 0 to 0.05%,
Ca: 0 to 0.05%,
REM: 0 to 0.50%,
V: 0 to 1.5%
B: 0 to 0.01%
Zr: 0 to 0.10%,
Hf: 0 to 1.0%
Ta: 0 to 8.0%,
Re: 0 to 8.0%,
Balance: Fe and impurities,
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 amount of Cr present as a precipitate obtained by extraction residue analysis satisfies the following formula (i):
Mechanical properties at room temperature satisfy the following formulas (ii) and (iii):
Austenitic heat-resistant alloy.
Cr PB / Cr PS ≦ 10.0 (i)
YS S / YS B ≦ 1.5 (ii)
TS S / TS B ≦ 1.2 (iii)
However, the meaning of each symbol in the above formula is as follows.
Cr PB : Cr amount existing as precipitates obtained by extraction residue analysis in the central portion Cr PS : Cr amount existing as precipitates obtained by extraction residue analysis in the outer surface portion YS B : 0.2% proof stress in the central portion YS S : 0.2% proof stress at the outer surface portion TS B : Tensile strength at the central portion TS S : Tensile strength at the outer surface portion
The chemical composition is mass%,
Mg: 0.0005 to 0.05%,
Ca: 0.0005 to 0.05%,
REM: 0.0005 to 0.50%,
V: 0.02 to 1.5%,
B: 0.0005 to 0.01%,
Zr: 0.005 to 0.10%,
Hf: 0.005 to 1.0%,
Ta: 0.01-8.0%, and
Re: 0.01 to 8.0%,
Containing one or more selected from
The austenitic 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 100 MPa or more,
The austenitic 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 1100 to 1250 ° C., holding 1000 D / T to 1400 D / T (min), and then performing a heat treatment of cooling with water,
A method for producing an austenitic 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 processing, the processing is performed at least once in a direction substantially perpendicular to the longitudinal direction.
The manufacturing method of the austenitic heat-resistant alloy of Claim 4.