JP6690359B2 - Austenitic heat-resistant alloy member and method for manufacturing the same - Google Patents

Description

  The present invention relates to an austenitic heat resistant alloy member and a method for manufacturing the same, and more particularly to an austenitic heat resistant alloy member having excellent creep rupture strength and a method for manufacturing the same.

  Conventionally, 18-8 austenitic stainless steel such as SUS304H, SUS316H, SUS321H, and SUS347H has been used as a material for equipment in boilers, chemical plants, and the like used in a high-temperature environment.

  However, in recent years, new installations of ultra-supercritical pressure boilers in which the temperature and pressure of steam have been increased in order to achieve high efficiency are being advanced all over the world. The operating conditions of the device in such a high temperature environment have become extremely severe, and the required performance of the materials used has become stricter accordingly. In addition, the 18-8 austenitic stainless steel that has been conventionally used is in a situation where not only the corrosion resistance but also the high temperature strength, particularly the creep rupture strength, is remarkably insufficient.

  Therefore, an austenitic stainless steel having improved creep rupture strength by containing optimal amounts of various alloying elements has been invented. However, recently, for example, in the field of boilers for thermal power generation, a plan for increasing the steam temperature to 700 ° C. or higher has been promoted. In this case, the temperature of the members used will be much higher than 700 ° C. Therefore, the creep rupture strength and corrosion resistance have become insufficient even with the newly improved austenitic stainless steel.

  In response to the above-mentioned strict requirements, Patent Document 1 discloses an austenitic heat-resistant alloy having excellent creep rupture strength and hot workability.

International Publication No. 2009/154161

  By the way, since large-scale structural members such as materials for equipment such as boilers and chemical plants are used by performing final heat treatment without hot working after hot rolling or hot forging, crystal grain size is used. Is relatively large. Therefore, there is a problem that 0.2% proof stress and tensile strength at room temperature, which are usually specified as material specifications, are lower than those obtained by performing final heat treatment after cold working.

  In addition, in a large-sized structural member, the cooling rate during heat treatment varies greatly depending on the site, so the amount of the solid solution element that contributes to strengthening as a precipitate when used at high temperatures varies depending on the site. Due to this, there is also a problem that the creep rupture strength varies. Therefore, it is difficult to apply the steel described in Patent Document 1 to a large structural member.

  The present invention solves the above problems and develops a 0.2% proof stress and tensile strength at room temperature which are sufficient for a large-scale structural member, and a 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 problems, and has as its gist the following austenitic heat-resistant alloy member and its manufacturing method.

(1) In mass%,
C: more than 0.02% and 0.15% or less,
Si: 2.0% or less,
Mn: 3.0% or less,
P: 0.030% or less,
S: 0.010% or less,
Cr: 28.0 to 38.0%,
Ni: more than 40.0% and 60.0% or less,
W: more than 3.0% and 15.0% or less,
Ti: 0.05 to 1.0%,
Zr: 0.005-0.2%,
Al: 0.01 to 0.3%,
N: 0.02% or less,
Mo: less than 0.50%,
B: 0.005% or less,
Co: 0 to 20.0%,
The balance: an alloy member obtained by hot working a steel ingot or a slab having a chemical composition of Fe and impurities,
In a cross section perpendicular to the longitudinal direction of the alloy member, the length from the central 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 Cr precipitation amount obtained by extraction residue analysis satisfies the following formula (i),
The mechanical properties at room temperature satisfy the following equations (ii) and (iii),
An austenitic heat resistant alloy member having a creep rupture strength of 10,000 MPa or more at 700 ° C. in the longitudinal direction at the central portion, of 130 MPa or more.
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 deposition amount obtained by extraction residue analysis at the center cr _PS: Cr deposition amount obtained by extraction residue analysis in the outer surface portion YS _B: 0.2% proof stress YS _S at the center: 0.2 the outer surface portion % Yield strength TS _B : Tensile strength in the central area TS _S : Tensile strength in the outer surface area

(2) The chemical composition of the steel ingot or the slab contains, in mass%, one or more elements belonging to one or more groups selected from the following groups <1> to <3>, The austenitic heat resistant alloy according to (1).
<1> Nb: 1.0% or less, V: 1.5% or less and Hf: 1.0% or less <2> Mg: 0.050% or less, Ca: 0.050% or less and REM: 0.50 % Or less <3> Ta: 8.0% or less, Re: 8.0% or less, Ir: 5.0% or less, Pd: 5.0% or less, Pt: 5.0% or less and Ag: 5.0. %Less than

(3) a step of hot working a steel ingot or a slab having the chemical composition according to (1) or (2) above,
Then, a heat treatment is performed to heat treatment temperature T (° C.) in the range of 1100 to 1250 ° C., hold at 1150 D / T to 1500 D / T (min), and then heat-treat by water cooling. Method.
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 of the cross section perpendicular to the longitudinal direction of the alloy member.

  (4) The method for producing an austenitic heat-resistant alloy member according to (3), wherein in the step of performing hot forging, forging is performed once or more in a direction substantially perpendicular to the longitudinal direction of hot forging.

  INDUSTRIAL APPLICABILITY The austenitic heat-resistant alloy member of the present invention has little variation in mechanical properties depending on the site and is excellent in creep rupture strength at high temperatures. Therefore, the austenitic heat-resistant alloy member of the present invention can be suitably used as a large-scale structural member such as a boiler and a chemical plant used in a high temperature environment.

  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 addition, in the following description, "%" regarding the content means "mass%".

C: more than 0.02% and 0.15% or less C has the function of forming a carbide and ensuring the tensile strength and creep rupture strength required when used in a high temperature environment. In order to exert this effect, it is necessary to contain C in an amount exceeding 0.02%. However, if the C content exceeds 0.15%, the amount of undissolved carbides after solution heat treatment does not increase, but does not contribute to the improvement of high temperature strength. Furthermore, other mechanical properties such as toughness and It also deteriorates weldability. Therefore, the C content is set to more than 0.02% and 0.15% or less. C is preferably contained in excess of 0.03%, more preferably in excess of 0.05%. The C content is preferably 0.13% or less, more preferably 0.12% or less.

Si: 2.0% or less Si is contained as a deoxidizing element. Further, Si is an effective element for enhancing the oxidation resistance, steam oxidation resistance and the like. However, when the Si content becomes high, and particularly when it exceeds 2.0%, the formation of intermetallic compounds such as σ phase is promoted, so that the stability of the structure at high temperature is deteriorated and the toughness and ductility are deteriorated. . Further, the weldability and hot workability are also reduced. Therefore, the Si content is 2.0% or less. When the toughness and ductility are important, the Si content is preferably 1.0% or less. In addition, when the deoxidizing action is sufficiently secured by another element, it is not necessary to set a lower limit for the Si content. However, when importance is attached to deoxidizing action, oxidation resistance, steam oxidation resistance, etc., the Si content is preferably 0.05% or more, and more preferably 0.1% or more.

Mn: 3.0% or less Mn has a deoxidizing effect similar to Si, and also has an effect of fixing S inevitably contained in the alloy as a sulfide to improve hot workability. However, when the Mn content exceeds 3.0%, precipitation of intermetallic compounds such as σ phase is promoted, and thus mechanical properties such as structural stability and high temperature strength are deteriorated. Therefore, the Mn content is 3.0% or less. The Mn content is preferably 2.0% or less, more preferably 1.5% or less. It is not necessary to set a lower limit for the Mn content, but when the hot workability improving action is emphasized, the Mn content is preferably 0.1% or more, and more preferably 0.2% or more. preferable.

P: 0.030% or less P is inevitably mixed in the alloy as an impurity and remarkably deteriorates hot workability. Therefore, the P content is set to 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: 0.010% or less S, like P, is unavoidably mixed in the alloy as an impurity and significantly deteriorates hot workability. Therefore, the S content is set to 0.010% or less. When it is desired to secure good hot workability, the S content is preferably 0.005% or less, and more preferably 0.003% or less.

Cr: 28.0-38.0%
Cr has an effect of improving corrosion resistance such as oxidation resistance, steam oxidation resistance, and high temperature corrosion resistance. Further, Cr is an essential element in the present invention because it precipitates as an α-Cr phase and enhances the creep rupture strength. However, if the content is less than 28.0%, these effects cannot be obtained. On the other hand, if the Cr content increases, particularly if it exceeds 38.0%, the hot workability deteriorates, and further the structure becomes unstable due to the precipitation of the σ phase. Therefore, the Cr content is 28.0 to 38.0%. In addition, it is preferable to contain Cr in an amount exceeding 30.0%.

Ni: More than 40.0% and 60.0% or less Ni is an essential element for ensuring a stable austenite structure. In the present invention containing 28.0 to 38.0% Cr, in order to suppress the precipitation of the σ phase and stably precipitate the α-Cr phase, Ni is contained in an amount exceeding 40.0%. There is a need. However, when the Ni content becomes excessive, particularly when it exceeds 60.0%, the α-Cr phase is not sufficiently precipitated depending on the Cr content, and further the economical efficiency is impaired. Therefore, the Ni content is more than 40.0% and 60.0% or less.

W: more than 3.0% and not more than 15.0% W not only contributes to the improvement of creep rupture strength as a solid solution strengthening element by forming a solid solution in the matrix, but also Fe ₂ W type Laves phase or Fe ₇ It is an extremely important element that precipitates as a W ₆ type μ phase and significantly improves the creep rupture strength. Further, W forms a solid solution in the α-Cr phase precipitated in the present invention containing 28.0 to 38.0% Cr, and causes growth coarsening of the α-Cr phase during long-term use at high temperature. Has the effect of suppressing the sudden decrease in creep rupture strength on the long-term side. However, if the W content is 3.0% or less, the above effects cannot be obtained. On the other hand, even if W is contained in an amount of more than 15.0%, the above effect is saturated and the cost is increased, and moreover, the structural stability and the hot workability are deteriorated. Therefore, the W content is set to more than 3.0% and 15.0% or less. The W content is preferably 13.0% or less. When the effect of improving the creep rupture strength is further emphasized, it is preferable to contain W in an amount exceeding 6.0%.

Ti: 0.05-1.0%
Ti is an important element that promotes the precipitation of the α-Cr phase and enhances the creep rupture strength. In particular, by including Ti in combination with Zr described later, the precipitation of the α-Cr phase is further promoted, and the creep rupture strength can be further increased. However, if the Ti content is less than 0.05%, a sufficient effect cannot be obtained, while if it exceeds 1.0%, the hot workability deteriorates. Therefore, the Ti content is 0.05 to 1.0%. The Ti content is preferably 0.1% or more, more preferably 0.2% or more. The Ti content is preferably 0.9% or less, more preferably 0.5% or less.

Zr: 0.005-0.2%
Like Ti, Zr is an important element that promotes the precipitation of the α-Cr phase and enhances the creep rupture strength. In particular, by containing Zr in combination with the above Ti, the precipitation of the α-Cr phase is further promoted, and the creep rupture strength can be further increased. However, if the Zr content is less than 0.005%, a sufficient effect cannot be obtained, while if it exceeds 0.2%, the hot workability deteriorates. Therefore, the Zr content is set to 0.005 to 0.2%. The Zr content is preferably 0.01% or more. Further, the Zr content is preferably 0.1% or less, more preferably 0.05% or less.

Al: 0.01 to 0.3%
Al is an element having a deoxidizing action, and its content is required to be 0.01% or more in order to exert its effect. It should be noted that by including a large amount of Al, the γ'phase can be precipitated and the creep rupture strength can be increased. However, in the present invention, an appropriate amount of W, Ti and Zr is contained to form an α-Cr phase. Since the creep rupture strength can be dramatically increased by the composite precipitation strengthening by the Laves phase and the like, the strengthening by the γ ′ phase is unnecessary. Moreover, if the Al content exceeds 0.3%, the hot workability, ductility and toughness may deteriorate. Therefore, the Al content is set to 0.01 to 0.3%.

N: 0.02% or less As described above, in the present invention, Zr and Ti are contained as essential elements for promoting the precipitation of the α-Cr phase. N, which is an element that is unavoidably included in the normal melting method, forms ZrN and TiN and consumes Zr and Ti. In order to avoid this, it is necessary to reduce the N content as much as possible. However, the extreme reduction of the N content requires application of a special dissolution method or the use of high-purity raw materials, which impairs the economical efficiency. Therefore, the N content is 0.02% or less. The N content is preferably 0.015% or less.

Mo: Less than 0.50% Conventionally, Mo has been considered to be an element having the same action as W as an element that forms a solid solution in the matrix and contributes to the improvement of creep rupture strength as a solid solution strengthening element. However, according to the study by the present inventors, in the case where Mo is compounded in the alloy containing W and Cr in the amounts described above, the σ phase may be precipitated when used for a long time, Therefore, it has been found that creep rupture strength, ductility and toughness may be deteriorated. Therefore, it is desirable that the Mo content be as low as possible, and it is less than 0.50%. The Mo content is more preferably limited to less than 0.20%.

B: 0.005% or less B exists in the grain boundary alone or in the carbonitride, and by the grain boundary strengthening during use at high temperature, grain boundary slip suppression and fine dispersion precipitation of carbonitride are promoted. , Has the effect of improving high temperature strength and creep rupture strength. However, if the B content exceeds 0.005%, the weldability deteriorates. Therefore, the B content is 0.005% or less.

Co: 0 to 20.0%
Like Ni, Co has an action of stabilizing the austenite structure and also contributes to the improvement of creep rupture strength. Therefore, Co may be contained in order to obtain the above effect. However, even if Co is contained in excess of 20.0%, the above effect is saturated and the cost is increased, and the hot workability is also deteriorated. Therefore, the Co content is 20.0% or less. The Co content is preferably 15.0% or less. On the other hand, in order to surely obtain the effect of stabilizing the austenitic structure of Co and the effect of improving creep rupture strength, the Co content is preferably 0.05% or more, and 0.5% or more. Is more preferable.

  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 5.0 to 20.0%. The term "impurity" as used herein refers to a component that is mixed by ores, raw materials such as scrap, and various factors in the manufacturing process when the alloy is industrially manufactured, within a range that does not adversely affect the present invention. Means acceptable.

The austenitic heat resistant alloy of the present invention may further contain one or more elements belonging to one or more groups selected from the following groups <1> to <3>.
<1> Nb: 1.0% or less, V: 1.5% or less and Hf: 1.0% or less <2> Mg: 0.050% or less, Ca: 0.050% or less and REM: 0.50 % Or less <3> Ta: 8.0% or less, Re: 8.0% or less, Ir: 5.0% or less, Pd: 5.0% or less, Pt: 5.0% or less and Ag: 5.0. %Less than

  Nb, V and Hf, which are elements of the <1> group, all have an action of improving high temperature strength and creep rupture strength. Therefore, in order to obtain higher high temperature strength and creep rupture strength, one or more of these elements may be positively contained within the following range.

Nb: 1.0% or less Nb has the functions of forming carbonitrides to improve high-temperature strength and creep rupture strength, and also to refine crystal grains to improve ductility. Therefore, Nb may be contained to obtain these effects. However, if the Nb content exceeds 1.0%, the hot workability and toughness deteriorate. Therefore, the amount of Nb, if contained, is 1.0% or less. The Nb content is more preferably 0.9% or less. On the other hand, in order to reliably obtain the above effects, the Nb content is preferably 0.05% or more, more preferably 0.1% or more.

V: 1.5% or less V has the effect of forming carbonitrides and improving high temperature strength and creep rupture strength. Therefore, V may be contained in order to obtain these effects. However, if the V content exceeds 1.5%, the high-temperature corrosion resistance decreases, and further the ductility and toughness deteriorate due to the precipitation of the embrittlement phase. Therefore, when V is contained, the amount of V is set to 1.5% or less. The V content is more preferably 1.2% or less. On the other hand, in order to reliably obtain the above effects, the V content is preferably 0.02% or more, more preferably 0.04% or more.

Hf: 1.0% or less Hf has a function of contributing to precipitation strengthening as a carbonitride and improving the high temperature strength and the creep rupture strength, so 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, further preferably 0.5% or less. On the other hand, in order to reliably obtain the above effects, the Hf content is preferably 0.01% or more, more preferably 0.02% or more.

  The total content of Nb, V, and Hf may be 3.5% or less, but it is more preferably 2.7% or less.

  The elements of the <2> group, Mg, Ca and REM, all have the effect of fixing S as a sulfide and improving the hot workability. Therefore, in order to obtain better hot workability, one or more of these elements may be positively contained within the following range.

Mg: 0.050% or less Since Mg has an effect of fixing S, which is unavoidably contained in the alloy, as a sulfide and improving hot workability, Mg should be contained in order to obtain this effect. Good. However, when the Mg content exceeds 0.050%, the cleanability is deteriorated, and the hot workability and ductility are deteriorated. Therefore, the amount of Mg, if contained, is 0.050% or less. The Mg content is more preferably 0.020% or less, further preferably 0.010% or less. On the other hand, in order to reliably obtain the above effects, the Mg content is preferably 0.0005% or more, more preferably 0.001% or more.

Ca: 0.050% or less Since Ca has an action of fixing S that inhibits hot workability as a sulfide and improving hot workability, Ca may be added to obtain this effect. . However, if the Ca content exceeds 0.050%, the cleanability is deteriorated, and the hot workability and ductility are deteriorated. Therefore, when Ca is contained, the amount of Ca is set to 0.050% or less. The Ca content is more preferably 0.020% or less, still more preferably 0.010% or less. On the other hand, in order to reliably obtain the above effects, the Ca content is preferably 0.0005% or more, more preferably 0.001% or more.

REM: 0.50% or less REM has an effect of fixing S as a sulfide and improving hot workability. Further, REM has an effect of improving the adhesion of the Cr ₂ O ₃ protective film on the steel surface, particularly, the effect of improving the oxidation resistance at the time of repeated oxidation, and further contributes to grain boundary strengthening to increase the creep rupture strength. It also has the function of improving the creep rupture ductility. However, if 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, further preferably 0.15% or less. On the other hand, in order to surely 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.

  In addition, REM refers to a total of 17 elements of Sc, Y and lanthanoid, and the content of the 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 further preferably 0.2% or less.

  Ta, Re, Ir, Pd, Pt, and Ag that are elements of the <3> group all have a solid solution strengthening action by forming a solid solution with austenite that is a matrix. Therefore, in order to obtain higher strength at high temperature and creep rupture strength by the solid solution strengthening action, one or more of these elements may be positively contained within the following range.

Ta: 8.0% or less Ta dissolves in austenite which is a matrix, forms carbonitrides, and has an effect of improving high-temperature strength and creep rupture strength. Therefore, Ta may be contained to obtain these effects. However, if the Ta content exceeds 8.0%, workability and mechanical properties are impaired. Therefore, if Ta is contained, the amount of Ta is set to 8.0% or less. The Ta content is more preferably 7.0% or less, and further 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 forming a solid solution in austenite which is a matrix to improve high temperature strength and creep rupture strength, so Re may be contained in order to obtain these effects. However, if the Re content exceeds 8.0%, the workability and mechanical properties are impaired. Therefore, when Re is contained, the amount of Re is set to 8.0% or less. The Re content is more preferably 7.0% or less, still more preferably 6.0% or less. On the other hand, in order to surely obtain the above effects, the Re content is preferably 0.01% or more, more preferably 0.1% or more, and further preferably 0.5% or more. preferable.

Ir: 5.0% or less Ir has a function of improving the high temperature strength and the creep rupture strength while forming a solid solution with austenite which is a matrix and partly forming a fine intermetallic compound depending on the content. . Therefore, Ir may be contained to obtain these effects. However, if the Ir content exceeds 5.0%, the workability and mechanical properties are impaired. Therefore, the amount of Ir, if contained, is 5.0% or less. The Ir content is more preferably 4.0% or less, further preferably 3.0% or less. On the other hand, in order to reliably obtain the above effects, the Ir content is preferably 0.01% or more, more preferably 0.05% or more, and further preferably 0.1% or more. preferable.

Pd: 5.0% or less Pd has a function of forming a solid solution with austenite which is a matrix, and partly forming a fine intermetallic compound depending on the content, thereby improving high temperature strength and creep rupture strength. . Therefore, Pd may be contained in order to obtain these effects. However, if the Pd content exceeds 5.0%, the workability and mechanical properties are impaired. Therefore, when Pd is contained, the amount is 5.0% or less. The Pd content is more preferably 4.0% or less, further preferably 3.0% or less. On the other hand, in order to surely obtain the above effect, the Pd content is preferably 0.01% or more, more preferably 0.05% or more, further preferably 0.1% or more. preferable.

Pt: 5.0% or less Pt also forms a solid solution in austenite as a matrix, and partially forms a fine intermetallic compound depending on the content, and has an effect of improving high temperature strength and creep rupture strength. Therefore, Pt may be contained in order to obtain these effects. However, if the Pt content exceeds 5.0%, workability and mechanical properties are impaired. Therefore, when Pt is contained, the amount of Pt is set to 5.0% or less. The Pt content is more preferably 4.0% or less, and further preferably 3.0% or less. On the other hand, in order to surely obtain the above effect, the Pt content is preferably 0.01% or more, more preferably 0.05% or more, and further preferably 0.1% or more. preferable.

Ag: 5.0% or less Ag has a function of forming a solid solution in austenite which is a matrix, and partly forming a fine intermetallic compound depending on the content to improve high temperature strength and creep rupture strength. . Therefore, Ag may be contained to obtain these effects. However, if the Ag content exceeds 5.0%, the workability and mechanical properties are impaired. Therefore, when it is contained, the amount of Ag is 5.0% or less. The Ag content is more preferably 4.0% or less, and further preferably 3.0% or less. On the other hand, in order to surely obtain the above effect, the Ag content is preferably 0.01% or more, more preferably 0.05% or more, and further preferably 0.1% or more. preferable.

  The total content of Ta, Re, Ir, Pd, Pt, and Ag is preferably 10.0% or less, more preferably 5.0% or less.

2. Grain size Austenite grain size number in the outer surface part: -2.0 to 4.0
If the austenite grain size on the outer surface is too coarse, the 0.2% proof stress and tensile strength at room temperature will be low, while if it is 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. Incidentally, in the manufacturing process of the austenitic heat-resistant alloy member, by appropriately adjusting the heat treatment temperature and holding time after hot working and the cooling method, the grain size number of the outer surface portion after the final heat treatment should be within the above range. You can

3. Dimension Length from center to outer surface: 40 mm or more As described above, in the case of large-scale structural members, 0.2% proof stress and tensile strength at room temperature are low, and in addition, variations in creep rupture strength depend on the site. There is also a problem that occurs. However, the austenitic heat-resistant alloy member according to the present invention exhibits sufficient 0.2% proof stress and tensile strength at room temperature and creep rupture strength at high temperature as a large structural member. That is, the effect of the present invention is remarkably exerted on a thick alloy member. Therefore, in the austenitic heat resistant alloy member of the present invention, the length from the central portion to the outer surface portion is 40 mm or more in the cross section perpendicular to the longitudinal direction.

  The alloy member according to the present invention is a steel ingot or a slab obtained by continuous casting or the like, which is subjected to hot working such as hot forging or hot rolling. When the steel ingot is used, the longitudinal direction of the alloy member refers to the direction connecting the top portion and the bottom portion of the steel ingot, and when the cast slab is used, the lengthwise direction.

4. Cr precipitation amount obtained by analysis of extraction residue Cr _PB / Cr _PS ≤10.0 (i)
However, the meaning of each symbol in the formula (i) is as follows.
Cr _PB : Cr precipitation amount obtained by extraction residue analysis in the central portion Cr _PS : Cr precipitation amount obtained by extraction residue analysis in the outer surface portion Crystal grains after heat treatment after hot working in the manufacturing process of the alloy member Undissolved Cr precipitates (mainly carbides) are formed in the boundaries or grains. Particularly, since the cooling rate is slower in the central portion of the alloy member than in the outer surface portion, the amount of precipitates tends to increase. Therefore, the amount of Cr precipitation in the central portion becomes large with respect to the outer surface portion of the alloy member, and when the value of Cr _PB / Cr _PS exceeds 10.0, it becomes impossible to maintain high creep rupture strength at high temperature. On the other hand, it is not necessary to set the lower limit of Cr _PB / Cr _PS , but it is preferable to set it to 1.0 or more because the amount of precipitates in the central portion tends to increase more than in the outer surface portion.

The Cr precipitation amount obtained by extraction residue analysis means the content (% by mass) of Cr contained as undissolved Cr precipitates in the alloy member, and the extraction residue is obtained by electrolyzing the alloy member. , Can be obtained by quantitatively analyzing this extraction residue. Further, the Cr _PB / Cr _PS value is obtained by measuring the Cr precipitation amount in each of the central portion and the outer surface portion of the alloy member.

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 in the central portion YS _S : 0.2% yield strength in the outer surface portion TS _B : Tensile strength in the central portion TS _S : Tensile strength in the outer surface portion Cooling during heat treatment for large structural members Due to the difference in speed depending on the site, there is a tendency for large variations in the mechanical properties from site to site. In a large-sized structural member, if the 0.2% proof stress and the tensile strength at room temperature are greatly different between the central portion and the outer surface portion, there arises a problem that the specifications are not satisfied depending on the portion. Therefore, the mechanical properties of the austenitic heat resistant alloy member according to the present invention at room temperature satisfy the above formulas (ii) and (iii). It is not necessary to set lower limit values for each, but since the mechanical properties of the central part tend to be inferior to the mechanical properties of the outer surface part, both (ii) and (iii) should be 1.0 or more. It is preferable.

6. Creep Rupture Strength Since the austenitic heat resistant alloy member of the present invention is used in a high temperature environment, it is required to have high high temperature strength, particularly high creep rupture strength. Therefore, the alloy member of the present invention needs to have a creep rupture strength of 130 MPa or more at the center in the longitudinal direction at 700 ° C. for 10,000 hours.

7. Manufacturing Method The austenitic heat resistant alloy member of the present invention is manufactured by subjecting a steel ingot or a slab having the above-described chemical composition to hot working. In addition, in the above-mentioned hot working step, the treatment is performed so that the longitudinal direction of the final shape of the alloy member coincides with the longitudinal direction of the steel ingot or slab as a raw material. The hot working may be performed only in the longitudinal direction, but in order to give a higher working degree and make a more homogeneous structure, the hot working is performed once or more in the direction substantially perpendicular to the longitudinal direction. May be given. Further, after the hot working, hot working by a different method such as hot extrusion may be further performed, if necessary.

  In the production of the austenitic heat-resistant alloy member of the present invention, after the above steps, in order to suppress the variation in the metal structure and mechanical properties of each site, and to maintain a high creep rupture strength, the final described below. Heat treatment is applied.

  First, the alloy member after hot working is heated to a heat treatment temperature T (° C.) in the range of 1100 to 1250 ° C., and maintained at 1150 D / T to 1500 D / T (min) within the range. Here, D is, for example, the diameter (mm) of the alloy member when the alloy member has a columnar shape, and the diagonal distance (mm) when the alloy member has a rectangular columnar shape. 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 of the cross section perpendicular to the longitudinal direction of the alloy member.

If the heat treatment temperature is less than 1100 ° C., undissolved chromium carbides and the like increase and the creep rupture strength decreases. On the other hand, when the temperature exceeds 1250 ° C, the grain boundary is melted or the crystal grains are significantly coarsened, so that the ductility is lowered and the creep rupture strength is also lowered. The heat treatment temperature is more preferably 1150 ° C. or higher, and more preferably 1230 ° C. or lower. If the holding time is less than 1150 D / T (min), the amount of undissolved chromium carbide in the central portion increases and Cr _PB / Cr _PS falls outside the range specified by the present invention. On the other hand, if it exceeds 1500 D / T (min), the crystal grains on the outer surface become coarse, and the austenite grain size number falls outside the range specified by the present invention.

  After heating and holding, the alloy member is immediately cooled with water. This is because if the cooling rate is slow, a large amount of undissolved Cr precipitates may be generated in the crystal grain boundaries or within the grains particularly in the central part of the alloy member, and the above formula (i) may not be satisfied.

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

  Alloys having the chemical compositions shown in Table 1 were melted in a high-frequency vacuum melting furnace to obtain steel ingots 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 and 4, after hot forging in the longitudinal direction and before final heat treatment, forging was performed in a direction substantially perpendicular to the longitudinal direction, and then final hot forging was performed in the longitudinal direction. .

  For each sample, a test piece for observing the structure was taken from the outer surface portion, the cross section in the longitudinal direction was polished with an emery paper and a buff, and then corroded with a mixed acid to observe with an optical microscope. The crystal grain size number of the observation surface was determined according to the determination method by the intersecting line segment (particle size) specified in JIS G 0551 (2013).

Next, test pieces for measuring Cr precipitates were taken from the central portion and the outer surface portion in the cross section perpendicular to the longitudinal direction of each sample. After determining the surface area of each of the above test pieces, only the base material of the alloy member was completely electrolyzed in a 10% acetylacetone-1% tetramethylammonium chloride-methanol solution under an electrolysis condition of 20 mA / cm ² to form precipitates. It is extracted as a residue, and the content (% by mass) of Cr contained as an undissolved Cr precipitate is measured by quantitatively analyzing the extracted residue, and the value of Cr _PB / Cr _PS is obtained based on the measured value. It was

  Also, from the center and outer surface of each sample, tensile test pieces with a parallel length of 40 mm were cut out by machining in parallel with the longitudinal direction, and subjected to a tensile test at room temperature to obtain 0.2% proof stress and tensile strength. I asked for strength. Further, a creep rupture test piece having a parallel portion length of 30 mm was cut out from the center portion of each sample in parallel with the longitudinal direction by machining. Then, a creep rupture test was carried out in the atmosphere of 700 ° C., 750 ° C. and 800 ° C., and the creep rupture strength at 700 ° C. and 10,000 hours was obtained using the Larson-Miller parameter method.

  The results are summarized in Table 3.

Alloys A and B have almost the same chemical composition as alloy 1 and have the same final shape by hot forging. However, the holding time during the heat treatment is out of the range of the manufacturing conditions specified in the present invention. Due to this, the grain size number of the outer surface portion of alloy A is out of the specified range of the present invention, and the values of YS _S / YS _B and TS _S / TS _B are outside the specified range of the present invention. The result is that the mechanical properties vary greatly depending on the part. Further, since alloy B has a high Cr _PB / Cr _{PS and} is outside the range specified by the present invention, the creep rupture strength is also outside the range specified by the present invention, which is significantly lower than that of alloy 1. .

Alloys C, D and E have almost 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, and compared with alloy 2. The result was that creep rupture strength was extremely low. Since the heat treatment temperature of Alloy D is higher than the specified range of the present invention, the 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. As a result, the creep rupture strength was remarkably lower than that of Alloy 2. Further, the alloy E had a cooling method at the time of the final heat treatment that was air cooling instead of water cooling, and the value of Cr _PB / Cr _PS was out of the specified range of the present invention due to the significantly slow cooling rate, and as a result, The creep rupture strength was significantly lower than that of Alloy 2. On the other hand, Alloys 1 to 4 satisfying all the requirements of the present invention had small variations in mechanical properties and good creep rupture strength.

INDUSTRIAL APPLICABILITY The austenitic heat-resistant alloy member of the present invention has little variation in mechanical properties depending on the site and is excellent in creep rupture strength at high temperatures. Therefore, the austenitic heat-resistant alloy member of the present invention can be suitably used as a large-scale structural member such as a boiler and a chemical plant used in a high temperature environment.

Claims (4)

In mass%,
C: more than 0.02% and 0.15% or less,
Si: 2.0% or less,
Mn: 3.0% or less,
P: 0.030% or less,
S: 0.010% or less,
Cr: 28.0 to 38.0%,
Ni: more than 40.0% and 60.0% or less,
W: more than 3.0% and 15.0% or less,
Ti: 0.05 to 1.0%,
Zr: 0.005-0.2%,
Al: 0.01 to 0.3%,
N: 0.02% or less,
Mo: less than 0.50%,
B: 0.005% or less,
Co: 0 to 20.0%,
The balance: an alloy member obtained by hot working a steel ingot or a slab having a chemical composition of Fe and impurities,
In a cross section perpendicular to the longitudinal direction of the alloy member, the length from the central 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 Cr precipitation amount obtained by extraction residue analysis satisfies the following formula (i),
The mechanical properties at room temperature satisfy the following equations (ii) and (iii),
An austenitic heat resistant alloy member having a 10,000-hour creep rupture strength of 130 MPa or more at 700 ° C. in the longitudinal direction in the central portion.
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 deposition amount obtained by extraction residue analysis at the center cr _PS: Cr deposition amount obtained by extraction residue analysis in the outer surface portion YS _B: 0.2% proof stress YS _S at the center: 0.2 the outer surface portion % Yield strength TS _B : Tensile strength in the central part TS _S : Tensile strength in the outer surface The chemical composition of the steel ingot or the slab contains, in mass%, one or more elements belonging to one or more groups selected from the following groups <1> to <3>. The austenitic heat resistant alloy member described.
<1> Nb: 1.0% or less, V: 1.5% or less and Hf: 1.0% or less <2> Mg: 0.050% or less, Ca: 0.050% or less and REM: 0.50 % Or less <3> Ta: 8.0% or less, Re: 8.0% or less, Ir: 5.0% or less, Pd: 5.0% or less, Pt: 5.0% or less and Ag: 5.0. %Less than A method for producing the austenitic heat-resistant alloy member according to claim 1 or 2,
A step of hot working a steel ingot or a slab having the chemical composition according to claim 1 or 2;
Then, a heat treatment is performed to heat treatment temperature T (° C.) in the range of 1100 to 1250 ° C., hold at 1150 D / T to 1500 D / T (min), and then heat-treat by water cooling. Method.
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 of the cross section perpendicular to the longitudinal direction of the alloy member.   The method for producing an austenitic heat resistant alloy member according to claim 3, wherein in the step of performing the hot forging, the forging is performed once or more in a direction substantially perpendicular to the longitudinal direction of the hot forging.