JP6520546B2 - Austenitic heat-resistant alloy member and method of manufacturing the same - Google Patents

Description

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

  Conventionally, in boilers and chemical plants used under high temperature environments, 18-8 series austenitic stainless steels such as SUS304H, SUS316H, SUS321H, and SUS347H have been used as apparatus materials.

  However, in recent years, construction of ultra-supercritical pressure boilers, in which the temperature and pressure of steam are increased for high efficiency, has been promoted all over the world. The operating conditions of the apparatus under such high temperature environment are extremely severe, and the performance requirements for the used materials are becoming strict accordingly. And in addition to corrosion resistance, the high temperature strength, in particular, the creep rupture strength is extremely insufficient in the conventionally used 18-8 austenitic stainless steel.

  Various studies have been made to solve the above problems. For example, Patent Documents 1 to 4 disclose good high corrosion resistant austenitic steels of 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 Cr content is increased to 20% or more, and W and / or Mo is contained to improve the high temperature strength.

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

  By the way, since large-scale structural members such as boiler and chemical plant materials are used by performing final heat treatment without cold working after hot rolling or hot forging, the crystal grain size Is relatively large. Therefore, there is a problem that 0.2% proof stress and tensile strength at normal temperature defined as specifications of materials are generally lower than those subjected to final heat treatment after cold working.

  In addition, in a large-sized structural member, since the cooling rate at the time of heat treatment largely varies depending on the site, the amount of solid solution element contributing to strengthening as precipitates at the time of use at high temperature 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 steels described in Patent Documents 1 to 5 to large-sized structural members.

  The present invention solves the above problems, and an austenitic heat-resistant alloy member exhibiting 0.2% proof stress and tensile strength at normal temperature sufficient as a large structural member, and creep rupture strength at high temperature, and a method for producing the same Intended to provide.

  The present invention has been made to solve the above-described problems, and the gist of the austenitic heat-resistant alloy member and the method for producing the same are as described below.

(1) mass%,
C: 0.020 to 0.120%,
Si: 2.00% or less,
Mn: 3.00% or less,
P: 0.030% or less,
S: 0.010% or less,
Cr: 20.0% or more and less than 28.0%,
Ni: more than 35.0% and less than 55.0%,
Co: 0 to 20.0%,
W: 4.0 to 10.0%,
Ti: 0.01 to 0.50%,
Nb: 0.01 to 1.00%,
Mo: less than 0.50%,
Al: 0.30% or less,
N: less than 0.100%,
Remainder: An alloy member obtained by hot working a steel ingot or slab having a chemical composition which is 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 deposition amount obtained by extraction residue analysis satisfies the following equation (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 100 MPa or more for 10,000 hours 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 in the central portion Cr _PS : Cr precipitation amount obtained by extraction residue analysis in the outer surface portion YS _B : 0.2% proof stress in the central portion YS _S : 0.2 in the outer surface portion % Proof stress TS _B : tensile strength at the center TS _S : tensile strength at the outer surface

(2) The chemical composition of the steel ingot or slab contains, in mass%, one or more elements belonging to one or more groups further selected from the following <1> to <3> groups: The austenitic heat-resistant alloy as described in (1).
<1> Mg: 0.0500% or less, Ca: 0.0500% or less, and REM: 0.50% or less <2> V: 1.5% or less, B: 0.0100% or less, Zr: 0.10 % Or less and Hf: 1.0% or less <3> Ta: 8.0% or less, Re: 8.0% or less

(3) a step of subjecting a steel ingot or slab having the chemical composition described in (1) or (2) above to hot working;
Thereafter, the material is heated to a heat treatment temperature T (° C.) in the range of 1100 to 1250 ° C., held at 1000 D / T to 1400 D / T (min), and then subjected to a heat treatment for water cooling; Method.
However, D is the maximum value (mm) of the linear distance between any point on the outer edge of the cross section and any other point on the outer edge in a 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 forging is performed once or more in the direction substantially perpendicular to the longitudinal direction of hot forging in the step of performing the hot forging.

  The austenitic heat-resistant alloy member of the present invention has less variation in mechanical properties depending on the part, 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-sized structural member such as a boiler and a chemical plant used under a high temperature environment.

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

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

C: 0.020 to 0.120%
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 member. Therefore, the C content needs to be 0.020% or more. However, when the content exceeds 0.120%, not only undissolved carbides are generated, but Cr carbides are also increased to deteriorate mechanical properties such as ductility and toughness and weldability. Therefore, the C content is set to 0.020 to 0.120%. The C content is preferably 0.050% or more, and preferably 0.100% or less.

Si: 2.00% or less Si is contained as a deoxidizing element. Further, Si is an element effective also for enhancing the oxidation resistance, the steam oxidation resistance and the like. It is also an element that improves the flow of the molten metal by casting material. However, if the Si content exceeds 2.00%, the formation of intermetallic compounds such as the σ phase is promoted, so that the stability of the structure at high temperatures is deteriorated, leading to a decrease in toughness and ductility. Furthermore, the weldability also decreases. Therefore, the Si content is 2.00% or less. When importance is placed on tissue stability, the Si content is preferably 1.00% or less. When the deoxidizing effect is sufficiently ensured by other elements, it is not necessary to set a lower limit in particular for the Si content. However, when importance is attached to deoxidation, oxidation resistance, steam oxidation resistance and the like, the Si content is preferably 0.05% or more, and more preferably 0.10% or more.

Mn: 3.00% or less Mn, as well as Si, has a deoxidizing action, and also has an action of fixing S contained inevitably in the alloy as a sulfide and improving the ductility at high temperatures. However, when the Mn content exceeds 3.00%, the precipitation of intermetallic compounds such as the σ phase is promoted, and thus the mechanical stability such as the structure stability and the high temperature strength is deteriorated. Therefore, the Mn content is 3.00% or less. The Mn content is preferably 2.00% or less, more preferably 1.50% or less. Although it is not necessary to set a lower limit for the Mn content, if importance is given to the ductility improving action at high temperatures, the Mn content is preferably 0.10% or more, and 0.20% or more. More preferable.

P: 0.030% or less P is inevitably mixed in the alloy as an impurity, and significantly reduces weldability and ductility at high temperatures. Therefore, the P content is made 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 inevitably mixed in the alloy as an impurity as in the case of P, and significantly reduces the weldability and the ductility at high temperatures. Therefore, the S content is made 0.010% or less. When the hot workability is important, the S content is preferably 0.005% or less, more preferably 0.003% or less.

Cr: 20.0% or more and less than 28.0% Cr is an important element that exerts an excellent action in improving the corrosion resistance such as oxidation resistance, water vapor oxidation resistance, high temperature corrosion resistance and the like. However, if the content is less than 20.0%, these effects can not be obtained. On the other hand, when the Cr content is increased, in particular, to 28.0% or more, destabilization of the structure due to precipitation of the σ phase or the like is caused, 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, more preferably 22.0% or more. Further, the Cr content is preferably 26.0% or less, more preferably 25.0% or less.

Ni: more than 35.0% and 55.0% or less Ni is an element that stabilizes the austenite structure, and is also an important element to ensure corrosion resistance. From the balance with the above-mentioned Cr content, Ni needs to be contained in excess of 35.0%. On the other hand, if the Ni content is excessive, the cost increases, so the content is made 55.0% or less. The Ni content is preferably 40.0% or more, more preferably 42.0% or more. Further, the Ni content is preferably 50.0% or less, more preferably 48.0% or less.

Co: 0 to 20.0%
Co does not necessarily have to be contained, but as it stabilizes the austenite structure like Ni and contributes to the improvement of creep rupture strength, it may be contained instead of a part of Ni. However, if the content exceeds 20.0%, the effect is saturated and the economic efficiency is also reduced. Therefore, the Co content is set to 0 to 20.0%. The Co content is preferably 15.0% or less. In addition, in order to acquire said effect, it is preferable to make Co content into 0.5% or more.

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

Ti: 0.01 to 0.50%
Ti is an element having the effect of forming carbonitrides to improve creep rupture strength. However, if the Ti content is less than 0.01%, a sufficient effect can not be obtained, while if it exceeds 0.50%, the ductility at high temperatures is reduced. Therefore, the Ti content is 0.01 to 0.50%. The Ti content is preferably 0.05 or more, more preferably 0.10% or more. The Ti content is preferably 0.40% or less, more preferably 0.35% or less.

Nb: 0.01 to 1.00%
Nb has the effect of forming carbonitrides to improve creep rupture strength. However, if the Nb content is less than 0.01%, a sufficient effect can not be obtained, while if it exceeds 1.00%, the ductility at high temperatures is reduced. Therefore, the Nb content is 0.01 to 1.00%. The Nb content is preferably 0.10% or more. The Nb content is preferably 0.90% or less, more preferably 0.70% or less.

Mo: less than 0.50% Conventionally, Mo has been considered as an element having a function equivalent to that of W as a solid solution strengthening element and contributing to the improvement of creep rupture strength as a solid solution strengthening element. However, according to the study of the present inventors, in the case where Mo is contained in combination in the alloy containing W and Cr in the above-mentioned amounts, the sigma phase may precipitate when it is used for a long time, For this reason, it was found that creep rupture strength, ductility and toughness may be reduced. Therefore, the Mo content is desirably as low as possible, and is less than 0.50%. In addition, it is more preferable to limit Mo content 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%, the ductility at high temperatures is deteriorated. Therefore, the Al content is 0.30% or less. The Al content is preferably 0.25% or less, more preferably 0.20% or less. In addition, in order to acquire said effect, it is preferable to make Al content into 0.01% or more, and it is more preferable to set it as 0.02% or more.

N: less than 0.100% N is an element having the effect of stabilizing the austenite structure, and is an element which is inevitably contained in the ordinary melting method. However, in the present invention where the inclusion of Ti is essential, it is better to reduce as much as possible to avoid the consumption of Ti due to the formation of TiN. However, in the case of atmospheric dissolution, the N content is less than 0.100% because it is extremely difficult to reduce it.

  In the chemical composition of the austenitic heat-resistant alloy of the present invention, the balance is Fe and impurities. Preferably, 0.1 to 40.0% of Fe is contained. Furthermore, the term "impurity" as used herein refers to a raw material such as ore, scrap, etc. when industrially producing an alloy, and a component to be mixed in by various factors of the manufacturing process, as long as the present invention is not adversely affected. Means something that is 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 <1> to <3> groups.
<1> Mg: 0.0500% or less, Ca: 0.0500% or less, and REM: 0.50% or less <2> V: 1.5% or less, B: 0.0100% or less, Zr: 0.10 % Or less and Hf: 1.0% or less <3> Ta: 8.0% or less, Re: 8.0% or less

  All of Mg, Ca and REM which are elements of the group <1> have the function of fixing S as a sulfide to improve high temperature ductility. For this reason, in order to obtain better high temperature ductility, one or more of these elements may be positively contained in the following range.

Mg: 0.0500% or less Mg has the function of fixing S as sulfide which inhibits ductility at high temperature as sulfide to improve the high temperature ductility, so Mg may be contained to obtain this effect. However, if the Mg content exceeds 0.0050%, the cleanliness is lowered and the high temperature ductility is impaired. Therefore, the amount of Mg in the case of being contained is made into 0.0050% or less. The Mg content is more preferably 0.0200% or less, further preferably 0.0100% 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 0.0010% or more.

Ca: 0.0500% or less Since Ca has an action of fixing S as sulfide which inhibits ductility at high temperature to improve high temperature ductility, Ca may be contained to obtain this effect. However, when the Ca content exceeds 0.0050%, the cleanliness is lowered and the high temperature ductility is impaired. Therefore, the amount of Ca in the case of being contained is made into 0.0050% or less. The Ca content is more preferably 0.0200% or less, still more preferably 0.0100% or less. On the other hand, in order to reliably obtain the above effects, the Ca content is preferably 0.0005% or more, and more preferably 0.0010% or more.

REM: 0.50% or less REM fixes S as sulfide to improve high temperature ductility. In addition, REM improves the adhesion of the Cr ₂ O ₃ protective film on the steel surface, and in particular, improves the oxidation resistance at the time of repeated oxidation, and contributes to grain boundary strengthening, and also creep rupture strength. And creep rupture ductility. However, when the REM content exceeds 0.50%, inclusions such as oxides increase and the processability and weldability are impaired. Therefore, the amount of REM in the case of containing is made 0.50% or less. The REM content is more preferably 0.30% or less, still more preferably 0.15% or less. On the other hand, in order to reliably obtain the above effects, 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 still more preferably 0.2% or less.

  The elements V, B, Zr and Hf which are elements of the <2> group all have the function of improving the high temperature strength and the creep rupture strength. For this reason, when it is desired to obtain 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 the function of forming carbonitrides to improve high-temperature strength and creep rupture strength. Therefore, V may be contained to obtain these effects. However, when the V content exceeds 1.5%, the high temperature corrosion resistance is lowered, and furthermore, the ductility and toughness are deteriorated due to the precipitation of the embrittled phase. Therefore, the amount of V in the case of making it contain is 1.5% or less. The V content is more preferably 1.0% or less. On the other hand, in order to reliably obtain the above effects, the V content is preferably 0.02% or more, and more preferably 0.04% or more.

B: 0.0100% or less B is present in the carbide or in the matrix and not only promotes the refinement of the precipitated carbide, but also has the effect of enhancing the creep rupture strength by strengthening the grain boundaries. However, when the B content exceeds 0.0100%, the ductility at high temperatures is lowered and the melting point is also lowered. Therefore, the amount of B in the case of containing is made 0.0100% or less. The B content is more preferably 0.0080% or less, still more preferably 0.0050% or less. On the other hand, in order to reliably obtain the above effects, the B content is preferably 0.0005% or more, more preferably 0.001% or more, and still more preferably 0.0015% or more. preferable.

Zr: 0.10% or less Zr is an element that promotes the refinement of carbonitrides 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 reduced. Therefore, the amount of Zr in the case of being contained is made 0.10% or less. The Zr content is more preferably 0.06% or less, still more preferably 0.05% or less. On the other hand, in order to reliably obtain the above effects, the Zr content is preferably 0.005% or more, and more preferably 0.01% or more.

Hf: 1.0% or less Hf has the function of contributing to precipitation strengthening as carbonitrides and improving the creep rupture strength, so Hf may be contained to obtain these effects. However, when the Hf content exceeds 1.0%, processability and weldability are impaired. Therefore, the amount of Hf to be contained is made 1.0% or less. The Hf content is more preferably 0.8% or less, still more preferably 0.5% or less. On the other hand, in order to reliably obtain the above effects, the Hf content is preferably 0.005% or more, more preferably 0.01% or more, and still more 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.

  Both elements Ta and Re, which are elements of the <3> group, have a solid solution strengthening action by solid solution in the matrix austenite. Therefore, 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 forms carbonitrides and has the effect of improving high temperature strength and creep rupture strength as a solid solution strengthening element. Therefore, Ta may be contained to obtain these effects. However, when the Ta content exceeds 8.0%, the processability and the mechanical properties are impaired. Therefore, the amount of Ta when it is contained is made 8.0% or less. The Ta content is more preferably 7.0% or less, and still more preferably 6.0% or less. On the other hand, in order to reliably obtain the above effects, the Ta content is preferably 0.01% or more, more preferably 0.1% or more, and still more preferably 0.5% or more. preferable.

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

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

2. Austenite grain size number in the outer surface part: -2.0 to 4.0
If the austenite grain size in the outer surface portion is too coarse, 0.2% proof stress and tensile strength at normal temperature will be low, while if too fine, high creep rupture strength at high temperature can not be maintained. Therefore, the austenite grain size number in the outer surface portion is -2.0 to 4.0. In the manufacturing process of the Ni-based alloy, 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 part after the final heat treatment can be made into the above range. .

3. Dimension Length from the center to the outer surface: 40 mm or more As mentioned above, in a large-sized structural member, in addition to the 0.2% proof stress and tensile strength becoming low at room temperature, the creep rupture strength varies depending on the part There is also a problem that However, the austenitic heat-resistant alloy member according to the present invention exhibits sufficient 0.2% proof stress and tensile strength at normal temperature as a large-sized structural member, and creep rupture strength at high temperature. That is, the effects of the present invention are remarkably exhibited for thick alloy members. Therefore, in the austenitic heat-resistant alloy member of the present invention, in the cross section perpendicular to the longitudinal direction, the length from the central portion to the outer surface portion is set to 40 mm or more.

  The alloy member according to the present invention is obtained by subjecting a slab obtained by steel ingot or continuous casting or the like to hot working such as hot forging or hot rolling. And when using a steel ingot, the longitudinal direction of an alloy member points out the direction which ties the top part and bottom part of a steel ingot, and when using a slab, it points out the length direction.

4. Cr precipitation amount 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 precipitation amount obtained by extraction residue analysis in the center part Cr _PS : Cr precipitation amount obtained by extraction residue analysis in the outer surface part In the manufacturing process of alloy members, crystal grains after heat treatment after hot working Precipitates (mostly carbides) of undissolved Cr form in the boundaries or in the grains. In particular, since the cooling rate is slower at the center of the alloy member than at the outer surface, the amount of precipitates tends to increase. Therefore, the amount of Cr deposition in the central portion with respect to the outer surface portion of the alloy member increases, and when the value of Cr _PB / Cr _PS exceeds 10.0, it is impossible to maintain high creep rupture strength at high temperature. On the other hand, it is not necessary to set the lower limit value of Cr _PB / Cr _PS , but it is preferable to set it to 1.0 or more because the amount of precipitates tends to increase in the central portion than in the outer surface portion.

The Cr deposition amount obtained by the extraction residue analysis means the content (mass%) of Cr contained as an undissolved Cr precipitate in the alloy member, and the above-mentioned alloy member is electrolyzed to obtain an extraction residue. This extraction residue can be determined by quantitative analysis. Further, the value of Cr _PB / Cr _PS is determined by measuring the amount of Cr deposition 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% proof stress at the center YS _S : 0.2% proof stress at the outer surface TS _B : Tensile strength at the center TS _S : Tensile strength at the outer surface For large structural members, cooling during heat treatment Due to the differences in speed between sites, 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 tensile strength at normal temperature largely differ between the central portion and the outer surface portion, there arises a problem that the specification is not satisfied depending on the part. Therefore, in the austenitic heat-resistant alloy member according to the present invention, the mechanical properties at normal temperature satisfy the above-mentioned equations (ii) and (iii). Although it is not necessary to set the lower limit value for each, the mechanical properties of the central portion tend to be inferior to the mechanical properties of the outer surface portion, so both of the formulas (ii) and (iii) Is preferred.

6. Creep rupture strength Since the austenitic heat-resistant alloy member of the present invention is used under a high temperature environment, high high temperature strength, particularly high creep rupture strength is required. Therefore, the alloy member of the present invention needs to have a creep rupture strength of 100 MPa or more for 10,000 hours at 700 ° C. in the longitudinal direction at the central portion thereof.

7. Manufacturing Method The austenitic heat-resistant alloy member of the present invention is manufactured by subjecting a steel ingot or slab having the above-mentioned chemical composition to hot working. In the above-mentioned hot working process, processing is performed so that the longitudinal direction in the final shape of the alloy member coincides with the longitudinal direction of the steel ingot or slab serving as the material. The hot working may be performed only in the longitudinal direction, but in order to give a higher degree of working and to make a more homogeneous structure, the hot working is performed once or more in the direction substantially perpendicular to the longitudinal direction. It may be applied. Moreover, after the said hot processing, you may further perform the hot processing of different methods, such as hot extrusion, as needed.

  When manufacturing the austenitic heat-resistant alloy member of the present invention, after the above steps, in order to suppress variations in the metallographic structure and mechanical properties of each part and maintain high creep rupture strength, the final described below. Apply heat treatment.

  First, the hot-worked alloy member is heated to a heat treatment temperature T (° C.) in the range of 1100 ° C. to 1250 ° C., and held in the range of 1000 D / T to 1400 D / T (min). Here, D is, for example, the diameter (mm) of the alloy member when the alloy member is cylindrical, and the diagonal distance (mm) when the alloy member is quadrangular prism. That is, D is the maximum value (mm) of the linear distance between any point on the outer edge of the cross section and any other point on the outer edge in the cross section perpendicular to the longitudinal direction of the alloy member.

If the heat treatment temperature is less than 1100 ° C., the 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 is lowered due to melting of grain boundaries and coarsening of crystal grains. The heat treatment temperature is more preferably 1150 ° C. or more, and more preferably 1230 ° C. or less. Further, if the holding time is less than 1000 D / T (min), the non-dissolved chromium carbide in the central portion increases, and the Cr _PB / Cr _PS is out of the range specified in the present invention. On the other hand, when it exceeds 1400 D / T (min), the crystal grains of the outer surface part become coarse, and the austenite grain size number is out of the range defined in the present invention.

  After heating and holding, the alloy member is immediately water-cooled. If the cooling rate becomes slow, a large amount of undissolved Cr precipitates may be generated in grain boundaries or grains particularly in the center of the alloy member, and the above equation (i) may not be satisfied.

  EXAMPLES Hereinafter, the present invention will be more specifically described by way of examples, but 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 a final heat treatment was performed 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, 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 observation of tissue was collected from the outer surface, and a cross section in the longitudinal direction was polished with an emery paper and a buff, corroded with mixed acid, and observed with an optical microscope. The grain size number of the observation surface was determined according to the determination method based on the intersecting line segment (grain size) defined in JIS G 0551 (2013).

Next, test pieces for measuring Cr deposits were taken from the center and the outer surface in a cross section perpendicular to the longitudinal direction of each sample. After the surface area of the above test piece was determined, only the base material of the alloy member was completely electrolyzed under an electrolysis condition of 20 mA / cm ² in 10% acetylacetone-1% tetramethyl ammonium chloride-methanol solution respectively to obtain a precipitate Extract the residue as a residue, and analyze the extracted residue quantitatively to measure the content (mass%) of Cr contained as an undissolved Cr precipitate, and determine the value of Cr _PB / Cr _PS based on the measured value. The

  In addition, a tensile test specimen with a parallel length of 40 mm is cut out from the center and outer surface of each sample in parallel to the longitudinal direction, and a tensile test is performed at room temperature, 0.2% proof stress and tension I asked for strength. Furthermore, a creep rupture test piece with a parallel portion of 30 mm in length was cut out by machining from the center of each sample in parallel to the longitudinal direction. Then, a creep rupture test was conducted in the atmosphere at 700 ° C., 750 ° C. and 800 ° C., and the creep rupture strength at 700 ° C. for 10,000 hours was determined 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 at the time of heat treatment is outside the range of the manufacturing conditions defined in the present invention. Due to that, the grain size number of the outer surface 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 out of the specified range of the present invention. The result was that the variation in mechanical characteristics increased depending on the part. Further, the creep rupture strength of the alloy B was out of the specified range of the present invention, and the result was extremely low as compared with the 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 alloy C has a heat treatment temperature 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 out of the range specified in the present invention. The creep rupture strength was extremely low. Since alloy D has a heat treatment temperature higher than the specified range of the present invention, the grain size number of the outer surface and the values of YS _S / YS _B and TS _S / TS _B are out of the specified range of the present invention, The creep rupture strength was significantly lower than that of alloy 2. In addition, since the cooling method at the time of final heat treatment in the alloy E is not water cooling but air cooling, and the cooling rate is extremely low, the value of Cr _PB / Cr _PS falls outside the specified range of the present invention. The creep rupture strength was significantly lower than that of alloy 2. On the other hand, in the alloys 1 to 4 satisfying all the requirements of the present invention, the variation in mechanical properties was small and the creep rupture strength was also good.

The austenitic heat-resistant alloy member of the present invention has less variation in mechanical properties depending on the part, 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-sized structural member such as a boiler and a chemical plant used under a high temperature environment.

Claims (4)

In mass%,
C: 0.020 to 0.120%,
Si: 2.00% or less,
Mn: 3.00% or less,
P: 0.030% or less,
S: 0.010% or less,
Cr: 20.0% or more and less than 28.0%,
Ni: more than 35.0% and less than 55.0%,
Co: 0 to 20.0%,
W: 4.0 to 10.0%,
Ti: 0.01 to 0.50%,
Nb: 0.01 to 1.00%,
Mo: less than 0.50%,
Al: 0.30% or less,
N: less than 0.100%,
Remainder: An alloy member obtained by hot working a steel ingot or slab having a chemical composition which is Fe and impurities,
In a cross section perpendicular to the longitudinal direction of the alloy member, the minimum value of 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 deposition amount obtained by extraction residue analysis satisfies the following equation (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 100 MPa or more for 10,000 hours 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 in the central portion Cr _PS : Cr precipitation amount obtained by extraction residue analysis in the outer surface portion YS _B : 0.2% proof stress in the central portion YS _S : 0.2 in the outer surface portion % Proof stress TS _B : tensile strength at the center TS _S : tensile strength at the outer surface The chemical composition of the steel ingot or slab contains, in mass%, one or more elements belonging to one or more groups further selected from the following <1> to <3> groups: The austenitic heat-resistant alloy member as described.
<1> Mg: 0.0500% or less, Ca: 0.0500% or less, and REM: 0.50% or less <2> V: 1.5% or less, B: 0.0100% or less, Zr: 0.10 % Or less and Hf: 1.0% or less <3> Ta: 8.0% or less, Re: 8.0% or less A method for producing an austenitic heat-resistant alloy according to claim 1 or 2, wherein
A step of subjecting a steel ingot or slab having the chemical composition according to claim 1 or 2 to hot working;
Thereafter, the material is heated to a heat treatment temperature T (° C.) in the range of 1100 to 1250 ° C., held at 1000 D / T to 1400 D / T (min), and then subjected to a heat treatment for water cooling; Method.
However, D is the maximum value (mm) of the linear distance between any point on the outer edge of the cross section and any other point on the outer edge in a cross section perpendicular to the longitudinal direction of the alloy member. In the step of performing processing between the hot, subjected to hot working at least once in the longitudinal direction substantially perpendicular to the direction of hot working, manufacturing method of austenitic heat resistant alloy member of claim 3.