CN110268079A - Austenitic heat-resistant alloy and manufacturing method thereof - Google Patents

Abstract

An austenitic heat-resistant alloy, wherein the chemical composition of the alloy is C: 0.02-0.12%, Si: 2.0% or less, Mn: 3.0% or less, P: 0.030% or less, S: 0.015% in mass % Below, Cr: 20.0% to less than 28.0%, Ni: more than 35.0% to 55.0%, Co: 0 to 20.0%, W: 4.0 to 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: less than 0.30%, 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: 0~0.01%, Zr: 0~0.10%, Hf: 0~1.0%, Ta: 0~8.0%, Re: 0~8.0%, balance: Fe and impurities, in the aforementioned alloy and In the section perpendicular to the length direction, the shortest distance from the center to the outer surface is more than 40 mm, the austenite grain size number of the outer surface is -2.0 to 4.0, and the amount of Cr existing in the form of precipitates satisfies [Cr _PB /Cr _PS ≤10.0], [YS _S /YS _B ≤1.5] and [TS _S /TS _B ≤1.2] at room temperature.

Description

Austenitic heat-resistant alloy and manufacturing method thereof

technical field

The invention relates to an austenitic heat-resistant alloy and a manufacturing method thereof.

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

However, in recent years, in order to increase efficiency, new construction of ultra-supercritical pressure boilers with increased steam temperature and pressure has been carried out all over the world. The service conditions of the device in such a high-temperature environment are obviously extremely severe, and the performance required for the materials used is also strict. In addition, the 18-8 series austenitic stainless steel used conventionally is in a situation where high temperature strength, especially creep rupture strength is remarkably insufficient in addition to corrosion resistance.

In order to solve the above-mentioned problems, various studies have been conducted so far. For example, Patent Documents 1 to 4 disclose a highly corrosion-resistant austenitic steel having excellent high-temperature strength. In addition, Patent Document 5 discloses an austenitic stainless steel excellent in high-temperature strength and corrosion resistance. According to Patent Documents 1 to 5, by increasing the amount of Cr to 20% or more and including W and/or Mo, high-temperature strength can be improved.

prior art literature

patent documents

Patent Document 1: Japanese Patent Application Laid-Open No. 61-179833

Patent Document 2: Japanese Patent Application Laid-Open No. 61-179834

Patent Document 3: Japanese Patent Laid-Open No. 61-179835

Patent Document 4: Japanese Patent Laid-Open No. 61-179836

Patent Document 5: Japanese Patent Laid-Open No. 2004-3000

Contents of the invention

The problem to be solved by the invention

However, since large structural members such as equipment materials for thermal power generation boilers and chemical equipment are not subjected to cold working after hot rolling or hot forging, but are used after final heat treatment, the crystal grain size is relatively large. Therefore, generally, there is a problem that the 0.2% conditional yield strength and tensile strength at room temperature specified as material specifications are lower than those when final heat treatment is performed after cold working.

In addition, in large structural members, the cooling rate during heat treatment greatly varies depending on the location, so the amount of solid-solution elements that contribute to strengthening as precipitates when used at high temperatures varies depending on the location. Due to this, there is also a problem that unevenness 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 object of the present invention is to solve the above-mentioned problems, and to provide an austenitic heat-resistant alloy and its method of manufacture.

solutions to problems

The present invention was made in order to solve the above-mentioned problems, and the gist of the following austenitic heat-resistant alloy and its production method is as its gist.

(1) An austenitic heat-resistant alloy, wherein the chemical composition of the alloy is calculated as

C: 0.02～0.12%,

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% to 55.0% or less,

Co: 0 to 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: 0～0.05%,

Ca: 0-0.05%,

REM: 0～0.50%,

V: 0～1.5%,

B: 0～0.01%,

Zr: 0～0.10%,

Hf: 0～1.0%,

Ta: 0 to 8.0%,

Re: 0～8.0%,

Balance: Fe and impurities,

In the cross-section perpendicular to the longitudinal direction of the aforementioned alloy, the shortest distance from the center part to the outer surface part is 40 mm or more,

The austenite grain size number of the aforementioned outer surface is -2.0 to 4.0,

The amount of Cr present in the form of precipitates obtained by extraction residue analysis satisfies the following formula (i),

The mechanical properties at room temperature of the austenitic heat-resistant alloy satisfy the following formulas (ii) and (iii).

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 said formula is as follows.

Cr _PB : The amount of Cr present in the form of precipitates in the central part obtained by analysis of the extraction residue

Cr _PS : The amount of Cr existing in the form of precipitates obtained by extraction residue analysis on the outer surface

YS _B : 0.2% conditional yield strength of the central part

YS _S : 0.2% conditional yield strength of the outer surface

TS _B : Tensile strength of the central part

TS _S : Tensile strength of the outer surface

(2) The austenitic heat-resistant alloy according to the above (1), wherein the aforementioned chemical composition contains, in mass %, selected from

Mg: 0.0005～0.05%,

Ca: 0.0005～0.05%,

REM: 0.0005～0.50%,

V: 0.02～1.5%,

B: 0.0005～0.01%,

Zr: 0.005～0.10%,

Hf: 0.005～1.0%,

Ta: 0.01 to 8.0%, and

Re: 0.01 to 8.0%

1 or more of them.

(3) The heat-resistant austenitic alloy according to (1) or (2) above, wherein the center portion has a creep rupture strength of 100 MPa or more at 700° C. in the longitudinal direction for 10,000 hours.

(4) A manufacturing method of an austenitic heat-resistant alloy, which has the following steps:

A step of hot working a steel ingot or slab having the chemical composition described in (1) or (2) above; and,

Thereafter, a heat treatment process of heating to a heat treatment temperature T (° C.) in the range of 1100 to 1250° C., maintaining 1000 D/T to 1400 D/T (minutes) and then water cooling is performed.

Wherein, D is the maximum value (mm) of the straight-line distance between any point on the outer edge of the section and any other point on the outer edge in the section perpendicular to the length direction of the alloy.

(5) The method for producing an austenitic heat-resistant alloy according to (4) above, wherein

In the step of performing the aforementioned hot working, one or more workings are performed in a direction substantially perpendicular to the longitudinal direction.

The effect of the invention

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

Detailed ways

Each feature of the present invention will be described in detail below.

1. Chemical composition

The reason for limitation of each element is as follows. In addition, in the following description, "%" concerning content means "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 required as 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 but also carbides of Cr will increase, deteriorating mechanical properties such as ductility and toughness, and weldability. Therefore, the C content is set to 0.02 to 0.12%. The C content is preferably 0.05% or more, preferably 0.10% or less.

Si: 2.0% or less

Si is contained as a deoxidizing element. In addition, Si is also an effective element for improving oxidation resistance, water vapor oxidation resistance, and the like. Furthermore, it is also an element that improves melt flow in cast materials. However, when the Si content exceeds 2.0%, the formation of intermetallic compounds such as σ is promoted, and thus the structural stability at high temperatures deteriorates, leading to a decrease in toughness and ductility. Furthermore, weldability also falls. Therefore, the Si content is made 2.0% or less. When emphasis is placed on structural stability, the Si content is preferably 1.0% or less. It should be noted that when the deoxidation effect is sufficiently ensured by other elements, there is no need to set the lower limit of the Si content. However, when deoxidation effect, oxidation resistance, water vapor oxidation resistance, etc. The content is preferably 0.05% or more, more preferably 0.10% or more.

Mn: 3.0% or less

Mn has a deoxidizing effect like Si, and has an effect of fixing S inevitably contained in the alloy in the form of sulfide to improve ductility at high temperature. However, when the Mn content exceeds 3.0%, the precipitation of intermetallic compounds such as σ is promoted, so mechanical properties such as structural stability and high-temperature strength deteriorate. Therefore, the Mn content is made 3.0% or less. The Mn content is preferably 2.0% or less, more preferably 1.5% or less. It should be noted that although the lower limit of the Mn content does not need to be set, when the ductility improvement effect at high temperature is emphasized, the Mn content is preferably 0.10% or more, more preferably 0.20% or more.

P: 0.030% or less

P is inevitably mixed into the alloy as an impurity, which significantly reduces weldability and ductility at high temperatures. Therefore, the P content is made 0.030% or less. The P content should be reduced as much as possible, and it is preferably 0.020% or less, more preferably 0.015% or less.

S: 0.015% or less

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

Cr: 20.0% or more and less than 28.0%

Cr is an important element that plays an excellent role in improving corrosion resistance such as oxidation resistance, water vapor oxidation resistance, and high-temperature corrosion resistance. However, when the content is less than 20.0%, the above effects cannot be obtained. On the other hand, when the Cr content increases, especially when it reaches 28.0% or more, the structure becomes unstable due to the precipitation of σ phase, etc., and the weldability also deteriorates. Therefore, the Cr content is set to 20.0% or more and less than 28.0%. The Cr content is preferably 21.0% or more, more preferably 22.0% or more. In addition, the Cr content is preferably 26.0% or less, more preferably 25.0% or less.

Ni: more than 35.0% and less than 55.0%

Ni is an element that stabilizes the austenite structure and is also an important element for securing corrosion resistance. In view of the balance with the above-mentioned Cr content, Ni needs to be contained in excess of 35.0%. On the other hand, when the Ni content is too large, the cost increases, so it is made 55.0% or less. The Ni content is preferably 40.0% or more, more preferably 42.0% or more. In addition, 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 since it stabilizes the austenite structure like Ni and contributes to the improvement of the creep rupture strength, it may be contained instead of a part of Ni. However, when the content exceeds 20.0%, the effect becomes saturated and the economic efficiency also decreases. Therefore, the Co content is set to 0 to 20.0%. The Co content is preferably 15.0% or less. In addition, when it is desired to obtain the above-mentioned effects, the Co content is preferably 0.5% or more.

W: 4.0 to 10.0%

W is not only dissolved in the matrix and contributes to the improvement of creep rupture strength as a solid solution strengthening element, but also precipitates as Fe ₂ W-type Laves phase or Fe ₇ W ₆ -type μ phase to greatly increase creep rupture strength. important element. However, when the W content is less than 4.0%, the aforementioned effects cannot be obtained. On the other hand, even if W is contained in an amount exceeding 10.0%, the strength improvement effect is saturated, and the structural stability and the ductility at high temperature deteriorate. 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. In addition, 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 that has the effect of forming carbonitrides and increasing the creep rupture strength. However, when the Ti content is less than 0.01%, sufficient effects cannot be obtained, and on the other hand, when the Ti content 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, more preferably 0.10% or more. In addition, the Ti content is preferably 0.40% or less, more preferably 0.35% or less.

Nb: 0.01 to 1.0%

Nb has the effect of forming carbonitrides to increase the creep rupture strength. However, when the Nb content is less than 0.01%, sufficient effects cannot be obtained, and on the other hand, when 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. In addition, the Nb content is preferably 0.90% or less, more preferably 0.70% or less.

Mo: less than 0.50%

Conventionally, Mo is an element that dissolves in a matrix and contributes to improvement of creep rupture strength as a solid solution strengthening element, and is considered to have an effect equivalent to that of W. However, the inventors of the present invention have found through studies that when Mo is contained in combination with the alloy containing the aforementioned amounts of W and Cr, the σ phase may precipitate during long-term use, which may cause damage to the creep rupture strength and ductility. and reduced toughness. Therefore, it is desirable to reduce the Mo content as much as possible and make it less than 0.50%. It should be noted 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 degrades hot workability and weldability. Therefore, it is desirable to reduce the Cu content as much as possible and make it less than 0.50%. It should be noted that the Cu content is preferably limited to less than 0.20%.

Al: 0.30% or less

Al is an element contained as a deoxidizer for molten steel. However, when the Al content exceeds 0.30%, the ductility at high temperature deteriorates. Therefore, the Al content is made 0.30% or less. The Al content is preferably 0.25% or less, more preferably 0.20% or less. In addition, when it is desired to obtain the above-mentioned effects, the Al content is preferably 0.01% or more, more preferably 0.02% or more.

N: Less than 0.10%

N is an element that stabilizes the austenite structure, and is an element that is unavoidably contained in a conventional melting method. However, in the present invention where Ti must be contained, it is preferable to reduce Ti as much as possible in order to avoid Ti consumption due to the formation of TiN. However, it is difficult to reduce extremely in the case of atmospheric melting, so the N content is set to be 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, the term "impurity" here refers to components that are mixed in due to raw materials such as ores and scraps, and various factors in the production process during the industrial production of alloys, and are allowed within the range that does not adversely affect the present invention.

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 in the form of sulfide to improve high temperature ductility. Therefore, when it is desired to obtain better high-temperature ductility, one or more of these elements can be positively contained within the following range.

Mg: 0.05% or less

Mg has the effect of fixing S, which inhibits ductility at high temperature, as sulfide to improve high temperature ductility, and therefore Mg may be contained in order to obtain this effect. However, when the Mg content exceeds 0.05%, the degree of cleanliness decreases and the high-temperature ductility is impaired instead. Therefore, the amount of Mg when contained is made 0.05% or less. The Mg content is more preferably 0.02% or less, still more preferably 0.01% or less. On the other hand, in order to securely obtain the above effects, the Mg content is preferably 0.0005% or more, more preferably 0.001% or more.

Ca: 0.05% or less

Ca has the effect of fixing S, which inhibits ductility at high temperatures, as sulfide to improve high temperature ductility, and therefore, Ca may be contained in order to obtain this effect. However, when the Ca content exceeds 0.05%, the degree of cleanliness decreases, and the high-temperature ductility is impaired instead. Therefore, the amount of Ca when contained is made 0.05% or less. The Ca content is more preferably 0.02% or less, still more preferably 0.01% or less. On the other hand, in order to securely obtain the above effects, the Ca content is preferably 0.0005% or more, more preferably 0.001% or more.

REM: less than 0.50%

REM has the effect of fixing S in the form of sulfide to improve high-temperature ductility. In addition, REM has the effect of improving the adhesion of the Cr ₂ O ₃ protective coating on the steel surface, especially improving the oxidation resistance during repeated oxidation, and further contributing to grain boundary strengthening, improving creep rupture strength and creep resistance. The role of variable fracture ductility. However, when the REM content exceeds 0.50%, inclusions such as oxides increase, and workability and weldability are impaired. Therefore, when REM is contained, the amount of REM 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 still more preferably 0.002% or more.

It should be noted that REM refers to a total of 17 elements including Sc, Y, and lanthanoid elements, and the aforementioned REM content represents the total content of these elements.

The total content of the aforementioned Mg, Ca and REM may be 0.6% or less, but is 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. Therefore, when it is desired to obtain greater high-temperature strength and creep rupture strength, one or more of these elements may be positively contained within the following ranges.

V: 1.5% or less

V has the effect of forming carbonitrides to increase high temperature strength and creep rupture strength. Therefore, V may be contained in order to obtain these effects. However, when the V content exceeds 1.5%, the high-temperature corrosion resistance decreases, and further, ductility and toughness deteriorate due to precipitation of brittle phases. Therefore, the amount of V when contained is made 1.5% or less. The V content is more preferably 1.0% or less. On the other hand, in order to securely obtain the above effects, the V content is preferably 0.02% or more, more preferably 0.04% or more.

B: less than 0.01%

The presence of B in carbides or in the matrix not only promotes the refinement of precipitated carbides, but also strengthens the grain boundaries to increase the creep rupture strength. However, when the B content exceeds 0.01%, the ductility at high temperature decreases and the melting point also decreases. Therefore, when B is contained, the amount of B is made 0.01% or less. The B content is more preferably 0.008% or less, still more preferably 0.006% 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.

Zr: 0.10% or less

Zr is an element that promotes the miniaturization of carbonitrides and increases the creep rupture strength as a grain boundary strengthening element. However, when the Zr content exceeds 0.10%, the ductility at high temperature decreases. Therefore, the amount of Zr when 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, more preferably 0.01% or more.

Hf: 1.0% or less

Hf has the effect of contributing to precipitation strengthening as a carbonitride and increasing the creep rupture strength, so Hf may be contained in order to obtain these effects. However, when the Hf content exceeds 1.0%, workability and weldability are impaired. Therefore, the amount of Hf when 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 securely 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.

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

Both Ta and Re are dissolved in austenite as a matrix, and have a solid-solution strengthening effect. Therefore, when it is desired to obtain higher high-temperature strength and creep rupture strength through solid solution strengthening, one or both of these elements may be positively contained within the following ranges.

Ta: 8.0% or less

Ta has the effect of forming carbonitrides and improving high temperature strength and creep rupture strength as a solid solution strengthening element. Therefore, Ta may be contained in order to obtain these effects. However, when the Ta content exceeds 8.0%, workability and mechanical properties are impaired. Therefore, the amount of Ta when contained is 8.0% or less. The Ta content is more preferably 7.0% or less, 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.

Re: below 8.0%

Re mainly functions as a solid solution strengthening element to increase high-temperature strength and creep rupture strength, so Re may be contained in order to obtain these effects. However, when the Re content exceeds 8.0%, workability and mechanical properties are impaired. Therefore, the amount of Re when 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 securely 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.

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

2. Grain size

Austenite grain size number on the outer surface: -2.0～4.0

When the austenite grains on the outer surface are too coarse, the 0.2% conditional yield strength and tensile strength at room temperature become low, and when the austenite grains are too fine, the high creep rupture strength at high temperature cannot be maintained. Therefore, the austenite grain size number of the outer surface is set to -2.0 to 4.0. It should be noted that in the production 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 after the final heat treatment can be within the above range.

3. Size

The shortest distance from the center to the outer surface: 40mm or more

As described above, in large structural members, in addition to the low 0.2% yield strength and tensile strength at room temperature, there is also a problem of uneven creep rupture strength depending on the location. However, the austenitic heat-resistant alloy of the present invention exhibits sufficient 0.2% yield strength and tensile strength at normal temperature and creep rupture strength at high temperature as a large structural member. That is, the effects of the present invention are remarkably exhibited for thick-walled members.

Therefore, in the austenitic heat-resistant alloy of the present invention, the shortest distance from the center portion to the outer surface portion in a cross section perpendicular to the longitudinal direction is set to be 40 mm or more. 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, more preferably 100 mm or more. Here, the shortest distance from the center to the outer surface is, for example, the radius (mm) of the section when the alloy is cylindrical, and half the length (mm) of the short side of the section when the alloy is rectangular.

It should be noted that, as will be described later, the heat-resistant alloy of the present invention is obtained by, for example, subjecting a steel ingot or a slab obtained by continuous casting to hot working such as hot forging or hot rolling. In addition, the longitudinal direction of the heat-resistant alloy is generally a direction connecting the top and bottom of the steel ingot when a steel ingot is used, and a longitudinal direction when a cast slab is used.

4. The amount of Cr existing in the form of precipitates obtained by extraction residue analysis

Cr _PB /Cr _PS ≤10.0...(i)

However, the meanings of the symbols in the formula (i) are as follows.

Cr _PB : The amount of Cr present in the form of precipitates in the central part obtained by analysis of the extraction residue

Cr _PS : The amount of Cr existing in the form of precipitates obtained by extraction residue analysis on the outer surface

In the production process of the alloy, after heat treatment after hot working, precipitates of Cr that are not solid-dissolved (mainly carbides) are generated at the crystal grain boundaries or in the crystal grains. Especially in the central part of the alloy, since the cooling rate is slower than that in the outer part, the amount of Cr precipitates tends to increase. Therefore, when the value of Cr _PB /Cr _PS exceeds 10.0, high creep rupture strength at high temperature cannot be maintained. On the other hand, although there is no need to set the lower limit of Cr _PB /Cr _PS , it is preferable to set it to 1.0 or more since there is a tendency that the amount of precipitates in the center portion is larger than that in the outer portion.

In addition, the extraction residue analysis was performed in the following procedure. First, a test piece for measuring Cr precipitates is collected from the central portion and the outer surface of a cross-section perpendicular to the longitudinal direction of the alloy sample. The surface area of the above-mentioned test piece was obtained, and then only the base material of the alloy sample was completely electrolyzed in 10% acetylacetone-1% tetramethylammonium chloride-methanol solution under the electrolysis conditions of 20 mA/cm ² . Then, the solution after electrolysis was filtered through a 0.2 μm filter, and a precipitate was extracted as a residue. Then, after the extraction residue was subjected to acid decomposition, it was analyzed using an inductively coupled plasma emission spectrometer (ICP-AES), thereby measuring the content (mass %) of Cr contained in the form of undissolved Cr precipitates, The value of Cr _PB /Cr _PS was calculated|required 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 said formula is as follows.

YS _B : 0.2% conditional yield strength of the central part

YS _S : 0.2% conditional yield strength of the outer surface

TS _B : Tensile strength of the central part

TS _S : Tensile strength of the outer surface

In a large structural member, since the cooling rate during heat treatment differs depending on the location, there is a tendency for the mechanical properties of each location to vary greatly. In large structural members, if the 0.2% yield strength and tensile strength at normal temperature are greatly different between the central part and the outer part, there will be a problem that the specification will not be satisfied depending on the part.

Therefore, the austenitic heat-resistant alloy of the present invention is an alloy whose mechanical properties at normal temperature satisfy the above formulas (ii) and (iii). It should be noted that although it is not necessary to set the lower limit values separately, it is preferable to set both the formulas (ii) and (iii) in view of the tendency that the mechanical properties of the central part are inferior to those of the outer part. 1.0 or more.

The 0.2% conditional yield strength and tensile strength were obtained by cutting out a round bar tensile test piece with a parallel portion length of 40mm from the central portion and the outer surface portion of the alloy by machining in a manner parallel to the longitudinal direction, A tensile test was carried out at room temperature, and it was calculated|required. In addition, the tensile test was performed based on 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, high high-temperature strength, particularly high creep rupture strength is required. Therefore, in the heat-resistant alloy of the present invention, the creep rupture strength at 700° C. in the longitudinal direction at 10,000 hours in the central portion thereof is preferably 100 MPa or more.

The creep rupture strength was obtained by the following method. First, a round bar creep rupture test piece with a diameter of 6 mm and a punctuation distance of 30 mm described in JIS Z 2241 (2011) was cut out from the center of the alloy by machining parallel to the longitudinal direction. Then, creep rupture tests were performed in the atmosphere at 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. In addition, the creep rupture test was performed based on JIS Z 2271 (2010).

7. Manufacturing method

The austenitic heat-resistant alloy of the present invention can be produced by hot working a steel ingot or slab having the above chemical composition. In addition, in the said hot working process, the process is performed so that the longitudinal direction of the final shape of an alloy may coincide with the longitudinal direction of the steel ingot or cast slab which is a raw material. Thermal processing may be performed only in the longitudinal direction, but in order to impart a higher processing rate and obtain a more uniform structure, thermal processing may be performed one or more times in a direction substantially perpendicular to the longitudinal direction. In addition, after this thermal processing, thermal processing of a different method, such as thermal extrusion, can be further implemented as needed.

When producing the austenitic heat-resistant alloy of the present invention, the final heat treatment described below is performed after the above steps in order to suppress variations in the metal structure and mechanical properties of various parts and maintain high creep rupture strength.

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

When the above-mentioned heat treatment temperature is lower 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 boundaries are melted or the crystal grains are remarkably coarsened, thereby reducing the ductility. The heat treatment temperature is more preferably 1150°C or higher, and more preferably 1230°C or lower. In addition, when the above-mentioned retention time is less than 1000 D/T (minutes), the undissolved chromium carbides in the central portion increase, and the Cr _PB /Cr _PS falls outside the range limited by the present invention. On the other hand, when it exceeds 1400 D/T (min), the crystal grains on the outer surface become coarse, and the austenite grain size number falls outside the range limited by the present invention.

Immediately after heating and holding, the alloy is water-cooled. This is because when the cooling rate becomes slow, especially in the center of the alloy, a large amount of undissolved Cr precipitates are formed at the grain boundaries or within the grains, which may result in failure to satisfy the above formula (i).

Hereinafter, although an Example demonstrates this invention more concretely, this invention is not limited to these Examples.

Example

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

[Table 1]

The obtained steel ingots were processed into a cylindrical shape with an outer diameter of 120 to 480 mm by hot forging, and the final heat treatment was performed under the conditions shown in Table 2 to obtain alloy member samples. It should be noted that, for alloys 1, 2, and 4, after hot forging in the longitudinal direction and before final heat treatment, forging is performed in a direction substantially perpendicular to the longitudinal direction, and then final hot forging is further performed in the longitudinal direction.

[Table 2]

Table 2

** indicates that it is outside the conditions of the production method defined in the present invention.

For each sample, a test piece for structure observation was collected from the outer surface, and the cross section in the longitudinal direction was ground with sandpaper and a grinding wheel, etched with mixed acid, and observed with an optical microscope. The grain size number of the observed surface was determined in accordance with the determination method using the intersection segment (grain diameter) defined in JIS G0551 (2013).

Next, test pieces for measuring Cr precipitates were collected from the central portion and the outer surface portion of the cross-section perpendicular to the longitudinal direction of each sample. The surface area of the above-mentioned test piece was obtained, and then only the base material of the alloy sample was completely electrolyzed in 10% acetylacetone-1% tetramethylammonium chloride-methanol solution under the electrolysis conditions of 20 mA/cm ² . Then, the electrolyzed solution was filtered through a 0.2 μm filter, and a precipitate was extracted as a residue. Then, after acid decomposition of the extraction residue, ICP-AES measurement was carried out to measure the content (mass %) of Cr contained in the form of undissolved Cr precipitates, and based on this measured value, the Cr _PB / Cr _PS value.

In addition, by machining, a tensile test piece having a parallel portion length of 40 mm was cut out from the center portion and the outer surface portion of each sample in parallel to the longitudinal direction, and a tensile test was performed at room temperature to obtain 0.2% Conditional yield strength and tensile strength. Furthermore, a creep rupture test piece having a parallel portion length of 30 mm was cut out from the center portion of each sample by machining so as to be parallel to the longitudinal direction. Then, creep rupture tests were performed in the atmosphere at 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.

These results are shown in Table 3 together.

[table 3]

table 3

* means outside the scope of the present invention.

# is the creep rupture strength at 700°C and 10,000 hours.

Alloys A and B are alloys having almost the same chemical composition as Alloy 1 and being hot forged into the same final shape. However, the holding time during the heat treatment is out of the range of the manufacturing conditions defined by the present invention. Due to this, for Alloy A, the grain size number of the outer surface is outside the limit of the present invention, the values of YS _S /YS _B and TS _S /TS _B are outside the limit of the present invention, according to The result of large deviations in the different mechanical properties of the parts. In addition, for Alloy B, the creep rupture strength was significantly lower than that of Alloy 1, which was outside the limits of the present invention.

Alloys C, D, and E are alloys that have almost the same chemical composition as Alloy 2 and are hot forged to the same final shape. Because the heat treatment temperature of alloy C is lower than the limited range of the present invention, the grain size number and the value of Cr _PB /Cr _PS on the outer surface are outside the limited range of the present invention, and the creep rupture strength is the same as that of alloy 2. than significantly lower results.

Because the heat treatment temperature of alloy D is higher than the limited 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 outside the limited range of the present invention, and the creep The result is that the fracture strength is significantly lower compared to Alloy 2.

In addition, the cooling method of alloy E in the final heat treatment is air cooling instead of water cooling, and the cooling rate is obviously slow. Due to this, the value of Cr _PB /Cr _PS is outside the limited range of the present invention. As a result, the creep rupture strength and Alloy 2 is significantly lower in comparison. On the other hand, in the alloys 1 to 9 that all satisfy the limitations of the present invention, the variation in mechanical properties is small, and the creep rupture strength is also good.

Industrial availability

The austenitic heat-resistant alloy of the present invention has little variation in mechanical properties depending on the location, 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 large-scale structural members such as boilers for thermal power generation and chemical equipment used in high-temperature environments.

Claims (5)

1. An austenitic heat-resistant alloy, wherein the chemical composition of the alloy is calculated as C: 0.02～0.12%, 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% to 55.0% or less, Co: 0 to 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: 0～0.05%, Ca: 0-0.05%, REM: 0～0.50%, V: 0～1.5%, B: 0～0.01%, Zr: 0～0.10%, Hf: 0～1.0%, Ta: 0 to 8.0%, Re: 0～8.0%, Balance: Fe and impurities, In a cross section perpendicular to the longitudinal direction of the alloy, the shortest distance from the center to the outer surface is 40 mm or more, The austenite grain size number on the outer surface is -2.0 to 4.0, The amount of Cr present in the form of precipitates obtained by extraction residue analysis satisfies the following formula (i), The mechanical properties of the austenitic heat-resistant alloy at room temperature satisfy the following formulas (ii) and (iii), Cr _PB /Cr _PS ≤10.0...(i) YS _S /YS _B ≤1.5...(ii) TS _S /TS _B ≤1.2...(iii) Among them, the meanings of the symbols in the above formula are as follows: Cr _PB : The amount of Cr existing in the form of precipitates obtained from the analysis of the extraction residue at the center, Cr _PS : The amount of Cr existing in the form of precipitates obtained by analysis of the extraction residue on the outer surface, YS _B : 0.2% conditional yield strength at the center, YS _S : 0.2% conditional yield strength of the outer surface, TS _B : Tensile strength of the central part, TS _S : Tensile strength of the outer surface. 2. The austenitic heat-resistant alloy according to claim 1, wherein the chemical composition contains, in mass %, selected from Mg: 0.0005～0.05%, Ca: 0.0005～0.05%, REM: 0.0005～0.50%, V: 0.02～1.5%, B: 0.0005～0.01%, Zr: 0.005～0.10%, Hf: 0.005～1.0%, Ta: 0.01 to 8.0%, and Re: 0.01 to 8.0% 1 or more of them. 3. The austenitic heat-resistant alloy according to claim 1 or claim 2, wherein the central portion has a creep rupture strength of 100 MPa or more at 700° C. in the longitudinal direction at 10,000 hours. 4. A method for manufacturing an austenitic heat-resistant alloy, comprising the following steps: A step of hot working a steel ingot or slab having the chemical composition described in claim 1 or claim 2; and, Afterwards, heat to the heat treatment temperature T (° C.) in the range of 1100 to 1250° C., hold 1000 D/T to 1400 D/T (minutes), and then carry out a heat treatment process of water cooling, Wherein, D is the maximum value (mm) of the straight-line distance between any point on the outer edge of the section and any other point on the outer edge in the section perpendicular to the length direction of the alloy. 5. The manufacturing method of the austenitic heat-resistant alloy according to claim 4, wherein, In the step of performing the thermal processing, one or more processings are performed in a direction substantially perpendicular to the longitudinal direction.