KR102090201B1 - Austenitic heat-resistant alloy and its manufacturing method - Google Patents

Abstract

Even in a high temperature environment, an austenitic heat-resistant alloy having high creep strength and high toughness is provided. The austenitic heat-resistant alloy according to the present embodiment is, in mass%, C: 0.03 to less than 0.25%, Si: 0.01 to 2.0%, Mn: less than 2.0%, Cr: less than 10 to 30%, and Ni: greater than 25 ~ Contains 45%, Al: more than 2.5 to less than 4.5%, Nb: 0.2 to 3.5%, N: 0.025% or less, the balance consists of Fe and impurities, and P and S in the impurities, respectively, P: 0.04% or less , And S: have a chemical composition of 0.01% or less. The total volume fraction of precipitates of 6 μm or more in the tissue is 5% or less.

Description

Austenitic heat-resistant alloy and its manufacturing method

The present invention relates to a heat-resistant alloy and a method for manufacturing the same, and more particularly, to an austenitic heat-resistant alloy and a method for manufacturing the same.

Conventionally, in facilities such as boilers and chemical plants used in a high temperature environment, 18-8 stainless steel is used as the heat resistant steel. The 18-8 stainless steel is an austenitic stainless steel containing about 18% Cr and about 8% Ni, and is, for example, SUS304H, SUS316H, SUS321H, and SUS347H according to JIS standards.

In recent years, the conditions of use of equipment under a high temperature environment have been severely severed, and a higher creep strength than 18-8 stainless steel is required. Recently, an advanced supercritical pressure power generation plan is being promoted to increase the conventional steam temperature of about 600 ° C to 700 ° C or more in a thermal power boiler. Moreover, in a chemical plant, an increase in the operation temperature is also planned to increase the operation efficiency. Steel materials used in these high temperature environments are required to have high creep strength and excellent corrosion resistance.

Heat-resistant materials with improved corrosion resistance are proposed, for example, in Japanese Patent Publication No. Hei 02-115348 (Patent Document 1) and Japanese Patent Publication No. Hei 07-316751 (Patent Document 2). Since the Al content is high in these heat-resistant alloys, an Al ₂ O ₃ film is formed on the surface during use and at a high temperature range. High corrosion resistance is obtained by this coating.

However, in the heat-resistant alloys disclosed in Patent Documents 1 and 2 described above, creep strength may be low in a high temperature environment of 700 ° C or higher.

As a heat-resistant material having high creep strength in a high temperature environment of 700 ° C. or higher, a heat-resistant alloy containing Ni and Co and a γ-phase (Ni ₃ Al) as a reinforcing phase has been developed. Such heat-resistant alloys are, for example, Alloy617,263, and 740 of Ni-based alloys. However, the alloy raw material of these heat-resistant alloys is expensive. Moreover, since the workability is low, the manufacturing cost is high.

Thus, a heat-resistant alloy that is cheaper than the Ni-based alloy and has excellent creep strength is proposed in Japanese Patent Publication No. 2014-43621 (Patent Document 3) and Japanese Patent Publication No. 2013-227644 (Patent Document 4). It is done.

The austenitic heat-resistant alloy disclosed in Patent Document 3 is C: less than 0.02%, Si: 2% or less, Mn: 2% or less, Cr: 15 to 26%, Ni: 20 to 35%, Al: 0.3% or less, P: 0.04% or less, S: 0.01% or less, N: 0.05% or less, Ti: 3.0% or less (including 0%), V: 3.0% or less (including 0%) ), Nb: less than 2.3% (including 0%) and Ta: 2.0% or less (including 0%), and at least one selected from f1 = 2Ti + 2V + Nb + (1/2) Ta Paper f1 satisfies 1.5 to 6.0, and the balance has a chemical composition composed of Fe and impurities. Patent Document 3 discloses that the austenite-based heat-resistant alloy has excellent high temperature strength and toughness by strengthening the precipitation of the Labeth phase and the γ ＇phase.

The austenitic heat-resistant alloy disclosed in Patent Document 4 is C: less than 0.02%, Si: 0.01 to 2%, Mn: 2% or less, Cr: 20% or more and less than 28%, Ni: more than 35% in mass% 50 % Or less, W: 2.0 to 7.0%, Mo: less than 2.5% (including 0%), Nb: less than 2.5% (including 0%), Ti: less than 3.0% (including 0%), Al ： 0.3% or less, P: 0.04% or less, S: 0.01% or less, and N: 0.05% or less, the balance is composed of Fe and impurities, and f1 represented by f1 = 1 / 2W + Mo is 1.0 to 5.0 , f2 = 1 / 2W + Mo + Nb + 2Ti and f2 represented by 2.0-8.0 and f3 = Nb + 2Ti represented by f3 have a chemical composition of 0.5-5.0. Patent document 4 describes that the austenite-based heat-resistant alloy has excellent high temperature strength and toughness by strengthening the precipitation of the Labeth phase and the γ ＇phase.

Japanese Patent Publication No. 02-115348

Japanese Patent Publication No. Hei 07-316751

Japanese Patent Publication 2014-43621

Japanese Patent Publication No. 2013-227644

However, as in the heat-resistant alloys of Patent Documents 3 and 4, in the case of an alloy using a reinforcing mechanism by a Laves phase and a γ-phase, creep strength and toughness after long-time aging may decrease.

An object of the present invention is to provide an austenitic heat-resistant alloy having high creep strength and high toughness even in a high temperature environment.

The austenitic heat-resistant alloy according to the present embodiment is, in mass%, C: 0.03 to less than 0.25%, Si: 0.01 to 2.0%, Mn: less than 2.0%, Cr: less than 10 to 30%, and Ni: greater than 25 ~ 45%, Al: more than 2.5 to less than 4.5%, Nb: 0.2 to 3.5%, N: 0.025% or less, Ti: 0 to less than 0.2%, W: 0 to 6%, Mo: 0 to 4%, Zr: 0 ~ 0.1%, B: 0 ~ 0.01%, Cu: 0 ~ 5%, rare earth elements: 0 ~ 0.1%, Ca: 0 ~ 0.05%, and Mg: 0 ~ 0.05%, the balance is Fe and impurities It is made, and P and S in an impurity have chemical composition of P: 0.04% or less, and S: 0.01% or less, respectively. Among the tissues, the total volume fraction of precipitates having an equivalent sugar diameter of 6 µm or more is 5% or less. Here, the precipitates are, for example, carbides, nitrides, NiAl and α-Cr.

The austenitic heat-resistant alloy according to the present embodiment has high temperature strength and excellent toughness for a long time even in a high-temperature environment.

The present inventors investigated and examined the creep strength and toughness of the austenitic heat-resistant alloy in a high temperature environment of 700 ° C or higher (hereinafter simply referred to as a high temperature environment), and obtained the following knowledge.

As described above, a heat-resistant alloy containing a γ ＇phase such as a Labes phase or Ni ₃ Al has a high creep strength in a high temperature environment. However, these precipitated phases become coarse when used for a long time in a high-temperature environment, so that the creep strength and toughness of the heat-resistant alloy decrease.

On the other hand, if a heat-resistant alloy is used in a high-temperature environment, if the precipitates such as carbides, nitrides, NiAl, α-Cr can be finely dispersed and precipitated, high creep strength and high toughness can be maintained even when used for a long time. These precipitates increase the grain boundary strength by covering the grain boundaries. In addition, when these precipitates precipitate in the mouth, the deformation resistance of the heat-resistant alloy increases, and the creep strength increases.

In order to increase creep strength and toughness by the fine precipitate described above, the structure of the heat-resistant alloy before use is controlled as follows.

[Limitation of the amount of precipitates whose original equivalent diameter is 6 μm or more]

In the solidification structure after casting the heat-resistant alloy, precipitates such as carbides, nitrides, NiAl, and α-Cr (hereinafter simply referred to as precipitates) are present. These precipitates are produced in a liquid state in which solute elements present between dendrites are concentrated. These precipitates usually have a coarse shape and are unevenly dispersed in the tissue. Therefore, the toughness of the heat-resistant alloy decreases.

Moreover, even if solution treatment is performed, these precipitates are hardly employed and are likely to remain in a coarse state. If these precipitates remain coarse in the heat-resistant alloy, it is difficult to form fine precipitates during use in a high temperature environment. Therefore, the total volume fraction of coarse precipitates in the heat-resistant alloy is preferably as low as possible.

If the total volume fraction of precipitates having a circular equivalent diameter of 6 μm or more (hereinafter referred to as coarse precipitates) is 5% or less among the structure of the heat resistant alloy, a sufficient amount of fine precipitates can be precipitated while using the heat resistant alloy in a high temperature environment, High creep strength and toughness can be obtained.

In order to make the total volume fraction of coarse precipitates in the tissue to be 5% or less, the C content in the heat-resistant alloy is made less than 0.25%. In addition, the cross-sectional reduction rate during hot forging is 30% or more. In this case, coarse precipitates are uniformly dispersed by hot forging. Therefore, at the time of solution treatment in a subsequent step, precipitates can be employed, and the total volume fraction of coarse precipitates is 5% or less.

The austenitic heat-resistant alloy according to the present embodiment, completed on the basis of the above findings, is in mass%, C: less than 0.03 to 0.25%, Si: 0.01 to 2.0%, Mn: less than 2.0%, Cr: 10 to 30%. Less than, Ni: more than 25 to 45%, Al: more than 2.5 to less than 4.5%, Nb: 0.2 to 3.5%, N: 0.025% or less, Ti: 0 to less than 0.2%, W: 0 to 6%, Mo: 0 Contains ~ 4%, Zr: 0 to 0.1%, B: 0 to 0.01%, Cu: 0 to 5%, rare earth elements: 0 to 0.1%, Ca: 0 to 0.05%, and Mg: 0 to 0.05% , The remainder is composed of Fe and impurities, and P and S in the impurities have chemical compositions of P: 0.04% or less and S: 0.01% or less, respectively. Among the tissues, the total volume fraction of precipitates having an equivalent sugar diameter of 6 µm or more is 5% or less.

The chemical composition is selected from the group consisting of Ti: 0.005 to less than 0.2%, W: 0.005 to 6%, Mo: 0.005 to 4%, Zr: 0.0005 to 0.1%, and B: 0.0005 to 0.01% in mass%. You may contain 1 type, or 2 or more types.

The said chemical composition may contain at least 1 sort (s) selected from the group which consists of Cu: 0.05-5% and rare earth element: 0.0005-0.1% in mass%.

The chemical composition may contain one or more selected from the group consisting of Ca: 0.0005 to 0.05% and Mg: 0.0005 to 0.05% by mass.

The above-described method for producing an austenitic heat-resistant alloy includes a step of hot forging a casting material having the above-described chemical composition at a cross-sectional reduction rate of 30% or more, and hot processing the material after hot forging to produce an intermediate material. And a step of subjecting the intermediate material to solution treatment at 1100 to 1250 ° C.

Hereinafter, the austenitic heat-resistant alloy of this embodiment is explained in full detail. "%" With respect to the element means mass% unless otherwise specified.

[Chemical composition]

The austenitic heat-resistant alloy according to the present embodiment is, for example, an alloy tube. The chemical composition of the austenitic heat-resistant alloy contains the following elements.

C: Less than 0.03 to 0.25%

Carbon (C) forms carbides and increases creep strength. Specifically, during use in a high temperature environment, C combines with crystal grain boundaries and alloy elements in the grains to form fine carbides. The fine carbide increases the deformation resistance and increases the creep strength. If the C content is too small, this effect is not obtained. On the other hand, if the C content is too large, a large number of coarse process carbides are formed in the solidification structure after casting of the heat-resistant alloy. Process carbide remains in the structure in a coarse state even after the solution treatment, so the toughness of the heat-resistant alloy is reduced. In addition, if coarse process carbides remain, fine carbides are unlikely to precipitate during use in a high temperature environment, and creep strength decreases. Therefore, the C content is less than 0.03 to 0.25%. The preferable lower limit of the C content is 0.05%, and more preferably 0.08%. The upper limit with preferable C content is 0.23%, More preferably, it is 0.20%.

Si ： 0.01 ~ 2.0%

Silicon (Si) deoxidizes the heat-resistant alloy. Si also improves the corrosion resistance (oxidation resistance and water vapor oxidation resistance) of the heat-resistant alloy. Si is an element inevitably contained, but when deoxidation can be sufficiently performed with other elements, the Si content is at least as much as possible. On the other hand, if the Si content is too large, the hot workability deteriorates. Therefore, the Si content is 0.01 to 2.0%. The preferable lower limit of the Si content is 0.02%, and more preferably 0.03%. The preferable upper limit of the Si content is 1.0%.

Mn: 2.0% or less

Manganese (Mn) is inevitably contained. Mn forms MnS by combining with S contained in the heat-resistant alloy, and increases the hot workability of the heat-resistant alloy. However, when the Mn content is too large, the heat-resistant alloy becomes too hard, and hot workability and weldability decrease. Therefore, the Mn content is 2.0% or less. The preferable lower limit of the Mn content is 0.1%, more preferably 0.2%. The preferable upper limit of the Mn content is 1.2%.

Cr: less than 10-30%

Chromium (Cr) improves the corrosion resistance (oxidation resistance, water vapor oxidation resistance, etc.) of the heat resistant alloy in a high temperature environment. Cr also precipitates finely as α-Cr during use in a high-temperature environment, and increases creep strength. If the Cr content is too small, these effects are not obtained. On the other hand, if the Cr content is too large, the stability of the structure decreases and creep strength decreases. Therefore, the Cr content is less than 10-30%. The preferable lower limit of the Cr content is 11%, more preferably 12%. The preferable upper limit of the Cr content is 28%, more preferably 26%.

Ni: Over 25 ~ 45%

Nickel (Ni) stabilizes austenite. Ni also increases the corrosion resistance of the heat-resistant alloy. If the Ni content is too small, these effects are not obtained. On the other hand, when the Ni content is too large, not only these effects are saturated, but hot workability is lowered. If the Ni content is too large, the raw material cost also increases. Therefore, the Ni content is more than 25 to 45%. The preferable lower limit of the Ni content is 26%, more preferably 28%. The upper limit with preferable Ni content is 44%, More preferably, it is 42%.

Al: more than 2.5 to less than 4.5%

Aluminum (Al), in use in a high temperature environment, combines with Ni to form fine NiAl and increases creep strength. Al also increases corrosion resistance in a high temperature environment of 1000 ° C or higher. If the Al content is too small, these effects are not obtained. On the other hand, when the Al content is too large, the structure stability decreases and the strength decreases. Therefore, the Al content is more than 2.5 to less than 4.5%. The preferable lower limit of the Al content is 2.55%, and more preferably 2.6%. The upper limit with preferable Al content is 4.4%, More preferably, it is 4.2%. In the austenitic heat-resistant alloy according to the present invention, the Al content means the total amount of Al contained in the steel material.

Nb: 0.2 to 3.5%

The niobium (Nb) forms a Laves phase and a Ni ₃ Nb phase, which are precipitation strengthening phases, and precipitates and strengthens the crystal grain boundaries and crystal grains, thereby increasing the creep strength of the heat-resistant alloy. If the Nb content is too small, the above effect cannot be obtained. On the other hand, if the Nb content is too large, the Labes phase and the Ni ₃ Nb phase are excessively generated, and the toughness and hot workability of the alloy decrease. When the Nb content is too large, the toughness after long-time aging also decreases. Therefore, the Nb content is 0.2 to 3.5%. The preferable lower limit of the Nb content is 0.35%, and more preferably 0.5%. The preferred upper limit of the Nb content is less than 3.2%, more preferably 3.0%.

N: 0.025% or less

Nitrogen (N) stabilizes austenite and is inevitably contained in conventional dissolution methods. In addition, during use in a high-temperature environment, N combines with crystal grain boundaries and alloy elements in the grain to form fine nitrides. The fine nitride increases the deformation resistance and increases the creep strength. However, if the N content is too large, a coarse nitride that remains unused even after the solution treatment is formed to lower the toughness of the alloy. Therefore, the N content is 0.025% or less. The upper limit of the preferable N content is 0.02%, and more preferably 0.01%.

P: 0.04% or less

Phosphorus (P) is an impurity. P deteriorates the weldability and hot workability of the heat-resistant alloy. Therefore, the P content is 0.04% or less. The preferable upper limit of the P content is 0.03%. The P content is preferably as low as possible.

S: 0.01% or less

Sulfur (S) is an impurity. S deteriorates the weldability and hot workability of the heat-resistant alloy. Therefore, the S content is 0.01% or less. The preferable upper limit of the S content is 0.008%. The S content is preferably as low as possible.

The balance of the chemical composition of the austenitic heat-resistant alloy of the present embodiment is made of Fe and impurities. Here, the impurity means that when austenite-based heat-resistant alloys are industrially manufactured, they are mixed from ores as raw materials, scraps, or manufacturing environments, and are acceptable within a range that does not adversely affect the present invention.

[About arbitrary elements]

The chemical composition of the austenite-based heat-resistant alloy described above may further contain one or two or more types selected from the group consisting of Ti, W, Mo, Zr and B in place of Fe. All of these elements are arbitrary elements and increase creep strength.

Ti: 0 to less than 0.2%

Titanium (Ti) is an arbitrary element and need not be contained. When contained, it forms a Laves phase and a Ni ₃ Ti phase that become precipitation strengthening phases, and increases creep strength by precipitation strengthening. However, when the Ti content is too large, the Labese phase and the Ni ₃ Ti phase are excessively generated, and high-temperature ductility and hot workability deteriorate. When the Ti content is too large, the toughness after aging for a long time also decreases. Therefore, the Ti content is less than 0 to 0.2%. The preferable lower limit of the Ti content is 0.005%, and more preferably 0.01%. The upper limit with preferable Ti content is 0.15%, More preferably, it is 0.1%.

W: 0-6%

Tungsten (W) is an optional element and need not be contained. When contained, it is employed in the austenite of the mother phase (matrix), and the creep strength is increased by strengthening the solution. W also forms a Laves phase in the grain boundaries and grain boundaries, and increases the creep strength by precipitation strengthening. However, when the W content is too large, the Labeth phase is excessively generated, deteriorating high temperature ductility, hot workability, and toughness. Therefore, the W content is 0 to 6%. The preferable lower limit of the W content is 0.005%, and more preferably 0.01%. The preferable upper limit of the content of W is 5.5%, more preferably 5%.

Mo: 0-4%

Molybdenum (Mo) is an arbitrary element and need not be contained. When contained, it is employed in the austenite of the mother phase and increases the creep strength by strengthening the solid solution. Mo also forms a Laves phase in the grain boundaries and grain boundaries, and increases the creep strength by strengthening precipitation. However, if the Mo content is too large, the Labeth phase is excessively generated, deteriorating high temperature ductility, hot workability, and toughness. Therefore, the Mo content is 0 to 4%. The preferable lower limit of the Mo content is 0.005%, and more preferably 0.01%. The preferable upper limit of the content of Mo is 3.5%, and more preferably 3%.

Zr: 0 to 0.1%

Zirconium (Zr) is an arbitrary element and need not be contained. When contained, Zr increases creep strength by grain boundary strengthening. However, if the Zr content is too large, the weldability and hot workability of the heat-resistant alloy decrease. Therefore, the Zr content is 0 to 0.1%. The preferable lower limit of Zr is 0.0005%, and more preferably, 0.001%. The preferable upper limit of the Zr content is 0.06%.

B: 0 to 0.01%

Boron (B) is an arbitrary element and need not be contained. When contained, creep strength is enhanced by grain boundary strengthening. However, if the B content is too large, weldability decreases. Therefore, the B content is 0 to 0.01%. The preferable lower limit of B is 0.0005%, and more preferably, 0.001%. The preferable upper limit of the B content is 0.005%.

The chemical composition of the austenite-based heat-resistant alloy described above may further contain one or more selected from the group consisting of Cu and rare earth elements in place of Fe. All of these elements are arbitrary elements, and the corrosion resistance of the heat-resistant alloy is enhanced.

Cu ： 0-5%

Copper (Cu) is an arbitrary element and need not be contained. When contained, it promotes the formation of an Al ₂ O ₃ film near the surface and enhances the corrosion resistance of the heat-resistant alloy. However, when the Cu content is too large, not only the effect is saturated, but the high temperature ductility decreases. Therefore, the Cu content is 0 to 5%. The preferable lower limit of the Cu content is 0.05%, and more preferably 0.1%. The upper limit with preferable Cu content is 4.8%, More preferably, it is 4.5%.

Rare earth elements: 0 to 0.1%

The rare earth element (REM) is an optional element and need not be contained. When contained, S is fixed as a sulfide, and hot workability is enhanced. REM also forms oxides and enhances corrosion resistance, creep strength, and creep ductility. However, if the REM content is too large, inclusions such as oxides increase, deteriorating hot workability and weldability, and manufacturing cost increases. Therefore, the REM content is 0 to 0.1%. The preferable lower limit of the REM content is 0.0005%, more preferably 0.001%. The upper limit with preferable REM content is 0.09%, More preferably, it is 0.08%.

In this specification, REM is a generic term for a total of 17 elements of Sc, Y and lanthanoid. REM content means content of the element when REM contained in a heat-resistant alloy is one of these elements. When the REM contained in the heat-resistant alloy is two or more, REM content means the total content of these elements. For REM, it is generally contained in micrometal. For this reason, for example, it may be added in the form of micrometal, and the REM content may be contained in the above range.

The chemical composition of the austenite-based heat-resistant alloy described above may further contain one or more selected from the group consisting of Ca and Mg in place of a part of Fe. All of these elements are arbitrary elements, and the hot workability of the heat-resistant alloy is enhanced.

Ca: 0 to 0.05%

Calcium (Ca) is an arbitrary element and need not be contained. When contained, S is fixed as a sulfide, and hot workability is enhanced. On the other hand, if the Ca content is too large, toughness, ductility and cleanliness decrease. Therefore, the Ca content is 0 to 0.05%. The preferred lower limit of Ca is 0.0005%. The preferable upper limit of the Ca content is 0.01%.

Mg: 0 to 0.05%

Magnesium (Mg) is an arbitrary element and need not be contained. When contained, S is fixed as a sulfide, and the hot workability of the heat-resistant alloy is enhanced. On the other hand, if the Mg content is too large, toughness, ductility and cleanliness decrease. Therefore, the Mg content is 0 to 0.05%. The preferred lower limit of Mg is 0.0005%. The upper limit with preferable Mg content is 0.01%.

[Total volume fraction of precipitates (coarse precipitates) with a circular equivalent diameter of 6 μm or more: 5% or less]

As described above, the austenitic heat-resistant alloy of the present embodiment precipitates fine precipitates during use in a high temperature environment, increases creep strength, and maintains toughness. Precipitates are, for example, carbides, nitrides, NiAl and α-Cr. When the precipitate is coarse, creep strength and toughness decrease. For this reason, among the heat-resistant alloys before use, it is preferable that there are few coarse precipitates. In the structure of the heat-resistant alloy, if the total volume fraction of precipitates (coarse precipitates) having a circle equivalent diameter of 6 μm or more is 5% or less, fine precipitates are precipitated during use in a high temperature environment, and creep strength and toughness are increased. The preferred upper limit of the total volume fraction of the coarse precipitate is 4%, more preferably 3%. Here, the equivalent equivalent diameter means the diameter (μm) when the area of the precipitate is converted into the area of the circle.

[Method for measuring total volume fraction of coarse precipitates in tissue]

The total volume fraction of coarse precipitates in the structure of the austenitic heat-resistant alloy of the present embodiment can be measured by the following method.

A test piece having a vertical cross section is taken from the surface of the heat resistant alloy material. For example, when the austenitic heat-resistant alloy material is an alloy tube, a test piece is taken from the center of the thickness of the cross section perpendicular to the axial direction.

After polishing the cross section (observation surface) of the collected test piece, the observation surface is etched with a mixed solution of hydrochloric acid and nitric acid. An SEM image (reflected electron image) is created by photographing an arbitrary 10 fields of view on a viewing surface using a scanning electron microscope (SEM). Each field of view is set to 100 μm × 100 μm.

In the SEM image, the contrast between the precipitate and the matrix is different. The area of the precipitate specified by the difference in contrast is determined, and the equivalent diameter of each precipitate is calculated. After calculation, precipitates (coarse precipitates) having a circle equivalent diameter of 6 µm or more are specified.

Find the total area of the specified coarse precipitate. Find the ratio (%) of the total area of the coarse precipitate to the field of view. Since the area ratio of the precipitate corresponds to the volume fraction, the ratio of the obtained coarse precipitate is defined as the total volume fraction (%) of the coarse precipitate.

The shape of the austenitic heat-resistant alloy according to the present embodiment is not particularly limited. The austenitic heat-resistant alloy is, for example, an alloy tube. The austenitic heat-resistant alloy pipe is used as a pipe for a boiler or a reaction pipe for a chemical plant. The austenitic heat-resistant alloy may be a plate material, a rod material, or a wire material.

[Manufacturing method]

As an example of a method for producing an austenitic heat-resistant alloy of this embodiment, a method for manufacturing an alloy tube will be described. The manufacturing method of the present embodiment is a step of preparing a material having the above-described chemical composition (preparation process), a step of hot forging the prepared material (hot forging process), and an intermediate material by performing hot processing on the hot forged material. It includes the process of manufacturing (hot working process) and the step of performing solution heat treatment on the intermediate material (solution heat treatment process). Hereinafter, each process is demonstrated.

[Preparation process]

A molten steel having the above-described chemical composition is prepared. Well-known degassing treatment is performed for molten steel as necessary. The material is produced by casting using molten steel. The material may be an ingot by the ingot method, or a slab, bloom, billet or other cast piece by the continuous casting method.

[Hot Forging Process]

A column material is manufactured by performing hot forging on the manufactured material. In hot forging, the cross-sectional reduction rate defined by equation (1) is 30% or more.

Section reduction rate = 100- (cross-sectional area of material after hot forging / cross-sectional area of material before hot forging) × 100 (%) (1)

As described above, precipitates such as process carbides are present in the structure of the material produced by casting. These precipitates are coarse, and many of them have a circular equivalent diameter of 6 μm or more. Such coarse precipitates are difficult to employ even in the solution treatment of a subsequent process.

If the cross-sectional reduction rate in the hot forging process is 30% or more, the coarse precipitates are destroyed during hot forging and the size is reduced. Therefore, the precipitate tends to be dissolved in the solution heat treatment in a subsequent step. As a result, the volume fraction of precipitates having an equivalent equivalent diameter of 6 μm or more becomes 5% or less.

The preferred cross-sectional reduction rate is 35% or more, and more preferably 40% or more. The upper limit of the cross-sectional reduction rate is not particularly limited, but considering productivity, it is 90%.

[Hot Processing Process]

Hot working is performed on the hot forged material (column material) to produce an alloy element pipe as an intermediate material. For example, a through hole is formed in the center of the cylindrical material by machining. Hot extrusion is performed on the cylindrical material in which the through-holes are formed, thereby producing an alloy element pipe. You may manufacture an alloy element pipe (intermediate material) by punching a cylindrical material. The intermediate material after hot working may be cold worked. Cold working is, for example, cold drawing. An intermediate material is manufactured by the above process.

[Solution heat treatment process]

Solution heat treatment is performed on the prepared intermediate material. Precipitation in the intermediate material is employed by solution heat treatment.

The heat treatment temperature in the solution heat treatment is 1100 to 1250 ° C. If the heat treatment temperature is less than 1100 ° C, the precipitate is not sufficiently dissolved, and as a result, the volume fraction of the coarse precipitate exceeds 5%. On the other hand, if the heat treatment temperature is too high, the austenite particles become coarse and the manufacturability decreases.

When the heat treatment temperature is 1100 to 1250 ° C, the precipitate is sufficiently solid-dissolved, so that the total volume fraction of the coarse precipitate is 5% or less.

The solution heat treatment time is not particularly limited. The solution heat treatment time is, for example, 1 minute to 1 hour.

The intermediate material after the solution heat treatment may be subjected to pickling treatment for the purpose of removing scale formed on the surface. For pickling, for example, a mixed acid solution of nitric acid and hydrochloric acid is used. The pickling time is, for example, 30 to 60 minutes.

In addition, the intermediate material after the pickling treatment may be subjected to a blast treatment using a projection material. For example, a blast treatment is performed on the inner surface of the alloy tube. In this case, by forming a working layer on the surface, corrosion resistance (such as oxidation resistance) is increased.

The austenite-type heat-resistant alloy of this embodiment is manufactured by the above manufacturing method. In addition, the manufacturing method of the alloy pipe was described above. However, you may manufacture a plate material, a rod material, a wire material, etc. by the same manufacturing method (preparation process, hot forging process, hot processing process, solution heat treatment process).

[Example]

[Manufacturing method]

The molten steel having the chemical composition shown in Table 1 was produced using a vacuum melting furnace.

[Table 1]

Using the molten steel, a cylindrical ingot having an outer diameter of 120 mm (30 kg) was prepared. The ingot was subjected to hot forging at a section reduction rate shown in Table 2 to prepare a rectangular material. Hot rolling and cold rolling were performed on the rectangular material to produce a plate-shaped intermediate material having a thickness of 1.5 mm. The intermediate material was subjected to a solution treatment for 10 minutes at the heat treatment temperature shown in Table 2. After holding for 10 minutes, the intermediate material was water-cooled to prepare an alloy plate material.

[Table 2]

[Creep fracture test]

Test pieces were prepared from the prepared alloy plates. The test piece was taken in parallel in the longitudinal direction (rolling direction) from the center of the thickness of the alloy plate. The test piece was a round bar test piece, the diameter of the parallel part was 6 mm, and the distance between the marks was 30 mm. A creep rupture test was conducted using the test piece. The creep rupture test was conducted in an atmosphere of 700 to 800 ° C. Based on the obtained breaking strength, the creep strength (MPa) at 1.0 × 10 ⁴ hours at 700 ° C. was determined by the Larson Miller parameter method.

[Charpy impact test]

The produced alloy plate material was subjected to an aging treatment held at 700 ° C for 8000 hours, followed by water cooling. The V-notch Charpy impact test specimen specified in JIS Z2242 (2005) was collected from the central portion in the thickness direction of the plate material after the aging treatment. The notch was produced parallel to the longitudinal direction of the alloy plate. The width of the test piece was 5 mm, the height was 10 mm, the length was 55 mm, and the notch depth was 2 mm. At 0 ° C, a Charpy impact test conforming to JIS Z2242 (2005) was conducted to obtain an impact value (J / cm ² ).

[Test result]

Table 2 shows the test results.

Referring to Table 2, the chemical composition of Test No. 1 to Test No. 11 was appropriate, and the volume fraction of coarse precipitates was 5% or less. As a result, the creep strength was 140 MPa or more, and showed excellent creep strength. In addition, the Charpy impact value was 40 J / cm ² or more, and it showed excellent toughness even after prolonged aging treatment.

On the other hand, in Test No. 12, the C content was too large. Therefore, the volume fraction of the coarse precipitate exceeded 5%. As a result, the creep strength was less than 140 MPa, and the Charpy impact value was less than 40 J / cm ² .

In Test No. 13, the Al content was too small. Therefore, the creep strength was less than 140 MPa. It is thought that the precipitation amount of NiAl was small.

In Test No. 14, the Al content was too large. Therefore, the creep strength was less than 140 MPa. Since the Al content was too large, it was thought that the structure was not stable and the creep strength was low.

In Test No. 15, the Cr content was too small. Therefore, the creep strength was less than 140 MPa. It is thought that this was because the amount of precipitation of α-Cr was small.

In Test No. 16, the Cr content was too large. Therefore, the creep strength was less than 140 MPa. Since the Cr content was too large, it was thought that the structure was not stable and the creep strength was low.

In Test No. 17, the cross-sectional reduction rate during hot forging was less than 30%. For this reason, the total volume fraction of coarse precipitates exceeded 5%. As a result, the creep strength was less than 140 MPa, and the Charpy impact value was less than 40 J / cm ² .

In Test No. 18, the solution heat treatment temperature was less than 1100 ° C. For this reason, the total volume fraction of coarse precipitates exceeded 5%. As a result, the creep breaking strength was less than 140 MPa, and the Charpy impact value was less than 40 J / cm ² .

In Test No. 19, the Nb content was too small. Therefore, the Charpy impact value was less than 40 J / cm ² .

In Test No. 20, the Nb content was too small. Therefore, the creep strength was less than 140 MPa.

The embodiments of the present invention have been described above. However, the above-described embodiment is only an example for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiments, and the above-described embodiments can be appropriately changed and carried out without departing from the spirit.

[Industrial availability]

The austenitic heat-resistant alloy of the present invention can be widely used in a high temperature environment of 700 ° C or higher. In particular, it is particularly suitable for use as an alloy tube in power generation boilers, chemical industry plants, etc. exposed to high temperature environments of 700 ° C or higher.

Claims (6)

In mass%,
C: 0.03% or more and less than 0.25%,
Si: 0.01-2.0%,
Mn: 2.0% or less,
Cr: 10% or more and less than 30%,
Ni: more than 25% and less than 45%,
Al: more than 2.5% and less than 4.5%,
Nb: 0.2 to 3.5%,
N: 0.025% or less,
Ti: 0% or more but less than 0.2%,
W: 0-6%,
Mo: 0-4%,
Zr: 0 to 0.1%,
B: 0 to 0.01%,
Cu: 0-5%,
Rare earth elements: 0 to 0.1%,
Ca: 0 to 0.05%, and
Mg: 0 ~ 0.05%,
The balance is made of Fe and impurities,
P and S in impurities are each,
P: 0.04% or less, and
S: Has a chemical composition of 0.01% or less,
Austenite-based heat-resistant alloy, characterized in that the total volume fraction of precipitates having a circular equivalent diameter of 6 μm or more among the tissues is 5% or less. The method according to claim 1,
The chemical composition, in mass%,
Ti: 0.005% or more but less than 0.2%,
W ： 0.005 ~ 6%,
Mo ： 0.005 ~ 4%,
Zr: 0.0005 to 0.1%, and
B: An austenitic heat-resistant alloy containing one or two or more selected from the group consisting of 0.0005 to 0.01%. The method according to claim 1,
The chemical composition, in mass%,
Cu: 0.05-5%, and
Rare earth element: Austenitic heat-resistant alloy containing at least one selected from the group consisting of 0.0005 to 0.1%. The method according to claim 2,
The chemical composition, in mass%,
Cu: 0.05-5%, and
Rare earth element: Austenitic heat-resistant alloy containing at least one selected from the group consisting of 0.0005 to 0.1%. The method according to any one of claims 1 to 4,
The chemical composition, in mass%,
Ca: 0.0005 to 0.05%, and
Austenitic heat-resistant alloy containing at least one selected from the group consisting of Mg: 0.0005 to 0.05%. A step of hot forging a material having a chemical composition according to claim 1 with a cross-sectional reduction rate of 30% or more,
A process of manufacturing an intermediate material by performing hot processing on the hot forged material,
A method for producing an austenitic heat-resistant alloy comprising a step of performing a solution treatment at 1100 to 1250 ° C for the intermediate material.