ES2865379T3 - NI-FE-CR alloy - Google Patents

Abstract

A Ni-Fe-Cr alloy comprising a chemical composition consisting of, in percent by mass: C: 0.005 to 0.015%; Yes: from 0.05 to 0.50%; Mn: 0.05 to 1.5%; P: 0.030% or less; S: 0.020% or less; Cu: 1.0 to 5.0%; Ni: 30.0 to 45.0%; Cr: 18.0 to 30.0%; Mo: 2.0 to 4.5%; Ti: 0.5 to 2.0%; N: 0.001 to 0.015%; and Al: from 0 to 0.50%, the rest being Fe and impurities, where an average grain size d (μm) satisfies Formula (1): d <4.386 / (Crel + 0.15) (1) where, Crel in Formula (1) is defined by Formula (2): Crel = C - 0.125Ti + 0.8571N (2) where, the symbols of the elements of Formula (1) and Formula (2) they must be replaced by the content of the corresponding elements (in% by mass).

Description

DESCRIPTION

NI-FE-CR alloy

Technical field

The present invention relates to a Ni-Fe-Cr alloy.

Background of the technique

Facilities, such as reheat furnace piping, in an oil refining plant and a petrochemical plant are used in high temperature environments. Furthermore, these installations come into contact with the process fluid that includes sulphides and / or chlorides. Therefore, a material used for installations is required to have excellent corrosion resistance. For installations, for example, Ni-based alloys or Ni-Fe-Cr alloys that have excellent corrosion resistance are used, typically Alloy825 (TM). Ni-based alloys used for the installations described above are proposed in Japanese Patent Application Publication No. 61-227148 (Patent Bibliography 1) and Japanese Patent Application Publication No. 6-240407 ( Patent Bibliography 2).

A high nickel content alloy disclosed in Patent Bibliography 1 consists of, in mass percent, C: 0.1% or less, Si: 1.0% or less, Mn: 1.5% or less, S : 0.015% or less, Ni: 30.0 to 30.5%, Cr: 19.0 to 25.0%, Cu: 1.0% or less, Al: 0.1 to 1.0% , Ti: from 0.05 to 1.0%, and Nb: from 0.05 to 1.0%, the remainder being iron and unavoidable impurities, and satisfies the conditions of (3Ti Nb) / S> 150 and (Ti Nb) / C> 15. Patent Bibliography 1 discloses that the high nickel content alloy can be endowed with excellent resistance to intergranular corrosion.

A high strength coated steel disclosed in Patent Bibliography 2 has a base metal composition consisting of, in mass percent, C: 0.03 to 0.12%, Si: 0.5% or less, Mn : 1 to 1.8%, Nb: 0.06% or less, Mo: 0.25% or less, V: 0.06% or less, and Al: 0.01 to 0.06%, where the rest Fe and unavoidable impurities. High strength coated steel is a Ni-based alloy having a coating material composition consisting of C: 0.05% or less, Si: 0.5% or less, Mn: 1% or less, Cr : 19.5 to 23.5%, Mo: 2.5 to 3.5%, Al: 0.2% or less, Ti: 0.6 to 1.2%, Cu: 1.5 at 3%, and Ni: from 38 to 46%, the rest being Fe and unavoidable impurities. Patent Bibliography 2 describes that high strength coated steel with excellent corrosion resistance can be provided by heating from 900 to 1030 ° C, then quenching and cooling from 500 to 630 ° C.

JP 2014-040669 discloses a high corrosion resistance alloy excellent in resistance to intergranular corrosion in an atmosphere containing nitrate differentiating the preferred correlation limitation between C content and Si content for resistance to corrosion. intergranular corrosion, and thereby effectively suppressing the deposition of an intermetallic compound such as a Cr carbide or an x-phase on an edge of the crystal grain. A high corrosion resistance alloy is disclosed which is excellent in resistance to intergranular corrosion and contains C: 0.015% by mass or less, Si: 0.45% by mass or less, Mn: 2.00% by mass or less, P: 0.040% by mass or less, S: 0.030% by mass or less, Cr: 19.00 to 25.00% by mass, Mo: 2.00 to 8.00% by mass, Fe : 20.00 to 40.00% by mass, N: 0.001 to 0.300% by mass, Cu: 0.01 to 3.00% by mass, and Al: 0.20% by mass or less, and the remainder Ni with unavoidable impurities, and satisfies the following expression. If 83C <0.51.

Appointment list

Patent bibliography

[Patent Bibliography 1] Japanese Patent Application Publication No. 61-227148

[Patent Bibliography 2] Japanese Patent Application Publication No. 6-240407

Summary of the invention

Technical problem

A Ni-based alloy or a Ni-Fe-Cr alloy can become sensitized in a heat affected zone of the weld when welding is performed. Sensitization is likely to cause intergranular corrosion. Therefore, a Ni-based alloy or a Ni-Fe-Cr alloy used in the high temperature environments described above is required to have excellent resistance to intergranular corrosion and which is achieved by inhibiting sensitization.

However, with the materials disclosed in Patent Bibliography 1 and Patent Bibliography 2 described Previously, intergranular corrosion may occur due to insufficient inhibition of sensitization.

An object of the present invention is to provide a Ni-Fe-Cr alloy having excellent resistance to intergranular corrosion.

Solution to the problem

A Ni-Fe-Cr alloy of the present embodiment has a chemical composition consisting of, in mass percent, C: 0.005 to 0.015%, Si: 0.05 to 0.50%, Mn: 0, 05 to 1.5%, P: 0.030% or less, S: 0.020% or less, Cu: 1.0 to 5.0%, Ni: 30.0 to 45.0%, Cr: 18, 0 to 30.0%, Mo: from 2.0 to 4.5%, Ti: from 0.5 to 2.0%, N: from 0.001 to 0.015% and Al: from 0 to 0.50%, where the rest Fe and impurities. An average grain size d (pm) satisfies Formula (1):

d <4.386 / (C ^rel + 0.15) (1)

where, C ^rel in Formula (1) is defined by Formula (2):

C ^rel = C - 0.125Ti 0.8571N (2)

where, the symbols of the elements of Formula (1) and Formula (2) are to be replaced by the content of the corresponding elements (in% by mass).

Advantageous effects of the invention

The Ni-Fe-Cr alloy according to the present invention has excellent resistance to intergranular corrosion.

Brief description of the drawings

[FIG. 1] FIG. 1 is a graph illustrating the relationship between the relative amount of dissolved C (C ^rel ), the average grain size d (pm), and the resistance to intergranular corrosion.

[FIG. 2] FIG. 2 is a graph illustrating the relationship between the average grain size d (pm), the subtraction (F1 - d) of d from F1 = 4.386 I (C ^rel + 0.15), and the resistance to intergranular corrosion.

Description of achievements

An embodiment of the present invention will now be described in detail with reference to the accompanying drawings. Hereinafter, the% sign that follows each item means "percent by mass."

The present inventors conducted studies on the sensitization and resistance to intergranular corrosion of Ni-Fe-Cr alloys. As a result, the present inventors obtained the following findings.

(A) Sensitization occurs by the following mechanism. When a Ni-Fe-Cr alloy is affected by heat due to welding or the like, the Cr carbide precipitates at the edges of the crystal grains. The precipitation of Cr carbide consumes the Cr present around the edges of the crystal grains. Therefore, when Cr carbide precipitates, Cr-depleted zones develop along the edges of the crystal grains. This phenomenon is called sensitization. In areas depleted in Cr, the adequate formation of passivation films fails, which leads to a decrease in corrosion resistance, leading to the appearance of intergranular corrosion. By reducing the amount of dissolved C in a Ni-Fe-Cr alloy, sensitization can be inhibited, thereby increasing resistance to intergranular corrosion.

(B) If the C content in a Ni-Fe-Cr alloy is reduced, the amount of dissolved C in the Ni-Fe-Cr alloy is reduced. Therefore, in the present embodiment, the C content is set to 0.005 to 0.015%.

(C) If C is immobilized by Ti in the form of Ti carbide, the amount of C dissolved in the Ni-Fe-Cr alloy can be further reduced. However, if N is present in the Ni-Fe-Cr alloy, the Ti nitride precipitates earlier than the Ti carbide on solidification because N has a higher affinity for Ti than for C. As a result, the Ti is depleted, and cannot immobilize C. Therefore, the N content is preferably low. Therefore, in the present embodiment, the N content is set to 0.015% or less.

As described above, the amount of C dissolved in an actual Ni-Fe-Cr alloy is a value determined from the content of C, Ti and N in relative form. Therefore, the theoretical amount of dissolved C is determined as follows.

Amount of dissolved C = C content in the alloy - C content immobilized by the Ti in the form of TiC.

Here, when N is present, Ti precipitates as Ti nitride and therefore the amount of Ti available for immobilization of C is determined as follows.

Amount of Ti available for immobilization of C = Ti - 48/14 3 N

Consequently, the theoretical amount of C dissolved in the alloy is determined as follows.

C ^totai = C - (Ti - 48/14 x N) x 12/48 = C - 0.250Ti 0.8571N

However, in a real industrial production process, chemical kinetics must be taken into account. Specifically, in an equilibrium condition, the amount of dissolved C is the theoretical amount of dissolved C ( ^total C) described above. On the contrary, in the actual production process, the reaction proceeds in a short time and can be completed before the equilibrium condition occurs. Therefore, not all Ti forms TiC in some cases and therefore it is necessary to adjust the Ti coefficient in the ^total C formula.

As a result of a study carried out by the inventors, the actual amount of C dissolved in a Ni-Fe-Cr ( ^actual C) alloy is as follows:

C ^real = C - 0.125Ti 0.8571N k ⁱ

where, k ⁱ is a constant of the amount of C dissolved.

From the actual amount of dissolved C ( ^actual C), an amount of C used in the precipitation of Cr carbide (total precipitation amount of C (C ^pre )) is as follows, with a solubility limit of the solid of C indicated by k ² (%).

C ^pre = C - 0.125Ti 0.8571N k ⁱ -k ²

(D) To increase the resistance to intergranular corrosion, furthermore, the refining of the grains is effective. The reason for this is the following. When the grains are refined, the total area of the grain edges increases. The total amount of precipitation of C (C ^pre ) in the alloy is constant and therefore the larger the total grain edge area, the lower the amount of C, per unit grain edge area, which is which contributes to the precipitation of Cr carbide (amount of precipitation per unit C (C ^unit )). As a result, precipitation and growth of Cr carbide per grain edge unit area is inhibited and the development of Cr depleted zones is inhibited. Consequently, sensitization is inhibited.

The relationship between the average grain size d and the amount of precipitation per unit of C (C ^unit ) is determined as follows. When the average grain size is d (pm), the edge area of the grain is determined as lo 3 d ² mm ² (k ³ is a constant). When the number of grains per unit volume is k ⁴ / d ³ (k ⁴ is a constant), the edge area of the total grain is determined as follows.

Total grain edge area = (k ³ xd ² ) x (k ⁴ / d ³ ) = k ³ k ⁴ , / d

Using the total grain border area and the amount of precipitation of C (C ^pre ), the amount of precipitation per unit of C (C ^unit ) is determined as follows.

C ^unit = C ^pre / (k ³ k ⁴ / d) = dx (C ^pre / Í ^O k ⁴ )

According to this formula, the average grain size d is in proportion to the amount of precipitation per unit of C (C ^unit ). In other words, the smaller the average grain size d, the smaller the amount of precipitation per unit of C (C ^unit ), and consequently, sensitization is inhibited.

(E) The present inventors introduced an idea of an intergranular corrosion resistance index based on the average grain size d and the amount of C that contributes to the precipitation of Cr carbide described above. As a result, the present inventors discovered that increasing the resistance to intergranular corrosion cannot be achieved simply by reducing the average grain size d, and there is an appropriate average grain size d in relation to the amount of C that contributes to the Cr carbide precipitation.

FIG. 1 is a graph illustrating the relationship between the amount of C that contributes to Cr carbide precipitation (relative amount of dissolved C (C ^rel )), the average grain size d (pm), and the corrosion resistance intergranular. In FIG. 1, the horizontal axis represents the formula for the total amount of precipitation of C (C ^pre ) from which the constants are omitted and lo (the relative amount of dissolved C (C ^rel ) to be described later). Fig. 1 is obtained by the Example which will be described later. In FIG. 1, those that showed excellent resistance to intergranular corrosion are represented as "O", and those that showed poor resistance to intergranular corrosion are represented as "x".

From Fig. 1, to inhibit sensitization, it is necessary to reduce the average grain size d and increase the amount of total precipitation of C (C ^pre ). Conversely, the smaller the amount of total precipitation of C (C ^pre ), the larger the average grain size d can be made to be. In other words, the amount of total precipitation of C (C ^pre ) is in an inversely proportional relationship with the average grain size, and is expressed as follows:

d - k ⁵ / (C ^pre + k ⁶ )

where, k ⁵ and k ⁶ are constants.

Consistent with the relationship between excellent intergranular corrosion resistances and poor intergranular corrosion resistances (O and x) illustrated in FIG. 1, F1 can be obtained by determining the constants k ⁱ , k ² , k ⁵ and k ⁶ , establishing the dashed line in FIG. 1 as a limit:

F1 - 4.386 / (C ^rel + 0.15)

where, C ^rel is the amount of dissolved C determined from the content of C, Ti, and N in a relative way (the relative amount of dissolved C (C ^rel )), and defined as follows:

C ^rel - C - 0.125Ti 0.8571N (2)

From Fig. 1, to inhibit sensitization, it is necessary to reduce the average grain size d and increase the relative amount of dissolved C (C ^rel ). Conversely, the smaller the relative amount of dissolved C (C ^rel ), the larger the average grain size d can be made.

F1 is an index of the resistance to intergranular corrosion. When the average grain size d is less than F1, the average grain size d is adequate for the relative amount of dissolved C (C ^rel ). In this case, the amount of precipitation per unit C (C ^unit ) is sufficiently reduced, which inhibits sensitization. As a result, the resistance to intergranular corrosion can be increased. On the contrary, when the average grain size d is not less than F1, the average grain size d is excessively large for the relative amount of dissolved C (C ^rel ). In this case, the amount of precipitation per unit C (C ^unit ) is not sufficiently reduced, which contributes to sensitization. As a result, the resistance to intergranular corrosion is reduced.

FIG. 2 is a graph illustrating the relationship between the average grain size d (pm), the subtraction (F1 - d) of d from F1, and the resistance to intergranular corrosion. FIG. 2 is obtained by Example to be described later, as with FIG. 1. In FIG. 2, those that showed excellent resistance to intergranular corrosion are represented as "O", and those that showed poor resistance to intergranular corrosion are represented as "x". With reference to FIG. 2, when the average grain size d satisfies Formula (1), that is, when F1-d has a positive value, excellent intergranular corrosion resistance can be provided even when the average grain size d is large. When the average grain size d does not satisfy Formula (1), that is, when F1 - d has a negative value, the resistance to intergranular corrosion decreases even when the average grain size d is small.

A Ni-Fe-Cr alloy according to the present embodiment that has been completed based on the above findings has a chemical composition consisting of, in mass percent, C: 0.005 to 0.015%, Si: 0.05 0.50%, Mn: 0.05-1.5%, P: 0.030% or less, S: 0.020% or less, Cu: 1.0-5.0%, Ni: 30.0 to 45.0%, Cr: from 18.0 to 30.0%, Mo: from 2.0 to 4.5%, Ti: from 0.5 to 2.0%, N: from 0.001 to 0.015% and Al: from 0 to 0.50%, the rest being Fe and impurities. An average grain size d (pm) satisfies Formula (1):

d <4.386 / (C ^rel + 0.15) (1)

where, C ^rel in Formula (1) is defined by Formula (2):

C ^rel - C - 0.125Ti 0.8571N (2)

where, the symbols of the elements of Formula (1) and Formula (2) are to be replaced by the content of the corresponding elements (in% by mass).

The above chemical composition may contain Al: from 0.05 to 0.50%.

[Chemical composition]

The chemical composition of the Ni-Fe-Cr alloy according to the present embodiment consists of the following elements.

C: 0.005 to 0.015%

Carbon (C) increases the strength of the alloy. In addition, the C deoxidizes the alloy. An excessively low C content prevents these effects from occurring. In contrast, excessively high C content leads to increased precipitation of Cr carbides at the edges of the grains, resulting in a decrease in resistance to intergranular corrosion. Consequently, the C content is 0.005 to 0.015%. A lower limit of the C content is preferably 0.008%. An upper limit of the C content is preferably 0.013%, more preferably 0.010%.

Yes: from 0.05 to 0.50%

Silicon (Si) deoxidizes the alloy. An excessively low Si content does not produce this effect. In contrast, an excessively high Si content makes inclusions likely to occur. Consequently, the Si content is 0.05 to 0.50%. A lower limit of the Si content is preferably 0.15%, more preferably 0.20%. An upper limit of the Si content is preferably 0.45%, more preferably 0.40%.

Mn: from 0.05 to 1.5%

Manganese (Mn) stabilizes an austenitic phase. In addition, Mn deoxidizes the alloy. An excessively low Mn content prevents these effects from occurring. In contrast, an excessively high Mn content causes S to combine with Mn to form a sulfide, which is converted to non-metallic inclusions, resulting in decreased resistance to pitting. Consequently, the Mn content is 0.05 to 1.5%. A lower limit of the Mn content is preferably 0.15%, more preferably 0.30%. An upper limit of the Mn content is preferably 1.2%, more preferably 1.0%.

P: 0.030% or less

Phosphorus (P) is an impurity. P segregates at the grain edges on solidification of the weld, increasing susceptibility to cracks that occur due to embrittlement of a heat-affected zone. Therefore, the P content is 0.030% or less. An upper limit of the P content is preferably 0.025%, more preferably 0.020%. The P content is preferably as low as possible.

S: 0.020% or less

Sulfur (S) is an impurity. As with P, S segregates at the grain edges on solidification of the weld, increasing susceptibility to cracks that occur due to embrittlement of a heat-affected zone. In addition, S forms MnS, resulting in decreased resistance to pitting. Therefore, the S content is 0.020% or less. An upper limit of the S content is preferably 0.010%, more preferably 0.005%. The content of S is preferably as low as possible.

Cu: 1.0 to 5.0%

Copper (Cu) increases the corrosion resistance of the alloy. An excessively low Cu content prevents this effect from occurring. On the contrary, an excessively high content of Cu results in a decrease in the hot processability of the alloy. Therefore, the Cu content is 1.0 to 5.0%. A lower limit of the Cu content is preferably 1.2%, more preferably 1.5%. An upper limit of the Cu content is preferably 4.0%, more preferably 3.0%.

Ni: 30.0 to 45.0%

Nickel (Ni) increases the resistance to pitting of the alloy. An excessively low Ni content prevents this effect. On the contrary, an excessively high Ni content leads to saturation of the effect. Therefore, the Ni content is 30.0 to 45.0%. A lower limit of the Ni content is preferably 35.0%, more preferably 38.0%. An upper limit of the Ni content is preferably 44.5%, more preferably 44.0%.

Cr: 18.0 to 30.0%

Chromium (Cr) increases the corrosion resistance of the alloy. An excessively low Cr content prevents this effect. On the contrary, an excessively high Cr content causes a decrease in the stability of austenite at high temperature, resulting in a decrease in the strength of the alloy at high temperature. Therefore, the Cr content is 18.0 to 30.0%. A lower limit of the Cr content is preferably 19.0%, more preferably 20.0%. An upper limit of the Cr content is preferably 26.0%, more preferably 24.0%.

Mo: 2.0 to 4.5%

Molybdenum (Mo) increases the corrosion resistance of the alloy. An excessively low Mo content does not produce this effect. In contrast, an excessively high Mo content causes Laves phases to precipitate at the grain edges in an alloy that has a high Cr content, resulting in a decrease in the corrosion resistance of the alloy. Therefore, the Mo content is 2.0 to 4.5%. A lower limit of the Mo content is preferably 2.4%, more preferably 2.8%. An upper limit of the Mo content is preferably 4.0%, more preferably 3.5%.

Ti: 0.5 to 2.0%

Titanium (Ti) forms Ti carbide, inhibiting alloy sensitization. An excessively low Ti content prevents this effect from occurring. In contrast, an excessively high Ti content results in a decrease in the hot processability of the alloy. Therefore, the Ti content is 0.5 to 2.0%. A lower limit of the Ti content is preferably 0.55%, more preferably 0.60%. An upper limit of the Ti content is preferably 1.5%, more preferably 1.3%.

N: 0.001 to 0.015%

Nitrogen (N) forms fine carbonitrides in grains, and therefore, as resistance increases, it may be present. In contrast, excessively high N content causes Ti to combine with N to form TiN, which prevents C from being immobilized as Ti carbide, resulting in decreased inhibition of sensitization. Therefore, the N content is 0.001 to 0.015%. A lower limit of the N content is preferably 0.002%, more preferably 0.005%. An upper limit of the N content is preferably 0.013%, more preferably 0.010%.

The remainder of the chemical composition of the Ni-Fe-Cr alloy according to the present embodiment is Fe and impurities. Here, impurities mean elements that are mixed from minerals and slag used as raw material, a production environment or the like when the Ni-Fe-Cr alloy is produced industrially, and are allowed to mix within ranges wherein the impurities have no adverse effect on the Ni-Fe-Cr alloy according to the present embodiment.

[Optional items]

The Ni-Fe-Cr alloy described above may additionally contain Al instead of Fe.

Al: 0 to 0.50%

Aluminum (Al) is an optional item and does not need to be included. When included, Al deoxidizes the alloy. However, an excessively high content of Al results in a decrease in the cleanliness of the alloy and a decrease in the processability and ductility of the alloy. Therefore, the Al content is 0 to 0.50%. A lower limit of the Al content is preferably 0.05%. An upper limit of the Al content is preferably 0.30%, more preferably 0.20%. In the present specification, Al content means sol.Al (Al acid soluble).

[Formula 1)]

F1 is defined as F1 = 4.386 / (C ^rel + 0.15). F1 is an index of the resistance to intergranular corrosion. When the average grain size d is less than F1, the average grain size d is adequate for the relative amount of dissolved C (C ^rel ). In this case, the amount of precipitation per unit C (C ^unit ) is sufficiently reduced, which inhibits sensitization. As a result, the resistance to intergranular corrosion can be increased. On the contrary, when the average grain size d is not less than F1, the average grain size d is excessively large for the relative amount of dissolved C (C ^rel ). In this case, the amount of precipitation per unit C (C ^unit ) is not sufficiently reduced, which promotes sensitization. As a result, the resistance to intergranular corrosion is reduced.

[Formula (2)]

Since the relative amount of dissolved C (C ^rel ) in Formula (1) is determined from the content of C, Ti, and N relative, as described above, the relative amount of dissolved C (C ^rel ) is defined as follows.

C ^rel = C - 0.125Ti 0.8571N (2)

[Method of production]

The Ni-Fe-Cr alloy according to the present embodiment can be produced by various production methods. As an example of the production methods, a production method for a Ni-Fe-Cr alloy tube will be described below.

First, a starting material having the following chemical composition is prepared. The starting material is, for example, a hollow billet. The hollow billet can be produced by, for example, machining or vertical drilling. The hollow billet is subjected to hot extrusion work. Hot extrusion work is, for example, the Ugine-Sejournet process. By the above steps, a Ni-Fe-Cr alloy pipe is produced. Ni-Fe-Cr alloy tube can be produced by hot work other than hot extrusion work. Hot work can be repeated several times.

Preferably, a cooling rate to reach a temperature of 900 ° C after final hot work is 0.3 ° C / sec or more. When the cooling rate to reach a temperature of 900 ° C after the final hot work is 0.3 ° C / sec or more, it is possible to adjust the average grain size d so that the average grain size d satisfies Formula (1). As a result, the Ni-Fe-Cr alloy can have excellent resistance to intergranular corrosion.

The cooling rate to reach a temperature of 900 ° C can be made at 0.3 ° C / sec or more by fog cooling after the final hot work.

On hot-worked Ni-Fe-Cr alloy tube, cold working including cold rolling and / or cold drawing can be performed. Performing cold work allows reduction of average grain size d. In this case, the resistance to intergranular corrosion is further increased.

In addition, on the Ni-Fe-Cr alloy tube subjected to hot working or cold working, a final heat treatment, such as solution treatment, can be performed to obtain a desired mechanical property. In the case of conducting heat treatment, a lower limit of the heat treatment temperature is preferably 900 ° C, more preferably 915 ° C, even more preferably 930 ° C. In case of performing the solution treatment, a lower limit of the heat treatment temperature is preferably 1020 ° C. In this case, Cr carbide can be dissolved. As a result, resistance to intergranular corrosion can be further inhibited.

An upper limit of the heat treatment temperature is preferably 1100 ° C, more preferably 1080 ° C, even more preferably 1060 ° C. In case of performing stabilization treatment, an upper limit of the heat treatment temperature is preferably less than 1000 ° C. A heat treatment temperature below 1000 ° C allows the precipitation of TiC. Furthermore, the heat treatment temperature lower than 1000 ° C allows the reduction of the average grain size d. In this case, sensitization can be further inhibited. As a result, resistance to intergranular corrosion can be further inhibited. As for the Ni-Fe-Cr alloy according to the present embodiment, sensitization can be inhibited even when the heat treatment is carried out at a high temperature of 1000 to 1100 ° C. A duration of the heat treatment of the final heat treatment is preferably 2 to 30 minutes.

The above example description of the production methods has been made based on the production method of a Ni-Fe-Cr alloy tube. However, the Ni-Fe-Cr alloy can be a plate product, a welded tube, a bar product, or the like. In summary, the Ni-Fe-Cr alloy is not limited to a particular product form.

The Ni-Fe-Cr alloy produced by the above production method has excellent resistance to intergranular corrosion.

[Examples]

The alloys from Test No. 1 to Test No. 23 shown in Table 1 were vacuum melted to produce the materials.

[Table 1]

TABLE 1

continuation

Columns "C ^rei " and "F1" of Table 1 include the C ^rei values and the F1 values of the Ni-Fe-Cr alloys of the respective test number.

From the materials, ingots were produced. For Test Number 1 to Test Number 21, each of the respective ingots was hot forged at 1200 ° C, then hot rolled at 1200 ° C with a 50% reduction in area, as It was then cold rolled with an area reduction of 67% to produce a plate product having a thickness of 5mm, a width of 80mm and a length of 650mm. For Test Number 22 and Test Number 23, each of the respective ingots was hot forged at 1200 ° C to produce a plate product with a thickness of 15mm, a width of 60mm and a length 290 mm. For Test Number 22 and Test Number 23, cold rolling was not performed. For each of the plate materials, the final heat treatment was performed at the heat treatment temperatures and for the heat treatment durations shown in Table 2. The heat treated plate products were quenched. (cooling in water).

[Table 2]

TABLE 2

[Measurement of average grain size]

The plate products were cut in a direction perpendicular to the rolling direction and specimens having a thickness of 5 mm, a width of 20 mm and a length of 10 mm were taken. Each of the specimens was embedded in resin such that a surface including the rolling direction of the plate product (longitudinal section of the specimen) becomes an observation surface, and the observation surface was polished. mirror. The polished surface was stripped with mixed acid. The pickled viewing surface was observed under a light microscope. Regarding the average grain size d, five visual fields were selected with a magnification of 100x to determine the average grain size d (| jm).

[Intergranular corrosion resistance test]

From the plate product of each Test Number, a specimen having a thickness of 5 mm, a width of 10 mm and a length of 50 mm was taken. The longitudinal direction of the specimen was parallel to the longitudinal direction of the plate product. The specimen was subjected to a sensitization heat treatment at 700 ° C for 60 minutes, which was a simulation of a heat affected zone of the weld. The surface of the specimen subjected to the sensitization heat treatment was finished by polishing with 600 # wet emery, degreased with acetone and dried. The test sample was subjected to the intergranular corrosion test according to the ASTM method at 262 C, and the intergranular corrosion resistance of the pickled specimen was evaluated. A test bath was a 65% boiling nitric acid, and three batches of a dip test were performed, each of the three batches lasting 48 hours. A corrosion loss was measured in each of the batches and, from the corrosion rate in the three batches, the mean corrosion rate was calculated.

In the evaluation of the resistance to intergranular corrosion, when the average corrosion rate of the three batches was not greater than 1 g / m ² h, it was determined that the resistance to intergranular corrosion was excellent (shown with "O" in Table 2). When the average corrosion rate was greater than 1 g / m ² h, the resistance to intergranular corrosion was determined to be poor (shown with "*" in Table 2).

[Test results]

Table 2 shows the test results.

With reference to Table 1, as for Test number 1 to Test number 9 and Test number 22, the content of the elements of the plate products was appropriate, and the chemical compositions and the average grain sizes d satisfied Formula (1). As a result, the grains became fine and excellent resistance to intergranular corrosion was demonstrated.

Regarding Test number 22, cold rolling was not performed and therefore the average grain size d was large compared to Test number 5. However, since the average grain size d satisfied the Formula (1), excellent resistance to intergranular corrosion was demonstrated.

On the contrary, regarding Test number 10, the N content was excessively high. As a result, Ti precipitated as Ti nitride, so C was not sufficiently immobilized. This made the relative amount of dissolved C (C ^rel ) high, making the average grain size d excessively large for the relative amount of dissolved C (C ^rel ). Consequently, the average grain size d was not less than F1 and the resistance to intergranular corrosion was low.

Regarding Test number 11, the Ti content was excessively low. This caused Ti to not sufficiently immobilize C, which made the relative amount of dissolved C (C ^rel ) high, making the average grain size d excessively large for the relative amount of dissolved C (C ^rel ) . Consequently, the average grain size d was not less than F1 and the resistance to intergranular corrosion was low.

Regarding Test number 12 to Test number 16, although their chemical compositions were appropriate, their average grain sizes d were not less than F1. As a result, the resistance to intergranular corrosion was low.

As for Test number 17, the Ti content was excessively low, and the N content was excessively high. This made the relative amount of dissolved C (C ^rel ) high, making the average grain size d excessively large for the relative amount of dissolved C (C ^rel ). Consequently, the average grain size d was not less than F1 and the resistance to intergranular corrosion was low.

Regarding Test number 18, the C content was excessively high. This made the relative amount of dissolved C (C ^rel ) high, making the average grain size d excessively large for the relative amount of dissolved C (C ^rel ). Consequently, the average grain size d was not less than F1 and the resistance to intergranular corrosion was low.

Regarding Test number 19, the Ti content was excessively high. This made the hot processability low, making processing impossible, and therefore Test 19 was out of the test.

As for Test 20, the cooling rate to reach 900 ° C after final hot work was less than 0.3 ° C / s. Therefore, even with the heat treatment temperature set to less than 1000 ° C, the average grain size d was large compared to Test number 2, not being less than F1. As a result, the resistance to intergranular corrosion was low.

As for Test number 21, the cooling rate to reach 900 ° C after final hot work was less than 0.3 ° C / s. Therefore, the average grain size d was large compared to Test number 3, not being less than F1. As a result, the resistance to intergranular corrosion was low.

As for Test number 23, the cooling rate to reach 900 ° C after final hot work was less than 0.3 ° C / s. Also, as for Test Number 23, cold rolling was not performed after hot processing. Therefore, even with the heat treatment temperature set to less than 1000 ° C, the average grain size d was large compared to Test number 5, not being less than F1. As a result, the resistance to intergranular corrosion was low.

The embodiment according to the present invention has been described above. However, the aforementioned embodiment is merely an example for practicing the present invention. Therefore, the present invention is not limited to the above-mentioned embodiment, and the above-mentioned embodiment can be modified and implemented as appropriate without departing from the scope of the present claims.

Claims (2)

1. A Ni-Fe-Cr alloy comprising a chemical composition consisting of, in percent by mass: C: 0.005 to 0.015%; Yes: from 0.05 to 0.50%; Mn: 0.05 to 1.5%; P: 0.030% or less; S: 0.020% or less; Cu: 1.0 to 5.0%; Ni: 30.0 to 45.0%; Cr: 18.0 to 30.0%; Mo: 2.0 to 4.5%; Ti: 0.5 to 2.0%; N: 0.001 to 0.015%; and Al: from 0 to 0.50%, the rest being Fe and impurities, where an average grain size d (| jm) satisfies Formula (1): d <4.386 / (C ^rel + 0.15) (1) where, C ^rel in Formula (1) is defined by Formula (2): C ^rel = C - 0.125Ti 0.8571N (2) where, the symbols of the elements of Formula (1) and Formula (2) are to be replaced by the content of the corresponding elements (in% by mass). 2. The Ni-Fe-Cr alloy according to claim 1, wherein the chemical composition contains Al: from 0.05 to 0.50%.