JP7261345B1 - Austenitic Ni-Cr-Fe alloy excellent in oxidation resistance and its production method - Google Patents

Abstract

An object of the present invention is to provide an austenitic Ni--Cr--Fe alloy having excellent oxidation resistance even in a severe high temperature environment. [Solution] C: 0.01 to 0.10%, Si: 0.02 to 0.45%, Mn: 0.10 to 0.85%, P: ≤ 0.015%, S: ≦0.0015%, Cr: 13.0 to 18.0%, Fe: 5.0 to 11.0%, Mo: ≦0.6%, Cu: ≦0.5%, B: ≦0.005 %, Al: 0.01 to 0.3%, Ti: 0.01 to 0.3%, Co: ≤ 1.00%, Nb: ≤ 1.00%, Ta: ≤ 0.05%, N: ≦0.01%, O: 0.0002 to 0.0040%, Ca: ≦0.002%, one or more of rare earth elements (REM) La, Ce, and Y Total weight: 0 An austenitic Ni--Cr--Fe alloy containing 0.001 to 0.010% with the balance being Ni and unavoidable impurities. [Selection drawing] Fig. 1

Description

TECHNICAL FIELD The present invention relates to an austenitic Ni--Cr--Fe alloy, and more particularly to an austenitic Ni--Cr--Fe alloy having excellent oxidation resistance in a high temperature environment.

Afterburner parts, nuclear power plants, and industrial heat treatment furnaces are used in harsh high-temperature environments of 1000 to 1200°C, so the materials used for these must have high-temperature strength, high-temperature corrosion resistance, and oxidation resistance. Excellent quality is required. In particular, with regard to oxidation resistance, the required characteristics are that the protective surface oxide scale, mainly composed of Cr ₂ O ₃ , formed on the material surface in the above-mentioned high-temperature environment is fine and has high adhesion to the material. It is mentioned as one of For example, SUS310S, NCF800, NCF600, etc. are used as materials for equipment used in the high-temperature environment.

As a technique for improving the high-temperature properties of materials used in such harsh environments, for example, Patent Document 1 describes the composition of surface oxide scale formed on the alloy surface by heat treatment in a predetermined atmosphere. Ni-based alloys and production methods thereof have been proposed in which the surface oxide scale properties are improved by appropriately controlling the , thickness, and crystal grain size of the oxide. Further, Patent Document 2 proposes a Ni--Cr--Fe alloy which is excellent in creep strength and stress relaxation cracking resistance due to combined addition of Ti, Al and REM (rare earth metal).

However, in the technique disclosed in Patent Document 1, the influence of S, which can form a compound with Cr in the material that is the main component of the surface oxide scale, and the adhesion of the surface oxide scale have a negative effect. No study has been made on the effects of Mo, N, and Mn, and it is considered that the application to the high-temperature environment described above, which requires excellent oxidation resistance, is insufficient.

In addition, in the technique disclosed in Patent Document 2, the composition of this alloy is adjusted in a high-frequency induction furnace in a laboratory, and a steel ingot is obtained and subjected to a hot rolling process. Such mass production was not possible. Furthermore, although it is stated that all REMs act effectively, the actual examples are mainly the addition of Nd, and in others, Ce, La, and Y are added to some alloys, and the REM target is Limited. In addition, there was no process for removing S and O in the manufacturing process, and this could not be achieved unless the raw materials were carefully selected. Therefore, in some cases, the added REM is lost due to oxidation or sulfurization, making it difficult to achieve the original purpose, and the proposal was industrially poor in productivity. Therefore, it is difficult to say that it is possible to accurately provide an alloy with improved creep strength by adding REM on an industrial scale.

JP-A-2002-121630 Table 2018-066579

The present invention has been made in view of the above circumstances, and an object thereof is to propose an austenitic Ni--Cr--Fe alloy which is excellent in oxidation resistance even in a severe high temperature environment.

The inventors have made intensive studies to solve the above problems. Until now, it has _been known that the addition of REMs La, Ce, and Y improves the adhesion of surface oxide scale formed on the alloy surface in a high-temperature environment. H ₂ O-12% CO ₂ -0.8% CO-0.1% NO ₂ -bal. Sufficient knowledge has not been obtained regarding the contribution to oxidation resistance evaluated by a cycle test in which temperatures from room temperature to 1000 to 1200° C. are repeated in a mixed gas atmosphere of N ₂ . Therefore, the correlation between La, Ce, Y and other elements contained in the alloy was investigated in detail. As a result, it was found that addition of La, Ce, and Y is extremely effective for improving oxidation resistance, and that Si, Cr, Al, and Ti are also effective as other elements. On the other hand, it was found that the inclusion of S, Mn, Mo, B, and N inhibits the oxidation resistance-improving action. From this, it was found that Si, Ni, Cr, Al, Ti, S, Mn, Mo, B, and N should be controlled in order to sufficiently ensure the effect of adding REM.

Furthermore, the content (mass%) of S contained in the alloy is subtracted from the total weight (mass%) of one or more of the REMs La, Ce, and Y contained in the alloy element alloy. It was found that the value obtained by this method has a good correlation with the oxidation weight loss in the high-temperature oxidation test, and the characteristic formula obtained by this was found to be 4.5≦REM/S.

In addition, as a result of repeated studies focusing on the form of surface oxide scale formed in a high-temperature environment, 5% O ₂ -16% H ₂ O-12% CO ₂ -0.8% CO-0.1% NO ₂ In the mixed gas atmosphere consisting of -bal _N2 , when the thickness of the surface oxide scale formed in the cycle test that repeats from room temperature to 1000 to 1200°C is in the range of 6 to 60 μm, the surface oxide scale is formed finely. , the adhesiveness was excellent, and the oxidation weight loss after the test also showed good results.

That is, the austenitic Ni-Cr-Fe alloy of the present invention has C: 0.01 to 0.10%, Si: 0.02 to 0.45%, Mn: 0.10 to 0.85 in mass%. %, P: ≤0.015%, S: ≤0.0015%, Cr: 13.0 to 18.0%, Fe: 5.0 to 11.0%, Mo: ≤0.6%, Cu: ≤0.5%, B: ≤0.005%, Al: 0.01-0.3%, Ti: 0.01-0.3%, Co: ≤1.00%, Nb: ≤1.00 %, Ta: ≤ 0.05%, N: ≤ 0.01%, O: 0.0002 to 0.0040%, Ca: ≤ 0.002%, rare earth elements (REM) La, Ce, The total weight of any one or more of Y: 0.001 to 0.010%, the balance being Ni and inevitable impurities, and satisfying the following formula (1) It is characterized by
60≧7.4Si+2.8×Cr+6.9×Al+3.2×Ti+3374×REM-5914×S-3.87×Mn-42.3×Mo-3072×B-1024×N≧30 (1)
(Here, each element symbol in the above formula indicates the content (mass%) of each element.)

Further, in the austenitic Ni-Cr-Fe alloy of the present invention, any one or two or more of the rare earth elements (REM) La, Ce, and Y preferably satisfy the following formula (2): and
4.5≦REM(La, Ce, Y)/S (2)
(Here, each element symbol in the above formula indicates the content (mass%) of each element.)

In the austenitic Ni--Cr--Fe alloy of the present invention, in addition to the above composition, 5% O ₂ -16% H ₂ O-12% CO ₂ -0.8% CO-0.1% NO ₂ -bal. The composition of the surface oxide scale formed in a cycle test in which the temperature is repeated from room temperature to 1000 to 1200 ° C in a mixed gas atmosphere consisting of N ₂ is, in terms of mass%, Cr: 50% or more, Fe: 0 to 10%, and Ni: 0. 5%, O: 10 to 40%, REM: 0.05 to 0.5%, and the balance contains Mn, Si, Al, and Ti as unavoidable elements. A preferable aspect is that the scale has a thickness of 6 to 60 μm.

The present invention also proposes a method for producing the austenitic Ni--Cr--Fe alloy. That is, the alloy composition is adjusted by refining after melting the alloy raw material, and in the refining, a mixed gas of oxygen and argon is blown into the melted alloy raw material (molten alloy) to decarburize, and the nitrogen concentration is reduced to 0. After controlling to .01% or less, Cr reduction, then aluminum, limestone and fluorite are added to the molten alloy to form CaO- _SiO2 - _{Al2O3 -} MgO-F system slag, and the molten alloy The oxygen concentration in the slab is set to 0.0002 to 0.0040 mass%, and then a raw material containing one or more of La, Ce, and Y is added to adjust the ingredients, and casting is performed to obtain a slab. It is used for the hot rolling process. In the hot rolling process, for example, a coil or the like is manufactured.

According to the present invention, the austenitic Ni--Cr--Fe alloy has excellent oxidation resistance in a high-temperature environment, and can greatly contribute to extending product life.

1 is a schematic diagram for explaining the surface structure of an austenitic Ni—Cr—Fe alloy plate according to an embodiment of the present invention in a high-temperature oxidation test. FIG.

Next, compositional components that should be included in the austenitic Ni--Cr--Fe alloy of the present invention will be described.
C: 0.01 to 0.10 mass%
C is an element that contributes to stabilization of the austenite phase. However, when added in a large amount, it combines with Cr, Mo, etc. to form carbides, and the amount of solid solution Cr in the vicinity thereof decreases, resulting in a decrease in oxidation resistance. On the other hand, C has the effect of increasing the alloy strength by solid solution strengthening, so the lower limit is made 0.01 mass%. Therefore, C is limited to 0.01 to 0.10 mass%. It is preferably 0.02 to 0.08 mass%, more preferably 0.03 to 0.07 mass%.

Si: 0.02 to 0.45 mass%
Si is an element effective in improving oxidation resistance and preventing peeling of an oxide film. Also, 0.02% is necessary to control the oxygen concentration to 0.0002 to 0.0040%. Furthermore, it also has the role of reducing CaO in the CaO-- _SiO.sub.2 --MgO-- _{Al.sub.2O.sub.3} --F system _slag and adjusting Ca in the molten metal to 0.0020% or less. From this point of view as well, 0.02% is necessary. However, excessive addition of Si promotes the precipitation of intermetallic compounds such as the σ phase and causes surface flaws caused by the intermetallic compounds. It is preferably 0.04 to 0.30 mass% or less, more preferably 0.06 to 0.20 mass% or less.

Mn: 0.10-0.85 mass%
Mn is an element that stabilizes the austenite phase and is also an element that has a deoxidizing effect. However, Mn, like Si, also causes the precipitation of intermetallic compounds such as the σ phase and the formation of Cr—Mn-based oxides, which lowers the oxidation resistance. Therefore, it is necessary to make it 0.10 to 0.85 mass%. It is preferably 0.15 to 0.70 mass%, more preferably 0.20 to 0.60 mass%.

P: 0.015% by mass or less P is an element that is inevitably mixed as an impurity, and is an element that impairs hot workability because it segregates as a phosphide at grain boundaries. Therefore, it is desirable to reduce it as much as possible. However, extremely reducing the P content invites an increase in manufacturing costs. Therefore, in the present invention, P is limited to 0.015 mass% or less. It is preferably 0.010 mass% or less, more preferably 0.006 mass% or less.

S: 0.0015% by mass or less S, like P, is an element that is unavoidably mixed as an impurity, and tends to segregate at grain boundaries, particularly significantly impairing hot workability. Furthermore, by forming a compound with Cr that contributes to oxidation resistance, which will be described later, the Cr necessary for forming a surface oxide scale is consumed, and the adhesion between the oxide film and the base material is reduced, causing the oxide film to come off. , is an element harmful to oxidation resistance because it promotes oxidation. When the content exceeds 0.0015 mass%, the harmfulness appears remarkably, so it is necessary to control the content to 0.0015 mass% or less. It is preferably 0.0010 mass% or less, more preferably 0.0006 mass% or less. As will be described later, the S content can be reduced by the addition of Al and the reaction between the slag components.

Cr: 13.0 to 18.0 mass%
Cr is an element that contributes to suppressing corrosion in a high-temperature environment, and also has the effect of suppressing high-temperature oxidation by forming a protective oxide film on the alloy surface in a high-temperature environment. In order to sufficiently obtain the above effect, it is necessary to contain 13.0 mass% or more. However, excessive addition of Cr results in the formation of an excessively large surface oxide scale, resulting in rather poor adhesion and deterioration in oxidation resistance. In addition, the stability of the austenite phase is lowered, so the content was made 13.0 to 18.0 mass%. It is preferably 14.0 to 17.5 mass%, more preferably 15.0 to 17.0 mass%.

Fe: 5.0 to 1 1.0 mass%
Fe is an element that affects hot workability and cold workability, and if less than 5.0%, the hot workability and cold workability deteriorate. Moreover, if it is added in excess of 11.0%, there is a possibility that the corrosion resistance will be impaired. Therefore, the range of Fe content is defined as 5.0 to 11.0 mass. Preferably, it is 6.0 to 10.5%. More preferably, it is 8.0 to 10.0%.

Mo: 0.6 mass% or less Mo dissolves in the alloy even when added in a small amount, and has the effect of increasing high-temperature strength. However, in a material to which a large amount of Mo is added, in a high-temperature environment and when the oxygen potential on the surface is low, Mo causes preferential oxidation and peels off the oxide scale, which rather adversely affects the material. Therefore, Mo is limited to 0.6 mass% or less from the viewpoint of ensuring the adhesion of the protective surface oxide scale. It is preferably 0.4 mass% or less, more preferably 0.2 mass% or less.

Cu: 0.5 mass % or less Cu is sometimes added as an element for improving corrosion resistance in a wet environment, but its effect is hardly recognized in a high-temperature environment as in the present invention. On the other hand, excessive addition forms a non-uniform film with a mottled pattern on the surface of the material, degrading corrosion resistance. Therefore, the amount of Cu added is limited to 0.5 mass% or less. It is preferably 0.3 mass % or less, more preferably 0.1 mass % or less.

B: 0.005 mass % or less B has an effect of assisting the effect of rare earth elements (REM) by grain boundary segregation, and is an element that also contributes to high-temperature strength. However, the addition of a large amount of Si results in a decrease in adhesion due to porous surface oxide scale and a decrease in weldability and hot workability of the alloy. In the present invention, the content of B is set to 0.005 mass% or less. It is preferably 0.004 mass% or less, more preferably 0.002 mass% or less.

Al: 0.01 to 0.3 mass%
Al is an element that promotes formation of a fine film and improves oxidation resistance, and this effect can be obtained by adding 0.01 mass% or more. Further, it is an element added as a deoxidizing agent, and is an important element for controlling the oxygen concentration within the range of the present invention: 0.0002 to 0.0040 mass% according to the formula (a).
2Al + _3O =( _Al2O3 ) (a)
Underlines represent elements in molten steel, and parentheses represent elements in slag.

By using CaO-- _SiO.sub.2 -- _{Al.sub.2O.sub.3} -- _MgO --F system slag during refining of _the alloy of the present invention, it is possible to effectively absorb the produced _{Al.sub.2O.sub.3} and control the oxygen concentration. In addition, as deoxidation progresses, the S concentration in the molten steel also decreases according to the formula (b).
2Al + 3S +3 ₍ CaO)=3(CaS)+( _Al2O3 ) (b)
As a result, the S concentration can be controlled to 0.0015 mass% or less, which is the range of the present invention. From this, 0.01 mass% or more of Al is required. However, excessive addition causes the equation (c) to proceed remarkably toward the right side, causing the Ca concentration to exceed 0.002 mass%, forming a large amount of Ca—Al oxide inclusions, and Al in the alloy. is consumed, the oxidation resistance is lowered.
₃ (CaO)+ 2Al = 3Ca +( _Al2O3 ) (c)
From this, the upper limit of Al was set to 0.3 mass%. It is preferably 0.01 to 0.2 mass%, more preferably 0.01 to 0.1 mass%.

Ti: 0.01 to 0.3 mass%
Ti is an element that promotes formation of a fine film and improves oxidation resistance, and this effect can be obtained by adding 0.01 mass% or more. However, excessive addition causes surface flaws due to the formation of a large amount of carbonitrides (TiN, TiC, TiCN), so the upper limit of Ti was made 0.3 mass%. It is preferably 0.01 to 0.2 mass%, more preferably 0.01 to 0.1 mass%. Furthermore, controlling the C and N concentrations within the range of the present invention is also a means of effectively suppressing carbonitrides.

Co: 1.00 mass% or less Co, like C and N, is an element effective in stabilizing the austenite phase. In addition, C and N cannot be added in large amounts because they form carbonitrides with Al, Ti, etc. and cause surface defects, but Co is advantageous because it does not form carbonitrides. However, since addition of a large amount causes an increase in raw material costs, the Co content is set to 1.00 mass% or less. It is preferably 0.90 mass% or less, more preferably 0.80 mass% or less.

Nb: 1.00 mass% or less Nb forms fine carbides and carbonitrides with C and N, and has the effect of increasing the high-temperature strength of the alloy. It will precipitate out, and the ductility and toughness will rather decrease. Therefore, the amount of Nb added is limited to 1.00 mass% or less. It is preferably 0.90 mass% or less, more preferably 0.80 mass% or less.

Ta: 0.05 mass% or less Ta, like Nb, forms fine carbides and carbonitrides, and has the effect of increasing the high-temperature strength of the alloy. However, the addition of a large amount of Ta results in the precipitation of a large amount of carbides and carbonitrides, which rather reduces the ductility and toughness. It is preferably 0.04 mass% or less, more preferably 0.03 mass% or less.

N: 0.01 mass % or less N is an element that is inevitably mixed as an impurity, but it is also an austenite phase forming element, so it contributes to structure stabilization. However, when adding Al, Ti, Zr, etc. as in the present invention, N combines with these elements to precipitate nitrides, and the hot deformation resistance is greatly increased, impairing hot workability. . In addition, the formation of the nitrides consumes Al and Ti, which are elements that contribute to improving the fineness of the surface oxide scale, thereby lowering the oxidation resistance. Therefore, in the present invention, the content of N is set to 0.01 mass% or less. It is preferably 0.009 mass% or less, more preferably 0.008 mass% or less.

Oxygen is blown during decarburization, and at that time, N can be controlled within the scope of the present invention by moving to CO gas bubbles as nitrogen gas and removing it out of the system.

O: 0.0002 to 0.0040 mass%
O in the alloy combines with Al, Ti, Si, La, Ce, and Y in molten steel to form oxides, which impairs the useful effects of these elements, such as oxidation resistance. In addition, a large amount of alumina-based oxide-based non-metallic inclusions are formed and adhere to the inside of the submerged nozzle for pouring molten steel from the tundish of the continuous casting machine into the mold. Therefore, it is desirable that the oxygen concentration is as low as 0.0040 mass% or less. In order to achieve this range, Al should be deoxidized by controlling the Al concentration according to the present invention as described above. On the other hand, if the O content in the alloy is reduced too much, the Ca concentration will exceed 0.002 mass% and increase according to the formula (c). Therefore, the lower limit is set to 0.0002 mass%. It is preferably 0.0003 to 0.0035 mass%, more preferably 0.0005 to 0.0030 mass%.

Ca: 0.002 mass % or less Ca is an element mixed from CaO in the slag in the alloy of the present invention, as described above. Ca forms a large amount of Ca—Al oxide-based inclusions and consumes Al in the alloy to lower the oxidation resistance, so it must be kept low. For that purpose, it is necessary to control the Al concentration to 0.01 to 0.3 mass% and the oxygen concentration to 0.0002 to 0.0040 mass%. Therefore, Ca should be 0.002 mass% or less.

Total weight of one or more of rare element (REM) La, Ce, and Y: 0.001 to 0.010 mass%
REM (La, Ce, Y) has the effect of enhancing the hot workability of the alloy and the adhesion between the surface oxide scale and the surface of the base material, and improving the oxidation resistance. Furthermore, by forming a compound with S that is dissolved in the alloy, it is expected that the formation of a compound of Cr and S, which is a constituent element of the surface oxide scale, can be suppressed, and the effect of preventing a local decrease in the amount of Cr can be expected. can. REM is generally used as a raw material as a misch metal, which is an alloy containing a plurality of REMs, but there are cases where a Ni--Fe alloy containing any one of REMs is used. Excessive addition, however, lowers the hot workability and weldability of the alloy, and excessive formation of REM inclusions lowers the adhesion of surface oxide scale. Furthermore, during continuous casting, clogging of the immerse nozzle is caused, and the productivity is remarkably deteriorated. Accordingly, in the present invention, the amount of REM added is set to 0.001 to 0.010 mass%. It is preferably 0.002 to 0.009 mass%, more preferably 0.003 to 0.008 mass%.

60≧7.4Si+2.8×Cr+6.9×Al+3.2×Ti+3374×REM-5914×S-3.87×Mn-42.3×Mo-3072×B-1024×N≧30 (1)
Formula (1) expresses the extent of the influence of elements that affect the surface oxide scale formed on the alloy surface in the oxidation resistance of an austenitic Ni-Cr-Fe alloy by weighted regression analysis. be. Si, Cr, Al, Ti, REM (La, Ce, Y) are 5% O ₂ -16% H ₂ O-12% CO ₂ -0.8% CO-0.1% NO ₂ -bal. It improves oxidation resistance evaluated by a cycle test in which room temperature and high temperature of about 1000 to 1200° C. are repeated in a mixed gas atmosphere of N ₂ . On the other hand, S degrades the adhesion between the oxide film and the base material, causing the oxide film to come off and promotes oxidation. Mn causes deterioration of oxidation resistance due to the formation of Cr--Mn-based oxides. When the Mo content is high, in a high-temperature environment and when the oxygen potential on the surface is low, Mo causes preferential oxidation and peels off the oxide scale. In addition, when the B content is large, the oxidized scale of the alloy becomes bolus-like, so that the oxidation rate at high temperatures increases, which promotes scale growth and exfoliation. N forms Al, Ti, AlN, and TiN, respectively, which contribute to the improvement of oxidation resistance, and reduces the effects of Al and Ti. In addition, the excessive addition of alloying elements that contribute to the improvement of oxidation resistance is not preferable because the excessive growth of surface oxide scale will rather decrease the adhesion and the formation of a large amount of inclusions that cause surface defects. Therefore, the lower limit of these elements is 30 or more and the upper limit is 60 or less based on the formula (1). It is preferably 31 or more and 59 or less, more preferably 32 or more and 58 or less.

4.5≦REM/S (2)
As an index for sufficiently obtaining the effect of improving the hot workability and oxidation resistance of the alloy, the relationship between the content of the rare element (REM) and the S that forms the compound is 4.5 ≤ REM / By satisfying S, the REM content is sufficient to fix S as inclusions, and the above effect can be obtained. On the other hand, if it is less than 4.5, the effect of REM cannot be sufficiently obtained, which is not desirable.

Definition of surface oxide scale As shown in FIG. 1, an austenitic Ni—Cr—Fe alloy sheet according to one embodiment of the present invention has an austenitic A Ni--Cr--Fe alloy is used as the base BM. 5% O ₂ -16% H ₂ O-12% CO ₂ -0.8% CO-0.1% NO ₂ -bal. In a cycle test in which temperatures from room temperature to 1000 to 1200° C. are repeated in a mixed gas atmosphere of N ₂ , an oxide scale mainly composed of Cr oxide is formed on the surface of the austenitic Ni—Cr—Fe alloy base material of the present invention. At this time, in the figure, the thickness of the surface oxide scale indicates the region LE from the outermost layer of the surface oxide scale to the scale/alloy interface in the cross-sectional microstructure observation after the above test.

5% O ₂ -16% H ₂ O-12% CO ₂ -0.8% CO-0.1% NO ₂ -bal. In the austenitic Ni --Cr--Fe alloy of the present invention, the surface oxide scale formed in a cycle test in which the temperature is repeated from room temperature to 1000-1200° C. in a mixed gas atmosphere consisting of N2 has a thickness of 6-60 μm _. 5% O ₂ -16% H ₂ O-12% CO ₂ -0.8% CO-0.1% NO ₂ -bal. In a cycle test in which temperatures from room temperature to 1000 to 1200° C. are repeated in a mixed gas atmosphere of N ₂ , an oxide scale mainly composed of Cr oxide is formed on the surface of the alloy base material to obtain oxidation resistance in the high temperature environment. At this time, if the thickness of the surface oxide scale is less than 60 μm, sufficient oxidation resistance cannot be obtained, while if it exceeds 60 μm, the peelability of the surface oxide scale increases, and the surface oxide scale and the base material surface are separated. Adhesion is impaired. Therefore, the protective surface oxide scale formed in the high temperature environment must have a thickness of 6 to 60 μm. It is preferably 8 to 55 μm, more preferably 10 to 50 μm.

A method of specifying the limiting formula of the above formula (1) is as follows.
Ni-17%Cr-9%Fe is the basic composition, and various alloys with varying amounts of Si, Cr, Al, Ti, La, Ce, Y, B, Mn, Mo, S, and N are vacuumed. After melting in a melting furnace and hot forging, a hot forged plate of 8 mmt×80 mmw was produced. The resulting hot forged plate was subjected to solution heat treatment under conditions of 1200° C.×10 minutes, surface grinding, cold rolling to 2 mmt, and solution heat treatment under conditions of 1150° C.×1 minute. After that, it was cut into a size of 20 mm×30 mm, and the surface was finished by wet polishing #320 to obtain a test piece. The obtained test piece was subjected to 5% O ₂ -16% H ₂ O-12% CO ₂ -0.8% CO-0.1% NO ₂ -bal. A repeated oxidation test was performed in a mixed gas atmosphere of _N2 , with one cycle of 1200°C x 10 minutes, 1000°C x 10 minutes, 1200°C x 10 minutes, and room temperature x 20 minutes. The test piece after 200 cycles was evaluated by the value obtained by dividing the change in mass excluding the weight of the peeled scale by the surface area before the test.

From the above test results, the degree of influence of the additive elements on the oxidation resistance of the austenitic Ni-Cr-Fe alloy was clarified, and the relational expression of the component composition represented by the formula (1) was obtained by multiple regression analysis. , and it can be seen that sufficient oxidation resistance can be obtained by making it 30 or more and 60 or less.

Next, a method for producing an austenitic Ni--Cr--Fe alloy according to the present invention will be described.
The austenitic Ni-Cr-Fe alloy of the present invention is produced by melting raw materials such as iron scraps, stainless steel scraps, ferronickel, and ferrochromium in an electric furnace, followed by an AOD (Argon Oxygen Decarburization) furnace or a VOD (Vacuum Oxygen Decarburization) furnace. , A mixed gas of oxygen and rare gas is blown for decarburization refining, quicklime, Fe—Si alloy, Al, etc. are added to reduce Cr oxide in the slag, and then fluorite is added to CaO A --SiO ₂ --Al ₂ O ₃ --MgO--F system slag was formed, deoxidized and desulfurized, and then a Ni-based alloy containing any one of La, Ce and Y was added. The reason for using CaO-- _SiO.sub.2 -- _{Al.sub.2O.sub.3} -- _MgO --F-based slag is that, as described above, deoxidation and desulfurization can be effectively carried out. The point is that it can be added effectively. At this time, the CaO concentration in the slag is preferably in the range of 40-80%. That is, if the content is less than 40%, the above desulfurization reaction does not proceed. If the content is 80% or more, more than 0.002% of Ca is mixed into the molten steel. Also, the Al ₂ O ₃ concentration is desirably 50% or less. The reason for this is that if the alumina activity in the slag is not low, deoxidation will be difficult to proceed, and desulfurization will also be difficult. After refining, a slab is produced by a continuous casting machine, and then the billet is hot-rolled or further cold-rolled into various steel materials such as thin steel plate, thick steel plate, shaped steel, bar steel, wire rod, etc. is preferable. It is not limited to a continuous casting machine, and a steel slab may be produced by an ingot casting-blooming rolling method.

Scrap, ferrochromium, ferronickel, stainless steel scrap, etc. are melted in a 70-ton scale electric furnace, and a mixed gas of oxygen and rare gas is blown in an AOD furnace or a VOD furnace. Decarburized and refined. Thereafter, quicklime, Fe—Si alloy, Al, etc. are added to reduce Cr oxide in the slag, and then fluorite is added to form CaO—SiO ₂ —A1 ₂ O ₃ —MgO—F system slag. was deoxidized and desulfurized. Thereafter, one or more of Ni-20% La, Ni-20% Ce, and Ni-20% Y were added in predetermined amounts, and cast slabs were obtained by continuous casting. After adjusting to various chemical compositions shown in Table 1, steel slabs were obtained by continuous casting. Each component shown in Table 1 was measured as follows. In the table, those that do not satisfy the scope of the present invention are parenthesized. It should be noted that invention examples in parentheses do not satisfy the scope of dependent claims, but do satisfy the scope of independent claims.

(1) The composition of C and S was measured using a carbon/sulfur simultaneous analyzer (combustion in an oxygen stream-infrared absorption method).
(2) The composition of N was analyzed using an oxygen/nitrogen simultaneous analyzer (inert gas-impulse heat melting method).
(3) Compositions other than C, S and N and slag components were analyzed by a calibration curve method using fluorescent X-ray analysis.

Next, the billet (slab) was hot-rolled to 8 mm, and cold-rolled, heat-treated and pickled repeatedly to produce a cold-rolled coil having a thickness of 2 to 3 mm. The final annealing temperature was 1150° C. for 1 minute. A test piece having a width of 20 mm, a length of 30 mm, and a thickness of 2 mm was taken from the plate.

<High temperature oxidation test>
In order to evaluate the oxidation resistance in a high-temperature environment, the surface of the test piece was wet-polished with #320 emery paper, and a high vacuum atmosphere heat treatment furnace was used to vacuum up to 5.0 × 10 ^-3 Pa. After drawing, 5% O ₂ -16% H ₂ O-12% CO ₂ -0.8% CO-0.1% NO ₂ -bal. 1200°C x 10 minutes in a mixed gas atmosphere consisting of _N2 , after temperature adjustment at a temperature decrease rate of 40°C/min, 1000°C x 10 minutes, after temperature adjustment at a temperature increase rate of 40°C/min, 1200°C x 10 minutes minutes, and then room temperature x 20 minutes as one cycle. For the test piece after 200 cycles, the value (mg/cm ² ) obtained by dividing the change in mass excluding the weight of the exfoliated scale by the surface area before the test was evaluated as the weight loss due to oxidation. Those with an oxidation weight loss of less than 30 mg/cm ² were judged to have good oxidation resistance (○), and those of 30 mg/cm ² or more were judged to have poor oxidation resistance (x). In addition, after 200 cycles, the test piece was cut and Cu-plated so that the cross section could be observed, then a buried sample was prepared, wet-polished, and finally puff-polished to a mirror finish for observation. The cross-sectional microstructure of this was observed by FE-SEM to measure the thickness of the surface oxide scale, and the attached EDS was used to identify the oxide.

No. shown in Tables 1 and 2. The steel sheets Nos. 1 to 17 were invention examples satisfying the conditions of the present invention and had excellent oxidation resistance. On the other hand, No. Steel sheets of 18 to 30 are comparative examples. No. The steel sheet of No. 18 satisfied the formula (1), but since the Mn content was high, the oxidation resistance was lowered due to the formation of Cr--Mn-based oxides. Furthermore, the CaO concentration in the slag was as high as 85%, and the oxygen concentration was too low. rice field.

No. Since the steel sheet No. 19 had a high Si content, surface flaws due to intermetallic compounds such as the σ phase occurred, and further, the formula (1) was not satisfied, resulting in poor oxidation resistance.

No. The steel sheet No. 20 satisfied the formula (1), but because of the high C content, a large amount of carbide precipitated, and the high Mo content reduced the adhesion of scale, resulting in oxidation resistance. I lost my temper.

No. The steel sheet of No. 21 satisfied the formula (1), but since the Cr content was high, the surface oxide scale grew excessively, forming a scale with poor adhesion, and the oxidation resistance was rather deteriorated. In addition, since the Al content was high, a large amount of carbonitrides that caused surface defects were formed, resulting in poor surface quality.

No. The steel sheet of No. 22 satisfied the formula (1), but since the N content was high, Cr, Al, and Ti, which contribute to oxidation resistance, precipitated as nitrides, and a sufficient surface oxide scale was formed. could not.

No. Since the steel sheet No. 23 had a low REM content, the formula (1) was not satisfied, and the effect of improving oxidation resistance and the effect of fixing S, which inhibits oxidation resistance, as inclusions could not be obtained sufficiently.

No. The steel sheet No. 24 had a low CaO concentration of 35% in the slag, so the desulfurization reaction did not progress. As a result, a large amount of inclusions are formed, and the Cr necessary for forming the surface oxide scale is consumed because the S content becomes high. The oxidation resistance was poor due to deterioration in adhesion.

No. The steel sheet No. 25 did not satisfy the formula (1) because the Al content was low, and the alumina concentration in the slag was as high as 54%, and the deoxidation was weak, resulting in a high oxygen concentration, which is useful for oxidation resistance. Oxidation resistance was poor because some elements formed oxides.

No. The steel sheet of No. 26 satisfies the range of the composition of each element, but exceeds the upper limit of the formula (1), so the oxidation resistance deteriorates due to the excessive formation of surface oxide scale.

No. The steel sheet of No. 27 had a low Cr content and did not satisfy the formula (1), so it was inferior in oxidation resistance. In addition, since the formula (2) is not satisfied, the effect of improving oxidation resistance and the effect of fixing S, which inhibits oxidation resistance, as inclusions cannot be obtained sufficiently.

No. Since the steel sheet No. 28 has a high REM content, it does not satisfy the formula (1). In addition, hot workability and weldability are deteriorated, and clogging of the immerse nozzle is caused during continuous casting, which significantly deteriorates productivity, resulting in poor productivity. Furthermore, since the Ti content was high, carbonitrides, which cause surface defects, were formed, resulting in poor surface quality.

No. Since the steel sheet No. 29 had a low Ti content, it did not satisfy the formula (1) and was inferior in oxidation resistance. In addition, although the Fe content is within the range, since it is rather low, the hot workability and cold workability are deteriorated, resulting in poor manufacturability.

No. The steel sheet No. 30 satisfied the composition range of each element, but did not satisfy the formulas (1) and (2), so it did not have sufficient oxidation resistance.

The austenitic Ni-Cr-Fe alloy of the present invention has excellent heat resistance in addition to the oxidation resistance in the high-temperature environment described above, so it is suitable for high-temperature environments such as industrial heat treatment furnaces and combustion parts. can be used.

1: Surface oxide scale 2: Surface oxide scale/base material interface

Claims (7)

% by mass, C: 0.01 to 0.10%, Si: 0.02 to 0.45%, Mn: 0.10 to 0.85%, P: ≤0.015%, S: ≤0. 0015%, Cr: 13.0 to 18.0%, Fe: 5.0 to 11.0%, Mo: ≤0.6%, Cu: ≤0.5%, B: ≤0.005%, AI : 0.01 to 0.3%, Ti: 0.01 to 0.3%, Co: ≤1.00%, Nb: ≤1.00%, Ta: ≤0.05%, N: ≤0. 01%, O: 0.0002 to 0.0040%, Ca: ≤ 0.002%, one or more of rare earth elements (REM) La, Ce, and Y, total weight: 0.001 to An austenitic Ni--Cr--Fe alloy containing 0.010% of Ni and the balance consisting of Ni and unavoidable impurities, satisfying the following formula (1).
60≧7.4Si+2.8×Cr+6.9×Al+3.2×Ti+3374×REM-5914×S-3.87×Mn-42.3×Mo-3072×B-1024×N≧30 (1)
(Here, each element symbol in the above formula indicates the content (mass%) of each element.) Furthermore, the austenitic Ni—Cr according to claim 1, wherein the total weight of one or more of the rare earth elements (REM) La, Ce, and Y satisfies the following formula (2): - Fe alloys.
4.5≦REM(La, Ce, Y)/S (2)
(Here, each element symbol in the above formula indicates the content (mass%) of each element.) 5% O ₂ -16% H ₂ O-12% CO ₂ -0.8% CO-0.1% NO ₂ -bal. The composition of the surface oxide scale formed on the alloy surface in a cycle test in which the temperature is repeated from room temperature to 1000 to 1200 ° C. in a mixed gas atmosphere consisting of N ₂ is Cr: 50% or more and Fe: 0 to 10% in mass%. , Ni: 0 to 5%, O: 10 to 40%, REM: 0.05 to 0.5%, and the balance containing Mn, Si, Al, and Ti as inevitable elements. 2. The austenitic Ni—Cr—Fe alloy according to 2. 4. The austenitic Ni--Cr--Fe alloy according to claim 3, wherein said surface oxide scale has a thickness of 6-60 μm. 3. The method for producing an austenitic Ni-Cr-Fe alloy according to claim 1 or 2, wherein the alloy composition is adjusted by refining after melting the alloy raw material, and in the refining, the melted alloy raw material (Molten alloy) is decarburized by blowing a mixed gas of oxygen and argon, the nitrogen concentration is controlled to 0.01% or less, Cr is reduced, and then aluminum, limestone and fluorite are added to the molten alloy, CaO—SiO ₂ —Al ₂ O ₃ —MgO—F system slag is formed, the oxygen concentration in the molten alloy is adjusted to 0.0002 to 0.0040 mass%, and then one or two of La, Ce, and Y A method for producing an austenitic Ni--Cr--Fe alloy characterized in that after adding the raw materials containing the above, casting is performed to obtain a slab, and the slab is subjected to a hot rolling process. The method for producing an austenitic Ni-Cr-Fe alloy according to claim 3, wherein the alloy composition is adjusted by refining after melting the alloy raw material, and in the refining, the melted alloy raw material (melting alloy) is decarburized by blowing a mixed gas of oxygen and argon, the nitrogen concentration is controlled to 0.01% or less, Cr is reduced, and then aluminum, limestone and fluorite are added to the molten alloy to form CaO —SiO ₂ —Al ₂ O ₃ —MgO—F system slag is formed, the oxygen concentration in the molten alloy is set to 0.0002 to 0.0040 mass%, and then one or more of La, Ce, and Y 1. A method for producing an austenitic Ni--Cr--Fe alloy, characterized in that after adding the contained raw material, casting is performed to obtain a slab, and the slab is subjected to a hot rolling process. The method for producing an austenitic Ni-Cr-Fe alloy according to claim 4, wherein the alloy composition is adjusted by refining after melting the alloy raw material, and in the refining, the melted alloy raw material (melting alloy) is decarburized by blowing a mixed gas of oxygen and argon, the nitrogen concentration is controlled to 0.01% or less, Cr is reduced, and then aluminum, limestone and fluorite are added to the molten alloy to form CaO —SiO ₂ —Al ₂ O ₃ —MgO—F system slag is formed, the oxygen concentration in the molten alloy is set to 0.0002 to 0.0040 mass%, and then one or more of La, Ce, and Y 1. A method for producing an austenitic Ni--Cr--Fe alloy, characterized in that after adding the contained raw material, casting is performed to obtain a slab, and the slab is subjected to a hot rolling process.

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