JP7158618B1 - Austenitic Fe-Ni-Cr alloy with excellent oxidation resistance and method for producing the same - Google Patents

Abstract

An object of the present invention is to provide an austenitic Fe--Ni--Cr alloy having excellent oxidation resistance even under severe high temperatures. [Solution] Mass% C: 0.004-0.13% Si: 0.15-1.0% Mn: 0.03-2.0% P: ≤0.040% S: ≤0.003% Ni: 20.0-38.0% Cr: 18.0-28.0%, Mo: ≤1.0%, Cu: ≤1.0%, N: ≤0.03%, B: ≤0.01%, Al: 0.10-1.0%, at least one of Ti and Zr: 0.10-1.0%, Zr: 0.01 to 0.6%, O: 0.0002 to 0.0030%, Ca: ≤ 0.002%, one or more of rare earth (REM) La, Ce, and Y total weight: 0.001 to 0.010%, An austenitic Fe--Ni--Cr alloy having a chemical composition with the balance being Fe and unavoidable impurities, wherein the mass % of each element satisfies the relationship defined by a specific formula. [Selection drawing] Fig. 1

Description

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

Thermal power boilers, chemical plants, and polysilicon refining reactors are used in harsh high-temperature environments of 700 to 900°C. Excellent oxidation resistance is required. In particular, with respect to oxidation resistance, the required properties 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 dense and has high adhesion to the material. It is mentioned as one. Fe--Cr--Ni alloys are attracting attention as a material to be used for equipment and the like used in the high-temperature environment. Of these, 18-8 stainless steels such as SUS304, SUS316, and SUS347 do not have sufficient properties in the above usage environment, so SUS310S and NCF800, which have higher Ni and Cr contents, are generally used. It is

As a technique for improving the high-temperature properties of materials used in such harsh usage environments, for example, Patent Document 1 discloses that a small amount of REM (Rare Earth Metal) is added to stainless steel, and Ni is added. An austenitic stainless steel sheet has been proposed in which the growth rate of the Cr ₂ O ₃ oxide film formed on the surface of the steel sheet is suppressed by defining the upper limit of the Mn content according to the amount of REM added. In addition, Patent Document 2 proposes a heat-resistant steel material for reformers in which the adhesion of Cr ₂ O ₃ generated on the surface of the steel material is enhanced by specifying the Si content corresponding to the Cr and Ni contents in the steel material. It is

However, none of the techniques disclosed in the above patent documents have studied the effects of REM in the material or S that can form a compound with Cr, and in the above-described high-temperature environment where excellent oxidation resistance is required. It is considered that the application to In addition, there are various elements in REM, and there is no mention of which of them exerts an effect, and it was practically difficult to implement.

In addition, none of the above patent documents discusses the effect of the inner oxide layer on the oxidation resistance, so it is considered that the technology applied in the above severe high temperature environment is insufficient.

Furthermore, in recent years, a Ni--Cr--Fe alloy has been proposed which is excellent in creep strength and resistance to stress relaxation cracking due to combined addition of Ti, Al, and REM (see, for example, Patent Document 3). However, 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 the hot rolling process. there were. Furthermore, although REM states that all REMs are effective, Nd is mainly added, and Ce, La, and Y are only added to some alloys. As a further big problem, there was no process for removing S and O, and it could not be achieved without careful selection of raw materials. Therefore, in some cases, the REM is oxidized or sulfurized, making it difficult to achieve the original purpose of the proposal, which is industrially fragile. Therefore, it is difficult to say that it is possible to quickly and accurately provide an alloy with improved creep strength by adding REM on an industrial scale.

JP 2003-171745 A JP-A-2002-256398 Retable 2018-066579

The present invention has been made in view of the above circumstances, and its object is to propose an austenitic Fe--Ni--Cr 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 La, Ce, and Y among _REMs improves the adhesion of surface oxide scale formed on the alloy surface in a high-temperature environment. _H2O -10%CO2-0.5%CO _- 0.1%NO2 _- bal. Sufficient knowledge has not been obtained regarding the contribution to oxidation resistance evaluated by a cycle test in which the temperature is repeated from room temperature to 700 to 900° C. 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 the addition of La, Ce, and Y is extremely effective for improving oxidation resistance, and that Si, Ni, Cr, Al, Ti, and Zr are also effective as other elements. . On the other hand, it was found that the inclusion of S, Mo, and B inhibits the oxidation resistance-improving action. From this, it was found that Si, Ni, Cr, Al, Ti, Zr, S, Mo, and B must be controlled in order to sufficiently ensure the effect of adding REM.

In addition, an internal oxide layer consisting of oxides of Cr, Si, Mn, Al, Ti, La, Ce, and Y is formed directly under the protective oxide scale that forms on the surface in a high-temperature environment. As a result of investigating the formation behavior of the oxide layer in detail, it was found that the area ratio of the inner oxide layer had a good correlation with the weight loss due to oxidation in the high-temperature oxidation test. Specifically, an improvement in oxidation resistance was observed when the area ratio of the internal oxide was 30% or more in 0.005 mm ² in the internal oxide layer immediately below the surface oxide scale. When the relationship between the formation behavior of these internal oxide layers and the alloying elements was investigated, La, Ce, Y, Si, Cr, Al, and Ti were effective, while the inclusion of S, Mn, and N was effective for the above-mentioned internal oxidation. It became clear that the formation behavior of the monolayer was inhibited. From this, it was found that it is necessary to control La, Ce, Y, Si, Cr, Al, Ti, S, Mn, and N in order to obtain the effect of improving oxidation resistance by controlling the internal oxide layer. .

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 is 3.2≦REM/S.

In addition, as a result of repeated studies focusing on the form of surface oxide scale formed in a high-temperature environment, 7% O ₂ -16% H ₂ O-10% CO ₂ -0.5% CO-0.1% NO ₂ - bal. When the thickness of the surface oxide scale formed in the range of 10 to 100 μm is formed in a cycle test in which the temperature is repeated from room temperature to 700 to 900° C. in a mixed gas atmosphere of N ₂ , the surface oxide scale is densely formed and adhesion is improved. was excellent, and showed good results in terms of oxidation weight loss after the test.

That is, the austenitic Fe-Ni-Cr alloy of the present invention has C: 0.004 to 0.13%, Si: 0.15 to 1.0%, Mn: 0.03 to 2.0% in mass%. , P: ≤ 0.040%, S: ≤ 0.003%, Ni: 20.0 to 38.0%, Cr: 18.0 to 28.0%, Mo: ≤ 1.0%, Cu: ≤ 1.0%, N: ≤0.03%, B: ≤0.01%, Al: 0.10 to 1.0%, at least one of Ti and Zr, Ti: 0.10 to 1.0%, Zr: 0.01 to 0.6%, O: 0.0002 to 0.0030%, Ca: ≤ 0.002%, one or two of rare earth elements (REM) La, Ce, Y Total weight of seeds and above: 0.001 to 0.010%, the balance being composed of Fe and inevitable impurities, and satisfying the following formulas (1) and (2) and
85≧0.3×Si+1.5×Ni+1.3×Cr+5.8×Al+7.7×Zr+2.7×Ti+2173×REM-3582×S-32.9×Mo-2448×B≧47 (1)
40≧0.6×Si+1.3×Cr+23.53×Al+5.88×Ti+3074×REM-5067×S-0.8×Mn-816×N≧0 (2)

Further, in the austenitic Fe—Ni—Cr alloy of the present invention, one or more of the rare earth elements (REM) La, Ce, and Y satisfies the following formula (3). there is
3.2≦REM(La, Ce, Y)/S (3)

In the austenitic Fe--Ni--Cr alloy of the present invention, in addition to the above composition, 7% O ₂ -16% H ₂ O-10% CO ₂ -0.5% 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 700 to 900°C in a mixed gas atmosphere consisting of N ₂ is, in terms of mass%, Cr: 40% or more, Fe: 10 to 20%, and Ni: 0. ~10%, O: 10 to 40%, REM: 0.05 to 0.5%, the balance contains Mn, Si, and Ti as unavoidable elements, and the scale has a thickness of 10 to 100 µm It is characterized by

Furthermore, the internal oxide layer formed directly under the surface oxide scale is composed of an internal oxide containing at least one of Cr, Si, Mn, Al, Ti, and REM, and the internal oxide layer at this time has an area ratio of 30% or more per 0.005 mm ² immediately below the surface oxide scale.

The present invention also proposes a method for producing the austenitic Fe--Ni--Cr 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 .03% or less, Cr reduction is performed, 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, and the molten alloy The oxygen concentration in the slab is set to 0.0002 to 0.0030 mass%, and then a raw material containing one or more of La, Ce, and Y is added to adjust the composition, and casting is performed to obtain a slab. A coil is manufactured in a hot rolling process.

According to the present invention, it has excellent oxidation resistance in a high-temperature environment, and can greatly contribute to extending the life of products.

FIG. 3 is a schematic diagram for explaining the surface structure of the Fe—Cr—Ni alloy plate according to one embodiment of the present invention in a high-temperature oxidation test.

Next, compositional components that should be included in the austenitic Fe--Ni--Cr alloy of the present invention will be described.
C: 0.004 to 0.13 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 0.004 mass%. Therefore, C is limited to 0.004 to 0.13 mass%. It is preferably 0.005 to 0.080 mass%, more preferably 0.006 to 0.070 mass%.

Si: 0.15 to 1.0 mass%
Si is an element effective in improving oxidation resistance and preventing peeling of an oxide film, and the above effect can be obtained by adding 0.15 mass % or more. However, excessive addition of Si promotes the precipitation of intermetallic compounds such as the σ phase and may cause surface flaws caused by the intermetallic compounds. It is preferably 0.16 to 0.8 mass% or less, more preferably 0.17 to 0.6 mass% or less.

Mn: 0.03-2.0 mass%
Mn is an austenite phase stabilizing element, and is also an element having a deoxidizing effect, so its content should be at least 0.03 mass % or more to obtain that effect. However, as with Si, Mn also causes precipitation of intermetallic compounds such as the σ phase and lowers the oxidation resistance. Therefore, it is necessary to make it 0.03 to 2.0 mass%. It is preferably 0.03 to 1.50 mass%, more preferably 0.03 to 1.00 mass%.

P: 0.040 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 0.040 mass% or less. It is preferably 0.030 mass% or less, more preferably 0.020 mass% or less.

S: 0.003 mass% or less S, like P, is an element that is unavoidably mixed as an impurity. 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 0.003 mass%, the harmfulness appears remarkably, so it is necessary to control the content to 0.003 mass% or less. It is preferably 0.002 mass% or less, more preferably 0.001 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.

Ni: 20.0 to 38.0 mass%
Ni is an austenite phase stabilizing element and has the function of suppressing the precipitation of intermetallic compounds such as the σ phase. It also has the effect of improving heat resistance and high-temperature strength. In order to sufficiently exert the above effects, it is added in an amount of 20 mass% or more. On the other hand, excessive addition causes deterioration of hot workability, increase in hot deformation resistance, and further increase in cost. Therefore, the Ni content is 20.0 to 38.0 mass%. It is preferably 21.0 to 36.0 mass%, more preferably 22.0 to 35.0 mass%.

Cr: 18.0-28.0 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 effects, it is necessary to contain 18.0 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 deteriorates, and it becomes necessary to add a large amount of Ni, so the Ni content was made 18.0 to 28.0 mass%. It is preferably 19.0 to 26.0 mass%, more preferably 20.0 to 25.0 mass%.

Mo: 1.0% by 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 1.0 mass% or less from the viewpoint of ensuring the adhesion of the protective surface oxide scale. It is preferably 0.8 mass% or less, more preferably 0.6 mass% or less.

Cu: 1.0 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, which reduces corrosion resistance. Therefore, the amount of Cu to be added is limited to 1.0 mass% or less. It is preferably 0.8 mass% or less, more preferably 0.6 mass% or less.

N: 0.03 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 the constituent elements of the internal oxide formed directly under the surface oxide scale, thereby reducing the area ratio of the internal oxide layer. Therefore, in the present invention, the content of N is set to 0.03 mass% or less. It is preferably 0.02 mass% or less, more preferably 0.01 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.

B: 0.01 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, if added in a large amount, the oxidized scale on the surface becomes porous, resulting in a decrease in adhesion, weldability of the alloy, and hot workability. In the present invention, the content of B is 0.01 mass% or less. It is preferably 0.008 mass% or less, more preferably 0.006 mass% or less.

Al: 0.10-1.0 mass%
Al is an element that promotes formation of a dense black film and improves oxidation resistance, and the effect can be obtained by adding 0.10 mass % or more of each. 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 0.0030 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 0.003 mass% or less, which is the range of the present invention. From this, 0.10 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 1.0 mass%. It is preferably 0.10 to 0.80 mass%, more preferably 0.10 to 0.60 mass%.

As with Al described above, Ti and Zr effectively act to form a dense black film and improve oxidation resistance, so it is necessary to add one or two of them.

Ti: 0.10 to 1.0 mass%
Ti is an element that promotes formation of a dense black film and improves oxidation resistance, and this effect can be obtained by adding 0.10 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 is set to 1.0 mass%. It is preferably 0.10 to 0.80 mass%, more preferably 0.10 to 0.60 mass%. Furthermore, controlling the C and N concentrations within the range of the present invention as described above is also a means for effectively suppressing carbonitrides.

Zr: 0.01 to 0.6 mass%
Zr is an element of the same group as Ti, and like Ti, it effectively acts to form a dense black film and improve oxidation resistance, so it can be used as a substitute element for Ti. The effect is superior to Ti, so even a small amount of addition is effective, but excessive addition causes surface defects due to the formation of a large amount of carbonitride, so the upper limit is limited to 0.6 mass%. . The range is preferably 0.01 to 0.4 mass%, more preferably 0.05 to 0.3 mass%.

O: 0.0002 to 0.0030 mass%
O in the alloy combines with Al, Ti, Zr, 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 0.0030 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 0.0027 mass%, more preferably 0.0005 to 0.0025 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 0.10 to 1.00 mass% and the oxygen concentration to 0.0002 to 0.0030 mass%. Therefore, Ca should be 0.002 mass% or less.

Total weight of one or more of La, Ce, and Y that are rare earth elements (REM): 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 an Fe--Ni alloy containing any one of REMs may be 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%.

85≧0.3×Si+1.5×Ni+1.3×Cr+5.8×Al+7.7×Zr+2.7×Ti+2173×REM-3582×S-32.9×Mo-2448×B≧47 (1)
The formula (1) expresses the degree of influence of the elements that affect the surface oxide scale formed on the alloy surface in the oxidation resistance of the Fe--Cr--Ni alloy by multiple regression analysis. Si, Ni, Cr, Al, TI, Zr, and REM (La, Ce, Y) are 7% O ₂ -16% H ₂ O-10% CO ₂ -0.5% CO-0.1% NO ₂ - bal. It improves oxidation resistance evaluated by a cycle test in which room temperature and high temperature of about 700 to 900° 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. 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, resulting in peeling of the oxide scale. In addition, when the B content is large, the oxidized scale of the alloy becomes porous, so that the oxidation rate increases at high temperatures, which promotes scale growth and exfoliation. In addition, excessive addition of alloying elements that contribute to the improvement of oxidation resistance is not preferable because the excessive growth of surface oxidized scales reduces the adhesion and produces a large amount of inclusions that cause surface defects. Therefore, the lower limit of these elements is 47 or more and the upper limit is 85 or less based on the formula (1). It is preferably 48 or more and 84 or less, more preferably 50 or more and 83 or less.

40≧0.6×Si+1.3×Cr+23.53×Al+5.88×Ti+3074×REM-5067×S-0.8×Mn-816×N≧0 (2)
Formula (2) expresses the degree of influence of elements that affect the formation behavior of the internal oxide layer formed directly under the surface oxide scale in the oxidation resistance of the Fe--Cr--Ni alloy by regression analysis. It is what I did. REM, Si, Cr, Al, and Ti are 7%O2-16% _{H2O -} 10%CO2-0.5%CO _- 0.1%NO2 _- bal. In the internal oxide layer formed directly under the oxide scale formed on the alloy surface in a cycle test that repeats room temperature and high temperature of about 700 to 900 ° C in a mixed gas atmosphere consisting of N ₂ , internal oxides are formed densely. It reduces the oxidation rate and improves the oxidation resistance by suppressing the inward diffusion of oxygen. On the other hand, by forming a compound with Cr, S consumes the internal oxide and the Cr required when it transitions to form a surface oxide scale. Mn likewise forms an internal oxide, but if the content is too large, the oxidation resistance rather decreases. N forms Al, Ti, AlN, and TiN, respectively, which contribute to improving oxidation resistance, and reduces the effects of Al and Ti. In addition, excessive addition of alloying elements that contribute to the improvement of oxidation resistance is not preferable because it produces a large amount of inclusions that cause surface defects. Therefore, these elements have a lower limit of 0 or more and an upper limit of 40 or less based on the formula (2). It is preferably 10 or more and 39 or less, more preferably 20 or more and 38 or less.

3.2≦REM/S (3)
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 earth element (REM) and the S that forms the compound is 3.2 ≤ REM / S By satisfying the requirements, 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 3.2, the effect of REM cannot be sufficiently obtained, which is undesirable.

Definition of Surface Oxide Scale and Internal Oxide Layer Formed Immediately Below As shown in FIG. An Fe--Cr--Ni alloy having a composition satisfying the formula (3) is used as the base BM. 7%O2-16% _{H2O -} 10%CO2-0.5%CO _- 0.1%NO2 _- bal. In a cycle test that repeats from room temperature to 700 to 900 ° C in a mixed gas atmosphere consisting of _N2 , an oxide scale mainly composed of Cr oxide is formed on the surface of the Fe-Cr-Ni alloy base material of the present invention. An internal oxide layer containing at least one of Cr, Si, Mn, Al, Ti and REM is formed directly under the interface. At this time, in the same 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 test, and the thickness of the internal oxide layer in the GDS analysis. Region LI is shown up to the position where the oxygen intensity is 1/4 of the intensity peak at the scale/alloy interface. ^In addition, the measurement range of the internal oxide layer area ratio described later is Cr, Si, The area ratio of internal oxides containing at least one of Mn, Al, Ti and REM is measured.

7% O2-16 % _{H2O -} 10% CO2-0.5%CO _- 0.1%NO2- _bal . The thickness of the surface oxide scale formed in a cycle test in a mixed gas atmosphere consisting of N2 from room temperature to 700 to 900° _C has a thickness of 10 to 100 µm. O2-16% _{H2O -} 10%CO2-0.5%CO _- 0.1%NO2 _- bal. In a cycle test in which temperatures from room temperature to 700°C to 900°C are repeated in a mixed gas atmosphere of _N2 , an oxide scale mainly composed of Cr oxide is formed on the surface of the alloy base metal to obtain oxidation resistance in the high-temperature environment. At this time, if the thickness of the surface oxide scale is less than 10 μm, sufficient oxidation resistance cannot be obtained, while if it exceeds 100 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 10 to 100 μm. It is preferably 12 to 90 μm, more preferably 12 to 80 μm.

[Internal oxide layer area ratio immediately below surface oxide scale]: ≧30%/0.005 mm ²
When exposed to a high-temperature environment, a layered surface oxide scale is formed on the alloy surface, and an internal oxide layer is formed immediately below the scale/alloy interface. The inner oxide layer is composed of a Cr-based oxide before transitioning to the surface oxide scale of the outer layer and an oxide containing at least one of Si, Mn, Al, Ti and REM. In the oxidation reaction, when oxygen present in a high-temperature environment diffuses into the alloy, it is slower to pass through the oxide than through the inner oxide and through the alloy base material. Thus, if the area ratio of the internal oxide layer is sufficiently ensured, the inward diffusion of oxygen can be suppressed. In the present invention, the above effect can be sufficiently obtained by setting the area ratio of the internal oxide in the internal oxide layer of 0.005 mm ² to 30% or more. On the other hand, if the area ratio is less than 30%, the above effect cannot be obtained sufficiently, which is undesirable.

A method of specifying the limiting formula of the above formula (1) is as follows.
The basic composition is Fe-30%Ni-20%Cr-0.8%Mn, and the amount of Si, Ni, Cr, Al, Ti, Zr, La, Ce, Y, B, Mo, and S added is changed. The various alloys obtained were melted in a vacuum melting furnace, hot forged, and then hot forged plates of 8 mmt×80 mmw were 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 resulting test piece was subjected to 7% O ₂ -16% H ₂ O-10% CO ₂ -0.5% CO-0.1% NO ₂ -bal. A repeated oxidation test was performed in a mixed gas atmosphere of _N2 , with one cycle of 900°C x 10 minutes, 700°C x 10 minutes, 900°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 Fe—Cr—Ni alloy was clarified, and the relational expression of the component composition represented by the formula (1) was obtained by multiple regression analysis. , 47 or more and 85 or less, the oxidation resistance is sufficient.

A method of specifying the limiting formula of the above formula (2) is as follows.
Various alloys having a basic composition of Fe-30%Ni-20%Cr-0.2%Zr with varying amounts of Si, Cr, Al, Ti, La, Ce, Y, N, Mn and S added. was melted in a vacuum melting furnace, and after 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 resulting test piece was subjected to 7% O ₂ -16% H ₂ O-10% CO ₂ -0.5% CO-0.1% NO ₂ -bal. A repeated oxidation test was performed in a mixed gas atmosphere of _N2 , with one cycle of 900°C x 20 minutes, 700°C x 10 minutes, 900°C x 10 minutes, and room temperature x 20 minutes. 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 buffed to a mirror finish for observation. The cross-sectional microstructure of this was observed by FE-SEM, and the oxide was identified and the area ratio of the inner oxide layer was measured by the attached EDS. The area ratio was obtained from an SEM image observed at a magnification of ×2000 at 0.005 mm ² in the internal oxide layer immediately below the surface oxide scale. The area of the massive oxide was obtained by measuring the length of two sides and approximating it to a circle. The long side and short side of the linear oxide were measured, and the area was determined as a rectangle. This was evaluated as the area ratio of the inner oxide layer in the observed area of 0.005 mm ² immediately below the surface oxide scale. It is preferable that the area ratio is 30% or more.

From the above test results, the degree of influence of the additive element on the formation behavior of the internal oxide layer in the oxidation resistance of the Fe—Cr—Ni alloy was clarified, and the result obtained by multiple regression analysis was expressed by the formula (2). It is a relational expression of the component composition, and it can be seen that sufficient oxidation resistance is obtained by making it 0 or more and 40 or less.

Next, a method for producing an austenitic Fe--Cr--Ni alloy according to the present invention will be described.
The austenitic Fe--Cr--Ni 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 —Al 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.

(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 evacuate to 5.0 × 10 ^-3 Pa. After drawing, 7% O ₂ -16% H ₂ O-10% CO ₂ -0.5% CO-0.1% NO ₂ -bal. 900 ° C. x 10 minutes in a mixed gas atmosphere consisting of N ₂ , after temperature adjustment at a temperature decrease rate of 40 ° C./min, 700 ° C. x 10 minutes, after temperature adjustment at a temperature increase rate of 40 ° C./min, 900 ° 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 50 mg/cm ² were judged to have good oxidation resistance (○), and those of 50 mg/cm ² or more were judged to have poor oxidation resistance (×). In addition, cross-sectional microstructure observation was performed after the test, and the thickness of the surface oxide scale and the area ratio of the internal oxide layer formed immediately below were measured.

Steel sheets Nos. 1 to 17 shown in Tables 1 and 2 are invention examples satisfying the conditions of the present invention and had excellent oxidation resistance. On the other hand, the steel sheets Nos. 18 to 37 are comparative examples.
No. The steel sheet No. 18 did not satisfy the formula (2) because the Al content was low, and the alumina concentration in the slag was as high as 54%, and the deoxidation was weak. Oxidation resistance was poor because some elements formed oxides. In addition, since the C content was high, inclusions causing surface flaws were formed, resulting in poor surface quality.
Since the steel sheet No. 19 has a high Si content, surface defects due to intermetallic compounds such as the σ phase are generated, and the high Mn content does not satisfy the formula (2). It has deteriorated in chemical resistance.
Steel sheet No. 20 did not satisfy the formula (1) because of its low Ni content. In addition, since the Al content was high, a large amount of carbonitrides that caused surface defects were formed, resulting in poor surface quality. Furthermore, the CaO concentration in the slag was as high as 85%, and the oxygen concentration was too low. rice field.
Since the steel sheet of No. 21 had a high B content, it did not satisfy the formula (1) and was inferior in oxidation resistance. In addition, since the Al ₂ O ₃ concentration in the slag was as high as 52% and the CaO concentration was as low as 28%, the O content increased and alumina-based inclusions adhered to the submerged nozzle and fell off. As a result, the number of surface defects increased.
Since the steel sheet No. 22 had a high Cr content, the formula (1) was not satisfied, and the surface oxide scale grew excessively to form a scale with poor adhesion, resulting in rather poor oxidation resistance. Moreover, since Cr nitrides that cause surface flaws are generated, the surface quality is inferior.
Steel plate No. 23 did not satisfy the formula (1) because of its high Ti content, and carbonitrides that cause surface flaws were formed, resulting in poor surface quality.
Steel plate No. 24 did not satisfy the formula (3) because the content of REM was low, and the effect of improving oxidation resistance and the effect of fixing S, which inhibits oxidation resistance, as inclusions was sufficiently obtained. I didn't.
Since the steel sheet of No. 25 had a high Zr content, it did not satisfy the formula (1) and had surface defects caused by the formation of a large amount of carbonitrides, resulting in poor surface quality.
Steel plate No. 26 did not satisfy the formula (1). Moreover, since the Mo content is high, the adhesion of scale is lowered, resulting in poor oxidation resistance.
Steel sheet No. 27 had a low Cr content and did not satisfy the formulas (1) and (2), and therefore was inferior in oxidation resistance. In addition, since the formula (3) 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.
Steel sheet No. 28 has a high REM content and does not satisfy the formulas (1) and (2). 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.
Since the steel sheet of No. 29 had a low CaO concentration of 35% in the slag, the desulfurization reaction did not progress. Therefore, since the S content becomes high, the formulas (1) and (2) are not satisfied, a large amount of inclusions are formed, and the Cr necessary for the formation of the surface oxide scale is consumed. , poor in oxidation resistance.
Steel sheet No. 30 had a high Ni content and did not satisfy the formula (1). could not. In addition, since the Mn content was low, the effects of deoxidizing and stabilizing the austenite phase were not sufficiently obtained. In addition, since the formula (3) was not satisfied, the effect of improving oxidation resistance and the effect of fixing S, which inhibits oxidation resistance, as inclusions could not be obtained sufficiently.
The steel sheet of No. 31 did not satisfy the formula (1), and the CaO concentration in the slag was as high as 83%, so the Ca content was high. A large amount of Ca—Al oxide-based inclusions was formed, Al in the alloy was consumed, and effective Al decreased, resulting in poor oxidation resistance.
Since the steel sheet of No. 32 had a high N content, it did not satisfy the expression (2), Cr, Al, and Ti, which contribute to oxidation resistance, precipitated as nitrides, and a sufficient internal oxide layer was formed. It could not be formed, resulting in poor oxidation resistance.
Steel sheet No. 33 had a high content of Ti and Zr and therefore did not satisfy the formula (1), resulting in the formation of a large amount of carbonitrides that cause surface flaws, resulting in poor surface quality.
Steel plate No. 34 did not satisfy the formulas (1) and (2) because the contents of Ti and Zr were low. was inferior to
Although No. 35 satisfied the component range of each element, it did not satisfy the formulas (1), (2) and (3), so it did not have sufficient oxidation resistance.
No. 36 satisfied the component range of each element, but did not satisfy the formulas (2) and (3), so it did not have sufficient oxidation resistance.
Although No. 37 satisfied the component range of each element, it did not satisfy the formula (1), so it could not have sufficient oxidation resistance.

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

1: surface oxide scale layer, 2: internal oxide layer, 3: interface

Claims (6)

C in mass %: 0.004 to 0.13%,
Si: 0.15 to 1.0%,
Mn: 0.03-2.0%,
P: ≤ 0.040%,
S: ≤ 0.003%,
Ni: 20.0 to 38.0%,
Cr: 18.0 to 28.0%,
Mo: ≤ 1.0%,
Cu: ≤ 1.0%,
N: ≤ 0.03%,
B: ≤ 0.01%,
Al: 0.10-1.0%,
At least one of Ti and Zr, Ti: 0.10 to 1.0%, Zr: 0.01 to 0.6%,
O: 0.0002 to 0.0030%,
Ca: ≤ 0.002%,
Total weight of one or more of La, Ce, and Y that are rare earth elements (REM): 0.001 to 0.010%
An austenitic Fe--Ni--Cr alloy characterized in that it has a component composition with the balance being Fe and unavoidable impurities, and contains the following formulas (1) and (2).
85≧0.3×Si+1.5×Ni+1.3×Cr+5.8×Al+7.7×Zr+2.7×Ti+2173×REM-3582×S-32.9×Mo-2448×B≧47 (1)
40≧0.6×Si+1.3×Cr+23.53×Al+5.88×Ti+3074×REM-5067×S-0.8×Mn-816×N≧0 (2)
(Here, each element symbol in the above formula indicates the content (mass%) of each element.) The austenitic system 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 (3): Fe--Ni--Cr alloy.
3.2≦REM(La, Ce, Y)/S (3)
(Here, each element symbol in the above formula indicates the content (mass%) of each element.) 7%O2-16% _{H2O -} 10%CO2-0.5%CO _- 0.1%NO2 _- bal. The composition of the surface oxide scale formed in a cycle test in which the temperature is repeated from room temperature to 700 to 900°C in a mixed gas atmosphere consisting of N ₂ is, in terms of mass%, Cr: 40% or more, Fe: 10 to 20%, and Ni: 0. Austenitic Fe- Ni-Cr alloy. 4. The austenitic Fe--Ni--Cr alloy according to claim 3, wherein said surface oxide scale has a thickness of 10-100 μm. The internal oxide layer formed directly under the surface oxide scale is composed of an internal oxide containing at least one of Cr, Si, Mn, Al, Ti, and REM, and the area of the internal oxide layer at this time 4. Austenitic Fe--Ni--Cr alloy according to claim 3, characterized in that it has a modulus of 30% or more per 0.005 mm ² directly below the surface oxide scale. A method for producing an austenitic Fe—Ni—Cr alloy according to any one of claims 1 to 5,
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 0.03. % or less, Cr reduction, then aluminum, limestone and fluorite are added to the molten alloy to form CaO—SiO ₂ —Al ₂ O ₃ —MgO—F system slag, After setting the oxygen concentration to 0.0002 to 0.0030 mass% and then adding a raw material containing one or more of La, Ce, and Y, casting is performed to obtain a slab, which is subjected to a hot rolling process. A method for producing an austenitic Fe—Ni—Cr alloy, characterized in that it is provided for.

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