Austenitic Fe-Ni-Cr alloy having excellent oxidation resistance and method for producing same

Abstract

Provided is an austenitic Fe-Ni-Cr alloy having excellent oxidation resistance even at harsh high temperatures, the austenitic Fe-Ni-Cr alloy comprising, in mass%, 0.004-0.13% of C, 0.15-1.0% of Si, 0.03-2.0% of Mn, less than or equal to 0.040% of P, less than or equal to 0.003% of S, 20.0-38.0% of Ni, 18.0-28.0% of Cr, less than or equal to 1.0% of Mo, less than or equal to 1.0% of Cu, less than or equal to 0.03% of N, less than or equal to 0.01% of B, 0.10-1.0% of Al, at least one of Ti and Zr (Ti: 0.10-1.0% and Zr: 0.01-0.6%), 0.0002-0.0030% of O, less than or equal to 0.002% of Ca, 0.001-0.010 and formulae (1) and (2) that satisfy prescribed relational expressions.

Description

Technical Field

[0001] 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. Background Art

[0002] Thermal power boilers, chemical plants, and polysilicon refining reactors are used in harsh high-temperature environments of 700 to 900°C, so the materials used for them are required to have excellent high-temperature strength, high-temperature corrosion resistance, and oxidation resistance. In particular, as one of the required characteristics, regarding oxidation resistance, the protective surface oxide scale mainly composed of Cr2O3 formed on the surface of the material under the above-mentioned high-temperature environment is dense and has high adhesion to the material. As a material used for equipment used in the above-mentioned high-temperature environment, Fe-Cr-Ni alloys have attracted attention. Among them, 18-8 series stainless steels such as SUS304, SUS316, and SUS347 do not have sufficient characteristics in the above-mentioned use environment, so SUS310S or NCF800 with further increased Ni or Cr content are usually used.

[0003] As a technology for improving various high temperature properties of materials used in such a severe use environment, for example, Patent Document 1 proposes an austenitic stainless steel sheet, which suppresses the generation of C on the surface of the steel sheet by adding a trace amount of REM (Rare Earth Metal) to the stainless steel and setting the upper limit of the Mn content in accordance with the Ni content and the REM addition amount. r2 Growth rate of O3 oxide film. In addition, Patent Document 2 proposes a heat-resistant steel material for a modified device that improves the adhesion of Cr2O3 generated on the surface of the steel material by specifying the Si content corresponding to the Cr and Ni contents in the steel material.

[0004] However, the technologies disclosed in the above patent documents do not study the effects of REM in the material or S that can form a compound with Cr, and are considered insufficient for application in the above high temperature environment requiring excellent oxidation resistance. Furthermore, REM contains various elements, and there is no mention of which element has an effect, which makes it difficult to implement in reality.

[0005] Furthermore, in none of the above-mentioned patent documents, the influence of the internal oxide layer on oxidation resistance is studied, and therefore it is considered that the technology is insufficient for application in the above-mentioned severe high-temperature environment.

[0006] In addition, in recent years, Ni-Cr-Fe alloys with excellent creep strength and stress relaxation cracking resistance obtained by composite addition of Ti, Al, and REM have been proposed (for example, refer to Patent Document 3). However, this alloy is a method of adjusting the composition in a high-frequency induction furnace in the laboratory to obtain a steel ingot, which is then supplied to the hot rolling process. After all, it is impossible to carry out mass production represented by a 60-ton scale. In addition, with respect to all effective descriptions of REM, the addition of Nd is the main body, and Ce, La, and Y are only added to a part of the alloy. As a larger issue, there is no process for removing S and O, which cannot be achieved if the raw materials are strictly selected. Therefore, REM will oxidize or sulfide depending on the situation, which is an industrial solution that cannot eliminate fragility and is difficult to achieve the original purpose. Therefore, it is difficult to say that an alloy with increased creep strength by adding REM can be provided quickly and reliably on an industrial scale.

[0007] Prior art literature

[0008] Patent Literature

[0009] Patent Document 1: Japanese Patent Application Publication No. 2003-171745,

[0010] Patent Document 2: Japanese Patent Application Publication No. 2002-256398,

[0011] Patent document 3: Japanese Patent Application No. 2018-066579. Summary of the invention

[0012] Problems to be solved by the invention

[0013] The present invention has been made in view of the above circumstances, and an object of the present invention is to propose an austenitic Fe-Ni-Cr alloy having excellent oxidation resistance even in a severe high-temperature environment.

[0014] Means for solving problems

[0015] The inventors have repeatedly conducted in-depth research to solve the above-mentioned problems. So far, the following insights have been obtained, that is, by adding La, Ce, and Y in REM, the adhesion of the surface oxide scale generated on the alloy surface under high temperature environment is improved, but no sufficient insights have been obtained about the contribution to oxidation resistance evaluated by repeated cycle tests from room temperature to 700-900°C in a mixed gas atmosphere composed of 7% O2-16% H2O-10% CO2-0.5% CO-0.1% NO2-bal.N2. Therefore, the correlation between La, Ce, Y and other elements contained in the alloy was investigated in detail. As a result, it was clarified that the addition of La, Ce, and Y is extremely effective in improving oxidation resistance, and Si, Ni, Cr, Al, Ti, and Zr are also effective as other elements. On the other hand, it was clarified that the effect of S, Mo, and B hindering the improvement of the above-mentioned oxidation resistance was hindered. Therefore, it was found that in order to fully ensure the effect of adding REM, Si, Ni, Cr, Al, Ti, Zr, S, Mo, and B need to be controlled.

[0016] In addition, an internal oxide layer composed of oxides of Cr, Si, Mn, Al, Ti, La, Ce, and Y is formed directly below the protective oxide scale formed on the surface under high temperature conditions. The formation behavior of the internal oxide layer was investigated in detail, and it was found that the area ratio of the internal oxide layer has a good correlation with the oxidation weight loss in the high temperature oxidation test. Specifically, the area ratio of the internal oxide layer within 0.005 mm2 directly below the surface oxide scale is 0.005 mm2. 2 When the area ratio of the internal oxide accounts for 30% or more, the oxidation resistance is improved. The relationship between the formation behavior of these internal oxide layers and the alloying elements was investigated, and it was found that La, Ce, Y, Si, Cr, Al, and Ti are effective, while on the other hand, the inclusion of S, Mn, and N hinders the formation behavior of the above-mentioned internal oxide layer. It was found that in order to obtain the effect of improving the oxidation resistance by controlling the internal oxide layer, La, Ce, Y, Si, Cr, Al, Ti, S, Mn, and N need to be controlled.

[0017] In addition, it was found that the value obtained by dividing the total weight (mass %) of any one or more of La, Ce, and Y contained in the alloy as REM by the content (mass %) of S contained in the alloy has a good correlation with the oxidation weight loss in the high-temperature oxidation test, and it was clarified that the characteristic formula obtained thereby is 3.2≤REM / S.

[0018] In addition, repeated studies were conducted with a focus on the morphology of surface oxide scales formed in high-temperature environments. The results showed that when the thickness of the surface oxide scale formed in repeated cycle tests from room temperature to 700-900°C was within the range of 10-100 μm, the surface oxide scale was densely formed and had excellent adhesion, and the oxidation weight loss after the test also showed good results.

[0019] That is, the austenitic Fe-Ni-Cr alloy of the present invention is characterized in that it is composed of the following components: in terms of mass%, it contains 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: ≤0.0 1%, Al: 0.10-1.0%, at least one of Ti and Zr (Ti: 0.10-1.0%, Zr: 0.01-0.6%), and O: 0.0002-0.0030%, Ca: ≤0.002%, any one of La, Ce, Y as rare earth elements (REM) or the total weight of two or more: 0.001-0.010%, the balance is Fe and unavoidable impurities; and the following formulas (1) and (2) are satisfied.

[0020] 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)

[0021] 40≥0.6×Si+1.3×Cr+23.53×Al+5.88×Ti+3074×REM-5067×S-0.8×Mn-816×N≥0…(2)

[0022] Furthermore, the austenitic Fe-Ni-Cr alloy of the present invention is characterized in that any one or two or more of La, Ce and Y as the rare earth elements (REM) satisfy the following formula (3).

[0023] 3.2≤REM(La,Ce,Y) / S…(3)

[0024] The austenitic Fe-Ni-Cr alloy of the present invention is characterized in that, in addition to the above-mentioned component composition, the composition of the surface oxide scale formed in a cycle test repeatedly performed from room temperature to 700-900°C in a mixed gas atmosphere composed of 7% O2-16% H2O-10% CO2-0.5% CO-0.1% NO2-bal.N2 contains, in terms of mass%, Cr: 40% or more, Fe: 10-20%, Ni: 0-10%, O: 10-40%, REM: 0.05-0.5%, and the balance contains Mn, Si, and Ti as inevitable elements, and the scale thickness has a thickness of 10 to 100 μm.

[0025] In addition, it is characterized in that the inner oxide layer formed directly below the surface oxide scale is composed of an inner oxide containing at least one of Cr, Si, Mn, Al, Ti, and REM, and the area ratio of the inner oxide layer is 0.005 mm2 per 0.005 mm2 directly below the surface oxide scale. 2 More than 30% of them.

[0026] In addition, the present invention also proposes a method for manufacturing the above-mentioned austenitic Fe-Ni-Cr alloy. That is, the alloy composition is adjusted by refining the alloy raw material after melting, and a mixed gas of oxygen and argon is blown into the melted alloy raw material (molten alloy) during refining to perform decarburization, and Cr reduction is performed after the nitrogen concentration is controlled to be below 0.03%, and then aluminum, limestone and fluorite are added to the molten alloy to form CaO-SiO2-Al2O3-MgO-F system slag, so that the oxygen concentration in the molten alloy is 0.0002-0.0030 mass%, and then, raw materials containing any one or more of La, Ce and Y are added to adjust the composition, and casting is performed to obtain a slab and then a coil is manufactured through a hot rolling process.

[0027] Effects of the Invention

[0028] According to the present invention, excellent oxidation resistance is achieved in a high temperature environment, and this can significantly contribute to extending the life of the product. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] [ Figure 1 ] A schematic diagram for illustrating the surface structure of a Fe-Cr-Ni alloy plate in a high temperature oxidation test according to one embodiment of the present invention. DETAILED DESCRIPTION

[0030] Next, the composition components that the austenitic Fe-Ni-Cr alloy of the present invention should have will be described.

[0031] C: 0.004-0.13% by mass

[0032] C is an element that contributes to the stabilization of the austenite phase. However, when added in large quantities, it combines with Cr and Mo to form carbides, and the amount of solid-solubilized Cr near it decreases, reducing oxidation resistance. On the other hand, C has the effect of improving the strength of the alloy by solid solution strengthening, so the lower limit is set to 0.004 mass%. Therefore, C is limited to 0.004-0.13 mass%. Preferably, it is 0.005-0.080 mass%, and more preferably 0.006-0.070 mass%.

[0033] Si: 0.15-1.0 mass%

[0034] Si is an element effective in improving oxidation resistance and preventing oxide film peeling, and the above effects are obtained by adding 0.15 mass % or more. However, excessive addition of Si promotes the precipitation of intermetallic compounds such as σ phases, and also causes surface defects caused by intermetallic compounds, so it is set to 0.15-1.0 mass %. It is preferably 0.16-0.8 mass % or less, and more preferably 0.17-0.6 mass % or less.

[0035] Mn: 0.03-2.0 mass%

[0036] Mn is an austenite stabilizing element and also an element having a deoxidizing effect, so in order to obtain its effect, it needs to be at least 0.03 mass %. However, Mn, like Si, also causes the precipitation of intermetallic compounds such as σ phases, and also causes a decrease in oxidation resistance, so it is not preferred to add more than the necessary amount. Therefore, it needs to be 0.03 to 2.0 mass %. It is preferably 0.03 to 1.50 mass %, and more preferably 0.03 to 1.00 mass %.

[0037] P: 0.040 mass% or less

[0038] P is an element that is inevitably mixed in as an impurity. Since it segregates at the grain boundary as a phosphide, it is an element that impairs hot workability. Therefore, it is desirable to reduce it as much as possible. However, an extreme reduction in the content of P will lead to an increase in manufacturing costs. Therefore, in the present invention, P is limited to 0.040 mass % or less. Preferably, it is 0.030 mass % or less, and more preferably, it is 0.020 mass % or less.

[0039] S: 0.003 mass % or less

[0040] S, like P, is an element that is inevitably mixed in as an impurity, and is easy to segregate at the grain boundary, especially significantly impairing hot workability. In addition, by forming a compound with Cr, which contributes to oxidation resistance described later, Cr required for forming a surface oxide scale is consumed, the adhesion between the oxide film and the substrate is reduced, thereby causing the oxide film to fall off and promoting oxidation, and therefore it is an element that is harmful to oxidation resistance. If the content exceeds 0.003% by mass, its harmfulness is significantly manifested, so it is necessary to control it to less than 0.003% by mass. It is preferably less than 0.002% by mass, and more preferably less than 0.001% by mass. As described later, the reduction of S can be reduced by the addition of Al and the reaction between the slag components.

[0041] Ni: 20.0-38.0 mass%

[0042] Ni is an austenite stabilizing element and has the effect of inhibiting the precipitation of intermetallic compounds such as σ phase. In addition, it also has the effect of improving heat resistance or high temperature strength. In order to fully exert the above effects, more than 20% by mass is added. On the other hand, excessive addition will lead to deterioration of hot workability or increase in thermal deformation resistance, as well as increased costs. Therefore, the Ni content is 20.0 to 38.0% by mass. Preferably, it is 21.0 to 36.0% by mass, and more preferably, it is 22.0 to 35.0% by mass.

[0043] Cr: 18.0-28.0 mass%

[0044] Cr is an element that helps to inhibit corrosion in high temperature environments. In addition, it forms a protective oxide film on the alloy surface in high temperature environments, which also has the effect of inhibiting high temperature oxidation. In order to fully obtain the above-mentioned effect, it is necessary to contain more than 18.0 mass%. However, excessive addition of Cr will form a large surface oxide scale, which will lack adhesion and deteriorate oxidation resistance. In addition, since the stability of the austenite phase is reduced, it is necessary to add a large amount of Ni, so it is set to 18.0 to 28.0 mass%. Preferably, it is 19.0 to 26.0 mass%, and more preferably 20.0 to 25.0 mass%.

[0045] Mo: 1.0 mass % or less

[0046] Even if Mo is added in small amounts, it can be dissolved in the alloy and has the effect of improving high temperature strength. However, in a material with a large amount of Mo added, Mo is preferentially oxidized under high temperature conditions and when the surface oxygen potential is low, resulting in peeling of the oxide scale, which has an adverse effect. Therefore, from the perspective of ensuring the adhesion of the protective surface oxide scale, Mo is limited to less than 1.0 mass %. Preferably, it is less than 0.8 mass %, and more preferably, it is less than 0.6 mass %.

[0047] Cu: 1.0 mass % or less

[0048] Cu is sometimes added as an element to improve corrosion resistance in a wet environment, but its effect is hardly seen in a high temperature environment as in the present invention. On the other hand, excessive addition forms an uneven film with a spot-like pattern on the surface of the material, which reduces corrosion resistance. Therefore, the addition amount of Cu is limited to 1.0 mass % or less. Preferably, it is 0.8 mass % or less, and more preferably, it is 0.6 mass % or less.

[0049] N: 0.03 mass % or less

[0050] N is an element that is inevitably mixed in as an impurity, but it is also an austenite phase generating element, so it contributes to the stabilization of the structure. However, when Al, Ti, Zr, etc. are added as in the present invention, N combines with these elements to precipitate nitrides, and the hot deformation resistance is significantly increased, which impairs the hot workability. In addition, due to the formation of the above-mentioned nitrides, Al or Ti, which is a constituent element of the internal oxide formed just below the surface oxide scale, is consumed, thereby reducing the area ratio of the internal oxide layer. Therefore, in the present invention, the content of N is set to 0.03% by mass or less. It is preferably 0.02% by mass or less, and more preferably 0.01% by mass or less.

[0051] Although oxygen blowing refining is performed during decarburization, N is transferred to CO gas bubbles in the form of nitrogen and removed to the outside of the system, so that it can be controlled within the scope of the present invention.

[0052] B: 0.01 mass % or less

[0053] B has the effect of assisting the effect of rare earth elements (REM) by grain boundary segregation, and is also an element that contributes to high temperature strength. However, a large amount of addition may lead to reduced adhesion or reduced weldability and hot workability of the alloy due to the porosity of the surface oxide scale. In the present invention, the content of B is set to 0.01% by mass or less. Preferably, it is 0.008% by mass or less, and more preferably, it is 0.006% by mass or less.

[0054] Al: 0.10-1.0 mass%

[0055] Al is an element that promotes the formation of a dense black film and improves oxidation resistance, and its effects are obtained by adding 0.10 mass % or more. In addition, it is an element added as a deoxidizing material 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 formula (a).

[0056] 2 Al +3 O =(Al2O3)…(a)

[0057] The underline indicates the elements in the molten steel, and the brackets indicate the components in the slag.

[0058] When refining the alloy of the present invention, the use of CaO-SiO2-Al2O3-MgO-F system slag can effectively absorb the generated Al2O3 and control the oxygen concentration. In addition, by deoxidation, the S concentration in the molten steel is also reduced according to formula (b).

[0059] 2 Al +3 S +3(CaO)=3(CaS)+(Al2O3)…(b)

[0060] Thus, the S concentration can be controlled to be below 0.003 mass % which is the range of the present invention. Therefore, Al needs to be above 0.10 mass %. However, excessive addition will significantly cause the formula (c) to move to the right, and the Ca concentration will exceed 0.002 mass %, forming a large amount of Ca-Al oxide inclusions, which consumes Al in the alloy and reduces oxidation resistance.

[0061] 3(CaO)+2 Al =3 Ca +(Al2O3)…(c)

[0062] Therefore, the upper limit of Al is set to 1.0 mass %, preferably 0.10 to 0.80 mass %, and more preferably 0.10 to 0.60 mass %.

[0063] Similar to Al described above, Ti and Zr effectively function to form a dense black film or improve oxidation resistance, and therefore, it is necessary to add one or both of them.

[0064] Ti: 0.10-1.0 mass%

[0065] Ti is an element that promotes the formation of a dense black film and improves oxidation resistance, and its effect can be obtained by adding 0.10 mass % or more. However, excessive addition will cause surface defects 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 %, and more preferably 0.10 to 0.60 mass %. In addition, controlling the concentration of C and N within the scope of the present invention as described above is also an effective means for suppressing carbonitrides.

[0066] Zr: 0.01-0.6 mass%

[0067] Zr is an element of the same group as Ti. Like Ti, Zr plays an effective role in forming a dense black film or improving oxidation resistance, so it can also be used as a substitute for Ti. Its effect is better than that of Ti, so even a small amount of addition is effective, but excessive addition will cause surface defects due to the formation of a large amount of carbonitrides, so the upper limit is limited to 0.6% by mass. It is preferably in the range of 0.01 to 0.4% by mass, and more preferably in the range of 0.05 to 0.3% by mass.

[0068] O: 0.0002-0.0030 mass%

[0069] O in the alloy combines with Al, Ti, Zr, Si, La, Ce, and Y in the molten steel to form oxides, which damages the useful effects of these elements, oxidation resistance, etc. In addition, a large number of oxide-based non-metallic inclusions of aluminum oxide are formed, which adhere to the submerged nozzle that injects molten steel into the mold from the tundish of the continuous casting machine, and they fall off and become the cause of surface defects. Therefore, the oxygen concentration is preferably as low as 0.0030% by mass or less. In order to achieve this range, it is sufficient to control Al to the concentration of the present invention as described above for deoxidation. On the other hand, if O in the alloy is excessively reduced, according to formula (c), the Ca concentration becomes high and exceeds 0.002% by mass. Therefore, the lower limit is set to 0.0002% by mass. It is preferably 0.0003 to 0.0027% by mass, and more preferably 0.0005 to 0.0025% by mass.

[0070] Ca: 0.002 mass % or less

[0071] As described above, in the alloy of the present invention, Ca is an element mixed from CaO in the slag. Ca forms a large amount of Ca-Al oxide inclusions, which consumes Al in the alloy and reduces oxidation resistance, so it needs to be suppressed to a low level. For this reason, the Al concentration needs to be controlled to 0.10-1.00 mass %, so that the oxygen concentration is 0.0002-0.0030 mass %. Therefore, Ca needs to be 0.002 mass % or less.

[0072] The total weight of any one or more of La, Ce, and Y as rare earth elements (REM): 0.001 to 0.010 mass % quantity%

[0073] REM (La, Ce, Y) has the effect of improving the hot workability of the alloy or the adhesion between the surface oxide scale and the substrate surface, and improving the oxidation resistance, and even a trace amount can obtain a significant effect. In addition, by forming a compound with S dissolved in the alloy, Cr, which is a constituent element of the surface oxide scale, and S are inhibited from forming a compound, it can be expected that the effect of preventing the local reduction of Cr can be prevented. In addition, REM is usually used as a raw material as an alloy containing a variety of REM, that is, mixed rare earth, but there are also cases where Fe-Ni alloys containing any of REM are used. However, excessive addition will reduce the hot workability and weldability of the alloy, and due to the excessive formation of REM inclusions, the adhesion of the surface oxide scale will be reduced. Moreover, it causes clogging of the submerged nozzle during continuous casting, and the manufacturability is significantly deteriorated. Therefore, in the present invention, the addition amount of REM is set to 0.001-0.010 mass%. Preferably, it is 0.002-0.009 mass%, and more preferably 0.003-0.008 mass%.

[0074] 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)

[0075] Formula (1) is a formula that expresses the degree of influence of the elements that affect the surface oxide scale formed on the surface of the Fe-Cr-Ni alloy in the oxidation resistance of the alloy in the form of a formula through multivariate regression analysis. Si, Ni, Cr, Al, TI, Zr, REM (La, Ce, Y) improve the oxidation resistance evaluated in a cycle test of repeated room temperature and high temperature of about 700 to 900°C in a mixed gas atmosphere composed of 7% O2-16% H2O-10% CO2-0.5% CO-0.1% NO2-bal.N2. On the other hand, S reduces the adhesion between the oxide film and the substrate, thereby causing the oxide film to fall off and promoting oxidation. When the Mo content is high, Mo is preferentially oxidized in a high temperature environment and when the surface oxygen potential is low, resulting in the peeling of the oxide scale. In addition, when the B content is high, since the alloy oxide scale is porous, the oxidation rate at high temperature increases, promoting the growth and peeling of the scale. In addition, excessive addition of alloying elements that contribute to the improvement of oxidation resistance is not preferred because the adhesion is reduced due to excessive growth of surface oxide scale, and a large number of inclusions that cause surface defects are generated. Therefore, based on formula (1), the lower limit of these elements is set to 47 or more, and the upper limit is set to 85 or less. Preferably, it is 48 or more and 84 or less, and more preferably, it is 50 or more and 83 or less.

[0076] 40≥0.6×Si+1.3×Cr+23.53×Al+5.88×Ti+3074×REM-5067×S-0.8×Mn-816 ×N≥0…(2)

[0077] Formula (2) is a formula that expresses the degree of influence of the elements that affect the formation behavior of the internal oxide layer formed just below the surface oxide scale in the oxidation resistance of Fe-Cr-Ni alloy in the form of a formula through regression analysis. In a cycle test at room temperature and high temperature of about 700 to 900°C repeatedly performed in a mixed gas atmosphere composed of 7% O2-16% H2O-10% CO2-0.5% CO-0.1% NO2-bal.N2, REM, Si, Cr, Al, and Ti each densely form an internal oxide in the internal oxide layer formed just below the oxide scale formed on the surface of the alloy, and inhibit the diffusion of oxygen into the interior, thereby reducing the oxidation rate and improving the oxidation resistance. On the other hand, S forms a compound with Cr, thereby consuming the internal oxide and its migration to form the surface oxide scale. In addition, Mn also forms an internal oxide, but when contained in large quantities, it reduces the oxidation resistance. N forms AlN and TiN with Al and Ti, which contribute to improving the oxidation resistance, respectively, and reduces the effect of Al and Ti. Moreover, excessive addition of alloying elements that contribute to improving oxidation resistance is not preferred because a large amount of inclusions that cause surface defects are generated. Therefore, based on formula (2), the lower limit of these elements is set to 0 or more, and the upper limit is set to 40 or less. Preferably, it is 10 or more and 39 or less, and more preferably, it is 20 or more and 38 or less.

[0078] 3.2≤REM / S…(3)

[0079] As an index for sufficiently obtaining the effect of improving the hot workability and oxidation resistance of the alloy, the above-mentioned effect can be obtained by making the relationship between the content of rare earth elements (REM) and S forming a compound satisfy 3.2≤REM / S, and having a REM content sufficient to fix S as an inclusion. On the other hand, if it is less than 3.2, the effect of REM cannot be sufficiently obtained, which is not desirable.

[0080] Definition of surface oxide scale and the inner oxide layer formed directly below it

[0081] like Figure 1As shown, the Fe-Cr-Ni alloy plate according to one embodiment of the present invention has a Fe-Cr-Ni alloy having a composition satisfying the above-mentioned formula (1), formula (2) and formula (3) as a matrix BM. In a cycle test repeatedly performed from room temperature to 700-900°C in a mixed gas atmosphere composed of 7% O2-16% H2O-10% CO2-0.5% CO-0.1% NO2-bal.N2, a scale mainly composed of Cr oxide is formed on the surface of the Fe-Cr-Ni alloy substrate of the present invention, and an internal oxide layer containing at least one of Cr, Si, Mn, Al, Ti, and REM is formed just below the scale / alloy interface. At this time, in the figure, the thickness of the surface scale represents the region LE from the outermost layer of the surface scale to the scale / alloy interface in the cross-sectional microstructure observation after the above-mentioned test, and the thickness of the internal oxide layer represents the region LI to the position where the oxygen intensity is 1 / 4 of the intensity peak at the scale / alloy interface in the GDS analysis. The measurement range of the internal oxide layer area ratio described below is 0.005 mm surrounded by a range of 0.05 mm in the interface direction and 0.1 mm in the depth direction with the skin / alloy interface as the upper end. 2 The area ratio of internal oxides containing at least one of Cr, Si, Mn, Al, Ti, and REM.

[0082] The reaction was carried out in a mixed gas atmosphere consisting of 7% O2-16% H2O-10% CO2-0.5% CO-0.1% NO2-bal.N2. The surface oxide scale formed by repeated cycle tests from room temperature to 700-900°C has a thickness of 10-100 μm.

[0083] In the Fe-Cr-Ni alloy of the present invention, in a cycle test repeatedly carried out from room temperature to 700-900°C in a mixed gas atmosphere composed of 7% O2-16% H2O-10% CO2-0.5% CO-0.1% NO2-bal.N2, an oxide scale mainly composed of Cr oxide is formed on the surface of the alloy substrate, and oxidation resistance in the above-mentioned high temperature environment is obtained. At this time, if the thickness of the surface oxide scale is less than 10μm, sufficient oxidation resistance cannot be obtained. On the other hand, if it exceeds 100μm, the peeling property of the surface oxide scale increases, and the adhesion between the surface oxide scale and the substrate surface is impaired. Therefore, the protective surface oxide scale formed in the above-mentioned high temperature environment needs to have a thickness of 10 to 100μm. Preferably, it is 12 to 90μm, and more preferably 12 to 80μm.

[0084] [Area ratio of inner oxide layer directly below the surface oxide scale]: ≥30% / 0.005mm 2

[0085] 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 just below the oxide scale / alloy interface. The internal oxide layer is composed of Cr-based oxides before the surface oxide scale migrates to the outer layer and oxides 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 diffuses slower through the internal oxide or through the alloy substrate. Therefore, if the area ratio of the internal oxide layer is sufficiently ensured, the diffusion of oxygen into the interior can be suppressed. In the present invention, by making 0.005mm 2 When the area ratio of the inner oxide in the inner oxide layer is 30% or more, the above-mentioned effect can be sufficiently obtained. On the other hand, when the area ratio is less than 30%, the above-mentioned effect cannot be sufficiently obtained, which is not preferable.

[0086] The method for determining the limiting formula of the above formula (1) is as follows.

[0087] Various alloys with a basic composition of Fe-30%NI-20%Cr-0.8%Mn and varying addition amounts of Si, Ni, Cr, Al, Ti, Zr, La, Ce, Y, B, Mo, and S were melted in a vacuum melting furnace, and hot forged to produce hot forged plates of 8mmt×80mmw. The hot forged plates were subjected to solution heat treatment at 1200°C×10 minutes, surface ground and then cold rolled to 2mmt, and then subjected to solution heat treatment at 1150°C×1 minute. Then, the plates were cut into 20mm×30mm, and the surfaces were finished by wet grinding #320 to produce test pieces. The obtained test piece was subjected to a repeated oxidation test in a mixed gas atmosphere consisting of 7% O2-16% H2O-10% CO2-0.5% CO-0.1% NO2-bal.N2, with one cycle of 900°C × 10 minutes, 700°C × 10 minutes, 900°C × 10 minutes, and room temperature × 20 minutes. The test piece after 200 cycles was evaluated by dividing the mass change after removing the peeled tare weight by the surface area before the test.

[0088] From the above test results, it can be seen that the degree of influence of the added elements on the oxidation resistance of the Fe-Cr-Ni alloy is clear. The relationship between the component composition shown in formula (1) is obtained through multivariate regression analysis. By setting it to 47 or more and 85 or less, sufficient oxidation resistance is achieved.

[0089] The method for determining the limiting formula of the above formula (2) is as follows.

[0090] Various alloys with a basic composition of Fe-30% Ni-20% Cr-0.2% Zr and varying amounts of Si, Cr, Al, Ti, La, Ce, Y, N, Mn, and S were melted in a vacuum melting furnace, and hot forged to produce 8mmt×80mmw hot forged plates. The resulting hot forged plates were subjected to solution heat treatment at 1200°C×10 minutes, surface ground and then cold rolled to 2mmt, and then subjected to solution heat treatment at 1150°C×1 minute. Then, they were cut into 20mm×30mm pieces, and the surfaces were finished by wet grinding #320 to produce test pieces. The obtained test piece was subjected to repeated oxidation test in a mixed gas atmosphere composed of 7% O2-16% H2O-10% CO2-0.5% CO-0.1% NO2-bal.N2, with 900℃×20 minutes, 700℃×10 minutes, 900℃×10 minutes, and room temperature×20 minutes as one cycle. The test piece after 200 cycles was cut, and Cu plating was applied so that the cross section could be observed. Then, an embedded sample was made, wet-ground, and finally polished to a finished mirror surface for observation. The cross-sectional microstructure was observed using FE-SEM, and the oxides were identified using the attached EDS and the area ratio of the internal oxide layer was measured. The area ratio is calculated from the 0.005mm in the internal oxide layer directly below the surface oxide scale. 2 The area of ​​the block oxide is obtained by measuring the length of both sides and approximating the circle. The area of ​​the linear oxide is obtained by measuring the long and short sides as a rectangle. 2 The area ratio of the internal oxide layer in the observed area is evaluated. The area ratio is preferably 30% or more.

[0091] From the above test results, it can be seen that the degree of influence of the added elements on the formation behavior of the internal oxide layer in the oxidation resistance of the Fe-Cr-Ni alloy is clear. The relationship between the component composition shown in formula (2) is obtained through multivariate regression analysis. By setting it to be greater than 0 and less than 40, sufficient oxidation resistance is achieved.

[0092] Next, the method for producing the austenitic Fe-Cr-Ni alloy of the present invention will be described.

[0093] The austenitic Fe-Cr-Ni alloy of the present invention is prepared by melting raw materials such as iron scraps, stainless steel scraps, ferronickel, ferrochrome, etc. in an electric furnace, blowing a mixed gas of oxygen and inert gas in an AOD (Argon Oxygen Decarburization) furnace or a VOD (Vacuum Oxygen Decarbutization) furnace for decarburization and refining, adding quicklime, Fe-Si alloy, Al, etc., reducing the Cr oxide in the slag, adding fluorite, forming a CaO-SiO2-Al2O3-MgO-F system slag, deoxidizing and desulfurizing, and then adding a Ni-based alloy containing any one of La, Ce, and Y. The reason for using the CaO-SiO2-Al2O3-MgO-F system slag is that, as mentioned above, deoxidation and desulfurization can be effectively carried out, and when REM is added, REM can be effectively added without being oxidized or sulfurized. At this time, the CaO concentration in the slag is preferably in the range of 40 to 80%. That is, if it is less than 40%, the above-mentioned desulfurization reaction cannot proceed. If it is more than 80%, Ca is mixed in the steel in excess of 0.002%. In addition, the Al2O3 concentration is preferably less than 50%. The reason is that if the activity of alumina in the slag is not low, deoxidation is difficult, and desulfurization also becomes difficult. It is preferred that after refining, a slab is manufactured using a continuous casting machine, and then the above-mentioned steel billet (steel sheet) is hot rolled, or further cold rolled to form various steel products such as thin steel plates, thick steel plates, steel sections, steel bars, wire rods, etc. It is not limited to a continuous casting machine, and a steel billet can also be manufactured by an ingot casting-blowing rolling method.

[0094] Example

[0095] The raw materials obtained by adjusting scrap iron, ferrochrome, ferronickel, stainless steel chips, etc. to a specified ratio are melted in a 70-ton electric furnace, and a mixed gas of oxygen and inert gas (rare gas) is blown into an AOD furnace or a VOD furnace for decarburization and refining. Then, quicklime, Fe-Si alloy, Al, etc. are added to reduce the Cr oxides in the slag, and then fluorite is added to form a CaO-SiO2-Al2O3-MgO-F system slag for deoxidation and desulfurization. Then, a specified amount of any one or more of Ni-20% La, Ni-20% Ce and Ni-20% Y is added, and a cast billet (cast sheet) is made by continuous casting. After adjusting to the various component compositions shown in Table 1, continuous casting is carried out to make a steel billet (slab). The components shown in Table 1 are measured as follows.

[0096] (1) The composition of C and S was measured using a carbon-sulfur simultaneous analyzer (combustion in oxygen flow-infrared absorption method).

[0097] (2) The N composition was analyzed using an oxygen-nitrogen simultaneous analyzer (inert gas-pulse heating fusion method).

[0098] (3) The composition other than C, S, and N and the slag components were analyzed by the calibration curve method using fluorescent X-ray analysis.

[0099] [Table 1]

[0100]

[0101] Next, the above-mentioned steel billet (slab) was hot rolled to 8 mm, and cold rolled, heat treated and pickled repeatedly to produce a cold rolled coil with a thickness of 2 to 3 mm. The final annealing temperature was 1150°C for 1 minute. A test piece with a width of 20 mm, a length of 30 mm and a thickness of 2 mm was collected from the plate.

[0102] <High temperature oxidation test>

[0103] In order to evaluate the oxidation resistance under high temperature environment, the surface of the test piece was wet-polished with #320 sandpaper and the high vacuum atmosphere heat treatment furnace was evacuated to 5.0×10 -3 After the test piece was heated to 200 ° C, the oxidation test was repeated in a mixed gas atmosphere of 7% O2-16% H2O-10% CO2-0.5% CO-0.1% NO2-bal.N2, with the temperature adjusted at 900 ° C for 10 minutes at a cooling rate of 40 ° C / min, and then at 900 ° C for 10 minutes at a heating rate of 40 ° C / min, and then at room temperature for 20 minutes. The value obtained by dividing the mass change after removing the peeled tare weight by the surface area before the test (mg / cm 2 ) was evaluated as oxidation weight loss. The oxidation weight loss was less than 50 mg / cm 2 The test piece was judged to have good oxidation resistance (○), and 50 mg / cm 2 The above test pieces were judged to be poor in oxidation resistance (×). At the same time, cross-sectional microstructure observation after the test was performed to measure the thickness of the surface oxide scale and the area ratio of the inner oxide layer formed directly below.

[0104] [Table 2]

[0105]

[0106] The steel sheets No. 1 to 17 shown in Tables 1 and 2 are invention examples satisfying the conditions of the present invention and have excellent oxidation resistance. On the other hand, the steel sheets No. 18 to 37 are comparative examples.

[0107] The steel sheet No. 18 does not satisfy the formula (2) because of its low Al content, and the alumina concentration in the slag is as high as 54%, which weakens the deoxidation. As a result, the oxygen concentration increases, and elements useful for oxidation resistance form oxides, resulting in poor oxidation resistance. In addition, since the C content is also high, inclusions that cause surface defects are formed, resulting in poor surface quality.

[0108] In the steel sheet No. 19, since the Si content was high, surface defects due to intermetallic compounds such as σ phase occurred. Furthermore, since the Mn content was high, the formula (2) was not satisfied and the oxidation resistance was poor.

[0109] The steel sheet No. 20 does not satisfy the formula (1) because of its low Ni content. In addition, due to its high Al content, a large amount of carbonitrides are formed, which are the cause of surface defects, and the surface quality is poor. In addition, the CaO concentration in the slag is as high as 85%, and the oxygen concentration is too low, resulting in a high Ca concentration, a large amount of Ca-Al oxide inclusions are formed, and the effect of Al is lost.

[0110] The steel plate No. 21 does not satisfy the formula (1) due to its high B content, and has poor oxidation resistance. In addition, since the Al2O3 concentration in the slag is as high as 52% and the CaO concentration is as low as 28%, the O content is high, and alumina inclusions adhere to the submerged nozzle, and their fall off increases the number of surface defects.

[0111] The steel sheet No. 22 has a high Cr content and therefore does not satisfy the formula (1). The surface oxide scale grows excessively to form a skin with poor adhesion, which deteriorates the oxidation resistance. In addition, the surface quality is poor due to the generation of Cr nitrides that cause surface defects.

[0112] The steel sheet No. 23 did not satisfy the formula (1) because of the high Ti content, and carbonitrides that caused surface defects were formed, resulting in poor surface quality.

[0113] The steel sheet No. 24 does not satisfy the formula (3) because of the low REM content, and cannot fully obtain the effect of improving the oxidation resistance or the effect of fixing S that hinders the oxidation resistance as inclusions.

[0114] The steel sheet No. 25 did not satisfy the formula (1) because of its high Zr content, and had surface defects due to the formation of a large amount of carbonitrides, resulting in poor surface quality.

[0115] The steel sheet of No. 26 does not satisfy the formula (1). In addition, due to the high Mo content, the skin adhesion is reduced and the oxidation resistance is poor.

[0116] The steel sheet No. 27 has a low Cr content and does not satisfy equations (1) and (2), so it has poor oxidation resistance. In addition, since it does not satisfy equation (3), it cannot fully obtain the effect of improving oxidation resistance or the effect of fixing S that hinders oxidation resistance in the form of inclusions.

[0117] The steel plate No. 28 does not satisfy equations (1) and (2) due to its high REM content. In addition, the hot workability and weldability are reduced, causing clogging of the submerged nozzle during continuous casting, significantly deteriorating the manufacturability, and thus poor in manufacturability.

[0118] In the steel sheet No. 29, the CaO concentration in the slag is as low as 35%, so the desulfurization reaction cannot proceed. Therefore, the S content becomes high, and the equations (1) and (2) are not satisfied, a large number of inclusions are formed, and the Cr required for the formation of the surface oxide scale is consumed, so the oxidation resistance is poor.

[0119] The steel sheet No. 30 does not satisfy the formula (1) because of the high Ni content, resulting in deterioration of hot workability or increase in hot deformation resistance, and increased manufacturing cost, so that the expected cost cannot be achieved. In addition, due to the low Mn content, the effects of deoxidation and austenite stabilization cannot be fully obtained. In addition, since the formula (3) is not satisfied, the effect of improving oxidation resistance or the effect of fixing S that hinders oxidation resistance in the form of inclusions cannot be fully obtained.

[0120] The steel sheet No. 31 does not satisfy the formula (1), and the CaO concentration in the slag is as high as 83%, so the Ca content is high. A large amount of Ca-Al oxide inclusions are formed, Al in the alloy is consumed, and the effective Al is reduced, so the oxidation resistance is poor.

[0121] The steel sheet No. 32 did not satisfy the formula (2) because of the high N content, and Cr, Al, and Ti contributing to oxidation resistance precipitated as nitrides, and a sufficient internal oxide layer could not be formed, resulting in poor oxidation resistance.

[0122] The steel sheet No. 33 did not satisfy the formula (1) because of the high contents of Ti and Zr, and a large amount of carbonitrides that caused surface defects were formed, resulting in poor surface quality.

[0123] The steel sheet No. 34 does not satisfy the equations (1) and (2) because the contents of Ti and Zr are low, and the effects of Ti and Zr that contribute to oxidation resistance cannot be fully obtained, resulting in poor oxidation resistance.

[0124] Although No. 35 satisfies the composition ranges of the elements, it does not satisfy the formulas (1), (2), and (3), and therefore does not have sufficient oxidation resistance.

[0125] No. 36 satisfies the composition ranges of the elements, but does not satisfy the equations (2) and (3), and therefore does not have sufficient oxidation resistance.

[0126] No. 37 satisfies the composition ranges of the elements but does not satisfy the formula (1), and therefore does not have sufficient oxidation resistance.

[0127] Industrial Applicability

[0128] The austenitic Fe-Ni-Cr alloy of the present invention has excellent heat resistance in addition to the oxidation resistance in the above-mentioned high temperature environment, and can therefore be suitably used in high temperature environments such as heat exchangers and combustion parts.

[0129] Explanation of symbols

[0130] 1: Surface oxide layer, 2: Internal oxide layer, 3: Interface.

Claims

1. Austenitic Fe-Ni-Cr alloy, characterized in that: The composition is composed of the following ingredients: in 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 (Ti: 0.10-1.0%, Zr: 0.01-0.6%), O：0.0002～0.0030％、 Ca: ≤0.002%, Any one of La, Ce, and Y as rare earth elements (REM) or the total weight of two or more thereof: 0.001 to 0.010%, The balance is composed of Fe and inevitable impurities; and the content satisfies the following formula (1) and formula (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 represents the content (mass %) of each element.

2. The austenitic Fe-Ni-Cr alloy according to claim 1, characterized in that: The composition is such that the total weight of any one or two or more of La, Ce, and Y as rare earth elements (REM) satisfies the following formula (3): 3.2≤REM(La,Ce,Y) / S…(3), Here, each element symbol in the above formula represents the content (mass %) of each element.

3. The austenitic Fe-Ni-Cr alloy according to claim 1, characterized in that: The composition of the surface oxide scale formed in a repeated cycle test from room temperature to 700-900°C in a mixed gas atmosphere composed of 7% O2-16% H2O-10% CO2-0.5% CO-0.1% NO2-bal.N2 contains, in mass%, Cr: more than 40%, Fe: 10-20%, Ni: 0-10%, O: 10-40%, REM: 0.05-0.5%, and the remainder contains Mn, Si, and Ti as unavoidable elements.

4. The austenitic Fe-Ni-Cr alloy according to claim 3, characterized in that: The surface oxide scale has a thickness of 10 to 100 μm.

5. The austenitic Fe-Ni-Cr alloy according to claim 3, characterized in that: The inner oxide layer formed directly below the surface oxide scale is composed of an inner oxide containing at least one of Cr, Si, Mn, Al, Ti, and REM, and the area ratio of the inner oxide layer is 0.005 mm2 per 0.005 mm2 directly below the surface oxide scale. 2 More than 30% of them.

6. A method for producing an austenitic Fe-Ni-Cr alloy, the method being the method for producing an austenitic Fe-Ni-Cr alloy according to any one of claims 1 to 5, characterized in that: The alloy composition is adjusted by refining the alloy raw materials after melting. During the refining, a mixed gas of oxygen and argon is blown into the molten alloy raw materials (molten alloy) to perform decarburization, and Cr reduction is performed after the nitrogen concentration is controlled to be below 0.03%. Then, aluminum, limestone and fluorite are added to the molten alloy to form a CaO-SiO2-Al2O3-MgO-F system slag, so that the oxygen concentration in the molten alloy is 0.0002~0.0030 mass%. Then, raw materials containing any one or more of La, Ce, and Y are added and cast to obtain a slab, which is supplied to a hot rolling process.

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