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

Abstract

Provided is an austenitic Ni-Cr-Fe alloy comprising a component composition containing, in mass%, 0.01 to 0.10% of C, 0.02 to 0.45% of Si, 0.10 to 0.85% of Mn, less than or equal to 0.015% of P, less than or equal to 0.0015% of S, 13.0 to 18.0% of Cr, 5.0 to 11.0% of Fe, less than or equal to 0.6% of Mo, less than or equal to 0.5% of Cu, less than or equal to 0.005% of B, 0.01 to 0.3% of Al, 0.01 to 0.3% of Ti, less than or equal to 1.00% of Co, less than or equal to 1.00% of Nb, less than or equal to 0.05% of Ta, less than or equal to 0.01% of N, 0.0002 to 0.0040% of O, less than or equal to 0.002% of Ca, 0.001 to 0.010%

Description

Technical Field

[0001] The present invention relates to an austenitic Ni-Cr-Fe alloy, and more particularly to an austenitic Ni-Cr-Fe alloy having excellent oxidation resistance in a high-temperature environment. Background Art

[0002] Since afterburner components, nuclear power generation equipment, and industrial heat treatment furnaces are used in a severe high-temperature environment of 1000 to 1200°C, the materials applied to them are required to have excellent high-temperature strength, high-temperature corrosion resistance, and oxidation resistance. In particular, regarding oxidation resistance, as one of the required characteristics, it can be cited that the protective surface oxide scale mainly composed of Cr2O3 formed on the material surface in the above high-temperature environment is dense and has a high adhesion to the material. As materials used in equipment and the like used in the above high-temperature environment, for example, SUS310S, NCF800, NCF600, etc. are used.

[0003] As a technique for improving the high-temperature characteristics of materials used in such a severe use environment, for example, Patent Document 1 proposes a Ni-based alloy and a manufacturing method thereof, in which the composition, thickness, and crystal grain size of the surface oxide scale formed on the alloy surface are appropriately controlled by performing heat treatment in a specified atmosphere, thereby improving the surface oxide scale characteristics. In addition, Patent Document 2 proposes a Ni-Cr-Fe alloy having excellent creep strength and stress relaxation crack resistance due to the combined addition of Ti, Al, and REM (rare earth metal).

[0004] However, in the technique disclosed in Patent Document 1, no study has been conducted on the influence of S that can form a compound with Cr in the material that is the main component of the surface oxide scale, or the influence of B, Mo, N, and Mn that have a negative effect on the adhesion of the surface oxide scale. It is considered that the application in the above high-temperature environment that requires excellent oxidation resistance is insufficient.

[0005] In addition, in the technique disclosed in Patent Document 2, this alloy is produced by adjusting the composition in a high-frequency induction furnace in the laboratory, obtaining an ingot, and subjecting it to a hot rolling process. In any case, mass production on a scale of 60 tons or more cannot be carried out. In addition, regarding the statement that all of REM effectively act, in the examples, the addition of Nd is the main body, and Ce, La, and Y are added to a part of the alloys. The object of REM is limited. In addition, there is no process for removing S and O during the manufacturing process, and it cannot be achieved without carefully selecting raw materials. Therefore, the added REM is lost due to oxidation and sulfidation in some cases, and it is a proposal that is industrially inefficient and difficult to achieve the original purpose. Therefore, it is difficult to say that an alloy having improved creep strength by adding REM can be reliably provided on an industrial scale.

[0006] Prior Art Documents

[0007] Patent Document

[0008] Patent Document 1: Japanese Patent Laid-Open No. 2002-121630

[0009] Patent Document 2: Japanese Re-Published Patent No. 2018-066579 Summary of the Invention

[0010] Problems to be Solved by the Invention

[0011] The present invention has been completed in view of the above circumstances, and an object thereof is to provide an austenitic Ni-Cr-Fe alloy having excellent oxidation resistance even in a severe high-temperature environment.

[0012] Means for Solving the Problems

[0013] The inventors conducted in-depth research repeatedly to solve the above problems. So far, although it has been found that by adding La, Ce, and Y as REMs, the adhesion of the surface scale formed on the alloy surface in a high-temperature environment can be improved, there is no sufficient understanding of the contribution to the oxidation resistance evaluated by a cyclic test in which the temperature is repeatedly cycled from room temperature to 1000 to 1200 °C in a mixed gas atmosphere composed of 5% O2 - 16% H2O - 12% CO2 - 0.8% CO - 0.1% NO2 - balance N2. Therefore, the correlation between La, Ce, Y and other elements contained in the alloy was investigated in detail. The results showed that in order to improve oxidation resistance, it is extremely effective to add La, Ce, and Y. As other elements, Si, Cr, Al, and Ti are also effective. On the other hand, it was clarified that the presence of S, Mn, Mo, B, and N hinders the above-mentioned effect of improving oxidation resistance. From this, it was found that in order to fully ensure the effect of adding REMs, it is necessary to control Si, Ni, Cr, Al, Ti, S, Mn, Mo, B, and N.

[0014] Furthermore, it was found that there is a good correlation between the value obtained by dividing the total weight (mass%) of any one or more of La, Ce, and Y as REMs contained in the alloy element alloy by the content (mass%) of S contained in the alloy and the oxidation weight loss in the high-temperature oxidation test, and it was clarified that the characteristic formula obtained therefrom is 4.5 ≤ REM / S.

[0015] In addition, through repeated research focusing on the morphology of the surface scale formed in a high-temperature environment, it was found that when the thickness of the surface scale formed in the cyclic test repeatedly carried out from room temperature to 1000 - 1200 °C in a mixed gas atmosphere composed of 5% O2 - 16% H2O - 12% CO2 - 0.8% CO - 0.1% NO2 - bal N2 is in the range of 6 - 60 μm, the surface scale is formed densely with excellent adhesion, and good results are also shown in terms of the oxidation weight loss after the test.

[0016] That is, the austenitic Ni - Cr - Fe alloy of the present invention is characterized in that it is composed of the following components. The component composition contains, by mass%, C: 0.01 - 0.10%, Si: 0.02 - 0.45%, Mn: 0.10 - 0.85%, P: ≤0.015%, S: ≤0.0015%, Cr: 13.0 - 18.0%, Fe: 5.0 - 11.0%, Mo: ≤0.6%, Cu: ≤0.5%, B: ≤0.005%, Al: 0.01 - 0.3%, Ti: 0.01 - 0.3%, Co: ≤1.00%, Nb: ≤1.00%, Ta: ≤0.05%, N: ≤0.01%. Further, it contains O: 0.0002 - 0.0040%, Ca: ≤0.002%, and the total weight of any one or more of La, Ce, and Y as rare earth elements (REM): 0.001 - 0.010%. The balance is composed of Ni and inevitable impurities, and satisfies the following formula (1):

[0017] 60 ≥ 7.4Si + 2.8×Cr + 6.9×Al + 3.2×Ti + 3374×REM - 5914×S - 3.87×Mn - 42.3×Mo - 3072×B - 1024×N ≥ 30…(1)

[0018] Here, each element symbol in the above formula represents the content (mass%) of each element.

[0019] In addition, a preferred embodiment of the austenitic Ni - Cr - Fe alloy of the present invention is that any one or more of La, Ce, and Y as the above-mentioned rare earth elements (REM) satisfy the following formula (2):

[0020] 4.5 ≤ REM(La, Ce, Y) / S…(2)

[0021] Here, each element symbol in the above formula represents the content (mass%) of each element.

[0022] In the austenitic Ni-Cr-Fe alloy of the present invention, a preferred embodiment is that, in addition to the above-described composition, the composition of the surface scale formed by repeated cycle tests from room temperature to 1000 to 1200 °C in a mixed gas atmosphere composed of 5% O2 - 16% H2O - 12% CO2 - 0.8% CO - 0.1% NO2 - the balance N2 contains, by mass%, Cr: 50% or more, Fe: 0 to 10%, Ni: 0 to 5%, O: 10 to 40%, REM: 0.05 to 0.5%, and the balance contains Mn, Si, Al, and Ti as inevitable elements; further, a preferred embodiment is that the scale thickness is 6 to 60 μm.

[0023] In addition, the present invention also provides a method for manufacturing the above-described austenitic Ni-Cr-Fe alloy. That is, the alloy composition is adjusted by refining after melting the alloy raw materials. In the refining, a mixed gas of oxygen and argon is blown into the molten alloy raw materials (molten alloy) for decarburization. After controlling the nitrogen concentration to 0.01% or less, Cr reduction is carried out. Then, aluminum, limestone, and fluorite are added to the molten alloy to form a CaO-SiO2-Al2O3-MgO-F-based slag, and the oxygen concentration in the molten alloy is made 0.0002 to 0.0040 mass%. Then, a raw material containing any one or two or more of La, Ce, and Y is added to adjust the composition, and casting is carried out. After obtaining a slab, it is supplied to a hot rolling process. In the hot rolling process, for example, a coil is manufactured. In addition, a preferred embodiment is that the composition of the CaO-SiO2-Al2O3-MgO-F-based slag is CaO: 55 to 70%, SiO2: 10% or less, and F: 10% or less.

[0024] Effects of the Invention

[0025] According to the present invention, the austenitic Ni-Cr-Fe alloy has excellent oxidation resistance in a high-temperature environment, which can greatly contribute to the high life of the product. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 Schematic diagram for explaining the surface structure of an austenitic Ni-Cr-Fe alloy plate according to an embodiment of the present invention in a high-temperature oxidation test. DETAILED DESCRIPTION OF THE INVENTION

[0027] The composition components that the austenitic Ni-Cr-Fe alloy of the present invention should have will be described below.

[0028] C: 0.01 to 0.10 mass %

[0029] ​C is an element that contributes to the stabilization of the austenite phase. However, when added in large amounts, it combines with Cr, Mo, etc. to form carbides, and the amount of solid-solution Cr in the vicinity thereof decreases, reducing the oxidation resistance. On the other hand, C has the effect of increasing the alloy strength by solid-solution strengthening, so the lower limit is set at 0.01% by mass. Therefore, C is limited to 0.01 to 0.10% by mass. Preferably, it is 0.02 to 0.08% by mass, and more preferably 0.03 to 0.07% by mass.

[0030] Si: 0.02 to 0.45 mass %

[0031] Si is an element effective in improving the oxidation resistance and preventing the peeling of the oxide film. In addition, in order to control the oxygen concentration to 0.0002 to 0.0040%, 0.02% is required. Furthermore, it has the effect of reducing CaO in the CaO-SiO2-MgO-Al2O3-F-based slag and adjusting the Ca in the melt to 0.0020% or less. From this viewpoint, 0.02% is also required. However, excessive addition of Si promotes the precipitation of intermetallic compounds such as σ phase, causing surface defects caused by intermetallic compounds, so it is set at 0.02 to 0.45% by mass. Preferably, it is 0.04 to 0.30% by mass or less, and more preferably 0.06 to 0.20% by mass or less.

[0032] Mn: 0.10 to 0.85 mass %

[0033] Mn is an austenite phase stabilizing element and also an element having a deoxidizing effect. Therefore, in order to obtain its effect, it needs to be at least 0.10% by mass or more. However, Mn also causes the precipitation of intermetallic compounds such as σ phase like Si, and in addition, it reduces the oxidation resistance due to the formation of Cr-Mn-based oxides. Therefore, it is not preferred to add more than necessary. Therefore, it needs to be 0.10 to 0.85% by mass. Preferably, it is 0.15 to 0.70% by mass, and more preferably 0.20 to 0.60% by mass.

[0034] P: 0.015 mass % or less

[0035] P is an element that is inevitably mixed in as an impurity. Since it segregates at the crystal grain boundaries as phosphide, it is an element that impairs the hot workability. Therefore, it is desirable to reduce it as much as possible. However, extremely reducing the content of P will increase the manufacturing cost. Therefore, in the present invention, P is limited to 0.015% by mass or less. Preferably, it is 0.010% by mass or less, and more preferably 0.006% by mass or less.

[0036] S: 0.0015 mass % or less

[0037] S, like P, is an element inevitably mixed in as an impurity, and is easy to segregate at the crystal grain boundary, especially significantly impairing hot workability. In addition, by forming a compound with Cr, which contributes to oxidation resistance described later, the Cr required for forming the surface oxide scale is consumed, and the adhesion between the oxide film and the base material is reduced, thereby causing the oxide film to fall off and promoting oxidation, so it is an element harmful to oxidation resistance. If it contains more than 0.0015% by mass, its harmfulness is significantly manifested, so it is necessary to control it below 0.0015% by mass. It is preferably below 0.0010% by mass, and more preferably below 0.0006% by mass. As described later, the reduction of S can be reduced by the reaction between the addition of Al and the slag component.

[0038] Cr: 13.0 to 18.0 mass %

[0039] Cr is an element that helps to inhibit corrosion in high temperature environments. In addition, it forms a protective oxide film on the surface of the alloy in a high temperature environment, 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 13.0 mass%. However, excessive addition of Cr will excessively form surface oxide scales, but lack of adhesion and deteriorate oxidation resistance. In addition, the stability of the austenite phase will be reduced, so it is set to 13.0-18.0 mass%. Preferably, it is 14.0-17.5 mass%, and more preferably 15.0-17.0 mass%.

[0040] Fe: 5.0 to 11.0 mass %

[0041] Fe is an element that affects hot workability and cold workability. If it is less than 5.0%, the hot workability and cold workability will be reduced. In addition, if it is added over 11.0%, there is a risk of damaging corrosion resistance. Therefore, the range of Fe content is specified to be 5.0 to 11.0 mass. Preferably, it is 6.0 to 10.5%. More preferably, it is 8.0 to 10.0%.

[0042] Mo: 0.6 mass % or less

[0043] 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, when a material with a large amount of Mo is added under high temperature conditions and when the surface oxygen potential is low, Mo is preferentially oxidized and causes the peeling of oxide scale, which has an adverse effect. Therefore, from the viewpoint of ensuring the adhesion of the protective surface oxide scale, Mo is limited to less than 0.6 mass %. Preferably, it is less than 0.4 mass %, and more preferably, it is less than 0.2 mass %.

[0044] Cu: 0.5 mass % or less

[0045] Cu is sometimes added as an element to improve the corrosion resistance in a humid environment, but in a high-temperature environment as in the present invention, its effect is hardly seen. On the other hand, excessive addition will form an uneven film with a mottled pattern on the material surface, reducing the corrosion resistance. Therefore, the addition amount of Cu is limited to 0.5% by mass or less. It is preferably 0.3% by mass or less, and more preferably 0.1% by mass or less.

[0046] B: 0.005 mass % or less

[0047] B has the effect of assisting rare earth elements (REM) through grain boundary segregation and is an element that also contributes to high-temperature strength. However, a large amount of addition will cause a decrease in the adhesion due to the porousness of the surface scale or a decrease in the weldability and hot workability of the alloy. In the present invention, the content of B is set to 0.005% by mass or less. It is preferably 0.004% by mass or less, and more preferably 0.002% by mass or less.

[0048] Al: 0.01 to 0.3 mass %

[0049] Al is an element that promotes the formation of a dense film and improves the oxidation resistance, and its effect can be obtained by adding 0.01% by 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 0.0002 to 0.0040% by mass according to formula (a).

[0050] 2 Al +3O=(Al2O3)…(a)

[0051] The underlined part represents the elements in the molten steel, and the content in the parentheses represents the components in the slag.

[0052] By using a CaO-SiO2-Al2O3-MgO-F-based slag during the refining of the alloy of the present invention, the generated Al2O3 can be effectively absorbed to control the oxygen concentration. In addition, by performing deoxidation, according to formula (b), the S concentration in the molten steel is also reduced.

[0053] 2 Al +3 S +3(CaO)=3(CaS)+(Al2O3)…(b)

[0054] Thus, the S concentration can be controlled within the range of the present invention, that is, 0.0015% by mass or less. Therefore, Al needs to be 0.01% by mass or more. However, excessive addition will cause formula (c) to shift significantly to the right, the Ca concentration to exceed 0.002% by mass, and a large amount of Ca-Al oxide-based inclusions to be formed, consuming Al in the alloy and thus reducing the oxidation resistance.

[0055] 3(CaO)+2 Al =3Ca +(Al2O3)…(c)

[0056] Therefore, the upper limit of Al is set to 0.3 mass%. It is preferably 0.01 to 0.2 mass%, more preferably 0.01 to 0.1 mass%.

[0057] Ti: 0.01 to 0.3 mass %

[0058] Ti is an element that promotes the formation of a dense film and improves oxidation resistance, and its effect can be obtained by adding 0.01 mass% or more. However, excessive addition will cause surface defects due to the formation of a large amount of carbonitrides (TiN, TiC, TiCN). Therefore, the upper limit of Ti is set to 0.3 mass%. It is preferably 0.01 to 0.2 mass%, more preferably 0.01 to 0.1 mass%. In addition, controlling the C and N concentrations within the scope of the present invention is also an effective means of suppressing carbonitrides.

[0059] Co: 1.00 mass % or less

[0060] Co, like C and N, is an element effective in stabilizing the austenite phase. In addition, C and N form carbonitrides with Al, Ti, etc., which become the cause of surface defects, so they cannot be added in large amounts. However, Co does not form carbonitrides, so it is advantageous. However, adding a large amount will increase the raw material cost. Therefore, the content of Co is set to 1.00 mass% or less. It is preferably 0.90 mass% or less, more preferably 0.80 mass% or less.

[0061] Nb: 1.00 mass % or less

[0062] Nb forms fine carbides and carbonitrides with C and N, and has the effect of improving the high-temperature strength of the alloy. However, excessive addition will cause a large amount of precipitation of carbides and carbonitrides, and the ductility and toughness will instead decrease. Therefore, the addition amount of Nb is limited to 1.00 mass% or less. It is preferably 0.90 mass% or less, more preferably 0.80 mass% or less.

[0063] Ta: 0.05 mass % or less

[0064] Ta, like Nb, forms fine carbides and carbonitrides and has the effect of improving the high-temperature strength of the alloy. However, excessive addition will cause a large amount of precipitation of carbides and carbonitrides, and as a result, the ductility and toughness will instead decrease. Therefore, the content of Ta is set to 0.05 mass% or less. It is preferably 0.04 mass% or less, more preferably 0.03 mass% or less.

[0065] N: 0.01 mass % or less

[0066] N is an element that inevitably mixes in as an impurity, but it is also an austenite-phase forming element, and thus contributes to the stabilization of the structure. However, in the case of adding Al, Ti, Zr, etc. as in the present invention, N combines with these elements to precipitate nitrides, and in addition, the hot deformation resistance also increases greatly, impairing the hot workability. In addition, by forming the above-mentioned nitrides, Al and Ti, which are elements that contribute to improving the denseness of the surface scale, are consumed, thus reducing the oxidation resistance. Therefore, in the present invention, the content of N is set to 0.01% by mass or less. Preferably it is 0.009% by mass or less, and more preferably 0.008% by mass or less.

[0067] During decarburization, oxygen blowing refining is carried out, and at this time, N is transferred to the CO gas bubbles in the form of nitrogen gas and removed from the system, whereby it can be controlled within the scope of the present invention.

[0068] O: 0.0002 to 0.0040 mass %

[0069] O in the alloy combines with Al, Ti, Si, La, Ce, Y in the molten steel to form oxides, which thus become the cause of impairing the useful effects, oxidation resistance, etc. of these elements. In addition, a large amount of alumina-based oxide-based non-metallic inclusions are formed and adhere to the immersion nozzle through which the molten steel is injected from the tundish of the continuous caster into the mold, and they become the cause of surface defects due to falling off. Thus, the oxygen concentration is preferably as low as 0.0040% by mass or less. In order to achieve this range, it is only necessary to deoxidize by controlling Al to the concentration of the present invention as described above. On the other hand, if the O in the alloy is excessively reduced, according to formula (c), the Ca concentration rises to exceed 0.002% by mass. Thus, the lower limit is set to 0.0002% by mass. Preferably it is 0.0003 - 0.0035% by mass, and more preferably 0.0005 - 0.0030% by mass.

[0070] Ca: 0.002 mass % or less

[0071] Ca is an element that mixes into the alloy of the present invention from CaO in the slag as described above. Ca forms a large amount of Ca - Al oxide-based inclusions, and reduces the oxidation resistance by consuming Al in the alloy, so it is necessary to suppress it to a low level. Therefore, it is necessary to control the Al concentration to 0.01 - 0.3% by mass and make the oxygen concentration 0.0002 - 0.0040% by mass. Thus, Ca needs to be 0.002% by mass or less.

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

[0073] REM (La, Ce, Y) has the effect of improving the hot workability of the alloy, the adhesion between the surface scale and the base metal surface, and the oxidation resistance. Even a trace amount can achieve a significant effect. In addition, by forming a compound with S dissolved in the alloy, it is expected to inhibit the formation of a compound between Cr, which is a constituent element of the surface scale, and S, and prevent the local reduction of the Cr content. In addition, REM is generally used as a raw material in the form of a rare earth mixture, which is an alloy containing multiple REMs, but there are also cases where a Ni-Fe alloy containing any one of REMs is used. However, excessive addition will reduce the hot workability and weldability of the alloy, excessively form REM-based inclusions, and thereby reduce the adhesion of the surface scale. In addition, it causes clogging of the submerged nozzle during continuous casting, and the manufacturability deteriorates significantly. Therefore, in the present invention, the addition amount of REM is set to 0.001 to 0.010% by mass. Preferably, it is 0.002 to 0.009% by mass, and more preferably, it is 0.003 to 0.008% by mass.

[0074] 60 ≥ 7.4Si + 2.8×Cr + 6.9×Al + 3.2×Ti + 3374×REM - 5914×S - 3.87×Mn - 42.3× Mo - 3072×B - 1024×N ≥ 30…(1)

[0075] Formula (1) is as follows: In the oxidation resistance of an austenitic Ni-Cr-Fe alloy, for the elements that affect the surface scale formed on the alloy surface, the degree of influence is expressed in the form of an arithmetic formula through multiple regression analysis. Si, Cr, Al, Ti, REM (La, Ce, Y) improve the oxidation resistance evaluated by a cyclic test that repeats room temperature and a high temperature of about 1000 to 1200 °C in a mixed gas atmosphere composed of 5% O2 - 16% H2O - 12% CO2 - 0.8% CO - 0.1% NO2 - balance N2. On the other hand, S promotes oxidation by reducing the adhesion between the oxide film and the base metal, causing the oxide film to peel off. Mn reduces the oxidation resistance by forming Cr-Mn-based oxides. When the Mo content is high, in a high-temperature environment with a low oxygen potential on the surface, Mo preferentially oxidizes and causes the peeling of the scale. In addition, when the B content is high, since the oxide scale of the alloy is porous, the oxidation rate at high temperature increases, promoting the growth and peeling of the oxide scale. N forms AlN and TiN with Al and Ti, which are helpful for improving the oxidation resistance, respectively, reducing the effects of Al and Ti. In addition, excessive addition of alloying elements that help improve the oxidation resistance is not preferred because the adhesion is reduced due to the excessive growth of the surface scale, and a large amount of inclusions that cause surface defects are generated. Therefore, for these elements, based on Formula (1), the lower limit is set to 30 or more, and the upper limit is set to 60 or less. Preferably, it is 31 or more and 59 or less, and more preferably, it is 32 or more and 58 or less.

[0076] 4.5 ≤ REM / S…(2)

[0077] As an index for sufficiently obtaining the effects of improving the hot workability and oxidation resistance of an alloy, the relationship between the content of rare earth elements (REM) and S that forms a compound can obtain the above effects by satisfying 4.5 ≤ REM / S and having a REM content sufficient to fix S in the form of inclusions. On the other hand, if it is less than 4.5, the effect of REM cannot be obtained sufficiently, so it is not preferred.

[0078] Definition of surface scale

[0079] As Figure 1 shown, the austenitic Ni - Cr - Fe alloy plate according to an embodiment of the present invention uses an austenitic Ni - Cr - Fe alloy having a composition satisfying the above formulas (1) and (2) as the matrix BM. In a cyclic test repeatedly from room temperature to 1000 - 1200 °C in a mixed gas atmosphere composed of 5% O2 - 16% H2O - 12% CO2 - 0.8% CO - 0.1% NO2 - balance N2, an oxide scale mainly composed of Cr oxide is formed on the surface of the austenitic Ni - Cr - Fe alloy base material of the present invention. At this time, the thickness of the surface oxide scale in the same figure represents the region LE from the outermost layer of the surface oxide scale to the oxide scale / alloy interface in the cross-sectional microstructure observation after the above test.

[0080] The surface scale formed by repeating the cycle test from room temperature to 1000 - 1200 °C in a mixed gas atmosphere composed of 5% O2 - 16% H2O - 12% CO2 - 0.8% CO - 0.1% NO2 - balance N2 has a thickness of 6 - 60 μm CaO: 55 - 70%

[0081] In the austenitic Ni - Cr - Fe alloy of the present invention, in a cyclic test repeatedly from room temperature to 1000 - 1200 °C in a mixed gas atmosphere composed of 5% O2 - 16% H2O - 12% CO2 - 0.8% CO - 0.1% NO2 - balance N2, an oxide scale mainly composed of Cr oxide is formed on the surface of the alloy base material, and oxidation resistance in the above high-temperature environment is obtained. At this time, when the thickness of the surface oxide scale is less than 60 μm, sufficient oxidation resistance cannot be obtained. On the other hand, if it exceeds 60 μm, the peelability of the surface oxide scale increases, damaging the adhesion between the surface oxide scale and the base material surface. Thus, the protective surface oxide scale formed in the above high-temperature environment needs to have a thickness of 6 - 60 μm. Preferably, it is 8 - 55 μm, and more preferably 10 - 50 μm.

[0082] The determination method of the limiting formula of the above formula (1) is as follows.

[0083] Alloys with Ni-17% Cr-9% Fe as the basic composition and with the addition amounts of Si, Cr, Al, Ti, La, Ce, Y, B, Mn, Mo, S, and N changed were melted in a vacuum melting furnace. After hot forging, a hot-forged plate with a thickness of 8 mm and a width of 80 mm was produced. The obtained hot-forged plate was solution heat-treated under the condition of 1200 °C × 10 minutes. After surface grinding, it was cold-rolled to a thickness of 2 mm and then solution heat-treated under the condition of 1150 °C × 1 minute. Then, it was cut into pieces of 20 mm × 30 mm, and the surface was finish-machined with wet grinding #320 to make test pieces. The obtained test pieces were subjected to a repeated oxidation test in a mixed gas atmosphere composed of 5% O2-16% H2O-12% CO2-0.8% CO-0.1% NO2- the balance N2 with cycles of 1200 °C × 10 minutes, 1000 °C × 10 minutes, 1200 °C × 10 minutes, and room temperature × 20 minutes. For the test pieces after 200 cycles, evaluation was carried out using the value obtained by dividing the mass change after removing the peeled oxide scale weight by the surface area before the test.

[0084] From the above test results, it can be seen that the influence degree of the added elements on the oxidation resistance of the austenitic Ni-Cr-Fe alloy becomes obvious. The relational expression of the composition represented by formula (1) was obtained through multiple regression analysis. By setting it to be 30 or more and 60 or less, it has sufficient oxidation resistance.

[0085] Next, the manufacturing method of the austenitic Ni-Cr-Fe alloy of the present invention will be described.

[0086] Regarding the austenitic Ni-Cr-Fe alloy of the present invention, raw materials such as iron scraps, stainless steel scraps, ferro-nickel alloy, and ferro-chromium alloy are melted in an electric furnace, and decarburization refining is carried out by blowing a mixed gas of oxygen and rare gas using an AOD (Argon Oxygen Decarburization) furnace or a VOD (Vacuum Oxygen Decarburization) furnace. After adding quicklime, Fe-Si alloy, Al, etc. to reduce the Cr oxide in the slag, fluorite is added to form a CaO-SiO2-Al2O3-MgO-F-based slag for deoxidation and desulfurization. Then, a Ni-based alloy containing any one of La, Ce, and Y is added. The reason for using the CaO-SiO2-Al2O3-MgO-F-based slag is that, as described above, deoxidation and desulfurization can be effectively carried out, and furthermore, when REM is added, REM can be effectively added without being oxidized or sulfided. At this time, the CaO concentration in the slag is preferably in the range of 40 to 80%. That is, if it is lower than 40%, the above-mentioned desulfurization reaction will not occur. If it is 80% or more, Ca will be mixed into the molten steel at a concentration exceeding 0.002%. In addition, the Al2O3 concentration is preferably 50% or less. The reason is that if the activity of alumina in the slag is not low, deoxidation is difficult to carry out, and furthermore, desulfurization becomes difficult. Preferably, after refining, a slab is manufactured using a continuous caster, and then the above-mentioned steel sheet is hot-rolled or further cold-rolled to produce various steel products such as thin steel plates, thick steel plates, sections, bars, and wire rods. It is not limited to a continuous caster, and steel sheets can also be made by the ingot-opening rolling method.

[0087] Here, although not particularly limited, the CaO-SiO2-Al2O3-MgO-F-based slag preferably has the following composition.

[0088] SiO2: 10% or less

[0089] If CaO is lower than 55%, deoxidation will not work and the oxygen concentration will increase beyond the scope of this application. On the other hand, if it is higher than 70%, the Ca concentration in the molten steel will increase beyond the scope of this application. Therefore, this range is preferred.

[0090] F: 10% or less

[0091] If SiO2 is higher than 10%, deoxidation cannot be carried out. As a result, the oxygen concentration will increase beyond the scope of the present invention of this application. Therefore, SiO2 should be 10% or less.

[0092] ​

[0093] Fluorine is important for melting the slag. As a fluorine source, fluorite is usually added. If it is higher than 10%, it will not only cause the brick to melt and shorten the brick life, but also be a cause of liquid leakage. Therefore, F should be suppressed below 10%.

[0094] Examples

[0095] Raw materials such as scrap iron, ferrochrome alloy, ferronickel alloy, and stainless steel chips that have been adjusted to a specified ratio are melted in an electric furnace with a scale of 70 tons, and decarburization refining is carried out by blowing a mixed gas of oxygen and rare gas with an AOD furnace or a VOD furnace. Then, after adding quicklime, Fe-Si alloy, Al, etc. to reduce the Cr oxide in the slag, fluorite is added to form a CaO-SiO2-A12O3-MgO-F-based 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 ingot casting is carried out by continuous casting. After adjusting to various component compositions shown in Table 1, continuous casting is carried out to make steel sheets (slabs). The respective components shown in Table 1 are measured as follows. In the table, items that do not meet the scope of the present invention are enclosed in parentheses. It should be noted that although the items enclosed in parentheses in the inventive examples do not meet the scope of the dependent claims, they meet the scope of the independent claims.

[0096] (1) The compositions of C and S are measured using a carbon-sulfur simultaneous analyzer (combustion in an oxygen stream - infrared absorption method).

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

[0098] (3) Compositions other than C, S, and N and slag components are analyzed by the standard curve method using fluorescent X-ray analysis.

[0099] [Table 1]

[0100]

[0101] Next, the above-mentioned steel sheet (slab) is hot-rolled to 8 mm, and cold rolling, heat treatment, and pickling are repeatedly carried out to manufacture a cold-rolled coil with a thickness of 2 - 3 mm. It is carried out at the final annealing temperature of 1150 °C for 1 minute. Test pieces with a width of 20 mm, a length of 30 mm, and a thickness of 2 mm are collected from this plate.

[0102] <High-temperature oxidation test>

[0103] In order to evaluate the oxidation resistance in a high-temperature environment, a sample obtained by wet-grinding the surface of the above-mentioned test piece with #320 sandpaper is prepared, and using a high-vacuum atmosphere heat treatment furnace, evacuation is carried out to 5.0×10 -3After Pa, in a mixed gas atmosphere composed of 5% O2 - 16% H2O - 12% CO2 - 0.8% CO - 0.1% NO2 - the balance N2, a repeated oxidation test with the following as one cycle was carried out: 1200 °C × 10 minutes, after adjusting the temperature at a cooling rate of 40 °C / min, 1000 °C × 10 minutes, after adjusting the temperature at a heating rate of 40 °C / min, 1200 °C × 10 minutes, and then room temperature × 20 minutes. For the test piece after 200 cycles, the value (mg / cm 2 ) obtained by dividing the mass change of the removed exfoliated scale weight by the surface area before the test was used as the oxidation reduction for evaluation. The test piece with an oxidation reduction lower than 30 mg / cm 2 was judged to have good oxidation resistance (○), and the test piece with 30 mg / cm 2 or more was judged to have poor oxidation resistance (×). At the same time, the test piece after 200 cycles was cut, and after performing Cu plating treatment to be able to observe the cross-section, an embedded specimen was made, wet grinding was carried out, and finally a finish mirror surface was made by polishing for observation. The cross-section microstructure was observed with FE-SEM, the thickness of the surface scale was measured, and the oxide was identified with the attached EDS.

[0104] [Table 2]

[0105]

[0106] The steel plates numbered 1 to 17 shown in Tables 1 and 2 are invention examples that meet the conditions of the present invention and have excellent oxidation resistance. On the other hand, the steel plates numbered 18 to 30 are comparative examples.

[0107] Although the steel plate numbered 18 satisfies formula (1), the Mn content is high, so the oxidation resistance is reduced due to the formation of Cr-Mn based oxides. In addition, the CaO concentration in the slag is as high as 80.9%, and the oxygen concentration is too low. As a result, the Ca concentration increases, forming a large amount of Ca-Al oxide based inclusions and losing the effect of Al. In addition, the fluorine in the slag is also high, and the melting loss of the brick is significant.

[0108] Since the Si content of the steel plate numbered 19 increases, surface defects caused by σ-phase and other intermetallic compounds are generated, and it does not satisfy formula (1), so the oxidation resistance is poor.

[0109] Although the steel plate numbered 20 satisfies formula (1), the C content is high, so a large amount of carbides are precipitated. In addition, since the Mo content is high, the adhesion of the scale is reduced, and the oxidation resistance is poor.

[0110] Although the steel plate numbered 21 satisfies formula (1), its high Cr content causes excessive growth of the surface scale, resulting in a scale lacking in tightness and a deterioration in oxidation resistance. Additionally, due to its high Al content, a large amount of carbonitrides that cause surface defects are formed, resulting in poor surface quality.

[0111] Although the steel plate numbered 22 satisfies formula (1), its high N content causes Cr, Al, and Ti, which contribute to oxidation resistance, to precipitate in the form of nitrides, preventing the formation of a sufficient surface scale.

[0112] Since the steel plate numbered 23 has a low REM content, it does not satisfy formula (1), and thus cannot fully achieve the effect of improving oxidation resistance and the effect of fixing S, which impairs oxidation resistance, in the form of inclusions.

[0113] Since the CaO concentration in the slag of the steel plate numbered 24 is as low as 35%, the desulfurization reaction does not occur. As a result, the S content increases, leading to the formation of a large amount of inclusions and consuming the Cr required for the formation of the surface scale. Additionally, due to the high addition amount of B, the tightness decreases due to the porosity of the surface scale, resulting in poor oxidation resistance.

[0114] Since the steel plate numbered 25 has a low Al content, it does not satisfy formula (1). The SiO2 concentration in the slag is as high as 18%, and the CaO concentration is as low as 45.3%. As a result, deoxidation weakens, leading to an increase in the oxygen concentration and the formation of oxides of elements useful for oxidation resistance, resulting in poor oxidation resistance.

[0115] Although the steel plate numbered 26 satisfies the composition ranges of each element, it exceeds the upper limit of formula (1), resulting in excessive formation of the surface scale and a consequent deterioration in oxidation resistance.

[0116] The steel plate numbered 27 has a low Cr content and does not satisfy formula (1), resulting in poor oxidation resistance. Additionally, since it does not satisfy formula (2), it cannot fully achieve the effect of improving oxidation resistance and the effect of fixing S, which impairs oxidation resistance, in the form of inclusions.

[0117] Since the steel plate numbered 28 has a high REM content, it does not satisfy formula (1). Additionally, its hot workability and weldability decrease, causing blockage of the submerged entry nozzle during continuous casting and a significant deterioration in manufacturability, resulting in poor manufacturability. Moreover, due to its high Ti content, carbonitrides that cause surface defects are formed, resulting in poor surface quality.

[0118] Since the steel plate numbered 29 has a low Ti content, it does not satisfy formula (1), resulting in poor oxidation resistance. Additionally, although the Fe content is within the range, it is relatively low, resulting in a decrease in hot workability and cold workability and poor manufacturability.

[0119] Although the steel plate numbered 30 satisfies the composition ranges of the respective elements, it does not satisfy Formula (1) and Formula (2), and thus does not have sufficient oxidation resistance.

[0120] Industrial applicability

[0121] Since the austenitic Ni-Cr-Fe alloy of the present invention is excellent not only in oxidation resistance in the above-mentioned high-temperature environment but also in heat resistance, it can also be suitably used in high-temperature environments such as industrial heat treatment furnaces and combustion components.

[0122] Symbol description

[0123] 1: Surface scale,

[0124] 2: Interface between surface scale and base metal.

Claims

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

2. The austenitic Ni-Cr-Fe alloy according to claim 1, which further has the following component composition, and the total weight of any one or more of La, Ce, and Y as rare earth elements (REM) satisfies the following formula (2): 4.5≤REM(La, Ce, Y) / S…(2) Here, each element symbol in the above formula represents the content (mass%) of each element.

3. The austenitic Ni - Cr - Fe alloy according to claim 1 or 2, characterized in that, In the surface scale formed on the alloy surface by repeatedly performing a cyclic test from room temperature to 1000 to 1200 °C in a mixed gas atmosphere composed of 5% O2 - 16% H2O - 12% CO2 - 0.8% CO - 0.1% NO2 - the balance N2, by mass%, it contains Cr: 50% or more, Fe: 0 to 10%, Ni: 0 to 5%, O: 10 to 40%, REM: 0.05 to 0.5%, and the balance contains Mn, Si, Al, and Ti as inevitable elements.

4. The austenitic Ni-Cr-Fe alloy according to claim 3, characterized in that, The surface scale has a thickness of 6 to 60 μm.

5. A method for manufacturing an austenitic Ni-Cr-Fe alloy, which is the method for manufacturing an austenitic Ni-Cr-Fe alloy according to claim 1 or 2, characterized in that, The alloy composition is adjusted by melting the alloy raw materials and then performing refining. In the refining, a mixed gas of oxygen and argon is blown into the molten alloy raw materials (molten alloy) for decarburization. After controlling the nitrogen concentration to be below 0.01%, Cr reduction is carried out. Then, aluminum, limestone, and fluorite are added to the molten alloy to form a CaO - SiO2 - Al2O3 - MgO - F - based slag, so that the oxygen concentration in the molten alloy is 0.0002 to 0.0040 mass%. Then, after adding a raw material containing any one or more of La, Ce, and Y, casting is carried out to obtain a slab, which is supplied to the hot rolling process.

6. A method for manufacturing an austenitic Ni-Cr-Fe alloy, which is the method for manufacturing an austenitic Ni-Cr-Fe alloy according to claim 3, characterized in that, The alloy composition is adjusted by refining after melting the alloy raw materials. In the refining process, a mixed gas of oxygen and argon is blown into the molten alloy raw materials (molten alloy) for decarburization. After controlling the nitrogen concentration below 0.01%, Cr reduction is carried out. Then, aluminum, limestone, and fluorite are added to the molten alloy to form a CaO-SiO2-Al2O3-MgO-F-based molten slag, and the oxygen concentration in the molten alloy is made 0.0002 to 0.0040% by mass. Then, after adding a raw material containing any one or more of La, Ce, and Y, casting is carried out to obtain a slab, which is supplied to the hot rolling process.

7. A method for manufacturing an austenitic Ni-Cr-Fe alloy, which is the method for manufacturing an austenitic Ni-Cr-Fe alloy according to claim 4, characterized in that, The alloy composition is adjusted by refining after melting the alloy raw materials. In the refining process, a mixed gas of oxygen and argon is blown into the molten alloy raw materials (molten alloy) for decarburization. After controlling the nitrogen concentration below 0.01%, Cr reduction is carried out. Then, aluminum, limestone, and fluorite are added to the molten alloy to form a CaO-SiO2-Al2O3-MgO-F-based molten slag, and the oxygen concentration in the molten alloy is made 0.0002 to 0.0040% by mass. Then, after adding a raw material containing any one or more of La, Ce, and Y, casting is carried out to obtain a slab, which is supplied to the hot rolling process.

8. The manufacturing method of the austenitic Ni-Cr-Fe alloy according to claim 5, characterized in that, The composition of the CaO-SiO2-Al2O3-MgO-F-based molten slag is CaO: 55 to 70%, SiO2: 10% or less, F: 10% or less.

9. The manufacturing method of the austenitic Ni-Cr-Fe alloy according to claim 6, characterized in that, The composition of the CaO-SiO2-Al2O3-MgO-F-based molten slag is CaO: 55 to 70%, SiO2: 10% or less, F: 10% or less.

10. The manufacturing method of the austenitic Ni-Cr-Fe alloy according to claim 7, characterized in that, The composition of the CaO-SiO2-Al2O3-MgO-F-based molten slag is CaO: 55 to 70%, SiO2: 10% or less, F: 10% or less.

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