Austenite-based ni-cr-fe alloy having excellent oxidation resistance, and method for producing same

Abstract

Austenitic Ni-Cr-Fe alloy which exhibits superior oxidation resistance even under harsh high-temperature environments is proposed. The austenitic Ni-Cr-Fe alloy, components of composition consisting of: in mass%, C: 0.01 to 0.10%, Si: 0.02 to 0.45%, Mn: 0.10 to 0.85%, P: ≤ 0.015%, S: ≤ 0.0015%, Cr: 13.0 to 18.0%, Fe: 5.0 to 11.0%, Mo: ≤ 0.6%, Cu: ≤ 0.5%, B: ≤ 0.005%, Al: 0.01 to 0.3%, Ti: 0.01 to 0.3%, Co: ≤ 1.00%, Nb: ≤ 1.00%, Ta: ≤ 0.05%, N: ≤ 0.01%, 0:0.0002 to 0.0040%, Ca: ≤ 0.002%, total weight of one or more kinds selected from La, Ce and Y being rare earth element (REM) : 0.001 to 0.010%, Ni as a remainder and inevitable impurities, wherein the following formula (1) is satisfied. 60 ≥ 7.4Si +2.8Cr +6.9A1 +3.2Ti +3374REM -5914S -3.87Mn -42.3Mo -3072B -1024N ≥ 30 (Here, each element symbol in the formula indicates content (mass%) of the element.)

Description

Technical Field

[0001] The present invention relates to an austenitic Ni-Cr-Fe alloys, and relates to the austenitic Ni-Cr-Fe alloys having excellent oxidation resistance in an environment at high temperature.Background Art

[0002] Afterburner parts, nuclear power plants, and industrial heat treatment furnaces are used in harsh high-temperature environments at 1000 to 1200°C, so the materials used in these applications are required to have excellent high-temperature strength, high-temperature corrosion resistance, and oxidation resistance. In particular, with respect to oxidation resistance, one of required properties is that the protective surface oxidation scale mainly composed of Cr 2 O 3 formed on the material surface in the high-temperature environment is dense and has high adhesion property to the material. Materials used in equipment used in the high-temperature environment include SUS310S, NCF800, NCF600, and the like, for example.

[0003] As a technique to improve high temperature properties of material which is used in such a harsh environment, for example, Patent document 1 discloses Ni alloy in which heat treatment is performed under a certain atmosphere so as to appropriately control composition and thickness of surface oxidation scale and crystal grain diameter of oxides, which are formed on the surface of alloy, thereby improve properties of the surface oxidation scale, and a method for production thereof. Furthermore, Patent Document 2 proposes Ni-Cr-Fe alloy having superior creep strength and stress relaxation cracking resistance by addition of complex of Ti, Al, REM (rare earth metal).

[0004] However, in the technique disclosed in the Patent Document 1, no consideration has been given to influence of S which can form compound with Cr in material being main component of the surface oxidation scale, or influence of B, Mo, N and Mn which cause negative action to adhesion properties of the surface oxidation scale. Therefore, the technique is considered to be insufficient to apply to the high-temperature environment in which superior oxidation resistance is required.

[0005] In addition, in the technique disclosed in the Patent Document 2, since the alloy is adjusted in its components in a high-frequency induction furnace in laboratory level so as to obtain steel slab and is supplied to hot rolling process, it is simply not possible to mass-produce on a scale of 60 tons. Furthermore, the technique discloses that all of REM act effectively, on the other hand, Examples of the technique mainly disclose addition of Nd. In other Examples, Ce, La and Y are added in part of alloys, thus object of REM is limited. Furthermore, the technique does not include a removing process of S and O during production process, therefore, the raw materials have to be carefully selected in order to achieve the present invention. Therefore, in some cases, the added REM is lost due to oxidation or sulfurization, making it difficult to achieve the original purpose, and the proposal was industrially unproductive. Therefore, it is difficult to say that alloys with improved creep strength due to the addition of REM can be provided accurately on an industrial scale.

[0006] The Patent Documents are as follows. Patent Document 1: Japanese Unexamined Patent Application Publication No. 2002-121630 Patent Document 2: Japanese Re-publication of PCT International Publication No. 2018-066579 Summary of Invention

[0007] The present invention is completed in view of the above problems, and an object of the present invention is to provide austenitic Ni-Cr-Fe alloy which has superior oxidation resistance even under harsh high-temperature environments.

[0008] The inventors had researched in order to solve above problems. A knowledge had been already known, in which adhesion property of surface oxidation scale generated on the surface of alloy under high temperature environment could be improved by adding La, Ce and Y being REM; however, a sufficient knowledge had not been obtained yet, regarding contribution to oxidation resistance which was evaluated by a cycle test in which temperature was repeatedly changed from room temperature to 1000 to 1200 °C under mixture gas atmosphere consisting of 5%O 2 - 16%H 2 O - 12%CO 2 - 0.8%CO - 0.1%NO 2 - bal N 2 . Then, the inventors had studied correlation relationship between La, Ce and Y and other elements contained in alloy in detail. As a result, it became obvious that addition of La, Ce and Y was extremely effective in order to improve oxidation resistance, and that Si, Cr, Al and Ti were also effective as another elements. On the other hand, it became obvious that action of improvement of oxidation resistance was inhibited by containing S, Mn, Mo, B and N. Accordingly, the inventors found that it was necessary to control Si, Ni, Cr, Al, Ti, S, Mn, Mo, B and N in order to maintain action of REM addition sufficiently.

[0009] Furthermore, the inventors found that there was a good correlation relationship between value in which total weight (mass%) of one or more kinds selected from La, Ce, and Y being REM contained in alloy element alloy was divided by S content (mass%) contained in the alloy, and weight reduction by oxidation during high temperature oxidation test, and they made it obvious that property formula obtained therefrom was 4.5 ≤ REM / S.

[0010] Furthermore, as a result of studying focusing on structure of surface oxidation scale formed under high temperature environments, surface oxidation scale was densely formed, adhesion property was superior, and good result was shown in weight reduction by oxidation after test, in a case in which the surface oxidation scale was formed to have thickness range of 6 to 60 µm in the cycle test in which temperature was repeatedly changed from room temperature to 1000 to 1200 °C under mixture gas atmosphere consisting of 5%O 2 - 16%H 2 O - 12%CO 2 - 0.8%CO - 0.1%NO 2 - bal N 2 .

[0011] That is, austenitic Ni-Cr-Fe alloy of the present invention has components of composition consisting of, in mass%, C: 0.01 to 0.10%, Si: 0.02 to 0.45%, Mn: 0.10 to 0.85%, P: ≤ 0.015%, S: ≤ 0.0015%, Cr: 13.0 to 18.0%, Fe: 5.0 to 11.0%, Mo: ≤ 0.6%, Cu: ≤ 0.5%, B: ≤ 0.005%, 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%, total weight of one or more kinds selected from La, Ce and Y being rare earth element (REM) : 0.001 to 0.010%, Ni as a remainder and inevitable impurities, wherein the following formula (1) is satisfied. (Here, each element symbol in the formula indicates content (mass%) of the element.)

[0012] It is desirable that in the austenitic Ni-Cr-Fe alloy of the present invention, one or more kinds selected from La, Ce and Y being rare earth element (REM) satisfies the following formula (2). 4.5 ≤ REM La Ce Y / S (Here, each element symbol in the formula indicates content (mass%) of the element.)

[0013] In addition to the above-mentioned composition of components, it is desirable that in the austenitic Ni-Cr-Fe alloy of the present invention, composition of surface oxidation scale which is formed during a cycle test in which temperature was repeatedly changed from room temperature to 1000 to 1200 °C under mixture gas atmosphere consisting of 5%O 2 - 16%H 2 O - 12%CO 2 - 0.8%CO - 0.1%NO 2 - bal N 2 contains, in mass%, Cr: not less than 50%, Fe: 0 to 10%, Ni: 0 to 5%, O: 10 to 40%, REM: 0.05 to 0.5% and inevitable elements Mn, Si, Al and Ti as a remainder, and that the surface oxidation scale has thickness of 6 to 60 µm.

[0014] Furthermore, the present invention proposes a method for production of the above-mentioned austenitic Ni-Cr-Fe alloy. That is, the method includes steps of: melting raw materials for alloy; as a refining process for adjustment of alloy composition, blowing mixture gas of oxygen and argon into melt alloy raw materials (molten alloy) so as to perform decarburization and control nitrogen concentration to be not more than 0.01%; performing Cr reduction; adding aluminum, lime and fluorite into the molten alloy so as to form CaO-SiO 2 -Al 2 O 3 -MgO-F type slag; rendering oxygen concentration in the molten alloy 0.0002 to 0.0040 mass%; adding raw material containing one or more kinds selected from La, Ce and Y so as to adjust components; casting to obtain slab; and hot-rolling the slab. In the hot rolling process, for example, coil or the like is produced. Furthermore, it is desirable that composition of the CaO-SiO 2 -Al 2 O 3 -MgO-F type slag is CaO: 55 to 70%, SiO 2 : not more than 10%, and F: not more than 10%.Effects of Invention

[0015] According to the present invention, the austenitic Ni-Cr-Fe alloy has superior oxidation resistance under high-temperature environments, and greatly contributes to a longer product life.Brief Description of Drawing

[0016] Fig. 1 is a schematic diagram for explaining surface structure in high-temperature oxidation test for the austenitic Ni-Cr-Fe alloy plate according to one embodiment of the present invention.Embodiments of Invention

[0017] Next, components of composition which should be contained in the austenitic Ni-Cr-Fe alloy of the present invention are explained.C: 0.01 to 0.10 mass%

[0018] C is an element which contributes to stabilization of austenitic phase. However, if it is added at large amount, it may combine Cr, Mo and the like to form carbides, amount of solid-solved Cr in the vicinity may be decreased, and oxidation resistance may be lowered. On the other hand, since C has an effect to increased strength of alloy due to solid solution strengthening, the lower limit is set to be 0.01 mass%. Therefore, C is limited to be 0.01 to 0.10 mass%. It is desirably 0.02 to 0.08 mass%, and more desirably 0.03 to 0.07 mass%.Si: 0.02 to 0.45 mass%

[0019] Si is an effective element to improve oxidation resistance and to avoid separation of oxide film. In addition, in order to control oxygen concentration to be 0.0002 to 0.0040%, it is necessary at least 0.02%. Furthermore, it has a role to reduce CaO in CaO-SiO 2 -MgO-Al 2 O 3 -F type slag and to control Ca to be not more than 0.0020%. From the viewpoints, it is necessary at least 0.02%. However, since excessive addition of Si may cause promotion of precipitation of intermetallic compounds such as σ phase and generation of surface damage due to the intermetallic compounds, it is set to be 0.02 to 0.45 mass%. It is desirably 0.04 to 0.30 mass%, and more desirably 0.06 to 0.20 mass%.Mn: 0.10 to 0.85 mass%

[0020] Since Mn is an element which stabilizes austenitic phase and also is an element which has action of deoxidation, it is necessary at least 0.10 mass% in order to obtain the effects. However, since as similar to Si, Mn may also cause precipitation of intermetallic compounds such as σ phase and deterioration of oxidation resistance due to formation of Cr-Mn type oxides, addition at more than a necessary amount is undesirable. Therefore, it is necessary to set to be 0.10 to 0.85 mass%. It is desirably 0.15 to 0.70 mass%, and more desirably 0.20 to 0.60 mass%.P: Not more than 0.015 mass%

[0021] P is an element which is inevitably mixed in as an impurity and is an element which harms hot workability due to segregation at crystal grain boundary as a phosphide. Therefore, it is desirable to reduce as much as possible. However, production cost may be increased if P content is tried to be reduced extremely. Therefore, in the present invention, P is limited to be not more than 0.015 mass%. It is desirably not more than 0.010 mass%, and more desirably not more than 0.006 mass%.S: Not more than 0.0015 mass%

[0022] S is an element which is inevitably mixed in as an impurity as similar as P, and is an element which is easily precipitated at crystal grain boundary and extremely inhibits hot workability in particular. Furthermore, it forms compound with Cr which contributes to oxidation resistance mentioned below, and therefore, Cr necessary for formation surface oxidation scale is consumed. Then, adhesion property between oxide film and base material is deteriorated thereby separate oxide film and promote oxidation. Therefore, it is a harmful element for oxidation resistance. Since harmfulness may be notably exhibited if the content is more than 0.0015 mass%, it is necessary to control it to be not more than 0.0015 mass%. It is desirably not more than 0.0010 mass%, and more desirably not more than 0.0006 mass%. As mentioned below, S can be reduced by addition of Al and reaction between Al and slag components.Cr: 13.0 to 18.0 mass%

[0023] Cr is an element which contributes to restraining corrosion under high-temperature environments, and has an effect in which protective oxide film is formed on the surface of alloy under high-temperature environments thereby restrain high-temperature oxidation. It is necessary to contain not less than 13.0 mass% in order to sufficiently obtain the above-mentioned effect. However, if Cr is added excessively, surface oxidation scale may be formed larger excessively, adhesion property may rather be deteriorated, and oxidation resistance may be deteriorated. In addition, stability of austenitic phase may be deteriorated. Therefore, it is set to be 13.0 to 18.0 mass%. It is desirably 14.0 to 17.5 mass%, and more desirably 15.0 to 17.0 mass%.Fe: 5.0 to 11.0 mass%

[0024] Fe is an element which has an influence on hot workability and cold workability. The hot workability and cold workability may be deteriorated if the content is less than 5.0%. Furthermore, if the content is more than 11.0%, corrosion resistance may be deteriorated. Therefore, range of the Fe content is set to be 5.0 to 11.0 mass%. It is desirably 6.0 to 10.5%, and more desirably 8.0 to 10.0%.Mo: Not more than 0.6 mass%

[0025] Mo has an effect which increases high-temperature strength by solid-solving in alloy even by addition of at a small amount. However, adverse effects may rather be exhibited in a material in which Mo is added at large amount, since Mo is oxidized preferentially and thereby occur separation of oxidation scale in a case of high-temperature environments and low oxygen potential on the surface. Therefore, from the viewpoint of maintaining adhesion property of protective surface oxidation scale, Mo is limited to not more than 0.6 mass%. It is desirably not more than 0.4 mass%, and more desirably not more than 0.2 mass%.Cu: Not more than 0.5 mass%

[0026] There is a case in which Cu is added as an element which improves corrosion resistance under wet environments; however, under high-temperature environments like in the present invention, the effect is hardly noticeable. On the other hand, an excessive addition may cause formation of uneven film having mottled pattern on the surface of material thereby deteriorate corrosion resistance. Therefore, the Cu content is limited to not more than 0.5 mass%. It is desirably not more than 0.3 mass%, and more desirably not more than 0.1 mass%.B: Not more than 0.005 mass%

[0027] B has an effect which helps effects of rare earth elements (REM) by grain boundary segregation, and is an element which contributes to high-temperature strength. However, addition at a large amount may render surface oxidation scale porous thereby deteriorate adhesion property, welding property and hot workability of alloy. In the present invention, the B content is set to be not more than 0.005 mass%. It is desirably not more than 0.004 mass%, and more desirably not more than 0.002 mass%.Al: 0.01 to 0.3 mass%

[0028] Al is an element which promotes formation of dense film and improves oxidation resistance. The effects can be obtained by addition of not less than 0.01 mass%. In addition, it is an element which is added as a deoxidizing material, and is an important element which controls oxygen concentration to be the range of the present invention: 0.0002 to 0.0040 mass% according to the formula (a). 2 Al ¯ + 3 O = Al 2 O 3 The underline indicates elements in molten steel and the brackets indicate components in slag.

[0029] It is possible to effectively absorb Al 2 O 3 generated and to control oxygen concentration by using CaO-SiO 2 -Al 2 O 3 -MgO-F type slag during refining alloy of the present invention. Furthermore, by proceeding deoxidation, S concentration in molten steel may also be decreased according to the formula (b). 2 Al ¯ + 3 S ¯ + 3 CaO = 3 CaS + Al 2 O 3 According to this, it is possible to control the S concentration to be not more than 0.0015 mass% which is the range of the present invention. From the formula, not less than 0.01 mass% of Al is necessary. However, an excessive addition may cause the reaction in the formula (c) to proceed significantly toward the right side, the Ca concentration may be more than 0.002 mass%. Ca-Al oxide type inclusions may be formed much and Al in alloy may be consumed thereby deteriorate oxidation resistance. 3 CaO + 2 Al ¯ = 3 Ca ¯ + Al 2 O 3 According to this, the upper limit of Al is set to be 0.3 mass%. It is desirably 0.01 to 0.2 mass%, and more desirably 0.01 to 0.1 mass%.Ti: 0.01 to 0.3 mass%

[0030] Ti is an element which promotes formation of dense film and improves oxidation resistance. The effect can be obtained by addition at not less than 0.01 mass%. However, since an excessive addition causes occurrence of surface damage by formation of large amount of carbonitrides (TiN, TiC, TiCN), the upper limit of Ti is set to be 0.3 mass%. It is desirably 0.01 to 0.2 mass%, and more desirably 0.01 to 0.1 mass%. Furthermore, a method in which C and N concentrations are controlled to be within the ranges of the present invention is also an effective means for restraining the carbonitrides.Co: Not more than 1.00 mass%

[0031] Co is an effective element in order to stabilize austenitic phase as similar as C and N. In addition, although C and N cannot be added at large amount since they form carbonitrides with Al, Ti or the like causing surface damage, Co is advantageous since it does not form carbonitrides. However, since raw material cost may be increased by addition at large amount, the Co content is set to be not more than 1.00 mass%. It is desirably not more than 0.90 mass%, and more desirably not more than 0.80 mass%.Nb: Not more than 1.00 mass%

[0032] Nb has an effect in which it forms fine carbides or carbonitrides with C and N thereby increase high-temperature strength of alloy; however, large amount of the carbides or carbonitrides may be precipitated by addition at large amount, and ductility and toughness may be rather decreased. Therefore, the Nb content is limited to not more than 1.00 mass%. It is desirably not more than 0.90 mass%, and more desirably not more than 0.80 mass%.Ta: Not more than 0.05 mass%

[0033] Ta has an effect in which it forms fine carbides or carbonitrides thereby increase high-temperature strength of alloy as similar as Nb; however, large amount of the carbides or carbonitrides may be precipitated by addition at large amount, and ductility and toughness may be rather decreased. Therefore, the Ta content is set to be not more than 0.05 mass%. It is desirably not more than 0.04 mass%, and more desirably not more than 0.03 mass%.N: Not more than 0.01 mass%

[0034] N is an element which is inevitably mixed in as an impurity; however, it is also an austenitic phase generating element, it contributes to stabilization of structure. However, in a case in which Al, Ti, Zr or the like is added like in the present invention, N may combine these elements thereby precipitate nitrides, hot deformation resistance maybe extremely increased, and hot workability may be inhibited. Furthermore, since Al or Ti which are the elements contributing to improvement on density of surface oxidation scale is consumed by the formation of nitrides, oxidation resistance may be deteriorated. Therefore, in the present invention, the N content is set to be not more than 0.01 mass%. It is desirably not more than 0.009 mass%, and more desirably not more than 0.008 mass%.

[0035] Oxygen is blown during performing decarburization, during this process, N can be controlled to be within the range of the present invention by moving N to CO gas bubble as nitrogen gas and removing the bubble toward outside of the system.O: 0.0002 to 0.0040 mass%

[0036] O in alloy may combine Al, Ti, Si, La, Ce and Y in molten steel and oxides thereof may be formed thereby cause losing advantageous effects of the elements and oxidation resistance. Furthermore, oxide type non-metallic inclusions of alumina type may be formed much, the inclusions may adhere on the inside of immerse nozzle for pouring molten steel from a tundish to a mold in continuous casting apparatus, and they may fall off thereby cause surface damage. Accordingly, it is more desirable that oxygen concentration is lower, being not more than 0.0040 mass%. To achieve this range, deoxidation should be performed by controlling Al to be within the range of the present invention as mentioned above. On the other hand, if O is reduced excessively in alloy, Ca concentration may be high, being more than 0.002 mass% according to the formula (c). Accordingly, the lower limit is set to be 0.0002 mass%. It is desirably 0.0003 to 0.0035 mass%, and more desirably 0.0005 to 0.0030 mass%.Ca: Not more than 0.002 mass%

[0037] Ca is an element which is mixed in from CaO in slag as mentioned above, in the alloy of the present invention. Ca should be reduced to a low level since Ca forms Ca-Al oxide type inclusions much and consumes Al in alloy thereby deteriorate oxidation resistance. Accordingly, Al concentration should be controlled to be 0.01 to 0.3 mass% so that oxygen concentration is 0.0002 to 0.0040 mass%. Therefore, it is necessary that Ca is not more than 0.002 mass%.Total weight of one or more kinds selected from La, Ce and Y which is rare earth element (REM): 0.001 to 0.010 mass%

[0038] REM (La, Ce and Y) has an effect in which hot workability of alloy and adhesion property between surface oxidation scale and surface of base material are increased thereby increase oxidation resistance, and significant effect can be obtained even at a small amount. Furthermore, an effect can be expected in which it forms compound with S which is solid-solved in alloy thereby restrain formation of compound of Cr being constituent element of surface oxidation scale and S and prevent reducing local Cr amount. In addition, misch metal which is an alloy containing multiple REM is generally used as raw material of REM, and there also may be a case in which Ni-Fe alloy containing one kind of REM is used. However, an excessive addition may cause deterioration of hot workability and welding properties of alloy, and adhesion property of surface oxidation scale may rather be deteriorated by forming REM type inclusions excessively. Furthermore, immerse nozzle may be blocked during continuous casting thereby greatly deteriorate productivity. Therefore, in the present invention, REM content is set to be 0.001 to 0.010 mass%. It is desirably 0.002 to 0.009 mass%, and more desirably 0.003 to 0.008 mass%.

[0039] In oxidation resistance of the austenitic Ni-Cr-Fe alloy, the formula (1) indicates degree of influence of elements on surface oxidation scale formed on the surface of alloy by multiple regression analysis as a formula. Si, Cr, Al, Ti and REM (La, Ce and Y) improves oxidation resistance which is evaluated in a cycle test in which temperature was repeatedly changed from room temperature to 1000 to 1200 °C under mixture gas atmosphere consisting of 5%O 2 - 16%H 2 O - 12%CO 2 - 0.8%CO - 0.1%NO 2 - bal N 2 . On the other hand, S deteriorates adhesion property between oxide film and base material thereby separate oxidation film and promote oxidation. Mn causes deterioration of oxidation resistance by forming Cr-Mn type oxides. Mo may be oxidized preferentially thereby separate oxidation scale in a case in which Mo content is large, and alloy is under high-temperature environments, and oxygen potential on the surface is low. Furthermore, since oxidation scale on alloy may become porous if B content is large, oxidation rate at high temperature may be increased and enlargement and separation of scale may be promoted. N reacts Al and Ti which contribute to improvement on oxidation resistance to form AlN and TiN, respectively, thereby reduce effects of Al and Ti. In addition, it is not desirable to add excessively alloy elements contributing to improvement on oxidation resistance, since adhesion property is rather deteriorated by excessive growth of surface oxidation scale and large amount of inclusions causing surface damage are generated. Therefore, these elements are set to be not less than 30 in lower limit and not more than 60 in upper limit based on the formula (1). It is desirably 31 to 59, and more desirably 32 to 58. 4.5 ≤ REM / S ¯

[0040] As an index in order to sufficiently obtain effect improving hot workability and oxidation resistance of alloy, if relationship of contents of rare earth element (REM) and S which forms compound with REM satisfies 4.5 ≤ REM / S, REM content sufficient for fixing S as inclusions can be maintained and the above effect can be obtained. On the other hand, if it is less than 4.5, it is not desirable because effect of REM cannot be obtained sufficiently.Definition of surface oxidation scale

[0041] As shown in Fig. 1, austenitic Ni-Cr-Fe alloy plate according to one Embodiment of the present invention has a base metal BM of austenitic Ni-Cr-Fe alloy having composition of components satisfying the above formulae (1) and (2). In a cycle test in which temperature was repeatedly changed from room temperature to 1000 to 1200 °C under mixture gas atmosphere consisting of 5%O 2 - 16%H 2 O - 12%CO 2 - 0.8%CO - 0.1%NO 2 - bal N 2 , oxidation scale mainly containing Cr oxide is formed on the surface of base material of austenitic Ni-Cr-Fe alloy of the present invention. At this time, thickness of the surface oxidation scale corresponds to region LE from the outermost layer of the surface oxidation scale to interface of scale / alloy in Fig. 1, in an observation of cross section microstructure after the test.Surface oxidation scale formed in a cycle test in which temperature is repeatedly changed from room temperature to 1000 to 1200 °C under mixture gas atmosphere consisting of 5%O 2 - 16%H 2 O - 12%CO 2 - 0.8%CO - 0.1%NO 2 - bal N 2 has thickness of 6 to 60 µm

[0042] In the austenitic Ni-Cr-Fe alloy of the present invention, the oxidation scale mainly containing Cr oxide is formed on the surface of base material of the alloy in the cycle test in which temperature was repeatedly changed from room temperature to 1000 to 1200 °C under mixture gas atmosphere consisting of 5%O 2 - 16%H 2 O - 12%CO 2 - 0.8%CO - 0.1%NO 2 - bal N 2 , and the alloy obtains oxidation resistance in the high-temperature environments. At this time, sufficient oxidation resistance may not be obtained if the thickness of the surface oxidation scale is less than 60 µm, and on the other hand, separating property of the surface oxidation scale may be increased and adhesion property between the surface oxidation scale and surface of base material may be deteriorated if the thickness is more than 60 µm. Accordingly, it is necessary that the protective surface oxidation scale formed in the above high-temperature environments have thickness of 6 to 60 µm. It is desirably 8 to 55 µm, and more desirably 10 to 50 µm.

[0043] A method for specifying limitation formula shown by the formula (1) is as follows.

[0044] Kinds of alloys in which Ni-17%Cr-9%Fe was a basic composition and amounts of addition of Si, Cr, Al, Ti, La, Ce, Y, B, Mn, Mo, S and N were varied were melt in a vacuum melting furnace. After hot forging, hot-forged plates having 8mmt x 80mmw were prepared. Solution heat treatment was performed on the obtained hot-forged plates under condition of 1200°C x10 minutes. After surface grounding, cold rolling was performed to have thickness of 2 mmt. After that, solution heat treatment was performed under condition of 1150°C x1 minute. After that, the plates were cut to have size of 20 mm x 30 mm, and finished by wet polishing of #320 on their surfaces to obtain test pieces. The test pieces obtained were subjected to the repeated oxidation test in which one cycle consists of 1200°C x 10 minutes, 1000°C x 10 minutes, 1200°C x 10 minutes and room temperature x 20 minutes under mixture gas atmosphere consisting of 5%O 2 - 16%H 2 O - 12%CO 2 - 0.8%CO - 0.1%NO 2 - bal N 2 . With respect to the test pieces after 200 cycles, a value of which mass change except for separated scale weight was divided by surface area before the test was evaluated.

[0045] From the above test results, degree of influence of added element on oxidation resistance of the austenitic Ni-Cr-Fe alloy became obvious, and relational formula of composition of components shown by the formula (1) was obtained by multiple regression analysis. It is obvious that sufficient oxidation resistance can be exhibited at 30 to 60.

[0046] Next, the method for production of the austenitic Ni-Cr-Fe alloy of the present invention is explained.

[0047] In the method for production of the austenitic Ni-Cr-Fe alloy of the present invention, raw materials such as iron scrap, stainless steel scrap, ferronickel and ferrochromium were melted in an electric furnace; mixture gas of oxygen and noble gas was blown in an AOD (Argon Oxygen Decarburization) furnace or VOD (Vacuum Oxygen Decarburization) furnace so as to perform decarburization and refining; quicklime, Fe-Si alloy, Al and the like were added so as to reduce Cr oxide in slag; fluorite was added to form CaO-SiO 2 -Al 2 O 3 -MgO-F type slag so as to perform deoxidation and desulfurization; and Ni alloy containing one kind of La, Ce and Y was added. The reason for using the CaO-SiO 2 -Al 2 O 3 -MgO-F type slag is that deoxidation and desulfurization can be effectively performed as mentioned above, and furthermore, REM can be effectively added without being oxidized or sulfurized during adding REM. At this time, it is desirable that CaO concentration in the slag is 40 to 80%. That is, if it is less than 40%, the above-mentioned desulfurization reaction may not proceed. If it is more than 80%, more than 0.002% of Ca may be mixed in molten steel. Furthermore, it is desirable that Al 2 O 3 concentration is not more than 50%. The reason is that if the alumina activity in slag is not low, deoxidation may not proceed smoothly, and as a result, desulfurization may not proceed smoothly. After refining, slab was produced by continuous casting apparatus, and it is desirable that the slab is hot-rolled, alternatively hot-rolled and cold-rolled to obtain kinds of steel material such as thin steel plate, thick steel plate, shaped steel, steel bar, wire and the like. The present invention is not limited to continuous casting apparatus, but ingot casting-slabbing rolling method can be applied to obtain slab.

[0048] Here, although it is not limited in particular, but it is desirable that CaO-SiO 2 -Al 2 O 3 -MgO-F type slag has the following composition.CaO: 55 to 70%

[0049] It the CaO is less than 55%, deoxidation may not proceed and oxygen concentration may be higher than the range of the present invention. On the other hand, if it is more than 70%, Ca concentration in molten steel may be higher than the range of the present invention. Therefore, the above range is desirable.SiO 2 : Not more than 10%

[0050] If SiO 2 is more than 10%, deoxidation may not proceed. As a result, oxygen concentration may be higher than the range of the present invention. Therefore, it is desirable that SiO 2 is not more than 10%.F: Not more than 10%

[0051] Fluorine is important to melt slag. As a source of fluorine, fluorite is usually added. If it is more than 10%, not only does this damage the bricks and shorten their lifetime, but it can also cause molten steel leaks. Therefore, F should be reduced to not more than 10%.Examples

[0052] Raw materials in which scrap, ferrochromium, ferronickel, stainless steel scrap were adjusted to be contained at a predetermined ratio were melted in an electric furnace of 70 t level, and mixture gas of oxygen and noble gas was blown in an AOD furnace or VOD furnace so as to perform decarburization and refining. Then, after quicklime, Fe-Si alloy, Al and the like were added to reduce Cr oxides in the slag, fluorite was added to form CaO-SiO 2 -Al 2 O 3 -MgO-F type slag so as to perform deoxidation and desulfurization. After that, one or more kinds selected from Ni-20%La, Ni-20%Ce and Ni-20%Y was added at a predetermined amount to obtain cast piece by continuous casting method. After adjusting to be kinds of compositions of components shown in Table 1, slab was obtained by continuous casting. Each component shown in Table 1 was measured as follows. In the Tables, value in brackets indicates that the value is out of the range of the present invention. It should be noted that value in brackets in Examples means that it satisfies the range of independent claims but does not satisfy the range of dependent claims. (1) Composition of C and S were measured by using carbon and sulfur simultaneous analyzer (combustion in oxygen flow - infrared absorption method). (2) Composition of N was analyzed using oxygen and nitrogen simultaneous analyzer (inert gas - impulse heating melting method) (3) Composition other than C, S and N and slag components were analyzed by calibration curve method using X-ray fluorescence analysis.

[0053] Next, the slab was hot-rolled to have thickness of 8 mm, and cold rolling, heat treatment and acid washing were repeated to produce cold-rolled coil having plate thickness of 2 to 3 mm. Final annealing temperature was 1150°C for 1 minute. Test piece of width: 20 mm, length: 30 mm, and thickness: 2 mm was obtained from the plate.<High temperature oxidation test>

[0054] In order to evaluate oxidation resistance under high-temperature environments, the surface of the test piece was wet-polished by emery paper of #320. After evacuating to 5.0×10 -3< Pa using high vacuum atmosphere heat treatment furnace, the repeated oxidation test was performed in which one cycle consists of processes maintaining 1200°C x 10 minutes, adjusting temperature at cooling rate of 40°C / min, maintaining 1000°C x 10 minutes, adjusting temperature at heating rate of 40°C / min, maintaining 1200°C x 10 minutes and maintaining room temperature x 20 minutes under mixture gas atmosphere consisting of 5%O 2 - 16%H 2 O - 12%CO 2 - 0.8%CO - 0.1%NO 2 - bal N 2 . With respect to the test pieces after 200 cycles, a value (mg / cm 2< ) of which mass change except for separated scale weight was divided by surface area before the test was evaluated as oxidation weight reduction. A sample in which oxidation weight reduction is less than 30 mg / cm 2< and a sample in which it is not less than 30 mg / cm 2< were decided as superior in oxidation resistance (O ("O" means superior)) and inferior in oxidation resistance (X ("X" means inferior)), respectively. In addition, the test piece after 200 cycles was cut and Cu-plated to enable cross-section observation, after which an embedded sample was prepared, wet-polished, and finally buff-polished to a mirror surface for observation. The cross-sectional microstructure of this sample was observed with an FE-SEM to measure the thickness of the surface oxidation scale, and oxides were identified with an attached EDS. Table 2Slag composition (mass%)NiFormula (1)Formula (2)Surface oxidation scaleOxidation resistanceCaOSiO 2 Al 2 O 3 MgOFThicknessCrFeNiOExamples163.65.912.912.74.576.37394.51681.42.23.212.30.50○262.32.311.815.97.473.744019.0577.51.11.519.40.32○362.32.311.815.97.469.77429.32457.96.53.032.40.10○463.22.011.813.69.177.274420.33176.58.13.311.70.22○563.52.311.815.96.276.855915.25654.23.84.237.10.50○662.32.311.815.97.473.84375.42270.42.9[5.8]23.60.25○762.32.311.815.97.473.81339.83249.38.82.538.90.32○861.32.311.815.98.473.414516.01956.26.71.635.10.17○962.32.311.815.97.473.065214.44372.68.00.218.60.47○1062.32.311.815.97.474.12436.75467.91.63.426.6[0.53]○1162.32.310.915.98.372.63325.63976.89.42.215.50.35○1262.32.311.815.97.471.904018.01571.66.73.417.50.44○1361.32.311.815.98.473.703126.84861.25.01.232.30.19○1462.32.311.815.97.472.93354.93664.68.43.922.60.28○1562.32.311.815.97.474.73355.9662.78.01.227.40.44○1662.32.311.814.88.574.895022.01472.70.52.923.40.10○1762.32.311.815.97.472.11327.36373.94.01.520.40.11○Compara tive Examples1880.91.01.25.311.370.2334[4.4]3263.82.13.128.50.30×1962.32.311.814.88.567.70

[64] 96.05172.85.23.715.40.36×2061.32.311.815.29.170.573034.020[47.2]7.03.040.00.39×2162.32.311.814.88.572.034932.51074.34.84.514.20.16×2262.32.311.814.88.572.783049.02278.73.70.214.60.43×2362.32.311.814.88.573.86

[27] [1.5]866.58.44.717.7[0.51]×2462.32.311.814.88.574.26414.7669.25.71.820.80.45×2545.318.311.814.89.571.38

[28] [3.5]3073.72.52.718.3[0.51]×2662.32.311.814.88.570.14

[69] 6.73667.19.52.218.40.40×2762.32.311.814.88.574.95

[20] [1.8]2972.56.73.415.30.06×2862.32.311.814.88.571.67

[77] 35.33061.56.52.127.70.15×2962.32.311.814.88.576.75

[28] [3.8]1256.94.10.036.4[0.51]×3062.32.311.814.88.572.63

[24] [3.8]962.39.92.922.30.42×

[0055] The steel plates of Nos. 1 to 17 shown in Tables 1 and 2 are Examples which satisfy conditions of the present invention, and exhibited superior oxidation resistance. On the other hand, the steel plates of Nos. 18 to 30 are Comparative Examples.

[0056] Although the steel plate of No. 18 satisfied the formula (1), since Mn content was high, oxidation resistance was deteriorated due to formation of Cr-Mn type oxides. Furthermore, as a result that CaO concentration in slag was high being 80.9% and oxygen concentration was too low, Ca concentration became high and large amount of Ca-Al oxide type inclusions were formed thereby lose effect of Al. Furthermore, fluorine content in slag was high thereby damage bricks considerably.

[0057] Since Si content was high in the steel plate of No. 19, surface damage due to intermetallic compounds such as σ phase occurred, the formula (1) was not satisfied, and oxidation resistance was deteriorated.

[0058] Although the steel plate of No. 20 satisfied the formula (1), large amount of carbides were precipitated since C content was high, and furthermore, adhesion property of scale was decreased thereby deteriorate oxidation resistance since Mo content was high.

[0059] Although the steel plate of No. 21 satisfied the formula (1), since Cr content was high, surface oxidation scale grew excessively to form scale having low adhesion property, and oxidation resistance was rather deteriorated. Furthermore, since Al content was high, large amount of carbonitrides causing surface damage was formed thereby deteriorate surface quality.

[0060] Although the steel plate of No. 22 satisfied the formula (1), since N content was high, Cr, Al and Ti which contribute to oxidation resistance were precipitated as nitrides and sufficient surface oxidation scale could not be formed.

[0061] Since REM content was low in the steel plate of No. 23, the formula (1) was not satisfied and an effect improving oxidation resistance and an effect in which S which inhibited oxidation resistance was fixed as inclusion could not be sufficiently obtained.

[0062] Since CaO concentration in slag was low being 35% in the steel plate of No. 24, desulfurizing reaction did not promote. Therefore, since S content became high, large amount of inclusions were formed, and Cr which was necessary for formation of surface oxidation scale was consumed. Furthermore, since B content was high, surface oxidation scale became porous and adhesion property was deteriorated thereby deteriorate oxidation resistance.

[0063] Since Al content was low in the steel plate of No. 25, the formula (1) was not satisfied. Since SiO 2 concentration and CaO concentration in slag were high being 18% and low being 45.3%, respectively, deoxidation was insufficient thereby increase oxygen concentration, and elements effective for oxidation resistance formed oxides thereby deteriorate oxidation resistance.

[0064] Although the steel plate of No. 26 satisfied range of component of each of elements, since the formula (1) was more than the upper limit, surface oxidation scale was formed excessively thereby deteriorate oxidation resistance.

[0065] Since Cr content was low in the steel plate of No. 27 and the formula (1) was not satisfied, oxidation resistance was deteriorated. Furthermore, since the formula (2) was not satisfied, an effect improving oxidation resistance and an effect in which S which inhibited oxidation resistance was fixed as inclusion could not be sufficiently obtained.

[0066] Since REM content was high in the steel plate of No. 28, the formula (1) was not satisfied. Furthermore, hot workability and welding property were deteriorated and immerse nozzle was blocked during continuous casting thereby extremely deteriorate productivity. Furthermore, since Ti content was also high, carbonitrides causing surface damage were formed thereby deteriorate surface quality.

[0067] Since Ti content was low in the steel plate of No. 29, the formula (1) was not satisfied and oxidation resistance was deteriorated. Furthermore, since the Fe content was within the range but slightly low, hot workability and cold workability were deteriorated thereby deteriorate productivity.

[0068] Although the steel plate of No. 30 satisfied range of component of each of elements, since the formulae (1) and (2) were not satisfied, sufficient oxidation resistance could not be obtained.Industrial Applicability

[0069] Since the austenitic Ni-Cr-Fe alloy of the present invention has superior heat resistance in addition to the above-mentioned oxidation resistance under high-temperature environments, it can be appropriately employed as industrial heat treatment furnaces or combustion parts used under high-temperature environments.Explanation of Reference Numerals

[0070] 1: Surface oxidation scale 2: Interface of surface oxidation scale / base material

Examples

examples

[0052]Raw materials in which scrap, ferrochromium, ferronickel, stainless steel scrap were adjusted to be contained at a predetermined ratio were melted in an electric furnace of 70 t level, and mixture gas of oxygen and noble gas was blown in an AOD furnace or VOD furnace so as to perform decarburization and refining. Then, after quicklime, Fe-Si alloy, Al and the like were added to reduce Cr oxides in the slag, fluorite was added to form CaO-SiO 2 -Al 2 O 3 -MgO-F type slag so as to perform deoxidation and desulfurization. After that, one or more kinds selected from Ni-20%La, Ni-20%Ce and Ni-20%Y was added at a predetermined amount to obtain cast piece by continuous casting method. After adjusting to be kinds of compositions of components shown in Table 1, slab was obtained by continuous casting. Each component shown in Table 1 was measured as follows. In the Tables, value in brackets indicates that the value is out of the range of the present invention. It should be noted that va...

Technical Field

[0001] The present invention relates to an austenitic Ni-Cr-Fe alloys, and relates to the austenitic Ni-Cr-Fe alloys having excellent oxidation resistance in an environment at high temperature.Background Art

[0002] Afterburner parts, nuclear power plants, and industrial heat treatment furnaces are used in harsh high-temperature environments at 1000 to 1200°C, so the materials used in these applications are required to have excellent high-temperature strength, high-temperature corrosion resistance, and oxidation resistance. In particular, with respect to oxidation resistance, one of required properties is that the protective surface oxidation scale mainly composed of Cr 2 O 3 formed on the material surface in the high-temperature environment is dense and has high adhesion property to the material. Materials used in equipment used in the high-temperature environment include SUS310S, NCF800, NCF600, and the like, for example.

[0003] As a technique to improve high temperature properties of material which is used in such a harsh environment, for example, Patent document 1 discloses Ni alloy in which heat treatment is performed under a certain atmosphere so as to appropriately control composition and thickness of surface oxidation scale and crystal grain diameter of oxides, which are formed on the surface of alloy, thereby improve properties of the surface oxidation scale, and a method for production thereof. Furthermore, Patent Document 2 proposes Ni-Cr-Fe alloy having superior creep strength and stress relaxation cracking resistance by addition of complex of Ti, Al, REM (rare earth metal).

[0004] However, in the technique disclosed in the Patent Document 1, no consideration has been given to influence of S which can form compound with Cr in material being main component of the surface oxidation scale, or influence of B, Mo, N and Mn which cause negative action to adhesion properties of the surface oxidation scale. Therefore, the technique is considered to be insufficient to apply to the high-temperature environment in which superior oxidation resistance is required.

[0005] In addition, in the technique disclosed in the Patent Document 2, since the alloy is adjusted in its components in a high-frequency induction furnace in laboratory level so as to obtain steel slab and is supplied to hot rolling process, it is simply not possible to mass-produce on a scale of 60 tons. Furthermore, the technique discloses that all of REM act effectively, on the other hand, Examples of the technique mainly disclose addition of Nd. In other Examples, Ce, La and Y are added in part of alloys, thus object of REM is limited. Furthermore, the technique does not include a removing process of S and O during production process, therefore, the raw materials have to be carefully selected in order to achieve the present invention. Therefore, in some cases, the added REM is lost due to oxidation or sulfurization, making it difficult to achieve the original purpose, and the proposal was industrially unproductive. Therefore, it is difficult to say that alloys with improved creep strength due to the addition of REM can be provided accurately on an industrial scale.

[0006] The Patent Documents are as follows. Patent Document 1: Japanese Unexamined Patent Application Publication No. 2002-121630 Patent Document 2: Japanese Re-publication of PCT International Publication No. 2018-066579 Summary of Invention

[0007] The present invention is completed in view of the above problems, and an object of the present invention is to provide austenitic Ni-Cr-Fe alloy which has superior oxidation resistance even under harsh high-temperature environments.

[0008] The inventors had researched in order to solve above problems. A knowledge had been already known, in which adhesion property of surface oxidation scale generated on the surface of alloy under high temperature environment could be improved by adding La, Ce and Y being REM; however, a sufficient knowledge had not been obtained yet, regarding contribution to oxidation resistance which was evaluated by a cycle test in which temperature was repeatedly changed from room temperature to 1000 to 1200 °C under mixture gas atmosphere consisting of 5%O 2 - 16%H 2 O - 12%CO 2 - 0.8%CO - 0.1%NO 2 - bal N 2 . Then, the inventors had studied correlation relationship between La, Ce and Y and other elements contained in alloy in detail. As a result, it became obvious that addition of La, Ce and Y was extremely effective in order to improve oxidation resistance, and that Si, Cr, Al and Ti were also effective as another elements. On the other hand, it became obvious that action of improvement of oxidation resistance was inhibited by containing S, Mn, Mo, B and N. Accordingly, the inventors found that it was necessary to control Si, Ni, Cr, Al, Ti, S, Mn, Mo, B and N in order to maintain action of REM addition sufficiently.

[0009] Furthermore, the inventors found that there was a good correlation relationship between value in which total weight (mass%) of one or more kinds selected from La, Ce, and Y being REM contained in alloy element alloy was divided by S content (mass%) contained in the alloy, and weight reduction by oxidation during high temperature oxidation test, and they made it obvious that property formula obtained therefrom was 4.5 ≤ REM / S.

[0010] Furthermore, as a result of studying focusing on structure of surface oxidation scale formed under high temperature environments, surface oxidation scale was densely formed, adhesion property was superior, and good result was shown in weight reduction by oxidation after test, in a case in which the surface oxidation scale was formed to have thickness range of 6 to 60 µm in the cycle test in which temperature was repeatedly changed from room temperature to 1000 to 1200 °C under mixture gas atmosphere consisting of 5%O 2 - 16%H 2 O - 12%CO 2 - 0.8%CO - 0.1%NO 2 - bal N 2 .

[0011] That is, austenitic Ni-Cr-Fe alloy of the present invention has components of composition consisting of, in mass%, C: 0.01 to 0.10%, Si: 0.02 to 0.45%, Mn: 0.10 to 0.85%, P: ≤ 0.015%, S: ≤ 0.0015%, Cr: 13.0 to 18.0%, Fe: 5.0 to 11.0%, Mo: ≤ 0.6%, Cu: ≤ 0.5%, B: ≤ 0.005%, 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%, total weight of one or more kinds selected from La, Ce and Y being rare earth element (REM) : 0.001 to 0.010%, Ni as a remainder and inevitable impurities, wherein the following formula (1) is satisfied. (Here, each element symbol in the formula indicates content (mass%) of the element.)

[0012] It is desirable that in the austenitic Ni-Cr-Fe alloy of the present invention, one or more kinds selected from La, Ce and Y being rare earth element (REM) satisfies the following formula (2). 4.5 ≤ REM La Ce Y / S (Here, each element symbol in the formula indicates content (mass%) of the element.)

[0013] In addition to the above-mentioned composition of components, it is desirable that in the austenitic Ni-Cr-Fe alloy of the present invention, composition of surface oxidation scale which is formed during a cycle test in which temperature was repeatedly changed from room temperature to 1000 to 1200 °C under mixture gas atmosphere consisting of 5%O 2 - 16%H 2 O - 12%CO 2 - 0.8%CO - 0.1%NO 2 - bal N 2 contains, in mass%, Cr: not less than 50%, Fe: 0 to 10%, Ni: 0 to 5%, O: 10 to 40%, REM: 0.05 to 0.5% and inevitable elements Mn, Si, Al and Ti as a remainder, and that the surface oxidation scale has thickness of 6 to 60 µm.

[0014] Furthermore, the present invention proposes a method for production of the above-mentioned austenitic Ni-Cr-Fe alloy. That is, the method includes steps of: melting raw materials for alloy; as a refining process for adjustment of alloy composition, blowing mixture gas of oxygen and argon into melt alloy raw materials (molten alloy) so as to perform decarburization and control nitrogen concentration to be not more than 0.01%; performing Cr reduction; adding aluminum, lime and fluorite into the molten alloy so as to form CaO-SiO 2 -Al 2 O 3 -MgO-F type slag; rendering oxygen concentration in the molten alloy 0.0002 to 0.0040 mass%; adding raw material containing one or more kinds selected from La, Ce and Y so as to adjust components; casting to obtain slab; and hot-rolling the slab. In the hot rolling process, for example, coil or the like is produced. Furthermore, it is desirable that composition of the CaO-SiO 2 -Al 2 O 3 -MgO-F type slag is CaO: 55 to 70%, SiO 2 : not more than 10%, and F: not more than 10%.Effects of Invention

[0015] According to the present invention, the austenitic Ni-Cr-Fe alloy has superior oxidation resistance under high-temperature environments, and greatly contributes to a longer product life.Brief Description of Drawing

[0016] Fig. 1 is a schematic diagram for explaining surface structure in high-temperature oxidation test for the austenitic Ni-Cr-Fe alloy plate according to one embodiment of the present invention.Embodiments of Invention

[0017] Next, components of composition which should be contained in the austenitic Ni-Cr-Fe alloy of the present invention are explained.C: 0.01 to 0.10 mass%

[0018] C is an element which contributes to stabilization of austenitic phase. However, if it is added at large amount, it may combine Cr, Mo and the like to form carbides, amount of solid-solved Cr in the vicinity may be decreased, and oxidation resistance may be lowered. On the other hand, since C has an effect to increased strength of alloy due to solid solution strengthening, the lower limit is set to be 0.01 mass%. Therefore, C is limited to be 0.01 to 0.10 mass%. It is desirably 0.02 to 0.08 mass%, and more desirably 0.03 to 0.07 mass%.Si: 0.02 to 0.45 mass%

[0019] Si is an effective element to improve oxidation resistance and to avoid separation of oxide film. In addition, in order to control oxygen concentration to be 0.0002 to 0.0040%, it is necessary at least 0.02%. Furthermore, it has a role to reduce CaO in CaO-SiO 2 -MgO-Al 2 O 3 -F type slag and to control Ca to be not more than 0.0020%. From the viewpoints, it is necessary at least 0.02%. However, since excessive addition of Si may cause promotion of precipitation of intermetallic compounds such as σ phase and generation of surface damage due to the intermetallic compounds, it is set to be 0.02 to 0.45 mass%. It is desirably 0.04 to 0.30 mass%, and more desirably 0.06 to 0.20 mass%.Mn: 0.10 to 0.85 mass%

[0020] Since Mn is an element which stabilizes austenitic phase and also is an element which has action of deoxidation, it is necessary at least 0.10 mass% in order to obtain the effects. However, since as similar to Si, Mn may also cause precipitation of intermetallic compounds such as σ phase and deterioration of oxidation resistance due to formation of Cr-Mn type oxides, addition at more than a necessary amount is undesirable. Therefore, it is necessary to set to be 0.10 to 0.85 mass%. It is desirably 0.15 to 0.70 mass%, and more desirably 0.20 to 0.60 mass%.P: Not more than 0.015 mass%

[0021] P is an element which is inevitably mixed in as an impurity and is an element which harms hot workability due to segregation at crystal grain boundary as a phosphide. Therefore, it is desirable to reduce as much as possible. However, production cost may be increased if P content is tried to be reduced extremely. Therefore, in the present invention, P is limited to be not more than 0.015 mass%. It is desirably not more than 0.010 mass%, and more desirably not more than 0.006 mass%.S: Not more than 0.0015 mass%

[0022] S is an element which is inevitably mixed in as an impurity as similar as P, and is an element which is easily precipitated at crystal grain boundary and extremely inhibits hot workability in particular. Furthermore, it forms compound with Cr which contributes to oxidation resistance mentioned below, and therefore, Cr necessary for formation surface oxidation scale is consumed. Then, adhesion property between oxide film and base material is deteriorated thereby separate oxide film and promote oxidation. Therefore, it is a harmful element for oxidation resistance. Since harmfulness may be notably exhibited if the content is more than 0.0015 mass%, it is necessary to control it to be not more than 0.0015 mass%. It is desirably not more than 0.0010 mass%, and more desirably not more than 0.0006 mass%. As mentioned below, S can be reduced by addition of Al and reaction between Al and slag components.Cr: 13.0 to 18.0 mass%

[0023] Cr is an element which contributes to restraining corrosion under high-temperature environments, and has an effect in which protective oxide film is formed on the surface of alloy under high-temperature environments thereby restrain high-temperature oxidation. It is necessary to contain not less than 13.0 mass% in order to sufficiently obtain the above-mentioned effect. However, if Cr is added excessively, surface oxidation scale may be formed larger excessively, adhesion property may rather be deteriorated, and oxidation resistance may be deteriorated. In addition, stability of austenitic phase may be deteriorated. Therefore, it is set to be 13.0 to 18.0 mass%. It is desirably 14.0 to 17.5 mass%, and more desirably 15.0 to 17.0 mass%.Fe: 5.0 to 11.0 mass%

[0024] Fe is an element which has an influence on hot workability and cold workability. The hot workability and cold workability may be deteriorated if the content is less than 5.0%. Furthermore, if the content is more than 11.0%, corrosion resistance may be deteriorated. Therefore, range of the Fe content is set to be 5.0 to 11.0 mass%. It is desirably 6.0 to 10.5%, and more desirably 8.0 to 10.0%.Mo: Not more than 0.6 mass%

[0025] Mo has an effect which increases high-temperature strength by solid-solving in alloy even by addition of at a small amount. However, adverse effects may rather be exhibited in a material in which Mo is added at large amount, since Mo is oxidized preferentially and thereby occur separation of oxidation scale in a case of high-temperature environments and low oxygen potential on the surface. Therefore, from the viewpoint of maintaining adhesion property of protective surface oxidation scale, Mo is limited to not more than 0.6 mass%. It is desirably not more than 0.4 mass%, and more desirably not more than 0.2 mass%.Cu: Not more than 0.5 mass%

[0026] There is a case in which Cu is added as an element which improves corrosion resistance under wet environments; however, under high-temperature environments like in the present invention, the effect is hardly noticeable. On the other hand, an excessive addition may cause formation of uneven film having mottled pattern on the surface of material thereby deteriorate corrosion resistance. Therefore, the Cu content is limited to not more than 0.5 mass%. It is desirably not more than 0.3 mass%, and more desirably not more than 0.1 mass%.B: Not more than 0.005 mass%

[0027] B has an effect which helps effects of rare earth elements (REM) by grain boundary segregation, and is an element which contributes to high-temperature strength. However, addition at a large amount may render surface oxidation scale porous thereby deteriorate adhesion property, welding property and hot workability of alloy. In the present invention, the B content is set to be not more than 0.005 mass%. It is desirably not more than 0.004 mass%, and more desirably not more than 0.002 mass%.Al: 0.01 to 0.3 mass%

[0028] Al is an element which promotes formation of dense film and improves oxidation resistance. The effects can be obtained by addition of not less than 0.01 mass%. In addition, it is an element which is added as a deoxidizing material, and is an important element which controls oxygen concentration to be the range of the present invention: 0.0002 to 0.0040 mass% according to the formula (a). 2 Al ¯ + 3 O = Al 2 O 3 The underline indicates elements in molten steel and the brackets indicate components in slag.

[0029] It is possible to effectively absorb Al 2 O 3 generated and to control oxygen concentration by using CaO-SiO 2 -Al 2 O 3 -MgO-F type slag during refining alloy of the present invention. Furthermore, by proceeding deoxidation, S concentration in molten steel may also be decreased according to the formula (b). 2 Al ¯ + 3 S ¯ + 3 CaO = 3 CaS + Al 2 O 3 According to this, it is possible to control the S concentration to be not more than 0.0015 mass% which is the range of the present invention. From the formula, not less than 0.01 mass% of Al is necessary. However, an excessive addition may cause the reaction in the formula (c) to proceed significantly toward the right side, the Ca concentration may be more than 0.002 mass%. Ca-Al oxide type inclusions may be formed much and Al in alloy may be consumed thereby deteriorate oxidation resistance. 3 CaO + 2 Al ¯ = 3 Ca ¯ + Al 2 O 3 According to this, the upper limit of Al is set to be 0.3 mass%. It is desirably 0.01 to 0.2 mass%, and more desirably 0.01 to 0.1 mass%.Ti: 0.01 to 0.3 mass%

[0030] Ti is an element which promotes formation of dense film and improves oxidation resistance. The effect can be obtained by addition at not less than 0.01 mass%. However, since an excessive addition causes occurrence of surface damage by formation of large amount of carbonitrides (TiN, TiC, TiCN), the upper limit of Ti is set to be 0.3 mass%. It is desirably 0.01 to 0.2 mass%, and more desirably 0.01 to 0.1 mass%. Furthermore, a method in which C and N concentrations are controlled to be within the ranges of the present invention is also an effective means for restraining the carbonitrides.Co: Not more than 1.00 mass%

[0031] Co is an effective element in order to stabilize austenitic phase as similar as C and N. In addition, although C and N cannot be added at large amount since they form carbonitrides with Al, Ti or the like causing surface damage, Co is advantageous since it does not form carbonitrides. However, since raw material cost may be increased by addition at large amount, the Co content is set to be not more than 1.00 mass%. It is desirably not more than 0.90 mass%, and more desirably not more than 0.80 mass%.Nb: Not more than 1.00 mass%

[0032] Nb has an effect in which it forms fine carbides or carbonitrides with C and N thereby increase high-temperature strength of alloy; however, large amount of the carbides or carbonitrides may be precipitated by addition at large amount, and ductility and toughness may be rather decreased. Therefore, the Nb content is limited to not more than 1.00 mass%. It is desirably not more than 0.90 mass%, and more desirably not more than 0.80 mass%.Ta: Not more than 0.05 mass%

[0033] Ta has an effect in which it forms fine carbides or carbonitrides thereby increase high-temperature strength of alloy as similar as Nb; however, large amount of the carbides or carbonitrides may be precipitated by addition at large amount, and ductility and toughness may be rather decreased. Therefore, the Ta content is set to be not more than 0.05 mass%. It is desirably not more than 0.04 mass%, and more desirably not more than 0.03 mass%.N: Not more than 0.01 mass%

[0034] N is an element which is inevitably mixed in as an impurity; however, it is also an austenitic phase generating element, it contributes to stabilization of structure. However, in a case in which Al, Ti, Zr or the like is added like in the present invention, N may combine these elements thereby precipitate nitrides, hot deformation resistance maybe extremely increased, and hot workability may be inhibited. Furthermore, since Al or Ti which are the elements contributing to improvement on density of surface oxidation scale is consumed by the formation of nitrides, oxidation resistance may be deteriorated. Therefore, in the present invention, the N content is set to be not more than 0.01 mass%. It is desirably not more than 0.009 mass%, and more desirably not more than 0.008 mass%.

[0035] Oxygen is blown during performing decarburization, during this process, N can be controlled to be within the range of the present invention by moving N to CO gas bubble as nitrogen gas and removing the bubble toward outside of the system.O: 0.0002 to 0.0040 mass%

[0036] O in alloy may combine Al, Ti, Si, La, Ce and Y in molten steel and oxides thereof may be formed thereby cause losing advantageous effects of the elements and oxidation resistance. Furthermore, oxide type non-metallic inclusions of alumina type may be formed much, the inclusions may adhere on the inside of immerse nozzle for pouring molten steel from a tundish to a mold in continuous casting apparatus, and they may fall off thereby cause surface damage. Accordingly, it is more desirable that oxygen concentration is lower, being not more than 0.0040 mass%. To achieve this range, deoxidation should be performed by controlling Al to be within the range of the present invention as mentioned above. On the other hand, if O is reduced excessively in alloy, Ca concentration may be high, being more than 0.002 mass% according to the formula (c). Accordingly, the lower limit is set to be 0.0002 mass%. It is desirably 0.0003 to 0.0035 mass%, and more desirably 0.0005 to 0.0030 mass%.Ca: Not more than 0.002 mass%

[0037] Ca is an element which is mixed in from CaO in slag as mentioned above, in the alloy of the present invention. Ca should be reduced to a low level since Ca forms Ca-Al oxide type inclusions much and consumes Al in alloy thereby deteriorate oxidation resistance. Accordingly, Al concentration should be controlled to be 0.01 to 0.3 mass% so that oxygen concentration is 0.0002 to 0.0040 mass%. Therefore, it is necessary that Ca is not more than 0.002 mass%.Total weight of one or more kinds selected from La, Ce and Y which is rare earth element (REM): 0.001 to 0.010 mass%

[0038] REM (La, Ce and Y) has an effect in which hot workability of alloy and adhesion property between surface oxidation scale and surface of base material are increased thereby increase oxidation resistance, and significant effect can be obtained even at a small amount. Furthermore, an effect can be expected in which it forms compound with S which is solid-solved in alloy thereby restrain formation of compound of Cr being constituent element of surface oxidation scale and S and prevent reducing local Cr amount. In addition, misch metal which is an alloy containing multiple REM is generally used as raw material of REM, and there also may be a case in which Ni-Fe alloy containing one kind of REM is used. However, an excessive addition may cause deterioration of hot workability and welding properties of alloy, and adhesion property of surface oxidation scale may rather be deteriorated by forming REM type inclusions excessively. Furthermore, immerse nozzle may be blocked during continuous casting thereby greatly deteriorate productivity. Therefore, in the present invention, REM content is set to be 0.001 to 0.010 mass%. It is desirably 0.002 to 0.009 mass%, and more desirably 0.003 to 0.008 mass%.

[0039] In oxidation resistance of the austenitic Ni-Cr-Fe alloy, the formula (1) indicates degree of influence of elements on surface oxidation scale formed on the surface of alloy by multiple regression analysis as a formula. Si, Cr, Al, Ti and REM (La, Ce and Y) improves oxidation resistance which is evaluated in a cycle test in which temperature was repeatedly changed from room temperature to 1000 to 1200 °C under mixture gas atmosphere consisting of 5%O 2 - 16%H 2 O - 12%CO 2 - 0.8%CO - 0.1%NO 2 - bal N 2 . On the other hand, S deteriorates adhesion property between oxide film and base material thereby separate oxidation film and promote oxidation. Mn causes deterioration of oxidation resistance by forming Cr-Mn type oxides. Mo may be oxidized preferentially thereby separate oxidation scale in a case in which Mo content is large, and alloy is under high-temperature environments, and oxygen potential on the surface is low. Furthermore, since oxidation scale on alloy may become porous if B content is large, oxidation rate at high temperature may be increased and enlargement and separation of scale may be promoted. N reacts Al and Ti which contribute to improvement on oxidation resistance to form AlN and TiN, respectively, thereby reduce effects of Al and Ti. In addition, it is not desirable to add excessively alloy elements contributing to improvement on oxidation resistance, since adhesion property is rather deteriorated by excessive growth of surface oxidation scale and large amount of inclusions causing surface damage are generated. Therefore, these elements are set to be not less than 30 in lower limit and not more than 60 in upper limit based on the formula (1). It is desirably 31 to 59, and more desirably 32 to 58. 4.5 ≤ REM / S ¯

[0040] As an index in order to sufficiently obtain effect improving hot workability and oxidation resistance of alloy, if relationship of contents of rare earth element (REM) and S which forms compound with REM satisfies 4.5 ≤ REM / S, REM content sufficient for fixing S as inclusions can be maintained and the above effect can be obtained. On the other hand, if it is less than 4.5, it is not desirable because effect of REM cannot be obtained sufficiently.Definition of surface oxidation scale

[0041] As shown in Fig. 1, austenitic Ni-Cr-Fe alloy plate according to one Embodiment of the present invention has a base metal BM of austenitic Ni-Cr-Fe alloy having composition of components satisfying the above formulae (1) and (2). In a cycle test in which temperature was repeatedly changed from room temperature to 1000 to 1200 °C under mixture gas atmosphere consisting of 5%O 2 - 16%H 2 O - 12%CO 2 - 0.8%CO - 0.1%NO 2 - bal N 2 , oxidation scale mainly containing Cr oxide is formed on the surface of base material of austenitic Ni-Cr-Fe alloy of the present invention. At this time, thickness of the surface oxidation scale corresponds to region LE from the outermost layer of the surface oxidation scale to interface of scale / alloy in Fig. 1, in an observation of cross section microstructure after the test.Surface oxidation scale formed in a cycle test in which temperature is repeatedly changed from room temperature to 1000 to 1200 °C under mixture gas atmosphere consisting of 5%O 2 - 16%H 2 O - 12%CO 2 - 0.8%CO - 0.1%NO 2 - bal N 2 has thickness of 6 to 60 µm

[0042] In the austenitic Ni-Cr-Fe alloy of the present invention, the oxidation scale mainly containing Cr oxide is formed on the surface of base material of the alloy in the cycle test in which temperature was repeatedly changed from room temperature to 1000 to 1200 °C under mixture gas atmosphere consisting of 5%O 2 - 16%H 2 O - 12%CO 2 - 0.8%CO - 0.1%NO 2 - bal N 2 , and the alloy obtains oxidation resistance in the high-temperature environments. At this time, sufficient oxidation resistance may not be obtained if the thickness of the surface oxidation scale is less than 60 µm, and on the other hand, separating property of the surface oxidation scale may be increased and adhesion property between the surface oxidation scale and surface of base material may be deteriorated if the thickness is more than 60 µm. Accordingly, it is necessary that the protective surface oxidation scale formed in the above high-temperature environments have thickness of 6 to 60 µm. It is desirably 8 to 55 µm, and more desirably 10 to 50 µm.

[0043] A method for specifying limitation formula shown by the formula (1) is as follows.

[0044] Kinds of alloys in which Ni-17%Cr-9%Fe was a basic composition and amounts of addition of Si, Cr, Al, Ti, La, Ce, Y, B, Mn, Mo, S and N were varied were melt in a vacuum melting furnace. After hot forging, hot-forged plates having 8mmt x 80mmw were prepared. Solution heat treatment was performed on the obtained hot-forged plates under condition of 1200°C x10 minutes. After surface grounding, cold rolling was performed to have thickness of 2 mmt. After that, solution heat treatment was performed under condition of 1150°C x1 minute. After that, the plates were cut to have size of 20 mm x 30 mm, and finished by wet polishing of #320 on their surfaces to obtain test pieces. The test pieces obtained were subjected to the repeated oxidation test in which one cycle consists of 1200°C x 10 minutes, 1000°C x 10 minutes, 1200°C x 10 minutes and room temperature x 20 minutes under mixture gas atmosphere consisting of 5%O 2 - 16%H 2 O - 12%CO 2 - 0.8%CO - 0.1%NO 2 - bal N 2 . With respect to the test pieces after 200 cycles, a value of which mass change except for separated scale weight was divided by surface area before the test was evaluated.

[0045] From the above test results, degree of influence of added element on oxidation resistance of the austenitic Ni-Cr-Fe alloy became obvious, and relational formula of composition of components shown by the formula (1) was obtained by multiple regression analysis. It is obvious that sufficient oxidation resistance can be exhibited at 30 to 60.

[0046] Next, the method for production of the austenitic Ni-Cr-Fe alloy of the present invention is explained.

[0047] In the method for production of the austenitic Ni-Cr-Fe alloy of the present invention, raw materials such as iron scrap, stainless steel scrap, ferronickel and ferrochromium were melted in an electric furnace; mixture gas of oxygen and noble gas was blown in an AOD (Argon Oxygen Decarburization) furnace or VOD (Vacuum Oxygen Decarburization) furnace so as to perform decarburization and refining; quicklime, Fe-Si alloy, Al and the like were added so as to reduce Cr oxide in slag; fluorite was added to form CaO-SiO 2 -Al 2 O 3 -MgO-F type slag so as to perform deoxidation and desulfurization; and Ni alloy containing one kind of La, Ce and Y was added. The reason for using the CaO-SiO 2 -Al 2 O 3 -MgO-F type slag is that deoxidation and desulfurization can be effectively performed as mentioned above, and furthermore, REM can be effectively added without being oxidized or sulfurized during adding REM. At this time, it is desirable that CaO concentration in the slag is 40 to 80%. That is, if it is less than 40%, the above-mentioned desulfurization reaction may not proceed. If it is more than 80%, more than 0.002% of Ca may be mixed in molten steel. Furthermore, it is desirable that Al 2 O 3 concentration is not more than 50%. The reason is that if the alumina activity in slag is not low, deoxidation may not proceed smoothly, and as a result, desulfurization may not proceed smoothly. After refining, slab was produced by continuous casting apparatus, and it is desirable that the slab is hot-rolled, alternatively hot-rolled and cold-rolled to obtain kinds of steel material such as thin steel plate, thick steel plate, shaped steel, steel bar, wire and the like. The present invention is not limited to continuous casting apparatus, but ingot casting-slabbing rolling method can be applied to obtain slab.

[0048] Here, although it is not limited in particular, but it is desirable that CaO-SiO 2 -Al 2 O 3 -MgO-F type slag has the following composition.CaO: 55 to 70%

[0049] It the CaO is less than 55%, deoxidation may not proceed and oxygen concentration may be higher than the range of the present invention. On the other hand, if it is more than 70%, Ca concentration in molten steel may be higher than the range of the present invention. Therefore, the above range is desirable.SiO 2 : Not more than 10%

[0050] If SiO 2 is more than 10%, deoxidation may not proceed. As a result, oxygen concentration may be higher than the range of the present invention. Therefore, it is desirable that SiO 2 is not more than 10%.F: Not more than 10%

[0051] Fluorine is important to melt slag. As a source of fluorine, fluorite is usually added. If it is more than 10%, not only does this damage the bricks and shorten their lifetime, but it can also cause molten steel leaks. Therefore, F should be reduced to not more than 10%.Examples

[0052] Raw materials in which scrap, ferrochromium, ferronickel, stainless steel scrap were adjusted to be contained at a predetermined ratio were melted in an electric furnace of 70 t level, and mixture gas of oxygen and noble gas was blown in an AOD furnace or VOD furnace so as to perform decarburization and refining. Then, after quicklime, Fe-Si alloy, Al and the like were added to reduce Cr oxides in the slag, fluorite was added to form CaO-SiO 2 -Al 2 O 3 -MgO-F type slag so as to perform deoxidation and desulfurization. After that, one or more kinds selected from Ni-20%La, Ni-20%Ce and Ni-20%Y was added at a predetermined amount to obtain cast piece by continuous casting method. After adjusting to be kinds of compositions of components shown in Table 1, slab was obtained by continuous casting. Each component shown in Table 1 was measured as follows. In the Tables, value in brackets indicates that the value is out of the range of the present invention. It should be noted that value in brackets in Examples means that it satisfies the range of independent claims but does not satisfy the range of dependent claims. (1) Composition of C and S were measured by using carbon and sulfur simultaneous analyzer (combustion in oxygen flow - infrared absorption method). (2) Composition of N was analyzed using oxygen and nitrogen simultaneous analyzer (inert gas - impulse heating melting method) (3) Composition other than C, S and N and slag components were analyzed by calibration curve method using X-ray fluorescence analysis.

[0053] Next, the slab was hot-rolled to have thickness of 8 mm, and cold rolling, heat treatment and acid washing were repeated to produce cold-rolled coil having plate thickness of 2 to 3 mm. Final annealing temperature was 1150°C for 1 minute. Test piece of width: 20 mm, length: 30 mm, and thickness: 2 mm was obtained from the plate.<High temperature oxidation test>

[0054] In order to evaluate oxidation resistance under high-temperature environments, the surface of the test piece was wet-polished by emery paper of #320. After evacuating to 5.0×10 -3< Pa using high vacuum atmosphere heat treatment furnace, the repeated oxidation test was performed in which one cycle consists of processes maintaining 1200°C x 10 minutes, adjusting temperature at cooling rate of 40°C / min, maintaining 1000°C x 10 minutes, adjusting temperature at heating rate of 40°C / min, maintaining 1200°C x 10 minutes and maintaining room temperature x 20 minutes under mixture gas atmosphere consisting of 5%O 2 - 16%H 2 O - 12%CO 2 - 0.8%CO - 0.1%NO 2 - bal N 2 . With respect to the test pieces after 200 cycles, a value (mg / cm 2< ) of which mass change except for separated scale weight was divided by surface area before the test was evaluated as oxidation weight reduction. A sample in which oxidation weight reduction is less than 30 mg / cm 2< and a sample in which it is not less than 30 mg / cm 2< were decided as superior in oxidation resistance (O ("O" means superior)) and inferior in oxidation resistance (X ("X" means inferior)), respectively. In addition, the test piece after 200 cycles was cut and Cu-plated to enable cross-section observation, after which an embedded sample was prepared, wet-polished, and finally buff-polished to a mirror surface for observation. The cross-sectional microstructure of this sample was observed with an FE-SEM to measure the thickness of the surface oxidation scale, and oxides were identified with an attached EDS. Table 2Slag composition (mass%)NiFormula (1)Formula (2)Surface oxidation scaleOxidation resistanceCaOSiO 2 Al 2 O 3 MgOFThicknessCrFeNiOExamples163.65.912.912.74.576.37394.51681.42.23.212.30.50○262.32.311.815.97.473.744019.0577.51.11.519.40.32○362.32.311.815.97.469.77429.32457.96.53.032.40.10○463.22.011.813.69.177.274420.33176.58.13.311.70.22○563.52.311.815.96.276.855915.25654.23.84.237.10.50○662.32.311.815.97.473.84375.42270.42.9[5.8]23.60.25○762.32.311.815.97.473.81339.83249.38.82.538.90.32○861.32.311.815.98.473.414516.01956.26.71.635.10.17○962.32.311.815.97.473.065214.44372.68.00.218.60.47○1062.32.311.815.97.474.12436.75467.91.63.426.6[0.53]○1162.32.310.915.98.372.63325.63976.89.42.215.50.35○1262.32.311.815.97.471.904018.01571.66.73.417.50.44○1361.32.311.815.98.473.703126.84861.25.01.232.30.19○1462.32.311.815.97.472.93354.93664.68.43.922.60.28○1562.32.311.815.97.474.73355.9662.78.01.227.40.44○1662.32.311.814.88.574.895022.01472.70.52.923.40.10○1762.32.311.815.97.472.11327.36373.94.01.520.40.11○Compara tive Examples1880.91.01.25.311.370.2334[4.4]3263.82.13.128.50.30×1962.32.311.814.88.567.70

[64] 96.05172.85.23.715.40.36×2061.32.311.815.29.170.573034.020[47.2]7.03.040.00.39×2162.32.311.814.88.572.034932.51074.34.84.514.20.16×2262.32.311.814.88.572.783049.02278.73.70.214.60.43×2362.32.311.814.88.573.86

[27] [1.5]866.58.44.717.7[0.51]×2462.32.311.814.88.574.26414.7669.25.71.820.80.45×2545.318.311.814.89.571.38

[28] [3.5]3073.72.52.718.3[0.51]×2662.32.311.814.88.570.14

[69] 6.73667.19.52.218.40.40×2762.32.311.814.88.574.95

[20] [1.8]2972.56.73.415.30.06×2862.32.311.814.88.571.67

[77] 35.33061.56.52.127.70.15×2962.32.311.814.88.576.75

[28] [3.8]1256.94.10.036.4[0.51]×3062.32.311.814.88.572.63

[24] [3.8]962.39.92.922.30.42×

[0055] The steel plates of Nos. 1 to 17 shown in Tables 1 and 2 are Examples which satisfy conditions of the present invention, and exhibited superior oxidation resistance. On the other hand, the steel plates of Nos. 18 to 30 are Comparative Examples.

[0056] Although the steel plate of No. 18 satisfied the formula (1), since Mn content was high, oxidation resistance was deteriorated due to formation of Cr-Mn type oxides. Furthermore, as a result that CaO concentration in slag was high being 80.9% and oxygen concentration was too low, Ca concentration became high and large amount of Ca-Al oxide type inclusions were formed thereby lose effect of Al. Furthermore, fluorine content in slag was high thereby damage bricks considerably.

[0057] Since Si content was high in the steel plate of No. 19, surface damage due to intermetallic compounds such as σ phase occurred, the formula (1) was not satisfied, and oxidation resistance was deteriorated.

[0058] Although the steel plate of No. 20 satisfied the formula (1), large amount of carbides were precipitated since C content was high, and furthermore, adhesion property of scale was decreased thereby deteriorate oxidation resistance since Mo content was high.

[0059] Although the steel plate of No. 21 satisfied the formula (1), since Cr content was high, surface oxidation scale grew excessively to form scale having low adhesion property, and oxidation resistance was rather deteriorated. Furthermore, since Al content was high, large amount of carbonitrides causing surface damage was formed thereby deteriorate surface quality.

[0060] Although the steel plate of No. 22 satisfied the formula (1), since N content was high, Cr, Al and Ti which contribute to oxidation resistance were precipitated as nitrides and sufficient surface oxidation scale could not be formed.

[0061] Since REM content was low in the steel plate of No. 23, the formula (1) was not satisfied and an effect improving oxidation resistance and an effect in which S which inhibited oxidation resistance was fixed as inclusion could not be sufficiently obtained.

[0062] Since CaO concentration in slag was low being 35% in the steel plate of No. 24, desulfurizing reaction did not promote. Therefore, since S content became high, large amount of inclusions were formed, and Cr which was necessary for formation of surface oxidation scale was consumed. Furthermore, since B content was high, surface oxidation scale became porous and adhesion property was deteriorated thereby deteriorate oxidation resistance.

[0063] Since Al content was low in the steel plate of No. 25, the formula (1) was not satisfied. Since SiO 2 concentration and CaO concentration in slag were high being 18% and low being 45.3%, respectively, deoxidation was insufficient thereby increase oxygen concentration, and elements effective for oxidation resistance formed oxides thereby deteriorate oxidation resistance.

[0064] Although the steel plate of No. 26 satisfied range of component of each of elements, since the formula (1) was more than the upper limit, surface oxidation scale was formed excessively thereby deteriorate oxidation resistance.

[0065] Since Cr content was low in the steel plate of No. 27 and the formula (1) was not satisfied, oxidation resistance was deteriorated. Furthermore, since the formula (2) was not satisfied, an effect improving oxidation resistance and an effect in which S which inhibited oxidation resistance was fixed as inclusion could not be sufficiently obtained.

[0066] Since REM content was high in the steel plate of No. 28, the formula (1) was not satisfied. Furthermore, hot workability and welding property were deteriorated and immerse nozzle was blocked during continuous casting thereby extremely deteriorate productivity. Furthermore, since Ti content was also high, carbonitrides causing surface damage were formed thereby deteriorate surface quality.

[0067] Since Ti content was low in the steel plate of No. 29, the formula (1) was not satisfied and oxidation resistance was deteriorated. Furthermore, since the Fe content was within the range but slightly low, hot workability and cold workability were deteriorated thereby deteriorate productivity.

[0068] Although the steel plate of No. 30 satisfied range of component of each of elements, since the formulae (1) and (2) were not satisfied, sufficient oxidation resistance could not be obtained.Industrial Applicability

[0069] Since the austenitic Ni-Cr-Fe alloy of the present invention has superior heat resistance in addition to the above-mentioned oxidation resistance under high-temperature environments, it can be appropriately employed as industrial heat treatment furnaces or combustion parts used under high-temperature environments.Explanation of Reference Numerals

[0070] 1: Surface oxidation scale 2: Interface of surface oxidation scale / base material

Examples

examples

[0052]Raw materials in which scrap, ferrochromium, ferronickel, stainless steel scrap were adjusted to be contained at a predetermined ratio were melted in an electric furnace of 70 t level, and mixture gas of oxygen and noble gas was blown in an AOD furnace or VOD furnace so as to perform decarburization and refining. Then, after quicklime, Fe-Si alloy, Al and the like were added to reduce Cr oxides in the slag, fluorite was added to form CaO-SiO 2 -Al 2 O 3 -MgO-F type slag so as to perform deoxidation and desulfurization. After that, one or more kinds selected from Ni-20%La, Ni-20%Ce and Ni-20%Y was added at a predetermined amount to obtain cast piece by continuous casting method. After adjusting to be kinds of compositions of components shown in Table 1, slab was obtained by continuous casting. Each component shown in Table 1 was measured as follows. In the Tables, value in brackets indicates that the value is out of the range of the present invention. It should be noted that va...

Claims

1. An austenitic Ni-Cr-Fe alloy, components of composition consisting of: in mass%, C: 0.01 to 0.10%, Si: 0.02 to 0.45%, Mn: 0.10 to 0.85%, P: ≤ 0.015%, S: ≤ 0.0015%, Cr: 13.0 to 18.0%, Fe: 5.0 to 11.0%, Mo: ≤ 0.6%, Cu: ≤ 0.5%, B: ≤ 0.005%, 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 0.004%, Ca: ≤ 0.002%, total weight of one or more kinds selected from La, Ce and Y being rare earth element (REM) : 0.001 to 0.010%, Ni as a remainder and inevitable impurities, wherein the following formula (1) is satisfied. (Here, each element symbol in the formula indicates content (mass%) of the element.)2. The austenitic Ni-Cr-Fe alloy according to claim 1, wherein the alloy has composition of components in which total weight of one or more kinds selected from La, Ce and Y being rare earth element (REM) satisfies the following formula (2). 4.5 ≤ REM La , Ce , Y / S (Here, each element symbol in the formula indicates content (mass%) of the element.)3. The austenitic Ni-Cr-Fe alloy according to claim 1 or 2, wherein composition of surface oxidation scale formed on the surface of the alloy during a cycle test in which temperature was repeatedly changed from room temperature to 1000 to 1200 °C under mixture gas atmosphere consisting of 5%O2 - 16%H2O - 12%CO2 - 0.8%CO - 0.1%NO2 - bal N2 contains, in mass%, Cr: not less than 50%, Fe: 0 to 10%, Ni: 0 to 5%, O: 10 to 40%, REM: 0.05 to 0.5% and inevitable elements Mn, Si, Al and Ti as a remainder.

4. The austenitic Ni-Cr-Fe alloy according to claim 3, wherein the surface oxidation scale has thickness of 6 to 60 µm.

5. A method for production of the austenitic Ni-Cr-Fe alloy according to claim 1 or 2, comprising steps of: melting raw materials for alloy, as a refining process for adjustment of alloy composition, blowing mixture gas of oxygen and argon into melt alloy raw materials (molten alloy) so as to perform decarburization and control nitrogen concentration to be not more than 0.01%, performing Cr reduction, adding aluminum, lime and fluorite into the molten alloy so as to form CaO-SiO2-Al2O3-MgO-F type slag, rendering oxygen concentration in the molten alloy 0.0002 to 0.0040 mass%, adding raw material containing one or more kinds selected from La, Ce and Y, casting to obtain slab, and hot-rolling the slab.

6. A method for production of the austenitic Ni-Cr-Fe alloy according to claim 3, comprising steps of: melting raw materials for alloy, as a refining process for adjustment of alloy composition, blowing mixture gas of oxygen and argon into melt alloy raw materials (molten alloy) so as to perform decarburization and control nitrogen concentration to be not more than 0.01%, performing Cr reduction, adding aluminum, lime and fluorite into the molten alloy so as to form CaO-SiO2-Al2O3-MgO-F type slag, rendering oxygen concentration in the molten alloy 0.0002 to 0.0040 mass%, adding raw material containing one or more kinds selected from La, Ce and Y, casting to obtain slab, and hot-rolling the slab.

7. A method for production of the austenitic Ni-Cr-Fe alloy according to claim 4, comprising steps of: melting raw materials for alloy, as a refining process for adjustment of alloy composition, blowing mixture gas of oxygen and argon into melt alloy raw materials (molten alloy) so as to perform decarburization and control nitrogen concentration to be not more than 0.01%, performing Cr reduction, adding aluminum, lime and fluorite into the molten alloy so as to form CaO-SiO2-Al2O3-MgO-F type slag, rendering oxygen concentration in the molten alloy 0.0002 to 0.0040 mass%, adding raw material containing one or more kinds selected from La, Ce and Y, casting to obtain slab, and hot-rolling the slab.

8. The method for production of the austenitic Ni-Cr-Fe alloy according to claim 5, wherein composition of the CaO-SiO2-Al2O3-MgO-F type slag is CaO: 55 to 70%, SiO2: not more than 10%, and F: not more than 10%.

9. The method for production of the austenitic Ni-Cr-Fe alloy according to claim 6, wherein composition of the CaO-SiO2-Al2O3-MgO-F type slag is CaO: 55 to 70%, SiO2: not more than 10%, and F: not more than 10%.

10. The method for production of the austenitic Ni-Cr-Fe alloy according to claim 7, wherein composition of the CaO-SiO2-Al2O3-MgO-F type slag is CaO: 55 to 70%, SiO2: not more than 10%, and F: not more than 10%.

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