An austenitic Fe—Ni—Cr alloy has superior oxidation resistance even under extreme high temperatures and includes, in mass %: C: 0.004 to 0.13%, Si: 0.15 to 1.0%, Mn: 0.03 to 2.0%, P:≤0.040%, S:≤0.003%, Ni: 20.0 to 38.0%, Cr: 18.0 to 28.0%, Mo:≤1.0%, Cu:≤1.0%, N:≤0.03%, B:≤0.01%, Al: 0.10 to 1.0%, at least one of Ti: 0.10 to 1.0% and Zr: 0.01 to 0.6%, O: 0.0002 to 0.0030%, Ca:<0.002%, total weight of one or more from La, Ce, and Y: 0.001 to 0.010%, Fe as a remainder and inevitable impurities, and wherein the chemical composition satisfies formulae (1) and (2): 85≥0.3×Si+1.5×Ni+1.3×Cr+5.8×Al+7.7×Zr +2.7×Ti+2173×REM −3582×S ×32.9×Mo -2448×B≥47 . . . (1) 40≥0.6×Si+1.3×Cr +23.53×Al+5.88×Ti+3074×REM −5067×S −0.8×Mn −816×N≥. . . (2), and each element symbols indicates content of each element.
Legal claims defining the scope of protection, as filed with the USPTO.
. The austenitic Fe—Ni—Cr alloy according to, wherein composition of surface oxidation scale which is formed in a cycle test in which temperature is repeatedly increased from room temperature to 700 to 900° C. in a mixed gas atmosphere consisting of 7% O-16% HO-10% CO-0.5% CO-0.1% NO2-bal.Nconsists of, in mass %: Cr: not less than 40%, Fe: 10 to 20%, Ni: 0 to 10%, O:10 to 40%, REM: 0.05 to 0.5%, and remainder Mn, Si, and Ti as an inevitable impurity.
. The austenitic Fe—Ni—Cr alloy according to, wherein the surface oxidation scale has a thickness of 10 to 100 μm.
. The austenitic Fe—Ni—Cr alloy according to, wherein an internal oxide layer which is formed immediately below the surface oxidation scale comprises internal oxides containing at least one kind of Cr, Si, Mn, Al, Ti, and REM, and at the same time, an area ratio of the internal oxide layer occupies not less than 30% per 0.005 mmimmediately below the surface oxidation scale.
. A method for production of the austenitic Fe—Ni—Cr alloy according to, wherein
Complete technical specification and implementation details from the patent document.
The present invention relates to austenitic Fe—Ni—Cr alloy, and relates to austenitic Fe—Ni—Cr alloy having superior oxidation resistance in high temperature environments.
Since thermal power generating boilers, chemical plants, and reacting furnaces for purifying polysilicon are used under severe high temperature conditions at 700 to 900° C., materials to be used should be superior in high temperature strength, high temperature corrosion resistance, and oxidation resistance. In particular, as one of the necessary properties regarding oxidation resistance, following properties can be mentioned in which surface protective oxidation scale which mainly contains CrOand which is formed on the material surface in such a high temperature environment is dense, and fit of the scale with respect to the material is high. As material used in an apparatus or the like used in such high temperature environments, Fe—Cr—Ni alloys are focused on. Among these alloys, since-stainless steels such as SUS304, SUS316 and SUS347 do not have sufficient properties in the above-mentioned use environment, SUS310S, NCF800 and the like in which content of Ni and Cr are increased are generally used.
As a technique to improve properties of materials used in such severe high temperature environments, for example, Patent Document 1 proposes austenitic stainless steel plate in which trace amounts of REMs (Rare Earth Metals) are added to stainless steel, and an upper limit of Mn content is defined according to Ni content and REM content so that rate of growth of CrOoxide film generated on the surface the steel plate is suppressed. Furthermore, Patent Document 2 proposes a heat resistant steel material for a reformer in which Si content is defined according to Cr and Ni contents in steel material so that fit of CrOgenerated on the surface of steel material is improved.
However, neither of the techniques disclosed in these Patent documents consider effects of S, which possibly forms compounds with REMs or Cr in the material, and they are insufficient to be used in high temperature environments in which superior oxidation resistance properties are required. Furthermore, although REMs include multiple elements, neither of the Patent documents disclose which element among them is effective, and the techniques are difficult to practice.
In addition, neither of the Patent documents consider the influence of an internal oxide layer affecting oxidation resistance properties, and they are insufficient as a technique to be used in severe high temperature environments.
Furthermore, in recent years, Ni—Cr—Fe alloys having superior creep strength and strain release cracking resistance properties by complex addition of Ti, Al, and REM is proposed (For example, see Patent document 3). However, the alloy is produced by adjusting compositions in a high frequency induction furnace, obtaining a slab, and performing hot rolling of the slab, in a laboratory scale. It is impossible to apply the technique for mass production such as a 60 t level or the like. Furthermore, although the technique mentions that all REMs are effective, on the contrary, only Nd is mainly added, and Ce, La, and Y are merely added in some of the alloys. Furthermore, as a major problem, the technique does not include a removing process of S and O, and therefore, the technique cannot be realized unless the raw materials are carefully selected. Then, in some cases, REM may be oxidized or sulfurized, it is not an industrially reliable proposal and cannot achieve the original purpose. Therefore, according to the technique, it is difficult to rapidly and accurately provide an alloy in which creep strength is improved by adding REM at an industrial level.
The Patent documents are as follows:
Patent document 1: Japanese Unexamined Patent Application Publication No. 2003-171745
Patent document 2: Japanese Unexamined Patent Application Publication No. 2002-256398
Patent document 3: WO2018-066579
The present invention has been completed in view of the above circumstances, and an object of the present invention is to provide austenitic Fe—Ni—Cr alloys having superior oxidation resistance even when exposed to severe high temperature environments.
The inventors have researched to overcome the above. So far, a fact is known in which fitting of the surface oxidation scale generated on the surface of alloy in high temperature environments can be improved by adding La, Ce, and Y among the REMs; however, regarding contribution to oxidation resistance which is evaluated by a cycle examination in which temperature is varied from room temperature to 700 to 900° C. in a mixed gas atmosphere consisting of 7% O2-16%H2O-10% CO2-0.5%CO −0.1%NO-bal. N, sufficient knowledge has not been obtained yet. Then, correlation of La, Ce, Y and other contained elements contained in alloy was researched in detail. As a result, it became obvious that addition of La, Ce, and Y was very effective to improve oxidation resistance, and that Si, Ni, Cr, Al, Ti, and Zr as another element were also effective. On the other hand, it became obvious that content of S, Mo, and B may interfere with improvement in oxidation resistance. According to this, the inventors found that it was necessary to control Si, Ni, Cr, Al, Ti, Zr, S, Mo, and B in order to sufficiently maintain action by REM addition.
In addition, an internal oxide layer consisting of oxides of Cr, Si, Mn, Al, Ti, La, Ce, and Y may be formed immediately below the protective oxidation scale formed on the surface in a high temperature environment, as a result of studying behavior of formation of the internal oxide layer in detail, the inventors found that there was a good correlation between area ratio of the internal oxide layer and weight reduction by oxidation in the high temperature oxidation test. Practically, in an area of 0.005 mmin the internal oxide layer immediately below the surface oxidation scale, oxidation resistance was improved in a case in which the area ratio of the internal oxide occupied not less than 30%. As a result of study of the relationship between the formation behavior of the internal oxide layer and alloy elements, the inventors found that La, Ce, Y, Si, Cr, Al, and Ti were effective; on the other hand, containing of S, Mn, and N inhibited the formation behavior of the internal oxide layer. According to these results, the inventors found that it was necessary to control La, Ce, Y, Si, Cr, Al, Ti, S, Mn, and N in order to improve oxidation resistance by control of the internal oxide layer.
Furthermore, the inventors found that a value in which total weight (mass %) of one or more kinds selected from La, Ce, and Y being REMs as alloy element contained in alloy is divided by content (mass %) of S contained in the alloy had good correlation with weight reduction by oxidation in high temperature oxidation tests, and it became obvious that the characteristics formula obtained by them was 3.2≤REM/S.
In addition, as a result of research focusing on structure of surface oxidation scale formed in high temperature environments, in a case in which surface oxidation scale is formed to have thickness of 10 to 100 μm in a cycle test in which temperature is repeatedly increased from room temperature to 700 to 900° C. in a mixed gas atmosphere consisting of 7%O-16%HO-10%CO-0.5%CO-0.1%NO-bal.N, the surface oxidation scale was formed densely, had superior fit, and showed good results in weight reduction by oxidation after the test.
That is, the austenitic Fe—Ni—Cr Alloy of the present invention consists of, in mass %, C: 0.004 to 0.13%, Si: 0.15 to 1.0%, Mn: 0.03 to 2.0%, P:≤0.040%, S:≤0.003%, Ni: 20.0 to 38.0%, Cr: 18.0 to 28.0%, Mo: ≤1.0%, Cu:≤1.0%, N:≤0.03%, B:≤0.01%, Al: 0.10 to 1.0%, at least one of Ti: 0.10 to 1.0% and Zr: 0.01 to 0.6%, O: 0.0002 to 0.0030%, Ca:≤0.002%, total weight of one or more kinds selected from La, Ce, and Y being rare earth metal elements (REMs): 0.001 to 0.010%, Fe as the remainder and inevitable impurities, and satisfies the following formulae (1) and (2).
Furthermore, in the austenitic Fe—Ni—Cr alloy of the present invention, one or more kinds selected from La, Ce and Y being rare earth metal elements (REMs) satisfies the following formula (3).
In the austenitic Fe—Ni—Cr alloy of the present invention, in addition to the above chemical compositions, composition of surface oxidation scale which is formed in a cycle test in which temperature is repeatedly increased from the room temperature to 700 to 900° C. in a mixed gas atmosphere consisting of 7%O-16%HO-10%CO-0.5%CO-0.1%NO-bal.Nconsists of, in mass %: Cr: not less than 40%, Fe: 10 to 20%, Ni: 0 to 10%, O:10 to 40%, REM: 0.05 to 0.5%, and remainder Mn, Si, and Ti as an inevitable element, and the surface oxidation scale has thickness of 10 to 100 μm.
Furthermore, internal oxide layer which is formed immediately below the surface oxidation scale comprises internal oxide containing at least one kind of Cr, Si, Mn, Al, Ti, and REM, and at the same time, area ratio of the internal oxide layer occupies not less than 30% per 0.005 mmimmediately below the surface oxidation scale.
Furthermore, the present invention also proposes a method for production of the austenitic Fe—Ni—Cr alloy. That is, alloy compositions are adjusted by melting alloy raw materials and refining, in the refining process, decarburizing is performed by blowing mixed gas of oxygen and argon into melted alloy raw materials (fused alloy) so as to control nitrogen concentration the be not more than 0.03%, Cr reduction is performed, and then, aluminum, limestone and fluorite are added in the fused alloy so as to form CaO—SiO-AlO-MgO—F slag, oxygen concentration in the fused alloy is controlled to 0.0002 to 0.0030 mass %, and after that, raw material containing at least one of La, Ce, and Y is added so as to adjust compositions, the fused alloy is cast so as to obtain a slab, and the slab is subjected to hot-rolling processing so as to produce a coil.
According to the present invention, superior oxidation resistance can be imparted to an alloy in high temperature environments, and the alloy can greatly contribute to increased service life of a product.
The chemical compositions which should be contained in the austenitic Fe—Ni—Cr alloy of the present invention are explained.
C is an element which contributes to stabilizing an austenitic phase. However, if it is added excessively, carbides may be formed by combining Cr, Mo, and the like, amount of Cr solid-solved therearound may be decreased, and oxidation resistance may be deteriorated. On the other hand, since C also has an effect of increasing alloy strength by solid solution strengthening, the lower limit is set to be 0.004 mass %. Therefore, C is limited to 0.004 to 0.13 mass %. It is desirably 0.005 to 0.080 mass %, and more desirably 0.006 to 0.070 mass %.
Si is an element effective for improving oxidation resistance and avoidance of separating of oxide film. The effects can be obtained by addition of not less than 0.15 mass %. However, if it is added excessively, precipitation of intermetallic compound such as o phase may be promoted and surface damage due to the intermetallic compounds may be generated, content is set to be 0.15 to 1.0 mass %. It is desirably 0.16 to 0.8 mass % and more desirably 0.17 to 0.6 mass %.
Since Mn is an element which stabilizes an austenitic phase and has action of deoxidation, it is necessary to add not less than 0.03 mass % to obtain the effects. However, similar to Si, Mn may also cause precipitation of intermetallic compounds such as o phase and deterioration of oxidation resistance, and it is not desirable to add more than the required amount. Therefore, it is necessary to limit it to 0.03 to 2.0 mass %. It is desirably 0.03 to 1.50 mass % and more desirably 0.03 to 1.00 mass %.
P is an element inevitably contained as an impurity, and an element which degrades hot workability since it may segregate at crystalline grain boundaries as a phosphide. Therefore, it is desirable to be reduced as much as possible. However, production cost may increase by attempting to extremely reduce P content. Therefore, in the present invention, P is limited to not more than 0.040 mass %. It is desirably not more than 0.030 mass % and more desirably not more than 0.020 mass %.
Similar to P, S is an element inevitably contained as an impurity. It may easily segregate at crystalline grain boundaries, and in particular, may extremely degrade hot workability. Furthermore, it may form compounds with Cr which contribute to oxidation resistance mentioned below so that Cr, which is necessary to form the surface oxidation scale, is consumed, fit between oxide film and parent material may be deteriorated so that the oxide film may separate, oxidation may be promoted, and it is a harmful element for oxidation resistance. Since the harmfulness is extremely exhibited if it is contained at more than 0.003 mass %, it is necessary to limit it to not more than 0.003 mass %. It is desirably not more than 0.002 mass %, and more desirably not more than 0.001 mass %. As mentioned below, S can be reduced by addition of Al and reaction with slag components.
Ni is an element which stabilizes the austenitic phase, and it has an action to restrain precipitation of intermetallic compounds such as a σ phase. Furthermore, it also has an action to improve heat resistance and high temperature strength. In order to obtain the above effects sufficiently, it is added at not less than 20 mass %. On the other hand, excessive addition may cause deterioration of hot workability, increase in hot deformation resistance and increase in cost. Therefore, Ni content is set to be 20.0 to 38.0 mass %. It is desirably 21.0 to 36.0 mass % and more desirably 22.0 to 35.0 mass %.
Cr is an element which contributes to preventing corrosion in high temperature environments, and also has effects of forming a protective oxide film on the surface of an alloy in high temperature environments and reducing high temperature oxidation. It is necessary to be contained at not less than 18.0 mass % in order to sufficiently obtain the above effects. However, if Cr is added excessively, surface oxidation scale may form excessively, and fit may be deteriorated and oxidation resistance may be deteriorated. In addition, since stability of austenitic phase may be decreased and thereby Ni may need to be added in large amounts, the content is set to be 18.0 to 28.0 mass %. It is desirably 19.0 to 26.0 mass % and more desirably 20.0 to 25.0 mass %.
Mo has an effect of being solid-solved into an alloy and increasing high temperature strength even in a small amount of addition. However, in a material in which Mo is added in large amount, in a case in which surface oxygen potential is small and in high temperature environments, Mo may be preferentially oxidized and oxidation scale may separate, which are regarded as adverse effects. Therefore, from the viewpoint of maintaining fit of protective surface oxidation scale, Mo is limited to not more than 1.0 mass %. It is desirably not more than 0.8 mass % and more desirably not more than 0.6 mass %.
Although there may be a case in which Cu is added as an element to improve corrosion resistance in wet environments, the effect is little exhibited in high temperature environments like in the present invention. On the other hand, if it is added excessively, an uneven film having mottled pattern may be formed on the surface of a material, thereby deteriorating corrosion resistance. Therefore, Cu content is limited to not more than 1.0 mass %. It is desirably not more than 0.8 mass % and more desirably not more than 0.6 mass %.
N is an element which is inevitably contained as an impurity; however, since it is also an element which generates an austenitic phase, 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 with these elements, thereby precipitating nitrides. Then, hot deformation resistance may be extremely increased and hot workability may be degraded. Furthermore, due to formation of the nitrides, since Al and Ti, which are constituent elements of internal oxide which is formed immediately below the surface oxidation scale are consumed, area ratio of the internal oxide layer may be decreased. Therefore, in the present invention, N content is set to be not more than 0.03 mass %. It is desirably not more than 0.02 mass % and more desirably not more than 0.01 mass %.
Oxygen is blown in during decarburization. During this, N moves to CO gas bubbles as nitrogen gas and is removed from the system, and thus, N can be controlled within the range of the present invention.
B has an effect of helping an effect of rare earth metals (REMs) by grain boundary segregation, and is an element which contributes to high temperature strength. However, if it is added excessively, the surface oxidation scale may be porous, thereby deteriorating fit, welding property, and hot workability of an alloy. In the present invention, B content is set to be not more than 0.01 mass %. It is desirably not more than 0.008 mass % and more desirably not more than 0.006 mass %.
Al is an element which promotes formation of dense black film and improves oxidation resistance, and each of these effects can be obtained by addition of not less than 0.10 mass %. Furthermore, it is an element which is added as a deoxidizing agent, and is an important element in order to control oxygen concentration within the range of the present invention: 0.0002 to 0.0030 mass % according to the formula (a).
Underlining indicates an element in melt steel and parentheses indicate composition in slag.
By using CaO—SiO-AlO-MgO—F slag during refining of alloy of the present invention, AlOwhich is generated is effectively absorbed, thereby enabling controlling oxygen concentration. Furthermore, by promoting deoxidation, S concentration in the melt steel may be also decreased according to the formula (b).
According to the above, S concentration can be controlled within the range of the present invention at not more than 0.003 mass %. In order to satisfy this condition, it is necessary to contain Al at not less than 0.10 mass %. However, if it is added excessively, reaction may be extremely promoted toward the right-hand side of the formula (c), Ca concentration may be more than 0.002 mass %, excessive Ca—Al oxides inclusions may be formed, and Al in the alloy may be consumed there by deteriorating oxidation resistance.
According to the above, the upper limitation of Al is set to be 1.0 mass %. It is desirably 0.10 to 0.80 mass % and more desirably 0.10 to 0.60 mass %.
Similar to the above mentioned Al, since Ti and Zr act effectively to form a dense black film and to improve oxidation resistance, it is necessary to add at least one of them.
Ti is an element which promotes formation of dense black film and improves oxidation resistance, and the effects can be obtained by addition of not less than 0.10 mass %. However, if it is added excessively, surface damage may occur due to formation of large amounts of carbonitrides (TiN, TiC, TiCN). Therefore, the upper limitation of Ti is set to be 1.0 mass %. It is desirably 0.10 to 0.80 mass % and more desirably 0.10 to 0.60 mass %. Furthermore, control of C and N concentrations within the range of the present invention as mentioned above is one means for restraining carbonitrides effectively.
Zr is a homologous element of Ti, since it acts effectively to form a dense black film and to improve oxidation resistance similarly to Ti, it can be used as an alternative element of Ti. Since effect of Zr is superior to that of Ti, an effect can be obtained even by adding a small amount. If it is added excessively, large amounts of carbonitrides may be formed, thereby causing surface damage, and therefore, the upper limit is set to be 0.6 mass %. It is desirably 0.01 to 0.4 mass % and more desirably 0.05 to 0.3 mass %.
O in alloy may combine with Al, Ti, Zr, Si, La, Ce, and Y in fused steel, thereby forming oxides thereof, which may cause impair desirable effects such as oxidation resistance of these elements. Furthermore, oxide type non-metallic inclusions of an alumina type may be excessively formed, they may adhere inside an immersed nozzle to pour melt steel from a tundish into a mold of a continuous casting apparatus, and they may fall off, causing surface damage. Therefore, it is desirable that oxygen concentration be lower as much as possible and be not more than 0.0030 mass %. In order to achieve the range, Al is controlled within the concentration range of the present invention as mentioned above to perform deoxidation. On the other hand, if O in an alloy is extremely reduced, according to the formula (c), Ca concentration may be higher than 0.002 mass %. Therefore, the lower limit is set to be 0.0002 mass %. It is desirably 0.0003 to 0.0027 mass % and more desirably 0.0005 to 0.0025 mass %.
Ca is an element which is a contaminant from CaO in the slag, as mentioned above, in the alloy of the present invention. Since Ca forms large amounts of Ca—Al oxide inclusions and consumes Al in an alloy, thereby reducing oxidation resistance, it is necessary that Ca be reduced to a low level. Therefore, it is necessary that Al concentration be controlled 0.10 to 1.00 mass % and oxygen concentration be controlled 0.0002 to 0.0030 mass %. Therefore, it is necessary that Ca be not more than 0.002 mass %.
REM (La, Ce and Y) has effects in which hot workability of alloy, fit of surface oxidation scale and a parent material surface and oxidation resistance are improved, and noticeable effects can be obtained by using even a small amount thereof. Furthermore, by forming compounds with S which is solid-solved in the alloy, effects can be expected in which Cr being a constituent element of surface oxidation scale and S are restrained from forming compounds, and that local reduction in amount of Cr is prevented. In addition, a REM is used as a raw material in a form of misch metal which is an alloy generally containing multiple REMs, and there may be a case in which Fe—Ni alloy containing one kind of REM is used. However, if it is added excessively, hot workability and welding property of alloys may be deteriorated and REM type inclusions may be excessively formed, thereby deteriorating fit of surface oxidation scale. Furthermore, an immersed nozzle may become blocked during continuous casting, and productivity may be extremely deteriorated. 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 %.
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October 16, 2025
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