Ni-cr-fe-mo alloys with excellent surface properties and their production methods

Abstract

An Ni—Cr—Fe—Mo alloy consists of: in mass %, C: 0.03 to 0.30%, Si: 0.05 to 1.50%, Mn: 0.05 to 2.00%, P: not greater than 0.05%, S: not greater than 0.005%, Cr: 18.0 to 28.0%, Mo: 6.0 to 15.0%, Cu: not greater than 1.0%, Al: 0.01 to 0.50%, Ti: 0.01 to 0.40%, Nb: 0.02 to 0.60%, Fe: 15.0 to 22.0%, Co: 0.5 to 4.0%, W: 0.10 to 2.00%, B: 0.0001 to 0.0100%, N: 0.005 to 0.100%, O: 0.0001 to 0.0060%, Mg: 0.0001 to 0.0300%, Ca: 0.0001 to 0.0080%, and Ni and inevitable impurities as the remainder, in which at least one kind selected from MgO, CaO, MgO—CaO type oxide and MgO—CaO—NbO—TiO2 type oxide is contained as non-metallic inclusion, and the MgO—CaO—NbO—TiO2 type oxide contains, in mass %, NbO: 0.1 to 5.0% and TiO2: not greater than 20%.

Description

TECHNICAL FIELD

[0001] The present invention relates to Ni—Cr—Fe—Mo alloys having excellent surface properties and their production methods, and in particular, relates to Ni—Cr—Fe—Mo alloys having excellent surface properties and their production methods in which slag composition and Si, Al, Mg, Ca and O in molten metal are controlled so that non-metallic inclusions in the molten metal are controlled to have harmless composition and the number of the inclusions on the surface is reduced. The present invention relates to Ni—Cr—Fe—Mo alloys which are used for a part for a combustor for gas turbines in which gas at high temperature more than 1000° C. flows and which has high creep property which can withstands harsh environment.BACKGROUND ART

[0002] A combustor for gas turbines is a part which plays a role in which fuel is combusted to generate combustion gas having high temperature and high pressure for driving the turbine and the combustion gas is guided to an inlet of the turbine. Generally, temperature of combustion gas used in the gas turbine is 1100° C. to 1300° C., and temperature of the combustor at this time is about 550 to 600° C. However, in recent years, temperature of combustion gas has been increased every year to improve power generation efficiency, and a type in which temperature exceeds 1500° C. has been also developed. Furthermore, in the future, it is considered that the gas turbine in which temperature of combustion gas is about 1600° C. will be realized, and accompanied by this, it is estimated that temperature of the combustor will be also about 1000° C. Therefore, development of Ni alloys is also demanded which exhibit creep property at temperature higher than Ni alloys conventionally used as material for a combustor.

[0003] Since Ni alloy having superior high temperature strength and corrosion resistance contains Cr, Mo, Nb and Ti being very expensive metals compared to iron in addition to Ni which is the main component, it is very important to improve yield and reduce cost in production. Here, if surface defect such as linear damage occurs on the surface of Ni—Cr—Fe—Mo alloy, since it may be needed to be removed by grinding or cutting and the yield may be greatly deteriorated, Ni—Cr—Fe—Mo alloys having superior surface properties are demanded.

[0004] Patent document 1 discloses a technique in which carbides and nitrides are precipitated by heat treatment so that superior room temperature strength, workability and creep property are realized in Ni alloy material. However, carbides and nitrides do not cause damage on the surface of products, the invention of the Patent Document 1 cannot be applied to the problem of surface properties due to oxide type non-metallic inclusions which are generated during refining, which is the object of the present invention. The problem of surface defects due to oxide type non-metallic inclusions still remains.

[0005] Patent Document 2 discloses a technique in which, in high Ni alloys for high temperature containing Al and Ti and their production methods, Ca / Al mass ratio in oxide type inclusions is set to be 1.0 to 1.5 so that composition of the oxide type inclusions is controlled to be CaO—Al2O3 type having low melting point, an immerse nozzle of a continuous casting apparatus is prevented from being blocked and surface defects are prevented from being generated on products.

[0006] However, an object of the Patent Document 2 is high Ni alloys having Mo at not greater than 5%, on the other hand, the present invention is for Ni—Cr—Fe—Mo alloy having Mo: 6.0 to 15.0%. Since Mo is a component which greatly increases activity of Si which is a deoxidizing material, composition of oxide type non-metallic inclusions may differ greatly even if minor components such as Ca, Mg, Al, Si, O and the like are the same, and a different technique may be necessary to control composition of inclusions of alloy having different Mo concentration. Furthermore, in the Patent Document 2, composition of inclusions is controlled by adding Ca alloy in molten metal. Ca is a deoxidizing material more powerful than Al, Si, Ti or the like, and it has ability in which composition of inclusions already generated is changed to CaO—Al2O3 type; however, a lot of inclusions may be generated at the same time. That is, if a powerful deoxidizing material such as Ca is added at last stage of refining, cleanliness may be deteriorated and surface quality of products may be deteriorated. That is, surface properties of the Ni—Cr—Fe—Mo alloys of the present invention cannot be improved sufficiently according to the Patent Document 2.

[0007] Patent Document 3 discloses a technique in which, in high Ni alloys, composition of non-metallic inclusions in alloy is controlled to generate low melting point inclusions having good stretching-dividing property during hot or cold rolling, so that surface defects are reduced. However, target of the Patent Document 3 is for high Ni alloy containing Cr: not greater than 0.5% or containing Cr: 3 to 10%, it is different from Ni—Cr—Fe—Mo alloy containing Cr: 18.0 to 28.0% of the present invention. Cr content has a great influence on control of inclusion composition, composition of oxide type non-metallic inclusions may differ greatly even if minor components such as Ca, Mg, Al, Si, O and the like are the same. That is, surface properties of the Ni—Cr—Fe—Mo alloy of the present invention cannot be improved sufficiently according to the method to control non-metallic inclusion composition disclosed in the Patent Document 3.

[0008] Patent Document 4 discloses a technique in which in stainless steel plate, inclusions are controlled to be harmless MgO, CaO—Al2O3—MgO type oxides so as to reduce surface defects. Nb that is contained in the Ni—Cr—Fe—Mo alloy of the present invention at 0.02 to 0.60% has oxidation ability almost as same as that of Si and Mn. However, the stainless steel plate which is disclosed in the Patent Document 4 does not contain Nb. Furthermore, the stainless steel plate of the Patent Document 4 also does not contain Ti which is contained at 0.01 to 0.40% in the Ni—Cr-Me-Mo alloy of the present invention. As similar to Nb, Ti is also a component which has a great influence on composition of non-metallic inclusions, surface properties of the Ni—Cr—Fe—Mo alloy of the present invention cannot be improved according to the technique disclosed in the Patent Document 4.

[0009] Patent Document 5 discloses a technique in which Nb is added to Fe—Ni—Cr alloy in high yield. However, Mo content of the Fe—Ni—Cr alloy which is an object is 1 to 5%, on the other hand, as mentioned above, the present invention is for the Ni—Cr—Fe—Mo alloy containing Mo at 6.0 to 15.0%. Mo is a component which greatly increases activity of Si which is a deoxidizing material, composition of oxide type non-metallic inclusions may differ greatly even if minor components such as Ca, Mg, Al, Si, O and the like are the same, and a different technique may be necessary to control composition of inclusions of alloy having different Mo concentration. Furthermore, alloy of the Patent Document 6 does not contain Ti. Ti is a component which has a great influence on composition of non-metallic inclusions, and control in the Patent Document 5 greatly differs from control of the non-metallic inclusions of the Ni—Cr—Fe—Mo alloy containing Ti: 0.01 to 0.40% of the present invention. That is, the technique in the Patent Document 5 cannot be applied to surface defects of the Fe—Ni—Cr alloy which is the object of the present invention.

[0010] Patent Document 6 discloses a technique in which non-metallic inclusions in Ni—Cr—Mo—Nb alloy are controlled to single MgO and complex oxynitride of MgO and (Ti, Nb)N so that large cluster is suppressed from forming, and a thin plate product of good quality having no surface defects is obtained. However, the technique for the Nb—Cr—Mo—Nb alloy disclosed in the Patent Document 6 is for Nb—Cr—Mo—Nb alloy containing Nb at 2.5 to 5%, and the alloy greatly differs from the present invention alloy containing Nb at 0.02 to 0.60%. Nb is a component which has a great influence on composition of non-metallic inclusions, control in the Patent Document 6 greatly differs from necessary control of non-metallic inclusions of the present invention. That is, the technique in the Patent Document 6 cannot be applied to surface defects of the Fe—Ni—Cr alloy which is the object of the present invention.

[0011] The Patent Documents are as follows.

[0012] Patent Document 1: Japanese Unexamined Patent Application Publication No. 2009-185352

[0013] Patent Document 2: Japanese Unexamined Patent Application Publication No. 2021-70838

[0014] Patent Document 3: Japanese Unexamined Patent Application Publication No. Heisei 11 (1999)-315354

[0015] Patent Document 4: Japanese Unexamined Patent Application Publication No. 2019-35124

[0016] Patent Document 5: Japanese Unexamined Patent Application Publication No. 2014-105341

[0017] Patent Document 6: Japanese Unexamined Patent Application Publication No. 2017-159449SUMMARY OF INVENTION

[0018] In view of the above problems, an object of the present invention is to provide Ni—Cr—Fe—Mo alloys in which composition of non-metallic inclusions having influence on surface properties is controlled and therefore surface properties are excellent, and furthermore, to provide production methods of the Ni—Cr—Fe—Mo alloy which realize such alloys.

[0019] The inventors have studied and researched in order to solve the above-mentioned problems. They have analyzed surface defects on Ni—Cr—Fe—Mo alloy and the alloy plate having the defects on the surface thereof in detail by a scanning electron microscope (SEM) and an energy dispersive X-ray analyzer (EDS), and then, they found that the surface defects were caused by non-metallic inclusions of MgO—CaO—TiO2 type oxide. FIGS. 1 and 2 show a mechanism in which the surface defects are generated during continuous casting. The reference numeral 1 in FIG. 1 is a ladle which holds molten metal 2. The molten metal 2 is moved to a tundish 3, and is poured into a mold 5 via an immerse nozzle 4. The molten metal 2 in the mold 5 is pulled out downwardly, is solidified to form solidifying shell 6, and cooled at spray cooling zone 7 so as to obtain a slab at downstream side. In this process, as shown in FIG. 2, non-metallic inclusions 8a which are contained in the molten metal 2 flow, and part of them adhere on inner wall of the immerse nozzle 4, so as to form an aggregation 8b. The non-metallic inclusions of this kind may easily adhere on the inner wall of the immerse nozzle which is for pouring molten metal from the tundish to the mold in continuous casting apparatus and increase in size. Inclusions 8c which fell off may flow into the mold 5 and may easily be captured in the solidification shell 6 and become origin point of surface defects. Actually, they have been the origin points of surface defects of Ni—Cr—Fe—Mo alloy plates. It should be noted that although a tundish and an immerse nozzle are not used in conventional casting (ingot casting), since a similar refractory flow path is used to guide molten metal to a mold, there is a problem similar in continuous casting, in which non-metallic inclusions adhere on inner wall of the flow path.

[0020] The inventors have further researched on relationship between inclusion composition and metal component in the Ni—Cr—Fe—Mo alloy. Practically, during the process for production of the Ni—Cr—Fe—Mo alloy, metal sample of the Ni—Cr—Fe—Mo alloy was collected from the tundish of the continuous casting apparatus. Twenty pieces of inclusions having size more than 5 μm were freely selected from the sample, and each composition thereof was measured by SED / EDS. Furthermore, the immerse nozzle for supplying molten metal from the tundish of the continuous casting apparatus to the mold was collected, and component of material adhered on the inner wall of the nozzle as shown in FIG. 2 was analyzed by SEM / EDS. Based on the above, the inventors have researched on relationship among composition of inclusion, metal component, and adhered material on the inner wall of the immerse nozzle.

[0021] As a result, the inventors found that at least one kind selected from MgO, CaO, MgO—CaO type oxide and MgO—CaO—NbO—TiO2 type oxide is contained as non-metallic inclusions, and the MgO—CaO—NbO—TiO2 type oxide contains, in mass %, NbO: 0.1 to 5.0% and TiO2: not greater than 20%, so that the non-metallic inclusions may be less likely to adhere and to precipitate on the inner wall of the immerse nozzle, that is, it may be less likely to increase in size and to cause generation of surface defects.

[0022] Furthermore, it became clear that MgO, CaO, and MgO—CaO type oxides do not adhere on the inner wall of the immerse nozzle of the tundish of the continuous casting apparatus, they are fine non-metallic inclusions, and they do not have influence on surface quality of the Ni—Cr—Fe—Mo-alloy plate, therefore, MgO, CaO, and MgO—CaO type oxides are one of desirable non-metallic inclusion compositions to which non-metallic inclusions should be controlled.

[0023] The inventors also found that refining is necessary to control minor components such as Si, Al, Ca, Mg and O and slag composition to be within an appropriate range, and that components of molten metal when Nb and Ti are added are important, in order to control to the above non-metallic inclusions.

[0024] Therefore, the Ni—Cr—Fe—Mo alloy of the present invention is completed according to the above knowledge, and one aspect of the invention is the Ni—Cr—Fe—Mo alloy consisting of: in mass %, C: 0.03 to 0.30%, Si: 0.05 to 1.50%, Mn: 0.05 to 2.00%, P: not greater than 0.05%, S: not greater than 0.005%, Cr: 18.0 to 28.0%, Mo: 6.0 to 15.0%, Cu: not greater than 1.0%, Al: 0.01 to 0.50%, Ti: 0.01 to 0.40%, Nb: 0.02 to 0.60%, Fe: 15.0 to 22.0%, Co: 0.5 to 4.0%, W: 0.10 to 2.00%, B: 0.0001 to 0.0100%, N: 0.005 to 0.100%, O: 0.0001 to 0.0060%, Mg: 0.0001 to 0.0300%, Ca: 0.0001 to 0.0080%, and Ni and inevitable impurities as the remainder, in which at least one kind selected from MgO, CaO, MgO—CaO type oxide and MgO—CaO—NbO—TiO2 type oxide is contained as non-metallic inclusion, and the MgO—CaO—NbO—TiO2 type oxide contains, in mass %, NbO: 0.1 to 5.0% and TiO2: not greater than 20%, in a case in which the MgO—CaO—NbO—TiO2 type oxide is contained.

[0025] In the present invention, it is desirable that number ratio of the MgO—CaO—NbO—TiO2 type oxide with respect to the entire non-metallic inclusions is not greater than 50%.

[0026] Furthermore, the present invention provides a method for production. That is, the method for producing the Ni—Cr—Fe—Mo alloy having superior surface properties includes steps of: melting raw materials in an electric furnace, decarburizing in AOD and / or VOD, adding lime and fluorite, adding one or both of ferrosilicon alloy and pure silicon, and Al, as primary deoxidation, adding Nb at a timing O concentration gets 0.0070 to 0.0120%, performing Cr reduction, secondary deoxidation and desulfurization by adding one or both of ferrosilicon alloy and pure silicon, and Al, using CaO—SiO2—MgO—Al2O3—F type slag consisting of CaO: 50 to 70%, SiO2: 1 to 8%, Al2O3: 10 to 30%, MgO: 5 to 15%, F: 2 to 8%, adding Ti at a timing O concentration gets not greater than 0.0060%, forming slab or ingot by continuous casing apparatus or conventional casting, optionally performing hot forging in a case in which the ingot is formed, and performing only hot rolling or both of hot rolling and cold rolling.BRIEF DESCRIPTION OF DRAWINGS

[0027] FIG. 1 is a schematic cross-sectional diagram showing continuous casting apparatus during casting.

[0028] FIG. 2 is a schematic cross-sectional diagram showing immerse nozzle.

[0029] FIG. 3 is a schematic diagram of non-metallic inclusion.

[0030] FIG. 4 is a schematic diagram of non-metallic inclusion melting point thereof is decreased.

[0031] FIG. 5 is a schematic diagram of non-metallic inclusion melting point thereof is increased.

[0032] FIG. 6 is a schematic diagram of non-metallic inclusion melting point thereof is increased.EMBODIMENTS OF INVENTION

[0033] First, reasons for limiting chemical components of the Ni—Cr—Fe—Mo alloy of the present invention are explained. It should be noted that “%” means “mass %” in the following explanation.(C: 0.03 to 0.30%)

[0034] C forms carbides of M6C type. In addition, it is an element which newly forms M6C type carbides and M23C type carbides during use at high temperature, strengthens crystal grain boundary and inside of crystal grain and improves in creep property. It is necessary to add at least 0.03% in order to obtain the effects. However, if it is added more than 0.30%, coarse carbides which are not solid-solved yet may be generated, remain, and deteriorate workability and creep property. Therefore, C is set to be within a range of 0.03 to 0.30%. It is desirably within a range of 0.04 to 0.20%, and is more desirably within a range of 0.05 to 0.10%.(Si: 0.05 to 1.50%)

[0035] Since Si is an element which is effective to improve oxidation resistance and effective for deoxidation, it is an important element in the present invention. In order to control oxygen concentration to be 0.0001 to 0.0060%, it is necessary to contain at least 0.05%. Furthermore, it has roles to reduce CaO and MgO in CaO—SiO2—MgO—Al2O3—F slag and to control Mg and Ca in molten metal to be 0.0001 to 0.030% and 0.0001 to 0.0080%, respectively. According to this, it has an effect to control composition of the non-metallic inclusions. It is necessary to contain at least 0.05% from this viewpoint. On the other hand, if it is contained more than 1.50%, CaO and MgO in the slag may be reduced excessively, and Mg and Ca may be supplied excessively exceeding 0.0300% in Mg content and 0.0080% in Ca content. If Mg and Ca are contained excessively in alloy, hot workability may be decreased and cracking may occur during hot rolling thereby cause surface defects. Therefore, Si content is set to be 0.05 to 1.50%. It is desirably 0.10 to 1.00%, and is more desirably 0.20 to 0.60%.(Mn: 0.05 to 2.00%)

[0036] Since Mn is an element which stabilizes an austenitic phase and contributes to deoxidation, it is necessary to add at least 0.05%. However, since oxidation resistance may be deteriorated if it is added at large amount, the upper limit is set to be 2.00%. It is desirably 0.10 to 1.50%, and is more desirably 0.30 to 1.00%.(P: Not Greater than 0.05%)

[0037] Since P is a harmful element which segregates at grain boundary and causes cracking during hot rolling, it is desirable to reduce it as much as possible, and it is limited to not greater than 0.05%. It is desirably not greater than 0.04%, and is more desirably not greater than 0.03%.(S: Not Greater than 0.005%)

[0038] Since S is a harmful element which segregates at grain boundary, forms low-melting point inclusions, and inhibits hot workability, it is desirable to reduce it as much as possible, and it is limited to be not greater than 0.005%. In order to achieve this, desulfurizing was promoted by limiting the lower limit of Al content at 0.01%, promoting deoxidation, and limiting oxygen concentration to 0.0001 to 0.0060%. It is desirably not greater than 0.003%, and is more desirably not greater than 0.001%.(Cr: 18.0 to 28.0%)

[0039] Cr is one of the main elements in the Ni—Cr—Fe—Mo alloy of the present invention. Cr component forms appropriate protection film so that oxidation resistance of the alloy is improved. In addition, it is an important element since it has an action to form carbides of M23C6 type during using at high temperature so that strength of crystal grain boundary is increased. However, sufficient oxidation resistance may not be obtained if Cr content is less than 18.0%. On the other hand, a phase may be generated and the alloy may be brittle if the content is more than 28.0%. According to the above reasons, Cr content is set to be 18.0 to 28.0%. It is desirably 19.0 to 26.0%, and is more desirably 20.0 to 24.0%.(Mo: 6.0 to 15.0%)

[0040] Mo is one of the main elements constituting the Ni alloy material of the present invention. It has an action improving creep property by solid-solving in parent phase, and it strengthens the alloy by forming M6C type carbides by combining C. Furthermore, it has an action in which M6C type carbides are formed during using at high temperature, the carbides are precipitated inside crystal grain, and creep property is improved. Sufficient creep property may not be obtained if Mo content is less than 6.0%. On the other hand, oxidation resistance may be deteriorated if Mo content is more than 15.0%. Therefore, range of Mo content is set to be 6.0 to 15.0%. It is desirably 7.0 to 12.5%, and is more desirably 8.0 to 10.0%.

[0041] Furthermore, Mo has an effect in which activity coefficient of Si which is a main deoxidation component is increased and deoxidation action is increased. Therefore, in order to control inclusion composition, it is necessary to consider amount of addition and timing of addition of Al and Si which are deoxidating material, with considering Mo component range of the present invention.(Cu: Not Greater than 1.0%)

[0042] Cu is an element which is contained due to addition of scrap being one of raw materials, and oxidation resistance may be decreased if the content is greater than 1.0%. Therefore, Cu content is set to be not greater than 1.0%. It is desirably not greater than 0.5%, and is more desirably not greater than 0.3%.(Al: 0.01 to 0.50%)

[0043] Al is a very effective element for deoxidation, and is a particularly important element in the present invention. In addition to an effect to control oxygen concentration to range of 0.0001 to 0.0060%, it also has effects in which MgO and CaO in CaO—SiO2—MgO—Al2O3—F type slag are reduced, not less than 0.0001% of Mg and not less than 0.0001% of Ca are supplied to molten metal, and non-metallic inclusions are controlled to harmless composition. These effects depend on the following reactions.3⁢(MgO)+2⁢Al_=3⁢Mg_+(Al2⁢O3)(1)3⁢(CaO)+2⁢Al_=3⁢Ca_+(Al2⁢O3)(2)

[0044] Compound in Brackets and compound with underline indicate component in slag and component in molten metal, respectively.

[0045] Deoxidation may not be promoted sufficiently and oxygen concentration may be greater than 0.0060% if Al content is less than 0.01%. Furthermore, desulfurization may be inhibited since deoxidation is not promoted, and S concentration may be greater than 0.005%. On the other hand, Mg concentration may be greater than 0.0300% by way of the above reaction formula (1) and the Ca concentration may be greater than 0.0080% by way of the above reaction formula (2) if Al concentration is greater than 0.50%. Therefore, range of Al content is set to be 0.01 to 0.50%. It is desirably 0.03 to 0.40%, and is more desirably 0.05 to 0.30%.(Ti: 0.01 to 0.40%)

[0046] Ti forms TiN nitride which remains after annealing, suppresses growth of crystal grain during annealing, and improves room temperature strength due to fine crystal grains. Furthermore, it has an effect to improve creep property since very fine TiN nitride is precipitated in the crystal grain during use at high temperature, and M6C type carbides are precipitated uniformly and finely using the nitride as a nucleus. It is necessary to contain not less than 0.010% in order to improve strength and creep property. However, thermal expansion coefficient may be increased and weld crack susceptibility may be increased if it is excessively added greater than 0.40%. Therefore, the Ti content is set to be 0.01 to 0.40%. It is desirably 0.02 to 0.30%, and is more desirably 0.03 to 0.20%.(Nb: 0.02 to 0.60%)

[0047] Nb is an important element in the present invention, it forms NbN nitride which remains after annealing, suppresses growth of the crystal grain during annealing, and improves room temperature strength due to fine crystal grains. Furthermore, it has an effect to improve creep property since very fine NbN nitride is precipitated in crystal grain during use at high temperature, and M6C type carbides are precipitated uniformly and finely using the nitride as a nucleus. It is necessary to contain not less than 0.02% in order to improve strength and creep property. However, thermal expansion coefficient may be increased and weld crack susceptibility may be increased if it is excessively added greater than 0.60%. Therefore, Nb content is set to be 0.02 to 0.60%. It is desirably 0.05 to 0.50%, and is more desirably 0.10 to 0.30%.(Fe: 15.0 to 22.0%)

[0048] Fe is one of the main elements which is contained due to addition of scrap being one of raw materials, and oxidation resistance may be decreased since the Ni content is decreased relatively if the content is greater than 22.0%. On the other hand, if the content is less than 15.0%, since the Ni content is increased relatively, raw material cost may be increased and hot workability may be deteriorated. Therefore, Fe content is set to be 15.0 to 22.0%. It is desirably 16.0 to 21.0%, and is more desirably 17.0 to 20.0%.(Co: 0.5 to 4.0%)

[0049] Co is an element which improves creep property by solid-solution strengthening action. However, Co is an expensive element, the above effect may be saturated if the content is greater than 4.0%, and the effect may be no longer commensurate with the amount added. Therefore, the Co content is set to be 0.5 to 4.0%. It is desirably 0.6 to 3.0%, and is more desirably 0.7 to 2.0%.(W: 0.10 to 2.00%)

[0050] W is an element which improves creep property by solid-solution strengthening action. In addition, it strengthens alloy by forming M6C type carbides by combining C. Furthermore, it has an action in which M6C type carbides are formed and precipitated inside of crystal grain during use even at high temperature and thereby improve creep property. However, W is an expensive element, the above effect may be saturated if the content is greater than 2.00%, and the effect may be no longer commensurate with the amount added. Therefore, W content is set to be 0.10 to 2.00%. It is desirably 0.18 to 1.80%, and is more desirably 0.20 to 1.50%.(B: 0.0001 to 0.0100%)

[0051] B is an element which strengthens crystal grain boundary and improves creep property. However, compound having low melting point may be precipitated and hot workability may be deteriorated if B content is greater than 0.0100%. Therefore, B content is set to be 0.0001 to 0.0100%. It is desirably 0.0005 to 0.0080%, and is more desirably 0.0010 to 0.0050%.(N: 0.005 to 0.100%)

[0052] Since N has an effect in which it solid-solved in parent phase thereby increase strength at room temperature or high temperature, it is considered as an effective element which should be positively added. In addition, strength at room temperature may be increased since it forms MN type fine nitrides with Ti and Nb, suppresses growth of crystal grain during annealing and forms fine crystal grain. Furthermore, it has an effect to improve creep property since very fine MN type nitrides are precipitated in crystal grain during use at high temperature, and M6C type carbides are precipitated uniformly and finely using the nitride as a nucleus. The above effects are exhibited if it is added not less than 0.005%. However, if N is added greater than 0.100%, workability at room temperature may be deteriorated since parent phase is hardened and nitrides are coarsened and the like. Therefore, N content in the present invention is set to be 0.005 to 0.100%. It is desirably 0.010 to 0.080% and more desirably 0.015 to 0.060%.

[0053] Furthermore, N is a component which should be controlled accurately since it has an influence on properties of non-metallic inclusions which are generated during refining process. N content in molten metal can be accurately controlled by blowing nitrogen gas into molten metal in AOD or VOD in order to add N, and on the other hand, by blowing Ar gas in AOD, VOD or LF in order to decrease N concentration.(O: 0.0001 to 0.0060%)

[0054] O is very important in the present invention since oxygen concentration closely relates to inclusions. The number of inclusions may be increased thereby cause surface defects and desulfurizing may be inhibited thereby increase S concentration if O exists greater than 0.0060% in alloy. However, ability of Al to reduce CaO and MgO in slag may be too high and Mg concentration may exceed 0.0300% of upper limit and Ca concentration may exceed 0.0080% of upper limit, if O content is less than 0.0001%. Therefore, O content is set to be 0.0001 to 0.0060%. It is desirably 0.0003 to 0.0050% and more desirably 0.0005 to 0.0040%.(Mg: 0.0001 to 0.0300%)

[0055] Mg is an element which is effective to control composition of non-metallic inclusions in molten metal to MgO and MgO—CaO type oxides which do not have adverse influence on surface properties. The effect may not be obtained if the content is less than 0.0001%, and on the other hand, cracking may easily occur during hot rolling process due to deteriorated hot workability and thereby cause surface defects in final products if the content is greater than 0.0300%. Therefore, the Mg content is set to be 0.0001 to 0.0300%. It is desirably 0.00050 to 0.0200%, and is more desirably 0.0010 to 0.0100%.

[0056] It is desirable that the reaction formula (1) is used in order to add Mg effectively in molten metal. Slag composition should be controlled to CaO: 50 to 70%, SiO2: 1 to 8%, Al2O3: 10 to 30%, MgO: 5 to 15% and F: 2 to 8% in order to control Mg to be within the above range.(Ca: 0.0001 to 0.0080%)

[0057] Ca is an element which is effective to control composition of non-metallic inclusions in molten metal to CaO and MgO—CaO type oxides which do not form cluster and do not have adverse influence on surface properties. The effect may not be obtained if the content is less than 0.0001%, and on the other hand, cracking may easily occur during hot rolling process due to deteriorated hot workability and thereby cause surface defects in final products if the content is greater than 0.0080%. Therefore, the Ca content is set to be 0.0001 to 0.0080%. It is desirably 0.0002 to 0.0050%, and is more desirably 0.0003 to 0.0030%.

[0058] It is desirable that the reaction formula (2) is used in order to add Ca effectively in molten metal. Slag composition should be controlled to CaO: 50 to 70%, SiO2: 1 to 8%, Al2O3: 10 to 30%, MgO: 5 to 15% and F: 2 to 8% in order to control Ca to be within the above range.(Ni: Remainder)

[0059] An object alloy of the present invention contains the above explained components, and its remainder is Ni.(Non-Metallic Inclusions)

[0060] In the present invention, it is desirable that at least one kind selected from MgO, CaO, MgO—CaO type oxide and MgO—CaO—NbO—TiO2 type oxide is contained as the non-metallic inclusion, and MgO—CaO—NbO—TiO2 type oxide contains, in mass %, NbO: 0.1 to 5.0% and TiO2: not greater than 20%.

[0061] Furthermore, it is desirable that number ratio of MgO—CaO—NbO—TiO2 type oxide is not greater than 50%. Hereinafter reasons for limiting the components and the number ratio of the non-metallic inclusions are explained.(At Least One Kind Selected from MgO, CaO, MgO—CaO Type Oxide and MgO—CaO—NbO—TiO2 Type Oxide is Contained)

[0062] The Ni—Cr—Fe—Mo alloy of the present invention contains at least one kind selected from MgO, CaO, MgO—CaO type oxide and MgO—CaO—NbO—TiO2 type oxide is contained, according to the Si, Al, Mg, Ca and O contents in Ni—Cr—Fe—Mo alloy and amount of addition and timing of addition of Nb and Ti.

[0063] Since MgO, CaO and MgO—CaO type oxide are non-metallic inclusions of solid phase and having high melting points, they do not adhere on inner wall of immerse nozzle of tundish of continuous casting apparatus, they are fine non-metallic inclusions, and they do not have an adverse influence on surface quality of Ni—Cr—Fe—Mo alloy of the present invention. MgO, CaO and MgO—CaO type oxide inclusions are desirable composition to which non-metallic inclusions should be controlled. Furthermore, regarding CaO and MgO—CaO type oxide inclusions, part of CaO reacts S in molten metal to generate CaS during solidifying process of Ni—Cr—Fe—Mo alloy in continuous casting apparatus. S segregates at grain boundary during solidifying process of Ni—Cr—Fe—Mo alloy, deteriorates hot workability, and occurs surface crack and edge crack. CaO and MgO—CaO type oxide inclusions have an effect in which S in molten metal which deteriorates hot workability is fixed as CaS so that surface quality are kept clean.

[0064] Furthermore, CaO inclusion existing on the surface becomes Ca(OH)2 by hydration reaction in wet environment and falls off from the surface, thereby form small pit and cause deterioration of corrosion resistance. However, since Ni—Cr—Fe—Mo alloy which is an object of the present invention is applied to parts for combustion tower for gas turbine in which gas flows at high temperature over 1000° C., there is no problem of deterioration of corrosion resistance by CaO inclusion under wet environment.(MgO—CaO—NbO—TiO2 Type Oxide Contains, in Mass %, NbO: 0.1 to 5.0% and TiO2: Not Greater than 20%)

[0065] MgO—CaO—NbO—TiO2 type oxide becomes configuration in which liquid phase MgO—CaO—TiO2 type oxide exists around MgO, CaO and MgO—CaO type oxide inclusions at 1600° C. of refining temperature of Ni—Cr—Fe—Mo alloy as shown in FIG. 3. If the liquid phase MgO—CaO—TiO2 type oxide is formed around MgO, CaO and MgO—CaO type oxide inclusions, the liquid phase inclusion may play a role of adhesive, furthermore, MgO, CaO and MgO—CaO type oxide inclusions which are solid phase may play a role of aggregate, and non-metallic inclusions are promoted to adhere on refractory of the inner wall of the immerse nozzle as shown in FIG. 2. The non-metallic inclusions fall off after being coarsened, they are carried to mold together with molten metal and they are captured in solidifying shell, thereby cause surface defects.

[0066] Then, the inventors have researched further on precipitated material on the inner wall of the immerse nozzle and non-metallic inclusions inside of surface defects of Ni—Cr—Fe—Mo alloy and plate of the alloy. Then, the inventors found that adhering on the inner wall of the immerse nozzle can be prevented and deterioration of surface properties can be prevented if MgO—CaO—TiO2 type oxide contains NbO: 0.1 to 5.0 mass %. Effect of NbO is explained as follows.

[0067] As shown in FIG. 4, NbO in MgO—CaO—TiO2 type oxide exists in liquid phase part, and has an effect decreasing melting point of MgO—CaO—TiO2 type oxide of liquid phase part. That is, NbO reduces effect which is like adhesive by liquid phase MgO—CaO—TiO2 type oxide and prevents MgO—CaO—TiO2 type oxide from adhering on the inner wall of the immerse nozzle. However, if MgO—CaO—NbO—TiO2 type oxide contains NbO greater than 5.0 mass %, part of NbO may change to NbN according to the reaction formula (3), and may be contained in non-metallic inclusion.[NbO]+N=O_+[NbN](3)

[0068] Compound in brackets and compound with underline indicate component in non-metallic inclusion and component in molten metal, respectively.

[0069] NbN is a nitride having melting point at 2573° C., and generates MgO—CaO—TiO2 type oxide+NbO (—NbN) liquid phase having high melting point again around MgO, CaO and MgO—CaO type oxide inclusions as shown in FIG. 5. The liquid phase inclusion having high melting point again acts as adhesive, and promotes non-metallic inclusions adhering on the inner wall of the immerse nozzle. The non-metallic inclusions adhere and precipitate on the inner wall, fall off after being coarsened, and cause deterioration in cleanliness. From the above reasons, it is desirable that MgO—CaO—NbO—TiO2 type oxide contains NbO at 0.1 to 5.0 mass %.

[0070] However, in MgO—CaO—NbO—TiO2 type oxide which contains TiO2 at greater than 20 mass %, a part of TiO2 may change to TiN according to the reaction formula (4) and may be contained in non-metallic inclusions.[TiO2]+N=2⁢O+[TiN](4)

[0071] Compound in brackets and compound with underline indicate component in non-metallic inclusion and component in molten metal, respectively.

[0072] TiN is a nitride having melting point at 2950° C., and generates MgO—CaO—NbO—TiO2 (—TiN) type oxide of liquid phase having high melting point around MgO, CaO and MgO—CaO oxide inclusions as shown in FIG. 6. If NbO is contained at 0.1 to 5.0 mass % in MgO—CaO—NbO—TiO2 type oxide having greater than 20 mass % of TiO2 content, the effect of lowering the melting point of liquid phase inclusions is lost, resulting in deterioration of cleanliness. From the above reasons, MgO—CaO—NbO—TiO2 type oxide is set to contain NbO at 0.1 to 5.0 mass % and TiO2 at not greater than 20 mass %.

[0073] The above-mentioned effects of NbO and TiO2, may not change if MgO—CaO—NbO—TiO2 type oxide contains SiO2: not greater than 2 mass % and Al2O3: not greater than 2 mass %.

[0074] In order to make MgO—CaO—NbO—TiO2 type oxide contain 0.1 to 5.0% NbO, timing at which Nb is added during refining of molten metal is important. One or both of ferrosilicon alloy and pure silicon, and Al are added as primary deoxidation and then Nb is added at a timing when 0 concentration gets 0.0070 to 0.0120%, so that NbO concentration in non-metallic inclusions can be controlled accurately.

[0075] Furthermore, in order to make MgO—CaO—NbO—TiO2 type oxide contain not greater than 20% of TiO2, timing at which Ti is added during refining of molten metal is important. It is important that Nb is added after the primary deoxidation; Cr reduction, secondary deoxidation and desulfurization are performed using CaO—SiO2—MgO—Al2O3—F type slag consisting of CaO: 50 to 70%, SiO2: 1 to 8%, Al2O3: 10 to 30%, MgO: 5 to 15% and F: 2 to 8% and adding one or both of ferrosilicon alloy and pure silicon and Al; and then Ti is added at a timing when 0 concentration gets not greater than 0.0060%. In this way, TiO2 concentration in non-metallic inclusion can be controlled to be not greater than 20%.

[0076] Furthermore, regarding control of N content, N content in molten metal can be accurately controlled by blowing nitrogen gas into molten metal in AOD or VOD in order to add N, and on the other hand, by blowing Ar gas in AOD, VOD or LF in order to decrease N concentration.

[0077] In the present invention, regarding composition of non-metallic inclusions consisting of oxides of two or more phases as shown in FIGS. 3 to 6, concentration (mass %) of each component of non-metallic inclusions is indicated based on average component of the entirety.(Number Ratio of MgO—CaO—NbO—TiO2 Type Oxide is not Greater than 50%)

[0078] As mentioned above, MgO—CaO—NbO—TiO2 type oxide is one of non-metallic inclusions which adheres on the inner wall of the immerse nozzle, falls off from the wall after being coarsened, and deteriorates cleanliness. However, it became clear that tendency to adhere is slight degree and number of surface defects occurring can be suppressed if the number ratio of MgO—CaO—NbO—TiO2 type oxide is not greater than 50%. Therefore, the number ratio of MgO—CaO—NbO—TiO2 type oxide is set to be not greater than 50%.(Method for Production)

[0079] In the present invention, a method for production of Ni—Cr—Fe—Mo alloy is also proposed as follows. First, raw materials are melted in an electric furnace to produce Ni—Cr—Fe—Mo alloy molten metal having a predetermined composition. Then, the molten metal is decarburized in AOD (Argon Oxygen Decarburization) singly or in VOD (Vacuum Oxygen Decarburization) after AOD. After that, lime and fluorite are added, and one or both of ferrosilicon alloy and pure silicon, and Al, are added as primary deoxidation. Nb is added at a timing O concentration gets 0.0070 to 0.0120%. Cr reduction, secondary deoxidation and desulfurization are performed by adding one or both of ferrosilicon alloy and pure silicon, and Al, using CaO—SiO2—MgO—Al2O3—F type slag consisting of CaO: 50 to 70%, SiO2: 1 to 8%, Al2O3: 10 to 30%, MgO: 5 to 15%, F: 2 to 8%. Ti is added at a timing O concentration gets not greater than 0.0060%. After that, the molten metal is poured into a ladle and temperature and components are adjusted in LF (Ladle Furnace). Then, slab or ingot is produced by continuous casing apparatus or conventional casting. In a case in which ingot is produced, hot forging is performed to form slab. According to this, at least one kind selected from MgO, CaO, MgO—CaO type oxide and MgO—CaO—NbO—TiO2 type oxide is contained as non-metallic inclusion, the MgO—CaO—NbO—TiO2 type oxide contains, in mass %, NbO: 0.1 to 5.0% and TiO2: not greater than 20%, and Ni—Cr—Fe—Mo alloy having superior surface properties can be obtained. The produced slab is ground on its surface and heated. Hot rolling is performed singly, or hot rolling and cold rolling are performed in series. The slab is annealed and washed with acid, surface scale is removed, and a plate is produced finally.

[0080] In the method for production of Ni—Cr—Fe—Mo alloy of the present invention, slag composition is characteristic as mentioned above. Hereinafter reasons for limiting the slag composition are explained.(CaO: 50 to 70%)

[0081] CaO concentration and SiO2 concentration in slag are elements in order to deoxidize and desulfurize efficiently and to control inclusions. Activity of CaO in slag may be high and reaction in the formula (2) may promote excessively if CaO content is more than 70%. As a result, Ca concentration which is reduced in molten metal may exceed 0.0080%, cracking may easily occur in hot rolling process due to deterioration in hot workability, and surface defects may occur in final products. Therefore, the upper limit is set to be 70%. On the other hand, deoxidation and desulfurization may not be promoted and S concentration and O concentration in the present invention may not be controlled to be within the range of the present invention if CaO concentration is less than 50%. Therefore, the lower limit is set to be 50%. It is desirably 53 to 67%, and is more desirably 55 to 65%.(SiO2: 1 to 8%)

[0082] Since SiO2 in slag is an element which is necessary to maintain appropriate flowability of the slag, it is necessary to contain at least 10%. However, if it is contained greater than 8%, since Al concentration, Mg concentration and Ca concentration in molten metal may be less than the range of the present invention, the upper limit is set to be 8%. It is desirably 2 to 7% and more desirably 3 to 6%.(Al2O3: 10 to 30%)

[0083] If Al2O3 content in slag is greater than 30%, deoxidation may not be promoted sufficiently, O concentration may not be controlled to be within the range of the present invention, and MgO—CaO—NbO—TiO2 type oxide may be generated at greater than 50% of number ratio as non-metallic inclusions. On the other hand, if Al2O3 content in slag is less than 10%, flowability of slag may not be maintained, desulfurizing may not be promoted and S concentration may exceed the range of the present invention. It is desirably 12 to 28% and more desirably 15 to 25%.(MgO: 5 to 15%)

[0084] MgO in slag is an important element in order to control Mg concentration in molten metal to be the concentration range of the present invention, and also is an important element in order to control non-metallic inclusions to be desirable composition in the present invention. Therefore, it is necessary that MgO is contained at least 5% in slag. On the other hand, if MgO concentration exceeds 15%, surface defects may occur in final products since reaction of formula (1) is excessively promoted, Mg concentration in molten metal is increased, and hot workability is deteriorated. Therefore, the upper limit of MgO concentration is set to be 15%. MgO in slag may be within the above range since dolomite brick or magnesia-chrome brick is partially melted into slag during AOD refining or VOD refining. Alternatively, one or both of recycled bricks of dolomite brick and magnesia-chrome brick can be added in order to control to be the range of the invention. It is desirably 7 to 14% and more desirably 9 to 13%.(F: 2 to 8%)

[0085] F is contained in fluorite which is added in refining process of slag, and it plays a role adjusting condition of melting of slag accurately. It is necessary to add at least 2%. If F concentration is less than 2%, slag may not melt and flowability may be deteriorated. On the other hand, if F concentration is greater than 8%, since flowability of slag may be excessively high, damage to the bricks may become noticeable. Therefore, it is set to be 2 to 8%.EXAMPLES

[0086] Next, effects of the present invention are explained further in detail by way of Examples. It should be noted that the present invention is not limited only to the following Examples. Using an electric furnace of content 60 t, raw materials such as ferronickel, pure nickel, ferrochromium, iron scrap, stainless steel scrap, Fe—Ni alloy scrap and Fe—Mo were melted. Then, oxygen blowing, that is, oxidizing refining was performed in AOD, or alternatively in AOD and VOD in order to remove C. Lime and fluorite were added to generate CaO—SiO2—Al2O3—MgO—F type slag, one or both of FeSi alloy and pure Si, and Al were added in order to reduce Cr, and then deoxidizing was performed. After that, desulfurizing was promoted by stirring with Ar. Nb and Ti were appropriately added after deoxidizing. In AOD and VOD, magnesia-chrome bricks were lined on furnace body. After that, molten metal was poured into a ladle, and temperature and components were adjusted by LF (Ladle Furnace). Slab and ingot were produced by continuous casting and conventional casting. The ingot was hot-forged to produce slab.

[0087] The slab which was produced was ground on its surface, and heated at 1200° C. so as to hot roll, and hot-rolled coil was produced. Then, annealing and acid washing were performed, scale on the surface was removed, and cold rolling was performed to have a predetermined thickness so as to produce cold-rolled coil. Tables 1 and 2 show chemical components of Ni—Cr—Fe—Mo alloy obtained, slag composition when finishing AOD or VOD refining, production process, composition of non-metallic inclusions, formation of inclusion, and evaluation of quality. In the Tables, values in brackets indicate that the value is out of the range of Claims. It should be noted that values of Examples in brackets satisfy the range of independent claims, although do not satisfy the range of dependent claims.TABLE 1Chemical components (remainder Ni) mass %CSiMnPSCrMoCuAlTiNbFeCoWExamples10.050.220.440.0120.000221.48.20.100.090.060.13619.70.960.4820.060.450.450.0140.000221.38.20.120.140.080.18719.50.900.4930.050.230.450.0130.000421.48.40.100.140.060.13619.70.830.4740.080.240.480.0120.000322.58.60.200.150.040.22018.51.100.6650.090.400.460.0140.000221.88.10.220.160.060.19018.80.650.3860.060.240.510.0130.000222.18.20.080.200.100.12219.60.840.5270.050.260.460.0100.000221.98.30.070.180.100.13519.50.830.4880.050.230.450.0130.000421.48.40.100.140.060.13619.70.830.4790.090.220.390.0180.000722.08.70.220.180.16518.10.990.39100.050.380.450.0140.000223.99.20.280.100.200.13218.50.800.38110.080.280.520.0130.000820.88.20.200.060.180.18817.51.020.38120.050.110.320.0130.001121.48.70.100.030.060.21120.01.560.33130.070.090.260.0130.002122.29.80.280.020.070.22319.52.100.32Comparative140.070.280.470.0130.000420.19.00.080.090.110.13919.61.100.49Examples150.060.230.450.0110.000222.08.20.080.150.090.12819.60.920.54160.090.480.550.0210.000820.98.80.210.150.080.16218.21.120.48170.080.250.480.0200.002920.68.20.18(0.008)0.080.16119.80.880.28180.05(1.55)0.450.0130.000122.39.50.30(0.52)0.210.32018.60.880.55190.060.580.440.0120.000122.39.50.300.450.180.35019.70.870.55200.06(0.04)0.450.011(0.0060)21.29.60.08(0.003)0.090.09819.60.920.54Chemical components (remainder Ni) mass %RemainderBNOMgCaNiProducing processExamples10.00210.0150.00060.00600.000848.17EF→AOD→LF→CC20.00200.0180.00080.00150.002548.13EF→VOD→LF→CC30.00220.0220.00050.00490.002947.93EF→AOD→LF→CC40.00320.0120.00060.00580.000947.20EF→AOD→LF→CC50.00230.0210.00080.00140.002448.64EF→VOD→LF→CC60.00220.0210.00050.00800.000747.38EF→AOD→LF→CC70.00230.0220.00050.00100.003047.78EF→AOD→VOD→LF→IC80.00220.0220.00050.00500.003047.93EF→AOD→LF→CC90.00200.0330.00420.00320.001048.41EF→AOD→LF→CC100.00330.0430.00380.00160.002345.55EF→VOD→LF→CC110.00320.0320.00410.00190.001050.54EF→AOD→LF→IC120.00300.0320.00450.00080.000247.03EF→AOD→LF→CC130.00380.0350.00480.00070.000245.01EF→AOD→LF→CCComparative140.00200.0230.00520.00300.001348.39EF→AOD→LF→ICExamples150.00210.0220.00530.00140.002047.54EF→VOD→LF→CC160.00220.0220.00050.00500.003148.72EF→AOD→LF→CC170.00300.024(0.0062)0.00020.000148.94EF→AOD→LF→CC180.00400.033(0.00003)(0.0310)0.007244.68EF→AOD→LF→CC190.00340.017(0.00003)0.0220(0.0092)44.55EF→AOD→LF→CC200.00060.022(0.0078)(0.00003)(0.00003)47.28EF→VOD→LF→CC indicates data missing or illegible when filedTABLE 2Oxygenconcentration inmolten metalduring addingSlag compositionsNb, Ti (mass %)Inclusions compositions (mass %) Analyzing 20 points by EDS(mass %)DuringDuringMgOCaOMgO—CaOCaOSiO2Al2O3MgOFadding Nbaddin TinMgOnCaOCaSnMgOCaOCaSExamples11010002925321342041004792105799133062010072097382009310011400101631011114401245013Comparative144100400Examples154231116124101718141002401915131120Evaluation ofsurface defectsNon-metallicinclusionsin 10 m2 andNumber ratio ofNumber ofnon-metallicsurfaceInclusions compositions (mass %) Analyzing 20 points by EDSinclusions (%)defectsMgO—CaO—NbO—TiO2 typeMgO—CaO—NbO—TiO2due to hotnMgOCaONbOTiO2SiO2Al2O3TiNNbNtypeworkabilityDecisionExamples141.52.10.00.50.00.0201⊚23.50.31.50.00.0301⊚32.30.01.20.00.0251⊚41.70.00.00.30.00.0401⊚50.00.31.50.00.0401⊚6 00⊚7 00⊚8 00⊚964.51.11.60.00.0303◯10617.23.50.91.70.00.0303◯11950.23.31.21.50.00.0454◯121344.32.41.50.00.0(65)7Δ132042.11.40.00.0(100) ΔComparative141.20.01.04014XExamples15104.20.91.00.050X1642.9(0.02)0.40.00.04012X172040.0(0.03)0.00.0(100) X18 020X19 022X20—27X indicates data missing or illegible when filed(1) Chemical components of alloy and slag composition: Quantitative analysis was performed using an X-ray fluorescence analyzer, and Quantitative analysis of oxygen concentration of alloy was performed using inert gas impulse fusion infrared absorption spectroscopy.(2) Composition of non-metallic inclusions: Sample which was collected in a tundish immediately after start of casting was mirror-polished, and inclusions with a size of 5 μm or more were measured at 20 random points using SEM / EDS.

[0090] (3) Number ratio of non-metallic inclusions: Based on the results of measurement (2), ratio of the number of MgO—CaO—NbO—TiO2 type oxide with respect to the number of all the non-metallic inclusions was evaluated.

[0091] (4) Evaluation of surface defects: Visual observation was performed on the surface of Ni—Cr—Fe—Mo alloy plate having thickness of 1 mm in which acid washing and annealing were performed and scale on the surface was removed. The number of surface defects due to non-metallic inclusions and hot workability in an area of width 1 m and length 10 m was counted. In the evaluation of quality, a case in which the number of the surface defects was not greater than 2 in 10 m2 was evaluated as “⊚ (two concentric circles means superior)”, a case in which the number was 3 to 5 was evaluated as “∘ (which means good)”, a case in which the number was 6 to 10 was evaluated as “Δ (which means adequate)”, and a case in which the number was not less than 11 was evaluated as “x (which means inferior)”.

[0092] Since Examples 1 to 13 satisfied the range of the present invention, surface defects on the plate were slight, and superior surface properties were obtained.

[0093] In Example 12, Si concentration was 0.110% and Al concentration was 0.03%, that is, slightly low within the range of the invention. Therefore, deoxidation was promoted slightly low and O concentration was 0.0045%, which was slightly high. Supplied amounts of Mg and Ca from the slag were small, and Mg content was 0.0008% and Ca content was 0.0002%, which was low. As a result, the number ratio of MgO—CaO—NbO—TiO2 type oxide was 65%, which was high, some coarse inclusions were generated, and the number of surface defects was 7, which was “Δ” evaluation.

[0094] In Example 13, Si concentration was 0.09% and Al concentration was 0.02%, that is, slightly low within the range of the invention. Therefore, deoxidation was promoted slightly low and O concentration was 0.0048%, which was slightly high. Supplied amounts of Mg and Ca from the slag were small, and Mg content was 0.0007% and Ca content was 0.0002%, which were low. As a result, only MgO—CaO—NbO—TiO2 type oxide was generated as non-metallic inclusions, some coarse inclusions were generated, and the number of surface defects was 9, which was “Δ” evaluation.

[0095] On the other hand, Comparative Examples 14 to 20 were out of the range of the present invention, surface defects were generated much, and surface properties were deteriorated. Hereinafter each of Comparative Examples is explained.

[0096] In Comparative Example 14, O concentration before adding Nb after primary deoxidation was 0.0142%, which was high, and NbO of MgO—CaO—NbO—TiO2 type oxide was 5.5%, which was high. NbN was also generated partially in liquid phase portion of MgO—CaO—NbO—TiO2 type oxide, and melting point was increased. There were a lot of adhesion on the immerse nozzle, and many coarse non-metallic inclusions fell off and the number of surface defects was 14, which was many. The evaluation was “x”.

[0097] In Comparative Example 15, O concentration before adding Ti after secondary deoxidation was 0.0075%, which was high, and TiO2 of MgO—CaO—NbO—TiO2 type oxide was 21.1%, which was high. TiN was also generated partially in liquid phase portion of MgO—CaO—NbO—TiO2 type oxide, and melting point was increased. There were a lot of adhesion on the immerse nozzle, and many coarse non-metallic inclusions fell off and the number of surface defects was 16, which was many. The evaluation was “x”.

[0098] In Comparative Example 16, O concentration before adding Nb after primary deoxidation was 0.0060%, which was low, and NbO of MgO—CaO—NbO—TiO2 type oxide was 0.02%, which was low. Effect of decreasing melting point of liquid phase portion of MgO—CaO—NbO—TiO2 type oxide is low since NbO is 0.02%, and melting point was increased. There were a lot of adhesion on the immerse nozzle, and many coarse non-metallic inclusions fell off and the number of surface defects was 12, which was many. The evaluation was “x”.

[0099] In Comparative Example 17, O concentration before adding Nb after primary deoxidation was 0.0058%, which was low, and NbO of MgO—CaO—NbO—TiO2 type oxide was 0.03%, which was low. Furthermore, Al was 0.008% which was slightly low, O concentration was high, Mg was 0.0002% and Ca was 0.00010% which were low, and only MgO—CaO—NbO—TiO2 type oxide was generated as non-metallic inclusions. Effect of decreasing melting point of liquid phase portion of MgO—CaO—NbO—TiO2 type oxide was low since NbO was 0.03%, and melting point was increased. There were a lot of adhesion on the immerse nozzle, and many coarse non-metallic inclusions fell off and the number of surface defects was 18, which was many. The evaluation was “x”.

[0100] In Comparative Example 18, Si was 1.55% which was high, Al was 0.52% which was high, O concentration was 0.00003% which was low, supplied amount of Mg from the slag was large, and Mg was 0.0310% which was high. As a result, non-metallic inclusions were only MgO and CaO, and there is no adhesion on the immerse nozzle. However, since Mg concentration was high and hot workability was deteriorated, crack occurred during hot rolling process and the number of surface defects generated was 20, which was many. The evaluation was “x”.

[0101] In Comparative Example 19, Al was 0.45% which was slightly high, O concentration was 0.00003% which was low, and furthermore, since Ca alloy was added during end term of refining, Ca concentration was 0.0092%, which was high. As a result, non-metallic inclusions were only CaO and MgO, and there is no adhesion on the immerse nozzle. However, since Ca concentration was high and hot workability was deteriorated, crack occurred during hot rolling process. Furthermore, since Ca alloy was added during end term of refining, non-metallic inclusions in molten metal could not be floated and separated, they were flown into the mold, thereby deteriorate cleanliness. The number of surface defects generated was 22, which was many. The evaluation was “x”.

[0102] In Comparative Example 20, since Si was 0.04% which was low and Al was 0.003% which was low, deoxidation and desulfurization were not promoted. Therefore, O concentration was 0.0078% which was high, S concentration was 0.0060% which was high, Mg concentration was 0.00003% and Ca concentration was 0.00003%. Although the number of non-metallic inclusions was not counted in Table 2, MgO—Al2O3 and Al2O3—SiO2—MnO—Cr2O3 type oxides were detected. MgO—Al2O3 may easily adhere inside of the immerse nozzle, which is one of non-metallic compounds to be avoided. Al2O3—SiO2—MnO—Cr2O3 type oxide is a lower grade oxide which may be generated in molten metal in which deoxidation is not performed sufficiently. Because of insufficient deoxidation, since many non-metallic inclusions are kept generated in molten metal and they are flown into the mold, it is one of non-metallic compounds to be avoided. The number of surface defects generated was 27, which was many. The evaluation was “x”.INDUSTRIAL APPLICABILITY

[0103] According to the technique of the present invention, formation of non-metallic inclusions are controlled thereby enable supplying Ni—Cr—Fe—Mo alloys which have superior surface properties and are appropriate for use in a part for a combustor for gas turbines, which has high creep property.EXPLANATION OF REFERENCE NUMERALS

[0104] 1: Ladle, 2: molten metal, 3: tundish, 4: immerse nozzle, 5: mold, 6: solidifying shell, 7: spray cooling zone, 8a: non-metallic inclusion, 8b: non-metallic inclusions which are adhered, and 8c: coarse non-metallic inclusions which are fallen off

Examples

examples

[0086]Next, effects of the present invention are explained further in detail by way of Examples. It should be noted that the present invention is not limited only to the following Examples. Using an electric furnace of content 60 t, raw materials such as ferronickel, pure nickel, ferrochromium, iron scrap, stainless steel scrap, Fe—Ni alloy scrap and Fe—Mo were melted. Then, oxygen blowing, that is, oxidizing refining was performed in AOD, or alternatively in AOD and VOD in order to remove C. Lime and fluorite were added to generate CaO—SiO2—Al2O3—MgO—F type slag, one or both of FeSi alloy and pure Si, and Al were added in order to reduce Cr, and then deoxidizing was performed. After that, desulfurizing was promoted by stirring with Ar. Nb and Ti were appropriately added after deoxidizing. In AOD and VOD, magnesia-chrome bricks were lined on furnace body. After that, molten metal was poured into a ladle, and temperature and components were adjusted by LF (Ladle Furnace). Slab and in...

TECHNICAL FIELD

[0001] The present invention relates to Ni—Cr—Fe—Mo alloys having excellent surface properties and their production methods, and in particular, relates to Ni—Cr—Fe—Mo alloys having excellent surface properties and their production methods in which slag composition and Si, Al, Mg, Ca and O in molten metal are controlled so that non-metallic inclusions in the molten metal are controlled to have harmless composition and the number of the inclusions on the surface is reduced. The present invention relates to Ni—Cr—Fe—Mo alloys which are used for a part for a combustor for gas turbines in which gas at high temperature more than 1000° C. flows and which has high creep property which can withstands harsh environment.BACKGROUND ART

[0002] A combustor for gas turbines is a part which plays a role in which fuel is combusted to generate combustion gas having high temperature and high pressure for driving the turbine and the combustion gas is guided to an inlet of the turbine. Generally, temperature of combustion gas used in the gas turbine is 1100° C. to 1300° C., and temperature of the combustor at this time is about 550 to 600° C. However, in recent years, temperature of combustion gas has been increased every year to improve power generation efficiency, and a type in which temperature exceeds 1500° C. has been also developed. Furthermore, in the future, it is considered that the gas turbine in which temperature of combustion gas is about 1600° C. will be realized, and accompanied by this, it is estimated that temperature of the combustor will be also about 1000° C. Therefore, development of Ni alloys is also demanded which exhibit creep property at temperature higher than Ni alloys conventionally used as material for a combustor.

[0003] Since Ni alloy having superior high temperature strength and corrosion resistance contains Cr, Mo, Nb and Ti being very expensive metals compared to iron in addition to Ni which is the main component, it is very important to improve yield and reduce cost in production. Here, if surface defect such as linear damage occurs on the surface of Ni—Cr—Fe—Mo alloy, since it may be needed to be removed by grinding or cutting and the yield may be greatly deteriorated, Ni—Cr—Fe—Mo alloys having superior surface properties are demanded.

[0004] Patent document 1 discloses a technique in which carbides and nitrides are precipitated by heat treatment so that superior room temperature strength, workability and creep property are realized in Ni alloy material. However, carbides and nitrides do not cause damage on the surface of products, the invention of the Patent Document 1 cannot be applied to the problem of surface properties due to oxide type non-metallic inclusions which are generated during refining, which is the object of the present invention. The problem of surface defects due to oxide type non-metallic inclusions still remains.

[0005] Patent Document 2 discloses a technique in which, in high Ni alloys for high temperature containing Al and Ti and their production methods, Ca / Al mass ratio in oxide type inclusions is set to be 1.0 to 1.5 so that composition of the oxide type inclusions is controlled to be CaO—Al2O3 type having low melting point, an immerse nozzle of a continuous casting apparatus is prevented from being blocked and surface defects are prevented from being generated on products.

[0006] However, an object of the Patent Document 2 is high Ni alloys having Mo at not greater than 5%, on the other hand, the present invention is for Ni—Cr—Fe—Mo alloy having Mo: 6.0 to 15.0%. Since Mo is a component which greatly increases activity of Si which is a deoxidizing material, composition of oxide type non-metallic inclusions may differ greatly even if minor components such as Ca, Mg, Al, Si, O and the like are the same, and a different technique may be necessary to control composition of inclusions of alloy having different Mo concentration. Furthermore, in the Patent Document 2, composition of inclusions is controlled by adding Ca alloy in molten metal. Ca is a deoxidizing material more powerful than Al, Si, Ti or the like, and it has ability in which composition of inclusions already generated is changed to CaO—Al2O3 type; however, a lot of inclusions may be generated at the same time. That is, if a powerful deoxidizing material such as Ca is added at last stage of refining, cleanliness may be deteriorated and surface quality of products may be deteriorated. That is, surface properties of the Ni—Cr—Fe—Mo alloys of the present invention cannot be improved sufficiently according to the Patent Document 2.

[0007] Patent Document 3 discloses a technique in which, in high Ni alloys, composition of non-metallic inclusions in alloy is controlled to generate low melting point inclusions having good stretching-dividing property during hot or cold rolling, so that surface defects are reduced. However, target of the Patent Document 3 is for high Ni alloy containing Cr: not greater than 0.5% or containing Cr: 3 to 10%, it is different from Ni—Cr—Fe—Mo alloy containing Cr: 18.0 to 28.0% of the present invention. Cr content has a great influence on control of inclusion composition, composition of oxide type non-metallic inclusions may differ greatly even if minor components such as Ca, Mg, Al, Si, O and the like are the same. That is, surface properties of the Ni—Cr—Fe—Mo alloy of the present invention cannot be improved sufficiently according to the method to control non-metallic inclusion composition disclosed in the Patent Document 3.

[0008] Patent Document 4 discloses a technique in which in stainless steel plate, inclusions are controlled to be harmless MgO, CaO—Al2O3—MgO type oxides so as to reduce surface defects. Nb that is contained in the Ni—Cr—Fe—Mo alloy of the present invention at 0.02 to 0.60% has oxidation ability almost as same as that of Si and Mn. However, the stainless steel plate which is disclosed in the Patent Document 4 does not contain Nb. Furthermore, the stainless steel plate of the Patent Document 4 also does not contain Ti which is contained at 0.01 to 0.40% in the Ni—Cr-Me-Mo alloy of the present invention. As similar to Nb, Ti is also a component which has a great influence on composition of non-metallic inclusions, surface properties of the Ni—Cr—Fe—Mo alloy of the present invention cannot be improved according to the technique disclosed in the Patent Document 4.

[0009] Patent Document 5 discloses a technique in which Nb is added to Fe—Ni—Cr alloy in high yield. However, Mo content of the Fe—Ni—Cr alloy which is an object is 1 to 5%, on the other hand, as mentioned above, the present invention is for the Ni—Cr—Fe—Mo alloy containing Mo at 6.0 to 15.0%. Mo is a component which greatly increases activity of Si which is a deoxidizing material, composition of oxide type non-metallic inclusions may differ greatly even if minor components such as Ca, Mg, Al, Si, O and the like are the same, and a different technique may be necessary to control composition of inclusions of alloy having different Mo concentration. Furthermore, alloy of the Patent Document 6 does not contain Ti. Ti is a component which has a great influence on composition of non-metallic inclusions, and control in the Patent Document 5 greatly differs from control of the non-metallic inclusions of the Ni—Cr—Fe—Mo alloy containing Ti: 0.01 to 0.40% of the present invention. That is, the technique in the Patent Document 5 cannot be applied to surface defects of the Fe—Ni—Cr alloy which is the object of the present invention.

[0010] Patent Document 6 discloses a technique in which non-metallic inclusions in Ni—Cr—Mo—Nb alloy are controlled to single MgO and complex oxynitride of MgO and (Ti, Nb)N so that large cluster is suppressed from forming, and a thin plate product of good quality having no surface defects is obtained. However, the technique for the Nb—Cr—Mo—Nb alloy disclosed in the Patent Document 6 is for Nb—Cr—Mo—Nb alloy containing Nb at 2.5 to 5%, and the alloy greatly differs from the present invention alloy containing Nb at 0.02 to 0.60%. Nb is a component which has a great influence on composition of non-metallic inclusions, control in the Patent Document 6 greatly differs from necessary control of non-metallic inclusions of the present invention. That is, the technique in the Patent Document 6 cannot be applied to surface defects of the Fe—Ni—Cr alloy which is the object of the present invention.

[0011] The Patent Documents are as follows.

[0012] Patent Document 1: Japanese Unexamined Patent Application Publication No. 2009-185352

[0013] Patent Document 2: Japanese Unexamined Patent Application Publication No. 2021-70838

[0014] Patent Document 3: Japanese Unexamined Patent Application Publication No. Heisei 11 (1999)-315354

[0015] Patent Document 4: Japanese Unexamined Patent Application Publication No. 2019-35124

[0016] Patent Document 5: Japanese Unexamined Patent Application Publication No. 2014-105341

[0017] Patent Document 6: Japanese Unexamined Patent Application Publication No. 2017-159449SUMMARY OF INVENTION

[0018] In view of the above problems, an object of the present invention is to provide Ni—Cr—Fe—Mo alloys in which composition of non-metallic inclusions having influence on surface properties is controlled and therefore surface properties are excellent, and furthermore, to provide production methods of the Ni—Cr—Fe—Mo alloy which realize such alloys.

[0019] The inventors have studied and researched in order to solve the above-mentioned problems. They have analyzed surface defects on Ni—Cr—Fe—Mo alloy and the alloy plate having the defects on the surface thereof in detail by a scanning electron microscope (SEM) and an energy dispersive X-ray analyzer (EDS), and then, they found that the surface defects were caused by non-metallic inclusions of MgO—CaO—TiO2 type oxide. FIGS. 1 and 2 show a mechanism in which the surface defects are generated during continuous casting. The reference numeral 1 in FIG. 1 is a ladle which holds molten metal 2. The molten metal 2 is moved to a tundish 3, and is poured into a mold 5 via an immerse nozzle 4. The molten metal 2 in the mold 5 is pulled out downwardly, is solidified to form solidifying shell 6, and cooled at spray cooling zone 7 so as to obtain a slab at downstream side. In this process, as shown in FIG. 2, non-metallic inclusions 8a which are contained in the molten metal 2 flow, and part of them adhere on inner wall of the immerse nozzle 4, so as to form an aggregation 8b. The non-metallic inclusions of this kind may easily adhere on the inner wall of the immerse nozzle which is for pouring molten metal from the tundish to the mold in continuous casting apparatus and increase in size. Inclusions 8c which fell off may flow into the mold 5 and may easily be captured in the solidification shell 6 and become origin point of surface defects. Actually, they have been the origin points of surface defects of Ni—Cr—Fe—Mo alloy plates. It should be noted that although a tundish and an immerse nozzle are not used in conventional casting (ingot casting), since a similar refractory flow path is used to guide molten metal to a mold, there is a problem similar in continuous casting, in which non-metallic inclusions adhere on inner wall of the flow path.

[0020] The inventors have further researched on relationship between inclusion composition and metal component in the Ni—Cr—Fe—Mo alloy. Practically, during the process for production of the Ni—Cr—Fe—Mo alloy, metal sample of the Ni—Cr—Fe—Mo alloy was collected from the tundish of the continuous casting apparatus. Twenty pieces of inclusions having size more than 5 μm were freely selected from the sample, and each composition thereof was measured by SED / EDS. Furthermore, the immerse nozzle for supplying molten metal from the tundish of the continuous casting apparatus to the mold was collected, and component of material adhered on the inner wall of the nozzle as shown in FIG. 2 was analyzed by SEM / EDS. Based on the above, the inventors have researched on relationship among composition of inclusion, metal component, and adhered material on the inner wall of the immerse nozzle.

[0021] As a result, the inventors found that at least one kind selected from MgO, CaO, MgO—CaO type oxide and MgO—CaO—NbO—TiO2 type oxide is contained as non-metallic inclusions, and the MgO—CaO—NbO—TiO2 type oxide contains, in mass %, NbO: 0.1 to 5.0% and TiO2: not greater than 20%, so that the non-metallic inclusions may be less likely to adhere and to precipitate on the inner wall of the immerse nozzle, that is, it may be less likely to increase in size and to cause generation of surface defects.

[0022] Furthermore, it became clear that MgO, CaO, and MgO—CaO type oxides do not adhere on the inner wall of the immerse nozzle of the tundish of the continuous casting apparatus, they are fine non-metallic inclusions, and they do not have influence on surface quality of the Ni—Cr—Fe—Mo-alloy plate, therefore, MgO, CaO, and MgO—CaO type oxides are one of desirable non-metallic inclusion compositions to which non-metallic inclusions should be controlled.

[0023] The inventors also found that refining is necessary to control minor components such as Si, Al, Ca, Mg and O and slag composition to be within an appropriate range, and that components of molten metal when Nb and Ti are added are important, in order to control to the above non-metallic inclusions.

[0024] Therefore, the Ni—Cr—Fe—Mo alloy of the present invention is completed according to the above knowledge, and one aspect of the invention is the Ni—Cr—Fe—Mo alloy consisting of: in mass %, C: 0.03 to 0.30%, Si: 0.05 to 1.50%, Mn: 0.05 to 2.00%, P: not greater than 0.05%, S: not greater than 0.005%, Cr: 18.0 to 28.0%, Mo: 6.0 to 15.0%, Cu: not greater than 1.0%, Al: 0.01 to 0.50%, Ti: 0.01 to 0.40%, Nb: 0.02 to 0.60%, Fe: 15.0 to 22.0%, Co: 0.5 to 4.0%, W: 0.10 to 2.00%, B: 0.0001 to 0.0100%, N: 0.005 to 0.100%, O: 0.0001 to 0.0060%, Mg: 0.0001 to 0.0300%, Ca: 0.0001 to 0.0080%, and Ni and inevitable impurities as the remainder, in which at least one kind selected from MgO, CaO, MgO—CaO type oxide and MgO—CaO—NbO—TiO2 type oxide is contained as non-metallic inclusion, and the MgO—CaO—NbO—TiO2 type oxide contains, in mass %, NbO: 0.1 to 5.0% and TiO2: not greater than 20%, in a case in which the MgO—CaO—NbO—TiO2 type oxide is contained.

[0025] In the present invention, it is desirable that number ratio of the MgO—CaO—NbO—TiO2 type oxide with respect to the entire non-metallic inclusions is not greater than 50%.

[0026] Furthermore, the present invention provides a method for production. That is, the method for producing the Ni—Cr—Fe—Mo alloy having superior surface properties includes steps of: melting raw materials in an electric furnace, decarburizing in AOD and / or VOD, adding lime and fluorite, adding one or both of ferrosilicon alloy and pure silicon, and Al, as primary deoxidation, adding Nb at a timing O concentration gets 0.0070 to 0.0120%, performing Cr reduction, secondary deoxidation and desulfurization by adding one or both of ferrosilicon alloy and pure silicon, and Al, using CaO—SiO2—MgO—Al2O3—F type slag consisting of CaO: 50 to 70%, SiO2: 1 to 8%, Al2O3: 10 to 30%, MgO: 5 to 15%, F: 2 to 8%, adding Ti at a timing O concentration gets not greater than 0.0060%, forming slab or ingot by continuous casing apparatus or conventional casting, optionally performing hot forging in a case in which the ingot is formed, and performing only hot rolling or both of hot rolling and cold rolling.BRIEF DESCRIPTION OF DRAWINGS

[0027] FIG. 1 is a schematic cross-sectional diagram showing continuous casting apparatus during casting.

[0028] FIG. 2 is a schematic cross-sectional diagram showing immerse nozzle.

[0029] FIG. 3 is a schematic diagram of non-metallic inclusion.

[0030] FIG. 4 is a schematic diagram of non-metallic inclusion melting point thereof is decreased.

[0031] FIG. 5 is a schematic diagram of non-metallic inclusion melting point thereof is increased.

[0032] FIG. 6 is a schematic diagram of non-metallic inclusion melting point thereof is increased.EMBODIMENTS OF INVENTION

[0033] First, reasons for limiting chemical components of the Ni—Cr—Fe—Mo alloy of the present invention are explained. It should be noted that “%” means “mass %” in the following explanation.(C: 0.03 to 0.30%)

[0034] C forms carbides of M6C type. In addition, it is an element which newly forms M6C type carbides and M23C type carbides during use at high temperature, strengthens crystal grain boundary and inside of crystal grain and improves in creep property. It is necessary to add at least 0.03% in order to obtain the effects. However, if it is added more than 0.30%, coarse carbides which are not solid-solved yet may be generated, remain, and deteriorate workability and creep property. Therefore, C is set to be within a range of 0.03 to 0.30%. It is desirably within a range of 0.04 to 0.20%, and is more desirably within a range of 0.05 to 0.10%.(Si: 0.05 to 1.50%)

[0035] Since Si is an element which is effective to improve oxidation resistance and effective for deoxidation, it is an important element in the present invention. In order to control oxygen concentration to be 0.0001 to 0.0060%, it is necessary to contain at least 0.05%. Furthermore, it has roles to reduce CaO and MgO in CaO—SiO2—MgO—Al2O3—F slag and to control Mg and Ca in molten metal to be 0.0001 to 0.030% and 0.0001 to 0.0080%, respectively. According to this, it has an effect to control composition of the non-metallic inclusions. It is necessary to contain at least 0.05% from this viewpoint. On the other hand, if it is contained more than 1.50%, CaO and MgO in the slag may be reduced excessively, and Mg and Ca may be supplied excessively exceeding 0.0300% in Mg content and 0.0080% in Ca content. If Mg and Ca are contained excessively in alloy, hot workability may be decreased and cracking may occur during hot rolling thereby cause surface defects. Therefore, Si content is set to be 0.05 to 1.50%. It is desirably 0.10 to 1.00%, and is more desirably 0.20 to 0.60%.(Mn: 0.05 to 2.00%)

[0036] Since Mn is an element which stabilizes an austenitic phase and contributes to deoxidation, it is necessary to add at least 0.05%. However, since oxidation resistance may be deteriorated if it is added at large amount, the upper limit is set to be 2.00%. It is desirably 0.10 to 1.50%, and is more desirably 0.30 to 1.00%.(P: Not Greater than 0.05%)

[0037] Since P is a harmful element which segregates at grain boundary and causes cracking during hot rolling, it is desirable to reduce it as much as possible, and it is limited to not greater than 0.05%. It is desirably not greater than 0.04%, and is more desirably not greater than 0.03%.(S: Not Greater than 0.005%)

[0038] Since S is a harmful element which segregates at grain boundary, forms low-melting point inclusions, and inhibits hot workability, it is desirable to reduce it as much as possible, and it is limited to be not greater than 0.005%. In order to achieve this, desulfurizing was promoted by limiting the lower limit of Al content at 0.01%, promoting deoxidation, and limiting oxygen concentration to 0.0001 to 0.0060%. It is desirably not greater than 0.003%, and is more desirably not greater than 0.001%.(Cr: 18.0 to 28.0%)

[0039] Cr is one of the main elements in the Ni—Cr—Fe—Mo alloy of the present invention. Cr component forms appropriate protection film so that oxidation resistance of the alloy is improved. In addition, it is an important element since it has an action to form carbides of M23C6 type during using at high temperature so that strength of crystal grain boundary is increased. However, sufficient oxidation resistance may not be obtained if Cr content is less than 18.0%. On the other hand, a phase may be generated and the alloy may be brittle if the content is more than 28.0%. According to the above reasons, Cr content is set to be 18.0 to 28.0%. It is desirably 19.0 to 26.0%, and is more desirably 20.0 to 24.0%.(Mo: 6.0 to 15.0%)

[0040] Mo is one of the main elements constituting the Ni alloy material of the present invention. It has an action improving creep property by solid-solving in parent phase, and it strengthens the alloy by forming M6C type carbides by combining C. Furthermore, it has an action in which M6C type carbides are formed during using at high temperature, the carbides are precipitated inside crystal grain, and creep property is improved. Sufficient creep property may not be obtained if Mo content is less than 6.0%. On the other hand, oxidation resistance may be deteriorated if Mo content is more than 15.0%. Therefore, range of Mo content is set to be 6.0 to 15.0%. It is desirably 7.0 to 12.5%, and is more desirably 8.0 to 10.0%.

[0041] Furthermore, Mo has an effect in which activity coefficient of Si which is a main deoxidation component is increased and deoxidation action is increased. Therefore, in order to control inclusion composition, it is necessary to consider amount of addition and timing of addition of Al and Si which are deoxidating material, with considering Mo component range of the present invention.(Cu: Not Greater than 1.0%)

[0042] Cu is an element which is contained due to addition of scrap being one of raw materials, and oxidation resistance may be decreased if the content is greater than 1.0%. Therefore, Cu content is set to be not greater than 1.0%. It is desirably not greater than 0.5%, and is more desirably not greater than 0.3%.(Al: 0.01 to 0.50%)

[0043] Al is a very effective element for deoxidation, and is a particularly important element in the present invention. In addition to an effect to control oxygen concentration to range of 0.0001 to 0.0060%, it also has effects in which MgO and CaO in CaO—SiO2—MgO—Al2O3—F type slag are reduced, not less than 0.0001% of Mg and not less than 0.0001% of Ca are supplied to molten metal, and non-metallic inclusions are controlled to harmless composition. These effects depend on the following reactions.3⁢(MgO)+2⁢Al_=3⁢Mg_+(Al2⁢O3)(1)3⁢(CaO)+2⁢Al_=3⁢Ca_+(Al2⁢O3)(2)

[0044] Compound in Brackets and compound with underline indicate component in slag and component in molten metal, respectively.

[0045] Deoxidation may not be promoted sufficiently and oxygen concentration may be greater than 0.0060% if Al content is less than 0.01%. Furthermore, desulfurization may be inhibited since deoxidation is not promoted, and S concentration may be greater than 0.005%. On the other hand, Mg concentration may be greater than 0.0300% by way of the above reaction formula (1) and the Ca concentration may be greater than 0.0080% by way of the above reaction formula (2) if Al concentration is greater than 0.50%. Therefore, range of Al content is set to be 0.01 to 0.50%. It is desirably 0.03 to 0.40%, and is more desirably 0.05 to 0.30%.(Ti: 0.01 to 0.40%)

[0046] Ti forms TiN nitride which remains after annealing, suppresses growth of crystal grain during annealing, and improves room temperature strength due to fine crystal grains. Furthermore, it has an effect to improve creep property since very fine TiN nitride is precipitated in the crystal grain during use at high temperature, and M6C type carbides are precipitated uniformly and finely using the nitride as a nucleus. It is necessary to contain not less than 0.010% in order to improve strength and creep property. However, thermal expansion coefficient may be increased and weld crack susceptibility may be increased if it is excessively added greater than 0.40%. Therefore, the Ti content is set to be 0.01 to 0.40%. It is desirably 0.02 to 0.30%, and is more desirably 0.03 to 0.20%.(Nb: 0.02 to 0.60%)

[0047] Nb is an important element in the present invention, it forms NbN nitride which remains after annealing, suppresses growth of the crystal grain during annealing, and improves room temperature strength due to fine crystal grains. Furthermore, it has an effect to improve creep property since very fine NbN nitride is precipitated in crystal grain during use at high temperature, and M6C type carbides are precipitated uniformly and finely using the nitride as a nucleus. It is necessary to contain not less than 0.02% in order to improve strength and creep property. However, thermal expansion coefficient may be increased and weld crack susceptibility may be increased if it is excessively added greater than 0.60%. Therefore, Nb content is set to be 0.02 to 0.60%. It is desirably 0.05 to 0.50%, and is more desirably 0.10 to 0.30%.(Fe: 15.0 to 22.0%)

[0048] Fe is one of the main elements which is contained due to addition of scrap being one of raw materials, and oxidation resistance may be decreased since the Ni content is decreased relatively if the content is greater than 22.0%. On the other hand, if the content is less than 15.0%, since the Ni content is increased relatively, raw material cost may be increased and hot workability may be deteriorated. Therefore, Fe content is set to be 15.0 to 22.0%. It is desirably 16.0 to 21.0%, and is more desirably 17.0 to 20.0%.(Co: 0.5 to 4.0%)

[0049] Co is an element which improves creep property by solid-solution strengthening action. However, Co is an expensive element, the above effect may be saturated if the content is greater than 4.0%, and the effect may be no longer commensurate with the amount added. Therefore, the Co content is set to be 0.5 to 4.0%. It is desirably 0.6 to 3.0%, and is more desirably 0.7 to 2.0%.(W: 0.10 to 2.00%)

[0050] W is an element which improves creep property by solid-solution strengthening action. In addition, it strengthens alloy by forming M6C type carbides by combining C. Furthermore, it has an action in which M6C type carbides are formed and precipitated inside of crystal grain during use even at high temperature and thereby improve creep property. However, W is an expensive element, the above effect may be saturated if the content is greater than 2.00%, and the effect may be no longer commensurate with the amount added. Therefore, W content is set to be 0.10 to 2.00%. It is desirably 0.18 to 1.80%, and is more desirably 0.20 to 1.50%.(B: 0.0001 to 0.0100%)

[0051] B is an element which strengthens crystal grain boundary and improves creep property. However, compound having low melting point may be precipitated and hot workability may be deteriorated if B content is greater than 0.0100%. Therefore, B content is set to be 0.0001 to 0.0100%. It is desirably 0.0005 to 0.0080%, and is more desirably 0.0010 to 0.0050%.(N: 0.005 to 0.100%)

[0052] Since N has an effect in which it solid-solved in parent phase thereby increase strength at room temperature or high temperature, it is considered as an effective element which should be positively added. In addition, strength at room temperature may be increased since it forms MN type fine nitrides with Ti and Nb, suppresses growth of crystal grain during annealing and forms fine crystal grain. Furthermore, it has an effect to improve creep property since very fine MN type nitrides are precipitated in crystal grain during use at high temperature, and M6C type carbides are precipitated uniformly and finely using the nitride as a nucleus. The above effects are exhibited if it is added not less than 0.005%. However, if N is added greater than 0.100%, workability at room temperature may be deteriorated since parent phase is hardened and nitrides are coarsened and the like. Therefore, N content in the present invention is set to be 0.005 to 0.100%. It is desirably 0.010 to 0.080% and more desirably 0.015 to 0.060%.

[0053] Furthermore, N is a component which should be controlled accurately since it has an influence on properties of non-metallic inclusions which are generated during refining process. N content in molten metal can be accurately controlled by blowing nitrogen gas into molten metal in AOD or VOD in order to add N, and on the other hand, by blowing Ar gas in AOD, VOD or LF in order to decrease N concentration.(O: 0.0001 to 0.0060%)

[0054] O is very important in the present invention since oxygen concentration closely relates to inclusions. The number of inclusions may be increased thereby cause surface defects and desulfurizing may be inhibited thereby increase S concentration if O exists greater than 0.0060% in alloy. However, ability of Al to reduce CaO and MgO in slag may be too high and Mg concentration may exceed 0.0300% of upper limit and Ca concentration may exceed 0.0080% of upper limit, if O content is less than 0.0001%. Therefore, O content is set to be 0.0001 to 0.0060%. It is desirably 0.0003 to 0.0050% and more desirably 0.0005 to 0.0040%.(Mg: 0.0001 to 0.0300%)

[0055] Mg is an element which is effective to control composition of non-metallic inclusions in molten metal to MgO and MgO—CaO type oxides which do not have adverse influence on surface properties. The effect may not be obtained if the content is less than 0.0001%, and on the other hand, cracking may easily occur during hot rolling process due to deteriorated hot workability and thereby cause surface defects in final products if the content is greater than 0.0300%. Therefore, the Mg content is set to be 0.0001 to 0.0300%. It is desirably 0.00050 to 0.0200%, and is more desirably 0.0010 to 0.0100%.

[0056] It is desirable that the reaction formula (1) is used in order to add Mg effectively in molten metal. Slag composition should be controlled to CaO: 50 to 70%, SiO2: 1 to 8%, Al2O3: 10 to 30%, MgO: 5 to 15% and F: 2 to 8% in order to control Mg to be within the above range.(Ca: 0.0001 to 0.0080%)

[0057] Ca is an element which is effective to control composition of non-metallic inclusions in molten metal to CaO and MgO—CaO type oxides which do not form cluster and do not have adverse influence on surface properties. The effect may not be obtained if the content is less than 0.0001%, and on the other hand, cracking may easily occur during hot rolling process due to deteriorated hot workability and thereby cause surface defects in final products if the content is greater than 0.0080%. Therefore, the Ca content is set to be 0.0001 to 0.0080%. It is desirably 0.0002 to 0.0050%, and is more desirably 0.0003 to 0.0030%.

[0058] It is desirable that the reaction formula (2) is used in order to add Ca effectively in molten metal. Slag composition should be controlled to CaO: 50 to 70%, SiO2: 1 to 8%, Al2O3: 10 to 30%, MgO: 5 to 15% and F: 2 to 8% in order to control Ca to be within the above range.(Ni: Remainder)

[0059] An object alloy of the present invention contains the above explained components, and its remainder is Ni.(Non-Metallic Inclusions)

[0060] In the present invention, it is desirable that at least one kind selected from MgO, CaO, MgO—CaO type oxide and MgO—CaO—NbO—TiO2 type oxide is contained as the non-metallic inclusion, and MgO—CaO—NbO—TiO2 type oxide contains, in mass %, NbO: 0.1 to 5.0% and TiO2: not greater than 20%.

[0061] Furthermore, it is desirable that number ratio of MgO—CaO—NbO—TiO2 type oxide is not greater than 50%. Hereinafter reasons for limiting the components and the number ratio of the non-metallic inclusions are explained.(At Least One Kind Selected from MgO, CaO, MgO—CaO Type Oxide and MgO—CaO—NbO—TiO2 Type Oxide is Contained)

[0062] The Ni—Cr—Fe—Mo alloy of the present invention contains at least one kind selected from MgO, CaO, MgO—CaO type oxide and MgO—CaO—NbO—TiO2 type oxide is contained, according to the Si, Al, Mg, Ca and O contents in Ni—Cr—Fe—Mo alloy and amount of addition and timing of addition of Nb and Ti.

[0063] Since MgO, CaO and MgO—CaO type oxide are non-metallic inclusions of solid phase and having high melting points, they do not adhere on inner wall of immerse nozzle of tundish of continuous casting apparatus, they are fine non-metallic inclusions, and they do not have an adverse influence on surface quality of Ni—Cr—Fe—Mo alloy of the present invention. MgO, CaO and MgO—CaO type oxide inclusions are desirable composition to which non-metallic inclusions should be controlled. Furthermore, regarding CaO and MgO—CaO type oxide inclusions, part of CaO reacts S in molten metal to generate CaS during solidifying process of Ni—Cr—Fe—Mo alloy in continuous casting apparatus. S segregates at grain boundary during solidifying process of Ni—Cr—Fe—Mo alloy, deteriorates hot workability, and occurs surface crack and edge crack. CaO and MgO—CaO type oxide inclusions have an effect in which S in molten metal which deteriorates hot workability is fixed as CaS so that surface quality are kept clean.

[0064] Furthermore, CaO inclusion existing on the surface becomes Ca(OH)2 by hydration reaction in wet environment and falls off from the surface, thereby form small pit and cause deterioration of corrosion resistance. However, since Ni—Cr—Fe—Mo alloy which is an object of the present invention is applied to parts for combustion tower for gas turbine in which gas flows at high temperature over 1000° C., there is no problem of deterioration of corrosion resistance by CaO inclusion under wet environment.(MgO—CaO—NbO—TiO2 Type Oxide Contains, in Mass %, NbO: 0.1 to 5.0% and TiO2: Not Greater than 20%)

[0065] MgO—CaO—NbO—TiO2 type oxide becomes configuration in which liquid phase MgO—CaO—TiO2 type oxide exists around MgO, CaO and MgO—CaO type oxide inclusions at 1600° C. of refining temperature of Ni—Cr—Fe—Mo alloy as shown in FIG. 3. If the liquid phase MgO—CaO—TiO2 type oxide is formed around MgO, CaO and MgO—CaO type oxide inclusions, the liquid phase inclusion may play a role of adhesive, furthermore, MgO, CaO and MgO—CaO type oxide inclusions which are solid phase may play a role of aggregate, and non-metallic inclusions are promoted to adhere on refractory of the inner wall of the immerse nozzle as shown in FIG. 2. The non-metallic inclusions fall off after being coarsened, they are carried to mold together with molten metal and they are captured in solidifying shell, thereby cause surface defects.

[0066] Then, the inventors have researched further on precipitated material on the inner wall of the immerse nozzle and non-metallic inclusions inside of surface defects of Ni—Cr—Fe—Mo alloy and plate of the alloy. Then, the inventors found that adhering on the inner wall of the immerse nozzle can be prevented and deterioration of surface properties can be prevented if MgO—CaO—TiO2 type oxide contains NbO: 0.1 to 5.0 mass %. Effect of NbO is explained as follows.

[0067] As shown in FIG. 4, NbO in MgO—CaO—TiO2 type oxide exists in liquid phase part, and has an effect decreasing melting point of MgO—CaO—TiO2 type oxide of liquid phase part. That is, NbO reduces effect which is like adhesive by liquid phase MgO—CaO—TiO2 type oxide and prevents MgO—CaO—TiO2 type oxide from adhering on the inner wall of the immerse nozzle. However, if MgO—CaO—NbO—TiO2 type oxide contains NbO greater than 5.0 mass %, part of NbO may change to NbN according to the reaction formula (3), and may be contained in non-metallic inclusion.[NbO]+N=O_+[NbN](3)

[0068] Compound in brackets and compound with underline indicate component in non-metallic inclusion and component in molten metal, respectively.

[0069] NbN is a nitride having melting point at 2573° C., and generates MgO—CaO—TiO2 type oxide+NbO (—NbN) liquid phase having high melting point again around MgO, CaO and MgO—CaO type oxide inclusions as shown in FIG. 5. The liquid phase inclusion having high melting point again acts as adhesive, and promotes non-metallic inclusions adhering on the inner wall of the immerse nozzle. The non-metallic inclusions adhere and precipitate on the inner wall, fall off after being coarsened, and cause deterioration in cleanliness. From the above reasons, it is desirable that MgO—CaO—NbO—TiO2 type oxide contains NbO at 0.1 to 5.0 mass %.

[0070] However, in MgO—CaO—NbO—TiO2 type oxide which contains TiO2 at greater than 20 mass %, a part of TiO2 may change to TiN according to the reaction formula (4) and may be contained in non-metallic inclusions.[TiO2]+N=2⁢O+[TiN](4)

[0071] Compound in brackets and compound with underline indicate component in non-metallic inclusion and component in molten metal, respectively.

[0072] TiN is a nitride having melting point at 2950° C., and generates MgO—CaO—NbO—TiO2 (—TiN) type oxide of liquid phase having high melting point around MgO, CaO and MgO—CaO oxide inclusions as shown in FIG. 6. If NbO is contained at 0.1 to 5.0 mass % in MgO—CaO—NbO—TiO2 type oxide having greater than 20 mass % of TiO2 content, the effect of lowering the melting point of liquid phase inclusions is lost, resulting in deterioration of cleanliness. From the above reasons, MgO—CaO—NbO—TiO2 type oxide is set to contain NbO at 0.1 to 5.0 mass % and TiO2 at not greater than 20 mass %.

[0073] The above-mentioned effects of NbO and TiO2, may not change if MgO—CaO—NbO—TiO2 type oxide contains SiO2: not greater than 2 mass % and Al2O3: not greater than 2 mass %.

[0074] In order to make MgO—CaO—NbO—TiO2 type oxide contain 0.1 to 5.0% NbO, timing at which Nb is added during refining of molten metal is important. One or both of ferrosilicon alloy and pure silicon, and Al are added as primary deoxidation and then Nb is added at a timing when 0 concentration gets 0.0070 to 0.0120%, so that NbO concentration in non-metallic inclusions can be controlled accurately.

[0075] Furthermore, in order to make MgO—CaO—NbO—TiO2 type oxide contain not greater than 20% of TiO2, timing at which Ti is added during refining of molten metal is important. It is important that Nb is added after the primary deoxidation; Cr reduction, secondary deoxidation and desulfurization are performed using CaO—SiO2—MgO—Al2O3—F type slag consisting of CaO: 50 to 70%, SiO2: 1 to 8%, Al2O3: 10 to 30%, MgO: 5 to 15% and F: 2 to 8% and adding one or both of ferrosilicon alloy and pure silicon and Al; and then Ti is added at a timing when 0 concentration gets not greater than 0.0060%. In this way, TiO2 concentration in non-metallic inclusion can be controlled to be not greater than 20%.

[0076] Furthermore, regarding control of N content, N content in molten metal can be accurately controlled by blowing nitrogen gas into molten metal in AOD or VOD in order to add N, and on the other hand, by blowing Ar gas in AOD, VOD or LF in order to decrease N concentration.

[0077] In the present invention, regarding composition of non-metallic inclusions consisting of oxides of two or more phases as shown in FIGS. 3 to 6, concentration (mass %) of each component of non-metallic inclusions is indicated based on average component of the entirety.(Number Ratio of MgO—CaO—NbO—TiO2 Type Oxide is not Greater than 50%)

[0078] As mentioned above, MgO—CaO—NbO—TiO2 type oxide is one of non-metallic inclusions which adheres on the inner wall of the immerse nozzle, falls off from the wall after being coarsened, and deteriorates cleanliness. However, it became clear that tendency to adhere is slight degree and number of surface defects occurring can be suppressed if the number ratio of MgO—CaO—NbO—TiO2 type oxide is not greater than 50%. Therefore, the number ratio of MgO—CaO—NbO—TiO2 type oxide is set to be not greater than 50%.(Method for Production)

[0079] In the present invention, a method for production of Ni—Cr—Fe—Mo alloy is also proposed as follows. First, raw materials are melted in an electric furnace to produce Ni—Cr—Fe—Mo alloy molten metal having a predetermined composition. Then, the molten metal is decarburized in AOD (Argon Oxygen Decarburization) singly or in VOD (Vacuum Oxygen Decarburization) after AOD. After that, lime and fluorite are added, and one or both of ferrosilicon alloy and pure silicon, and Al, are added as primary deoxidation. Nb is added at a timing O concentration gets 0.0070 to 0.0120%. Cr reduction, secondary deoxidation and desulfurization are performed by adding one or both of ferrosilicon alloy and pure silicon, and Al, using CaO—SiO2—MgO—Al2O3—F type slag consisting of CaO: 50 to 70%, SiO2: 1 to 8%, Al2O3: 10 to 30%, MgO: 5 to 15%, F: 2 to 8%. Ti is added at a timing O concentration gets not greater than 0.0060%. After that, the molten metal is poured into a ladle and temperature and components are adjusted in LF (Ladle Furnace). Then, slab or ingot is produced by continuous casing apparatus or conventional casting. In a case in which ingot is produced, hot forging is performed to form slab. According to this, at least one kind selected from MgO, CaO, MgO—CaO type oxide and MgO—CaO—NbO—TiO2 type oxide is contained as non-metallic inclusion, the MgO—CaO—NbO—TiO2 type oxide contains, in mass %, NbO: 0.1 to 5.0% and TiO2: not greater than 20%, and Ni—Cr—Fe—Mo alloy having superior surface properties can be obtained. The produced slab is ground on its surface and heated. Hot rolling is performed singly, or hot rolling and cold rolling are performed in series. The slab is annealed and washed with acid, surface scale is removed, and a plate is produced finally.

[0080] In the method for production of Ni—Cr—Fe—Mo alloy of the present invention, slag composition is characteristic as mentioned above. Hereinafter reasons for limiting the slag composition are explained.(CaO: 50 to 70%)

[0081] CaO concentration and SiO2 concentration in slag are elements in order to deoxidize and desulfurize efficiently and to control inclusions. Activity of CaO in slag may be high and reaction in the formula (2) may promote excessively if CaO content is more than 70%. As a result, Ca concentration which is reduced in molten metal may exceed 0.0080%, cracking may easily occur in hot rolling process due to deterioration in hot workability, and surface defects may occur in final products. Therefore, the upper limit is set to be 70%. On the other hand, deoxidation and desulfurization may not be promoted and S concentration and O concentration in the present invention may not be controlled to be within the range of the present invention if CaO concentration is less than 50%. Therefore, the lower limit is set to be 50%. It is desirably 53 to 67%, and is more desirably 55 to 65%.(SiO2: 1 to 8%)

[0082] Since SiO2 in slag is an element which is necessary to maintain appropriate flowability of the slag, it is necessary to contain at least 10%. However, if it is contained greater than 8%, since Al concentration, Mg concentration and Ca concentration in molten metal may be less than the range of the present invention, the upper limit is set to be 8%. It is desirably 2 to 7% and more desirably 3 to 6%.(Al2O3: 10 to 30%)

[0083] If Al2O3 content in slag is greater than 30%, deoxidation may not be promoted sufficiently, O concentration may not be controlled to be within the range of the present invention, and MgO—CaO—NbO—TiO2 type oxide may be generated at greater than 50% of number ratio as non-metallic inclusions. On the other hand, if Al2O3 content in slag is less than 10%, flowability of slag may not be maintained, desulfurizing may not be promoted and S concentration may exceed the range of the present invention. It is desirably 12 to 28% and more desirably 15 to 25%.(MgO: 5 to 15%)

[0084] MgO in slag is an important element in order to control Mg concentration in molten metal to be the concentration range of the present invention, and also is an important element in order to control non-metallic inclusions to be desirable composition in the present invention. Therefore, it is necessary that MgO is contained at least 5% in slag. On the other hand, if MgO concentration exceeds 15%, surface defects may occur in final products since reaction of formula (1) is excessively promoted, Mg concentration in molten metal is increased, and hot workability is deteriorated. Therefore, the upper limit of MgO concentration is set to be 15%. MgO in slag may be within the above range since dolomite brick or magnesia-chrome brick is partially melted into slag during AOD refining or VOD refining. Alternatively, one or both of recycled bricks of dolomite brick and magnesia-chrome brick can be added in order to control to be the range of the invention. It is desirably 7 to 14% and more desirably 9 to 13%.(F: 2 to 8%)

[0085] F is contained in fluorite which is added in refining process of slag, and it plays a role adjusting condition of melting of slag accurately. It is necessary to add at least 2%. If F concentration is less than 2%, slag may not melt and flowability may be deteriorated. On the other hand, if F concentration is greater than 8%, since flowability of slag may be excessively high, damage to the bricks may become noticeable. Therefore, it is set to be 2 to 8%.EXAMPLES

[0086] Next, effects of the present invention are explained further in detail by way of Examples. It should be noted that the present invention is not limited only to the following Examples. Using an electric furnace of content 60 t, raw materials such as ferronickel, pure nickel, ferrochromium, iron scrap, stainless steel scrap, Fe—Ni alloy scrap and Fe—Mo were melted. Then, oxygen blowing, that is, oxidizing refining was performed in AOD, or alternatively in AOD and VOD in order to remove C. Lime and fluorite were added to generate CaO—SiO2—Al2O3—MgO—F type slag, one or both of FeSi alloy and pure Si, and Al were added in order to reduce Cr, and then deoxidizing was performed. After that, desulfurizing was promoted by stirring with Ar. Nb and Ti were appropriately added after deoxidizing. In AOD and VOD, magnesia-chrome bricks were lined on furnace body. After that, molten metal was poured into a ladle, and temperature and components were adjusted by LF (Ladle Furnace). Slab and ingot were produced by continuous casting and conventional casting. The ingot was hot-forged to produce slab.

[0087] The slab which was produced was ground on its surface, and heated at 1200° C. so as to hot roll, and hot-rolled coil was produced. Then, annealing and acid washing were performed, scale on the surface was removed, and cold rolling was performed to have a predetermined thickness so as to produce cold-rolled coil. Tables 1 and 2 show chemical components of Ni—Cr—Fe—Mo alloy obtained, slag composition when finishing AOD or VOD refining, production process, composition of non-metallic inclusions, formation of inclusion, and evaluation of quality. In the Tables, values in brackets indicate that the value is out of the range of Claims. It should be noted that values of Examples in brackets satisfy the range of independent claims, although do not satisfy the range of dependent claims.TABLE 1Chemical components (remainder Ni) mass %CSiMnPSCrMoCuAlTiNbFeCoWExamples10.050.220.440.0120.000221.48.20.100.090.060.13619.70.960.4820.060.450.450.0140.000221.38.20.120.140.080.18719.50.900.4930.050.230.450.0130.000421.48.40.100.140.060.13619.70.830.4740.080.240.480.0120.000322.58.60.200.150.040.22018.51.100.6650.090.400.460.0140.000221.88.10.220.160.060.19018.80.650.3860.060.240.510.0130.000222.18.20.080.200.100.12219.60.840.5270.050.260.460.0100.000221.98.30.070.180.100.13519.50.830.4880.050.230.450.0130.000421.48.40.100.140.060.13619.70.830.4790.090.220.390.0180.000722.08.70.220.180.16518.10.990.39100.050.380.450.0140.000223.99.20.280.100.200.13218.50.800.38110.080.280.520.0130.000820.88.20.200.060.180.18817.51.020.38120.050.110.320.0130.001121.48.70.100.030.060.21120.01.560.33130.070.090.260.0130.002122.29.80.280.020.070.22319.52.100.32Comparative140.070.280.470.0130.000420.19.00.080.090.110.13919.61.100.49Examples150.060.230.450.0110.000222.08.20.080.150.090.12819.60.920.54160.090.480.550.0210.000820.98.80.210.150.080.16218.21.120.48170.080.250.480.0200.002920.68.20.18(0.008)0.080.16119.80.880.28180.05(1.55)0.450.0130.000122.39.50.30(0.52)0.210.32018.60.880.55190.060.580.440.0120.000122.39.50.300.450.180.35019.70.870.55200.06(0.04)0.450.011(0.0060)21.29.60.08(0.003)0.090.09819.60.920.54Chemical components (remainder Ni) mass %RemainderBNOMgCaNiProducing processExamples10.00210.0150.00060.00600.000848.17EF→AOD→LF→CC20.00200.0180.00080.00150.002548.13EF→VOD→LF→CC30.00220.0220.00050.00490.002947.93EF→AOD→LF→CC40.00320.0120.00060.00580.000947.20EF→AOD→LF→CC50.00230.0210.00080.00140.002448.64EF→VOD→LF→CC60.00220.0210.00050.00800.000747.38EF→AOD→LF→CC70.00230.0220.00050.00100.003047.78EF→AOD→VOD→LF→IC80.00220.0220.00050.00500.003047.93EF→AOD→LF→CC90.00200.0330.00420.00320.001048.41EF→AOD→LF→CC100.00330.0430.00380.00160.002345.55EF→VOD→LF→CC110.00320.0320.00410.00190.001050.54EF→AOD→LF→IC120.00300.0320.00450.00080.000247.03EF→AOD→LF→CC130.00380.0350.00480.00070.000245.01EF→AOD→LF→CCComparative140.00200.0230.00520.00300.001348.39EF→AOD→LF→ICExamples150.00210.0220.00530.00140.002047.54EF→VOD→LF→CC160.00220.0220.00050.00500.003148.72EF→AOD→LF→CC170.00300.024(0.0062)0.00020.000148.94EF→AOD→LF→CC180.00400.033(0.00003)(0.0310)0.007244.68EF→AOD→LF→CC190.00340.017(0.00003)0.0220(0.0092)44.55EF→AOD→LF→CC200.00060.022(0.0078)(0.00003)(0.00003)47.28EF→VOD→LF→CC indicates data missing or illegible when filedTABLE 2Oxygenconcentration inmolten metalduring addingSlag compositionsNb, Ti (mass %)Inclusions compositions (mass %) Analyzing 20 points by EDS(mass %)DuringDuringMgOCaOMgO—CaOCaOSiO2Al2O3MgOFadding Nbaddin TinMgOnCaOCaSnMgOCaOCaSExamples11010002925321342041004792105799133062010072097382009310011400101631011114401245013Comparative144100400Examples154231116124101718141002401915131120Evaluation ofsurface defectsNon-metallicinclusionsin 10 m2 andNumber ratio ofNumber ofnon-metallicsurfaceInclusions compositions (mass %) Analyzing 20 points by EDSinclusions (%)defectsMgO—CaO—NbO—TiO2 typeMgO—CaO—NbO—TiO2due to hotnMgOCaONbOTiO2SiO2Al2O3TiNNbNtypeworkabilityDecisionExamples141.52.10.00.50.00.0201⊚23.50.31.50.00.0301⊚32.30.01.20.00.0251⊚41.70.00.00.30.00.0401⊚50.00.31.50.00.0401⊚6 00⊚7 00⊚8 00⊚964.51.11.60.00.0303◯10617.23.50.91.70.00.0303◯11950.23.31.21.50.00.0454◯121344.32.41.50.00.0(65)7Δ132042.11.40.00.0(100) ΔComparative141.20.01.04014XExamples15104.20.91.00.050X1642.9(0.02)0.40.00.04012X172040.0(0.03)0.00.0(100) X18 020X19 022X20—27X indicates data missing or illegible when filed(1) Chemical components of alloy and slag composition: Quantitative analysis was performed using an X-ray fluorescence analyzer, and Quantitative analysis of oxygen concentration of alloy was performed using inert gas impulse fusion infrared absorption spectroscopy.(2) Composition of non-metallic inclusions: Sample which was collected in a tundish immediately after start of casting was mirror-polished, and inclusions with a size of 5 μm or more were measured at 20 random points using SEM / EDS.

[0090] (3) Number ratio of non-metallic inclusions: Based on the results of measurement (2), ratio of the number of MgO—CaO—NbO—TiO2 type oxide with respect to the number of all the non-metallic inclusions was evaluated.

[0091] (4) Evaluation of surface defects: Visual observation was performed on the surface of Ni—Cr—Fe—Mo alloy plate having thickness of 1 mm in which acid washing and annealing were performed and scale on the surface was removed. The number of surface defects due to non-metallic inclusions and hot workability in an area of width 1 m and length 10 m was counted. In the evaluation of quality, a case in which the number of the surface defects was not greater than 2 in 10 m2 was evaluated as “⊚ (two concentric circles means superior)”, a case in which the number was 3 to 5 was evaluated as “∘ (which means good)”, a case in which the number was 6 to 10 was evaluated as “Δ (which means adequate)”, and a case in which the number was not less than 11 was evaluated as “x (which means inferior)”.

[0092] Since Examples 1 to 13 satisfied the range of the present invention, surface defects on the plate were slight, and superior surface properties were obtained.

[0093] In Example 12, Si concentration was 0.110% and Al concentration was 0.03%, that is, slightly low within the range of the invention. Therefore, deoxidation was promoted slightly low and O concentration was 0.0045%, which was slightly high. Supplied amounts of Mg and Ca from the slag were small, and Mg content was 0.0008% and Ca content was 0.0002%, which was low. As a result, the number ratio of MgO—CaO—NbO—TiO2 type oxide was 65%, which was high, some coarse inclusions were generated, and the number of surface defects was 7, which was “Δ” evaluation.

[0094] In Example 13, Si concentration was 0.09% and Al concentration was 0.02%, that is, slightly low within the range of the invention. Therefore, deoxidation was promoted slightly low and O concentration was 0.0048%, which was slightly high. Supplied amounts of Mg and Ca from the slag were small, and Mg content was 0.0007% and Ca content was 0.0002%, which were low. As a result, only MgO—CaO—NbO—TiO2 type oxide was generated as non-metallic inclusions, some coarse inclusions were generated, and the number of surface defects was 9, which was “Δ” evaluation.

[0095] On the other hand, Comparative Examples 14 to 20 were out of the range of the present invention, surface defects were generated much, and surface properties were deteriorated. Hereinafter each of Comparative Examples is explained.

[0096] In Comparative Example 14, O concentration before adding Nb after primary deoxidation was 0.0142%, which was high, and NbO of MgO—CaO—NbO—TiO2 type oxide was 5.5%, which was high. NbN was also generated partially in liquid phase portion of MgO—CaO—NbO—TiO2 type oxide, and melting point was increased. There were a lot of adhesion on the immerse nozzle, and many coarse non-metallic inclusions fell off and the number of surface defects was 14, which was many. The evaluation was “x”.

[0097] In Comparative Example 15, O concentration before adding Ti after secondary deoxidation was 0.0075%, which was high, and TiO2 of MgO—CaO—NbO—TiO2 type oxide was 21.1%, which was high. TiN was also generated partially in liquid phase portion of MgO—CaO—NbO—TiO2 type oxide, and melting point was increased. There were a lot of adhesion on the immerse nozzle, and many coarse non-metallic inclusions fell off and the number of surface defects was 16, which was many. The evaluation was “x”.

[0098] In Comparative Example 16, O concentration before adding Nb after primary deoxidation was 0.0060%, which was low, and NbO of MgO—CaO—NbO—TiO2 type oxide was 0.02%, which was low. Effect of decreasing melting point of liquid phase portion of MgO—CaO—NbO—TiO2 type oxide is low since NbO is 0.02%, and melting point was increased. There were a lot of adhesion on the immerse nozzle, and many coarse non-metallic inclusions fell off and the number of surface defects was 12, which was many. The evaluation was “x”.

[0099] In Comparative Example 17, O concentration before adding Nb after primary deoxidation was 0.0058%, which was low, and NbO of MgO—CaO—NbO—TiO2 type oxide was 0.03%, which was low. Furthermore, Al was 0.008% which was slightly low, O concentration was high, Mg was 0.0002% and Ca was 0.00010% which were low, and only MgO—CaO—NbO—TiO2 type oxide was generated as non-metallic inclusions. Effect of decreasing melting point of liquid phase portion of MgO—CaO—NbO—TiO2 type oxide was low since NbO was 0.03%, and melting point was increased. There were a lot of adhesion on the immerse nozzle, and many coarse non-metallic inclusions fell off and the number of surface defects was 18, which was many. The evaluation was “x”.

[0100] In Comparative Example 18, Si was 1.55% which was high, Al was 0.52% which was high, O concentration was 0.00003% which was low, supplied amount of Mg from the slag was large, and Mg was 0.0310% which was high. As a result, non-metallic inclusions were only MgO and CaO, and there is no adhesion on the immerse nozzle. However, since Mg concentration was high and hot workability was deteriorated, crack occurred during hot rolling process and the number of surface defects generated was 20, which was many. The evaluation was “x”.

[0101] In Comparative Example 19, Al was 0.45% which was slightly high, O concentration was 0.00003% which was low, and furthermore, since Ca alloy was added during end term of refining, Ca concentration was 0.0092%, which was high. As a result, non-metallic inclusions were only CaO and MgO, and there is no adhesion on the immerse nozzle. However, since Ca concentration was high and hot workability was deteriorated, crack occurred during hot rolling process. Furthermore, since Ca alloy was added during end term of refining, non-metallic inclusions in molten metal could not be floated and separated, they were flown into the mold, thereby deteriorate cleanliness. The number of surface defects generated was 22, which was many. The evaluation was “x”.

[0102] In Comparative Example 20, since Si was 0.04% which was low and Al was 0.003% which was low, deoxidation and desulfurization were not promoted. Therefore, O concentration was 0.0078% which was high, S concentration was 0.0060% which was high, Mg concentration was 0.00003% and Ca concentration was 0.00003%. Although the number of non-metallic inclusions was not counted in Table 2, MgO—Al2O3 and Al2O3—SiO2—MnO—Cr2O3 type oxides were detected. MgO—Al2O3 may easily adhere inside of the immerse nozzle, which is one of non-metallic compounds to be avoided. Al2O3—SiO2—MnO—Cr2O3 type oxide is a lower grade oxide which may be generated in molten metal in which deoxidation is not performed sufficiently. Because of insufficient deoxidation, since many non-metallic inclusions are kept generated in molten metal and they are flown into the mold, it is one of non-metallic compounds to be avoided. The number of surface defects generated was 27, which was many. The evaluation was “x”.INDUSTRIAL APPLICABILITY

[0103] According to the technique of the present invention, formation of non-metallic inclusions are controlled thereby enable supplying Ni—Cr—Fe—Mo alloys which have superior surface properties and are appropriate for use in a part for a combustor for gas turbines, which has high creep property.EXPLANATION OF REFERENCE NUMERALS

[0104] 1: Ladle, 2: molten metal, 3: tundish, 4: immerse nozzle, 5: mold, 6: solidifying shell, 7: spray cooling zone, 8a: non-metallic inclusion, 8b: non-metallic inclusions which are adhered, and 8c: coarse non-metallic inclusions which are fallen off

Examples

examples

[0086]Next, effects of the present invention are explained further in detail by way of Examples. It should be noted that the present invention is not limited only to the following Examples. Using an electric furnace of content 60 t, raw materials such as ferronickel, pure nickel, ferrochromium, iron scrap, stainless steel scrap, Fe—Ni alloy scrap and Fe—Mo were melted. Then, oxygen blowing, that is, oxidizing refining was performed in AOD, or alternatively in AOD and VOD in order to remove C. Lime and fluorite were added to generate CaO—SiO2—Al2O3—MgO—F type slag, one or both of FeSi alloy and pure Si, and Al were added in order to reduce Cr, and then deoxidizing was performed. After that, desulfurizing was promoted by stirring with Ar. Nb and Ti were appropriately added after deoxidizing. In AOD and VOD, magnesia-chrome bricks were lined on furnace body. After that, molten metal was poured into a ladle, and temperature and components were adjusted by LF (Ladle Furnace). Slab and in...

Claims

1. A Ni—Cr—Fe—Mo alloy consisting of:in mass %, C: 0.03 to 0.30%, Si: 0.05 to 1.50%, Mn: 0.05 to 2.00%, P: not greater than 0.05%, S: not greater than 0.005%, Cr: 18.0 to 28.0%, Mo: 6.0 to 15.0%, Cu: not greater than 1.0%, Al: 0.01 to 0.50%, Ti: 0.01 to 0.40%, Nb: 0.02 to 0.60%, Fe: 15.0 to 22.0%, Co: 0.5 to 4.0%, W: 0.10 to 2.00%, B: 0.0001 to 0.0100%, N: 0.005 to 0.100%, O: 0.0001 to 0.0060%, Mg: 0.0001 to 0.0300%, Ca: 0.0001 to 0.0080%, and Ni and inevitable impurities as the remainder, whereinat least one kind selected from MgO, CaO, MgO—CaO type oxide and MgO—CaO—NbO—TiO2 type oxide is contained as non-metallic inclusion, andthe MgO—CaO—NbO—TiO2 type oxide contains, in mass %, NbO: 0.1 to 5.0% and TiO2: not greater than 20%, in a case in which the MgO—CaO—NbO—TiO2 type oxide is contained.

2. The Ni—Cr—Fe—Mo alloy according to claim 1, wherein number ratio of the MgO—CaO—NbO—TiO2 type oxide with respect to the entire non-metallic inclusions in the Ni—Cr—Fe—Mo alloy is not greater than 50%.

3. A method for producing the Ni—Cr—Fe—Mo alloy according to claim 1, comprising steps of:melting raw materials in an electric furnace,decarburizing in AOD and / or VOD,adding lime and fluorite,adding one or both of ferrosilicon alloy and pure silicon, and Al, as primary deoxidation,adding Nb at a timing O concentration gets 0.0070 to 0.0120%,performing Cr reduction, secondary deoxidation and desulfurization by adding one or both of ferrosilicon alloy and pure silicon, and Al, using CaO—SiO2—MgO—Al2O3—F type slag consisting of CaO: 50 to 70%, SiO2: 1 to 8%, Al2O3: 10 to 30%, MgO: 5 to 15%, F: 2 to 8%,adding Ti at a timing O concentration gets not greater than 0.0060%,forming slab or ingot by continuous casing apparatus or conventional casting,optionally performing hot forging in a case in which the ingot is formed, andperforming only hot rolling or both of hot rolling and cold rolling.

4. A method for producing the Ni—Cr—Fe—Mo alloy according to claim 2, comprising steps of:melting raw materials in an electric furnace,decarburizing in AOD and / or VOD,adding lime and fluorite,adding one or both of ferrosilicon alloy and pure silicon, and Al, as primary deoxidation,adding Nb at a timing O concentration gets 0.0070 to 0.0120%,performing Cr reduction, secondary deoxidation and desulfurization by adding one or both of ferrosilicon alloy and pure silicon, and Al, using CaO—SiO2—MgO—Al2O3—F type slag consisting of CaO: 50 to 70%, SiO2: 1 to 8%, Al2O3: 10 to 30%, MgO: 5 to 15%, F: 2 to 8%,adding Ti at a timing O concentration gets not greater than 0.0060%,forming slab or ingot by continuous casing apparatus or conventional casting,optionally performing hot forging in a case in which the ingot is formed, andperforming only hot rolling or both of hot rolling and cold rolling.

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