Fe-cr-ni alloys with excellent surface properties and their production methods

Abstract

A composition of non-metallic inclusions influence surface properties is controlled providing Fe—Cr—Ni alloys having surface properties and production methods. The Fe—Cr—Ni alloy having surface properties including, in mass %, C: 0.020 to 0.150%, Si: 0.05 to 0.80%, Mn: 0.10 to 1.50%, P: not greater than 0.035%, S, 0.0050%, Ni: 34.0 to 48.0%, Cr: 22.0 to 29.0%, Mo: 0.20 to 1.20%, Al: 0.005 to 0.180%, B: 0.0001 to 0.0080%, Mg: 0.0001 to 0.0100%, Ca: 0.0001 to 0.0100%, Nb: 0.20 to 0.80%, N: 0.050 to 0.500%, O: 0.0001 to 0.0060%, Cu, than 0.80%, Ti, 0.100%, Co, 0.50%, and Fe and inevitable impurities as the remainder, wherein non-metallic inclusions contain at least one of MgO, CaO, CaO—MgO type oxide, CaO—Al2O3—MgO type oxide and MgO·Al2O3, in which number ratio of the MgO·Al2O3 with respect to the total oxide type non-metallic inclusions is not greater than 50%.

Description

TECHNICAL FIELDS

[0001] The present invention relates to Fe—Cr—Ni alloys having excellent surface properties and their production methods, and in particular, relates to Fe—Cr—Ni 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 thereby control non-metallic inclusions in molten metal to be harmless composition and reduce number of inclusions on the surface, and furthermore, relates to Fe—Cr—Ni alloys for which corrosion resistance and high temperature strength are highly required as a structural material such as for reactors.BACKGROUND ART

[0002] During production of solar power installations, as material of reactors which are used for purifying polysilicon which is to be raw material for power generating elements, Fe—Cr—Ni alloy plate is often used. The reactor is used under high temperature and high pressure, that is, very severe environment from the viewpoints of high temperature strength and corrosion resistance. Since service life of the reactor may be shortened if high temperature strength and corrosion resistance are not sufficient, there is a demand for Fe—Cr—Ni alloys which satisfy such properties.

[0003] Since Fe—Cr—Ni alloys having superior high temperature strength and corrosion resistance contain Cr, Ni and Mo in addition to Fe which is a main component and such metals are very expensive compared to iron, it is very important to improve yields and reduce production costs. Here, if surface defects such as linear damage occur on the surface of Fe—Cr—Ni alloy, since the surface defects should be removed by grinding or cutting thereby greatly deteriorate yields, Fe—Cr—Ni alloys having superior surface properties are demanded.

[0004] Patent Document 1 discloses a technique in which size and number of nitrides or carbides of Nb and Ti are controlled in Fe—Cr—Ni alloy having superior high temperature strength thereby suppress crystal grains from being coarsened at annealing temperature of products and high creep rapture property is realized. However, the nitrides or carbides of Nb and Ti do not cause occurring damage of the surface of products. The invention according to 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 a target in the present invention, and the problem of surface defects due to oxide type non-metallic inclusions still remains.

[0005] Patent Document 2 discloses a technique in which mass ratio Ca / Al in oxide type inclusions is set to be in a range of 1.0 to 1.5 in high Ni alloys containing Al and Ti for high temperature and their production methods so that composition of the oxide type inclusions is controlled to be CaO—Al2O3 type having low melting point, an immerse nozzle of continuous casting apparatus is prevented from being blocked and surface damage on products is prevented. However, since 0.15 to 1.5% of Ti is contained in the Patent Document 2, it is considered that CaO—Al2O3—TiO2 is generated as oxide type inclusions thereby occur blocking in nozzle. Since Ti content in the present invention is not greater than 0.10%, blocking in nozzle by CaO—Al2O3—TiO2 does not occur. Surface properties of Fe—Cr—Ni alloys of the present invention is not sufficiently improved according to the technique disclosed in the Patent Document 2.

[0006] 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 the Fe—Cr—Ni alloy containing Cr: 22.0 to 29.0% 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 Fe—Cr—Ni 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.

[0007] 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 Fe—Cr—Ni alloy of the present invention at 0.20 to 0.80% has oxidation ability almost as same as that of Si and Mn. That is, Nb is an important element to control surface defects, since Nb oxides are contained in inclusions although in small amount, and stretching-dividing property during hot or cold rolling is improved by decreasing melting point of inclusions. However, since the stainless steel plate which is disclosed in the Patent Document 4 does not contain Nb, the technique disclosed therein is different from the technique of the present invention to obtain inclusion properties that are less likely to cause surface defects.

[0008] Patent Document 5 discloses a technique in which number ratio of MgO·Al2O3 is maintained at not greater than 50% with respect to the total of non-metallic inclusions in stainless steel plate so as to prevent surface defects. However, although Al is mainly used for deoxidation of molten metal in the present invention, Si is used in the Patent Document 5. Accordingly, basicity of slag in the Patent Document 5 is 2 to 5, which is lower than that of the present invention. Since the basicity is a factor which has a great influence on properties of inclusions, the technique which is disclosed in the Patent Document 5 is different from that of the present invention.

[0009] Patent Document 6 discloses a technique in which Nb is added to Fe—Ni—Cr alloy in high yield. However, in the present invention, Nb is added at a timing at which primary deoxidation of molten metal is performed, that is, a timing at which oxygen concentration is high to some extent being 0.0070 to 0.0120% so that Nb concentration in non-metallic inclusions is controlled, on the other hand, in the Patent Document 6, it is said that Nb should be added after preliminary deoxidation with Si, followed by deoxidation with Al to sufficiently reduce the oxygen concentration. Furthermore, it is said that Nb2O5 inclusions should be avoided since it may cause surface damages in the Patent Document 6; however, in the present invention, by adding Nb by the above-mentioned method, Nb is contained at small amount as NbO in CaO—Al2O3—MgO inclusions and stretching-dividing property during hot or cold rolling is improved so as to prevent surface defects. Therefore, the technique which is disclosed in the Patent document 6 is different from that of the present invention.

[0010] The Patent Documents are as follows.

[0011] Patent Document 1: Japanese Unexamined Patent Application Publication No. 2017-057461

[0012] Patent Document 2: Japanese Unexamined Patent Application Publication No. 2021-70838

[0013] Patent Document 3: Japanese Unexamined Patent Application Publication No. Heisei 11 (1999)-315354

[0014] Patent Document 4: Japanese Unexamined Patent Application Publication No. 2019-35124

[0015] Patent Document 5: Japanese Unexamined Patent Application Publication No. 2015-074807

[0016] Patent Document 6: Japanese Unexamined Patent Application Publication No. 2014-105341SUMMARY OF INVENTION

[0017] In view of the above problems, an object of the present invention is to provide Fe—Cr—Ni alloy in which composition of non-metallic inclusions having an influence on surface properties is controlled and surface properties are superior. Furthermore, the present invention also provides a method for production of such Fe—Cr—Ni alloy.

[0018] The inventors have researched and investigated in order to solve the above problems, and they found that cause of surface defects is non-metallic inclusions of oxides of MgO·Al2O3 type, CaO and CaO—MgO type by analyzing in detail by scanning electron microscope (SEM) and energy dispersive X-ray analyzer (EDS) with respect to surface defects of Fe—Cr—Ni alloy plates. This type of non-metallic inclusion tends to adhere to inner wall of an immerse nozzle used to pour molten metal from a tundish into a mold in a continuous casting apparatus, and tends to grow in size. When the inclusion falls off, it may be easily captured by solidified shell and may become the origin point of surface defects. In addition, since the non-metallic inclusion has a high melting point, it is difficult to be stretched during hot rolling, it is not dispersed into small particles, and therefore it becomes the origin point of surface defects in Fe—Cr—Ni alloy plates.

[0019] The inventors have further researched on relationship between composition of inclusions and metal components, in Fe—Cr—Ni alloy. Practically, during production of Fe—Cr—Ni alloy, metal samples of Fe—Cr—Ni alloy were collected from the tundish which was inside of the continuous casting apparatus, twenty inclusions having size greater than 5 μm were freely selected in the sample, and inclusion composition thereof was measured by SEM / EDS. Furthermore, the immerse nozzle for supplying molten metal from the tundish to the mold inside of the continuous casting apparatus was collected, and components of adhered material on the inner wall of the nozzle were analyzed by SEM / EDS. Based on the above, the inventors have researched on relationship among composition of inclusions, metal components and adhered material on the inner wall of the immerse nozzle.

[0020] As a result, a guideline was obtained that basically enables the inclusion composition to be controlled to MgO or CaO—Al2O3—MgO type oxide if non-metallic inclusions of Fe—Cr—Ni alloy contained at least one kind selected from MgO, CaO, CaO—MgO type oxide, CaO—Al2O3—MgO type oxide and MgO·Al2O3, and furthermore, Si concentration and Al concentration were controlled to be 0.05 to 0.80 mass % and 0.005 to 0.180 mass % respectively, and Mg concentration, Ca concentration and O concentration were controlled to be 0.0001 to 0.0100 mass %, 0.0001 to 0.0100 mass % and 0.0001 to 0.0060 mass % respectively. Furthermore, if number ratio of MgO·Al2O3 is not greater than 50% and number ratio of the total of CaO and CaO—MgO type oxide is not greater than 50% with respect to the total of the oxide type non-metallic inclusions, it was found that the non-metallic inclusions are less likely to adhere to and deposit on the inner wall of the immerse nozzle, that is, are less likely to grow in size and become a cause of surface defects, and it was also found that such non-metallic inclusions have excellent cleanliness because they are finely divided during hot rolling and cold rolling.

[0021] Accordingly, the Fe—Cr—Ni alloy of the present invention was completed in view of above knowledge, the feature of the present invention is the Fe—Cr—Ni alloy consisting of, in mass %, C: 0.020 to 0.150%, Si: 0.05 to 0.80%, Mn: 0.10 to 1.50%, P: not greater than 0.035%, S: not greater than 0.0050%, Ni: 34.0 to 48.0%, Cr: 22.0 to 29.0%, Mo: 0.20 to 1.20%, Al: 0.005 to 0.180%, Mg: 0.0001 to 0.0100%, Ca: 0.0001 to 0.0100%, Nb: 0.20 to 0.80%, N: 0.050 to 0.500%, O: 0.0001 to 0.0060%, Cu: not greater than 0.80%, Ti: not greater than 0.100%, Co: not greater than 0.50%, and Fe and inevitable impurities as the remainder, in which non-metallic inclusions contain at least one of MgO and CaO—Al2O3—MgO type oxide as an essential component and contain optionally selected from CaO, CaO—MgO type oxide and MgO·Al2O3 as an optional component, and number ratio of the MgO·Al2O3 with respect to the total oxide type non-metallic inclusions is not greater than 50%, and number ratio of the total of the CaO and CaO—MgO type oxide with respect to the total oxide type non-metallic inclusions is not greater than 50%.

[0022] In the present invention, it is desirable that the CaO—Al2O3—MgO type oxide in the non-metallic inclusions contains 0.01 to 0.60 mass % of NbO.

[0023] In the present invention, it is desirable that in the non-metallic inclusions, the CaO—MgO type oxide contains, in mass %, CaO: 20 to 80% and MgO: 20 to 80%, the CaO—Al2O3—MgO type oxide contains CaO: 10 to 60%, Al2O3: 5 to 60%, MgO: 10 to 80% and SiO2: not greater than 10%, and the MgO·Al2O3 contains MgO: 10 to 40% and Al2O3: 60 to 90%.

[0024] Furthermore, a method for production is also provided in the present invention. That is, the method for producing the Fe—Cr—Ni alloy having excellent surface properties includes steps of: melting raw materials in an electric furnace; decarburizing in AOD, or VOD after AOD; 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, after using CaO—SiO2—MgO—Al2O3—F type slag consisting of CaO: 45 to 75%, SiO2: 1 to 15%, Al2O3: 10 to 30%, MgO: 5 to 20%, F: 1 to 15%; forming slab or ingot by continuous casing apparatus or conventional ingot 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.EMBODIMENTS OF INVENTION

[0025] First, reasons for limiting chemical components of Fe—Cr—Ni alloy of the present invention are explained. It should be noted that in the following explanation, “%” means “mass %”.(C: 0.020 to 0.150%)

[0026] C is an austenitic phase stabilizing element. If it exists at large amount, it may combine Cr and Mo to form carbide, amounts of Cr and Mo of solid solution contained in base material may be decreased, and corrosion resistance may be deteriorated. Therefore, C content is set to be 0.020 to 0.150%. It is desirably 0.030 to 0.100% and more desirably 0.040 to 0.070%.(Si: 0.05 to 0.80%)

[0027] Since Si is an effective element for deoxidation, it is an important element in the present invention. It is necessary at least 0.05% in order to control oxygen concentration to be 0.0001 to 0.0060%. Furthermore, it also has a role to reduce CaO and MgO in CaO—SiO2—MgO—Al2O3—F type slag and a role to adjust Mg and Ca in molten metal to be 0.0001 to 0.0100% and 0.0001 to 0.0100%, respectively. Accordingly, it has an effect in which inclusions are maintained as harmless MgO and CaO—Al2O3—MgO type. From the viewpoint, it is necessary at least 0.05%. On the other hand, if it is contained greater than 0.80%, CaO and MgO in slag may be reduced too much, and Mg at greater than 0.0100% and Ca at greater than 0.0100% may be supplied. As a result, CaO and CaO—MgO type oxide may be generated in the total number ratio exceeding 50%, large number of surface defects and pits may be generated on products, and surface properties thereof may be deteriorated. Furthermore, if Mg is contained excessively in alloy, hot workability may be deteriorated and crack may be generated during hot rolling thereby occur surface defects. Therefore, Si content is set to be 0.05 to 0.80%. It is desirably 0.08 to 0.60% and more desirably 0.10 to 0.40%.(Mn: 0.10 to 1.50%)

[0028] Since Mn is an austenitic phase stabilizing element and contributes to deoxidation, it is necessary to add at least 0.10%. However, oxidation resistance may be deteriorated if added at large amount, the upper limit is set to be 1.50%. It is desirably 0.30 to 1.00% and more desirably 0.05 to 0.80%.(P: Not greater than 0.035%)

[0029] Since P is a harmful element which segregates at grain boundary and causes crack during hot processing, it is desirable to reduce as much as possible, and it is limited to not greater than 0.035%. It is desirably not greater than 0.030% and more desirably not greater than 0.025%.(S: Not greater than 0.0050%)

[0030] Since S is a harmful element which segregates at grain boundary, forms low-melting point compounds, and deteriorates hot workability, it is desirable to reduce as much as possible, and it is limited to not greater than 0.0050%. In order to achieve this, the lower limit of Al content was set to be 0.005%, deoxidation was promoted, and O concentration was controlled to be within a range of 0.0001 to 0.0060% so that desulfurization was promoted. It is desirably not greater than 0.0030% and more desirably not greater than 0.0010%.(Ni: 34.0 to 48.0%)

[0031] Ni is a main element in Fe—Cr—Ni alloy of the present invention, and is an element which stabilizes austenitic phase, maintains high temperature strength and increases corrosion resistance. If it is contained at not less than 34.0%, it may be possible to obtain pitting corrosion resistance and acid resistance that can withstand use in severe corrosive environments. However, since Ni is a very expensive material compared to Fe and causes high production cost, it is not desirable to add at large amount. Therefore, the upper limit is set to be 48.0%. It is desirably 35.0 to 45.0% and more desirably 37.0 to 40.0%.(Cr: 22.0% to 29.0%)

[0032] Cr is an element which forms passivation film on the surface of Fe—Cr—Ni alloy, and the most important element as a constituent component of base material in order to improve resistance to acid, pitting corrosion, crevice corrosion and stress corrosion cracking. However, if the Cr content is less than 22.0%, sufficient corrosion resistance cannot be obtained. On the other hand, if the content is greater than 29.0%, a phase may be generated and alloy may be brittle. For these reasons, Cr content is set to be 22.0 to 29.0%. It is desirably 23.0 to 27.0% and more desirably 24.0 to 26.0%.(Mo: 0.20 to 1.20%)

[0033] Mo has an effect in which corrosion resistance under wet environments and high temperature atmosphere environments existing chlorides is greatly improved even by addition at small amount and the corrosion resistance is improved in proportion to the added amount. Furthermore, although the upper limit of Si which is effective for deoxidation is set to be 0.80%, Mo has a useful element since it has an effect in which activity coefficient of Si is increased and deoxidizing power is compensated. Therefore, it is necessary to add at not less than 0.20%. On the other hand, in a material in which Mo is added at large amount, if oxygen potential of the surface is low under high temperature atmosphere environment, Mo may be oxidized preferentially and oxide film may be separated thereby cause surface defects. Therefore, the upper limit is set to be 1.20%. It is desirably 0.30 to 1.00% and more desirably 0.40 to 0.80%(Al: 0.005 to 0.180%)

[0034] Al is an element which is very effective for deoxidation, and is a particularly important element in the present invention. It can control oxygen concentration to be within a range of 0.0001 to 0.0060%, and has an effect in which MgO and CaO in CaO—SiO2—MgO—Al2O3—F type slag are reduced, Mg of not less than 0.0001% and Ca of not less than 0.0001% are supplied into molten metal, and inclusions are controlled to be harmless MgO, CaO—Al2O3—MgO type. These effects are based on the following reactions.3⁢(MgO)+2⁢Al_=3⁢Mg_+(Al2⁢O3)(1)3⁢(CaO)+2⁢Al_=3⁢Ca_+(Al2⁢O3)(2)

[0035] An element in brackets indicates component in slag and an element with underline indicates component in molten metal.

[0036] If Al concentration is less than 0.005%, deoxidation may not be promoted sufficiently and oxygen concentration may be greater than 0.0060%. Furthermore, since deoxidation is not promoted, desulfurization may be inhibited, and the S concentration may be greater than 0.0050%. On the other hand, if Al concentration is high, being greater than 0.180%, Mg concentration may be greater than 0.0100% according to the reaction formula (1), and Ca concentration may also be greater than 0.0100% according to the reaction formula (2). Therefore, the range of Al content is set to be 0.005 to 0.180%. It is desirably 0.010 to 0.120% and more desirably 0.020 to 0.100%.(Mg: 0.0001 to 0.0100%)

[0037] Mg is an element which is effective for controlling composition of non-metallic inclusions in molten metal to be MgO, CaO—Al2O3—MgO type oxide which have no adverse influence on surface properties. The effect may not be obtained if the content is less than 0.0001%, and on the other hand, if it is contained at greater than 0.0100%, hot workability may be deteriorated, crack may easily occur during hot rolling process and surface defects may occur in final products. Therefore, Mg content is set to be 0.0001 to 0.0100%. It is desirably 0.0002 to 0.0050% and more desirably 0.0003 to 0.0020%.

[0038] It is desirable to use the reaction indicated by the formula (1) in order to effectively add Mg in molten metal. Slag composition should be controlled to be CaO: 45 to 75%, SiO2: 1 to 15%, Al2O3: 10 to 30%, MgO: 5 to 20%, and F: 1 to 15% in order to control Mg to be within the above range.(Ca: 0.0001 to 0.0100%)

[0039] Ca is an element which is effective for controlling composition of non-metallic inclusions in molten metal to be CaO—Al2O3—MgO type oxide which does not form clusters and which has no adverse influence on surface properties. The effect may not be obtained if the content is less than 0.0001%, and on the other hand, if it is contained at greater than 0.0100%, inclusions of singe CaO and / or CaO—MgO type oxide may be formed much, and surface defects and pits may occur in final products. Therefore, Ca content is set to be 0.0001 to 0.0100%. It is desirably 0.0002 to 0.0030% and more desirably 0.0003 to 0.0020%.

[0040] It is desirable to use the reaction indicated by the formula (2) in order to effectively add Ca in molten metal. Slag composition should be controlled to be CaO: 45 to 75%, SiO2: 1 to 15%, Al2O3: 10 to 30% MgO: 5 to 20%, and F: 1 to 15% in order to control Ca to be within the above range.(Nb: 0.20 to 0.80%)

[0041] Nb is an important element in the present invention, since it improves strength of Fe—Cr—Ni alloys, it is necessary to add at least not less than 0.20%. However, if it is added excessively, thermal expansion coefficient and welding crack susceptibility may be increased. Therefore, the upper limit is set to be 0.80%. It is desirably 0.30 to 0.70% and more desirably 0.40 to 0.60%.

[0042] In order to yield Nb in molten metal efficiently, since Nb is an element which is easily oxidized, Al is added at not less than 0.005% to promote deoxidation so that 0 in molten metal is controlled to be not greater than 0.0060%.(N: 0.050 to 0.500%)

[0043] Since N is an element which strengthens solid solution and has effect to increase strength of Fe—Cr—Ni alloys, it is added at least not less than 0.050%. However, if it is added excessively, it may form NbN which is nitride with Nb, and may reduce solid-solved Nb which is effective for improvement in strength of Fe—Cr—Ni alloys. Therefore, the upper limit is set to be 0.500%. It is desirably 0.100 to 0.400% and more desirably 0.200 to 0.300%.(O: 0.0001 to 0.0060%)

[0044] Oxygen concentration is very important in the present invention since it is closely related to inclusions. If O exists at greater than 0.0060% in alloy, number of inclusions may be increased thereby occur surface defects, and desulfurizing may be inhibited thereby increase S concentration. However, if it is less than 0.0001%, ability of Al reducing CaO and MgO in slag may be too high, and each of the Mg concentration and Ca concentration may be more than 0.0100% which is the upper limit. 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%.(Cu: Not Greater than 0.80%)

[0045] Cu is effective for improvement in resistance to sulfuric acid corrosion; however, if it is added excessively, hot workability may be deteriorated and crack may occur thereby cause surface defects. It is set to be not greater than 0.80%. It is desirably not greater than 0.50% and more desirably not greater than 0.30%.(Co: Not Greater than 0.50%)

[0046] Co is one of elements which stabilize austenite; however, if it is added excessively, raw material cost may be increased. Therefore, it is limited to not greater than 0.50%. It is desirably not greater than 0.40% and more desirably not greater than 0.30%.(Ti: Not greater than 0.100%)

[0047] Since Ti is an element which is effective for deoxidation of molten metal, it can be added. However, if it is added excessively, oxide thereof may adhere and block inside of an immerse nozzle inside of a continuous casting apparatus, and this may result in casting having to be stopped. Therefore, range of the Ti content is set to be not greater than 0.100%. It is desirably not greater than 0.050% and more desirably not greater than 0.020%.(Non-Metallic Inclusions)

[0048] In the present invention, it is desirable that the non-metallic inclusions composition contains at least one kind selected from MgO, CaO, CaO—MgO type oxide, CaO—Al2O3—MgO type oxide, and MgO·Al2O3, and number ratio of the MgO·Al2O3 is not greater than 50%, and number ratio of the total of the CaO and CaO—MgO type oxide is not greater than 50%.

[0049] Furthermore, it is desirable that the CaO—Al2O3—MgO type oxide contains 0.01 to 0.60 mass % of NbO.

[0050] Hereinafter reasons for limiting the components and the number ratio of the non-metallic inclusions are explained.(Non-Metallic Inclusions Composition Contains at Least One Kind Selected From MgO, CaO, CaO—MgO Type Oxide, CaO—Al2O3—MgO Type Oxide, and MgO·Al2O3)

[0051] The Fe—Cr—Ni alloy according to the present invention contains at least one kind selected from MgO, CaO, CaO—MgO type oxide, CaO—Al2O3—MgO type oxide, and MgO·Al2O3 depending on the contents of Si, Al, Mg and Ca in the Fe—Cr—Ni alloy. It should be noted that in notations of the above non-metallic inclusion composition, the notation in which elements are connected with “-” means that the inclusion kinds form uniform melt at 1600° C. of refining temperature of the Fe—Cr—Ni alloy, and the notation in which elements are connected with “-” means that the inclusion kinds form solid intermediate compounds at 1600° C. of refining temperature of the Fe—Cr—Ni alloy. Regarding CaO—MgO type oxide, CaO and MgO form eutectic composition at 1600° C. in a binary state diagram of CaO and MgO; however, since CaO and MgO are dispersed finely in a wide range of components in CaO—MgO type oxide, it is described by “-” indicating solid solution. A reason for MgO and CaO—Al2O3—MgO type oxide among the above non-metallic inclusions can be limitlessly contained in number ratio with no problem is that MgO and CaO—Al2O3—MgO type oxide do not adhere on inner wall of the immerse nozzle for pouring molten metal from the tundish to the mold in the continuous casting apparatus, they do not form large adhered depositions or occur surface defects.(Number Ratio of MgO·Al2O3 is not Greater than 50%)

[0052] MgO·Al2O3 can cause surface defects when large adhered depositions adhere to the immerse nozzle inside the continuous casting apparatus and fall off, are carried into the mold together with the molten metal, and are captured by solidified shell. However, it was found that if number ratio of MgO·Al2O3 is not greater than 50%, the adhesion tendency is low and number of surface defects is reduced. Therefore, the number ratio of MgO·Al2O3 is set to be not greater than 50%.(NbO in CaO—Al2O3—MgO Type Oxide is 0.01 to 0.60%)

[0053] Since NbO which is contained in CaO—Al2O3—MgO type oxide has an effect in which it decreases melting point of inclusions thereby make stretching and dividing property during hot or cold rolling superior, it is desirable to contain it at not less than 0.01%. However, if NbO is contained excessively, not only melting point of inclusion may be rather increased and the stretching and dividing property during hot or cold rolling may be deteriorated, but also inclusion of single Nb2O5 may be easily generated, surface defects may occur due to the inclusions, and surface properties may be deteriorated. Furthermore, Nb may not be yielded effectively in Fe—Cr—Ni alloys. Therefore, the upper limit is set to be 0.60%.

[0054] In order that NbO was contained at 0.01 to 0.60% in CaO—Al2O3—MgO type oxide, one or both of ferrosilicon alloy and pure silicon, and Al were added as primary deoxidation, and Nb was added at a timing at which the O concentration was 0.0070 to 0.0120%. After that, by adding one or both of ferrosilicon alloy and pure silicon, and Al in order to perform secondary deoxidation, final O concentration can be controlled to be 0.0001 to 0.0060% and that Nb concentration in inclusions can be controlled accurately.(Component Ratio in CaO—MgO Type Oxide is CaO: 20 to 80%, MgO: 20 to 80%)

[0055] Concentration ratio of CaO and MgO in CaO—MgO type oxide corresponds to phase ratio of CaO and MgO in the CaO—MgO type oxide. If CaO concentration is greater than 80%, influence by CaO phase is large, the oxide may behave similar to CaO inclusion. If MgO concentration is greater than 80%, influence by MgO phase is large, the oxide may behave similar to MgO inclusion. Therefore, CaO concentration and MgO concentration in CaO—MgO type oxide is set to be 20 to 80% and 20 to 80%, respectively.(Component Ratio in CaO—Al2O3—MgO Type Oxide is CaO: 10 to 60%, Al2O3: 5 to 60%, MgO: 10 to 80%, and SiO2: Not Greater than 10%)

[0056] It is more desirable that composition of CaO, Al2O3 and MgO in CaO—Al2O3—MgO type oxide is within the above ranges since melt condition is maintained at temperature inside of the immerse nozzle. If the composition is out of the ranges, the oxide may behave as a solid thereby tend to be adhered on the inner wall of the immerse nozzle in the continuous casting apparatus and cause surface defects. Furthermore, if SiO2 content is greater than the above range, coarse large inclusions may be generated much thereby cause surface defects. Therefore, CaO, Al2O3, MgO and SiO2 are set to be 10 to 60%, 5 to 60%, 10 to 80% and not greater than 10%, respectively.(Constituent Component Ratio in MgO·Al2O3 is MgO: 10 to 40% and Al2O3: 60 to 90%)

[0057] MgO·Al2O3 is a compound which has a solid solution of comparatively wide range, and the range is set since it may be a solid solution if the content is within the above ranges.(Number Ratio of Total of CaO and CaO—MgO Type Oxide is not Greater than 50%)

[0058] CaO is an inclusion which reacts with moisture in the atmosphere to be hydrate on the surface of product and falls off from the surface, thereby cause pit. CaO—MgO type oxide is an inclusion in which a single inclusion appears to contain a mixture of CaO and MgO phases. Compared to MgO, CaO—MgO type oxide may easily become hydrate and fall off from the surface of product thereby cause pit. If there is a pit on the surface of product, corrosion may be easily promoted from the pit as an origin point under corrosive environment thereby cause a large hole-like defect on the surface of product. In addition, CaO and CaO—MgO type oxide may adhere on the inner wall of the immerse nozzle for pouring molten metal from the tundish to the mold in the continuous casting apparatus, coarsened adhered depositions may fall off, and it may be carried into the mold with molten metal and captured by solidified shell, thereby cause surface defects. On the other hand, it was found that if number ratio of total of CaO and CaO—MgO type oxide is not greater than 50%, tendency for adhesion to the nozzle may be low, number of surface defects generated may be suppressed, and in addition, pit which is generated by hydrate falling off from the surface of product can also be suppressed. Therefore, the number ratio of total of CaO and CaO—MgO type oxide is set to be not greater than 50%.Method for Production

[0059] In the present invention, the method for production of Fe—Cr—Ni alloy is also provided. First, raw materials are melted in an electric furnace so as to prepare melt metal of Fe—Cr—Ni alloy having predetermined composition. Next, decarburization is performed using AOD (Argon Oxygen Decarburization) or in VOD (Vacuum Oxygen Decarburization) after AOD. Then, 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 at which O concentration gets 0.0070 to 0.0120%, and one or both of ferrosilicon alloy and pure silicon, and Al are added as secondary deoxidation. The molten metal is refined using CaO—SiO2—MgO—Al2O3—F type slag consisting of CaO: 45 to 75%, SiO2: 1 to 15%, Al2O3: 10 to 30%, MgO: 5 to 20%, F: 1 to 15%. After that, the molten metal is put into a ladle to adjust temperature and components, and slab or ingot is formed by continuous casing apparatus or conventional ingot casting. Hot forging is performing to produce slab in a case in which the ingot is formed. In this way, the non-metallic inclusions can be controlled to contain at least one of MgO, CaO, CaO—MgO type oxide, CaO—Al2O3—MgO type oxide and MgO·Al2O3, number ratio of total of the CaO and CaO—MgO type oxide can be controlled to be not greater than 50% and number ratio of the MgO·Al2O3 can be controlled to be not greater than 50%, thereby obtain the Fe—Cr—Ni alloy having excellent surface properties. The produced slabs have their surfaces ground, are heated, and then are hot-rolled, or hot-rolled and then cold-rolled, are annealed, and are washed with acid. Scale on the surface is removed, to finally produce plates.

[0060] In the method for production of Fe—Cr—Ni alloy of the present invention, as mentioned above, the slag has a characteristic composition. Hereinafter basis for slag composition defined in the present invention is explained.(CaO: 45 to 75%)

[0061] CaO concentration and SiO2 concentration in slag are factors for efficient deoxidation and desulfurization, and control for inclusions. If CaO concentration is greater than 75%, activity of CaO in the slag may be high and the reaction of the formula (2) may be promoted excessively. Therefore, concentration of Ca which is reduced in molten metal may be high, being greater than 0.0100%, non-metallic inclusions of single CaO and / or CaO—MgO type oxide may be generated and adhered inside of the immerse nozzle inside of the continuous casting apparatus, the adhered depositions may fall off and be carried into the mold with molten metal, and they may be captured by solidified shell thereby cause surface defects in final products. In addition, CaO and CaO—MgO type oxide may cause deterioration of surface properties if they exist excessively since CaO and CaO—MgO type oxide is an inclusion which reacts with moisture in the atmosphere to form hydrate, the hydrate falls off from the surface of final product and causes pit on the surface thereof. Therefore, the upper limit is set to be 75%. On the other hand, if CaO concentration is less than 45%, deoxidation and desulfurization may not be promoted and S concentration and O concentration in the present invention cannot be controlled to be the specified range. Therefore, the lower limit is set to be 45%. It is desirably 50 to 70% and more desirably 53 to 68%.(SiO2: 1 to 15%)

[0062] Since SiO2 in slag is a necessary element in order to maintain appropriate flowability of the slag, at least 1% is necessary. However, if it is greater than 15%, Al concentration, Mg concentration and Ca concentration in molten metal may be lower than the specified range, the upper limit is set to be 15%. It is desirably 3 to 10% and more desirably 5 to 8%.(Al2O3: 10 to 30%)

[0063] If Al2O3 content in slag is high, deoxidation may not be promoted sufficiently, O concentration may not be controlled to be within the specified range, and MgO·Al2O3 may be generated at greater than 50% in number ratio as non-metallic inclusion. Furthermore, Al2O3 inclusion which is easily clustered may also be formed. On the other hand, if Al2O3 content in slag is low, number ratio of total of CaO and CaO—MgO type oxide with respect to non-metallic inclusions may be greater than 50%. Therefore, the Al2O3 concentration is set to be 10 to 30%. It is desirably 13 to 27% and more desirably 15 to 25%.(MgO: 5 to 20%)

[0064] MgO in slag is an important element in order to control Mg concentration which is contained in molten metal to be within concentration range disclosed in Claims, and is an important element to control non-metallic inclusions to be a desirable composition for the present invention. Therefore, it is necessary that MgO in slag is at least not less than 5%. On the other hand, if the MgO concentration is greater than 20%, the reaction of formula (1) may be promoted excessively, Mg concentration in molten metal may be high and hot workability may be deteriorated thereby cause surface defects in final products. Therefore, the upper limit of MgO concentration is set to be 20%. The MgO content in slag may be within the specified range as a result of dolomite bricks or magnesia-chrome bricks used in AOD refining or VOD refining being dissolved in slag. Alternatively, in order to control to be within the specified range, one or both of recycled bricks of dolomite bricks and magnesia-chrome bricks can be added. It is desirably 6 to 18% and more desirably 8 to 16%.(F: 1 to 15%)

[0065] Since F has a role to maintain slag in fused state during performing slag refining, it is necessary to add at least not less than 1%. If the F concentration is less than 1%, slag cannot be melted and flowability may be low. On the other hand, if F concentration is greater than 15%, flowability of the slag increases significantly, causing noticeable damage to the bricks. Therefore, it is set to be 1 to 15%.EXAMPLES

[0066] Next, the effects of the present invention are explained further in detail with reference to Examples. It should be noted that the present invention is not limited only to the following Examples. Ferronickel, pure nickel, ferrochromium, iron scrap, stainless steel scrap, Fe—Ni alloy scrap, Fe—Mo and the like as raw materials were melted in an electric furnace of content 60 t. Then, oxygen was blown (oxidation refining) in order to remove C in AOD, or VOD after AOD, lime and fluorite were added, CaO—SiO2—Al2O3—MgO—F type slag was generated, one or both of FeSi alloy and pure Si, and Al were added, Cr reduction was performed, and then deoxidation was performed. After that, desulfurization was promoted by stirring with Ar. In AOD and VOD, magnesia-chrome bricks were lined on furnace body. Then, molten metal was put into a ladle, temperature and components were adjusted, slab and ingot were produced by continuous casting or conventional ingot casting. In a case in which ingot was produced, hot forging was performed to produce slab.

[0067] The surface of the produced slab was ground, and then hot rolling was performed to produce a hot strip. Then, annealing and acid washing were performed to remove surface scale, and a plate with a thickness of 20 mm was produced and the quality was evaluated. Next, cold rolling was performed to produce a cold strip, and then annealing and acid washing were performed to remove surface scale, and a plate with a thickness of 1 mm was produced and the quality was evaluated.

[0068] Table 1 shows chemical components of the obtained Fe—Cr—Ni alloy and slag composition after finishing refining in AOD or VOD. Table 2 shows composition of non-metallic inclusions, formation of the inclusions and evaluation of quality. Here, refining was performed in VOD in Example 5, refining was performed in AOD and then VOD in Example 6, and refining was performed in AOD in the other Examples. Furthermore, slab was casted in conventional ingot casting in Example 3 and in continuous casting in the other Examples. A value shown in bracket “[ ]” indicates that the value is out of the range of Claims of the present invention. It should be noted that there are some Examples with “[ ]” in Tables 1 and 2, they mean that they do not satisfy the range of dependent claims, but they satisfy the range of independent claims.TABLE 1Chemical components (mass %)No.CSiMnPSNiCrMoCuCoAlTiFeMgExamples123——45—6———78——910—11—12—131415Comparative16Examples17—18——19—20—21—Chemical components (mass %) composition (mass %)No.CaNbNOExamples123456789101112131415Comparative16Examples1718192021 indicates data missing or illegible when filedTABLE 2Examples110021003100410051006100789100101110012100131001410015Comparative16100Examples1718191002010021Examples1⊚⊚⊚⊚⊚⊚2⊚⊚⊚⊚⊚⊚3⊚⊚⊚⊚⊚⊚4⊚⊚⊚⊚⊚⊚5⊚⊚⊚⊚⊚⊚6◯ΔΔ◯ΔΔ7Δ⊚◯Δ⊚◯8◯⊚◯◯⊚◯9Δ⊚◯Δ⊚◯10Δ⊚◯Δ⊚◯11◯⊚◯◯⊚◯12◯⊚◯◯⊚◯13◯ΔΔ◯ΔΔ14◯ΔΔ◯ΔΔ15◯ΔΔ◯ΔΔComparative16XXXXXXExamples17XΔXXΔX18X◯XX◯X19X◯XX◯X20X◯XX◯X21XXXXXX 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 the oxygen concentration of the alloy was quantitatively analyzed by an inert gas impulse fusion infrared absorption method.(2) Non-metallic inclusions composition: Immediately after starting casting, a sample was collected in the tundish. The sample was mirror-polished, and inclusions having a size of not less than 5 μm were measured at 20 random points using SEM / EDS.

[0071] (3) Number ratio of inclusions: From the measurement results of the above (2), number ratio of MgO·Al2O3 (spinel type, in Table) and number ratio of the total of CaO and CaO—MgO type oxide with respect to the total number of non-metallic inclusions were evaluated.

[0072] (4) Evaluation of surface defects on hot-rolled plate: The surface of a plate having a thickness of 20 mm produced by hot rolling was visually observed over the entire length, and number of surface defects caused by nonmetallic inclusions or hot workability was counted within a width of 1 m and a length of 30 m. In evaluation of quality, a case in which number of surface defects was not greater than 2 is evaluated as “⊚ (concentric circle means superior)”, a case in which the number was 3 to 5 is evaluated as “o (which means good)”, a case in which the number was 6 to 10 is 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)”.

[0073] (5) Evaluation of pits on hot rolled plate: A test piece was collected from the plate of the above (4) having thickness of 20 mm, a mirror finish was applied and the sample was kept in an atmosphere of 60% humidity and 40° C. temperature for 24 hours, the surface of the sample was washed with water and further buffed to a depth of about 1 μm. After that, number of pits exceeding 10 μm in depth and 40 μm in diameter was counted on the surface of sample in an area of 10 cm×10 cm using a 3D laser microscope. Here, a case in which the number of pits was 0 is evaluated as “⊚”, a case in which the number was 1 to 2 was evaluated as “o”, a case in which the number was 3 to 5 is evaluated as “Δ”, and a case in which the number was not less than 6 was evaluated as “x”.

[0074] (6) Total evaluation of hot-rolled plate: The results of the hot-rolled plate surface defects evaluation and the hot-rolled plate pits evaluation were scored as follows.Hot-Rolled Plate Surface Defects Evaluation:⊚ 3 points o 2 points Δ 1 point x 0 pointHot-Rolled Plate Pits Evaluation:⊚ 3 points o 2 points Δ 1 point x 0 pointThen, as a total evaluation, a case in which total score for hot-rolled plate surface defects and hot-rolled plate pits evaluations was 6 points was evaluated as ⊚, a case in which the total score was 4 to 5 points was evaluated as o, a case in which the total score was 3 points was evaluated as Δ, and a case in which the total score was not greater than 2 or at least one of the hot-rolled plate surface defects evaluation and the hot-rolled plate pits evaluation was x was evaluated as x.(7) Evaluation of surface defects on cold-rolled plate: After the hot rolling, a plate having thickness of 1 mm was produced by cold rolling. The surface of the plate was visually observed over its entire length, and number of surface defects caused by non-metallic inclusions or hot workability was counted within a width of 1 m and a length of 100 m. In the evaluation of quality, a case in which the number of surface defects was not greater than 2 was evaluated as ⊚, a case in which the number was 3 to 5 was evaluated as o, a case in which the number wase 6 to 10 was evaluated as Δ, and a case in which the number was not less than 11 was evaluated as x.

[0079] (8) Evaluation of pits on cold-rolled plate: A test piece was collected from the plate of the above (8) having thickness of 1 mm, a mirror finish was applied and the sample was kept in an atmosphere of 60% humidity and 40° C. temperature for 24 hours, the surface of the sample was washed with water and further buffed to a depth of about 1 μm. After that, number of pits exceeding 10 μm in depth and 40 μm in diameter was counted on the surface of test piece in an area of 10 cm×10 cm using a 3D laser microscope. Here, a case in which the number of pits was 0 was evaluated as “⊚”, a case in which the number was 1 to 2 was evaluated as “o”, a case in which the number was 3 to 5 was evaluated as “Δ”, and a case in which the number was not less than 6 was evaluated as “x”.

[0080] (9) Total evaluation of cold-rolled plate: The results of the cold-rolled plate surface defects evaluation and the cold-rolled plate pits evaluation were scored as follows.Cold-Rolled Plate Surface Defects Evaluation:⊚ 3 points o 2 points Δ 1 point x 0 pointCold-Rolled Plate Pits Evaluation:⊚ 3 points o 2 points Δ 1 point x 0 pointThen, as a total evaluation, a case in which total score for cold-rolled plate surface defects and cold-rolled plate pits evaluations was 6 points was evaluated as ⊚, a case in which the total score was 4 to 5 points was evaluated as o, a case in which the total score was 3 points was evaluated as Δ, and a case in which the total score was not greater than 2 or at least one of the hot-rolled plate surface defects evaluation and the cold-rolled plate pits evaluation was x was evaluated as x.

[0084] Since Examples 1 to 15 satisfied the range of the present invention, the plate had only few surface defects, and almost no large pits exceeding 10 μm in depth and 40 μm in diameter were observed, resulting in good surface properties.

[0085] In Example 6, since Si concentration was 0.66% and Al concentration was 0.121%, which were within the specified ranges but high, as a result of slightly strong deoxidation, supply of Mg and Ca from slag increased slightly, and the total number ratio of CaO and CaO—MgO oxide increased slightly. As a result, a few pits exceeding 10 μm in depth and 40 μm in diameter were observed on the surface of the 10 cm×10 cm test piece.

[0086] In Example 7, since Si concentration was 0.07% and Al concentration was 0.008%, which were within the specified ranges but low, as a result of slightly insufficient deoxidation, supply of Mg and Ca from slag was slightly insufficient, and number ratio of MgO·Al2O3 increased slightly. As a result, MgO·Al2O3 was easily adhered to the inner wall of the immerse nozzle, and the coarsened MgO·Al2O3 was captured in alloy, causing slight surface defects.

[0087] In Example 8, oxygen potential was high before deoxidation and Si which also functioned as deoxidizing agent was oxidized much, and SiO2 concentration in slag was slightly high, being 9.0%. As a result, Si concentration was 0.08%, Mg concentration was 0.0002%, and Ca concentration was 0.0001%, which were within the specified ranges but low. As a result, SiO2 in CaO—Al2O3—MgO type oxide increased to 15.9%, making it easier for inclusions to become larger, and as a result, slight surface defects occurred.

[0088] In Example 9, since Si concentration was 0.09% and Mn concentration was 0.29%, which were within the specified ranges but slightly low, deoxidation was slightly insufficient, the supply of lime was slightly insufficient, CaO concentration in slag was slightly low, and supply of Ca from slag was slightly low. As a result, CaO content in the CaO—Al2O3—MgO type oxide was as low, being 9.2%, Al2O3 content was high, being 60.9%, and MgO·Al2O3 was generated. As a result, inclusions were adhered to the inner wall of the immerse nozzle in the continuous casting apparatus, making it easier for inclusions to become larger, and as a result, slight surface defects occurred.

[0089] In Example 10, when Al was added just before the end of refining, Al2O3 concentration in slag was slightly high, being 27.4%, and Al concentration was also slightly high, being 0.122%. As a result, Al2O3 concentration in MgO·Al2O3 increased to 91.1%, which made the properties similar to those of single Al2O3 and made it easier to form clusters. However, number ratio of the generated MgO·Al2O3 was not greater than 50%, so only a few surface defects occurred.

[0090] In Example 11, when Mg was added directly just before the end of refining, Mg concentration was slightly, high being 0.0067%. As a result, MgO concentration in MgO·Al2O3 was increased to 44.7%, and melting point of MgO·Al2O3 was lowered, making it easier for clusters to form. However, number ratio of the generated MgO·Al2O3 was not greater than 50%, so only a few surface defects occurred.

[0091] In Example 12, melting damage on refractory of furnace body was slightly large, and supply of Mg from slag to molten metal was increased, resulting in a slightly high Mg concentration, being 0.0068%. As a result, MgO concentration in MgO·Al2O3 was high, being 42.8%, and the melting point of MgO·Al2O3 was lowered, making it easier for clusters to form. However, number ratio of the generated MgO·Al2O3 was not greater than 50%, so only a few surface defects occurred.

[0092] In Example 13, since Al concentration was 0.136%, which was within the specified range but slightly high, and deoxidation reaction was promoted excessively. As a result, Mg and Ca were supplied from slag to molten metal excessively, and Mg concentration and Ca concentration were high. As a result, CaO—MgO type oxide was generated at a number ratio slightly exceeding 50%, and the ratio of MgO in the CaO—MgO type oxide was higher than the specified range, which lowered the melting point and made clusters more likely to form. As a result, a few pits exceeding 10 μm in depth and 40 μm in diameter were observed on the surface of the 10 cm×10 cm test piece.

[0093] In Example 14, since Si concentration was 0.61% and Al concentration was 0.127% which were within the specified ranges but slightly high, and deoxidation reaction was promoted excessively. As a result, Mg and Ca were supplied from slag to molten metal excessively, and Mg concentration and Ca concentration were high. As a result, CaO—MgO type oxide was generated at number ratio slightly exceeding 50%, and the ratio of MgO in the CaO—MgO type oxide was higher than the specified range, which lowered the melting point and made clusters more likely to form. As a result, a few pits exceeding 10 μm in depth and 40 μm in diameter were observed on the surface of the 10 cm×10 cm test piece.

[0094] In Example 15, since amount of addition of lime during refining was slightly large, CaO concentration in slag was slightly high being 70.8%. Accordingly, activity of CaO in slag was high, Ca was supplied excessively to molten metal, and Ca concentration was slightly high being 0.0032%. As a result, number ratio of the total of CaO inclusion and CaO—MgO type oxide exceeded 50%, and CaO content in CaO—MgO oxides exceeded the specified range, making hydrates more likely to occur. A few pits exceeding 10 μm in depth and 40 μm in diameter were observed on the surface of the 10 cm×10 cm test piece.

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

[0096] In Comparative Example 16, since Al concentration was 0.182% and Si concentration was 0.82% being greater than the specified ranges, deoxidation reaction was promoted excessively, and O concentration was less than the specified range, being 0.00006%. As a result, Mg and Ca were supplied from slag to molten metal excessively, and Mg concentration and Ca concentration were greater than the specified range. As a result, non-metallic inclusions of CaO and CaO—MgO type oxide were generated much, large number of pits exceeding 10 μm in depth and 40 μm in diameter were observed on the surface of the 10 cm×10 cm test piece, and surface properties were deteriorated. In addition, since the O concentration was lower than expected, Nb was hardly oxidized, and NbO content in the CaO—Al2O3—MgO type oxide was low, being 0.002%, which reduced stretching-dividing property of the inclusions during hot and cold rolling, resulting in occurrence of numerous surface defects.

[0097] In Comparative Example 17, Since Si concentration was 0.03%, Mn concentration was 0.080% and Al concentration was 0.004%, all of which were lower than the specified range, deoxidation was not promoted sufficiently, and O concentration was high, being 0.076%. As a result, although CaO—Al2O3—MgO type oxide is mainly contained, the high O concentration caused a large number of non-metallic inclusions, resulting in occurrence of many surface defects caused by the inclusions. In addition, the high O concentration caused Nb to be oxidized and not be retained sufficiently in the molten metal, resulting in a low yield being 0.15%, while NbO content in the CaO—Al2O3—MgO type oxide was high, being 0.74%, resulting in a high melting point, which meant that the inclusion was not stretched or divided during hot and cold rolling, resulting in surface defects in products. In addition, as the oxidation of Nb in molten metal progressed, surface defects due to inclusions of single Nb2O5 also occurred, deteriorating surface properties.

[0098] In Comparative Example 18, since granular Al was added from above the slag, Al which was added contacted the slag directly, Al is not yielded in molten metal thereby become oxide, and Al2O3 concentration in the slag was high, being 30.2%. Furthermore, as a result of the shortage of Al in the molten metal and insufficient deoxidation, supply of Mg and Ca from slag was insufficient, and Mg concentration and Ca concentration became lower than the specified concentration. As a result, MgO·Al2O3 were generated at number ratio of greater than 50% and clustered, and non-metallic inclusions of single Al2O3 were also generated and clustered, resulting in numerous surface defects in final product. In addition, the oxidation of Nb in molten metal progressed due to insufficient deoxidation, and surface defects due to inclusions of single Nb2O5 also occurred, deteriorating surface properties.

[0099] In Comparative Example 19, since melting damage of the refractory was extremely large, MgO concentration in slag was greater than the specified range, being 20.3%, Mg was supplied excessively to molten metal, and Mg concentration was greater than the specified range, being 0.0136%. As a result, hot workability was extremely deteriorated, surface defects due to hot workability occurred much in final products, and surface properties were deteriorated.

[0100] In Comparative Example 20, large amount of Mg was added just before the end of refining for adjusting components, Mg reacted with Al2O3 in slag, and MgO·Al2O3 inclusions were generated much. As a result, MgO·Al2O3 inclusions adhered to and deposited on the immerse nozzle inside the continuous casting apparatus, and coarsened inclusions fell off and were captured in the solidified shell, causing numerous surface defects. In addition, as Mg concentration was 0.0150%, which exceeded the specified range, hot workability was significantly deteriorated, and numerous surface defects due to hot workability occurred in final product, deteriorating the surface properties.

[0101] In Comparative Example 21, since lime was excessively added, CaO concentration in slag was greater than the specified range, being 76.5%, and SiO2 concentration and Al2O3 concentration were less than the specified range, being 1.1% and 9.7%, respectively. Accordingly, CaO activity in slag was increased, Ca was supplied excessively in molten metal, and Ca concentration was high, being 0.0158%. As a result, since CaO inclusions were generated much and CaO in CaO—MgO type oxide was greater than the specified range, surface defects due to inclusions occurred and large number of pits exceeding 10 μm in depth and 40 μm in diameter were observed on the surface of the 10 cm×10 cm test piece, and surface properties were deteriorated.INDUSTRIAL APPLICABILITY

[0102] According to the technique of the present invention, by controlling formation of non-metallic inclusions, Fe—Cr—Ni alloy having superior surface properties can be supplied, which is appropriate for use in a reactor in which corrosion resistance and high temperature strength are required.

Claims

1. A Fe—Cr—Ni alloy having excellent surface properties consisting of:in mass %, C: 0.020 to 0.150%, Si: 0.05 to 0.80%, Mn: 0.10 to 1.50%, P: not greater than 0.035%, S: not greater than 0.0050%, Ni: 34.0 to 48.0%, Cr: 22.0 to 29.0%, Mo: 0.20 to 1.20%, Al: 0.005 to 0.180%, Mg: 0.0001 to 0.0100%, Ca: 0.0001 to 0.0100%, Nb: 0.20 to 0.80%, N: 0.050 to 0.500%, O: 0.0001 to 0.0060%, Cu: not greater than 0.80%, Ti: not greater than 0.100%, Co: not greater than 0.50%, and Fe and inevitable impurities as the remainder, whereinnon-metallic inclusions contain at least one of MgO and CaO—Al2O3—MgO type oxide as an essential component and contain optionally selected from CaO, CaO—MgO type oxide and MgO·Al2O3 as an optional component,number ratio of the MgO·Al2O3 with respect to the total oxide type non-metallic inclusions is not greater than 50%, and number ratio of total of the CaO and CaO—MgO type oxide with respect to the total oxide type non-metallic inclusions is not greater than 50%.

2. The Fe—Cr—Ni alloy having excellent surface properties according to claim 1, wherein the CaO—Al2O3—MgO type oxide contains 0.01 to 0.60 mass % of NbO.

3. The Fe—Cr—Ni alloy having excellent surface properties according to claim 1, wherein the CaO—MgO type oxide contains, in mass %, CaO: 20 to 80% and MgO: 20 to 80%, the CaO—Al2O3—MgO type oxide contains CaO: 10 to 60%, Al2O3: 5 to 60%, MgO: 10 to 80% and SiO2: not greater than 10%, and the MgO·Al2O3 contains MgO: 10 to 40% and Al2O3: 60 to 90%.

4. A method for producing the Fe—Cr—Ni alloy having excellent surface properties 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, after using CaO—SiO2—MgO—Al2O3—F type slag consisting of CaO: 45 to 75%, SiO2: 1 to 15%, Al2O3: 10 to 30%, MgO: 5 to 20%, F: 1 to 15%,forming slab or ingot by continuous casing apparatus or conventional ingot casting,performing hot forging in a case in which the ingot is formed, andperforming only hot rolling or both of hot rolling and cold rolling.

5. A method for producing the Fe—Cr—Ni alloy having excellent surface properties according to claim 3, 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: 45 to 75%, SiO2: 1 to 15%, Al2O3: 10 to 30%, MgO: 5 to 20%, F: 1 to 15%,forming slab or ingot by continuous casing apparatus or conventional ingot casting,performing hot forging in a case in which the ingot is formed, andperforming only hot rolling or both of hot rolling and cold rolling.

6. The Fe—Cr—Ni alloy having excellent surface properties according to claim 2, wherein the CaO—MgO type oxide contains, in mass %, CaO: 20 to 80% and MgO: 20 to 80%, the CaO—Al2O3—MgO type oxide contains CaO: 10 to 60%, Al2O3: 5 to 60%, MgO: 10 to 80% and SiO2: not greater than 10%, and the MgO·Al2O3 contains MgO: 10 to 40% and Al2O3: 60 to 90%.

7. A method for producing the Fe—Cr—Ni alloy having excellent surface properties 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, after using CaO—SiO2—MgO—Al2O3—F type slag consisting of CaO: 45 to 75%, SiO2: 1 to 15%, Al2O3: 10 to 30%, MgO: 5 to 20%, F: 1 to 15%,forming slab or ingot by continuous casing apparatus or conventional ingot casting,performing hot forging in a case in which the ingot is formed, andperforming only hot rolling or both of hot rolling and cold rolling.

8. A method for producing the Fe—Cr—Ni alloy having excellent surface properties according to claim 6, 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, after using CaO—SiO2—MgO—Al2O3—F type slag consisting of CaO: 45 to 75%, SiO2: 1 to 15%, Al2O3: 10 to 30%, MgO: 5 to 20%, F: 1 to 15%,forming slab or ingot by continuous casing apparatus or conventional ingot casting,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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