Fe-Cr-Ni-based alloy having excellent surface properties and method for producing same

Abstract

A Fe-Cr-Ni alloy comprising 0.020 to 0.150% of C, 0.05 to 0.80% of Si, 0.10 to 1.50% of Mn, 0.035% or less of P, 0.0050% or less of S, 34.0 to 48.0% of Ni, 22.0 to 29.0% of Cr, 0.20 to 1.20% of Mo, 0.005 to 0.180% of Al, 0.0001 to 0.0100% of Mg, 0.0001 to 0.0100% of Ca, 0.20 to 0.80% of Nb, 0.050 to 0.500% of N, 0.0001 to 0.0060% of O, 0.80% or less of Cu, 0.100% or less of Ti, and 0.50% or less of Co, with the remainder being Fe and unavoidable impurities, non-metallic inclusions including one or more of MgO, CaO, CaO-MgO oxides, CaO-Al2O3- and the number ratio of MgO.Al2O3 is 50% or less.

Description

Technical Field

[0001] The present invention relates to an Fe-Cr-Ni alloy having excellent surface properties and a method for manufacturing the same, and more particularly to an Fe-Cr-Ni alloy having excellent surface properties in which the non-metallic inclusions in the melt are controlled to a harmless composition by controlling the slag composition and Si, Al, Mg, Ca, and O in the melt, thereby reducing the number of inclusions on the surface, and a method for manufacturing the same. The present invention relates to an Fe-Cr-Ni alloy that is strictly required to have corrosion resistance and high-temperature strength as a structural material for reaction towers and the like. Background Art

[0002] When manufacturing a solar power generation device, an Fe-Cr-Ni alloy plate is mainly used as a raw material for a reaction tower for purifying polysilicon as a raw material for a power generation element. Since this reaction tower is operated at high temperature and high pressure, it is a very harsh environment from the viewpoints of high-temperature strength and corrosion resistance. If the high-temperature strength and corrosion resistance are insufficient, the service life of the reaction tower becomes short, and thus an Fe-Cr-Ni alloy that satisfies such characteristics is required.

[0003] In an Fe-Cr-Ni alloy having excellent high-temperature strength and corrosion resistance, in addition to Fe as a main component, it contains Cr, Ni, and Mo, and these metals are extremely expensive metals compared with iron. Therefore, it is very important to improve the yield and suppress the manufacturing cost. Here, if surface defects such as linear flaws are generated on the surface of the Fe-Cr-Ni alloy, it is necessary to remove them by grinding or cutting, and the yield is greatly reduced. Therefore, an Fe-Cr-Ni alloy having excellent surface properties is required.

[0004] In Patent Document 1, the following technique is disclosed: In an Fe-Cr-Ni alloy having excellent high-temperature strength, by controlling the size and number of nitrides or carbides of Nb and Ti, grain coarsening is suppressed at the annealing temperature of the product, and high creep rupture characteristics are achieved. However, nitrides or carbides of Nb and Ti are not the cause of surface flaws in the product, and the invention of Patent Document 1 cannot solve the problem of surface properties caused by oxide-based non-metallic inclusions generated in the refining process targeted by the present invention, and the problem of surface defects caused by oxide-based non-metallic inclusions still exists.

[0005] In Patent Document 2, the following technique is disclosed: In a method for manufacturing a high-Ni alloy for high temperature and a high-Ni alloy containing Al and Ti, by making the Ca / Al mass ratio in the oxide-based inclusions in the range of 1.0 to 1.5, the composition of the oxide-based inclusions is controlled to a low-melting-point CaO-Al 2 O 3system, preventing clogging of the immersion nozzle of the continuous casting machine, thereby preventing surface defects of the product. However, in Patent Document 2, since it contains 0.15 to 1.5% of Ti, CaO-Al 2 O 3 -TiO 2 is generated as an oxide-based inclusion, resulting in nozzle clogging. Since the Ti content in the invention of the present application is 0.10% or less, nozzle clogging caused by CaO-Al 2 O 3 -TiO 2 does not occur. Therefore, the content described in Patent Document 2 cannot be said to be a technology that sufficiently improves the surface properties of the Fe-Cr-Ni alloy of the present application invention.

[0006] In Patent Document 3, the following technology is disclosed: in a high-Ni alloy, by controlling the composition of non-metallic inclusions in the alloy, low-melting-point inclusions with good tensile and truncation properties are formed during hot rolling or cold rolling, thereby reducing surface defects. However, the high-Ni alloy in Patent Document 3 targets alloys containing 0.5% or less or 3 to 10% of Cr, which is different from the Fe-Cr-Ni alloy of the present invention containing 22.0 to 29.0% of Cr. The Cr content has a great influence on the control of the inclusion composition. Even if trace components such as Ca, Mg, Al, Si, and O are the same, the composition of the oxide-based non-metallic inclusions is very different. That is, the method for controlling the composition of non-metallic inclusions described in Patent Document 3 cannot be said to be a technology that sufficiently improves the surface properties of the Fe-Cr-Ni alloy of the present application invention.

[0007] In Patent Document 4, a technology for reducing surface defects by controlling inclusions in a stainless steel plate to harmless MgO, CaO-Al 2 O 3 -MgO-based oxides is reported. However, Nb contained in the Fe-Cr-Ni alloy of the present application invention at 0.20 to 0.80% has an oxidation ability similar to that of Si and Mn. That is, even if Nb oxide is contained in trace amounts in the inclusions, it will lower the melting point of the inclusions, thereby improving the tensile and truncation properties during hot rolling or cold rolling. Therefore, it is an important element in the control of surface defects. However, since the stainless steel plate described in Patent Document 4 does not contain Nb, it is a technology different from the technology for obtaining inclusion properties that are difficult to generate surface defects in the present application invention.

[0008] In Patent Document 5, it is reported that by making MgO·Al in the non-metallic inclusions in a stainless steel plate 2 O 3A technique for preventing surface defects at a ratio of 50% or less by number. However, in the present invention, Al is mainly used for deoxidizing the melt, while Si is used in Patent Document 5. Therefore, the basicity of the slag in Patent Document 5 is 2 to 5, which is lower than that of the present invention. Since the basicity is a factor that has a great influence on the inclusion properties, the technique described in Patent Document 5 is different from the present invention.

[0009] In Patent Document 6, a technique for adding Nb to an Fe-Ni-Cr alloy with a high yield is reported. However, in the present invention, the Nb concentration in non-metallic inclusions is controlled by adding Nb at the time of primary deoxidation of the melt, that is, at a relatively high oxygen concentration of 0.0070 to 0.0120%, while in Patent Document 6, it is necessary to add Nb after pre-deoxidation with Si and then deoxidation with Al to sufficiently reduce the oxygen concentration. In addition, in Patent Document 6, since Nb 2 O 5 inclusions cause surface defects and should be avoided, but in the present invention, by adding Nb by the above method, it is contained in trace amounts in the form of NbO in the CaO-Al 2 O 3 -MgO inclusions, which can improve the tensile and cut-off properties during hot rolling or cold rolling and prevent surface defects. Therefore, the technique described in Patent Document 6 is different from the present invention.

[0010] Prior art documents

[0011] Patent documents

[0012] Patent Document 1: Japanese Patent Application Laid-Open No. 2017-057461,

[0013] Patent Document 2: Japanese Patent Application Laid-Open No. 2021-70838,

[0014] Patent Document 3: Japanese Patent Application Laid-Open No. 11-315354,

[0015] Patent Document 4: Japanese Patent Application Laid-Open No. 2019-35124,

[0016] Patent Document 5: Japanese Patent Application Laid-Open No. 2015-074807,

[0017] Patent Document 6: Japanese Patent Application Laid-Open No. 2014-105341. Summary of the invention

[0018] Problems to be solved by the invention

[0019] In view of the above problems, the purpose of the present invention is to control the composition of non-metallic inclusions that affect the surface properties and provide a Fe-Cr-Ni alloy with excellent surface properties. In addition, a method for producing a Fe-Cr-Ni alloy to achieve this purpose is also provided.

[0020] Means of solving problems

[0021] The inventors have conducted in-depth research and investigation to solve the above problems. By using a scanning electron microscope (SEM) and an energy dispersive X-ray analyzer (EDS) to analyze the surface defects of Fe-Cr-Ni alloy plates with surface defects, they found that the cause of the surface defects is MgO·Al 2 O 3 Non-metallic inclusions of Fe-Cr-Ni alloy sheets include Fe-Cr-Ni alloys, CaO and CaO-MgO oxides. Such non-metallic inclusions are attached to the inner wall of the submerged nozzle used for casting from the tundish in the continuous casting machine to the mold and tend to be large-scale. The detached inclusions are captured by the solidified shell and tend to become the starting point of surface defects. In addition, due to the high melting point, they are difficult to stretch during hot rolling and are not finely dispersed, thus becoming the starting point of surface defects of Fe-Cr-Ni alloy sheets.

[0022] The inventors further conducted an in-depth study on the relationship between the composition of inclusions and metal components in Fe-Cr-Ni alloys. Specifically, in the manufacturing process of Fe-Cr-Ni alloys, metal samples of Fe-Cr-Ni alloys were collected from the tundish inside the continuous casting machine, and 20 inclusions exceeding 5 μm in the sample were randomly selected, and the inclusion composition was determined by SEM / EDS. In addition, the submerged nozzle used to supply the melt from the tundish inside the continuous casting machine to the mold was collected, and the components of the attachments on the inner wall of the nozzle were analyzed by SEM / EDS. Based on the above, the relationship between the composition of inclusions, metal components, and attachments on the inner wall of the submerged nozzle was studied in depth.

[0023] As a result, the following guidelines were obtained: by making the non-metallic inclusions of Fe-Cr-Ni alloys contain MgO, CaO, CaO-MgO oxides, CaO-Al 2 O 3 -MgO-based oxides, MgO·Al 2 O 3 The inclusion composition can be basically controlled to be MgO or CaO-Al by adjusting the Mg concentration to 0.0001-0.0100 mass%, the Ca concentration to 0.0001-0.0100 mass%, and the O concentration to 0.0001-0.0060 mass% while controlling the Si concentration to 0.05-0.80 mass% and the Al concentration to 0.005-0.180 mass%.2 O 3 -MgO-based oxides. It has also been found that when the number ratio of MgO·Al 2 O 3 is 50% or less with respect to all oxide-based non-metallic inclusions, and the combined number ratio of CaO and CaO-MgO-based oxides is 50% or less, it is difficult for such non-metallic inclusions to adhere and accumulate on the inner wall of the submerged nozzle, that is, it is difficult to grow large, and thus it is difficult to become a cause of surface defects. In addition, it has been found that since such non-metallic inclusions are finely truncated during hot rolling and cold rolling, the cleanliness is excellent.

[0024] Therefore, the Fe-Cr-Ni-based alloy of the present invention is completed based on the above insights. It is an Fe-Cr-Ni-based alloy composed of, by mass%, C: 0.020 to 0.150%, Si: 0.05 to 0.80%, Mn: 0.10 to 1.50%, P: 0.035% or less, S: 0.0050% or less, 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: 0.80% or less, Ti: 0.100% or less, and Co: 0.50% or less, with the balance being Fe and unavoidable impurities. It is characterized in that the non-metallic inclusions contain any one or both of MgO and CaO-Al 2 O 3 -MgO-based oxides as essential components, and may contain any substance in CaO, CaO-MgO-based oxides, MgO·Al 2 O 3 as optional components. With respect to all oxide-based non-metallic inclusions, the number ratio of MgO·Al 2 O 3 is 50% or less, and the combined number ratio of CaO and CaO-MgO-based oxides is 50% or less.

[0025] In the present invention, the CaO-Al 2 O 3 -MgO-based oxides in the non-metallic inclusions preferably contain 0.01 to 0.60 mass% of NbO.

[0026] In the present invention, the CaO-MgO-based oxides in the non-metallic inclusions are preferably, by mass%, CaO: 20 to 80%, MgO: 20 to 80%, and CaO-Al 2 O3 - The MgO-based oxide is composed of 10 - 60% CaO, Al 2 O 3 : 5 - 60%, MgO: 10 - 80%, SiO 2 : 10% or less, and MgO·Al 2 O 3 is composed of 10 - 40% MgO, Al 2 O 3 : 60 - 90%.

[0027] In addition, the present invention also provides a manufacturing method. That is, a method for manufacturing an Fe-Cr-Ni alloy with excellent surface properties, characterized in that raw materials are melted in an electric furnace, and then, after decarburization by AOD or by VOD after AOD, lime and fluorite are added. Then, one or two of ferrosilicon alloy and pure silicon and Al are added as primary deoxidizers. Nb is added when the O concentration reaches 0.0070 - 0.0120%. A CaO-SiO 2 : 1 - 15%, Al 2 O 3 : 10 - 30%, MgO: 5 - 20%, F: 1 - 15% composed of CaO-SiO 2 -MgO-Al 2 O 3 -F-based slag is then added, and then one or two of ferrosilicon alloy and pure silicon and Al are added to perform Cr reduction, secondary deoxidation, and desulfurization. A slab or ingot is manufactured by a continuous casting machine or ordinary ingot making. In the case of an ingot, hot forging is performed, and then hot rolling or cold rolling is performed after hot rolling. Detailed implementation mode

[0028] First, the reasons for limiting the chemical composition of the Fe-Cr-Ni alloy of the present invention are shown. It should be noted that in the following description, "%" refers to "mass%".

[0029] (C: 0.020 - 0.150%)

[0030] C is an austenite phase stabilizing element. However, if it exists in a large amount, it combines with Cr and Mo to form carbides, reducing the amount of solid-solution Cr and Mo contained in the base material and deteriorating the corrosion resistance. Therefore, the C content is set to 0.020 - 0.150%. Preferably 0.030 - 0.100%. More preferably 0.040 - 0.070%.

[0031] (Si: 0.05 - 0.80%)

[0032] Si is an element effective for deoxidation and is thus an important element in the invention of this application. In order to control the oxygen concentration within the range of 0.0001 to 0.0060%, it is required to be 0.05%. In addition, it also has the effect of reducing CaO - SiO 2 -MgO - Al 2 O 3 -F - based slag. CaO and MgO in the slag respectively adjust Mg in the melt to 0.0001 to 0.0100% and Ca to 0.0001 to 0.0100%. Thus, it has the effect of maintaining inclusions as harmless MgO, CaO - Al 2 O 3 -MgO - based. From this perspective, it is also required to be 0.05%. On the other hand, if it contains more than 0.80%, it will excessively reduce CaO and MgO in the slag, so that Mg is supplied in excess of 0.0100% and Ca also exceeds 0.0100%. As a result, CaO and CaO - MgO - based oxides are generated in an amount exceeding 50% in terms of the total number ratio, causing many surface defects or pits on the product and reducing the surface quality. In addition, if the alloy contains an excessive amount of Mg, the hot workability is reduced, and cracks are generated during hot rolling, resulting in surface defects. Therefore, the Si content is specified to be 0.05 to 0.80%. Preferably, it is 0.08 to 0.60%. More preferably, it is 0.10 to 0.40%.

[0033] (Mn: 0.10 to 1.50%)

[0034] Mn is an austenite phase stabilizing element and contributes to deoxidation, so it is required to add more than 0.10%. However, if a large amount is added, the oxidation resistance is damaged, so the upper limit is 1.50%. Preferably, it is 0.30 to 1.00%. More preferably, it is 0.50 to 0.80%.

[0035] (P: 0.035% or less)

[0036] P is a harmful element that segregates at grain boundaries and causes cracks during hot working, so it is desired to minimize it and limit it to 0.035% or less. Preferably, it is 0.030% or less. More preferably, it is 0.025% or less.

[0037] (S: 0.0050% or less)

[0038] S is a harmful element that segregates at grain boundaries to form low - melting - point compounds, thus hindering hot workability. Therefore, it is desired to minimize it and limit it to 0.0050% or less. To achieve this, the lower limit of the Al content is set to 0.005%, and deoxidation is carried out to control the O concentration within the range of 0.0001 to 0.0060%, thereby performing desulfurization. Preferably, it is 0.0030% or less. More preferably, it is 0.0010% or less.

[0039] (Ni: 34.0 - 48.0%)

[0040] Ni is the main element in the Fe - Cr - Ni alloy of the present invention. It is an element that stabilizes the austenite phase to maintain high - temperature strength and has high corrosion resistance. By containing more than 34.0% of Ni, pitting corrosion resistance and acid resistance that can withstand use in a severe corrosion environment can be obtained. However, since Ni is a very expensive raw material compared to Fe, adding a large amount will increase the manufacturing cost, so it is not preferred. Therefore, the upper limit is specified as 48.0%. It is preferably 35.0 - 45.0%. More preferably, it is 37.0 - 40.0%.

[0041] (Cr: 22.0% - 29.0%)

[0042] Cr is an element that forms a passive film on the surface of the Fe - Cr - Ni alloy. As a constituent of the base material for improving acid resistance, pitting corrosion resistance, crevice corrosion resistance, and stress corrosion cracking resistance, it is the most important element. However, if the Cr content is less than 22.0%, sufficient corrosion resistance cannot be obtained. On the contrary, if the content exceeds 29.0%, the σ phase is formed, resulting in embrittlement. For the above reasons, the Cr content is specified as 22.0 - 29.0%. It is preferably 23.0 - 27.0%. More preferably, it is 24.0 - 26.0%.

[0043] (Mo: 0.20 - 1.20%)

[0044] Mo has the effect of significantly improving corrosion resistance in a wet environment containing chlorides and a high - temperature atmospheric environment even when added in a small amount, and improving corrosion resistance in proportion to the addition amount. In addition, for Si, which is effective for deoxidation, the upper limit is 0.80%, and Mo has the effect of increasing the activity coefficient of Si to supplement the deoxidation ability, so it is a useful element. Therefore, it is necessary to add more than 0.20%. On the other hand, in materials with a large amount of Mo added, in a high - temperature atmospheric environment and when the oxygen potential on the surface is low, Mo will be preferentially oxidized, resulting in the peeling of the oxide film, which can be the cause of surface defects. Therefore, the upper limit is set at 1.20%. It is preferably 0.30 - 1.00%. More preferably, it is 0.40 - 0.80%.

[0045] (Al: 0.005 - 0.180%)

[0046] Al is an element that is very effective for deoxidation and is a particularly important element in the present invention. It can control the oxygen concentration in the range of 0.0001 - 0.0060%, and reduce CaO - SiO 2 -MgO - Al 2 O 3MgO and CaO in the -F series slag supply Mg above 0.0001% and Ca above 0.0001% to the melt respectively, and have the effect of controlling inclusions into harmless MgO and CaO-Al 2 O 3 -Effect of the MgO system. These are based on the following reactions.

[0047] 3(MgO) + 2 Al = 3 Mg +(Al 2 O 3 )…(1)

[0048] 3(CaO) + 2 Al = 3 Ca +(Al 2 O 3 )…(2)

[0049] The content in parentheses represents the components in the slag, and the underlined part represents the components in the melt.

[0050] If the Al concentration is lower than 0.005%, deoxidation will not proceed sufficiently, and the oxygen concentration will increase to more than 0.0060%. In addition, since deoxidation is not carried out, desulfurization is hindered, and the S concentration will increase to more than 0.0050%. On the other hand, if the Al concentration is higher than 0.180%, the Mg concentration will increase to more than 0.0100% through the reaction of formula (1) above, and the Ca concentration will also increase to more than 0.0100% through the reaction of formula (2) above. Therefore, the range of the Al content is specified as 0.005 - 0.180%. Preferably 0.010 - 0.120%. More preferably 0.020 - 0.100%.

[0051] (Mg: 0.0001 - 0.0100%)

[0052] Mg is an effective element for controlling the composition of non-metallic inclusions in the melt into MgO and CaO-Al 2 O 3 -MgO-based oxides that have no adverse effect on the surface properties. If the content is lower than 0.0001%, this effect cannot be obtained. Conversely, if it contains more than 0.0100%, the hot workability decreases, and thus cracks are likely to occur in the hot rolling process, bringing surface defects to the final product. Therefore, the Mg content is specified as 0.0001 - 0.0100%. Preferably 0.0002 - 0.0050%. More preferably 0.0003 - 0.0020%.

[0053] To effectively add Mg to the melt, it is preferable to utilize the reaction shown in formula (1). In order to control Mg within the above range, as long as the slag composition is controlled to CaO: 45 - 75%, SiO 2: 1 to 15%, Al 2 O 3 : 10 to 30%, MgO: 5 to 20%, F: 1 to 15% is sufficient.

[0054] (Ca: 0.0001 to 0.0100%)

[0055] Ca is an effective element for controlling the composition of non-metallic inclusions in the melt so that they do not form clusters and have no adverse effect on the surface quality. It is a CaO - Al 2 O 3 -MgO series oxide. If the content is less than 0.0001%, this effect cannot be obtained. On the contrary, if it contains more than 0.0100% of Ca, many inclusions of single-component CaO and / or CaO - MgO series oxides will be formed, resulting in surface defects or pits on the final product. Therefore, the Ca content is specified as 0.0001 to 0.0100%. Preferably, it is 0.0002 to 0.0030%. More preferably, it is 0.0003 to 0.0020%.

[0056] In order to effectively supply Ca to the melt, it is preferable to utilize the reaction shown in formula (2). In order to control Ca within the above range, as long as the slag composition is controlled to CaO: 45 to 75%, SiO 2 : 1 to 15%, Al 2 O 3 : 10 to 30%, MgO: 5 to 20%, F: 1 to 15% is sufficient.

[0057] (Nb: 0.20 to 0.80%)

[0058] Nb is an important element in the invention of this application, which improves the strength of the Fe - Cr - Ni series alloy. Therefore, at least 0.20% or more needs to be added. However, if it is added in excess, in addition to the increase in the coefficient of thermal expansion, the welding crack sensitivity will also increase. Therefore, the upper limit is specified as 0.80%. Preferably, it is 0.30 to 0.70%. More preferably, it is 0.40 to 0.60%.

[0059] Since Nb is an element that is easily oxidized, in order to effectively retain Nb in the melt, as long as 0.005% or more of Al is added for deoxidation and the O in the melt is controlled below 0.0060%.

[0060] (N: 0.050 to 0.500%)

[0061] N is a solid solution strengthening element and has the effect of increasing the strength of the Fe-Cr-Ni alloy system. Therefore, it contains at least 0.050% or more of N. However, if N is contained in excess, nitrides of Nb such as NbN are formed, and the dissolved Nb effective for increasing the strength of the Fe-Cr-Ni alloy system decreases. Therefore, the upper limit is specified as 0.500%. It is preferably 0.100 to 0.400%. More preferably, it is 0.200 to 0.300%.

[0062] (O: 0.0001 to 0.0060%)

[0063] Since the oxygen concentration is closely related to inclusions, it is very important in the present invention. If O exists in the alloy in an amount exceeding 0.0060%, the number of inclusions increases, which is related to the occurrence of surface defects and hinders desulfurization, increasing the S concentration. However, if it is less than 0.0001%, the ability of Al to reduce CaO and MgO in the molten slag is excessively increased, and the Mg concentration and Ca concentration respectively exceed the upper limit of 0.0100% and 0.0100%. Therefore, the O content is specified as 0.0001 to 0.0060%. It is preferably 0.0003 to 0.0050%. More preferably, it is 0.0005 to 0.0040%.

[0064] (Cu: 0.80% or less)

[0065] Cu is effective for improving sulfuric acid corrosion resistance. However, if it is added in excess, the hot workability is reduced, cracks are generated, which causes surface defects. Therefore, it is specified as 0.80% or less. It is preferably 0.50% or less. More preferably, it is 0.30% or less.

[0066] (Co: 0.50% or less)

[0067] Co is one of the austenite stabilizing elements. However, a large amount of addition leads to an increase in raw material costs. Therefore, it is limited to 0.50% or less. It is preferably 0.40% or less. More preferably, it is 0.30% or less.

[0068] (Ti: 0.100% or less)

[0069] Ti is an element effective for deoxidizing the melt, so it can be added. However, if it is added in excess, it adheres in the form of oxides inside the immersion nozzle in the continuous casting machine and blocks the nozzle, resulting in a situation where casting has to be aborted. Therefore, the range of the Ti content is specified as 0.100% or less. It is preferably 0.050% or less. More preferably, it is 0.020% or less.

[0070] (Non-metallic inclusions)

[0071] In the invention of the present application, a preferred embodiment is that the non-metallic inclusion composition contains one or more of MgO, CaO, CaO-MgO-based oxides, CaO-Al 2 O 3 -MgO-based oxides, MgO·Al 2 O 3 Among them, the number ratio of MgO·Al 2 O 3 is 50% or less, and the total number ratio of CaO and CaO-MgO-based oxides is 50% or less.

[0072] In addition, a preferred embodiment is that the CaO-Al 2 O 3 -MgO-based oxide contains 0.01 to 0.60% of NbO.

[0073] Hereinafter, the basis for defining the composition and number ratio of non-metallic inclusions is shown.

[0074] (The non-metallic inclusion composition contains one or more of MgO, CaO, CaO-MgO-based oxides, CaO-Al 2 O 3 -MgO-based oxides, MgO·Al 2 O 3 Among them)

[0075] In the Fe-Cr-Ni-based alloy related to the invention of the present application, according to the contents of Si, Al, Mg, and Ca in the Fe-Cr-Ni-based alloy, it contains one or more of MgO, CaO, CaO-MgO-based oxides, CaO-Al 2 O 3 -MgO-based oxides, MgO·Al 2 O 3 Among them. It should be noted that in the above description method of the non-metallic inclusion composition, the composition expressed by connecting with "-" indicates that these inclusion types become a uniform melt at the refining temperature of 1600 °C of the Fe-Cr-Ni-based alloy; the composition expressed by connecting with "·" indicates that these inclusion types form a solid intermediate compound at the refining temperature of 1600 °C of the Fe-Cr-Ni-based alloy. Regarding the CaO-MgO-based oxide, on the binary phase diagram of CaO and MgO, it is the eutectic composition of CaO and MgO at 1600 °C; in the CaO-MgO-based oxide, since CaO and MgO are finely dispersed in a wide composition range, it is described by "-" representing a solid solution. Among the above non-metallic inclusions, MgO and CaO-Al 2 O 3- The reason why it is no problem to contain MgO-based oxides without restricting the number ratio is that: Since MgO and CaO-Al 2 O 3 -MgO-based oxides do not adhere to the inner wall of the immersion nozzle used for casting from the tundish to the mold in the continuous casting machine, so large attached deposits will not be generated, and surface defects will not occur.

[0076] (The number ratio of (MgO·Al 2 O 3 is 50% or less)

[0077] MgO·Al 2 O 3 adheres to the immersion nozzle inside the continuous casting machine, and the large attached deposits fall off and are transported into the mold together with the melt and are caught by the solidified shell, which can thus become the cause of surface defects. However, it has been found that if the number ratio of MgO·Al 2 O 3 is 50% or less, this adhesion tendency is mild and the occurrence number of surface defects is suppressed. Therefore, the number ratio of MgO·Al 2 O 3 is specified to be 50% or less.

[0078] (In CaO-Al 2 O 3 -MgO-based oxides, NbO is 0.01 - 0.60%)

[0079] CaO-Al 2 O 3 - The NbO contained in MgO-based oxides has the effect of improving the drawability and cut-off property during hot rolling or cold rolling by lowering the melting point of inclusions, so it is preferably 0.01% or more. However, if NbO is contained in excess, not only will the melting point of the inclusions rise instead, deteriorating the drawability and cut-off property during hot rolling or cold rolling, but also single-component inclusions of Nb 2 O 5 are likely to be generated, resulting in surface defects caused by inclusions and deteriorating the surface quality. In addition, Nb does not stay effectively in the Fe-Cr-Ni alloy system. Therefore, the upper limit is specified to be 0.60%.

[0080] In order to make CaO-Al 2 O 3- The -MgO based oxide contains 0.01 to 0.60% of NbO. One or two of ferrosilicon alloy and pure silicon and Al are added as primary deoxidizers, and Nb is added at the moment when the O concentration reaches 0.0070 to 0.0120%. Then one or two of ferrosilicon alloy and pure silicon and Al are added for secondary deoxidization, whereby while controlling the final O concentration to 0.0001 to 0.0060%, the Nb concentration in the inclusions can be finely controlled.

[0081] (The component ratio of the CaO-MgO based oxide is CaO: 20 to 80%, MgO: 20 to 80%)

[0082] The concentrations of CaO and MgO in the CaO-MgO based oxide correspond to the ratio of the CaO phase and the MgO phase in the CaO-MgO based oxide. If the CaO concentration is higher than 80%, the influence of the CaO phase is large and it has the same behavior as CaO inclusions; if the MgO concentration is higher than 80%, the influence of the MgO phase is large and it has the same behavior as MgO inclusions. Therefore, the CaO concentration of the CaO-MgO based oxide is specified as 20 to 80%, and the MgO concentration is specified as 20 to 80%.

[0083] (CaO-Al 2 O 3 -MgO based oxide has a component ratio of CaO: 10 to 60%, Al 2 O 3 : 5 to 60%, MgO: 10 to 80%, SiO 2 : 10% or less)

[0084] In the CaO-Al 2 O 3 -MgO based oxide, if the compositions of CaO, Al 2 O 3 and MgO are within the above ranges, it remains in a molten state at the temperature in the immersion nozzle, so it is more preferable. If it is outside this range, it shows the behavior of a solid, so it shows a tendency to adhere to the inner wall of the immersion nozzle in the continuous caster, becoming the cause of surface defects. In addition, if SiO 2 is more than this range, the number of coarsened large inclusions increases, becoming the cause of surface defects. Therefore, CaO is specified as 10 to 60%, Al 2 O 3 is specified as 5 to 60%, MgO is specified as 10 to 80%, and SiO 2 is specified as 10% or less.

[0085] (The constituent ratio of MgO·Al 2 O 3 is MgO: 10 to 40%, Al 2O 3 : 60 to 90%)

[0086] MgO·Al 2 O 3 is a compound with a relatively broad solid solution, forming a solid solution within the above range, and thus is specified in this way.

[0087] (The total number ratio of CaO and CaO-MgO-based oxides is 50% or less)

[0088] CaO is an inclusion that peels off from the surface by reacting with moisture in the atmosphere on the surface of the product to form a hydrate, causing pits. CaO-MgO-based oxides are inclusions in the form of a mixture of CaO phase and MgO phase present in one inclusion. Compared with MgO, CaO-MgO-based oxides are more likely to form hydrates and are more likely to peel off from the surface of the product, causing pits. If the surface of the product has pits, corrosion is likely to occur starting from the pits in a corrosive environment, bringing large pore-like defects to the surface of the product. In addition, CaO and CaO-MgO-based oxides will adhere to the inner wall of the submerged nozzle used for casting from the tundish to the mold in the continuous casting machine. The large-sized adhered deposits will fall off and be transported into the mold together with the melt, and be captured by the solidified shell, which can thus become the cause of surface defects. On the other hand, it has been found that if the total number ratio of CaO and CaO-MgO-based oxides is 50% or less, the tendency to adhere to the nozzle is mild, the occurrence number of surface defects is suppressed, and the generation of pits caused by peeling off as hydrates on the surface of the product is also suppressed. Therefore, the total number ratio of CaO and CaO-MgO-based oxides is specified to be 50% or less.

[0089] (Manufacturing method)

[0090] In the present invention, a manufacturing method of Fe-Cr-Ni-based alloy is also proposed. First, raw materials are melted in an electric furnace to melt an Fe-Cr-Ni-based melt with a specified composition. Then, after decarburization using AOD (Argon Oxygen Decarburization) or using VOD (Vacuum Oxygen Decarburization) after AOD, lime and fluorite are added, and one or two of ferrosilicon alloy and pure silicon and Al are added as primary deoxidizers. Nb is added at the moment when the O concentration reaches 0.0070 to 0.0120%, and then one or two of ferrosilicon alloy and pure silicon and Al are added for secondary deoxidization. CaO: 45 to 75%, SiO 2 : 1 to 15%, Al 2 O 3 : 10 to 30%, MgO: 5 to 20%, F: 1 to 15% are used to form CaO-SiO2 -MgO-Al 2 O 3 -F-based slag refining melt. Then, the molten steel is tapped into a ladle, and the temperature and composition are adjusted. Slabs or ingots are manufactured by a continuous casting machine or ordinary ingot making. The ingots are hot forged to manufacture slabs. Thus, non-metallic inclusions can be controlled to be one or more of MgO, CaO, CaO-MgO-based oxides, CaO-Al 2 O 3 -MgO-based oxides, MgO·Al 2 O 3 and the total number ratio of CaO and CaO-MgO-based oxides is 50% or less, and at the same time, the number ratio of MgO·Al 2 O 3 can be suppressed to 50% or less. Therefore, an Fe-Cr-Ni-based alloy with excellent surface properties can be obtained. This method is a method of grinding the surface of the manufactured slab, performing hot rolling after heating or cold rolling after hot rolling, annealing, pickling, removing the scale on the surface, and finally manufacturing a plate.

[0091] In the method for manufacturing an Fe-Cr-Ni-based alloy according to the present invention, as described above, the composition of the slag has characteristics. Hereinafter, the basis for the slag composition specified in the present invention will be described.

[0092] (CaO: 45 - 75%)

[0093] The CaO concentration and SiO 2 concentration in the slag are elements for effectively deoxidizing and desulfurizing and controlling inclusions. If the CaO concentration exceeds 75%, the activity of CaO in the slag increases, and the reaction of formula (2) proceeds excessively. Therefore, the reduced Ca concentration in the melt rises to more than 0.0100%, and non-metallic inclusions of CaO single component and / or CaO-MgO-based oxides are generated. If they adhere to the immersion nozzle inside the continuous casting machine, the adhered deposits fall off, are transported into the mold together with the melt, and are captured by the solidified shell, thereby causing surface defects in the final product. In addition, CaO and CaO-MgO-based oxides are inclusions that react with moisture in the atmosphere to form hydrates and fall off from the surface of the final product, causing pits. Therefore, if they are present in excess, they become a cause of deteriorating the surface properties. Therefore, the upper limit is specified as 75%. On the other hand, if the CaO concentration is less than 45%, deoxidation and desulfurization cannot be carried out, and the S concentration and O concentration in the present invention cannot be controlled within the specified range. Therefore, the lower limit is specified as 45%. Preferably, it is 50 - 70%. More preferably, it is 53 - 68%.

[0094] (SiO 2 : 1 - 15%)

[0095] SiO in the slag 2 is an element necessary to ensure appropriate fluidity of the slag, so it needs to be at least 1%. However, if it rises above 15%, the Al concentration, Mg concentration, and Ca concentration in the melt will drop below the specified range, so the upper limit is specified as 15%. It is preferably 3 - 10%. More preferably 5 - 8%.

[0096] (Al 2 O 3 : 10 - 30%)

[0097] If the Al 2 O 3 in the slag is high, deoxidation cannot be sufficiently carried out, the O concentration cannot be controlled within the specified range, and more than 50% by number ratio of MgO·Al 2 O 3 is formed as non-metallic inclusions. In addition, Al 2 O 3 inclusions that are prone to clustering are also formed. On the other hand, if the Al 2 O 3 in the slag is low, the total number ratio of CaO and CaO - MgO based oxides in the non-metallic inclusions will exceed 50%. Therefore, the Al 2 O 3 concentration is specified as 10 - 30%. It is preferably 13 - 27%. More preferably 15 - 25%.

[0098] (MgO: 5 - 20%)

[0099] MgO in the slag is an important element for controlling the Mg concentration contained in the melt within the concentration range described in the claims, and is also an important element for controlling the non-metallic inclusions to the preferred composition of the present invention. Therefore, MgO in the slag needs to be at least 5% or more. On the other hand, if the MgO concentration exceeds 20%, the reaction of formula (1) will proceed excessively, the Mg concentration in the melt will increase, and the hot workability will decrease, thus causing surface defects in the final product. Therefore, the upper limit of the MgO concentration is specified as 20%. MgO in the slag becomes the specified range by being dissolved into the slag from dolomite bricks or magnesia-chrome bricks used during AOD refining or VOD refining. Or, in order to control within the specified range, one or both of waste bricks of dolomite bricks and magnesia-chrome bricks can also be added. It is preferably 6 - 18%. More preferably 8 - 16%.

[0100] (F: 1 - 15%)

[0101] F serves the function of keeping the slag in a molten state during slag refining, so it needs to be added at least at 1% or more. If the concentration of F is less than 1%, the slag will not melt and the fluidity will decrease. On the other hand, if the concentration of F increases to more than 15%, the fluidity of the slag will increase significantly, so the melting loss of the brick becomes significant. Therefore, it is specified to be 1 - 15%.

[0102] Examples

[0103] Examples are presented below to further clarify the effects of the invention of this application. However, the invention of this application is not limited to the following examples. Using an electric furnace with a capacity of 60 tons, nickel-iron alloy, pure nickel, ferrochrome alloy, iron filings, stainless steel filings, Fe-Ni-based alloy filings, Fe-Mo, etc. are melted as raw materials. Then, oxygen blowing refining (oxidative refining) for removing C is carried out by AOD or by VOD after AOD, limestone and fluorite are added, and CaO-SiO 2 -Al 2 O 3 -MgO-F-based slag is produced, one or two of FeSi alloy and pure Si and Al are added for Cr reduction, and then deoxidation is carried out. Then, Ar stirring is further carried out for desulfurization. The furnace body in AOD and VOD is lined with magnesia-chrome bricks. Then, it is poured into a ladle, temperature adjustment and composition adjustment are carried out, and slabs or ingots are manufactured by continuous casting or ordinary ingot casting. The ingots are hot forged to manufacture slabs.

[0104] After grinding the surface of the manufactured slabs, hot rolling is carried out to manufacture hot strip. Then, annealing and pickling are carried out to remove the scale on the surface, a plate with a thickness of 20 mm is manufactured and its quality is evaluated. Then, cold rolling is carried out to manufacture cold strip, and then annealing and pickling are carried out to remove the scale on the surface, a plate with a thickness of 1 mm is made and its quality is evaluated.

[0105] The chemical composition of the obtained Fe-Cr-Ni-based alloy and the slag composition at the end of AOD or VOD refining are shown in Table 1, and the non-metallic inclusion composition, the morphology of inclusions and the quality evaluation are shown in Table 2. Here, Invention Example 5 is refined by VOD, Invention Example 6 is refined by VOD after AOD, and the others are refined by AOD. In addition, Invention Example 3 blocks the slab by ordinary ingot casting, and the others block the slab by continuous casting. The values shown in [] are outside the scope of the claims of the invention of this application. It should be noted that in Tables 1 and 2, there are examples with [] although they are invention examples, but they refer to those that do not meet the dependent claims but meet the scope of the independent claims.

[0106] [Table 1]

[0107]

[0108] [Table 2]

[0109]

[0110] (1) Chemical composition of the alloy and slag composition: Quantitative analysis was carried out using a fluorescent X-ray analyzer, and the oxygen concentration of the alloy was quantitatively analyzed by the inert gas fusion-infrared absorption method.

[0111] (2) Non-metallic inclusion composition: The sample collected in the tundish immediately after the start of casting was mirror-polished, and 20 inclusions with a size of 5 μm or more were randomly measured using SEM / EDS.

[0112] (3) Inclusion number ratio: Based on the measurement results in (2) above, the number ratio of MgO·Al 2 O 3 (in the table, spinel system) and the total number ratio of CaO and CaO-MgO system oxides with respect to the total number of all non-metallic inclusions were evaluated.

[0113] (4) Evaluation of surface defects of hot-rolled plate: The surface of a 20-mm-thick plate manufactured by hot rolling was visually observed over the entire length, and the number of surface defects caused by non-metallic inclusions and hot workability in a 1-m-wide and 30-m-long area was counted. When evaluating the quality, if the number of surface defects is 2 or less, it is evaluated as ◎; if it is 3 - 5, it is evaluated as ○; if it is 6 - 10, it is evaluated as △; if it is 11 or more, it is evaluated as ×.

[0114] (5) Evaluation of pits on hot-rolled plate: Test pieces were collected from the 20-mm-thick plate in (4) above, mirror-finished, kept in an atmosphere with a humidity of 60% and a temperature of 40 °C for 24 hours, then the surface of the test pieces was washed with water, and further polished to a depth of about 1 μm. After that, the number of pits with a depth exceeding 10 μm and a diameter of 40 μm was measured on the surface of a 10-cm × 10-cm test piece using a 3D laser microscope. Here, if the number of pits is 0, it is evaluated as ◎; if it is 1 - 2, it is evaluated as 〇; if it is 3 - 5, it is evaluated as △; if it is 6 or more, it is evaluated as ×.

[0115] (6) Comprehensive evaluation of hot-rolled plate: The results of the evaluation of surface defects of hot-rolled plate and the evaluation of pits on hot-rolled plate were scored as follows.

[0116] Evaluation of surface defects of hot-rolled plate: ◎ 3 points, 〇 2 points, △ 1 point, × 0 point

[0117] Evaluation of pits on hot-rolled plate: ◎ 3 points, 〇 2 points, △ 1 point, × 0 point

[0118] Then, as a comprehensive evaluation, if the total score of the evaluation of the surface defects of the hot-rolled sheet and the pits of the hot-rolled sheet is 6 points, it is evaluated as ◎; if it is 4 - 5 points, it is evaluated as 〇; if it is 3 points, it is evaluated as △; if it is less than 2 points or the evaluation of the surface defects of the hot-rolled sheet or the pits of the hot-rolled sheet is ×, it is evaluated as ×.

[0119] (7) Evaluation of surface defects of cold-rolled sheet: Visually observe the surface of a 1-mm-thick sheet manufactured by cold rolling after hot rolling within the entire length, and count the number of surface defects caused by non-metallic inclusions and hot workability in a 1-m-wide and 100-m-long area. When conducting quality evaluation, if the number of surface defects is 2 or less, it is evaluated as ◎; if it is 3 - 5, it is evaluated as ○; if it is 6 - 10, it is evaluated as △; if it is 11 or more, it is evaluated as ×.

[0120] (8) Evaluation of pits of cold-rolled sheet: Collect test pieces from the 1-mm-thick sheet in (8) above, perform mirror finishing, keep them in an atmosphere with a humidity of 60% and a temperature of 40°C for 24 hours, then wash the surface of the test pieces, and further perform polishing with a depth of about 1 μm. On the surface of a 10-cm × 10-cm test piece, use a 3D laser microscope to measure the number of pits with a depth exceeding 10 μm and a diameter of 40 μm. Here, if the number of pits is 0, it is evaluated as ◎; if it is 1 - 2, it is evaluated as 〇; if it is 3 - 5, it is evaluated as △; if it is 6 or more, it is evaluated as ×.

[0121] (9) Comprehensive evaluation of cold-rolled sheet: Score the results of the evaluation of surface defects of cold-rolled sheet and the evaluation of pits of cold-rolled sheet as follows.

[0122] Evaluation of surface defects of cold-rolled sheet: ◎ 3 points, 〇 2 points, △ 1 point, × 0 points

[0123] Evaluation of pits of cold-rolled sheet: ◎ 3 points, 〇 2 points, △ 1 point, × 0 points

[0124] Then, as a comprehensive evaluation, if the total score of the evaluation of the surface defects of the cold-rolled sheet and the pits of the cold-rolled sheet is 6 points, it is evaluated as ◎; if it is 4 - 5 points, it is evaluated as 〇; if it is 3 points, it is evaluated as △; if it is less than 2 points or the evaluation of the surface defects of the cold-rolled sheet or the pits of the cold-rolled sheet is ×, it is evaluated as ×.

[0125] In Examples 1 - 15 of the invention, since the scope of the invention of the present application is satisfied, there are few surface defects in the sheet, and hardly any large pits with a depth exceeding 10 μm and a diameter of 40 μm can be seen, and good surface properties can be obtained.

[0126] In Invention Example 6, the Si concentration was 0.66% and the Al concentration was 0.121%. Although both were within the specified ranges, they were high, so the deoxidation was slightly stronger. As a result, the supply of Mg and Ca from the slag increased slightly, and the total number ratio of CaO and CaO-MgO-based oxides increased slightly. As a result, pits deeper than 10 μm and with a diameter of 40 μm were observed on the surface of the 10 cm × 10 cm test piece, although only a few.

[0127] In Invention Example 7, the Si concentration was 0.07% and the Al concentration was 0.008%. Although both were within the specified ranges, they were low, so the deoxidation was slightly insufficient, and the supply of Mg and Ca from the slag was slightly insufficient. The number ratio of MgO·Al 2 O 3 increased slightly. As a result, MgO·Al 2 O 3 that was likely to adhere to the inner wall of the immersion nozzle and grow larger was captured in the alloy, resulting in surface defects, although only a few.

[0128] In Invention Example 8, the oxygen potential before deoxidation was high, and the oxidation of Si, which also served as a deoxidizer, increased. The concentration of SiO 2 in the slag increased slightly to 9.0%. As a result, the Si concentration was 0.08%, the Mg concentration was 0.0002%, and the Ca concentration was 0.0001%. Although all were within the specified ranges, they were decreased. Therefore, the SiO 2 O 3 in the CaO-Al 2 -MgO-based oxides increased to 15.9%, and the inclusions were likely to grow larger, resulting in surface defects, although only a few.

[0129] In Invention Example 9, the Si concentration was 0.09% and the Mn concentration was 0.29%. Although both were within the specified ranges, they were slightly low, and the deoxidation was slightly insufficient. In addition, the amount of lime added was small, the CaO concentration in the slag was slightly low, and the supply of Ca from the slag was slightly insufficient. As a result, the CaO in the CaO-Al 2 O 3 -MgO-based oxides was as low as 9.2%, and the Al 2 O 3 increased to 60.9%, and MgO·Al 2 O 3 was generated. As a result, it adhered to the inner wall of the immersion nozzle inside the continuous casting machine, and the inclusions were likely to grow larger, resulting in surface defects, although only a few.

[0130] In Invention Example 10, Al was added near the end of refining, and the Al 2 O 3The concentration slightly increased to 27.4%, and the Al concentration also slightly increased to 0.122%. As a result, the Al in MgO·Al 2 O 3 increased to 91.1%, thereby having properties similar to those of a single Al 2 O 3 component and being prone to forming clusters. However, since the number ratio of the generated MgO·Al 2 O 3 is 50% or less, only a few surface defects are generated. 2 O 3

[0131] In Invention Example 11, Mg was directly added near the end of refining, and the Mg concentration slightly increased to 0.0067%. As a result, the MgO concentration in MgO·Al 2 O 3 increased to 44.7%, and the melting point of MgO·Al 2 O 3 decreased, thereby being prone to forming clusters. However, since the number ratio of the generated MgO·Al 2 O 3 is 50% or less, only a few surface defects are generated.

[0132] In Invention Example 12, the melting loss of the refractory of the furnace body was slightly larger, and more Mg was supplied from the slag to the melt. As a result, the Mg concentration slightly increased to 0.0068%. As a result, the MgO concentration in MgO·Al 2 O 3 increased to 42.8%, and the melting point of MgO·Al 2 O 3 decreased, thereby being prone to forming clusters. However, since the number ratio of the generated MgO·Al 2 O 3 is 50% or less, only a few surface defects are generated.

[0133] In Invention Example 13, the Al concentration was 0.136%, which was slightly high although within the specified range, and the deoxidation reaction proceeded excessively. As a result, Mg and Ca were excessively supplied from the slag to the melt, and the Mg concentration and Ca concentration increased. As a result, CaO-MgO-based oxides with a number ratio slightly exceeding 50% were generated, and the proportion of MgO in the CaO-MgO-based oxides increased beyond the specified range. Thereby, the melting point decreased, and clusters were easily formed. As a result, pits with a depth exceeding 10 μm and a diameter of 40 μm were observed on the surface of a 10 cm × 10 cm test piece, although only a few.

[0134] ​In Invention Example 14, the Si concentration was 0.61% and the Al concentration was 0.127%. Although both were within the specified ranges, they were slightly high, and the deoxidation reaction proceeded excessively. As a result, Mg and Ca were excessively supplied from the slag to the melt, and the Mg concentration and Ca concentration increased. As a result, CaO-MgO-based oxides with a number ratio slightly exceeding 50% were formed, and the MgO in the CaO-MgO-based oxides increased beyond the specified range, thereby lowering the melting point and easily generating clusters. As a result, pits deeper than 10 μm and with a diameter of 40 μm were observed on the surface of the 10 cm × 10 cm test piece, although only a few.

[0135] In Invention Example 15, since the amount of lime input during refining was slightly increased, the CaO concentration in the slag slightly increased to 70.8%. As a result, the activity of CaO in the slag increased, and Ca was excessively supplied to the melt, and the Ca concentration slightly increased to 0.0032%. As a result, CaO inclusions and CaO-MgO-based oxides with a total number ratio exceeding 50% were generated, and the CaO in the CaO-MgO-based oxides increased beyond the specified range. As a result, hydrates were easily generated, and pits deeper than 10 μm and with a diameter of 40 μm were observed on the surface of the 10 cm × 10 cm test piece, although only a few.

[0136] On the other hand, since the Comparative Example deviated from the scope of the present invention, many surface defects and / or pits were generated, and the surface properties deteriorated. Hereinafter, each example will be described.

[0137] In Comparative Example 16, the Al concentration was 0.182% and the Si concentration was 0.82%, both of which were higher than the established ranges, and the deoxidation reaction proceeded excessively. The O concentration was 0.00006%, which decreased below the specified range. As a result, Mg and Ca were excessively supplied from the slag to the melt, and the Mg concentration and Ca concentration increased beyond the specified range. As a result, a large number of non-metallic inclusions of CaO and CaO-MgO-based oxides were formed, and many pits deeper than 10 μm and with a diameter of 40 μm were observed on the surface of the 10 cm × 10 cm test piece, and the surface properties deteriorated. In addition, since the O concentration was lower than expected, Nb was hardly oxidized, and NbO in the CaO-Al 2 O 3 -MgO-based oxides decreased to 0.002%, and the tensile and truncation properties of inclusions in hot rolling and cold rolling decreased, resulting in many surface defects.

[0138] In Comparative Example 17, the Si concentration was 0.03%, the Mn concentration was 0.080%, and the Al concentration was 0.004%, all of which were lower than the established ranges. Therefore, the deoxidation did not proceed sufficiently, and the O concentration increased to 0.0076%. As a result, although with CaO-Al 2 O 3- The -MgO based oxide is the main component. However, due to the high O concentration, the number of non-metallic inclusions increases, resulting in many surface defects caused by inclusions. In addition, due to the high O concentration, Nb is oxidized and cannot stay in the melt sufficiently, dropping to 0.15%, and CaO-Al 2 O 3 - The NbO in the -MgO based oxide rises to 0.74%, thus becoming high melting point, and is not stretched or cut during hot rolling and cold rolling, bringing surface defects to the product. In addition, Nb in the melt is oxidized, and thus surface defects caused by Nb 2 O 5 single-component inclusions also occur, deteriorating the surface properties.

[0139] In Comparative Example 18, since granular Al was added from above the slag, the added Al directly contacted the slag and became an oxide that could not stay in the melt, and the Al 2 O 3 concentration in the slag rose to 30.2%. In addition, the Al in the melt was insufficient, resulting in insufficient deoxidation. As a result, the supply of Mg and Ca from the slag was insufficient, and the Mg concentration and Ca concentration were also lower than the specified concentrations. Therefore, MgO·Al with a number ratio exceeding 50% 2 O 3 agglomerated, and in addition, Al 2 O 3 single-component non-metallic inclusions also agglomerated, resulting in many surface defects in the final product. In addition, due to insufficient deoxidation, Nb in the melt was oxidized, and surface defects caused by Nb 2 O 5 single-component inclusions also occurred, deteriorating the surface properties.

[0140] In Comparative Example 19, due to severe melting loss of the refractory, the MgO concentration in the slag was 20.3%, rising to exceed the specified range, and Mg was excessively supplied to the melt. The Mg concentration was 0.0136%, rising to exceed the specified range. As a result, the hot workability deteriorated significantly, and many surface defects caused by hot workability occurred in the final product, deteriorating the surface properties.

[0141] In Comparative Example 20, in order to adjust the composition, a large amount of Mg was added near the end of refining, reacting with Al 2 O 3 in the slag, generating many MgO·Al 2 O 3 inclusions. As a result, MgO·Al 2 O 3Inclusions adhere and accumulate on the immersion nozzle inside the continuous casting machine. Larger inclusions fall off and are captured by the solidified shell, resulting in many surface defects. Additionally, the Mg concentration is 0.0150%, which rises above the specified range. As a result, the hot workability deteriorates significantly, and many surface defects caused by hot workability occur in the final product, deteriorating the surface properties.

[0142] In Comparative Example 21, due to excessive addition of lime, the CaO concentration in the slag is 76.5%, rising above the specified range. The SiO 2 concentration is 1.1%, and the Al 2 O 3 concentration is 9.7%, both of which drop below the specified range. Thereby, the activity of CaO in the slag increases, supplying an excessive amount of Ca to the melt, and the Ca concentration rises to 0.0158%. As a result, many CaO inclusions are generated, and the CaO in the CaO-MgO system oxide rises above the specified range. Thereby, surface defects caused by inclusions occur, and many pits deeper than 10 μm and with a diameter of 40 μm are observed on the surface of a 10 cm × 10 cm test piece, deteriorating the surface properties.

[0143] Industrial applicability

[0144] The technology of the present invention can supply an Fe-Cr-Ni alloy with excellent surface properties suitable for use in a reaction tower requiring corrosion resistance and high-temperature strength by controlling the morphology of non-metallic inclusions.

Claims

1. An Fe-Cr-Ni alloy with excellent surface properties, which is composed of, by mass %, C: 0.020 - 0.150%, Si: 0.05 - 0.80%, Mn: 0.10 - 1.50%, P: 0.035% or less, S: 0.0050% or less, Ni: 34.0 - 48.0%, Cr: 22.0 - 29.0%, Mo: 0.20 - 1.20%, Al: 0.005 - 0.180%, Mg: 0.0001 - 0.0100%, Ca: 0.0001 - 0.0100%, Nb: 0.20 - 0.80%, N: 0.050 - 0.500%, O: 0.0001 - 0.0060%, Cu: 0.80% or less, Ti: 0.100% or less, and Co: 0.50% or less, with the balance being Fe and unavoidable impurities, characterized in that, The non-metallic inclusions contain MgO and any one or both of CaO-Al 2 O 3 -MgO-based oxides as essential components, and may contain CaO, CaO-MgO-based oxides, MgO·Al 2 O 3 any substance therein as an optional component. With respect to all oxide-based non-metallic inclusions, the number ratio of MgO·Al 2 O 3 is 50% or less, and the total number ratio of CaO and CaO-MgO-based oxides is 50% or less.

2. The Fe-Cr-Ni alloy with excellent surface properties according to claim 1, characterized in that, The CaO-Al 2 O 3 -MgO-based oxide contains 0.01 to 0.60 mass% of NbO.

3. The Fe-Cr-Ni alloy with excellent surface properties according to claim 1 or 2, characterized in that, By mass percentage, in the CaO-MgO-based oxide, CaO: 20 to 80%, MgO: 20 to 80%, CaO-Al 2 O 3 -MgO-based oxide, CaO: 10 to 60%, Al 2 O 3 : 5 to 60%, MgO: 10 to 80%, SiO 2 : 10% or less, MgO·Al 2 O 3 In, MgO: 10 to 40%, Al 2 O 3 : 60 to 90%.

4. A method for manufacturing an Fe-Cr-Ni alloy with excellent surface properties, which is the method for manufacturing an Fe-Cr-Ni alloy with excellent surface properties according to claim 1 or 2, characterized in that, The raw materials are melted in an electric furnace. Then, after decarburization by AOD and / or VOD, lime and fluorite are added. Then, one or two of ferrosilicon alloy and pure silicon and Al are added as primary deoxidizers. Nb is added at the moment when the O concentration reaches 0.0070 - 0.0120%. A CaO - SiO 2 : 1 - 15%, Al 2 O 3 : 10 - 30%, MgO: 5 - 20%, F: 1 - 15% composed CaO - SiO 2 -MgO - Al 2 O 3 -F - based slag is used. Then, one or two of ferrosilicon alloy and pure silicon and Al are added for Cr reduction, secondary deoxidation, and desulfurization. A slab or ingot is produced by a continuous caster or ordinary ingot making. In the case of an ingot, hot forging is carried out, and then hot rolling or hot rolling and cold rolling are carried out.

5. A method for manufacturing an Fe-Cr-Ni alloy with excellent surface properties, which is the method for manufacturing an Fe-Cr-Ni alloy with excellent surface properties according to claim 3, characterized in that, The raw materials are melted in an electric furnace. Then, after decarburization through AOD and / or VOD, lime and fluorite are added. Then, one or two of ferrosilicon alloy and pure silicon and Al are added as primary deoxidizers. Nb is added at the moment when the O concentration reaches 0.0070 to 0.0120%. A CaO-SiO 2 : 1 to 15%, Al 2 O 3 : 10 to 30%, MgO: 5 to 20%, F: 1 to 15% composed CaO-SiO 2 -MgO-Al 2 O 3 -F-based slag is then added, and then one or two of ferrosilicon alloy and pure silicon and Al are added for Cr reduction, secondary deoxidation, and desulfurization. A slab or ingot is produced by a continuous caster or ordinary ingot making. In the case of an ingot, hot forging is carried out, followed by hot rolling or hot rolling and cold rolling.

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