Ni-Cu alloy having excellent surface properties and method for producing same

Abstract

[Problem] To provide a Ni-Cu alloy having excellent surface properties and a method for producing the same by reducing non-metallic inclusions that affect the surface properties and controlling the composition. [Solution] A Ni-Cu alloy comprising, in mass%, 0.01 to 0.20% of C, 0.01 to 0.5% of Si, 0.01 to 2% of Mn, 0.03% or less of P, 28 to 40% of Cu, 0.01 to 1% of Cr, 0.3 to 3% of Fe, 0.01 to 0.5% of Al, 0.01 to 0.40% of Ti, 0.010% or less of N, 0.005 to 0.04% of Mg, 0.0005 to 0.04% of Ca, 0.0003 to 0.005% of O, 0.0001 to 0.002% of S, and the balance Ni and unavoidable impurities, the mass concentrations of Ti, N, and C satisfying the following formula: [% Ti] * ([% N] + 1 / 100 * [% C]) lt; and 0.0003, 0.0003.

Description

Technical Field

[0001] The present invention relates to a Ni-Cu alloy having excellent surface properties and a method for manufacturing the same. Background Art

[0002] Ni-Cu alloys typified by Monel alloys (containing about 65% Ni and about 30% Cu by mass%) are used in applications such as spectacle frames, heat exchangers, and coating materials for marine structures that require high corrosion resistance, and thus excellent surface properties are required. If there are defects on the surface of the alloy plate, corrosion develops starting from these defects. Therefore, in order to maximize the corrosion resistance of the alloy, it is necessary to suppress them as much as possible.

[0003] In actual Ni-Cu alloys, in addition to Ti(N,C) carbonitrides that are likely to aggregate / coalesce to form large clusters, MgO·Al2O3 oxide-based non-metallic inclusions are also formed, resulting in linear surface defects on the surface after rolling. In particular, Ti(N,C) adheres, accumulates, and then falls off on the inner walls of submerged entry nozzles in continuous casting and pouring nozzles in ordinary ingot making, thereby becoming serious surface defects. Through various corrosion tests, it is known that corrosion develops significantly starting from surface defects, leading to defective products. If such surface defects occur, in order to ensure excellent surface properties, grinding is performed using a grinding machine, which significantly reduces the yield. Ni and Cu, which are the main raw materials of Ni-Cu alloys, are extremely expensive metals compared to Fe or Cr. Therefore, it is very important to increase the yield and suppress the manufacturing cost. Against this background, there is a need for a Ni-Cu alloy with excellent surface properties after casting that minimizes grinding.

[0004] In order to fully exhibit the corrosion resistance of high corrosion-resistant alloys, a technique for reducing surface flaws caused by non-metallic inclusions has been disclosed in Ni-Cr-Mo-Nb alloys (for example, refer to Patent Document 1). In addition, a Ti-containing Fe-Cr-Ni alloy with excellent surface properties has been disclosed (for example, refer to Patent Documents 2 to 4). Since any alloy system contains Ti, TiN nitride becomes the main cause of flaws, and improvement methods based on optimization of composition, slag composition, etc. have been provided. In Ni-based alloys, a technique has been provided for making non-metallic inclusions that form surface defects harmless and reducing them by controlling the slag composition and then controlling the concentrations of trace elements such as Mg, Ca, and O in the alloy (for example, refer to Patent Documents 5 and 6). However, the above alloy systems are very different from the composition system of Ni-Cu alloys, and in addition, the morphology and composition of the non-metallic inclusions to be controlled are also different. Therefore, it is very difficult to apply these existing technologies to manufacture Ni-Cu alloys with excellent surface properties.

[0005] Regarding a method for improving the surface properties related to Ni-Cu alloys, a method for improving bubble-like surface defects caused by gas defects and poor ultrasonic flaw detection by adding a trace amount of Ti has been disclosed, but it cannot be applied to the improvement of surface defects which are the technical problems of the present invention (for example, refer to Patent Document 7). In addition, in order to obtain a Ni-Cu alloy with excellent hot workability and good surface quality, a refining method for adjusting the slag composition to accurately control the concentrations of Mg, Ca, O, and S in the alloy has been disclosed (for example, refer to Patent Document 8). This is a technology aimed at reducing the concentration of S that deteriorates hot workability, and further binding Ca and Mg with S to fix them to improve hot workability, but it cannot be applied to the improvement of surface defects which are the technical problems of the present invention. That is, the problem of surface properties caused by non-metallic inclusions in Ni-Cu alloys still exists.

[0006] Prior Art Documents

[0007] Patent Documents

[0008] Patent Document 1: Japanese Patent No. 6937190,

[0009] Patent Document 2: Japanese Patent Application Laid-Open No. 2003-147492,

[0010] Patent Document 3: Japanese Patent No. 4542079,

[0011] Patent Document 4: Japanese Patent No. 6791711,

[0012] Patent Document 5: Japanese Patent No. 7015410,

[0013] Patent Document 6: Japanese Patent No. 6990337,

[0014] Patent Document 7: Japanese Patent Application Laid-Open No. 8-120375,

[0015] Patent Document 8: Japanese Patent Application Laid-Open No. 2009-114544. Summary of the Invention

[0016] Problems to be Solved by the Invention

[0017] In view of the above problems, the invention of the present application provides a Ni-Cu alloy with excellent surface properties and a manufacturing method thereof by reducing non-metallic inclusions that affect surface properties and controlling the composition.

[0018] Means for Solving the Problems

[0019] In order to solve the above problems, the inventors repeatedly conducted in-depth research and investigations. First, linear surface defects observed on the surface of Ni-Cu alloy plates manufactured using actual machines were collected and analyzed in detail using a scanning electron microscope (SEM) and an energy dispersive X-ray spectroscopy device (EDS). As a result, non-metallic inclusions of Ti(N,C) were detected in most of the surface defects. In addition, when further investigated in detail, non-metallic inclusions of MgO·Al2O3 were detected in some of the surface defects.

[0020] Furthermore, detailed investigations were carried out in each process of actual operation. The results showed that in the case where surface defects were detected, Ti(N,C) inclusions in the molten alloy were significantly attached and accumulated on the inner wall of the immersion nozzle that poured from the tundish into the mold during the continuous casting process. Moreover, it was found that during casting, clustered Ti(N,C) inclusions partially detached and were transported into the mold, and as a result, were captured by the solidified shell and remained in the subsequent rolling process, generating linear surface defects. In addition, it was found that these phenomena were the same in ordinary ingot making, causing attachment, accumulation, and detachment on the inner wall of the pouring nozzle of the ladle, resulting in surface defects. Therefore, it was found that preventing the generation of Ti(N,C) inclusions was extremely effective in preventing linear surface defects. To achieve this, it was necessary to control the component concentrations of trace amounts of Ti, N, and C within a specified range.

[0021] In addition, in addition to Ti(N,C), MgO·Al2O3 inclusions that were the main cause of generation in some surface defects were likely to aggregate and combine in the molten alloy to form clusters. Therefore, it was known that clustered MgO·Al2O3 was captured by the solidified shell during casting, resulting in linear surface defects. In order to improve the surface defects associated with the generation of these inclusions, it was known that by controlling the slag composition in secondary refining, using one or more of the CaO-CaS, MgO-MgS, CaO-MgO-CaS-MgS systems in which aggregation / combination was difficult to occur as the main body, and the number ratio of these to all non-metallic inclusions was 85% or more, this could be achieved. Based on the above insights, the present invention was completed.

[0022] That is, a Ni-Cu alloy, characterized in that, by mass %, it consists of C: 0.01 to 0.20%, Si: 0.01 to 0.5%, Mn: 0.01 to 2%, P: 0.03% or less, Cu: 28 to 40%, Cr: 0.01 to 1%, Fe: 0.3 to 3%, Al: 0.01 to 0.5%, Ti: 0.01 to 0.40%, N: 0.010% or less, Mg: 0.005 to 0.04%, Ca: 0.0005 to 0.04%, O: 0.0003 to 0.005%, S: 0.0001 to 0.002%, the balance being Ni and inevitable impurities, and the mass concentrations of Ti, N, and C satisfy the following formula:

[0023] [%Ti] × ([%N] + 1 / 100 × [%C]) < 0.0003.

[0024] In addition, the Ni-Cu alloy is characterized in that, with respect to all non-metallic inclusions contained in the Ni-Cu alloy, the proportion of one or more non-metallic inclusions in sulfur oxides of the CaO-CaS system, MgO-MgS system, and CaO-MgO-CaS-MgS system is 85% or more by number ratio.

[0025] Furthermore, in the present invention, a method for manufacturing the Ni-Cu alloy is also provided. The method for manufacturing the Ni-Cu alloy is characterized in that raw materials are melted in an electric furnace, and then, in secondary refining, blowing oxygen refining is carried out by AOD and / or VOD to adjust the C and N concentrations, and then Ti is added. Lime, fluorite, Si, and / or Al are added, and the composition of the generated slag is, by mass %, CaO: 50 to 75%, Al2O3: 5 to 25%, SiO2: 1 to 10%, MgO: 2 to 15%, F: 1 to 15% of the CaO-Al2O3-SiO2-MgO-F system slag. While stirring, deoxidation and desulfurization are carried out to adjust the O and S concentrations, and a slab or ingot is manufactured by continuous casting or ordinary ingot making. The ingot is hot forged to form a slab, and then hot rolling is carried out.

[0026] Effects of the Invention

[0027] According to the Ni-Cu alloy and its manufacturing method provided by the technology of the present invention, by preventing the generation of Ti(N, C) and MgO·Al2O3 inclusions, which are the main causes of surface defects, and mainly generating harmless non-metallic inclusions, a Ni-Cu alloy with excellent surface properties can be obtained. Brief Description of the Drawings

[0028] Figure 1 A graph showing the influence of Ti concentration, N concentration, and C concentration in the present invention on surface quality. Detailed Description of the Invention ​

[0029] First, the reasons for limiting the chemical composition of the Ni-Cu alloy of the present invention are shown. It should be noted that the following % refers to mass%.

[0030] C: 0.01 to 0.20%

[0031] Since it has the effect of improving the strength of the alloy by solid solution strengthening, it is an essential element for ensuring strength at normal and high temperatures. If the C concentration is lower than 0.01%, the strength cannot be obtained sufficiently. However, C concentrates between dendrite arms during solidification, that is, microsegregation is significant. Therefore, if the concentration is higher than 0.20%, severe longitudinal cracks will occur in continuous casting slabs and ordinary ingots, and in the worst case, steel leakage during casting called breakout will occur. Therefore, in the present invention, it is specified as 0.01 to 0.20%. Preferably, it is 0.02 to 0.19%. The C mixed in from the raw materials is removed by oxygen blowing refining (oxidative refining) in secondary refining, thereby controlling the concentration within the specified range. In the case of reducing to below the specified range, a C source such as anthracite can also be added for adjustment.

[0032] Si: 0.01 to 0.5%

[0033] Si is an important element in the present invention. It helps with deoxidation and adjusts the oxygen concentration to 0.0003 to 0.005%. In addition, it also has the effect of adjusting the Mg concentration in the alloy to 0.005 to 0.04% and the Ca concentration to 0.0005 to 0.04%. These effects are based on the following reaction formulas.

[0034] 2(MgO) + Si = 2 Mg +(SiO2)…(1)

[0035] 2(CaO) + Si = 2 Ca +(SiO2)…(2)

[0036] Here, the content in parentheses is the composition in the slag, and the underlined part represents the composition in the molten alloy. If the Si concentration is lower than 0.01%, the oxygen concentration rises above 0.005%. In addition, if the Si concentration is higher than 0.5%, the oxygen concentration drops below 0.0003%, and the reactions of (1) and (2) proceed further. As a result, the Mg concentration rises above 0.04%, and at the same time, the Ca concentration also rises above 0.04%. Therefore, the Si concentration is specified as 0.01 to 0.5%. Preferably, it is 0.03 to 0.45%.

[0037] Mn: 0.01 to 2%

[0038] Mn is an austenite phase stabilizing element that is cheaper than Ni and Cu, so more than 0.01% is added. However, since the oxidation resistance is impaired by a large addition, the upper limit is set at 2%. Preferably, it is 0.05 - 1.8%.

[0039] P: 0.03% or less

[0040] P is a harmful element that segregates at grain boundaries and causes hot working cracks, so it is desirable to minimize it and limit it to 0.03% or less. Preferably, it is 0.025% or less.

[0041] Cu: 28 - 40%

[0042] Cu is an element effective in improving the sulfuric acid corrosion resistance and the corrosion resistance in chloride environments such as seawater, so more than 28% needs to be added in the Ni - Cu alloy. However, if more than 40% is added, segregation occurs between dendrite arms during casting, that is, microsegregation becomes significant, and severe longitudinal cracks occur in continuous casting slabs and ordinary ingots, so the Cu concentration is set at 28 - 40%. Preferably, it is 29 - 38%.

[0043] Cr: 0.01 - 1%

[0044] Cr is an element that improves oxidation resistance by forming an oxide film at high temperatures, so more than 0.01% is added. However, if a large amount is added, Cr carbides precipitate at grain boundaries and cause intergranular corrosion, so the upper limit is set at 1%. Preferably, it is 0.01 - 0.7%.

[0045] Fe: 0.3 - 3%

[0046] Fe is an element that dissolves in the Ni - Cu alloy and increases the strength, so more than 0.3% is added. However, if a large amount is added, the corrosion resistance decreases, so the upper limit is set at 3%. Preferably, it is 0.5 - 2.5%.

[0047] Al: 0.01 - 0.5%

[0048] Al is an element effective for deoxidation and desulfurization and is an important element in the present invention. It has the effect of adjusting the oxygen concentration to 0.0003 - 0.005% and the S concentration to 0.0001 - 0.002%. In addition, it also has the effect of adjusting the Mg concentration in the alloy to 0.005 - 0.04% and the Ca concentration to 0.0005 - 0.04%. These effects are based on the following reactions.

[0049] 3(MgO)+2 Al =3 Mg +(Al2O3)…(3)

[0050] 3(CaO)+2 Al =3Ca +(Al2O3)…(4)

[0051] If the Al concentration is lower than 0.01%, deoxidation cannot proceed, and the oxygen concentration rises to more than 0.005%. In addition, the S concentration also rises to more than 0.002%. Conversely, if it is higher than 0.5%, due to the reactions of (3) and (4) above, the Mg concentration rises to more than 0.04%, and the Ca concentration also rises to more than 0.04%. Therefore, it is specified to be 0.01 - 0.5%. Preferably, it is 0.02 - 0.48%.

[0052] Ti: 0.01 - 0.40%

[0053] Ti is an element that generates fine precipitates such as TiC at grain boundaries and improves the strength of the alloy. Therefore, it is necessary to add more than 0.01%. However, if more than 0.40% is added, segregation occurs between dendrite arms during casting, that is, microsegregation becomes significant, and severe longitudinal cracks are generated in continuous casting slabs and ordinary ingots. Preferably, it is 0.01 - 0.38%.

[0054] N: 0.010% or less

[0055] N is an element inevitably mixed in from the atmosphere and forms non-metallic inclusions of Ti(N, C) carbonitrides by combining with Ti and C in the Ni - Cu alloy. This inclusion is prone to agglomeration, adheres to and falls off on the inner wall of the immersion nozzle, becoming the starting point of surface defects. Therefore, it is necessary to minimize the N concentration. If it is higher than 0.010%, processing cracks starting from pores caused by N2 gas are generated. Therefore, it is specified to be 0.010% or less. The N mixed in from the atmosphere is removed by oxygen blowing refining in secondary refining to reduce it to the specified range.

[0056] Mg: 0.005 - 0.04%

[0057] Mg is an element required to form inclusions of MgO - MgS system and CaO - MgO - CaS - MgS system sulfur oxides, and to react with S that deteriorates hot workability during solidification in the casting process to fix S in the form of fine MgS. Therefore, in order to obtain these effects, it is necessary to be 0.005% or more. On the other hand, if it is too high, low - melting - point Ni - Mg - based intermetallic compounds are formed, deteriorating hot workability. Therefore, the Mg concentration is specified to be 0.005 - 0.04%. Regarding its addition, it can be adjusted by the reactions of the above (1) - (4) formulas, or auxiliary raw materials such as Ni - Mg alloys can also be added.

[0058] Ca: 0.0005 - 0.04%

[0059] Ca is an element required for inclusions that generate sulfur oxides in the CaO-CaS system and the CaO-MgO-CaS-MgS system, and for fixing S in the form of fine CaS by reacting with S that deteriorates hot workability during solidification in the manufacturing process. Therefore, in order to obtain these effects, it is required to be 0.0005% or more. On the other hand, if it is too high, porosity defects caused by Ca vapor will occur during the welding of the product, deteriorating the quality of the welded part. Therefore, the Ca concentration is specified to be 0.0005 to 0.04%. Regarding its addition, it can be adjusted by the reactions of the above formulas (1) to (4), or auxiliary raw materials such as Ca-Si alloys can also be added.

[0060] O: 0.0003 to 0.005%

[0061] If the oxygen concentration is higher than 0.005%, desulfurization becomes weak, the S concentration rises above 0.002%, and hot workability deteriorates. On the other hand, if it is too low, below 0.0003, the reactions of the above formulas (1) to (4) proceed excessively, Mg rises above 0.04%, and Ca also exceeds 0.04%. Therefore, it is set in the range of 0.0003 to 0.005%. This control is achieved by the concentrations of the above Si and Al.

[0062] S: 0.0001 to 0.002%

[0063] S is a harmful element that segregates at grain boundaries to form low-melting-point compounds and causes hot cracks during processing. The adjustment of the S concentration by desulfurization in secondary refining is achieved by controlling Al at 0.01 to 0.5% and further controlling the slag composition within the specified range of the present invention. This is based on the following reaction formula.

[0064] 3 S +2 Al +3(CaO) = 3(CaS)+(Al2O3)…(5)

[0065] If the S concentration rises above 0.002%, hot rolling cracks will occur. In addition, if it is lower than 0.0001%, it cannot react with Ca and Mg in the molten alloy to form CaS and MgS.

[0066] Next, the reasons why the mass concentrations of Ti, N, and C in the Ni-Cu alloy of the present invention need to satisfy the following formula (6) will be explained.

[0067] [%Ti]×([%N]+1 / 100×[%C]) < 0.0003…(6)

[0068] Here, [%Ti], [%N], and [%C] respectively represent the mass % of Ti, N, and C in the Ni-Cu alloy.

[0069] As described above, the present inventors conducted a detailed investigation on the relationship between the linear surface defects generated on the Ni-Cu alloy plate and the actual operating conditions, and as a result, Ti(N, C) inclusions were detected from most of the surface defects. Preventing the formation of these inclusions is extremely effective in preventing surface defects. To achieve this, it was found that for the component concentrations of trace amounts of Ti, N, and C, it is necessary to control them to satisfy formula (6). Referring to previous insights (for example, Patent Document 2), the reaction for generating TiN in the molten alloy that is likely to agglomerate and cluster during casting is basically based on the following reaction formula.

[0070] TiN = Ti + N …(7)

[0071] K / f Ti / f N = [%Ti][%N]…(8)

[0072] Here, K is the equilibrium constant of the chemical reaction formula (7), and f Ti and f N are the activity coefficients of Ti and N, respectively (refer to THERMODYNAMIC DATA FOR STEELMAKING, the 19th Committee of the Japan Society for the Promotion of Science, published by Tohoku University (2010)). According to formula (8), it shows an inverse proportional relationship with the concentrations of Ti and N. That is, when expressing this relationship as a curve graph, it means that TiN is generated within the composition range above the concentration curve, and Ti and N dissolve in the alloy within the composition range below. To prevent the formation of TiN, it is necessary to control the concentrations of Ti and N so that they are below this curve. However, since the surface defects of the Ni-Cu alloy in this problem are caused by Ti(N, C), C, which has a high affinity for Ti, must also be considered in the same way as N. In addition, from the Ti(N, C) detected from the defects, based on the analysis by SEM-EDS, it is considered that it is not a two-phase separation of TiN and TiC, but a solid solution, so it is considered that the concentrations of Ti, N, and C are interrelated. Therefore, based on the above insights, the component concentrations of the molten alloy during casting in actual operation, the generation of Ti(N, C), and the relationship with surface defects were analyzed in detail. As a result, it was found that by controlling the component concentrations of Ti, N, and C to satisfy formula (6), the generation of Ti(N, C) can be prevented, which is extremely effective in preventing the surface defects of the Ni-Cu alloy plate.

[0073] In addition, in the present invention of this application, a preferred mode is that, with respect to all non-metallic inclusions (which can also be simply referred to as inclusions), the proportion of one or more non-metallic inclusions in the CaO-CaS system, MgO-MgS system, and CaO-MgO-CaS-MgS system sulfur oxides is 85% or more in terms of the number ratio.

[0074] As inclusions that can exist in Ni-Cu alloys, depending on the composition concentration and slag composition, there are Ti(N,C) carbonitrides, CaO-CaS-based, MgO-MgS-based, and CaO-MgO-CaS-MgS-based sulfur oxides, and MgO·Al2O3 oxide-based inclusions. Here, non-metallic inclusions refer to the general term for oxides, sulfides, and carbonitrides generated in the molten alloy from the secondary refining to the casting process, and most of them are dispersed in the mother alloy in the form of several μm to several tens of μm in size after solidification.

[0075] Ti(N,C) is a solid solution, and the content in parentheses indicates that N and C replace each other in the crystal structure. Regardless of the atomic ratio of C and N in Ti(N,C), it has the property of easily adhering to the inner wall of the nozzle and aggregating / coalescing.

[0076] In addition, in the representation method of the composition of sulfur oxide-based inclusions, the compounds represented by connecting with "-" form a solid solution at a refining temperature exceeding 1600°C. Referring to the phase diagram, it means that it is composed of fine CaO, MgO, CaS, and MgS dispersed in a wide composition range during casting.

[0077] For MgO·Al2O3 oxide-based inclusions, they are represented by connecting with "·", which indicates the formation of an intermediate compound. MgO·Al2O3 has a solid solution range of MgO: 10 - 40% and Al2O3: about 60 - 90% by mass, and has the properties of being hard and easily coalescing and aggregating in the molten alloy.

[0078] As described above, Ti(N,C) and MgO·Al2O3 aggregate / coalesce to form clusters, which are inclusions that cause surface defects, so it is desirable to prevent their generation. In particular, since Ti(N,C) adheres and accumulates on the inner wall of the nozzle during casting and they fall as large clusters, resulting in a large number of surface defects, it is extremely important to prevent their generation. For CaO-CaS-based, MgO-MgS-based, and CaO-MgO-CaS-MgS-based sulfur oxides, regardless of the composition range of CaO, MgO, CaS, and MgS, they do not have the property of aggregating / coalescing, so they do not form clusters and do not cause surface defects. Since these sulfur oxide-based inclusions are generated by the reaction of CaO, MgO, and CaO-MgO oxides generated during deoxidation with S in the molten alloy, they also have the function of absorbing and fixing harmful S in the molten alloy. Therefore, the non-metallic inclusions in Ni-Cu alloys are preferably one or more of CaO-CaS-based, MgO-MgS-based, and CaO-MgO-CaS-MgS-based sulfur oxides. However, if the proportion of the above three types of inclusions in all non-metallic inclusions is less than 85% by number ratio, the grinding amount after hot rolling increases, so it is preferably 85% or more.

[0079] Next, a method for manufacturing the Ni-Cu alloy of the present invention will be described. A preferred method is as follows.

[0080] When manufacturing the Ni-Cu alloy, first, raw materials are melted in an electric furnace to produce a Ni-Cu alloy with a specified composition. The raw materials include pure nickel, pure copper, Ni-Cu alloy chips, iron chips, stainless steel chips, nickel-iron alloy, ferromanganese alloy, etc. Next, in secondary refining, oxygen blowing refining, i.e., oxidative refining, is carried out by AOD (Argon Oxygen Decarburization) and / or VOD (Vacuum Oxygen Decarburization). At this time, while carrying out a decarburization reaction to remove C mixed in from the raw materials, a denitrification reaction to remove N mixed in from the atmosphere is carried out, and the C and N concentrations can be adjusted. Then, Ti is added to satisfy the formula (6) described above.

[0081] Then, lime, fluorite, Si, and / or Al are added, and a CaO-Al2O3-SiO2-MgO-F-based slag with a composition by mass% of CaO: 50 - 75%, Al2O3: 5 - 25%, SiO2: 1 - 10%, MgO: 2 - 15%, F: 1 - 15% is used. While blowing and stirring with a gas such as Ar, deoxidation and desulfurization are carried out to adjust the O and S concentrations. Here, the formation and concentration adjustment of the molten slag will be described. CaO is adjusted by adding lime and fluorite, Al2O3 is adjusted by adding Al and Al2O3 powder, SiO2 is adjusted by adding Si or SiO2 contained in fluorite, MgO is appropriately added by melting from the MgO-based refractories (dolomite, MgO-C, magnesia-chrome bricks) used for the linings of AOD and VOD into the slag, or MgO-based waste bricks are added to inhibit excessive melting loss and adjust the concentration. F is adjusted by adding fluorite.

[0082] Furthermore, the temperature and component concentration are precisely adjusted through subsequent ladle refining. Finally, a slab or ingot is manufactured by continuous casting or ordinary ingot making, the ingot is hot forged, and then hot rolled to manufacture a Ni-Cu alloy plate.

[0083] Here, the reasons for limiting the chemical composition of the slag composition described above are shown. It should be noted that the following % refers to mass% (mass%).

[0084] CaO: 50 - 75%

[0085] CaO is an essential component of the slag for desulfurization. If it is less than 50%, a high CaO activity cannot be obtained, so the desulfurization reaction cannot proceed, and the S concentration in the alloy increases to more than 0.002%. On the other hand, if it exceeds 75%, a large amount of CaO solid with a melting point exceeding 2500 °C crystallizes out from the molten slag, resulting in a significant increase in the slag viscosity and deterioration of fluidity. Therefore, the desulfurization and deoxidation reactions cannot proceed, and the S concentration increases to more than 0.002%.

[0086] Al2O3: 5 - 25%

[0087] Al2O3 is a component required to control the Al concentration in the molten alloy and to lower the melting point of the slag to form a molten slag. If more than 25% is added, it combines with Mg and O in the molten alloy to form MgO·Al2O3 inclusions that cause surface defects. In addition, if it is less than 5%, deoxidation proceeds excessively, and the Mg concentration increases to more than 0.04%, and the Ca concentration also increases to more than 0.04%. Therefore, it is limited to 5 - 25%.

[0088] SiO2: 1 - 10%

[0089] SiO2 is a component required to control the Si concentration in the molten alloy and to lower the melting point of the slag to form a molten slag. If more than 10% is added, it acts as an oxidant for the molten alloy, hindering deoxidation and desulfurization, so that the oxygen concentration exceeds 0.005% and the S concentration increases to more than 0.002%. In addition, if it is too little, less than 1%, deoxidation proceeds excessively, and the Mg concentration increases to more than 0.04%, and the Ca concentration also increases to more than 0.04%. Therefore, it is limited to 1 - 10%.

[0090] MgO: 2 - 15%

[0091] MgO is effective in forming inclusions in the MgO - MgS system and the CaO - MgO - CaS - MgS system and in lowering the melting point of the slag to form a molten slag. If added in excess, it combines with Al and O in the molten alloy to form MgO·Al2O3 inclusions that cause surface defects. If too little, the melting point of the slag increases and a molten slag cannot be formed, and the desulfurization and deoxidation reactions cannot proceed. Therefore, it is limited to 2 - 15%.

[0092] F: 1 - 15%

[0093] When refining the slag, F has the effect of increasing the reaction rates of desulfurization and deoxidation by reducing the viscosity of the molten slag and improving fluidity. If the concentration is less than 1%, high fluidity cannot be maintained, and the desulfurization and deoxidation reactions cannot proceed. In addition, if the concentration is as high as more than 15%, the fluidity is too high, the melting loss of refractories becomes severe, and the operating cost surges. Therefore, it is specified as 1 - 15%.

[0094] Examples

[0095] Examples are shown below to further clarify the constitution and effects of the invention of the present application. However, the invention of the present application is not limited to the following examples. Using an electric furnace with a melting weight of 60 tons, pure nickel, pure copper, Ni-Cu alloy chips, iron chips, stainless steel chips, nickel-iron alloy, ferromanganese alloy, etc. are melted. Then, in secondary refining, in order to adjust C and N to within the specified concentration, oxygen blowing refining is carried out by AOD or VOD to decarburize and denitrify, and then the component concentrations are confirmed, and Ti is added. Lime, fluorite, Si and / or Al are added to generate a CaO-Al2O3-SiO2-MgO-F-based slag, and deoxidation and desulfurization are carried out by Ar gas blowing and stirring. Then, samples of the molten alloy and slag are collected to confirm the component concentrations. It should be noted that in AOD and VOD, the inner lining is a MgO-based refractory (dolomite, MgO-C, magnesia-chrome brick). After secondary refining, the temperature and chemical composition are adjusted with a ladle, and a slab with a thickness of 200 mm or an ingot is manufactured by continuous casting (CC) or ordinary ingot casting (IC). The ingot is hot forged into a slab, and after grinding the surface of the manufactured slab, hot rolling is carried out to manufacture an alloy plate with a width of 1200 mm and a plate thickness of 2 mm. This process is carried out for Invention Examples 1 to 7 and Comparative Examples 8 to 15 shown in Table 1.

[0096] [Table 1]

[0097]

[0098] [Table 2]

[0099]

[0100] Regarding the chemical components shown in Table 1, the slag composition shown in Table 2, the number ratio of inclusion types, and the evaluation methods for the surface defects of the alloy plate are carried out as follows.

[0101] (1) Chemical components of the alloy and slag composition: Quantitative analysis is carried out using a fluorescent X-ray analyzer. For the concentrations of C, S, and O in the alloy, quantitative analysis is carried out by inert gas fusion-non-dispersive infrared absorption method, and N in the alloy is quantitatively analyzed by inert gas fusion-thermal conductivity method. It should be noted that for the alloy, the total concentration being less than 100% is caused by inevitable impurities in the balance. In addition, for the slag, the total concentration being less than 100% is because trace amounts of FeO, S, etc. are contained in the balance.

[0102] (2) Analysis of non-metallic inclusions: After casting started, molten alloy samples were collected from the tundish in the CC and from the ladle in the IC and analyzed. Using SEM-EDS, 20 non-metallic inclusions with a size of 5 μm or more were randomly measured, and the number ratio % of each inclusion type was obtained. The detected inclusion types were Ti(N,C), CaO-CaS system, MgO-MgS system, CaO-MgO-CaS-MgS system sulfur oxides, and MgO·Al2O3, but Ti(N,C) was not observed in the examples applying the invention of this application.

[0103] (3) Evaluation of surface properties: The surface of the alloy plate with a rolling direction length of 100 m was photographed with an automatic inspection camera. After detecting surface defects, the part was visually confirmed, and the number of linear surface defects with a length of 50 mm or more was counted.

[0104] ○: No defects

[0105] △: The number of defects is 4 or less

[0106] ×: The number of defects is 5 or more

[0107] The results of the inventive examples and comparative examples are shown in Tables 1 and 2. [] in the table indicates outside the scope of the present invention. Among them, Nos. 4 and 11 were refined by VOD and cast by ordinary ingot making (IC). In addition, Figure 1 shows a concentration curve with the vertical axis being [%Ti], the horizontal axis being ([%N] + 1 / 100 × [%C]), and the curve being [%Ti] × ([N] + 1 / 100 × [%C]) = 0.0003, and the surface quality results of the inventive examples and comparative examples are plotted. It should be noted that the dotted line shows the upper and lower limits of the specified range of Ti concentration.

[0108] Since all of the inventive example Nos. 1 to 5 satisfied the scope of the invention of this application, there were no surface defects, the surface quality was good, and the evaluation was ○.

[0109] In inventive example No. 6, the formation of Ti(N,C) was not confirmed, but the concentration of slag Al2O3 in secondary refining was higher than the specified range, so MgO·Al2O3 inclusions were formed at a number rate of 10%, resulting in surface defects. However, it was judged that the generated surface defects could be ground off within an acceptable range of manufacturing cost, that is, they were minor, and the evaluation was △.

[0110] In inventive example No. 7, the formation of Ti(N,C) was not confirmed, but the concentration of slag MgO in secondary refining was higher than the specified range, so MgO·Al2O3 was formed at a number rate of 15%, resulting in surface defects, but the quality was of the same level as No. 6, and the evaluation was △.

[0111] In Comparative Examples Nos. 8 to 11, the value of [%Ti]×([%N]+1 / 100×[%C]) increased to exceed 0.0003, generating Ti(N, C). Due to clusters that adhered / accumulated and fell on the inner wall of the nozzle during casting, an unacceptable number of surface defects were produced, and the evaluation was ×.

[0112] In Comparative Example No. 12, the value of [%Ti]×([%N]+1 / 100×[%C]) increased to exceed 0.0003, generating Ti(N, C). In addition, regarding the slag composition, the concentrations of Al2O3 and MgO exceeded the specified range, so MgO·Al2O3 was also generated, resulting in an unacceptable number of surface defects. Therefore, the evaluation was ×.

[0113] In Comparative Example No. 13, the value of [%Ti]×([%N]+1 / 100×[%C]) was lower than 0.0003, and the slag composition was also within the specified range. Therefore, the generation of Ti(N, C) and MgO·Al2O3 was not confirmed. However, due to the excessive addition of Ti exceeding the specified range, longitudinal cracks occurred throughout the entire length of the slab during casting. Although the yield rate decreased significantly, the surface grinding of the slab was carried out and it was advanced to the hot rolling process. However, the remaining flaws of the longitudinal cracks that could not be completely removed by grinding became an unacceptable large number of surface defects, and the evaluation was ×.

[0114] In Comparative Example No. 14, the value of [%Ti]×([%N]+1 / 100×[%C]) was lower than 0.0003, and the slag composition was also within the specified range. Therefore, the generation of Ti(N, C) and MgO·Al2O3 was not confirmed. However, since denitrification could not be sufficiently carried out during secondary refining, the N concentration exceeded the specified upper limit, and a large number of bubble (porosity) defects occurred throughout the entire length of the slab. After the surface grinding of the slab was carried out, it was advanced to the hot rolling process. However, since a large number of bubble defects occurred throughout the slab interior, rolling cracks were caused on the entire surface starting from them, resulting in the suspension of rolling.

[0115] In Comparative Example No. 15, the value of [%Ti]×([%N]+1 / 100×[%C]) was lower than 0.0003, and the slag composition was also within the specified range. Therefore, the generation of Ti(N, C) and MgO·Al2O3 was not confirmed. However, the composition adjustment before the casting process failed, and the C concentration exceeded the specified range, resulting in longitudinal cracks occurring throughout the entire length of the slab during casting. Although the yield rate decreased significantly, the surface grinding of the slab was carried out and it was advanced to the hot rolling process. However, the remaining flaws of the longitudinal cracks that could not be completely removed by grinding became an unacceptable large number of surface defects, and the evaluation was ×.

Claims

1. A Ni-Cu alloy, characterized in that, By mass percentage, it consists of C: 0.01 - 0.20%, Si: 0.01 - 0.5%, Mn: 0.01 - 2%, P: 0.03% or less, Cu: 28 - 40%, Cr: 0.01 - 1%, Fe: 0.3 - 3%, Al: 0.01 - 0.5%, Ti: 0.01 - 0.40%, N: 0.010% or less, Mg: 0.005 - 0.04%, Ca: 0.0005 - 0.04%, O: 0.0003 - 0.005%, S: 0.0001 - 0.002%, with the balance being Ni and unavoidable impurities, and the mass concentrations of Ti, N, and C satisfy the following formula: [%Ti] × ([%N] + 1 / 100 × [%C]) < 0.0003.

2. The Ni-Cu alloy according to claim 1, characterized in that, With respect to all non-metallic inclusions contained in the Ni-Cu alloy, the proportion of one or more non-metallic inclusions among CaO-CaS-based, MgO-MgS-based, and CaO-MgO-CaS-MgS-based sulfur oxides is 85% or more by number ratio.

3. A method for manufacturing a Ni-Cu alloy, which is the method for manufacturing a Ni-Cu alloy according to claim 1 or 2, characterized in that, The raw materials are melted in an electric furnace. Then, in secondary refining, oxygen blowing refining is carried out through AOD and / or VOD to adjust the C and N concentrations, and then Ti is added. Lime, fluorite, Si, and / or Al are added. The composition of the generated slag is, by mass percentage, CaO: 50 - 75%, Al2O3: 5 - 25%, SiO2: 1 - 10%, MgO: 2 - 15%, F: 1 - 15% of the CaO-Al2O3-SiO2-MgO-F-based slag. Deoxidation and desulfurization are carried out while stirring to adjust the O and S concentrations. A slab or ingot is manufactured by continuous casting or ordinary ingot casting. The ingot is hot forged to form a slab, and then hot rolling is carried out.

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