Mold powder for continuous casting of ni-cu-system ni-based alloy and continuous casting method

Abstract

To provide mold powder for continuous casting of an Ni-Cu based alloy which suppresses cracks under a slab skin and has excellent surface properties, and to provide a continuous casting method using the powder.SOLUTION: A mold powder for use in continuous casting of a Ni-Cu based superalloy having a liquidus temperature of 1320 to 1400 °C, the mold powder comprising, as chemical components by mass%, 25 to 40% of CaO, 25 to 40% of SiO2, 10 to 20% of Na2O, 1 to 10% of Al2O3, 3 to 10% of F, 0.1 to 4% of MgO, 0.1 to 3% of Cr2O3, 0.3 to 4% of C, and an inevitable impurity, mold powder for continuous casting, having a basicity (CaO / SiO2 in terms of mass% ratio) of 0.8 to 1.5, a viscosity at 1300 °C of 0.5 to 3. 0poise, and a solidification temperature of 900 to 1200 °C.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a continuous casting mold powder for obtaining good surface quality in the production of a low-melting-point Ni-Cu-based Ni-based alloy with a liquidus temperature in the range of 1320 to 1400°C, and a continuous casting method using the continuous casting mold powder. [Background technology]

[0002] Ni-Cu alloys (hereinafter referred to as "Ni-Cu alloys") have excellent corrosion resistance and high-temperature strength, and are therefore used in severe corrosive and high-temperature environments where stainless steels cannot be used. Therefore, they require excellent surface quality, where even minute surface defects are not acceptable.

[0003] In the Ni-Cu alloy manufacturing process, raw materials such as scrap, pure metals, and alloys are melted in an electric furnace, decarburized, desulfurized, and refined using Cr reduction in an Argon Oxygen Decarburization Furnace (AOD) or Vacuum Decarburization Furnace (VOD), and finally, slabs are produced in a continuous casting machine. The resulting cold-rolled sheet is then produced through hot rolling and cold rolling processes. During this process, linear defects can develop on the surface of the final cold-rolled sheet, which can act as a starting point for corrosion and fracture, resulting in product defects. Such surface defects require an additional grinding process to ensure excellent surface quality, significantly reducing yield. Because Ni, the primary raw material for Ni-Cu alloys, is a more expensive metal than Fe and Cr, improving yield and reducing manufacturing costs are important industrially. These issues are also important from the perspective of resource conservation.

[0004] One of the main causes of these surface defects is fine cracks (hereinafter also referred to as "subsurface cracks") under the slab surface during continuous casting. If subsurface cracks are not removed after slab surface grinding, they will be stretched during hot and cold rolling, forming long linear defects measuring several centimeters to several meters. The subsurface cracks in slabs are so minute that they cannot be visually detected before grinding. They are cracks with an opening width of less than 0.1 mm that can only be detected by performing penetrant testing and enlarging the crack with a camera after grinding, or by eddy current testing. Therefore, to solve the above problems and obtain cold-rolled sheets with excellent surface quality, it is necessary to suppress not only conventional surface defects such as longitudinal slab cracks and depressions (dent defects), but also subsurface cracks.

[0005] Ni-Cu alloys have a single-phase austenite structure, which means that impurity elements such as P and S tend to concentrate between dendrites during solidification, making them prone to solidification cracking. Therefore, if the alloy is subjected to strong or uneven cooling in the mold during casting, the initially solidified shell will deform due to thermal stress, easily causing casting defects such as vertical cracks, depressions (dented defects), and subsurface cracks.

[0006] Ni-Cu alloys have a lower liquidus temperature (e.g., 1320–1400°C) than common steels and stainless steels, making it difficult to apply conventional continuous casting techniques in the steel industry. Several techniques for improving slab surface defects in the continuous casting of Ni-based, Fe-Ni, and Fe-Cr-Ni alloys have been disclosed (e.g., Patent Documents 1–6). Continuous casting mold powders (hereinafter simply referred to as “powder”) and continuous casting techniques have been proposed to prevent surface or internal defects in slabs. While these techniques control the powder's melting properties, crystallization behavior, and powder flow characteristics between the mold and solidified shell, they only aim to reduce or prevent visually noticeable vertical cracks and depressions. Therefore, they are unable to suppress subsurface cracking in Ni-Cu alloys, which is the objective of the present invention. In other words, defects resulting from subsurface cracking that cannot be completely removed by slab grinding remain in the cold-rolled sheet, resulting in reduced yields and increased manufacturing costs due to additional processes. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2007-061845 [Patent Document 2] Japanese Patent Application Laid-Open No. 2008-207187 [Patent Document 3] Japanese Patent Application Publication No. 2018-144055 [Patent Document 4] Patent No. 7032600 [Patent Document 5] Japanese Patent Application Laid-Open No. 2008-272786 [Patent Document 6] Patent No. 7158614 Summary of the Invention [Problem to be solved by the invention]

[0008] In view of the above problems, the present invention provides a mold powder for continuous casting of Ni-Cu alloys that suppresses subsurface cracking of slabs and has excellent surface properties, and a continuous casting method using the powder. [Means for solving the problem]

[0009] To solve the above problems, the inventors conducted extensive research and development. First, they investigated subsurface cracks that occurred in slabs of Ni-Cu alloys produced using a continuous casting machine. After grinding off the oxide scale on the slab surface, they performed penetrant testing and magnified observations with a camera. Furthermore, to detect cracks with narrow openings that are difficult to detect with penetrant testing, they applied eddy current testing to investigate the occurrence of subsurface cracks. As a result, the subsurface cracks were found to be several millimeters long and scattered. Subsurface cracks were then sampled from several slabs and observed and analyzed in detail using an optical microscope, a scanning electron microscope (SEM) with an attached energy dispersive X-ray spectrometer (EDS), and an electron probe microanalyzer (EPMA).

[0010] The results indicated that the subsurface cracks were cracks that opened along the interdendritic boundaries of the solidification structure. Furthermore, significant concentrations of elements such as P and S were detected on the fracture surface, confirming previous knowledge that they were solidification cracks. The opening width was narrow, less than 0.1 mm, and the depth was approximately 1-5 mm from the surface. Based on these results, it is believed that the subsurface cracks were caused by thermal stress and deformation due to strong or uneven cooling in the solidified shell inside the mold during casting, which opened the interdendritic boundaries, which are weak in strength, but not enough to cause vertical cracks or depression. Furthermore, a correlation was found between the number of subsurface cracks detected on the slab surface and the crack depth.

[0011] Therefore, the inventors focused on mold powder for continuous casting and engaged in research and development to improve it. The powder is applied to the surface of the molten alloy (hereinafter also referred to as the "molten alloy surface") supplied to the mold from the tundish via a submerged nozzle during the continuous casting process. It maintains the temperature of the molten alloy surface and prevents atmospheric oxidation. It melts at an appropriate rate when heated by the molten alloy surface and flows into the copper plate between the mold and solidified shell, acting as a lubricant. The molten powder that flows into the mold / solidified shell space is cooled by the mold and solidifies, forming a glassy powder film. If a crystalline phase does not form quickly on the mold side of the powder film, the solidified shell will be cooled rapidly or unevenly, potentially causing vertical cracks, depression, bleeding (bleeding of the molten alloy), and, in the worst case, breakout (leakage of the molten alloy). This is because the glassy powder film is highly transparent to radiation, which increases radiative heat transfer from the solidified shell to the mold and tends to result in uneven contact with the mold. The crystalline phase blocks radiant heat transfer and has good thermal insulation properties, which allows for slow cooling. In addition, it adheres evenly to the mold wall, which allows for uniform cooling. Therefore, this crystallization behavior is an important characteristic for optimizing cooling to prevent subsurface cracking, which is the issue at hand.

[0012] The Ni-Cu alloy targeted by the present invention is composed of 26 to 36 wt.% Cu, the balance being Ni and unavoidable impurities (e.g., JIS H 4551 NW4400), and has a liquidus temperature in the range of 1320 to 1400°C, which is characterized by being much lower than the liquidus temperatures of ordinary steels, stainless steels, etc. produced by continuous casting machines.

[0013] Mold powders for continuous casting melt at an appropriate rate when heated by the molten metal surface, flowing between the mold and solidified shell to provide lubrication and optimize cooling of the solidified shell. Therefore, the Ni-Cu alloy powder for continuous casting that is the subject of the present invention must melt at an appropriate rate even at the low alloy molten metal temperatures described above, flow into the mold and solidified shell, and rapidly form a crystalline phase on the mold side while forming a glassy powder film. Furthermore, the objective of the present invention is to not only prevent large defects such as vertical cracks and depressions that occur in conventional Ni-Cu alloys, but also to suppress subsurface cracking by promoting crystallization behavior that promotes slower and more uniform cooling.

[0014] Therefore, the inventors conducted various investigations and experimental studies on powders that satisfy the above-mentioned properties. First, with regard to the appropriate powder properties, it was found that if the viscosity at 1300°C is 0.5 to 3.0 poise and the solidification temperature is 900 to 1200°C, the powder will have good flowability between the mold and solidified shell even with the heat of molten Ni-Cu alloy, which has a relatively low liquidus temperature.

[0015] Next, the inventors discovered a powder composition that can achieve the above viscosity and solidification temperature range by adding C as an aggregate and further adding MgO and CrO in appropriate concentration ranges to the previously investigated CaO-SiO-NaO-AlO-(LiO)-F powder composition. LiO may be added as needed to adjust the viscosity.

[0016] In addition, within the powder composition range that yielded the above viscosity and solidification temperature range, high-temperature experiments such as thermal analysis were conducted with various compositions to simulate the thermal environment inside the mold, and the recovered samples equivalent to the powder film were analyzed using SEM / EDS, transmission electron microscope (TEM), etc. to investigate the crystallization behavior.

[0017] As a result, it was found that the appropriate powder film thickness and the appropriate proportion of crystalline phases within the total thickness were formed, and that in addition to the previously reported cuspidine (3CaO·2SiO2·CaF2), nepheline (Na2O·Al2O3·2SiO2), and calcium fluoride (CaF2), magnesiochromite (MgO·Cr2O3), a compound of MgO and Cr2O3, was also formed within the crystalline phase structure. Further research revealed that magnesiochromite crystallizes more easily than the other crystals mentioned above, and that MgO-Cr2O3 oxides have such good insulating properties that they are commonly used in steelmaking refractories as magnesiochromite bricks. Therefore, the formation of magnesiochromite resulted in slow and uniform cooling that suppressed subsurface cracking.

[0018] Further research was conducted to improve the results, and it was found that the powder melting rate at the surface of the molten Ni-Cu alloy at 1440°C was 40 to 90 g / m 2 It was found that if the melting speed is set to / sec, the heat supply from the surface of the molten alloy will melt it at an appropriate rate, promoting the uniform flow of molten powder between the mold and solidified shell and reducing uneven cooling of the solidified shell.

[0019] Furthermore, the inventors also investigated the continuous casting conditions under which the powder of the present invention exerts a strong effect of suppressing subsurface cracking. The present invention was completed based on the above research results, and is as follows.

[0020] The present invention is a mold powder used in the continuous casting of a Ni-Cu based Ni-based alloy having a liquidus temperature of 1320-1400°C, the chemical composition of which is, in mass %, CaO: 25-40%, SiO2: 25-40%, Na2O: 10-20%, Al2O3: 1-10%, F: 3-10%, MgO: 0.1-4%, Cr2O3: 0.1-3%, aggregate C: 0.3-4%, unavoidable impurities, and, if necessary, contains 1.5 mass% or less Li2O for viscosity adjustment, the mold powder for continuous casting is characterized by having a basicity (CaO / SiO2 in mass %) of 0.8-1.5, a viscosity at 1300°C of 0.5-3.0 poise, and a solidification temperature of 900-1200°C.

[0021] The mold powder for continuous casting is also suitable for forming the Ni-Cu alloy, which has a melting rate of 40 to 90 g / m on the surface of the molten metal at 1440°C. 2 / second.

[0022] The present invention also proposes a continuous casting method using the above-mentioned mold powder for continuous casting, which is characterized by casting a molten Ni-Cu alloy having a composition, by mass%, of 0.5% or less C, 0.5% or less Si, 3% or less Mn, 3% or less Fe, 26-36% Cu, and the remainder being Ni and unavoidable impurities, and a liquidus temperature of 1320-1400°C, under conditions of a withdrawal speed of 300-900 mm / min and a superheat of the molten alloy of 20-60°C. [Effects of the Invention]

[0023] According to the present invention, it is possible to suppress subsurface cracks that occur in slabs produced by continuous casting of Ni-Cu based Ni-based alloys, and as a result, it is possible to prevent linear defects that occur in cold-rolled sheets during the cold rolling process. As a result, it is possible to provide products with good surface quality at low cost, with high yields and no additional grinding process required in the cold rolling process, thereby reducing manufacturing costs. DETAILED DESCRIPTION OF THE INVENTION

[0024] First, the reasons for limiting the chemical composition of the powder for continuous casting of the present invention will be described. In the following explanation, "%" means mass %. Note that, since the powder is basically melted by heat from the surface of the molten alloy, the heat supply in the mold is important for the powder to exert its effects.

[0025] CaO: 25~40%, SiO2: 25~40%, Na2O: 10~20%, Al2O3: 1~10%, F: 3~10%, MgO: 0.1~4%, Cr2O3: 0.1~3%, Li2O: 1.5% or less If the oxides and fluorides are within the above ranges, the viscosity at 1300°C will be 0.5 to 3.0 poise and the solidification temperature will be 900 to 1200°C. For example, F is added as CaF2, NaF, LiF, etc. Furthermore, MgO and Cr2O3 may be added as impurities, such as simple oxides, magnesiochromite, or other oxides and fluorides. When the present powder melts and flows between the mold and solidified shell as molten powder, it forms a glassy powder film. The appropriate thickness is 0.5 to 3.0 mm, and it is preferable that a crystalline phase is formed in a portion of the film that corresponds to 15 to 75% of the total thickness on the side in contact with the mold. If the powder film thickness is less than 0.5 mm, cooling will be too strong, while if it exceeds 3.0 mm, uneven cooling will be promoted. If the crystalline phase formation rate is less than 15%, the cooling will be strong, similar to that of a glassy material. However, if it exceeds 75%, the cooling will be too slow, resulting in insufficient strength of the solidified shell and promoting vertical cracks. Furthermore, the crystalline phases that make up the material are cuspidine (3CaO·2SiO2·CaF2), nepheline (Na2O·Al2O3·2SiO2), calcium fluoride (CaF2), and magnesiochromite (MgO·Cr2O3), which are the most preferable because they allow proper control of the heat flux from the solidified shell to the mold.

[0026] In particular, magnesiochromite (MgO·Cr2O3), a compound of MgO and Cr2O3, is important because it crystallizes more easily than the other crystals mentioned above. It not only acts as a nucleus for heterogeneous nucleation, promoting the crystallization of other crystals, but also has excellent insulating properties, thereby suppressing subsurface cracking in the solidified shell during the early stages of solidification. Magnesiochromite (MgO·Cr2O3) exerts its effects when present in the crystalline phase of the powder film at a concentration of 0.1% by mass or more, so the MgO and Cr2O3 content must be 0.1% or more. Furthermore, if the MgO concentration exceeds 4%, not only magnesiochromite but also MgO is formed, which causes excessive slow cooling, slowing the solidified shell's development of sufficient strength and actually promoting subsurface cracking. On the other hand, if the Cr2O3 concentration exceeds 3%, not only magnesiochromite but also Cr2O3 is formed, which promotes subsurface cracking due to excessive slow cooling. If the contents of both MgO and Cr2O3 are outside the specified range, the formation of magnesiochromite (MgO·Cr2O3) will be excessive, causing excessive slow cooling, which will promote subsurface cracking for the reasons mentioned above.

[0027] Therefore, to simultaneously achieve the appropriate viscosity, solidification temperature, and crystallization behavior of the powder to suppress subsurface cracking of the slab, it is necessary to control the composition within the above ranges. From these perspectives, the following are specified: CaO: 25-40%, SiO2: 25-40%, Na2O: 10-20%, Al2O3: 1-10%, F: 3-10%, MgO: 0.1-4%, Cr2O3: 0.1-3%, and Li2O: 1.5% or less, which is added as needed to adjust viscosity.

[0028] C: 0.3 to 4% C particles exist between the oxide and fluoride particles, preventing melting due to contact. In other words, when the C is burned and gasified by heat supplied from the surface of the molten alloy, the C particles disappear, and the oxide and fluoride particles come into contact and begin to melt. Controlling the C content can control the powder melting rate and uniformity of the melting reaction. At less than 0.3%, oxide and fluoride particles come into contact, resulting in uneven and rapid melting, which causes excessive molten powder to flow between the mold and solidified shell, resulting in uneven cooling. At higher C content levels (over 4%), melting is uneven and slow, resulting in many areas where molten powder cannot flow between the mold and solidified shell, resulting in uneven cooling. This not only promotes subsurface cracking, but can also lead to sticking (seizure of the solidified shell to the mold) and, in the worst case, breakout. For these reasons, the C content is limited to 0.3-4%.

[0029] Basicity (mass concentration ratio of CaO / SiO2): 0.8 to 1.5 If the basicity is too low, the solidification temperature will be lower than 900°C and the viscosity at 1300°C will be lower than 0.5 poise. Furthermore, the powder film will be thicker than 3.0 mm, and the crystalline phase will account for less than 15% of the total thickness within the glassy film, resulting in strong and uneven cooling and promoting subsurface cracking. On the other hand, if the basicity is too high, the solidification temperature will be higher than 1200°C and the viscosity at 1300°C will be higher than 3.0 poise. Furthermore, the powder film will be too thin (less than 0.5 mm) and the crystalline phase will be thicker (more than 75%), reducing the lubrication effect and causing surface defects such as sticking and vertical cracks. In other words, when the basicity (CaO / SiO2) is in the range of 0.8 to 1.5, a glassy powder film will form when the molten powder flows between the mold and solidified shell, and its thickness will be within the appropriate range of 0.5 to 3.0 mm. Then, within this film, a crystalline phase is formed on the mold side, accounting for 15 to 75% of the total thickness. This reduces strong and uneven cooling of the solidified shell, making it possible to suppress subsurface cracking. Therefore, the basicity range is specified as 0.8 to 1.5.

[0030] Viscosity at 1300℃: 0.5 to 3.0 poise If the viscosity at 1300°C is less than 0.5 poise, excessive flow of molten powder into the mold / solidified shell gap occurs. This causes deeper oscillation marks when the solidified shell is pulled out of the vibrating mold in the casting direction, resulting in uneven cooling of the solidified shell and promoting subsurface cracking. On the other hand, if the viscosity at 1300°C is higher than 3.0 poise, the fluidity of the molten powder deteriorates, resulting in many areas where the molten powder cannot flow into the mold / solidified shell gap, leading to uneven cooling. This not only promotes subsurface cracking, but can also lead to sticking (the solidified shell sticking to the mold) and, in the worst case, breakout. For this reason, the viscosity range at 1300°C is specified as 0.5 to 3.0 poise.

[0031] Solidification temperature: 900~1200℃ If the solidification temperature is below 900°C, the powder melts too much, causing excessive inflow of molten powder between the mold and solidified shell, deepening oscillation marks, leading to uneven cooling of the solidified shell and promoting subsurface cracking. On the other hand, if the solidification temperature is higher than 1200°C, the powder melts poorly, creating many areas where the molten powder cannot flow between the mold and solidified shell, resulting in uneven cooling. For this reason, the solidification temperature range was specified as 900 to 1200°C.

[0032] Melting rate of Ni-Cu alloy at the surface of molten alloy at 1440℃: 40-90g / m 2 / sec When the oxides and fluorides that make up the powder melt due to the heat from the surface of the molten alloy, if the melting rate is slow and below the lower limit of the specified range, localized areas of slow melting will be formed. On the other hand, if the melting rate is fast and above the upper limit of the specified range, localized areas of excessive melting will be formed. From these perspectives, in order to reduce the uneven cooling of the solidified shell caused by the uneven flow of molten powder between the mold and solidified shell and to obtain the effect of suppressing subsurface cracking, the powder melting rate should be 40 to 90 g / m 2 / sec is preferable. The melting speed can be adjusted not only by the aggregate C but also by the constituent oxide and fluoride components. However, since the aggregate C has a significant effect on the contact state between oxide and fluoride particles, i.e., the uniformity of the melting reaction, it is necessary to keep the content within the specified range above.

[0033] The present invention also proposes a continuous casting method using the above-mentioned powder for continuous casting. First, the powder for continuous casting of the present invention is suitable for use in the continuous casting of Ni-Cu alloys having a liquidus temperature of 1320 to 1400°C. When applied to a molten alloy having a liquidus temperature below the lower limit of the specified range, the powder of the present invention does not melt sufficiently. Furthermore, when applied to a molten alloy having a liquidus temperature above the upper limit of the specified range, the powder will melt excessively. Therefore, a powder having the above-mentioned physical properties is preferably applied to continuous casting having a liquidus temperature of 1340 to 1400°C.

[0034] The Ni-Cu alloy having the above liquidus temperature range is optimal for casting an Ni-Cu alloy having the following composition, although not particularly limited thereto: an alloy consisting of, by mass, 0.5% or less of C, 0.5% or less of Si, 3% or less of Mn, 3% or less of Fe, 26-36% of Cu, and the remainder being Ni and unavoidable impurities.

[0035] Let us explain the above Ni-Cu alloy. C is an element that is effective in maintaining strength, while Si and Mn act as deoxidizers and are useful for adjusting the O and S concentrations. Ni is an element necessary to maintain the alloy structure as austenite, and Cu is an important element for ensuring corrosion resistance in marine applications.

[0036] The proposed method for continuous casting of the Ni-Cu alloy is characterized by continuously casting a molten alloy using the powder for continuous casting at a drawing speed of 300 to 900 mm / min and a superheat of the molten alloy of 20 to 60°C.

[0037] Pulling speed: 300-900mm / min If the withdrawal speed is slow, less than 300 mm / min, the supply of molten alloy to the mold is significantly reduced, resulting in a decrease in heat supply from the molten alloy surface to the powder. This leads to poor powder melting, which causes the molten powder to flow unevenly between the mold and the solidified shell, promoting subsurface cracking. Conversely, if the withdrawal speed is fast, exceeding 900 mm / min, excessive heat supply results, leading to excessive powder melting. This results in deeper oscillation marks, which in turn leads to uneven cooling of the solidified shell and promotes subsurface cracking. Therefore, a withdrawal speed in the range of 300 to 900 mm / min is preferable.

[0038] Superheat of molten alloy: 20~60℃ The superheat of the molten alloy is controlled by adjusting the temperature of the molten alloy in the ladle refining furnace (LF), lifting the ladle, pouring the molten alloy into a tundish, and measuring the temperature of the molten alloy in the tundish. The superheat here is defined as the superheat temperature above the liquidus temperature of the Ni-Cu alloy. If this value is below 20°C, the powder melting is impaired, causing uneven flow of the molten powder between the mold and the solidified shell, promoting subsurface cracking. Conversely, if it is higher than 60°C, excessive heat supply results, resulting in excessive powder melting. This results in deeper oscillation marks, which causes uneven cooling of the solidified shell and promotes subsurface cracking. Therefore, a superheat of the molten alloy in the range of 20 to 60°C is preferable. [Example]

[0039] The following examples will clarify the configuration and effects of the present invention, but the present invention is not limited to these examples. Alloy raw materials, such as nickel, copper, and Monel scrap (NW4400 scrap), were melted in an electric furnace with a melting capacity of 60 tons. They were refined using an AOD or VOD. After adjusting the temperature and chemical composition in an LF, the ladle was lifted and poured into a tundish. The molten alloy was then continuously cast from the tundish into a mold through an immersion nozzle (IEN) to produce slabs. The mold size of the continuous casting machine was 200 mm thick and 1200 mm wide. Table 1 shows the chemical compositions (mass%) and physical properties of Inventive Powders 1-6 and Comparative Powders 1-10. Table 2 shows the chemical compositions (mass%), liquidus temperatures, casting conditions, powder film properties after casting, and slab surface quality evaluation results for Ni-Cu alloys cast using these powders for Inventive Examples 1-12 and Comparative Examples 13-23. The components and the methods for measuring physical properties are shown below.

[0040] Continuous casting mold powder composition: Components other than carbon contained in the powder were quantitatively analyzed by chemical analysis. The carbon concentration was determined from the weight ratio of the carbon raw material added as the carbon source. The total of each component shown in Table 1 is less than 100% because it contains unavoidable impurities such as Fe2O3, P, and S. To clarify the effects of MgO and Cr2O3, the mass percentages of these oxide components are accurate to two decimal places.

[0041] Powder viscosity at 1300°C: The viscosity of the powder was measured by the rotating cylinder method, in which a predetermined amount of powder was placed in an iron crucible, melted in a vertical resistance furnace at 1300°C, and the viscosity was determined from the torque generated when an iron rotor was inserted into the crucible.

[0042] Powder solidification temperature: After the viscosity measurement at 1300°C, the rotor was rotated while gradually decreasing the temperature, and the temperature at which the viscosity suddenly increased was taken as the solidification temperature.

[0043] Melting rate of Ni-Cu alloy at the surface of the molten alloy at 1440℃: In a high-frequency induction furnace, a Ni-Cu alloy placed in an MgO crucible with an inner diameter of 120 mm is melted and the temperature is adjusted to 1440°C, after which 100g of powder is added to the surface of the melt. The time from when the powder spreads over the entire surface of the melt until the aggregate C burns and the powder is completely melted is measured, and the powder weight is calculated by dividing the measured time and the inner cross-sectional area of ​​the crucible.

[0044] Alloy composition: Quantitative analysis was performed using an X-ray fluorescence analyzer, and the oxygen concentration of the alloy was quantitatively analyzed using an inert gas impulse fusion infrared absorption method. The remainder consists of inevitable impurities such as P, S, O, H, and N.

[0045] Liquidus temperature of the alloy: Alloy samples having the same alloy composition were subjected to differential scanning calorimetry (DSC). After melting the sample, the temperature was gradually lowered and the temperature at which the exothermic peak of the latent heat of solidification appeared was taken as the liquidus temperature.

[0046] The slab surface quality was evaluated as follows. Slabs (8 m long) from the start of continuous casting to a casting length of 10 to 18 m were ground with a grinder, with a uniform grinding amount of 1% of the slab thickness. Then, penetrant testing was performed, magnified observation with a camera, and the surface was scanned with an eddy current probe to evaluate the number of subsurface cracks per slab. When subsurface cracks were absent, good yields were achieved because minimal grinding was required to remove surface oxide scale and oscillation marks. Furthermore, since no additional grinding steps were required in the cold rolling process, good manufacturing costs were achieved. Furthermore, various studies conducted during the research and development of the powder of the present invention revealed that a correlation between the number of subsurface cracks and reduced crack depth was observed. If the number of cracks was within the range of 1 to 20 per slab, an additional 1% of grinding could be used to completely remove all cracks. Although the grinding yield was slightly reduced, the absence of additional grinding steps in the cold rolling process ensured satisfactory manufacturing costs. When the number of cracks per slab is 21 or more, the correlation between the number of cracks and their depth weakens, and even if the amount of grinding aimed at completely removing the cracks is reduced, some cracks remain. As a result, the yield drops significantly, additional steps are required in the cold rolling process, and the manufacturing cost is not acceptable. From these perspectives, the slab surface quality was evaluated on the following three-point scale. ○: No subepidermal cracks △: Number of subsurface cracks: 1-20 / slab (8m) ×: Number of subsurface cracks ≥ 21 / slab (8m)

[0047] In this invention, we also investigated the powder film after casting. First, we measured the powder film thickness with a micrometer. Then, we embedded the powder film in resin, polished the cross section, and performed microanalysis using an SEM and attached EDS. We calculated the percentage of the crystalline phase in the total powder film thickness from the thickness of the crystalline phase, and confirmed the constituent crystalline phases. The appropriate crystalline structures were cuspidine (3CaO 2SiO2 CaF2), nepheline (Na2O Al2O3 2SiO2), calcium fluoride (CaF2), and magnesiochromite (MgO Cr2O3).

[0048] [Table 1]

[0049] [Table 2]

[0050] The results of the examples and comparative examples shown in Table 2 are explained below. In the table, values ​​in parentheses ( ) indicate values ​​that do not satisfy the essential elements of the present invention (defined in the independent claims), and values ​​in parentheses [ ] indicate values ​​that do not satisfy the preferred elements (defined in the dependent claims). First, examples Nos. 1 to 12 are explained below. Nos. 1 to 4 used invention powders 1 to 4, whose powder composition and physical properties met the ranges of the present invention. Furthermore, the alloy casting conditions and post-cast powder film properties all met the ranges of the present invention. Therefore, no subsurface cracking was detected in the slab, resulting in a favorable rating of ○. Nos. 5 to 12 all met the specified ranges for powder composition, viscosity at 1300°C, and solidification temperature, but some items were outside the preferred ranges. Nos. 5 and 6 were rated △ because the powder melting speed was outside the preferred range. Nos. 7 and 8 were rated △ because the drawing speed of the casting conditions was outside the preferred range. Nos. 9 and 10 were rated △ because the superheat of the molten alloy was outside the preferred range. In addition, Nos. 11 and 12 were evaluated as fair because the liquidus temperature range of the Ni-Cu alloy was outside the preferred range.

[0051] Next, we will explain Comparative Examples Nos. 13 to 23. In No. 13, the concentrations of CaO, SiO2, Na2O, Al2O3, Li2O, and F in the powder were outside the specified range, resulting in a low basicity, and therefore a low viscosity and solidification temperature at 1,300°C. As a result, the oscillation marks were deep, the powder film was thick (over 3.0 mm), and only a small proportion of the crystalline phase formed, resulting in strong and uneven cooling of the solidified shell, resulting in an × result, and vertical cracks also occurred.

[0052] For No. 14, the concentrations of CaO, SiO2, Na2O, Al2O3, and F in the powder, as well as the basicity, were outside the specified range, resulting in a high viscosity and solidification temperature at 1,300°C. As a result, the powder's melting and flow properties deteriorated, and the powder film thickness was thin and the crystalline phase ratio was too high, resulting in uneven cooling of the solidified shell and a negative result. Vertical cracks also occurred.

[0053] In No. 15, the MgO concentration in the powder was low and fell outside the specified range. As a result, magnesiochromite could not be generated in the crystalline phase of the powder film, and sufficient slow and uniform cooling effect could not be achieved, resulting in an × result.

[0054] In No. 16, the Cr2O3 concentration in the powder was low and fell outside the specified range. As a result, magnesiochromite could not be generated in the crystalline phase of the powder film, and sufficient slow and uniform cooling effect could not be achieved, resulting in an X result.

[0055] For No. 17, the MgO concentration in the powder was too high and fell outside the specified range. As a result, MgO was generated in addition to magnesiochromite in the crystalline phase of the powder film, which caused excessive slow cooling, making it take too long for the solidified shell to develop sufficient strength, resulting in significant subsurface cracking and resulting in an X rating.

[0056] For No. 18, the Cr2O3 concentration in the powder was too high and fell outside the specified range. As a result, Cr2O3 was generated in addition to magnesiochromite in the crystalline phase of the powder film, which caused excessive slow cooling, making it take too long for the solidified shell to develop sufficient strength, resulting in significant subsurface cracking and resulting in an X rating.

[0057] In No. 19, the MgO and Cr2O3 concentrations in the powder were too high and fell outside the specified range. As a result, excessive magnesiochromite was formed in the crystalline phase of the powder film, which caused excessive slow cooling, making it take too long for the solidified shell to develop sufficient strength, resulting in significant subsurface cracking and resulting in an X.

[0058] No. 20 had a low carbon concentration in the powder, which fell outside the specified range. As a result, the powder melted too quickly, causing excessive flow of molten powder between the mold and solidified shell, which promoted uneven cooling and resulted in an X.

[0059] For No. 21, the powder had a high C concentration that fell outside the specified range. As a result, melting was too slow, resulting in many areas where the molten powder could not flow between the mold and solidified shell, which promoted uneven cooling and resulted in an X result.

[0060] For No. 22, the concentrations of components other than C and Li2O in the powder and the physical properties other than the melting rate were outside the specified range, and the solidified shell was cooled so strongly and unevenly that deep vertical cracks that could be seen with the naked eye were formed, resulting in an × rating.

[0061] In No. 23, the MgO and Cr2O3 concentrations in the powder were high and outside the specified range, and the casting conditions were outside the optimum ranges for liquidus temperature, withdrawal speed, and superheat of the molten metal, which resulted in poor flow of the molten powder between the mold and solidified shell, causing significant uneven cooling to the point that vertical cracks were visible to the naked eye, and the product was therefore marked as x. [Industrial Applicability]

[0062] According to the technology of the present invention, it is possible to obtain slabs in which subsurface cracking of the slabs is suppressed, which contributes to improving the yield of Ni-Cu based Ni-based alloys and reducing the manufacturing costs.

Claims

1. A mold powder used in continuous casting of a Ni-Cu alloy with a liquidus temperature of 1320 to 1400°C, the chemical composition of which is, in mass %, CaO: 25 to 40%, SiO ２ :25-40%, Na ２ O: 10-20%, Al ２ O ３ :1~10%, F:3~10%, MgO:0.1~4%, Cr ２ O ３ : 0.1 to 3%, C: 0.3 to 4%, and unavoidable impurities, and the basicity (in mass % ratio CaO / SiO ２ ) is 0.8 to 1.5, viscosity at 1300°C is 0.5 to 3.0 poise, and solidification temperature is 900 to 1200°C.

2. The above chemical composition further contains 1.5 mass % or less of Li ２ 2. The molding powder for continuous casting according to claim 1, further comprising O.

3. The melting rate of the molten alloy at the surface of the Ni-Cu alloy at 1440°C is 40 to 90 g / m ２ 2. The molding powder for continuous casting according to claim 1, wherein the melt melting rate is 1 / second.

4. The melting rate of the molten alloy at the surface of the Ni-Cu alloy at 1440°C is 40 to 90 g / m ２ 3. The molding powder for continuous casting according to claim 2, wherein the melt melting rate is 1 / second.

5. A method for continuously casting a molten Ni-Cu-based Ni-base alloy, the method comprising using the mold powder for continuous casting according to any one of claims 1 to 4, and continuously casting a molten Ni-Cu-based Ni-base alloy having a component composition, in mass%, of 0.5% or less C, 0.5% or less Si, 3% or less Mn, 3% or less Fe, 26 to 36% Cu, and the balance being Ni and unavoidable impurities, and having a liquidus temperature of 1320 to 1400°C, under conditions of a drawing speed of 300 to 900 mm / min and a superheat of the molten alloy of 20 to 60°C.

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