Mold powder for continuous casting of al-containing ni-based alloy and continuous casting method

Abstract

To provide mold powder for continuous casting of an Al-containing Ni-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 used for continuous casting of Ni-based alloys containing 0.8 to 2.0 mass% of Al, the mold powder having a chemical composition containing, in mass%, 20 to 30% of CaO, 30 to 40% of SiO2, 1 to 10% of Na2O, 0.5 to 5% of Al2O3, 4 to 10% of Li2O, 0.5 to 5% of MnO, 4 to 10% of BaO, 10 to 15% of F, 0.1 to 4% of MgO, 0.1 to 3% of Cr2O3, 0.5 to 4% of C, and an unavoidable impurity, mold powder for continuous casting, having a basicity (CaO / SiO2 in terms of mass% ratio) of 0.5 to 1.0, a viscosity at 1300 °C of 0.4 to 2. 0poise, and a solidification temperature of 850 to 1100 °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 Ni-based alloy containing 0.8 to 2.0 mass % of Al, and a continuous casting method using the continuous casting mold powder. [Background technology]

[0002] Ni-based alloys containing 0.8 to 2.0 mass% Al (hereinafter also referred to as "Al-containing Ni-based alloys") have excellent corrosion resistance, oxidation resistance, and high-temperature strength, and are therefore used in severe corrosive environments and high-temperature environments where stainless steels cannot be used. Therefore, they require good surface quality, where even minute surface defects are not acceptable.

[0003] In the manufacturing process of Al-containing Ni-based alloys, raw materials such as scrap, pure metals, and alloys are melted in an electric furnace. Then, decarburization, desulfurization, and Cr reduction are performed in an Argon Oxygen Decarburization Furnace (AOD) or Vacuum Decarburization Furnace (VOD). Finally, slabs are produced in a continuous casting machine. The 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 the 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 main raw material for Al-containing Ni-based 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] Because Al-containing Ni-based alloys have a single-phase austenite structure, impurity elements such as P and S tend to concentrate between dendrites during solidification, making them prone to solidification cracking. Therefore, if they are subjected to strong or uneven cooling in the mold during casting, the initially solidified shell deforms due to thermal stress, easily resulting in casting defects such as vertical cracks, depressions (dent defects), and subsurface cracks. Furthermore, Al-containing Ni-based alloys have a lower liquidus temperature (e.g., 1340–1400°C) than general steels and stainless steels, making it difficult to apply conventional continuous casting techniques used in the steel industry. Therefore, various technologies for continuous casting mold powders (hereinafter simply referred to as “powder”) have been developed to achieve good surface quality in slabs produced during continuous casting. However, because Al-containing Ni-based alloys contain chemically active Al, if the molten alloy is continuously cast using powders typically used for continuous casting of ordinary steel or stainless steel, which contain oxides such as CaO, SiO2, Al2O3, Na2O, and F, as well as C as an aggregate, the Al in the molten alloy will undergo a redox reaction with the SiO2 in the molten powder (powder that has been molten by the heat of the molten alloy) to form Al2O3 (oxide), which will then be incorporated into the powder. Therefore, changes in the powder composition affect physical properties such as viscosity and solidification temperature, as well as the crystallization characteristics of the powder film (a lubricating film approximately 0.5 to 3 mm thick that forms when molten powder flows between the mold and solidified shell) that promotes uniform, slow cooling.

[0006] Several techniques for improving slab surface defects in the continuous casting of Fe-Cr-Ni alloys and Ni-based alloys containing active Al or Al and Ti have been disclosed (e.g., Patent Documents 1-3). Continuous casting mold powders and continuous casting techniques have also been proposed to prevent slab surface defects. While these techniques control appropriate powder melt properties, crystallization behavior, and powder flow characteristics between the mold and solidified shell, taking into account changes in powder composition due to reactions between the molten alloy and the molten powder during casting, they aim to reduce or prevent vertical cracks and depressions that are visible to the naked eye, making it difficult to suppress subsurface cracks in Al-containing Ni-based alloys, which is the subject of the present invention. In other words, defects caused by subsurface cracks that could not 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. 2003-94150 [Patent Document 2] Japanese Patent Application Laid-Open No. 2006-312200 [Patent Document 3] Japanese Patent Application Laid-Open No. 2017-18978 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 an Al-containing Ni-based alloy 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 Al-containing Ni-based alloys produced using a continuous casting machine. After polishing off the oxide scale on the slab surface and conducting penetrant testing, they conducted magnified observations using a camera. Furthermore, for cracks that are difficult to detect with penetrant testing, i.e., cracks with narrow openings, they used eddy current testing to investigate the occurrence of subsurface cracks. The results showed that each subsurface crack was 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). The results indicated that the subsurface cracks were cracks that opened along the interdendritic boundaries of the solidification structure. Furthermore, the presence of enriched elements such as P and S on the fracture surface confirmed solidification cracking, in line with prior knowledge. The opening width was narrow, less than 0.1 mm, and the depth reached approximately 1 to 5 mm from the surface. Based on the above results, it is believed that the thermal stress and deformation caused by strong or uneven cooling in the solidified shell inside the mold during casting were not enough to cause vertical cracks or depression, but were enough to open up the spaces between the weak dendrites, leading to subsurface cracks. Furthermore, a correlation was found between the number of subsurface cracks detected on the slab surface and the depth of the cracks.

[0010] Therefore, the inventors focused on mold powder for continuous casting and engaged in research and development to improve the process. The powder is applied to the surface of the molten alloy (hereinafter referred to as the "molten alloy surface") supplied to the mold from the tundish via a submerged entry nozzle during the continuous casting process. It maintains the temperature of the molten alloy surface and prevents atmospheric oxidation. It receives heat from the molten alloy surface, melts at an appropriate rate, 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 and 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.

[0011] The Ni-based alloy targeted by the present invention preferably contains 0.8 to 2.0 mass % of Al and has a liquidus temperature in the range of 1340 to 1400°C, which is significantly lower than that of ordinary steel, stainless steel, etc. produced by continuous casting machines.

[0012] 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 Al-containing Ni-based alloy powder for continuous casting, which is the subject of the present invention, must melt at an appropriate rate even at the low alloy molten metal temperature, 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 present invention aims to achieve a crystallization behavior that promotes slower and more uniform cooling to suppress subsurface cracking, as well as to prevent large defects such as vertical cracks and depressions that occur in conventional Al-containing Ni-based alloys. Furthermore, the powder film must maintain its properties even when the molten powder reacts with Al in the alloy molten metal during continuous casting.

[0013] Therefore, the inventors conducted various investigations and experimental studies to determine what kind of powder of Al-containing Ni-based alloy satisfies the above-mentioned properties. First, regarding the appropriate powder properties, it was found that if the viscosity at 1300°C is 0.4 to 2.0 poise and the solidification temperature is 850 to 1100°C, the powder will have good flowability between the mold and solidified shell even with the heat of the molten Al-containing Ni-based alloy, which has a relatively low liquidus temperature.

[0014] Next, the inventors discovered a powder composition that could achieve the above viscosity and solidification temperature range by adding the aggregate C to a previously investigated CaO-SiO2-Na2O-Al2O3-Li2O-MnO-BaO-F powder composition as a base, and further adding MgO and Cr2O3 in appropriate concentration ranges. Furthermore, within the powder composition range that could achieve the above viscosity and solidification temperature range, high-temperature experiments such as thermal analysis and solidification experiments were performed with various compositions simulating the thermal environment inside a mold, and the recovered samples equivalent to powder films were analyzed using SEM / EDS, transmission electron microscope (TEM), etc., to investigate their crystallization behavior. 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 the crystalline phase structure contained one or more of the previously reported cuspidine (3CaO·2SiO2·CaF2), CaO-SiO2-Al2O3, Li2O-SiO2-Al2O3, or Li2O-BaO-CaO-SiO2-Al2O3 crystalline phases, as well as magnesiochromite (MgO·Cr2O3), a compound of MgO and Cr2O3. Further research revealed that magnesiochromite crystallizes more easily than the other crystals mentioned above. Furthermore, because MgO-Cr2O3 oxides have such good insulating properties that they are commonly used in steelmaking refractories as magnesiochromite bricks, the formation of magnesiochromite resulted in slow and uniform cooling that suppressed subsurface cracking. Furthermore, it was found that by controlling the basicity (mass concentration ratio of CaO / SiO2) of the powder of the present invention to 0.5 to 1.0, the Al2O3 concentration in the powder when the Al in the molten alloy reacts with the molten powder during continuous casting can be controlled within the range suitable for casting (30% or less by mass of Al2O3 in the powder film), and the above-mentioned crystallization behavior can be maintained.

[0015] Further research was conducted to improve the results, and it was found that the powder melting rate at the surface of the molten alloy of Al-containing Ni-based alloy at 1420℃ was 100-200g / m 2 / sec, the molten alloy melts at an appropriate rate due to the heat supplied from the surface, promoting uniform flow of the molten powder between the mold and the solidified shell and reducing uneven cooling of the solidified shell. Furthermore, we investigated the casting conditions for continuous casting 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, as described below.

[0016] The present invention is a mold powder used in the continuous casting of a Ni-based alloy containing 0.8 to 2.0 mass% Al, the mold powder having the following chemical components by mass: CaO: 20 to 30%, SiO2: 30 to 40%, Na2O: 1 to 10%, Al2O3: 0.5 to 5%, Li2O: 4 to 10%, MnO: 0.5 to 5%, BaO: 4 to 10%, F: 10 to 15%, MgO: 0.1 to 4%, Cr2O3: 0.1 to 3%, and C: 0.5 to 4%; the mold powder for continuous casting is characterized by having a basicity of 0.5 to 1.0, a viscosity at 1300°C of 0.4 to 2.0 poise, and a solidification temperature of 850 to 1100°C.

[0017] The powder for continuous casting is characterized in that the melting rate of the Ni-based alloy containing 0.8 to 2.0 mass % of Al at the surface of the molten alloy at 1420°C is 100 to 200 g / m 2 / second.

[0018] The present invention also proposes a continuous casting method using the above powder for continuous casting, specifically, a continuous casting method for an Al-containing Ni-based alloy, characterized by casting a molten alloy of a Ni-based alloy having a liquidus temperature of 1340 to 1400°C and containing 0.8 to 2.0 mass% Al at a drawing speed of 300 to 900 mm / min and a superheat of the molten alloy of 20 to 60°C. [Effects of the Invention]

[0019] According to the present invention, it is possible to suppress subsurface cracks that occur in slabs produced by continuous casting of Al-containing 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. This makes it 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 production costs. DETAILED DESCRIPTION OF THE INVENTION

[0020] First, the reasons for limiting the chemical composition of the mold powder for continuous casting of the present invention (hereinafter also referred to simply as "powder") 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 within the mold is important for the powder to exert its effects. CaO:20~30%, SiO2:30~40%, Na2O:1~10%, Al2O3:0.5~5%, Li2O:4~10%, MnO:0.5~5%, BaO:4~10%, F:10~15%, MgO:0.1~4%, Cr2O3:0.1~3% If the oxides and fluorides are within the above ranges, the viscosity at 1300°C will be 0.4 to 2.0 poise and the solidification temperature will be 850 to 1100°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 formation rate of the above crystalline phase 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 phase is preferably one or more of cuspidine (3CaO·2SiO2·CaF2), CaO-SiO2-Al2O3, Li2O-SiO2-Al2O3, or Li2O-BaO-CaO-SiO2-Al2O3, or magnesiochromite (MgO·Cr2O3), which allows for proper control of the heat flux from the solidified shell to the mold.

[0021] 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 concentrations 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.

[0022] 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 range. From these perspectives, the following was specified: CaO: 20-30%, SiO2: 30-40%, Na2O: 1-10%, Al2O3: 0.5-5%, Li2O: 4-10%, MnO: 0.5-5%, BaO: 4-10%, F: 10-15%, MgO: 0.1-4%, Cr2O3: 0.1-3%.

[0023] C: 0.5 to 4% C particles exist between the oxide and fluoride particles, preventing melting due to contact. In other words, when the C burns and gasifies due to 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 0.5%, 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. This can promote subsurface cracking. At levels above 4%, melting is uneven and too 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 (the solidified shell sticking to the mold) and, in the worst case, breakout. For these reasons, the C content is specified as 0.5-4%.

[0024] Basicity (mass concentration ratio of CaO / SiO2): 0.5 to 1.0 If the basicity is less than 0.5, the SiO2 activity in the powder increases, and the Al2O3 concentration in the powder film during continuous casting exceeds 30% due to the chemical reaction shown in Equation (1) below. As a result, the powder's physical properties and crystallization behavior change significantly, resulting in a deviation from the range of target casting conditions for which the powder is applicable, resulting in surface defects such as vertical cracks and, in the worst case, breakout (a problem with molten metal leaking during casting). On the other hand, if the basicity exceeds 1.0, the viscosity at 1300°C increases to over 2.0 poise. Furthermore, the powder film becomes too thin (less than 0.5 mm) and the crystalline phase ratio exceeds 75%, resulting in a decrease in lubrication and surface defects such as sticking and vertical cracks. Therefore, the basicity range is specified as 0.5 to 1.0. 4 Al +3(SiO2)=2(Al2O3)+3 And …(1) Here, the parentheses in formula (1) indicate the oxide components in the powder, and the underlined parts indicate the components in the molten alloy.

[0025] Viscosity at 1300℃: 0.4 to 2.0 poise If the viscosity at 1300°C is less than 0.4 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 2.0 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 0.4 to 2.0 poise.

[0026] Solidification temperature: 850~1100℃ If the solidification temperature is below 850°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 1100°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 850 to 1100°C.

[0027] Melting rate of Al-containing Ni-based alloy at the surface of the molten alloy at 1420℃: 100-200g / 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 100 to 200 g / m 2 / sec is preferable. The melting speed can be adjusted not only by the amount of C as an aggregate, but also by the constituent oxide and fluoride components and the particle size of the powder particles. However, since the C as an aggregate affects the contact state between the oxide and fluoride particles, i.e., the uniformity of the melting reaction, it is necessary to keep the content within the specified range above.

[0028] 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 an Al-containing Ni-based alloy having a liquidus temperature of 1340 to 1400°C. When applied to a molten alloy having a liquidus temperature of less than 1340°C, the powder of the present invention does not melt sufficiently. Furthermore, when applied to a molten alloy having a liquidus temperature exceeding 1400°C, the powder melts 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.

[0029] The Al-containing Ni-based alloy having the liquidus temperature range described above is ideal for casting a Ni-based alloy having the following composition, but not limited to: an alloy consisting of, by mass, 0.01-2% Si, 0.01-2% Mn, 20-30% Cr, 0.1-20% Fe, 0.8-2% Al, with the balance being Ni and unavoidable impurities.

[0030] The above Ni-based alloys will now be explained. Si and Mn are useful as deoxidizers during refining, adjusting the O and S concentrations. Ni is useful as an element that stabilizes the austenite phase, and Cr improves corrosion resistance, such as pitting and crevice corrosion resistance. Fe is an inexpensive element that adjusts the liquidus temperature and strength. Al improves oxidation resistance at temperatures above 400°C, and is useful for maintaining strength in high-temperature environments. Note that Al content of less than 0.8% does not provide sufficient oxidation resistance, and above 2.0% causes strong microsegregation during solidification, making the alloy more susceptible to hot-rolling cracking.

[0031] The proposed continuous casting method for the Al-containing Ni-based alloy is characterized by continuously casting a molten alloy using the continuous casting powder at a drawing speed of 300 to 900 mm / min and a superheat of the molten alloy of 20 to 60°C.

[0032] 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.

[0033] 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-based 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, the superheat of the molten alloy is preferably in the range of 20 to 60°C. [Example]

[0034] The following examples will clarify the configuration and effects of the present invention, but the present invention is not limited to these examples. Nickel, ferrochromium, aluminum, Ni-based alloy scrap, etc. were melted in an electric furnace with a melting capacity of 60 tons. Subsequently, in secondary refining, refining was performed using an AOD or VOD, and the temperature and chemical composition were adjusted in an LF. The ladle was then lifted and poured into a tundish. The molten alloy was then continuously cast from the tundish into a mold through an immersion nozzle to produce a slab. 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 the inventive powders Nos. 1 to 7 and the comparative powders Nos. 1 to 12. Table 2 shows the chemical composition (mass %), liquidus temperature, casting conditions, powder film properties after casting, and evaluation results of slab surface quality of the Al-containing Ni-based alloys cast using these powders for Examples 1 to 13 and Comparative Examples 14 to 26.

[0035] The components and the methods for measuring physical properties are shown below. 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.

[0036] 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.

[0037] 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.

[0038] Powder Melting Speed: In a high-frequency induction furnace, an Al-containing Ni-based alloy placed in an MgO crucible with an inner diameter of 120 mm is melted and the temperature is adjusted to 1420°C, after which 100g of powder is added to the surface of the molten metal. The time from when the powder spreads over the entire surface of the molten metal until the aggregate carbon burns and the powder is completely melted is measured, and the melting point is calculated by dividing the weight of the powder added by the measured time and the inner cross-sectional area of ​​the crucible.

[0039] Alloying Elements: 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, N, and Cu.

[0040] 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.

[0041] The slab surface quality was evaluated as follows. Slabs (8 m long) from the start of continuous casting to the casting length of 10 to 18 m were ground using a grinder, with a uniform grinding amount of 1% of the slab thickness. Then, penetrant testing was performed, magnified observation was performed 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. 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 △: Subsurface cracks: 1-20 per 8m of slab ×: Number of subsurface cracks ≥ 21 per 8m of slab

[0042] 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 analyzed it using an SEM and an 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 and the average Al2O3 concentration throughout the film. The correct crystalline phase composition is one or more of the above-mentioned cuspidine (3CaO·2SiO2·CaF2), CaO-SiO2-Al2O3, Li2O-SiO2-Al2O3, or Li2O-BaO-CaO-SiO2-Al2O3 systems, as well as magnesiochromite (MgO·Cr2O3).

[0043] [Table 1]

[0044] [Table 2]

[0045] 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 13 are explained below. Nos. 1 to 5 used invention powders 1 to 5, whose powder composition and physical properties met the ranges of the present invention, and all alloy casting conditions met the ranges of the present invention. Therefore, no subsurface cracking was detected in the slab, resulting in a favorable rating of ○. Nos. 6 to 13 all met the specified ranges for powder composition, viscosity at 1300°C, and solidification temperature, but some items were outside the preferred ranges. Nos. 6 and 7 were rated △ because invention powders 6 and 7 were used, whose melting speeds were outside the preferred ranges. Nos. 8 and 9 were rated △ because the drawing speed of the casting conditions was outside the preferred range. Nos. 10 and 11 were rated △ because the superheat of the molten alloy was outside the preferred range. In addition, Nos. 12 and 13 were rated as fair because the liquidus temperature range of the Ni-based alloy was outside the preferred range.

[0046] Next, we will explain the comparative examples Nos. 14 to 26. In No. 14, the concentrations of CaO, Na2O, Al2O3, Li2O, MnO, BaO, and F in the powder were outside the specified range, resulting in a low viscosity and solidification temperature at 1300°C. As a result, the oscillation marks were deep, the powder film was thick (over 3.0 mm), and only a low proportion of crystalline phase was formed. Furthermore, the appropriate crystalline phase was not obtained. As a result, the solidified shell was cooled excessively and unevenly, resulting in not only subsurface cracks (×), but also vertical cracks.

[0047] For No. 15, the concentrations of SiO2, Na2O, Al2O3, Li2O, MnO, BaO, and F in the powder were outside the specified range, resulting in a high viscosity and solidification temperature at 1300°C. As a result, the powder's melting and flow properties deteriorated, the proportion of crystalline phase in the powder film became too high, and the solidified shell cooled unevenly, resulting in subsurface cracking (×). Some sticking also occurred.

[0048] In No. 16, the powder basicity was low and outside the specified range, so the Al2O3 concentration in the powder film exceeded 30%, which is the optimum range for casting, resulting in subsurface cracks and a "×" rating. Vertical cracks also occurred.

[0049] In No. 17, the powder basicity was too high and outside the specified range, resulting in a high viscosity, a thin powder film, and an excessively high crystalline phase ratio. This resulted in uneven cooling of the solidified shell, resulting in subsurface cracking (×). The lubrication effect also decreased, causing sticking.

[0050] For No. 18, the MgO concentration in the powder was low and fell outside the specified range. As a result, magnesiochromite could not be formed in the crystalline phase of the powder film, and sufficient slow and uniform cooling was not achieved, resulting in subsurface cracking.

[0051] For No. 19, the Cr2O3 concentration in the powder was low and fell outside the specified range. As a result, magnesiochromite could not be formed in the crystalline phase of the powder film, and sufficient slow and uniform cooling was not achieved, resulting in subsurface cracking.

[0052] For No. 20, 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 a rating of ×.

[0053] For No. 21, 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 a rating of ×.

[0054] For No. 22, the powder had high concentrations of MgO and Cr2O3, which fell outside the specified range. As a result, excessive magnesiochromite was formed in the crystalline phase of the powder film, which led to excessive slow cooling. It took too long for the solidified shell to develop sufficient strength, resulting in significant subsurface cracking and resulting in an X rating.

[0055] No. 23 had a low carbon concentration in the powder, which fell outside the specified range. As a result, the powder melted too quickly, causing excessive molten powder to flow between the mold and solidified shell, promoting uneven cooling and resulting in subsurface cracking.

[0056] For No. 24, the powder had a high C concentration, which was outside the specified range. As a result, melting was too slow, and there were many areas where the molten powder could not flow between the mold and solidified shell, which promoted uneven cooling and resulted in subsurface cracking.

[0057] For No. 25, the powder had all of its component concentrations, basicity, and physical properties outside of the specified range, except for C. This resulted in the solidified shell being cooled too strongly and unevenly, causing a breakout and forcing the casting to be halted.

[0058] In No. 26, the MgO and Cr2O3 concentrations in the powder were high and outside the specified range, and the casting conditions, such as the liquidus temperature, withdrawal speed, and superheat of the molten metal, were outside the optimum ranges, which resulted in poor flow of the molten powder between the mold and solidified shell, and the crystalline phase of the powder film becoming too thick, resulting in subsurface cracks (×). Vertical cracks also occurred. [Industrial Applicability]

[0059] 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 Al-containing Ni-based alloys and reducing the manufacturing costs.

Claims

1. A mold powder used in continuous casting of a Ni-based alloy containing 0.8 to 2.0 mass % of Al, the chemical composition of which is as follows in mass %: CaO: 20 to 30%, SiO ２ :30-40%, Na ２ O: 1-10%, Al ２ O ３ :0.5~5%, Li ２ O: 4-10%, MnO: 0.5-5%, BaO: 4-10%, F: 10-15%, MgO: 0.1-4%, Cr ２ O ３ : 0.1 to 3%, C: 0.5 to 4%, and unavoidable impurities, and the basicity (in mass % ratio CaO / SiO ２ ) is 0.5 to 1.0, viscosity at 1300°C is 0.4 to 2.0 poise, and solidification temperature is 850 to 1100°C.

2. The melting rate of the Ni-based alloy at the surface of the molten alloy at 1420°C is 100 to 200 g / m ２ 2. The molding powder for continuous casting according to claim 1, wherein the melt melting rate is 1 / second.

3. A method for continuous casting of an Al-containing Ni-based alloy, comprising using the mold powder for continuous casting according to claim 1 or 2, and casting a molten alloy of the Ni-based alloy having a liquidus temperature of 1340 to 1400°C and containing 0.8 to 2.0 mass% of Al 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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