Mold Powder for Continuous Casting of Ni-Based Alloys Containing Al and Continuous Casting Method

Abstract

Provided are a mold powder for continuous casting of an Al-containing Ni-based alloy that suppresses cracks under the slab skin and has excellent surface properties, and a continuous casting method using the powder. A mold powder for continuous casting used for continuous casting of a Ni-based alloy containing 0.8 to 2.0% by mass of Al, wherein the chemical components are as follows in 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%, C: 0.5 to 4%, and inevitable impurities, and the basicity (CaO / SiO2 in mass% ratio) is 0.5 to 1.0, the viscosity at 1300 °C is 0.4 to 2.0 poise, and the solidification temperature is 850 to 1100 °C. The mold powder for continuous casting is characterized by the above.

Description

Technical Field

[0001] The present invention relates to a mold powder for continuous casting for obtaining good surface quality in the production of a Ni-based alloy containing 0.8 to 2.0% by mass of Al, and a continuous casting method applying the mold powder for continuous casting.

Background Art

[0002] A Ni-based alloy containing 0.8 to 2.0% by mass of Al (hereinafter also referred to as "Al-containing Ni-based alloy") has excellent corrosion resistance, oxidation resistance, and high-temperature strength, and is therefore used in severe corrosion environments and high-temperature environments where stainless steel cannot be applied. Therefore, good surface quality that does not allow even fine surface defects is required.

[0003] In the manufacturing process of the Al-containing Ni-based alloy, raw materials such as scrap, pure metal, and alloy are melted in an electric furnace, decarburization, desulfurization, and Cr reduction refining are performed in an AOD (argon oxygen decarburization furnace) or a VOD (vacuum decarburization furnace), and finally a slab is manufactured by a continuous casting machine. Thereafter, a cold-rolled sheet is manufactured through a hot rolling process and a cold rolling process. Here, it has been found that linear defects that become defects may occur on the surface of the cold-rolled sheet that is the final product, leading to product defects as the starting points of corrosion and fracture. When such surface defects occur, a grinding process using a grinder is added to ensure excellent surface properties, which greatly reduces the yield. Since Ni, which is a main raw material of the Al-containing Ni-based alloy, is a more expensive metal than Fe and Cr, improving the yield and suppressing the manufacturing cost are industrially important. It is also an important issue from the perspective of resource conservation.

[0004] One of the main causes of such surface defects is the fine cracks under the slab skin in continuous casting (hereinafter also referred to as "subsurface cracks"). After surface grinding of the slab, if the subsurface cracks are not removed, they will extend during hot and cold rolling, forming long linear flaws ranging from several centimeters to several meters. The subsurface cracks of the slab are so fine that they cannot be visually confirmed before grinding, and they are cracks with an opening width of less than 0.1 mm that can only be detected after grinding by performing a penetrant inspection test and observing it magnified with a camera or by performing eddy current testing. Therefore, in order to solve the above problems and obtain a cold-rolled sheet with excellent surface properties, it is necessary to suppress not only conventional surface defects such as slab longitudinal cracks and depressions (concave defects), but also subsurface cracks.

[0005] Since the Al-containing Ni-based alloy has a single-phase austenite structure, impurity elements such as P and S are likely to concentrate between the dendrite arms during solidification, and it has the characteristic of being prone to solidification cracking. Therefore, when it is strongly cooled or unevenly cooled in the mold during casting, the initial solidification shell is deformed by thermal stress, easily causing casting defects such as longitudinal cracks, depressions (concave defects), and subsurface cracks. In addition, the Al-containing Ni-based alloy has a lower liquidus temperature (for example, 1340 - 1400 °C) compared to general steels and stainless steels, etc., and it is difficult to apply the continuous casting technology in the conventional steel field. Therefore, in order to obtain good surface quality in the slab for continuous casting, various technologies of continuous casting mold powder (hereinafter also simply referred to as "powder") have been developed. However, since the Al-containing Ni-based alloy contains chemically active Al, when using the powder for continuous casting of general carbon steel and stainless steel containing oxides such as CaO, SiO2, Al2O3, Na2O, F, etc. and C as an aggregate for the continuous casting of the alloy melt, Al in the alloy melt undergoes an oxidation-reduction reaction with SiO2 in the molten powder (powder that has become molten state by receiving the heat of the alloy melt) to generate Al2O3 (oxide), which is taken into the powder. Therefore, the change in the composition of the powder affects not only physical property values such as viscosity and solidification temperature, but also the crystallization characteristics of the powder film (a lubricating film with a thickness of about 0.5 - 3 mm formed by the inflow of molten powder between the mold / solidification shell) that promotes uniform slow cooling.

[0006] So far, some techniques for improving slab surface defects in the continuous casting of Al or Fe-Cr-Ni alloys and Ni-based alloys containing Al and Ti, which have been active, have been disclosed (for example, Patent Documents 1 to 3), and mold powders for continuous casting and continuous casting techniques for preventing slab surface defects have been proposed. All of them are techniques that control appropriate powder melting physical properties, crystallization behavior, powder inflow characteristics between the mold / solidified shell, etc., considering the change in powder composition due to the reaction between the alloy melt and the molten powder during casting. However, they aim to reduce or prevent vertical cracks and depressions at a level that can be visually confirmed, and it is difficult to suppress subsurface cracks in Al-containing Ni-based alloys, which is the problem of the present invention. That is, it can be said that the defects caused by subsurface cracks that could not be completely removed by slab grinding remain in the cold-rolled sheet, and the problems of yield reduction and increased manufacturing costs caused by adding processes still remain.

Prior Art Documents

Patent Documents

[0007]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems 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 slab subsurface cracks and has excellent surface properties, and a continuous casting method using the powder.

Means for Solving the Problems

[0009] To solve the above problems, the inventors conducted extensive investigations and research and development. First, they investigated the subsurface cracks that occurred in the slabs of Al-containing Ni-based alloys produced by a continuous casting machine. After grinding and removing the oxide scale on the slab surface and performing a penetrant inspection test, they performed magnified observation with a camera. Furthermore, for cracks that were difficult to detect by the indication pattern of penetrant inspection, that is, cracks with a narrow opening, eddy current inspection was applied to investigate the appearance of subsurface crack generation. As a result, the length of one subsurface crack was about several millimeters, and they were scattered. Subsequently, subsurface crack portions were collected from several slabs and were carefully observed and analyzed using an optical microscope, a scanning electron microscope (SEM) and an attached energy dispersive X-ray spectrometer (EDS), and an electron probe microanalyzer (EPMA). As a result, the subsurface cracks were cracks that opened along the dendrite arms of the solidification structure. In addition, since enrichment of elements such as P and S was detected from the fracture surface, it was found to be solidification cracking in comparison with conventional findings. The opening width was narrow, less than 0.1 mm, and the depth reached about 1 to 5 mm from the surface layer. From the above results, it was considered that in the solidification shell in the mold during casting, although it did not cause longitudinal cracks or depressions, thermal stress and deformation occurred due to strong cooling or non-uniform cooling that opened the dendrite arms with weak strength, leading to subsurface cracks. In addition, a correlation was also found that the crack depth decreased as the number of subsurface cracks detected on the slab surface decreased.

[0010] Therefore, the inventors focused on the mold powder for continuous casting and carried out research and development for improvement. The powder is introduced onto the surface of the molten alloy (hereinafter also referred to as the "molten metal surface") supplied from the tundish of the continuous casting process to the mold through the immersion nozzle. It serves to keep the molten metal surface warm, prevent atmospheric oxidation, melt at an appropriate rate by receiving heat from the molten metal surface, and flow into the gap between the mold and the solidified shell of the copper plate to play a lubricating role. Further, the molten powder that has flowed into the gap between the mold and the solidified shell is cooled by the mold to become a solid phase, forming a glassy powder film. At this time, if a crystal phase is not rapidly formed on the mold side of the powder film, the solidified shell is subjected to strong and non-uniform cooling, making it easy for longitudinal cracks, depressions, bleeding (seepage of molten alloy), etc. to occur in the slab, and in the worst case, breakout (molten alloy leakage trouble) occurs. The reason for this is that the glassy powder film easily transmits radiant light, resulting in enhanced radiant heat transfer from the solidified shell to the mold, and the contact with the mold is likely to become non-uniform. In addition to the fact that the crystal phase blocks radiant heat transfer and has a slow cooling effect due to its good heat insulation properties, it adheres evenly to the mold wall surface, also bringing about a uniform cooling effect. Therefore, this crystallization behavior is an important characteristic in optimizing cooling to suppress subsurface cracking, which is the present problem.

[0011] The Ni-based alloy targeted by the present invention desirably contains 0.8 to 2.0 mass% of Al and has a liquidus temperature in the range of 1340 to 1400°C. This is characterized by being very low compared to that of ordinary steel, stainless steel, etc. manufactured by continuous casting machines.

[0012] The mold powder for continuous casting has the function of melting at an appropriate rate upon receiving heat from the molten metal surface, optimizing the cooling of the solidification shell along with the lubricating action that flows into the mold / solidification shell interface. Therefore, the powder for continuous casting of an Al-containing Ni-based alloy targeted by the present invention is required to melt at an appropriate rate even at the above-mentioned low molten alloy temperature, flow into the mold / solidification shell interface, and have the property of rapidly forming a crystal phase on the mold side while forming a glassy powder film. In addition, in the problems of the present invention, not only large defects such as longitudinal cracks and depressions that occurred in conventional Al-containing Ni-based alloys are prevented, but also a crystallization behavior that promotes slower and uniform cooling is required to suppress subsurface cracks. Further, it is required that the characteristics of the powder film can be maintained even when the molten powder reacts with Al in the molten alloy during continuous casting.

[0013] Therefore, the inventors conducted various investigations and experimental studies on what kind of powder of an Al-containing Ni-based alloy satisfies the above properties. First, regarding the properties of the appropriate powder, 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 inflowability into the mold / solidification shell interface is good even with the heat of the molten alloy of an Al-containing Ni-based alloy having a relatively low liquidus temperature.

[0014] Next, the inventors added C, which is an aggregate, based on the conventionally studied CaO-SiO2-Na2O-Al2O3-Li2O-MnO-BaO-F powder composition, and further added MgO and Cr2O3 within an appropriate concentration range to find a powder composition that can obtain the above viscosity and solidification temperature range. Also, in the powder composition range where the above viscosity and solidification temperature range can be obtained, high-temperature experiments such as thermal analysis and solidification experiments simulating the thermal environment in the mold with various compositions were conducted. Samples equivalent to the recovered powder film were analyzed by SEM / EDS, transmission electron microscope (TEM), etc., and the crystallization behavior was investigated. As a result, an appropriate powder film thickness was formed, and in the total thickness thereof, a crystal phase was formed at an appropriate ratio. In the crystal phase structure, there are one or more crystal phases of the conventionally reported cuspidine (3CaO·2SiO2·CaF2), CaO-SiO2-Al2O3 system, Li2O-SiO2-Al2O3 system, or Li2O-BaO-CaO-SiO2-Al2O3 system. Furthermore, it was found that a crystal phase of magnesiochromite (MgO·Cr2O3), which is a compound of MgO and Cr2O3, is generated. As a result of further research, it was found that magnesiochromite is more likely to crystallize than the other above-mentioned crystals, and since MgO-Cr2O3-based oxides generally have good heat insulation properties enough to be used as steelmaking refractories for magnesite bricks, the generation of magnesiochromite brings about a slow cooling and uniform cooling effect at a level that suppresses subsurface cracking. Also, the powder of the present invention described above can control the Al2O3 concentration in the powder when Al in the molten alloy reacts with the molten powder during continuous casting within an appropriate casting range (30% or less by mass% of Al2O3 in the powder film) by controlling the basicity (CaO / SiO2 by mass concentration ratio) to 0.5 to 1.0, and it was found that the above crystallization behavior can be maintained.

[0015] Furthermore, as a result of repeated research for further improvement, the melting rate of the powder on the surface of the molten alloy of the Al-containing Ni-based alloy at 1420°C is 100 to 200 g / m 2If it is in seconds, it has been clarified that it can be melted at an appropriate speed by heat supply from the surface of the molten alloy, promoting uniform inflow of the molten powder between the mold / solidified shell, and reducing non-uniform cooling of the solidified shell. Also, investigations were conducted on the casting conditions for continuous casting in which the effect of suppressing subsurface cracks in the powder of the present invention is strongly exhibited. The present invention has been completed based on the above research results and is as follows.

[0016] The present invention is a mold powder used for continuous casting of a Ni-based alloy containing 0.8 to 2.0 mass% Al, and its chemical composition is as follows in 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%, C: 0.5 to 4%, and it is characterized as a mold powder for continuous casting with 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] Also, the above powder for continuous casting is characterized in that the melting rate at the surface of the molten alloy at 1420 °C of the Ni-based alloy containing 0.8 to 2.0 mass% Al is 100 to 200 g / m 2 / second.

[0018] Furthermore, the present invention also proposes a continuous casting method using the above powder for continuous casting. That is, it is a continuous casting method for an Al-containing Ni-based alloy, characterized in that the liquidus temperature is 1340 to 1400 °C, and the molten alloy of the Ni-based alloy containing 0.8 to 2.0 mass% Al is cast under the conditions of a drawing speed of 300 to 900 mm / min and a superheat degree of the molten alloy of 20 to 60 °C.

Effects of the Invention

[0019] According to the present invention, it is possible to suppress surface cracks occurring in a slab produced by continuous casting of an Al-containing Ni-based alloy. As a result, it is possible to prevent linear defects occurring in a cold-rolled sheet in the cold rolling process. Therefore, it is possible to provide a product with good yield, low manufacturing cost without an additional grinding process in the cold rolling process, and good surface quality at low cost.

Embodiments for Carrying Out the Invention

[0020] First, the reasons for limiting the chemical composition of the mold powder for continuous casting of the present invention (hereinafter, also simply referred to as "powder") are shown. In the following description, "%" means mass%. Basically, since the powder is melted by the heat from the surface of the molten alloy, heat supply in the mold is important for exerting the effect of the powder. 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% If the above oxides and fluorides are within this range, they satisfy a viscosity of 0.4 to 2.0 poise at 1300 °C and a solidification temperature of 850 to 1100 °C. For example, F is added as CaF2, NaF, LiF, etc. Also, MgO and Cr2O3 may be added as simple oxides, magnesiochromite, or as impurities in other oxides and fluorides. When the powder of the present application melts and flows into the mold / solidification shell as a molten powder, a glassy powder film is formed, and its appropriate thickness is 0.5 to 3.0 mm. Among them, it is preferable to form a crystal phase in a portion corresponding 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, the cooling becomes too strong, and if it exceeds 3.0 mm, non-uniform cooling is promoted. If the formation ratio of the above crystal phase is less than 15%, it is the same as a glassy substance and is strong cooling. If it exceeds 75%, the cooling is too slow, resulting in insufficient strength of the solidification shell and promoting longitudinal cracking. Furthermore, the crystal phase is composed of one or more crystals of cuspidine (3CaO·2SiO2·CaF2), CaO-SiO2-Al2O3 system, Li2O-SiO2-Al2O3 system, or Li2O-BaO-CaO-SiO2-Al2O3 system, or magnesiochromite (MgO·Cr2O3), which is most preferable because it can appropriately control the heat flux from the solidification shell to the mold.

[0021] In particular, magnesiochromite (MgO·Cr2O3), which is a compound of MgO and Cr2O3, is more likely to crystallize than the other crystals mentioned above. It not only serves as nuclei for heterogeneous nucleation and promotes the crystallization of other crystals, but also has excellent heat insulation properties. Therefore, it is important because it has the effect of suppressing the occurrence of subcutaneous cracks in the solidification shell at the initial stage of solidification. Magnesiochromite (MgO·Cr2O3) exhibits the above effects when it is generated at 0.1% or more by mass in the crystal phase within the powder film. Therefore, MgO and Cr2O3 need to be contained at 0.1% or more. Also, when the concentration of MgO is too high, exceeding 4%, not only magnesiochromite but also MgO is generated, resulting in excessive slow cooling and taking a long time for the solidification shell to develop sufficient strength. On the contrary, it promotes subcutaneous cracks. On the other hand, when the concentration of Cr2O3 is too high, exceeding 3%, not only magnesiochromite but also Cr2O3 is generated, and subcutaneous cracks are promoted by excessive slow cooling. When the concentrations of both MgO and Cr2O3 deviate significantly from the specified range, the generation of magnesiochromite (MgO·Cr2O3) becomes excessive, leading to excessive slow cooling and promoting subcutaneous cracks for the reasons mentioned above.

[0022] Therefore, in order to simultaneously satisfy the appropriate viscosity, solidification temperature, and crystallization behavior of the powder for suppressing subcutaneous cracks in the slab, it is necessary to control it within the above composition range. From these viewpoints, it is specified as 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 - 4% Particle C exists between the particles composed of the above-mentioned oxides and fluorides and has the effect of preventing melting by contact. That is, when C burns and gasifies due to heat supply from the surface of the molten alloy, particle C disappears, and the oxide and fluoride particles come into contact and start to melt. By controlling the C content, the melting rate of the powder and the uniformity of the melting reaction can be controlled. If it is less than 0.5%, the oxide and fluoride particles will come into contact, and the melting will be non-uniform and too fast, resulting in excessive inflow of the molten powder between the mold / solidified shell and causing non-uniform cooling. As a result, it promotes subcutaneous cracking. Also, if it is higher than 4%, the melting is non-uniform and too slow, and there will be many places where the molten powder cannot flow into the mold / solidified shell, resulting in non-uniform cooling. As a result, it not only promotes subcutaneous cracking, but in severe cases, it causes sticking (adhesion of the solidified shell to the mold), and in the worst case, it causes breakout. For these reasons, it is specified to be 0.5 - 4%.

[0024] Basicity (CaO / SiO2 by mass concentration ratio): 0.5 - 1.0 When the basicity is less than 0.5, the activity of SiO2 in the powder increases, and due to the chemical reaction of the following formula (1), the Al2O3 concentration in the powder film during continuous casting exceeds 30% and becomes too high. As a result, the physical properties and crystallization behavior of the powder change greatly, deviating from the range of casting conditions for the intended purpose to which the powder can be applied, causing surface defects such as longitudinal cracks and, in the worst case, breakout (molten metal leakage trouble during casting). On the other hand, when the basicity exceeds 1.0 and becomes too high, the viscosity at 1300 °C exceeds 2.0 poise and becomes too high. Furthermore, the thickness of the powder film becomes too thin, less than 0.5 mm, and the crystal phase ratio becomes too thick, exceeding 75%, resulting in a decrease in the lubricating effect and causing surface defects such as sticking and longitudinal cracks. Therefore, the range of basicity is specified to be 0.5 - 1.0. 4 Al +3(SiO2)=2(Al2O3)+3 Si …(1) Here, the content in the parentheses of formula (1) is the oxide component in the powder, and the underlined part indicates the component in the molten alloy.

[0025] Viscosity at 1300 °C: 0.4 - 2.0 poise If the viscosity at 1300 °C is less than 0.4 poise, the inflow of the molten powder between the mold / solidified shell will be excessive, and the depth of the oscillation marks formed when the solidified shell is pulled out in the casting direction from the vibrating mold will increase, resulting in non-uniform cooling of the solidified shell and promoting subcutaneous cracking. On the other hand, if the viscosity at 1300 °C is too high, exceeding 2.0 poise, the fluidity of the molten powder will deteriorate, and there will be many places where the molten powder cannot flow into the mold / solidified shell, resulting in non-uniform cooling. As a result, not only does it promote subcutaneous cracking, but in severe cases, sticking (adhesion of the solidified shell to the mold) may occur, and in the worst case, breakout will be caused. Therefore, the viscosity range at 1300 °C is defined as 0.4 - 2.0 poise.

[0026] Solidification temperature: 850 - 1100 °C If the solidification temperature is less than 850 °C, the melting of the powder progresses too far, and the inflow of the molten powder between the mold / solidified shell becomes excessive, and the depth of the oscillation marks increases, resulting in non-uniform cooling of the solidified shell and promoting subcutaneous cracking. On the other hand, if the solidification temperature is too high, exceeding 1100 °C, the meltability of the powder deteriorates, and there will be many places where the molten powder cannot flow into the mold / solidified shell, resulting in non-uniform cooling. Therefore, the solidification temperature range is defined as 850 - 1100 °C.

[0027] Melting rate on the surface of the molten alloy of the Al-containing Ni-based alloy at 1420 °C: 100 - 200 g / m 2 / s When the oxides and fluorides constituting the powder are melted by the heat from the surface of the molten alloy, if the melting rate is slow, less than the lower limit of the specified range, locally slow melting areas will be formed. Also, if the melting rate is too fast, exceeding the upper limit of the specified range, locally excessive melting areas will be formed. From these viewpoints, to reduce the non-uniform cooling of the solidified shell associated with the non-uniform inflow of the molten powder between the mold / solidified shell and obtain the effect of suppressing subcutaneous cracking, the melting rate of the powder should be 100 - 200 g / m 2It is preferably in seconds. Note that the melting rate may be adjusted not only by C which is the aggregate but also by the components of the constituent oxides and fluorides and the particle size of the powder particles. However, since C which is the aggregate also affects the contact state between the oxide and fluoride particles, that is, the uniformity of the melting reaction, it is necessary to set the content within the above specified range.

[0028] In the present invention, a continuous casting method using the above powder for continuous casting is further proposed. First, the powder for continuous casting of the present invention is preferably used for continuous casting of an Al-containing Ni-based alloy having a liquidus temperature of 1340 to 1400 °C. When applied to a molten alloy with a liquidus temperature of less than 1340 °C, the powder of the present invention will not melt sufficiently. Also, when applied to a molten alloy with a temperature exceeding 1400 °C, excessive melting will occur. Therefore, it is a preferred embodiment that the powder having the above physical properties is applied to continuous casting with a liquidus temperature of 1340 to 1400 °C.

[0029] The Al-containing Ni-based alloy having the above liquidus temperature range is not particularly limited, but is most suitable for casting a Ni-based alloy having the following composition. That is, in mass %, it is an alloy composed of Si: 0.01 to 2%, Mn: 0.01 to 2%, Cr: 20 to 30%, Fe: 0.1 to 20%, Al: 0.8 to 2%, and the balance Ni and inevitable impurities.

[0030] The above Ni-based alloy will be described. Si and Mn are useful for adjusting the O and S concentrations as deoxidizers during refining. Ni is useful as an element for stabilizing the austenite phase, and Cr improves corrosion resistance such as pitting corrosion resistance and crevice corrosion resistance. Fe is an inexpensive element and has a role in adjusting the liquidus temperature and strength. Al improves oxidation resistance at high temperatures exceeding 400 °C and is useful for maintaining strength in a high-temperature environment. Note that when Al is less than 0.8%, sufficient oxidation resistance cannot be exhibited, and when it exceeds 2.0%, microsegregation during solidification becomes strong and hot rolling cracks are likely to occur.

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

[0032] Drawing speed: 300 to 900 mm / min If the drawing speed is less than 300 mm / min and is slow, the supply of the alloy melt to the mold significantly decreases, so the heat supply from the molten metal surface to the powder decreases. Along with this, the melting of the powder deteriorates, causing non-uniform inflow of the molten powder between the mold / solidified shell, and promoting subcutaneous cracking. Conversely, if it is faster than 900 mm / min, the heat supply becomes excessive, leading to excessive melting of the powder. As a result, the depth of the oscillation marks increases, resulting in non-uniform cooling of the solidified shell and promoting subcutaneous cracking. Therefore, a drawing speed in the range of 300 to 900 mm / min is suitable.

[0033] Superheat degree of the alloy melt: 20 to 60°C The superheat degree of the alloy melt is controlled by adjusting the temperature of the alloy melt in an LF (ladle furnace), then lifting the ladle, continuously pouring the molten metal into the tundish, and measuring the molten metal temperature in the tundish. The superheat degree here is defined as the superheat temperature from the liquidus temperature of the Ni-based alloy. When this value is lower than 20°C, the melting of the powder deteriorates, causing non-uniform inflow of the molten powder between the mold / solidified shell, and promoting subcutaneous cracking. Conversely, if it is higher than 60°C, the heat supply becomes excessive, resulting in excessive melting of the powder. As a result, the depth of the oscillation marks increases, resulting in non-uniform cooling of the solidified shell and promoting subcutaneous cracking. Therefore, a superheat degree of the alloy melt in the range of 20 to 60°C is suitable.

Example

[0034] Next, examples are presented to clarify the configuration, working effects of the invention of the present application. However, the invention of the present application is not limited only to the following examples. An electric furnace with a melting weight of 60 tons was used to melt nickel, ferrocromium, aluminum, Ni-based alloy scraps, etc. Then, in secondary refining, refining was carried out by AOD or VOD, and after adjusting the temperature and chemical composition by LF, the ladle was lifted and poured into the tundish. Subsequently, the molten alloy was supplied from the tundish into the mold through the immersion nozzle to perform continuous casting, and slabs were manufactured. The mold size of the continuous casting machine was 200 mm in thickness and unified to 1200 mm in width. Table 1 shows the chemical compositions (mass%; mass%) and physical property values of the invention powders No. 1 to 7 and the comparative powders No. 1 to 12. Table 2 shows the chemical compositions (mass%; mass%), liquidus temperatures, casting conditions, powder film properties after casting, and evaluation results of the 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 measurement methods of each component and physical property value are shown below. Mold powder components for continuous casting: Components other than C contained in the powder were quantitatively analyzed by chemical analysis. The C concentration was determined from the weight ratio of the C raw material added as the C source. The reason why the total of each component shown in Table 1 is less than 100% is that it contains inevitable impurities such as Fe2O3, P, and S. Also, in order to clarify the working effects of MgO and Cr2O3, the mass% of these oxide components is accurate to two decimal places.

[0036] Viscosity of the powder at 1300 °C: The viscosity of the powder was measured by the rotating cylinder method. This is a method in which a predetermined amount of powder is charged into an Fe crucible, held at 1300 °C in a vertical resistance furnace to be melted, and an iron rotor is inserted to obtain the viscosity from the torque during rotation.

[0037] Solidification temperature of the powder: After measuring the viscosity at 1300°C as described above, the rotor was rotated while gradually decreasing the temperature, and the temperature at which the viscosity rapidly increased was defined as the solidification temperature.

[0038] Melting rate of the powder: In a high-frequency induction furnace, an Al-containing Ni-based alloy charged into an MgO crucible with an inner diameter of 120 mm was melted and the temperature was adjusted to 1420°C. Then, a powder weighing 100 g was introduced onto the molten metal surface. The time from when the powder spread over the entire molten metal surface until the aggregate C burned and the powder was completely melted was measured, and the melting rate of the powder was obtained by dividing the powder input weight by the measured time and the cross-sectional area of the crucible inner diameter.

[0039] Alloy composition: Quantitative analysis was performed using a fluorescent X-ray analyzer, and the oxygen concentration of the alloy was quantitatively analyzed by the inert gas impulse fusion infrared absorption method. The remainder is inevitable impurities such as P, S, O, H, N, and Cu.

[0040] Liquidus temperature of the alloy: An alloy sample having the same alloy composition was subjected to differential scanning calorimetry (DSC). Once the sample was melted, the temperature at which the exothermic peak of the latent heat of solidification appeared while gradually decreasing the temperature was defined as the liquidus temperature.

[0041] The method for evaluating the slab surface quality is as follows. For the slab (8 m in length) at the position of 10 to 18 m in casting length from the start of withdrawal in continuous casting, after performing surface grinding on the entire surface with a grinder with the grinding amount being uniformly 1% of the slab thickness, a penetrant flaw detection test is conducted, followed by enlarged observation with a camera and surface scanning with an eddy current flaw detection probe to evaluate the number of occurrences of subsurface cracks per slab. When there are no subsurface cracks, a good yield can be obtained as the grinding is limited to the minimum, such as removing surface oxidation scale and oscillation marks, and furthermore, since no additional grinding process occurs in the cold rolling process, a good manufacturing cost can be achieved. In addition, through various investigations during the research and development of the powder of the present invention, it has been found that the fewer the number of occurrences of subsurface cracks, the more the crack depth decreases. If the number of occurrences is in the range of 1 to 20 per slab, all cracks can be removed by adding another 1% of grinding. Although the grinding yield will slightly decrease, since no additional grinding process occurs in the cold rolling process, the manufacturing cost is quite satisfactory. When the number of occurrences reaches 21 or more per slab, the correlation between the number of occurrences and the depth of cracks becomes weak, and residual cracks will occur for the grinding amount aimed at completely removing the cracks. As a result, the yield significantly decreases, an additional process occurs in the cold rolling process, and the acceptable manufacturing cost cannot be achieved. From these viewpoints, the slab surface quality is evaluated in the following three levels. 〇: No subsurface cracks △: Number of subsurface cracks: 1 to 20 per 8 m of slab ×: Number of subsurface cracks ≧ 21 per 8 m of slab

[0042] In addition, the powder film after casting is also being investigated. First, the powder film thickness was measured with a micrometer. Then, the powder film was embedded in resin, the cross-section was polished, and analyzed by SEM and the attached EDS. The ratio of the crystal phase thickness to the total powder film thickness was determined, and the constituent crystal phases and the average Al2O3 concentration of the entire film were confirmed. Note that the appropriate crystal phase composition is one or more of the above-mentioned cuspidine (3CaO·2SiO2·CaF2), CaO-SiO2-Al2O3 system, Li2O-SiO2-Al2O3 system, or Li2O-BaO-CaO-SiO2-Al2O3 system crystals and magnesiochromite (MgO·Cr2O3).

[0043]

Table 1

[0044]

Table 2

[0045] The results of the examples and comparative examples shown in Table 2 will be described. In the table, the numerical values within parentheses ( ) do not satisfy the essential components (specified in the independent claims) of the present invention, and the numerical values within brackets [ ] do not satisfy the preferable components (specified in the dependent claims). First, Examples Nos. 1 to 13 will be described below. In Nos. 1 to 5, since the powder components and physical property values satisfied the scope of the present invention, and all the casting conditions of the alloy satisfied the scope of the present invention, no subsurface cracks in the slab were detected, and a good ○ evaluation was obtained. In the examples of Nos. 6 to 13, although the powder components, viscosity at 1300°C, and solidification temperature all satisfied the specified ranges, there were some items that deviated from the preferable ranges. Nos. 6 and 7 received a △ evaluation because invention powders 6 and 7 with melting rates outside the preferable range were used. Nos. 8 and 9 received a △ evaluation because the drawing speed of the casting conditions deviated from the preferable range. Nos. 10 and 11 received a △ evaluation because the superheat degree of the molten alloy in the casting conditions deviated from the preferable range. Also, Nos. 12 and 13 received a △ evaluation because the liquidus temperature range of the Ni-based alloy deviated from the preferable range.

[0046] Next, Comparative Examples Nos. 14 to 26 will be described. In No. 14, the concentrations of CaO, Na2O, Al2O3, Li2O, MnO, BaO, and F in the powder deviated from the specified ranges, and the viscosity and solidification temperature at 1300°C were low and deviated. As a result, the oscillation mark depth became deep, the thickness of the powder film exceeded 3.0 mm and became thick, and the crystal phase was formed at a low ratio. Also, an appropriate crystal phase could not be obtained. Therefore, the solidification shell was strongly cooled and unevenly cooled, resulting not only in subsurface cracks × but also longitudinal cracks.

[0047] In No. 15, since the concentrations of SiO2, Na2O, Al2O3, Li2O, MnO, BaO, and F in the powder deviated from the specified ranges, the viscosity and solidification temperature at 1300°C were high and deviated. As a result, the meltability and fluidity of the powder deteriorated, the crystal phase ratio in the powder film became too high, and the solidification shell was unevenly cooled, resulting in subsurface cracks ×. Also, some sticking occurred.

[0048] For No.16, since the basicity of the powder was low and out of the specified range, the Al2O3 concentration in the powder film exceeded 30% which is the proper range for casting, resulting in subsurface cracks ×. Also, longitudinal cracks occurred.

[0049] For No.17, since the basicity of the powder was high and out of the specified range, the viscosity was too high and out of range, the powder film thickness was thin, the crystal phase ratio was too high, and the solidification shell underwent non-uniform cooling, resulting in subsurface cracks ×. Also, the lubricating effect decreased and sticking occurred.

[0050] For No.18, the MgO concentration of the powder was low and out of the specified range. As a result, magnesiochromite could not be formed in the crystal phase of the powder film, and sufficient slow cooling and uniform cooling effects could not be obtained, resulting in subsurface cracks ×.

[0051] For No.19, the Cr2O3 concentration of the powder was low and out of the specified range. As a result, magnesiochromite could not be formed in the crystal phase of the powder film, and sufficient slow cooling and uniform cooling effects could not be obtained, resulting in subsurface cracks ×.

[0052] For No.20, the MgO concentration of the powder was high and out of the specified range. As a result, in addition to magnesiochromite, MgO was also formed in the crystal phase of the powder film, causing excessive slow cooling and taking a long time for the solidification shell to exhibit sufficient strength, resulting in the significant manifestation of subsurface cracks as ×.

[0053] For No.21, the Cr2O3 concentration of the powder was high and out of the specified range. As a result, in addition to magnesiochromite, Cr2O3 was also formed in the crystal phase of the powder film, causing excessive slow cooling and taking a long time for the solidification shell to exhibit sufficient strength, resulting in the significant manifestation of subsurface cracks as ×.

[0054] For No.22, the concentrations of MgO and Cr2O3 in the powder were high and outside the specified range. As a result, excessive magnesiochromite was generated in the crystal phase of the powder film, leading to excessive slow cooling and taking a long time for the solidification shell to develop sufficient strength, and the subsurface cracks became prominent and turned into ×.

[0055] For No.23, the concentration of C in the powder was low and outside the specified range. As a result, the melting was too fast, causing excessive inflow of molten powder between the mold / solidification shell and promoting non-uniform cooling, resulting in subsurface cracks ×.

[0056] For No.24, the concentration of C in the powder was high and outside the specified range. As a result, the melting was too slow, with many places where molten powder could not flow into the mold / solidification shell, and non-uniform cooling was promoted, resulting in subsurface cracks ×.

[0057] For No.25, the component concentrations, basicity, and physical property values of the powder other than C were all outside the specified range. The solidification shell was cooled too strongly and non-uniformly, resulting in breakout and the casting being aborted.

[0058] For No.26, the concentrations of MgO and Cr2O3 in the powder were high and outside the specified range, and the casting conditions were outside the suitable ranges of liquidus temperature, drawing speed, and superheat of the molten metal. As a result, poor inflow of molten powder between the mold / solidification shell occurred, and the crystal phase of the powder film became too thick, resulting in subsurface cracks ×. Also, longitudinal cracks occurred.

Industrial Applicability

[0059] According to the technology of the present invention, it is possible to obtain a slab with suppressed subsurface cracks in the slab, contributing to an improvement in the yield of the Al-containing Ni-based alloy and a reduction in manufacturing costs.

Claims

1. A mold powder for use in continuous casting of a Ni-based alloy containing 0.8 to 2.0 mass% of Al, the chemical components being 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 0.1-3%, C: 0.5-4%, and unavoidable impurities. ２ ) is 0.5 to 1.0, the viscosity at 1300°C is 0.4 to 2.0 poise, and the 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 mold powder for continuous casting according to claim 1, wherein the melting rate is 1 / sec.

3. A method for continuous casting of an Al-containing Ni-based alloy, comprising the steps of: 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 the conditions of a drawing speed of 300 to 900 mm / min and a degree of superheat of the molten alloy of 20 to 60°C.

Images