Mold Powder for Continuous Casting of Ni-Cr-Mo-Fe Series Ni-Based Alloys and Continuous Casting Method

Abstract

Provided are a mold powder for continuous casting of a Ni-Cr-Mo-Fe-based 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 of a Ni-Cr-Mo-Fe-based Ni-based alloy having a liquidus temperature of 1320 to 1400 °C, the chemical composition of which is, by mass%, CaO: 30 to 40%, SiO2: 30 to 40%, Na2O: 10 to 15%, Al2O3: 1 to 5%, Li2O: 0.2 to 1.5%, F: 3 to 10%, MgO: 0.1 to 4%, Cr2O3: 0.1 to 3%, C: 0.3 to 5%, and inevitable impurities, and having a basicity (CaO / SiO2 in mass% ratio) of 0.80 to 1.30, a viscosity at 1300 °C of 0.5 to 2.0 poise, and a solidification temperature of 900 to 1200 °C.

Description

Technical Field

[0001] The present invention relates to a mold powder for continuous casting for obtaining good surface quality and a continuous casting method using the mold powder for continuous casting in the production of Ni-Cr-Mo-Fe series Ni-based alloys.

Background Art

[0002] Ni-Cr-Mo-Fe series Ni-based alloys (hereinafter also referred to as "Ni-based alloys") have excellent corrosion resistance and high-temperature strength, and are 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 Ni-based alloys, raw materials such as scrap, pure metals, and alloys 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 slabs are manufactured using a continuous casting machine. Thereafter, cold-rolled sheets are 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, which may cause 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 major raw material of Ni-based alloys, 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 (hereinafter also referred to as "subsurface cracks") under the slab skin in continuous casting. If the subsurface cracks cannot be removed after surface grinding of the slab, they will be stretched 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 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 magnifying observation with a camera or by eddy current inspection. 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 Ni-based alloys have a single-phase austenite structure, impurity elements such as P and S are likely to concentrate between the dendrites during solidification, and they have the characteristic of being prone to solidification cracking. Therefore, when the mold undergoes strong cooling or non-uniform cooling in the mold during casting, the initial solidified shell is deformed by thermal stress, easily causing casting defects such as longitudinal cracks, depressions (concave defects), and subsurface cracks. In addition, Ni-Cr-Mo-Fe series Ni-based alloys have a lower liquidus temperature (for example, 1320 - 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.

[0006] So far, several improvement techniques for slab surface defects in the continuous casting of Ni-based and Fe-Ni, Fe-Cr-Ni series alloys have been disclosed (for example, Patent Documents 1 - 5), and a continuous casting mold powder (hereinafter also simply referred to as "powder") and a continuous casting technology for preventing surface defects or internal defects of the slab have been proposed. Although all of them are technologies that control appropriate powder melting physical properties, crystallization behavior, powder inflow characteristics between the mold / solidified shell, etc., they aim to reduce or prevent longitudinal cracks and depressions at a visually confirmable level, and it is difficult to suppress the subsurface cracks of Ni-Cr-Mo-Fe series Ni-based alloys, which is the problem of the present invention. That is to say, it can be said that the defects caused by the subsurface cracks that could not be completely removed by slab grinding remain until 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

Patent Document 4

Patent Document 5

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 a Ni-Cr-Mo-Fe-based Ni-based alloy that suppresses cracks under the slab surface 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 Ni-Cr-Mo-Fe-based Ni-based alloys produced by a continuous casting machine. After grinding and removing the oxide scale on the slab surface, they performed a penetrant inspection test and magnified observation with a camera. Furthermore, for cracks that were difficult to detect by the indication pattern of the penetrant inspection, that is, cracks with a narrow opening, eddy current testing was applied to investigate the occurrence pattern of subsurface cracks. As a result, the length of one subsurface crack was about several millimeters, and they were scattered. After that, subsurface crack parts were collected from several slabs and observed and analyzed in detail 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 a significant enrichment of elements such as P and S was detected from the fracture surface, it was found to be a solidification crack in comparison with conventional knowledge. The opening width was narrow, less than 0.1 mm, and the depth reached about 1 to 5 mm from the surface layer.

[0010] 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.

[0011] Therefore, the inventor focused on the mold powder for continuous casting and carried out research and development for improvement. The powder is put on the surface of the molten alloy supplied from the tundish of the continuous casting process to the mold through the immersion nozzle (hereinafter also referred to as the "molten metal surface"), and while keeping the molten metal surface warm and preventing atmospheric oxidation, it melts at an appropriate speed by receiving heat from the molten metal surface and flows into the space between the mold and the solidified shell of the copper plate to play a lubricating role. In addition, the molten powder that has flowed into the space between the mold and the solidified shell is cooled by the mold and becomes solid phase, forming a glassy powder film. At this time, if a crystal phase is not formed quickly on the mold side of the powder film, the solidified shell will be strongly cooled and unevenly cooled, and longitudinal cracks, depressions, bleeding (seepage of molten alloy), etc. are likely to occur in the slab, and in the worst case, breakout (molten alloy leakage trouble) will occur. The reason for this is that the glassy powder film is easily permeable to radiant light, so the radiant heat transfer from the solidified shell to the mold becomes strong, and the contact with the mold is likely to become uneven. In addition to the fact that the crystal phase blocks radiant heat transfer and has a slow cooling effect because of its good heat insulation, it adheres evenly to the mold wall surface and also brings about the effect of uniform cooling. Therefore, this crystallization behavior is an important characteristic in optimizing cooling to suppress subsurface cracking, which is the subject of this problem.

[0012] The Ni-Cr-Mo-Fe-based Ni-based alloys targeted by the present invention are, for example, in mass%, Ni-15%Cr-16%Mo-5%Fe-4%W (JIS NW 0276, UNS N10276), Ni-21%Cr-14%Mo-4%Fe-3%W (JIS NW 6022, UNS N06022), Ni-21%Cr-9%Mo-18%Fe-1.2%Co (JIS NW 6002, UNS N06002), Ni-22%Cr-9%Mo-3%Fe-4%Nb (JIS NCF 625, UNS N06625), etc., and the liquidus temperature is in the temperature range of 1320~1400°C. This is characterized by a very low liquidus temperature compared with that of ordinary steel, stainless steel, etc. manufactured by a continuous casting machine.

[0013] 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 between the mold / solidification shell, and flowing into the space between the mold and the solidification shell. Therefore, the mold powder for continuous casting of Ni-based alloys 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 space between the mold / solidification shell, and have the property of rapidly forming a crystal phase on the mold side along with the formation of a glassy powder film. Further, in the problems of the present invention, in addition to preventing large defects such as longitudinal cracks and depressions that have occurred in conventional Ni-based alloys, a crystallization behavior that promotes slower and uniform cooling is required to suppress subsurface cracks.

[0014] Therefore, the inventor conducted various investigations and experimental studies on powders that satisfy the above properties. First, regarding the properties of an appropriate powder, if the viscosity at 1300 °C is 0.5 to 2.0 poise and the solidification temperature is 900 to 1200 °C, it was found that the flowability between the mold / solidification shell is good even with the heat of the molten alloy of a Ni-based alloy having a relatively low liquidus temperature.

[0015] Next, the inventor added C, which is an aggregate, based on the conventionally studied powder composition of CaO - SiO2 - Na2O - Al2O3 - Li2O - F system, 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 are obtained, high-temperature experiments such as thermal analysis were conducted to simulate the thermal environment in the mold with various compositions, and samples equivalent to the recovered powder film were analyzed by SEM / EDS, transmission electron microscope (TEM), etc. to investigate the crystallization behavior. As a result, an appropriate powder film thickness is formed, and a crystal phase is formed at an appropriate ratio in the total thickness. In the crystal phase structure, in addition to the conventionally reported cuspidine (3CaO·2SiO2·CaF2), nepheline (Na2O·Al2O3·2SiO2), calcium fluoride (CaF2), it was found that a crystal phase of magnesiochromite (MgO·Cr2O3), which is a compound of MgO and Cr2O3, is generated.

[0016] As a result of further research, it was found that magnesiochromite is more likely to crystallize than the other crystals mentioned above, and since MgO-Cr2O3-based oxides generally have good heat insulation properties and are used as magnesite bricks for steelmaking refractories, the formation of magnesiochromite brings about a slow cooling and uniform cooling effect at a level that suppresses subsurface cracking.

[0017] As a result of further research for improvement, it was found that particles with a particle size in the range of 0.15 to 0.85 mm in the above-mentioned developed powder are contained in an amount of 90% or more by weight, and thus they melt uniformly by heat supply from the surface of the molten alloy. As a result, it was clarified that it promotes the uniform inflow of the molten powder between the mold and the solidified shell and has the effect of reducing the non-uniform cooling of the solidified shell. Also, an investigation was conducted on the casting conditions for continuous casting in which the effect of suppressing subsurface cracking of 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.

[0018] The present invention relates to a mold powder for continuous casting of a Ni-Cr-Mo-Fe-based Ni-based alloy having a liquidus temperature of 1320 to 1400 °C, the chemical components of which are, by mass%, CaO: 30 to 40%, SiO2: 30 to 40%, Na2O: 10 to 15%, Al2O3: 1 to 5%, Li2O: 0.2 to 1.5%, F: 3 to 10%, MgO: 0.1 to 4%, Cr2O3: 0.1 to 3%, C: 0.3 to 5%, and inevitable impurities, and having a basicity (CaO / SiO2 in mass% ratio) of 0.80 to 1.30, a viscosity at 1300 °C of 0.5 to 2.0 poise, and a solidification temperature of 900 to 1200 °C.

[0019] Further, the mold powder for continuous casting is characterized in that particles in the particle size range of 0.15 to 0.85 mm are contained in an amount of 90% or more by weight.

[0020] Furthermore, the present invention also proposes a continuous casting method using the above-mentioned mold powder for continuous casting. That is, it is a continuous casting method for Ni-Cr-Mo-Fe series Ni-based alloys, characterized in that an alloy melt of Ni-Cr-Mo-Fe series Ni-based alloy with a liquidus temperature of 1320 to 1400 °C is cast under the conditions of a drawing speed of 300 to 900 mm / min and a superheat of the alloy melt of 20 to 60 °C.

Advantages of the Invention

[0021] According to the present invention, it is possible to suppress subcutaneous cracks generated in the slab produced by continuous casting of Ni-Cr-Mo-Fe series Ni-based alloys. As a result, it is possible to prevent linear defects generated in the cold-rolled sheet in the cold rolling process. Therefore, the yield is good, and it is possible to provide a product with good surface quality at low cost by suppressing the manufacturing cost without generating an additional grinding process in the cold rolling process.

Embodiments for Carrying Out the Invention

[0022] First, the reasons for limiting the chemical composition of the mold powder for continuous casting of the present invention are shown. In the following description, “%” means mass %. Basically, since the powder is melted by the heat from the surface of the alloy melt, heat supply in the mold is important for exerting the effect of the powder.

[0023] CaO: 30 to 40%, SiO2: 30 to 40%, Na2O: 10 to 15%, Al2O3: 1 to 5%, Li2O: 0.2 to 1.5%, F: 3 to 10%, MgO: 0.1 to 4%, Cr2O3: 0.1 to 3% If the above oxides and fluorides are within this range, the viscosity at 1300 °C satisfies 0.5 to 2.0 poise, and the solidification temperature satisfies 900 to 1200 °C. As an example, F is added as CaF2, NaF, LiF, etc. Also, MgO and Cr2O3 may be added as simple oxides, magnesiochromite, or impurities of other oxides and fluorides.

[0024] When the powder of the present application melts and flows into the mold / solidification shell as 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 strong cooling indistinguishable from glassy, and 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 cuspidine (3CaO·2SiO2·CaF2), nepheline (Na2O·Al2O3·2SiO2), calcium fluoride (CaF2), and magnesiochromite (MgO·Cr2O3), which is most preferable because it can appropriately control the heat flux from the solidification shell to the mold.

[0025] 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, so it is important because it has the effect of suppressing the occurrence of subsurface cracks in the solidification shell at the initial stage of solidification. Magnesiochromite (MgO·Cr2O3) exhibits the above-mentioned efficacy when it is generated at 0.1% by mass or more 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 higher than 4%, not only magnesiochromite but also MgO is generated, resulting in excessive slow cooling and taking a long time for the solidification shell to exhibit sufficient strength. On the contrary, it promotes subsurface cracks. On the other hand, when the concentration of Cr2O3 is higher than 3%, not only magnesiochromite but also Cr2O3 is generated, and subsurface 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 subsurface cracks for the above reasons. Therefore, in order to simultaneously satisfy the appropriate viscosity, solidification temperature, and crystallization behavior of the powder for suppressing subsurface cracks in the slab, it is necessary to control it within the above composition range. From these viewpoints, it is specified as CaO: 30 - 40%, SiO2: 30 - 40%, Na2O: 10 - 15%, Al2O3: 1 - 5%, Li2O: 0.2 - 1.5%, F: 3 - 10%, MgO: 0.1 - 4%, Cr2O3: 0.1 - 3%.

[0026] C: 0.3 - 5% 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 can be controlled. If it is less than 0.3%, the oxide and fluoride particles will come into contact, and the melting will be too fast, resulting in excessive inflow of molten powder between the mold / solidification shell and causing non-uniform cooling. As a result, it promotes subsurface cracking. Also, if it is higher than 5%, the melting is too slow, and there will be many places where the molten powder cannot flow into the mold / solidification shell, resulting in non-uniform cooling. As a result, it not only promotes subsurface cracking, but in severe cases, sticking (adhesion of the solidification shell to the mold) may occur, and in the worst case, breakout will be caused. For these reasons, it is specified to be 0.3 - 5%.

[0027] Basicity (CaO / SiO2 in mass concentration ratio): 0.80 - 1.30 If the basicity is too low, the solidification temperature will be lower than 900 °C, and at the same time, the viscosity at 1300 °C will also be lower than 0.5 poise. Furthermore, the thickness of the powder film will exceed 3.0 mm, and the crystal phase in the glassy film will be less than 15% of the total thickness, resulting in strong and non-uniform cooling and promoting sub-surface cracking. On the other hand, if the basicity is too high, the solidification temperature will exceed 1200 °C, and at the same time, the viscosity at 1300 °C will exceed 2.0 poise. Furthermore, the thickness of the powder film will be too thin, less than 0.5 mm, and the crystal phase ratio will exceed 75%, reducing the lubricating effect and causing surface defects such as sticking and longitudinal cracking. That is, when the basicity (CaO / SiO2) is in the range of 0.80 - 1.30, when the molten powder flows into the mold / solidification shell, a glassy powder film is formed, and its thickness is an appropriate 0.5 - 3.0 mm. And within this film, a crystal phase of 15 - 75% of the total thickness is formed on the mold side. Therefore, it is possible to reduce the strong and non-uniform cooling of the solidification shell and suppress sub-surface cracking. Therefore, the range of basicity is defined as 0.80 - 1.30.

[0028] Viscosity at 1300 °C: 0.5 - 2.0 poise If the viscosity at 1300 °C is less than 0.5 poise, the inflow of the molten powder between the mold / solidification shell will be excessive, and the depth of the oscillation marks formed when the solidification shell is pulled out in the casting direction from the vibrating mold will increase, resulting in non-uniform cooling of the solidification shell and promoting sub-surface cracking. On the other hand, if the viscosity at 1300 °C is higher than 2.0 poise, the fluidity of the molten powder will deteriorate, and there will be more places where the molten powder cannot flow into the mold / solidification shell, resulting in non-uniform cooling. As a result, it not only promotes sub-surface cracking but also, in severe cases, causes sticking (adhesion of the solidification shell to the mold), and in the worst case, breakout. Therefore, the range of viscosity at 1300 °C is defined as 0.5 - 2.0 poise.

[0029] Solidification temperature: 900 - 1200 °C When the solidification temperature is less than 900 °C, the melting of the powder progresses too far, resulting in excessive inflow of the molten powder between the mold / solidification shell, increasing the oscillation mark depth, leading to non-uniform cooling of the solidification shell and promoting subsurface cracking. On the other hand, when the solidification temperature is too high, exceeding 1200 °C, the meltability of the powder deteriorates, and there are more locations where the molten powder cannot flow into the mold / solidification shell, resulting in non-uniform cooling. Therefore, the range of the solidification temperature is defined as 900 - 1200 °C.

[0030] Particle size: Weight percentage of particles in the range of 0.15 - 0.85 mm: 90% or more The particle size of the powder particles plays an important role in the uniformity of powder melting. For particles that are too small, with a particle size less than 0.15 mm, the specific surface area represented by the surface area per unit volume is large, and the heat input efficiency from the surface of the molten alloy is too high, forming locally over-melted areas. Also, for particles with a particle size exceeding 0.85 mm, due to the small specific surface area, the heat input efficiency is poor, forming areas where the melting is locally slow. From these perspectives, to reduce the non-uniform cooling of the solidification shell associated with the non-uniform inflow of the molten powder between the mold / solidification shell and obtain the effect of suppressing subsurface cracking, it is preferable that the powder contains 90% or more, by weight percentage, of oxide and fluoride particles in the range of 0.15 - 0.85 mm in particle size.

[0031] In the present invention, a continuous casting method using the above-mentioned mold powder for continuous casting is also proposed. First, the mold powder for continuous casting of the present invention is preferably used for the continuous casting of Ni-Cr-Mo-Fe-based Ni-based alloys with a liquidus temperature of 1320 - 1400 °C. When applied to a molten alloy with a liquidus temperature less than 1320 °C, the powder of the present invention does not melt sufficiently. Also, when applied to a molten alloy with a temperature exceeding 1400 °C, it will be over-melted. Therefore, it is a preferred embodiment that the powder having the above physical properties is applied to continuous casting with a liquidus temperature of 1320 - 1400 °C.

[0032] The Ni-Cr-Mo-Fe based Ni-based alloy having the above liquidus temperature range is, for example, in mass %, Ni-15%Cr-16%Mo-5%Fe-4%W (JIS NW 0276, UNS N10276), Ni-21%Cr-14%Mo-4%Fe-3%W (JIS NW 6022, UNS N06022), Ni-21%Cr-9%Mo-18%Fe-1.2%Co (JIS NW 6002, UNS N06002), Ni-22%Cr-9%Mo-3%Fe-4%Nb (JIS NCF 625, UNS N06625), etc. Although not particularly limited here, it is optimal for casting Ni-based alloys with the following compositions. That is, it is an alloy composed of Cr: 10 to 30%, Mo: 3 to 20%, Fe: 1 to 30%, Nb: 5% or less, W: 4% or less, Co: 1.2% or less, C: 0.2% or less, Si: 0.5% or less, Mn: 1.0% or less, and the balance being Ni and unavoidable impurities.

[0033] The above Ni-based alloy will be described. C, Nb, W, and Co are elements effective for maintaining strength, and Si and Mn are useful for adjusting the O and S concentrations as deoxidizers. Ni is useful as an element for stabilizing the austenite phase, and Cr and Mo improve corrosion resistance such as pitting corrosion resistance and crevice corrosion resistance, for example. Fe is an inexpensive element and has a role in adjusting the liquidus temperature and strength.

[0034] The continuous casting method of the above Ni-based alloy proposed here is a continuous casting method characterized by continuously casting the molten alloy at a drawing speed of 300 to 900 mm / min and a superheat degree of the molten alloy of 20 to 60°C using the above mold powder for continuous casting.

[0035] Drawing speed: 300 to 900 mm / min If the drawing speed is less than 300 mm / min and thus slow, the supply of the molten alloy to the mold will significantly decrease, resulting in a reduction in the heat supply from the molten metal surface to the powder. Accordingly, the melting of the powder deteriorates, leading to non-uniform inflow of the molten powder between the mold / solidification shell, and promoting subcutaneous cracking. Conversely, if it is faster than 900 mm / min, the heat supply will be excessive, causing excessive melting of the powder. As a result, the depth of the oscillation marks increases, resulting in non-uniform cooling of the solidification shell and promoting subcutaneous cracking. Therefore, the drawing speed preferably ranges from 300 to 900 mm / min.

[0036] Superheat of the molten alloy: 20 - 60°C The superheat of the molten alloy is controlled by lifting the ladle after adjusting the temperature of the molten alloy in the LF (ladle furnace), then continuously pouring the molten metal into the tundish, and measuring the molten metal temperature in the tundish. The superheat here is defined as the superheat temperature from the liquidus temperature of the Ni-based alloy. When this value is less than 20°C and low, the melting of the powder deteriorates, leading to non-uniform inflow of the molten powder between the mold / solidification shell, and promoting subcutaneous cracking. Conversely, if it is higher than 60°C, the heat supply will be 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 solidification shell and promoting subcutaneous cracking. Therefore, the superheat of the molten alloy preferably ranges from 20 to 60°C.

Example

[0037] 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, ferrochrome, chromium, molybdenum, niobium, tungsten, 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, and then the molten alloy was supplied from the tundish into the mold through the immersion nozzle for continuous casting to produce slabs. 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 6 and the comparative powders No. 1 to 10. 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 Ni-Cr-Mo-Fe-based Ni-based alloy cast using these powders for Examples 1 to 12 and Comparative Examples 13 to 23.

[0038] 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.

[0039] 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.

[0040] 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.

[0041] Powder particle size: Weight ratio of particles in the range of 0.15 to 0.85 mm Using metal test sieves (JIS Z 8801) with a mesh opening of 0.15 mm and 0.85 mm for the wire mesh, approximately 200 g of the powder was sieved to obtain it.

[0042] 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 melting infrared absorption method. The remainder is inevitable impurities such as P, S, O, H, N, and Cu. Also, in the components of Table 2, "-" indicates no addition.

[0043] 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.

[0044] The evaluation method for the slab surface quality is as follows. Regarding the slab (length: 8 m) in the range of 10 to 18 m in casting length from the start of withdrawal in continuous casting, after performing full-surface grinding on the grinder with the grinding amount being uniformly 1% of the slab thickness, a penetrant flaw detection test was conducted, followed by enlarged observation with a camera and surface scanning with an eddy current flaw detection probe to evaluate the number of subsurface cracks per slab. When there are no subsurface cracks, it is possible to obtain a good yield as the grinding is limited to the minimum for removing surface oxide scale and oscillation marks, etc., and furthermore, since no additional grinding process occurs in the cold rolling process, a good manufacturing cost can be achieved. In addition, from various investigations during the powder research and development of the present invention, it is known that there is a correlation that the smaller the number of subsurface cracks generated, the lower the crack depth. If the number of occurrences is in the range of 1 to 20 pieces / 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 sufficiently satisfactory. When the number of occurrences reaches 21 pieces / slab or more, the correlation between the number of cracks generated and the depth becomes weak, and residual removal occurs for the grinding amount aimed at completely removing the cracks. As a result, there is a significant decrease in yield, an additional process occurs in the cold rolling process, and an acceptable manufacturing cost cannot be achieved. From these viewpoints, the slab surface quality was evaluated in the following three stages. 〇: No subsurface cracks △: Number of subsurface cracks: 1 - 20 pieces / slab (8 m) ×: Number of subsurface cracks ≧21 pieces / slab (8 m)

[0045] In addition, in the present invention, an investigation of the powder film after casting is also being carried out. 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 attached EDS. The ratio of the powder film total thickness occupied by the crystal phase thickness was obtained from the crystal phase thickness, and the constituent crystal phases were also confirmed. The appropriate crystal structure is cuspidine (3CaO·2SiO2·CaF2), nepheline (Na2O·Al2O3·2SiO2), calcium fluoride (CaF2), and magnesiochromite (MgO·Cr2O3).

[0046]

Table 1

[0047]

Table 2

[0048] The results of the examples and comparative examples shown in Table 2 will be described. In the table, the numerical values within parentheses ( ) indicate those that do not satisfy the essential components (specified in the independent claims) of the present invention, and the numerical values within brackets [ ] indicate those that do not satisfy the preferable components (specified in the dependent claims). First, Examples Nos. 1 to 12 will be described below. In Nos. 1 to 5, Invention Powders 1 to 5 whose powder components and physical property values satisfied the scope of the present invention were used, and since all of the casting conditions of the alloy and the properties of the powder film after casting 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 12, although the powder components, the viscosity at 1300 °C, and the solidification temperature all satisfied the specified ranges, there were some items that deviated from the preferable ranges. In No. 6, since Invention Powder 6 that did not satisfy the particle weight ratio in the range of 0.15 to 0.85 mm of the powder particle size was used, it was evaluated as △. In Nos. 7 and 8, since they deviated from the preferable range of the drawing speed of the casting conditions, they were evaluated as △. In Nos. 9 and 10, since they deviated from the preferable range regarding the degree of superheat of the molten alloy of the casting conditions, they were evaluated as △. Also, in Nos. 11 and 12, since the range of the liquidus temperature of the Ni-based alloy deviated from the preferable range, they were evaluated as △.

[0049] Next, Comparative Examples Nos. 13 to 23 will be described. In No. 13, since the concentrations of CaO, SiO2, Na2O, Al2O3, Li2O, and F in the powder deviated from the specified ranges and the basicity was low and deviated, the viscosity and solidification temperature at 1300 °C deviated. As a result, the oscillation mark depth became deep, and furthermore, the thickness of the powder film exceeded 3.0 mm and was thick, and the crystal phase was formed at a low ratio, so that the solidification shell was strongly cooled and unevenly cooled, resulting in an × result. Also, longitudinal cracks occurred.

[0050] For No.14, since the concentrations of CaO, SiO2, Na2O, Al2O3, Li2O, and F in the powder deviated from the specified ranges and the basicity was extremely high, the viscosity and solidification temperature at 1300°C deviated significantly. As a result, the fusibility and fluidity of the powder deteriorated. Moreover, the powder film thickness was too thin and the crystal phase ratio was too high, resulting in non-uniform cooling of the solidification shell and the outcome of ×. Additionally, longitudinal cracks occurred.

[0051] For No.15, the concentration of MgO in the powder was low and deviated from the specified range. As a result, magnesiochromite could not be formed in the crystal phase of the powder film. Consequently, due to the inability to obtain sufficient slow cooling and uniform cooling effects, the evaluation was ×.

[0052] For No.16, the concentration of Cr2O3 in the powder was low and deviated from the specified range. As a result, magnesiochromite could not be formed in the crystal phase of the powder film. Therefore, due to the inability to obtain sufficient slow cooling and uniform cooling effects, the result was ×.

[0053] For No.17, the concentration of MgO in the powder was high and deviated from the specified range. As a result, in addition to magnesiochromite, MgO was also formed in the crystal phase of the powder film. Therefore, the slow cooling progressed too far, and it took a long time for the solidification shell to exhibit sufficient strength, resulting in the prominent appearance of subsurface cracks and the outcome of ×.

[0054] For No.18, the concentration of Cr2O3 in the powder was high and deviated from the specified range. As a result, in addition to magnesiochromite, Cr2O3 was also formed in the crystal phase of the powder film. Therefore, the slow cooling progressed too far, and it took a long time for the solidification shell to exhibit sufficient strength, resulting in the prominent appearance of subsurface cracks and the outcome of ×.

[0055] For No.19, the concentrations of MgO and Cr2O3 in the powder were high and deviated from the specified range. As a result, an excessive amount of magnesiochromite was formed in the crystal phase of the powder film. Therefore, the slow cooling progressed too far, and it took a long time for the solidification shell to exhibit sufficient strength, resulting in the prominent appearance of subsurface cracks and the outcome of ×.

[0056] For No.20, the concentration of C in the powder was low and outside the specified range. As a result, the melting was too fast, so the inflow of molten powder between the mold / solidification shell became excessive and non-uniform cooling was promoted, resulting in ×.

[0057] For No.21, the concentration of C in the powder was high and outside the specified range. As a result, the melting was too slow and there were many places where molten powder could not flow into the mold / solidification shell, and non-uniform cooling was promoted, resulting in ×.

[0058] For No.22, the concentration of components other than C in the powder and the physical property values were all outside the specified range. The solidification shell was cooled too strongly and non-uniformly, resulting in deep longitudinal cracks and ×.

[0059] For No.23, 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 degree of the molten metal. As a result, poor inflow of molten powder between the mold / solidification shell occurred, causing significant non-uniform cooling that led to longitudinal cracks and ×.

Industrial Applicability

[0060] According to the technology of the present invention, it is possible to obtain a slab with suppressed subcutaneous cracks in the slab, which contributes to improving the yield and reducing the manufacturing cost of Ni-Cr-Mo-Fe-based Ni-based alloys.

Claims

1. A mold powder for use in continuous casting of a Ni-Cr-Mo-Fe system Ni-based alloy having a liquidus temperature of 1320 to 1400°C, the chemical components being, in mass%, CaO: 30 to 40%, SiO ２ :30-40%, Na ２ O: 10-15%, Al ２ O ３ : 1 to 5%, Li ２ O: 0.2-1.5%, F: 3-10%, MgO: 0.1-4%, Cr ２ O ３ 0.1-3%, C: 0.3-5%, and unavoidable impurities. ２ ) is 0.80 to 1.30, the viscosity at 1300°C is 0.5 to 2.0 poise, and the solidification temperature is 900 to 1200°C.

2. 2. The mold powder for continuous casting according to claim 1, characterized in that particles having a particle size in the range of 0.15 to 0.85 mm account for 90% or more by weight.

3. A method for continuous casting of a Ni-Cr-Mo-Fe-based 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-Cr-Mo-Fe-based Ni-based alloy having a liquidus temperature of 1320 to 1400°C 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.

Images