Mold powder for continuous casting of ni-cr-mo-fe ni-based alloy and continuous casting method

Abstract

To provide mold powder for continuous casting of an Ni-Cr-Mo-Fe-based 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-Cr-Mo-Fe-based Ni-based alloys having a liquidus temperature of 1320 to 1400 °C, the mold powder comprising, as chemical components by mass%, 30 to 40% of CaO, 30 to 40% of SiO2, 10 to 15% of Na2O, 1 to 5% of Al2O3, 0.2 to 1.5% of Li2O, 3 to 10% of F, 0.1 to 4% of MgO, 0.1 to 3% of Cr2O3, 0.3 to 5% of C, and an unavoidable impurity, A mold powder for continuous casting having a basicity (CaO / SiO2 in terms of mass% ratio) of 0.80 to 1.30, a viscosity at 1300 °C of 0.5 to 2. 0poise, and a solidification temperature of 900 to 1200 °C.SELECTED DRAWING: None

Description

[Technical Field]

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

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

[0003] In the Ni-based alloy manufacturing process, raw materials such as scrap, pure metals, and alloys are melted in an electric furnace, decarburized, desulfurized, and refined using argon oxygen decarburization (AOD) or vacuum decarburization (VOD), and finally, slabs are produced in a continuous caster. The resulting material then undergoes hot rolling and cold rolling to produce cold-rolled sheets. During this process, linear defects can develop on the surface of the final cold-rolled sheet, which can act as a starting point for corrosion and fracture, resulting in product defects. Such surface defects require an additional grinding process to ensure excellent surface quality, significantly reducing yield. Because Ni, the primary raw material for Ni-based alloys, is more expensive than Fe and Cr, improving yield and reducing manufacturing costs are important industrially. These challenges 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 only detected by penetrant testing after grinding, where they are magnified and observed with a camera, or by eddy current testing, with an opening width of less than 0.1 mm. Therefore, to solve the above problems and obtain cold-rolled sheets with excellent surface quality, it is necessary to suppress not only conventional surface defects such as longitudinal slab cracks and depressions (dent defects), but also subsurface cracks.

[0005] Ni-based alloys have a single-phase austenite structure, which means that impurity elements such as P and S tend to concentrate between dendrites during solidification, making them prone to solidification cracking. Therefore, if the alloy is subjected to strong or uneven cooling in the mold during casting, the initial solidification shell deforms due to thermal stress, easily resulting in casting defects such as vertical cracks, depressions (dent defects), and subsurface cracks. Furthermore, Ni-Cr-Mo-Fe-based Ni-based alloys have a lower liquidus temperature (e.g., 1320–1400°C) than general steels and stainless steels, making it difficult to apply conventional continuous casting techniques used in the steel industry.

[0006] Several techniques for improving slab surface defects in the continuous casting of Ni-based, Fe-Ni, and Fe-Cr-Ni alloys have been disclosed (e.g., Patent Documents 1 to 5). Continuous casting mold powders (hereinafter simply referred to as "powder") and continuous casting techniques have been proposed to prevent surface or internal defects in slabs. While these techniques control the powder's melting properties, crystallization behavior, and powder flow characteristics between the mold and solidified shell, they aim to reduce or prevent vertical cracks and depressions that are visible to the naked eye. Therefore, they are unable to suppress subsurface cracking in Ni-Cr-Mo-Fe-based Ni-based alloys, which is the subject of the present invention. In other words, defects resulting from subsurface cracking that cannot be completely removed by slab grinding remain in the cold-rolled sheet, resulting in reduced yields and increased manufacturing costs due to additional processes. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2007-61845 [Patent Document 2] Japanese Patent Application Publication No. 2018-144055 [Patent Document 3] Patent No. 7032600 [Patent Document 4] Japanese Patent Application Laid-Open No. 2008-272786 [Patent Document 5] Patent No. 7158614 Summary of the Invention [Problem to be solved by the invention]

[0008] In view of the above problems, the present invention provides a mold powder for continuous casting of a Ni-Cr-Mo-Fe-based 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 Ni-Cr-Mo-Fe Ni-based alloys produced using a continuous casting machine. After grinding and removing the oxide scale from the slab surface, they performed penetrant testing and magnified observations with 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 showed that the subsurface cracks were cracks that opened along the interdendritic boundaries of the solidification structure. Furthermore, significant concentrations of elements such as P and S were detected on the fracture surface, confirming prior knowledge that they were solidification cracks. The opening was narrow, less than 0.1 mm wide, and its depth ranged from the surface to about 1 to 5 mm.

[0010] From the above results, we believe 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 weak dendrites, leading to subsurface cracks. We also found a correlation between the number of subsurface cracks detected on the slab surface and the depth of the cracks.

[0011] 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.

[0012] Examples of Ni-Cr-Mo-Fe alloys that are the subject of the present invention include, 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), and Ni-22%Cr-9%Mo-3%Fe-4%Nb (JIS NCF 625, UNS N06625), and have liquidus temperatures in the range of 1320 to 1400°C. These alloys are characterized by a significantly lower liquidus temperature than ordinary steels and stainless steels produced by continuous casting machines.

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

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

[0015] 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-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 simulating the thermal environment inside a mold were performed with various compositions, 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 were formed within the total thickness, and that in addition to the previously reported cuspidine (3CaO·2SiO2·CaF2), nepheline (Na2O·Al2O3·2SiO2), and calcium fluoride (CaF2), a crystalline phase of magnesiochromite (MgO·Cr2O3), a compound of MgO and Cr2O3, was also formed within the crystalline phase structure.

[0016] Further research revealed that magnesiochromite crystallizes more easily than the other crystals mentioned above, and that MgO-Cr2O3 oxides have such good insulating properties that they are commonly used in steelmaking refractories as magnesiochromite bricks. Therefore, the formation of magnesiochromite brings about slow and uniform cooling to a level that suppresses subsurface cracking.

[0017] Further research aimed at further improvement revealed that when the developed powder contains 90% or more particles by weight with a particle size in the range of 0.15 to 0.85 mm, it melts uniformly with heat supplied from the surface of the molten alloy. As a result, it was found that this powder promotes the uniform flow of molten powder between the mold and solidified shell, thereby reducing uneven cooling of the solidified shell. Furthermore, the casting conditions for continuous casting under which the powder of the present invention exerts its strong effect of suppressing subsurface cracking were investigated. The present invention was completed based on the above research results, as described below.

[0018] The present invention provides a mold powder for use in continuous casting of a Ni-Cr-Mo-Fe-based Ni-based alloy having a liquidus temperature of 1320 to 1400°C, the mold powder having chemical components, in mass %, of 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 unavoidable impurities, and characterized in that the mold powder for continuous casting has a basicity (CaO / SiO2 ratio by mass %) 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] The mold powder for continuous casting is characterized in that it contains particles with a particle size in the range of 0.15 to 0.85 mm in an amount of 90% or more by weight.

[0020] The present invention also proposes a continuous casting method using the above-mentioned mold powder for continuous casting, namely, a continuous casting method for a Ni-Cr-Mo-Fe-based Ni-based alloy, characterized by casting a molten alloy of the Ni-Cr-Mo-Fe-based Ni-based alloy having a liquidus temperature of 1320 to 1400°C under conditions of a withdrawal speed of 300 to 900 mm / min and a superheat of the molten alloy of 20 to 60°C. [Effects of the Invention]

[0021] According to the present invention, it is possible to suppress subsurface cracks that occur in slabs produced by continuous casting of Ni-Cr-Mo-Fe Ni-based alloys, and as a result, it is possible to prevent linear defects that occur in cold-rolled sheets during the cold rolling process. As a result, it is possible to provide products with good surface quality at low cost, with high yields and no additional grinding process required in the cold rolling process, thereby reducing manufacturing costs. DETAILED DESCRIPTION OF THE INVENTION

[0022] First, the reasons for limiting the chemical composition of the mold powder for continuous casting of the present invention will be described. In the following explanation, "%" means mass %. Note that, since the powder is basically melted by heat from the surface of the molten alloy, the heat supply within the mold is important for the powder to exert its effects.

[0023] 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% If the oxides and fluorides are within this range, the viscosity at 1300°C will be 0.5 to 2.0 poise, and the solidification temperature will be 900 to 1200°C. As an example, F is added as CaF2, NaF, LiF, etc. Furthermore, MgO and Cr2O3 may be added as simple oxides, magnesiochromite, or as impurities in other oxides and fluorides.

[0024] 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 form in the portion of the film that contacts the mold, corresponding to 15 to 75% of the total thickness. A powder film thickness of less than 0.5 mm results in excessive cooling, while a thickness exceeding 3.0 mm promotes uneven cooling. If the crystalline phase formation rate is less than 15%, the cooling is as strong as a glassy film, while if it exceeds 75%, the cooling is too slow, resulting in insufficient strength of the solidified shell and promoting vertical cracks. Furthermore, the crystalline phase is most preferably composed of cuspidine (3CaO 2SiO2 CaF2), nepheline (Na2O Al2O3 2SiO2), calcium fluoride (CaF2), or magnesiochromite (MgO Cr2O3), which allows for appropriate control of the heat flux from the solidified shell to the mold.

[0025] 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 this effect 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, excessive magnesiochromite (MgO·Cr2O3) formation will occur, causing excessive slow cooling, which will promote subsurface cracking for the reasons mentioned above. Therefore, to simultaneously achieve the appropriate viscosity, solidification temperature, and crystallization behavior of the powder to prevent subsurface cracking of the slab, it is necessary to control the composition within the above range. From this perspective, the following values ​​were specified: CaO: 30-40%, SiO2: 30-40%, Na2O: 10-15%, Al2O3: 1-5%, Li2O: 0.2-1.5%, F: 3-10%, MgO: 0.1-4%, and Cr2O3: 0.1-3%.

[0026] C: 0.3 to 5% 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 allows for control of the powder melting rate. At less than 0.3%, the oxide and fluoride particles come into contact, resulting in excessive melting, which causes excessive flow of molten powder between the mold and solidified shell, resulting in uneven cooling. This promotes subsurface cracking. At a carbon content of more than 5%, melting is too slow, resulting in many areas where the 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 carbon content is specified as 0.3-5%.

[0027] Basicity (mass concentration ratio of CaO / SiO2): 0.80-1.30 If the basicity is too low, the solidification temperature will be lower than 900°C and the viscosity at 1300°C will be lower than 0.5 poise. Furthermore, the powder film will be thicker than 3.0 mm, and the crystalline phase will account for less than 15% of the total thickness within the glassy film, resulting in strong and uneven cooling and promoting subsurface cracking. On the other hand, if the basicity is too high, the solidification temperature will be higher than 1200°C and the viscosity at 1300°C will be higher than 2.0 poise. Furthermore, the powder film will be too thin (less than 0.5 mm) and the crystalline phase will be thicker (more than 75%), reducing the lubrication effect and causing surface defects such as sticking and vertical cracks. In other words, when the basicity (CaO / SiO2) is in the range of 0.80 to 1.30, a glassy powder film will form when the molten powder flows between the mold and solidified shell, and its thickness will be within the appropriate range of 0.5 to 3.0 mm. Then, within this film, a crystalline phase is formed on the mold side, accounting for 15 to 75% of the total thickness. This reduces strong and uneven cooling of the solidified shell and makes it possible to suppress subsurface cracking. Therefore, the basicity range is specified as 0.80 to 1.30.

[0028] Viscosity at 1300℃: 0.5 to 2.0 poise If the viscosity at 1300°C is less than 0.5 poise, excessive flow of molten powder into the mold / solidified shell gap occurs. This leads to deeper oscillation marks formed 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. As a result, not only does this promote subsurface cracking, but in severe cases it can also cause sticking (the solidified shell sticking to the mold) and, in the worst case, breakout. For this reason, the viscosity range at 1300°C is specified as 0.5 to 2.0 poise.

[0029] Solidification temperature: 900~1200℃ If the solidification temperature is below 900°C, the powder melts too much, causing excessive inflow of molten powder between the mold and solidified shell, deepening oscillation marks, leading to uneven cooling of the solidified shell and promoting subsurface cracking. On the other hand, if the solidification temperature is higher than 1200°C, the powder melts poorly, creating many areas where the molten powder cannot flow between the mold and solidified shell, resulting in uneven cooling. For this reason, the solidification temperature range was specified as 900 to 1200°C.

[0030] Particle size: 0.15 to 0.85 mm Weight percentage of particles: 90% or more The particle size of the powder particles plays an important role in the uniformity of powder melting. Particles with a particle size of less than 0.15 mm have a large specific surface area, which is the surface area per volume. This increases the heat input efficiency from the surface of the molten alloy, resulting in localized areas of excessive melting. Particles with a particle size of more than 0.85 mm have a small specific surface area, resulting in poor heat input efficiency and localized areas of slow melting. From these perspectives, to reduce uneven cooling of the solidified shell due to uneven flow of molten powder between the mold and solidified shell and to suppress subsurface cracking, it is preferable that the powder contain 90% or more oxide and fluoride particles with particle sizes in the range of 0.15 to 0.85 mm by weight.

[0031] The present invention also proposes a continuous casting method using the above-mentioned mold powder for continuous casting. First, the mold powder for continuous casting of the present invention is suitable for use in the continuous casting of Ni-Cr-Mo-Fe-based Ni-based alloys with a liquidus temperature of 1320 to 1400°C. When applied to a molten alloy with a liquidus temperature of less than 1320°C, the powder of the present invention does not melt sufficiently. Furthermore, when applied to a molten alloy with a liquidus temperature exceeding 1400°C, excessive melting occurs. Therefore, a powder having the above physical properties is preferably applied to continuous casting with a liquidus temperature of 1320 to 1400°C.

[0032] Examples of Ni-Cr-Mo-Fe based Ni-based alloys having the above liquidus temperature range include, 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), and Ni-22%Cr-9%Mo-3%Fe-4%Nb (JIS NCF 625, UNS N06625). Although not particularly limited, these alloys are ideal for casting Ni-based alloys having the following compositions: That is, the alloy is composed of Cr: 10-30%, Mo: 3-20%, Fe: 1-30%, 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 alloys will now be explained. C, Nb, W, and Co are elements that are effective in maintaining strength, while Si and Mn are deoxidizing agents and are useful for adjusting the O and S concentrations. Ni is useful as an element that stabilizes the austenite phase, while Cr and Mo improve corrosion resistance, such as pitting corrosion and crevice corrosion resistance. Fe is an inexpensive element that plays a role in adjusting the liquidus temperature and strength.

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

[0035] 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.

[0036] 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]

[0037] 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, chromium, molybdenum, niobium, tungsten, 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 6 and the comparative powders Nos. 1 to 10. 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 Ni-Cr-Mo-Fe-based Ni-based alloys cast using these powders for Examples 1 to 12 and Comparative Examples 13 to 23.

[0038] 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.

[0039] 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.

[0040] 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.

[0041] Powder particle size: Weight percentage of particles in the range of 0.15 to 0.85 mm Approximately 200 g of powder was sieved using metal test sieves (JIS Z 8801) with wire mesh openings of 0.15 mm and 0.85 mm.

[0042] 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 unavoidable impurities such as P, S, O, H, N, and Cu. In addition, "-" in the components of Table 2 indicates that no additives were added.

[0043] 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.

[0044] The evaluation method for slab surface quality is as follows. Slabs (8 m long) from the start of continuous casting to the casting length of 10 to 18 m were ground with a grinder at a uniform grinding amount of 1% of the slab thickness. Then, penetrant testing was performed, magnified observation with a camera, and the surface was scanned with an eddy current probe to evaluate the number of subsurface cracks per slab. When subsurface cracks were absent, good yields were achieved because minimal grinding was required to remove surface oxide scale and oscillation marks. Furthermore, since no additional grinding steps were required in the cold rolling process, good manufacturing costs were achieved. Furthermore, various studies conducted during the research and development of the powder of the present invention have shown that the fewer subsurface cracks there are, the lower the crack depth. For cracks between 1 and 20 per slab, an additional 1% of grinding was sufficient to remove all cracks. Although the grinding yield was slightly reduced, the absence of additional grinding steps in the cold rolling process ensured satisfactory manufacturing costs. When the number of cracks per slab is 21 or more, the correlation between the number of cracks and their depth weakens, and even if the amount of grinding aimed at completely removing the cracks is reduced, some cracks remain. As a result, the yield drops significantly, additional steps are required in the cold rolling process, and the manufacturing cost is not acceptable. From these perspectives, the slab surface quality was evaluated on the following three-point scale. ○: No subepidermal cracks △: Number of subsurface cracks: 1-20 / slab (8m) ×: Number of subsurface cracks ≧ 21 / slab (8m)

[0045] In this invention, we also investigated the powder film after casting. First, we measured the powder film thickness with a micrometer. Then, we embedded the powder film in resin, polished the cross section, and analyzed it with an SEM and attached EDS. We calculated the percentage of the crystalline phase in the total powder film thickness from the thickness of the crystalline phase, and confirmed the constituent crystalline phases. The appropriate crystalline structures are 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 are explained below. In the table, parentheses ( ) indicate values ​​that do not satisfy the essential elements of the present invention (defined in the independent claims), and parentheses [ ] indicate values ​​that do not satisfy the preferred elements (defined in the dependent claims). First, examples Nos. 1 to 12 are explained below. Examples Nos. 1 to 5 used invention powders 1 to 5, whose powder composition and physical properties met the ranges of the present invention. Furthermore, the alloy casting conditions and post-cast powder film properties all met the ranges of the present invention. Therefore, no subsurface cracks were detected in the slab, resulting in a favorable rating of ○. Examples Nos. 6 to 12 all met the specified ranges for powder composition, viscosity at 1300°C, and solidification temperature, but some items were outside the preferred ranges. Example No. 6 was rated △ because it used invention powder 6, which did not satisfy the powder particle size (particle weight ratio) range of 0.15 to 0.85 mm. Examples Nos. 7 and 8 were rated △ because the drawing speed of the casting conditions was outside the preferred range. Nos. 9 and 10 were rated △ because the superheat of the molten alloy in the casting conditions was outside the preferred range. Additionally, Nos. 11 and 12 were rated △ because the liquidus temperature range of the Ni-based alloy was outside the preferred range.

[0049] Next, we will explain Nos. 13 to 23, which are comparative examples. In No. 13, the concentrations of CaO, SiO2, Na2O, Al2O3, Li2O, and F in the powder were outside the specified range, resulting in a low basicity, and therefore a low viscosity and solidification temperature at 1,300°C. As a result, the oscillation marks were deep, the powder film was thick (over 3.0 mm), and only a small proportion of the crystalline phase formed, resulting in strong and uneven cooling of the solidified shell, resulting in an × result. Vertical cracks also occurred.

[0050] For No. 14, the concentrations of CaO, SiO2, Na2O, Al2O3, Li2O, and F in the powder were outside the specified range, and the basicity was high, which resulted in high viscosity and solidification temperature at 1300°C. As a result, the powder's melting and flow properties deteriorated, and the powder film thickness was thin and the crystalline phase ratio was too high, resulting in uneven cooling of the solidified shell and resulting in a "×" result. Vertical cracks also occurred.

[0051] In No. 15, the MgO concentration in the powder was low and fell outside the specified range. As a result, magnesiochromite could not be generated in the crystalline phase of the powder film. As a result, sufficient slow and uniform cooling effect could not be achieved, and the sample was rated as x.

[0052] In No. 16, the Cr2O3 concentration in the powder was low and fell outside the specified range. As a result, magnesiochromite could not be generated in the crystalline phase of the powder film, and sufficient slow and uniform cooling effect could not be achieved, resulting in an X result.

[0053] For No. 17, the MgO concentration in the powder was too high and fell outside the specified range. As a result, MgO was generated in addition to magnesiochromite in the crystalline phase of the powder film, which caused excessive slow cooling, making it take too long for the solidified shell to develop sufficient strength, resulting in significant subsurface cracking and resulting in an X rating.

[0054] For No. 18, the Cr2O3 concentration in the powder was too high and fell outside the specified range. As a result, Cr2O3 was generated in addition to magnesiochromite in the crystalline phase of the powder film, which caused excessive slow cooling, making it take too long for the solidified shell to develop sufficient strength, resulting in significant subsurface cracking and resulting in an X rating.

[0055] For No. 19, 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 caused 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.

[0056] No. 20 had a low carbon concentration in the powder, which fell outside the specified range. As a result, the powder melted too quickly, causing excessive flow of molten powder between the mold and solidified shell, which promoted uneven cooling and resulted in an X.

[0057] For No. 21, the powder had a high C concentration that fell outside the specified range. As a result, melting was too slow, resulting in many areas where the molten powder could not flow between the mold and solidified shell, which promoted uneven cooling and resulted in an X result.

[0058] For No. 22, the concentrations of all the components and physical properties of the powder except for C were outside the specified range, and the solidified shell was cooled so strongly and unevenly that deep vertical cracks occurred, resulting in an X rating.

[0059] In No. 23, the MgO and Cr2O3 concentrations in the powder were high and outside the specified range, and the casting conditions were outside the optimum ranges for liquidus temperature, withdrawal speed, and superheat of the molten metal, which resulted in poor flow of the molten powder between the mold and solidified shell, causing significant uneven cooling to the point of vertical cracks, and was therefore rated as x. [Industrial Applicability]

[0060] According to the technology of the present invention, it is possible to obtain slabs in which subsurface cracking of the slabs is suppressed, which contributes to improving the yield of Ni-Cr-Mo-Fe Ni-based alloys and reducing the manufacturing costs.

Claims

1. A mold powder used in 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, 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 to 3%, C: 0.3 to 5%, and unavoidable impurities, and the basicity (in mass % ratio CaO / SiO ２ ) is 0.80 to 1.30, viscosity at 1300°C is 0.5 to 2.0 poise, and 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. 3. A method for continuous casting of a Ni-Cr-Mo-Fe-based 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-Cr-Mo-Fe-based Ni-based alloy having a liquidus temperature of 1320 to 1400°C under the conditions of a withdrawal speed of 300 to 900 mm / min and a superheat of the molten alloy of 20 to 60°C.

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