Casting powder for inhibiting stainless steel black belt defect and preparation process thereof

Abstract

The invention relates to the technical field of casting powder, in particular to casting powder for inhibiting stainless steel black strip defects and a preparation process thereof, and the casting powder comprises the following raw materials: wollastonite, spodumene, composite carbon black, yttrium oxide stabilized rare earth microspheres, calcium borate and calcium phosphate. The SiO2 coating layer physically isolates carbon from being in contact with molten steel in a low-temperature stage, so that early-stage carbon migration is effectively blocked; and in the high-temperature stage, the coating layer reacts with CaO to gradually release carbon black, the slag forming rhythm is matched, the melting speed fluctuation is reduced, and the situation that a casting blank makes contact with a carbon-rich layer due to insufficient supplementing of a liquid slag layer is avoided, so that the color difference / peeling-off-shaped black belt defect caused by carbide precipitation and grain boundary corrosion in subsequent rolling is reduced. The cerium / lanthanum active component is wrapped by the yttrium oxide shell, the synchronous effect of high-temperature slow-release deoxidation and sulfur fixation is achieved, Al2O3 and MnS inclusions are effectively reduced, and therefore the inclusion enrichment type black belt defect is reduced.

Description

Technical Field

[0001] The present invention relates to the technical field of protective slag, in particular to a protective slag for suppressing black belt defects in stainless steel and a preparation process thereof. Background Art

[0002] During the continuous casting process of stainless steel, the protective slag forms a lubricating layer between the crystallizer wall and the solidified billet shell, effectively reducing the friction coefficient between the two, thereby reducing the resistance to billet drawing, preventing the billet shell from sticking to the crystallizer wall, and avoiding accidents such as leakage; at the same time, the lubricating layer helps the billet to be pulled out of the crystallizer evenly and smoothly, improving the surface quality of the billet and reducing the occurrence of defects such as surface cracks; in addition, the protective slag has good adsorption capacity and can capture and accommodate non-metallic inclusions precipitated from the molten steel, thereby reducing the inclusion content in the molten steel, improving the purity of the molten steel, and reducing the risk of surface defects such as black bands caused by inclusions.

[0003] However, when the free carbon content in the mold slag is too high, the melting rate will slow down significantly, resulting in insufficient replenishment of the liquid slag layer, causing the ingot to directly contact the carbon-rich layer, triggering carbide precipitation or grain boundary corrosion during the subsequent rolling process, manifesting as black band defects with color difference or peeling. In view of this, we propose a mold slag and its preparation process for suppressing black band defects in stainless steel. Summary of the Invention

[0004] The purpose of the present invention is to provide a protective slag and a preparation process for suppressing black band defects in stainless steel, so as to solve the problem proposed in the above background technology that when the free carbon content in the protective slag is too high, the melting rate will be significantly slowed down, resulting in insufficient replenishment of the liquid slag layer, causing the ingot to directly contact the carbon-rich layer, causing carbide precipitation or grain boundary corrosion in the subsequent rolling process, and manifesting as color-difference or peeling-like black band defects.

[0005] To achieve the above object, the present invention provides a protective slag for suppressing black band defects in stainless steel, comprising the following raw materials: wollastonite, spodumene, composite carbon black, yttrium oxide stabilized rare earth microspheres, calcium borate and calcium phosphate; The composite carbon black is prepared by coating carbon black with silica sol; Yttrium oxide stabilized rare earth microspheres are prepared by atomizing a mixed solution of cerium nitrate, lanthanum nitrate and yttrium nitrate and then pyrolyzing it at high temperature.

[0006] Preferably, the wollastonite comprises 30-45 parts by weight, spodumene 8-12 parts by weight, composite carbon black 4-6 parts by weight, yttrium oxide stabilized rare earth microspheres 5-8 parts by weight, calcium borate 8-12 parts by weight and calcium phosphate 2-3 parts by weight.

[0007] Preferably, the preparation steps of the composite carbon black are: Add carbon black with a particle size of 40-60 nm to a 20% ethanol solution, and perform ultrasonic treatment at a power of 200-300 W at 25° C. for 20-30 minutes; add ethyl orthosilicate to the carbon black, and dropwise add 28% ammonia water at a rate of 1-2 mL / min, adjust the pH to 4, and stir at a speed of 180-220 rpm at 60° C. for 5-6 hours; heat to 80° C., continue stirring at a speed of 180-220 rpm for 1.5-2 hours to obtain a reaction solution; centrifuge the reaction solution at a speed of 2000-3000 rpm for 10-15 minutes, wash with ethanol 3-4 times, and finally obtain a composite carbon black with a coating layer thickness of 50-100 nm by spray drying.

[0008] In the low-temperature stage, the dense SiO2 coating on the surface of carbon black forms a physical barrier, blocking the early contact between carbon black and molten steel / slag, and preventing carbon from migrating to the surface of the ingot in advance in the low-temperature zone; in the high-temperature stage, the coating reacts with CaO in the protective slag, gradually releasing the internal carbon black, achieving a precise match with the slag formation rhythm, ensuring the continuous replenishment of the liquid slag layer, and preventing the ingot from contacting the carbon-rich layer.

[0009] Preferably, the added amount of ethyl orthosilicate accounts for 18-22% of the mass of carbon black.

[0010] Preferably, the inlet air temperature of the spray drying is 150-170° C., the outlet air temperature is 75-85° C., and the atomization pressure is 0.2-0.3 MPa.

[0011] Preferably, the preparation steps of the yttrium oxide stabilized rare earth microspheres are: Cerium nitrate, lanthanum nitrate, and yttrium nitrate are dissolved in deionized water in a molar ratio of 6:3:1 to obtain a nitrate solution with a concentration of 0.5 mol / L; polyvinyl pyrrolidone is added to the nitrate solution, and the mixture is stirred at a speed of 150-250 rpm for 1.5-2.5 hours to obtain a mixed solution; the mixed solution is sprayed into a preheating zone at 200°C through an ultrasonic atomizer to form droplets with a particle size of 20-50 μm; the droplets enter a tube furnace, and the temperature is increased to 500°C at a rate of 1.8-2.2°C / min and maintained for 20-40 minutes, and then increased to 900°C and maintained for 0.8-1.2 hours to obtain a product; the product is ball milled at a ball-to-material ratio of 6:1, milled at a speed of 180-220 rpm for 1.8-2.2 hours, and then sieved through a 500-1500 mesh sieve to obtain yttrium oxide-stabilized rare earth microspheres with a thickness of 1-2 μm.

[0012] The yttrium oxide shell releases the internal cerium / lanthanum active components in the high-temperature zone of continuous casting, prolongs the deoxidation time, and continuously captures free oxygen in the molten steel, thereby reducing the formation of Al2O3 inclusions; at the same time, lanthanum combines with sulfur to form a stable La2O2S phase, effectively blocking the grain boundary corrosion-type black band caused by MnS inclusions.

[0013] Preferably, the added amount of polyvinyl pyrrolidone accounts for 1.8-2.2% of the mass of the nitrate solution.

[0014] Preferably, the atomization frequency of the ultrasonic atomizer is 1.7-1.9 MHz, and the nitrogen flow rate is 0.8-1.2 m 3 / h.

[0015] On the other hand, the present invention provides a process for preparing a protective slag for suppressing black band defects in stainless steel, which is used to prepare the protective slag for suppressing black band defects in stainless steel, comprising the following steps: S1.1. Weigh the following raw materials in parts by weight: 30-45 parts by weight of wollastonite, 8-12 parts by weight of spodumene, 4-6 parts by weight of composite carbon black, 5-8 parts by weight of yttrium oxide-stabilized rare earth microspheres, 8-12 parts by weight of calcium borate, and 2-3 parts by weight of calcium phosphate; S1.2. Mix wollastonite and spodumene at a speed of 20-25 rpm for 5-10 minutes; add calcium borate and calcium phosphate and continue mixing for 10-15 minutes to obtain a dry mix base material; The composite carbon black and yttrium oxide stabilized rare earth microspheres were mixed for 5-10 minutes, and then added to the dry mixed base material three times with an interval of 2 minutes each time to obtain a mixture; S1.3. Place the mixed material into a rotary kiln and pre-calcine it at 640-660°C for 25-35 minutes at a speed of 2-3 rpm under nitrogen to obtain a pre-calcined material; S1.4. Add the pre-burned material to deionized water and ball mill for 1.5-2 hours, then centrifugal spray drying with an air inlet temperature of 280-300°C, an outlet temperature of 100-110°C, and an atomizing disk speed of 12000-15000 rpm to obtain a protective slag that suppresses black band defects in stainless steel.

[0016] Preferably, in S1.4, the volume ratio of the pre-burned material to the deionized water is 30-35%.

[0017] Compared with the prior art, the present invention has the following beneficial effects: 1. In the protective slag for suppressing black band defects in stainless steel and its preparation process, the SiO2 coating layer physically isolates carbon from contact with molten steel in the low-temperature stage, effectively blocking early carbon migration; in the high-temperature stage, the coating layer reacts with CaO to gradually release carbon black, matching the slag formation rhythm, reducing melting rate fluctuations, and avoiding the ingot contacting the carbon-rich layer due to insufficient replenishment of the liquid slag layer, thereby reducing color difference / flaking black band defects caused by carbide precipitation and grain boundary corrosion in subsequent rolling.

[0018] 2. In this protective slag for suppressing black band defects in stainless steel and its preparation process, the cerium / lanthanum active components are wrapped in an yttrium oxide shell to achieve simultaneous high-temperature slow-release deoxidation and sulfur fixation, effectively reducing Al2O3 and MnS inclusions, thereby reducing inclusion-enriched black band defects; at the same time, yttrium oxide delays rare earth deactivation, maintains stable slag viscosity, and avoids carburization and uneven slag film problems caused by local lubrication failure. DETAILED DESCRIPTION

[0019] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0020] The present invention provides a protective slag for suppressing black band defects in stainless steel, comprising the following raw materials: wollastonite, spodumene, composite carbon black, yttrium oxide stabilized rare earth microspheres, calcium borate and calcium phosphate; The composite carbon black is prepared by coating carbon black with silica sol; Yttrium oxide stabilized rare earth microspheres are prepared by atomizing a mixed solution of cerium nitrate, lanthanum nitrate and yttrium nitrate and then pyrolyzing it at high temperature.

[0021] The preparation steps of yttrium oxide stabilized rare earth microspheres are as follows: Cerium nitrate, lanthanum nitrate and yttrium nitrate were dissolved in deionized water at a molar ratio of 6:3:1 to obtain a nitrate solution with a concentration of 0.5 mol / L. Polyvinylpyrrolidone (2.0% by mass of the nitrate solution) was added to the nitrate solution and stirred at 200 rpm for 2 h to obtain a mixed solution. The mixed solution was sprayed into a preheating zone at 200°C using an ultrasonic atomizer with an atomization frequency of 1.8 MHz and a nitrogen flow rate of 1.0 m / s. 3 / h to form droplets with a particle size of 30 μm; the droplets enter a tubular furnace, the temperature is increased to 500°C at a rate of 2.0°C / min and kept warm for 40 minutes, and then increased to 900°C and kept warm for 1 hour to obtain the product; the product is ball-milled at a ball-to-material ratio of 6:1, ground at a speed of 200 rpm for 2 hours, and then sieved through an 800-mesh sieve to obtain yttrium oxide-stabilized rare earth microspheres with a thickness of 2 μm.

[0022] Example 1: A process for preparing a mold slag for suppressing black band defects in stainless steel, comprising the following steps: S1.1. Weigh the following raw materials in parts by weight: 30 parts by weight of wollastonite, 8 parts by weight of spodumene, 4 parts by weight of composite carbon black, 5 parts by weight of yttrium oxide-stabilized rare earth microspheres, 8 parts by weight of calcium borate, and 2 parts by weight of calcium phosphate; S1.2. Mix wollastonite and spodumene at 20 rpm for 10 min; add calcium borate and calcium phosphate and continue mixing for 15 min to obtain a dry mix base; The composite carbon black and yttrium oxide stabilized rare earth microspheres were mixed for 10 minutes, and then added to the dry mixed base material three times with an interval of 2 minutes each time to obtain a mixture; S1.3. Place the mixed material in a rotary kiln and pre-calcine it at 660°C at 3 rpm for 35 min under nitrogen to obtain a pre-calcined material; S1.4. The pre-calcined material was added to deionized water (35% by volume) and ball-milled for 2 h. The material was then centrifugally spray-dried at an inlet air temperature of 290°C, an outlet air temperature of 105°C, and an atomizing disk speed of 13,000 rpm to obtain a protective slag that suppresses black band defects in stainless steel.

[0023] The preparation steps of composite carbon black are: Carbon black with a particle size of 50 nm was added to a 20% mass concentration ethanol solution and ultrasonically treated at a power of 200 W at 25°C for 30 min; ethyl orthosilicate (accounting for 18% of the mass of carbon black) was added to the carbon black, and 28% mass concentration of ammonia water was added dropwise at a rate of 2 mL / min, the pH was adjusted to 4, and stirred at a speed of 200 rpm at 60°C for 5 h; the temperature was raised to 80°C and the stirring was continued at a speed of 200 rpm for 2 h to obtain a reaction liquid; the reaction liquid was centrifuged at a speed of 2000 rpm for 15 min, washed with ethanol 3 times, and finally spray-dried with an inlet air temperature of 160°C, an outlet air temperature of 80°C, and an atomization pressure of 0.3 MPa to obtain a composite carbon black with a coating layer thickness of 50 nm.

[0024] Example 2: A process for preparing a mold slag for suppressing black band defects in stainless steel, comprising the following steps: S1.1. Weigh the following raw materials in parts by weight: 37 parts by weight of wollastonite, 10 parts by weight of spodumene, 5 parts by weight of composite carbon black, 6.5 parts by weight of yttrium oxide-stabilized rare earth microspheres, 10 parts by weight of calcium borate, and 2.5 parts by weight of calcium phosphate; S1.2. Mix wollastonite and spodumene at 20 rpm for 10 min; add calcium borate and calcium phosphate and continue mixing for 15 min to obtain a dry mix base; The composite carbon black and yttrium oxide stabilized rare earth microspheres were mixed for 10 minutes, and then added to the dry mixed base material three times with an interval of 2 minutes each time to obtain a mixture; S1.3. Place the mixed material in a rotary kiln and pre-calcine it at 660°C at 3 rpm for 35 min under nitrogen to obtain a pre-calcined material; S1.4. The pre-calcined material was added to deionized water (35% by volume) and ball-milled for 2 h. The material was then centrifugally spray-dried at an inlet air temperature of 290°C, an outlet air temperature of 105°C, and an atomizing disk speed of 13,000 rpm to obtain a protective slag that suppresses black band defects in stainless steel.

[0025] The preparation steps of composite carbon black are: Carbon black with a particle size of 50 nm was added to a 20% ethanol solution and ultrasonically treated at a power of 200 W at 25°C for 30 min; ethyl orthosilicate (accounting for 20% of the mass of carbon black) was added to the carbon black, and ammonia water with a mass concentration of 28% was added dropwise at a rate of 2 mL / min, the pH was adjusted to 4, and stirred at a speed of 200 rpm at 60°C for 5 h; the temperature was raised to 80°C and the stirring was continued at a speed of 200 rpm for 2 h to obtain a reaction liquid; the reaction liquid was centrifuged at a speed of 2000 rpm for 15 min, washed with ethanol 3 times, and finally spray-dried with an inlet air temperature of 160°C, an outlet air temperature of 80°C, and an atomization pressure of 0.3 MPa to obtain a composite carbon black with a coating layer thickness of 70 nm.

[0026] Example 3: A process for preparing a mold slag for suppressing black band defects in stainless steel, comprising the following steps: S1.1. Weigh the following raw materials in parts by weight: 45 parts by weight of wollastonite, 12 parts by weight of spodumene, 6 parts by weight of composite carbon black, 8 parts by weight of yttrium oxide-stabilized rare earth microspheres, 12 parts by weight of calcium borate, and 3 parts by weight of calcium phosphate; S1.2. Mix wollastonite and spodumene at 20 rpm for 10 min; add calcium borate and calcium phosphate and continue mixing for 15 min to obtain a dry mix base; The composite carbon black and yttrium oxide stabilized rare earth microspheres were mixed for 10 minutes, and then added to the dry mixed base material three times with an interval of 2 minutes each time to obtain a mixture; S1.3. Place the mixed material in a rotary kiln and pre-calcine it at 660°C at 3 rpm for 35 min under nitrogen to obtain a pre-calcined material; S1.4. The pre-calcined material was added to deionized water (35% by volume) and ball-milled for 2 h. The material was then centrifugally spray-dried at an inlet air temperature of 290°C, an outlet air temperature of 105°C, and an atomizing disk speed of 13,000 rpm to obtain a protective slag that suppresses black band defects in stainless steel.

[0027] The preparation steps of composite carbon black are: Carbon black with a particle size of 50 nm was added to a 20% ethanol solution and ultrasonically treated at a power of 200 W at 25°C for 30 min; ethyl orthosilicate (accounting for 22% of the mass of carbon black) was added to the carbon black, and 28% ammonia water was added dropwise at a rate of 2 mL / min, the pH was adjusted to 4, and the mixture was stirred at 200 rpm at 60°C for 5 h; the temperature was raised to 80°C and the stirring was continued at 200 rpm for 2 h to obtain a reaction solution; the reaction solution was centrifuged at 2000 rpm for 15 min, washed with ethanol 3 times, and finally spray-dried with an inlet air temperature of 160°C, an outlet air temperature of 80°C, and an atomization pressure of 0.3 MPa to obtain a composite carbon black with a coating layer thickness of 100 nm.

[0028] Example 4: A process for preparing a mold slag for suppressing black band defects in stainless steel, comprising the following steps: S1.1. Weigh the following raw materials in parts by weight: 37 parts by weight of wollastonite, 10 parts by weight of spodumene, 8 parts by weight of composite carbon black, 6.5 parts by weight of yttrium oxide-stabilized rare earth microspheres, 10 parts by weight of calcium borate, and 2.5 parts by weight of calcium phosphate; S1.2. Mix wollastonite and spodumene at 20 rpm for 10 min; add calcium borate and calcium phosphate and continue mixing for 15 min to obtain a dry mix base; The composite carbon black and yttrium oxide stabilized rare earth microspheres were mixed for 10 minutes, and then added to the dry mixed base material three times with an interval of 2 minutes each time to obtain a mixture; S1.3. Place the mixed material in a rotary kiln and pre-calcine it at 660°C at 3 rpm for 35 min under nitrogen to obtain a pre-calcined material; S1.4. The pre-calcined material was added to deionized water (35% by volume) and ball-milled for 2 h. The material was then centrifugally spray-dried at an inlet air temperature of 290°C, an outlet air temperature of 105°C, and an atomizing disk speed of 13,000 rpm to obtain a protective slag that suppresses black band defects in stainless steel.

[0029] The preparation steps of composite carbon black are: Carbon black with a particle size of 50 nm was added to a 20% ethanol solution and ultrasonically treated at a power of 200 W at 25°C for 30 min; ethyl orthosilicate (accounting for 20% of the mass of carbon black) was added to the carbon black, and ammonia water with a mass concentration of 28% was added dropwise at a rate of 2 mL / min, the pH was adjusted to 4, and stirred at a speed of 200 rpm at 60°C for 5 h; the temperature was raised to 80°C and the stirring was continued at a speed of 200 rpm for 2 h to obtain a reaction liquid; the reaction liquid was centrifuged at a speed of 2000 rpm for 15 min, washed with ethanol 3 times, and finally spray-dried with an inlet air temperature of 160°C, an outlet air temperature of 80°C, and an atomization pressure of 0.3 MPa to obtain a composite carbon black with a coating layer thickness of 70 nm.

[0030] Example 5: A process for preparing a mold slag for suppressing black band defects in stainless steel, comprising the following steps: S1.1. Weigh the following raw materials in parts by weight: 37 parts by weight of wollastonite, 10 parts by weight of spodumene, 5 parts by weight of composite carbon black, 10 parts by weight of yttrium oxide-stabilized rare earth microspheres, 10 parts by weight of calcium borate, and 2.5 parts by weight of calcium phosphate; S1.2. Mix wollastonite and spodumene at 20 rpm for 10 min; add calcium borate and calcium phosphate and continue mixing for 15 min to obtain a dry mix base; The composite carbon black and yttrium oxide stabilized rare earth microspheres were mixed for 10 minutes, and then added to the dry mixed base material three times with an interval of 2 minutes each time to obtain a mixture; S1.3. Place the mixed material in a rotary kiln and pre-calcine it at 660°C at 3 rpm for 35 min under nitrogen to obtain a pre-calcined material; S1.4. The pre-calcined material was added to deionized water (35% by volume) and ball-milled for 2 h. The material was then centrifugally spray-dried at an inlet air temperature of 290°C, an outlet air temperature of 105°C, and an atomizing disk speed of 13,000 rpm to obtain a protective slag that suppresses black band defects in stainless steel.

[0031] The preparation steps of composite carbon black are: Carbon black with a particle size of 50 nm was added to a 20% ethanol solution and ultrasonically treated at a power of 200 W at 25°C for 30 min; ethyl orthosilicate (accounting for 20% of the mass of carbon black) was added to the carbon black, and ammonia water with a mass concentration of 28% was added dropwise at a rate of 2 mL / min, the pH was adjusted to 4, and stirred at a speed of 200 rpm at 60°C for 5 h; the temperature was raised to 80°C and the stirring was continued at a speed of 200 rpm for 2 h to obtain a reaction liquid; the reaction liquid was centrifuged at a speed of 2000 rpm for 15 min, washed with ethanol 3 times, and finally spray-dried with an inlet air temperature of 160°C, an outlet air temperature of 80°C, and an atomization pressure of 0.3 MPa to obtain a composite carbon black with a coating layer thickness of 70 nm.

[0032] Example 6: A process for preparing a mold slag for suppressing black band defects in stainless steel, comprising the following steps: S1.1. Weigh the following raw materials in parts by weight: 37 parts by weight of wollastonite, 10 parts by weight of spodumene, 5 parts by weight of composite carbon black, 6.5 parts by weight of yttrium oxide-stabilized rare earth microspheres, 10 parts by weight of calcium borate, and 2.5 parts by weight of calcium phosphate; S1.2. Mix wollastonite and spodumene at 20 rpm for 10 min; add calcium borate and calcium phosphate and continue mixing for 15 min to obtain a dry mix base; The composite carbon black and yttrium oxide stabilized rare earth microspheres were mixed for 10 minutes, and then added to the dry mixed base material three times with an interval of 2 minutes each time to obtain a mixture; S1.3. Place the mixed material in a rotary kiln and pre-calcine it at 660°C at 3 rpm for 35 min under nitrogen to obtain a pre-calcined material; S1.4. The pre-calcined material was added to deionized water (35% by volume) and ball-milled for 2 h. The material was then centrifugally spray-dried at an inlet air temperature of 290°C, an outlet air temperature of 105°C, and an atomizing disk speed of 13,000 rpm to obtain a protective slag that suppresses black band defects in stainless steel.

[0033] The preparation steps of composite carbon black are: Carbon black with a particle size of 50 nm was added to a 20% ethanol solution and ultrasonically treated at a power of 200 W at 25°C for 30 minutes; ethyl orthosilicate (accounting for 15% of the mass of carbon black) was added to the carbon black, and 28% ammonia water was added dropwise at a rate of 2 mL / min, the pH was adjusted to 4, and the mixture was stirred at 200 rpm at 60°C for 5 hours; the temperature was raised to 80°C and the stirring was continued at 200 rpm for 2 hours to obtain a reaction solution; the reaction solution was centrifuged at a speed of 2000 rpm for 15 minutes, washed with ethanol three times, and finally spray-dried with an inlet air temperature of 160°C, an outlet air temperature of 80°C, and an atomization pressure of 0.3 MPa to obtain a composite carbon black with a coating layer thickness of 70 nm.

[0034] Comparative Example 1: The method of Example 2 was adopted, but carbon black was used directly instead of composite carbon black in a process for preparing protective slag for suppressing black band defects in stainless steel.

[0035] Comparative Example 2: Using the method of Example 2, in a process for preparing protective slag for suppressing black band defects in stainless steel, yttrium oxide-stabilized rare earth microspheres were not used, and cerium oxide / lanthanum oxide rare earth oxides were directly used.

[0036] Comparative Example 3: The method of Example 2 was adopted, but calcium borate was not added in the preparation process of a protective slag for suppressing black band defects in stainless steel.

[0037] The present invention adds a protective slag prepared by composite carbon black and yttrium oxide stabilized rare earth microspheres to the stainless steel during the preparation process, and the performance index inspection items and inspection standards of the stainless steel obtained by the protective slag for suppressing the stainless steel black band defect are as follows: Three hot-rolled or cold-rolled stainless steel plates (1m×1m) from the same batch were pickled (HNO3:HF=3:1, 60℃, 5min) to remove the oxide film. A 5% potassium ferricyanide solution was then sprayed on the plates and allowed to stand for 3min to cause the black band defects to appear blue due to carburization or inclusion enrichment. Software was used to identify the blue area and calculate the black band defect area ratio (black band defect rate = black band defect area / total surface area × 100%).

[0038] A 20×20 mm sample was taken by wire cutting, and the surface was ground to Ra ≤ 0.1 μm (using sandpaper in stages to 2000#), and ultrasonic cleaning was performed with ethanol to avoid sampling the heat-affected zone (> 10 mm from the edge). Subsequently, a glow discharge spectrometer was used to perform sputtering analysis on the surface, and the carbon signal intensity was simultaneously detected. The parameters of the glow discharge spectrometer were argon pressure 600 Pa, power 35 W, and sputtering rate 0.3 μm / min (calibrated with a pure iron standard sample). The carbon concentration-depth curve was converted by the sputtering crater depth (measured using a step profiler), and a quantitative integral calculation was performed in the depth range of 0-50 μm to obtain the surface carbon content.

[0039] According to the above standards, the stainless steels prepared by the mold slags for suppressing the black band defects of stainless steel in the above Examples 1-6 and Comparative Examples 1-3 were tested, and the obtained data are shown in Table 1: Table 1 Performance data of stainless steel of Examples 1-6 and Comparative Examples 1-3

[0040] It can be seen from Examples 1-3 and 4 that: when the other components in the protective slag for suppressing black band defects in stainless steel remain unchanged and the weight of the composite carbon black continues to increase, the black band defect rate and the surface carbon content of the stainless steel first decrease and then increase; an appropriate increase in the composite carbon black can improve the slag foaming efficiency, promote the reaction of SiO2 and CaO to form calcium silicate, enhance the stability and covering ability of the liquid slag layer, and thus effectively isolate the ingot from the carbon-rich layer; however, excessive carbon black will lead to insufficient CaO required for the SiO2 coating layer, premature exposure of part of the carbon black, premature carbon migration, and increased risk of the ingot contacting the carbon-rich layer, causing the surface carbon content to rebound; in addition, unreacted SiO2 remains as solid particles, significantly increasing the slag viscosity, reducing fluidity, forming a local covering blind area, and inducing slag curls and inclusion-enriched black bands.

[0041] Furthermore, by comparing Examples 1-3 with Example 5, it can be seen that: when the other components in the protective slag for suppressing black band defects in stainless steel remain unchanged and the weight of yttrium oxide stabilized rare earth microspheres continues to increase, the black band defect rate and surface carbon content of the stainless steel first decrease and then increase; an appropriate increase in the amount of microspheres can increase the release of active Ce / La, thereby strengthening the deoxidation and desulfurization effects, reducing Al2O3 and MnS inclusions, improving the cleanliness of the molten steel, and effectively reducing the inclusion-enriched black bands caused by slag rolls; in addition, the continuous stable slag of the yttrium oxide shell The structure helps to evenly cover the liquid slag layer and block the carbon migration channel; however, excessive microspheres are locally enriched in the slag, and the yttrium oxide shell is not completely dissolved and sintered and agglomerated, which hinders the release of Ce / La ions and causes a decrease in deoxidation efficiency; unconsumed oxygen combines with aluminum in the steel to form Al2O3 cluster inclusions, which trigger inclusion-type black bands; at the same time, excessive rare earth oxides neutralize the alkalinity of CaO, reduce the effective alkalinity, cause a sudden increase in the viscosity of the high Al2O3 slag, deteriorate the fluidity, reduce the crystallization rate of the slag film, fail to lubricate, and induce longitudinal cracking and carburization.

[0042] Furthermore, by comparing Examples 1-3 with Example 6, it can be seen that: in the protective slag for suppressing black band defects in stainless steel, as the addition amount of ethyl orthosilicate continues to increase, the black band defect rate and the surface carbon content of the stainless steel first decrease and then increase; when the addition amount of ethyl orthosilicate reaches a certain value, the SiO2 generated by hydrolysis can completely cover the surface of the carbon black to form a uniform coating layer, thereby extending the release delay time of the carbon black and effectively isolating the ingot from the carbon-rich layer; SiO2 reacts quantitatively with CaO to generate calcium silicate, maintains the alkalinity of the slag, and ensures the solubility of Al2O3; however, excessive ethyl orthosilicate will cause the SiO2 layer to be too thick, requiring a higher temperature to completely react with CaO, and the actual continuous casting temperature is insufficient, resulting in part of the carbon black being permanently embedded and the melting rate being slowed down; unreacted SiO2 particles remain, causing the slag viscosity to increase, thereby inducing the slag curling phenomenon.

[0043] According to the above test experiments, embodiment 2 is regarded as the optimal embodiment; By comparing Example 2 with Comparative Example 1, it can be seen that: when carbon black is used directly without using composite carbon black, the surface carbon content of the stainless steel is significantly increased; free carbon is released prematurely in the low-temperature zone and migrates to the surface of the ingot through gas phase infiltration and slag-molten steel interface diffusion paths, resulting in a sharp increase in the surface carbon content; excess carbon precipitates carbides at the grain boundaries, forming chromium-poor areas, which preferentially corrode the grain boundaries during pickling, appearing as continuous network-like black bands; in addition, the carbides have weak bonding with the matrix and are easily peeled off during rolling, forming peeled black bands; the high-carbon surface layer also reduces the pitting potential of the stainless steel, causing the black band defect to continue to worsen during subsequent heat treatment or pickling; traditional carbon black also leads to an increase in the black band rate, but the main reason is the indirect effect caused by carbon, such as the unstable combustion rate of free carbon and the fluctuation of the thickness of the liquid slag layer, which increases the probability of slag coiling.

[0044] By comparing Example 2 with Comparative Example 2, it can be seen that: when yttria-stabilized rare earth microspheres are not used and cerium oxide / lanthanum oxide rare earth oxides are directly used, the black band defect rate of stainless steel is significantly improved; traditional rare earth oxides are prone to internal sintering and agglomeration at high temperatures, resulting in a decrease in deoxidation efficiency; residual oxygen combines with aluminum to form Al2O3 cluster inclusions, and sulfur cannot be fixed as La2O2S, forming MnS chain precipitation (extended into strips after rolling), resulting in a surge in inclusion density, directly causing inclusion-rich black bands; in addition, the agglomerated rare earth oxides act as solid particles to increase the slag viscosity, reduce the slag film crystallization rate, induce lubrication failure and induce longitudinal cracks, and enrich the protective slag at the cracks to form carburized black bands; although traditional rare earth oxides lead to an increase in carbon content, its impact is relatively weak, mainly manifested in increased slag viscosity, uneven coverage of the liquid slag layer, local contact of the ingot with the carbon-rich layer, limited increase, and shallow depth of grain boundary corrosion.

[0045] A comparison of Example 2 and Comparative Example 3 shows that the black band defect rate of the stainless steel is significantly improved when calcium borate is not added. Pre-melted calcium borate can quickly dissolve floating Al2O3 inclusions in the molten steel by providing a low-temperature eutectic phase, thereby preventing their enrichment. In the absence of calcium borate, the solubility of Al2O3 will decrease, and solid Al2O3 clusters will easily form at the slag-steel interface. These clusters are entrained into the billet shell during solidification and extend into gray-black strips after rolling, namely black band defects. In addition, high-melting-point Al2O3 inclusions in the slag film form an insulating barrier, leading to localized overheating zones, triggering shrinkage cracks in the billet shell, and entering carburizing channels. At the same time, low-temperature zones lead to abnormal slag crystallization and lubrication failure, ultimately forming carbon penetration at the cracks, superimposed on the entrained slag, and forming a composite black band (color difference + spalling). Although the lack of calcium borate will lead to a slight increase in carbon content, its impact is relatively small.

[0046] The above shows and describes the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The above embodiments and descriptions are merely preferred examples of the present invention and are not intended to limit the present invention. Various changes and improvements may be made to the present invention without departing from the spirit and scope of the present invention. Such changes and improvements fall within the scope of the present invention. The scope of protection claimed in the present invention is defined by the appended claims and their equivalents.

Claims

1. A protective slag for suppressing black band defects in stainless steel, characterized in that: The raw materials include: wollastonite, spodumene, composite carbon black, yttrium oxide stabilized rare earth microspheres, calcium borate and calcium phosphate; The composite carbon black is prepared by coating carbon black with silica sol; Yttrium oxide stabilized rare earth microspheres are prepared by atomizing a mixed solution of cerium nitrate, lanthanum nitrate and yttrium nitrate and then pyrolyzing it at high temperature.

2. The mold slag for suppressing black band defects in stainless steel according to claim 1, characterized in that: The wollastonite is 30-45 parts by weight, the spodumene is 8-12 parts by weight, the composite carbon black is 4-6 parts by weight, the yttrium oxide stabilized rare earth microspheres are 5-8 parts by weight, the calcium borate is 8-12 parts by weight and the calcium phosphate is 2-3 parts by weight.

3. The mold slag for suppressing black band defects in stainless steel according to claim 2, characterized in that: The preparation steps of the composite carbon black are: Add carbon black with a particle size of 40-60 nm to a 20% ethanol solution, and perform ultrasonic treatment at a power of 200-300 W at 25° C. for 20-30 minutes; add ethyl orthosilicate to the carbon black, and dropwise add 28% ammonia water at a rate of 1-2 mL / min, adjust the pH to 4, and stir at a speed of 180-220 rpm at 60° C. for 5-6 hours; heat to 80° C., continue stirring at a speed of 180-220 rpm for 1.5-2 hours to obtain a reaction solution; centrifuge the reaction solution at a speed of 2000-3000 rpm for 10-15 minutes, wash with ethanol 3-4 times, and finally obtain a composite carbon black with a coating layer thickness of 50-100 nm by spray drying.

4. The mold slag for suppressing black band defects in stainless steel according to claim 3, characterized in that: The added amount of the ethyl orthosilicate accounts for 18-22% of the mass of the carbon black.

5. The mold slag for suppressing black band defects in stainless steel according to claim 3, characterized in that: The inlet air temperature of the spray drying is 150-170° C., the outlet air temperature is 75-85° C., and the atomization pressure is 0.2-0.3 MPa.

6. The mold slag for suppressing black band defects in stainless steel according to claim 2, characterized in that: The preparation steps of the yttrium oxide stabilized rare earth microspheres are: Cerium nitrate, lanthanum nitrate, and yttrium nitrate are dissolved in deionized water in a molar ratio of 6:3:1 to obtain a nitrate solution with a concentration of 0.5 mol / L; polyvinyl pyrrolidone is added to the nitrate solution, and the mixture is stirred at a speed of 150-250 rpm for 1.5-2.5 hours to obtain a mixed solution; the mixed solution is sprayed into a preheating zone at 200°C through an ultrasonic atomizer to form droplets with a particle size of 20-50 μm; the droplets enter a tube furnace, and the temperature is increased to 500°C at a rate of 1.8-2.2°C / min and maintained for 20-40 minutes, and then increased to 900°C and maintained for 0.8-1.2 hours to obtain a product; the product is ball milled at a ball-to-material ratio of 6:1, milled at a speed of 180-220 rpm for 1.8-2.2 hours, and then sieved through a 500-1500 mesh sieve to obtain yttrium oxide-stabilized rare earth microspheres with a thickness of 1-2 μm.

7. The mold slag for suppressing black band defects in stainless steel according to claim 6, characterized in that: The added amount of the polyvinyl pyrrolidone accounts for 1.8-2.2% of the mass of the nitrate solution.

8. The mold slag for suppressing black band defects in stainless steel according to claim 6, characterized in that: The atomization frequency of the ultrasonic atomizer is 1.7-1.9 MHz, and the nitrogen flow rate is 0.8-1.2 m 3 / h.

9. A process for preparing a mold slag for suppressing black band defects in stainless steel, for preparing the mold slag for suppressing black band defects in stainless steel as claimed in any one of claims 1 to 8, characterized in that: The preparation method of the protective slag for suppressing black band defects in stainless steel is as follows: S1.

1. Weigh the following raw materials in parts by weight: 30-45 parts by weight of wollastonite, 8-12 parts by weight of spodumene, 4-6 parts by weight of composite carbon black, 5-8 parts by weight of yttrium oxide-stabilized rare earth microspheres, 8-12 parts by weight of calcium borate, and 2-3 parts by weight of calcium phosphate; S1.

2. Mix wollastonite and spodumene at a speed of 20-25 rpm for 5-10 minutes; add calcium borate and calcium phosphate and continue mixing for 10-15 minutes to obtain a dry mix base material; The composite carbon black and yttrium oxide stabilized rare earth microspheres were mixed for 5-10 minutes, and then added to the dry mixed base material three times with an interval of 2 minutes each time to obtain a mixture; S1.

3. Place the mixed material into a rotary kiln and pre-calcine it at 640-660°C for 25-35 minutes at a speed of 2-3 rpm under nitrogen to obtain a pre-calcined material; S1.

4. Add the pre-burned material to deionized water and ball mill for 1.5-2 hours, then centrifugal spray drying with an air inlet temperature of 280-300°C, an outlet temperature of 100-110°C, and an atomizing disk speed of 12000-15000 rpm to obtain a protective slag that suppresses black band defects in stainless steel.

10. The process for preparing mold slag for suppressing black band defects in stainless steel according to claim 9, characterized in that: In the above-mentioned S1.4, the volume ratio of the pre-burned material to the deionized water is 30-35%.

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