A low thermal conductivity coating for the meniscus region of a continuous casting mold and a method for making the same

Abstract

This invention discloses a low thermal conductivity coating for the meniscus region of a continuous casting crystallizer. Its microstructure consists of a continuous metallic matrix phase and uniformly dispersed low thermal conductivity nanoparticles. The metallic matrix phase is either nickel-cobalt or a nickel-cobalt-chromium alloy, and the low thermal conductivity nanoparticles are zirconium oxide. The volume fraction of the low thermal conductivity nanoparticles in the coating is 5%–20%, and the average particle size is 20 nm–100 nm. This invention introduces nanoscale second-phase particles into a wear-resistant metallic matrix, which can significantly improve the coating hardness through pinning effects, etc. Furthermore, utilizing the high-density interfacial thermal resistance and dispersion strengthening effect formed by these particles, the equivalent thermal conductivity of the coating at the operating temperature is reduced to 8–20 W / (m·K), while maintaining a hardness of not less than 550 HV. This coating can effectively suppress heat flow in the meniscus region and slow down the uneven shrinkage of the initial billet shell, thereby significantly reducing the probability of surface depressions and longitudinal cracks in the billet, ensuring the wear-resistant life of the crystallizer while improving the surface quality of the billet.

Description

Technical Field

[0001] This invention relates to a low thermal conductivity and high wear-resistant coating on the surface of copper material in a continuous casting mold, specifically to a high wear-resistant composite coating for reducing the thermal conductivity of the meniscus region of a continuous casting mold and its preparation method, belonging to the field of steel continuous casting technology. Background Technology

[0002] The continuous casting mold is the core component for the initial solidification and forming of molten steel, playing a crucial role in both. In its meniscus region, the molten steel and the initial billet shell have low thermal resistance, rapid heat transfer, and fast solidification rates, leading to concentrated local heat flux and excessively high cooling intensity. This rapid solidification causes a drastic temperature drop and high thermal stress within the initial billet shell, interfering with the uniform filling of the protective slag melt and weakening the heat transfer and lubrication functions of the slag film, ultimately resulting in uneven circumferential and longitudinal growth of the billet shell. Existing research and practice have confirmed that microcracks on the billet surface often originate from non-ideal initial solidification behavior in the meniscus region, and this problem is particularly prominent in peritectic steels and other steels with significant shrinkage and low high-temperature strength. Surface cracks not only reduce the yield rate but can also induce leaks in severe cases. Therefore, appropriately suppressing the heat flux density in the meniscus region and delaying the initial solidification rate of the molten steel is of great significance for improving the surface quality of the billet and ensuring smooth continuous casting.

[0003] To reduce the heat flux density in the meniscus region, various technical approaches have been explored. For example, adjusting the cooling water slit structure and forced flow parameters of the crystallizer can influence heat transfer; however, controlling the cooling structure independently in the meniscus region is extremely difficult and impractical in industrial applications. Furthermore, under forced convection conditions, further increasing water pressure or flow velocity has very limited effect on reducing heat transfer efficiency. Another approach is to alter the structural parameters of the submerged entry nozzle (such as the inclination angle of the upward-sloping side holes and reducing the insertion depth) to change the molten steel flow field, thereby lowering the temperature of the molten steel in the upper part of the crystallizer and thus reducing heat transfer. However, this method has minimal effect on reducing heat flux density, and the decrease in molten steel temperature may adversely affect the melting and inflow of the protective slag, or even disrupt the normal flow field distribution within the crystallizer. In addition, the concept of a hot-top crystallizer, which increases thermal resistance by locally thickening the copper plate of the crystallizer, adding a thermal barrier coating, or machining surface grooves, is theoretically an effective method to reduce the initial solidification rate. However, these technologies have not yet achieved widespread industrial application due to reasons such as the large size of their mechanical structures making on-site deployment difficult, or their unsatisfactory thermal resistance reduction effects and complex processing and preparation. Although electromagnetic soft contact continuous casting technology can improve the initial solidification uniformity, its direct effect on reducing thermal resistance is not significant, and its high application cost has also prevented its industrialization.

[0004] In summary, existing technologies lack a functional coating that can simultaneously meet the requirements of high wear resistance and long service life for crystallizers while effectively reducing the thermal conductivity of the meniscus region. Therefore, developing a novel nanocomposite coating with both high wear resistance and low thermal conductivity for the meniscus region of continuous casting crystallizers is crucial to filling this technological gap. This coating has significant engineering application value in addressing the surface quality problems of cast billets caused by excessively rapid and uneven initial solidification. Summary of the Invention

[0005] In view of the above-mentioned shortcomings of the existing technology, the purpose of this invention is to provide a wear-resistant coating that reduces the thermal conductivity of the meniscus region of a continuous casting crystallizer, solves the problem that the crystallizer coating cannot simultaneously achieve wear resistance and low thermal conductivity, and further solves the problem of surface defects of the billet caused by excessive heat flow in the meniscus.

[0006] To achieve the above objectives, the present invention adopts the following technical solution:

[0007] A low thermal conductivity coating for the meniscus region of a continuous casting crystallizer is characterized in that its microstructure consists of a continuous metal matrix phase and uniformly dispersed low thermal conductivity nanoparticles therein; the metal matrix phase is one of nickel-cobalt or nickel-cobalt-chromium alloy, and the low thermal conductivity nanoparticles are zirconium oxide.

[0008] Furthermore, the volume fraction of the low thermal conductivity nanoparticles in the coating is 5% to 20%, and the average particle size of the nanoparticles is 20 nm to 100 nm.

[0009] Furthermore, the thickness of the wear-resistant coating is 0.1 mm to 1.0 mm.

[0010] The present invention also provides a method for preparing a low thermal conductivity coating in the meniscus region of a continuous casting crystallizer. The low thermal conductivity coating in the meniscus region of the continuous casting crystallizer has the composition described above and is prepared on the surface of a copper plate substrate in the meniscus region of the crystallizer using an ultrasonic-assisted composite electrodeposition process.

[0011] Furthermore, the electrodeposition process includes:

[0012] The pretreated (degreasing, pickling) copper plate of the crystallizer is used as the cathode and the nickel plate as the anode for electrodeposition in the plating bath; the electrodeposition temperature is 45℃~60℃, the cathode current density is 2~5 A / dm², and the electrodeposition time is 1~4 hours; ultrasonic-assisted dispersion is used during the electrodeposition process, with an ultrasonic frequency of 20 kHz~40 kHz and a power of 100W~300W.

[0013] Among them, the nickel plate is the metallic matrix phase, that is, one of nickel-cobalt or nickel-cobalt-chromium alloy.

[0014] The plating solution was prepared as follows: ZrO2 nanoparticles pre-dispersed with surfactant (sodium dodecyl sulfate) were added to the chloride salt plating solution, and the concentration of the nanoparticles in the plating solution was 10~20 g / L; the pH value of the plating solution was maintained at 4.0±0.2.

[0015] The chloride salt plating solution is composed of the following components: nickel sulfate (NiSO4·6H2O) 180 g / L; cobalt sulfate (CoSO4·7H2O) 20 g / L; nickel chloride (NiCl·6H2O) 40 g / L; boric acid: 30 g / L; sodium saccharin: 0.05 g / L; sodium dodecyl sulfate: 0.5 g / L.

[0016] Furthermore, the optimized electrodeposition process conditions were as follows: the electrodeposition temperature was controlled at 45±2℃, and the cathode current density was 4A / dm²; throughout the electrodeposition process, an ultrasonic wave with a frequency of 28 kHz and a power of 200W was used to continuously oscillate the plating solution to break up the agglomeration of nanoparticles and promote their uniform co-deposition in the coating; the electrodeposition time lasted for 4 hours, resulting in a coating thickness of approximately 0.8 mm.

[0017] Compared with the prior art, the present invention has the following beneficial effects:

[0018] 1. This invention introduces low thermal conductivity nanoparticles into a wear-resistant metal matrix to form a novel composite coating. By introducing nanoparticles to construct high-density interfacial thermal resistance and phonon scattering centers in the coating, the thermal conductivity of the coating is significantly reduced from the intrinsic structure of the material, effectively weakening the heat flux density in the meniscus region. At the same time, the composite coating embeds high-hardness nano-ceramic particles into the metal matrix, which plays a dispersion strengthening role, so that while reducing thermal conductivity, it maintains wear resistance comparable to or even higher than that of pure metal coatings, ensuring the service life of the crystallizer.

[0019] 2. This invention introduces nanoscale second-phase particles into a wear-resistant metal matrix, which can significantly improve the coating hardness through pinning effects and other means. Furthermore, utilizing the high-density interfacial thermal resistance and dispersion strengthening effect formed by these particles, the equivalent thermal conductivity of the coating at operating temperature is reduced to 8-20 W / (m·K), while maintaining a hardness of not less than 550 HV. This coating can effectively suppress heat flow in the meniscus region and slow down the uneven shrinkage of the initial billet shell, thereby significantly reducing the probability of surface depressions and longitudinal cracks in the billet. While ensuring the wear-resistant life of the crystallizer, it improves the surface quality of the billet. The coating of this invention can effectively increase the surface temperature of the billet in the meniscus region of the crystallizer, reduce the temperature gradient within the initial solidification shell, and improve the uniformity of temperature distribution, thereby reducing the probability of surface depressions and cracks in the billet, significantly improving the surface quality of the billet and the efficiency of continuous casting production.

[0020] 3. This coating is prepared using a composite electrodeposition process, which is highly compatible with existing copper plate electroplating production lines for crystallizers. The process parameters are highly controllable, making it easy to achieve industrial production and application. By adjusting the electrodeposition temperature, current density, and ultrasonic dispersion process, a high-quality coating with a dense structure, smooth surface, and uniform composition can be obtained, avoiding the risk of stress concentration or peeling caused by coating defects. Attached Figure Description

[0021] Figure 1 The microstructure of the Ni-Co-ZrO2 (ZrO2 diameter 50nm) coating in Example 1 of this invention;

[0022] Figure 2 The microstructure of the Ni-Co-ZrO2 (ZrO2 diameter 20nm) coating in Example 2 of this invention;

[0023] Figure 3 The image shows the microstructure of the Ni-Co coating in Comparative Example 1, where (a) is the microstructure and (b) is the energy spectrum. Detailed Implementation

[0024] The present invention will be further described below with reference to specific embodiments, but it should not be construed as limiting the scope of the subject matter of the present invention to the following embodiments. All modifications, substitutions and alterations made based on ordinary technical knowledge and common practices in the art without departing from the above-described technical concept of the present invention are included within the scope of the present invention.

[0025] A low thermal conductivity coating for the meniscus region of a continuous casting crystallizer is a metal-based nanocomposite coating. Its microstructure consists of a continuous metal matrix phase and uniformly dispersed low thermal conductivity nanoparticles. It is prepared on the surface of a copper plate substrate in the meniscus region of the crystallizer using an ultrasonic-assisted composite electrodeposition process.

[0026] Example 1:

[0027] In this embodiment, a nickel-cobalt alloy is used as the metal substrate, and zirconium oxide (ZrO2) particles with an average particle size of 50 nm are used as the low thermal conductivity nanoparticles. The nanoparticles are prepared on the surface of the copper plate substrate of the crystallizer using an ultrasonic-assisted composite electrodeposition process.

[0028] Plating bath system: Nickel sulfate (NiSO4·6H2O): 180 g / L; Cobalt sulfate (CoSO4·7H2O): 20 g / L; Nickel chloride (NiCl·6H2O): 40 g / L; Boric acid: 30 g / L; Sodium saccharin: 0.05 g / L; Sodium dodecyl sulfate: 0.5 g / L.

[0029] Plating solution preparation: ZrO2 nanoparticles pre-dispersed with surfactant were added to the above plating solution at a concentration of 15 g / L. The pH of the plating solution was maintained at 4.0 ± 0.2.

[0030] Electrodeposition process: The pretreated (degreasing, pickling) copper plate of the crystallizer is used as the cathode, and the nickel plate as the anode. The electrodeposition temperature is controlled at 45±2℃, and the cathode current density is 4 A / dm². Throughout the electrodeposition process, ultrasonic waves at a frequency of 28kHz and a power of 200W are used to continuously oscillate the plating solution to break up the agglomeration of nanoparticles and promote their uniform co-deposition in the coating.

[0031] Post-treatment: Electrodeposition time lasted for 4 hours, resulting in a coating thickness of approximately 1.0 mm; its SEM microstructure can be found in [reference needed]. Figure 1 As shown in the figure, the nano ZrO2 particles are evenly distributed in the nickel-cobalt alloy matrix without obvious agglomeration. The coating structure is dense and forms a large number of metal-ceramic interfaces, which is beneficial to enhance phonon scattering to reduce thermal conductivity. At the same time, the coating hardness is improved through dispersion strengthening.

[0032] The equivalent thermal conductivity of the nickel-zirconia nanocomposite coating prepared in this embodiment was only 10.2 W / (m·K) as measured by laser scintillation, which is much lower than the 78 W / (m·K) of the traditional pure nickel coating. This is attributed to the formation of numerous high-density interfaces between the large number of dispersed ZrO2 nanoparticles and the nickel matrix, which greatly enhances phonon scattering and thus effectively suppresses the rapid heat conduction in the coating thickness direction.

[0033] Example 2:

[0034] The difference between this embodiment and Example 1 is that ZrO2 nanoparticles of different sizes were selected, and the particle concentration in the plating solution was adjusted, in order to investigate the effect of nanoparticle size on the thermal conductivity and hardness of the coating. The preparation process is basically the same.

[0035] Plating solution preparation: basically the same as in Example 1, except that: ZrO2 nanoparticles with an average particle size of 20 nm are used; the ZrO2 concentration in the plating solution is 20 g / L; and the amount of surfactant (SDS) is adjusted to 3 g / L. The remaining components and preparation methods are the same as in Example 1.

[0036] Matrix pretreatment: Same as in Example 1.

[0037] Electrodeposition process: Electrodeposition temperature was 50 ℃, cathode current density was 4 A / dm², and electrodeposition time was 2 hours. Ultrasonic auxiliary parameters: frequency 30 kHz, power 250 W. Other parameters were the same as in Example 1.

[0038] Post-processing: Same as in Example 1.

[0039] Coating characterization: Average thickness is 0.8 mm. Its SEM microstructure can be found in [reference needed]. Figure 2 As shown in the figure, the ZrO2 particles are uniformly dispersed, the interface is dense, the coating is dense and smoother, and the volume fraction of ZrO2 in the coating is about 14%.

[0040] Microstructure: The particles are evenly distributed and no obvious agglomeration is observed.

[0041] Performance testing: Equivalent thermal conductivity: 8.3 W / (m·K). Hardness: 580HV.

[0042] Results show that compared with Example 1, this example uses smaller nanoparticles and a higher particle volume fraction, which further increases the interfacial density and continues to decrease the thermal conductivity; at the same time, the hardness is improved due to the enhanced dispersion strengthening effect. However, it should be noted that when the volume fraction exceeds 20%, the internal stress of the coating increases, and the thickness needs to be controlled to prevent cracking.

[0043] Example 3: (Changing the composition of the metal matrix).

[0044] The difference between this embodiment and Embodiments 1 and 2 is that a nickel-cobalt-chromium (Ni-Co-Cr) ternary alloy is used as the metal matrix to further improve the coating's resistance to high-temperature oxidation.

[0045] Plating solution system: Nickel sulfate (NiSO4·6H2O): 6 g / L; Cobalt sulfate (CoSO4·7H2O): 1 g / L; Chromium sulfate (Cr2(SO4)3xH2O): 40 g / L; Boric acid: 30 g / L; Sodium saccharin: 0.2 g / L; Sodium dodecyl sulfate: 0.5 g / L.

[0046] Plating solution preparation: Adjust the pH to 4.9 and add ZrO2 nanoparticles with an average particle size of 20 nm, with a concentration of 20 g / L in the plating solution. Ultrasonic pre-dispersion is the same as in Example 1.

[0047] Matrix pretreatment: Same as in Example 1.

[0048] Electrodeposition process: Electrodeposition temperature: 45 ℃; Cathode current density: 6A / dm²; Electrodeposition time: 8 hours; Ultrasonic frequency / power: 30kHz / 250W.

[0049] Post-processing: Same as in Example 1.

[0050] Performance testing: Equivalent thermal conductivity: 8.6 W / (m·K); Coating thickness: approximately 0.2 mm; Microhardness: 620 HV.

[0051] The results show that the coating hardness of this embodiment is significantly better than that of Embodiments 1 and 2 after the introduction of Cr. Meanwhile, since the addition of Cr has little effect on the thermal conductivity of the substrate, the thermal conductivity remains at a low level of 8.5 W / (m·K). This embodiment is suitable for the meniscus region of continuous casting molds under high casting speed conditions.

[0052] Comparative Example 1:

[0053] The most typical pure Ni-Co alloy coating (without nanoparticles) in the existing technology was prepared on the surface of the same crystallizer copper plate according to the same electrodeposition process parameters as in Example 1 (but without adding ZrO2 particles and without applying ultrasound).

[0054] Plating solution preparation: Compared with Example 1, no ZrO2 nanoparticles and SDS surfactant were added, but the other components were exactly the same.

[0055] Matrix pretreatment: Same as in Example 1.

[0056] Electrodeposition process: Electrodeposition temperature: 40 ℃; Cathode current density: 4 A / dm²; Electrodeposition time: 2 hours; No ultrasonic waves are applied, only magnetic stirring (300 rpm) is used.

[0057] Post-processing: Same as in Example 1.

[0058] Performance testing: Equivalent thermal conductivity: 60 W / (m·K); Microhardness: 417 HV. Its SEM microstructure can be found in [reference needed]. Figure 3 ,Depend on Figure 3 (a) It can be seen that the coating surface is smooth, but the grains are large and there are no nanoparticle precipitates; Figure 3 (b) shows only characteristic peaks of Ni and Co in the energy spectrum, with a ratio of approximately 1:1.

[0059] Performance test comparison table of the examples and comparative examples:

[0060]

[0061] Therefore, this invention significantly reduces the equivalent thermal conductivity of the coating from the intrinsic structure of the material, effectively weakening the heat flux density in the meniscus region. At the same time, the coating embeds high-hardness nano-ceramic particles in the metal matrix, which plays a role in dispersion reinforcement. This allows the coating to maintain wear resistance comparable to or even higher than that of pure metal coatings while reducing thermal conductivity, ensuring the service life of the crystallizer. This is based on the fact that a large number of dispersed ZrO2 nanoparticles form numerous high-density interfaces with the nickel matrix, which greatly enhances phonon scattering and thus effectively suppresses the rapid conduction of heat in the coating thickness direction.

[0062] The coating of this invention can effectively increase the surface temperature of the billet in the meniscus region of the crystallizer, reduce the temperature gradient in the initial solidification shell, and improve the uniformity of temperature distribution, thereby reducing the probability of surface depressions and cracks on the billet, reducing the risk of surface crack formation, improving billet quality, obtaining high-quality billets, reducing continuous casting costs, avoiding steel leakage, ensuring smooth continuous casting, and significantly improving the surface quality of the billet and the efficiency of continuous casting production.

[0063] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described with reference to preferred embodiments, those skilled in the art should understand that various changes in form and detail can be made without departing from the spirit and scope of the invention as defined in the appended claims.

Claims

1. A low thermal conductivity coating for the meniscus region of a continuous casting crystallizer, characterized in that, Its microstructure consists of a continuous metal matrix phase and uniformly dispersed low thermal conductivity nanoparticles therein; the metal matrix phase is one of nickel-cobalt or nickel-cobalt-chromium alloy, and the low thermal conductivity nanoparticles are zirconium oxide.

2. The low thermal conductivity coating for the meniscus region of the continuous casting crystallizer according to claim 1, characterized in that, The low thermal conductivity nanoparticles have a volume fraction of 5% to 20% in the coating, and the average particle size of the nanoparticles is 20 nm to 100 nm.

3. The low thermal conductivity coating for the meniscus region of the continuous casting crystallizer according to claim 1, characterized in that, The thickness of the wear-resistant coating is 0.1 mm to 1.0 mm.

4. A method for preparing a low thermal conductivity coating in the meniscus region of a continuous casting crystallizer, characterized in that, The low thermal conductivity coating in the meniscus region of the continuous casting crystallizer, as described in claim 1, 2, or 3, is prepared on the surface of the copper plate substrate in the meniscus region of the crystallizer using an ultrasonic-assisted composite electrodeposition process.

5. The method for preparing the low thermal conductivity coating in the meniscus region of a continuous casting crystallizer according to claim 4, characterized in that, The ultrasonic composite electrodeposition process includes: The pretreated copper plate of the crystallizer is used as the cathode and the nickel plate as the anode for electrodeposition in the plating bath. The electrodeposition temperature is 45℃~60℃, the cathode current density is 2~5 A / dm², and the electrodeposition time is 1~4 hours. Ultrasonic dispersion is used during the electrodeposition process, with an ultrasonic frequency of 20 kHz~40 kHz and a power of 100W~300W.

6. The method for preparing the low thermal conductivity coating in the meniscus region of a continuous casting crystallizer according to claim 5, characterized in that, in, Nickel plates are metallic matrix phases, namely nickel-cobalt or nickel-cobalt-chromium alloys.

7. The method for preparing the low thermal conductivity coating in the meniscus region of a continuous casting crystallizer according to claim 5, characterized in that, The plating solution was prepared as follows: ZrO2 nanoparticles pre-dispersed with surfactant were added to the chloride salt plating solution, and the concentration of the nanoparticles in the plating solution was 10~20g / L; the pH value of the plating solution was maintained at 4.0±0.

2.

8. The method for preparing the low thermal conductivity coating in the meniscus region of a continuous casting crystallizer according to claim 5, characterized in that, The electrodeposition temperature was controlled at 45±2℃, and the cathode current density was 4 A / dm². During the entire electrodeposition process, an ultrasonic wave with a frequency of 28kHz and a power of 200W was used to continuously oscillate the plating solution to break up the agglomeration of nanoparticles and promote their uniform co-deposition in the coating. The electrodeposition time lasted for 4 hours, and a coating thickness of about 0.8mm was obtained.

9. The method for preparing a low thermal conductivity coating in the meniscus region of a continuous casting crystallizer according to claim 5, 7, or 8, characterized in that, The plating solution is composed of the following components: nickel sulfate (NiSO4·6H2O) 180 g / L; cobalt sulfate (CoSO4·7H2O) 20 g / L; nickel chloride (NiCl·6H2O) 40 g / L; boric acid: 30 g / L; sodium saccharin: 0.05 g / L; sodium dodecyl sulfate: 0.5 g / L.

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