A method for hot delivery of a high carbon steel defect-free continuous casting billet

Abstract

The application discloses a high-carbon steel defect-free continuous casting billet hot feeding method, wherein, in a slab continuous casting process, the working surface material of a crystallizer is Inconel 600 alloy, and the thickness range is 10-20 mm; the plating layer thickness range of the crystallizer close to the molten steel side is 2.5-5.0 mm, and the plating layer material is one of pure Ni, Ni-Cr, Ni-Co or Co-Ni alloy; in the continuous casting process, the superheat of the molten steel in a tundish is controlled to be 50-100 DEG C, the conventional slab continuous casting speed of high-carbon steel is controlled to be 1.8-2.6 m / min, the water amount of the wide surface of the crystallizer is 5000-6000 L / min, the crystallizer nozzle insertion depth is controlled to be 80-150 mm, the narrow surface taper of the crystallizer is 1.25-1.35%, the secondary cooling zone cooling specific water amount is 0.10-0.30 L / kg, and the high-carbon steel hot feeding billet surface temperature is above 1100 DEG C, thereby meeting the requirements of not entering a hot rolling heating furnace and being directly rolled.

Description

Technical Field

[0001] This invention relates to a hot delivery method for defect-free continuous casting billets of high-carbon steel, which aims to meet the process requirement of direct rolling of the billets without entering the hot rolling furnace, and belongs to the field of metallurgical technology. Background Technology

[0002] High-carbon steel with a carbon content exceeding 0.77 wt% has a lower melting point and a liquidus temperature more than 100°C lower than that of ordinary low-carbon steel. This makes melting of the continuous casting protective slag difficult, significantly increasing friction between the billet and the mold. Consequently, high-carbon steel continuous casting billets often develop surface depressions, longitudinal cracks, and even leaks, severely impacting subsequent product quality. Defects in high-carbon steel billets often force them to be removed from the production line, severely affecting hot rolling, hot delivery, and hot charging rates.

[0003] The crystallizer is a key piece of equipment in a continuous casting machine, and most surface defects in the cast billets originate there. The working surface of the crystallizer must withstand the scouring and thermal erosion of high-temperature molten steel, as well as the wear and stress generated by the movement of the initial solidified billet shell, making it prone to wear, hot cracking, corrosion, and deformation. Therefore, the material of the working surface must possess good thermal conductivity, high strength and hardness, a high recrystallization temperature, creep resistance, thermal fatigue strength, and corrosion resistance.

[0004] The initial material commonly used for the working face of the crystallizer was pure copper. Pure copper was widely used initially due to its excellent thermal conductivity, but it was generally only suitable for low-speed continuous casting machines with casting speeds below 1.0 m / min and crystallizer working face temperatures below 230℃. This was mainly because the material's strength would significantly decrease after prolonged use at temperatures above 230℃. Subsequently, Ag was added to increase the softening temperature of the copper plate, but the operating temperature had to be controlled below 350℃. With the increase in continuous casting machine speeds and the requirement for longer crystallizer lifespans, the performance requirements for copper plate materials also became increasingly stringent. This led to the development of precipitation-strengthened copper alloys such as CuCr, CuCrZr, and CuNiBe. These materials possess higher strength and hardness, and softening temperatures can reach 450℃.

[0005] CuCr material has high electrical conductivity and high strength at both room temperature and high temperature. However, due to its poor high-temperature ductility, it is prone to hot cracking when the working surface temperature of the crystallizer reaches 400℃.

[0006] CuCrZr material maintains the high electrical conductivity of CuCr material while improving its high-temperature ductility, resulting in strong high-temperature strength and good resistance to deformation, thus extending the service life of copper plates. However, with the increasing demands for high casting speeds in continuous casting, the operating conditions of the crystallizer become more stringent. To meet these requirements, CuNiBe material, which has higher strength than CuCrZr, is used. CuNiBe material is generally applied to crystallizers with particularly high thermal loads, such as thin-strip continuous casting crystallizers.

[0007] In addition, the working surfaces of crystallizers mainly suffer from edge wear, wide-face hot cracking, narrow-face shrinkage, and corrosion during use. Previous research on crystallizer copper plates focused on using appropriate surface treatment techniques to coat the copper plate surface with one or more materials to obtain special functional surfaces. This aimed to achieve various coatings that are firmly bonded to the substrate, have good wear resistance, and strong resistance to hot corrosion, while ensuring that the thermal conductivity is not significantly affected by the coating. The application of surface modification technologies such as electroplating, chemical plating, electroforming, thermal spraying, high-temperature self-propagating coating, and composite plating on continuous casting crystallizers can solve problems related to corrosion resistance and wear resistance of structural components. The characteristics and shortcomings of various coatings are as follows:

[0008] Cr plating: The extremely thin oxide film formed on the surface of the Cr plating layer is very beneficial for the removal of splashed molten steel particles generated during casting after solidification. Generally, a plating layer of about 0.1 mm thickness is used. However, due to the large difference in the coefficients of thermal expansion between Cu and Cr, thicker plating layers are prone to peeling. Moreover, the Cr plating process in the crystallizer is complex and generates chemical waste acid, causing serious environmental pollution.

[0009] Ni+Cr composite coating: Due to the close proximity of Ni's coefficient of thermal expansion to Cu, peeling issues are significantly improved, and the thickness can be increased. Ni solves the problems of adhesion and thickness, while Cr solves the problems of splashing and surface hardness. To address the poor thermal conductivity of Ni, a bevel layer process is introduced, allowing the Ni layer thickness at the meniscus to be controlled at approximately 0.2 mm, and at the exit point at approximately 3 mm. The Cr layer is extremely thin, while the thicker Ni layer is very soft, failing to fully resolve the wear resistance issue at the bottom of the crystallizer. On one hand, due to the increased casting speed, the thinner billet shell adheres tightly to the copper plate surface under the hydrostatic pressure of the molten steel, increasing the mechanical friction load; on the other hand, the defect-free casting process places increasingly stringent requirements on the crystallizer's shape accuracy, and the crystallizer replacement cycle depends on the amount of wear at the bottom.

[0010] Ni-Fe coating: Adding Fe or W and Fe to Ni significantly increases the hardness of the coating. Depending on the Fe content, the hardness can be adjusted within the range of HV250–550. Its high-temperature strength (400℃) is 2.5 times that of Ni, and a relatively thick coating can be achieved with a single electroplating layer. Although Ni-Fe coatings offer significantly improved wear resistance and steel throughput compared to Ni coatings, they suffer from low electroplating efficiency, complex processes, and severe environmental pollution.

[0011] Ni-Co or Co-Ni coatings: Ni-Co and Co-Ni coatings further improve the wear resistance of the coating and extend the service life of the copper plate in the crystallizer. These are actually two high-temperature alloys with different substrates: Ni-Co is a nickel-based alloy containing Co, and Co-Ni is a cobalt-based alloy containing Ni. Their performance is further improved compared to Ni-Fe layers, with Co-Ni coatings offering even better performance, but it suffers from low electroplating efficiency, complex processes, and severe environmental pollution.

[0012] Ni-Cr alloy coating: In slab crystallizers, narrow-edge copper plates experience more severe wear than wide-edge copper plates. To improve the service life of narrow-edge copper plates, a Ni-Cr alloy layer is sprayed onto the surface using a supersonic flame, replacing the electroplating layer. Its service life is comparable to that of the Co-Ni plating on wide-edge copper plates. However, this technology easily generates significant thermal stress; and the new heat treatment process during thermal spraying leads to uneven microstructure, making it prone to thermal deformation. The melting and resolidification of the metal during thermal spraying easily causes oxidation, and the increased oxygen content significantly reduces the material's thermal conductivity. Furthermore, these oxygen elements are difficult to remove during subsequent processing. Vacuum spraying can alleviate oxidation to some extent, but it is costly and difficult to control.

[0013] Composite coatings: such as the early Ni-P-fluorinated graphite, which used chemical plating to obtain a self-lubricating coating with a fluorinated graphite volume fraction of up to 9%, significantly reducing the coefficient of friction and extending the life of the crystallizer. However, the bonding between solid particles and the substrate in composite coatings is difficult to control.

[0014] Ceramic coating: Ceramic spraying technology is used. The ceramic coating has a strong affinity with molten steel and is easily adhered and worn, which may lead to adhesion and steel leakage accidents.

[0015] In the continuous casting production of high-carbon steel, uneven flow of the protective slag between the inner wall of the crystallizer and the billet shell leads to uneven heat transfer and growth rate of the billet shell, resulting in longitudinal depressions and cracks on the billet. This is one of the main problems encountered in the continuous casting production of this type of steel. Reducing the heat transfer rate and slowing down the heat transfer in the crystallizer to create "weak cooling" can increase the surface temperature of the billet, slow down the growth rate of the billet shell, and thus promote the uniformity of billet shell growth, reducing the tendency for defects such as longitudinal cracks and depressions to form. However, the traditional materials and coatings used in the working surface of the copper plate in the crystallizer mentioned above are difficult to meet the requirements for continuous casting of high-carbon steel. Summary of the Invention

[0016] The technical problem this invention aims to solve is to overcome the shortcomings of existing technologies in the continuous casting production of high-carbon steel, which suffer from longitudinal depressions and cracks in the billet due to high cooling intensity. By improving the material of the crystallizer working surface and combining it with appropriate coating materials and continuous casting processes, the heat transfer rate of the crystallizer is reduced, resulting in "weak cooling." This increases the surface temperature of the billet, slows down the growth rate of the billet shell, and promotes the uniformity of shell growth, reducing the tendency for defects such as longitudinal cracks and depressions to form. Furthermore, this invention provides a technical method that plays a crucial role in the continuous casting process by improving the surface quality and hot delivery of high-carbon steel continuously cast billets. Finally, it further meets the process requirements for direct rolling of billets without entering a hot rolling furnace.

[0017] The technical problem to be solved by this invention can be implemented through the following technical solutions.

[0018] A hot delivery method for high-carbon steel defect-free continuous casting billets is proposed to meet the process requirements of direct rolling of the billets without entering the hot rolling furnace. The method is characterized by the use of Inconel 600 alloy as the crystallizer material in the slab continuous casting process, with a thickness range of 10-20 mm.

[0019] To improve the lifespan of the crystallizer, an additional coating is applied to the side of the crystallizer closest to the molten steel. Traditional crystallizers have a coating thickness that gradually increases from top to bottom, with a thinner upper coating and a thicker lower coating. However, for the crystallizer used in this invention for producing high-carbon steel, the original design cannot meet the requirements for slow cooling. The coating thickness on the side of the crystallizer closest to the molten steel in this invention ranges from 2.5 to 5.0 mm, and the coating material is one of pure Ni, Ni-Cr, Ni-Co, or a Co-Ni alloy.

[0020] The chemical composition of the Inconel 600 alloy used in the crystallizer of this invention and the commonly used copper-based materials on the working surface of the crystallizer are shown in Table 1 below.

[0021] Table 1: wt%

[0022]

[0023] The mechanical properties of commonly used copper-based materials and Inconel 600 alloy for the working surface of crystallizers are shown in Table 2; Inconel 600 alloy has better strength and ductility than copper-based materials.

[0024] Table 2:

[0025]

[0026]

[0027] The thermal conductivity of Inconel 600 alloy is 1 / 20 that of copper. Table 3 shows a comparison of the physical properties of Inconel 600 alloy with those of copper alloy commonly used in crystallizer working surfaces.

[0028] Table 3:

[0029]

[0030] In the continuous casting process, the superheat of the molten steel in the tundish is controlled at 50-100℃, the casting speed of conventional continuous casting of high carbon steel is controlled at 1.8-2.6m / min, the water flow rate of the wide face of the crystallizer is 5000-6000L / min, the insertion depth of the tundish nozzle of the continuous casting crystallizer is controlled at 80-150mm, the taper of the narrow face of the crystallizer is 1.25-1.35%, the cooling water flow rate of the secondary cooling zone is 0.10-0.30L / kg, and the surface temperature of the hot-delivered high carbon steel billet is above 1100℃.

[0031] As a preferred option, the thickness of the entire crystallizer coating ranges from 2.5 to 4.5 mm.

[0032] As a preferred embodiment, the superheat of the molten steel in the tundish is controlled at 50-80°C, and the casting speed is controlled at 2.0-2.4 m / min.

[0033] As a preferred embodiment, the water flow rate of the wide face of the crystallizer is 5250-5750 L / min.

[0034] As a preferred embodiment, the insertion depth of the sprue in the continuous casting crystallizer is controlled between 80 and 120 mm.

[0035] As a preferred embodiment, the taper of the narrow face of the crystallizer is 1.25–1.30%.

[0036] As a preferred embodiment, the cooling water volume of the secondary cooling zone is 0.15 to 0.3 L / kg.

[0037] As a preferred embodiment, the surface temperature of the hot-cast high-carbon steel billet is 1100–1200°C.

[0038] The technical mechanism of this invention and the reasons for its limitations are as follows:

[0039] The working surface material of the crystallizer is Inconel 600 alloy, which possesses excellent resistance to stress corrosion cracking, pitting and crevice corrosion, oxidation, and non-oxidizing hot acid properties, and exhibits good mechanical properties even at high temperatures up to 550℃. Its thermal conductivity is only 1 / 20th that of copper, fully meeting the requirements for weak cooling in the crystallizer. Extensive experimental verification has shown that a thickness below 10mm is insufficient to withstand the static pressure of molten steel; a thickness above 20mm affects heat transfer rate, and the billet shell becomes too thin. Therefore, the thickness range of the working surface material for the crystallizer in this invention is 10–20mm.

[0040] The coating material near the molten steel side: Pure Ni, Ni-Cr, Ni-Co, or Co-Ni alloys have expansion coefficients close to Inconel 600 alloy, and the coating thickness can be easily and precisely controlled by controlling the electroplating process, resulting in less wear and a longer service life. This invention selects one of pure Ni, Ni-Cr, Ni-Co, or Co-Ni alloys as the coating material.

[0041] Coating thickness: When the coating thickness is less than 2.5 mm, the coating's wear resistance is insufficient, leading to wear of the crystallizer coating by the cast billet and a reduction in crystallizer life. When the coating thickness is greater than 5.0 mm, it reduces the heat transfer rate of the crystallizer meniscus, and the continuously cast billet shell becomes too thin, causing collapse under the shrinkage force of the billet shell and resulting in a heavy scale on the billet surface. The coating thickness range of this invention is 2.5–5.0 mm.

[0042] Superheat in the tundish: Previously, high-carbon steel continuous casting tended to use low superheat, typically below 30°C, to ensure uniform solidified billet shell thickness. Using Inconel 600 alloy as the working material for the crystallizer, with a thermal conductivity only 1 / 20 that of copper, achieves a weak cooling effect in the crystallizer and ensures uniform billet shell thickness. Superheat in continuous casting has always been the most difficult technology to control at the steelmaking interface. This invention allows for greater control over superheat, increasing production efficiency while ensuring effective cooling and solidification of high-carbon steel. However, superheat cannot be unlimited. If the superheat exceeds 100°C, the initial billet shell in the crystallizer becomes too thin, increasing the tendency to crack, and resulting in prolonged solidification time and excessive crystallization, exacerbating billet segregation and porosity. This invention controls the superheat of the molten steel in the tundish between 50°C and 100°C.

[0043] Continuous casting speed: The casting speed design of this invention takes into account the direct rolling of hot-charged high-carbon steel. Through experiments and on-site temperature measurements, it was found that if the casting speed is below 1.8 m / min, the surface temperature of the billet exiting the casting machine is below 1100℃, which cannot meet the requirements for direct rolling. Due to the metallurgical length limitation of the casting machine, the upper limit of the conventional slab continuous casting speed is 2.6 m / min. This invention controls the continuous casting speed between 1.8 and 2.6 m / min.

[0044] Crystallizer water flow rate: The billet shell thickness at the wide outlet of the crystallizer needs to be greater than 12mm. Considering the high superheat and the low thermal conductivity of Inconel 600 alloy, strong cooling measures are required to ensure the billet shell thickness of high-carbon steel at the crystallizer outlet. After extensive experimental analysis, the water flow rate at the wide outlet of the crystallizer is no less than 5000L / min; at the same time, to improve heat transfer and billet shell thickness uniformity, the water flow rate at the wide outlet of the crystallizer is no more than 6000L / min. The water flow rate at the wide outlet of the crystallizer in this invention is 5000-6000L / min.

[0045] Mold nozzle insertion depth: If the mold nozzle insertion depth is too shallow, the nozzle stream will have a stronger impact on the steel-slag interface, increasing the probability of reaction between the molten steel and the mold flux, and also increasing the probability of slag entrapment at the meniscus; if the mold nozzle insertion depth is too deep, the cast billet is prone to cracking. Considering the low pouring temperature and difficulty in slag formation for high-carbon steel, the mold nozzle insertion depth of this invention is controlled at 80–150 mm.

[0046] Secondary cooling: In high-carbon steel (C>0.77wt%) composition systems, high-carbon steel continuously cast billets are pure austenitic with high plasticity in the secondary cooling zone. Considering that direct rolling requires a billet surface temperature higher than 1100℃, after extensive calculations and temperature measurements, the specific water content in the secondary cooling zone cannot exceed 0.30L / kg; simultaneously, to prevent damage to the casting machine under high-temperature conditions, the specific water content in the secondary cooling zone cannot be lower than 0.10L / kg. The specific water content in the secondary cooling zone of this invention is 0.1~0.3L / kg.

[0047] Crystallizer taper: During solidification, air gaps are generated between the crystallizer and the billet, leading to uneven heat transfer and uneven billet shell growth thickness. This can easily cause surface depressions or cracks. Therefore, an inverted taper is set to compensate for the shrinkage of the solidified billet shell. If the crystallizer taper is too small, the billet shell is prone to bulging; if the taper is too large, the friction between the billet shell and the copper plate of the crystallizer increases. The crystallizer narrow face taper of this invention is 1.25-1.35%.

[0048] Compared with the prior art, the present invention has the following beneficial effects:

[0049] 1. Defect-free manufacturing technology.

[0050] This invention improves the material of the crystallizer and adds a coating to the crystallizer. By coordinating with key process parameters of continuous casting, it reduces the heat transfer rate and slows down the heat transfer in the crystallizer, forming a "weak cooling" effect. This slows down the growth rate of the billet shell, thereby promoting the uniformity of the billet shell growth and achieving the goal of eliminating surface depressions and cracks in high-carbon steel billets. This enables the production of defect-free high-carbon steel billets.

[0051] 2. Cost reduction and carbon reduction effects.

[0052] This invention achieves direct rolling of high-carbon steel billets by setting the billet temperature above 1100℃, significantly reducing manufacturing costs and carbon emissions. Existing hot-charging temperatures for high-carbon steel continuously cast slabs range from 200 to 600℃. Assuming a saving of 3 kg / t of standard coal and a reduction of 8 kg / t of CO2 for every 100℃ increase in hot-charging temperature, and considering a minimum increase of 500℃, this invention's direct rolling of high-carbon steel can save 15 kg / t of standard coal and reduce CO2 by 40 kg / t.

[0053] 3. It has significant economic benefits.

[0054] Existing copper-based crystallizers produce 100,000 tons of steel per service cycle. Using the Inconel 600 alloy of this invention significantly extends service life and cycle time, increasing steel production to over 300,000 tons, reducing replacement frequency, and improving operational efficiency. The cost of a copper-based crystallizer is approximately 70,000 RMB / ton, while the cost of Inconel 600 alloy is approximately 140,000 RMB / ton. Therefore, although the cost of Inconel 600 alloy is twice that of copper crystallizers, its steel production is three times that of copper-based crystallizers, and it significantly improves operational efficiency, billet quality, hot delivery rate, and direct rolling performance. Detailed Implementation

[0055] The specific embodiments of the present invention will be further described in detail below with reference to specific examples.

[0056] The following embodiments will help those skilled in the art to further understand the present invention, but do not limit the invention in any way. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention. These all fall within the scope of protection of the present invention.

[0057] Table 4 below provides specific technical details and effects of various embodiments of the present invention.

[0058] Table 4: Embodiments and Effects of the Invention

[0059]

[0060] By employing the high-carbon steel defect-free continuous casting billet and hot delivery method of the present invention, defect-free high-carbon steel billets can be achieved. The high-carbon steel billet exiting the casting machine of the present invention has a temperature higher than 1100℃, realizing the direct rolling of high-carbon steel billets, which significantly reduces manufacturing costs and carbon emissions.

[0061] Finally, it should be noted that the above embodiments are only used to illustrate the present invention and are not intended to limit the technical solutions described in the present invention. Therefore, although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the present invention. All technical solutions and improvements that do not depart from the spirit and scope of the present invention should be covered within the scope of the claims of the present invention.

Claims

1. A hot delivery method for defect-free continuous casting billets of high-carbon steel, used to meet the process requirement of direct rolling of the billets without entering a hot rolling furnace, characterized in that, In the slab continuous casting process, the working surface of the crystallizer is made of Inconel 600 alloy with a thickness ranging from 10 to 20 mm; the coating thickness on the side of the crystallizer near the molten steel ranges from 2.5 to 5.0 mm, and the coating material is one of pure Ni, Ni-Cr, Ni-Co, or Co-Ni alloy. In the continuous casting process, the superheat of the molten steel in the tundish is controlled at 50-100℃, the casting speed of conventional high-carbon steel slabs is controlled at 1.8-2.6m / min, the water flow rate of the wide face of the crystallizer is 5000-6000L / min, the insertion depth of the sprue of the continuous casting crystallizer is controlled at 80-150mm, the taper of the narrow face of the crystallizer is 1.25-1.35%, the cooling water flow rate of the secondary cooling zone is 0.10-0.30L / kg, and the surface temperature of the hot-delivered high-carbon steel slab is above 1100℃.

2. The hot delivery method for defect-free continuous casting billets of high-carbon steel according to claim 1, characterized in that, The coating thickness on the side of the crystallizer closest to the molten steel ranges from 2.5 to 4.5 mm.

3. The hot delivery method for defect-free continuous casting billets of high-carbon steel according to claim 1, characterized in that, The superheat of the molten steel in the tundish is controlled at 50-80°C, and the casting speed of high-carbon steel conventional slabs is controlled at 2.0-2.4 m / min.

4. The hot delivery method for defect-free continuous casting billets of high-carbon steel according to claim 1, characterized in that, The water flow rate of the wide face of the crystallizer is 5250-5750 L / min.

5. The hot delivery method for defect-free continuous casting billets of high-carbon steel according to claim 1, characterized in that, The insertion depth of the sprue in the continuous casting crystallizer is controlled at 80–120 mm.

6. The hot delivery method for defect-free continuous casting billets of high-carbon steel according to claim 1, characterized in that, The taper of the narrow face of the crystallizer is 1.25–1.30%.

7. The hot delivery method for defect-free continuous casting billets of high-carbon steel according to claim 1, characterized in that, The cooling water volume in the secondary cooling zone is 0.15–0.30 L / kg.

8. The hot delivery method for defect-free continuous casting billets of high-carbon steel according to claim 1, characterized in that, The surface temperature of the hot-cast high-carbon steel billet is 1100–1200℃.

Images