A continuous casting process for improving the quality of ultra-high carbon steel cast slab

Abstract

The application relates to a continuous casting process for improving the quality of ultra-high-carbon-steel casting billets and belongs to the technical field of metallurgy. The continuous casting process is as follows: liquid ultra-high-carbon-steel liquid steel is poured from a ladle into a tundish through a long nozzle, then poured into a crystallizer through a submerged nozzle, the liquid steel is cooled and solidified in the crystallizer to form a billet shell with uniform thickness and a liquid core, the billet shell is completely solidified in a secondary cooling chamber and is bent and straightened, the continuous casting is carried out, the casting machine is finally pulled out, and a flame cutting machine is used to cut the casting billets into required sizes, and the casting billets are loaded into a hot-rolling heating furnace after cooling control. The obtained casting billets are free of surface cracks, the center segregation is less than or equal to C1.0, and the center porosity is less than or equal to 1.0.

Description

Technical Field

[0001] This invention belongs to the field of metallurgical technology, specifically relating to a continuous casting process for improving the quality of ultra-high carbon steel billets. Background Technology

[0002] Ultra-high carbon steel is a new type of high-performance steel material that has been developed in recent years both domestically and internationally, and has important development prospects. It is a high-value-added and high-tech steel product, mainly used for tool steel, die steel, saw blade steel, and other applications.

[0003] The carbon content in ultra-high carbon steel molten steel is generally 0.6%-1.2%. During continuous casting solidification, under the action of selective crystallization, a large amount of low-melting-point solutes accumulate at the end of solidification. In addition, the strong suction effect generated by the solidification shrinkage of the two-phase region at the end of the continuous casting billet causes the solutes enriched with low-melting-point impurities to flow towards the center of the billet, forming central segregation, accompanied by central porosity, and in severe cases, central shrinkage cavities. These internal defects of the continuous casting billet are difficult to effectively eliminate during subsequent heating and rolling processes, thus affecting the quality of the final product.

[0004] The numerous technical challenges in the continuous casting production of ultra-high carbon steel are primarily determined by its solidification and metallurgical characteristics. As shown in the iron-carbon phase diagram, high-carbon steel has a low liquidus line and a relatively wide solid-liquid two-phase region. When the carbon mass fraction exceeds 0.053%, austenite precipitates directly from the liquid phase without peritectic transformation, resulting in relatively small overall solidification shrinkage. The small solidification shrinkage of the initial billet shell means a narrower channel for slag inflow between the mold and the solidified shell, leading to a smaller inflow volume. These factors result in poor flow lubrication of the protective slag within the mold, high sensitivity to surface and internal cracks in the billet, and a high likelihood of quality defects such as longitudinal cracks and depressions on the billet surface, central segregation and porosity, as well as production accidents such as mold adhesion and leaks. Therefore, the continuous casting production of ultra-high carbon steel is extremely difficult and is considered a "no-man's land" in carbon steel production. It has become a major technical challenge and hot topic in continuous casting production in the metallurgical industry.

[0005] Currently, most ultra-high carbon steel casting in the industry adopts traditional ingot casting, with a small portion using special methods such as spray forming and vacuum casting. However, there is very little literature on the continuous casting of ultra-high carbon steel billets, and very few manufacturers have actually successfully produced high-carbon steel billets through continuous casting. Detailed reports on their production practices are lacking, and systematic research on the cooling process of ultra-high carbon steel continuous casting is also unavailable. Summary of the Invention

[0006] The purpose of this invention is to provide a continuous casting process that improves the quality of ultra-high carbon steel billets, corrects the biases of traditional continuous casting technology, and effectively solves the technical problems that have long plagued the continuous casting production and billet quality of ultra-high carbon steel in the industry.

[0007] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows:

[0008] A continuous casting process for improving the quality of ultra-high carbon steel billets includes the following steps: molten ultra-high carbon steel is poured from a ladle to a tundish through a long nozzle, and then poured into a crystallizer through a submerged entry nozzle. The molten steel cools and solidifies in the crystallizer to form a billet shell with a uniform thickness and a liquid core. The billet is then cooled and solidified in a secondary cooling chamber and bent and straightened. Finally, it is pulled out of the casting machine and cut into billets of the required length by a flame cutter. After controlled cooling during transportation, the billets are loaded into a hot rolling furnace.

[0009] The process includes the following control parameters: the molded crystallizer uses molded crystallizer protective slag, and the basicity (CaO / SiO2) of the molded crystallizer protective slag is 1.10-1.20; the secondary cooling chamber uses a continuous casting secondary cooling process with strong cooling at the top and weak cooling at the bottom: strong cooling at the head of the secondary cooling zone, with a specific water volume of 0.9-1.10 L / kg; and weak cooling in the middle and lower part of the secondary cooling zone, with a specific water volume of 0.4-0.6 L / kg.

[0010] The properties of the crystallizer protective slag described in this invention: viscosity (η) 1300℃ 0.06-0.10 Pa·s, solidification temperature 1095-1135℃.

[0011] The crystallizer protective slag described in this invention is a crystallizer protective slag with added heating agent. The composition of the heating agent and its content in the crystallizer protective slag are: CaSi: 4%-6%, FeO: 6%-8%.

[0012] The consumption of the protective slag for the crystallizer described in this invention is ≥0.4 kg / t.

[0013] The inverted taper of the crystallizer described in this invention is as follows: narrow face inverted taper of the crystallizer: 1.15% ± 0.05%; wide face shrinkage of the crystallizer: 1 mm ± 0.02 mm.

[0014] The superheat of the molten steel in the tundish described in this invention is controlled at 20℃-30℃, and the nitrogen content in the molten steel in the tundish is controlled at [N]≤30ppm.

[0015] The diameter of the submerged entry nozzle described in this invention is 50-55mm, which is 5%-10% smaller than the diameter of the submerged entry nozzle for other steel grades on the same continuous casting machine.

[0016] The present invention describes controlled cooling during transportation. The billet cooling must be carried out according to the hot delivery organization. If slow cooling is required, the following must be met: billet temperature drop rate ≤ 10℃ / h; billet finishing temperature ≥ 150℃; the billet needs to be slowly preheated in the hot rolling furnace, and the temperature should be slowly increased below 800℃, with a heating rate ≤ 10℃ / min.

[0017] The billets described in this invention are strictly prohibited from cold cutting operations such as longitudinal cutting and transverse cutting, and are prohibited from contact with water in a hot state; the billets should be rolled in a heating furnace within 5 hours after continuous casting cutting; the billets that are not slowly cooled in the heat preservation pit must be rolled in a heating furnace within 48 hours after continuous casting cutting.

[0018] The chemical composition and mass percentage of the ultra-high carbon steel molten steel described in this invention are as follows: C: 1.0%-2.10%, Si: 0.10%-0.50%, Mn: 0.30%-1.80%, with the remainder being Fe, alloying elements, and unavoidable impurities.

[0019] The quality of the billet described in this invention is as follows: the billet has no surface cracks; central segregation ≤ C1.0, and central porosity ≤ 1.0.

[0020] The design concept of this invention is as follows:

[0021] 1. Continuous casting crystallizer protective slag process:

[0022] Traditional continuous casting processes assume that ultra-high carbon steel, lacking peritectic reaction, exhibits low shrinkage behavior within the crystallizer. Therefore, ultra-high carbon protective slag is typically a low-viscosity, low-melting-point acidic slag, primarily designed to provide good lubrication due to its slightly lower melting point. However, poor heat transfer control easily leads to quality defects such as sink marks and longitudinal cracks. Furthermore, during continuous casting, the protective slag readily forms slag rings, requiring frequent slag removal and cleaning operations. This affects both billet quality and production flow, potentially triggering yellow card alarms (a pre-leakage alarm requiring speed reduction, which can lead to severe leakage) and sticky leakage.

[0023] This invention corrects traditional technical biases by using an alkaline slag with high basicity, high solidification temperature, low viscosity, and added exothermic agent as the protective slag for ultra-high carbon steel continuous casting crystallizers. The principle is that the protective slag of this invention has an exothermic effect, keeping the molten steel in the crystallizer (mainly near the meniscus) at a constant temperature. Furthermore, the high basicity and high solidification temperature of the protective slag lead to the early formation of a large amount of opaque protective slag film (the protective slag liquid layer is a crystallized layer, which has the effect of preventing heat radiation), reducing heat loss within the crystallizer and allowing the solidified shell to cool slowly.

[0024] The main indicators of the protective slag for crystallizers of this invention are as follows: (1) basicity (CaO / SiO2) = 1.10-1.20%; (2) solidification temperature = 1095-1135℃; (3) viscosity (η) 1300℃ =0.06-0.08Pa.s; (4) The composition and content of the heating agent: CaSi: 4-6%, FeO: 6-8%; (5) The consumption of protective slag in the crystallizer is ≥0.4kg / t.

[0025] 2. Secondary cooling process for continuous casting

[0026] Traditional continuous casting processes consider that ultra-high carbon steel is sensitive to surface longitudinal cracks, surface corner cracks, and transverse cracks, so a weak cooling secondary cooling process is required. However, a few steel mills use a strong cooling secondary cooling process to control the center segregation of ultra-high carbon steel, in order to control the billet shell bulging and separation crystallization.

[0027] This invention reveals that ultra-high carbon steel continuously cast billets are prone to internal cracks. This is related to the low deformation rate of this steel grade at 1350℃ and the sharp decrease in strength above 1250℃. Therefore, ultra-high carbon steel billets tend to bulge when they first emerge from the crystallizer, and internal cracks easily form in the upper part of the secondary cooling zone. Thus, the casting speed for high-carbon steel continuous casting should be low, the secondary cooling water ratio should be high, and the water distribution at the head of the secondary cooling zone should be high, with a secondary cooling water ratio within the range of 0.9-1.10 L / kg. If the casting speed is too high, the secondary cooling strength will be insufficient, leading to internal cracks. However, the temperature at the bending and straightening points of the billet must be controlled between 880-1250℃ to avoid surface cracks (longitudinal and corner cracks). Therefore, the billet should be weakly cooled, the secondary cooling water ratio should be low, and the water distribution in the lower part of the secondary cooling zone should be low, with a secondary cooling water ratio within the range of 0.4-0.6 L / kg. The spray water at the corners of the billet should also be turned off.

[0028] This invention corrects traditional technical biases. The secondary cooling process for ultra-high carbon steel takes into account both the control of central segregation and surface cracks. It adopts a continuous casting secondary cooling process with strong cooling at the head of the secondary cooling zone and weak cooling in the middle and lower parts of the secondary cooling zone.

[0029] 3. Control of crystallizer taper

[0030] Traditional continuous casting processes assume that the initial shell of ultra-high carbon steel has low solidification shrinkage, therefore a smaller crystallizer with a narrow face taper (Taper ≦ 1.0%) should be used.

[0031] The present invention has found that although the initial shell of ultra-high carbon steel has a small solidification shrinkage, the solid-liquid two-phase region of ultra-high carbon steel is wide and the solidification rate is slow. Therefore, the initial shell thickness is thin (only about 1 / 2 of that of conventional carbon steel), and it is very easy to bulge. If the inverted taper of the crystallizer is not enough to fully support the initial shell and eliminate bulging, it is easy to produce surface longitudinal cracks and shell tearing that results in steel leakage.

[0032] This invention corrects traditional technical biases by using a larger inverted taper of the crystallizer for ultra-high carbon steel: the narrow face inverted taper of the crystallizer is 1.15% ± 0.05%; the wide face of the crystallizer shrinks by 1 mm ± 0.02 mm.

[0033] 4. Control of nitrogen content in molten steel

[0034] Traditional continuous casting processes assume that the content of secondary phase precipitation elements such as aluminum, niobium, boron, and vanadium in ultra-high carbon steel is very low, making it difficult to form aluminum, niobium, boron, and vanadium carbon compounds at the austenite grain boundaries, thus weakening the grain boundary strength and causing surface cracks in the cast billet.

[0035] The present invention has found that although the content of secondary phase precipitation elements such as aluminum, niobium, boron and vanadium in ultra-high carbon steel is very low, nitrogen atom segregation itself can weaken the grain boundary strength of the billet, leading to surface cracks in the billet.

[0036] This invention corrects traditional technical biases by strictly controlling the nitrogen content in ultra-high carbon steel, requiring the nitrogen content in the molten steel in the continuous casting tundish to be ≤30ppm.

[0037] 5. Control of superheat in continuously cast steel

[0038] Traditional continuous casting theory holds that when molten steel solidifies near the liquidus temperature with a superheat of zero, the equiaxed crystal zone in the center of the billet can reach over 60%, eliminating central porosity and segregation. Therefore, the superheat requirement for ultra-high carbon steel continuous casting is controlled at a low superheat of 15±5℃.

[0039] This invention, through research, has discovered that the degree of center segregation in continuously cast billets is not necessarily better the lower the superheat of the molten steel (which results in a higher equiaxed crystal ratio at the billet center). For specific steel grades, continuous casting machines, and processes, it is necessary to find a suitable superheat to achieve the optimal match between the equiaxed crystal ratio and the density (dendritic spacing, etc.) of the central equiaxed crystal zone. At high superheat, the central columnar crystal structure grows to the center, and under light pressure, the leading edges of the two columnar crystals are pressed together, reducing the space for solute accumulation and thus decreasing the number and size of semi-macroscopic segregation points, thereby reducing semi-macroscopic segregation. Under existing light pressure conditions, appropriately increasing the superheat can also achieve a smaller degree of center segregation, and the central segregation points are distributed within a very narrow area at the center, improving the uniformity of solute concentration in the thickness direction of the steel. Simultaneously, it can avoid problems such as difficulty in the flotation of inclusions in the molten steel, poor melting of the protective slag, nodule formation in the tundish, and even interruption of casting caused by excessively low pouring temperatures.

[0040] This invention corrects traditional technical biases and improves the superheat of ultra-high carbon steel, requiring the superheat of molten steel in the continuous casting tundish to be controlled at a high superheat of 25±5℃.

[0041] 6. Process optimization of continuous casting submerged entry nozzle

[0042] The present invention produces an immersion nozzle with a diameter of 53mm for ultra-high carbon steel, which is 5%-10% smaller than the 58mm diameter of immersion nozzles for other steel grades on the same continuous casting machine. This increases the distance between the nozzle and the copper plate of the crystallizer, which is beneficial for slag removal, reduces uneven heat conduction on the copper plate of the crystallizer, and controls longitudinal cracking defects in the billet.

[0043] Through research, this invention has found that reducing the volume of the submerged nozzle reduces heat transfer from the molten steel in the crystallizer to the nozzle, increases the thickness of the protective slag molten layer between the nozzle and the wide face of the crystallizer, increases the thickness of the opaque layer of the protective slag film near the center of the wide face, and makes the film thickness uniform. This eliminates the uneven heat conduction of the copper plate in the crystallizer during the growth of the initial solidified billet shell, and can control longitudinal crack defects.

[0044] 7. Cooling control of continuously cast billets

[0045] During the cooling process, ultra-high carbon steel billets undergo martensitic transformation. Due to the varying shrinkage stresses in different microstructures after solidification, the billets exhibit high low-temperature brittleness. Improper cooling and handling methods can easily lead to low-temperature cracks, which can result in severe cases such as billet fracture, furnace breakage, or rolling failure. Therefore, billets must be hot-delivered, and all billets must be transferred to the furnace within 5 hours of cutting. Billets requiring offline processing or secondary cutting must be slowly cooled in an insulation pit, with a temperature drop rate ≤10℃ / h; the finishing temperature of defective billets must be ≥150℃.

[0046] The billet needs to be preheated slowly in the hot rolling furnace. The temperature should be increased slowly below 800℃, with a heating rate of ≤10℃ / min. The billet is strictly prohibited from cold cutting operations such as longitudinal cutting and transverse cutting, and it is prohibited to contact water in the hot state. The billet should be put into the heating furnace for rolling within 5 hours after continuous casting cutting. The billet that is not slowly cooled in the heat preservation pit must be put into the heating furnace for rolling within 48 hours after continuous casting cutting.

[0047] The beneficial effects of adopting the above technical solution are as follows: This invention addresses the unique solidification and metallurgical characteristics of ultra-high carbon steel, corrects the biases of traditional continuous casting technology, creatively adjusts and changes the characteristics of the original mold flux, adopts a mold flux with high basicity, high solidification temperature, low viscosity and added exothermic agent, and a continuous casting secondary cooling process with strong upper part and weak lower part. It uses a larger inverted taper of the mold, a higher superheat of molten steel, and a lower nitrogen content of molten steel for process control of the submerged entry nozzle and billet cooling process. The resulting billet has no surface cracks, central segregation ≤C1.0 grade, and central porosity ≤1.0, effectively solving the long-standing technical problems in steel production that have plagued the continuous casting production and billet quality of ultra-high carbon steel. Attached Figure Description

[0048] Figure 1 Example 1: Low-magnification image of internal quality;

[0049] Figure 2 Example 2: Low-magnification image of internal quality;

[0050] Figure 3 Example 3: Low-magnification image of internal quality;

[0051] Figure 4 Example 4: Low-magnification image of internal quality;

[0052] Figure 5 Example 5: Low-magnification image of internal quality. Detailed Implementation

[0053] The invention will be further described in detail below through embodiments.

[0054] Example 1

[0055] The chemical composition and mass percentage of the ultra-high carbon steel in this embodiment are as follows: C: 1.0%, Si: 0.40%, Mn: 1.10%, P: 0.008%, S: 0.002%, Als: 0.035%, Cr: 0.2%, with the remainder being Fe, alloying elements, and unavoidable impurities.

[0056] A continuous casting process for improving the quality of ultra-high carbon steel billets includes the following steps: molten ultra-high carbon steel is poured from the ladle to the tundish through a long nozzle, and then poured into the crystallizer through an immersion nozzle. The molten steel cools and solidifies in the crystallizer to form a billet shell with a uniform thickness and a liquid core. The billet is then cooled and solidified in the secondary cooling chamber and bent and straightened. Finally, it is pulled out of the casting machine and cut into billets of the required length by a flame cutter. After controlled cooling during transportation, the billets are loaded into the hot rolling furnace within 5 hours.

[0057] The continuous casting process includes the following control parameters: tundish steel superheat 30℃, tundish steel nitrogen content [N] 25ppm, and submerged entry nozzle diameter 53mm.

[0058] The narrow face taper of the crystallizer is 1.15%; the wide face shrinkage is 1mm. The crystallizer protective slag is a slag with added exothermic agent to increase the temperature of the meniscus, with a consumption of 0.45 kg / t. Properties of the crystallizer protective slag: basicity (CaO / SiO2) 1.15, viscosity (η) 1300℃ 0.06 Pa·s, solidification temperature 1115℃, the composition of the exothermic agent and its content in the protective slag of the crystallizer are: CaSi: 5%, FeO: 7%.

[0059] The secondary cooling chamber uses a continuous casting secondary cooling process with strong cooling at the top and weak cooling at the bottom: strong cooling at the head of the secondary cooling zone with a specific water volume of 1.10 L / kg; and weak cooling in the middle and lower part of the secondary cooling zone with a specific water volume of 0.6 L / kg.

[0060] Cooling is controlled during billet transportation. Billets are delivered hot, and all billets are transferred to the heating furnace within 5 hours after cutting. Billets requiring offline processing or secondary cutting must be slowly cooled in an insulation pit at a temperature drop rate of 5-8℃ / h. The finishing temperature of the billets is 200℃.

[0061] The billet is slowly preheated in a hot rolling furnace, and the temperature is slowly increased below 800℃, with the heating rate controlled at 10℃ / min.

[0062] The billet quality in this embodiment (see Table 1) is as follows: no surface cracks; central segregation C 0.5, central porosity 0.5, as shown in the attached table. Figure 1 .

[0063] Example 2

[0064] The chemical composition and mass percentage of the ultra-high carbon steel in this embodiment are as follows: C: 1.10%, Si: 0.20%, Mn: 0.50%, P: 0.007%, S: 0.0020%, Als: 0.005%, Cr: 0.2%, Cu: 0.1%, Nb: 0.002%, with the remainder being Fe and unavoidable impurities.

[0065] A continuous casting process for improving the quality of ultra-high carbon steel billets includes the following steps: molten ultra-high carbon steel is poured from the ladle to the tundish through a long nozzle, and then poured into the crystallizer through an immersion nozzle. The molten steel cools and solidifies in the crystallizer to form a billet shell with a uniform thickness and a liquid core. The billet is then cooled and solidified in the secondary cooling chamber and bent and straightened. Finally, it is pulled out of the casting machine and cut into billets of the required length by a flame cutter. After controlled cooling during transportation, the billets are loaded into the hot rolling furnace within 5 hours.

[0066] The continuous casting process includes the following control parameters: tundish steel superheat 28℃, tundish steel nitrogen content [N] 23ppm, and submerged entry nozzle diameter 50mm.

[0067] The narrow face taper of the crystallizer is 1.15%; the wide face shrinkage is 1mm. The crystallizer protective slag is a slag with added exothermic agent to increase the temperature of the meniscus, with a consumption of 0.51 kg / t. Properties of the crystallizer protective slag: basicity (CaO / SiO2) 1.10, viscosity (η) 1300℃ 0.09 Pa·s, solidification temperature 1110℃, the composition of the exothermic agent and its content in the protective slag of the crystallizer are: CaSi: 5%, FeO: 7%.

[0068] The secondary cooling chamber uses a continuous casting secondary cooling process with strong cooling at the top and weak cooling at the bottom: strong cooling at the head of the secondary cooling zone with a specific water volume of 1.05 L / kg; and weak cooling in the middle and lower part of the secondary cooling zone with a specific water volume of 0.5 L / kg.

[0069] Cooling is controlled during billet transportation. Billets are delivered hot, and all billets are transferred to the heating furnace within 4 hours after cutting. Billets requiring offline processing or secondary cutting must be slowly cooled in an insulation pit at a temperature drop rate of 5-8℃ / h. The finishing temperature of the billets is 280℃.

[0070] The billet is slowly preheated in a hot rolling furnace, and the temperature is slowly increased below 800℃, with the heating rate controlled at 8℃ / min.

[0071] The billet quality in this embodiment (see Table 1) is as follows: no surface cracks; central segregation C 0.5, central porosity 0.5, as shown in the attached table. Figure 2 .

[0072] Example 3

[0073] The chemical composition and mass percentage of the ultra-high carbon steel in this embodiment are as follows: C: 1.50%, Si: 0.20%, Mn: 0.5%, P: 0.006%, S: 0.0010%, Als: 0.005%, Cr: 1.6%, Mo: 0.2%, Ni: 0.1%, with the remainder being Fe and unavoidable impurities.

[0074] A continuous casting process for improving the quality of ultra-high carbon steel billets includes the following steps: molten ultra-high carbon steel is poured from the ladle to the tundish through a long nozzle, and then poured into the crystallizer through an immersion nozzle. The molten steel cools and solidifies in the crystallizer to form a billet shell with a uniform thickness and a liquid core. The billet is then cooled and solidified in the secondary cooling chamber and bent and straightened. Finally, it is pulled out of the casting machine and cut into billets of the required length by a flame cutter. After controlled cooling during transportation, the billets are loaded into the hot rolling furnace within 5 hours.

[0075] The continuous casting process includes the following control parameters: tundish steel superheat 20℃, nitrogen content [N] in tundish steel controlled at 22ppm, and submerged entry nozzle diameter 55mm.

[0076] The narrow face taper of the crystallizer is 1.15%; the wide face shrinkage is 1mm. The crystallizer protective slag is a slag with added exothermic agent to increase the temperature of the meniscus, consuming 0.6 kg / t. Properties of the crystallizer protective slag: basicity (CaO / SiO2) 1.10, viscosity (η) 1300℃ 0.08 Pa·s, solidification temperature 1095℃, the composition of the exothermic agent and its content in the protective slag of the crystallizer are: CaSi: 4%, FeO: 8%.

[0077] The secondary cooling chamber uses a continuous casting secondary cooling process with strong cooling at the top and weak cooling at the bottom: strong cooling at the head of the secondary cooling zone with a specific water volume of 0.9 L / kg; and weak cooling in the middle and lower part of the secondary cooling zone with a specific water volume of 0.4 L / kg.

[0078] Cooling is controlled during billet transportation. Billets are delivered hot, and all billets are transferred to the heating furnace within 4 hours after cutting. Billets requiring offline processing or secondary cutting must be slowly cooled in an insulation pit at a temperature drop rate of 8-10℃ / h. The finishing temperature of the billets is 300℃.

[0079] The billet is slowly preheated in a hot rolling furnace, and the temperature is slowly increased below 800℃, with the heating rate controlled at 6℃ / min.

[0080] The billet quality in this embodiment (see Table 1) is as follows: no surface cracks; central segregation C 0.5, central porosity 1.0, as shown in the attached table. Figure 3 .

[0081] Example 4

[0082] The chemical composition and mass percentage of the ultra-high carbon steel in this embodiment are as follows: C: 1.05%, Si: 0.45%, Mn: 0.80%, P: 0.007%, S: 0.0020%, Als: 0.005%, Cr: 0.3%, Cu: 0.1%, Nb: 0.002%, with the remainder being Fe and unavoidable impurities.

[0083] A continuous casting process for improving the quality of ultra-high carbon steel billets includes the following steps: molten ultra-high carbon steel is poured from the ladle to the tundish through a long nozzle, and then poured into the crystallizer through an immersion nozzle. The molten steel cools and solidifies in the crystallizer to form a billet shell with a uniform thickness and a liquid core. The billet is then cooled and solidified in the secondary cooling chamber and bent and straightened. Finally, it is pulled out of the casting machine and cut into billets of the required length by a flame cutter. After controlled cooling during transportation, the billets are loaded into the hot rolling furnace within 5 hours.

[0084] The continuous casting process includes the following control parameters: tundish steel superheat 25℃, nitrogen content [N] in tundish steel controlled at 25ppm, and submerged entry nozzle diameter 52mm.

[0085] The narrow face taper of the crystallizer is 1.15%; the wide face shrinkage is 1mm. The crystallizer protective slag is a slag with added exothermic agent to increase the temperature of the meniscus, with a consumption of 0.54 kg / t. Properties of the crystallizer protective slag: basicity (CaO / SiO2) 1.10, viscosity (η) 1300℃ 0.07 Pa·s, solidification temperature 1110℃, the composition of the exothermic agent and its content in the protective slag of the crystallizer are: CaSi: 6%, FeO: 6%.

[0086] The secondary cooling chamber uses a continuous casting secondary cooling process with strong cooling at the top and weak cooling at the bottom: strong cooling at the head of the secondary cooling zone with a specific water volume of 1.0 L / kg; and weak cooling in the middle and lower part of the secondary cooling zone with a specific water volume of 0.5 L / kg.

[0087] Cooling is controlled during billet transportation. Billets are delivered hot, and all billets are transferred to the heating furnace within 4 hours after cutting. Billets requiring offline processing or secondary cutting must be slowly cooled in an insulation pit at a temperature drop rate of 6-9℃ / h. The finishing temperature of the billets is 260℃.

[0088] The billet is slowly preheated in a hot rolling furnace, and the temperature is slowly increased below 800℃, with the heating rate controlled at 7℃ / min.

[0089] The billet quality in this embodiment (see Table 1) is as follows: no surface cracks; central segregation C1.0, central porosity 0.5, as shown in the attached table. Figure 4 .

[0090] Example 5

[0091] The chemical composition and mass percentage of the ultra-high carbon steel in this embodiment are as follows: C: 1.10%, Si: 0.50%, Mn: 0.80wt%, P: 0.006%, S: 0.0013%, Als: 0.050%, Cr: 0.5%, with the remainder being Fe and unavoidable impurities.

[0092] A continuous casting process for improving the quality of ultra-high carbon steel billets includes the following steps: molten ultra-high carbon steel is poured from the ladle to the tundish through a long nozzle, and then poured into the crystallizer through an immersion nozzle. The molten steel cools and solidifies in the crystallizer to form a billet shell with a uniform thickness and a liquid core. The billet is then cooled and solidified in the secondary cooling chamber and bent and straightened. Finally, it is pulled out of the casting machine and cut into billets of the required length by a flame cutter. After controlled cooling during transportation, the billets are loaded into the hot rolling furnace within 5 hours.

[0093] The continuous casting process includes the following control parameters: tundish steel superheat 25°C, nitrogen content in tundish steel controlled at [N] 24ppm, and submerged entry nozzle diameter 53mm.

[0094] The narrow face taper of the crystallizer is 1.15%; the wide face shrinkage is 1mm. The crystallizer protective slag is a slag with added exothermic agent to increase the temperature of the meniscus, with a consumption of 0.53 kg / t. The properties of the crystallizer protective slag are: basicity (CaO / SiO2) 1.20, viscosity (η1300℃) 1.0 Pa·s, solidification temperature 1135℃. The exothermic agent composition and its content in the crystallizer protective slag are: CaSi: 5%, FeO: 7%.

[0095] The secondary cooling chamber uses a continuous casting secondary cooling process with strong cooling at the top and weak cooling at the bottom: strong cooling at the head of the secondary cooling zone with a specific water volume of 1.08 L / kg; and weak cooling in the middle and lower part of the secondary cooling zone with a specific water volume of 0.52 L / kg.

[0096] Cooling is controlled during billet transportation. Billets are delivered hot, and all billets are transferred to the heating furnace within 4 hours after cutting. Billets requiring offline processing or secondary cutting must be slowly cooled in an insulation pit at a temperature drop rate of 5-7℃ / h. The finishing temperature of the billets is 305℃.

[0097] The billet is slowly preheated in a hot rolling furnace, and the temperature is slowly increased below 800℃, with the heating rate controlled at 6℃ / min.

[0098] The billet quality in this embodiment (see Table 1) is as follows: no surface cracks; central segregation C 0.5, central porosity 0.5, as shown in the attached table. Figure 5 .

[0099] Table 1. Results of Internal Quality Assessment of Cast Billets in Examples 1-5

[0100]

[0101] The above embodiments are only used to illustrate and not limit the technical solutions of the present invention. 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 without departing from the spirit and scope of the present invention. Any modifications or partial substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A continuous casting process for improving the quality of ultra-high carbon steel billets, characterized in that, The process includes the following steps: molten ultra-high carbon steel is poured from the ladle to the tundish through a long nozzle, and then poured into the crystallizer through an immersion nozzle. The molten steel cools and solidifies in the crystallizer to form a uniformly thick billet shell with a liquid core. The billet is then cooled and solidified in the secondary cooling chamber and bent and straightened. Finally, it is pulled out of the casting machine and cut into billets of the required length by a flame cutting machine. After controlled cooling during transportation, the billets are loaded into a hot rolling furnace. The process includes the following control parameters: a mold flux is used in the mold, and the basicity of the mold flux is 1.10-1.20; the secondary cooling chamber uses a continuous casting secondary cooling process with strong cooling at the top and weak cooling at the bottom: strong cooling at the head of the secondary cooling zone with a specific water volume of 0.9-1.10 L / kg; weak cooling in the middle and lower part of the secondary cooling zone with a specific water volume of 0.4-0.6 L / kg, and the spray water at the corner of the billet is turned off; The properties of the crystallizer protective slag: viscosity (η) 1300℃ 0.06-0.10 Pa·s, solidification temperature 1095-1135℃; The crystallizer protective slag is a crystallizer protective slag with added heating agent. The composition of the heating agent and its content in the crystallizer protective slag are: CaSi: 4%-6%, FeO: 6%-8%; The consumption of the protective slag in the crystallizer is ≥0.4 kg / t; The carbon content (C) of the ultra-high carbon steel molten steel is 1.0%-2.10%. The quality of the cast billet is as follows: no surface cracks; central segregation ≤ C1.0, central porosity ≤ 1.

0.

2. The continuous casting process for improving the quality of ultra-high carbon steel billets according to claim 1, characterized in that, The inverted taper of the crystallizer: narrow face inverted taper: 1.15%±0.05%; wide face shrinkage of the crystallizer: 1mm±0.02mm.

3. A continuous casting process for improving the quality of ultra-high carbon steel billets according to claim 1 or 2, characterized in that, The superheat of the molten steel in the tundish is controlled at 20℃-30℃, and the nitrogen content in the molten steel in the tundish is controlled at [N]≤30ppm.

4. A continuous casting process for improving the quality of ultra-high carbon steel billets according to claim 1 or 2, characterized in that, The diameter of the immersion nozzle is 50-55mm.

5. A continuous casting process for improving the quality of ultra-high carbon steel billets according to claim 1 or 2, characterized in that, Cooling is controlled during transportation. The billet cooling must be organized as hot delivery. If slow cooling is required, the following must be met: billet temperature drop rate ≤ 10℃ / h; billet finishing temperature ≥ 150℃; the billet needs to be slowly preheated in the hot rolling furnace, and the temperature should be slowly increased below 800℃, with a heating rate ≤ 10℃ / min.

6. A continuous casting process for improving the quality of ultra-high carbon steel billets according to claim 1 or 2, characterized in that, The chemical composition and mass percentage of the ultra-high carbon steel molten steel are as follows: C: 1.0%-2.10%, Si: 0.10%-0.50%, Mn: 0.30%-1.80%, with the remainder being Fe, alloying elements, and unavoidable impurities.

Images