Method for manufacturing high-carbon medium-alloy large-size continuous casting round billet for cold rolling rolls

Abstract

This invention discloses a method for manufacturing a large-size continuous casting round billet of high-carbon medium alloy steel for cold rolling mills, comprising the following steps: S1, providing molten high-carbon medium alloy steel; S2, performing continuous casting to obtain a continuous casting round billet; S3, subjecting the continuous casting round billet to stress-relief annealing treatment; wherein, during the continuous casting process, the straightening temperature is controlled to be no lower than the upper limit of the brittle temperature range of the high-carbon medium alloy steel, and multi-stage electromagnetic stirring is applied. The method for manufacturing a large-size continuous casting round billet of high-carbon medium alloy steel for cold rolling mills of this invention, by controlling the straightening temperature to be no lower than the upper limit of the brittle temperature range of the high-carbon medium alloy steel, ensures that the straightening deformation occurs in the temperature range where the material has good plasticity. This improves the core quality of the billet, significantly reduces the risk of central cracks directly caused by straightening stress, reduces the formation of central cracks, and meets the requirements for producing φ700-1200mm rolling mill steel.

Description

Technical Field

[0001] This invention belongs to the field of high carbon steel production technology. Specifically, this invention relates to a method for manufacturing large-size continuous casting round billets of high carbon medium alloy for cold rolling rolls. Background Technology

[0002] Cold rolling rolls are key components in cold rolling equipment, and their performance directly affects the quality of rolled products, production efficiency, and equipment stability. To meet the high requirements for hardness, wear resistance, and fatigue resistance of cold rolling rolls, the industry currently widely adopts the electroslag remelting process for manufacturing. Electroslag remelting can effectively improve the purity, density, and compositional uniformity of steel ingots, thereby obtaining high-quality roll blanks.

[0003] However, with the increasing size of rolling mills, the demand for large-diameter, high-carbon, medium-alloy cold rolling rolls (φ700mm and above) is growing. Market feedback and production practice indicate that the core quality of steel ingots produced using existing processes for these large-diameter rolls remains significantly inadequate. Specifically, severe central cracks easily appear in the core of the ingot during the subsequent electroslag remelting process. These cracks not only reduce the material's performance but also induce localized "scraping" during production due to thermal stress, seriously threatening the stability and safety of the electroslag remelting process, leading to production interruptions, increased costs, and difficulty in guaranteeing the final product's pass rate and reliability.

[0004] Analysis suggests that the root cause of these problems lies in the inadequate quality control of the core of large-section billets in traditional smelting and casting processes for high-carbon medium-alloy steel. Existing processes are insufficient to completely eliminate macroscopic segregation and porosity in the central region of the billet, making this area a weak point for crack initiation and propagation during subsequent remelting.

[0005] Chinese patent CN106563780A discloses a continuous casting method for large round billets of medium-high carbon and medium-high alloy tool steel. This method primarily overcomes the drawbacks of high alloy tool steel, such as high hardness and difficulty in straightening, by setting and optimizing process parameters. The composition includes: C content ≥ 0.30%, Cr content ≥ 3.0%, and total alloy content ≥ 5%. It mainly targets most die steels, and the patent primarily describes round billets with a diameter of φ700mm or less. This method mainly addresses the continuous casting of high-carbon and medium-alloy round billets for large-diameter cold rolling rolls; however, it does not address the difficulty in controlling center cracks ≤ 2.0 mm in these larger sizes.

[0006] Chinese Patent CN115652181A discloses a method for manufacturing a large round billet of high-carbon low-alloy hot-roll steel, mainly applicable to low-alloy hot-roll steel with a specification of φ800mm, and the corresponding grade 60CrMnMo has a carbon content of about 0.60% and a Cr content of about 1.05%; it increases the tension leveling temperature and controls the temperature at 55~105℃ above the liquidus line. In contrast, this invention is for steel with a carbon content of about 0.80% and a Cr content of 4.80~5.20%, and controls the temperature at 20~35℃ above the liquidus line, applicable to the preparation of high-carbon medium-alloy cold-roll steel with a specification of φ700-1200mm.

[0007] This invention provides a method for manufacturing large-size continuous casting round billets of high carbon medium alloy steel for cold rolling rolls, particularly concerning how to improve the core quality of the billet, reduce central cracks, and meet the requirements for producing φ700-1200mm roll steel. Summary of the Invention

[0008] The present invention aims to at least solve one of the technical problems existing in the prior art. To this end, the present invention provides a method for manufacturing a large-size continuous casting round billet of high carbon medium alloy for cold rolling rolls, with the purpose of improving the core quality of the billet, reducing central cracks, and meeting the requirements for producing φ700-1200mm roll steel.

[0009] To achieve the above objectives, the technical solution adopted by the present invention is as follows: a method for manufacturing large-size continuous casting round billets of high-carbon medium alloy for cold rolling rolls, comprising the following steps: S1. Provides molten steel for high-carbon medium alloy steel; S2. Perform continuous casting to obtain a continuously cast round billet; S3. Perform stress-relief annealing on the continuously cast round billet; During the continuous casting process, the straightening temperature is controlled to be no lower than the upper limit of the brittle temperature range of the high-carbon medium alloy steel, and multi-stage electromagnetic stirring is applied.

[0010] Step S1 includes sequential hot metal pretreatment, converter smelting, LF furnace refining, and RH vacuum refining.

[0011] The sulfur content of the molten iron after pretreatment is ≤0.005%.

[0012] During the converter smelting process, the phosphorus content at the final stage is controlled by adding slag in batches and using slag-blocking operations, so that the phosphorus content of the finished product is ≤0.012%.

[0013] During the refining process in the LF furnace, the alkalinity of the refining slag is controlled at 3-4, the refining time is ≥40 min, and the white slag retention time is ≥20 min.

[0014] During the RH vacuum refining process, the vacuum degree is maintained at no more than 100 Pa for ≥25 minutes, and after the vacuum is broken, weak stirring is performed for ≥20 minutes.

[0015] In step S2, the straightening temperature is ≥840℃.

[0016] In step S2, the superheat of the molten steel in the continuous casting process is controlled at 30℃~50℃, the casting speed is controlled at 0.08~0.35m / min, and the specific water content is controlled at 0.05~0.18L / kg.

[0017] The multi-stage electromagnetic stirring includes crystallizer electromagnetic stirring, secondary cooling zone electromagnetic stirring and solidification end electromagnetic stirring. The current of the electromagnetic stirring in the crystallizer is 300A to 500A, and the frequency is 1.0Hz to 2.0Hz. The electromagnetic stirring current in the second cooling zone is 50A to 70A, and the frequency is 0.5Hz to 1.5Hz. The current of the electromagnetic stirring at the solidification end is 1150A to 1250A, and the frequency is 4.0Hz to 6.0Hz.

[0018] In step S2, a protective slag with an alkalinity of 0.8 to 1.0, a melting temperature of 1105°C to 1185°C, and a viscosity of 0.7 to 1.1 Pa•s is used.

[0019] In step S2, the ladle and the tundish are connected by a long nozzle, and argon gas is introduced into the connection for protection, with the argon gas flow rate not exceeding 13 Nm. 3 / h.

[0020] In step S3, the stress-relieving annealing process includes: heating a continuously cast round billet with a surface temperature ≥500℃ to 650℃~700℃ at a rate of ≤60℃ / h and holding it at that temperature, and then cooling it to below 400℃ at a rate of ≤25℃ / h.

[0021] The present invention relates to a method for manufacturing large-size continuous casting round billets of high-carbon medium alloy steel for cold rolling rolls. By controlling the straightening temperature to be no lower than the upper limit of the brittle temperature range of high-carbon medium alloy steel, the straightening deformation occurs in the temperature range where the material has good plasticity. This method can improve the core quality of the billet, significantly reduce the risk of central cracks caused directly by straightening stress, reduce the formation of central cracks, and meet the requirements for producing φ700-1200mm roll steel. Attached Figure Description

[0022] This manual includes the following figures, which illustrate the following: Figure 1 This is a flowchart of the manufacturing method of high-carbon medium alloy large-size continuous casting round billet for cold rolling rolls according to the present invention; Figure 2 This is a low-magnification acid-etched cross-section of the center cross section of a φ700mm round blank obtained according to the manufacturing method of the present invention; Figure 3 This is a low-magnification acid-etched cross-section of the center cross section of a φ800mm round blank obtained according to the manufacturing method of the present invention. Detailed Implementation

[0023] The specific embodiments of the present invention will be further described in detail below with reference to the accompanying drawings, in order to help those skilled in the art to have a more complete, accurate and in-depth understanding of the inventive concept and technical solution of the present invention, and to facilitate its implementation.

[0024] like Figure 1 As shown, this embodiment of the invention provides a method for manufacturing large-size continuous casting round billets of high-carbon medium alloy for cold rolling mills, comprising the following steps: S1. Provides molten steel for high-carbon medium alloy steel; S2. Perform continuous casting to obtain a continuously cast round billet; S3. Perform stress-relief annealing on the continuously cast round billet; During the continuous casting process, the straightening temperature is controlled to be no lower than the upper limit of the brittle temperature range of the high-carbon medium alloy steel, and multi-stage electromagnetic stirring is applied.

[0025] Specifically, the present invention relates to a manufacturing process for high-carbon medium alloy steel continuous casting round billets used to prepare electroslag remelting base materials for cold rolling rolls.

[0026] The main chemical composition of this steel ranges as follows: C: 0.70%–1.00%, Cr: 4.80%–5.20%, P: ≤0.012%, S: ≤0.005%, O: ≤0.0015%, N: ≤0.0080%. This composition system is particularly suitable for manufacturing electroslag remelted base material for large-diameter (φ700mm to φ1200mm) cold rolling rolls.

[0027] In the existing technology, when using conventional processes to produce the above-mentioned large-size high-carbon medium alloy cold rolling roll steel, the continuously cast billets often have the problem of poor core quality. This is mainly manifested in the subsequent electroslag remelting process, where the billet center has serious crack defects, which easily leads to localized spalling during remelting. This seriously affects the stability and yield of the electroslag remelting process and restricts the reliable production of large-size high-performance cold rolling rolls.

[0028] To address the aforementioned issues, this invention provides a process flow based on "converter smelting + LF furnace refining + RH vacuum degassing + continuous casting" by optimizing composition control and matching corresponding manufacturing processes. This process aims to significantly improve the core quality of large-section continuously cast round billets, effectively control the generation and development of central cracks, and thus meet the stringent requirements for the internal quality of the base material in the manufacture of large-size cold rolling rolls ranging from φ700mm to 1200mm.

[0029] In this embodiment of the invention, step S1 includes sequential hot metal pretreatment, converter smelting, LF furnace refining, and RH vacuum refining.

[0030] In this embodiment of the invention, the sulfur content of the molten iron after pretreatment is ≤0.005%.

[0031] In this embodiment of the invention, during the converter smelting process, the phosphorus content at the final stage is controlled by adding slag in batches and using slag blocking operation, so that the phosphorus content of the finished product is ≤0.012%.

[0032] In this embodiment of the invention, during the LF furnace refining process, the amount of slag is increased, and the basicity of the refining slag is controlled to be 3-4; the carbon content is precisely controlled within the composition range; the refining time is ≥40 min, and the white slag retention time is ≥20 min.

[0033] In this embodiment of the invention, during the RH vacuum refining process, the vacuum degree is maintained for ≥25 minutes at a level not exceeding 100 Pa, and after the vacuum is broken, weak stirring is performed for ≥20 minutes.

[0034] In this embodiment of the invention, in step S2 above, the straightening temperature is ≥840℃, and the upper limit of the brittle temperature range of high carbon medium alloy steel is 840℃.

[0035] In this embodiment of the invention, in step S2 above, the superheat of the molten steel in the continuous casting process is controlled at 30℃~50℃, the casting speed is controlled at 0.08~0.35m / min, and the specific water content is controlled at 0.05~0.18L / kg.

[0036] Multi-stage electromagnetic stirring includes electromagnetic stirring in the crystallizer, electromagnetic stirring in the secondary cooling zone, and electromagnetic stirring at the end of solidification. The current of the electromagnetic stirrer in the crystallizer is 300A to 500A, and the frequency is 1.0Hz to 2.0Hz. The electromagnetic stirring current in the secondary cooling zone is 50A to 70A, and the frequency is 0.5Hz to 1.5Hz. The current of the electromagnetic stirrer at the end of solidification is 1150A~1250A, and the frequency is 4.0Hz~6.0Hz.

[0037] The manufacturing method of this invention is particularly applicable to the continuous casting production of high-carbon medium-alloy cold-rolled roll steels such as 8Cr5Mo. The high-temperature thermoplasticity curve of this steel exhibits a typical "C"-shaped or bell-shaped characteristic, with significant brittle valleys within the temperature range of 750-950℃, and the temperature range with a higher risk of low plasticity is approximately 800-840℃. Within this range, austenite undergoes a phase transformation to thin-film ferrite, leading to weakened grain boundaries; simultaneously, the pinning effect of solute atoms and the uneven distribution of undissolved carbides further reduce the deformation uniformity and plasticity of the material. If hot deformation operations such as continuous casting straightening are performed in this brittle temperature region, it is highly likely to induce central cracks in the cast billet.

[0038] To address the inherent characteristics of the aforementioned 8Cr5Mo steel, this invention implements a synergistic control process strategy in the continuous casting process. First, by strictly setting the straightening temperature above 840℃, its low-plasticity brittleness temperature range is avoided, thus suppressing crack initiation from the deformation condition perspective. Second, a three-stage electromagnetic stirring system is employed, combining electromagnetic stirring in the crystallizer, electromagnetic stirring in the secondary cooling zone, and electromagnetic stirring at the end of solidification, with stirring parameters optimized for the solidification characteristics of this steel. Specifically, electromagnetic stirring in the crystallizer (current 300~500A, frequency 1.0~2.0Hz) aims to refine the initial solidification structure; electromagnetic stirring in the secondary cooling zone (current 50~70A, frequency 0.5~1.5Hz) aims to reduce banded segregation; and electromagnetic stirring at the end of solidification (current 1150~1250A, frequency 4.0~6.0Hz) aims to enhance feeding and homogenization at the end of solidification, ultimately controlling the liquid core diameter to below 80mm. These measures work together to improve the solidification structure of 8Cr5Mo steel billets, thereby controlling the tendency for central cracks to be below level 2.0.

[0039] Furthermore, the continuous casting process employs constant casting speed operation, with the casting speed controlled between 0.08 and 0.35 m / min according to the billet specifications, and the matching specific water volume is 0.05 to 0.18 L / kg. The superheat of the molten steel is also controlled between 30 and 50°C. The comprehensive control of these parameters aims to create stable solidification conditions for the aforementioned key processes (straightening temperature control and electromagnetic stirring), further ensuring the core quality of large-size 8Cr5Mo continuously cast round billets.

[0040] In this embodiment of the invention, to reduce secondary oxidation and heat loss of molten steel during continuous casting, the tundish is covered with a ladle cover, and the surface of the molten steel inside is covered with a double-layer structure consisting of a tundish covering agent and carbonized rice husks. A protective slag compatible with 8Cr5Mo steel is used in the crystallizer.

[0041] In step S2 above, the basicity of the protective slag is 0.8 to 1.0, the melting temperature is 1105℃ to 1185℃, and the viscosity is 0.7 to 1.1 Pa•s.

[0042] In step S2 above, the ladle and tundish are connected by a long nozzle, and argon gas is introduced into the joint between the long nozzle and the ladle sliding nozzle for sealing protection. The argon gas flow rate does not exceed 13 Nm. 3 / h. The initial insertion depth of the long nozzle into the molten steel surface of the tundish should be controlled between 110 mm and 160 mm; when using an argon-blown stopper rod, this insertion depth should not be less than 115 mm. The addition of the protective slag should follow the principle of small, uniform, and frequent addition, and the amount added at one time should ensure that the slag layer thickness fluctuates by no more than 10 mm. Throughout the entire casting cycle, the total slag layer thickness in the crystallizer should be maintained between 40 mm and 60 mm, and the thickness fluctuation should be controlled within ±10 mm. The slag layer thickness must be monitored using a dedicated measuring tool, and the measurement frequency should not be less than once every 5 heats of molten steel.

[0043] The continuous casting process described above employs a continuous straightening process to straighten the cast billets. After being cut into lengths online, the continuously cast round billets are immediately subjected to pit annealing. Given that the Ac1 transformation point of this steel grade is between 810℃ and 830℃, to avoid austenitic transformation, the introduction of new structural stresses, and a decrease in hardness during annealing, the stress-relieving annealing temperature must be set below the Ac1 point. A holding temperature range of 650℃ to 700℃ is selected, as atoms at this temperature have sufficient diffusion capacity to effectively eliminate most of the macroscopic residual stress. The holding time must ensure that the core of the cast billet reaches the set temperature and completes sufficient stress relaxation.

[0044] In this embodiment of the invention, the stress-relieving annealing process in step S3 includes: heating a continuously cast round billet with a surface temperature ≥500℃ to 650℃~700℃ at a rate of ≤60℃ / h and holding it at that temperature, and then cooling it to below 400℃ at a rate of ≤25℃ / h.

[0045] In step S3 above, a continuously cast round billet with a surface temperature of not less than 500°C is placed into the annealing pit. It is then heated to 700°C at a heating rate not exceeding 60°C / h, and held at this temperature according to the billet specifications. After holding, the billet is slowly cooled to 400°C at a cooling rate not exceeding 25°C / h, then the lid is removed, and the billet is removed from the pit after its temperature drops below 200°C.

[0046] This annealing process, by controlling the heating and cooling rates and holding the temperature below the Ac1 phase transformation point, aims to fully release the structural and thermal stresses inside the billet, thereby reducing hardness and preventing the propagation of central cracks caused by residual stress.

[0047] The present invention relates to a method for manufacturing large-size continuous casting round billets of high-carbon medium alloy for cold rolling rolls. By taking measures such as reasonable straightening temperature, controlling electric stirring parameters, reasonable superheat, stable casting speed, reasonable water content, and reasonable annealing process, the central cracks are reduced and the solidification structure is improved, thereby ensuring that the central cracks of the billet are ≤2.0 grade, the central porosity is ≤1.5 grade, and the shrinkage cavity is ≤0.5 grade.

[0048] The method of the present invention can be used for continuously cast round billets with a diameter of φ700-1200mm, improving the core quality of the billet, reducing central cracks, and meeting the requirements for use of continuously cast round billets for cold rolling rolls.

[0049] To further understand the present invention, the manufacturing method of high-carbon medium alloy continuous casting round billet for cold rolling rolls provided by the present invention will be described in detail below with reference to embodiments. Example

[0050] This embodiment provides a method for manufacturing high-carbon medium-alloy continuous casting round billets for cold rolling rolls. Taking the production of 8Cr5Mo steel for rolling rolls by smelting in a 150t converter as an example, the method includes the following steps performed in sequence: Determine the furnace charge conditions and charging system: Select high-quality blast furnace molten iron with a Si content of 0.30%~0.50%, an S content of ≤0.050%, and a molten iron temperature of ≥1250℃.

[0051] Hot metal pretreatment: KR hot metal pretreatment desulfurizer uses lime and fluorite, and the S content after treatment is 0.003%. The hot metal undergoes two slag removal treatments. Steel tapping: The smelting cycle is 33 minutes. When the blowing starts, add 1 / 3 of the slag first. After blowing for about three minutes, pour out the slag first, and then add the remaining slag for blowing. When tapping, do a good job of blocking the slag to prevent the slag from falling back into the phosphorus. The P of the tapped steel is 0.008%.

[0052] LF furnace refining: The Al2O3 content in the LF refining slag is 20%~30%, and the basicity is 3.4; the refining time is 42 min, the white slag holding time is 21 min, the deoxidation and slag formation are good, and argon blowing promotes the flotation of inclusions and reduces secondary oxidation; the composition after refining is: C content is 0.81%, and Cr content is 5.10%.

[0053] RH refining: vacuum treatment time 25min, vacuum degree 67Pa, argon blowing throughout the vacuum treatment; after the RH furnace is broken, ladle covering agent is added for hydrogen determination.

[0054] Continuous casting: The continuous casting machine produces φ700mm 8Cr5Mo rolls using a three-stage electromagnetic stirring system (M-EMS+S-EMS+F-EMS). The ladle and tundish are connected by a long nozzle. Argon gas is introduced into the gap between the long nozzle of the ladle and the sliding nozzle of the ladle. The argon gas flow rate is controlled at 6 Nm3 / h. The initial insertion depth of the nozzle into the molten steel surface is 120mm. During continuous casting, the molten steel was superheated to 25℃, the electromagnetic stirring current in the crystallizer was controlled at 300A and the frequency at 1.0Hz; the electromagnetic stirring current in the secondary cooling zone was controlled at 50A and the frequency at 0.5Hz; the electromagnetic stirring current at the solidification end was controlled at 1150A and the frequency at 4.0Hz; the casting speed was controlled at 0.23m / min; the straightening outlet temperature was 848℃, corresponding to a specific water content of 0.09L / kg; the tundish was covered with a ladle cover; the molten steel inside the tundish was covered with a double layer of tundish covering agent and carbonized rice husk; a special 8Cr5Mo crystallizer protective slag was used; the basicity of the protective slag was 0.83; the melting temperature of the protective slag was 1150℃; and the viscosity of the protective slag was 0.8Pa•S.

[0055] Annealing: After being straightened by 11 straightening machines, the continuously cast round billets are cut into fixed lengths and directly hoisted into a slow cooling pit for hot charging annealing. The slow cooling pit is surrounded by insulation walls and covered with an insulation cover. There are 8 nozzles around the pit for uniform heating. Before charging, the surface temperature of the round billet is 539℃. It is heated to 700℃ at a rate of ≤60℃, held for 30 hours, and then cooled to 400℃ at a rate of 25℃. Then the cover is removed and the billet is taken out of the pit at ≤200℃.

[0056] In this embodiment, the cross-sectional morphology of the continuously cast round billet for φ700mm 8Cr5Mo cold rolling rolls is as follows: Figure 2 As shown, the surface quality is good. The low-magnification structural defects are rated according to the Grade A rating chart in Appendix A of YB / T 153: central porosity ≤ Grade 1.5, shrinkage cavities ≤ Grade 0.5, and central cracks 0; there are no intermediate cracks. Example

[0057] This embodiment provides a method for manufacturing high-carbon medium-alloy continuous casting round billets for cold rolling rolls. Taking the production of 8Cr5Mo steel for rolling rolls by smelting in a 150t converter as an example, the method includes the following steps performed in sequence: Determine the furnace charge conditions and charging system: Select high-quality blast furnace molten iron with a Si content of 0.30%~0.50%, an S content of ≤0.050%, and a molten iron temperature of ≥1250℃.

[0058] Hot metal pretreatment: KR hot metal pretreatment desulfurizer uses lime and fluorite, and the S content after treatment is 0.004%. The hot metal undergoes two slag removal treatments. Loading amount: Add materials according to the ratio of molten iron to scrap steel of 3:1. Add appropriate amounts of molten iron and scrap steel according to the condition of the molten iron. Add lime and sinter in a timely manner according to the composition analysis and temperature to enhance dephosphorization and appropriately reduce the temperature to prevent the temperature from being too high and causing the phosphorus to be too high. Steel tapping: The smelting cycle is 32 minutes. When the blowing starts, add 1 / 3 of the slag first. After blowing for about three minutes, pour out the slag first, and then add the remaining slag for blowing. When tapping, do a good job of blocking the slag to prevent the slag from falling back into the phosphorus. The P of the tapped steel is 0.007%.

[0059] LF furnace refining: The Al2O3 content in the LF refining slag is 20%~30%, and the basicity is 3.2; the refining time is 42 min, the white slag holding time is 21 min, the deoxidation and slag formation are good, and argon blowing promotes the flotation of inclusions and reduces secondary oxidation; the composition after refining is: C content is 0.82%, and Cr content is 5.11%.

[0060] RH refining: vacuum treatment time 26min, vacuum degree 66Pa, argon blowing throughout the vacuum treatment; after the RH furnace is broken, ladle covering agent is added for hydrogen determination; Continuous casting: The continuous casting machine produces φ800mm 8Cr5Mo rolls using a three-stage electromagnetic stirring system (M-EMS+S-EMS+F-EMS). The ladle and tundish are connected by a long nozzle. Argon gas is introduced into the gap between the long nozzle of the ladle and the sliding nozzle of the ladle. The argon gas flow rate is controlled at 6 Nm3 / h. The initial insertion depth of the nozzle into the molten steel is 120mm. During continuous casting, the superheat of the molten steel is controlled at 30℃, the electromagnetic stirring current in the crystallizer is controlled at 500A and the frequency is controlled at 1.5Hz; the electromagnetic stirring current in the secondary cooling zone is controlled at 70A and the frequency is controlled at 1.5Hz; the electromagnetic stirring current at the end of solidification is controlled at 1250A and the frequency is controlled at 6.0Hz; the casting speed is controlled at 0.18m / min, the straightening outlet temperature is 844℃, the corresponding specific water content is 0.07L / kg, the tundish is covered with a ladle cover, and the molten steel in the tundish is covered with a double layer of tundish covering agent plus carbonized rice husk, using 8Cr5Mo special crystallizer protective slag, with a basicity of 0.83, a melting temperature of 1150℃, and a viscosity of 0.8Pa•S.

[0061] Annealing: After being straightened by 11 straightening machines, the continuously cast round billets are cut into fixed lengths and directly hoisted into a slow cooling pit for hot charging annealing. The slow cooling pit is surrounded by insulation walls and covered with an insulation cover. There are 8 nozzles around the pit for uniform heating. Before loading into the furnace, the surface temperature of the round billet is 540℃. It is heated to 700℃ at a rate of ≤60℃, held for 36 hours, and then cooled to 400℃ at a rate of 25℃. Then the cover is removed and the billet is removed from the pit at ≤200℃.

[0062] In this embodiment, the cross-sectional morphology of the φ800mm 8Cr5Mo roll steel continuously cast round billet is as follows: Figure 3As shown, the surface quality of the cast billet is good. According to the rating chart of Appendix A of YB / T 153, the low magnification structural defects are: central porosity ≤ 1.0 grade, shrinkage cavity ≤ 0 grade, and central crack ≤ 1.5 grade; there are no intermediate cracks.

[0063] The present invention has been described above by way of example with reference to the accompanying drawings. Obviously, the specific implementation of the present invention is not limited to the above-described manner. Any non-substantial improvements made using the inventive concept and technical solution; or the direct application of the inventive concept and technical solution to other situations without modification, are all within the protection scope of the present invention.

Claims

1. A method for manufacturing large-size continuous casting round billets of high-carbon medium alloy for cold rolling rolls, characterized in that, Includes the following steps: S1. Provides molten steel for high-carbon medium alloy steel; S2. Perform continuous casting to obtain a continuously cast round billet; S3. Perform stress-relief annealing on the continuously cast round billet; During the continuous casting process, the straightening temperature is controlled to be no lower than the upper limit of the brittle temperature range of the high-carbon medium alloy steel, and multi-stage electromagnetic stirring is applied.

2. The manufacturing method according to claim 1, characterized in that, Step S1 includes sequential hot metal pretreatment, converter smelting, LF furnace refining, and RH vacuum refining.

3. The manufacturing method according to claim 2, characterized in that, The sulfur content of the molten iron after pretreatment is ≤0.005%.

4. The manufacturing method according to claim 2, characterized in that, During the converter smelting process, the phosphorus content at the final stage is controlled by adding slag in batches and using slag-blocking operations, so that the phosphorus content of the finished product is ≤0.012%.

5. The manufacturing method according to claim 2, characterized in that, During the refining process in the LF furnace, the alkalinity of the refining slag is controlled at 3-4, the refining time is ≥40 min, and the white slag retention time is ≥20 min.

6. The manufacturing method according to claim 2, characterized in that, During the RH vacuum refining process, the vacuum degree is maintained at no more than 100 Pa for ≥25 minutes, and after the vacuum is broken, weak stirring is performed for ≥20 minutes.

7. The manufacturing method according to any one of claims 1 to 6, characterized in that, In step S2, the straightening temperature is ≥840℃.

8. The manufacturing method according to any one of claims 1 to 6, characterized in that, In step S2, the superheat of the molten steel in the continuous casting process is controlled at 30℃~50℃, the casting speed is controlled at 0.08~0.35m / min, and the specific water content is controlled at 0.05~0.18L / kg.

9. The manufacturing method according to any one of claims 1 to 6, characterized in that, The multi-stage electromagnetic stirring includes crystallizer electromagnetic stirring, secondary cooling zone electromagnetic stirring and solidification end electromagnetic stirring. The current of the electromagnetic stirring in the crystallizer is 300A to 500A, and the frequency is 1.0Hz to 2.0Hz. The electromagnetic stirring current in the second cooling zone is 50A to 70A, and the frequency is 0.5Hz to 1.5Hz. The current of the electromagnetic stirring at the solidification end is 1150A to 1250A, and the frequency is 4.0Hz to 6.0Hz.

10. The manufacturing method according to any one of claims 1 to 6, characterized in that, In step S2, a protective slag with an alkalinity of 0.8 to 1.0, a melting temperature of 1105°C to 1185°C, and a viscosity of 0.7 to 1.1 Pa•s is used.

11. The manufacturing method according to any one of claims 1 to 6, characterized in that, In step S2, the ladle and the tundish are connected by a long nozzle, and argon gas is introduced into the connection for protection, with the argon gas flow rate not exceeding 13 Nm. 3 / h.

12. The manufacturing method according to any one of claims 1 to 6, characterized in that, In step S3, the stress-relieving annealing process includes: heating a continuously cast round billet with a surface temperature ≥500℃ to 650℃~700℃ at a rate of ≤60℃ / h and holding it at that temperature, and then cooling it to below 400℃ at a rate of ≤25℃ / h.

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