A method for improving the macrostructure quality of 460 mm thickness medium carbon steel continuous cast billets

Abstract

This invention discloses a method for improving the low-magnification quality of a 460mm thick medium-carbon steel continuous casting billet, comprising: S1, molten steel is sequentially smelted in a converter, refined in an LF furnace, and subjected to RH vacuum degassing and calcium treatment to control the sulfur content in the steel to ≤0.006% and the phosphorus content to ≤0.013%; S2, the liquid level in the tundish is 1200±50mm, the casting superheat is 10~20℃, the liquid level fluctuation in the crystallizer is ≤±3mm, and argon gas is introduced into the long nozzle for sealing; the casting speed is 0.10~0.15m / min during the initial casting stage and 0.40~0.55m / min after steady state; S3, the overall specific water volume in the secondary cooling zone is 0.25~0.35L / kg, and the surface temperature of the billet exiting the fan-shaped section is above 750℃; S4, continuous reduction is performed within the range of 0.4~1.9 solid fraction at the center of the billet, with a total reduction of 15~20mm. The advantage of this invention is that it avoids porosity and segregation in the center of the continuously cast billet and improves the internal low-magnification quality of the billet.

Description

Technical Field

[0001] This invention belongs to the field of steel continuous casting technology, and particularly relates to a method for improving the low-magnification quality of a 460mm thick medium carbon steel continuous casting billet. Background Technology

[0002] Currently, high-end extra-thick plates are core materials used in key fields such as large pressure vessels, deep-sea engineering equipment, and heavy engineering machinery. As market demands for their internal purity and microstructure uniformity continue to rise, 460mm-class ultra-thick continuous casting billets have become the core raw material for the production of high-end extra-thick plates due to their ability to provide a high compression ratio for subsequent rolling. However, under the existing conventional continuous casting production system, this type of ultra-thick billet has long faced an internal quality bottleneck that is difficult to overcome.

[0003] The cross-sectional dimensions of a 460mm ultra-thick continuously cast billet are much larger than those of a conventional slab, directly resulting in a significantly extended solidification path and a markedly increased depth of the liquidus cavity. For medium carbon steel, a mainstream steel grade, the selective crystallization effect of solute elements during solidification is already significant. Under the long-cycle solidification conditions of an ultra-thick section, the solute redistribution process caused by selective crystallization is further amplified. Simultaneously, the slow overall solidification rate and long high-temperature residence time of the thick billet provide ample time and space for the continuous directional growth of columnar crystals. These columnar crystals easily extend from both sides of the wide face of the billet towards the center, eventually causing a "bridging" phenomenon in the central region. This bridging structure forms a closed, solute-rich residual liquid phase channel in the center. The solute-rich molten steel within this channel cannot be adequately fed back during subsequent solidification, ultimately leading to severe central segregation and central porosity defects in the billet center. This directly results in the billet exceeding the low-magnification rating and a significant decrease in internal microstructure uniformity.

[0004] However, existing conventional continuous casting processes generally suffer from systemic shortcomings when adapting to the production of such 460mm ultra-thick sections. On the one hand, to avoid problems such as nozzle clogging and obstructed floating of inclusions caused by insufficient molten steel flow during ultra-thick section casting, a relatively high superheat is generally used in production. However, excessive superheat further inhibits nucleation, accelerates the directional extension of columnar crystals, and significantly reduces the proportion of equiaxed crystal zone in the center of the billet, making it easier for columnar crystals to penetrate to the core and cause bridging. On the other hand, to accelerate the solidification speed of ultra-thick billets and improve production line efficiency, conventional processes generally use a relatively high secondary cooling intensity. This leads to a large temperature gradient between the inside and outside of the billet, which not only further strengthens the unidirectional growth trend of columnar crystals but also easily induces bulging deformation on the wide side of the billet, causing the solidification front to form a typical W-shaped structure. This further widens the solidification progress difference between different locations, causing more residual liquid phase enriched with solute to accumulate towards the center of the billet, continuously deteriorating the low-magnification quality inside the billet.

[0005] The combination of the aforementioned process shortcomings and the solidification characteristics of ultra-thick billets will ultimately be directly transmitted to subsequent rolling processes, resulting in a series of quality problems in the finished ultra-thick plates, such as significant fluctuations in mechanical properties, uneven properties in the thickness direction, steel plate delamination, and failure of internal flaw detection. These issues completely fail to meet the stringent requirements of high-end equipment materials for steel mechanical uniformity, low-temperature toughness, and long-term service stability. Summary of the Invention

[0006] The purpose of this invention is to solve the problem of poor low-magnification quality of existing 460mm thick medium carbon steel continuous casting billets. It provides a method to improve the low-magnification quality of 460mm thick medium carbon steel continuous casting billets, which can inhibit excessive growth of columnar crystals from the solidification source, uniform solute distribution in the core, effectively compensate for solidification volume shrinkage, avoid central porosity and segregation of extra-thick section continuous casting billets, and improve the low-magnification quality inside the billet.

[0007] To achieve the above objectives, the present invention adopts the following technical solution: A method for improving the low-magnification quality of a 460mm thick medium-carbon steel continuous casting billet, the specific implementation steps of which are as follows: The first step, steel smelting and cleanliness control: After the molten steel undergoes basic composition and temperature control in the converter primary smelting, it sequentially enters the LF refining station and the RH vacuum degassing station for further treatment. Specifically: The RH vacuum treatment process strictly controls the vacuum level to ≤3mbar and the pressure holding and degassing time to be no less than 15min, so as to fully remove harmful gases such as hydrogen and nitrogen from the molten steel and promote the flotation and removal of inclusions in the steel. Immediately after the RH vacuum treatment, calcium treatment is carried out. The amount of calcium added is precisely controlled at a feeding rate of 0.5 to 1.8m per ton of steel. Through the modification effect of calcium, the high melting point aluminum inclusions in the steel are spheroidized and modified to avoid nozzle nodule formation during subsequent casting. After calcium treatment, the molten steel is subjected to soft argon blowing, with the blowing time stably controlled between 10 and 25 minutes. This provides sufficient kinetic conditions for inclusions to float to the surface without exposing the molten steel to slag.

[0008] This process strictly controls the content of harmful elements in molten steel, ultimately controlling the mass fraction of sulfur in the steel to ≤0.006% and the mass fraction of phosphorus to ≤0.013%, reducing the tendency of solute elements to segregate from the source, while ensuring the overall high cleanliness of the molten steel and avoiding the accumulation of large particles in the core of the billet to form low-magnification defects.

[0009] The second step is low-superheat steady-state casting: After the ladle is hoisted to the continuous casting station, it is covered and insulated throughout the process to minimize temperature fluctuations during the transfer of molten steel; an alkaline low-carbon covering agent is used in the tundish for full protection to isolate it from air and prevent secondary oxidation of the molten steel. At the same time, the liquid level in the tundish is stably controlled at 1200±50mm, and the stable static pressure of the molten steel suppresses slag entrapment during the casting process.

[0010] The core of this process is controlling the superheat of the casting, limiting the superheat of the molten steel in the tundish to a narrow range of 10-20°C, which significantly reduces the initial superheat of the molten steel, promotes uniform nucleation of the entire cross-section of the billet, and inhibits the preferential directional growth trend of columnar crystals from the initial solidification stage.

[0011] The crystallizer adopts a high-precision automatic liquid level control system to control the liquid level fluctuation within the range of ≤±3mm, avoiding the problems of slag entrapment and uneven growth of the primary billet shell caused by large liquid level fluctuations; the long nozzle is sealed with argon gas throughout the process, and the argon gas flow rate is stably controlled at 5~10L / min, completely eliminating secondary oxidation caused by air entering the steel flow.

[0012] The casting speed control adopts a segmented steady-state mode: during the initial casting stage, the casting speed is controlled at 0.10 to 0.15 m / min to ensure uniform and stable growth of the initial billet shell and avoid the risk of steel leakage during initial casting; after entering the steady-state casting stage, the casting speed is stably controlled within a narrow range of 0.40 to 0.55 m / min to avoid large fluctuations in casting speed throughout the process and ensure the continuity and stability of the billet solidification process.

[0013] The third step is ultra-weak dynamic secondary cooling water distribution: an ultra-weak cooling regime is adopted, setting the overall specific water volume of the secondary cooling zone to 0.25–0.35 L / kg, which reduces the total cooling water volume by 25%–35% compared to the traditional process. Based on the continuous casting two-stage control model, the cooling water volume of each section of the secondary cooling is dynamically adjusted according to the real-time casting speed and billet surface temperature feedback, achieving precise matching of cooling intensity, and ultimately ensuring that the surface temperature of the billet after exiting the fan-shaped section is maintained above 750℃.

[0014] The cooling process in this step significantly reduces the temperature gradient between the inside and outside of the billet, fundamentally suppressing the unidirectional rapid growth rate of columnar crystals. This creates ample space for the nucleation and growth of equiaxed crystals in the core of the billet, effectively increasing the proportion of the central equiaxed crystal zone. From the solidification structure level, this reduces the probability of columnar crystal "bridging" and avoids the formation of liquid phase channels rich in solute in the core.

[0015] The fourth step is dynamic large-scale reduction at the end of solidification: Within the solidification range of 0.4 to 1.9 at the center of the billet, a continuous large-scale reduction operation is performed on the billet, with the reduction area continuously covering 4 to 6 sector segments of the continuous casting machine. Within this range, some incompletely solidified solute-rich residual liquid phase still exists in the core of the billet. Through continuous mechanical reduction, the total reduction is controlled at 15 to 20 mm, the reduction of a single sector segment is set at 3 to 5 mm, and the reduction rate is controlled at 0.5 to 0.8 mm / min.

[0016] This process, while avoiding internal cracks in the billet, fully compensates for the volume shrinkage during the solidification of the thick billet, directly presses together the tiny shrinkage cavities and porosity in the core, and at the same time blocks the flow channels of the residual liquid phase enriched with solute, fundamentally inhibiting the formation of central segregation and achieving densification of the core structure of the billet.

[0017] The fifth step is to implement steady-state production control throughout the entire casting process. This involves real-time online monitoring of all key process parameters, such as casting speed, steel temperature, secondary cooling zone water pressure, cooling water volume, and argon pressure and flow rate. This prevents significant abnormal fluctuations in parameters, ensures the continuous and stable solidification morphology of the billet throughout the process, and avoids abrupt changes in the solidification front position and abnormal solute flow and enrichment caused by process disturbances. Ultimately, this achieves stable and controllable low-magnification quality of 460mm thick medium carbon steel continuous casting billets.

[0018] Through the coordinated control of multiple processes in steps one to five above, it is possible to stably achieve low-magnification center segregation of 460mm medium carbon steel continuous casting billet of grade C ≤1.5 and center porosity ≤0.5, significantly improving the internal quality uniformity of the billet. This results in a significant increase in the pass rate of flaw detection for subsequent high-end extra-thick plates, fully meeting the stringent requirements of high-end equipment for the mechanical uniformity, low-temperature toughness, and long-term service stability of steel.

[0019] Compared with the prior art, the advantages of the technical solution of the present invention are as follows: (1) This invention targets the solidification characteristics of 460mm ultra-thick medium carbon steel. Through the deep coordination of multiple processes such as smelting cleanliness control, low superheat casting, ultra-weak cold water distribution, and large end-stage pressure reduction, it fundamentally matches the inherent characteristics of this type of thick billet, such as long solidification path, strong solute selection and crystallization effect, and easy bridging of columnar crystals, and achieves stable control of central segregation and central porosity of this type of billet. (2) The present invention significantly inhibits columnar crystal growth from the solidification initiation stage by combining low superheat steady-state casting with ultra-weak dynamic secondary cooling. The nucleation uniformity of the billet cross section is significantly improved, the equiaxed crystal ratio in the center of the billet is increased, and the probability of columnar crystals extending to the center and "bridging" is greatly reduced from the microstructure level. This reduces the formation of liquid phase channels for solute enrichment in the center from the source. (3) The present invention adopts the large pressure reduction at the end of solidification to compensate for the volume shrinkage of the solidified billet, which avoids the risk of internal cracks caused by pressure reduction under high solid fraction, effectively compresses the small shrinkage cavities in the core, and blocks the solute enrichment channel, and finally achieves the center segregation C class ≤1.5 grade and center porosity ≤0.5 grade of the billet, and the internal density of the billet is significantly improved. (4) The entire system of the present invention can be implemented on existing continuous casting production lines without the need for new large-scale equipment. The transformation cost is low and the system is highly replicable. It can be quickly promoted in domestic ultra-thick plate continuous casting production lines and is suitable for large-scale, batch, and high-quality production needs. (5) This invention significantly improves product quality, and the pass rate of subsequent rolling of high-end extra-thick plates is steadily increased by more than 5%. The uniformity of mechanical properties in the thickness direction of steel plates is greatly improved, which fully meets the stringent requirements of high-end fields such as marine engineering equipment, high pressure vessels, and large heavy-duty equipment for the long-term service stability of thick plates. Attached Figure Description

[0020] Figure 1 This is a flowchart of the low-magnification quality control method for a 460mm thick medium carbon steel continuous casting billet according to the present invention. Detailed Implementation Example 1

[0021] To make the present invention clearer, the following description, in conjunction with the accompanying drawings, further illustrates a method for improving the low-magnification quality of a 460mm thick medium-carbon steel continuous casting billet. The specific embodiments described herein are for illustrative purposes only and are not intended to limit the scope of the invention.

[0022] Taking the existing straight-arc 460mm×2200mm ultra-thick plate continuous casting production line as an example, see... Figure 1 The specific implementation steps for producing medium carbon steel continuous casting billets of grade Q355D using this process are as follows: (1) Smelting process: After tapping from the 150t converter, the steel enters the LF refining station for initial composition adjustment, and then is sent to the RH vacuum degassing station. The vacuum degree is stably controlled at 2.2mbar, and continuous pressure degassing is carried out for 17min to fully remove hydrogen and nitrogen harmful gases from the steel. After the RH treatment, calcium wire is fed in at a uniform rate of 1.2m / t to spheroidize and modify the aluminum inclusions in the steel. After the calcium treatment, soft blowing argon is carried out for 18min. There is no obvious turbulence of the molten steel surface during the soft blowing process. The sulfur content of the final finished steel is controlled at 0.004%, the phosphorus content is controlled at 0.011%, and the overall cleanliness of the steel meets the requirements for ultra-thick billet production.

[0023] (2) Casting: The ladle was covered and insulated throughout the process and transported to the continuous casting station. The tundish was sealed and protected with an alkaline low-carbon covering agent. The liquid level in the tundish was kept stable at 1200 mm. The superheat of the molten steel was controlled within a narrow range of 14 ± 1℃ throughout the casting process. The crystallizer was equipped with a high-precision automatic liquid level control system. The liquid level fluctuation was controlled within ± 2 mm throughout the process. Argon gas with a flow rate of 8 L / min was introduced through the long nozzle and sealed throughout the process to prevent secondary oxidation of the steel flow. The casting speed was controlled at 0.12 m / min during the initial casting stage. After the billet was completely pulled out of the crystallizer and entered steady-state production, the casting speed was locked at 0.48 m / min. There were no large fluctuations in casting speed throughout the process.

[0024] (3) Secondary cooling water distribution: The overall specific water volume of the secondary cooling zone is set at 0.29 L / kg, and the total cooling water volume is reduced by 31% compared with the traditional 460 mm thick billet production process of the plant. Based on the continuous casting secondary control model, the cooling water volume of each section of the secondary cooling zone is dynamically adjusted according to the real-time casting speed and infrared temperature measurement data of the billet surface. Finally, the surface temperature of each sector of the billet is stably maintained above 780℃, the temperature gradient inside and outside the billet is greatly reduced, and the unidirectional growth of columnar crystals is effectively suppressed.

[0025] (4) End-of-line pressing: Through real-time calculation using the solidification heat transfer model of the billet, five fan-shaped segments are continuously applied to implement gradient pressing within the range of 0.5~1.8 solid fraction at the center of the billet. The total pressing amount is set to 17 mm, and the pressing amount of a single fan-shaped segment is evenly distributed at 3.4 mm. The pressing rate is stably controlled at 0.6 mm / min. Under the premise of not causing internal cracks, the solidification volume shrinkage is fully compensated, the small shrinkage cavities in the core are pressed together, and the channels of the solute-rich liquid phase are blocked.

[0026] In this embodiment, the continuous casting secondary system locks and monitors key parameters such as casting speed, steel temperature, secondary cooling water pressure, and argon flow rate. The fluctuation range of all parameters is controlled within ±6% of the set value. There is no process disturbance during the entire casting process, and the solidification morphology of the billet is continuous and stable throughout the entire process.

[0027] The 460mm×2200mm Q355D continuous casting billet produced in this embodiment, after low-magnification pickling inspection, showed center segregation of Class C, grade 1.0, and center porosity of grade 0.5, with no visible internal cracks. Subsequent rolling of 150mm thick Q355D extra-thick plates using this billet achieved a flaw detection pass rate of 99.5%. Example 2

[0028] according to Figure 1 The process flow shown takes the production of medium carbon steel with a carbon content of 0.16% as an example, with a billet size of 460mm × 2000mm. The specific steps are as follows: (1) Smelting process: converter → LF refining → RH vacuum treatment, RH vacuum for 25 min, calcium feed line 1.5 m / t, argon blowing for 20 min, finished product S=0.004%, P=0.010%.

[0029] (2) Casting: superheat 15℃, liquid level in tundish 1165mm, casting speed 0.52m / min, argon seal flow rate of long nozzle 8L / min.

[0030] (3) Secondary cooling water supply: specific water volume 0.32L / kg, surface temperature of the billet in the fan-shaped section above 780℃.

[0031] (4) End reduction: Reduce the solidification interval from 0.4 to 1.8 mm, with a total reduction of 15 mm, implemented in 5 sector segments, with a single segment reduction of 3 mm and a reduction rate of 0.6 mm / min.

[0032] The test results of the continuously cast billet prepared by the method of this embodiment show that the central segregation of the billet is Class C 1.0 grade, the central porosity is 0.5 grade, and the flaw detection pass rate of the finished plate is 99.5%. Example 3

[0033] according to Figure 1 The process flow shown takes the production of medium carbon steel with a carbon content of 0.18% as an example, with a billet size of 460mm × 2500mm. The specific steps are as follows: (1) Smelting process: converter → LF refining → RH vacuum treatment, RH vacuum for 20 min, calcium feed line 1.2 m / t, argon blowing for 15 min, finished product S=0.002%, P=0.008%.

[0034] (2) Casting: superheat 10℃, liquid level in tundish 1205mm, casting speed 0.50m / min, argon seal flow rate of long nozzle 6L / min.

[0035] (3) Secondary cooling water supply: specific water volume 0.32L / kg, surface temperature of the billet in the fan-shaped section above 750℃.

[0036] (4) End reduction: Reduce the solidification interval from 0.4 to 1.9 mm, with a total reduction of 20 mm. This is carried out in 5 sector segments, with a single segment reduction of 4 mm and a reduction rate of 0.55 mm / min.

[0037] The continuous casting billet prepared using the method of this embodiment has the following test results: center segregation grade C 1.5, center porosity grade 1.0, and the flaw detection pass rate is 99.7%.

[0038] In addition to the embodiments described above, the present invention may have other implementations. All technical solutions formed by equivalent substitution or equivalent transformation fall within the protection scope claimed by the present invention.

Claims

1. A method for improving the low-magnification mass of a 460mm thick medium-carbon steel continuous casting billet, characterized in that: S1, Steel smelting and cleanliness control: After being smelted in a converter, the molten steel undergoes LF refining and RH vacuum degassing treatment in sequence. The RH vacuum degree is ≤3mbar and the holding time is not less than 15min. After RH vacuum, calcium treatment is carried out with a calcium wire feed rate of 0.5~1.8m / t. After the calcium treatment is completed, argon is blown for 10~25min. Finally, the sulfur content in the steel is controlled to be ≤0.006% and the phosphorus content is controlled to be ≤0.013%. S2, Low Superheat Steady-State Casting: After continuous casting on the ladle, the entire process is covered and insulated. The tundish is protected with an alkaline low-carbon covering agent. The liquid level in the tundish is kept stable at 1200±50mm. The casting superheat is controlled at 10~20℃. The liquid level fluctuation in the crystallizer is ≤±3mm. Argon gas is introduced into the long nozzle for sealing, and the argon gas flow rate is 5~10L / min. The casting speed is controlled at 0.10~0.15m / min during the initial casting stage, and stabilized at 0.40~0.55m / min after steady-state production. S3, ultra-weak dynamic secondary cooling water distribution: the overall specific water volume of the secondary cooling zone is set to 0.25~0.35L / kg, the total cooling water volume is reduced by 25%~35%, and the cooling water volume of each zone is dynamically adjusted to keep the surface temperature of the billet exiting the fan-shaped section above 750℃. S4, dynamic large reduction at the end of solidification: continuous reduction is carried out in the range of 0.4 to 1.9 solid fraction at the center of the billet, the reduction area covers 4 to 6 fan-shaped segments, the total reduction is 15 to 20 mm, the reduction of a single fan-shaped segment is 3 to 5 mm, and the reduction rate is controlled at 0.5 to 0.8 mm / min. S5, steady-state control of the entire production process: keeping key parameters such as casting speed, steel temperature, water pressure, water volume, and air pressure stable throughout the entire casting process.

2. The method for improving the low-magnification mass of a 460mm thick medium-carbon steel continuous casting billet according to claim 1, characterized in that: In step S1, the calcium wire feeding rate for calcium treatment is 0.8–1.5 m / t, and the soft argon blowing time after calcium treatment is controlled at 15–20 min.

3. The method for improving the low-magnification mass of a 460mm thick medium-carbon steel continuous casting billet according to claim 1 or 2, characterized in that: In step S2, the control deviation of the superheat during casting shall not exceed ±2℃, and the intermediate ladle shall be controlled with constant liquid level throughout the process to avoid large fluctuations in liquid level of more than ±30mm.

4. The method for improving the low-magnification mass of a 460mm thick medium-carbon steel continuous casting billet according to claim 1 or 2, characterized in that: In step S3, the dynamic adjustment accuracy error of the cooling water volume in each section of the secondary cooling zone is ≤ ±5%, and the surface temperature of the billet exiting the fan-shaped section is continuously maintained above 780℃.

5. The method for improving the low-magnification mass of a 460mm thick medium-carbon steel continuous casting billet according to claim 1 or 2, characterized in that: In step S3, the overall specific water volume of the secondary cooling zone is set to 0.25–0.29 L / kg.

6. The method for improving the low-magnification mass of a 460mm thick medium-carbon steel continuous casting billet according to claim 1 or 2, characterized in that: In step S4, the billet is continuously pressed down within the range of 0.5 to 1.8 solid fraction at the center, with a total reduction of 15 to 17 mm.

7. The method for improving the low-magnification mass of a 460mm thick medium-carbon steel continuous casting billet according to claim 1 or 2, characterized in that: In step S5, the fluctuation range of key process parameters is controlled within ±6% of the set value.

8. The method for improving the low-magnification mass of a 460mm thick medium-carbon steel continuous casting billet according to claim 1 or 2, characterized in that: The 460mm medium carbon steel continuous casting billet finally prepared by steps S1 to S5 has a low magnification center segregation of Class C ≤ 1.5 and a center porosity ≤ 0.5.

Images