A sulfur-containing steel continuous casting billet and a method for controlling surface microcracks thereof

Abstract

This invention relates to a method for controlling microcracks on the surface of sulfur-containing steel continuously cast billets, belonging to the field of iron and steel smelting and continuous casting technology. To address the problem of microcracks easily forming on the surface of sulfur-containing steel continuously cast billets, this invention provides a method for controlling microcracks on the surface of sulfur-containing steel continuously cast billets, including control methods for the primary refining furnace process, the refining LF process, the vacuum treatment process, and the continuous casting process. This invention, through full-process process control, suppresses Class II MnS inclusions in molten steel, enhances the deformation and ductility of oxygen-sulfide composite inclusions by utilizing specific oxide particles, controls their morphology ratio to 2.5~3.0, and improves the castability of continuously cast billets; the surface longitudinal cracks of the continuously cast billets are ≤0.1mm, with a 100% pass rate, achieving low-magnification quality standards, reducing grinding and scrap rates, improving production efficiency, reducing costs, and facilitating the large-scale, high-quality production of sulfur-containing steel.

Description

Technical Field

[0001] This invention belongs to the field of iron and steel smelting and continuous casting technology, and particularly relates to a method for controlling microcracks on the surface of a sulfur-containing steel continuous casting billet. Background Technology

[0002] Sulfur-containing steel, as a special-performance steel, is widely used in many key fields such as machinery manufacturing and automotive industry due to its excellent machinability and good processing performance. It is often used to manufacture core components such as automotive transmissions, differentials, engine crankshafts, connecting rods and suspension system springs, which is of great significance to promoting the lightweight and high-performance development of related industries.

[0003] However, the presence of sulfur in steel easily induces surface microcracks during continuous casting, becoming a key bottleneck restricting the high-quality production of sulfur-containing steel. Sulfur readily combines with iron and other alloying elements in steel to form low-melting-point sulfides such as FeS, with a melting point of only about 988℃, far below the solidification temperature of steel. During continuous casting, these low-melting-point sulfides tend to segregate and accumulate in the grain boundary regions of steel, damaging the grain boundary bonding.

[0004] When a billet is in the solidification shrinkage stage or subjected to external thermal stress, the sulfides enriched at the grain boundaries become stress concentration points, leading to the formation of microcracks on the billet surface. These surface microcracks not only directly affect the appearance quality of the billet, but also further expand during subsequent hot rolling, cold rolling, and other processing, evolving into macrocracks. This severely reduces the final product qualification rate, resulting in raw material waste and a significant increase in production costs, thus limiting the further promotion and application of sulfur-containing steel. Summary of the Invention

[0005] To address the problem of microcracks easily forming on the surface of sulfur-containing steel continuous casting billets, this invention provides a method for controlling microcracks on the surface of sulfur-containing steel continuous casting billets.

[0006] The technical solution of the present invention:

[0007] A method for controlling microcracks on the surface of sulfur-containing steel continuous casting billets, comprising the following steps:

[0008] Step 1, Primary Refining Furnace Process: Control the Si content of the molten iron to be 0.60~0.85%, S≤0.050%, Ti≤0.060%, P≤0.080%, and the molten iron temperature to be 1300~1350℃; when tapping, the C content should be 0.07~0.09%, the tapping temperature should not be lower than 1620℃, and P≤0.012%; during the tapping process, aluminum blocks are added for pre-deoxidation and deoxidation alloying, followed by the addition of quicklime;

[0009] Step 2, LF refining process: First, add quicklime by powering on, and use C powder and aluminum granules for deoxidation and slag formation. Take samples to confirm the composition and adjust it to meet the standards for sulfur-containing steel. Second, add quicklime by powering on again to form white slag. The white slag should be maintained for no less than 20 minutes. After treatment, the S content of the molten steel should be ≤0.005%. After refining, the temperature at the station should be increased by 75~85℃ according to the liquid phase temperature of the steel grade, and then enter the vacuum treatment process.

[0010] Step 3, Vacuum treatment process: The refined qualified molten steel is sent into the vacuum device and kept in vacuum at a vacuum degree ≤67pa. After the molten steel is vacuumed, it is fed into the S line, then fed into the sulfur line and soft blown. After sampling and inspection, it is transferred to the continuous casting process.

[0011] Step 4, Continuous Casting Process: First, complete the debugging of the crystallizer and cooling system, and control the water quality and protective slag, then proceed with casting; the casting superheat is controlled at 30±5℃, and the continuous casting process parameters are controlled as follows: casting speed 0.60~0.70m / min, amplitude 2.8~3.2mm, vibration frequency 140~146 times / min, skewness 0.08~0.12, negative slip time 0.14~0.18s, negative slip rate 37~39%, specific water 0.18~0.22L / kg Cooling water distribution ratio: 34~38% / 37~41% / 23~27%; The continuous casting process integrates MEMS+FEMS electromagnetic stirring and light reduction comprehensive control technology. The first-end stirring is continuous stirring with parameters of 150A / 2Hz, and the end stirring is alternating stirring with parameters of 220A / 6Hz. The alternating stirring cycle is 10s running-3s pause-10s running-3s pause. The light reduction parameters are 0mm / 2mm / 4mm / 4mm, and the total reduction is 10mm.

[0012] Furthermore, when tapping steel from the primary smelting furnace in step one, the steel output is 95~105t. Pre-deoxidation is carried out using 7~9kg / t aluminum blocks. When the steel output reaches 38~42t, deoxidation and alloying are carried out, followed by the addition of 380~420kg of quicklime.

[0013] Furthermore, in step two, the refining LF process involves adding 180-220 kg of quicklime during the first power-on process and adding another 180-220 kg of quicklime during the second power-on process to create white slag.

[0014] Furthermore, in step three, the vacuum treatment process requires a vacuum holding time of no less than 10 minutes and a soft blowing time of no less than 15 minutes.

[0015] Furthermore, during the crystallizer debugging in step four of the continuous casting process, the water gap in the crystallizer is controlled at 3.8~4.2mm, the temperature difference between the inlet and outlet water of each crystallizer is ≤±0.2℃, and the number of times the copper tube of the crystallizer is used is ≤50 firings.

[0016] Furthermore, during the commissioning of the cooling system in step four of the continuous casting process, the specific water volume for continuous casting is 0.18~0.20L / kg, the accuracy of the large arc alignment in continuous casting is -0.5mm~0mm, and the deviation between the actual pressure and the set pressure of the support roller conveyor is <2bar.

[0017] Furthermore, in the water quality and protective slag control of the fourth continuous casting process, the turbidity of the soft water in the crystallizer is <20 NTU, the turbidity of the spray water is <30 NTU, and the inlet pressure is not lower than 0.95 MPa; the tundish uses an integral immersion nozzle with an insertion depth of 100~120 mm; the protective slag composition, by mass fraction, includes CaO 20~30%, SiO2 25~35%, Al2O3 6~10%, MgO 1~3%, F 3~7%, the melting point of the special protective slag is 1126±5℃, and the viscosity at 1300℃ is 0.77±0.3 Pa·s; the reducing agent in the protective slag, by mass fraction, is a mixture of 1~3% SiC and 0.5~2% aluminum powder.

[0018] Furthermore, in step four of the continuous casting process, the amount of residual steel in the ladle shall not be less than 2.5t, the amount of residual steel in the tundish during ladle replacement shall not be less than 16t, and the liquid level shall not be less than 950mm.

[0019] The sulfur-containing steel continuous casting billet prepared by the method provided by this invention has the following chemical composition by weight percentage: C 0.36~0.40%, Si 0.50~0.65%, Mn 1.30~1.55%, P≤0.025%, S 0.040~0.065%, Cr 0.10~0.20%, Ni≤0.15%, Mo≤0.05%, Cu≤0.20%, Al≤0.020%, with the remainder being Fe and unavoidable impurities.

[0020] The beneficial effects of this invention are:

[0021] This invention effectively suppresses Type II MnS inclusions in molten steel through full-process process control. By utilizing specific oxide particles as nucleation sites, it significantly improves the deformation and ductility of oxysulfide composite inclusions during hot rolling, successfully maintaining the aspect ratio (length-to-width ratio) of CaS-MnS composite inclusions within a reasonable range of 2.5–3.0, thus achieving precise control over MnS inclusions in rolled steel. This optimized inclusion morphology also effectively improves the continuous casting castability of sulfur-containing steel.

[0022] With effective control over inclusion morphology, the surface and internal quality of the sulfur-containing steel continuous casting billet prepared by this invention are fundamentally improved. The depth of surface cracks on the continuous casting billet is strictly controlled to ≤0.1mm, with a pass rate of 100%, completely solving the long-standing problem of surface microcracks in sulfur-containing steel production. The billet exhibits excellent low-magnification microstructure quality, with central porosity ≤1 grade, shrinkage cavities ≤0.5 grade, core density stable above 96, and segregation index controlled within a reasonable range of 0.93~1.08. This effectively avoids potential defects such as internal porosity, shrinkage cavities, and compositional segregation, providing high-quality base material with uniform quality and stable performance for the steel rolling process.

[0023] The sulfur-containing steel continuously cast billet provided by this invention, as a high-quality base material, significantly reduces the number of grinding cycles for both cast billets and rolled products, lowering material losses and labor costs during the grinding process, while also reducing the scrap rate due to quality defects. The number of continuous casting heats of sulfur-containing steel consistently reaches and exceeds 10 heats, significantly improving the continuity and stability of continuous casting production and further enhancing overall production efficiency. Ultimately, through full-process optimization, while effectively addressing quality defects in sulfur-containing steel, this invention achieves both cost reduction and efficiency improvement, enhancing the product's market competitiveness and providing a feasible path for the large-scale, high-quality production of sulfur-containing steel. Attached Figure Description

[0024] Figure 1 Comparative diagram of MnS inclusion morphology in sulfur-containing steel rolled products obtained in Comparative Example 1 and Example 1;

[0025] Figure 2 Comparative diagram showing the surface quality of sulfur-containing steel continuous casting billets obtained in Comparative Example 1 and Example 1;

[0026] Figure 3 This is a trend diagram showing the segregation index variation across the entire cross-section of a five-strand sulfur-containing steel continuous casting billet prepared by five-strand continuous casting in Example 1.

[0027] Figure 4 This is a low-magnification microstructure diagram of the sulfur-containing steel continuous casting billet obtained in Example 1. Detailed Implementation

[0028] The technical solution of the present invention will be further described below with reference to embodiments, but it is not limited thereto. Any modifications or equivalent substitutions to the technical solution of the present invention without departing from the spirit and scope of the technical solution of the present invention should be covered within the protection scope of the present invention. In the following embodiments, the process equipment or apparatus not specifically specified are all conventional equipment or apparatus in the art. Unless otherwise specified, the raw materials used in the embodiments of the present invention are all commercially available; unless otherwise specified, the technical means used in the embodiments of the present invention are all conventional means well known to those skilled in the art.

[0029] Example 1

[0030] This embodiment provides a method for controlling microcracks on the surface of sulfur-containing steel continuous casting billets, employing an electric furnace / converter-refining LF-vacuum-continuous casting process. The specific steps are as follows:

[0031] Step 1, Primary Refining Furnace Process:

[0032] The molten iron conditions were controlled as follows: Si content 0.72%, S 0.048%, Ti 0.052%, P 0.075%, and molten iron temperature controlled at 1325℃. During tapping, the C content was controlled at 0.08%, the tapping temperature at 1625℃, and the tapping quantity at 100t. During tapping, pre-deoxidation was performed using 8kg / t aluminum blocks. When the tapping quantity reached 40t, deoxidation and alloying treatment was carried out, followed by the addition of 400kg of quicklime.

[0033] Step 2, Refining LF process:

[0034] Moving to the refining LF process, the core of this step is to optimize the steel composition and improve its purity, while also creating conditions for subsequent vacuum sulfur feeding and inclusion control. During the first power-on refining cycle, 200 kg of quicklime is added, and deoxidation and slagging are performed using carbon powder and aluminum granules. After slagging is completed, samples are taken for composition confirmation. Based on the test results, the steel composition is precisely adjusted to ensure it meets the standards for sulfur-containing steel.

[0035] After the composition was adjusted to meet the requirements, 200 kg of quicklime was added during the second power-on to create white slag. The white slag was kept for 22 minutes. After treatment, the S content in the molten steel was controlled at 0.004%.

[0036] Meanwhile, to ensure the high stability and uniformity of the billet quality and achieve the "constant temperature and constant speed" goal in subsequent continuous casting processes, the refining process time is precisely controlled. The flow field characteristics and structural design of the ladle are optimized to improve overall metallurgical reaction efficiency, ensuring that the controllability of key process parameters such as refining time perfectly matches the continuous casting cycle. After refining, the outlet temperature is increased by 80°C based on the liquidus temperature of the steel grade, and then the billet enters the vacuum treatment process.

[0037] Step 3: Vacuum treatment process:

[0038] The refined molten steel is fed into a vacuum device and kept under a vacuum of 60 Pa. After the molten steel is under vacuum, it is fed into the S-line and the vacuum time is maintained for 12 minutes. After the sulfur feeding line is completed, it is soft blown for 18 minutes.

[0039] This process further removes gases and inclusions from the molten steel in a vacuum environment, while precisely controlling the sulfur content to ensure proper inclusion morphology control. After soft blowing, samples are taken for inspection to ensure the molten steel meets quality standards. Once qualified, the steel is hoisted and transported to the continuous casting process for pouring.

[0040] Step 4: Continuous casting process:

[0041] Before the continuous casting process begins, all equipment and process parameters must be debugged to ensure that production requirements are met.

[0042] (1) Crystallizer debugging: Ensure that no Cr plating layer falls off above the meniscus of the crystallizer, and that the copper tube of the crystallizer is used ≤50 times; check that the cooling spray of the foot roller section is in good condition, the nozzle opening angle meets the requirements, and the water gap of the crystallizer is controlled at 4mm; adopt a high-precision crystallizer that does not require width adjustment, and ensure that the temperature difference between the inlet and outlet water of each crystallizer is controlled at 0.15℃ to ensure the uniformity of primary cooling, so that the thickness and strength of the initial billet shell meet the process requirements.

[0043] (2) Cooling system debugging: Replace the secondary cooling nozzles before production, and replace all nozzles and bag filters with poor water spraying and atomization. Remeasure the spray ring spacing and alignment to ensure uniform spraying. Verify the arc alignment of the continuous casting large arc with a large arc ruler, and the accuracy meets the standard requirement of -0.4mm. Level the continuous casting support roller table to ensure that all roller tables rotate without abnormality, there is no water leakage at the joints, and the actual pressure deviates from the set pressure by 1.5 bar. Debug the vibration system to control the vibration level error ≤5‰, the polarization along the inner and outer arc directions <0.2mm, and the transverse direction <0.2mm. Control the continuous casting specific water volume at 0.20L / kg, and control the temperature difference between the inlet and outlet water of each flow within 0.2℃.

[0044] (3) Water quality and protective slag control: The turbidity of the soft water in the crystallizer is 18 NTU, the turbidity of the spray water is 28 NTU, and the inlet pressure is 0.98 MPa; the water is drained before pouring to ensure that the soft water inlet temperature is 26℃ before pouring and the temperature is controlled at 30℃ during continuous pouring, in accordance with the lower limit of the process; the cold test mode is turned on to ensure that the water pressure and flow rate after each flow bag are consistent; the water inlet depth is controlled at 110 mm, and a special protective slag with high alkalinity, low melting rate and added specific reducing agent components is used, which is developed for the solidification characteristics of sulfur-containing steel. The components include the following mass percentages: CaO 25%, SiO2 30%, Al2O3 8%, MgO 2%, F 5%. The melting point of the special protective slag is 1126±5℃, and the viscosity at 1300℃ is 0.77±0.3 Pa·s; the reducing agent in the protective slag is a mixture of 2% SiC and 1% aluminum powder by mass fraction. To ensure slag removal effect and suppress the formation of longitudinal cracks on the surface of the cast billet.

[0045] During continuous casting, the superheat is controlled at 28℃, the remaining steel in the ladle is 2.8t, and the remaining steel during the tundish changeover is 16.5t. The liquid level is 960mm. The continuous casting process parameters are controlled as follows: casting speed 0.65m / min, amplitude 3.0mm, frequency 143 times / min, skewness 0.1, negative slip time 0.16s, negative slip rate 38.11%, specific water volume 0.20L / kg, and cooling water distribution ratio 36% / 39% / 25%.

[0046] To address the issues of central porosity, carbon-sulfur segregation, and magnetic trace defects in large billets containing sulfur, this embodiment integrates a comprehensive control technology combining MEMS (Multi-Mode Electromagnetic Stirring) and FEMS (Final Solidification Electromagnetic Stirring) with light reduction in the continuous casting process: the initial stirring is set to continuous stirring with parameters of 150A / 2Hz, the final stirring is set to alternating stirring with parameters of 220A / 6Hz, the alternating cycle is 10s-pause-10s-pause, and the light reduction parameters are controlled at 0mm / 2mm / 4mm / 4mm, with a total reduction of 10mm.

[0047] The sulfur-containing steel continuous casting billet obtained in this embodiment has the following chemical composition by weight percentage: C 0.38%, Si 0.60%, Mn 1.4%, P 0.020%, S 0.050%, Cr 0.15%, Ni 0.10%, Mo 0.05%, Cu 0.20%, Al 0.020%, with the remainder being Fe and unavoidable impurities.

[0048] Example 2

[0049] This embodiment provides a method for controlling microcracks on the surface of sulfur-containing steel continuous casting billets, employing an electric furnace / converter-refining LF-vacuum-continuous casting process. The specific steps are as follows:

[0050] Step 1, Primary Refining Furnace Process:

[0051] The molten iron conditions were controlled as follows: Si content 0.60%, S 0.035%, Ti 0.040%, P 0.060%, and molten iron temperature controlled at 1300℃. During tapping, the C content was controlled at 0.07%, the tapping temperature at 1620℃, and the tapping quantity at 95t. During tapping, pre-deoxidation was performed using 7kg / t aluminum blocks. When the tapping quantity reached 38t, deoxidation and alloying treatment was carried out, followed by the addition of 380kg of quicklime.

[0052] Step 2, Refining LF process:

[0053] Entering the refining LF process, the core of this process is to optimize the steel composition and improve its purity, while also creating conditions for subsequent vacuum sulfur feeding and inclusion control. During the first power-on refining cycle, 180 kg of quicklime is added, and deoxidation and slagging are carried out using C powder and aluminum granules. After slagging is completed, samples are taken for composition confirmation. Based on the test results, the steel composition is precisely adjusted to ensure that it meets the standard requirements for sulfur-containing steel.

[0054] After the composition was adjusted to meet the requirements, 180 kg of quicklime was added during a second power-on process to create white slag. The white slag was maintained for 20 minutes. After treatment, the sulfur content in the molten steel was controlled at 0.005%.

[0055] After refining, the temperature at the station is increased by 80°C according to the liquid phase temperature of the steel grade, and then it enters the vacuum treatment process.

[0056] Step 3: Vacuum treatment process:

[0057] The refined molten steel is fed into a vacuum device and held at a vacuum level of 67 Pa for 10 minutes. During this time, sulfur wire is fed in, and after the sulfur wire is fed in, soft blowing is performed for 15 minutes. After soft blowing, samples are taken for inspection to ensure that the molten steel meets the quality standards. Once qualified, the steel is hoisted and transferred to the continuous casting process for pouring.

[0058] Step 4: Continuous casting process:

[0059] Before the continuous casting process begins, all equipment and process parameters must be debugged to ensure that production requirements are met.

[0060] (1) Crystallizer debugging: Ensure that no Cr plating layer falls off above the meniscus of the crystallizer, and that the copper tube of the crystallizer is used ≤40 times; check that the cooling spray of the foot roller section is in good condition, the nozzle opening angle meets the requirements, and the water gap of the crystallizer is controlled at 3.8mm; adopt a high-precision crystallizer that does not require width adjustment, and ensure that the temperature difference between the inlet and outlet water of each crystallizer is controlled at 0.18℃.

[0061] (2) Cooling system debugging: Replace the secondary cooling nozzles before production, and replace all nozzles and bag filters with poor water spraying and atomization. Remeasure the spray ring spacing and alignment to ensure uniform spraying. Verify the arc alignment of the continuous casting large arc with a large arc ruler, and the accuracy meets the standard requirement of -0.5mm. Level the continuous casting support rollers to ensure that all rollers rotate without abnormality, there is no water leakage at the joints, and the actual pressure deviates from the set pressure by 1.8 bar. Debug the vibration system, control the vibration level error to ≤5‰, the polarization along the inner and outer arc directions to <0.2mm, and the transverse direction to <0.2mm. The continuous casting specific water volume is 0.18L / kg, and the temperature difference between the inlet and outlet water of each flow is controlled within 0.2℃.

[0062] (3) Water quality and protective slag control: The turbidity of the soft water in the crystallizer is 16 NTU, the turbidity of the spray water is 26 NTU, and the inlet pressure is 0.95 MPa; before pouring, the water is drained to ensure that the soft water inlet temperature is 26℃ before pouring, and the temperature is controlled at 30℃ during continuous pouring, in accordance with the lower limit of the process; the cold test mode is turned on to ensure that the water pressure and water flow rate after each flow bag are consistent; the water inlet depth is controlled at 100 mm, and a special protective slag with high alkalinity, low melting rate and added specific reducing agent components is used, which is developed for the solidification characteristics of sulfur-containing steel. The components include the following mass percentages: CaO 25%, SiO2 30%, Al2O3 8%, MgO 2%, F 5%. The melting point of the special protective slag is 1126±5℃, and the viscosity at 1300℃ is 0.77±0.3 Pa·s; the reducing agent in the protective slag is a mixture of 2% SiC and 1% aluminum powder by mass fraction. To ensure slag removal effect and suppress the formation of longitudinal cracks on the surface of the cast billet.

[0063] During continuous casting, the superheat is 30℃, the remaining steel in the ladle is 2.5t, and the remaining steel during the tundish changeover is 16t. The liquid level is 950mm. The continuous casting process parameters are controlled as follows: casting speed 0.65m / min, amplitude 3.0mm, frequency 143 Hz / min, skewness 0.1%, negative slip time 0.16s, negative slip rate 38.11%, specific water volume 0.20L / kg, and cooling water distribution ratio 36% / 39% / 25%.

[0064] To address the issues of central porosity, carbon-sulfur segregation, and magnetic trace defects in large billets containing sulfur, this embodiment integrates a comprehensive control technology combining MEMS (Multi-Mode Electromagnetic Stirring) and FEMS (Final Solidification Electromagnetic Stirring) with light reduction in the continuous casting process: the initial stirring is set to continuous stirring with parameters of 150A / 2Hz, the final stirring is set to alternating stirring with parameters of 220A / 6Hz, the alternating cycle is 10s-pause-10s-pause, and the light reduction parameters are controlled at 0mm / 2mm / 4mm / 4mm, with a total reduction of 10mm.

[0065] The sulfur-containing steel continuous casting billet obtained in this embodiment has the following chemical composition by weight percentage: C 0.36%, Si 0.50%, Mn 1.30%, P 0.015%, S 0.040%, Cr 0.10%, Ni 0.15%, Mo 0.05%, Cu 0.20%, Al 0.020%, with the remainder being Fe and unavoidable impurities.

[0066] Example 3

[0067] This embodiment provides a method for controlling microcracks on the surface of sulfur-containing steel continuous casting billets, employing an electric furnace / converter-refining LF-vacuum-continuous casting process. The specific steps are as follows:

[0068] Step 1, Primary Refining Furnace Process:

[0069] The hot metal conditions were controlled as follows: Si content 0.85%, S 0.032%, Ti 0.060%, P 0.080%, and hot metal temperature controlled at 1350℃. During tapping, the C content was controlled at 0.09%, the tapping temperature at 1630℃, and the tapping quantity at 105t. During tapping, pre-deoxidation was performed using 9kg / t aluminum blocks. When the tapping quantity reached 42t, deoxidation and alloying treatment was carried out, followed by the addition of 420kg of quicklime.

[0070] Step 2, Refining LF process:

[0071] Entering the refining LF process, the core of this process is to optimize the steel composition and improve its purity, while also creating conditions for subsequent vacuum sulfur feeding and inclusion control. During the first power-on refining cycle, 220 kg of quicklime is added, and deoxidation and slagging are carried out using C powder and aluminum granules. After slagging is completed, samples are taken for composition confirmation. Based on the test results, the steel composition is precisely adjusted to ensure that it meets the standard requirements for sulfur-containing steel.

[0072] After the composition was adjusted to meet the requirements, 220 kg of quicklime was added during a second power-on process to create white slag. The white slag retention time was controlled to be 25 minutes. After treatment, the sulfur content in the molten steel was controlled to be 0.003%.

[0073] After refining, the temperature at the station is increased by 80°C according to the liquid phase temperature of the steel grade, and then it enters the vacuum treatment process.

[0074] Step 3: Vacuum treatment process:

[0075] The refined molten steel is fed into a vacuum device and held at a vacuum level of 55 Pa for 15 minutes. During this time, sulfur wire is fed in, and after the sulfur wire is fed in, soft blowing is performed for 20 minutes. After soft blowing, samples are taken for inspection to ensure that the molten steel meets the quality standards. Once qualified, the steel is hoisted and transferred to the continuous casting process for pouring.

[0076] Step 4: Continuous casting process:

[0077] Before the continuous casting process begins, all equipment and process parameters must be debugged to ensure that production requirements are met.

[0078] (1) Crystallizer debugging: Ensure that no Cr plating layer falls off above the meniscus of the crystallizer, and that the copper tube of the crystallizer is used ≤50 times; check that the cooling spray of the foot roller section is in good condition, the nozzle opening angle meets the requirements, and the water gap of the crystallizer is controlled at 4.2mm; adopt a high-precision crystallizer that does not require width adjustment, and ensure that the temperature difference between the inlet and outlet water of each crystallizer is controlled at 0.20℃.

[0079] (2) Cooling system debugging: Replace the secondary cooling nozzles before production, and replace all nozzles and bag filters with poor water spraying and atomization. Remeasure the spray ring spacing and centering status to ensure uniform spraying. Use a large arc ruler to verify the arc alignment of the continuous casting large arc, and the accuracy meets the 0mm standard requirement. Level the continuous casting support roller table to ensure that all roller tables rotate without abnormality, there is no water leakage at the joints, and the actual pressure deviates from the set pressure by 1.2 bar. Debug the vibration system and control the vibration level error to ≤5‰, the polarization along the inner and outer arc directions to <0.2mm, and the transverse direction to <0.2mm. Control the continuous casting specific water volume to 0.20L / kg, and control the temperature difference between the inlet and outlet water of each flow to within 0.2℃.

[0080] (3) Water quality and protective slag control: The turbidity of the soft water in the crystallizer is 19 NTU, the turbidity of the spray water is 29 NTU, and the inlet pressure is 1.0 MPa; the water is drained before pouring to ensure that the soft water inlet temperature is 26℃ before pouring and the temperature is controlled at 30℃ during continuous pouring, in accordance with the lower limit of the process; the cold test mode is turned on to ensure that the water pressure and flow rate after each flow bag are consistent; the water inlet depth is controlled at 120 mm, and a special protective slag with high alkalinity, low melting rate and added specific reducing agent components is used, which is developed for the solidification characteristics of sulfur-containing steel. The components include the following mass percentages: CaO 25%, SiO2 30%, Al2O3 8%, MgO 2%, F 5%. The melting point of the special protective slag is 1126±5℃, and the viscosity at 1300℃ is 0.77±0.3 Pa·s; the reducing agent in the protective slag is a mixture of 2% SiC and 1% aluminum powder by mass fraction. To ensure slag removal effect and suppress the formation of longitudinal cracks on the surface of the cast billet.

[0081] During continuous casting, the superheat is controlled at 28℃, the remaining steel in the ladle is 3.0t, and the remaining steel during the tundish changeover is 17t. The liquid level is 970mm. The continuous casting process parameters are controlled as follows: casting speed 0.65m / min, amplitude 3.0mm, frequency 143 times / min, skewness 0.1, negative slip time 0.16s, negative slip rate 38.11%, specific water volume 0.20L / kg, and cooling water distribution ratio 36% / 39% / 25%.

[0082] To address the issues of central porosity, carbon-sulfur segregation, and magnetic trace defects in large billets containing sulfur, this embodiment integrates a comprehensive control technology combining MEMS (Multi-Mode Electromagnetic Stirring) and FEMS (Final Solidification Electromagnetic Stirring) with light reduction in the continuous casting process: the initial stirring is set to continuous stirring with parameters of 150A / 2Hz, the final stirring is set to alternating stirring with parameters of 220A / 6Hz, the alternating cycle is 10s-pause-10s-pause, and the light reduction parameters are controlled at 0mm / 2mm / 4mm / 4mm, with a total reduction of 10mm.

[0083] The sulfur-containing steel continuous casting billet obtained in this embodiment has the following chemical composition by weight percentage: C 0.40%, Si 0.65%, Mn 1.55%, P 0.020%, S 0.065%, Cr 0.20%, Ni 0.15%, Mo 0.05%, Cu 0.20%, Al 0.020%, with the remainder being Fe and unavoidable impurities.

[0084] Comparative Example 1

[0085] This comparative example uses the same electric furnace / converter-refining LF-vacuum-continuous casting process as Example 1, and the specific steps are as follows:

[0086] Step 1, Primary Refining Furnace Process: Control the content of Si in the molten iron to be 0.70%, S to be 0.030%, Ti to be 0.050%, and P to be 0.070%, with the molten iron temperature at 1320℃; when tapping, the C content is 0.08%, the tapping temperature is 1620℃, and the tapping amount is 100t; when tapping, pre-deoxidize with 6kg / t aluminum blocks, and deoxidize and alloy when 35t of steel is tapped, followed by the addition of 350kg of quicklime.

[0087] Step 2, LF refining process: 150kg of quicklime is added during the first power-on process, and deoxidation and slag formation are carried out using single C powder. The composition is sampled and adjusted to meet the sulfur content steel standard. 150kg of quicklime is added during the second power-on process to form white slag. The white slag is maintained for 15 minutes. After treatment, the sulfur content of the molten steel is 0.008%. After refining, the temperature at the station is increased by 70°C according to the liquid phase temperature of the steel grade, and the steel enters the vacuum treatment process.

[0088] Step 3, Vacuum treatment process: The refined qualified molten steel is sent into the vacuum device and kept under a vacuum of 80 Pa for 8 minutes. During this period, S line is fed in, and after the sulfur line is fed in, soft blowing is carried out for 10 minutes. After sampling and inspection, the qualified steel is transferred to the continuous casting process.

[0089] Step 4. Continuous casting process: Debug the mold, control the water gap at 3.5 mm, the temperature difference between the inlet and outlet water of each strand mold is 0.3 °C, and the copper tube of the mold is used for 55 heats; Debug the cooling system, control the specific water volume of continuous casting at 0.22 L / kg, the alignment accuracy of the large arc of continuous casting is -0.8 mm, and the deviation between the actual pressure and the set pressure of the support roller table is 3 bar; Control the water quality and mold powder, the turbidity of soft water in the mold is 25 NTU, the turbidity of spray water is 35 NTU, the inlet water pressure is 0.90 Mpa, the submerged nozzle depth is 90 mm, and ordinary mold powder (without special component ratio) is used; Control the superheat at 28 °C, continuous casting process parameters: casting speed 0.75 m / min, amplitude 2.5 mm, vibration frequency 135 times / min, offset slope 0.06, negative slippage time 0.12 s, negative slippage rate 35%, specific water volume 0.16 L / kg, cooling water volume distribution ratio 30% / 35% / 35%; Adopt a single stirring mode (180 A / 4 Hz), without soft reduction operation; The remaining steel in the tundish is 2.0 t, the remaining steel in the intermediate ladle during ladle change is 15.0 t, and the liquid level height is 900 mm.

[0090] Figure 1 It is a comparison diagram of the morphology of MnS inclusions in the sulfur-containing steel rolled products obtained in Comparative Example 1 and Example 1; From Figure 1 It can be seen that the inclusions in the sulfur-containing steel of Comparative Example 1 are large in size, and the morphology is mostly irregular blocks and near-spherical shapes, with obvious compositional segregation. Large-sized inclusions are likely to become stress concentration sources, which will significantly reduce the plasticity, toughness and fatigue properties of the steel. The size of the inclusions in the sulfur-containing steel of Example 1 is significantly refined, and the morphology changes to small and dispersed strip or dot shapes, without obvious composite oxide coating. The surface scanning element distribution is more uniform, and the MnS inclusions are effectively fragmented and dispersed, avoiding the harm of large-sized inclusions, which is beneficial to improving the mechanical properties and processing performance of the steel. This shows that the control method of the present invention effectively realizes the refinement and dispersion of MnS inclusions, and significantly improves the morphology and distribution of inclusions.

[0091] Figure 2 It is a comparison diagram of the surface quality of the continuous casting billets of sulfur-containing steel obtained in Comparative Example 1 and Example 1; From Figure 2 It can be seen that there are obvious defects such as cracks, depressions and scabs on the surface of the continuous casting billet in Comparative Example 1, and the surface is rough and uneven. The defect area is marked with a white frame line, indicating poor surface quality. Such defects will expand during the subsequent rolling process, resulting in problems such as surface cracks and peeling of the finished steel, reducing the成材率. The surface of the continuous casting billet in Example 1 is flat and smooth, without obvious visible defects such as cracks and scabs, and the surface quality is significantly improved. Good surface quality can reduce the subsequent finishing workload, improve the成材率, and at the same time avoid the inheritance of surface defects to the final product. This shows that the control method of the present invention effectively improves the surface quality of the continuous casting billets of sulfur-containing steel and eliminates surface defects.

[0092] Figure 3This is a trend chart showing the segregation index variation across the entire cross-section of a five-strand sulfur-containing steel continuous casting billet prepared by five-strand continuous casting in Example 1; (The chart is derived from...) Figure 3 As can be seen, the segregation indices of each flow fluctuate slightly between 0.93 and 1.08, generally approaching 1.0, indicating that the carbon element is evenly distributed and the degree of segregation is low. This shows that the control method of the present invention can effectively control carbon segregation, achieve low segregation and high uniformity in continuous casting billet production, and exhibit excellent stability in multi-flow production.

[0093] Figure 4 This is a low-magnification microstructure diagram of the sulfur-containing steel continuous casting billet obtained in Example 1. Figure 4 As can be seen, the low-magnification microstructure of the continuously cast billet is uniform and dense, without obvious low-magnification defects such as central porosity, central segregation, cracks, or inclusions. The crystalline structure is uniformly distributed, without abnormally large grains or segregation bands, indicating that the solidification process was well controlled. The sulfur-containing steel continuously cast billet prepared by this invention has excellent low-magnification microstructure quality, with a dense and uniform internal structure and no obvious defects.

Claims

1. A method for controlling surface microcracks of a continuous casting billet of a sulfur-containing steel, characterized by, The steps are as follows: Step one, primary refining furnace process: control the Si content of molten iron 0.60~0.85%, S≤0.050%, Ti≤0.060%, P≤0.080%, the temperature of molten iron ; the C content at tapping is 0.07~0.09%, the tapping temperature is not lower than 1620℃, P≤0.012%; in the tapping process, aluminum block is added for pre-deoxidization and deoxidization alloying, and then lime is added; Step 2, LF refining process: First, add quicklime by powering on, and use C powder and aluminum granules for deoxidation and slag formation. Take samples to confirm the composition and adjust it to meet the standards for sulfur-containing steel. Second, add quicklime by powering on again to form white slag. The white slag should be maintained for no less than 20 minutes. After treatment, the S content of the molten steel should be ≤0.005%. After refining, the temperature at the station should be increased by 75~85℃ according to the liquid phase temperature of the steel grade, and then enter the vacuum treatment process. Step 3, Vacuum treatment process: The refined qualified molten steel is sent into the vacuum device and kept in vacuum at a vacuum degree ≤67pa. After the molten steel is vacuumed, it is fed into the S line, then fed into the sulfur line and soft blown. After sampling and inspection, it is transferred to the continuous casting process. Step 4, Continuous Casting Process: First, complete the debugging of the crystallizer and cooling system, and control the water quality and protective slag, then proceed with casting; the casting superheat is controlled at 30±5℃, and the continuous casting process parameters are controlled as follows: casting speed 0.60~0.70m / min, amplitude 2.8~3.2mm, vibration frequency 140~146 times / min, skewness 0.08~0.12, negative slip time 0.14~0.18s, negative slip rate 37~39%, specific water 0.18~0.22L / kg Cooling water distribution ratio: 34~38% / 37~41% / 23~27%; The continuous casting process integrates MEMS+FEMS electromagnetic stirring and light reduction comprehensive control technology. The first-end stirring is continuous stirring with parameters of 150A / 2Hz, and the end stirring is alternating stirring with parameters of 220A / 6Hz. The alternating stirring cycle is 10s running-3s pause-10s running-3s pause. The light reduction parameters are 0mm / 2mm / 4mm / 4mm, and the total reduction is 10mm.

2. The method for controlling microcracks on the surface of sulfur-containing steel continuously cast billets according to claim 1, characterized in that, When tapping steel from the primary smelting furnace in step one, the steel output is 95~105t. Pre-deoxidation is carried out at 7~9kg / t of aluminum blocks. When the steel output reaches 38~42t, deoxidation and alloying are carried out, followed by the addition of 380~420kg of quicklime.

3. The method for controlling microcracks on the surface of sulfur-containing steel continuously cast billets according to claim 1 or 2, characterized in that, Step 2: In the refining LF process, 180~220kg of quicklime is added during the first power-on process, and another 180~220kg of quicklime is added during the second power-on process to create white slag.

4. The method for controlling microcracks on the surface of sulfur-containing steel continuously cast billets according to claim 3, characterized in that, Step 3: Vacuum treatment process. Vacuum holding time shall not be less than 10 minutes, and soft blowing time shall not be less than 15 minutes.

5. The method for controlling microcracks on the surface of sulfur-containing steel continuously cast billets according to claim 4, characterized in that, In the fourth step of the continuous casting process, during the crystallizer debugging, the water gap in the crystallizer is controlled at 3.8~4.2mm, the temperature difference between the inlet and outlet water of each crystallizer is ≤±0.2℃, and the number of times the copper tube of the crystallizer is used is ≤50 firings.

6. The method for controlling microcracks on the surface of sulfur-containing steel continuously cast billets according to claim 5, characterized in that, In step four, during the commissioning of the cooling system for the continuous casting process, the specific water volume for continuous casting is 0.18~0.20L / kg, the accuracy of the large arc alignment in continuous casting is -0.5mm~0mm, and the deviation between the actual pressure and the set pressure of the support roller conveyor is <2bar.

7. The method for controlling microcracks on the surface of sulfur-containing steel continuously cast billets according to claim 6, characterized in that, In step four of the continuous casting process, the water quality and protective slag control are as follows: the turbidity of the soft water in the crystallizer is <20 NTU, the turbidity of the spray water is <30 NTU, and the inlet pressure is not lower than 0.95 MPa; the tundish uses an integral immersion nozzle with an insertion depth of 100~120 mm; the protective slag composition, by mass fraction, includes CaO 20~30%, SiO2 25~35%, Al2O3 6~10%, MgO 1~3%, and F 3~7%; the melting point of the special protective slag is 1126±5℃, and the viscosity at 1300℃ is 0.77±0.3 Pa·s; the reducing agent in the protective slag, by mass fraction, is a mixture of 1~3% SiC and 0.5~2% aluminum powder.

8. The method for controlling microcracks on the surface of sulfur-containing steel continuously cast billets according to claim 7, characterized in that, In step four of the continuous casting process, the amount of residual steel in the ladle shall not be less than 2.5t, the amount of residual steel in the tundish shall not be less than 16t, and the liquid level shall not be less than 950mm.

9. The sulfur-containing steel continuous casting billet prepared by the method according to any one of claims 1-8, characterized in that, The chemical composition by weight percentage includes: C 0.36~0.40%, Si 0.50~0.65%, Mn 1.30~1.55%, P≤0.025%, S 0.040~0.065%, Cr 0.10~0.20%, Ni≤0.15%, Mo≤0.05%, Cu≤0.20%, Al≤0.020%, with the remainder being Fe and unavoidable impurities.

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