A low-crack and low-segregation continuous casting method for 37Mn5 small round billets

Abstract

The present application relates to a kind of 37Mn5 small round billet low crack low segregation continuous casting method, and the chemical composition of round billet includes by weight percentage:C:0.35%~0.39%, Si:0.22%~0.32%, Mn:1.35%~1.55%, P≤0.016%, S≤0.008%, Al:0.010%~0.030%, N≤55ppm;Crystallizer copper pipe upper outer wall is equipped with heat insulation plating, from the upper mouth to the lower mouth direction of crystallizer copper pipe, the taper of crystallizer copper pipe decreases from no heat insulation plating place;The solidification of the casting blank in crystallizer is carried out three-stage soft cooling after two cooling zone;It can solve the problem such as subcutaneous crack, center segregation, center loose, developed columnar crystal of φ180mm diameter 37Mn5 small round billet without light press-down equipment, so that round billet surface does not need large area to be directly entered subsequent rolling process after grinding, to realize stable production, improve yield.

Description

Technical Field

[0001] This invention belongs to the field of special steel continuous casting technology, specifically relating to a low-crack, low-segregation continuous casting method for φ180mm 37Mn5 small round billets. Background Technology

[0002] 37Mn5 steel, as a typical medium-carbon manganese steel, significantly improves the strength and toughness of the steel by adding appropriate amounts of manganese and other alloying elements, while maintaining good processing performance and corrosion resistance. It is widely used in fields such as oil well drilling casing and machinery manufacturing. To match the pipe specifications, φ180mm 37Mn5 small round billets are used as rolling and pipe-threading blanks. Their quality directly affects the yield and subsequent processing performance. However, 37Mn5 steel has a C content of 0.34%~0.40% and a Mn content of 1.30%~1.60%, which has a high crack sensitivity and a tendency for central segregation. If the round billet has problems such as subcutaneous cracks, central segregation, and well-developed columnar crystals, it will cause defects such as internal and external folding during pipe threading, which may lead to scrapping in severe cases. If large-area grinding is required before rolling, it will further affect production costs and efficiency. Therefore, it is necessary to develop a low-crack and low-segregation continuous casting method for φ180mm 37Mn5 small round billets, so that the surface of the round billet does not need to be ground over a large area and can directly enter the subsequent rolling process to achieve stable mass production.

[0003] The existing methods for developing high-quality continuous casting of 37Mn5 small round billets still face the following technical bottlenecks:

[0004] I. Existing continuous casting technology for medium carbon manganese steel round billets mostly relies on a combination of strong electromagnetic stirring and light reduction to control center segregation and porosity. Compared with φ150mm medium carbon manganese steel round billets, φ180mm 37Mn5 round billets have larger cross sections, are more sensitive to cooling, have longer solidification paths and wider solidification ranges during continuous casting. In addition, most on-site equipment is equipped with electromagnetic stirring but lacks light reduction equipment. Under the condition of relying solely on electromagnetic stirring without light reduction, the difficulty of controlling center segregation, center porosity and surface cracks is significantly increased, making it difficult to meet quality standards.

[0005] Second, the cooling process of existing medium carbon manganese steel round billet continuous casting technology is mostly a general strong cooling mode for large-scale billets with a diameter of φ250mm and above. This mode shortens the liquid core length and suppresses center segregation through high-intensity cooling, and then relies on light pressing to compensate for solidification shrinkage. However, when using this strong cooling mode to produce 37Mn5 small round billets with a larger specific surface area, it is impossible to avoid the high-temperature brittle zone. Not only is the improvement of center segregation limited, but the conventional process cannot be adapted to the current situation of φ180mm small round billets without light pressing equipment. Without light pressing, problems such as subcutaneous cracks, central porosity, and well-developed columnar crystals are easily caused, which cannot meet the requirements of stable mass production. Summary of the Invention

[0006] This invention aims to at least partially solve one of the aforementioned technical problems. This invention provides a low-crack, low-segregation continuous casting method for 37Mn5 small round billets, which can solve the problems of subcutaneous cracks, central segregation, central porosity, and well-developed columnar crystals in φ180mm diameter 37Mn5 small round billets without light-pressure equipment. This allows the round billet surface to directly enter the subsequent rolling process without large-area grinding, so as to achieve stable mass production and improve the yield.

[0007] The technical solution adopted by this invention to solve its technical problem is:

[0008] A method for continuous casting of 37Mn5 small round billets with low cracking and low segregation, wherein the chemical composition of the billets, by weight percentage, includes: C: 0.35%~0.39%, Si: 0.22%~0.32%, Mn: 1.35%~1.55%, P≤0.016%, S≤0.008%, Al: 0.010%~0.030%, N≤55ppm, with the remainder being Fe and unavoidable impurity elements; the method includes:

[0009] (1) Steel transfer and pouring: The molten steel is transferred to the tundish for pouring, and the temperature of the molten steel in the tundish is controlled to be >1495℃ and the superheat of the tundish is ≤26℃;

[0010] (2) Solidification in the crystallizer: The molten steel is poured into the crystallizer and initially solidifies to form a billet. The upper outer wall of the copper tube of the crystallizer is provided with a heat insulation coating. From the upper opening to the lower opening of the copper tube of the crystallizer, the inverted taper of the copper tube of the crystallizer decreases from the part without the heat insulation coating. A protective slag with an alkalinity of 1.15~1.25 is used.

[0011] (3) Secondary cooling and solidification: The billet solidified in the crystallizer undergoes three-stage soft cooling in the secondary cooling zone: along the continuous casting direction, the first zone is located 0~1.8m after the crystallizer, and the billet surface temperature is controlled at ≥960℃; the second zone is located 1.8~3.8m after the crystallizer, and the billet surface temperature is controlled at 820~920℃; the third zone is located 3.8m after the crystallizer, and the billet surface temperature is controlled at ≥680℃. The total water volume of the secondary cooling zone is 0.78~0.96L / kg.

[0012] (4) Straightening: The billet, after being cooled and solidified in the secondary cooling zone, is pulled out of the continuous casting machine, and the pulling speed is controlled at 2.0~2.2m / min;

[0013] (5) Online slow cooling after casting: The billet pulled out of the continuous casting machine immediately enters the heat preservation and slow cooling pit. The surface temperature of the billet entering the pit is controlled to be ≥680℃, the slow cooling rate is ≤38℃ / h, the heat preservation and slow cooling time is ≥4h, and the billet is removed from the pit when the billet temperature is ≤280℃ to obtain a φ180mm round billet.

[0014] In a preferred embodiment, the Mn / Si ratio in the chemical composition of the round blank is controlled at 4.8~5.2.

[0015] In the preferred technical solution, the temperature of the molten steel in the tundish during the steel transfer and pouring is 1510~1530℃, with fluctuations ≤±3℃; the superheat of the tundish is 18~26℃.

[0016] In the preferred technical solution, when the superheat of the tundish is 18~22℃, the pulling speed is controlled at 2.1~2.2m / min; when the superheat of the tundish is 23~26℃, the pulling speed is controlled at 2.0~2.1m / min; and the pulling speed fluctuation is controlled at ≤±0.05m / min.

[0017] In the preferred technical solution, the total cooling water volume during solidification in the crystallizer is controlled to be 200~230L / min, the water volume in the upper 1 / 2 region of the crystallizer is 18%~22% lower than that in the lower region, and the outlet water temperature difference is controlled to be ≤8℃.

[0018] In the preferred embodiment, the height of the copper tube in the crystallizer during solidification is 850 mm, the wall thickness is 20 mm, and a heat-insulating coating is provided on the outer wall from the top of the copper tube to 400 mm. The thickness of the heat-insulating coating gradually decreases from 0.65 mm to 0.25 mm from top to bottom. The inverted taper of the copper tube from the top of the copper tube to 400 mm is 1.0%, and the inverted taper of the copper tube from 400 mm to the bottom of the copper tube is 0.8%.

[0019] In the preferred embodiment, the composition of the protective slag during solidification in the crystallizer, by weight percentage, includes: CaO: 32.0%~38.0%, SiO2: 28.0%~33.0%, Al2O3: 2.5%~4.5%, MgO: 3.0%~5.5%, Na2O: 4.0%~6.5%, F: 3.0%~5.5%, B2O3: 1.0%~2.5%, MnO: ≤1.0%, Fe2O3: ≤1.0%, solid C: 5.0%~7.5%, with the remainder being unavoidable impurities.

[0020] In the preferred embodiment, the viscosity of the protective slag during solidification in the crystallizer is 0.3~0.6 Pa·s at 1300℃, the melting point is 1100~1140℃, and the thickness of the liquid slag layer is 9~11 mm.

[0021] In the preferred embodiment, the vibration during solidification in the crystallizer is a non-sinusoidal vibration with an amplitude of ±3.2 mm, a frequency of 180~200 cpm, a negative slip time of 0.14~0.16 s, and a vibration mark depth of ≤0.25 mm.

[0022] In a preferred embodiment, the electromagnetic stirring current of the crystallizer during solidification is 180~220A, and the frequency is 5~7Hz.

[0023] In the preferred technical solution, water cooling is used for cooling and solidification in the two cooling zones. The water volume in the first zone is 0.32~0.38L / kg, the water volume in the second zone is 0.28~0.34L / kg, and the water volume in the third zone is 0.18~0.24L / kg.

[0024] In the preferred technical solution, the cooling and solidification in the secondary cooling zone employs electromagnetic stirring at the solidification end. The electromagnetic stirring at the solidification end is located 4.5~5.5m behind the crystallizer, the solid content of the billet is 30%~50%, the current is 160~200A, and the frequency is 4~6Hz.

[0025] A 37Mn5 small round billet, wherein the billet is manufactured by the low-crack, low-segregation continuous casting method for 37Mn5 small round billets described in any one of the above-mentioned methods.

[0026] In the preferred technical solution, the subsurface cracks of the round blank are ≤0.3%, and there are no surface cracks with a length ≥2mm; the low magnification structure is: central porosity ≤1.0 grade, central shrinkage cavity ≤0.5 grade, and there are no obvious segregation bands; the compositional uniformity is: central carbon segregation index ≤1.08; the grain structure is: equiaxed crystal ratio ≥50%, columnar crystal ratio ≤50%, and there is no obvious transgranular phenomenon in the columnar crystals.

[0027] Compared with the prior art, the beneficial effects of the present invention are at least as follows:

[0028] (1) No new equipment required, adaptable to existing working conditions: Relying on the existing EMS continuous casting equipment without light pressure, there is no need to increase alloy consumption, it can replace the light pressure function, and there is no need to modify the continuous casting machine, thus reducing the modification cost.

[0029] (2) Simultaneous solution of cracks and segregation: In response to the defects of no light pressing, this invention combines the "weak at the top and strong at the bottom" of the crystallizer with the "weak at the front, stable in the middle, and slow at the back" three-stage soft cooling of the secondary cooling system. This can avoid the high-temperature brittle zone of 37Mn5 and solve the crack problem caused by thermal stress concentration when there is no light pressing. It is adapted to the solidification characteristics of 37Mn5 steel. Combined with the functions of compaction and loosening and segregation suppression under light pressing, the φ180mm 37Mn5 small round billet can achieve: subcutaneous crack rate ≤0.3%, no obvious corner cracks on the surface, no surface cracks with a length ≥2mm; central carbon segregation index ≤1.08; columnar crystal rate ≤50%, equiaxed crystal rate ≥50%, and columnar crystals without obvious transgranular phenomena. This means that the surface of the round billet does not need to be ground over a large area, the amount of grinding can be reduced by more than 80%, and it can directly enter the subsequent rolling process, increasing the yield by 3%~5%.

[0030] (3) The method is stable and reliable and has strong mass production capability: abandoning the existing strong stirring mode, it can be further combined with online slow cooling after casting and EMS. The phase transformation stress is eliminated by slow cooling, and the microstructure uniformity is improved by low-strength EMS. The φ180mm 37Mn5 small round billet can achieve: central porosity ≤1.0 grade, central shrinkage cavity ≤0.5 grade, and no obvious segregation zone; it has strong operability, greatly improves production stability, and is suitable for large-scale stable production. Attached Figure Description

[0031] The above and / or additional aspects and advantages of the present invention will become apparent and readily understood from the description of the embodiments taken in conjunction with the following drawings, in which:

[0032] Figure 1 This is a low-magnification microstructure photograph of the cross-section of a φ180mm 37Mn5 small round billet according to Embodiment 1 of the present invention.

[0033] Figure 2 This is a photograph of the surface of a φ180mm 37Mn5 small round billet from Embodiment 1 of the present invention.

[0034] Figure 3 This is a low-magnification microstructure photograph of the cross-section of a φ180mm 37Mn5 small round billet, which is Comparative Example 1 of this invention.

[0035] Figure 4 This is a photograph of the surface cracks of a φ180mm 37Mn5 small round billet, which is Comparative Example 1 of this invention. Detailed Implementation

[0036] The embodiments described below with reference to the accompanying drawings are exemplary and intended to explain the invention, and should not be construed as limiting the invention.

[0037] Existing continuous casting technologies for medium-carbon manganese steel round billets rely heavily on a combination of strong electromagnetic stirring, light reduction, and strong cooling, which are unsuitable for the continuous casting of φ180mm 37Mn5 small round billets in scenarios with electromagnetic stirring (EMS) but without light reduction equipment. This can easily lead to problems such as subcutaneous cracks, central segregation, central porosity, and well-developed columnar crystals, severely affecting yield and subsequent processing performance. To solve the cracking and segregation problems in the continuous casting process of this size of small round billet and achieve stable mass production, this invention provides a continuous casting method adapted to EMS and without light reduction equipment without adding new equipment or increasing alloy costs.

[0038] A preferred embodiment of the low-crack, low-segregation continuous casting method for 37Mn5 small round billets of the present invention comprises, by weight percentage: C: 0.35%~0.39%, Si: 0.22%~0.32%, Mn: 1.35%~1.55%, P≤0.016%, S≤0.008%, Al: 0.010%~0.030%, N≤55ppm, with the remainder being Fe and unavoidable impurity elements; belonging to 37Mn5 medium carbon manganese steel, this composition is designed to control the Al content to avoid AlN precipitation along grain boundaries leading to grain boundary embrittlement; reducing the N content reduces nitride precipitation, suppresses crack sensitivity, eliminates the need for additional alloys, and meets the microstructure uniformity requirements without light pressure.

[0039] The low-crack, low-segregation continuous casting method for 37Mn5 small round billets includes:

[0040] (1) Steel transfer and pouring: The steel is transferred to the tundish for pouring. The temperature of the steel in the tundish is controlled to be >1495℃ and the superheat of the tundish is ≤26℃. Specifically, the steel that meets the chemical composition of the billet is hoisted to the ladle turret of the continuous casting machine and aligned with the tundish nozzle. After the tundish is ready for pouring, the ladle nozzle is opened and the steel flows into the tundish. When the steel in the tundish reaches the target liquid level, the tundish nozzle is opened to start pouring. By controlling the temperature of the steel in the tundish and the superheat of the tundish, the columnar crystals can be prevented from becoming too coarse due to excessive superheat of the tundish, and the element segregation can be reduced. At the same time, it is compatible with the liquidus line of 37Mn5 steel to ensure the fluidity of the steel and the solidification uniformity when there is no light pressure.

[0041] (2) Solidification in the crystallizer: The molten steel in the tundish is poured into the crystallizer and initially solidifies to form a billet with a shell. The upper outer wall of the copper tube of the crystallizer is provided with a heat insulation coating. From the upper opening to the lower opening of the copper tube of the crystallizer, the inverted taper of the copper tube of the crystallizer decreases from the part without the heat insulation coating. A protective slag with an alkalinity of 1.15~1.25 is used.

[0042] In the above method, the upper region of the copper tube in the crystallizer has a lower cooling intensity due to the thermal resistance of the heat-insulating coating and the larger inverted taper of the copper tube. This results in a smoother and more uniform growth of the initial billet shell formed by the molten steel in the upper part of the copper tube, preventing premature and rapid solidification and shrinkage of the billet shell due to excessive cooling. The lower part of the copper tube without the heat-insulating coating has a smaller inverted taper, which adapts to the reduced shrinkage of the lower billet shell, maintaining its original strong cooling capacity and achieving axial gradient cooling with a weaker upper part and a stronger lower part. Through this gradient cooling method, the billet shell gradually thickens and shrinks uniformly from top to bottom in the crystallizer, effectively alleviating the superposition of thermal and mechanical stresses caused by premature solidification of the billet shell in the upper part of the crystallizer, and avoiding early stress concentration and crack initiation. At the same time, controlling the basicity of the protective slag reduces friction between the billet shell and the copper tube, thereby significantly improving the surface and internal quality of the 37Mn5 continuously cast round billet.

[0043] (3) Secondary cooling and solidification: The billet solidified in the crystallizer undergoes three-stage soft cooling in the secondary cooling zone: along the continuous casting direction, the first zone is located 0~1.8m after the crystallizer, and the billet surface temperature is controlled at ≥960℃; the second zone is located 1.8~3.8m after the crystallizer, and the billet surface temperature is controlled at 820~920℃; the third zone is located 3.8m after the crystallizer, and the billet surface temperature is controlled at ≥680℃. The total water volume of the secondary cooling zone is 0.78~0.96L / kg.

[0044] Unlike existing full-area forced cooling systems, the above method, tailored to the characteristics of φ180mm 37Mn5 small round billets with large specific surface area and sensitive cooling, employs a three-stage soft cooling approach: weak initial cooling, stable middle cooling, and gentle final cooling. Along the continuous casting direction, the first zone, located in the foot roll area 0-1.8m after the crystallizer, controls the billet surface temperature to ≥960℃, allowing weak cooling in this zone to prevent billet shell cracking caused by rapid cooling. The second zone, located 1.8-3.8m after the crystallizer, controls the billet surface temperature to 820-920℃. At 0℃, stable cooling is possible, avoiding the high-temperature brittle zone of 700~800℃ for 37Mn5 steel, thus preventing cracks or excessive growth of columnar crystals; Zone 3 is located at the end of solidification 3.8m after the crystallizer, controlling the surface temperature of the billet to ≥680℃. This can reduce phase transformation stress through extremely slow cooling, preventing premature solidification of the pasty zone due to excessively rapid cooling, which would prevent the molten steel from being fully fed and forming central porosity and shrinkage cavities. Combined with a total water content of 0.78~0.96L / kg for secondary cooling, stress cracks in the billet shell are avoided.

[0045] (4) Straightening: The billet that has been cooled and solidified in the secondary cooling zone is pulled out of the continuous casting machine by the straightening machine. The straightening speed is controlled at 2.0~2.2m / min, which can avoid the billet shell being too thin due to excessive straightening speed. It is suitable for weak cooling process and realizes high-speed and stable continuous casting. The billet is then cut into fixed length by the cutting machine.

[0046] (5) Online slow cooling after casting: The billet pulled out from the continuous casting machine and cut into fixed length is immediately conveyed by the roller conveyor into the heat preservation and slow cooling pit. The surface temperature of the billet entering the pit is controlled to be ≥680℃, the slow cooling rate is ≤38℃ / h, the heat preservation and slow cooling time is ≥4h, and the billet is removed from the pit when the billet temperature is ≤280℃ to obtain a φ180mm round billet.

[0047] During the research process, the inventors discovered that 37Mn5 steel is highly sensitive to cracking. Under the conventional slow cooling temperature of 400~600℃, surface microcracks are prone to occur during slow cooling. Therefore, after the billet leaves the continuous casting machine, it can immediately enter a sealed heat preservation slow cooling pit. The surface temperature of the billet entering the pit is controlled to be ≥680℃, and the slow cooling rate is ≤38℃ / h to avoid phase transformation stress caused by excessive cooling. The heat preservation slow cooling time is controlled to be ≥4h, and the billet is removed from the pit when the billet temperature is ≤280℃. This eliminates the internal phase transformation stress and thermal stress of the billet, adapts to the process without light reduction, inhibits the propagation of subcutaneous microcracks, and further homogenizes the composition and improves the center segregation.

[0048] Furthermore, the following components are strictly controlled, with a focus on optimizing the Mn / Si ratio. The Mn / Si ratio in the chemical composition of the round billet is controlled at 4.8~5.2, which can further improve the sulfide morphology, reduce stress concentration sources, and thus suppress segregation and grain boundary embrittlement from the root, adapting to the segregation control requirements in scenarios without light pressure.

[0049] Furthermore, the temperature of the molten steel in the tundish during the transfer and casting of molten steel is 1510~1530℃, with fluctuations ≤±3℃, which can be higher than the liquidus line of 37Mn5 steel to maintain good fluidity of molten steel; preferably, the superheat of the tundish is 18~26℃, which can further adapt to the solidification uniformity without light pressure.

[0050] Furthermore, when the superheat of the tundish is 18~22℃, the casting speed is controlled at 2.1~2.2m / min; when the superheat of the tundish is 23~26℃, the casting speed is controlled at 2.0~2.1m / min; the casting speed fluctuation is controlled to be ≤±0.05m / min. Matching the casting speed with the superheat can further avoid the billet shell being too thin due to excessive casting speed and the billet shell being repeatedly subjected to tension and compression due to excessive casting speed fluctuation, while improving the solidification uniformity of the cast billet.

[0051] Furthermore, the total cooling water volume during solidification in the crystallizer is controlled at 200~230L / min, the water volume in the upper half of the crystallizer is 18%~22% lower than that in the lower half, and the outlet water temperature difference is controlled at ≤8℃, which can further promote uniform growth of the billet shell and eliminate local unevenness in thickness.

[0052] Furthermore, based on the solidification shrinkage characteristics of 37Mn5 steel in the crystallizer, it is preferred that the height of the crystallizer copper tube during solidification is 850mm and the wall thickness is 20mm. A heat-insulating coating is provided on the outer wall from the top of the crystallizer copper tube to 400mm, and the thickness of the heat-insulating coating gradually decreases from 0.65mm to 0.25mm from top to bottom. There is no heat-insulating coating from 400mm to the bottom of the crystallizer copper tube, i.e., from 400mm to 850mm. The inverted taper of the crystallizer copper tube from the top of the crystallizer copper tube to 400mm is 1.0%, and the inverted taper of the crystallizer copper tube from 400mm to the bottom of the crystallizer copper tube is 0.8%, so as to achieve gradient cooling with weaker upper part and stronger lower part, and avoid early stress concentration of the billet shell.

[0053] Furthermore, the composition of the protective slag during solidification in the crystallizer, by weight percentage, includes: CaO: 32.0%~38.0%, SiO2: 28.0%~33.0%, Al2O3: 2.5%~4.5%, MgO: 3.0%~5.5%, Na2O: 4.0%~6.5%, F (calculated as CaF2): 3.0%~5.5%, B2O3: 1.0%~2.5%, MnO: ≤1.0%, Fe2O3: ≤1.0%, solid C: 5.0%~7.5%, with the remainder being unavoidable impurities; it can achieve a basicity R=1.15~1.25, balancing lubrication and heat transfer, and is further adapted to the continuous casting method of φ180mm small round billets, casting speed of 2.0~2.2m / min, and 37Mn5 medium carbon manganese steel.

[0054] Furthermore, the protective slag in the crystallizer has a viscosity of 0.3~0.6 Pa·s and a melting point of 1100~1140℃ at 1300℃, and a liquid slag layer thickness of 9~11mm, which can improve lubrication and inclusion adsorption capacity, reduce friction between the billet shell and the copper tube, and avoid surface cracks.

[0055] Furthermore, the vibration during solidification in the crystallizer is non-sinusoidal vibration with an amplitude of ±3.2 mm, a frequency of 180~200 cpm, a negative slip time of 0.14~0.16 s, and a vibration mark depth of ≤0.25 mm. By controlling the crystallizer vibration, stress concentration at the vibration mark can be further reduced.

[0056] Furthermore, the current of the crystallizer electromagnetic stirring (M-EMS) during solidification in the crystallizer is 180~220A and the frequency is 5~7Hz; a low-intensity M-EMS is used, and the stirring time runs through the entire crystallizer stage. Compared with conventional strong stirring, it can further refine the grains and break the initial columnar crystals, while avoiding internal cracks caused by excessive stirring.

[0057] Furthermore, water cooling is used in the secondary cooling and solidification processes. The specific water volume in Zone 1 is 0.32~0.38 L / kg, in Zone 2 it is 0.28~0.34 L / kg, and in Zone 3 it is 0.18~0.24 L / kg. For example, the secondary cooling uses uniform atomized water spraying, avoiding localized overcooling and water impact, thus preventing stress cracks in the billet shell. The specific water volume is the ratio of the total water consumed per unit time in the secondary cooling zone of the continuous casting machine to the mass of the billet passing through the secondary cooling zone per unit time. By controlling the specific water volume in each zone, the surface temperature of the billet can be further adjusted to ≥960℃ in Zone 1 (weak cooling), ≥820~920℃ in Zone 2 (stable cooling), and ≥680℃ in Zone 3 (extremely slow cooling). At the same time, the total specific water volume is lower than that of conventional processes, controlling the cooling intensity and temperature distribution, avoiding high-temperature brittle zones, inhibiting columnar crystal growth, and preventing thermal stress concentration.

[0058] Furthermore, the solidification process in the secondary cooling zone employs front-end electromagnetic stirring (F-EMS). The front-end electromagnetic stirring is located 4.5–5.5 m from the crystallizer, with a billet solids content of 30%–50%, a current of 160–200 A, and a frequency of 4–6 Hz. This method abandons the existing strong stirring mode and optimizes the EMS current and frequency to address defects without light pressure. Low-intensity F-EMS continuous stirring replaces the compaction and loosening / segregation suppression functions of light pressure. The stirring position is located 4.5–5.5 m from the crystallizer, i.e., at the solidification end. The stirring intensity is designed to refine grains and promote uniform composition, achieving a segmented adaptation of "crystallizer stirring + solidification end stirring." This avoids internal cracks caused by excessive stirring, while simultaneously breaking up columnar crystals, further suppressing central segregation and loosening. Slow cooling eliminates phase transformation stress, and combined with low-intensity EMS, improves microstructure uniformity, replacing the compaction and loosening / segregation suppression functions of light pressure without requiring equipment modifications.

[0059] The specific embodiments of the present invention are further illustrated below through examples and comparative examples. The chemical composition of the round blanks of each example and comparative example by weight percentage is shown in Table 1 below, with the remainder being Fe and unavoidable impurities.

[0060] Table 1. Chemical composition of φ180mm round billets in the examples and comparative examples

[0061]

[0062] In Examples 1 and 2, the crystallizer copper tube has a height of 850 mm and a wall thickness of 20 mm. A heat-insulating coating is applied to the outer wall from the top of the copper tube to 400 mm, with the thickness gradually decreasing from 0.65 mm to 0.25 mm from top to bottom. There is no heat-insulating coating from 400 mm to the bottom of the copper tube (400-850 mm). The taper of the copper tube from the top to 400 mm is 1.0%, and the taper from 400 mm to the bottom of the copper tube is... The degree is 0.8%; in Comparative Example 1, the height of the copper tube in the crystallizer during solidification is 850mm, the wall thickness is 20mm, the outer wall has no heat insulation coating, and the inverted taper of the copper tube from the top to the bottom of the crystallizer is 1%; after the billet is cut into fixed lengths, it is directly sent to the production line for natural air cooling and relies on the ambient temperature to dissipate heat freely; the main difference between Comparative Example 2 and Example 1 is that (3) the cooling and solidification process parameters of the second cooling zone are different; the continuous casting parameters of each example and comparative example are shown in Table 2 below.

[0063] Table 2. Continuous casting parameters for φ180mm round billets in the examples and comparative examples

[0064]

[0065] The test results of the φ180mm round blanks in each embodiment and comparative example are shown in Table 3 below.

[0066] Table 3. Inspection results of φ180mm round billets in the examples and comparative examples

[0067]

[0068] Depend on Figure 1 The round blank in Example 1 shown in the low magnification display has no defects such as subcutaneous cracks and central shrinkage cavities. Figure 2 The surface of the round blank in Example 1 shown is free of cracks; Figure 3 The round billet shown in Comparative Example 1 exhibits well-developed low-magnification columnar crystals, with central porosity and central shrinkage defects; Figure 4 The round billet in Comparative Example 1 shows surface cracks. Comparison of Examples 1-2 with Comparative Example 1 shows that this invention, by controlling the tundish steel temperature to >1495℃ and the tundish superheat to ≤26℃, avoids excessive tundish superheat leading to coarse columnar crystals. It also adapts to the liquidus line of 37Mn5 steel, ensuring steel fluidity and adapting to solidification uniformity without light pressure. The upper outer wall of the crystallizer copper tube is provided with a heat-insulating coating. From the upper to the lower end of the crystallizer copper tube, the tapered angle decreases from the point without the heat-insulating coating, creating a gradient cooling effect. This allows the billet shell to gradually thicken and shrink uniformly from top to bottom within the crystallizer. Combined with a protective slag with a basicity of 1.15-1.25, this effectively alleviates the problem of the billet shell sticking to the crystallizer. The thermal and mechanical stresses generated by premature solidification are superimposed, avoiding early stress concentration and crack initiation in the billet shell; at the same time, controlling the basicity of the protective slag reduces friction between the billet shell and the copper tube, thereby significantly improving the surface and internal quality of 37Mn5 continuously cast round billets; appropriate casting speed is used to avoid excessively slow casting speed, resulting in a longer liquid core, longer solidification time, excessive residence time in the secondary cooling zone, and over-cooling, which can be further optimized and improved by combining EMS; online slow cooling after casting eliminates internal phase transformation stress and thermal stress in the billet, adapts to the process without light reduction, inhibits the propagation of subcutaneous microcracks, and further homogenizes the composition, improving center segregation; this invention can produce φ180mm 37Mn5 small round billets without adding new equipment or increasing alloy costs.

[0069] Subcutaneous cracks: Subcutaneous crack rate ≤0.3%, no obvious corner cracks on the surface, and no surface cracks with a length ≥2mm; Low magnification structure: Central porosity ≤1.0 grade, central shrinkage cavity ≤0.5 grade, and no obvious segregation bands; Compositional uniformity: Central carbon segregation index ≤1.08; Grain structure: Columnar crystal ratio ≤50%, equiaxed crystal ratio ≥50%, and columnar crystals without obvious transgranular phenomena; The surface of the round billet does not require large-area grinding, and the grinding amount can be reduced by more than 80%, allowing it to directly enter the subsequent rolling process, increasing the yield by 3%~5%, suitable for mass production, filling the technical gap of high-quality continuous casting of φ180mm 37Mn5 small round billets without light pressure, without the need to modify the continuous casting machine, reducing modification costs, and can be directly implemented on site.

[0070] As can be seen from the comparison results of Examples 1-2 and Comparative Example 2, compared with the cracks caused by strong secondary cooling, the thermal stress and phase transformation stress caused by uneven secondary cooling, and the aggravation of porosity, the present invention combines the gradient cooling of "weak at the top and strong at the bottom" in the crystallizer and the three-stage soft cooling of "weak at the front, stable in the middle, and slow at the back" in the secondary cooling. That is, the first zone is located 0-1.8m after the crystallizer, controlling the surface temperature of the billet ≥960℃; the second zone is located 1.8-3.8m after the crystallizer, controlling the surface temperature of the billet 820-920℃; and the third zone is located after 3.8m after the crystallizer, controlling the surface temperature of the billet ≥680℃. The total water content of the secondary cooling is 0.78-0.96L / kg, avoiding the high-temperature brittle zone of 37Mn5, solving the problem of cracks caused by thermal stress concentration without light pressure, achieving excellent levels of central segregation and porosity, improving the density of the round billet, and greatly improving production stability.

[0071] The detailed descriptions listed above are merely specific descriptions of feasible embodiments of the present invention and are not intended to limit the scope of protection of the present invention. For example, molten steel for continuous casting can be refined using the following process: electric furnace melting, LF refining, and VD vacuum degassing. The VD vacuum degassing controls the vacuum degree to ≤67Pa and the holding time to ≥18min to ensure the purity of the molten steel. All equivalent embodiments or modifications made without departing from the spirit of the present invention should be included within the scope of protection of the present invention.

Claims

1. A continuous casting method for 37Mn5 small round billets with low cracking and low segregation, characterized in that, The chemical composition of the billet, by weight percentage, includes: C: 0.35%~0.39%, Si: 0.22%~0.32%, Mn: 1.35%~1.55%, P≤0.016%, S≤0.008%, Al: 0.010%~0.030%, N≤55ppm, with the remainder being Fe and unavoidable impurity elements; the method includes: (1) Steel transfer and pouring: The molten steel is transferred to the tundish for pouring, and the temperature of the molten steel in the tundish is controlled to be >1495℃ and the superheat of the tundish is ≤26℃; (2) Solidification in the crystallizer: The molten steel is poured into the crystallizer and initially solidifies to form a billet. The upper outer wall of the copper tube of the crystallizer is provided with a heat insulation coating. From the upper opening to the lower opening of the copper tube of the crystallizer, the inverted taper of the copper tube of the crystallizer decreases from the part without the heat insulation coating. A protective slag with an alkalinity of 1.15~1.25 is used. (3) Secondary cooling and solidification: The billet solidified in the crystallizer undergoes three-stage soft cooling in the secondary cooling zone: along the continuous casting direction, the first zone is located 0~1.8m after the crystallizer, and the billet surface temperature is controlled at ≥960℃; the second zone is located 1.8~3.8m after the crystallizer, and the billet surface temperature is controlled at 820~920℃; the third zone is located 3.8m after the crystallizer, and the billet surface temperature is controlled at ≥680℃. The total water volume of the secondary cooling zone is 0.78~0.96L / kg. (4) Straightening: The billet, after being cooled and solidified in the secondary cooling zone, is pulled out of the continuous casting machine, and the pulling speed is controlled at 2.0~2.2m / min; (5) Online slow cooling after casting: The billet pulled out of the continuous casting machine immediately enters the heat preservation and slow cooling pit. The surface temperature of the billet entering the pit is controlled to be ≥680℃, the slow cooling rate is ≤38℃ / h, the heat preservation and slow cooling time is ≥4h, and the billet is removed from the pit when the billet temperature is ≤280℃ to obtain a φ180mm round billet.

2. The low-crack, low-segregation continuous casting method for 37Mn5 small round billets according to claim 1, characterized in that, The Mn / Si ratio in the blank is controlled at 4.8~5.

2.

3. The low-crack, low-segregation continuous casting method for 37Mn5 small round billets according to claim 1, characterized in that, The temperature of the molten steel in the tundish during the transfer and pouring of molten steel is 1510~1530℃, with fluctuations ≤±3℃; the superheat of the tundish is 18~26℃.

4. The low-crack, low-segregation continuous casting method for 37Mn5 small round billets according to claim 3, characterized in that, When the superheat of the tundish is 18~22℃, the pulling speed should be controlled at 2.1~2.2m / min; when the superheat of the tundish is 23~26℃, the pulling speed should be controlled at 2.0~2.1m / min; the pulling speed fluctuation should be controlled at ≤±0.05m / min.

5. The low-crack, low-segregation continuous casting method for 37Mn5 small round billets according to claim 1, characterized in that, The total cooling water flow rate during solidification in the crystallizer is controlled at 200~230L / min. The water flow rate in the upper half of the crystallizer is 18%~22% lower than that in the lower half, and the outlet water temperature difference is controlled at ≤8℃.

6. The low-crack, low-segregation continuous casting method for 37Mn5 small round billets according to claim 1, characterized in that, The composition of the protective slag during solidification in the crystallizer, by weight percentage, includes: CaO: 32.0%~38.0%, SiO2: 28.0%~33.0%, Al2O3: 2.5%~4.5%, MgO: 3.0%~5.5%, Na2O: 4.0%~6.5%, F: 3.0%~5.5%, B2O3: 1.0%~2.5%, MnO: ≤1.0%, Fe2O3: ≤1.0%, solid C: 5.0%~7.5%, with the remainder being unavoidable impurities; the viscosity of the protective slag at 1300℃ is 0.3~0.6 Pa·s, the melting point is 1100~1140℃, and the thickness of the liquid slag layer is 9~11 mm.

7. The low-crack, low-segregation continuous casting method for 37Mn5 small round billets according to claim 1, characterized in that, The solidification vibration in the crystallizer is non-sinusoidal vibration with an amplitude of ±3.2 mm, a frequency of 180~200 cpm, a negative slip time of 0.14~0.16 s, and a vibration mark depth of ≤0.25 mm; the electromagnetic stirring current of the crystallizer is 180~220 A, and the frequency is 5~7 Hz.

8. The low-crack, low-segregation continuous casting method for 37Mn5 small round billets according to claim 1, characterized in that, Water cooling is used for cooling and solidification in the two cooling zones. The water volume in zone one is 0.32~0.38L / kg, the water volume in zone two is 0.28~0.34L / kg, and the water volume in zone three is 0.18~0.24L / kg.

9. The low-crack, low-segregation continuous casting method for 37Mn5 small round billets according to claim 1, characterized in that, The cooling and solidification process in the secondary cooling zone employs electromagnetic stirring at the solidification end, which is located 4.5 to 5.5 m behind the crystallizer. The solid content of the billet is 30% to 50%, the current is 160 to 200 A, and the frequency is 4 to 6 Hz.

10. A small round billet of 37Mn5, characterized in that, The round billet is manufactured by the low-crack, low-segregation continuous casting method for 37Mn5 small round billets as described in any one of claims 1 to 9.

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