Medium-carbon manganese steel continuous casting small round billet and quality control production method thereof

Abstract

The application provides a medium-carbon manganese steel continuous casting small round billet and a quality control production method thereof, the method comprises a converter smelting, LF refining and continuous casting process, when a crystallizer electromagnetic stirring is combined with a terminal electromagnetic stirring, low parameter terminal electromagnetic stirring is used for conventional steady-state casting billet, large current high frequency terminal electromagnetic stirring is switched under the condition of furnace lifting speed of a pouring tail; a hot billet drawing and straightening pressure is controlled in stages, a slightly high clamping pressure is used for a casting transition section, and extrusion pressure is reduced in a steady-state casting section; the method is suitable for Φ180-200mm small round billet production of medium-carbon manganese steel such as 30Mn2 and 25Mn2 for seamless steel pipe, can improve the cleanliness of molten steel, the internal quality of the casting billet, the size precision of the round billet and the surface quality, and can reach the following indexes: the center loose is less than or equal to 1.5 levels, there is no center crack, subsurface crack, white band, scarring and pit type defects, the A, B, C, D and DS type inclusions are less than or equal to 1.5 levels, and the method is beneficial to reducing the subsequent flaw detection rejection rate of the seamless pipe.

Description

Technical Field

[0001] This invention belongs to the field of special steel smelting and continuous casting technology, specifically relating to a medium carbon manganese steel continuous casting small round billet and its quality control production method, applicable to the production of Φ180-200mm small round billets for seamless steel pipes of medium carbon manganese steel such as 30Mn2 and 25Mn2. Background Technology

[0002] Typical grades of medium carbon manganese steel, such as 30Mn2 and 25Mn2, are widely used in seamless steel pipe production and mechanical structural component manufacturing due to their high strength, high hardness, and excellent wear resistance. Improving the quality of continuously cast small round billets of Φ180-200mm medium carbon manganese steel is crucial for increasing billet yield, reducing scrap in the continuous casting process, lowering scrap and grinding costs in hot working and piercing processes, reducing the forging ratio, and ultimately achieving cost reduction and efficiency improvement. However, currently, Φ180-200mm continuously cast small round billets of medium carbon manganese steel differ significantly from Φ150mm small round billets and large-diameter round billets in terms of cooling rate, liquid core length, phase transformation range, and billet shell mechanical properties. The following technical challenges still exist in actual production: 1. Difficulty in controlling the stability of molten steel cleanliness: Slag discharge from the converter can easily cause phosphorus reversion in the molten steel, increasing the oxidizability of the molten steel in the LF process. 30Mn2 and 25Mn2 are high-manganese steels with high Mn content in the molten steel. Under high oxidizability conditions, deoxidation will generate a large number of inclusions with high melting points that are not easy to polymerize and float. The LF soft blowing time is prone to fluctuation, resulting in poor stability of molten steel cleanliness. At the same time, compared with large-sized round billets, small round billets have a faster drawing speed, making it difficult for inclusions to float, which in turn affects the quality of the billet and makes it a source of crack initiation, increasing the scrap rate of seamless pipes in subsequent flaw detection.

[0003] II. Difficulty in simultaneously controlling negative segregation of bright bands and central porosity in billets cast in different pouring cycles: Existing technologies generally adopt a constant stirring regime throughout the entire pouring cycle. Compared to Φ150mm round billets, Φ180-200mm specifications have a larger cross-section and a longer liquid core at the same casting speed. If the process of Φ150mm round billets is directly adopted, high-current stirring throughout the process, especially in the tail furnace, can easily induce negative segregation of bright bands and roundness deviations, damaging the uniformity of the internal structure of the billet and affecting subsequent processing, resulting in poor product performance consistency. On the other hand, if the production process of large round billets is directly copied, it can easily cause obstruction of molten steel feeding, significantly increasing the risk of central porosity and excessive inclusions. At the same time, the defect welding conditions of this specification of billet in subsequent processing are not as good as those of large-specification round billets, which will lead to more prominent defect inheritance problems and even product scrap.

[0004] 3. Insufficient dimensional accuracy and excessive roundness of billets: Compared with large round billets of Φ450-600mm, billets of Φ180-200mm specification have thinner shells and poorer resistance to deformation, making them more prone to extrusion deformation. This results in a larger width of the flat area and excessive roundness of the billet, which in turn makes it easy for uneven wall thickness defects to occur during the piercing and rolling process of subsequent seamless steel pipe production.

[0005] Fourth, surface quality defects such as deepening oscillation marks and surface cracks in the billet exist: Under the existing protective slag conditions, stress concentration and increased oscillation mark depth are easily caused; and since this steel grade has poor high-temperature plasticity, oscillation marks will become the source of crack initiation, which will then lead to surface quality defects such as subcutaneous cracks and surface transverse cracks. Ultimately, this will not only increase the processing difficulty of subsequent finishing processes, but also cause a decrease in the yield of round billets. If the defects remain in the product, they may even cause service risks and quality problems in the finished parts. Summary of the Invention

[0006] This invention aims to at least partially solve one of the above-mentioned technical problems. This invention provides a method for producing medium carbon manganese steel continuous casting small round billets and its quality control, which is applicable to the production of Φ180-200mm small round billets for seamless steel pipes made of medium carbon manganese steel such as 30Mn2 and 25Mn2. It can improve the cleanliness of molten steel, the internal quality of the billet, the dimensional accuracy and surface quality of the round billet, and reduce the scrap rate of seamless pipes after subsequent flaw detection.

[0007] The technical solution adopted by this invention to solve its technical problem is: The first aspect of the present invention is to provide a quality control production method for medium carbon manganese steel continuously cast small round billets, the method comprising the following steps during the continuous casting process for producing Φ180-200mm small round billets: (1) A combination of electromagnetic stirring in the crystallizer and electromagnetic stirring at the end is used: The current for controlling the electromagnetic stirring of the crystallizer is 150-250A, and the frequency is 1-3Hz; In each casting cycle, for the billet that has entered the stable production stage, the steady-state casting speed is controlled according to the billet cross-section and superheat, and the current of the end electromagnetic stirrer is controlled to be 150-250A and the frequency is 1-3Hz; for the billet in the tail furnace of the casting cycle, the casting speed is controlled to be increased by 0.1-0.2m / min relative to the steady-state casting speed, and the current of the end electromagnetic stirrer is controlled to be 400-450A and the frequency is 8-10Hz. (2) Grading control of hot billet pressure during straightening: The temperature of the billet entering the straightening machine should be ≥950℃; An auxiliary straightening machine is installed in the straight section of the continuous casting machine; If the cumulative length of the casting after the start of casting does not exceed that of the auxiliary straightening machine, then along the straightening direction, control the pressure of the first frame of the casting transition section to 20-25 bar, the pressure of the second frame to 20-25 bar, and the pressure of the auxiliary straightening machine to 0 bar. If the cumulative length of the casting exceeds that of the auxiliary straightening machine after the start of casting, then along the straightening direction, switch the pressure of the steady-state casting section to 13-19 bar for frame one, 13-19 bar for frame two, and 5-10 bar for frame three.

[0008] Among the preferred technical solutions, converter smelting is included: using BH type molten iron to avoid BF type self-produced scrap steel.

[0009] In the preferred technical solution, converter smelting is included: controlling the final carbon content to be 0.06%-0.10% and the tapping temperature to be ≥1600℃.

[0010] In the preferred technical solution, the converter smelting is included: the steel output is 52±1.5 tons, calcium aluminate and lime are added during the steel output process to implement the slag return process, the amount of calcium aluminate is 400±50 kg / furnace, and the amount of lime is 300±50 kg / furnace.

[0011] In the preferred technical solution, LF refining is included: the ladle baking temperature is ≥800℃, and the transfer time from converter tapping to LF entry is ≤12min.

[0012] In the preferred technical solution, the following are included: LF refining: the white slag holding time after white slag formation is ≥20min; when phosphorus return occurs in the converter slag, 25-30kg of lime is added to continue white slag smelting; soft blowing time is ≥20min.

[0013] In the preferred technical solution, LF refining is included: pure calcium wire is fed in before leaving the station after refining. The amount of pure calcium wire fed in the start-up heat is 55-65m, and the amount of pure calcium wire fed in the continuous casting heat is 20-30m, with 52±1.5 tons of molten steel per heat; the ladle Als is controlled at 0.020-0.030%, and the ladle P≤0.018%; the gas composition of molten steel is controlled at T[O]≤0.0020%, and [N]≤0.0055%.

[0014] In the preferred technical solution, during the continuous casting process, argon gas purging is performed after the tundish is baked.

[0015] In the preferred technical solution, the erosion amount of the submerged entry nozzle in the continuous casting process is controlled to be 3-5 mm.

[0016] In a preferred embodiment, the mold flux composition of the continuous casting process is required to include, by mass percentage: SiO2: 24.5%-31.3%, CaO: 22.6%-30.5%, Al2O3: 4.5%-11.4%, Fe2O3: ≤3%, MgO: 2%-5%, F - : 2%-6.5%, MnO: ≤3%, R2O: 4%-11.3%; TC: 4%-11.3%, H2O: ≤0.5%; the remainder are unavoidable impurities.

[0017] In the preferred technical solution, when the billet steel grade is 30Mn2, the CaO / SiO2 ratio is 0.96±0.1.

[0018] In the preferred technical solution, the continuous casting process adopts superheat and casting speed linkage control: the superheat of the first furnace is controlled at 30-45℃, and the superheat of the continuous casting furnace is controlled at 20-35℃.

[0019] In the preferred technical solution, the continuous casting process adopts a linkage control between superheat and casting speed: when the superheat is >45℃, the casting speed is 1-1.3m / min; when the superheat is 36-45℃, the casting speed is 1.2-1.5m / min; when the superheat is 20-35℃, the casting speed is 1.2-1.7m / min; and when the superheat is <20℃, the casting speed is 1.6-1.9m / min.

[0020] In the preferred technical solution, when the cross-sectional specification of the billet is Φ180mm, the drawing speed is 1.2-1.3m / min when the superheat is >45℃; 1.2-1.5m / min when the superheat is 36-45℃; 1.6-1.7m / min when the superheat is 20-35℃; and 1.8-1.9m / min when the superheat is <20℃.

[0021] In the preferred technical solution, when the cross-sectional specification of the cast billet is Φ200mm, the drawing speed is 1-1.1m / min when the superheat is >45℃; 1.2-1.3m / min when the superheat is 36-45℃; 1.2-1.5m / min when the superheat is 20-35℃; and 1.6-1.7m / min when the superheat is <20℃.

[0022] In the preferred technical solution, the continuous casting process before straightening adopts foot roller water cooling and secondary cooling mist cooling, with a total water content of 0.6-0.9 L / kg steel.

[0023] In the preferred technical solution, the total secondary cooling water volume in the continuous casting process is 240-270 L / min, and the water volume ratio along the straightening direction is as follows: foot roll water cooling: 30%-36%; secondary cooling zone 1: 39%-45%; secondary cooling zone 2: 16%-21%; secondary cooling zone 3: 2%-8%.

[0024] In the preferred technical solution, during the continuous casting process, the pressure of the first stand in the steady-state casting section is 13-18 bar, the pressure of the second stand is 13-18 bar, and the pressure of the third stand is 5-10 bar.

[0025] In a preferred embodiment, the continuous casting process controls the thermal shrinkage coefficient of the billet to be 1.012-1.014.

[0026] A second aspect of the present invention is to provide a medium-carbon manganese steel continuous casting small round billet, said billet being produced by the quality control production method for medium-carbon manganese steel continuous casting small round billets described in any one of the preceding claims.

[0027] In a preferred embodiment, the round billet comprises 30Mn2 or 25Mn2 steel grades.

[0028] In the preferred technical solution, the inclusion ratings of the round blank for types A, B, C, D, and DS are all ≤1.5.

[0029] In the preferred technical solution, the width of the flat area of ​​the round blank is ≤65mm, and the roundness is stably controlled within 1-2mm.

[0030] In the preferred technical solution, the central porosity of the round blank is ≤1.5 grade, and there are no defects such as central cracks, subcutaneous cracks, white bright bands, scars, or pits.

[0031] Compared with the prior art, the beneficial effects of the present invention are at least as follows: (1) Improve the internal quality of the billet: It is applicable to the production of Φ180-200mm small round billets for seamless steel pipes of medium carbon manganese steel such as 30Mn2 and 25Mn2. When the casting furnace is accelerated, the end electromagnetic stirring parameters are switched. Low-parameter end electromagnetic stirring is used for conventional steady-state billets to avoid the formation of white bright band defects. When the casting furnace is accelerated, high-current and high-frequency end electromagnetic stirring is switched to break up the solidified end dendrites. The central porosity of the billet is stably controlled to ≤1.5 level, with no obvious white bright band defects. It takes into account the synergistic control of negative segregation of white bright band and central porosity under different casting conditions, and improves the uniformity of the internal structure of the billet.

[0032] (2) Improve the dimensional accuracy of the billet: Under the combined process of tension straightening hot billet pressure graded control, the billet shell in the casting transition section is thin, and a higher clamping pressure is used to prevent the billet from slipping; the billet shell is thickened in the steady state stage, reducing the extrusion pressure; the stress conditions of the thin billet shell small round billet are improved, and the round billet non-roundness is effectively controlled. The round billet non-roundness is reduced to 1-3mm, and the width of the flat area is ≤65mm, which improves the dimensional accuracy of the billet and can reduce the uneven wall thickness defects in subsequent piercing and rolling.

[0033] (3) Improve the surface quality of round billets: The continuous casting process controls the composition of the mold flux, adopts the linkage control of superheat and casting speed, and uses foot roller water cooling and secondary cooling air mist cooling. The surface quality of round billets is better, with no central cracks, subcutaneous cracks, scabs, pits and other defects. The vibration mark state is controllable, reducing the risk of finished product service.

[0034] (4) Improve the cleanliness of molten steel: Control the tapping endpoint and the slag return process of calcium aluminate and lime during converter smelting, control the return of phosphorus to the converter slag during LF refining, and feed pure calcium line for calcium treatment before leaving the station after refining, control T[O]≤0.0020% and [N]≤0.0055%, so that the rating of inclusions of type A, B, C, D and DS are all ≤1.5, so as to achieve stable and controllable cleanliness of molten steel and reduce the risk of inclusion-induced cracks and scrapping during flaw detection. Attached Figure Description

[0035] 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: Figure 1 This is a low-magnification photograph of Embodiment 1 of the present invention.

[0036] Figure 2 This is a low-magnification photograph of Comparative Example 1 of this invention.

[0037] Figure 3 This is a photograph of Embodiment 2 of the present invention where the roundness does not exceed the standard.

[0038] Figure 4 This is a photograph of the non-roundness exceeding the standard in Comparative Example 2 of this invention.

[0039] Figure 5 This is a low-magnification photograph of the bright white band produced in Comparative Example 3 of this invention.

[0040] Figure 6 This is a photograph of the deep vibration marks in Comparative Example 3 of this invention. Detailed Implementation

[0041] 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.

[0042] To address the challenges of simultaneously controlling negative segregation of the bright band and central porosity in the production of seamless steel pipes made from Φ180-200mm small round billets for medium carbon manganese steels such as 30Mn2 and 25Mn2, as well as excessive non-roundness of the billets in different casting batches, the inventors discovered during their research that a single parameter cannot be adapted to the entire casting process. I. In the current production process, the electromagnetic stirring at the end of the entire casting process uses fixed parameters. Compared with the Φ150mm small round billet, after increasing the size to Φ180-200mm, the liquid core of the billet is longer. Under the condition of increasing the pull speed of the tail furnace in the casting process, the position of the liquid core of the billet moves forward, and the forward movement is more significant than that of the Φ150mm small round billet. The solidification two-phase zone is out of the optimal range of action of the end electromagnetic stirring. Under the same stirring parameters, the stirring efficiency is reduced, and the fixed stirring parameters will cause the central porosity to be aggravated. On the other hand, if a high current stirring is used throughout the process, it will produce white bright band negative segregation defects. It is difficult to achieve both. It is necessary to dynamically coordinate and control the internal working conditions of the billet under the conditions of conventional steady-state casting and casting under the condition of increasing the pull speed of the tail furnace in the casting process.

[0043] Second, the existing production process uses constant straightening pressure. Compared with large round billets, Φ180-200mm billets have smaller cross-sections and thinner shells, resulting in poorer resistance to deformation. Furthermore, the shell thickness of the billet exhibits unsteady changes throughout the casting cycle, especially during the casting transition phase when the shell thickness is relatively thin, making it prone to extrusion deformation. As the casting enters the steady-state casting stage, the shell gradually thickens, improving its resistance to deformation. If the same set of process parameters is used for straightening the hot billet pressure throughout the process, when the extrusion load of the straightening rollers exceeds the tolerance limit of the thin shell, it will lead to excessive roundness of the billet. However, if the pressure is reduced simultaneously, it is difficult to meet the normal straightening requirements. Therefore, the production process with constant straightening pressure needs to be improved by implementing graded control of the straightening hot billet pressure.

[0044] Therefore, in order to achieve the production goals of low center porosity, good billet dimensional accuracy, and avoidance of bright white band defects, a preferred embodiment of the quality control production method for medium carbon manganese steel continuously cast small round billets described in this invention includes the following steps during the continuous casting process for producing Φ180-200mm small round billets: (1) A combination of electromagnetic stirring in the crystallizer and electromagnetic stirring at the end is used: The current for controlling the electromagnetic stirring of the crystallizer is 150-250A, and the frequency is 1-3Hz; In each casting cycle, for the billet that has entered the stable production stage, the steady-state casting speed is controlled according to the billet cross-section and superheat, and the current of the end electromagnetic stirrer is controlled to be 150-250A and the frequency is 1-3Hz; for the billet in the tail furnace of the casting cycle, the casting speed is controlled to be increased by 0.1-0.2m / min relative to the steady-state casting speed, and the current of the end electromagnetic stirrer is controlled to be 400-450A and the frequency is 8-10Hz. (2) Grading control of hot billet pressure during straightening: The temperature of the billet entering the straightening machine should be ≥950℃; An auxiliary straightening machine is installed in the straight section of the continuous casting machine; If the cumulative length of the casting after the start of casting does not exceed that of the auxiliary straightening machine, then along the straightening direction, control the pressure of the first frame of the casting transition section to 20-25 bar, the pressure of the second frame to 20-25 bar, and the pressure of the auxiliary straightening machine to 0 bar. If the cumulative length of the casting exceeds that of the auxiliary straightening machine after the start of casting, then along the straightening direction, switch the pressure of the steady-state casting section to 13-19 bar for frame one, 13-19 bar for frame two, and 5-10 bar for frame three.

[0045] The above method, for small round billets with a diameter of Φ180-200mm, employs a combination of electromagnetic stirring in the crystallizer and electromagnetic stirring at the end: the first-end electromagnetic stirring (M-EMS) is installed in the crystallizer position with uniformly set parameters to stir the molten steel and improve the solidification structure within the crystallizer; the last-end electromagnetic stirring (F-EMS) is installed at the end of the solidification of the billet in the secondary cooling zone. Considering the relatively long liquid core under steady-state casting conditions, if high-current, high-frequency stirring is used throughout the process, it will scour the solidified dendrites, resulting in white bright band negative segregation. Therefore, for billets that have entered the stable production stage after casting, i.e., the intermediate normal heats after initial casting, when the tundish liquid level is stable and the superheat of the molten steel fluctuates little, this method is used. In conventional steady-state billet casting, low-current frequency parameters are used for end-of-cast electromagnetic stirring to avoid the formation of bright white band defects. However, for the tail furnace speed-up operation, which is the last heat of molten steel in the casting cycle, the casting speed is increased by 0.1-0.2 m / min compared to the steady-state casting speed to match the production rhythm. At this time, the characteristics of the liquid core shortening and the two-phase region narrowing under the tail furnace operation can be utilized. High-current, high-frequency end-of-cast electromagnetic stirring can be switched and applied for a short time. This can not only break up the dendrites at the solidification end, achieve solidification feeding, and suppress central porosity, but also prevent the formation of bright white bands. The central porosity of the billet can be stably controlled at ≤1.5 level, solving the problem of simultaneously controlling the deterioration of porosity in the tail furnace and the negative segregation of bright white bands.

[0046] The above method employs graded control of hot billet pressure for small round billets with diameters of Φ180-200mm. The casting transition section is the billet stage from the crystallizer to the auxiliary straightener after casting begins. During this stage, the billet shell is relatively thin and prone to extrusion deformation. Frames 1 and 2 in the casting transition section use relatively high clamping pressure to prevent the billet from slipping and to avoid the auxiliary frame rolling the thin billet shell, which would cause the billet to have excessive roundness. After casting begins, the cumulative casting length exceeds that of the auxiliary straightener and enters the steady-state casting stage. In the steady-state stage, the billet shell thickens. Frames 1 and 2 in the steady-state casting section reduce the extrusion pressure to decrease the radial extrusion load on the billet and avoid roller pressing that could cause indentations and micro-cracks on the billet surface. When the billet shell is thick and strong enough in the steady-state stage, frame 3 applies low pressure to complete straightening and constrain the billet shape. This can reduce the roundness of the billet from the original 4mm to 1-3mm, and the width of the flat area ≤65mm, thereby reducing the uneven wall thickness defects in subsequent piercing and rolling.

[0047] Furthermore, in converter smelting: BH type molten iron is used to avoid BF type self-produced scrap steel; BH molten iron is blast furnace molten iron with low sulfur, low phosphorus and few impurities, while BF type self-produced scrap steel is molten iron with large composition fluctuations and carries inclusions, which stabilizes the original purity of molten steel from the source and can avoid the external inclusions brought in by scrap steel causing internal inclusions and excessive defects in round billets.

[0048] Furthermore, including converter smelting: controlling the final carbon content to 0.06%-0.10% and the tapping temperature ≥1600℃, it can be adapted to 30Mn2 and 25Mn2 steel grades, avoiding excessive oxidation of molten steel due to excessively low carbon content, and avoiding difficulty in hitting the medium carbon manganese steel range due to excessively high carbon content. Controlling the tapping temperature can ensure sufficient LF inlet temperature and avoid poor slag formation due to low temperature.

[0049] Considering that slag removal during converter tapping can easily cause phosphorus return in molten steel, and that the oxidizability of molten steel increases in the LF process, making inclusion control more difficult, further measures are taken, including converter smelting: the tapping volume is 52±1.5 tons, and calcium aluminate and lime are added during the tapping process to implement a slag return process. The amount of calcium aluminate is 400±50 kg / heat and the amount of lime is 300±50 kg / heat. Calcium aluminate and lime are added to the ladle during the tapping process and then refined in the LF furnace. The calcium aluminate and lime slag return can form a low melting point and highly active initial slag in advance, so as to quickly adsorb deoxidation products, promote the aggregation and flotation of inclusions, and reduce the number of large inclusions in molten steel.

[0050] Furthermore, including LF refining: the ladle baking temperature is ≥800℃, and the transfer time from converter tapping to LF entry is ≤12min, which can ensure controllable temperature drop of molten steel and reduce secondary oxidation; normal turnover packages are preferred, and the use of overhaul packages, minor repair packages, and non-LF process building material packages is prohibited.

[0051] Considering the volatile nature of LF soft blowing time and the poor stability of molten steel cleanliness, further measures are taken, including LF refining: the white slag maintenance time after white slag formation should be ≥20 min; when phosphorus return occurs in the converter slag, 25-30 kg of lime should be added to continue white slag smelting; the soft blowing time should be ≥20 min, and for abnormal slag return furnaces, the soft blowing time should be no less than 15 min, and lime should be added to continue white slag smelting to ensure slag reduction capacity; molten steel should not be exposed throughout the soft blowing process to avoid secondary oxidation and further ensure the cleanliness of molten steel.

[0052] Furthermore, this includes LF refining: before leaving the station after refining, pure calcium wire is fed in. The feed rate for each start-up heat is 55-65m, and for each continuous casting heat it is 20-30m, with a steel volume of 52±1.5 tons per heat; ladle Als is controlled at 0.020-0.030%, and ladle P≤0.018%; the gas composition of the molten steel is controlled at T[O]≤0.0020%, [N]≤0.0055%; compared to the feed rate... The silicon-calcium wire undergoes light calcium treatment, resulting in a higher inclusion content in the first furnace run. Feeding a larger amount of pure calcium wire can fully modify various hard inclusions, ensuring stable production conditions in the first furnace run. Reducing the amount of pure calcium wire fed can prevent excessive calcium from causing new impurities and defects. Controlling the Al in the ladle can stabilize the deoxidation level of the molten steel. Controlling the P in the ladle can prevent phosphorus segregation from causing uneven microstructure. Controlling T[O] and [N] can reduce the amount of oxide and nitride inclusions generated, reducing the sources of internal defects in the billet.

[0053] Furthermore, in the continuous casting process, after the tundish is baked, argon gas is purged to reduce secondary oxidation of the molten steel.

[0054] Furthermore, in the continuous casting process, the erosion amount at the bottom of the immersion nozzle is controlled to be 3-5mm to avoid insufficient erosion causing nozzle nodules or excessive erosion causing refractory material to peel off and be trapped in the molten steel. Considering that improper matching of the mold copper tube and the protective slag can easily lead to surface quality defects such as deepened oscillation marks and surface cracks in the cast billet, the composition of the mold protective slag in the continuous casting process is further required to include, by mass percentage: SiO2: 24.5%-31.3%, CaO: 22.6%-30.5%, Al2O3: 4.5%-11.4%, Fe2O3: ≤3%, MgO: 2%-5%, F - : 2%-6.5%, MnO: ≤3%, R2O: 4%-11.3%; TC: 4%-11.3%, H2O: ≤0.5%; the remainder are unavoidable impurities; this slag system is suitable for medium carbon manganese steel round billets, with SiO2 and CaO used to regulate the basic basicity. When the billet steel grade is 30Mn2, the CaO / SiO2 ratio is 0.96±0.1; when the billet steel grade is 25Mn2, the CaO / SiO2 ratio is 0.81-1.06; F -R2O is used to regulate the melting point and viscosity of the slag, with the preferred melting point being 1050-1190℃ and the preferred viscosity being 0.15-0.68 Pa·s; TC controls the melting rate of the protective slag, thereby suppressing excessively deep oscillation marks and surface longitudinal cracks, and achieving uniform heat transfer and good lubrication in the crystallizer.

[0055] Furthermore, the continuous casting process adopts superheat and casting speed linkage control: the superheat of the first heat is controlled at 30-45℃, and the superheat of the continuous casting heat is controlled at 20-35℃ to prevent insufficient fluidity of molten steel caused by the cold ladle of the first heat. The continuous casting heat adopts low superheat, which is conducive to improving central porosity and segregation.

[0056] Furthermore, the continuous casting process employs a linkage control between superheat and casting speed: when the superheat is >45℃, the casting speed is 1-1.3 m / min; when the superheat is 36-45℃, the casting speed is 1.2-1.5 m / min; when the superheat is 20-35℃, the casting speed is 1.2-1.7 m / min; and when the superheat is <20℃, the casting speed is 1.6-1.9 m / min. Through linkage control, the length of the liquid core of the billet and the position of the solidification end are stabilized, and the working range of the end electromagnetic stirring is matched.

[0057] Furthermore, when the billet cross-section is Φ180mm, the casting speed is 1.2-1.3m / min when the superheat is >45℃; 1.2-1.5m / min when the superheat is 36-45℃; 1.6-1.7m / min when the superheat is 20-35℃; and 1.8-1.9m / min when the superheat is <20℃. The Φ180mm cross-section is smaller, resulting in faster heat exchange and a higher solidification rate. Appropriately increasing the casting speed can improve production efficiency, while also matching the effective range of electromagnetic stirring.

[0058] Furthermore, when the billet cross-section is Φ200mm, the casting speed is 1-1.1m / min when the superheat is >45℃; 1.2-1.3m / min when the superheat is 36-45℃; 1.2-1.5m / min when the superheat is 20-35℃; and 1.6-1.7m / min when the superheat is <20℃. The Φ200mm cross-section is larger and dissipates heat more slowly, requiring a lower casting speed to ensure uniform billet shell growth and prevent excessively long liquid cores that could induce central porosity and shrinkage cavities. Furthermore, before the straightening process in the continuous casting process, foot roller water cooling and secondary cooling mist cooling are adopted, with a total water content of 0.6-0.9 L / kg steel. This can be adapted to the solidification characteristics of medium carbon manganese steel round billets, achieving slow cooling and uniform solidification, and suppressing the generation of internal and external cracks in the billet.

[0059] Furthermore, in the continuous casting process, the total secondary cooling water volume is 240-270 L / min, and the water volume proportion along the straightening direction is as follows: foot roll water cooling: 30%-36%; secondary cooling zone 1: 39%-45%; secondary cooling zone 2: 16%-21%; secondary cooling zone 3: 2%-8%. Through the gradient water distribution mode of strong cooling in the front section and gradual weak cooling in the middle and rear sections, the billet shell can be rapidly thickened and solidified uniformly after the billet exits the crystallizer, avoiding the stress concentration in the structure caused by excessive cooling in the middle and later stages.

[0060] Furthermore, the continuous casting process controls the hot and cold shrinkage coefficient of the billet to be 1.012-1.014. The hot and cold shrinkage coefficient is the length of the hot billet divided by the length of the cold billet. Preferably, the hot and cold billet shrinkage coefficient is controlled at 1.013. When the red billet is cut to length, the length of the cold billet is 9000mm, and the length of the hot billet is controlled to be 9110-9130mm.

[0061] Taking an example, the specific steps of the present invention are further illustrated: its production follows a process flow of converter → LF refining → continuous casting → sheltered cooling → inspection → finishing → weighing → marking → warehousing. Specifically: Step 1: Converter smelting (1) Raw materials: BH-type molten iron is used in the furnace to avoid using BF-type self-produced scrap steel. Molten iron conforming to the composition of 30Mn2 and 25Mn2 steel grades is added to the converter for smelting. The chemical composition of 30Mn2, by mass percentage, includes: C: 0.27%-0.34%, Si: 0.17%-0.37%, Mn: 1.4%-1.8%, P≤0.030%, S≤0.030%, Cr≤0.3%, Ni≤0.3%. The chemical composition of 25Mn2, by mass percentage, includes: C: 0.22%-0.28%, Si: 0.17%-0.37%, Mn: 1.4%-1.8%, P≤0.035%, S≤0.035%, Cr≤0.3%, Ni≤0.3%, Cu≤0.3%, with the remainder being Fe and unavoidable impurities.

[0062] (2) Endpoint control: The carbon content at the end point is controlled to be 0.06%-0.10%, and the tapping temperature is ≥1600℃.

[0063] (3) Tapping: The tapping volume is 52±1.5 tons. During the tapping process, calcium aluminate (calcium oxide content in calcium aluminate is 45%-50% and aluminum oxide content is 25%-30%) and lime (calcium oxide content in lime is ≥88% and activity is ≥350) are added to the ladle to implement the slag return process. The amount of calcium aluminate is 400±50 kg / furnace and the amount of lime is 300±50 kg / furnace. The initial slag is pre-made and then refined in the LF furnace.

[0064] Step 2: LF Refining (1) Steel ladle management: Use normal turnover ladles and prohibit the use of overhaul ladles, minor repair ladles and non-LF process building material ladles; the actual temperature of the ladle after baking is ≥800℃, and the transfer time from the converter tapping to the LF station is ≤12min to reduce the secondary oxidation of molten steel.

[0065] (2) Smelting with white slag: The molten steel after being smelted in the converter is refined in the LF furnace of the LF station along with the ladle. After the white slag is formed, the white slag is maintained for ≥20 minutes. If phosphorus is returned to the converter slag, 25-30 kg of lime is added after the first sample is taken in the refining process. After the lime is added, the white slag smelting is maintained. The soft blowing time is ≥20 minutes. The molten steel is prohibited from being exposed during the soft blowing process to avoid secondary oxidation and allow fine inclusions to fully aggregate and float.

[0066] (3) Calcium treatment wire feeding: Five minutes before the end of refining and exit of the station, the pure calcium wire feeding amount for the start-up furnace is 55-65m, and the pure calcium wire feeding amount for the continuous casting furnace is 20-30m, with each furnace of molten steel being 52±1.5 tons.

[0067] (4) Outgoing composition control: control the ladle Als to 0.020-0.030%, ladle P≤0.018%; control the gas composition of molten steel to T[O]≤0.0020%, [N]≤0.0055%, and send it out of the station to the intermediate ladle after passing the test.

[0068] Specifically, the process parameters for the converter smelting and LF refining processes in each embodiment and comparative example are shown in Table 1.

[0069] Table 1. Process parameters for converter smelting and LF refining processes in each embodiment and comparative example.

[0070] Step 3: Continuous Casting After the steel is smelted in a converter and refined in an LF furnace, round billets are continuously cast using an R8m arc-shaped continuous casting machine. The continuous casting machine, from top to bottom, consists of: a ladle turret, tundish, crystallizer, secondary cooling unit, straightening machine, flame cutter, conveyor roller conveyor, steel turning machine, steel transferring machine, and walking beam cooling bed. Specifically: (1) Preparation of intermediate tundish and nozzle The tundish is preheated beforehand, and then purged with argon gas to replace the air inside the ladle, remove refractory dust, and reduce secondary oxidation of the molten steel. After leaving the LF refining station, the ladle is hoisted to the ladle turret. The turret rotates the ladle to the casting position, and the bottom nozzle of the ladle is aligned with the tundish, so that the molten steel is injected into the tundish through the nozzle, and then flows into the crystallizer through the submerged entry nozzle. After the casting is completed, the submerged entry nozzle is inspected offline. The slag line erosion of the submerged entry nozzle is controlled to be 3-5mm to avoid nodule formation or refractory spalling and inclusion.

[0071] (2) Crystallizer and protective slag Molten steel enters the crystallizer through a submerged entry nozzle. A matching protective slag is added to the crystallizer, and the protective slag melts to form a liquid slag layer. This slag system is used to achieve lubrication, heat transfer, and air isolation. The superheat of the first furnace is controlled at 30-45℃, and the superheat of the continuous casting furnace is controlled at 20-35℃. Electromagnetic stirring is used in the crystallizer. The molten steel cools on the inner wall of the copper tubes of the crystallizer to form a primary billet shell with a certain thickness, while the interior is still a liquid core.

[0072] The composition requirements for protective slag A, by mass percentage, include: SiO2: 27.5±3.0%, CaO: 26.5±3.0%, Al2O3: 9.5±1.9%, Fe2O3: ≤3%, MgO: 3.2±1.0%, F - : 3.0±1.0%, MnO: ≤3%, R2O: 6.0±2.0%; TC: 12.0±3.0%; the remainder are unavoidable impurities; CaO / SiO2 is 0.96±0.1, melting point is 1150±40℃, viscosity is 0.58±0.10Pa·s, and particle size is (0.15-2mm)≥85%.

[0073] The composition requirements for protective slag B, by mass percentage, include: SiO2: 28.3±3.0%, CaO: 25.6±3.0%, Al2O3: 6.0±1.5%, Fe2O3: ≤3%, MgO: 3.5±1.5%, F - 5.0±1.5%, MnO: ≤3%, R2O: 9.3±2.0%; TC: 15.0±2.0%; the remainder are unavoidable impurities; CaO / SiO2 ratio is 0.91±0.10, melting point is 1080±30℃, viscosity is 0.30±0.15Pa·s, and particle size is (0.15-2mm)≥85%.

[0074] (3) A combination of crystallizer electromagnetic stirring (M-EMS) and terminal electromagnetic stirring (F-EMS) is adopted to implement a superheat and pulling speed linkage matching system: The current for controlling the electromagnetic stirring in the crystallizer is 150-250A, and the frequency is 1-3Hz. In each casting cycle, for the billet that has entered the stable production stage, the steady-state casting speed is controlled according to the billet cross-section and superheat. That is, the current for controlling the electromagnetic stirring at the end of the conventional section is 150-250A, and the frequency is 1-3Hz. For the billet in the tail furnace of the casting cycle, the casting speed is controlled to be 0.1-0.2m / min higher than the steady-state casting speed, and the current for controlling the electromagnetic stirring at the end is 400-450A, and the frequency is 8-10Hz. Taking advantage of the shortening of the liquid core and the narrowing of the two-phase region under the tail furnace condition, residual dendrites are broken up, central porosity is suppressed, and no bright white band is produced.

[0075] (4) Secondary cooling system The billet, carrying the liquid core, is pulled downwards from the crystallizer and first enters the foot roll section for foot roll water cooling. This process intensifies the cooling of the thin billet shell as it exits the crystallizer, rapidly thickening the shell. The billet continues downwards into the secondary cooling zone, passing through secondary cooling zone one, secondary cooling zone two, and secondary cooling zone three in sequence. The secondary cooling zone uses air mist cooling. A secondary cooling water meter with a corresponding cross-section is used, with a total specific water volume of 0.6-0.9 L / kg steel. The total secondary cooling water volume is 240-270 L / min. The water volume of each section accounts for the following percentages of the total secondary cooling water volume: foot roll water cooling: 30%-36%; secondary cooling zone one: 39%-45%; secondary cooling zone two: 16%-21%; secondary cooling zone three: 2%-8%. Electromagnetic stirring is installed at the corresponding position in the secondary cooling zone.

[0076] (5) Grading control of hot billet pressure during straightening Before casting begins, the dummy bar is fed downwards from the straightening machine and inserted into the bottom of the crystallizer to receive the molten steel and pull the freshly solidified billet head out of the crystallizer. When the dummy bar is held and conveyed by the straightening rollers, the cold billet pressure (dummy bar pressure) is 85 bar. The billet temperature is controlled to be ≥950℃ when it enters the straightening machine. To ensure that the billet does not slide down when the machine starts and to control the roundness of the billet during continuous casting, the R8m arc continuous casting machine adopts a three-stand straightening machine. An auxiliary straightening machine device is designed in the straight straightening section. After the billet exits the second cooling zone, it passes through stand one, stand two, and the auxiliary straightening machine. The distance from the crystallizer to stand one is about 12 meters, the distance between stand one and stand two is about 2 meters, the distance between stand two and the auxiliary straightening machine is about 5 meters, and the distance from the crystallizer to the auxiliary straightening machine is about 19 meters. Along the straightening direction, there are three stands after the auxiliary straightening machine, namely stand one, stand two, and stand three in the steady-state casting section.

[0077] If the cumulative casting length after the start of casting does not exceed that of the auxiliary straightening machine, i.e., the first 19m of the casting stage after the start of casting, the billet shell is relatively thin and prone to extrusion deformation. Therefore, along the straightening direction, the pressure of the first stand of the casting transition section is controlled at 20-25 bar, the pressure of the second stand is 20-25 bar, and the pressure of the auxiliary straightening machine is 0 bar. If the cumulative casting length after the start of casting exceeds that of the auxiliary straightening machine, i.e., the cumulative casting length exceeds 19m and enters the steady-state casting stage, then along the straightening direction, the pressure of the first stand of the steady-state casting section is switched to 13-19 bar, the pressure of the second stand is 13-19 bar, and the pressure of the third stand is 5-10 bar.

[0078] (6) Hot-state fixed-length cutting After leaving the straightening machine, the billet is conveyed to the flame cutting station. The length of the hot billet is controlled at 9110-9130mm according to the thermal shrinkage coefficient of 1.013. The high-temperature billet after cutting is conveyed backward by the conveyor roller. After the billet is conveyed to the position, the billet flipping machine completes the flipping action, and then the billet transfer machine transfers the billet to the walking beam cooling bed. The billet enters the walking beam cooling bed and moves step by step to air cool to room temperature. After the billet is cooled to room temperature, the target cold billet length of 9000mm is obtained.

[0079] Specifically, the process parameters for the continuous casting process in each embodiment and comparative example are shown in Table 2.

[0080] Table 2. Process parameters of continuous casting process in each embodiment and comparative example

[0081] (7) Offline sampling Sampling objects include samples from the cut-off head and tail, as well as key heat samples from each casting cycle, covering the first, intermediate, and final heats. This allows for a comprehensive evaluation of the overall casting billet quality. Each casting cycle requires pickling, low-magnification cross-section inspection, and axial longitudinal section inspection. Low-magnification inspection is conducted according to GB / T226 standard, and non-metallic inclusions in the steel are rated according to Method A of GB / T10561. Specifically, samples are taken from each heat for inspection and rating, and the average value is taken. The test results of each example and comparative example are shown in Table 3.

[0082] Table 3. Detection results of each embodiment and comparative example

[0083] For Φ180-200mm small round billets of medium carbon manganese steel such as 25Mn2 and 30Mn2, existing technologies lack dynamic and coordinated control processes for changes in the internal working conditions of each casting cycle. This makes it impossible to simultaneously achieve the production goals of high steel purity, low central porosity, and good billet dimensional accuracy. As seen in the results of Examples 1-4, this invention employs a combined process of switching end-of-line electromagnetic stirring parameters and graded control of hot billet pressure under tail furnace speed-up conditions. Low-parameter end-of-line electromagnetic stirring is used for conventional steady-state billets to avoid white bright band defects. During the tail furnace speed-up conditions of each casting cycle, high-current, high-frequency end-of-line electromagnetic stirring is switched to break up dendrites at the solidification end, ensuring stable control of central porosity of the billet ≤1. Grade 0.5, free from defects such as central cracks, subcutaneous cracks, bright white bands, scabs, and pits, solving the problem of porosity deterioration in the tail furnace; and can further provide a high-cleanliness and low-porosity control production method through a complete set of smelting, refining, and continuous casting processes. The converter and LF work together to control oxygen and suppress inclusions, making the total oxygen and nitrogen content of molten steel controllable. The A, B, C, D, and DS type inclusion ratings of round billets are all ≤1.5. Without modifying existing equipment, it can simultaneously improve the cleanliness of molten steel, the internal quality of billets, the dimensional accuracy of round billets, and the surface quality, making it suitable for the mass production of seamless tube round billets, especially for 30Mn2 steel, and reducing subsequent scrap due to flaw detection.

[0084] Compared to Comparative Example 1, which did not switch the end electromagnetic stirring parameters according to the tail furnace operating conditions, the white bright band defects in the tail furnace casting section were aggravated and the central porosity was significantly worsened. The present invention adapts the current and frequency of the end electromagnetic stirring during the tail furnace speed-up stage, which can effectively improve the solidification structure of the billet and inhibit the deterioration of the tail furnace quality.

[0085] Compared to Comparative Example 2, which eliminated differentiated pressure control in the initial casting transition section, the thin billet shell in the early casting stage was subjected to unreasonable stress, resulting in severe non-roundness of the billet and large-sized flat defects in some areas. In this invention, the billet shell is thin in the casting transition section, and a higher clamping pressure is used to prevent the billet from slipping. In the steady-state stage, the billet shell is thickened, reducing the extrusion pressure. The non-roundness of the billet is reduced from the original 4mm to 1-3mm, and the width of the flat area is ≤65mm, which can reduce the uneven wall thickness defects in subsequent piercing and rolling.

[0086] Compared to Comparative Example 3, which used a protective slag with poor compatibility with the steel grade, resulting in deeper oscillation marks on the billet and aggravated white bright band defects, this invention can further control the composition requirements of the mold protective slag in the continuous casting process, suppress excessively deep oscillation marks and surface longitudinal cracks, and achieve uniform heat transfer and good lubrication within the mold.

[0087] The detailed descriptions listed above are merely specific illustrations of feasible embodiments of the present invention and are not intended to limit the scope of protection of the present invention. 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 quality control production method for medium carbon manganese steel continuously cast small round billets, characterized in that, The method includes the following steps during the continuous casting process for producing small round billets with a diameter of Φ180-200mm: (1) A combination of electromagnetic stirring in the crystallizer and electromagnetic stirring at the end is used: The current for controlling the electromagnetic stirring of the crystallizer is 150-250A, and the frequency is 1-3Hz; In each casting cycle, for the billet that has entered the stable production stage, the steady-state casting speed is controlled according to the billet cross-section and superheat, and the current of the end electromagnetic stirrer is controlled to be 150-250A and the frequency is 1-3Hz; for the billet in the tail furnace of the casting cycle, the casting speed is controlled to be increased by 0.1-0.2m / min relative to the steady-state casting speed, and the current of the end electromagnetic stirrer is controlled to be 400-450A and the frequency is 8-10Hz. (2) Grading control of hot billet pressure during straightening: The temperature of the billet entering the straightening machine should be ≥950℃; An auxiliary straightening machine is installed in the straight section of the continuous casting machine; If the cumulative length of the casting after the start of casting does not exceed that of the auxiliary straightening machine, then along the straightening direction, control the pressure of the first frame of the casting transition section to 20-25 bar, the pressure of the second frame to 20-25 bar, and the pressure of the auxiliary straightening machine to 0 bar. If the cumulative length of the casting exceeds that of the auxiliary straightening machine after the start of casting, then along the straightening direction, switch the pressure of the steady-state casting section to 13-19 bar for frame one, 13-19 bar for frame two, and 5-10 bar for frame three.

2. The quality control production method for medium carbon manganese steel continuously cast small round billets according to claim 1, characterized in that, Including converter smelting and LF refining: control the final carbon content to be 0.06%-0.10%, tapping temperature ≥1600℃; tapping volume is 52±1.5 tons, calcium aluminate and lime are added during the tapping process to implement the slag return process, calcium aluminate dosage is 400±50kg / heat, lime dosage is 300±50kg / heat; ladle baking temperature ≥800℃, and the transition time from converter tapping to LF entry is ≤12min.

3. The quality control production method for medium carbon manganese steel continuously cast small round billets according to claim 1, characterized in that, Including LF refining: after the white slag is produced, the white slag holding time is ≥20min; when phosphorus return occurs in the converter slag, add 25-30kg of lime to continue white slag smelting; soft blowing time is ≥20min.

4. The quality control production method for medium carbon manganese steel continuously cast small round billets according to claim 1, characterized in that, Including LF refining: Pure calcium wire is fed in before leaving the station after refining. The amount of pure calcium wire fed in the start-up heat is 55-65m, and the amount of pure calcium wire fed in the continuous casting heat is 20-30m. The amount of molten steel per heat is 52±1.5 tons. The ladle Als is controlled at 0.020-0.030%, and the ladle P≤0.018%. The gas composition of molten steel is controlled at T[O]≤0.0020%, and [N]≤0.0055%.

5. The quality control production method for medium carbon manganese steel continuously cast small round billets according to claim 1, characterized in that, In the continuous casting process, argon gas is purged after the tundish is baked; the erosion of the submerged entry nozzle is controlled to be 3-5 mm.

6. The quality control production method for medium carbon manganese steel continuously cast small round billets according to claim 1, characterized in that, The mold flux composition for the continuous casting process is required to include, by mass percentage: SiO2: 24.5%-31.3%, CaO: 22.6%-30.5%, Al2O3: 4.5%-11.4%, Fe2O3: ≤3%, MgO: 2%-5%, F - : 2%-6.5%, MnO: ≤3%, R2O: 4%-11.3%; TC: 4%-11.3%, H2O: ≤0.5%, the remainder being unavoidable impurities; when the round billet steel grade is 30Mn2, the CaO / SiO2 ratio is 0.96±0.

1.

7. The quality control production method for medium carbon manganese steel continuously cast small round billets according to claim 1, characterized in that, The continuous casting process employs a linkage control between superheat and casting speed: the superheat for the first casting is controlled at 30-45℃, and the superheat for subsequent castings is controlled at 20-35℃; when the superheat is >45℃, the casting speed is 1-1.3 m / min; when the superheat is 36-45℃, the casting speed is 1.2-1.5 m / min; when the superheat is 20-35℃, the casting speed is 1.2-1.7 m / min; and when the superheat is <20℃, the casting speed is 1.6-1.9 m / min.

8. The quality control production method for medium carbon manganese steel continuously cast small round billets according to claim 1, characterized in that, Before straightening in the continuous casting process, foot roller water cooling and secondary cooling mist cooling are used, with a total water volume of 0.6-0.9 L / kg steel. The water volume ratio along the straightening direction is as follows: foot roller water cooling: 30%-36%; secondary cooling zone 1: 39%-45%; secondary cooling zone 2: 16%-21%; secondary cooling zone 3: 2%-8%.

9. The quality control production method for medium carbon manganese steel continuously cast small round billets according to claim 1, characterized in that, The continuous casting process controls the thermal shrinkage coefficient of the billet to be 1.012-1.

014.

10. A medium-carbon manganese steel continuously cast small round billet, characterized in that, The round billet is produced by the quality control production method for medium carbon manganese steel continuous casting small round billets as described in any one of claims 1-9. The chemical composition of the round billet, by mass percentage, includes: C: 0.27%-0.34%, Si: 0.17%-0.37%, Mn: 0.14%-0.18%, P≤0.035%, S≤0.035%, with the remainder being Fe and unavoidable impurities.

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