A production method for avoiding longitudinal cracks in medium carbon manganese steel

Abstract

The application belongs to the technical field of carbon steel preparation, and discloses a production method for avoiding longitudinal cracks of medium-carbon manganese steel. The method comprises the following steps: converter smelting, obtaining molten steel and carrying out double-block slag tapping, and adding functional materials into the ladle to obtain outgoing molten steel; LF refining is carried out on the outgoing molten steel; protective pouring is carried out to obtain straightened casting blank; the straightened casting blank is sent into a first slow cooling pit for slow cooling to obtain slow cooling casting blank; the slow cooling casting blank is heated to obtain heated casting blank; the heated casting blank is sequentially subjected to rough rolling, continuous rolling, sawing, cold bed stepping and second slow cooling pit slow cooling to obtain medium-carbon manganese steel without longitudinal cracks. The application solves the problem of longitudinal cracks of medium-carbon manganese steel without increasing cost and affecting the quality of steel under the premise of strictly controlling the nitrogen content in the steel, using appropriate high secondary cooling intensity (i.e. high specific water quantity) in continuous casting, straightening casting blank slow cooling and using low-temperature preheating technology in steel rolling heating.

Description

Technical Field

[0001] This invention belongs to the field of carbon steel preparation technology, and more specifically, relates to a production method for avoiding longitudinal cracks in medium carbon manganese steel. Background Technology

[0002] In recent years, the booming development of the automotive and construction machinery industries has led to an increase in the demand for steel used in automotive and construction machinery parts, and the quality requirements have become increasingly stringent. 40Mn2 and 45Mn2, as typical grades of medium-carbon quenched and tempered manganese steel, are widely used in the manufacture of automotive and construction machinery parts operating under heavy loads, such as wheel bodies for four-wheel drive systems, scrapers for coal mining machinery, control levers, worm gears, and reinforcing rings. As these are automotive and construction machinery parts operating under heavy loads, 40Mn2 and 45Mn2 have strict requirements for surface quality, hardness uniformity, wear resistance, and fatigue performance. 40Mn2 has a carbon content of 0.37%–0.44% and a manganese content of 1.40%–1.80%, while 45Mn2 has a carbon content of 0.42%–0.49% and a manganese content of 1.40%–1.80%. Due to the characteristics of these medium-carbon manganese steel grades, longitudinal surface cracks are easily generated during the cooling and heating process of the cast billet, seriously affecting the quality of the steel.

[0003] Chinese patent application No. 200910301243.8 discloses a "production method for preventing longitudinal cracks on the surface of medium carbon manganese steel billets." This invention proposes adding a titanium-iron alloy to the original chemical composition after final deoxidation, controlling the titanium content to be 0.005%–0.0145%; the steel is then continuously cast after adding the titanium-iron alloy. This improvement achieves the goal of avoiding surface longitudinal cracks in medium carbon manganese steel billets. However, while the method provided in this invention can prevent longitudinal cracks in medium carbon manganese steel billets, it requires the addition of the chemical element titanium to the steel, which easily produces titanium nitride and titanium carbonitride inclusions, affecting the cleanliness of the steel and increasing production costs. Furthermore, it does not address how to prevent crack formation during the rolling process of medium carbon manganese steel billets. Currently, there is no simple and effective production method to avoid longitudinal cracks in medium carbon manganese steel. Summary of the Invention

[0004] The purpose of this invention is to address the shortcomings of existing technologies by proposing a production method to avoid longitudinal cracks in medium carbon manganese steel. This invention solves the problem of longitudinal cracks in medium carbon manganese steel without increasing costs or affecting steel quality. This is achieved through strict control of the nitrogen content in the steel, using appropriately high secondary cooling strength (i.e., high specific water content) in continuous casting, slow cooling of the straightened billet, and low-temperature preheating technology for rolling.

[0005] To achieve the above objectives, the present invention provides a production method for avoiding longitudinal cracks in medium carbon manganese steel, the method comprising the following steps:

[0006] S1: Steelmaking raw materials are fed into the converter for converter smelting to obtain molten steel, which is then tapped using a double-slag-blocking method, allowing the molten steel to flow into a ladle filled with bottom-blown argon gas. Within the ladle, when the weight percentage of the molten steel fed into the converter reaches a first threshold value relative to the total weight percentage of the molten steel in the converter before tapping, functional materials are added to the ladle while the molten steel continues to flow into the ladle. The addition of the functional materials is completed when the weight percentage of the molten steel fed into the converter reaches a second threshold value relative to the total weight percentage of the molten steel in the converter before tapping. After the double-slag-blocking tapping is completed, the converter exits the station, yielding molten steel.

[0007] S2: LF refining of the molten steel leaving the station: The molten steel leaving the station is moved from the soft blowing station to the refining station, where it is subjected to electric current, white slag refining, composition conditioning and wire feeding treatment to obtain LF refined molten steel; The LF refined molten steel is returned to the soft blowing station and a covering agent is added to the surface of the LF refined molten steel for heat preservation and to prevent secondary oxidation of the molten steel.

[0008] S3: Protective casting: Under argon protection, the LF refined steel flows into the tundish through a large ladle protective sleeve; the tundish uses an internal water inlet to form a closed steel channel between the tundish and the crystallizer, and the LF refined steel in the tundish is sent into the crystallizer to obtain a billet; the billet is then straightened after passing through a secondary cooling zone to obtain a straightened billet.

[0009] S4: The straightened billet is sent into the first slow cooling pit for slow cooling to obtain a slow-cooled billet;

[0010] S5: The slow-cooled billet is fed into a heating furnace for heating to obtain a heated billet; the heated billet is subjected to initial rolling, continuous rolling, sawing, cooling bed stepping and second slow cooling pit slow cooling in sequence to obtain medium carbon manganese steel without longitudinal cracks.

[0011] According to the present invention, preferably, the grade of the medium carbon manganese steel without longitudinal cracks is 40Mn2.

[0012] According to the present invention, preferably, in step S1:

[0013] Based on the total mass of the molten steel, the carbon content in the molten steel is ≥0.08%;

[0014] The double-slag tapping method involves using slag-blocking plugs and slag-blocking valves to tap steel, strictly controlling the amount of slag discharged to prevent top slag from returning to phosphorus.

[0015] The first threshold is 0.20-0.30% of the total weight of molten steel in the converter before tapping;

[0016] The second threshold is 0.70-0.80% of the total weight of molten steel in the converter before tapping;

[0017] The functional materials include materials for deoxidation and alloying, low-nitrogen carbon raisers, and materials for top slag modification.

[0018] The addition of functional materials to the ladle includes: sequentially adding the materials for deoxidation and alloying, the low-nitrogen carbon raiser, and the materials for top slag modification to the ladle, while allowing the molten steel to continue flowing into the ladle;

[0019] Based on the total mass of the molten steel leaving the station, the carbon content in the molten steel leaving the station is 0.30-0.36%, and the manganese content is 1.55-1.61%.

[0020] According to the present invention, preferably, the material used for deoxidation alloying is at least one of aluminum ingot, silicon-manganese alloy and ferromanganese.

[0021] According to the present invention, preferably, the material used for top slag modification is pre-melted calcium aluminate and / or refined lime.

[0022] In this invention, no alloys or slag are allowed to be added to the bottom of the ladle before the molten steel flows into it. All added functional materials are preheated and baked.

[0023] In this invention, the double-permeable brick of the ladle is well-designed, and argon is circulated throughout the entire process from steel tapping to ladle hoisting.

[0024] According to the present invention, preferably, in step S2:

[0025] When the molten steel leaving the station is in the soft blowing station, the soft blowing station sequentially performs strong argon blowing and weak argon blowing on the molten steel leaving the station until it enters the refining station.

[0026] The strong argon blowing is performed at an argon flow rate of 600-700 NL / min, and the strong argon blowing time is 0.9-1.2 min. After the gas permeability is normal, the flow is adjusted to weak argon blowing.

[0027] The argon gas weak blowing is an argon gas flow rate of 200-300 NL / min;

[0028] The refining station performs argon gas blowing, and the argon gas blowing is at an argon gas flow rate of 400-500 NL / min.

[0029] The white slag refining process also includes shutting down the power supply and taking samples for testing when the temperature inside the LF refining furnace reaches 1530-1540℃; the sampling and testing includes taking samples of molten steel and slag; when the results of the molten steel test show that the aluminum content in the molten steel inside the LF refining furnace is ≤0.020%, the aluminum content in the molten steel inside the LF refining furnace is adjusted to above 0.025% (to avoid adjusting aluminum later); the obtained slag samples are then assessed for slag condition and their properties are controlled;

[0030] White residue retention time ≥ 20 min

[0031] The composition conditioning includes adding at least one of medium-carbon ferromanganese, ferrosilicon and low-nitrogen carbon raiser to the molten steel in the LF refining furnace.

[0032] The wire feeding speed of the wire feeding process is 1.2-2.0 m / s;

[0033] The LF refining of molten steel takes ≥20 minutes at the soft blowing station. Argon gas is adjusted to make the surface of the molten steel creep, and the molten steel must not be exposed.

[0034] Based on the total mass of the LF refined molten steel, the carbon content in the LF refined molten steel is 0.38-0.41%, and the manganese content is 1.47-1.53%.

[0035] In this invention, the voltage and current of the energizer are selected according to the temperature and production rhythm to reasonably control the heating rate of LF refining.

[0036] In this invention, the white slag refining process includes: adding slag-forming materials (lime, pre-melted calcium aluminate, etc.) in batches, with each batch weighing no more than 100 kg, to prevent slag agglomeration. A deoxidizer (calcium carbide, aluminum granules, and silicon carbide mixed in bulk and added in batches) is added to the slag surface for diffusion deoxidation. After energizing for 10 minutes, the slag melts and becomes highly fluid, forming white slag. Then, when the temperature in the LF refining furnace reaches 1530-1540℃, the power is cut off and samples are taken for analysis. In this invention, after the power is cut off, argon gas is blown in for 2 minutes.

[0037] According to the present invention, preferably, the carbon content in the low-nitrogen carbon raiser is ≥96.0% and the nitrogen content is ≤0.025% based on the total mass of the low-nitrogen carbon raiser.

[0038] According to the present invention, preferably, in step S3:

[0039] Since 3-15 heats of molten steel need to be prepared for one run of the above method, the temperature of the first heat of LF refining molten steel in the ladle protective sleeve is 1572-1582℃, and the temperature of the second and subsequent heats of LF refining molten steel is 1539-1549℃.

[0040] The temperature of the molten steel in the tundish is 1510-1525℃;

[0041] The water flow rate of the crystallizer is 80-86 m. 3 / h, narrow face is 68-74m 3 / h;

[0042] The electromagnetic stirring current of the crystallizer is 370-410A, and the electromagnetic stirring frequency of the crystallizer is 1.4-1.6Hz; the electromagnetic stirring current at the solidification end is 390-410A, and the electromagnetic stirring frequency at the solidification end is 5.2-5.8Hz.

[0043] The dimensions of the cast billet are (380-400)mm × (500-520)mm, preferably 390mm × 510mm;

[0044] The specific water content is 0.175-0.185 L / kg; the throwing speed is 0.40-0.42 m / min;

[0045] The secondary cooling zone includes a first secondary cooling zone, a second secondary cooling zone, a third secondary cooling zone, and a fourth secondary cooling zone arranged sequentially from the crystallizer to the straightening device; the ratio of the cooling water usage of the first secondary cooling zone, the second secondary cooling zone, the third secondary cooling zone, and the fourth secondary cooling zone is (20-30):(35-40):(20-30):(10-15).

[0046] In this invention, the protective slag for the crystallizer uses a special protective slag for 40Mn2 steel billets.

[0047] In this invention, as a preferred embodiment, a water content of 0.18 L / Kg is used instead of the traditional 0.15 L / Kg water content for medium carbon steel. The preferred distribution ratio of cooling water usage in the four secondary cooling zones is 25:38:24:13 for the first, second, third, and fourth secondary cooling zones. Under the premise of avoiding the brittle range of the steel grade in the billet straightening area, a process to improve cooling intensity is adopted to ensure that the billet has sufficient shell strength to resist stress in the straightening area and avoid cracking.

[0048] According to the present invention, preferably, in step S4:

[0049] Before the straightened billet is fed into the first slow cooling pit for slow cooling, the first slow cooling pit needs to be warmed up; the number of warmed-up billets used for the warming-up treatment is ≥8, the warming-up treatment time is ≥4h, and the bottom temperature of the first slow cooling pit is ≥100℃.

[0050] The temperature at which the straightened billet enters the first slow cooling pit is ≥600℃, and the temperature at which the straightened billet exits the first slow cooling pit is ≤350℃; the time for the straightened billet to be slow-cooled in the first slow cooling pit is ≥24h.

[0051] According to the present invention, preferably, in step S5:

[0052] The heating furnace is a regenerative walking beam heating furnace;

[0053] The heating furnace is divided into a preheating section, a heating section II, a heating section III, and a soaking section from its inlet to its outlet.

[0054] The preheating section does not have a burner and its temperature is ≤820℃;

[0055] The temperature of the heating section II is 850℃-1050℃;

[0056] The temperature of the heating section III is 1180℃-1250℃;

[0057] The temperature of the heat spreader is 1160℃-1240℃;

[0058] The residence time of the slow-cooling billet in the heating furnace shall not be less than 380 minutes, wherein the residence time of the slow-cooling billet in the preheating section shall not be less than 100 minutes, and the total residence time in the heating section III and the soaking section shall not be less than 150 minutes.

[0059] The initial rolling temperature is 1080-1120℃;

[0060] The final temperature of the continuous rolling mill is 920-1000℃;

[0061] The second slow-cooling pit has a slow-cooling time of ≥36 hours.

[0062] In this invention, the initial rolling and continuous rolling are performed using a φ1150mm two-roll reversible billet mill, and the rolling is carried out by a 4-stand φ850mm + 4-stand φ750mm finishing mill.

[0063] According to the present invention, preferably, the grade of the medium carbon manganese steel without longitudinal cracks is 45Mn2.

[0064] The beneficial effects of the technical solution of this invention are as follows: Without increasing costs or causing other impacts on steel quality, this invention solves the problem of longitudinal cracks in medium-carbon manganese steel by strictly controlling the nitrogen content in the steel, using appropriately high secondary cooling strength (i.e., high specific water content) in continuous casting, slow cooling of the straightened billet, and using low-temperature preheating technology for rolling heating. Specifically:

[0065] 1. The carbon raiser used in the converter smelting and LF refining processes of this invention is a low-nitrogen carbon raiser, which effectively reduces the nitrogen content in the steel, improves the plasticity of the steel, and avoids the nitrides precipitated at the austenite grain boundaries during the cooling process of the billet from hindering grain boundary movement during the hot deformation of the billet, thereby generating stress and cracks at these locations.

[0066] 2. For crack-sensitive steel grades, the traditional approach to secondary cooling water volume design is to reduce the specific water volume and slow down cooling to avoid cracking. This invention, through research on the crack formation mechanism of 40Mn2 and combined with actual working conditions, breaks with the traditional approach. For billets with a cross-section of (380-400)mm × (500-520)mm, a secondary cooling water volume of 0.18L / kg is set. Under the premise of avoiding the brittle range of the steel grade in the billet straightening zone, a process of increasing cooling intensity (increasing specific water volume) is adopted to ensure that the billet has sufficient shell strength to resist stress in the straightening zone, thus avoiding cracking.

[0067] 3. In this invention, the straightened billet is fed into the first slow cooling pit for slow cooling. The straightened billet is cooled slowly and evenly in the slow cooling pit, which fully releases stress and avoids cracking.

[0068] 4.40Mn2 is a crack-sensitive steel. When heating a slab with a cross-section of (380-400)mm × (500-520)mm, excessively high preheating temperatures and rapid heating rates can easily lead to large temperature differences between the inside and outside of the slab, resulting in high thermal stress and stress cracks. In this invention, for 40Mn2, a novel approach is adopted: the preheating section does not use burners, and the residual heat of the furnace gas in the other three high-temperature sections is used for low-temperature preheating of the slab. This creates a slow preheating environment for the slab before it reaches 500℃, thus preventing the formation of longitudinal cracks.

[0069] Other features and advantages of the present invention will be described in detail in the following detailed description section. Detailed Implementation

[0070] Preferred embodiments of the invention will now be described in more detail. While preferred embodiments of the invention are described below, it should be understood that the invention can be implemented in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided so that the invention will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art.

[0071] Example 1

[0072] This embodiment provides a production method for avoiding longitudinal cracks in medium carbon manganese steel. The furnace number is 23501155, producing medium carbon manganese steel 40Mn2 with a carbon content of 0.37%–0.44% and a manganese content of 1.40%–1.80%. The method includes the following steps:

[0073] S1: Steelmaking raw materials are fed into the converter for converter smelting to obtain molten steel. The steel is then tapped at 1663°C using a double-slag-blocking method, allowing it to flow into a ladle filled with bottom-blown argon gas. In the ladle, when the weight of the molten steel fed into the converter accounts for 0.25% of the total weight of the molten steel before tapping, functional materials are added (i.e., 117 kg of aluminum ingots, 1302 kg of silicon-manganese alloy, 1169 kg of ferromanganese, 30 kg of low-nitrogen carbon raiser, 466 kg of pre-melted calcium aluminate, and 302 kg of refined (top) lime are added sequentially). Simultaneously, the molten steel continues to flow into the ladle. The addition of these functional materials is completed when the weight of the molten steel fed into the converter accounts for 0.75% of the total weight of the molten steel before tapping. After the double-slag-blocking tapping is completed, the converter exits the station, yielding molten steel.

[0074] The steelmaking raw materials consist of 125 tons of molten iron and 6.5 tons of scrap steel.

[0075] Based on the total mass of the molten steel, the carbon content in the molten steel is 0.131%, and the phosphorus content is 0.023%.

[0076] The double-slag-blocking steel tapping method involves tapping steel using slag-blocking plugs and slag-blocking valves;

[0077] S2: LF refining of the molten steel leaving the station: The molten steel is brought to the soft-blowing station, where strong argon blowing (argon flow rate 650 NL / min) is initiated for 1 minute. Once normal permeability is achieved, the argon flow rate is adjusted to weak blowing (argon flow rate 300 NL / min) before entering the refining station. The ladle is then introduced to the refining station with argon blowing adjusted to medium blowing (argon flow rate 500 NL / min). Temperature is measured, and the voltage and current are adjusted according to the temperature and production rhythm to reasonably control the heating rate. Slag-forming materials (321 kg lime, 363 kg pre-melted calcium aluminate) are added in batches, each batch not exceeding 100 kg to prevent slag agglomeration. Deoxidizer (120.9 kg calcium carbide, 23.2 kg aluminum granules, and 60 kg silicon carbide, mixed in bulk and added in batches) is added to the slag surface for diffusion deoxidation. After 10 minutes of energization, the slag melted and formed white slag with good fluidity at a temperature of 1539℃. After power was cut off and argon was blown in for 2 minutes, the first sample was taken for testing (including steel and slag samples; the slag sample showed white slag; when the steel analysis results showed that the aluminum content in the molten steel in the LF refining furnace was ≤0.020%, the aluminum content in the molten steel in the LF refining furnace was adjusted to above 0.025%). Diffusion deoxidation continued, maintaining a reducing atmosphere until exiting the station, with the white slag maintained for 32 minutes. After good deoxidation, medium-carbon ferromanganese (120 kg), ferrosilicon (36 kg), and low-nitrogen carbon raiser (80 kg) were added according to the target composition to adjust the composition to the target. Once the composition was qualified and the temperature was suitable, 120 m of calcium wire was fed at a feeding speed of 1.5 m / s. After the calcium feeding line is completed, the steel is moved to the soft blowing station and 60 kg of covering agent is added for heat preservation and to prevent secondary oxidation of the molten steel. At the same time, the argon gas is adjusted to ensure that the molten steel is not exposed due to surface peristalsis. The soft blowing argon time is 22 minutes to obtain LF refined molten steel. Based on the total mass of the LF refined molten steel, the carbon content in the LF refined molten steel is 0.40% and the manganese content is 1.49%.

[0078] S3: Protective casting: Under argon protection, the LF refined steel flows into the tundish through a large ladle protective sleeve; the tundish uses an internal water inlet to form a closed steel channel between the tundish and the crystallizer, and the LF refined steel in the tundish is sent into the crystallizer to obtain a billet; the billet is then straightened after passing through a secondary cooling zone to obtain a straightened billet.

[0079] Wherein: to carry out the above method once, 4 heats of molten steel need to be prepared. The platform temperature of the ladle protective sleeve is 1543℃. The temperature of the first heat of LF refined molten steel in the tundish is 1518℃. The temperatures of the molten steel in the tundish for the second and subsequent heats are 1518℃, 1517℃ and 1516℃, respectively.

[0080] The water flow rate of the crystallizer is 80-86m. 3 / h, narrow face is 68-74m 3 / h;

[0081] The electromagnetic stirring current of the crystallizer is 390A, and the electromagnetic stirring frequency of the crystallizer is 1.5Hz; the electromagnetic stirring current at the solidification end is 400A, and the electromagnetic stirring frequency at the solidification end is 5.5Hz.

[0082] The specific water content is 0.18 L / kg; the throwing speed is 0.41 m / min;

[0083] The secondary cooling zone includes a first secondary cooling zone, a second secondary cooling zone, a third secondary cooling zone, and a fourth secondary cooling zone arranged sequentially from the crystallizer to the straightening device; the ratio of cooling water usage in the first secondary cooling zone, the second secondary cooling zone, the third secondary cooling zone, and the fourth secondary cooling zone is 25:38:24:13;

[0084] Step S3 ultimately yields 14 390mm×510mm cross-section castings, with a total weight of 124.88t. Based on the total mass of one cross-section casting, the C content in each casting is 0.399% and the Mn content is 1.49%.

[0085] S4: The straightened billet is sent into the first slow cooling pit for slow cooling to obtain a slow-cooled billet;

[0086] Before the straightened billet is fed into the first slow cooling pit for slow cooling, the bottom temperature of the first slow cooling pit must be ≥100℃. If the bottom temperature of the first slow cooling pit is <100℃, the first slow cooling pit needs to be warmed up. The number of warmed-up billets used for the warming-up treatment must be ≥8, the warming-up treatment time must be ≥4h, and the bottom temperature of the first slow cooling pit must be ≥100℃.

[0087] The temperature at which the straightened billet enters the first slow cooling pit is 617-625℃, and the temperature at which the straightened billet exits the first slow cooling pit is 100-300℃; the time for the straightened billet to be slow-cooled in the first slow cooling pit is ≥24h.

[0088] The slow-cooled billet, after being inspected by surface examination, showed no visible longitudinal cracks, and the surface of the billet after low-magnification pickling showed no longitudinal cracks.

[0089] S5: The slowly cooled billet is fed into a regenerative walking beam furnace for heating. The furnace is divided into a preheating section (without burners, temperature 745℃-756℃), a heating section II (978℃-991℃), a heating section III (1202℃-1217℃), and a soaking section (1196℃-1199℃) from the inlet to the outlet. The residence time of the slowly cooled billet in the furnace is not less than 380 minutes, of which the residence time in the preheating section is 134 minutes. The total dwell time in the heating stage III and soaking stage is 293 minutes, resulting in a heated billet. The heated billet is then subjected to initial rolling and continuous rolling (the initial rolling and continuous rolling are performed using a φ1150mm two-roll reversible billet mill, followed by rolling on a 4-stand φ850mm + 4-stand φ750mm finishing mill; the initial rolling temperature is 1083-1086℃; the continuous rolling final temperature is 935-939℃). After rolling, the steel is rapidly collected on the cooling bed and promptly placed in the second slow cooling pit for slow cooling for ≥36 hours, ensuring the hot steel is placed at the bottom of the slow cooling pit. The resulting bars are φ100mm and φ120mm in size. Upon leaving the warehouse, the bars show no visible longitudinal cracks after surface inspection, and the billet surface after low-magnification pickling shows no longitudinal cracks.

[0090] In this embodiment, based on the total mass of the low-nitrogen carbon raiser, the carbon content in the low-nitrogen carbon raiser is ≥96.0%, and the nitrogen content is ≤0.025%.

[0091] The various embodiments of the present invention have been described above. These descriptions are exemplary and not exhaustive, nor are they limited to the disclosed embodiments. Many modifications and variations will be apparent to those skilled in the art without departing from the scope and spirit of the described embodiments.

Claims

1. A production method for avoiding longitudinal cracks in medium carbon manganese steel, characterized in that, The method includes the following steps: S1: Steelmaking raw materials are fed into the converter for converter smelting to obtain molten steel, which is then tapped using a double-slag-blocking method, allowing the molten steel to flow into a ladle filled with bottom-blown argon gas. Within the ladle, when the weight percentage of the molten steel fed into the converter reaches a first threshold value relative to the total weight percentage of the molten steel in the converter before tapping, functional materials are added to the ladle while the molten steel continues to flow into the ladle. The addition of the functional materials is completed when the weight percentage of the molten steel fed into the converter reaches a second threshold value relative to the total weight percentage of the molten steel in the converter before tapping. After the double-slag-blocking tapping is completed, the converter exits the station, yielding molten steel. The first threshold is 0.20-0.30% of the total weight of molten steel in the converter before tapping; The second threshold is 0.70-0.80% of the total weight of molten steel in the converter before tapping; The functional materials include materials for deoxidation and alloying, low-nitrogen carbon raisers, and materials for top slag modification. S2: LF refining of the molten steel leaving the station: The molten steel leaving the station is moved from the soft blowing station to the refining station, where it is subjected to electric current, white slag refining, composition conditioning and wire feeding treatment to obtain LF refined molten steel; The LF refined molten steel is returned to the soft blowing station and a covering agent is added to the surface of the LF refined molten steel for heat preservation and to prevent secondary oxidation of the molten steel. White residue retention time ≥ 20 min; The time for the LF refining molten steel in the soft blowing station is ≥20 min; Based on the total mass of the LF refined steel, the carbon content in the LF refined steel is 0.38-0.41%, and the manganese content is 1.47-1.53%. S3: Protective casting: Under argon protection, the LF refined steel flows into the tundish through a large ladle protective sleeve; the tundish uses an internal water inlet to form a closed steel channel between the tundish and the crystallizer, and the LF refined steel in the tundish is sent into the crystallizer to obtain a billet; the billet is then straightened after passing through a secondary cooling zone to obtain a straightened billet. The specific water content is 0.175-0.185 L / kg; the throwing speed is 0.40-0.42 m / min; The secondary cooling zone includes a first secondary cooling zone, a second secondary cooling zone, a third secondary cooling zone, and a fourth secondary cooling zone arranged sequentially from the crystallizer to the straightening device; the ratio of cooling water usage in the first secondary cooling zone, the second secondary cooling zone, the third secondary cooling zone, and the fourth secondary cooling zone is (20-30):(35-40):(20-30):(10-15). S4: The straightened billet is sent into the first slow cooling pit for slow cooling to obtain a slow-cooled billet; Before the straightened billet is fed into the first slow cooling pit for slow cooling, the first slow cooling pit needs to be warmed up; the number of warmed-up billets used for the warming-up treatment is ≥8, the warming-up treatment time is ≥4h, and the bottom temperature of the first slow cooling pit is ≥100℃. The temperature at which the straightened billet enters the first slow cooling pit is ≥600℃, and the temperature at which the straightened billet exits the first slow cooling pit is ≤350℃; the time for the straightened billet to be slow-cooled in the first slow cooling pit is ≥24h. S5: The slow-cooled billet is fed into a heating furnace for heating to obtain a heated billet; the heated billet is subjected to initial rolling, continuous rolling, sawing, cooling bed stepping and second slow cooling pit slow cooling in sequence to obtain medium carbon manganese steel without longitudinal cracks; The heating furnace is divided into a preheating section, a heating section II, a heating section III, and a soaking section from its inlet to its outlet. The preheating section does not have a burner and its temperature is ≤820℃; The residence time of the slow-cooling billet in the heating furnace shall not be less than 380 minutes, wherein the residence time of the slow-cooling billet in the preheating section shall not be less than 100 minutes, and the total residence time in the heating section III and the soaking section shall not be less than 150 minutes.

2. The production method for avoiding longitudinal cracks in medium carbon manganese steel according to claim 1, wherein, The grade of the medium carbon manganese steel without longitudinal cracks is 40Mn2.

3. The production method for avoiding longitudinal cracks in medium carbon manganese steel according to claim 1 or 2, wherein, In step S1: Based on the total mass of the molten steel, the carbon content in the molten steel is ≥0.08%; The double-slag-blocking steel tapping method involves tapping steel using slag-blocking plugs and slag-blocking valves; The addition of functional materials to the ladle includes: sequentially adding the materials for deoxidation and alloying, the low-nitrogen carbon raiser, and the materials for top slag modification to the ladle, while allowing the molten steel to continue flowing into the ladle; Based on the total mass of the molten steel leaving the station, the carbon content in the molten steel leaving the station is 0.30-0.36%, and the manganese content is 1.55-1.61%.

4. The production method for avoiding longitudinal cracks in medium carbon manganese steel according to claim 3, wherein, The material used for deoxidation alloying is at least one of aluminum ingot, silicon-manganese alloy and ferromanganese; The materials used for top slag modification are pre-melted calcium aluminate and / or refined lime.

5. The production method for avoiding longitudinal cracks in medium carbon manganese steel according to claim 1 or 2, wherein, In step S2: When the molten steel leaving the station is in the soft blowing station, the soft blowing station sequentially performs strong argon blowing and weak argon blowing on the molten steel leaving the station until it enters the refining station. The argon gas forced blowing is performed at an argon gas flow rate of 600-700 NL / min, and the argon gas forced blowing time is 0.9-1.2 min. The argon gas weak blowing is an argon gas flow rate of 200-300 NL / min; The refining station performs argon gas blowing, and the argon gas blowing is at an argon gas flow rate of 400-500 NL / min. The white slag refining process also includes shutting down the power supply and taking samples for testing when the temperature inside the LF refining furnace reaches 1530-1540℃; the sampling and testing includes taking samples of molten steel and slag; when the results of the molten steel test show that the aluminum content in the molten steel inside the LF refining furnace is ≤0.020%, the aluminum content in the molten steel inside the LF refining furnace is adjusted to above 0.025%; the slag samples are then assessed for slag condition and their properties are controlled. The composition conditioning includes adding at least one of medium-carbon ferromanganese, ferrosilicon and low-nitrogen carbon raiser to the molten steel in the LF refining furnace. The wire feeding speed of the wire feeding process is 1.2-2.0 m / s.

6. The production method for avoiding longitudinal cracks in medium carbon manganese steel according to claim 4, wherein, Based on the total mass of the low-nitrogen carbon raiser, the carbon content in the low-nitrogen carbon raiser is ≥96.0%, and the nitrogen content is ≤0.025%.

7. The production method for avoiding longitudinal cracks in medium carbon manganese steel according to claim 1 or 2, wherein, In step S3: Since 3-15 heats of molten steel need to be prepared for one run of the above method, the temperature of the first heat of LF refining molten steel in the ladle protective sleeve is 1572-1582℃, and the temperature of the second and subsequent heats of LF refining molten steel is 1539-1549℃. The temperature of the molten steel in the tundish is 1510-1525℃; The water flow rate of the crystallizer is 80-86 m. 3 / h, narrow face is 68-74m 3 / h; The electromagnetic stirring current of the crystallizer is 370-410A, and the electromagnetic stirring frequency of the crystallizer is 1.4-1.6Hz; the electromagnetic stirring current at the solidification end is 390-410A, and the electromagnetic stirring frequency at the solidification end is 5.2-5.8Hz. The dimensions of the cast billet are (380-400) mm × (500-520) mm.

8. The production method for avoiding longitudinal cracks in medium carbon manganese steel according to claim 1 or 2, wherein, In step S5: The heating furnace is a regenerative walking beam heating furnace; The temperature of the heating section II is 850℃-1050℃; The temperature of the heating section III is 1180℃-1250℃; The temperature of the heat spreader is 1160℃-1240℃; The initial rolling temperature is 1080-1120℃; The final temperature of the continuous rolling mill is 920-1000℃; The second slow-cooling pit has a slow-cooling time of ≥36 hours.

9. The production method for avoiding longitudinal cracks in medium carbon manganese steel according to claim 1, wherein, The grade of the medium carbon manganese steel without longitudinal cracks is 45Mn2.