Smelting process for regulating and controlling C-class and D-class inclusions of heavy rail steel through magnesium treatment

Abstract

The invention discloses a smelting process for regulating and controlling C-class and D-class inclusions of heavy rail steel through magnesium treatment, and belongs to the technical field of ferrous metallurgy. Smelting is carried out according to given components and a production process, the smelting process comprises the steps of converter smelting, LF refining, VD vacuum treatment, magnesium treatment and continuous casting, and the generation type and distribution of inclusions in the heavy rail steel are accurately regulated and controlled mainly by adding a proper amount of magnesium-containing cored wires in the last stage of smelting. Most inclusions in the heavy rail steel exist in the form of fine dispersed calcium-silicon-magnesium-aluminum-oxygen composite inclusions. According to the smelting process, large-size C-class and D-class inclusions with the size larger than 10 micrometers can be eliminated, intragranular acicular ferrite can be induced to be formed in the solidification process, grains are refined, the performance is improved, and the performance and quality of the heavy rail steel are improved.

Description

Technical Field

[0001] The invention belongs to the technical field of iron and steel metallurgy, and in particular relates to a smelting process for regulating C-type and D-type inclusions in heavy rail steel by magnesium treatment. Background Art

[0002] Heavy rail steel is a type of steel with high cleanliness, high strength and high toughness. It is the main component of railway tracks. During railway transportation, it needs to withstand huge vertical pressure from the wheels and strong lateral friction. For heavy rail steel, inclusions in the steel are the main cause of internal flaw detection and fatigue damage of heavy rail steel. Heavy rail steel adopts non-aluminum deoxidation process. Due to strict control of sulfur content, large particle inclusions in the steel are often mainly C and D inclusions. For the production process using VD vacuum degassing, due to the full mixing of steel slag under VD high vacuum conditions, its inclusions are multi-phase composite inclusions. This type of inclusion has a low melting point, large size, and is liquid at high temperature. It is not easy to remove and is easy to gather and grow in the center of the ingot. In particular, large-sized inclusions of Class C and Class D often exceed the standard. These large-sized inclusions of Class C and Class D are likely to eventually cause defects such as inclusions, looseness, and segregation in the rails, which significantly affect the service performance of the rails.

[0003] Type A inclusions, also known as sulfide inclusions, are prone to sticky deformation during processing; Type B inclusions, also known as alumina inclusions, are granular inclusions that are grouped in the processing direction and are not continuously arranged; Type C inclusions, also known as silicates, are single black or dark gray inclusions with high ductility and a wide range of morphology ratios (generally ≥3), and generally have sharp ends. Type D inclusions, also known as spherical oxides, are usually not easy to deform, are angular or round, and have a relatively small shape. The control of non-metallic inclusions has always been the focus and difficulty in the production of heavy rail steel. The technical production conditions of TB / T3276-2011 "Steel Rails for High-Speed ​​Railways" require that the inclusions in heavy rail steel be Class A ≤ Grade 2.0, Class B ≤ Grade 1.0, Class C ≤ Grade 1.0, and Class D ≤ Grade 1.0. In actual production, heavy rail steel manufacturers have stricter control standards for inclusion control. Usually, the sum of the four types of inclusions A, B, C, and D is less than Grade 2.0.

[0004] As my country's railway transportation continues to move towards high speed and heavy load, the demand for high-speed rails is increasing, requiring rails to have better resistance to wear, impact, fatigue, fracture and other properties. The cleanliness of heavy rail steel has a great impact on its fatigue and other properties. Several domestic steel companies such as Baosteel, Panzhihua Steel, and Anshan Iron and Steel produce heavy rail steel on a large scale, all using aluminum-free deoxidation technology. The production process is based on the different equipment of each factory. The general process is: BOF converter → LF refining → VD / RH vacuum degassing → continuous casting.

[0005] Magnesium (Mg) is a metal element belonging to Group ⅡA in the periodic table. When metallic magnesium is added to molten steel, it will quickly vaporize into magnesium vapor, form bubbles and float up, indirectly driving the floating of inclusions in the steel. And magnesium also plays a very obvious modifying role on the inclusions in steel. Dispersed nano-scale MgAl2O4 and MgO inclusions (most of which are below 2μm) will be formed in the molten steel. Since the precipitates have a simple sodium chloride crystal structure, these inclusions can serve as nucleation points for other precipitate phases (such as sulfides, alumina, carbides, nitrides, etc.) to form composite inclusions. Since the magnesium deoxidation products are minutely dispersed, numerous magnesium deoxidation products can provide sufficient heterogeneous nucleation points, thereby effectively dispersing the oxide inclusions, sulfide inclusions, and carbonitride inclusions in the steel, thereby reducing the size of the inclusions and precipitate phases in the steel, making them dispersed and precipitated, improving the morphology and distribution of the inclusions and precipitates, and eliminating large particles, especially large-sized inclusions above 10μm.

[0006] Comparative Document 1: A low alloy steel and its smelting method_202110764914.5. This patent uses magnesium to modify the Class A inclusions in low alloy steel, so that conventional sulfides are transformed into fine sulfides with cores, which then become clean nucleation particles to refine the steel and improve the quality of steel. This patent is mainly aimed at modifying low melting point, large-sized Class B and C inclusions in heavy rail steel (with high alloy content), and uses magnesium to affect the components of Class B and C inclusions, making them easier to float up and remove and to be fine and dispersed. While solving the problem of large-sized inclusions, the steel structure is refined by using the modified inclusions as nucleation particles, thereby improving the quality of steel from two aspects.

[0007] Comparative Document 2: A smelting process for improving the size and distribution morphology of Class A inclusions in heavy rail steel_202111395455.4, the Class A inclusions in heavy rails were optimized by process adjustment and the addition of low-aluminum slag washing methods. The size and number of Class A inclusions were controlled, but the Class A inclusions were not denatured and did not have the ability to become nucleation points and improve the steel structure. This method also did not propose an optimization contribution to Class B and Class C inclusions in steel.

[0008] Comparative Document 3: A method for producing high-cleanliness high-manganese steel_202210946337.6, which is suitable for high-manganese steel with high requirements for the size of inclusions such as sulfides and silicates. By designing a reasonable alloying process route and refining process system, the number, type, size and distribution of inclusions in the steel can be effectively controlled. At the same time, a large number of fine and dispersed nucleation particles are provided for the precipitation of MnS inclusions during the solidification process of continuous casting molten steel, thereby achieving the purpose of effectively controlling the size, morphology and distribution of Class A and Class C inclusions in the steel in the product. The composite alloy mentioned in this patent is a three-layer structure, the alloy preparation is complex, and the use process requires a new vacuum silo, which has great limitations in use; at the same time, the control of silicate (Class C) inclusions in this patent is mainly through reducing the oxygen content in molten steel, rather than the method of modifying and denaturing Class B and C inclusions proposed in this patent. Therefore, the process proposed in this patent is essentially different from the process in Document 3.

[0009] Comparative Document 4: A method for preparing ingots to control inclusions in high-titanium steel_202110676659.9. This patent improves the distribution and morphology of inclusions by forming fine inclusions of magnesium-aluminum spinel or magnesium oxide, and then forming titanium nitride nucleation particles. Its essence requires the formation of fine magnesium-aluminum spinel or magnesium oxide with a high melting point and first nucleation, and then promotes the dispersed distribution and nucleation of titanium nitride during the solidification process. The risk of flocculation of magnesium-aluminum spinel or magnesium oxide during the casting process is not considered. The process proposed in this patent requires the denaturation and floating of large-sized B and C type inclusions in the steel before casting. During the casting and solidification process, the liquid B and C type inclusions of the steel have already wrapped the magnesium-aluminum spinel core in liquid form, thus avoiding the flocculation problem and improving the morphology and distribution of inclusions. Summary of the invention

[0010] In order to solve the above technical problems, the purpose of the present invention is to provide a smelting process for regulating the C and D inclusions of heavy rail steel by magnesium treatment. By adding an appropriate amount of magnesium-containing cored wire during the smelting process, the type and distribution of inclusions in heavy rail steel can be accurately regulated, so that the inclusions in heavy rail steel are mostly in the form of fine and dispersed calcium-silicon-magnesium-aluminum-oxygen composite inclusions. This can not only eliminate large-sized inclusions larger than 10 microns, but also induce the formation of intracrystalline acicular ferrite during the solidification process, thereby achieving the effect of refining the grains and improving the performance, thereby realizing the improvement of the performance and quality of heavy rail steel.

[0011] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0012] The present invention discloses a smelting process for regulating Class C and Class D inclusions in heavy rail steel by magnesium treatment, the smelting process comprising: converter smelting → LF refining → VD vacuum treatment → magnesium treatment → continuous casting; the specific production steps are as follows:

[0013] (1) Converter smelting: The converter adopts the double slag method "high-drawing and supplementary blowing" process, uses silicon calcium barium for deoxidation, and the deoxidation strength is 2.8-3.2kg / ton steel. The control end point C% is ≥0.08%, P% is ≤0.020%, and the tapping temperature is 1620-1650℃; after using silicon calcium barium for deoxidation during the tapping process, alloying treatment is carried out, silicon-manganese alloy, ferrosilicon and carburizer are added, and finally lime and refined slag are added;

[0014] (2) LF refining: The LF furnace is nitrogen-blown throughout the process, with a flow rate of 50.0-75.0 L / min. During the LF refining process, low-carbon ferrosilicon is used for slag surface deoxidation. The LF refining time is controlled to be greater than 50 min, and the slag basicity is 4.0-7.0. After the refined slag turns white, the white slag is kept for 30-50 min. Temperature measurement and sampling are performed. Carbon powder, ferrosilicon, ferromanganese, ferrochrome, and ferroniobium are added to the LF furnace according to the sampling to fine-tune the composition of the molten steel so that the molten steel meets the composition requirements and the tapping temperature meets 1620-1650°C.

[0015] (3) VD vacuum refining: During vacuum refining, the slag layer thickness is ensured to be 10-20 mm when the molten steel enters the VD furnace, and the VD furnace is kept at a high vacuum for ≥15 min. After breaking the vacuum, nitrogen-manganese wire is used to increase nitrogen to ensure that the nitrogen element meets the standard. Temperature measurement and sampling are performed, and magnesium treatment process is carried out after the composition is qualified;

[0016] (4) Feeding magnesium cored wire; To ensure the effect of magnesium modification, the following method is used to ensure the modification effect;

[0017] ① The composition of the refined slag must be strictly controlled during the LF smelting process. The basicity of the refined slag must be ensured to be between 3.5 and 4.5, and the sum of the contents of iron oxide and manganese oxide in the slag must not be higher than 1.5%, so as to achieve the effect of high-basicity reducing slag and ensure the adsorption of inclusions by the steel slag;

[0018] ②After VD breaks through the air, the active oxygen in the molten steel should be measured to ensure that [O] ≤ 6ppm to ensure the modification effect of magnesium. If [O] is higher than 6ppm, silicon-calcium-barium alloy should be added for further deoxidation;

[0019] ③ Due to the violent reaction of magnesium cored wire, an intermittent wire feeding method is adopted. When the feeding amount of cored wire is less than 200m, the magnesium cored wire is fed in two times, each time feeding about half of the amount, and pausing for 30 seconds in between to reduce the intensity of feeding the magnesium cored wire; if the feeding amount of cored wire is greater than 250m, the magnesium cored wire is fed in three times, and the three feeding amounts account for 1 / 3 of the total feeding length respectively, to reduce the intensity of the reaction;

[0020] ④ Add magnesium-containing cored wire at a magnesium yield of 15%-30%, and ensure that the magnesium content in the steel meets 0.0005-0.0050%;

[0021] ⑤ After feeding the cored wire, soft blowing is performed for 10 minutes without breaking the liquid surface to avoid oxygen increase, so that the composition of the steel is uniform and the inclusions float fully; carbonized rice husks are added to keep the molten steel warm and prevent oxidation of the molten steel;

[0022] Continuous casting: oxygen is fixed and temperature is measured. After the composition is qualified, the superheat is ensured to be 25-35℃, and the ladle is used for casting. The casting speed is 0.6-1.0m / min. The casting is protected by argon blowing throughout the process from the ladle to the tundish long water nozzle;

[0023] The heavy rail steel comprises the following components by mass percentage: C: 0.70-0.75%, Si: 0.30-0.40%, Mn: 0.90-0.99%, P≤0.025%, S≤0.025%, Cr≤0.15%, Ni≤0.1%, Cu≤0.15%, Mo≤0.02%, Ti≤0.025%, V≤0.03%, Nb≤0.01%, Al≤0.004%, Mg: 0.0005-0.0050%, and the remainder of Fe and unavoidable impurities.

[0024] Furthermore, silicon calcium barium is used for deoxidation, and the deoxidation strength is 3kg / ton of steel.

[0025] Furthermore, the first furnace has an overheating degree of 35-40°C.

[0026] Furthermore, the heavy rail steel composition is as follows by mass percentage: C: 0.70-0.74%, Si: 0.30-0.39%, Mn: 0.90-0.98%, P≤0.022%, S≤0.024%, Cr≤0.14%, Ni≤0.09%, Cu≤0.14%, Mo≤0.018%, Ti≤0.022%, V≤0.028%, Nb≤0.009%, Al≤0.0038%, Mg: 0.00013%, and the remainder of Fe and unavoidable impurities.

[0027] Furthermore, the heavy rail steel composition is as follows by mass percentage: C: 0.71-0.73%, Si: 0.32-0.36%, Mn: 0.92-0.96%, P≤0.020%, S≤0.021%, Cr≤0.12%, Ni≤0.08%, Cu≤0.13%, Mo≤0.017%, Ti≤0.020%, V≤0.025%, Nb≤0.008%, Al≤0.0035%, Mg: 0.00020%, and the remainder of Fe and unavoidable impurities.

[0028] Furthermore, inclusions can be rated according to national standards: Class A <1.0, Class B <1.0, Class C ≤1.0, Class D <1.0, and large-sized Class C and D inclusions larger than 10μm produced during the smelting process of heavy rail steel are basically eliminated, and the mechanical properties of the steel are improved.

[0029] Compared with the prior art, the beneficial technical effects of the present invention are:

[0030] The present invention provides a smelting method for eliminating large-sized inclusions of Class C and Class D in heavy rail steel by magnesium treatment. By adding an appropriate amount of magnesium cored wire during the smelting process, the type and distribution of inclusions in the heavy rail steel can be accurately regulated. The type of inclusions can be accurately regulated by magnesium treatment, so that the inclusions in the heavy rail steel are mostly in the form of fine dispersed calcium-silicon-magnesium-aluminum-oxygen composite inclusions, as well as single MgO-Al2O3 inclusions, etc., the cleanliness of the molten steel is improved, and the inclusions can meet the national standard rating: Class A <1.0, Class B <1.0, Class C ≤1.0, Class D <1.0, and the large-sized inclusions of Class C and D above 10μm generated during the smelting process of heavy rail steel are basically eliminated, and various mechanical properties of the steel are improved. The present invention has the advantages of simple process, strong operability, and significant effect, improves the internal quality of heavy rail steel and the safety of railway transportation, and has significant effect. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] The present invention will be further described below in conjunction with the accompanying drawings.

[0032] Figure 1 The length statistics of large-sized inclusions in heavy rail steel treated with and without magnesium treatment are shown in Figure 2.

[0033] Figure 2 This is a scanned photo of a typical composite inclusion after electrolytic etching in Example 1 of the present invention;

[0034] Figure 3 This is a scanned photo of a typical composite inclusion after electrolytic etching in Example 2 of the present invention;

[0035] Figure 4 This is a scanned photo of a typical large-sized composite inclusion in a comparative example of the present invention. DETAILED DESCRIPTION

[0036] The technical solution of the present invention is described in detail below through examples. The following examples are only used to explain and illustrate the technical solution of the present invention, and cannot be interpreted as limiting the technical solution of the present invention.

[0037] Two specific embodiments and one comparative example of the present invention are given below to clearly and completely describe the technical solution of the present invention. The smelting production processes of the three embodiments are as follows:

[0038] (1) Converter smelting: The converter adopts the double slag method "high-drawing and supplementary blowing" process, uses silicon calcium barium for deoxidation, the deoxidation intensity is 3kg / ton steel, the control end point C% ≥ 0.08%, P% ≤ 0.020%, and the steel tapping temperature is 1620-1650℃; after using silicon calcium barium for deoxidation during the steel tapping process, alloying treatment is carried out, silicon-manganese alloy, ferrosilicon and carburizer are added, and finally lime and refined slag are added;

[0039] (2) LF refining: The LF furnace is nitrogen-blown throughout the process, with a flow rate of 50.0-75.0 L / min. Low-carbon ferrosilicon is used for slag surface deoxidation during the LF refining process. The LF refining time is controlled to be greater than 50 min, and the slag basicity is 4.0-7.0. After the refined slag turns white, the white slag is kept for 30-50 min. Temperature measurement and sampling are performed. According to the sampling, carbon powder, ferrosilicon, ferromanganese, ferrochrome, ferroniobium and other raw materials are added to the LF furnace to fine-tune the composition of the molten steel so that the molten steel meets the composition requirements and the tapping temperature meets 1620-1650°C.

[0040] (3) VD vacuum refining: During vacuum refining, the slag layer thickness is ensured to be 10-20 mm when the molten steel enters the VD furnace, and the VD furnace is kept at a high vacuum for ≥15 min. After breaking the vacuum, nitrogen-manganese wire is used to increase nitrogen to ensure that the nitrogen element meets the standard. Temperature measurement and sampling are performed, and magnesium treatment process is carried out after the composition is qualified;

[0041] (4) Feeding magnesium cored wire; To ensure the effect of magnesium modification, the following method is used to ensure the modification effect;

[0042] ① The composition of the refined slag must be strictly controlled during the LF smelting process. The basicity of the refined slag must be ensured to be between 3.5 and 4.5, and the sum of the contents of iron oxide and manganese oxide in the slag must not be higher than 1.5%, so as to achieve the effect of high-basicity reducing slag and ensure the adsorption of inclusions by the steel slag;

[0043] ②After VD breaks through the air, the active oxygen in the molten steel should be measured to ensure that [O] ≤ 6ppm to ensure the modification effect of magnesium. If [O] is higher than 6ppm, silicon-calcium-barium alloy should be added for further deoxidation;

[0044] ③Since the magnesium cored wire reacts violently, an intermittent wire feeding method is adopted. When the amount of cored wire fed is less than 200m, the magnesium cored wire is fed in two times, each time feeding about half of the amount, with a 30-second pause in between to reduce the intensity of feeding the magnesium cored wire; if the amount of cored wire fed is greater than 250m, the magnesium cored wire can be fed in three times, and the three feeding amounts account for 1 / 3 of the total feeding length respectively, to reduce the intensity of the reaction;

[0045] ④ Add magnesium-containing cored wire at a magnesium yield of 15%-30%, and ensure that the magnesium content in the steel meets 0.0005-0.0050%;

[0046] ⑤ After feeding the cored wire, perform soft blowing for 10 minutes without breaking the liquid surface to avoid oxygen increase, so that the composition of the steel is uniform and the inclusions float fully; add carbonized rice husks to keep the molten steel warm and prevent oxidation of the molten steel;

[0047] (5) Continuous casting: oxygen is determined and temperature is measured. After the composition is qualified, the superheat is ensured to be 25-35℃ (the first furnace superheat is 35-40℃), and the ladle is used for casting. The casting speed is 0.6-1.0m / min. The casting is protected by argon blowing from the ladle to the long water inlet of the tundish.

[0048] Example 1

[0049] The composition of the magnesium-treated heavy rail steel provided in this embodiment is shown in Table 1. In this production, 10 ppm of Mg was added to the steel to modify the inclusions in the steel into MgO-Al 2 O 3 , Ca-Si-Al-O is modified to Ca-Si-Mg-Al-O, and the large-sized inclusions of type C and type D are significantly reduced, and the modification effect is obvious. Typical composite inclusions are as follows Figure 2 The distribution of inclusions after modification is shown in Table 2. The rating of inclusions in modified heavy rail steel is: C 细 , C 粗 , D 细 , D 粗 All are grade 0.5, and the proportion of inclusions larger than 10μm is 0.4%. Figure 1 1 is a statistical diagram of the length of large-sized inclusions in the heavy rails of Example 1 (magnesium treatment) and Comparative Example 1 (without magnesium treatment). Figure 2 It is the composite inclusion after modification in this embodiment.

[0050] Example 2

[0051] The composition of magnesium treated heavy rail steel provided in this embodiment is shown in Table 1. In this production, 18 ppm of Mg, Al and 2 O 3 Basically modified to MgO-Al 2 O 3 , Ca-Si-Al-O is basically transformed into Ca-Si-Mg-Al-O, and MgO-Al 2 O 3 -MnS, Ca-Si-Mg-Al-O-MnS composite inclusions, and MgS inclusions are also generated. The large-sized inclusions of type C and type D are significantly eliminated. The distribution of inclusions is shown in Table 2. The rating of the inclusions in the modified heavy rail steel is: C 细 , C 粗 , D 细 , D 粗 All are grade 0.5, and the proportion of inclusions larger than 10μm is 0.2%. Figure 3It is the composite inclusion after modification in this embodiment.

[0052] Comparative Example 1

[0053] The composition of the heavy rail steel provided in this embodiment is shown in Table 1. In this production, magnesium is not added to the steel. There are a large number of large-sized inclusions of type C and D in the steel. The distribution of inclusions is shown in Table 2. The rating of inclusions in the heavy rail steel that has not been modified by magnesium is: C 细 =1.0, C 粗 =1.0, D 细 =1.5, D 粗 =0.5, the proportion of inclusions larger than 10μm is: 2.8%, Figure 4 This is a scan of the unmodified large inclusions in Comparative Example 1.

[0054] Table 1 Specific composition (wt%) of heavy rail steel in Examples 1 to 3 and Comparative Example 1

[0055]

[0056] The size changes of inclusions before and after magnesium treatment are shown in Table 2. After magnesium treatment, large-sized inclusions, especially those larger than 10 microns, are basically eliminated.

[0057] Table 2 Changes in inclusion size before and after magnesium treatment

[0058]

[0059] In summary, through the production process of the present invention: converter / electric furnace smelting → LF refining → VD / RH refining → Mg treatment → continuous casting, an appropriate amount of magnesium cored wire is added during the smelting process to accurately control the type and distribution of inclusions in the heavy rail steel, so that the large-sized C and D inclusions in the steel are transformed into the form of fine and dispersed composite inclusions, and the large-sized C and D inclusions of more than 10 μm generated in the heavy rail steel smelting process are basically eliminated, the cleanliness of the molten steel is improved, and the rating of various inclusions in the steel is guaranteed to be less than or equal to level 1.0, which is suitable for the production of high-quality heavy rail steel.

[0060] The embodiments described above are only descriptions of the preferred modes of the present invention, and are not intended to limit the scope of the present invention. Without departing from the design spirit of the present invention, various modifications and improvements made to the technical solutions of the present invention by ordinary technicians in this field should all fall within the protection scope determined by the claims of the present invention.

Claims

1. A smelting process for regulating Class C and Class D inclusions in heavy rail steel by magnesium treatment, the smelting process comprising: Converter smelting → LF refining → VD vacuum treatment → magnesium treatment → continuous casting; it is characterized by: the specific production steps are as follows: (1) Converter smelting: The converter adopts the double slag method "high-drawing and supplementary blowing" process, uses silicon calcium barium for deoxidation, and the deoxidation strength is 2.8-3.2kg / ton steel. The control end point C% is ≥0.08%, P% is ≤0.020%, and the tapping temperature is 1620-1650℃; after using silicon calcium barium for deoxidation during the tapping process, alloying treatment is carried out, silicon-manganese alloy, ferrosilicon and carburizer are added, and finally lime and refined slag are added; (2) LF refining: The LF furnace is nitrogen-blown throughout the process, with a flow rate of 50.0-75.0 L / min. During the LF refining process, low-carbon ferrosilicon is used for slag surface deoxidation. The LF refining time is controlled to be greater than 50 min, and the slag basicity is 4.0-7.

0. After the refined slag turns white, the white slag is kept for 30-50 min. Temperature measurement and sampling are performed. Carbon powder, ferrosilicon, ferromanganese, ferrochrome, and ferroniobium are added to the LF furnace according to the sampling to fine-tune the composition of the molten steel so that the molten steel meets the composition requirements and the tapping temperature meets 1620-1650°C. (3) VD vacuum refining: During vacuum refining, the slag layer thickness is ensured to be 10-20 mm when the molten steel enters the VD furnace, and the VD furnace is kept at a high vacuum for ≥15 min. After breaking the vacuum, nitrogen-manganese wire is used to increase nitrogen to ensure that the nitrogen element meets the standard. Temperature measurement and sampling are performed, and magnesium treatment process is carried out after the composition is qualified; (4) Feeding magnesium cored wire; To ensure the effect of magnesium modification, the following method is used to ensure the modification effect; ① The composition of the refined slag must be strictly controlled during the LF smelting process. The basicity of the refined slag must be ensured to be between 3.5 and 4.5, and the sum of the contents of iron oxide and manganese oxide in the slag must not be higher than 1.5%, so as to achieve the effect of high-basicity reducing slag and ensure the adsorption of inclusions by the steel slag; ②After VD breaks through the air, the active oxygen in the molten steel should be measured to ensure that [O] ≤ 6ppm to ensure the modification effect of magnesium. If [O] is higher than 6ppm, silicon-calcium-barium alloy should be added for further deoxidation; ③ Due to the violent reaction of magnesium cored wire, an intermittent wire feeding method is adopted. When the feeding amount of cored wire is less than 200m, the magnesium cored wire is fed in two times, each time feeding about half of the amount, and pausing for 30 seconds in between to reduce the intensity of feeding the magnesium cored wire; if the feeding amount of cored wire is greater than 250m, the magnesium cored wire is fed in three times, and the three feeding amounts account for 1 / 3 of the total feeding length respectively, to reduce the intensity of the reaction; ④ Add magnesium-containing cored wire at a magnesium yield of 15%-30%, and ensure that the magnesium content in the steel meets 0.0005-0.0050%; ⑤ After feeding the cored wire, perform soft blowing for 10 minutes without breaking the liquid surface to avoid oxygen increase, so that the composition of the steel is uniform and the inclusions float fully; Continuous casting: oxygen is fixed and temperature is measured. After the composition is qualified, the superheat is ensured to be 25-35℃, and the ladle is used for casting. The casting speed is 0.6-1.0m / min. The casting is protected by argon blowing throughout the process from the ladle to the tundish long water nozzle; The heavy rail steel comprises the following components by mass percentage: C: 0.70-0.75%, Si: 0.30-0.40%, Mn: 0.90-0.99%, P≤0.025%, S≤0.025%, Cr≤0.15%, Ni≤0.1%, Cu≤0.15%, Mo≤0.02%, Ti≤0.025%, V≤0.03%, Nb≤0.01%, Al≤0.004%, Mg: 0.0005-0.0050%, and the remainder of Fe and unavoidable impurities.

2. The smelting process for regulating Class C and Class D inclusions in heavy rail steel by magnesium treatment according to claim 1, characterized in that: Silicon calcium barium is used for deoxidation, and the deoxidation strength is 3kg / ton of steel.

3. The smelting process for regulating Class C and Class D inclusions in heavy rail steel by magnesium treatment according to claim 1, characterized in that: The first furnace superheat is 35-40℃.

4. The smelting process for regulating Class C and Class D inclusions in heavy rail steel by magnesium treatment according to claim 1, characterized in that: The heavy rail steel comprises the following components by mass percentage: C: 0.70-0.74%, Si: 0.30-0.39%, Mn: 0.90-0.98%, P≤0.022%, S≤0.024%, Cr≤0.14%, Ni≤0.09%, Cu≤0.14%, Mo≤0.018%, Ti≤0.022%, V≤0.028%, Nb≤0.009%, Al≤0.0038%, Mg: 0.00013%, and the remainder of Fe and unavoidable impurities.

5. The smelting process for regulating Class C and Class D inclusions in heavy rail steel by magnesium treatment according to claim 1, characterized in that: The heavy rail steel comprises the following components by mass percentage: C: 0.71-0.73%, Si: 0.32-0.36%, Mn: 0.92-0.96%, P≤0.020%, S≤0.021%, Cr≤0.12%, Ni≤0.08%, Cu≤0.13%, Mo≤0.017%, Ti≤0.020%, V≤0.025%, Nb≤0.008%, Al≤0.0035%, Mg: 0.00020%, and the remainder of Fe and unavoidable impurities.

6. The smelting process for regulating Class C and Class D inclusions in heavy rail steel by magnesium treatment according to claim 1, characterized in that: In the step ⑤, carbonized rice husks are added to keep the molten steel warm and prevent oxidation of the molten steel.

7. The smelting process for regulating Class C and Class D inclusions in heavy rail steel by magnesium treatment according to claim 1, characterized in that: The inclusions are rated according to national standards: Class A <1.0, Class B <1.0, Class C ≤1.0, Class D <1.0, and large-sized Class C and D inclusions larger than 10μm produced during the smelting process of heavy rail steel are basically eliminated.

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