A multi-stage synergistic control method for reducing the size of class a inclusions in heavy rail steel

Abstract

The application discloses a kind of multi-stage synergic control methods for reducing the size of heavy rail steel A inclusion, including converter smelting, LF refining, RH refining, continuous casting and rolling steps;The RH refining step: before breaking empty, add complex nucleating agent, and the amount is 0.8-2.0 kg / t steel;The composition of the complex nucleating agent is: Zr 10%-20%, Ti 15%-25%, Mg 5%-10%, Al 10%-20%, the balance is Fe and unavoidable impurities;The continuous casting step: the primary dendrite arm spacing in the center region of continuous casting billet is controlled at 200-400 μm, and the solidification front cooling rate is controlled at 0.5-3.0 K / s.This method reduces the average size of MnS to below 2.0 μm, and the A inclusion rating is ≤1.5 level, realizes the precise control of A inclusion size and distribution, significantly reduces the A inclusion rating, and significantly improves the transverse impact toughness and fatigue performance of heavy rail steel.

Description

Technical Field

[0001] This invention relates to the field of iron and steel metallurgy technology, and in particular to a multi-stage collaborative control method for reducing the size of Class A inclusions in heavy rail steel. Background Technology

[0002] Heavy rail steel, such as U75V and U71Mn, has extremely strict requirements for the cleanliness of molten steel. The rating of Class A inclusions (mainly MnS) is generally required to be ≤2.5, and for high-speed rails, it is even required to be ≤2.0. MnS, the main component of Class A inclusions, precipitates between dendrites at the end of solidification and extends into long strips along the rolling direction during the rolling process, which will significantly deteriorate the transverse impact toughness and fatigue performance.

[0003] In existing technologies, the main approaches to reducing the size of Class A inclusions include: 1. Deep desulfurization process: By pretreating molten iron and performing deep desulfurization (LF), the sulfur content is reduced to below 0.002%, thereby reducing the total amount of MnS precipitation. However, this method has high process costs and produces a large amount of slag. Furthermore, under high Mn / C conditions, even with very low sulfur content, numerous fine MnS particles will still form, which is still detrimental to toughness.

[0004] 2. Calcium / Magnesium Treatment Modification: By feeding in CaSi and passivated magnesium spheres, CaS and MgO·Al2O3 are generated as nucleation sites for MnS, causing MnS to precipitate in the form of complex sulfides, which to some extent refines MnS and improves its morphology. However, calcium and magnesium have low solubility in steel, unstable yield, and easily generate a large amount of dust. Furthermore, CaS and MgO·Al2O3 easily form brittle complex inclusions in steel, affecting fatigue performance.

[0005] 3. Single oxide nucleation (TiO2, ZrO2, etc.): Publication No. CN107699659A proposes a method for modifying sulfide inclusions in heavy rail steel. This method involves adding TiO2 at the end of RH refining to modify MnS. Related studies indicate that oxides such as ZrO2 can serve as heterogeneous nucleation nuclei for MnS, refining the MnS structure. However, considering only one oxide and a single addition results in a limited variety and size distribution of nucleation particles, and the nucleation is not coupled with solidification structure control, leading to less than ideal MnS size and distribution.

[0006] 4. Solidification Structure Control: Publication No. CN108213369A proposes a method for controlling the solidification structure of continuously cast billets to reduce the Class A inclusion rating in heavy rail steel. This method controls the MnS size by controlling the primary dendrite spacing and establishes a quantitative relationship between MnS size, S content, and primary dendrite spacing. However, this method only considers the cooling regime and dendrite spacing, without introducing nucleation point regulation, thus limiting the refinement of MnS.

[0007] 5. Electromagnetic stirring and light reduction technology: Literature shows that in the continuous casting of heavy rail steel large billets, the use of crystallizer electromagnetic stirring (M-EMS) and solidification end electromagnetic stirring (F-EMS), combined with light reduction, can significantly improve the central equiaxed crystal ratio and reduce central segregation. However, existing technologies mainly focus on segregation and porosity defects, and do not conduct integrated design of nucleation particles, solidification structure, and cooling regime for the size of Class A inclusions.

[0008] In summary, the existing technologies have the following shortcomings: 1) Deep desulfurization is costly and cannot completely avoid the impact of fine MnS on toughness; 2) The yield of calcium / magnesium treatment is unstable, easily forming brittle composite inclusions and causing significant environmental pollution; 3) The size and distribution of single oxide nucleation particles are poorly controllable and not coordinated with solidification structure control; 4) Solidification structure control technologies focus more on segregation and do not take MnS size control as a core objective in the system design.

[0009] Therefore, developing a full-process collaborative control method that balances cost, stable yield, environmental friendliness, and significant refinement of MnS size is of great significance for improving the control level of Class A inclusions in heavy rail steel. Summary of the Invention

[0010] The technical problem to be solved by the present invention is to provide a multi-stage collaborative control method for reducing the size of Class A inclusions in heavy rail steel.

[0011] To solve the above-mentioned technical problems, the technical solution adopted by the present invention includes the following steps: converter smelting, LF refining, RH refining, continuous casting and rolling. The RH refining step involves adding a composite nucleating agent before breaking the air void, at a rate of 0.8–2.0 kg / t of steel. The composite nucleating agent consists of: Zr 10%–20%, Ti 15%–25%, Mg 5%–10%, Al 10%–20%, with the balance being Fe and unavoidable impurities. The continuous casting process involves controlling the dendrite spacing λ1 in the central region of the continuously cast billet to be between 200 and 400 μm, and controlling the cooling rate Vc at the solidification front to be between 0.5 and 3.0 K / s.

[0012] Furthermore, in the RH refining step, the composite nucleating agent is added 5-10 minutes before the cavitation is broken, and the circulation time after addition is ≥5 minutes.

[0013] Furthermore, in the RH refining step, the vacuum degree is ≤100Pa, the holding time is ≥12min, the gas flow rate is 1200~1600NL / min, and the soft blowing argon flow rate is adjusted to 100~200NL / min before rupture, and the soft blowing time is ≥3min.

[0014] Furthermore, in the continuous casting step, the secondary cooling water ratio is controlled at 0.25–0.40 L / kg.

[0015] Furthermore, the continuous casting step employs electromagnetic stirring and light pressure.

[0016] Furthermore, the electromagnetic stirring includes electromagnetic stirring in the crystallizer and electromagnetic stirring at the solidification end; the electromagnetic stirring current in the crystallizer is 300-500A and the frequency is 4-8Hz; the electromagnetic stirring current at the solidification end is 250-400A and the frequency is 4-8Hz.

[0017] Furthermore, the light reduction refers to the light reduction at the end of solidification, with the solid fraction corresponding to the starting position being fs = 0.30 to 0.80, and the total reduction being 6 to 12 mm.

[0018] The beneficial effects of adopting the above technical solution are as follows: (1) Compared with simple calcium / magnesium treatment, the present invention adds a special composite nucleating agent at the end of RH refining to generate nano-sized ZrO2-MgO·Al2O3-TiO in situ in molten steel. x Composite oxides serve as heterogeneous nucleation cores for MnS; pre-implantation with composite nucleating agents is employed, while simultaneously introducing ZrO2, MgO·Al2O3, and TiO2. x Various oxides are used to form small, composite nucleation cores with matching interfaces, avoiding the growth of single CaS or MgO·Al2O3 into brittle inclusions, and improving the number density and stability of nucleation particles.

[0019] 2. Compared with single TiO2 or ZrO2 treatment, this invention, through the design of composite components of nucleating agents and the control of the timing of their addition, enables the composite oxides to be fully dispersed in the solid-liquid two-phase region and coupled with subsequent solidification structure control, to achieve a synergistic refinement effect of multi-scale nucleation, rapid solidification, and structure refinement.

[0020] 3. The continuous casting process of this invention controls the primary dendrite spacing to 200–400 μm by optimizing the secondary cooling process, coupling electromagnetic stirring, and dynamic light pressing. Compared with technologies that only control the primary dendrite spacing, this invention, while controlling the primary dendrite spacing, introduces composite nucleation particles, electromagnetic stirring, and light pressing to synergistically control the MnS size from the three stages of nucleation, growth, and segregation, reducing the average MnS size by 30%–50% and stabilizing the Class A inclusion rating to ≤1.5.

[0021] 4. Compared with existing continuous casting processes for heavy rail steel, this invention takes MnS size control as one of its core objectives. Through an integrated design of pre-implantation of composite nucleation particles, optimization of the secondary cooling model, dynamic light pressing, and multi-stage electromagnetic stirring, it achieves coordinated control of the entire process from nucleation to growth to segregation, reducing the average MnS size to below 2.0 μm and stabilizing the Class A inclusion rating to ≤1.5. This enables precise control of the size and distribution of Class A inclusions, significantly reducing the Class A inclusion rating and significantly improving the transverse impact toughness and fatigue performance of heavy rail steel. Moreover, the process is stable and has significant industrial application value. Detailed Implementation

[0022] The present invention will now be described in further detail with reference to specific embodiments.

[0023] This multi-stage collaborative control method for reducing the size of Class A inclusions in heavy rail steel includes the following composition control (mass fraction): C 0.65%–0.80%, Si 0.20%–0.60%, Mn 0.80%–1.20%, S 0.005%–0.020%, Als 0.020%–0.050%, O ≤0.0020%, N ≤0.0080%; composite nucleating element: Zr 0.005%–0.020%, Ti 0.010%–0.030%, Mg 0.0008%–0.0030%; the balance being Fe and unavoidable impurities.

[0024] This multi-stage collaborative control method for reducing the size of Class A inclusions in heavy rail steel adopts a process route of hot metal pretreatment → converter smelting → LF refining → RH refining → continuous casting → rolling. The process steps are described below: S1. Hot metal pretreatment: Hot metal is pretreated to remove sulfur, and the sulfur content of the hot metal entering the converter is controlled to be ≤0.020wt%.

[0025] S2. Converter smelting: molten iron is smelted.

[0026] S3, LF Refining: Molten iron undergoes LF refining.

[0027] S4, RH refining: vacuum degree ≤100Pa, holding time ≥12min; argon flow rate during refining process 1200~1600NL / min; before breaking the vacuum, adjust the soft blowing argon flow rate to 100~200NL / min, soft blowing time ≥3min.

[0028] The composite nucleating agent is added at the end of the RH refining process, preferably 5-10 minutes before the RH refining process begins. The addition amount is 0.8-2.0 kg / t of steel, and the circulation time after addition is ≥5 minutes to ensure sufficient dispersion of the composite oxide. The composite nucleating agent composition is: Zr 10%-20%, Ti 15%-25%, Mg 5%-10%, Al 10%-20%, with the balance being Fe and unavoidable impurities. In this way, by adding the composite nucleating agent to the molten steel, nano-sized ZrO2-MgO·Al2O3-TiO2 is pre-formed in the molten steel. x Composite oxide particles serve as heterogeneous nucleation sites for MnS. Simultaneously, by controlling the composition and amount of the composite nucleating agent, the contents of O, Al, Mg, Zr, and Ti in the steel are controlled, ensuring the composite oxide size is maintained between 0.1 and 1.5 μm and the number density is ≥1×10⁻⁶. 6 / mm 3 .

[0029] S5. Continuous casting: Large square billets with cross-sections of 280mm×380mm or 320mm×410mm; casting speed 0.65~0.75m / min, superheat 15~35℃, secondary cooling water ratio 0.25~0.40L / kg; using electromagnetic stirring in the crystallizer and electromagnetic stirring at the solidification end; the electromagnetic stirring in the crystallizer (M-EMS) has a current of 300~500A and a frequency of 4~8Hz; the electromagnetic stirring at the solidification end (F-EMS) has a current of 250~400A and a frequency of 4~8Hz; light reduction is used at the solidification end, with a total reduction of 6~12mm, and the solid fraction fs corresponding to the reduction position is 0.30~0.80. By optimizing the secondary cooling model of continuous casting and applying dynamic light pressure, the primary dendrite spacing λ1 in the central region of the continuously cast billet is controlled within 200–400 μm, while the cooling rate Vc at the solidification front is controlled within 0.5–3.0 K / s. This shortens the growth time of MnS in the liquid phase and reduces the degree of microsegregation of Mn and S. M-EMS and F-EMS are respectively placed in the crystallizer and at the solidification end. Combined with dynamic light pressure, this increases the central equiaxed crystal ratio, interrupts the continuous network precipitation of MnS along the grain boundaries, and results in a dispersed spherical or short rod-shaped distribution of MnS.

[0030] S6: Rolling: Heating temperature 1180~1260℃, holding time ≥2h; rolling adopts conventional heavy rail steel rolling process, final rolling temperature ≥850℃.

[0031] Example 1: Taking U75V heavy rail steel, with S=0.012%, as an example for illustration.

[0032] (1) Process route: hot metal composite injection desulfurization → converter smelting → LF refining → RH refining → 320mm×410mm large square billet continuous casting → heating → rolling.

[0033] RH refining: Vacuum degree ≤80Pa, maintained for 15 min; increase gas flow rate to 1400 NL / min; soft argon blowing at 150 NL / min for 5 min before RH rupture. Add 1.2 kg / t steel of composite nucleating agent 6 min before RH rupture, circulate for 7 min, and then rupture. The composite nucleating agent composition is: Zr 15%, Ti 20%, Mg 7%, Al 12%, with the balance being Fe and unavoidable impurities.

[0034] Continuous casting: casting speed 0.70 m / min; superheat 28℃; secondary cooling water ratio 0.32 L / kg; M-EMS: current 380 A, frequency 5 Hz; F-EMS: current 320 A, frequency 6 Hz; light reduction: total reduction 9 mm, starting reduction position corresponds to fs≈0.40.

[0035] (2) The composition of the obtained U75V heavy rail steel is: C 0.72%, Si 0.28%, Mn 0.95%, S 0.012%, Als 0.035%, O 0.0015%, N 0.0060%, Zr 0.010%, Ti 0.018%, Mg 0.0015%, with the balance being Fe and unavoidable impurities.

[0036] (3) Results: The primary dendrite spacing at the center of the continuously cast billet: λ1≈320μm; the average size of MnS: about 1.8μm (about 3.5μm in the traditional process); the inclusion rating of Class A: 0.5 to 1.5, with an average of 1.2; the transverse impact energy of the rolled material was increased by about 18% compared with the traditional process.

[0037] Example 2: Taking U71Mn heavy rail steel with S=0.008% as an example for illustration.

[0038] (1) Process route: hot metal composite injection desulfurization → converter smelting → LF refining → RH refining → 320mm×410mm large square billet continuous casting → heating → rolling.

[0039] RH refining: Vacuum degree ≤90Pa, maintain for 18min; increase gas flow rate to 1500NL / min; soft argon blowing at 120NL / min for 4min. Add 1.5kg / t of composite nucleating agent 8min before RH vacuum breaking, and circulate for 6min. The composite nucleating agent composition is: Zr 15%, Ti 20%, Mg 7%, Al 12%, with the balance being Fe and unavoidable impurities.

[0040] Continuous casting: casting speed 0.68m / min; superheat 25℃; secondary cooling water ratio 0.35L / kg; M-EMS: current 400A, frequency 6Hz; F-EMS: current 350A, frequency 5Hz; light reduction: total reduction 10mm, fs initial ≈ 0.35.

[0041] (2) The composition of the obtained U71Mn heavy rail steel is: C 0.68%, Si 0.25%, Mn 1.05%, S 0.008%, Als 0.032%, O 0.0018%, N 0.0055%, Zr 0.008%, Ti 0.020%, Mg 0.0012%, with the balance being Fe and unavoidable impurities.

[0042] (3) Results: λ1≈300μm; average size of MnS is about 1.6μm; Class A inclusion rating ≤1.5; proportion of center segregation ≤0.5 is ≥90%.

[0043] Example 3: Taking U75V heavy rail steel, with S=0.006%, as an example for illustration.

[0044] (1) Process route: hot metal composite injection desulfurization → converter smelting → LF refining → RH refining → 320mm×410mm large square billet continuous casting → heating → rolling.

[0045] RH refining: Vacuum degree ≤70Pa, maintain for 20min; increase gas flow rate to 1300NL / min; soft argon blowing at 180NL / min for 6min. Add 1.0kg / t steel of composite nucleating agent 5min before RH vacuum breaking, and circulate for 8min. The composite nucleating agent composition is: Zr 15%, Ti 20%, Mg 7%, Al 12%, with the balance being Fe and unavoidable impurities.

[0046] Continuous casting: casting speed 0.72m / min; superheat 30℃; secondary cooling water ratio 0.38L / kg; M-EMS: current 420A, frequency 7Hz; F-EMS: current 360A, frequency 6Hz; light reduction: total reduction 11mm, fs initial ≈ 0.38.

[0047] (2) The composition of the obtained U75V heavy rail steel: C 0.75%, Si 0.30%, Mn 0.98%, S 0.006%, Als 0.040%, O 0.0012%, N 0.0050%, Zr 0.015%, Ti 0.025%, Mg 0.0020%, with the balance being Fe and unavoidable impurities.

[0048] (3) Results: λ1≈280μm; average size of MnS is about 1.4μm; 100% of inclusions are rated as Class A inclusions ≤1.5.

[0049] Example 4: Taking U71Mn heavy rail steel with S=0.015% as an example for illustration.

[0050] (1) Process route: hot metal composite injection desulfurization → converter smelting → LF refining → RH refining → 320mm×410mm large square billet continuous casting → heating → rolling.

[0051] RH refining: Vacuum degree ≤100Pa, maintained for 12min; increase gas flow rate to 1450NL / min; soft argon blowing at 160NL / min for 5min. Add 1.8kg / t steel of composite nucleating agent 7min before RH vacuum breaking, and circulate for 5min. The composite nucleating agent composition is: Zr 15%, Ti 20%, Mg 7%, Al 12%, with the balance being Fe and unavoidable impurities.

[0052] Continuous casting: casting speed 0.70 m / min; superheat 32℃; secondary cooling water ratio 0.30 L / kg; M-EMS: current 350 A, frequency 5 Hz; F-EMS: current 300 A, frequency 5 Hz; light reduction: total reduction 9 mm, fs initial ≈ 0.42.

[0053] (2) The composition of the obtained U71Mn heavy rail steel is: C 0.70%, Si 0.22%, Mn 1.10%, S 0.015%, Als 0.028%, O 0.0016%, N 0.0065%, Zr 0.006%, Ti 0.015%, Mg 0.0010%, with the balance being Fe and unavoidable impurities.

[0054] (3) Results: λ1≈340μm; average size of MnS is about 2.0μm; 100% of inclusions are rated as Class A inclusions ≤1.5.

[0055] Example 5: Using U75V heavy rail steel, where S=0.010%, and high-speed operation as an example, this example illustrates the process.

[0056] (1) Process route: hot metal composite injection desulfurization → converter smelting → LF refining → RH refining → 320mm×410mm large square billet continuous casting → heating → rolling.

[0057] RH refining: Vacuum degree ≤85Pa, maintained for 16min; increase gas flow rate to 1350NL / min; soft argon blowing at 140NL / min for 4min. Add 1.3kg / t steel of composite nucleating agent 6min before RH vacuum breaking, and circulate for 6min. The composite nucleating agent composition is: Zr 15%, Ti 20%, Mg 7%, Al 12%, with the balance being Fe and unavoidable impurities.

[0058] Continuous casting: casting speed 0.74 m / min; superheat 26℃; secondary cooling water ratio 0.34 L / kg; M-EMS: current 390 A, frequency 6 Hz; F-EMS: current 330 A, frequency 5 Hz; light reduction: total reduction 10 mm, fs initial ≈ 0.36.

[0059] (2) The composition of the obtained U75V heavy rail steel: C 0.74%, Si 0.32%, Mn 1.00%, S 0.010%, Als 0.038%, O 0.0014%, N 0.0058%, Zr 0.012%, Ti 0.022%, Mg 0.0018%, with the balance being Fe and unavoidable impurities.

[0060] (3) Results: λ1≈310μm; average size of MnS is about 1.7μm; Class A inclusion rating ≤1.5.

[0061] Example 6: Taking U71Mn heavy rail steel with low superheat as an example for illustration.

[0062] (1) Process route: hot metal composite injection desulfurization → converter smelting → LF refining → RH refining → 320mm×410mm large square billet continuous casting → heating → rolling.

[0063] RH refining: Vacuum degree ≤90Pa, maintained for 17min; increase gas flow rate to 1400NL / min; soft argon blowing at 150NL / min for 5min. Add 1.4kg / t steel of composite nucleating agent 8min before RH vacuum breaking, and circulate for 7min. The composite nucleating agent composition is: Zr 15%, Ti 20%, Mg 7%, Al 12%, with the balance being Fe and unavoidable impurities.

[0064] Continuous casting: casting speed 0.68m / min; superheat 18℃; secondary cooling water ratio 0.36L / kg; M-EMS: current 410A, frequency 6Hz; F-EMS: current 340A, frequency 5Hz; light reduction: total reduction 9.5mm, fs initial ≈ 0.40.

[0065] (2) The composition of the obtained U71Mn heavy rail steel is: C 0.69%, Si 0.24%, Mn 1.08%, S 0.009%, Als 0.030%, O 0.0017%, N 0.0052%, Zr 0.009%, Ti 0.019%, Mg 0.0013%, with the balance being Fe and unavoidable impurities.

[0066] (3) Results: λ1≈290μm; MnS average size is about 1.5μm; Class A inclusion rating ≤1.5.

[0067] Example 7: U75V heavy rail steel is used as an example for illustration.

[0068] (1) Process route: hot metal composite injection desulfurization → converter smelting → LF refining → RH refining → 320mm×410mm large square billet continuous casting → heating → rolling.

[0069] RH refining: Vacuum degree ≤80Pa, maintained for 14min; increase gas flow rate to 1450NL / min; soft argon blowing at 170NL / min for 5min. Add 1.6kg / t steel of composite nucleating agent 7min before RH vacuum breaking, and circulate for 6min. The composite nucleating agent composition is: Zr 12%, Ti 18%, Mg 8%, Al 15%, with the balance being Fe and unavoidable impurities.

[0070] Continuous casting: casting speed 0.70 m / min; superheat 29℃; secondary cooling water ratio 0.33 L / kg; M-EMS: current 380 A, frequency 5 Hz; F-EMS: current 310 A, frequency 6 Hz; light reduction: total reduction 9 mm, fs initial ≈ 0.38.

[0071] (2) The composition of the obtained U75V heavy rail steel: C 0.73%, Si 0.26%, Mn 0.96%, S 0.011%, Als 0.034%, O 0.0015%, N 0.0062%, Zr 0.013%, Ti 0.021%, Mg 0.0016%, with the balance being Fe and unavoidable impurities.

[0072] (3) Results: λ1≈330μm; average size of MnS is about 1.7μm; Class A inclusion rating ≤1.5.

[0073] Example 8: Taking U71Mn heavy rail steel with high Mg content as an example for illustration.

[0074] (1) Process route: hot metal composite injection desulfurization → converter smelting → LF refining → RH refining → 320mm×410mm large square billet continuous casting → heating → rolling.

[0075] RH refining: Vacuum degree ≤75Pa, maintained for 19 min; increase gas flow rate to 1500 NL / min; soft argon blowing at 160 NL / min for 6 min. Add 1.7 kg / t steel of composite nucleating agent 9 min before RH vacuum breaking, and circulate for 8 min. The composite nucleating agent composition is: Zr 15%, Ti 20%, Mg 7%, Al 12%, with the balance being Fe and unavoidable impurities.

[0076] Continuous casting: casting speed 0.66m / min; superheat 22℃; secondary cooling water ratio 0.37L / kg; M-EMS: current 400A, frequency 6Hz; F-EMS: current 350A, frequency 5Hz; light reduction: total reduction 11mm, fs initial ≈ 0.35.

[0077] (2) The composition of the obtained U71Mn heavy rail steel is: C 0.68%, Si 0.23%, Mn 1.06%, S 0.007%, Als 0.031%, O 0.0019%, N 0.0048%, Zr 0.011%, Ti 0.020%, Mg 0.0025%, with the balance being Fe and unavoidable impurities.

[0078] (3) Results: λ1≈270μm; average size of MnS is about 1.3μm; Class A inclusion rating ≤1.5.

[0079] Example 9: Taking U75V heavy rail steel with a medium reduction as an example.

[0080] (1) Process route: hot metal composite injection desulfurization → converter smelting → LF refining → RH refining → 320mm×410mm large square billet continuous casting → heating → rolling.

[0081] RH refining: Vacuum degree ≤85Pa, maintain for 15min; increase gas flow rate to 1400NL / min; soft argon blowing at 150NL / min for 5min. Add 1.5kg / t steel of composite nucleating agent 6.5min before RH vacuum breaking, and circulate for 6min. The composite nucleating agent composition is: Zr 15%, Ti 20%, Mg 7%, Al 12%, with the balance being Fe and unavoidable impurities.

[0082] Continuous casting: casting speed 0.71m / min; superheat 27℃; secondary cooling water ratio 0.32L / kg; M-EMS: current 370A, frequency 5Hz; F-EMS: current 300A, frequency 5Hz; light reduction: total reduction 8mm, fs initial ≈ 0.42.

[0083] (2) The composition of the obtained U75V heavy rail steel: C 0.74%, Si 0.29%, Mn 0.99%, S 0.009%, Als 0.036%, O 0.0013%, N 0.0056%, Zr 0.010%, Ti 0.019%, Mg 0.0017%, with the balance being Fe and unavoidable impurities.

[0084] (3) Results: λ1≈320μm; MnS average size is about 1.6μm; Class A inclusion rating ≤1.5.

[0085] As can be seen from the above nine examples, compared with traditional deep desulfurization and calcium treatment processes, the main effects of this method are as follows: Significantly reduced MnS size: The average MnS size decreased from 3.0–4.5 μm in traditional processes to 1.3–2.0 μm, a reduction of approximately 40%–55%. Improved Class A inclusion rating: The proportion of Class A inclusions ≤1.5 increased from approximately 70% to over 95%, with some batches reaching 100%. Improved transverse impact toughness: Under the same composition conditions, the transverse impact energy at -20℃ increased by an average of 15%–25%, with significant improvement in anisotropy. Improved process stability: The yield of the composite nucleating agent is stable, avoiding problems such as large amounts of smoke and dust and large fluctuations in yield during calcium treatment. Cost and environmental benefits: The S content can be controlled within the range of 0.005%–0.015%, eliminating the need for excessive deep desulfurization, reducing slag volume, lowering overall costs, and alleviating environmental impact.

Claims

1. A multi-stage collaborative control method for reducing the size of Class A inclusions in heavy rail steel, characterized in that: This includes converter smelting, LF refining, RH refining, continuous casting, and rolling steps; The RH refining step involves adding a composite nucleating agent before breaking the air void, at a rate of 0.8–2.0 kg / t of steel. The composite nucleating agent consists of: Zr 10%–20%, Ti 15%–25%, Mg 5%–10%, Al 10%–20%, with the balance being Fe and unavoidable impurities. The continuous casting process involves controlling the dendrite spacing λ1 in the central region of the continuously cast billet to be between 200 and 400 μm, and controlling the cooling rate Vc at the solidification front to be between 0.5 and 3.0 K / s.

2. The multi-stage collaborative control method for reducing the size of Class A inclusions in heavy rail steel according to claim 1, characterized in that: In the RH refining step, the composite nucleating agent is added 5-10 minutes before the cavitation is broken, and the circulation time after addition is ≥5 minutes.

3. The multi-stage collaborative control method for reducing the size of Class A inclusions in heavy rail steel according to claim 1, characterized in that: The RH refining step has a vacuum degree ≤100Pa and a holding time ≥12min; a gas flow rate of 1200~1600NL / min; before breaking the vacuum, the soft blowing argon flow rate is adjusted to 100~200NL / min and the soft blowing time is ≥3min.

4. The multi-stage collaborative control method for reducing the size of Class A inclusions in heavy rail steel according to claim 1, characterized in that: In the continuous casting step, the secondary cooling water ratio is controlled at 0.25–0.40 L / kg.

5. A multi-stage collaborative control method for reducing the size of Class A inclusions in heavy rail steel according to any one of claims 1-4, characterized in that: The continuous casting step employs electromagnetic stirring and gentle pressure.

6. The multi-stage collaborative control method for reducing the size of Class A inclusions in heavy rail steel according to claim 5, characterized in that: The electromagnetic stirring includes electromagnetic stirring in the crystallizer and electromagnetic stirring at the solidification end; the electromagnetic stirring current in the crystallizer is 300-500A and the frequency is 4-8Hz; the electromagnetic stirring current at the solidification end is 250-400A and the frequency is 4-8Hz.

7. The multi-stage collaborative control method for reducing the size of Class A inclusions in heavy rail steel according to claim 5, characterized in that: The light reduction refers to the light reduction at the end of solidification, with a solid fraction fs of 0.30 to 0.80 at the starting position and a total reduction of 6 to 12 mm.