A method for controlling oxygen in RH decarburization front high oxygen liquid steel refining heating and slag phase

Abstract

This invention provides a method for refining high-oxygen molten steel in LF (Left-Stage Refining) and dynamically controlling oxygen in the slag phase before RH (Reverse Hydrocarbon) decarburization. The method includes: a synergistic process of segmented slag formation, gradient heating matched with staged bottom blowing dynamics control, and progressive slag surface deoxidation. The molten steel is mixed with two batches of slag-forming materials to construct a high-basicity slag system; at low temperatures, only slag formation occurs without deoxidation. After 1580℃, the bottom blowing intensity is adjusted in stages: strong stirring and uniform slag formation in the early stage, followed by weak stirring at medium and high temperatures to suppress oxygen transfer from the slag; carbon powder, ferrosilicon powder, and aluminum-based auxiliary materials are sequentially sprinkled onto the slag surface for deoxidation according to the temperature gradient, without depleting dissolved oxygen in the molten steel; after 1650℃, the temperature is maintained with weak stirring and dynamic fine-tuning to stabilize the oxygen content in the molten steel at 400-800 ppm and the FeO content in the slag at 5-8%. This invention eliminates defects such as RH vacuum re-oxygenation and nozzle nodule formation, ensures decarburization driving force, reduces auxiliary material consumption, and improves the cleanliness of the molten steel and the stability of continuous casting.

Description

Technical Field

[0001] This invention relates to the field of iron and steel smelting technology, specifically to a method for heating and dynamic oxygen control of slag phase in the LF refining of high-oxygen molten steel before RH decarburization. Background Technology

[0002] In the production of special steel grades such as ultra-pure iron, ultra-low carbon steel, and high scrap ratio green steel, the process route of "converter → LF refining → RH vacuum treatment → continuous casting" is usually adopted. Among them, the LF refining process is responsible for heating the molten steel and slag removal, while the RH vacuum treatment process is responsible for core refining tasks such as decarburization, deoxidation, alloy composition adjustment, and inclusion control.

[0003] For ultrapure iron and other steel grades that require vacuum decarburization in an RH furnace, the LF refining stage must maintain the molten steel in a high-oxygen state (total oxygen content [O] = 400–800 ppm). This is a crucial prerequisite for ensuring sufficient thermodynamic driving force for the subsequent RH vacuum decarburization reaction. Simultaneously, to compensate for the temperature drop between converter tapping and RH treatment and to match the temperature requirements of the RH process, the LF process needs to rapidly raise the temperature of the molten steel from approximately 1500–1560°C to 1650–1700°C.

[0004] However, existing technologies suffer from the following irreconcilable technical contradictions:

[0005] First, during the high-power heating process of LF (Liquid Fluidized Iron) energized molten steel, the dissolved oxygen [O] in the molten steel is easily transferred to the slag and generates (FeO) due to the high-temperature radiation of the electric arc, the arc force, and the strong convection of the molten steel. As the temperature rises, the solubility of oxygen in the molten steel decreases, further aggravating the oxygen removal behavior into the slag, resulting in a sharp increase in the (FeO) content in the refining slag to about 25-30%.

[0006] Secondly, when ladle slag containing high levels of FeO (e.g., 25-30%) enters the RH process, a strong slag-steel interface re-oxidation reaction occurs under vacuum conditions: (FeO) → Fe + [O]. This reaction severely interferes with the precise control of the RH decarburization endpoint, leading to excessive carbon content and uncontrolled oxygen content at the endpoint. Simultaneously, during the RH deoxidation and alloying stage, the returned oxygen reacts with deoxidizing elements to generate a large number of clustered Al2O3 and other deoxidation products, deteriorating the cleanliness of the molten steel and causing severe nodule formation at the submerged entry nozzle during continuous casting, even leading to casting interruption.

[0007] Third, if a strong deoxidizer (such as aluminum wire or aluminum powder) is added directly to the molten steel and slag phase for deep deoxidation in the early stage of LF heating, although it can reduce the (FeO) in the slag, it will inevitably consume the [O] in the molten steel, causing the oxygen content of the molten steel to drop below the lower limit of 400ppm. At this time, the RH decarburization reaction will stop prematurely due to the lack of sufficient oxygen driving force, and the carbon content at the end of decarburization will not meet the stringent requirements of ultrapure iron (C≤0.005%). Additional oxygen blowing will also easily deteriorate the cleanliness of the molten steel.

[0008] Therefore, how to maintain a high oxygen content (400-800ppm) in molten steel to meet the RH decarburization requirements during the process of LF significant heating (ΔT≥100℃), while precisely suppressing the formation of (FeO) in the slag and controlling it within a reasonable range of 5-8%, is a core technical problem that has long existed in this field and urgently needs to be solved.

[0009] In summary, the existing technology has the following problems: how to maintain a high oxygen content (400-800 ppm) in the molten steel during the LF heating process to meet the RH decarburization requirements, while accurately suppressing the formation of (FeO) in the slag and controlling it within the range of 5-8%. Summary of the Invention

[0010] This invention provides a method for heating and dynamically controlling oxygen in slag phase during the LF refining of high-oxygen molten steel before RH decarburization. The technical problem solved is how to maintain a high oxygen content (400-800 ppm) in the molten steel to meet the requirements of RH decarburization during the LF heating process, while accurately suppressing the formation of (FeO) in the slag and controlling it within the range of 5-8%.

[0011] To achieve the above objectives, this invention proposes a method for heating and dynamic oxygen control in the slag phase during LF refining of high-oxygen molten steel before RH decarburization, comprising the following steps:

[0012] Step 1: Transfer the molten steel after tapping from the converter to the LF refining furnace at a temperature of 1500-1560℃, and at the same time add the first batch of slag-forming material to the slag surface.

[0013] Step 2: Use electrode heating to control the molten steel temperature at 1500-1580℃, control the bottom blowing argon flow rate at 100-200 NL / min, add the second batch of slag-forming material to adjust the slag basicity to 4.0-7.0, do not add deoxidizer for the time being, and slag formation time is 8-15 min;

[0014] Step 3: Increase the temperature of the molten steel. When the temperature of the molten steel is greater than 1580℃, the reaction at the interface between the steel and slag is controlled by adjusting the flow rate of bottom-blown argon in stages and stirring. Deoxidation is carried out in steps according to the FeO content in the slag.

[0015] Step 4: After the molten steel temperature reaches 1650℃, reduce the electrode heating power and adjust the bottom-blowing argon flow rate to 100-150 NL / min. Detect the oxygen content of the molten steel and assess the FeO in the slag every 5-10 minutes, dynamically fine-tuning the amount of deoxidizer added to stabilize the oxygen content of the molten steel at 400-800 ppm and the FeO in the slag at 5-8%. This scheme solves the core contradiction of traditional LF heating processes—maintaining high oxygen levels (400-800 ppm) in the molten steel and inhibiting the increase of FeO in the slag—through step-by-step slag formation, segmented heating, gradient bottom blowing, and temperature-matched stepped slag surface deoxidation. The deoxidizer reacts only on the slag surface and does not enter the molten steel to consume dissolved oxygen. The oxygen content in the molten steel is stably maintained within the range required for RH decarburization, ensuring the thermodynamic driving force for RH vacuum decarburization and avoiding excessive carbon at the decarburization endpoint.

[0016] Furthermore, in step three, the phased adjustment of the bottom-blown argon flow rate and the control of the steel-slag interface reaction specifically includes:

[0017] When the temperature of molten steel is 1580~1620℃, control the bottom blowing argon flow rate to be 300~500NL / min;

[0018] When the molten steel temperature is 1620–1650℃, the bottom-blown argon flow rate is controlled at 150–250 NL / min. By segmenting and controlling the bottom-blown argon flow rate in two stages, strong stirring in the early stage accelerates slag formation and temperature uniformity, while weak stirring in the middle and later stages suppresses oxygen transfer from the high-temperature steel slag, thus balancing slag formation efficiency and oxygen control objectives in the slag phase.

[0019] Furthermore, in step three, the stepwise deoxidation based on the FeO content in the slag specifically includes:

[0020] When the temperature of molten steel is 1580~1620℃, if the FeO content in the slag is greater than 15%, then 0.3~0.6kg / t of carbon powder should be evenly sprinkled on the slag surface.

[0021] When the temperature of molten steel is 1620-1650℃, if the FeO content in the slag is greater than 10%, 0.2-0.4 kg / t of ferrosilicon powder should be evenly sprinkled on the slag surface.

[0022] When the temperature of molten steel rises to 1650℃, if the FeO content in the slag is greater than 8%, 0.1 to 0.2 kg / t of aluminum slag balls or aluminum powder should be evenly sprinkled on the slag surface.

[0023] Gradient slag surface deoxidation is adopted using carbon powder, ferrosilicon powder, aluminum slag balls, etc. The deoxidation intensity is gradually adapted to the FeO level in the slag at each temperature range, which steadily reduces FeO without consuming dissolved oxygen in the molten steel, ensuring the high oxygen conditions of the molten steel required for RH decarburization.

[0024] Furthermore, the first batch of slag-forming materials includes: 3-5 kg / t lime and 0.5-1.5 kg / t fluorite;

[0025] The second batch of slag-forming material consists of 1-2 kg / t of lime. This limits the ratio of the two slag-forming materials, constructs a high-alkalinity initial slag system, optimizes slag fluidity and inclusion adsorption capacity, reduces auxiliary material consumption, and minimizes erosion of ladle refractory materials.

[0026] Furthermore, the composition of the final LF refining slag is controlled as follows by mass percentage: CaO 50–55%, SiO2 8–15%, Al2O3 15–25%, MgO 5–10%, FeO 5–8%, CaF2 3–8%, MnO ≤ 1.5%, and basicity R = 3.0–7.0. This forms an alkaline slag that combines high-temperature stability, reducing properties, and inclusion adsorption capacity, thereby inhibiting RH vacuum re-oxygenation and improving the cleanliness of the molten steel.

[0027] Furthermore, in step one, the first batch of slag-forming materials added consists of 4 kg / t lime and 1.0 kg / t fluorite. This results in a moderate slag-forming rate, easily controllable slag system, and reduced total consumption of slag-forming auxiliary materials.

[0028] Furthermore, in step two, the bottom-blown argon gas is controlled at 150 NL / min; the second batch of slag-forming material added is 1.5 kg / t lime; the slag basicity is adjusted to 4.0; no deoxidizer is added for the time being, and the slag formation time is 12 min. Weak stirring is used to prevent oxygen absorption by the molten steel, and the optimal equilibrium basicity is precisely adjusted to fully melt the slag material, providing a stable slag phase basis for subsequent staged oxygen control.

[0029] Furthermore, in step three,

[0030] When the temperature of the molten steel reaches 1580-1620℃, control the bottom blowing argon flow rate to 400NL / min;

[0031] When the molten steel temperature reaches 1620–1650℃, the bottom-blown argon flow rate is controlled at 200 NL / min. By limiting the fixed flow rate of bottom-blown argon in the two stages of the heating phase, standardized operation reduces human fluctuations, maximizes the effect of uniform slag formation, and significantly suppresses the rise of FeO in the slag during the high-temperature stage.

[0032] Furthermore, in step three, the stepwise deoxidation based on the FeO content in the slag specifically includes:

[0033] When the temperature of molten steel reaches 1600℃, if the FeO content in the slag is greater than 15%, 0.4 kg / t of carbon powder should be evenly sprinkled on the slag surface.

[0034] When the temperature of molten steel reaches 1630℃, if the FeO content in the slag is greater than 10%, 0.3 kg / t of ferrosilicon powder should be evenly sprinkled on the slag surface.

[0035] When the molten steel temperature reaches 1650℃, if the FeO content in the slag is greater than 8%, evenly sprinkle 0.15 kg / t of aluminum slag balls or aluminum powder onto the slag surface. This limits the optimal dosage of deoxidizer at each temperature point, matching the deoxidation intensity to the FeO level at each stage, minimizing auxiliary material consumption, and stably maintaining the oxygen content in the molten steel and the FeO content in the slag to meet standards, thus ensuring the accuracy of RH decarburization.

[0036] Furthermore, the composition of the final LF refining slag is controlled as follows by mass percentage: CaO 50%, SiO2 11%, Al2O3 20%, MgO 7%, FeO 6.2%, CaF2 5%, MnO 0.8%, with an alkalinity R=4.5. This completely eliminates the RH re-oxygenation problem, significantly reduces the risk of nodule formation at the continuous casting nozzle, and achieves optimal billet cleanliness.

[0037] The beneficial technical effects of the above technical solution are as follows:

[0038] This invention employs a synergistic process of segmented slag formation, gradient heating matched with staged bottom blowing kinetic control, and temperature-progressive slag surface deoxidation. Throughout the entire process of raising the temperature of molten steel from 1500–1560℃ to 1650–1700℃ in the LF stage, the dissolved oxygen in the molten steel is stably maintained at 400–800 ppm, ensuring sufficient thermodynamic driving force for vacuum decarburization in the RH stage. Simultaneously, the FeO in the slag is stably controlled at 5–8%, suppressing oxygen transfer from the high-temperature steel slag at the kinetic source. This completely solves the problems of vacuum oxygen re-oxygenation after high-oxidizing slag enters the RH stage, excessive carbon at the decarburization endpoint, large-scale alumina inclusion formation, and continuous casting nozzle blockage. The final slag composition is optimized to enhance inclusion adsorption capacity, enabling stable production of ultra-low carbon steel and high scrap ratio green steel, reducing the consumption of auxiliary materials and refractory materials, and improving the cleanliness and castability of molten steel. Attached Figure Description

[0039] Figure 1 This is a flowchart of a method for heating and dynamic oxygen control of slag phase in LF refining of high oxygen molten steel before RH decarburization according to an embodiment of the present invention.

[0040] Figure 2 This is a flowchart of the first implementation method of a method for heating and dynamic oxygen control of slag phase in LF refining of high oxygen molten steel before RH decarburization, according to an embodiment of the present invention. Detailed Implementation

[0041] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0042] This invention provides a method for temperature rise and dynamic oxygen control in slag phase during LF refining of high-oxygen molten steel before RH decarburization, such as... Figure 1 As shown, by optimizing heating process parameters, designing a staged bottom-blowing stirring regime, and dynamically controlling the oxygen potential of the slag phase, stable heating of molten steel can be achieved while precisely controlling the final oxygen content. This includes the following steps:

[0043] Step 1: Transfer the molten steel from the converter to the LF refining furnace at a temperature of 1500–1560℃. Simultaneously, add the first batch of slag-forming material to the slag surface to establish an initial basic slag system. After the molten steel enters the LF refining furnace, temperature, oxygen levels, and samples are measured and analyzed. Based on the oxygen content upon entry, it is determined whether pre-oxygen adjustment is necessary to maintain the oxygen content of the molten steel within the range of 400–800 ppm. The first batch of slag-forming material includes 3–5 kg / t lime and 0.5–1.5 kg / t fluorite; preferably, the first batch of slag-forming material added in Step 1 is 4 kg / t lime and 1.0 kg / t fluorite. It should be noted that the unit of measurement for the amount of slag-forming material added is kilograms per ton of molten steel. Specifically, this step is the initial preparation stage of the refining process. The temperature of the molten steel after tapping from the converter is typically between 1500 and 1560℃. The primary function of adding the first batch of slag-forming material at this temperature is not to immediately perform deep deoxidation or alloying, but rather to utilize the residual heat of the molten steel to initially melt the slag material, forming a liquid or semi-liquid slag layer covering the surface of the molten steel. This initial slag layer effectively isolates the air, reducing secondary oxidation of the molten steel in the early stages of heating, while also providing the necessary submerged arc medium for subsequent electrode heating, preventing the electric arc from directly radiating the molten steel and causing localized over-oxidation.

[0044] Step Two: Electrode heating is used to control the molten steel temperature at 1500–1580℃, and the bottom-blown argon flow rate is controlled at 100–200 NL / min. A second batch of slag-forming material is added to adjust the slag basicity to 4.0–7.0. No deoxidizer is added at this stage, and the slag formation time is 8–15 minutes. The second batch of slag-forming material consists of 1–2 kg / t lime. Preferably, in Step Two, the bottom-blown argon flow rate is controlled at 150 NL / min; the second batch of slag-forming material is 1.5 kg / t lime; the slag basicity is adjusted to 4.0; no deoxidizer is added at this stage, and the slag formation time is 12 minutes. Specifically, this step is a crucial preparatory stage for constructing the basic slag system and maintaining a high-oxygen state in the molten steel. Unlike the traditional LF refining process, this embodiment does not add a deoxidizer at this stage. This is because the subsequent RH vacuum decarburization process requires a high dissolved oxygen content in the molten steel (typically in the range of 400–800 ppm) as the driving force for the reaction. If the deoxidizer is added too early in the early stage of LF, the oxygen content in the molten steel will drop below the lower limit, causing the RH decarburization reaction to stagnate. Therefore, this step uses gentle bottom-blowing argon stirring (100–200 NL / min) combined with electrode heating to promote the melting of the second batch of slag-forming material and adjust the basicity of the slag system to the target range of 4.0–7.0, thereby establishing an alkaline slag system environment with a high capacity to absorb inclusions. The slag formation time is controlled at 8–15 minutes, which ensures both sufficient melting and homogenization of the slag material and avoids excessive temperature drop in the molten steel caused by prolonged low-temperature heating. This "slag-forming only, no deoxidation" strategy creates the necessary thermodynamic and kinetic conditions for dynamic oxygen control in the high-temperature range in subsequent steps.

[0045] Step 3: Increase the molten steel temperature. When the molten steel temperature exceeds 1580℃, control the steel-slag interface reaction by adjusting the bottom-blown argon flow rate in stages and performing stepwise deoxidation based on the FeO content in the slag. Specifically, as the molten steel temperature rises above 1580℃, the rate of dissolved oxygen in the molten steel transferring to the slag phase to generate FeO increases exponentially. At this point, if constant stirring or one-time deoxidation is used, it is very easy to cause uncontrolled FeO in the slag or over-deoxidation of the molten steel. This embodiment adopts a dynamic control strategy that combines "stage-wise adjustment of bottom-blown argon flow rate" with "stepwise deoxidation based on the FeO content in the slag". Among them, stage-wise adjustment of bottom-blown argon flow rate refers to differentiate the bottom-blowing intensity according to temperature changes, and optimize the mass transfer behavior of the slag-steel interface in different temperature ranges using fluid dynamics principles. This ensures the kinetic conditions required for slag formation and temperature homogenization while suppressing unnecessary oxygen transport in the high-temperature range. Simultaneously, deoxidation is performed in stages based on the FeO content in the slag, abandoning the model of adding a single deoxidizer all at once. Instead, based on real-time monitoring of the slag phase oxidation state, deoxidation methods suitable for the current temperature and reaction conditions are selected to gradually and progressively reduce the FeO content in the slag. This dynamic synergistic mechanism ensures that the molten steel maintains its high oxygen state during significant temperature increases while effectively preventing excessive FeO formation in the slag, achieving simultaneous oxygen preservation during temperature increases and slag phase oxygen reduction.

[0046] In step three, the phased adjustment of the bottom-blown argon flow rate and the control of the steel-slag interface reaction specifically includes:

[0047] When the molten steel temperature is 1580–1620℃, the bottom-blown argon flow rate is controlled at 300–500 NL / min; when the molten steel temperature is 1620–1650℃, the bottom-blown argon flow rate is controlled at 150–250 NL / min. Preferably, when the molten steel temperature reaches 1580–1620℃, the bottom-blown argon flow rate is controlled at 400 NL / min; when the molten steel temperature reaches 1620–1650℃, the bottom-blown argon flow rate is controlled at 200 NL / min.

[0048] The stepwise deoxidation based on the FeO content in the slag specifically includes:

[0049] When the temperature of molten steel is 1580~1620℃, if the FeO content in the slag is greater than 15%, then 0.3~0.6kg / t of carbon powder should be evenly sprinkled on the slag surface.

[0050] When the temperature of molten steel is 1620-1650℃, if the FeO content in the slag is greater than 10%, 0.2-0.4 kg / t of ferrosilicon powder should be evenly sprinkled on the slag surface.

[0051] When the temperature of molten steel rises to 1650℃, if the FeO content in the slag is greater than 8%, 0.1 to 0.2 kg / t of aluminum slag balls or aluminum powder should be evenly sprinkled on the slag surface.

[0052] Among them, the principle of phased bottom blowing dynamic control is as follows:

[0053] The intensity of bottom-blown argon stirring directly affects the mass transfer rate at the slag-steel interface. Based on mass transfer theory, this invention establishes a linkage control model between bottom-blowing flow rate and temperature:

[0054] Strong stirring stage (1580–1620℃): During this stage, the slag is not yet fully melted, requiring strong stirring to promote melting and temperature homogenization. A bottom-blowing flow rate of 300–500 NL / min can effectively renew the slag-steel interface, accelerating slag formation. Simultaneously, because the temperature is relatively low at this stage (<1620℃), the oxygen transfer rate from the molten steel to the slag is not yet intense, and strong stirring will not lead to an excessive increase in (FeO).

[0055] Weak stirring stage (1620–1650℃): When the temperature exceeds 1620℃, the oxygen transfer rate from molten steel to slag increases exponentially with temperature. At this point, reducing the bottom blowing flow rate to 150–250 NL / min significantly weakens the slag-steel interface renewal rate, kinetically cutting off the oxygen transfer channel between steel and slag under high temperature and high oxygen conditions, and inhibiting further formation of FeO.

[0056] As one implementation method, when the molten steel temperature reaches 1600℃, if the FeO content in the slag is greater than 15%, 0.4 kg / t of carbon powder is evenly sprinkled on the slag surface.

[0057] When the temperature of molten steel reaches 1630℃, if the FeO content in the slag is greater than 10%, 0.3 kg / t of ferrosilicon powder should be evenly sprinkled on the slag surface.

[0058] When the temperature of molten steel reaches 1650℃, if the FeO content in the slag is greater than 8%, 0.15 kg / t of aluminum slag balls or aluminum powder should be evenly sprinkled on the slag surface.

[0059] Step 4: After the molten steel temperature reaches 1650℃, reduce the electrode heating power and adjust the bottom-blown argon flow rate to 100-150 NL / min. Detect the oxygen content of the molten steel and assess the FeO in the slag every 5-10 minutes, dynamically fine-tuning the amount of deoxidizer added to stabilize the oxygen content of the molten steel at 400-800 ppm and the FeO in the slag at 5-8%. Specifically, when the molten steel reaches the target outlet temperature of 1650℃, the system enters a heat preservation and waiting state. At this time, the electrode heating power is reduced and the bottom-blown argon flow rate is decreased to 100-150 NL / min to reduce the disturbance of the steel-slag balance system by external energy input and mechanical stirring, preventing fluctuations in the already stable oxygen distribution relationship. Through high-frequency detection and assessment every 5-10 minutes, closed-loop feedback control is implemented for the oxygen content of the molten steel and the FeO in the slag, dynamically fine-tuning the amount of deoxidizer added to compensate for any possible minor deviations. The ultimate goal is to ensure that the oxygen content of the molten steel is precisely and stably maintained within the golden range of 400–800 ppm before entering the RH station, and that the FeO in the slag is suppressed within the safe range of 5–8%. This steady-state control verifies the effectiveness of the aforementioned dynamic regulation strategy and provides reliable and consistent raw material conditions for efficient decarburization and cleanliness control in the RH process, eliminating the risk of RH treatment anomalies caused by fluctuations in the LF outlet status.

[0060] The composition of the final LF refining slag, by mass percentage, is controlled as follows: CaO 50-55%, SiO2 8-15%, Al2O3 15-25%, MgO 5-10%, FeO 5-8%, CaF2 3-8%, MnO ≤1.5%, and basicity R = 3.0-7.0. Preferably, the composition of the final LF refining slag, by mass percentage, is controlled as follows: CaO 50%, SiO2 11%, Al2O3 20%, MgO 7%, FeO 6.2%, CaF2 5%, MnO 0.8%, and its basicity R = 4.5.

[0061] Example 1:

[0062] This invention provides a method for heating and dynamic oxygen control in the slag phase during LF refining of high-oxygen molten steel before RH decarburization. The initial molten steel temperature is 1500–1560℃, and the total oxygen content of the molten steel is 400–800 ppm. Figure 2 As shown, it includes the following steps:

[0063] Step 101, Pre-conditioning and initial slag formation: After the molten steel enters the LF refining furnace, temperature measurement, oxygen determination and sampling analysis are carried out. Based on the oxygen content upon entering the furnace, it is determined whether oxygen pre-conditioning is required to keep the oxygen content of the molten steel in the range of 400-800 ppm. At the same time, the first batch of slag-forming material is added to the slag surface to establish the initial alkaline slag system.

[0064] Step 102, Low-temperature slag formation stage: Low-power electrode heating is used, bottom blowing argon flow rate is 100-200 NL / min, and a second batch of slag-forming material is added during the slag formation process to adjust the slag system basicity to 4.0-7.0. No deoxidizer is added in this stage, and the slag formation time is 8-15 min.

[0065] Step 103, Heating and Oxygen Transfer Control Stage: When the molten steel temperature reaches 1580℃, increase the heating power and adopt a staged bottom blowing control strategy: the bottom blowing argon flow rate is 300-500 NL / min during the period of 1580-1620℃, and the bottom blowing argon flow rate is 150-250 NL / min during the period of 1620-1650℃.

[0066] Deoxidation is carried out in stages according to the FeO content in the slag: if FeO in the slag is greater than 15% at 1580℃, carbon powder of 0.3-0.6 kg / t steel is evenly sprinkled on the slag surface; if FeO in the slag is greater than 10% at 1620℃, ferrosilicon powder of 0.2-0.4 kg / t steel is evenly sprinkled on the slag surface; if FeO in the slag is greater than 8% at 1650℃, aluminum slag balls of 0.1-0.2 kg / t steel are evenly sprinkled on the slag surface.

[0067] Step 104, High-Temperature Steady-State Control Stage: After the molten steel temperature reaches 1650℃, the electrode heating power is reduced to a medium-low setting, the bottom-blowing argon flow rate is 100-150 NL / min, the oxygen content of the molten steel is detected every 5-10 minutes and the FeO in the slag is assessed, and the amount of deoxidizer added is dynamically adjusted to stabilize the oxygen content of the molten steel at 400-800 ppm and the FeO in the slag at 5-8%.

[0068] Step 105, Final Adjustment Before Leaving the Station: Before the LF refining is completed, make the final adjustment to confirm that the molten steel temperature is 1650-1700℃, the oxygen content is 400-800ppm, and the FeO in the slag is 5-8%. Then, reduce the bottom blowing flow rate to 50-80NL / min and blow softly for 5-8min. Then, hoist the steel to the RH vacuum treatment station.

[0069] The first batch of slag-forming materials includes 3-5 kg / t steel of lime and 0.5-1.5 kg / t steel of fluorite, while the second batch of slag-forming materials includes 1-2 kg / t steel of lime.

[0070] In step 103, the deoxidizer for stepwise deoxidation is only evenly sprinkled on the slag surface, and the deoxidation reaction takes place at the slag-gas interface without directly contacting the molten steel body.

[0071] The composition of the final LF refining residue by mass percentage is as follows: CaO 50-55%, SiO2 8-15%, Al2O3 15-25%, MgO 5-10%, FeO 5-8%, CaF2 3-8%, MnO≤1.5%, and basicity R=3.0-7.0.

[0072] The above method is applicable to the smelting of steel grades such as ultra-pure iron, ultra-low carbon steel, and high scrap ratio green steel that require vacuum decarburization in the RH process. The process route is converter → LF refining → RH vacuum treatment → continuous casting.

[0073] The core principle of this invention lies in achieving the dual objectives of preserving oxygen in molten steel and reducing oxygen in the slag phase through the synergistic effect of "stepwise slag phase deoxidation with temperature gradient matching" and "staged bottom blowing dynamics control".

[0074] The principle of stepwise slag phase deoxidation with temperature gradient matching in this invention is as follows: Traditional LF refining usually uses a single deoxidizer added at once for deep deoxidation, the purpose of which is to reduce the (FeO) content in the slag to below 1% (white slag). However, under special conditions where molten steel needs to maintain high oxygen (400-800ppm), once a strong deoxidizer (such as aluminum) comes into contact with the molten steel, it will rapidly consume the [O] in the molten steel, disrupting the thermodynamic conditions for RH decarburization.

[0075] This invention employs a progressive deoxidation strategy of carbon powder → ferrosilicon powder → aluminum slag balls / aluminum powder, based on the following:

[0076] 1. Low-temperature range (1500~1580℃): Use carbon powder as a weak deoxidizer. The reaction between carbon powder and FeO in the slag is: C+(FeO)→[Fe]+CO↑. This reaction can be carried out at a relatively low temperature, with moderate deoxidation capacity, without excessively reducing the FeO in the slag. At the same time, the reaction product CO gas forms bubbles in the slag layer, which plays a role in foaming and submerging the arc, effectively isolating air and inhibiting oxygen transfer from the arc zone to the molten steel.

[0077] 2. Medium-temperature range (1580~1620℃): Use ferrosilicon powder with moderate deoxidizer. The Si in ferrosilicon powder can react with (FeO): Si + 2(FeO) → SiO2 + 2Fe. Ferrosilicon has a stronger deoxidizing ability than carbon powder but weaker than aluminum slag balls / powder, making it suitable for stages with moderate (FeO) content (10~15%). The generated SiO2 can adjust the slag composition and improve slag fluidity.

[0078] 3. High-temperature range (1620~1650℃): Precise fine-tuning is performed using Al powder, a strong deoxidizer. 2Al + 3(FeO) → Al2O3 + 3Fe. Al powder has the strongest deoxidizing ability, but the generated Al2O3 may increase the risk of inclusions. Therefore, only when (FeO) is close to the target value (8~12%) is a small amount of aluminum slag balls or Al powder used for fine-tuning, and it is strictly limited to being sprinkled only on the slag surface to avoid Al powder being directly consumed by the molten steel [O].

[0079] Through the above-mentioned progressive deoxidation, the FeO content in the slag will gradually decrease from the initial 15% to 5-8%, avoiding the problem of a sharp drop in [O] in molten steel caused by traditional one-time strong deoxidation.

[0080] Example 2

[0081] This invention provides a method for heating and dynamic oxygen control in the slag phase during LF refining of high-oxygen molten steel before RH decarburization. The steel grade is IF steel (target C ≤ 0.002%, final [O] ≤ 20 ppm).

[0082] The amount of molten steel is 150 tons;

[0083] The process route is: converter → LF refining → RH vacuum treatment → continuous casting;

[0084] Initial conditions: After tapping from the converter, the molten steel enters the LF station with an initial temperature of 1520℃ and a total oxygen content [O] of 680ppm. Target: Exit temperature 1680℃, [O] maintained at 400-800ppm, and (FeO) in slag ≤8%.

[0085] Step 1: Temperature and oxygen levels are measured at the station. [O] = 680ppm, which is within the range of 400-800ppm, so no pre-adjustment of oxygen is required. Add the first batch of slag-forming materials: 4kg / t steel of lime and 1.0kg / t steel of fluorite to establish the initial slag layer;

[0086] Step 2: Low-power electrode heating, bottom-blown argon gas 150 NL / min, add the second batch of slag-forming material: lime 1.5 kg / t steel, adjust the slag basicity to 4.0. No deoxidizer is added, slag formation time is 12 min;

[0087] Step 3, Heating and Oxygen Transfer Control Stage: When the molten steel temperature reaches 1580℃, increase the heating power. In the 1580~1620℃ range, the bottom-blown argon flow rate is 400NL / min; in the 1620~1650℃ range, the bottom-blown argon flow rate is 200NL / min.

[0088] Stepwise deoxidation: When the temperature reaches 1600℃, the slag sample evaluation (FeO) is 18% (>15%), and 0.4 kg / t steel of carbon powder is evenly sprinkled on the slag surface; when the temperature rises to 1630℃, the slag sample evaluation (FeO) is 12% (>10%), and 0.3 kg / t steel of ferrosilicon powder is evenly sprinkled on the slag surface; when the temperature rises to 1650℃, the slag sample evaluation (FeO) is 9% (>8%), and 0.15 kg / t steel of aluminum slag balls / aluminum powder is evenly sprinkled on the slag surface.

[0089] Step 4, High-Temperature Steady-State Control Stage: When the molten steel temperature reaches 1650℃, the electrode power is reduced to medium level for heat preservation, and bottom blowing is maintained at 100 NL / min. FeO in the slag is checked every 8 minutes; if it exceeds 8%, aluminum powder is added at a rate of 0.05 kg / t of steel, and the steady-state condition is maintained for 10 minutes. Ultimately, [O] stabilizes at 580 ppm, and (FeO) stabilizes at 6.2%.

[0090] Step 5, Final Adjustment Before Leaving the Station: Finally confirm the molten steel temperature as 1680℃, [O] = 580ppm, and (FeO) in the slag as 6.2%. Reduce the bottom blowing flow rate to 60NL / min for soft blowing for 6 minutes, and hoist it to the RH station.

[0091] Results: The decarburization reaction during the RH vacuum treatment process was stable with no violent splashing. The RH endpoint [O] was precisely controlled, and there were no nozzle nodule alarms throughout the continuous casting process. The final slag composition was: CaO 50%, SiO2 11%, Al2O3 20%, MgO 7%, FeO 6.2%, CaF2 5%, MnO 0.8%, and basicity R=4.5.

[0092] Example 3

[0093] This invention provides a method for heating and dynamic oxygen control in the slag phase during LF refining of high-oxygen molten steel before RH decarburization. The steel volume is 200 tons.

[0094] Initial conditions: Initial temperature 1500℃, total oxygen content in molten steel [O] = 780ppm. Target: Outgoing temperature 1700℃, [O] maintained at 400~800ppm, slag (FeO) ≤8%.

[0095] S1, [O] at the station = 780ppm, add the first batch of slag-forming materials: lime 5kg / t steel, fluorite 1.5kg / t steel;

[0096] S2, low-power heating, bottom blowing 200NL / min, add second batch of slag-forming material: lime 2kg / t steel, slag formation 15min, basicity reaches 5.0;

[0097] S3, Temperature Rise and Oxygen Transfer Control Stage: 1580~1620℃ bottom blowing 500NL / min; 1620~1650℃ bottom blowing 250NL / min; 1600℃ (FeO) = 20%, add 0.6kg / t carbon powder; 1630℃ (FeO) = 14%, add 0.4kg / t ferrosilicon powder; 1650℃ (FeO) = 10%, add 0.2kg / t Al powder. Oxygen concentration during temperature rise: 1580℃ [O] = 750ppm; 1630℃ [O] = 680ppm; 1650℃ [O] = 620ppm; 1700℃ [O] = 560ppm. Throughout the process, [O] remains between 400~800ppm, requiring no additional oxygen blowing.

[0098] S4. High-temperature steady-state control stage: 1700℃ high-temperature holding, bottom blowing 150NL / min, fine-tuning slag phase, final slag (FeO) = 7.5%;

[0099] S5, Final adjustment before departure: Soft blowing for 8 minutes before departure.

[0100] Results: Even under harsh conditions of extremely high temperature of 1700℃ and high oxygen content of nearly 800ppm, the present invention still successfully suppressed (FeO) in the slag to below 8% without causing destructive consumption of oxygen content in the molten steel.

[0101] Comparative Example 1

[0102] The conventional LF heating process is adopted (without stepwise oxygen control).

[0103] Initial conditions: Same as in Example 2, initial temperature 1520℃, [O] = 680ppm.

[0104] Operation process: Upon entering the station, 5 kg / t of lime and 1 kg / t of fluorite are added at once for slag formation. A medium bottom-blowing flow rate of 250 NL / min is used throughout the process. To prevent excessive FeO levels, 0.8 kg / t of aluminum wire is added to the molten steel at 1600℃ for deep deoxidation. The steel is then heated to 1680℃ at high power before exiting the station. No stepwise slag phase deoxidation is performed throughout the entire process.

[0105] Results: After adding aluminum, the [O] in the molten steel dropped rapidly to 80 ppm, causing the subsequent RH vacuum decarburization to stop prematurely due to the lack of reaction-driving [O], and the endpoint [C] exceeded the standard. During the heating process, due to the failure to inhibit the steel-slag mass transfer and the consumption of aluminum wire in the early stage, the [O] in the molten steel in the high-temperature section rose again, and the (FeO) in the slag soared to 26% before leaving the station. After the highly oxidizing slag containing 26% (FeO) entered the RH, it strongly re-oxidized under vacuum. During the RH deoxidation and alloying, a large number of clustered Al2O3 inclusions were generated. The continuous casting submerged entry nozzle was severely blocked by nodules when casting the second heat, forcing the machine to stop.

[0106] This invention employs a stepwise slag phase deoxidation strategy with temperature gradient matching. Breaking away from the traditional single-agent addition approach, this invention innovatively adopts a progressive deoxidation process: carbon powder → ferrosilicon powder → aluminum powder / aluminum slag balls. In the low-temperature stage, a weak deoxidizer, carbon powder, is used, whose deoxidation product, CO gas, can assist in foaming and submerging the arc, isolating air and inhibiting oxygen transfer from the arc. In the medium-temperature stage, a medium-strength deoxidizer, ferrosilicon powder, is used, balancing deoxidation with improved slag fluidity. In the high-temperature stage, a strong deoxidizer, Al powder, is used for precise fine-tuning. This strategy avoids the problems of high burn-off of strong deoxidizers in the early stages and the potential for Al buildup that disrupts the high-oxygen state of molten steel, while also achieving a gradual and stable reduction in FeO.

[0107] Precise control of oxygen transfer kinetics between steel and slag: By controlling the bottom blowing in stages (strong stirring at 1580-1620℃ to promote slag formation and temperature uniformity, and weak stirring at 1620-1650℃ to inhibit macroscopic mass transfer from high-oxygen molten steel to the slag phase), the generation pathway of (FeO) under high temperature and high oxygen conditions is cut off from the source of kinetics.

[0108] Achieving a win-win situation of maintaining molten steel [O] and reducing slag (FeO): By limiting the deoxidizer to be sprinkled on the slag surface for reaction, it ensures that the molten steel [O] is always maintained in the golden range of RH decarburization of 400-800ppm, while suppressing slag (FeO) to 5-8%, thus completely eliminating the stubborn problem of high (FeO) oxygen reversion in the RH process.

[0109] The final slag composition is designed reasonably: through the precise proportioning of two batches of slag-forming materials, the resulting final slag (R=3.0~7.0, Al2O315~25%, MgO5~10%) has a suitable melting point and viscosity. It can effectively coat the electric arc at high temperatures, absorb a large amount of deoxidation products, and reduce the transfer of oxygen from molten steel to the slag at high temperatures, thus ensuring the cleanliness of molten steel at extremely high temperatures (1650~1700℃).

[0110] Significantly improves RH decarburization efficiency and product quality: FeO in LF slag is controlled at 5-8%, avoiding interference from high FeO slag on the RH decarburization process, and the carbon content at the decarburization endpoint can be stably controlled below 0.002%; the molten steel has high cleanliness after RH treatment, the risk of nozzle nodule formation during continuous casting is greatly reduced, and the total oxygen content of the billet is ≤20ppm.

[0111] The above description is merely an illustrative embodiment of the present invention and is not intended to limit the scope of the invention. The various components of the present invention can be combined with each other without conflict. Any equivalent changes and modifications made by those skilled in the art without departing from the concept and principles of the present invention should fall within the scope of protection of the present invention.

Claims

1. A method for temperature rise and dynamic oxygen control in slag phase during LF refining of high-oxygen molten steel before RH decarburization, characterized in that, Includes the following steps: Step 1: Transfer the molten steel after tapping from the converter to the LF refining furnace at a temperature of 1500~1560℃, and at the same time add the first batch of slag-forming material to the slag surface. Step 2: Use electrode heating to control the molten steel temperature at 1500~1580℃, control the bottom blowing argon flow rate at 100~200NL / min, add the second batch of slag-forming material to adjust the slag basicity to 4.0~7.0, do not add deoxidizer for the time being, and the slag forming time is 8~15min; Step 3: Increase the temperature of the molten steel. When the temperature of the molten steel is greater than 1580℃, the reaction at the interface between the steel and slag is controlled by adjusting the flow rate of bottom-blown argon in stages and stirring. Deoxidation is carried out in steps according to the FeO content in the slag. Step 4: After the molten steel temperature reaches 1650℃, reduce the electrode heating power, adjust the bottom blowing argon flow rate to 100~150NL / min, detect the oxygen content of the molten steel and evaluate the FeO in the slag every 5~10 minutes, dynamically fine-tune the amount of deoxidizer added, so that the oxygen content of the molten steel is stable at 400~800ppm and the FeO in the slag is stable at 5~8%.

2. The method for temperature rise and dynamic oxygen control of slag phase in LF refining of high-oxygen molten steel before RH decarburization according to claim 1, characterized in that, Step three, which involves adjusting the bottom-blown argon flow rate and stirring to control the steel-slag interface reaction in stages, specifically includes: When the temperature of molten steel is 1580~1620℃, control the bottom blowing argon flow rate to be 300~500NL / min; When the molten steel temperature is 1620~1650℃, control the bottom blowing argon flow rate to be 150~250NL / min.

3. The method for temperature rise and dynamic oxygen control of slag phase in LF refining of high-oxygen molten steel before RH decarburization according to claim 1, characterized in that, Step three, based on the FeO content in the slag, involves stepwise deoxidation, specifically including: When the temperature of molten steel is 1580~1620℃, if the FeO content in the slag is greater than 15%, then 0.3~0.6 kg / t of carbon powder should be evenly sprinkled on the slag surface. When the temperature of molten steel is 1620~1650℃, if the FeO content in the slag is greater than 10%, then 0.2~0.4 kg / t of ferrosilicon powder should be evenly sprinkled on the slag surface. When the temperature of molten steel rises to 1650℃, if the FeO content in the slag is greater than 8%, 0.1~0.2 kg / t of aluminum slag balls or aluminum powder should be evenly sprinkled on the slag surface.

4. The method for temperature rise and dynamic oxygen control of slag phase in LF refining of high-oxygen molten steel before RH decarburization according to claim 1, characterized in that, The first batch of slag-forming materials includes: 3~5 kg / t lime and 0.5~1.5 kg / t fluorite; The second batch of slag-forming material is 1~2 kg / t of lime.

5. The method for temperature rise and dynamic oxygen control of slag phase in LF refining of high-oxygen molten steel before RH decarburization according to claim 1, characterized in that, The composition of the final slag from LF refining is controlled as follows by mass percentage: CaO 50~55%, SiO2 8~15%, Al2O3 15~25%, MgO 5~10%, FeO 5~8%, CaF2 3~8%, MnO≤1.5%, and basicity R=3.0~7.

0.

6. The method for temperature rise and dynamic oxygen control of slag phase in LF refining of high-oxygen molten steel before RH decarburization according to claim 1, characterized in that, In step one, the first batch of slag-forming materials added is: 4 kg / t lime and 1.0 kg / t fluorite.

7. The method for temperature rise and dynamic oxygen control of slag phase in LF refining of high-oxygen molten steel before RH decarburization according to claim 1, characterized in that, In step two, the bottom-blown argon gas is controlled at 150 NL / min; The second batch of slag-forming material added was 1.5 kg / t lime; Adjust the slag basicity to 4.0; Do not add deoxidizer for now, slag formation time is 12 minutes.

8. The method for temperature rise and dynamic oxygen control of slag phase in LF refining of high-oxygen molten steel before RH decarburization according to claim 1, characterized in that, In step three, When the temperature of the molten steel reaches 1580~1620℃, control the bottom blowing argon flow rate to 400NL / min; When the temperature of the molten steel reaches 1620~1650℃, control the bottom blowing argon flow rate to 200NL / min.

9. The method for temperature rise and dynamic oxygen control of slag phase in LF refining of high-oxygen molten steel before RH decarburization according to claim 1, characterized in that, Step three, based on the FeO content in the slag, involves stepwise deoxidation, specifically including: When the temperature of molten steel reaches 1600℃, if the FeO content in the slag is greater than 15%, 0.4 kg / t of carbon powder should be evenly sprinkled on the slag surface. When the temperature of molten steel reaches 1630℃, if the FeO content in the slag is greater than 10%, 0.3 kg / t of ferrosilicon powder should be evenly sprinkled on the slag surface. When the temperature of molten steel reaches 1650℃, if the FeO content in the slag is greater than 8%, 0.15 kg / t of aluminum slag balls or aluminum powder should be evenly sprinkled on the slag surface.

10. The method for heating and dynamic oxygen control of slag phase in LF refining of high-oxygen molten steel before RH decarburization according to claim 1, characterized in that, The composition of the final LF refining slag by mass percentage is as follows: CaO 50%, SiO2 11%, Al2O3 20%, MgO 7%, FeO 6.2%, CaF2 5%, MnO 0.8%, with an alkalinity R=4.5.

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