A method for controlling the process of leak-proof steel smelting in a 25 kg vacuum induction furnace

Abstract

This invention discloses a process control method for leak-proof steel melting in a 25 kg vacuum induction furnace, belonging to the field of vacuum induction melting technology. The method includes leak-proof steel pretreatment during the charging stage, vacuuming and atmosphere control, staged power curve control, real-time monitoring of crucible wall condition and leak warning, and leak-proof steel control during the casting stage. The purpose of this invention is to provide a process control method for leak-proof steel melting in a 25 kg vacuum induction furnace. Through the coordinated control of the entire process—charging strategy, staged power curve, real-time monitoring of crucible condition, and emergency handling mechanism—it significantly reduces the leakage rate and extends crucible lifespan while ensuring melting efficiency.

Description

Technical Field

[0001] This invention belongs to the field of vacuum induction melting technology, and particularly relates to a method for preventing steel leakage during steel melting in a 25 kg vacuum induction furnace. Specifically, it relates to a method that prevents steel leakage through the synergistic effect of charging strategies, staged power control, and crucible status monitoring. Background Technology

[0002] The 25 kg vacuum induction furnace is a key piece of equipment for materials science research, special alloy development, and small-batch production, offering advantages such as rapid melting, short experimental cycles, and flexible operation. However, in practical use, this type of equipment suffers from significant steel leakage problems, primarily due to the following reasons: First, the furnace structure results in weak refractory materials. The small size and low thermal inertia of the 25 kg furnace, coupled with crucible wall thicknesses typically only 15-25 mm, significantly reduces its thermal shock resistance compared to large industrial furnaces (50-100 mm thick). Under repeated rapid heating and cooling conditions, micro-cracks easily form on the inner wall of the crucible, allowing molten steel to penetrate along these cracks and eventually break through the crucible. Second, power control relies heavily on experience. Small furnaces are often operated manually; if the temperature rises too quickly, causing localized overheating, or if bridging occurs and is not handled properly, the local temperature of the crucible wall can easily exceed the refractory material's tolerance limit (typically 1650-1700℃), accelerating corrosion. Third, there is a lack of systematic monitoring methods. Existing processes primarily focus on smelting effects (such as composition control and yield), lacking real-time assessment and lifespan management of the crucible, often resulting in no clear warning before steel leakage. While existing technologies such as CN118086763A address rare earth yield control in a 25kg vacuum induction furnace, they do not address steel leakage prevention; CN111023806A proposes a crucible preparation method, but it is not linked to the smelting process parameters for integrated control. Therefore, a systematic steel leakage prevention smelting process for a 25kg vacuum induction furnace is urgently needed. Summary of the Invention

[0003] To overcome the shortcomings of existing technologies, such as the high risk of steel leakage and lack of systematic preventive measures in 25 kg vacuum induction furnaces, the purpose of this invention is to provide a process control method for preventing steel leakage in a 25 kg vacuum induction furnace. Through the coordinated control of the entire process, including charging strategy, staged power curves, real-time monitoring of crucible status, and emergency handling mechanism, the method can significantly reduce the steel leakage rate and extend the service life of the crucible while ensuring smelting efficiency.

[0004] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0005] This invention discloses a process control method for leak-proof steel smelting in a 25 kg vacuum induction furnace, comprising the following steps:

[0006] Step 1: Leak-proof steel pretreatment during the loading stage

[0007] Raw material size control: Cut steel into strips with a length of 300-400mm and a cross-sectional size of 20mm×30mm, and remove surface iron oxide scale and oil stains; This size design ensures that the furnace charge does not form a lateral support force when it naturally accumulates in the crucible, and avoids the crucible breaking due to the lever effect generated by the long bar material obliquely pressing against the crucible wall during the melting process.

[0008] Crucible condition assessment and repair: Perform a macroscopic inspection of the inner wall of the crucible and record the current furnace age; if the furnace age exceeds 15 times, use fused magnesia repair material to locally repair the bottom of the crucible and severely corroded areas, with a repair thickness of 3-5 mm, and allow it to air dry naturally for more than 24 hours; the repaired crucible must be baked at a low temperature (200-300℃, 2 hours) before it can be put into use.

[0009] Optimized charging method: The "tight at the bottom, loose at the top" principle is adopted for layered charging—large pieces of material (single piece weight >1kg) are placed at the bottom of the crucible, small pieces of material (<0.5kg) fill the gaps in the middle layer, and extremely fine fragments (<0.1kg) are evenly sprinkled on the top layer; this method ensures that the bottom molten pool forms quickly in the early stage of melting, and the upper furnace charge sinks slowly, avoiding "bridging"; the total charging height does not exceed 80% of the height of the induction coil, leaving space for the melt to boil;

[0010] Step 2: Vacuuming and Atmosphere Control

[0011] Close the furnace door, start the mechanical pump and Roots pump, and evacuate to below 5 Pa; purge with high-purity argon gas to 0.05-0.06 MPa, repeat 2-3 times, and then adjust the argon gas pressure to 0.03-0.04 MPa as a protective atmosphere for melting; this pressure range can effectively suppress metal volatilization without affecting the induction heating efficiency due to excessive pressure;

[0012] Step 3: Staged Power Curve Control

[0013] A four-stage power control strategy is adopted, with the following specific parameters:

[0014] stage Power range Duration Control Target Leak-proof steel mechanism Preheating stage 30-40kw 3-5 minutes The furnace charge is heated evenly, releasing adsorbed gases. Avoid thermal stress cracks in the inner wall of the crucible caused by rapid thermal shock. Melting stage 80-100kw 10-150 minutes Rapid melting to form a molten pool Step-by-step power increase to avoid localized overheating Refining stage 50-60kw 5-8 minutes homogenization of components, with impurities rising to the surface. Reduce heat load and lessen thermal erosion of the crucible wall. Heating and tapping steel 70-80kw 2-3 minutes Heat to tapping temperature Short-duration high power output reduces the duration of high temperatures.

[0015] During the melting stage, it is necessary to closely observe the sinking of the furnace charge. If "bridging" occurs (the upper part of the furnace charge is stuck and does not sink, while the lower part has formed a molten pool), an emergency operation should be performed immediately: reduce the power to 50kw, use a stainless steel tamping rod to gently touch the furnace charge through the observation hole, and restore the power after the bridging collapses. This operation can prevent a large amount of cold material from crashing into the molten pool due to the sudden collapse of the bridging, causing the molten material to splash and impact the crucible wall.

[0016] Step 4: Real-time monitoring of crucible wall condition and early warning of steel leakage

[0017] 1) Monitoring frequency and indicators: After the melting stage begins, check the condition of the crucible wall every 2 minutes through the observation window, focusing on three areas: the bottom (where the static pressure of molten steel is the greatest), the slag line (where slag erosion is the most severe), and the area corresponding to the induction coil (where the heat load is the highest). The observation indicators include: local reddening (abnormal temperature), steel seepage (small steel beads seeping out), and crack propagation.

[0018] 2) Tiered Early Warning and Handling: Level 1 Warning (Localized Redness): Immediately reduce power to 30kW, prepare for emergency pouring, and observe whether the redness area expands; if the redness subsides within 2 minutes, low power operation can continue until pouring; if the redness persists, execute emergency pouring; Level 2 Warning (Signs of Steel Leakage): Immediately cut off the power, activate the emergency tilting mechanism, and pour molten steel into the ingot mold within 10 seconds; it is strictly forbidden to attempt to maintain low power, because once steel leakage occurs, the breakdown time is only a few seconds to a dozen seconds;

[0019] 3) Furnace life management: After each furnace smelting is completed, the location and extent of crucible erosion are recorded, and a life file is established for each crucible. When the furnace life reaches 25 times, it enters the end-of-life management stage—the single furnace charge is reduced from 25kg to 15-20kg, and the monitoring frequency is increased to once every 1 minute.

[0020] Step 5: Leakage prevention steel control during the casting stage

[0021] Pouring temperature control: The superheat should be determined based on the liquidus temperature of the steel grade and controlled between 50-80℃; excessively high pouring temperatures will increase the heat load on the crucible wall and increase the risk of steel leakage; excessively low temperatures will affect the quality of the casting.

[0022] Tilting speed control: A three-stage pouring strategy of "slow first, then fast, then slow again" is adopted—initially, slow tilting (angular velocity 1-2° / s, lasting 10-15 seconds) allows the molten steel to flow smoothly into the pouring cup, preventing splashing onto the upper edge of the crucible; in the middle stage, rapid tilting (3-4° / s, lasting 30-40 seconds) completes the main pouring; in the final stage, slow tilting (1-2° / s, lasting 5-10 seconds) controls the feeding. The entire tilting process is stable, avoiding sudden stops and turns that could cause the molten steel to slosh and scour the crucible wall.

[0023] Vacuum pressure holding: After casting, continue to maintain the vacuum state for 10-15 minutes; this operation prevents the high-temperature crucible from being exposed to air and causing severe oxidation, and also uses the negative pressure environment to accelerate the cooling of the crucible and reduce thermal stress.

[0024] Furthermore, during the melting stage, closely observe the sinking of the furnace charge. If "bridging" occurs, meaning the upper part of the furnace charge is stuck and does not sink while a molten pool has formed at the bottom, immediately perform emergency operations: reduce the power to 50 kW, use a stainless steel tamping rod to gently touch the furnace charge through the observation hole, and restore the power after the bridging collapses. This operation can prevent a large amount of cold material from crashing into the molten pool due to the sudden collapse of the bridging, causing the molten material to splash and impact the crucible wall.

[0025] Furthermore, place nickel plates, 2-3 kg / piece, at the bottom of the crucible; electrolytic nickel and metallic chromium, 0.3-0.5 kg / piece, fill the middle; and aluminum-titanium alloy shavings, <0.1 kg, are sprinkled on the top layer.

[0026] Furthermore, in step two, the vacuum is evacuated to 3 Pa, followed by argon purging twice, with a working argon pressure of 0.035 MPa.

[0027] Furthermore, the power control is as follows: 35 kW / 4 min for the preheating stage; 90 kW / 15 min for the melting stage; 55 kW / 6 min for the refining stage; and 75 kW / 2 min for the heating and tapping stage.

[0028] Furthermore, the tapping temperature is 1540℃.

[0029] Furthermore, the total tilting and pouring time was 55 seconds, consisting of 10 seconds slow, 38 seconds fast, and 7 seconds slow; vacuum pressure was maintained for 12 minutes after pouring.

[0030] Compared with the prior art, the beneficial technical effects of the present invention are as follows:

[0031] Effectively reducing the risk of steel leakage accidents: This invention establishes a comprehensive steel leakage prevention control system, encompassing source control of charging, management of the smelting process, monitoring of crucible status, and standardized casting operations. After 150 heats of continuous tracking and verification, there were zero steel leakage accidents, a significant improvement over conventional processes (historical occurrence rate of 0.94%), effectively enhancing equipment operational safety.

[0032] Significantly extends crucible lifespan: By avoiding thermal shock through staged power control, combined with furnace age classification management and local repair technology, the average lifespan of 25kg vacuum induction furnace crucibles is extended from 21.5 times in conventional processes to 34.2 times, an increase of 59%. The maximum number of times a single crucible can be used reaches 38 times, greatly reducing consumable costs and furnace construction frequency.

[0033] A standardized anomaly handling mechanism was established: This invention establishes a tiered early warning and standardized emergency response process to address common anomalies during the smelting process, such as bridging and localized reddening. During the verification period, bridging was handled 8 times and early warnings were triggered 3 times, all of which were successfully handled according to the process, changing the traditional "experience-based" approach and reducing the risk of human error.

[0034] 4. Improve process stability and replicability: Transform traditional experience-based smelting operations into a standardized parameter system (power-time curve, monitoring frequency, early warning threshold) to ensure process stability between different operators and different furnace batches, providing a reliable guarantee for the research and development and small-batch production of special alloys. Detailed Implementation

[0035] Example 1

[0036] The method of this invention was used to smelt nickel-based alloy GH4169 in a ZG-25 type 25 kg vacuum induction furnace, with a total charge of 25 kg. The specific operation is as follows:

[0037] 1) Loading: Place nickel plates (2-3 kg / piece) at the bottom of the crucible, fill the middle with electrolytic nickel and metallic chromium (0.3-0.5 kg / piece), and sprinkle aluminum-titanium alloy shavings (<0.1 kg) on ​​top. The crucible was used for the 10th time, and the inner wall was inspected and found to be smooth and without cracks.

[0038] 2) Evacuation: Evacuate to 3 Pa, purge with argon twice, and maintain the working argon pressure at 0.035 MPa.

[0039] 3) Power control: 35kw / 4min during the preheating stage; 90kw / 15min during the melting stage (during which the furnace charge sinks normally and there is no bridging); 55kw / 6min during the refining stage; 75kw / 2min during the heating and tapping stage, with a tapping temperature of 1540℃.

[0040] 4) Monitoring: During the melting stage, observe every 2 minutes. The crucible wall should be a uniform dark red color with no localized redness.

[0041] 5) Pouring: The total pouring time was 55 seconds (10 seconds slow - 38 seconds fast - 7 seconds slow), followed by vacuum pressure holding for 12 minutes. Result: No steel leakage occurred throughout the process. After the crucible was removed from the furnace, no new cracks were found on the inner wall, and it can continue to be used.

[0042] Example 2

[0043] The crucible was being used for the 28th time (end of its service life). Following the method of this invention, the loading was reduced to 18 kg, with other parameters remaining the same as in Example 1, but the monitoring frequency was increased to once every minute. During the smelting process, a slightly reddish area with a diameter of approximately 5 mm was observed near the slag line at the 12-minute mark. A Level 1 warning was immediately issued: power was reduced to 30 kW. After observing for 2 minutes, the redness subsided, and power was then maintained at 50 kW until casting. Result: Smelting was successfully completed without any steel leakage.

[0044] Example 3

[0045] Bridging occurred during the smelting process: At the 8-minute mark of the melting stage, it was discovered that the upper charge was stuck and not sinking, while the lower molten pool had formed. The power was immediately reduced from 90kW to 50kW using the method described in this invention. A stainless steel tamping rod was used to gently touch the bridging area, and the bridging collapsed before the power was restored. There was no molten steel splashing throughout the process, the crucible wall showed no abnormalities, and the smelting was successfully completed.

[0046] Experimental verification

[0047] To verify the effectiveness of the method of this invention, process verification was conducted on a 25kg vacuum induction furnace (model ZG-0.025JC) in the rolling mill laboratory of Baogang Steel Technology Center. The verification was carried out in two stages:

[0048] 1. Historical Data Statistics (Control Group): Statistics were compiled on smelting records using conventional, experience-based operations within the 12 months prior to the implementation of this invention, involving a total of 320 heats (crucible usage cycles covering 1-25 cycles). The data are as follows: Steel leakage incidents: 3 incidents (occurrence rate 0.94%), occurring on the 19th, 23rd, and 24th heats, all caused by severe localized corrosion of the crucible wall that was not detected in time. Average crucible lifespan: 21.5 cycles (referring to the average number of times the crucible was used from its initial use until it was scrapped due to severe corrosion / cracks). Abnormal situations: Bridging occurred 15 times, of which 4 were caused by improper handling resulting in molten steel sloshing and impacting the crucible wall.

[0049] 2. Verification of the Method of this Invention (Experimental Group): The method of this invention was used to continuously track and record 150 heats (covering the entire life cycle of 5 crucibles). The results are as follows: Steel leakage accidents: 0 times. Average crucible lifespan: 34.2 times (59% improvement compared to historical data), with the highest number of uses for a single crucible reaching 38 times. Anomaly handling: Bridging occurred 8 times, all handled according to the standardized procedures of this invention, with no secondary impact; crucible wall warnings were triggered 3 times (local reddening), all of which returned to normal after power reduction control.

[0050] 3. Typical comparative cases were selected, comparing smelting records of the same operator and the same steel grade (304 stainless steel): Conventional process (before implementation of this invention): Localized steel leakage occurred after 22 furnace cycles, and the crucible was scrapped after emergency casting. Invention process (after implementation): The crucible was used continuously up to 35 times. During the 28th smelting cycle, localized red-hot areas appeared. Following the graded warning system of this invention, it was used normally up to 35 times. Finally, it was proactively scrapped due to bottom erosion reaching the safety threshold. There was no steel leakage throughout the entire process. The verification results show that the method of this invention, through optimized charging, staged power control, graded warning, and furnace age management, can effectively reduce the incidence of steel leakage accidents, extend the service life of the crucible by more than 30%, and provide standardized handling procedures when abnormalities occur, reducing the risk of human error.

[0051] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. A method for controlling the leakage prevention process of steel smelting in a 25 kg vacuum induction furnace, characterized in that: Includes the following steps: Step 1: Leak-proof steel pretreatment during the loading stage Raw material size control: Cut the steel material into strips with a length of 300-400mm and a cross-sectional size of 20mm×30mm, and remove the surface iron oxide scale and oil stains; Crucible condition assessment and repair: Perform a macroscopic inspection of the inner wall of the crucible and record the current furnace age; if the furnace age exceeds 15 times, use fused magnesia repair material to locally repair the bottom of the crucible and severely corroded areas, with a repair thickness of 3-5 mm, and allow it to air dry naturally for more than 24 hours; the repaired crucible must be baked at a low temperature before it can be put into use. Optimized charging method: The charging method adopts the principle of "tight at the bottom and loose at the top" for layered charging. Large pieces weighing more than 1 kg are placed at the bottom of the crucible, small pieces weighing less than 0.5 kg are placed in the middle layer gaps, and extremely fine fragments weighing less than 0.1 kg are evenly scattered on the top layer. This method ensures that the bottom molten pool forms quickly in the early stage of melting, and the upper furnace charge sinks slowly, avoiding "bridging". The total charging height does not exceed 80% of the height of the induction coil, leaving space for the melt to boil. Step 2: Vacuuming and Atmosphere Control Close the furnace door, start the mechanical pump and Roots pump, and evacuate to below 5 Pa; purge with high-purity argon gas to 0.05-0.06 MPa, repeat 2-3 times, and then adjust the argon gas pressure to 0.03-0.04 MPa as a protective atmosphere for melting; this pressure range can effectively suppress metal volatilization without affecting the induction heating efficiency due to excessive pressure; Step 3: Staged Power Curve Control A four-stage power control strategy is adopted, with the following specific parameters: Preheating stage: power range 30-40kw; duration 3-5 minutes; Melting stage: power range 80-100kw; duration 10-150 minutes; Refining stage: Power range 50-60kw; Duration 5-8 minutes; Heating and tapping: Power range 70-80kw; duration 2-3 minutes; Step 4: Real-time monitoring of crucible wall condition and early warning of steel leakage 1) Monitoring frequency and indicators: After the melting stage begins, check the condition of the crucible wall through the observation window every 2 minutes, focusing on three areas: the bottom, the slag line area, and the area corresponding to the induction coil; The observation indicators include: localized reddening, steel penetration, and crack propagation; 2) Tiered Early Warning and Handling: Level 1 Warning - Localized Red-hot Area: Immediately reduce power to 30kW, prepare for emergency pouring, and observe whether the red-hot area expands; if the red-hot area subsides within 2 minutes, low power operation can continue until pouring; if the red-hot area persists, execute emergency pouring; Level 2 Warning - Steel Leakage Signs: Immediately cut off the power supply, activate the emergency tilting mechanism, and pour molten steel into the ingot mold within 10 seconds; it is strictly forbidden to attempt to maintain low power, because once steel leakage occurs, the breakdown time is only a few seconds to a dozen seconds; 3) Furnace life management: After each furnace smelting is completed, record the location and degree of crucible erosion and establish a "one furnace, one file" life record; when the furnace life reaches 25 times, it enters the end-of-life management stage, the single furnace loading amount is reduced from 25kg to 15-20kg, and the monitoring frequency is increased to once every 1 minute. Step 5: Leakage control of steel during the pouring stage 1) Pouring temperature control: Determine the superheat based on the liquidus temperature of the steel grade and control it between 50-80℃; 2) Tilting speed control: A three-stage pouring strategy of "slow first, then fast, then slow again" is adopted. In the initial stage, the molten steel is slowly tilted at an angular velocity of 1-2° / s for 10-15 seconds to ensure that the molten steel flows smoothly into the pouring cup and prevents splashing onto the upper edge of the crucible. In the middle stage, the molten steel is rapidly tilted at 3-4° / s for 30-40 seconds to complete the main pouring. In the final stage, the molten steel is slowly tilted at 1-2° / s for 5-10 seconds to control the feeding. The tilting is stable throughout the process to avoid sudden stops and turns that could cause the molten steel to slosh and scour the crucible wall. 3) Vacuum pressure holding: After pouring, continue to maintain the vacuum state for 10-15 minutes.

2. The method for controlling the leak-proof steel smelting process in a 25 kg vacuum induction furnace according to claim 1, characterized in that: During the melting stage, closely observe the sinking of the furnace charge. If "bridging" occurs, that is, the upper part of the furnace charge is stuck and does not sink, while the lower part has formed a molten pool, immediately perform emergency operation: reduce the power to 50kw, use a stainless steel tamping rod to gently touch the furnace charge through the observation hole, and restore the power after the bridging collapses. This operation can prevent a large amount of cold material from falling into the molten pool due to the sudden collapse of the bridging, causing the molten material to splash and impact the crucible wall.

3. The method for controlling the leak-proof steel smelting process in a 25 kg vacuum induction furnace according to claim 1, characterized in that: Place nickel plates, 2-3 kg / piece, at the bottom of the crucible; electrolytic nickel and metallic chromium, 0.3-0.5 kg / piece, in the middle; and aluminum-titanium alloy shavings, <0.1 kg, on the top layer.

4. The method for controlling the leak-proof steel smelting process in a 25 kg vacuum induction furnace according to claim 1, characterized in that: In step two, vacuuming is performed: the vacuum is evacuated to 3 Pa, followed by argon purging twice, with a working argon pressure of 0.035 MPa.

5. The process control method for leak-proof steel smelting in a 25 kg vacuum induction furnace according to claim 1, characterized in that: Power control: 35kw / 4min for preheating stage; 90kw / 15min for melting stage; 55kw / 6min for refining stage; 75kw / 2min for heating and tapping stage.

6. The process control method for leak-proof steel smelting in a 25 kg vacuum induction furnace according to claim 5, characterized in that: The tapping temperature is 1540℃.

7. The method for controlling the leak-proof steel smelting process in a 25 kg vacuum induction furnace according to claim 1, characterized in that: The total time for tilting and pouring is 55 seconds, consisting of 10 seconds of slow pouring, 38 seconds of fast pouring, and 7 seconds of slow pouring; vacuum pressure is maintained for 12 minutes after pouring.