A method for controlling the thickness of a special steel slag skin smelted by a lifting mold type ingot drawing electroslag furnace

Abstract

This invention relates to a method for controlling the slag thickness in special steel smelting using a crystallizer-type electroslag remelting furnace, comprising the following steps: 1) In the arc initiation and slag formation stage, full slag arc initiation is adopted, with constant current and voltage to form a constant melting rate; 2) In the smelting stage, a constant melting rate is maintained by using voltage swing amplitude; in the constant melting rate stage, the electrode insertion speed change rate is specified by setting a sinusoidal periodic function to the system, so that the electrode moves downward at a constant speed relative to the slag pool; the system detects voltage fluctuations in real time, compares the voltage swing amplitude signal with the set value, determines whether the safe time interval is met, and then drives the increase or decrease of the electrode speed to stabilize the slag pool voltage. The advantages are: it reduces frequent fluctuations in the slag surface, improves the thermal stability of the solidification process, and is conducive to forming a thin and uniform slag shell. It is also more conducive to achieving constant melting rate control, with stable heat input to the molten pool at a constant speed, consistent solidification rate of the molten slag on the inner wall of the crystallizer, and uniform slag shell thickness.

Description

Technical Field

[0001] This invention belongs to the field of electroslag remelting, and particularly relates to a method for controlling the slag thickness in special steel smelting using a crystallizer-lifting electroslag furnace. Background Technology

[0002] The electroslag remelting furnace with a lift-crystallizer (ESR-MM) is a special metallurgical technology whose core principle is based on the traditional electroslag remelting (ESR) process. A consumable electrode (the metal to be refined) is immersed as the negative electrode in a conductive molten slag pool (such as a mixture of CaF2 and Al2O3). When electricity is applied, the molten slag generates high temperatures (above 1700℃) due to resistance, melting the electrode tip and forming molten metal droplets. As the droplets pass through the molten slag layer, impurities are adsorbed by the slag, achieving refining. Subsequently, the molten metal solidifies at the bottom of the crystallizer to form steel ingots.

[0003] The difference between ESR-MM and traditional ESR lies in its use of hydraulic or mechanical devices to synchronously elevate the crystallizer, maintaining a relatively stable molten pool level. Initially, the crystallizer is positioned low and in contact with the ingot slab. As the electrodes melt and the ingot grows from bottom to top, the crystallizer slowly rises at a rate matching the ingot's solidification rate, maintaining a constant molten pool depth and thermal distribution. This dynamic adjustment offers numerous advantages: it avoids the temperature gradient changes in the molten pool caused by the ingot's rise, reducing defects such as shrinkage cavities and segregation; it extends the contact time between the molten metal and the slag, further removing inclusions such as sulfides and oxides; and the thin slag layer formed between the ingot surface and the crystallizer's inner wall improves surface finish and reduces the need for subsequent processing. The entire process, from arc ignition and slag pool formation to synchronous melting and crystallizer elevation, ultimately produces high-quality steel ingots with uniform composition and dense microstructure.

[0004] This technology is widely used in the preparation of aerospace high-temperature alloys, nuclear power corrosion-resistant materials, and high-end tool steels. It boasts advantages such as strong refining effect, low energy consumption, and adaptability to large-size steel ingot production, making it one of the core processes in modern special metallurgy. However, current research on electroslag remelting furnaces with lifting crystallizers is limited. Fluctuations in melting rate can lead to changes in the depth of the molten pool, causing defects such as inclusion aggregation and compositional segregation, especially affecting high-alloy special steels (such as high-temperature alloys and die steels). The thickness of the slag skin (solidified slag layer) directly affects the thermal balance of the molten pool and the surface quality of the steel ingot. Traditional methods rely on manual experience to adjust the electrode speed, resulting in a delayed response and easily causing the slag skin to be too thick (increased thermal resistance) or too thin (risk of steel leakage).

[0005] Traditional voltage feedback adjusts electrode speed based solely on instantaneous voltage values, neglecting dynamic changes in slag resistivity and hysteresis effects, making precise control difficult. Existing voltage control methods employ voltage swing, which works as follows: when the actual voltage Vrms deviates from the set voltage V during smelting, the consumable electrode moves in the direction of the deviation. If Vrms is greater than the set voltage V, the electrode moves downwards; conversely, it moves upwards. Simultaneously, the swing voltage Vs is calculated; if it exceeds a preset threshold, a negative voltage is added to suppress oscillations; otherwise, positive deviation fine-tuning is used to approximate the swing voltage set value. While the voltage swing method works well for traditional bottom-pull water-pan type electroslag furnaces, for crystallizer-lifting type electroslag furnaces, the control system must simultaneously consider the electrode insertion speed and the crystallizer lifting speed while maintaining a constant melting rate in the electroslag system. This leads to more frequent electrode control, but the irregular and frequent movement of the consumable electrode often causes uncontrolled and unstable electrode movement, disrupting the thermal stability within the slag pool and severely impacting slag thickness and ingot quality. Summary of the Invention

[0006] To overcome the shortcomings of existing technologies, the purpose of this invention is to provide a method for controlling the slag thickness in special steel smelting using a crystallizer-type electroslag remelting furnace. By employing different processes and control methods at different stages of electroslag remelting, optimal smelting quality can be achieved. Simplifying the relationship between the crystallizer lifting speed and the electrode insertion speed helps maintain a stable molten droplet velocity, resulting in a slag shell with uniform thickness and a dense structure.

[0007] To achieve the above objectives, the present invention provides the following technical solution:

[0008] A method for controlling the slag thickness in special steel smelting using a crystallizer-type electroslag remelting furnace includes the following steps:

[0009] 1) In the arc initiation and slag formation stage, full slag arc initiation is adopted, constant current and voltage are used to form a constant melting rate;

[0010] 2) During the smelting stage, the melting rate is maintained constant by using voltage swing.

[0011] During the constant melting rate stage, the system sets a sinusoidal periodic function to specify the rate of change of electrode insertion speed, so that the electrode moves downward at a constant speed relative to the slag pool. The system detects the voltage fluctuation value in real time, i.e., the voltage swing signal Vswing. The voltage swing signal Vswing is compared with the set value Vsetpoint to further determine whether the safe time interval is met, thereby driving the increase or decrease of the electrode speed to stabilize the voltage in the slag pool.

[0012] The comparison between the voltage swing signal Vswing and the setpoint Vsetpoint is described below:

[0013] If the voltage swing signal Vswing-set value Vsetpoint > the judgment value, and the time interval since the last control action > the safety time interval, then increase the electrode speed;

[0014] If the voltage swing signal Vswing-set value Vsetpoint ≯ judgment value, and the interval between the last control action is greater than the safe time interval, then reduce the electrode speed;

[0015] If the voltage swing signal Vswing does not exceed the limit or the safe time interval is insufficient, the electrode will not operate.

[0016] The determination value is 5% to 10% of the set value Vsetpoint.

[0017] The aforementioned safety time interval is at least one half of a sine cycle.

[0018] The relationship between the melting rate and the slag shell satisfies:

[0019]

[0020] In equation (1), H 渣壳 and v 熔速 They are inversely proportional; that is, the faster the melting rate, the thinner the slag shell; the slower the melting rate, the thicker the slag shell.

[0021] The constant melting rate stage is the stage that occurs after the arc initiation and slag formation stage, when all the solid slag has melted and the consumable electrode gradually forms molten droplets. At this point, the smelting process enters the constant melting rate stage.

[0022] Compared with the prior art, the beneficial effects of the present invention are:

[0023] 1. Compared with traditional electroslag remelting control, the voltage swing control method of the present invention reduces frequent fluctuations in the slag surface, improves the thermal stability of the solidification process, and is conducive to the formation of a thin and uniform slag shell.

[0024] 2. The method of this invention is more conducive to achieving constant melting rate control. Under constant rate, the heat input of the molten pool is stable, the solidification rate of the slag on the inner wall of the crystallizer is consistent, and the thickness of the slag shell formed is uniform. This avoids the problems of local excessive thinness (easy to crack) or excessive thickness (impeding heat transfer), ensuring stable heat conduction efficiency.

[0025] 3. The method of the present invention will significantly reduce the repeated melting and solidification of the slag shell, greatly reduce the generation of pores or cracks, make the slag shell structure more compact, and enhance its heat insulation and gas isolation capabilities. Attached Figure Description

[0026] Figure 1 This is a schematic diagram of periodic voltage swing.

[0027] Figure 2This is a schematic diagram of the periodic voltage swing control principle.

[0028] Figure 3 This is a schematic diagram of melting rate control. Detailed Implementation

[0029] The present invention will now be described in detail with reference to the accompanying drawings, but it should be noted that the implementation of the present invention is not limited to the following embodiments.

[0030] A method for controlling slag thickness in special steel smelting using a crystallizer-type electroslag remelting furnace consists of two parts: an arc-starting and slag-forming stage and a normal smelting stage. It is applicable to protective atmosphere electroslag remelting furnaces. Specifically:

[0031] 1) The arc initiation and slag formation stage adopts full slag arc initiation and constant current to form a constant melting rate; this is conducive to the melting of solid slag into liquid slag and rapid formation of slag shell, thereby reducing heat loss in the slag pool.

[0032] 2) During the smelting stage, a periodic voltage swing method is used to maintain a constant melting rate;

[0033] Based on voltage swing control, the electrode is driven downwards at a relatively constant speed, with the downward rate fluctuating sinusoidally. This periodic fluctuation generates a discernible voltage swing signal. This signal is compared to a pre-set value, and the downward speed of the electrode is dynamically adjusted based on the comparison result. Considering that the crystallizer and slag pool are relatively stationary, the slag pool experiences slight fluctuations when the crystallizer is lifted, causing subtle and repetitive changes in the electrode insertion depth. These changes are very frequent and have little impact on the smelting process. Without proper shielding, frequent changes in the electrode insertion speed can easily lead to larger fluctuations in the slag surface, negatively affecting the smelting process. Therefore, to ensure the stability of electrode control and reduce the number of electrode controls, an amplitude error threshold and time interval are set for electrode control to ensure the thermal stability of the slag pool.

[0034] During normal smelting, a constant melting rate is maintained through periodic voltage swings. This is achieved through the surface effect of the slag. In the electroslag remelting process, there is a certain relationship between voltage fluctuations (actual voltage Vrms) and immersion depth, as shown in the attached figure. Figure 1 As shown, when the consumable electrode is inserted at position 1, it is deeply inserted into the slag pool, resulting in minimal voltage fluctuations. The main fluctuations originate from the dripping of molten metal. When the consumable electrode is raised—that is, when its insertion position is increased from 1 to 2 or from 2 to 3—the actual voltage Vrms increases. At this point, the voltage fluctuation is caused by the surface of the slag. Therefore, the actual voltage is an indicator of the insertion depth, and its fluctuation amplitude is the swing voltage Vs.

[0035] The core of periodic voltage swing operation lies in the periodic variation of the electrode insertion rate, which generates the voltage swing signal. The control flow is as follows: Figure 2 As shown, by setting a sinusoidal periodic function to define the rate of change of electrode insertion speed, the electrode moves downward at a relatively constant speed. The periodic fluctuations in the electrode insertion speed generate a identifiable voltage swing signal. The system detects the voltage swing value (Vswing) in real time and compares it with a preset value (Vsetpoint). If it exceeds the preset value and the error is large, it further determines whether the safe time interval (dead time) is met. When all the above conditions are met, the electrode is driven to rise or fall, stabilizing the slag pool voltage by dynamically increasing or decreasing the speed deviation (e.g., accelerating insertion or decelerating retraction). If the fluctuation does not exceed the limit or the time interval is insufficient, the electrode does not move.

[0036] That is: a comparison between Vswing and Vsetpoint:

[0037] If Vswing-Vsetpoint > decision value, and the time interval since the last control action > safety time interval, then increase electrode speed;

[0038] If Vswing-Vsetpoint ≯ decision value, and the interval between the last control action is greater than the safe time interval, then reduce the electrode speed;

[0039] If the voltage swing signal Vswing does not exceed the limit or the safe time interval is insufficient, the electrode will not operate.

[0040] The judgment value is 5% to 10% of the setpoint Vsetpoint. The safety time interval is at least half a sine cycle.

[0041] The principle of slag shell thickness control in electroslag remelting furnaces for special steel production using a crystallizer-type ingot-drawing method: The main factor affecting slag shell thickness during electroslag remelting is the amount of heat input. The amount of heat input is equivalent to the amount of Joule heat generated. High Joule heat accelerates the melting rate and localized remelting of the slag shell, resulting in a thinner shell. Conversely, insufficient Joule heat leads to excessively low slag pool temperature and a slow melting rate, easily resulting in a thicker slag shell, which is detrimental to producing a uniform and thin shell. The relationship between Joule heat and slag shell thickness can be indirectly derived through the heat conduction equation and solidification rate model.

[0042] Q = I 2 Rt (2)

[0043] In equation (2), the Joule heat Q is related to the current I, resistance R, and time t. Input power (I) 2 When R increases or decreases, changes in heat flow distribution will affect the slag shell solidification rate (v). 凝固 ) and melting rate (v)熔速 The dynamic balance of current, voltage, and melting rate is disrupted, ultimately leading to variations in slag shell thickness. During production, the settings for current, voltage, and melting rate are derived through repeated summaries of actual production. Therefore, maintaining stable current and voltage values, and achieving a stable melting rate, plays a crucial role in obtaining a thin and uniform slag shell.

[0044] The relationship between melting rate and slag shell satisfies:

[0045]

[0046] In equation (1), H 渣壳 and v 熔速 The slag shell thickness is inversely proportional to the melting rate; that is, the faster the melting rate, the thinner the slag shell, and the slower the melting rate, the thicker the slag shell. Therefore, in production, keeping the melting rate stable within the set range is beneficial for achieving the desired slag shell thickness.

[0047] Example

[0048] In actual production at a certain steel plant (Plant A), the method for controlling the slag thickness in special steel smelting using a crystallizer-type electroslag remelting furnace is as follows:

[0049] Plant A uses a lift-crystallizer type electroslag remelting furnace to smelt H13 steel. The slag system is F60F, and the inlet and outlet of the crystallizer are on the same side. During the arc-starting and slag-forming stage, the electroslag remelting furnace adopts full-slag start-up, with both current and voltage set to given values. This stage is divided into 5 parts, and the duration and current and voltage set values ​​of each part are shown in Table 1.

[0050] Table 1

[0051] runtime 5min 10min 10min 5min 12min Current 5.8kA 6.8kA 7.8kA 9.8kA 12kA Voltage 54V 62V 71V 80V 80V

[0052] After the arc initiation and slag formation stage, the solid slag has completely melted, and the consumable electrode gradually forms molten droplets, marking the start of the constant melting rate stage in the smelting process. In this stage, a sinusoidal periodic function is set to regulate the rate of change of the electrode insertion speed, ensuring the electrode moves downwards at a relatively constant speed. The periodic fluctuations in the electrode insertion speed generate a identifiable voltage swing signal. The system monitors the voltage fluctuation value (Vswing) in real time and compares it with a preset value (Vsetpoint). If the voltage swing exceeds the preset value and the error is significant, it further determines whether the safe time interval (dead time) is met. When all conditions are met, the electrode is driven to rise or fall, stabilizing the slag pool voltage by dynamically increasing or decreasing the speed deviation (e.g., accelerating insertion or decelerating retraction). If the fluctuation does not exceed the limit or the time interval is insufficient, the electrode remains stationary. The melting rate is adjusted as follows: Figure 3 As shown.

[0053] After using this control method, Plant A saw a significant improvement in the surface quality of its steel ingots, a reduction in the thickness of the slag shell during smelting, and a substantial increase in the uniformity of the slag shell.

[0054] The method of this invention effectively avoids the slag pool fluctuations caused by the repeated up-and-down movement of the electrodes during traditional voltage swing. The electrodes only move downward at different rates, which also simplifies the relationship between the crystallizer lifting speed and the electrode insertion speed. It is more suitable for crystallizer-type electroslag furnaces, which helps to maintain a stable dripping speed of molten droplets and obtain a slag shell with uniform thickness and dense structure.

[0055] The method of this invention is more conducive to maintaining a constant melting rate, which has a decisive influence on the formation and quality of the slag shell. A constant melting rate, by stabilizing the heat input and solidification conditions of the molten pool, ensures that the slag solidifies at a uniform rate on the inner wall of the crystallizer, thereby forming a slag shell of uniform thickness and dense structure. This uniformity not only optimizes the heat transfer efficiency from the molten pool to the cooling system, reducing the internal temperature gradient and defects (such as shrinkage cavities and cracks) of the steel ingot, but also effectively isolates air, preventing metal oxidation and reducing the risk of gas entrapment. Furthermore, a stable slag shell can buffer the thermal shock of the molten metal to the crystallizer, extending equipment life, avoiding slag shell cracking or slag leakage accidents caused by melting rate fluctuations, and significantly improving process safety and product quality consistency.

Claims

1. A method for controlling the slag thickness in special steel smelting using a crystallizer-type electroslag remelting furnace, characterized in that, Includes the following steps: 1) In the arc initiation and slag formation stage, full slag arc initiation is adopted, constant current and voltage are used to form a constant melting rate; 2) During the smelting stage, the melting rate is maintained constant by using voltage swing. During the constant melting rate stage, the system sets a sinusoidal periodic function to specify the rate of change of the electrode insertion speed, so that the electrode moves downward at a constant speed relative to the slag pool. The system detects the voltage fluctuation value in real time, i.e. the voltage swing signal Vswing. The voltage swing signal Vswing is compared with the set value Vsetpoint to further determine whether the safe time interval is met, thereby driving the increase or decrease of the electrode speed to stabilize the slag pool voltage. The comparison between the voltage swing signal Vswing and the setpoint Vsetpoint is described below: If the voltage swing signal Vswing-set value Vsetpoint > the judgment value, and the time interval since the last control action > the safety time interval, then increase the electrode speed; If the voltage swing signal Vswing-set value Vsetpoint ≤ judgment value, and the interval of the last control action > safety interval, then reduce the electrode speed; If the voltage swing signal Vswing does not exceed the limit or the safe time interval is insufficient, the electrode will not operate; The relationship between the melting rate and the slag shell satisfies: （1） In equation (1), H 渣壳 and v 熔速 They are inversely proportional; that is, the faster the melting rate, the thinner the slag shell; the slower the melting rate, the thicker the slag shell. The constant melting rate stage is the stage that occurs after the arc initiation and slag formation stage, when all the solid slag has melted and the consumable electrode gradually forms molten droplets. At this point, the smelting process enters the constant melting rate stage.

2. The method for controlling the slag thickness in special steel smelting using a crystallizer-type electroslag remelting furnace according to claim 1, characterized in that, The determination value is 5% to 10% of the set value Vsetpoint.

3. The method for controlling the slag thickness in special steel smelting using a crystallizer-type electroslag remelting furnace according to claim 1, characterized in that, The aforementioned safety time interval is at least one half of a sine cycle.

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