Analysis on transmission behavior characteristics and key technologies in high-speed continuous casting process

High-speed continuous casting is the prerequisite for achieving direct rolling and casting rolling, the concrete embodiment of achieving efficient and green steel production processes, and the theme of developing a new generation of efficient continuous casting.

The biggest challenge facing high-speed continuous casting is frequent steel leakage and cracks. The uniform growth control of the solidified billet shell should be the core connotation of its realization. The focus is on the integration of crystallizer technology, including crystallizer flow control, protective slag type, crystallizer lubrication, crystallizer cavity structure, crystallizer cooling structure and system, etc. The key lies in the stable control of the crystallizer liquid level. The transmission behavior characteristics of the high-speed continuous casting process and the key technologies for its realization are explained.

The first prerequisite for high-speed continuous casting is safe and smooth operation. Therefore, the safe thickness of the bottom shell of the crystallizer is the most important factor limiting the increase in continuous casting speed.As the pulling speed increases, the solidification time of the molten steel in the crystallizer is shortened, and the solidified shell becomes thinner. Studies have shown that when the pulling speed increases by 10%, the thickness of the shell out of the crystallizer decreases by about 5%. This also makes the adverse factors affecting solidification heat transfer more apparent. With the increase in pulling speed, the heat flux increases, the consumption of protective slag decreases, the lubrication between the solidified shell and the copper wall of the crystallizer becomes worse and worse, and the ability of the high-temperature solidified shell to withstand various stresses and strains becomes weaker and weaker. Steel leakage and cracks become the biggest challenges.Therefore, how to ensure the uniformity and safety of the solidified shell in the crystallizer under high pulling speed conditions and the quality of the ingot is a technical problem that must be faced and solved to achieve high-speed continuous casting.Starting from the analysis of the transmission behavior characteristics of the high-speed continuous casting process, this paper explains the key technologies such as mold flow control, efficient heat transfer crystallizer, ingot bulging and mold liquid level control, and provides guidance for the development and production of high-speed continuous casting technology.

Flow characteristics and control of high casting speed mold

1.1  Crystallizer flow characteristics

Under high drawing speed conditions, the flow stream discharged from the side hole of the submerged nozzle impacts the narrow surface of the crystallizer at high speed, causing the solidified billet shell to remelt and thin. The upward reverse flow stream intensifies the fluctuation of the molten steel level, which will cause the molten steel to absorb air and slag, and the downward flow stream brings inclusions and bubbles into the deep liquid phase hole (Figure 1).

The effect of pulling speed on the liquid surface behavior in the crystallizer is shown in Figure 2. Through physical simulation, it is found that for every 0.20 m/min increase in pulling speed, the liquid surface velocity near the meniscus increases by about 0.01 m/s, the fluctuation increases by about 0.05 mm (Figure 2 (a), (b)), and the liquid surface vortex frequency increases by 1 to 2 min-1 (Figure 2 (c)). When the pulling speed is increased to 2.0 m/min, the vortex frequency is as high as 12 min-1; when the depth of the submerged nozzle is increased from 117 to 100 mm, the liquid surface velocity increases by 17%, and the fluctuation of the meniscus increases by 29% (Figure 2 (b) pulling speed 1.8 m/min). It can be seen that with the increase in pulling speed, the crystallizer liquid surface develops in an unfavorable direction in terms of flow rate and calmness, and it is very necessary to implement flow control.

The effect of mold slag viscosity on the slag interface velocity in the crystallizer is shown in Figure 3. As can be seen from Figure 3, the maximum interface velocity and meniscus velocity at high viscosity are reduced from 0.203 m/s (point a) and 0.152 m/s (point c) to 0.143 m/s (point b) and 0.112 m/s (point d) at low viscosity. Therefore, high viscosity mold slag should be an effective means to prevent the occurrence of mold slag, but its important role in lubrication and heat transfer must also be considered comprehensively.

1.2  Crystallizer electromagnetic flow control

Electromagnetic technology has become an important means of mold flow control, such as M-EMS with bilateral traveling wave magnetic field, EMBr with local magnetic field, LMF with full-width one-segment magnetic field, FC-Mold with full-width two-segment magnetic field, EMLA of electromagnetic horizontal accelerator, EMLS of electromagnetic horizontal stabilizer, etc.

The primary purpose of electromagnetic flow control in the mold is to stabilize its liquid surface, while taking into account the flow velocity near the meniscus to improve its heat transfer. The flow field distribution on the central symmetry plane of the slab mold (FC-Mold) under different flow control conditions when the pulling speed is 1.8 m/min is shown in Figure 4, and the velocity distribution on the center line of the mold width direction at 20 mm below the free liquid surface is shown in Figure 5. After applying electromagnetic braking, the flow rate of the molten steel decreases as a whole, especially the speed at the liquid surface and the meniscus becomes gentle, but under the dual effects of electromagnetic braking and argon blowing, the liquid surface of the mold appears a wavy flow state, and the maximum speed appears in the vortex area near the water inlet, increasing from 0.02 when only electromagnetic braking is used to 0.12 m/s.

The horizontal velocity distribution along the center line of the mold width direction at 20 mm below the free liquid surface at different pulling speeds under the action of electromagnetic braking and argon blowing is shown in Figure 6.

As can be seen from the figure, when the pulling speed increases from 1.5 to 2.4 m/min, the maximum horizontal vortex velocity of the liquid surface decreases from 0.14 to 0.03 m/s. It can be seen that electromagnetic braking can only play its positive role at higher pulling speeds.

Research by Nippon Steel in Japan shows that single-circulation flow in the mold will lead to many defects in the product, while balanced development of double-circulation flow is the key to improving the quality of slabs and thin plate coils. At present, computational fluid dynamics (CFD) has developed rapidly. CFD knowledge and electromagnetic theory can be used to establish a computational model, and real-time adjustment and reasonable regulation can be carried out according to the size of the billet, pulling speed, nozzle parameters and argon gas volume, so as to transform the unstable and single-circulation steel flow into a stable and optimized double-circulation flow to ensure the stability of the free liquid surface and prevent the occurrence of slag rolls.

From the perspective of on-site control, it is best to have an index that comprehensively reflects the flow characteristics of the high-speed continuous casting mold.

Through laboratory tests and field results analysis, it is believed that the optimal F value is 1.7~3.0. Through numerical simulation research, it is found that under the combined action of electromagnetic braking and argon blowing, the F value shows a linear increasing relationship with the pulling speed, argon blowing amount, and electromagnetic coil current intensity. The F value can effectively reflect the fluctuation of the steel/slag interface near the meniscus. Controlling the reasonable variation range of the F value is more practical for avoiding the violent fluctuation of the steel/slag interface in the crystallizer and the occurrence of slag rolls. The flow control with the calmness of the steel/slag interface as the main goal in the high-pulling speed crystallizer can be converted into the control of the F value. The online control of the F value can be achieved by real-time monitoring of the entire liquid level fluctuation of the crystallizer.

High casting speed and high efficiency heat transfer crystallizer

As the casting speed increases, although the heat flux in the crystallizer increases (Figure 7), the solidification time of the molten steel and the consumption of protective slag decrease, resulting in a thinning of the shell thickness out of the crystallizer, increased unevenness, and increasing risks of leakage and cracks.

Therefore, in order to achieve high-speed continuous casting, the uniformity and safety of the solidified shell in the crystallizer must be solved, and the fundamental solution is to solve the problem of efficient heat transfer in the crystallizer under high casting speed conditions.

2.1 Safe thickness of crystallizer

At present, the quantitative determination of the safe thickness δme of the billet shell at the outlet of the crystallizer is still basically based on traditional experience or common sense, that is, the square billet with a small section is not less than 8 mm, and the slab and square billet with a large section is not less than 15 mm. However, the latest research by Ito Y et al. shows that the billet shell thickness of the high-speed slab out of the crystallizer can be in the range of 10~15 mm.

Therefore, it is of practical significance to accurately quantify δme for different sections and steel grades. Recently, QIAN Hai-tao et al. studied the repair mechanism of bonding leakage and proposed an expression for determining the safe thickness of the billet shell through the force analysis of the billet shell out of the crystallizer.

Obviously, the size of δme is mainly determined by the tensile strength of the cast steel at this time. If its strength can be improved, δme can be reduced accordingly. The yield strength of steel decreases with increasing temperature. Therefore, in order to achieve a higher drawing speed, reducing the temperature of the solidified shell of the cast steel when it leaves the crystallizer should be one of the effective ways.

To this end, we can start from two aspects. One is to improve the heat transfer efficiency of the crystallizer, especially the heat transfer at the corners, to solve the problem of uneven growth of the shell; the other is to implement rapid cooling of the shell leaving the crystallizer.

The occurrence of steel leakage and cracks often originates from the weakest part of the shell. If the uniform growth of the shell in the crystallizer can be better controlled, the so-called safe thickness can be effectively reduced, and the increase in the pulling speed will have a prerequisite guarantee.

The key to controlling the uniform growth of the shell in the crystallizer lies in the uniform heat transfer between the shell and the copper plate in the crystallizer, which mainly depends on the interfacial thermal resistance, involving the state and thickness distribution of the protective slag, the distribution of the air gap between the shell and the protective slag caused by the solidification shrinkage of the shell, the cooling structure and cooling system of the crystallizer, etc. The influence of the performance and behavior of the protective slag on the interfacial heat transfer is obvious, but from the perspective of the uniformity of the shell growth, the first consideration should be the shape and structure of the inner wall of the crystallizer copper plate.

Voestalpine, Concast, Daline, etc. process the inner wall of the crystallizer into parabolas, diamonds and other types, the purpose of which is to reduce the air gap, improve heat transfer, promote the uniform growth of the shell, and reduce surface thermal cracks. Therefore, to achieve high-speed continuous casting, the reasonable design of the inner cavity structure of the crystallizer is particularly important, which is the basis for ensuring efficient heat transfer in the crystallizer.

The starting point of the design of the shape and structure of the inner cavity of the crystallizer is to ensure that the inner wall of the copper plate of the crystallizer can highly cater to the shrinkage and growth characteristics of the solidified shell during the casting process. This requires first solving the problem of quantitative description of the extremely complex solidification heat transfer in the crystallizer.

The author established a thermal-mechanical coupling finite element model of the shell-crystallizer system that takes into account the solute micro-segregation, the dynamic distribution of the protective slag and the air gap, and the high-temperature creep behavior of the shell. The dynamic distribution behavior of the protective slag film and the air gap between the solidified shell and the copper wall of the crystallizer during the solidification of peritectic steel was quantitatively described and revealed. Based on this, a new curved crystallizer with efficient heat transfer was developed (Figure 9). The characteristics of the inner cavity structure of the crystallizer are “fast compensation in the upper part, slow compensation in the middle and lower parts, and multiple compensation in the corners”, which highly caters to the growth and shrinkage of the solidified shell.

Compared with the traditional flat crystallizer, it can be found that the distribution of air gap and slag layer in the new curved crystallizer has changed significantly. The air gap and protective slag film accumulation in the corners are basically eliminated (Figure 10), and the cooling rate in the middle and lower parts of the crystallizer is increased from the traditional 3.5 to more than 10 ℃/s. The shell temperature out of the crystallizer is significantly reduced (Figure 11), and the shell growth in the corner area of ​​the crystallizer is more uniform (Figure 12)

Crystallizer solidification coefficient

For high-speed continuous casting, in addition to the reasonable design and manufacture of the shape and structure of the inner cavity of the crystallizer, a very important task is to fully utilize and give full play to the lubrication and heat transfer effects of the protective slag.

As the pulling speed increases, the lubrication between the solidified billet shell and the copper wall of the crystallizer becomes less and less ideal. The structure and thickness of the protective slag film are directly related to the lubrication state and heat transfer between them, and have an extremely important impact on the growth of the billet shell.Therefore, the liquid protective slag on the surface of the crystallizer steel liquid should be allowed to flow evenly into the gap between the copper wall of the crystallizer and the billet shell (slag channel) to obtain good heat transfer and a decrease in the surface temperature of the billet shell. A large number of studies and practices have shown that the viscosity of the protective slag has the greatest impact on the longitudinal cracks of the ingot. The protective slag with too high viscosity is difficult to flow into the gap between the meniscus of the molten steel and the copper wall of the crystallizer, thereby increasing the longitudinal cracks.Increasing the basicity of the protective slag can reduce the viscosity, but it will increase the solidification temperature and crystallization temperature of the slag and affect the lubrication. For this reason, Li2O with a mass fraction of 2% to 5% or MgO and BaO are used to replace part of the CaO in the slag. In order to reduce costs, protective slag without Li2O is also being developed in China. Too high or too low viscosity of the liquid slag will cause uneven thickness of the slag film on the surface of the billet shell, resulting in poor lubrication and heat transfer.

For this reason, the protective slag should maintain an appropriate viscosity value, depending on the type of steel, section, drawing speed, and casting temperature. Ogibayashi S et al. believe that when the product of viscosity and drawing speed (η·Vc) is controlled at 0.10-0.40 (Pa·s·m)/min, the change in the thickness of the protective slag film and the fluctuation of heat flow are minimal.

The Km values ​​of two protective slags under different drawing speed conditions are shown in Figure 13. It can be seen that the performance of the protective slag does have a great influence on the Km value and solidification uniformity in the crystallizer. Therefore, in order to achieve high-drawing speed continuous casting, the change of the solidification coefficient Km is indispensable, and it is urgent to develop a continuous casting protective slag that meets the requirements of high drawing speed.

Mold lubrication

Lubrication between the solidified billet shell and the copper plate is the key to ensuring smooth continuous casting and billet quality, and is also the key to achieving high-speed continuous casting. The better the lubrication, the smaller the friction between the solidified billet shell and the copper wall of the crystallizer, otherwise it will cause serious problems such as cracks and bonding leakage.

Obviously, the friction force increases with the increase of the pulling speed and the viscosity of the protective slag and the decrease of the thickness of the liquid slag film, which is directly related to the consumption of the protective slag. It can be seen from Figures 14 and 15 that the consumption of protective slag decreases with the increase of the pulling speed, so that the thickness of the slag film becomes thinner and the lubrication of the crystallizer becomes worse. Under high pulling speed conditions, the slag consumption of traditional slabs is generally not less than 0.30 kg/m2 (Figure 14), while the slag consumption of thin slabs at a pulling speed of 6 m/min is about 0.10 kg/m2 (Figure 15).

The consumption of mold slag is closely related to its performance, casting process, mold vibration, and mold section. The consumption decreases as the viscosity and melting temperature of the mold slag increase; the consumption decreases as the casting speed increases and the superheat decreases; the consumption of mold slag increases as the positive slip time increases, and the consumption of non-sinusoidal vibration is greater than that of sinusoidal vibration.

Crystallizer vibration has become a very effective means to improve the lubrication state of the crystallizer. The characteristics of non-sinusoidal vibration are reflected in: gentle vibration, significantly reducing the tensile stress on the surface of the billet shell; increased positive slip time, increased protection slag consumption; appropriate negative slip time, easy to reduce the depth of vibration marks; a certain amount of negative slip, which is conducive to the demolding of the continuous casting billet and healing of the torn billet shell.Since the first use of non-sinusoidal vibration in the Fukuyama plant of NKK in Japan in 1984, this technology has become an indispensable process technology for high-speed continuous casting machines. At present, research is being carried out around non-sinusoidal vibration waveforms involving negative slip time, positive slip time, negative slip amount, positive slip relative speed difference, non-sinusoidal vibration factors, etc., as well as microscopic interface behaviors such as lubrication and friction behavior during vibration and slag channel pressure, in order to further improve the contact state between the copper plate of the crystallizer and the billet shell surface and meet the development requirements of high-speed continuous casting.

High-speed casting mold liquid level control

Causes of crystallizer liquid level fluctuation

During the continuous casting process, the change of the mold level not only affects the slag mixing, but also has an extremely important impact on the lubrication and heat transfer behavior of the mold slag, which directly affects the uniformity of the solidified shell growth and the smoothness of the continuous casting production. It can be seen that to achieve high-speed continuous casting, the stability control of the mold level is crucial.

The process of crystallizer liquid level fluctuation is extremely complex and should be a resonant process. There are many influencing factors involved. In addition to factors such as unstable injection speed, water outlet deviation, crystallizer turbulence, etc., a very important reason is the bulging of the ingot (Figure 16). This phenomenon is more prominent during the continuous casting of peritectic steel, which directly limits the increase of the pulling speed. At present, most of the domestic speeds are 1.2~1.3 m/min, and the highest pulling speed of Nippon Steel is 1.7 m/min.When the peritectic reaction occurs, the primary shell in the crystallizer shrinks irregularly in the meniscus and corner areas, resulting in uneven growth. Under the action of the static pressure of the molten steel in the secondary cooling zone, the shell bulges, causing the crystallizer liquid level to drop. When the bulging shell reaches the next guide roller, it is squeezed back and the liquid level rises again. This repetition forms the crystallizer liquid level fluctuation.

Studies have shown that the frequency of liquid level fluctuation caused by bulging due to uneven growth of the peritectic phase transformation shell is 0.06~0.15 Hz. Su Zhijian et al. believe that the main frequency of abnormal liquid level fluctuation in the crystallizer is about 0.3 Hz, and the change of solidification shrinkage coefficient leads to changes in the bulging frequency of different peritectic steels.Studies have found that the increase in the fluctuation of the pulling speed, the increase in the amount of non-steady-state bulging, and the increase in the amount of liquid level fluctuation, the main frequency of interference caused by bulging is linearly related to the pulling speed. Therefore, the root cause of the crystallizer liquid level fluctuation in the continuous casting process lies in the bulging caused by the uneven growth of the solidified shell in the crystallizer, which will be more prominent during high-speed continuous casting. To control the stability of the liquid level, the first task is to promote the uniform growth of the initial solidified shell, and the most targeted and effective control should be the bulging control of the casting.

Crystallizer level stability control

The amount of bulging of the billet is closely related to factors such as the static pressure of the molten steel, the distance between the nip rolls, and the cooling intensity. The amount of bulging increases with the fourth power of the distance between the rolls and decreases with the third power of the billet shell thickness.

Changing the arrangement of rollers to change the periodicity of billet shell extrusion reduces the generation of non-steady-state bulging. For example, in the first few stages of secondary cooling, the roller pitches in the same sector vary greatly, or the roller pitches in the same sector are similar but the adjacent roller pitches vary greatly. At any time, the billet within a certain length is in an asynchronous state of partial bulging and partial depression, resulting in the superposition and mutual offset effect of the changes in different cycles, so that the liquid level of the crystallizer remains basically stable.

Increase the cooling intensity of the secondary cooling zone to reduce the temperature of the billet shell, increase its thickness and high-temperature strength.

Aiming at the control of transverse cracks in the corners of micro-alloy steel continuous casting, the author cooperated with many domestic companies to develop a continuous casting machine foot roller and a secondary cooling strong spray structure in the vertical zone (Figure 17) to achieve ultra-strong cooling of the corners of the billet (cooling rate greater than 15 ℃/s), forming a “γ→α→γ cyclic phase transformation” grain ultra-fine cooling control technology for the corners of the billet in the high-temperature zone of the secondary cooling of continuous casting (Figure 18), thereby enhancing the plasticity of the solidified billet shell and improving its crack resistance.

This idea is also suitable for the control of billet bulging during high-speed continuous casting, and is a very effective technical means to deal with non-steady-state bulging. In the actual implementation process, in order to coordinate the heat transfer between the wide and narrow sides of the ingot, the cooling strength of the wide side full roller, the second and third zones of the wide side should be greatly enhanced at the same time, which is helpful to strengthen the shell strength of the crystallizer and reduce the shell’s anti-deformation bulging ability in the vertical zone of the bending section. After the bending point of the bending section, the water volume of the fourth zone of the wide side is appropriately reduced to meet the temperature recovery requirements.

In general, the water volume of the bending section increases by 30%~35%, which is conducive to strengthening the shell, reducing the bulging between the rollers, and stabilizing the liquid level of the crystallizer.

In addition to the fundamental control of the bulging of the ingot, the stable control of the crystallizer liquid level also needs to consider the actual conditions of the transient, nonlinear, and multi-interference of the liquid level fluctuation. Therefore, to truly achieve stable control, an important task is to implement effective monitoring.

The main methods for monitoring the liquid level of the crystallizer include thermocouple method, isotope method, eddy current method, industrial television photography method, light wave measurement method and electromagnetic method.

The electromagnetic method for measuring the molten steel level mainly uses the principle of electromagnetic induction. The transmitting coil and the receiving coil are placed side by side in the electromagnetic box and installed on the top of the crystallizer copper plate. The transmitting coil generates a high-frequency electromagnetic field and induces eddy currents on the surface of the copper plate and near the surface of the molten steel. The distribution of the eddy current is affected by the height of the molten steel liquid level. The potential generated by the eddy current in the receiving coil changes with the height of the molten steel liquid level, thereby determining the position of the molten steel liquid level.

Compared with other detection devices, this method has many advantages in terms of installation method and detection performance, but the detection results are affected by the temperature of the crystallizer copper plate and the surface coating.

In recent years, fuzzy control has been widely used in liquid level control. Many European steel mills have adopted fuzzy control, and its control effect is good, which reduces the increase of the molten steel level in the crystallizer by about 40%, shortens the time of liquid level interference by about 80%, improves the safety of mechanical equipment, and reduces the situation of steel change due to flow interruption by about 80%.

The expert system used by Japan JEF can judge the status, causes and parameters of the entire operation process based on the data collected by various sensors, such as pouring speed, molten steel level in the crystallizer, the status of the sliding gate, and the quality of molten steel in the tundish, and formulate countermeasures and issue adjustment instructions in time to optimize the PID control parameters. The fluctuation range of the steel liquid level in the double-stream round billet continuous casting crystallizer is only 4~5 mm; the gain-scheduled H∞ control system is used on the slab continuous casting machine. Compared with the conventional PID control system, the amplitude of the crystallizer liquid level is reduced by more than 30%.

By adopting the algorithm of high-frequency jitter compensation theory, a high-frequency jitter signal is introduced into the control signal. Its amplitude and period can be automatically adjusted to cause the liquid level actuator to jitter, so as to overcome and eliminate the non-steady nonlinear changes caused by the friction and dead zone characteristics of the hydraulic actuator, such as the stopper rod or the sliding nozzle, so that the molten steel level deviation is controlled within the minimum range to meet the process requirements.

The control of the crystallizer liquid level can be considered as the core of the entire continuous casting process control, but the current control is basically focused on how to monitor the liquid level online with high precision and the subsequent rapid response control, which is a typical “afterthought”.

Recently, Tian Li et al. found that the crystallizer liquid level fluctuation is coupled with the torque of a certain driving roller. By analyzing the torque of the straightening roller or the spectrum of the current curve, the characteristic curve and the liquid level phase deviation are obtained, and the stopper compensation amount is obtained to make advance compensation and reduce the periodic fluctuation of the liquid level.

Therefore, to achieve high-speed continuous casting, the intelligent control of the crystallizer liquid level is particularly critical. To achieve “foreknowledge and foreknowledge”, it is necessary to organically combine the metallurgical knowledge database of the continuous casting process, such as the billet bulging, crystallizer flow field, and peritectic phase transformation, with the industrial big data such as steel grade composition, drawing speed, stopper opening, and roll gap, and implement control through AI intelligent prediction, prejudgment of the liquid level compensation amount and concurrent instructions, so as to achieve “foreknowledge and foreknowledge”. The intelligent control block diagram of the crystallizer liquid level is shown in Figure 19, thereby truly achieving stable control of the crystallizer liquid level and breakthroughs in drawing speed.

High-speed continuous casting reflects the requirements for efficient and green development of the steel manufacturing process and is the core connotation of the development of a new generation of efficient continuous casting technology.

The main factor limiting the continuous casting speed is the safe thickness of the solidified shell of the mold. The core focus of achieving high-speed continuous casting is the uniform growth control of the solidified shell in the mold, which involves mold flow, protective slag type, lubrication, inner cavity shape, cooling structure and system. The key is the stable control of the mold liquid level. The focus is on the effective control of the bulging of the billet through processes and equipment such as roller arrays and secondary cooling. Combined with the metallurgical knowledge database of billet bulging, mold flow field, peritectic phase transformation, and industrial big data such as steel composition, drawing speed, stopper opening, and roll gap, through AI intelligent prediction, the “foresight” control of stable liquid level is achieved.