Earticle

In this study, the molten steel flow inside the mold according to the shape change of the submerged entry nozzle (SEN) was simulated in the continuous casting process, and the effects of the immersion depth and long side length of the mold on the molten steel flow were compared and analyzed. The heat flow analysis was performed using the ANSYS Fluent software, and the turbulence model used the standard k-ε model. Based on the analysis results, the meniscus velocity decreased with deeper immersion depth, but it was slightly faster with lower immersion depth. In addition, the flow velocity of the molten steel of the outlet was increased because the molten steel exiting the upper outlet spread directly without colliding with the short side of the mold when the long side of the mold length was increased.

본 연구는 연속주조 설비에서 턴디쉬 내의 용강이 유출구를 통해 유출될 때 발생되는 와류 유동에 의해 슬래그가 침지 노즐(SEN) 내로 혼입되는 것을 억제하기 위해 턴디쉬 하부의 소정의 위치에 장착하는 내화 조립체에 관한 발명의 사상을 형상 설계로 구체화한 것이다. 이 내화 조립체는 턴디쉬에서 유출되는 용강이 유입되는 유입구와 침지 노즐이 부착되는 유출구가 서로 대향되게 형성되어 있는 하우징과 이 하우징을 경사방향으로 관통하여 미끄럼 이동하는 다수의 구멍이 형성되어 있는 슬라이드 부재로 구성되어, 그 내부 구조는 전체적으로 S자형 용강 유동경로가 형성되도록 되어 있고 그 외부 형상은 턴디쉬 하부의 소정의 위치에서 그 상부 일부가 턴디쉬 용탕 내로 매입되는 상태에서 턴디쉬 내부와 외부 사이에 장착시켜 턴디쉬 내의 용강 유출 시에 어떠한 규모의 와류도 발생하지 않도록 한 것을 주요 특징으로 한다. 이에 따라, 침지 노즐로 유입되는 용강 유량의 조절은 슬라이드 부재의 미끄럼 이동 제어나 종래의 스토퍼에 의해 달성되면서 슬래그의 혼입은 적극 억제하여 용강청정도 향상과 잔류용탕(잔탕)을 최소화하여 실수율을 향상시킬 수 있는 효과를 발휘한다.

This study is an embodiment of the spirit of the invention into a geometrical shape design, which is related to a refractory assembly for preventing carried-over slag by suppressing the vortex flows of the molten steel discharging from tundish in continuous casting process. The refractory assembly is characterized in comprising a housing having a inlet and a outlet, and a slide member having a plurality of holes; the inlet and oulet being opposedly disposed each other, the inlet for the molten steel discharging from a tundish, the outlet for connecting to a submerged entry nozzle (SEN), and the slide member for sliding with being inserted through the slits on the front and rear walls of the assembly; and the overall internal configuration of the assembly for defining a certain “S" shaped flow path for molten steel, the outer shape being suitable to installed snugly between the interior and exterior of the tundish, and the upper part of the assembly being embedded into the molten steel bath within the tundish.

연속주조 공정에서 턴디쉬는 불순물(슬래그)를 용강에서 부상 분리시켜 용강이 청정하게 주조되게 하는 데 필수적이다. 턴디쉬는 또한 레이들 교체 중에는 주조 공정이 중단되지 않도록 용강을 저장하는 용기 기능도 한다. 따라서 턴디쉬 내의 유동 제어 부재 (즉, 배플, 댐, 위어 등)의 배열과 그 배열이 용강의 유동 패턴에 미치는 효과를 이해하는 것이 고청정도 강을 달성하는 기본적이고도 본질적인 부분이다. 본 연구에서는 1/3 축소 턴디쉬 수모델로 유동 제어 부재의 4가지 배열을 시뮬레이션하여 각각에 상응하는 실제 턴디쉬에서의 용강 유동 패턴의 특징을 비교하였다.

The tundish is necessary to separate impurities (slag) from the molten steel and to keep the steel to be cast pure in continuous casting (CC) process. The tundish also serves as a storage vessel for the molten steel during a ladle change in order not interrupt the casting process. Therefore, understanding of the arrangement of the flow control devices (i.e. baffles, dams, weirs and so on) and thus its effect to the flow pattern of the molten steel within the tundish is basic and essential parts of accomplishing the highest cleanliness of the steel. In this study, a 1/3 reduced tundish water model with four arrangements of flow control devices within the tundish was used to simulate and compare the characteristics of corresponding molten steel flow behaviors in prototype.

In order to obtain semi-solid aluminum alloy billets of high quality, an investigation was carried out of imposing multiple magnetic fields from the outside of a cold-crucible copper mold in a continuous casting process. AlSi6Mg2 (wt.%) alloy designed for a semi-solid metal (SSM) process was continuously cast through a submerged entry nozzle under various conditions. By means of optimizing the distribution of multiple magnetic fields in the mold, the effect of multiple magnetic fields on both the meniscus motion and billet quality was examined. The experimental results showed that meniscus disturbance could be controlled efficiently and the surface quality of a semi-solid aluminum alloy billet was improved greatly by imposing multiple magnetic fields, and also a uniformly fine, globular microstructure across the transverse section of the billet was achieved by optimizing the distribution of the multiple magnetic field.

Mould level control system for continuous casting process involves stick-slip friction in the sliding gate , time-delay, non-linearity, and certain uncertainties such as friction force variations between molten steel and the inner wall of mould. In this paper, sliding mode control technique was used to solve these complex control problem. The controller is then designed and implemented onto the continuous casting process. Testing result shows that sliding mode controller can decrease the fluctuating magnitude of the mould level and is superior to the existing PID controller.

To develop the hydraulic mold oscillator in continuous casting machine, the guiding mechanism of mold was studied. The main topics of this study were to design the guiding mechanism of mold which oscillates to prevent the sticking and to reduce the friction resistant force between the solidified shell and mold on casting. We studied many guiding types to analyze the features of worldwide mold oscillator and developed the new model of hydraulic mold oscillator. On the basis of the mold oscillating experiment, the capability of guiding system was proofed by the position error measuring system. The experiment was carried out up to $50{\sim}500cpm$ frequencies and $2{\sim}10mm$ stroke in the variable waveform and the casting results was analyzed by the oscillation mark of slab surface which was formed unavoidably by oscillation.

Thermal shock behavior of porous ceramic nozzles with various pore sizes for continuous casting process of steel was investigated in terms of physical properties and microstucture. Porous nozzle samples with a composition of $Al_2O_3$-$SiO_2$-$ZrO_2$ were fabricatedby adding various sizes of graphite as the pore forming agent. As the graphite size increased from 45~75 to 150~180 ${\mu}m$, both the resulting pore size and the flexural strength also increased. A thermal shock test was carried out at temperatures (${\Delta}$T) of 600, 700, 800, and 900$^{\circ}C$. Microstructure analysis revealed a small number of cracks on the sample with the largest mean pore size of 22.32 ${\mu}m$. In addition, increasing the pore size led to a smaller decrease in both pressure drop and elastic modulus. In conclusion, controlling the pore size can enhance thermal shock behavior.

연속주조공정에서 용강의 통로, 산화방지 및 유체 흐름을 용이하게 하는 역할을 하는 다공성 노즐(porous nozzle)은 용강과의 직접적인 접촉으로 인한 화학 반응 및 용강의 침투현상을 방지하기 위해 불활성 가스를 주입하여 청정강을 제조하는데 이용된다. 공정 중 노즐 막힘으로 인한 배압상승과 열충격에 의한 크랙(crack) 발생이 문제되고 있으며 신뢰성 향상 연구가 요구되고 있다. 따라서 본 연구에서는 기공크기와 기공분포가 고온안정성 및 내열충격성에 미치는 영향을 알아보고, 내구성 시험 및 고장분석을 통하여 노즐의 신뢰성 향상 방안을 고찰 하였다. 기공을 제어한 시편을 제조하여 기공분포에 따른 고온안정성을 확인하기 위해 실제 사용 조건인 용강온도($1550^{\circ}C$)와 보다 높은 온도($1700^{\circ}C$)에서 각각 고온 시험을 수행하였다. 열충격을 스트레스 인자로 한 내구성 시험을 수행한 후 고장원인을 분석하였으며 열화정도를 확인하기 위해 열처리 온도에 따른 차압 및 굽힘 강도 변화를 비교하였다. 또한 결정상 분석을 통해 온도에 대한 상변화를 확인하였고, 시편의 표면 및 파단면의 미세구조 분석을 통해 크랙 발생여부를 확인하였다. 다공성 노즐의 기공분포가 균일 할수록 고온안정성 및 내열충격성이 향상됨을 확인하였고, 이를 통해 Porous Nozzle의 열화원인으로 판단되는 기공 크기 및 분포에 따른 크랙 발생에 대해 열응력 고찰을 수행하였다.

There are some methods that have been used to manage a degasing process in recent years, such as an injection method that uses aluminum molten metal powder and chemicals supplier and input method that supplies argon and nitrogen, or chlorine gas by using a gas blow-tube. However, these methods show some problems, and it shows that it is a difficult process to handle, pollution due to producing a lot of toxic gases like chlorine and fluoride gas, irregular effects, and lowering work efficiency due to the excessive processing time. The problems that are the most fatal are the producing a lot of sludge due to the reaction of aluminum molten metal with chemicals, loss of metals, and decreasing the life of refractory materials. In order to solve these problems, this paper develops a technology that is related to aluminum continuous casting molten metal and monolithic degasing apparatus. A degasing apparatus developed in this study improved the existing methods and prevented environmental pollution with smokeless, odorless, and harmlessness by using a new method that applies argon and nitrogen gas in which the methods used in the West and Japan are eliminated. The method developed in this study decreases the molten metal processing and settling time compared to the existing methods and improves the workers' health, safety, and environment because there is no pollution in processes.

Although continuous casting process has highly developed, there still remain many problems to be considered. Specifically, two vortex flows resulting from impingement against narrow walls make a flow field unstable in a mold, and it is directly related to internal and external defects of steel products. To cope with this instability, EMBR (Electromagnetic Brake Ruler) technique has been lately studied for the stability of molten steel flow, and it is revealed that molten steel flow in a mold can be controlled with applied magnetic field. However, it is still difficult to clarify flow pattern in an EMBR caster due to complex correlations among variables such as geometric factors, casting conditions, and the place and the intensity of charged magnetic field. In the present study, flow field in a mold is focused with different conditions of electromagnetic effect. To accurately analyze the case, three dimensional low Reynolds turbulent model and appropriate boundary conditions are chosen. To evaluate the electromagnetic effect in molten steel flow, dimensionless numbers are employed. The results show that the location and the intensity of the applied magnetic field significantly influence the flow pattern. Both impingement and internal flow pattern are changed remarkably with the change of the location of applied magnetic field. It turns out that an insufficient magnetic force yields adverse effect like channeling, and rather lowers the quality of steel product.

The effects of the solid solution and aging treatment on the microstructure, hardness and electrical conductivity of Cu alloys containing 0.7 wt% and 1.8 wt% Be were investigated. These Cu-Be alloys contain Co, Si, Ni and Mn as alloying elements. An horizontal continuous casting process was applied to obtain unidirectionally solidified billets. A solid solution treatment of the sample was carried out at 865 oC and aging treatments were performed in the range 270-550 oC. For the Cu-1.8 wt% Be alloys, the electrical conductivity generally increased as the aging temperature increased in the range of 270-370 oC. For the Cu-0.7 wt% Be alloys, the electrical conductivity increased as the aging time increased in the range of 10-180 min.

In order to investigate the variation of solidification structure due to the variation of heated model temperature and casting speed, the aluminum alloys which had the diameter to 6mm from 3mm and the composition of 5wt.%Si and 11.6wt.%Si have been fabricated by the horizontal continuous casting process which has a heated mold. As a result of this reasearch, the microstructures have been unidirectional structures parallel to the casting direction, and the secondary dendrite arm spacing and cell spacing decreased according as casting speed increased in all the compositions. In the eutectic composition, the dendrite structure developed in comparision with hypoeutectic composition. Also, it was confirmed that the mushy zone amplitude decreased with the increase of casting speed.

In order to improve the quality of the billet continuous casting, a two-phase orthogonal power supply (TPOPS) for electromagnetic stirrer is researched, which is composed of three-phase PWM rectifier and three-leg inverter. According to the power analysis of system, the ripple of dc-link voltage is analyzed and its analytical expression is derived. In order to improve the performance of electromagnetic stirring, an integrated control method with feedforward control is proposed for PWM rectifier to suppress the fluctuations of dc-link voltage and provide a stable dc source for inverter. According to the simplified equivalent model, a composite current control method is proposed for inverter. This proposed method can combine the merits of feedforward control with feedback control to effectively improve the dynamic output performance of TPOPS. Finally, a 300kVA prototype of TPOPS is developed, and the results have verified the analysis and control method.

In the continuous casting process, mold level fluctuation Is the major cause of the surface and sub-surface defects. In the No.3 bloom continuous caster at Kobe Works, we ensured that the major cause of mold level fluctuation was mechanical hysteresis which existed in the driving system of mold level control. Moreover, we found out that it was possible to greatly Improve the stability of mold level by estimating this mechanical hysteresis and compensating It on-line. As a result of applying a new level control system based on this method, we got accurate control over good stability.

This paper gives a simulation study of a new fuzzy logic control(FLC) approach for the mold level control in continuous casting processes. The proposed FLC is PID type hybridizing the conventional fuzzy PI control and Fuzzy PD control with a simplified design scheme. It is shown that, compared with the conventional control, this new control strategy can achieve superior performance for steady-state response and is more robust against process parameter variations and disturbances.

Mold level control has been a major control task for continuous casting plants. The system involves nonlinearities such as stick-slip friction in the sliding gate, time-delay, friction force variations between molten steel and the inner wall of mold, and nozzle logging/unclogging. These complex problems should be solved to control mold level for steel cast. In this paper, we propose a neural fuzzy mold level control technique for solving these complex problems and give experiment studies to show the mold level control in continuous casting process.