Mold Level Research Articles

Re-start Technology for Reducing Sticking-type Breakout in Thin Slab Caster

In order to increase the production rates and to secure the base production technology for the hard casting materials, a re-start casting technology as an active curing methodology has been developed to reduce the number of sticking type breakouts in the thin slab caster with high speed. It consists of the healing casting conditions of breakouts, the mold level control method for preventing the overflow or big change of molten steel near meniscus and the control of cooling rate at second cooling zone for rolling under the limits of rolling force and torque at the directly linked roughing mill. By applying the proposed technology, the automatic restarting rate of breakout is remarkably increased from 0 to 92%.

Development and investigation of electroslag continuous casting

Abstract The principle and technological design of electroslag continuous casting (ESCC), including bifilar mode, T-shaped mould, and metal level detecting system, are detailed. Remelting was carried out for 1Cr18Ni9Ti stainless steel with ESCC. The surface finish, chemical composition, macro- and microstructures, and inclusions of the remelted billets were characterized. ESCC reduces the cost and increases the productivity in comparison to traditional ESR, while achieves comparably excellent products.

Slab quality improvement by controlling mould fluid flow

Cleanliness of steel is a primary requirement for flat products. It is obtained with minimum of defects by controlling the liquid flow characteristics in the mould and fluctuations over the meniscus surface. Liquid flow in the mould region is due to the momentum of the pouring stream which in turn is related to the clogging of submerged entry nozzle and argon flow in the mould. This makes control of liquid steel flow dynamics in the mould important. The mould level fluctuation index, flow fraction or clogging percentage and optimised gas flow models have been developed and are correlated for minimised surface fluctuations throughout the casting sequence. Tundish weight, casting speed, casting section and immersion depth of the nozzle which primarily change the flow profile inside the mould are the key operational variables considered for model calculations. The operational parameters were adjusted to follow the developed models criteria for different casting conditions. Online application of these operational control models contributed to stabilise the mould fluid flow and have helped in decision making for pumping, flushing and tube changing. The present paper describes the mathematical approach adopted in calculation of optimum casting parameters for controlling flow of liquid steel, nozzle clogging and gas injection rate at JSW Steel Ltd. This has resulted in considerable reduction in mould level fluctuations and production of superior quality slabs even at higher casting speeds.

FUZZY NEURON HYBRID CONTROL FOR CONTINUOUS STEEL CASTING

The fuzzy neuron hybrid control method is proposed for mold level control of the continuous steel casting in this paper. The dynamic characteristics of the continuous steel casting process is described. To the plant with big uncertainties and grave nonlinearities, the fuzzy neuron hybrid control system is set up and the new model-free controller is designed. In this control system, the fuzzy neuron hybrid controller is constructed by the fuzzy PI controller and the neuron controller, the gain of the neuron controller is tuned by using a fuzzy algorithm. The simulation tests of mold level control are made under various conditions with different casting speeds, new gate valve and old gate valve, changing in the valve position loop gain. The results demonstrate that the new controller has good performance, very strong robustness and adaptability.

MOULD LEVEL CONTROL FOR THE CONTINUOUS STEEL CASTING

Existing systems for the control of mould level of continuous steel casting in Czech Steelworks have had problems with the stability of the steel level position. A new control algorithm is designed on the basis of mathematical modelling and digital simulation. This should better satisfy the actual demand on the control quality. A PI controller with a variable gain and dither signal was used. During several months of practical tests and the experimental tuning of the controller, parameters were determined regarding the required shape time diagram of the steel level for all types of steels, resulting in very good slab surface quality.

連続鋳造機鋳型内湯面レベル周期性変動制御の自動調整系

連続鋳造機鋳型内湯面レベル周期性変動制御の自動調整系

Investigation of strand surface defects using mould instrumentation and modelling

The surfaces of continuously cast steel blooms exhibit a variety of surface features and defects, which were investigated to reveal the interactions at the meniscus between the steel shell and interfacial flux layers that caused them. One such defect formed at periodic intervals along the surface of first and second blooms in a sequence. It was characterised by gradually deepening oscillation marks, followed immediately by longitudinal striations or 'glaciation marks'. In severe cases, deep depressions were clearly visible within the glaciated region. These defects were investigated through plant trials and both physical and mathematical modelling. The defects were found to exhibit a characteristic temperature history: temperature troughs that move down the mould at the casting speed. These defects may be monitored in much the same way as sticker breakouts, thereby allowing existing thermocouple based breakout detection systems to be modified to include a quality alarm. This study attributes these defects to high amplitude, low frequency, mould level fluctuations. A mechanism is proposed which ascribes the generation of these defects to the interaction of the meniscus with the slag rim at peaks in the mould level cycle. Installing an improved mould level control system eliminated the defects.

연속 주조 공정에서 평균 토크전류 추종에 의한 인발력 분배 제어

연속 주조 공정에서 설정한 속도로 주편을 인발하는 스트랜드 구동롤에 인발력이 적절하게 분배가 되지 않으면 주형 레벨의 주기적인 변동을 유발시킴과 동시에 주편의 품질을 저하하게 된다. 따라서 본 논문에서는 주편의 속도를 제어함과 동시에 각 구동롤에 운전자가 원하는 비율로 인발력을 분배할 수 있는 알고리즘을 제안한다. 제안된 방법은 전동기에 작용하는 토크 전류를 상위 제어기로 귀환하여 평균 토크 전류를 산출하고 각 구동롤 전동기의 토크 전류와 비교하여 전동기의 속도 기준값을 미소하게 변경함으로써 각 구동롤 전동기의 토크 전류가 평균 토크 전류에 추종하도록 하여 인발력을 분배한다. 그리고 과도상태 개선을 위하여 안티와인더업(anti-wind up) 제어기를 가진 인발력 제어기를 적용하였으며, 제안된 알고리즘의 확인을 위하여 포스코의 시험연주기에 적용하여 우수성을 입증하였다.

Technologically indicative properties of straw fractions of flax, linseed ( Linum usitatissimum L.) and fibre hemp ( Cannabis sativa L.)

In this study of the behaviour of the fractions of unretted and frost-retted fibre straws in damp air, a production scale method to separate fibre and shive from fibre plants was introduced and tested on bast fibre plants ( Linum usitatissimum L. and Cannabis sativa L.). The method consists of optional drying of stalks, unloading bales, milling the straws with a hammer mill, separating the fractions from air stream with a cyclone and finally separating fibres from shives with a screening drum. Fractions were characterized focusing on technologically indicative properties such as equilibrium moisture content, ash and microbiological quality. Unretted fractions of the bast fibre plant stem reached higher equilibrium moisture contents than the retted fractions, and hemp fibres absorbed more moisture from air than did the Linum fibres. In very humid air, all fractions began to lose weight due to moulding. The weight decrease during the first week was lower in frost-retted than in unretted fractions. The frost-retted fractions appeared to be more resistant to humidity in the short term. The total number of microbes and especially the numbers of yeasts and moulds can be used as a criterion of hygienic level. For green fractions, the mould level was similar in fibres and in shives, but frost-retted shives contained more moulds than the unretted shives. The mould content of a fraction had no direct correlation with the moulding tendency of the fraction. The ash contents of fibres were somewhat higher than those of shives, due to a probable soil contamination. Ash content did not have significant correlation with microbiological quality, although ash is a possible risk factor for hygienic quality. According to the results of this study it is highly important to study the quality of the production chain of bast fibre plants to ensure the quality of industrial products. From the producer's point of view, raw material with defined quality can be directed to the most suitable application. The behaviour of fractions in various ambient atmospheres, and other quality aspects such as hygienic level can be used as criteria for defining the most appropriate product applications.

Mould Temperature Control in Continuous Casting for the Reduction of Surface Defects

Temperature control in the continuous casting mould can have considerable implications for the eradication of surface defects that arise in the process. This paper presents auto regression with external inputs as a modelling tool to describe the effect of casting speed, mould level and water inlet temperatures on the temperature distribution in the mould. The accuracy of the modelling strategy is determined. The model can be used to design a closed-loop controller to maintain the mould temperature at a predefined value. The defects caused by temperature are therefore controlled implicitly through mould temperature, and the delay caused by measurement of defects is thus eliminated. Three controllers are designed and compared through simulation. It is found that an optimal regulator type controller delivers the best performance among the three controllers. Control of only one loop and a switching type controller show marginal improvements over the uncontrolled case.

Revamping of mould level control equipments at the Dunkirk steelplant

Mould level variations, which are a significant cause of slab downgrading and production troubles at the continuous casters, can result from bad performances and low reliability of control equipments. A revamping project was realized at Dunkirk, after studies and prototype trials which enabled us to select the different components: stopper mechanism and actuating cylinder, level sensor, control electronics. A specific start-up organization, allowing for daily analysis of signals and results and frequent contacts with the engineering team, has permitted to reach quickly the targeted performances.

Investigations of Flow Conditions and Solidification in Continuous Casting Moulds by Advanced Simulation Techniques

Concerning further improvement of product quality in continuous casting, advanced simulation techniques are successfully used to investigate and optimise the complex flow and temperature conditions and the physical phenomena related to them. The work presented here focuses on the mould and discusses the corresponding application of selected mathematical and physical simulation techniques. Transient numerical simulations considering multiphase flow conditions and solidification with mould oscillation provide detailed information on the so-called solidified hook formation directly below the meniscus. Concerning mould powder performance, the numerical simulation approach is carried out considering the flux flow in the meniscus region in order to obtain information e.g. on the mould powder consumption. With physical model investigations performing simultaneous and time dependent measurements of flow field and mould level behaviour, an optimised SEN design for the thin slab casting could be developed, which allows significantly higher casting velocities and yield, respectively. Operational plant trials confirm these results.

High performance hybrid mold level controller for thin slab caster

This paper proposes a hybrid type controller that stabilizes the molten steel level in a reliable manner in a high speed thin slab caster despite various disturbances such as casting speed change, tundish weight variation, clogging/unclogging of submerged entry nozzle, periodic bulgings, etc. The proposed control terms include a mass flow compensation term, a proportional term, a differential term, and a fuzzy logic term. In addition, special filters are designed to eliminate high frequency noise and periodic disturbances. When applied to a commercial thin slab caster, the proposed controller showed good performance in minimizing the molten steel variation.

Some aspects of behaviour of calcium in silicon killed steels

Irregular flow can occur randomly in a casting sequence and mostly occurs in Si-Mn killed high C steels. Irregular flow increases reoxidation, mould level fluctuations and slag entrainment, all of which cause deterioration of the surface quality of the billets. The effect of calcium on irregular flow in billet casting of Si-Mn killed steels is discussed based on experience at two Corus steelplants, Scunthorpe and IJmuiden. Some theoretical aspects of calcium treatment are also discussed.

Investigations of Unsteady and Asymmetric Flow Phenomena in Continuous Casting Moulds by Advanced Simulation Techniques

For further improvement of the continuous casting process a more detailed insight into the unsteady flow conditions in the mould is of great importance. Due to the difficulties to obtain this information directly in an operational plant, simulation techniques can be successfully used to extend the relevant knowledge. Therefore, simultaneously measurements of both, unsteady flow velocity using particle‐image‐velocimetry and mould level fluctuations using ultrasonic sensors, were carried out in physical models providing correlation between casting parameters and asymmetric flow behaviour. The numerical simulation of unsteady multiphase flow conditions shows time‐dependent varying local mould level fluctuations indicating an asymmetrical distribution across the mould. Both simulation approaches provide important information for increase in process stability, productivity and product quality.

Q-パラメータアプローチによる連続鋳造機鋳型内湯面レベル周期性変動の制御

Q-パラメータアプローチによる連続鋳造機鋳型内湯面レベル周期性変動の制御

Neural Fuzzy Mold Level Control for Continuous Steel Casting

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.

Analysis of Mold Level Hunching by Unsteady Bulging during Thin Slab Casting.

In order to analyze the phenomenon of mold level hunching (MLH) during thin slab casting process, the variation of the mold level and the bulging of strand were measured and analyzed using Fast Fourier Transform (FFT) spectrum analysis. Both of mold level hunching and bulging had the same frequency and the specific frequency corresponded to the main roll pitch of the thin slab caster. The unsteady bulging was found to be a main reason of mold level hunching. The unsteady bulging profile through the full range of the caster and the height of mold level hunching were calculated using a heat transfer model and a continuous beam model. The effects of process variables on mold level hunching and the process condition of reducing mold level hunching were studied. The amount of mold level hunching increased as unsteady bulging increased due to the hunching of casting speed. An aperiodic roll pitch between segments could be effective in reducing mold level hunching.

Minimax Control of Discrete-Time Stochastic Systems

This paper gives a unified, self-contained presentation of minimax control problems for discrete-time stochastic systems on Borel spaces, with possibly unbounded costs. The main results include conditions for the existence of minimax strategies for finite-horizon problems and infinite-horizon discounted and undiscounted (average) cost criteria. The results are specialized to control systems with unknown disturbance distributions---also known as games against nature. Two examples illustrate the theory, one of them on the mold level control problem, which is a key problem in the steelmaking industry.

A Novel Control Scheme for Precise Positioning of Mechanical Positioning Systems

Utilizing the energy-dissipated characteristic of friction, this paper presents a pulse positioning energy model, which extends the application of pulse positioning to disturbance-unmatched systems. The corresponding pole-placement adaptive control algorithm is also given. Based on the energy model and a switch rule, the adaptive control and conventional proportion control are integrated to realize the whole stroke precise positioning of mechanical-positioning systems. Simulations based on the hydraulic-stopper positioning system in mold level control show that the energy model presented in this paper captures the mechanism of pulse positioning and that the control law presented in this paper is simple and effective.