Mold Level Fluctuation Research Articles

Study of Inclusion’s Source and Character in Different T[O] Content

Inclusion’s source and character in different T[ content are studied in the paper, molten slag and refractory material that leave in steel are also calculated. Inclusion in steel are principally come from deoxidation, accompany with the decrease of T[ content, influence of molten slag and refractory materials to inclusions are getting more and more serials. Inclusions which come from molten slag and refractory material are 88.9%, 24.6% and 18.1% defenetly. Great influence on non steady large inclusions in T[ steel, when the mold level fluctuation is large, almost 100% of inclusions in steel have slag inclusion of covering slag. When tundish liquid level fluctuation is large, about 30%~60% inclusions in steel have tundish slag.

Control of mould level fluctuation through the modification of steel composition

Periodic mould level fluctuation (MLF) during slab casting is a bottleneck for upgrading the surface quality and casting speed especially for hypoperitectic (HP) or ultralow carbon steels. The uneven growth of the initially solidified shell is verified to be one of the important inducements to MLF due to related unsteady bulging in the secondary cooling zone. It is shown that the solidification mode of steels and the contraction behavior can be modified through chemical composition optimization within given composition limits. For high strength low alloy (HSLA) steels, the actual peritectic points calculated by Thermo-Calc software may change remarkably with the slight variations of alloying element contents. Accordingly, the narrow limit of chemical composition of HP steels through optimization is proven to be one of the effective factors to control the popular MLF phenomenon during slab casting.

A Study about the Influence of Carbon Content in the Steel on the Casting Behavior

AbstractIn the casting process of steels with a C‐content ranging from 0.09 to 0.53 mass%, austenite is formed as secondary crystal phase by peritectic reaction between crystal of δ ferrite and residual melt. For unalloyed or micro‐alloyed steels the C‐content or C‐equivalent influences the casting behavior of steel in the mould, such as strand shell growth, crack formation, heat transfer, temperature fluctuation in the copper plate, mould level fluctuation and oscillation marks formation. The negative casting behavior like the uneven strand shell growth, the deep oscillation mark formation, the high mould level fluctuation, the crack formation on the strand surface were found mostly for steel with C‐content or Cp between 0.10–0.13 mass%. The strand shell structure (strand shell growth, mushy zone, δ + γ phase transformation) and shrinkage of the strand shell were simulated depending on the C‐content by means of mathematical simulation. On the basis of the simulation results and of the measured high temperature strength of steel the dependence of stiffness and the irregularity of the shrinkage of strand shell on the C‐content was investigated. It was found that the stiffness and irregularity of the shrinkage of the strand shell reach the maximum value at a C‐content of about 0.12 mass%.

Effect of Feeding Modes of Molten Steel on the Mould Metallurgical Behavior for Round Bloom Casting

Three kinds of modes for molten steel delivery have been studied in the paper for better understanding of their mould metallurgical effects during round bloom casting. The melt flow, free surface fluctuation, temperature field and solidification behavior in the mould region have been numerically analyzed upon the respective adoption of conventional straight single SEN, quad-furcated SEN with outlets in radial direction and a new type of quad-furcated SEN with outlets at tangential direction. For the latter, a strong horizontal swirling flow has been observed along with the upper and lower recirculation region in the mould. It is shown that the horizontal swirling flow in the mould can reduce the impingement depth of molten steel remarkably, inhibit the mould level fluctuation, and create an active bulk flow below the meniscus, which can also move the hot spot upward and promote superheat dissipation of the molten steel. Meanwhile, the temperature of molten steel near the free surface can be increased by 2.6 K to 4.4 K as compared with the two other normal nozzles. Moreover, compared with quad-furcated SEN with outlets in radial direction, the jet impingement on shell is much weaker, while the thickness of solidified shell at the cross-section of the mould is more even. Furthermore, the shell thickness at exit of the mould under the new type of SEN can reach approximately 18.6 mm, while that under quad-furcated SEN with outlets at radial direction is about 17.4 mm, which leave much allowance for the speed enhancement of present round bloom castings.

Compensator design for the mould level of a thin slab caster to suppress periodic fluctuations

The design of a parameter-dependent controller to reject periodic mould level fluctuations at a thin slab caster is presented. These fluctuations can be related to bulging of the solidified steel shell between the supporting rolls. They deteriorate the steel quality and prohibit high speed casting. A state of the art controller has been designed to suppress these casting speed dependent fluctuations and is based on a H∞-optimal controller design technique. The controller has been implemented at caster of the Corus Direct Sheet Plant in IJmuiden. Measurement results show a significant reduction of these periodic mould level fluctuations.

Effect of SEN structure on the mold level fluctuation and heat transfer for a medium thin slab caster

It is important to select suitable parameters of a submerged entry nozzle (SEN) for optimizing the flow and temperature patterns in a mold. The effect of SEN design on the mould level stability, meniscus steel flow velocity, and heat transfer of the mold of a medium thin slab caster was studied by means of 1:1 water modeling and industrial testing. The advantages of a 2-port SEN compared with a 3-port SEN are the following: more optimal flow patterns with a lower mold level fluctuation and a lower meniscus steel flow velocity; proper powder consumption without slag bears due to a reasonable liquid powder thickness. The argon flow rate can be reduced and the mold average heat flux and temperature near the edges of the copper plate are reduced. At a casting speed of 2.5 m-min −1, the mold level fluctuation lies within ±5 mm. In addition, soft cooling of the steel shell in the mold is realized, which is suitable for casting crack susceptible steel grades.

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.

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.

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

Q-Parameter Design Control for Periodic Mold Level Fluctuation of Continuous Caster

A controller design technique to shape frequency response of a controlled system is proposed. The controller consists of an initial controller stabilizing a plant, a set of coprime factorization of the nominal plant model, and a parameter transfer function shaping the frequency response of the closed loop system. A design of the paiametei using the model matching problem of Is provided. Examples of the frequency shaping of the sensitivity function and numerical simulation show the eifect of the technique. The performance of rejection of the periodic disturbance is shown by an experimental result.

Mould heat transfer and continuously cast billet quality with mould flux lubrication Part 2 Quality issues

With many billet producers adopting mould powder lubrication, there is a need to clarify the gains in quality that can be achieved with this practice. Over the past three decades considerable research has been conducted to establish the relationship between mould behaviour and defect formation for billets continuously cast with oil lubrication, but little has been done to compare oil cast billets with powder cast billets. In this study, conducted at a Canadian minimill, four faces of a copper mould were instrumented with thermocouples and mould temperatures and billet quality were monitored with mould powder lubrication during casting of 208 × 208 mm billets. In the first part of this two part series (in Ironmaking & Steelmaking No. 1 2000), the results of the mould heat transfer analysis and the influence of variables were presented, together with a comparison between oil and powder lubrication. In the present paper, Part 2, billet quality is examined in detail. The difference in turbulence at the meniscus between oil and powder lubrication is established, and the need to tune mould level sensors when switching to mould powders is demonstrated. Previous work has shown that mould level fluctuations have a strong influence on defects such as offsquareness and transverse depressions, both of which are markedly reduced when casting with mould powders. The inherent stability of the meniscus is improved when employing mould powder lubrication and a submerged entry nozzle. Furthermore, the significant reduction in mould heat transfer at the meniscus, when mould powders are employed, particularly for medium carbon steels has been shown to correlate well with the observed reduction in offsquareness. The paper also elucidates the reasons for the reduction, and in most cases, elimination of transverse depressions in B–Ti grades when casting with mould powders. The mechanism of longitudinal depression formation and subsurface cracking observed in many of the powder cast, medium carbon billets has also been established.

Mold Level Fluctuation Caused by Bulging During High Speed Continuous Slab Casting.

Mold Level Fluctuation Caused by Bulging During High Speed Continuous Slab Casting.