Oscillation Marks Research Articles

Recent Topics of Research and Development in Continuous Casting

Three major issues in continuous casting are nonmetallic inclusions, cracking and segregation. By the application of electromagnetic stirring (EMS) in mold the inclusion entrapment by the growing initial solid shell is hindered and the number of inclusions near the surface of slabs is much reduced. By the application of level magnetic field (LMF) the downward liquid flow induced by the exit flow from submerged entry nozzle is much decelerated and the entrapment of large size inclusions inside the slabs is eliminated, which otherwise cause inner defects. Electromagnetic casting (EMC) prevents from the formation of oscillation mark and hook mark, concomitantly eliminates the entrapment of inclusions and bubbles at the hook mark and also prevents from the formation of surface depression and longitudinal surface cracking in hypo-peritectic steels. Resonance-like unsteady bulging with oscillation of molten metal level in mold causes internal cracking and can be prevented by unequally spaced support rolls. Macrosegregation can be reduced by the proper reduction of casting thickness or, in the case of bloom, the minimization of the size of equiaxed solids, which can be realized by the application of EMS.

Several slag rims and lubrication behaviours in slab casting

Phenomena that occur at the meniscus in the CC mould have a major effect on product quality. The mechanism of slag rim behaviour and flux infiltration are reported based on the results of industrial characterization and of fluid dynamics modelling of the effect of oscillation and shell velocity. Lubrication stability is not established as the slag film happens to be broken down and renewed several times during casting. A mechanism of slag infiltration is put forward and the main factors for oscillation marks formation are pointed out.

Mould investigations on a thick slab caster

As regards conditions in the mould, Continuous Casting of 400 mm thick slabs can differ from conventional slab casting. Specific features of thick slab casting are reported, concerning heat fluxes, both integral and local. Local heat flux has been determined through an instrumented mould along with thermal modeling of the copper plate. Integral heat withdrawal data are compared with data from conventional slab casting. The effect of steel composition on integral and local heat extraction has been evaluated and interpreted in relation to residence times in the mould and to the relevant thermal resistances. The influence of tundish temperature on superheat in the mould and on heat removal was quantified. Oscillation mark depth and mould powder consumption have been related to heat withdrawal. Cracking defects were characterized in dependence on mould heat flux densities.

A mold simulator for continuous casting of steel: Part II. The formation of oscillation marks during the continuous casting of low carbon steel

The restrictions on quality for low carbon continuously cast slab products require that surface defects be kept to a minimum. Currently, the steel industry has developed a wealth of experience on how to apply slabs with oscillation marks to very demanding applications. However, these practices circumvent the problem, rather than solving it. By understanding the formation mechanism of oscillation marks, one can then develop casting practices that can minimize their effect on slab surface quality. The techniques developed in this study allowed a more detailed examination of the mold heat-transfer interactions during continuous casting, such that the variations of heat flux due to irregular solidification near the meniscus could be measured. It is shown that the mechanisms proposed in the literature are not individually sufficient for the formation of an oscillation mark, but that several are necessary and must occur in sequence for an oscillation mark to form. A mechanism is proposed for the formation of oscillation marks that is shown to be in agreement with the trends observed and reported in the literature. Additionally, it is shown that the success of practices used in industry to reduce the severity of oscillation marks can be explained using this proposed hypothesis.

A mold simulator for the continuous casting of steel: Part I. The development of a simulator

Surface defects, such as oscillation marks, ripples, and cracks that can be found on the surface of continuously cast steel, originate in the continuous casting mold. Therefore, a detailed knowledge of initial solidification behavior of steel in a continuous casting mold is necessary because it determines the surface quality of continuously cast slabs. In order to develop an understanding of the initial solidification of continuous cast steels, a “mold simulator” was designed and constructed to investigate heat-transfer phenomena during the initial phase of strand solidification. The mold simulator was used to obtain solidified steel shells of different grades of steel under conditions similar to those found in industrial casting operations. The resulting cast surface morphologies were compared with industrial slabs and were found to be in good agreement, indicating that it is possible to simulate the continuous casting process by a laboratory scale simulator.

Performance and properties of mould fluxes

The role of mould fluxes in continuous casting is reviewed. The main functions of the flux are described and the importance of the flux in providing lubrication and the right level of heat transfer between the shell and the mould is described. The major problems of longitudinal, transverse and star cracking, gas and slag entrapment and deep oscillation marks in the steel product and 'sticker breakouts' are analysed. The important physical properties of the flux in combating these various problems are identified. Physical property data for the fluxes are given.

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.

Phenomenological Description of the Surface Morphology and Crack Formation of Continuously Cast Peritectic Steel Slabs

The surface quality of continuously cast material is strongly depending on the initial solidification of the steel. Oscillation marks are formed at the very early stages of the strand shell growing process, thus influencing the microstructure and cracking behaviour of the surface and subsurface region. An industrial study of the oscillation mark morphology and the surface structure of peritectic medium carbon steel slabs was performed. The formation of oscillation marks and their effect on the surface quality was examined by metallographic investigations of slab samples. Although constant casting and oscillation conditions were applied, a variation of oscillation mark geometry along the narrow faces of the slabs was measured. A relation between the depth of the oscillation marks and the thickness of a layer of segregated melt situated inside the bottom of the marks was found. Measuring the distribution and length of surface and subsurface segregated cracks in the vicinity of the marks, existing theories of oscillation mark formation could be confirmed. The austenite grain size was found to increase with increasing oscillation mark depth. There was no clear correlation between the austenite and the δ‐ferrite grain size.

Magnetohydrodynamic solidification calculation in darcy flow

The free surface velocity strongly influences steel quality in the casting process, so an accurate calculation method is required. The velocity at the free surface is imparted through the wall of the continuous casting mold in an electromagnetic stirring operation. The wall friction in the solidification calculation model is modeled by the Darcy flow coefficient, which was found to be 0.7 from the operation data, according to the oscillation mark made by the free surface deformation. The calculated velocity is in good agreement with the operational velocity.

A Review on Causes of Surface Defects in Continuous Casting

A review on the causes of the foremost defects in continuous casting, based on an extensive literature survey, is given. The specific defects which are considered are transversal and longitudinal cracking, inclusions, sticking, bleeding, oscillation marks, stopmarks and depressions.

Heat-transfer and solidification model of continuous slab casting: CON1D

A simple, but comprehensive model of heat transfer and solidification of the continuous casting of steel slabs is described, including phenomena in the mold and spray regions. The model includes a one-dimensional (1-D) transient finite-difference calculation of heat conduction within the solidifying steel shell coupled with two-dimensional (2-D) steady-state heat conduction within the mold wall. The model features a detailed treatment of the interfacial gap between the shell and mold, including mass and momentum balances on the solid and liquid interfacial slag layers, and the effect of oscillation marks. The model predicts the shell thickness, temperature distributions in the mold and shell, thickness of the resolidified and liquid powder layers, heat-flux profiles down the wide and narrow faces, mold water temperature rise, ideal taper of the mold walls, and other related phenomena. The important effect of the nonuniform distribution of superheat is incorporated using the results from previous three-dimensional (3-D) turbulent fluid-flow calculations within the liquid pool. The FORTRAN program CONID has a user-friendly interface and executes in less than 1 minute on a personal computer. Calibration of the model with several different experimental measurements on operating slab casters is presented along with several example applications. In particular, the model demonstrates that the increase in heat flux throughout the mold at higher casting speeds is caused by two combined effects: a thinner interfacial gap near the top of the mold and a thinner shell toward the bottom. This modeling tool can be applied to a wide range of practical problems in continuous casters.

Trials for Reducing Depth of Oscillation Marks in Continuous Casting

Improvement of surface defects in a continuously cast slab or billet is important for saving surface scarfing and for carrying out direct rolling. Deep oscillation marks, one type of the surface defects, sometimes cause surface cracking and positive segregation. In this study, the mechanism by which oscillation marks are formed was investigated by using a continuously cast simulator, which is a billet‐type machine. Then attempts were made to reduce the depth of oscillation marks by two methods in which electromagnetic force, which is the most effective means for reducing depth, was not used. The two methods were use of an adiabatic board for preventing solidification of a meniscus and the use of a board for suppressing the flow of flux.Overlapping of a molten metal on a meniscus resulted in formation of oscillation marks in tin. On the other hand, bending of a solidified shell also resulted in formation of oscillation marks in a tin‐lead alloy. The depth of the oscillation marks formed by the overlapping mechanism was greater than that formed by the bending mechanism. Both mechanisms depended on the strength of the solidified shell. Therefore, two trials to reduce the depth of oscillation marks formed by the overlapping mechanism were carried out. In one trial, an adiabatic board was inserted into the molten metal. Reduction in depth of the oscillation marks reduced up to about 86% was achieved when high viscosity flux was used. However, the adiabatic board was not effective when low viscosity flux was used. In the other trial, a board was inserted into a molten flux layer with a depth of 10 mm in depth in order to suppress the flow of the flux and to change the direction of flux flow, and the depth of oscillation marks was reduced by about 33%. Therefore, both of these methods are effective for reducing the depth of oscillation marks in a continuously cast billet.

Continuous Casting of Steel Billet with High Frequency Electromagnetic Field

In order to develop the high frequency electromagnetic continuous casting technology for applying to steel, numerical analysis of the magnetic field and casting experiments using various parameters have been conducted. The method of using cold inserts was well established to lead to a reliable numerical model. According to this numerical model, it was predicted that while casting, the magnetic field would be concentrated on the common region occupied by the coil and the melt and further its maximum value would be seen just below the melt level. Casting experiments have been carried out using tin as a simulating material for steel. No oscillation mark was observed on the billets because the solidification started without hook. Under an optimum condition, billet surface roughness was improved to 1/10 of the conventionally cast billets. The surface quality of the billet was heavily dependent on the melt level, the casting speed, and the coil current. In case of the excessive coil current, wave marks other than the oscillation mark appeared on the billet surface. The billet with proper electromagnetic conditions showed a thinner solid shell at the early stage of the solidification and a thicker shell at the mold bottom in comparison with the conventional cast billet. It has been concluded that the Joule heat is a more dominant factor than the magnetic pressure in determining the surface quality of cast products in the high frequency electromagnetic continuous casting process.

Thoughts about the initial solidification process during continuous casting of steel

The initial solidification during continuous casting is very important for the surface quality of the strand. Oscillation marks are formed by the movement of the mould. One mainly distinguishes between two different types of marks, namely folding marks and overflow marks. The formation of these can be described by the heat flow and the surface tension balance. A theoretical analysis outlining the most important parameters controlling the formation of these marks is presented. A metallographic analysis of the formation was also performed. The theory and observations have been used to analyse the effect of casting speed and oscillation frequency on the formation of different types of marks. It is shown that a meniscus is formed at the top of the growing shell. The maximum height of the meniscus determines the ideal distance between the marks. The mould frequency is related to this height. A frequency that is far from ideal causes overflow marks and folding marks with macrosegregation and cracks. The distance between the marks is, in such cases, not constant. The depth of the marks is calculated from the heat flow and the surface tension balance.

Development of mechanical drive type non-sinusoidal oscillator for continuous casting of steel

A relatively simple mechanical drive type non-sinusoidal oscillator has been developed, based on mechanical theory. In this oscillator, a non-circular gearbox is additionally installed between the electromotor and eccentric cam. Compared with a servohydraulic type mould oscillation unit, this oscillator has the advantage that a non-sinusoidal oscillation mode can be used without the need for a large modification to the existing sinusoidal oscillator. In the present paper, the mechanical realisation of the oscillator is analysed, and the features of mechanical drive type non-sinusoidal oscillation are discussed. The design of the oscillator has been verified by measuring mould oscillation displacement. Plant experimental results have shown that the depth of the oscillation mark is reduced, and the surface quality of the cast product is improved, by using the oscillator.

Cooling Behavior and Slab Surface Quality in Continuous Casting with Alloy 718 Mold.

In order to decrease surface defects of continuously cast slab, a newly designed innovative mold to realize uniform mild cooling has been developed. Nickel base super alloy, Alloy 718, characterized by high strength at elevated temperature and low thermal conductivity was applied as the material of mold plates instead of conventional copper. Casting experiments of medium carbon steel have been performed with pilot scale continuous caster. The following advantages of the new mold with 40% decrease of meniscus heat extraction and 75% decrease of its fluctuation were confirmed in comparison with conventional copper mold. 1) Index of longitudinal surface crack decreased by about 50%, 2) Frequency of hook (over flow type oscillation marks) appearance on which non-metallic inclusions are likely entrapped decreased by 50%, 3) Depth of "hook" decreased by 20 to 45%, and 4) Average of oscillation mark depth decreased by 35% and its fluctuation also decreased by about 50%.

Improvement of Billet Surface Quality by Ultra-high-frequency Electromagnetic Casting.

Electromagnetic casting using ultra high frequency magnetic field (100 kHz) was investigated. At the initial stage, laboratory experiments have been made, and it is clear that oscillation marks can be totally suppressed and the homogeneity of the initial solidified shell is greatly improved. From these results, EMC has beneficial effects on preventing surface and sub-surface cracks. Furthermore, accumulation of inclusions and pinholes to the surface due to electromagnetic force does not occur, together with the electromagnetic stirring in the mold, when EMC frequency enhances to 100 kHz. Based on these results, plant trials were put into practice. As far as operational conditions are concerned, a new method of meniscus control was developed, and lubrication between the mold and the initial solidified shell is improved as the reason that friction force decreases, and mold powder consumption increases. From an investigation of the quality of rolled bars, it was confirmed that the quality of bars rolled from EMC billet without surface conditioning is the same as that of bars rolled from conventionally cast billets with surface conditioning.

Depth of oscillation marks forming in continuous casting of steel

The surface of continuously cast slabs is characterized by the presence of oscillation marks. Direct linkage of the continuous casting process and hot rolling process requires that cast slabs should be free of surface defects. In the present work, a mechanical model has been developed for the prediction of the depth of oscillation marks of the depression type. It is based on the beam bending theory and on viscoplastic material behavior. The downward movement of the strand is taken correctly into account, which has not been done in previous models. Auxiliary parts of the model are the models for the determination of the temperatre field and of the fluid flow and pressure in the meniscus region and in the gap between strand and mold. The deflection of the shell is computed as a function of time and distance from the shell tip. The retained deflection, which corresponds to the depth of oscillation marks observed on the slab surface, is determined for different values of stroke, frequency, and casting velocity. The theoretical data are compared with the measured data as available in the literature.

Simulation of transverse crack formation on continuously cast peritectic medium carbon steel slabs

Reliable data are limited to the critical strain for the formation of transverse cracks on the slabs, owing to experimental difficulty to simulate temperature gradient in solidified shell in continuous casting mold. The present study is to determine the critical strain, ɛc, for the formation of transverse cracks on continuously cast slabs. A convenient and simple hot tensile test using rectangular test pieces with either V‐notch or semi‐circle notch or oscillation marks has been developed by placing the specimen under similar temperature gradient to that in solidified shell in the mold. The ɛc has been determined at a better accuracy and reproducibility, and the ɛc at a strain rate of 5•10−4s−1 is found to be a high 35% for test pieces without notch. It sharply decreases, however, to 10% for those with V‐ and semi‐circle‐notches, slightly decreases with increasing notch depth, and further decreases for those with oscillation marks that accompany solute segregation. Reduction of the oscillation mark depth is shown to be important measure to prevent the occurrence of transverse cracking of continuously cast slabs.

Analysis of flux flow and the formation of oscillation marks in the continuous caster

In the industrial process of continuous steel casting, flux added at the top of the casting mould melts and forms a lubricating layer in the gap between the steel and the oscillating mould walls. The flow of flux in the gap plays an essential role in smoothing the casting operation. The aim of the present work is to better understand the mechanics of flux flow, with an emphasis on such problems as how the flux actually moves down the mould, the physical parameters governing the consumption rate of the flux and the geometry of the lubricating layer. The problem considered is a coupled problem of liquid flow and multi-phase heat transfer. In the first part of the paper, the formation of the lubricating layer is analysed and a set of equations to describe the flux flow is derived. Then, based on an analysis of the heat transfer from the molten steel through the lubricating layer to the mould wall, a system of equations correlating the temperature field in the steel and flux with the geometry of the lubricating layer is derived. Subsequently, the equations for the flux flow are coupled with those arising from heat-transfer analysis and then a numerical scheme for the calculation of the consumption rate of flux, the geometry of the lubricating layer and the solidification surface of the steel is presented.