High Speed Continuous Casting Research Articles

Effects of Li<sub>2</sub>O on Properties of Low Fluoride Content Mould Flux for High Speed Continuous Casting

In order to reduce fluorine pollution, partial fluoride could be replaced by Li2O in mould flux. The effects of Li2O on properties of low fluoride content mould flux for high speed continuous casting were studied. The properties included melting temperature, viscosity, fluidity, surface tension and Al2O3 inclusion adsorption. The results showed that with the increase of Li2O content, melting temperature and viscosity of mould flux decreased obviously, fluidity increased, surface tension changed little, and Al2O3 inclusion adsorption of mould flux increased. In order to play an effective role, the appropriate Li2O content should be less than 4 per cent. The crystallization state of mould flux changed after slag adsorbed Al2O3. There were some small cuspidine and wollastonite crystals in the sample.

Effects of BaO on Properties of Low Fluoride Content Mould Flux for High Speed Continuous Casting

In order to reduce fluorine pollution, partial fluoride could be replaced by BaO in mould flux. The effects of BaO on melting temperature, viscosity, fluidity, surface tension, Al2O3 inclusion and adsorption of low fluoride content mould flux for high speed continuous casting were studied, as well as the effect of basicity on crystallization property. The results showed that with the increase of BaO content, melting temperature, viscosity and surface tension of mould flux decreased obviously, and fluidity and Al2O3 inclusion adsorption increased. Partial fluoride could be replaced by BaO in mould flux. The BaO content should be less than 4 per cent so that it can play an effective role. The basicity had great influence on crystallization property.

Effects of K<sub>2</sub>O on Properties of Low Fluoride Content Mould Flux for High Speed Continuous Casting

In order to reduce fluorine pollution, partial fluoride could be replaced by K2O in mould flux. The effects of K2O on melting temperature, viscosity, fluidity, surface tension, Al2O3 inclusion and adsorption of low fluoride content mould flux for high speed continuous casting were studied. The results showed that with the increase of K2O content, melting temperature, viscosity and surface tension of mould flux decreased obviously, and fluidity and Al2O3 inclusion adsorption increased. When basicity was 0.81, 1.03 and 1.22, the suitable content of K2O was 5%, 5% and 4% respectively, which could play an effective role. Partial fluoride could be replaced by K2O in mould flux.

Study and Application on Crystallization Temperature of Mould Powder for High-Speed Continuous Casting

Crystallization rule of mould powder for high-speed continuous casting was studied and the suitable composition powder was done. The analysis of experiment shows: in the given range, the influencing order of different fluxing agents for the crystallization temperature is: Li2O> BaO> B2O3> Na2O> CaF2> MgO. According to different solidification characteristics of steel and the requirements to the performance of mould power, component design of mould powder for high-speed continuous casting was done from the perspective of the crystallization characteristics.

Effects of Li<sub>2</sub>CO<sub>3</sub> on Properties of Mould Flux for High Speed Continuous Casting

The effects of Li2CO3 on melting temperature, viscosity, fluidity, surface tension and Al2O3 inclusion adsorption of mould flux for high speed continuous casting were studied. The results showed that with the increase of Li2CO3 content, melting temperature, viscosity and viscous activation energy of mould flux decreased obviously, fluidity increased, surface tension changed little, and Al2O3 inclusion adsorption of mould flux increased. In order to play an effective role, the Li2CO3 content should be less than 4 per cent. The results will play a guiding role in designing of mould flux for high speed continuous casting.

Properties of slag-forming mixtures for high-speed continuous casting of steel

Properties of slag-forming mixtures for high-speed continuous casting of steel

Growth of Solidified Shell Just below the Meniscus in Continuous Casting Mold

In the high speed continuous casting of hypo-peritectic steel slabs, growth of solidified shell just below the meniscus in the mold was researched. Rate of cooling and solidification was estimated on the basis of experimental results of thickness profiles of solidified shell, heat flux in the mold and dendrite arm spacing in the solidified structure beneath the surface of the slabs cast at 3–5 m/min.As a result, it was found that there is a delaying period of solidification growth at the beginning, till the shell grows up to about 1 mm thick. After that, it grows in linear relation to square root of solidification time. Dealing with the thickness profiles as a function of solidification time and solidification rate was obtained by differential. Furthermore, cooling rate of the shell was estimated with this solidification rate and heat flux in the mold. This cooling rate showed good agreement with that estimated from dendrite secondary arm spacing of the slabs.Estimated cooling rates were compared with the slabs by conventional continuous caster at 1.1–1.6 m/min and the influence of casting speed was discussed. It resulted in that cooling rate increases with casting speed and the difference by casting speed begins to be remarkable in the period of 0.1–1 s for solidification time, when uneven solidification just begins to occur. This agreement was considered to be one of the reasons why uneven solidification or longitudinal surface cracking of slabs tends to occur with increase of casting speed.

Investigation of Friction Force between Mould and Strand Shell under Sinusoidal and Non‐sinusoidal Oscillation in Continuous Slab Casting

Friction force is an important parameter to evaluate powder lubrication and reflect the interaction between mould and strand shell. A non‐sinusoidal oscillation pattern emerges under high speed continuous casting, the characteristic of which is that the time of upward motion is longer than that of the downward motion within one cycle. In the present research experiments were performed on a slab continuous caster with a strand size of 1000 mm ×120 mm. The friction forces of sinusoidal and non‐sinusoidal oscillation under the same parameters were calculated and comparatively analysed. The results show that the friction forces of both oscillation patterns approximately change along with velocity, and present a trend to a trapezoid wave, but there is a phase difference between the friction force and relative velocity. Compared with sinusoidal oscillation, the non‐sinusoidal oscillation pattern exhibits some important advantages as follows; the maximal and average compressive forces and the compressive force work acting on the strand shell are all higher, the maximal tension is lower, and the actual amount of negative strip is higher. These advantages effect better healing of surface cracks and strand demoulds, and reduce the generation and expansion of cracks, consequently improve the surface quality.

Modelling of steel shrinkage and optimisation of mould taper for high speed continuous casting

In high speed continuous casting, optimisation of mould taper is key for intensifying heat transfer and for improving the quality of the cast products. Mathematical modelling has been carried out by combining heat transfer, steel shrinkage and parabolic continuous taper model in order to optimise the mould taper profile. These models have been assembled to a set of software, the inputs of which include the steel grade, casting speed, casting temperature, length and the cross-section of mould tube, while the outputs consist of surface temperature of the strand, thickness of solidified shell, thermal linear expansion coefficient, steel shrinkage, distortion of the mould tube wall, the actual air gap, total taper and the continuous taper profile. Optimum mould taper has a parabolic profile which is tapered inwards that changes continuously along the length of the strand in order to achieve reduction in air gap while avoiding distortion of the mould tube.

Shrinkage of carbon steel by thermal contraction and phase transformation during solidification

In high speed continuous casting, optimisation of mould taper is important for intensifying heat transfer and improving the quality of cast products. In order to calculate the shrinkage during cooling, the thermal linear expansion coefficient (TLE) model has been developed and combined with phase transformation relevant to continuous casting of steel. In the present paper, a model to predict the shrinkage and to optimise mould taper for high speed casting is presented by taking into account variations in the TLE of steel and the effect of phase transformation during solidification of steel with varying carbon content. The TLE of steel purely from thermal contraction is nearly independent of carbon content when no δ→γ phase transformation is involved. For example, the TLE of 0·05% carbon steel is calculated to be 21·3 × 10−6 K−1, while the TLE of 0·60% carbon steel is shown to be 19·88 × 10−6 K−1. However, phase transformation processes which are greatly dependent upon the carbon content account for large difference in the shrinkage behaviour between the various grades of steel and extremely high apparent TLEs are calculated for low carbon steels; for example the apparent TLE for a 0·05 wt-%C steel is calculated to be 111·81 × 10−6 K−1.

Ladle furnace on-line reckoner for prediction and control of steel temperature and composition

In the present day competitive market scenario, steel producers are striving for high speed continuous casting with stringent and consistent quality as well as reduced production cost. The ladle furnace (LF) is a key unit for achieving the above objectives during secondary steelmaking. Proper control of process parameters during LF processing of liquid steel is essential. The paper reports development of a model based advisory system, called ladle furnace on-line reckoner (LFOR), for the prediction and control of temperature and composition of steel in a LF. The thermal and chemistry models employed in the LFOR system are based on simplified physics, material and heat balance, and statistical analysis of plant data. The LFOR system is provided with graphical user interface (GUI) to display the predicted composition and temperature profile. The control advice displayed on the screen provides guidance for arcing and addition to achieve the target temperature and composition. The LFOR system has been commissioned at LF No. 2 of the LD2 and Slab Caster Shop of Tata Steel, Jamshedpur and is designed to meet the requirements of sequence casting as well as to optimise the energy input and cost of alloying additions. The performance of the LFOR system was analysed and validated with data taken over 100 heats and was noted to be satisfactory.

Mold flux for high speed continuous casting of hypoperitectic steel slabs

A new mold flux has been developed to favour cuspidine crystallisation at the upper level of the mold. This reduces the local heat flux and allows an even solidification of the slab shell, thus preventing any occurrence of longitudinal cracks. Pilot tests have established that the new flux make it possible to cast hypoperitectic steel slabs at casting speeds up to 5 m/min.

Principles of Billet Soft-reduction and Consequences for Continuous Casting

The production of continuously cast products that show very low center segregation and are thus more homogeneous is an important objective, particularly for high-speed continuous casting of billets, in meeting quality requirements that are becoming increasingly stringent. In addition to the well-known methods of reducing macro-segregation in continuous casting, inline thickness reduction of the partially solidified strand, already used successfully in slab and bloom casting, offers an alternative for producing material with low segregation level. In the present report, which considers the feasibility and effectiveness of a mechanical soft-reduction for the continuous casting of billets, the influence of the soft-reduction position from the metallurgical point of view, the reduction parameter and the roll diameter in combination with the structural design of the soft-reduction unit was assessed comparatively. One strand of the billet caster S4 was modified for this purpose and soft-reduction trials were carried out on high carbon steel grade D72C for wire rod application and spring steel grade 54SiCr6 at casting speeds between 2.80 and 3.60m/min. The conclusions drawn from the test results with respect to required solid fraction in the liquid core, effective reduction rate and necessary roll diameter formed the basis for designing the soft-reduction unit of billet caster S0 and for predicting favourable process parameters. First results of billet soft-reduction trials on the caster S0 are presented.

High-speed continuous casting of peritectic carbon steels

Productivity of an integrated steel plant is improved by high-speed casting of hypo-peritectic steels that makes the productivity of caster meet basic oxygen furnace. In high-speed casting, however, strands of the steels tending to form cracks on the shell in the mold, require off-line conditioning that limits the plant productivity and premium yield. Hypo-peritectic transformation occurring on solidification in mold results in irregular shell-surface roughness that causes non-uniform heat transfer to the mold, causing local lifting of the shell from the mold, recalescence, and surface cracks. Influential factors are summarized on the development of the surface roughness, the non-uniform heat transfer and the decline of mechanical properties of the shell upon recalesce. Effective means are presented to reduce the non-uniformity and the cracks at high casting speeds by controlling the properties of mold flux film infiltrating into the solidifying shell/mold boundary.

Mould powder consumption of continuous casting operations

Mould powder consumption is an important parameter in continuous casting. Previous investigations, involving regression analysis or estimation of powder consumption, have identified the factors controlling powder consumption. However, those data were obtained for casting speeds less than 2 m min -1, and some relate to speeds less than 1 m min -1. Sumitomo Metal Industries has developed a medium thick high speed continuous casting operation for speeds between 2 and a maximum of 8 m min -1. Some of the powder consumption data obtained using the new process could not be explained on the basis of the former investigations: these data are discussed in the present report. The conclusions drawn are: (i) gravity is the main driving force for inflow of liquid powder between mould wall and solidified shell; (ii) to increase the powder consumption per unit strand surface area for speeds of ~8 m min -1 is difficult; and (iii) a powder consumption of under 0·1 kg m -2 is sufficient when casting at 8 m min -1.

High speed continuous casting technologies of peritectic medium thickness steel slabs

It is necessary for a machine casting products of medium thickness to cast at a speed as high as possible for productivity. But, longitudinal surface cracks and fluctuation of molten steel level increase as the casting speed increases. The peritectic steel grade is most sensitive to such defects and phenomena. Thanks to various technological developments, the peritectic steel grade could be cast at a casting speed of 5.0 m/min, which is the highest used ever.

High speed continuous casting of steel billets: Part 2: Mould heat transfer and mould design

A high speed instrumented mould trial was conducted under industrial conditions to study the heat transfer at the midfaces and corners of the mould and to clarify mould taper requirements in high speed continuous casting. The predicted peak heat transfer in this high speed system was found to be up to 2·5 times that reported for conventional speeds, and up to 1·5 times that for other reported high speed systems. The average heat transfer was found to be up to 45% greater than in conventional systems, and comparable with other high speed systems. The effect of casting speed was analysed in detail and was found to be dependent on carbon content. Increased casting speed was also found to increase the metal level standard deviation but to have less of an impact on the heat transfer than similar changes at conventional speeds. A mathematical billet thermal and solidification model was applied to these heat transfer results to determine the response of the current mould to high speed casting conditions. Using this assessment of the mould distortion and billet dimensions, new mould tapers were designed on the basis of minimising any mould-strand interaction and/or binding. New mould taper designs for high and low carbon grades were recommended for casting speeds of 3·0, 3·5, 4·0, and 4·5 m min -1. The design sensitivity to changes in casting speed is discussed.

High speed continuous casting of steel billets: Part 1: General overview

This paper is the first of a series of two, providing an overview of the results of mould heat transfer analyses conducted over years of industrial trials. The effects of casting speed, carbon content, superheat, water flowrate, electromagnetic stirring, and mould lubricant are discussed in terms of the midface mould heat transfer. Mould corner heat transfer is also discussed. Finally, an overview of mould design is given, as well as an analysis that reveals that both excessively steep and shallow meniscus tapers cause high heat transfer. Alternative designs to the conventional straight tapered moulds are discussed briefly. The focus is on the primary cooling zone, and therefore billet quality issues are not extensively covered.

Decomposition of Coarse Grained Austenite During Accelerated Cooling of C-Mn Steels.

The simulation of microstructural evolution of a range of C–Mn steels following high-speed continuous casting and controlled cooling was carried out using quench dilatometry. An initial coarse-grained austenitic microstructure (as expected during casting of low carbon steel strip) was produced by a high temperature austenitising treatment followed by cooling at rates up to 600°C/s. Continuous cooling transformation (CCT) diagrams for a range of cooling schedules were constructed. It was found that both cooling rate and steel composition produces a range of final microstructures from polygonal ferrite to martensite with a concomitant range in hardness. Coarse grained austenite was found to promote the formation of degenerate pearlite and bainitic microstructures. The role of alloying additions, in combination with cooling rate is discussed in the context of microstructural development and mechanical properties of as-cast low carbon steel strip.

亜包晶鋼スラブの高速連続鋳造用モールドフラックス

High speed continuous casting technique for hypo-peritectic medium thick slab was developed. For mild-cooling of solidified shell, the mold flux was designed to crystallize effectively by new method which gives accurate phase relation between mold flux composition and cuspidine (3CaO·2SiO2·CaF2).The results were as follows;(1) It was possible to increase casting speed to 5.0 m/min without surface longitudinal cracking on slabs by the developed mold flux of mild-cooling.(2) The stability of phase equilibrium between mold flux composition and cuspidine was important to promote crystallization in flux film between mold and solidified shell. Then interaction between F- and Na+ was necessary to be considered for accurate prediction about phase relation between them.(3) Because of crystallization in flux film, the top of solidified shell was cooled mildly to be uniform and that resulted in prevention of surface longitudinal crack of slabs.