Slab Surface Quality Research Articles

Development of mould fluxes based on lime–alumina slag system for casting high aluminium TRIP steel

With excellent strength and ductility properties, transformation induced plasticity (TRIP) steels are superior to ordinary commercial steels and have great application potential in the automobile industry. However, the continuous casting of TRIP steels is associated with challenges owing to its high aluminium content, approximately 0⋅5–1⋅8%. The reaction of aluminium near the steel/slag interface continuously changes the basicity of conventional lime–silica based mould fluxes causing variation in the flux physical properties between the solidifying shell and mould wall. Therefore, the heat transfer process is unstable and the surface quality of the as-cast slab is not guaranteed.To reduce the reactivity of aluminium in TRIP steel during casting, it is suggested to replace the lime–silica based fluxes with a lime–alumina based mould flux, based on laboratory measurements and industrial trials.The trials showed great improvement in the slab surface quality and no depression defect was noticed so far.

Effect of casting process on mould friction during wide, thick slab continuous casting

Real time detection of the complex friction behaviour and reasonable regulation of lubrication between mould and initial solidified shell are key factors in quality improvement, steady operation and efficient production in continuous casting. In the present work, mould/slab friction was detected based on an online mould friction monitoring system on a wide, thick slab continuous caster equipped with hydraulic oscillators. The influence of the main casting parameters on mould friction is discussed including the casting temperature, slab dimensions and mould level. The response and sensitivity of friction to the changes of steel grade and powder were analysed, as well as the effects of different oscillation modes and oscillation parameters on friction. The results show that the friction is sensitive to variations of casting parameters, and steel grades and powder have significant effect on mould/slab friction behaviour. A reasonable match of oscillation modes and oscillation parameters can significantly optimise the friction behaviour at high casting speed, which also has an important role in improving the slab surface quality. These experimental results provided foundations for further understanding and controlling on the mould/slab friction behaviour.

Investigation and Application of Non-sinusoidal Oscillation Technique of Mold

To improve the slab quality, a kind of non-sinusoidal oscillation technique of mold driven by elliptic gears was developed. The waveform functions of non-sinusoidal oscillation were determined, and the calculation method and curves of technological parameters for non-sinusoidal oscillation were given. At the case of waveform deviation factor α equal to 0. 31 and oscillation amplitude h equal to ±2.7 mm, the relationship between oscillation frequency and casting speed is determined, and the technological parameters for non-sinusoidal oscillation are calculated. The testing results of industrial application indicated that this technique could not only improve the slab surface quality, but also reduce the steel breakout.

Heat-Transfer Behavior of Mold Fluxes for Continuous Casting of Invar Alloy

The heat-transfer behavior across mold fluxes for Invar alloy Fe-36Ni would introduce significant influence on the slab surface quality. A study on the heat-transfer property of mold flux film for Invar alloy Fe-36Ni was carried out by an interaction between laboratory simulation and field trial. The study results indicate that great effect on heat transfer across flux film is caused by chemical compositions of mold fluxes. An increase of basicity and CaF2 content suppresses heat transfer across flux film; heat transfer across flux film increases when the Al2O3 content increases from 4 pct to 8 pct but decreases when Al2O3 content is above 8 pct. The crystalline phases of both the conventional mold fluxes and the improved mold fluxes are all cuspidine phases. However, crystallization capability of the improved mold fluxes decreases as the result of the increase of basicity and CaF2 content as well as the decrease of Al2O3 content. The average thickness of flux film taken from mold is about 1.6 mm, and the crystalline fraction is only 21.4 pct. All these promote heat transfer across the flux film. The field trial of the improved mold fluxes shows that the properties of liquid slag are steady during continuous casting; comprehensive heat transfer across flux film meets the needs of continuous casting of Fe-36Ni. Border solidification structures of solidified shell are refined remarkably, and hot cracking gets avoidance eventually.

Online Measurement for Transient Mold Friction Based on the Hydraulic Oscillators of Continuous-Casting Mold

The interaction of the strand shell surface and mold copper plates has significant effects on the slab surface quality and casting productivity. This article focuses on developing a reliable approach to measure the transient friction force between the slab and the mold for the purpose of the investigation of lubrication and friction behavior inside a mold. This method is presented to monitor transient mold frictions for the slab continuous caster equipped with hydraulic oscillators. A mathematical model is also developed to calculate the empty working force of the no casting state, and a new algorithm, based on the particle swarm optimization, is proposed to predict the dynamic characteristic parameters of mold oscillation. The results have shown that the method has a sufficient sensitivity to variation, especially to the periodical variation of the mold friction, and it has been identified that the transient mold friction can be used as an effective index with regard to detecting mold oscillation and optimizing the casting parameters for process control. It may lay the practical foundation for the online detection of powder lubrication and the visualization of the continuous-casting mold process.

Introduction of a system for automatically feeding a slag-forming mixture into the mold of a continuous caster

The quality and safety of the operation of continuous casting depend to a large extent on making the correct choices for the slag-forming mixture and the technology to use it. Automated systems developed to feed such mixtures make it possible to introduce a mixture into the mold in small and precisely measured amounts, which improves the surface quality of continuous-cast slabs, decreases the number of metal products rejected for defects of metallurgical origin, and reduces the consumption of slag-forming mixtures.

Effect of High Frequency Electromagnetic Fields on Liquid Steel in Soft-Contact Mold

Using finite element method, the molten steel flow process in soft-contact electromagnetic continuous casting mold is simulated. The results show that the molten steel surface were affected by the electromagnetic force and shrink to the centre, near meniscus zone the flow velocity of steel increases and appears an obvious circumfluence, a large number of inclusion float and are absorbed by flux; on the other hand, the turbulent kinetic energy significant increases in circumfluence area, this exacerbates the reaction between flux and liquid steel, the initial size of the solidification organization are refined, and the surface quality of slab are enhanced.

Problems concerning the surface quality of continuous-cast slabs

This article examines the main types of surface defects on slabs and their causes. A general explanation is given for the nucleation and growth of surface cracks. An analysis is made of the possibility of applying criteria from the fracture mechanics of deformable solids to the conditions that exist during the continuous casting of steel slabs.

Three-Dimensional Numerical Simulation of Molten Steel Flow Behavior in Soft-Contact Electromagnetic Continuous Casting Mold

Using finite element method, the molten steel flow process in soft-contact electromagnetic continuous casting mold is simulated. The results show that the molten steel surface were affected by the electromagnetic force and shrink to the centre, near meniscus zone the flow velocity of steel increases and appears an obvious circumfluence, a large number of inclusion float and are absorbed by flux; on the other hand, the turbulent kinetic energy significant increases in circumfluence area, this exacerbates the reaction between flux and liquid steel, the initial size of the solidification organization are refined, and the surface quality of slab are enhanced.

The on-line continuous casting slab surface quality prediction expert system

The on-line continuous casting slab surface quality prediction expert system

DESENVOLVIMENTO DE FLUXANTE ISENTO DE FLÚOR APLICADO EM AÇOS BAIXO CARBONO

The mould flux is a mixture of non-metallic oxides that, in contact with liquid steel melts, becomes a liquid slag which the mainly function is to lubricate and control heat transfer between mould and strand during the continuous casting process. The mould flux without fluoride has the advantage of decreasing the wear of machine and the SEN in comparison to common mould flux. The application in Continuous Casting of Slabs has been a great challenge in relation to the operational viability together with internal and surface quality of slabs. Another differential is the decrease of environmental issues on account of the contamination of secondary cooling water by the fluorides. It is considered that properties of mould flux as chemical composition, viscosity, softening, melting flowing temperatures, fusion rate, etc, should be suitable to the chemical composition and the mechanical properties at elevated temperatures of steel and also the operational parameters such as casting temperature, casting speed, mould frequency, among others. This work presents a preliminary analysis in relation to operational viability, analysis of surface quality of slabs, measurements of fluorides content in the water of secondary cooling of machine. In addition to that, the analyses of operational features as measurements of wear of SEN, mould flux consumption, slag pool and behavior of thermocouples of detection system break outs (MSD) are considered

Impact of Mould Powder Density on Surface Quality and Near-Surface Zone Microstructure of Cast Slab

Impact of Mould Powder Density on Surface Quality and Near-Surface Zone Microstructure of Cast Slab

Measurement and Research of Oscillating State of Hydraulic Oscillation in Slab Continuous Casting

The stability of mold oscillation can directly influence on slab surface quality and operational safety in continuous casting. In recent years, the hydraulic oscillation is developed and applied as the driven equipment of mold oscillation. In the present work, based on the slab continuous caster of hydraulic oscillations, the displacement and other parameters of hydraulic oscillator are measured and the evaluation method of oscillation is studied. The displacement difference and phase difference of oscillation with sinusoidal waveform and non-sinusoidal waveform are analyzed. Especially, the dynamic characteristics of the driving force of left and right cylinders are evaluated. The results indicated that the hydraulic oscillator is quite a good device in terms of precision. The method proposed may be useful for evaluating oscillators.

Present Situation and Development Perspective of Soft-Contact Electromagnetic Continuous Casting Technology

Soft-contact electromagnetic continuous casting is one of innovative technologies for the development of steel industry. In this paper, the present situation was reviewed from five aspects: the soft-contact mold material and structure parameter design, the electromagnetic field distribution, the meniscus shape and movement behavior, casting slab surface quality improvement mechanism and the method of magnetic fields imposed. The main questions of the technology were analysed, and the remarhable in the process of industrialization were pointed out.

The Influence of Thermal Stresses and Strand Bending on Surface Defects Formation in Continuously Cast Strands

The Influence of Thermal Stresses and Strand Bending on Surface Defects Formation in Continuously Cast StrandsImprovement of slab surface quality is a very important goal in continues casting process of steel. It is closely coupled with casting line productivity and tendency to increase casting speed. In such a case rapid cooling of strand surface mast be employed. It results in thermal stresses development and formation of surface cracks if casting speed, cooling conditions or the arc of casting machine are not appropriate. The strain and stress fields in continuously cast strand have been determined based on the developed thermo-mechanical model and finite element software. It allowed to calculate variation of selected criterions integrals over the hole casting line starting from solidification process in the mould and ending at cutting section. Steady solution to heat transfer equation has been used to calculate strand temperature field. Mould temperature has been calculated from the three dimensional transient model. Finite element method has been employed to build steady and transient heat transfer models. Finite element solution accuracy to the temperature field has been improved. New algorithm of the solidification heat handling has been developed to stabilize a steady solution to the heat transport equation. Damageability of the strand has been evaluated based on four fracture criterions.

Modification and Minimization of Spinel(Al2O3·xMgO) Inclusions Formed in Ti-Added Steel Melts

High-melting-point inclusions such as spinel(Al2O3·xMgO) are known to promote clogging of the submerged entry nozzle (SEN) in a continuous caster mold. In particular, Ti-alloyed steels can have severe nozzle clogging problems, which are detrimental to the slab surface quality. In this work, the thermodynamic role of Ti in steels and the effect of Ca and Ti addition to the molten austenitic stainless steel deoxidized with Al on the formation of Al2O3·xMgO spinel inclusions were investigated. The sequence of Ca and Ti additions after Al deoxidation was also investigated. The inclusion chemistry and morphology according to the order of Ca and Ti are discussed from the standpoint of spinel formation. The thermodynamic interaction parameter of Mg with respect to the Ti alloying element was determined. The element of Ti in steels could contribute to enhancing the spinel formation, because Ti accelerates Mg dissolution from the MgO containing refractory walls or slags because of its high thermodynamic affinity for Mg \( ( {e_{\text{Mg}}^{\text{Ti}} = - 0. 9 3 3}). \) Even though Ti also induces Ca dissolution from the CaO-containing refractory walls or slags because of its thermodynamic affinity for Ca \( \left( {e_{\text{Ca}}^{\text{Ti}} = - 0.119} \right), \) dissolved Ca plays a role in favoring the formation of calcium aluminate inclusions, which are more stable thermodynamically in an Al-deoxidized steel. The inclusion content of steel samples was analyzed to improve the understanding of fundamentals of Al2O3·xMgO spinel inclusion formation. The optimum processing conditions for Ca treatment and Ti addition in austenitic stainless steel melts to achieve the minimized spinel formation and the maximized Ti-alloying yield is discussed.

Crystallization Extent and Phase Constitution of Slag Films Taken from Continuous Casting Moulds for Stainless Steels

Slag film samples were taken from the mould wall after casting of stainless steel slabs of grade 304, 321, 409L and 430, which represent austenitic, ferritic and titanium stabilized stainless steels. The chemical compositions of the samples were analysed and their phases were identified using a combination of scanning electron microscope (SEM), energy dispersive X-ray analysis (EDX) and X-ray diffraction (XRD). Finally the crystallization ratio was determined by SEM. The results show that the chemical compositions of the films during casting differ from the original designed flux composition. Concerning the heat transfer in the mould, the film taken during casting of steel grade 304 showed the highest insulation ability whereas the film taken during casting of steel grade 409L had the lowest insulation ability. The films taken during casting of steel grades 321 and 430 showed average heat insulation abilities. The solidification conditions in the mould during casting of steel grades 304, 321, and 409L matched the solidification requirements of the corresponding steel grades. However, the basicity of the powder for steel grade 430 should be decreased a little in order to improve the lubrication function and counteract sticking tendencies. Perovskite crystals, which usually precipitate at relatively high temperature, were found in the film taken during casting of steel grade 321. This is harmful in view of the safety of the casting process and the surface quality of the slab.

Mould lubrication and friction behaviour with hydraulic oscillators in slab continuous casting

It is very important to obtain reliable lubrication from casting powder both at the meniscus and in the gap between strand and mould as it affects slab surface quality and caster productivity. With knowledge of mould friction, a quantitative insight into the behaviour of powder during caster operation is possible. In the present research, the friction was studied based on a slab continuous caster equipped with hydraulic oscillators. The effects of mould oscillation and the abrupt change of casting speed on mould friction force were evaluated, and the characteristics of lubrication behaviour in a casting sequence were investigated. In particular, a comparison between the mould friction force between sinusoidal oscillation mode and non-sinusoidal oscillation mode was made. Finally, the characteristics of friction before a breakout are discussed. The experimental and analytical results may contribute to the development of mould friction online measurement and more clearly learn the lubrication behaviour in different conditions.

RELATION BETWEEN MIX PROPORTION OF CONCRETE AND FINISHABILITY

Finishing work greatly changes slab surface quality which influences floor performance. And, characteristics of concrete also greatly influence finishability. From such backgrounds, relations between mix proportion and finishiability were examined.60 kinds of mix proportion used on actual sites and prepared for the examination were set, and finishability of them were measured. As a result, those were shown that unit water content and admixture addition ratio influenced finishability directly, and finishability was predictable clearly by a combination of both. Moreover, it was generally thought that water-bond ratio and fluidity were causes, the combination turned out to be the direct cause.

Development of Swirling-flow Submerged Entry Nozzles for Slab Casting

We began development of swirling-flow submerged entry nozzles in 1997 as a fundamental and effective measure for controlling the flow pattern in continuous casting molds. As a first step, we developed a swirling-flow submerged entry nozzle for round billet casting at the Wakayama works. We then began developing swirling-flow submerged entry nozzles for slab casting. The main purpose of the present work was to demonstrate that the formation of swirling flow in submerged entry nozzle improves productivity and the quality of products in continuous casting. We examined swirling-flow submerged entry nozzles with a swirl blade in these main bodies because such an arrangement is the easiest way to apply swirling flow to submerged entry nozzles in continuous casters without investment by facilities. We had only to change the submerged entry nozzle in the experiment. Swirling-flow submerged entry nozzles for slab casting were developed and their operation examined at the Wakayama and Kashima works. It was found that the proposed submerged entry nozzles increased the casting speed and improved the surface quality of slabs and steel sheets.