| In the current thesis, water modeling and plant experiments were conducted to investigate the fluid flow in a wide slab continuous caster ?with 2040 mm in width and 200 mm in thickness?. During water modeling experiments, particle image velocimetry ?PIV? was used to measure the turbulent characteristics and DJ800 was introduced to monitor the level fluctuations in the water model of mold. The slag entrapment and its critical condition was investigated using a high speed camera. During plant experiments, a kind of nailboard made of three rows of nails was designed to simultaneously measure the surface velocities and level fluctuations of the loose side, the centerplane and the fixed side in an actual continuous wide slab casting mold. The distribution of fold oscillation marks?OMs? on the surface of corresponding slabs ?in common with nailboard experiments strand? was systematic studied. In order to investigate the relationship between fold OMs and entrapment of inclusions to the solidified shell, samples were taken at ten locations on one relevant slab, and it was polished 1.5 ?m each time. Inclusions larger than 10 ?m were analyzed using the optical microscope.Some optimization measures have been proposed through the results of flow pattern, symmetric flow, level fluctuations, surface velocities, turbulent characteristics and considering slag entrapment critical condition in mold. The submergence depth of the submergence entry nozzle?SEN? increased from 160 ?m to 180?200 mm. The angle of entry nozzle changed from +5° to 0°. The cast speed improved from 0.8-0.85 m·min-1 to 0.9-1.0 m·min-1. After the measures taken in the steel plant, the grinding ratio of slab decreased from 100% to 30%.From the water-oil modeling experiments, the main way that slag entrainment is causing by fluctuating flow on the meniscus of narrow face. When the submergence depth of SEN is 50 mm and if the viscosity of silicone oil is 5×10-5 m2·s-1, the critical cast speed of slag entrainment is 0.60 m·min-1; if the the viscosity of silicone oil is 1×10-4 m2·s-1, the critical cast speed of slag entrainment is 0.62 m·min-1; if the viscosity of silicone oil is 2×10-4 m2·s-1,the critical cast speed of slag entrainment is 0.65 m·min-1, When the cast speed is 0.425 m·min-1 and if the viscosity of silicone oil is 5×10-5 m2·s-1, the critical submergence depth of SEN is 44 mm; if the viscosity of silicone oil is 1×10-4 m2·s-1, the critical submergence depth of SEN is 35 mm; if the viscosity of silicone oil is 2×10-4 m2·s-1, the critical submergence depth of SEN is 23 mm.From the nailboard experiments in the plant, it can be found that there is a little difference on level position between loose sied and fixed side during casting. In another word, the surface flow between loose side and fixed side in the mold is asymmetric. As a further consequence, symmetric flow includes not only the flow in width taking SEN as symcenter, but also the flow in thickness taking half of thickness as centerline. The change of angle between OMs and casting direction on slab corner is causing by impingeing of the upflow, oscillation of the mold and solidfication of steel. The change of distance between two neighbouring OMs is the variation of meniscus position recorded in the surface of slab during CC. The ideal range of distance between two neighbouring OMs is 5.96-9.96 mm.The formation mechanism of fold OMs on the surface wide slab is concluded and verified by the results of distribution of inclusions in the sub-surface of slab. The mechanism can be described as follow:the meniscus is in the gas-liquid interface and contact with the copper, making it good heat transfer condition. The temperature somewhere in meniscus is in solid-liquid two phase zone, increasing the local viscous force of liquid steel, weakening its liquidity. This part may adhesion in the copper for a while and fluctuate synchronously with the mold but other locations are still in their own pace, then it will fall off from the copper because of oscillation of the mold and join in the main frequency of fluctuating. With the continuous casting, the process mentioned above are recorded on the surface of slab making the fold OMs forming.One corollary of the fold OMs formation principle is that more larger inclusions may be entrapped by the fold OMs formation zone nearby meniscus. The result of number density of inclusions is in accordance with the result of fold OMs. The formation mechanism of fold OMs is verified by the distribution of inclusions in sub-surface of slab. In addition, at the location where the thickness of slab is 5 mm in mold during casting, the inclusions are more likely to be entrapped by the solidification front. |
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