제강연주공정 레이들 및 턴디시 슬라이드 게이트 내화물의 열응력 균열 거동 전산모사

Abstract

The slide-gate plate refractories in a cassette assembly control the steel flow through the ladle- and tundish-nozzles to maintain a meniscus level stable. Molten steel flow from the ladle into the tundish is often controlled by an assembly of two refractory plates located between the upper nozzle and the shroud nozzle. Also the molten steel flow from the tundish into the continuous-casting mold is controlled by the assembly of three refractory plates placed between the upper tundish nozzle (UTN) and the submerged entry nozzle (SEN). The horizontally moving plate back and forth is connected to a hydraulic cylinder to adjust the eye-shaped opening area. The moving plate according to feedback from the eddy current sensor measurement can control the flow rate through the nozzle to maintain a constant molten steel level in next vessels. Therefore the slide-gate refractory plates may experience through-thickness cracks or rare cracks under the severe conditions during the casting, caused by rapid temperature change, mechanical loading provided by bolt or spring tightening of the cassette, friction forces generated by the horizontal movements, and ferrostatic pressure due to the height difference between the free surface of the molten steel and the slide-gate location, leading to air aspiration, safety, and productivity concerns. Different mechanisms for common and rare crack formation are investigated with the aid of a three-dimensional finite-element model using a commercial software package ABAQUS of thermal and mechanical behavior of the ladle- and tundish-nozzle slide-gate plate assembly during spring/bolt pretensioning, preheating, ladle refining or tundish filling, casting, and cooling stages. Input material properties are all measured as a function of temperature. The model was validated with previous plant temperature measurements of the ladle plate during preheating and casting, and then applied to a typical ladle- and tundish-nozzle slide-gate assembly. The formation mechanisms of different types of cracks in the slide-gate plates are investigated using the finite-element analysis (FEA) and evaluated with actual slide-gate plates at POSCO. Common through-thickness radial cracks, found in every used plate, are caused during continuous casting by high tensile stress on the outside surfaces of the plates when temperature difference between an inner bore and outside surface are the greatest at the beginning of preheating or casting stage, due to different internal thermal expansion. Excessive spring/bolt tightening, combined with thermal expansion during casting may cause rare radial cracks in the plates. Rare radial and transverse cracks in middle plate appear to be caused during tundish filling by impingement of molten steel on the middle of the plate that generates tensile stress in the surrounding refractory in case of not using filler powder in the nozzle inside. The mechanical properties of the refractory, the spring/bolt tightening conditions, and the cassette/plate design are all important to service life. Additionally, this research applied reusable technology based on a patent published by JFE Steel Corporation into conventional ladle-nozzle slide-gate plates to investigate how the plates are thermally or mechanically behaved differently compared to conventional one.