Study On Slow Solidification Process And Quality Characteristics Of Ultra-thick Slab

Ultra-thick slab is widely used in various fields,and with the development of economy,the thickness of ultra-thick slab tends to be thicker.As the main raw material in the production of ultra-thick slab,the quality of the slab has attracted much attention.The internal defects of continuous casting slab are related to its solidification speed.The larger the solidification rate,the greater the tendency of internal cracks,and the stronger the forced cooling strength is,the more internal cracks will be induced.However,due to the large section and slow heat transfer,the solidification rate of the extra thick slab is slower,which leads to the improvement of the quality of the Lite thick slab.It is necessary to study the slow solidification of ultra-thick slab in order to develop thicker and thicker slab and control the quality of ultra-thick slab.Based on the design of continuous casting mold,the specific size of 420 mm thick slab mold in practical production is calculated,and the mold size and mold material which accord with slow solidification are calculated according to the similarity principle.According to the metallurgical criterion of continuous casting,the cooling stage and water distribution of the ultra-thick slab are calculated.The 105 mm×270 mm model of ultra-thick slab is established by using the finite element numerical simulation software ProCAST,and the slow solidification temperature field of the ultra-thick slab is analyzed under the condition of theoretical distribution of water.The changing law of solidified shell and solid-liquid two-phase zone,and according to the design content,the mold is made and carried out.The hot casting experiment was conducted to find out the quality of the ultra-thick slab formed by slow solidification.The specific research contents are as follows:(1)Design and test of slow solidification mold and water distribution of super thick slab.In this paper,according to the basic parameters of the production of 420 mm thick slab,the size of the alloy mold produced in practice is calculated.Under the condition that the similarity ratio is 4:1 and the solidification time ratio is 1/8-1,the mold size of the laboratory thick slab is determined to be 105 mm×270 mm × 550 mm and the material is clay,according to the premise that the temperature gradient is similar to the solidification rate,The theoretical initial slab shell is 13.6 mm,the secondary cooling zone is divided into three sections,the lengths of each section are0.3 m?3.3 m and 3.58 m respectively,and the water allocation is obtained.18.8 L /min,8.38 L / min,2.28 L / min,respectively.(2)The model of 105 mm × 270 mm thick slab was established.According to the design parameters,the boundary conditions required for numerical simulation are calculated,and the slow solidification process of ultra-thick slab is simulated by the thin slice boundary moving method,and the temperature field in the direction of the section of the slab is obtained.The variation law of solidified shell and solid-liquid two-phase zone is verified by the temperature change of special position and the theoretical calculation value.The most reasonable water distribution and model are obtained by adjusting the distribution of water when the metallurgical criterion is satisfied.(3)Combined with reasonable distribution of water and parameters of design crystallization,the hot slow solidification test was carried out to analyze the macro and micro structure of the slab formed by slow solidification,and to explore the effect of slow solidification on the quality of super thick slab.At the same time,the solidification experiments at different cooling rates were carried out by using high temperature confocal microscope,and the microstructure and precipitates of the slab were obtained at different cooling rates,which verified the conclusion that the slow solidification of the slab obtained from the hot state experiment would also produce relatively fine grains.Using the designed parameters as boundary conditions,the temperature field,solidified shell and solid-liquid two-phase region of 105 mm × 270 mm ultra-thick slab during slow solidification are numerically simulated by using finite element numerical simulation software ProCAST.The temperature and shell thickness at each cooling point of the slab can be obtained directly,which reflects the slow solidification process of the super thick slab.The fine grains under slow solidification conditions are obtained,which provides a certain reference for the development of ultra-thick slabs in the thicker direction.