Improving surface cracks of slabs based on second cooling water optimization
The surface quality of the casting billet directly affects the efficiency of hot delivery and hot charging,as well as the quality of the steel plate.Surface cracks are the main quality problem currently existing in the slabs of Xiangtan Iron and Steel Company.For A36 steel grade,the surface crack morphology of 300 mm thick cast billets was observed using an optical microscope.Using scanning electron microscopy and energy spectrum analysis,it was determined that there are a large number of Si,Mn,and Fe oxides present inside and near the crack.The temperature of different parts of the casting machine was simulated and analyzed,and based on the types of oxides near the cracks and the generation temperature,the location of crack generation or propagation was determined to be in zone 2 from the mold foot roller to the bending section.Optimization experiments were conducted on the secondary cooling water at the location of crack generation or propagation.The microstructure changes at different distances from the edge of the casting billet were analyzed using an optical microscope,and the ferrite grain size and pearlite volume fraction were statistically analyzed using Image J.The optimization experiment of secondary cooling water in relevant parts shows that after increasing the water content of the 300 mm thick mold foot roller to 260 L/min and the secondary cooling water in zone 2 to 170 L/min,the overall change pattern of the microstructure on the surface of the casting bil-let from the edge to the center remains unchanged,but after optimizing the secondary cooling water,the position of the microstructure transformation moves towards the edge;the grain size of ferrite in different parts is significantly reduced,and the reduction of ferrite grain size increases the number of grain boundaries and enhances the hindering effect on crack propagation;the volume percent of pearlite decreases,and the morphology changes from sharp lath to smooth,reducing the contact stress between different tissues.The decrease in volume percent of pearlite reduces the contact stress between different tissues.The optimization of secondary cooling water has significantly reduced the sensitivity of slab cracks.A method and means to solve the cracks in the casting billet are provided.
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