Production Practice Of Ultra-Low Phosphorus Steel Smelting

Phosphorus is apt to segregate in grain boundaries,causing the cold brittleness of steel.With deterioration of steel service conditions,low-temperature steel and marine steel have more and more strict requirements on the phosphorus content in steel.Generally,the phosphorus content of the final product is required to be controlled below 0.005%.The development of stable and efficient deep dephosphorization technology is of great practical significance for improving the hit rate of dephosphorization.The thesis mainly investigates the dephosphorization effect and the applicability of deep dephosphorization of duplex slag process,duplex converter process,duplex slag combined LF refining process by the industrial test methods.The results show:For the duplex slag process,the proportion of the final product whose phosphorus content was controlled below 0.005%was about 35.8%.It was more difficult to smelt ultra-low phosphorus steel,and it is suitable for smelting steel grades with phosphorus content less than 0.008%.The optimized conditions of smelting process were as follows:The FeO content in the slag was controlled at 20%～30%,the basicity was controlled at 2.5-3.5,and the tapping temperature was controlled at 1600～1640℃.The abovementioned smelting process is beneficial to the stable control of the phosphorus content of molten steel.For the duplex converter process,the proportion of final steel whose phosphorus content was controlled below 0.005%could reach 98.93%,but this process leads to long smelting cycles and high operating costs.The optimized smelting process conditions were as follows:The tapping temperature in the first half smelting process was controlled at 1400～1499℃,the basicity was controlled at 2.0～2.5,the iron oxide content in the slag was controlled at 25～3 5%;the tapping temperature in the second half smelting was controlled below 1640℃,the basicity of final slag was controlled at 5.5～6.5,and the iron oxide content in the slag was controlled at 30～35%.For the duplex slag combined LF refining deep dephosphorization process,the control target for the phosphorus content of final steel to be less than 0.005%could be fully achieved.Compared with the duplex converter process,the average phosphorus content was reduced by 0.0012%,and the smelting cycle was shortened.The optimized smelting process conditions were follows:The converter end temperature was controlled at 1640～1660℃,the slag iron oxide content was controlled at 20-30%,the basicity of final slag was controlled within the range of 2.5～3.0,the oxygen content of molten steel was between 800 and 900ppm,and the basicity of the top slag treated by LF was controlled in the range of 5.5～7.5.