Research On The Control Technology Of Inclusions In BNS Pipe

With the vigorous development of oil and natural gas,the performance requirements of conveying pipes are becoming increasingly demanding.By detailed analyzing the phenomenon that the inclusions of BNS steel have more unqualified flaw detection and basing on the actual production situation,this article research the formation and removal mechanism of inclusions in steel.The new process of producting clean steel is designed based on the optimization idea of reducing the steel inclusions to improve the properties of the steel.The inclusion defects of unqualified steel are located by ultrasonic flaw detection.The inclusions are analyzed qualitatively by metallographic microscope and scanning electron microscope,and the types and sources of inclusions are determined.Optimizing the original deoxidization,slag making and soft blowing process,combining the theoretical and field test results,finally determine the smelting process to reduce the inclusions in the steel,and get the following conclusions:(1)The deoxidization rate of molten steel was increased by 5.1%and the total oxygen content of the end point was 20×10-6 by the deoxidation of silicon calcium barium and aluminum.The yield of Mn and Al alloys increased greatly.The use of aluminum deoxidizer is reduced,the amount of aluminum per ton steel is only 0.25 kg,the type,quantity and size of inclusions in steel are effectively controlled.The main composition of calcium aluminate composite inclusions in steel is close to that of 12CaO 7Al2O3.(2)The basicity of the slag is increased,the deoxidation of molten steel,the desulfurization rate are significantly increased,and the LF refining cycle is significantly shortened.When the slag basicity is 3.5～4.0,the number of inclusions in the steel is small,the sulfur content of the molten steel drops to 15×10-6,the refining desulfurization rate reaches 96%,and Inclusions in the steel are CaO-Al2O3-based calcium aluminate composite inclusions containing MgO,SiO2,or CaS.(3)The flow rate of soft argon blowing is controlled at 40 L/min～50 L/min,and the soft blow time is over 15 min,which obviously improves the effect of soft blow mixing inclusion removal.(4)By using the new clean steel smelting process,the comprehensive index of steel was obviously improved:the proportion of steel w[O]under 30×10-6 was increased by 73.7%,and the proportion of w[S]under 20×10-6 was increased by 66.4%;the steel is mainly D non-metallic inclusions,the coarse and fine grade rating was increased to 100%and 98.2%respectively;the cost of ton steel is low,the harmful elements in steel are low,and BNS has better corrosion resistance.(5)The new clean steel smelting process is feasible in the actual production,and can be widely applied to the other steel smelting production,instead of imported products,reflecting the application value of the new clean steel smelting process.