| In order to realize the stable control of the quality of high-grade pipeline steel in Jingtang company of Shougang and improve the cleanliness of slab and corresponding rolled products,firstly,the control of molten steel sulfur content in smelting process is studied to control the formation of MnS inclusions in products from the source.Secondly,the metallurgical reaction characteristics of CO2 are studied from the aspects of thermodynamics and bubble characteristics,the converter steelmaking process based on CO2 compound injection is optimized,and the control of end-point P,N and O content is realized,which creates conditions for the control of cleanliness and purity.Then,from the three dimensions of inclusion density,size and composition control in liquid steel,it is confirmed that low denatured Al2O3-CaS composite inclusions are the key influencing factors for inclusion size control in pipeline steel production.The Al2O3-CaS inclusion control process in the production process of pipeline steel RH,calcium treatment and billet was optimized,and the product quality and production stability of high-grade pipeline steel such as X70 pipeline steel and X65MS acid resistant pipeline steel were improved.The industrial desulfurization effects of fluorine-free and fluorine-containing hot metal desulfurizer in the production of pipeline steel were compared by industrial experiments,and the effects of charging,hot metal slag raking and converter smelting process on sulfur recovery were quantitatively studied.It is found that the sulfur content of molten steel can be reduced to less than 1Oppm by using CaO-CaF2 system fluorine-containing desulfurizer and lime+aluminum slag system fluorine-free deoxidizer for KR hot metal pretreatment desulfurization.Through the research on the sulfur recovery control of converter,it is found that increasing the slag raking rate of KR tapping can realize more effective sulfur recovery control.At the same time,it is concluded that the sulfur content of converter tapping in conventional smelting process is lower than that in"dephosphorization+decarbonization" dual steelmaking process,which can be stably controlled at 20? 35ppm.For the endpoint composition control of converter steelmaking based on CO2 compound injection,the metallurgical reaction characteristics of CO2 participation in steelmaking are studied,and the thermodynamic law and kinetic mechanism of converter injection of CO2 on dephosphorization,denitrification and oxygen control in converter steelmaking are analyzed.It is found that the control and optimization of phosphorus,nitrogen and oxygen content in liquid steel at the end of converter can be realized by adjusting the heating rate of molten bath,increasing the contact area between bubble and liquid steel and reducing the equilibrium co partial pressure.The industrial test results show that the phosphorus content in the endpoint liquid steel is reduced by 19 ppm,the denitrification rate is increased by 1.9 times,and the end-point carbon and oxygen product is reduced from 16.59×10-4 to 14.45×10-4Through continuous sampling and analysis of LF,RH and refining soft blowing processes of pipeline steel,the effects of different refining processes on the removal of smelting inclusions of pipeline steel were studied.It is found that the influence of LF deep and light desulfurization treatment on inclusions is mainly the change of inclusion type.The inclusion type after light desulfurization has higher removal efficiency in RH refining process.Therefore,treatment of LF light desulfurization process is more conducive to the removal of inclusions.The removal efficiency of inclusions in RH refining process is Al2O3,Al203-MgO,Al203-Cao-MgO and CaO-Al2O3-CaS from high to low,and Al2O3-CaO inclusions can not be removed.The inclusion removal effect of 40NL/min soft blowing flow is obviously better than 60NL/min and standing conditions.However,when the soft blowing time exceeds 15 minutes,the T.O content does not change obviously.After LF refining,the T.O and N contents of liquid steel increase significantly.Through continuous sampling analysis of slab and coil in the production process of pipeline steel and slab thermal simulation rolling experiment,the distribution law of inclusions in pipeline steel and the rolling deformation behavior of different types of inclusions are analyzed.It is found that the size of oxygen sulfide inclusion in slab is smaller than that of oxide inclusion,and the rolling deformation degree of oxygen sulfide inclusion is lower than that of oxide inclusion The type of large-size inclusions is mainly calcium aluminate composite inclusions with good modification.Reducing CaO content and increasing CaS content in composite inclusions can make the inclusion size in slab smaller.Low denatured Al2O3-CaS composite inclusions are the key factors affecting the inclusion size control in the production of pipeline steel,which provides a basis for the accurate control of the target composition of inclusions in high-grade pipeline steel.Based on thermodynamic calculation and industrial measured data,the influence of Ca,S and O content in liquid steel on inclusion denaturation is studied.The formation mechanism of Al2O3-CaS inclusions in pipeline steel is emphatically analyzed.It is found that increasing Ca content in liquid steel can reduce MgO content in inclusions.At the same time,when T.Ca/S is increased and T.Ca/T.O is between 0.5-1.5,Al2O3-CaS composite inclusions are the main inclusions.When Ca/S is greater than 0.5,it is difficult to precipitate large MnS inclusions in steel.Based on this research,taking X70 pipeline steel and X65MS acid resistant pipeline steel as the research objects,the Al2O3-CaS inclusion control process in RH,calcium treatment and billet production process are proposed,and the industrial control of class A,B,C and D inclusions in X70 pipeline steel and acid resistant pipeline steel is controlled within class 1.0,the qualified rate is increased from 97%before optimization to 99%,and the product quality stability is significantly improved. |
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