Study On The Technology Of Top-slag Modification For IF Steel

In recent years,with the rapid development of automobile industry in our country,the steel used in automobiles has presented a situation of?large quantity,high quality?.IF steel which is widely used in automotive panel production,demands extra-low carbon content,excellent deep draw ability,aging and high elongation properties.At present,there are some problems in the process of producing IF steel in China.The non-metallic inclusions in molten steel are difficult to remove efficiently.The high loss of the aluminum,and strongly oxidized of the top-slag may lead to less continuous casting times,secondary oxidation of molten steel during the continuous casting process,and the?linear defects?in the surface of the final products.This paper tries to figure out the large pollution during production,surface?linear defects?,and nozzle nodulation during the continuous casting process.With the on-the-spot tracking investigation,the reasons for these problems mainly include the flowing aspects.The effect of modification in the aspect of modification agent is not satisfactory,because a large amount of dust will be generated when the modifier is added into the ladle.After the RH process,the deep oxidized slag lead to the secondary oxidation of the molten steel,the increasing loss of aluminum,and serious flocculation.The unreasonable control of C/A results in poor slag-adsorption Al₂O₃inclusions.Combined with the CaO-SiO₂-Al₂O₃ ternary phase diagram and the activity curve of slag system,the thermodynamics and kinetics of slag-adsorption Al₂O₃inclusions were analyzed.It was found that the target slag composition was:w(CaO)_%=45%?52%,w(Al₂O₃)_%=26%?35%,w(SiO₂)_%=5%?12%,C/A=1.3?1.7,liquidus is under 1450°C,0.1<a(Al₂O₃)<0.2.In the production practice of IF steel,the content of FeO in the top-slag after RH process was normally higher than 10%,resulting in serious secondary oxidation of the molten steel.Through theoretical calculation and analysis,it concluded that if the aluminum loss was been controlled below 0.01%,the content of FeO in the top-slag after RH process should be lower than 6.67%.This provided a theoretical basis for the implementation of the secondary reforming process.Based on the experiment,the new ladle top-slag modifier composition and density range was set.The new modifier has the characteristics of less pollution and good quality improvement.The C/A in slag was basic stable at 1.3?1.7.Through the industrial trials of the primary reforming process and the secondary reforming process,it was found that the continuous casting process was more stable with the secondary reforming process.The w(FeO)_%in tundish slag was lower than 5%.The nozzle clogging area was decreased from 70%in the primary reforming process to 30%in the secondary reforming process.The average aluminum loss in steel was reduced from 0.013%to 0.005%,and the number density of Al₂O₃ inclusion of the cold-rolled sheet decreased from 80#/mm² to the current 40#/mm².