Control Technology Of Oscillation Marks Based On Thermomechanical Behaviors Near The Meniscus Of Continuous Casting Mold

The inherent oscillation of the mold during continuous casting causes periodic oscillation marks on the surface of the billets,which is often accompanied by defects such as slag,micro-cracks and segregation.Hence,surface treatment before rolling is usually applied to ensure the quality of rolled products.It's essential especially for high value-added steel and special steel However,surface grinding not only hinder the process of continuous casting and rolling,but also causes a lot of energy consumption and metal loss.Therefore,it's necessary to investigate and develop technologies to suppress oscillation marks,which is a major topic and challenge for modern continuous casting.In this dissertation,a new control method of oscillation marks based on thermomechanical behaviors near the meniscus of continuous casting mold was proposed,which is termed as "thermal barrier coating above mold meniscus(TBCMM)".A series of investigations including one-dimensional heat transfer model experiments,hot-dip experiments using low melting point Sn-12.5wt.%Pb alloy,heat transfer analysis on the meniscus region with additional thermal barrier coating and continuous casting experiments of low carbon steel using mold with thermal barrier coating were carried out to study the method of oscillation marks forming and the corresponding theory.The experimental results showed that:(1)The metal-mold interfacial heat transfer resistance is dramatically increased up to 15%-50%because of the interface effect between TBCMM and mold flux.Consequently,the chilling effect of mold wall on the initially solidifying shell is decreased effectively which causes the suppression of oscillation marks.(2)The results of hot-dip experiments using Sn-12.5wt.%Pb alloy indicated that temperature fluctuations in the mold meniscus region was reduced by 50%when TBCMM was applied.In the meantime,oscillation marks were almost suppressed on the hot-dip samples.(3)The mold heat transfer model under the circumstance of mold oscillation was established,by which the fluctuation of temperature and heat flow was confirmed,and the mechanism of TBCMM on suppressing oscillation marks was revealed.(4)Pilot-scale tests of continuous casting of low carbon steel were carried out and billets without oscillation marks were fabricated while the thermal barrier coating was applied to the outer wall of the mold.Secondly,investigations on the control technology of oscillation marks based on the pressure change in the mold flux channel were carried out in this dissertation.The effect of additional external field force on the meniscus shape and pressure changes in the mold channel was theoretically analyzed.And then the induction heating(Joule heat)generated by soft-contact electromagnetic continuous casting within the initially solidifying shell in the meniscus region was modelled.The inhibition mechanism of soft-contact electromagnetic continuous casting on the formation of oscillation marks was explained accordingly.Furthermore,pilot-scale tests of continuous casting of stainless steel round billets with 100 mm in diameter were carried out in the laboratory,which validates the feasibility of soft-contact electromagnetic continuous casting of stainless steel,and then a series of production tests of soft-contact electromagnetic continuous casting of stainless steel were carried out.The experimental results showed that:(1)The maximum pressure in the mold flux channel are 80 kPa and 5 kPa respectively without(in the case of traditional continuous casting)and with additional external field force,which suggests that the pressure in the mold flux channel can be greatly decreased while applying additional external field force.Accordingly,the bending deformation of the initially solidifying shell was decreased during negative strip period,which is in favor of depression of formation of oscillation marks.(2)The analysis of induced Joule heating effect showed that both additional external field force and Joule heat were applied to the meniscus region during soft-contact electromagnetic continuous casting.It has a prevailed advantage with respect to suppression of oscillation marks compared to TBCMM.(3)Pilot tests of soft-contact electromagnetic continuous casting of stainless steel were carried out and round billets with 100 mm in diameter were fabricated.It was found that there were no oscillation marks on the surface of the billets which indicates the feasibility of soft-contact electromagnetic continuous casting of stainless steel(4)Based on a round billet(182 mm in diameter)continuous casting production line,a whole set system of soft-contact electromagnetic continuous casting of stainless steel was developed and industrial production experiments were successfully carried out.The oscillation marks on the round billets of stainless steel were totally suppressed when the power frequency is 20 kHz and the power reaches the range of 120 kW to 150 kW.(5)Based on a square billet(160 mm×160 mm)continuous casting production line,a whole set system for industrially applicable soft-contact electromagnetic continuous casting of stainless steel were developed and industrial production of several different steel grades were successfully implemented.The oscillation marks on the square billets of stainless steel were totally suppressed when the power frequency is 20 kHz and the power reaches 150 kW.Stainless steel billets produced by soft-contact electromagnetic continuous casting can be exempt from grinding and can be transported for direct rolling,which makes the comprehensive yield of wire was increased by more than 3%.