Evolution Of Hook-like Shell And Its Effect On Surface Cleanness Of IF Steel Slab

The surface cleanliness of IF steel continuous casting slab is the key to restrict manufacturing cost and product quality.In order to control the surface quantity of IF steel slab,metallographic experiment,thermodynamic analysis,numerical simulation,physical simulation,vacuum melting were adopted to study the evolution behavior and distribution characteristics of inclusions in IF steel slabs.And,the formation mechanism of hook-like shell in the initial solidification process was analyzed as well.What's more,orderliness of inclusions in hook-like shell was revealed,and the effect of Mg treatment on the inclusions in IF steel was also explored.The evolution of inclusions in the IF steel production process and the distribution of the inclusions beneath the slab surface were analyzed by automatic scanning electron microscope(Auto-SEM).After RH refining,some Al2O3 inclusions gradually changed into Al2O3-Ti Ox inclusions,and the proportion of Mg inclusions increased gradually.The large size inclusions were mainly Al2O3.The number of inclusions in the slab surface is obviously higher than that of other locations.The number of inclusions on the broad face center and corners is slightly higher than that of other locations.The morphology of hook-like structure and the distribution of inclusions in different regions were studied by metallographic microscope.Under the complex environment,the curvature of the center line of the hook shell was fluctuated,which was generally close to the meniscus described by the Bikerman equation.The depth of hook shell was 1.2 mm~2.9 mm,and the depth decreased with the increase of casting speed and temperature.There were more inclusions in the lower part of hook and in the overflow area,and there were only a small amount of inclusions less than 100 ?m in the upper part of hook and the area of unsolidified hook shell.Based on the relative motion and heat transfer characteristics of the continuous casting slab and themold,the steady heat transfer model of the three-dimensional mold and the transient heat transfer model of the continuous casting slab were established by using 3D NTI and IHCP.The temperature field of the copper wall and the continuous casting slab was analyzed synchronously.According to the measured data of the copper wall temperature of mould,the distribution of heat flux on the shell surface of mould was studied.In the vicinity of the meniscus,the heat flux at the center of the wide surface was the highest,which leaded to the rapid growth of the hook-like shell and the increase of the probability of inclusion.According to the principle of the solidification and overflow of the meniscus,a twodimensional numerical model of longitudinal slice heat transfer was established.Taking the heat flux distribution determined by the 3D NTI-IHCP model as the boundary condition,the formation and evolution process of the hook-like shell during the initial solidification process were systematically analyzed.The pouring temperature rose from 1550 oC to 1565oC,and the depth of hook was shortened from 3.5mm to 1.8 mm;the casting speed increased from 1.1m min-1 to 1.7m min-1,and hook depth decreased from 3.5mm to 2.2 mm.The results of model analysis were basically consistent with the measured values of hook shell depth.With the increase of casting speed of hook shell,“buried point” decreased and the hook shell residual area expanded.When the pouring temperature increased,the “buried point” position increased,and the remnant area of the hook shaped shell decreased.Based on the numerical simulation results of the hook shell morphology,a physical model of the capturing inclusions was established by combining with the laser particle velocity measurement technology.The floating inclusions in the molten steel were most likely to be trapped on the lower side of the original hook-like shell.The inclusions near the meniscus were easily carried by the molten steel into the upper side of the hook shaped shell,and the capture amount was 70% of the hook lower part.When the casting speed increased from 1.1 m·min-1 to 1.7 m·min-1,the amount of captured inclusions decreased by 54.7%.A force analysis model for inclusions in molten steel near hook-like shell was established.In the lower and overflow areas of the center line of the coagulant hook,the inclusion in the size of 0um~300um pointed to the solidification front,and the resultant force increased with the increase of the inclusion size;the inclusions tended to move parallel to the solidification front along the solidification front,and the inclusions tended to move parallel to the solidification front.On the upper side of the line,inclusions with the size greater than 30 ?m were separated from the solidification front by the direction of resultant force,and the ability to trap inclusions was the weakest.The smelting experiment of IF steel processed by Mg has been carried out by vacuum induction furnace,and the effect of Mg on the size control of inclusions was also studied.The Mg content in steel increased to 8 ppm and 15 ppm,and the inclusion sizes were 4.8 m and 4.5 m respectively,which were significantly lower than those of IF steel without Mg treatment.The number of inclusions in IF steel increased by 60.8 %~ 84.7% after Mg treatment.The rolling and annealing experiments showed that Mg treatment had no obvious effect on the mechanical properties of IF steel.The equilibrium thermodynamics and melting thermodynamics of inclusions in Al-Mg-Ti-O system were calculated and compared with experimental results.The rationalization of Mg timing for industrial production of IF steel was discussed as well.