| Except for the good machining property,the high surface quality is required for the automobiles external panels to meet the ever demanding market.The steelmaking and casting process are reported to be responsible for sliver defects.Therefore,the metallurgical technology is the key research topic for automobile steels.Shougang Group is a key domestic automobile steel manufacturer whose output is about 3 Mts annually.Main clients include BMW,BENZ,Volkswagen,GM,Ford,FIAT,Hyundai,FAW,SAW,DFAC,BAW,and GWM and so on.A series of experiments were carried out in Shougang to improve the quality&efficiency as well as reducing the cost of automobile steels.As an on-job doctoral students,the author attend some of these experiments,which include the characterizations and cause analysis of sliver defects,key technologies on slag reduction practice,high speed casting technology to produce the high quality steel sheets and the effect of unsteady casting on the macro inclusions on the surface layer of slabs.The main conclusions list as follows:1、Some key technologies are developed:(1)Top slag reduction after tapping,dynamic adjustments of the amount of aluminum conditioners were proposed based on the slag reduction theory.The FeO content of top slag below 6%reached over 80%.(2)The effect of pure cycling time on the cleanliness was evaluated,and the most appropriate cycling time was determined to 6～8 min with the consideration of temperature loss and RH refining period.(3)The procedure of Ti alloying was designed to eliminate the nozzle clogging due to the formation of TiOx-Al2O3,(4)10%CaF2 was added to the tundish flux to decrease its melting point by 65 ℃,and the CaO/Al2O3 ratio was set to be 1.5 to increase the inclusion absorption ability and thus improve the steel cleanliness.(5)The viscosity and the melting rate of the mold flux was increase to decrease the sliver ratio.2.The macro-inclusion characterization method on a very large area of the slabs was proposed,and systematical investigation on macro inclusions in the surface layer of steady-state cast slabs were determined.The results show that most of the macro inclusions in steady-state cast slabs are alumina clusters,and the number density of inclusions over 50 μm ranged from 0.201～0.434N/cm2(average 0.315 N/cm2).No mold powder inclusion was detected for the steady-cast slabs.The number density of macro inclusions decline with the rise of casting speed at the casting speed of 1.0～1.5m/min.3、The macroinclusions on the surface layer of several kinds of unsteady-cast slabs were determined,including the cast start,cast end,ladle change and nozzle change.The results show that different from the steady-cast slabs,more macro inclusions were detected in unsteady-cast slabs,and mold powder inclusions were detected in all kinds of unsteady-cast slabs.The degrade extent of top slabs is far more severe than any other kinds of unsteady cast slabs,which show that the effect of severe re-oxidation is more severe than the effect of flow pattern change.4、The effect of unsteady casting on the surface cleanliness of slabs(affected length and extent)were quantitatively evaluated by the analysis method mentioned above.These results provide the criteria to evaluate different kinds of unsteady cast slabs,which are beneficial to improve the quality and reduce the degrade amount of automobile steel sheets.5、Compared to the samples taken from the tundish,the number density of inclusions between 20 to 50μm in the surface layer of slabs sharply decrease,while the macro inclusions larger than 50μm dramatically increased.This is due to the fact that many small inclusions agglomerate to large-sized inclusions during the continuous casting process.6、The water modelling and plant trials proved that using the well bottom,20° angled SEN with the SEN immersion depth of 170mm can depress the level fluctuation and the surface velocity in the mold at the high speed casting.The casting practice has also shown that the mold level can be controlled within ±2mm by using the well-bottom,20°SEN combined with the FC mold at the casting speed of 2.4m/min.7、Four kinds of macro inclusions were detected on the surface layer of slabs at high speed casting.Namely alumina clusters,bubble plus alumina clusters,brick alumina and CaO-Al2O3-MgO inclusions.The amount of macro inclusions slowly increase withinthe 3.5 mm zone from the slab surface,rapidly increasein the zone 3.5-6 mm from the slab surface,then sharply decrease within the 6-10 mm layer from theslab surface.The distribution of macro inclusions were in accord with the dendrite deflection surface layer.During the initial stage of solidification about 0～3.5 mm from the slab surface(corresponding to 0～60mm from the meniscus),the flow velocity at the solidification front is as high as 0.17m/s.As the shell thickness increase to 3.5～6mm(corresponding to 60～180mm),the local velocity could be as low as 0.03m/s.Within this zone,the inclusion amount over 50 μ m reached 0.25 N/cm2.As the steel shell further solidified to 6～10mm,corresponding to a distance of 180～450mm from the meniscus,the flow velocity at the S/L interface rise from 0.03m/s to 0.07m/s and the number density of macro inclusions decrease to 0.05 N/cm2.The inclusions distribution mechanisms were illustrated by a typical double roll flow in the mold.8、The maximum casting speed reached 2.5m/min when casting LCAK steels,top three in the world.The speed for casting IF steels increase to 2.0m/min,similarto that of JFE Corporation(highest level in the world).In addition,the tapping temperature decrease from 1690 ℃(1.6m/min)to 1660 ℃(2.5m/min),and the RH refining time drop from 35min(2.0m/min)to below 25min(2.5m/min).9、This paper has some significant innovations in the area of science and technology of metallurgy.(1)Steady and unsteady cast slabs were continuously sampled along the casting speed directions and quantitative evaluations of surface macro inclusions on these slabs were determined.The affected length and extent of slabs by unsteady casting were determined to provide a criteria to evaluate different kinds of unsteady cast slabs,which are beneficial to improve the quality and reduce the degrade amount of automobile steel sheets.(2)A new method to characterize the macro inclusions based on ASPEX was proposed.Firstly,the samples with large size were scanned by ASPEX to obtain the location,size,morphology,chemistry etc.,and then the relocation function equipped with ASPEX was used to distinguish the single alumina particles and alumina clusters.(3)The casting speed increase to 2.5m/min when casting the 237mm-thick slabs.The amount of macro inclusions at high speed cast slabs are less than the slabs at conventional speed casting.Thus in the present work,high speed casting was recommended to produce the steel grades with sound surface quality requirements. |
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