Quantitative Evaluations of Surface Cleanliness in IF Steel Slabs at Unsteady Casting
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CHENXI JI, GUOSEN ZHU, QINGMEI LIU, ZHIHONG TIAN AND XINHUA WANG are with the Shougang Group. Co, Ltd., Research Institute of Technology, No. 69, Yangzhuang Road, Shijingshan District, Beijing, 100043, China. Contact e-mails: [email protected], [email protected] FUXIANG HUANG is with the Beijing Shougang Co., LTD, Tangshan, 064404, China Manuscript submitted December 10, 2018. Article published online May 7, 2019. 1974—VOLUME 50B, AUGUST 2019
INTRODUCTION
IF steels are widely used for car panels in the automobile industry or household electrical appliances where high surface quality should be guaranteed.[1,2] Although many influencing factors exist, macro-inclusions distributed in the surface layer of the slabs are mainly responsible for the slivers or blisters in the cold-rolled steel sheets.[3–5] Due to the large reduction ratio of IF steels, these large-sized inclusions will be deformed, crushed and exposed to the surface during the rolling process, deteriorating the surface quality and subsequent coating properties. The situation becomes even worse at the so-called ‘‘unsteady-state casting.’’ METALLURGICAL AND MATERIALS TRANSACTIONS B
Unsteady-state casting is inevitable for continuous sequential casting. Usually it includes the casting start, ladle change, nozzle change and casting cap-off, etc., in the whole casting sequence. Compared with the normal casting condition, the unsteady state involves the flow pattern change due to the casting speed variations and re-oxidation by air or refractory. In the steelmaking plant, downgrading is often used to deal with these kinds of ‘‘unsteady-state slabs.’’ However, if they are simply judged only by certain casting parameters, some good slabs would be downgraded while defective slabs would be wrongly treated as good slabs.[6] Therefore, a systematic investigation of the surface cleanliness of the unsteady slabs should be carried out to treat different kinds of unsteady-state slabs accurately. Several publications have reported the steel cleanliness at different unsteady-state casting conditions in some steel works, and part of the results are listed in Table I. For the casting start condition, at Dillinger steel,[7] the TO contents in the mold during the casting start were 25~50 ppm compared with 10~20 ppm for the steady state, and the argon flushing before casting decreased to 12~35 ppm. Dofasco[8] reported the TO contents in the tundish during casting start were twice as high as those of normal levels, and they proposed bell shrouds and baffles to eliminate the transitional effects. Furthermore, for the ladle exchange process, Dofasco[8] also reported the TO in the mold fluctuated violently at the first 100 tons cast. At POSCO,[9] the Flexible Drawability Test (FDT) defect index during the ladle change process was twice the normal states, and slag entrapment of the ladle and tundish were the main factors for steel contamination. As discussed above, the former investigation on the steel cleanliness at unsteady states mainly focused on the liquid steels, such as the TO contents in t
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