ArcelorMittal Ruhrort (Duisburg, Germany):

U. Leuschke, Dr. Eng., Quality control / Technology development
D. Kirsch, Head of Steelmaking shop

SMS Group (Düsseldorf, Germany):
U. Grafe, Dr. Eng., Head of Direction “Technology of materials”
U. Plociennik, Head of Direction “Grounds and simulation of continuous casting”, e-mail: uwe.plociennik@sms-group.com
M. Reifferscheid, Dr. Eng., Head of Technological development

The experiment based on stopping of continuous casting has shown that crack origination occurred among the 4^th and 5^th roll pairs in the section of straightening and soft reduction. After the 5^th roll pair cracks are forming on free side and two chamfers in transversal direction. Analysis of plastic elongation using FEM calculation has displayed that maximal elongation is achieved on side or chamfer. Content of Al, Mn, C and V has been analyzed using Sheil – Gulliver model, and it was revealed that Al is extracted in dendrites, while V, C and Mn — in inter-dendrite space. Mn is also extracted in the melt as MnS. As a result, ductility of all three examined steel grades doesn’t exceed 70% in the range of increased temperatures. Al and V extraction is occurred mainly as AlN and V(C,N), while vanadium nitride was not found in the researched areas. As soon as extraction of AlN secondary phase is occurred mainly in dendrites, it can’t be considered as the cause of crack origination. At the same time, extraction of V(C,N) secondary phases in combination with lowered temperature of billet chamfers decrease ductility and can be the cause of crack origination and propagation. Secondary cooling was decreased in such way that it was possible to provide increase temperature of billet straightening with sufficient crust thickness and cast length at preset casting rate. Just the first casting experiment with crack-sensitive steel 20MnV6 and billet surface temperature decreased by 100° has displayed the desired effect: essential elevation of ductility and lowering of extraction of secondary phase presented by vanadium carbonitrides.

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