Linear Contraction Behavior of Low-Carbon, Low-Alloy Steels During and After Solidification Using Real-Time Measurements
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THE continuous casting (CC) process is the most common route to produce primary and semi-finished steel products for subsequent processing. Although the process has been continually improved since its emergence, the ongoing increase in casting speeds and demanding dimensions of slabs still cause casting defects such as uneven shell growth, surface marks, surface and internal cracks and breakouts to mention but a few. The CC process involves complicated phenomena like heat and mass transport, solidification and shell formation, structure development, evolution of thermophysical and thermomechanical properties, etc., a better understanding of which are essential for treating those defects and increasing productivity of the CC technology.[1–3] Hot cracking is one of the prevalent problems in CC practice of low-carbon and low-alloy steels. It forms when stresses and strains built up during solidification exceed strength and ductility developed in the solidifying material. It is well accepted that such conditions are most likely to occur at high solid fractions where solid grains have essentially formed a coherent dendritic network capable HOSSEIN MEHRARA, Ph.D. Student, and MEHDI LALPOOR, Postdoctoral Researcher, are with the Materials Innovation Institute, Delft, The Netherlands and also with the Department of Materials Science and Engineering, Delft University of Technology, Delft, The Netherlands. DMITRY G. ESKIN, Professor, is with the Brunel Centre for Advanced Solidification Technology, Brunel University, Uxbridge, U.K. Contact e-mail: [email protected] ROUMEN H. PETROV, Associate Professor, is with the Department of Materials Science and Engineering, Delft University of Technology and also with the Department of Materials Science and Engineering, Gent University, Gent, Belgium. LAURENS KATGERMAN, Professor, is with the Department of Materials Science and Engineering, Delft University of Technology. Manuscript submitted May 21, 2013. Article published online November 8, 2013 METALLURGICAL AND MATERIALS TRANSACTIONS A
of transferring stresses but films of liquid still remain at grain boundaries, thereby weakening the material and making it vulnerable to cracking if the material is exposed to tension.[4] A systematic treatment of the crack formation process requires knowledge of structure formation within the solidification range, mushy zone coherency and rigidity, solidification shrinkage, feeding of growing solid along with its thermal contraction, which are all interrelated phenomena. The term ‘‘coherency’’ has not been always used in the same meaning by researchers through hot cracking studies. The obtained values of coherency temperature and fraction solid depend on the type of testing and on grain structure.[5] In the context of solidification shrinkage and contraction testing, terminology used in literature to describe structure and mechanical behavior of mushy zone is mostly suited for equiaxed and mixed morphologies—usually observed in aluminum alloys.[6–8] However, the microstructure is predominantly
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