A new criterion for internal crack formation in continuously cast steels
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I. INTRODUCTION
THE continuous casting process has been adopted worldwide by steel industries over the last 2 decades owing to its inherent advantages of low cost, high yield, flexibility of operation, and ability to achieve a high quality cast product.[1] In recent years, internal crack has again become an important concern as it was 20 years ago. To understand internal cracks, an accurate description of the thermomechanical behavior of solidifying shell is required. Many mathematical models have been developed to describe the thermomechanical behavior of the solidifying shell during continuous casting. But a major obstacle to the accurate mathematical analysis is the lack of the proper constitutive equations, which can adequately describe the relationship between stress and strain in the mushy zone of steels.[2] The strength and ductility of steel have a small value below the equilibrium solidus temperature because of the high temperature and the existence of interdendritic liquid film. All internal cracks observed in continuously cast steels originate and propagate along the interdendrites in the mushy zone. Especially, during continuous casting of steels, internal cracks tend to occur in a brittle temperature range, DTB , due to the thermal contraction, mechanical deformation, and d/g phase transformations.[3–9] The ductility loss of the mushy zone is associated with microsegregation of solute elements between solidifying dendrites.[5] This solute enrichment locally lowers the solidus temperature and gives rise to a zero ductility temperature below the equilibrium solidus temperature. A tensile deformation at the brittle temperature range can cause YOUNG MOK WON, formerly Ph.D. with the School of Materials Science and Engineering and Research Institute of Advanced Materials, Seoul National University, is now Postdoctoral Student, University of Illinois at Urbana-Champaign, Urbana, IL 61801. TAE-JUNG YEO, Postdoctor, DONG JIN SEOL, Postdoctoral Student, and KYU HWAN OH, Professor, are with the School of Materials Science and Engineering and Research Institute of Advanced Materials, Seoul National University, Seoul 151-742, Korea. Manuscript submitted March 15, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS B
the separation of dendrites. Hot tears can take place under a small strain when the interdendritic liquid film is isolated to resist feeding of the surrounding liquid through the dendrite arms. It is difficult to model this phenomenon because it depends on steel composition, microstructure, dendrite arm spacing, and so on.[10] The mathematical model describing the thermomechanical behavior of the mushy zone as a function of temperature and steel composition has to be developed to calculate the temperature and stress distribution in the solidifying shell accurately and to examine their influence on the crack formation. Thus, thermomechanical models are essentially required, which can explain both the experimental values of critical strain and fracture stress for crack initiation to determine the fracture cr
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