Study of Hot Tearing During Steel Solidification Through Ingot Punching Test and Its Numerical Simulation
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INTRODUCTION
THE present paper focuses on a major defect encountered during secondary cooling in steel Continuous Casting (CC) known as hot tearing. Also named hot cracking or solidification cracking, this defect takes place deep in the mushy zone in regions close to the end of solidification, typically exceeding 90 pct of solid. A comprehensive description of the physical phenomena involved in hot tearing is found in Reference 1. The liquid phase remaining at the late stage of solidification takes the form of thin interdendritic and intergranular films that cannot resist possible mechanical deformations. These are tensile stresses arising from external solicitations exerted by the immediate environment, for instance solidification shrinkage, thermal contraction of the neighboring regions, or boundary conditions TAKAO KOSHIKAWA, Manager, is with Nippon Steel & Sumitomo Metal Corporation Oita Works, Equipment division, Oita, Japan, and also Ph.D. Student at MINES ParisTech, Centre de Mise en Forme des Mate´riaux (CEMEF), UMR CNRS 7635, 06904 Sophia Antipolis, France. Contact e-mail: [email protected]. com MICHEL BELLET, Professor and Group Leader, and CHARLES-ANDRE´ GANDIN, CNRS Researcher and Group Leader, are with MINES ParisTech, CEMEF UMR CNRS 7635. HIDEAKI YAMAMURA, formerly Chief Researcher with Nippon Steel & Sumitomo Metal Corporation, Steelmaking R&D Division, is now Secretary General with The Japan Institute of Metals and Materials, 1-14-32, Ichibancho, Aoba-ku, Sendai 980-8544, Japan. MANUEL BOBADILLA, Group Manager, is with ArcelorMittal Maizie`res, Research and Development, BP 30320, 57283 Maizie`res-le`sMetz, France. Manuscript submitted January 3, 2016. Article published online June 6, 2016 METALLURGICAL AND MATERIALS TRANSACTIONS A
restraining the product. Even low local deformations cannot be compensated by liquid feeding because of the extremely low permeability of the solid network at such high solid fractions. This leads to the rupture of the remaining liquid films, thus creating cracks. The material appears particularly vulnerable in a critical range of solid fraction, which is known as the Brittleness Temperature Range (BTR). Processing by CC leads to hot tearing formation during the bending or unbending of the strand, or as a consequence of bulging of the shell between support rolls under the effect of metallostatic pressure. Locally and at small scale, various parameters influence the occurrence of hot tearing. The first of them is the chemical composition of the alloy. The larger the solidification interval is, the higher is its hot tearing sensitivity. Indeed, fragility strongly depends on the solidification path through the kinetic of liquid transformation near the end of solidification. In steels, some chemical elements such as sulfur, phosphorus or boron tend to increase the hot tearing sensitivity as they increase the solidification interval and thus decrease the solidus temperature.[2] Second, the microstructure also influences the hot tearing sensitivity. A fine microstructure decreases t
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