Investigation of panel crack formation in steel ingots: Part II. Off-corner panel cracks
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I.
INTRODUCTION
OFF-comer panel cracks run in a discontinuous manner along the edges of the wide faces of large, low-carbon, steel ingots. These cracks often form rough oval patterns on the wide face of the ingot and sometimes also on the transverse section as seen in Figure 2 of Part I f~Jand in Figure 1. While the exact time of crack formation is unknown, it is associated with reheating since off-comer panel cracks are discovered only after removal from the soaking pit. The cracks also are affected greatly by the extent of cooling prior to reheating. Ingots that experience more than two hours of air cooling, or are allowed to grow completely cold prior to reheating, seldom experience problems. I2'3J Like mid-face panel cracks, the defect is believed to arise through a complex combination of reduced elevated-temperature ductility and stress generation. In Part II of this paper, the results of a metallurgical study of off-corner panel cracks and model predictions of thermal and stress evolution in affected ingots are presented. From these, a detailed mechanism for the formation of the cracks is formulated and solutions to the problem are proposed.
II.
i \l i 1, Ill'l,'t"
Fig. 1 - - Location of off-corner panel cracks found by Sussman et al. m transverse cross-section near the top of a large, corrugated ingot subjected to 108 min air cooling. 4
METALLURGICAL INVESTIGATION
To develop a more complete understanding of how offcomer panel cracks are manifested, a metallurgical investigation was conducted prior to mathematical modeling. Since the cracks are easily discernible only after rolling has started, obtaining a sample from an unrolled ingot containing offcomer panel cracks is exceedingly rare. Nonetheless, such an ingot was found (Figure 2 in Part I), t31 the cross-section B. G. THOMAS is Assistant Professor, Department of Mechanical and Industrial Engineering, University of Illinois at Urbana-Champaign, 1206 West Green Street, Urbana, IL 61081. I.V. SAMARASEKERA, Associate Professor, and J. K. BRIMACOMBE, Stelco/NSERC Professor and Director, are with The Centre for Metallurgical Process Engineering at the University of British Columbia, Vancouver, BC, Canada, V6T 1W5. Manuscript submitted August 19, 1987. METALLURGICAL TRANSACTIONS B
of which is traced in Figure 2 (which also illustrates the terminology used for specific locations). The internal crack pattern differs somewhat from that found by Sussman et al. ,laj Figure I. Samples containing cracks were taken from the lower left-hand comer region at approximately mid-height of the ingot shown in Figure 2. The composition of this ingot was typical of crack-prone steels and contained 0.14 pct C, 1.40 pct Mn, 0.008 pct P, 0.005 pct S, 0.28 pct Si, 0.054 pct V, 0.039 pct Nb, 0.031 pct ASA, 0.28 pct Ca, 0.358 pct Ni, 0.033 pct Cr, and 0.002 pct Mo. An HC1 macroetch of a sectioned sample containing a complete off-corner panel crack is presented in Figure 3. The association of the cracks with mold corrugations can be seen as the crack intersects the surface of
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