Investigation of panel crack formation in steel ingots: Part I. Mathematical analysis and mid face panel cracks
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I.
INTRODUCTION
PANEL crack formation in static-cast steel ingots is a problem that has plagued the steel industry for several decades. Although the defect is intermittent and affects less than two percent of susceptible steel grades, the problem is persistent and expensive since affected ingots must be scrapped. Panel cracks are manifested as two distinct types of cracking problems, referred to as "mid-face" and "off-corner" panel cracks, respectively) 3) "Mid-face" panel cracks are found exclusively in small (1500 to 6000 kg), medium carbon (0.4 to 0.7 pct C), hypo-eutectoid, pearlitic steel ingots and usually exhibit a single, continuous, longitudinal fracture down the center of one of the ingot faces as shown in Figure 1.I31Certain alloy steels are particularly prone to this defect and are affected at slightly lower carbon contents. I3"41 "Off-corner" panel cracks often form rough oval, discontinuous, crack patterns on the wide faces of large (18,000 to 30,000 kg) ingots as seen in Figure 2. I4) They affect only low-carbon steels (0.1 to 0.2 pct C) with high Mn content (0.7 to 1.5 pct Mn) and are usually first observed when they open up during hot rolling. Both types of defect affect only killed, aluminum-treated steels (0.015 to 0.6 pct A1) and appear as deep, intergranular cracks that follow prior austenite grain boundaries. In a previous review, f41it was revealed that these cracks arise due to a combination of lowered intermediate temperature ductility and thermal stress generation. The loss of ductility in steel at intermediate temperatures has received a great deal of study which was reviewed as a preliminary step to the present investigation) 5) However, the generation of stresses in ingots arising from both changing thermal gradients and phase transformation is a complex subject 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 December 8, 1986. METALLURGICAL TRANSACTIONS B
that has received relatively little attention. Thus, the objective of the present investigation was to determine the mechanism(s) for panel crack formation, primarily through application of mathematical models developed to predict heat flow and stress generation in a static-cast ingot during the various processing stages prior to rolling.
Fig. 1 --Mid-face panel crack in 350 • 350 mm square Enl8 (0.4 pct C, 1.0 pct Cr) steel ingot) 3! VOLUME 19B, APRIL 1988--277
Table I. Input Data for Model Simulation of Mid-Face Panel Cracks
Ingot size Steel composition
Fig. 2--Off-comer panel cracks in a 760 x 1520mm, rectangular, corrugated (0.14 pct C, 1.4 pct Mn, Si-killed, A1grain refined)steel ingot.I41 The models, which are described briefly in the next section, we
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