The development, verification, and application of a steady-state thermal model for the pusher-type reheat furnace
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INTRODUCTION
HOT rolling of steel requires an initial material temperature of about 1150 ~ in order to allow deformation at economically low levels of roll force. Although the potential exists for the steel to leave the casting operation with a temperature at, or greater than, that required for rolling, there remain formidable difficulties in achieving routine direct rolling, and this feat has only recently been accomplished, m For a variety of reasons, such as defect detection and plant scheduling, it is standard practice for the steel to enter the rolling mill at ambient temperature or, when hot charged, at a temperature above ambient but well below the level required for rolling. For most of the world's mills, direct rolling is likely to remain a tantalizing but elusive goal for at least several decades. Thus, as shown in Figure 1, the reheating furnace will continue to provide the essential link between the casting and rolling operations. Disregarding obsolescent soaking pit designs, all current reheating furnaces are either of the walking-beam P.V. BARR, Associate Professor, The Centre for Metallurgical Process Engineering, is with The University of British Columbia, Vancouver, BC, Canada V6T 1Z4. Manuscript submitted February 9, 1993. METALLURGICAL AND MATERIALS TRANSACTIONS B
type, in which the steel is advanced by the periodic stroking action of the walking beams, or of the pusher type, in which the steel is moved through the furnace by the action of charging the next piece. In a typical pushertype furnace, such as that illustrated in Figure 2, the slabs or billets about one another over the furnace length, and as shown in Figure 3(a), only four surfaces are exposed to thermal radiation and convection from the furnace chamber. This is in contrast to the walking-beam-type furnace, in which the pieces are separated by a gap, and as shown in Figure 3(b), all six sides of each slab or billet are exposed to the furnace chamber. This fundamental dissimilarity implies that the modeler should approach each furnace type differently. The current article focuses on the modeling of the pusher-type furnace and leaves a similar effort directed at the walking-beam-type furnace as the subject of a subsequent report. Regardless of the furnace type employed, the task of the reheating operation is to raise the steel to a uniform temperature of about 1150 ~ and to do so at a minimum cost in both energy and material loss through scale formation. In practice, this is not a simple task, and all reheating operations share, to varying degrees, the common problems of nonuniform temperature in the discharged product, scale formation, and excessive energy VOLUME 26B, AUGUST 1995--851
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Fig. 1--Placement of the reheating furnace between the casting and rolling operations.
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