Microstructural engineering applied to the controlled cooling of steel wire rod: Part I. Experimental design and heat tr
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INTRODUCTION
M I C R O S T R U C T U R A L engineering is an interdisciplinary approach to the quantitative prediction of the thermal, microstructural, and mechanical property evolution of a metal subjected to a given thermomechanical process. Recent demands on the metals industry to improve product quality and performance, while at the same time reducing cost, have spurred the development of this methodology. The root of the microstructural engineering approach is imbedded in the mathematical model, which links the basic principles of heat and mass transfer and microstructural phenomena to the operating process. In addition, both laboratory experiments and industrial trials are necessary to obtain empirical and semiempirical relationships characterizing transport phenomena by which the model can be tuned to operating variables. In the present study, microstructural engineering has been applied to the Stelmor cooling of steel wire rod, Ill a process situated after the finishing stand of a rod mill which provides controlled cooling of the steel through the temperature range of austenite decomposition. The process was developed to replace lead patenting, which
P.C. CAMPBELL, formerly Graduate Student, The University of British Columbia, is with BHP Central Research Laboratories, Wallsend, New South Wales 2287, Australia. E.B. HAWBOLT, Professor, Department of Metals and Materials Engineering and The Centre for Metallurgical Process Engineering, and J.K. BRIMACOMBE, Stelco/NSERC Professor and Director, The Centre for Metallurgical Process Engineering, are with the University of British Columbia, Vancouver, BC V6T 1Z4, Canada. Manuscript submitted February 14, 1990. METALLURGICAL TRANSACTIONS A
utilized a molten lead bath to impart controlled thermal changes and desired properties to wire rod. In the Stelmor line, forced air is the cooling medium, but more recently, other processes have exploited water and molten salt baths to develop desired cooling characteristics, t2,3,aJ Nonetheless, since its development nearly 25 years ago, the Stelmor process has become the most popular patenting technique in the world. In 1982, there were 69 mills with 153 Stelmor lines operating in 26 countries, tSJ Global capacity for the production of wire rod through this process has been estimated to be 21 million tonnes per year. [2] II.
PROCESS DESCRIPTION
In the Stelmor process, rods exiting the last stand of the rod finishing mill travel through an intermediate zone of water cooling boxes prior to arriving at the laying head (Figure 1). The water boxes provide control over rod temperature prior to continuous cooling, thus affecting prior austenite grain size, while the high-velocity water jets remove surface scale. At the laying head, the rod is looped continuously into coils and placed on the line where the chain conveyor, seen in Figure 1, pulls them through the successive cooling zones. Air is forced up from below the loops by a series of fans in zones to effect control of the rate of rod cooling. For lower carbon grades, where
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