Microstructural model for hot strip rolling of high-strength low-alloy steels
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I. INTRODUCTION WE dedicate this article to the late Professor J. Keith Brimacombe, who passed away suddenly on December 16, 1997. His leadership and vision were instrumental in mounting the hot strip–process modeling research in the Centre for Metallurgical Process Engineering at the University of British Columbia. This research was conducted in collaboration with the American Iron and Steel Institute, the United States Department of Energy, and the National Institute of Standards and Technology. The present article summarizes the results of a physical metallurgy investigation on several high-strength low-alloy (HSLA) steels. The HSLA steels were developed in the 1960s by microalloying low-carbon steels with Nb, V, and Ti in the 0.01 to 0.1 wt pct range. The increased strength of HSLA steels is attributed to a combination of ferrite grain refinement and precipitation strengthening. The HSLA steels have become a widely used material, in particular for automotive applications and as linepipe grades. Interestingly, while the weight fraction of steel and iron in an average family vehicle has decreased from 74 pct in 1978 to 67 pct in 1997, that of high- and medium-strength steels has increased from 3.7 to 9.1 pct in the same time period.[1] With this increase of approximately 150 pct, higher-strength steels show the biggest gain of any material class in automotive applications, even ahead of aluminum or plastics and plastic composites, which are widely discussed as alternatives to steel for the development of more-fuel-efficient, lightweight vehicles. A significant component of these higher-strength steels are hotrolled HSLA steels, which are used in high-strength vehicle components such as wheel rims or bumpers. Microstructural engineering has been increasingly gaining attention, with the goal being to quantitatively link the operational parameters of a hot-strip mill with the properties of the M. MILITZER, Assistant Professor, E.B. HAWBOLT, Professor Emeritus, and T.R. MEADOWCROFT, Professor, are with the Centre for Metallurgical Process Engineering, The University of British Columbia, Vancouver, BC, Canada V6T 1Z4. Manuscript submitted July 13, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS A
hot band.[2–8] These microstructural process models, which focused initially on plain carbon steels, have only recently been extended to include medium-strength HSLA steels.[8] These model extensions, however, still incorporate a great deal of empiricism, with comparatively tight process ranges of applicability. More fundamentally based process models are required to develop predictive tools to optimize the production of high-quality HSLA steel grades for a wide range of mill designs and processing conditions. The processing in a hot-strip mill can be subdivided into three principal stages: (1) reheating, (2) rolling (in both the roughing and finishing mill), and (3) cooling (water cooling on the run-out table and coiling). The metallurgical phenomena which occur in these three processing steps are summarized in Table I and can
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