Simulation of the hot rolling and accelerated cooling of a C-Mn ferrite-bainite strip steel
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I.
INTRODUCTION
THE mechanical properties of many steels produced in hot strip mills are determined by a combination of the hot-rolling and runout-table-cooling conditions. For this reason, the simulation of rolling and accelerated-cooling processes has become a necessary step in the development of new steel grades as well as in studies related to the improvement of product quality. Such simulations are usually performed by rolling on laboratory or pilot mills and rapid cooling by means of oil or water quenching. However, the cooling rates obtained from this method are often difficult to control and can differ considerably from those observed in hot strip mills. The strains, strain rates, and interpass times pertaining to experimental mills can also differ appreciably from their industrial counterparts. In an attempt to find an alternative to this conventional approach, a new method based on hot torsion testing followed by gas cooling was developed at McGill University. It was used successfully to simulate the hot-rolling and accelerated-cooling behavior of a high-strength, highformability, ferrite-bainite sheet steel. The torsion technique allows strains, strain rates, and particularly interpass times to be employed that are closer to the ones used in hot strip mills than can be obtained from experimental mills. Similarly, the use of gas cooling permits ready control of the cooling rate and its interruption at precise moments. It also follows a Newtonian heattransfer law similar to the one produced by spray cooling B. DEBRAY, Graduate Student, and J.J. JONAS, Professor, are with the Department of Metallurgical Engineering, McGill University, Montreal, PQ, Canada, H3A 2A7. P. T E R A C H E R , Research Metallurgist, is with the Centre de Recherche des Produits fi Chaud, Sollac, 13776 Fos sur Mer Cedex, France. Manuscript submitted June 16, 1993. METALLURGICAL AND MATERIALS TRANSACTIONS A
in hot strip mills. The cooling rates obtained in this way ranged from 10 to 90 ~ as measured in the austenite temperature range. Once deformed and cooled, the samples could be used directly as tensile specimens for measurement of the mechanical properties produced. Information about the transformation kinetics was also obtained from the temperature recordings. This technique was used to study the influence of process parameters, such as finishing temperature, cooling rate, and coiling temperature, on the mechanical properties of a ferrite-bainite steel. This type of steel was developed in the last decade and is mainly directed toward the automotive wheel market. Several studies have shown that the replacement of pearlite by bainite leads to increased strength without degradation of the formability. "-4] The bainite is produced by accelerated cooling and coiling at temperatures between 500 ~ and 300 ~ However, within this range, the exact influence of coiling temperature on bainite morphology and on the mechanical properties was not known precisely. The same applied to the effects of cooling rate and finishing temperature. II.
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