METALLURGICAL AND MATERIALS TRANSACTIONS
VOLUME 36B, JUNE 2005—355

A. BADRI, T.T. NATARAJAN, C.C. SNYDER, K.D. POWERS, F.J. MANNION, and A.W. CRAMB

initial source: http://doc.tms.org/ezMerchant/prodtms.nsf/ProductLookupItemID/MMTB-0506-355/$FILE/MMTB-0506-355F.pdf

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A Mold Simulator for the Continuous Casting of Steel:

Part I. The Development of a Simulator

Surface defects, such as oscillation marks, ripples, and cracks that can be found on the surface of continuously cast steel, originate in the continuous casting mold. Therefore, a detailed knowledge of initial solidification behavior of steel in a continuous casting mold is necessary because it determines the surface quality of continuously cast slabs. In order to develop an understanding of the initial solidification of continuous cast steels, a “mold simulator” was designed and constructed to investigate heat-transfer phenomena during the initial phase of strand solidification. The mold simulator was used to obtain solidified steel shells of different grades of steel under conditions similar to those found in industrial casting operations. The resulting cast surface morphologies were compared with industrial slabs and were found to be in good agreement, indicating that it is possible to simulate the continuous casting process by a laboratory scale simulator.

Introduction

One of the difficulties in studying the effects of operational parameters on the initial solidification behavior of steel in a continuous casting mold is the interdependence among different variables. It is not always feasible to conduct controlled experiments on an industrial continuous caster that will allow the effects of different operational parameters on the initial solidification of steel to be studied due to practical constraints. Therefore, most of the information developed on the formation of defects during the continuous casting of steels is collected under uncontrolled conditions. In the past, this constraint has led to the development of different types of mold simulators to study various aspects of continuous casting.

Mold simulators can generally be divided into four types— dip tests, static molds, dip simulators, and small-scale casters. The major issue in designing mold simulators is to ensure that the apparatus and the experiment are a true simulation of reality. This has led to the development of experimentspecific simulators that simulate the conditions in a casting mold to different degrees. For example, to study the effects of mold fluxes on the heat transfer between steel and a copper mold, Machingawuta developed a dip-type simulator specifically for that purpose. Another dip-type simulator was used by Bouchard to investigate the effects of mold surface conditions on the heat-transfer rate and attendant surface quality of the cast product. These dip simulators involved chilled plates that were immersed into a molten metal bath without any of the sophistication of continuous caster systems, such as oscillation and shell extraction. The dip simulators are very useful for determining fundamental interactions in the continuous casting process, but are not true simulators since they do not mimic the dynamic nature of continuous casting.

Related to the dip test simulators are the bottom-pouring molds, which are in essence similar to dip-type mold simulators, with the exception that the bottom-pouring simulators have the metal contained in the mold, instead of having the mold dipped into the metal. This configuration has the advantage that it is easier to observe the surface of the casting during solidification. Tomono used a bottom-filling mold to investigate the behavior of the liquid steel meniscus during casting and projected the results to explain the formation of oscillation marks. Wray developed a simulator to determine the mechanisms by which surface features formed on chill cast surfaces and provided a classification of the different types of features that could be formed. Stemple used a bottom-pouring configuration to investigate the formation of ripple marks on the surfaces of continuously cast products. It was emphasized that the bottom- pouring simulator could only be used to investigate phenomena unrelated to mold oscillation, since the experimental apparatus did not have provision for oscillation. Even so, Stemple et al. were able to observe the motion of the meniscus and provided an explanation for the formation of ripple marks. Nishida developed a mold simulator with the novel addition of an in-situ tool to measure the distortion of the shell from the mold wall. This was done to determine the dynamics of air gap formation and the resulting effect on the steel-mold heat transfer. Again, these were incomplete simulators of the continuous casting process.

To incorporate further sophistication into the experiment, several researchers have constructed more complex dip-type experiments in which the mold is equipped with oscillation drives and a mechanism for the extraction of the solidified shell to simulate continuous casting. This type of mold simulator is quite versatile and has been used by Saucedo to investigate the initial solidification phenomena. The simulated castings exhibited the typical surface morphologies of industrial cast slabs, and the results were used to propose a mechanism of oscillation mark formation. The work also included a comprehensive survey of the various hypotheses.