A Study of the Crystallization Behavior of a New Mold Flux Used in the Casting of Transformation-Induced-Plasticity Stee
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TRANSFORMATION-INDUCED-PLASTICITY (TRIP) steels are a new generation of steel grades that can be used in automotive manufacture. The TRIP steels exhibit a better ductility at a given strength when compared to other advanced high-strength steels. This enhanced formability is due to the transformation of retained austenite to martensite during plastic deformation. Typical TRIP steels possess a three-phase microstructure, i.e., ferrite, bainite, and retained austenite. One microstructure of TRIP steel, from Mittal Steel USA (East Chicago, IN), is shown in Figure 1. Because of the improved formability, TRIP steels could be used to produce more complicated parts than other highstrength steels. Aluminum is added to TRIP steels for two reasons. The first reason is to increase austenite thermal stability such that it can be retained upon thermomechanical loading. The addition of aluminum is widely employed, because it promotes the carbon enrichment in austenite, which in turn favors austenite stability.[1–3] The second reason is as a substitute for silicon in TRIP grades that are to be galvannealed. Silicon contents higher than 0.1 pct do not permit the formation of a uniform galvannealed product, while aluminum does not have a detrimental effect on the galvanealing process. Therefore, the content of aluminum (1 pct or higher) in galvannealed TRIP steels is significantly higher than in other commercial carbon steels (0.005 to 0.08 pct), and one example of the chemical composition of a highaluminum TRIP steel is listed in Table I.[4] WANLIN WANG, Research Scientist, is with the Reckitt Benckiser Innovation Center, Montvale, NJ 07645, USA. Contact e-mail: [email protected] KENNETH BLAZEK, Principal Research Engineer, is with the Mittal Steel USA Research Laboratory, East Chicago, IN 46312, USA. ALAN CRAMB, Dean of Engineering and the John A. Clark and Edward T. Crossan Professor, is with Rensselaer Polytechnic Institute, Troy, NY 12180, USA. Manuscript submitted July 10, 2007. Article published online December 12, 2007. 66—VOLUME 39B, FEBRUARY 2008
Because of the addition of significant quantities of aluminum, the great challenge of casting high-aluminum TRIP steels is to design a mold flux that is chemically compatible with this steel chemistry. As all mold fluxes are currently based on the lime-silica system, the casting of steel grades containing significant quantities of highly reactive elements, such as aluminum or titanium, is problematic, due to the reduction by aluminum of any oxides having a metallic component nobler than aluminum during casting. The dynamically changing chemistry of a mold slag leads to varying viscosity and to a varying solidification path during casting. This dynamic change has led to various casting problems: sticking of the mold flux to copper mold, increased crack frequency, nonuniform heat transfer across the mold flux, reduced-mold heat transfer, reduced consumption of mold flux, and, hence, reduced lubrication, etc. The reasons for these casting problems are not yet completely understood. In this a
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