Effect of liquid phase on scale formation during high-temperature oxidation of AlSi-transformation-induced plasticity st
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MODERN steels need to be light, tough, and available at a reasonable cost. A new class of steels, referred to as transformation-induced plasticity (TRIP) steels, meets these requirements. They are superior to other steels because they combine excellent formability and very high strength.[1,2] A special two-step annealing treatment is used to retain the high-temperature phase (-austenite) at room temperature. This retained austenite phase is responsible for the outstanding formability. During deformation, an irreversible strain-induced austenite-to-martensite phase transformation occurs. This results in ultrahigh strength levels (590 to 780 MPa) that are significantly greater than those of conventional steels (390 to 440 MPa).[3,4,5] The typical chemical composition of standard TRIP steels is 0.1 to 0.2 wt pct carbon (C), 1.0 to 2.2 wt pct silicon (Si), and 1.0 to 3.0 wt pct manganese (Mn). Carbon and, to some extent, manganese contribute to stabilizing the -austenite. Silicon is added to prevent the formation of iron carbides. Due to the high strength of TRIP steels, thinner and lighter body parts for cars can be produced. Cars utilizing these steels will be lighter and more energy effiB. SAUERHAMMER, Research Associate, and M. SPIEGEL, Group Leader, are with the Department of Interface Chemistry and Surface Engineering, Max-Planck-Institut für Eisenforschung, 40237 Düsseldorf, Germany. D. SENK, Professor, is with the Department of Ferrous Metallurgy, RWTHAachen University, 52056 Aachen, Germany. E. SCHMIDT, Graduate Student, and S. SRIDHAR, Professor, are with the Department of Materials Science and Engineering, Carnegie-Mellon University, Pittsburgh, PA 15213. Contact e-mail: [email protected] M. SAFI, former Graduate Student, Department of Ferrous Metallurgy, RWTH-Aachen University, is Engineer with Mittal-Steel, Hamburg, D-21129 Hamburg, Germany. Manuscript submitted August 6, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS B
cient, while the excellent formability will reduce production costs. The production of TRIP sheets involves continuous casting, hot rolling, and cold rolling, followed by a unique annealing cycle.[6] In order to develop a cost-effective continuous production of TRIP steel sheets for the car industry, it is necessary to control the oxide-scale structure that evolves during processing. To expand the market for thin-gage TRIP steels into body parts for cars that are exposed to corrosive atmospheres, it is crucial to be able to put a protective coating onto the steel surface. This can be difficult in these steels, where a high Si content leads to a surface oxide layer that is difficult to remove using acidic treatment. A good Zn coating cannot be achieved, as liquid Zn alloys do not readily wet oxide surfaces. Uncoated regimes, called bare spots, are the result. As a consequence, economical production of TRIP steels using existing galvanizing lines may not be possible. Maki and co-workers[7] studied oxidation during annealing and identified a stable Mn2SiO4 oxide film on the su
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