Phase transformation and mechanical properties of si-free CMnAl transformation-induced plasticity-aided steel
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INTRODUCTION
IN order to reduce the automotive body weight without loss of crash-worthiness, the automotive industry has increased its use of formable high-strength steel. In general, standard high-strength sheet steel does not have good formability, and considerable research has been carried out to develop steel grades with both a high-strength and a high uniform elongation.[1,2,3] The transformation-induced plasticity (TRIP) effect is a very promising solution for obtaining high-strength steels which have a better formability than conventional high-strength steels. The standard CMnSi TRIP-aided steel typically contains about 0.15 mass pct C, 1.5 mass pct Mn, and 1.5 mass pct Si. The presence of these elements results in the formation of a very stable Mn2SiO4 oxide film on the surface during the annealing process. This oxide inhibits the galvanizability,[4] and, as a result, the TRIP-aided steels are generally electrogalvanized rather than hot-dip galvanized. Figure 1 shows the thermal cycle used to obtain the typical TRIP-aided steel microstructure consisting of ferrite, bainite, and retained austenite. It consists of a twostep process: an intercritical anneal (IA) and an isothermal bainitic transformation (IBT). At the end of the IA, the steel contains 50 vol pct austenite and 50 vol pct ferrite. The C content in austenite increases to about twice the initial C content, since the C solubility in ferrite is approximately 0.02 mass pct and the C solubility in the austenite at typical intercritical temperatures is in the range of 0.3 J. MAHIEU, Graduate Student, and B.C. De COOMAN, Professor, are with the Laboratory for Iron and Steelmaking, Ghent University, 9052 Ghent, Belgium. Contact e-mail: [email protected] J. MAKI, Visiting Scientist, Laboratory for Iron and Steelmaking, Ghent University, is on leave from the Yawata R&D Laboratory, Nippon Steel Corporation, Tobihata-cho, Japan. S. CLAESSENS, Product Research Manager, is with OCAS NV, Research Centre of the Sidmar Group, ARBED Group Flat Rolled Products Division, B-9060 Zelzate, Belgium. Manuscript submitted June 15, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
to 0.4 mass pct. After the IA, TRIP-aided steels are quenched to an IBT temperature in the range between the bainite start (Bs) and martensite start (Ms) temperatures, in order to transform the austenite to a low-C bainite and a highly C-enriched metastable austenite, for which the Ms temperature is below room temperature. The bainite transformation involves a diffusionless mechanism, the critical limit of which is the allotropic phase-boundary line (T0). The free energy of the ferrite and the austenite is a function of both the temperature and their composition. At any temperature (T ), there is a common composition at which the free energy of ferrite equals the free energy of austenite of identical composition. The bainitic transformation can only take place at a temperature below the T0 line. As Si effectively suppresses the formation of cementite, the IBT in high-Si alloys leads to the
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