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two decades, research has been devoted to developing the austempered high silicon cast steel[1–10] with a chemical composition similar to the matrix of the austempered ductile iron (ADI). This kind of steel demonstrates a combination of higher mechanical properties such as strength, ductility, fatigue strength, fracture toughness, impact strength, and wear resistance. Austempered high silicon cast steel has great potential to be a new structural material of low cost and high reliability.[5] Although the tensile strength and yield strength of austempered high silicon cast steel have reached the level of high strength steel, the toughness of the steel is still an obstacle to meet the demands of some service conditions under high stress. Also, the dendrite structure in austempered high silicon cast steel is usually coarse due to the primary crystallization, and thus causes severe dendrite segregation. The content of carbon and other alloy elements between the dendrites is at a high level and the dendritic structure and segregation could not be eliminated at the following heat treatments; consequently, there is a large amount of blocky retained austenite in the austempering structure. Literature[11–16] has shown that the blocky retained austenite is less thermal and mechanically stable. This instability can lead to brittle plate shape martensite occurring in the microstructure, which degrades the toughness of the steel. However, the thin-film-shaped retained austenite has a much higher thermal and mechanical stability, and transformation of austenite to martensite in the plastic zone at the tip of a propagating crack should reduce the energy available for crack propagation and hence improve brittle fracture resistance. So, some effective methods must be developed to reduce or eliminate the coarse dendrite structure and increase the volume of thin-filmshaped retained austenite and thus improve the toughness of austempered high silicon cast steel. It is shown that a XIANG CHEN, Ph.D., and YANXIANG LI, Professor, are with the Department of Mechanical Engineering, Tsinghua University, and Key Laboratory for Advanced Materials Processing Technology, Ministry of Education, Beijing 100084, People’s Republic of China. Contact e-mail: [email protected] Manuscript submitted April 13, 2006. METALLURGICAL AND MATERIALS TRANSACTIONS A

modifier containing titanium, vanadium, and rare earth metals (Ti-V-RE modifier) can effectively reduce the dendritic structure and segregation of carbon and alloy elements, and refine the austenitic grains.[17–20] The aim of this study is to investigate the microstructure and mechanical properties of austempered high silicon cast steel with and without treating with the Ti-V-RE modifier and to develop a practical method for improving the mechanical properties of the steel. II.

EXPERIMENTAL PROCEDURES

The chemical composition of the tested steels is presented in Table I. The steels were melted in a 500-kg medium-frequency coreless induction furnace with siliceous lining, with charge materials of s