《Lathlike upper bainite in a silicon steel》.pdf

Materials Science and Engineering A 434 (2006) 289–293 Lathlike upper bainite in a silicon steel Cheng Liu a,b,∗, Zhenbo Zhao c , S.D. Bhole a a Department of Mechanical and Industrial Engineering, Ryerson University, 350 Victoria St., Toronto, Ontario M5B 2K3, Canada b School of Materials Engineering, Changchun University of Technology, 17 Yanan Road, Changchun 130012, China c Unimotion Gear, Magna Powertrain Inc., 245 Edward Street, Aurora, Ontario L4G 3M7, Canada Received 26 May 2006; received in revised form 26 June 2006; accepted 27 June 2006 Abstract The morphology and mechanical properties of upper bainite formed isothermally at 400 ◦C for different holding times in a 1.83 wt.% silicon steel have been investigated by optical metallograph, X-ray diffraction and transmission electron microscopy (TEM). In the early stage of upper bainitic transformation, lathlike bainite whose individual lath ferrite is separated by the thin film type of retained austenite is obtained. As the isothermal holding times are increased, the blocky region consisting of retained austenite and martensite is also found. The stability of retained austenite in lathlike upper bainite is studied in relation to the isothermal treatment times, and the heat treatment conditions. The results show that an optimum combination of strength and ductility is attributed to the formation of bainitic ferrite (BF) and a large amount of thin film carbon-enriched retained austenite in the upper bainite. © 2006 Elsevier B.V. All rights reserved. Keywords: Bainitic ferrite; Silicon steel; Retained austenite; Mechanical property 1. Introduction ite in conjunction with BF can pr