Microstructure Evolution of a Pearlitic Steel during Hot Deformation of Undercooled Austenite and Subsequent Annealing
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INTRODUCTION
MATERIALS with ultrafine grains exhibit ultrahigh hardness, outstanding compressive and tensile strengths, and superplastic properties at relatively high temperatures. However, the ductility of such materials at room temperature is low due to the diminishing strain-hardening capacity and the inadequate strain-rate hardening. Recently, much effort has been spent on improving the ductility of materials with ultrafine grains by microstructure adjustment, e.g., pure copper maintaining the majority of the grains in a nanocrystalline to ultrafine range with some coarser grains,[1] Ti-based alloys with a composite microstructure of a nanostructured matrix and ductile dendritic phase,[2] Al-base alloy,[3] and plain carbon steels[4–19] manipulated by the dispersed secondphase particles. Particularly in the case of ultrafinegrained steels with average grain sizes of less than 1 lm, it has been proven that a certain uniform elongation can be obtained by the dispersion of martensite[4,5] or cementite.[6–19] Since an excellent combination of high yield strength and low ductile-brittle transition temperature can be achieved by grain refinement without any costly alloying, the improvement of the ductility of plain carbon steels with ultrafine grains has its advantage for the potential application in structural materials over the steels with high alloy contents.
LONGFEI LI, Assistant Professor and ZUQING SUN, Professor, State Key Laboratory for Advanced Metals and Materials and WANGYUE YANG, Professor, School of Materials Science and Engineering, are with the University of Science and Technology Beijing, Beijing 100083, P.R. China. Contact e-mail: [email protected]. edu.cn Manuscript submitted October 31, 2006. Article published online January 19, 2008 624—VOLUME 39A, MARCH 2008
For high-carbon steels, lamellar pearlite, which consists of ferrite and lamellar cementite, is usually formed by the transformation of austenite during slow cooling from the austenite phase region. It is well established that the lamellar pearlite reduces the ductility of steels and thus significantly limits their applications. On the other hand, spheroidized cementite is more beneficial for toughness, cold formability and machinability. Therefore, pearlitic steels commonly undergo annealing treatment at temperatures below Ac1 in the temperature range of 873 to 1023 K with the aim of spheroidizing the original lamellar structure. The main disadvantage of this process, generally by bath annealing, is the exceptionally long throughput time of up to 2 days. Thus, it is of great interest to accelerate the spheroidization process. Special heat treatments[14,20,21] and thermomechanical processes[15–19,22–26] have been developed to accelerate the spheroidization of lamellar pearlite. Moreover, among these investigations, the microstructure with ultrafine ferrite matrix and spheroidized cementite particles, with both grain sizes less than 1 lm (also called ‘‘the microduplex structure’’) demonstrates a good balance between strength and ductility,[14–19] which is f
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