Reverse transformation of ferrite and pearlite to austenite in an ultrafine-grained low-carbon steel fabricated by sever
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TERCRITICAL annealing, in which low- or mediumcarbon steels are reheated from ambient temperature to the temperature range of ferrite/austenite two-phase region and held for desired duration, is useful to produce various kinds of multiphase steels such as ferrite/martensite dual-phase steels and transformation-induced plasticity steels.[1] One of the major advantages of such steels is that the desired combination of mechanical properties can be achieved by tailoring the characteristics of constituent phases, i.e., volume fraction, size, distribution, etc., with a selection of appropriate intercritical annealing conditions. In such a case, understanding of reverse transformation characteristics during intercritical annealing becomes important to optimize the intercritical annealing conditions. Meanwhile, recent development of ultrafine-grained (UFG) ferrite/pearlite steels using various severe plastic deformation techniques is of great interest due to their attractive ultrahigh strength.[2,3] In addition, it was recently reported that UFG ferrite/martensite dual-phase steel with a very good combination of ultrahigh strength and extended ductility was successfully produced by intercritical annealing of UFG ferrite/pearlite steel.[4] The factors influencing reverse transformation kinetics, such as driving force, austenite YOUNG IL SON, Senior Researcher, is with Technology Research Center of Agency for Defense Development, Taejon 305–600, Korea. YOUNG-KOOK LEE, Associate Professor, is with Department of Metallurgical Engineering, Yonsei University, Seoul 120-749, Korea. KYUNGTAE PARK, Associate Professor, is with Division of Advanced Materials Science and Engineering, Hanbat National University, Taejon 305-719, Korea. Contact e-mail: [email protected] Manuscript submitted November 24, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS A
nucleation sites, etc., of UFG ferrite/pearlite steel fabricated by severe plastic deformation would not be the same as those of their coarse-grained (CG) counterpart due to an UFG structure and large accumulation of strain energy in the former. Therefore, in this study, reverse transformation characteristics of UFG and CG ferrite/pearlite steels were compared by analyzing their dilatational response during heating. A plain low-carbon steel (Fe-0.15C-0.25Si-1.1Mn (wt pct)) was austenitized at 1200 °C for 1 hour and then air cooled: hereafter, un-ECAPed steel. After austenitization, some of the steel was subjected to equal channel angular pressing (ECAP) at 500 °C up to the effective strain of ;4 with route C[2]: hereafter, ECAPed steel. After machining the samples of 1 3 3 3 10 mm3 from both ECAPed and unECAPed steels, a series of dilatometric tests was conducted by heating the samples from room temperature to 1100 °C with a heating rate of 10 °C/s, followed by cooling to room temperature with a cooling rate of 50 °C/s. The dilatational strain to temperature data were collected and processed by a data acquisition system. Microstructures were examined by optical microscopy, scanning electron
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