Increase of Austenite Grain Coarsening Temperature in Banded Ferrite/Pearlite Steel by Cold Deformation
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ibition of austenite grain growth above the eutectoid temperature in steels is of great importance in the steel production processes. The presence of second phase particles substantially changes the grain growth behavior of austenite. Early attempts at grain refinement have centered on the formation of AlN precipitates by the addition of Al and N in steels.[1] The other additives such as Ni, V, Ti, and Nb have also been shown to lead to grain refinement due to the formation of carbides or nitrides.[2–5] At lower temperatures, the precipitates are stable, and the austenite grain boundaries can be effectively pinned, and the grain growth is thus suppressed. At elevated temperatures, however, the second phase particles are unstable, and they readily coarsen or dissolve.[6] Then, unpinning might occur, and the grain growth becomes abnormal. During abnormal grain growth (AGG), a few large grains grow discontinuously and consume the neighboring matrix of smaller grains, which results in the development of extraordinarily large grains.[7] A critical temperature exists above which the grain growth kinetics changes from normal to abnormal. This temperature is defined as the grain coarsening temperature, Tc. Furthermore, when the temperature is much higher than Tc, normal grain growth (NGG) again
XIANGUANG ZHANG, Ph.D. Student, is with the Graduate School of Engineering, Hokkaido University, Kita 13 Nishi 8, Kita-ku, Sapporo, Hokkaido 060-8628, Japan. Contact e-mail: [email protected] KIYOTAKA MATSUURA, Professor, and MUNEKAZU OHNO, Associate Professor, are with the Division of Materials Science and Engineering, Faculty of Engineering, Hokkaido University. Manuscript submitted August 25, 2014. Article published online October 16, 2014 32—VOLUME 46A, JANUARY 2015
occurs due to the nearly complete dissolution of pinning particles.[6] AGG of austenite during manufacturing processes should be avoided because the mechanical properties of steels are degraded due to a bimodal grain size distribution of prior austenite caused by AGG.[8] The Tc of steels is an important factor in determining the processing parameters so as to avoid the occurrence of AGG. Increasing Tc is an effective way to increase the stability of the steels against abnormal grain coarsening during thermal processing.[9] There are numerous studies about the austenite grain growth behavior in many types of microalloyed steels. While AGG takes place in all steels alloyed with Al, Ti, V, and Nb during austenitizing treatment, Tc is quite different depending on the type and concentration of microalloying elements.[10] Ferrite/pearlite (F/P) banding is commonly observed in hot-rolled low-alloy steels.[1,11–17] Steels with the F/P banded structure, called the F/P banded steel in this paper, are usually reheated prior to further processing such as carburization, nitriding, and ausforming etc. In the authors’ recent work,[18] it was found that Tc of the F/P banded 0.2 mass pct steel is about 1173 K (900 °C) which is quite lower than that of the non-banded steel (1323 K (1
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