Dynamic recrystallization of ferrite in a low-carbon steel
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INTRODUCTION
RECENTLY, warm rolling (ferritic rolling) of lowcarbon steels has received much attention due to its potential to broaden the product range and reduce the production costs.[1] The deformation and recrystallization of ferrite are therefore an integral part of the physical metallurgy of warm rolling. It is generally considered that dynamic recovery (DRV) is the sole dynamic restoration process of ferrite phase (bcc) during deformation at elevated temperature. However, since the 1970s, dynamic recrystallization (DRX) of ferrite has been found to occur in highly pure irons,[2,3] interstitial-free (IF) steels,[4,5,6] ferritic stainless steels,[7,8,9] and low-carbon steels, which deformed in the (F A) twophase range.[10,11,12] Further, in our study of deformationenhanced transformation of austenite to ferrite in a low-carbon steel,[13] it was found that the occurrence of DRX of ferrite led to the further refinement of ferrite grains after the transformation of austenite to ferrite. In order to develop a novel process to refine the ferrite grains, it is necessary to make clear the mechanism of DRX of ferrite that occurred in the low-carbon steel. In the present study, the hot deformation behavior of a low-carbon steel is investigated by plane strain compression tests at temperatures below Ac1, i.e., in the ferrite-phase region, and the microstructure evolution is investigated by means of scanning electron microscopy, transmission electron microscopy (TEM), and electron backscattered diffraction (EBSD). It is found that the DRX of ferrite does occur in this low-carbon steel under a wide range of hot deformation conditions. II. EXPERIMENTAL The material used in this study is a low-carbon steel containing 0.171C, 0.013S, 0.017P, 0.09Si, 0.36Mn, 0.02Cr, LI LONGFEI and YANG WANGYUE are with the School of Materials Science & Engineering, University of Science and Technology Beijing, 100083 Beijing, People’s Republic of China. Contact e-mail: wyyang@ mater.ustb.edu.cn SUN ZUQING is with The State Key Laboratory for Advanced Metals and Materials, University of Science & Technology Beijing, 100083 Beijing, People’s Republic of China. Manuscript submitted January 13, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS A
0.03Ni, 0.01Cu, 0.025Al, 0.01Mo, and the balance Fe (wt pct). The specimens of 10-mm height 15-mm width 20-mm length were machined from hot-forge and air-cooled bars of 25-mm height 15-mm width 1-m length; the forge temperature range was 1000 °C to 850 °C. The plane strain compression tests were performed on a Gleeble 2000 hot simulation machine (Figure 1(a)). In order to avoid the influence of → transformation on the study of DRX, the specimens were directly heated to 700 °C (under the Ac1, 740 °C), and held for 5 minutes; they were then cooled to the deformation temperatures of 700 °C to 550 °C and compressed at a strain rate of 1 101 to 5 104 s1 (Figure 1(b)). The specimens were quenched immediately after deformation by spraying water, which is automatically controlled by computer. Sc
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