Microstructural evolution of submicron sized ferrite in bimodal structural ultrafine grained ferrite/cementite steels by
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ON-MANGANESE (C-Mn) steels are economical structural materials because they involve no costly alloying. The ultrafine grained ferrite/cementite microstructure, generally characterized by an ultrafine grained ferrite matrix and nanosized globular cementite particles, has been developed very recently in these economical structural steels (hereafter, UGF/C steels).[1–4] Among the different strengthening mechanisms of steels, grain refinement is generally believed to improve the strength and decrease ductile-brittle transition temperature (DBTT) simultaneously.[5,6,7] Therefore, UGF/C steels are commercially attractive as advanced structural materials due to their potential for high strength and low DBTT as well as no costly alloying. Much work concerning UGF/C steels has focused on the refinement of the ferrite grain size, and average ferrite grain sizes of smaller than a few microns and even smaller than 1 mm have been successfully developed by a heavy deformation in the laboratory or on a pilot scale.[1–4] Meanwhile, the microstructure evolution during deformation and annealing was investigated.[3,4] In this type of cementite particle spheroidized UGF/C steels, the size and the distribution of cementite particles were believed to be the critical factors affecting the microstructural evolution, especially the cementite particle space. However, there are currently little explicit results. On the other hand, a number of investigations have been made on the microstructural evolutions of bainite, martensite, and normal ferrite at elevated temperatures during annealing.[8–15] The UGF/C microstructures MING-CHUN ZHAO, Postdoctoral Fellow, and TOSHIHIRO HANAMURA, and HAI QIU, Senior Researchers, are with the Steel Research Center, National Institute for Materials Science, Tsukuba, Ibaraki 3050047, Japan. Contact e-mail: [email protected] or [email protected] KE YANG, Professor, is with the Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, People’s Republic of China. Manuscript submitted July 24, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS
should have some different characteristics in the microstructural evolution during annealing from these microstructures because of their extraordinary fine ferrite grain size and the characteristic distribution of nanosized cementite particles within their microstructures. This work attempts to investigate the microstructural evolution of the submicron sized ferrite in UGF/C steels by annealing below the austenized temperature, with a special emphasis on the role of the cementite particle spacing. II.
EXPERIMENTAL PROCEDURE
The chemical compositions of the test steels, identified as steel A and steel B, are listed in Table I. The UGF/C microstructures were obtained by the caliber warm rolling followed by annealing below the austenized temperature. The ingots were melted in vacuum in a laboratory scale, homogenized for 60 minutes at 1473 K, and hot forged to 115-mm-diametral rods. Then the hot-forged rods were reheated for 60 minutes at 1173 K; caliber groove rolled
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