Effects of Si and Al on acicular ferrite formation in C-Mn steel
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I. INTRODUCTION
BAINITE has been the subject of interest for many decades, since it may exhibit numerous microstructural variants and resulting mechanical properties of a wide range.[1–4] In recent years, considerable efforts[4,5] have been made to develop bainitic steels with high strength and good toughness for structural applications as alternatives to conventional ferrite-pearlite steels. Ultra low carbon bainitic steel is a good example of the bainitic structural steels.[6,7] The exceptional microstructure consisting of intragranularly nucleated ferrite laths 1- to 3-m wide has been often observed in steels inoculated with deliberately dispersed nonmetallic inclusions.[8–12] It exhibits an interwoven nature of ferrite laths, and these ferrite laths are believed to be nucleated at fine nonmetallic inclusions.[8–12] This structure has been usually referred to as “acicular ferrite.” This acicular ferrite should be discriminated from so-called acicular ferrite in high strength low alloy steels containing very low carbon contents, which is bainitic ferrite formed predominantly at prior austenite grain boundaries.[13] Although acicular ferrite has been regarded as intragranularly nucleated bainite, it shows a pronounced difference in microstructure from conventional bainite structures. The interwoven structure of nonparallel laths of acicular ferrite is in contrast to the sheaf structure of parallel laths that are formed predominantly at austenite grain surfaces of conventional bainite. Acicular ferrite has been known to provide an optimal combination of high strength and good toughness due to its refined and interwoven structure.[8,10,14] Although the mechanical properties of acicular ferrite are suitable for JAE-HYEOK SHIM, Postdoctoral Fellow, YOUNG WHAN CHO and JAE-DONG SHIM, Principal Researchers, and YOUNG-JOO OH, Senior Researcher, are with the Metals Processing Research Center, Korea Institute of Science and Technology, Seoul 136-791, Korea. JUNG-SOO BYUN, Graduate Student, and DONG NYUNG LEE, Professor, are with the School of Materials Science and Engineering, Seoul National University, Seoul 151-742, Korea. Manuscript submitted January 31, 2000.
METALLURGICAL AND MATERIALS TRANSACTIONS A
structural applications, the application of this structure has been rather limited.[5] This is presumably because acicular ferrite structures have not been found frequently in wrought steels that have relatively low densities of nonmetallic inclusions such as oxides, which can serve as the nucleation sites of acicular ferrite.[3] However, several attempts[8–12] have been made recently to obtain acicular ferrite structures in wrought steels by inoculating with selected nonmetallic inclusions as nucleants of acicular ferrite. It has been reported that Ti2O3 particles, which are formed intrinsically in Ti-containing low carbon steels in the steelmaking process, are very effective for the nucleation of acicular ferrite.[8,9,10] It has also been known that the formed Ti2O3 particles absorb a substantial amount of Mn from th
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