Solidification Microsegregation and Hot Ductility of Fe-Mn-C-Al-xNb TWIP Steels
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HIGH Mn twinning-induced plasticity (TWIP) steel is a type of structural steel, characterized by both high strength and superior formability.[1–4] Deformation twinning is known to be the main strain hardening enhancing mechanism in TWIP steels via the dynamic Hall–Petch effect.[5–7] TWIP steels generally show a large strength-ductility balance, i.e., the product of ultimate tensile strength (UTS) and total elongation (TE), in excess of 50,000 MPa pct. During straightening operation of continuous casting processes, casting defects such as transverse cracks are easily formed in steel.[8,9] Hot ductility from high-temperature tensile tests can assess susceptibility to cracking during continuous casting.[8] Hot ductility of TWIP steels has been an important focus of research due to high sensitivity of surface cracking in TWIP steels.[10–22] MIN HYEOK KWON is with the Graduate Institute of Ferrous Technology, Pohang University of Science and Technology, Pohang, 37673, South Korea. JIN-KYUNG KIM is with the Graduate Institute of Ferrous Technology, Pohang University of Science and Technology and also with the Department of Energy Science, Sungkyunkwan University (SKKU), Suwon, 16419, Korea. Contact e-mail: [email protected] JIAN BIAN is with the CBMM Asia PTE Ltd, Singapore, 049315, Singapore. HARDY MOHRBACHER is with the NiobelCon bvba, 2970, Schilde, Belgium. TAEJIN SONG and SUNG KYU KIM is with the Steel Products Research Group, Technical Research Laboratories, POSCO, Gwangyang, 57807, South Korea. BRUNO C. DE COOMAN is with the Graduate Institute of Ferrous Technology, Pohang University of Science and Technology and also with the NLMK Group, Moscow, Russia, 119017. Manuscript submitted March 8, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
In TWIP steels, reduced Mn concentration resulted in an increase in hot ductility.[10,17] Bleck et al.[10] suggested that the wide solidification range of high Mn steels promoted micro-shrinkage and inter-dendritic segregation. Lan et al.[16] reported that reduced matrix homogeneity by Mn micro-segregation and micro-porosity led to a decrease in hot ductility of a Fe-22Mn-0.6C (wt pct) TWIP steel. Kang et al.[12,18–22] investigated the effect of various alloying elements such as Al, B, Ti, Nb, V, P, N, and S on hot ductility of TWIP steels. Micro-alloying has been considered as a way to increase strength of TWIP steels.[23–26] Previous works reported increases of strength of TWIP steels by Nb addition.[24,26] Nb addition in TWIP steels also noticeably retards grain recrystallization as compared to micro-alloying with Ti or V.[25] Mejia et al.[15] reported beneficial influence of Nb addition on the hot ductility of a Fe-21Mn-1.3Al-1.5Si-0.5C (wt pct) TWIP steel, particularly in a temperature range of 1073 K to 1173 K (800 °C to 900 °C). However, the role of Nb addition in hot ductility of TWIP steels has not been clearly described. Development of Nb-added TWIP steels requires understanding of the impact of Nb addition on castability, solidification microstructures, and high-tempera
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