Strengthening Mechanisms in Thermomechanically Processed NbTi-Microalloyed Steel
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MICROSTRUCTURE and mechanical properties of steel evolve from a combined effect of chemical composition and parameters of thermomechanical processing (TMP). In high-strength low-alloy steels, addition of precipitate-forming elements, such as Ti, Nb, or V, is commonly applied. High-temperature precipitation retards austenite (c) recrystallization via grain boundary pinning,[1,2] which facilitates ferrite (a) grain refinement. Low-temperature precipitation results in precipitation strengthening[3] and may stimulate dislocation generation which enhances work hardening.[4,5] Following TMP, the steel strength usually increases with (1) an increase in reheating temperature,[6,7] which can be attributed to particle dissolution, further precipitation during rolling at a finer scale, stronger grain boundary pinning, and grain refinement; (2) an increase in deformation strain,[6,8] due to ANDRII G. KOSTRYZHEV, Research Fellow, is with the Faculty of Engineering and Information Sciences, School of Mechanical, Materials and Mechatronics Engineering, University of Wollongong, Northfields Avenue, Wollongong, NSW 2522, Australia. Contact e-mail: [email protected] OLEXANDRA O. MARENYCH, Honorary Research Fellow, is with the UOW Electron Microscopy Centre, University of Wollongong, Squires Way, Wollongong, NSW 2519, Australia. CHRIS R. KILLMORE, Product Innovation Specialist: Manufacturing, Construction, and Mining, is with the BlueScope Steel Limited, Five Islands Road, Port Kembla, NSW 2505, Australia. ELENA V. PERELOMA, Professor of Physical Metallurgy, Director, is with the Faculty of Engineering and Information Sciences, School of Mechanical, Materials and Mechatronic Engineering, University of Wollongong, and also with the UOW Electron Microscopy Centre, University of Wollongong. Manuscript submitted February 4, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS A
grain refinement, work hardening (dislocation density increase), and strengthening by strain-induced precipitates; (3) a decrease in finish rolling temperature,[7,9–11] due to grain refinement, work hardening (especially if deformation is carried out in two-phase a + c region), and transformation strengthening (formation of bainite or acicular ferrite); and (4) an increase in cooling rate,[10] due to grain refinement and transformation strengthening. Although these qualitative trends of microstructure property development during TMP are well understood, comparative contributions of each of the strengthening mechanisms (grain refinement, solid solution strengthening, precipitation strengthening, and work hardening) and their variations with steel composition and TMP parameters still require clarification. For example, in a steel containing 0.044C-0.25Mo-0.0554Nb-0.014Ti, the ferrite grain refinement, solid solution strengthening, and work hardening together contributed up to 65 pct to the yield stress, and the precipitation and transformation (presence of bainite) strengthening contributed 35 pct.[12] In another example, in a steel containing 0.064C-0.0625Nb-0.0435Ti the grai
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