Effect of Composition and Deformation on Coarse-Grained Austenite Transformation in Nb-Mo Microalloyed Steels
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VALUE-ADDED applications of microalloyed steels continue to be developed to meet the increasing material demands of a variety of structural applications. Most of the required properties can be obtained via two main strategies: chemical alloying and optimization of the processing parameters. Suitable combinations of alloying element additions, such as Nb, Ti, Mo, or B, contribute to an increase in strength directly through microstructural refinement, solid solution strengthening, and precipitation hardening, as well as, indirectly, through enhanced hardenability and associated modification of the resultant microstructure.[1] Nevertheless, toughness may be impaired depending on the selected processing strategy followed to achieving the strength requirements. Nowadays, combinations of high resistance and high toughness are required for applications such as gas and oil transportation pipes, offshore facilities, and naval technologies.[2–4] Reducing grain size improves both strength and toughness, and the best way to obtain fine microstructures in the final product is by applying thermomechanN. ISASTI, Graduate Student, and D. JORGE-BADIOLA, B. LO´PEZ and P. URANGA, Researchers, are with CEIT and TECNUN (University of Navarra), Donostia-San Sebastian, 20018, Basque Country, Spain. Contact e-mail: [email protected] M.L. TAHERI, Hoeganaes Assistant Professor of Metallurgy, is with Materials Department, Drexel University, Philadelphia, PA 19104. Manuscript submitted November 16, 2010. Article published online February 12, 2011 METALLURGICAL AND MATERIALS TRANSACTIONS A
ical processes to microalloyed grades. When controlled rolling is applied to Nb microalloyed steels, the austenite is rolled at temperatures low enough to produce straininduced precipitation of carbonitrides during deformation to inhibit recrystallization.[5] As is well known, the addition of Nb as an alloying element can retard or inhibit recrystallization due to two mechanisms: the solute drag effect owed to Nb atoms in solid solution and the pinning effect due to strain-induced precipitation, the latter usually exerting the strongest effect.[6,7] Some authors reported that the reduction of austenite grain size and an increase in retained strain provide a higher density of nucleation sites, which leads to a refinement of the final transformed microstructure[8,9] and a modification of the balance between the different ferrite morphologies. Occasionally, rolled products show heterogeneous austenite structure prior to transformation. For example, the presence of microstructural banding in rolled plates was reported as an origin for final heterogeneities in the microstructure in low-carbon microalloyed steels.[10] The microstructural bands are coarse bainitic phases formed in a uniform matrix of ferrite and are transformed from coarse austenite grains, which are developed when conditions favor abnormal grain growth or strain-induced grain boundary migration in the rolling passes.[11,12] It is also possible to have inappropriate austenite refinement before strain begins accumul
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