Effects of Solute Nb Atoms and Nb Precipitates on Isothermal Transformation Kinetics from Austenite to Ferrite
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NG steel manufacturing, many alloying elements are added for various purposes. Nb is one of the most important micro-alloying elements in low–carbon steels. The large misfit between Nb atoms and the austenite matrix makes the solubility of Nb in austenite low, and thus the Nb content in low-carbon steels is normally quite low, i.e., less than 0.05 wt pct. Nb has a strong tendency to form a carbo-nitride, and thus there are two typical forms of Nb present in steels during steel manufacturing, which are solute Nb atoms and Nb carbo-nitride precipitates.[1–5] Nb addition in steels is beneficial for mechanical properties by grain size refinement and precipitation strengthening. Nb can also be used to control the transformation kinetics, and thus hardenability is improved by the presence of solute Nb.[1–7] The effects of Nb on steel manufacturing have been studied widely by many researchers. Many of them state that Nb has a retardation effect on phase transformation kinetics in steels,[6–18] and even a small amount of Nb
LI WANG, formerly PhD student with the Department of Materials, Loughborough University, Loughborough LE11 3TU, UK, is now Engineer with Jaguar Land Rover, Coventry. Contact email: [email protected] SALLY PARKER, Principal Research Metallurgist, and ANDREW ROSE, Principal Investigator, are with the Materials Design Department, Tata Steel R\&D, Swinden Technology Centre, Rotherham S60 3AR, UK. GEOFF WEST, formerly Research Fellow with Loughborough University, is now Senior Research Fellow with the University of Warwick, Coventry, UK. RACHEL THOMSON, Professor, is with the Department of Materials, Loughborough University. Manuscript submitted September 2, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS A
(i.e., less than 0.05 wt pct) can result in significant effects on transformation behavior and mechanical properties. The presence of Nb in steels also affects the final microstructure after phase transformations.[18] However, the effects of Nb on isothermal transformation kinetics from austenite to ferrite have not yet been fully characterized and quantified. In particularly, the two forms of Nb in steels, solute Nb atoms and Nb precipitates, can affect phase transformation kinetics via different mechanisms, which are the solute drag effect and particle pinning effect. Since there is still controversy about the exact nature of the retardation mechanism,[7,16] advanced microscopy techniques can be utilized for samples with different heat treatments to characterize the solute drag effect and the particle pinning effect. The solute drag effect has been demonstrated by many previous researchers.[19–23] Its basic requirement is the segregation of solute atoms to the transformation interface, and thus the grain boundary mobility is inhibited. Felfer et al.[23] have utilized atom probe study to find that the interfacial excess Nb atoms at prior austenite grain boundaries are more than those at ferrite-ferrite grain boundaries. Fazeli and Militzer[24] have successfully studied the solute drag effect in Fe-C-Mn ternary syste
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