A quantitative atom probe study of the Nb excess at prior austenite grain boundaries in a Nb microalloyed strip-cast steel Peter J. Felfer a,⇑ , Chris R. Killmore b , Jim G. Williams b , Kristin R. Carpenter b , Simon P. Ringer a , Julie M. Cairney a a Australian Centre for Microscopy and Microanalysis, The University of Sydney, Madsen Building F09, NSW 2006, Camperdown, Australia b Metallurgical Technology, BlueScope Steel, Five Islands Rd, NSW 2500, Port Kembla, Australia Received 6 March 2012; received in revised form 5 June 2012; accepted 6 June 2012 Available online 22 July 2012 Abstract Most modern HSLA steels rely on the effect of Nb in steels to achieve the properties desired for a specific application. While the role of Nb in forming precipitates has been well characterized, its role in a solid solution is less well understood due to the difficulty of obtain- ing quantitative experimental data. In the current work, site-specific atom probe tomography was used to quantify the amount of Nb present at prior austenite grain boundaries in a commercial strip-cast steel, produced via the Castrip Ò process. This was compared to the amount of Nb found at ferrite–ferrite grain boundaries that had formed during the transformation from austenite to ferrite. With the interfacial excess Nb measured, thermodynamic calculations were carried out and compared to the change in transformation temperature obtained by dilatometry, with reference to a comparable Nb free, strip-cast steel. Ó 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Atom probe tomography; Bainitic steel; Grain boundary segregation; Nucleation of phase transformations; Strip-casting 1. Introduction In many microalloyed steels, Nb is used to improve the mechanical properties through grain refinement, precipita- tion hardening and the production of microstructures containing acicular ferrite, bainite and martensite. This happens through conditioning of the austenite grain structure and/or shifting of the austenite (c)–ferrite (a) transformation to lower temperatures. The role of Nb in controlling the austenite microstructure through precipita- tion of Nb(C,N) is firmly established and understood on the micrometer length scale, as reviewed by DeArdo [1]. The microstructural evolution of these precipitates has largely been characterized using transmission electron microscopy. It has been established that Nb(C,N) precipi- tation in the austenite regime takes place almost exclusively at lattice defects such as grain boundaries and dislocations, owing to the large lattice mismatch between Nb(C,N) and both a and c Fe [2]. These lattice defects are usually intro- duced by hot deformation, such as rolling and forging, and precipitation typically takes >100 s to occur. In steels where the processing time/temperature/defor- mation scheme is too rapid for precipitation to occur, Nb will act in solid solution through segregation to lattice defects, mainly grain boundaries. Nb in solid solution lowers the c!a transformation temperature, leading to non-polygonal ferrite microstructures and increased yield strength, with a pronounced dependency on cooling rates. This effect has generally been attributed to solute drag of the prior austenite grain boundaries. Suehiro et al. [3] found that, in a 0.045 at.% Nb steel, solute drag signifi- cantly lowered the transformation temperature if the cool- ing rate was >10 K s 1 . The effect of Nb in solution is more difficult to characterize than particle precipitation, and is therefore less well understood. Its impact on transformation and microstructural evolution in the austenite regime has been 1359-6454/$36.00 Ó 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.actamat.2012.06.013 ⇑ Corresponding author. Tel.: +61 2 9351 7679. E-mail address: peter.felfer@sydney.edu.au (P.J. Felfer). www.elsevier.com/locate/actamat Available online at www.sciencedirect.com Acta Materialia 60 (2012) 5049–5055