Influence of Deformation on the Precipitation Behavior of Nb(CN) in Austenite and Ferrite
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INTRODUCTION
MICRO-ALLOYED steels play an important role in the automotive, pipeline, and high strength structural steel industry. A key micro-alloying element is Nb. By adding small amounts of less than 0.1 wt pct Nb, a significant improvement of the mechanical properties can be achieved.[1,2] At temperatures, where Nb is in solid solution, it retards the recrystallization and grain growth of the austenite by the so-called solute drag effect.[3–5] Nb precipitates, i.e., carbides, nitrides or carbonitrides, also retard recrystallization and grain growth.[2,5–8] Investigations by Vervynckt et al.[5] have shown that Nb precipitates are more effective than Nb in solid solution. The most effective retardation is obtained with a fine dispersion of Nb precipitates with a diameter of about 20 nm.[9]
MATTHIAS NO¨HRER, Ph.D. Student, formerly Christian Doppler Laboratory for Early Stages of Precipitation, Department of Physical Metallurgy and Materials Testing, Montanuniversita¨t Leoben, Franz-Josef-Straße 18, 8700 Leoben, Austria, is now with buntmetall amstetten Ges.m.b.H., 3300 Amstetten, Austria. WALTER MAYER, Ph.D. Student, and ERNST KOZESCHNIK, Professor, are with the Christian Doppler Laboratory for Early Stages of Precipitation, Vienna University of Technology, Favoritenstrasse 9-11, 1040 Vienna, Austria. SOPHIE PRIMIG, Scientist, is with the Christian Doppler Laboratory for Early Stages of Precipitation, Department of Physical Metallurgy and Materials Testing, Montanuniversita¨t Leoben. Contact e-mail: [email protected] SABINE ZAMBERGER, Ph.D. Student, is with the voestalpine Stahl Donawitz GmbH & Co KG, Kerpelystraße 199, 8700 Leoben, Austria, and also with the Christian Doppler Laboratory for Early Stages of Precipitation, Vienna University of Technology. HARALD LEITNER, Scientist, formerly with the Christian Doppler Laboratory for Early Stages of Precipitation, Department of Physical Metallurgy and Materials Testing, Montanuniversita¨ Leoben, is now with Bo¨hler Edelstahl GmbH & Co KG, 8605 Kapfenberg, Austria. Manuscript submitted April 8, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS A
The formation of Nb precipitates in the austenite can be facilitated by deformation.[10–13] While austenite grain boundaries act as nucleation sites without deformation,[14] deformation-induced nucleation occurs at dislocation networks which are introduced during deformation in deformed samples.[10–12,15,16] Due to the generation of dislocations, the number of nucleation sites and, thus, the absolute number of precipitates increases. Furthermore, the diffusion of Nb is accelerated by pipe diffusion along dislocation lines.[10–12] The incubation time for nucleation, the growth, and the coarsening kinetics are influenced by deformation. The higher the accumulated strain, the shorter is the incubation time[6,13,14] and the higher is the coarsening rate.[7,12] Nb precipitates formed in the austenite do not have an as-pronounced strengthening effect as the particles formed in the ferrite, because they lose their coherence whe
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