Characterization of clusters and ultrafine precipitates in Nb-containing C–Mn–Si steels
Introduction
The strength of TRansformation Induced Plasticity (TRIP) steels typically ranges from 500 to 800 MPa. The conventional composition of these steels is 0.15–0.2 wt.%C–1.5 wt.%Mn–1.5 wt.% Si (0.68–0.91 at.%C–1.54 at.%Mn–3.01 at.%Si) with possible substitution of Si by Al and/or addition of Mo and P [1], [2], [3], [4]. Further increases in strength may be achieved by microalloying with Nb, Ti or V. These additions lead to a refinement of the microstructure and the precipitation of carbides or carbo-nitrides.
Recently, with advances in atom probe tomography, attention has been drawn to the formation of solute clusters in steels and light alloys [4], [5], [6], [7], [8], [9], [10], [11], [12]. Clusters play an important role also in nucleation processes of second-phase precipitates [11], [12] and in determining the mechanical properties of materials. It has been suggested that these clusters present a network of elastically soft and relatively diffuse obstacles for the movement of dislocations [9]. As a result, an increase in strength is achieved while maintaining good toughness [9].
This paper addresses the formation of nanosized particles and clusters in Nb-containing TRIP steels with additions of Mo and/or Al after laboratory simulated thermomechanical processing.
Section snippets
Experimental
The compositions of C–Mn–Si TRIP steels with additions of Nb, Al and Mo are shown in Table 1. All the steels were subjected to laboratory simulated thermomechanical processing described in detail elsewhere [13], [14], [15]. After the final rolling pass at 875 °C, the steels were slowly cooled through the ferrite formation region, followed by accelerated cooling from TAC (Table 1) to avoid the formation of pearlite. After an isothermal hold at 450–500 °C (Table 1) during which bainite was formed,
TEM characterization
The microstructure of these steels comprises ∼50% polygonal ferrite (PF), ∼30% of bainite, (including upper bainite (UB) and carbide-free morphologies of granular bainite (GB) and acicular ferrite (AF)) and some retained austenite (RA) and martensite (M). A detailed description of the microstructure of these steels is given elsewhere [13], [14], [15]. The presence of fine particles was observed by TEM in PF and bainite in all the Nb-alloyed steels (Fig. 1a and b). These particles were
Conclusions
In addition to the strain-induced Nb-containing precipitates detected by TEM, a high number density of uniformly distributed ultrafine (0.8 to ∼7 nm) clusters and precipitates was observed by APT in all the steels studied. Their number density was the lowest in the ferrite in the Nb–Al steel, but still exceeded the number density of strain-induced precipitates by several orders of magnitude. The number density of clusters increased by an order of magnitude in the martensite compared to the
Acknowledgements
The authors would like to thank Prof. P.D. Hodgson, Deakin University for discussion of the paper. Research at the SHaRE User facility was sponsored by the Division of Materials Sciences and Engineering, US Department of Energy, under Contract DE-AC05-00OR22725 with UT-Battelle, LLC (atom probe, MKM, EVP).
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On Outside Study Program at Oak Ridge National Laboratory.