Characterization of NbC and (Nb,Ti)N nanoprecipitates in TRIP assisted multiphase steels

Abstract

Multiphase steels utilising composite strengthening may be further strengthened via grain refinement or precipitation by the addition of microalloying elements. In this study a Nb microalloyed steel comprising martensite, bainite and retained austenite has been studied. By means of transmission electron microscopy (TEM), we have investigated the size distribution and the structural properties of (Nb,Ti)N and NbC precipitates, their occurrence in the various steel phases, and their relationship with the Fe matrix. (Nb,Ti)N precipitates were found in ferrite, martensite, and bainite, while NbC precipitates were found only in ferrite. All NbC precipitates were found to be small (5 -20 nm in size) and to have a face centred cubic crystal structure with lattice parameter a = 4.36 ± 0.05 Å. In contrast, the (Nb,Ti)N precipitates were found in a broader size range (5 -150 nm) and to have a face centred cubic crystal structure with lattice parameter a = 8.09 ± 0.05 Å. While the NbC precipitates were found to be randomly oriented, the (Nb,Ti)N precipitates have a well-defined Nishiyama Wasserman (N-W) orientation relationship with the ferrite matrix. An analysis of the lattice mismatch suggests that the latter precipitates have a high potential for effective strengthening. DFT calculations were performed for various stoichiometries of NbC X and Nb X Ti Y N Z phases and the comparison with experimental data indicates that both the carbides and nitrides are deficient in C and N content.

Novel ultrafine Fe(C) precipitates strengthen transformation-induced-plasticity steel

“…The retained austenite constitutes 9.5% and martensite, bainite ferrite constitute 1.2%, 19.9% and 62% while the remaining 7.4% of the area fraction is due to the grain boundaries. It can be seen from Figure 1 that the average ferrite grain size of this steel is about 8 m µ , which is large in comparison with Nb alloyed TRIP steel having an average ferritic grain size of 5 m µ [6]. Furthermore, the fractions of austenite are considerably higher than Nb microalloyed TRIP steel [6].…”

“…In general, microalloying is adopted for this purpose by using elements such as Nb, V or Ti [4]. It is well known that transition metals like Nb, V or Ti have the tendency to interact with interstitial elements such as C and N, forming precipitates [5,6,7]. Among Nb, V and Ti, Ti is thermodynamically the first element to precipitate during solidification.…”

Strengthening Mechanisms in Thermomechanically Processed NbTi-Microalloyed Steel

“…For the measured particle volume fractions (Table I) and the unit cell sizes a Ti3Nb1C2N2 = 0.64 nm (on average from [42] ), a NbC = 0.44 nm, [43][44][45] and a bccFe = 0.286 nm, the C contents in all particles were calculated to be 0.063 at. pct (0.014 wt pct), 0.068 at.…”

“…[42] Due to the nitrogen content being relatively low in the steel composition, the N/C atomic ratio in these particles was assumed to be 2/2. For such Ti 3 Nb 1 C 2 N 2 precipitates with fcc crystal structure, [42] the >20 nm particle volume fraction can be related to the Nb atomic fraction in these particles in the following way…”

Unravelling the structural and chemical features influencing deformation-induced martensitic transformations in steels

“…The samples for transmission electron microscopy (TEM) were mechanically pre-thinned along the ND-TD plane using SiC paper with roughness from 350 down to 4000. Electropolishing was thereafter carried out in a twin-jet polisher using 5 percent perchloric acid solution at a temperature of -20 0 C [22]. TEM analysis was performed using Philips CM30T and FEI Tecnai F20ST/STEM microscopes operating at 300 and 200 kV, respectively.…”