Precipitation in V bearing microalloyed steel containing low concentrations of Ti and Nb

Abstract

The paper describes the precipitation behaviour in a thermomechanically processed V bearing microalloyed steel containing small additions of Ti and Nb (0?007-0?008 wt-%) using analytical transmission electron microscopy. An intriguing aspect is the significant precipitation of titanium and niobium at these low concentrations, contributing to strength. A high density of multimicroalloyed precipitates of (V,Nb,Ti)(C,N) are observed instead of simple TiN, TiC, and NbC precipitates. They are characterised as cuboidal (45-70 nm), spherical (20-45 nm), irregular (20-45 nm), and fine (10-20 nm). Estimation of solubility products of carbides and nitrides of V, Nb, and Ti implies that the precipitation of titanium occurs primarily in austenite. Interphase precipitation of niobium occurs during austenite to ferrite transformation, while complete precipitation of vanadium takes place in the austenite-ferrite region close to completion of transformation. Substoichiometric concentrations of Ti and Nb, the presence of nitrogen, and the mutual extensive solubility of microalloying carbonitrides explains the formation of core shell (triplex/duplex) precipitates with highly stable nitrides ((Ti,Nb,V)N) in the core and carbides ((Ti,Nb,V)C) in the shell. The qualitative stochiometric ratios of triplex and duplex carbonitrides were Ti 0 . 53 Nb 0 . 35 V 0 . 12 and Ti 0 . 6 V 0 . 4 , Nb 0 . 51 V 0 . 49 and Ti 0 . 64 Nb 0 . 36 . Extensive precipitation of fine carbides on dislocation substructures, and sub-boundaries occurred. They were generally characterised as vanadium carbide precipitates with ordered cubic L1 2 structure and exhibited a Baker-Nutting orientation relationship with the ferrite matrix. M 4 C 3 types of carbides were also observed similar to the steel, having high concentrations of Ti and Nb.

Processes, microstructure and properties of vanadium microalloyed steels

“…Evidence of some particles with a Ti core and V/Nb shell was presented. 157 In a latter paper, 158 again using EDX, the particles were described as having a core of (V,Nb,Ti)N and a shell of (V,Nb,Ti)C. The authors noted that the density of precipitation in the Ti-Nb-V steel was significantly higher than in the V steel, 157 but surprisingly, the difference in yield stress between the two steels was only 12 MPa. 156 Many of the microstructural features observed in low carbon multimicroalloyed steels are also present in steels with medium carbon levels 0?3to0?40 wt-%C.…”

“…155 This decrease in dispersion strengthening also occurred in Ti-Nb-V-N steels. 154 A series of papers [156][157][158][159] have compared in detail the precipitate morphologies and compositions of steels containing y0?09C-0?017Ti-0?016Nb-0?044V (wt-%) with y0?07C-0?044V (wt-%) steels. Unfortunately, the N and Si levels were not given.…”

Atom Probe Tomography Study of Multi-microalloyed Carbide and Carbo-Nitride Precipitates and the Precipitation Sequence in Nb-Ti HSLA Steels

“…[1][2][3][4][5][6][7][8] The combination of these microalloying elements with interstitial solutes, such as C and N, leads to the formation of carbides, nitrides, and complex carbo-nitrides. [7,[9][10][11][12][13] In general, Ti, Nb, and V facilitate grain refinement through precipitation in the austenite and contribute to dispersion hardening through carbide precipitation in ferrite. Nb can retard recovery and recrystallization during hot rolling resulting in ferrite grain refinement, and Ti combines with nitrogen to form TiN precipitates at high temperature in austenite which prevent grain growth.…”

The influence of niobium content on austenite grain growth in microalloyed steels

“…Numerous attempts have been made to develop an empirical model based on the general equation developed by Sellars and Whiteman (1979). Many of these models do not account for the direct effects of the microalloying elements such as Nb in austenite grain growth control [Fu et al, 2011;Wang and Wang, 2008;Wang et al, 2006;Shanmugama et al, 2005;Banerjee et al, 2010;Pous-Romeroa et al, 2013). The current work has considered this limitation, taking into account the direct effect of niobium in grain growth control during thermal processing.…”