August 2020 Volume 2

FORGING RESEARCH depressing of the recrystallization temperature. Figure 11 below demonstrates both the effects of Ti on the grain coarsening and recrystallization temperatures, and the effects of the N level, which will be explained shortly.

2.2.3 Vanadium Vanadium is a prominent microalloying addition, being potentially involved with austenite conditioning, hardenability, and precipitation hardening of the final microstructure. The primary purpose of V in the steels proposed herein is to supply a substantial quantity of precipitation strengthening. Vanadium carbides may form in the steel under suitable transformation and cooling conditions, but in the presence of sufficient quantities of N, vanadium nitride precipitates may form and substantially increase the strength evenmore. [14] However, since in the current experiments the N content is kept constant at approximately 60 ppm, this may not be a factor. An additional benefit of V additions in the Ti-V-N steels studied in the literature is the refinement of the final microstructure through the intragranular nucleation of ferrite upon inclusions, especially the V precipitates which form on MnS inclusions. [19] Traditionally, ferrite nucleation during the austenite to ferrite transformation occurs predominantly upon the prior austenite grain boundaries. With the increased nucleation rates from the intragranular nucleation of the ferrite, a higher quantity of individual ferrite grains is formed, and thus an overall smaller ferrite grain size is observed. [19] These methods of intragranular ferrite nucleation upon inclusions in V-bearing steels were studied by several authors [20-22] and were found to be effective means of refining the final microstructure of the steel. 2.2.4 Titanium Titanium is the other prominent microalloying element in the proposed steels.The role of Ti is primarily in the control of austenitic grain size, through the Zener pinning of austenite grain boundaries by stable, high-temperature TiN precipitate particles .[17] These TiN particles, when subjected to proper conditions, can significantly lower the potential for grain coarsening, providing the optimal conditions for recrystallization controlled forging. With regards to austenite conditioning and control, an addition of Ti to a N containing alloy results in the high temperature precipitation of TiNparticles, which pin austenitic grain boundaries and impede growth, significantly raising the grain coarsening temperature. [17] These TiN particles were observed by various authors in the literature, and are well documented to be a key austenitic conditioner. [14, 17, 23-26] TiN particles also have a significant effect upon the recrystallization kinetics of the steel, which is a very core component in the recrystallization controlled forging process proposed. Zheng et al. [17] demonstrated that additions of Ti to the V and N steels resulted in a depressing of the recrystallization temperature. Figure 11 below demonstrates both the effects of Ti on the grain coarsening and recrystallization temperatures, and the effects of the N level, which will be explained shortly.

Figure 11: Influence of N composition and Ti resence on the grain coarsening and recrystallization temperatures in a V microalloyed steel [17] Though Ti has a very positive effect on the conditioning of the austenite in the RCF process, the composition of the element is limited practically due to coarsening of the TiN particles. For the grain size control to be most effective, the dispersion of the TiN particles should be very fine. [14, 27] Such a small distribution is achievable through adjusting the Ti/N ratio in the composition. As Ti is a slower diffusing element than N, limiting the quantity of Ti in the steel to a hypostoichiometric Ti/N ratio (Ti < 3.42N) will limit the coarsening of the TiN particles. Such was confirmed in the literature by several authors. [14, 17, 24-28] The key role of titanium in the steels, as stated, is the formation of fine TiN precipitation, in order to pin austenite grain boundaries at high temperatures, preventing austenite grain growth in the steels. The pinning pressure of a precipitate distribution in steel is dependent on several variables, shown in the rigid boundary model, which assumes a fully rigid austenite boundary, allowing for no flexibility: [29-33] = 6 v (2-5) Where σ = 0.8 J/m 2 in austenite [34] , v is the volume fraction of precipitates, and r is the average particle radius. Other derived pinning models include the flexible boundary model, [30, 35] which assumed an infinitely flexible boundary which interacted with all particles in the steel until completely pinned, and the subgrain boundary model, [30, 36] which considered the impact of the interactions of austenite subgrain boundaries and precipitates: = 3 v 2/3 (2-6) = 3 v 2 (2-7) 2 Where l is the average austenite subgrain intercept in the microstructure. The overall effect of the Ti in the system can be seen in Figure 12 below, which is taken from the work of Zheng et al. [17] This figure shows the effect of additions of N and Ti to a V steel on the austenitic grain size and the grain coarsening behavior of the steel. 20 Figure 11: Influence of N composition and Ti resence on the grain coarsening and recrystallization temperatures in a V microalloyed steel [17]

FIA MAGAZINE | AUGUST 2020 71