Some Metallurgical Issues Concerning Austenite Conditioning in Nb-Ti and Nb-Mo Microalloyed Steels Processed by Near-Net-Shape Casting and Direct Rolling Technologies

Titanium microalloyed steels

“…372 After casting 50 mm thin slabs, Nagata et al 387 reported TiN mean particle sizes between 20 nm and 27 nm, while Arribas et al 386 obtained TiN mean particle size,2r, of between 12 nm and 41 nm following reheating Ti steels for 10mins at 1220 ºC,produced by'beam blank' casting. The effect of cooling rate on the mean particle size of TiN, collated by López and Rodriguez-Ibabe 388 is shown in Fig.59: for 1000°C min =1 , a 2r of <10nm, is predicted by extrapolation, which compares well with ~7nm measured by Li et al 381,382…”

“…In the case of Ti-Nb-N steels, the results in Fig.61 from Sage et al 385 and the Rodriguez-Ibabe group, 388,399 show experimental data from several hypostoichiometric Ti-MA steels, having a range of fine TiN particle sizes, for the time to obtain 50% static recrystallization plotted as a function of deformation temperature, prior to hot deformation.Data corresponding to TiN particle sizes of 12 and 41nm is corrected for comparison with an initial austenite grain size of 700µm.The solid lines represent the behaviour associated with conventional C-Mn grades or solute drag effect, while dashed lines correspond to the delay exerted by fine particles.With a mean particle size of 99nm, the Ti steel behaves in a similar manner to a C-Mn steel, showing no additional delay. After deformation at 1050ºC, when some SIP can occur, there is a delay in recrystallization.…”

“…The authors concluded that the fine TiN particles introduced a delay which should be added to that produced by the solute drag effect of Nb in solution. 388 An investigation was undertaken by Nagata et al 387 showing dramatic coarsening to >90 nm after hot rolling. Hypostoichiometric steels, which contain excess carbon and/or nitrogen, exhibited no coarsening in the end product.…”

“…After casting 50 mm thin slabs, Nagata et al [388] reported TiN mean particle sizes between 20 nm and 27 nm, while Arribas et al [387] obtained TiN mean particle size, 2r, of between 12 and 41 nm following reheating Ti steels for 10 mins at 1220°C, produced by 'beam blank' casting. The effect of cooling rate on the mean particle size of TiN, collated by López and Rodriguez-Ibabe [389] is shown in Figure 59: for 1000°C min −1 , a 2r of <10 nm, is predicted by extrapolation, which compares well with ∼7 nm measured by Li et al [382,383] Liu and Jonas [390] developed a technique for determining precipitation-time-temperature relations particularly applicable to MA steels, such as Ti steels [391]. Their stress relaxation technique can investigate precipitation phenomena during cooling, allowing a parallel to be made with controlled rolling and fast cooling rates, such as occur during TSDC operations.…”

“…Investigations on stress relaxation related to SIP include those made on Ti(C, N) [389,394], TiN [386,393], TiC [280], (Nb, Ti)C [156], (Ti, Mo)C [395] and Ti 4 C 2 S 2 [396]. Arribas et al [387] found that the fine TiN particles present in the steel prior to rolling, in addition to decreasing grain growth by pinning the austenite grains, produced a beneficial delay in both static and dynamic recrystallisation kinetics of austenite (Figure 61).…”