An investigation on the influence of various heat treatments of Nb-Ti-V and V-N microalloyed steels for high strength and low-temperature impact toughness

Abstract

The requirement for a combination of high yield strength (≥345 MPa)and impact toughness at low temperatures, typically, CVN 40 to 60 J at –20 °C, is stringent in wind tower applications of normalised Nb-Ti-V and VN microalloyed plate steel grades. In many cases, coarse and banded as-hot rolled microstructures persist even after normalising heat treatment and this leads to inconsistencies and some scatter in the mechanical properties. Hence, homogeneous microstructures are a requirement for consistent mechanical properties in normalised microalloyed plate steels. Therefore, this work focused on the response of the study-microalloyed steels to different heat treatment cycles while considering the effects of the starting as hot-rolled microstructural banding (ferrite-pearlite). The study was conducted on two grades of hot rolled microalloyed plate steels, namely; 0.13C-NbTiV 30 mm plate steel and 0.12C-VN 16 mm plate steel (mass %) which were industrially hot rolled differently but subjected to the same heat treatment cycles to study their response to the applied heat treatment routes. In other words, this study focused on the microstructural evolution and the subsequent mechanical properties of these steel grades after various heat treatment cycles that simulated their respective cooling rates in air. Three thermal cycles were applied, namely; normalising heat treatment (NHT), a double austenitisation heat treatment (DAHT) that involved full austenitisation followed by intercritical-austenitisation, and intercritical annealing heat treatment (IAHT).

Characterisation of the as hot-rolled material showed that both steel grades’ microstructures were banded, with anisotropy indices of 1.13 and 1.31 for the 30 mm thick NbTiV and 16 mm thick VN-steel plates, respectively, showing that the VN-steel was more banded than the NbTiV-steel. A ferrite grain size gradient was also evident from the surface to the centre of the steel plates in the as-hot rolled condition. The average ferrite grain sizes of the as-hot rolled microstructures were 25.8±2.8 µm and 20.3±2 µm for the NbTiV and VN-steel respectively. In the as-hot rolled condition, the VN plate steel underwent larger deformation (93.3% reduction) than the NbTiV plate steel (87.5% reduction). This can explain the finer ferrite grain size and the higher degree of banding of the VN-steel in the as-hot rolled condition.

After applying the three heat treatments, the grain size gradient was no longer visible in both steel grades, and ferrite grain refinement was evident in the NHT and DAHT, which showed comparable mean ferrite grain size distributions. The NbTiV-steel exhibited a finer average ferrite grain size distribution after the NHT and DAHT than the VN-steel. In the NbTiV-steel, the mean ferrite grain sizes after the NHT, DAHT and IAHT were; 11±1 µm, 12±1 µm, and 18±2 µm respectively. The VN-steel’s mean ferrite grain sizes after the NHT, DAHT and IAHT were; 18±1 µm, 16±1.8 µm, and 27±2 µm respectively. Regarding pearlite banding in the NbTiV-steel, the DAHT and IAHT processes produced a more uniform pearlite distribution, while the NHT retained the pearlite banded microstructure, however, the IAHT produced a bimodal ferrite grain size distribution and the coarsest mean ferrite grain size relative to those from the NHT and DAHT. The VN-steel exhibited a similar trend but to a lesser extent, that is, it had a stronger memory of the as-hot rolled microstructure. This led to the deduction that a higher degree of banding in the as-hot rolled condition correlated to greater retention of microstructural banding after applied heat treatment. In general, of the three heat treatments, the DAHT cycle proved to be the most effective in eliminating the microstructural banding for both steel grades despite the initial as-hot rolled conditions of the study-steels.

Regarding mechanical properties of both steel grades, the NHT produced the highest yield strength, Rp (NbTiV– 404±9 MPa & VN– 377±1.5 MPa) and fracture elongation, A5 (NbTiV– 31±0.6% & VN– 33±0.5%), which could be attributed to the ferrite grain size refinement. Despite yielding similar average ferrite grain size distributions with the NHT, the DAHT produced a lower yield strength relative to the NHT which ascribes to some loss of precipitation strengthening associated with the second annealing process of the DAHT. The lowest yield strength was observed after the IAHT due to the bimodal and coarse mean ferrite grain structure. In the NbTiV, the DAHT produced the Rp of 355±12 MPa and A5 of 27±1%; while the IAHT produced Rp and A5 values of 350±1 MPa and 27±0.5% respectively. In the VN, the DAHT produced the Rp of 345±20 MPa and A5 of 29±2.5%; while the IAHT produced Rp and A5 values of 359±3.5 MPa and 32±1.5% respectively. The highest Rm was achieved in the IAHT for both study-steels; with NbTiV– 543±4 MPa and VN– 512±3 MPa. These high ultimate tensile strength results produced by the IAHT could be attributed to pearlite strengthening by carbon enrichment of the austenite during intercritical austenitisation. Numerically the DAHT produced the highest CVN upper shelf energy values (though with a minimal difference to that of the NHT) in both steel grades;- NbTiV– 259 J and VN – 293 J, possibly due to the refined ferrite grain structure. However, the best impact toughness at –40 °C was achieved with the NHT (NbTiV– 196 J & VN– 210 J). In general, the best combination of strength and impact toughness was achieved with the NHT process. The IAHT produced the lowest impact toughness properties in both steel grades which can be ascribed to the coarseness and bimodality of the ferrite grain structure after the IAHT. Finally, the results after the heat treatment cycles showed higher impact toughness properties for the VN-steel than the NbTiV-steel, and contrastingly, higher tensile properties for the NbTiV-steel than the VN-steel. The yield strength specification requirements for structural applications in the NbTiV-steel were well met after the NHT and DAHT, and just marginally met after the IAHT. However, all the heat treatment cycles adequately met the CVN impact toughness specifications for structural applications in both steel grades. In the VN-steel on the other hand, the yield strength specification requirements were again well met after the NHT, followed by the IAHT and failed to be met after the DAHT.