The influence of a high aluminium addition on the strength and impact behaviour of hot-rolled Nb containing steels

Abstract

An addition of 0.3 wt-% Al has been made to a Nb containing 0.1 wt-% C, 1.4 wt-% Mn steel in an attempt to improve its impact behaviour on hot rolling to 15 mm thick plate; the intention being to replace some of the specifications that currently can only be achieved by the more expensive methods of control rolling or normalising. Previous work had shown that a 0.2 wt-% Al addition to a hot-rolled plain C–Mn steel resulted in the impact transition temperature decreasing by 40°C with no significant change in strength making such a proposition possible. Unfortunately, the coarse austenite grain size of the hot-rolled Nb containing steel resulted on transformation in colonies of lower transformation products and Widmanstätten ferrite forming in the ferrite/pearlite microstructure resulting in poor impact behaviour. Reducing the cooling rate by working with thicker plate, 30 mm, led to little change in the mechanical behaviour on hot rolling. In contrast, on control rolling, the finer-grained steels which were virtually free of these lower transformation products showed a small benefit from the high Al addition even when Nb was present, particularly for the slower cooling rate, the 27J impact transition temperature decreasing by ∼20°C together with a small increase in yield strength. Further work is suggested to decrease the hardenability so that the original aim might be achieved.

The effect of Al and Nb contents, cooling rate and rolling condition on the microstructure and corrosion behaviour of HSLA steel

“…The current authors [8] reported that adding 0.16%Al to HSLA steels enhances toughness considerably without impairing strength. A further study [9] revealed that the addition of 0.018%Nb is beneficial to improve the strength through grain size refinement and precipitation hardening [10,11].…”

“…In the case of the 0.02%Al bearing steel, N remains in solution rather than precipitating out since no AlN precipitates were observed under the SEM examination. However, increasing the Al content to 0.16% encouraged precipitation of AlN as shown in Figure 3(e,f), as the driving force for precipitation becomes higher and thus Al can remove N from solution [8].…”

“…To identify optimisation potential in corrosion resistance, H2 steel was further enriched with an additional 0.018%Nb, for H3 steel, whilst it is common to find 0.015-0.03%Nb in commercial steels. The contents of Al and Nb were selected based on their previous reported benefits to mechanical properties; 0.16%Al is the optimum content needed to improve impact behaviour while its combination with 0.018%Nb promotes strength considerably [8][9][10]. This is to explore any further benefits to corrosion behaviour.…”

“…The key concept of the current work emerges as a continuation of our research on the effect of Al, Nb and their combination along with variation in the cooling and rolling conditions on the changes in the microstructural features and mechanical performance of HSLA steel [8][9][10]. The inherent protection capabilities of Al and Nb are well-documented, but the impact of small alloying addition on the corrosion behaviour of steel requires further attention.…”

“…Nb is known to encourage carbide formation at grain boundaries, while Al is effective in refining carbide thickness [13]. In addition, Nb reduces the Ar3 temperature, the temperature at which austenite begins to transform to ferrite during cooling, so encouraging martensite formation, while Al increases the Ar3 temperature which suppresses martensite formation, so both elements have a contradictory influence on the microstructure [8]. Moreover, identifying the best heat treatment during processing has been studied to compensate for the low alloying composition, to obtain the optimum microstructure and to achieve the desired corrosion behaviour.…”