Critical Materials:Underlying Causes and Sustainable Mitigation Strategies: 5 (World Scientific Series in Current Energy Issues) by S Erik Offerman

Author:S Erik Offerman [S Erik Offerman]
Language: eng
Format: epub
Publisher: World Scientific Publishing Company
Published: 2019-02-26T16:00:00+00:00

Fig. 10.1. Microstructures of (a) nano-steel and (b) conventional AHSS.

Nano-steels use considerable microalloying additions, including Niobium (Nb, ≤ 0.1 wt.%), Titanium (Ti, ≤ 0.35 wt.%), Vanadium (V, ≤ 0.45 wt.%) and Molybdenum (Mo, ≤ 0.7 wt.%), to form the essential precipitating species, 4 – 10 that is, the corresponding carbides and/or carbonitrides (MC and/or M(C,N), where M=Nb, Ti, V and Mo). Albeit these contents do not seem very high at first, they imply significant amounts of microallying additions considering the large volume of steel produced worldwide. The microalloying elements are usually expensive, meaning that the commercial and industrial implementation of nano-steels is subjected to the efficient use of these elements, and hence, to the better understanding of the precipitation reactions involved. The first nano-steels that have been developed for automotive applications are based on additions of Ti and Mo 11 : the Ti precipitates in the form of (Ti,Mo)C particles and the Mo suppresses TiC-precipitate coarsening (Ostwald ripening), keeping the precipitates small during processing. Other studies have also demonstrated a similar effect of Mo on NbC-precipitates. 12 , 13 Yet, quantitative data about the Mo effect on the TiC- and NbC-precipitation kinetics remains insufficient. Moreover, the EU has recently identified Nb as a critical raw material because its supply risk is high. 14 The challenge is thus to design nano-steels that are: (1) more “resource-efficient”, by reducing and optimizing the microalloying additions, (2) containing less or no critical raw materials, and (3) at the same time maintaining or improving their excellent mechanical properties.

Vanadium is a promising candidate to (partially) replace Nb and Ti in nano-steels. The advantages of this element will be enumerated through this chapter. In contrast to Nb and Ti, the larger solubility of V can be exploited for different purposes, but mainly to optimize ferrite strengthening via precipitation of V(C,N). Literature on V-precipitation has provided compelling evidence that by using high Nitrogen (N) levels precipitate coarsening can be suppressed due to the reduced solubility of V(C,N) compared with VC in ferrite, which further enhances the precipitate hardening contribution. By contrast, little or nothing has been reported so far about the interaction between V and Mo, and the question to answer in the near future is if V, N, and Mo is the perfect combination for nano-steels.

From the abovementioned it can be concluded that dispersion of V(C,N) nanometer-sized precipitates is the most promising strategy to maximize precipitation strengthening and at the same time minimize the addition of alloying elements. This, however, is not straightforward and requires a deep understanding of how precipitation and the austenite to ferrite ( γ → α ) phase transformation interact, since both phenomena occur within the same range of temperatures during thermomechanical processing of nano-steels. In some cases, the precipitates will form at the austenite/ferrite ( γ / α ) interface as phase transformation proceeds. This is known as interphase precipitation (IP) and usually recognized by rows of aligned precipitates. In other cases, the precipitates will nucleate randomly from supersaturated ferrite. Different microalloying

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