Page 173 - AWSAR 2.0

given load and the ease with which it can be shaped. This also reflects that stronger the steel thinner it can be to support a given load. This, in turn, necessitates the development of stronger steels to reduce fuel consumption for different sectors, such as automotive and transportation, leading to a decrease in greenhouse emission, which is the need of the hour. One of the ways to make steel stronger is by adding alloying elements such as Manganese, Nickel, Chromium, and Niobium. However, the inclusion of more and more alloying elements generally leads to challenges in industrial processing, deterioration of the formability, toughness, and weldability. The ductility of steel is measured by its ability to deform plastically, whereas the toughness is a measure of the ability to resist fracture. Hence, low ductility and toughness lead to easy crack initiation and its propagation leading to early failure of the structures. The disastrous failure of ships during and after World
War I and the tragic end of the Titanic ship are some of the examples highlighting the importance of high toughness and ductility required for structural applications.
The enhancement of the
strength while maintaining the
desired ductility, toughness,
and weldability requires
strategic modifications in the
processing to achieve the
desired microstructure without
adding much of the alloying
elements. In this regard,
microalloying addition (namely,
Niobium, Vanadium, and
Titanium) to lean-alloyed steel
has been an accepted strategy
to improve the strength-ductility
trade-off to a certain level.
Among these, Niobium is more effective for
Mr. Gaurav Bansal || 149
strengthening through fine precipitation and grain refinement. The refinement of grains and precipitates leads to increased number of boundaries, which act as an obstacle to the dislocation movement when external forces are applied on the material. The widely accepted practice in most of the Niobium- containing steel is to have the entire Niobium in the precipitate form to get the maximum benefit from precipitation hardening and fine grain structure. However, in many cases, fine precipitates and an increased number of grain boundaries due to excessive grain refinement have shown to promote the formation of ferrite- dominated microstructure, which limits the achievement of superior strength. On the contrary, a bainitic microstructure, a mixture of relatively finer ferrite and carbides/martensite/ austenite, can achieve superior strength- ductility combinations. This is due to a lower temperature for bainite formation than ferrite,
restricting its growth during transformation.
One of the possible approaches to promote bainitic transformation in lean-alloyed steel could be to control the state of Niobium during industrial processing, that is, either as a precipitate, or as a solute, or in combination. In this regard, only a few studies have been conducted in the recent past, which have diverse opinions on the role of Niobium (as solute or precipitate) on bainitic transformation. For example, some of the studies have reported a delay in bainitic transformation, whereas others have shown enhancement of bainite formation due to Niobium as solute and/or
precipitate. There are also some studies,
Steel is one of the most versatile materials used in different industrial sectors, mainly due to its cost-effectiveness and ease of recyclability as compared to other materials. The significance of steel for a country like India is easily manifested in the recent announcement of the National Steel Policy 2017 for the enhancement of steel production capacity to 300 million tons (MT) by 2030 from its current capacity of ~125 MT.