Microalloyed steels reviewed
A newly published review is a timely and significant analysis of the processing and properties of microalloyed steels, as David Price writes.
The IOM3 journal Ironmaking and Steelmaking (I&S) has published an extensive review considering the compositions, production processes, microstructure, and structural properties of microalloyed steels. It is written by one of the world’s leading metallurgists in the subject, Professor Neville Baker. More general, strategic, and broader in content than the journal’s characteristic peer-reviewed papers, Baker’s review, Microalloyed steels, is available freely online in I&S Volume 43, Issue 4, published in June 2016.
Professor Baker, from the Metallurgy and Engineering Materials Research Group, University of Strathclyde, UK, has been involved with microalloyed steels for more than 50 years. The review acts as a reference source for engineers and metallurgists, covering the basis of steel strength, toughness and ductility, as well as the practicalities of steel production and welding of this class of steels.
Microalloyed steels comprise a family of steels typified by a yield strength range of 350–800Mpa, compared with 150–200Mpa for mild steels. Of the 1.65 billion tonnes of liquid steel produced in 2014, some 200 million tonnes – around 12% – were microalloyed.
Dr Norman McPherson, Knowledge Exchange Associate at the University of Strathclyde, told Materials World that ‘the use of microalloyed steels is almost taken for granted nowadays. For example, the increased strength developed by a niobium microalloyed steel, such as DH36, can be used to reduce the weight in particular applications. The benefits include using less steel, which, in turn, has environmental benefits.’
Current examples include the Queen Elizabeth-class aircraft carriers, where almost all areas used DH36 steel, while the flight deck and hanger deck employ the niobium vanadium microalloyed steel EH46. The use of EH46 has a weight-reducing benefit over the base case, as well as lowering the cost. In addition, welding processes are now suitably well developed to ensure that microalloyed steels are readily weldable. The Queensferry Crossing, which is being constructed alongside the Forth Road Bridge and will be completed by May 2017, also makes use of microalloyed steel in its girders and supports.
Baker’s review starts with the history of development of these steels. Much of the early work on niobium microalloyed steels, which emerged in the late 1950s and early 1960s, was concentrated in the USA and UK, but such was the interest that research, development and use became global.
It then describes the chemical compositions used and the role of individual and multiple alloying elements in precipitation strengthening and grain size control. In controlled rolling and controlled cooling, the key to obtaining a fine grain size in a low niobium steel (~0.02wt%) is the low finishing rolling temperature and controlled rolling or rolling over a deliberately lowered temperature range – known as thermomechanical processing. Compared with conventional hot rolling, this is now a widely accepted technique for the production of microalloyed steels and a description of how this process works is included.
The use of modelling of deformation, strain recrystallisation, recovery and microstructurure are all described, with detail on the solubility of particles that control grain size and provide dispersion hardening, precipitation, and recrystallisation.
The review describes the metallurgy of high-strength, bainitic, and martensitic microalloyed steels, and the impact of thin slab casting and direct charging on the development and use of these steels. A change in practices was needed and this is well described. Such is the impact of thin slab casting that, today, around 12% of all strip is produced by this route worldwide.
The opportunities that microalloying gives for improving the weldability of microalloyed steels, by reducing carbon content, was fairly quickly recognised by researchers and became another major driving force for the development and understanding of these steels. For instance, the presence of the nitride forming elements aluminium, titanium, zirconium, niobium and vanadium can considerably mitigate the negative effect of nitrogen, indicated by the relaxation of the limit of nitrogen in the presence of these elements. The potential benefits of friction stir welding are also mentioned.