Microalloyed (MA) steels have matured during the past 40 years into an important class of high-strength structural materials. Their cost-effectiveness has been enhanced by the growth of electric-arc-fumace (EAF) steelmaking and the thin-slab-casting process.
Microalloying is a term that is used to describe the deliberate addition of very small amounts of elements, usually individually below 0.1wt% to improve the steel properties. This is different from trace elements, which are usually undesirable. The development of microalloyed steels eliminated the need for heat treatments, such as normalizing. Yield strengths ranging up to 550 to 600 MPa can be attained through small additions (less than 0.1%) of selected carbonitride formers without requiring costly alloying elements. Recent technological developments in steel melting and hot rolling further reduced the cost and enhanced the competitiveness of microalloyed steels.
In such steels Niobium, Vanadium and Titanium are added either singularly or in combination. Since the solubility product and the physical properties of each element and each compound are different, there exist characteristic differences which cause each of these elements to have specific merits. Sometimes boron is also added in smaller quantity (0.00596 /0.00396) for improving the steel properties.
The effects of various additives are given below:
Titanium forms nitrides, which are stable at high temperatures and these titanium nitrides provide control of the austenite grain size at the reheating temperature before hot working and also in the weldment, in particular in the heat affected zone close to the fusion boundary. The elimination of free nitrogen due to the formation of TiN is positive for the toughness and indirectly makes niobium more effective.
Vanadium forms almost no austenite precipitates and is plentifully available for precipitation hardening. Even though the specific efficiency of vanadium compounds is comparably low, the high volume fraction of fine precipitates compensates for this, especially in steels with relatively high carbon content.
Niobium is the most effective microalloy for grain refinement by controlling the austenite grain size during the reheating processes for heat treatments like normalizing, quenching or carburi,ng, acting additionally to the traditional AIN technology. Furthermore it has an outstanding status in retarding recrystallisation during austenite processing via thermomechanical rolling, resulting in grain refinement, which cannot be obtained by any heat treatment process.
Ti, V and Nb can be added in to liquid steel in the form of Ferro alloys either lumps or cored wire. Use of cored wire results in higher recovery of additions. Ferroalloys (lumps and cored wires) manufactured by MINEX for microalloying of steels has following specifications and can also be tailor made as per the specific requirement of customers.