Development of Zr Bearing Micro Alloyed Cast Steels and Investigations on Their Microstructure and Mechanical Properties

Abstract

or the last two decades, a considerable research work has been carried out on the development of high strength low alloy steels for melting the stringent requirements in specific applications. In particular, these high strength low alloy steels find applications in structural and automotive industry because of the outstanding mechanical properties combined with good formability. Based on the modifications in the chemistry (both substitution elements and micro-alloying elements), microstructure, grain refinement, thermo-mechanical treatments and by many strengthening mechanisms, a number of high strength low alloy steels have developed with a wide range of mechanical properties. Micro alloyed steel is a class of advanced high strength steel containing micro-alloying elements such as Vanadium (V), Titanium (Ti), Niobium (Nb), Aluminium (Al) and Boron (B). The total amount of these elements in these steels is maintained upto 0.20 wt %. These steels are not only used for structural applications, but also used in automotive industry because of better mechanical properties, very good fabricability and good resistance to atmospheric corrosion compared to the conventional carbon steels. The selection of micro-alloyed steels largely depends on the end user requirements such as (i) reduction in thickness, (ii) weldability, (iii) formability and (iv) environmental degradation. However, for many applications, the selection of micro-alloyed steels is in favor of high strength to weight ratio, i.e. high specific strength as compared to the conventional structural steels. The strength to weight ratio of the micro-alloyed steels can be accomplished by one or more of many strengthening mechanisms along the micro-alloying elements. The primary strengthening mechanisms that operative in these steel are as follows: (i) solid solution, (ii) grain boundary, (iii) precipitation and (iv) phase transformation. Off these strengthening mechanisms, grain refinement strengthening mechanism is a very powerful in the micro-alloyed steels because it increases not only strength but also toughness without sacrificing weldability and ductility. With Hall-Petch equation, the quantitative relationship between yield strength of the micro-alloyed steels and grain size can be evaluated. The grain refinement in the micro-alloyed steels can be accomplished by three factors, which are as follows: (i) pin down the austenite grains during reheating by undissolved precipitates of carbides or carbo-nitrides and nitrides, (ii) prevent recrystallization and grain growth by reprecipitation of precipitates of carbides and carbo-nitrides in austenite during hot rolling and (iii) refine the ferrite grains during transformation of austenite to ferrite by presence of carbides and carbo-nitrides (V(C, N), Nb(C, N), Ti(C, N), AlN, Zr(C,N) and B(C,N). Addition of Al as micro alloying element leads to the formation of aluminium nitride which has a strong capability to enhance the mechanical properties of the developed steel. In addition, Boron has also similar effect on refinement of grain size and improvising strength of the steel newline