Mechanical properties of as cast microalloyed steels containing V, Nb and Ti H. Najafi*, J. Rassizadehghani and A. Halvaaee Tensile, hardness and room temperature Charpy V notch impact tests were used to evaluate the variations in the mechanical properties of a low carbon cast steel containing combinations of vanadium, niobium and titanium in the as cast condition. Tensile and hardness test results indicate that good combinations of strength and ductility can be achieved by microalloying additions. Based on the TEM studies, random and interphase fine scale microalloy precipitates play a major role in the strengthening of the microalloyed heats. However, the presence of titanium leads to some reduction in the strength of the microalloyed heat. Coarse TiN particles can be responsible for this behaviour. On the other hand, microalloying additions significantly decrease the impact energy and lead to the dominance of cleavage facets on the fracture surfaces. It seems that heterogeneous nucleation of microalloy carbonitrides on dislocations along with coarse ferrite grains and pearlite colonies has triggered the brittle fracture in the microalloyed heats. Keywords: Cast steel, Microalloyed steel, Microalloy precipitates, Titanium, Niobium, Vanadium Introduction Wrought grades of microalloyed steels containing vanadium, niobium, titanium or combination of these elements have been available for years. These steels, which are produced through several thermomechanical processes, exhibit a combination of high strength and good toughness due to the fine ferrite grain size and precipitation hardening. Fine scale carbonitrides play a major role in grain refinement through precipitation in austenite and contribute to dispersion hardening through precipitation in ferrite during or after c–a transformation. 1–3 From the 1980s, similar demands for producing low cost, higher strength steel castings with good toughness and weldability have encouraged some researchers to focus on microalloyed cast steels. Microalloyed cast steels are basically low to medium carbon steels with manganese levels in the range of 1?2–2 wt-%, and additions of conventional microalloying elements such as titanium, niobium and vanadium. 4,5 Nowadays, these steels have found many applications in manufacturing industrial parts such as offshore platform nodes, centrifugal cast pipes, machinery supports, nuclear reactor support frames, natural gas compressor housing, ingot moulds and buckets which were all being produced by expensive manufacturing processes. 5,6 Because most of these parts have to be heat treated before use, the effects of different heat treatment variables including the effects of previous homogenisation, austenitising time and temperature, cooling rate, intercritical heat treat- ment, and tempering time and temperature on the mechanical properties of microalloyed cast steels, have been the subject of many investigations. 6–10 However, the effects of microalloying additions on the mechanical and microstructural properties of cast steels in the as cast condition have not been investigated yet. Therefore, it seems valuable to study the mechanical properties of the cast microalloyed steels in the as cast condition to examine the possibility of achieving good combinations of properties and substituting some industrial parts produced by expensive processes with these inexpensive steels. The objective of this study was to assess the influence of vanadium, niobium and titanium additions on the strength and room temperature impact toughness of a low carbon steel in the as cast condition. Experimental Materials A 100 kg capacity, 125 kW, 3 kHz basic lined induction furnace was used for melting the alloys. After complete melting of base material, graphite and ferromanganese were added to the melts to adjust carbon and manganese contents. The heats were appropriately deoxidised with ferrosilicon and Al shot. Microalloying elements were added to the melts in the form of ferrovanadium, ferroniobium and ferrotitanium. The base composition for all heats was adjusted to be y0?15 wt-% carbon and 1?5 wt-% manganese and all microalloyed heats con- tained 0?08–0?1 wt-% vanadium. Titanium and niobium levels were selected to be respectively about 0?02 and Department of Metallurgy and Materials Engineering, Faculty of Engineering, Tehran University, PO Box 14395-731, Tehran, Iran *Corresponding author, email hnajafi@ut.ac.ir ß 2007 Institute of Materials, Minerals and Mining Published by Maney on behalf of the Institute Received 25 January 2007; accepted 29 January 2007 DOI 10.1179/174328407X179755 Materials Science and Technology 2007 VOL 23 NO 6 699