Materials Science and Engineering A 486 (2008) 1–7 As-cast mechanical properties of vanadium/niobium microalloyed steels Hamidreza Najafi a,∗ , Jafar Rassizadehghani a,1 , Siroos Asgari b,1 a School of Metallurgy and Materials Engineering, University College of Engineering, University of Tehran, P.O. Box 14395-731, Tehran, Iran b Department of Materials Science and Engineering, Sharif University of Technology, P.O. Box 11365-9466, Tehran, Iran Received 17 April 2007; received in revised form 16 August 2007; accepted 27 August 2007 Abstract Tensile and room temperature Charpy V-notch impact tests along with microstructural studies were used to evaluate the variations in the as-cast mechanical properties of low-carbon steels with and without vanadium and niobium. Tensile test results indicate that good combinations of strength and ductility can be achieved by microalloying additions. While the yield strength and UTS increase up to respectively 370–380 and 540–580 MPa in the microalloyed heats, their total elongation range from 20 to 25%. TEM studies revealed that random and interphase fine-scale microalloy precipitates play a major role in the strengthening of the microalloyed heats. On the other hand, microalloying additions significantly decreased the impact energy and led 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 trigger the brittle fracture in the microalloyed heats. © 2007 Elsevier B.V. All rights reserved. Keywords: Cast steel; Microalloyed steel; Niobium; Vanadium 1. Introduction Although wrought grades of microalloyed steels have been available for years, demands for producing low-cost, higher strength steel castings with good toughness and weldability have encouraged some researchers to focus on cast grades of these steels. Microalloyed cast steels are basically low to medium carbon steels with manganese levels in the 1.2–2 wt% range, and additions of conventional microalloying elements such as titanium, niobium, and vanadium [1,2]. Some grades of microal- loyed cast steels, especially French grades, are alloyed with nickel [3]. These steels exhibit a combination of high strength with good toughness and weldability. Nowadays, microalloyed cast steels have found many applications in the manufacturing of industrial parts such as offshore platform nodes, centrifugal cast pipes, machinery supports, nuclear reactor support frames, nat- ural gas compressor housing, ingot moulds and buckets which were all produced by expensive manufacturing processes before [2,4]. Since most of these parts have to be heat treated before use, the effects of different heat treatment variables have been the ∗ Corresponding author. Tel.: +98 21 8208 4066; fax: +98 21 8800 6076. E-mail addresses: hnajafi@ut.ac.ir (H. Najafi), jghani@ut.ac.ir (J. Rassizadehghani), sasgari@sharif.edu (S. Asgari). 1 Tel.: +98 21 8208 4066; fax: +98 21 8800 6076. subject of many investigations [3–7]. The heat treatment of these steels has been generally performed in three stages: homogeniza- tion, austenitization followed by quenching or air cooling, and tempering at subcritical temperatures. It has also been reported that special intercritical heat treatments can be used to improve the toughness of these materials. Basically, the ultimate goal of these heat treatments is to benefit from fine ferrite grains by controlling austenite grain growth and precipitation harden- ing. Hence, the microalloying elements niobium and vanadium are added to microalloyed cast steels primarily to provide grain refinement and response to aging. Niobium, often at levels of less than 0.05%, effectively prevents undesirable grain growth and can also contribute to precipitation strengthening. Vanadium, in particular, at levels of less than 0.1% forms strengthening carbonitride precipitates [8–12]. In contrast to the heat-treated grades, mechanical and microstructural properties of as-cast microalloyed steels have not been investigated yet. Therefore, it seems valuable to study the mechanical properties of the cast microalloyed steels in the as-cast condition in order to examine the possibility of achieving good combinations of properties and producing some industrial parts with these inexpensive steels. The objective of this study was to assess the influence of vana- dium and niobium as microalloying additions on the strength and impact toughness of a low-carbon steel in the as-cast con- dition. 0921-5093/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2007.08.057