Vol.:(0123456789) 1 3 Metals and Materials International https://doi.org/10.1007/s12540-018-0084-9 Contributions of Rare Earth Element (La,Ce) Addition to the Impact Toughness of Low Carbon Cast Niobium Microalloyed Steels Hadi Torkamani 1  · Shahram Raygan 1  · Carlos Garcia Mateo 2  · Jafar Rassizadehghani 1  · Yahya Palizdar 3  · David San‑Martin 2 Received: 4 September 2017 / Accepted: 20 December 2017 © The Korean Institute of Metals and Materials 2018 Abstract In this research Rare Earth elements (RE), La and Ce (200 ppm), were added to a low carbon cast microalloyed steel to disclose their infuence on the microstructure and impact toughness. It is suggested that RE are able to change the interac- tion between the inclusions and matrix during the solidifcation process (comprising peritectic transformation), which could afect the microstructural features and consequently the impact property; compared to the base steel a clear evolution was observed in nature and morphology of the inclusions present in the RE-added steel i.e. (1) they changed from MnS-based to (RE,Al)(S,O) and RE(S)-based; (2) they obtained an aspect ratio closer to 1 with a lower area fraction as well as a smaller average size. Besides, the microstructural examination of the matrix phases showed that a bimodal type of ferrite grain size distribution exists in both base and RE-added steels, while the mean ferrite grain size was reduced from 12 to 7 μm and the bimodality was redressed in the RE-added steel. It was found that pearlite nodule size decreases from 9 to 6 μm in the RE-added steel; however, microalloying with RE caused only a slight decrease in pearlite volume fraction. After detailed fractography analyses, it was found that, compared to the based steel, the signifcant enhancement of the impact toughness in RE-added steel (from 63 to 100 J) can be mainly attributed to the diferences observed in the nature of the inclusions, the ferrite grain size distribution, and the pearlite nodule size. The presence of carbides (cementite) at ferrite grain boundaries and probable change in distribution of Nb-nanoprecipitation (promoted by RE addition) can be considered as other reasons afecting the impact toughness of steels under investigation. Keywords RE addition · Cast microalloyed steel · Impact toughness · Grain size · Inclusion 1 Introduction Although wrought grades of microalloyed steels have been available for decades, producing low-cost high strength cast steels is still highly demanded. The carbon content of these steels can be reduced to improve both weldability and tough- ness resulting in strength reductions, which can be compen- sated by microalloying additions [1–4]. So far, low carbon cast microalloyed steels have found many applications in the production of complex internal shapes and the manu- facturing of industrial parts where a high impact toughness at room temperature is desired [1, 5]. Generally, the solidifcation path followed by these low carbon microalloyed steels comprises a peritectic reaction/ transformation. Although this transformation is regarded as an important source of crack-formation [6–8], it has been studied with less detail compared to other widespread transformations in steels. According to John and Hogan [9] this transformation * Hadi Torkamani H.torkamani@ut.ac.ir * Shahram Raygan Shraygan@ut.ac.ir * Carlos Garcia Mateo cgm@cenim.csic.es 1 School of Metallurgy and Materials Engineering, College of Engineering, University of Tehran, P.O. Box 11155-4563, Tehran, Iran 2 Materalia Research Group, National Center for Metallurgical Research (CENIM), Consejo Superior de Investigaciones Cientifcas (CSIC), Avda Gregorio del Amo, 8, E-28040 Madrid, Spain 3 Materials and Energy Research Center, Karaj 3177983634, Iran