Prediction of intragranular ferrite nucleation from TiO, TiN, and VN inclusions Wangzhong Mu 1 • Pa ¨r Go ¨ran Jo ¨nsson 1 • Keiji Nakajima 1 Received: 15 July 2015 / Accepted: 22 October 2015 / Published online: 3 November 2015 Ó Springer Science+Business Media New York 2015 Abstract The current study presents a method to calculate the critical diameters of TiO, TiN, and VN inclusions for intragranular ferrite (IGF) nucleation in steels. Based on the calculation results, it was noted that the critical diameters of TiO, TiN, and VN inclusions for IGF nucleation were 0.192, 0.355, and 0.810 lm. The calculation results agreed with the experiment data of a minimum inclusion size for IGF nucleation in the actual steel samples. Moreover, the effects of Mn, C, and S contents on the critical diameters of inclu- sions were investigated. It was found that the critical diam- eters of TiO, TiN, and VN inclusions increased with the increasing Mn and C contents. In addition, it was found that S does not have a direct effect on the critical diameters of TiO, TiN, and VN inclusions. However, the increasing S content led to an increased amount of MnS precipitation in the actual steels. This is negative, since MnS is ineffective nucleation site for IGF nucleation. When the amount of MnS increases in steels, the area fraction of IGF slightly decreases. This fact has been investigated by in situ observation experiments. Introduction It is recognized that intragranular ferrite (IGF) is the desired microstructure in weld metals and low-carbon low- alloy steels. When the IGF nucleates from the inclusion surface, the nucleation of grain boundary ferrite (GBF) is suppressed. This leads to the improvement of the mechanical properties of steels [1–4]. This positive use of inclusions is the basis for the concept of oxide metallurgy [1]. Specifically, this concept concerns the utilization of fine inclusions, such as: Ti–oxides, TiN, VN, etc. [2–4]. Recently, several reports related to the oxide metallurgy have been published, and the focus is the effect of the inclusion composition on the IGF nucleation [5–12]. Besides the effect of inclusion composition, the inclu- sion size also affects the heterogeneous nucleation of IGF [13–19]. Lee et al. [13], Furuhara et al. [14], and Mu et al. [15, 16] carried out experimental study to investigate the probability of IGF nucleation at each inclusion size. In their studies, the number of nuclei inclusions is divided by the number of the total inclusions at each inclusion size, in order to calculate the probability of IGF nucleation. Their results showed that complex oxides containing Ti [13], TiO x ? MnS [15], TiN ? Mn–Al–Si–TiO ? MnS [16], and V(C,N) ? MnS [14] have different effective sizes to induce IGF nucleation. Besides these experimental studies, several theoretical studies have been carried. These were based on the classical nucleation theory, and the results showed that the normalized energy barrier for IGF nucle- ation (DG het. * /DG hom. * ) decreases systematically with an increased inclusion size [17, 18]. However, the nuclei inclusions in their studies were only considered to be inactive, incoherent, and non-deformable, based on the assumption provided by Ricks et al. [17]. Specifically, the interfacial energy between inclusion and austenite, r Ic , is assumed to be the same as the value between inclusion and ferrite, r Ia (r Ic = r Ia )[17]. Therefore, the inclusion sur- face is always less advantageous as the nucleation site than that of the prior austenite grain boundary, as long as r Ic equals to r Ia . However, if the specific inclusion surface is effective for IGF nucleation, the value of r Ia is lower than & Wangzhong Mu wmu@kth.se & Keiji Nakajima keiji@kth.se 1 Division of Applied Process Metallurgy, Department of Materials Science and Engineering, KTH Royal Institute of Technology, Brinellva ¨gen 23, 10044 Stockholm, Sweden 123 J Mater Sci (2016) 51:2168–2180 DOI 10.1007/s10853-015-9527-6