Doctor of Philosophy
Engineering Materials Institute
Xu, Xiaofeng, Sulphide formation in low-carbon, low-manganese steel, Doctor of Philosophy thesis, Engineering Materials Institute, University of Wollongong, 2014. http://ro.uow.edu.au/theses/4071
By radically departing from the prevailing mainstream steel design philosophy, the industrial and business drives for the present overarching research program were to develop a new family of steels by exploiting the attributes of an innovative alloy design based on low manganese, but relatively high sulphur levels. In addition to the industrial importance and new innovative business initiatives, it also provided a huge intellectual challenge and an exciting opportunity to gain a better understanding of the scientific principles underpinning this stimulating and largely un-researched technological innovation. Forming an integral part of the program, a key component was a study of the impact of sulphide inclusions on this new steel design. Whereas high-manganese steels used in the linepipe industry contain only minute amounts of sulphur, the new alloy design called for a significant increase in the sulphur content of the steel, with the concomitant danger of FeS formation and hence, hotshortness. In order to assess the potential industrial application of these steels, a comprehensive understanding of the formation of sulphides is required. This requirement triggered the present project, which aimed at providing a sound scientifically-based understanding of the mechanisms by which sulphide inclusions form, the exact nature and morphology of these sulphide inclusions and how they can be modified by the addition of small amounts of titanium.
The size, composition, distribution and crystal structures of sulphide inclusions in lowcarbon, low-manganese steels have been determined in detail. In addition to traditional metallographic studies, carbon extraction replica studies and thin foil transmission electron microscopy techniques were used as tools to study sulphide formation in these steels. In addition, extensive use was made of TEM selected area electron diffraction of thin foils prepared by focused ion-beam milling techniques (FIB), selected from large inclusions. A much improved understanding of the sequence of precipitation in the steels under investigation has been developed and new insights have been gained. It has been possible to explain the formation of a variety of sulphide inclusions in the course of solidification of the steels investigated by a judicious application of stable and meta-stable phase diagrams.
It was necessary, as a first step to determine and understand equilibrium sulphide precipitation in the solid state in the proposed low-carbon, low-manganese steels containing varying amounts of titanium and niobium. MnS, CuxS, TiN, (Ti,Nb)(C,N), and NbCN/NbC precipitates with different morphologies and size distributions have been observed and the respective mechanisms by which they form have been clarified.
In order to determine the mechanisms of sulphide inclusion formation, due account was taken of the known thermodynamics such as the iron-sulphide phase diagram and the thermodynamics of MnS formation. However, it was necessary to construct new metastable Fe-MnS-FeS, Fe-MnS-TiS phase diagrams to interpret the observations of the various sulphide phases, particularly the complex sulphide mixtures that were identified at the centreline region of five low-carbon low manganese steels.
Sulphide inclusion formation in low-manganese steel without titanium additions was explained by using a metastable Fe-MnS-FeS phase diagram. It is proposed that the last remaining highly segregated liquid steel, just before final solidification will bypass the stable eutectic Fe/MnS valley and reach the metastable miscibility gap boundary. Subsequently, various sulphur-rich liquids form as a result of the metastable monotectic reactions. Following separation of these sulphur-enriched liquids from the remaining liquid steel, they are isolated between the surrounding steel dendrites and solidify as separate melts at their respective liquidus temperatures. Consequently the different sulphur-rich liquids, which formed at progressively lower temperature, solidify to form sulphide inclusions of different compositions, sizes and morphologies. Some sulphide liquid may fall into the FeS/MnS eutectic valley and solidify as a complex eutectic mixture of ((Mn,Fe)S+(Fe,Mn)S) while some may fall into the FeS corner, and thus form iron manganese sulphide (Fe,Mn)S.
In the steel containing 0.008 wt % titanium, sulphide mixtures formed at higher temperatures and the iron content in the (Mn,Fe,Ti)S phase of the sulphide mixture reduced. Most importantly, iron sulphide does not form in this steel. It was necessary to construct a metastable Fe-MnS-TiS phase diagram to explain the observed sulphide formation in the steels containing more than 0.01 wt % titanium. Due to the higher melting point of TiS and the higher eutectic temperature in this system, metastable monotectic liquid sulphides form at higher temperatures and lead to sulphide inclusion modification in phase, composition and morphology.
In conclusion, the present study has revealed that sulphide formation in low-carbon, lowmanganese steel is decidedly different from sulphide formation in traditional highmanganese steel, and the effect of small titanium additions on the sulphide formation have been clarified. New insights have been gained of the mechanisms of sulphide formation in these steels. Importantly, it has been shown that liquid sulphides that form through metastable reactions can reasonably explain the sulphide formation and modification that have been observed in low-carbon, low-manganese steel.