Morphology control of copper sulfide in strip casting of low carbon steel

Copper and sulfur are the typical residual elements and impurities in steel. Previously, we reported the precipitation of very tiny particles of CU2S in copper and sulfur containing steel by strip casting process. In the present paper, the morphologies of copper sulfides in strip casting low carbon steels were distinguishably investigated by Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). Four kinds of copper sulfide with different morphology were observed, namely duplex inclusion of oxide and sulfide (OS), plate-like copper sulfide (PS), shell-like copper sulfide Surrounding the inclusions (SS), and nano-scale copper sulfide (NS), and their formation mechanisms were discussed. The OS is considered to firstly form as molten manganese silicate in molten steel, and grow up with the formation of sulfide inside of the silicate after the solidification of steel. The PS is considered to precipitate from the gamma-Fe phase with plate like shape due to semi coherency with the gamma-Fe matrix. The SS is considered to precipitate in lower temperature ranges on the other pre-formed inclusions such as MnS, oxide and also CU2-xS. The NS is considered to form in the low temperature range of gamma-Fe and especially in alpha-Fe phase as very tiny particles due to the high supersaturation, low diffusivity of component elements and the coherency with the alpha-Fe matrix. Based on this classification, formation stage of oxide, MnS and CU2S was clarified and described as like TTT diagram.