Copper and sulfur are typical residual elements or impurity elements in steel. Sufficient removal of them during steelmaking process is difficult for copper and costly for sulfur. Utilization of copper and sulfur in steel, especially in steel scrap, has been an important issue for a long period for metallurgists.Copper and sulfur may combine to form a copper sulfide, which may provide a prospect to avoid the detrimental effects of copper and sulfur in steel. Unfortunately the formation mechanism of a copper sulfide in steel has not been completely clarified so far. In the present paper, solution treatment of samples containing copper and sulfur are firstly performed at 1 623 K for 2.7ϫ10 3 s followed by quenching into water. The samples are then isothermally heat-treated at 673 K, 873 K, 1 073 K, 1 273 K and 1 373 K for different time followed by quenching into water again. The size, morphology, constituent and crystallography of sulfide precipitates in these samples are investigated by SEM and TEM equipped with EDS. Fine copper sulfides (less than 100 nm) are observed to co-exist with silicon oxide in samples even isothermally heattreated at 1 373 K for 1.44ϫ10 4 s; Film-like copper sulfides are generally observed to co-exist with iron sulfide in all samples; Plate-like copper sulfides are observed especially in sample isothermally heat-treated at 1 073 K for 1.44ϫ10 4 s. The formation mechanisms of these copper sulfides have been discussed in detail.KEY WORDS: copper sulfide; isothermal precipitation; morphology; crystallography; ultra low carbon steel.up to 780 K at 36.60 at% S and down to 345 K at 35.65 at% S. Therefore, the high Digenite is the most probable phase that is expected in steel at high temperature. But people have confusedly presumed and reported several crystal structures and phases, 11,[13][14][15]20) for example cubic Cu 1.8 S or Cu 1.6 S or CuS 2 , hexagonal CuS and rhombohedral Cu 1.6 S and so on, for copper sulfide in steel.In the previous papers, 5,6,8) the authors found that copper sulfides could be formed in strip casting steel. Differently from the formerly reported copper sulfides in steel/ iron, [10][11][12][13][14][15] which usually coexisted with MnS, almost pure copper sulfides with various morphologies were found in the previous strip casting steels. In the present paper, the formation mechanism such as precipitation temperature, stability and crystallography of various copper sulfides have been investigated during isothermal heat treatment process. Experimental Procedures Materials and Heat Treatment ConditionsThe chemical composition of the present steel is shown in Table 2. The steel is firstly prepared in an induction heating furnace under flowing argon gas. About 350 g of electrolytic iron is melted at 1 873 K. After the alloying elements (Cu, S) are added to the melted iron, the melt is cooled to room temperature with the furnace. An ingot with size of f40ϫH50 mm (hereafter named as sample H00) then could be obtained for the following heat treatment.The solution treatment o...