This report corresponds to the third paper in three consecutive papers to know the whole sulfide precipitation in Ti-added steel by adding a new knowledge that even a trace level of copper (0.01%Cu) acts as a sulfide-former. The authors show the strange phenomenon of copper sulfide in Ti-added steel in long-time-annealing in ferrite region. When annealed for short time (ex. 90 s), much of Cu-S is precipitated, but the amount of Cu-S is small at longer-time-annealing, while TiS reduces and Ti₄C₂S₂ increases monotonically. This phenomenon does not mean that Cu-S is initially formed and then dissolved away, but the Cu-S precipitation should be interpreted as causal evidence of “(re-)solute sulfur and its re-precipitation” along the same concept in the first and the second papers. Long-time-annealing means that our proposed reaction “4TiS + 2[C] → Ti₄C₂S₂ + 2[S]” proceeds rightward and the re-solute sulfur is re-precipitated as Ti₄C₂S₂ (and MnS). Consequently, the amount of Cu-S is small. In contrast, short-time-annealing is not long enough to stabilize re-solute sulfur as Ti₄C₂S₂ (and MnS) and much of sulfur still remains solute. Therefore the amount of Cu-S is increased.
Furthermore, the phenomena in three consecutive papers is organized into an integrated concept to understand the whole sulfide precipitation in Ti-added steel in both austenite and ferrite region.