Sulfide Precipitations in Steel and its Role of a Residual Level of Copper

A new concept of sulfide precipitation in steel is proposed that 'a residual level of Copper (0.01 %Cu)' plays an important role in the formation of copper sulfide (Cu-S), although such a small amount of Cu has not been regarded as a sulfide-former and is often regarded as an inevitable impurity, even during Blast-Furnace & LD-converter steelmaking process. The role of Cu and sulfide precipitation is investigated by using samples from ultra low carbon steel and Ti-stabilized IF steel. In ultra low carbon steel, Copper Sulfide (Cu-S) is observed as two types: (1)'Coating' Cu-S on MnS and (2)'Free-standing' Cu-S. The precipitated Sulfur as Cu-S composes about one-third of the entire amount of precipitated Sulfur as sulfides. The temperature to form Cu-S is estimated to be lower than 700 deg.C in the ferrite region. In Ti-stabilized IF-steel, Cu-S also plays an important role in sulfide precipitation in the ferrite region. It is related to a morphological change of TiS to Ti4C2S2 during annealing. The amount of precipitated Sulfur as Cu-S is affected by heat treatment, but it is NOT negligible. For example, the precipitated Sulfur in the form of Cu-S corresponds to about 20% of the all sulfides in steel at 680 deg.C (coiling temp) & 830 deg.C (annealing temp). Cu-S, which is formed by Copper included as a tramp element, should be precisely accounted for to understand the phenomena of sulfide precipitations in steel.