Isothermal precipitation behavior of copper sulfide in ultra low carbon steel

Abstract

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 2.7×103 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 heat-treated at 1 373 K for 1.44×104 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 1.44×104 s. The formation mechanisms of these copper sulfides have been discussed in detail. © 2007 ISIJ.