Precipitation of copper sulfide in ultra low carbon steel containing residual level of copper

Abstract

The influence of residual copper (≒0.01 mass%) on sulflde precipitation in ultra low carbon steel was studied with microanalysis using Field Emission-Transmission Electron Microscopy (FE-TEM), X-Ray Diffraction (XRD) of the extracted precipitates and quantitative chemical analysis of extraction residue. A small amount of copper (≒0.01 mass%) plays an important role in sulfide formation in steel, as does manganese. Two types of sulfides were found in ultra low carbon steel with residual copper: (1) MnS covered with copper sulfide (Cu-S) and (2) "free-standing" Cu-S, with sizes of 0100∼300nm and less than about 040 nm, respectively. The atomic ratio of Cu/S was determined to be 1.8±0.3 by Energy Dispersive X-ray spectroscopy (EDX), and the phase was determined to be Cu8S 5 from XRD of the extracted precipitates. Quantitative chemical analysis revealed that almost all sulfur (71 mass ppm) was precipitated as sulfides in steel containing 73ppm of sulfur. The concentration of sulfur as Cu8S5 was 24ppm, which accounts for about 35% of the total quantity of sulfur. This indicates that the formation of copper sulfide has to be taken into account in precise analysis of sulfides in low Mn and S systems (Mn ≒ 0.20%, S ≒ 0.007%). The precipitation of Cu-S in water-quenched (WQ) samples and the morphologies of MnS-cored BN & Cu-S-covered MnS in hot-rolled and WQ samples suggest that Cu-S is formed at temperatures between "below 750°C" and 620°C and is also precipitated even during a short time in water-quenching. ©2005 The Japan Institute of Metals.