Effect of Cooling Rate on Distribution of Cu and S Concentrations at Residential Levels in Low Iron Carbon Steel

The effect of cooling rate on distribution of Cu and S concentration at residential levels in low iron carbon steel was investigated to discuss the formation mechanism of fine copper sulfide precipitates. Quenching molten samples for Fe-0.1mass%Cu-0.02mass%S-0.1mass%C and Fe-0.1mass%Cu-0.1mass%S-0.1mass%C were conducted with water (WQ), flowing water (FWQ), air (AQ), argon gas (ARQ) and furnace (FQ). Subsequently, the samples were analyzed for the composition of precipitates and the distribution of Cu and S concentration of the iron matrix. Increasing cooling rate increased molar ratio of Cu to S in precipitates as decreasing S concentration, indicating that higher cooling rate would cause less formation of iron sulfide of the precipitates. Moreover, the distribution of Cu concentration over the solidification fraction was reproduced with the Clyne–Kurz segregation model regardless of cooling rate; however, below 99% solidification fraction, the S concentration of the WQ sample was higher and increased more moderately than the Scheil model while that of the FQ sample was close to the Scheil model. The sulfur of the WQ samples was supposed to form finer sulfides dispersed in the iron matrix than the FQ samples. Thus, the precipitation of fine copper sulfide in solid iron is probably due to fast cooling rate, which more suppresses local segregation of sulfur and prevents the formation of sulfides other than copper sulfide.