Effect of atmosphere on decarburization and internal oxidation after coiling of hot-rolled steel

Abstract

This study was conducted to investigate the effect of atmosphere on decarburization and internal oxidation of hot-rolled pearlitic steel after coiling. Specimens for simulating the condition of steel coil (CONTACT) and steel strip (SINGLE) were isothermally heated for 1 hour with the atmosphere as dry air. Depth of decarburization is higher in SINGLE test, while that of internal oxidation is higher in CONTACT test. Decarburization is faster in kinetics than internal oxidation in CONTACT test, with relatively small difference compared to SINGLE test. At about 2.0mm from the edge of specimen, external oxides block the gap of CONTACT test and aspect of decarburization is divided, which means the external oxides isolate the atmosphere in the gap. Gas composition in the gap is found from analysis of equilibrium oxygen pressure from Ellingham diagram, resulting that the pressure of CO increases as temperature increases. It indicates that high CO pressure in CONTACT test increases carbon activity at Fe-wustite interface and the depth difference of decarburization between SINGLE and CONTACT test is getting larger as temperature increases due to increasing carbon activity. Quasi-steady-state approximation is well suited for predicting internal oxidation depth and the change of grain boundary density by grain growth determines kinetics of internal oxidation. These results provide an insight on setting new atmosphere condition to apply existing model of decarburization and internal oxidation.