Selective Oxidation of Manganese and Chromium during Annealing of Low Carbon Strip Steels

Abstract

Surface graphitization is a well known defect that occurs when low carbon steel strip is batch annealed. A small addition of chromium (about 0.04 wt %) can be used to minimize the surface graphitization. However chromium and some other alloy elements, such as manganese used in this class of steel, have higher affinities for oxygen than iron. Therefore it is possible for them to be oxidized during batch annealing in a reducing environment to iron. Selective oxidation of these two elements gives rise to a risk of forming different surface defects that may affect the quality of the tinned surface. The edge defect is characterized as a region of low reflectivity on the tin plate product caused by grain boundary precipitates. A porous tin coating with a dull appearance is produced in the affected areas. Not only are the aesthetical values of the finished tin plate product affected but the corrosion resistance is also reduced. In this investigation surface oxides formed at grain boundary of low carbon steels annealed at 700°C in 5% hydrogen 95% nitrogen atmosphere were characterized using scanning electron microscopy and transmission electron microscopy. Two different oxide particles (Fe,Mn)O and MnCr2O4 were observed at the grain boundaries with the former five-fold coarser than the latter. It was found at the annealing temperature of 700°C, that the mean particle size of the (Fe,Mn)O depends on the manganese content, while the mean particle size, and distribution of the MnCr2O4 was dependent on chromium, but independent of manganese. The coarse (Fe,Mn)O precipitates pose no potential risks to electrolytic tinning as they will be removed by the pickling operation prior to tinning. Controlling the chromium content of the steel can minimize the potential risks posed by the MnCr2O4 to the quality of electrolytic tinning.