Formation of Surface and Subsurface Oxides during Ferritic, Intercritical and Austenitic Annealing of CMnSi TRIP Steel

Abstract

The equilibrium external oxidation of CMnSi TRIP steel at different annealing temperatures in a low dew point N-2-10%H-2 atmosphere of -30 degrees C was investigated for the first time by means of high resolution transmission electron microscopy of cross-sectional samples. Annealing in the ferrite stability temperature range below the Ae1 temperature resulted in the formation of 300-600nm size crystalline internal MnO in the matrix, In the subsurface region, 10-20 nm size SiO2 particles were detected. Large amorphous lens-shaped xMnO center dot SiO2, oxides, with x<0.5, were present at the surface, and thin films of crystalline xMnO center dot SiO2 oxides, with 1<x<2 were formed between these large lens-shaped oxides. The amorphous lens-shaped xMnO center dot SiO2 oxides were covered by a continuous thin layer of crystalline xMnO center dot SiO2 oxide, with x>2. Amorphous 15-50 nm size internal SiO2 particles covered with a thin layer of crystalline MnO center dot SiO2 and MnO center dot Al2O3 oxides were found in the subsurface matrix region and at the grain boundaries after annealing in the intercritical and the fully austenitic temperature ranges. A discontinuous amorphous SiO2 layer covered by a layer of crystalline 2MnO center dot SiO2 + MnO center dot SiO2 mixed oxide was present at the surface after annealing in the intercritical Ae1-Ae3 temperature range. This changed to a continuous layer of amorphous SiO2 on the steel surface covered by a continuous layer of crystalline 2MnO center dot SiO2+ MnO center dot SiO2 mixed oxide after annealing in the austenitic stability range. The results clearly show an increased tendency for the xMnO center dot SiO2 and SiO2 oxides to form two separate surface oxide films covering the entire steel surface during continuous annealing at the higher annealing temperatures used to process advanced high strength steels. The presence of these film-forming oxide layers will prevent the formation of the inhibition layer and its wetting by the liquid Zn, and cause galvanizing surface defects.