Internal Oxidation during Intercritical Annealing of CMnSi TRIP Steel

Abstract

The equilibrium internal oxidation of CMnSi TRIP steel at intercritical annealing temperatures in a +3°C dew point N₂+10%H₂ atmosphere was investigated by means of high resolution transmission electron microscopy of cross-sectional samples. The experimental conditions are considered to lead to the selective internal oxidation of Mn and Si. The intercritical annealing however resulted in the formation of three types of isolated particles on the surface: 200–340 nm size single crystal MnO oxide particles, crystalline 30–60 nm size xMnO·SiO₂ (1≤x≤4) and amorphous α-xMnO·SiO₂ (0<x<0.9) oxide particles. A thin 25–35 nm film of crystalline xMnO·SiO₂ (1≤x≤2) was present between these particles. Thin discontinuous transition oxide layers with a thickness of 25–55 nm were formed between the MnO particles and the Fe matrix. The composition of this xMnO·SiO₂ layer varied from 1≤x≤2 for crystalline xMnO·SiO₂ to 0<x<0.9 for amorphous α-xMnO·SiO₂. In the subsurface region, two distinct types of internal oxidation zones were observed. In the first internal oxidation zone, within the 1 μm range below the surface, fine ferrite grains with a grain size of ~550 nm were observed. Crystalline xMnO·SiO₂ (1≤x≤2) oxide particles with a diameter of 80–250 nm were present in this internal oxidation zone. In the second internal oxidation zone, from 1 to 4 μm below the surface, the ferrite grain size was about 4 μm, and similar to the grain size in the bulk. Inter-granular amorphous α-SiO₂ particles, 180–300 nm in diameter, and an extensive, apparently continuous, grain boundary network of amorphous α-SiO₂ were observed in this second internal oxidation zone. The observed oxide distribution is related to the difference in selective oxidation behavior of the main alloying elements Si and Mn and very likely due to the difference in their diffusivities, oxide stability and oxide solubility during intercritical annealing. The decarburization of the matrix during annealing in a high dew point atmosphere is also likely to play a role. The observed selective oxidation is relevant for the continuous galvanizing of high strength steels. It is expected that the presence of a thinner oxide film is less likely to prevent the formation of the inhibition layer and the wetting of the steel surface by the liquid Zn during hot dip galvanizing. Operation of the annealing furnace at a higher dew point during the processing of Si-bearing steels such as conventional CMnSi TRIP steel may therefore lead to a decrease of galvanizing surface defects.