Internal Oxidation during Intercritical Annealing of CMnSi TRIP Steel

Abstract

The equilibrium internal oxidation of CMnSi TRIP steel at intercritical annealing temperatures in a ϩ3°C dew point N 2 ϩ10%H 2 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 · SiO 2 (1ՅxՅ4) and amorphous a-xMnO · SiO 2 (0ϽxϽ0.9) oxide particles. A thin 25-35 nm film of crystalline xMnO · SiO 2 (1ՅxՅ2) 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 · SiO 2 layer varied from 1ՅxՅ2 for crystalline xMnO · SiO 2 to 0ϽxϽ0.9 for amorphous a-xMnO · SiO 2 . In the subsurface region, two distinct types of internal oxidation zones were observed. In the first internal oxidation zone, within the 1 mm range below the surface, fine ferrite grains with a grain size of ϳ550 nm were observed. Crystalline xMnO · SiO 2 (1ՅxՅ2) 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 mm below the surface, the ferrite grain size was about 4 mm, and similar to the grain size in the bulk. Inter-granular amorphous a-SiO 2 particles, 180-300 nm in diameter, and an extensive, apparently continuous, grain boundary network of amorphous a-SiO 2 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.

Effect of Dew Point and Gas Flow Rate on the Surface Oxidation of Advanced High Strength Steels

“…[3][4][5][6][7][8][9][10][11] To identify ways to reduce surface oxides, studies were carried out in various annealing conditions; such as the influence of oxygen partial pressure or H 2 /N 2 ratio in atmosphere, Si/Mn ratio or trace elements in steel compositions etc. Most of these investigations were focused on the reaction of metals with oxygen in atmospheres that were assumed to be under (or near) thermodynamic equilibrium conditions wherein the oxygen partial pressure in atmosphere was determined by gas phase equilibrium.…”

Prediction of Oxide Phases Formed upon Internal Oxidation of Advanced High-Strength Steels

“…[5][6][7][8][9]). However, only a few studies have been reported on the oxidation of Cr-alloyed Fe-Mn-based steels [10,11].…”

Effects of Oxygen Partial Pressure on Oxidation Behavior of CMnSi TRIP Steel in an Oxidation-Reduction Scheme

“…Therefore, extensive work has been performed to suppress the external oxidation of the alloying elements in AHSSs during intercritical annealing by controlling the dew point (DP) and composition of the annealing gases [6][7][8][9][10][11][12][13]. Increasing the DP in the annealing furnace could promote the alloying elements to shift from undergoing external oxidation to internal oxidation, and thus improve the galvanizability of AHSS sheets.…”