High-resolution topochemical analysis and thermochemical simulations of oxides and nitrides at grain boundaries and within the grains of a low alloy Mn-Cr hot-rolled steel sheet

Vera G Praig,Michael Stöger-Pollach,Arno Schintlmeister,Michael Auinger,Herbert Danninger

doi:10.1016/j.jallcom.2021.160069

Abstract

The selective oxidation underneath the scale layer of an industrially hot rolled Fe-1.8Mn-0.8Cr steel at temperatures between 600 and 700 °C has been investigated. The spatial distribution and composition of formed precipitates has been studied by high-resolution topochemical analysis via TEM-EELS and NanoSIMS and revealed heterogeneities in chemical composition, especially along grain boundaries. It could be shown that grain boundary oxides are predominantly composed of aluminium, chromium or silicon oxides/nitrides, surrounded by manganese-rich oxides. Experimental results of phase stability have been compared to numerical simulations, considering the distribution of more than 40 potentially stable oxide-, nitride- and carbide phases, and differences are critically discussed.

Highlights

P High temperature oxidation studies and numerical simulations of phase stabilities for an industrial low-alloy Mn-Cr steel have been carried out
The values of the oxidation depth agree reasonably well between theoretia cal predictions and experimental observations but deviate for the amount of formed nitrides. This deviation originates from the presence of an external wustite scale at process temperatures, which serves as an oxygen reservoir for selecn tive oxidation but blocks further nitrogen uptake from the reaction gas. r High resolution images of internal precipitates clearly show heterogeneities in their chemical composition instead of a homogeneous spinel-type oxide
Grain boundary oxides often seem to be composed of a layered structure with u chromium, aluminium and nitrogen in the center, surrounded by manganese-rich oxides

Summary

Ea RT

For the upper limit of the oxygen partial pressure (p(O2) = 10−22 bar) defined by the iron/wustite equilibrium at the alloy/oxide scale interface, the calculated nitrogen partial pressure to stabilise aluminium nitrides is much higher than p(N2) = 10+5 bar This means that based on bulk thermodynamic principles, no nitride phases are expected to be stable near the surface because the thermodynamic nitrogen partial pressure required would be far too high. 20 30 40 50 60 70 80 distance d / nm ro Figure 6: Relative concentration profiles of the alloy elements manganese, aluminium and silicon across a grain boundary oxide (i, left) and p two closely adjacent internal oxides (ii, iii, right) of a hot-rolled Fe-1.8Mn-0.8Cr industrial-grade steel sheet after the completed coil cooling programme.

Conclusions

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