Selective Oxidation of a Medium-Mn Third Generation Advanced High Strength Steel during Austenitizing and Intercritical Annealing

Abstract

The effect of the simulated continuous galvanizing line N2-5 vol% H2 process atmosphere oxygen partial pressure (pO2) on the external and internal selective oxidation of a prototype medium-Mn third generation (3G) advanced high strength steel was determined during a two-stage heat treatment cycle (i.e., austenitizing and intercritical annealing) which had previously yielded 3G properties. Thick external oxides (~ 200 nm) were observed after the austenitizing heat treatment, regardless of the process atmosphere pO2 employed. An intermediate flash pickling step was successful in reducing the external oxide thickness significantly (to ~ 30 nm) along with revealing some extruded metallic Fe nodules on the surface. The austenitizing heat treatment also resulted in a solute-depleted surface layer with a minimum thickness of 2 µm. This solute-depleted layer inhibited the formation of external oxides during intercritical annealing, resulting in a surface similar to that observed after flash pickling comprising a near-pure Fe surface with isolated, nodular external oxides. These surfaces are promising in terms of successful reactive wetting of this prototype medium-Mn steel during subsequent continuous hot-dip galvanizing.

Continuous Hot‐Dip Galvanizing of Medium‐Manganese Third‐Generation Advanced High‐Strength Steels

“…However, an additional mechanism for Zn ingress into the substrate is proposed whereby the reactive wetting was facilitated by the selective dissolution of MnO, MnSiO 3 , and Mn 2 SiO 4 during flash pickling. Previous work by the present authors [ 51,52 ] showed that the flash pickling used to restore the substrate surfaces also dissolved some of the internal oxide structures directly connected to the surface. These partially dissolved structures were preserved during the intercritical annealing step and could allow for bath metal infiltration and mechanical interlocking of the coating with the substrate.…”

“…SEM images of the preimmersion surfaces (Figure 2 and 3) show discrete and dispersed nanoscaled oxides that were previously identified as MnCr 2 O 4 [ 51 ] and MnAl 2 O 4 [ 52 ] for steels A and B, respectively. The surfaces were also covered with extruded Fe nodules, which provided additional oxide‐free Fe surfaces to react with the galvanizing bath metal to form the desired Fe 2 Al 5– x Zn x interfacial layer, as shown in Figure 10.…”

“…Previous selective oxidation studies revealed that, for steel A, the discrete nodular oxides were approximately 30 nm thick and comprised mostly MnCr 2 O 4 with some MnAl 2 O 4 and MnSiO 3 /Mn 2 SiO 4 oxides. [ 51 ] In the case of steel B, the external oxide nodules were approximately 50 nm thick and largely comprised MnAl 2 O 4 . [ 52 ] The extruded Fe nodules were a result of the volumetric expansion associated with internal oxidation during the 600 s austenitizing pretreatment.…”

“…Figure adapted under the term of CC‐BY license. [ 51 ] Copyright 2022, the Authors, published by IOP Publishing.…”

“…The chemical speciation and morphology of the internal and external oxides formed during these annealing cycles have also been previously documented by the present authors. [51,52]…”