12th International Conference on Zinc & Zinc Alloy Coated Steel Sheet - GALVATECH 2021

The present paper will summarize the selective oxidation and reactive wetting results obtained for a prototype medium-Mn 0.15C-6Mn-2Al-1Si 3G AHSS after CGL-compatible thermal processing. It was previously determined that 3G AHSS mechanical property targets of 24,000 UTS x total elongation < 40,000 MPa% could be achieved using CGL-compatible heat treatments by annealing from a 80% martensite + 20% ferrite starting microstructure. This work summarizes the subsequent interrogation conducted to determine the composition, morphology, and spatial distribution of the selective surface and subsurface oxide species formed during the martensitic pre-treatment and subsequent intercritical annealing cycles. Samples were annealed in a N2-5% H2 process atmosphere with a controlled dew point of either -30°C or -10°C. Auger elemental maps indicated that, for selected samples intercritically annealed for 120s, significant area fractions of the surfaces remained rich in Fe and were, therefore, available for the interfacial reaction required for reactive wetting of the substrate by the conventional GI bath. Auger maps also revealed that Si-rich oxide films were dispersed among Mn-rich surface oxide nodules. X-ray photoelectron spectroscopy results showed surface enrichment of Mn and Si and subsurface enrichment of Al. Reactive wetting test results will be discussed as a function of annealing temperature and process atmosphere.