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Selective Oxidation of Advanced High Strength Steels (AHSS) during Annealing in Continuous GI/GA Lines

Selective Oxidation of Advanced High Strength Steels (AHSS) during Annealing in Continuous GI/GA Lines
Gong, Yong Feng
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The selective oxidation of Advanced High Strength Steels (AHSS), such as TRIP steels and TWIP steels, during annealing in continuous Galvanizing/Galvannealing lines was investigated. The selective oxidation of alloying elements, such as Mn, Si and Al occurred in the 10%H2+N2 atmosphere at different dew points. High Resolution Transmission Electron Microscopy (HR-TEM) with Focused Ion Beam (FIB) sample preparation and Energy Dispersive Spectrometer (EDS) was used to analyze the composition, crystal structure, morphology, size, spatial distribution and interfacial structure of the oxides on the steel surface and in the subsurface. For Si-bearing TRIP steels, after intercritical annealing in external atmosphere, two kinds of oxide layers: amorphous a-SiO2 and crystalline c-xMnO.SiO2 (12) and amorphous a-xMnO.SiO2 (x<0.9) particles were found at the MnO/steel interface. In the subsurface, a narrow Mn-depleted zone was formed due to selective oxidation of Mn. After austenitic annealing in an internal oxidation promoting atmosphere, internal MnO and crystalline c-xMnO.SiO2 (x>1) particles were formed in the Mn-depleted ferritic subsurface. MnO oxide layer was formed at the surface and no c-xMnO.SiO2 (x>2) or amorphous a-xMnO.SiO2 (x<0.9) particles were formed at the MnO/steel interface. For Al-added TWIP steel, after austenitic annealing in external oxidation atmosphere, three kinds of oxide layers were formed on the surface: an outer MnO layer, an intermediate amorphous a-xMnO.SiO2 (x<0.9) and a thin inner crystalline c-CxMnO.Al2O3 (0.8
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