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

Abstract

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 (1<x<2) oxides were formed on the surface. In the subsurface, complex multi-layer structure oxides particles with an amorphous a-SiO2 core and a crystalline c-xMnO.SiO2 (x=1)/c-xMnO.Al2O3 (x=1) shell were found. After intercritical annealing in an internal oxidation promoting atmosphere, MnO particles, crystalline c-xMnO.SiO2 (2<x<4) and small amorphous a-xMnO.SiO2 (x<0.9) particles and a thin crystalline c-xMnO.SiO2 (1<x<2) film were formed on the surface. In the subsurface, two zones with characteristic oxides were found: crystalline c-xMnO.SiO2 (1<x<2) particles and a network of amorphous a-SiO2 oxides. The effects of selective oxides on the wettability of Si-bearing TRIP steels in hot dip processes were studied by means of a galvanizing simulator and the results show that the amorphous oxides, especially a-SiO2 and a-xMnO.SiO2 (x<0.9), cannot be reduced by alumino-thermic reaction in the zinc bath and prevent the galvanizability of Si-bearing TRIP steels. It implies that an internal oxidation promoting atmosphere can improve the galvanizability of Si-bearing TRIP steels by suppressing the formation of amorphous a-xMnO.SiO2 (x<0.9) or a-SiO2 oxides.For Al-free TWIP steel, after austenitic annealing in external oxidation atmosphere, MnO layer was formed on the surface and small crystalline c-xMnO.SiO2 (x>2) 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<x<1.2).Smaller voids were formed between the a-xMnO.SiO2 (x<0.9) and the c-CxMnO.Al2O3 (0.8<x<1.2) transition layers. In the subsurface, crystalline c-xMnO.Al2O3 (x<0.5) particles were formed in the Mn-depleted ferritic subsurface. After austenitic annealing in an internal oxidation promoting atmosphere, a MnO layer was formed at the surface and there were no voids or transition oxides layer formation at the MnO/steel interface. In the Mn-depleted ferritic subsurface, two oxidation zones were observed: a void zone and a network of c-CxMnO.Al2O3 (0.6<x<2) and c-CxMnO.SiO2 (2<x<5). The voids were surrounded by a-CxMnO.Al2O3 (x<0.2) oxides. The thick MnO layer and the voids formed at the surface are believed to affect the galvanizability of TWIP steels in hot dipping process negatively.