Nanoscale surface analysis on second generation advanced high strength steel after hot dip galvanizing

Abstract

Second generation advanced high strength steel is one promising material of choice for modern automotive structural parts because of its outstanding maximal elongation and tensile strength. Nonetheless there is still a lack of corrosion protection for this material due to the fact that cost efficient hot dip galvanizing cannot be applied. The reason for the insufficient coatability with zinc is found in the segregation of manganese to the surface during annealing and the formation of manganese oxides prior coating. This work analyses the structure and chemical composition of the surface oxides on so called nano-TWIP (twinning induced plasticity) steel on the nanoscopic scale after hot dip galvanizing in a simulator with employed analytical methods comprising scanning Auger electron spectroscopy (SAES), energy dispersive X-ray spectroscopy (EDX), and focused ion beam (FIB) for cross section preparation. By the combination of these methods, it was possible to obtain detailed chemical images serving a better understanding which processes exactly occur on the surface of this novel kind of steel and how to promote in the future for this material system galvanic protection.

Nanoscale analysis of the influence of pre-oxidation on oxide formation and wetting behavior of hot-dip galvanized high strength steel

“…In contrast, without pre-oxidation the same TWIP steel could not be covered with Zn, because of the formation of a 100-200 nm thick MnO layer at the steel surface [21]. Fig.…”

“…We know from our previous study [21] that the surface of the conventionally Zn coated sample contains Mn oxide which prevents the formation of the Fe 2 Al 5 inhibition layer.…”

“…6b is a mixture of the RGB overlay of Mn, Al and Fe, plus the yellow color from the Zn mapping. Details of the coloring method can be found in our previous paper [21]. The Zn layer has a homogeneous thickness of approximately 30 lm (see Fig.…”

“…These difficulties are linked to the formation of manganese oxides on the surface during the technologically necessary annealing step in production. These Mn based oxides were found to hinder the growth of a continuous Zn layer on the steel surface during galvanizing [21]. http However, there are some approaches to improve the wettability of high Mn steel with Zn: Blumenau et al [22] suggest an additional pre-oxidation step in order to create a Mn/Fe mixed oxide on the steel surface.…”

Microstructure Control of Dual-Phase Steels through Hot-Dip Al–Mg–Si Alloy Coating Process

“…In order to solve these issues, optimizing the alloying elements is needed to control two kinetics of α→γ and γ→α transformations as well. Thus, further alloy modification is desirable, especially when it comes to controlling the multiphase microstructures of advanced high strength steels such as TRIP and TWIP 22) through a combination of heat treatment and hot-dipping.…”