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

In order to meet the demands for improved safety, fuel economy and comfort from the automotive industry, various 3rd Generation (3G) advanced high strength steels (AHSS) with the product of TS (Tensile Strength) and TEL (Total Elongation) higher than 10GPa% have been developed. To guarantee their use as auto body parts, corrosion resistance has been assured by zinc coating on the steel surface by continuous galvanizing process. New chemistries to produce 3G-AHSS involve the addition of several alloying elements into the steel, amongst them Si, Mn, Al and Cr are selectively oxidized externally on the steel surface during annealing in the continuous galvanizing lines when industrial dry dew point is maintained. This external selective oxidation of alloying elements renders the steel surface unsuitable for subsequent galvanizing. To produce a steel surface suitable for galvanizing, the selective oxidation behavior of an alloying element can be changed from external to internal (below the steel surface) in the presence of a high partial pressure of oxygen by maintaining a high dew point annealing atmosphere. These internal oxides mostly form along the grain boundaries as well as inside the grains of the steel sub-surface area. It is also well known that carbon plays a key role in determining the tensile properties of 3G-AHSS. The presence of a high partial pressure of oxygen associated with the high dew point atmosphere reacts with steel alloying element Carbon and forms a decarburized layer at the steel sub-surface area. Thus, the annealing atmosphere necessary to produce the galvanized coated 3G-AHSS modifies the steel sub-surface area. The decarburized layer along with network of internal oxides has a considerable effect in determining the tensile strength as well as the elongation of 3G-AHSS. The present study deals with the systematic evaluation of the effect of an annealing condition suitable for a good galvanized coating on the tensile properties of 3G-AHSS. It has been observed that the sub-surface structure not only deteriorates the tensile strength but also affects the elongation of the steel. This effect has been more prominently observed with the increase of the alloying elements added to the steel. This study also evaluates through interrupted tensile test, the effect of network of internal oxides along with decarburized layer present in the steel sub-surface on the crack initiation and propagation.