Heating Rate Effect on Liquid Zn-assisted Embrittlement of Advanced High Strength Steels

Abstract

The aim of the study is to investigate the heating rate effect on Liquid Metal Embrittlement (LME) of galvanized Advanced High Strength steels (AHSSs) by using hot tensile tests. Three galvanized steels were heated with different rates and strained at 600 and 700 ℃ to examine the embrittlement by liquid Zn. In all steel grades, the microstructures at coating/substrate interfaces change as Fe-Zn intermetallic compounds form during heating. These intermetallic compounds have higher melting points than that of pure Zn, and thereby can prevent the substrate from contacting with the embrittling liquid metal. Hence, the embrittlement possibility is reduced with slower heating rate by more active formation of Fe-Zn intermetallics. The formation kinetics varies among the steels due to the different characteristics of inhibition layer rather than the crystal structures or compositions of the substrates. In TRansformation Induced Plasticity (TRIP) steel, it is likely that liquid metal exists at both temperature, because Fe-Zn intermetallic formation kinetics is slowest. Thus, the TRIP steel exhibits LME at both temperatures regardless of heating rates. Since Fe-Zn reaction is faster in TWining Induced Plasticity (TWIP) steel than that in the TRIP steel, LME occurs at 700 ℃, but not at 600 ℃. Although the Fe-Zn reaction in the Press Hardening Steel (PHS) is slower than that in TWIP steel and liquid metal is available at 700 ℃, the PHS is not embrittled; this is because the PHS experiences a lower tensile stress than the other steel grades due to its lower tensile strength. In overall, the heating rate plays an important role in LME by influencing the presence of a liquid Zn at high temperature. Moreover, it is proved that a sufficient tensile stress is required for LME.