Mechanical Property Development and Galvanizing of a Medium-Mn Third-Generation Advanced High Strength Steel

Abstract

Medium-Mn third-generation advanced high strength steels (3G AHSS) are promising candidates for vehicle lightweighting due to their desirable balances of specific strength and ductility. However, the continuous galvanizing of these steels is challenging because their thermal processing routes are not always compatible with existing continuous galvanizing line (CGL) infrastructure and the selective oxidation of the main alloying elements during annealing can prevent reactive wetting of the substrate by the galvanizing bath. The conflicting objectives of achieving the desired mechanical properties and high-quality galvanized coatings were addressed in this work on a prototype 0.15C-5.6Mn-1.9Al-1.1Si (wt%) steel. The alloy achieved the general mechanical property targets for 3G AHSS using a two-stage CGL-compatible thermal cycle. The martensitic pre-treatment resulted in a 80% martensitic-20% ferritic microstructure with no detectable retained austenite. Subsequent intercritical annealing for 120 s resulted in significant (0.2-0.3) volume fractions of lamellar retained austenite whose chemical composition was a strong function of intercritical annealing temperature. Hence, several combinations of yield strength, ultimate tensile strength (UTS), and total elongation (TE) were obtained due to differences in TRansformation-Induced Plasticity (TRIP) kinetics. An intercritical annealing temperature of 710°C resulted in the highest UTS × TE combination, which was attributed to these samples having a retained austenite chemical composition which enabled gradual exhaustion of the TRIP effect as well as extensive mechanical twinning. This processing route was selected for subsequent selective oxidation and reactive wetting trials. The evolution of the selective oxide morphology, distribution, and chemistry during each annealing stage was determined as a function of process atmosphere pO2. After the martensitic pre-treatment at dew points of –30°C, –10°C, and +5°C, a compact and complex external oxide layer (approximately 200 nm thick) was formed. An extensive internal oxide network ranging from v 6-10 μm deep was also formed, depleting the local matrix of solute and leaving a metallic Fe surface layer that was populated with extruded Fe nodules. Flash pickling with a hydrochloric acid-based solution dissolved the external MnO, MnSiO3, and Mn2SiO4, leaving only dispersed MnAl2O4 nodules (approximately 50 nm thick) remaining on the surface. No significant evolution of the selective oxides occurred during intercritical annealing due to the large diffusion distances through the solute-depleted layer to the surface from the bulk matrix. The experimental steel was successfully reactively wetted by a conventional 0.2 wt% Al (dissolved) galvanizing bath held at 460°C after an immersion time of 4 s. The primary wetting mechanism was direct wetting of the solute-depleted metallic Fe surface layer (which was aided by the additional surface area provided by the extruded Fe nodules). Evidence of several secondary reactive wetting mechanisms (such as oxide wetting, oxide lift-off, and bath metal infiltration) was also documented, showing that the antecedent MnAl2O4 nodules were not deleterious to reactive wetting in this case. The coatings were high-quality and adherent, as they resisted cracking or flaking after bending. Overall, this research shows that the two-stage processing route is promising for the production of Zn-coated third-generation advanced high strength steels.