Third generation advanced high-strength steels (3G-AHSS) have received significant interest from leading auto steel industries and OEMs as candidate materials for reduced mass Body In White (BIW) components due to their unique combination of high specific strength and ductility. However, the continuous hot-dip galvanizing of these steels is challenging due to selective oxidation of the main alloying elements such as Mn, Si, Al and Cr at the steel surface during the annealing step prior to immersion in the galvanizing Zn(Al, Fe) bath, as extensive coverage of the substrate surface by these oxides is detrimental to reactive wetting, good coating adhesion and integrity. Simulated galvanizing treatments were conducted on two prototype Fe-0.1C-6Mn-2Si (wt pct) 3G steels; one as the reference steel and the other with 0.05 wt pct Sn added to the composition. The combined effects of annealing temperature, time, process atmosphere oxygen partial pressure and 0.05 wt pct Sn addition on the selective oxidation of the steel substrates were determined. Subsequently, the reactive wetting of the steels with respect to the pre-immersion surface structures of the samples annealed for 120 s was examined. Annealing heat treatments were carried out at 800˚C and 690˚C in a N2-5 vol pct H2 process atmosphere under three dew points of –50˚C, –30˚C and +5˚C, covering process atmosphere oxygen partial pressures within the range of 1.20  10-27 atm to 1.29  10-20 atm. MnO was present at the outmost layer of the external oxides on all samples after annealing. However, the morphology, distribution, thickness and surface coverage were significantly affected by the experimental variables. Annealing the reference steel under the low dew point process atmospheres, i.e. –50˚C and –30˚C, resulted in the highest Mn surface concentration as well as maximum surface oxide coverage and thickness. The oxides formed under these process atmospheres generally comprised coarse, compact and continuous film forming nodules, whereas the surface morphologies and distributions obtained under the +5˚C dew point process atmosphere, which was consistent with the internal oxidation mode, exhibited wider spacing between finer and thinner MnO nodules. The grain boundary internal oxide networks had a multi layer structure with SiO2 and MnSiO3 at the oxide cores and shells, respectively. Significant morphological changes were obtained as a result of Sn addition. The continuous film-like external MnO nodules were modified to a fine and discrete globular morphology, with less surface coverage by the oxides and reduced external oxide thickness. Both the external and internal oxidations followed parabolic growth kinetics, where the depth of the internal oxidation zone decreased with Sn addition and decreasing oxygen partial pressure. Poor reactive wetting was observed for the reference steel substrates that were annealed for 120 s under the –50˚C and –30˚C dew point process atmospheres at 800˚C and under the –50˚C dew point atmosphere at 690˚C, such that no integral metallic coating was formed after the 4 s immersion in the Zn(Al, Fe) bath. By contrast, excellent coating quality was obtained for the Sn-added steels when the –30˚C and +5˚C dew point process atmospheres were employed when annealing at 690˚C. The remainder of the experimental conditions demonstrated good reactive wetting with intermediate coating quality. For the two reference steels annealed at 800˚C under the –50˚C and –30˚C dew point process atmospheres, poor reactive wetting was due to full coverage of the surface by 116 nm and 121 nm thick and continuous MnO films. In the case of the 690˚C  –50˚C reference steel with the external layer thickness of only 35 nm, however, poor wetting was attributed to substantial coverage of the surface by continuous, film-like oxides. In both cases, exposure of the underlying substrate to the bath alloy and an intimate contact between the substrate Fe and the bath dissolved Al could not take place and the formation of the Fe2Al5Znx interfacial layer was hidered. For the processing conditions that satisfactory reactive wetting was obtained despite the pre-immersion selective oxidation of the surfaces, several reactive wetting mechanisms were determined. For the samples with a sufficiently thin external MnO layer, good reactive wetting was attributed to partial reduction of MnO by the bath dissolved Al, as well as bridging of the Mn sub-oxides by the Zn coating or Fe2Al5Znx interfacial intermetallics. Partial or full formation of the Fe2Al5Znx interfacial layer was observed in the successfully galvanized substrates with Fe-Al crystals formed between, underneath and also on top of the reduced oxides. Furthermore, for cases with widely-spaced, fine oxide nodules, it was found that the liquid bath alloy was able to infiltrate the external oxide/substrate interface, resulting in surface oxide lift-off and enhanced coating adhesion. It was globally concluded that the thin, discrete and fine globular morphology of external MnO, resultant of annealing the steel substrates with 0.05 wt pct Sn addition under the process atmosphere oxygen partial pressures consistent with internal oxidation, allowed for an enhanced reactive wetting by the Zn(Al, Fe) galvanizing bath which was manifested by increased amount of Al uptake and population of the Fe2Al5Znx intermetallics at the coating/steel interface. / Thesis / Doctor of Science (PhD)

Aluminium is usually added to the continuous hot-dip galvanizing bath to improve coating ductility and adhesion through the rapid formation of a thin Fe-Al intermetallic layer at the substrate-liquid interface, thereby inhibiting the formation of brittle Fe-Zn intermetallic compounds. On the other hand, Mn is essential for obtaining the desired microstructure and mechanical properties in advanced high strength steels, but is selectively oxidized in conventional continuous galvanizing line annealing atmospheres. This can deteriorate reactive wetting by the liquid Zn(Al,Fe) alloy during galvanizing and prevent the formation of a well developed Fe-Al interfacial layer at the coating/substrate interface, resulting in poor zinc coating adherence and formability. However, despite Mn selective oxidation and the presence of surface MnO, complete reactive wetting and a well developed Fe-Al interfacial layer have been observed for Mn-containing steels. These observations have been attributed to the aluminothermic reduction of surface MnO in the galvanizing bath. According to this reaction, MnO is reduced by the bath dissolved Al, so the bath can have contact with the substrate and form the desired interfacial layer. Heat treatments compatible with continuous hot-dip galvanizing were performed on four different Mn-containing steels whose compositions contained 0.2-3.0 wt% Mn. It was determined that substrate Mn selectively oxidized to MnO for all alloys and process atmospheres. Little Mn surface segregation was observed for the 0.2Mn steel, as would be expected because of its relatively low Mn content, whereas the 1.4Mn through 3.0Mn steels showed considerable Mn-oxide surface enrichment. In addition, the proportion of the substrate surface covered with MnO and its thickness increased with increasing steel Mn content.A galvanizing simulator equipped with a He jet spot cooler was used to arrest the reaction between the substrate and liquid zinc coating to obtain well-characterized reaction times characteristic of the timescales encountered while the strip is resident in the industrial continuous galvanizing bath and short times after in which the Zn-alloy layer continues to be liquid (i.e. before coating solidification). Two different bath dissolved Al contents (0.20 and 0.30 wt%) were chosen for this study. The 0.20 wt% Al bath was chosen as it is widely used in industrial continuous galvanizing lines. The 0.30 wt% Al bath was chosen to (partially) compensate for any dissolved Al consumption arising from MnO reduction in the galvanizing bath.The Al uptake increased with increasing reaction time following non-parabolic growth kinetics for all experimental steels and dissolved Al baths. For the 0.20 wt% dissolved Al bath, the interfacial layer on the 1.4Mn steel showed the highest Al uptake, with the 0.2Mn, 2.5Mn and 3.0Mn substrates showing significantly lower Al uptake. However, increasing the dissolved bath Al to 0.30 wt% Al resulted in a significantly increased Al uptake being observed for the 2.5Mn and 3.0Mn steels for all reaction times. These observations were explained by the combined effects of the open microstructures associated with the multi-phase nature of an oxide-containing interfacial layer and additional Al consumption through MnO reduction. For instance, in the case of the 1.4Mn steel, the more open interfacial layer structure accelerated Fe diffusion through the interfacial layer and increased Al uptake versus the 0.2Mn substrate for the same bath Al. However, in the case of the 2.5Mn and 3.0Mn substrates and 0.20 wt% Al bath, additional Al consumption through MnO reduction caused the interfacial layer growth to become Al limited, whereas the very open structure dominated growth in the case of the 0.30 wt% Al bath and resulted in the changing the growth kinetics from mixed diffusion-controlled to a more interface controlled growth mode. A kinetic model based on oxide film growth (Smeltzer et al. 1961, Perrow et al. 1968) was developed to describe the Fe-Al interfacial layer growth kinetics within the context of the microstructural evolution of the Fe-Al interfacial layer for Mn-containing steels reacted in 0.20 wt% and 0.30 wt% dissolved Al baths. It indicated that the interfacial layer microstructure development and the presence of MnO at the interfacial layer had significant influence on the effective diffusion coefficient and interfacial layer growth rate. However, in the cases of the 2.5Mn and 3.0Mn steels in 0.20 wt% Al bath, the kinetic model could not predict the interfacial layer Al uptake, since the Fe-Al growth was Al limited. In fact, in these cases, additional Al was consumed for reducing their thicker surface MnO layer, resulted in limiting the dissolved Al available for Fe-Al growth. / Dissertation / Doctor of Science (PhD)

Medium Mn (med-Mn) third generation advanced high strength steels (3G AHSSs) are promising candidates for meeting automotive weight reduction requirements without compromising passenger safety. However, the thermal processing of these steels should be compatible with continuous galvanizing line (CGL) processing capabilities as it provides cost-effective, robust corrosion protection for autobody parts. Hence, the main objective of this Ph.D. research is to develop a CGL-compatible thermal processing route for a prototype 0.2C-6Mn-1.5Si-0.5Al-0.5Cr-xSn (wt%) (x = 0 and 0.05 wt%) med-Mn steel that will result in the 3G AHSS target mechanical properties (24,000 MPa%  UTS × TE  40,000 MPa%) and high-quality galvanized coatings via enhanced reactive wetting. It was found that the starting microstructure, intercritical annealing (IA) time/temperature, and Sn micro-alloying had a significant effect on the retained austenite volume fraction and stability and, thereby, the mechanical properties of the prototype med-Mn steel. For the as-received cold-rolled (CR) starting microstructure, the intercritical austenite nucleated and grew on dissolving carbide particles and resulted in blocky retained austenite. However, Sn micro-alloying significantly effected the intercritical austenite chemical stability by segregating to the carbide/matrix interface and retarding C partitioning to the intercritical austenite. This resulted in lower volume fractions of low stability retained austenite which transformed to martensite (via the TRIP effect) at low strains, thereby quickly exhausting the TRIP effect and resulting in a failure to sustain high work hardening rates and delay the onset of necking. Consequently, the Sn micro-alloyed CR starting microstructure was unsuccessful in achieving 3G AHSS target mechanical properties regardless of the IA parameters employed. Contrastingly, the CR starting microstructure without Sn micro-alloying was able to meet target 3G mechanical properties via intercritical annealing at 675 °C × 60 s and 120 s, and at 690 °C × 60 s owing to sufficiently rapid carbide dissolution and C/Mn partitioning into the intercritical austenite such that it had sufficient mechanical and chemical stability to sustain a gradual deformation-induced transformation to martensite and maintain high work hardening rates. On the other hand, the martensitic (M) starting microstructure produced higher volume fractions of chemically and mechanically stable lamellar retained austenite regardless of Sn micro-alloying. Intercritical annealing at 650 °C × 60 s and 675 °C × 60 s and 120 s produced 3G AHSS target mechanical properties. It was shown that the stable lamellar retained austenite transformed gradually during deformation. Furthermore, deformation-induced nano-twin formation in the retained austenite was observed, suggesting the TWIP effect being operational alongside the TRIP effect. As a result, a continuous supply of obstacles to dislocation motion was maintained during deformation, which aided in sustaining a high work hardening rate and resulted in a high strength/ductility balance, meeting 3G AHSS target properties. Based on these results, the martensitic starting microstructure without Sn micro-alloying and the M-675 °C × 120 s IA condition were chosen for the selective oxidation and reactive wetting studies. The selective oxidation study determined the effect of a N2-5H2-xH2O (vol%) process atmosphere pO2 (–30, –10, and +5 °C dew point (Tdp)) on the composition, morphology, and spatial distribution of the external and internal oxides formed during the austenitizing and subsequent intercritical annealing cycles. The objective of this study was to identify the process atmosphere for the promising M-675 °C × 120 s heat treatment that would result in a pre-immersion surface that could be successfully galvanized in a conventional galvanizing (GI) bath. The austenitizing heat treatment (775 °C × 600 s) used to produce the martensitic starting microstructure resulted in thick (~ 200 nm) external oxides comprising MnO, MnAl2O4, MnSiO3/Mn2SiO4, and MnCr2O4, regardless of the process atmosphere pO2. However, intermediate flash pickling was successful in dissolving the external oxides to a thickness of approximately 30 nm along with exposing metallic Fe in areas which contained relatively thin external oxides. Furthermore, extruded Fe nodules that were trapped under the external oxides were revealed during the flash pickling process. Overall, flash pickling resulted in a surface consisting of dispersed external oxide particles with exposed metallic substrate and extruded Fe nodules. This external surface remained unchanged during IA owing to the multi-micron (~ 2–8 µm) solute-depleted layer that formed during the austenitizing heat treatment. Subsequent galvanizing in a 0.2 wt% (dissolved) Al GI bath with an immersion time of 4 s at 460 °C was successful in achieving high-quality, adherent galvanized coatings through multiple reactive wetting mechanisms. The dispersed nodule-type external oxides along with exposed substrate and extruded Fe nodules on the pre-immersion surface facilitated direct wetting of the steel substrate and promoted the formation of a robust and continuous Fe2Al5Znx interfacial layer at the steel/coating interface. Additionally, oxide lift-off, oxide wetting, bath metal ingress, and aluminothermic reduction were operational during galvanizing. The galvanized med-Mn steels met 3G AHSS target mechanical properties. Overall, this Ph.D. research showed that it is possible to employ a CGL-compatible thermal processing route for med-Mn steels to successfully produce 3G AHSS target mechanical properties as well as robust galvanized coatings. / Thesis / Doctor of Philosophy (PhD) / One of the largest challenges associated with incorporating the next generation of advanced high strength steels into the automotive industry lies in processing these steels in existing industrial production lines. In that regard, a two-stage heat treatment with an intermediate flash pickling stage and process atmosphere compatible with existing industrial continuous galvanizing line technology was developed for a prototype medium-Mn steel. The heat-treated prototype steel met the target mechanical properties outlined for the next generation of advanced high strength steels. Furthermore, the heat treatment and process atmosphere utilised in this research produced a surface that facilitated the successful galvanizing of the prototype medium-Mn steel. This adherent and high-quality galvanized coating will provide robust corrosion protection if the candidate medium-Mn steel is used in future automotive structural applications.

Recently, Zn-XAl-YMg coatings have emerged as lighter-weight substitutes for traditional Zn-based coatings for the corrosion protection of steels; however, little is currently known concerning the interactions between the oxides present on advanced high strength steel (AHSS) surfaces and the Zn-Al-Mg bath. In the current contri- bution, the selective oxidation and reactive wetting of a series of C-Mn AHSS were determined with the objective of providing a quantitative description of this pro- cess. The process atmosphere pO2 was varied using dew points of −50◦C, −30◦C and −5◦C. The surface oxide chemistry and morphology were analysed by means of SEM and XPS techniques. Reactive wetting of the selectively oxidized surfaces using a Zn-1.6 wt.% Al-1.6 wt.% Mg bath was monitored as a function of annealing time at 60 s, 100 s and 140 s at 800◦C. The resulting bare spot defects in the Zn-1.6 wt.% Al-1.6 wt.% Mg coating were assessed by means of SAM-AES and FIB, while coating adhesion was analysed by 180◦ bend tests. Annealing the steel substrates resulted in the formation of surface MnO, which varied based on pO2 and Mn alloy content, and that this MnO greatly reduced the wettability of the steel by the Zn-1.6 wt.% Al- 1.6 wt.% Mg bath, resulting in bare spot defects. It was determined that the reactive wetting of the steel substrate was dependant on the oxide morphology and oxidation mode, which was a function of both alloying content of Mn in the steel and annealing pO2 process atmosphere (dew point). Finally, it was concluded that the bare spot area percentage on the coated panels was statistically invariant for annealing times of between 60 s and 140 s at 800◦C. / Thesis / Master of Applied Science (MASc) / Metallic coatings are applied to steels that are not naturally corrosion resistant. The aim of this research was to determine how well a coating containing zinc, aluminum and magnesium adhered to high strength automotive steel. It was deter- mined that manganese oxides formed on the steel during heating prior to applying the metallic coating. The manganese oxides prevented good adhesion between the steel and the coating, resulting in bare spot defects in the coating. The bare spot defects are undesirable as they leave the steel exposed and therefore susceptible to corrosion and are unsightly when painted.