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.

Medium Mn TRIP steels are amongst the most widely researched third generation advanced high strength steels (3G-AHSSs) as they are ideal candidates for automotive light-weighting applications owing to their superior strength and ductility balance. However, the thermal processing cycles of these steels need to be compatible with the industrial continuous galvanizing line (CGL) in order to successfully employ them in the automotive manufacturing industry. The main objective of the present research was to develop a CGL compatible thermal processing cycle for a prototype medium Mn steel that would produce significant volume fractions of chemically stable retained austenite and exhibit mechanical properties consistent with established 3G-AHSS targets. In that regard, the effects of intercritical annealing (IA) time and temperature and starting microstructure were determined in the first part of this research. The as-received tempered martensite (S-TM) and heat treated martensite (S-M) were the two different starting microstructures studied in this research. In this case, the overaging temperature (OT) treatment (460°C for 20s) was kept constant. It was found that high volume fractions (≥ 0.30) of retained austenite were achieved for S-M samples intercritically annealed at 675°C for shorter times (i.e. 60 to 120s) compared to S-TM samples. TEM analysis of the S-M samples showed that most of the retained austenite was present in a film type morphology, which is known to be more stable chemically and mechanically compared to the block type morphology. The tensile test results showed that although both the S-TM and S-M samples exhibited a high strength/ductility balance, the S-M samples, particularly the S-M 675°C + 120s samples, showed more potential in terms of CGL compatibility and achieving 3G-AHSS target mechanical properties. The effect of OT holding time was determined in the second part of this research. In that regard, the OT holding time was varied form 20s to 120s for selected S-TM and S-M samples. The S-TM 710°C samples with increased OT holding times (60s and 120s) had a significant increase in retained austenite volume fraction compared to the baseline 20s OT samples. However, the retained austenite volume fractions did not change for S-M samples regardless of OT holding time. It was also found that the mechanical properties of the annealed S-TM and S-M steels depended on the OT holding time. For the S-TM samples with > 120s IA holding times, longer OT holding times (60s and 120s) produced chemically unstable retained austenite which transformed rapidly at low strain resulting in low UTS × TE products. However, although longer OT holding times significantly increased the yield strength of the annealed S-M samples, the UTS × TE product decreased significantly owing to decreased retained austenite stability. Finally, based on the results of this research, it was concluded that the prototype medium Mn TRIP steel can achieve 3G-AHSS target mechanical properties using CGL-compatible thermal processing cycles. Moreover, depending on successful reactive wetting, it may be possible to perform both thermal processing and galvanizing of this steel in the industrial CGL. / Thesis / Master of Applied Science (MASc)