In response to more stringent global CO2 emissions, automotive manufacturers have increased the use of advanced high strength steels (AHSS). Ultra-high strength steels are often used within the body-in-white (BIW) for safety critical parts and structural reinforcements, such as roof rails and side impact beams. Currently, the most commonly used press hardened steel (PHS) grade for these applications is 22MnB5, with a typical composition of 0.22C 1.2Mn 0.25Si 0.005B (wt%). Automotive OEMs have expressed a desire to use Zn-based coatings as they are compatible with the current painting system and have the potential to provide robust cathodic protection. The steel blanks generally undergo direct hot press forming (DHPF) to achieve the necessary martensitic microstructure and target mechanical properties, but this presents challenges for Zn-coated 22MnB5. The adoption of Zn-based coatings within the automotive industry has been inhibited by the prospect of liquid metal embrittlement (LME) resulting from DHPF, as well as the desire to provide robust cathodic protection.
Previous literature has reported that a zinc ferrite (α-Fe(Zn)) coating with a global Zn content of at least 30 wt% will provide cathodic protection to the underlying substrate. The main goal of this work was to determine the microstructural evolution and electrochemical properties of galvanized (GI70 – 70 g/m2/side) 22MnB5 substrates as a function of the annealing time at a typical austenization temperature of 900°C. It was found that the Zn-based coatings annealed at 700°C consisted to a mixture of small volume fraction of α-Fe(Zn) and Г-Fe3Zn10. After heating to 900°C, the coating comprised varying volume fractions of α-Fe(Zn) and Zn(Fe) liquid, which transformed to Г-Fe3Zn10 after solidification. The relative fraction of Г Fe3Zn10 was found to decrease with increasing annealing time until the coating completely transformed to α-Fe(Zn) after annealing at 900°C for 240 s. GDOES results found that, when the sample was annealed at 900°C for 240 s, the global Zn content of the coating was less than 30 wt%. Coatings comprising varying fractions of Г-Fe3Zn10 were subjected to uniaxial tensile tests to determine how the coating microstructure affected the mechanical properties in comparison to the uncoated substrate material. It was found that the uncoated substrate material met the mechanical property requirements of σ(UTS)min ≥ 1500 MPa regardless of annealing time. However, σ(UTS) was found to decrease with increasing annealing times for the GI70 coated samples until the target mechanical properties were not met when the sample was annealed at 900°C for 180 s. This was attributed to increased coating thicknesses leading to a decrease in the martensitic cross-sectional area to support the load.
Furthermore, the coatings were subjected to a variety of electrochemical characterization techniques, including potentiodynamic and galvanostatic polarization scans, potentiostatic scans, and electrochemical noise tests. Potentiodynamic polarization scans indicated a higher driving force for cathodic protection when the coating contained some fraction of Г-Fe3Zn10. Furthermore, a limiting current density for these samples was observed, demonstrating that Г-Fe3Zn10 corrodes at a slower rate in comparison to α Fe(Zn). Galvanostatic polarization measurements indicated that, when the fraction of Г Fe3Zn10 within the coating was below 15 vol%, the protective properties of the phase were not exhibited. XRD and TEM analysis revealed the formation of three corrosion products on the surface: simonkolleite, hydrozincite, and akaganeite. It was found that, when samples contained greater than 15 vol% Г-Fe3Zn10 in the coating, the predominant corrosion products were a combination of simonkolleite and hydrozincite. When the Г Fe3Zn10 content was below this value, the dominant corrosion product was found to be akaganeite. Furthermore, substrate attack was observed on a sample annealed at 900°C for 420 s when the coating layer was intact, indicating that the α-Fe(Zn) only containing coating obtained at this time does not provide cathodic protection.
Based upon the current results, it was determined that a minimum volume fraction of 15 vol% Г-Fe3Zn10 must be present within the coating layer to obtain robust cathodic protection. Furthermore, it was determined that the processing window to develop cathodically protective Zn based coatings while mitigating LME is extremely narrow. This is a result of the fact that it is necessary for at least 15 vol% Г-Fe3Zn10 to be present within the coating microstructure at room temperature, which is liquid at the forming temperatures of 900°C. From the current findings, it was found that it is unlikely that a cathodically protective Zn-based coating can be obtained for DHPF steel parts using 22MnB5 as a substrate material. This is due to the high forming temperature resulting in liquefication of the coating and the rapid cooling rates necessary to achieve the target mechanical properties of σ(UTS)min ≥ 1500 MPa. Thus, it is recommended that the current substrate material be altered such that the part may be formed below the peritectic temperature of 782°C. / Thesis / Master of Applied Science (MASc)
| Identifer | oai:union.ndltd.org:mcmaster.ca/oai:macsphere.mcmaster.ca:11375/23441 |
| Date | January 2018 |
| Creators | Dever, Caitlin |
| Contributors | McDermid, Joseph, Kish, Joseph, Materials Science and Engineering |
| Source Sets | McMaster University |
| Language | English |
| Detected Language | English |
| Type | Thesis |
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