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)
| Identifer | oai:union.ndltd.org:mcmaster.ca/oai:macsphere.mcmaster.ca:11375/21476 |
| Date | January 2017 |
| Creators | Bhadhon, Kazi |
| Contributors | McDermid, Joseph, Materials Science and Engineering |
| Source Sets | McMaster University |
| Language | English |
| Detected Language | English |
| Type | Thesis |
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