Intercritically annealed medium manganese steels are a promising third-generation advanced high-strength steel candidate, relying on large fractions of Mn-enriched retained austenite for excellent strength-ductility combinations. The present study proposes a novel cyclic intercritical annealing to promote nucleation and efficient stabilization of austenite in a medium Mn microstructure. Design of the heat treatment is driven by the hypothesis that the distribution of ductile austenite is key in mitigating the strain incompatibility that accelerates failure in these steels. Development and preliminary testing of the heat treatment are first detailed and compared with literature results for equivalent isothermal annealing. The effects of cyclic annealing parameters on the amount and stability of retained austenite are also explored through diffraction methods and mechanical testing. Finally, steps are taken towards quantifying austenite formation during the cyclic treatment, and recommendations are made for adapting the designed heat treatment to thicker gauges.

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.

The third-generation advanced high strength steels, AHSS, represent an opportunity for today’s steel development, where lighter materials with maintained strength and toughness are in demand. The unique properties of these materials often stem from a tailored microstructure. In the continued development of these steels, without relying on expensive alloying methods, process design in the form of precise heat treatments plays an increasingly important role. This work focuses on Medium Mn AHSS with the aim of investigating one of these heat treatments, intercritical annealing, which is essential for achieving the desired material properties. Experimental testing of annealing effects is acknowledged to be a challenging process, and this study aims to present a novel approach for these types of tests. During experimental testing of intercritical annealing, the thermomechanical testing system Gleeble 3800 is a recognized tool. The mounting technique employed in the Gleeble results in an inhomogeneous heat distribution in the samples, generating a thermal gradient. This report aims to utilize this gradient as an opportunity to test the effect of several intercritical annealing temperatures on one sample, thereby increasing the efficiency of experimental work. The method is based on data retrieved from thermocouples attached to the specimen during Gleeble trials with the intent to identify the thermal gradient. This data is combined with x-ray diffraction measurements where the retained austenite fraction is measured. Thermodynamic calculations of expected retained austenite fraction following intercritical annealing are performed parallel to experimental work. The results of this work show that it is possible to utilize the thermal gradient to retrieve extensive data regarding the effect of intercritical annealing using only one sample. The results show a distinct thermal gradient and a corresponding gradient of retained austenite fraction along the specimen. The results for retained austenite fraction at room temperature can be rationalized on the basis of computational predictions. These variations potentially arise due to the material not reaching equilibrium within the annealing timeframe. This conclusion is supported by other computational results concerning austenite composition. In summary, the present work illustrates a new approach streamlining experimental work that, with some refinements, has the potential to benefit the broader scienitific community, in addition to providing a powerful new tool for rapid technological advancement in the steel industry / Tredje generationens avancerade höghållfasta stål representerar en möjlighet för dagens stålutveckling där lättare material med bibehållen styrka och seghet efterfrågas. De unika egenskaperna hos dessa material härrör ofta frän en skräddarsydd mikrostruktur. Vid fortsatt utveckling av dessa stål är det önskvärt att minimera användningen av legeringsämnen, vilket betyder att processdesign i form av korrekta värmebehandlingar blir  av  stor  betydelse.  Detta  arbete  fokuserar  på  Medium  Mn  avancerade höghållfasta stål med syftet att undersöka en av dessa värmebehandlingar, interkritisk glödgning,  vilken  har  en  avgörande  betydelse  för  att  uppnå önskad  prestanda. Experimentell  testning  av  glödgningseffekter  anses  vara  en  utmanande  process och avsikten med denna studie är att presentera ett nytt tillvägagångssätt för denna typ av test. Under experimentell utvärdering av glödgningseffekter används ofta det termomekaniska testsystemet  Gleeble  3800.  Provmonteringen  i  Gleeblen  resulterar i en  inhomogen värmefördelning i proverna vilket medför en temperaturgradient. Denna rapport syftar till att använda gradienten som en möjlighet att testa effekten av flera  glödgningstemperaturer  på  ett  enda  prov  och  därigenom  öka  effektiviteten i det  experimentella  arbetet.  Metoden  grundas  på  data  från  termoelement  fästa  på provet under Gleebleförsök, med avsikt att identifiera den termiska gradienten. Denna data kombineras sedan med XRD-mätningar där austenitfraktion efter värmebehandling utvärderas.  Termodynamiska  beräkningar  av  förväntad  austenitfraktion  efter interkritisk glödgning genomförs parallellt med experimentellt arbete. Resultaten  från  detta  arbete  påvisar  att  den  presenterade  metoden  är  genomförbar då omfattande data gällande interkritisk glödgningseffekt grundat på endast ett prov erhålls. Resultaten visar en tydlig termisk gradient och en motsvarande gradient av austenitfraktion längs provet, vilka är i överensstämmelse med tidigare experimentella resultat för samma material. Resultaten för austenitfraktion vid rumstemperatur uppvisar  betydande  likheter  med  de  termodymiska  beräkningarna, med  några  undantag. Orsaken till dessa variationer ¨ar troligen en otillräcklig glödgningstid, vilket gör att  materialet  inte  når  jämvikt.  Denna  hypotes  stöds  av andra  beräkningsresultat gällande austenitens sammansättning. Sammanfattningsvis presenterar denna rapport ett nytt tillvägagångssätt för att effektivisera experimentellt arbete, som med vissa förbättringar har potential att gynna det bredare forskarsamhället.