Tensile Behavior of Intercritically Annealed Medium Manganese Steels
Ahmed, Shahroz (2026)
Tampere University
2026
Teknisten tieteiden tohtoriohjelma - Doctoral Programme in Engineering Sciences
Tekniikan ja luonnontieteiden tiedekunta - Faculty of Engineering and Natural Sciences
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Väitöspäivä
2026-09-18
Julkaisun pysyvä osoite on
https://urn.fi/URN:ISBN:978-952-03-4749-9
https://urn.fi/URN:ISBN:978-952-03-4749-9
Tiivistelmä
The automotive industry is a major contributor to carbon dioxide emissions in the atmosphere. To address this issue, the automotive industry is favoring the use of advanced high strength steels for body-in-white components in passenger vehicles. This class of steel offers a high strength-to-weight ratio for vehicle weight reduction without compromising strength. Medium manganese steels have been suggested as one of the candidates suitable for this application. Prior studies have indicated that these materials offer high strength with a good balance of elongation. The final step in the manufacturing process of medium manganese steels includes heat treatment after cold rolling. This is a crucial step in designing these grades, as it dictates the final phase fractions and hence the mechanical properties of these steels. The usual heat treatment cycle performed on medium manganese steels includes: (1) quenching and partitioning or (2) intercritical annealing and cooling. However, there have been only a limited number of studies on the effect of intercritical annealing and subsequent quenching and partitioning on the final properties and usability of the steels studied in this thesis.
The thesis deals with the effects of changing the intercritical annealing quenching and partitioning treatment parameters on the microstructure, mechanical properties, and behavior of cold rolled low carbon medium manganese aluminum alloyed steels. Particular attention has been given to the effects of different intercritical annealing temperatures and their impact on the microstructure and mechanical behavior. Furthermore, in order to optimize the final properties, the effect of varying the cooling route after hot rolling of the studied steels was investigated. The characterization methods used in this study include various electron microscopy techniques, X-ray diffraction, dilatometry, and uniaxial tensile tests. In-situ X-ray diffraction tensile tests were conducted to investigate the transformation of austenite to martensite during uniaxial deformation, enabling real time monitoring of the phase transformation as a function of applied strain.
The study showed that the microstructure, mechanical properties and mechanical behavior of cold rolled medium manganese steels are affected by the heat treatment parameters and starting microstructure. Intercritical annealing close to the Ac1 phase transformation temperature and subsequent quenching and partitioning, resulted in a microstructure majorly consisting ferrite and retained austenite. In addition, in these samples, serrations were observed in the stress-strain curve at room temperature at the applied strain rate of 2.5 × 10-4 s-1. In the same samples, stepwise austenite transformation, i.e., a transformation burst followed by a pause, was observed in the in-situ tests at the area exposed to the beam. The serrations arise due to the dynamic strain aging phenomenon, which was proposed to be due to the interaction of moving dislocations and interstitial atoms present in ferrite; although, the origin could not be conclusively proved. Dynamic strain aging resulted in the nucleation of so-called Portevin-Le-Chatelier bands. These bands possess an inherently higher strain than the sample bulk and propagate under load along the sample gauge length. The continuous propagation of these bands leads to strain induced stepwise transformation of austenite. Furthermore, it was also observed that serrations did not occur when the applied strain rate was increased to 1 × 10-1 s-1. This was either due to the direct effects of increasing strain rate or adiabatic heating. Either of the reasons reduce the interaction time between interstitial atoms and dislocations, hence the condition for dynamic strain aging is eliminated and no serrations are observed.
Intercritical annealing at higher temperatures followed by quenching and partitioning, produced a multiphase microstructure comprising ferrite, martensite and retained austenite which resulted in a favorable combination of strength and ductility. Furthermore, these samples did not show serrations in the stress-strain curve, nor was the stepwise transformation observed.
The study showed that different cooling routes after hot rolling resulted in different starting microstructures, which in turn affects the final microstructure and mechanical properties of the steel. The results indicated that a higher amount of austenite is formed at the same annealing temperature from a starting microstructure comprising bainite and ferrite or a mixture of martensite, ferrite, and retained austenite, compared to a starting microstructure of ferrite and cementite. Combined results from the study suggested that the starting microstructure affects the prior austenite fraction at the intercritical annealing temperature, which dictates the fraction of martensite at the initial quenching stage, hence affecting the final properties of the samples. Furthermore, the results indicated that a higher fraction of martensite formation at the quench stop temperature is required for sufficient stabilization of austenite.
The findings of this study indicated that carbon partitioning to austenite is facilitated when the partitioning temperature is 400 °C, leading to a higher fraction of stable retained austenite. A lower fraction of retained austenite fraction is obtained at the partitioning temperature of 450 °C, indicating that more carbon is combined to carbides, thus leading to retained austenite with lower stability.
The thesis deals with the effects of changing the intercritical annealing quenching and partitioning treatment parameters on the microstructure, mechanical properties, and behavior of cold rolled low carbon medium manganese aluminum alloyed steels. Particular attention has been given to the effects of different intercritical annealing temperatures and their impact on the microstructure and mechanical behavior. Furthermore, in order to optimize the final properties, the effect of varying the cooling route after hot rolling of the studied steels was investigated. The characterization methods used in this study include various electron microscopy techniques, X-ray diffraction, dilatometry, and uniaxial tensile tests. In-situ X-ray diffraction tensile tests were conducted to investigate the transformation of austenite to martensite during uniaxial deformation, enabling real time monitoring of the phase transformation as a function of applied strain.
The study showed that the microstructure, mechanical properties and mechanical behavior of cold rolled medium manganese steels are affected by the heat treatment parameters and starting microstructure. Intercritical annealing close to the Ac1 phase transformation temperature and subsequent quenching and partitioning, resulted in a microstructure majorly consisting ferrite and retained austenite. In addition, in these samples, serrations were observed in the stress-strain curve at room temperature at the applied strain rate of 2.5 × 10-4 s-1. In the same samples, stepwise austenite transformation, i.e., a transformation burst followed by a pause, was observed in the in-situ tests at the area exposed to the beam. The serrations arise due to the dynamic strain aging phenomenon, which was proposed to be due to the interaction of moving dislocations and interstitial atoms present in ferrite; although, the origin could not be conclusively proved. Dynamic strain aging resulted in the nucleation of so-called Portevin-Le-Chatelier bands. These bands possess an inherently higher strain than the sample bulk and propagate under load along the sample gauge length. The continuous propagation of these bands leads to strain induced stepwise transformation of austenite. Furthermore, it was also observed that serrations did not occur when the applied strain rate was increased to 1 × 10-1 s-1. This was either due to the direct effects of increasing strain rate or adiabatic heating. Either of the reasons reduce the interaction time between interstitial atoms and dislocations, hence the condition for dynamic strain aging is eliminated and no serrations are observed.
Intercritical annealing at higher temperatures followed by quenching and partitioning, produced a multiphase microstructure comprising ferrite, martensite and retained austenite which resulted in a favorable combination of strength and ductility. Furthermore, these samples did not show serrations in the stress-strain curve, nor was the stepwise transformation observed.
The study showed that different cooling routes after hot rolling resulted in different starting microstructures, which in turn affects the final microstructure and mechanical properties of the steel. The results indicated that a higher amount of austenite is formed at the same annealing temperature from a starting microstructure comprising bainite and ferrite or a mixture of martensite, ferrite, and retained austenite, compared to a starting microstructure of ferrite and cementite. Combined results from the study suggested that the starting microstructure affects the prior austenite fraction at the intercritical annealing temperature, which dictates the fraction of martensite at the initial quenching stage, hence affecting the final properties of the samples. Furthermore, the results indicated that a higher fraction of martensite formation at the quench stop temperature is required for sufficient stabilization of austenite.
The findings of this study indicated that carbon partitioning to austenite is facilitated when the partitioning temperature is 400 °C, leading to a higher fraction of stable retained austenite. A lower fraction of retained austenite fraction is obtained at the partitioning temperature of 450 °C, indicating that more carbon is combined to carbides, thus leading to retained austenite with lower stability.
Kokoelmat
- Väitöskirjat [5374]