Control Upstream Austenite Grain Coarsening during Thin Slab Casting Direct Rolling (TSCDR) Process

Thin-slab cast direct-rolling (TSCDR) has become a major process for flat-rolled production. However, the elimination of slab reheating and limited number of thermomechanical deformation passes leave fewer opportunities for austenite grain refinement, resulting in some large grains persisting in the final microstructure. In order to achieve excellent ductile to brittle transition temperature (DBTT) and drop weight tear test (DWTT) properties in thicker gauge high-strength low-alloy products, it is necessary to control austenite grain coarsening prior to the onset of thermomechanical processing. This contribution proposes a suite of methods to refine the austenite grain from both theoretical and practical perspectives, including: increasing cooling rate during casting, liquid core reduction, increasing austenite nucleation sites during the delta-ferrite to austenite phase transformation, controlling holding furnace temperature and time to avoid austenite coarsening, and producing a new alloy with two-phase pinning to arrest grain coarsening. These methodologies can not only refine austenite grain size in the slab center, but also improve the slab homogeneity

Process Maps for Predicting Austenite Fraction (vol.%) in Medium-Mn Third-Generation Advanced High-Strength Steels

Process maps were developed using a combination of microstructural analysis and DICTRA-based modeling to predict the austenite vol.% as a function of the intercritical annealing parameters and starting microstructure. The maps revealed a strong dependence of the calculated austenite fraction (vol.%) on the Mn content (4–12 wt.%) and intercritical annealing temperatures (600 °C to 740 °C). The calculations were carried out for constant carbon, Al, and Si contents of 0.2 wt.%, 1.5 wt.%, and 1.0 wt.%, respectively. A modified empirical equation proposed by Koistinen and Marburger was employed to calculate the room-temperature retained austenite vol.% as a function of the intercritical annealing temperature, including the effect of the austenite composition. The process maps offer valuable insights for designing intercritical treatments of medium-Mn steels, aiding in the optimization of steel properties for automotive applications

A New Alloy System Having Autogenous Grain Pinning at High Temperature

This contribution proposes a new alloy in which a small volume fraction of austenite particles is used to pin ferrite grain growth at high temperatures. During the reheating process, when the temperature is higher than 1200 °C, the coarsening of austenite particles is driven by volume-diffusion-controlled behaviour and ferrite grain growth is dominated by the pinning effect of austenite particles. At low temperature (\u3c1280 °C), grain growth occurred at a rate which is proportional to the particle coarsening rate; while at high temperature (\u3e1280 °C), grain growth is much lower than that expected without pinning. During the solidification process, austenite particles nucleate along ferrite grain boundaries and retard grain growth. Grain growth can be completely arrested with more austenite particle precipitates. This new alloy can be applied to control grain coarsening in the thin slab casting direct rolling process, grain size control in the HAZ of welds and grain growth resistance at high temperature

Model Fe-Al Steel with Exceptional Resistance to High Temperature Coarsening. Part I: Coarsening Mechanism and Particle Pinning Effects

The mechanism by which austenite particles coarsen in a delta-ferrite matrix was investigated in a model Al-containing steel. Special emphasis was placed on the effect of volume fraction on the coarsening kinetics as well as the ability of the particles to pin the growth of delta-ferrite grains. The specimens were heated to temperatures in the range of 1123 K to 1583 K (850 °C to 1305 °C) in the austenite plus delta-ferrite two-phase region and held for times between 5 minutes and 288 hours, followed by water quenching. When the reheating temperature was higher than 1473 K (1200 °C), the coarsening of austenite particles was found to evolve as t1/3, which is typical of volume diffusion-controlled behavior. For lower temperatures, the particle coarsening behavior followed t1/4 kinetics which is consistent with a grain boundary diffusion-controlled process. The observations were interpreted in terms of the modified Lifshitz–Slyozov–Wanger theory by considering multi-component diffusion, particle volume fraction, and the fact that this two-phase material is a non-ideal solid solution. Three types of interaction between particle coarsening and grain growth were observed. Grain growth was completely pinned when the particle pinning force was much larger than the driving force for grain growth. When the particle pinning force was comparable to the driving force for grain growth, the delta-ferrite grains were observed to grow at a rate which is controlled by the kinetics of coarsening of the austenite particles. Finally, when the particle pinning force was smaller than the driving force for grain growth, significant grain growth occurred but its rate was lower than that expected in the absence of particle pinning. The results point to an effective approach for controlling grain growth at high temperatures

Interface Segregation and Nitrogen Measurement in Fe–Mn–N Steel by Atom Probe Tomography

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Effect of C and N and their absence on the kinetics of austenite-ferrite phase transformations in Fe-0.5Mn alloy

Investigating the partitioning effect of substitutional and interstitial elements on the migration of transformation interfaces during austenite-ferrite phase transformation in steels with the conventional experimental method is extremely challenging due to interaction between the solute atoms and the transformation interfaces. Additionally, the simultaneous nucleation of new phases during phase transformations limits the accuracy of extracted growth rates from experimental kinetics measurements of phase fractions. In a novel experimental approach, the cyclic partial phase transformation concept is used to avoid the effect of nucleation on total kinetics of phase transformations. In this study, a Fe-0.5Mn alloy in the presence and absence of interstitial C and N additions is subjected to different cyclic transformation routes to examine the possible interaction between solute atoms and migrating interfaces. The experimental results are in semi-quantitative agreement with modelling predictions made by the local equilibrium approach and provide indirect evidence of Mn partitioning at austenite/ferrite interface in absence of any interstitial elements. It is also confirmed that the presence of interstitial elements promotes Mn interaction with the interface, whereas N promotes more Mn partitioning at transformation interface compared to C.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Novel Aerospace Material