Modeling of Transformation Behavior and Compositional Partitioning in TRIP Steel.

Minote, Toru; Torizuka, Shiro; Ogawa, Atsushi; Niikura, Masakazu

doi:10.2355/isijinternational.36.201

Cited by 26 publications

(13 citation statements)

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“…For industrial processing of AHSS, stages 1 and 2 appear primarily of interest. In contrast, more recent studies by Minote et al 125) for a classical TRIP steel (0.2wt%C-1.5wt%Mn-1.5wt%Si) suggest that for specimens annealed for 5 min at 800°C full equilibrium with redistribution of Si and Mn is attained, i.e. they found excellent agreement with the experimentally observed austenite fraction and that predicted by Thermo-Calc.…”

Section: Austenite Formationsupporting

confidence: 59%

“…Models have been developed based on both proposed transformation mechanisms. 125,126) The analysis by Minote et al 125) for classical CMnSi TRIP steels suggests that the diffusional model provides a better fit for bainite transformation at higher temperature (Ͼ350°C) whereas for lower temperatures (Ͻ350°C) the displacive mechanism seems to provide a better description. Further, a more in-depth analysis revealed that both model approaches employ a number of adjustable parameters that can be tuned such that both transformation mechanisms may be used to equally well describe experimental data for the bainite transformation.…”

Section: )mentioning

confidence: 99%

“…127) As a result, both model frameworks have been used to address the bainite transformation stage in TRIP steels. 50,123,125) One of the underlying assumptions of these models is that bainitic ferrite forms in these steels without cementite precipitation. In reality, however, some cementite may also form and this will affect the bainite transformation kinetics.…”

Section: Austenite Decompositionmentioning

confidence: 99%

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Computer Simulation of Microstructure Evolution in Low Carbon Sheet Steels

Militzer

2007

ISIJ Int.

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Models of microstructure evolution in steels are reviewed. The emphasis of the review is on low carbon sheet steels both hot-rolled and cold-rolled and annealed. First the state-of-the-art on industrial microstructure process models is presented. The individual model concepts for grain growth, recrystallization, precipitation and phase transformations are briefly discussed. The development from empirically-based models to physically-based models is identified as a key issue to have increased predictive capabilities for these models over a wider range of steel grades and operational conditions. The challenges in the development of the next generation of models are delineated. In particular, new aspects of microstructure evolution associated with novel processing routes and advanced high strength steels are evaluated. Further, the majority of the currently employed models are on the macro-scale but future microstructure models will increasingly be meso-scale models that predict actual microstructures rather than a number of average parameters (e.g. grain size, fraction transformed) to describe microstructure evolution.KEY WORDS: mathematical model; microstructure; steel; hot rolling; annealing; grain growth; recrystallization; precipitation; phase transformation. © 2007 ISIJ Reviewvates the development of advanced microstructure models that provide an increased insight into the underlying physical metallurgy principles. Further, AHSS are frequently produced as cold-rolled annealed and/or coated sheets. Here, the intercritical annealing step assumes a crucial role to obtain the desired complex, multi-phase microstructures. Intercritical annealing constitutes a paradigm shift from annealing of conventional steels where no cycling through the transformation region occurs. The added complexity of the annealing routes for AHSS requires the development of novel annealing models that also address austenite formation which has received comparatively little attention in process models.Currently available microstructure models for the production of sheet and other steels are usually formulated on the macro-scale, i.e. the microstructure is described with a number of so-called internal state variables such as grain size, fraction recrystallized and fraction transformed. However, the advances in computer technology make it now feasible to routinely conduct modelling of the microstructure on the meso-scale if not on the atomistic level. In particular the meso-scale approach appears to open a new era of modelling the microstructure evolution in sheet steels. In this methodology actual microstructures can be predicted rather than mean values which have traditionally been used to characterize microstructures. The significance of these new modelling strategies is also fueled by the realization that material properties may markedly depend on morphology and spatial distributions of microstructure constituents. [22][23][24] In this paper, first the models proposed for hot strip rolling are reviewed. Special attention is given to the ...

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Section: Austenite Formationsupporting

confidence: 59%

Section: )mentioning

confidence: 99%

Section: Austenite Decompositionmentioning

confidence: 99%

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Computer Simulation of Microstructure Evolution in Low Carbon Sheet Steels

Militzer

2007

ISIJ Int.

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“…Austenite stability is favored by carbon enrichment (chemical stabilization) as well as austenite particle size refinement 10,42,[46][47][48][49] and NF formation can contribute in both aspects. 45) Several authors 14,21,27,29,34,38,41,42,47,50) have shown that formation of NF during cooling from IAT to IHT (generally considered as epitaxial growth under paraequilibrium conditions) enhances carbon enrichment and subsequent retention of austenite. On the other hand, the austenite particles decrease in size during cooling 31) and very stable small retained austenite can be isolated by the local growth of ferrite at the expense of austenite.…”

Section: The Role Of New Ferrite On Retained Austenite Stabilization mentioning

confidence: 99%

The Role of New Ferrite on Retained Austenite Stabilization in Al-TRIP Steels

2010

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Microstructure and mechanical properties of two high Al, low-Si TRIP steels with different Cr and Mo contents were studied using continuous galvanizing line (CGL) laboratory simulation. Combined use of specific etching methods, optical and electron microscopy observations and EBSD characterization led to verify the epitaxial growth of ferrite during cooling at a moderate rate from the intercritical annealing to the isothermal holding temperature. The amounts of "new" ferrite formed during cooling and retained austenite obtained after processing are much higher in the steel with lower content of hardenability-promoting elements. Measured tensile properties and mechanical behavior of the steel strongly depend on the amount of new ferrite and retained austenite. It is found that the formation of new epitaxial ferrite from intercritical austenite can effectively contribute to the chemical and particle size stabilization of untransformed austenite as well as to obtain the desired TRIP effect under processing conditions highly compatible with industrial practice, i.e. cooling rates near 15°C/s and isothermal holding times at 460°C shorter than 60 s.

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“…The austenite formed in the intercritical annealing was transformed to ferrite or bainite during cooling and austempering annealing, and the untransformed austenite remained as the retained austenite after annealing. 18) During transformation of austenite to bainite, the precipitates may retard the transformation rate by dragging the grain boundaries, and the irregular grain boundaries of retained austenite (Fig. 11(b)) can be explained by this reaction between the precipitates and grain boundaries during transforming of austenite.…”

Section: Effect Of Aln Precipitation On the Retained Austenite Characmentioning

confidence: 99%