Computer Simulation of Microstructure Evolution in Low Carbon Sheet Steels

Abstract

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 ...

Assessment of Steel Subjected to the Thermomechanical Control Process with Respect to Weldability

“…Tests of thin foils performed using a transmission electron microscope revealed that the hardening of steel S700MC was primarily affected by dispersive (Ti,Nb)(C,N)-type precipitates sized from several nanometres to between ten and twenty nanometres. The precipitation hardening was mainly triggered by dispersive precipitates of several nanometres in size, precipitated in the steel (in ferrite) during cooling ( Figure 2) [28][29][30][31][32]. Particles sized above twenty nanometres did not significantly contribute to precipitation hardening, yet they restrained the recrystallised austenite grain growth, favouring the formation of the fine-grained structure [28,[33][34][35][36][37].…”

Modeling of Microstructure Evolution during Hot Strip Rolling of Dual Phase Steels

“…Models for this transformation had first been developed for plain carbon steels and subsequently applied to microalloyed steels. 32,48,49) These models remain applicable to AHSS with higher alloying contents. 33,35,38) Following the previously established approach the austenite-to-ferrite transformation kinetics is described with a model consisting of three submodels, i.e.…”

“…In particular, the description of the bainite reaction for run-out table cooling requires further work. 49) Figure 5 shows the results for the critical cooling rates of bainite and martensite formation in the present steels. These cooling rates tend to decrease with increasing ferrite fraction since the associated increase of carbon concentration in the remaining austenite leads to higher stability of the austenite phase.…”

Multiscale Modeling of Phase Transformations in Steels

“…2 To take advantage of the potential of these phase transformations it is critical to optimize processing paths for producing materials with the desired properties. Although significant progress has been made in developing knowledge-based process models for the steel industry, [3][4][5][6] these models still require numerous empirical parameters that are typically determined from time-consuming laboratory experiments. Thus, these models are, strictly speaking, limited to the investigated steel chemistries and processing conditions.…”