Effect of alloying elements and heating rates on ferrite recrystallization in dual phase steels

The Dual Phase Steels (DP Steels) have attracted interest of steel manufacturers, especially automotive industry, because of a good combination of high strength and ductility. The final DP steel microstructures are formed during continuous heating to intercritical temperatures, then isothermal holding and quenching to form a mix of ferrite and martensite. Final martensite content, morphology and spatial distribution depends on the austenite formed during the isothermal holding. The austenite formation is affected by the progress of ferrite recrystallization during continuous heating. Therefore, it is important to understand the process of ferrite recrystallization.This thesis aims to study the effect of Si and Mn, which are common alloying elements in steels, on the ferrite recrystallization. The present study extensively uses in-situ 2D X-ray Diffraction (XRD) to investigate the ferrite recrystallization kinetics. This technique is faster and records more data than the other conventional methods used to study the kinetics of recrystallization until now. This study introduces a method to quantify the recrystallized grains using Scanning Electron Microscope (SEM) and Backscattered Electron Detector (BED). This method results in faster analysis of microstructure. The microstructure analysis helped in quantifying the grain growth, its dimensions and type of nucleation. Afterwards, those have been used as input parameters for modified non-isothermal JMAK model. This model is used to obtain the kinetic parameters from the experimental in-situ 2D XRD results by model fitting. The parameters obtained are: rate constant, activation energy and Avrami exponent. In all the cases, the microstructure analysis show site-saturated nucleation and predominantly 2D grain growth. The model fitting reveals that most nucleation cases show site-saturation. The density of nucleation sites is dependent on the pearlite content of the alloys. The boundaries between deformed pearlite and ferrite being the preferred nucleation sites. In some cases, the recrystallization nuclei are also seen at the boundaries between the ferrite grains which correspond to areas with micro-segregation of Mn. Presence of Mn is leading to an increase recrystallization start temperatures. The solute drag effect of Mn is quite high. Similarly, Si is also retarding the recrystallization. The retardation effect of Si is not as significant as Mn. The solute drag effect seems to be dependent on the velocity of the grain boundaries and consequently, on progress of recrystallization. In presence of Mn, the Si is able to interact with the moving grain boundaries. This points to co-segregation effect between Mn and Si. This effect was found to be strongest for 1:1 Si to Mn atomic fractions, where the lowest growth rates were observed. KEYWORDS: DP steels, ferrite recrystallization, solute drag effect, in-situ 2D XRD, recrystallization kinetics, SEM, JMAK model, activation energy, co-segregation.<br/

Effect of silicon, manganese and heating rate on the ferrite recrystallization kinetics

This study presents the effects of silicon (Si) and manganese (Mn) concentration and of heating rate on the ferrite recrystallization kinetics in seven model alloys with different Si and Mn concentrations, which are of relevance for the development of Advanced High Strength Steels (AHSS). The recrystallization kinetics were studied with in-situ 2D X-ray Diffraction (2D-XRD) and ex-situ microstructure observations using Scanning Electron Microscopy (SEM). The experimentally observed differences in the recrystallization start temperature (Ts), dependent on the Si and Mn concentrations and the heating rate, can be described by combining the non-isothermal JMAK-model with a modified version of Cahn's solute drag model. The modified Cahn model takes into account - in an approximate manner - that the interaction energy of the solute atoms with the grain boundary depends on the Si and Mn concentrations in the boundary and the Wagner interaction parameters. The collective contribution of the Si and Mn atoms to the increase in the Ts with respect to the reference alloy (without Si and with very little Mn) is higher than would be expected from the simple addition of the effects of the Si and Mn concentrations alone. This means that the interaction between Si and Mn atoms leads to an additional increase in Ts, i.e. a coupled solute drag effect. For the later stages of recrystallization, we have studied the change in the number density and the growth rates of the recrystallizing grains using SEM. The observations show non-random nucleation, early impingement of the grains in the normal-direction and non-constant growth rates of recrystallizing grains. (OLD) MSE-1Materials Science and Engineering(OLD) MSE-

Large-deformation crystal plasticity simulation of microstructure and microtexture evolution through adaptive remeshing

The capability of high-resolution modeling of crystals subjected to large plastic strain is essential in predicting many important phenomena occurring in polycrystalline materials, such as microstructure, deformation localization and in-grain texture evolution. However, due to the heterogeneity of the plastic deformation in polycrystals, the simulation mesh gets distorted during the deformation. This mesh distortion deteriorates the accuracy of the results, and after reaching high local strain levels, it is no longer possible to continue the simulation. In this work, two different adaptive remeshing approaches are introduced for simulating large deformation of 3D polycrystals with high resolution under periodic boundary conditions. In the first approach, a new geometry with a new mesh is created, and then the simulation is restarted as a new simulation in which the initial state is set based on the last deformation state that had been reached. In the second approach, the mesh is smoothened by removing the distortion part of the deformation, and then the simulation is continued after finding a new equilibrium state for the smoothed mesh and geometry. The first method is highly efficient for conducting high-resolution large-deformation simulations. On the other hand, the second method's primary advantage is that it can overcome periodicity issues related to shear loading, and it can be used in conjunction with complex loading conditions. The merits of the methodologies are demonstrated using full-field simulations performed using a dislocation-density-based crystal plasticity model for Interstitial free (IF-) steel. Particular emphasis is put on studying the effect of resolution and adaptive meshing. The algorithms presented have been implemented into the free and open-source software package, DAMASK (Düsseldorf Advanced Material Simulation Kit).Team Jilt Sietsm

Current challenges and opportunities in microstructure-related properties of advanced high-strength steels

This is a viewpoint paper on recent progress in the understanding of the microstructure–property relations of advanced high-strength steels (AHSS). These alloys constitute a class of high-strength, formable steels that are designed mainly as sheet products for the transportation sector. AHSS have often very complex and hierarchical microstructures consisting of ferrite, austenite, bainite, or martensite matrix or of duplex or even multiphase mixtures of these constituents, sometimes enriched with precipitates. This complexity makes it challenging to establish reliable and mechanism-based microstructure–property relationships. A number of excellent studies already exist about the different types of AHSS (such as dual-phase steels, complex phase steels, transformation-induced plasticity steels, twinning-induced plasticity steels, bainitic steels, quenching and partitioning steels, press hardening steels, etc.) and several overviews appeared in which their engineering features related to mechanical properties and forming were discussed. This article reviews recent progress in the understanding of microstructures and alloy design in this field, placing particular attention on the deformation and strain hardening mechanisms of Mn-containing steels that utilize complex dislocation substructures, nanoscale precipitation patterns, deformation-driven transformation, and twinning effects. Recent developments on microalloyed nanoprecipitation hardened and press hardening steels are also reviewed. Besides providing a critical discussion of their microstructures and properties, vital features such as their resistance to hydrogen embrittlement and damage formation are also evaluated. We also present latest progress in advanced characterization and modeling techniques applied to AHSS. Finally, emerging topics such as machine learning, through-process simulation, and additive manufacturing of AHSS are discussed. The aim of this viewpoint is to identify similarities in the deformation and damage mechanisms among these various types of advanced steels and to use these observations for their further development and maturation.(OLD) MSE-