Recrystallization Of Interstitial Free Steel Research Articles

Cellular Automata Simulation of Textural Evolution during Primary Static Recrystallization of IF Steel

A two dimensional probabilistic cellular automata model is used to simulate primary static recrystallization of interstitial free (IF) steel. The present study is to investigate the effect of curvature-driven pressure that is induced by protrusions/retrusions of recrystallization fronts on the microstructural and textural evolution during recrystallization. It was found that local interface migration of protrusions/retrusions of recrystallization fronts could significantly affect the kinetics, grain morphology and annealing texture according to the present investigation.

Deformation and Recrystallization of Interstitial Free (IF) Steel

The rolling texture of Body Centered Cubic (BCC) low carbon and interstitial free (IF) steel consists of two crystallographic fibers, the α fiber (⟨110⟩ //RD) and the γ fiber (⟨111⟩ //ND). Cells, subgrains, and microbands, where these are not associated with significant lattice curvature, provide the driving forces for recrystallization. However, when these microstructures are associated with significant short range lattice curvature, as they are in deformation or shear bands, they can provide the nucleation sites. The process of recrystallization in IF steel is shown to occur in two stages: the first where the nuclei are contained in the original rolled-out grains belonging to the γ fiber, and where the essential lattice curvature is derived from shear and deformation bands. The second stage involves the impingement/coalescence of several recrystallized grains in the as-rolled envelope of an original hot band grain which provides a super-nucleation event, in which the as rolled boundaries bow out to give an equiaxed microstructure. An elegant proof is provided for this process, along with a simple model based on Strain Induced Boundary Migration (SIBM). The well-known optimal cold rolling, beyond which the drawability is known to degrade, is explained by deformation banding of a particular subset of the grains belonging to the α fiber. Finally, a novel experiment is described, based on the observed deformation banding behavior of crystals belonging to the γ fiber, which leads to very strong γ recrystallization textures, which are desirable in deep drawing.

Microstructure and Texture Evolution during Recrystallization of Interstitial Free Steel

The evolution of microstructure and texture due to recrystallization was investigated in cold rolled interstitial free (IF) steel. Samples taken from the cold band of a 0.07%Ti bearing IF steel with total cold rolling reduction of 75% were investigated by using optical microscopy, X-ray diffraction and electron back-scattered diffraction etc. The aim of this study was to obtain a real picture of the formation of the recrystallization texture of IF steel, which would contribute to proper texture control for improving deep drawability. The mechanism responsible for the evolution of texture is discussed experimentally from four aspects. First of all, the microstructure of partially annealed cold rolled specimen is obtained. The recrystallized grain form earlier in more darkly etched regions from the micrographs, which belong to the ND//<111> fiber components. In addition, the fractions of the {111}<110> and {111}<112> components increase slowly during the early stage of recrystallization, because the γ-fiber recrystallized texture is growing, while at the same time the γ-fiber deformed texture is being consumed. Moreover, The texture formation has been discussed taking into consideration of the stored energy and the misorientation between the orientation of the recrystallized grain and that of the surrounding deformed matrix. The work indicates that the recrystallized grains, which migrate into the deformed grains, are mainly with the high misorientation angles. The large percentage of the recrystallized grains, whose misorientation angles with deformed grains exceed 15°, are corresponding to the {111} transformation texture. Nucleation first starts at colonies that have the highest stored energy of deformation, which has the same orientation as the deformed grains. By analysis, the formation of recrystallization texture was well explained by oriented nucleation mechanism rather than by selective growth mechanism.

Orientation imaging microscopy studies of recrystallization in interstitial-free steel

Orientation imaging microscopy studies of recrystallization in interstitial-free steel

Orientation imaging microscopy studies of recrystallization in interstitial-free steel

Orientation imaging microscopy studies of recrystallization in interstitial-free steel

Orientation imaging microscopy studies of recrystallization in interstitial-free steel

The origin of the γ fiber recrystallization texture in interstitial-free (IF) steel developed during continuous annealing has been investigated by scanning electron microscopy (SEM) and orientation imaging microscopy (OIM). Nucleation of {111} «uvw» oriented crystals occurs in deformation banded γ grains and therefore a comprehensive study of microstructure of cold-rolled IF steel in the sections perpendicular to the rolling and transverse directions (TDs) and the rolling plane (RP) has been carried out to understand the formation, geometry, and microstructural features of recrystallization. The RP section gave abundant evidence of orientation gradients formed in γ oriented grains that had been subject to orientation splitting to give deformation bands. These orientation gradients across a single grain are around 5 to 30 deg and this orientation difference is sufficient to form nuclei with mobile interfaces during annealing and hence to create chains of γ oriented new grains in the original hot band γ grain envelopes. A grain impingement model requiring orientation pinning is then proposed to explain how these grains, contained in deformed γ grain envelopes, grow out into their neighbors to dominate the final recrystallization texture of IF steel. The α deformed grains contain only small lattice curvatures, and therefore in-grain nucleation is rare. These grains are mostly consumed by invading γ grains toward the end of the recrystallization process.

Effect of Initial Texture on the Evolution of Texture and Stored Energy during Recrystallization of Interstitial Free Steel

By applying a double cold rolling and annealing treatment, the evolution of the α and γ fiber components differed from the ones observed in conventional processing. This is attributed to the difference of the initial texture. An increased reduction of stored energy of the {111}<112> component was found by monitoring the change of the stored energy during annealing, indicating that the onset of the nucleation stage of recrystallization by relaxation and annihilation of dislocations occurred mainly on the {111}<112> component with its higher stored energy. The detailed texture evolution of the double cold rolled specimen during 2nd annealing is described by coupling the theory of oriented nucleation and orientation pinning, which is experimentally confirmed by OIM scan.

Evaluation of Stored Energy in Cold-Rolled IF Steels from EBSD Data and Its Application to Recrystallization Modeling

This study has been conducted to evaluate the orientation-dependent stored energy of deformed grains from the data measured using electron back-scattered diffraction (EBSD) analysis. The stored energy in deformed grains is an important term as a driving force for static recrystallization of cold-rolled steels. Subgrain method based on subgrain structure is used for quantitative analysis of the stored energy developed in cold-rolled interstitial free (IF) steels to a reduction of 80%. The orientation dependent stored energy term was used in Monte Carlo technique to simulate static recrystallization of IF steels.

Static and Metadynamic Recrystallization of Interstitial Free Steels during Hot Deformation

A rapid method was used to identify kinetics of the recrystallization for two IF (Interstitial Free) steels which have different phosphorous and boron contents. The static and metadynamic softening behaviour of the materials for a range of strain rates and temperatures were quantified. The critical strain for initiation of strain independent softening was estimated for the IF steels in respect to the time for 50 percent softening after deformation. The results showed that the strain for the initiation of strain independent softening (often referred to as metadynamic recrystallization) varies with the Zener Hollomon parameter. Classic static recrystallization was observed at strains below the strain independent softening for all processing conditions and the strain rate had a strong effect on the time for strain independent softening. Results also revealed that static and metadynamic recrystallization was delayed owing to the phosphorous and boron alloying elements. Hence, the large strain at above no-recrystallization temperature may be required for the early stage of Finishing Stands Unit (FSU) in hot strip rolling mills to initiate austenite grain refinement of phosphorous and boron added IF steels.

Modeling of texture development during recrystallization of interstitial free steel

A new model of texture development in recrystallization of cold-rolled interstitial free (IF) steel is proposed in this paper. This model assumes that oriented nucleation takes place in the vicinity of grain boundaries, where the orientation of an area rotates to a common orientation during deformation. The orientation of recrystallized nuclei is selected between the orientation of the deformed grain and the orientation of the grain boundary area. Nucleation first starts in grains that have the highest stored energy of deformation, and then progresses in grains that have lower stored energy. The development of γ fiber (ND//<111>) annealing texture can be explained using the model. The predicted texture differences resulting from differences in cold rolling reduction correspond to the experimental results.

Modeling texture change during the static recrystallization of interstitial free steels

A recrystallization model was developed based on the principal features of oriented nucleation and selective growth. The nucleation mechanism that is adopted assumes that nucleation takes place in deformed grains with relatively high Taylor factors. The selective growth law is based on the assumed high mobility of {110} plane matching (PM) boundaries and also includes a variant selection rule. The two submodels enable the full recrystallization process to be simulated. The theory is applied to the annealing of a Ti-Nb alloyed interstitial free (IF) steel. It is demonstrated that the nucleation and growth algorithms can successfully account for the annealing textures observed, including the presence of a strong {111} component. An important feature of the model is that it permits the role of variant selection to be assessed; it also shows the effect on the texture of the low mobility of low-angle grain boundaries (LAGBs).