Cold Rolled IF Steel Research Articles

Recrystallization behaviour of IF steel at the interface of Al junction

The recrystallization behavior of an IF steel was investigated at the interface with aluminum junction, using cold-rolled IF steel sheets with high recrystallization temperature. In the region surrounded by the tongue-like η phase, recrystallization of the steel was fast at first glance, and the equiaxed structure was formed, but actually, it had a subgrain structure. The η phase grew preferentially to the c axis, and the growth of the η-Fe2Al5 phase distorted the surrounding iron due to the difference in the molar volume; compressive deformation was expected in the a-axis and b-axis directions, and tensile deformation was expected in the c-axis. The growth of η phase obviously changed the recrystallization behavior of the IF steel. The development of γ-fiber crystalline texture (<111>/ND), which was a typical recrystallization texture of IF steel, was not observed in the region surrounded by η phase.

Study of Methods for Increasing Ductility and Formability of Cold-Rolled Ti-Stabilized IF Steels

Study of the influence of the chemical composition and parameters of the complex treatment technology on characteristics of the microstructure, phase precipitates, the solid solution state, and mechanical properties was carried out for hot-rolled and cold-rolled Ti-stabilized IF steels. Cold-rolled products were annealed according to the regime of continuous hot-dip galvanizing units. The following methods of investigation were used: light, scanning and transmission electron microscopy, mechanical properties testing, thermodynamic analysis of phase stability conditions, and determination of the interstitial impurities content in a solid solution by internal friction. The possibility and conditions of a significant increase in ductility, formability of cold-rolled steels are established. They are obtained not by the traditional way consisting in reducing the content of interstitial elements and impurities, but by giving them a favorable form of existence. This is achieved by optimizing the chemical composition, first of all, the sulfur content, and parameters of thermo-deformation treatment of steel, which ensure complete carbon binding in Ti4C2S2 as a result of TiS transformation already at the hot-rolling stage. To reduce the yield and tensile strength, it is necessary to decrease the content of interstitial elements and to increase the temperature of recrystallization annealing of cold-rolled steel.

Effect of flash processing on recrystallization behavior and mechanical performance of cold-rolled IF steel

Flash processing (FP) has attracted considerable attention due to its high efficiency, economic advantages, and the extraordinary opportunity if offers to improve the mechanical properties of steel. In this study, we investigated the influences of FP on the recrystallization (REX) behavior and mechanical performance of cold-rolled IF steel. Using a thermomechanical simulator, we performed both single-stage FPs, at heating rates of 200°C/s and 500°C/s, and two-stage FP, with an initial preheating to 400°C at a rate of 5°C/s and then to peak temperatures at a rate of 200°C/s. In comparison to continuous annealing (CA), single-stage FP can effectively refine the recrystallized grain sizes and produce a similar or even sharper γ (ND (normal direction)//{111}) texture component. In particular, the heating rate of 500°C/s led to an increase in the yield strength of about 23.2% and a similar ductility. In contrast, the two-stage FP resulted in a higher REX temperature as well as a certain grain refinement due to the stored strain energy, i.e., the driving force of REX, which was largely consumed during preheating. Furthermore, both stronger {110} and weaker γ texture components appeared in the two-stage FP and were believed to be responsible for the early necking and deterioration in ductility.

Assessment of recovery and recrystallisation behaviours of cold rolled IF steel through non-destructive electromagnetic characterisation

The recovery and recrystallisation behaviours of cold rolled IF steel have been investigated by destructive (optical microscopy and hardness) and non-destructive electromagnetic sensor, (which allows direct measurement of strip samples with no surface preparation) techniques. The onset and completion of recrystallisation are clearly monitored through destructive techniques of optical microscopy and hardness measurements. The nucleation of new recrystallised grains is observed in the sample annealed at 600 °C/15 min, while completion of recrystallisation takes place at 700 °C/15 min. The destructive techniques are not very accurate in monitoring recovery, for example, changes in hardness of <20% are seen. In contrast, the magnetic properties of annealed steel show the onsets of both recovery and recrystallisation, with recovery accounting for ≈60% change in the coercivity value. Therefore, the measurement of magnetic softening through an electromagnetic sensor acts a crucial role for understanding recovery and recrystallisation behaviours of steels during industrial processing. The present investigation is aimed not only for controlling product quality but also saving characterisation time through off line monitoring during steel processing at industry.

Microstructure and Property Modifications of Cold Rolled IF Steel by Local Laser Annealing

Laser annealing experiments are performed on cold rolled IF steel whereby highly localized microstructure and property modification are achieved. The microstructure is seen to develop by strongly heterogeneous recrystallization to provide steep gradients, across the submillimeter scale, of grain size and crystallographic texture. Hardness mapping by microindentation is used to reveal the corresponding gradients in macroscopic properties. A 2D level set model of the microstructure development is established as a tool to further optimize the method and to investigate, for example, the development of grain size variations due to the strong and transient thermal gradient. Particular focus is given to the evolution of the beneficial γ-fiber texture during laser annealing. The simulations indicate that the influence of selective growth based on anisotropic grain boundary properties only has a minor effect on texture evolution compared to heterogeneous stored energy, temperature variations, and nucleation conditions. It is also shown that although the α-fiber has an initial frequency advantage, the higher probability of γ-nucleation, in combination with a higher stored energy driving force in this fiber, promotes a stronger presence of the γ-fiber as also observed in experiments.

Heterogeneous multi-layered IF steel with simultaneous high strength and good ductility

Multi-layered IF steel samples were designed and fabricated by hot compression followed by cold forging of an alternating stack of cold-rolled and annealed IF steel sheets, with an aim to improve the strength of the material without losing much ductility. A very good combination of strength and ductility was achieved by proper annealing after deformation. Microstructural analysis by electron back-scatter diffraction revealed that the good combination of strength and ductility is related to a characteristic hierarchical structure that is characterized by layered and lamella structures with different length scales.

Structural characterization of plasma nitrided interstitial-free steel at different temperatures by SEM, XRD and Rietveld method

Plasma nitriding processes are widely used to improve surface properties of several steels and alloys. In this work, the formation of nitrides in the surface of plasma nitrided IF steels as a function of the temperature was investigated. Three cold-rolled IF steel plates were nitrided for 4h after shot peening at three different temperatures: 450°C, 475°C, and 500°C. The resultant nitrided layers were then characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), Rietveld method, and hardness measurements. Through SEM images, it was possible to visualize two main sublayers: a compound layer and a diffusion zone. Through XRD, two phases were identified in the compound layer, which were ɛ-Fe2–3N and γ′-Fe4N. The diffusion zone presented a ferritic matrix with fine precipitates, possibly α″-Fe16N2. By Rietveld, the calculated quantity of γ′-Fe4N was 68wt.% for the sample treated at 475°C and 58wt.% for the one treated at 500°C. These values were consistent with the hardness measurements. Thus, it is suggested that higher nitriding temperatures facilitate the decreasing of γ′-Fe4N and, consequently, the increasing of ɛ-Fe2–3N in the compound layer.

Mechanical Properties and Crystallographic Texture of Symmetrical and Asymmetrical Cold Rolled IF Steels

The crystallographic texture developed during cold rolling and subsequent annealing of interstitial free sheet steels aims to increase conformability. For this, it is necessary to obtain partial α-fiber and continuous and homogeneous γ-fiber texture components. In this work, the influence of symmetric (SR) and asymmetric (AR) cold rolling on crystallographic texture and mechanical properties of an interstitial free steel (IF) was investigated. Symmetric cold rolling yields α- and γ-fibers, which are enhanced as deformation increases. Moreover, α-fiber weakening occurs due to recrystallizations, improving formability. The same fibers are produced by asymmetric cold rolling, but in this case, the γ-fiber is slightly shifted in psi, which is one of Euler angles second ROE's notation1,2, and more homogeneous than in symmetric rolling. The best mechanical properties were achieved by asymmetric cold rolling/annealed with about 80% deformation.

Kinetika nastanka segregacije nečistoč po mejah zrn med žarjenjem po hladnem valjanju jekel 08C-Al in IF za globoki vlek

Kinetika nastanka segregacije nečistoč po mejah zrn med žarjenjem po hladnem valjanju jekel 08C-Al in IF za globoki vlek

The evolution with strain of the stored energy in different texture components of cold-rolled IF steel revealed by high resolution X-ray diffraction

During the deformation of low carbon steel by cold-rolling, dislocations are created and stored in grains depending on local crystallographic orientation, deformation, and deformation gradient. Orientation dependent dislocation densities have been estimated from the broadening of X-ray diffraction lines measured on a synchrotron beamline. Different cold-rolling levels (from 30% to 95% thickness reduction) have been considered. It is shown that the present measurements are consistent with the hypothesis of the sole consideration of screw dislocations for the analysis of the data. The presented evolutions show that the dislocation density first increases within the α fiber (={hkl}<110>) and then within the γ fiber (={111}<uvw>). A comparison with EBSD measurements is done and confirms that the storage of dislocations during the deformation process is orientation dependent and that this dependence is correlated to the cold-rolling level. If we assume that this dislocation density acts as a driving force during recrystallization, these observations can explain the fact that the recrystallization mechanisms are generally different after moderate or large strains.

Forming Limit Analyses of Cold Rolled IF Steel Sheet Using Differential Work Hardening Model

A servo-controlled multiaxial tube expansion testing machine was used to measure the multiaxial plastic deformation behavior of a cold rolled IF steel sheet for a range of strain from initial yield to fracture. The testing machine is capable of applying arbitrary principal stress or strain paths to a tubular specimen using an electrical, closed-loop servo-control system for an axial force and an internal pressure. Tubular specimens with an inner diameter of 44.6mm were fabricated from a cold rolled interstitial-free steel sheet with a thickness of 0.7mm by roller bending and laser welding. Many linear stress paths in the first quadrant of stress space were applied to the tubular specimens to measure the forming limit curve and forming limit stress curve of the as-received sheet sample, in addition to the contours of plastic work and the directions of the plastic strain rates. It was found that the shapes of the measured work contours changed with increasing plastic work. The observed differential work hardening behavior was approximated by changing the material parameters and the exponent of the Yld2000-2d yield function (Barlat et al., 2003) as a function of the equivalent plastic strain. The forming limit curve and forming limit stress curve were calculated using the Marciniak-Kuczyński-type approach and the differential work hardening model. The calculated results were in fair agreement with the measurement.

21112 冷延IF鋼板の異方硬化挙動の定式による成形シミュレーションの高精度化

The deformation behavior of cold-rolled IF steel sheet under biaxial tension was investigated for a strain range from initial yielding to fracture. The test material was bent and laser welded to fabricate a tubular specimen. Multiaxial tube expansion tests were performed using a servo-controlled tension-internal pressure testing machine. Many linear stress paths in the first quadrant of stress space were applied to the tubular specimens to measure the counters of plastic work and the directions of the plastic strain rates. It was found the shapes of the work counters changed with an increase in ε^P_0 and the Yld2000-2d yield function could reproduce the differential work hardening behavior (DWH) of the test material by changing the material parameters and exponent as functions of ε^P_0. Finite element analysis of hydraulic bulge forming has verified the effectiveness of proposed DWH model based on Yld2000-2d yield function for improving the accuracy of the FEA.

Formation of Nuclei With {111} and {111} Orientations of IF Steel at Early Stage of Recrystallization Using EBSD Analysis

The excellent deep drawability of interstitial free steel (IF steel) is closely related to its texture formed during recrystallization. The nucleation process of cold rolled IF steel at the early stage of recrystallization was investigated by electron back scattered diffraction (EBSD). The characteristics of the microstructure after deformation and the orientation of nucleation were observed. The results show that the deformed microstructure with 80% reduction could be subdivided into two groups. These two types of microstructure were characterized by their orientation and internal local misorientations. The nuclei with γ-orientation preferred to form in deformed bands with γ-orientation and at the boundaries between deformed grains with different orientations. The recrystallized grains with {111}<110> orientation appeared firstly in deformed matrix with {111}<112> orientation and consumed the matrix with {111}<112> to grow up, while the recrystallized grains with {111}<112> orientation were observed secondly in deformed matrix with {111}<110> orientation and consumed matrix with {111}<110> to grow up.

Origin of Copious Recrystallization in Cold Rolled Interstitial Free (IF) Steels

Copious recrystallization occurs during the very early stages of annealing in some deformed grains in 85% cold rolled IF steel. This event happens to certain regions in γ oriented grains while the α oriented grains remain unrecrystallized. The orientations of the newly forming grains are near either {111}&lt;123&gt; or {554}&lt;225&gt;, and as a result the intensity at these orientations are high in the recrystallization texture. An investigation by scanning electron microscope (SEM) and electron backscatter diffraction (EBSD) shows that such copious recrystallization event originates from the frequent presence of high crystallographic gradients, at the locations where two set shear bands intersect.

Effect of Micro-Alloying Elements of Ti, Nb and B on Recrystallization Behavior of Cold-Rolled Interstitial-Free Steel Sheets

The effect of micro-alloying elements of niobium and boron and titanium on recrystallization behavior is researched after the cold rolled IF steels are annealed at high temperature. The results show that there is high density dislocation in the cold rolled steel and the microstructure is fibrous tissue. The recrystallization grains appear when the cold steel annealed at 655 °C and then the grains grow up with the annealing temperature increased to 840 °C. The recrystallization temperature and time of B-Ti-IF steel is lower than that of Nb-Ti-IF and Ti-IF steels. The recrystallization activation energy of Nb-Ti-IF steel is 181.7KJ/mol which is 56.6KJ/mol larger than that of B-Ti-IF steel.

Structure of IF steel in continuous annealing

The recovery and recrystallization kinetics of IF titanium steel is studied by investigating the microstructure, measuring the coercive force, and analyzing the residual stress. The results are familiar, except for the initial and final temperatures of these processes, which are specific to the steel employed. We assume that, on heating cold-rolled IF steel in the range 550–650°C, the dislocation density is unchanged on account of retardation by small TiC particles. Formulas are derived for the coercive force as a function of the grain size and may be used for monitoring purposes.

Effect of Electric Field Strength on Texture Evolution of IF Steel Sheet during Annealing

Specimens cut from a cold-rolled IF steel sheet of 1 mm thickness were respectively annealed at 750°C for 20min under a range of DC electric fields (1kV/cm~4kV/cm). The Effect of electric field strength on recrystallization texture of IF steel sheet was studied by mean of X-ray diffraction ODF analysis. It was found that γ-fiber textures were notably enhanced as electric field strength increased. The strength of γ-fiber textures got their peak values as the applied electric field reached to 4kV/cm. The possible reason for such phenomena was discussed in the viewpoint of interaction between the applied electric field and the orientation-dependent stored-energy in deformed metals which is known as the driving force for recrystallization during annealing.

二軸バルジ試験による冷延IF鋼板の加工硬化特性の測定と材料モデリング

二軸バルジ試験による冷延IF鋼板の加工硬化特性の測定と材料モデリング

Control of Surface Quality of if Steel Sheet

In order to solve the problem of mold powder entrapment which is one of the affecting factors on faint-sliver defect in IF cold-rolled steel sheet in casting process, continuous casting process is optimized. Based on the analysis of the defect, the composition of mold powder is optimized. Otherwise, surface tension and viscosity of mold powder are raised. With theses countermeasures, mold powder entrapment and casting subcutaneous slag is decreased obviously. In addition, the rate of faint-sliver defect in cold-rolled IF steel sheet has been reduced from 5.21% to below 1.0%.

Control for Surface Faint-Sliver Defects in Cold-Rolled IF Steel Sheet

The samples of cold-rolled sheets, hot-rolled plates and continuously cast slabs have been analyzed with the condition of faint-sliver defects appeared in IF steel of Pangang, and it was shown that faint-sliver defects in cold-rolled steel sheet were mainly caused by micro-cracks in the surface of strands, slag entrapment in mold during casting and inclusions in sub-surface of strand. Faint-sliver defects were controlled effectively with the technical measures, which were decreasing oxidation of ladle slag, controlling the ratio of CaO/Al2O3 in slag, extending circulation time of deoxidization and alloying in RH process, controlling the casting speed and increasing the viscosity of mold fluxes. Therefore, the ratio of downgrade induced by faint-sliver defects reduced from 10.97% to 1.0%.