Cold-rolled Interstitial-free Steel Research Articles

The Role of Parent Phase Topology in Double Young–Kurdjumow–Sachs Variant Selection during Phase Transformation in Low-Carbon Steels

The present paper investigates the role of parent phase topology on a crystallographic variant selection rule. This rule assumes that product phase nuclei appear at certain grain boundaries in the parent structure, such that a specific crystallographic orientation relationship is observed with both parent grains at either side of the grain boundary. The specific crystallographic orientation correspondence considered here is the Young–Kurdjumow–Sachs (YKS) orientation relationship <112>90° (which exhibits 24 symmetrical equivalents). The aforementioned relationship is characteristic of phase transformations in low-carbon steel grades. It is shown that, for different parent phase textures, ~20% of the grain boundaries comply with the double YKS condition allowing for a tolerance of 5°, ignoring the presence of topology in the parent phase microstructure. The presented model allows for connecting the presence of a specific parent phase topology with the condition of the double YKS variant selection rule in a number of practical cases: (i) for hot rolled Ti–Interstitial Free (IF) steel with and without Mn addition, (ii) for cold rolled IF steel exhibiting very strong texture memory after forward and reverse α ⇌ γ phase transformation and (iii) for a martensitic transformation in a Fe–8.5% Cr steel. It is shown that the double YKS variant selection criterion may explain several specific features of the observed transformation textures, while assuming a non-correlated arbitrary pair topology of the parent austenite structure (implying that for N parent orientations N/2 pairs are selected in an arbitrary manner).

Characterisation of the plane anisotropy and its effect on interstitial free steel thin sheet metal forming simulation

The main purpose of this paper is to study the orthotropic plastic behaviour of the cold-rolled interstitial free steel HC260Y when it is submitted to various loading directions under monotonic tests. The experimental database included tensile tests carried out on specimens (in the as-received condition and after undergoing an annealing heat treatment) cut in different orientations according to the rolling direction. A model was proposed, depending on a plasticity criterion, a hardening law and an evolution law, which takes into account the anisotropy of the material. To validate the proposed identification strategy, a comparison with the experimental results of the planar tension tests, carried out on specimens cut parallel to the rolling direction, was considered. The obtained results allowed the prediction of the behaviour of this material when it is subjected to other solicitations whether simple or compound.

Topological aspects responsible for recrystallization evolution in an IF-steel sheet – Investigation with cellular-automaton simulations

A cellular automaton algorithm for curvature-driven coarsening is applied to a cold-rolled interstitial-free steel’s microstructure - obtained through electron backscatter diffraction (EBSD). Recrystallization nucleation occurs naturally during the simulation, due to the highly heterogeneous and hence competitive growth among pre-existing (sub) grains. The spatial inhomogeneity of the subgrain growth that takes place derives from the large local variations of subgrain sizes and misorientations that comprise the prior deformed state. The results show that capillary-driven selective growth takes place to the extent that the prior elongated and deformed grains are replaced by equiaxed grains with no interior small-angle boundaries. Additionally, during the simulation certain texture components intensify and others vanish, which indicates that preferential growth occurs in a fashion that relates to the crystal orientations’ topology. The study of the early stages of recrystallization (i.e. nucleation) shows that the pre-existing subgrains that eventually recrystallize, exhibit certain topological characteristics at the prior deformed state. Successful nucleation occurs mostly for pre-existing matrix subgrains abutting shear bands or narrow deformation bands and particularly at regions where the latter intersect.

True stress-control ratcheting behaviour of cold-rolled interstitial-free steel sheet

True stress-control ratcheting behaviour of 2.3 mm thick titanium stabilised interstitial-free steel has been investigated under constant maximum stress, constant mean stress and constant stress amplitude conditions until complete failure of the specimens. The ratcheting strain to failure, and cycles to failure under different stress conditions have been rationalised by considering the evolution of back stresses vis-a-vis total and net cyclic plastic strain response. It is found that higher stress asymmetry due to increase of stress ratio at constant maximum stress results in lower ratcheting deformation. This happens due to both mean stress assisted hardening and lower back stress amplitude resulting from decreasing stress amplitude. On the other hand, mean stress induced hardening improves ratcheting-fatigue life at constant stress amplitude. The amplification of cyclic plastic strain response with increase of stress amplitude at constant mean stress results in higher ratcheting strain accumulation at higher stress amplitude.

Effect of grain boundary character distribution and grain boundary phosphorus segregation on the brittleness of an interstitial-free steel

Grain boundary character distribution and grain boundary P segregation in an interstitial-free steel annealed for up to 4h at 700°C after cold rolling are investigated using electron backscatter diffraction and Auger electron spectroscopy. During annealing, the recrystallized grain size and hardness of the steel do not change apparently, being ~10µm and 90HV, respectively, but the number percentage of random high-angle grain boundaries initially increases with increasing annealing time until reaching a maximum at 3h and then decreases slightly after that. The random high-angle grain boundaries play an important role in the steel embrittlement. Both P grain boundary segregation and ductile-to-brittle transition temperature (DBTT) of the steel increase in the same trend with increasing number percentage of random high-angle grain boundaries. The relationship between DBTT and P boundary concentration (Cp) is linear, being expressed as DBTT(°C)=3.12CP−118.87 (Cp, at%), showing that the annealing treatment of cold-rolled interstitial-free steel should be precisely designed to avoid embrittlement.

Effect of Equivalent Stress-Strain Relation and Differential Hardening on Accuracy of Forming Limit Analyses

Multiaxial tube expansion tests (MTETs) were performed to measure the multiaxial plastic deformation behavior of a cold rolled interstitial-free (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 tubular specimens using an electrical, closed-loop servo-control system for an axial force and an internal pressure. Tubular specimens with an inner diameter of 44.6 mm were fabricated from a cold rolled IF steel sheet with a thickness of 0.7 mm 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 strains and forming limit stresses 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 hardening behavior was approximated by changing the material parameters and the exponent of the Yld2000-2d yield function (Barlat et al, 2003) as functions of the reference plastic strain. The hydraulic bulge tests were also conducted to measure the forming limit strain and forming limit stress for equibiaxial tension and to determine the equivalent stress-equivalent plastic strain relation for a larger strain range. The forming limit curve and forming limit stress curve were calculated using the Marciniak-Kuczyński-type approach. The calculated results were in fair agreement with the measurement.

Observation of co-segregation of titanium and boron at the interface between recrystallized and unrecrystallized grains in cold-rolled interstitial-free steel sheets

It has been reported that the addition of ppm levels of B strongly retarded the growth of recrystallized grain into unrecrystallized grains in the process of cold-rolling and annealing of Ti-added interstitial-free (IF) ferritic steels. This phenomenon was explained by solute drag effect based on the assumption that, during annealing, B atoms segregate at the interface between recrystallized and unrecrystallized grains where they interact with Ti atoms. To verify this, atom probe tomography analysis of the interface was performed in Ti-added IF steels with and without B addition. Needle tips containing the interface identified from electron backscattering diffraction analysis, were produced by focused ion beam milling with the lift-out method. To increase the experiment reliability, the misorientation angle of the aimed interface was compared with that estimated by field ion microscopy analysis. Considerable amount of Ti segregation was observed at the interface in the steel without B addition, which increased with increasing amount of B segregation in the steel with B addition. The results suggest that the retardation of the interface migration was caused by solute drag effect based on the simultaneous co-segregation of Ti and B due to their attractive interaction.

Formation of Highly Misoriented Fragments at Hot Band Grain Boundaries During Cold Rolling of Interstitial-Free Steel

The deformation heterogeneities that form in the vicinity of prior hot band grain boundaries in a 75 pct cold-rolled interstitial-free steel have been investigated by 3D electron backscatter diffraction. Grain boundary-affected regions occupy a large fraction of the overall material volume. The coexistence of several features, such as steep orientation gradients up to 5 deg/μm, high-angle boundary networks, and thin, elongated grain boundary fragments, has confirmed the highly complex nature of these regions. Most notably, these thin boundary fragments were found to be significantly misoriented from any of the deformed grains immediately adjacent to the boundary. Overall, grain boundary regions adopt the so-called ‘deformation banding’ mode of deformations on both the micro (e.g., steep gradients)- and nano (e.g., thin fragments)-length scales. Grain boundary structures comprise the essential features to act as preferred sites for recrystallization. The discovery of numerous thin grain boundary fragments in the deformation microstructure provides a plausible explanation for the origin of recrystallized grains with orientations other than those found within the adjoining deformed grains in the vicinity of grain boundaries; this phenomenon has been commonly observed in texture data for many years but remained unexplained.

In-Situ Annealing Study of Transformation of α and γ Texture of Interstitial-Free Steel Sheet by High-Energy X-Ray Diffraction

High-energy synchrotron diffraction offers great potential for experimental study of recrystallization kinetics. An experimental design to study the recrystallization mechanism of interstitial-free (IF) steel was implemented. The whole annealing process of cold-rolled IF steel with 80% reduction was observed in situ using high-energy X-ray diffraction (HEXRD). The results show how the main texture component of IF steel change, i.e. the α [<110>// rolling direction (RD)] fiber texture decreases and the γ [<111>//normal direction (ND)] fiber texture increases. The important part of the α fiber texture is that both the {100}<011> and {112}<011> texture decrease at the beginning of recrystallization. The γ fiber texture increases at the early stage of recrystallization which stems from the increase of {111}<112>. Nevertheless, the {111}<110> does not change after recrystallization. The dynamic evolution of the main texture components {100}<011>, {112}<011>, {111}<112> and {111}<110> is given by in-situ HEXRD.

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.

Designing cold rolled IF steel sheets with optimized tensile properties using ANN and GA

Artificial neural network (ANN) models, correlating the mechanical properties (yield strength, tensile strength, %elongation and r ¯ ) of the cold rolled interstitial free (IF) steel sheets with compositional and processing parameters, are used to find the importance of different variables. Further the above models are used as objective functions for the evolutionary multi-objective optimization algorithms by evolving tradeoffs, which gives a range of combination of strength and ductility. The optimal solutions are also analysed to extract further knowledge on the role of various parameters, which could be used for designing the chemistry of the alloy as well as process parameters for producing cold rolled strips of IF steel with tailor made property.

Microstructure and Texture Evolution in Cold Rolled Interstitial Free (IF) Steel Sheet during Annealing under AC Magnetic Field

An AC magnetic field (0.5Tesla) is applied with the field direction perpendicular to the rolling direction during annealing of a 76% cold-rolled IF steel sheet. Microstructure and texture evolution in the as-annealed specimens were determined using SEM based OIM technique. It is found that the recrystallization is noticeably retarded by AC magnetic field annealing. At the early stage of recrystallization (annealing at 650°C for 30min), the development of (111) &lt;123&gt; orientations was favored by the AC magnetic field. With progress of recrystallization (annealing at 700°C and 750°C for 30min), the applied AC magnetic field suppressed the development of γ-fiber recrystallization textures to some extent.

Effects of High Magnetic Field Annealing on the Nucleation Sites for Recrystallized Nuclei with {111} &amp;lt;112&amp;gt; Orientation in Cold-Rolled IF Steel Sheet

The effect of high magnetic field annealing on the nucleation sites for recrystallized nuclei with {111}&lt;112&gt; orientation in the initial stage of recrystallization in as-annealed interstitial-free (IF) steel sheet were investigated by the SEM-EBSD analysis. Specimens of cold rolled IF steel sheet were annealed at 650°C for 0min and 10min respectively with a 12-tesla magnetic field to obtain a partially recrystallized microstructure. During the magnetic field annealing, they were respectively placed at the center of the applied field with their rolling planes parallel to the magnetic field direction (MD), and with their rolling direction (RD) parallel and normal to the MD, respectively. It was found that different to the non-field annealed specimen, in the field annealed specimens, the {111}&lt;112&gt; oriented recrystallized nulei favored to nucleate in the {111}&lt;112&gt; deformed matrix and not in the {111}&lt;110&gt; deformed matrix. This phenomenon may be attributed to the magnetic ordered state induced by the magnetic field, which might suppress the formation of high-energy grain boundaries between the newly formed nuclei and the deformed matrix.

Composition–Processing–Property Correlation of Cold-Rolled IF Steel Sheets Using Neural Network

Artificial neural network (ANN) models have been developed to establish relationships between the mechanical properties of the cold-rolled sheets of interstitial free (IF) as a function of the chemical composition of the steel, the rolling and the batch annealing parameters. The mechanical properties studied include yield and . The technology of rolling mill automation necessarily embraces a broad spectrum of interests, ranging from fundamental process analysis to the solution of special control-theoretic problems. Determining the relationship between various compositional and processing parameters and the mechanical properties of cold-rolled sheets is important but not easy to establish. The complex and highly nonlinear interactions between the variables make the system difficult to assess from the prediction point of view. ANN model is found to be capable of developing the inherent relationship between the variables and could be successfully used to develop better understanding on the metallurgical principles of cold rolled IF steel.

Study of recrystallisation in interstitial free (IF) steel by ultrasonic measurements

The purpose of understanding recrystallisation in interstitial free (IF) steel through non-destructive ultrasonic measurements was to develop an online system to monitor specific grain conditions, textures and mechanical properties during annealing of such steels. Ultrasonic measurements for microstructure and property evaluation samples had larger volumes of the material when compared with other conventional methods. In order to make it an alternative to the conventional destructive methods using microscopes, X-ray diffraction, mechanical testing, hardness testing, etc., correlation between the ultrasonic surface wave velocities in 45° to rolling direction and average velocity v-Bar and those of materials parameters like plastic strain ratio r-Bar, hardness, texture, in cold rolled IF steel sheets with 79% thickness reduction and annealed at 650°C for different soaking times were developed. The ultrasonic surface wave velocity measurements at 45° to rolling direction and average velocity v-Bar were found to be the ultrasonic parameters to be monitored to control recrystallisation in IF steels.

Effects of magnetic field strength on microstructure and texture evolution in cold-rolled interstitial-free steel by magnetic field annealing

Effects of magnetic field strength on the evolution of recrystallization microstructure and recrystallization texture in cold-rolled interstitial-free steel were investigated after annealing at 660°C in a magnetic field up to 12 T. Magnetic annealing was found to retard recrystallization and induce recrystallized grains elongated along the magnetic field direction. An interesting phenomenon is that the magnetic field shows the strongest effect at 1 T.

Effects of High Magnetic Field and Field Direction on Recrystallization and Recrystallization Texture in Cold-Rolled IF Steel Sheet

The effects of magnetic annealing on recrystallization and texture evolution in asannealed interstitial-free (IF) steel sheet were investigated by means of X-ray diffraction ODF analysis, SEM-EBSD analysis, and optical microstructure observation. During the magnetic annealing, specimens were placed at the center of the applied magnetic field, with their rolling planes parallel to the field direction (MD) and their rolling direction (RD) normal to the field direction (MD). It was found that the magnetic annealing retards the recrystallization process, but promotes the nucleation at the initial stage of recrystallization. Magnetic annealing did not change the mechanism of recrystallization texture evolution but improved the development of γ-fiber texture during the process of recrystallization, and the magnetically annealed specimen had stronger γ-fiber texture compared with the conventionally annealed specimen; this interesting finding is quite different from the previous work on magnetically annealed IF steel.

Three-dimensional morphology of microbands in a cold-rolled steel

A three-dimensional (3-D) reconstruction of a typical microband in cold-rolled interstitial-free steel was generated by 3-D electron backscatter diffraction. The microband contained irregular curved surfaces that reconcile findings that microbands appear straight and aligned along slip planes when viewed in normal direction–rolling direction sections but are wavy in transverse direction–rolling direction sections. Three slip planes were found within the angular range of the curved surface of the microband, which indicates that multiple planes is operative during deformation.

Effect of High Magnetic Field Annealing on Microstructure and Texture at the Initial Stage of Recrystallization in a Cold-Rolled Interstitial-Free Steel

(mass%): 0.0023C, 0.056Ti, 0.014Si, 0.16Mn, 0.011P, 0.0064S, 0.052Al, 0.0018N. Specimens with the dimensions of 20 mm � 10 mm � 1 mm were respectively subjected to isothermal annealing at 650 � C for different time (0, 10 and 30 min) in a furnace installed in a cryocooler-cooled superconducting magnet at a heating rate of 5 � C/min, with and without a 12-tesla magnetic field, and then cooled in the furnace. During the magnetic field annealing, a magnetic field was kept being applied in the whole heating and cooling processes. The specimens were placed at the center of the applied field with their rolling direction (RD) paralleling to the magnetic field direction (MD). The experimental setup is schematically shown in Fig. 1. Microstructures of longitudinal sections of the specimens were examined by orientation imaging microscopy (OIM). In the maps, the recrystallized grains are highlighted (dark grains). The scan was carried out over the area of 1191 � 878 measuring points with a 0.07 mm step size, and the corresponding ODFs were calculated using the Channel 5

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.