IF Steel Sheet Research Articles

Effect of Cold Reduction Ratio and Annealing Temperature on Microstructure and Mechanical Property of 240MPa Grade High Strength if Steel

Hot rolled 240MPa grade high strength IF steel sheet was used to study the effect of cold reduction ratio and annealing temperature on the microstructures and mechanical properties of experimental steel. The experimental results showed that the recrystallization has finished under different annealing temperature followed 75% to 85% cold reduction ratio. To the experimental steel of high coiling temperature, The yield strength, tensile strength ,elongation percentage, yield point elongation, the plastic strain ratio and strain hardening exponent were approximate 260MPa, 445MPa, 37.5%, 2.14%, 0.25, 2.0. To the experimental steel of low coiling temperature, that were approximate 235MPa, 369MPa, 38.8%, 1.73%, 0.26, 2.1. Therefore, the suggested the optimal annealing temperature is 840 °C, cold reduction ratio is 75%, and it obtains optimally match of high strength and punching property.

Influence of Annealing Temperature on Properties and Texture of High Strength IF Steel

Using the cold rolled High Strength IF Steel sheet of 1.0 mm thickness as the test material,the r value and recrystallization texture were investigated by the tensile test and X Radial Diffraction Device.The results show that the test steel were annealed by heating to 760°C,800°C,840°C and holding sixty seconds,and with the increase of the annealing temperature ,the recrystallization finished and the texture to obtain the full evolution,and the r value of the steel grown.After the annealing,the recrystallization texture shows strong γ-fiber texture,homogeneous γ-fiber and weak α-fiber texture,and the texture strength change rule with the annealing temperature change was gained.

Effect of Magnetic Field Direction on Texture Evolution in Cold-Rolled IF Steel Sheet at the Initial Stage of Recrystallization

Sheets of cold rolled (76%) IF steel were annealed at 650°C for 30min under a 12-tesla magnetic field. During the magnetic field annealing, they were placed at the center of the applied field respectively, being oriented differently with respect to the magnetic field direction. The results show that the high magnetic field annealing prevents the evolution from deformed {111}<110> texture component to recrystallized {111}<112> texture component at the initial stage of recrystallization. For the field annealed specimens, altering the specimen orientation to the magnetic field direction during annealing does not change the final annealing textures. The intensity of main {111} texture components presents a similar periodic variation with respect to the specimen orientation to the magnetic field.

Formability of ultrafine-grained interstitial-free steel fabricated by accumulative roll-bonding and subsequent annealing

The miniaturized Erichsen test was successfully developed for evaluating the stretch formability of the ARB and annealed IF steel sheets. The Erichsen value tends to decrease as the average grain size becomes small. However, it should be emphasized that even in the specimen with ultrafine grain, a high Erichsen value is obtained. This result indicates that the ultrafine grained IF steel sheet could be highly deformed under multiaxial stress.

Determinition of Optimal Coiling Temperature and Cold Reduction Ratio of Adding P High Strength if Steel

Hot rolled adding P high strength IF steel sheet was used to study the effect of coiling temperature and cold reduction ratio on the microstructures , mechanical properties and textures of experimental steel. The experimental results showed that the recrystallization has finished after cold rolling followed diffrenent cold reduction ratio and coiling temperature was from 550 °C to 600 °C under experimental condition. When the cold reduction ratio was increased, the grain changed fine and uniformity under diffrenent coiling temperature; The yield strength, tensile strength and elongation percentage were approximate 160 MPa, 345 MPa and 35.0 % respectively, under coiling temperature 600 °C and diffrenent cold reduction ratio condition, the plastic strain ratio r value and the strain hardening exponent n value were reached the maximum, respectively, the r value was approximate 1.5, the n value was approximate 0.30. When the coiling temperature was increased, the {111}＜110＞ texture of α skeleton lines was stronger. Therefore, the suggested the optimal coiling temperature is 600 °C, cold reduction ratio was 60 % to 70 %, and it obtains optimally match of high strength and punching property.

Elasto–plasticity Behavior of IF Steel Sheet with Planar Anisotropy and Its Macro–Meso Modeling

Elasto–plasticity behavior of a IF steel sheet was investigated by performing uniaxial tension tests in three directions (0°, 45° and 90‚ to the rolling direction of the sheet), in-plane cyclic tension–compression test and bi-axial tension test. The sheet has strong planar r-value anisotropy (r0=2.15, r45=2.12 and r90=2.89) but very weak flow stress directionality. Equi-biaxial flow stress is as large as 1.23 times of the uniaxial flow stress. These elasto–plasticity deformation characteristics, as well as the Bauschinger effect and cyclic hardening behavior, are well described by a macro–plasticity model (Yoshida–Uemori model incorpolating with the 4th-order anisotropic yield function). Further, the simulation of elasto–plasticity stress–strain responses of the sheet were conducted by two types of crystal plasticity models, i.e., Taylor hypothesis based model and CPFEM, using the crystallographic orientation distribution data measured by neutron diffraction method. The models capture most of the above-mentioned deformation characteristics qualitatively, but the predicted anisotropy and the Bauschinger effect are weaker than those of the real material. The CPFEM gives more realistic results than the Taylor model.

Structure and Mechanical Properties of Asymmetrically Rolled IF Steel Sheet

As-received hot-rolled 5.6 mm thick IF steel sheet was symmetrically/asymmetrically cold rolled at room temperature down to 1.9 mm. The asymmetric rolling was carried out in monotonic (an idle roll is always on the same side of the sheet) and reversal (the sheet was turned 180º around the rolling direction between passes) modes. Microstructure, texture and mechanical properties were analysed. The observed differences in structure and mechanical properties were modest, and therefore further investigation of the effects of other kinds of asymmetry is suggested.

A mixed experimental/numerical approach to sheet metal plastic modeling, with application to the prediction of the forming limit curves

An accurate description of anisotropic plasticity is required for obtaining reliable predictions in numerical simulations of sheet metal forming processes. An approach to material parameters identification is developed in this paper, which combines classical results obtained in uniaxial tension tests performed along different orientations from the rolling direction, and strain fields measurements obtained by an optical method on cruciform specimens tested in biaxial tension. Material behaviour is described using an 8-parameter anisotropic yield function with the assumption of isotropic hardening. Uniaxial tension tests are used to determine strain-hardening parameters and yield function parameters related to strain- and stress-anisotropy. Strain fields measured in the biaxial tests are used as input data in an optimization procedure that consists of fitting simulated fields with experimental ones in order to determine the material parameters describing the shape of the yield surface in the biaxial stretching range. The method is applied to the determination of the yield surfaces of IF steel and HSLA steel sheets. The identified material parameters also are introduced in an elasto-viscoplastic Marciniak-Kuczinski localization analysis for predicting the Forming Limit Curves (FLCs). The predictions are compared with experimental FLCs determined with a Bragard-type method.

Evaluation of Formability and Mechanical Behavior of Laser-Welded Tailored Blanks Made of Interstitial-Free and Dual-Phase Steels

An attempt has been made to produce laboratory scale laser-welded tailored blanks (LWTBs) with interstitial-free (IF, 300 MPa) and dual-phase (DP, 590 MPa) steel sheets with a thickness ratio of 2. Microstructures and textures of the base, weld, and heat-affected zones (HAZs) were examined using the scanning electron microscope and electron backscattered diffraction technique. The blanks were deformed using the standard limiting dome height (LDH) test setup, and the forming limit diagrams (FLDs) for base IF, DP, and welded blanks with two different weld orientations were constructed. The findings suggest that the FLDo of the welded blank lies in between that of parent IF and DP steel sheets. The formability of the welded blank is highly influenced by the weld orientation. The location of crack formation in the blanks changed with changing the weld orientation in the blanks. The formation of crack on different locations of the blank has been correlated with the local textural differences.

Microstructural evolution and mechanical properties of accumulative roll bonded interstitial free steel

Severe deformation processing is an emerging technique, for the production of submicron grain structures in metallic material, which involves plastic deformation to ultra-high strains. An ultra-fine grained (UFG) material was prepared from an IF steel sheet using Accumulative Roll Bonding (ARB) process. After initial preparing to achieve good sheet bonding, ten cycles of ARB at 500 °C were successfully performed. Atomic Force Microscopy (AFM) and Transmission Electron Microscopy (TEM) were used for the characterization of subgrain and grain structures of ARB processed samples. The mechanical attributes after rolling and cooling were examined. Also, the fracture surfaces were studied by Scanning Electron Microscopy (SEM). It was concluded that metal’s tensile strengths increased by 334% while the ductility dropped from a pre-rolled value of 50.5% to 2.6%. The rolling process was stopped in cycle 10, when cracking of the edges became pronounced.

Effect of Initial Texture on the Deformed Microstructure of IF Steel

The development of deformation texture and microstructure was examined for four different initial textures. IF steel sheets with a majority of α-, ε-, and γ-fiber and near random texture were prepared and cold rolled. The specimens exhibited characteristic behaviors in rolling texture evolution and deformation-induced misorientation development, according to their initial textures, especially at small strain levels. Due to the orientation dependence of intra-granular misorientation accumulation, the different texture evolutions affected the induced misorientation distribution. A larger fraction of γ-fiber orientations was related to more prominent misorientation development, while the initial texture stability simultaneously affected the misorientation development. The unstable, initial ε-fiber texture showed a stronger tendency of misorientation accumulation than the stable α-fiber during the subsequent cold rolling.

Deformation and Damage Evolution of the Microstructure around Ti-Particles in IF Steel during Tensile Deformation

The microstructure evolution in the deformation zone around second phase particles of IF steel sheets subjected to tensile deformation has been investigated in order to correlate it to the damage at microstructural scale. The experimental set-up consisted of a series of interrupted tensile tests which were carried out at different tensile deformations up to fracture. The microstructure of the deformed samples was investigated by EBSD analysis in which the EBSD scans were focused on the areas containing Ti (C, N) particles of cubical shape. It was found that at tensile strains below 25%, the ferrite matrix exhibited the evolution of slip bands inside specific grains depending on their crystallographic orientation although no special strain localization around the particles was observed. After 35% of tensile strain, the strain concentrates around the particles and particle-matrix decohesion was observed. The lattice rotations of the matrix surrounding the particles as well as the selective deformation of the grains are analyzed and discussed.

Evaluation of Annealing Texture in IF and EDD Steel Sheets

The textural and microstructural studies of cold-rolled-batch annealed (CRBA) interstitial free-Ti (IF-Ti) stabilized steel and extra deep drawing steel (EDD) produced at Tata Steel have been investigated. In the IF steels, α-fiber has been found to be strong even after batch annealing, which is not desirable for deep drawing applications. The presence of a stronger recrystallization texture in the surface region rather than in the central region accounts for the deformation and orientation path history of cold-rolling texture components and the texture inherited from hot band. The hot band of IF steel has been observed to have mixture of grain structure along with inhomogeneous texture throughout the thickness. While the inhomogeneity in EDD steel, pancake grain structure in the through-thickness direction, milder pancake grains in the transverse section, and polygonal grain structure in the longitudinal section have led to the inference that the undesirable α -fiber is present. This observation has been explained by the early precipitation of AlN during coiling. The results of all the steels have been explained with the help of the orientation distribution functions, volume fraction of alpha and gamma fiber along with their components, volume fraction ratio (γ /α), and the microstructure. The analysis of the present data suggests that the deep drawability of both steels can be improved by modifying the hot-rolling and annealing parameters.

Effects of Electric Field Annealing on Recrystallization Texture and Plastic Strain Ratio (r-Value) of IF Steel Sheet

Effects of electric field annealing on recrystallization texture and average plastic strain ratio (r value) of IF steel sheet were studied by mean of X ray diffraction ODF analysis and the tensile test. Specimens of 75% cold-rolled IF steel sheet were annealed at 850flC for 25min,with and without electric field annealing respectively. Results show that the electric field annealing does not change the type of recrystallization texture and its formation mechanism in IF deep-drawing steel sheet, but improves the formation and development of recrystallization i-fiber texture, enhances the intensity of i-fiber texture, and retards the formation and development of c-fiber texture. Comparing with the conventional annealing, electric field annealing improves the average plastic strain ratio (r value). Possible reasons for the intensification of recrystallization i-fiber texture with the electric field applied may attribute to the reduction of stored energy, thus hindering the formation of other orientated nuclei and enhancing the nucleating ratio of i-oriented nuclei. And the intensification of recrystallization i-fiber texture was considered as the main reason for the enhancement of the plastic strain ratio (r value) in IF steel sheet.

Damage analysis of sheet metal forming for IF steel

Hill’s quadratic yield criterion and flow theory is employed to describe anisotropic plasticity of sheet metal during forming procedure. Based on Continuum Damage Mechanics, an isotropic damage model is developed to give a damage-coupled localized necking criterion. Experiments of formality of sheet metal in three plane states are performed on IF steel. By measuring the density, the damage curves obtained are compared with calculation results based on the damage-coupled model, which shows a good coincidence. Furthermore, for the uniaxial and biaxial tension states, the relations between the damage of density and strain in thickness are analyzed, which helps to discuss the forming mechanism of IF steel sheet metal.

Characterization of tensile properties of tailor welded IF steel sheets and their formability in stretch forming

Use of laser welded blanks of multiple sheets of material prior to forming is one of the current interests of automotive industries as it reduces manufacturing cost and weight and also improves the quality of the components. As the demand for different types of tailor welded blanks is increasing, the effects of difference in material properties, surface characteristics, weld properties and its orientation on blank formability become important in various forming processes. In this study the properties of laser welded interstitial-free steel sheet samples of different thickness and samples of same thickness but with different surface characteristics (galvanized and un-galvanized) have been characterised. The tensile properties of the welded specimens have been determined in both longitudinal and transverse directions. Weld quality has been evaluated with the help of microstructure and micro-hardness tests. Formability of these two types of tailor welded blanks has been studied by performing out-of-plane stretch forming tests using a hemispherical punch. Both experimental work and finite element analysis have been done to study the effect of thickness difference and surface properties on the formability and strain distribution. It has been found that the limiting dome height and strain distribution are greatly influenced by the difference in thickness within the blank but the difference in surface condition has a very minor effect on the overall formability.

Interface fracture behavior of zinc coatings on steel: Experiments and finite element calculations

Hot-dipped galvanized steels are widely used in the automotive industry. The formability and damage resistance of zinc coatings depend strongly on their microstructure and adhesion to the steel substrate. In order to improve the mechanical performance of zinc coatings, the influence of their thickness, grain orientation and grain size on the zinc coating/steel substrate interface cracking behavior was studied. To this end, scanning electron microscopic observations during in situ tensile testing of zinc coated IF steel sheets were performed. After partial delamination of the zinc coating, cross sections of zinc coated steel were prepared to determine the location and extent of the interface cracking and the crystallographic orientation of the delaminated zinc grains. A two-grain model using a finite element method is proposed to analyze the zinc coating/steel substrate interface cracking behavior. In addition, the coating adhesion strength can be estimated based on this model. Both calculations and experimental observations show that: (i) a preferential zinc grain orientation with the [0001] direction parallel to the interface and (ii) a small grain size mitigate zinc coating/steel substrate interface cracking.

Study on wrinkling limit of interstitial free steel sheets of different thickness when drawn through Conical and Tractrix dies

This paper deals with the wrinkling limit diagrams of cold rolled and close annealed of different grades of IF Steel sheets, which are used in Automotive Industries. The study pertains to deep drawing into cylindrical cups through Conical and Tractrix dies using a flat bottom punch. It is well established that, the use of either a Conical die or a Tractrix die can enhance the limiting draw ratio compared with that obtainable in a conventional drawing operation. When a Conical or a Tractrix die is employed the need for a hold down or clamping ring is eliminated. However, this enhances the propensity of the blank to fail by wrinkling or buckling, particularly in the early stages of a drawing process in which thin sheet blanks are used. It is proved by these researchers and others that the onset of wrinkling takes place when the ratio of strain increments (d ε r/d ε θ ) reaches a critical value during the drawing process. This value, which could be, determined experimentally over which the wrinkling takes place has been shown in the form of wrinkling limit diagrams for the above different grades of IF Steel sheet. An attempt is also made to rank the IF Steel sheet grade that appears to suppress the wrinkling based on results obtained in the form of wrinkling limit diagrams, established for the above different IF Steel sheet grades, in terms of the strain increments ratio and the stress ratio parameters. Further, it is noticed that as a thickness of IF Steel sheet increases the safe region also increases. This means that higher is the thickness better is the resistance against wrinkling. Drawing through the Tractrix die shows better resistance against wrinkling comparing with the Conical die.

Material Identification Using a Bi-Axial Test Machine

This paper shows the identification of material parameters for a DC06 IF steel sheet of 0.8 mm by mechanical tests. The experimental equipment used consists of a tensile test machine, a bi-axial test machine able to perform plane-strain and simple shear tests separately or simultaneously and an optical strain gauge. Tensile, plane-strain and simple shear tests were performed at 0°, 45° and 90° from the sheet rolling direction in order to identify Hill 1948 and Hosford 1979 yield criteria. Two identification methods are used: one based on strain measurements (anistropy coefficients) and the other one based on stress measurements (plastic contours). The results confirm that mechanical tests applying other stress-states than tensile are required to obtain accurate material parameters identification.

Diffusion Reactivity between Zinc Plate Layer and Nanocrystalline Surface Layer of IF Steel by Near Surface-Severe Plastic Deformation

The effect of near surface-ultrafine grain (NSUFG) layer with grain size of about 90nm on the solid reaction between Ti-added ultra-low carbon interstitial free steel sheet and electroplated zinc layer (ZP) was basically investigated at 473K and 573K, in comparison with the reactions of the coarse grains IF steel sheet (CG-IF) and the ZP. The NSUFG structure dramatically changes the reactions between ZP and IF steel, as follows; (1) the incubation times for the formation of reaction layers, pseudo-z-phase, are quite shorter for the NSUFG/ZP reactions than the CG/ZP ones, (2) in the early stage of annealing the former has the smooth interfaces of pseudo-z-phase layers but the latter has wavier ones like a stone wall, (3) the thickness of the pseudo-z-phase layer of the former is thicker than that of the latter, (4) some cracks are observed in NSUFG layers while no crack in coarse grain IF steel, and (5) the subsequent layer appears in the reaction between pseudo-z-phase and NSUFG layers former after long annealing by the blast-like break of the interface.