Interstitial Free Steel Sheets Research Articles

Multiaxial tube expansion test method for measurement of sheet metal deformation behavior under biaxial tension for a large strain range

A servo-controlled multiaxial tube expansion testing machine was developed to measure the multiaxial plastic deformation behavior of sheet metals for the 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 axial force and internal pressure, in addition to having a novel strain measurement apparatus for tubular specimens. Tubular specimens with an inner diameter of 44.6mm were fabricated from 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 (FLC) and forming limit stress curve (FLSC) of the as-received sheet material, in addition to contours of plastic work and directions of plastic strain rates. Results calculated using the Yld2000-2d yield function with an exponent of 6 provided the closest agreement with the measured work contours and directions of plastic strain rates for an equivalent plastic strain range of 0.005–0.36. Moreover, a Marciniak–Kuczyński-type forming limit analysis was performed using the most appropriate yield function with the assumption of isotropic hardening; the calculated and measured FLC and FLSC were in fair agreement. Therefore, the multiaxial tube expansion test is effective to measure the multiaxial deformation behavior of sheet metals for a large range of plastic strain.

Effect of texture on grain growth in an interstitial-free steel sheet

Abstract The effect of crystallographic texture on grain growth was studied in two samples from an interstitial-free steel sheet with two different primary recrystallization textures, one with a pronounced {111} fiber texture and the other with an almost random texture. Upon grain growth annealing, the sample with the random texture showed slow continuous grain growth. In the strongly textured material discontinuous grain growth prevailed at a higher growth rate. The effect of texture on the rate of grain growth is discussed in terms of the motion of the grain boundary triple junctions in the two samples with different distributions of grain-to-grain misorientations. Numerical simulations with a 2D vertex model confirmed the influence of the triple junctions on the development of microstructure and texture observed during grain growth in the two differently textured steel samples.

Bilinear Coffin–Manson Relationship in Thin Sheets of Interstitial-Free Steel

Bilinear Coffin–Manson (C-M) as well as cyclic stress–strain (CSS) relationship is observed during low-cycle fatigue study (strain amplitude ∆e t/2 = 0.0015 to 0.004) of an interstitial-free (IF) steel sheet unlike some of the earlier reports. In this work, an attempt has been made to correlate the observed bilinearity with the evolution of dislocation substructure and the nature of cyclic strain hardening in the selected steel. To achieve this goal, some of the low-cycle fatigue (LCF) tests were interrupted after the elapse of 2, 5, 10, 20, and 50 pct of fatigue life, and the microstructures at various stages were examined using TEM. Cyclic hardening at low-strain amplitudes (∆e t/2 ≤ 0.0020) is predominantly due to dislocation–dislocation and dislocation–precipitation interaction. On the other hand, at high-strain amplitudes (∆e t/2 > 0.0020), subgrains start forming much earlier in fatigue life, and there is an additional contribution of subgrains toward the total hardening. The above phenomenon leads to a difference in the values of cyclic strain-hardening exponents, e.g., 0.24 at low (∆e t/2 ≤ 0.0020) and 0.45 at high ∆e t/2, respectively. The above difference is reflected in the bilinear C-M plot around the transition ∆e t/2 of 0.0020 as also observed in the CSS plot.

Influence of cooling rate and boron content on the microstructure and mechanical properties of hot-rolled high strength interstitial-free steels

A pilot hot strip rolling and cooling test that simulates an actual hot strip rolling and continuous cooling process was performed. We then examined the effect of cooling rates ranging from 0.1 °Cs−1 to 100 °Cs−1 on the microstructure and mechanical properties of high strength interstitial-free (IF) steels containing 0.003 wt% of boron, 0.0005 wt% of boron and no boron. The mechanical properties and microstructures of the boron-added high strength IF steels were analyzed using uni-axial tensile test and electron back-scattered diffraction (EBSD) following the pilot hot strip rolling and cooling test. Results show that the microstructure of high strength IF steel with no boron is influenced significantly by cooling rates. There is a critical cooling rate for building up polygonal ferrite (PF) grains. PF grains do not occur when high strength IF steels with a boron content of 0.0005 wt% and 0.003 wt% undergo a cooling rate of 5.0 °Cs−1, however widmanstatten ferrite (WF), granular ferrite (GF) and quasi-polygonal ferrite (QF) grains are present. Under the same hot rolling and slow cooling conditions, high strength IF steel with no boron has recrystallized PF grains. On the contrary, high strength IF steel with boron cooled at above 3 °Cs−1 doesn’t have GF or QF grains, and subsequently generates the unrecrystallized ferritic grains and WF grains, which increase the yield and tensile strengths. It is deduced that we need to control both the cooling rate and coiling temperature when boron-added high strength IF steel sheet is manufactured in an actual hot strip rolling mill.

Weldability of electrogalvanized versus galvanized interstitial free steel sheets by resistance seam welding

In this research, joining of interstitial free (IF) steel with two kinds of galvanized and electrogalvanized coatings was investigated using resistance seam welding in an automotive fuel tank. In this welding process, heat was generated by resistance against the flow of current; then the joint was made by simultaneous application of pressure. Therefore, the most important factors in this process were welding current, welding time, welding speed and electrode force. This research, was based on the effects of two parameters, welding current and speed on microstructure and mechanical properties (shear-tensile strength and hardness) of the weld joint. The results, showed the decrease of nugget size and increase of joining zone thickness in each galvanized and electrogalvanized sheet by raising welding current at low speeds. With the increase of welding speed in the constant current, nugget size decreased and thickness of joining zone increased. In each galvanized and electrogalvanized sheet, hardness grew from the base metal towards the center of the weld. Therefore, the maximum hardness always was in the center of the weld. In a comparative mode and constant current of 23,600A, the electrogalvanized sheet needed more welding speed for obtaining approximately equal strength to the galvanized sheet. Also, at equal speed, the electrogalvanized sheet needed lower welding current in order to gain approximately equal strength to the galvanized sheet. With increasing the welding current, grain size increased and as a result, the brittle fracture increased.

CO2 laser welding of interstitial free galvanized steel sheets used in tailor welded blanks

Recently, laser welding technologies have been widely utilized to weld different automobile panels. In this research, the laser beam welding (LBW) process of interstitial free (IF) steel sheets used in the manufacturing of the car body was investigated on the basis of mathematical models. The quality indexes of LBW joints were estimated from Erichsen Cupping Test results including strength and Erichsen Cupping Index. Furthermore, three process parameters, namely laser power (P), welding speed (S), and focal position (F) were considered as the factors influencing the quality indexes. A 2.2-kW CO2-laser beam was utilized to weld 1.2- and 0.8-mm-thick IF steel sheets. The modeling is done using experimental data which were gathered using design of experiments approach based on central composite face centered design matrix. The final regression models can be used to predict the quality indexes of laser beam-welded IF steel sheets joints at 95% confidence level. Optical metallography was utilized to characterize the weld profile and microstructures. In the second phase of this research, multi-objective genetic algorithm with the fitness function based on regression models was employed as an optimization procedure; as a result, the best quality indexes were obtained. Optimization results showed high compatibility with the actual experimental data.

Forming Limit Diagram Prediction of Tailor-Welded Blank Using Experimental and Numerical Methods

The forming limit diagram (FLD) is a useful method for characterizing the formability of sheet metals. In this article, different numerical models were used to investigate the FLD of tailor-welded blank (TWB). TWBs were CO2 laser-welded samples of interstitial-free (IF) steel sheets with difference in thickness. The results of the numerical models were compared with the experimental FLD as well as with the empirical model proposed by the North American Deep Drawing Research Group. The emphasis of this investigation is to determine the performance of these different approaches in predicting the FLD. These numerical models for FLD are: second derivative of thinning (SDT), effective strain rate (ESR), major strain rate (MSR), thickness strain rate (TSR), and thickness gradient (TG). Results of this research show necking will be happened, when the value of MSR, TSR, ESR criteria is maximum, TG ≤ 0.78 and SDT criterion has the first peak in forming process time. The value of dome height of TWB samples at failure was predicted based on the numerical models for samples with different widths. These numerical predictions were compared with the experimental results. The SDT model indicates a better agreement with experimental results in prediction of both the FLD and the limit dome height (LDH) in comparison to the other numerical models. Both numerical and experimental results show that minimum of LDH is happened in plane strain condition.

Impact toughness of spot welds on prestrained interstitialfree steel sheets

A series of impact tensile tests of spot welded joints made on interstitialfree steel sheets have been carried out using a fabricated fixture. Spot weldedsamples were made from as received as well as prestrained steel sheets withthe aim of examining the effect of prestrain on the impact tensile toughnessof spot welds. The impact tensile toughness values of spot welds were foundto be higher than their corresponding quasi-static toughness determined usingtensile shear specimens. While the quasi-static toughness of spot welds decreaseswith increasing prestrain, the impact tensile toughness exhibits an oppositetrend with prestrain within the range of investigation. The observed resultshave been discussed with considerations of specimen configurations, test procedure,microstructure, substructure, hardness profiles along the welds and post-failureexaminations.

Comparison of Gradients in Orientation and Stress between Experiment and Simulation

We examine the relationship between local gradients in orientation, which are quantified with the Kernel Average Misorientation, and the grain boundary network in an interstitial-free steel sheet, before and after 12% tensile strain. A portion of the unstrained microstructure is used as input to a full-field spectral viscoplastic code that simulates the same deformation. The orientation gradients are concentrated near grain boundaries in both experiments and simulation. Mapping out stress gradients in the simulation suggests that the development of orientation gradients is strongly correlated with such gradients.

Texture Evolution of IF-Steel Sheets Processed by Equal-Channel Angular Sheet Extrusion (ECASE)

Interstitial-Free steel (IF-steel) sheets were severe plastically deformed using a continuous equal-channel angular extrusion/pressing technique called “Equal-Channel Angular Sheet Extrusion (ECASE). After processing, texture development as well as microstructural alteration and tensile properties were investigated. The microstructural investigations revealed that the processed sheets exhibited a dislocation cell and/or subgrain structures with mostly low angle grain boundaries. It was also observed that the strength of the processed sheets increased substantially after ECASE processing in the expense of ductility. It was shown that the ECASE has moderate influence on the texture of IF-steel sheets through route A. Intially there was θ partial fiber which changes to {110}θ with straining.

Henry Granjon Prize Competition 2010 Winner Category a: “joining and Fabrication Technology” Study On Friction Stir Spot Welding Of Dual-Phase High-Strength Steel Sheets

Friction stir spot welding (FSSW) was applied to interstitial free (IF) steel sheet and dual-phase high-strength steels for automobiles with an uncoated Si3N4 tool. The tool with TiN-coating and Ar shielding gas were also attempted. Defect-free joints were successfully obtained. Microstructure distribution of the joint and its evolution during FSSW were systematically examined as well as joint strength. The joint exhibited the heterogeneous microstructure distribution, which could be mainly explained by phase transformation from the different peak temperatures during FSSW. However, contaminations of oxygen, silicon and nitrogen were detected in an elliptical region observed near the pin hole, and strongly affected the microstructural evolution. Additionally, they caused the increase in hardness in the elliptical zone. On the other hand, by using coated tool and shielding gas, contaminations and hardness increase in elliptical region were suppressed and joint strength was improved.

Amorphous Fe 1−xC x coatings as carbon reservoirs for diffusion strengthening of steel sheets

In situ transmission electron microscopy–electron energy loss spectroscopy heating experiments were carried out to study the composition, structure and thermal stability of physical-vapour-deposited amorphous-Fe 1−x -C x films. Compositionally graded interstitial-free steel sheets were then produced by diffusion annealing specimens coated with thin layers of amorphous-Fe 0.4 C 0.6 . After annealing at 425 °C, tensile samples exhibited a + 86 MPa increase in yield strength and + 41 MPa in ultimate tensile strength with no significant reduction in elongation compared to the unannealed condition. Furthermore, specimens annealed at 425 °C did not show any yield point elongation.

The modeling and process analysis of resistance spot welding on galvanized steel sheets used in car body manufacturing

In this study, the resistance spot welding (RSW) process of the galvanized interstitial free (IF) steel sheets and galvanized bake hardenable (BH) steel sheets, used in the manufacturing of car bodies, has been modeled and optimized. The quality measure of a resistance spot welding joint is estimated from the tensile–shear strength. Furthermore, four important process parameters, namely welding current (WC), welding time (WT), electrode force (EF), and holding time (HT) are considered as the factors influencing the quality of the joints. In order to develop an accurate relationship between the process inputs (4-component vectors) and the response output (tensile–shears strength) at first a linear regression model was utilized but the residuals analysis revealed a non-linear behavior. Therefore, an artificial neural network (ANN) was proposed because the ANNs are capable of mapping the non-linear systems. A back propagation neural network model was developed to analyze RSW process and the interaction effects of the parameters. In the second phase of this research, Genetic Algorithm with the fitness function based on an ANN model was employed as an optimization procedure for determining a set of process parameters; as a result, the maximum joint strength was obtained. Optimization results showed high compatibility with the actual experimental data.

Texture evolution of monolithic-phase and dual-phase steel sheets during a deep-drawing process

Interstitial-free high-strength steel (IF-HSS) sheets consisting of a monolithic α-ferrite phase and dual-phase (DP) steel sheets consisting of α-ferrite and hard martensite phases were used to investigate the evolution of texture during a deep-drawing process. Electron back-scattered diffraction (EBSD) was used for microtexture analysis at different heights from the bottom of a deep-drawn cup. EBSD analysis revealed that a stable orientation depends on the measurement heights and direction with respect to sample symmetry. The volume fraction of a stable orientation was lower in a DP steel sheet than in an IF-HSS sheet. Rotation rate maps and the orientation stability were used to calculate the kinematic stability of the initial texture components during the deep-drawing process. The stability parameters and the kernel average misorientation (KAM) distribution explain why the volume fraction of the stable orientation is lower in the DP steel sheet than in the IF-HSS steel sheet.

Structural Integrity of Spot-Welds Used for Auto-Body Fabrication

The effects of weld nugget size, base metal strength, mode of loading and pre-strain of base metal on the load bearing capacity of spot-welds have been investigated using two varieties of interstitial free steel sheets. The effect of bake hardening treatment after spot-welding of prestrained sheets has also been examined using an ultra low carbon bake hardening grade steel. The major results infer that: (i) the load bearing capacity of spot-welds increases with increasing nugget size and base metal strength, (ii) the load bearing capacity of spot-welds increases with increasing pre-strain of base metal, (iii) the remnant dislocation density around the spot-weld increases with increasing pre-strain, (iv) the strength of spot-welds on pre-strained sheets does not alter significantly after the baking treatment, but baking treatment increases the strength of pre-strained base metals, and (v) the location of failure is commonly at the interface of HAZ/base metal for tensile-shear specimens and at the HAZ for cross-tension specimens.

Equal-channel angular sheet extrusion of interstitial-free (IF) steel: Microstructural evolution and mechanical properties

Abstract Interstitial-free steel (IF-steel) sheets were processed at room temperature using a continuous severe plastic deformation (SPD) technique called equal-channel angular sheet extrusion ( ECASE ). After processing, the microstructural evolution and mechanical properties have been systematically investigated. To be able to directly compare the results with those from the same material processed using discontinuous equal channel angular extrusion, the sheets were ECASE processed up to eight passes. The microstructural investigations revealed that the processed sheets exhibited a dislocation cell and/or subgrain structures with mostly low angle grain boundaries. The grains after processing have relatively high dislocation density and intense micro-shear band formation. The electron backscattering diffraction (EBSD) examination showed that the processed microstructure is not fully homogeneous along the sheet thickness due probably to the corner angle of 120° in the ECASE die. It was also observed that the strengths of the processed sheets increase with the number of ECASE passes, and after eight passes following route-A and route-C, the yield strengths reach 463 MPa and 459 MPa, respectively, which is almost 2.5 times higher than that of the initial material. However, the tensile ductility considerably dropped after the ECASE. The limited ductility was attributed to the early plastic instability in the tensile samples due to the inhomogeneous microstructure. The specimen orientation with respect to the ECASE direction did not have a considerable effect on the stress–strain response. Appropriate low temperature annealing of ECASE-processed IF-steel resulted in a good strength–ductility balance.

Effects of Finish Rolling Temperature on Microstructure and Mechanical Properties of Ferritic-Rolled P-Added High Strength Interstitial-Free Steel Sheets

The steels were rolled at 3 different finishing temperatures. The mechanical properties were tested by tensile tests. The results show that as finish rolling temperature decreases from 620 to 560 °C in ferrite region, the deep drawability of ferritic-rolled P-added high strength IF steels is improved, and r value rises from 1.14 to 1.37. Finish rolling temperature (FT) has less influence on other mechanical properties, such as yield strength, tensile strength and elongation. Microstructures of hot rolled and annealed steel sheets and precipitates of annealed steel sheets were also analyzed.

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.

Morphology and texture of high speed galvanized coatings on interstitial free steel sheet

The morphology and texture of zinc coatings deposited on interstitial free (IF) steel sheets were investigated by means of scanning electron microscopy (SEM) and orientation distribution function. It was shown that the microstructure of the coatings consisted of thin hexagonal platelets tilted with respect to the substrate surface. Zinc coatings exhibited low angle pyramidal {11.5} nonfiber texture component resulting from epitaxial growth via two-dimensional (2D) nucleation. The 2D nucleation was attributed to severe zinc hydroxide adsorption on the substrate surface during the nucleation stage, which can inhibit three-dimensional (3D) nucleation and promote nonfiber texture. The pyramidal texture was beneficial for plastic deformation of zinc coatings because a significant amount of resolved shear stress can be obtained when the uniaxial stress is applied.

Study of Recrystallization in Interstitial Free (IF) Steel by Ultrasonic Techniques

The article highlights the correlations between the ultrasonic parameters like attenuation and velocity and those of materials parameters like plastic strain ratio r-Bar, texture, grain shape and size, etc. in cold-rolled interstitial free (IF) grade steel sheets with 79% thickness reduction and annealed at 650°C for different soaking times. The ultrasonic parameter, attenuation was utilized to describe the grain details including stress condition, since it was found to be well correlated with the Larsen Miller Parameter (LMP), whereas a parameter K (which is a function of longitudinal and polarized shear waves), normal ultrasonic longitudinal wave velocity, and ultrasonically measured Poisson's ratio from the longitudinal as well as polarized shear waves were utilized to describe the materials parameters like plastic strain ratio r-Bar, recrystallization, and texture. The data from ultrasonic normal longitudinal waves and ultrasonically measured Poisson's ratio could be fitted in Avrami's equation for the nucleation and growth of the recrystallized phase with an exponent 7. An empirical equation was derived to correlate ultrasonic attenuation with grain size, grain aspect ratio, and x-ray intensity ratio between the (222) and (002) crystallographic planes; 15 MHz normal probe was used for generating longitudinal whereas 20 MHz delay line probe was used for generating polarized shear waves.