Interstitial Free Steel Sheets Research Articles

Thermodynamic Analysis of Precipitation Behaviors in Hot-Rolled Ti-stabilized Interstitial-Free Steel Sheets

Thermodynamic Analysis of Precipitation Behaviors in Hot-Rolled Ti-stabilized Interstitial-Free Steel Sheets

Corrosion Resistance of Interstitial Free Steel and Mg Alloys Sheets Joined by Mechanical Clinching

Corrosion Resistance of Interstitial Free Steel and Mg Alloys Sheets Joined by Mechanical Clinching

Rapid Strengthening of Interstitial Free Steel Using Amorphous FeC Thin Films and Induction Heating

A new process to rapidly obtain high-strength interstitial free (IF) steel was investigated. Thin sheets of IF steel were coated on one or both sides with an amorphous FeC film and subjected to a two-step induction heating cycle (1100 °C followed by an isothermal hold at 780 °C for 2 or 4 min) and a rapid quench in water. Tensile mechanical properties were measured, and a yield stress of 374 MPa and an ultimate tensile strength of 448 MPa were achieved after 2 min of induction heating. After 4 min of induction heating, the yield stress and the ultimate tensile strength drop at 206 and 320 MPa, respectively. During tensile testing, the specimens induction heated for 2 min show Lüdering, which is suppressed when the induction heating is extended to 4 min. Vickers microhardness measurements through thickness confirm that higher mechanical properties are obtained after 2 min of induction heating. Transmission electron microscopy reveals that strengthening results from dislocations, carbon in solid solution, and the precipitation of nanosized TiC particles. A fine microstructure with an average grain size of 15 μm is preserved after the induction heat treatment.

Improvement of the strength-ductility-toughness balance in interstitial-free steel by gradient microstructure

High strength-ductility-toughness is required for the usage of interstitial-free (IF) steel sheets in the automotive industry. However, strength and ductility are contradictory. In the current research, gradient microstructure is produced to enhance the strength of interstitial-free steel without compromising ductility and toughness. Interstitial-free steel sheet with gradient microstructure was fabricated via accumulative roll bonding (ARB) followed by annealing treatment. Mechanical measurements (tensile and hardness tests) were conducted to study the influence of annealing temperature on the properties. Compared with the ac-received interstitial-free steel sheet, the sample with gradient microstructure exhibited enhancing strength-ductility-toughness balance. By increasing the temperature of the annealing treatment, the strength and hardness were decreased while the ductility and toughness were increased. The ultimate tensile strength, yield strength, and toughness of the annealed sample at 700 °C were remarkably improved (212.7 MPa, 391.1 MPa, and 147.1 J/cm3, respectively) without sacrificing the ductility (40.8%) as compared to the as-received interstitial-free steel sheet. With increasing the annealing temperature, the quality of bonding between layers had been improved and the interface debonding decreased. After the annealing at 700 °C, the interface debonding and delamination induced by the accumulative roll bonding had been invisible.

Work hardening under shear and tensile deformation modes

The flow and work hardening behaviour of pure copper (FCC), interstitial-free steel (BCC) and AZ31 magnesium (HCP) sheets were investigated under tensile and shear deformation modes. The sheets were deformed via tensile and shear punch testing (SPT) at room and 0.4Tm temperatures. The work hardening rate of sheet metals quantified by the Kocks–Mecking modelling approach was found to be significantly lower in the shear mode than in tension. Taylor factor values for the deformed samples were computed from the measured textures after tensile and shear tests to reveal the role of textural evolution on the work hardening rate. The difference in the work hardening under two strain paths was proposed to be associated with the lower Taylor factor values.

Investigation of Streaky Marks Formed on the Coating Surface of Hot-Dip Galvannealed Steel Sheets

Abstract Interstitial-free High strength (IF-HS) steels, the tensile strength ranging from 340MPa to 440MPa, are widely used in automobile industry. The major strengthening mechanism employed to increase the tensile strength of ultra-low carbon steels is solution hardening by the additional elements of P, Mn, Si, Cr and Cu. It has been well known that the surface condition of steel sheet substrates before dipping into a molten zinc pot significantly affects the final products quality of galvannealed (GA) steel sheets, i. e., the occurrence of the visual defects so-called “streaky mark” and others in GA process are strongly related to the surface enrichment of oxides in terms of the chemical composition of the steel sheet substrate. In the present study, high strength interstitial-free steel sheets with a relatively high amount of Mn and P were galvannealed in an industrial continuous galvanizing line and streaky-marks formed on the coating surface were characterized by optical microscopy, scanning electron microscopy, electron probe micro analysis and transmission electron microscopy. Effects of streaky marks on the mechanical properties and paintability of GA coating were also investigated.

Resistance spot welding of high-strength DP steel and nano/ultrafine-grained IF steel sheets

In the current study, microstructural characteristics, mechanical behavior, and failure analysis of resistance spot welded joints for high-strength dual-phase (DP) steel and nano/ultrafine-grained interstitial free (IF) steel was investigated. The DP steel side indicated three different heat-affected zones including intercritical, fine-grained, and coarse-grained. The volume fraction of martensite in the coarse-grained heat-affected zone was higher than in other regions due to more growth of γ grains. The IF steel side exhibited three different subregions in the heat-affected zone, namely, ultrafine-grained, fine-grained, and coarse-grained heat-affected zone. In the coarse-grained heat-affected zone, the iron oxide was effective on the microstructure by suppressing the grain growth. The uniformity of microhardness in the fusion zone of 10 kA welding current was more than other samples due to faster homogenization of melt. All welded samples indicate hardening at fine-grained heat-affected zone (on both sides) and ultrafine-grained heat-affected zone (on the interstitial free steel side). With increasing the current of welding from 6 kA to 10 kA, the peak load increased from 7.38 kN to 11.23 kN and the energy absorption improved from 6.01 kJ to 11.72 kJ due to increasing the diameter of the fusion zone. All samples failed through partial thickness-partial pullout mode. Cleavage facets and shear dimples were observed in all samples, which indicated a mixture of brittle and ductile fractures.

Effect of Pre-Strain and Strain Rate on Deformation and Fracture Behavior of Automotive Grade Interstitial Free Steel Sheets

Effect of Pre-Strain and Strain Rate on Deformation and Fracture Behavior of Automotive Grade Interstitial Free Steel Sheets

An equivalent work-hardening description of an interstitial-free steel sheet based on uniaxial tensile and hydraulic bulge tests

In this work, the plastic behavior of an interstitial-free steel sheet was first evaluated by standard uniaxial tensile and hydraulic bulge tests to reproduce the intermediate and high straining levels commonly observed in sheet metal forming processes. Next, a simple procedure was proposed to obtain an equivalent work-hardening curve, representing the high strain domain instead of the conventional uniaxial tensile data. This procedure involves an anisotropy parameter calculated based on Hill's quadratic criterion using the measured r-values in the rolling and transverse sheet directions. Under the assumption of an isotropic work-hardening, the equivalent curve is defined from the true stress and true thickness plastic-strain results of the hydraulic bulge test. The transformed data were fitted to usual power-law type equations to obtain an adequate work-hardening description for finite element numerical simulation purposes. Based on the goodness-of-fit measures, namely, R-squared, root-mean-square error, and root-mean-square of residual percentage, the combined Swift–Hockett–Sherby equation has proved to be the most suitable choice to describe the plastic behavior of the investigated interstitial-free steel under both intermediate and high strains conditions.

Characterization of Ductile Crack Growth Resistance Behaviour of Interstitial-Free Steel Sheet using Energy Dissipation Rate Parameter

The present study aims at charactering the crack growth resistance parameter in an 1 mm thick interstitial-free (IF) steel sheet using the energy dissipation rate parameter, R. The R-Δa plots of single-edge and double-edge notched tension specimens have been determined with only additional computations of mechanical and optical test data already generated in the course of determining the variation of crack tip opening angle with Δa following the optical method of ASTM E 2472. A new method has been adopted for determining the stationary value of energy dissipation rate, R∞, This has been done by analyzing mechanical test data generated in the course of essential work fracture (EWF) testing, with additional inputs on crack initiation points identified in those EWF tests. An attempt has been made to normalize the R-Δa plots in order to reduce the specimen geometry dependence of the energy dissipation results.

Study of Corrosion Behavior in Resistance Spot Welds of Thin Sheets of Zinc-Coated Interstitial-Free Steel

The aim of this research is to investigate the corrosion behavior of the resistance spot-welded joints of the zinc-coated interstitial free steel (IF steel) in contrast to the steel. For this purpose, the corrosion behavior of the welds and base interstitial-free steel (IF steel) was investigated in two solutions including 0.1 M NaCl + 0.1 M NaOH and 0.1 M NaCl + 0.1 M H2SO4. Potentiodynamic polarization curves and electrochemical impedance spectroscopy test results showed that the corrosion resistance of the base metal and welds in 0.1 M NaCl + 0.1 M NaOH solution is higher than their corrosion resistance in 0.1 M NaCl + 0.1 M H2SO4 solution. It was found that the corrosion resistance of the IF steel base metal in both solutions is higher than that of the welds obtained by the steel. According to the cyclic polarization curves, it can be said that the passive film formed on the specimens in 0.1 M NaCl + 0.1 M NaOH solution is highly susceptible to pitting corrosion while the specimens do not show any passivation behavior in 0.1 M NaCl + 0.1 M H2SO4 solution and undergo severe uniform corrosion.

Analysis of the pre-straining effects on the limit strains of interstitial-free steel: experiments and elasto-plastic modeling of strain and stress-based forming limit curves

In this work, the limit strains under as-received and pre-strained conditions of an interstitial-free steel sheet were investigated by mechanical tests and numerical modeling. An original testing procedure was proposed to reproduce two equivalent pre-straining amounts under equibiaxial stretching (EB-PS) and uniaxial tension (UT-PS) deformation modes. The limit strains were defined from Nakajima’s hemispherical punch tests using digital image analysis according to the ISO 12004:2 standard. The plastic behavior of the interstitial-free steel sheet was evaluated by uniaxial tensile, disk compression, and hydraulic bulge tests. Moreover, an elasto-plastic framework based on the Marciniak–Kuczynski (M–K) model is proposed to forecast both the strain-based and stress-based forming limits. The parameters of the Yld-2000-2d yield criterion were identified using the plastic anisotropy coefficients with the flow-stress values determined from two distinct amounts of plastic-work per unit volume. For both deformation modes and pre-straining levels defined by the von Mises effective plastic-strain (4.8% and 9.0% EB-PS; 5.0% and 10% UT-PS), the transformed polar effective plastic-strain (PEPS) and forming limit stress (FLSC) results shown to be more strain-path insensitive than the conventional strain-based forming limit curve (FLC). The proposed elasto-plastic M–K model successfully forecasted the linear strain-path limit strains defined using the FLC, PEPS, and FLSC representations. The M–K model also provided quite conservative predictions for the pre-strained PEPS and FLSC. The Yld-2000-2d parameters identified using the flow-stress values defined from an upper-level of the plastic-work per unit volume proved to be the best choice in the present M–K modeling framework.

A Study on the Resolution of Pin-dent Defects in Automotive Door Panels

The automotive customers have been down-gauging steel sheets for making door panels to reduce weight of the passenger cars and improve fuel efficiency under “Go Green” initiative. Tata Steel has been extending support in weight reduction initiative of the automakers through supplying thinner gauge interstitial free (IF) steel sheets. It was reported that in some cases, the thinner sheets could not withstand the intricate forming and stretching operation and resulted in pin-dent or press crack defect generation in deep drawn areas of the components. The defect was reported for the first time to TSL by auto customer, and in the sense, it was a unique defect. At times, the defect impacted 20–40% of deep drawn door components and led to a near line stoppage situation in the press shop. Representative samples containing the defects were collected from the site, and detailed investigation was carried out employing metallographic techniques such as scanning electron microscope (SEM) attached with energy-dispersive x-ray analysis (EDX). The investigation revealed non-metallic inclusions (NMI) to be the cause of such defects. A thorough process and data analysis was carried out to identify the sources of such inclusions and arrive at appropriate countermeasures to fix this problem. Subsequently, countermeasures were implemented in the steel manufacturing process, and such pin-dent defects were eliminated bringing much respite to the customer. The paper will highlight the systematic approach in dealing with the problem and successful resolution of the intriguing quality issue.

Modelling of spring-back in interstitial free steel sheet using Taguchi design of experiments

ABSTRACT Spring-back is one of the vital problems associated with sheet metal forming. To satisfy the customer with a high-quality product at a low cost and quick delivery, the awareness, and proper control of the spring-back after forming is a primary aspect in achieving near-net-shape products. This work aims at developing empirical models using techniques such as the Taguchi design of experiments to predict spring-back in air-bending process of the Interstitial Free (IF) steel sheet. Using the five identified independent predominant bending parameters such as punch travel, strain hardening exponent, punch radius, punch velocity and width of the sheet that has greater influences on the spring-back is used to establish the prediction models, using MINITAB v13 with the help of the experimental data. It is used effectively to analyze the cause and effect of process parameters on response and provides insights for the selection of process variables to achieve minimum spring-back.

Variation in Mechanical Properties of SAE 1006 Interstitial Free (IF) Steel Sheets During Cold Rolling

This research aims to study the variation in properties of hot-rolled SAE 1006 IF steel sheets during cold rolling to reveal the factors causing the decrease in formability of SPCG steel sheets during deep drawing. Results have shown that pickling does not affect the mechanical properties; however, cold rolling results in an increase in hardness, tensile strength and yield strength. Stress relieve annealing results in coarsening and homogenizing of grains which reduces brittleness and impart ductility. Temper rolling causes an increment in mechanical parameters again, improve surface hardness and flatness. By analyzing the microstructure and comparing it with the HESCO sample, it is revealed that there are still some elongated grains found along with coarse grains after annealing. Soaking time is not sufficient for the growth of homogenized structure, resulting in decreased ductility and formability during deep drawing.

Experimental investigation on resistance spot welding of dissimilar weld joints

In the present study, galvanized High Strength Interstitial Free (HIF) steel sheets, and Dual Phase (DP780) steel sheets were used for the investigations. Resistance spot weld joints were fabricated between dissimilar steel sheets. The variation in dynamic resistance (DR) with the change in welding process parameters such as weld current, weld time and electrode force were used for establishing the range of adequate weld nugget formation parameters. Effect of these parameters over tensile strength, nugget diameter and the observed failure mode was studied using one factor at a time (OFAT) approach. Microstructure and hardness of parent metal, fusion & HAZ region has also been studied.

Creep Deformation and Dynamic Grain Growth in an Interstitial-Free Steel

Dynamic grain growth is demonstrated to be much faster than static grain growth in a body-centered-cubic, interstitial-free steel sheet material at 850 $$\,^\circ {\rm{C}}$$ . Dynamic grain growth occurs during concurrent plastic deformation at elevated temperature, whereas static grain growth occurs during static annealing. Grain growth during steady-state plastic flow in tension at 850 $$\,^\circ {\rm{C}}$$ to a true strain of 0.2 at a true-strain rate of $$10^{-4}$$ $${\rm{s}}^{-1}$$ doubled grain size, while static annealing for the same time produced no increase in grain size. This is described as dynamic normal grain growth (DNGG) because no abnormally large grains were observed. The recrystallized microstructure of the steel demonstrated a log-normal distribution of grain sizes. DNGG produced bimodal grain size distributions that deviate from the theoretical expectation of a simple shift to larger sizes during normal growth. The bimodal distributions contained a remnant of small grains that were not consumed during grain growth. DNGG produced a crystallographic texture that is unique from both the recrystallized material and that produced by lattice rotation alone. DNGG strengthened the $$\{111\} \langle 110 \rangle $$ and $$\{111\} \langle 112 \rangle $$ components of the strong $$\gamma $$ -fiber component in the original recrystallization texture. Lattice rotation from tensile deformation, by contrast, strengthened the $$\alpha $$ -fiber components that intersect the original $$\gamma $$ -fiber.

Study of White Patch Defect in Automotive Grade Interstitial Free Steel

Surface of cold-rolled steel intended for automotive body panel applications is expected to be defect free. The presence of surface defects hampers the aesthetics and average surface roughness and thereby deteriorates the paintability of the steel sheet. This study aims to investigate the source of white patch defect in cold-rolled and skin-passed interstitial free steel sheets. Macroscopic observations of as-received white patch sample revealed the presence of blackish deposit having cracks inside the deposits, suggesting that these deposits were heated up possibly during annealing process. Further during macroscopic observation, it was revealed that the black deposits were of near round shape. If deposit is present before rolling, then it is expected that it will elongate during rolling and will not retain round shape. Round shape of the deposits under investigation suggested that it is not before rolling. Electrolytic cleaning line (ECL) is the only in-between process. Surface characterization techniques such as SEM–EDS, Raman spectroscopy, Fourier transform infrared spectroscopy were used to analyze the defect and good region of the steel sheet. Overall analysis indicated grease contamination on cold-rolled steel sheet at the entry of ECL caused generation of white patch defect. Regular cleaning and preventive maintenance of plant machinery were recommended to avoid such leakages to arrest the defect generation in the future.

Forming Limit Diagrams of Low-Carbon Steels Obtained Using Digital Image Correlation Technique and Enhanced Formability Predictions Incorporating Microstructural Developments

In the present work, drawing quality (DQ) and interstitial-free (IF) steel sheets were subjected to limiting dome height tests for determining strain-based forming limit diagrams (FLDs) experimentally and effect of dynamic (variable) material properties, such as work hardening ‘n’ and plastic anisotropy ( $$\bar{r}$$ ), on FLDs has been studied and simulated by PAMSTAMP™ finite element software. Dot prints on the steel specimens and an efficient optical strain measurement system GOM™ which works on the digital image correlation technique principle were used to measure limiting strains accurately instead of conventional circle grid analysis technique and strain measurement by traveling microscope. The dynamic (variable) material properties (n) and ( $$\bar{r}$$ ) were estimated by studying the microstructural developments in terms of changes in grain average misorientation and crystallographic texture, respectively, at different strains and strain paths, during deformation. In our proposed work, a novel technique of incorporating dynamic (variable) material properties (n and $$\bar{r}$$ ) in FE simulations was carried out during FLDs predictions. Though marginal but improved predictions in FLDs were observed in both IF and DQ steels. In addition to strain-based FLDs, stress-based FLDs were also determined for both grades. Interestingly, in both cases it was noticed that IF steel had higher formability than DQ steel.

Evaluation of ductile tearing resistance of an interstitial free steel sheet using SENT specimens

Ductile tearing resistance of a 1 mm thick interstitial free steel sheet has been characterized using single edge notched tensile (SENT) specimens with or without fatigue precracks at quasistatic strain rates. In this study, the critical Crack Tip Opening Angle (CTOA), and also crack initiation parameters, namely the initiation energy per unit ligament area (wi), critical value for the J-integral (Jc), and critical crack tip opening displacement (δc) have been examined and compared with the corresponding parameters from the Essential Work of Fracture (EWF) method. Dependence of these parameters on specimen geometry has also been examined. The EWF paradigm has been extended to the initial mixed-constraint regime of crack extension. 3D cohesive zone modelling for flat fracture becomes reasonably successful in simulating crack growth in the SENT specimens.