Interstitial Free High Strength Research Articles

Elimination of Wettability Issues in Interstitial Free High Strength (IFHS) Steel by Controlling the Annealing Atmosphere

Elimination of Wettability Issues in Interstitial Free High Strength (IFHS) Steel by Controlling the Annealing Atmosphere

Effect of heat input on corrosion behavior of automotive zinc-coated steel joint

Abstract Lately, coated steel sheets have replaced bare steel in the automotive sectors due to their corrosion protection. Among them, zinc-coated high strength interstitial free (HIF) steel is widely used due to its durability, formability, good surface finish, impact resistance, and corrosion resistance. However, joining galvannealed (GA) steel using conventional fusion welding processes is challenging as it results in porosity and potentially harmful fumes. To overcome these challenges, this work uses a pulse MIG (metal inert gas) brazing technique to join the HIF-GA steel sheet with a lap joint configuration utilizing Cu-Si base filler wire (CuSi3Mn1) in an argon atmosphere. The study aims to evaluate the effect of heat input on bead geometry and Zn degradation in MIG brazed joints by adjusting the current from 40 to 60 A and brazing speed from 400 to 500 mm min−1. It is observed that the heat input is inversely proportional to the wetting angle and directly proportional to the bead width, leg length, and cross-sectional area. Heat input influences Zn degradation on both the front (at the toe) and back sides of the lap joint. The corrosion behavior of joints is studied using an electrochemical test (in 3.5% NaCl solution) and a salt spray test (in 5% NaCl solution). Further, the Zn degradation is analyzed with EDS line scanning to understand the corrosion resistance of the MIG brazed joints. A potentiostatic polarization study is also conducted to determine the corrosion rate using a Tafel plot. It is found that the lowest corrosion rate in base metal (BM) increased from 0.88 to 12.26 mm year−1 with the increase in heat input from 61 to 113 J mm−1.

Experimental and numerical investigation of differential hardening of cold rolled steel sheet under non-proportional loading using biaxial tensile test

Strain path change is common phenomenon during industrial sheet metal forming processes. These strain paths can be proportional and non-proportional in nature. To enable strain path change in a single experiment is key challenge faced by many researchers. To predict deformation behavior accurately during such strain paths, it is required to represent initial yielding, the evolution of subsequent yield surface and hardening behavior precisely. This work deals with the mechanical characterization of Interstitial Free High Strength Steel (IFHS) during multi-axial proportional and non-proportional loading for the determination of yield surface evolution and strain hardening behavior. A set of tensile tests at various orientations with respect to the rolling direction and in-plane biaxial tests using cruciform specimens for various linear and bi-linear stress ratios were performed on as-received materials. It was found that at the onset of stress path change, the stress strain curve shows a change in the hardening rate for rolling and transverse directions of the biaxial specimen due to the change in active slip systems. To predict this behaviour various yield criterion like von Mises, Hill's 48 and Yld2000-2d were used. The experimental results show that the mechanical behavior of the subjected specimen deviates from isotropic behavior predicted by the von Mises and anisotropic behavior predicted by the Hill's 48 yield criterion. The initial yield loci measured by biaxial testing machine is in good agreement with what is predicted by Yld2000-2d yield criterion. However, it was observed that such phenomenological models were unable to capture the observed deformation behaviour. It is also shown that classical Crystal Plasticity Finite Element Method (CPFEM) can capture change in strain hardening rate during non-proportional deformation for few cases reasonably well while in few cases it could not predict.

Comparative Study of the Cleanliness of Interstitial‐Free Steel with Low and High Phosphorus Contents

Herein, a study comparatively investigating the total oxygen (TO) and nitrogen (N) contents, evolution of inclusions, variations in slag composition, and nozzle clogging during the production of an interstitial‐free (IF) steel and a P‐containing high‐strength (HS) IF steel is presented. The results show that during the production of both steels, the TO content gradually decreases after the deoxidation, with the TO content in the molten steel in the tundish below 30 ppm. There is no significant variance in the N content. The two steels have similar quantities of small inclusions of similar types and sizes. However, the P‐containing HS IF steel has a much higher content of large inclusions, which mainly come from the entrapped mold powder and peeled‐off nozzle clogging. Compared to the common IF steel, the P‐containing HS IF steel has a higher MnO content in the slag during the Ruhrstahl–Heraeus process, compromising cleanliness control; the P‐containing HS IF steel has more significant mold liquid‐level fluctuations and more nozzle clogging, which mainly consists of Al2O3 and TiO2. The cause of these issues during the production of the P‐containing HS IF steel is concluded to be related to the high P content.

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.

On the Study of Batch Annealing Parameter Optimization for Higher Lankford Value in High Phosphorus Interstitial Free High Strength Steel

IFHS grades best known for their superior strength and high formability are conventionally processed through batch annealing. However, high phosphorus contents, which are responsible for rendering the high strength property to IFHS grades, have also been proved to impair the formability of these steels, mainly because of the formation of FeTiP precipitates during batch annealing process. In the present study, two different categories of high phosphorus-containing IFHS steels, viz. (i) Ti and (ii) Ti + Nb stabilized, were subjected to batch annealing simulations targeting $$ \bar{r} $$ > 2.0. Uniqueness in the present effort lies in the usage of a custom-designed annealing simulator wherein annealing conditions akin to those prevailing in steel industry could be maintained. Processed specimens were characterized for microstructure, texture, tensile properties, and $$ \bar{r} $$ measurements. Through careful and iterative optimization of annealing parameters and physical simulations, it was possible to achieve $$ \bar{r} $$ values higher than 2.0 in both the categories of IFHS steels. The optimization of parameters was based upon maximization of γ-fiber texture along with proper recrystallized microstructure. It has been shown that soaking temperature of 710 and 740 °C leads to the achievement of $$ \bar{r} $$ > 2.0 in case of Ti-stabilized and Ti + Nb-stabilized grades, respectively.

Experimental Study of High Temperature Phase Equilibria in the Iron-Rich Part of the Fe-P and Fe-C-P Systems

During the solidification of steel, phosphorus strongly segregates in the interdendritic liquid phase. In the continuous casting process, even low levels of P may have a detrimental effect on the final product quality. However, phosphorus is partly added up to 0.10 wt pct to improve the mechanical properties of advanced steel grades nowadays, e.g., High-Strength Interstitial-Free (HSIF). To provide new experimental data for the development of thermodynamic databases and solidification models for P alloyed steel grades, phase equilibria in the Fe-P and Fe-C-P key systems were studied up to 1550 °C using differential scanning calorimetry (DSC) and high temperature laser scanning confocal microscopy (HT-LSCM). Special focus was placed on solid/liquid equilibrium temperatures in the Fe-rich part of the binary Fe-P system between 0.025 and 9 wt pct P. In the ternary system, three isoplethal sections with 0.10 mass pct. P, 0.20 mass pct. C and constant mass percent ratio P/C of 2 were investigated. In the latter section, HT-LSCM observations were linked with DSC signals to optically identify present phase stabilities. Particularly at [pct P] < 1, significant differences between performed measurements and calculated phase equilibrium temperatures using thermodynamic assessments from the literature were identified. In all ternary sections, the experiments indicate less influence of P on the hypo-peritectic range compared to the thermodynamic calculations.

Cruciform Specimen Machining Using EDM and a New Design Verification for Biaxial Testing

Cruciform flat specimen is mostly used to investigate experimentally the in-plane biaxial mechanical behavior of sheet metals. In this study, a modified cruciform specimen design is proposed and validated against the performance of current standard design. The existing standard cruciform specimen prepared through laser cutting for sheet metal can accurately provide biaxial stress–strain curve through biaxial tensile test by ensuring superior homogeneous stress and strain field in gauge section. However, preparation of such specimen using laser cut is cumbersome, expensive and prone to errors. The current standard cruciform specimen design features slotted arm of fixed length and width with specific gauge. Few other designs also feature reduced gauge section to localize failure. Both need high precision of laser cutting and machining to prepare specimens which can achieve biaxial stress–strain curve for more than 4% equivalent plastic strain level. The proposed specimen design features running slits in all four arms of specimen with uniform gauge section machined using electrical discharge machining (EDM) owing to higher accuracy and minimum heat-affected zone (HAZ). This technique also avoids machining effects with an understanding that proposed specimen design is to determine the yield behavior of material only. Also, specimen fabricated from as received sheets and without any thickness reduction in gauge area allows the user to keep full thickness in biaxial condition. The modified design qualifies the commonly accepted performance criteria for cruciform specimen, i.e., stress and strain distribution in the gauge section is found to be homogeneous and the biaxial stress–strain curve can be generated until 4% of equivalent plastic strain. The proposed specimen design is evaluated experimentally by conducting biaxial test at various stress ratios, and it is compared with alike experiments using standard specimen design. One automotive grade material, interstitial-Free high-strength (IFHS) steel sheet of thickness 0.70 mm is tested in this study to validate the specimen design.

Assessment of fracture toughness using EWF of IFHS steel

The aim of the present work is to experimentally investigate the essential work of fracture of 1 mm thick interstitial free high strength steels in cold rolled condition. Essential Work of Fracture (EWF) is done experimentally using Double edge notched tension (DENT) specimens of 140 mm in length and 25 mm in width along both rolling and transverse direction in tensile testing machine. Using a universal testing machine, the specimens were strained along their longitudinal direction until completion of cracking. Totally 23 samples with different ligament lengths are tested to obtain essential work of fracture. The specific essential work was determined by extrapolating the straight-line relationship between the specific work of fracture (wf) and ligament length (/) to zero ligament length. The magnitude of essential work of fracture (we) along rolling direction is 133.56 kJ/m2 and for transverse direction is 17.85 kJ/m2.

Effect of Rare Earth Ce on Deep Stamping Properties of High-Strength Interstitial-Free Steel Containing Phosphorus.

The influence of rare earth Ce on the deep stamping property of high-strength interstitial-free (IF) steel containing phosphorus was analyzed. After adding 120 kg ferrocerium alloy (Ce content is 10%) in the steel, the inclusion statistics and the two-dimensional morphology of the samples in the direction of 1/4 thickness of slab and each rolling process were observed and compared by scanning electron microscope (SEM). After the samples in each rolling process were treated by acid leaching, the three-dimensional morphology and components of the second phase precipitates were observed by SEM and energy dispersive spectrometer (EDS). The microstructure of the sample was observed by optical microscope, and the grain size was compared. Meanwhile, the content and strength of the favorable texture were analyzed by X-ray diffraction (XRD). Finally, the mechanical properties of the product were analyzed. The results showed that: (1) The combination of rare earth Ce with activity O and S in steel had lower Gibbs free energy, and it was easy to generate CeAlO3, Ce2O2S, and Ce2O3. The inclusions size was obviously reduced, but the number of inclusions was increased after adding rare earth. The morphology of inclusions changed from chain and strip to spherical. The size of rare earth inclusions was mostly about 2–5 μm, distributed and dispersed, and their elastic modulus was close to that of steel matrix, which was conducive to improving the structure continuity of steel. (2) The rare earth compound had a high melting point. As a heterogeneous nucleation point, the nucleation rate was increased and the solidification structure was refined. The grade of grain size of products was increased by 1.5 grades, which is helpful to improve the strength and plasticity of metal. (3) Rare earth Ce can inhibit the segregation of P element at the grain boundary and the precipitation of Fe(Nb+Ti)P phase. It can effectively increase the solid solution amount of P element in steel, improve the solid solution strengthening effect of P element in high-strength IF steel, and obtain a large proportion of {111} favorable texture, which is conducive to improving the stamping formability index r90 value.

Insitu investigation of tensile deformation behaviour of cold-rolled interstitial-free high-strength steel in scanning electron microscope

The progress of tensile deformation of cold rolled interstitial-free high-strength (IFHS) steel in the as-received and 10 pct tensile pre-strained conditions has been studied through testing of miniature sized specimens in scanning electron microscope. Strength and ductility of the steel obtained through insitu tests have been compared with the macro tensile test results. The insitu test results show that the yield strength and tensile strength of miniature sized specimens are less as compared to that of macro tensile test results. But, uniform strain in case of insitu tests is higher than that obtained in case of macro tensile specimens. The insitu tests reveal that intergranular cracking of surface grains and formation of both intergranular and transgranular voids precede the final ductile fracture. The areal density of voids is found to increase with increase of Von Mises equivalent plastic strain. Under pre-strained condition void nucleation strain of the steel is found to be lower as compared to the as-received condition.

Correlation between Lüders band formation and precipitation kinetics behaviour during the industrial processing of interstitial free high strength steels

Lüders band formation in steels is critical to surface finish during automobile panel manufacturing. This research reports on the problem of Lüders band formation in interstitial free high strength steel compositions (IFHS-steels). The study investigates the effect of chemical composition and processing parameters on the formation of Lüders bands in IFHS-steels. It correlates the problem of Lüders band formation with precipitation kinetics behaviour during the industrial processing of IFHS-steels. Four different compositions viz. Ti-stabilized, Ti-Nb stabilized, low Ti-low Nb, and high Ti-low Nb with high Al were investigated. Annealing parameters were similar to industrial practice followed for batch and continuous annealing lines in steel manufacturing plants. Stabilized IFHS-steel compositions possessing excess of stabilizing elements (Ti, Nb, etc.) for stabilization of interstitial elements (C, N) also showed the problem of Lüders band formation. The new type of IFHS composition containing high Al, investigated in this research, showed no Lüders band formation during batch annealing cycles along with adequate mechanical properties (YS: 190–202MPa; Δr¯-value: 1.57; Δr-value: 0.25). Thus, steel compositions with high Al content processed through batch annealing cycle offer a practical solution to the problem of Lüders band formation in IFHS-steels.

Approximation of initial shape of the weld line in the stamping of tailor welded blanks

Tailor welded blanks consisting of interstitial free steel and high strength dual phase steel are laser welded with non-linear welds. The influence of parameters viz. non-linearity in the weld, strength of the parent materials and variable blank holder forces on the movement of weld line were studied. It was found that the effect of non linearity in weld dominated the other parameters in restricting the weld line movement. It was also noticed that the height of the formed cup obtained was comparatively more in non-linear weld than a straight weld.

Texture characteristics of a warm rolled 0.085 wt%-P interstitial free (IF) steel with and without lubrication

A 0.085 wt% P micro - alloyed and Ti-stabilized high strength IF (interstitial free) steel was warm rolled under two conditions: One with lubrication and the other without. The results show that lubrication has a significant effect on texture evolution during warm rolling and the subsequent annealing process. Without lubrication, severe shear deformation occurs in the surface layer, which results in recrystallization and the formation of a //ND (normal direction) texture, while the central layer undergoes plane strain deformation and forms a weak //ND deformation texture (typical of bcc iron). With good lubrication, both the surface and central layer experience plane strain deformation and a strong //ND texture is normally formed. The warm rolling texture (which contains both recrystallized and deformed grains) has a profound effect on the subsequent annealing texture. The final textures influence the draw - ability of the plates, i.e. the r - value (r-value: The ratio of the true width strain to the true thickness strain in a sheet tensile test) and |Dr|-value (planar anisotropy coefficient). After annealing, the lubricated plates are characterized by a higher r - value (1.27) and a much lower |Dr|-value (0.075) than those rolled without lubrication (i.e. 0.96 for the r - value and 0.465 for the |Dr|-value).

Microstructures and strengthening mechanisms in high-carbon titanium-microalloyed interstitial-free steels

ABSTRACTWe report the mechanical properties and microstructures of titanium-microalloyed interstitial-free (IF) steels with different carbon contents smelted and rolled in the laboratory. The results show an increase of yield strength and tensile strength with carbon content, which are enhanced with lower coiling temperature (CT). Microstructure analysis reveals that the ferrite grain size varies from 11 to 23 μm, while the average size of TiC as the main second-phase particle ranges from 10 to 35 nm. The sample with 80 ppm carbon and 853 K CT reaches a mechanical strength close to that of high-strength IF steels. The increase of yield strength is mainly attributed to both grain-refining and precipitation strengthening.

A Study on Microstructure, Texture and Precipitation Evolution at Different Stages of Steel Processing in Interstitial Free High Strength Steels

The formability characteristics of interstitial free high strength (IFHS) steels are significantly governed by processing parameters of steel at consecutive processing stages, chiefly hot rolling of slabs followed by cold reduction of hot rolled coils and finally annealing treatment of cold reduced coils. The present work focuses on investigating the steel processing parameters for a chosen set of processing parameters which in turn influence the final formability of the steel. The underlying objective is to study the effect of steel processing parameters, which in turn favorably control texture, microstructure, precipitate morphology and its distribution at each step to impart desired properties in the steel sheet in the end. In the current work, an IFHS steel chemistry is chosen with an objective to evaluate the contribution of texture, microstructure and precipitates generated at each processing step to the formability in relation with the chosen processing parameters.

Precipitation Behavior of FeTiP in Ti-added Interstitial Free High Strength Steels

The precipitation behavior of FeTiP in interstitial free high strength (IFHS) steels has been studied by using a transmission electron microscope (TEM). The results show that the TiC particles, appearing at earlier stage, are more stable than the FeTiP ones during recrystallization annealing at the two given temperatures (810 °C and 840 °C). Therefore, the FeTiP particles can only be observed in the steels with sufficient amount of Ti. There is a critical forming time for the FeTiP, which is between 90 – 120 s at 810 °C and 60 – 90 s at 840 °C. The precipitation of FeTiP involves two steps, i.e. the formation of FeTi precursors and the diffusion of P. The formier step determines the reliance of Ti content for the precipitation of FeTiP, whereas the latter step leads to the difference in the critical annealing time.

Ductile Behaviour Characterization of Low Carbon Steel: a CDM Approach

In this paper, the ductile behaviour of two different low carbon steels, C-Mn-440 and interstitial free high strength (IFHS), is presented using a continuum damage mechanics (CDM) approach. The damage growth law is adopted to predict the ductile response of the specified materials. Cyclic load-unload tensile tests in combination with standard uniaxial tensile tests helped to estimate the necessary parameters: damage variable, D, fracture stress, σf, threshold damage strain, e0, and strain hardening exponent, n, required to apply the model. The strain hardening exponent estimated from the cyclic test data is used to predict the damage variable, D. Increase of damage shows deterioration of the hardening exponent magnitudes varying nonlinearly. The simulated flow curve by the damage variable, D, corresponding to the loadunload test is observed to approximate the experimental true stress-true strain curve very closely up to the onset of necking for both the materials. The experimental values of D, as obtained for C-Mn-440 and IFHS steels, vary from 0.10 to 0.44 and 0.09 to 0.45, respectively. The critical damage parameters, DC, for the considered materials are 0.44 and 0.45, representing their good ductile response.

Digital image correlation for grain scale strain measurement in interstitial free high strength steel

Deformation measurement of materials and structures subjected to various loading conditions is an important task of experimental solid mechanics. Apart from the widely used point wise strain gauge technique, various full field non-contact optical methods are used for this purpose. In this work, an automated scheme to measure the grain level deformation in tensile deformed interstitial free high strength steel has been introduced. The method is based on digital image correlation technique. The proposed scheme utilised high resolution scanning electron images of the specimen surface that are sequentially captured during tensile loading. It is found that grain to grain there is large variation in deformation response at a given load. The present work also reveals point to point variation of strain within a grain interior.

High Temperature Mechanical Properties of P-Containing High Strength IF Steel

The high temperature mechanical properties of P- contained high strength interstitial free(IF) steel were investigated using Gleeble3500 thermal simulation testing machine, in terms of zero ductility temperature(ZDT), zero strength temperature(ZST) and the relationship of tensile strength with temperature. The fractured surfaces at different tensile temperatures were characterized by means of scanning electron microscope. The phase transition temperature interval was deduced by the THERMOCALC software. The results show that the ZDT and ZST of the steel are 1420℃ and 1445℃, respectively. The brittleness temperature interval Ⅰ is from 1400℃ to the melting point, and there is no the brittleness temperature interval Ⅲ. The transverse cracks on the surface of casting blank did not occur during the straightening process. The tensile strengths decrease with the increasing temperature in the test temperature range, and which are lower than 5.3 MPa above 1300℃. The result of THERMO-CALC calculation shows that, with the specimens cooling from 500℃ to 400℃, Fe3 P precipitates out in the α-Fe matrix,which may results in cold short. The transverse cracks on the continuous casing(CC) slab could dramatically be reduced by Hot-charging Technology.