High Strength Interstitial Free Steel 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.

Effect of Al addition on the fatigue properties of interstitial-free steel and interstitial-free high-strength steel grades

Effect of Al addition on the fatigue properties of interstitial-free steel and interstitial-free high-strength steel grades

Comprehensive Observations and Interpretations in Al-Rich Interstitial-Free High-Strength Steel via Process-Induced Structure Evolution

Comprehensive Observations and Interpretations in Al-Rich Interstitial-Free High-Strength Steel via Process-Induced Structure Evolution

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.

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.

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.

Enhancements in the Bonding Properties of a Friction Stir Lap-Welded Interstitial Free Steel and Al Alloy by Introducing a Ni Interlayer

A pure Ni interlayer with a thickness of 0.1 mm was introduced between high-strength interstitial free steel and Al–Mg–Si alloy, which were friction stir lap welded, producing an excellent welded joint. The interface layer consisted of a γ-Ni solid solution, and the mixed stirring zone contained alternate lamellae of γ-Ni and α-Fe solid solutions. The addition of a Ni interlayer strongly suppressed the reaction between Al and Fe because of the atomic arrangement of Ni. Furthermore, the insertion depth of the stirring pin has a significant influence on the Al/steel interfacial reaction. Under shallow insertion depth, the intermetallic compounds of both FeAl and Fe2Al5 were observed at the interface layer. A maximum tensile-shear fracture load of 4.3 kN was achieved, with fractures being present in the steel substrate far away from the Al/steel weld.

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.

Precipitation behaviour of phosphorus in high strength IF steel during laminar cooling

ABSTRACT In order to study the precipitation behaviour of phosphorus-containing precipitates in the rolling and cooling process of phosphorus-containing high-strength interstitial free steels, and to solve the problem of the tensile-straightening broken belt in the actual production process, the GLEEBLE-3500 thermal simulator was used to simulate the rough rolling, finishing and laminar cooling processes, in which different cooling speeds were used to simulate the laminar cooling process, and the transmission electron microscope was used to study the microstructure of steel and the precipitation behaviour of phosphorus. Studies have shown that as the cooling rate increases, the size and quantity of phosphorus-containing precipitates at the grain boundaries gradually decrease, and the content of phosphorus in the precipitates decreases significantly.

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.

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.

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.

Resistance spot welding of galvannealed high strength interstitial free steel

Variation in dynamic contact resistance with the change in welding process parameters such as weld current, weld time and electrode force were taken into account for establishing the range of adequate nugget formation parameters. Influence of the welding process parameters on the shear – tensile strength, nugget diameter and the observed failure mode was analysed. The adequate resistance spot welding process parameters for galvannealed high strength interstitial free steel sheets of 1.6mm thickness were estimated as 8kA weld current, 250ms weld time and 3.5kN electrode force. Increase in the mean dynamic contact resistance led to a significant reduction in nugget diameter. A critical nugget diameter distinguishing between the IF and PF mode was experimentally determined by failure mode analysis. Different numerical models for estimation of critical nugget diameter were evaluated.

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.