IF Steel Research Articles

More than 25 Years of Experience in Thin Slab Casting and Rolling Current State of the Art and Future Developments

Industrial thin slab casting and direct rolling processing started more than 25 years ago with the world's first compact strip production (CSP®). Since that time, CSP® and competing thin slab casting and direct rolling concepts have been developed to a standard process for hot strip production. Typical features of this process are the homogeneous structural and mechanical properties all along the strip. The product range covers low carbon, as well as medium and high carbon steel grades comprising IF, HSLA, API, electrical, and AHSS multiphase steel grades. Hot strip from thin slab can be easily further processed to cold rolled and/or surface treated strip. A special feature is thin hot strip that may substitute cold rolled strip. Today, process and material developments, for example, go for rise in productivity, thinner gauge hot strip, as well as thicker line pipe and 3rd generation AHSS grades combining higher strength and ductility. The quality- and product-related advantages of thin slab casting and direct rolling technology are explained in this contribution at the example of the CSP® process.

Fabricating interstitial-free steel with simultaneous high strength and good ductility with homogeneous layer and lamella structure

Annealed interstitial-free steel (IF steel) and deformed IF steel sheets were stacked alternatively into multi-layers to produce laminated IF steel through thermal-mechanical processing. After proper processing, a yield strength of 500MPa, an ultimate tensile strength of 600MPa (comparable to cold rolled one) and a uniform elongation around 17% can be realized. Microstructural observation by electron back-scatter diffraction revealed a characteristic hierarchical layer+heterogeneous lamella structure, namely L2 structure. The reasons for the good mechanical properties were discussed.

Microstructure and Property Modifications of Cold Rolled IF Steel by Local Laser Annealing

Laser annealing experiments are performed on cold rolled IF steel whereby highly localized microstructure and property modification are achieved. The microstructure is seen to develop by strongly heterogeneous recrystallization to provide steep gradients, across the submillimeter scale, of grain size and crystallographic texture. Hardness mapping by microindentation is used to reveal the corresponding gradients in macroscopic properties. A 2D level set model of the microstructure development is established as a tool to further optimize the method and to investigate, for example, the development of grain size variations due to the strong and transient thermal gradient. Particular focus is given to the evolution of the beneficial γ-fiber texture during laser annealing. The simulations indicate that the influence of selective growth based on anisotropic grain boundary properties only has a minor effect on texture evolution compared to heterogeneous stored energy, temperature variations, and nucleation conditions. It is also shown that although the α-fiber has an initial frequency advantage, the higher probability of γ-nucleation, in combination with a higher stored energy driving force in this fiber, promotes a stronger presence of the γ-fiber as also observed in experiments.

Heterogeneous multi-layered IF steel with simultaneous high strength and good ductility

Multi-layered IF steel samples were designed and fabricated by hot compression followed by cold forging of an alternating stack of cold-rolled and annealed IF steel sheets, with an aim to improve the strength of the material without losing much ductility. A very good combination of strength and ductility was achieved by proper annealing after deformation. Microstructural analysis by electron back-scatter diffraction revealed that the good combination of strength and ductility is related to a characteristic hierarchical structure that is characterized by layered and lamella structures with different length scales.

EBSD characterization of deformed lath martensite in IF steel

Rolling deformation results in the transformation of a lath martensite structure to a lamellar structure characteristic to that of IF steel cold-rolled to medium and high strains. The structural transition takes place from low to medium strain, and electron backscatter diffraction analysis shows that the frequency of medium angle boundaries with misorientation angles of 5-10° decreases with increasing strain, while the frequencies of boundaries with angles in the ranges of 1-5° and 10-25° increase, resulting in the evolution of a bimodal misorientation angle distribution. The microstructural evolution and the strength are characterized for lath martensite rolled to a thickness reduction of 30%, showing that large changes in the misorientation take place, while the strain hardening rate is low.

비대칭 압연한 IF 강 판의 Goss 집합조직 발달

비대칭 압연한 IF 강 판의 Goss 집합조직 발달

Abrasion resistance characterization of low alloy construction steels: A comparison between three different scratch test protocols

In the present work, three different scratch tests are compared on their ability to rank the abrasion resistance of low alloy steels for industrial applications where the abrasion play a key role, e.g., in earthmoving, agricultural and mining equipment. The first test involves single pass scratching of pristine surfaces with a relatively large rigid indenter. The second test involves multi-pass scratching along a fixed track using the same large indenter. The third test involves the creation of a multi-pass scratch track using the same large indenter followed by final scratching of the abrasion track with a sharp indenter, i.e., Multi-Pass Dual Indenter (MPDI) scratch test. The three test protocols activate different abrasion mechanisms. Five low alloy construction steel grades with different strain hardening capabilities, i.e., Interstitial-free Ferritic steel (IF steel), Fully Martensitic steel (FM steel), Dual Phase steel (DP steel), Quench Partitioning steel (Q&P steel) and TWining Induced Plasticity steel (TWIP steel), are used. The results show that for both single (large) indenter scratch test protocols, the scratch depths always increase with indenter load and the failure mechanism is pure ploughing except for the IF steel due to the nature of softness. While for the dual-indenter scratch test, the scratch depth is a more complex function of the load applied during creation of the work hardened surface layer. The conventional scratch test protocols cannot well reveal micro-cracks and defect in (sub) surface and cannot well reflect the real response of strain hardening of steels. However, the MPDI test can distinguish between different steels with different initial hardness and different strain hardening behaviour and reveal the damage and, equally important, can indicate how average loading conditions may affect the relative abrasion resistance of construction steels during steady state conditions.

Model for partitioning slip patterns at triple junctions of grains

A rate insensitive model based on interfacial compatibility of plastic strains is formulated to predict partitioning slip patterns in interaction of polyhedral grains at triple junctions. The model imposes macroscopic deformation on a skeleton of grain boundaries that, in turn, prescribes a particular common projection of plastic strains on both sides of any boundary facet. To accommodate three independent components of this projection, each of two adjacent domains deform with three slip systems. Unlike Taylor's model, such slip patterns at different facets of the same grain prove to be dissimilar and hence trigger the crystal orientation splitting near triple lines. For a case study of this phenomenon, the model applies to a grain junction of low deformed IF steel regardless of slip patterns in grain cores. Allowance for the latter is finally discussed that will be indispensable for the model extension to entire grains and, eventually, the entire polycrystal.

Strategies for nitrogen control during the production of interstitial-free steel

ABSTRACTInterstitial-free (IF) steel is a micro-alloyed steel that contains significantly low concentrations of interstitial elements such as carbon and nitrogen. In this context, nitrogen control in liquid steel is a major issue, particularly in cases where the nitrogen content in hot metal or other charged material is high. This study describes the practice of IF steel production at No.3 steelmaking plant of WISCO using hot metal containing high nitrogen due to hot metal desulphurisation treatment with nitrogen as the carrier gas. By adapting new technology for nitrogen removal in a combined blown converter and an Ruhrstahl Heraeus vessel as well as controlling nitrogen pick-up during transfer operations, final product can be generated with an average nitrogen content of about 18 ppm and with some values as low as 10 ppm.

Interrelationship between Hot Ductility and Particle Size in TiNb IF Steel

The aim of this work was to observe the relationship between hot ductility and morphology, distribution and size of particles in TiNb IF steel after hot torsion testing at the critical temperatures of deformation with low as well as maximum values of plasticity. Transmission electron microscopy showed that the particles at all temperatures of deformation with minimum number of turns to failure e.g. 1132°C (33.72 rev.), 946°C (6.24 rev.), 637°C (5.54 rev.) as well as with maximum value of plasticity at 844°C (1726 rev.) were of globular, cuboid or elliptical shape. EDX analysis revealed that there were different types of particles such as carbides, sulfides, and carbonitrides of Ti and Nb, Al and Si oxides, Mn sulfides, and phosphides. Quantitative evaluation of particle size in the carbon extracted replicas showed 20% of total number of particles with size 2r = 30-39 nm and an average linear dimension = 42 nm at the deformation temperature of 1132°C. There were 28% of the particles with size 2r = 20-29nm and = 41nm at the temperature of 946°C while at the temperature of 637°C there were 29% of particles with size 2r = 20-29 and 26% with size 2r=10-19 nm and = 41 nm. In the case of maximum plasticity (1726 rev.) at 844°C, the presence of large particles was confirmed with = 105 nm size and 9% distribution in three size categories of 20-29, 30-39, 90-99 nm.

Tensile and Fatigue Behaviour of AA6022-T4 to IF Steel Resistance Spot Welds

Vehicle light weighting plays an integral role in achieving the mandatory fuel efficiency requirements as established in the Corporate Average Fuel Economy Standards (CAFE), USA. To optimize vehicle structures for both performance and mass savings, materials such as aluminum alloys may be used in combination with steels, especially advanced high strength steels. Although joining dissimilar materials is a challenge, General Motors (GM) has recently succeeded in developing a proprietary resistance spot welding process using a multi-ring, domed (MRD) electrode geometry that is capable of producing welds between aluminum alloys and steel materials with acceptable joint strength. In this work, tensile and fatigue properties are presented of welds produced between dissimilar materials in lap-shear configuration. The dissimilar metal resistance spot welds (RSWs) were composed of wrought aluminum 1.2-mm thick AA6022-T4 sheet and 2.0-mm thick IF steel. For comparison, similar metal lap-shear RSWs were made and tested of 1.2-mm and 2.0-mm thick AA6022-T4. Average lap-shear strengths of 4400 N and 3200 N were observed for AA6022-T4 to IF steel welds and AA6022-T4 to AA6022-T4 welds, respectively. In load-controlled fatigue testing, limited scatter in fatigue life was observed for both stack-ups tested at maximum fatigue loads below 1500 N. The overall fatigue life was lower for the AA6022-T4 to AA6022-T4 welds compared to AA6022-T4 to IF steel welds. Failure modes for both stack-ups, particularly at long lives, were primarily a result of crack growth through the thickness direction in the AA6022-T4 sheet around the outer edge of the weld nugget. The superior performance of the AA6022-T4 to IF steel welds was most likely primarily due to the larger size of the weld nuggets created in that joint. Also contributing to the superior performance of the Al-steel welds was a more favourable notch root opening geometry that reduced stress concentration as well as a microstructure in the region of the crack path that contained fine, columnar grains alloyed with iron that SEM analyses indicated slowed fatigue crack growth. Using the structural stress concept, it was found that the fatigue life of lap-shear AA60222-T4 to AA6022-T4 and AA6022-T4 to IF steel welds falls onto a master curve indicating that the nugget size dominates the fatigue life.

Undercooling and Wettability Behavior of Interstitial-Free Steel on TiN, Al2O3 and MgAl2O4 Under Controlled Oxygen Partial Pressure

The heterogeneous undercooling degree of interstitial-free steel (IF steel) on various substrates was measured through observing the recalescence phenomenon by sessile drop method at the oxygen partial pressure of 10−23 atm, where there was no obvious interaction between the steel and the substrate. The wetting behavior of liquid IF steel against different substrates was also studied during the sessile drop test. The results suggested that the undercooling degree increased with the increase of cooling rate, and the effective contact surface area between the liquid steel and the solid substrate could not fully explain the undercooling behavior of the designed cases as there is no interaction layer. Furthermore, a method to calculate the critical contact angle for IF steel against its substrate was developed in this study, which could be adapted to explain the undercooling variance of IF steel with the change of its substrate. The undercooling degree increased with the increase of the critical contact angle, and the critical contact angle was also improved with the increase of lattice disregistry.

Mechanical properties, formability and corrosion resistance of thermomechanically controlled processed Ti-Nb stabilized IF steel

Aim of the present study is to examine the possibilities of achieving a better combination of yield strength (YS), ductility and formability with a better corrosion resistance of a Ti-Nb stabilize IF steel through microstructural refinement by simple phase controlled thermomechanical rolling. The phase controlled thermomechanical rolling could be an industrially reliable method for the production of hot strips, which can substitute cold rolled steel sheets. Accordingly, the phase controlled multi-pass rolling was performed in 3 selected phase regimes (γ-recrystallization, γ→α transition & α-regions) on the basis of Ac3/Ar3, Ac1/Ar1 (obtained through Thermo-Calc & Gleeble-3800) and Tnr (from Boratto equation) followed by air cooling. The volume fraction of precipitate(s) correspond to the deformation temperature was estimated using Thermo-Calc Software and the morphology of the precipitates was analyzed by TEM. The strain induced phase transformation of unstable γ occurred during rolling at a high reduction of ~80%, ɛ=1.6 at γ→α transition region. Thus, dynamically recovered stable bimodal equiaxed ferrite structures (fine ferrite ~5µm embedded with larger size ferrite grains ~32µm) were obtained after air cooling to room temperature. In case of the rolling at α-region, improvement of the YS (>3-fold) is attributed to the formation of ultrafine ferrite grains (1–3µm) through subgrain structures, strain-induced precipitation of nanosize NbC and/or TiC and micro-shear bands. Very short-annealing (~100s) at 850°C followed by forced air cooling was employed in order to simulate continuous annealing process and was found to improve the formability without much affecting its YS. The avoidance of FeTiP phase formation (which deteriorates to form {111} recrystallization texture) and nucleation of ferrite grains within the deformation bands (studied through EBSD study) by a short-annealing treatment are accountable for regaining the formability. The role of strain hardening exponent (n) and plastic-strain-ratio (r) on the deformation characteristics of the thermomechanically treated IF steel were also investigated to correlate the YS and uniform elongation. Furthermore, the rolled (at α-region) + short-annealed samples showed an excellent corrosion resistance due to the formation of dense oxide film on the surface. This is attributed to the dissolution of Fe(Ti+Nb)P precipitates (which are the potential sites for initiation of pits), and formation of fine grains (which facilitate to form dense oxide film on the large surface area).

Bulk Ultrafine-Grained Interstitial-Free Steel Processed by Equal-Channel Angular Pressing Followed by Flash Annealing

Interstitial-free steel workpieces are deformed by equal-channel angular pressing (ECAP) for equivalent strain εvm = 3 and εvm = 21 followed by flash annealing. Microstructures are analyzed by optical microscopy, scanning electron microscopy and transmission electron microscopy. Mechanical properties are evaluated by hardness testing. Yield strength of materials is calculated from hardness values. Flash annealing (at 675 °C) of ECAPed samples for εvm = 3 and εvm = 21 results in abnormal subgrain growth and abnormal grain growth, respectively. Flash annealing at 700 °C of ECAPed (at εvm = 3) IF steel converts abnormally grown subgrains to grains which serve as nuclei for recrystallization and that result in bimodal grain size distribution. Bimodal grain size distribution is also produced when ECAPed IF steel for εvm = 21 is flash annealed at 675 °C due to abnormal grain growth or secondary recrystallization. Flash annealing of IF steel samples ECAPed for low εvm, in the temperature range 600-675 °C, decreases the hardness continuously with increase in the annealing temperature but it increases at high εvm. The former is due to annihilation of defects but the later is caused by ordering of nonequilibrium boundaries. The hardening and strengthening behaviors are similar.

Structural characterization of plasma nitrided interstitial-free steel at different temperatures by SEM, XRD and Rietveld method

Plasma nitriding processes are widely used to improve surface properties of several steels and alloys. In this work, the formation of nitrides in the surface of plasma nitrided IF steels as a function of the temperature was investigated. Three cold-rolled IF steel plates were nitrided for 4h after shot peening at three different temperatures: 450°C, 475°C, and 500°C. The resultant nitrided layers were then characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), Rietveld method, and hardness measurements. Through SEM images, it was possible to visualize two main sublayers: a compound layer and a diffusion zone. Through XRD, two phases were identified in the compound layer, which were ɛ-Fe2–3N and γ′-Fe4N. The diffusion zone presented a ferritic matrix with fine precipitates, possibly α″-Fe16N2. By Rietveld, the calculated quantity of γ′-Fe4N was 68wt.% for the sample treated at 475°C and 58wt.% for the one treated at 500°C. These values were consistent with the hardness measurements. Thus, it is suggested that higher nitriding temperatures facilitate the decreasing of γ′-Fe4N and, consequently, the increasing of ɛ-Fe2–3N in the compound layer.

Statične in dinamične natezne lastnosti pločevine iz S420 in IF jekla

Statične in dinamične natezne lastnosti pločevine iz S420 in IF jekla

Mechanical Properties and Crystallographic Texture of Symmetrical and Asymmetrical Cold Rolled IF Steels

The crystallographic texture developed during cold rolling and subsequent annealing of interstitial free sheet steels aims to increase conformability. For this, it is necessary to obtain partial α-fiber and continuous and homogeneous γ-fiber texture components. In this work, the influence of symmetric (SR) and asymmetric (AR) cold rolling on crystallographic texture and mechanical properties of an interstitial free steel (IF) was investigated. Symmetric cold rolling yields α- and γ-fibers, which are enhanced as deformation increases. Moreover, α-fiber weakening occurs due to recrystallizations, improving formability. The same fibers are produced by asymmetric cold rolling, but in this case, the γ-fiber is slightly shifted in psi, which is one of Euler angles second ROE's notation1,2, and more homogeneous than in symmetric rolling. The best mechanical properties were achieved by asymmetric cold rolling/annealed with about 80% deformation.

Theoretical and experimental investigation of deep drawing of tailor-welded IF steel blanks with non-uniform blank holder forces

Tailor-welded blank is one of the promising technologies in the application of lightweight materials for auto body parts manufacturing. The material discontinuity across the weld line results in an inhomogeneous deformation and weld line displacement. In this study, a two-dimensional analytical model is proposed to predict the tension distribution along the cross section. An energy method is used to calculate the restraining force due to bending, sliding, and unbending phenomena on the die and punch radii. To control the weld line movement, a blank holder force control strategy is proposed to achieve force equilibrium at the bottom of the part across the weld line. Finite element simulations are performed to study the effect of die and punch radii, friction coefficient, thickness ratio, and blank holder forces on the weld line displacement in strip drawing process. Under a uniform blank holder force, the weld line moves toward the thicker/stronger side implying a higher blank holder force is required for the thinner/weaker side. The results show that the weld line displacement can be controlled by an appropriate blank holder force adjustment. In order to control the weld line movement in square cup deep drawing, blank holder force on the thinner side is increased and its influences on the deformation process are investigated. Comparisons of material draw-in, weld line movement, and forming force show a good agreement between the theoretical, numerical, and experimental results.

The Study on the Relationship between the Deformation of Resistance and Temperature and Rate of Ti-IF Steel

With the rapid development of automobile industry, automobile panel material has also gained great attention, the IF steel has got more and more applications in automobile panel production for its excellent deep drawing propertie s, such as, high strain ratio, high extensibility, high yield ratio, etc. In this paper, through the analysis of thermal simulation results and microstructure of IF steel, we studied the influences of microstructure changes on IF steel with different deformation parameters, obtained the influence laws of the deformation parameters on deformation resistanc e of IF steel. So there is a certain directive significance to control the rolling process of IF steel in the actual production of IF steel.

Elasto-Plastic Modeling of the Limit Strains in Metallic Sheets

In this work, the model of Marciniak and Kuczynski, hereafter referred to as the M-K model, was extended to account for the elastic strains to forecast the Forming Limit Curve (FLC) of metallic sheets. The return mapping algorithm is adopted where an elastic predictor step is performed with the generalized Hooke’s law and the plastic correction step is performed assuming the isotropic work-hardening together with the associated flow rule. The current M-K model predicted quite well both experimental and rigid-plastic results obtained for an IF steel sheet. Also, the principal stresses predicted by the elasto-plastic M-K model provided a good agreement with the experimental Forming Limit Stress Diagram (FLSD) of a Bake-Hardening steel sheet. Therefore, the elastic strains should be taken into account in the modeling of limit strains in metallic sheets, especially in the case of advanced high strength steels for which the Young modulus decreases with the plastic strain.