IF Steel Research Articles

Asymmetric Rolling of Interstitial-Free Steel Using Differential Roll Diameters. Part I: Mechanical Properties and Deformation Textures

IF steel sheets were processed by conventional symmetric and asymmetric rolling (ASR) at ambient temperature. The asymmetry was introduced in a geometric way using differential roll diameters with a number of different ratios. The material strength was measured by tensile testing and the microstructure was analyzed by optical and transmission electron microscopy as well as electron backscatter diffraction (EBSD) analysis. Texture was also successfully measured by EBSD using large surface areas. Finite element (FE) simulations were carried out for multiple passes to obtain the strain distribution after rolling. From the FE results, the velocity gradient along selected flow lines was extracted and the evolution of the texture was simulated using polycrystal plasticity modeling. The best mechanical properties were obtained after ASR using a roll diameter ratio of 2. The textures appeared to be tilted up to 12 deg around the transverse direction, which were simulated with the FE-combined polycrystal plasticity modeling in good agreement with measurements. The simulation work revealed that the shear component introduced by ASR was about the same magnitude as the normal component of the rolling strain tensor.

Microstructure evolution and mechanical properties of ultrafine grained IF steel via multipass differential speed rolling

The microstructural evolution and mechanical properties of ultrafine grained interstitial free steel fabricated by multipass differential speed rolling (DSR) technique were investigated. A series of DSR operations were performed up to four passes under a roll speed ratio of 1∶4 for lower and upper rolls. Microstructural observations using a transmission electron microscope showed that the deformed microstructure comprised the severely deformed ferrite grains with a mean size of ∼0·7 μm having a relatively high dislocation density. In addition, the nearly equiaxed grains were effectively formed due to the sample rotation of 180° around the transverse axis between each passage where the macroscopic shearing was reversed. Electron backscattered diffraction analysis revealed an increase in the fraction of the high angle grain boundaries in the severely deformed samples with increasing number of DSR operations. The mechanical properties were also examined utilising microhardness and tensile tests, and these were discussed in relation to the shearing and microstructural characteristics.

The Influence of Friction on the Texture Formation of a IF Steel during Hot Rolling in the Ferrite Region

AbstractThe effect of friction on evolution of the microstructure and texture during hot rolling in the ferrite region was studied with secondary electromicroscopy (SEM), X‐ray diffractometer, and FEM simulation. The friction between a roll and a steel sheet surface leads to the formation of notable through‐thickness texture gradients. The finite element method (FEM) simulation shows that the large friction coefficient between a roll and sample results in a pronounced variation of ${\dot {\varepsilon }}_{13} $ (shear strain rate) and large ε13/ε11 value which accounts for strong {110}<001> texture components and weak γ‐fiber components at rolled sheet surface; whereas the friction condition do not exert notable effect on texture formation at the sheet center.

Effect of Austenite Morphology on Ferrite Refinement in Microalloyed Steel

The Strain-Induced Dynamic Transformation (SIDT) is an efficient way to overcome the limitation of grain refinement during the conventional thermomechanical controlled processing (TMCP) of steels. The present study deals with the effects of austenite morphology on the SIDT in microalloyed and IF steels. The discussion of the processing route in terms of chemical composition, deformation schedules, heating and cooling conditions is carried out by the means of torsion tests with deformation in metastable conditions. As it was expected, the microstructure of microalloyed steel was clearly controlled by the microalloying elements what, in turn, directly affectedthe SIDT products. Examination of water-quenched microstructures, just after deformation, revealed the morphology of "strain-induced ferrite". The kinetics of SIDT were observed and analyzed using the strain-stress curves. It is shown that presence of strain-induced precipitations in microalloyed steels accelerates kinetic of SIDT - by reducing the amount of Nb in solution. The changes in ferrite refinement of the experimental steels were explained from the view of the austenite morphology and processing parameters.

Effect of Micro-Alloying Elements of Ti, Nb and B on Recrystallization Behavior of Cold-Rolled Interstitial-Free Steel Sheets

The effect of micro-alloying elements of niobium and boron and titanium on recrystallization behavior is researched after the cold rolled IF steels are annealed at high temperature. The results show that there is high density dislocation in the cold rolled steel and the microstructure is fibrous tissue. The recrystallization grains appear when the cold steel annealed at 655 °C and then the grains grow up with the annealing temperature increased to 840 °C. The recrystallization temperature and time of B-Ti-IF steel is lower than that of Nb-Ti-IF and Ti-IF steels. The recrystallization activation energy of Nb-Ti-IF steel is 181.7KJ/mol which is 56.6KJ/mol larger than that of B-Ti-IF steel.

HIGH TEMPERATURE EMBRITTLEMENT EVALUATION OF SAMPLES FROM TINB IF STEEL AFTER HOT TORSION TEST

The aim of this paper was to analyze the high temperature embrittlement of TiNb IF steel using plastometric torsion tests. High temperature embrittlement in the temperature range 600 - 1200°C is a problem mainly connected with additions of microalloying elements to steel and also with inhomogeneous and coarse-grained microstructure formed during continuous casting. Reduction of high temperature ductility can lead to cracks in the slab. These surface cracks can easily become oxidised, and then they do not fuse during forming operations. A significant decrease in hot ductility of the analyzed steel occurred in the stable austenite just above the temperature of 1100°C, where the number of rotations to failure (N f ) in the sample was lowest, at 28.4. The cavities ratio was 1.9%, which occurred mainly below the fracture line. At the bottom of the dimple transcrystalline ductile failure (DTDF), complex particles based on Al, Ti, Mn, S, N were observed. In this temperature range the embrittlement is mainly attributed to fine precipitation of sulfides, oxides, particulates based on microalloys as well as segregation of impurities on the austenite grain boundaries.

EFFECT OF GRAIN SIZE ON ATMOSPHERIC CORROSION RESISTANCE OF ULTRA--LOW CARBON IF STEEL

Three kinds of ultra-low carbon IF steel with different grain sizes,and same chemical composition were prepared by different rolling and heat treat process.The relationship between grain size and atmospheric corrosion resistance of IF steel was investigated by immersion corrosion test,cyclic immersion corrosion test,AFM/SEM micro-analysis and electrochemical test. The results show that the local corrosion in grain boundary increases after immersion corrosion test,the depth of crack in grain boundary becomes deeper and the width of crack becomes wider with grain sizes of IF steel increase from 15μm to 220μm.The crack and cavity in the rust after cycle immersion corrosion test are increased and the atmospheric corrosion resistance is decreased with IF steel grain size coarsing from 15μm to 46μm.As grain size increase from 15μm to 220μm,the whole compactness of rust are increased,the rust resistance and the atmospheric corrosion resistance are increased.The effect of grain size on the corrosion current density of local grain boundary was analysed and the mechanics of corrosion was discussed.The total quantity of corrosion surface defect is decreased due to the decrease of grain boundary energy with the increase of grain size and the atmospheric corrosion resistance is increased.Meanwhile,the local corrosion near the grain boundary is increased duo to the increase of local corrosion current density with the increase of grain size and the atmospheric corrosion resistance is decreased.The atmospheric corrosion resistance is influenced by the two factors simultaneously.

A user-friendly 3D yield function to describe anisotropy of steel sheets

A sixth-order polynomial type 3D yield function, which has a high flexibility of describing anisotropic behavior of steel sheet, is proposed. The yield function is constructed as a sum of several components of the Cazacu and Barlat, 2001 which was derived as an extension of the J2–J3 Drucker yield criterion (Drucker, 1949) to orthotropy using the linear transformation of the stress deviator, so that the convexity of the yield locus is perfectly guaranteed. The model was validated by comparing the numerical predictions of planar anisotropy of r-values and flow stress directionality, as well as the shape of yield loci, with the corresponding experimental data on several types of steel sheets (high r-valued IF steel and SPCE, and high strength steel sheets of 440–980MPa TS grades). For most of steel sheets, the model using the sum of two J2 components, which involve eight anisotropic coefficients, is sufficient for the description of their anisotropies. Further it was found that the consideration of J3 effect improves the description of anisotropy for some steel sheets (e.g., high r-valued IF steel and 980MPa grade dual phase HSS).

The Effect of Strain Rate on the Mechanical Properties of Automotive Steel Sheets

The automotive industry is currently seeking detailed information about various types of materials and their behavior under dynamic loading. Dynamic tensile testing of sheet steels is growing in importance. The experimental dynamic tensile technique depends on the strain rate. Each type oftest serves for a specific range of strain rates, and provides specific types of information. This workdeals with the influence of the strain rate on the mechanical properties of automotive steel sheets.Three different types of steel: IF steel, DP steel, and micro-alloyed steel (S 460) were used to compare static and dynamic properties.

402 IF鋼のひずみ時効硬化異方性特性評価(OS9 オーガナイズドセッション《計算力学と数値シミュレーション》)

402 IF鋼のひずみ時効硬化異方性特性評価(OS9 オーガナイズドセッション《計算力学と数値シミュレーション》)

OS0208 IF鋼における加工硬化とBACK STRESS : 4点曲げによるバウシンガー効果の測定

OS0208 IF鋼における加工硬化とBACK STRESS : 4点曲げによるバウシンガー効果の測定

Recrystallization Behavior of IF Steel Sheets Immediately after Hot-rolling in Ferrite Region

Recrystallization Behavior of IF Steel Sheets Immediately after Hot-rolling in Ferrite Region

Influence of microstructure on hydrogen diffusion and impedance of IF steel

Influence of microstructure on hydrogen diffusion and impedance of IF steel

The Development of Annealing Time for Super Fine Grain and High Strength IF Steel

The hot rolling, cold rolling and simulative continuous annealing experiments were carried out in the laboratory on the base of new type SFG HSS (super fine grain, high strength steel sheet). The results show that the microstructure which contains a number of cake shaped grain can be refined and homogenized by the feasible annealing holding time. Contrast to the conventional steel, the SFG steel have the characters of super fine grain, high tensile strength, low yield strength/tensile strength rate, good elongation and high r-value high（the plastic strain ratio）．

Accelerated Potts model for grain growth – Application to an IF steel

A modification of the Potts model for grain growth is presented. We add to the site energy a grain size factor that favors growth of large grains and whose magnitude has been optimized with respect to various aspects. The motivation for such a modification as well as the consequences in term of calculation speed are presented. The capabilities of this modified model are checked by comparing several characteristics of the microstructures obtained after various grain growth simulations with experimental data obtained for an IF steel. One large EBSD map, made at the end of the primary recrystallization, is used directly as the initial image for the calculations. A second EBSD map, obtained after grain growth (with a mean diameter of the grains about twice the initial value) was used for comparisons between the simulations and the experiment. The grain size distribution, the grain boundary character distribution and the crystallographic texture are reproduced rather well with this modified model using a moderate grain size factor effect. Also the speedup of the calculation is quite significant.

Effect of Phosphorus on the Mechanical Properties and Microstructure of Interstitial Free Steel

Effect of phosphorus content on the mechanical properties and microstructure of IF steel sheets was investigated. Average grain size and recrystallization texture were measured by electron backscatter diffraction (EBSD). The results showed that the higher P resulted in higher tensile strength and lowered the elongation and r-value. The average grain size increased with decreasing P content. The //ND (γ-fiber) pole intensity had a lowest value for IF steel with the highest P content which in turn deteriorate r-value. The element P played an important role in recrystallization process which affected the mechanical properties and microstructure of IF steels.

The Study on the Precipitation Behavior of the Second Phase Particles for Fine Grain and High Strength IF Steel

The new type SFG HSS (super fine grain, high strength steel sheet) has been developed by adding solid-solution strengthening elements to conventional IF steel such as Si, Mn. The precipitation behavior of the second phase particles was observed and studied by a carbon extraction replica technique used transmission electron microscopy (TEM). The shape of the small second particles is similar to spherical and ellipse, the sizes of which are 10~30nm. It is seen that the particles are dispersed on the matrix. The precipitate composition of small particles is Nb (CN) and the precipitate composition of large particles is NbC examined by energy dispersive X-ray (EDX). It is noted that the yield strength is low as well as the tensile strength is high by the PFZ which is free of precipitate called precipitated free zone on the one side of the grain boundary. The results of microstructure shows that the second phase particles pinned on grain boundary not only can inhibit the grain growth, but also the grain can be fined.

The Distribution of Grain Boundary Planes in Interstitial Free Steel

The grain boundary character distribution in a commercial IF steel has been measured as a function of lattice misorientation and boundary plane orientation. The grain boundary plane distribution revealed a relatively low anisotropy with a tendency for grain boundaries to terminate on low index planes having relatively low surface energy and large interplanar spacings. Although the most common grain boundary plane orientation was (111), grain boundaries terminated on higher index planes were sometimes found. For instance, at a misorientation angle of 60 deg about [111], symmetric {112} tilt boundaries were far more populous than [111] twist boundaries. The current observation revealed an inverse relationship between the measured populations and the previously reported grain boundary energies.

Microstructural Evolution with Annealing of Ultralow Carbon IF Steel Severely Deformed by Six-Layer Stack ARB Process

A sample of ultra low carbon IF steel was processed by six-layer stack accumulative roll-bonding (ARB) and annealed. The ARB was conducted at ambient temperature after deforming the as-received material to a thickness of 0.5 mm by 50% cold rolling. The ARB was performed for a six-layer stacked, i.e. a 3 mm thick sheet, up to 3 cycles (an equivalent strain of ~7.0). In each ARB cycle, the stacked sheets were, first, deformed to 1.5 mm thickness by 50% rolling and then reduced to 0.5 mm thickness, as the starting thickness, by multi-pass rolling without lubrication. The specimen after 3 cycles was then annealed for 0.5 h at various temperatures ranging from 673 to 973 K. The microstructural evolution with the annealing temperature for the 3-cycle ARB processed IF steel was investigated in detail by transmission electron microscopy observation. The ARB processed IF steel exhibited mainly a dislocation cell lamella structure with relatively high dislocation density in which the subgrains were partially observed. The selected area diffraction (SAD) patterns suggested that the misorientation between neighboring cells or subgrains was very small. The thickness of the grains increased in a gradual way up to 873 K, but above 898 K it increased drastically. As a result, the grains came to have an equiaxed morphology at 898 K, in which the width and the thickness of the grains were almost identical. The grain growth occurred actively at temperatures above 923 K.

Research on the Controlling Technic of Conclution in If Steel

Combining with the production experience in steel making and rolling plant, the oxide inclusions of IF steel were investigated. In the operation process, the inclusions of IF steel are main Al2O3-TiN. They are granular and dispersed in the steel, and most of sizes are 1~5μm. In the inclusion center, the Al2O3 plays the role of TiN heterogeneous nucleation center. So reducing the Al2O3 inclusion is essential to stabilize the Ti content and strengthen the fixtation to C and N in steel. The quantity of Al2O3 inclusion was closely relation to the operation process. The Al2O3 inclusions can be significantly reduced by taking some measure.