Cold-Rolled IF Steel Sheet Research Articles

Forming Limit Analyses of Cold Rolled IF Steel Sheet Using Differential Work Hardening Model

A servo-controlled multiaxial tube expansion testing machine was used to measure the multiaxial plastic deformation behavior of a cold rolled IF steel sheet for a range of strain from initial yield to fracture. The testing machine is capable of applying arbitrary principal stress or strain paths to a tubular specimen using an electrical, closed-loop servo-control system for an axial force and an internal pressure. Tubular specimens with an inner diameter of 44.6mm were fabricated from a cold rolled interstitial-free steel sheet with a thickness of 0.7mm by roller bending and laser welding. Many linear stress paths in the first quadrant of stress space were applied to the tubular specimens to measure the forming limit curve and forming limit stress curve of the as-received sheet sample, in addition to the contours of plastic work and the directions of the plastic strain rates. It was found that the shapes of the measured work contours changed with increasing plastic work. The observed differential work hardening behavior was approximated by changing the material parameters and the exponent of the Yld2000-2d yield function (Barlat et al., 2003) as a function of the equivalent plastic strain. The forming limit curve and forming limit stress curve were calculated using the Marciniak-Kuczyński-type approach and the differential work hardening model. The calculated results were in fair agreement with the measurement.

21112 冷延IF鋼板の異方硬化挙動の定式による成形シミュレーションの高精度化

The deformation behavior of cold-rolled IF steel sheet under biaxial tension was investigated for a strain range from initial yielding to fracture. The test material was bent and laser welded to fabricate a tubular specimen. Multiaxial tube expansion tests were performed using a servo-controlled tension-internal pressure testing machine. Many linear stress paths in the first quadrant of stress space were applied to the tubular specimens to measure the counters of plastic work and the directions of the plastic strain rates. It was found the shapes of the work counters changed with an increase in ε^P_0 and the Yld2000-2d yield function could reproduce the differential work hardening behavior (DWH) of the test material by changing the material parameters and exponent as functions of ε^P_0. Finite element analysis of hydraulic bulge forming has verified the effectiveness of proposed DWH model based on Yld2000-2d yield function for improving the accuracy of the FEA.

Effect of Electric Field Strength on Texture Evolution of IF Steel Sheet during Annealing

Specimens cut from a cold-rolled IF steel sheet of 1 mm thickness were respectively annealed at 750°C for 20min under a range of DC electric fields (1kV/cm~4kV/cm). The Effect of electric field strength on recrystallization texture of IF steel sheet was studied by mean of X-ray diffraction ODF analysis. It was found that γ-fiber textures were notably enhanced as electric field strength increased. The strength of γ-fiber textures got their peak values as the applied electric field reached to 4kV/cm. The possible reason for such phenomena was discussed in the viewpoint of interaction between the applied electric field and the orientation-dependent stored-energy in deformed metals which is known as the driving force for recrystallization during annealing.

二軸バルジ試験による冷延IF鋼板の加工硬化特性の測定と材料モデリング

二軸バルジ試験による冷延IF鋼板の加工硬化特性の測定と材料モデリング

Control of Surface Quality of if Steel Sheet

In order to solve the problem of mold powder entrapment which is one of the affecting factors on faint-sliver defect in IF cold-rolled steel sheet in casting process, continuous casting process is optimized. Based on the analysis of the defect, the composition of mold powder is optimized. Otherwise, surface tension and viscosity of mold powder are raised. With theses countermeasures, mold powder entrapment and casting subcutaneous slag is decreased obviously. In addition, the rate of faint-sliver defect in cold-rolled IF steel sheet has been reduced from 5.21% to below 1.0%.

Control for Surface Faint-Sliver Defects in Cold-Rolled IF Steel Sheet

The samples of cold-rolled sheets, hot-rolled plates and continuously cast slabs have been analyzed with the condition of faint-sliver defects appeared in IF steel of Pangang, and it was shown that faint-sliver defects in cold-rolled steel sheet were mainly caused by micro-cracks in the surface of strands, slag entrapment in mold during casting and inclusions in sub-surface of strand. Faint-sliver defects were controlled effectively with the technical measures, which were decreasing oxidation of ladle slag, controlling the ratio of CaO/Al2O3 in slag, extending circulation time of deoxidization and alloying in RH process, controlling the casting speed and increasing the viscosity of mold fluxes. Therefore, the ratio of downgrade induced by faint-sliver defects reduced from 10.97% to 1.0%.

Effect of Mold Powder on Faint-Sliver Defect in if Cold-Rolled Steel Sheet

Mold powder entrapment is one of the affecting factors on faint-sliver defect in IF cold-rolled steel sheet in casting process. Based on the analysis of the defect, the composition of mold powder is adjusted. Otherwise, surface tension and viscosity of mold powder are raised. After taking these measures, mold powder entrapment and casting subcutaneous slag is decreased obviously. In addition, the rate of faint-sliver defect in cold-rolled IF steel sheet has been reduced from 5.21% to below 1.0%.

Effect of Magnetic Field Direction on Texture Evolution in Cold-Rolled IF Steel Sheet at the Initial Stage of Recrystallization

Sheets of cold rolled (76%) IF steel were annealed at 650°C for 30min under a 12-tesla magnetic field. During the magnetic field annealing, they were placed at the center of the applied field respectively, being oriented differently with respect to the magnetic field direction. The results show that the high magnetic field annealing prevents the evolution from deformed {111}<110> texture component to recrystallized {111}<112> texture component at the initial stage of recrystallization. For the field annealed specimens, altering the specimen orientation to the magnetic field direction during annealing does not change the final annealing textures. The intensity of main {111} texture components presents a similar periodic variation with respect to the specimen orientation to the magnetic field.

Effects of Electric Field Annealing on Recrystallization Texture and Plastic Strain Ratio (r-Value) of IF Steel Sheet

Effects of electric field annealing on recrystallization texture and average plastic strain ratio (r value) of IF steel sheet were studied by mean of X ray diffraction ODF analysis and the tensile test. Specimens of 75% cold-rolled IF steel sheet were annealed at 850flC for 25min,with and without electric field annealing respectively. Results show that the electric field annealing does not change the type of recrystallization texture and its formation mechanism in IF deep-drawing steel sheet, but improves the formation and development of recrystallization i-fiber texture, enhances the intensity of i-fiber texture, and retards the formation and development of c-fiber texture. Comparing with the conventional annealing, electric field annealing improves the average plastic strain ratio (r value). Possible reasons for the intensification of recrystallization i-fiber texture with the electric field applied may attribute to the reduction of stored energy, thus hindering the formation of other orientated nuclei and enhancing the nucleating ratio of i-oriented nuclei. And the intensification of recrystallization i-fiber texture was considered as the main reason for the enhancement of the plastic strain ratio (r value) in IF steel sheet.

{111}熱延集合組織を有するIF鋼からの強い {111} 冷延集合組織形成機構

{111}熱延集合組織を有するIF鋼からの強い {111} 冷延集合組織形成機構