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Authors: Karin Yvell, Göran Engberg
Abstract: The evolution of the deformation structure with strain has been studied using electron backscatter diffraction (EBSD). Samples from interrupted uniaxial tensile tests and from a cyclic tension/compression test were investigated. The evolution of low angle boundaries (LABs) was studied using boundary maps and by measuring the LAB density. From calculations of local misorientations, smaller orientation changes in the substructure can be illustrated. The different orientations developed with strain within a grain, due to operation of different slip systems in different parts of the grain, were studied using a misorientation profile showing substantial orientation changes after a true strain of 0.24. The texture evolution with increasing strain was followed by using inverse pole figures (IPFs). The observed substructure development in the ferritic and austenitic phases could be successfully correlated with the stress-strain curve from a tensile test. LABs were first observed in the different phases when the strain hardening rate changed in appearance indicating that cross slip started to operate as a significant dislocation recovery mechanism. The evolution of the deformation structure is concluded to occur in a similar manner in the austenitic and ferritic phases but with different texture evolution for the two phases.
Authors: Seiji Furusako, Shinji Kodama, Masanori Yasuyama, Yasunobu Miyazaki
Abstract: High-strength steel sheets (HSSs) have been increasingly used to reduce the weight of automobile bodies. It is necessary to enhance the reliability of their joint strength in order to realize optimal usage. Projection welding (PW) is one of the important welding methods for HSSs; however, research on PW is insufficient. In this research, therefore, the effect of welding conditions on maximum load in the press-peel test for joints, made by PW HSS of 590 MPa grade and square weld nuts, was investigated. It is found that the maximum load showed a tendency to increase with an increase in the current and a decrease in the electrode force. In several cases of the press-peel test, four welds of one joint fractured one by one in sequence and showed a peak load each time it broke. The first or second peak load became the maximum load of each joint. SEM (Scanning Electron Microscope) observation of the welds fractured at the early stage in the press-peel test revealed that the fractured surface can be classified into a good weld region and a bad one. The former was a dimple fracture surface or cleavage one and the latter was a relatively flat fracture surface with many oxides. The same analysis was conducted for joints with several welding conditions. It was consequently found that there is a good correlation between the area of good weld region and the maximum load. In addition, it was considered that the scattering of moderate expulsion during PW was effective for reducing oxides.
Authors: Dávid Molnár, Göran Engberg, Wei Li, Levente Vitos
Abstract: In FCC metals a single parameter – stacking fault energy (SFE) – can help to predict the expectable way of deformation such as martensitic deformation, deformation twinning or pure dislocation glide. At low SFE one can expect the perfect dislocations to dissociate into partial dislocations, but at high SFE this separation is more restricted. The role of the magnitude of the stacking fault energy on the deformation microstructures and tensile behaviour of different austenitic steels have been investigated using uniaxial tensile testing and electron backscatter diffraction (EBSD). The SFE was determined by using quantum mechanical first-principles approach. By using plasticity models we make an attempt to explain and interpret the different strain hardening behaviour of stainless steels with different stacking fault energies.
Authors: Aniruddha Dutta, Dirk Ponge, Stefanie Sandlöbes, Dierk Raabe
Abstract: We address the differences in yield stresses between hot and cold rolled medium manganese steel showing continuous yielding. Continuous yielding in both, the hot and cold rolled samples were resulting from reverted austenite islands plastically deforming first and less strain in the tempered martensite matrix. At higher global strains, strain was taken up not only by the reverted austenite, but also by tempered martensite and fresh martensite formed from the austenite through martensitic phase transformation during deformation. Strain localization was also observed in the hot rolled samples. This localization is caused by cumulative deformation of colonies of lamellar reverted austenite islands. It is interpreted in terms of the spatial alignment of austenite colonies to the loading direction in addition to the crystallographic orientation.
Authors: Wen Qiang Zhou, Li Bo Pan, Kuan Hui Hu, Wei Hua Sun, Rong Dong Han
Abstract: The press hardening steel sheets WHF1500H with different original structures produced by compact strip production (CSP) line in Wuhan Iron and Steel Ltd. were austenitized at 950 °C for 5 min, and then hot stamped and quenched by using the flat die. The microstructure and mechanical properties were investigated by optical microscope, transmission electron microscope and universal testing machine. The results show that the microstructure after stamping and quenching is fully composed of lath martensite. The mechanical properties of the steel samples with different original structures are different after hot stamping and quenching, and this difference is smaller than that before hot stamping and quenching. When the original structure consists of ferrite and pearlite, the austenite grains after austenitization are fine and uniform. In addition, the martensite structure obtained after hot stamping and quenching is also uniform and fine, leading to higher mechanical properties. With the increase of the strength of the original steel, when the original structure is martensite, the austenite grains after austenitization are coarse, and the martensite structures obtained after quenching is also coarse, and thereby decreasing the strength.
Authors: Yoshiteru Aoyagi, Atsushi Sagara, Chihiro Watanabe, Masakazu Kobayashi, Yoshikazu Todaka, Hiromi Miura
Abstract: In this study, yield surfaces of austenitic stainless steel produced by a cold-rolling process are measured using uniaxial and biaxial tensile tests. Using results obtained by electron backscatter diffraction, information on crystal orientation is introduced into a computational model for a multiscale crystal plasticity simulation. Finite element simulations for polycrystal of fine-grained austenitic stainless steel under biaxial tension are performed in order to predict yield surfaces of fine-grained austenitic stainless steel. Validity of predicted yield surfaces is evaluated by comparison between yield surfaces obtained by numerical simulations and experimental tensile tests.
Authors: Kyoichi Ishida, Taku Matsuo, Muneyuki Imafuku
Abstract: We investigated texture evolution features in fine-grained austenitic stainless steels (JIS-SUS304) under tensile deformation. Three kinds of fine-grained specimens having average grain sizes, d =0.5 μm, 2 μm and 9 μm were prepared. The mechanical properties and phase transformation behaviors of fine grained specimens were compared to those of commercially available SUS304 ( d =16 μm). Tensile test showed increase of yield and tensile strength and decrease of work hardening rate and significant martensitic phase transformation in earlier stage of deformation for 2 μm specimen. These results suggest that simultaneous progress of the dynamic recovery of dislocations in γ-phase and the evolution of harder α’-phase owe the elongation and hardening respectively in fine-grained specimen. From the result of texture analysis, the texture evolution process was different from for each specimen, although the final orientation was almost the same. At the plastic instability stage, an increase of {112}<111>γ phase and a decrease of {112}<113>α’ phase occurred simultaneously for fine-grained 2 μm specimen whereas the opposite tendency was observed for 16 μm specimen. Such a grain size dependence of texture affects an extra ductility of SUS304 at the latter stage of deformation.
Authors: Takahiro Izumi, Tatsuya Kobayashi, Ikuo Shohji, Hiroaki Miyanaga
Abstract: Microstructures and mechanical properties of lap fillet welded joints of several high and ultra-high tensile strength steel by arc welding were investigated. Steel plates having tensile strength of 400 (SPH400W), 590 (SPC590Y, SPC590R), 980 (SPC980Y) and 1500 MPa (SAC1500HP) class with 2 mm thickness were prepared. Four types of joints were formed by MAG welding; SPH400W/SPH400W, SPC590Y/SPC590Y, SPC980Y/SPC980Y and SAC1500HP/SPC590R. In joints with SPC590Y, SPC980Y and SAC1500HP steel which matrixes are martensitic microstructures, the HAZ softens due to transformation of martensite into ferrite with precipitating cementite. By using high and ultra-high tensile strength steel, the weld metal is strengthened due to dilution of the matrix into the weld metal and thus tensile shear strength of the welded joint increases. In the fatigue test, similar S-N diagrams were obtained in the all welded joints investigated. It seems that the effect of stress concentration due to the shape of the welded joint on fatigue properties is larger than that of the strength of the matrix.
Authors: Ling Ling Yang, Tatsuya Nakagaito, Yoshimasa Funakawa, Katsumi Kojima
Abstract: Yield strength of low carbon mild steel decreases when temper-rolling is applied to release yield point elongation. Generally mobile dislocation used to be considered as the cause of the YS lowering. However from Bailey-Hirsch theory, strength should be higher with temper-rolling because of the increase of dislocation density. To newly explain the lowering yield strength by temper-rolling, standing at the point that a few ppm carbon change Hall-Petch coefficient , decrease in yield strength by temper-rolling is investigated using an ultra-low carbon steel. Yield strength of steel with the small amount of solute carbon increased after 2% temper-rolling and didn’t change after aging. On the other hand, yield strength of steel with the high amount of solute carbon decreased after 2% temper-rolling and increased again after aging. Despite solute carbon content, the Hall-Petch σ0 increased by dislocation strengthening of temper-rolling. Hall-Petch coefficient ky of low solute carbon steel remained at the low level even after temper-rolling or aging , however, that of high solute carbon steels significantly decreased after temper-rolling and increased again after aging. Yield strength reduction of the high solute carbon steel can be attributed to the decrease of ky.
Authors: Chrysoula Ioannidou, Zaloa Arechabaleta, Arjan Rijkenberg, Robert M. Dalgliesh, A.A. van Well, S. Erik Offerman
Abstract: Nanosteels are used in automotive applications to accomplish resource-efficiency while providing high-tech properties. Quantitative data and further understanding on the precipitation kinetics in Nanosteels can contribute to fulfil this goal. Small-Angle Neutron Scattering measurements are performed on a Fe-C-Mn-V steel, previously heat-treated in a dilatometer at 650°C for several holding times from seconds to 10 hours. The evolution of the precipitate volume fraction, size distribution and number density is calculated by fitting the experimental Small-Angle Neutron Scattering curves. The effect of phase transformation on precipitation kinetics is also discussed. Complementary Transmission Electron Microscopy, Scanning Electron Microscopy and Inductively Coupled Plasma Optical Emission Spectroscopy measurements are performed to support the Small-Angle Neutron Scattering data analysis.

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