In this study, an experimental investigation has been carried out for analysis of influence of molten steel flow on homogenization of initial solidification continuously cast slabs. By applying EMS to produce medium carbon steel, it is confirmed that EMS remarkably improves the uniformity of solidification shell thickness over the width of slabs. In the present study, experimental investigation and 3-D magnetohydrodynamic calculation have been carried out for analysis of the influence of molten steel flow on temperature in mold, distribution of solute in slab, heat flux to mold plate, and frequency of nucleus formation. The following results are obtained. (1) A steady level of initial solidification uniformity is possible to be maintained by providing steady molten steel flow along the solidification front of continuously cast slabs. (2) According to 3-D magnetohydrodynamics calculation taking into consideration of heat transfer and solidification, electromagnetic stirring technique is useful for improvement of initial solidification uniformity. In the large-scale application to the commercial production, EMS contributes to reduce surface defects originating from longitudinal cracks.
We have proposed a technology of swirling flow formation in submerged entry nozzles as a fundamental and effective measure for controlling flow pattern in continuous casting molds. A joint study started in 1997 between Nippon Institute of Technology, Osaka University, Kyushu Refractories and Sumitomo Metal Industries to develop swirling flow submerged entry nozzles with a swirling blade for steel casting. As a first step, we have developed a swirling flow submerged entry nozzle for round billet casting in Wakayama works. Generally, single port submerged entry nozzles are applied for round billet casting with small section molds. Outlet flow from the single port intrudes deeply in strands, which causes lack of equiaxed crystals at the center part of billets and low temperature of molten steel surface in the mold. Low temperature of molten steel surface in the mold induces solidified deckle and retard melting of mold fluxes. As a countermeasure of these problems, M-EMS (Electromagnetic stirring in mold) is widely applied for round billet casters. The basic effect of the swirling flow in the single port submerged entry nozzle for the round billet casting was evaluated by a wood metal experiment, then we applied the swirling flow submerged entry nozzle for the round billet casting. As a result of steel casting in Wakayama, we confirmed same phenomena as the wood metal experiment as follows; (1) The surface temperature of molten steel in the mold rose in the case that the swirling flow in the nozzle and the M-EMS were combined in the opposite direction of rotations by appropriate balance of intensity. (2) The surface temperature of molten steel in the mold also rose in the case without the M-EMS. (3) The surface temperature of molten steel in the mold was thought to rise as a result of the upward flow formation along the mold wall.
The Spark OES-PDA (Pulse height Distribution Analysis) method has been widely used in the field of steel making processes, as a rapid composition analysis of elements, such as aluminum in molten steel, according to the chemical states respectively: acid soluble (Sol.), insoluble (Insol.) and the Total (Insol.+Sol.). When a sample surface is sparked, several thousands of elemental emission intensities are stored and transformed to the frequency distribution of PDA histogram. In literature, symmetric normal distribution of low intensity pulses on the histogram was identified to be the acid soluble (Sol.) component in the metal, whereas the high intensity pulses were attributed to the acid insoluble inclusions (Insol.). In this study, the collapse process of inclusions on the sample surface was investigated using optical microscope, SEM-EDS, EPMA, and spark-OES. It was found that the number of inclusions decreased sharply before the stable discharge region over 500 pulses, because the inclusions were initially destroyed and dispersed finely into the metal matrix. As the result, the emission intensities of the symmetric normal distribution region are superimposed on those of finely dispersed inclusions (Fine Insol.) and the elements dissolved in the metal (Sol.). It is proposed in this study that symmetric normal distribution of the PDA histogram should be reassigned to the Total (Insol.+Sol.).
T.M.C.P. (Thermo Mechanical Control Process) has been widely used in the steel industry. We have produced fine grain hot strips and hot plates through high reduction and low temperature rolling. However austenite structure before ferrite transformation has not been clarified, since it is impossible to freeze the austenite structure in carbon steels. The fine ferrite grains are generally inherited from parent austenite grains through phase transformation at high temperature. Electron backscattering diffraction with field-emission scanning electron microscopy (EBSD) was recently used to analyze the martensite structure with Kurdjumov-Sachs orientation relationship by the use of the pole figures. In this work, the computational variant analysis method using the rotational matrix of Euler angle data measured by EBSD was applied to determine the prior austenite grain boundaries. The prior austenite structures were constructed from the ferrite/pearlite structures of the multiple hot-compression test samples and of the hot strips at the actual tandem hot strip mill. The accuracy of the prior austenite grain size obtained by the present method is coincided well with the results from corresponding compression experiments. By employing the numerical method as an effective tool, a comprehensive investigation into the fine grain ferrite structure would be very useful.
Gigacycle fatigue tests were conducted for hydrogen charged SCM440 low-alloy steel in order to investigate the effects of hydrogen on fatigue properties of high-strength steel. 200 and 550°C tempered samples were prepared and an ultrasonic fatigue testing machine was used for the fatigue tests. The base steel without hydrogen charging showed only fish-eye fractures in case of the 200°C tempered version, while the 550°C tempered version showed surface and fish-eye fractures in short and long life regions, respectively. On the other hand, the hydrogen charged steel of both versions showed only fish-eye fracture and the fatigue strength largely dropped. These results meant that the hydrogen effects were larger on the fish-eye fracture than on the surface fracture. However, no extension of ODAs was observed in the hydrogen charged steels, so the degradation of fatigue strength due to hydrogen had nothing to do with ODAs. Moreover, in this research, it was confirmed that the ultrasonic fatigue testing machine was effective in evaluating gigacycle fatigue properties of the hydrogen charged steel. The ultrasonic fatigue testing machine enabled us to finish the gigacycle fatigue test before the specimen lost a large amount of hydrogen due to emission, and no frequency effects were observed in case of fish-eye fracture even in the hydrogen charged steel.
Fatigue crack growth under mixed mode loading is an important problem in railway fields. Therefore, a method was developed and prepared to observe the fatigue crack propagation under mixed loading of tensile and in-plane shear modes by an in-plane biaxial fatigue machine. In the experiments, sequential and overlapping mode I and mode II cycles were applied to the crack in the cruciform specimen made of wheel or rail steel. Growth rate laws for wheel and rail steel were obtained by means of least square regression analysis in terms of both the effective stress intensity factor ranges. The fracture surfaces near the crack tip region of tested specimens were observed by SEM and discussed to relate the superiority of pearlite structure over tempered martensite structure in detail.