Recent applications of Magnetohydrodynamics to the steelmaking process are reviewed. Many applications have been developed in the field of steelmaking because of the demand to employ clean and efficient remote actions for various states of the liquid and solid steels by using various functions such as heating, stirring, braking and shaping. Not only the endeavor of the technological development but of the theoretical investigations play important roles. Plenty of R&D has been tried, and some of the technologies are already established and some are still in the very fundamental stage or interrupted because of their difficulties. In this review, some of the latter cases are also picked up, because the recent or future progress in the surrounding technologies or the change in the demand might lead to the retrial of the development.
An investigation of the reduction of Cr2O3 and Fe2O3 by graphite was carried out using a nonisothermal gravimetric technique under an argon atmosphere, in a temperature range from 600 to 1973K. In the case of Cr2O3-C system, the reduction reaction proceeds through two stages. At first, Cr2O3 is reduced by graphite to form chromium carbide, and then the rest of Cr2O3 by the chromium carbide. The activation energies of the reaction are evaluated from 270 to 293 kJ/mol in the first stage and from 327 to 359 kJ/mol in the second stage. Excess graphite addition increases the reduction degree, and chromium carbide is obtained. Magnesia addition delayed the reduction, while hematite addition accelerates it. In the case of Fe2O3-C system, the reduction proceeds through three steps. In the first stage, Fe2O3 is reduced by graphite to form Fe3O4. In the second stage, Fe3O4 is reduced to form FeO. In the third stage, FeO is reduced to form Fe. The activation energies of the reaction are 504 kJ/mol in the first stage, 858 kJ/mol in the second stage and 243-312 kJ/mol in the third stage. Excess graphite addition increases the reduction degree.
Two types of iron ores were reduced partially until some levels of metallization in a laboratory fluidized bed. These fines were briquetted under cold or hot press machines. Then the briquettes were isothermally reduced in a thermobalance at 900-1200°C with CO-CO2 mixture to elucidate their kinetics, imaging semi reduced iron burdens in a blast furnace. The briquettes from Australian are indicated a severe reduction delay in whole periods above 1100°C due to the formation of liquid slag. The addition of 2% MgO into briquettes released fairly this delay. The briquettes from Brazilian are poor in gangues indicated no reduction delay even at 1200°C. The briquettes seemed to be reduced according to the first order reaction equation on remained wustite. The obtained rate parameters kM were almost similar between cold and hot briquettes in spite of more density and crushing strength for the latter than the former, because smooth gas diffusion was ensured insides due to both briquettes having relatively large porosities. Crushing strength of hot briquettes reached more than 1000 N/briquette.
In recent years, the promotion of recycling of steels is desired from the viewpoints of effective usage of resources and restraint of CO2 emission. On the other hand, some elements (As, Sb, Bi, B, Zn and so on) affect seriously the characteristics of recycled steel and it is difficult to refine away these elements from recycled steel in the iron recycling process. These elements are called tramp elements. For that reason, a new technique to determine the tramp elements in steel is demanded. In this paper, tramp elements, As and Sb, in iron certified reference materials were determined by neutron activation analysis with multiple gamma-ray detection method. For determination of As, two samples (JSS001-5, JSS168-7) were irradiated for 10 min at a neutron flux of 5.2×1017 m-2 s-1 and measured by the multiple gamma-ray detector, GEMINI-II, which consists of 16 Ge detectors with BGO Compton suppressor, for 12 h. For determination of Sb, seven samples (JSS001-5, JSS003-2, 4 and so on) were irradiated for 5 h at a neutron flux of 9.6×1017 m-2 s-1 and measured by GEMINI-II for 24 h. The determined values were in good agreement with the certified and reference values. The lower limit of determination values for As and Sb in high purity iron were 0.01 and 0.003 ppm, respectively. The demand on the limit of determination values for As and Sb is 0.1 ppm. From these results, this method has turned out to be useful for determining As and Sb in recycled steel.
The surface crack is fatal for the spring which high fatigue characteristic is required. The surface crack of drawn wire has big influence on the quality of product. As a cause of a surface crack, rolling of a wire rod and the poor handling at the time of conveyance are cited. The authors carried out multi-pass drawing of the stainless steel wire with artificial transversal crack, and have investigated growth and disappearance of crack from both sides of experiment and FEM. If drawing of the wire rod with a surface crack is repeated, it will become the check mark which the width of a concave crack spreads. Furthermore, if drawing is repeated, it will become long and slender in the direction of an axis by stress, and will become quite small. However, the crack of the direction of an axis remained and it was shown clearly that quality is affected. Moreover, the influence which the surface treatment of peeling has on a wire was also considered.
Work hardening behavior of IF and low carbon Al-killed steel sheets (SPCE and SPHC) under biaxial tension is investigated using biaxial tension tests and hydraulic bulge tests. The test materials have different r-values: an average r-value, r, is 1.87 for the IF steel, and 1.76 and 0.95 for SPCE and SPHC, respectively. In the biaxial tension tests detailed measurements are made of the initial yield locus, contours of plastic work for different levels of work hardening, and the directions of the incremental plastic strain vectors for linear stress paths. Only the IF steel and SPCE, which have relatively large r-values, exhibit the differential hardening, while SPHC work-hardens almost isotropically. The results are consistent with the author's earlier research. True stress-logarithmic thickness strain curves measured in the bulging tests are also consistent with the biaxial tension test results. Accordingly, it emerges that the differential hardening is characteristic of steel sheets with large r-values (about 1.5 or more). The work-hardening behavior of the test materials predicted using Yld2000 yield function (Barlat et al., Int. J. Plasticity, 19 (2003), 1297) agrees closely with the observations.
It has been reported that the characteristic of rolling contact wear of rail steels varies depending on the microstructure of steels. However, the reasons for such variations have not been fully elucidated yet. Accordingly, a two cylinder rolling contact wear test was conducted by using pearlitic steels, tempered martensite steels, spheroidal carbide steels and pro eutectoid cementite steels which contain pro-eutectoid cementite structures in pearlite structure. The relation between micro-structures and wear in high carbon steels was investigated, and the dominating factor of the rolling contact wear and the wear mechanism in high carbon steels was discussed. The main findings are as follows:(1) The wear property of high carbon steels is greatly influenced by the micro-structural morphology, and the wear resistance of pearlitic steels and pro eutectoid cementite steels with the lamella structure as pearlite are higher than the tempered martensite steels and spheroidal carbide steels with the carbide dispersion structure (the carbide are distributed in the matrix ferrite). (2) The reason why the wear resistance of steels with the lamella structure as pearlite improves compared with steels with the carbide dispersion structure is attributable to the prevention of adhesive wear due to increasing hardness (strength) of the rolling contact surface and to the prevention of the occurrence of fatigue wear due to flaking in the rolling contact surface.
In order to find out the effect of cementite decomposition on delamination, decomposition behavior of cementite in high carbon steel wires during drawing and aging was investigated by atom probe field ion microscope, transmission electron microscope and torsion testing. Fracture mode of normal fracture in torsion test is Mode III and that of delamination is Mode II. Microvoids or coarse grain boundary ferrite are not observed in the delaminated wires. Cementite decomposition proceeds with the drawing strain and aging, and the carbon concentration of lamellae ferrite remarkably increases, which can only be due to the dissolution of cementite and concurrent migration of carbon atoms into ferrite. The measured carbon concentration varies from region to region, depending on the lamellae spacing or the location within wires. It is considered that cementite decomposition proceeds heterogeneously in macroscopic and microscopic scale. Delamination occurs in the regions in which the maximum carbon concentration of ferrite exceeds 1 at%, irrespective of the dry drawn, the wet drawn and the aged wires. The heterogeneous decomposition of cementite suggests that the strength of ferrite is non-uniform. It may be concluded that delamination is caused by the non-uniformity of ferrite strength. Namely, delamination results from the local shear stress concentration in the low ferrite strength regions, which correspond to the locally low carbon concentration areas.
The residual stress of a quenched steel bar with induction heating was measured by neutron diffraction. The conventional X-ray sin2Ψmethod was also used and three dimensional stress condition was estimated from plane-stress data obtained by a progressive polishing technique. Good agreements were found between the results by the neutron and X-ray methods. Improvements of the neutron stress measurement are discussed.
We performed a high-speed tensile test at a strain rate of 103s-1 below room temperature for ultrafine-grained ferrite-cementite (FC) steels with ferrite grain sizes of 0.47, 0.7, 1.1, and 1.5 μm, which were obtained by using a low carbon steel. In the high-speed tensile tests, with decreasing of ferrite grain size, flow stress increased but uniform and total elongations decreased. When the effects of temperature and strain rate on flow stress between the FC steels and the FP steels are compared at the investigated range of temperature and strain rate, thermal stress component of the FC steels is bigger than that of the FP steels. The effect of ferrite grain size on flow stress of the FC steels is almost independent of temperature and strain rate including the high strain rate. That means the ultrafine-grain strengthening depends on the increasing of athermal stress component. The effect of ferrite grain size on absorbed energy obtained by the high-speed test was discussed in various steels.