This report presents the simulation results of flow patterns of liquid metal in a twin roll strip caster. To analyze the velocity field in liquid pool, the influence of the nozzle flow and free surface were considered. The large eddy simulation-L.E.S.-model was applied to handle the turbulence of flow. The fluctuation of the fluid was measured using the full size water model test apparatus as well. The influence of nozzle flow on the flow pattern was clarified. In the pool the fluctuation of flow depends on casting speed, submerged nozzle type, and so on. The fluctuation of flow changes with the hole type of submerged nozzle.
Severe demand for thickness and flatness of steel plate has been increased, as users of steel plates are actively promoting the automation and abbreviation of processes. It is important to establish a shape control technology with higher accuracy in rolling process for TMCP steel plate that is produced by controlled rolling and accelerated cooling control process. In this study, to develop a new shape control technology in plate mill concerning the thermal profile control of a work roll, the thermal profile prediction model of a work roll with high accuracy has been firstly constructed by solving the two dimensional heat transfer equation with the perturbation method on thermal conductivity. Secondary, the change of thermal profile of a work roll in a rolling campaign has been investigated by using the newly developed thermal profile prediction model. It is verified that the thermal profile control as a new shape control technology can be realized with the variable roll cooling of work roll inplate mill.
Effect of glass lubrication on metal flow in hot extrusion of stainless tube in an example of Ugine-Sejournet process has been investigated experimentally and theoretically, which has been closely related to the surface defect of products. First, a relationship between viscosity of glass lubricants and friction coefficient is measured with a hot friction test. Friction coefficient is proportional to logarithmic viscosity of glass lubricant. Glass lubrication for hot tube extrusion is found in hydrodynamic condition. Then finite element analyses of metal flow during hot tube extrusion is carried out to simulate metal flows, strains and stresses. Computed extrusion forces and metal flows are in good agreement with experimental measurements. Quantitative flow characteristics such as redundant work and abrupt change of metal flow in bearing portion are evaluated with finite element analyses. It may be possible to optimize an extrusion condition of hot tube extrusion to reduce surface defects such as transversal crack of products.
The effects of trace inorganic additives on the morphology and crystal orientation of electrodeposited Zn have been investigated by using a polycrystal commercial low carbon Al-killed steel sheet as a base material and the sulfate bath with inorganic compounds of 4.8×10-64.1×10-2mol/l to bath. The current density, flow rate and bath temperature are 50 A/dm2, 1.2 m/s and 60°C, respectively. The in organic additives can be classified into 3 groups by the effects on the morphology and crystal orientation of electrodeposited Zn. When the inorganic compounds of Fe, Ni, Co, Al, Mg and Mn (Group I) are added to the bath, the relative intensity of (0002)Zn and Zn crystal size do not change because Group I additives do not offend the heteroepitaxial growth of Zn and the overpotential for Zn deposition. When the inorganic componds of Cr, W, Mo and Zr (Group II) are added to the bath, the relative intensity of (0002)Zn increases because Group II additives do not change the heteroepitaxial growth of Zn but decreases the overpotential for Zn deposition. When the inorganic compounds of Cu, Pb, Cd, In, Sn, Ge, Sb and Ag (Group III) are added to the bath, the relative intensity of (0002)Zn increases and Zn crystal size becomes small because these additives co-deposit with Zn and prevent the heteroepitaxial growth of Zn.
Rare earth elements are effective for improving the oxidation resistance of stainless steels and chromia forming alloys at high temperatures. The reasons for the improvement, however, are not well understood and the effect of the content of rare earth elements is not clear. In this study, Type 430 stainless steel covered with a lanthanum hydroxide overlayer with various lanthanum contents was oxidized at 1373K in a 1.671×104Pa O2-2.026×104Pa H2O-balanced N2 atmosphere. It was found that the critical amount of lanthanum atoms, at which reduction of the oxidation rate becomes obvious, was about 1016 atoms cm-2. This value is similar to published data obtained from studies of Fe-Cr alloys coated with different rare earth elements by surface coating or ion implantation. Observation of the oxide surfaces at the initial stages of oxidation showed that lanthanum oxide does not provide a physical barrier for oxidant migration through oxide films nor preferential nucleation sites for Cr rich oxides. An explanation for the increased oxidation resistance with lanthanum is a change in the growth mode of the oxide from a predominance of cation to anion diffusion, at the critical amount of lanthanum of 1016 atoms cm-2 in the surface oxide film.
Mechanism of decarburization in ultra low carbon steel sheet at the continuous annealing process has been investigated. Thermo-Calc and DICTRA programs were used for calculation analysis of decarburization and the calculation results were compared with those observed by chemical analysis. Decarburization was observed in the ultra low carbon steel sheets in a short continuous annealing treatment at 750°C for 10s. Decarburization and nitriding were observed in 3%H2+97%N2 gas atmospheres. Precipitation of Nb(C, N) affected decarburization kinetics, and the amount of decarburization decreased with the increase of Nb content because precipitates did not resolve during annealing. The decarburization rate obtained by this experiment is in a good agreement with those reported by Collin et al. The rate was dependent upon gas atmosphere and annealing temperature, but independent of carbon content in steel. Control of decarburization kinetics in continuous annealing process is indispensable for quality control of materials, because Yield Strength of ultra low carbon steel sheet after continuous annealing is closely related with the carbon content of steels.
The creep behaviour of alpha iron and its solid-solution Fe-Mo and/or W alloys and Fe-Mo and/or W with MX alloys was investigated at temperature of 600°C (0.48Tm, Tm: the melting temperature) under stress of 30 to 120 MPa. The results showed that the creep curves of these alloys were similar to those observed for ferritic steels. The minimum creep rate of εmin for solid-solution Fe-Mo and/or W alloys was three orders of magnitude lower than εmin in alpha iron. Also εmin for Fe-Mo and/or W with MX alloys was five orders of magnitude lower than εmin in alpha iron. By the dispersed MX particles the stress exponent of, n, increased to 11, while it was 7 in alpha iron. The observed substructure revealed that sub-boundaries with well-knitted dislocations were always formed and the subgrain size in steady state was inversely proportional to the applied stress. The subgrain size and the dislocation density within subgrains during creep decreased unequivocally with strain, while the misorientation of subgrains increased with strain. The rate of refinement for the subgrains of these alloys, however, was dependent on the solute Mo and W, its concentration and the dispersed MX particles. It is suggested that creep of these alloys is controlled by the mobility of dislocations in the subgrains and sub-boundaries under the present experimental condition, though the subgrains are refined due to creep.
A SUS308 stainless steel weldment of which main phases are ferrite and austenite was aged at 450°C for a time period up to 11000 h in order to investigate the compositional evolution of the ferrite by Mossbauer spectroscopy. Changes in the internal magnetic field of Fe with aging time suggest that the ferrite decomposes by spinodal process finally to produce the Fe-rich α and the Cr-rich α' phase. According to the mean internal magnetic field measurement, the Cr content of the former is about 9 at% Cr which is consistent with a calculated phase diagram. The Cr content of the α' phase is evaluated to be about 83 at% Cr by the isomer shift measurement, which shows a slight deviation from the calculated phase diagram. The phase decomposition is accompanied by an increase in the hardness of ferrite, while that of austenite is remained unchanged.
Long term creep strength property of a Mod.9Cr-1Mo steel has been investigated in conjunction with changes in microstructure, in order to understand a degradation mechanism at the long term conditions. The predicted creep rupture lives from short term creep deformation data whose rupture lives are less than 1000 h, are consistent with the experimental data up to about 10000 h. In the long term region longer than about 10000 h, however, creep rupture lives become shorter than the predicted one with increase in time to rupture. In the specimens creep ruptured at short term region less than about 10000 h, homogeneous progress in recovery of tempered martensitic microstructure, such as increase in lath width and coarsening of subgrain, has been observed. On the other hand, remarkable progress in such recovery of the microstructure has been observed at the vicinity of prior austenite grain boundary for the specimen creep ruptured at long term region under the low stresses. Such preferential recovery along a prior austenite grain boundary can promote the beginning of tertiary creep under the low stress condition. It has been concluded that preferential recovery along a prior austenite grain boundary is a main factor of a degradation of the Mod.9Cr-1Mo steel during long term creep deformation.
Effect of initial microstructure on the long term creep deformation properties of 2.25Cr-1Mo steel has been investigated and a correlation between creep strength and changes in microstructure during creep deformation has been discussed. Three different heat treatments of(1)quenching and tempering (QT), (2) normalizing and tempering (NT) and (3) annealing (Ann) were employed to obtain different initial microstructure. There was not clear difference in both creep rupture strength and microstructure of the creep ruptured specimens for NT and Ann steels. Creep strength of the QT steel is higher than those of NT and Ann steels at high stress and short term conditions. Very high dislocation density was observed on the creep ruptured QT steel at higher stress condition and that is distinctly different from those of the creep ruptured NT and Ann steels. In the lower stress conditions less than about 100 MPa, however, no difference in creep rupture strength and microstructure of those steels was observed. The differences in creep deformation behaviour of these steels were explained by the differences in the parameterα of a modified θ projection method. It has been shown that the magnitude of α depends on the stability of microstructure during creep deformation. Disappearance of the differences in creep rupture strength of three 2.25Cr-1Mo steels in the long term region is caused by decrease increep strength due to microstructural change, and the common long term creep rupture strength for three steels is the inherent creep strength of the 2.25Cr-1Mo steel. It has been concluded that a proper assessment on a stability of microstructure at the elevated temperature is very important to evaluate a long term creep strength property. It has been proposed that α parameter of a modified θ projection method is a candidate indicator to describe a stability of microstructure.
Fatigue hardening-softening behavior of a newly developed "TRIP-aided bainitic (TB) steel" which is composed of a bainitic ferrite matrix and retained austenite films was examined. A significant fatigue hardening, similar to "TRIP-aided dual-phase (TDP) steel" and conventional ferrite-martensite dual-phase steel, was exhibited in the TB steel, particularly when austempered at temperatures higher than the martensitestart temperature. This was mainly caused by an increase in the strain-induced martensite content and development of severe cell structure inthe matrix. Compressive internal stress in the matrix resulting from the stable and strain-hardened retained austenite films was expected tomake a relatively small contribution to the fatigue hardening, iffering from thatof the TDP steel.