The effect of chromium on elevated temperature properties of ferritic heat resisting steels containing Cr, Mo, V and Nb was studied. The chromium content was varied from 3 to 15% in order to find the optimum value. The tempering characteristics and the tensile strength were measured and creep rupture tests were carried out at 550-700°C. The microstructures were observed by a transmission electron microscope.
The results indicated that the low chromium steels have a good tempering resistance during the short term tempering, but have a tendency to soften markedly after the long term tempering. With the increase of chromium content up to 8%, the tensile strength increases but the ductility decreases. In high chromium steels, it is observed by a transmission electron microscope that the coalescence of precipitates is accelerated, but that the recovery of matrix is markedly retarded.
It is concluded that the optimum chromium content is 10% at the testing temperature of 600°C.

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A mathematical model of liquid flow in cylindrical vessels has been developed and applied to the case of ladle tapping operations. The results of the work indicate that significant three dimensional effects are to be expected when jets penetrate a bath of steel in an off centre and/or inclined mode, and these are described in the text. Comparisons with experimental flow fields demonstrate the predictive capability of the present model.

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For predicting effective diffusivities of bi-disperse porous media, an improved model in which tortuosity factors of the macro- and micropores are taken into account is proposed on the basis of the Wakao and Smith random pore model. The diffusion fusion rates of binary gases at constant pressure and the effective diffusivity are derived according to the model.
By the use of the experimental data of Wakao and Smith and of Henry et al., tortuosity factors of their porous solids are determined approximately on the basis of the model. The theoretical effective-diffusivities calculated by using the tortuosity factors agree better with the experimental ones over the pressure range 0.0006 to 12atm than those calculated on the basis of the random pore model.
In order to examine the influence of an inert gas on the diffusion process, solutions to the Stefan-Maxwell equations for equimolar counter diffusion in a ternary gas mixture through a bi-disperse porous medium are derived on the basis of the model; the diffusion rate and the effective diffusivity in this system are determined reasonably.
The expressions for the simple one-dimensional diffusion mentioned above are extended to those for the radial diffusion in a spherical shell.

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A mathematical model is proposed for the mill arrangement equivalent to semi continuous type in a hot strip mill. The rolling schedule is arranged so that a slab is rolled from 230mm into 60mm in bar thickness by 3 passes roughing with only one reverse mill and 60mm into 1.2mm, 2.0mm, and 2.5mm in final thickness by 7 passes finishing. The possibility of rolling in this case is analytically examined in due consideration of slab weight, temperature change, rolling power, rolling capacity, and rolling speed. As a result, the rolling of the comparably heavy slab with 30t/m in unit weight is enabled, and the temperature condition is sufficiently fulfilled. The length of the rolling mill line can be geometrically shortened very much in comparison with the case of other type hot strip mill.

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Hot ductility in steels was studied. Special emphases were placed on the effects of thermal history, strain rate and fracture mode in order to clarify the sensitivity of surface cracking during both continuous casting operation and direct hot rolling.
There exist three temperature regions where typical embrittlement is noticed, i.e., Tm-1200°C (I), 1200-900°C (II), and 900-600°C (III). The cause of the embrittlement in the region I is the existence of residual liquid film along the dendritic interfaces. The ductility is found to be independent of the strain rate. In the region II, the precipitation of finely distributed oxy-sulfides at the austenite grain boundary weakens the boundary strength, and thus overaging treatments such as slow cooling, holding for certain time, or slow rate of straining result in good ductility. On the other hand, the embrittlement in the III region is manifested by the slower strain rate of test. Controlling factors of this embrittlement are precipitation of oxides, sulfides and nitrides, precipitation of proeutectoid ferrite film along austenite grain boundary as well as grain boundary sliding. Detailed mechanism is discussed.

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Single hematite pellets having not so large porosity are reduced over a temperature range 600-1000°C and a flow-rate range 0.1-20Nl/min. In the initial stage of reduction, hydrogen is diluted by nitrogen which flows through a reactor until it reaches a set temperature, and the reaction retards especially at low flow rates. In the course of reduction, hydrogen is also diluted by the product gas. For these dilution processes, the dispersion model is introduced into the unreacted-core shrinking (UCS) models for one and three interfaces in consideration of the resistance due to the rate of gas flow proposed by Clair. With appropriate selection of kinetic constants, the modified models produce greatly improved fit to experimental data.
Porous wustite pellets are reduced and their reduction rates are analyzed by using modified grain and intermediate models; for this case the latter analysis gives rather satisfactory results. Then, multi-stage diffusional (MSD) model is derived on the basis of the intermediate model, and the calculated results are compared with hematite-reduction data. Both reduction curves based on this model and the three interface UCS model reproduce experimental ones very well, while interface radii based on the MSD model simulate cross-sectional view much better than those based on the three interface model. When basic hematite pellets are reduced at 1000°C, the reduction rate decreases extremely in the final stage. An equation of solid-state diffusion is introduced into the MSD model; fairly good agreement is seen up to the final stage.
Packed beds of Hamersley pellets are reduced over a temperature range 600-1000°C and a flow-rate range 1-45Nl/min. Not only reduction curves and exit hydrogen flow-rate but core radii and local fractional-reduction in beds reduced partially are measured. The equations of mass balance for a fixed bed are combined with the UCS models for one and three interfaces and numerical calculations are done by the method of characteristics or by an approximate approach; the results calculated by both methods are much the same. Calculated results based on the three interface model reproduce experimental data much better than those based on the Rist model or the one interface model.
Porous pure-hematite pellets and basic pellets (fluxed pellets) are reduced at 800 and 1000°C, respectively, and their rates are analyzed by the approximate approach based on the MSD models with and without the solid-state diffusion; each analysis reproduces respective experimental data very well.

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