The reduction processes of sintered ore in the fixed bed with CO gas have been studied.

The reduction rates of sintered ore in the temperature range from 800 to 1000℃ have been measured by analysing the waste gas composition or by weighing the bed weight continuously ; the following results are obtained.

(1) The reduction curves obtained by gas analysis are in good agreement with those by the thermobalance in the same experimental conditions.

(2) At a constant ratio of the bed weight to the gas flow rate (M/V), the same reduction curves were obtained at different gas flow rates (4.6, 9.1 and 36 Nl/min) ; the gas film resistance seems to be unimportant.

(3) The analysis based on the unreacted core model for a single pellet applied to the reduction of sintered ore with CO gas gives the following expression for the calculated chemical reaction rate constant k_s=exp(5.23-14000/RT). As the calculated constant F defind as r₀k_s/De is dependent of the diameter of sintered ore, it is considered that the reduction process is controlled by the chemical reaction rate for small sizes (Dp=3 mm), but is controlled by mixed rates of the chemical reaction and the gas diffusion through ash layer for large sizes (Dp=7-13 mm).

The present paper is concerned with the effect of surface movement on the rate of gas-metal reactions. Experimental studies were made by blowing argon-oxygen mixtures onto the silver melt heated electrically both by resistance and by high frequency induction. During the process of oxygen desorption from the melt of high oxygen concentration into the atmosphere of pure argon or the argon-oxygen mixture of very low oxygen potentials, the retardation of surface movement occurred and the surface became immobile. When the oxygen concentration of the melt became low, the surface movement recovered. On the other hand, in the case of oxygen absorption, the retardation of surface movement did not occur. The mass transfer coefficient for the immobile surface was smaller by a factor of about 7 than that for the mobile surface.

An analysis was made to explain the observed phenomena relating to the surface movement. By considering the shear stress due to the uneven distribution of the concentration of oxygen adsorbed on the surface leading to the uneven distribution of surface tension, the momentum boundary layer equation was solved. The analytical result gave a reasonable explanation for the experimental findings.

The rate of nitrogen removal from liquid iron and iron alloys containing oxygen, sulphur, silicon, carbon, nickel, and chromium, respectively, was studied at 1600℃ using argon as a carrier gas. The results obtained are as follows:

1. The reaction of nitrogen removal is of the second-order, and it is considered that the chemical reaction on the surface of the liquid iron is the rate-determining step. The rate of the reaction, 2N (in liquid iron) ↔ N₂(gas), is expressed by the following equation:

−d [%N] ⁄ dt = A ⁄ V k₂′ [%N]²

where, A and V are the surface area and the volume of liquid iron.

2. The activation energy of the nitrogen removal reaction is 35.2 K cal.

3. The surface active elements decrease the rate markedly, and the effects of other alloying elements on the rate depend on the effect of the activity of nitrogen in the liquid iron. A mechanism of nitrogen removal from liquid iron is proposed to describe the rate of reaction quantitatively by considering the effect of the absorption of the surface active elements. The apparent rate constant, k₂′ is expressesed as follows at 1600℃:

k₂′ = 0.974 f ²_N (1 ⁄ 1 + 28.4 [%O] + 5.34 [%S])²

where f_N is the activity coefficient of nitrogen in liquid iron.

Unusual zones with columnar structure were observed near the bottom of the 100 t ingot. Metallographic examination of the unnsual zones has shown that the growth direction of one zone differs from those of other zones. This means that unusual zones originally solidify elsewhere and finally settle in the sedimental zone.

As it has previously been proposed that the top surface of the ingot is the place where the unusual zones originally solidify, the existence of the frozen top crust was checked at the hot top of a 120 t ingot. Some of the top crust could be picked up in the relatively early stage of solidification of the ingot and their structure was also columnar.

A small scale experiment using Al-0.2%Cu alloy proved that the top crust formed by putting a chill block on the top surface of the ingot sank to near the bottom of the ingot.

The process analysis of the oxidizing period was carried out by using a 2 ton arc furnace. The changes of and the relationship among C, Si, Mn, Cr, Fe in the melt and the bath-temperature during oxygen blowing were analyzed.

A method of controling the carbon content after oxygen blowing was established by using a mathematical model obtained according to that analysis.

The controling accuracy of carbon content obtained by that mathematical model was twofold better than that of conventional method.

The greatest merit of the controling method by means of the mathematical model lies on the fact that even an unexperienced operator can control the carbon content after oxygen blowing easily and accurately. Furthermore, an operator-guide-control-system by a computer can be introduced into stainless-steel-melting.

The effect of boron on hardenability was studied in Al-B-N low alloy steels. It was clarified that the effect of boron on hardenability depends upon the amount of soluble boron which is in equilibrium in austenite and that the maximum hardenability can be attained at 3∿5ppm soluble boron. Based on the equilibrium theory, the amounts of Al, B, and N which can bring the maximum hardenability were calculated.

It was concluded that the addition of 0.06 to 0.08% Al gives rise to the maximum hardenability due to boron even with considerable variations of B and N content.

The relation between strength and toughness of tempered martensite and bainite was investigated for 5%Ni-0.5%Mo steels containing 0.15% or 0.30%C with various austenite grain size. When toughness of martensite is compared with that of bainite in the same strength level, former is more affected by austenite grain size and less affected by carbon content.

The surface and cross sections of fractured specimens after impact tests were examined by scanning and ordinary electron microscopy in order to investigate the structural factors affecting toughness. These microscopic observation indicated that the fracture facets were formed at the large martensites or bainites and at the packets of relatively thick laths which were found when the austenite grains were coarse or the C content was low. Although the concept of the effective grain size seemed to be valid in the martensite and bainite structure as the fracture facet size coincided with those of packets in these structures, behavior of toughness was not completely explained by this factor only. The role of carbide to impair touhgness is emphasized from the point that the increase of C content decreases the toughness of the bainite, in spite of the structure becoming fine, where the carbides become coarse and abundant.

It is shown that the recrystallization texture of low carbon steel containing a small amount of copper depends on the heating rate of the final annealing. In a steel containing 0.61%Cu, the (222) pole intensity shows a maximum at about 100℃/hr and a minimum at about 50℃/hr. This behavior is interpreted qualitatively by separating the (222) pole intensity into two components; one from {111}<110> orientation and another from {554}<225> orientation. The microstructures after recrystallization are also interpreted by considering the difference in the condition for the selection of the recrystallized grains of each orientation. In a steel containing 0.29% Cu, the (222) pole intensity also shows a weak maximum at about 100℃/hr and the {111}<110> component is weak in the recrystallization texture of this steel.

High pressure and temperature steam condition of recent steam turbine demands development of 12%Cr steel having superior elevated temperature strength. So, the effects of individual addition of alloying element such as Mo, W, Ti, V and Nb have been investigated in order to improve the elevated temperature strength of 12%Cr steel.

The elevated temperature strength of 12%Cr steel is increased by individual addition of alloying element such as Mo, W, Ti, V and Nb. Additions of Mo or W results in solid solution strengthening as well as precipitation hardening, whereas those of Ti, V or Nb results in precipitation hardening. The strengthening effect can be explained by the difference in atomic volume between Fe and the alloying elements. It is observed under transmission electron microscope that the dislocation density becomes higher, as the difference in atomic volume is larger. As a result, the creep rupture strength of the steel is increased.

The effects of molybdenum and heat treatment on the micro-structure and hardness of 0.15C-15Cr-14Ni heat resisting steels have been investigated in both solution treated and aged conditions. Structural examination has been made using optical and electron microscopies and X-ray diffraction analysis of electrolytically extracted residues. The results are as follows:

The carbon solubilities of the steels after solution treatments were determined by electrolytical isolation method. It was shown that the dissolved carbon content decreased progressively with molybdenum content. In these conditions the undissolved carbidies, mainly M₂₃C₆, tended to coarsen with increase of solution temperature and of molybdenum content. Below 1% molybdenum, the coasening was not remarkable but over this content there was progressive increase in the diameter of undissolved carbide. It was founed that the distribution of this particle increased with increase of molybdenum content and with decrease of solution temperature, and had an important effect on the hardeness and the austenite grain size.

During the aging of the steels, massive, cubic, and ribbon-like M₂₃C₆ and rod-like M₆C precipitated. From the results of the microstructural observation and hardness testing, it was found that there was a linear relationship between the logarithm of distribution density of cubic M₂₃C₆ plus rod-like M₆C and the logarithm of amount of age-hardening.

Diffusion coefficients of vanadium, copper, silicon, phosphorus, and sulfur in molten iron saturated with carbon have been measured by the capillary-reservoir technique in the temperature range from 1350° to 1550℃.

The results are summerized by the follwing Arrhenius' equation:

D_V = (6.45 ± 1.08) × 10⁻³ exp (− 18700 ± 600 ⁄ RT) cm²/sec

D_Cu = (41.5 ± 7.5) × 10⁻³ exp (− 21700 ± 700 ⁄ RT) cm²/sec

D_Sl = (1.85 ± 0.28) × 10⁻³ exp (− 13300 ± 500 ⁄ RT) cm²/sec

D_P = (7.05 ± 0.95) × 10⁻³ exp (− 17700 ± 500 ⁄ RT) cm²/sec

D_S = (0.98 ± 0.10) × 10⁻³ exp (− 10000 ± 300 ⁄ RT) cm²/sec

The present results have been discussed together with the previous results on the diffusivities of chromium, manganese, cobalt, and nickel in molten iron saturated with carbon (Refer to Trans. ISIJ, 11 (1971), p.275). A definite correlation has been found between the diffusivity and the thermodynamic interaction of the diffusing element with the solvent as expressed by the standard free energy of solution of the diffusing element in molten iron saturated with carbon.