This is a review of kinetic studies on fundamental reactions pertinent to steelmaking processes. With citing research results mainly obtained in theauthor's laboratory, description of the paper is made aiming at showing the present state of the knowledge on the selected subjects. Results of studies on the behavior of bubbles and jets at the submerged orifice (nozzle) are reviewed. The mixing characteristics in a gas-stirred molten metal is discussed. Kinetic studies on carbon-oxygen reactions and nitrogen reactions in molten iron are presented. Comments are made on the rate-controlling mechanism of slag-metal reactions. A summary is made of theresults of studies on the rate of mass transfer between molten slag and metal influenced by gas injection. Kinetic problems in scrap melting and the rates of dissolutions of solid iron and Cr₂O₃ in liquid iron with formation of CO are handled.

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Bubble dispersion and mass transfer between gas and liquid in a gas-stirred system have been studied experimentally and theoretically.
Nitrogen gas was injected into water through a nozzle located at the bottom center of a cylindrical vessel. Local gas-holdup distributions were measured by an electrical resistivity probe. The volumetric coefficient in the bubble-dispersion zone for the absorption of CO₂-water system was measured. Experimental conditions were as follows: gas-flow rate (q_G) =(16.7-167)×10^-6m³/s, radius of vessel (r₁)=O.055-0.50m, height of water (z₁)=0.1-0.4m and diameter of nozzle =6mm.
A mathematical model based on the boundary-layer theory is proposed. The model consists of equation of flow with uniform effective kinematic viscosity υ_e and equations of bubble and solute diffusion with uniform effective diffusivities, D_{e, B} and D_{e, S}, respectively. Equations were solved numerically assuming υ_e=D_{e, B}=D_{e, S}, and the theoretical distribution of local gas holdup, axial velocity, and solute concentration were obtained. By comparing the theoretical distributions of local gas holdup with the measured ones, values of υ_e could be obtained for various q_G, r₁ and z₁. The values of υ_e were correlated with q_G on the basis of dimensional analysis. This correlation was consistent with related data available in the literature. Volumetric coefficients, calculated by the present model, were in agreement with the observed ones.

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The size distribution of bubbles in iron melt is one of the fundamental knowledges to analyze the gas injection processes. The method to estimate the size distribution from the chord length distribution, which is obtainable experimentally, is derived from stochastic consideration. The derived method is confirmed by Monte Carlo simulation using micro-computor. An example of the size distribution of dispersed bubbles in iron melt is shown.

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The penetrating behavior of a single particle impinged on liquid surface has been studied with cold model experiments.
The velocity of a polystyrene particle at impact on distilled water or glycerin liquid was obtained by detectors consisted of laser diode photo sensors. The behavior of the particle, entrained gas bubbles and liquid flow were observed by using a high speed video and camera and analyzed by a mathematical model.
At impact of the particle on liquid surface, a cavity was formed and gas was entrained as a columnar bubble, when the particle impact velocity (V_po) exceeded a critical velocity.
Although the penetrating length of the particle (L_p) increased with increasing V_po, the rate of increase of L_p gradually decreases with increasing V_po.
A mathematical model was developed to predict how the initial kinetic energy of the particle was distributed and consumed by several energy dissipation mechanisms.
It was inferred that most of the kinetic energy was consumed as dissipation works, mainly spitting and heat generation in a higher region of V_po and hence the attainment of a higher particle velocity could not be an effective means to increase penetration length into the liquid. The effect of reduction of the apparent liquid density by increasing gas holdup in the liquid with entrained gas or gas jet was discussed as an effective way to obtain deep penetration of powders into the liquid.

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In order to understand the optimal condition of the design and operation of the lance equipped with secondary nozzles as well as main ones, the behavior of the inclined oxygen jet was theoretically analyzed and the post combustion ratio was calculated after combining the previous experimental data with the theoretical jet model.
Equations of continuity, conservations for momentum, enthalpy and gas species inside the jet were derived from quasi one dimensional assumption. The numerical calculation of the inclined jet by using Runge-Kutta method enables us to describe the jet trajectory, the distributions of velocity, temperature and mass fractions. Under the assumption that CO₂ or O₂ in the impinging jet reacts with C at the interface, under the rate-determing step of gas diffusion, the relation between the reaction rate and the theoretically calculated physical quantities is obtained statistically from the data in the real converters.
The calculation of this model shows that the optimal secondary oxygen flowrate is determined by the factors such as the lance height, nozzle diameter, nozzle angle, etc. in order to obtain the maximum post combustion ratio.

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New methods have been developed for accelerating the nitrogen removal in stainless steel under reduced pressure. In the first method, nitrogen can be removed effectively by reducing the ambient pressure at high carbon levels after interrupting oxygen top blowing in the VOD process. In the second method, the efficiency of nitrogen removal increased twice as much as that in the oxygen gas blowing by blowing the powdered oxidizer into the molten steel from a top lance under reduced pressure. In these methods, the favourable conditions for increase of nitrogen removal were satisfied, such as the increase of gas-metal interfacial area where the oxygen content is kept at low level and so on.

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In order to investigate the decarburization behavior in the RH-reactor, the samples of bulk steels and splashed metals were directly taken from the vacuum vessel by the use of a specially designed sampler in addition to the conventional ladle samples.
The analyses of samples have revealed that (i) the carbon content of the bulk steel is uniform throughout the vacuum vessel and (ii) the carbon content of the splashed metal is almost the same as that of the bulk steel. On the basis of the results obtained, several decarburization mechanisms were examined and a new decarburization model has been proposed. In this model, it is assumed that the C-O reaction in the bulk steel is affected by the static pressure of the vessel and also by the evolution pressure of CO bubbles, therefore the reaction gradually decreases with the progress of decarburization. This mechanism covers the reason of low reaction rate in the final stage of decarburization.
Along with the discussions of the mechanisms, a new method to measure the circulation flow rate of molten steel has been established and also a new equation which is able to make more precise estimation of the flow rate has been derived. A few experiments have been made with the 100t RH-reactor in Hirohata Works, NSC to accelerate the decarburization rate. It has been found that the use of oval shape snorkels is effective in the early stage, and that argon blowing from the bottom of vacuum vessel is effective in the final stage of decarburization.

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The rate of dephosphorization of liquid iron-carbon alloys by FeO-CaO-SiO₂ slaps and the effect o f NaCl or CaCl₂ added to the slaps on the reaction rate have been investigated on a laboratory scale at temperatures of 1300-1460°C. The results show that the rate and degree of dephosphorization increase by the addition of NaCl or CaCl₂ to the slag. But the degree of dephosphorization decreases with increasing temperature.
It is found that the rate of dephosphorization of iron-carbon melt can be explained by the modified mass transfer controlled model in which mass transfer of iron and phosphorus across the slag-metal interface is controlled by diffusion while the evolution of CO gas is the chemical reaction controlled.

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Removal of copper and tin in molten steel was investigated in a laboratory scale plasma furnace. It was found that the removal of copper and tin proceeded by both argon-hydrogen plasma and argon plasma even at 10⁵Pa. The degrees of removal of those elements were much greater at reduced pressures, at higher hydrogen contents in the plasma gas, and at higher plasma gas, flow rates. Maximum degrees of removal of copper and tin were about 90 and 60% respectively in 2h.
The removal of copper is thought to take place through accelerated evaporation at the very high temperature hot spot created by the plasma. However, the removal rate is assumed to be controlled by the transport of gaseous species evaporating from the melt surface.

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The Daido Steel Co., Ltd, has been developing a new scrap melting process using the reaction heat of powder carbon as an energy source in place of electric power. The experimental furnace is called the reactor, and the melting rate is 0.5t/h. Up to about 50% of the CO gas, which is generated in the iron bath by blown-in powder carbon and oxygen, is oxidized into CO₂ by top-blowing oxygen in the upper space of the furnace. In order to improve the melting heat efficiency, scrap is pre-heated by the waste gas from the furnace before being changed into the iron bath. Melting heat efficiency for scrap has been increased as much as 40% by making various improvements in the furnace structure, giving some prospect for application to the full-scale operation.

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