Showing papers in "Isij International in 1984"

Mathematical Analysis of Segregations in Continuously-cast Slabs

Abstract: A method of mathematically analyzing interdendritic microsegregation was established using finite difference method and taking into consideration the diffusion of the solute in the solid and liquid phases. The cross-sectional shape of dendrites and the fact that the enrichment of the solute in the liquid phase at the solid-liquid interface restrains the advancement speed of the solid-liquid interface were considered. Directional solidification tests to examine interdendritic segregation were made to verify the mathematical analysis method established. The advantages of the new method over other methods were discussed. Then, spot-like segregations were mathematically analyzed applying the same method, and the results were in good agreement with the observations in continuously-cast slabs.

Embrittlement of Steels Occurring in the Temperature Range from 1 000 to 600°C

Abstract: Characteristic features of the embrittlement occurring in the temperature range from 1 000 to 600°C in various kinds of steels were examined by hot stage tensile test as well as fractographic analysis, and the mechanism of this embrittlement is discussed.The embrittlement is caused by the slower strain rate of the tensile test and fracture occurs along the austenite grain boundary by either grain boundary sliding or by localization of strain in the film-like proeutectoid ferrite produced by the γ-α transformation.Steels such as ferritic steel and electrolytic iron do not have this embrittlement, while carbon steels containing 0.05 to 0.4% carbon and fully austenitic steels reveal this embrittlement. In the case of carbon steels, sulfur, nitrogen and niobium are detrimental elements while aluminum and phosphorus suppress it depending on their content and state of existence in the steels.The stress-strain analysis by an Instron type machine shows that the restoration process either by dynamic recovery or by dynamic recrystallization plays an important role in this embrittlement. Another finding is that proeutectoid ferrite deforms preferentially in the austenite plus ferrite region because of very low levels of flow stress and work hardening rate in this ferrite.Thus, factors governing this embrittlement are the degree of ease of the recrystallization in austenitic steel relating to the grain boundary sliding, and the formation of film-like proeutectoid ferrite produced by the γ-α transformation in the case of carbon steels. Grain boundary precipitates such as sulfides and carbonitrides act as nuclei of voids and thus promote this embrittlement.

Calculation of Fe-C, Co-C and Ni-C Phase Diagrams

Abstract: The phase diagrams of Fe-C, Co-C and Ni-C systems have been studied by computer calculations combined with thermodynamic analysis. The Gibbs energy of the individual phase was described by the regular solution-sublattice model, and the empirical parameters were determined from selected data on the activity of carbon as well as the phase boundary composition. A special care has been taken to account for the effect of magnetic transition in bcc Fe, fcc Co and Ni. The calculated phase diagrams differ only slightly from others previously published, but this revision will permit the calculation of phase equilibria for multi-component ferrous alloys with greater accuracy.

Improvement of Hot Ductility of Continuously Cast Carbon Steels

Abstract: To improve the quality of continuously cast carbon steels, the characteristics of the hot ductility were studied via in-situ melted and solidified tensile testing method. Special emphases were placed on the effect of C and P on the hot ductility in the temperature range between melting temperature and 700°C.It was found that zero strength temperature (ZSTc) and zero ductility temperature (ZDTc) have a linear relationship with the equilibrium solidus temperature in Fe-C binary alloys ranging from 0.003 to 1.6% C and does not have the singularity between 0.1 to 0.2% C content. The embrittlement due to P exists only in the carbon steels having C content higher than 0.25%, which are subjected to the melting thermal history. This embrittlement is caused by the segregation of P along the austenite grain boundary.Mild cooling in the secondary cooling stage is very effective to diminish the embrittlement resulting in producing defect free cast slabs.

Dissolution rate of burnt dolomite in molten FetO-CaO-SiO2 slags.

Abstract: Cylinders of burnt dolomite with the apparent porosities of 20 to 35% were immersed into molten FetO-CaO-SiO2 slags at temperatures of 1350 to 1425°C and rotated at 100 to 400rpm. The dissolution rates were determined by measuring the decrease in radius of cylinder.The dissolution rate increased with increases in temperature and revolutional speed of cylinder, but was not affected by the difference in apparent porosity of burnt dolomite. From the analysis of experimental results, it was concluded that the dissolution rate was controlled by the mass transport of CaO in burnt dolomite for low FetO slag (slag A) and that of MgO for other slags in the boundary layer of molten slag. Mass transport coefficients calculated were in the range of 4.7×10-4 to 1.7×10-3cm/s at 1400°C.It was found that the burnt dolomite after the experiment was widely covered with (Fe, Mg) O solid solution in the case of high FetO slag.

Experimental Model Study of the Physical Mechanisms Governing Blast Furnace Hearth Drainage

Abstract: Previous studies concerned with the drainage of the blast furnace hearth have assumed that the iron-slag interface remains horizontal and fixed at the level of the taphole during the outflow of the slag phase. As a consequence they have ignored the presence of the iron phase and its effect on the drainage characteristics of the slag phase.In this study we examine the validity of the above assumption using a novel two-dimensional experimental drainage apparatus which allows a clear visualisation o f the drainage behaviour of two immiscible fluids. The results of the study suggest that in most cases the iron and slag phases may be expected to flow simultaneously from the taphole and that the iron-slag interface will not remain horizontal and fixed at the level of the taphole as has been previously assumed.It is shown that the drainage process is characterised by a down-ward tilting of the gas-slag interface towards the taphole and a simultaneous downward tilting of the iron-slag interface away from the taphole. The experiments show that it is possible to drain the iron phase down to levels considerably below the level of the taphole. The volume of iron removed below the level of the taphole increases with increasing slag drainage rate. Calculations are presented which suggest that under actual blast furnace conditions this volume may be of the same order as the volume of residual slag remaining above the level of the taphole at the end of tapping.Finally, it is concluded that existing predictions of slag residual ratio based on studies which have neglected the presence of the lower iron phase may lead to a serious underprediction of the residual ratio particularly at higher slag drainage rates.

Investigation of bell-less charging based on full scale model experiments.

Abstract: The burden distribution in bell-less charging was investigated by the use of a full scale experimental apparatus and simulation models were developed on the basis of experimental results.The discharge behavior of burden from the bunker was found to be primarily funnel flow and was quantitatively evaluated by the simulation model considering the distribution of particle velocity in the flow region above discharge hole. Sinter showed stronger tendency to funnel flow than coke.The burden trajectory in the chute and outside the chute was successively estimated by solving the equation of motion on a single particle.The mixed layer formation was observed in bell-less charging as well as bell-armor charging. The extent of the mixed layer formation in bell-less charging was found to be expressed by the same experimental formula for bell-armor charging through the proper evaluation of “formation energy of mixed layer”.A simulation model of burden distribution was developed on the basis o f experimental results. The validity of the simulation model was confirmed by the comparison with experimental results in various charging schedules. The simulation model was successfully applied to the flat profile charging.

Factors Affecting the Formation of Shell and Longitudinal Cracks in Mold during High Speed Continuous Casting of Slabs

Abstract: Factors affecting the formation of shell and longitudinal cracks on the shell have been investigated for high speed continuous casting of slabs.Withdrawal speed of slab, stream velocity from submerged nozzle, mold taper, and mold oscillation have shown respective influence on the formation of shell. Their effects have been analyzed quantitatively. Also, characteristics of the heat extraction from and resulting stress in the shell have been examined with respect to the crack formation.These observations have resulted in countermeasures to enhance uniform shell formation in mold and prevent the cracks on the slabs.

Plastic Instability of Sheet Metals under Simple and Complex Strain Paths

Abstract: Forming limit diagrams (FLDs) in simple and complex deformation paths are determined using a theoretical model of localized necking due to an initial heterogeneity of the sheet. The influence of the strain path upon the formability of the sheet is investigated using different types of FLDs under complex strain paths.The effect of material strain hardening and strain-rate hardening is examined for several strain paths. The influence of the imperfection level on the FLDs is presented. Computer simulation of the evolution of rheological parameters during deformation and their dependence on the strain path is carried out.The model is compared with previous experimental works, and a good agreement is obtained between theoretical results and experimental forming limit diagrams.The mathematical model developed in this work is shown to be a powerful tool to understand and predict the plastic behaviour of metal under simple and complex strain paths.

Effect of Grain Size and Solid Solution Strengthening Elements on the Bake Hardenability of Low Carbon Aluminum-killed Steel

Abstract: Effect of metallurgical factors on the bake hardenability (BH) of low-C Al-killed steel is studied. The _yield stress change due to baking (ΔYS) is determined by grain size and dissolved (C+N) content. On the other hand, the ultimate tensile stress change due to baking (ΔTS) depends mostly on the dissolved (C+N) content. The locking mechanism of dislocation is discussed in relation to grain size dependence of these properties. The influence of alloying elements (Mn, Si, P) on BH through their metallurgical factors is also studied. Mn decreases ΔYS through the reduction of dissolved C content. Si gives rise to the increase of dissolved C content and grain refining up to 0.5% Si content. Over 0.5% Si content, the grain becomes coarser in turn. Consequently, ΔYS increases up to 0.5% Si content and levels of after that. Though the addition of 0.06% P does not affect the dissolved (C+N) content, it gives rise to a little increase of ΔYS through grain refining. Larger grain size dependence of ΔYS is observed at -5ppm of dissolved (C+N) content for 0.06% P containing steel.

The Deoxidation of Low-alloy Steel Ingots during ESR

Abstract: The mechanisms of deoxidation in the electroslag remelting process have been studied, with particular reference to the reactions involving calcium. ESR ingots made with varying deoxidation rates have been examined with respect to their bulk composition and oxide inclusion composition. We find that at high oxygen potentials (% FeO>0.7 in the slag), reactions between slag FeO and the deoxidant predominate. At low oxygen potentials (% FeO<0.2 in the slag) exchange reactions between deoxidant and the slag components control the inclusion composition. At intermediate oxygen potentials, the inclusion composition is strongly influenced by the SiO2 content of the slag. The manufacture of low-oxygen, low-sulphur steels by ESR will hence require slags with small contents of both SiO2 and Al2O3.

Behavior of Semi-macroscopic Segregation in Continuously Cast Slabs and Technique for Reducing the Segregation

Abstract: The authors highlighted the small spot segregation present in the center portion of a continuously cast slab, i.e., the semi-macroscopic segregation, and carried out various studies on the subject.The center portion of daily-cast slabs was first investigated in detail. The results reveal that the semi-macroscopic segregation is present in high-temperature cast slabs as well as in low-temperature cast ones. Concentrations of chemical components were measured by EPMA to demonstrate the far higher degree of segregation of semi-macroscopic segregation as compared with that of microscopic segregation, and the dependency of segregation ratio on the size of spot-segregation.Application of soft-reduction casting proved the possibility of reducing the spot size of semi-macroscopic segregation, decreasing the number of spots, and minimizing segregation of constituent elements. By driving a steel pin enclosing tracer element into the center portion of slab, application of soft-reduction casting proved prevention of molten steel from flowing toward the end of solidification crater, resulting in a reduced semi-macroscopic segregation.

Viscosity Measurements on Liquid Slags in the System CaO-FeO-Fe2O3-SiO2

Abstract: For viscosity measurements in CaO-FeO-Fe2O3SiO2 slags at temperatures up to 1700°C a viscosimeter with high accuracy was constructed and built. Measurements were undertaken mainly at 1600°C under air and carbon dioxide atmospheres at CaO/SiO2-ratios of 0.66, 1.00 and 1.50. Additional measurements were carried out at 1500 and 1700°C under air. The results are:(1) At low iron oxide contents the viscosity strongly depends on the basicity. The highest viscosities are found in acid slags.(2) Increasing iron oxide content decreases the viscosity. The decrease is stronger in acid than in basic slags since at high iron oxide contents the viscosity values approach each other.(3) The degree of oxidation influences the viscosity only a little.(4) At Fe2O3/(FeO+CaO)=0.5 the viscosity changes due to a change in the atomic structure of the slag.The results could be interpreted by the slag structure. The viscosity depends on the size and the concentration of the atomic structural units of viscous flow. These units are the silicate and the ferrite anions. Their size and concentration are influenced by the slag composition.

Reduction of Molten Iron Oxide and FeO Bearing Slags by H2-Ar Plasma

Abstract: (I) Reduction of molten iron oxide and FeO bearing slag by H2-Ar plasma was studied using water cooled Cu crucible. The sample weights were 25 to 75g, the flow rate of mixture-gas was 20l/mm and DC electric power of plasma was 8.3kW. Results obtained were as follows:(1) The reduction of molten iron oxides proceeds linearly with time and the reaction rate is proportional to the partial pressure of atomic hydrogen. Therefore, it is considered that the rate determining step is the chemical reaction between FeO and the atomic hydrogen formed by thermal dissociation in the plasma.(2) The rate of reduction of FeO bearing slag is lower than that of molten iron oxide and is proportional to the FeO concentration in slag. It is presumed that the reduction rate is controlled by both the chemical reaction rate of FeO with atomic hydrogen at the gas-solid interface and the mass transport rate of FeO across the boundary layer between the interface and the molten slag bulk.(3) The reduction of molten iron oxide and FeO bearing slag by H2-Ar plasma takes place only on the cavity formed at the surface of melt by the momentum of plasma jet gas.(II) Continuous melting of pre-reduced ore powder, obtained by a fluidized bed reduction was examined using MgO crucible and H2-Ar plasma.Following results were obtained:(1) Carry-over loss of the pre-reduced ore powder during the melting in plasma arc furnace was small, when the condition of powder feeding and plasma arc were properly chosen.(2) Reduction of FeO in slag, accompanied in fed material, by H2-Ar plasma, could be described by a simple model of continuous melting and reduction, based on experimental results of the reduction of FeO bearing slag as described (1-2).With this model, the rate of reduction during continuous melting was determined.

Physicochemical Aspects of Reactions in Ironmaking and Steelmaking Processes

Abstract: This is a critical review of the physical chemistry of selected aspects of ironmaking and steelmaking processes. The equilibrium data from gas-slag-metal reactions are presented in a form suitable for easy application in the study of the state of reactions in ironmaking and steelmaking. Plant data indicate departures from equilibrium for most reactions in the blast-furnace bosh and hearth. In steelmaking, most reactions are close to equilibrium at the time of furnace tapping. Comments are made on reaction equilibria in relation to the ladle refining of hot metal. A summary is made of new equilibrium data for desulfurization of liquid steel with calcium aluminate slags. Various examples are given of the role of vapor species in pyrometallurgical processes, e.g., silicon- and sulfur-bearing species and alkali cyanide vapors in the blast furnace, causes of fume emission during furnace tapping, and consequences of fume emission in coal gasification using liquid iron as the reaction medium.

Mold powder technology for continuous casting of aluminum-killed steel.

Abstract: Mold powder technology for continuous casting of aluminum-killed steel was developed to produce the clean and crack free steel slabs.The following items were investigated:1) Mass balance of Al2O3 in the molten powder pool.2) Al2O3 absorption capacity o f the molten powder.3) Uniformization of heat removal through the mold during continuous casting.4) Change in powder viscosity with absorption of Al2O3.5) Design of optimum powder viscosity by consideration of Al2O3 absorption during continuous casting.The results obtained were as follows:(1) The Al2O3 content of the molten powder pool increases with the elapse of casting time in accordance with the Al2O3 absorption capacity of the powder, but after some time, reaches a constant value that matches the Al2O3 absorption capacity of the powder.(2) The Al2O3 absorption rate of the molten powder increases as the basicity index Bi of the powder increases.(3) The viscosity of the molten powder generally increases with increasing Al2O3 content.(4) To uniformize heat removal through the mold during casting, it is effective to control the parameter ηV over the range of 1.0 to 3.5, where η is the viscosity of the molten powder in poise at 1300°C and V is the casting speed of the machine in m/min.(5) A method for designing the optimum powder viscosity that allows for the absorption of Al2O3 during casting has been established by the combination of the results mentioned above.(6) Powders designed by the above method have been applied to continuous slab casters to prevent the longitudinal surface cracking of medium-carbon aluminum-killed steel slabs and to realize the hot direct rolling of unconditioned slabs.

Effects of Composition and Processing Factors on the Mechanical Properties of As-hot-rolled Dual-phase Steels

Abstract: As-hot-rolled, ferrite-martensite dual-phase steels of rather simple composition can be produced by the “Dual Phase Rolling (DPR) process” which involves a low finish rolling temperature and a very low coiling temperature. Laboratory DPR experiments have been carried out using C-Mn steels and those with Cr or Si additions, to examine the effects of alloying and processing factors on the structure and mechanical properties of the processed steels. Major results obtained are as follows:(1) To attain a sufficiently low yield-to-tensile strength ratio, the final finish pass temperature should be at about Ar3 point which varies depending on the composition, so as to bring about early separation of the alpha phase from the gamma phase before cooling starts. The coiling after a rapid cooling should be done at a temperature lower than 200°C, almost regardless of the steel composition, to suppress auto-tempering of the transformed martensite and aging of the ferrite.(2) Both Cr and Si additions enhance the hardenability of partitioned austenite, allowing a relaxed cooling rate to obtain the martensite phases. However, Cr addition is prone to hinder the early phase separation making the gamma-to-alpha transformation sluggish. Silicon addition accelerates the phase separation, so that a wide range of finishing temperature is available.

Analysis of Flatness of Hot Rolled Steel Strip after Cooling

Abstract: To analyze non-flatness (edge wave) of hot rolled steel strip after cooling, a numerical method for the prediction of temperature and thermal stress has been developed by taking phase transformation into consideration. It has been confirmed that the temperature during cooling and the residual stress calculated by the present method agree well with the measured ones. Using this method, the occurrence of edge wave in a thin and wide hot strip (3.24×2 173mm) has been studied.The results obtained are as follows:(1) The edge wave occurs when compressive residual stress in the edge becomes greater than the critical buckling stress.(2) The residual stress becomes small when the transverse distributions of finishing temperature and of water-cooling rate after rolling are small. Water-cooling of the hot strip in the latter stage of the runout table and high coiling temperature are beneficial to reduce the residual stress.(3) The most effective measure of preventing the occurrence of edge wave is to cool the strip uniformly in the transverse direction by controlling the distribution of water-supply of the cooling device after rolling.

Dephosphorization of Liquid Iron by CaF2-base Fluxes

Abstract: Dephosphorization of liquid iron by CaF2-base fluxes was studied at 1600°C. Iron oxide was added to metal or flux as the oxidizing agent. Dephosphorizing reaction proceeded rapidly and the rate increased with increasing CaF2 content of flux. The rate of reaction was analyzed on the assumption that the reaction was controlled by the simultaneous transport of phosphorus, oxygen and sulfur.The effect of CaF2 on the distribution ratio of phosphorus between liquid iron and flux was examined.

Investigation of Recovery and Recrystallization during Hot Rolling of Stainless Steels with High Speed Laboratory Mill

Abstract: It is very difficult to investigate how recovery, dynamic or static recrystallization proceeds during hot rolling of steel. In the present study, a unique combination of high speed laboratory rolling mill and water cooling facility enables quenching the microstructure immediately after rooling. By using this method, process of recovery and recrystallization was studied in hot rolling of austenitic and ferritic stainless steels. Inhomogeneities of equivalent strain and recrystallization are observed through the thickness of rolled specimens. A layer of finely recrystallized grains is formed at the position of maximum equivalent strain just below the surface. The fine grains are thought to originate from dynamic recrystallization, which occurs during high reduction rolling at high temperature.

Formation of {111} Recrystallization Texture in Polycrystalline Iron

Abstract: Development of sharp {111} recrystallization texture in polycrystalline iron can not be explained by simply synthesizing the results obtained so far on single crystals Since this strongly suggests that this phenomenon is related with the presence of grain boundaries, it was tried to clarify the origin of this texture component by studying the rolling and recrystallization textures of polycrystalline specimens having sharp initial texturesIt was found that the interaction between differently oriented neighbouring grains during rolling was quite aniotropic This suggested that orientations of recrystallized grains nucleated near grain boundary should not be random It was further suggested that, if the stable end orientations of two neighboring grains were the same, their boundary regions were forced during rolling to rotate into these common end orientations, so that stress and strain compatibilities could be satisfied concurrently Since these regions are highly strained, they could provide recrystallization nuclei Such interactions among the members of fiber texture having axes parallel to the normal direction would produce highly strained regions having {111} common end orientations Among them, the interaction between {111} and {111} initial orientations provided the most potential nuclei of {111} recrystallization texture which is typically found in the commercial Al-killed steel

Precipitation of M23C6 and .SIGMA.-phase in .DELTA./.GAMMA. duplex stainless steels.

Abstract: Precipitation of M23C6 and σ-phase during isothermal aging of δ/γ duplex stainless steels has been studied mainly by means of transmission electron microscope coupled with energy dispersive X-ray analyser. M23C6 particles form at both γ/γ and γ/δ boundaries. The particles are crystallo-graphically related to either one of the γ grains or the γ grain corresponding to the γ/γ or the γ/δ boundary. The M23C6 particles nucleating at γ/δ interphase boundaries grow into the untransformed δ-ferrite as an M23C6/new γ eutectoid structure dragging the γ/δ boundaries. The habit plane is the {111}γ//{111}M23C6 which is most parallel to the edgewise growth direction. Since C atoms to form M23C6 are supplied from the γ behind the eutectoid structure, the reaction will finally stop after consumption of C atoms. Then σ-phase particles nucleate on the δ/new γ interphase boundary in Nenno orientation relationship with the new γ, and grow into the δ-ferrite as a σ/γ cellular structure with the (001)σ//(111)γ habit plane. In some cases, inpingements between σ and γ grains nucleating separately occur, and result in a tangled eutectoid structure without any specific orientation relationship. Since the partition of alloying elements between γ and δ-ferrite phases accelerates precipitations of both M23C6 and σ-phase, the solution treatment in the δ-ferrite single phase region largely retards these reactions.