Surface analysis methods, such as Auger electron spectroscopy, X-ray photoelectron spectroscopy, secondary ion mass spectrometry, glow discharge optical emission spectrometry and so on, have become indispensable to characterize surface and interface of many kinds of steel. Although a number of studies on characterization of steel by these methods have been carried out, several problems still remain in quantification and depth profiling. Nevertheless, the methods have provided essential information on the concentration and chemical state of elements at the surface and interface. Recent results on characterization of oxide layers, coated films, etc. on the surface of steel are reviewed here.
The reduction rate of hematite powder (0.15mm in diameter) with CO-CO2 gas mixture was measured by a thermobalance of which sensitivity was 1μg. The sample was put into a Pt-basket and the basket was suspended in a reaction tube from the balance. The weight of sample should be less than 0.2g in order to obtain the reduction rate of a single hematite particle accurately. The reduction rate was controlled by interface chemical reaction because the reduction product layer was porous. The chemical reaction rate constant kc was obtained at 600°C-900°C. The value of kc decreased with a rise of temperature because of sintering of hematite layer. The obtained value of kc was applied to the analysis of fluidized bed reduction of hematite powder with CO-CO2 gas mixture. The calculated reduction curves agreed well with the measured ones by fluidized bed.
The oxidation behavior of low-alloy steels with Ni of 0-4.8% has been investigated in the temperature range of 1173-1573K in the Air. The results obtained are summarized as follows. Scale properties changed remarkably whether the steel contained Ni or not. On the Ni bearing steels, subscales consisting of oxide and Ni enriched metal grew even if the steel contained less than 1.0% Ni. Total oxidation rate and external scale thickness decreased in the case of Ni bearing steels, but the dependence of the Ni content was small. On the other hand, the subscale thickness increased with an increase in the Ni content. At all temperatures, total oxidation, external scale and the subscale thickness grew according to the parabolic rate law. At 1573K, the external scale thickness of Ni bearing steels decreased compared with the thickness at 1373K. The subscale thickness, however, increased with the rise in the temperature, and particles of oxide and metal in subscale became coarse. This was because the supply of Fe2+ for the external scale was retarded and the diffusion of Fe2+ in subscale oxide was promoted, according to the formation of liquid Fe2SiO4.
A new process for dephosphorization of molten pig iron was developed, which is characterized by a single-hole lance used to inject gaseous oxygen with powder flux containing iron oxide. Experiments with a 200-ton torpedo car were carried out. The ratio of gaseous oxygen to total oxidizing component in gas and flux was changed from 0% to 40%. The suppression of temperature drop per one percent of gaseous oxygen was 1.44°C/%. The oxygen utilization efficiency for dephosphorization was not affected with the gaseous oxygen ratio in the range from 0% to 25%. The lance can be used more than 3 heats when the gaseous oxygen ratio was less than 25%, which is sufficient for industrial use. The lance wear rate depended significantly on the ratio of the input heat by molten metal around lance to the output heat by flux and gas in the lance pipe. It is suggested that a stable and uniform flux flow in the lance pipe is effective to decrease the lance wear rate.
The kinetics of reaction between MnO based slag and Fe-C-P-Si-S alloy was examined at 1450°C and the results were analyzed using a kinetic model. MnO in the slag was found to be a good agent for desulfurization, and oxidation of other impurities in the iron alloys; Phosphorus and silicon present in the metal did not affect the rates of decarburization and desulfurization, but the reduction rate of MnO in the slag increased. Dephosphorization rate was more rapid with increasing the initial content of sulfur in the metal, while the rate was suppressed as the content of silicon in the metal increased. These results were well simulated by a kinetic model developed for a slag-metal reaction. The present modeling study suggested that decarburization was controlled by the chemical reaction at slag-metal interface, and the other reactions were controlled by the mass transports in slag and/or metal phases.
The flow pattern of the molten steel in the immersion nozzle have a important effect on the quality of the slabs, billets or blooms produced. Uneven flow in the nozzle developed after passing through a sliding gate, sometimes result in formation of vortex near the nozzle and entrapment of CC powder in the molten steel. The purpose is to suppress those uneven flow as soon as possible after passing through the sliding gate. We proposed a new type nozzle with the contraction and a step. Numerical and water model studies revealed that rectification was accelerated significantly using the new type nozzle, which would lead to the suppression of the vortex generation and turbulence on the meniscus in the mold.
Combination of deoxidation, Zr addition after Mn-Si deoxidation, was applied for the dispersion of fine Mn-Si oxides which provide sufficient nucleation sites of MnS. It was revealed that Zr addition led to decrease in diameter and increase in specific gravity of oxides by forming MnO-SiO2-ZrO2 complex oxides, resulting in the suppression of floatation. Thus, higher number of oxides and MnS is obtained in Mn-Si-Zr deoxidation than in Mn-Si deoxidation. Both model calculation and experiments were conducted to clarify the upper limit of Zr amount for the formation of MnO-SiO2-ZrO2 complex oxides, without complete reduction of MnO-SiO2 by Zr, and calculated results yielded in good agreement with experimental ones. Moreover, both results indicate that Zr addition has wider range of amount than Al addition for obtainig complex oxide: MnO-SiO2-X.
The effect of Zr addition in the steel deoxidized with Ti was investigated to obtain uniform distribution of oxides and MnS. The dependence of the oxide distribution on cooling rate in Ti deoxidized steel became smaller and the number of oxide particles in the center of an ingot increased by the addition of Zr. It can be considered that Zr oxides may work as crystallization sites of Ti oxides during solidification of steel, in addition to the effects of Zr described in previous report; that is, refinement of the oxide particles and increase in their specific gravity to suppress their decrease in molten steel by flotation. Large number of fine MnS precipitated on Mn-Si-Ti-Zr complex oxide particles in the steel deoxidized with Mn, Si, Ti and Zr under the condition based on the result of a model calculation. Fine acicular ferrite formed in the steel sample after heat cycle test, and small Mn-Si-Ti-Zr oxides were observed in the center of the acicular ferrite.
In order to calculate the stress and strain states in the subsurface layer of work roll for hot rolling, the numerical analysis model for stresses and strains was derived under the thermal elastic-plastic and isotropic hardening conditions. A new numerical analysis model for temperature was also proposed to estimate the steady state of heating and cooling. Then, the stress, residual stress and residual displacement in the subsurface layer of work roll, where the pressure and thermal loads were applied, were calculated by coupled analysis of these two models. It has been found that the compressive stresses σxx(absolute values of σxx) at the IMR side are larger than those at the WR side, and that the residual stresses (σxx), and the residual displacements (Ux)r at the WR surface increse with increase of the friction coefficients μB between WR and IMR.
Effects of atomizing condition on surface characteristics of nitrogen atomized Alloy 625 (high Ni alloy) powder and properties of the consolidated material have been investigated. The surface characteristics of the gas atomized powder depend largely upon the cooling rate after solidification even in an inert gas atmosphere. The amount of desorbed water increases with decreasing the powder size, whereas the dependence of cooling rate after solidification on the water content is negligibly small. The adsorbed water on the powder surface can be desorbed by evacuation at elevated temperatures. In addition, especially in case of slow cooling, significantly large amounts of oxides and/or hydrides, such as Cr2O3, Nb2O5, Ni(OH)2, are formed resulting in blowholes in the welding process of the consolidated material. Finally we have confirmed that the consolidated material of rapidly cooled gas atomized powder encapsulated and evacuated at 400°C has no blowhole in the welded metal. The tensile test and impact testing give almost the same results as the wrought materials.
The microstructures in several soft-nitrided steels have been examined by using the electron probe microanalysis on cross-section samples and the X-ray diffraction method combined with the polishing technique from sample surface. The volume fraction changes of ε-, γ'-, and/or α-Fe phases in the direction of the depth could be estimated from the integral intensities of X-ray diffraction peaks. The compound layers which included the porous layers, were characterized the following cases: (1) In the case of fused bath soft-nitriding, the compound layer was mainly constructed with ε-phase. At the upper of it, there was the porous layer which included M3O4 and γ'-phase. Then at the inner of it, there was evidently γ'-phase. The ε-phase lattice constants were clearly decreased with depth. (2) In the case of gas soft-nitriding, the compound layer was mainly constructed with ε-phase. At the upper of it, there was the porous layer which included M3O4. The ε-phase lattice constants were decreased with depth, though the changes were slight compared to the case (1). (3) In the case of ion nitriding, the compound layer was mainly γ'-phase. α-Fe phase and ε-phase existed at the upper of it, and the volume fraction of ε-phase increased with depth. The grain boundaries in the diffusion layers of all samples were nitrides. Therefore it was thought that the grain boundaries in the matrices were preferentially nitrided in the nitriding processes. As the nitriding processes for the samples were discussed by using the analytical results and the Fe-N-C phase diagram near the temperature of 600°C, it was suggested that the above cases of (1) and (2) were treated in the carbon-poor and rich atmospheres respectively.
Dynamic and static recrystallization behavior of γ grains of low carbon steels have been studied by means of hot working simulator. The obtained results are the following. Dynamic recrystallization is initiated after some critical degree of deformation εc which decreases as the temperature increases and the initial grain size becomes finer. Dynamically recrystallized fraction increases as increasing the strain, the strain rate and the temperature. On the other hand, statically recrystallized grain size becomes finer with decreasing the initial grain size and with increasing the strain, and statically recrystallized fraction increases markedly after deformation as increasing the temperature and the strain. On the basis of these results, a prediction model of γ structure changes after deformation is developed and its availability in the case of hot mill rolling was certified.
Behavior of inert gas entrapment into powder in the gas atomization process and its effects on the mechanical properties after consolidation have been investigated using Alloy 625 (high Ni alloy). By using supplemental gas jet impinging onto the accumulated powder in the tank bottom, the atomized powder can be rapidly cooled without any contamination. When the inert gas with no solubility in the material e.g., Ar, is used for the atomization, the powder inevitably contains Ar gas. The Ar content entrapped in the powder is largely influenced by the atomization condition, e.g., it is increased significantly by increasing the gas/metal ratio. In addition, the Ar gas content decreases with decreasing the particle size. The Ar gas concentration was about 0.5 mass-ppm in case of fine powder less than 22 microns. The Ar gas content increases with increasing the fraction of porosity in the cross section, resulting in the cavity after hot extrusion. Although certain cavities are detected in the consolidated materials, almost the same tensile and impact properties as wrought materials were obtained.
Influence of alloying elements (Ni, Mn and Cu) and cooling conditions on toughness of as-thermal cycled martensite simulating the microstructure of coarse-grained weld heat-affected zone (HAZ) of quenched and tempered high strength steels was investigated. Regardless of the cooling conditions, toughness of as-thermal cycled martensite increases with increasing Ni content and deteriorates with increasing Mn content, whereas toughness is not influenced by Cu content. Toughness change of as-thermal cycled martensite is mainly attributed to change in cleavage facet size and amount of cementite precipitated due to auto tempering in the laths of martensite. Cleavage facet size at the initiation point of brittle fracture tends to be refined with increasing Ni content. The amount of cementite in the laths of martensite precipitated during auto tempering decreases with increasing Mn content and cooling rate. Therefore, soluble C is presumably detrimental to the toughness of as-thermal cycled martensite.