In the sintering process, productivity and strength are essential indicators, and it is known that they are closely related to the sintering time and temperature. Since the sintering time and high temperature holding time vary greatly depending on the combustion behavior of coke breeze, various studies have been conducted on the influence of oxygen which is critical element in the sintering phenomena. However, the influence of oxygen enrichment on sinter strength and yield has not yet been unified. In this study, the sintering productivity, strength, and yield were investigated when the oxygen concentration of the inlet gas was increased to 40 vol%. In addition, the heat profile on pot tests and the effect of oxygen partial pressure on the melting property were analyzed from the viewpoint of thermodynamics. As a result, it was found that the yield showed the maximum value at 30 vol% oxygen concentration, and the strength increased with oxygen enrichment. As for the yield, it was found that the effect of the heat profile was significant, and the deviation between the heat transfer speed and the combustion speed started when the oxygen concentration exceeded 30 vol%. On the other hand, it was suggested that the strength improvement was caused by the increase in the amount of melt produced by oxygen enrichment in addition to the effect of high temperature holding time.
The effect of Al2O3 addition on the reaction between 2CaO·SiO2 (C2S) and iron ore has been elucidated from the perspective of silico-ferrite of calcium and aluminum (SFCA) formation. Hematite iron ore, synthesized C2S, and reagent grade CaO, Al2O3, and SiO2 were mixed so that the Fe2O3/SiO2 (mass%) ratio was 14.43. The mixed powders were uniaxially pressed into the shape of a tablet, which was sintered in air at 1250°C for 5 min. The constituent phases were investigated using XRD and EPMA for the samples where CaO/SiO2 (mass%) ratio was varied from 0.22 to 2.60 and for the samples where Al2O3 concentration was varied from 1.44 mass% (T1) to 3.98 mass% (T2). (i) The constituent phases are mainly Fe2O3 and SiO2 for the samples with a CaO/SiO2 (mass%) ratio of 0.22, and Fe2O3, SiO2, and SFCA for the samples with ratios above 1.97. (ii) The compositions of the SFCA phases are all on the plane connecting CaFe6O10, CaAl6O10 and Ca4Si3O10. The Al2O3 concentrations of SFCA in the T1 and T2 samples are in the range of 1-5 mass% and 2-8 mass%, respectively, and the Fe2O3 concentration tends to decrease with increasing Al2O3 concentration. (iii) The Rietveld analyses of XRD profiles have revealed that adding CaO and Al2O3 increases the fraction of the SFCA phase in the sintered sample. Hence, it is suggested that the placement of CaO and Al2O3 near C2S would promote SFCA formation, which leads to mitigating the decrease in strength relevant to C2S-added iron ore sinters.
X-ray Line Profile Analysis (XLPA) is a powerful and convenient method to investigate the dislocation density, crystallite size and nature (screw or edge) and arrangement of dislocation in a material. Microstructural observation by Scanning Transmission Electron Microscope (STEM) and XLPA were performed on low-energy ball milled iron powder. It was found that XLPA can be applied to low-energy and short-time ball milled iron powder. Dislocations were found to be concentrated on the surface of the particles by STEM observation, and XLPA was also evaluated on the surface of the particles.
X-ray Line Profile Analysis (XLPA) is a powerful and convenient method to investigate dislocation density, crystallite size and nature (screw or edge) and arrangement of dislocation in a material. However, few studies have been done concerning the application limit of dislocation density measurements for localized regions or curved materials. In this study, X-ray Diffraction measurements of iron powders formed into flat plates and cylinders were performed using the micro-focus method, and XLPA was performed. As a result, it was found that almost the same dislocation density as that of the Bragg-Brentano method could be derived for the flat specimen by micro-focus method, and that the dislocation density of the cylindrical specimen was almost the same as that of the flat specimen up to a radius in 10 mm.
The effect of brighteners on the deposition behavior of Zn–Ni alloys ant its microstructure was investigated. Zn–Ni alloys were electrodeposited on Cu electrode at 10–5000 A·m−2, 105 C·m−2, 308 K in unagitated zincate solutions containing various brighteners. Although the degree of suppression of hydrogen evolution was different depending on the kind of brightener, the transition current density at which the deposition behavior shifted from normal to anomalous was almost same in solutions containing brighteners. The current efficiency for alloy deposition significantly decreased with an addition of brighteners which had suppression effect on the Zn deposition. Since the brighteners more suppressed the Ni deposition than Zn deposition, the Ni content in deposited films decreased with an addition of brighteners. When the brightener of a straight-chain polymer composed of a quaternary ammonium cation (PQ) which can suppress the diffusion of ZnO22− and Ni ions in solution was added, the Ni content in deposited films increased with increasing current density at high current density region. This is attributed to that Zn which is preferentially deposited over Ni earlier reached the diffusion limitation of ZnO22− ions and Ni deposition didn’t reach the diffusion-limited current density. When both PQ and a quaternary ammonium salt with a benzene ring were added in solution, the films obtained at the diffusion-limited current density of ZnO22− ions exhibited the smooth surfaces composed of fine crystals. With an addition of brighteners to increase the overpotential for deposition, the γ-phase (intermetallic compound of Ni2Zn11) of the deposited films was formed easily.
Zn–Ni alloys were electroplated on an Fe plate with thicknesses of 40 μm at 500 A·m−2 and 293 K in unagitated zincate solutions. The reaction product of epichlorohydrin and imidazole (IME) was added in the solution as a brightener at the concentration of 0-5 ml/dm−3. The corrosion resistance of the obtained Zn–Ni alloys films was investigated from the polarization curve in 3 mass% NaCl solution before and after the corrosion treatment (formation of corrosion products) in NaCl solution for 48 h. Before the corrosion treatment, the corrosion current density of plated films rarely changed regardless of addition of IME into the zincate solution because the reduction reaction of dissolved oxygen rarely changed. However, in films plated from the solution containing IME, the anode reaction was suppressed and the corrosion potential shifted to noble direction. The suppression of anode reaction with an addition of IME in plating solution is attributed to the increase in γ-phase in plated films. On the other hand, after the corrosion treatment, the morphology of Zn chloride hydroxide of corrosion product was uniformly formed on the surface with increasing the concentration of IME. The reduction reaction of dissolved oxygen was suppressed with increasing the concentration of IME, resulting in decrease in corrosion current density.
To investigate the effect of the combined addition of V to Mo-added steel on the hydrogen trapping behavior, 0.1C-2Mn-1.6Mo mass% steel (Steel A) and 0.1C-2Mn-1.6Mo-0.2V mass% steel (Steel B) were prepared, quenched, and tempered at 873 K for 0.5 to 10 hours. The thermal desorption analysis of hydrogen-charged specimens confirmed that Steel B showed a higher hydrogen trapping capacity than Steel A. According to thermodynamic equilibrium calculations, it was predicted that only M2C precipitated in Steel A and B after tempering at 873 K. However, according to TEM observation of specimens tempered for 4 hours, coarse M2C and fine MC precipitated in Steel A, whereas only fine MC precipitated in Steel B. Based on the three-dimensional atom probe analysis, MC of both Steel A and B show a composition close to MC0.5, in which Mo is the primary element in metal sites. It was found that the carbon-site vacancy (C vacancy) ratio of MC in the present work is higher than that of V4C3 (VC0.75). The hydrogen trapping capacity showed a good correlation with the product of the area of interface of MC and the C vacancy ratio in MC. The reason of the higher hydrogen trapping capacity of Steel B than that of Steel A is considered to be below; 1) The combined addition of V suppressed the precipitation of M2C and increased the amount of MC. 2) C vacancies in MC which act as hydrogen trapping sites increased by the partitioning of Mo into MC.
Effect of alloying elements on the composition distribution, morphology and volume fraction of eutectic carbides were investigated for high-speed tool steels with uniform hardness from 68 to 70HRC after hot working and quenching and tempering. The stability of eutectic carbides at high temperature was also evaluated. M2C, M6C and MC eutectic carbide are observed in as-cast samples similar to the general high-speed tool steels. M2C type carbide increases in volume with increasing Si and V contents, and M6C and MC carbides appear with increasing W and V contents. The amount of M2C eutectic carbide varies with the composition in liquid phase just prior to eutectic solidification. The morphology of the eutectic carbide changes from fine fiber or lamellar type to coarse lamellar or feather type, and the interlayer spacing of eutectic carbide tends to increase with increasing the area fraction of M2C eutectic carbide. Moreover, after heat treatment at 1140°C for 16 hours, some M2C carbides remain stabled but MC and M6C carbide appears.
The change in precipitates during cold rolling of Nb-added interstitial-free (IF) steel was studied by an electrochemical analysis and 3D atom probe (3DAP) measurement. The amount of dissolved Nb and C increased with the rolling reduction ratio. In the 3DAP analysis, the size distribution of NbC was changed by cold rolling, as the extremely coarse precipitates and fine precipitates observed in the sheets before cold-rolling decreased markedly. The C/Nb atomic ratio in NbC near the precipitate/matrix interface was also lowered by cold rolling, which corresponded to a change to a high-energy state, as indicated by thermodynamic calculation. A molecular dynamics calculation suggested that the crystal structure of the precipitate surface was randomized by shear deformation and dislocations increased in the matrix steel around the precipitates. This may be due to the interaction between carbon and dislocations, as reported in the dissolution of cementite under heavy deformation. It is inferred that NbC is not completely dissolved in the matrix phase by cold rolling, but rather that the accumulated energy and dislocation propagation due to cold deformation cause structural collapse near the precipitate surface and heavier deformation of finer precipitates, which is evaluated as the resultant solid solution content by electrochemical analysis.
Effect of carbon (C) content (0.05 mass% to 0.3 mass%) on critical intergranular fracture stress of tempered martensite steel was investigated using 3 mass% manganese (Mn) steel. The critical intergranular fracture stress was obtained by calculating the maximum principal stress at fracture in a circumferential notched specimen tensile test using elastoplastic finite element analysis. As a result, critical intergranular fracture stress of tempered martensite steel increased with increasing C content. Therefore, the dominant factors of critical intergranular fracture stress were examined from the viewpoints of the amount of segregation of each element on prior-austenite grain boundaries and the grain size of the martensite substructure. The first result was found to be the effect of reducing the amount of Mn segregation by increasing C content. This was thought to be because cementite acts as a solid solution site for Mn, and the amount of Mn in solid solution in the matrix phase is reduced by increasing the C content. The second result was found to be the effect of refining the substructure of martensite surrounded by high angle grain boundaries by increasing the C content. This indicates that grain size also affects crack initiation resistance in intergranular fracture by determining the stress concentration at the grain boundaries as the distance of dislocation accumulation.
Fe-0.4C-1.2Si-0.8Mn (mass%) alloys austenitized at different temperatures, ranging from 1103 to 1473 K, were subjected to interrupted quenching (IQ) at 473 K and then maintained at that temperature to induce the partitioning of carbon from martensite to austenite (one-step quenching and partitioning (Q&P) process). The initial austenite grain size before the IQ was varied from 20 to 573 μm. As the initial austenite grain size becomes finer, the enrichment of carbon in the untransformed austenite during the partitioning treatment is enhanced, which leads to a greater increase in the volume fraction of retained austenite. The reasons for the increased carbon enrichment were explained by the effective carbon partitioning as well as the promoted bainitic transformation, which were both caused by the increase in the area of the martensite/austenite interface. Tensile tests of the specimens with different initial austenite grain sizes revealed that the mechanical properties of the one-step Q&P specimens improved in both strength and elongation by the refinement of the initial austenite grains.