Journal of Iron and Steel Research(International)最新文献

Abstract

Erosion corrosion causes significant problems in various industrial environments through a synergistic effect which results in much greater weight loss than the sum of the weight losses in the individual processes. The erosion-corrosion behavior of three low-alloy steels was investigated in a simulated concrete slurry using the rotation method. The key influencing factors and mechanism of material degradation were analyzed. The experimental results indicate that the weight loss increases with the linear velocity according to a nearly exponential relationship (W = KVⁿ), where n is 1. 40–2. 14. This weight loss is mainly caused by erosion in the alkaline slurry, and steels with higher tensile strengths show higher erosion-corrosion resistance. The formation of many platelets and ring cracks and their removal from the sample surface during erosion corrosion in the slurry are thought to constitute the mechanism responsible for this weight loss. These platelets and ring cracks are formed by solid particles striking the sample surface. Craters are initially produced and subsequently disappear as they grow and come in contact with each other. Fewer craters were observed on the surfaces of samples that exhibited higher weight loss. The surface of the material became work-hardened because of the effect of the particles striking and scratching, and a deformed layer was produced on the surface for steels of lower strengths, leading to deeper and more abundant gouges.

Abstract

Baosteel No. 3 blast furnace hearth was divided into tuyere area, taphole area, taphole upper side wall and taphole lower side wall according to different working situations. Through chemical composition analysis, scanning electron microscopy, X-ray diffraction, energy dispersive spectrometry and other means, chemical composition and microstructure of different parts of hearth carbon brick were analyzed and markedly different corrosion mechanisms of these areas were found. Zn element in form of ZnO mainly deposited on the hot side of carbon brick. There was no obvious evidence that Zn permeates into carbon bricks and erodes them. Except for taphole area, K, Na, and Fe contents from hot side to cold side gradually rise and fall, resulting in the decrease of apparent porosity, the increase of density and the higher thermal conductivity compared with those of new carbon brick. The higher content of Fe in carbon brick leads to more serious erosion because Fe has greatly changed the physical properties of carbon brick. In the taphole area, the contents of Si and Al present obvious concentration gradient because of the mechanical souring of molten iron and slag. The SiO₂ and Al₂O₃ particles that have different expansion factors with carbon brick damaged the carbon substrate because of temperature fluctuation. The graphitized carbon found on H4 where is the most serious corrosion site means that the carbon brick ever directly contacts with molten iron.

Abstract

The sintering performance of three typical specular hematite ores (coarse SO-A, intermediate SO-B and ultrafine SO-C) was compared in an industrial ore blend through pilot-scale sinter pot tests. The effect of particle size of specular hematite ores on their granulation and sintering performance was revealed. Compared with the coarse SO-A fine and ultrafine SO-C concentrate, the intermediate SO-B showed inferior granulation and sintering performance characterized with poorer bed permeability and productivity, lower sinter strength and higher fuel rates. A new material preparation method was hence proposed and verified at both pilot and industrial scales. The proposed method by mixing SO-B with a high amount of goethite-type iron ore fines was found to be an effective way in improving the granulation and assimilative characteristics of ore blend comprising 31% intermediate SO-B, leading to improved sinter productivity and lowered fuel rates. The metallurgical properties and microstructure of sinters were also investigated. The sinters obtained through the proposed preparation method were generally stronger and more reducible on account of better sinter structure with more relict hematite ultimately connected with needle-like silico-ferrite of calcium and aluminum and lower porosity.

Abstract

A new hot-dip galvanizing method was employed on hot-rolled low carbon steel. The effects of Al contents on microstructure, micro-hardness and corrosion resistance of Zn-Al alloy coatings were systematically investigated. Phase composition, microstructure and element distribution in Zn-Al alloy coatings were analyzed using X-ray diffraction (XRD) and electron probe micro analysis (EPMA), respectively. It is found that Al content (0. 6–6. 0 wt. %) in galvanizing zinc affects surface quality and adhesion between coatings and matrix in the newly developed method. In addition, with increasing Al content, micro-hardness significantly increased due to the increase in Zn-Al eutectoid phases. Potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) also revealed that increase in Al plays a noticeable role in improving the corrosion resistance of Zn-Al alloy coatings.

Abstract

A nickel-based single crystal superalloy containing Re and Ru was cast in a directional solidification furnace. The single crystal specimens after standard heat treatment were grit blasted with different pressures and then heat treated at 1100 °C for 4 h under vacuum condition. The evolution of recrystallized microstructure and its effect on the tensile properties at 850 and 980 °C were investigated. After heat treatment, the cellular microstructure was observed, and the thickness of the cellular recrystallization zone increases with the increase in grit blasting pressure. The appearance of the cellular structure undermines the tensile properties. Both the tensile strength and elongation decrease with increasing the thickness of the cellular structure. The recrystallized grain boundaries can act as the channels for the crack initiation and propagation during tensile test. The low bearing capacity of recrystallized layers and the local stress concentration resulting from the notch effect of cracking were the main reasons for the decrease of tensile properties.

Abstract

Diffusion bonding is a near net shape forming process that can join dissimilar materials through atomic diffusion under a high pressure at a high temperature. Titanium alloy TC4 (Ti-6Al-4V) and 4J29 Kovar alloy (Fe-29Ni-17Co) were diffusely bonded by a vacuum hot-press sintering process in the temperature range of 700–850 °C and bonding time of 120 min, under a pressure of 34. 66 MPa. Interfacial microstructures and intermetallic compounds of the diffusion-bonded joints were characterized by optical microscopy, scanning electron microscopy, X-ray diffraction (XRD) and energy dispersive spectroscopy (EDS). The elemental diffusion across the interface was revealed by electron probe microanalysis. Mechanical properties of joints were investigated by micro Vickers hardness and tensile strength. Results of EDS and XRD indicated that (Fe, Co, Ni)-Ti, TiNi, Ti₂Ni, TiNi₂, Fe₂Ti, Ti₁₇Mn₃ and Al₆ Ti₁₉ were formed at the interface. When the bonding temperature was raised from 700 to 850 °C, the voids of interface were reduced and intermetallic layers were widened. Maximum tensile strength of joints at 53. 5 MPa was recorded by the sintering process at 850 °C for 120 min. Fracture surface of the joint indicated brittle nature, and failure took place through interface of intermetallic compounds. Based on the mechanical properties and microstructure of the diffusion-bonded joints, diffusion mechanisms between Ti-6Al-4V titanium and Fe-29Ni-17Co Kovar alloys were analyzed in terms of elemental diffusion, nucleation and growth of grains, plastic deformation and formation of intermetallic compounds near the interface.

Abstract

Non-isothermal method was used to study gasification characteristics of three coal chars and one biomass char. Four chars were made from anthracite coal (A), bituminous coal (B), lignite coal (L), and wood refuse (W), respectively. The gasification process was studied by random pore model (RPM), unreacted core model (URCM) and volumetric model (VM). With an increase in metamorphic grade, the gasification reactivity of coal char decreased, and the gasification reactivity of biomass char was close to that of low metamorphic coal char. With an increase in heating rate, the gasification of all samples moved towards high temperature zone, and the whole gasification time decreased. It was concluded from kinetics analysis that the above-mentioned three models could be used to describe the gasification process of coal char, and the RPM fitted the best among the three models. In the RPM, the activation energies of gasification were 193. 9, 225. 3 and 202. 8 kJ/mol for anthracite coal char, bituminous coal char and lignite coal char, respectively. The gasification process of biomass char could be described by the URCM and VM, while the URCM performed better. The activation energy of gasification of wood refuse char calculated by the URCM was 282. 0 kJ/mol.

In order to control the CaO-Al₂O₃-SiO₂-MgO system inclusions in 50CrVA spring steel in a lower melting temperature region, high temperature equilibrium experiments between steel and slag were performed in the laboratory, under the conditions of the initial slag basicity within 3–7 and the content of Al₂O₃ between 18–35 mass%, to investigate the formation and evolution of this type of inclusion. The results indicate that the total oxygen content in the steel decreases with the increase of slag basicity and the decrease of Al₂O₃ content in slags, and CaO-Al₂O₃-SiO₂-MgO inclusions tend to deviate from the low melting point region with the increase of Al₂O₃ content in slags. The most favorable composition for the refining slag is composed of 51–56 mass% CaO, 9–13 mass% SiO₂, 20–25 mass% Al₂O₃ and 6 mass% MgO. In this case, the inclusions in 50CrVA spring steel are mostly in the low melting point regions, in which their plasticities are expected to improve during steel rolling. The MgO-based inclusions were observed in the steel matrix and the formation mechanism was theoretically and schematically revealed. It is also found that adding around 11 mass% of MgO into the refining slags is beneficial to reducing the refractory corrosion. Further work should be carried out focusing on the evolution rates of MgO-based inclusions.

The integrated steelmaking cycle based on the blast furnace-basic oxygen furnace (BOF) route plays an important role in the production of plain and ultra-low carbon steel, especially for deep drawing operations. BOF steelmaking is based on the conversion of cast iron in steel by impinging oxygen on the metal bath at supersonic speed. In order to avoid the addition of detrimental chemical elements owing to the introduction of uncontrolled scrap and in order to decrease environmental impact caused by the intensive use of coke for the production of cast iron, HBI (hot briquetted iron) can be used as a source of metal and a fraction of cast iron. Forty industrial experimental tests were performed to evaluate the viability of the use of HBI in BOF. The experimental campaign was supported by a thermal prediction model and realized through the estimation of the oxidation enthalpy. Furthermore, the process was thermodynamically analyzed based on oxygen potentials using the off-gas composition and the bath temperature evolution during the conversion as reference data.

Isothermal hot compression tests on the as-cast high-Cr ultra-super-critical rotor steel with columnar grains were carried out in the temperature range from 1223 to 1523 K and at strain rates from 0.001 to 1 s^–1. The compression direction was parallel to the longitudinal direction of columnar grains. The constitutive equation based on Arrhenius model was presented, and the processing maps based on the dynamic material model were developed, correlating with microstructure observation. The main softening mechanism was dynamic recovery at 1223 K under strain rates from 0.1 to 1 s^–1, whereas it was dynamic recrystallization under other deformation conditions. The constitutive equation modified by strain compensation reasonably predicted the flow stresses. The processing maps and microstructure evolution mechanism schematic indicated that the optimum hot working parameters lay in the zone defined by the temperature range from 1423 to 1473 K and the strain rate range from 0.001 to 1 s^–1.