Journal of Iron and Steel Research International最新文献

Controlling the adhesion of potentially corrosive substances from flue gas on grate bar is crucial for extending the operational lifespan of the equipment. The adhesive behaviour and mechanism of ultrafine particulate matters (UPM) throughout the sintering process were elucidated, and measures to control adhesion on grate bars were developed. Research findings indicated that a small quantity of UPM were found on grate bar during the initial sintering stages (ignition stage and middle stage I and II). The main compositions of UPM were Fe_xO_y-rich, CaO-rich, and aluminium silicate-rich particles. In contrast, corrosive substances like alkali metal compounds were almost absent. These UPM adhered onto grate bar primarily through inertial impaction. When moving to the final sintering stages (middle stage III and temperature rising stage), many UPM rich in corrosive substances like NaCl and KCl adhered to the grate bar. These UPM adhered to grate bar through thermal diffusion and vortex deposition. Solid waste water washing technology can greatly decrease the quantity of UPM (rich in NaCl and KCl) on the grate bar due to vortex deposition and thermal diffusion, and it represents a potentially promising way to control adhesion and corrosion on grate bars.

The effects of austenite grain size on the deformed microstructure and mechanical properties of an Fe–20Mn–6Al–0.6C–0.15Si (wt.%) low-density steel were investigated. The microstructure of the experimental steel after solution treatment was single austenitic phase. The austenite grain size increased with solution temperature and time. A model was established to show the relationship between temperature, time and austenite grain size for the experimental steel. In addition, as the solution temperature increased, the strength decreased, while the elongation first increased and then decreased. This decrease in elongation after solution treatment at 1100 °C for 90 min is contributed to the over-coarse austenite grains. However, after solution treatment at 900 °C for 90 min, the strength–elongation product reached the highest value of 44.4 GPa%. As the austenite grain size increased, the intensity of <111>//tensile direction fiber decreased. This was accompanied by a decrease in dislocation density, resulting in a lower fraction of low-angle grain boundaries and a lower work hardening rate. Therefore, the austenite grain size has a critical influence on the mechanical properties of the low-density steels. Coarser grains lead to a lower yield strength due to the Hall–Petch effect and a lower tensile strength because of lower dislocation strengthening.

Based on a thermodynamic study of 5 wt.% Si high-silicon austenitic stainless steel (SS-5Si) smelting using CaF₂–CaO–Al₂O₃–MgO–SiO₂ slag to obtain a low oxygen content of less than 10 × 10⁻⁴ wt.%, a kinetic mass transfer model for deep deoxidation was established through laboratory studies, and the effects of slag components and temperature on deoxidation during the slag–steel reaction process of SS-5Si were systematically studied. The experimental data verified the accuracy of the model predictions. The results showed that the final oxygen content in the steel at 1873 K was mainly controlled by the oxygen content derived from the activity of SiO₂ regulated by the [Si]–[O] equilibrium reaction in the slag system; in particular, when the slag basicity R (R = w(CaO)/w(SiO₂), where w(CaO) and w(SiO₂) are the contents of CaO and SiO₂ in the slag, respectively) is 3, the Al₂O₃ content in the slag needs to be less than 2.7%. The mass transfer rate equation for the kinetics of the deoxidation reaction revealed that the mass transfer of oxygen in the liquid metal is the rate-controlling step under different slag conditions at 1873 K, and the oxygen transfer coefficient k_O,m increases with increasing the slag basicity from 4.0 × 10⁻⁶ m s⁻¹ (R = 1) to 4.3 × 10⁻⁵ m s⁻¹ (R = 3). k_O,m values at R = 2 and R = 3 are almost the same, indicating that high slag basicity has little effect. The integral of the mass transfer rate equation for the deoxidation reaction of SS-5Si under different slag conditions is obtained. The total oxygen content of the molten steel decreases with increasing basicity from an initial content of 22 × 10⁻⁴ to 3.2 × 10⁻⁴ wt.% (R = 3), consistent with the change in k_O,m with slag basicity. At R = 2, the slag–steel reaction takes 15 min to reach equilibrium (w[O] = 5.5 × 10⁻⁴ wt.%), whereas at R = 3, the slag–steel reaction takes 30 min to reach equilibrium (w[O] = 3.2 × 10⁻⁴ wt.%). Considering the depth of deoxidation and reaction time of SS-5Si smelting, it is recommended the slag basicity be controlled at approximately 2. Similarly, the effect of temperature on the deep deoxidation of SS-5Si was studied.

The precipitation of secondary Laves phases and its effect on notch sensitivity are systematically studied in Thermo-Span alloy. The results show that the precipitation peak temperature of secondary Laves phases is 925 °C. Below 925 °C, the volume fraction of secondary Laves phases increases with the rise of the temperature, and its morphology changes from granular to thin-film; above 925 °C, the volume fraction of secondary Laves phases shows an opposite trend to temperature, and its morphology changes from thin-film to granular. A detailed explanation through linear density (ρ) is provided that the influence of secondary Laves phases at the grain boundaries (GBs) on notch sensitivity depends on the coupling competition effect of their size, quantity, and morphology. Notably, the granular Laves phases are more beneficial to improving the notch sensitivity of the alloy compared with thin-film Laves phases. Granular secondary Laves phases can promote the formation of γ′ phases depletion zone to improve the ability of GBs to accommodate high strain localization, and effectively inhibit the crack initiation and propagation.

Considering the dynamic variation of roll gap and the transverse distribution of dynamic rolling force along the work roll width direction, the movement and deformation of rolls system, influenced by the coupling of vertical chatter and transverse bending vibration, may cause instability and also bring product defect of thickness difference. Therefore, a rigid-flexible coupling vibration model of the rolls system was presented. The influence of dynamic characteristics on the rolling process stability and strip thickness distribution was investigated. Firstly, assuming the symmetry of upper and lower structures of six-high rolling mill, a transverse bending vibration model of three-beam system under simply supported boundary conditions was established, and a semi-analytical solution method was proposed to deal with this model. Then, considering both variation and change rate of the roll gap, a roll vertical chatter model with structure and process coupled was constructed, and the critical rolling speed for self-excited instability was determined by Routh stability criterion. Furthermore, a rigid-flexible coupling vibration model of the rolls system was built by connecting the vertical chatter model and transverse bending vibration model through the distribution of dynamic rolling force, and the dynamic characteristics of rolls system were analyzed. Finally, the strip exit thickness distributions under the stable and unstable rolling process were compared, and the product shape and thickness distribution characteristics were quantitatively evaluated by the crown and maximum longitudinal thickness difference.

A novel Al-alloyed press-hardening steel (PHS) was developed, which exhibits excellent tensile, bending and antioxidation properties. Al is a ferrite-forming element that can hinder the formation of cementite and enhance the stability of austenite. The incorporation of Al not only induces the formation of ferrite within martensitic matrix but also enhances the stability of retained austenite (RA). The microstructure of novel steel consists of martensite, ferrite, and RA after press hardening. Investigations into the role of Al in RA development were supported by thermo-kinetic calculations. The simultaneous introduction of ferrite and RA into the martensitic matrix via tailored chemical compositions significantly enhances the elongation and bending toughness of the novel PHS. Additionally, Al can form a dense Al oxide at the bottom of oxide layer, resulting in the improved antioxidant properties. Compared to 22MnB5 steel, it is an exciting discovery as there is a significant improvement in total elongation and bending toughness of novel PHS without compromising strength. The novel PHS, with its exceptional balance of strength and ductility, will play a crucial role in reducing weight when it replaces the existing class 22MnB5 PHS in different structural components of vehicle bodies.

Combined with the hydrogen pre-charging and tensile testing methods, the effect of charged hydrogen content on the microstructure and mechanical behavior of an as-forged Ti–6Al–4V alloy was investigated. After performing hydrogen charging for 2, 4, 6, 8 and 10 h at a constant cathodic current density value of 75 mA/cm² in a corrosion medium of 3.5 wt.% NaCl solution, the hydrogen contents in the charged samples increased gradually from 73 × 10⁻⁴ to 230 × 10⁻⁴ wt.%. When the hydrogen content was less than 190 × 10⁻⁴ wt.%, the charged hydrogen atoms were present as the solute atoms in the matrix, resulting in the enhanced tensile strength due to the solid solution strengthening of hydrogen atoms. Moreover, the reduced axial ratio c/a for α-Ti matrix due to the hydrogen dissolution was beneficial to improving the ductility of the hydrogenated samples. The critical hydrogen content for simultaneously improving the ductility and strength is determined to be 99 × 10⁻⁴ wt.%. When the hydrogen content was 230 × 10⁻⁴ wt.%, a small number of δ-TiH_x hydrides and micro cracks formed in the localized areas of α-Ti matrix, resulting in the simultaneous decrease of ductility and strength.

To reveal the intricate mechanisms underlying the melting and dissolution processes of scraps in the iron ladle, the melting characteristics of three carbon steels with different C concentrations at the bath temperatures of 1623 and 1723 K were studied. Upon immersing scraps into the molten metal, the liquid metal immediately froze around the submerged parts of scrap cylinders. Whereafter, the solid shell completely melted at both bath temperatures after the immersion time of 5 s. The maximum thickness of solidified steel shells significantly decreased with increasing the bath temperature. The findings also suggested that the melting rate of scrap cylinder exhibited a positive correlation with the C concentration in the scrap and the bath temperature. Quantitatively, the mass transfer coefficients of C for the low carbon (0.18 wt.%), medium carbon (0.32 wt.%), and high carbon (0.61 wt.%) concentrations in the scrap cylinders at 1723 K were determined by a kinetic model, which were 8.78 × 10⁻⁵, 9.57 × 10⁻⁵ and 10.00 × 10⁻⁵ m s⁻¹, respectively, and those corresponding values decreased to 3.87 × 10⁻⁵, 4.49 × 10⁻⁵ and 3.54 × 10⁻⁵ m s⁻¹ at 1623 K. However, there was little difference observed among the heat transfer coefficients of hot metal for the three carbon steels, which were estimated to have an average value of 16.36 and 18.82 kW m⁻² K⁻¹ at the experimental temperatures of 1623 and 1723 K, respectively. The results from the experiments and mathematical models showed good consistency at both bath temperatures, providing feasible guidance for efficient melting of steel scraps in the iron ladle.

The municipal solid waste incineration fly ash (MSWI-FA) contains a large amount of heavy metals, and the process of iron ore sintering and treating fly ash needs to pay attention to the migration characteristics of heavy metals. The impact of the application of MSWI-FA in the sintering process on the emission law of heavy metals in the collaborative treatment process was studied, and corresponding control technologies were proposed. The results showed that the direct addition of water washing fly ash (WM-FA) powder resulted in varying degrees of increase in heavy metal elements in the sinter. As the amount of WM-FA added increases, the content of heavy metal elements correspondingly increases, and an appropriate amount of WM-FA added is 0.5%–1.0%. The migration mechanism of heavy metals during the sintering treatment of WM-FA was clarified. Heavy metals are mainly removed through direct and indirect chlorination reactions, and Cu and Cr can react with SiO₂ and Fe₂O₃ in the sintered material to solidify in the sinter. Corresponding control techniques have been proposed to reduce the heavy metal elements in WM-FA through the pre-treatment of WM-FA. When the WM-FA was fed in the middle and lower layers of the sintered material, the high temperature of the lower layer was utilized to promote the removal of heavy metals. The Ni element content has decreased from 130 to 90 mg kg⁻¹, and the Cd removal rate has increased by 23%. The removal rates of Cd and Cr elements increase by 2.4 and 5.5 times, respectively. There is no significant change in sintering indexes.

The La_1.7Pr_0.3Mg₁₆Ni hydrogen storage alloy was prepared by medium-frequency induction melting, and then the composite hydrogen storage alloy powder of La_1.7Pr_0.3Mg₁₆Ni + x wt.% (x = 0, 2, 4, and 6) graphene was prepared by ball milling for 10 h. The effect of the addition of graphene on the activation and hydrogen de/absorption properties of La_1.7Pr_0.3Mg₁₆Ni alloy was studied. The result demonstrated that these composite alloys were composed of La₂Mg₁₇, La₂Ni₃, and Mg₂Ni phases. After saturated hydrogen absorption, it was composed of LaH₃, Mg₂NiH₄, and MgH₂ phases, while during the dehydrogenation process, it was composed of LaH₃, Mg, and Mg₂Ni phases. The addition of graphene can help get a more homogeneous granule after ball milling and accelerate the first activation of dehydrogenation/hydrogen absorption. The hydrogen release activation energy of the alloys first decreases and then increases as the graphene content increases from x = 0 wt.% to x = 6 wt.%. The minimum activation energy of the composite hydrogen storage alloy is 51.22 kJ mol⁻¹ when x = 4 wt.%.