Journal of Iron and Steel Research International最新文献

Utilizing ultrafine iron ore concentrate for pellet production can expand domestic iron ore resources in China and promote the utilization of low-grade ores. However, a challenge arises with the low decrepitation temperature and reducibility in the preparation process of ultrafine iron ore concentrate pellets. To address the challenge, a novel approach was proposed, which incorporated straw powder as an additive to enhance pellet porosity, thereby improving the decrepitation temperature and reducibility of ultrafine iron ore concentrate pellets. The effect of varying proportions of straw powder (0.0–2.0%) on the characteristics of ultrafine iron ore concentrate pellets was examined. Results indicate that at a 2.0% straw powder ratio, pellet decrepitation temperature notably rises from 380 to 540 °C, while the reducibility index escalates from 25.7% to 48.1%. Nevertheless, the addition of straw powder results in diminished drop strength, compressive strength of green pellets, and cold crushing strength of fired pellets. In addition, enhanced pellet reducibility leads to exacerbated reduction swelling index and reduction degradation index. Despite these effects, all parameters remain within an acceptable range.

The effect of TiB₂ addition on microstructure refinement of the as-cast and reheated A517 steel has been investigated. 0.1 wt.% TiB₂ addition resulted in a reduction in equiaxed γ grain size from 990 ± 183 to 116 ± 35 μm and an increase in the volume fraction of equiaxed γ grain region from 5% to 67% in the as-cast A517 steel ingots. Microstructure analysis identified TiN particles rather than TiB₂. This is attributed to the low thermodynamic stability of TiB₂, leading to its decomposition into free Ti and B elements at an elevated temperature. Then, chemical reaction between the free Ti and residual nitrogen in the liquid resulted in the formation of TiN. Hence, it is considered that TiN acted as heterogeneous nucleation sites for the δ-ferrite. This initiated the refinement and columnar to equiaxed transition of δ-dendrites. As a result, the subsequently formed γ grains were correspondingly refined. Such microstructure refinement led to improvement of the yield strength and ultimate tensile strength of the as-cast A517 steel. However, the reheating of the as-cast A517 steel resulted in a marginal microstructure refinement in the samples with low TiB₂ addition. This is attributed to the limited pinning effect of the coarse TiN particles formed during casting process. Consequently, the tensile properties of the reheated A517 steel remained unaffected by the TiB₂ addition.

To address the current issues with the conventional slide gate system utilized in the steel teeming process, a unique electromagnetic induction controlled automated steel teeming (EICAST) technology has been developed. Cooling means of spiral coil in this technology is directly related to its service life. Firstly, heat transfer processes of air cooling and spray cooling were compared and analyzed. Secondly, the impacts of water temperature, water flow rate and air flow rate were examined in order to maximize the spray cooling effect. To maintain coil temperature at a low value consistently throughout the entire thermal cycle process of the ladle, a combined cooling mode was finally employed. Numerical simulation was applied to examine the coil temperature variation with different cooling systems and characteristics. Before coil operation, spray cooling is said to be more effective. By controlling the water flow rate and air flow rate, the spray cooling effect is enhanced. However, water temperature has little or no impact when using spray cooling. Air cooling during the secondary refining process and spray cooling prior to coil operation are combined to further lower coil temperature. When the direction of the spray cooling is from bottom to top, the coil temperature is lowered below 165 °C. A practical induction coil cooling plan was provided for the EICAST technology’s production process.

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.