Journal of Iron and Steel Research International最新文献

MgO–CaO (40 wt.% CaO) refractory aggregates were prepared using the calcined dolomite and light-burned magnesia fine powder as raw materials and TiO₂ as additive. The effect of TiO₂ on their phase composition, microstructures and properties was investigated by X-ray diffraction and scanning electron microscopy. The properties such as bulk density, apparent porosity, relative aggregate tube strength and hydration resistance were also investigated. The results showed that the CaTiO₃ generated by the reaction between CaO and TiO₂ was distributed around the CaO grain boundaries and intermittently distributed with MgO, which formed an isolation layer around CaO and greatly improved the hydration resistance. Meanwhile, the introduction of TiO₂ promoted sintering and increased the grain size, further improving the strengths and hydration resistance of the materials. In addition, the most significant enhancement in the hydration resistance and strengths of the samples was achieved when 1.0–2.0 wt.% TiO₂ was added. In this case, the relative strength of aggregate increased from 33.3% to 37.3%–43.1%, and the mass gain after the hydration test decreased from 3.13% to 1.26%–1.45%.

The reaction behavior between CaO–Al₂O₃–La₂O₃-based slags and La-bearing FeCrAl melt was quantitatively characterized, which was further compared with the reaction behavior of CaO–SiO₂-based slags. Based on this, the new type of mold flux for La-bearing FeCrAl alloy continuous casting was designed and its basic properties were evaluated. The results showed that the order of reaction degree of fluxing agents in CaO–Al₂O₃–La₂O₃-based slags is (Na₂O) > (B₂O₃) > (Li₂O), and the percentages of mass change of fluxing agents were 85.8, 54.29 and 42.35 wt.%, respectively. Moreover, the addition of (Li₂O) and (Na₂O) promoted the reaction between (CaO) and [Al], and the reaction degree of the former was weaker than that of the latter, which was due to the greater effect of (Na₂O) on the activity of (CaO) and (Al₂O₃) than (Li₂O). Compared with the reactivity of CaO–SiO₂-based slags, the percentages of mass change of Al and La caused by slag–steel reaction decreased by 10.63–14.36 and 39.78–50.49 wt.%, respectively. The percentages of mass change of (Al₂O₃), (La₂O₃) and (CaO) in slags highest increased by 17.71, 17.98, and 7.81 wt.%, respectively. The reactivity of CaO–Al₂O₃–La₂O₃-based slags was significantly weakened. Ultimately, the new type of mold flux was designed and the composition range was determined. The fundamental properties of new mold flux basically meet the theoretical requirements for La-bearing FeCrAl alloy continuous casting.

In order to increase the utilization rate of vanadium–titanium magnetite in blast furnace smelting, the viscosity and potassium removal capacity of CaO–SiO₂–Al₂O₃–MgO–BaO–TiO₂ slag (CaO/SiO₂ = 1.05, 1–5 wt.% BaO, 2–20 wt.% TiO₂) were studied for slag optimization using the cylinder method and slag–metal equilibrium technique, respectively. Also, the structural properties of the slag were characterized by Fourier transform infrared spectroscopy. The concept of “a ring structure of Ti–O–Si” was proposed to express the change in the viscosity of the blast furnace slag. The results showed that the viscosity of slag increased with the increase in BaO content while the potassium removal capacity decreased. Furthermore, an increase in TiO₂ content from 2 to 20 wt.% resulted in a decrease in viscosity and an increase in potassium removal capacity. The Fourier transform infrared spectroscopy results showed that the charge compensation of Ba²⁺ can form complex aluminosilicate structure and increase the viscosity of slag. Meanwhile, with the increase in TiO₂ content, Ti⁴⁺ ions replace Si⁴⁺ in the silicon-oxygen tetrahedral structure, thereby reducing the degree of polymerization of the silicate network and decreasing the viscosity.

The microstructure evolution and mechanical properties of a Fe–0.12C–0.2Si–1.6Mn–0.3Cr–0.0025B (wt.%) steel with different initial microstructures, i.e., hot rolled (HR) and cold rolled–annealed (CRA), were studied through optical microscopy, scanning electron microscopy, electron channeling contrast imaging, microhardness and room temperature uniaxial tensile tests. After water quenching from 930 °C to room temperature, a fully martensitic microstructure was obtained in both as-quenched HR and CRA specimens, which shows a microhardness of 480 ± 5 HV, and no significant difference in microstructure and microhardness was observed. Tensile test results show that the product of tensile strength and total elongation (UTS × TE) of the as-quenched HR specimen, i.e., 24.1 GPa%, is higher than that of the as-quenched CRA specimen, i.e., 18.9 GPa%. While, after being tempered at 300 °C, the martensitic microstructures and mechanical properties of the two as-quenched specimens change significantly due to the synergy role of the matrix phase softening and the precipitation strengthening. Concerning the maximum UTS × TE, it is 18.9 GPa% obtained in the as-quenched CRA one, while that is 24.4 GPa% obtained in the HR specimen after tempered at 300 °C for 5 min.

SiO₂ is the main component of gangue in sinters and a crucial constituent in the formation of the SiO₂–Fe₂O₃–CaO (SFC) system. The non-isothermal crystallization kinetics of the SFC system were investigated using differential scanning calorimetry. The crystallization process of SFC was studied under different cooling rates (5, 10, 15, and 20 K/min), and the crystalline phases and microstructures of the SFC crystals were verified through X-ray diffraction and scanning electron microscopy. The results indicate that when the SiO₂ content is 2 wt.%, increasing the cooling rate promotes the precipitation of CaFe₂O₄ (CF) in the SFC system, thereby inhibiting the precipitation of Ca₂Fe₂O₅ (C₂F). In contrast to the CaO–Fe₂O₃ (C–F) system, the addition of SiO₂ does not alter the precipitation mechanisms of C₂F and CF. By further adding SiO₂, the precipitation of Ca₂SiO₄ (C₂S) significantly increases. Simultaneously, the CaO content in the liquid phase decreases. This leads to the crystallization process of the CF₄S (4 wt.% SiO₂) system bypassing the precipitation of C₂F and directly forming CF and CaFe₄O₇ (CF₂). In the case of the CF₈S (8 wt.% SiO₂) system, the crystallization process skips the precipitation of C₂F and CF, directly yielding CF₂. The crystallization process of both CF₂S (2 wt.% SiO₂) and CF is similar, comprising two reaction stages. The Ozawa method was used to calculate the activation energy for the crystallization of C₂F and CF as − 329 and − 419 kJ/mol, respectively. Analysis using the Malek method reveals model functions for both stages.

Metallographic microscopy, scanning electron microscopy and TiN growth thermodynamic and kinetic equations were used to investigate the morphology, quantity, and size of TiN in the center of high-titanium high-strength steels under different solidification cooling rates. The results showed that TiN in the center of the experimental steels mainly existed in three forms: single, composite (Al₂O₃–TiN), and multi-particle aggregation. TiN began precipitating at around 1497 °C (solidification fraction of 0.74). From the end of melting to solidification for 180 s, the cooling rates in the center of the experimental steels for furnace cooling, air cooling, refractory mold cooling, and cast iron mold cooling tended to stabilize at 0.17, 0.93, 1.65, and 2.15 °C/s, respectively. The size of TiN in the center of the experimental steel cooled using furnace cooling was mainly concentrated in the 5–15 µm range. In contrast, the size of TiN in the center of the experimental steels cooled using air cooling, refractory mold cooling, and cast iron mold cooling were mainly concentrated in the 1–5 µm range. In addition, their density of TiN in the center of the experimental steels is significantly higher than that of the furnace-cooled experimental steel. Thermodynamic and kinetic precipitation models of TiN established predicted the growth size of TiN in a high-titanium high-strength steel when the solidification cooling rates are not below 0.93 °C/s.

With the depletion of high-quality iron ore resources, high-phosphorus oolitic hematite (HPOH) has attracted great attention due to its large reserve and relatively high iron content. However, HPOH is very difficult to be used in ironmaking process due to its special structure. A two-step method of gas-based direct reduction and magnetic separation was thus proposed to recover iron and reduce phosphorus. The results showed that the powdery reduced iron produced contained 92.31% iron and 0.1% phosphorus, and the iron recovery was 92.65% under optimum reduction condition, which is suitable for following steelmaking. The apatite will be reduced under long reduction time and a large reducing gas flow rate, resulting in more phosphorus found in the metallic iron. Increasing the hydrogen–carbon ratio will inhibit the formation and growth of iron particles and prevent the breakage of oolitic structure. Careful adjustment of reduction temperature is recommended as it affects the oolitic structure and reduction.

A novel Fe–Cr–Mo amorphous coating, a high-temperature corrosion-resistant material for water wall protection of power plant ultra-supercritical boilers, has been prepared via arc spraying. A systematic study was conducted to evaluate the high-temperature corrosion behavior of this coating, and its resistance to corrosion at high temperatures was scientifically assessed. The results indicate that the thickness of Fe–Cr–Mo amorphous coating is approximately 350 μm, exhibiting typical amorphous characteristics as confirmed by X-ray diffraction and transmission electron microscope characterization. During each stage of the 750 °C corrosion test, the oxygen content of the amorphous coating was significantly lower than that of the contrast coating (PS45 alloy coating), indicating a superior corrosion protection effect at high temperature. After 100 h of continuous testing, the corrosion mass gain of the amorphous coating was only 28.62% that of PS45 coating and 3.89% that of T12 steel substrate, indicating significantly depressed high-temperature corrosion kinetics. The excellent high-temperature corrosion resistance of Fe–Cr–Mo amorphous coating is primarily attributed to the stable Fe/Cr oxide film generated by the metastable state of the amorphous state, which serves as an excellent barrier. Furthermore, under the influence of heat in a high-temperature environment, the amorphous structure gradually transforms into a nanocrystalline structure. In contrast, the oxide film of the amorphous/nanocrystalline coating has low thermal stress, leading to better adhesion with the coating and resistance to cracking and peeling, thus providing excellent sustained protection.

Slag corrosion is one of the main factors of the damage of refractory, and its primary manifestations involve the melting of refractory in slag and the slag penetration into refractory, both of which are highly related to the wetting behavior between slag and refractory. The high-temperature wettability could be characterized by parameters including the surface tension, adhesion work, and spreading coefficient of the slag on refractory surface, and it could be suppressed by altering the slag/refractory interface, thus resulting in an improved anti-corrosion performance. From this, the key knowledges of the slag corrosion, theory of wetting behavior and test of high-temperature contact angle were firstly summarized. Then, the major factors influencing the high-temperature slag wetting behavior were discussed based on the aspects of slag composition, refractory composition, and surface microstructure. Finally, the future research direction was proposed in this field.

Herein, the effect of direct current (DC) attached the mold on refining the microstructure and alleviating the central segregation of a tin–bismuth (Sn–10 wt.% Bi) alloy ingot during the solidification process has been investigated. The experiment used a self-made device, which can achieve the effect of refining the solidified structure and alleviate the segregation of the metal casting. Numerical simulations were performed to calculate the Lorentz force, Joule heating and induced melt vortex flow for the magneto-hydrodynamic case. Our results show that the maximum velocity of the global electro-vortex reached 0.017 m s^–1. The DC-induced electro-vortex was found to be the primary reason of refining the equiaxed grain and alleviating the segregation of the β-Sn crystal boundary. The grain refining effect observed in these experiments can be solely attributed to the forced melt flow driven by the Lorentz force. DC field attached the mold can lead to grain refinement and alleviate the segregation of the ingot via a global vortex. The technology can be applied not only to opened molds, but also toward improving the quality in closed molds.