Journal of Iron and Steel Research International最新文献

Breakout is the most serious production accident in continuous casting and must be detected and predicted by stable and reliable methods. The sticking region, which forms on the local copper plate and expanded into a “V” shape, is the typical precursor of breakout. Therefore, computer vision technology was exploited to visualize the temperature change rate of the copper plate based on the temperature signals from thermocouples; then, the static and dynamic features of the abnormal sticking region were extracted. Meanwhile, logistic regression and Adaboost models were used to study and identify these features, resulting in the development of a mold breakout prediction model based on computer vision and machine learning. The test results demonstrate that the proposed model can effectively distinguish anomalous temperature patterns and considerably reduce false alarms without any missing reports. As a result, the proposed method could offer valuable insights into the realm of abnormality detection and prediction during continuous casting process.

The thick layer and graded feeding technology of a belt roasting machine is an effective method for improving the production efficiency and quality index of pellet production, and a reasonable design of the mechanical structure of the layer is the basis for optimizing the heat and mass transfer performance of the layer. Janssen effect and von Mises yield criterion were used to establish a simplified mathematical model describing the elastic and plastic deformation of the green pellet under the action of an external force. The mechanical characteristics of extrusion, contact, and elastic–plastic deformation between green pellet particles in the material layer of the belt roasting machine were modeled using EDEM software. For a green pellet size of 12 mm, as the layer height increases from 300 to 1000 mm, the maximum vertical pressure on the pellets increases from 11.64 to 24.01 N, and the porosity decreases from 27.04% to 22.01%. As the layer height increases, the contact between the green pellets becomes more intense, and the force chain structure of the layer becomes more stable; the Janssen effect is observed when the layer reaches 700 mm. The compressive strength of the green pellets is linearly related to the particle size, and the compressive strength increases with an increase in particle size. At a layer height of 600 mm, as the particle size of the green pellets increases from 8 to 20 mm, the maximum vertical pressure increases from 7.54 to 44.16 N, and the porosity increases from 23.20% to 31.47%, while the yield per unit of the layer decreased by 12.1%. Small particles have a more stable force chain structure, larger comparative area, and higher production efficiency; however, their compressive strength is lower. Large particles have higher compressive strength and good permeability in the layer, but the production efficiency is relatively low. In actual production, a variety of factors should be integrated to optimize the feeding, and a multi-granularity graded feeding is the most ideal feeding.

After the heavy reduction (HR) process was carried out at the solidification end of the continuous casting slab, the austenite grains were refined by recrystallization, which improved the thermoplasticity of the slab. However, the reduction in deformation during the HR process initiated stress concentration at the slab surface, and the crack risk increased. To effectively evaluate the risk of slab surface cracks under these complex conditions, the effect of the HR on the austenite recrystallization and thermoplasticity of a microalloyed slab surface was investigated by 15-pass reduction thermal simulation according to the wide and thick slab continuous casting process. The softening fraction was introduced as a global internal variable to quantitatively analyze various recrystallized re-refined grains. After the critical strain reaches the critical strain of dynamic recrystallization, a variety of recrystallization modes alternately occur. Among them, the contribution rate of dynamic crystallization to the later refinement reaches more than 50%. The contribution rates of static recrystallization and metadynamic recrystallization to grain refinement are almost the same. The thermoplasticity of the slab surface first increases and then decreases with increasing reduction pass. It was verified by transmission electron microscopy that the main reason for the decrease in thermoplasticity is that the dislocation multiplication rate increases, resulting in a sharp increase in stress and a decrease in thermoplasticity.

In order to investigate the modification behavior and regularity of inclusions in X80 pipeline steel by Mg-treatment, the comparative industrial experiments of Mg-treatment and Ca-treatment in X80 pipeline steel were carried out. Mg and Ca were added to the steel in the form of cored wire after RH (Ruhrstahl-Hereaeus vacuum degassing) process. After adding Ca-containing cored wire, the inclusions were transformed into CaO–Al₂O₃ in the steel. With the progress of smelting, the cleanliness of molten steel became worse, and the equivalent diameter of inclusions was at a higher level. Mg-treatment had a good effect on the modification of inclusions. After Mg-containing cored wire was added to the steel, Al₂O₃ and CaO–Al₂O₃ in the steel were transformed into MgO, MgO·Al₂O₃, and CaO–MgO–Al₂O₃ inclusions, which were basically spherical. The initial precipitated MgO became the core of other inclusions in the steel, which promotes the precipitation of MgO·Al₂O₃. After Mg-treatment, almost no unmodified calcium aluminate inclusions existed in the hot rolled plate, and the cleanliness of the steel was improved. The effect of Ca and Mg on the transformation of inclusions in pipeline steel was studied by thermodynamic calculation, the result of which is consistent with the experimental results.

The two-fluid model coupled with population balance model was used for simulating the gas–liquid flow in the Ruhrstahl–Heraeus (RH) degasser. The predicted circulation flow rate was compared with that measured from a water model experiment to validate the mathematical model. Then, influence of snorkel immersion depth on liquid circulation flow rate was numerically investigated under an atmospheric pressure of 101 and 84 kPa, respectively. Predicted result indicates that the circulation flow rate of the RH degasser in the high-altitude area was severely reduced because of the decrease in atmospheric pressure. However, increasing the snorkel immersion depth from 0.5 to 0.7 m can compensate for the decrease in atmospheric pressure. Industrial test result indicates that decarburization rate is significantly enhanced by increasing the snorkel immersion depth. Through optimization, the percentage of heats with a final carbon content less than 0.002 wt.% is significantly increased from 22.0% to 96.4%.

The corrosion behavior of an Al_0.75CoFeCr_1.25Ni high-entropy alloy (HEA) in 0.5 mol/L NaOH solution was investigated using a series of electrochemical and analytical techniques, including X-ray photoelectron spectroscopy, X-ray diffraction, scanning electron microscopy, potentiodynamic polarization measurement and electrochemical impedance spectroscopy. The results showed that the Al_0.75CoFeCr_1.25Ni HEA exhibited a typical columnar dendritic structure, which is composed of face-centered cubic, body-centered cubic (BCC), and ordered BCC phases (B2 phase). The corrosion resistance of this HEA in 0.5 mol/L NaOH solution is comparable to that of 304 SS, attributed to the change in the composition of the passive film formed on the surface. Although the passive film formed was generally rich in Al, the proportion of Cr₂O₃ inside it increased with the increasing immersion time, enhancing the stability of the passive film and thus improving the corrosion performance of this HEA in 0.5 mol/L NaOH solution.

Refill friction stir spot welding (RFSSW) provides a novel method to join similar and/or dissimilar metallic materials without a key-hole in the center of the joint. Having the key-hole free characterization, the similar/dissimilar RFSSW joint exhibits remarkable and endurable characteristics, including high shear strength, long fatigue life, and strong corrosion resistance. In the meanwhile, as the key-hole free joint has different microstructures compared with conventional friction stir spot welding, thus the RFSSW joint shall possess different shear and fatigue fracture mechanisms, which needs further investigation. To explore the underlying failure mechanism, the similar/dissimilar metallic material joining parameters and pre-treatment, mechanical properties, as well as fracture mechanisms under this novel technology will be discussed. In details, the welding tool design, welding parameters setting, and the influence of processing on the lap shear and fatigue properties, as well as the corrosion resistance will be mainly discussed. Moreover, the roadmap of RFFSW is also discussed.

In order to elucidate the mechanism of the effect of Ta content on the high temperature behaviour of the alloys, the high temperature oxidation and thermal corrosion experiments were carried out on the three alloys with different Ta contents (2.72, 3.10 and 4.00 wt.%). The results of high temperature oxidation and hot corrosion show that because Ta has a higher valence state than Al, it can reduce the indiffusion of O, and the rate at which Ta diffuses within the alloy matrix is relatively slow since it has a larger atomic radius. As a result, the diffusion of the Al element is inhibited as the Ta content increases. Therefore, adding Ta inhibits the formation of Al₂O₃ in the surface oxide and promotes the formation of Cr₂O₃. Thus, Ta promotes oxidised film growth on the sample surface, which inhibits the diffusion of S, O and other elements into the matrix. Additionally, Cr₂O₃ is not easy to dissolve in molten salt, which ultimately makes the alloy have high oxidation resistance and thermal corrosion resistance.

To address the inhomogeneous microstructure and improve the mechanical properties of DT300 ultra-high strength steel specimens fabricated by laser powder bed fusion, different post-heat treatment schedules are performed. With the increase in austenitizing temperature and time, the migration rate of austenite grain boundaries continuously increases with the dissolution of nano-carbides, and the formation of nano-oxides and twin martensite is also inhibited accordingly. The rapid growth in the size of prior austenite grains and martensite laths, as well as the decrease in the content of nano-oxides and twin martensite, led to a rapid decrease in the strength (yield strength and ultimate tensile strength) from HT2 to HTF specimens. The HT1 specimens (austenitizing at 830 °C for 30 min, then oil quenching and tempering at 300 °C for 120 min and finally air cooling) display excellent mechanical properties of yield strength of 1572 MPa, ultimate tensile strength of 1847 MPa, elongation of 9.84%, and fracture toughness of 106 MPa m^1/2, which are counterparts to those of conventional DT300 steel forgings after heat treatment.

NiCoFe alloy, a medium-entropy alloy, shows potential for applications in extreme environments. However, there is a theoretical barrier concerning the unclear understanding of its high-temperature dislocation motion mechanism. The load response exhibits distinct signatures relevant to thermal activation, most notably a decrease in critical force (i.e., softening) from cryogenic to elevated temperatures, e.g., from 200 to 1000 K. The onset of plasticity is characterized by the nucleation of stacking faults and prismatic loops at low temperatures, whereas the surface nucleation of Shockley partial dislocations dominates plasticity at elevated temperatures. We show that thermal effects lead to non-uniform atom pile-ups and control the rate of phase transformation with increasing indentation depth. The findings in this work extend the understanding of the mechanical response of NiCoFe alloys under indentation at different temperatures, shedding light on the underlying dislocation motion mechanisms and surface deformation characteristics. The observed transformation-induced plasticity mechanism has implications for the properties of medium-entropy alloys and their potential applications in extreme environments.