Journal of Iron and Steel Research International最新文献

A new flow control technology in continuous casting process named permanent magnet flow control-mold (PMFC-Mold) was proposed, in which the permanent magnets are arranged in Halbach array near the narrow region of the mold. The behavior of molten steel flow and the fluctuation of molten steel/slag interface in the PMFC-Mold under different continuous casting speeds were investigated. Firstly, a physical experiment of liquid Ga–In–Sn alloy circulating flow was carried out in Perspex mold with Halbach’s permanent magnets (HPMs) to investigate the magnetic field distribution of HPMs and its impactful electromagnetic braking effect. The numerical simulation of 1450 mm × 230 mm slab shows that a stronger magnetic field over 0.3–0.625 T is formed at the wide surface and the narrow surface of the mold, which provides an effective electromagnetic braking for controlling the impingement of molten steel jet and suppressing the fluctuation of molten steel/slag interface. The numerical simulation results show that in the PMFC-Mold, the region with the turbulent kinetic energy greater than 0.01 and 0.04 m² s⁻² on the upper backflow zone and near the narrow surface of the mold are significantly reduced. The maximum turbulent kinetic energy of the submerged entry nozzle (SEN) jet in front of the narrow surface is significantly reduced, and the SEN jet moves downward before impacting the narrow surface of the mold. In the PMFC-Mold, the region with the surface velocity greater than 0.2 m s⁻¹ on the steel/slag interface is eliminated, the flow pattern and fluctuation profiles on the molten steel/slag interface become regular on both sides of SEN, and the vortex near SEN disappears. The maximum fluctuation height of molten steel/slag interface is controlled below 2.59 and 5.40 mm corresponding to the casting speed of 1.6 and 2.0 m min⁻¹, respectively.

The variations in the mechanical and magnetic properties of cold-rolled 20Mn23AlV non-magnetic structural steel after annealing at different temperatures were investigated. The microstructure and precipitation changes during annealing were studied by optical microscopy, scanning electron microscopy, and transmission electron microscopy. The results show that recrystallization completed after annealing at 620 °C, resulting in grain sizes of approximately 800 nm and the best combination of strength and plasticity. The yield-to-tensile ratio of the non-magnetic structural steel after cold rolling continuously decreases from low to high temperatures after annealing, with the highest value being 0.89 and the lowest value being 0.43, indicating a wide range of yield-to-tensile ratio adjustment. The introduction of numerous dislocations during cold rolling provided favorable nucleation sites for precipitation, leading to abundant precipitation of the fine second-phase V(C, N). The phase composition of the samples remained unchanged as single-phase austenite after annealing, and the relative permeability values were calculated to be less than 1.002, meeting the requirements for non-magnetic steel in terms of magnetic properties.

The substantial influences of Mo contents varying from 0 to 0.26 and 0.50 wt.% on the microstructural evolution and MX (M = Nb, V and Mo; X = C and N) precipitation characteristics of Nb–V–N microalloyed steels processed by hot deformation and continuous cooling were studied using a Gleeble 3800 thermomechanical simulator. Metallographic analysis showed that the ferrite microstructure transformed from polygonal ferrite (PF) in 0Mo steel to both acicular ferrite (AF) and PF in 0.26Mo and 0.50Mo steels, and AF content first increased and then decreased. The thermodynamic calculations and the experimental results proved that the quantity of solid solution of Mo in austenite obviously increased, which reduced the austenite (γ) to ferrite (α) transformation temperature, consequently promoting AF formation in 0.26Mo steel and bainite transformation in 0.50Mo steel. Moreover, the submicron Nb-rich MX particles that precipitated at the temperature of the austenite region further induced AF heterogeneous nucleation with an orientation relationship of ((011)_{{{text{MX}}}} //(100)_{{{text{Ferrite}}}}) and ([1overline{1}1]_{{{text{MX}}}} //[001]_{{{text{Ferrite}}}}). The interphase precipitation of the nanosized V-rich MX particles with Mo partitioning that precipitated during γ → α transformation exhibited a Baker–Nutting orientation relationship of (left( {100} right)_{{{text{MX}}}} //left( {100} right)_{{{text{Ferrite}}}}) and (left[ {001} right]_{{{text{MX}}}} //left[ {01overline{1}} right]_{{{text{Ferrite}}}}) with respect to the ferrite matrix. With increasing Mo content from 0 to 0.26 and 0.50 wt.%, the sheet spacing decreased from 46.9–49.0 to 34.6–38.6 and 25.7–28.0 nm, respectively, which evidently hindered dislocation movement and greatly enhanced precipitation strengthening. Furthermore, facilitating AF formation and interphase precipitation was beneficial to improving steel properties, and the optimal Mo content was 0.26 wt.%.

During the continuous casting process of low carbon steel, the solidified hook formed in the mold has great effects on the surface quality of the cast slab. Some factory experiments have been conducted to investigate the microscopic characteristics and reveal the influence of process parameters on solidified hooks. The depth of the hooks showed a positive correlation with the deflection angle, length, and oscillation mark (OM) depth, which indicates that the OM depth can serve as an approximate indicator for evaluating the depth of the solidified hooks. On the wide and narrow faces of the cast slab, the depth of the solidified hooks and the temperature distribution in the mold show opposite trends, with lower depths of solidified hooks at positions with higher temperatures. In addition, the influence of process parameters on solidified hooks was analyzed. With the increase in superheat, not only the depth of solidified hooks gradually decreases, but also the ratio of depression-typed marks increases. Increasing casting speed and decreasing immersion depth of the submerged entry nozzle will both lead to a decrease in the depth of the solidified hook.

The second phase dissolution and elements migration behavior of a nickel-based single crystal superalloy during solution heat treatment with direct current were investigated for simplifying and shortening the solution heat treatment of the Ni-based single crystal superalloy. The results showed that the electric current solution heat treatment improved microstructural homogenization as well as the distribution of alloying elements, especially for the refractory metal W and Mo. The microsegregation ratios for Mo and W after electric current solution heat treatment at 1230 °C for 4 h are near those without electric current at 1250 °C for 4 h. The electric current accelerated the γ′ phase dissolution process, and the γ′ phase could be completely dissolved at a lower treatment temperature or within a shorter treatment time under electric current solution heat treatment with direct current. A microcosmic current model was proposed to analyze the effect of the electric current on the solution heat treatment of the Ni-based single crystal superalloy.

The impact-abrasive wear behavior of high-C martensitic steel was investigated, taking into account varying carbon (C) contents and different tempering temperatures. The evaluation was done through comprehensive microstructural characterization, analysis of worn surface morphology, and measurement of key performance like impact toughness and surface hardening. The findings demonstrate that increasing C content and tempering temperature both has a positive effect on wear resistance, with C content exhibiting a more pronounced influence compared to the tempering temperature. The improved wear resistance of the steel with higher C content and tempering at a higher temperature can be attributed to its enhanced impact toughness. This increase in impact toughness is primarily a result of microstructural refinement and alterations in carbide morphology. Moreover, cyclic impact loading induces surface hardening due to dislocation strengthening within the martensite and the retained austenite, leading to an increase in surface hardness. The combination of surface hardening and excellent impact toughness synergistically contributes to the overall improved wear resistance observed in the experimental steel with higher C content after tempering at a higher temperature. Additionally, the dominant features observed on the worn surface are scratches and substrate delamination, indicative of a wear mechanism of the experimental steels characterized by micro-cutting/ploughing and fatigue wear.

Over the years, the high magnetic induction of industrial Mn-added electrical steel is assumed to be the enhancement of {100} texture derived from its austenite–ferrite phase transformation during hot rolling (phase transformation (PT) method). However, it is still undetermined without straightforward experimental evidence. The reason for {100} texture improvement of Mn-added electrical steel is experimentally confirmed due to the recrystallization induced by the austenite–ferrite phase transformation during hot rolling. Moreover, a more promising methodology to further improve {100} texture and formability of hot-rolled electrical steel is promoted by the control of hot rolling deformation condition (shear deformation (SD) method). The results show that the nucleation mechanisms of {100} oriented recrystallized grains are different in the samples by SD and PT methods, which are in-depth shear deformation and austenite–ferrite phase transformation, respectively. In this case, coarse {100} oriented recrystallized grains and low residual stress are obtained in the sample by SD method, which is responsible for its superior {100} texture and formability. In contrast, the sample by PT method forms fine recrystallized grains with random orientations and accumulates severe residual stress.

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.