Shuhuan Wang

Insufficient ductility and processing-dependent mechanical properties limit laser powder bed-fused (LPBF) high-nitrogen stainless steels. This study investigated short-term solution treatment of an Fe–Cr–Mn–Mo–N duplex stainless steel using two as-built samples with distinct microstructures and elongations. Samples were treated at 1050 and 1100 °C for 10 min and water quenched. Microstructure and deformation behavior were characterized by electron backscatter diffraction, tensile testing, loading–unloading–reloading experiments, and transmission electron microscopy. Treatment increased the austenite fraction from 16.0 and 21.1% to 70.8–77.0%, producing an austenite-dominated duplex microstructure with residual ferrite as short strips or isolated islands. Differences in tensile properties between the initial states decreased substantially, with ultimate tensile strengths of 1030.9–1049.9 MPa and elongations of 29.45–33.82%. A representative sample treated at 1100 °C achieved 1030.9 MPa and 33.82% elongation. Its back stress showed an overall increase during plastic deformation, indicating a contribution of long-range internal stresses to strain hardening. Early deformation involved planar slip and localized stacking faults in austenite. Further deformation produced denser planar defects and local 9R structures in austenite, with pronounced dislocation tangles in ferrite. Short-term solution treatment thus enhanced ductility while retaining approximately 1 GPa strength and reducing property discrepancies between initial LPBF-built states.

Investigating pressure-dependent laser powder bed fusion (LPBF) requires equipment capable of performing repeated layer cycles in a sealed pressurized environment. This study describes a pressurized LPBF platform integrating internal powder feeding and recoating, external laser delivery, synchronized motion control, and recirculating gas flow. The platform has a nominal design-pressure limit of 20 bar absolute, which was not experimentally demonstrated as a multilayer operating condition. Ar builds comprising 200 programmed layers with a nominal height of 6 mm were completed at 1 and 3 bar in approximately 2 h per build. One block was characterized at each Ar pressure; repeated Archimedes measurements yielded relative densities of 99.58 ± 0.19% and 94.08 ± 0.50%, respectively, without establishing between-build variability. Separate experiments demonstrated multilayer LPBF processing in N₂ up to 9 bar and stationary laser melting up to 15 bar. The mean final nitrogen content increased from 0.05985 to 0.09270 wt.% between 1 and 9 bar for multilayer specimens and from 0.1415 to 0.3505 wt.% between 1 and 15 bar for stationary-melted specimens. Pressure retention was assessed through mechanical-gauge observation rather than continuous time-resolved acquisition. The results document the machine’s initial multilayer processing capability and its potential for investigating pressure-dependent processing behavior; they do not establish full-range performance or pressure-induced densification enhancement.

In order to investigate and mitigate carbon segregation behaviour in the curved mould region during the continuous casting of 46MnVS5 steel, this study developed a multiphysics coupled model for an R10m curved mould. Four operating variables, namely current intensity, frequency, casting speed, and cooling intensity, were systematically manipulated to investigate their individual and interactive effects on carbon distribution and segregation behaviour in the curved mould under electromagnetic stirring. Based on a Box-Behnken design, response surface methodology was employed to establish predictive models for the positive segregation index (PSI) and negative segregation index (NSI), which were taken as the optimisation responses for carbon macrosegregation characteristics at the mould outlet. It shows that increasing the intensity from 200 A to 400 A reduced the PSI by 0.86%, with the optimal NSI of 0.7924 achieved at 300 A. Increasing the frequency from 2 Hz to 4 Hz decreased the PSI by 0.54%, with the minimum NSI of 0.7924 observed at 3 Hz. Casting speed had the most significant effect on segregation: increasing the speed from 2.4 m·min ⁻¹ to 3.6 m·min ⁻¹ reduced the positive and negative segregation indices by 9.94% and 6.94%, respectively. Although rising the cooling intensity effectively suppressed positive segregation, it concurrently aggravated negative segregation. Through multi-objective optimisation, the optimal process parameters were obtained, including a current intensity of 351.062 A, a frequency of 3.867 Hz, a casting speed of 3.6 m·min ⁻¹ , and a cooling intensity of 1200 kW·m ⁻² . The relative influence of each parameter on segregation decreased in the following order: casting speed > cooling intensity > current intensity > frequency. These findings provide a theoretical basis and practical guidance for controlling carbon segregation at the mould outlet.

The blast furnace–basic oxygen furnace long process is the dominant steel production route in China. Increasing the scrap ratio is an effective way to reduce cost and carbon emissions, and scrap preheating is a key technology to achieve a high scrap ratio. To improve the low thermal efficiency and poor deep-bed melting performance of converter gas-based scrap preheating, an innovative process using oxygen-enriched combustion in a hot metal ladle is proposed. Numerical simulation is essential for capturing the complex multiphysics phenomena, as real-time monitoring of melting inside the packed scrap bed is extremely difficult. In this study, a novel multiphysics approach based on a User-Defined Function (UDF) is developed to dynamically track the progressive melting of the scrap skeleton, overcoming the key limitation of conventional enthalpy–porosity models that cannot capture the feedback between phase change and porous medium property evolution. A three-dimensional transient model was established, integrating turbulent combustion, gas–solid convective heat transfer in porous media, and solid–liquid phase change. The effects of impact pit depth, scrap porosity, and converter gas flow rate on temperature distribution, melting behavior, and thermal efficiency were systematically investigated. Results showed that porosity had the strongest influence; thermal efficiency increased from 33.92% to 65.59% as porosity rose from 0.6 to 0.8, due to a transition from conduction-dominated to coupled convection–conduction heat transfer. Converter gas flow rate exhibited a non-monotonic effect, peaking at 3688.14 m³·h⁻¹, highlighting a trade-off between energy input and gas residence time, while impact pit depth showed a limited effect with diminishing returns. A 600 s full-process simulation revealed stage-dependent melting, and the initial phase was crucial for process optimization. The optimal condition, with a pit depth of 64 cm, porosity of 0.8, and converter gas flow rate of 3688.14 m³·h⁻¹, achieved a 1.23% melting fraction and 65.59% thermal efficiency within 120 s. These findings clarify the combined roles of geometric confinement, permeability, and energy-residence time interactions, providing guidance for industrial scrap preheating design.

The demand for high-nitrogen austenitic stainless steel (HNASS) powders has become increasingly urgent due to the rapid development of advanced manufacturing processes. However, it still remains a challenge to accurately control the nitrogen content and its chemical state. In this work, an innovative process combining high-pressure metallurgy and solid-state powder nitriding is proposed to prepare ultra-high nitrogen austenitic stainless steel powders. The prepared powders not only exhibit fine austenite grains, but also achieve a high nitrogen content of up to 5.63 wt.% at 1000 °C, 2.5 h, and 2.5 MPa. The results demonstrate that the phase composition of the powders, as well as the size and distribution of nitrides can be effectively regulated by carefully controlling the key processing parameters including temperature, time and pressure. Nitrogen is predominantly uniformly distributed as solid solution, with minor nanoscale nitride precipitates. XPS analysis of the powder surface indicates that the peak area ratios of N1s (N 1s core-level) in the form of solid solution and nitrides are ~82.95% and ~17.05%, respectively. And the peak area ratios of N1s at different depths of the powder do not show significant changes. Furthermore, the high-pressure nitriding mechanism reveals that the synergy between a high-pressure nitrogen atmosphere and solid-state nitriding enhances nitrogen diffusion flux and increases nitride nucleation density, enabling precise control of nitrogen content and precipitate size. Moreover, the high-pressure nitriding process can effectively keep nitrogen in a solid solution, prevent the precipitation of coarse nitrides, and consequently improve the quality of the powders. This research provides in-depth guidance and insights into the design and preparation of HNASS powders.

This study investigates the evolution and influencing factors of non-metallic inclusions in industrial pipeline steel during the vacuum degassing (VD) process. Steel and slag samples were systematically collected at multiple intervals throughout the vacuum treatment using an automated robotic sampler, which integrated temperature measurement and sampling functions. The results indicate that the molten steel temperature and the concentrations of nitrogen, oxygen, and sulfur exhibited an overall decreasing trend, with removal kinetics characterized by a rapid initial reduction followed by a gradual stabilization. The dominant inclusion phases were identified as Mg-Al spinel and calcium aluminates. Specifically, the top slag composition was distributed within the C₃A phase field, trending toward the liquid C₁₂A₇ region, while the endogenous inclusions transitioned from CA₂ toward CA. With prolonged vacuum treatment, the relative fraction of calcium aluminates progressively increased at the expense of Mg-Al spinel due to continuous slag-metal reactions. Furthermore, quantitative analysis reveals that the population density of small equivalent circular diameter (ECD) inclusions continuously decreased, while the average inclusion size increased, indicating that the VD process promotes the collision and coalescence of inclusions.

22MnCrNiMo steel, a high-strength low-alloy material, is primarily used in the production of mooring chains for offshore oil platforms, offshore wind turbines, and ships. The application of additive manufacturing technology allows for the direct fabrication of seamless mooring chains. This paper investigates the selective laser melting (SLM) process parameters for 22MnCrNiMo mooring chain steel, analyzing the effects of different process parameters on the microstructure, phase composition, and mechanical properties of the steel. The experimental results demonstrate that under the laser parameters of 200 W laser power, 800 mm/s scanning speed, 30 μm layer thickness, and 110 μm scanning spacing, the SLM-formed parts exhibit the best comprehensive mechanical properties, with a microhardness of 513.2 HV0.5, a tensile strength of 1223 MPa, a yield strength of 1114 MPa, an elongation of 8.5%, and an impact energy of 127 J. This study reveals the microstructure evolution and the mechanism of enhanced mechanical properties in SLM-fabricated 22MnCrNiMo steel, providing a new approach for the preparation of high-performance mooring chains using 22MnCrNiMo steel.

In the steel industry, small billets have become the main type of billet for steel production due to the efficiency of the continuous casting process. However, the segregation that occurs during solidification remains a significant issue affecting billet quality. This study conducted a macroscopic segregation analysis on 172 mm × 172 mm small square billets and investigated the influence of various process parameters on the distribution of carbon within the cast billets. The results showed that an increase in superheat led to a 0.036% rise in the carbon difference and an increase in the central segregation value from 0.357% to 0.364%. Increasing the cooling intensity resulted in a 0.037% rise in the carbon difference and a decrease in the negative segregation value from 0.266% to 0.250%. Higher casting speeds caused the carbon difference to reach a minimum of 0.107% at a speed of 1.6 m·min⁻¹, while the central segregation value reached its lowest point of 0.353% at a casting speed of 2.6 m·min⁻¹.