When the casting speed of the thin slab continuous caster is increased, the ratios of the solid and liquid phases in the solidification front of the molten steel in the mold change, which affects the thickness of the solidified shell. In order to accurately calculate the thickness of the solidified shell and determine the value range of the mushy zone coefficient suitable for the mathematical model of solidification heat transfer at high casting speed, this paper established the solidification heat transfer mathematical model in thin slab funnel mold, and the influence of different mushy zone coefficients on the accuracy of solidification heat transfer mathematical model was analyzed and compared with the actual solidified shell thickness. The results showed that, when the casting speed was increased to 4~6 m/min and the coefficient of the mush zone coefficient was 3 × 10⁸~9 × 10⁸ kg/(m³⋅s), the thickness of solidified shell calculated by the solidification heat transfer model was in good agreement with that measured in practice. The research in this paper provides an important reference for the establishment of the solidification heat transfer mathematical model at high casting speed in the future.

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.

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.