Xuemin Wang

Fire‐resistant structural steels must retain load‐bearing capacity under fire exposure, and their elevated‐temperature performance is governed not only by alloy design but also by the thermal stability of the microstructure. This work investigates the effects of Mo on the microstructural evolution, mechanical properties, and fire resistance of steels subjected to air‐cooling and water‐cooling. SEM observations reveal that Mo promotes a limited fraction of bainite under air‐cooling and refines the bainitic/martensitic lath structure under water‐cooling. Mechanical testing demonstrates that Mo enhances room‐temperature strength under both cooling conditions; however, its contribution to fire resistance is strongly microstructure‐dependent. In the air‐cooled steel, the yield‐strength retention at 600 °C remains essentially unchanged upon Mo addition, whereas in the bainitic/martensitic steel, the yield strength at 600 °C increases markedly from ~162.9 to ~254.4 MPa with the retention ratio rising from 0.29 to 0.38. TEM analysis after holding at 600 °C identifies coarse alloyed cementite along lath boundaries, without a high number density of fine Mo‐rich precipitates, indicating that precipitation strengthening is unlikely to dominate. EBSD characterization, prior austenite reconstruction, and boundary statistics confirm that Mo mitigates dislocation annihilation and preserves block/sub‐block boundary density during elevated‐temperature exposure, thereby enhancing resistance to recovery, coarsening, and softening.

Central segregation, a typical internal defect in continuous casting slabs, significantly deteriorates the mechanical properties of steel products. However, traditional manual defect evaluation methods rely heavily on experience, are highly subjective and inefficient, making it difficult to meet the quality assessment requirements of today’s high-end steel materials. In this study, an approach which combines an unsupervised image enhancement algorithm and Otsu algorithm analysis was proposed to achieve automatic recognition and quantitative features extracting of central segregation in continuous casting slabs. The challenges posed by insufficient brightness and low contrast in central segregation images were addressed using unsupervised image enhancement algorithms. Following this enhancement, batch objective quantification of the segregation images was conducted through Otsu processing. Comparative experimental results showed that the enhanced images yielded an average Dice Similarity Coefficient of 0.965 for segregation recognition, representing a 38% improvement over unprocessed images, with consistent accuracy gains across complex segregation scenarios. This intelligent detection method eliminates the need for manually labeling a training set, substantially improves the consistency of segregation quantification and reduces the time cost significantly. Consequently, multiple parameters can be employed to quantify segregation characteristics, offering a more comprehensive and precise approach than current simplified rating methods. This advancement holds promise for enhancing quality control in steel processing and advancing Artificial Intelligence-driven technological progress within the metallurgical sector.

In the domain of fire-resistant steels, the characteristics of precipitates significantly influence material properties. This study developed a novel heat treatment protocol to concurrently achieve both interphase precipitation and random precipitation. Samples were subjected to isothermal treatments at various temperatures and durations, while techniques such as scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were employed to thoroughly analyze the coarsening behavior of the two types of precipitate and reveal their thermal stability differences. The results show that the growth and coarsening rates of interphase precipitates are substantially lower than random precipitates. Coarsening kinetics analysis reveals that the radius of random precipitates follows a 1/3 power law with time at 600 °C and 650 °C, whereas the radius of interphase precipitates adheres to a 1/6 power law at 600 °C and a 1/5 power law at 650 °C. Furthermore, interphase precipitation demonstrates excellent size uniformity, which hinders the formation of a concentration gradient, thereby reducing the coarsening rate and enhancing thermal stability. After prolonged tempering treatment, interphase precipitation maintains a higher strengthening contribution than random precipitation. This study provides novel insights and theoretical foundations for the design and development of fire-resistant steels.

Designing multi-element alloy compositions to achieve target performance was the first step in the development of modern materials, but the traditional trial-and-error experiment seriously restricted the development of new materials due to its low efficiency. In this investigation, the composition design of 316LN austenitic stainless steel for enhanced liquid-hydrogen storage using multi-crucible synchronous metallurgy in a high-flux experiment was proposed. Sixteen groups of as cast austenitic stainless steel samples with different compositions were smelted using high-flux material preparation. Then, through the observation of their microstructures, the chemical composition that best matched the target performance was finally selected. The results show that high-throughput experiments can greatly improve the efficiency of composition design optimization of new stainless steel products. At the same time, this investigation also analyzed the elemental composition of δ-ferrite and the method for effectively controlling the δ-ferrite content. In 316LN stainless steel, Cr and Mo were easily enriched in ferrite grains or grain boundaries, forming Cr and Mo enriched regions. This resulted in a gradient transition of Cr and Mo from the ferrite region to the austenite region, forming galvanic corrosion. In this study, the distribution of Cr, Mo and other elements in 316LN stainless steel was studied by means of a metallographic microscope, electron probe microanalysis, transmission electron microscope and scanning electron microscope. In addition, the relationship between the ferrite content and chemical composition was explored. Finally, it was determined that high-temperature, long-term sensitization treatment is an effective method for controlling the δ-ferrite content.