Dendrite Structure, Spot Segregation and Banded Defect of Gear Steel in Continuous Casting and Rolling Process

The types, distribution, morphology, orientation, growth, and aggregation mechanism of primary carbides in the continuous casting slab of 6Cr13 high‐carbon martensitic stainless steel are investigated. In the results, it is shown that M7C3 carbides precipitate from the liquid steel at the end of solidification. The area fraction of primary carbides in the slab thickness direction shows an overall decreasing trend from the center to the edge. And, the closer to the center region, the greater the difference in carbide distribution due to the presence of spot segregation defects. At 1/8 thickness and edges in the slab, the fast cooling rate and fine dendrites make the carbide morphology to be a small‐sized brain like that aggregates a large number of spherical carbides. At 1/4 thickness in the slab, blocky, fibrous, and spherical carbides precipitate successively and eventually aggregate together. At 3/8 thickness and center in the slab, the cooling rate is slower and spot segregation defects are randomly distributed in this region. The carbide morphology is mostly small‐sized brain like in the non‐spot segregation region. However, the alloying elements are highly supersaturated and the carbides can grow sufficiently to form less oriented, large‐sized brain‐like carbide in the spot segregation region.

Section: Discussionmentioning

confidence: 99%

“…Spot segregation defects can be classified into two types: loose and speckled. [ 10,31,32 ] The speckled type spot segregation defects are larger in size and appear more in the center of the continuous casting slab, while the spot segregation defects appearing in the 3/8 thickness of the slab are basically of the loose type.…”

Section: Discussionmentioning

confidence: 99%

Precipitation, Growth, and Aggregation Behavior of Primary Carbides during Continuous Casting of 6Cr13 Slabs

Li,

Pan

et al. 2024

Harmful Effects, Origin, and Control of Banded Structures in Electroplated Fasteners of High‐Carbon Steel

Zhang,

Long,

Cheng

et al. 2024

In this study, the harmful effects of a banded structure on the bending fracture of electroplated fasteners are investigated. The origins and ways to control these structures are also explored. The fracture surfaces of the fasteners exhibit stepwise cracks and numerous intergranular facets associated with ductile tearing, which is a characteristic of hydrogen embrittlement. The origin of these stepwise cracks can be traced to the internal banded structures. Therefore, the internal banded structures promote fracturing during the bending test. The internal banded structures in the fasteners can be traced back to the internal cracks in the bloom, and inappropriate soft‐reduction processes lead to internal cracks. Notably, the carbon segregation at the internal cracks is severe. This is attributed to the proximity of the internal cracks to the center of the bloom, causing the enriched liquid in the interdendritic region to be sucked into these internal cracks. The optimal reduction position and amount are determined by employing heat‐transfer and soft‐reduction models. Fluctuations in superheat and casting speed significantly influence this optimal position and reduction amount.

Numerical Study on the Effects of Variable Mushy Zone Constants on Fluid Flow and Heat Transfer in Bloom Mold

Yao,

Wang,

Cao

et al. 2024

In previous studies, the mushy zone constant was assigned a fixed value or a value associated with dendrite characteristics at room temperature. However, elucidating the constitutive behaviors of mushy dendrites at high temperatures using solid dendrite characteristics at room temperature poses challenges. In this study, a numerical model was constructed to investigate the effects of variable mushy zone constants on fluid flow and heat transfer in the mold. The results revealed that “Zone Z‐1/2” exhibited a thinner solidified shell compared with the other zones, indicating a close correlation between heat transfer and thickness differences. Under default simulation conditions, the liquid steel cooled rapidly, leading to the dissipation of superheat. The mushy zone exhibited a wide range, with almost no pure liquid phase present in the mold. In cases 2, 3, and 4 within “Zone Z‐1/2”, the average thickness of the solidified shell gradually increased during solidification, indicating the absence of evident remelting phenomena in the mold. As the variable mushy zone constants increased, the local temperature gradient increased, while the local cooling rate decreased. The accuracy of the numerical model was validated through actual measurements and empirical formulas. The simulation results for case 4 exhibited a high degree of fitting.