Sheng Yu

Effects of Partition Coefficients, Diffusion Coefficients, and Solidification Paths on Microsegregation in Fe-Based Multinary Alloy

To quantitatively study the effects of partition coefficients, diffusion coefficients, and solidification paths on solute microsegregation, an analytical model was developed combined with the calculation of thermodynamic software FactSage. This model, applied with variational partition coefficients and temperature-dependent diffusion coefficients, is based on the Voller–Beckermann model and is extended to take into account the effects of multiple components and the peritectic phase transformation using FactSage. The predictions agree well with a range of measured data and the results of other numerical solutions. As the results indicate, the partition coefficients of solutes are functions of temperature and phase fraction during the solidification process, and the solute microsegregation increases significantly with decreasing partition coefficients. The calculations of solute microsegregation ratio (CL/C0) in the interdendritic region are related to solidification paths. The microsegregation ratios of P and S increase as the initial C concentration increases, while they reduce with increasing initial C contents for solutes C and Si. Parameter sensitivity analysis was performed, and the results indicate that the solute microsegregation shows larger variation with partition coefficients and solidification paths than diffusion coefficients.

Hot Ductility of X70 Pipeline Steel in Continuous Casting

Transverse cracks in continuous cast steel may easily form if the hot ductility of the continuous cast steel is poor at the straightening stage. In this paper, the hot ductility of X70 pipeline steel was studied at various temperature from 600 to 1300 °C. The results show that the steel has a good hot ductility (RA above 70%) at 800–1000 and 600–650 °C. And the steel exhibits a hot ductility trough (RA below 80%) at 650–800 °C, which is mainly contributed to the austenite–ferrite transformation. For further understanding the low ductility, the fracture surface was examined by scanning electron microscope, and the second phase particles was examined by energy spectrum analysis. The results indicate that the low ductility of the steel is affected by the equiaxed (Fe, Mn)S precipitate as well as austenite transformation at the third brittle zone.

Stress and Friction Distribution around Slab Corner in Continuous Casting Mold with Different Corner Structures

The non-uniform friction and thermal stress in the mold are important as causes of the transverse cracks around strand corner. To analyze the stress distribution features around strand corner, a three-dimensional thermo-elastoplastic finite-element mold model with different corner structures (right-angle, big-chamfer, multi-chamfer, and fillet) was established. The temperature field in the mold was indirectly coupled through a three-dimensional fluid flow and heat transfer model. In addition, the non-uniform mold friction stress loaded on the strand surface was calculated through a friction model. The results show that the stress distribution on the shell is similar to the temperature distribution. The stress concentration appears in the strand corner and the lower part of wide face. The friction stress enhances the corner stress around the edge of the air-gap. For chamfered molds, the stress around the corner between the wide face and chamfer face is larger than that between the narrow face and chamfer face. Around the corner region, both the stress peak and the area of the large stress zone of the right-angle strand are the largest, while those of big-chamfered, multi-chamfered, and fillet strands decrease in that order. The stress peak position of the chamfered strands is closer to the mold exit than that of the right-angle strand. Compared with the use of the right-angle mold, the application of chamfered molds is able to reduce the stress concentration around the strand corner.

Fluid flow and heat transfer behavior of liquid steel in slab mold with different corner structures. Part 2: Fluid flow, heat transfer, and solidification characteristics

ABSTRACT In this article, the complex transmission behavior was discussed in the slab mold with different corner structures. Results show that the up backflow is stronger than the down backflow. The cooling water temperature rise and heat flux through the wide face in the right-angle mold are the largest, while those through the narrow face in the multichamfered mold are larger than those in the big-chamfered mold. The corner temperature at mold exit of the right-angle, big-chamfered, and multichamfered strand increases. The shell thickness at the narrow face center in the chamfered mold is thinner than that in the right-angle mold.

Fluid flow and heat transfer behavior of liquid steel in slab mold with different corner structures. Part 1: Mathematical model and verification

ABSTRACT A three-dimensional model considering the fluid and temperature field of the liquid steel and cooling water, along with the copper plate temperature was established to study the fluid flow and heat transfer behavior of liquid steel in slab mold with different corner structures. Then, a two-dimensional stress–strain model was established to calculate the strand shrinkage. The two-dimensional stress and three-dimensional temperature were connected through the thermal resistance. The model was validated by the measured solidification shell, copper plate temperature, and cooling water temperature rise. Results show that this model is suitable to study the complex transmission behavior in slab mold.

Genetic Influence of Mold Corner Structure on the Strand Corner Temperature in Secondary Cooling Zone During Slab Continuous Casting

Mold corner structure will influence the strand corner temperature by changing fluid flow and heat transfer of liquid steel at mold corner. A three-dimensional model coupling fluid flow and heat transfer in the mold and a two-dimensional moving-slice heat transfer model in secondary cooling zone was established to investigate slab corner temperature under different mold corner structure: right-angle, big-chamfer, multi-chamfer and fillet. Results show that strand corner temperature increases obviously at mold exit through using chamfered mold. Compared with the corner temperature of right-angle strand, corner temperature of big-chamfered, multi-chamfered and fillet strand are promoted during the straightening process, which will avoid the low ductility temperature zone. When mold corner structure changed from right-angle to big-chamfer, the rise of corner temperature is much higher than that changed from big-chamfer to multi-chamfer or fillet.