In the field of metallurgy, the production of large-scale steel ingots is critical for applications in heavy machinery, energy, and transportation sectors. However, traditional casting methods often lead to significant internal defects, such as shrinkage porosity and cracks, which compromise the mechanical integrity and performance of the final product. These metal casting defects arise from uneven solidification, thermal gradients, and inadequate feeding during the process. To address these challenges, we investigate a liquid-solid composite casting technique through numerical simulation, focusing on how this approach can optimize temperature distribution and solidification sequences to reduce metal casting defects in large ingots. By pre-placing a solid steel core within the mold and encapsulating it with molten metal, we aim to achieve metallurgical bonding and minimize defects like shrinkage and porosity. This study employs a three-dimensional finite element model to simulate the filling and solidification processes, providing insights into the thermal behavior and defect formation mechanisms. The results demonstrate that the liquid-solid composite process significantly enhances the quality of large-scale steel ingots by controlling heat transfer and promoting sequential solidification, thereby mitigating common metal casting defects.
The liquid-solid composite casting process involves introducing a pre-fabricated solid metal core into the mold before pouring the molten metal. This core acts as an internal chill, altering the heat flow and solidification patterns. A key advantage is its ability to reduce the volume of liquid metal that solidifies at once, which helps in managing thermal stresses and preventing defects. However, achieving proper metallurgical bonding at the interface between the core and the cladding layer is crucial; insufficient heat can lead to mechanical adhesion and weak interfaces, while excessive heat can cause complete melting of the core, reintroducing metal casting defects. Therefore, optimizing parameters such as core size, preheating temperature, and pouring temperature is essential to balance these factors and minimize metal casting defects.
To model the casting process, we consider the mass, momentum, and energy conservation equations for an incompressible, viscous fluid with a free surface. The continuity equation, derived from mass conservation, is given by:
$$ \frac{\partial \rho}{\partial t} + \nabla \cdot (\rho \mathbf{v}) = 0 $$
where \( \rho \) is the density, \( t \) is time, and \( \mathbf{v} \) is the velocity vector. For incompressible flow, this simplifies to \( \nabla \cdot \mathbf{v} = 0 \). The momentum conservation equation, based on Newton’s second law, is expressed as the Navier-Stokes equation:
$$ \rho \frac{D\mathbf{v}}{Dt} = -\nabla p + \mu \nabla^2 \mathbf{v} + \rho \mathbf{W} $$
where \( p \) is the pressure, \( \mu \) is the dynamic viscosity, and \( \mathbf{W} \) represents body forces. The energy conservation equation, accounting for heat transfer during solidification, is:
$$ \rho c_p \left( \frac{\partial T}{\partial t} + \mathbf{v} \cdot \nabla T \right) = \lambda \nabla^2 T + Q $$
Here, \( c_p \) is the specific heat capacity, \( T \) is the temperature, \( \lambda \) is the thermal conductivity, and \( Q \) is the internal heat source. These equations are solved numerically to simulate the temperature and flow fields, which are critical for predicting metal casting defects like shrinkage porosity and hot tears.
The initial and boundary conditions are set to reflect real casting scenarios. The core is preheated to 800°C, while the mold and environment start at 27°C. The pouring temperature is maintained at 1,560°C with a pouring rate of 2,180 kg/s. Boundary conditions include no-slip walls at the mold interface, and heat transfer coefficients are defined for different surfaces: 900 W/(m²·K) for the core-molten metal interface and mold inner wall, and 50 W/(m²·K) for the riser to account for insulation. These parameters help in accurately capturing the thermal gradients that influence metal casting defects.
For the simulation, a 400-ton large-scale steel ingot is modeled, comprising a mold, riser, and base. The core is placed centrally with a diameter of 1,200 mm and height of 4,100 mm, based on thermal equilibrium calculations to ensure metallurgical bonding without excessive melting. The mesh is refined near boundaries to resolve steep gradients, with element sizes ranging from 5 mm to 40 mm. The material properties for the steel (17Cr2Ni2Mo) are summarized in Table 1, which includes chemical composition and thermal parameters relevant to predicting metal casting defects.
| Element | Content (wt%) |
|---|---|
| C | 0.18 |
| Cr | 1.67 |
| Mn | 0.54 |
| Mo | 0.28 |
| Ni | 1.62 |
| Si | 0.29 |
| V | 0.01 |
The core size is determined using equations derived from casting principles, treating it as a fusion chill. The maximum mass \( m \) of the core that can be melted is given by:
$$ m = f \rho V_0 \left( \frac{M_0 – M_1}{M_0} \right) $$
where \( f \) is a coefficient dependent on pouring temperature, \( \rho \) is density, \( V_0 \) is volume, and \( M_0 \) and \( M_1 \) are the original and equivalent modulus of the ingot, respectively. The coefficient \( f \) is calculated as:
$$ f = \frac{\left( \frac{1}{3} L + q_s \right)}{\left( c_s T_s + \frac{1}{2} L \right) + \left( \frac{1}{3} L + q_s \right)} $$
Here, \( L \) is the latent heat of fusion, \( q_s \) is the superheat, \( c_s \) is the specific heat of the solid, and \( T_s \) is the solidus temperature. The maximum diameter \( d_{\text{max}} \) of the core for effective bonding is:
$$ d_{\text{max}} = 3.05 f_u M_r^2 $$
where \( f_u \) is an overheating-related factor and \( M_r \) is the practical modulus of the casting section. For the 400-ton ingot, these calculations yield a core mass of approximately 24.25 tons and a maximum diameter of 1,235 mm, ensuring that the interface reaches the solidus temperature for bonding without causing metal casting defects.
In traditional casting simulations, the temperature distribution shows significant gradients, with rapid cooling at the mold walls and slower solidification in the center. This leads to shrinkage porosity and voids, as the liquid metal cannot compensate for volumetric shrinkage. The Niyama criterion, a common indicator for metal casting defects, predicts defect-prone regions based on local thermal gradients and solidification rates. For traditional casting, defects concentrate in the upper central region near the riser, due to premature closure of feeding channels. This highlights the inherent limitations of conventional methods in controlling metal casting defects.
In contrast, the liquid-solid composite process alters the solidification sequence. The pre-placed core absorbs heat from the molten metal, reducing the superheat in the central region and promoting directional solidification from the core outward. This minimizes thermal hotspots and ensures that shrinkage is directed toward the riser, where it can be managed. The temperature fields from simulations reveal that the core surface reaches the solidus temperature, enabling metallurgical bonding. For instance, at a height of 5,200 mm, the interface remains above the solidus for over 36,474 seconds, allowing sufficient time for diffusion and bonding. This controlled solidification reduces the incidence of metal casting defects like macroporosity and segregation.
To quantify the improvement, we analyze the porosity distribution using the Niyama criterion. In traditional casting, the porosity volume fraction is high in the center, whereas in the composite process, it is significantly reduced and confined to the riser area. This is because the core acts as a heat sink, stabilizing the solidification front and improving feeding. The reduction in metal casting defects is further evidenced by the temperature profiles at different heights along the core surface. Table 2 summarizes the time intervals during which the interface remains molten, indicating effective bonding and minimized defects.
| Height (mm) | Time Above Solidus (s) |
|---|---|
| 2,200 | 9,842 |
| 3,200 | 18,842 |
| 4,200 | 29,842 |
| 5,200 | 36,474 |
The energy balance during the process can be expressed as:
$$ \int_V \rho c_p \frac{\partial T}{\partial t} dV = \int_S \lambda \nabla T \cdot d\mathbf{S} + \int_V Q dV $$
This equation highlights how heat extraction through the core and mold influences solidification. By optimizing the core dimensions and preheating, we achieve a balance where the core surface melts partially without complete dissolution, thus preventing the reintroduction of metal casting defects. The liquid-solid ratio, defined as the volume of molten metal to solid core, plays a critical role; our simulations show that a ratio derived from the above equations ensures proper bonding while minimizing defects.
In conclusion, the liquid-solid composite casting process offers a robust solution to mitigate metal casting defects in large-scale steel ingots. Through numerical simulation, we demonstrate that pre-placing a heated core alters the thermal dynamics, promoting sequential solidification and reducing shrinkage porosity. The model predictions align with practical casting principles, showing that parameters like core size, preheating temperature, and pouring temperature must be optimized to achieve metallurgical bonding and minimize metal casting defects. This approach not only enhances the quality of ingots but also provides a framework for designing composite casting processes in industrial applications, where controlling metal casting defects is paramount for performance and safety.
Future work could explore multi-core configurations or varying core materials to further optimize the process and address other types of metal casting defects, such as inclusion-related issues. Overall, this study underscores the importance of integrated numerical modeling in advancing casting technologies and reducing metal casting defects in heavy steel production.