In a continuous casting and rolling (CC&R) production line, the induction holding furnace (typically a line-frequency or medium-frequency induction holding furnace) plays a critical bridging role at the front end. It functions not only as a “buffer reservoir” and thermal buffer between the melting system and the continuous casting machine, but also serves as core equipment to ensure a continuous supply of molten steel or copper characterized by “constant temperature, constant flow, and minimal oxidation.”
To guarantee a continuous and constant-temperature supply of molten metal throughout complex production processes, the induction holding furnace primarily relies on core control technologies and structural designs across the following dimensions:
1. Intelligent Closed-Loop PID Automatic Temperature Regulation System
The primary task of the holding furnace is to compensate for thermal energy lost during molten metal standing, transportation, and thermal radiation.
- High-Precision Temperature Sensing Arrays: Dual-element armored thermocouples (typically platinum-rhodium thermocouples with protective sleeves for molten steel, and high-precision thermocouples or continuous infrared pyrometers for molten copper) are deployed to monitor multi-point molten metal temperatures inside the furnace in real time.
- Precise Power Adjustment via Power Electronics: Temperature feedback signals are transmitted to a Programmable Logic Controller (PLC) or specialized temperature controller. By employing phase-shift triggering or zero-crossing triggering techniques, the output power of the thyristor (SCR) variable-frequency/variable-voltage power supply is adjusted in real time.
- Smooth Stepless Heating: The power adjustment operates on a continuous, stepless basis, eliminating the response lag and temperature overshoot associated with traditional resistance or gas-fired furnaces. This maintains molten metal temperature fluctuations within $$\pm 2^\circ\text{C$$.
2. Electromagnetic Stirring and Thermal Homogenization
A key inherent advantage of induction heating is that “the heat generator acts as the stirrer.”
- Elimination of Thermal Gradients: Alternating current passing through the induction coil generates an alternating magnetic field, inducing eddy currents and Lorentz forces within the molten metal. This electromagnetic force drives the directional or turbulent recirculation of the molten metal.
- Uniform Composition and Temperature: Continuous electromagnetic stirring completely eliminates temperature gradients between the top and bottom layers, as well as between the inlet and outlet ports, preventing localized freezing/crystallization from undercooling or furnace lining erosion from overheating.
3. Dynamic Power Compensation and Load Adaptation
During continuous production, the upstream melting furnace periodically transfers freshly melted metal into the holding furnace, introducing thermal perturbations.
- High-Power Rapid Response: Modern furnace electrical control systems feature wide-range power regulation capabilities. When the system detects a temperature drop caused by incoming fresh liquid, it rapidly switches to “boost heating” mode, using high power density to quickly compensate for thermal losses.
- Holding/Heating Dual-Mode Switching: During normal steady-state supply, the system switches to a low-power “holding mode,” maintaining thermal equilibrium while achieving energy savings.
4. High-Efficiency Furnace Lining Thermal Insulation and Structural Optimization
Minimizing heat dissipation is the foundational requirement for maintaining a constant temperature.
- Multi-Layer Composite Refractories: The lining utilizes high-purity, high-density refractory castables or dry-vibratable mixes, paired with microporous nano-insulation panels and lightweight insulating bricks with superior thermal performance, significantly reducing heat conduction through the furnace wall.
- Furnace Cover and Sealing Design: A tightly sealed furnace cover structure allows the introduction of inert protective gases (such as nitrogen or argon). This prevents oxidation and gas absorption on the molten metal surface (e.g., oxygen absorption in molten copper or nitrogen pickup in molten steel) while effectively reducing radiant heat loss from the furnace mouth.
5. Stable Fluid Flow and Liquid Level Control (Front-End Synergies)
A constant-temperature supply must ultimately be realized through constant flow (a stable flow rate).
- Synergy Between Launders, Tilting Mechanisms, and Stopper Rods:
- For continuous copper rod casting and rolling, medium-frequency holding furnace tilting pouring or launder stopper-rod flow control is commonly applied, integrated with closed-loop liquid level controls to ensure a constant flow rate into the crystallizer or distribution launder.
- A stable flow velocity combined with a constant temperature completely eliminates temperature non-uniformity or crystallizer chill effects caused by erratic flow rates.
Summary
Through an integrated control framework consisting of “precise temperature measurement + variable-frequency automatic power regulation + electromagnetic thermal homogenization + high-efficiency lining insulation + dynamic load compensation,” the induction holding furnace resolves technical challenges related to asynchronous supply-demand rhythms and variable heat losses at the front end, providing reliable thermal process assurance for downstream dense billet forming and high-speed rolling.
