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Induction heating technology of metal plates and strips

Date：

2023-08-24

The heat treatment of metal plates and strips mainly includes the applications of preheating, annealing, quenching+tempering, surface softening, painting and drying, and strip laminating. Online induction heating of metal plates and strips has advantages of good controllability, high efficiency, energy conservation, safety and environmental protection. It is an ideal heating method to replace traditional annealing furnaces and resistance furnaces mainly using coal and gas, with wide applications in the market.

Induction heating technology of metal plates and strips

Applicable scope

The heat treatment of metal plates and strips mainly includes the applications of preheating, annealing, quenching+tempering, surface softening, painting and drying, and strip laminating. Online induction heating of metal plates and strips has advantages of good controllability, high efficiency, energy conservation, safety and environmental protection. It is an ideal heating method to replace traditional annealing furnaces and resistance furnaces mainly using coal and gas, with wide applications in the market.

Transverse magnetic flux induction heating and longitudinal magnetic flux induction heating technology

Induction heating technology can be divided into two types based on the making method of the inductor and the position relationship between the alternating magnetic flux and the workpiece: longitudinal magnetic flux heating and transverse magnetic flux heating.

Fig. 1 longitudinal magnetic flux induction heating (left) and transverse magnetic flux induction heating (right)

Longitudinal magnetic flux induction heating technology

Fig. 2 Longitudinal magnetic flux induction heating

The thickness, heating temperature, and material of the heated workpiece have strict requirements for heating frequency. The thinner the metal strip, the higher the heating frequency is required.
For very thin non-ferrous metal strips or thin steel strips heated above the Curie point, using longitudinal magnetic field heating technology needs very high heating frequency, the production of induction heating power supply is difficult and expensive
Due to the skin effect, for longitudinal magnetic field heating, the outer surface of the strip is first heated, and the heat is transmitted to the core of the strip through thermal conduction, which means that the heating is "from the outside to the inside". The heated strip has a "sandwich" phenomenon and a large temperature difference existed between the core and surface.
The width direction of the plate and strip material is uniformly heated, with high heating efficiency.
The inductor has a simple structure, easy manufacturing, and convenient maintenance.
Suitable for surface heating applications of plate and strip materials.

Transverse magnetic flux induction heating technology
Fig. 3 Transverse magnetic flux induction heating
There is no strict correspondence relationship between the thickness, heating temperature, workpiece material, and heating frequency of the heated workpiece, i.e., the induction heating power supply with lower frequency can be used to heat very thin non-ferrous metal strips or thin steel strips that exceed the Curie point.2) The induction current flows horizontally along the entire width direction of the strip, having uniform heating of the strip cross section and a small
temperature difference between the core and surface.
The magnetic field accumulates at the edge of the strip, leading to overheat at the edge. The area near the edge of the strip has a low temperature area due to magnetic field scattering, which is "Cat's ears effect".
The width direction of the plate and strip material is not uniformly heated and special measures need to be taken to improve
The structure of the inductor is complex with high cost, and difficult to be maintained.
Suitable for heating non-ferrous metal plates and strips, as well as steel plates and strips with heating temperatures exceeding the Curie point.

Fig. 4 Transverse magnetic flux inductor integration

High power induction heating power supply for plate and strip

The high-power plate and strip induction heating power supply is inverter power supply that converts 3-phase power frequency 50Hz/60Hz AC into single-phase medium frequency/ultra-audio frequency/high-frequency AC (500Hz~300kHz). The power supply adopts an AC-DC-AC topology.
Due to the high power of a single machine, the high-power induction heating power supply for plate and strip materials generally adopts 12-pulse rectification on the rectifier side to reduce the harmonic current on the grid side and improve the power factor on the grid side.

Fig. 5 Schematic diagram of high-power induction heating power supply for plate and strip

Technical indicators
Power range: 1000kW-6000kW
Frequency range: 10kHz-300kHz
Rated DC voltage: 450V~850V
Grid voltage: 400V~660V, 50/60Hz, 3-phase system
Technical characteristics
12-pulse rectifier circuit with high power factor and low harmonics on the grid side.
MOSFET/IGBT series inverter circuit, adopting pulse shielding technology on inverter side for power regulation, with excellent load matching ability.
Fully digital control technology, with perfect protection functions, fiber optic drive, and Ethernet communication.
Adopt perfect process structure, which is safe and reliable, with convenient maintenance and repair.
Customized design can be adopted for different heating process requirements of customers.