Low-temperature Cast Billet Heating Technology for Oriented Silicon Steel
Dec 21, 2023
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Why Slab Reheating Decides CRGO Quality
Grain-oriented silicon steel carries roughly 3.0% to 3.3% silicon, and its magnetic performance depends on the Goss texture developed during secondary recrystallization. That texture can only form if inhibitor phases stay finely dispersed through rolling and annealing. Slab reheating is the step where those phases are taken into solution, so the heating curve constrains the loss level and magnetic induction that the finished coil can reach.
From High-temperature Furnace Heating to Low-temperature Casting
The traditional route relied on a high-temperature furnace treatment to bring the inhibitor elements into solution before rolling. The modern route combines low-temperature casting with ordinary walking-beam reheating and, where additional thermal input is required, high-frequency induction heating for short-time exposure at higher temperature. Reported low-temperature reheating is carried out in the range of 1150 C to 1250 C, appreciably lower than the traditional high-temperature route. The benefits are lower fuel consumption, reduced slab oxidation loss and less refractory wear, which is why the practice spread quickly across oriented silicon steel plants.
Thermal Path of a Low-temperature CRGO Route
| Process step | Thermal condition | Process objective |
|---|---|---|
| Slab reheating | 1150-1250 C, controlled soaking | Bring inhibitor elements into controlled solution while limiting scale loss |
| Hot rolling | Finish rolling completed in the austenite range | Obtain a uniform, recrystallized hot band with the right precipitate state |
| Normalizing | Controlled austenitizing and cooling | Set grain size and precipitation before cold reduction |
| Cold rolling | Two-stage reduction with intermediate anneal | Build up the deformation texture that later feeds Goss growth |
| Decarburizing anneal | Wet hydrogen atmosphere | Reduce carbon to very low levels and form the oxide layer used as an inhibitor base |
| Secondary recrystallization | High-temperature batch anneal | Develop the Goss orientation and remove the inhibitor |
Inhibitor Chemistry and the Nitrogen Balance
Aluminium nitride and manganese sulphide are the classic inhibitor systems. In the low-temperature route nitrogen is deliberately made available, either as an alloying addition or through processing, because AlN precipitation depends on the nitrogen that can combine with aluminium. Carbon and sulphur must also be held in narrow ranges: carbon is removed later in the decarburizing anneal, while excess sulphur forms coarse MnS particles that can no longer pin grain boundaries effectively. The practical consequence for steelmakers is a tighter specification on aluminium, nitrogen, manganese and sulphur, and tighter control of slab temperature uniformity, because a cold slab corner behaves differently from the slab centre.
Effect on Final Magnetic Properties
When the thermal path is controlled, low-temperature processed strip reaches the loss and induction ranges expected of commercial high-permeability grades, with product thickness normally 0.23 mm, 0.27 mm or 0.30 mm and loss quoted at 1.7 T and 50 Hz. Poor reheating control shows up as an incomplete secondary recrystallization texture: loss rises, magnetic induction drops, and the strip becomes sensitive to bending and clamping stress during core building. Transformer builders usually observe it as higher no-load loss and a wider spread between finished cores rather than as an obvious material defect.
Commercial and Inspection Notes
Grain-oriented strip is supplied to GB/T 2521.1-2016, IEC 60404-8-7, EN 10107 or ASTM A876/A876M, with loss and induction measured by the Epstein method of IEC 60404-2. A purchasing specification that cares about reheating practice should require a coil-to-coil loss record, the insulation coating type and thickness, lamination factor, and a residual curvature or flatness limit, because these parameters dominate the building factor of the finished core.
Frequently Asked Questions
Q: What slab reheating temperature is used in a low-temperature CRGO route?
A: Reported low-temperature reheating is carried out around 1150 C to 1250 C, which is below the traditional high-temperature route and reduces energy consumption and scale loss.
Q: Why is nitrogen important in oriented silicon steel?
A: Nitrogen combines with aluminium to form AlN, the inhibitor that pins grain boundaries until secondary recrystallization. Too little nitrogen weakens the inhibitor; too much can leave coarse particles that no longer pin effectively.
Q: Does low-temperature processing reduce the magnetic quality of the finished coil?
A: No. With correct chemistry and annealing control the route reaches the loss and induction ranges of high-permeability commercial grades; the process window is simply narrower than on the high-temperature route.
Q: At what induction and frequency is grain-oriented loss quoted?
A: Standard commercial quotation is specific total loss at 1.7 T and 50 Hz for 50 Hz markets, or at 1.7 T and 60 Hz for 60 Hz markets, measured to IEC 60404-2.
Q: Which thicknesses are available for transformer cores?
A: 0.23 mm, 0.27 mm and 0.30 mm are the common commercial thicknesses, with 0.18 mm and 0.20 mm used for very low loss designs. Thinner strip reduces eddy-current loss but raises handling and stacking cost.
Q: How does a buyer verify that a coil meets the loss class ordered?
A: Compare the mill's coil-by-coil Epstein test record with the grade limits, then confirm coating type, lamination factor and flatness on arrival. Testing finished cores only shows the total building factor, not the raw material class.