Annealing is a process in which cold rolled structures are converted to a recrystallized structure through heat treatment. For grain-oriented electrical steel, two different annealing methods are available: decarbonization annealing and high temperature annealing. Decarbonization annealing removes excess carbon from the steel and apply an MgO coating. High temperature annealing produces secondary recrystallized structures having superior magnetic properties.
In this process, insulation coating is applied by a continuous coater roll in order to minimize eddy current losses, which are proportional to the sheet thickness. Grain-oriented electrical steel has two layers of coating, a base coating of dark brown Forsterite as the main ingredient and a transparent insulating coating containing phosphates. For non-oriented electrical steel, various coating methods with various thicknesses and ingredients are used depending on end usage and specific user’s requirements.
During the preliminary annealing process, scale which may have formed on the hot rolled steel is removed as delayed passing through scale breaker and a hydrochloric acid bath. This initial heat treatment process improves cold rolling properties of steel as well as it’s magnetic properties.
In order to obtain specific thickness and material properties, reduction ratio of 40~90% is applied normally. Rolling and edge trimming machines are automatically controlled to obtain uniform thickness and width.