The purpose of high temperature annealing is as follows:
(1) secondary recrystallization.
When annealing at high temperature, the oriented nucleus in the primary recrystallization structure grows abnormally, which makes the steel strip become a single oriented secondary recrystallization structure. Secondary recrystallization is the most basic and important process for the production of oriented silicon steel.
(2) purify steel.
In order to recrystallize the silicon steel strip, inclusions such as sulfide and nitride are necessary. But these inclusions are harmful to magnetism if they remain in the finished product. Therefore, after the secondary recrystallization is completed, the steel must be removed and purified, which requires high temperature and long time heat treatment of the silicon steel strip.
(3) the bottom layer of magnesium silicate is generated.
In order to improve the insulation performance of the silicon steel strip, a compact film with good adhesion to the base metal needs to be formed on the surface of the steel strip. For this reason, high temperature was used to react the annealing isolator (MgO) with SiO2 film on the surface of the steel belt, forming a uniform glass thin film -- magnesium silicate bottom layer with 2MgO·SiO2 as the main component on the surface of the steel belt.
Because the oriented steel in high temperature annealing, to further a MgO crystal water decomposition and exclusion, but also purification steel annealing insulation time is long, so the oriented silicon steel belt high temperature annealing, do not use steel belt continuous annealing furnace, usually in the periodic operation of the cover type annealing furnace. However, there are a series of shortcomings in the production, quality and management of the hooded furnace. In recent years, a number of projects have adopted continuous operation of steel coil annealing furnace as a high temperature annealing furnace, which has the advantages of energy saving, continuous production, stable product quality, less occupation, less personnel, low production cost, etc. The main furnace types are ring furnace and tunnel furnace. However, the continuous annealing furnace with steel coil has its limitation and certain application range.