Bright annealing and requirements for stainless steel plates

When performing bright annealing on the stainless steel plate, the stainless steel plate can obtain a bright surface without oxidation, and can also eliminate the work hardening formed in the previous process to obtain a satisfactory metallographic structure. In order to achieve these two purposes, there must be corresponding requirements for bright annealing.

Bright annealing of stainless steel plate
No oxidation bright surface
The so-called bright annealing treatment is to heat treat the stainless steel plate under the H2 protective atmosphere, and to meet the requirements of the BA plate, the protective atmosphere in the furnace must be strictly controlled to prevent oxidation as much as possible.
Another key issue is how to maintain the purity of the protective gas in the furnace. In this regard, the muffle cover has good sealing performance, and will not cause pollution to the protective atmosphere such as the electric heating furnace body material, which can ensure a relatively pure environmental space. In addition, the strip steel entrance and exit of the vertical furnace are located at the bottom of the furnace, and the furnace pressure stability is high, so compared with the horizontal furnace, the risk of air entering is much smaller. However, the sealing box at the entrance and exit of the bright annealing furnace, the strip steel conveying section, the tension adjustment roller and the furnace top guide roller box should all meet the sealing requirements of a few parts per million to prevent oxygen and water vapor from entering, so that the protective gas will not leak .
Eliminate work hardening
The bright annealing furnace is mainly used for bright heat treatment of finished stainless steel plates. When the performance requirements of stainless steel materials are different, the requirements for the metallographic structure after bright annealing are also different, and the bright heat treatment process is also different.
The typical heat treatment process in 300 series austenitic stainless steel is solution treatment. During the heating process, the carbides are dissolved into the austenite structure, and heated to 1050-1150°C, and after a short period of proper heat preservation, the carbides can be completely dissolved into the austenite structure, and then rapidly cooled to Below 350°C, a supersaturated solid solution, that is, a uniform unidirectional austenite structure, is obtained. The focus of this heat treatment process is to be able to cool rapidly, which requires a cooling rate of 55°C/s, which can quickly pass through the re-precipitation temperature zone after carbide solid solution. The holding time during this period should be as short as possible, otherwise it is easy to cause coarse grains and affect the surface finish of the stainless steel plate.
The heating temperature of 400 series ferritic stainless steel is lower, and the annealed softening structure is obtained more by slow cooling. Martensitic stainless steel uses annealing method, and can also be processed by segmental quenching and tempering.
From the above, it can be known that the heat treatment system of 300 series and 400 series stainless steel is very different, and in order to obtain a qualified metallographic structure, it is required that the cooling section of the bright annealing furnace has a large enough adjustment space. Therefore, the modern advanced bright annealing furnace usually adopts strong convection cooling in the cooling section, and is equipped with three cooling sections, so that the air volume can be adjusted independently. Along the width direction of the stainless steel plate, it is divided into three sections, and the cooling rate in the width direction of the stainless steel plate is adjusted by the air flow guide, and finally the shape of the stainless steel plate is controlled.