The process of bright annealing of stainless steel aims to obtain sufficient metallographic structure by eliminating the work hardening and, on the other hand, to obtain an oxide free bright surface. Here is a detailed discussion of the role of these two aspects.
1. Excellent microstructure of stainless steel can be obtained by eliminating hardening
The purpose of a bright annealing furnace is to heat the finished product of stainless steel under protective atmosphere. According to the requirements of the application performance, the requirements of the microstructure of the annealed stainless steel are different, so the processing technology of the stainless steel with bright heat treatment is different.
Process of 300 austenitic stainless steel heat treatment process is a typical solution treatment, in the heating process will dissolve in austenite carbide, heated to 1050 to 1150 DEG C after appropriate heat preservation for a short period of time, can make the carbide completely dissolved in austenite, and then rapidly cooled to 350 DEG C to get under the supersaturated solid solution is uniform one-way austenite. The focus of this heat treatment stainless steel processing technology is rapid cooling, requiring cooling rate of 55 degrees /s, fast through the solid solution after precipitation of precipitation temperature zone (550~850 degrees Celsius). The heat preservation time should be as short as possible, otherwise the stainless steel grain will be coarse, which will affect the surface smoothness of stainless steel.
The heating temperature of the 400 series ferritic stainless steel heat treatment is relatively low, which is about 900 DEG C, and more is obtained by annealing and softening with slow cooling. Martensitic stainless steel adopts this kind of annealing method, and also adopts subsection quenching and re – tempering heat treatment method.
Seen from the above 300 series and 400 series stainless steel in heat treatment process have very big difference, so if you want to obtain the satisfactory microstructure of stainless steel, it is necessary to regulate room for the bright annealing furnace cooling equipments is large and flexible. Therefore, the excellent bright annealing furnace is usually cooled by strong convection in the cooling section, and has three cooling sections, which can adjust the air flow independently. Three sections are divided along the width direction of strip steel, and the cooling speed of strip width direction is adjusted by air flow diversion. Finally, the shape of stainless steel plate is controlled.
Another important aspect of the heat treatment cold rolled stainless steel strip is that the entire strip is uniformly distributed in width and length. The large size of muffle tube and muffle type bright annealing furnace, muffle tube from uniform external heating air flow organization of the spiral around over the strip, uniform heating. To ensure that the stainless steel strip along the length of the organization is uniform, it is necessary to maintain the strip in the heating furnace line speed unchanged. Therefore, the advanced vertical bright heat treatment furnace is equipped with adjustable roller tension adjusting device both front and rear. The device not only to strip the import and export of heat treatment speed to meet the speed requirements, should not affect the amount of empty or full sets of sets of sets, and also according to the situation of stainless steel strip plate is established and precision adjustment strip small tension, so as to achieve the demand of stainless steel plate.
2. Make the stainless steel band get the bright surface without oxidation
Generally, bright annealing is the heat treatment of stainless steel strip under H2 protective atmosphere. And stainless steel processing technology to meet the requirements of BA panels, we must strictly control the bright furnace protection atmosphere, purity, shall avoid oxidation. And what about the oxidation under the protective atmosphere of H2?
Because of the chemical composition of stainless steel belt mainly iron, chromium, nickel, manganese, titanium, silicon and so on. In the range of annealing temperature, the oxidation of iron and nickel is not important, but the oxidation temperature range of chromium, manganese, silicon and titanium is just in the range of heating temperature. So, their oxidation affects the surface brightness of stainless steel strip. In particular, chromium oxidation causes stainless steel strip surface chrome removal, thereby reducing corrosion resistance of stainless steel. When the chromium content in the stainless steel band is 17% ~ 18% and the content of Ti element is 0.5%, the H2 dew point must be less than 60 DEG C in order to avoid chromium and titanium, and the oxidation occurs at the temperature range of 800C to 1150 DEG C.
In addition, we must pay attention to how to maintain the protection of the atmosphere in the furnace, the purity of the gas. In this respect, good sealing and muffle, does not occur as pollution material of electric heating furnace for the protection of the atmosphere, so as to ensure the relatively pure furnace environment. Also, the vertical furnace stainless steel strip entrance is at the bottom of the furnace, which will ensure the stability of the furnace pressure, so the risk of air intake is much smaller than the horizontal furnace. But the bright annealing furnace and the import and export of sealing box, strip steel, tension roller and the feeding roller box should ensure top guide to a few parts per million (ppM) of the sealing requirements, so that the oxygen and water vapor can not enter, gas can not leak protection.
Source: China Stainless Steel Pipes Manufacturer – Yaang Pipe Industry Co., Limited (www.yaang.com)