How to control the surface quality of ferritic stainless steel

The carbon and nitrogen content in superferritic stainless steel is very small. In many aspects, it can be directly used as a substitute for austenitic stainless steel such as 304 and 316L. Moreover, the cost of this type of stainless steel is low, so its application is developing rapidly.
Ferritic stainless steel cold-rolled plates not only have good performance, but also have very strict requirements on the surface condition of the plates. Defects on the surface of stainless steel will be corroded first in a corrosive environment, shortening the service life of stainless steel products. For decorative stainless steel, some visible defects will cause the entire stainless steel plate to be unusable, and some surface defects may cause cracks during stamping and processing. Therefore, it is very critical to enhance the surface quality of ferritic stainless steel.
The surface quality control of ferritic stainless steel mainly includes the following aspects.
The first is to control inclusions. For superferritic stainless steel, large particle inclusions and spinel-type hard inclusion aggregates can easily cause the stainless steel to form scale defects during the hot rolling process, which is harmful to the surface quality of the stainless steel and will also reduce the quality of the stainless steel. Product corrosion resistance. As a common surface defect in stainless steel production, scale defects will appear as linear or mountain-shaped white marks after annealing and pickling of stainless steel cold-rolled plates, and there is a color difference compared to the normal plate surface. Serious scale defects are prone to material cracking when processing stainless steel parts. Through analysis, it can be determined that the main composition is titanium oxide, which is formed by inclusions entering the continuous casting billet during the continuous casting process and being extruded to the surface of the stainless steel plate and extended along the rolling direction after rolling.
The second is to control the grain structure. The data shows that there is a close relationship between the grain structure state and surface wrinkles. The wrinkling degree of the columnar crystal structure sample after annealing is more serious than that of the equiaxed crystal structure sample, so reducing the columnar crystals of the continuous casting billet and increasing the equiaxed crystal structure can reduce the surface wrinkles of the stainless steel plate. In order to improve the equiaxed crystal, the main methods include electromagnetic stirring, applying current pulses and using the continuous casting secondary cooling section cooling process.
The third is to control the rolling process. Because the deformation resistance of superferritic stainless steel is small, the material texture is relatively soft, and it is easy to bond with the rollers, so the rolling plan must use a mixed rolling mode, that is, superferritic stainless steel and conventional carbon steel with low surface quality requirements. Mixed rolling. The drawing temperature of superferritic stainless steel is controlled in the range of 1000 to 1100°C. Using a relatively low drawing temperature can improve the deformation resistance and surface hardness of this steel type during the rolling process, and alleviate the friction between the roll and the rolled material. Adhesion problem. In addition, the reduction rate of each pass cannot be too high, otherwise it will cause the rolling load to increase or the local temperature of the strip to be too low. Because the local rolling force is large, a rough band will be formed on the surface of the roll, which will form on the surface of the roll after contact with the strip. Surface defects occur on the strip surface. The work rolls of the finishing rolling should be made of high-speed steel. The high-speed steel rolls have higher strength and hardness and are less likely to bond with the strip during hot rolling at high temperatures. In addition, the roughness of the roll should be reduced as much as possible while ensuring that the strip can bite smoothly, so as to reduce the frequency of super-ferritic stainless steel sticking to the roll.