Main factors and countermeasures affecting the bright annealing quality of stainless steel strip

The main factors affecting the quality of bright annealing of stainless steel strip are: the composition and purity of the protective gas, the heating and cooling system, the degreasing and dehydration of the steel strip, and the production process of the steel strip before BA treatment. The composition and purity of the protective gas used in the production of BA panels are the primary conditions for bright annealing. BA uses H2 and N2 as protective gas. Shi is a strong reducing agent, such as Cr2O3+3H2-2Cr+3H2O, so the higher the concentration of the protective gas neutral line, the better the brightening effect. Due to the high price and explosiveness of the game, the mixture of nitrogen and hydrogen was widely used in the past, adding 1% to 5% of pure nitrogen (volume fraction); or using 75% and 25% from (volume fraction) Ammonia decomposition gas. With the advancement of technology, in order to improve the surface quality and production capacity of stainless steel strips, pure hydrogen is widely used as protective gas on large bright furnaces, and ammonia decomposition gas is generally used for protection on some small furnaces due to the preparation of protective gas. Gas, ammonia decomposition gas contains 75% practice (volume fraction), and its brightening effect and productivity are also very good.
As the treatment temperature and dew point decrease, the oxidation temperature decreases. For example, the oxidation (Cr2O3) that occurs at 1000°C can be reduced by the treatment temperature with a higher dew point of -30°C, but it will be oxidized again when the temperature of the steel strip drops to 600°C. Therefore, under the condition of constant dew point, increasing the treatment temperature, or decreasing the dew point temperature under the condition of constant treatment temperature, can achieve better brightening effect. In order to obtain 3~4 BA board, when it is greater than 85% (volume fraction), the dew point can be -35℃; but when it is 55%-75%h2 (volume fraction), the dew point temperature should be lowered to below -40℃ . The dew point of the protective gas is directly related to the gas moisture content. Since the reaction of 2Cr+3H2O2–Cr2O3+3H oxidizes the surface of the steel strip, the protective gas dew point should be controlled within the range of -40~-60℃, and the moisture content (mass fraction) should be 1.21%-0.23%. Similarly, the residual oxygen content (volume fraction) of the protective gas should also be less than 0.001%. Determined by the Cr/Cr2O3 equilibrium temperature, the quality of the BA plate has a great relationship with the cooling rate of the steel strip after heating. At the normal processing temperature, in the heating process from room temperature to high temperature, although oxidation occurs in the range of 300-800°C, it is quickly reduced when the processing temperature is reached. Only the oxidation that occurs during the cooling process after the treatment can no longer be reduced. Therefore, rapid intensive cooling is extremely important. The cooling rate should be about 20°C/s below 800°C; the cooling method is to spray protective gas directly from the bottom up to the surface of the treated steel strip.