Why does bell type annealing furnace produce black strip? How to solve it?

Today, Suzhou jinruier Electric Furnace Co., Ltd., the manufacturer of bell type annealing furnace in Suzhou, shares with you the reason why there is black belt in steel coil during annealing in bell type annealing furnace? How to solve it?

The steel coil annealed in bell type annealing furnace has good plasticity, but the yield strength and tensile strength decrease slightly. In the whole process, the role of hydrogen is to provide strong circulation medium for heating and cooling of strip steel and purge the residues on the surface of strip steel. The residue on the surface of strip steel mainly comes from the cold rolling process. With the coiling of coil, it is entrained in the inside of coil. Cold rolled products are generally not degreased after cold rolling, but directly to the bell type annealing furnace for bright annealing. The emulsion left on the surface of the steel plate must be removed. Now the commonly used method is to use offline or online degreasing treatment, or annealing to remove the dirt. If the emulsion is removed by annealing, the annealed coil will often appear black belt, which seriously affects the surface quality.

So what is the reason for the appearance of black strip in bell type annealing furnace? How to solve this situation?

In the early stage of annealing, the rolling oil in the emulsion on the surface of the steel strip is heated and volatilized. With the increase of temperature, part of the rolling oil not discharged with the hot gas will produce pyrolysis reaction. If it is not blown clean in time, it will deposit on the surface of the steel strip and form carbon black in the cooling process.

At the beginning, with the increase of temperature, the moisture in the residual emulsion of the rolling mill began to evaporate (about 100 degrees Celsius), and the residual emulsion on the surface of the steel plate began to be decomposed (about 300 degrees Celsius). Hydrocarbons first react with oxygen in water vapor to form carbon monoxide between the rings of the steel strip. With the increase of temperature, the long chain begins to crack and form atomic clusters. Small hydrogen molecules are easy to diffuse between the steel strips, and the open end of these atomic clusters is in the hydrogen saturation stage, which will form volatile light hydrocarbons. At this time, hydrogen is conducive to reducing carbon deposition, and some carbon will remain on the surface of steel plate. At 700 ℃, hydrogen will directly react with carbon deposition to generate methane. Therefore, the purpose of hydrogen purging is to replace the emulsion volatilized on the coil surface during heating in the furnace.

So how can we solve the problem of coil black belt? In fact, the solution is very simple, just need to improve the hydrogen purge system.

For some common CQ steel, from the mechanical properties, the heating speed will not be limited, can play the potential of the equipment. In the actual production process, the heating rate is 60 to 100 ℃ per hour, and the heating temperature is 680 ℃. According to the characteristics of bell type annealing furnace and rolling oil of rolling mill, the temperature of high flow hydrogen blowing starts at 450 ℃. According to the above analysis, combined with the original hydrogen blowing system.