Pickling of high strength steel by trolley annealing furnace

With the advent of some new CR series stainless steels with high strength and good processing and welding performance, such as super martensite, martensite, ferritic aging stainless steel, etc. with the addition of Cu and Nb elements, the process of neutral salt electrolytic pickling plus nitric acid electrolysis has not been able to adapt to the pickling treatment of these new steels. Recently, a new process has been developed to increase alkali electrowinning on the basis of neutral salt electrolytic pickling + nitric acid electrolysis Nitric acid, hydrofluoric acid, sulfuric acid, hydrochloric acid, etc. are used alone or in combination in the hydrolysis process. Nitric acid is an oxidant, which can make stainless steel passivated, but it has no descaling effect when used alone. Hydrofluoric acid is toxic. It not only corrodes iron scales, but also matrix metals, making the surface rough. Therefore, these two kinds of acids can not act alone, and the mixture effect of them is the first, good surface can be obtained in a short time, and the risk of pitting is also small, so no matter what kind of steel, this kind of mixed acid is mostly used.

In mixed acid solution, the mechanism of acid pickling is very complex. It is generally considered that HF acts as a mediator to promote the reaction between HNO3 and oxide layer. Sulfuric acid has a good pickling effect at a higher temperature, but it is very poor at a lower temperature, so it is generally only used in the following two situations: one is to improve the descaling performance of chromium steel hot rolling coil, sometimes a hot sulfuric acid tank is set up, which is used in combination with a mixed acid tank of nitric acid and hydrofluoric acid; the other is to wash in the salt bath water on the cold rolling coil deacidification line to neutralize the alkali brought out of the strip after alkali tank treatment A sulphuric acid tank is arranged between the tanks to improve the effectiveness of the back pickling tank. When sulfuric acid pickling is used, the key is to master the acid temperature. Hydrochloric acid is generally not used for stainless steel pickling. Because hydrochloric acid not only erodes scales, but also erodes metal matrix. Even if it is used, it should be mixed with nitric acid. However, this mixed acid metal matrix is still corrosive, resulting in rough metal surface, so it is rarely used in general stainless steel plants.

Electrolytic pickling is a method to accelerate descaling when only acid leaching is difficult to completely remove. The combination of trolley annealing furnace and acid leaching can improve pickling efficiency. The characteristics of this method are: due to the effect of electrochemistry, the uniform glossiness of the surface is good. Two methods are usually used in electrolytic pickling, nitric acid electrolysis and sulfuric acid electrolysis. When nitric acid is used as acid leaching solution alone, it has no descaling effect. However, when current flows through the electrode (silicon cast iron), due to the electrochemical effect, the oxide film can be ionized and acid pickling can be carried out. However, due to the low corrosiveness and low phosphorus removal capacity, it is generally not used alone. The descaling ability of sulfuric acid electrolysis is worse. In addition, in addition to electrolytic charging, this method also needs to recycle waste acid and sulfur oxide in waste gas, so the equipment cost is high. The combination of various pickling methods on the continuous pickling line, strip pickling is carried out continuously, and various pickling methods, including pretreatment, are combined in different ways. The combination mode is determined according to the steel grade, strip surface state and equipment conditions.

The pickling equipment of trolley type annealing furnace and pickling process control pickling equipment can be divided into three categories: acid storage equipment, pickling equipment, final washing equipment and waste acid treatment equipment. This paper mainly introduces the pickling equipment. The pickling equipment consists of acid tank, brush washer, dryer and other auxiliary equipment. According to the type of acid, the acid tank is divided into sulfuric acid tank, hydrofluoric acid mixed acid tank, nitric acid electrolytic tank, etc. According to the characteristics of acid, lining is different. Generally, the acid tank is made of steel plate, lined with acid resistant plate (such as polyethylene resin plate), and then built with acid resistant brick (ceramic tile for sulfuric acid and nitric acid electrolysis, carbon brick for nitric acid hydrofluoric acid mixed with acid). The acid is added from the upper part of the acid tank through the metering tank. There are two ways to heat the acid solution of the trolley annealing furnace: one is to directly heat the tank with steam; the other is to indirectly heat it with a heat exchanger. Indirect heating can be divided into two ways: one is to heat in the tank, the other is to set a heat exchanger outside the tank, so that the acid liquid circulates between the acid tank and the heat exchanger.

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