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[Cold Rolling Technology] Analysis of the Causes of Broken Strips in Hot-dip Galvanizing Lines


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The broken strip in the continuous annealing furnace of the hot-dip galvanizing line not only seriously affects the stability of the unit, but also damages the equipment and increases the operating cost of the unit. Although it is said that the broken belt in the furnace is a sudden situation, this sudden situation can be avoided by finding the cause. Therefore, in order to achieve high-speed, high-efficiency and high-quality continuous annealing production, reducing the probability of strip breaking in the furnace has become an urgent problem to be solved. Today we will talk to you about the reasons for the broken belt in the furnace of the hot-dip galvanizing production line.

There are many reasons for the broken belt in the hot-dip galvanizing line furnace, but from the statistics of a large number of broken belts, there are mainly the following reasons: the change of the roll crown of the furnace roll, the shape of the strip, the ability to correct the deviation and the specification of the strip. We will analyze the reasons for the belt breakage from the above aspects.

  • The change of the crown of the furnace roll

Generally, the shape of the furnace rolls of the vertical annealing furnace is designed as a double cone with a slight positive convexity. As the furnace temperature of each section of the furnace is different, the original convexity of each furnace roll is also different. There should be an optimal value for the furnace rolls corresponding to different positions in the furnace. Generally speaking, the furnace rolls in the heating section of the vertical annealing furnace adopt the design of the ladder rolls. In production, with the change of the temperature in the furnace, the temperature difference between the strip and the furnace roll, and the tension effect, the original roll shape will change, and the original roll crown will be superimposed with the temperature-affected roll crown to produce the actual roll crown. Spend. The relationship between the crown of the furnace roll and the thermal buckling and deviation of the strip is shown in Figure 1. When the strip temperature is lower than the furnace roll temperature, the actual roll crown is the reverse superposition of the two, and the resulting macroscopic phenomenon is deviation [2]. When the strip temperature is higher than the furnace roll temperature, the actual roll crown is the positive superposition of the two crowns, and the resulting macroscopic phenomenon is the thermal buckling of the strip.

(The relationship between the crown of the furnace roll and the thermal buckling and deviation of the strip)

  • Influence of strip type

Theoretically, the shape of the strip is straight, but in the actual rolling process, the deviation of the control accuracy of the rolling mill equipment or the production control accuracy will lead to different reduction rates on the cross section during the rolling process, resulting in the stripping of the strip. The phenomenon of inconsistent extension of steel on the cross section, the macroscopic phenomenon is the wave-shaped defect. According to different positions, wave defects are divided into middle wave, unilateral wave, bilateral wave, rib wave, etc. In the continuous production of hot-dip galvanizing line, the wave-shaped defect that has the greatest impact on it is the single-sided wave-shaped defect. The unilateral wave defect is caused by the strip on the side with the wave extending significantly larger than the middle and the other side. When entering the annealing furnace, this defect will be infinitely amplified under the action of thermal stress and tension in the furnace. At this time, the running position of the strip will deviate from the center line of the furnace and deviate to the side without waves. If the deviation exceeds the capacity of the deviation correction unit in the furnace, the strip will scrape against the furnace wall, causing the strip to break.

  • The influence of correction ability

The deviation correction unit is divided into single-roller deviation correction and double-roller deviation correction according to the number of deviation correction rollers. The principle of single-roller rectification is to use the helical offset effect of strip steel on the rectification roller to rectify the deviation. The principle of double roller rectification is to rectify the deviation by the left and right swing of the roller group. These two deviation correction methods are very common deviation correction methods in vertical annealing furnaces. According to the actual situation of the annealing furnace process design, different types of deviation correction methods are allocated to the corresponding positions of the annealing furnace to ensure that the strip will not deviate during the production and operation process. Usually, a single roller is installed near the entrance of the annealing furnace, and the 1# deflection roller is installed in the furnace. In the middle or end of the annealing furnace, the double-roller deviation correction is installed. Although both single-roller rectification and double-roller rectification can play a role in rectification, from the perspective of rectification ability, the ability of single-roller rectification is far less than that of double-roller rectification. Judging from the situation of the broken belt accident, it is because the deviation of the strip steel is far greater than the ability of the single roll to correct the deviation, and the deviation correction device has no effect of correcting the deviation, and the strip steel will hang on the furnace wall, causing the belt breaking accident to occur.

  • Influence of strip specification

Judging from the specifications of the strip steel calculated from a large number of strip-breaking accidents, the strip-breaking occurs mostly when the specifications of the steel grades change, that is, the cross-sectional area of the strip steel changes. After analysis, it is believed that the change of the cross-sectional area of the strip steel causes the heat conduction between the strip steel and the furnace roll to change, resulting in the belt breakage accident. The temperature of the furnace rolls is mainly affected by the ambient temperature in the furnace (gas heat transfer) and the strip temperature (strip contact heat transfer). Due to the different heat transfer methods, the effect is also very different. The heat transfer value αs between the strip and the surface of the furnace roll is much larger than the furnace gas heat transfer value αj. Investigation of gas conduction to the interface between the strip and the furnace rolls according to the heat conduction model. When αs is larger than αj, the heat taken away when the strip is in contact with the furnace roll is far greater than the heat of the furnace roll to maintain the crown of the roll by the furnace gas. As a result, the roll crown of the contact surface between the furnace roll and the strip disappears, and the contact surface becomes flat. This causes the strip steel to not be tightly wrapped on the furnace rolls under the action of tension, and the strip steel will swing left and right on the roll surface, thereby deviating in the furnace.

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