Cause analysis and countermeasures of warped head in rough rolling

Since the rough-rolled slab is thick, temperature unevenness is prone to occur on the upper and lower surfaces, so warping generally occurs more often in rough rolling. The slab with serious head warping will hit the insulation cover, affecting the flying shear head, and even affecting the difficulty of strip threading in finish rolling and affecting the rolling rhythm.

 

Cause Analysis and Adjustment Measures for Head Warping

The root cause of warping is the uneven elongation of the upper and lower surfaces of the strip. Since the rough-rolled slab is thicker, temperature unevenness tends to occur on the upper and lower surfaces, so warping generally occurs more frequently in rough-rolled slabs. The slab with serious head warping will hit the heat preservation cover, affecting the flying shear head, and even affecting the difficulty of strip threading in finish rolling and affecting the rolling rhythm.

 

1. The influence of the temperature difference between the upper and lower surfaces of the slab

a. The initial temperature difference between the upper and lower surfaces is large, resulting in uneven elongation of the upper and lower surfaces.

b. The temperature drops during rough rolling, and the upper surface drops faster than the lower surface.

The upper surface of the strip is in contact with more water than the lower surface: if the effect of the rough rolling scraper is not good, the roll cooling water will cool the upper surface of the strip; the upper surface is exposed to strip purging water, descaling water, The compressed dust water and the cooling water between the racks will accumulate on the upper surface of the steel strip, and the heat on the upper surface will be taken away by evaporation and conduction, while the lower surface will fall directly due to gravity. As a result, the temperature of the upper surface is lower than that of the lower surface, and the deformation degree of the metal on the upper and lower surfaces is different. The side with a higher temperature deforms more, so the strip bends to the side with a lower temperature, that is, warping occurs. Improving the heating conditions, under the condition that the rolling conditions of the upper and lower surfaces are the same, and reducing the temperature difference of the same plate can obviously improve warping. Optimizing the structure of the rolling mill can reduce the water leakage of the wiper board and the water removal effect of the rack roll, and can also reduce the temperature drop of the upper surface, thereby reducing the amount of head warping.

 

2. The influence of rolling line elevation

The rolling line is high and easy to warp. The height of the rolling line is too high, the reduction of the lower surface is larger than that of the upper surface, so the elongation is larger than that of the upper surface, and the head warping is obvious. Adjust the lower step pad height. According to the different diameters of work rolls and backup rolls, the lower step pad is used to adjust the height of the rolling line, and the level of the step pad generally has a certain calculation method according to the required compensation amount, which cannot be changed arbitrarily. Because adjusting the height of the rolling line may cause the steel plate to be inserted between the guide plate and the roll, causing a rolling accident. However, when the amount of head warping is large, the height of the step pad can be appropriately reduced, and the effect of adjusting the head warping is obvious. Generally, do not adjust too much, otherwise, steel rolling accidents will easily occur.

 

3. The influence of the diameter difference between the upper and lower rollers

In order to prevent the buckle from hitting the roller table, the diameter of the upper roll of the work roll is generally smaller than that of the lower roll, so that the rolling speed of the upper roll is slightly lower than that of the lower roll, and the elongation of the upper surface is smaller than the elongation of the lower surface so that the strip A small amount of head warping occurs in steel. However, if the diameter difference between the upper and lower rollers is too large, the possibility of head warping will be greatly increased.

 

4. The influence of roll speed ratio on buckle head

When the thickness of the rolled piece is 227mm, 66mm, and 38mm, the radius of the upper and lower rolls is 600mm, the average temperature of the rolled piece is 1180°C, and the reduction rate is 20%, only the speed ratio of the rolls is changed. When the thickness of the slab is 227mm, as the speed ratio of the upper and lower rolls increases, the bending rate of the buckle head decreases linearly. When the speed ratio of the upper and lower rolls is less than 1, the head warping phenomenon occurs, and when the speed ratio of the upper and lower rolls is greater than 1, the head buckling phenomenon occurs.
When the thickness of the slab is 66 mm, the speed ratio of the upper and lower rolls is between 0.9 and 1.1, which has little effect on the buckling phenomenon of the rolled piece. When the thickness of the slab is 38mm, the bending of the buckle head first increases and then decreases. When the roll speed of the upper and lower rolls is the same, the rolled piece remains in the best horizontal state. And when the roller speed ratio is greater than or less than 1, it will cause the buckling phenomenon. The upper and lower work rolls of rough rolling are driven by two independent speed-adjustable motors, and the principle of the sled value is to adjust the speed ratio of the upper and lower rolls.

Rolling operators generally adjust the sled value to adjust the amount of warping online during the rolling process.

When the thickness of the slab is 227mm and the reduction rate increases, the bending rate of the buckle head becomes larger. This is because the diameter of the upper roll is generally smaller than that of the lower roll when the rolls are matched. The increase in the reduction rate leads to an increase in the elongation of the upper and lower surfaces. But the lower surface extends more. When the thickness of the slab is 66 mm and 38 mm, the bending rate of the buckle head decreases when the reduction rate becomes larger. For thin slab climbing and biting, the reduction of the upper work roll is larger than that of the lower work roll, so that the elongation of the upper surface of the slab is greater than the elongation of the lower surface of the slab, resulting in the upper surface of the slab at the exit of the rolled piece. The surface speed is greater than the lower surface speed so that the thickness of the slab exiting the rolling mill is bent downward, that is, the buckling phenomenon. This phenomenon becomes more serious when the reduction ratio increases.

 

Epilogue

Adjusting the sled value, controlling the temperature difference between the upper and lower surfaces, adjusting the height of the lower step pad, and adjusting the amount of depression can effectively improve the head-warping phenomenon. Through the above adjustment measures, the accident rate of skidding and warping in the production of this factory has been significantly reduced, and good results have been received.

 

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