Cause analysis and countermeasure of cold rolling slippage

Cause analysis and countermeasure of cold rolling slippage

The concept of skid

In the process of continuous cold rolling, the circular velocity of the roll exceeds the exit velocity of the strip, and the relative sliding occurs between the strip and the roll. In essence, the deformation zone of strip steel is completely replaced by the back slip zone, and the neutral surface appears outside the deformation zone, resulting in the rolling process instability.

Harm and cause of skid

The hazard of skid

Strip slippage will cause the rolling force fluctuation of the frame, resulting in the increase of the thickness of the exit of the frame, thus increasing the load of the downstream frame. If it happens in the end frame, it will produce waste products, affecting the yield of the material. If it happens in S1 frame, it will cause the strip of S1 frame to run away, resulting in broken strip pile steel accident.

The cause of the skid

There are many reasons for skid, including mechanical equipment reasons and technological reasons, the following two aspects of analysis.

Effect of mechanical equipment on skidding

The main drive of the rolling mill has a corresponding natural frequency. When the strip is rolling at high speed, if the speed frequency is close to the natural frequency of the main drive of the rolling mill, resonance will occur, resulting in skid.

Skid caused by process conditions

(1) The influence of work roll roughness.

The smaller the roughness of the work roll, the smaller the friction coefficient between the work roll and the strip. When the friction coefficient is less than a certain degree, there will be skid phenomenon. Figure 2 shows the surface roughness of the work roll when it is skidding. It can be seen from Figure 2 that the surface roughness of the work roll when it is skidding is generally as low as about 0.3μm, while the roughness of the work roll when it is just on in frame S1 is generally 1.0μm, and that of the work roll when it is just on in frame S2~S4 is generally 0.6~0.8μm.

Effect of tension

Due to the essence of skid is the neutral surface offset, when the current post-tension fluctuation is large, it will cause the change of metal flow velocity in the deformation area, resulting in the center surface offset, resulting in skid phenomenon.

The effect of the amount of pressure

When rolling thick gauge or high strength steel, S1 frame is easy to slip. The number of slip of each frame since the 180mm production line was put into operation two years ago. S1 frame skid frequency is the highest reason: when rolling high strength steel or wide and thick plate, S1 frame pressure rate is very high, its load is large, coupled with raw material plate shape is not good, thickness fluctuation and other factors, easy to cause S1 frame skid.

The effect of emulsion concentration

In high speed rolling, the emulsion is needed for cooling and lubrication. If the emulsion concentration is too large, it will cause the strip to slip.

The effect of rolling miles

The relationship between the rolling distance of the work roll and the friction factor. It can be known that the friction coefficient decreases quickly after the new roll change and becomes stable after about 20km rolling. If the roll runs seriously beyond kilometers, the friction factor will drop to 0.15 after rolling 400km, and it is easy to slip accident.

The influence of work roll diameter.

The size of the diameter of the work roll is directly related to the size of the rolling force, when the small diameter of the work roll is close to scrap (less than 490mm), then rolling thick plate or high strength steel, it is easy to slip, especially the first two frames, because of the large amount of pressure, if the load distribution is unreasonable, the probability of slipping is very high.

Solutions to the skid problem

Mechanical equipment caused by skid countermeasures

For the skid caused by mill resonance, mill operator must always pay attention to the tension fluctuation and thickness fluctuation of strip steel between the rack, as well as the rolling force fluctuation, can be judged by listening to the sound, general resonance, mill noise is particularly large, different specifications of resonance speed is different, avoid resonance speed segment, can avoid the skid caused by mill resonance.

The countermeasure of process slip

Make a reasonable work roll roughness use strategy.

In production, the roughness of S1 frame work roll should be ensured to be no less than 1.0μm, generally 1.1~1.2μm; The roughness of work rolls in S2 and S3 racks is set to 0.8~1.0μm, 0.6~0.8μm in S4 rack, and 3.2~3.6μm in S5 rack, which can effectively avoid skidding caused by small roughness of work rolls.

Adjust front and rear tension.

When a frame appears to slip, it can be appropriate to increase the front tension of the frame, or reduce the rear tension of the frame, but it is easy to cause the next frame to slip, because the rear tension of the frame is the front tension of the next frame, so the adjustment amount must be small.

Adjust down rate allocation

Rolled deformation resistance will increase with the increase of pass reduction. Table 1 shows the deformation resistance parameters of typical steel types. MSO is the initial value of deformation resistance, MSI is the incremental value of deformation resistance, and MSE is the work hardening index. It can be seen that the change rate of work hardening of high-strength steel with the increase of cumulative deformation is low, so when the pressure reduction of S1 frame is distributed, it can avoid slipping by appropriately reducing the pressure reduction.

Develop a reasonable roll change system.

It is known that the surface roughness of skidding work roll is generally below 0.5, especially the skidding frequency of S1 frame, so the rolling kilometers of S1 frame work roll should not be too large, and the work roll replacement cycle is set as rolling 150km and the small roll diameter work roll replacement cycle is set as rolling 100km.

Make a reasonable production plan.

When the production plan is arranged, the steel with higher strength cannot be rolled immediately after the new roll change of the mill, and then the high strength steel can be rolled after rolling 10 rolls of ordinary carbon steel.

Adopt reasonable emulsion concentration system.

When the 180mm unit was put into operation, the emulsion concentration of S1-S4 frame was controlled to be 3.0%~5.0%, and that of S5 frame was controlled to be 0.5%~1.0%. The emulsion concentration of S1-S4 frame is controlled to be 2.5%~3.0%, and that of S5 frame is controlled to be 0.5%~1.0%. When rolling thin specifications, the emulsion concentration of S1~S4 frame can be increased appropriately.

Adjust the anti-skid strategy according to the steel type.

When producing low-strength steel, such as DC04 steel, increasing the pressure of S1 frame can reduce the burden on subsequent frames and prevent subsequent frames from slipping. In the production of high-strength steel, such as HC380LA steel, the press down of S1 frame can be appropriately reduced, while the front and rear tension between the frames can be adjusted. For high-strength steel, the work roll with small diameter should be avoided due to the large rolling force.