Causes and Control Measures of Black Spot Defects on Cold-rolled Strip Surface

Abstract: Aiming at the problem of black spot defects in the products of cold-rolled sheet factory, the causes of black spot defects were analyzed from the aspects of raw materials and processes, and the control of strip shape, optimization of coiling tension and annealing process, and strengthening Emulsion management and other measures have better controlled black spot defects, and the product qualification rate has reached 96% .
Key words: cold-rolled steel strip; black spot defect; annealing process; plate shape; emulsion

Causes Analysis of Black Spot Defects on the Surface of Cold Rolled Strip and Its Countenneasures

1. Strip Surface Roughness

The experimental results show that the greater the surface roughness R* of the strip steel, the more beneficial it is to avoid black spots and black band defects, because the roughness increases the gap between the coil layers and the layer interface, which is conducive to the volatilization of the emulsion .

1. Rolling product pass coiling tension

According to the theory of elastic mechanics, the radial stress between steel coil layers should be equal to the coiling tension. Reducing the coiling tension can reduce the compressive stress between the layers of the steel coil, thereby increasing the gap between the layers, protecting the gas from flowing between the layers of the steel coil, and ensuring the full volatilization of the emulsion.
The smaller the coiling tension value is, the better it is to reduce black spots and black belt defects. However, if the coiling tension is too small, it will cause defects such as poor coil shape, flat coil, loose coil, loose coil, etc., and affect smooth production at the same time. injury defect. Therefore, the tension value should be selected according to the strip thickness and width.

1. Volatility of the emulsion

Generally, cold rolling oil is composed of base oil (mineral oil and grease) and various additives (extrusion agent, antioxidant and emulsifier, etc.). The thermal volatility of rolling oil during the annealing process is crucial to the cleanliness of the plate surface after annealing, and its good thermal volatility can greatly reduce the probability of black spots and black bands on cold-rolled coils.
( 5) ;Strip surface cleanliness
The residues on the strip surface (metal debris, mechanical inclusions and oily adhesives) mainly come from the cold rolling process, and are entrained into the interior of the coil as the coil is coiled. The higher the thermal stability of the rolling oil on the surface of the strip, the less volatile it is when heated, and it is easy to pyrolyze to form carbon black at high temperature. In addition, the emulsified liquid is not cleaned during the rolling process, resulting in too much emulsified liquid on the surface of the strip steel. During the normal annealing process, the emulsified liquid cannot be completely volatilized, and finally black spots and black bands will be formed on the steel strip. If the emulsion is mixed with high-pressure, low-pressure oil and lubricating oil, these oils belong to heavy molecular organic matter, which are decomposed into hard-to-volatile substances during the annealing process, and finally deposited on the surface of the strip to form black spots and black belts.
(6) ;Annealing process
In the annealing process, a reasonable hydrogen blowing process can reduce the black spots and black band defects of cold rolled coils.
Fig . 1 is a typical hydrogen annealing furnace shielding gas composition change curve with time and temperature. It can be seen from Figure 1 that the methane in the atmosphere in the furnace reaches its maximum value twice (that is, the peak value of methane in Figure 1 ), and the first peak value appears when the temperature reaches about 500° C during the heating process . At this time, CH 4 The C in CH comes from rolling oil and should be blown off; another peak appears in the heat preservation section at the end of heating, the C in CH ( mainly comes from steel, in order not to de- C the strip steel, the methane produced at this time cannot be blown blown away.

Therefore, in the heating stage (that is, at 300 ~ 500 ° C ), it is the emulsion evaporation stage, and a large flow of hydrogen can be used for purging; when the second peak of methane arrives , it should not be purged.
Figure 2 is the change curve of the dew point of the atmosphere in the furnace during the annealing process. After the pre-purging of the steel strip in the hearth space, the measured dew point is usually -50 ° C ; after the final N 2 gas purging, the dew point can usually reach -73 ° C . During the coil heating process, water and oil vapor evaporate, and the dew point of the annealing atmosphere depends on the total oil content on the surface of the strip, the heating rate and the H2 purge rate, and the increase in the dew point is not conducive to the decomposition of the residual emulsion, resulting in the residual emulsion at high temperature. Cracks to form carbon black.

 

1 - dew point a (open stove ); 2 - dew point b (closed stove - control temperature
( 7) Residual chlorine ions on the surface of the strip steel
Clean and dry the surface of the steel plate with black spots, and boil the surface with black spots with 5% KOH solution for 20 minutes. It is found that most of the black spots can be cleaned (the main component is carbon), but some black spots have not been cleaned. Therefore, using XPSCX photoelectron spectroscopy analysis) to analyze the components of this part of the black spots, the results show that the black spots are mainly iron oxides ( FqOQ .
Analyzing the annealing process, iron oxides will not be produced during the annealing process, and iron oxides should be reduced. Therefore, the oxide is from before annealing.
Check the steel coil before annealing. Black spots and sometimes yellow spots are found on the surface of the strip where the emulsion remains. Further analysis shows that chloride ions are enriched around the black spots.
There should be no chloride ions in the emulsion, but Jinan Steel’s cold rolling production is pickling with hydrochloric acid. If the rinsing is not clean, chloride ions will remain on the surface of the strip and will be brought into the emulsion to cause pollution of the emulsion. If the purge at the outlet of the rolling mill is out of control during production, the emulsion will be entrained into the steel coil. In addition, the low temperature of the finished coil will also cause the entrainment of the emulsion. Steel coils (especially thin-gauge products) are tightly wound, and electrochemical corrosion will occur on the plate surface (destruction due to electrochemical interaction between the metal surface and the ion-conducting medium) when the plate gap is small and at a certain temperature. Medium C1 - and its high conductivity rinsing water will accelerate the electrochemical corrosion process, and the product may be FeO (yellow ), Fe 2 O 3 (red) or Fe 3 O 4 ( black), in the cracks covered by the oil film, Due to insufficiency, black Fe 3 0. lo is often generated
Therefore, it is necessary to conduct regular tests on the emulsion to reasonably control the chloride ion content in the emulsion.

2 Measures to eliminate black spot defects
(1) Improve chilled coil shape

According to the effect of plate shape on the surface quality of strip steel after annealing, micro-edge wave rolling should be used when rolling the annealed substrate.
( 2) ;Optimize the working radial roughness
The cold rolling mill of Jinan Iron and Steel Co., Ltd. is a double-stand reversing rolling mill (2), and its working radial roughness cannot be too high. If it is too high, the rolling load will be too large and the steel cannot be rolled. After exploration, the working spoke roughness of the rolling mill is 0. 8~1. Ofxm is ideal.
( 3) ;Optimizing the coiling tension of rolled products
In order to improve the volatilization effect of emulsion and other residues in the annealing process, and reduce the coiling tension of the finished product pass as much as possible under the premise of ensuring rolling stability, different thickness specifications adopt different coiling tensions, and the value range of coiling tensions 20 ~ 40MPa .
( 4) ;Select high-quality rolling oil and strengthen emulsion management
On the premise of ensuring rolling lubrication, reduce the concentration of emulsion appropriately, avoid mixing low-chain stick oils in rolling oil as far as possible, and strictly control the content of miscellaneous oil to improve the volatility of rolling oil and ensure the purging of rolling mill The stability of the effect to reduce the residual emulsion on the surface of the steel plate after rolling. Jinan Iron and Steel Cold Rolling Plant selects Quaker N680-2DPD emulsion, which has a volatilization temperature of about 330° C and a low volatilization temperature, which is conducive to the complete decomposition of the emulsion.
At the same time, strengthening the emulsion management is also an important link to improve the cleanliness of the strip surface. After practice, the emulsion management indicators are set, see Table 1 .
(5) Adjust the mill emulsion purging system
The cold rolling mill adopts the emulsion purging system combined with anti-winding guide plate air wall, nozzle purging and suction device. After production practice, the following improvements have been made to the purging system:

1. The original design compressed air pressure was 0. 3 ~ 0. 4MPa, and it was found that the purge pressure was insufficient and the emulsion was not purged cleanly. Therefore, the compressed air pressure was increased to 0. 7MPa .
2. Adjust the clearance of the air wall of the anti-winding guide plate of the rolling mill. If the gap of the air wall is unreasonable, the pressure of the air wall is not enough, and the emulsion is easy to drip onto the strip.
3. Adjust the gap between the anti-winding guide plate with air wall and the rolling spoke in the rolling mill. According to the design, the gap between the anti-winding guide plate and the rolling spoke is 3 ~ 5mm, but the actual gap is about 10 ~ 20mm. Therefore, the gap should be readjusted and controlled by the lower limit.

Table 1 Emulsion Management Indicators

project	Control range	Detection frequency (during normal use)
concentration/%	3. 0~5. 0	1 time/class
pH value	4. 5 ~ 7. 0	1 time/class
Conductivity /ps • mm -1	W 300	1 time/class
Saponification value /mgKOH • gT	> 135	1 time/day
Chlorine content / mg - L- 1	< 20	2 times/week
Acid value / mgKOH • g7	7~13	1 time/week
Bacterial content	< 103	1 time/month
Ash content /mg • L _1	< 500	1 time/week
Iron content mg • L -1 /	< 300	1 time/week
ESI/%	40 ~ 70	1 time/day

1. In order to prevent the gap between the air wall of the anti-wrapping guide plate from being blocked and the gap between the anti-wrapping guide plate and the rolling spokes from changing, it should be checked and maintained regularly to ensure the blowing effect.

( 6) Optimizing the hydrogen blowing process
Proper hydrogen blowing process can not only ensure that the residual emulsion volatile gas is blown off, so that there is no carbon black deposition on the surface of the steel plate, but also the hydrogen consumption can be minimized. After repeated practice, optimized hydrogen blowing process .
( 7) Control the chloride ion content in the emulsion
The chloride ions in the emulsion are residual chloride ions on the strip surface after pickling. After practice, the following control measures can be taken:

1. Formulate a strict inspection system for the conductivity of rinse water, requiring the conductivity to be less than 20us/mm. If the conductivity of the rinse water exceeds the standard, discharge the rinse water in time and replenish new water;
2. Strengthen the management of the squeezed spokes in the process section. If the squeezed spokes are worn out, the strip cannot be squeezed well, and chloride ions will remain on the surface of the strip;
3. Strengthen the monitoring of chloride ions in the emulsion, requiring the content of chloride ions to be less than 20mg/L. If it exceeds the standard, it must be re-prepared in time;
4. Speed up the circulation of steel coils. Under normal circumstances, the steel coils should be furnace-loaded and annealed within 2 days to avoid long-term stacking of steel coils in the warehouse to reduce black spots and black belt defects on strip steel caused by corrosion.

Improvement effect
After adopting the above-mentioned measures in the cold rolling mill, the black spot and black belt problem of the cold rolled coil has basically been solved, and the qualified rate of the cold rolled coil has reached about 96% .

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