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Annealing defects and cause analysis of cold-rolled strip steel, cold-rolling technology

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Desicription: Bonding, oxidation color and performance incompatibility are the three major problems in the annealing process of cold-rolled sheets, especially the two defects of bonding and oxidation color are more prominent.

Keyword: Adhesion, oxidation, Carbon black edge, Reasons for unqualified performance, cold-rolled strip steel

Causes of adhesion

1. Excessive tension. Tension is one of the main causes of steel coil bonding. The tension includes the rolling tension and the coiling tension of the rolling mill. When the tension is too high, the air circulation of the shielding gas will not be good, resulting in thermal blockage, and bonding of the steel plate.

2. Poor plate shape, bad plate shape will make the strip appear thick on both sides, thin in the middle or thin on both sides, thick in the middle or side waves, middle waves, multiple waves and periodic partial waves, etc., after high temperature annealing Afterwards, adhesion can be produced.

3. Uneven overflow edges appear during coiling, and uneven coiling of strip steel, especially thinner plates, is prone to adhesion.

4. The emulsion is not pure. There are impurities in the emulsion, which remain between the steel plate and the steel plate after annealing and evaporation. If it is not cleanly blown off, it will also cause adhesion.

5. Over temperature. If severe over-temperature occurs in the furnace, it will inevitably cause adhesion. The main reason is that the temperature measurement thermocouple is out of control.

6. Poor circulation of protective gas in the furnace. Because the stacking and stacking of the furnace does not meet the requirements, the shielding gas does not circulate in the furnace well, and the temperature of the furnace is uneven, and the thermal retardation of individual parts is serious, and the temperature difference is large and adhesion occurs.

7. The deformation of the convection plate and the poor surface flatness and smoothness affect the gas circulation at the edge of the steel coil and form a bond.

8. The stacking is too heavy. Mainly refers to very thin plates, if the stack pressure is too much, it is very likely to cause adhesion.

9. Improper selection of annealing parameters. Different annealing process parameters should be adopted for steel coils of different specifications and steel grades.

Causes of oxidation

1. The system is not closed.

2. The protective cover is “bulging”, ruptured, and the flange is deformed.

3. The sealing ring is aging and damaged.

4. The cooler is broken.

5. Hot out of the oven. If it is not produced in accordance with the process requirements, and the output is rushed and the high temperature is released, it will also oxidize the steel coil.

6. The temperature control system is malfunctioning. Whether it is measurement, transmission, or recording, as long as there is a problem in one link, the temperature recorded by the annealing curve will not be accurate, which is also an important factor in oxidation.

7. Insufficient purging of protective gas, and unclean removal of air in the inner cover and the external fast cooling system. In addition, if it is a steel coil with too much residual emulsion or high humidity, the blowing time should be longer than normal, otherwise the bright annealing will not be achieved.

8. At the beginning of rapid cooling by spraying, the shunted protective gas suddenly shrinks in volume after cooling, causing the pressure difference in the furnace to cause negative pressure on the inner cover, which leads to air intrusion.

9. The protective gas has a high dew point. The high dew point indicates that the water content in the protective gas is high, which will inevitably bring oxidation color to the annealed steel coil.

10. There are other failures, such as compression device failure, solenoid valve failure, outlet valve failure, circulation fan failure, etc., as well as power failure, water failure, and protection gas failure, which are all factors that affect the oxidation color of annealed steel coils.

Carbon black edge

“Carbon black edge” refers to the residual rolling oil in the steel coil, which is not completely purged after annealing in the bell furnace, and a black mark is formed on the edge of the strip.

Cause: The bell-type furnace itself does not cause carbon black edge defects, but is only caused by the rolling oil remaining on the uncleaned rolling piece. The viscosity, concentration, volatilization point of the selected rolling oil, as well as the foreign matter brought in during the rolling process, are all factors that cause the carbon black edge.

Treatment method: It is not feasible to change the annealing process of the bell-type annealing furnace to adapt to various rolling oils. The use of suitable rolling oil will have a significant effect on eliminating carbon black edges.

Reasons for unqualified performance

1. There is an error in the process system.

2. The furnace is not installed properly. During the annealing of the steel coil, if the inner diameters are not aligned, and the pressure is misaligned with each other, the protective gas will not circulate smoothly, resulting in a large temperature difference in the furnace, and the small air volume of the circulating fan will also cause some performance inconsistencies.

3. Low temperature and time. The heating ends before the rated annealing temperature is reached, and the performance is prone to be inconsistent, which is mainly affected by the failure of the metering and control equipment. If the heating time is not reached or the holding time is too short, the heating ends, which also easily affects the performance.

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