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Optimizing the rolling and annealing process to improve the quality of cold-rolled products


Description: Laiwu Steel’s cold rolling production line is composed of a pickling line, two single-stand cold rolling mills, 24 bell annealing furnaces, a four-roll leveling line and a tension leveling line. The cold-rolling production line is mainly used for pickling hot-rolled strips of electrical steel, ordinary carbon steel, and low-alloy high-quality steel with a thickness of 1.8 to 3.0 mm under normal temperature conditions. After rolling, annealing, and The flattening and straightening process makes it into cold-rolled coils of various specifications with the required thickness. The purging effect of the purging system in the rolling process and the quality of the steel coil after annealing have a great influence on the product quality, comprehensive yield and production cost of the cold-rolled finished product. Therefore, the optimization of the cold-rolled production line process system and the transformation of key equipment are of special significance to the production and marketing of Laiwu Steel’s cold-rolled products.

Keyword: Rolling roll,  Cold rolling, annealing process to improve the quality

  • Process and defect analysis

The pickled hot-rolled coil is hoisted by the crane to the coil receiving table of the rolling mill uncoiler. The winding trolley puts the coil on the receiving table on the reel of the uncoiler. The rotating strip of the uncoiler passes through the front steering roller, the front sweeping beam, the upper and lower anti-winding guide plates in the front of the machine, the six-roller cold rolling host, the upper and lower anti-winding guide plates at the rear of the machine, the accident handling shear, the rear steering roller, and finally enters the machine. The post-coiler performs the coiling of the rolled strip. During reverse rolling, the strip passes from the back coiler, passes through the back equipment, the six-roll cold rolling main machine, and the front equipment enters the front coiler, and then reciprocates rolling until the strip of the required specifications is rolled. Material so far. When the steel strip passes through the rolls, it needs emulsion for cooling and lubrication. The coils from the rolling mill are transported to the bell-type annealing furnace for annealing. The coils are annealed for 72 hours in the hood retreat area. When they reach room temperature after cooling for 24 hours, they are then transported to the leveler and tension leveler to eliminate the annealing caused by the strip. Deformation, and finally get the required bright annealed cold plate.

  • Defects of the air purge system

In the rolling process of cold-rolled strip steel, emulsion as a rolling lubrication and cooling medium plays a very important role in the rolling process. At the same time, whether the emulsion can be sufficiently purged from the strip steel surface during the rolling process has a great impact on the surface quality after rolling. At present, the biggest problem facing the reversible rolling mill of the cold rolling production line is that the emulsion on the surface of the plate cannot be effectively purged after rolling, resulting in the defect of emulsion spotting. Emulsification stains cannot be washed off in the degreasing section. And in the mask annealing stage, emulsion stains are also difficult to remove. Therefore, emulsion stains are an important defect that plagues the improvement of product quality in cold-rolled production lines. For processes that mainly produce annealed plates, emulsion stains on the surface of the plate are a major bottleneck restricting the improvement of product quality.

  • Annealing adhesion

Adhesion is the bonding phenomenon between the inner layer of the steel coil in the high temperature and high pressure bell furnace. Generally, in the annealing process in the bell furnace, the steel coil is prone to sticking after 3 hours of cooling. The annealing adhesion produced by the steel coil has a great influence on the product quality and the yield rate.

  • Modification of process and equipment defects

Optimization and transformation of air purge system [2]

The stains in the rolling mill production process are mainly caused by the large design defects of the current strip surface sweeping system. In order to fundamentally solve the surface quality problems caused by the current emulsion entrainment, it is necessary to blow the current surface The sweeping system, the emulsion injection system, and the belt-breaking anti-wrapping and protection system are comprehensively modified.

Design and layout of anti-wrap guide

The upper and lower anti-wrap guides before the transformation are fixed guides. After the transformation, the anti-wrap guide is moved in and out by hydraulic control. The anti-wrap guide moves out through the limit to detect, after the moved out limit is activated, it indicates that the anti-wrap guide has withdrawn; the position of the anti-wrap guide when it moves in is adjusted by the mechanical position, after the anti-wrap guide is installed according to the actual maximum roller Diameter to determine the maximum mechanical stroke and position of the guide plate.

Renovation of the panel purge system

The purge system before the transformation is only equipped with a purge beam at the front and back of the rolling mill. The purge beams are equipped with side blowing nozzles and plate surface purge nozzles. It is difficult to clean the emulsion and cannot withstand the splashing emulsion. The following figure shows the schematic diagram of anti-wrap board, emulsion spraying and purging before modification. Therefore, the transformation of the plate surface purging system should be based on the actual situation of the rolling mill purging, and the rolling mill purging system must be comprehensively modified. A purge mechanism and a suction device integrating the anti-wrap guide plate are added to the frame. Two rows of purge devices are arranged on one side of the upper anti-wrap guide plate. The emulsion brought out by the cooling of the intermediate roller and the supporting roller splashes on the surface of the rolled strip, as shown in the figure below, after the transformation, the anti-wrapping plate, emulsion spraying and purging

Air knife anti-overflow purge system attached to the upper surface

Before the transformation, there was no design of the anti-overflow purge system. Because in the rolling reversal phase, the emulsion accumulated on the upper surface of the strip is very easy to roll into the steel coil, causing serious emulsion spots on the head and tail. In order to effectively eliminate the emulsion entrainment caused by the reversing process, cylinders are added on both sides of the rolling mill to drive a liftable air knife anti-overflow purge mechanism.

Control of annealing adhesion [1]

During the annealing process of the steel coil, the internal thermal stress is tensile stress, and the tensile stress will not cause adhesion between layers. Therefore, the heating stage and the heat preservation stage will not affect the adhesion between the layers.

The cooling stage of the steel coil during annealing is opposite to the heating stage. The inner and outer surfaces and end surfaces are cooled first, and the temperature of the inner and outer surfaces is lower than the temperature of the core of the steel coil. At this time, compressive stress will be generated inside the steel coil. Compressive stress will promote the occurrence of interlayer bonding between steel coils. With the different cooling rate, the difference between the core temperature and the surface temperature is different, that is, the temperature gradient is different, and the resulting compressive stress is also different.

To calculate the internal compressive stress of the steel coil, firstly determine the convective heat transfer coefficient between the steel coil and the surrounding medium, and then interpolate the temperature of different coil layers according to the coil compactness coefficient and the test curve to calculate the radial heat conduction of different position units. Coefficient, elastic modulus and thermal expansion coefficient; then call the Supper-SAP software to calculate the internal temperature field of the steel coil, compare the calculated result with the measured annealing temperature curve, and then re-correct the convective heat transfer coefficient of the coil boundary to recalculate. Finally, the internal thermal stress is calculated according to the calculated temperature field. During the annealing process of the steel coil in the bell-type furnace, 3 hours after the start of cooling, the temperature of the coil core is above 670℃. The layer of the steel coil is easily bonded under high temperature and high pressure. The radial stress of the steel coil caused by the coiling tension and the radial stress generated during the annealing process are distributed along the radial direction of the steel coil.

It can be seen that pressure, temperature and time are the three main reasons for adhesion. Solving the adhesion problem also mainly starts from the three aspects of pressure, temperature and time. Due to the requirements of the annealing process, the requirements for temperature and time are relatively strict, and the temperature generally does not decrease much. The annealing time is a problem worthy of further study. The calculation of the thermal stress shows that a certain amount of thermal stress can be reduced by appropriately extending the time at the beginning of the cooling phase, thereby reducing the bonding tendency. Therefore, different annealing systems should be formulated according to different steel grades. However, due to the limitations of the specific conditions on site, an effective method to solve the adhesion problem can start from the aspect of reducing the pressure between the layers of the steel coil. The contact pressure between layers depends on the tightness of the coil coil, which is closely related to the coil tension, plate shape, plate convexity and surface roughness.

Through the above analysis, not only the annealing process system of various products of CQ and DQ materials is optimized, but also thin gauge steel coils with a thickness of less than 0.4 mm and rolled steel with poor shape, crown and surface roughness are required. Before the coil enters the hood unwinding, the coil is re-coiled and uncoiled, and a series of annealing procedures suitable for the production line have been formulated

After optimization and improvement, through actual production and product quality inspection, the performance of the newly formulated annealing program is better than the original annealing program in all aspects, the annealing cycle is reduced by about 5 hours on average, the actual hydrogen consumption per ton of steel is reduced by about 0.5m3, and the surface of the product is clean. The degree of increase is about 10%, especially to eliminate the phenomenon of board surface adhesion.

  • Conclusion

After the optimization and transformation of Laiwu Steel’s 1500 mm cold rolling mill, the process equipment is operating well, the production cost is well controlled, the production operation rate has increased by 5%, and the yield rate has increased by 5%. The quality of the products has also been greatly improved, has been well received by users, has produced significant economic benefits, and has good promotion value in the same industry.

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The world No. 1 in crude steel production steel plant, over 70% of the HSS for bar and wire are from us.

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