Comparison between hot rolling and leveling process and straightening and leveling process

 

1. Different product ranges covered

The hot rolling and leveling unit mainly processes thin-gauge plain carbon steel, and the main process goal is to improve the flatness of thin-gauge plain carbon steel. With the expansion of the product range of hot-rolled pickled plates and the impact of Luders strips on product quality, some manufacturers use hot rolling and leveling units as the pre-process of pickling units to eliminate the yield platform of thin-gauge plain carbon steel hot-rolled products. Therefore, the products processed by traditional hot rolling and leveling units are mainly thin-gauge plain carbon steel hot-rolled products, and the main process goals are to improve the flatness of thin-gauge products and eliminate the yield platform of plain carbon steel.

The straightening and leveling process is a process developed to address the problem of insufficient flatness improvement capabilities of traditional hot rolling and leveling units when processing thick-gauge products, high-strength steel products, and products with strict post-processing deformation restrictions in downstream processing. Therefore, the products processed by the straightening and leveling process are mainly conventional thickness and ultra-thick products, high-strength steel products, and plain carbon steel products with strict post-processing deformation restrictions in downstream processing.

Therefore, compared with the traditional hot rolling leveling process, the straightening and leveling process greatly broadens the range of products covered in terms of thickness specifications and strength range, and the straightening and leveling process also greatly improves the flatness accuracy of the final product.

2. Different product flatness indicators

The straightening and leveling process uses the straightening machine to reduce the residual stress level of the product and improve the flatness of the product; at the same time, with the help of the real-time dynamic control capability of the leveling machine, the performance uniformity of the processed product is controlled. Under the condition of reasonable selection of the straightening machine roller diameter and roller speed, the straightening and leveling process can fully reduce the residual stress level of the product and improve the flatness index of the product.

Since the straightness index represents the residual stress level inside the product in a sense, the smaller the straightness index, the lower the residual stress level inside the product. In the downstream processing process, drilling, shearing, welding, etc. will cause the internal stress of the strip to be released. After the internal stress is released, the strip will deform, reduce the dimensional accuracy of the downstream products, and even cause scrapping. At present, the requirements of automobile factories for the straightness of hot-rolled strip are: the wave height per meter is less than or equal to 3mm, which is converted to 2.22I in I units. For this type of product, the traditional hot rolling leveling process is completely powerless. Only the straightening and leveling process can meet the requirements of the downstream industry for flatness indicators.

3. Different surface quality of products

Since the traditional hot rolling leveling process cannot solve the problem of secondary pressing of detached oxides, surface pitting defects always restrict the surface quality of hot rolled products. The WISDRI straightening and leveling process uses independent core technologies: the theory of self-generated air flow field of rolling/leveling equipment and active air flow field control technology, and adopts a combination of blowing and suction to remove detached oxides, avoiding quality defects on the surface of the product.

4. Different effects on reducing acid consumption

In order to avoid the Luders band defect in the pickled products of plain carbon steel, many customers use the method of leveling and then pickling to eliminate the yield platform of plain carbon steel. Since leveling can only break the oxides on the surface of the strip, but cannot remove the oxides, it is impossible to reduce the amount of acid reaction in subsequent pickling and reduce acid consumption.

In the straightening and leveling process, after leveling and crushing the oxides, the straightening machine is used to continue to bend the surface oxides, and the brush roller is used to clean the oxides in the critical detachment state, partially removing the oxides on the surface of the strip, reducing the amount of acid reaction in the subsequent pickling, and achieving the goal of reducing acid consumption.

5. Different working roll consumption indicators

The hot rolling leveling process uses conventional chromium 3 or chromium 5 working rolls. It takes one roll change to produce about 700 tons of products, and the roll loss is very high. The straightening and leveling process develops rolls based on the characteristics of severe oxidation of the edges of hot-rolled strip steel, greatly extending the roll service life, and increasing the roll change cycle of the working roll from 700 tons to 2100 tons to 2800 tons, reducing roll consumption.

6. Different product quality stability

The flatness control of conventional hot rolling leveling units depends entirely on the production experience of the operator, resulting in poor product quality stability. The straightening and leveling process uses plate shape detection + automatic pre-set mode of the straightening machine to control the plate shape, so that the stability of the product flatness accuracy is improved.

7. Different unit layouts

The conventional hot rolling skin-pass mill has a single layout, and it is difficult for the equipment in the units to leverage each other. The straightening and skin-passing process can flexibly configure the process equipment on the unit according to the product positioning of the unit and the process requirements of the upstream and downstream units to give full play to the combined efficiency of the equipment.

Since the content of interstitial atomic carbon and nitrogen in high-strength steel and IF steel is less than 30ppm, there is no yield platform, and no Luders band defects will be generated during processing. Therefore, we use a straightening + skin-passing configuration on the unit. The advantages of this configuration are:

• Deep bending rolls and straightening units are combined to avoid the time lag effect during the straightening process and fully improve the flatness of the strip;
• After straightening, skin-passing can be used to eliminate some residual curvature and appropriately reduce the number of rollers of the straightening machine;
• The skin-passing machine can fully utilize the ability of the skin-passing machine to improve the surface quality of the strip and improve the surface quality of the product;
• Reduce the capacity bottleneck of the straightening machine.