Cause Analysis of Red Oxide Scale Defects on the Surface of Hot Rolled Steel Sheet

The surface of hot-rolled coils is usually blue-gray, and the surface is smooth with a certain luster. However, due to the different chemical composition and rolling process of different steel types, sometimes red oxide scale (commonly known as red rust) will appear on the surface of the steel plate, which not only affects the appearance of the product, but also causes increased wear of the roll, resulting in the steel plate being damaged by the iron scale. In the hot rolling process, the iron oxide scale mainly composed of FeO is basically formed on the surface of the strip. FeO has high plasticity under higher temperature conditions and can deform with the matrix without breaking.

However, during low-temperature rolling, FeO will be broken and the specific surface area in contact with air will increase, which will continue to be oxidized into Fe2O3.

2 Chemical composition of test steel

Analysis of red oxide scale defect This defect covers the entire steel plate surface, elongates along the rolling direction, and has obvious directionality. There are obvious pits after pickling in some positions, and the thicker the specification, the more serious the defect.

Through the analysis, it is found that the structure of the iron oxide scale of this material is relatively complex, and there are pits at the interface between the iron oxide scale and the substrate, indicating that there is an indentation phenomenon of the iron oxide scale. According to the analysis results, it is inferred that the cause of the red rust defect may be related to Si.

In order to further confirm the corresponding relationship between the defect and the surface quality of the slab, the slab was tracked. Through observation, it was found that there were black spot defects on the surface of the slab after the furnace descaled.

In order to analyze the influence of the “black spots” on the slab of the heating furnace on the scale, the slab was removed after the furnace was descaled, and the surface oxide scale of the slab was analyzed by scanning electron microscopy. analyze. Through the analysis, it is determined that the types of iron oxide scale are FeO and FeSiO4. It can be seen that there is a corresponding relationship between the red oxide scale defect and the “black spot” defect on the surface of the slab. In order to remove this defect, it is necessary to focus on controlling the black spot on the surface of the slab.

3 Control measures for red scale defects

3.1 Mechanism of scale formation in hot rolling

The formation process of the iron oxide scale is the diffusion process of two elements, iron and oxygen. Oxygen diffuses from the surface to the interior of the iron, while iron diffuses to the outside. Oxidation reaction When the concentration of oxygen in the outer layer is large and the concentration of iron is small, high-valent oxides of iron are generated. The concentration of iron in the inner layer is large, while the concentration of oxygen is small, and low-valent oxides of oxygen are generated. O2 reacts with steel.

3.2 Effect of heating temperature on red iron oxide scale

According to the literature, the most effective way to eliminate or reduce the oxide scale defects on the surface of the slab is to increase the furnace temperature, so that the surface temperature of the slab is higher than the melting point of FeSiO4 when the slab is descaled after the furnace, making it liquid. For this reason, it is necessary to ensure that the FeSiO4 (fayalite) in the slab is completely liquid, and the furnace temperature must be controlled above 1205°C. Therefore, under the premise of ensuring the temperature at the entrance of finishing rolling, it is necessary to increase the number of descaling passes and reduce the descaling speed of each pass.

3.3 Effect of finishing temperature and rolling speed on red iron sheet

According to on-site observation, it is found that thin-gauge products have fewer red oxide scale defects. Through the comparison of hot-rolling processes with different thickness specifications, the finish-rolling inlet temperature of steel plates with thickness specifications greater than 3.0mm is reduced by 30°C, and the finish-rolling temperature is increased by 10°C. The rolling speed increased by 2m/s, thereby reducing the contact time of the steel coil with the air before coiling, and the red oxide scale defects were obviously alleviated through on-site observation.

3.4 Effect of laminar cooling and coiling process on red iron sheet

According to the following reaction formula of H2O and steel, it can be seen that the steel plate will not produce Fe2O3 during the layer cooling process, because the surface of the steel plate is covered with a layer of film before coiling, which prevents the oxygen in the air from contacting the steel plate, which is beneficial to prevent the red color Iron oxide scale. The reaction of H20 with steel is as follows:
Since the steel plate still reacts with oxygen in the air after coiling, the proportion of Fe2O3 in the reaction product is relatively large at this time. In order to reduce the contact area between the steel plate and oxygen, the coiling process adopts a high-tension coiling process to reduce the gaps in each layer of the steel coil , effectively reduce the air intake, and reduce the red oxide scale defects.

4 Conclusion

(1) The red rust defect on the surface of the hot-rolled sheet is due to the high proportion of Fe2O3 in the iron oxide scale, and its essence is that FeSiO4 adheres to the steel matrix and FeO, resulting in red oxidation in contact with cooling water and air during hot rolling and cooling. Iron sheet.

(2) Ensure that the descaling temperature after the slab furnace is above 1205°C so that FeSiO4 is in a molten state, which is the most effective way to remove FeSiO4 from the slab surface.

(3) Increasing the descaling passes of rough rolling and reducing the descaling speed can remove FeO on the surface of the steel plate and reduce the subsequent generation of the red oxide scale.

(4) For erythema defects on the surface of thick-gauge steel plates, it can be used to reduce the finish rolling inlet temperature and increase the finish rolling temperature, and then increase the finish rolling speed.

(5) Appropriately increasing the coiling tension, reducing the gap between layers of the steel coil after coiling, and reducing the amount of air entering, can reduce the defects of the red oxide scale.

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