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Occurrence and Countermeasures of Surface Defects on Scale of Hot Rolled Strip Steel

1 Classification of iron oxide scale

Iron oxide scale is a common product quality defect of hot-rolled steel strips. According to the different parts of production, it is generally divided into furnace pig iron scale, rough rolling and finishing rolling iron oxide scale, and coiled iron oxide scale and mold slag to remove dirty iron scale.

2 Iron oxide scale production mechanism

The formation of the iron oxide scale is generally due to the chemical reaction of the billet after being heated in a heating furnace or in contact with an oxidizing atmosphere at a high temperature to form a mixture of Fe304, Fe203, and FeO. When the temperature is higher than 700℃, FeO is formed in the inner layer closest to the billet, accounting for 95%; Fe304 is formed in the middle layer, accounting for 4%; Fe2O3 is formed in the outermost layer, accounting for 1%.

3 furnace pig iron scale

Furnace-generated oxidation occurs in the heating furnace, which is related to the chemical composition, heating temperature, time in the furnace, and atmosphere in the furnace. The higher the heating temperature, the longer the time in the furnace, and the stronger the oxidizing atmosphere in the furnace, the easier it is to generate iron scale. C, Si, Ni, Cu, and other elements in the chemical composition promote the formation of the iron oxide scale, while Mn, Al, and Cr can slow down the formation of iron oxide scale. For example, Si-containing steel, high-carbon steel, and high-strength steel commonly used in production have iron oxide scales distributed throughout the length of the steel strip, and the lower surface is heavier than the upper surface. Due to the low melting point of Si-containing steel (1170 ℃), The compound FeSi2O4 is produced between the iron oxide scale and the steel matrix, and this wedge-shaped oxide is retained during the subsequent rolling process to form a brown-red iron oxide scale.

4 Iron oxide scale during rolling

The removal of the rough rolling scale is related to the rough rolling descaling water pressure, nozzle angle, water quality, and vertical roll side pressure capacity.

The iron oxide scale in the finishing rolling area is divided into the water system iron scale and the roll formed iron scale. The iron sheet in the water system refers to the insufficient pressure of descaling water, side spray water, dust removal water, etc., the angle and height of the nozzle are incorrect, or not put in or blocked. At high temperatures, the steel strip combines with the oxygen in the air to form an iron oxide scale that cannot be timely. The iron oxide scale formed by the pressing of the work rolls is swept away. In addition, side water spray can also inhibit the formation of scale. During normal production, the descaling water and dedusting water for finishing rolling must be put into use. However, sometimes when producing thin-gauge products, in order to ensure the shape of the plate, reduce the temperature drop at the edge of the steel plate, improve the rolling stability, and prevent tail flicking, the side water spray is often not used, resulting in the iron sheet generated in the finishing rolling stand cannot be removed in time. The iron oxide scale is pressed into the surface of the steel plate.

Another scale defect in the finishing mill is the so-called roll pig scale, and its production mechanism is shown in Figure 3. The main factors affecting the scale of the roll iron oxide are the material of the roll and the temperature of the roll. When the surface of the roll is in contact with the surface of the steel plate, the instantaneous high temperature, the surface temperature rises sharply and expands (generally, the instantaneous temperature of the hot-rolled roll is 600-800°C), showing high compressive stress; when the rolled piece leaves the roll, the roll is due to cooling water. The temperature of the rolls of the finishing stand is generally 60-90°C, and the surface turns into tensile stress. Repeatedly, fatigue cracks are prone to appear on the surface of the rolls, resulting in damage to the surface oxide film, and the damaged surface is printed on the surface of the steel plate to form Rolled iron oxide scale defects.

Generally, the roll raw iron scale occurs in the first three stands of finishing rolling, namely F1, F2, and F3, mainly due to the high surface temperature of the first three rolls, which leads to the rupture of the oxide film on the surface of the roll, resulting in roll raw iron scale. It can be seen from Figure 4 that when the heating temperature is 1230°C and the finishing rolling temperature is 950-1010°C, the shadow in the figure is the iron-free area. When the finishing rolling temperature is between 1030 and 1080℃, the scale is serious, and the finishing rolling temperature is between 950 and 1030℃. There is no scale or the scale is lighter. According to the different equipment and steel quality of each hot rolling mill, when the finishing rolling temperature is controlled at 950℃ to produce high-strength steel or high-carbon steel, the rolling force of the first three stands is too high, which may damage the equipment. The rolling temperature should be controlled at 950 ~ 1030 ℃, which can effectively reduce the temperature of the upstream stand roll and reduce the occurrence of roll iron oxide scale.

5 Coiling produces iron oxide scale

After coiling, the iron oxide scale transforms very quickly. When the steel coil just comes out of the coiler, the surface shows white powdery strips with different widths. After ten minutes, it transforms into a dark iron oxide scale. . The contrast of the surface morphology of the iron oxide scale formed on the upper surface of the walking beam is very obvious when the same steel coil is taken out of the coiler and after 15 minutes. Under normal circumstances, due to the low coiling temperature, the resulting iron skin layer is relatively thin, and because the surface tension of the steel strip formed by coiling and the compressive stress during the formation of the iron skin cancel each other out, the iron oxide skin and the surface of the steel plate are very densely bonded.

6 Surface defects formed by the involvement of mold slag

The inclusion of mold slag can also generate scale, which is difficult to remove. Figure 5 and Figure 6 show the comparison of photos before and after pickling of the SPA-H scale of container steel produced in a short process. It can be seen from photo 5 that the iron oxide scale is mainly concentrated on the edge of the steel strip, which is brown-red. According to scanning electron microscope analysis, the main components of dark gray strips are K+, Ca2+, and Na+, which are the main components of mold slag.

7 Measures to reduce the defects of the iron oxide scale

(1) By reducing the heating temperature, reducing the time in the furnace, and adjusting the atmosphere in the furnace to a partial reducing atmosphere, the generation of furnace pig iron oxide is suppressed;

(2) By increasing the descaling water pressure, adjusting and optimizing the height and angle of the nozzle, improving the lateral pressure capacity of the vertical roll, and reducing the rough rolling scale;

(3) Measures to reduce the raw iron oxide scale of the rolls: adopt the roll material with good thermal crack resistance, and adopt a reasonable grinding system to remove the residual cracks on the surface of the roll in a timely and thorough manner; adopt lubricating rolling to improve the surface quality of the roll and reduce the rack unit. Rolling force to prevent roll raw oxide scale due to the expansion of micro-cracks on the surface of the roll due to excessive unit rolling force; the inlet water volume of the roll cooling water stand is less than the outlet water volume, and the cooling water volume of the intermediate stand roll is increased to ensure the roll is cooled quickly; The temperature of the finishing rolling is less than or equal to 1030℃, and the roll temperature of the upstream stand of the finishing rolling is reduced.

(4) Water spraying on the side of the finishing stand can reduce the pressing of the iron oxide scale;

(5) Improve the quality of mold slag, reduce the involvement of mold slag, and ensure the descaling effect of the billet can reduce mold slag and iron scale

(6) Optimize the chemical composition of the steel, on the basis of not affecting the mechanical properties and other properties, try to reduce the content of C and Si elements, and increase the content of Mn and A1, which is more practical and effective for reducing scale in hot rolling mills.

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