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Integrated analysis of the characteristics and causes of common quality defects in steel plates

Views:15     Author:Site Editor     Publish Time: 2021-08-22      Origin:Site


1. Roll printing: It is a group of irregularities with periodic, uniform size and shape, and irregular appearance and shape.

Reasons: 

(1) On the one hand, due to fatigue or insufficient hardness of the roller, a part of the roll surface has a concave edge; on the other hand, there may be foreign matter on the surface of the roller, which makes the surface part protruding;

(2) When rolling or finishing, press into the surface of the steel plate to form uneven defects.


2. Surface inclusions: There are irregular point-like block or strip-like non-metallic inclusions on the surface of the steel plate. The color is generally reddish-brown, yellowish-brown, off-white, or gray-black.

Reasons: 

(1) The subcutaneous inclusions of the slab are exposed after rolling, or the original surface inclusions of the slab remain on the surface of the steel plate after rolling;

(2) Non-metallic objects such as refractory materials and silt in the heating furnace fall on the surface of the slab and are pressed into the surface of the slab during rolling.


3. Iron oxide scale: Iron oxide scale generally adheres to the surface of the steel plate, distributed on part or all of the board surface, and is black or reddish-brown; some of the iron skin is loose and peeling, and some are not easy to fall off when pressed into the surface of the board; according to the appearance and shape, there are: Red iron sheet, lumpy iron sheet, strip iron sheet, linear iron sheet, wood grain iron sheet, meteoroid iron sheet, spindle iron sheet, drag iron sheet and scattered iron sheet, etc., the pressing depth is deep and shallow.

Reasons:

 (1) The formation of the pressed iron oxide scale depends on the heating conditions of the slab. If the heating time is longer, the heating temperature is higher, the stronger the oxidizing atmosphere, the more iron oxide scale is generated, and it is not easy to fall off, and it is difficult to remove the iron scale. It is pressed into the surface of the steel plate during rolling;

(2) The setting of the large vertical roller is unreasonable, and the iron sheet is not loosened and it is difficult to remove;

(3) Due to the low water pressure of the high-pressure descaling water pipe, the blockage of the water nozzle, the wrong angle of the water nozzle and improper use, etc., the iron sheet on the surface of the steel plate is not removed, and it is pressed into the surface of the steel plate after rolling;

(4) Iron oxide scale occurs more frequently in boiling steel, and red iron scale is easily produced in steel with a higher silicon content.


4. Uneven thickness: The thickness of each part of the steel plate is inconsistent. It is called uneven thickness. For steel plates with uneven thickness, the deviation is generally too large, and the thickness of the local steel plate exceeds the specified allowable deviation.

Reasons: 

(1) Improper roll gap adjustment and roll type configuration;

(2) The wear of the bearing bushes on both sides of the roller and the roller is different;

(3) Uneven heating temperature of the slab


5. Pockmarks: There are partial or continuous pits on the surface of the steel plate called pockmarks, which vary in size and depth.

Reason:

During the heating process, the slab is seriously oxidized, and the iron sheet is pressed into the surface during rolling, and small pits are formed after falling off


6. Bubbles: There are irregularly distributed circular convex hulls on the surface of the steel plate, sometimes in the form of an earthworm-like straight line, with relatively smooth outer edges and gas inside; when the bubbles are broken, irregular cracks appear; some The air bubbles are not raised. After leveling, the surface is bright, and the shear section is layered.

Reasons: 

(1) Because there are many bubble-like defects on the slab, they are not healed after multiple rolling and remain on the steel plate;

(2) The slab is in the furnace for a long time and the bubbles are exposed.


7. Folding (folding, wrinkling, folding, cornering): The surface of the steel plate has a double-layer metal fold that is partially folded to each other. The shape is similar to the crack, and the depth is different, and the cross-section generally presents an acute angle. Straight folds along the rolling direction are called forward folds; folds perpendicular to the rolling direction are called horizontal folds; edge folds are called folds; the difference between folds and folds and wrinkles is mainly due to defects The shape and degree are different, and the degree of folding and corner folding is different according to the size of the angle. Lateral folding occurs mostly in thin-gauge strip steel. In mild steel with a carbon content of less than 0.08, it is easy to produce wrinkles because the leveling machine is not equipped with a tension roller.

Reasons: 

(1) The rolled piece is scratched and folded during rolling, which mostly appears on the lower surface of the steel plate;

(2) The vertical roll is squeezed too much, and the roll ring gnaws on the lower surface of the rolling piece;

(3) The aspect ratio of slab defect cleaning is too large;

(4) Uneven slab temperature or unreasonable finish roll profile configuration and unreasonable rolling load distribution, etc. The strip during rolling is deformed into large waves due to unevenness and then pressed together;

(5) The extrusion of the vertical roll ring or the rolling piece has serious scratches, large sickle bends due to rough rolling, poor centering, etc., after the frame is scraped, it is rolled into a press-fit;

(6) The side guide plate in front of the coiler is severely worn and grooved, the opening is too small, and the pinch roll gap is wedge-shaped, which is easy to cause the strip to deviate, and the reached position at the side guide spot groove is pressed by the pinch roll ;

(7) The coiling temperature is too low or the coiling speed setting is inappropriate for some reason;

(8) The curling edge of the steel coil is misaligned, or due to the looseness of the steel coil, when the steel coil is lifted by a crane, it is easy to produce hemming and corner folding when falling to the ground. At this time, it often occurs on the steel coil with a relatively thin thickness;

(9) The uncoiling temperature of the strip is too high, or the tension of the uncoiling and the pressure of the pressing roller are not set properly.


8. Tower shape (rolling edge misalignment): The upper and lower ends of the steel coil are uneven, and one circle is higher (or lower) than the other circle, which is called a tower shape. The curling up and down movement is called the curling up and down movement.

Reasons: 

(1) Improper adjustment of the gap between the roll-up rollers;

(2) The pinch roller gap is wedge-shaped;

(3) Poor alignment when the strip enters the coiler;

(4) The winding tension setting is inappropriate;

(5) Improper adjustment of the gap of the forming guide;

(6) The action time of the side guides in front of the coiler is not synchronized;

(7) There is a gap between the roll and the push roll;

(8) The drive end of the reel is severely worn, and there is a large centrifugal difference when rotating;

(9) The strip steel has a large sickle bend or a bad plate shape.


9. Loose coil: The steel coil is not tightly coiled, and there is a gap between layers, which is called a loose coil.

Reasons: 

(1) The winding tension setting is inappropriate;

(2) The strip steel has serious waves or is deformed on the roller table due to coiling failure;

(3) The yield strength of steel is high, but the coiling temperature is too low;

(4) After the coiling is completed, the reel is reversed for some reason;

(5) The strap is not tightened or the strap is broken.


10. Flat coil: The end of the steel coil is oval and is called a flat coil.

Reason: 

It easily occurs in softer or thinner steel coils.

(1) The steel coil has withstood a big impact during the hoisting process;

(2) The steel coils are too tight, the temperature is high, and the steel coils are stacked flat on the ground or top


11. Sickle bend: The two longitudinal sides of the steel plate bend to the same side, resembling a sickle.

Reasons: 

(1) The slab has a sickle bend or serious uneven thickness;

(2) The roughing and finishing rolls have uneven wear, and wedge shapes appear between the roll gaps;

(3) Uneven temperature or uneven heating temperature on both sides of the rolling mill;

(4) The rolling mill is poorly adjusted, and the reductions on both sides are inconsistent;

(5) The centerline of the vertical roller has deviation;

(6) Axial string movement of the roll or uneven wear of the bearings on both sides;

(7) The opening of the side guide plate is too large, the rolling piece is off-track or the rolling piece is not well aligned;

(8) When using a disc cutter to cut the edge, the overlap of the two cutting blades is inconsistent.


12. Wedge shape: The steel plate is thick on one side and thin on the other side. Seen from the cross-section of the steel plate in the width direction, it resembles a wedge, with different degrees of the wedge.

Reasons: 

(1) The rolls are severely worn;

(2) The roll gap adjustment is inappropriate;

(3) The rolling stock is off-track; the temperature of the rolling stock is uneven.


13. Convexity: The steel plate is thick in the middle and thin on both sides. From the transverse end surface of the steel plate in the width direction, it is similar to an arc, and the degree of the arc is large or small.

Reasons:

(1) The rolls are severely worn;

(2) Uneven cooling due to thermal expansion of the roll;

(3) The roller shape design is unreasonable;

(4) Uneven rolling load distribution and excessive roll spring deformation;

(5) The roll bending device is not good;

(6) Uneven heating temperature or uneven rolling temperature.


14. Buckling: The warping of the longitudinal and vertical parts of the steel plate in the same direction at the same time is called buckling.

Reasons: 

(1) The cooling conditions on both sides of the steel plate are inconsistent, and the temperature difference between the upper and lower surfaces is large;

(2) The temperature of the rolled piece is uneven, and the deformation of the rolling process is uneven;

(3) The steel strip is unevenly cooled by spraying water on the rolling table;

(4) The final rolling reduction rate is too small;

(5) When the steel strip is being finished, the setting of the straightening machine is poor, the uncoiling temperature is too high, and the pressure roller and the straightening roller are seriously worn.