Analysis and Control methods of Common Appearance Defects of Hot-Rolled Steel

During the production process of hot-rolled steel, appearance defects may occur due to various factors such as process, equipment, and raw materials. The following are some common appearance defects of hot-rolled steel:

Insufficient filling of steel angles

Defect characteristics: Insufficient filling of steel angles is caused by insufficient filling of finished product holes, resulting in lack of metal at the edges and corners of steel. The surface is rough, mostly along the entire length, and some appear partially or intermittently.

Causes: Due to the inherent characteristics of the hole type, the edges and corners of the rolled piece cannot be processed; the rolling mill is improperly adjusted and operated, and the reduction distribution is unreasonable. The reduction at the corner is small, or the extension of each part of the rolled piece is inconsistent, resulting in excessive shrinkage; the hole type or the guide plate is seriously worn, the guide plate is too wide or installed incorrectly; the temperature of the rolled piece is low, the metal plasticity is poor, and the corners of the hole type are not easy to fill; the rolled piece has serious local bending, and it is easy to produce local corners that are not fully filled after rolling.

Control method: Improve the hole design, strengthen the adjustment operation of the rolling mill, and reasonably distribute the reduction; correctly install the guide device, and replace the hole and guide plate that are severely worn in time; adjust the reduction according to the temperature of the rolled piece to ensure good filling of the edges and corners.

Steel section size out of tolerance

Defect characteristics: A general term for the geometric dimensions of the steel section that do not meet the requirements of the standard. When the difference from the standard size is too large, it will appear deformed. There are many types of defects, most of which are named according to the location and degree of out of tolerance. For example, out of roundness out of tolerance, length out of tolerance, etc.

Causes: unreasonable hole design; uneven hole wear, improper matching of new and old holes; poor installation of various parts of the rolling mill (including guide devices), broken safety mortar; improper adjustment of the rolling mill; uneven temperature of the steel billet, uneven temperature of a single piece causes local specifications to be inconsistent, and the entire low-temperature steel causes the entire length specifications to be inconsistent and too large.

Control method: correctly install all parts of the rolling mill; improve the hole design and strengthen the adjustment operation of the rolling mill; pay attention to the wear of the hole. When replacing the new finished hole, consider replacing the finished front hole and other related hole types at the same time according to the specific situation; improve the heating quality of the steel billet to achieve uniform temperature of the steel billet; some special-shaped materials may affect a certain size that does not meet the regulations due to the change of the cross-sectional shape after straightening. At this time, re-straightening can be used to eliminate defects.

Steel rolling scar

Defect characteristics: Metal lumps adhered to the surface of steel due to rolling. Its appearance is similar to scars, but the main difference from scars is that the shape of rolling scars and their distribution on the surface of steel have certain regularity. There are usually no non-metallic oxide inclusions under the defect.

Causes: The rough rolling mill hole is severely worn, resulting in intermittent active scars on the fixed surface of the steel section; foreign metal objects (or metal scraped off the workpiece by the guide device) are pressed into the surface of the workpiece to form scars; periodic bumps or pits are generated on the surface of the workpiece before the finished hole, and periodic scars are formed after rolling. The specific reasons are: poor groove notching; sand holes or meat loss in the groove; the groove is hit by a “black head” workpiece or has protrusions such as scars; the workpiece slips in the hole, causing metal to accumulate on the surface of the deformation zone, and then forming scars after rolling; the workpiece is partially stuck (scratched) or bent by mechanical equipment such as the surrounding plate, roller table, and steel turning machine, and then scars will be formed.

Control methods: timely replace the rolling groove that is severely worn or has foreign objects on it; carefully check the surface of the rolling groove before changing the rolls, and do not use the rolling groove with sand holes or poor scoring; it is strictly forbidden to roll black steel to prevent the rolling groove from falling off or being hit; when dealing with steel clamping accidents, be careful not to damage the rolling groove; keep all mechanical equipment before and after the rolling mill smooth and flat, and install and operate them correctly to avoid damaging the rolled piece; be careful not to press foreign objects into the surface of the rolled piece during rolling; the heating temperature of the steel billet should not be too high to avoid the rolled piece slipping in the hole.

Steel section lacks meat

Defect characteristics: Metal is missing along the length of one side of the steel section. There is no hot rolling mark of the finished rolling groove at the defect, the color is darker, and the surface is rougher than normal. It usually appears throughout the length, but sometimes it appears locally.

Causes: The rolling groove is misaligned or the guide is improperly installed, resulting in metal missing in a certain section of the rolled piece, and the hole is not fully filled during further rolling; the hole design is poor or the turning is wrong and the rolling mill is improperly adjusted, the amount of rolled piece metal entering the finished hole is insufficient, so that the finished hole is not fully filled; the different degrees of wear of the front and rear holes can also cause meat missing; the rolled piece is twisted or partially bent, resulting in local meat missing after further rolling.

Control method: Improve the hole design, strengthen the adjustment operation of the rolling mill, and ensure that the finished hole is well filled; tighten the various components of the rolling mill to prevent the axial movement of the rollers, and correctly install the guide device; replace the severely worn hole in time.

Scratches on steel sections

Defect characteristics: Scratches caused by sharp edges of equipment and tools during hot rolling and transportation. The depth varies, and the bottom of the groove is visible. Generally, there are sharp edges and corners. They are usually straight-line, and some are also curved. Single or multiple, distributed throughout or partially on the surface of the steel section.

Causes: The hot rolling area floor, rollers, steel transfer and steel turning equipment have sharp edges, which scratch the workpiece when it passes through; the guide plate is poorly processed, the edge is not smooth, or the guide plate is severely worn, and there are foreign objects such as iron oxide on it, which scratch the surface of the workpiece; the guide plate is improperly installed and adjusted, and the pressure on the workpiece is too great, which scratches the surface of the workpiece; the edge of the surrounding plate is not smooth, and it is scratched when the workpiece jumps out of the sleeve.

Control method: The guide devices, enclosures, floors, ground rollers and other equipment should be kept smooth and flat without sharp edges; strengthen the installation and adjustment of the guide plates, which should not be skewed or too tight to avoid excessive pressure on the rolled piece.

Steel wave

Defect characteristics: The undulations along the length direction of the local section of the steel due to uneven rolling deformation are called waves. There are local and full-length undulations. The longitudinal undulations of the waist of I-beams and channel steels are called waist waves; the longitudinal undulations of the legs of I-beams, channel steels, and angle steels are called leg waves. The longitudinal thickness of the waist of I-beams and channel steels with waist waves is uneven. In severe cases, metal overlap and tongue-shaped cavities occur.

Causes: The wave is mainly caused by the inconsistency of the elongation coefficients of various parts of the rolled piece, resulting in serious shrinkage, which generally occurs in the parts with larger elongation. The main factors that cause the elongation changes in various parts of the rolled piece are as follows: improper distribution of the reduction amount; roller skew, groove misalignment; serious wear of the groove of the front hole or the second front hole of the finished product; uneven temperature of the rolled piece.

Control method: When replacing the finished product hole in the middle of rolling, the finished product front hole and the finished product re-front hole should be replaced at the same time according to the product characteristics and specific conditions; strengthen the rolling adjustment operation, reasonably distribute the reduction amount, and tighten the various parts of the rolling mill to prevent the groove from being misaligned. Make the extension of each part of the rolled piece uniform.

Steel section torsion

Defect characteristics: Different angles of sections around the longitudinal axis along the length are called torsion. When the twisted steel is placed on a horizontal inspection stand, one side of one end is tilted up, and sometimes the other side of the other end is also tilted up, forming a certain angle with the table surface. When the torsion is very serious, the whole steel even becomes a “twisted shape”.

Causes: Improper installation and adjustment of the rolling mill, the center lines of the rolls are not in the same vertical or horizontal plane, the rolls move axially, and the grooves are misaligned; the guide plates are not installed correctly or are severely worn; the temperature of the rolled piece is uneven or the amount of reduction is uneven, resulting in uneven extension of each part; the straightening machine is improperly adjusted; when the steel, especially large pieces, are hot, they are turned over at one end of the cooling bed, which can easily cause end torsion.

Control method: Strengthen the installation and adjustment of the rolling mill and the guide plate. Do not use the severely worn guide plate to eliminate the torsional moment on the rolled piece; strengthen the adjustment of the straightening machine to eliminate the torsional moment added to the steel during straightening; try not to turn the steel at one end of the cooling bed when the steel is hot to avoid torsion at the end.

Bending of steel

Defect characteristics: longitudinal unevenness is generally called bending. Named according to the bending shape of the steel, the uniform bending in the shape of a sickle is called a sickle bend; the overall repeated bending in the shape of a wave is called a wave bend; the overall bending at the end is called an elbow; one side of the end angle is warped inward or outward (curled up in severe cases) is called an angle bend.

Causes: Before straightening: Improper adjustment of steel rolling operation or uneven temperature of rolled pieces, which makes the extension of each part of the rolled piece inconsistent, can produce sickle bend or elbow; The difference between the upper and lower roller diameters is too large, and the finished product exit guide plate is improperly designed and installed, which can also produce elbows, sickle bends or wave bends; The cooling bed is uneven, the speed of each roller of the roller cooling bed is inconsistent, or the cooling after rolling is not uniform, which can produce wave bends; The metal distribution of each part of the product section is uneven, and the natural cooling speed is inconsistent. Even if the steel is straight after rolling, sickle bends in a fixed direction will be produced after cooling; When hot sawing steel, the saw blade is severely worn, the saw is too fast, or the hot steel hits the baffle at high speed on the roller, and the end of the steel collides with some protrusions during the transverse movement, which can produce elbows or corners; Improper storage of steel during lifting and intermediate storage, especially when operating in a red-hot state, can produce various bends.

After straightening: In addition to corners and elbows, the wave bends and sickle bends in the normal state of steel should be able to achieve a straight effect after the straightening process

Control methods: Strengthen the adjustment operation of the rolling mill, correctly install the guide device, and control the rolled piece to not have excessive bending during rolling; strengthen the operation of the hot saw and cooling bed process to ensure the cutting length and prevent the steel from being hit and bent; strengthen the adjustment operation of the straightening machine, and replace the straightening rollers or roller shafts that are severely worn in time; to prevent the steel from being hit and bent during transportation, a spring baffle can be installed in front of the cooling bed roller; strictly control the temperature of the straightened steel according to the regulations, and stop straightening when the temperature is too high; strengthen the storage of steel in the intermediate warehouse and finished product warehouse to prevent the steel from being pressed and bent or being bent by the crane rope.

The shape of the steel section is not correct

Defect characteristics: There is no metal defect on the surface of the steel section, and the cross-sectional shape does not meet the specified requirements. There are many names for this type of defect, which vary with the variety. For example, the oval of round steel; the diamond of square steel; the slanted legs, wavy waist, and missing legs of channel steel; the top angle of angle steel is large, the angle is small, and the legs are not equal; the legs of I-beam are slanted and the waist is uneven; the shoulders of channel steel are collapsed, the waist is convex, the waist is concave, the legs are expanded, and the legs are parallel.

Causes: Improper design, installation, adjustment or severe wear of the straightening roller; unreasonable design of the straightening roller hole type; severe wear of the straightening roller; improper design and wear of the rolling hole type and guide device, or poor installation of the finished hole guide device.

Control method: Improve the hole design of the straightening roller, and select the straightening roller according to the actual size of the rolled product; when the channel steel and automobile wheel mesh are bent and rolled, the second (or third) lower straightening roller in the forward direction of the straightening machine can be made into a convex shape (convexity height 0.5~1.0mm), which is beneficial to eliminate the concave waist defect; for steel sections that need to ensure the unevenness of the working surface, control should be started from the rolling process; strengthen the adjustment operation of the straightening machine.

Cutting defects of steel sections

Defect characteristics: Various de fects caused by poor cutting are collectively referred to as cutting defects. When a small steel is cut with a flying shear in a hot state, the scars of varying depths and irregular shapes on the surface of the steel are called cut scars; in a hot state, the surface is damaged by a saw blade, which is called saw scars; after cutting, the cutting surface is not perpendicular to the longitudinal axis, which is called bevel cutting or saw bevel; the hot-rolled shrinkage part at the end of the rolled piece is not cut cleanly, which is called short cut head; after cold shearing, a small local crack on the shear surface is called tearing; after sawing (shearing), the metal flash left on the end of the steel is called burrs.

Causes: The sawed steel is not perpendicular to the saw blade (shear blade) or the head of the rolled piece is bent too much; in terms of equipment: the saw blade has a large curvature, the saw blade is worn out or improperly installed, the gap between the upper and lower shear blades is too large; the flying shear is out of adjustment; in terms of operation: too many steel roots are sheared (sawn) at the same time, too little is cut off at the end, the hot-rolled shrinkage part is not cut cleanly, and various misoperations.

Control methods: Improve the incoming material conditions, take measures to avoid excessive bending of the rolled piece head, keep the incoming material direction perpendicular to the shearing (sawing) plane; improve the equipment conditions, use saw blades with no or small curvature, select the saw blade thickness appropriately, replace the saw blade (shear blade) in time when it is worn, correctly install and adjust the shearing (sawing) equipment; strengthen the operation, and at the same time, do not shear (sawing) too many roots to avoid the steel from falling and bending. The necessary end removal must be ensured, the hot-rolled shrinkage part must be cut cleanly, and various misoperations must be avoided.

Steel straightening marks

Defect characteristics: surface scars caused during the cold straightening process. This defect has no traces of hot processing and has a certain regularity. There are three main types. Pit type (or straightening pit), fish scale type, and damaged type.

Causes: The hole of the straightening roller is too shallow, the steel is severely bent before straightening, the steel is not fed correctly during straightening, or the straightening machine is improperly adjusted, which can cause damaged straightening marks; local damage to the straightening roller or bonding metal blocks, local bulges on the roller surface, severe wear of the straightening roller or high roller surface temperature, bonding metal, can cause fish scale-shaped straightening marks on the steel surface.

Control method: Do not continue to use the straightening roller when it is severely worn or has serious straightening marks. When the straightening roller is partially damaged or adhered to metal blocks, it should be polished and cleaned in time. When straightening angle steel and other steel sections, the temperature of the straightening roller may rise and scrape due to the large relative movement between the contact surface of the straightening roller and the steel (caused by the difference in linear speed), resulting in straightening marks on the surface of the steel. Therefore, cooling water should be poured on the surface of the straightening roller to cool it. Improve the material of the straightening roller or quench the straightening surface to increase the surface hardness and wear resistance.