In a steel cold rolling mill, the surface defects of the roll bring great troubles to the production, which greatly affect the product quality and production rhythm. Roll surface defects mainly include vibration marks, roll marks, knife scratches, cracks and spalling, which will affect the surface of the product. Exploring the causes and solutions of these defects is of great significance to on-site production.
1. Vibration marks
In the rolling process of strip steel, the strip surface often appears a light and dark, and the direction of the strip movement perpendicular to the stripe, this strip steel defect is called strip vibration. Practice has proved that most of the reasons for the vibration of the strip steel come from the roll, that is, the roll vibration is copied to the strip surface.
1.1 Causes of roll vibrations
The causes and manifestations of roll striations are as follows: the roll striations caused by the vibration of the grinding wheel spindle are distributed on the roll surface in a spiral shape; The roll vibrations caused by poor shape of the grinding wheel or poor threshing of the grinding wheel are distributed on the surface of the roll in a spiral shape. The spacing of the vibrations can usually be measured by the rotational speed of the roll and the rotational speed of the grinding wheel, and then compared with the actual situation, it can be accurately identified as the grinding wheel vibrations. The roll vibration marks caused by the poor contact of the roll bracket (support) are distributed on the roll surface in the form of parallel to the roll bus. Roll vibration marks caused by poor shape of tailstock center, the distribution of vibration marks is parallel to roll bus; The distribution of roll vibration marks caused by poor rotation of the head frame is parallel to the roll bus; The roll vibration pattern caused by resonance between the vibration of the grinding wheel head frame and the vibration of the headstock is distributed on the roll surface in the form of parallel bus.
1.2 Elimination method of roll vibration marks
To eliminate roll vibration, first of all, to eliminate the abnormal vibration source, and then choose the grinding method according to the obvious degree of vibration. Obvious vibromarks should be eliminated from rough grinding, and non-obvious vibromarks can be eliminated from semi-rough grinding.
2. Roll printing
① The expression form and cause of roll marks. Roll marks are mainly manifested as pinholes, dents, indentations and holes on the roll surface. The irregular distribution of dents on the surface of the roll, generally circular, and the surface state or texture of the roll is usually retained within the dent, usually caused by some debris (foreign matter) entering the bite zone or contact friction between rollers. The preventive measures include: identifying the source of debris, cleaning the frame, improving the process lubrication and cooling conditions, improving the surface quality and cutting edge quality of the pickling coil, increasing the surface hardness and hardening depth of the work roll, and rationally distributing the hardness difference between the work roll and the intermediate roll.
The elimination method of roll marks. Roll marks are removed by grinding or turning according to different degrees of roll marks.
3. Knife flower
3.1 Reasons for roll cutting flowers
Due to the roller speed, grinding wheel speed, Z axis longitudinal moving speed and grinding wheel feed amount of four do not match. Sharp edge of the grinding wheel or the edge of the grinding wheel caused by defects; The number of semi-fine grinding passes is not enough, and the defects of rough grinding passes are not eliminated. Fine grinding pass due to excessive load caused by the knife flower; Too large roll crown can also produce a knife.
3.2 Preventive measures and elimination methods of roll cutting flowers
① Preventive measures. According to a=, the rotation speed of the roll, the longitudinal moving speed of the Z axis, the feed amount of the grinding wheel and the rotation speed of the grinding wheel are reasonably configured. Where, a is the grinding amount (mm); Vw is the workpiece circumference speed (m/min); Vs is the circular speed of the grinding wheel (m/s); Ap is the amount of back cutting (mm); Ds is the diameter of grinding wheel (mm); Dw is the workpiece diameter (mm); m is the number of effective grinding edges within the unit length of the wheel circumference. Chamfer the edge of the sharp grinding wheel and grind the tail seat of the defective grinding wheel until the defect disappears. Select the standard grinding program according to different roll types, grinding in strict accordance with the standard grinding program, every start to jump to the next grinding program, to ensure that the previous step of grinding defects are eliminated. Under the premise of ensuring that the defects of the semi-finishing pass are eliminated, the number of passes and load of the finishing pass are controlled, and the softer grinding wheel is selected or the crown of the grinding wheel is reduced.
The elimination method of roll cutting flower. The method of repeated grinding is generally used to eliminate roll cutting.
4. Cracks
① Causes of roll cracks. When the roll is rolling or the hot roll is grinding, it is easy to crack because of the local stress concentration. The accidents such as belt breaking and stranding lead to cracks. The uneven thickness of the incoming material causes the roll to be impacted in the rolling process, which causes the roll to crack due to excessive rolling force in a short time. Fatigue crack of roll.
2. Elimination method of roll crack. In order to prevent the roll from falling flesh or larger defects caused by cracks, it must be treated in time and ensure that the cracks are removed clean. When grinding, check the roll carefully, and mark the end of the roll neck if there is a crack, so as to facilitate the next inspection. If the crack is small, after grinding for a period of time, the naked eye can not determine whether the crack has been worn out, then it is necessary to do imaging test or flaw detection to determine whether the crack has been worn out, and continue grinding if it still exists; If the crack is large and deep, it is still obvious after a long time of grinding, fixed-point cut-in grinding can be carried out to determine the depth of the crack.
5. Flaking (shedding meat)
Roll stripping is the phenomenon that a certain area of the roll body is separated from the roll body.
5.1 Roll surface peeling
① During rolling, due to strip breaking, tail dumping, steel strip overlap and so on, the clamping or slipping occurs, causing thermal shock to the contact area of the roll surface, causing different degrees of burn on the roll surface, resulting in stress concentration factors on the roll surface. If the rolling stress acting on the roll is greater than the tensile strength of the roll, surface cracks may be dispersed during the rolling cycle, resulting in roll stripping.
(2) High-period fatigue spalling, caused by cracks under the skin, is generally more common in the middle of the two and support rolls, and usually does not occur in the mill accident. Generally, this type of fatigue fracture is described as a "brittle" type of spalling, indicating that the entire process is initiated under the roll skin. The number of stress acts is very important, even if the stress is lower than the strength of the roll material, but due to repeated action can also lead to cracks, that is, fatigue cracks lead to spalling. Because very small cracks are distributed parallel to the tangential lines of the roll surface, fatigue spread of high periodic contact stress can occur in many places within the stress region. Repeated cyclic stress then diffuses these cracks to the surface until spalling occurs.
5.2 Roll end flesh off
The abnormal thrust bearing of the rolling mill causes the roll end to fall off. The position of roll changing trolley in the mill is wrong or abnormal, which leads to the abnormal situation in the roll changing process, which causes the roll end to drop meat. The end of the roll is damaged by impact due to lifting or other accidents.
5.3 Treatment method of roll peeling
Roll surface spalling shall be roughed and inspected until the defect is eliminated. Roll end spalling shall be treated by grinding for mild spalling and scrap for severe spalling.
6. Tova Seal
① Causes of roll bearing. Roll bearing marks generally exist on rolls with small diameters that require the support of roll body brackets during grinding. Rolls with bearing marks in cold rolling plants generally have 20-roll mill work rolls, a middle roll and two middle rolls. The Tovar mark is produced because the tovar is uneven and produces burrs, there are impurities in the Tovar material, or the grinding wheel particles, iron filings and other debris are mixed between the tovar and the roll, and the roll surface is scratched during grinding.
The elimination method of roll bearing imprint. It is generally used to eliminate tovar marks by pad paper, half-pass grinding or repeated grinding.
