In the process of cold rolling, the roll has a great influence on the three indicators of strip steel output, plate surface quality and cost consumption per ton of steel. Defects in the use of cold-rolled rolls will cause the quality of batches to be degraded or even scrapped, resulting in lower yields and possibly damage to related equipment. This largely affects product quality and production rhythm. Roll surface defects mainly include chatter marks, roll marks, knives, cracks and spalling, which will affect the product surface. Exploring the causes and solutions of these defects is of great significance to on-site production.
1 Vibration pattern
During the rolling process of strip steel, stripes with alternating light and dark often appear on the surface of strip steel, which are perpendicular to the direction of strip steel movement. This strip steel defect is called strip steel chatter. Practice has proved that most of the vibration lines of this strip come from the rolls, that is, the vibration lines of the rolls are copied to the surface of the strip.
1.1 Causes of roll chatter lines
The causes and manifestations of roll vibration lines are mainly as follows: roll vibration lines caused by vibration of the grinding wheel spindle, and the vibration lines are distributed on the surface of the roll in a spiral form; roll vibration lines caused by poor shape of the grinding wheel or poor threshing of the grinding wheel. The lines are distributed on the surface of the roll in a spiral form. Usually, the spacing of the vibration lines can be measured by the speed of the roll and the speed of the grinding wheel, and then compared with the actual situation, it can be accurately judged that it is the grinding wheel. The vibration pattern of the roll caused by the poor contact of the bracket) is distributed on the surface of the roll in the form of parallel to the busbar of the roll; the vibration pattern of the roll caused by the poor shape of the tip of the tailstock, the distribution of the vibration pattern is parallel to the busbar of the roll; The vibration pattern of the roll caused by the poor rotation of the headstock, the distribution of the vibration pattern is parallel to the busbar of the roll; the vibration pattern of the roll caused by the resonance between the vibration of the grinding wheel headstock and the vibration of the headstock is distributed parallel to the busbar on the roll surface.
1.2 Elimination method of roll chatter marks
To eliminate the roll vibration pattern, the first is to eliminate the abnormal vibration source, and then select the grinding method according to the obvious degree of the vibration pattern. Obvious vibration lines should be eliminated from rough grinding, and insignificant vibration lines can be eliminated from semi-coarse grinding.
2 roll print
①The manifestations and causes of roll marks. The roll marks are mainly manifested as pinholes, dents, indentations, and holes on the roll surface. Irregularly distributed dents on the surface of the rolls, generally in the shape of a circle, the surface state or the texture of the rolls are usually retained in the dents, generally caused by some debris (foreign matter) entering the bite area or contact friction between the rolls. The preventive measures are: to identify the source of debris and foreign matter, clean the frame, improve the lubricating and cooling conditions of the process, improve the surface quality and trimming quality of the pickling coil, increase the surface hardness and hardening depth of the work roll, and reasonably configure the work roll and the intermediate roll. the difference in hardness.
②The elimination method of the roll mark. According to the different degrees of the roll marks, the roll marks are removed by grinding or turning.
3 knives
3.1 Reasons for the occurrence of roll knives
The knife pattern is caused by the mismatch between the rotational speed of the roller, the rotational speed of the grinding wheel, the longitudinal movement speed of the Z axis, and the feed amount of the grinding wheel; the knife pattern is caused by the sharp edge of the grinding wheel or the defect of the edge of the grinding wheel; semi-finishing grinding The number of cuts is not enough, and the cuttings formed by the defects of the rough grinding passes are not eliminated; the cuttings are caused by the excessive load of the fine grinding passes; the roll crowns are too large, which will also produce cuttings.
3.2 Preventive measures and elimination methods of roll cutter
①Precautionary measures. According to a= reasonably configure the rotational speed of the roll, the longitudinal movement speed of the Z axis, the feed amount of the grinding wheel, and the rotational speed of the grinding wheel. Among them, a is the grinding amount (mm); Vw is the peripheral speed of the workpiece (m/min); Vs is the peripheral speed of the grinding wheel (m/s); Ap is the amount of back cutting (mm); Ds is the diameter of the grinding wheel (mm); Dw is the diameter of the workpiece (mm); m is the number of effective grinding points within the unit length of the grinding wheel circumference. Chamfer the sharp edge of the wheel and tailstock the defective wheel until the defect disappears. Choose a standard grinding program according to different roll types, strictly follow the standard grinding program, and ensure that the defects of the previous grinding are eliminated every time you start to jump to the next grinding program. Under the premise of ensuring that the defects of the semi-finishing pass are eliminated, control the number of passes and load of the finishing pass, and select a softer grinding wheel or reduce the convexity of the grinding wheel.
②The elimination method of roll knife flower. Generally, the method of repeated grinding is used to eliminate the rolling cutter.
4 cracks
① Causes of roll cracks. When the roll is rolling or the hot roll is grinding, cracks are easy to occur due to local stress concentration; cracks are caused by accidents such as broken strips and strands in the rolling mill; uneven thickness of incoming materials causes the rolls to be impacted during the rolling process. The rolling force of the roll is too large in a short time to cause cracks; the fatigue cracks of the roll.
②The elimination method of roll cracks. In order to prevent the roller from falling off meat or larger defects due to cracks, it must be treated in time and the cracks must be removed cleanly. Carefully check the rolls during grinding, and if any cracks are found, mark the end of the roll neck to facilitate the next inspection. If the cracks are small, the naked eye cannot determine whether the cracks have been ground after a period of grinding. Like test or flaw detection, determine whether the crack has been ground, and if it still exists, continue grinding; if the crack is large and deep, and it is still obvious after long-term grinding, fixed-point plunge grinding can be carried out to judge the depth of the crack.
5 flakes (drop the meat)
Roll spalling refers to the phenomenon that a certain area of the roll body is separated from the roll body.
5.1 Peeling of the roll surface
①During rolling, due to strip breakage, tail flicking, steel strip overlapping, etc., the steel is stuck or slipped, which will cause thermal shock to the contact area of the roll surface, causing different degrees of burns on the roll surface, resulting in stress concentration on the roll surface. factor. If the rolling stress acting on the roll is greater than the tensile strength of the roll, the surface cracks may spread during the rolling cycle and cause the roll to peel.
②High-cycle fatigue spalling, caused by sub-skin cracks, is generally common in the second intermediate and backup rolls, and usually occurs without rolling mill accidents. Typically, this type of fatigue fracture is described as a “friable” type of spalling, indicating that the entire process is initiated under the roll skin. The number of times of stress is very important. Even if the stress is lower than the strength of the roll material, repeated action can lead to crack initiation, that is, fatigue cracks and spalling occur. Since very small cracks are distributed parallel to the tangent to the roll surface, fatigue propagation of high-cycle contact stress can occur at many places within the stress region. Repeated cyclic stress then propagates these cracks to the surface until spalling occurs.
5.2 Roller ends drop meat
An abnormality in the thrust bearing of the rolling mill causes the roll end to drop meat; the wrong position or abnormality of the roll changing trolley of the rolling mill leads to an abnormal situation during the roll change, which causes the roll end to drop the meat; damage.
5.3 Treatment method of roll peeling
The peeling of the surface of the roll is carried out by rough grinding + flaw detection until the defect is eliminated. The peeling of the roll end depends on the situation, and the mild peeling is treated by grinding. If the peeling is serious, the roll is scrapped.
6 towa seals
①The cause of the roller support printing. Roll backing marks generally exist on rolls with small diameters that need to be supported by roll body brackets during grinding. Rolls with backing marks in cold rolling mills generally include work rolls of twenty-high mills, one middle, two middle. The generation of the support plate is due to the unevenness of the support plate and the burr, the impurities in the support plate material, or the grinding wheel abrasive particles, iron filings and other impurities are mixed between the support plate and the roll, and the surface of the roll is scratched during grinding. produced.
②The elimination method of roller support tile mark. Generally, underlay paper, half-pass grinding or repeated grinding are used to eliminate underlay marks.
