Description: Roll surface defects mainly include vibration lines, roll marks, knife marks, cracks and peeling, and these defects will affect the product surface. Exploring the causes and solutions of these defects is of great significance to on-site production.
Keyword: cold rolling，cold rolled rolls; roll defects;
What is cold rolling?
Cold rolling is an important metal processing technology, which refers to pressing metal sheets or strips on a cold rolling mill at room temperature. The purpose of cold rolling is to compress heavy metal sheets or strips into thin sheets or strips while enhancing the material’s hardness, flexural strength, and corrosion resistance. Compared with other metal processing processes, cold rolling has many advantages, such as low processing cost, simple production process, high flatness and dimensional stability of processed products, good surface quality, and changes in the microstructure of metals, which improves the quality of metal materials. performance etc. Therefore, cold rolling is widely used in steel manufacturing, automobile manufacturing, electronic equipment manufacturing, aerospace, building decoration and other fields.
In the cold rolling process, the rolls have a great influence on the three major indicators of strip production, plate surface quality, and per ton steel cost consumption. Defects during the use of cold-rolled rolls will cause quality degradation or even scrapping of batch products, resulting in reduced yields and possible damage to related equipment. Therefore, domestic cold-rolling manufacturers all pay great attention to the research on the use of rolls, and are committed to effectively reducing the consumption of cold-rolled rolls. Aiming at the common defects of rolls in the stainless steel cold rolling field production process, the article briefly analyzes the causes of common roll defects and puts forward the treatment measures.
Mill Rolls defects and treatment measures during cold rolling
During the rolling process of the strip steel, there will often be a kind of light and dark stripes on the surface of the strip steel, which are perpendicular to the direction of movement of the strip steel. This defect of the strip steel is called the strip steel vibration pattern. It has been proved by practice that most of the chatter marks of the strip come from the rolls, that is, the chatter marks of the rolls are copied to the surface of the strip steel.
Causes of roll chatter marks
The causes and manifestations of roll chatter are mainly as follows: roll chatter caused by the vibration of the main shaft of the grinding wheel, which is distributed on the surface of the roll in a spiral form; roll chatter caused by poor shape of the grinding wheel or poor threshing of the grinding wheel, chatter The grooves are distributed on the surface of the roll in a spiral form. Usually, the distance between the vibration lines can be calculated by the rotation speed of the roll and the rotation speed of the grinding wheel, and then compared with the actual situation, it can be accurately judged that it is the vibration lines of the grinding wheel; because the roll bracket ( The roll chatter caused by the poor contact of the paddle), the chatter is distributed on the roll surface in the form of parallel to the roll generatrix; the roll chatter caused by the poor shape of the tailstock tip, the chatter is distributed parallel to the roll generatrix; Roll chatter caused by poor headstock rotation, the distribution of chatter is parallel to the roll generatrix; roll chatter caused by resonance between the vibration of the grinding wheel headstock and the vibration of the headstock is distributed parallel to the generatrix on the roll surface.
Elimination method of roll chatter marks
To eliminate roll chatter marks, the first thing is to eliminate the abnormal vibration source, and then choose the grinding method according to the obvious degree of chatter marks. Obvious vibration lines should be eliminated from coarse grinding, and inconspicuous vibration lines can be eliminated from semi-coarse grinding.
①The manifestation 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 roll are generally circular, and the surface state or texture of the roll is usually retained in the dents, which are generally caused by some debris (foreign matter) entering the bite area or the contact and friction between the rolls. The preventive measures are: identify the source of debris and foreign matter, clean the frame, improve the process lubrication and cooling conditions, 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 gap between the work roll and the intermediate roll. difference in hardness.
② Elimination method of roll marks. According to the different degrees of roll marks, the roll marks are removed by grinding or turning.
Causes of Roll Knives
Knives caused by the mismatch between the rotation speed of the roll, the rotation speed of the grinding wheel, the longitudinal movement speed of the Z axis, and the feed rate of the grinding wheel; the knives caused by the sharp edge of the grinding wheel or the defect of the edge of the grinding wheel; semi-fine grinding The number of cuts is not enough, and the knives formed by the defects of the rough grinding pass are not eliminated; the knives are caused by the excessive load of the fine grinding pass; the roll crown is too large to produce knives.
Preventive measures and elimination methods of roll knives
①Precautionary measures. According to a=reasonably configure the rotation speed of the roll, the longitudinal movement speed of the Z axis, the feed amount of the grinding wheel, and the rotation speed of the grinding wheel. Among them, a is the amount of grinding (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 back cutting amount (mm); Ds is the diameter of the grinding wheel (mm) ; Dw is the workpiece diameter (mm); m is the number of effective sharpening points within the unit length of the grinding wheel circumference. Chamfer the edge of the sharp grinding wheel and tailstock the defective grinding wheel until the defect disappears. Select the standard grinding program according to different roll types, and grind in strict accordance with the standard grinding program. Every time you start to jump to the next grinding program, you must ensure that the defects of the previous grinding step are eliminated. On the premise of ensuring that the defects of the semi-finishing pass are eliminated, the number of passes and the load of the fine grinding pass are controlled, and a softer grinding wheel is selected or the crown of the grinding wheel is reduced.
② Elimination method of roll knives. Generally, the method of repeated grinding is used to eliminate the roll knives.
① Causes of roll fissure.
When the roll is rolled or when the hot roll is ground, cracks are prone to occur due to local stress concentration; accidents such as broken strips and twisted steel in the rolling mill cause cracks; the uneven thickness of the incoming material causes the roll to be impacted during the rolling process, The rolling force of the roll is too large in a short time to cause cracks; fatigue cracks of the roll.
② Elimination of roll fissure.
In order to prevent the roll from losing meat or larger defects due to cracks, it is necessary to deal with it in time and ensure that the cracks are removed. Check the roll carefully during grinding, if any crack is found, it should be marked at the end of the roll neck for the convenience of next inspection. Like testing or flaw detection, it is determined whether the crack has been ground, and if it still exists, continue grinding; if the crack is large and deep, it is still obvious after long-term grinding, and fixed-point plunge grinding can be performed to judge the depth of the crack.
Flaking (dropping flesh)
Roll spalling refers to the phenomenon that a certain area of the roll body is separated from the roll body.
Mill Roll surface spalling
① Steel jamming or slipping due to strip breakage, tail flicking, and overlapping of steel strips during rolling will cause thermal shock to the contact area of the roll surface, causing burns to varying degrees 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, surface cracks may disperse during the rolling cycle and cause the roll to peel off.
② High-cycle fatigue spalling and subcutaneous cracks are generally common in the secondary and backup rolls, and usually occur without mill accidents. Typically, this type of fatigue fracture is described as a “brittle” type of spalling, indicating that the process is initiated beneath the roll skin. The number of times of stress action is very important. Even if the stress is lower than the strength of the roll material, it can cause crack initiation due to repeated action, that is, fatigue cracks lead to spalling. Due to the parallel distribution of very small cracks on the tangent of the roll surface, the fatigue propagation of high cyclic contact stress can occur in many places in the stress area. Repeated cyclic stress then propagates these cracks to the surface until spalling occurs.
Flesh off the end of the roll
The abnormality of the thrust bearing of the rolling mill causes the roll end to drop meat; the wrong position or abnormality of the roll changing trolley in the rolling mill leads to an abnormal situation during the roll change, which causes the roll end to drop meat; lifting or other accidents cause the end of the roll to be impacted and damaged. damage.
Treatment method for roll spalling
Coarse grinding + flaw detection is adopted for the peeling of the roll surface until the defects are eliminated. The peeling of the roll end depends on the situation. The slight peeling is treated by grinding, and the severe peeling is used to scrap the roll.
Roller pad printing
① The cause of the roll support tile. Roll support tiles generally exist on rolls with small diameters that need to be supported by roll body brackets during grinding. Rolls with support tiles in cold rolling mills generally include work rolls, first middle, and second rolls of 20-high rolling mills. middle. The occurrence of backing tiles is due to the unevenness of the backing tiles, burrs, impurities in the backing tile material, or grinding wheel abrasive grains, iron filings and other sundries mixed between the backing tiles and the roll, which scratch the surface of the roll during grinding. produced.
② The method of eliminating the roll backing mark. Generally, padding paper, half-pass grinding or repeated grinding are used to eliminate supporting tile marks.