What are the quality problems that often occur in mill roll grinding? How to eliminate them?
|Straight waveform (polygonal)||① The grinding wheel is blunt and slippery and repaired in time; ② The grinding wheel is unbalanced and the dressing is not good; ③ Grinding machine spindle or bearing damage; ④ The peripheral speed of the grinding wheel garden is too large; ⑤ The hardness of the grinding wheel is too large;||①Repair the grinding wheel in time; ② Balance the grinding wheel; ③ Eliminate the clearance between the bearing and the main shaft; ④ Adjust the speed; ⑤ Select the appropriate hardness.|
|Spiral||①The grinding wheel spindle has axial serial movement; ②The axis line of the grinding wheel is not perpendicular to the axis line of the workpiece; ③ The surface of the grinding wheel is uneven; ④Longitudinal feed is too large.||① Eliminate the axial series movement of the grinding wheel spindle; ② Adjust the balance between the axis line of the grinding wheel and the axis line of the workpiece; ③ Dress the grinding wheel; ④ The longitudinal feed is appropriate.|
|scratches and scratches||①Abrasive grains fall off between the grinding wheel and the workbench; ②The coolant filter is not clean.||① Eliminate the grinding wheel; ② Filter the coolant and clean the filter.|
|Roll surface burns||① The amount of grinding is too large, such as the peripheral speed of the sand office is too high or the peripheral speed of the workpiece is too low, and the grinding depth is too large; ②The hardness of the grinding wheel is too high, and the abrasive grains become pure and cannot fall off during grinding, resulting in high temperature on the surface of the workpiece; ③The grain size of the grinding wheel is too large, the structure is tight, and the grinding wheel is blocked during grinding; ④ The contact surface between the grinding wheel and the workpiece is too large or the contact arc is too long, and the grinding wheel is easy to block; ⑤ The material of the workpiece itself has poor thermal conductivity and small thermal conductivity; ⑥ The cooling method is poor, the coolant is insufficient, and the heat cannot be discharged.||① Correspondingly increase the peripheral speed of the roll and reduce the cutting depth; ② Choose the grinding wheel correctly. When the heat dissipation conditions are poor, a grinding wheel with low hardness and small particle size should be selected; ③ Check the grinding wheel in time to make it have good cutting ability; ④ Reduce the contact surface between the grinding wheel and the workpiece; ⑤ Select the appropriate coolant for sub-cooling.|
|Roll body is tapered||① The centerlines of the headstock and the tailstock do not form an angle on the horizontal plane; ②The axis of the roll is not adjusted well, and it is not perpendicular to the axis of the grinding wheel; ③The grinding wheel is too soft, which causes wear when the grinding wheel moves from the workpiece end to the other end.||①Use a dial gauge to calibrate the headstock and tailstock so that the centers coincide; ② Adjust the roll so that its axis coincides with the centerline of the thimble; ③ Choose the grinding wheel reasonably.|
|Waist-shaped roller body||① The centerlines of the headstock and tailstock do not overlap to form an angle in the vertical plane; ②The axis of the roll is not adjusted well, and forms an angle in the vertical plane.||① Correct the headstock and tailstock so that the center lines coincide; ② Adjust the roll so that its axis coincides with the center of the thimble.|
How to conduct failure analysis of mill roll?
Through roll failure analysis, finding the main cause of roll damage is a topic of common concern for roll manufacturers and cold rolling mills. After problems such as roll peeling, breakage, and surface cracks occur, the user should pay attention;
(1) Protect the fracture of the roll
(2) Collect peeled pieces
(3) Collect and protect the roll change records, eddy current flaw detection records and other relevant materials of the roll
Through the macro-judgment of spalling fracture, professionals can preliminarily judge the crack generation area and expansion, and the instantaneous fracture area. The crack generation area should be given special attention. At the same time, you can compare the roll change records to find out whether there are abnormal roll changes in the latest roll change records, whether there is abnormal damage to the roll surface, and preliminarily determine the cause of the spalling.
If a single visual judgment cannot determine the cause of crack formation, samples can be taken from the crack generation area for analysis. The analysis items include metallographic structure analysis, acid attack test, chemical composition analysis, etc., to judge whether the original structure of the roll is normal, whether the distribution of carbides is reasonable, and whether the surface of the roll has been subjected to abnormal mechanical and thermal damage. This work can be carried out jointly by the roll manufacturer and the user, or it can be analyzed by a third party.
What are the main causes of mill roll spalling?
The causes of roll spalling can be categorized as:
(1) Spalling of high-hardness rolls caused by surface damage. Due to rolling accidents or improper maintenance of the rolls during use, the surface of the rolls is partially subjected to thermal shock, which changes the surface structure of the rolls and produces cracks or soft areas. If it is not cleaned in time, the crack will expand during use and cause spalling. This is the most common type of roll spalling, especially on high-hardness rolls.
(2) Surface peeling caused by fatigue cracks. A common type of peeling on intermediate rolls and backup rolls. Due to the long roll change cycle, the fatigue cracks initiated on the surface of the rolls propagate and cause peeling.
(3) Spalling caused by metallurgical defects. The slag inclusions in the steel exceed the standard, or the distribution of carbides is not good, and cracks sprout during use to cause spalling.
How to distinguish roll spalling?
Roll peeling is very harmful to cold rolling. Among all peeling accidents, the peeling caused by cracks on the roll surface accounts for a high proportion. Usually, this kind of peeling first produces a small crack on the surface, and then the crack expands obliquely along the opposite direction of the roll rotation to the transition layer and then expands along the circumference, forming a “cat tongue” or “rail” trace. When the internal cracking is large enough to a certain extent, an instantaneous cracking zone is formed.
|Typical features||Main reason||Improvement missed|
|Zoning||Fracture smooth gray or dark gray massive or banded fatigue area.||Surface cracks such as steel rolling accidents and grinding damage were not removed in time.|
1. Reduce rolling accidents. 2. surface flaw detection 3. Improve grinding process
1. 减少轧制事故2. 表面缺陷检测3. 改进磨削工艺
|conchoidal exfoliation||Concentric conchoidal striations are evident at the bottom of the fracture, and material defects are sometimes visible.||Metallurgical defects such as inclusions near the surface are exfoliated as crack sources expand during rolling.||1. Using ESR material 2. Guaranteed forging ratio 3. control manufacturing process|
|Other types of peeling|
Thermal Shock Peeling
|Spalling occurs directly after accidents such as sticking to steel.||Locally suffer strong thermal shocks such as broken belts and steel sticking.||1. Controlling rolling accidents|
exfoliation at the end
|The entire hardened layer at the end of the roll body peels off in a ring.||Quenching residual stress and working stress are too large.|
1. Reduce residual stress 2. Improve the end structure
1. 减少残余应力; 2. 改进端部结构
|area punctate||A large number of spot-like peelings appear in a certain area.||Contact fatigue is high, and the roll changing cycle is too long.|
1. Adjust the roll changing cycle 2. Adjust technical requirements
1. 调整换辊周期2. 调整技术要求
How to prevent peeling problem?
To prevent these peelings, the manufacturer and the user must work together. The roll manufacturer must use electroslag remelting materials to reduce the total amount of slag inclusions, improve the distribution of carbides, careful heat treatment, and strict inspection of the total surface amount of the factory. ;As the user, it is necessary to operate carefully, establish a grinding system for rolls, a treatment system for accidental rolls, promote surface flaw detection, and prevent rolls from being damaged on the machine.
(1) Reduce rolling accidents
Forged steel cold rolls are generally made of Cr alloy steel after quenching and low-temperature tempering. The temperature of low-temperature tempering usually does not exceed 170ºC. When major accidents such as sticking steel occur, the local temperature can reach 800ºC or even higher. After the roll surface is heated, the martensite matrix will decompose into carbide and ferrite, and the volume will shrink, causing local tensile stress on the surface and inducing surface cracks. The cracks are not formed immediately, and the strength of the heat-affected zone is greatly reduced. Fatigue cracks are generated early in subsequent rolling, which is the main source of cracks on the roll surface. Therefore, improving the use environment of the roll is the premise of improving the service life of the roll.
(2) Strengthen detection
Whether there are defects on the surface of the roll, it is not enough to rely on the naked eye, especially the roll that has been subjected to overheating shock in a rolling accident, often has no obvious cracks, but the surface or shallow surface of the roll has been damaged, such as heat-affected zones and microcracks , these defects can only be detected by surface flaw detection. Commonly used roll surface flaw detection methods are as follows.
|Flaw detection method||Inspection scope||Applicable defects|
|double erosion||The outer surface||Surface cracks, heat affected zone, segregation, etc.|
|surface wave method||outer surface and superficial layer||Point defects such as surface cracks and pinholes.|
|Eddy current testing||outer surface and superficial layer||Point defects such as surface cracks and pinholes.|
|Colored flaw detection||The outer surface||Surface opening cracks, pinholes and other point defects.|
|Magnetic particle inspection||The outer surface||Surface and near-surface cracks.|
In small and medium-sized cold rolling mills, due to the lack of special equipment and technical personnel, eddy current flaw detection and surface wave flaw detection are currently less used.
(3) Maintenance of accidental rolls
Due to the balance of on-site workload and on-site personnel allocation, it is required to carry out surface flaw detection on all replaced rolls, which will obviously be more difficult for small and medium-sized cold rolling mills. However, it is necessary to establish the “Accidental Roller Grinding Inspection Procedure” and implement tracking for the accidental rolls to improve the service life of the rolls. The accidental rolls should be continuously tracked and detected for more than three times.
(4) Roll body grinding
A large number of practices have proved that improper grinding is also an important source of surface burns and cracks. It is necessary to carry out surface flaw detection on the rolled rolls after grinding to evaluate the rationality of the grinding process and the correctness of the operation.
The following is the recommended grinding process: the grinding amount of a conventional roll change is recommended to be at least 0.1mm on one side, and the accidental roll is required to completely grind away the cracks and heat-affected zones. The grinding parameters are as follows.
Commonly used grinding specifications:
|Name||coarse grinding||fine grinding|
|Feed rate (mm)||<0.02||<0.005|
|Axial feed (mm/min)||2000-3000||300-2000|
|Grinding wheel speed (m/min)||1500-1700||1500-1700|
|Grinding wheel model||Corundum material, 60-100 grains, medium soft, resin binder.|
|Coolant volume (L/min)||>30||>30|
(5) Management system
The life cycle of the roll is a closed-loop system from
design→purchasing→manufacturing→storage→use→maintenance→statistics→comprehensive evaluation→modification of design→purchasing. Each link should have clear operating procedures to ensure that the process is under effective control. The main links are:
a) The design should be based on the annual comprehensive performance evaluation, according to the requirements of the production outline in the coming year, and combined with the manufacturer’s suggestion, propose a modified design, as the basis for procurement in the next year.
b) The procurement of spare parts should ensure that the roll manufacturer has a reasonable production cycle to ensure production quality.
c) A special person shall be appointed to be responsible for the maintenance of the rolls, and the “Roll History Filing Report” shall be established.
d) Organize technical exchanges and vocational training to improve the technical level and operational skills of employees.
e) Reasonably determine the rolling cycle and the grinding amount of the roll body, and formulate the grinding specifications.
f) Regularly check the quality status of cold-rolled raw materials and the stability of the rolling mill.
What is double erosion testing and penetrant testing, how to operate?
Double erosion and penetrant testing is simple, economical and convenient, and it is a commonly used method in the grinding roll room. The specific operation steps of the testing method are as follows for your reference.
Double erosion flaw detection
|1||to clean up||Clean and dry the inspected area with methanol.||Self made Nitric acid + methanol solution; Hydrochloric acid + methanol solution.|
|2||first erosion||Etch the roll surface with a solution of nitric acid: methanol = 1:3.|
|3||to clean||Wipe the surface clean with a methanol cotton ball.|
|4||second erosion||Use a cotton ball dipped in hydrochloric acid: methanol = 1:1 solution to erode the surface.|
|5||to clean||Wipe the surface clean with a methanol cotton ball.|
|6||observation record||The color of the normal surface is lighter, the color of the heat-affected zone is obviously darker, and the cracks can be clearly displayed.。|
|7||anti-rust||Clean the surface of the roller body with gasoline or acetone and treat it with rust prevention.|
Penetrant Testing (PT)
|1||to clean up||After rough cleaning by mechanical means, wash with gasoline or acetone.||Solvent-based colored penetrant flaw detector. 5-10 times magnifying glass.|
|2||to clean||Special cleaning agent for spraying penetrant flaw detection|
|4||to clean||Spray cleaning agent after 10-15 minutes to remove excess penetrant|
|6||observation record||Penetration testing is used for non-magnetic materials or magnetic materials, observe after 5-10 minutes, and pay attention to distinguishing false phenomena such as scratches.|
|7||anti-rust||Clean the surface of the roller body with gasoline or acetone and treat it with rust prevention.|
Grinding inspection procedure for accidental mill rolls
How to prevent breakage of backup rollers?
Back-up roll fractures often occur on 4-high or 6-high rolling mills for small strip steel cold rolling. The fracture area has certain rules, and usually occurs at the place where the stress concentration is most obvious at the junction of the roll body and roll neck. Usually, the reasons for the fracture are :
(1) The strength of the raw material of the roll. Due to the insufficient metallurgical purity of the steel used, high slag inclusions, or improper forging and heat treatment processes, the strength of the roll neck is low.
(2) Improper design and processing. Most of the back-up roll fractures are fatigue fractures, and the cracks first appear in the form of fatigue cracks at the junction of the roll body and neck, and then propagate along the circumference and radius directions simultaneously (the difference in the growth rate between the two can be used to interpret the degree of stress concentration). Stress concentration caused by improper design, such as too large diameter ratio and too small transition fillet, will accelerate the formation and expansion of fatigue cracks. Improper processing refers to the phenomenon of sharp corners and serious tool marks in the stress concentration area during processing. It will also promote the fatigue fracture of the roll.
(3) Serious overload operation in use. This is a common problem in the operation of small and medium rolling mills.
To sum up, improve the design, reduce stress concentration, arrange reasonable diameter transition and transition fillet, improve the surface roughness at the fillet, and properly increase the quenching and tempering strength; choose appropriate materials and smelting methods to prevent overload operation, It is the main way to prevent roll breakage.
How to carry out the surface inspection of the mill roll?
There are currently four common methods for surface inspection of rolls
(1) Eddy current flaw detection method.
Eddy current flaw detection has developed into a special surface inspection equipment that can be equipped on the grinding machine to detect the surface cracks, soft spots and other defects of the roll. Its characteristic is that the detection speed is fast, and it can detect surface and shallow surface defects. The disadvantage is that the investment is large, sometimes false signals appear, and high-level technical support is required. It is suitable for the requirements of large-scale steel mills for rapid and continuous action.
(2) Ultrasonic surface flaw detection uses ordinary ultrasonic equipment for surface detection. It is characterized by small investment and high detection progress, but relatively slow detection speed, which requires professional operation. Large, medium and small cold rolling can be popularized and used.
(3) Magnetic particle inspection.
The surface of the roll is magnetized by permanent magnet or electromagnetic, and the magnetic powder is sprayed to show cracks. It is characterized by small investment and convenient on-site operation. The disadvantage is that it can only detect open cracks, with false signals and residual magnetism on the roll. It is suitable for small and medium-sized steel mills, and can also be used as a confirmation of crack defects after eddy current flaw detection in large steel mills.
(4) Colored flaw detection.
It is beneficial for different reagents to clean-color-display the surface area of the roll to find defects. It is characterized by small investment and good on-site operability. The disadvantage is that the operation process is slow and there are missed inspections. It is suitable for local surface inspection of rolls. It can also be used as confirmation of crack defects after eddy current inspection in large steel plants.