Rolling mill rolls use with fault prevention

Description：The performance and quality of the rolling roll directly affect the output and product quality of the cold rolling mill, which is an important cost factor in the production, so after using a certain time or through a certain amount of rolling, the surface fatigue layer, the roughness has also changed greatly, to timely grinding, and eliminate the hidden trouble;

Keyword：rolling mill rolls, rolling mill rolls performance

• The performance and quality of roll directly affect the production and product quality of cold rolling mill in rolling production;
• Roll is an important cost factor in the production of aluminum strip;
• After the roll is used for a certain time or through a certain amount of rolling, the surface fatigue layer, the roughness has also changed greatly, so it is necessary to grind in time, and eliminate the hidden trouble;
• A pair of qualified cold rolls must not only meet the technical requirements of roughness, roundness, cylindrical degree, diameter difference, and convexity, but also have no obvious surface defects.

Basic item of mill roll files

Number file

The content includes the number, manufacturer, entry date, original hardness, inspection certificate, original hardness, tolerance and other original information

Dynamic course

Grinding record, rolling pass quantity record, abnormal condition record, image data, etc

General requirements for mill roll lifting movement

• Move only one roll each time to avoid contact with the roll;
• Use a suitable sling at the roll neck suitable for lifting;
• During transportation, the roll body shall not contact with any hard surface;
• Do not destroy the roller by welding the “accessories” to move the roll.
• No electromagnetic crane is allowed to move the roll.

General requirements for roll storage

• Place a partition pad (wooden wedge, rubber pad) between the roller body and the roller body to avoid contact with each other.
• Roller neck, sealing area and roll body shall be treated with rust prevention.
• Store in an appropriate environment to avoid the sudden temperature change of the roll body.
• Ensure no residual magnetization from storage, support and equipment.
• The damaged (peeling) roll shall be covered with protective felt. At least, the damage shall be turned to the ground. An acoustic radiation test can also be performed to help determine whether the damaged roll is in equilibrium (no significant acoustic radiation);
• The grinding must be close to the ambient temperature; prevent rapid cooling;

General requirements for roll use

• In the process of use, the operator should pay attention to various sound changes from the rolling mill to ensure the safe state at any time;
• Implement the point inspection system to prevent the parts above the mill from falling into the rolled aluminum plate and damaging the roll;
• Implement lubrication measures to prevent bearing dry friction and hold the roll;
• The pressure down procedure does not exceed the design scope;

Common roll failure

• Abnormal roll surface (aluminum, orange wrinkled surface, roller printing, soft point, hot crack);
• Pepping (peeling caused by roll material, peeling caused by contact stress);
• Roll neck fracture (roller neck fracture caused by metal fatigue, instantaneous stimulation caused by material mass, instantaneous roller neck fracture caused by rolling mill overload) roller body fracture (roller body fracture caused by metal fatigue, instantaneous roller body fracture caused by material quality, instantaneous roller body fracture caused by rolling mill overload)

Roll surface abnormal

Thermal crack (stress crack)

In appearance, thermal cracks range from tight, longitudinal small thermal cracks to various patterns (“cracks”, dry riverbed cracks) and various densities, which depend on mill operation and production (hot / cold rolling). Generally speaking, thermal cracks also appear with the soft point area (see abnormal signs of roll surface- -soft point).

Roll peeling

Mechanisation

1. The soft point area (see the abnormal signs of the roll surface- -soft point) is the formation of hot cracks

The first stage. When re-annealed martensitic steel shrinks from its surrounding material, uneven stress occurs in the soft spot zone. When the soft spot area is released by stress, the thermal crack (stress crack) is excited. As the mill continues to run, the thermal crack spreads in radial and annular directions, resulting in spoff.

2, hot shock (roll surface caused by rapid heating and cooling peeling phenomenon), is a more serious form of hot crack, is produced in an instantaneous way. Hot shock is usually due to the failure of the mill in the roller (sticky roller; see legend below), which increases the temperature of the roll surface, resulting in instantaneous cracking and peeling of the roll.

Preventive measure

Reducing the conditions that produce the soft point zone or cause the hot shock can reduce the possibility of a hot crack formation on the roll surface.

Pepping caused by roll surface (metal fatigue cracks, cracks, scratches)

Stripping of roller excitation

• Metal fatigue “broken” cracks will occur on the fracture surface. The metal fatigue cracks are annular cracks, which can sometimes be seen extending back to the roll surface.
• Metal fatigue cracks are characterized by metal fatigue retention marks (sand wave marks) and “fan” fracture lines on metal fatigue fracture surfaces. The metal fatigue cracks range from a few inches to several weeks around the roll, sometimes with a bright (wiped) or dim (oxidized) appearance.
• Roller surface excitation is often accompanied by thermal cracks (thermal crack-see abnormal signs of roll surface-thermal crack) or mechanical force excitation crack (roller seal-see abnormal signs of roll surface-roller seal).
• The metal fatigue “breakage” crack will spread in the opposite direction to the roll rotation. Radial crack diffusion mode is the diffusion characteristic of roller crack on reversible rolling mill.

Scale fication of the fracture surface of the metal fatigue trajectory

The peeling of roller surface excitation occurs in several different stages

• Stage 1-3: Roll surface excitation crack. This crack can be generated by a soft point (such as abnormal signs of the roll surface-introduction and sample 1 diagram) or one unrelated to the soft point. For example, roller printing, scratches and so on play the role of stress concentration and produce roller surface cracks. Each time the roll turns, the whole roll surface experiences high tensile stress and high compressive stress replacement. Any stress concentration at a certain point will cause an extension of the roller surface crack.
• Stage 4: For each rotation of the roll, the crack spreads radially and in rings through the excess zone (hardness depth) in a metal fatigue mode. In the fourth stage, the gauge and ring metal fatigue tracks appear, which are marked with retention marks and “fan” broken lines.
• Stage 5: Crcrack continue to circulate in the excessive area under the roll surface. In the fifth stage, the ring metal fatigue trajectory has continuity, showing obvious retention marks and “fan shape” rupture streamline within the metal fatigue trajectory.
• Stage 6: The yield strength of the surrounding matrix decreases to the extent that the peeling occurs. The phenomenon of stage 6 can occur at any time between stages 4 and 5, depending on the roll material strength and the magnitude of the rolling stress produced. The final stage of this rupture is instantaneous and is actually brittle, which originates from the metal fatigue trajectory on the rupture surface.

Preventive measure

• The following methods can be used to prevent the peeling stimulated by the roll surface Avoid mill-related roll damage, such as soft points, cracks, roller marks, scratches, or any factor that may concentrate on stress.

• If the rolling stress generated on the roll is greater than the roll material strength, the whole process of crack excitation, diffusion and peeling of the roll surface may occur in a rolling cycle. Shortening the rolling cycle and reducing the rolling force can prevent the roll surface damage. These injuries are caused by a certain rolling cycle, and if not removed by grinding, they will stimulate the crack and enter the metal fatigue trajectory stage.
• Ensure any surface damage caused during the final grinding cycle.
• After the grinding operation is completed, each roll should be inspected by eddy current flaw detection and ultrasonic inspection technology. This will ensure that the rolls put into production do not have any potential hazards that may stimulate the metal fatigue trajectory.

Contact stress (crushing) fatigue flaking

Contact stress (crushing) fatigue flaking

Contact stress (crushing) fatigue stripping

• Due to the load of the mill and the local flattening of the roll at the contact point, the maximum shear stress (usually called “Hertz stress”) in the shallow position below the roll surface (Figure 1-2 illustrates the stress conditions) can stimulate multiple cracks and diffuse in the shallow position under the roll surface when the Hertz stress exceeds the compressive strength of the roll. This will occur in two modes.
• Instantaneous: Due to the sudden increase of Hertz stress, this phenomenon occurs in winding rollers, slipping, or sudden shutdown. The Hertz stress is significantly increased and easily exceeds the compressive strength of the roll. Therefore, the cracks under the roll surface can be instantly formed, and repeated with the rolling stress. The cracks spread through the fatigue mode, and may produce peeling. In the extreme case of too high contact stress, the crack under the roll surface can be instantly excited and spread to cause peeling.
• High frequency metal fatigue rupture: this mode of crack excitation under the roller surface occurs more in the supporting roll, and usually without rolling mill accidents. Generally speaking, this metal fatigue rupture is called “fragile” type peeling, which is produced by a long excitation crack under the roll surface. This can be easily explained in a typical S-N fatigue diagram. From the diagram, the fatigue frequency of stripping is equivalent to 1 million times. If the stress action frequency is sufficient, even if the repeated stress is lower than the intrinsic material strength of the roll, it can also lead to crack excitation phenomenon. At many sites in these stress zones, high frequency contact stress fatigue flaking stimulates many very small cracks that are arranged in tangent parallel with the roller surface. Therefore, the stress repeated cycle, so that these small cracks to the roll surface expansion, until the roll surface peeling. Sometimes, contact stress fatigue cracks can be radial and annular diffusion, forming a fatigue “damage” trajectory, at this time, peeling is inevitable (see peeling- -roll surface signs). The favorable reasons can produce contact junction metal glass milk drop, such as the use of the roller roller grinding weight is not enough, the rolling pressure, the diameter difference between the working roller and the supporting roller.

Distribution diagram of contact stress between working roller and supporting roller

Instantaneous contact stress stripping

Since the maximum residual shear stress is located under the roller surface, when the mill suddenly stops or slips under special circumstances, the maximum shear stress exceeds the compressive strength of the roller.

This sudden increase of shear stress can cause transient formation of cracks under the roll surface. In extreme cases, the sharp increase in contact stress is sufficient to cause cracks under the roller surface and spread to cause immediate peeling phenomenon.

Preventive measure

Avoid rolling mill faults, such as: skid, winding roll, sticky roll, etc.

Roller height frequency contact stress metal fatigue

Due to the high hardness of the roll body, the high frequency of contact stress metal fatigue on the roll body is usually the result of the the point load caused on the roll.

This point load can be a small piece of metal in the roller joint, can be a broken welding residue through the roller joint, or can be any stress concentration on a certain independent point of the roller body, they play a role in concentrating the stress at a point in the roller body.

If this stress is greater than the compressive strength of the material, small cracks can be generated on the roll surface, which can be “damaged” through fatigue

The trajectory patterns spread rapidly and eventually produced exfoliation.

• Prevention measures for high frequency contact stress metal fatigue peeling on cold rolling roller Ⅰ

Avoid increasing the maximum shear stress due to the abrasive or solder going into the roller joint and exceeding the compressive metal fatigue strength of the roll.

• Prevention measures for high frequency contact stress metal fatigue peeling on cold rolling roller Ⅱ

After grinding, each roll shall be inspected with a dual-probe ultrasonic inspection technique (“Pitch / Catch”) and a surface wave conveyor. Ensure no crack under each roll surface and roll surface on the return mill.

• Preventive measures for high frequency contact stress metal fatigue peeling on cold rolling roller Ⅲ

Enough grinding of the roll during the grinding process can ensure any crack formed by the roller surface and keeps the position of the crack under the roller surface away from the maximum shear stress. These repositioned cracks would be in low stress conditions and they would barely diffuse.

• Preventive measures for high frequency contact stress metal fatigue peeling on cold rolling roller Ⅳ

Shorten the roll changing cycle and reduce the stress frequency of the roll. The stress exceeding the metal fatigue strength of the roll pressure is repeated, and the stress frequency is sufficient to stimulate the crack under the roll surface.

• Preventive measures for high frequency contact stress metal fatigue peeling on cold rolling roller Ⅴ

Reduce the rolling force to reduce the maximum shear stress.

• Preventive measures for high frequency contact stress metal fatigue peeling on cold rolling roller Ⅵ

The surface shape of the roll (working roller / support roller convex) is properly designed to ensure a uniform contact pattern along the entire working roller / support roller (intermediate roller) contact area.

• Preventive measures for high frequency contact stress metal fatigue peeling on cold rolling roller Ⅶ

The side part of the working roll is designed to reduce the stress concentration around the roll.

• Prevention measures for high frequency contact stress metal fatigue peeling on cold rolling roller Ⅷ

The side part of the supporting roller is designed to have rounded corners and appropriate rear corners (about 0.5) to reduce the stress concentration at the contact point of the working roller and the side part of the supporting roller.

Contact stress fatigue stripping

When the roll is put into rolling, the contact area between the edge of the plate belt, the working roll and the supporting roll or the edge of the working roll will become the stress concentration place;

With each rotation of the roll, if the maximum residual shear stress located under the roll surface exceeds the compressive metal fatigue strength of the material, the crack will form in this position;

The high frequency of the stress makes the crack spread to the roll surface, so that a small “broken” peeling appears on the roll surface;

In many cases, the metal fatigue “breakage” trajectory is excited under the roller surface where the contact stress metal fatigue cracks have been formed. Then the metal fatigue “damage” trajectory is radial and annular diffusion, so that the strength of the material around it to decline, resulting in large peeling phenomenon;

Contact stress stripping of supporting roller

Supporting roller usually does not occur instantaneous contact stress stripping; but high frequency contact stress metal fatigue stripping is quite common;

High frequency contact stress metal fatigue peeling can occur at any position of the roll body and often has a “broken” peeling appearance. The “broken” peeling is usually larger due to the low hardness of the supporting rollers, and their appearance is usually more severe compared to the high frequency contact fatigue peeling of the working rollers. The contact stress metal fatigue crack under the roller surface can develop silently before exposing to the roller surface (peeling off).

High frequency contact stress metal fatigue mechanism of supporting roller

Pepping can occur when the maximum shear stress under the roller surface exceeds the compressive fatigue stress of the material. The most common position of high frequency contact stress metal fatigue on the supporting roller is in the center protrusion of the roller body, the side part of the roller and the contact point between the supporting roller and the edge part of the working roller. These areas become the stress concentration point during rolling, and obviously increase the maximum shear stress under the roll surface.

Unreasonable angle design causes high frequency contact metal fatigue “crushing” flaking on the support roller

The sharp edge of the working roller (or middle roller) cuts into the roller surface near the edge of the supporting roll, which concentrates the stress. In this case, this peeling phenomenon can be eliminated by using the cone rear corner instead of a sudden change in diameter.

High frequency contact stress metal fatigue in the support roller prevents with the following measures

• After grinding, each roll adopts a two-probe ultrasonic inspection technique (“” PitcyCatc?”) And surface wave transmitters. This ensures that each support roller served on the return mill has no roll surface and cracks under the roll surface.
• Sufficient grinding of the roller during the grinding process guarantees any cracks formed on the roller surface or keeps the crack under the roller surface away from the maximum shear stress area. These repositioned cracks are in low stress environments and barely diffuse.
• Shorten the roll change cycle time, reduce the frequency of roller bearing stress. The stress exceeding the compressive strength of the roller repeats itself, and when the frequency number is large enough, the crack under the roller surface will be stimulated.
• Reduce the rolling force to reduce the maximum shear stress.
• Increase the hardness of the supporting roller to improve the fatigue resistance strength of the roll.
• If the contact stress metal fatigue stripping occurs in the late life of the roller, the fatigue strength increases the diameter of the roller layer.
• The side part of the supporting roller is designed with rounded corners and appropriate “rear angle” to reduce the stress concentration of the working roller and the contact area of the supporting roller. This design also reduces the stress concentration at the side of the supporting roller.
• Reduce the roll body convex measure in order to reduce the maximum shear stress at the center of the roll body.

Instantaneous fracture — mill overload

Roll neck fracture

The instantaneous roller neck fracture caused by the rolling mill overload generally occurs along the transverse shear (45) surface. It is characterized by a single point excitation from the roller surface and the fracture streamline across the entire fracture surface from the excitation level. The middle part of the fracture surface has a typical plastic appearance. Close inspection of the cause of the fracture, and the absence of any metal fatigue trace (metal fatigue retention mark), which indicates that the cracks did not occur in a long time, but was instantaneous.

Instsient roller neck fracture mechanism

Instantaneous roller neck fracture usually occurs when mill-related faults suddenly increase the bending stress applied to the roller neck. If the applied bending stress exceeds the strength of the roller neck material, the instantaneous fracture will occur. The fracture is excited at the highest bending stress on the roller neck and then spreads across the transverse shear surface in radial and longitudinal directions.

Preventive measures for instantaneous roller neck fracture caused by rolling mill overload

• Avoid mill failure
• Eliminate any possibility of applying a sudden increased bending pressure to the roller neck

Side view of the instantaneous fracture roller

Note that the broken transverse shear surface (450) The arrow refers to the roll surface excitation position.

Instant fracture of roller body

The instantaneous fracture of the roll body caused by the rolling mill overload generally occurs along the transverse shear (45) surface. This fracture is characterized by a single point excitation from the roller surface and a typical plastic appearance in the middle of the fracture surface. The cause of the fracture is checked in a close range, without any trace of metal fatigue (metal fatigue retention mark), which indicates that the crack does not occur in a long time, but is instantaneous.

Instantaneous fracture mechanism of the roller body

The instantaneous fracture of the roller body generally occurs when the mill related fault makes the stress applied to the roller body increases. If the stress force exceeds the material strength of the roller body, instantaneous fracture will occur. The fracture is excited at a point with the highest stress on the roller surface and is diffused radially and lengthwise on the transverse shear plane.

Preventive measures for the instantaneous fracture of the roller body

Control the load and operation condition of the rolling mill to avoid the abnormal stress concentration on the roll body and prevent the instantaneous fracture of the roll body caused by the rolling mill overload.

Influence of roller grinder parameter selection on roller grinding quality

In addition to the relative rotation movement of the grinding wheel and the workpiece, while the relative longitudinal motion, there is also the radial relative displacement, which is different from the compound motion of the grinding taper. So its transmission structure is more complex, the machine tool work accuracy requirements are also higher.

In recent years, with the continuous development of technology, a series of new requirements for modern roller grinder, can grind more complex roller type; the roller tolerance (eccentricity, roundness, circle jump, taper) is stricter; the roller tolerance is smaller; the roller detection technology is higher and more comprehensive (size, flaw, hardness, roughness).

In order to obtain higher roll grinding accuracy, in addition to the relationship between roll grinding accuracy and surface quality and grinder working accuracy, the relationship between the corresponding grinding wheel, coolant and grinding process parameters is also studied.

Selection of the grinding wheel

The grinding time should be as short as possible, with the grinding wheel, and no self-excitation during grinding.

Abrasive selection

Corundum grinding wheel is used for hardened or non-hardened steel rolls; silicon carbide grinding wheel is used for cold hard cast iron rolls, rubber rolls, copper rolls and granite rollers. For different materials, the matching steel corundum grinding wheel should be selected to obtain higher grinding precision and surface quality. For example, chromium corundum (PA) is used for alloy steel rollers, zirconium corundum (ZA) is used for heat resistant alloy steel rollers, and single crystal corundum (SA) grinding wheel is used for stainless steel grinding rollers.

Selection of granularity

Even for the sharp and protruding grinding particles, the cutting process can be roughly divided into three stages: the abrasive scratches from the surface of the workpiece, only elastic deformation and no chips; the grinding particles cut into the surface of the workpiece, cut the groove and raise; the cutting thickness increases to a certain critical value, cut the chips.

The grinding process of the grinding wheel is the combination of cutting, cutting and scraping. The sharpness of the grinding wheel is round and blunt grinding grain, the cutting ability decreases, and the force acting on the grinding grain is increasing. When the force exceeds the grinding strength limit, the grinding particles will break, creating new sharp edges and corners instead of the old blunt grinding particles; if the force exceeds the bonding force of the grinding wheel, the blunt grinding particles will fall off from the surface of the grinding wheel, exposing a layer of fresh and sharp grinding. This kind of grinding wheel innovation, to maintain their own sharp performance, called “self-sharp”. Although the grinding wheel itself has self-sharp properties, it will block the cutting, and the grinding of the grinding. Therefore, coarse grained (24-60) grinding wheel for coarse grinding, fine grained (60~100) grinding wheel for fine grinding, fine grinding, fine grinding (W64~W14).

Hardness selection

The harder the grinding roll surface, the softer the grinding wheel.

(1) If the grinding wheel wears too fast, indicating that the grinding wheel selected for the processed roller is too soft, the following measures can be taken to improve the linear speed of the grinding wheel; improve the longitudinal feed speed of the tow board, that is, increase the longitudinal feed of the workpiece per turn to 2 / 3~3 / 4 of the width of the grinding wheel, and reduce the speed of the roller. If the above measures do not achieve obvious results, the hard grinding wheel should be replaced.

(2) If the grinding wheel is obviously blocked or very dirty when grinding roller, the passive sand is not easy to fall off, the grinding wheel is easy to stick to the grinding chips, and the grinding roller surface is burns, it means that the grinding wheel is too hard, the following measures can be taken: reduce the speed of the grinding wheel; improve the roller speed. If the action is invalid, a soft grinding wheel should be selected.

Selection of the binder

• The effect of the binder is to bond the abrasive together to give the grinding wheel the necessary shape and strength.
• Ceramic binder (code A):, made of clay, feldspar, talc, boron glass, silica and other ceramic materials. Good chemical stability, water resistance, acid resistance, heat resistance, low cost, but more brittle. Therefore, except for cutting off the grinding wheel, most of the ceramic grinding wheel is used.
• Resin binding agent (code S): the main composition is phenolic resin, also useful epoxy resin resin binding agent of high sand wheel strength, good elasticity, mostly used for high-speed grinding, cutting, and groove and other processes. Resin grinding wheel has poor heat resistance.
• Rubber binder (code X): use mostly artificial rubber. Rubber grinding wheel is more elastic than resin grinding wheel, which can make the grinding wheel have a very good polishing effect. Rubber grinding wheel is mostly used for the guide wheel of careless grinder, but also used for cutting, grooving and polishing grinding wheel, but not suitable for rough grinding wheel.
• Metal binder (code J):, the common is the bronze binder (code Q), mainly used for making diamond grinding wheel. Bronze binding diamond grinding wheel is characterized by good molding, high strength and certain toughness. However, the grinding wheel made of metal binding agent is poor.

Therefore, in addition to the mirror grinding resin binding agent grinding wheel, other grinding surface are better to use the ceramic grinding wheel.

Selection of grinding coolant

2.1 Effect of grinding coolant

(1) cooling effect; (2) cleaning action; (3) rust prevention effect.

2.2 Common abrasive Coolant

(1) Saponification liquid: good lubricity, poor rust prevention, general cooling performance, short service cycle.

(2) Chemical grinding coolant: good rust resistance and cooling ability. General chemical solution contains sodium nitrite, so it has certain harm.

(3) New grinding fluid: at present, new environmentally friendly grinding fluid is constantly developed at home and abroad, with no odor, excellent cooling and cleaning performance, good rust and corrosion prevention effect, but the price is high.