Causes and elimination methods of surface quality defects of high-speed wire rods

1. High-speed wire cracks

Cracks refer to defects on the surface of the wire that are straight or curved along the rolling direction or penetrate into the inside of the wire at a certain angle. Cracks vary in length and depth and can be found along the length of the wire. There are inclusions in some cracks and decarburization on both sides.

 

1) Causes of cracks on the surface of wires

The main reason for cracks on the surface of the wire rod is that unresolved cracks on the steel billet (whether longitudinal or transverse), subcutaneous air bubbles and non-metallic inclusions will cause cracks on the surface of the wire rod. If the pinholes on the continuous casting billet are not eliminated, they will be stretched, oxidized, and dissolved after rolling, which will cause linear hair lines in the finished product. Pinholes are one of the important defects of continuous casting billets. They are difficult to detect when they are not exposed, so special attention should be paid to them. If the high-carbon steel wire rod cools too quickly after rolling, it may also cause cracks in the finished product, and the latter may also cause transverse cracks. Cracks formed by improper air cooling after rolling are not accompanied by decarburization, and there is generally no iron oxide scale in the cracks. In addition, poor cleaning of the blank can also cause such phenomena.

 

The main reasons for the formation of cracks during the rolling process are as follows:

(1) The rolling groove is inappropriate, mainly due to problems with the sharp corners and the size of the rolling groove.

(2) The surface of the rolling groove is too rough or damaged.

(3) Scratches on the guides before rough rolling.

(4) Coarse iron oxide scale is rolled into the surface and inside of the rolled piece, and this usually occurs in the first few passes of rough rolling.

(5) Improper use of guides is mainly due to their large size.

 

2) If cracks occur, check and troubleshoot from the following aspects:

(1) Whether high-pressure water descaling is working normally, and whether the cooling water path of a certain rolling mill roll is blocked or deviated from the rolling groove.

(2) Whether the guides deviate from the rolling line and whether there is an iron oxide scale blocked in a certain guide.

(3) Whether the rolling groove is excessively worn or the rolling groove is damaged during the handling of steel piles.

(4) Whether the finishing mill has the wrong roll, whether the guide is centred and whether the size corresponds to the rolling specifications.

 

2. Folds on the wire rod

Thin wires on the surface of the wire that are straight or curved along the rolling direction penetrate into the surface of the wire at any angle. Defects that intersect with the surface at a small angle on the cross-section are mostly folds. Usually, the folds are long, but there are also discontinuous ones. It is discontinuous and distributed in the length direction of the wire. There are decarburization layers or partial decarburization layers on both sides of the fold, and iron oxide inclusions often exist in the middle of the fold.

 

1) If there are defects such as shrinkage cavities, segregation, and inclusions in the blank, or the blank is not trimmed well, defects similar to folding may occur.

The possible reasons for folding during the rolling process are:

(1) Overfilling of the rolled piece in the past is the main cause of folding.

(2) Too much tension between the racks is also one of the reasons for folding.

(3) The guide centring is not good, and if it deviates from the rolling centre line, it may be filled on one side and cause folding.

(4) The guide is abnormal. Check whether there is foreign matter accumulation in the sliding guide and whether the guide roller in the rolling guide is normal.

(5) Improper adjustment of the rolling mill, incorrect rolling mill dimensions or severe wear of the guides may also cause intermittent folding.

(6) Uneven heating temperature of the billet and uneven temperature along the length of the billet can also lead to intermittent overfilling.

(7) The amount of cooling water for the rolls and excessive accumulation of iron oxide scale in the roughing mill.

 

2) If the wire is folded, you should check the following aspects to troubleshoot:

(1) Check the roll cooling. Excessive accumulation of iron oxide scale in the roughing mill may also be the cause of intermittent folding.

(2) Whether a guide deviates from the rolling centre line causing overfilling.

(3) The guide is abnormal. Check whether there is foreign matter accumulation in the sliding guide and whether the guide roller in the rolling guide is normal.

(4) Whether the size of the rolled piece passing through the rolling mill is correct and whether it is overfilled.

(5) Check the tension. Check the billet exit temperature. Uneven temperatures along the length of the billet can also cause intermittent overfilling.

 

3. Protrusions on the wire surface

The protrusions on the wire surface along the rolling direction are called ears, which are mainly caused by overfilling of the rolling groove. If there are defects such as shrinkage cavities, segregation and inclusions in the billet, it will cause the rolled piece to be overfilled during rolling.

 

1) The reasons why ears may occur during the rolling process are:

(1) The size of the rolled piece is incorrect or the roll gap is improperly adjusted.

(2) Excessive tension causes ears to appear at the head and tail of the wire.

(3) The upper guide of the finished rolling mill is not properly aligned or adjusted improperly.

(4) The temperature of the billet is uneven. The size of the finished product in the high-temperature section is appropriate, while ears appear in the low-temperature section.

(5) The shaking of the rolled piece will produce intermittent ears, one section on one side of the wire and another section on the other side of the wire.

 

2) If the finished product has ears, it should be checked from the following aspects to troubleshoot:

(1) The size of the rolled piece at the entrance of the finishing rolling unit must be correct.

(2) It is also important to match the working roll diameter and set the roll gap correctly.

(3) Check the tension conditions in the rolling mill. If the speed is appropriate and the entrance rolling stock and work roll diameter are correct, the tension in the finishing mill should be very small.

(4) If there are ears only on one side of the wire rod, check whether the centring and alignment of the entrance guide of the finished rolling mill are good.

(5) If you want to eliminate rolling mill jitter, you need to check and make the following points: the working diameter of the roll ring of the finishing rolling unit is correct; the size of the entrance rolling mill is correct; the roll gap is adjusted correctly; the guides are adjusted correctly; and the shock-absorbing guides are used; If the steel type changes, the width of the rolled piece will also change, and corresponding adjustments should be made.

 

4. Scratches on the surface of the wire

Scratches are defects like grooves on the surface of the wire along the length direction. Their shapes and sizes vary. Some scratches have overlapping edges that turn up on the groove side, and there are also small sharp cracks like scratches. The scratches are mainly on the finished product. Caused by defective equipment such as guides, loopers, water-cooling boxes, pinch rollers, laying heads, loose coil conveying lines, collectors and balers.

 

1) The reasons for scratches during the rolling process are as follows:

(1) There is accumulation in the guide.

(2) The guide guard is improperly installed or the guide roller is broken.

(3) The guide has burrs.

(4) The neutralization of the rolling mill is not good, or the neutralization of the guide is not good.

(5) The opening of the guide is enlarged.

 

2) If scratches occur on the rolled piece, the following aspects should be inspected to troubleshoot:

(1) Inspection must be carried out after each rolling mill is shut down. Usually, the guides on each stand must be checked. During inspection, it is necessary to use a flashlight for clear observation.

(2) Poor hole shape or guide alignment is the most common cause of scratches and should be checked regularly.

 

5. Cracks and “meat loss” of tungsten carbide roller rings

Cracks in the tungsten carbide roller ring usually form periodic defects on the surface of the wire rod, and sometimes hair lines are formed on the surface of the wire rod. The “meat loss” of the roller ring sometimes forms bumps or scars on the surface of the wire rod.

 

1) The reasons for the rupture and “meat loss” of the tungsten carbide roller ring are:

(1) Due to the characteristics of the tungsten carbide roller ring, it is easy to break if not cooled properly.

(2) The “meat loss” in the roll ring groove may be caused by improper cooling, improper roll ring grinding groove technology, etc.
(3) The pH of the cooling water is inappropriate.

 

2) If the tungsten carbide roller ring is cracked or “flesh-off”, the following aspects should be checked to troubleshoot:

(1) The use of the roll ring should be checked every time the machine is stopped. If the rolling surface peels off prematurely, the cause should be analyzed and investigated immediately.

(2) Be careful when transporting and handling, and have appropriate transportation and storage tools.

(3) If there are bumps or scars on the surface of the wire, measure the distance from one bump or scar to the next to determine the number of faults.

(4) If there is peeling in the work roll ring or guide, it will be quite obvious.

(5) Improper grinding of the roll ring will cause the surface of the roll ring to be overcooked and the surface of the roll ring will be purple-blue. Such roll rings should not be installed on the rolling mill.

(6) Regularly check the quality of the cooling water of the rolling mill. The content of suspended solids must be controlled. The pH value of brands with a bonding phase composed solely of cobalt should be maintained between 7.5 and 8.5. The pH value of brands with a bonding phase composed of cobalt, nickel and chromium should be maintained between 7.5 and 8.5. The pH value should be less than 7.5.

 

6. Pitting defects

Under a magnifying glass, it can be clearly seen that irregular convex and concave defects are continuously distributed on the surface of the wire, which is called pitting.

 

1) The main reasons for pitting defects during the rolling process are:

(1) The rolling groove is improperly cooled or severely worn.

(2) The pH value of the cooling water is inappropriate, and the binder in the WC roll ring is corroded, causing the WC particles to fall out during the rolling process.

 

2) If pockmarks appear, you should check and troubleshoot from the following aspects:

(1) Replace the rolling groove strictly according to the plan.

(2) During shift handover, a comprehensive inspection of the roll cooling and groove surface conditions must be carried out.

(3) Regularly check the water quality.

(4) Check whether the cooling water pipe of the roll is blocked.

 

7. Scarring (warped skin or scales)

The metal flakes on the surface of the wire that are partially bonded or completely unbonded to the wire matrix are called scabs. The former is formed by the protruding objects on the previous several passes of the finished product being rolled into the base body, while the latter is formed by the metal debris that has separated from the rolled piece being rolled on the surface of the rolled piece.

 

1) The surface quality of the billet is not good, the original scars on the billet are missed, or the warped skin, flying fins, etc. on the surface of the continuous casting billet that are not cleaned can form scars. The reasons for scarring during rolling are:

 

(1) The blank is overheated.

(2) The blank is not well-ground.

(3) Excessive wear of rolling groove.

(4) The roller ring “drops meat”.

(5) The rolling mill guide has burrs.

 

2) If scarring (warped skin or scales) occurs, check the following aspects to troubleshoot:

(1) Overheating of the billet will cause segregation and excessive iron oxide scale.

(2) During the rolling process, larger pieces of iron oxide scale are rolled into the surface of the rolled piece to form scarring defects. Therefore, high-pressure water must be used to remove scales and eliminate the iron oxide scale.

(3) Segregation makes the surface of the billet uneven, and the broken fragments are ready to be rolled into the rolled piece. They will break and form scars in the later stages of rolling, so the heating temperature must be controlled.

(4) After the rolling mill stops, carefully check whether there are burrs, wear, meat loss and other problems on the surface of the rollers and guides.

 

8. Sticky roller and sticky guide

The defects along the length direction of the wire surface have the same shape and size, and they are regular. For example, the distance between the sticking roller defects is approximately the circumference of the roller (the length of the sticking guide is the circumference of the guide wheel).

 

1) Reasons for formation:

(1) The material of the roller (guide wheel) is not good and it is caused by iron sticking.

(2) The wire temperature is too high.

 

2) If sticky rollers (guides) occur, check the following aspects to troubleshoot:

(1) Control the wire temperature to the lower limit.

(2) Replace the roller (guide).

 

9. Fried lines

Frills are protruding stripes along the diameter of the surface of the wire.

1) Cause: cracks in the roll.

2) If cracks occur, the machine should be stopped to check the rolls, and the rolls or grooves should be replaced after confirming the number of runs.

 

10. Pinch nose

Nose pinching is a regular and intermittent slight scratch on the surface of the wire.

 

1) Reasons for formation:

(1) The size of the semi-finished product is large, and steel is piled between the two frames.

(2) The last two roll gaps are improperly set.

(3) The exit guide is too large.

 

2) If nose pinching occurs, check the following aspects to troubleshoot:

(1) Adjust the size of the semi-finished product.

(2) Replace the entrance and exit guides.

11. The size is out of tolerance

The diameter or ovality of the wire (except the head and tail) exceeds the allowable tolerance specified in the standard is called dimensional out-of-tolerance.

 

1) Reasons for formation:

The dimensional accuracy of high-speed wire rod rolling mill products can basically meet the A-level accuracy of the GB/T14981-2004 standard. However, dimensional deviation is still a common surface defect in actual production because there are many influencing factors, mainly including:

(1) The temperature of the rolled piece is uneven.

(2) Tension changes.

(3) The adjustment is not timely or the method is inappropriate.

(4) The roller turning accuracy is low, the installation is not in place, and the guides are installed incorrectly.

(5) Automatic detection, automatic control, instrument misalignment, malfunction, etc.

 

2) Control measures:

(1) Strictly control the temperature of the rolling stock, and make the temperature of the rolling strip and the same batch as consistent as possible.

(2) Achieve micro-tension or tension-free rolling as much as possible during rolling.

(3) Accurately adjust after the pass wear changes to ensure the size of the rolled piece.

(4) Strictly check whether the guide is installed correctly, and strictly inspect the surface quality, surface hardness, outer diameter, and control dimensions of the roll.

(5) Always check the accuracy of the instrument and pay attention to the accuracy of control execution.

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