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Yangchun New Steel Co., Ltd. is located in Yangchun City, Guangdong Province, in order to meet the needs of the market and improve the production efficiency of the company, the company's rolling steel mill high line (the scale of φ5.5~φ16mm high-speed wire 1.2 million tons) was officially put into production in December 2010.

The high line is mainly composed of rough rolling unit, medium rolling unit, pre-finishing mill group and finishing mill group. The roughing mill unit is arranged in single line, and the other units are arranged in double line, and a total of 41 rolling mills are composed. A total of 5 roughing mill units (φ550×5) are arranged in a flat and vertical manner. There are 8 middle rolling units (φ550×2+φ450×4+φ350×2), which are full horizontal rolling mills. The pre-finishing mill group of 8 sets (double lines, 4 sets per line) is two-high cantilever type (φ285×4×2), and the horizontal/vertical alternating arrangement. Finishing mill group of 20 (double lines, 10 each line), for the two-high cantilever type (collective drive, φ228×5+φ170×5), 45° top crossing arrangement.

Due to the climate and many other reasons, the roller ring and cone sleeve of the double high line in the rolling mill are affected by moisture and rust, and the roller ring explodes the roller. Through the field tracking, find out the reasons affecting the roller ring explosion, and develop the corresponding control measures, in order to solve the problem of the roller ring explosion, so as to achieve the purpose of stable production operation.
 

The billet first goes in from the roughing mill unit, then to the middle rolling mill unit, then to the pre-finishing mill group, and finally to the finishing mill. The main production process is as follows:

1) After the billet is rolled out in the roughing mill, the head and tail of the rolling piece are cut respectively in the 1# flying shear (the rolling piece can be broken in the accident), and then separated from the roughing mill, the steel separator is introduced into the position of line A (or line B) in the double-wire rolling line, and the roller table is sent to the middle mill for double-wire rolling. Small tension control rolling is adopted inside the rough and medium rolling units.

2) After the rolled pieces of the middle rolling unit are divided by the guide groove, the head and tail of the rolled pieces are cut respectively in the 2# flying shear (double wire) (the rolled pieces can also be broken in the event of an accident), and then rolling into the pre-finishing mill group, and the pneumatic vertical looping tubes are arranged between each stand of the pre-finishing mill group, so that the rolled pieces can be rolled in this area without torsion and tension. Thus to ensure the precision of the size of the rolling pieces entering the finishing mill;

3) After the rolling piece is taken out of the pre-finishing mill group, the head and tail are cut at the 3# flying shear after the pre-water cooling. Rolled pieces are rolled through side looping into finishing mill group. If an accident occurs in the finishing mill group, the breaking shear at the entrance of the finishing mill group will be activated immediately to cut the rolled piece and prevent subsequent rolled pieces from continuing into the finishing mill. At the same time, the 3# cutting function will be activated to break the rolled piece.

4) The finishing mill group is a 45° torsion-free mill with 10 frames of collective drive and cantilever tungsten carbide roller ring. The rolled pieces are rolled by single-line torsion-free micro-tension between the finishing mill groups, and the rolled pieces are rolled into wire products with high dimensional precision and high surface quality. According to different product specifications, the rolled pieces are rolled several times in the finishing mill group. When producing φ5.5 ~ 7mm wire, the finishing mill group guarantees the final rolling speed of 95m/s.

In order to optimize the cost, the roller ring is made of tungsten carbide. Through the analysis of the roller ring explosion site, the reasons are as follows:

1) Due to the wet rain in Guangdong Yangchun, the site environment is relatively humid, and the roller ring and cone sleeve are exposed to the air affected by moisture corrosion, resulting in the roller ring explosion;

2) The temperature change curve at any point on the surface of the rolling groove during rolling is shown in Figure 1. As can be seen from Figure 1, the surface temperature of the rolling groove rises and falls repeatedly, resulting in alternating thermal stress and thermal fatigue cracks, and high temperature has thermal corrosion on the surface of the rolling groove. In order to prevent the crack of the roller ring, it is very important to delay the expansion of micro crack, prolong the life of the roller ring and improve the effect of the roller ring cooling. The effect of roller ring cooling is not good, the maximum temperature of roller ring, the average temperature of roller ring body and the temperature difference △T will further increase, which will cause the hot crack and hot corrosion expansion of rolling groove to accelerate, reduce the amount of rolling, and even lead to crack expansion, which is the main reason for the roller ring explosion.

According to the content, type and chemical composition of the adhesive phase, the grain size of WC, the porosity of the material, the content of carbon, the microstructure and grain morphology of the raw materials and the preparation process, different grades of roller rings can be designed. Different grades of cemented carbide roller rings have different hardness and strength, which are suitable for different frames, and should be correctly selected.

The hardness and wear resistance of the roller ring are derived from tungsten carbide, while the toughness and strength depend on the bonding phase metal. High binder phase content, good toughness and strength, good impact and hot crack resistance, suitable for the front frame with large rolling force and poor rolling precision requirements; Tungsten carbide content is high, good hardness and wear resistance, toughness and strength is relatively poor, suitable for rolling force is small, high precision rolling mill behind the frame, can ensure the dimensional accuracy of the finished product.

According to the geographical location of Yangchun, Guangdong Province, the selection of YGR series carbide roller ring, the series of grades of alloy are added Ni, Cr elements, can significantly change the oxidation resistance of cemented carbide in 700 ~ 1000℃ when exposed to the air, corrosion resistance in acidic solution is also improved, but also improve the high temperature performance of cemented carbide. The alloy has high thermal crack resistance and corrosion resistance. Suitable for PH≤7.2 slightly acidic environment and high rolling speed.

Cooling water is the life of the roller ring, according to the actual situation of the site, the following requirements for cooling water supervision:

1) Water quantity and pressure: The groove width is less than 0.5 "and the water quantity is controlled at more than 18m³/ shelf · hour, the groove width is 0.5" ~ 1.0 "and the water quantity is controlled at more than 20.5m³/ shelf · hour, the groove width is 1.0" ~ 1.5 "and the water quantity is controlled at more than 22.7m³ / shelf · hour, and the groove width is 1.5" ~ 2.0 "and the water quantity is controlled at more than 25m³ / shelf · hour. Strengthen the spot check of the water valve to ensure that the water pressure at the cooling water pipe nozzle is required to be 3 to 7bar.

2) The content of solid particles in the water, the solid particles in the cooling water in the rolling as abrasive, the adhesive phase in the roller ring (drill or nickel drill) has an abrasive effect, due to the high rolling speed and large rolling pressure, this effect is serious. It will make the microcrack of the groove expand and prolong faster. Therefore, the fluid is required to check the water quality every month to ensure the quality of cooling water.

3) Strengthen spot inspection, stop rolling and repair in the workshop, there must be a special post personnel to check the water pipe, and ensure that the cooling water pipe is on the rolling groove.

Generally speaking, when the micro-crack of the rolling groove reaches 0.2~0.4mm, the line grinding should be carried out, this crack depth is considered to be safer, and the rolling amount is more appropriate at this time, usually the following range:

1, 2 1,800 ~ 2500 tons

Three or four 1,800-2,500 tons

5, 6 1500 ~ 2000 tons

7, 8 1500 ~ 2000 tons

9, 10 600 ~ 1000 tons

Roll cone sleeve and machine assembly should be strictly in accordance with the design process requirements of the precision, the roller and frame roller and cone sleeve between the requirements of suitable fit, can not be too tight or loose. The gap between the alloy of the roller and the steel sleeve should be appropriate so that there is a certain gap between the steel sleeve and the alloy to fill the binder.

During the assembly of the roll cone sleeve and the machine, the assembly surface of the roll, shaft and sleeve should be scrubbed clean with cleaning agent, and the surface should be free of oil, rust and other debris. It is forbidden to knock with a hammer and other heavy objects during assembly to prevent the assembly surface from being damaged.

The cone sleeve must be tested after the production line, and the cone sleeve that cannot meet the requirements should be scrapped. The inspection, processing, storage, loading, dismounting, and grinding of the rolls after unpacking should be placed in a solid wooden box or a large wooden box with a solid partition, one for each box, and carried out. Grinding processing should also be stored separately, not overlapping or contact.

The technical team started from the four aspects of cemented carbide roller ring grade, cooling water, rolled steel amount and roller ring process quality, combined with the site situation to carry out a series of work, formulated very effective control measures, successfully solved the problem of roller ring explosion, and improved the stable operation of production. This method can also provide some technical reference for similar roller ring blasting problems.

In view of the above work, put forward a number of points that need attention:

1) When selecting the cemented carbide roller ring grade, it is necessary to combine geographical location to avoid corrosion of the roller ring.

2) To ensure the quality of cooling water, to avoid abrasion of the roller ring.

3) Control the amount of rolled steel so that the micro-crack depth of the rolling groove is within the safe area.

4) Reasonable use of roller rings, pay attention to maintenance and maintenance.