The high-speed bar production line is the first phase of the new steel wire bar upgrading project. It was officially put into operation in October 2013. The design scale is to produce 1 million tons of qualified hot-rolled steel bars per year. This production line is currently the most advanced domestic high-speed bar production line. The rolling equipment is a fully continuous bar rolling mill with a maximum final rolling speed of 18m/s. There are a total of 18 main rolling mills, which are arranged alternately between horizontal and vertical rolling mills. They are divided into three rolling mills: preliminary, intermediate and finishing rolling mills. Each rolling mill is equipped with 6 rolling mills, and the finishing rolling mill loopers are designed to have 6 loopers. The finishing mill looper plays a very important role in the rolling process of high bars and is an important part of the entire production line. Once a problem occurs, it will not only directly affect product quality and yield, but the entire rolling line will also be forced to stop producing.
1. The composition and working principle of the looper
1.1 Production process flow
The production process of Xingang’s high-speed bars is as follows: raw material acceptance→hot roller table→furnace heating→No. 1 to No. 6 rough rolling mill→No. 1 shear→No. 7 to No. 12 intermediate rolling mill→No. 2 shear→No. 1 live Set → No. 13 finishing mill → No. 2 looper → No. 14 finishing mill → No. 3 looper → No. 15 finishing mill → No. 4 looper → No. 16 flat-to-vertical conversion mill → No. 5 looper → No. 17 finishing mill → 6 No. 1 looper → No. 18 flat-to-vertical conversion rolling mill → No. 3 double-length shear → Step-by-step cooling bed → Cold shear → Point count → Bundling → Collection → Stacking → Shipping.
1.2 Composition of the looper
The looper of the finishing mill is mainly composed of a front pressure roller, a front idler roller, a lifting roller, a rear pressure roller, a rear idler roller and a looper scanner. The schematic diagram of its structure is shown in the figure.
1.3 How the looper works
1-front pressure roller, 2-front roller, 3-lifting roller, 4-rear pressure roller, 5-rear roller, 6-loop scanner
The looper is installed between the finishing rolling stands and is used to control the micro-tension between the two stands to prevent excessive tension between the strips from breaking the strips. The working principle of the looper is that when the rolled piece enters the looper, the scanner sends a signal, starts the cylinder, lifts the looper, and drops the looper after the rolling is completed. During this process, the scanner displays the position of the looper roller and controls the looper. The height of the casing is adjusted, and the information is fed back at any time for adjustment to ensure that the second flow rate of the front and rear racks is equal, thereby improving the dimensional accuracy of the product and improving the rolling accuracy.
2. Causes and preventive measures of finishing rolling looper failure
2.1 The looper roller is slow to set or cannot set.
2.1.1 Fault phenomenon
When the rolled piece passes through the front pressure roller and the front supporting roller and enters the casing roller, the casing roller is slow to casing or simply cannot casing, causing steel accumulation.
2.1. 2 Causes and preventive measures
The main reasons that cause the looper roller to be sleeved slowly or cannot be sleeved are as follows:
1) The signal reception of the looper scanner is not good, the cooling water vapour of the cooling roller is too large, the scanner lens is dirty due to the accumulation of iron oxide chips, and the solenoid valve of the looper cylinder does not receive feedback information from the scanner;
2) The sealing performance of the looper cylinder is not good, and there is air blow-by, resulting in insufficient pressure for the looper to lift;
3) The diameter of the air pipe of the looper cylinder is small, and the instantaneous lifting pressure is insufficient;
4) Due to the moisture in the compressed air, water vapour is brought into the solenoid valve of the looper cylinder, making the solenoid valve insensitive;
5) The solenoid valve core is clogged or the wire package is burned out, and the valve cannot operate;
6) The electrical circuit is disconnected due to poor contact or other reasons, and the set-up signal cannot enter the solenoid valve.
To address the above main reasons, the following preventive measures can be taken:
1) Blow the lens by installing a compressed air pipe, and clean the lens frequently;
2) Regularly replace the looper cylinder, and replace all the replaced looper cylinders with new seals. For the online looper cylinder, use the opportunity of online machine completion to pour a small amount of lubricating oil into the cylinder to increase the piston rod seal and the cylinder. The tightness of the barrel;
3) Improve the cylinder diameter and increase the original cylinder diameter appropriately;
4) Purchase a set of small dryers to dry the gas entering the loop area. At the same time, install a water drain ball valve under the gas main pipe entering the loop and drain the water regularly;
5) Regularly purge and clean the valve core of the loop solenoid valve, and replace the damaged lines to ensure that the solenoid valve is intact;
6) Check the lines regularly and connect the lines with poor contact in time.
2.2 The looper roller is stuck
2.2.1 Fault phenomenon
During the rolling process, when the rolled piece passes through the looper rollers (including the front pressure roller, the rear pressure roller, the front idler roller, the rear idler roller, and the lifting roller), the rollers do not rotate, causing steel accumulation.
2.2.2 Causes and Preventive Measures
The main reasons causing the looper roller to not rotate are as follows:
1) The bearing in the looper roller bearing seat is not lubricated in place, causing the bearing inside to be burned out due to lack of oil and the roller will not rotate;
2) When the rolled piece enters the inside of the looper, the rollers of the looper (especially the front pressure roller and the front idler roller) receive a large instantaneous impact force, causing the bearings in the rollers to receive a large impact force and break;
3) Due to material limitations, when rolling certain specifications of rolled parts, the friction between the roll surface of the looper roller (especially the lifting roller) and the rolled parts is very large, resulting in deep grooves on the roll surface. The rolled piece sinks into the groove so that the roller does not turn and causes steel accumulation.
In response to the above main reasons, preventive measures should be taken as follows:
1) Increase the oil and gas supply of the looper, change the oil supply cycle from 2 times within 35 seconds to 3 times within 30 seconds, and change the oil quality to BP220 to increase the viscosity of the oil and prolong the internal lubrication of the bearing. The oil storage time ensures lubrication efficiency;
2) Since the looper is more affected by radial loads, the original bearings were changed to radial ball bearings, thereby improving the looper’s ability to withstand radial loads and making the bearings less likely to break when impacted;
3) Change the material of the looper roller from ordinary cast iron to medium nickel-chromium cold cast material to increase its wear resistance and hardness.
2. 3 The looper vibrates greatly
2.3.1 Fault phenomenon
The looper vibrates greatly during the rolling process, causing the rolled piece to deviate from the rolling line, causing steel accumulation.
2.3. 2 Causes and preventive measures
The main reasons causing the looper to vibrate are as follows:
1) The looper base bolts become loose after long-term rolling;
2) Since the entire rolling line is only basically aligned, when rolling certain specifications of rolled parts, there will be a certain deviation in the size of the rolled parts, resulting in a certain amount of friction between the rolled parts and the looper body.
In response to the above main reasons, preventive measures should be taken as follows:
1) Tighten all the bolts of the looper regularly, replace some easily broken bolts, and change the material of the bolts from the original grade 8.8 to grade 9.8;
2) Regularly align the entire rolling line to ensure that the error is within the allowable range.
After taking a series of preventive measures against common failures of the new steel high-bar finishing rolling looper, the failure time of the looper was reduced, the number of replacement looper rollers was reduced, the steel pile phenomenon was significantly reduced, and the equipment operation rate was improved, which promoted Stability of product quality.