Types and Causes of Strip Defects in Hot Rolled 2250mm Rolling Mill

2250mm hot-rolling mill of Jingtang Company , the types of defects in the quality of the strip mainly include defects such as tower shape, scale pits , indentations , scratches , and waves ; the analysis and control of the causes of defects are expounded measures .
Key words surface defect type surface defect cause analysis surface quality control quality defect statistics

1. overview

2250mm hot-rolled steel strip production line of Shougang Jingtang Iron and Steel Plant has an annual output of 5.5 million tons of hot-rolled steel coils and 5.48 million tons of finished products , including 1 million tons of flat-cut steel coils and 200,000 tons of pipeline steel coils , which are used as raw materials for cold rolling Steel coil 301 ． 370,000 tons , 126 hot-rolled commercial steel coils . 630,000 tons . _ The annual demand for slabs is 5.67 million tons ; the main steel types produced are : carbon structural steel , high-quality carbon structural steel , steel for boilers and pressure vessels , steel for shipbuilding, steel for bridges, steel for pipelines, weathering steel, IF steel, dual-phase ( DP ) and multi-phase ( MP ) and transformation-induced plasticity steel ( TRIP ) , ultra-fine grain high-strength steel , etc.
After the successful hot test on December 10 , 2008 , the common quality defects in the trial production stage include : tower shape , iron oxide scale indentation , indentation , scratches , wave shape, etc.

1. Defect type and cause

2 . 1 tower
2 . 1 . 1 Influence of tower shape on quality
The tower shape is a few turns or dozens of turns at the head or tail of the steel coil to one side , resulting in a poor appearance of the finished coil , and the edge quality of the steel coil is easily damaged during transportation and lifting . The tower shape is divided into outer tower and inner tower . In the trial production stage , the number of tower shapes is too large , which seriously affects the appearance quality of steel coils .

2 . 1 . 2 Reasons for the tower shape

1. The expansion timing of the coiler shaft is unreasonable .
2. The presetting of the front guide plate of the coiler is not straight , and the head and tail swing .
3. Take-up pinch roll set uneven roll gap or uneven pinch roll wear .
4. The roll gap established by the take-up help roll and the mandrel is uneven .
5. The head and tail of the strip are side-bent and swing when entering the coiler .

2 . 2 oxide scale
2 . 2 . 1 Effect of iron oxide scale on quality Oxidized iron scale forms small pits on the surface of strip steel , which has a greater impact on the surface of strip steel . For cold-rolled substrates , black spots are easily formed on the surface of strip steel after pickling before cold rolling .
2 . 2 . 2 Reasons for iron oxide scale pressing
1 ) The residual oxygen analyzer of the heating furnace has not been put into use , and the oxide scale of the furnace is thick , which is not easy to remove .

1. Part of the descaling water nozzle of the descaling machine is blocked , and the spraying angle of the descaling water nozzle is not suitable , which cannot cover the entire slab surface , resulting in partial descaling in the width direction of the strip .
2. Because the sizing machine hammers the secondary oxide scale generated on the side of the slab into the slab , the iron scale embedded in the side cannot be removed when the slab is descaled in the first pass of R1 , thus causing the oxide scale to be pressed into the surface of the strip .
3. Due to the inaccurate material tracking system , the control timing of the opening and closing of the descaler is unreasonable , so that the descaling effect of the head and tail of the slab is not good .
4. The unreasonable height of the descaling nozzle from the slab surface leads to poor descaling effect .
5. The reason for the descaling pump is that the descaling water pressure is low , or the descaling effect is not ideal because the descaling nozzle is too large . 2 . 3 indentation 2 . 3 . 1 The impact of indentation on quality Indentation is the formation of periodic pits or protrusions on the surface of the strip .

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The surface quality of the strip steel has a great influence . When such quality defects occur , it is necessary to stop the car for inspection and find out the cause of the defects before proceeding to production .
2 . 3 . 2 Causes of indentation

1. There are foreign objects on the finished work rolls of the rolling mill .
2. There is foreign matter stuck to the take-up pinch roller .
3. There is foreign matter on the pinch roller before winding .
4. Sampling equipment There is foreign matter on the pinch roller or straightening pinch roller or reverse pressure roller or on the idler roller .

2 . 4 scratches
2 . 4 . 1 scratched state
Scratch is the formation of thinner and shallower scratches on the surface of the strip , one or several streaks appear continuously or intermittently along the rolling direction , and the scratches sometimes appear metallic luster , sometimes without luster , and are dark stripes .
2 . 4 . 2 causes

1. There are protruding foreign objects on the output roller table , and there are non-rotating rollers on the roller table .
2. After the pinch roll is aged , it is easy to form scratches on the surface of the strip .
3. The guide plate on the front side of the pinch roller is severely worn , and the scraped iron filings fall on the surface of the strip .
4. After the winding aid roll is aged , it is easy to form scratches on the tail of the strip .
5. There are foreign objects on the rollers of the unloading trolley , which cause scratches when the tail is fixed .

2 . 5 waves
2 . 5 . 1 Wave shape The cause of wave shape is uneven deformation . Common wave shapes include unilateral waves , medium waves , double-sided waves , and ribbed waves .
2 . 5 . 2 causes

1. If the positive roll bending force of the rolling mill is too large or too small, medium waves or double-sided waves will be generated .
2. the CVC roll value is too large or too small, medium waves or double-sided waves will be generated .
3. The unilateral wave is caused by the level error of the roll .

1. Ribbing is caused by uneven wear of the work rolls due to the aging of the backup rolls .
2. The learning of the shape control model is abnormal .

6 ) After rolling too much narrow material , the wide material is rolled without changing the roll .

1. control measures

3 . 1 control tower shape

1. Ensure that the temperature of the center and edge of the slab is uniform when it comes out of the furnace , try to keep the slab staying at the furnace door for less than 10 minutes, and ensure a slight positive pressure in the furnace to avoid excessive temperature difference between the driving side and the operating side of the slab due to cold wind back suction . Therefore, in the rolling deformation process , uneven deformation is easy to occur , and the head is bent sideways , so when the strip steel enters the coiler , it forms an internal tower shape .
2. Ensure that the guide plate on the entrance side of the roughing mill is well centered , avoid rolling the slab off the center line , and ensure that the work rolls of the roughing mill are properly leveled . For the camber found during rolling in the roughing mill , it is necessary to correct the deviation in the next pass in time to ensure the supply of qualified intermediate slabs for finishing rolling .
3. Ensure the normal function of the finish rolling side guide plate , ensure the normal function of the finish rolling work roll leveling , reduce the probability of uneven deformation , improve the leveling ability of the operator , and avoid camber .
4. Appropriate clamping force is adopted for the side guide plate of the coiler , and different clamping force control should be adopted for different steel grades and different specifications . The operator adopts the appropriate opening and closing timing of the side guide plate , and sets the appropriate guide plate opening value from the second level .

3 . 2 Control of iron oxide scale

1. Use the residual oxygen analyzer of the heating furnace to detect the oxygen content in the atmosphere in the heating furnace , control the residual oxygen content below 3% , reduce the thickness of the oxide scale , and facilitate removal .
2. Try to control the time of slab in the furnace as much as possible . The time of cold slab in the furnace is controlled at 160min ~ 180min. Take 600°C as an example for hot - loading slab , control it at about 120min . be warm .
3. Strengthen the maintenance of descaling equipment , and use the weekly maintenance period to check and clean the descaling nozzles to ensure that the descaling water pressure is normal . Carry out a target test on the descaling machine every two weeks to check the descaling effect and coincidence of the descaling nozzle .
4. Adjust the finish rolling exit temperature and coiling temperature , and under the premise of ensuring performance , reduce the finish rolling entrance and exit temperature as much as possible , and control the amount of secondary oxide scale in the process as much as possible .

3 . 3 control indentation

1. In the process of producing thin-gauge strip steel , if there is a rotten end in the rolling mill , pull out the work roll of the rolling mill where the rotten end has occurred , check the surface of the work roll , and enter the frame to check the surface of the backup roll . Replace the work roll , if the back-up roll is damaged , grind or replace it according to the situation .
2. According to the periodicity of the indentation, calculate the diameter of the roll that produces the indentation , check the corresponding coiling pinch roller , pressing roller , and winding helper roller , and use oil stone and angle grinder for grinding after discovery .
3. Check the idler and pinch rollers on the inspection line to ensure that there are no foreign objects on the surface of the idler and pinch rolls , so as to avoid indentations on the upper surface of the strip during uncoiling inspection .

3 . 4 control scratches

1. The dead roll on the output roller table before the coiler is the main area for scratches , especially the roller table under the side guide plate of the coiler . The roller table is easy to rub against the side guide plate , causing the roller table to rotate ineffectively . produce scratches . Strengthen the maintenance of the output roller table , ensure that the dry oil is well lubricated , and deal with it in time if there is a jamming situation .
2. The coiling front guide plate is pressure controlled , so the coiling side guide plate should be replaced regularly , generally every 24 hours , to avoid grooves formed on the edge of the side guide plate and scratches on the edge of the strip steel .
3. When the strip steel enters the coiler , the head impacts the pinch roller and the auxiliary winding roller . The thick and hard strip steel is easy to damage the surface of the pinch roller and the auxiliary winding roller . The rollers should be inspected and repaired during the roll change time .
4. Strengthen the maintenance of coil unloading equipment and sampling equipment , and use roll change

During inspection and grinding , ensure that the surface of the roller has no protrusions.
Figure 2 Histogram of various quality defects in March 2009
3 . 5 control wave shape
1 ) To improve the operation level of the finish rolling operator , the finish rolling operator can adjust the roll bending force through the industrial TV , and can also input parameters in the PCFC shape control screen to control the middle wave and double side wave of the strip .

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Figure 3 Pie chart of various quality defects in February 2010
2 ) Optimizing the shape control model , using the roll changing period , measuring

Roll temperature , roll thermal crown calculation in the optimization model .
3 ) Reasonably arrange the rolling plan , try to arrange the rolling plan according to the width - narrow arrangement , and avoid the narrow - width - narrow arrangement , so as to make full use of the rolls and reduce the width steel rolling after the rolls appear stepped wear , which will cause Bad board shape .

1. Conclusion In February of the year, the tower shape of the steel coil was 18 ． 85% reduced to 0 . 9% ; Iron oxide sheet is pressed in by 15 . 57% reduced to 1 ． 2% ; scratched by 8 . 2% down to 0 . 7% ; indentation by 9 . 02% reduced to 0 ; wave shape from 15 . 57% down to 0 . See Figure 2 , 3 .

Practice has proved that the analysis of the causes of several defects that affect quality and the improvement measures are correct and effective . With the further deepening of production and the continuous expansion of varieties and specifications , quality problems will be tracked at any time to ensure the production of high-quality hot-rolled strip products .
references

1. Lin Lianghuai , Xie Jinsong . Reducing Tower Shape Improvement Heat for Rolling Thin Gauge Coils

Coil Appearance J ] . Metallurgy Series , 2004(3 ) : 21-22

1. Yu Qian , Yang Jian , Yu Wei , Wang Lun , Li Ruiheng . Causes and preventive measures of hot-rolled strip coiling tower J ] . Rolling Steel , 2007,24(3 ): 59-60
2. Xia Xianping , Sun Yezhong . Scale defects on the surface of hot-rolled strip steel in the finishing rolling area

Causes and Prevention D. Shanghai Institute of Metallurgy, Chinese Academy of Sciences , 2002

1. Qi Huibin , He Xiaoming , Qian Yuhai , Zhang Hua , Xu Jiachun . Oxygen in hot-rolled strip

Types and Causes of Melting Defects C. Proceedings of the Seventh ( 2009 ) China Iron and Steel Annual Conference . 2009
( Received date : 2011 -03 -23 )

Industry dynamics
Ansteel's wire rod water cooling technology fills the gap in China
The first domestic " high-speed wire rod online water bath toughening treatment ( EDC ) technology " has recently been successfully applied in the wire rod factory of Anshan Iron and Steel Co., Ltd. , and the first batch of nearly 60t water-cooled products has been sent to customers . This technology fills a gap in domestic high-speed wire production technology .
At present , when wire rod manufacturers conduct online cooling for wire products , most of them use air cooling method . The disadvantage of this method is that it easily leads to low tensile properties of products , poor strength and toughness , and seriously affects the stability of product performance after drawing. . At the same time , the surrounding environment will be polluted when customers reprocess and do lead bath quenching . Anshan Iron and Steel Co., Ltd. wire rod factory decided to adopt water cooling technology for wire rod cooling , and successfully realized the mass production of online water bath toughening processing wire rod in late June this year . Although the online water bath toughening treatment technology uses water cooling , but because it uses clean circulating water , the fresh water consumption per ton of steel will not increase ; the wire products after water cooling no longer need lead bath treatment , and the " green " will be transmitted to downstream users . and reduce costs .

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