Cause Analysis and Countermeasures of Contusion in 410S Recoiling

Aiming at the quality problems of contusion defects often occurring in the 410S stainless steel produced by the 1 780 mm production line of the stainless steel division when the tempering unit is recoiled, the causes of contusion defects are analyzed from the aspects of technology, equipment, raw materials and measurement. Combined with the characteristics of the 1 780 mm skin-passing unit, the splicing operation mode of the inch uncoiler, the uncoiler tension parameters, the operation mode of the tension roller in the tension unit, and the working pressure of the uncoiler oil cylinder were adjusted and applied to actual production to effectively control 410S stainless steel surface contusion defects.
Key words: 410S ; contusion; unwinding; recoil parameters; skin-pass decoiler

410S is the ferritic stainless steel (containing a small amount of martensite) with the lowest chromium content. It has good plasticity, toughness and cold formability, and is superior to other ferritic stainless steels with higher chromium content [ 1 ] . In recent years, due to its good performance and relatively low production cost, it has been widely used in the civil stainless steel product industry, and it has also brought competition among stainless steel manufacturing industries. In order to further reduce costs, users have higher and higher requirements for the surface quality of 410 S.
Due to the characteristics of 410S stainless steel hot-rolled production, some quality defects of hot-rolled coils must be repaired and re-coiled to meet user requirements. However, 410S stainless steel often produces strip surface contusion defects during rework and recoiling in the skin pass process. Therefore, research on how to reduce the tail strip contusion problem during rework and recoiling of 410S is of general significance for improving the surface quality of steel coils and reducing rework costs.

1. Overview of leveling unit

The 1 780 mm hot-rolled strip skinning and coiling unit mainly performs skinning operations on high-quality carbon structural steel, weathering steel, ordinary carbon structural steel and low-alloy steel at room temperature to improve the surface quality of the plate, improve the shape and mechanical performance; also carry out recoiling operations such as repairing, sampling, and coiling of carbon steel at normal temperature to improve the coil shape and cut off the parts that are inconsistent with the quality of the steel coil in hot rolling production to meet the contract requirements, and also to meet the needs of different users for steel coils According to the different requirements of weight, the annual processing capacity of steel coils is designed to be 790,000 tons .

1. Morphological characteristics of contusion defects

Since the 1780 mm hot rolling production line was developed to roll 410S stainless steel varieties in 2005 , about 10% of the hot rolled 410S steel coils cannot meet the factory requirements. For this reason, the 410S stainless steel rework and recoil operation in the skin pass unit has been developed , mainly focusing on coil shape treatment and edge quality defect treatment. According to user feedback and on-site follow-up investigation, it is found that surface contusions are often found at about 100 m from the tail of 410S strip steel . From the appearance point of view, the defects are concentrated in the middle of the plate surface, both on the upper and lower surfaces. The change of the coil diameter and the defect cycle also change, which seriously affects the output of the reworked qualified products of this steel type.

1. Mechanism of Contusion Defects

For steel coils, contusion refers to the damage caused by relative movement where there is a gap between layers of steel coils under the joint action of uncoiling tension, coiling tension and unit operation. In order to illustrate this defect visually, Call it a contusion defect. This kind of defect exists on the upper and lower surfaces of the tail of the strip, and is mostly concentrated in the middle of the strip.
When the tempering unit is reworked and recoiled, the steel coil is first introduced into the uncoiler, and then the strip threading operation is carried out, and the strip is guided from the uncoiler to the coiler. Set the corresponding tension to meet the job requirements. During production, the set value of the uncoiling tension remains basically unchanged. Because the inner ring of the steel coil is unwound, there is a gap between the layers, and this part of the strip will move under the level of tension. There are contusion defects on the surface of the strip steel plate due to unevenness.

Microscopically, due to the certain convexity of the board surface, which is thick in the middle and thin on both sides, the middle part of the strips is in close contact. When the distance reaches the distance of mutual attraction between the molecules, the middle of the strips will stick together. During the production process, due to the difference in the strength of the strip steel, those parts with lower strength on the leveling decoiler are pulled off, resulting in contusion defects, which is the reason why the contusion defects are severe in the middle and less or absent at the edges.

1. Causes of contusions According to the principle of quality management, the causes of contusions are analyzed from the aspects of man, machine, material, method, and measurement. Combined with the mechanism of contusions and the actual tracking situation on site, the causes of contusions are made as follows analyze.

4 . 1 Process
There are two main process parameters for rework and recoiling of 410S stainless steel in the skin pass unit: tension and speed. The tension value is the condition to keep the strip straight and provide guarantee for the automatic width control device of the temper mill in production. It is an important parameter to ensure the quality of the recoiling coil. Generally, it is determined by the specification and strength of the strip. At the same time, it is also the driving force for the formation of interlayer frustration in the strip steel production process. On the one hand, the speed is the guarantee to increase the production capacity of the skin-passing unit, and it also increases the kinetic energy of the steel coil on the uncoiler. These two parameters are mainly determined by the capacity of the skin-passing equipment.
4 . 1 . 1 Analysis of uncoiling process The entrance steel coil trolley transports the steel coils that have been trimmed at the preparation station to the uncoiler, and automatically completes the alignment of the width and height of the steel coils with the help of the grating and the position sensor on the trolley. After the steel coil is sent to the uncoiler reel, the support bearing is put in, and the uncoiler reel moves laterally into the support bearing. This kind of lateral movement is also used to adjust the center of the strip steel. At this time, the reel of the uncoiler expands to fix the steel coil on the reel, the pinch roller presses down, presses the outer ring of the coil, and the steel coil trolley descends and exits. During this process, the expansion of the uncoiler drum cannot guarantee to eliminate all the gaps in the inner ring, and can only fill the 2~5 rings close to the drum. However, the on-site tracking found that there is no contusion defect in the 2~5 rings near the drum, so Failure to take full advantage of the expansion of the reel is one of the main causes of contusion defects.
4 . 1 . 2 Tension cutting mode When the strip steel is threaded and the jaws of the coiler reel bite into the tape head, it will stop coiling on the coiler drum for 1~2 circles. At this time, the strip threading operation is completed, press The size of the steel specification is provided by the upper computer of the computer to provide the uncoiling tension setting parameters. After the uncoiling tension is put in, the operating conditions of the unit are established, and the tension setting value of the coiler is also automatically given by the computer according to the uncoiling tension value. The size is the uncoiling tension value. 2 times . Frustration is most likely to occur at the moment of putting in the tension, because when the set tension is put in, the steel coil is subjected to two opposite moments at the same time, that is, the inner ring of the loose coil is relatively moved under the simultaneous action of the uncoiling tension and the coiling tension. Produce contusion defects. Therefore, one of the reasons for the bruising defect is that the tension roller of the skin pass mill does not carry out the tension cutting operation before applying the tension.
4 . 1 . 3 leveling speed
During the production process, the operating speed gradually increases to the maximum value, and as the speed of the unit increases, the contusion range between the strip layers will also gradually increase. This is because during the running process of the strip steel, its mass remains unchanged, and the speed is increased, which increases the kinetic energy of the strip steel, so that the internal energy transformed by the friction between the relatively moving layers increases, which intensifies the layer friction of the strip steel. friction. Therefore, the size of the speed is also one of the reasons for the degree of strip contusion.
4 . 2 equipment aspects
The expansion and contraction range of the tension decoiler reel of the 1 780 mm leveling unit is # 680 ~ 790 mm , and the design maximum working pressure of the oil cylinder of the uncoiler reel is 12.5 MPa . The damage is relatively large, and oil leakage or other equipment damage often occurs. Therefore, the working pressure is lowered to 8.0 MPa , which basically solves the problem that the reel equipment of the uncoiler is fragile. But this also reduces the expansion and contraction force of the uncoiler reel, so that the uncoiler's ability to tighten the steel coil during the expansion and contraction process is reduced, which is also prone to contusion defects.

1. 3 raw materials
2. Inner ring loose coil staggered layer

of 410S hot rolling is about 800T , after cooling to normal temperature, the temperature is 60T for repair . Due to thermal expansion, the strip itself has a certain shrinkage, which causes a certain unwinding between the strip layers. The loose coil phenomenon of the inner ring of the steel coil (which is the hot-rolled head) is the root cause of the contusion defect. Because of the unwinding, when the tension is put in, there will be relative friction between the layers. Due to the roughness of the two surfaces in contact with each other, friction will occur, and as the speed gradually increases, the degree of dampening defects will gradually increase.

1. Strip surface finish

410S stainless steel is relatively "soft " in steel , and there will be some dust particles on the surface of the strip steel in the hot-rolled state. In addition, some stainless steels have edge wire defects, and the fallen edge wires stay in the gaps in the uncoiled part, so the upper During the rework operation of the tempering machine, some hard particles become the source of contusion defects.

1. 4 Judgment on the formation of on-site contusion

Based on the experience accumulated in the field, the method of measuring whether the contusion defect occurs is summarized. Draw a line on the steel coil on the walking beam at the entrance of the tempering machine, and observe whether there is a shift between the layers of the steel coil according to the deformation of the line.

5 410S rework and recoil anti-damage countermeasures

1. 1 Adjustment of the splicing mode of the uncoiler When the strip steel is transported from the coil trolley to the coiler, the operation

The personnel first convert the uncoiler from automatic to manual, and then operate it so that the reel of the uncoiler expands and rotates forward at the same time. When the reel expands and rotates forward to drive the outer ring of the steel coil to rotate, stop the operation immediately and operate the coil The barrel retracts back to its original position. Repeat the above operation 3 to 5 times to wind up the steel coil layer by layer.

1. 2 Tension value parameter adjustment

On the main operation panel of the tempering machine, the uncoiling tension fine-tuning increase and decrease buttons are newly added. Each time the increase button is pressed, the tension increases by 5 kN , and each time the decrease button is pressed, the tension decreases by 1 kN . Before threading, set the initial value of uncoiling tension to 20 kN. After the initial tension is established, gradually increase the tension to the tension setting value issued by the upper-level computer to avoid instantaneous application of large tension and increase the size of the steel coil. Potential for contusion defects.
5.3 Adjustment of the operation mode of the tension roller in the tension unit
When the strip threading is completed, the jaws of the coiler will bite the head of the strip steel, and stop after coiling 2~3 laps under slight tension. Before the unit puts in the tension, the tension roller is first pressed down to adjust the uncoiling tension and the coiling tension. Put in the initial tension after cutting, run in linkage to the center position of the strip steel, then adjust the tension to the set value according to the tension application method, and then start the operation.
5 . 4 Uncoiler oil cylinder pressure adjustment
In order to take into account the damage to the equipment in the uncoiler area due to excessive oil pressure and improve the coiling ability of the uncoiler on the inner ring of the steel coil during the splicing process, the working pressure of the oil cylinder of the uncoiler will be appropriately increased after consultation with the equipment maintenance personnel.
After adjusting according to the above method, a group of 410S stainless steel was followed up and tested, and the results are shown in Table 1 . From the test results in the table, it can be seen that the adjustment by the above method can effectively control the occurrence of contusion defects.

Table 1 A group of 410S stainless steel (width 1 250 mm ) test contusion tracking situation

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