Key word: Rolling roll, quality control system, rolls
Hot-rolled strip steel is one of the most important steel varieties and is widely used in various fields. With the increasingly severe steel market situation, market competition is becoming more and more fierce, and customer demand for products is also getting higher and higher. The common defects of hot-rolled steel can be divided into surface defects, composition characteristic defects, plate defects, geometric size defects and volume appearance defects. This article studies and summarizes the common quality defects of hot-rolled steel strip products. Through the study of product defects, systematic defect identification and cause analysis are carried out, quality control is carried out for corresponding defects, and necessary preventive measures are formulated.
1 Defect identification and cause analysis
(1) Defect characteristics
The linear mechanical damage on the surface of the steel strip lower than the rolling surface is called scratch. Scratches at high temperature are often gray-black; scratches at room temperature are gray-white with metallic luster. Longitudinal linear defects along the rolling direction; with narrow, clear, and regular boundaries; often distributed along the width direction of the strip and the entire length direction.
(2) Causes and hazards
There are sharp corners in the work roller table, side guide plate, guard plate and other equipment in the steel rolling process or finishing process. The scratches in the steel rolling process are mainly the scratches of the F7 export guard plate flower stand of the finishing rolling and the scratches of the layer cold roller table. The former is often a continuous scratch; the latter is often an intermittent scratch. Hazard: The iron oxide scale on the scratch is difficult to remove by pickling; it may cause bending and cracking.
(3) Prevention and elimination methods
Check the work roller table, side guide plate, guard plate and other equipment in contact with the strip steel to ensure that the surface is smooth and has no edges or corners.
(4) Inspection and judgment
Check with naked eyes;
Slight scratch defects are allowed, but their depth shall not exceed the range specified by the standard .
2 Iron sheet
(1) Iron sheet (without descaling)
This defect covers a large area, the concentration of iron oxide particles inside the defect is not uniform, and the surface around the defect is in good condition. Due to the failure of the descaling water system, high pressure water was not normally injected during the rolling process to wash the surface of the rolled piece, resulting in a large surface The range of metal oxides is milled in.
Causes: Blockage of the descaling faucet leads to a decrease in flow rate; The angle deviation of the descaling faucet leads to poor scouring effect in a certain area; The descaled water on the head is delayed due to the process.
(2) Iron sheet (small boat shape)
This defect is common on the edges, less in the middle, discontinuous, with a large aspect ratio, and looks like a willow leaf. There are dense iron oxide particles in the defect. The oxide extends in the same direction as the rolling direction and has different sizes. The surface around the defect is in good condition, and the distribution position is irregular;
Based on past experience analysis, due to the large aspect ratio and inferring from the degree of oxide particle fragmentation, the defect occurred in the front line; since the defect periphery is in good condition, the possibility of flaking of the oxide film on the surface of the roll is ruled out;
According to the results after pickling in the past, the areas with large oxidized particles and dark colors in the defects are seriously compressed, and attention needs to be paid.
(3) Iron sheet (fine holes)
This defect is diffusely distributed and covers a large area, which is a common oxide scale pore;
There are pinholes in the oxide film of the work roll of the finishing rolling F2-4 stand, the surface of the steel plate is relatively rough, and there are a lot of small black spots. From the effect of pickling, individual spots will also have a certain depth;
After rolling in the next process, the dense pores will show small white bars after rolling, indicating that the defects are difficult to eliminate in the cold rolling process; the determination of defects should not only consider whether they can be cleaned by pickling, but also the degree of density.
(4) Iron sheet (oxidized film)
During the rolling process, the establishment and decomposition of the oxide film are carried out all the time, so we hope to establish a dynamic balance. Of course, this is too ideal.
For the oxide film, both too thick and too thin are unfavorable. What we need is a delicate and stable structure.
(5) Iron sheet (sticky iron)
Adhesive iron defects generally appear at the head and tail parts of the strip. The defects are distributed randomly, with varying degrees of density, and may occur on the upper and lower surfaces;
The sticky iron is mainly the iron oxide particles or dust accumulated in the finishing rolling stand. Due to the poor effect of the finishing rolling FS-7 dust removal facility, it cannot be removed. It adheres to the surface of the steel plate and is rolled into the surface by the rollers. In severe cases, it can be clearly seen Embedded shiny metal particles;
High-pressure water cleaning during shutdown is an effective control measure. This defect is easily confused with the traditional oxide scale defect in the cold rolling area.
(6) Iron sheet (point-like pressing against people)
The defects are meteor-shaped, with a length of about 30mm. They are randomly distributed over the entire length of the upper surface of the strip. The degree of defects gradually aggravates from beginning to end, and the occurrence frequency of silicon steel is relatively high.
Since silicon steel rolling produces a lot of dust after finishing rolling, the dust removal system does not clean the dust during the working process. Part of the dust adheres to the surface of the equipment and mixes with water vapor to fall on the surface of the strip and is produced by the rolling of the rear rack.
(7) Iron sheet (red iron sheet)
Defect characteristics: as shown in the figure below, it is generally reddish-brown, dot-shaped, strip-shaped or flake-shaped, distributed on the entire steel strip, and may occur on the upper and lower surfaces. It is common in special steel grades.
Causes: high silicon content and high slab heating temperature.
Corrective measures: optimize the quality design and reduce the silicon content; control the neutral or micro-oxidizing atmosphere in the furnace; reduce the furnace temperature during failure
3 Edge cracks
The source of the defect is related to the finish rolling guide plate and the coiling guide plate. There are three reasons:
(1) The finishing rolling guide and guard system is in direct contact with the edge of the steel strip, the edge of the steel strip and the guide plate are violently rubbed, and the edge is damaged;
(2) In the later period of use of the coiling side guide, the side surface is rubbed by the strip steel edge to create a groove, and the strip steel edge will be caught on the edge of the groove, thereby breaking;
(3) The edge of some strips is not broken, but not far from the edge, there are pits. This defect will also cause edge tearing during cold rolling; it is speculated that the cause of this defect is the metal produced by the friction between the edge of the strip and the side guide plate. The particles, not all falling off, are crushed by the take-up pinch roller, causing the surface layer to crack.
Mechanism: There are physical damages on the surface of the slab, such as mechanical damage, slab cracks, etc., during the hot rolling process, it cannot be rolled and part of the metal is turned up; or the slab is caused by slag inclusion.
Macroscopic characteristics: generally distributed in a single strip in the length direction, with a fixed position in the width direction and a skin-like shape.
The occurrence of peeling: When the export descaling of rough rolling cannot be put into use, it can only be descaled by population (odd pass). Because of the existence of circular cracks and uneven extension at the tail of the strip, the peeling will occur.
Preventive measures: make full use of red check and other equipment to find out and control the influence of ring-shaped head and tail.
5 Waves (medium waves, bilateral, unilateral waves)
(1) Defect characteristics
The wave-shaped bending phenomenon that shows ups and downs along the rolling direction of the steel strip is called wave-shaped. According to different parts of the distribution, it is divided into middle waves, rib waves and side waves.
(2) Causes and hazards
Causes: unreasonable roll shape, uneven roll wear; poor fit between roll shape and incoming plate shape, excessive rack reduction after finishing rolling; deviation of the steel strip or improper roll gap adjustment; poor roll cooling all;
(3) The temperature of the steel strip is uneven.
Hazard: Affect the appearance quality of the steel plate, and the subsequent procedures cannot be processed and used in severe cases.
(4) Prevention and elimination methods
Reasonable configuration of roll shapes and reasonable replacement of rolls; reasonable arrangement of rolling plans to ensure a reasonable transition of different specifications of strip steel and a reasonable distribution of the reduction of each stand; strengthen the inspection of the wedge difference of the roll gap, and keep the rolled pieces aligned during the rolling process. Moderately good; strengthen roll cooling control; strengthen slab heating and cooling control during rolling to ensure uniform temperature of the steel strip.
6 roll printing
(1) Defect characteristics
The convex and concave defects on the surface of the steel strip with periodic distribution and irregular appearance are called roll marks.
Causes and hazards
The roller is cracked, part of the meat is dropped, or there is foreign matter on the surface. The part of the roller surface is concave or convex. During rolling or finishing, the surface of the pressed steel strip will form convex and concave defects.
Hazard: Convex roll printing may cause folding defects in the subsequent rolling process; concave roll printing may cause holes in the subsequent rolling process.
7 Prevention and elimination methods
(1) Regularly check the surface quality of the roll, and deal with it in time when it is found that the roll has lost meat or foreign matter;
(2) When there are abnormal conditions such as steel jam, tail-flicking, and rolling during the rolling process, check the surface quality of the roll in time to prevent damage to the roll surface or adhesion of foreign matter;
(3) When the roll anilox is found in the inspection, immediately stop the machine to check the surface quality of the roll.
8 concluding remarks
Hot-rolled strip steel products are one of the very important steel varieties, which are widely used in various fields. With the increasingly severe steel market situation and increasingly fierce competition, customers have higher and higher requirements for products. The common defects of hot-rolled steel strip products can be divided into several categories: surface defects, component property defects, plate shape defects, geometric size defects and overall coil appearance defects. A large amount of data shows that at present, more than 70% of the quality objections of hot-rolled steel coils are related to the surface quality of the strip steel. Such as: peeling, roll printing, pitting, iron oxide, etc. It can be seen that how to improve the surface quality of hot-rolled steel coils has a very important impact on improving the market competitiveness of hot-rolled steel.