Surface defects generated during the hot rolling process, if not properly handled, will be directly inherited by the cold-rolled steel plates, greatly affecting the surface quality of the cold-rolled plates. This article summarizes several common defects and preventive measures of hot rolled steel plates.
1. Defects of hot rolled steel plates
(1)Oxide Scale Inclusion
①Definition
The inclusion of oxide scale is the most common surface defect in hot rolling. The defects usually appear as small spots, fish-scale patterns, strips, or lumps, irregularly distributed on the entire or partial upper and lower surfaces of the steel strip, with some defects being loose and easy to fall off. Some are pressed into the plate surface, and after pickling or sandblasting treatment, different degrees of pits appear. The microstructure of the oxide scale inclusion defect is uneven and broken, and the boundary of the defect is not obvious, with the composition at the defect site being Fe and O.
Oxide scale inclusion defects are further divided into primary oxide scale inclusion and secondary oxide scale inclusion. Primary oxide scale inclusion is due to the production of severe oxide scale during the heating of the slab, which is not completely removed before rough rolling and is pressed into the plate surface during rolling. Secondary oxide scale inclusion is formed when secondary oxide scale is pressed into the surface of the steel strip during finishing rolling.
②Preventive Measures
The main measures to eliminate hot rolling oxide scale inclusion are to increase the descaling pressure during hot rolling, improve the descaling effect, and optimize the heating and rolling systems.
Since hot rolling oxide scale inclusion defects will be inherited by the cold-rolled plate surface, once hot rolling oxide scale inclusion defects are detected, the pickling time should be appropriately increased to completely remove the oxide scale. In this way, during the cold rolling process, due to the three-dimensional flow of the metal, especially the metal flow in the thickness direction, the convex and concave pits that appear in the matrix after the oxide scale is pressed in can be repaired, flattening and welding the defects, and ultimately eliminating them. Once hot rolling oxide scale inclusion steel plates are under-pickled, the residual oxide scale does not have plasticity at room temperature and does not undergo plastic deformation with the matrix. This will lead to defects deep inside the inclusion of oxide scale, affecting the surface quality of the steel plate.
(2)Pockmarks
①Definition
During the hot rolling process, the working rolls of the finishing mill on one hand come into contact with the backup rolls, forming contact fatigue, and on the other hand, they come into contact with the rolled piece, forming friction wear. Moreover, due to the action of cooling water, the working rolls are also subjected to repeated thermal cycling. This leads to thermal fatigue, gradually roughening the surface of the working rolls. When the rough working rolls come into contact with the rolled piece, if the surface of the rolled piece has a thick layer of tertiary oxide scale, it will cause the scale to break and form tertiary oxide scale inclusion, also known as pockmark defects. The microstructure of pockmark defects is uneven and broken.
②Causes and Hazards
Generally, pockmark defects are prone to occur in the first three stands of the finishing mill, mainly because the surface temperature of the first three stands is high, leading to the rupture of the oxide film on the surface of the rolls. Once the number of pockmark defects increases significantly, the pickling speed in the cold rolling continuous pickling line must be reduced, which will lead to a loss of pickling capacity.
③Preventive Measures
The occurrence of pockmark defects is directly related to the chemical composition of the steel, rolling temperature, and the surface condition of the finishing mill rolls. Generally, when the Si content in the steel exceeds 0.05%, a 2FeO·SiO2 layer will form on the surface of the steel strip, which significantly improves the prevention of steel oxidation, making it less likely for the oxide scale to break during rolling. The main measure to eliminate pockmark defects is to increase the Si content in the steel to above 0.05% while meeting the standard requirements, or to lower the temperature at the entrance of the finishing mill, and improve the surface condition of the finishing mill rolls, etc.
(3)Bright Bands
①Definition
Hot rolling bright bands are a common defect in hot-rolled steel strips and occur on most hot rolling production lines. Minor bright bands generally have little impact on the normal use of the steel strip, but when severe, they can also affect the cold rolling process. Bright bands appear as high gloss and circumferential protrusions in the coiling state. After the coil is unrolled, severe bright bands will show slight wavy defects.
②Occurrence Location
Bright bands generally occur on soft steel thin gauge steel strips, mainly at the tail of the steel strip, with a few occurring at the head. Bright bands appear simultaneously on the upper and lower surfaces of the steel strip, with completely corresponding positions. However, the gloss of the bright band on the lower surface is slightly higher than that on the upper surface, and the width is also greater than that on the upper surface.
③Formation Reasons
Through production practice and inspection analysis research, it has been concluded that the bright band defect is caused by the local contact between the steel strip and the coiling tension roll during the coiling process. Under the combined action of the tension roll pressure and the tension between the tension roll and the coiling cylinder, the steel strip in the contact area undergoes plastic deformation.
④Preventive Measures
Controlling the flatness accuracy of the steel strip during production, reducing the tension between the tension rolls and the coiling cylinder, and shortening the usage cycle of the tension rolls can prevent the occurrence of bright band defects.
(4)Scratches
① Formation Reasons
Scratch defects can occur during both hot rolling and transportation processes. If foreign objects adhere to the working rolls or coiling machine gripper rolls during the rolling process, scratch defects will occur. During hot rolling roller transportation or coiling and uncoiling, due to the relative motion between the rolled piece and mechanical equipment parts, it is also easy to produce. This defect has a periodic nature and is easy to identify in the hot rolled state.
②Appearance
Due to the high temperature, most hot rolled scratch defects will undergo secondary oxidation, and the defect surface appears iron skin-colored. If the hot rolled scratch defect is deep, it will be inherited to the cold rolled plate. After pickling and rolling deformation, the shape of the defect will change, and it is easy to be confused with other cold rolled defects macroscopically.
③Preventive Measures
The measures to solve scratch defects are very simple, which is to strengthen equipment maintenance and immediately stop production for investigation as soon as defects are found.
(5)Holes
①Definition
Hole defects are easily identifiable to the naked eye and can be observed on both hot-rolled and cold-rolled plates. On hot-rolled plates, hole defects are characterized by various shapes, appearing individually or in groups as pits without peeling characteristics. The depth of the pits varies, especially at the edges of the steel plate where they are more likely to occur. On cold-rolled plates, hole defects appear as a series of holes of different sizes, with uneven deformation at the hole sites; generally, the position of the hole on the cold-rolled plate corresponds to the position of the pit on the hot-rolled plate.
②Formation Reasons
The cause of hole defects is due to the rolling of silicon steel and other products with poor plasticity during the hot rolling finishing stage. During the process of the steel plate head passing through the roll, rolling, and tail leaving the roll, there is an impact of the side of the steel plate hitting the side guide plate of the finishing mill. The impact splashes fall on the surface of the steel plate and are pressed into the steel plate in the subsequent frame. Because the rolled piece is scratched at high speed, the falling foreign objects can splash on different parts of the surface of the steel plate, especially at the edges. When the overheated falling objects are pressed onto the steel plate, they cause pits of various shapes. These pits, varying in number, become holes of different sizes after subsequent cold rolling.
③Preventive Measures
Eliminating hole defects can be achieved by improving rolling stability, avoiding steel plate deviation during rolling, increasing side guide plate lubrication, and grinding and maintaining the side guide plate before production, which has a significant effect on the prevention of hole defects.
2.Hot Rolled Steel Strip Oxide Scale Defect Causes
(1)Causes of Generation
The main reasons for the presence of oxide scale inclusion defects on the surface of hot-rolled steel coils are:
①The slab is kept in the heating furnace for too long, resulting in too thick oxide scale on the surface of the slab, which is not easy to remove;
②The descaling water pressure is low, or the nozzle is blocked, aged, and falls off;
③The descaling sequence is inappropriate, resulting in unclean descaling at the head and tail of the steel coil;
④Due to the small angle of the finishing water beam, the water flow overlap is small, reducing the distance between the surface of the slab and the water beam, resulting in a blind area of descaling at the overlap, thus causing strip-like oxide scale inclusion defects at the head;
⑤When the roll usage is high and the wear is severe, it will cause "salt and pepper" oxide scale residue on the surface of the steel strip.
3. Preventive measures for hot-rolled steel plate defects
(1)Optimize the slab heating system and formulate a reasonable slab waiting rolling insulation system.
In response to the process requirements of cold-rolled materials, combined with the mechanism of oxide scale formation, a reasonable cold and hot slab heating system has been established. The furnace time for cold slabs is 180-200 minutes, and the uniform heating section temperature is (1250±20) ℃; the furnace time for direct-loaded slabs is 120-140 minutes, and the uniform heating section temperature is (1250±20) ℃. When the rolling line fault exceeds 10 minutes, it must be cooled down at a rate of 2℃/min according to the slab waiting rolling insulation system, and after the rolling line is ready, the heating furnace is heated again to prevent the furnace temperature from being too high, which would thicken the oxide scale and make it difficult to remove the rough descaling. The optimized slab heating temperature needs to ensure that the temperature at the entrance of the finishing mill is within the range of 990-1020℃, and at the same time, the heating temperature should be uniform, with a head-tail temperature difference not greater than 20℃.
(2)Optimize the descaling process
Modify the descaling pump acceleration point from the original 19.3MPa to 21.0MPa to ensure the descaling system pressure; both the post-furnace and pre-finishing descaling can use double descaling beams. At present, the post-furnace and pre-rough descaling pressures have reached 21-22MPa, and the finishing descaling pressure has reached more than 21MPa. Two pumps are put into use in the post-furnace, pre-rough, and pre-finishing descaling systems, and the rolling rhythm is controlled to avoid the descaling system pressure dropping after multiple-point descaling; adjust the descaling sequence, increase the head descaling opening advance and tail delay closure to avoid the phenomenon of two points (including two points) descaling at the same time, causing unclean head and tail descaling problems; appropriately reduce the post-furnace descaling speed from the original 1.5m/s to 1.3m/s, and increase the unit area striking water volume.
(3)Stabilize equipment operation to ensure descaling effect
Use maintenance to conduct descaling striking force tests, check the overlap of descaling machine nozzles to ensure the descaling effect along the width direction of the steel strip; each maintenance checks and replaces the post-furnace, rough, and finishing descaling nozzles, and the small class uses roll replacement to check the descaling nozzles, and uses low-pressure water to check the nozzle blockage, and replaces the nozzles with problems; strengthen the maintenance of descaling pumps, descaling nozzles, and descaling motors, and take a person responsible for the inspection system of roll cooling water nozzles to ensure that the working roll cooling water is normally invested; ensure that the rolling lubricating oil is normally used according to the usage regulations. To ensure good surface quality of the working rolls.
(4)Formulate a reasonable roll replacement cycle and side guide plate inspection system
Aging working rolls will form oxide scale on the surface of the steel strip. Therefore, a reasonable roll replacement cycle has been established, and the finishing working roll replacement cycle has been adjusted from the original 2300-2500t to below 1500t. And the working roll cooling water nozzle is checked three times a day, and the water nozzle is dredged in time when blockage is found to ensure that the working roll works under good cooling conditions; when replacing the side guide plate lining plate, it is necessary to adjust the height to ensure smooth transition, and there is no excessive wear to form steps. If excessive wear steps are found, they must be replaced in time to prevent the steel strip from being scratched and forming oxide scale.
(5)Hot Rolled Steel Strip Scab, Pitted Surface, Bubble and Other Surface Defect Control Measures
Hot rolled steel strip surface has scab, pitted surface, bubble and other surface defects. In order to eliminate these defects, it is necessary to consider comprehensively from the three processes of smelting, pouring, and rolling. The following measures are taken in production:
①Strictly implement the smelting and pouring process system, bake the raw materials, steel ladle, intermediate ladle, etc., strictly implement the deoxidation system, reduce the subcutaneous bubble defects of the continuous casting slab, and the continuous casting slab with severe bubbles should be scrapped and not allowed to enter the next process;
②Stabilize the pulling speed to ensure the stability of the intermediate ladle and the crystallizer liquid surface, avoid the formation of slag inclusions on the surface of the continuous casting slab during pouring, ensure that the end cutting of the continuous casting slab is neat without burrs and no flame cleaning residue to prevent scab defects;
③ During rolling, strictly implement the heating temperature control system, use high-pressure water descaling in both rough and finishing rolling, check the descaling water pressure and descaling nozzle to prevent pitted surface defects caused by low water pressure or nozzle blockage.
1. Defects of hot rolled steel plates
(1)Oxide Scale Inclusion
①Definition
The inclusion of oxide scale is the most common surface defect in hot rolling. The defects usually appear as small spots, fish-scale patterns, strips, or lumps, irregularly distributed on the entire or partial upper and lower surfaces of the steel strip, with some defects being loose and easy to fall off. Some are pressed into the plate surface, and after pickling or sandblasting treatment, different degrees of pits appear. The microstructure of the oxide scale inclusion defect is uneven and broken, and the boundary of the defect is not obvious, with the composition at the defect site being Fe and O.
Oxide scale inclusion defects are further divided into primary oxide scale inclusion and secondary oxide scale inclusion. Primary oxide scale inclusion is due to the production of severe oxide scale during the heating of the slab, which is not completely removed before rough rolling and is pressed into the plate surface during rolling. Secondary oxide scale inclusion is formed when secondary oxide scale is pressed into the surface of the steel strip during finishing rolling.
②Preventive Measures
The main measures to eliminate hot rolling oxide scale inclusion are to increase the descaling pressure during hot rolling, improve the descaling effect, and optimize the heating and rolling systems.
Since hot rolling oxide scale inclusion defects will be inherited by the cold-rolled plate surface, once hot rolling oxide scale inclusion defects are detected, the pickling time should be appropriately increased to completely remove the oxide scale. In this way, during the cold rolling process, due to the three-dimensional flow of the metal, especially the metal flow in the thickness direction, the convex and concave pits that appear in the matrix after the oxide scale is pressed in can be repaired, flattening and welding the defects, and ultimately eliminating them. Once hot rolling oxide scale inclusion steel plates are under-pickled, the residual oxide scale does not have plasticity at room temperature and does not undergo plastic deformation with the matrix. This will lead to defects deep inside the inclusion of oxide scale, affecting the surface quality of the steel plate.
(2)Pockmarks
①Definition
During the hot rolling process, the working rolls of the finishing mill on one hand come into contact with the backup rolls, forming contact fatigue, and on the other hand, they come into contact with the rolled piece, forming friction wear. Moreover, due to the action of cooling water, the working rolls are also subjected to repeated thermal cycling. This leads to thermal fatigue, gradually roughening the surface of the working rolls. When the rough working rolls come into contact with the rolled piece, if the surface of the rolled piece has a thick layer of tertiary oxide scale, it will cause the scale to break and form tertiary oxide scale inclusion, also known as pockmark defects. The microstructure of pockmark defects is uneven and broken.
②Causes and Hazards
Generally, pockmark defects are prone to occur in the first three stands of the finishing mill, mainly because the surface temperature of the first three stands is high, leading to the rupture of the oxide film on the surface of the rolls. Once the number of pockmark defects increases significantly, the pickling speed in the cold rolling continuous pickling line must be reduced, which will lead to a loss of pickling capacity.
③Preventive Measures
The occurrence of pockmark defects is directly related to the chemical composition of the steel, rolling temperature, and the surface condition of the finishing mill rolls. Generally, when the Si content in the steel exceeds 0.05%, a 2FeO·SiO2 layer will form on the surface of the steel strip, which significantly improves the prevention of steel oxidation, making it less likely for the oxide scale to break during rolling. The main measure to eliminate pockmark defects is to increase the Si content in the steel to above 0.05% while meeting the standard requirements, or to lower the temperature at the entrance of the finishing mill, and improve the surface condition of the finishing mill rolls, etc.
(3)Bright Bands
①Definition
Hot rolling bright bands are a common defect in hot-rolled steel strips and occur on most hot rolling production lines. Minor bright bands generally have little impact on the normal use of the steel strip, but when severe, they can also affect the cold rolling process. Bright bands appear as high gloss and circumferential protrusions in the coiling state. After the coil is unrolled, severe bright bands will show slight wavy defects.
②Occurrence Location
Bright bands generally occur on soft steel thin gauge steel strips, mainly at the tail of the steel strip, with a few occurring at the head. Bright bands appear simultaneously on the upper and lower surfaces of the steel strip, with completely corresponding positions. However, the gloss of the bright band on the lower surface is slightly higher than that on the upper surface, and the width is also greater than that on the upper surface.
③Formation Reasons
Through production practice and inspection analysis research, it has been concluded that the bright band defect is caused by the local contact between the steel strip and the coiling tension roll during the coiling process. Under the combined action of the tension roll pressure and the tension between the tension roll and the coiling cylinder, the steel strip in the contact area undergoes plastic deformation.
④Preventive Measures
Controlling the flatness accuracy of the steel strip during production, reducing the tension between the tension rolls and the coiling cylinder, and shortening the usage cycle of the tension rolls can prevent the occurrence of bright band defects.
(4)Scratches
① Formation Reasons
Scratch defects can occur during both hot rolling and transportation processes. If foreign objects adhere to the working rolls or coiling machine gripper rolls during the rolling process, scratch defects will occur. During hot rolling roller transportation or coiling and uncoiling, due to the relative motion between the rolled piece and mechanical equipment parts, it is also easy to produce. This defect has a periodic nature and is easy to identify in the hot rolled state.
②Appearance
Due to the high temperature, most hot rolled scratch defects will undergo secondary oxidation, and the defect surface appears iron skin-colored. If the hot rolled scratch defect is deep, it will be inherited to the cold rolled plate. After pickling and rolling deformation, the shape of the defect will change, and it is easy to be confused with other cold rolled defects macroscopically.
③Preventive Measures
The measures to solve scratch defects are very simple, which is to strengthen equipment maintenance and immediately stop production for investigation as soon as defects are found.
(5)Holes
①Definition
Hole defects are easily identifiable to the naked eye and can be observed on both hot-rolled and cold-rolled plates. On hot-rolled plates, hole defects are characterized by various shapes, appearing individually or in groups as pits without peeling characteristics. The depth of the pits varies, especially at the edges of the steel plate where they are more likely to occur. On cold-rolled plates, hole defects appear as a series of holes of different sizes, with uneven deformation at the hole sites; generally, the position of the hole on the cold-rolled plate corresponds to the position of the pit on the hot-rolled plate.
②Formation Reasons
The cause of hole defects is due to the rolling of silicon steel and other products with poor plasticity during the hot rolling finishing stage. During the process of the steel plate head passing through the roll, rolling, and tail leaving the roll, there is an impact of the side of the steel plate hitting the side guide plate of the finishing mill. The impact splashes fall on the surface of the steel plate and are pressed into the steel plate in the subsequent frame. Because the rolled piece is scratched at high speed, the falling foreign objects can splash on different parts of the surface of the steel plate, especially at the edges. When the overheated falling objects are pressed onto the steel plate, they cause pits of various shapes. These pits, varying in number, become holes of different sizes after subsequent cold rolling.
③Preventive Measures
Eliminating hole defects can be achieved by improving rolling stability, avoiding steel plate deviation during rolling, increasing side guide plate lubrication, and grinding and maintaining the side guide plate before production, which has a significant effect on the prevention of hole defects.
2.Hot Rolled Steel Strip Oxide Scale Defect Causes
(1)Causes of Generation
The main reasons for the presence of oxide scale inclusion defects on the surface of hot-rolled steel coils are:
①The slab is kept in the heating furnace for too long, resulting in too thick oxide scale on the surface of the slab, which is not easy to remove;
②The descaling water pressure is low, or the nozzle is blocked, aged, and falls off;
③The descaling sequence is inappropriate, resulting in unclean descaling at the head and tail of the steel coil;
④Due to the small angle of the finishing water beam, the water flow overlap is small, reducing the distance between the surface of the slab and the water beam, resulting in a blind area of descaling at the overlap, thus causing strip-like oxide scale inclusion defects at the head;
⑤When the roll usage is high and the wear is severe, it will cause "salt and pepper" oxide scale residue on the surface of the steel strip.
3. Preventive measures for hot-rolled steel plate defects
(1)Optimize the slab heating system and formulate a reasonable slab waiting rolling insulation system.
In response to the process requirements of cold-rolled materials, combined with the mechanism of oxide scale formation, a reasonable cold and hot slab heating system has been established. The furnace time for cold slabs is 180-200 minutes, and the uniform heating section temperature is (1250±20) ℃; the furnace time for direct-loaded slabs is 120-140 minutes, and the uniform heating section temperature is (1250±20) ℃. When the rolling line fault exceeds 10 minutes, it must be cooled down at a rate of 2℃/min according to the slab waiting rolling insulation system, and after the rolling line is ready, the heating furnace is heated again to prevent the furnace temperature from being too high, which would thicken the oxide scale and make it difficult to remove the rough descaling. The optimized slab heating temperature needs to ensure that the temperature at the entrance of the finishing mill is within the range of 990-1020℃, and at the same time, the heating temperature should be uniform, with a head-tail temperature difference not greater than 20℃.
(2)Optimize the descaling process
Modify the descaling pump acceleration point from the original 19.3MPa to 21.0MPa to ensure the descaling system pressure; both the post-furnace and pre-finishing descaling can use double descaling beams. At present, the post-furnace and pre-rough descaling pressures have reached 21-22MPa, and the finishing descaling pressure has reached more than 21MPa. Two pumps are put into use in the post-furnace, pre-rough, and pre-finishing descaling systems, and the rolling rhythm is controlled to avoid the descaling system pressure dropping after multiple-point descaling; adjust the descaling sequence, increase the head descaling opening advance and tail delay closure to avoid the phenomenon of two points (including two points) descaling at the same time, causing unclean head and tail descaling problems; appropriately reduce the post-furnace descaling speed from the original 1.5m/s to 1.3m/s, and increase the unit area striking water volume.
(3)Stabilize equipment operation to ensure descaling effect
Use maintenance to conduct descaling striking force tests, check the overlap of descaling machine nozzles to ensure the descaling effect along the width direction of the steel strip; each maintenance checks and replaces the post-furnace, rough, and finishing descaling nozzles, and the small class uses roll replacement to check the descaling nozzles, and uses low-pressure water to check the nozzle blockage, and replaces the nozzles with problems; strengthen the maintenance of descaling pumps, descaling nozzles, and descaling motors, and take a person responsible for the inspection system of roll cooling water nozzles to ensure that the working roll cooling water is normally invested; ensure that the rolling lubricating oil is normally used according to the usage regulations. To ensure good surface quality of the working rolls.
(4)Formulate a reasonable roll replacement cycle and side guide plate inspection system
Aging working rolls will form oxide scale on the surface of the steel strip. Therefore, a reasonable roll replacement cycle has been established, and the finishing working roll replacement cycle has been adjusted from the original 2300-2500t to below 1500t. And the working roll cooling water nozzle is checked three times a day, and the water nozzle is dredged in time when blockage is found to ensure that the working roll works under good cooling conditions; when replacing the side guide plate lining plate, it is necessary to adjust the height to ensure smooth transition, and there is no excessive wear to form steps. If excessive wear steps are found, they must be replaced in time to prevent the steel strip from being scratched and forming oxide scale.
(5)Hot Rolled Steel Strip Scab, Pitted Surface, Bubble and Other Surface Defect Control Measures
Hot rolled steel strip surface has scab, pitted surface, bubble and other surface defects. In order to eliminate these defects, it is necessary to consider comprehensively from the three processes of smelting, pouring, and rolling. The following measures are taken in production:
①Strictly implement the smelting and pouring process system, bake the raw materials, steel ladle, intermediate ladle, etc., strictly implement the deoxidation system, reduce the subcutaneous bubble defects of the continuous casting slab, and the continuous casting slab with severe bubbles should be scrapped and not allowed to enter the next process;
②Stabilize the pulling speed to ensure the stability of the intermediate ladle and the crystallizer liquid surface, avoid the formation of slag inclusions on the surface of the continuous casting slab during pouring, ensure that the end cutting of the continuous casting slab is neat without burrs and no flame cleaning residue to prevent scab defects;
③ During rolling, strictly implement the heating temperature control system, use high-pressure water descaling in both rough and finishing rolling, check the descaling water pressure and descaling nozzle to prevent pitted surface defects caused by low water pressure or nozzle blockage.
