Keyword：Steel Plate Production , Rolls
Desicription：
The surface defects of medium and thick plates are divided into steel defects, which are defects of the steel body itself. Another type of defect known as an operational defect is a defect caused during the production process. How can we prevent these defects?
Steel Plate production process
Common Defects of Medium and Heavy Steel Plates
The surface defects of medium and thick plates can be divided into two categories according to the source, one is the defects caused by the billet itself, which is called steel defects; the other is the defects caused during the production process from billets to finished products, called operational defects .
Steel defects
Cracks and fissures
Defect characteristics:
Such defects appear as cracks of different shapes, depths, and lengths on the surface of the steel plate, generally in the shape of fish scales or water ripples. Individual cracks and cracks can occur anywhere, and dense cracks are mostly distributed on the edge of the steel plate.
cause:
(1) This kind of defect is generally that the cracks in the continuous casting billet are not cleaned thoroughly, and the original surface longitudinal cracks, surface transverse cracks and star cracks are exposed after rolling.
(2) The subcutaneous bubbles or subcutaneous inclusions of the slab are broken or exposed during rolling to form point or star-shaped cracks. The subcutaneous air bubbles or subcutaneous inclusions of the slab are difficult to find before cleaning. Some gas overflows during the heating process, and some gas overflows the surface during the rolling process, forming cracks, which are shaped like cracks after rolling.
(3) Uneven heating temperature of the slab, excessive heating speed, etc. cause uneven plasticity of the steel plate, and cracks occur during rolling.
(4) For steel types with low thermal conductivity, the holding time is insufficient, or the temperature is uneven, the internal stress after deformation is too large, and the cracking occurs during rolling or the internal stress releases during cooling.
Precaution:
(1) Strengthen the quality inspection of the slab, and clean up the cracks and cracks on the surface of the slab.
(2) Prevent the billet from overheating, control the heating speed, and achieve uniform temperature.
(3) When rolling at high temperature, use a large reduction as much as possible to weld the subcutaneous bubbles.
Bubbles
Defect characteristics:
This type of defect is distributed irregularly on the surface of the steel plate, and in some parts it appears as a circular bulge, and the outer edge of the bulge is relatively smooth. The bubbles are caused by the gas inside the steel plate. This place cannot be welded during rolling, and some are torn during rolling.
cause:
More gas has accumulated inside the slab.
Precaution:
(1) Take measures to remove the gas in the steel plate as much as possible during steelmaking.
(2) Strengthen the quality inspection of slabs, and do not use slabs exposed to air bubbles.
Surface inclusions
Defect characteristics:
This kind of defect has non-metallic inclusions visible to the naked eye on the surface of the steel plate, which are distributed in obvious spots, blocks and strips, and the color is generally reddish brown, light yellow or off-white. Severe surface inclusions can transform into cracks during rolling.
cause:
(1) The raw material itself has non-metallic inclusions, which are exposed after rolling. Fluctuations in the liquid steel level in the crystallizer are involved in mold slag or defective intrusive nozzles are damaged in molten steel, which will cause billet inclusions.
(2) The numbered bricks in the rolling mill and the refractory materials of the furnace roof or furnace wall fall on the surface of the raw material during heating, and are pressed into the surface of the steel plate during rolling to produce non-metallic inclusions.
Precaution:
(1) Strengthen the cleaning and acceptance of slabs, and do not use slabs with surface inclusions.
(2) Ensure the high pressure water pressure, and remove all the oxide scale and sundries on the surface of the billet.
Operational defects
Convex hull and indentation
Defect characteristics:
Convex defects are regular local protrusions on the steel plate, and indentations are pits of different shapes and sizes that are pressed out on the surface of the steel plate.
cause:
(1) The convex hull is produced when the surface of the mill rolls or straightening roll loses meat or the surface is pressed out of a pit by a hard object to roll the steel plate.
(2) Indentation means that hard objects such as steel slag and iron sheet are stuck on the mill rolls surface during the rolling process or straightening process, and pits of different shapes and sizes are pressed out on the surface of the steel plate.
(3) There are foreign objects falling on the surface of the steel plate, which are pressed into the surface of the steel plate during straightening, and form indentations after falling off.
Precaution:
(1) Strengthen the steel rolling operation, do not roll “black stamped steel”, and keep the rolls cooled evenly.
(2) Frequently check the rolls and straightening rolls, and replace the rolls in time if foreign matter is pressed or meat is dropped.
(3) Prevent foreign matter from falling on the rolled piece during rolling.
Pitting
Defect characteristics:
There are local or continuous pits on the surface of the steel plate, and the defects can be divided into the following types according to the cause of formation.
cause:
(1) When the raw material is heated, the fuel spray stains and erodes the surface of the steel billet. After rolling, the surface of the steel plate is partially black honeycomb-shaped rough pit surface, which is generally small or densely pitted, so it is called black spots.
(2) Due to the severe oxidation of the raw material during heating, the oxide scale falls off during rolling, forming massive or continuous rough planes or off-white smooth pits on the surface of the steel plate, so it is called light pitting.
(3) When the steel plate is normalizing in the roller hearth heat treatment furnace, due to the high temperature, the surface of the steel plate is very soft, and the gas combustion products with high melting point bonded to the furnace bottom roller are pressed out on the surface of the steel plate in regular or large areas. pits.
Precaution:
(1) It is necessary to control the temperature fluctuation of the heating furnace, the amount of fuel injection, and the temperature, atmosphere and time of the high temperature stage.
(2) It is necessary to strengthen descaling during rolling to ensure high-pressure water pressure and remove all oxide scales and foreign matter on the surface of the billet.
(3) The hearth rollers of the heat treatment furnace should be regularly inspected to reduce the adhesion of foreign matter to the hearth rollers.
Iron oxide scale pressing
Defect characteristics:
The steel plate is pressed into the iron oxide scale, which is generally taupe or reddish brown, distributed in lumps or strips.
cause:
(1) This type of defect is mainly due to the oxide scale on the surface of the raw material or the secondary oxide scale produced during the rolling process, which is not clean and pressed into the surface of the steel plate during rolling. After the slab comes out of the heating furnace, the scale is removed by high-pressure water at the descaling point, and then rough rolling is carried out. The scale is not completely removed during the rough rolling process.
(2) During the rough rolling process and the cold steel process after rough rolling, secondary oxidation occurs on the surface of the steel plate, and the interface stress between the steel plate and the secondary oxide scale is small, so the stripping property is poor. If the high-pressure water injection in the finishing rolling cannot completely remove the secondary scale, it is called secondary scale pressing after pressing.
Precaution:
(1) Control the heating temperature, the atmosphere in the furnace and the time in the furnace to reduce the oxidation of the billet.
(2) Ensure high-pressure water pressure, and strengthen descaling during rolling, especially the high-pressure water dephosphorization of the four-high finishing mill, to remove the oxide scale on the surface of the billet.
Overburn
Defect characteristics:
Slightly overburned cracks appear in the edges and corners of the steel plate in the shape of diamonds, chicken feet or snakes. In severe cases, there are rough fibrous structures visible to the naked eye in the cracks, and the grain boundaries can be seen to be oxidized under a microscope.
cause:
(1) The heating temperature is too high, and the holding time in the high temperature zone is too long, causing the grains to grow and the intergranular to be oxidized, resulting in a decrease in plasticity.
(2) Improper adjustment of the heating flame, the flame is directly sprayed onto the surface of the billet, causing local overburning.
(3) The temperature in the furnace is uneven, and a high temperature appears in a certain part, and the billet stays here for too long, resulting in local overburning.
(4) High manganese and sulfur content in steel can easily cause overburning.
Precaution:
(1) Reasonably formulate the heating temperature and heating time in the furnace according to the steel type and furnace conditions.
(2) Control the atmosphere in the furnace and the length of the flame, and it is strictly forbidden to spray the flame directly on the surface of the billet.
(3) When the rolling time is long, the temperature should be lowered according to the regulations.
