Hello, Everyone today Hua steel international company introudce some common defects of cold rolled sheets, here is our opinion:
First :Surface inclusions
Defect characteristics:
The surface of stainless steel cold-rolled sheet shows non-metallic inclusions in the form of dots, blocks or lines, which are distributed intermittently or continuously along the rolling direction, and the color is reddish brown, dark gray or white. In severe cases, stainless steel cold-rolled sheet will have holes, cracks, and broken strips.
Causes:
1) Poor slag formation during steelmaking, high viscosity of molten steel, poor fluidity, slag cannot float, and there are many non-metallic inclusions in the steel.
2) Low casting temperature, poor boiling, and inclusions do not float.
3) During continuous casting, the protective slag band is mixed into the steel.
4) Non-metallic materials in the molten steel tank, ingot mold or injection pipe are not cleaned up.
Second:Scarring
Defect characteristics:
Irregular tongue-shaped and fish-scale black spots appear on the surface of stainless steel cold-rolled plates, with different distribution areas and uneven depths. This type of iron scale is difficult to wash off in the pickling process, and pits are formed when the iron scale falls off.
Causes:
1) In steelmaking, the inner wall of the ingot mold is not cleaned properly, and the mold wall is peeled off. When pouring, steel corrosion splashes and sticks to the mold wall, causing oxidation. The casting temperature is low, and sometimes the injection flow is interrupted. When pouring steel, the ingot is flipped and poured. The improper addition of protective slag causes steel liquid to splash.
2) In steel rolling, the residual scars on the surface of the slab are not cleaned up, and they remain on the stainless steel cold-rolled plate after rolling.
Third:Pressing into iron oxide
Defect characteristics:
Defects are black spots in the form of dots, strips or fish scales, with different distribution areas and uneven depths. This type of iron scale is difficult to wash out during the pickling process, and pits are formed when the iron scale falls off.
Causes:
1) The slab heating temperature is too high, the time is too long, and the furnace is in a strong oxidizing atmosphere, and the furnace-generated iron scale is pressed in during rolling.
2) The high-pressure water pressure is insufficient, and the iron scale is not cleaned before continuous rolling.
3) The high-pressure water nozzle is blocked, and the local iron scale is not removed.
4) The number of scale collection pipes opened is insufficient, and the scale removal is not clean.
Forth: Under-pickling
Defect characteristics:
When the upper and lower surfaces of the stainless steel cold-rolled plate are serious, the entire plate surface will appear strip-like, black-gray stripes, and dull.
Causes:
1) The temperature and cooling rate of each part of the stainless steel cold-rolled plate are different, that is, the temperature and cooling rate of the head, middle, and tail along the length direction of the stainless steel cold-rolled plate and the edge and middle along the width are different, which makes the iron sheet structure and thickness of each part of the same stainless steel cold-rolled plate different. Generally speaking, the temperature of the head and middle of the stainless steel cold-rolled plate is 30-50℃ higher than that of the tail. Therefore, the iron sheet of the head is thicker and the iron sheet of the tail is thinner. Therefore, under the same pickling speed, it is easy to produce local uncleanness.
2) Improper pickling process, such as low pickling concentration and temperature, too fast pickling speed, insufficient pickling time, or high ferrous concentration and no timely replenishment of acid solution.
3) The tensile coefficient of the stretching descaling machine is not enough, so that the iron sheet is not fully broken and peeled, affecting the pickling effect.
4) The appearance of cold-rolled stainless steel sheets is poor, such as crescent bends, waves, etc., which makes the mechanical descaling effect poor and easily causes local under-pickling.
Fifth: Adhesion mark
Defect characteristics:
The adhesion between layers of annealed steel coils is called bonding. Point-shaped arc folds after flattening are point bonding. Arc-shaped folds in rows along the rolling direction are strip bonding. Severe surface bonding, flat unwinding is torn or holes appear, becoming waste coils.
Causes:
1) The rolling tension is too large or the tension fluctuates, the plate shape is not good, resulting in bulges, and bonding occurs at the part where the interlayer pressure is high.
2) The surface roughness is too small.
3) The steel is too soft, with little carbon and silicon.
4) The heat treatment furnace temperature is too high or the annealing cooling speed is too fast.
5) The steel coil is damaged by collision before loading.
Sixth:Oxidation color
Defect characteristics:
The surface of the stainless steel cold-rolled sheet is oxidized, and its color gradually changes from dark blue at the edge to light blue, brown, and light yellow, collectively referred to as oxidation color.
Causes:
1) The protective cover is not sealed tightly or leaks during annealing, resulting in oxidation of the stainless steel cold-rolled coil.
2) The hood furnace annealing process, high temperature out of the furnace (steel coil temperature is greater than 110°C), leads to oxidation of the stainless steel cold-rolled coil.
3) The dew point of the protective gas is too high or the hydrogen content is too low, the pre-purge time before heating is insufficient, there is residual oxygen in the furnace, and the stainless steel cold-rolled coil is annealed in an oxidizing atmosphere.
Seventh:Emulsion spots
Defect characteristics:
The surface of the annealed stainless steel cold-rolled sheet presents irregular or island-like black and brown patterns.
Causes:
1) The emulsion is not fully added at the mill outlet, and it carbonizes during heating, forming spots.
2) The blower pressure at the outlet of the final stand is low and cannot blow cleanly.
3) The blower is not turned on during the belt threading, and the blower is turned off during the tail swinging.
4) After loading the furnace, the pre-purge time is insufficient, and the emulsion carbonizes during heating to form spots.
Eighth:Scratching
Defect characteristics:
The surface of stainless steel cold-rolled sheet presents straight and thin grooves of varying depths. Parallel to the rolling direction, continuous or intermittent, with varying densities and no certain rules. The scratched area is relatively smooth before leveling, and the color of the groove is gray-black. After leveling, it is scratched, has burrs, and is bright metallic.
Causes:
1) The parts of the pickling, rolling, leveling, and finishing units that contact the stainless steel cold-rolled sheet have sharp edges or hard objects, resulting in relative movement.
2) The various rollers of the finishing line (pinch rollers, pressure rollers, guide plates, etc.) do not operate and cause scratches. 3) When unwinding or coiling, the speed of the stainless steel cold-rolled sheet changes or the layers move relative to each other.
Ninth:Wrinkle
Defect characteristics:
The surface of stainless steel cold-rolled plate shows uneven wrinkles, which mostly occur in stainless steel cold-rolled plates less than 0.8mm. The edges of the wrinkles are at a certain angle, and they are severely wrinkled into folds.
Causes:
1) The stainless steel cold-rolled plate deviates, stretches on one side, and wrinkles on the other side.
2) The plate shape is poor, with large side waves or middle waves. When the stainless steel cold-rolled plate passes through the leveling machine, straightening machine or pinch roller, wrinkles are generated in the wave-shaped area.
3) Improper adjustment of the straightening machine causes uneven deformation.
Tenth:Roller Printing
Defect characteristics:
Periodic pits or convex bulges appear on the surface of stainless steel cold-rolled plates, and severe roller marks cause stainless steel cold-rolled plates to be rolled through.
Causes:
1) The weld of the stainless steel cold-rolled plate is too high or unevenly cleaned, causing roller sticking during continuous rolling.
2) Hard metal objects (weld beads, metal chips, etc.) or dirt are stuck on the rollers. When rolling or leveling, hard objects or dirt are pressed on the surface of the stainless steel cold-rolled plate, leaving indentations.
Eleventh:Imprint
Defect characteristics:
The pits of a certain depth on the surface of stainless steel cold-rolled plates are embossed, which are periodic and vary in number. The color of the defect is brighter.
Causes:
1) During the production process, various auxiliary rollers (tension rollers, clamping rollers, clamping rollers, straightening rollers, etc.) are sticky with iron filings and dirt.
2) Iron filings and foreign objects fall into the stack of stainless steel cold-rolled plates.
Twelveth:Tower Shape
Traps:
Defects in the shape of the steel coil, one circle of the end face of the stainless steel cold-rolled coil is higher (or lower) than the other circle, continuous, shaped like a pagoda, and mostly appears in the inner (outer) circle of the stainless steel cold-rolled coil.
Causes:
1) The centering device of the coiler fails, and the strip runs off.
2) The stainless steel cold-rolled coil is not straight, such as sickle bend, narrowing, etc.
3) The plate shape is poor, and large edge waves appear, causing the stainless steel cold-rolled coil to exceed the control limit of the photoelectric tube.
4) Improper operation adjustment, small reel shrinkage, and the inner ring is pulled out when the coil pusher pushes out the stainless steel cold-rolled coil.
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