304L stainless steel manufacturer Shandong Baowu Steel Group will share with you today a summary of twelve common defects in stainless steel cold-rolled plates. Stainless steel people must read! Common defects include surface inclusions, scarring, pressed iron oxide scale, under pickling, sticking Knot marks, oxidized color, emulsion spots, scratches, wrinkles, roll marks, embossing, towers, etc.
Surface inclusions
Defect characteristics:
The surface of the stainless steel cold-rolled sheet presents point-like, block-like or line-like non-metallic inclusions, distributed intermittently or continuously along the rolling direction, and its color is reddish-brown, dark gray or white. In severe cases, holes, cracks, and broken strips will appear in the stainless steel cold-rolled plate.
cause:
1) Poor slagging during steelmaking, high viscosity of molten steel, poor fluidity, slag cannot float up, and many non-metallic inclusions in steel.
2) The casting temperature is low, the boiling is poor, and the inclusions do not float up.
3) During continuous casting, the mold slag band is mixed into the steel.
4) The non-metallic materials in the molten steel tank, steel ingot or injection pipe have not been cleaned
Scarring
Defect characteristics:
Irregular tongue-shaped and fish-scale black spots appear on the surface of the stainless steel cold-rolled plate, with different distribution areas and uneven indentation depths. This type of iron sheet is difficult to wash off in the pickling process, and pits are formed when the iron sheet falls off.
cause:
1) In steelmaking, the inside of the ingot mold is not clean, and the mold wall loses flesh. When pouring, steel rot splashes, sticks to the mold wall, and oxidizes. The casting temperature is low, and sometimes the injection flow is interrupted. Improper addition of mold slag during ingot injection will cause molten steel to splash.
2) In terms of steel rolling, the remaining scars on the surface of the slab have not been cleaned, and remain on the stainless steel cold-rolled plate after rolling
Pressed iron oxide scale
Defect characteristics:
Defects are black spots in the form of dots, strips or fish scales, with different distribution areas and uneven depth of pressure. This type of iron sheet is difficult to wash off in the pickling process, and pits are formed when the iron sheet falls off.
cause:
1) The heating temperature of the slab is too high and the time is too long. There is a strong oxidizing atmosphere in the furnace, and the furnace oxide iron sheet is pressed in during rolling.
2) The pressure of the high pressure water is insufficient, and the oxide scale has not been cleaned before continuous rolling.
3) The high-pressure water nozzle is blocked, and the local oxide scale is not removed.
4) The scale-collecting pipes are not opened enough, and the descaling is not clean.
Under pickling
Defect characteristics:
When the upper and lower surfaces of the stainless steel cold-rolled plate are severe, the entire plate surface appears flaky, black-gray streaked, and dull.
cause:
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, as well as the edges and middle parts along the width are different, making it the same as the stainless steel cold-rolled plate The iron sheet structure and thickness of each part of the plate are different. Generally speaking, the temperature of the head and middle of the stainless steel cold-rolled plate is 30-50 degrees Celsius higher than that of the tail. Therefore, the iron sheet at the head is thicker and the iron sheet at the tail is thinner. In the same situation, it is easy to cause local uncleanness.
2) The pickling process is not appropriate, such as the concentration and temperature of pickling are low, the pickling speed is too fast, the pickling time is insufficient, or the acid solution is not replenished in time due to the high concentration of ferrous iron, etc.
3) The tensile coefficient of the stretching descaling machine is not enough, so that the iron sheet is not fully broken, peeled off, and the pickling effect is affected.
4) The shape of stainless steel cold-rolled plate is poor, such as sickle crescent, wave shape, etc., so that the effect of mechanical descaling is poor, and it is easy to cause partial pickling.
Bond marks
Defect characteristics:
The layers of annealed steel coil are bonded to each other, which is called bonding. Point-shaped arc creases are formed after flattening. Point-shaped arc-shaped creases along the rolling direction are strip-shaped bonding. Severe surface bonding, flattening The uncoil is torn or there are holes and becomes a waste roll
cause:
1) The rolling tension is too large or the tension fluctuates, and the shape of the plate is not good, resulting in bulging, and bonding occurs at the part where the interlayer pressure increases.
2) The surface roughness is too small.
3) The steel is too soft, and the amount of carbon and silicon is low.
4) The heat treatment furnace temperature is too high or the annealing cooling rate is too fast.
5) The steel coil is injured by collision before loading the furnace
Oxidation color
Defect characteristics:
The surface of stainless steel cold-rolled sheet is oxidized, and its color gradually transitions from dark blue at the edge to light blue, brown, and light yellow, collectively referred to as oxidation color.
cause:
During annealing, the protective cover is not tightly sealed or leaks air, resulting in oxidation of stainless steel cold-rolled coils.
Bell furnace annealing process, high temperature out of the furnace (steel coil temperature up to 110 ° C), resulting in oxidation of stainless steel cold rolled coils.
The dew point of the protective gas is too high or the hydrogen content is too low, the pre-purging 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.
Emulsion spot
Defect characteristics:
The surface of the annealed stainless steel cold-rolled sheet presents irregular or island-like black and brown patterns
cause:
1) The emulsion at the exit of the rolling mill is not fully filled, and it is carbonized during heating to form spots
2) The pressure of the air blower at the outlet of the last rack is low and the blowing is not clean.
3) The fan is not turned on when the belt is threaded, and the fan is turned off when the tail is flicked.
4) After the furnace is installed, the pre-purging time is insufficient, and the emulsified liquid is carbonized to form spots during heating.
Scratch
Defect characteristics:
The surface of the stainless steel cold-rolled plate presents straight and thin grooves of different depths. Parallel to the rolling direction, continuous or intermittent, varying in density and irregular, the scratches are smooth before leveling, the frontal color of the grooves is gray-black, after leveling, scratches, burrs, and metallic bright color.
cause:
1) There are sharp edges and corners or hard objects in the parts of the pickling, rolling, leveling and finishing units that are in contact with the stainless steel cold-rolled plate, resulting in relative motion.
2) Various rollers (pinch rollers, pinch rollers, guide plates, etc.) of the finishing line do not run and cause scratches.
3) When uncoiling or coiling, the speed of the stainless steel cold-rolled plate changes or the relative movement between layers.
To wrinkle
Defect characteristics:
The surface of the stainless steel cold-rolled plate presents uneven wrinkles, which mostly occur in the stainless steel cold-rolled plate less than 0.8mm.
cause:
1) The stainless steel cold-rolled sheet deviates, one side is stretched, and the other side is wrinkled.
2) The shape of the plate is bad, with large side waves or middle waves. When the stainless steel cold-rolled plate passes through the tempering machine, straightening machine or pinch roll, there will be wrinkles in the wave shape.
3) Improper adjustment of the straightening machine, resulting in uneven deformation.
Roll printing
Defect characteristics:
Periodic pits or convex hulls appear on the surface of the stainless steel cold-rolled sheet, and severe roll marks cause the stainless steel cold-rolled sheet to roll through.
cause:
1) The welding seam of the stainless steel cold-rolled plate is too high or the cleaning is uneven, causing sticking to the roll during continuous rolling
2) There are hard metal objects (welding beads, metal shavings, etc.) or dirt 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.
3) Work rolls lose meat.
Embossed
Defect characteristics:
The pits of a certain depth on the surface of the stainless steel cold-rolled plate are imprints, which are periodic and vary in number, and the color of the defect is brighter.
cause:
1) Various auxiliary rollers (tension rollers, pinch rollers, nip rollers, straightening rollers, etc.) in the production process are caused by iron filings and dirt.
2) Iron filings and foreign matter fall into the stack of stainless steel cold-rolled plates.
Tower shape
Defect characteristics:
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 appear in the inner (outer) circle of the stainless steel cold-rolled coil
cause:
1) The centering device of the coiler fails, and the strip runs off
2) Stainless steel cold-rolled coils are not straight, such as sickle crescent, narrowing, etc.
3) The shape of the plate is poor, and there are large side waves, which make the stainless steel cold-rolled coil exceed the control limit of the photoelectric cell.
4) Improper operation and adjustment, the shrinkage of the reel is small, and when the pusher rolls out the stainless steel cold-rolled coil, the inner ring is pulled out.
Shandong Baowu Steel Group is mainly engaged in 304L stainless steel, 316L stainless steel, 317L stainless steel, 409L stainless steel, 436L stainless steel, 904L stainless steel, 301L stainless steel and other low-carbon stainless steel plates, stainless steel coils, stainless steel strips, stainless steel flat steel, and more than 1,000 tons of materials in stock , complete specifications and sufficient supply. At the same time, various stainless steel plate processing services can be provided according to customer requirements: flattening, slitting and drawing, mirror surface, oil grinding, polishing, punching, bending, leveling, slitting, cutting, rolling, punching, etc.
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