If the defects such as chemical composition and mechanical properties of
welded steel tube billets that require certain detection methods to know are collectively referred to as invisible defects; then, those defects that can be seen only by vision can be called visible defects of steel tube billets. The visible defects of welded steel tube billets generally include cracked edges, broken edges, serrations, pitting, scratches, roller marks, black spots, edge burrs, sickle bends, wrinkles, waves, edge warping, delamination, warping, pinholes, sticking, soft spots, rust, oxide layer, tower shape, loose coils, etc.
First, cracked edges
Cracked edges refer to fine cracks of varying widths on the edges of steel tube billets, which mainly occur on the edges of cold-rolled steel tube billets and can be single-sided, double-sided, continuous or intermittent.
1. Causes of cracked edges:
(1) Chemical composition segregation is very easy to occur at the edges of hot-rolled steel strips, and brittle cracks are easy to occur at the segregated edges during the cold rolling process.
(2) Unreasonable cold rolling process. Excessive reduction in a single pass causes the plate to harden rapidly and its elongation to decrease sharply. Under the subsequent thinning and longitudinal tension, cracks or even cracks will appear at the edge of the plate where the strength is weakest.
(3) The trimming amount is too small, and cracks of varying widths or narrows will still remain on the edge of the longitudinally sheared steel tube billet.
2. The impact of cracked steel tube billets on welded steel pipe production:
(1) During the forming process, cracks on the edge of the steel tube billet will crack under the action of longitudinal tensile stress.
(2) The density of the weld cannot be guaranteed.
(3) The strength of the weld cannot be guaranteed and cannot withstand deformation such as bending, flaring, flattening, and flanging.
Second, serrations Serrations refer to the protrusions on the outer edge of the steel tube billet. Both cold-rolled and hot-rolled steel tube billets may have serrations, but hot-rolled steel tube billets are more common.
1. Causes of serrations on steel tube billets:
(1) During the hot rolling process, the edge of the steel tube billet scrapes against the abnormally rotating vertical rollers and side guides of the rolling mill, and is directly put into the production of welded steel pipes without trimming.
(2) When the steel tube billet is stripped, the longitudinal shear blades jump out of the blade; or when it is rolled, it scrapes against the separator.
2. Effects of serrated steel tube billets on welded steel pipes:
(1) According to the proximity effect principle of high-frequency welding, steel tube billets with serrations will approach in advance during the welding process, and "pre-welding" will occur at the tooth tip, causing the weld to be over-burned at the serration part or even burn through.
(2) Larger serrations will be pressed into the steel pipe cavity by the guide ring, pulling away the magnetic rod and causing production to stop.
(3) This will cause the guide ring to be distorted and scrapped in advance.
(4) Contact welding will cause leaks, and the contacts will be instantly disconnected or short-circuited.
Third, pitting and roughness
Pitting and roughness refer to small pits on the surface of steel tube blanks. Pits with a small number and scattered distribution are defined as pitting, while pitting is defined as a surface if the number of pits is large and dense.
1. Causes of pitting and roughness:
(1) Rust on steel tube blanks.
(2) Severely rusted steel strips, after pickling and cold rolling, rust pits become small pits.
(3) Incomplete pickling, some rust is pressed into the plate surface during cold rolling, and then the rust falls off to form small pits.
(4) Iron oxide scale is pressed into the plate surface during hot rolling.
2. Impact of pitting and roughness on the quality of welded steel pipes:
Pitting and roughness mainly affect the surface quality of welded steel pipes. For users who need high-quality surface treatment (chrome plating, nickel plating, electrophoresis, powder spraying) and users of medium and high-end furniture steel pipes, they are unacceptable defects.
Fourth, scratches
Scratches refer to scratches on the surface of steel tube blanks. The characteristics of scratches are mostly longitudinal and relatively fixed in position, and there are certain patterns to follow.
1. Causes of scratches:
(1) During cold rolling, hard foreign objects enter the tension plate of the rolling mill and scratch the surface of the steel pipe billet.
(2) Hard foreign objects enter the clamping plate of the slitting tension station and scratch it.
(3) When the welding steel pipe is stored, the material roller rotates inflexibly and scratches the plate surface.
(4) Scratches on the outer ring of the billet are mainly caused by careless collisions during storage, lifting, and transportation.
2. The impact of scratches on the quality of welded steel pipes: The impact of scratches on the quality of welded steel pipes is similar to that of pitting and roughness.
Fifth. Burrs and edge warping
Steel pipe billet burrs refer to the parts that remain on the edges of the trimmed steel pipe billet on both sides, protruding from the plane, and are not smooth when touched horizontally with the hand (for safety reasons, it must be touched horizontally). Edge warping is the part of the steel pipe billet on both sides that is warped relative to the plane. Edge burrs and edge warping of steel tube billets are twin brothers. Generally, if the edge burrs are large, the edge warping is severe; conversely, if the edge warping is severe, the edge burrs must be large. The two coexist with the stripped steel tube billet, but the degree is different. Slight edge burrs and edge warping can be partially corrected by forming rollers, but warping cannot be eradicated.
1. Causes of burrs and edge warping:
(1) The longitudinal shear blade is not sharp.
(2) The longitudinal shear blade shaft has poor accuracy, or there are impurities on the spacer surface, causing the slitting blade plane to swing on the blade shaft, affecting the side clearance, and periodic burrs are generated on the edge of the steel tube billet.
(3) The side clearance between the upper and lower blades of the slitting blade is too large.
2. The impact of burrs and edge warping on the quality of welded steel pipes:
(1) Slight burrs and edge warping of steel pipe billets do not have much impact on thick-walled steel pipes and standard-walled steel pipes. For some steel pipes that require smaller or extra-small internal burrs, the purpose can be achieved by reverse feeding the steel pipe billet.
(2) When the edge warping is severe, a "corner joint" will be formed in the weld seam, that is, when the A side is rolled outside, an outer corner joint as shown in Figure A will appear, and when the A side is rolled inside, an inner corner joint as shown in Figure B will appear. Both edge joint forms are undesirable in the welding process.
(3) The outer burrs of thin-walled steel pipes are difficult to remove. In the production of thin-walled steel pipes, steel pipe billets with large edge burrs and edge warping should be avoided as much as possible.
Sixth, sickle bend
Sickle bend refers to the phenomenon of lateral bending of the steel pipe billet. The standard stipulates that the sickle bend of hot-rolled steel pipe billets shall not exceed 3mm/m, and that of cold-rolled steel pipe billets shall not exceed 2mm/m. Hot rolling of narrow steel pipes without trimming is most likely to cause this.
1. Causes of sickle bending:
(1) The amount of reduction at both ends of the roll fluctuates during the production of steel pipes.
(2) The temperature difference between the two sides of the strip is large when rolling.
(3) The center of the longitudinal shearing machine's receiving device is inconsistent with the tension station and shearing machine.
2. The impact of sickle bending on the production of welded steel pipes:
(1) During the forming process, the long side will be "excess" and it is easy to produce a forming bulge.
(2) During the welding process, it is easy to produce "lap welding" or weld misalignment, and the weld strength cannot be guaranteed.
(3) The weld deflects left and right, making it difficult to control the depth of external burr removal.
(4) The forming force on both sides of the sickle-bent steel pipe is different. During the forming deformation process, the steel pipe tends to bend forward, which makes it easy for the formed steel pipe to "deviate", causing production to stop.
Seventh, transverse wrinkles
Transverse wrinkles refer to the transverse fold lines on the surface of the steel pipe. Whether wrinkles can be eliminated depends on the size of the R angle at the bottom of the wrinkles. When R is less than 0.5 mm, it is considered that the wrinkles are difficult to completely eliminate during the production of welded steel pipes. The ideal situation for wrinkle elimination is: there is visual effect but no hand feeling. Transverse wrinkles mostly occur on thin-walled mild steel pipe billets.
1. Causes of transverse wrinkles:
(1) The steel pipe billet is too soft. For example, steel pipe billets with a thickness of less than 0.8 mm and a width of more than 100 mm are very prone to transverse wrinkling when the hardness is less than 100 HV.
(2) Cold-rolled steel pipe billets have not been subjected to finishing and straightening, and the surface is prone to yielding.
(3) There are too many materials stored in the cage-type storage bin, and they are squeezed too tightly against each other, resulting in "dead folds" at the folding part of the steel pipe billet.
2. The impact of transverse wrinkles on the quality of welded steel pipes:
(1) It affects the surface quality of welded steel pipes. Slightly thicker steel pipe walls can achieve visual effect but no hand feeling, but this will increase the load on the equipment.
(2) Transverse wrinkles that cannot be eliminated will form irregular "O"-shaped or "C"-shaped rings on thin-walled steel pipes or steel pipes with a wall thickness of less than 0.7 mm, making post-processing of the steel pipe surface difficult or increasing the processing cost. Therefore, steel pipe billets with transverse wrinkles that cannot be eliminated must be avoided in the production of welded steel pipes.
(3) The welding process is unstable, and each wrinkle is equivalent to a "micro-bulge", and the weld strength is not guaranteed.
Eighth, delamination Delamination refers to the phenomenon that two or more layers appear in the thickness direction of the steel pipe billet. Delamination can be divided into explicit and implicit. Explicit delamination can be judged by the naked eye without deformation, while implicit delamination can only be seen after deformation. The harm of implicit delamination is greater than that of explicit delamination.
1. Causes of delamination:
(1) The oxide film on the surface of the molten steel is rolled into the interior of the ingot and formed during subsequent rolling.
(2) There are slag inclusions in the billet, which forms delamination after rolling.
(3) During the solidification process, the steel liquid will not be filled with enough liquid steel, resulting in the formation of looseness at the necking and the area below the necking. The necking and looseness will form layers or interlayers after rolling.
(4) Non-metallic inclusions. During casting, the slag or refractory materials in the steel fail to float to the surface of the steel liquid and are mixed in the steel ingot, forming layers during subsequent rolling.
2. The impact of layering on the quality of welded steel pipes:
(1) The steel pipe billet is prone to deviation during the forming process, resulting in the inability to carry out normal production. This is because when the layered steel pipe billet is rolled by the upper and lower forming rollers of the open hole mold, the linear speed of the upper and lower forming rollers is not absolutely synchronized, so the layered steel pipe billet will slip irregularly.
(2) Affecting the quality of the weld. If the edge of the steel pipe billet is layered, after forming into the open steel pipe tube to be welded, the inner layer of the steel pipe tube will be compressed and the outer layer will be extended, resulting in the layered edge misalignment, which will then form the weld misalignment and the external burrs will be difficult to remove.
(3) Even if steel pipes are produced from steel pipes with hidden delamination, if they are not discovered in time during the production process, the harm will be greater, making it difficult for the welded steel pipe to withstand the pressure of normal wall thickness.
Ninth, Warping
Warping refers to the iron sheet on the surface of the steel pipe billet. It is connected to the steel pipe base at one end and separated from the base at the other end. It is high and has different shapes and sizes. It is different from delamination in that the local delamination is obvious, but the formation mechanism is similar to delamination. It is an extreme manifestation of local delamination and is located on the surface and is rolled out.
The impact of warping on welded steel pipe production is as follows:
(1) It is easy to damage the induction coil or contact, carry away the magnetic rod, and force shutdown.
(2) It leads to a reduction in the pressure bearing capacity of the steel pipe.
(3) It has poor sensory properties and does not meet the surface quality requirements of the welded steel pipe.
Tenth, Sticking
Sticking refers to the adhesion phenomenon between layers or coils of cold-rolled annealed steel pipe billets. Slight sticking can be pulled apart smoothly when the welded steel pipe is unwound, but a small tearing sound can be heard; moderate local sticking can be pulled apart, but it will tear at the bonding point or even open a hole, leaving rough and uneven marks on both sides of the steel pipe billet; severe sticking will cause the entire steel pipe billet to be scrapped and unable to be unfolded.
1. Causes of sticking: Sticking mainly occurs on cold-rolled annealed steel pipe billets.
(1) The cold rolling coolant is not filtered cleanly, and a lot of metal powder remains on the cold-rolled plate surface, and the metal powder melts during the annealing process.
(2) During tension reduction rolling (direct annealing without unwinding after rolling), the tension control is still very high, and sticking is easy to occur during the annealing process.
(3) If the annealing process is not strictly followed, point-shaped local sticking will occur at the burner.
(4) If the annealing process is not strictly followed, the cooling rate is too fast above the recrystallization temperature, resulting in too fast and violent cold shrinkage of the outer ring of the steel coil, which forms a large compressive stress on the inner ring.
(5) During annealing, the temperature rises too fast above the recrystallization temperature (400~450℃).
(6) Mixed thick and thin materials, mixed wide and narrow materials, and mixed large and small coils make it difficult to achieve the best of both worlds in the annealing process. Small coils, narrow coils, and thick materials conduct heat quickly and are easy to heat through, resulting in uneven recrystallization recovery. By the time thin, wide, and large materials are also heated through, the former may have been over-burned.
(7) The cleaning coiling tension is too large.
(8) The annealing stack is too high.
2. The impact of adhesive tape steel billets on the quality of welded steel pipes:
(1) Welded steel pipes produced with adhesive tape steel billets cannot be used for products with high surface requirements.
(2) For medium-grade adhesive tape steel billets, strict selection is required. Not only should the steel billets be selected, but also those with holes should be picked out and cut off to make short steel pipes. This is time-consuming and labor-intensive and also affects the yield rate.
Eeventh. Rust Rust refers to the phenomenon that the surface of the steel tube billet is damaged by being soaked in liquids such as rainwater, acid, alkali, or oxidized by air. Rust can be divided into light rust and severe rust. For slightly rusted steel tube billets, gently wipe them with 240 sandpaper and blow away the rust powder. There is no trace on the surface of the steel tube billet. For severely rusted steel tube billets, after wiping with sandpaper, there are still corrosion depressions on the surface of the steel tube billet. Rust is one of the common defects of steel tube billets.
1. Causes of rust:
(1) The steel tube billet is exposed to the air for a long time and oxidized by contact with water vapor, oxygen, etc. in the air.
(2) It is directly corroded by liquids such as water, acid, and alkali. 2. The impact of rust on the quality of welded steel pipes:
(1) Even with slight rust, after the welded steel pipe is formed, welded, and rolled by the sizing roller, the surface of the welded steel pipe will be left with marks due to the intrusion of rust powder, which will damage the surface quality of the welded steel pipe and increase the cost of subsequent surface treatment of the welded steel pipe.
(2) The weld strength is low and the weld structure is not dense. The chemical formula of rust powder is mFe2O3,·nH2O. The rust in the thickness direction of the steel pipe will undergo a series of chemical reactions during high-temperature welding, generating CO gas and H2, which will be retained in the weld during solidification, forming pores and loose structure.
(3) Severe rust will thin the steel pipe wall and reduce the pressure bearing capacity of the steel pipe.
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