Quality problems in the production process of seamless steel pipes - quality defects of tube billets and their prevention

1. Quality defects and prevention of tube billets: The tube billets used in the production of seamless steel pipes can be either continuous casting round tube billets, rolled (forged) round tube billets, centrifugally cast round hollow tube billets, or steel ingots can be used directly. In the actual production process, continuous casting round tube billets are mainly used because of their low cost and good surface quality.
1.1 Appearance shape and surface quality defects of tube billets
1.1.1 Appearance shape defects: For round tube billets, the appearance shape defects of tube billets mainly include the diameter and ovality of the tube billets, the end face bevel deviation, etc. For steel ingots, the appearance shape defects of tube billets mainly include the incorrect shape of the steel ingot due to wear of the steel ingot mold, etc. Diameter and ovality deviation of round tube billets: It is generally believed in practice that when the tube billet is pierced, the size of the pressure reduction rate before the piercing head is proportional to the amount of inward folding of the piercing rough tube. The larger the pressure reduction rate of the head, the more premature formation of the cavity of the tube billet is conducive, and the rough tube is prone to internal surface cracks. During normal production, the hole parameters of the piercing machine are determined according to the nominal diameter of the tube and the outer diameter and wall thickness of the rough tube. When the hole type is adjusted, if the outer diameter of the tube exceeds the positive tolerance, the front pressure reduction rate of the plug increases, and the perforated rough tube produces an inward fold defect; if the outer diameter of the tube exceeds the negative tolerance, the front pressure reduction rate of the plug decreases, and the first bite point of the tube moves toward the throat, which makes the perforation process difficult to achieve. Ovality out of tolerance: When the ovality of the tube is uneven, the tube will rotate unstably after entering the perforation deformation zone, and the roller will scratch the surface of the tube, resulting in surface defects of the rough tube. The end face of the round tube is out of tolerance: The wall thickness of the front end of the perforated rough tube of the tube is uneven. The main reason is that when the tube does not have a centering hole, the plug meets the end face of the tube during the perforation process. Because there is a large bevel on the end face of the tube, it is difficult for the nose of the plug to center the center of the tube, resulting in uneven wall thickness on the end face of the rough tube.
1.1.2 Surface quality defects (continuous casting round tube billet): Cracks on the tube billet surface: longitudinal cracks, transverse cracks, and mesh cracks. Causes of longitudinal cracks:
A. The offset flow caused by the misalignment of the nozzle and the crystallizer scours the solidified shell of the tube billet;
B. The melting property of the protective slag is poor, and the liquid slag layer is too thick or too thin, resulting in uneven thickness of the slag film, making the local solidified shell of the tube billet too thin.
C. Fluctuation of the crystallization liquid level (when the liquid level fluctuation is ﹥±10mm, the crack occurrence rate is about 30%);
D. P and S content in steel. (P﹥0.017%, S﹥0.027%, longitudinal cracks tend to increase);
E. When C in steel is 0.12%-0.17%, longitudinal cracks tend to increase.
Preventive measures: A. Ensure that the nozzle and the crystallizer are aligned; B. The crystallization liquid level fluctuation should be stable; C. Use appropriate crystallization taper; D. Select protective slag with excellent performance; E. Use hot top crystallizer.
Causes of transverse cracks: A. Too deep vibration marks are the main cause of transverse cracks; B. The increase in the content of (niobium, and aluminum) in steel is the inducing cause. C. The tube billet is straightened at a temperature of 900-700℃. D. The secondary cooling intensity is too large.
Preventive measures: A. The crystallizer adopts high frequency and small amplitude to reduce the vibration mark depth on the inner arc surface of the billet; B. The secondary cooling zone adopts a stable weak cooling system to ensure that the surface temperature is greater than 900 degrees during straightening. C. Keep the crystallization liquid level stable;
D. Use protective slag with good lubrication performance and low viscosity.
Causes of surface network cracks: A. The high-temperature ingot absorbs the copper of the crystallizer, and the copper becomes liquid and then seeps out along the austenite grain boundary; B. Residual elements in the steel (such as copper, tin, etc.) remain on the surface of the tube and seep out along the grain boundary;
Preventive measures:
A. Chrome plating on the surface of the crystallizer to increase the surface hardness; B. Use appropriate secondary cooling water volume; C. Control the residual elements in the steel. D. Control the Mn/S value to ensure that Mn/S>40. It is generally believed that when the surface crack depth of the tube does not exceed 0.5mm, the crack will be oxidized during the heating process and will not cause cracks on the surface of the steel pipe. Since the surface cracks of the tube will be severely oxidized during the heating process, the cracks are often accompanied by oxidation particles and decarburization after rolling.
Scarring and heavy skin of tube:
Causes: Too low molten steel temperature, too viscous molten steel, nozzle blockage, injection deviation, etc. Due to the scarring and heavy skin on the surface of the tube billet, the steel tube external fold is different from the scarring and external folding defects of the rough tube produced during tube rolling. It has very obvious oxidation characteristics, accompanied by oxidation particles and severe decarburization, and ferrous oxide exists at the defects.
Tube billet pores: Generally, some small pores are formed on the surface of the tube billet due to the rupture of subcutaneous bubbles during the casting of molten steel. After the tube billet is rolled, small flying skin will form on the surface of the steel pipe.
Pit and groove on the tube billet:
The reasons for the pits and grooves on the tube billet: On the one hand, it may be generated during the crystallization process of the billet, which is related to the large taper of the crystallizer or the uneven cooling of the secondary cooling zone; on the other hand, it may be caused by mechanical bumps or scratches on the surface of the tube billet when the billet has not been completely cooled. Folds or scars (pits) and large external folds (grooves) are formed on the surface of the rough tube after perforation.
"Ears" of tube billets: mainly because the roll gap (drawing and straightening rolls of continuous casting machines, rolling rolls of rolling mills) is not closed. When the tube billet is drawn or rolled, the draw and straightening rolls or rolling rolls are pressed too much or the roll gap is too small. This causes too much wide metal to enter the roll gap. After perforation, the rough tube surface produces a spiral fold. No matter what kind of surface defect of the tube billet, it is possible to form defects on the surface of the steel pipe during the tube rolling process. In severe cases, the rolled steel pipe will be scrapped. Therefore, it is necessary to strengthen the control of the surface quality of the tube billet and the removal of surface defects. Only tube billets that meet the standard requirements can be put into tube rolling production.
1.2 Low-power organizational defects of tube billets: Visual subcutaneous bubbles in tube billets: The reasons for the generation are insufficient deoxidation of molten steel and the gas content (especially hydrogen) in molten steel, which is also an important reason for the generation of subcutaneous bubbles in tube billets. This defect forms flying skin (without rules) on the outer surface of the steel pipe after perforation or rolling. The shape is similar to "fingernails". In severe cases, it will cover the outer surface of the steel pipe. This type of defect is small and shallow and can be removed by grinding.
Subsurface cracks in tube billets: The main reason is that the temperature of the surface layer of the continuous casting round tube billet changes repeatedly and forms after multiple phase changes. Generally, no defects are produced, if there are any, they are slight external folds.
Intermediate cracks and center cracks in tube billets: The intermediate cracks and center cracks in the continuous casting round tube billet are the main reasons for the internal folding of the seamless steel pipe. The causes of cracks are very complicated, involving the effects of solidification heat transfer, penetration, and stress of the billet, but generally speaking, they are controlled by the solidification process of the billet in the secondary cooling zone.
Porosity and shrinkage holes in tube billets: Mainly due to the advanced grain effect of the billet during the solidification process, the movement of liquid metal is based on the shrinkage caused by cooling in the solidification direction. If the continuous casting round tube billet has looseness and shrinkage holes, it will not have much effect on the quality of the rough tube of the oblique rolling perforation.
1.3 Microstructure defects of tube billets: High power or electron microscope: When the composition and structure of the tube billet are uneven and severe segregation occurs, the rolled steel pipe will present a severe banded structure, which will affect the mechanical properties and corrosion properties of the steel pipe and make its performance not meet the requirements. When the content of inclusions in the tube billet is too much, it will not only affect the performance of the steel pipe but also may cause cracks in the steel pipe during the production process.
Factors: harmful elements in steel, composition, and segregation of tube billets, and non-metallic inclusions in tube billets.

2. Heating defects of tube billets
In the production of hot-rolled seamless steel pipes, two heatings are generally required from tube billets to finished steel pipes, namely, heating before perforation of the tube billet and reheating of the rough tube after rolling before sizing. When producing cold-rolled steel pipes, intermediate annealing is required to eliminate the residual stress of the steel pipe. Although the purpose of each heating is different and the heating furnace may be different, if the process parameters and heating control of each heating are improper, the tube billet (steel pipe) will produce heating defects and affect the quality of the steel pipe. The purpose of heating the tube billet before perforation is to improve the plasticity of the steel, reduce the deformation resistance of the steel, and provide a good metallographic structure for the rolled tube. The heating furnaces used are annular, walking beam heating furnaces, inclined bottom heating furnaces, and car bottom heating furnaces. The purpose of reheating the rough tube before sizing is to increase and even the temperature of the rough tube, improve the plasticity, control the metallographic structure, and ensure the mechanical properties of the steel tube. The heating furnaces mainly include walking beam reheating furnaces, continuous roller bottom reheating furnaces, inclined bottom reheating furnaces, and electric induction reheating furnaces. The purpose of annealing heat treatment of steel pipes during cold rolling is to eliminate the work hardening phenomenon caused by the cold processing of steel pipes, reduce the deformation resistance of steel, and create conditions for the continued processing of steel pipes. The heating furnaces used for annealing heat treatment mainly include walking beam heating furnaces, continuous roller bottom heating furnaces, and car bottom heating furnaces.
Common defects of tube billet heating include: uneven heating of tube billets (steel pipes) (commonly known as yin and yang surfaces), oxidation, decarburization, heating cracks, overheating and overburning, etc.
The main factors affecting the heating quality of the tube billet: are heating temperature, heating speed, heating and holding time, and furnace atmosphere.
Tube billet heating temperature: mainly manifested as too low or too high temperature or uneven heating temperature. If the temperature is too low, it will increase the deformation resistance of the steel and reduce the plasticity. Especially when the heating temperature cannot ensure that the metallographic structure of the steel is completely transformed into austenite grains, the tendency of cracks in the tube billet during hot rolling will increase. When the temperature is too high, the surface of the tube billet will undergo severe oxidation, decarburization, and even overheating or overburning.
Tube billet heating speed: The size of the tube billet heating speed is closely related to the generation of tube billet heating cracks. When the heating speed is too fast, the tube billet is prone to heating cracks. The main reason is: that when the temperature of the tube surface rises, the metal inside the tube and the metal on the surface have a temperature difference, resulting in inconsistent thermal expansion of the metal and generating thermal stress. Once this thermal stress exceeds the fracture stress of the material, cracks will occur; the heating cracks of the tube may exist on the surface of the tube or inside. When the tube with heating cracks is pierced, it is easy to form cracks or folds on the inner and outer surfaces of the rough tube. Prevention: When the tube is still at a low temperature after entering the heating furnace, a lower heating rate is used. As the temperature of the tube increases, the heating rate can be increased accordingly.
Tube heating time and insulation time: The length of the tube heating time and insulation time are related to heating defects (surface oxidation, decarburization, coarse grain size, overheating or even overburning, etc.). Generally speaking, the longer the tube is heated at high temperature, the more likely it is to cause severe oxidation, decarburization, overheating, or even overburning on the surface, which may cause the steel pipe to be scrapped in severe cases. Preventive measures: A. Ensure that the tube billet is heated evenly and completely transformed into austenite structure; B. Carbides should be dissolved into austenite grains; C. Austenite grains should not be coarse and mixed crystals should not appear; D. After heating, the tube billet should not be overheated or overburned.
In short, to improve the heating quality of the tube billet and prevent heating defects, the following requirements are generally followed when formulating the tube billet heating process parameters: A. Accurate heating temperature to ensure that the perforation process is carried out within the temperature range with the best permeability of the tube billet; B. Uniform heating temperature, strive to make the heating temperature difference of the tube billet along the longitudinal and transverse directions not greater than ±10℃; C. Less metal burning, during the heating process, the tube billet should be prevented from overoxidation, surface cracks, bonding, etc. D. The heating system is reasonable, and the heating temperature, heating speed, and heating time (insulation time) should be reasonably coordinated to prevent the tube billet from overheating or even overburning.

3. Heating defects of rough pipes and their prevention
After being rolled by the tube rolling mill, the rough pipes generally cannot ensure the uniformity of their temperature, and it is difficult to meet the requirements of the final rolling temperature of the rough pipes. Therefore, it is necessary to reheat the metallographic structure before sizing. The main heating furnaces are walking reheating furnaces, continuous roller bottom reheating furnaces, inclined bottom reheating furnaces, and induction reheating furnaces. The main quality defects of the rough pipe during the heating process include uneven heating of the rough pipe, too high or too low heating temperature, unreasonable metallographic structure, severe surface oxidation, and decarburization, overheating or overburning, and mechanical scratches on the pipe body in the heating furnace. Uneven heating of rough pipe: Generally speaking, there is a temperature difference between the head and tail of the rough pipe after it is rolled out of the rolling mill. The slower the rolling speed of the rolling mill, the longer the rolling time, and the greater the temperature difference between the head and tail of the rough pipe (reflecting the most obvious re-cycle pipe rolling unit). When the rough pipe with a temperature difference between the head and the tail enters the reheating furnace, if the heating time is not enough, the temperature difference is difficult to eliminate. Another situation is that if the furnace of the reheating furnace is wide, and the burner is unevenly heated, it is also easy to produce a longitudinal temperature difference of the rough pipe. When the wall thickness of the rough pipe is thicker and the heating time is shorter, this temperature unevenness is more serious. To ensure the uniformity of the heating temperature of the rough pipe, the heating time of the rough pipe and the uniformity of the heating and atmosphere in the furnace should be ensured, and the burner type should be selected.
Unreasonable microstructure of the steel pipe: The performance of the steel pipe that does not need to be heat treated after sizing is achieved through reheating before the rough pipe sizing and the cooling system after sizing. If the reheating temperature and heating time of the rough pipe are not appropriate, there may be situations such as uneven austenite grain size or carbides in the steel not completely entering the austenite grains, which will cause unreasonable microstructure of the rough pipe and affect the performance of the steel pipe.
Oxidation and decarburization of the rough pipe surface: During the reheating process of the rough pipe, a defect that often occurs is severe surface oxidation (when the iron oxide scale is thick, the steel pipe after sizing (reduction) will have a pitted surface) and decarburization. The effective way to eliminate the rough surface and severe decarburization of steel pipes is to ensure that the atmosphere in the furnace is weakly reduced. According to the requirements of the heating process, the heating time and temperature should be controlled. Under the premise of not causing defects in the rough pipe, a faster heating speed and a shorter heating time should be used.
Surface scratches on rough pipes: mainly caused by the rough pipe sliding relative to the roller or spiral rod on the step-type heating furnace or the spiral rod of the heating furnace. There is currently no way to eliminate the surface scratches on rough pipes. The following points are mainly used to reduce scratches: A. Coating the surface of the roller and spiral rod to improve the surface hardness and finish; B. Use high temperature resistant and alloy steel to make rollers and spiral rods; C. Use roller cooling technology;