1. Quality defects of tube billets and their prevention.
The tube billets used to produce 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. 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 cutting bevel tolerance, 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.
Out-of-tolerance of round tube diameter and ovality: It is generally believed in practice that when the tube is pierced, the size of the pressure reduction rate before the piercing head is proportional to the amount of inward folding of the pierced raw tube. The larger the pressure reduction rate of the head, the more likely the hole cavity of the tube is formed prematurely, and the raw tube is prone to inner surface cracks. In normal production, the hole type parameters of the piercing machine are determined according to the nominal diameter of the tube and the outer diameter and wall thickness of the raw tube. When the hole type is adjusted, if the outer diameter of the tube exceeds the positive tolerance, the pressure reduction rate before the head increases, and the pierced raw tube produces an inward fold defect; if the outer diameter of the tube exceeds the negative tolerance, the pressure reduction rate before the head decreases, and the first bite point of the tube moves to the throat, which makes the piercing process difficult to achieve. Out-of-tolerance of ovality: When the ovality of the tube is uneven, the tube will rotate unstably after entering the piercing deformation zone, and the roller will scratch the surface of the tube, resulting in surface defects of the raw tube. The bevel of the end face of the round tube billet is out of tolerance: the wall thickness of the front end of the tube billet perforation rough tube is uneven. The main reason is that when the tube billet does not have a centering hole, the plug meets the end face of the tube billet during the perforation process. Due to the large bevel on the end face of the tube billet, the nose of the plug is not easy to center the center of the tube billet, resulting in uneven wall thickness of the rough tube end face.
1. 1. 2 Surface quality defects (continuous casting round tube billet): tube billet surface cracks: longitudinal cracks, transverse cracks, mesh cracks Causes of longitudinal cracks:
A. The offset flow caused by the misalignment of the nozzle and the crystallizer erodes the solidified shell of the tube billet; B. The melting property of the protective slag is poor, 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. Crystallization liquid level fluctuation (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 fluctuation of the crystallization liquid level should be stable; C. Use a suitable crystallization taper; D. Select protective slag with excellent performance; E. Use a hot top crystallizer.
Causes of transverse cracks: A. Too deep vibration marks are the main cause of transverse cracks; B. Increased (niobium, aluminum) content 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 depth of vibration marks on the inner arc surface of the ingot; 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 surface stable; D. Use protective slag with good lubrication 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 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 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 surface cracks on the steel pipe. Since the cracks on the surface of the tube billet will be severely oxidized during the heating process, the cracks are often accompanied by oxidation particles and decarburization after rolling.
Tube billet scarring and heavy skin:
Causes: The temperature of the molten steel is too low, the molten steel is too viscous, the nozzle is blocked, the injection flow is deviated, etc. The steel pipe external folding caused by the surface scarring and heavy skin of the tube billet 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 defect.
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, a small flying skin will be formed on the surface of the steel pipe.
Tube billet pits and grooves:
Causes of pits and grooves in the tube billet: On the one hand, it may be generated during the crystallization process of the casting, 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 damage or scratches on the surface of the tube billet when the casting billet has not been completely cooled. After perforation, folds or scars (pits) and large external folds (grooves) are formed on the surface of the rough tube.
“Ears” of the tube billet: mainly because the roll gap (the straightening roll of the continuous casting machine and the rolling roll of the rolling mill) is not closed. When the tube billet is straightened or rolled, the straightening roll or the rolling roll is pressed too much or the roll gap is too small. This causes too much wide metal to enter the roll gap. After perforation, a spiral external fold is generated on the surface of the rough tube. No matter what kind of surface defect of the tube billet is, 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 is 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 formation are insufficient deoxidation of molten steel and the gas content (especially hydrogen) in molten steel, which is also an important reason for the formation 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, and 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 for the formation 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 generated, and if there are any, it is a slight external fold.
Middle cracks and center cracks in tube billets: The middle cracks and center cracks of 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.
Loose 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 impact on the quality of the rough tube after oblique rolling and perforation.
1.3 Microstructure defects of tube billet: high magnification or electron microscope
When the composition and structure of the tube billet are uneven and severe segregation occurs, the steel pipe after rolling 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 billet, and non-metallic inclusions in tube billet.
2. Heating defects of tube billet: In the production of hot-rolled seamless steel pipes, two heatings are generally required from tube billet to finished steel pipe, namely heating before tube billet perforation and reheating of rough pipe 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 pipe. 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 the steel pipe annealing heat treatment during the cold rolling process is to eliminate the work hardening phenomenon caused by the cold processing of the steel pipe, reduce the deformation resistance of the steel, and create conditions for the continued processing of the steel pipe. 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 billet (steel pipe) (commonly known as yin and yang surface), oxidation, decarburization, heating cracks, overheating, and overburning. The main factors affecting the heating quality of 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. Too low a temperature will increase the deformation resistance of steel and reduce plasticity. Especially when the heating temperature cannot ensure that the metallographic structure of steel is completely transformed into austenite grains, the tendency of cracking of tube billet will increase during hot rolling. When the temperature is too high, the surface of the tube billet will undergo severe oxidation, decarburization, and even overheating or overburning.
Heating speed of tube billet: The size of the heating speed of tube billet is closely related to the generation of heating cracks in tube billet. When the heating speed is too fast, heating cracks are easy to occur in the tube billet. The main reason is: that when the temperature of the tube billet surface rises, the temperature difference between the metal inside the tube billet and the metal on the surface is generated, resulting in inconsistent thermal expansion of the metal and thermal stress. Once this thermal stress exceeds the fracture stress of the material, cracks will occur; the heating cracks of the tube billet may exist on the surface of the tube billet or inside. When the tube billet 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 billet is still at a low temperature after entering the heating furnace, a lower heating speed is adopted. As the temperature of the tube billet increases, the heating speed can be increased accordingly.
Heating time and holding time of tube billet: The length of the heating time and holding time of the tube billet is 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 and even overburning on the surface, which may cause the steel pipe to be scrapped in severe cases. Preventive measures: A. Ensure that the tube is heated evenly and completely transformed into an austenite structure; B. Carbides should be dissolved into austenite grains; C. Austenite grains should not be coarse and mixed crystals should not appear; D. The tube should not be overheated or overburned after heating.
In short, to improve the heating quality of the tube and prevent heating defects, the following requirements are generally followed when formulating the heating process parameters of the tube: A. Accurate heating temperature to ensure that the perforation process is carried out within the temperature range with the best permeability of the tube; B. Uniform heating temperature, strive to make the heating temperature difference of the tube along the longitudinal and transverse directions not more than ±10℃; C. There is less metal burnout, during the heating process, and the tube should be prevented from overoxidation, surface cracks, and bonding. D. The heating system should be reasonable, and the heating temperature, heating speed, and heating time (holding time) should be reasonably coordinated to prevent the tube blank from overheating or even overburning.
Post time: Sep-29-2024