First, the production standards of seamless steel pipes:
1. Seamless steel pipe quality requirements.
① Chemical composition of steel: The chemical composition of steel is the most important factor affecting the performance of seamless steel pipes. It is also the main basis for formulating the process parameters of rolling tubes and heat treatment of steel pipes. In the seamless steel pipe standard, according to the different uses of steel pipes, corresponding requirements are put forward for the smelting of steel and the manufacturing methods of tube blanks, and strict regulations are made on the chemical composition. In particular, requirements are put forward for the content of certain harmful chemical elements (arsenic, tin, antimony, lead, bismuth) and gases (nitrogen, hydrogen, oxygen, etc.). To improve the uniformity of chemical composition in steel and the purity of steel, reduce non-metallic inclusions in tube blanks, and improve their distribution morphology, refining equipment outside the furnace is often used to refine molten steel, and even electro-slag furnaces are used to remelt and refine tube blanks.
② Steel pipe geometric size accuracy and outer diameter: steel pipe outer diameter accuracy, wall thickness, ovality, length, steel pipe curvature, steel pipe end face cutting slope, steel pipe end face groove angle and blunt edge, cross-sectional dimensions of special-shaped steel pipes
A. Steel pipe outer diameter accuracy: The outer diameter accuracy of seamless steel pipes depends on the method of sizing (reducing) diameter (including tension reduction), equipment operation, process system, etc. The outer diameter accuracy is also related to the hole processing accuracy of the sizing (reducing) machine and the deformation distribution and adjustment of each frame. The outer diameter accuracy of cold-rolled (rolled) seamless steel pipes is related to the accuracy of the die or rolling hole.
B. Wall thickness: The wall thickness accuracy of seamless steel pipes is related to the heating quality of the tube blank, the process design parameters and adjustment parameters of each deformation process, the quality of the tool, and its lubrication quality. The uneven distribution of steel pipe wall thickness is uneven transverse wall thickness and uneven longitudinal wall thickness.
③ Surface quality of steel pipes: The standard stipulates the “surface finish” requirements of steel pipes. However, there are as many as 10 kinds of surface defects in steel pipes due to various reasons during the production process. Including surface cracks (cracks), hairline, inner fold, outer fold, puncture, inner straight, outer straight, delamination, scar, pit, convex, pitting (pit), abrasion (scratches), inner spiral, outer spiral, blue line, straightening, roller mark, etc. The main reasons for these defects are caused by surface defects or internal defects of the tube blank. On the other hand, they are generated during the production process, that is, if the rolling process parameters are not designed reasonably, the surface of the tool (mold) is not smooth, the lubrication conditions are not good, the hole design and adjustment are unreasonable, etc., it may cause surface quality problems of the steel pipe; or if the tube blank (steel pipe) is heated, rolled, heat treated and straightened if the heating temperature is improperly controlled, the deformation is uneven, the heating and cooling speed is unreasonable, or the straightening deformation is too large, it may also cause the steel pipe to have surface cracks.
④ Physical and chemical properties of steel pipes: The physical and chemical properties of steel pipes include mechanical properties at room temperature, mechanical properties at a certain temperature (thermal strength or low-temperature performance), and corrosion resistance (anti-oxidation, anti-water erosion, acid, and alkali resistance, etc.). Generally speaking, the physical and chemical properties of steel pipes mainly depend on the chemical composition, organizational structure, purity of steel, and the heat treatment method of steel pipes. Of course, in some cases, the rolling temperature and deformation system of steel pipes also affect the performance of steel pipes.
⑤ Process performance of steel pipes: The process performance of steel pipes includes flattening, flaring, curling, bending, ring pulling, and welding of steel pipes.
⑥ Metallographic structure of steel pipes: The metallographic structure of steel pipes includes low-magnification and high-magnification structures of steel pipes.
⑦ Special requirements for steel pipes: special conditions required by customers.
Second, quality problems in the production process of seamless steel pipes – quality defects of tube billets and their prevention.
1. Quality defects of tube billets and their prevention: 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 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.
① The diameter and ovality of the round tube billet are out of tolerance: In practice, it is generally believed 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 pierced raw tube. The larger the pressure reduction rate of the head, the more likely the cavity of the tube billet is to be 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 billet and the outer diameter and wall thickness of the raw tube. When the hole type is adjusted, if the outer diameter of the tube billet 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 billet is over-negative, the pressure reduction rate before the head decreases, and the first bite point of the tube billet moves toward the throat, which makes the piercing process difficult to achieve. Ovality out of tolerance: When the ovality of the tube billet is uneven, the tube billet rotates unstably after entering the piercing deformation zone, and the roller will scratch the surface of the tube billet, resulting in surface defects of the raw tube.
② The end face bevel of the round tube billet is out of tolerance: The wall thickness of the front end of the pierced raw tube of the tube billet is uneven. The main reason is that when the tube billet has no centering hole, the plug meets the end face of the tube billet during the piercing process. Due to the large slope 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): Surface cracks of tube billet: 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 erodes 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 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. 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 depth of vibration marks 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 the tube: the temperature of the molten steel is too low, the molten steel is too sticky, the nozzle is blocked, the injection flow deviates, etc. Due to the scarring and heavy skin on the surface of the tube billet, the steel tube folding is different from the scarring and 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.
Causes of pits and grooves in 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 bruises or scratches on the surface of the tube billet before the billet is completely cooled. Folds or scars (pits) and large folds (grooves) are formed on the surface of the rough tube after perforation.
Tube billet “ears”: 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 has too much pressure 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 steel tube surface during the tube rolling process. In severe cases, the rolled steel tube 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 structural defects of tube billets:
Visual subcutaneous bubbles in tube billets: The reasons for the occurrence are insufficient deoxidation of molten steel and 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 tube after perforation or rolling. The shape is similar to “fingernails”. In severe cases, it will cover the outer surface of the steel tube. This type of defect is small and shallow and can be removed by grinding.
The main reason for the subcutaneous cracks in the tube billet is that the temperature of the surface layer of the continuous casting round tube billet changes repeatedly and forms multiple phase changes. Generally, no defects are produced, and if there are any, they are slight folds.
Middle cracks and center cracks in tube billets: Middle cracks and center cracks in continuous casting round tube billets are the main causes of the inward bending of seamless steel pipes. 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.
Looseness and shrinkage of tube billets: mainly due to the advanced grain effect of the billet during the solidification process, the movement of liquid metal is formed based on the shrinkage resistance caused by cooling in the solidification direction. If the continuous casting round tube billet has looseness and shrinkage, it will not have much effect on the quality of the rough tube with oblique rolling and perforation.
1.3 Microstructural defects of tube billets: high magnification or electron microscope.
When the composition and organization of the tube billet are uneven and severe segregation occurs, the rolled steel pipe will present a severe banded organization, thereby affecting the mechanical properties and corrosion properties of the steel pipe, and making its performance not meet the requirements. When the inclusion content of 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 organization 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 heating processes are generally required from tube billets to finished steel pipes, namely, heating of tube billets before perforation and reheating of rough tubes after rolling before sizing. When producing cold-rolled steel pipes, intermediate annealing is required to eliminate the residual stress of steel pipes. 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 billets (steel pipes) will produce heating defects and affect the quality of steel pipes. The purpose of heating the tube billets before perforation is to improve the plasticity of steel, reduce the deformation resistance of steel, and provide a good metallographic structure for rolling pipes. The heating furnaces used are annular, walking beam heating furnaces, inclined bottom heating furnaces, and car bottom heating furnaces. The purpose of reheating rough tubes before sizing is to increase and even the temperature of rough tubes, improve plasticity, control metallographic structure, and ensure the mechanical properties of steel pipes. 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 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.
① 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.
② The main factors affecting the heating quality of tube billet: are heating temperature, heating speed, heating and holding time, and furnace atmosphere.
③ 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 steel and reduce 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.
④ 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 billet surface rises, the metal inside the tube billet and the metal on the surface have a temperature difference, resulting in inconsistent metal thermal expansion 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 perforated, 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 used. 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 heating time and holding time of tube billet is related to heating defects (surface oxidation, decarburization, coarse grain size, overheating or even overburning, etc.). Generally speaking, the longer the tube billet is heated at high temperatures, the more likely it is to cause severe surface oxidation, decarburization, overheating, or even overburning, 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 an austenite structure; B. Carbides should be dissolved into austenite grains; C. Austenite grains cannot be coarse and mixed crystals cannot appear; D. The tube billet cannot be overheated or overburned after heating.
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; C. Less metal burning, during the heating process, the tube billet should be prevented from overoxidation, surface cracks, adhesion, etc. D. Reasonable heating system, the heating temperature, heating speed and heating time (holding time) should be reasonably coordinated to prevent the tube billet from overheating or even overburning.
Post time: Jan-17-2025