Defects of Seamless Steel Tubes and Their Prevention (Part One)
1. Defects and prevention of tube blanks
The tube blanks used in the production of seamless steel tubes can be continuous casting round tube blanks, rolling (forging) round tube blanks, centrifugal casting round hollow tube blanks and steel ingots. In the actual production process, the continuous casting round tube blank is mainly used, because the continuous casting round tube blank has low costs and good surfaces.
1.1 The appearance and surface defects of the tube blank
1.1.1 Defects on appearance
For the round tube blank, defects on the appearance of the tube blank mainly include the diameter and ovality of the tube blank, and the out-of-tolerance end face cut inclination. For the steel ingot, defects on the appearance of the tube blank mainly are the incorrect shape of the steel ingot due to the wear of the ingot mold.
The diameter and ovality of the round tube blank are out of tolerance.
In practice, it is generally believed that when the tube blank is perforated, the size of the reduction rate before the perforated head is proportional to how much the perforated capillary is folded inward. The greater the reduction rate of the perforated head is, the easier the inner surface cracks of the capillary tube become. In the manufacturing process, the hole parameters of the piercing mill are determined according to the nominal diameter of the tube blank and the outer diameter and wall thickness of the capillary. When the hole type is adjusted, if the outer diameter of the tube blank exceeds the positive tolerance, the reduction rate before the perforated head increases; internal folding will occur for the perforated capillary tube. If the outer diameter of the tube blank exceeds the positive allowance, the reduction rate before the perforated head decreases, resulting in the bite point of a tube blank moveing to the pore throat, which will make the perforation process difficult. Out-of-tolerance ovality: When the ovality of the tube blank is uneven, the tube blank will rotate and be unstable after entering the perforation deformation zone, and the roller will scratch the surface of the tube blank, resulting in surface defects of the capillary.
The cut slope of the end face of the round tube blank is out of tolerance: The wall thickness of the front end of the perforated capillary of the tube blank is uneven. The main reason is that when the tube blank does not have centering holes, the head meets the end face of the tube blank in the perforation process. Because the end face of the tube blank has a large slope, it is difficult for the nose of the head to center the center of the tube blank, resulting in the uneven wall thickness of the end face of the capillary tube.
1.1.2 Surface defects of continuously casting round tube blanks
Cracks on the surface of the tube blank: longitudinal cracks, transverse cracks and net cracks
Causes of longitudinal cracks
A. The scouring of the solidified shell of the tube blank is caused by the misalignment of the nozzle and the mold.
B. The melting property of the mold slag is poor, and the liquid slag layer is too thick or too thin, resulting in an uneven thickness of the slag film, and too thin partial solidification shells of the tube blank.
C. When the crystal liquid level fluctuates, that is, the liquid level fluctuation is greater than ± 10mm, the crack occurrence rate is about 30%.
D. P and S content in steel: when P is greater than 0. 017% and S 0. 027%, the longitudinal crack increases.
E. When C in the steel is 0.12% to 0.17%, the longitudinal crack tends to increase.
Measures
The tube blanks used in the production of seamless steel tubes can be continuous casting round tube blanks, rolling (forging) round tube blanks, centrifugal casting round hollow tube blanks and steel ingots. In the actual production process, the continuous casting round tube blank is mainly used, because the continuous casting round tube blank has low costs and good surfaces.
1.1 The appearance and surface defects of the tube blank
1.1.1 Defects on appearance
For the round tube blank, defects on the appearance of the tube blank mainly include the diameter and ovality of the tube blank, and the out-of-tolerance end face cut inclination. For the steel ingot, defects on the appearance of the tube blank mainly are the incorrect shape of the steel ingot due to the wear of the ingot mold.
The diameter and ovality of the round tube blank are out of tolerance.
In practice, it is generally believed that when the tube blank is perforated, the size of the reduction rate before the perforated head is proportional to how much the perforated capillary is folded inward. The greater the reduction rate of the perforated head is, the easier the inner surface cracks of the capillary tube become. In the manufacturing process, the hole parameters of the piercing mill are determined according to the nominal diameter of the tube blank and the outer diameter and wall thickness of the capillary. When the hole type is adjusted, if the outer diameter of the tube blank exceeds the positive tolerance, the reduction rate before the perforated head increases; internal folding will occur for the perforated capillary tube. If the outer diameter of the tube blank exceeds the positive allowance, the reduction rate before the perforated head decreases, resulting in the bite point of a tube blank moveing to the pore throat, which will make the perforation process difficult. Out-of-tolerance ovality: When the ovality of the tube blank is uneven, the tube blank will rotate and be unstable after entering the perforation deformation zone, and the roller will scratch the surface of the tube blank, resulting in surface defects of the capillary.
The cut slope of the end face of the round tube blank is out of tolerance: The wall thickness of the front end of the perforated capillary of the tube blank is uneven. The main reason is that when the tube blank does not have centering holes, the head meets the end face of the tube blank in the perforation process. Because the end face of the tube blank has a large slope, it is difficult for the nose of the head to center the center of the tube blank, resulting in the uneven wall thickness of the end face of the capillary tube.
1.1.2 Surface defects of continuously casting round tube blanks
Cracks on the surface of the tube blank: longitudinal cracks, transverse cracks and net cracks
Causes of longitudinal cracks
A. The scouring of the solidified shell of the tube blank is caused by the misalignment of the nozzle and the mold.
B. The melting property of the mold slag is poor, and the liquid slag layer is too thick or too thin, resulting in an uneven thickness of the slag film, and too thin partial solidification shells of the tube blank.
C. When the crystal liquid level fluctuates, that is, the liquid level fluctuation is greater than ± 10mm, the crack occurrence rate is about 30%.
D. P and S content in steel: when P is greater than 0. 017% and S 0. 027%, the longitudinal crack increases.
E. When C in the steel is 0.12% to 0.17%, the longitudinal crack tends to increase.
Measures
A. Ensure the nozzle and the mold are centered.
B. The fluctuation of the crystal liquid level should be stable.
C. Use the appropriate crystal taper.
D. Select the casting powder with excellent performance.
E. Use the hot top crystallizer.
Causes of transverse cracks
Causes of transverse cracks
A. Too deep vibration marks are the main reason for transverse cracks.
B. The content of niobium or aluminum in the steel increases, which is the cause.
C. The tube blank is straightened at a temperature of 700 to 900°C.
D. The secondary intensity of cooling is too great.
Measures
A. Adopt crystallizers with high frequency and small amplitude to reduce the depth of vibration marks on the inner arc surface of the casting blank.
B. The secondary cooling area adopts a stable weak cooling system to ensure that the surface temperature is greater than 900 degrees during straightening.
C. Keep the crystal liquid level stable.
D. Use casting powder with good lubricity and low viscosity.
Causes of mesh cracks on surfaces
A. The high-temperature casting blank absorbs the copper of the crystallizer, and after the copper becomes liquid, it seeps out along the austenite grain boundary.
B. Residual elements such as copper and tin in the steel remain on the surface of the tube blank and seep out along the grain boundary.
Measures
A. The surface of the mold is chrome plated to increase the surface hardness.
B. Use an appropriate amount of secondary cold water.
C. Control residual elements in steel.
D. Control Mn or S value to ensure it is greater than 40. It is generally believed that when the surface crack depth of the tube blank does not exceed 0.5 mm, the cracks will be oxidized in the heating process, and the surface cracks of the steel tube will not appear. Since the surface cracks of the tube blank are severely oxidized in the heating process, the cracks are often accompanied by oxidized material points and decarburization after rolling.
Scarring and double skin of tube blanks
Causes of scarring and double skin
The temperature of molten steel is too low. The molten steel is too viscous, and the nozzle is blocked; the injection flow deviates. The outward folds of the steel tube formed by the scarring on the surface of the tube blank and double skin are different from that in the rolling process. They have characteristics of obvious oxidation, accompanied by oxidized material points and serious decarburization, and ferrous oxide appears at the defective part.
Gas holes of the tube blank
Generally, some small holes are formed on the surface of the tube blank due to the rupture of subcutaneous bubbles in the casting process of molten steel. After the tube blank is rolled, small flying skin will be formed on the surface of the steel tube.
D. The secondary intensity of cooling is too great.
Measures
A. Adopt crystallizers with high frequency and small amplitude to reduce the depth of vibration marks on the inner arc surface of the casting blank.
B. The secondary cooling area adopts a stable weak cooling system to ensure that the surface temperature is greater than 900 degrees during straightening.
C. Keep the crystal liquid level stable.
D. Use casting powder with good lubricity and low viscosity.
Causes of mesh cracks on surfaces
A. The high-temperature casting blank absorbs the copper of the crystallizer, and after the copper becomes liquid, it seeps out along the austenite grain boundary.
B. Residual elements such as copper and tin in the steel remain on the surface of the tube blank and seep out along the grain boundary.
Measures
A. The surface of the mold is chrome plated to increase the surface hardness.
B. Use an appropriate amount of secondary cold water.
C. Control residual elements in steel.
D. Control Mn or S value to ensure it is greater than 40. It is generally believed that when the surface crack depth of the tube blank does not exceed 0.5 mm, the cracks will be oxidized in the heating process, and the surface cracks of the steel tube will not appear. Since the surface cracks of the tube blank are severely oxidized in the heating process, the cracks are often accompanied by oxidized material points and decarburization after rolling.
Scarring and double skin of tube blanks
Causes of scarring and double skin
The temperature of molten steel is too low. The molten steel is too viscous, and the nozzle is blocked; the injection flow deviates. The outward folds of the steel tube formed by the scarring on the surface of the tube blank and double skin are different from that in the rolling process. They have characteristics of obvious oxidation, accompanied by oxidized material points and serious decarburization, and ferrous oxide appears at the defective part.
Gas holes of the tube blank
Generally, some small holes are formed on the surface of the tube blank due to the rupture of subcutaneous bubbles in the casting process of molten steel. After the tube blank is rolled, small flying skin will be formed on the surface of the steel tube.
