Defects of Seamless Steel Tubes and Their Prevention (Part Two)
Pits and grooves of tube blanks
The reasons for the pits and grooves in the tube blank: on the one hand, they may be caused by the crystallization process of the casting blank, which is related to the too large taper of the mold or the uneven cooling of the secondary cooling zone. On the other hand, when the casting blank is not completely cooled, the surface of the tube blank is mechanically bumped or scratched. Folds or scars (pits) and large outward folds (grooves) are formed on the surface of the capillary after perforation.
Ears of tube blanks: the roll gap (the tension leveling roll of the continuous casting machine or the roll of the rolling mill) is not closed. When the tube blank is stretched or rolled, the tension leveling roll or the roll is pressed down too much or too little, causing too much widening metal to enter the roll gap. After perforation, the surface of the capillary produces a spiral outward fold. No matter what kind of surface defects of the tube blank, there may be defects on the surface of the steel tube in the rolling process, and the rolled steel tube will be scrapped in serious cases. Therefore, it is necessary to strictly control the surface quality of the tube blank and remove defects on the surface. Only the tube blanks that meet the requirements can be processed.
Low magnification structure defects of tube blanks
The subcutaneous air bubbles of the tube blank by the visual inspection: The first reason is that the deoxidation of molten steel is insufficient, and the gas content, especially hydrogen in the molten steel is the second reason. This defect forms irregular flying skin on the outer surface of the steel pipe after perforation or rolling, and the shape is similar to the nail; in severe cases, it will cover the outer surface of the steel pipe. Such defects are small and shallow and can be removed by grinding.
Subcutaneous cracks of tube blanks: The main reason is that the temperature of the surface layer of the continuous casting round tube blank changes repeatedly and undergoes multiple phase transformations. Generally, no defect occurs. If there is a defect, it will be a slight outward fold.
Intermediate cracks and center cracks of tube blanks: The middle crack and center crack of the continuous casting round tube blank are the main reasons for the inward folding of the seamless steel tube. The causes of cracks are very complex, involving the effects of heat transfer, penetration, and stress in the solidification of the billet, which are generally controlled by the solidification process of the billet in the secondary cooling zone.
Porosity and shrinkages of tube blanks: They are caused by the effect of advanced grains in the solidification process of the casting blank; the liquid metal’s movement is formed based on the cooling to the solidification direction and the shrinkage is hindered. If the continuous casting round tube blank has porosity and shrinkage cavity, it does not have much influence on the quality of the capillary tube that is cross-rolled and pierced.
Microstructure defects of tube blanks: high magnification or electron microscope
When the composition and structure of the tube blank are uneven and serious segregation occurs, the rolled steel tube will show a serious banded structure, which will affect the mechanical properties and corrosion properties of the steel tube, and make its performance unsatisfactory. When the inclusion content of the tube blank is too much, it will not only affect the performance of the steel tube, but also may cause the steel tube to crack in the manufacturing process.
Factors: Harmful elements in steel, composition and microstructure segregation of tube blank, and non-metallic inclusions in tube blank.
The reasons for the pits and grooves in the tube blank: on the one hand, they may be caused by the crystallization process of the casting blank, which is related to the too large taper of the mold or the uneven cooling of the secondary cooling zone. On the other hand, when the casting blank is not completely cooled, the surface of the tube blank is mechanically bumped or scratched. Folds or scars (pits) and large outward folds (grooves) are formed on the surface of the capillary after perforation.
Ears of tube blanks: the roll gap (the tension leveling roll of the continuous casting machine or the roll of the rolling mill) is not closed. When the tube blank is stretched or rolled, the tension leveling roll or the roll is pressed down too much or too little, causing too much widening metal to enter the roll gap. After perforation, the surface of the capillary produces a spiral outward fold. No matter what kind of surface defects of the tube blank, there may be defects on the surface of the steel tube in the rolling process, and the rolled steel tube will be scrapped in serious cases. Therefore, it is necessary to strictly control the surface quality of the tube blank and remove defects on the surface. Only the tube blanks that meet the requirements can be processed.
Low magnification structure defects of tube blanks
The subcutaneous air bubbles of the tube blank by the visual inspection: The first reason is that the deoxidation of molten steel is insufficient, and the gas content, especially hydrogen in the molten steel is the second reason. This defect forms irregular flying skin on the outer surface of the steel pipe after perforation or rolling, and the shape is similar to the nail; in severe cases, it will cover the outer surface of the steel pipe. Such defects are small and shallow and can be removed by grinding.
Subcutaneous cracks of tube blanks: The main reason is that the temperature of the surface layer of the continuous casting round tube blank changes repeatedly and undergoes multiple phase transformations. Generally, no defect occurs. If there is a defect, it will be a slight outward fold.
Intermediate cracks and center cracks of tube blanks: The middle crack and center crack of the continuous casting round tube blank are the main reasons for the inward folding of the seamless steel tube. The causes of cracks are very complex, involving the effects of heat transfer, penetration, and stress in the solidification of the billet, which are generally controlled by the solidification process of the billet in the secondary cooling zone.
Porosity and shrinkages of tube blanks: They are caused by the effect of advanced grains in the solidification process of the casting blank; the liquid metal’s movement is formed based on the cooling to the solidification direction and the shrinkage is hindered. If the continuous casting round tube blank has porosity and shrinkage cavity, it does not have much influence on the quality of the capillary tube that is cross-rolled and pierced.
Microstructure defects of tube blanks: high magnification or electron microscope
When the composition and structure of the tube blank are uneven and serious segregation occurs, the rolled steel tube will show a serious banded structure, which will affect the mechanical properties and corrosion properties of the steel tube, and make its performance unsatisfactory. When the inclusion content of the tube blank is too much, it will not only affect the performance of the steel tube, but also may cause the steel tube to crack in the manufacturing process.
Factors: Harmful elements in steel, composition and microstructure segregation of tube blank, and non-metallic inclusions in tube blank.
