Defects Caused by Heating and Their Prevention
The uniform temperature of the pierced billet after rolling by the pipe mill generally cannot be guaranteed, and it is difficult to meet the requirements of the final rolling temperature of the pierced billet. Therefore, the pierced billet needs to be reheated metallographically before sizing. Heating furnaces mainly include walking reheating furnaces, continuous roller hearth reheating furnaces, inclined hearth reheating furnaces and electric induction reheating furnaces. The main quality defects of the pierced billet in the heating process include uneven heating of the pierced billet, too high or too low heating temperature, unreasonable metallographic structure, serious surface oxidation and decarburization, overheating or overburning and mechanical scratches in heating furnaces.
Uneven heating of the pierced billet: Generally speaking, there is a temperature difference between the head and tail of the pierced billet after it is rolled by the pipe mill. The slower the rolling speed of the pipe mill is, the longer the rolling time becomes, and the greater the temperature difference between the head and tail of the pierced billet is. When the pierced billet with temperature difference between the head and tail enters the reheating furnace, if the heating time is not enough, the temperature difference is difficult to be eliminated. Another situation is that if the furnace chamber of the reheating furnace is wide and uneven heating happens for the burner, it is easy to produce longitudinal temperature difference for the pierced billet. When the wall thickness of the pierced billet is thicker and the heating time is shorter, the temperature unevenness is more serious. In order to ensure the uniformity of the heating temperature of the pierced billet, it is necessary to ensure the heating time of the pierced billet and the uniformity of the heating and atmosphere in the furnace. In addition, the verified burner type should be selected.
The unreasonable microstructure of the steel pipe: the performance of the steel pipe that does not need to be heat treated after sizing is realized by the reheating before sizing and the cooling system after sizing. If the reheating temperature and heating time of the pierced billet are not suitable, the grain size of austenite may be different, or the carbides in the steel do not completely enter the austenite grain, which will cause the microstructure of the pierced billet to be unreasonable and affect the performance of the steel pipe.
Oxidation and decarburization of the surface of the pierced billet: In the process of reheating the pierced billet, one of the defects that often occurs is the serious oxidation and decarburization of the surface. When the iron oxide scale is thick, a pitted surface will happen for the steel pipe after sizing. The effective way to eliminate the serious decarburization and pitted surface of the steel pipe 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 well controlled, and a faster heating speed and shorter heating time should be adopted on the premise of not causing defects in the pierced billet.
Scratches on the surface of the pierced billet: they are mainly caused by the relative sliding of the pierced billet on the screw rod of the heating furnace. At present, there is no way to eliminate the scratches on the surface of the pierced billet. The following aspects are mainly used to reduce the scratches:
A. Coat the surface of the roller table and the screw rod to improve the surface hardness and smoothness.
B. Use high-temperature resistant alloy steel to make roller tables and screw rods.
C. Adopt the cold-water technology in the roller table.
Uneven heating of the pierced billet: Generally speaking, there is a temperature difference between the head and tail of the pierced billet after it is rolled by the pipe mill. The slower the rolling speed of the pipe mill is, the longer the rolling time becomes, and the greater the temperature difference between the head and tail of the pierced billet is. When the pierced billet with temperature difference between the head and tail enters the reheating furnace, if the heating time is not enough, the temperature difference is difficult to be eliminated. Another situation is that if the furnace chamber of the reheating furnace is wide and uneven heating happens for the burner, it is easy to produce longitudinal temperature difference for the pierced billet. When the wall thickness of the pierced billet is thicker and the heating time is shorter, the temperature unevenness is more serious. In order to ensure the uniformity of the heating temperature of the pierced billet, it is necessary to ensure the heating time of the pierced billet and the uniformity of the heating and atmosphere in the furnace. In addition, the verified burner type should be selected.
The unreasonable microstructure of the steel pipe: the performance of the steel pipe that does not need to be heat treated after sizing is realized by the reheating before sizing and the cooling system after sizing. If the reheating temperature and heating time of the pierced billet are not suitable, the grain size of austenite may be different, or the carbides in the steel do not completely enter the austenite grain, which will cause the microstructure of the pierced billet to be unreasonable and affect the performance of the steel pipe.
Oxidation and decarburization of the surface of the pierced billet: In the process of reheating the pierced billet, one of the defects that often occurs is the serious oxidation and decarburization of the surface. When the iron oxide scale is thick, a pitted surface will happen for the steel pipe after sizing. The effective way to eliminate the serious decarburization and pitted surface of the steel pipe 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 well controlled, and a faster heating speed and shorter heating time should be adopted on the premise of not causing defects in the pierced billet.
Scratches on the surface of the pierced billet: they are mainly caused by the relative sliding of the pierced billet on the screw rod of the heating furnace. At present, there is no way to eliminate the scratches on the surface of the pierced billet. The following aspects are mainly used to reduce the scratches:
A. Coat the surface of the roller table and the screw rod to improve the surface hardness and smoothness.
B. Use high-temperature resistant alloy steel to make roller tables and screw rods.
C. Adopt the cold-water technology in the roller table.
