The production of hot-rolled seamless steel pipes generally requires two heating processes from the billet to the finished steel pipe, that is, the heating before the piercing of the billet and the pierced billet before sizing. When producing cold-rolled steel pipes, intermediate annealing is required to eliminate the residual stress of the steel pipe. The purpose of each heating of the tube is different, and the heating furnace may also be different. If the process parameters and heating control of each heating are improper, defects will occur for the tube blank (steel tube) and the quality of the steel tube will be affected.
 
The purpose of heating the tube blank 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 heating furnaces, inclined bottom heating furnaces and car bottom heating furnaces.
 
The purpose of reheating the pierced billet pipe before sizing is to increase and uniformize the temperature of the pierced billet, improve the plasticity, control the metallographic structure, and ensure the mechanical properties of the steel pipe. Heating furnaces mainly include walking reheating furnaces, continuous roller hearth reheating furnaces, inclined hearth reheating furnaces and electric induction furnaces. The purpose of the annealing heat treatment of the steel pipe in the cold rolling process is to eliminate the work hardening caused by the cold working 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 in annealing heat treatment mainly include walking furnaces, continuous roller hearth furnaces and car hearth furnaces.
 
Common defects of heating tube blanks are uneven heating of tube blank (steel pipes), oxidation, decarburization, heating cracks, overheating, overburning, etc.
The main factors affecting the heating quality of the tube blank are heating temperatures, heating speeds, heating and holding time, and furnace atmosphere.
 
Heating temperatures of tube blanks
The temperature is too low or too high, or the heating temperature is not uniform. If the temperature is too low, it will increase the deformation resistance of the steel and reduce the plasticity. 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 blank in the hot rolling process will increase. When the temperature is too high, serious oxidation, decarburization and even overheating or overburning will occur on the surface of the tube blank.
 
The heating speed of the tube blank
The heating speed of the tube blank is closely related to the generation of heating cracks in the tube blank. When the heating speed is too fast, the tube blank is prone to heating cracks. The main reason is that when the temperature of the surface of the tube blank increases, the metal inside the tube blank and on the surface produce a temperature difference, resulting in inconsistent thermal expansion of the metal and thermal stress. Once the thermal stress exceeds the fracture stress of the material, cracks will occur. The heating crack of the tube blank may exist on the surface of the tube blank, or may exist inside. When the tube blank with the heating crack is perforated, it is easy to form cracks or folds in the inner and outer surfaces of the tube blank.

Preventive measures: When the tube blank is still at a low temperature after entering the heating furnace, a lower heating rate should be adopted, and the heating speed can be increased with the increase of the temperature of the tube blank.
 
The heating time and holding time of the tube blank
The heating time and holding time of the tube blank is related to defects caused by heating, such as surface oxidation, decarburization, coarse grain sizes, overheating and even overburning. Generally speaking, if the heating time of the tube blank at a high temperature is longer, it is more likely to cause serious oxidation, decarburization, overheating and even overburning of the surface, and in serious cases, the steel tube will be scrapped.
 
Preventive measures
A. Ensure that the tube blank is heated evenly and completely transformed into austenite structure.
B. Carbides should be dissolved into austenite grains.
C. Austenite grains should not be coarse and mixed crystals cannot appear.
D. The tube blank cannot be overheated or overburned after heating.
 
In short, to improve the heating quality of the tube blank and prevent defects caused by heating, the following aspects should be paid attention to when formulating the heating process parameters of the tube blank:
A. The heating temperature should be accurate to ensure that the perforation process is within the best temperature range for the tube blank.
B. The heating temperature should be uniform, and strive to make the heating temperature difference between the longitudinal and transverse directions of the tube blank not greater than ±10°C.
C. The metal burning loss is less, and overoxidation, surface cracks and bonding should be avoided for the tube blank in the heating process.
D. The heating system is reasonable, and the reasonable coordination of the heating temperature, heating speed and heating time (holding time) should be done to prevent the tube blank from overheating or even overburning.