The production of hot-rolled seamless tube generally requires two heatings from the billet to the finished steel pipe, that is, the heating of the billet before piercing and the reheating of the blank pipe after rolling before sizing. When producing cold-rolled steel tubes, it is necessary to use intermediate annealing to eliminate the residual stress of the steel pipes. Although the purpose of each heating is different, the heating furnace may also be different, but if the process parameters and heating control of each heating are improper, heating defects will occur in the tube blank (steel pipe) and affect the quality of the steel pipe.
The purpose of heating the tube billet before piercing 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 include annular heating furnaces, walking heating furnaces, inclined bottom heating furnaces and car bottom heating furnaces.
The purpose of reheating the billet pipe before sizing is to increase and uniform the temperature of the blank pipe, improve the plasticity, control the metallographic structure, and ensure the mechanical properties of the steel pipe. The heating furnace mainly includes walking reheating furnace, continuous roller hearth reheating furnace, inclined bottom type reheating furnace and electric induction reheating furnace. The steel pipe annealing heat treatment in the cold rolling process is to eliminate the work hardening phenomenon caused by the cold working of the steel pipe, reduce the deformation resistance of the steel, and create conditions for the continuous processing of the steel pipe. The heating furnaces used for annealing heat treatment mainly include walking heating furnaces, continuous roller hearth heating furnaces and car bottom heating furnaces.
The common defects of seamlesss tube billet heating are: uneven heating of tube billet, oxidation, decarburization, heating crack, overheating and overburning, etc. The main factors affecting the heating quality of tube billets are: heating temperature, heating speed, heating and holding time, and furnace atmosphere.
1. Tube billet heating temperature:
The main performance is that the temperature is too low or too high, or the heating temperature is uneven. If the temperature is too low, it will increase the deformation resistance of the steel and reduce the 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 will increase during the hot rolling process of the tube blank. When the temperature is too high, severe oxidation, decarburization and even overheating or overburning will occur on the surface of the tube blank.
2. Tube billet heating speed:
The heating speed of the tube billet is closely related to the occurrence of heating cracks of the tube blank. When the heating rate is too fast, the tube blank is prone to heating cracks. The main reason is: when the temperature on the surface of the tube blank rises, there is a temperature difference between the metal inside the tube blank and the metal on the surface, 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 cracks of the tube blank may exist on the surface of the tube blank or inside. When the tube blank with heating cracks is perforated, it is easy to form cracks or folds on the inner and outer surfaces of the capillary. Prevention prompts: When the tube blank is still at low temperature after entering the heating furnace, a lower heating rate is used. As the tube blank temperature increases, the heating rate can be increased accordingly.
3. Tube billet heating time and holding time:
The heating time and holding time of the tube billet are related to heating defects (surface oxidation, decarburization, coarse grain size, overheating or even overburning, etc.). Generally speaking, if the heating time of the tube blank at high temperature is longer, it is more likely to cause severe oxidation, decarburization, overheating or even overburning of the surface, and in severe cases, the steel tube will be scrapped.
Precaution:
A. Ensure that the tube billet is heated evenly and completely transformed into austenite structure;
B. Carbide should dissolve into austenite grains;
C. Austenite grains cannot be coarse and mixed crystals cannot appear;
D. After heating, the tube blank cannot be overheated or overburned.
In short, in order 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. The heating temperature is accurate to ensure that the piercing process is carried out in the temperature range with the best penetrability of the tube blank;
B. The heating temperature is 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. There is less metal burning loss, and the tube billet should be prevented from over-oxidation, surface cracks, bonding, etc. during the heating process.
D. The heating system is reasonable, and the reasonable coordination of heating temperature, heating speed and heating time (holding time) should be done well to prevent the tube billet from overheating or even overburning.