The directly forming to square tube mills will go through heat treatment steps when making steel pipes. Heat treatment is a common metal material processing technology, which generally includes three processes: heating, heat preservation, and cooling. These processes are connected with each other to achieve different processing effects according to the different requirements of metal materials.

There are many types of steel pipes, and the steel grades (species) used are not the same. The chemical composition of the same variety of steel pipes may also be different, but the steel pipes can meet the relevant technical requirements after the heat treatment of the directly forming to square tube mills. There are mainly 5 types of heat treatment processes for steel pipes, and we introduce types 4-5.

4. Annealing

After the steel pipe is heated to the annealing temperature and kept for a certain period of time, it is slowly cooled to a certain temperature with the furnace and then discharged from the furnace for cooling.

The role of the process:

①Reduce the hardness of the steel pipe and increase its plasticity to facilitate subsequent cutting or cold deformation processing;

②Refine grains, eliminate structural defects, uniform internal structure and composition, improve the performance of steel pipes or prepare for subsequent processes;

③ Eliminate the internal stress of the steel pipe to prevent deformation or cracking.

According to the steel pipe processing method, it is divided into weld seam annealing, full tube annealing and stainless steel bright annealing.

①Weld annealing

②Full tube annealing

③Bright annealing of stainless steel

5. Solution treatment (stainless steel):

The stainless steel tube is heated to the solid solution temperature, so that the carbides and various alloying elements are fully and uniformly dissolved in the austenite, and then quickly cooled, so that the carbon and alloying elements are too late to precipitate, and the heat treatment process of obtaining a single austenite structure.

The role of the process:

① Uniform the internal structure of the steel pipe and the composition of the steel pipe;

②Eliminate hardening during processing to facilitate subsequent cold deformation processing;

③Restore the corrosion resistance of stainless steel.

Generally speaking, the solid solution temperature should be increased correspondingly for the grades with many kinds of alloying elements and high content. Especially for steels with high manganese, molybdenum, nickel, and silicon content, the softening effect can only be achieved by increasing the solid solution temperature and making it fully dissolved. However, stabilized steel, such as 1Cr18Ni9Ti, when the solid solution temperature is high, the carbides of the stabilizing elements are fully dissolved in austenite, and will precipitate in the grain boundaries in the form of Cr23C6 during subsequent cooling, causing intergranular corrosion. In order to prevent the carbides of stabilizing elements (TiC and Nbc) from decomposing and solid solution, the lower limit solid solution temperature is generally adopted.

The directly forming to square tube mills can be used to make steel pipes according to the actual situation to perform the above heat treatment process operations.