How are low-temperature seamless steel pipes produced?

In modern industry, equipment and piping systems operating in low-temperature environments place higher demands on materials. As critical pressure-bearing components, seamless steel pipes must possess excellent low-temperature toughness, resistance to brittle fracture, and stable mechanical properties. So, how are low-temperature seamless steel pipes produced? This article will provide a comprehensive explanation of several aspects, including raw material selection, production processes, heat treatment, and testing and inspection.

1. Raw Material Selection

The performance of low-temperature seamless steel pipes depends primarily on the type of steel used. Commonly used steels include low-carbon steel, low-alloy steel, and special low-temperature-resistant steels. For example:

10Mn2V, 16Mn, 09Mn2Si, and low-alloy high-strength steels maintain excellent toughness in environments of tens or even hundreds of degrees Celsius below zero.

To improve low-temperature impact toughness, the carbon content in the steel must be strictly controlled to a low level, and alloying elements such as nickel, manganese, and vanadium are added to improve the grain structure and low-temperature performance.

At the raw material stage, the steel billet undergoes rigorous composition testing and ultrasonic testing to ensure it is free of serious internal defects.

II. Billet Preparation and Piercing

Seamless steel pipes for low-temperature use are typically produced using hot rolling or cold drawing. The key process is piercing:

The heated round billet is placed in a piercing mill, where the action of a mandrel and rollers forms a hollow shell.

This process requires strict temperature control, generally around 1150°C, to prevent coarse grains that reduce toughness.

For low-temperature pipes, controlling the temperature and speed during piercing is particularly critical to ensure uniform microstructure and reduce inclusions and cracks.

III. Hot Rolling and Cold Drawing

Hot Rolling Process

The shell is fed into a continuous rolling mill or sizing mill to be rolled into the desired outer diameter and wall thickness.

During hot rolling, the rolling temperature must be strictly controlled to avoid overheating or overcooling to ensure fine and uniform grains.

Cold Drawing Process

For low-temperature steel pipes requiring more precise specifications or higher performance, cold drawing or cold rolling is performed in addition to hot rolling.

After cold working, the dimensional accuracy and surface quality of the pipe are significantly improved, while the microstructure is further refined, which helps improve low-temperature impact toughness.

IV. Heat Treatment Process

Heat treatment is a crucial step in the production of seamless steel pipes for low-temperature use. Common methods include:

Normalizing: Heating to a certain temperature followed by air cooling refines the grain size and improves microstructure uniformity.

Quenching and tempering: Improves the strength and toughness of the steel and prevents brittle fracture at low temperatures.

Controlled Rolling and Controlled Cooling: Controls the temperature and cooling rate during the rolling process to maintain a fine grain size and enhance low-temperature performance.

Proper heat treatment can effectively improve the impact resistance of steel pipes in temperatures as low as -20°C, -40°C, and even -70°C.

5. Inspection and Quality Control

Quality inspections for low-temperature seamless steel pipes are extremely stringent. Common tests include:

Chemical composition analysis: Ensures that the alloying element ratio meets the requirements for low-temperature steel.

Nondestructive testing (UT/ET/RT): Inspects the steel pipe for defects such as cracks and inclusions.

Mechanical property testing: Tests include tensile strength, yield strength, and elongation.

Low-temperature impact testing (Charpy V-notch impact testing): Performed under specified low-temperature conditions to ensure that the steel pipe does not undergo brittle fracture.

Dimensional accuracy and surface inspection: Ensure that the pipe is free of defects such as folds, delamination, and pits.

6. Product Applications

Low-temperature seamless steel pipes, produced through rigorous processes, are widely used in:

Liquefied natural gas (LNG) storage and transportation systems

Cryogenic pressure vessels

Petrochemical plants

Cryogenic engineering equipment and piping systems

These applications place extremely high demands on the safety and stability of steel pipes, and therefore production standards must strictly adhere to international standards such as ASTM A333, GB 5310, and EN 10216-4.

VII. Conclusion

The production of low-temperature seamless steel pipes involves more than just a single rolling process; it encompasses meticulously controlled processes from raw material selection, smelting, piercing, hot rolling/cold drawing, heat treatment, and final testing. Each step directly impacts the pipe's reliability and safety in cryogenic environments. It is this rigorous process management that enables low-temperature seamless steel pipes to maintain stable performance in extreme environments, safeguarding the energy, chemical, and cryogenic storage and transportation industries.