Definition and classification of steel pipes

1. Definition and classification of steel pipes
(I) Basic concept of steel pipes
Steel pipes are long steel strips with hollow structures, which are widely used in many fields such as fluid transportation, machinery manufacturing, building structures and military industry. Because steel pipes have high strength and low weight, they are lighter and easier to process than solid steel under the same bending and torsional strength. Therefore, they are widely used in the manufacture of various mechanical parts, engineering structures, etc. In addition, steel pipes are also used to manufacture weapons and equipment, such as gun barrels and cannon barrels, showing their importance in the fields of industry and national defense.

(II) Classification of steel pipes
Steel pipes can be divided into two categories according to the manufacturing process: seamless steel pipes and welded steel pipes (seamless steel pipes). According to the cross-sectional shape, they can be further divided into round pipes and special-shaped pipes. Among them, round steel pipes are the most widely used because of their uniform force, mature manufacturing process and wide range of applications. However, according to different usage requirements, square pipes, rectangular pipes, semi-circular pipes, hexagonal pipes, equilateral triangle pipes, octagonal pipes and other special-shaped steel pipes can also be manufactured to meet the needs of special application scenarios.

For steel pipes that are subjected to fluid pressure, hydraulic tests must be carried out to test their pressure resistance and quality to ensure that there will be no leakage, wetting or expansion within the specified pressure range, otherwise they will be deemed unqualified. In addition, according to industry standards or customer needs, some steel pipes also need to undergo curling tests, expansion tests, flattening tests, etc. to further test their processing performance and structural stability.

2. Seamless steel pipes
(I) Manufacturing process of seamless steel pipes
Seamless steel pipes are made of steel ingots or solid tubes through a perforation process to form rough tubes, and then further processed and formed by hot rolling, cold rolling or cold drawing. The specifications of seamless steel pipes are usually expressed in outer diameter × wall thickness (unit: mm) to distinguish different models and uses.

According to the manufacturing method, seamless steel pipes can be divided into hot-rolled seamless steel pipes and cold-rolled (or cold-drawn) seamless steel pipes.

(II) Hot-rolled seamless steel pipes
Hot-rolled seamless steel pipes refer to steel billets that are heated to high temperatures and then perforated, rolled and sized to form steel pipes of the required size. The outer diameter of this type of steel pipe is usually greater than 32mm, and the wall thickness ranges from 2.5mm to 75mm. Hot-rolled seamless steel pipes have high processing temperatures, relatively uniform internal structures, and excellent mechanical properties. They are mainly used for pipelines and structural parts that withstand high pressure, high temperature or corrosive media.

According to the purpose, hot-rolled seamless steel pipes can be divided into:

General steel pipes: used for fluid transportation, machinery manufacturing, structural parts, etc.;

Low-medium pressure boiler steel pipes: used for low-medium pressure pipeline systems for boilers and heat exchangers;

High-pressure boiler steel pipes: used for steam transportation pipelines under high temperature and high pressure environments;

Alloy steel pipes: used in occasions with high requirements for strength, heat resistance, and corrosion resistance, such as petrochemicals, nuclear power and other fields;

Stainless steel pipes: mainly used in corrosion-resistant environments, such as food, medicine, chemical and other industries;

Petroleum cracking pipes: pipes used to transport high-temperature fluids during petroleum refining;

Geological steel pipes: used for geological drilling equipment, able to withstand wear and pressure in complex environments.

(III) Cold-rolled (cold-drawn) seamless steel pipe
Cold-rolled (cold-drawn) seamless steel pipe refers to a steel pipe that is further processed and shaped by cold rolling or cold drawing at room temperature. Compared with hot-rolled seamless steel pipes, cold-rolled steel pipes have higher dimensional accuracy and better surface quality, and are suitable for precision equipment with strict dimensional requirements. The outer diameter of cold-rolled seamless steel pipes can reach 6mm, and the thinnest wall thickness can be as thin as 0.25mm, while the outer diameter of thin-walled pipes can reach 5mm, and the wall thickness is less than 0.25mm.

The categories of cold-rolled seamless steel pipes include:

General steel pipes: similar to hot-rolled steel pipes, used for general fluid transportation and structural parts manufacturing;

Low-medium pressure boiler steel pipes, high-pressure boiler steel pipes: used for hot water and steam transportation in boiler systems;

Alloy steel pipes: used for special applications that withstand high temperature, high pressure, and high strength;

Stainless steel pipes: suitable for fields with high corrosion resistance requirements, such as food, medicine, and chemicals;

Petroleum cracking pipes: used for high-temperature fluid transportation in the petrochemical industry;

Carbon thin-walled steel pipes, alloy thin-walled steel pipes, stainless thin-walled steel pipes: suitable for precision instruments, medical equipment and other industries;

Special-shaped steel pipes: including square pipes, rectangular pipes, polygonal pipes, etc., suitable for structural engineering and special equipment manufacturing.

3. Materials and applications of seamless steel pipes
The production materials of seamless steel pipes mainly include carbon steel, alloy steel, stainless steel, etc., and their selection depends on the application environment and performance requirements.

(I) Carbon steel seamless pipe
Common grades: 10#, 20#, 30#, 35#, 45# and other high-quality carbon structural steel.

Low carbon steel (such as 10#, 20#): Mainly used for fluid transportation, such as pipeline systems for transporting water, oil, natural gas and other media.

Medium carbon steel (such as 45#, 40Cr): Commonly used to manufacture mechanical parts, such as load-bearing components of automobiles and tractors.

(II) Low alloy steel seamless pipe
Common grades: 16Mn, 5MnV, etc.

Compared with carbon steel, low alloy steel seamless pipe has higher strength and corrosion resistance, and is widely used in engineering structures and machinery manufacturing.

(III) Alloy steel seamless pipe
Common grades: 40Cr, 30CrMnSi, 45Mn2, 40MnB, etc.

These materials are usually used to manufacture mechanical parts and engineering components with high requirements for strength, wear resistance and high temperature resistance, such as high-pressure boiler tubes, pressure vessel tubes, power equipment tubes, etc.

4. Delivery status of seamless steel pipes
Depending on the production process and customer needs, seamless steel pipes can be delivered in different states:

Hot-rolled steel pipes: generally delivered in hot-rolled state or after heat treatment (normalizing, annealing, tempering, etc.) to ensure good organizational properties;

Cold-rolled (cold-drawn) steel pipes: usually delivered in annealed or normalized state to improve mechanical properties and reduce internal stress, and improve the processing adaptability of products.

5. Summary
Seamless steel pipes play an important role in petrochemical, machinery manufacturing, aerospace, military industry, power engineering and other fields due to their excellent mechanical properties, high temperature resistance, corrosion resistance and other characteristics. With the continuous development of technology, the manufacturing process and application fields of seamless steel pipes are also expanding to meet the needs of modern industry for high-performance steel pipes.