In the modern industrial system and infrastructure construction, carbon steel pipes, as alloy pipes with iron (Fe) and carbon (C) as the core components, have become the core basic materials penetrating multiple fields such as energy, chemical industry, construction, and machinery, relying on their mature manufacturing processes, excellent comprehensive performance and high cost performance. From deep-sea oil and gas transportation to urban pipeline laying, from large-scale machinery manufacturing to national major project construction, carbon steel pipes are everywhere. Their technological iteration and application expansion have always resonated with the progress of industrial civilization.

I. Core Definition and Classification System of Carbon Steel Pipes

The core definition of carbon steel pipes is: a general term for pipes with iron and carbon as the main components, the carbon content usually between 0.05% and 2.0% by mass, containing a small amount of impurity elements such as silicon, manganese, sulfur and phosphorus, and no large amount of alloy elements. There are various classification methods, which can be mainly divided from two core dimensions: carbon content and manufacturing process. The performance and applicable scenarios of different types of products are significantly different.

(1) Classification by Carbon Content

Carbon content is the core factor determining the performance of carbon steel pipes. According to this, it can be divided into three categories, which are suitable for scenarios with different strength requirements:

• Low-carbon steel (C≤0.25%): It has excellent plasticity, easy weldability and good toughness, but relatively low strength. It is mainly used for building structural parts, ordinary fluid transportation pipelines, etc. The common grade is Q235B, which is widely used in scaffolding, building support columns and other scenarios.

• Medium-carbon steel (0.25%<C≤0.6%): It has balanced strength and toughness, and its mechanical properties are better than low-carbon steel. It is mostly used for mechanical parts, medium-pressure pipelines, etc. Its hardness can be further improved through heat treatment, which is suitable for industrial scenarios with certain strength requirements.

• High-carbon steel (C>0.6%): It has high hardness and strong wear resistance, but poor plasticity and weldability. It is mainly used for tools, springs, high-pressure wear-resistant pipe fittings, etc., and is widely used in the field of mechanical processing.

(2) Classification by Manufacturing Process

The manufacturing process directly determines the pressure-bearing capacity, precision and applicable scenarios of carbon steel pipes, which are mainly divided into two categories: seamless steel pipes and welded steel pipes:

• Seamless steel pipes: Formed by hot rolling or cold drawing process, with no weld structure, high pressure-bearing capacity and high dimensional accuracy. Among them, hot-rolled seamless pipes need to heat the steel billet to above 1200℃, which are made by piercing and continuous rolling, and the tensile strength can exceed 480MPa; cold-drawn seamless pipes are processed twice at room temperature, with dimensional accuracy up to ±0.05mm and surface roughness controlled within Ra0.8μm. Seamless steel pipes are mainly used in high-pressure systems, such as boiler pipes, oil drilling pipes, deep-sea oil and gas pipelines and other high-end scenarios.

• Welded steel pipes: Formed by curling steel plates and then welding, which are divided into straight seam welding, spiral welding and other types, with simple manufacturing process and low cost. Its pressure-bearing capacity is lower than that of seamless steel pipes, and it is mainly used for low-pressure fluid transportation (such as tap water, gas), building structure support and other scenarios, such as urban gas pipe networks and scaffolding steel pipes. Among them, spiral welded steel pipes can also be used for medium and low-pressure oil and gas transportation pipelines due to their high welding strength.

II. Core Performance of Carbon Steel Pipes: Advantages and Disadvantages Coexist

The wide application of carbon steel pipes comes from their prominent performance advantages, and their inherent shortcomings also promote the industry to continuously carry out technological innovation to achieve performance upgrading.

(1) Core Advantages

• Outstanding cost performance: The raw materials (iron and carbon) are abundant in reserves, the production process is mature, and the manufacturing cost is much lower than that of stainless steel pipes and alloy steel pipes. On the premise of meeting the basic performance requirements, it can greatly reduce the project investment cost, and is one of the preferred pipes for large-scale industrial applications.

• Excellent mechanical properties: It has high overall strength and good pressure-bearing capacity, can bear high internal pressure and external load, and has good plasticity and weldability. It is easy to carry out on-site processing such as cutting, bending and welding, with flexible and convenient installation, and is suitable for various complex construction scenarios.

• Improved supply and standards: It has complete specifications and sufficient market supply. The relevant standard systems for design, manufacturing, installation and acceptance are mature, covering Chinese national standards (GB/T 8163), American standards (ASTM A53, A106), Japanese standards (STKM11A), etc. The technical threshold is low, and it is easy to purchase and maintain later.

(2) Inherent Shortcomings and Improvement Directions

• Easy to corrode: This is the core shortcoming of carbon steel pipes. In humid environments, acid-base media or buried conditions, it is prone to electrochemical corrosion, leading to thinning of the pipe wall, leakage and even pipe explosion. Anti-corrosion treatments such as hot-dip galvanizing, epoxy coating and cathodic protection are needed to extend the service life. For example, the new anti-corrosion process can make carbon steel pipes free of red rust for 1500 hours in the neutral salt spray test, which is far higher than the standard of 400~600 hours for traditional hot-dip galvanized steel pipes.

• Low-temperature brittleness: In the environment below -20℃, the toughness of carbon steel pipes will decrease significantly, the brittleness will increase, and it is prone to fracture. Therefore, when applied in extremely cold areas, it is necessary to select low-temperature steel modified by adding nickel elements, or take thermal insulation protection measures.

• Large weight: The high density of carbon steel leads to large self-weight of the pipeline, which increases the cost of transportation, hoisting and supporting structures. In the construction of large-scale pipe networks, it is necessary to optimize the construction scheme in combination with lightweight design.

III. Application Scenarios of Carbon Steel Pipes: Covering Multiple Industries and Full Scenarios

The application scenarios of carbon steel pipes almost cover the core fields of the national economy. From basic people's livelihood to high-end industry, from conventional scenarios to extreme working conditions, they meet the differentiated needs of different industries with their customizable performance.

(1) Construction and Infrastructure Field

This is the most basic application scenario of carbon steel pipes, mainly using low-carbon steel welded pipes and grades such as Q235B. It is used for building scaffolding, support columns, bridge skeletons, urban water supply and drainage pipelines, gas pipelines, etc. Among them, the crimped carbon steel pipes rely on the "double crimping connection technology", the construction speed is increased by more than 30% compared with the traditional method, and there is no hot work. It is suitable for the construction of gas pipe networks in densely populated residential areas and commercial core areas, and greatly reduces the maintenance cost and safety risks.

(2) Energy Field

The energy field is the core application scenario of carbon steel pipes, mainly using seamless steel pipes, which are suitable for the transportation needs of high temperature, high pressure and complex media. In the oil and gas field, seamless steel pipes of API 5L standard form the main artery of energy transportation. For example, the Φ1219×26.4mm X80 steel grade pipeline used in the third West-East Gas Pipeline Project has a working pressure of 12MPa and an annual gas transmission capacity of 30 billion cubic meters; in the power field, it is used for steam transmission and cooling water pipelines in power plants, which need to bear high temperature and high pressure working conditions. In some scenarios, microalloyed modified carbon steel pipes are used to improve low-temperature toughness and creep performance.

(3) Chemical and Manufacturing Field

In the chemical field, carbon steel pipes are used for transporting crude oil, chemicals, reaction media, etc. Some pipes are lined with rubber, enamel and other protective materials to enhance corrosion resistance, which are suitable for complex working conditions such as refining and coal-to-ethylene glycol; in the mechanical manufacturing field, they are used for hydraulic cylinder pipes, high-speed rail bogie pipe fittings, etc. For example, the 27SiMn material hydraulic pipe after cold drawing + honing processing has a roundness error of no more than 0.02mm and a surface hardness controlled in the HRC28-32 range; in addition, in industries such as food and beverage, pharmaceutical, automobile manufacturing, pulp and paper making, carbon steel pipes can be used for fluid transportation, waste treatment and other scenarios after appropriate anti-corrosion treatment to meet the specific requirements of the industry.

(4) Special Scenario Applications

With the technological upgrading, carbon steel pipes are gradually adapting to more extreme scenarios. For example, carbon steel pipes used for deep-sea oil and gas drilling need to meet the requirements of high collapse resistance and hydrogen sulfide corrosion resistance; the X70 grade pipeline steel used in the extremely cold section of the China-Russia Eastern Route Natural Gas Pipeline has a low-temperature toughness of 220J at -40℃; in the hydrogen energy storage and transportation pilot project, customized carbon steel pipe fittings are used for hydrogen transportation, promoting the development of the clean energy industry.