What is Lined Pipe?

The development of steel pipe

In the 1940s, due to the limitations of metallurgical technology and process, C, Mn and Si were added to the steel for pipes to increase the strength of steel.

Line Pipes

Specification & Standard – API Spec 5L . ASTM/ASME A106 . ASTM/ASME A53 .

Steel Grades – PSL1 . PSL2 . X42 . X52 . X60 . X65 . X70 . X80 

The chemical composition of this carbon is: C 0.1 to 0.25%, Mn: 0.4 to 0.7%, Si: 0.1 to 0.5%, part of the S, P content and other residues. Then, by increasing the manganese content, decreasing the C content and adding a small amount of alloying content (usually less than 3%), the upper limit of C content is reduced by 0.25% to 0.2%, the upper boundary. Limit the Mn content to increase from 0.7% to 1.8%. Strong metals; as with plain carbon, this steel is delivered in the hot rolled or normalized condition; this steel is called C+Mn steel, e.g., 20#, X42, X46, X52; these steels are usually designed to meet strength requirements.

With the high carbon content and the increase in pipe diameter, it is impossible to improve the strength of the steel pipe by relying only on increasing the content of C and Mn. Solving the problem of improving strength will make the steel pipe weldable and durable. . Meet the requirements. The project.

In the 1960s, the steel industry broke through the production process of C-Mn alloying and normalizing, adding the right amount of Nb, V, Ti (no more than 0.2%) and other trace elements in steel to reduce the cost of steel C elements control rolling, cooling control (TMPC) and other processes successfully improved the quality of steel, this steel is known as micro-alloyed high-strength low-alloy steel, a new level of cold project.

Early micro-alloyed steels usually had only one micro-alloy element, such as Mn-Nb steels, Mn-V steels and Mn-Ti steels. Better pipes could be provided, hence the emergence of Mn-Nb-V steels, such as X60 and X65.

In the 1970s, based on the Mn-Nb series, the Mn-Mo-Nb series of micro-alloyed high-strength steels were produced, such as X70 and X80 steels.

With the demand of marine and polar pipelines, ultra-low carbon Mn-Nb-Mo-B-Ti series of high strength steels such as X100 and X120 were developed in 1990s.

China’s West-East gas transmission project first used X70 pipeline steel, and now there are several steel mills have successfully piloted X80, X100 pipeline steel.

The current development of oil and gas transmission to large diameter, high transmission pressure direction, which requires pipeline steel with strength, high durability and good welding process, which leads to the development of pipeline steel also to the ultra-high pressure direction. . – Low carbon and a small amount of alloy.

Second, the role of chemical composition and carbon balance
A, the role of chemical products
Chemical composition is an important factor in determining the performance of steel pipe, the following is a brief description of the role of many important chemical substances in the steel pipe.

Carbon (C): is the main element to improve the strength of steel, is also the cheapest material, because the carbon content increases, yield strength and tensile strength increases, but ductility and impact strength decreases, and weldability. Decrease

The weldability of the steel deteriorates when the carbon content exceeds 0.23%.

GB/T9711 in PSL2 steel pipe maximum C content, depending on the steel grade, the range is: C ≤ 0.12 ~ 0.24%.

Manganese (Mn): Manganese is an important element to reduce the C content and improve the strength of steel. Mn can reduce the temperature change, change the organization, adjust the grain size and improve the strength and hardness of steel by strengthening the material, intergranular strengthening strength and phase change, for example, 16Mn steel is 40% higher than A3. Yield strength . Manganese also reduces temperature change and increases the strength of steel.

In steelmaking, manganese is a good desulfurizer and weak deoxidizer, which forms MnS with S, removes sulfur and oxygen from the hot embrittlement of steel, improves the cold embrittlement of steel and increases the hardness of metal. However, if the manganese content is too high, manganese segregation will occur in the billet, which reduces the workability of the steel.

The content of manganese in pipeline steel is usually controlled at 1.1% to 2.0%.

GB/T9711 in PSL2 steel pipe maximum Mn content, depending on the steel grade, the range is: Mn ≤ 1.2 ~ 1.8%

Silicon (Si): the addition of silicon in carbon steel can refine the grain and improve the strength and hardness of steel. Excessive silicon content can reduce the plasticity, toughness and weldability of steel. In the steelmaking process, silicon is a reducing agent and a strong deoxidizer, so the steel usually contains 0.15-0.30% silicon.

GB/T9711 in PSL2 steel pipe maximum Si content, depending on the steel grade, the range is: Si ≤ 0.4 ~ 0.45%.

Phosphorus (P): In steel pipe, phosphorus is a harmful substance that reduces the plasticity and hardness of the steel, increases cold brittleness, and makes the welding performance and cold bending performance worse.

GB/T9711 in PSL2 steel pipe maximum P content: P ≤ 0.025%.

Sulfur (S): sulfur is also a problem. Causes thermal embrittlement and delamination of steel, reducing the ductility and durability of steel, easy to fracture during forging and rolling; sulfur also affects the welding properties of steel.

The maximum S content of PSL2 steel pipe in GB/T9711: S ≤ 0.015%.

Niobium (Nb): in the rolling control and cooling process control, has the role of preventing austenite grain growth, delaying austenite recrystallization, refining the organization and grain, strengthening precipitation and reducing plasticity and brittleness . The temperature change. Can improve the strength and hardness of steel, but the effect of preventing the growth of the welded hot forming zone and improve the heat-affected zone is not very effective.

GB/T9711 in PSL2 steel pipe maximum niobium content: Nb ≤ 0.05%.

Vanadium (V): in the rolling control and cooling process control, has the effect of reducing temperature changes, increasing precipitation and weakening the grain. It increases the strength and durability of the steel. In steelmaking, vanadium is an excellent deoxidizer and vanadium forms carbides with carbon to improve the corrosion resistance of steel at high temperatures. Generally pipeline steel composite design, V is not used separately.

GB/T9711 PSL2 steel pipe maximum vanadium content, depending on the steel grade, the range is: V ≤ 0.04 ~ 0.1%; L360 steel grades below, Nb + V ≤ 0.06%.

Titanium (Ti): important precipitation improvement effect in the control of rolling and the control of cooling process, improve the average effect of grain and the power to reduce the temperature. Sulfide distribution pattern; it has a special contribution to the durability of the welded heat-affected zone, it can reduce the sensitivity of the welded heat-affected zone to cracking and improve welding performance, but if the content of Ti is large, a large number of will form TiC particles, which will reduce toughness.

GB/T9711 in PSL2 steel pipe maximum titanium content: Ti ≤ 0.04%, while the highest grade of L415 steel is specified as Nb + V + Ti ≤ 0.15%.

Molybdenum (Mo): has the role of preventing the formation of large ferrite, promoting the transformation of needle ferrite, increasing the grain size; can also reduce the temperature and improve the precipitation force of Nb. to improve the strength and durability of steel.

Mo can also compensate for energy through the Bauschinger effect (the strengthening of plastic materials caused by subsequent loading during plastic processing of metals, so that the metal material sees a loss of soft plastic, i.e., product, during back loading). . Force) .

The maximum molybdenum content of PSL2 steel pipe in GB/T9711, depending on the steel grade, ranges from: Mo ≤ 0.15 to 0.5%.

Nickel (Ni): The phase change behavior of nickel in steel is similar to that of Mo, which can lower the phase change temperature of steel, improve the organization, refine the grain, and increase the strength of steel, while maintaining good plasticity and durability. . It also improves the corrosion resistance of steel.

The maximum nickel content of PSL2 steel pipe in GB/T9711, depending on the steel grade, ranges from: Ni ≤ 0.3 to 0.5%.

Chromium (Cr): improve the strength and hardness of steel, but also causes changes in steel, reducing hardness; chromium can improve the oxidation resistance and corrosion resistance of steel.

GB/T9711 in PSL2 steel pipe maximum chromium content, depending on the steel grade, the range is: Cr ≤ 0.3 ~ 0.5%.

Copper (Cu): the right amount of copper can improve the corrosion resistance and hydrogen cracking resistance of pipeline steel. It can prevent hydrogen atoms from penetrating into the metal and reduce the average crack length. When the copper content is greater than 0.2%, it also forms a dense protective layer on the steel, reducing the average corrosion of the HIC and steel plate and making the average crack length zero. Copper also reduces the phase change temperature and increases the strength and durability of the metal. However, when the copper content is greater than 0.5%, the plasticity of the steel is greatly reduced; it also affects the weldability.

The maximum copper content of PSL2 steel pipe in GB/T9711: Cu ≤ 0.5%.