-100°C low temperature seamless steel pipe

Through chemical composition design, steelmaking, rolling, heat treatment and other process control, the low-temperature impact toughness and corrosion resistance of -100 t low-temperature seamless steel pipes are guaranteed. It is important to point out that the elbow products processed by seamless steel pipes should be subjected to matching heat treatment to ensure that the quality of the elbow is not lower than that of the base material. The developed low-temperature seamless steel pipe has the advantages of wide size range, excellent low-temperature toughness and corrosion resistance, which ensures its safety in low-temperature environments.

Key words: low temperature seamless steel pipe; -100 steel grade design; process control; performance evaluation; corrosion resistance test; elbow heat treatment

3.5Ni low-temperature seamless steel pipe has relatively moderate strength and good low-temperature toughness, and is widely used in the manufacture of low-temperature equipment in industries such as petroleum, chemical, and fertilizer industries with operating temperatures ranging from -45 to -101 t . With the development of related industries, the requirements for materials are not limited to basic requirements such as mechanical properties and welding properties, and some customers have also put forward the requirements for the H2S stress corrosion resistance of low-temperature pipes.

• Development of low temperature seamless steel pipe

1. Component Design

Based on the customer's technical requirements for low-temperature seamless steel pipes, and considering the production organization and cost of steel pipes, the composition design of low-temperature seamless steel pipes must ensure that the product has appropriate strength, high low-temperature toughness, convenient heat treatment process, and excellent welding performance , The ability to resist hydrogen-induced cracking.
American Standard ASTM A 333/A 333M — 2011 “Seamless and Welded Steel Pipes for Low Temperature Operations” and Japanese Standard JIS G 3460 — 1988 ( Steel Steel Pipes for Low Temperature Pipelines) only stipulate the upper limit of w ( C , respectively no more than 0.19% and 0.18% . C element is an element that increases steel strength and reduces toughness, especially for products with low temperature toughness requirements, the C content in the product should be reduced as much as possible . Necessary. But for electric furnace smelting, the C content cannot be reduced indefinitely
, otherwise the production cost will be increased. After testing, w ( C ) is designed at 0.06% ~ 0.12% .
Mn element mainly acts as solid solution strengthening in steel, which can make up for part of the strength loss caused by C reduction. Some studies in recent years have shown that: when w (Mn <2.0% , the strength of steel increases with the increase of Mn content, while the trend of impact toughness decline is very small, and does not affect its brittle transition temperature . However, if the Mn content is too high, it will cause Severe banded structure, reduced toughness and anisotropy, etc., so w ( Mn ) should be controlled at 0.4% ~ 0.6% .
S element is one of the most harmful elements in steel, which seriously deteriorates the HIC and SSC resistance of pipeline steel . French Schaw winhold D et al. showed that with the increase of S content in steel, the crack sensitivity rate increased significantly; only when w ( S < 0.001 2% , HIC hydrogen-induced cracking was significantly reduced, or even negligible , so S element is One of the key elements of control.
P element is a kind of easy segregation element in pipeline steel, especially when w (P A 0.015% , the segregation of P increases sharply, and promotes the increase of hardness of segregation zone, which reduces the HIC performance; at the same time, P element also deteriorates the welding performance , significantly reduce the low-temperature impact toughness of steel, increase the brittle transition temperature of steel, and cause cold and brittle steel pipes, so the P content should be controlled in production.
For the performance requirements of low-temperature steel, it must first ensure that it has sufficient impact toughness at the service temperature. Metal materials with body-centered cubic lattices have the phenomenon of low-temperature brittleness, but the low-temperature toughness of steel can be improved by refining grains, alloying, and improving the purity of steel. Adding Ni element to the steel can increase the content of retained austenite, promote the formation of reverse transformed austenite, and improve the low temperature toughness of the matrix. The alloying cost of Ni element accounts for a large proportion of the cost of Ni -based low-temperature steel, and the price of Ni fluctuates greatly. In the design, it should be considered to reduce the cost of Ni -based low-temperature steel, so choose w ( N ) = 3.4% ~ 3.6% .

Based on the above chemical composition design ideas and drawing on the successful experience of 3.5Ni steel plate (4), the chemical composition design of 3.5Ni low temperature seamless steel pipe is shown in Table 1 .
Table 1 Chemical composition design of 3.5Ni low temperature seamless steel pipe (mass fraction %

C	Si	mn	P	S	Ni
0.06 ~ 0.12	0.15 ~ 0.35	0.4 ~ 0.6	£ 0.012	£ 0.005	3.4 ~ 3.6

1. Process control of low temperature seamless steel pipe steelmaking

Since the low-temperature seamless steel pipe is mainly used to transport chemical raw materials such as liquid ethylene and liquid propylene, its boiling point temperature is below -50 °C. At the same time, chemical companies should try their best to improve the transmission efficiency, so the transmission pressure will be increased accordingly, which will increase the pressure on low-temperature pipes or components under low-temperature conditions. The low-temperature toughness of seamless steel pipes is an important indicator to ensure the safety of transportation. Therefore, during the steelmaking process , there are extremely strict requirements on composition control, steel purity , inclusion content (all types of inclusion grades are not greater than 2.0 , w (S = £ 0.005% and w ( P = £ 0.012%) , and it is necessary to take Special technological measures.
In response to this situation, it was decided to take the following process control measures:

• Control the C content. The matching of electric furnace and refining is an effective means to achieve precise control of C content. Strictly control the C content at the end of electric furnace smelting. Refining outside the furnace is carried out by increasing the flow rate of argon gas, the diameter of the dipping tube, and blowing oxygen on the VD or RH furnace. Vacuum de- C , which can ensure the precise C content requirements of pipeline steel.

The connection management between the upper and lower furnaces, as well as the protective casting, can effectively reduce the non-metallic inclusions in the steel.

• Rolling process control

Since the specifications of the low-temperature seamless steel pipes purchased by customers are ① 60 ~ 720 mm x 4 ~ 10 mm , the geometric dimensions of products with thin wall thickness

The accuracy will directly affect the customer's welding quality and construction efficiency at the construction site, so the PQF continuous rolling tube unit is used for rolling, and the heating temperature and time of the tube billet are adjusted to reduce the eccentricity during the piercing process and reduce the uneven wall thickness.

Heat treatment process control of low temperature seamless steel pipe
The heat treatment of seamless steel pipes is carried out within a reasonable temperature range, which is determined by the CCT curve (continuous cooling transformation curve) of the steel. The designed steel is austenitized at 840 r and kept for 3 minutes to test the CCT curve , Observe the relationship between the cooling rate and the metallographic structure, so as to determine the heat treatment process during production.

CCT curve of steel and equipment conditions, it is better to adopt normalizing or quenching and tempering process for low-temperature seamless steel pipe, so that the production organization is easier. Since the specifications include ① 60 mmx4 mm slender pipes, if the water quenching and tempering process is used, it is easy to bend, so the normalizing or normalizing + tempering process is more applicable.

The actual performance of low temperature seamless steel pipe
The heat treatment of low-temperature seamless steel pipe needs to consider the heating rate and holding time to avoid the burning of the steel pipe and the generation of oxide scale, and at the same time control the austenite grain size to improve the strength and toughness matching and corrosion resistance of the steel pipe.

Low temperature seamless steel pipe strength
Strength is the premise to ensure the service safety of steel pipes, but too high strength and hardness will lead to a decrease in low temperature toughness and corrosion resistance; therefore, on the premise of meeting the requirements of the ASTM A 333 / A 333M standard [ 6 ] , try to reduce and stabilize the strength of steel pipes .

Low temperature seamless steel pipe impact toughness
The customer requires that the Charpy V -notch impact value of the full-scale sample of the seamless steel pipe is not less than 50 J at -100 r . After testing , the impact value of the 3.5Ni low-temperature seamless steel pipe produced is higher than the customer's requirement, ensuring its Safety for use in low temperature environments.

Corrosion resistance of low temperature seamless steel pipe
Since cryogenic pipelines may be used to transport substances such as liquid carbonyl sulfide, or there is a certain corrosive environment during the transportation process, customers should also be concerned about the ability of cryogenic pipelines to resist hydrogen-induced cracking.
NACE TM 0284 standard [ 7 ] was used to carry out HIC test on low temperature seamless steel pipe , the solution was solution A , the test temperature was ( 25±3 ) r , and the test period was 96 h . After the test, the 3 samples were cut and examined under a microscope, no cracks were found.

Result and analysis of low temperature seamless steel pipe
Product design and actual performance
The two main performance indicators of cryogenic pipelines are low temperature toughness and corrosion resistance. The good low-temperature toughness brought by Ni element can be improved by reducing C , controlling P and S , and controlling the generation of inclusions , but the selection of Ni content should comprehensively consider cost and performance. It is necessary to rely on reasonable heat treatment methods and systems to control the strength and hardness of the product to ensure that it has good corrosion resistance.
Key points of elbow manufacturing
Judging from the results of the mechanical properties of the elbow before and after heat treatment, even if it is a hot simmer bending process, if no subsequent heat treatment is performed, the performance of the elbow will seriously deviate from the base material, resulting in an increase in strength and hardness, but a decrease in corrosion resistance and low temperature toughness. Reduced; therefore, the same heat treatment process as the base metal can be used for post-bending treatment. From the actual results, the performance has been significantly improved, which is close to the base metal, thereby reducing the difficulty of on-site welding construction [ 9 ] and improving the reliability of production and operation.

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