3.5Ni low-temperature seamless steel tubes have relatively moderate strength and good low-temperature toughness, and are widely used in the manufacture of cryogenic equipment for petroleum, chemical, fertilizer, and other industries with operating temperatures of -45 to -101°C. With the development of related industries, the requirements for materials are not limited to basic requirements such as mechanical properties and welding performance. Some customers have also proposed the requirement for anti-H2S stress corrosion resistance of cryogenic pipes.
A well-known steel pipe manufacturing company in China is based on new technical requirements. Starting from the design of steel grades, the steelmaking, rolling, and heat treatment processes for 3.5Ni low-temperature seamless steel pipes are strictly controlled to ensure 3.5Ni cold-seam seamless steel pipes. Good toughness matching and resistance to H2S stress corrosion. Based on the customer's technical requirements for low temperature seamless steel tubes, considering the production organization and cost of the steel tubes, the composition design of the low temperature seamless steel tubes needs to ensure that the products have appropriate strength, high low temperature toughness, convenient heat treatment process, and excellent welding performance. Resistance to hydrogen induced cracking.
The low temperature seamless steel tube steelmaking process control measures are as follows:
(1) Control C content. Matching the electric furnace with refining is an effective means to achieve accurate control of the C content. Strictly controls the C content at the end of the smelting of the electric furnace. Furnace refining is performed on a VD or RH furnace by increasing the argon flow rate and the diameter of the impregnation tube and the oxygen blowing method. Remove C to ensure the exact C content of pipeline steel;
(2) Reduce P, S content. Through the use of high-quality scrap steel and appropriate refining slag to meet the needs of desulfurization, through the high alkalinity slag made of electric furnace and strict control of steel slag to reduce the P content in the steel.
(3) To ensure the Ca content in the steel and weak stirring time, to strengthen the connection management between the upper and lower heat of the continuous casting, and to protect the pouring, can effectively reduce the non-metallic inclusions in the steel.
Rolling process control. Because the specifications of low-temperature seamless steel tubes purchased by customers are Φ60 to 720mm×4 to 10mm, the geometrical accuracy of thin-walled products will directly affect the welding quality and construction efficiency of customers at the construction site. Therefore, the PQF tandem tube mill is used. Rolling, and adjusting the billet heating temperature and time, reduce the eccentricity of the perforation process, reducing the uneven wall thickness.
Heat treatment process control. The heat treatment of the seamless steel pipe is based on a reasonable temperature range, which is determined by the steel CCT curve. The designed steel grades were austenitized at 840°C and incubated for 3 min to test the CCT curve. The relationship between the cooling rate and metallographic structure was observed to determine the heat treatment process during production.
According to the CCT curve and equipment conditions of the steel, the low-temperature seamless steel pipe is preferably normalized or tempered so that the production of the tissue is relatively easy. As the specifications include slender tubes of Φ60mm×4mm, if water quenching and tempering processes are used, bending tends to occur. Therefore, normalizing or normalizing and tempering processes are more suitable.
The heat treatment of low-temperature seamless steel tubes needs to consider the heating rate and holding time to avoid the burning of steel tubes and the production of oxide scales. At the same time, the austenite grain size is controlled to improve the toughness matching and corrosion resistance of steel tubes.
The -100°C low-temperature seamless steel tubes developed through the above-mentioned process measures have the following characteristics:
(1) Wide range of sizes, outer diameter from 60mm to 720mm;
(2) It has excellent low temperature toughness and corrosion resistance, and can pass strict corrosion resistance evaluation test;
(3) The bending heat treatment of the bent pipe has improved the performance of the bent pipe.