3.5Ni low-temperature seamless steel pipes have moderate strength and good low-temperature toughness, and are widely used in the manufacturing of low-temperature equipment in industries such as petroleum, chemical, and fertilizer at working temperatures of -45 to -101°C. The two main indicators for low-temperature pipelines are low-temperature toughness and corrosion resistance.
For its development, it is necessary to control processes such as chemical composition design, steelmaking, rolling, and heat treatment to ensure its low-temperature impact toughness and corrosion resistance. Specifically:
1. Composition Design
The chemical composition is designed as w(C)=0.06-0.12%, w(Mn)=0.4-0.6%, w(S)≤0.005%, w(P)≤0.012%, w(Ni)=3.4~3.6%.
2. Steelmaking
1) Control C content. Matching electric furnace with refining is an effective means to achieve precise control of C content; strictly control the C content at the end of electric furnace smelting, and use increased argon flow rate and immersion tube diameter during vacuum decarburization in VD or RH furnaces to ensure precise C content requirements.
2) Reduce P and S content.
Meet the desulfurization needs by using high-quality scrap steel and appropriately composed refining slag; reduce P content in steel by producing high-basicity slag in the electric furnace and strictly controlling the slag during tapping.
3) Ensure Ca content in steel and weak stirring time; strengthen management between continuous casting upper and lower ladles, as well as protect casting, which can effectively reduce non-metallic inclusions in steel.
3. Rolling
Use PQF continuous rolling pipe mill for rolling, adjusting the heating temperature and time of pipe blanks to reduce eccentricity during perforation process and minimize wall thickness unevenness.
4. Heat Treatment
Low-temperature seamless steel pipes are best treated using normalizing or quenching processes. For slender pipes, using water quenching may lead to bending; normalizing or normalizing + tempering processes are more applicable. | 
