The influence law of cooling rate on microstructure and properties of low-carbon high-grade AF pipeline steel

To explore influence laws of cooling rate on microstructure and properties of low-carbon high-grade acicular ferrite (AF) pipeline steel. The microstructure and properties influence of alloy composition and super-cooled austenite continuous cooling transformation (CCT) has been investigated by thermodynamic analysis and calculation. Studies have shown that the optimised composition was C0.06, Si0.2, Mn1.85, Nb0.04, Ni0.33, Cr0.4, Mo0.15, P ≤ 0.01 and S ≤ 0.002, when the cooling rate was less than 1°C/s and pearlite (P) transition has been suppressed. When the cooling rate exceeds 20°C/s, structure mainly consists of AF and small amount of retained austenite (RA), and the mechanical properties partially met and exceed X90 performance levels of API SPEC 5L-45th. This paper offers a novel research method of low-carbon AF high-grade steel. The conclusion provides theoretical and practical guidance for production and study of such types of steel.