Decomposition of Supercooled Austenite in Low-Carbon Pipe Steel

Abstract

Controlled rolling may facilitate the transition from ferrite–pearlite structure to bainite structure and hence improve the strength of low-carbon alloy steel. A literature review shows the lack of detailed research regarding the type of bainite structure that ensures optimal properties of low-carbon alloy steel. To determine the temperature, time, and other parameters of treatment to produce such structure, it is expedient to establish the influence of the cooling rate on the structure and properties of low-carbon alloy steel. In the present work, we investigate the decomposition of supercooled austenite in low-carbon alloy pipe steel with 0.062% C, 1.80% Mn, 0.120% Mo, 0.032% Cr, 0.90% Ni, and other elements (Al, Cu, V, Nb, Ti). The treatment conditions producing bainite structure with elevated steel are determined for such steel. At low cooling rates (no more than 6°C/s), not only ferrite is formed in the microstructure but also granular (or globular) bainite consisting of bainitic α phase and an insular martensite–austenite component (1–6 μm). At a rate of 6°C/s, transition to rack bainite is observed; around the boundaries of the racks, carbides and residual austenite are seen. At cooling rates above 16°C/s, the bainite has packet–rack structure, Between 50 and 150°C/s, the mean rack width of the bainitic α phase declines from 2.24 to 1.32 μm.