Abstract
The influence of two-stage accelerated cooling of hot-rolled strip on the microstructure of X70 steel with different alloying (Cequ = 0.36 and 0.43%) is investigated, in laboratory conditions. The types of bainitic-transformation products present are found to depend on the final accelerated-cooling temperature in the first cooling section (Twi1) and the initial temperature of bainitic transformation (Bs), which are determined experimentally. The final accelerated-cooling temperature in the second cooling section (Twi2) determines the type and morphology of the secondary (carbon-bearing) phase. Reducing Twi2 increases the content of acicular ferrite when Twi1 ≤ \(T_{wil} \leqslant B_{S_e } \). Such accelerated-cooling conditions are used to produce industrial batches of steel strip with guaranteed mechanical properties and DWTT data. The formation of disperse ferritic-bainitic microstructure results in an extension diagram with a degenerate yield area and consequently ensures the required strength of straight-seam K60 pipe (diameter 1020 mm).
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