Effect of bainite component morphology on the microstructure of X70 low-alloyed steel on thick plate cold resistance

The effect of controlled rolling and accelerated cooling production parameters on the structure and properties of plates 25 mm thick or more of strength category X70 is studied. A change is shown for the proportion of structural components in steel X70 with a ferrite-bainite microstructure in relation to CR regime. The presence in the steel microstructure of areas of bainitic ferrite, forming at the boundaries of coarse austenite grains as a result of austenite nonuniformity, leads to a reduction in the proportion of ductile component in the fracture of a DWTT specimen. An effect is revealed for an increase in the volume fraction of bainitic ferrite on plate cold brittleness during DWTT. The amount of reduction at the end of the roughing rolling stage should be less than 13–14 % in order to prevent a difference in austenite grain size. CR regimes with limited intensity make it possible to obtain a steel microstructure consisting mainly of quasipolygonal ferrite with a minimum number of areas of bainitic ferrite, which is favorable for steel DWTT properties.