Abstract
The effects of thermo-mechanical controlled processing (TMCP) and accelerated cooling (AC) on the structure and mechanical properties of 40 mm thick sheets of X70 steel made of heavy slabs (300 mm thick) are studied in this work. A TMCP with a rolling finish in a two-phase temperature interval followed by AC to 480−530 °C was found optimal, ensuring full compliance with the API X70 grade in tensile and Charpy tests, while the result of a drop weight tear test (DWTT) at −20 °C was close to the standard level. This was due to the forma tion of fine-grained quasi-polygonal⁄acicular ferrite strengthened by the martensite⁄austenite islands and nanosized (Nb,V)C precipitates. The advanced combination of mechanical proper ties in 40 mm thick sheets refers to using slabs of increased thickness, allowing sufficient rolling reduction to refine a cast structure of the billet effectively. The DWTT fractures concerning steel microstructure are discussed.

Key words
pil⁄gas pipe, X70 steel grade, thermo-mechanical controlled processing (TMCP), accelerated cooling, microstructure, mechanical properties

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