Scientific basis for cold brittleness of structural BCC steels and their structural degradation at below zero temperatures

The paper considers the physics of cold brittleness of structural bcc steels and methods of reducing the ductile-brittle fracture temperature. A complex study was performed to examine the degradation of structural phase state of pipe steel 09Mn2Si from the main gas pipeline of Yakutia after long-term (over 3 0 years) operation. Important regularities of degradation of pearlite colonies with carbide precipitation on ferrite grain boundaries were revealed. This phenomenon is associated with brittle fracture of gas pipelines. It is shown that the low-temperature kinetic processes in main pipelines which define the degradation of their structure and properties are related to interstitial athermal structural states in the zones of local crystal structure curvature. This is a fundamentally new, as yet unknown, mechanism. Pipe steels in warm rolling acquire a longitudinal textured band structure with alternating bands of initial ferrite grains and bands of fine grains with carbide precipitates formed during lamellar pearlite degradation. This type of structure allows for a shift of ductile-brittle transition temperature down to -80°C and ductility δ = 22% at this temperature. The production of high-curvature vortex structure in pipe steel surface layers results in a 3.5-fold increase in their service life.