Abstract
Abstract Structure, phase transformation features and the strengthening of nitrogen-microalloyed structural C(0.2–0.5)CrNiMoNV steels from alloying and high-temperature thermomechanical treatment (HTMT) by various hot deformation routes have been investigated using X-ray diffraction analysis, optical microscopy, hardness measurements and tensile testing. The nitrogen alloying retards recrystallization processes during hot deformation widens ɛ-carbonitride region and allows more effective thermomechanical treatment application for realization of high-strength state after low temperature tempering of quenched martensitic steels. Heat and thermomechanical treatment regimes providing increase of deformation and fracture resistance after quenching and tempering were determined. A complete dissolution of special carbonitrides during HTMT and following low-temperature tempering provide high strength level (by 300–500 MPa higher than for nitrogen-free analogs) combined with sufficient ductility and fracture toughness. HTMT comprising hot deformation by the radial-shear rolling with the same strain and deformation temperature as under longitudinal rolling, allows obtaining polygonized substructure, peculiar “twisted” in a definite direction grain structure and texture favorable for strength and plasticity characteristics in an axis direction.
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