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Effect of Nitrogen Addition on Tempering and Strain Aging Processes of Thermomechanically Strengthened Structural Steels

Mechanical behavior of structural nitrogen-containing steels with various structures and compositions, including the same steels with different summary C+N content and C/N ratio were studied using pressing tests in a wide temperature range, tensile tests, impact bending tests, hardness measurements and shock-wave loading resistance. The tempering and aging under load processes after quenching or thermomechanical treatment with various regimes have been investigated using optical and electron microscopies, X-ray diffraction analysis, calorimetric and dilatometric analyses. Hot strain resistance of the austenite is determined essentially by the steel composition, while the final structure and mechanical properties of hot-deformed austenite are determine mainly by hot deformation conditions. The higher the nitrogen content and C/N ratio, the higher hot strain resistance was and earlier the softening processes start, especially recrystallization process. The nitrogen microalloying of low-alloyed structural steels changes kinetics of the martensite tempering. Application of the high temperature thermomechanical treatment or combined thermomechanical strengthening with following tempering under load allows the use of these steels in a high-strength state after low-temperature tempering.