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Knowledge of mechanical properties and fracture behaviour of finegrainedlow-alloyed and structural steels in wide range of yield stresses is importantfrom the manufacturer, as well as user point of view. Especially, the behaviour of steelsbelow the nil ductility temperature (NDT±20 °C) is very important. Mechanicalproperties of selected structural and fine-grained low-alloy steels were determined in asdelivered and as strain-aged condition. In the present paper for selected steels thefollowing characteristics are given: chemical compositions, impact Charpy toughnessin temperature region between 80°C and -196 °C, nil temperature ductility TNDTdetermined by drop weight test, tensile properties at room temperature and attemperatures ±20°C higher and lower than TNDT, respectively. For selected steelsfracture toughness KIC in nil ductility temperature region, as well as J-integral weredetermined. KIC was determined with cylindrical V-notched precracked tensilespecimens and J-integral was determined with CT samples by a modified chevron notch.Detailed microstructure characterisation and fractography of fractured samples werealso performed. The effect of strain-aging on the impact Charpy toughness and quasystaticfracture toughness of ten structural steels was investigated in the nil-ductilitytemperature range. Strain–aging provokes shifts of Charpy curves to highertemperatures, but it decreases the nil-ductility temperatures regarding to as-receivedsteels. The correlation between KIC and conventional mechanical properties valid forlow temperatures confirms that KIC values of as strain-aged steels are higher than thosein as received condition at the same Charpy energy level.