The effect of different relaxation time before online quenching on mechanical strength and toughness of high-strength medium-thickness martensitic steel plate was analysed in this work. Increasing the relaxation time from 15 s to 135 s resulted in an increase in the impact energy at −20 °C from 33 J to 148 J. However, the yield strength was decreased from 1050 MPa to 1002 MPa. Dislocation density and martensitic lath width are the main factors causing the strength change. Moreover, the dislocation density caused by deformation drops first sharply and then gently during the relaxation, resulting in an inverse trend of martensitic lath width. Additionally, there appears to be a linear relation between impact energy at −20 °C and dislocation density−1/2.
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