Abstract: Automatic flame cutting is widely employed in slitting mechanical testing samples of heavy plates,while the validity of mechanical properties is always affected by the heat affected zone of the flame cutting.Using the high strength B-HARD400 steel plates with DQ and low-temperature tempering technology,which is even more liable to be affected by automatic flame cutting,a set of experiments with cold and hot cutting samples were designed and carried out.The results show that when the test samples were 5 mm away from the flame cutting seam,there was no significant properties difference between the sample 5 mm away from the flame cutting seam and the normal position sample,such as room temperature tensile strength,Vickers hardness,microstructure,impact absorbed energy and ductile-brittle transition temperature.That is to say,the automatic flame cutting heat affected zone depth of B-HARD400 steel plates was about 5 mm.