Recently, high strength steel becomes more and more important material from the viewpoint of lightening of the structures and decreasing the amount of welding. It is important to know the behavior of material steels after plastic deformation because many structures need plastic deformation for material steels in the manufacturing process of steel product. On the other hand, high strength steel plate for commercial use is mainly composed of Martensite microstructure by low carbon content in order to balance weldability and strength. So far, the relation between stress and strain after plastic deformation has been investigated only for Ferrite or the mixture of Ferrite, Pearite and Bainite structure. In this paper, by using 780 MPa steel plate, behavior of deformation of Martensite material after plastic deformation including the repeated patterns is surveyed comparing of behavior of Ferrite steel plate. Main results can be summarized as follows,
1) In case of large prestrain, over 3%, configuration of stress-strain curve is decided by the last prestrain condition even in repeated pattern.
2) Uniform elongation in the tensile properties vanishes after 6% pretension: This phenomenon is caused by many dislocations in the Martensite structure.
3) Baushinger effect becomes greater, as prestrain to reverse direction increases up to 2%. This amount corresponds to stress constant range of virgin material. It can be thought that piling-up of dislocations propagates all over the structure by taking 2% strain.
4) Baushinger effect ratio α0.2 of HT780 is approximately 0.3 under sufficient large reverse prestrain.
5) Configuration of stress-strain curve after prestrain in both direction hardly change by aging heat treatment (250°C). This is caused by difficulties of movement of dislocations because of mutual binding due to the tangle of many dislocations characteristic of Martensite transformation.
6) HT780 steel composed of Martensite is hardly changed in condition of damage by strain during practical use. This characteristic nature is unlike Ferrite steel.
