Investigation on enhancement of uniform elongation in 780 MPa class high tensile strength steel plate - Studies of safety against fracture on 780 MPa class high tensile strength steel plate after prestrain (report 2)

As reported before, there are many dislocations in the martensite matrix due to the mechanism of transformation itself. The actual structure of pipes includes the tensile prestrained part of the outside layer and it has become clear that the outside layer with prestrained HT780 has remarkable deformation behavior even without uniform elongation. The initial amount of uniform elongation must be important in preventing the disappearance of that characteristic after prestraining. Accordingly, in this report several methods of developing steels with higher initial uniform elongation are investigated. 1) By using finer microstructure (γ grain size is more than 9.4), clear uniform elongation is shown even after 3% prestraining. However, the effect of grain size on the configuration of stress-strain curve is smaller than the effect of ferrite structure. This is presumed to be because one prior γ grain is composed of so many substructures that function as grains, i.e.; packet or lath. However, from the point of view of the ability of piling-up dislocation, γ grain boundary is superior to the others because γ grain boundary has more differences of crystallographic orientation. 2) By increasing carbon content (more than 0.26%), uniform elongation can be enhanced through precipitating carbides in laths. However, ordinary HT780 steels for structures where welding is carried out with not more than 0.17% carbon shows a constant value of uniform elongation. In this carbon range, uniform elongation is controlled only by tempering temperature and independent of chemical composition. According to observation of microstructure, there are few carbide precipitates in laths but many precipitates in the boundary between laths. Therefore, the increase of carbon content does not effectively contribute to enhance the ability of work hardening.