Tensile behavior of fine-grained steels

With decreasing of grain size in ferritic steels, Luders elongation becomes larger while work-hardening is lowered, finally resulting in loss of uniform elongation. This drawback can be overcome by introducing the second phase like martensite or metastable austenite. The Improvement of strength and uniform elongation balance by the second phase can well be estimated by applying the secant method of micromechanics approach. The stress partitioning between two constituents brings high work-hardening, which is verified by in situ neutron diffraction. The influences of strain rate and temperature are described by using the Kocks-Mecking model. It is found that the grain refinement and the above stress partitioning contribute mainly to the athermal stress component of flow stress. Hence the tensile properties obtained at a high speed deformation like 10(3)/s is excellent in fine-grained multi-phase steels. As an example of ultrafine-microstructure with 20-30 nm in size, the tensile behavior of severely drawn pearlite steel wires with tensile strength larger than 4 GPa is investigated. In spite of such ultra-high strength, the wire deforms plastically by dislocation motion resulting in dimple fracture. The strengthening consists of isotropic hardening due to microstructure refinement and anisotropic hardening caused by residual intergranular stresses which are determined by neutron diffraction.