The preparation and performance of grain size gradient TWIP steel fabricated by laser heat treatment

The grain size gradient structure is a promising solution to resolve the problem of strength-ductility trade-off. In this work, a gradient micron-structure (GMS) sample was fabricated by applying a laser heat treatment on the surface of a cold-rolled TWIP steel plate. The recrystallized microstructure showed a linear gradient in grain size from similar to 25.5 mu m to similar to 13.3 mu m throughout the cross section. The yield and ultimate strengths of the GMS sample were 306.3 MPa and 674.1 MPa, respectively, which are lower than those of a sample annealed at 850 degrees C for 60 min but higher than those of the one annealed at 950 degrees C for 60 min. It suggests the strengths of the GMS sample obey the rule of mixtures as the coarse grains in the upper surface can be considered as the soft part and the fine grains as the hard ones. However, the GMS sample exhibited a similar tensile elongation (60.7%) as the annealed (850 degrees C, 1 h) sample does (61.0%). When the strong strain concentration is increased, a large number of small cracks in the lower surface may connect and propagate to the upper side rapidly, resulting in a premature failure.