A Tailored Preparation Method of Variable Strength for Ultra-High-Strength Steel Sheet and Mapping Mechanism between Process and Property

Abstract

The spatiotemporal phase transformation during hot stamping would considerably effect the microstructure and mechanical properties of steels. In order to manufacture hot-stamping components of ultra-high-strength steel with tailored mechanical properties, the effect of the quenching time and the temperature of the die on the phase-transformation characteristics and mechanical property of ultra-high-strength steel was deeply studied. A finite element (FE) model coupled with a thermomechanical phase was employed to perform a succession of simulations for hot stamping corresponding to different quenching times and the temperatures of die, and the corresponding hot stamping experiments were performed. The 3D mapping surfaces of the temperature; quenching time; and three microstructures, namely austenite, bainite, and martensite, were constructed, and the mapping relationships in such surfaces were further explained by microstructural observations. Subsequently, based on the test results of the mechanical properties, the relationship curves of hardness and tensile strength, hardness, and elongation at break were fitted respectively, and then the 3D mapping surfaces were constructed for hardness, tensile strength, and elongation at break, which varied with the temperature die and quenching time. Finally, the quenching parameters of the automobile B-pillar were designed according to the constructed mapping relationship, and the hot-stamping FE simulation of the automobile B-pillar was developed. The result shows that those constructed mapping surfaces are helpful for adjusting the local mechanical property of the steels by designing the parameters.

Evolution of Microstructures and Mechanical Properties of Nb-V Alloyed Ultra-High Strength Hot Stamping Steel in Austenitizing Process

“…In recent years, the process parameters (such as austenitizing temperature, cooling rate, pressure magnitude, holding time and die exit temperature) in the hot forming process, which directly affect the important factors of its mechanical properties, have been studied intensively and extensively [8][9][10][11][12][13]. Quan et al [14] investigated in depth the effect of the quenching time and the temperature on the phase-transformation characteristics and mechanical property of ultra-high-strength steel. Jiang et al [15] closely examined the microstructure and mechanical properties of 22MnB5 steel during hot stamping, and found that the microstructure was uniformly distributed martensite when the austenitization temperature was 930 • C and the holding time was 4.5 min.…”