Microstructure and Mechanical Properties of Intercritical Annealed Multiphase Ultrahigh Strength Steel

In recent years, more and more ultrahigh strength steels are used in automotive industry, including dual-phase steels and complex phase steels, which are formed using cold stamp forming and roll-forming process. In this paper, a cold rolled ultrahigh strength steel was studied, and different intercritical annealing parameters were adopted to get different microstructures of the steels. Scanning electronic microscopy (SEM) and transition electronic microscopy (TEM) were used to analyze the microstructure evolution of the multiphase ultrahigh strength steels. Tensile tests and hole expansion tests were also adopted to evaluate the mechanical properties and hole expansion properties of the multiphase ultrahigh strength steels. The results show that with increasing intercritical annealing temperatures, the yield strength and ultimate tensile strength of the steels increase with elongation decrease simultaneously, furthermore the yield ratio is also improved. The microstructure of the steels contain a mixture of bainite, martensite and ferrite, and with increasing annealing temperatures, the content of martensite decrease. Meanwhile, the content of tempered martensite increase gradually. With increasing annealing temperatures, the hole expansion ratio increases first and then slightly decreases.