High strength dual phase steel with low yield ratio, high toughness and superior weldability

摘要

A dual phase, high strength steel having a composite microstructure of soft and hard phases providing a low yield ratio, high strain capacity, superior weldability, and high toughness is provided. The dual phase steel includes from, about 10 percent by volume to about 60 percent by volume of a first phase or constituent consisting essentially of fine-grained ferrite. The first phase has a ferrite mean grain size of about 5 microns or less. The dual phase steel further includes from about 40 percent by volume to about 90 percent by volume of a second phase or constituent comprising fine-grained martensite, fine-grained lower bainite, fine-grained granular bainite, fine-grained degenerate upper bainite, or any mixture thereof. Methods for making the same are also provided.