Microstructural control of hot rolled strips and their tensile strengths after hot stamping process

摘要

The influence of microstructure of hot rolled strips on tensile property after hot stamping was investigated using boron-bearing steel of 1470 MPa grade for hot stamping. Hot rolled strips with two kinds of microstructures were produced by mill trial based on a time-temperature transformation (TTT) diagram that resulted from a dilatometric experiment. The first had a ferrite-pearlite banded microstructure and the other had a bainitic microstructure. The strip with bainite had a higher tensile strength after hot stamping compared with the tensile strength of the strip with ferrite-pearlite banded microstructure after hot stamping. The former almost fully transformed to martensite after hot stamping, while the latter transformed to partial martensite and ferrite that originated from the inhomogeneous hardenability of the ferrite-pearlite banded structure due to segregation of alloying elements such as carbon and manganese.