The Development of High-Strength Steel of 980MPa Grade with Higher Hole Expansibility

摘要

Recently, due to the requirements for lightweight and safer automobile structures, the grade of 980MPa highstrength steel has the demand of high hole expansibility and high yield strength. Due to the large difference in hardness between the soft ferrite and hard martensite, the traditional DP980Y dual phase steel has poor hole expansibility. In order to improve the hole expansibility of DP980Y dual phase steel, the best way is to modify the microstructure into a single-phase to eliminate the large difference in hardness. In this paper, the steel of nearly full bainite microstructure with a small amount of ferrite and martensite-austenite (M/A) constituents was studied. Compared to the DP980Y dual phase steel, it was found that this modified steel with a single-phase microstructure has the same grade of 980MPa of tensile strength, but can achieve the demand of higher yield strength and hole-expansion ratio. This study shows that reducing the amount of ferrite can increase the homogeneity of the matrix with the single phase to improve the hole expansibility. In addition, the use of a lower bainite transformation temperature and lower carbon content has the higher hole-expansion ratio due to a lesser amount of M/A constituents.