Effects of Alloying on the Microstructure and Hole-expansion Property of QSTE460 Hot Rolling Automobile Frame Steel Plate

Based on the existing hot rolling automobile frame steel SAPH370, a new dual-phase steel QSTE460 was designed by increasing the content of Mn, reducing the content of C, and introducing micro-alloying elements of Ti and Nb. The microstructure and hole expansion property of the two steels were compared and analyzed, and the effect of microalloying on hole expansion performance of SAPH370 steel was investigated by means of scanning electron microscopy, transmission electron microscopy, and electron backscatter diffraction. The results indicate that the microstructure of SAPH370 is characterized by ferrite (F), granular bainite (B), and a small amount of pearlite (P). However, the increase of Mn content in QSTE460 suppresses the pearlite transformation, resulting in a dual-phase structure composed of ferrite and granular bainite.Moreover, the granular B in QSTE460 is finer, more abundant, and exhibits a morphology closer to equiaxed. Furthermore, the reduction in C content and the precipitation of (Ti, Nb)(C, N) secondary phase in QSTE460 contribute to improved phase compatibility, grain refinement, a notable increase in the proportion of low angle grain boundaries, and the formation of a favorable 111〈110〉 texture. These microstructural characteristics collectively lead to the superior hole expansion performance of QSTE460, with its average hole expansion rate being 6.6% higher than that of SAPH370. This achievement demonstrates a synergistic improvement in strength and formability, thereby providing a theoretical basis and technical support for the development of high-performance automotive steels.