Influence of Manganese Content and Finish Rolling Temperature on the Martensitic Transformation of Ultrahigh-Strength Strip Steels

The effects of manganese content and finish rolling temperature (FRT) on the transformed microstructures and properties of two low-alloyed thermomechanically rolled and direct-quenched (TM-DQ) steels were investigated. The materials were characterized in respect of microstructures and tensile properties. In addition, microhardness measurements were made both at the surface and centerline of the hot-rolled strips to help characterize the phase constituents. Detailed microstructural features were further revealed by laser scanning confocal microscopy (LSCM) and field emission scanning electron microscopy combined with electron backscatter diffraction (FESEM-EBSD). It was apparent that a decrease in the temperature of controlled rolling, i.e., the finish rolling temperature (FRT), resulted in reduced martensite fractions at the surface, as a consequence of strain-induced fine ferrite formation. The centerline of the strip, however, comprised essentially martensite and upper bainite. In contrast, high FRT and higher manganese content resulted in essentially a fully martensitic microstructure due to enhanced hardenability. The paper presents a detailed account of the hot rolling and hardenability aspects of TM-DQ ultra-high-strength strip steels and corresponding microstructures and properties.