| The rolling process parameters of ER70S-6 steel for obtaining a ferrite plus martensite dual phase (DP) microstructure for high strength fasteners graded 8.8 were investigated. Thermal simulating experiments were carried out using Gleeble 3500 simulator. Referring to the simulating results, the productive experiment was conducted in a continuous no-twist mill, and the mechanical properties of the as-rolled materials were tested. The results showed that deformed at 845 °C, followed by cooling to 780 °C at 5 °C/s and holding for 30 s, then quenching in water led to DP microstructure consisting of fine grained ferrite matrix plus about 20% martensite particles (in volume fraction). The wire rod rolled in this process route had a heterogeneous DP microstructure. Tensile strengths of the hot rolled DP wire rod and the drawn wire rod with a 19% reduction in area were 675-720 MPa and 915-970 MPa, respectively. ER70S-6 steel with DP microstructure could meet the tensile strength specification for grade 8.8 components. |
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