Effect of Two-Phase Region Quenching Temperature on Microstructure Evolution and Microhardness of HSLA100 Steel

The microstructure evolution characteristics and those effects on microhardness of HSLA (high strength low alloy) 100 steel secondary quenched in the two-phase region were investigated. The results show that the mixed microstructure of ferrite and the M-A（mastenite-austenite）islands can be obtained in the intercritical quenching region. A small amount of island structure distributing along the lath ferrite quenched at 700°C is observed by transmission electron microscope (TEM). With the quenching temperature increasing, the island structure increases in quantity and coarsens in shape, at the same time, the ferrite gradually transform from single lath morphology to polygonal shape with the dislocation density lowing. When quenched at 820°C, the microstructure reverts to lath bainite. There is a good correlation between Vickers hardness value and the volume fraction of martensite or bainite HSLA100 steel quenched in the two-phase region. The microhardness value of the steel continually increase from 240HV to 320HV quenched at the range of 700°C to 820°C, and then keep a very small fluctuation around 320HV when the temperature exceeds to 820°C.