Effects of low-temperature tempering on the microstructure and precipitating evolution of newly developed high-strength microalloyed steel

The effect of tempering temperature on the microstructure and precipitating evolution and the resultant mechanical properties of newly developed high-strength microalloyed steel plate was investigated by optical microscopy(OM) and transmission electron microscopy(TEM).The steel mainly consists of fine lath martensite and lower bainite.The width of the martensitic laths in as-hot-rolled state is about 120 nm,and increases from 120 nm to 150 nm and 180 nm after tempering at 200 ℃ and 250 ℃ for 2 h respectively with no change in its morphology.Of special interest is the phenomenon that both tensile strength and impact toughness of the steel plate decrease with the increase of the tempering temperature,which might be attributed to the combination of lath martensite broadening and the coarsening of needle-like carbides located on the boundaries of lath martensite and within bainitic ferrite.It is suggested that the existence of the complex carbonitride larger than 100 nm in bainitic ferrite is one of the reasons.