Abstract: A low-alloy high-strength steel Q960 for construction machinery was designed.Under the condition of homogeneous composition,homogeneous rolling and quenching techniques,a mechanism was researched to describe the influence of tempering temperature on low angle grain boundaries in Q960 steel.It is shown that with the rising of tempering temperature,the frequency of low angle grain boundaries as well as the tensile strength and yield strength of Q960 steel gradually decreases,but the tensile ratio increases.At different tempering temperatures,the peak value of the frequency of low angle grain boundaries is about at a misorientation of 5°,so the misorientation is small.Because of the small misorientation,the low angle grain boundaries are quite stable and the atomic mobility ratio is also very small.According to the calculation of bond order integrals(BOI),B element can strengthen low angle grain boundaries in Q960 steel.