The precipitation and control of boron nitrogen(BN) inclusions in 42CrMo steel were investigated and their efect on machinability was analyzed.First,the precipitation regularity of BN in 42CrMo steel was studied by theoretical calculation.Then,the machinability of the steel was investigated through contrast cutting experiments,and the composition and cooling rate of the steel were controlled to analyze the variation laws of the size,distribution,and area ratio of BN inclusions.Finally,the results were combined with the machinability of the steel to analyze the relationship among them.It is found that the machinability of the steel is mainly influenced by the diameter and quantity of BN inclusions.Fine and dispersedly distributed BN inclusions are more benefcial for the improvement in machinability of 42CrMo steel than coarse and sparse BN inclusions. The precipitation and control of boron nitrogen (BN) inclusions in 42CrMo steel were investigated and their efect on machinability was analyzed. First, the precipitation regularity of BN in 42CrMo steel was studied by theoretical calculation. Chen, the machinability of the steel was investigated through contrast cutting experiments, and the composition and cooling rate of the steel were controlled to analyze the variation laws of the size, distribution, and area ratio of BN inclusions .Finally, the results were combined with the machinability of the steel to analyze the relationship among them.It is found that the machinability of the steel is primarily determined by the diameter and quantity of BN inclusions. Fine and dispersedly distributed BN inclusions are more benefial for the improvement in machinability of 42CrMo steel than coarse and sparse BN inclusions.