Most of BN was precipitated around the spherical MnS and formed the compound of MnS and BN during the isothermal treatment at 850℃after austenizing at 950-1050℃.Most of MnS was transformed into polygon when austenized at 1150℃ or above,which could not be the nuclei of BN anymore.During the insulation process at 850℃,MnS kept the state of monomer.The compound of MnS and BN at earlier time was of fine polycrystalline in spherality and BN was formed as a plate-like monocrystal on a certain part of the polycrystalline when its dimension reached 0.6μm. Most of BN was precipitated around the spherical MnS and formed the compound of MnS and BN during the isothermal treatment at 850 ° C after austenizing at 950-1050 ° C.Most of MnS was transformed into polygon when austenized at 1150 ° C or above, which could not be the nuclei of BN anymore. During the insulation process at 850 ° C, MnS kept the state of the monomer. The compound of MnS and BN at earlier time was the fine polycrystalline in spherality and BN was formed as a plate-like monocrystal on a certain part of the polycrystalline when its dimension reached 0.6 μm.