In this paper the structure of air cooled bainitic steels which contain the series of Mn Si B has been observed and analysed on different heat treatment condition.The results showed that the main structure is bainite or bainite/martensite after casting,normalizing and forging air cooled condition.By microalloyed inoculant treatment,the austenitic grain size and microstructure can be remarkably fined.The amount,kind and dispersity of carbide in steel can increase by regulating the content and kind of microalloying elements.So its hardness and impact toughness can be increased.The structure of studied steels can alter finer and more homogeneous by multicomponent microalloyed inoculant treatment and the steels became the ultrafine structure bainitic steels.The steels possess the advantages of simple technology and cheap price.The plate hammer using in the field to test showed that its service life is higher twice than high Mn steels which is used in the field.