Abstract: An air cooled bainitic steel without quenching-tempering treatment,22Mn2SiVBS steel used as semi-axis housing material was designed.By observing the microstructure of the hot rolled tube,certain amount of intermittent reticular ferrite in 22Mn2SiVBS is found near original austenitic grain boundaries,which decrease the mechanical properties of the steel.By reheating samples and controlling cooling rate,the relations of the microstructure,mechanical properties and controlling cooling rate are analyzed,and a measure is put forward to obtain uniform bainite and good mechanical properties under air cooling condition after the steel was rolled.The tensile strength of the steel with uniform bainite is increased to 900 MPa and hardness increased to 301 HB.