| Hot-rolled ferrite and bainite dual-phase(FBDP)steels have excellent stretch-flangeability,high strength and a certain cold formability,which are suitable for the manufacture of products with complex shape by coldforming.When they are used for fabricating automotive parts,the steels can effectively reduce vehicle weight and thus save energy consumption to deduce the emission of exhaust gas.Therefore,it is very significance to research and develop the ultra high strength hot-rolled FBDP steel at an industrial large-scale.In this article,the mechanical property testing and microstructure analyzing was performed on the hot-rolled FBDP steel firstly produced by a steel company.Test results indicate that the strength is not too high and the hole expansion ratio is also less than80% in the steel sheet.The pearlite,martensite and lath bainite were found in the microstructure of the steel by SEM analysis,which are is injurious to the property of stretch-flangeability.In order to eliminate the harmful microstructure and to target strength properties,the chemical composition and hot rolling procedure of steel have been designed,including decreasing the carbon content,adding some Si,Mn and Nb element in the steel.Also,the final rolling temperature has been adjusted to 870℃.After the designed steel are stimulated to figure out the behaviors of the cooling transformation,the designed steel was fabricated at industrial scale,which shows an excellent properties.The tensile strength is around 600 MPa,the elongation is higher than 20%,and hole expansion ratio is over 80% with maxim μ m up to 115%.The polygonal ferrite of FBDP steel can helpful to improve the hole expansion ratio,and so do M/A islands in small size.They are capable to relax the spread of cracks in the steel. |
PDF Full Text Request