Controlled Rolling and Cooling of Ultra-High Strength Ferrite-Bainite Dual Phase Steel

Controlled rolling and cooling of ultra-high strength ferrite-bainite dual phase steel in the different conditions were conducted by a laboratory hot rolling mill.Effect of various processing parameters on the microstructure and mechanical properties of the steel was investigated.The results show that ultra-high strength dual phase steel with ferrite-bainite microstructure can be obtained by applying controlled rolling and cooling.The tensile strength values are nearly the same in all cases and always exceed 1 000 MPa,yield ratio varies from 0.54 to 0.62 and the total elongation varies from 13% to 17% for the specimens which are cooled at different cooling rates after various finish rolling temperatures.The ferrite grains are refined with decreasing finish rolling temperature and accelerating cooling,and the amount of bainite increases with decreasing finish cooling temperature.Tensile strength,total elongation and the product of tensile strength and total elongation for the specimen,which is finished rolling at 800 ℃ and subsequent laminar cooling to 560 ℃ and then air cooling to room temperature,reach maximum values(1 130 MPa,16% and 18 080 MPa%,respectively).This is attributed to the ferrite-grain refinement,strengthening of granular bainite,carbide free bainite and a small amount of twinning martensite.The presence of retained austenite improves the ductility of the steel.Multiphase microstructures contain ferrite and bainite as main phase with a small amount of retained austenite and martensite,contribute to enhance mechanical properties of tested steel.