High-temperature mechanical properties of Al-TRIP steel

The high-temperature mechanical properties of Al-TRIP steel with 0.16% C,1.49% Mn and 1.35% Al,which is applied to automobiles,were investigated by using a Gleeble 3500 machine. The phase transition temperature interval was measured by differential scanning calorimetry(DSC). The fractographs and microstructures near the tensile fracture of the steel at different tensile temperatures were analyzed by means of a scanning electron microscope and an optical microscope. The zero ductility temperature(ZDT) and the zero strength temperature(ZST) of the steel were measured to be 1425℃ and 1430℃,respectively. The brittleness temperature interval Ⅰ is from 1400℃ to the melting point,and the brittleness temperature interval Ⅲ is from 800℃ to 925℃.Ferrite precipitation from austenite grain boundaries is the main cause of the presence of the brittleness temperature interval Ⅲ. With specimens cooling from 975℃ to 700℃,the proportion of ferrite increases continuously,while the reduction of area(ψ) decreases firstly and then increases. When the proportion of ferrite reaches to 8.1%(at 850℃),the value of ψ decreases to 28.9%; but when being stretched at 800℃,the proportion of ferrite is greater than 16.7%,and the value of ψ is more than 38.5%. A small number of Al N particles precipitate in the steel at 1275.6℃. These Al N particles are coarse,but they have no influence on the ductility of the steel.