Typical continuous annealing technology of cold-rolled 700 MPa ultra-high strength low-alloy steel

In order to develop a new generation cold-rolled ultra-high strength low-alloy steel,continuous annealing experiments of a Ti-0. 12%,Nb-0. 076% cold-rolled ultra-high strength low-alloy steel were done on a continuous annealing test machine. The influence of annealing temperature on the phase composition,grain size,and mechanical properties of the steel was investigated at four different annealing temperature ranges from 760 ℃ to 830 ℃. The final composition is ferrite and a small amount of bainite under 800 ℃annealing and 400 ℃ over-aging conditions,the ferrite grain size is about 1. 4 μm,and the yield strength reaches up to 700 MPa. A large number of nanometer size sub-crystal structures,a small number of dislocations,and nano-Ti and Nb precipitates are observed by scanning electron microscopy and transmission electron microscopy. Those microstructures improve the strength a lot.