Influence of nanoparticle reinforcements on the strengthening mechanisms of an ultrafine-grained dual phase steel containing titanium

To reduce cost, optimise mechanical properties and improve process tolerance, a series of 1000 MPa grade ultrafine-grained dual phase (DP) steels with nanosized precipitates have been developed based on the C–Si–Mn–Ti alloy system. The grain size of ferrite in ultra-high strength DP steels ranges from 1.1 to 1.7 μm. The amount of precipitations in the annealed sheet is much more than that in the hot-rolled plates with the highest distribution frequency being between 5 and 10 nm. The grain refinement and precipitation strengthening interact significantly and have a considerable effect on yield strength. Therefore, the strengthening effects cannot be expressed as a simple linear relationship. A modified root-mean-square (RMS) relationship has been proposed to express the yield strength for dual phase steel with obvious grain refinement and precipitation strengthening.

► A series of 1000 MPa grade ultrafine-grained DP steels have been developed.
► The grain size of ultrafine ferrite is 1.1–1.7 μm.
► Continuous annealing intensifies precipitation behaviour.
► TiC precipitates between 5 nm and 10 nm dominate in the annealing sheet.
► A modified root-mean-square relationship has been expressed for the yield strength.