Hyun Jo Jun

Effects of Nb on Microstructural Evolution and Mechanical Properties of Low-Carbon Cold-Rolled Dual-Phase Steels

The effects of Nb additions, up to 0.06%, on the microstructure and properties of dual-phase (DP) steels containing 0.08% C and 2.2% Mn, were evaluated after hot rolling at different coiling temperatures, and after intercritical annealing of cold-rolled steels. During hot rolling, Nb precipitation was promoted by higher coiling temperature. During annealing of cold-rolled steels, microstructural characterization revealed that recovery and recrystallization processes overlapped with austenite formation. The addition of Nb increased the ferrite recrystallization start temperature, but had no significant effect on the start temperature of austenite formation during heating. Nb additions accelerated austenite formation once the transformation started, and also resulted in the formation of a finer and more homogeneous microstructure after annealing. In comparison with the steel without Nb, the ultimate tensile strength increased with the addition of up to 0.04% Nb, whereas addition of 0.06% Nb did not further increase the strength level of cold-rolled DP steels. Hot rolling at lower coiling temperatures improved both the strength and the ductility of the Nb-containing DP steels after annealing.

Effect of Nb on Delayed Fracture Resistance of Ultra-High Strength Martensitic Steels

Ultra-high strength steels are materials of considerable interest for automotive and structural applications and are increasingly being used in those areas. Higher strength, however, makes steels more prone to hydrogen embrittlement (HE). The effects of Nb and other alloying elements on the hydrogen-induced delayed fracture resistance of cold rolled martensitic steels with ultra-high strength 2000 MPa were studied using an acid immersion test, thermal desorption analysis (TDA) and measuring of permeation. The microstructure was characterized by high resolution field emission Scanning Electron Microscopy (SEM) with Electron Backscattered Diffraction (EBSD) and Transmission Electron Microscopy (TEM). It was shown that the combined addition of Nb significantly improved the delayed fracture resistance of investigated steel. The addition of Nb to alloyed martensitic steels resulted in very apparent grain refinement of the prior austenite grain size. The Nb microalloyed steel contained a lower diffusible hydrogen content during thermal desorption analysis as compared to the base steel and had a higher trapped hydrogen amount after charging. The reason that Nb improved the delayed fracture resistance of steels can be attributed mostly to both hydrogen trapping and grain refinement.