Influence of Nb and Ti Microalloying on Microstructure and TensileProperties of δ-Transformation Induced Plasticity Steel

The influence of Nb and Ti microalloying on the microstructure and tensile properties of δ-transformation induced plasticity (δ-TRIP) steel was investigated by adding Nb and Ti element into the steel. The results show that the microstructure of the test steel had a banding distribution along the rolling direction, and was composed of δ-ferrite bands and α-ferrite/retained austenite bands. After the test steel was microalloyed by Nb and Ti, the ferrite grain in the test steel was refined; a large number of low angle boundaries were formed inside the ferrite, and the content of retained austenite decreased. Rectangle (Nb, Ti)(C, N) phase with size of about 5 μm and oval Ti(C, N) phase with size of 50-200 nm precipitated in the Nb/Ti-microalloyed test steel. Compared with those of test steel without adding Nb and Ti, the yield strength and tensile strength of Nb/Ti-microalloyed test steel increased by 65, 37 MPa, respectively, and the percentage elongation after fracture decreased from 33.3% to 30.4%. The cracks in the Nb/Ti-microalloyed test steel initiated at the interface between ferrite and martensite and at (Nb, Ti)(C, N) phase, and the fracture type was a composite fracture composed of ductile fracture and cleavage fracture.