Effects of Ti-Nb Microalloying on the Microstructure and Mechanical Properties of Ultra-pure 30% Cr Super Ferritic Stainless Steel

The effects of Ti-Nb microalloying on the microstructure and mechanical properties of ultra-pure 30% Cr super ferritic stainless steel were studied by optical microscope (OM), scanning electron microscope (SEM) and transmission electron microscope (TEM). The results show that the inclusions in the matrix of ultra-pure material are mainly composed of Cr-rich oxides with size of 2 similar to 6 mu m. With Ti-Nb addition, Cr-rich oxides are transformed into Ti-O-N-rich composite oxides with size of 1 similar to 4 mu m. Meanwhile, the nano-scale (Ti, Nb)(C, N) precipitations are formed in the matrix. The strong growth inclination of ferrite grain is the unique feature for the ultra-pure material during microstructure evolution. Ti-Nb microalloying can apparently refine the microstructure by weakening the grain growth inclination and promote the room temperature strength and hardness. Moreover, the susceptibility of room temperature elongation of material to grain size is enhanced by Ti-Nb microalloying. In addition, Ti-Nb microalloying has the double effect on the impact toughness for the ultra-pure material. On the one hand, the toughness can be improved by refining the grains; on the other hand, brittle inclusions formed in matrix deteriorate the toughness of ultra-pure material, especially at low temperature.