Abstract: Effect of Nb and V microalloying on microstructure and mechanical properties of Q355B steel after normalizing, quenching, and tempering were studied. The results show that Nb and V microalloying introduces undissolved spherical MC carbides at austenite grain boundaries, which inhibits boundary movement at high temperature, leading to significant grain refinement of the 900 ℃ normalized and 900 ℃ quenched steel. Additionally, Nb and V microalloying enhances the martensitic fraction of the quenched steel, thereby improving hardenability. After quenching at 900 ℃ and tempering at 450-530 ℃, the strength of the tested steel increases first and then decreases with the tempering temperature increases, while the elongation remains consistently above 25%, and the strength is the highest when the tempering temperature is 480 ℃. Combined with the microstructure and mechanical properties analysis results, 480 ℃ is determined as the optimal tempering temperature. At this temperature, Fe₃C carbides are uniformly dispersed within the martensitic matrix, significantly enhancing the precipitation strengthening effect. Furthermore, Nb and V microalloying further optimizes the microstructure of the Q355B steel, promoting the precipitation of rod-like MC-type carbides. After tempering at 480 ℃, the yield strengths of the Q355B steel and the Nb-V-Q355B tested steel reach 469 MPa and 582 MPa, respectively, with elongations of 26.5% and 25.0%. These results indicate that Nb and V microalloying, while ensuring good ductility, leads to a 113 MPa increase in yield strength.

Key words: Q355B steel, heat treatment, Nb, V microalloying, MC carbide, mechanical properties