Effects of Nb microalloying and quenching rate on microstructure and mechanical properties of hot formed steels

Abstract

A new type of Nb containing hot formed steel was designed based on traditional 22MnB5 steel, and the effect of Nb on the microstructure and mechanical properties of the hot formed steel at different quenching rates was studied. The new Nb microalloyed hot formed steel and the widely used 22MnB5 commercial steel were both subjected to solution treatment at 900 ℃ for 3 min, followed by water quenching and oil quenching to room temperature, respectively. The mechanical properties of the two steels were tested under two quenching conditions. And the alloy microstructure was analyzed by scanning electron microscope (SEM), backscattered electron diffraction (EBSD), X-ray diffraction (XRD) and transmission electron microscope (TEM). The results show that Nb microalloyed hot formed steel and 22MnB5 steel can undergo significant self tempering during oil quenching, but the yield strength of the former is about 130 MPa higher than that of the latter, and the elongation is also slightly improved. The quantitative calculations of the strengthening mechanism indicate that this is due to the fine grain strengthening formed by grain refinement of Nb containing steel, as well as the combined effect of dislocation strengthening and precipitation strengthening. Under water quenching conditions, the yield strength and elongation of both steels are similar, suggesting that the high cooling rate inhibits self tempering, making the residual stress generated by martensitic transformation the dominant factor affecting yield strength. When the water quenched sample is tempered at 170 ℃ to reduce internal stress, the yield strength of Nb containing steel is once again higher than that of 22MnB5 steel.

Keywords

• nb microalloying
• hot formed steel
• mechanical property
• residual stress