Study Of Mechanical Properties Fluctuation Of Nb-Bearing HSLA

With the development of metallurgy industry, HSLA steel has become one of the most important steel with high quality. At present, HSLA steel is the best choice when pursuing high quality with low cost. As a microalloyed element, niobium could condition the microstructure and permit attractive mechanical properties. Based on the production of commercial steel, the fluctuation of mechanical properties of a Nb-bearing HSLA steel were studied in this paper.By testing mechanical properties and analyzing the microstructure of cold rolled Nb-bearing HSLA steel, we found that the difference of the recrystallization degree is the main reason which cause the fluctuation of mechanical properties of cold rolled Nb-bearing HSLA steel. By re-annealing the cold rolled Nb-bearing HSLA steel, we found that the decrease of unrecrystallized structure result in the decline of HRB.By annealing the head & tail end of hot rolled Nb-bearing HSLA coil, with the annealing time of 20min, we found that the strength decreased first when increasing the heating temperature from 500℃. The strength increased after the heating temperature reach 560℃and decreased again after 600℃. We speculated that the decrease of strength was due to the coarsening of cementite and the release of residual stress. The increase of strength was due to the precipitation of NbC which had been in solid-solution after coiling.It is found that the strengths are noticeably higher at ends when the U-cooling standard isΔT=50℃, L=50m by testing the head and tail samples of hot rolled Nb-bearing HSLA coil. When the U-cooling standard isΔT=80℃, L=80m, the strength of tail increased after a shortly decrease from the end. We speculated that the cooling condition and the coiling temperature made the precipitation hardening fully develop.Continuous annealing experiment results showed that both soaking temperature and soaking time had prominent effect on the strength of cold rolled Nb-bearing HSLA steel. As the recrystallized structure accumulated, the effect of soaking time on the strength weakened. Soaking temperature always had prominent effect. The uneven distributing of Nb and NbC intensified the nonhomogeneity of the microstructure during annealing and intensified the fluctuation of mechanical properties.