Investigation on the Strengthening and Toughening Mechanism of 500 MPa V-Nb Microalloyed Anti-Seismic Rebars

Abstract

Two types of 500 MPa anti-seismic rebars were produced by V-Nb microalloyed combined with controlled rolling and cooling technology, the strengthening and toughening mechanism of which were investigated. The complex phase microstructures of specimens consist of ferrite, pearlite and bainite (6-10 %). Furthermore, a large number of V(C,N) and Nb(C,N) precipitates with size of 5-30 nm formed in the ferrite matrix, grain boundaries and on dislocation lines, promoting the precipitation strengthening and inhibiting grain coarsening to controlled cooling microstructure. The mechanical performance of the steels was improved by solution and grain refinement strengthening, precipitation and microstructure strengthening. And the best strengthening effect was obtained by grain refinement, which increased the yield strength more than 35 % strength increment contribution ratio to yield strength. Moreover, about 16.5 % microstructure strengthening increment was obtained due to bainite formation. The plasticity and low-temperature toughness enhancement were mainly attributed to ferrite grain refinement improvement.

Effect of heat treatment process on precipitation of V microalloyed steel

“…Figure 7(a) shows that the precipitates are relatively uniform and finely dispersed in the matrix. The size of the precipitates is about 10∼50 nm, which plays the role of dispersion precipitation strengthening [46][47][48]. It can be seen from Figure 7(b) that the precipitates were also distributed between the lamellae in the pearlite phase.…”

Post-Rolling Cooling Phase Transformation and Microstructure of High-Strength Anti-Seismic Rebars with Different Solute Nb and Austenite Microstructure

“…Zeng's [21] studies have shown that when the content of Nb in high-strength anti-seismic rebars reaches 0.035% to 0.060%, a large number of bainite microstructures will appear, resulting in rebars with high tensile strength but no yield phenomenon. The research results of Chen [22] on niobium show that the high reheating temperature will make the prior austenite grains more coarse, and promote the formation of bainite. The brittleness of the rebar increases, the elongation decreases, and the brittle fracture occurs when the bainite content reaches 50%.…”

Revisão Do Efeito Do Vanádio Em Vergalhões Para Dois Processos De Laminação

“…O refino de grão é a única forma de aumentar a resistência mecânica e ao mesmo tempo a tenacidade do aço. Os precipitados e os contornos de grãos refinados servem como barreiras para a movimentação das discordâncias no interior dos grãos ferríticos, contribuindo para o aumento de resistência ao escoamento [2][3][4]. O principal objetivo aço microligado reside no refino do tamanho de grão ferrítico, imposto pela precipitação.…”

“…A adição de Nb e V retarda a recuperação e recristalização e suprime o crescimento de grão pela precipitação de tamanhos pequenos durante a deformação a quente. Um grande número de precipitados induzidos por deformação no processo de laminação pode retardar a recristalização da austenita e refinar a microestrutura pela supressão do crescimento de grão da austenita e ferrita durante o resfriamento [3]. Os elementos microligantes que são segregados para os contornos de grão da austenita tendem a reduzir a energia interfacial e a mobilidade dos contornos, reduzindo a taxa de recristalização, ou seja, aumentando-se o teor desses microligantes, o valor da temperatura de não-recristalização (Tnr) do aço aumenta.…”