Strengthening Effects of Niobium on the Tensile-to-Yield ratio of VN-Strengthened High-Strength Anti-seismic Rebar

Reinforcing bars (abbreviated as rebars) have been widely used for reinforcement of concrete, and total yearly production output has topped other steel products resulted from progressive urbanization process, accounting for around 20.0 percent of total crude steel output in China. In order to relieve the constraints of natural resources and environment protection to realize low carbon emission, the application of rebars with a minimum yield strength of 500MPa or higher is being promoted due to its ability to reduce the overall amount of steel needed compared with 400MPa grade. Meanwhile, the minimum value of 1.25 in tensile-to-yield ratio is required for regions prone to earthquake. Originally, VN strengthened process has been employed to produce high strength rebars, but the test results of tensile-to-yield ratio for 500MPa and above grade sometimes fail to meet required minimum value of 1.25, which affected the promotion and application of higher strength rebars. Through trial and error, it was found that the addition of Nb can improve the seismic performance of VN-strengthened rebars, which is contradictory with the grain refinement effect of Nb microalloying in flat products produced by thermomechanical process (TMP). In order to figure out the reason behind this metallurgical phenomenon, the steels with and without Nb addition had been organized focusing on 500MPa and 600MPa grade, and the strengthening mechanisms of Nb in VN strengthened rebars had been investigated through thermal simulation test and M(C,N) phase analysis [1].

Tensile-to-yield problem for VN-strengthened rebars

Based on the physical characteristics such as atom position, size and the affinity with free C and N in steel, metallurgical actualization had been established for each microalloying element. Due to high solid solutions in austenite and low dependence on TMP, V has been regarded more applicable to medium and high carbon steels, in particular long products. In addition, high N contents can promote precipitation of V in steels, so VN strengthened has been considered the most practical metallurgical design scenario for the hot rolled rebars. Based on the regression equations of VN strengthened 400MPa, it is found that per 0.01%V can contribute to 19.94MPa in yield strength and contribute to 17.08MPa in tensile strength from 20MnSi. With the increase of strength levels to 500MPa and even 600MPa in yield strength, the tensile-to-yield shall decrease accordingly.

Where σ_s(20MnSi) and σ_b(20MnSi) are the yield strength and tensile strength of 20MnSi (0.21%C-0.45%Si-1.40%Mn), in which 0.03%V was added to achieve 400MPa in yield strength [2].

Table 1 lists the tensile properties of HRB500E and HRB600E grade on industrial scale. As shown, for the rebar with diameters less than 22mm, the average values of tensile-to-yield ratio are lower than required minimum value of 1.25. With the decrease of rebar size, high strain rate and rolling penetration in finishing rolling shall result in fine austenite grain, which shall cause high ferrite faction and fine ferrite grain size.

Table 1:Tensile test results of HRB500E and HRB600E grade.

The trial results are shown in Table 2, as shown, adding of 0.015%Nb is very effect to increase the tensile-to-yield ratio for both HRB500E and HRB600E.

Table 2:Chemical compositions of 12mm HRB500E and HRB600E.

Figure 1 & Figure 2 show the optical microstructure of HRB500E and HRB600E respectively. Obviously, the pearlite fraction of Nbbearing rebars are higher than those of Nb-free rebars. In addition, the ferrite grain sizes between Nb-bearing and Nb-free rebars show no big difference [3].

Figure 1:The optical microstructure of HRB500E.

Figure 2:The optical microstructure of HRB600E.

Figure 3 shows the TEM analysis results of HRB500E, as shown, most of the fine particles are composed of N-rich V(C, N) nanosized precipitates. While most of the V(C, N) precipitates in the Nb-free HRB500E sample are in the range of 5 to 20nm, the large amount of 5nm V(C, N) precipitates were found in Nb-bearing HRB500E samples [4].

Figure 3:TEM analysis results of HRB500E.

Figure 4 shows the TEM-EDS analysis results of HRB600E samples. As shown, the nanosized precipitates of Nb-bearing HRB600E are higher than that of Nb-free HRB600E sample. Based on analysis results of HRB500E and HRB600E, it is very clear that adding small amount of Nb can promote nanosized VN precipitation, which can contribute to precipitation hardening. Nb(C, N) precipitation will fix part of free nitrogen in austenite zone, which will decrease the supersaturation of VN in austenite, and leave more V contents to precipitate at low temperature. What is more, low γ→α transformation temperature resulted from the niobium in solution will decrease the diffusion rate of VN particles precipitated at lower temperature. At last, thermal stability of nitrides of microalloy elements are more stable than those of carbides [5].

Figure 4:TEM-EDS analysis results of HRB600E samples.

Figure 5 & Figure 6 show the M(C, N) analysis results of HRB500E and HRB600E respectively. As shown, big precipitates are composed of complex phase of Nb, V and N, which are the precipitates during hot rolling at relatively high austenite temperature zone. Due to limited solubility in austenite, NbN and NbC will precipitate in austenite in turn, which will act as the nucleation site for subsequent VN particles.

Figure 5:TEM-EDS analysis results of big precipitates of HRB500E.

Figure 6:TEM-EDS analysis results of big precipitates of HRB600E.

The role of niobium in microstructure adjustment is more complicated than that of V, depending mainly on its existence status as precipitated or in solid solution. Nb remaining in solid solution will increase the hardenability to decrease γ→α start temperature. The undissolved and strain induced Nb(C, N) during reheating and rolling will retard austenite grain growth, resulting in low hardenability to improve ferrite transformation start temperature. In order to investigate the strengthening effect of niobium in VNstrengthened rebars, the phase transformation via dilatometry tests had been conducted for trial HRB500E and HRB600E. Figure 7 & Figure 8 show the CCT curves and transformation products of HRB500E steels. As shown, addition of small amount of Nb can decrease ferrite transformation start temperature, and enlarge pearlite transformation zone, which can increase pearlite fraction. Because Nb’s atom radius is 15.6 percent higher than that of Fe, the niobium in solution will prefer to exist at defect positions like austenite boundary, stacking faults and defects, which can drag the movement of phase interface to decrease γ→α transformation start temperature. Due to lower γ→α transformation start temperature, leaving more austenite to transform into pearlite microstructure [6,7].

Figure 7:CCT curves of HRB500E.

Figure 8:Transformation products of HRB500E under 1℃/s and 2℃/s cooling rate.

Figure 9 and Figure 10 show the CCT curves and transformation products of HRB600E steels. As we can see, ferrite transformation start temperature will increase with the increase of V addition. In addition, ferrite grain sizes of HRB600E steels are finer than those of two HRB500E experimental steels. Based on CCT curves and transformation products, high V and N contents will refine ferrite grain sizes, which can be explained by intragranular ferrite nucleation in medium-carbon VN steels. Compared with Nb-free HRB600E, addition of small amount of Nb can decrease ferrite transformation temperature, which can inhibit ferrite transformation and promote pearlite transformation product.

Figure 9:CCT curves of HRB600E steels.

Figure 10:Transformation products of HRB600E under 1℃/s and 2℃/s cooling rate.

Based on the classical Pickering’s regression equations of yield strength (3) and tensile strength (4) for low alloy, high alloy steels, it is found that high pearlite fraction is favorable to increase tensileto- yield ratio.

Figure 11 shows the calculated equilibrium conditions of Nb(C,N), VN and VC in studied 500MPa and 600MPa experimental steels respectively. As we can see, there is no doubt that Nb’s carbonitrides shall precipitate in austenite firstly, and then V’s nitrides shall precipitate in austenite and ferrite in turn. Unless free nitrogen is exhausted, it is impossible for VC to precipitate in austenite. However, it is more complicated for actually industrial production, high final rolling temperature and quick cooling after rolling will keep more Nb in solution, which can increase hardenability and decrease ferrite transformation start temperature. As a consequence of low ferrite transformation start temperature resulting from the Nb in solid solution, ferrite transformation will be inhibited, and more pearlite fraction will be obtained.

Figure 11:Equilibrium conditions of Nb(C,N), VN and VC in 500MPa and 600MPa.

Up to now, the addition of small amounts of niobium is considered the key to increase tensile-to-yield ratio for VNstrengthened rebars with the minimum yield strength of 500MPa and 600MPa. While some questions need further research, the follow conclusions can be obtained:
1) Adding small amount of Nb can decrease ferrite transformation start temperature, and at the same time enlarge pearlite transformation zone. As a consequence, high pearlite fractions shall contribute more to tensile strength and higher tensile-toyield ratio.
2) Precipitation of N-rich Nb(C,N) will fix part of N contents, which will decrease supersaturated precipitation of V in austenite zone, leaving more V to precipitate in ferrite zone, which can explain why more 5nm nanosized precipitates for Nb-bearing VN-strengthened steels.

Compared with HRB400E with a minimum yield strength of 400MPa, the promotion of HRB500E and HRB600W will save steel materials for building and infrastructure, especially large-span construction, but welding for HRB600E will decrease the strength level sharply due to coarsening of ferrite grain and ripening of fine precipitated carbonitrides of Nb and V.

Support is gratefully acknowledged from the sponsors of CBMM.

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