Vanadium microalloyed high strength rebar is a safe, reliable and cost effective solution for reinforced concrete construction in earthquake prone regions.
There are different options for the producing high strength rebars, the most popular being microalloying (MA) and quenching and self-tempering (QST) and they have different performance and fabrication characteristics. QST is the lower cost process, but the properties of QST rebar are inferior in several ways compared with microalloying, which can restrict its application in seismic situations. Of the microalloyedoptions, Vanadium provides the best combinations of properties and also provides ease of use benefits for the steelmaker during casting and rolling.
The production of vanadium MA rebar is very straightforward and, despite the increased cost of alloying compared with QST, the ease-of-use of vanadium is an important consideration for steel producers.
The high solubility of vanadium carbonitrides in austenite minimises the risk of cracking during continuous casting, and permits the use of economical hot rolling practices compared to the other microalloying choices. Also, vanadium rebars do not require sophisticated cooling lines, and the required microstructures and mechanical properties are achieved directly during air cooling after rolling.
Typically a low reheating temperature of 1100°C is adequate to dissolve all of the vanadium, and this compares favourably with other less soluble microalloying elements. Most of the vanadium is retained in solution during rolling and does not precipitate before ferrite is formed during cooling, therefore allowing the full microalloying potential to be utilised and avoiding property variation due to premature precipitation which can occur with less soluble microalloying elements.
Precipitation of V(C,N) or Nb(C,N) in Microalloyed Reinforcing Steels at Equilibrium Condition
Vanadium carbonitrides tend not to precipitate in austenite and the rolling loads for vanadium reinforcing steels are found to be lower than those for niobium reinforcing steels and similar to those for C-Mn steels when measured in the same temperature range. Therefore, vanadium rebars can be easily hot rolled in the mills designed for C-Mn rebars.
During hot rolling of rebars, high finishing rolling temperatures are inevitable and the conditions of low temperature controlled rolling, which is normally used for niobium steels, are impossible to meet. In addition, low temperature controlled rolling increases the process time and consequently reduces productivity. High finish rolling temperature can be used in vanadium containing steels since grain refinement is obtained by repeated recrystallization.
Unlike niobium rebars, which must be rapidly cooled after hot rolling to control the subsequent concentration of solute Nb in solution to form fine Nb precipitates in ferrite, vanadium rebars can be air cooled after hot rolling to achieve required properties. In addition, niobium present in solution in relatively coarse grained austenite may contribute to bainitic transformation during cooling which results in poor bendability and lower toughness.
Vanadium increases the yield strength through precipitation of vanadium carbonitrides in the ferrite and by grain refinement. The addition of nitrogen increases further the precipitation strengthening since it increases the driving force for nucleation of vanadium nitride (VN) during cooling, resulting in more nucleation sites and a much finer dispersion of VN precipitates.
Effect of vanadium and nitrogen on the yield strength
In order to avoid situations in which reinforced concrete sections become overloaded and risk brittle shear failure during seismic loading, it is considered important to restrict the maximum yield strength of rebars to match the specified level considered in the design. This approach to safe aseismic design criteria, intended to promote ductile failure, is taken into account in modern seismic rebar standards, which specify both minimum and maximum yield strengths. Vanadium microalloyed rebars exhibit the lowest sensitivity to processing parameters, compared to other microalloyed rebars, and have low variation in yield strength.
The Tensile Strength/Yield Strength Ratio (TS/YS) ratio is a specific requirement for seismic design and is required to ensure that the inelastic performance of a structure is in a predictable manner. The TS/YS ratio is a measure of the rebars’ ability to work harden (i.e. strengthen) when undergoing plastic deformation, and a large value means a greater energy absorption capability before failure. This parameter is to ensure that yielding will not be confined to where it first commences, thereby permitting greater elongation of the rebar before fracture and hence greater ductility of the structural member. Additions of vanadium and nitrogen in reinforcing steels increase the yield strength, which results in a slight decrease in the TS/YS ratio. However, vanadium MA rebars have TS/YS ratio values higher or at least equal to 1.25 (the minimum TS/YS ratio is specified in the Chinese rebar standard GB 1499.2-2018) for anti-seismic rebars, and normally less than 1.40.
Effect of vanadium and nitrogen on the ratio of tensile to yield strength
Uniform elongation is also a measure of ability to deform prior to fracture. In general terms, there is an inverse relationship between strength and ductility. However, vanadium strengthened rebars maintain adequate ductility to meet meet the rebar specifications.
Effect of vanadium on uniform elongation
In seismic areas, rebars must have adequate energy dissipation capacity, which characterizes the resistance to failure. The total energy dissipation capacity is evaluated as the sum of the areas formed within the hysteresis loops from a high strain low cycle fatigue test. It has been reported that vanadium microalloyed rebars have superior high strain low cycle fatigue properties than C-Mn rebars. The better seismic resistant property in the vanadium containing rebars was suggested due to non-strain aging and lower ductile to brittle transition temperature compared to the C-Mn steel.
High strain low cycle fatigue stress-strain hysteresis loop of a vanadium microalloyed (0.22C-0.078V-0.0034N) rebar
Vanadium containing rebar has shown no strain aging