Microstructure and Mechanical Properties of V-Alloyed Rebars Subjected to Tempcore Process

Abstract

Two B400B-R and B500B grade rebars were industrially produced through a Tempcore process. The standard chemical composition of B500B grade was additionally alloyed with 0.067 wt.% V to enhance its mechanical properties. A set of optimized processing parameters were applied to manufacture two different diameters D20 (Ø 20 mm) and D32 (Ø 32 mm). The microstructure -mechanical properties relationships were evaluated using optical and scanning electron microscopes, hardness, and tensile testing. In addition, a thermal model was developed to define the thermal cycle evolution during cooling in the quenching & tempering box (QTB) to simulate the kinetics of V(C,N) precipitation. The microstructure observations showed a typical graded microstructure consisting of ferrite-pearlite core and outer tempered martensite ring for both grades of both diameters. The optimized processing parameters for B400B-R of D32 (compared with D20) resulted in softening of the core (from 160 to 135 HV10) and tempered martensite surface (from 220 to 200 HV10) as well as in decreasing the yield strength (from 455 to 413 MPa) and tensile strength (from 580 to 559 MPa). On the contrary, an increase in hardness of the core (from 165 to 175 HV10) and the outer tempered martensite (from 240 to 270 HV10), in addition to an increase in yield strength (from 510 to 537 MPa) at almost the same level of tensile strength of 624–626 MPa are observed for B500B grade D32 compared with D20. The modeling and simulation calculations suggest that the manufacturing D32 rebars of B500B grade involves longer quenching time in the QTB which allow deeper tempered martensite surface along with a relatively higher core temperature that renders faster kinetics and larger volume fraction of V(C,N) precipitates. The current study demonstrates that the full potential of V-alloying can be exploited when a sufficient quenching time at the equalization temperature is achieved, which is valid for D32 rebars.

A comparative study on the microstructure, hardness and corrosion resistance of epoxy coated and plain rebars

“…It has been observed from figure 4 that the peripheral hardness is maximum due to the presence of tempered martensite which reduces gradually as the centre approaches, exhibiting the classical U-shaped profile for both the rebar samples which is similar to the earlier observations [2,5,48,49]. The hardness of the outer rim is around 300 HV for ECR samples which indicates the presence of tempered martensite phase [2,48]. Since the carbon concentration of the ECR sample is very low (≈0.22 wt%), therefore, in this case, the hardness is expected to be low because the hardness of martensite mainly depends on the carbon concentration [2,50].…”

“…Figure 4 represents the hardness profiles of the plain and ECR sample surfaces (along the cross-sectional diameter) from the centre to the outer surfaces at varying distances. It has been observed from figure 4 that the peripheral hardness is maximum due to the presence of tempered martensite which reduces gradually as the centre approaches, exhibiting the classical U-shaped profile for both the rebar samples which is similar to the earlier observations [2,5,48,49]. The hardness of the outer rim is around 300 HV for ECR samples which indicates the presence of tempered martensite phase [2,48].…”

Study on the Perspective of Mechanical Properties and Corrosion Behaviour of Stainless Steel, Plain and TMT Rebars

“…In this context, it is noteworthy to mention here that the pearlite microstructure that is shown in Figure 2(d) reveals the presence of degenerated pearlite in the case of the Fe 600 rebar sample, which can be correlated with inadequate carbon diffusion during cooling. Detailed discussion on the formation of degenerated pearlite and bainite is available in the literature [23,33,[39][40][41][42][43][44]. Figure 3(d) primarily reveals pearlite microstructure at a higher magnification with greyish ferrite and whitish cementite flake-like structures, which is completely distinguishable for galvanized rebar samples.…”

“…Figures 2 and 3 reveal the SEM microstructures of the Fe 600 rebar and galvanized rebar samples, respectively, consisting of the tempered martensitic rim at the periphery as shown in Figures 2(c) and 3(c), along with a transition zone consisting of bainite as shown in Figures 2(a) and 3(a), followed by a ferritepearlite mixed microstructure in the core as shown in Figures 2(b) and 3(b).In this context, it is noteworthy to mention here that the pearlite microstructure that is shown in Figure2(d) reveals the presence of degenerated pearlite in the case of the Fe 600 rebar sample, which can be correlated with inadequate carbon diffusion during cooling. Detailed discussion on the formation of degenerated pearlite and bainite is available in the literature[23,33,[39][40][41][42][43][44]. Figure3(d)primarily reveals pearlite microstructure at a higher magnification with greyish ferrite and whitish cementite flake-like structures, which is completely distinguishable for galvanized rebar samples.…”

Artificial Neural Network Modelling of the Effect of Vanadium Addition on the Tensile Properties and Microstructure of High-Strength Tempcore Rebars

“…Recrystallization behaviour depends on several conditions in the steel manufacturing process, such as the recrystallisation temperature, deformation temperature, deformation amount, deformation rate and cooling rate, thereby affecting the final strength [ 14 ]. However, previous studies on strength improvement by V carbonitride have mainly focused on flat-rolled products, and studies on rebars are limited [ 15 , 16 ]. Rebars have a substantially high hot deformation rate, therefore producing a recrystallisation behaviour different from that of flat-rolled products.…”