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.
Highlights
Searching for more energy efficient solutions has led to an increased interest in the development of structural steels using cost effective production processes [1,2]
The quenching and self-tempering (QST) or Tempcore process has been widely applied in the production of low C-Mn steel rebars due to its relatively low cost compared to processes based on microalloying addition or conventional cold working [3,4,5]
V-alloyed B500B grade aims to improve its whole mechanical property profile through V(C,N) precipitation during adopting processing parameters that render higher productivity than that can be obtained under normal conditions in the quenching and tempering box (QTB)
Summary
Searching for more energy efficient solutions has led to an increased interest in the development of structural steels using cost effective production processes [1,2]. The Tempcore technique allows the production of concrete reinforcing bars with high mechanical properties alongside excellent weldability and superior ductility and bendability. This technique can guarantee excellent process controllability and high flexibility. The Tempcore process allows lower final temperatures compared to normalizing rolling route resulting in a graded microstructure consisting of outer hard self-tempered martensite case and soft ferrite-pearlite core. The Tempcore process that take place in the quenching and tempering box (QTB) has many input variables that must be kept under control to obtain the desired properties These input variables are the finishing temperature, number of coolers, number of strippers, water pressure, water flow, finishing speed and cooler size relative to the bar size. The differences in microstructure and mechanical properties due to manufacturing different of the examined rebars grades are investigated and discussed and correlated with the potential of V(C,N)
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