Microstructure and properties of thermomechanically strengthened reinforcement bars: a comparative assessment of plain-carbon and low-alloy steel grades

Abstract

An extensive investigation has been carried out to study structure-property characteristics and corrosion behavior in three varieties of thermomechanically treated (TMT) reinforcement bars (rebars) produced in an integrated steel plant under the Steel Authority of India Limited. Three experimental steel heats--one of plain-carbon and two of low-alloy chemistry--were chosen for the study. Of the two lowalloy heats, one was copper-bearing and the other contained both copper and chromium for improved corrosion resistance. Hot-rolled bars for each specific chemistry were subjected to in-line thermomechanical treatment, where quenching parameters were altered to achieve different yield strength levels.All the TMT rebars, regardless of chemistry and strength level, exhibited a composite microstructure consisting of ferrite-pearlite at the core and tempered martensite at the rim. Although a tendency toward formation of Widmanst/itten ferrite was evident in bars of 500 and 550 MPa yield strength levels, no adverse effect on their strength and ductility was observed. From the standpoint of mechanical properties, the rebars not only conformed to minimum yield strength requirements, but also exhibited high elongation values (21 to 28 %) and excellent bendability. Corrosion studies of both TMT and cold-twisted and deformed (CTD) rebars subjected to different laboratory tests indicated that corrosion resistance increased in this order: CTD, plain-carbon TMT, copper-bearing TMT, and copper/chromium-bearing TMT.

The steel–concrete interface

“…), and/or by the exposure related influences during the service life listed below. Columns ''lab'' and ''on-site'' indicate where these characteristics occur primarily standards, and these have led to differences in steel microstructure [15][16][17]. Established processes are micro-alloying (MA), cold working (CW), stretching (STR), and thermomechanical strengthening, e.g.…”

“…Thermomechanical strengthening thus leads to a composite microstructure ( Fig. 3) with a tempered martensite surface layer and a more ductile ferrite/pearlite core [15][16][17][18]. The classic hot-rolled and cold-worked, as well as micro-alloyed, reinforcing steel bars, on the other hand, have a relatively uniform microstructure across the cross section.…”

A Finite Element Method for the Prediction of Thermal, Metallurgical, and Mechanical Behavior of Rebars in the TempCore Process

“…The TempCore, or Quench and Self Temper (QST) process, as it is commonly referred to, is an in‐line heat treatment process that is commonly applied for the production of high quality rebar …”

“…In rebar produced by this method, the microstructure, and hence the mechanical properties, continuously varies from the surface to the centre of the rebar. The overall yield strength of the rebar depends on the volume fraction of the individual phases present and is, therefore, in turn dependent on the process parameters, the most important being the water flow rate, the quenching time (rebar finishing speed), the finishing temperature, and the steel chemical composition …”

“…The tempered martensite surface layer imparts high thermal resistance to rebars even at temperatures of up to 600 °C. However, prolonged exposure at these conditions will result in a compromise of the mechanical properties . Core process, expensive and prolonged, experiments are required to see the range of the quality materials produced by various quenching time and bar diameter, etc.…”