Soaking temperature, drafting schedule, finish rolling and coiling temperatures all play important roles in processing of low carbon plate and strip. They control the kinetics of various physical and metallurgical processes, viz. austenitization, recrystallization and precipitation behaviour. The final transformed microstructures depend upon these processes and their interaction with each other. In view of increasing cost of input materials, new processing techniques such as recrystallized controlled rolling and warm rolling have been developed for production of plates and thinner hot bands with very good deep drawability respectively. Besides hybrid computer modelling is used for production of strip products with tailor made properties. Although there have been few reviews on low carbon microalloyed steels in the past the present one deals with new developments.

The effect of copper, phosphorus, and chromium present in a semikilled reinforcing bar steel produced by in-line quenching [thermomechanical treatment (TMT)] process on the tensile properties, microstructure, and corrosion resistance of steel in simulated chloride environment has been investigated. The results have been compared with that of a semikilled C-Mn reinforcing bar steel without these alloying elements produced by the same process route. Though the amount of phosphorus (0.11 wt.%) was higher than that specified by ASTM A 706 standard, the Cu-P-Cr steel exhibited a composite microstructure, and good balance of yield stress, tensile stress, elongation, and ultimate tensile to yield stress ratio. Two conventional test methods, namely, the salt fog, and potentiodynamic polarization tests, were used for the corrosion test. The rust formed on Cu-P-Cr steel was adherent, and was of multiple colors, while the corrosion products formed on the C-Mn steel were weakly adherent and relatively darker blue. Also, the free corrosion potential of the Cu-P-Cr steel was nobler, and the corrosion current was markedly lower than that of a C-Mn rebar. The Cu-P-Cr steel did not develop any pits/deep grooves on its surface even after the prolonged exposure to salt fog. The improved corrosion resistance of the Cu-P-Cr steel has been attributed to the presence of copper, phosphorus, and small amount of chromium in the dense, adherent rust layer on the surface of reinforcing steel bar. A schematic mechanism of charge transfer has been proposed to explain the improved corrosion resistance of the Cu-P-Cr alloyed TMT rebar.

Charpy V-notch impact toughness of 600 MPa yield stress TMT rebars alloyed with copper, phosphorus, chromium and molybdenum has been evaluated. Subsize Charpy specimens were machined from the rebar keeping the tempered martensite rim intact. The copper-phosphorus rebar showed toughness of 35 J at room temperature. The toughness of copper-molybdenum and copper-chromium rebars was 52 J. The lower toughness of phosphorus steel is attributed to solid solution strengthening and segregation of phosphorus to grain boundaries. Due to superior corrosion resistance, copper-phosphorus TMT rebar is a candidate material in the construction sector.

Microstructures and properties of weldable quality low-alloy fire resistant structural steels (YS: 287-415 MPa) and TMT rebar (YS: 624 MPa) have been investigated. The study showed that it is possible to obtain two-thirds of room temperature yield stress at 600°°C with 0⋅ ⋅20-0⋅ ⋅25% Mo and 0⋅ ⋅30-0⋅ ⋅55% Cr in low carbon hot rolled structural steel. Microalloying the Cr-Mo steel by niobium or vanadium singly or in combination resulted in higher guaranteed elevated temperature yield stress (250-280 MPa). The final rolling temperature should be maintained above austenite recrystallization stop temperature (~ 900°°C) to minimize dislocation hardening. In a quenched and self-tempered 600 MPa class TMT reinforcement bar steel (YS: 624 MPa), low chromium (0⋅ ⋅55%) addition produced the requisite yield stress at 600°°C. The low-alloy fire resistant steel will have superior thermal conductivity up to 600°°C (> 30 W/m⋅ ⋅k) compared to more concentrated alloys.

Mechanical properties particularly Charpy impact toughness of two low-carbon [(a) 0.11% phosphorus and (b) 0.009% niobium] thermomechanically treated reinforcing bar steels were investigated. The phosphorus and niobium steels showed tensile to yield strength ratio of 1.25 and 1.19, ductile-brittle transition temperature of 223 K and below 193 K at yield strength levels of 428 and 472 MPa, respectively. The improved toughness of phosphorus steel is attributed to a mixed transformation microstructure comprising lowcarbon bainite and fine polygonal ferrite. Lowest ductile-brittle transition temperature was observed in the niobium steel due to overall fineness of microstructure consisting mainly of low-carbon bainite, acicular ferrite, and polygonal ferrite.