Effects of sulphur and phosphorus on the grain structure of phases in steel.

The presence of sulphur and phosphorus in TMT steel has a significant impact on its grain structure. The amount of these elements present can affect the formation and rate of growth for ferrite, pearlite, and bainite grains. This, in turn, will influence both the strength and ductility of the material.

For high-strength deformed (HSD) bars, this effect is particularly important as they are more sensitive to changes in composition due to their increased resistance to yielding. If too much sulphur or phosphorus is present, it leads to coarser grain size which reduces strength - whereas lower levels result in finer grains which increases strength.

During the rolling process, the grains in the phase of a hot bar are not uniformly arranged and are larger in size compared to when the rolled bar exits the rolling mill. The grains restructure themselves during recrystallisation, and as the bar is quenched in high-pressure water, through various nozzles, the phases start to differentiate at the core, the transition zone, and the out layer forming uniform grain sizes and linear grain boundaries. This enhances the ductility of the bar as well as the increased yield strength.

There are three important areas of the rebar where different microstructures play different roles in the properties of steel. A – The rim or the outer layer is usually known as the Martensite layer; B – the transition zone or a.k.a. the Bainite (ring); C – the inner core or a.k.a. Ferrite-Pearlite soft core of TMT bars.

In the cover image, we can refer to the (b) alloy rebar, wherein recrystallisation is more homogenous, and grain boundaries are seen to be protected with larger pearlites. Alloying elements like P, Cu, and Nb allow for the uniform formation of grain boundaries.  

**Cover Image outsourced from Journal of Materials Engineering and Performance – Panigrahi, B. K., Srikanth, S., & Sahoo, G. (2009, November). Effect of Alloying Elements on Tensile Properties, Microstructure, and Corrosion Resistance of Reinforcing Bar Steel (thesis). Journal of Materials Engineering and Performance. Image is not a property of SKNMIPL. Image is outsourced for information purposes only.

Manufacturers should also understand how changing the composition affects other properties such as corrosion resistance, ductility, and hardness so that an optimal balance between cost-effectiveness and safety assurance can be met when selecting materials. By taking into account all aspects mentioned above during their selection process manufacturers can ensure they make cost-effective decisions that meet industry standards while guaranteeing safety.

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