Good quality steel should possess optimum mechanical, chemical, thermal, as well as, some other additional qualities. Here, some necessary qualities are described in a nutshell.
• Mechanical properties: Tensile and fatigue properties lie in this group. Our main concern is to maintain sufficient tensile properties of deformed bar. Yield strength, Ultimate tensile strength, % Elongation, Ductility etc. are measured while tensile test is conducted.
► Yield Strength (YS): The yield strength or yield point of a material is defined in engineering and materials science as the stress at which a specific amount of plastic deformation is produced, usually taken as 0.2 percent of the unstressed length. Our infrastructure and structural designs are mainly based on YS while higher YS provides lower consumption of reinforcing steel bars which is necessary to resist several catastrophes like earthquake.
► Ultimate Tensile Strength (UTS): Ultimate tensile strength (UTS), often shortened to tensile strength (TS) or ultimate strength, is the maximum stress that a material can withstand while being stretched or pulled before failing or breaking.
► %Elongation (E): The amount of extension of an object under stress, usually expressed as a percentage of the original length. This is one of the key characteristics for reinforcing steel bars since higher value of %elongation ensures the longevity of the structure
► Ductility: Ductility is a mechanical property that describes the extent in which solid materials can be plastically deformed without fracture. Lack of ductility of a deformed MS bar could affect its bending property.
► Tensile properties can be understood by stress-strain curve. Typical stress-strain curve for several deformed MS bars are provided here;
• Chemical Properties: Chemical composition of deformed steel bars plays a key role since entire mechanical properties are subordinated to it. Since steel is an alloy, lots of alloying elements are present in it. Common alloying elements are C, Mn, Si, Cr, Ni, Mo, P, Sn, Cu etc and we have to maintain the optimum range for all elements since slight alteration of any element can cause severe alteration in mechanical properties.
• ther Qualities (TMT): The new addition of BSI to our country is BSI STANDARD TMT 500W which reveals some unique characteristics. These are given here in brief;
• Resists Fire: Survives high temperatures.
• Resists Corrosion: The TMT process gives the bar anti-corrosive properties. Twisting operation is not involved in the production of the TMT steel bar. Hence no residual stress resulting in high corrosion resistance to chloride. Resists fire: Survives high temperatures.
• Earthquake Resistance: The soft ferrite-pearlite core enables the TMT bar to have high strength, higher %elongation, as well as, high ductility. It can easily survive seismic zone of 5.
• Enables Welding: TMT steel has approximately 0.2 percent of carbon. It gives high weldability without compromising on strength.
• Safety: Since higher Strength combined with higher Ductility in TMT steel, it becomes safer reinforcing agent.
• Formability: TMT rebars have excellent workability and bendability due to very high elongation values and consistent properties throughout the length of the bar.
• Malleability: TMT bars are most preferred because of their flexible nature.
• Bonding Strength: External ribs running across the entire length of the TMT bar give superior bonding strength between the bar and the concrete.
• Cost-effective: A high tensile strength and better elongation value give you great savings. In addition to this, it’s a low alloy steel, so requirement for various chemical elements is less in production process of TMT steel.
• Microstructural Analysis: If we conduct microstudy of MS deformed bars, we will find several phases which practically control entire properties. There are two phases for 300MPa (40 grade), as well as, 400MPa (60 grade) deformed bar; ferrite and pearlite. Ferrite is soft which provides toughness of the material whereas pearlite is harder & provides strength. There is one additional phase presents in quenched and self-tempered MS bars (locally known as TMT Steel), which is martensite. This phase is extremely hard and it drastically increases the strength of the material. Since this product is low alloy steel, higher portion of ferritic region provides necessary elongation of the material. Some microstructures are added below:
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