Continuous Galvanizing of Steel Strip

Continuous Galvanizing of Steel Strip is an important manufacturing activity which requires constant attention to quality and cost parameters. Many production and quality control engineers are tasked with this.An appreciation of the issues is also needed by those who design and manufacture Continuous Galvanizing Lines.

A very good resource which deals thoroughly with all the intricacies of the process is given here:http://www.galvinfo.com/galv_info_notes.htm. 
This is a gateway to 20 separate articles on galvanizing; all very informative, and of profound importance to those responsible for the design and operation of Continuous Galvanizing Lines.

Hot Rolling Practice-an Attempted Recollection

 Saral Dutta,B.Tech(Hons),IIT
Executive Director(Retd),ISP&RMD ,SAIL

https://drive.google.com/open?id=0B3F5vmJLyAu_Z0tnZnlhQVg5bm8

Seminar on TMT Bars

Saral Dutta,B.Tech(Hons)IIT

Executive Director(Retd) ISP&RMD,SAIL

Preamble

Thermo-Mechanical Treatment (TMT) is a metallurgical process that integrates work hardening and heat treatment into a single process resulting in bars having a hard outer surface with a softer core. This quenching process produces high strength bars from low carbon steel. TMT Bars are extra high strength reinforcing bars which eliminate any form of cold twisting, the technology of yesteryears. The need for cutting down the cost of production of high strength re-bars has initiated the involvement of a more economical and competitive Thermo Mechanical Treatment Process. TMT bars are produced as per IS1786:2008 for Fe 415 & 500 grade. The need for reduction in the steel used for concrete re-enforcement has prompted to switch to re-bars of higher yield strengths of 500 and 550 MPa.

TMT Bars are much stronger than conventional CTD Bars and provide up to 20% stronger concrete structure with same quantity of steel and shows up to 50% more elongation than conventional CTD Bars without compromising on strength which makes it safe in earthquake prone areas. Re-bars of yield strength up to 500 N/sq. mm. produced either by cold twisting or micro-alloying or a combination of both adds considerably to the cost of the re-enforcement bars. These cold twisted bars have inferior ductility, weld-ability and increased rate of corrosion. Production of re-bars by the addition of micro-alloys gives the desired results of high strengths but at a cost, which is prohibitive.

Thermo Mechanical Treatment process produces re-bars of high yield strength, superior ductility, weld-ability, bend-ability, better corrosion resistance and thermal resistance creating a revolution in re-enforcement engineering.

More strength with higher elongation makes a concrete structure sound and safe. TMT steel bars compliments “Reinforced Cement Concrete” (RCC) which has become an integral part of every structure, be it a multi-storeyed building, a tunnel, a flyover, or a TV tower. With TMT, RCC can be molded into any desired shape with which the steel rears will gain the ability to withstand any load made to act upon them. In composite RCC, the re-enforcing steel is the costliest constituent (30 to 40% per cu. m. of concrete). This cost can be substantially reduced by using higher grades of steel re-enforcing bars. The higher yield strength of re-bars lowers the steel requirement, which results in reduced cost of construction.

Steel billets are heated to approximately 1100°C in a reheating furnace and then progressively rolled to reduce to the final diameter of the reinforcing bar. After the ultimate rolling pass the hot steel bars are passed through a specially designed water-cooling system to receive a short & intensive cooling. A microprocessor controls the water flow to the quench box to manage the temperature difference across the cross-section of the bars. The correct temperature difference assures that all the metallurgical changes occur and bars attain the necessary mechanical properties

Thermo-Mechanical Treatment - Quenching Stages.

• Surface quenching first stage begins when the hot rolled bar leaves the ultimate rolling stand, it is rapidly and intensively quenched by a special water spray system and the temperature is suddenly reduced drastically;. This drastic cooling measure converts the surface layer of the bar to a hardened structure called martensite while the hot core remains austenite.
• Self Tempering second stage begins when the bar leaves the quenching box with a temperature gradient across its cross section, the temperature of the core being higher than that of the surface. This allows heat to flow from the core to the surface, resulting in tempering of the surface, producing a structure called tempered martensite which is both strong and tough. The core is still austenitic at this stage.
• Atmospheric cooling final stage takes place on the cooling bed. The temperature difference between the core (which is still hot) and the cooled peripheral surface layer is equalized at around 600⁰ C. the austenitic core is transformed to a ductile ferrite-pearlite core.
• Controlling parameters of Quenching

• The equalizing temperature together with the finishing rolling temperature is the most important parameter to achieve the required mechanical properties.
• Quenching time and Water flow rate

Effects of Quenching & Cooling

• The resultant soft core forms about 65-75 % of the cross-sectional area (depending upon the desired minimum yield strength) and the rest is the hardened periphery.
• There is a variation across the cross section, having a combination of strong, tough, tempered martensite in the surface layer of the bar, an intermediate layer of martensite and bainite, and a refined, tough and ductile ferrite and pearlite core.
• TMT bars have undesired brittleness, superior tensile strength (high yield strength), ductility, toughness, hardness, stress free and resistance to corrosion,
• Thus a high strength bar is obtained from inexpensive low carbon steel.

Microstructure Changes in TMT Process

Austenite

• In iron-carbon alloys austenite is the solid solution formed when carbon is dissolved in face centered cubic gamma iron (γ-iron) having a maximum of about 2% C at 1130⁰ C.
• Coarse-grained austenite transforms to pearlite when it is cooled slowly below the Ar critical temperature.
• However, when more rapidly cooled, this transformation is retarded.
• Faster cooling rate to the temperature at which the transformation occurs result in the micro-constituent as mentioned.

Constituents	Temperature range
Pearlite	7050C to 5350C
Bainite	5350C to 2300C
Martensite	Below 2300C

.

Pearlite

• It is very fine plate like or laminar in aggregate of ferrite and cementite.
• It is the result of a eutectoid reaction which takes place at 720⁰ C when plain carbon steel of approximately 0.8% carbon is cooled very slowly from the temperature range where austenite is stable.
• The white ferritic matrix makes up most of the eutectoid mixture together with the plates of cementite..
• Average properties are tensile strength 120,000psi: elongation 20% in 2 in: hardness Rockwell C 20, Rockwell B-95-100, or B.H.N 250-300.

Ferrite

• Ferrite is a solid solution - an interstitial solid solution of a small amount of carbon dissolved in α (B.C.C.) iron.
• It is the softest structure in the iron-carbon diagram
• The maximum solubility is 0.025% C at 720⁰ C and it dissolves only 0.008% C at room temperature.
• Average properties are tensile strength 40.000psi elongation, 40 % in 2 in: hardness, less than Rockwell Co or less than Rockwell B 90.

Cementite or Iron Carbide

• Interstitial compound of iron and carbon, Fe₃C.
• A very hard compound.
• Tensile strength 5000 psi approximately.
• Elongation in 2 inch is 0

Bainite

• A decomposition of austenite bainite consists of an aggregate of ferrite and carbide.
• Its appearance is featherlike, if formed on the upper part of the temperature range and acicular if formed on the lower part.
• The hardness increases as the transformation temperature decreases.
• This is due to a finer distribution of carbide in bainite formed at lower temperature.

Marten site - Transformation & Tempering

• The martensitic reaction begins on quenching when the austenite reaches the martensite start temperature (M_s) and the parent austenite becomes unstable.
• Austenite transformation to martensite continues till the temperature (M _f) is reached, at which the martensitic transformation is completed. 
• The martensite transformation occurs almost instantaneously - the proportion of austenite transformed to martensite depends only on the temperature to which it is quenched.
• Martensite is not shown in the equilibrium phase diagram of the iron-carbon system because it is a metastable phase. It is the kinetic product of a rapid cooling of steel containing sufficient carbon.

• Equilibrium phases are formed by slow cooling rates that allow sufficient time for diffusion, whereas martensite is formed by extremely high cooling rates.
• Martensite is formed of austenite at such a high rate that carbon atoms do not have time to diffuse out of the crystal structure in large enough quantities to form cementite (F_{e 3} C).
• Face-centered cubic austenite transforms to a highly strained body-centered tetragonal form of martensite that is supersaturated with carbon.
•  Martensite has a lower density than austenite, so that the martensitic transformation results in a relative change (increase) in volume.
• Transformation induces a great deal of internal stress, often manifesting itself as cracks.
• However, shear strain is more significant and produce large numbers of dislocations, which is a primary strengthening mechanism of steels.
• More the number of dislocations, greater are the interlocking of dislocations causing obstruction to the movement of dislocations pin[ng the dislocations in place, which results in increase in strength.
• The highest hardness of a pearlitic steel is 400 Brinell, whereas martensite can achieve 700 Brinell. 
• Since quenching can be difficult to control, many steels are quenched to produce an overabundance of martensite.
• The needle-like microstructure of martensite leads to brittle behavior of the material. Too much martensite leaves steel brittle, too little leaves it soft.

Retained Austenite

• In many steels, the martensite transformation does not go to completion due to insufficient quenching (not quenched to the Mf temperature), resulting in varying amounts of retained austenite.

• Mechanical properties are affected by a high percentage of retained austenite.
• If the cooling rate is slower than the critical cooling rate, some amount of pearlite is formed, starting at the grain boundaries where it will grow into the grains until the M_s temperature is reached when the remaining austenite transforms into martensite.
• Moreover, the percentage of retained austenite increases from insignificant for less than 0.6% C to 13% retained austenite at 0.95% C

• The amount of retained austenite is a function of carbon and alloy contents and the quenching temperature
• Austenite is the normal phase of steel at high temperatures, but not at room temperature. Because retained austenite exists outside of its normal temperature range, it is metastable and given the opportunity, transforms from austenite into martensite.
• Higher amount of retained austenite after quenching and martensitic transformation at the surface increases the chance of distortion and danger of forming cracks in the specimen
• However, a combination of austenite (soft and tough) and martensite (hard, strong and brittle) creates a composite material that has some of the benefits of each, while compensating for the shortcomings of both.

Tempering

• It is necessary to modify the very strong but normally very brittle properties of Martensite by heat treatment (tempering) in the range 150-700°C.

• Martensite is a highly supersaturated solid solution of carbon in iron, which, during tempering, rejects carbon in the form of finely divided carbide phases.
• The end result of tempering is a fine dispersion of carbides in an α-iron matrix, which often bears little structural similarity to the original as-quenched martensite. 
• The needed quantum of tempering is carried out until the right structure for the intended application is achieved.

• Retained austenite does not remain stable during the tempering process
• The as-quenched martensite possesses a complex structure. The first formed martensite, i.e. the martensite formed near Ms has the opportunity of tempering during the remainder of the quench.
• This is auto-tempering, which is more likely to occur in steels with a high Ms.
• Tempering takes place in distinct but overlapping stages:

Stage 1

• Martensite formed in medium and high carbon steels (0.3-1.5% C) is not stable at room temperature because interstitial carbon atoms can diffuse in the tetragonal martensite lattice at this temperature.
• These instability-increases between room temperature and 250°C, when iron carbide precipitates in the martensite.

Stage 2

• Austenite retained during quenching is decomposed, usually in the temperature range 230-300°C.
• Retained austenite decomposes to bainite, ferrite and cementite.

Stage 3

• During this stage cementite first appears in the microstructure
• This reaction commences as low as 100°C, and is fully developed at 300°C.
• During tempering, there is replacement of low-temperature martensite by cementite and ferrite.
• During the third stage of tempering it is, essentially, ferrite, not supersaturated with respect to carbon.

Stage 4

• The cementite particles undergo a coarsening process and essentially lose their crystallographic morphology, becoming spheroidized.
• The coarsening commences between 300 and 400°C, while spheroidization takes place increasingly up to 700°C.
• The final result is an equiaxed array of ferrite grains with coarse spheroidized particles of Fe₃C partly, but not exclusively, in the grain boundaries.

Role of Carbon Content

• The hardness of the as-quenched martensite is largely influenced by the carbon content.
• Carbon has a profound effect on the behavior of steels during tempering.
• The Ms temperature is reduced as the carbon content increases, and thus the probability of the occurrence of auto-tempering is less.

Microstructure of TMT Bars

Three distinct rings appear when the cut ends of TMT bars are etched in Nital 

• Tempered surface layer / outer ring of martensite.
• Semi-tempered middle ring of martensite and bainite.
• Circular core of bainite, ferrite and pearlite.

BIS - Mechanical Properties

Fe 500 Fe 500-D

Yield Stress- YS (N/mm²) 500 500

Ultimate Tensile Stress- UTS (N/mm²) 545 565

UTS/YS Ratio 1.08 1.08

Grades

• The grades of TMT bars depend on the various chemical compositions the steel,

which determine the various characteristics such as , malleability, hardness, etc.

• Carbon is restricted to below 0.20% for imparting better ductility and bend-ability and to ensure better weld-ability.
• The carbon equivalent of the steel is controlled by the addition of manganese (from 0.50% to 1.2% depending on the grade of the TMT bar being produced.
• In corrosion resistant TMT bars, corrosion resisting elements are suitably added in the steel.
• Sulphur and phosphorus maintained below 0.05 %

BIS - Chemical Analysis

Fe 500 Fe 500-D

% Carbon 0.300 0.250

% Carbon Equivalent (CE) 0.420 0.420

% Sulphur (S) 0.055 0.040

% Phosphorus (P) 0.055 0.040

% Sulphur & Phosphorus (S&P) 0.105 0.075

% Nitrogen (PPM) 120 120 1

The consistency in strength across the rebar is maintained by reducing the impurities like sulphur and phosphorous to a level below 0.075%. '500' refers to the strength of the rebar in MPa and 'D' refers to ductility of the rebar.

CTD & TMT Bars Compared

PROPERTIES	CTD/Plain Bars	TMT Bars
Strength	Low strength.	Higher strength even at elevated temperature with high ductility. Do not need more work hardening. and so torsional stress cannot form surface defects
Formability	Lower formability.	Excellent formability due to uniform elongation
Weldability	Welding avoided due to weak welded joints	No loss strength on welding.
Formability	Bend 3D to 5D Rebend 5D to 8D (D=Diameter of bar).	Very High Bendability Bend ID and Rebend 4D.
Fire Hazards	Loss of strength due to temperature rise.	No loss of strength up to 5000C.
Ductility & Fatigue Strength	High.	Very high. Most suited for earth quake resistant structures and equipment foundations.
Corrosion Resistant	Scales fall down during cold twisting.	Better corrosion resistant. Absence of cold stress means longer life of concrete structure.
Workability		(i) Pre-welded meshes is used to eliminate manual binding at site; saving construction time. (ii) Easy working at site due to excellent features of ductility and bendability reduces fabrication time.
Transportation Cost	Higher manufacturing cost	Comparatively lower manufacturing cost. .
Overall Economy		(i) Availability of higher grade like 500N/sq.mm and 550N/sq.mm. (ii) Lesser requirement of bar length in welding as compared to mechanical anchorages and results in overall saving. (iii) Material saving, Saving in labour cost of bending binding etc.

TMT Rolling Process.

Applications

• General purpose concrete re-enforcement structures
• Bridges
• Flyovers
• Dams
• High rise buildings
• Industrial structures
• Concrete roads
• Underground structures

Why choose TMT BARS?

• Ductility & Strength

Higher yield strengths combined with better elongation values and toughness as compared to conventional CTD bars resulting in saving of steel and cost of transportation.

The process used ensures combination of tempered martensite on the surface with fine grain ferrite-pearilite and austenite in the core providing for higher tensile strength, toughness ductility and imparts quality of fatigue resistance on dynamic loading on account of the high strength of the surface layer.

Better Safety of structures because of higher Strength combined with higher Ductility.

Strength of the TMT product depends on :

• Carbon equivalent.
• Temp after finishing pass.
• Water pressure.
• Diameter of the TMT Bar being rolled.
• Cooling tube condition i.e. if tube is worn out, then pressure required is more.

• Bendability

Easy working at site owing to better Ductility and Bendability.

Controlled quenching results in adherence to martensite ring formation with fine grain ferrite-pearilite and austenite soft core of TMT bars causing uniform elongation and excellent bendability. These bars can be subjected to Bend ID and Rebend 4D easily. This results in lot of advantages formability during construction work. Due to very high elongation values and consistent properties throughout the length of bar, TMT rebars have excellent workability and bendability.

• Higher Weldability

TMT bars have excellent weldable properties due to carbon being restricted to below 0.20% together with low carbon equivalent

They can be butt-welded or lap-welded using ordinary coated electrodes of matching strength. In manual arc welding no-pre warming or post-welding treatment is necessary.

TMT Bars are produced from IS 2830 Billets. Hence, due its lower carbon range it can be used in making of pre-welded meshes. The same is done without reduction in strength of weld joints. Pre-welded meshes eliminate manual binding at site. Reduces construction and fabrication time.

higher manganese results in, exceptional ductility and improved scope for welding. Low sulphur & low phosphorous nullifies the problems of “Hot Shortness” & “Cold Shortness”.

• Thermal Stability

Resists fire. It can be used successfully even till 600⁰ C without any significant sacrifice in strength. Unlike Tor steel/ CTD Reinforcement bars, TMT bars have high thermal stability. They are the preferred choice when elevated temperatures of 400-600⁰ C may be encountered (Chimneys, fires).

• Superior Corrosion Resistance

The TMT process gives the bar superior strength and anticorrosive properties. Bars produced by Thermo Mechanical treatment show virtually no rusting even after a long time, as there is absence of any tensional residual stress.Controlled water-cooling prevents the formation of coarse carbides, which has been cited as the main cause for the corrosive nature of common bar. The absence of surface stresses caused by the cold twisting process] contributes to the anticorrosive properties.

• Malleability:

TMT bars are most preferred because of their flexible nature

• Monolithic Bond Concrete Strength

• Despite steel and concrete are two different materials, it is desired that these should form as a single unit in a reinforced structure.
• Concrete grips the TMT steel bars to form the strongest bond because of the unique pattern, greater depth, closer and uniform rib spacing. External ribs running across the entire length of the TMT bar adds to the superior bonding strength between the bar and the concrete.
• TMT bars make structures strong, safe and to last for generations. Fulfils bond requirements as per IS: 456/78 and IS: 1786/85.
• Two main loading conditions that concrete under goes are compression and tension.

• Steel is weak under compression but is strong at withstanding tensile stress (bending forces).
• Concrete is superior at bearing compressive stress (squeezing forces) but is very weak in Tension (pulling) and can crack under tensile stress.
• Moreover, concrete's weakest rating is in its shear strength. To increase concretes shear strength, TMT steel bars are used because of its high shear strength characteristic.
• Concrete reinforced, i.e., strengthened with steel combine these qualities to create a material that is stronger than either material alone.
• The strength of one offsets the weakness of the other.
• Reinforced concrete is made by forming the concrete inside a metal or timber framework or by casting the concrete around ridged steel rebars called (reinforcing bars).
• Stressed or pre-stressed concrete involves molding wet concrete around pre-tensioned steel wires. The wires compress the concrete as it sets, making it much harder.
• The coefficient of thermal expansion of concrete is similar to that of steel, eliminating large internal stresses due to differences in thermal expansion or contraction.
• When the cement paste within the concrete hardens, this conforms to the surface details of the steel, permitting any stress to be transmitted efficiently between the different materials. Usually steel bars are roughened or corrugated to further improve the bond or cohesion between the concrete and steel.
• The reinforcement steel bar has to undergo the same strain or deformation as the surrounding concrete in order to prevent discontinuity, slip or separation of the two materials under load.
• Maintaining composite action requires transfer of load between the concrete and steel. The direct stress is transferred from the concrete to the bar interface so as to change the tensile stress in the reinforcing bar along its length, this load transfer is achieved by means of bond (anchorage) and is idealized as a continuous stress field that develops in the vicinity of the steel-concrete interface.

• Earthquake/ Seismic Resistance

• The soft ferrite-pearlite core enables the bar to bear dynamic and seismic loading.

• Excellent cold forming higher ductility, bend-ability and higher Fatigue strength makes it suitable for structures and foundations subject to dynamic and seismic loading

• In earthquake prone zones Indian Code does not permit use of steel bars with less than 14.5% elongation.
• TMT bars guarantee higher UTS to Yield Strength ratio and elongation percentage well above 15% - 18% even for FE 500 grade.
• High UTS/YS ratio and more percentage elongation signify that the steel is capable to strain harder, in the event of an earthquake, i.e., have high fatigue resistance to seismic loads

• Thermo mechanically treated rebars impart strength and ductility to RCC structure to withstand various kinds of loads impacting a building.
• Superior resistance to sustain stress without failure prevents buildings from collapsing

• Steel Economy

Quality Compared TMT Fe-415  TMT Fe- 500 TMT Fe- 550

IS: 2062 Gr.              40%              44%              48%

Plain Bar 

IS: 1786 Fe: 415        12%           14%               19%

HSD Steel Bar 

• Cost-effective:

Section       Grade of Steel         Cost Saving w.r.t. (in %)

                                                                    Plain Bars     CTD Bars

Doubly Re-inforced CTD 415        28 - 33

Beam                   TMT Fe-500      34 - 37           12 - 14

                                  TMT Fe-550      38 - 42

Axially Loaded       CTD 415         32.0                

Columns                TMT Fe-415    39.0                 10.4

Uni-axial Bending CTD 415           28.0                 

with Compression TMT Fe-415     34.0                  8.0

Conclusion

• Higher tensile strength and ductility (superior elongation) values enable economy in design, construction of high risers with improved earthquake resistance (better seismic resilience).
• TMT bars have the added advantage of superior weldability, corrosion resistance) and durability.
• Desired properties are attained with lesser amounts of alloying elements thereby reducing the production cost.
• Achieves great savings in usage of steel to the extent of about 17% as compared to ordinary steel bars and thus reducing transportation costs.