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Tempcore process for the Production of TMT Reinforcement bars

• satyendra
• February 19, 2020
• 1 Comment
• CCT diagram, Ferrite, pearlite, quenching, Rebars, Tempcore process, Tempered martensite, Thermo-mechanical treatment, TMT bars,

Tempcore process for the Production of TMT Reinforcement bars

Tempcore process for the production of reinforcement bar (rebar) is a patented process in which the hot rolled bar is intensively surface quenched by water, immediately as the bar emerges from the last hot rolling stand of the rolling mill and during the subsequent air cooling the quenched outer layer is tempered by the dissipation of retained heat from the core.

Tempcore process is one of the thermo-mechanical processes which was developed in early 1970s by Centre de Rechaerche Metallurgiques (CRM), at Liege Belgium to produce high yield strength weldable rebars from mild steel billets without impairing their ductility and without the addition of micro-alloying elements.

Property requirements of rebars

The strength together with ductility, weldability and formability are the most essential quality requirements of rebars. Thus yield strength, together with tensile strength, is the first requirement for reinforcement steel in standards and the grade of steel is classified according to the specified minimum yield strength. Use of the higher grade steel rebars is economical since less steel is needed for the same loading condition, and the total cost is reduced. When the guaranteed yield strength of the rebars is increased, the weight of rebars to be used for a given civil construction is reduced and, consequently, the reinforcing costs are cut down. This reduction in quantity also reduces the cost in transporting, handling and fixing of the bars during the civil construction.

In many assembling techniques of rebars at the construction site, welding of the rebars is a necessity for ensuring the required dimensional accuracy. In many cases, tied connections are not strong enough while mechanical splices are not always applicable. Because of these reasons welding becomes a requirement and a considerable amount of field welding takes place. Hence, the weldability of the rebars is also a major concern. It is well known that weldability of rebars requires low carbon content (around 0.25 % or even around 0.22 % for the tack resistance process) and a low carbon equivalent (around 0.45 %) in the steels. Several types of welding methods can be used such as manual metal arc welding (MMAW), gas metal arc welding (GMAW), flash welding (FW) and resistance welding (RW). Amongst the several welding techniques, lap and cross welding are the most sensitive to heat affected zone cracking due to the low heat input and the fast heat dissipation.

Another important property is the bendability. Rebars displaying a good bendability make possible the use of an optimum design and, hence, bring a further reduction of the costs. In case of the presence of connecting reinforcements requires a good rebending ability. In fact, some standards prescribe that rebars have to succeed in bending and rebending operations and this on small diameter mandrels (down to 3 or 4 times the rebar diameter) or in cold weather (- 20 deg C).

Earlier, the design of a concrete construction was based on a stress concept, i.e. on the assumption that the loads which normally appear in service induce only elastic stresses in the reinforcement steel. In such a case, the safety of a construction can be defined in terms of a stress ratio, e.g. the ratio between the service stress and the yield stress of the rebar (Fig 1a). However, recent studies have led to the conclusion that a stress based calculation of the reinforced concrete is not sufficient because, in some cases, local plastic deformations of a given extent have to be absorbed without failure of the rebars (for instance, when tamping occurs). In such cases, the safety of a concrete construction is expressed in terms of a strain ratio, e.g. the ratio between the local plastic deformation which can occur and the uniform elongation of the rebar (Fig 1b). Such a way of design is now adopted in many countries and it requires ductile rebars and a guaranteed uniform elongation of upto 4 % is prescribed for as-received rebars and for welded rebars.

Fig 1 Safety concepts for the use of rebars during construction

Another important aspect for the safety of concrete constructions is to prevent the loss of ductility after rebending. This again emphasizes the necessity of a good rebending ability. Additionally, the use of high yield strength steel rebars can also permit a reduction in the width of major girders or the size of columns in high-rise buildings, thus it can reduce the cost of concrete and handling. Adequate ductility is needed during the fabrication for safety considerations. This is particularly important for structures where the possibility of earthquakes is part of the design consideration. From the fabrication point of view, where bending and rebending of rebars are taking place, good ductility is very important. The concern regarding ductility is reflected in all the standards, and normally the minimum diameter of the mandrel for 180 degrees bend and the total elongation are generally specified.

Other requirements include fatigue resistance, high and low temperature properties, impact properties, corrosion resistance, but these are generally not specified by various national standards although the concern is increasing.

Hence, it can be seen that from the requirements of the rebars and from the users point of view, the important properties which the rebars is required to have are (i) high yield strength, (ii) good weldability, (iii) good bendability and rebendability, and (iv) ductility.

Production processes for rebars

There are several conventional processes which can be used for the production of high strength weldable rebars. These methods can be classified into two distinct categories (Fig 2) as given below.

Rebars can be used in as-rolled condition after slow cooling in air. For the production of these rebars, the yield strength is increased by modifying the chemical composition but the carbon and manganese contents are to be kept low in order to avoid a significant decrease in weldability. This is done by micro-alloyirig, i.e. by adding appropriate quantities of micro-alloying elements such as niobium or vanadium.

Rebars can be produced by putting the rebars to a strain hardening after hot rolling, for instance by cold deformation. For such bars, the yield strength can be increased by increasing the extent of strain hardening. This method enables the production of high strength weldable rebars from low carbon and manganese steels. Example of such rebars is cold twisted deformed rebars popularly known as CTD rebars.

Fig 2 Processes used for production of high strength weldable rebars

Tempcore process

In the rolling mill producing rebars, steel billets are heated to around 1150 deg C in reheating furnace and rolled through a sequence of rolling strands which progressively reduce the billets to the final size and shape of reinforcement bars. On leaving the last strand, a controlled cooling is applied in such a way that the bar undergoes three stage metallurgical transformations as it is shown by a typical continuous cooling transformation (CCT) diagram at Fig 3. As shown in CCT diagram, the Tempcore process is dependent on temperature and time.

Fig 3 Tempcore process on a CCT diagram

An important feature of the Tempcore process is its great versatility. The properties of the rebar for a given diameter and steel composition, can be varied to a large extent by correctly choosing the duration of the first stage (cooling time) and the water flow in the quenching installation (intensity of cooling). Properly applied, the process allows an increase of the yield strength of 150 MPa to 230 MPa without a prohibitive and significant decrease in ductility.  High strength (weldable) grades and gain in steel strength is achieved in the Tempcore steel rebars without any addition of the alloying elements.

Tempcore process is able to produce high strength weldable steel rebars with low carbon and low manganese contents without any addition of costly micro-alloying elements such as vanadium or niobium. Since the same billet composition can be used for different steel grades and diameters, a significant rationalization can be done in the plant.  The water quenching and self tempering treatment is applied directly in line after the finishing stand without any reduction of rolling speed or loss of productivity. By a judicious combination of Tempcore treatment and micro-alloying chemistry, new higher grades (yield strength higher than 700 MPa and tensile strength higher than 800 MPa) can be produced, especially for large diameter rebars (upto 75 mm).

The Tempcore process for the production of rebars has three stages. These stages of the Tempcore process are (i) quenching of the surface layer, (ii) self-tempering of the martensite, and (iii) transformation of the core. The process, properly applied, leads to an increase of the yield strength of rebars and this increase depends on the cooling intensity. Schematics of the process are shown in Fig 3.

In the first stage, the rebar leaving the last stand of the hot rolling mill passes through a special water cooling section. The cooling efficiency of this installation is such that the surface layer of the rebar is quenched into martensite while the core remaining austenitic. The quenching treatment is stopped when a determined thickness of martensite has been formed under the skin (outer part of the rebar section dropping below the martensite transformation starting temperature Ms).

In the second stage, the rebar leaves the intense cooling section and a temperature gradient is established in its cross section which causes heat to release from the centre to the surface. This increasing of the surface layer temperature results in the self-tempering of the martensite.  The name Tempcore has been chosen to illustrate the fact that the martensitic layer is TEMPered by the heat left in the CORE at the end of the quenching stage.

Finally, in the third stage, during the slow cooling of the rebar on the cooling bed, the austenitic core transforms into ferrite and pearlite or into bainite, ferrite and pearlite.  Hence, a Tempcore steel rebar is essentially a composite material consisting of concentrically disposed hard outer layer and soft core with an intermediate layer which is intermediately hardened. With relatively low carbon content, Tempcore steel rebars provides high strength, excellent ductility and weldability amongst other advantages.

Fig 4 Schematics of the Tempcore process for rebar production

There are two process models which have been developed for Tempcore process by CRM (Fig 5). Both are continuously being used and improved upon with the commissioning results of the new installations.  The first model is used for the design of installations. It computes the quenching time necessary to obtain the grade to produce minimum yield strength (i.e. yield strength + safety margin) from (i) the rebar data (diameter, finishing temperature), (ii) by selection of the internal diameter of cooling nozzles, and (iii) the specific water flow rate.  The second model links the mechanical properties to the chemical composition of the steel and with rebar diameter.

The optimization of each new Tempcore installation, takes into account the mill constraints, the rebar straightness, and the controllability of the process, and a best compromise is achieved between (i) overall length of the equipment, (ii) total cooling water flow rate (at nominal pressure of 1.2 MPa), and (iii) number of ranges of cooling nozzles required to cover the whole range of rebars diameters.

Fig 5 Models for the design of the Tempcore installation

Another important point is to reduce the time and manpower needed during changes in diameter or to produce non Tempcore products. When the available space is sufficient, a laterally movable water collecting box is chosen. For long cooling line, a segmentation in two or three water boxes (each individually laterally movable) is also possible as has been done in a recent installation for a large diameter (upto 75 mm) rebar. It is the longest Tempcore installation of 54 m length (3 sections of 18 m each). It is equipped with remote on/off valve on each cooling nozzle. When the available space (width, pass line level) is the main constraint, the water collecting box can be fixed with one range of cooling nozzles.

Tempcore installation can also be designed with 4 parallel cooling lines for slit-rolling of 8 mm to 12 mm diameter rebars. This line has a compact water collecting box, equipped on each strand with individual control of cooling length and water flow rate.  Some other specific features are also used such as the protection of the not used cooling nozzles against overheating (mainly at entry of the cooling line) by external full cone nozzles, or by means of water jackets along the downstream cooling pipes with water going by two full cone nozzles to cool the injector. An anti-sucking device is used in front of cooling nozzle (for rebar diameters from 25 mm) to prevent sucking of air by the nozzles and to avoid the corresponding decrease in the cooling efficiency at the front end of the billets.

Control of the Tempcore process

The control procedure is based on the yield strength/tempering temperature relationships, which are obtained from the results of the commissioning trials. If there is a variation of the thickness of martensite layer versus the yield strength, then the cooling power of the quenching lines has to be adjusted in order to obtain the value required for the yield strength (YS from grade + safety margin).

The pyrometers are the heart of the control of the process. The location of the tempering pyrometer is of prime importance to get measurements as close as possible to the maximum recovery temperature (maximum reached by the surface). The pyrometer located at the entry of the Tempcore box is also important to measure the variation of finishing temperature along the billet or between successive billets. A variation of 50 deg C of the finishing temperature corresponds to a variation of around 8 % of the cooling length at equal specific water flow rate, and thus to variation of the tensile properties after treatment.

The two easy to adjust and powerful control variables in the Tempcore process are water flow rate and the quenching time (Fig 6). The control of the cooling power of a Tempcore line is performed in two steps. In the first step (main control), the length of the quenching line (i.e. the number of nozzles in use) is adjusted. In order to perform an efficient control and to maintain a good homogeneity of the cooling, the nozzles to be switched off are the upstream ones. In the second step (fine tuning), the water flow rate is adjusted by acting on the main modulating valve.

Fig 6 On-line control of the Tempcore process

The cooling length and quenching time are linked by the rolling speed at the finishing stand. This speed is required to be maintained constant along the billet during its crossing of the quenching equipment, and more particularly when the tail of the billet is leaving the finishing stand. That is assumed by the pinch-roll located between the exit of the Tempcore box and the dividing shear. Without using this pinch-roll, when the tail leaves the finishing stand, small rebar diameters are decelerated by the water while the largest diameters are accelerated.

For a given diameter, the input parameters (cooling length, water flow rate, finishing temperature, and chemical composition of the steel) have influence on the output parameter of the process (tempering temperature, yield strength, tensile strength, and TS/YS ratio). This influence is shown in Fig 7.

Fig 7 Influence of the input parameters on the output parameters

Proper use of the Tempcore installation

To obtain a homogenous treatment, the intensity of the cooling (given by the water flow rate per meter of line) is to be high enough to obtain a complete and regular martensite outer ring, constant along the length of the billet. The good straightness of the rebars is obtained with homogenous Tempcore treatment. If the martensite ring is open, wavy bars are observed on the cooling bed.  Some of the important points which are not to be done for the proper use of the Tempcore installation are given below.

• When rebars of too small diameter are treated through too large cooling nozzle, then the filling coefficient F is very low. This leads to an open martensite ring. This phenomenon is also observed when the cooling nozzles have an excessive wear, often due to misalignment of these cooling nozzles.
• When using of the full cooling length available with low water flow rate per meter then the cooling intensity is too low.
• The previous method is more striking when the designed cooling length is more than twice the necessary one.
• Using an active cooling nozzle always at the entry of the cooling line followed by some close cooling nozzles and finally a given active cooling length upto the stripper is helpful. Then the cooling rate is interrupted and becomes too low to get a uniform martensite layer.
• The previous wrong method if often used to protect the closed cooling nozzles from overheating in case of the hot bar (around 1000 deg C) at the entry, leads to water leakages. Hence, a correct design is to be there to present a serious protection of these cooling nozzles to avoid burning of the gaskets.
• Partial or complete clogging of some cooling nozzles can cause irregular cooling around the product.
• Partial or complete clogging of the strippers can cause over-cooling of the rebar because the cooling is not stopped at the exit of the installation. Water in the downstream guides can also affect the photocell detection which drives the pinch-roll or the shear.
• Wrong location of the tempering pyrometer can result into no visibility to control the process.
• In case there is no pyrometer maintenance (electronic protection by cooling, periodic cleaning of lens, periodic checking in front of a black body furnace) then it causes a temperature drift and then the produced rebars gets rejected.
• If there is no pinch-roll at exit of the quenching line, or there is wrong pinching / speed setting, then it leads to large variations of mechanical properties along the length of the billet.

Characteristics properties of Tempcore steel rebar

Various characteristics properties of the Tempcore steel rebar are described below.

Type of steel – Steels for the Tempcore rebars are basically plain low carbon steels specified for yield strength, ductility, carbon or carbon equivalent and yield to tensile ratio. The maximum and minimum specified carbon content intends to ensure weldability and hardenability. With too low carbon content, hardenability of the steel is generally not sufficient and hence more severe quenching is needed which affects the rolling mill design, e.g., speed of rolling mill, as well as the length and efficiency of cooling chamber. Carbon steel with carbon content in the range of 0.13 % to 0.24 % and the carbon equivalent (CE) of less than 0.45 % has been proved to be the best balance to satisfy the above considerations.

Metallurgical phases and microstructure – Tempered martensite in the form of packets of thin plates with martensitic morphology characterizes the hardened layer. A mixture of bainite and polygonal ferrite is in the intermediate hardened layer and polygonal ferrite and pearlite develops in the core.

The microstructure is usually fine due to a relative fast cooling in the core and because of the thermo-mechanical treatment involved in Tempcore process, e.g., polygonal ferrite grains in the core region can be as small as 8 micrometers in diameter and even 3 micrometers in diameter when lower tempering temperature is used. However, coarse conglomerate of pseudo-eutectoid and Widmanstatten ferrite in the core are also possible outcome of the process. The microstructure of the Tempcore steel rebar is shown in Fig 8.

Fig 8 Microstructure of Tempcore steel rebar

Sometimes Widmanstatten ferrite is formed due to the higher equalization temperature. It is possible that high finishing temperature and perhaps also insufficient rolling deformation are the major reasons for forming this type of microstructure. High finishing temperature and insufficient rolling deformation results in large austenitic grains at the end of rolling, and thus coarse martensite and bainite develops in the hardened layer and in the intermediate hardened layer during the subsequent quenching. Large austenite grain size in the core prevents the impingement of grain boundary ferrite, thus allowing Widmanstatten ferrite to grow.

Typical etched cross section showing the three metallurgical regions and typical microstructures and the homogeneity of the microstructure of Tempcore steel rebar are shown in Fig 9.

Fig 9 Microstructure and the homogeneity of the microstructure of Tempcore steel rebar

Effects of process parameters and steel composition -The process parameters and steel compositions play part in the final properties. Normally if the martensite layer is thicker the retained heat is less and thus the tempering is more modest so that the rebar shows higher yield strength and lower elongation. Longer quenching time, lower finishing temperature and higher intensity of quenching result in thicker martensitic layer and lower tempering temperature. Higher carbon and manganese content increases the hardenability of the steel, and hence more martensite is formed. Additionally, the strength of the tempered martensite increases as the carbon content increases.

The models in Fig 5 describe the relationship between yield strength and all influencing parameters. The model was originally used for the design of installations. Although tempering temperature does not appear in this model directly, the finishing temperature, quenching time, bar diameter and water flow rate relate to it quite strongly. Elongation of Tempcore steel rebars has a virtually linear correspondence with the yield strength.

Tensile properties – The Tempcore process can increase the yield stress for a given composition by 150 MPa to 200 MPa without losing much elongation. The tensile properties of the bars depend on the process parameters and steel composition. The range of typical yield strength of Tempcore steel rebar is between 415 MPa to 550 MPa and elongation on a 5d gauge length is 30 % down to 25 % in the same order. The features of typical stress strain curve shows (i) elastic modulus is 200,000 MPa, (ii) the bar has marked yield point and a Luders type of yield and hence the 0.01 % proof stress coincides with 0.2 % proof stress, (iii) the ratio of yield stress to tensile strength is around 0.85, and (iv) the bar has large elongation (25 % to 30 %), large Luders strain and large uniform strain.

Tempcore steel rebar has two major features when the tensile properties are compared with those of conventional steel bar. These features are (i) higher ratio of yield strength to tensile strength, 0.85 versus 0.65 in case of conventional bar, and (ii) larger elongation, 25 % versus 4.5 % to 22 % in case of conventional steel bar.

Formability – Other remarkable properties of Tempcore steel rebar is that it has excellent bending and rebending properties. Despite the hardened outside layer, minimum bend diameter for a 180 degrees single bend is specified as 1d for 12 mm to 28 mm diameter bars and 2d for 32 mm and 36 mm diameter bars.

Some of the Tempcore rebars (20 mm and 28 mm diameter) can even be bent without mandrel. The bars can also withstand all the bending and rebending tests after aging, satisfying the standard requirements. Further, bending operation requires less energy when compared with other types of rebars due to the low tensile strength to yield strength ratio. It is estimated that 10 % to 20 % energy is saved in bending. Comparison of the bending properties of Tempcore rebars with conventional steel bars is given in Tab 1.

Weldability – Weldability of steel is very sensitive to the chemical composition, especially to the carbon content and carbon equivalent (CE). Two popular formulas are used, one is followed by the International Institute for Welding (IIW) and the other is given by Ito and Besseyo, covering the high and low ranges of carbon respectively.

IIW formula is CE = C + (Mn + Si)/6 + (Cr + Mo + V)/5 + (Cu + Ni)/15 for carbon higher than 0.18 %, while the formula of Ito and Besseyo is CE = C + Si/30 + (Mn + Cu + Cr)/20 + Ni/60 + Mo/15 + V/10 + 5B for carbon less than 0.18 %. The IIW carbon equivalent formula shows less tolerance to substitutional alloying elements than the Ito-Besseyo equation. For the weldability of steel re bars, normally the IIW formula or simplified IIW formula (CE = C + Mn/6) is used because of the carbon content.

With the IIW formula, when CE is less than 0.45 % the steel is considered weldable with modern techniques. The CE of the Tempcore steel rebars is well below the critical value of 0.45 % and thus again is superior to other types of rebars.

The excellent weldability of the Tempcore steel rebars is well demonstrated by the bend test on a cross weld (Fig 10). There is no sign of HAZ cracking in the weld of a 20 mm diameter bar when it is bent at an angle of 180 degrees on a 3d mandrel. The Tempcore steel rebars in low temperature and/or in wet state show remarkable weldability. No preheat and no post heat is necessary.

Fig 10 Bending of the Tempcore rebar with a cross weld

The excellent weldability is also demonstrated by the tensile properties obtained after welding. In flush butt weld no decrease in yield strength is generally noticed with the fracture located outside the weld. Also, it is seen that under different weld and welding processes, no cracks occur in the weld.

Other properties – In addition to high tensile strength, excellent ductility and remarkable weldability, the Tempcore steel rebars show good low temperature toughness, less sensitivity to surface damage, and the fatigue resistance and sensitivity to heat are also very competitive. It has been demonstrated that at -60 deg C a 20 mm diameter rebar with a 1 mm deep cut absorbed 190 calories in drop weight test without breaking. Drop weight tests on arc strike damaged 12 mm, 16 mm and 20 mm diameter Tempcore steel rebars show that there is no fracture at -75 deg C. Similarly, notch damaged and strain aged bars survive in drop weight test at -60 deg C.

A limited number of fatigue tests have been conducted on the Tempcore steel rebars. These tests indicate that the fatigue properties of the Tempcore steel rebars meet the requirements of standards. A fatigue test conducted on a 12 mm diameter Tempcore steel rebars has shown superior fatigue properties over cold worked bars. Investigations have also been carried out on the fatigue properties of the Tempcore steel rebars with interest in the effects of galvanizing. All test results suggested that the fatigue strength of the Tempcore steel rebars is as good as those of other types of steel rebars with equivalent yield strength.

Properties of heat resistance of the Tempcore steel rebars are of importance because of the possibility of fire damage. This resistance has been evaluated by two ways namely (i) tensile strength loss at room temperature after previous heat application, and (ii) tensile strength loss at elevated temperature. It has been shown that after heating in laboratory conditions at temperatures between 250 deg C and 900 deg C for half an hour, the room temperature tensile strength increases slightly with preheating upto 500 deg C and significant drop occurs above that temperature. This property is as good as cold twisted rebars and better than those shown by some hot rolled bars. Cold -worked rebars start to lose strength at 300 deg C to 400 deg C. Hot rolled steel bars lose considerable strength from 350 deg C onwards and hot-rolled low carbon micro-alloyed steel rebar starts to show loss in strength from 600 deg C. Tensile strength of Tempcore steel rebars at elevated temperature is similar to cold worked and micro-alloyed rebars with a 20 % and 40 % reduction in yield strength at 300 deg C and 500 deg C respectively.