Optimum Temperature of Hot Rolled Reinforced Bars at the Cooling Bed

Maintaining high accuracy temperature measurements at the cooling section is essential in order to attain the overall quality of the finished product, and to realise the correct properties. A series of “heat” numbers or batches of molten steel from an Electric Arc Furnace (EAF) for the production of Y12 mm reinforced bars (rebars) were observed at a steel plant to establish the optimum temperature of the rebar at the cooling bed. The casting was done in billet casters and the billets with 100mm×100mm cross-section were then hot rolledto the required size. The finish rolling temperature was between 850-900°C at 11m/s rolling speed. There bars were water quenched in the water box, and lastlysent for cooling on the cooling bed. Tensile tests and bend tests were carried out on rebars every after 15 minutes during the production to ensure that correct mechanical properties were achieved. It was observed that 850°C was the best finishing temperature and 250 °C was the optimum temperature at the cooling bed after equalization. The results for the tensile tests and microstructures were consistent with prescribed standards. The rebar samples were all of low carbon steel.

Rebars are one of the important construction materials and control of the temperature during the production of such a material in a hot rolling process can ensure good quality product. During the hot working process of steel, the flow stress, strain-rate and recrystallization are controlled by the temperature [1].The changes in the microstructure of rolled product are influenced by a series of dynamic events usually accompanied by heat transfer between the work piece, the work roll and the surrounding. All these events have a role in determining the mechanical properties of the final product.It has also been observed that controlling the temperatureat the finishing stand guarantees a good final product than controlling temperature during the roughing stage [1].In a hot rolling process the ultimate is to obtain a fine ferrite grain size. However, whilst temperature control is important at the finishing stand, control of temperature at the cooling bed is even more crucial because this is where the final mechanical properties are established.So in order to achieve the required mechanical properties of rebar, monitoring with precision measuring tools becomes imperative.
The microstructureof the billet before rolling is initially composed of coarse grains of austenite. The austenite grain structure begins to change when the billet passes through the rollers and is compressed. During this stage the austenite grains are elongated into pancaked structure and each grain experiences a change in dimension and usually with deformation bands induced within the grains [2,3]. During these dynamic events, recrystallization becomes an important and powerful tool for achieving significant grain size refinement. The dynamics of this recrystallization process involves dynamic recovery, dynamic recrystallization (DRX) occurs during deformation, it is referred to as 'dynamic recrystallization' (DRX), whereas the term 'static' is applied when it happens after by the applied strain, the strain rate, the temperature and the interpass time. These process parameters, together with the material characteristics have an influence on the recrystallization kinetics and the resulting grain size. recrystallized fraction with time is represented in equation (1) as articulated by In equation (1),X is the recrystallized fraction after a time expressed through . which is the time for deformation before, the temperature of material constant. A typical recrystallization process du As soon asrecrystallizationis completed, temperature developed during the deformation process as long as the interpass time is sufficient evolution of the austenite grain size after described in equation (2) asfollows [4] In equation (2), is recrystallized grain size, whereas has completed and this is usually . , being the interpass time). In this equation, growth and also depending on the chemical composition of the material recovery, dynamic recrystallization (DRX) and static recrystallization (SRX) [4].When this process occurs during deformation, it is referred to as 'dynamic recrystallization' (DRX), whereas the term en it happens after deformation [4]. During hot rolling, each pass is characterised applied strain, the strain rate, the temperature and the interpass time. These process parameters, together with the material characteristics usually involving chemical composition and initial grain size on the recrystallization kinetics and the resulting grain size.The evolution of the recrystallized fraction with time is represented in equation (1)  is the recrystallized fraction after a time t. The kinetics of recrystallisation is which is the time for 50% recrystallization and this time deformation before, the temperature of deformation, and the initial microstructure. The value A typical recrystallization process during hot rolling is shown in figure 1. completed, the growth of austenite grains will be facilitated by the high temperature developed during the deformation process as long as the interpass time is sufficient evolution of the austenite grain size after recrystallization, under isothermal conditions, is usually follows [4]: is recrystallized grain size, whereas ! represents the time once recrystallization usually considered as a 95% recrystallized fraction ( being the interpass time). In this equation, % && denotes the activation energy for grain epending on the chemical composition of the material n and B in the equation When this process occurs during deformation, it is referred to as 'dynamic recrystallization' (DRX), whereas the term During hot rolling, each pass is characterised applied strain, the strain rate, the temperature and the interpass time. These process parameters, position and initial grain size, The evolution of the The kinetics of recrystallisation is this time depends on ure. The value 'n' is a in figure 1.
uniform grains, (2) Coarse equiaxed (grains elongated), (4) Static recrystallization, (5) Complete recrystallization, (6) Grain will be facilitated by the high temperature developed during the deformation process as long as the interpass time is sufficient. The , under isothermal conditions, is usually taken as constant values. The time and the temperature are not the only parameters that affect grain growth. It should be further noted that, the recrystallized grain size is also asignificant variable, considering thatthe tendency for larger grains to grow is lower compared to the smaller grains. It can therefore be deduced that, the kinetics of grain growthconsequently depends on the recrystallized grain size to a large extent [4].
Upon completion of the rolling process, the rebar in the austenitic state enters a water boxwhere the surface is superficially cooled by water at a pressure and flow rate enough to decrease the temperature of a surface layer below the martensite start temperature. The dwell time for this quenching process is less than one second. When the rebar leaves the water box, the heat accumulated in the core is driven outward and this results into self-tempering of the martensite periphery. Eventually the austenitic core is transformed into ferrite and pearlite at the cooling bed [5].The temperature difference between the core and the outer surface results in the equalization process at the cooling bed. It should also be noted that a substantial increase in the ultimate tensile strength and yield strength is also achieved due to self-tempering of the martensite without compromising the ductility.The core can transform to pearlite and ferrite or indeed a mixed microstructure including bainite can be formed. These variations in microstructure are, however, influenced by the composition of the material,the finishing temperature and cooling rate.

Methodology
Seven Y12 mm rebar samples were selected from every "heat"and twenty eight (28) samples were subjected to tensile tests and bending tests using a computerised 60 metric tons TUE-C-600 Universal Testing Machine. ASTM E-290 standard was used to conduct the bending test. This standard requires that the testing is done primarily to assess the extent of ductility in the material.Other conditions for this standard are that, after testing the curved surface of the bend specimen should have no cracks or any open defects after a visual inspection [7]. Optimum tensile strength and yield strengths were then recordedfor analysis. The standard guide used to prepare the samples for observation in the microscopy was ASTM E3-11.The samples were mirror polished and etched using 2% Nital. The etching time was in the range of 15 to 20 seconds and samples where then viewed in the optical microscopy to identify the grain structure.Monitoring of temperature at the cooling bed was done using a hold peak infra-red (hp-1300) thermometer with the temperature range of (-50℃ to 1300℃) and distance to diameter ratio of 4:1.

Tensile test reports and cooling bed temperature.
Tensile test reports for heat numbers A and B, and the cooling bed temperature were compiled and are represented in table 1 and table 2. The relationship between mechanical properties and the cooling bed temperature were derived from these tables and this relationship is illustratedgraphically in figure 2(a) and (b) respectively. The interpretation of these graphs is discussed under section 4 of this paper.    The tensile test reports compiled in table 1 and table 2 together with the graphs shown in figure 2(a) and (b) clearly show that the cooling bed temperature was fluctuating between 180 that there is a direct relationship between the cooling bed temperature and the mechanical properties. In all cases (figure 2 (a) and (b)), the yield stress and tensile strength i maximum of 550 MPa for yield stress and 650 MPa maximum for tensile strength was 21% on average indicating that the steel is able to satisfy the main qualities looked for in a steel rebar such as 'yield point',weldability, fatigue resistance, and sufficient and tensile strengths values obtained the optimum cooling bed temperature range is 200 are consistent and in agreement with the cooling bed within ten seconds. Since the core temperature remains at 80 implies that there will be equalization of the temperatureto room temperature.

Conclusions
This study has established that, the surface rebar is in the range 190℃-250℃.Within this temperature range and hence a good quality product can be produced. It should also be noted that the water flow rate and quenching dwell time was set at 645m³/h and 0.8 seconds respectively. These two variables are easy to control but very important for proper heat treatment of steel.