Temperature Controlled Rolling of Wire Rod and Bar

Technical Paper - Originally written for the 1995 AISE Annual Convention Program and Iron and Steel Exposition, Pittsburgh, Pennsylvania, USA, September 25-28, 1995.

Last edited and updated April 27, 2005.

Authors

Contents

Overview

This article describes the development and operation of the patented and highly efficient ThermoQuench® Cooling nozzles used in interstand and post-finishing cooling trains. A review of basic cooling principles will reveal that under given operating conditions, effective product cooling is primarily a function of the cooling nozzle’s heat transfer coefficient.

back to top

Temperature Effects on Material Metallurgy

• Yield Stress: > 500 Nmm-2 [>72 kpsi]
• Ultimate Tensile Strength: > 550 Nmm-2 [>79 kpsi]
• Elongation: > 10%

These material-strength characteristics are optimized by reducing the carbon and alloy content and using controlled water cooling as opposed to the standard practice of air cooling and adding carbon and other strength-increasing alloys such as vanadium and niobium. Table 1 compares the chemical compositions required to achieve similar mechanical characteristics.

More specifically, improvements in material properties result as the intense cooling of rebar transforms the outer case into a martensitic state while maintaining a high internal core temperature. The retained heat in the core subsequently tempers the martensitic case resulting in rebar with the following cross-sectional characteristics:

• annealed martensitic case
• intermediary structure
• fine ferritic-pearlitic core (black/white structure)

Dual-Phase Ferritic Martensitic Steel (FERMAR) - A further example of the benefits of temperature controlled rolling is seen in the production of FERMAR. FERMAR is a low carbon, low alloy steel containing generally two phases, ferrite and martensite. FERMAR exhibits optimized mechanical and durability characteristics and has proven to be nearly corrosion-free. These characteristics are attained by

• controlling the initial constituent content (carbon, alloys),
• controlling parameters such as type, size, shape and distribution of martensite particles

• by the sudden quenching of the bar following the last rolling pass.

The controlled water cooling of the bar (combined with the low carbon content) prevents the formation of coarse carbides which has been cited as the cause for the corrosive nature of common rebar

Austenitization - The use of residual heat after controlled water quenching can be used to austenitize Cr-Ni bar products directly on the cooling bed. Conventional post finishing heat treating processes, involving the re-heating to austenitization temperature and subsequent quenching, can therefore be eliminated.

Rod Mills - Rod-mill temperature-controlled rolling systems are generally more complex and are extremely important in obtaining homogeneous structural characteristics and the necessary material properties for each product and grade of steel rolled. Each product and grade of steel has a) its own rolling characteristics, i.e. critical speed, deformation behaviour, etc., which effect the temperature profile through the mill and b) specific cooling rates to achieve desired properties. Each product must subsequently adhere to temperature range restrictions, i.e., a minimum bar surface rolling temperature, a maximum bar core temperature and a maximum temperature difference between the core and surface, while attempting to meet a specific laying cone temperature. The laying cone temperature is the primary determinant of grain size and scale characteristics and also influences product microstructure. Excessive bar temperatures can result in undesirable microstructural changes in the bar, i.e., segregations (tool steel, martensitic stainless steel, spring and bearing steel), precipitation of lattice carbon in the form of carbides (high-speed steel) and micromeltings on the grain boundaries (austenetic valve steels).

Rolling Mill Heat Balance Variables

• loss of heat to the surroundings due to radiation and convection
• loss of heat to the rolls due to conduction
• heat gain of the product due to deformation

Heat Loss due to Radiation (Tr) - can be determined by solving a complicated differential equation through iterative means and relates temperature drop as a function of time. Factors in the equation that affect the quantity of heat loss include shape and surface area factors, ambient temperature, radiative and emissivity constants and material-specific variables such as specific heat and density.

Conductive Heat Loss (Tc) - is determined by the following parameters:

• surface contact between the roll and rolled product
• temperature of the rolled product
• temperature of the rolls
• heat-transfer coefficient which is a function of:
• the different thermophysical properties of the rolls and rolled product
• roll/product contact time
• scale thickness
• specific heat [kcal/kg/ oC]
• density [kg/mm3]
• volumetric product flow (A rod x Vmill)

It can be readily seen that both the radiative and conductive heat loss to the surroundings are dependent on mill speed as all other variables are constant for each product rolled. As mill speeds increase, the time available to dissipate heat decreases until a point is reached where the product temperature actually increases as it passes through the mill.

Temperature Increase due to Deformation (Td) - is where the mechanical work equivalent of deforming the bar profile is transferred into heating of the bar or rod.

The following parameters are in the equation:

• deformation resistance [kg/mm2] is a function of the flow stress, which is a material constant, and includes the internal shearing losses that depend on the geometry of deformation and associated friction. Flow stress varies with temperature, strain and strain rate.
• logarithmic shape factor [ln (d1/d2)]
• specific heat [kcal/kg/ oC]
• density [kg/mm3]
• mechanical thermal equivalent [mkg/kcal]

The prediction of actual rod and bar temperatures throughout the mill is complex. All three heat balance equations must be numerically solved beginning with the mill entry temperature conditions and re-evaluated at each roll pass. It can also be concluded from the equations that for a given product final diameter, the resulting temperature increase or decrease at each stand is a function of the mill speed, deformation resistance and the draught taken on each pass .

back to top

ThermoQuench® Cooling Nozzle Development

Analysis of Rod and Bar Cooling - The heating and cooling of rolled product can be approximated by the solution of Newton’s First Law; parameters in Newton’s First Law:

• average temperature of the rolled product [oC] after cooling
• temperature of the cooling water [oC]
• temperature of the rolled product before cooling
• cooling time

The available cooling time (i.e. cooling train length/rolling speed) is usually prescribed by the physical mill layout. It follows then, that for both new and existing mill configurations, the decisive parameter for the effective cooling of a given product diameter and material, is the heat transfer coefficient h, of the cooling nozzle.

Heat Transfer Coefficient h - Published values of the heat transfer coefficients for different cooling arrangements vary over a broad range as many different cooling nozzle designs exist. Through research and field experimentation of new nozzle designs, the highly efficient cooling nozzle, turboQunech, was developed. The heat transfer coefficient for the ThermoQuench® nozzle was found to be represented by the following parameters:

• temperature of the cooling water [oC]
• relative velocity between rolled product and the cooling water
• constants K, a, b

The relative velocity of the water passing through the cooling nozzle is the dominant variable effecting the heat transfer coefficient.

Cooling Water Relative Velocity - The relative velocity of the cooling water is the difference between the water velocity and the rolled product velocity.

• water velocity = Q/A
• water flow rate Q
• area A between the rolled product o.d. and nozzle i.d.

One can assume, that effective cooling is obtained when the relative velocity of the water within the nozzle, i.e. water flow rate, is maximized. It is necessary, however, to consider the relative directions of the water and the rod. In design attempts to realize a high relative velocity within the nozzle, a detrimental situation was revealed in cases where the water and product flowed in the same direction (parallel flow) and the product speed was larger than the water velocity. It was found that increases in cooling water can actually decrease cooling effectiveness. A 100% full counterflow (water and product in opposite directions) situation also has a disadvantage where, although cooling is maximized, the braking effect of the water on the rod may promote cobbles on smaller diameter product. Flow within the ThermoQuench® nozzle is divided in an approx. 50% parallel and 50% counterflow distribution, maximizing the cooling effect of increased water quantities and promoting the smooth travel of product.

D/d Ratio - An important ratio frequently used in cooling, D/d is obtained from the parameters in the area, A,

• cooling nozzle diameter [mm]
• rolled product outside diameter [mm]

The selection of a D/d ratio is important in that a small D/d ratio maximizes the cooling capacity of the nozzle but can also affect the smooth travel of small diameter product through the nozzle. A large D/d ratio will require considerably more water to achieve the same cooling rate. The final D/d ratio selected for a particular application is decided by considering the range of product diameters to be cooled and the associated required cooling water quantities. A suitable ratio is selected by maximizing the cooling effect and minimizing water and nozzle size requirements within the constraints of the ratio.

ThermoQuench® Description - The ThermoQuench® nozzle consists of a common spring coil compressed between two guide bushings. Its overall length depends on the cooling application. Cooling water enters the nozzle through the successive circular channels or gaps formed by the compressed spring coil. The spring coil used in the turboQunech nozzle is essentially a combination of the drilled hole and slotted type cooling nozzles currently used. This provides an advantage over existing configurations in that both the current drilled hole or slot designs are typically prone to contamination. The amount of spring gap and the number of spring coils (i.e. spring gaps) is determined after an iterative analysis of the specific cooling requirements and the resulting water flow rates.

The entry spring is instrumental in achieving high cooling rates as it causes the initial water contact to contain a very high kinetic energy and impact perpendicular to the product axis. The high kinetic energy prevents the initial steam jacket from forming around the bar surface. Laminar water flow, with high heat transfer characteristics, then occurs inside the guide bushings. Experimental data shows that the cooling effectiveness quickly diminishes after approx. 4-6 inches of travel (steam boundary layer formation), where in the turboQunech, the water is quickly and efficiently vented.

In determining a solution to a particular cooling application, an iterative analysis evaluates the desired temperature drop, the cooling time available (ie. nozzle or train length), the required water quantities, the D/d ratio, and range of product grades. The optimized solution takes into account the implications of both D/d ratio extremes, as well as the concern of having a long nozzle with a minimal amount of water. A long nozzle with relatively small amounts of water may promote the premature formation of steam (through Leidenfrost phenomena), and dramatically reduces the heat transfer coefficient while potentially inducing severe vibrations in the rolled product. This results in not only product waviness but can lead to cobbles.

back to top

ThermoQuench® Cooling Nozzle for post-finishing cooling

The success in implementing the ThermoQuench® nozzle for interstand cooling lead to further design developments resulting in the emergence of the ThermoQuench® cooling nozzle for post-finishing cooling.It was found that on long cooling train applications, numerous ThermoQuench® nozzles were required to sustain the necessary heat transfer to cool a hot bar.The ThermoQuench® nozzle for post-finishing cooling was developed to obtain a high heat transfer coefficient over a greater length so as to minimize the number of cooling elements required. The ThermoQuench® nozzle for post-finishing cooling is therefore equipped with secondary cooling chambers, which contains turbulence chambers. These secondary chambers facilitate the continued intense cooling by introducing turbulence and preventing unwanted steam formation. The ThermoQuench® nozzle for post-finishing cooling is designed in different length, which allows the water cooling effect to be maximized and then quickly vented. This characteristic is instrumental in controlling steam formation before its adverse effects are realized.

This renewed cooling capacity i.e. secondary turbulence is a particularly important feature in the post finishing cooling of rebar. The characteristic ribs on rebar tend to trap cooling water as is travels through the nozzle promoting the formation of local steam pockets. The additional turbulence prevents these local steam pockets from adversely effecting the rebar material characteristics and promotes the smooth bar travel through the cooling train.

back to top

Cooling Systems

• high heat transfer capabilities using minimal water at low pressures
• high heat transfer capabilities to minimize the cooling train length in both rod and bar mills and must also facilitate,
• the precise control of the cooling rate and possess the ability to maintain the temperature setpoint based on product, diameter, material grade and rolling speed
• the precise control of the cooling water supply, especially on small diameter rod lead and tail ends

Most importantly, the results obtained must be reproducible from product to product, for every scheduled rolling.

Bar Mill Cooling Trains - A ThermoQuench® post rolling cooling system is comprised of different numbers of cooling elements located in series after the last finishing pass, a water supply system and electrical control equipment. A pinch roll may also be required depending on the existing shear location.

The cooling elements and spray wipes are housed in modular free standing, enclosed water box. Each cooling element is equipped with an individual shut off valve for precise cooling water control.

The cooling train length consists of the required number of ThermoQuench® nozzles located back to back in a modular arrangement. This modular design has the advantage that if any rolling variable is changed the cooling train length can easily be modified or entirely replaced to accommodate the new parameters. For example, a bar mill producing rebar will typically require 2 - 3 cooling trains to accommodate the entire size range of rebar. The number of trains required is determined after considering the range of D/d ratios, mill speed and the desired cooling effects.

Rod Mill Cooling Trains - Water cooling systems for rod mills consist of water boxes located before and after the finishing mill/block. Each water box contains 2 to 3 ThermoQuench® cooling nozzles as well as 1 entry and 2 exit water strippers and 1-2 air strippers to dry the rod product upon the exit of the cooling chamber. An engineered free area follows each cooling box to allow for bar core/case temperature equalization to occur. Each ThermoQuench® cooling nozzle is intentionally designed with a minimum amount of space (volume) surrounding the entry spring. This, along with quick response shut off valves, ensures optimum on/off control of the water flow which minimizes the cold lead end length and avoids cold tail end situations.

Interstand Cooling - The ThermoQuench® nozzle is typically used in interstand cooling applications on the intermediate and/or finishing trains i.e. high speed finishing blocks. The cooling nozzle is situated immediately after each round pass. The purpose of interstand cooling is to provide intense cooling immediately after the roll pass in order to keep the product temperature to within the allowable limits.

Water Supply Systems - Water supply systems for cooling trains consist of centrifugal water pumps, water control valves and flow feedback devices. ThermoQuench® cooling trains, for either rod or bar mills, are arranged with parallel water supply feeds, one per water box, which ensures optimum control of the water. The water supply for each box contains a throttle valve and a flow meter feedback. The water is then further sub-divided to each cooling nozzle via a header located in or near the cooling box. When required, a return flow control valve is utilized to maintain the water inertia in the header for quick response situations.

On ThermoQuench® installations, each nozzle is equipped with independent control, i.e. shut off valve, throttle valve and flow meter feedback. A diverter valve is an option when quick response is required.

Electrical Control - ThermoQuench® Cooling trains are equipped with electrical control systems which permit precise product cooling and variable monitoring. Both the rod mill ThermoQuench® water box and finishing block have entry and exit pyrometers which provides feedback to regulate the water flow. Similarly, a bar mill ThermoQuench® cooling train utilizes pyrometer feedback (both entry and exit) for water flow regulation.

The water cooling systems utilize a PLC for field device control and monitoring. The water flow rates are regulated in real time via the pyrometer feedback over the length of the bar. Flow meter feedback ensures accurate throttle valve control.

An operator interface is supplied to provide set point adjustment, real-time temperature monitoring and facilitate cooling reproducibility through product cooling recipes.

back to top

Conclusions