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Rolling Mill Cooling Bed


Rolling Mill Cooling Bed

A rolling mill cooling bed is used for the uniform air cooling of the rolled materials (bars or profiles usually referred as ‘bars’) and transport it in a phased manner from the entry side of the cooling bed to its discharge side. It transfers the bars one by one to the roller table, on which they are transported to the finishing section. It is important equipment in the hot rolling of long products in a rolling mill. A cooling bed carefully moves and cools the hot steel rolled bars after the hot rolling process. Typical view of a cooling bed in a rolling mill is at Fig 1

Fig 1 Typical view of a cooling bed

Cooling beds are to be specifically designed considering the smallest and the maximum size of the bars being rolled in a particular rolling mill. They are designed for receiving, transferring, and cooling of the rolled material. They are sized so that the product cools within a particular cycle time. The length of the cooling bed is determined by the maximum run-out bar length, optimized by the selling lengths to minimize crop losses. The width of a cooling bed is determined on the basis of mill productivity (tons/hour) and the time required for cooling. Cooling beds normally have natural air cooling. However, for achieving faster cooling of the bars, blowers can be installed for blowing air from the bottom of the hot bars.

The rolled bars after their rolling in the rolling mill are generally above 700 deg C and are to be cooled to below 150 deg C for subsequent finishing process. The cooling beds are to provide adjustability to the size, shape, and alloy of the profiles with appropriate cooling rate and minimized distortion. This results into high quality profiles and throughput.

The cooling bed is located in the rolling mill after the rolling of the steel material is completed. It supports and permits the hot rolled bars from the last stand of the mill to cool. Cooling bed cools the bars as well as cross transfers them towards the discharge end. In a cooling bed the temperature of the entire length of the bar is to be cooled at the same time. If not, it develops stresses in the bar.

After the last stand of the rolling mill, a shear arrangement is used to cut the bar which is moving to the cooling bed. This shear is normally a dual ratio crop and dividing shear. The lengths cut are to suit the length of the cooling bed and is based on the multiples of the finished saleable lengths. The cut length optimization software adjusts the primary cut lengths to the cooling bed to minimize yield losses. The tail end of the last bar can be either cobble cut at the crop and dividing shear, or if the length is greater than 3 m (metres), collected at the short bar recovery after the abrasive saws.

Cooling beds are longer than they are wide because of the bar dimensions which they are to typically cool. The length of the cooling bed is an important parameter. The length is to be such that it is able to accommodate the multiple of the saleable length. The length of the cooling bed is determined by the maximum run-out bar length, optimized by the selling lengths to minimize crop losses. The width of a cooling bed is determined on the basis of mill productivity (tons/hour) and the time required for cooling. Higher is the number of saleable length bars per cooling bed length of the bar, there is better yield of the rolling mill.

The bar delivery system to the cooling bed is to enable the bars to be transported onto the cooling bed at a speed matching with the speed of the rolling in the rolling mill. There need to have a soft braking system for the bars so that the surface of the bars remains in a perfect condition despite the high delivery speed. The system is required to adjust the braking pressure to the individual diameter and temperature of the bars. The bars are to be transferred onto the cooling bed for which the suitable arrangement needs to be provided. This ensures that only one bar is deposited per rake. A typical schematic diagram of a cooling bed is shown in Fig 2.

Fig 2 Typical schematic diagram of a cooling bed

Types of the cooling beds

Several types of cooling beds are used in the hot rolling mills for the long products. These are (i) manual cooling bed, (ii) mechanical cooling bed, (iii) walking beam type cooling bed, and (iv) turnover type cooling bed.

Manual cooling bed – It is being used in very low capacity cross country rolling mills. It is normally of sliding type and is capable of accommodating one full length of the bar. It has slope for the bar to move forward by sliding action due to gravity. The bars are cooled on the cooling bed through natural cooling by atmospheric air. In a cooling bed the temperature of the entire length of the bar is required to cool at the same time. If not, it develops stresses in the bar. If part of the bar stays continually in touch with the metal supports, then it cools at a different rate in comparison to the parts of the bar which remains continually open to the ambient air. In manual cooling bed since the bar is resting on the cooling bed, the uniform cooling of the bar along its entire length at the same time is difficult to achieve. Hence, the probability of the bar developing stresses is very high.

Mechanical cooling bed – Several types of mechanical cooling beds are used.  The three most common designs for mechanical cooling beds are rake type, disc type, and chain type.

The rake type cooling bed has a saw tooth pattern which is why it is also known as a rake type cooling bed. The purpose of the cooling bed of a movable rake design is to uniformly air-cool the rolled bars or light sections and transport the same in a phased manner from the entry of the cooling bed to discharge side. The rolled bar as it enters the cooling bed slides onto the first notch on the rakes. The initial notches provide continuous support for the bar on a casting called a grid casting. Long plates with notches set at some distance apart, support the bar after it moves beyond the grid castings. The bar moves across the cooling bed by the movement of alternative plates moving in a cycle of lift, move, and retract, by the action of eccentric cams. Repeating of this cycle moves the bars as they are delivered from the mill. The front ends of the bars and light sections are also leveled at the discharge side and a fixed number of cooled rolled bars are sent for final length cutting by cold shear for subsequent bundling or piling. Cooling beds can also use a chain transfer as the traversing method.

Disc type cooling beds are normally used for sheets and they carry sheet products using series of discs fixed to rotating shafts.

Chain type cooling beds are used for cooling of hot pipes and tubes using raised links to keep each product separate. In the chain type cooling bed, the rolled product is moved by the chain on the cooling bed. It is cooled while it is transported by chain from hot side to the cold side on the cooling bed.

Walking beam type cooling bed – They are commonly used in modern large capacity long product rolling mills. Walking beam type of cooling bed utilizes two sets of rakes, one moving and one fixed, to carry the bars across the cooling bed. The hot side of the cooling bed begins with grid castings to absorb much of the initial heat from the hot bars while offering additional support as the steel bar is still malleable. The moving rake lifts, traverses, and lowers the bar numerous times so that where it touches continually changes permitting the entire length of the bars to cool at the same rate. The bar walks its way to the end of the bed where it is placed on a set of roller tables. Aligning rolls are used close to the cool side of the cooling bed to align the product which is needed for the finishing side of the mill. Walking beam cooling bed has a saw tooth pattern which is why it is also known as a walking rake type cooling bed. The mechanism ensures that the bars are uniformly positioned over the toothed rakes. The cooling bed is usually designed considering the smallest and the maximum size of the bars being rolled, delivered from the finishing rolling stand of the mill, and the cooling time required for the various sizes of the bars.

Waking beam cooling bed increases the production efficiency. However, the structure of the walking beam cooling bed is complex and needs extra efforts in its production and installation.

Turn over type cooling bed – This type of the cooling bed is basically used for the cooling of hot billets produced in a billet mill or in a billet continuous casting machine. This cooling bed is basically a walking beam type of cooling bed but has a feature which continually rotates the billet by 90 degrees as it moves forward. Because of this rotation, the position of the billet as it touches the bed changes continuously, thus the billet does not distort its shape in the cooling process.

Major components of a cooling bed

Cooling bed is very crucial to the process of rolling in a rolling mill since it is to carefully transfer and cool the hot rolled steel bar after the rolling process. As this is an integral part of a rolling mill, any mechanical problem or failure to the cooling bed has a huge consequence for the rolling mill. Hence, the cooling bed and its components are required to be produced to the highest quality standards and thoroughly tested before installation. The installation of the cooling bed also need close precision since the location of tooth of each rake is to match along the entire length of the cooling bed.  The components of a cooling bed are cast, forged or fabricated to meet the design requirements. The major components of a typical cooling bed are described below.

Fixed straightening rakes – These are cast Iron toothed blocks, which receive the hot bars from the bar receiving mechanism. These fixed rakes help to straighten the bars, which are at a temperature of above 700 deg C. 

Movable rake mechanism – These are fabricated steel tooth rakes which are mounted on eccentric rollers to enable them to move in the x and y axis. The rakes have profile cut to suit the smallest and maximum size of bars. This mechanism collects the bars from the fixed straightening rakes and transfers them towards the delivery side of the cooling bed. 

Fixed rake mechanism – These are also fabricated steel toothed rakes, which receive the bars from movable rake mechanism as the bars are transferred toward the delivery side of the cooling bed. 

Drive for movable rake mechanism – The drive to the movable rack mechanism is generally through two motors driving two parallel sets of drive shafts. Each motor transmits drive through two worm reduction gear drives. The shaft incorporates eccentrics, which provide the two-axis movement to the movable rack mechanism. 

Bar aligning rollers – These are chain driven profile cut rollers which are fitted at the end of the rakes but before the bars leaves the rakes. They are driven by motors to align the bars front end before delivery to the run-out roller table. 

Throw off device – At the end of the collecting grid the bars pack is transported on the delivery roller table by means of throw off fingers. 

Run out roller table – These are normally chain driven roller table at the end of the cooling rake. Run out roller table is provided to transport the pack of the bars to the cold shear. 

The cooling bed also has a large amount of supporting structure. The drives of several system can also be hydraulic drives.

Functioning of cooling bed

The functioning of a typical cooling bed is described here.

Immediately after the dividing shear, the cut bar is moved on the approach and run-in roller tables to the cooling bed. The approach roller table is generally provided with individually driven horizontal rollers which are gradually inclined to match the incline of the run-in roller table. The approach table is usually designed to operate faster than the finishing speed of the bar being rolled in order to create a gap between the divided materials at the shear prior to entering the brake slide to the cooling bed.

The run-in table rollers are also individually driven and inclined at the same angle as the brake slide. The brake slide lifts the cut bar off the run-in table rollers to allow braking of the cut bar under friction before being discharged onto the first notch of the cooling bed. A hydraulic cylinder cushions the cut bar against the brake slides while lowering it for a smoother control. The entry section of the run-in table is equipped with drop down walls to prevent the larger bar from sliding down the brake slide before the previous bar is discharged. The number of drop down walls used depends on the product size. Special wear-resistant plates along the approach table and brake slide prevent scratching of the bars prior to the lifting of the brake slide.

The long product bar as it enters the cooling bed slides onto the first notch on the rakes. The initial notches provide continuous support for the bar on a casting called a grid casting. Long plates with notches set at some distance apart, support the bar after it moves beyond the grid castings. The bar moves across the cooling bed ( say from right to left) by the movement of alternative plates moving in a cycle of lift, move, and retract, by the action of eccentric cams. Repeating of this cycle moves the bars as they are delivered from the rolling mill.

For normal conventional operation, the bars are deposited from the brake slide onto the straightening grid of the cooling bed. Moving rakes, operating automatically by two variable speed motors, advance the bars one notch at a time from the straightening grids to the fixed rakes. The speed of the rake section can be varied to control the temperature of the bar leaving the cooling bed. A set of aligning rollers are located at the end of the rake section to align the front of bars in layers before cutting at the cold saw for better yield.

During normal operation, the bars are cooled on the moving rakes and then discharged one by one, onto a grouping chain transfer, driven electromechanically, which shifts layers of bars in required numbers. A lifting tray system picks up each layer of bars and moves them onto the cooling bed run-out roller table feeding the cold saws.

Fast transfer across the cooling bed is also sometimes provided to deliver products to the slow cooling bins at elevated temperatures. In this case, the lifting tray system picks up the layer of bars immediately after they have been deposited on the moving rakes after the straightening grid and quickly transfers them to the cooling bed run-out table for cutting the final commercial lengths, before depositing them into the slow cooling bins.

 

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