Metalworking or metal forming processes are very important aspects of the manufacturing of the metal part industry. Also, one of these processes is the metal rolling process. In the rolling processes, we produce different kinds of metal shapes. Here, we explain the general process characteristics of metal rolling processes and classifications, etc.
Metal Rolling Process
Metal rolling is a metal forming technology in that we shape starting metal parts or we reduce the thickness of that metal with the action of two or more opposing roll sets. Also, these rolls apply pushing and compression force on the thicker metal to obtain more thin metal parts.
There are two types of metal rolling processes;
- Flat Rolling Processes: In flat metal rolling processes, we reduce the starting metal workpiece’s thickness with the action of rolls. So, we can use the produced thinner metal part in subsequent metal forming processes.
- Shape Rolling Processes: In shape rolling of metal workpieces, we obtain shaped and useful cross-sections such as I-beams, rails, etc. The output of these processes is generally the product.
Temperature of The Metal Rolling Processes
Furthermore, like in the other metalworking processes, application temperature in metal rolling processes is a very important process parameter. Also, we can apply rolling processes in very high temperatures which are hot rolling. If we perform the rolling processes at room temperature, we call it cold rolling. So, we call the rolling processes that we perform in intermediate temperatures warm rolling.
The application temperature of the metal rolling processes is very important. With the increasing temperature, the formability of metal increases, which also means we can form metal parts more extensively. But in cold rolling processes, the formability of metal is limited because of the strain hardening of metal parts.
The strength of the cold-rolled parts is better than the hot-rolled metal parts because of the strain hardening phenomena. But the isotropy of the hot-rolled products is better.
The dimensional accuracies of cold rolling processes are better than the hot rolling processes.
Because of these facts, we roll different types of metal parts and start workpieces in hot and cold rolling processes.
Classification of Starting Metal Workpieces in Metal Rolling
In general, the starting forms of metal workpieces in metal rolling processes are classified into three categories;
- Bloom: Bloom is the thickest type of these three types of metal workpieces. Also, they are starting a bulk metal workpiece that has 150×150 mm of cross-sections.
- Slab: We roll slabs from blooms and we can use them in subsequent metal rolling or forming processes. 250×40 mm of the cross-section can be a good example for slabs. So, you can understand from these dimensions, we can use the bloom to a thinner shape.
- Billet: Billet is another subsequent bulk metal workpiece that has 40 mm of rectangular cross-section.
The types of final products that we produce from these bulk metal workpieces change. We roll slabs into plates and strips and these plates and we can roll strips into more thin products such as metal sheets.
We produce heavy structural final products with hot rolling processes.
General Rolling Mill Configurations
These are the general rolling mill configurations that we use in industry;
- Two-high Rolling Mills: This configuration is the most general rolling mill configuration. Two rolls are opposing each other. So, they rotate in different directions and the work part’s thickness reduces.
- Non-reversing Rolling Mills: Actually, this type of rolling mill configuration is a type of two-high rolling mill. In non-reversing rolling mills, opposing rollers are rotating in the same direction.
- Reversing Rolling Mills: This is the other type of two-high rolling mill. And the opposing rolls are rotating in opposite directions. The advantage of the reversing rolling mill is that the work part can pass in either direction. So work part can pass through several times in the same rolling mill.
Other Important Configurations
- Three-high Rolling Mills: In a three-high rolling mill configuration, we place three rolls face to face. With the reverse rotations of these rolls, we can pass the work partly through these two rolling mechanisms consecutively. After each pass, we must orientate the work part to another pass. We require some moving and elevator mechanisms.
- Four-high Rolling Mills: For the higher rolling forces, we can use four-high rolling mills. We use one small-diameter roll set and the other two big-diameter rolls to support them. With this support, we prevent elastic deformation due to the high rolling forces of the small-diameter rolls.
- Cluster Rolling Mills: The purpose of the use of the cluster rolling mechanisms is the same as the four-high rolling mills. For much higher forces, we can use cluster rolling mills.
- Tandem Rolling Mills: In tandem rolling mill mechanisms, we place multiple roll sets consecutively. This series of rolling sets can be very good for serial production. Generally, we use this rolling mill configuration with continuous casting processes. We can use the output of the continuous casting process in tandem rolling mills to obtain shapes constantly.
Shape Metal Rolling Processes
The difference in the shape metal rolling process is the shape of the final products. The produced final shapes are not flat generally. We produce them in intended shapes such as rail cross-section, I-beam cross-section, etc. Bars, rods, and different railroad tracks are the products of the shape metal rolling process.
The design of the required shape rolling machinery is very important. Obtaining the required cross-section with one set of rolls is not possible. You need to design the required rolling process with sets of rolls, in which there must be a smooth transition between cross-sections, from start to end. We call this design process a roll-pass design.
This roll-pass design principle was important because of the uniform shaping of the metal cross-section. The starting cross-section of the metal billet is generally rectangular.
Warping and cracking of the work part are the general problems if we do not embrace the roll-pass design.
Flat Metal Rolling Process
To start to design a flat rolling process, you need to start from the calculation of the draft. This parameter gives the total squeeze rate of the work between two rolls. You can find this value by calculating the thickness difference between thicknesses before the rolls and after the rolls.
Formulations
If there are a series of rolls in the production line, we can find the total reduction of the thickness with the summation of drafts which we divide by the original thickness of starting part. We must make this calculation for each roll sets in a series of rolls. The formula of reduction for individual sets of rolls;
The velocity of the input workpiece and output metal is also a very important parameter to calculate. According to basic physics, the total volume of the input metal and output metal is the same in rolling operations. By using this principle, we can calculate the input and output speeds of the metal in a flat metal rolling operation. Consider this equation;
In this equation, we denote the values with ‘t’ as the input and output thicknesses of workpieces. The values which we denote with ‘v’ are the input and output values of velocities. ‘w’ is the width of the input and output metal in the rolling process.
From classical mechanics, we can calculate the rotational or circumferential speed of the rolls easily. You just need to multiply the angular velocity by the radius of the roll.
The velocity of the input metal to roll is smaller than the rotational velocity of the roll. In the meantime, the velocity of the output metal in the rolling application is bigger than the rotational velocity of the roll. Because of these speed differences, a physical phenomenon takes place in flat metal rolling processes called slip. This slip takes place in the forward direction and we calculate it with this formula again;
In this equation, ‘Vf’ is the velocity of the exiting work and the ‘Vr’ is the rotational velocity of the rolls.
Flow Stress in Flat Metal Rolling Process
We can calculate the flow stress of the metal in the designed rolling process by using the calculator below. Flow stress is very important because we calculate other process parameters with this value.
Flow Stress Calculator for Metal Rolling
The use of the calculator above is very simple. Just enter the required values inside the brackets then click on the ‘Calculate!’ button to see the result. If you want to make another calculation, click on the ‘Reset’ button.
In this calculation, the entrance thickness and exiting thickness of the work part is very simple. You can find information about the other parameters in this article about flow stress.
You can use the recommended units inside the parentheses. If you do not have a consistent value of units, you can use the MB-Unit Converter to convert your units into consistent sets of values.
Maximum Allowed Draft in Flat Metal Rolling Processes
Sticking can be a very important problem in flat metal rolling processes. We must make the design with the consideration of sticking. Sticking is about the friction between the work part and the rollers. If the friction force exceeds the flow stress of the metal at that temperature, the metal will stick to the roller which will demolish the production process.
To avoid sticking, you need to calculate the maximum draft for your flat metal rolling process. Use the calculator below to calculate it;
Maximum Allowed Draft Calculator for Metal Rolling
Required Rolling Force, Torque, and Power Calculation
The calculation of the required rolling force, torque, and power is very simple. Use the calculator below.
Torque, Force And Power Calculator For Metal Rolling
In this calculator; You just need to enter the radius of the rollers additionally.
Ring Rolling Process
As you understand from its name, ring-shaped metal parts are processed in ring-rolling processes. In ring rolling, the thicker and smaller diameter of metal rings is rolled into bigger diameter and thinner parts. In general, the ring rolling process is applied as hot for the bigger rings and cold for the smaller rings.
The cross-section of the ring is not limited to only a rectangular cross-section. Various kinds of cross-sections can be produced with the ring rolling process.
Parts of rotating types of machinery such as roller and ball bearings, pipe rings, rings for pressure vessels, and railroad wheels are examples of the products in ring rolling processes.
The general advantage of the ring rolling process while production the products like above, is less material loss and very good grain orientation for the physical characteristics of the produced part. If this process is applied in a cold environment, strength enhancement due to strain hardening is another plus of this process.
Thread Rolling Process
Thread rolling is a process that threads geometries as obtained by rolling the cylindrical parts between the sets of rollers. Threaded parts are very important in mechanics such as bolts and screws. With thread rolling, serial and mass production of these machinery parts is possible.
The thread rolling process is applied at room temperature generally. Because of this fact, the thread rolling process can be classified as a cold working process.
The geometry of the thread die surface, which came in contact with the starting blank, defines the thread geometry of the finished part.
Two types of dies are used in thread rolling processes generally. The first type is the flat dies which are doing reciprocating motions according to each other to form threaded forms. Round dies are the second type that threaded geometries are obtained with the rotation of these dies.
Thread cutting is another alternative to the thread rolling process, in which the machining operations are applied. There are very important advantages of the thread rolling process over the thread cutting process;
- Stronger thread structures are obtained because of the work hardening of the work part between dies.
- Better surface characteristics can be achieved with the thread-rolling process.
- Material use in thread rolling is more efficient.
- Because of the action of compression forces applied to the work part, the fatigue resistance of the produced part increased.
Powder Rolling Process
In the powder rolling processes, metal powders are brought to the metal rolls. These metal rolls are giving the required thickness and shape to the metal powder. With this compacting of the metal powder with the rolls, strict metal strips are obtained.
These metal strips are sent to sintering furnaces. These sintering operations are applied to the newly produced metal strips to obtain a more rigid structure. Because the sintering operation creates inter-particular bonding between the metal parts that are compacted. But it is not to be confused with the melting of the metal particles. Sintering temperatures are around 0.7-0.9 times the melting temperature of the used metal.
Metal Roll Piercing Process
As we stated above, it has a slightly different principle to obtain hollow metal parts. In the roll piercing process, two rotating rolls are opposed to each other. And they were placed at a certain angle to provide the metal cylinder from moving on.
Rolls are applying compressive forces to the advancing cylinder. This compressive force initiates a crack in the center of the cylinder. Because of this crack, we obtain a hollow shape.
There is a mandrel on the other side of the rolls to control the dimension and finish of the obtained hole in the cylinder.
In seamless tube production from metals, this roll-piercing process is one of the most used processes.
Conclusion
This can be summarized as information about the general characteristics of metal rolling processes.
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