Metal Rolling

Metal rolling is the process of metal forming in which metal stock is passed through one or more pairs of rolls that reduce the thickness and make the thickness uniform, similar to the rolling of dough. Rolling mills compress metal into a uniform thickness, bend it into custom shapes, and create custom-sized stripping to generate products such as structural steel (I-beams, angle stock, channel stock, and so on), bar stock, and rails. It is used for manufacturing of shafts, rods, tubes, axles, and spindles, producing workpieces with desired cross-sections, gear manufacturing, thread Rolling for threaded parts, bolts, and screws, production of bearings and turbine rings through ring rolling, manufacturing metal sheets, plates, and panels and other wide applications in the automotive industry.

The process of rolling is broadly classified into two based on the temperature of the metal being rolled:

• Hot-rolling, is when metal is heated above its recrystallization temperature (usually above 1000 °F). Hot-rolled metals generally have little directionality in their mechanical properties and deformation-induced stresses. A hot rolling mill heats up the metal to shape it into pieces of sheet metal. This rolling process is often used to create steel I-beams for building construction or framing for new automobiles. Hot-rolling is used to make rail tracks, truck frames, water heater, agricultural equipment, metal buildings, etc.
• Cold-rolling, is when the temperature of the metal is below its recrystallization temperature. In a cold rolling mill, metal is pressed through the rollers, and the cold temperature hardens the metal. The metal is usually worked at room temperature which increases its strength through strain hardening, by up to 20%. They also have a better surface finish and tighter tolerances than hot rolled ones. This type of roll mill is often used for materials like stainless steel, aluminum, and titanium.

In terms of usage, hot rolling processes have more tonnage than any other manufacturing process, and cold rolling processes have the most tonnage out of all cold working processes.  

There are many types of rolling processes, including ring rolling, roll bending, roll forming, profile rolling, and controlled rolling.

History

The invention of the rolling mill in Europe may be attributed to Leonardo da Vinci’s drawings. The basic principles of the earliest rolling mills in crude form were found in Middle East and South Asia as early as 600 BCE. Earliest rolling mills were slitting mills, which were introduced from what is now Belgium to England in 1590. These rolling mills passed flat bars between rolls to form a plate of iron, which was then passed between grooved rolls (slitters) to produce rods of iron.

Continuous Rolling Mill (Source: MFVSL)

Types of Rolling mills

The main types of rolling mills are as follows:

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  Planetary rolling mill (Source: MFVSL)
  Continuous rolling mill, also known as a tandem rolling mill, in which  sheet metal is passed through several rolling stands, each pressing the metal thinner. This  allows multiple rolls of sheet metal to pass through the machine without interruption. Continuous rolling mills are helpful for large projects and are often used to produce a large quantity of heavy-duty material like I-beams for buildings or guardrails for highways or roads. These mills are best suited for mass production as they can handle a large rolling load. However, they aren’t cost effective for small jobs due to the labour needed to set them up and the utilities required to run them.
• Cluster rolling mill, in which a cluster of metal rolls are lined up to be thinned out. Cluster rolling mills are massive pieces of machinery, typically a specialized type of four-high rolling mill that allows addition of larger and smaller work rolls of sheet metal during the rolling process. Thanks to their strength, they’re often used for hard metals like titanium alloys.
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  Two-, Three- and Four-High rolling mill (Source: MFVSL)
  Planetary rolling mill is mainly used to crush or grind down raw metal rolls. These are used for processes like steel rolling and stripping. The mill uses a grinding jar to create friction and grind down the sheet metal that’s fed into it. Although they don’t produce as much output as continuous rolling mills, they don’t require as much heating and result in lower material loss.
• Two-, three-, and four-high rolling mills, which roll metal in opposite directions to press and finish the metal.

Terminology used In Rolling Process

Following are some of the common terminologies used in rolling process:

• Ingot: The initial metal input provided to the rolling process, extracted from casting with various defects.
• Bloom: The first rolled product of an ingot, having a cross-sectional area greater than 230 cm².
• Billet: A product obtained by further rolling of a bloom with a cross-sectional area greater than 1600 mm².

• Slab: A hot-rolled ingot with a cross-sectional area greater than 100 cm² and a width equal to or greater than twice its thickness.
• Plate: A mill product with a thickness exceeding 6 mm.
• Sheet: A mill product with a thickness less than 6 mm and a width greater than 600 mm.
• Strip: A mill product with a thickness less than 6 mm and a width less than 600 mm.