Hot rolling

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Hot-rollingSemi-finished products called blooms, billets and slabs are transported from the steelmaking plant to the rolling mills. In many plants steelmaking and rolling are both carried out on the same site. However there are also many stand-alone rolling mills in the UK (some are independently owned while others are part of a larger group but located away from the steelmaking works).

Steel products can be classified into two basic types according to their shape: flat products and long products. Slabs are used to roll flat products, while blooms and billets are mostly used to roll long products. Billets are smaller than blooms, and therefore are used for the smaller type of long product. Uniquely, at Corus' Teesside works, slabs are used ingeniously to roll large long products (such as beams).

Semi-finished products are first heated in a re-heat furnace until they are red hot (around 1200o C). On all types of mill the semi-finished products go first to a roughing stand. A stand is a collection of steel rolls (or drums) on which pressure can be applied to squeeze the hot steel passing through them, and arranged so as to form the steel into the required shape. The roughing stand is the first part of the rolling mill. The large semi-finished product is often passed backwards and forwards through it several times. Each pass gradually changes the shape and dimension of the steel closer to that of the required finished product.

Plate mills

Slabs are used to make plate. Typically, after leaving the plate mill's roughing stand, they are passed through a finishing stand. This is a reversing mill: like on the roughing stand, the steel is passed backwards and forwards through the mill. It is also turned 900 and rolled sideways at one stage during the process.

Plate is a large, flat piece of steel perhaps 10mm or 20mm thick (although it can be up to 50mm thick) and up to 5 metres wide. It is used for example to make the hulls and decks of ships or to make large tanks and boilers. It can also be rolled up and welded to form a large steel tube, used for oil and gas pipelines.

Strip mills

Slabs are also used to make steel strip, normally called hot rolled coil. After leaving the roughing stand, the slab passes continuously through a series of finishing stands which progressively squeeze the steel to make it thinner.

As the steel becomes thinner, it also of course becomes longer, and starts moving faster. And because the single piece of steel will be a whole range of different thicknesses along its length as each section of it passes through a different stand, different parts of the same piece of steel are travelling at different speeds.

This requires very close control of the speeds at which each individual stand rolls; and the entire process is controlled by computer. By the time it reaches the end of the mill, the steel is travelling at about 40 miles per hour. Finally the long strip of steel is coiled and allowed to cool.

 

Hot rolled strip is a flat product which has been coiled to make storage and handling easier. It is a lot thinner than plate, typically a few millimetres thick, although it can be as thin as 1mm. Its width can vary from 150mm to nearly 2 metres. It frequently goes through further stages of processing such as cold rolling and is also used to make tubes (smaller tubes than those made from plate).

Long product mills

Blooms and billets are used to make long products. After leaving the roughing stand, the piece of steel passes through a succession of stands which do not just reduce the size of the steel, but also change its shape. In a universal mill, all faces of the piece of steel are rolled at the same time. In other mills, only two sides of the steel are rolled at any one time, the piece of steel being turned over to allow the other two sides to be rolled.

Long products are so called because they come off the mill as long bars of steel. They are however produced in a vast range of different shapes and sizes. They can have cross-sections shaped like an H or I (called joists, beams and columns), a U (channels) or a T. These types of steel "section" are used for construction. Bars can have cross-sections the shape of squares, rectangles, circles, hexagons, angles.

These bars can also be used for construction, but many types of bar are also used for engineering purposes. Rod is coiled up after use and is used for drawing into wire or for fabricating into products used to reinforce concrete buildings, as are some types of bar.

Other types of long product include railway rails and piling. Some long product mills make unique shapes of steel to a customer's individual specification. These are known as special sections.

Cooling

In all rolling processes, cooling the steel is a critical factor. The speed at which the rolled product is cooled will affect the mechanical properties of the steel. Cooling speed is controlled normally by spraying water on the steel as it passes through and/or leaves the mill, although occasionally the rolled steel is air-cooled using large fans.

Further processing

Hot rolled products can undergo many forms of further processing before they are finally used to make an end-product (such as a steel-framed building or a consumer product). Such processing includes:

  • Cold rolling and drawing

  • Fabricating: Steel sections are cut, welded and otherwise prepared to form the steel frame of a building. Rods and bars are similarly cut and shaped to form the steel reinforcement for concrete buildings.

  • Coating

  • Cutting and slitting: Service centres cut steel into many complex shapes.

  • Profiling: Sheet steel may be pressed into the correct shape for crash barriers or the cladding of buildings (known as profiling).

End-uses-of-steel How is steel used?

This blog from UK Steel lists some of the many end uses of steel in transport, engineering, construction and the home.

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