There are various methods of forming steel into finished products, including hot forging, hot and cold rolling, seamless tube making and welded tube making. The most widely used process is hot rolling, which accounts for over 90% of all steel production.

Hot Rolling Steel

Rolling is a forming process, which causes permanent change of shape (set) by plastic deformation of the material as it passes between sets of steel rolls.Sets of cylindrical rolls reduce the cross-sectional thickness of the metal whilst simultaneously causing it to become elongated. Other rolling processes are employed to change the cross-sectional shape of the metal using shaped rolls while other methods form the metal into a specific shape for a particular purpose.

C. Hot Rolling

C.1 Description of the Rolling Operation

Hot rolling involves reheating of ingots, slabs, blooms or billets to the region of 1200 - 1300°C and passing the material between two rolls (Figure 8). The piece of steel may be passed repeatedly back and forth through the same rolls with the roll gap being reduced progressively.

This operation is done in the hot state because the yield strength of steel decreases as temperature rises. Large deformations can thus be obtained with modest roll forces. It is necessary to control both the total reduction, which defines the degree to which the steel is worked, and the reduction in each pass in order to avoid excessive deformation leading to metal cracking or breakage.

Hot Rolling Processing

The number of passes depends upon the input material and the size of the finished product; it can be as high as 70 before the material becomes too cold to roll down further. Plain barrel rolls are used for flat products such as plate, strip and sheet, while grooved rolls are used for structural sections, rails, rounds, squares, beams, sheet piles, etc.

Hot Rolling Principle

The basic rolling unit is called a stand and consists of the rolls and a support structure (housing). The rolling mill comprises the stand or group of stands, complete with auxiliary facilities for control and regulation, such as roll drive motors, roller tables for entering and removing the metal, shears, scarfers, etc.

Hot rolling Billets

The simplest type of mill consists of a two-high stand. Generally, the two rolls can turn in both directions, which permits reversible operation such that the hot metal is passed repeatedly through the mill in opposite directions achieving progressive reduction in thickness.

Comparison Hot Rolling and Cold Rolling Principle

When large reductions are required, four-high stands are used to achieve the required high roll forces. The cylindrical work rolls, through which the hot metal passes, are of relatively small diameter and are supported above and below by a second set of larger diameter backing rolls that transmit the force to the work rolls. A four-high stand may also be reversible.

Comparison Conventional Rolling and Thermo Mechanical Rolling

The reduction in thickness of the hot material results in both length increase and sideways spread. The spreading, which depends mainly upon the amount of reduction, temperature, and roll diameter, must be controlled to give the correct dimensions and cross-section. Universal Mills have a set of vertical rolls at the delivery side of the horizontal rolls. In parallel face beam mills, they serve to provide a good dimensional finish to the final product and, in flat product mills, to edge the plates, improving finish and mechanical characteristics.

In addition to its function of shaping the steel into the required size, hot rolling improves the mechanical properties. Correct control of the cast steel chemical composition, final rolling temperature and amount of material reduction is necessary to give products the required physical properties. For certain steel qualities (e.g. high strength with good impact properties at low temperatures) “controlled rolling” or the QST process of quenching and self-tempering of the material during rolling is employed. This process involves either delaying or cooling until a specified lower temperature is reached before the final passes through the mill.

The main product routes for structural steel grades are summarized in Figure 9.

C.2. Primary Rolling

The first hot-rolling operation is to convert ingots into the basic shapes shown in Figure 10. This is generally carried out on a large single-stand, two-high reversing mill, known as a primary or roughing mill. In between the steelmaking plant and the primary mill there is a bay for stripping moulds from the ingots and a battery of soaking pits. Each pit may hold up to 150 tons of ingots and serves to bring the ingots up to a uniform temperature for rolling and to act as a reservoir to accommodate fluctuations in the flow of ingots. Normal practice is to charge ingots into the soaking pits immediately after stripping from the moulds whilst they are still hot. Soaking pit temperatures are generally controlled at 1300°C.

Primary mills are equipped with manipulators for positioning and turning the ingots to enable work to be done on each face as rolling proceeds. Roll grooves (Figure 11) are arranged to enable a variety of basic shapes to be made. Leaving the primary mill, the ends of the bar must be removed (cropping), as they have an irregular shape and this zone concentrates segregation, piping and other defects. The amount to be cropped varies depending on the type of steel (rimmed, killed, etc), the type of casting (direct, bottom casting, hot topping, etc.) and above all on the quality of the finished product.

The bar or beam blank, after cropping, is fed in some cases is directly for rolling into another mill to produce billets or finished sections such as rails or structural sections. More usually the bar is sheared to a set length and passed into stock to be inspected and conditioned, prior to reheating and rolling into finished products at other mills. Primary mill outputs typically range from 500.000 tonnes to 5 million tonnes per year.

C.3. Finish Rolling

The finish rolling of products for construction work divides broadly into four groups: plates, structural sections, merchant bar and strip. Structural sections comprise standard shapes, e.g. beams, channels, angles, bulb flats, and special sections. As a general rule, large sections are rolled directly from ingots, intermediate-size ones from reheated blooms, and small sections from reheated billets. In all cases the process begins with roughing down, in which the initial square or rectangular cross-section is gradually shaped in successive roll passes into an outline of the required product. This process is followed by finish rolling in successive passes to give the final standard shape and dimensions after cooling. Finishing mill rolling temperatures are usually in the region of 900 - 1000°C. An example of the pass sequence for angle rolling is given in Figure 12. The rather more specialized method for universal beams and columns is shown in Figure 13. Subject to mill size and type, section mill outputs are typically from 200.000 to 1 million tons per year.

Merchant bar is a traditional term for small cross-sections such as rounds, squares, hexagons, flats, etc. which are rolled from reheated billets from continuous in-line mills with as many as 23 rolling stands. Feedstock is generally 100 mm square billet and pass sequences are of a square, diamond, or oval type, culminating at the last mill stand with the finished cross-section. The production of hot-rolled strip is, in many aspects, an extension of plate rolling, with thicknesses in the range of 2-16 mm and widths up to 2 m. Modern mills are fully instrumented and computer-controlled to give a high standard of dimensional accuracy and finished properties.

C.4. Hot Rolling Processes

These processes can be divided into two basic groups, traditional hot rolling and controlled rolling. In traditional hot rolling, the object is to produce the required shape with the minimum number of roll passes. In controlled rolling, the objective is to increase the strength and toughness of the steel by careful control of temperature and deformation during rolling.

Hot rolling

In traditional hot rolling, temperatures are kept to a maximum so as to reduce the hot strength of the steel and allow large reductions in each roll pass. Because of the high temperature, rapid recrystallization and grain growth occurs between consecutive passes and consequently no grain refinement is achieved. Today, this process is only used for primary reduction and for low-quality steels where there are no specific requirements for strength and resistance against brittle fracture.

Controlled rolling

In the 1960′s and 1970′s, new application fields, such as nuclear power stations and offshore platforms, demanded structural steel components having improved properties and higher reliability than had been previously available. For North Sea offshore structures, erected in hostile environments including deep waters, severe storms and low service temperatures, not only was strength important, but so was resistance to brittle fracture. Attention was also focused on fabrication properties; easy weldability of steel components under difficult conditions had to be guaranteed. At that time, it became clear that the traditional hot rolling process was unlikely to achieve these requirements and so new production technologies, such as controlled rolling, appeared.

Controlled rolling is a generic term for rolling procedures in which the temperature and deformation during rolling are controlled to achieve desired material properties.
Controlled rolling includes:

  1. Normalizing rolling (N).
  2. Thermomechanical controlled rolling (TMCR). This procedure includes the following processes which employ increased cooling rates, with or without tempering:
    1. Accelerated cooling
    2. Quenching and Self-Tempering

Normalizing Rolling

Normalizing rolling is a thermomechanical treatment during which the final deformation is carried out in the normalizing temperature range (» 950°C). The austenite phase completely recrystallises between passes but, because of the reduced temperature, does not experience grain growth. Consequently, after the final pass, air cooling produces a material condition equivalent to that obtained after normalizing. The abbreviated designation of this delivery condition is N. Normalizing rolling can be performed on nearly all mills because the final rolling takes place at relatively high temperatures (³ 950°C) such that the power and load capacity of the rolling mill is not exceeded.

Thermomechanical Controlled Rolling

Thermomechanical Controlled Rolling (TMCR) is a thermomechanical treatment in which the final deformation is carried out in a temperature range where austenite does not recrystallise significantly. On subsequent cooling, the deformed austenite grain structure leads to a final fine grain ferrite-pearlite microstructure. Usually, the final forming takes place at temperatures just above that at which austenite begins to transform into ferrite. Thermomechanical controlled rolling leads to a material condition which cannot be achieved by heat treatment alone. The resulting grain refined steel shows very desirable toughness properties down to low temperatures for a medium range of product thicknesses and yield strengths.

For several years there has been an increased demand for rolled steel products with yield strengths up to 500 N/mm2 and in large thicknesses, combined with improved fabrication properties. As TMCR cannot be exploited any further because the mechanical power of the rolling mills is limited, new production technologies have had to be introduced.

Accelerated Cooling

Accelerated (water) cooling is performed after the final deformation in order to improve mechanical properties by refining the microstructure. This process has a positive influence on strength as well as on toughness properties and allows the alloy content to be lowered compared to TMCR alone. The microstructure of accelerated cooled steels consists mainly of fine-grained ferrite + pearlite and ferrite + bainite, showing low ductile to brittle transition temperatures, i.e. good toughness.

Quenching and Self-tempering

In the Quenching and Self-Tempering (QST) process, intense waterspray cooling is applied to the surface of the product after the last rolling pass, so that the skin is quenched. Cooling is interrupted before the core is affected by quenching and the outer layers are then tempered by the heat flow from the core to the surface during a temperature equalization phase. The QST process has resulted in the creation of a new generation of steel products with high yield strengths up to 500 N/mm2 and excellent low temperature toughness properties, which are weldable without preheating. Such steels offer important advantages in terms of weight savings and fabrication costs compared to conventionally produced grades.

Influence of rolling conditions on mechanical properties of the steel

The dominant mechanical properties of steel are tensile properties, i.e. yield strength, tensile strength and elongation, and toughness or resistance against brittle fracture. Both properties can be influenced to a large degree by the applied rolling conditions which determine the final grain size and structure (ferrite/pearlite or tempered martensite/bainite).

The main parameters which influence the microstructure and properties are as follows:

  • the finish rolling temperature, combined with the deformation rate per pass, influences the grain size of the finished product : fine grain results if this temperature is situated in the non-recrystallising region (TMCR process) and coarse grain if the rolling temperature is above that region (Hot rolling)
  • the cooling rate of the finished product immediately after the last rolling pass decides its structure and grain size. Three different types of cooling can be distinguished :

a. slow (air) cooling at a rate of less than 1°C/s has little influence on mechanical properties : grain size and structure are determined by the preceding rolling

b. accelerated (water) cooling at a rate higher than 1°C/s but not high enough to quench the product to form martensite. This process produces a further refinement of the grain size of the ferrite/pearlite structure, substantially improving toughness and increasing tensile properties

c. quenching and self-tempering (QST), which produces tempered martensite in the surface layers and a fine-grained bainite/ferrite/pearlite structure in the core area. This process increases tensile strength by 120 to 150 N/mm2 relative to the untreated state and substantially improves toughness.

Depending on the rolling process, the chemical composition of the steel has to be adjusted to obtain the different steel grades. Figure 14 shows, in terms of carbon equivalent, the alloy content of the steel necessary to reach yield strengths of 255 to 500 N/mm2 for product thicknesses up to 140 mm. The traditional hot rolling process demands not only the highest alloy contents but it is also not able to cover the whole range of product thicknesses. A lower alloy content and practically the whole range of product thicknesses can be obtained by combining TMCR rolling and accelerated cooling.The lowest alloy content, as well as the full range of modern structural steel products, can be obtained by a combination of TMCR rolling and quenching and self-tempering (QST). By this process route it is not only possible to produce high strength steels in a most economic way but these steels also have excellent weldability due to their low alloy content.

Concerning toughness or resistance to brittle fracture, the poorest characteristics are obtained by traditional hot rolling, which produces steels with ductile to brittle transition temperatures limited to 0°C and higher. Such material characteristics are inadequate for many applications in modern steel construction, especially in cases of larger product thicknesses and higher yield strengths. By combining TMCR rolling with accelerated cooling or with quenching and self-tempering, it is now possible to fulfil these demands. With the accelerated cooling route, and especially the TMCR/QST route, steel can be produced with yield strengths up to 500 N/mm2 and transition temperatures lower than -60°C. These characteristics are sufficient to cover the most stringent specifications arising from high technology areas such as the offshore industry or high-rise building construction.

CONCLUDING SUMMARY

  • Steel production involves the refining of molten iron, the removal of impurities and the addition of alloying elements.
  • The process may take place in an oxygen converter, yielding low impurity steel from molten iron (derived mainly from iron ore) at high rates of productivity. Alternatively, electric arc furnaces may be used to process scrap steel.
  • Further refinement of the steel to achieve the required composition is carried out in the secondary or ladle steelmaking unit by the addition of appropriate elements and removal of unwanted products.
  • Molten steel is solidified using either continuous casting into semi-finished products or by casting ingots.
  • Structural steel products are most commonly manufactured by hot rolling - squeezing the steel between rollers to achieve the required cross-section shape.
  • Cold rolling produces a wide range of thin steel products, often with surface coatings, that have good surface quality and forming qualities.
  • Special techniques are required for the manufacture of structural hollow sections.
  • By controlling the temperature regime during rolling, improved steel characteristics can be obtained.

References :

  1. The Making, Shaping and Treating of Steel Edited by Harold E McGannon USS (United States Steel) 10th Edition Published 1985.
  2. Brockenbrough, R. L., Metallurgy Chapter 1.1 Constructional Steel Design, an International Guide, 1992.
  3. Alexander, W., Metals in the Service of Man, Penguin Books, London, 1989.
  4. Tamura, I, Theromechanical Processing of High Strength Low Alloy Steels, Butterworths, 1988.
  5. http://www.fgg.uni-lj.si/
  6. http://gadgetflashstore.com/
  7. http://gadgetflash.com/

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