When we observe steel goods, structures, or mechanisms, we often overlook the intricate process involved in their creation.
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Like all other stages of steel production, the rolling process employs various methods to create rolled steel.
On a global scale, steel can be categorized as either hot rolled or cold rolled, with hot-rolled steel representing approximately 80% of worldwide commercial rolled steel production. This article delves into the specifics of hot rolling.
The metallurgy process begins with the utilization of raw materials such as iron ore and coke. Traditional methods involve converting these materials into iron through blast furnaces (BF). Following this, iron is transformed into steel and semi-finished products, including slabs, blooms, round or square billets, and beam blanks. In certain instances, the BF process is bypassed, resorting instead to electric arc furnaces (EAF) that utilize ferrous scrap and direct reduction iron (DRI) as feedstock.
While many global manufacturers offer slabs and square billets for sale, these products cannot be directly employed in the manufacturing of automobiles, machinery, or steel structures. Semi-finished items must undergo additional processing in rolling mills, enabling the production of rolled steel that possesses specific shapes, geometries, and physical and mechanical characteristics.
Every semi-finished steel product initially goes through hot rolling. This process, conducted at hot-rolling mills, involves exposing the shaped material to plastic deformation as it passes between two rolls, altering its initial form. The temperature of the material during this phase must exceed the steel recrystallization temperature.
In essence, the material needs to be relatively soft to allow deformation by rolls, akin to how dough is flattened beneath a rolling pin. Additionally, the steel is quite hot; typically, its temperature during rolling starts above 1,100°F and finishes at no less than 900°F. Hence, the term "hot-rolled" steel.
- Hot rolling is utilized to produce:
- Plates or heavy plates ranging from 4 to 350 mm in thickness.
- Sheets measuring 1.2 to 4 mm thick.
- Coils up to 25 mm thick.
- Simple section shapes such as round, square, rebar, flat bar, and wire rod.
- Structural shapes including rails, angles, H-beams, channels, and special profiles.
Hot-rolled steel is produced at rolling mills equipped with stands. For instance, hot-strip mills may comprise up to 15 "quarto" stands (each featuring two pairs of rolls) alongside edgers that shape the strip edges.
The production process for hot-rolled plates originating from slabs follows these steps:
- Feeding slabs into reheating furnaces
- Reheating
- Passing through the mill stands and rolling in multiple passes
- Levelling at a leveller
- Cooling on cooling beds
- Performing non-destructive testing
- Trimming the longitudinal edges and ends, followed by cutting to length
- Finishing, testing, approval, and marking
What are the benefits of hot rolling? If one attempted to convert a cold steel slab into a flat product, success would be unlikely due to the hard, thick, and strong nature of such a slab, even with specialized rolling equipment. Therefore, prior reheating of semi-finished products is essential. Hot steel becomes malleable and easier to work with. Consequently, rolling mills minimize the time and effort needed for producing steel plates and strips.
In the CIS, finished hot-rolled plates are categorized based on:
- Accuracy (increased accuracy, normal accuracy);
- Flatness (extremely high, high, improved, normal flatness);
- Edge type (mill edge, trimmed edge), etc.
The principal advantage of hot-rolled steel is its lower cost compared to cold-rolled steel, leading to widespread usage in applications that do not require meticulous attention to surface quality or dimensional precision, such as support steel structures for large construction projects or in the engineering sector. Metinvest Group stands out as a major supplier of hot-rolled steel, with its facilities in Ukraine and Europe equipped with rolling gear capable of producing a diverse range of hot-rolled flat products. For instance, the plate mill at Azovstal Iron and Steel Works and the plate mill at Ilyich Iron and Steel Works in Mariupol represent significant production hubs. The mills located in Italy and the UK manufacture plates for construction, shipbuilding, and pipe manufacturing, while the HSM at Ilyich Iron and Steel Works, the mill at Zaporizhstal, and the Steckel mill at Ferriera Valsider (Italy) focus on producing steel coils. Notably, Metinvest Group invested $110 million in modernizing the HSM at Ilyich Steel between specific years.
Presently, the typical cycle of hot-rolled flats manufacturing involves the reheating and rolling of slabs. Increasingly, global steelmakers are adopting strip-casting plants, which effectively merge steel casting and hot rolling into a unified production flow, eliminating the need for a semi-finished production phase. This technology allows for utility savings and accelerates the overall production process.
Even though this technology remains relatively novel, Metinvest Group is exploring its implementation possibilities at one of its steel plants.
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