Rolling into thin sheet is carried out in long rolling mills in which the sheet passes through a series of stand, one after the other. The sheet is reduced in thickness on every pass through the stands.
This increases the sheet’s length. While the plate is still glowing the process starts with a roughing stand and a hot rolling mill.
Some of the hot rolled plate can then be cold rolled into thinner and smoother sheet.
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The steel slabs are heated up to about 1,250°C in two furnaces, using oil or LP gas as the energy source. The slab is then rolled in a roughing stand, in which the thickness is reduced to about 30 mm in five passes, back and forth. The length of the glowing slab will then have increased from 11 meters to an intermediate slab which is about 80 meters
long. This is wound onto a coil in a coil box and is transferred to the adjacent hot strip mill. The material is then rolled down to between 16 mm and 1.8 mm in only one pass through six stands.
At the end of the hot rolling mill, the speed of the strip is 120 km/h. Due to the reduction in thickness, the plate must expand in length. The speed increases after every stand and is highest at the end. When rolled down to 2 mm thick, the sheet will have grown to about 1,300 meters in length, i.e., 1.3 kilometers.
After the last stage, the sheet is quenched. For certain steel grades, the quenching rate is very high. Other steel grades require slower cooling. In a future quenching line in the hot rolling mill, the quenched sheet steel produced will achieve the very highest strength among hot rolled strip grades. In the quenching line, the sheet is quenched at a rate of up to 1,000°C per second, which imparts both hardness and strength to the sheet. SSAB can roll the world’s hardest hot rolled strip in Borlänge.
After quenching, the sheet passes through a cleaning bath in one of two pickling lines. During rolling, millscale is formed on the surface of the sheet on contact with the air. In the pickling baths, the strip is cleaned to remove the millscale. A large proportion of the material produced by SSAB is delivered to customers as hot rolled sheet.
If the customer specifies thinner sheet than that obtainable by hot rolling, the sheet must be cold rolled. Cold rolling produces a more accurate thickness and a smoother surface. In the tandem rolling mill, the sheet is cold rolled in five stands arranged one after the other. Every stand consists of two working rolls and back-up rolls. High roll forces and also tension between the stands are needed to produce thinner sheet. All of this is done automatically at high speed under computer control. Cold rolling causes the sheet to become brittle and hard in the initial stage. This is because the grains in the structure of the steel have become elongated and deformed. Cold rolled sheet must therefore be annealed.
Heating to a certain temperature causes new grains to grow at the expense of the deformed grains, which restores the formability of the sheet.
Heating and quenching
An SSAB specialty is extra-high strength steel grades. To achieve higher strength, the sheet steel must be hardened. This is done by heating and quenching. The extremely fast cooling modifies the atomic structure of the steel grains. A displacement makes it difficult for the carbon atoms to move, which is what causes the high increase in strength. The thickness of cold rolled sheet steel may vary from 0.3 mm to 3 mm, as specified in the order. Cold rolled sheet steel that is 0.6 mm thick will then be 4 kilometers long on a coil.
Cold rolled sheet steel can be further processed by coating. Hot-dip galvanizing provides good protection against corrosion. For hot-dip galvanizing, the strip is first annealed and is then cooled to 470°C before being passed through a molten zinc bath at 455°C. After final cooling, the steel strip passes through trimming and leveling rolls to provide it with the final properties. The sheet may also receive a number of paint coats in a continuous process. The paint is applied to the moving steel strip by means of rubber rollers, and the sheet then runs through drying ovens before being coiled again. SSAB has sheet steel painting lines in Borlänge, Finspång and Luleå. Laminated sheet steel is produced in Ronneby.
Sheet steel can be delivered on coils or as flat sheet, cut-to-length. A large proportion of the production tonnage is cut-to-length, so that it immediately meets the needs of customers. This process is carried out in shearing lines in which the forces are sufficiently high enough to cut the new, stronger steel grades. To divide the wide strip into narrower strip, the material runs through a slitting line.