SSAB’s success in high strength steels began in Oxelösund with a special quenching technique. This plant also specializes in quenched and tempered steels. A rolling mill is needed to produce heavy plate. The thick, heavy slab must be rolled into plate of the correct thickness and properties. Oxelösund represents SSAB’s Swedish rolling mill for heavy plate.
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The slabs are heated to 1,250°C in a furnace and are cleaned to remove the millscale. Rolling is carried out in the four-high rolling mill. In four-high rolling, four heavy rolls – two working rolls and two back-up rolls – roll the plate with enormous force in a number of passes, back and forth, through the stand. A pass is one passage of the plate through the stand.
Oxelösund has the world’s most powerful four-high rolling mill, in which the roll forces are 100,000 kN (10,000 tonnes). The mill rolls 290 mm thick slabs down to plate ranging in thickness between 150 mm and 4 mm. The plates are always flat and can be up to 40 meters long.
Heavy plate on coils
In the USA, Steckel mills are used for producing heavy plate in the two rolling mills. Steckel mills are similar to four-high mills and have the same arrangement of four rolls, with working and back-up rolls in a stand, but they also have coilers with a coil box for heating the plate on each side. The plate is run from one coiler through the rolls to the other coiler, back and forth in a number of passes until the plate is of the correct thickness. Plate up to 3 meters wide can be rolled in Mobile and Montpelier, and they can be supplied either as flat plate or in coils.
Quenching produces extreme strength
Final treatment of the cooled steel is carried out in SSAB hardening lines. Hardening to extremely high strength is achieved by quenching at a rate of up to 1,000°C per second. These steel grades are very strong andhard and wear resistant. Quenching is carried out using very high water pressure. SSAB is one of the pioneers in quenched steels. SSAB is increasing its quenching capacity to six hardening lines – five in Sweden and one under construction in the USA. Production is growing for quenched, highstrength steels.
Quenched and tempered steels
Certain hard and wear resistant steel grades are tempered after hardening in order to restore the toughness and adjust the strength of the product. The actual procedure depends on the application.
Wear steels, such as those for the blades of excavator buckets, must be prevented from cracking and are tempered at a lower temperature. Structural steels that must be much tougher and are tempered at a higher temperature.
Heat treatment is an important part of the SSAB recipe for providing the steel with its final properties.