Minimills

The coke making, ironmaking, and steelmaking facilities have provided steel companies with good service over many years; however, many are now old and inefficient. Simple repairs and maintenance cannot restore them to be as efficient as those found in newer steel mills. Since the 1960s, the integrated mills (big steel mills), that have been facing fierce competition from offshore steel manufacturers, have been looking for technology which would enable them to produce a better quality product at lower cost. Minimills have provided manufacturers with a less expensive, more flexible electric furnace steelmaking process. Minimills are smaller, faster, and more efficient than the large-size integrated mills. They use DC electric arc furnaces and avoid the coke and iron making steps used in larger steel mills. The furnaces are fast, producing 180 to 200 tons of steel per hour at 20% lower capital and operating costs. They have become so popular that by the early 1990s 40% of the steel was produced in minimills.

The Minimill Process

The minimill process of steelmaking is basically the same as that of integrated mills; raw materials, furnacing, casting tower, soaking furnace, and the finishing mill.

• Raw materials

Most of the steel produced in minimills is made of scrap steel. Because the supply and the cost vary so greatly, many steelmakers are using pig iron, hot-briquette iron, direct-reduced iron, and iron carbide, to provide them with a steady supply of raw materials at fixed prices.

• Furnaces

Direct current, electric-arc furnaces are generally used in minimills, Fig. 9-15. They are 22 feet (6.7 m) diameter bottom-tap furnaces that use 24 electrodes and have water-cooled roofs and sidewalls. These furnaces, which can produce from 50 to 200 tons per hour, are generally smaller than the furnaces used by integrated mills, cost less, are more flexible, and create less pollution. It is felt that electric arc furnaces will continue to produce more steel, while the blast furnace and basic oxygen furnaces will find less and less use.

• The melting process

The charge of scrap iron and iron supplements is placed into the DC electric-arc furnace, Fig. 9-15, and brought to approximately 2800° to 2900°F (1538° to 1593°C) temperature and held there for about one hour. The molten metal is moved from the furnace to a ladle metallurgical station with a lid-type vacuum degassing unit for stirring, removing impurities, alloy additions, and temperature control. The vacuum degassing unit is used only when steel with very low carbon and/or nitrogen is required.

• Casting tower

The metal goes to the casting tower from the ladle station and is fed into the tundish. As the liquid steel flows out of the tundish, it passes through a water-cooled mold where it begins to solidify and take shape. The temperature of the slab coming out of the containment section is about 1800°F (980°C) and is traveling at approximately 13 ft/min. (4 m/min).

• Soaking furnace

As the slabs go into the soaking furnace, they are sheared to lengths of 138 to 150 feet (42 to 46 m). After leaving the soaking furnace, the slabs pass a water spray to remove scale.

• Finishing mill

The finishing mill reduces the slab thickness from 2 to .100 in. (50 to 2.5 mm), Fig. 9-16. The strip then passes through a cooling section where it is cooled from the top and bottom by water sprays. The strip, which is at a temperature of 986 to 1290°F (530 to 700°C), is rolled into 76 in. (1930 mm) diameter coils.

1. Why is steel called our most versatile metal?
2. Name two reasons for the changes in the iron and steel industries during the past 20 years.

Raw Materials

3. Where are the main sources of iron ore in North America?
4. Name and describe three types of iron ore.
5. Briefly describe the iron ore pelletizing process.
6. How is coal converted into coke?
7. What purpose does limestone serve in steelmaking?

Manufacture of Pig Iron

8. What type of furnace is used to manufacture pig iron?
9. Briefly describe the operation of a blast furnace.

Direct Ironmaking

10. What is the goal of the direct ironmaking process?
11. Name three main advantages of coal-based ironmaking or bath smelting.

Manufacture of Cast Iron

12. In what type of furnace is cast iron manufactured?
13. Name four types of cast iron and give one use for each.

Manufacture of Steel

14. Name two furnaces that are used to convert pig iron into steel.
15. List the main steps in the operation of the basic oxygen furnace.
16. Explain why the electric furnace is used to produce fine alloy and tool steels.
17. List the main steps of producing steel in an electric furnace.

Direct Steelmaking

18. What is the purpose of the direct-steelmaking process?
19. Name the four main steps in direct steelmaking.

Steel Processing

20. Briefly describe the continuous casting process used to convert molten steel into slabs or billets.

Minimills

21. Name five advantages of minimills over integrated mills.
22. What is the purpose of the ladle metallurgical station?

Copyright 2004, Industrial Press, Inc., New York, NY