Production of Mild Steel
In a blast furnace, fuel and ore are continuously supplied through the top of the furnace, while hot air is blown into the bottom of the chamber, so that the chemical reactions take place throughout the furnace as the material moves downward. The end products are usually molten metal and slag phases tapped from the bottom, and waste gases exiting from the top of the furnace.
The blast furnace remains an important part of modern iron production. Modern furnaces are highly efficient, including Cowper stoves to pre-heat the blast air and employ recovery systems to extract the heat from the hot gases exiting the furnace. Competition in industry drives higher production rates. The largest blast furnaces have the volume to fit the water from 2 standard swimming pools, and can produce around 80,000 tons of iron per week. This is a great increase from the typical 18th century furnaces, which averaged about 400 tons per year. Variations of the blast furnace, such as the Swedish electric blast furnace, have been developed in countries which have no native coal resources.
The molten iron is initially formed through a continuous casting process to produce semi-finished shapes known as ingots, slabs and blooms. These shapes are often refined to final specifications utilizing one of two main rolling processes:
Hot rolling consists of three main types of rolls: strip rolls; sheet rolls; and rod rolls. This method is used to reduce the size of wrought-iron ingots or bars. The Bessemer process was made mandatory in the rolling technique so manufacturers could keep pace with the high demand of these materials needed for the market. Hot rolling is used for larger or thicker amounts of metal. The machines used for this application are very similar to cold rolling, but the requirements are not as important - such as keeping a precise thickness or exact edge dimensions. Hot rolling is mainly used with non-ferrous metals during the beginning stages of the breakdown of alloys.
Cold rolling steel is generally used at the end of a metal forming process. The following functions are applied to obtain and enable accurate dimensions in the finished product. Cold rolling also ensures a smooth clean appearance or finish and the straightness of the edges of the product. Cold rolling is to help obtain a degree of hardness known otherwise as temper in alloys. Once the desired temper is obtained it cannot be re-heated or it will lose properties intended for specific jobs.
Cold rolling is applied during the end process of production for strip and section rolled metals, such as foil. The cold roll mills are to obtain the desired thickness and also to limit and prevent any excess thinning of the already thin material. When rolling these alloys they are made long enough to make sure there are no creases or tears in the metal. The end result after the process of cold rolling will acquire the metal thickness and temper which is needed for each type of work.
Hot Rolled Pickle and Oil – Guarantee’s Sheet Flatness
1. Uncoiler - Receives coils from hot strip mill and unwraps them for processing.
2. Processor (fig. 3.1)- Minimizes tendency for coil breaks by flexing the steel around small rolls.
3. Shear and Welder (fig. 3.1) - Coil ends are cut and welded together to provide a continuous roll.
4. Temper Mill (fig. 3.2) - Provides a degree of improved strip flatness.
5. Pickle/Rinse Tanks (fig. 3.3) - Uses Hydrochloric Acid (HCL) to remove surface oxides and rinses excess acid from strip.
6. Dryer - Removes excess water to prevent rust or staining of the strip.
7. Shear - Cuts coils to customers desired weight.
8. Oiler - Applies a film of oil for rust protection, as well as handling gouges.
9. Side Trimmer (fig. 3.4) - Provides uniform edges and excellent width control.
The pickling operation results in coils that have a bright grayish surface. The coils ends are cropped back to gauge. These products may be ordered as dry, oiled, mill edge or side trimmed edge. When ordered as side trimmed edge the product will exhibit a uniform edge with excellent width control. These edges result in better conditions for welding, as compared to a mill edge. When product is ordered as oiled, a film of oil will be applied for rust and surface protection. Special precautions to prevent rust must be taken on any dry product or material before they are shipped out the factory.
Types of Mild Steel
A36 is the most popular mild steel specification for carbon steel. The most common shapes are plates and bars for welded and bolted construction.
HSLA Steel (High Strength Low Alloy Steel) is a type of steel alloy that is stronger than ordinary plain carbon steels. They are used in cars, trucks, cranes, bridges and other structures that are designed to handle stress at low temperatures. HSLA steels usually contain >15% carbon. Common forms of HSLA are Corten and Exten material.
AR or Abrasion Resistant is a very hard mild steel that is abrasion or wear resistant commonly used in truck and hopper bodies, shoots, and wear.
PVQ or Pressure Vessel Quality (A514, T1) is a high strength, low weight mild steel commonly used in earth moving, transport equipment such as booms, buckets, and pressure vessels.
4140 and 8620 perform well under high heat, heavy load wear-resistant material. Used for dyes and molds.
1045 is a common mild steel that has more durability than A36 but is harder to machine and weld. It is commonly used for axles, bolts, connecting rods, hydraulic rams, pins, rolls, spindles, shafts, shocks, etc.
Free Machine Steel (FM 45) is a mild steel that has reasonable durability in strength and is very easy to machine.