Know where does steel come from:
Steel is obtained from Rocks by processing it. Iron ore [red rocks] is loaded into a Bessemer furnace along with limestone, coke [pure carbon chunks], and some scrap iron/steel. The ore [iron oxide] is ‘reduced’ back to pure iron [unoxidised] with a lot of heat and pure oxygen [blown up through the furnace. The process is controlled to the point that the molten iron had only the necessary 1%(+) carbon to make it steel left in the mix; the oxygen burns out the extra carbon. Then the needed chrome, manganese, vanadium, etc. is added to adjust the blend of steel needed to make the specified type and grade of steel. This furnace batch is called a ‘Heat’, and it is poured into a mold – ingot – to cool and solidify. At this point, it is steel, and the huge ingot is rolled or forged into the needed shape, usually while still red-hot, but solid [not liquid].
How is steel obtained?
There are 2 major methods of steel production.
Place ferrous scrap (steel scrap, iron scrap) in an electric arc furnace (EAF), lower the 3 graphite electrodes and turn on the electric current. Generally, in a modern EAF, the transformer capacity will be in the area of 100MVA – 200MVA. The electric current will start to melt the scrap, and then oxygen and fuel (usually natural gas) will be blown in from sidewall burners to impinge on the scrap. Once the steel is melted, alloying elements such as Mn, Mo, Cr, Si, Ni, etc., are added, according to the grade of steel being produced. Some 40 minutes after the start, the steel may be tapped from the furnace.
This method is more complicated, but can produce higher quality steel.
A mix of high-volatile, medium volatile and low-volatile coal (volatile meaning the relative content of xylene, toluene, butadiene and other aromatic hydrocarbons) is coked (meaning heated without air) for 17 or 18 hours to drive off the volatiles. The result is called “coke” and it consists of carbon and about 9% to 12% ash.
The coke is dumped into a blast furnace, together with iron ore Fe2O3 and some limestone as flux (flux to form slag, containing the impurities), and hot air is blown in to burn the coke and also to use it to reduce the Fe2O3 to Fe plus CO and CO2. The result is liquid iron, tapped from the blast furnace at approximately 1400⁰C. This iron contains 4.2 wt. % carbon, because that’s the saturation level for carbon in iron.
This molten iron is taken to a steelmaking furnace, where it is dumped into a furnace containing 15% to 25% ferrous scrap (meaning 15% to 25% of the total metallic charge is ferrous scrap). Oxygen is then blown into the liquid iron at high velocity, to reduce the carbon content from 4.2% to 0.10% – 0.40% by removing carbon as CO (90%) and CO2 (10%). The result is now called steel. Manganese Mn and other elements such as Mo, Cr, Si, Ni, etc., are added, according to the grade of steel being produced. For example, if it is intended to produce stainless steel, then a minimum of 10.5 wt.% must be chromium.
Steel can be categorized into four groups – Carbon, Alloy, Stainless, and Tool.
Carbon steels only contain trace amounts of elements besides carbon and iron. This group is the most common, accounting for 90% of steel production. Carbon Steel is divided into three subgroups depending on the amount of carbon in the metal: Low Carbon Steels/Mild Steels (up to 0.3% carbon), Medium Carbon Steels (0.3–0.6% carbon), and High Carbon Steels (more than 0.6% carbon).
While it is the carbon content of steel that determines the degree to which it can be hardened, certain alloying elements added to the steel can make heat treatment less traumatic, a benefit when it comes to reducing quenching distortion in complex, thin-walled parts, for example. The term hardenability refers to how deep a steel can be hardened, and alloy steels loosely fall into two camps around this measure: carburizing steel, which mostly hardens near the surface, and through-hardening steel, which can extend the hardening down into the metal’s core.
In the AISI numbering system, manganese steels are designated 13xx, nickel steels, 2xxx, nickel-chromium steels, 3xxx, molybdenum steels, 4xxx, and so on up to 9xxx for silicon-manganese steels.
Hardening of alloy steels can usually be done in oil for a slower quench than with water as required for plain carbon steels. This can reduce distortion and permit hardening to penetrate deeper into the material’s core.
Stainless steel is an alloy of Iron with a minimum of 10.5% Chromium. Chromium produces a thin layer of oxide on the surface of the steel known as the ‘passive layer’. This prevents any further corrosion of the surface. Increasing the amount of Chromium gives an increased resistance to corrosion.
Stainless steel also contains varying amounts of Carbon, Silicon and Manganese. Other elements such as Nickel and Molybdenum may be added to impart other useful properties such as enhanced formability and increased corrosion resistance.
Tool steels contain tungsten, molybdenum, cobalt and vanadium in varying quantities to increase heat resistance and durability, making them ideal for cutting and drilling equipment.
Steel products can also be divided by their shapes and related applications:
- Long/Tubular Products include bars and rods, rails, wires, angles, pipes, and shapes and sections. These products are commonly used in the automotive and construction sectors.
- Flat Products include plates, sheets, coils, and strips. These materials are mainly used in automotive parts, appliances, packaging, shipbuilding, and construction.
- Other Products include valves, fittings, and flanges and are mainly used as piping materials.
What do all steel types have in common?
The primary common feature for all types of steel is that they are Fe-based alloys with carbon less than 2.1 wt.% (when carbon content is higher than 2.1 wt.% it is cast iron). Strictly speaking, it is not possible to come up with other common features for different types of steels because their microstructures are so versatile and their properties are very diverse. However, we can still summarize some common features for most types of the steels. Simply speaking, steels are strong, hard, brittle at low temperature, showing different properties with different amount of phase (austenite, ferrite, bainite, martensite and cementite) inside even though the chemical composition is the same. So nowadays, steels are still serve as the most widely used metal in the world not only because they are cheap but also because they can be easily tailored to achieve the expected properties such as ultra-high strength, corrosion-resistance, high temperature resistance and so on.