You come in contact with steel appliances and products daily without even realizing it. Steel, especially in its stainless form, has several applications, like culinary uses with knives, cookware and cutlery, washing machines, refrigerators, surgical and medical equipment, bridges, airport roofs, aircraft, and rail cars, to mention but a few.
Steel isn’t a pure metal occurring naturally but an alloy. For reference, an alloy is a combination of two or more metallic elements, making a superior product. Steel alloy comes from combining iron, less than 2% carbon, 1% manganese, and small bits of oxygen, silicon, and phosphorus.
Iron ore occurs naturally, but it’s not in its pure form. Iron ore contains impurities like sulfur, phosphorus, and silica. Since these impurities weaken iron, their removal is essential in steel making. The extracted pure form of iron combined with carbon makes a strong metal (steel).
Let’s look into the process of making steel to understand exactly what this special metal is made of, from the experts at Connors Industrial.
Steel Making Process
Metal smelting began thousands of years ago, and it involves subjecting the extracted ores to extremely high temperatures in a process known as rudimentary furnaces. Furnaces have improved tremendously with additional technological improvements making it possible to mass-produce steel.
The types of steel created in the process have different chemical compositions, shapes, applications, microstructures, and surface finishes.
The process of steel making involves six main steps:
Step 1: Iron Making
In steel making, the iron ore melts at approximately 1,6000C or 2,9000F. The molten iron used in steel making comes from smelted iron ore, coke, and lime. The process involves several chemical reactions till you arrive at the required chemical composition. The main components required in making steel are pig iron, direct-reduced iron (DRI), and steel scrapes.
Step 2: Primary Steel Making
Pig iron is derived from iron ore as it contains the correct percentage of carbon useful in steelmaking. However, iron contains impurities that weaken it, like manganese, silicon, sulfur, and phosphorus. DRI is derived from iron ore in its solid-state using carbon monoxide (CO) and hydrogen (H2). On the other hand, steel scraps come from metallic residue from iron, such as chromium, tin, and copper.
Oxygen plays a vital role in steelmaking as it oxidizes the materials. For example, carbon forms carbon monoxide, silicon forms silica, phosphorus into phosphate, manganese into manganous oxide, and iron into ferrous oxide.
The primary steel-making stage occurs from the top to almost mid-level of the furnace at approximately 3000 Fahrenheit. After the oxidation process, the molten slag floats in a pool of molten iron, waiting for separation.
Step 3: Secondary Steel Making
Secondary steelmaking takes place in the ladle. In addition, several metallurgical processes take place in the ladle, including electrical heating, chemical heating, deslagging, and reslagging.
The ladle is heated and cooled to temperatures ideal for refining and alloying steel. Then, it receives the liquid steel and enables it to move it by ladling cars, cranes, or turrets.
Step 4: Casting
The solidification process of steel and slag takes place in the shell. The liquid steel is poured into heavy iron ingot molds, and the mold extracts heat from the melt, leading to solidification. Solid shells form depending on the heat flux between the cooling medium and the already solidified shell.
Once the liquid has cooled, you can cut it into desired sizes depending on its intended purpose, meaning slabs for plates, billets for wires, and thin pipes and blooms for beams.
Step 5: Primary Forming
Primary forming, also known as first forming, is where the initially formed shapes like slabs, beams, billets, and blooms are manipulated by hot rolling. Hot rolling makes it easier to divide the steel into seamless tubes, flat and long products.
Step 6: Finishing Process
The final process of steel making involves heat treating, annealing, quenching and tempering, and surface treating. First, the steel is subjected to hot temperatures and then cooled afterward. The heating and cooling rates depend on the chemical composition, shape, and size of steel.
Annealing is the process of making steel malleable so that it may perform various functions. For example, for steel to create some products, the plates, rails, and tubes are subjected to the quench and temper process that involves equal heating and quenching.
Finally, surface treating determines the type and thickness of the steel product. Additionally, it plays a part in the adaptation of steel to atmospheric corrosion, surface patterns, and paintability.
What Is Steel Slag?
Steel slag occurs as a result of steel making and forms during the separation of molten steel and other impurities. The impurities contained in the slag include silicates, manganese, sulfur, carbon monoxide, and phosphates.
The grades of steel are high, medium, and low. Steel slag contains different properties depending on the quality of steel. Slag removal takes place in the secondary steel-making step. If the extraction is delayed, some impurities like phosphorus reverse back into the liquid steel. For you to avoid slag extraction delays, monitoring is vital, so the slag extraction takes place at slag skimming stations.
Slag removal is done with a rake or by pouring the liquid steel into the ladle and retained in separate slag pots. Other steel companies prefer a vacuum system that sucks the slag off the steel and instantaneously granulates it.
After slag removal, the liquid steel is covered with an insulating layer or slag formers to prevent heat loss and oxidation.
Steel-making processing depends heavily on temperature control as the iron ore, carbon, and other materials are exposed to very high heat. In addition, liquid steel also needs cooling to convert it into steel products.