Technology of metallic materials - Steels

Steels

Importance and manufacturing methods

Steels are the most important alloys in mechanical engineering. There are three ways of manufacturing them:

Integral production (from minerals)

When mined, iron ores have a lot of impurities that must be removed. The removal of these impurities is called extraction. The extraction process is carried out in a high temperature furnace called a blast furnace. At first, cast iron is produced, which is very rich in carbon content. Then, to produce steels from cast iron, we need an intermediate process, which is called decarburization. In the BOF (basic oxygen furnace) converter, the cast iron is converted into steels. Since iron is melted at 1538 degrees, the walls of the blast furnace are made of temperature-resistant ceramics.

Secondary production (scrap and pig iron)

We can produce steels also by exploiting scrap from steels. Firstly, we must melt the scrap. The BOF will be just a furnace when we have scrap. When they only use the BOF as a converter, it converts cast iron into steel.

Direct production

Once again, we make steel from scrap. The electric furnace melts the scrap very rapidly, either by electricity or by electromagnetic induction. At most, this process takes 30 minutes (the process is so fast that the iron does not interact with the furnace walls, which are made of ceramics). It is used to produce high-quality steels like stainless steels.

Pig iron - Molten cast iron or cast iron in the liquid phase. (There is actually a difference, but stick with this for now.)

Classification of steels

Steels may be classified into two main families: high and low alloy steels.

Low alloy steels (less than 5% of alloying elements)

• Low carbon steels, for example, have less than 0.25% C. They are ductile but not suitable for heat treatments. Strengthening is accomplished by cold working.
• Intermediate range of carbon content: medium carbon steels have 0.25% to 0.6% carbon steels. They are more suitable for heat treatments.
• High carbon steels: 0.6% to 1.4% C. They are the hardest, strongest, and the least ductile. Used in hardened and tempered condition. Wear-resistant.

When we have only carbon steels (no other alloying elements), we call them plain carbon steels. IF free: Interstitial free steels (pure iron).

High alloy steels (more than 10% of alloying elements)

• A good example is stainless steels with 11% chromium content.
• Tool steels: two subfamilies. Steels are used to make tools. They have a strong cutting edge, resistant to abrasion and deformation. Carbon steels or carbon and other alloying elements.

Production of steels: The big scheme

The integral production involves adding iron ore, coke (fuel with high carbon content and few impurities made by heating coal), and limestone in the blast furnace. The iron ore is the primary ingredient, from which we want to extract the iron. The coke serves as a reducing agent for the oxides, while the limestone serves to refine the liquid (remove some impurities). We add them to the blast furnace, which has a temperature of 2000°C at the bottom.

The materials should be hot (around 300 or 400°C) before entering the blast furnace. They are pre-heated above the blast furnace, so the gases of the blast furnace are recycled to reheat them. The hottest point (2500°C) is where the oxygen is injected into the blast furnace. The combustion reaction happens there. Then the temperature decreases as you go to the top. The hot gases will heat up the raw materials to the highest temperature. The topmost material will just start to heat up. The layers will shift downward as the materials at the bottom of the ‘pile’ melt. The liquid is collected at the bottom. To keep this process continuous, we need to pour the liquid out of the furnace and continue adding more raw materials. Most of the pig iron produced in a blast furnace will produce steels. In a BOF (Basic Oxygen Furnace), the pig iron collected from the blast furnace will be converted into steel (decarburization).

If the second production is to be considered, it is worth noting that if we want stainless steels, we need to select only stainless steels in the scrap, and not random scrap. This is valid for other high alloy steels too.

When we produce steels with scrap, we place it at the bottom of a different furnace, called an electric arc furnace. Here the scrap will act as a charged material that will interact with electrodes made of graphene, which are lowered close to it by a position system in order to allow the formation of an electric arc once the furnace is powered with a current. Thanks to the very high temperatures provided by the electric arc, the scrap is rapidly smelted.

During the melting process, limestone is added to catch the impurities. After the melting, the slag is removed. But still, the liquid steel requires more refining, either removing or adding alloying elements.

The liquid steel is then purified in one of the liquid stations (the type varies on the element we desire to add or remove). After the last step, the steel is ready for rolling. It will be poured and then solidified. There are four intermediate metallurgical products: slabs, thin slabs, blooms, and billets. They are characterized by their different cross-sections. Blooms have much larger square cross-sections compared to billets. Thin slabs may be used in pipes, while billets may be used in rolls and wires.

Reactions in the blast furnace

Exam question

There are three main reactions that happen inside a blast furnace:

• Combustion reaction: (Incomplete combustion) C+O → CO₂ (Complete combustion) C+O → CO₂. The purpose of this reaction is to produce enough heat needed for melting. If necessary, we can control the combustion to be incomplete if we don’t want the blast furnace to overheat. We can have incomplete combustion if we don’t supply the furnace with too much oxygen. In the incomplete reaction, carbon and oxygen gas react to form carbon monoxide. (Before furnaces used air, but to avoid nitrogen in the molten steel, now only oxygen gas is used.)
• Reduction of mineral to pure Fe: The purpose of this reduction is to get pig iron. The reducing agent in this reduction equation is the carbon monoxide (CO). We either get carbon monoxide when the carbon (raw coke) reacts with the formed carbon dioxide (also a reduction reaction) or from the incomplete combustion. The carbon monoxide reacts with the iron oxides to produce pig iron and carbon dioxide.
• Purification: The limestone (calcium carbonate) is added in the furnace to purify the molten pig iron from the impurities. It captures them, and the slag then will float on top of the molten pig iron. The slag will serve as a filter for the layers that melt above because it still has some CaCO₃. The calcium carbonate will react with the iron that is still melting and will get rid of its impurities. The reaction happens in that layer, getting rid of all impurities. The slag also protects the liquid pig iron from oxidation and hydrogen. (Liquid iron may react with the oxygen in the air). Hydrogen, on the other hand, can make the steel brittle (we have a critical situation due to hydrogen embrittlement).

BOF operations for primary and secondary processes

The pig iron from the blast furnace is poured into a large container called a ladle. Even though during transportation the temperature decreases a bit, in the BOF a combustion reaction will reheat the liquid again. From the ladle, it is either sent directly for basic oxygen steelmaking or to a pretreatment phase. The pretreatment usually consists of reducing the sulfur or phosphorus in the liquid before putting it in the converter. (Magnesium is used for this purpose).

Pure oxygen gas is then inserted inside the converter. The carbon dissolved in the liquid iron is ignited, and carbon dioxide and monoxide are formed, which raise the heat up to 1700°C. This lowers the carbon content (decarburization) and helps to remove unwanted impurities. The carbon content is significantly reduced, turning the cast iron into steel. This process lasts up to 20 minutes in general.

The steel is ready to make. We may send the material to another station. For example, if I want to reduce the S levels, I send it to a desulfurization furnace. It depends on the impurities I want to remove. If I want to increase the carbon content, I add some rods of cast iron. The carbon inside of cast iron will spread, and the carbon content will increase. If we want to increase the level of another element, I add an iron alloy with that element.

Impurities

The most harmful elements found in iron ores are sulfur (S) and phosphorus. Their tolerated level in a steel is very low. However, there are some special alloys that include them. They provide embrittlement. The question would be, why do we want the alloy to be brittle? We need some sulfur alloys, for example, just for automatic machining, since it’s easier for the machines to fragment the piece. (Pb was used before, but it had a low melting point, and it would melt, so the chips would melt too.)

In general, phosphorus and sulfur are eliminated because they form a eutectic point with iron. The compounds Fe₃P or FeS are formed in grain boundaries; therefore, they will melt in hot processes. Lead (Pb) used to be used but not anymore because it caused immense pollution and was dangerous for health when spread in the environment. The same goes for cadmium (Cd) and beryllium (Be). Sometimes even chromium (Cr), but not in all cases since chromium exists in different oxidating states.

Even if copper (Cu) is a good element to protect steel from corrosion, we can’t tolerate a large amount of it. We get it from copper wires mixed with scrap. Tin (Sn) is harmful because it has a very low melting point. That makes it dangerous to hot roll the material. Tin will melt, and it will join the grains into larger ones, making the material more brittle.

Nickel (Ni) isn’t so bad, but it is costly. When we want to increase toughness, we put nickel. Nickel will austenitize the element at room temperature; that’s why we don’t tolerate much of it. In heat treatments, we don’t want any austenizing element. Chromium (Cr) will make the steel corrosion-resistant. It encourages the production of carbides. If the steel is rolled or extruded, the carbides will scratch the tools, causing a lot of damage.

Phosphorous (P) is tolerated for low carbon steels because it has a large atomic size (compressive stress); it can act as a substitutional atom. Too much phosphorus in steels will make the steel brittle in welding if there is more than 0.04%.

Silicon (Si) and ...