Technology of metallic materials - Steels

Chemical Reactions in Blast Furnace

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 the 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).

SLIDE 6 (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 the 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 of 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℃. 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.

SLIDE 7 (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 FeP 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 the health when spread in the environment. Same thing 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 becomes brittle in welding if the content of silicon (Si) and manganese (Mn) exceeds 0.04%. Normally, steels contain small amounts of silicon and manganese, up to 0.4%, which is considered acceptable. Both silicon and manganese are oxidizing elements, with manganese being a strong desulfurizing element. However, an excess of manganese can make the material brittle during welding. When combined with sulfur, manganese forms manganese sulfites, which have plastic properties. During rolling, these sulfites elongate and create a strong texture, resulting in strong anisotropic behavior of the steel. This means that the strength and toughness of the steel will vary in the rolling and transversal directions, making large amounts of manganese undesirable. On the other hand, silicon generates silica compounds. If the steel needs to be shaped while cold, these silica compounds can damage the tools.

Slide 8 (Deoxidation): After decarburizing, it is necessary to minimize the presence of trapped oxygen in the melt. This is achieved through a degassing process using argon gas or, in more expensive cases, high vacuum technology. However, some residual oxygen will still be present.

remain(it remains in the liquid in atomic form). Even 100 parts per million of Oand N are enough to induce embrittlement.

The most dangerous gas in steels is hydrogen. It can easily diffuse in the atomic sites because hydrogen has a very small atom. The volume of the steel will increase dramatically, and it will cause a lot of stress in the interstitial sites. That causes cracks and subsequently the embrittlement of the material.

Even oxygen and nitrogen can cause embrittlement too. To remove interstitials we use degassing, or in the case of oxygen, deoxidating. Nitrogen is also present in a huge amount. In the past in blast furnaces, air was used instead of oxygen gas. So, a lot of nitrogen was inserted in the blast furnace and therefore steel. But today we only use oxygen in blast furnaces and not air anymore. To remove all interstitials, vacuum degassing is very efficient but also very expensive. Nitrogen in steels can be trapped using aluminum, since it is a very good oxidizer, but this method

won't be as effective as the vacuum degassing.

SLIDE 9 (PRODUCTION OF STAINLESS STEELS) *See slide. Annotations there.

SLIDE 10 (CONTINUOUS CASTING AND HOT ROLLING)

The solidification of steel can be done by pouring the steel in ingots or other forms. However, it will bring other problems like segregation, etc.

A long time ago, a continuous casting machine was built. It was vertical at first, but they needed bigger ones, so in order to not make the machines taller, they made them curvilinear.

The ladle is the special container where the liquid steel is poured into. The ladle is the container that is moved by the liquid station and brought there.

Tundish is another container which has the role of maintaining the piezometric height constant. The height is exploited since it means that the potential energy is high (Of course, while pouring that potential energy turns into kinetic energy).

Below the tundish, there is another container, open on top and on bottom. It is made by copper walls, and it is

Cooled down by flowing water. The copper will extract heat rapidly from the surface. It is likely to have a solid skin of copper, but the inside of that skin is liquid. This is done so that the material can be shapeable. The skin contains the liquid and prevents it from being spread. Only then, the material is ready for rolling. After straightening, the roll cylinders get thinner and thinner, and so the thickness of the layer decreases as well. If we don't have much space, the rolls can revert their motion to repass the sheets and make them thinner. This happens in rough mills while finishing mills are those who produce fine products with very low tolerances.

SLIDE 12 TMCP of HSLA

First rectangle: In the reheating furnace, the most important parameter is the soaking temperature (the one at which I want the material to be). By increasing this temperature what happens to the material (already shaped and refined by rough milling)? We control the yield stress and the brittle to ductile transition.

temperature (ITT). This is a measure of toughness: the lower this temperature, the tougher the material. If it is low, the material changes from brittle to ductile at a very low temperature (maybe cryogenic). We also notice that the yield strength start increasing and then decreasing (look at the picture). When the temperature is low, both ITT and yield strength are low. When we increase the temperature until the red dot, precipitates get bigger faster. High temperature will increase diffusion and precipitates grow. When they're getting bigger and bigger, the yield stress will increase, reaching the maximum when they are semi coherent. After the maximum, we have overaging. Ductile transition temperature increases because precipitates got bigger, making the material more brittle. (We build points of this graph by inspecting samples at different temperature). If the holding temperature increases, the yield strength goes up to a maximum value while ITT continues to increase. This must be related.gth and toughness. However, as the rolling process continues, the grains will become smaller and the material will regain its strength and toughness. To achieve the best rolling condition, it is important to control the size of the grains and combine both high yield strength and impact toughness. This can be achieved by maintaining fine grains in the material. In summary, grain size plays a crucial role in determining the strength and toughness of a material. By controlling the grain size, we can optimize the rolling process and achieve the desired mechanical properties.