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The role of carbon in steel

overview

Carbon is the most important alloy component in steel and cast iron, mainly determining the wide range of mechanical properties that forgings and castings can obtain. When the carbon content of iron-based alloys exceeds 2.0% (excluding ferroalloys), they are classified as cast iron. According to metallurgical classification, ordinary carbon steel is divided into hypoeutectoid steel grade and hypereutectoid steel grade based on whether the carbon content is lower or higher than 0.80%. Adding other alloying elements such as manganese, silicon, nickel, etc. can alter the carbon content of the eutectic point, and may even completely eliminate it. The limitations of carbon have spurred the development of low-carbon microalloyed steels. However, this chapter only discusses steel and cast iron with carbon as the main alloying element, ignoring the presence of other elements remaining after deoxidation or necessary for controlling sulfur.

carbon

Atomic number: 6

Density (20 ° C): 2.3 grams per cubic centimeter

Atomic weight: 12.01

Melting point:> 3550°C（6422°F）

Boiling point: 4827 ° C (8721 ° F)

 Carbon is present in almost all steel materials from the beginning of the steel manufacturing process. Steelmaking raw materials (such as molten iron, pig iron, scrap steel, ferroalloys, etc.) typically contain higher carbon content than required for the final product. During the steelmaking process, carbon element is removed through oxidation reaction. When the target carbon content is reached (if high carbon ferroalloy needs to be added to the ladle, it can be slightly lower than the target value), the steel can be produced. In the BOF process, a common operation is to "blow refine" the carbon content to below 0.10%, and then add carbon through a ladle. In the production process of electric arc furnace, foam slag will be formed by carbon injection, while the carbon core wire is used to precisely control the carbon content in the ladle refining station. 

When using high carbon ferroalloys, these alloys themselves become the "additive medium" for carbon. If the carbon content is too low and cannot be supplemented by ferroalloys, the following materials can be used for carbonization: graphite, coke, calcined petroleum coke, anthracite, and in rare cases, high carbon scrap steel such as cast iron or cold pig iron will be used. Special attention should be paid: using cast iron for carbon supplementation may lead to excessive phosphorus, while coke carbon supplementation should choose varieties with low sulfur and low volatile matter. 

The production of cast iron in electric arc furnaces often requires a carbonization step, as the raw materials often use low-cost low-carbon scrap steel. Although high carbon scrap steel, high carbon ferroalloys, and even pig iron can be used as carbon sources, specialized carburizers such as graphite or coke are usually used based on process requirements or cost considerations. Natural graphite (mainly produced in Mexico) is widely used in North America, with a carbon content of 70-85% and high impurities, which limits its application; Synthetic graphite (mostly from electrode waste in electric arc furnaces) has higher purity, and its crystal structure affects the structure of cast iron. Although metallurgical coke is low-cost, its high ash content of up to 9% limits its application; The purity of calcined petroleum coke can reach 99%, but the sulfur content may exceed 1%.

Carbon Enhancement Operation Practice

When carbon addition is carried out in the form of high carbon ferroalloys, it can be completed in the furnace, before steelmaking, or in the ladle. The conventional operation is to slightly excess decarburization in the furnace and supplement it to the target carbon content range through ferroalloys. The carbon oxygen temperature balance during steelmaking is a key control factor, but the specific operation usually depends on on-site experience. Due to the high processing cost inside the furnace, the common practice is to "open" steelmaking (allowing the steel to come into contact with air), followed by necessary deoxidation and composition adjustment in the ladle. Special attention should be paid: using cast iron to adjust carbon content may lead to excessive sulfur and phosphorus. Unless the background values of these elements in the molten steel are extremely low or the final composition allows for their increase, the phosphorus and sulfur content of the raw iron should be minimized as much as possible.

 Carbonizing agents (graphite, coke, anthracite) have a low density and are prone to floating on the surface of the slag layer, causing ineffective burning damage. Therefore, they should be added or pre installed at the bottom of the empty ladle during the initial steelmaking stage. The steelmaking process requires sufficient turbulence to accelerate carbon dissolution. Carbonizing agents are also added to the ladle during casting production, following the above operating procedures.

Rolling/forging

The carbon content in steel affects the deformation process in various ways. Generally speaking, as the carbon content increases, the processing difficulty will increase. The impact of carbon is first reflected in the homogenization furnace or reheating furnace. High carbon steel is more sensitive to thermal shock and must be heated slowly to avoid cracking. Ladder heating (i.e. allowing the steel ingot to stay at multiple temperature platforms to achieve temperature uniformity before reaching the rolling or forging temperature) may be necessary, especially for large section steel. Steel with a carbon content exceeding 0.30% is more prone to "overburning" (deep surface oxidation), which can lead to cracks or substandard surface conditions of the final product, almost always requiring the overburning steel ingot to be scrapped. Therefore, high carbon steel should be heated slowly and evenly to avoid local overheating caused by direct flame impact. 

The rolling force of both hot and cold rolling increases with the increase of carbon content. In hot rolling, this effect is more significant near the final rolling temperature. For example, adding an additional 0.15% carbon to regular carbon steel can increase energy consumption by up to 20% at 870 ° C (1600 ° F). The energy required for cold processing is highly dependent on the carbon content, which is related to the proportion of pearlite in its microstructure. All other conditions being equal, the demand for intermediate annealing increases with the increase of carbon content. 

It is worth noting that carbon has a strong tendency to segregate in thick sections (such as steel ingots) and will accumulate in the final solidified metal (along with manganese, phosphorus, and sulfur). This may lead to uneven carbon distribution in the final product, such as the common "banded structure" in hot-rolled plates (caused by phosphorus segregation: high phosphorus areas repel carbon). However, this is not necessarily harmful. For steel containing microalloying elements, the ratio of atomic percentage of microalloying elements (MAE) to carbon content determines the amount of MAE precipitates formed at low temperatures. At this point, cold-rolled and annealed thin plate steel requires a carbon content of less than 0.01%.

heat  treatment

Carbon can increase the strength of hot-rolled steel, but reduce its notch toughness, ductility, and weldability. Regarding the application details of carbon in continuous casting and hot-rolled steel, reference can be made to the relevant content of vanadium, niobium, and titanium.

 The maximum solubility of carbon in ferrite is about 0.025% (at 723 ° C/1333 ° F). The carbon solubility of ferrite at room temperature is less than 0.008%. The iron carbon balance diagram (Figure 1) shows three reactions and indicates the formation of cementite (Fe3C) at a carbon content of 6.67%. At 1492 ° C (2718 ° F), δ - ferrite with a carbon content exceeding 0.10% undergoes transgranular reaction with liquid metal to form austenite. Iron with a carbon content exceeding 2.0% undergoes eutectic reaction at 1130 ° C (2066 ° F), forming pearlite - a rod-shaped distribution of carbides in austenite. At 723 ° C (1333 ° F), austenite decomposes into layered composite pearlite through eutectoid reaction. 

Carbon will lower the temperature of the gamma → alpha allotrope transition from 910 ° C (1670 ° F) in pure iron to the eutectoid temperature (at 0.80% carbon). Below the eutectoid temperature (723 ° C/1333 ° F), carbon has a significant effect on the kinetics (rate) of pearlite formation and reacts with iron to form non-equilibrium phases of bainite and martensite. Pearlite forms at high temperatures ranging from approximately 550 ° C (1020 ° F) to the eutectoid temperature range, and its structure gradually refines as the transition temperature decreases. Between approximately 220 ° C (425 ° F) and the lower limit of pearlite formation range, austenite transforms into bainite. There are two main types of bainite:

upper bainite: formed at higher temperatures, with a needle like structure and carbide particles oriented along the boundaries of the ferrite region. 

Lower bainite: also needle like but finer, with carbide particles distributed laterally in the ferrite region, giving it higher toughness. The temperature boundary between upper and lower bainite mainly depends on its composition (especially carbon content). The growth rate of the two types of bainite is mainly determined by the diffusion of carbon in iron.

The transformation of austenite into martensite through non diffusive shear mechanism below approximately 220 ° C (425 ° F) is the most important phase transformation in commercial heat treatment. As the eutectic composition approaches, the temperature at which martensite begins to form (Ms) will decrease. If the part requires a hard and wear-resistant surface and a tough core with better toughness, carburizing technology can be used: carbon is diffused to the surface of low carbon steel (usually to a depth of no more than a few thousandths of an inch). The carburizing temperature is approximately 925 ° C (1700 ° F), and the steel composition needs to maintain fine grains at this temperature. Conventional heat treatment is required after carburizing.

application

Carbon steel currently accounts for the largest tonnage of all steel sales, and its wide range of applications cannot be listed one by one. Carbon steel is used as castings and forgings, pipes and tubes, thin and thick plates, wires, bars, rails, and structural profiles. Of course, carbon steel is the cheapest iron-based alloy, and designers will prioritize carbon steel unless special performance requirements require the use of more expensive alloy steel grades. 

Carbon steel can be classified in various ways, and composition classification is the most intuitive method, usually using standards published by organizations such as the Society of Automotive Engineers (SAE) and the American Iron and Steel Institute (AISI). The American Society for Testing and Materials (ASTM) and the American Society of Mechanical Engineers (ASME) mainly specify the performance indicators of steel, and the composition is only used as supplementary information. 

Many standards identify the same steel grade through their respective specifications, and users can add specific requirements based on their needs on the general standards. Some large users, such as automobile and construction machinery manufacturers, tend to establish their own standards that are stricter than national standards.

The application of steel grades with different carbon contents varies: 

thin plate steel usually has the lowest carbon content (less than 0.10%), 

ultra-low carbon steel (carbon content less than 0.02%) includes high formability thin plate steel; 

Low carbon steel (carbon content 0.05% -0.20%) covers hot-rolled steel strips, thick plates, and pipes; 

Medium carbon steel (carbon content 0.25% -0.55%) is mainly used for forging; 

High carbon steel (with a carbon content exceeding 0.6%) includes steel used for rails.

 In specific industrial applications, such as the selection of friction pair materials for oil pumps, carbon steel (such as 45 steel) is often improved in wear resistance through chrome plating or laser treatment, while the oxide scale control technology for hot-rolled low-carbon steel (such as SPHC, 510L) directly affects surface quality.