Definition of Carbon Steel and the Five Elements of Steel
Iron-carbon alloys containing less than 2% carbon are called steel. The five elements of carbon steel refer to the main constituents of its chemical composition, that is, C (Carbon), Si (Silicon), Mn (Manganese), S (Sulphur) and P (Phosphorus).
Furthermore, during the steel manufacturing process, gases such as O (oxygen), H (hydrogen) and N (nitrogen) inevitably mix.
Furthermore, in the Aluminum-Silicon deoxidation process, Al (aluminum) is inevitably present in the molten steel, and when Als (acid-soluble aluminum) is equal to or greater than 0.020%, it plays a role in refining the size of the grain.
The effects of chemical elements on the properties of steel
1. Carbon (C):
As the carbon content in steel increases, the yield strength and tensile strength increase, but plasticity and impact resistance decrease. When the carbon content exceeds 0.23%, the weldability of the steel deteriorates.
Therefore, for low-alloy structural steels used for welding, the carbon content generally does not exceed 0.20%. The higher carbon content also reduces the steel's resistance to atmospheric corrosion; high carbon steel in outdoor storage is prone to rust. Additionally, carbon can increase the cold brittleness and aging sensitivity of steel.
2. Silicon (Si):
Silicon is added in the steel manufacturing process as a reducer and deoxidizer, so calm steel contains 0.15-0.30% silicon. If the silicon content in steel exceeds 0.50-0.60%, silicon is considered an alloying element. Silicon can significantly increase the elastic limit, yield point and tensile strength of steel, making it widely used in spring steel.
By adding 1.0-1.2% silicon to quenched and tempered structural steel, its strength can be increased by 15-20%. Silicon, in combination with elements such as molybdenum, tungsten and chromium, increases resistance to corrosion and oxidation, useful for making heat-resistant steel.
Low carbon steel containing 1-4% silicon has extremely high magnetic permeability and is used in the electrical industry for silicon steel sheets. An increase in silicon content reduces the weldability of the steel.
3. Manganese (Mn):
In the steelmaking process, manganese acts as an excellent deoxidizer and desulfurizer, with steel generally containing 0.30-0.50% manganese. When more than 0.70% is added to carbon steel, it is called “manganese steel”.
This type of steel not only has sufficient toughness compared to general steel, but also has higher strength and hardness, improving the steel's hardenability and thermal processing properties.
For example, the yield point of 16Mn steel is 40% higher than that of A3 steel. Steel containing 11-14% manganese has extremely high wear resistance, making it suitable for excavator buckets, ball mill linings, etc. An increase in manganese content weakens the corrosion resistance of steel and decreases its weldability.
4. Phosphorus (P):
Phosphorus is generally harmful to steel. It increases the cold brittleness of steel, deteriorates its weldability, reduces plasticity and worsens its cold bending performance. Consequently, the phosphorus content in steel must generally be less than 0.045%, with high-quality steel requiring even lower levels.
5. Sulfur (S):
Sulfur is normally harmful to steel. It induces hot brittleness, reducing the ductility and toughness of the steel, leading to cracking during forging and rolling. Sulfur is also detrimental to welding performance and reduces corrosion resistance.
Therefore, it is generally necessary for the sulfur content to be less than 0.055%, with high-quality steel requiring less than 0.040%. Adding 0.08-0.20% sulfur to steel can improve machinability; this steel is often referred to as free-cutting steel.
6. Chromium (Cr):
In structural and tool steels, chromium significantly increases strength, hardness and wear resistance, but simultaneously reduces plasticity and toughness. Chromium increases the oxidation and corrosion resistance of steel, making it an integral element of stainless and heat-resistant steels.
7. Nickel (Ni):
Nickel increases the strength of steel, maintaining good plasticity and toughness. Nickel has high resistance to corrosion by acids and alkalis and is resistant to rust and heat at high temperatures.
However, as it is a scarce resource, nickel should be replaced by other alloying elements whenever possible, especially in nickel-chromium steels.
8. Molybdenum (Mo):
Molybdenum refines the grain structure of steel, increases hardenability and thermal resistance, and maintains sufficient strength and creep resistance at high temperatures (creep refers to deformation under long-term stress at high temperatures).
Adding molybdenum to structural steel improves mechanical properties and suppresses heat-caused brittleness in alloy steel. In tool steels, it increases hardness when hot.
9. Titanium (Ti):
Titanium is a strong steel deoxidizer. Densifies the internal structure of the steel, refines the grain size, reduces sensitivity to aging and cold brittleness and improves weldability. Adding appropriate titanium to 18Cr-9Ni austenitic stainless steel can prevent intergranular corrosion.
10. Vanadium (V):
Vanadium is an excellent deoxidizer for steel. Adding 0.5% vanadium to steel refines the grain structure, increasing strength and toughness. Carbides formed from vanadium and carbon can improve the corrosion resistance of hydrogen under high temperature and pressure.
11. Tungsten (W):
Tungsten has a high melting point, high density and is an expensive alloying element. Tungsten carbide has high hardness and wear resistance. Adding tungsten to tool steel significantly increases hot hardness and heat resistance, making it suitable for cutting tools and forging dies.
12. Niobium (Nb):
Niobium refines grain size and reduces overheat sensitivity and temper brittleness of steel, increasing strength but reducing plasticity and toughness. The addition of niobium to common low-alloy steel increases resistance against atmospheric corrosion and corrosion by hydrogen, nitrogen and ammonia at high temperatures. Niobium improves weldability. When added to austenitic stainless steel, it can prevent intergranular corrosion.
13. Cobalt (Co):
Cobalt is a rare precious metal often used in special steels and alloys such as heat-resistant steel and magnetic materials.
14. Copper (Cu):
Steel refined from Daye ore by Wuhan Iron and Steel generally contains copper. Copper increases strength and toughness, especially resistance to atmospheric corrosion. The disadvantage is that it tends to cause heat starvation during hot processing, and if the copper content exceeds 0.5%, the plasticity decreases significantly. When the copper content is less than 0.50%, it does not affect the weldability.
15. Aluminum (Al):
Aluminum is a common deoxidizer in steel. Adding a small amount of aluminum to steel can refine the grain and improve impact resistance, such as in 08Al steel used for deep drawing of thin sheets.
Aluminum also has oxidation resistance and corrosion resistance. When used together with chromium and silicon, it can significantly improve the scale and high-temperature corrosion resistance of steel. The disadvantage of aluminum is that it affects the hot workability, weldability and machinability of steel.
16. Boron (B):
Adding trace amounts of boron can improve the density and hot-rolling properties of steel, increasing its strength.
17. Nitrogen (N):
Nitrogen can increase the strength, low temperature toughness and weldability of steel, as well as increase its sensitivity to aging.
18. Rare Earths (Xt):
Rare earth elements refer to the 15 lanthanide elements with atomic numbers 57-71 on the periodic table. These elements are all metals, but their oxides are like “earth,” which is why they are commonly called rare earths.
Adding rare earths to steel can change the composition, shape, distribution and properties of inclusions in the steel, thereby improving various properties such as toughness, weldability and cold workability. Adding rare earths to plow steel can improve its wear resistance.
Production process
1. How is steel made?
The main task of the steel industry is to adjust the carbon content and alloy elements of steel within the specified range in accordance with the quality requirements of the type of steel being produced and to reduce the content of impurities such as P, S, H, O, N below permitted limits.
The steel manufacturing process is essentially an oxidation process. Excess carbon in the furnace charge is oxidized and burned into CO gas and escapes, while other elements such as Si, P, Mn are oxidized and enter the slag. Some of the S enters the slag and some is discharged as SO2.
When the composition and temperature of the molten steel meet the process requirements, the steel can be threaded. To remove excess oxygen from the steel and adjust the chemical composition, deoxidizers and ferroalloys or alloying elements can be added.
2. Brief introduction to converter steel production
The hot metal transported from the torpedo car, after desulfurization and slag blocking treatments, can be dumped into the converter as main charge, together with less than 10% steel scrap. Then oxygen is blown into the converter to burn, the excess carbon in the hot metal is oxidized and releases a large amount of heat. When the probe detects the predetermined low carbon content, the oxygen blow is stopped and the steel is used.
Deoxygenation and composition adjustment operations generally occur in the shell; Then, carburized rice husks are dropped onto the surface of the molten steel to prevent it from oxidizing, ready to be sent to the continuous casting or mold casting area.
For high-demand steel types, blown argon, RH vacuum treatment and powder spraying treatment (Si-Ca powder spraying and modified lime) can effectively reduce gases and inclusions in steel and further reduce carbon and sulfur . After these secondary refining measures, the composition can be precisely adjusted to meet the requirements of high-quality steel materials.
3. Preliminary Launch
Molten steel ingots are heated in a reheating furnace using the new hot loading and hot delivery process, and then rolled into slabs, billets, small square billets and other preliminary rolled products through a roughing mill and rolling mill continuous.
After head and tail cutting, surface cleaning (flame cleaning, grinding), high-quality products also require peeling and flaw detection for preliminary rolled billets. After passing inspection, they are stored in the warehouse.
At present, the products of preliminary rolling are preliminary slabs, square rolled billets, oxygen cylinder steel billets, gear round tube billets, railway vehicle axle billets, and plastic mold steel.
The preliminarily rolled slab mainly supplies the hot rolling mill as raw material; The rolled square billet, in addition to being supplied externally, is mainly sent to the high-speed wire rod rolling mill as raw material. Due to the advancement of continuous casting slabs, the demand for pre-rolled slabs has been greatly reduced and has therefore shifted to the other above products.
4. Continuous Hot Lamination
Using continuous casting slabs or thinning slabs as raw materials, they are heated in a stepwise heating furnace and enter the raw rolling mill after descaling with high pressure water.
The raw rolled materials are cut at the head and tail and then enter the finishing mill, where computer-controlled rolling is implemented. After final lamination, they undergo laminar cooling (computer-controlled cooling rate) and winding by a winder, forming a hot coil.
The head and tail of the hot coil often appear in tongue and fishtail shapes, with poor thickness and width accuracy, and defects such as curling, bent edges and tower shapes are common on the edges.
The coil is relatively heavy, with an internal diameter of 760 mm (which is generally preferred in the tube manufacturing industry). The hot coil, after being slit into the head, tail and edges, and undergoing multiple rounds of straightening and flattening on the finishing line, is further slit into plates or rewound, forming products such as hot-rolled steel plates, hot-rolled sheets hot flattened coils and longitudinal strips.
If the finished hot-rolled coil is acid washed to remove scale and then lubricated, it becomes a hot-rolled pickled coil. This product, with its trend of local replacement of cold-rolled sheets and its moderate price, is widely preferred by users.
5. Continuous Cold Rolling
Hot-rolled steel coils are used as raw materials, which are first acid washed to remove the oxide film and then cold rolled. The product is a hard laminated coil. Continuous cold deformation causes work hardening, which increases the strength and hardness of the hard rolled coil and reduces its toughness and plasticity.
As a result, its stamping performance deteriorates and it can only be used for parts with simple deformation. Hard rolled coils can be used as raw material for hot dip galvanizing plants since these plants are equipped with annealing lines. The weight of hard rolled coils generally ranges from 6 to 13.5 tons, with an internal diameter of 610 mm.
Standard continuous cold rolling plates and coils must undergo continuous annealing (in a CAPL unit) or annealing in a bell furnace to eliminate work hardening and rolling stresses, achieving the mechanical performance indicators established by the respective standards.
Cold-rolled steel plates have superior surface quality, appearance and dimensional accuracy compared to hot-rolled plates, with rolled product thicknesses up to about 0.18mm, therefore, they are highly preferred by users.
Deep processing of products based on cold-rolled steel coils results in products with high added value. Examples include galvanized galvanizing, hot-dip galvanizing, fingerprint-resistant electroplating, color-coated steel sheet coils, vibration-damped composite steel plates, and PVC laminated steel plates.
These products, with their aesthetic qualities and high resistance to corrosion, have wide application.
After annealing, cold-rolled steel coils must undergo finishing, including head and tail cutting, edge cutting, leveling, flattening, rewinding or longitudinal shear coating. Cold rolled products are widely used in the manufacture of automobiles, household appliances, instrument switches, construction, office furniture and other industries.
The weight of each bundled steel sheet is 3 to 5 tons, while the weight of flattened subrolls generally ranges from 3 to 10 tons per roll, with an inner diameter of 610 mm.
Most steel processing is carried out using pressure-based methods, causing the steel part (e.g. billets or ingots) to undergo plastic deformation. Steel processing can be divided into cold working and hot working based on the temperature applied. The main methods for processing steel include:
Rolling: This is a pressure processing method in which a piece of metal passes through a gap between a pair of rotating rollers of various shapes. Roller compression reduces the cross-sectional area of the material and increases its length. This is the most common method of steel production, mainly used for the production of profiles, sheets and tubes. Includes cold and hot lamination.
Forging: This pressure processing method uses the reciprocating impact of a forging hammer or pressure from a press to form the part into the desired shape and size. It is generally divided into free forging and die forging, often used to produce large materials, and open die forging with larger cross-sectional dimensions.
Drawing: This involves pulling already rolled metal parts (profiles, tubes, products, etc.) through holes in the die in a process that reduces cross-sectional area and increases length. This method is widely used in cold working.
Extrusion: This process involves placing the metal in a sealed extrusion cylinder and applying pressure to one end. The metal is extruded through a specific die hole to produce finished products of the same shape and size. This method is mainly used for the production of non-ferrous metal materials.
6. Mechanical properties of steel
6.1 Yield Rate
The relationship between the yield strength is the quotient between the yield strength and the tensile strength (σs/σb). The higher the yield rate, the more resistant the material. On the other hand, the lower the yield strength ratio, the better the plasticity and stamping formability. For example, the yield strength ratio of deep drawing steel sheet is ≤0.65.
Spring steel is generally used within the elastic limit range and cannot undergo plastic deformation under load. Therefore, it is necessary for the spring steel to have as high a yield strength and yield strength ratio as possible after quenching and tempering (σs/σb≥0.90). Furthermore, fatigue life is often strongly correlated with tensile strength and surface quality.
6.2 Plasticity
Plasticity refers to the ability of a metallic material to sustain permanent deformation before failing under stress. Plasticity is typically represented by stretching and reducing area ratios. The greater the elongation and reduction in area ratios, the better the plasticity.
7. Impact Resistance
Impact toughness, represented by αk, refers to the impact work expended per unit cross-sectional area in the notch of a metal specimen when it fractures under a specified impact test load.
The common specimen is 10×10×55mm with a 2mm deep V-notch, and the standard directly adopts the impact work (J Joule value) AK, not the αK value, because the impact work per unit of area has no practical meaning.
Impact work is more sensitive to examine the brittleness transformation of metallic materials at different temperatures, and catastrophic fracture accidents under actual service conditions are often related to material impact work and service temperature.
Therefore, standards often stipulate specific impact work values at a given temperature and require that the FATT (Fracture Appearance Transition Temperature) be lower than a given temperature.
The so-called FATT is the temperature corresponding to the brittle fracture that occupies 50% of the total area after a group of impact specimens is broken at different temperatures. Due to the influence of the thickness of the steel plate, for plates with thickness ≤10mm, 3/4 size impact specimens (7.5×10×55mm) or 1/2 size impact specimens (5×10×55mm) can be obtained.
However, it should be noted that only impact work values under the same specifications and the same temperature can be compared.
Only under the conditions stipulated in the standard can the impact work be converted into the impact work of the standard impact sample according to the standard conversion method and then compared.
8. Hardness Test
The ability of a metallic material to resist penetration by an indenter (a hardened steel ball or diamond indenter with a 120-degree cone or angle) is called hardness. Depending on the applicable testing methods and scopes, hardness can be classified into Brinell hardness, Rockwell hardness, Vickers hardness, Shore hardness, as well as microhardness and high temperature hardness. Metallurgical products generally use Brinell hardness and Rockwell hardness.
9. Baosteel Corporate Standard (Q/BQB)
The steel grades in Baosteel's corporate standards can be roughly divided into three sources: those transplanted from the Japanese JIS standard, the German DIN standard, and those developed and produced by Baosteel itself.
Steel grades transplanted from the JIS standard generally begin with S (Steel); those transplanted from the DIN standard usually begin with ST (Stahl, the German word for “steel”); Steel grades developed and produced by Baosteel itself generally begin with B, the initial in Baosteel's phonetic spelling.
10. Hot and cold rolled structural steel plates and strips
Structural steel is generally graded by strength, and the numbers in the steel grade generally represent the minimum tensile strength. Because this type of steel is commonly used to manufacture structural components, it is called structural steel.
Structural steel strengthening mechanisms tend to favor decarbonization and strengthening of ferrite with solid manganese solutions, refinement of pearlite, and the addition of microalloys for precipitation strengthening, sediment strengthening, and fine-grain strengthening.
This ensures that, while increasing strength, the steel maintains good toughness, plasticity levels and excellent weldability.
