There are various chemical elements in steel, which make up hundreds of different grades, so what role do these chemical elements play in steel?
- Carbon (Carbon):
The role of carbon in steel is a double-edged sword. As the carbon content increases, the yield point and tensile strength of the material will gradually increase, but the plasticity and impact resistance will decrease. Therefore, the carbon content needs to be tailored according to different material uses.
When the carbon content exceeds 0.23%, the welding performance will be greatly reduced, so that the carbon content of low-alloy structural steel used for welding occasions cannot exceed 0.20%. Excessive carbon content will also lead to a decline in the atmospheric corrosion resistance of steel, and high-carbon steel in open stockyards is easily corroded.
High carbon content is also a disadvantage. High carbon content can improve the cold brittleness and aging sensitivity of steel.
- Silicon (Silicon):
Silicon is added as a reducing agent and deoxidizer in the steelmaking process, so the killed steel will contain 0.15-0.30% silicon. When the silicon content in steel exceeds 0.50-0.60%, silicon is considered an alloying element.
Silicon can significantly improve the elastic limit, yield point, and tensile strength of steel, so it is widely used in spring steel, such as 65Mn and 82B, which contain 0.15-0.37% silicon. Adding 1.0-1.2% silicon to quenched and tempered structural steel can increase the strength of steel by 15-20%; bonding silicon with molybdenum, tungsten, chromium, etc. can improve corrosion resistance and oxidation resistance, and can produce heat-resistant steel; Low-carbon steel containing 1.0-4.0% silicon, has extremely high magnetic permeability, and is used in the electrical industry to make silicon steel sheets.
Of course, silicon is not all beneficial and harmless, it will reduce the weldability of steel.
- Manganese (Manganese):
In the steelmaking process, manganese is a good deoxidizer and desulfurized, and generally, steel contains 0.30-0.50% manganese. When more than 0.70% is added to carbon steel, it is considered “manganese steel”. Compared with ordinary steel, it not only has sufficient toughness but also has higher strength and hardness, which improves the hardenability of steel and improves the hot workability of steel, such as 16Mn The steel has a 40% higher yield point than A3. Steel containing 11-14% manganese has extremely high wear resistance and is used for excavator buckets, ball mill liners, etc.
High manganese content also has disadvantages. When the manganese content is high, the steel has obvious temper brittleness; manganese element can promote the growth of grains, so it needs to be paid attention to in the heat treatment process; when the mass fraction of manganese exceeds 1%, it will reduce the welding performance of the steel.
- Sulfur:
Sulfur is derived from steelmaking ore and fuel coke, and is a harmful element in steel.
Sulfur exists in steel in the form of iron sulfide (FeS). FeS and Fe form a compound with a low melting point (985°C), and the hot working temperature of steel is generally 1150-1200°C, so when the steel is hot worked, the FeS compound will Premature melting leads to cracking of the workpiece, a phenomenon known as “hot embrittlement”. The higher the sulfur content, the more serious the hot embrittlement phenomenon, so the sulfur content in the steel must be controlled.
High-grade high-quality steel S<0.02-0.03%, high-quality steel S<0.03-0.045%, ordinary steel S<0.055%-0.07%. In individual cases, it is necessary to add sulfur. For example, adding 0.08-0.20% sulfur to steel can improve machinability, and it is usually called free-cutting steel.
Sulfur is also detrimental to weldability and reduces corrosion resistance.
- Phosphorus:
Phosphorus is brought into steel by ore, generally speaking, phosphorus is also a harmful element. Although phosphorus can increase the strength and hardness of steel, it causes a significant decrease in plasticity and impact toughness. Especially at low temperatures, it makes the steel significantly brittle, a phenomenon called “cold brittleness”. Cold brittleness makes the cold working and weldability of steel worse. The higher the phosphorus content, the greater the cold brittleness, so the control of phosphorus content in steel is stricter. High-quality steel: P<0.025%; high-quality steel: P<0.04%; common steel: P<0.085%.
- Oxygen:
Oxygen is a harmful element in steel. It naturally enters into steel during the steelmaking process. Although manganese, silicon, iron, and aluminum are added for deoxidation at the end of steelmaking, it is impossible to remove them. Oxygen is in the form of FeO, MnO, SiO2, Al2O3, and other inclusions in steel, which reduces the strength and plasticity of steel. In particular, it has a serious impact on fatigue strength and impact toughness.
- Nitrogen:
The ability of ferrite to dissolve nitrogen is very low. When supersaturated nitrogen is dissolved in steel, nitrogen will precipitate in the form of nitrides after it is placed for a long time or heated at 200-300 ° C, which will increase the hardness and strength of the steel, decrease the plasticity, and cause aging. Add Al, Ti, or V to molten steel for nitrogen fixation treatment, so that nitrogen can be fixed in AlN, TiN, or VN, which can eliminate the aging tendency.
- Chromium:
In structural steel and tool steel, chromium can significantly improve the strength, hardness, and wear resistance, so that the steel has good oxidation resistance and corrosion resistance, so it is an important alloy element of stainless steel and heat-resistant steel; it can also improve steel Hardenability is a very important alloying element.
However, chromium will also increase the brittle transition temperature of steel and increase the temper brittleness of steel, which will cause unnecessary troubles in the processing process.
- Nickel:
Nickel can increase the strength of steel while maintaining good plasticity and toughness. Nickel has high corrosion resistance to acid and alkali, and has anti-rust and heat resistance at high temperatures. However, since nickel is a relatively scarce resource, other alloying elements should be used instead of nickel-chromium steel. Manufacturers need to detect the Ni content in stainless steel and prevent suppliers from shoddy products. GNR Handheld Spectrometer P5 can effectively detect Ni content (hyperlink on the right)–GNR Handheld Spectrometer | Nickel Alloy Test Report
- Molybdenum (Molybdenum):
Molybdenum can refine the grains of steel, improve hardenability and thermal strength, and maintain sufficient strength and creep resistance at high temperatures (it is stressed and deformed at high temperatures for a long time, which is called creep). Adding molybdenum to structural steel can improve mechanical properties. It can also suppress the brittleness of alloy steel due to fire. Adding molybdenum to tool steel can improve redness.
- Titanium:
Titanium is a strong deoxidizer in steel. It can make the internal structure of steel dense, refine grain strength; reduce aging sensitivity and cold brittleness. Improve welding performance. Adding appropriate titanium to Cr18Ni9 austenitic stainless steel can avoid intergranular corrosion.
- Vanadium:
Vanadium is an excellent deoxidizer for steel. Adding 0.5% vanadium to the steel can refine the grain of the structure and improve its strength and toughness. Carbides formed by vanadium and carbon can improve hydrogen corrosion resistance under high temperatures and high pressure.
- Tungsten (Tungsten):
Tungsten has a high melting point and a large specificity and is an alloying element for Guisheng. Tungsten and carbon form tungsten carbide, which has high hardness and wear resistance. Adding tungsten to tool steel can significantly improve red hardness and thermal strength, and can be used as cutting tools and forging dies.
- Niobium:
Niobium can refine the grain and reduce the overheating sensitivity and temper the brittleness of the steel, and increase the strength, but the plasticity and toughness decrease. Adding niobium to ordinary low-alloy steel can improve the resistance to atmospheric corrosion and the resistance to hydrogen, nitrogen, and ammonia corrosion at high temperatures. Niobium improves solderability. Adding niobium to austenitic stainless steel can prevent intergranular corrosion.
- Cobalt (Cobalt):
Cobalt is a rare and precious metal, which is mostly used in special steels and alloys, such as heat-strength steels and magnetic materials.
- Copper (Copper):
The steel made by WISCO from Daye ore often contains copper. Copper improves strength and toughness, especially atmospheric corrosion resistance. The disadvantage is that hot embrittlement is easy to occur during hot working, and the plasticity is significantly reduced when the copper content exceeds 0.5%. When the copper content is less than 0.50%, it does not affect weldability.
- Aluminum:
Aluminum is a commonly used deoxidizer in steel. Adding a small amount of aluminum to steel can refine grains and improve impact toughness, such as 08Al steel for deep drawing thin plates. Aluminum also has anti-oxidation and anti-corrosion properties. The combination of aluminum, chromium, and silicon can significantly improve the high-temperature non-skinning performance and high-temperature corrosion resistance of steel. The disadvantage of aluminum is that it affects the hot workability, welding performance, and cutting performance of steel.
- Boron:
Adding a small amount of boron to the steel can improve the compactness and hot-rolling performance of the steel, and increase its strength.
- Rare Earth Element:
Rare earth elements refer to the 15 lanthanide elements with atomic numbers 57-71 in the periodic table of elements. These elements are all metals, but their oxides are very similar to “earth”, so they are customarily called rare earth. Adding rare earth to steel can change the composition, shape, distribution, and properties of inclusions in steel, thereby improving various properties of steel, such as toughness, weldability, and cold workability. Adding rare earth to plowshare steel can improve wear resistance.
