There are a variety of chemical elements in steel, which make up hundreds of different grades.
So what role do these chemical elements play in steel?
Today I will take you into the world of chemical elements in steel.
Effect of carbon in steel
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 customized according to different material uses.
When the carbon content exceeds 0.23%, the welding performance will decline significantly, so that the carbon content of the low-alloy structural steel used for welding cannot exceed 0.20%.
Excessive carbon content will also cause the steel’s resistance to atmospheric corrosion to decline.
High-carbon steel in open-air yards is vulnerable to corrosion.
Actually, high carbon content is not all disadvantage because that high carbon content can improve the cold brittleness and aging sensitivity of steel.
Effect of silicon in steel
Silicon is added as a reducing agent and deoxidizing agent in the steelmaking process, so the sedated steel will contain 0.15-0.30% silicon.
When the silicon content in steel exceeds 0.50-0.60%, then silicon is considered an alloying element.
Silicon element can significantly improve the elastic limit, yield point and tensile strength of steel, so it is widely used in spring steels, such as 65Mn and 82B. These kind of spring steels contain 0.15-0.37% silicon.
Adding 1.0-1.2% silicon to the quenched and tempered structural steel can increase the strength of the steel by 15-20%.
Silicon, combined with molybdenum, tungsten, chromium, etc., has the effect of improving corrosion resistance and oxidation resistance, and can manufacture heat-resistant steel.
Low-carbon steel containing 1.0-4.0% silicon, with extremely high magnetic permeability, is used for silicon steel sheets in the electrical industry.
Of course, silicon is not without its shortcomings, it will reduce the welding performance of steel.
Effect of manganese in steel
In the steelmaking process, manganese is a good deoxidizer and desulfurizer.
Generally, steel contains 0.30-0.50% manganese.
When adding more than 0.70% of manganese to carbon steel, it is considered as “manganese steel”.
Manganese steel has not only sufficient toughness but also higher strength and hardness than ordinary steel.
Improve the hardenability and the hot workability of the steel, for example, the yield point of 16Mn steel is 40% higher than that of A3 steel.
Steel containing 11-14% of 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 more obvious tempering brittleness.
Manganese has the effect of promoting grain growth, which needs attention during heat treatment.
When the mass fraction of manganese exceeds 1%, the welding performance of steel will be reduced.
Effect of sulfur in steel
Sulfur is derived from steelmaking ore and fuel coke and is a harmful element in steel.
Sulfur exists in steel in the form of FeS, and FeS and Fe form compounds with a low melting point (985 °C), and the hot working temperature of steel is generally 1150-1200 °C.
Therefore, when the steel is hot-worked, the FeS compound will melt prematurely and cause the workpiece to crack.
This phenomenon is called “hot brittleness”.
The higher the sulfur content, the more severe the hot brittle phenomenon, so the sulfur content in the steel must be controlled.
For high-quality steel, the sulfur content is less than 0.02-0.03%; for quality steel, the sulfur content is less than 0.03-0.045%; for ordinary steel, the sulfur content is less than 0.055% -0.07%.
In some cases, it is necessary to add sulfur. For example, adding 0.08-0.20% sulfur to steel can improve cutting workability, which is usually called free cutting steel.
Sulfur is also detrimental to welding performance and can reduce corrosion resistance.
Effect of phosphorus in steel
Phosphorus is brought into steel by ore.
Generally speaking, phosphorus is also a harmful element.
Phosphorus can increase the strength and hardness of steel, but cause plasticity and impact toughness to decrease significantly.
Especially at low temperatures, it makes the steel significantly brittle, a phenomenon called “cold brittleness”.
Cold brittleness deteriorates the cold working and weldability of the steel.
The higher the phosphorus content, the greater the cold brittleness.
Therefore, the control of phosphorus content in steel is stricter.
High quality steel: P <0.025%, quality steel: P <0.04%, ordinary steel: P <0.085%.
Effect of oxygen in steel
Oxygen is a harmful element in steel and it naturally enters the steel during the steelmaking process.
Although manganese, silicon, iron, and aluminum are added for deoxidation at the end of steelmaking, but it is impossible to remove all the oxygen.
Oxygen appears in the steel as FeO, MnO, SiO2, Al2O3 and other inclusions, which reduces the strength and plasticity of steel.
In particular, it has a serious impact on fatigue strength and impact toughness.
Effect of nitrogen in steel
Ferrite has a low ability to dissolve nitrogen.
When supersaturated nitrogen is dissolved in the steel, after being left for a long time or after heating at 200-300 °C, nitrogen will precipitate in the form of nitrides, which will increase the hardness and strength of the steel, and reduce the plasticity as well as cause aging.
Adding Al, Ti, or V to the molten steel for nitrogen fixation treatment to fix nitrogen in AlN, TiN, or VN can eliminate the aging tendency.
Effect of chromium in steel
Chromium can significantly improve the strength, hardness and wear resistance in structural steel and tool steel, so that the steel has good oxidation resistance and corrosion resistance.
Therefore, chromium is an important alloying element for stainless steel and heat-resistant steel.
It can also improve the hardenability of steel and is an important alloying element.
However, chromium will also increase the brittle transition temperature of the steel, increase the tempering brittleness of the steel, and cause unnecessary trouble to the processing process.
Effect of nickel in steel
Nickel can increase the strength of steel while maintaining good plasticity and toughness.
Nickel has high corrosion resistance to acids and alkalis, and rust and heat resistance at high temperatures.
However, since nickel is a scarce resource, other alloy elements should be used instead of nickel-chrome steel.
Effect of molybdenum in steel
Molybdenum can refine the grains of steel, improve hardenability and hot strength.
It can also maintain sufficient strength and creep resistance at high temperature (under long-term stress at high temperature, deformation occurs, which is called creep).
The addition of molybdenum to structural steel can improve mechanical properties and can also suppress the brittleness of alloy steel due to fire.
Adding molybdenum to tool steel can improve red hardness.
Effect of titanium in steel
Titanium is a strong deoxidizer in steel.
It can make the internal structure of steel dense, refine grain power, reduce aging sensitivity and cold brittleness, as well as improve welding performance.
Adding proper titanium to Cr18Ni9 austenitic stainless steel can avoid intergranular corrosion.
Effect of vanadium in steel
Vanadium is an excellent deoxidizer for steel.
Adding 0.5% vanadium to the steel can refine the grain structure and improve strength and toughness.
The carbides formed by vanadium and carbon can improve the resistance to hydrogen corrosion under high temperature and pressure.
Effect of tungsten in steel
Tungsten has a high melting point and a high specific gravity, and is an important alloying element.
Tungsten carbide formed from tungsten and carbon has high hardness and wear resistance.
Effect of niobium in steel
Niobium can refine the grains and reduce the overheating sensitivity and temper brittleness of steel, and also improve the strength, but the plasticity and toughness will decrease.
Adding niobium to ordinary low alloy steel can improve the resistance to atmospheric corrosion and hydrogen, nitrogen and ammonia corrosion resistance at high temperature.
Niobium can improve welding performance.
Adding niobium to austenitic stainless steel can prevent intergranular corrosion.
Effect of cobalt in steel
Cobalt is a kind of rare and precious metal. It is mostly used in special steels and alloys, such as heat-resistant steel and magnetic materials.
Effect of copper in steel
WISCO, steel made from Daye Ore, often contains copper.
Copper can increase strength and toughness, especially atmospheric corrosion performance.
The disadvantage is that hot brittleness is easy to occur during hot processing.
When the copper content exceeds 0.5%, the plasticity is significantly reduced, when the copper content is less than 0.50%, it has no effect on weldability.
Effect of aluminum in steel
Aluminum is a common deoxidizer in steel.
Adding a small amount of aluminum to the steel can refine the grains and improve impact toughness, such as 08Al steel for deep drawing sheet.
Aluminum also has oxidation resistance and corrosion resistance.
When aluminum is used with chromium and silicon, it can significantly improve the high temperature non-skinning performance and the ability to withstand high temperature corrosion.
The disadvantage of aluminum is that it affects the hot workability, welding performance and cutting performance of steel.
Effect of boron in steel
Adding a small amount of boron to the steel can improve the compactness and hot rolling properties of the steel, and increase the strength.
Effect of rare earth element in steel
Rare earth elements refer to 15 lanthanides with an atomic number of 57-71 in the periodic table.
These elements are all metals, but their oxides are much like “earth”, so they are customarily called rare earths.
Adding rare earth to the steel can change the composition, morphology, distribution, and properties of the inclusions in the steel, thereby improving various properties of the steel, such as toughness, weldability, and cold workability.
Adding rare earth to plowshare iron can improve wear resistance.