1 C (Carbon) 

C is the second most important element after iron. It directly affects the strength, plasticity, toughness and welding properties of steel. 

When the carbon content in steel is below 0.8%, as the carbon content increases, the strength and hardness of the steel increase, while the plasticity and toughness decrease. 

However, when the carbon content exceeds 1.0%, as the carbon content increases, the strength of the steel actually decreases. 

As the carbon content increases, the welding performance of steel deteriorates (for steel with a carbon content greater than 0.3%, weldability significantly decreases), cold brittleness and age hardening sensitivity increase, and the resistance to atmospheric corrosion decreases. 

2 Si (Silicon) 

Si is an important reducing agent and deoxidizer in the steelmaking process: For many materials in carbon steel, there is generally less than 0.5% of Si. This Si is usually introduced during the steelmaking process as a reducing agent and deoxidizer. 

Silicon can dissolve in ferrite and austenite to enhance the hardness and strength of steel. Its effect is second only to that of phosphorus, and it is stronger than elements such as manganese, nickel, chromium, tungsten, molybdenum, and vanadium. However, when the silicon content exceeds 3%, it will significantly reduce the plasticity and toughness of the steel. 

Silicon can enhance the elastic limit, yield strength and yield ratio of steel, as well as fatigue strength and fatigue ratio. This is why silicon or silicon-manganese steel can be used as spring steel. 

Silicon can reduce the density, thermal conductivity and electrical conductivity of steel. It can promote the coarsening of ferrite grains and reduce the coercive force. It has the tendency to reduce the anisotropy of crystals, making magnetization easier and reducing magnetic resistance. It can be used to produce electrical steel, so the magnetic hysteresis loss of silicon steel sheets is relatively low. Silicon can increase the magnetic permeability of ferrite, making the steel sheet have a higher magnetic flux density in a weaker magnetic field. However, in a strong magnetic field, silicon reduces the magnetic flux density of the steel. Because silicon has strong deoxidizing ability, it reduces the magnetic aging effect of iron. When silicon-containing steel is heated in an oxidizing atmosphere, a layer of SiO2 film will form on the surface, thereby improving the oxidation resistance of the steel at high temperatures. Silicon can promote the growth of columnar crystals in cast steel and reduce plasticity. If silicon steel is heated and cooled too quickly, due to its low thermal conductivity, there will be a large temperature difference between the inside and outside of the steel, resulting in fracture. 

Silicon can reduce the welding performance of steel. Because silicon has a stronger binding ability with oxygen than iron, during welding, it is prone to form low-melting-point silicates, which increase the fluidity of slag and molten metal, causing spattering and affecting the welding quality. 

Silicon is a good deoxidizer. When using aluminum for deoxidation, adding a certain amount of silicon as appropriate can significantly enhance the deoxidizing property of aluminum. 

Silicon is inherently present in steel to a certain extent, which was brought in as a raw material during iron and steel production. In bath steel, the silicon content is limited to less than 0.07%. When it is intentionally added, silicon iron alloy is added during the steelmaking process. 

3 Mn (manganese) 

Manganese can enhance the strength of steel: Due to its relatively low price and the ability to form an unlimited solid solution with iron, manganese can increase the strength of steel while having a relatively minor impact on plasticity. Therefore, manganese is widely used as a strengthening element in steel. 

It can be said that manganese is present in almost all carbon steels. The common types of press-formed mild steel, duplex steel (DP steel), transformation-induced plasticity steel (TR steel), and martensitic steel (MS steel) all contain manganese. 

Generally, the content of manganese in mild steel does not exceed 0.5%. 

The manganese content in high-strength steel increases as the strength grade rises. For example, in martensitic steel, the manganese content can reach up to 3%. 

Mn enhances the hardenability of steel and improves its heat processing properties: A typical example is 40Mn and 40-grade steel. 

Mn can counteract the influence of S (sulfur): In steel production, Mn can combine with S to form a high-melting-point compound called Mns, thereby weakening and eliminating the adverse effects of S. 

However, the content of Mn is also a double-edged sword. The higher the manganese content, the worse it is. An increase in manganese content will reduce the plasticity and welding performance of the steel. 

4 Cr (Chromium) 

Chromium can enhance the hardenability of steel and has the effect of secondary hardening. It can increase the hardness and wear resistance of carbon steel without making the steel brittle. 

When the content exceeds 12%, it enables the steel to have excellent high-temperature oxidation resistance and corrosion resistance to oxygen, and also enhances the steel's heat strength. Chromium is the main alloying element for stainless steel, acid-resistant steel and heat-resistant steel. Chromium can increase the strength and hardness of carbon steel in the rolled state, and reduce the elongation and cross-sectional contraction rate. 

When the chromium content exceeds 15%, the strength and hardness will decrease, while the elongation and reduction of area will increase accordingly. The parts made of chromium-containing steel can easily achieve a high surface processing quality after grinding. 

The main function of chromium in the quenched and tempered structure is to enhance the quenching penetration, enabling the steel to have better comprehensive mechanical properties after quenching and tempering. In carburized steel, chromium can also form chromium-containing carbides, thereby improving the wear resistance of the material's surface. 

Spring steel containing chromium is not prone to decarburization during heat treatment. Chromium can enhance the wear resistance, hardness and red hardness of tool steel, and has good temper stability. In electrically heated alloys, chromium can improve the oxidation resistance, electrical resistance and strength of the alloy. 

5 Mo (Molybdenum) 

Molybdenum in steel can enhance the quenching toughness and heat resistance, prevent temper brittleness, increase remanence and coercivity, as well as enhance the corrosion resistance in certain media. 

In quenched and tempered steel, molybdenum can enhance the quenching depth and penetration of parts with larger cross-sections, improve the steel's resistance to rehardening or rehardening stability, enable the parts to be rehardened at higher temperatures, thereby more effectively eliminating (or reducing) residual stress and enhancing plasticity. 

In case-hardening steel, molybdenum not only has the aforementioned functions, but also can reduce the tendency of carbides to form a continuous network on the grain boundaries in the case-hardening layer, decrease the residual austenite in the case-hardening layer, and relatively increase the wear resistance of the surface layer. 

In wrought steel, molybdenum can also ensure that the steel maintains relatively stable hardness, and enhance its resistance to deformation, cracking and wear. 

In stainless acid-resistant steel, molybdenum can further enhance the corrosion resistance to organic acids (such as formic acid, acetic acid, oxalic acid, etc.) as well as hydrogen peroxide, sulfuric acid, sulfurous acid, sulfates, acid dyes, and bleaching powder solutions, etc. Especially due to the addition of molybdenum, the tendency of pitting corrosion caused by the presence of chloride ions is prevented. 

The W12Cr4V4Mo high-speed steel containing about 1% molybdenum has properties such as wear resistance, temper hardness and red hardness.