Vanadium
Vanadium is one of the elements that strengthen ferrite. It has a strong affinity with C, N, and O to form corresponding stable compounds. Vanadium and α-Fe form a continuous solid solution, but it mainly exists in the form of carbides in steel. The role of vanadium in low-alloy steel is mainly to refine the grains, enhance the strength of steel and inhibit aging; in alloy structural steel, it is to refine the grains, enhance the strength and toughness of steel; in spring steel, it is used in combination with chromium and manganese to increase the elastic limit of steel; in tool steel, it is mainly to refine the organization and grains of steel, increase the tempering stability of steel, enhance the secondary hardening effect, improve wear resistance, and extend the service life of tools; in heat-resistant steel and hydrogen-resistant steel, vanadium can also play some beneficial effects. Although vanadium is also a good deoxidizer, its deoxidation ability is not as good as aluminum and titanium, and it is more expensive, so it is mainly used for alloying steel.
Aluminum
Aluminum has a strong affinity for oxygen and nitrogen. It is a good deoxidizing and nitrogen fixing agent, and can refine grains and inhibit the aging of low-carbon steel, and improve the toughness of steel at low temperatures. Aluminum is added to steel as an alloying element to improve the oxidation resistance of steel, improve the electromagnetic properties of steel, and improve the wear resistance and fatigue strength of nitrided steel. Aluminum does not combine with hydrogen when added to steel. The aluminum oxide formed by nitrogen absorbs oxygen to a large extent, thus hindering the precipitation of hydrogen during solidification. Steel with a high aluminum content will reduce lateral impact and ductility during hot working deformation, and the welding and cutting performance of such steel is not good.
Titanium
Titanium has a strong affinity for N, O, and C, and its affinity for sulfur is also stronger than that of iron. Therefore, it is a good deoxidizing, denitrifying, and desulfurizing agent. In some special cases, titanium, like manganese, can form titanium sulfide in steel to avoid hot brittleness of steel caused by iron sulfide. Titanium is a strong carbide forming element, but it only forms TiC carbide with carbon. Tic has a strong and stable bonding ability and is not easy to decompose. It has the function of organizing grain growth and coarsening in steel, so titanium can be used to refine the grains of steel. In addition, it can eliminate or reduce the intergranular corrosion tendency of stainless steel and improve the corrosion resistance of steel. Titanium is also one of the strong ferrite forming elements. When it exists in a solid solution state, it has the effect of improving the hardenability of steel, but when it exists in the state of titanium carbide particles, it reduces the hardenability of steel. Titanium can also improve the thermal strength when added to steel, but it is also easy to generate titanium nitride. Although its density is small, it is easy to float to the slag. A part of nitrogen will always remain in the steel to form angular inclusions and promote the generation of fatigue cracks.
Copper
The solubility of copper in iron is not large, and it cannot form a continuous solid solution with iron. The solubility of copper in α-Fe decreases sharply with the decrease of temperature, so in ferrite and low carbon steel, after appropriate heat treatment, precipitation strengthening can be produced. Copper does not form carbides with carbon. Some of its effects in steel, such as its influence on critical temperature and hardenability and solid solution strengthening, are quite similar to those of nickel, so it can be used to replace part of the nickel in steel. The prominent role of copper in steel is to improve the corrosion resistance of low-alloy steel to the atmosphere and seawater. Especially when used in combination with phosphorus, copper-containing steel can be used to manufacture ship themes, because copper can hinder the adhesion of sinks, shells, etc. in the underwater parts, and has good adhesion to protective paints and other coatings. In addition, copper can also increase the strength and yield point of steel and improve fluidity, which is of great significance for casting steel castings. When steel with a high copper content is heated at high temperature for a long time in a strong oxidizing atmosphere, a thin layer of copper-rich alloy with a melting point below 1100°C will be enriched under the surface iron oxide of the steel as a result of selective oxidation. This layer of alloy melts at about 1100°C and erodes the grain boundaries of the steel surface layer, causing the steel to crack during rolling, forging and other processing. In order to solve this problem, in addition to taking necessary measures during heating and processing, the nickel in the steel can also be adjusted during the smelting process to make the nickel-copper content ratio about 1:3 to 1:2, and change the copper-enriched layer on the steel surface to a copper-nickel-enriched layer with a melting point of more than 1200°C. Copper cannot be removed during the steel smelting process, and it is one of the scarce strategic materials. Therefore, unless there is a special requirement, it should not be intentionally added during the refining process.
