The main influence of alloying elements on steel properties

Dec 14, 2023

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The carbon content in steel has a great influence on the temperature regime of smelting, rolling and heat treatment. Low steel with a carbon content below 0.25% has good plasticity, no hardening tendency, and good weldability. Medium-carbon steel with a carbon content of 0.25% to 0.60% has good overall properties (that is, good strength and toughness). Carbon content ≥0.60% is high-carbon steel with high hardness and poor durability. Carbon forms a variety of high-hardness carbides in bearing steel and tool and die steel. Can improve the hardness and wear resistance of steel.
Silicon is the main reducing agent and deoxidizer in the steelmaking process. Killed steel generally contains 0.15% to 0.30% silicon. The silicon in steel can dissolve in ferrite at room temperature and has a certain strengthening effect on steel. If the silicon content in steel exceeds 0.50% to 0.60%, the elastic limit, yield point and tensile strength of the steel can be significantly increased, so it can be used in spring steel. Silicon is combined with molybdenum, tungsten, chromium, etc. to improve corrosion resistance and oxidation resistance, and can be used to make heat-resistant steel. Low carbon steel with a silicon content of 1% to 4% has extremely high magnetic permeability and is the raw material for electrical silicon steel sheets. When the silicon content is high, it is easy to cause cold brittleness, and it is easy to produce graphitization when tempered in medium carbon steel and high carbon steel.
The content of manganese in carbon structural steel is 0.50% to 1.50%, and in high-quality carbon structural steel is 0.20% to 1.20%. It is the main deoxidation and sulfur removal element. For killed steel, manganese can improve the deoxidation ability of silicon and aluminum. Some of the oxides formed by manganese in the steel can be combined with the iron to form spherical, high-melting point manganese sulfide, which has certain plasticity at high temperatures, thus It can reduce the thermal brittleness caused by sulfur and eliminate the harmful effects of sulfur in steel to a certain extent. The other part of manganese dissolves in ferrite to cause solid solution strengthening, so that when the steel is cooled after rolling, it will obtain relatively fine and high-strength pearlite, which can improve the hardness and strength of the steel after hot rolling, and has a significant impact on the area shrinkage (Z) and Impact toughness (Akv) is slightly affected. Manganese is an element that strongly expands the γ phase zone and can be used in high manganese austenitic wear-resistant steel, high-strength non-magnetic steel, austenitic stainless steel and wear-resistant steel.
Phosphorus enters the steel with the raw materials. Phosphorus has a strong solid solution strengthening effect and can be completely dissolved in the ferrite, which increases the strength and hardness of the steel but significantly reduces its plasticity and toughness. This embrittlement phenomenon is more serious at low temperatures and is called "cold brittleness". In particular, during the crystallization process of phosphorus, intracrystalline segregation is easy to occur, resulting in a high local phosphorus content, which causes the cold-brittle transition temperature to increase, causing greater harm. In addition, the segregation of phosphorus also causes the steel to form a band-like structure after hot rolling. The phosphorus content in steel should be reduced as much as possible (general steel is less than 0.045, and high-quality steel requires lower instantaneous content). Under certain conditions, the combined use of phosphorus and copper will improve the atmospheric corrosion resistance of low-alloy high-strength steel.
Sulfur enters steel along with raw materials and fuels. In the solid state, sulfur in steel exists in the form of FeS, and its solubility is extremely small. Due to the poor plasticity of FeS, steel with higher sulfur content is more brittle. In particular, FeS and Fe can form a low melting point eutectic distributed on the grain boundaries of austenite. When the steel is heated to 1200°C for pressure processing, due to the melting of the eutectic at the grain boundaries, the intergranular bonding is destroyed, causing the steel to Cracking along grain boundaries during processing is a phenomenon known as "hot embrittlement." In order to eliminate the harmful effects of sulfur, the sulfur content must be strictly limited and the manganese content in steel should be appropriately increased. Generally, sulfur is considered a harmful component, but steel with more sulfur content can form more MnS, which can play a role in lubrication and chip breaking during cutting, and can improve the cutting processability of steel, so it is easy to Commonly used additives for cutting steel.
Chromium is a precious metal. It has the effect of solid solution strengthening, making steel hot-hard, and can improve high-temperature performance, oxidation resistance and corrosion resistance. It is an important alloy element in high-temperature alloys and super-hard high-speed steel. In structural and tool steels, chromium can significantly increase strength, hardness and wear resistance, but reduces plasticity and toughness. It can improve the oxidation resistance and corrosion resistance of steel, so it is also an important alloy element of stainless steel and heat-resistant steel.
Nickel has high corrosion resistance to acids and alkalis, and is rust-proof and heat-resistant at high temperatures. However, it is expensive and is a scarce resource in my country. It is often used in combination with chromium and molybdenum in high-grade alloy steel to form a heat-strength steel. And the main alloying elements of stainless steel and high-temperature alloys. Nickel can increase the strength of steel and maintain good plasticity and toughness.
When the copper content is high, it is detrimental to hot deformation processing. If it exceeds 0.3%, it will cause high-temperature copper brittleness during hot deformation processing. When the content is higher than 0.75%, aging strengthening can occur after solid solution treatment and aging. In low carbon alloy steel, especially the coexistence of copper and phosphorus can improve the atmospheric corrosion resistance of the steel. 2% -3% copper in stainless steel can improve the corrosion resistance of sulfuric acid, phosphoric acid and hydrochloric acid.
Tungsten has a high melting point and high density. It is an alloy with abundant reserves in China. Tungsten and carbon form tungsten carbide, which has high hardness and wear resistance. Adding tungsten to tool steel can significantly improve the red hardness and thermal strength, and is suitable for manufacturing tools, mold steels and cemented carbide, etc.
Molybdenum can refine the grains of steel, improve hardenability and thermal strength properties, and maintain sufficient strength and creep resistance at high temperatures. Adding molybdenum to structural steel can improve the mechanical properties, inhibit the brittleness of alloy steel due to tempering, and improve the red hardness and wear resistance of tool steel.
Vanadium solid solution in ferrite produces a strong solid solution strengthening effect, which can refine the grains. Vanadium solid solution in austenite can improve the hardenability of steel and improve low-temperature impact toughness. However, the presence of vanadium in the combined state will reduce the hardenability of steel, increase the tempering stability of steel, and have a strong secondary hardening effect. Vanadium carbide is a metal hardener with extremely high hardness and excellent wear resistance. It can significantly extend the life of tool steel and improve the creep and lasting strength of steel.
Titanium is a strong deoxidizer for steel. It can make the internal structure of steel dense, refine the grains, and reduce aging sensitivity and cold brittleness. Titanium has strong solid solution strengthening effect, and its solid solution in austenite improves the hardenability of steel, but it also reduces the toughness of the solid solution. Titanium compounds reduce the hardenability of steel, improve tempering stability and have a secondary hardening effect. It can improve the oxidation resistance, thermal strength, creep and lasting strength of heat-resistant steel, and has a good effect on improving the weldability of steel.
Microalloyed steel (steel with an alloying element content of less than 0.1%) developed in recent years mainly uses niobium, vanadium, and titanium as alloying elements. Among them, niobium plays a prominent role in improving the strength of steel. Its characteristic is that it can combine with carbon and nitrogen to form nitrides and carbonitrides. These compounds dissolve at high temperatures and precipitate at low temperatures. Its function is to hinder the growth of original austenite grains during heating, inhibit recrystallization and grain growth after recrystallization during rolling, and precipitate at low temperatures to strengthen. The trace elements added to microalloy steel can improve the strength, but the controlled rolling process must be used for pressure processing, otherwise the toughness will deteriorate. This is because the controlled rolling process can refine the grains and offset the deterioration in toughness caused by precipitation strengthening.
Aluminum is one of the elements with extremely active chemical properties and has a strong affinity with oxygen and nitrogen. In order to deoxidize, aluminum is usually added to steelmaking. It can refine grains, inhibit the aging of low carbon steel, and improve the toughness of steel at low temperatures. When used as an alloy element, it can improve the oxidation resistance of steel. It can also be used to improve the electromagnetic properties of steel and improve nitriding. Wear resistance and fatigue strength of steel. Therefore, it is widely used in nitrided steel, heat-resistant peeling steel, magnetic steel, and electrothermal alloys.
Boron is one of the elements with extremely active chemical properties. It has a strong affinity with nitrogen, oxygen and carbon. It is added to steel mainly to improve the hardenability. Tempering at 300~400℃ can improve the impact resistance. It is often used for Produces gear steel, spring steel, heat-resistant steel, etc. However, when it is used in high carbon steel or when the residual oxygen content in the steel is high, its proper function will be affected.
The nitrogen in steel comes from the furnace charge. During smelting and casting, the liquid steel absorbs nitrogen from the furnace gas and the atmosphere. Nitrogen causes the quenching and deformation aging of carbon steel, which has a significant impact on the properties of carbon steel. Due to the aging effect of nitrogen, although the hardness and strength of steel have increased, the plasticity and toughness will decrease. Especially in the case of deformation aging, the plasticity and toughness decrease significantly. For ordinary low-alloy steels, the aging phenomenon is harmful, so nitrogen is considered a harmful element. However, when applied to some fine-grained steels, steels containing vanadium and niobium, and super stainless steels, nitride has the effect of strengthening and refining grains, so its beneficial effects have been discovered in recent years. In addition, as an alloying element, nitrogen is used in some stainless acid-resistant steels and in nitriding treatment. Nitriding treatment can enable machine parts to obtain excellent comprehensive mechanical properties and extend the service life of parts, so nitriding treatment is the best choice for tool steel. A method used to increase hardness.
The melting point of lead is very low. It is distributed in the grain boundaries in the form of fine metal particles with low melting point in steel, causing brittleness. It is a harmful element to general steel. However, when it is used to make lead free-cutting steel, because the lead will adhere to the surrounding sulfide, the molten lead will seep out during cutting, which will lubricate and break the chips, reducing the winding of the tool; moreover, while improving the cutting performance of the steel, it will It has little effect on the mechanical properties at room temperature.
Rare earth elements refer to the 15 lanthanide elements with atomic numbers from 57 to 71 in the periodic table, as well as a total of 17 elements including yttrium and scandium. Rare earth elements can improve the as-cast structure of steel, change the composition, shape, distribution and properties of inclusions in steel, thereby improving various properties of steel, such as toughness, weldability, cold working performance, and improving oxidation resistance and high-temperature strength. and creep strength, increasing corrosion resistance.
The hydrogen in the steel is brought in by the water-containing or rusty charge, or absorbed from the air containing water vapor. Hydrogen is very harmful to steel and can cause "hydrogen embrittlement", that is, when the allowable stress of the steel is lower than the allowable stress of the steel, after a certain operating time, the steel will suddenly break without any warning, causing catastrophic damage. Consequences; it will also cause a large number of fine cracks inside the steel - white spots, that is, smooth silvery white spots on the cross section of the steel, and hairline-like cracks on the longitudinal section after pickling. This kind of hairline significantly reduces the elongation, area shrinkage and impact toughness of the steel. This type of defect often occurs in alloy steel and is seriously harmful.
The solubility of oxygen in steel is very low, and almost all of it exists in the form of oxide inclusions, such as Fe0, AL2O3, MnO, CaO, MgO, etc. In addition, FeS, MnS, silicates, nitrides and phosphides also exist in steel. These inclusions destroy the continuity of the steel matrix and become sources of cracks under static and dynamic loads. The various states of these non-metallic inclusions affect the plasticity, toughness, fatigue strength and corrosion resistance of steel to varying degrees.

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