1. Pig iron:
In addition to iron, pig iron also contains elements such as carbon, silicon, manganese, phosphorus, and sulfur. These elements have a certain influence on the properties of pig iron.
Carbon (C): exists in two forms in pig iron, one is free carbon (graphite), which mainly exists in casting pig iron, and the other is compound carbon (iron carbide), which mainly exists in steelmaking pig iron, carbonized Iron is hard and brittle, with low plasticity. Appropriate content can improve the strength and hardness of pig iron. Too much content makes it difficult to cut pig iron, which is the reason for the poor cutting performance of steel-making pig iron. Graphite is soft and low in strength, and its presence can increase the castability of pig iron.
Silicon (Si): It can promote the separation of carbon contained in pig iron into graphite, can deoxidize, reduce air holes in castings, improve the fluidity of molten pig iron, and reduce the shrinkage of castings, but it contains too much silicon. It also makes pig iron hard and brittle.
Manganese (Mn): soluble in ferrite and cementite. When the pig iron is smelted in the blast furnace, the manganese content is appropriate, which can improve the casting performance and cutting performance of the pig iron. In the blast furnace, manganese can also form manganese sulfide with harmful impurities sulfur and enter the slag.
Phosphorus (P): It is a harmful element, but phosphorus can increase the fluidity of molten iron. This is because sulfur reduces the melting point of pig iron, so some products often contain high phosphorus content. However, the presence of phosphorus increases the hardness and brittleness of iron. Excellent pig iron should contain less phosphorus. Sometimes, to increase fluidity, the phosphorus content can reach 1.2%.
Sulfur (S): It is a harmful element in pig iron, which promotes the combination of iron and carbon, makes iron hard and brittle, and combines with iron to form low-melting iron sulfide, making pig iron hot and brittle. To reduce the fluidity of molten iron, pig iron with high sulfur content is not suitable for casting small parts. The content of sulfur in cast pig iron shall not exceed 0.06% at most (except for wheel pig iron).
2. Steel:
In addition to carbon, steel also contains a small number of elements such as manganese (Mn), silicon (Si), sulfur (S), phosphorus (P), oxygen (O), nitrogen (N), and hydrogen (H). These elements are not intentionally added to improve the quality of steel but are brought in by the ore and smelting process, so they are called impurity elements. These impurities have a certain influence on the performance of steel. To ensure the quality of steel, strict regulations are made on the chemical composition of various types of steel in national standards.
1) Sulfur
Sulfur comes from ore and fuel coke for steelmaking. It is a harmful element in steel. Sulfur exists in steel in the form of iron sulfide (FeS), and FeS and Fe form low melting point (985°C) compounds. The hot working temperature of steel is generally above 1150-1200 °C, so when the steel is hot worked, the workpiece is cracked due to the premature melting of the FeS compound, which is called “hot embrittlement”. The higher the sulfur content, the more serious the hot embrittlement phenomenon, so it is necessary to control the sulfur content in the steel. High-grade high-quality steel: S < 0.02% ~ 0.03%; high-quality steel: S < 0.03% ~ 0.045%; ordinary steel: S < 0.055% ~ 0.7% or less.
2) Phosphorus
Phosphorus is brought into steel by ore, and it is generally said that 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 deteriorates the cold working and weldability of steel. The higher the phosphorus content, the greater the cold brittleness, so the control of phosphorus content in steel is stricter. High-quality high-quality steel: P < 0.025%; high-quality steel: P < 0.04%; ordinary steel: P < 0.085%.
3) Manganese
Manganese is added to steel as a deoxidizer during steelmaking. Since manganese can form MnS with a high melting point (1600℃) with sulfur, the harmful effect of sulfur is eliminated to a certain extent. Manganese has good deoxidation ability and can form MnO into slag with FeO in steel, thereby improving the quality of steel, especially reducing the brittleness of steel and improving the strength and hardness of steel. Therefore, manganese is a beneficial element in steel. It is generally believed that when the manganese content in steel is below 0.5% to 0.8%, manganese is regarded as a permanent impurity. The technical conditions stipulate that the normal content of manganese in high-quality carbon structural steel is 0.5% to 0.8%; in structural steel with higher manganese content, the amount can reach 0.7% to 1.2%.
4) Silicon
Silicon is also an element added to steel as a deoxidizer during steelmaking. Silicon and FeO in molten steel can form silicate slag with lower density and be removed, so silicon is a beneficial element. Silicon dissolves in ferrite in steel to increase the strength and hardness of the steel, and reduce the plasticity and toughness. The content of silicon in killed steel is usually 0.1% to 0.37% and only 0.03% to 0.07% in boiling steel. Since the silicon content in steel generally does not exceed 0.5%, it has little effect on the performance of steel.
5) Oxygen
Oxygen is a harmful element in steel. It enters the steel naturally during the steelmaking process. Although manganese, silicon, iron, and aluminum are added for deoxidation at the end of the steelmaking process, it is impossible to remove them completely. Oxygen in the form of FeO, MnO, SiO2, Al2O3, and other inclusions in the steel reduces the strength and plasticity of the steel. In particular, it has a serious impact on fatigue strength and impact toughness.
6) Nitrogen
The ability of ferrite to dissolve nitrogen is very low. When supersaturated nitrogen is dissolved in the steel, the precipitation of nitrogen in the form of nitrides will occur after being placed for a long time or heated at 200-300 °C, and the hardness and strength of the steel will be increased, the plasticity will be decreased, and aging will occur. Add Al, Ti, or V to the molten steel for nitrogen fixation, so that nitrogen is fixed in AlN, TiN, or VN, which can eliminate the aging tendency.
7) Hydrogen
Dissolved hydrogen in steel can cause defects such as hydrogen embrittlement and a white spot of steel. White spots are often found in rolled thick plates and large forgings. Round or oval white spots can be seen in the longitudinal section; slender hairline cracks can be seen in the cross-section. There are white spots in the forgings, which will suddenly break during use, resulting in unexpected accidents. Therefore, the steel used for chemical containers is not allowed to have white spots. The main reason why hydrogen produces white spot cold cracking is because the solubility of hydrogen in steel decreases sharply when the high-temperature austenite is cooled to a lower temperature. When the cooling is faster, the hydrogen atoms do not have time to diffuse to the surface of the steel and escape, and just at some defects in the steel, the hydrogen in the atomic state changes into the hydrogen in the molecular state. Under the condition that the hydrogen molecules cannot diffuse, a large pressure is generated in the local area, which exceeds the strength limit of the steel and forms cracks, that is, white spots.
3. Alloy steel:
The most commonly used alloying elements for alloying are silicon, manganese, chromium, nickel, molybdenum, tungsten, vanadium, titanium, niobium, boron, aluminum, etc. Now respectively explain their role in steel.
1) Silicon
①Improving the strength of the solid solution and the degree of cold work hardening in the steel reduces the toughness and plasticity of the steel;
②Silicon can significantly improve the elastic limit, yield limit, and yield ratio of steel;
③ Corrosion resistance. The mass fraction of silicon is 15% to 20% of high silicon cast iron, which is a good acid-resistant material. When the steel containing silicon is heated in an oxidizing atmosphere, a layer of SiO2 film will also be formed on the surface, thereby improving the oxidation resistance of the steel at high temperatures.
Disadvantages: Deteriorating the weldability of steel.
2) Manganese
①Manganese can improve the hardenability of steel.
②Manganese has a significant effect on improving the strength of low-carbon and medium-carbon pearlitic steels.
③Manganese improves the high-temperature instantaneous strength of steel.
shortcoming:
① When the manganese content is high, there is a more obvious phenomenon of temper brittleness;
②Manganese has the effect of promoting grain growth, so manganese steel is more sensitive to overheating, and attention must be paid to the heat treatment process. This disadvantage can be overcome by adding grain refining elements such as molybdenum, vanadium, titanium, etc.:
③ When the mass fraction of manganese exceeds 1%, the welding performance of the steel will have deteriorated
3) The role of chromium in steel
①Chromium can improve the strength and hardness of steel.
②Chromium can improve the high-temperature mechanical properties of steel.
③ Make the steel have good corrosion resistance and oxidation resistance
④ Prevent graphitization
⑤ Improve hardenability.
Shortcoming:
① is to significantly increase the brittle transition temperature of steel
②Chromium can promote the temper brittleness of steel.
4) The role of nickel in steel
①It can improve the strength of steel without significantly reducing its toughness;
② Nickel can reduce the brittle transition temperature of steel, which can improve the low-temperature toughness of steel;
③Improve the workability and weldability of steel;
④Nickel can improve the corrosion resistance of steel, not only acid resistance but also alkali and atmospheric corrosion.
5) The role of molybdenum in steel
①Molybdenum has a solid solution strengthening effect on ferrite.
②Improve the thermal strength of steel
③ The role of resistance to hydrogen corrosion.
④ Improve the hardenability of steel.
Shortcoming:
The main adverse effect of molybdenum is its tendency to graphitize low-alloy molybdenum steels.
6) The role of tungsten in steel
①Increase strength
②Improve the high-temperature strength of steel.
③ Improve the hydrogen resistance of steel.
④ is to make the steel have hot hardness. Therefore, tungsten is the main alloying element in high-speed tool steel.
7) The role of vanadium in steel
① Thermal strength.
②Vanadium can significantly improve the welding performance of ordinary low carbon and low alloy steel.
8) The role of titanium in steel
①Titanium can improve the thermal strength of steel, improve the creep resistance and high temperature lasting strength of steel;
②And can improve the stability of steel in high temperature and high-pressure hydrogen. The stability of the steel to hydrogen under high pressure is as high as 600 ℃ or more, and in the pearlitic low alloy steel, titanium can prevent the graphitization of molybdenum steel at high temperatures. Therefore, titanium is one of the important alloying elements in heat-strength steels used in high-temperature components of boilers.
9) The role of niobium in steel
①Niobium has a strong bonding force with carbon, nitrogen, and oxygen, and forms corresponding extremely stable compounds, which can refine grains and reduce the overheating sensitivity and temper brittleness of steel.
②It has excellent hydrogen resistance.
③Niobium can improve the thermal strength of steel
10) The role of boron in steel
①Improve the hardenability of steel.
②Improve the high-temperature strength of steel. Strengthen the role of grain boundaries.
11) The role of aluminum in steel
①It is used as a deoxidation and nitrogen determination agent in steelmaking, refines grains, inhibits the aging of low carbon steel, improves the toughness of steel at low temperature, and especially reduces the brittle transition temperature of steel;
②Improve the oxidation resistance of steel. Many studies have been done on the oxidation resistance of iron and aluminum alloys; 4% AI can change the structure of the oxide scale, and adding 6% A1 can make the steel have oxidation resistance below 980C. When aluminum and chromium are used together, their oxidation resistance is greatly improved. For example, an alloy containing 50% to 55% of iron, 30% to 35% of chromium, and 10% to 15% of aluminum still has quite good oxidation resistance at a high temperature of 1400C. Due to this effect of aluminum, in recent years, aluminum is often added to heat-resistant steel as an alloying element.
③ In addition, aluminum can improve the corrosion resistance of hydrogen sulfide and V2O5.
Shortcoming:
①If too much aluminum is used during deoxidation, it will promote the graphitization tendency of steel.
② When the aluminum content is high, its high-temperature strength and toughness are low.
Similar Articles:
- 15 residual elements in steel
- Characteristics of 18 Cast Alloying Elements
- Analysis of Commonly Used Casting Alloy Elements
Recent Articles：
- Casting technical requirements and analysis of common problems of steel castings
- What is the role of Mo in high manganese steel casting production?
- What effect does carbon have on cast steel and iron during smelting?
- Prevention and Analysis of Stainless Steel Casting Defects
- Causes of cracks in steel castings