In addition to iron, pig iron also contains elements such as carbon, silicon, manganese, phosphorus and sulfur. These elements have certain influence on the properties of pig iron.
Carbon (C): exists in two forms in pig iron, one is free carbon (graphite), which is mainly found in casting pig iron, and the other is compound carbon (iron carbide), which is mainly found 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 pig iron difficult to cut, 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, can reduce the air holes of castings, can 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 impurity 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, and the phosphorus content of excellent pig iron should be small. Sometimes, in order to increase the fluidity, the phosphorus content can reach 1.2%.
Sulfur (S): It is a harmful element in pig iron. It promotes the combination of iron and carbon, makes iron hard and brittle, and combines with iron to form low-melting iron sulfide, which makes pig iron hot and brittle. It reduces the fluidity of molten iron. Gu Pig iron with high sulfur content is not suitable for casting fine parts. The content of sulfur in cast iron shall not exceed 0.06% (except for wheel pig iron).
steel
The role of elements in steel
The influence of frequently existing impurity elements on the properties of steel
In addition to carbon, steel also contains a small amount 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 change the quality of the steel, but are brought in from the ore and the smelting process, so they are called impurity elements. These impurities have a certain influence on the performance of steel. In order 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 ores 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 ℃, 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. Excellent steel: S＜0.02%～0.03%; Excellent steel: S＜0.03%～0.045%; Ordinary steel: S＜0.055%～0.7% or less.
2) Phosphorus
Phosphorus is brought into steel by ore. Generally speaking, 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. Excellent steel: P＜0.025%; Excellent 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 °C) with sulfur, the harmful effect of sulfur is removed to a certain extent. Manganese has good deoxidation ability, and can form MNO into slag with FEO in steel, thereby changing 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 be removed as silicate slag with less density, 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 silicon content in steel is usually 0.1% to 0.37%, and the boiling steel only contains 0.03% to 0.07%. 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 it 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.
Chapter 4: Influence of various chemical components in common metal materials on properties 2
6) The ability of nitrogen 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 increase, the plasticity will decrease, 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 remove the aging tendency.
7) Hydrogen
Dissolved hydrogen in steel will cause hydrogen embrittlement and 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 that the solubility of hydrogen in steel decreases sharply when the high temperature austenite is cooled to a lower temperature. When the cooling is fast, 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 becomes the hydrogen in the molecular state. The hydrogen molecules are not able to diffuse under the condition that a large pressure is generated in the local area, which exceeds the strength limit of the steel and forms cracks, that is, white spots.
combine
gold
The role of elements in alloy steel
The alloying elements added for alloying are commonly used silicon, manganese, chromium, nickel, molybdenum, tungsten, vanadium, titanium, niobium, boron, aluminum, etc. Now respectively explain their role in steel.
1) Silicon
①Improve the strength of the solid solution in the steel and the degree of cold work hardening to reduce the toughness and plasticity of the steel;
②Silicon can significantly improve the elastic grade limit, yield grade limit and yield-strength ratio of steel;
③Corrosion 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.
Disadvantage: Deteriorating the connection performance 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:
①When 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. Attention should be paid to the heat treatment process. This disadvantage can be overcome by adding grain refining elements such as molybdenum, vanadium, titanium, etc.:
③When the mass fraction of manganese exceeds 1%, the connection performance of steel will be 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.
③Make the steel have good corrosion resistance
④ Prevent graphitization
⑤Improve 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;
②Ni can reduce the brittle transition temperature of steel, which can improve the low temperature toughness of steel;
③Change the workability and weldability of steel;
④Nickel can improve the corrosion resistance of steel, it is not resistant to acid, but also resistant to 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
③Anti-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.
③Improve the hydrogen resistance of steel.
④It 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 change the connection properties of ordinary low-carbon and low-alloy steels.
8) The role of titanium in steel
①Titanium can change the thermal strength of steel, improve the creep resistance and high temperature long-term 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. In pearlitic low alloy steel, titanium can prevent the graphitization of molybdenum steel at high temperature. 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 strong bonding force with carbon, nitrogen and oxygen, and forms corresponding very stable compounds, which can refine grains and reduce the overheating sensitivity and temper brittleness of steel.
②It has good 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 deoxidation and nitrogen determination agent in steelmaking, refines grains, controls the aging of low carbon steel, changes the toughness of steel at low temperature, especially reduces the brittle transition temperature of steel;
②In 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.
②When the aluminum content is high, its high temperature strength and toughness are low.
