In addition to iron, pig iron also contains elements such as carbon, silicon, manganese, phosphorus and sulfur. These elements have a certain effect on the performance of pig iron.
Carbon (C): It exists in two forms in pig iron. One is free carbon (graphite), which is mainly found in cast pig iron, and the other is compound carbon (iron carbide), which is mainly present in steel pig iron. 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. This is the reason for poor cutting performance of steel 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, deoxidize, reduce the gas holes in the casting, improve the flowability of molten pig iron, and reduce the shrinkage of the casting, but it contains too much silicon. It also makes pig iron hard and brittle. Manganese (Mn): soluble in ferrite and cementite. When smelting pig iron in a blast furnace, the appropriate manganese content can improve the castability and cutting performance of 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 it is often high in some products. However, the presence of phosphorus makes iron harder and more brittle. Excellent pig iron should contain less phosphorus. Sometimes, in order to increase 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 synthesizes low melting point iron sulfide with iron to make pig iron hot brittle. It reduces the fluidity of molten iron, and 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% (except wheel pig iron). 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 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 impact on the properties of steel. In order to ensure the quality of steel, the chemical composition of various types of steel is strictly regulated in national standards.
Sulfur comes from steelmaking ore and fuel coke. It is a harmful element in steel. Sulfur exists in steel in the form of iron sulfide (FeS), and FeS and Fe form a low melting point (985 ° C) compound. The hot working temperature of steel is generally above 1150 ～ 1200 ℃. Therefore, when the steel is hot processed, the workpiece is cracked due to the premature melting of FeS compounds. This phenomenon is called "hot brittleness". The higher the sulfur content, the more severe the hot brittleness phenomenon, so the sulfur content in the steel must be controlled. 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. Phosphorus is brought into steel by ore. Generally speaking, phosphorus is also a harmful element. Phosphorus can increase the strength and hardness of steel, but cause plasticity and impact toughness to decrease significantly. Especially at low temperatures, it makes the steel significantly brittle, a phenomenon called "cold brittleness". Cold brittleness deteriorates the cold working and weldability of the steel. The higher the phosphorus content, the greater the cold brittleness. Therefore, the control of phosphorus content in steel is stricter. High quality steel: P <0.025%; high quality steel: P <0.04%; ordinary steel: P <0.085%. Manganese is added to steel as a deoxidizer during steelmaking. Because manganese can form high melting point (1600 ° C) MnS with sulfur, the harmful effect of sulfur is eliminated to a certain extent. Manganese has very good deoxidation ability, and can become MnO with FeO in the steel and enter the slag, thereby improving the quality of the steel, especially reducing the brittleness of the steel and increasing the strength and hardness of the 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 manganese content of high-quality carbon structural steel is 0.5% to 0.8%; while the structural steel with higher manganese content can reach 0.7% to 1.2%. Silicon is also an element added to steel as a deoxidizer during steelmaking. Silicon and FeO in molten steel can be formed by removing silicate slag with a lower density, so silicon is a beneficial element. Silicon dissolves in the ferrite in the steel to increase the strength and hardness of the steel and reduce the plasticity and toughness. The silicon content in the killed steel is usually 0.1% to 0.37%, and the boiling steel contains only 0.03% to 0.07%. Since the silicon content in steel generally does not exceed 0.5%, it has little effect on the properties of the steel. Oxygen is a harmful element in steel. It naturally enters the steel during the steelmaking process. Although it is necessary to add manganese, silicon, iron, and aluminum for deoxidation at the end of steelmaking, it is impossible to remove it. 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 ferrite has a low ability to dissolve nitrogen. When supersaturated nitrogen is dissolved in the steel, after a long period of time or subsequent heating at 200 to 300 ° C, nitrogen will precipitate in the form of nitrides, which will increase the hardness and strength of the steel, reduce plasticity, and cause aging. . Adding Al, Ti, or V to the molten steel for nitrogen fixation treatment to fix nitrogen in AlN, TiN, or VN can eliminate the aging tendency. Dissolving hydrogen in steel can cause defects such as hydrogen embrittlement and white spots. White spots are often found in rolled thick plates and large forgings, and round or oval white spots can be seen in the longitudinal section; on the cross section are elongated hairline cracks. With white spots in the forgings, sudden breaks will occur during use, resulting in unexpected accidents. Therefore, white spots are not allowed for steel used in chemical containers. The main reason for the white point cold cracking of hydrogen is that when the high temperature austenite is cooled to a lower temperature, the solubility of hydrogen in steel decreases sharply. When cooling is faster, the hydrogen atoms have no time to diffuse to the surface of the steel and escape, and some of the defects in the steel change from atomic hydrogen to molecular hydrogen. Under the condition that the hydrogen molecules cannot diffuse, a large pressure is generated in a local area. This pressure exceeds the strength of the steel and a crack is formed there, that is, a white point.
Alloy steel Role of elements in alloy steel The alloying elements added for alloying include silicon, manganese, chromium, nickel, molybdenum, tungsten, vanadium, titanium, niobium, boron, and aluminum. Their role in steel will now be explained separately.
① Improving the strength of the solid solution in the steel and the degree of cold work hardening will reduce the toughness and plasticity of the steel; ② Silicon can significantly improve the elasticity, yield and yield ratio of steel; ③ Corrosion resistance. High silicon cast iron with a silicon mass fraction of 15% -20% is a very good acid-resistant material. When silicon-containing steel is heated in an oxidizing atmosphere, a SiO2 film will also form on the surface, thereby improving the oxidation resistance of the steel at high temperatures. Disadvantages: The welding performance of steel is deteriorated. ① Manganese can improve the hardenability of steel. ② Manganese has a significant effect on improving the strength of low- and medium-carbon pearlitic steels. ③ The high-temperature transient strength of manganese on steel has been improved. ① When the manganese content is high, there is a more pronounced tempering brittleness phenomenon; ② Manganese has the effect of promoting grain growth, so manganese steel is more sensitive to overheating. It must be paid attention to in the heat treatment process. This disadvantage can be overcome by adding fine grain elements such as molybdenum, vanadium and titanium: ③ When the mass fraction of manganese exceeds 1%, the welding performance of steel will be deteriorated 3) The role of chromium in steel ① Chromium can improve the strength and hardness of steel. ② Chromium can improve the high temperature mechanical properties of steel. ③ make steel have good corrosion resistance and oxidation resistance ① It significantly increases the brittle transition temperature of steel ② Chromium can promote the tempering brittleness of steel. 4) The role of nickel in steel ① can increase 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; ③ Improve the workability and weldability of steel; ④ Nickel can improve the corrosion resistance of steel, not only resistant to acid, but also resistant to alkali and atmospheric corrosion. 5) The role of molybdenum in steel ① Molybdenum has a solid solution strengthening effect on ferrite. ②Improve the thermal strength of steel ③ the role of resistance to hydrogen erosion. ④ Improve the hardenability of steel. The main adverse effect of molybdenum is its tendency to graphitize low alloy molybdenum steel. 6) The role of tungsten in steel ② Improve the high temperature strength of steel. ③ Improve the hydrogen resistance of steel. ④ is to make the steel hot. Therefore, tungsten is the main alloying element in high-speed tool steel. 7) The role of vanadium in steel ②Vanadium can significantly improve the welding performance of ordinary low carbon low alloy steel. 8) The role of titanium in steel ① Titanium can improve the thermal strength of steel, increase the creep resistance of steel and high-temperature durability; ② And can improve the stability of steel in high temperature and high pressure hydrogen. It makes the stability of steel to hydrogen under high pressure as high as 600 ℃. In pearlite low alloy steel, titanium can prevent the graphitization of molybdenum steel at high temperature. Therefore, titanium is one of the important alloying elements in hot-strength steels used in high-temperature components of boilers. 9) The role of niobium in steel ① Niobium has a binding force with carbon, nitrogen, and oxygen, and forms a corresponding stable compound with it, so it can refine the grains and reduce the overheat sensitivity and temper brittleness of steel. ③ Niobium can improve the thermal strength of steel 10) The role of boron in steel ① Improve the hardenability of steel. ② Improve the high temperature strength of steel. Strengthen the role of grain boundaries. 11) The role of aluminum in steel ① Used as a deoxidizing and nitrogen-fixing agent during steelmaking to refine the grains, suppress the aging of low-carbon steel, improve the toughness of steel at low temperature, especially reduce the brittle transition temperature of steel; ② Improve the oxidation resistance of steel. There have been many studies on the oxidation resistance of iron and aluminum alloys; 4% AI can change the structure of the scale, and the addition of 6% A1 can make the steel have oxidation resistance below 980C. When aluminum and chromium are used in combination, their oxidation resistance is greatly improved. For example, alloys containing 50% to 55% iron, 30% to 35% chromium, and 10% to 15% aluminum still have fairly good oxidation resistance at a high temperature of 1 400C. Because of this role of aluminum, aluminum has often been added to heat-resistant steel as an alloying element in recent years. ③ In addition, aluminum can also improve the corrosion resistance to hydrogen sulfide and V2O5. ① If too much aluminum is used during deoxidation, the graphitization tendency of steel will be promoted. ② When aluminum is high, its high temperature strength and toughness are low.
Nanjing Kirin Scientific Instrument Group Co., Ltd.