Mechanical properties of low carbon steel coils
Mild steel is carbon steel with a carbon content of less than 0.25%. It is also called mild steel because of its low strength, low hardness and softness. It includes most ordinary carbon structural steel and some high-quality carbon structural steel, most of which are used for engineering structural parts without heat treatment, and some are used for mechanical parts that require wear resistance after carburizing and other heat treatment.
Low carbon steel coil characteristics
Mild steel has limited use due to its lower strength. Appropriately increasing the manganese content in carbon steel and adding trace amounts of vanadium, titanium, niobium and other alloying elements can greatly improve the strength of the steel. If the carbon content in the steel is reduced and a small amount of aluminum, a small amount of boron and carbide-forming elements are added, the ultra-low carbon bainite group can be obtained with high strength and good ductility and toughness.
Low carbon steel has a large aging tendency, including quenching aging (when the steel is rapidly cooled from a high temperature, iron carbonitride can also be slowly formed at room temperature, so the strength and hardness of the steel increase, while the plasticity and toughness decrease. ) tendency, as well as deformation aging (low carbon steel is deformed to produce a large number of dislocations, and the resulting old Coriolis air mass is the Coriolis air mass, which will increase the strength and hardness of the steel and reduce the plasticity and toughness). Low carbon steel will age even if it is not quenched and air cooled. Deformation aging is more harmful to the plasticity and toughness of low carbon steel than quenching aging, and there are two obvious upper and lower yield points on the tensile curve of low carbon steel. From the upper yield point to the end of the yield extension, a Lüders band appears on the surface of the specimen. Many stamping parts are often scrapped because of this. There are two ways to prevent it. A high pre-deformation method, the pre-deformed steel should not be placed for too long before stamping. The other is to add aluminum or titanium to the steel to form a stable compound with nitrogen.
Types of low carbon steel coils
Low carbon steel is generally rolled into angle steel, channel steel, I-beam, steel pipe, steel strip or steel plate, and is used to make various building components, containers, boxes, furnace bodies and agricultural machinery. High-quality low carbon steel is rolled into sheets to make deep-drawn products such as car cabs and engine covers; it is also rolled into bars to make mechanical parts with low strength requirements. Low carbon steel is generally not heat treated before use, and carbon content above 0.15% is carburized or cyanided. It is used for shafts, bushings, sprockets and other parts that require high surface temperature and good wear resistance.
Mechanical properties of low carbon steel coil
Mild steel is a ductile material. The stress-strain curve during stretching is mainly divided into four stages: elastic stage, yield stage, strengthening stage, and local deformation stage. In the local deformation stage, there are obvious yielding and necking phenomena. At the beginning, it is the elastic stage, which completely obeys Hooke's law and rises along a straight line. After the proportional limit, the deformation accelerates, but there is no obvious yield stage.
Medium Carbon Steel Coil Usage and Welding
Medium carbon steel coil refers to carbon steel with a carbon content of 0.25% to 0.60%, which has good hot working and cutting performance, but poor welding performance.
Use of medium carbon steel coil
Medium carbon steel is mainly used to manufacture high-strength moving parts, such as air compressors, pump pistons, steam turbine impellers, heavy machinery shafts, worms, gears, etc., wear-resistant parts, crankshafts, machine tools Spindles, rollers, fitter tools and more.
Application in medium carbon steel coil welding
(1) Preheating is beneficial to reduce the maximum hardness of the heat-affected zone of medium carbon steel and prevent cold cracks. This is the main technological measure for welding medium carbon steel. Preheating can also improve joint plasticity and reduce residual stress after welding. Usually, the preheating temperature of 35 and 45 steel is 150~250℃. If the carbon content is higher or the crack tendency is large due to the large thickness and stiffness, the preheating temperature can be increased to 250~400℃. If the weldment is too large and the overall preheating is difficult, local preheating can be performed. The heating range of local preheating is 150~200mm on both sides of the weld.
(2) When the conditions of the electrode permit, the alkaline electrode is preferred.
(3) Groove form The weldment should be opened into a U-shaped groove for welding as much as possible. If it is a casting defect, the shape of the groove excavated by the shovel should be smooth. The purpose is to reduce the proportion of the base metal melting into the weld metal, so as to reduce the carbon content in the weld and prevent cracks.
⑷Welding process parameters Since the proportion of the base metal melted into the first layer of weld metal is up to about 30%, when the first layer of weld is welded, low current and slow welding speed should be used as much as possible to reduce the melting of the base metal. deep.
⑸ Post-weld heat treatment It is best to perform stress relief heat treatment on the weldment immediately after welding, especially for the weldment with large thickness, high-rigidity structural parts and those that work under severe conditions (dynamic load or impact load). The tempering temperature for stress relief is 600~650℃.
If stress relief heat treatment cannot be carried out after welding, post heat treatment should be carried out immediately.
Introduction to the advantages, disadvantages and performance of high carbon steel coils
High carbon steel coils are often called tool steel coils, with carbon content ranging from 0.60% to 1.70%, and can be quenched and tempered. Hammers, crowbars, etc. are made of steel with a carbon content of 0.75%; cutting tools such as drills, taps, reamers, etc. are made of steel with a carbon content of 0.90% to 1.00%.
High carbon steel coil advantages:
1. High hardness (HRC60-65) and better wear resistance can be obtained after heat treatment.
2. The hardness is moderate in the annealed state and has good machinability.
3. The raw materials are easily available and the production cost is low.
High carbon steel coil disadvantages:
1. Poor thermal hardness. When the working temperature of the tool is greater than 200 °C, its hardness and wear resistance drop sharply.
2. Low hardenability. The diameter of complete hardening during water quenching is generally only 15-18mm; the maximum diameter or thickness (95% martensite) of complete hardening during oil quenching is only about 6mm, and it is easy to deform and crack.
Introduction to the characteristics of high carbon steel coils
Carbon tool steel is a high carbon steel that basically does not add alloying elements, and it is also a steel with low cost, good cold and hot workability, and a wide range of use among tool steels. Its carbon content is 0.65 to 1.35%, and it is a steel specially used for making tools. The density of high carbon steel is 7.81g/cm³. Can be used for the production of fishing gear. Due to the high carbon content, cracks are prone to occur during water quenching, so double-liquid quenching (water quenching + oil cooling) is often used, and oil quenching is often used for small-section parts. This type of steel is generally tempered or normalized at medium temperature after quenching or used in a surface quenched state. Mainly used in the manufacture of springs and wear-resistant parts.
Characteristics of high carbon steel coil
The hardness and strength of high carbon steel mainly depend on the amount of solid solution carbon in the steel, and increase with the increase of the amount of solid solution carbon. When the solid solution carbon content exceeds 0.6%, the hardness does not increase after quenching, but the amount of excess carbides increases, the wear resistance of the steel increases slightly, while the plasticity, toughness and elasticity decrease. Generally, high carbon steel can be produced by electric furnace, open hearth and oxygen converter. When high quality or special quality is required, electric furnace smelting plus vacuum self-consumption or electroslag remelting can be used. When smelting, strictly control the chemical composition, especially the content of sulfur and phosphorus. In order to reduce segregation and improve isotropic properties, ingots can be subjected to high temperature diffusion annealing (especially important for tool steels). During hot working, the stop forging (rolling) temperature of hypereutectoid steel is required to be low (about 800 ℃), and the precipitation of coarse network carbides should be avoided after forging and rolling, and attention should be paid to slow cooling below 700 ℃ to prevent thermal stress. cause cracks. Prevent surface decarburization during heat treatment or hot working (especially important for spring steel). There should be enough compression ratio during hot working to ensure the quality and performance of the steel.
How to choose low carbon steel medium carbon steel high carbon steel
The annealed structure of low carbon steel is ferrite and a small amount of pearlite, which has low strength and hardness, and good plasticity and toughness. Therefore, its cold formability is good, and cold forming can be carried out by methods such as hemming, bending, and stamping. This steel also has good weldability. Low carbon steel with very low carbon content has low hardness and poor machinability, and normalizing treatment can improve its machinability. Low carbon steel generally refers to steel with carbon content between 0.10 and 0.25%. This kind of steel has low hardness and good plasticity, which is convenient for cold plastic deformation forming process, welding and cutting. It is often used in the manufacture of chains, rivets, bolts, shafts Wait. Some large steel mills or steel trading companies are actively working closely with large rigging companies to jointly develop a series of high-tech, high-precision and high-quality rigging products. Different steel grades are often selected according to the conditions of use and the strength and toughness of the steel. For example, for the manufacture of springs or spring-loaded parts that are less stressed, 65 steel with a lower carbon content can be selected.
The strength and hardness of medium carbon steel coils are higher than those of low carbon steel, while the plasticity and toughness are lower than those of low carbon steel. Hot-rolled and cold-drawn materials can be used directly without heat treatment, or they can be used after heat treatment. The quenched and tempered medium carbon steel has good comprehensive mechanical properties. The highest hardness that can be achieved is about HRC55 (HB538), and σb is 600~1100MPa. Therefore, among various uses of medium strength level, medium carbon steel is the most widely used, in addition to being used as a building material, it is also widely used in the manufacture of various mechanical parts.
The high carbon steel coil has high strength and hardness, high elastic limit and fatigue limit (especially notch fatigue limit) after proper heat treatment or cold drawing hardening, and the cutting performance is acceptable, but the welding performance and cold plastic deformation ability Difference.
