Carbon steel rods: the basic skeleton of industrial manufacturing

In the huge system of modern industrial manufacturing, carbon steel bars, as a basic material, support the development of many key areas, just like the bones of the human body. It is based on iron and carbon is the main alloying element (the carbon content is usually between 0.05% and 2.1%). With its unique properties, it plays an irreplaceable role in mechanical processing, construction, automobile manufacturing and other industries.
1. Classification system of carbon steel bars
There are many ways to classify carbon steel bars, and different classification bases correspond to different performance characteristics and application scenarios.
Classification by carbon content
Low carbon steel bars: The carbon content is generally less than 0.25%, with good plasticity and toughness, and its texture is relatively soft, which is easy to cold process, such as bending, stamping and other operations. Common low carbon steel bar materials include Q195, Q215, etc. In daily life, many of the various types of thin plate stamping parts we commonly see, such as some small metal containers, shells, etc., are made of low carbon steel bars. In the field of construction, low-carbon steel bars are often used to make some parts that have relatively low strength requirements but require good formability, such as connectors in building structures, some light brackets, etc.
Medium carbon steel bars: The carbon content is between 0.25% and 0.6%. This type of carbon steel bar combines the advantages of strength and toughness and has good comprehensive mechanical properties. For example, 45 steel is a very representative type of medium carbon steel bar and is widely used in the field of mechanical manufacturing. Like common shaft parts, since they need to withstand certain torque and bending moments during work, they have high requirements for the strength and toughness of the material. After proper heat treatment, 45 steel bars can meet these requirements well. In automobile manufacturing, some key parts of the engine, such as crankshafts, are also often made of medium carbon steel bars.
High carbon steel bars: The carbon content is higher than 0.6%, and its hardness and strength are higher, but the plasticity and toughness are relatively poor. After quenching and tempering, high carbon steel bars can obtain extremely high hardness and wear resistance, which is suitable for manufacturing tools and parts with strict requirements on hardness and wear resistance. For example, the files widely used in the metal processing industry need to have extremely high hardness to cut other metal materials, and high carbon steel bars are ideal materials for making files. In addition, some springs also use high carbon steel bars, which can produce elastic deformation when subjected to external forces and return to their original state after unloading due to their high strength and good elastic properties.
Classification by production process
Hot-rolled carbon steel bars: After the steel billet is heated to a high temperature (generally 1000℃ - 1200℃), it is rolled through a rolling mill. During the hot rolling process, the internal structure of the steel undergoes dynamic recrystallization, which refines the grains and improves the performance of the steel. Hot-rolled carbon steel bars have high production efficiency and relatively low cost. The surface generally presents the color of oxide scale and has relatively low dimensional accuracy, but for some application scenarios that do not require high surface quality and dimensional accuracy, it can fully meet the needs. In the construction industry, many of the construction steel bars used in large quantities are hot-rolled carbon steel bars, which are used to construct the load-bearing structure of the building. In the manufacturing of basic structural parts of some large machinery, such as the bed and base of large machine tools, hot-rolled carbon steel bars are also widely used due to their good comprehensive performance and cost advantages.
Cold-rolled carbon steel bars: Hot-rolled carbon steel bars are used as raw materials and rolled at room temperature. During the cold rolling process, the steel undergoes obvious work hardening, which significantly improves its strength and hardness. At the same time, the surface quality and dimensional accuracy are also greatly improved, the surface is smooth, and the dimensional tolerance can be controlled within a small range. Cold-rolled carbon steel bars are often used to manufacture parts with extremely high requirements for surface quality and dimensional accuracy, such as shaft parts in precision machinery and precision gears in automobile engines. Since the cold rolling process increases the processing cost, the price of cold-rolled carbon steel bars is higher than that of hot-rolled carbon steel bars.
Cold-drawn carbon steel bars: By stretching the hot-rolled carbon steel bars through a die at room temperature, the cross-sectional size is reduced and the length is increased. The cold drawing process also causes work hardening of the steel, improves its strength and hardness, and the cold-drawn carbon steel bars have good surface quality, high dimensional accuracy, and good straightness. It is suitable for manufacturing some parts that require high precision and good surface quality, such as rollers in textile machinery, precision shaft parts in some instruments and meters, etc. Compared with cold-rolled carbon steel bars, cold-drawn carbon steel bars focus more on stretching in processing technology, and have unique advantages for the processing of some slender shaft parts.
Classification by shape
Round bar: The cross section is round, which is the most common shape of carbon steel bars. Round bars are widely used in various fields. Because of their symmetrical shape, they are evenly stressed when subjected to loads such as axial force and torque, and can give full play to the performance of the material. In mechanical manufacturing, most shaft parts use round bars as raw materials, and shafts that meet different needs are manufactured through turning, grinding and other processing processes. In the construction industry, some rods used for support and tie-in also often use round bars, such as tie rods in some light steel structures.
Square bar: The cross section is square. Square bars have good stability and load-bearing capacity, and are suitable for the manufacture of some structural parts and parts with special requirements for shape. In the field of architectural decoration, some carbon steel materials used to make door and window frames and decorative lines often use square bars. In mechanical manufacturing, some square pillars, connectors, etc. will also be processed with square bars, and their square cross-sections can be better connected and assembled with other parts.
Hexagonal bar: The cross-section is a regular hexagon. The special shape of the hexagonal bar gives it unique advantages in some specific application scenarios. For example, when manufacturing fasteners such as nuts and bolts, hexagonal bars are ideal raw materials. Through cold heading, cutting and other processing processes, standard hexagonal nuts and bolt heads can be efficiently produced. In addition, in some occasions where rotation operations are required and relative rotation of parts is prevented, such as the coupling connection parts in some mechanical transmission systems, the use of hexagonal bars can provide better anti-rotation effects.
2. Performance Analysis
The performance of carbon steel bars is significantly affected by the carbon content and processing technology, which determine its applicability in different fields.
Mechanical properties
Strength: As the carbon content increases, the strength of carbon steel bars shows an upward trend. Low-carbon steel bars have relatively low strength due to their low carbon content, but they have good plasticity and can undergo large deformation without breaking when subjected to a certain external force. Medium carbon steel bars have achieved a good balance between strength and plasticity, with moderate strength, which can meet the requirements of many mechanical parts and structural parts to withstand various loads under normal working conditions. The high strength of high carbon steel bars enables them to withstand greater external forces, and are suitable for manufacturing parts that withstand heavy loads and high stresses. Processing technology also has an important influence on strength. The hot rolling process improves the strength to a certain extent by improving the internal structure; cold rolling and cold drawing significantly improve the strength of carbon steel bars due to work hardening.
Hardness: There is a positive correlation between carbon content and hardness. High carbon steel bars have higher hardness due to their higher carbon content. This high hardness makes high carbon steel bars have irreplaceable advantages in the manufacture of tools and parts that require wear resistance. Low carbon steel bars have lower hardness, which is convenient for cutting, drilling and other processing operations. Processing technology will also change the hardness. The carbon steel bars after cold rolling and cold drawing have significantly higher hardness due to work hardening, while the hardness of hot rolled carbon steel bars is relatively moderate, which is conducive to subsequent mechanical processing.
Toughness: Toughness is an important indicator to measure the ability of a material to resist fracture under impact load. Low carbon steel bars have good toughness and can absorb more energy without brittle fracture when impacted. As the carbon content increases, the toughness of carbon steel bars gradually decreases, and high carbon steel bars have relatively poor toughness and are more likely to break when impacted. Therefore, in some application scenarios that may be subjected to impact loads, such as earthquake-resistant components in building structures and car bumpers, low carbon steel or medium carbon steel bars are usually preferred, and their toughness is further optimized through appropriate heat treatment processes.
Physical properties
Density: The density of carbon steel is generally around 7.85g/cm³, which makes the carbon steel bar have a certain strength and hardness while having a relatively moderate mass. Compared with some metal materials with higher density, carbon steel bars can reduce the weight of the overall structure and reduce transportation and installation costs while meeting the structural strength requirements. For example, in some non-critical structural parts in the aerospace field, carbon steel bars are used to replace metal materials with higher density to achieve the purpose of weight reduction while meeting the performance requirements. In the field of construction, the reasonable selection of carbon steel bars can also help control the deadweight of the building on the basis of ensuring the structural safety of the building.
Thermal expansion coefficient: The thermal expansion coefficient of carbon steel is about 1.2×10⁻⁵/℃, which means that when the temperature changes, the carbon steel rod will have a certain thermal expansion and contraction phenomenon. In some occasions where the dimensional accuracy is extremely high and the working environment temperature changes greatly, such as parts in precision instruments, connectors in high-temperature industrial equipment, etc., it is necessary to fully consider the thermal expansion characteristics of carbon steel rods, and compensate for the dimensional changes caused by thermal expansion and contraction through reasonable structural design or selection of appropriate material combinations to ensure the normal operation and accuracy requirements of the equipment.
Electrical conductivity and thermal conductivity: The electrical conductivity and thermal conductivity of carbon steel are relatively good, but lower than those of pure metals such as copper and aluminum. Carbon steel rods can play a role in some occasions where there are certain requirements for electrical conductivity and thermal conductivity but do not require extremely high performance. For example, in the grounding system of some electrical equipment, carbon steel rods are used as grounding electrodes, and their certain electrical conductivity is used to introduce current into the earth. In some heat exchange equipment, such as small radiators, carbon steel rods can be used as part of the heat dissipation element to transfer heat. Although its thermal conductivity is not as good as that of special thermal conductive materials, it has certain application value under the comprehensive consideration of cost and performance.
Chemical properties
Corrosion resistance: Carbon steel rods are prone to chemical reactions with oxygen and moisture in the air in general atmospheric environments, resulting in rust. This is because the iron element in carbon steel will undergo electrochemical corrosion in humid air to form rust (the main component is Fe₂O₃). The rust is loose and porous and cannot prevent further corrosion, resulting in a gradual decline in the performance of carbon steel rods. In order to improve the corrosion resistance of carbon steel rods, some protective measures are usually adopted, such as surface painting, galvanizing, bluing treatment, etc. In some humid or highly corrosive environments, such as marine engineering and chemical companies, higher corrosion resistance is required for carbon steel rods. In addition to the above-mentioned surface protection measures, low-alloy steel rods with better corrosion resistance may be selected or special corrosion-resistant coatings may be added to the surface of carbon steel rods.
Oxidation resistance: In a high temperature environment, carbon steel bars will react with oxygen to form oxide scale. As the temperature rises and the time increases, the degree of oxidation will gradually intensify, which will not only affect the surface quality of the carbon steel bars, but also reduce its strength and other properties. In order to improve the high-temperature oxidation resistance of carbon steel bars, alloying elements (such as chromium, aluminum, etc.) can be added to form a dense oxide film to prevent oxygen from further diffusing inward, thereby improving its oxidation resistance. In some high-temperature industrial furnaces, heat treatment equipment and other occasions, the carbon steel bars used need to have good high-temperature oxidation resistance to ensure the normal operation and service life of the equipment.
3. Decryption of the production process
The production of carbon steel bars is a complex and precise process involving multiple key links, each of which has an important impact on the quality of the final product.
Steelmaking link
Raw ​​material preparation: The main raw materials for the production of carbon steel bars are iron ore, scrap steel and coke. After the iron ore is processed by beneficiation, high-grade iron concentrate is obtained as the main raw material for ironmaking. Scrap steel is a recycled material in the steel production process, which is recycled by remelting. Coke mainly acts as a heating agent and reducing agent in the iron-making process, providing heat and carbon monoxide gas for the reduction of iron ore. In addition, some auxiliary materials, such as limestone, need to be added for slag making and removal of impurities in iron ore.
Iron-making process: In the iron-making blast furnace, pre-treated iron ore, coke and limestone are added to the furnace in a certain proportion. At high temperatures, the combustion of coke generates a large amount of heat, which makes the temperature in the furnace reach about 1500℃. At the same time, coke reacts with oxygen to generate carbon monoxide, which reduces the iron oxide in the iron ore to metallic iron. Limestone decomposes at high temperatures to generate calcium oxide, which reacts with impurities such as sulfur and phosphorus in the iron ore and gangue components to form slag, which floats on the surface of the molten iron and is discharged from the furnace through the slag outlet. After the iron-making process, the molten iron obtained contains more carbon and other impurities, which need to be further processed for steelmaking.
Steelmaking refining: The molten iron obtained from iron-making is poured into a converter or electric furnace for steelmaking. In converter steelmaking, oxygen is blown into the molten iron to cause oxidation reactions of carbon, silicon, manganese and other elements in the molten iron, reduce the carbon content, and remove harmful impurities such as sulfur and phosphorus. In electric furnace steelmaking, the heat generated by electricity is mainly used to melt scrap steel and molten iron, and the composition and properties of the molten steel are adjusted by adding slag-making materials and alloying elements. In order to further improve the purity and quality of the molten steel, refining treatment is also carried out, such as the use of out-of-furnace refining technology, including LF (ladle refining furnace), VD (vacuum degassing device), etc. In the LF refining process, through heating, stirring and adding refining agents, the inclusions in the molten steel are further removed, the composition and temperature of the molten steel are adjusted, and the molten steel is made more uniform and pure. VD refining mainly reduces the gas content in the molten steel, such as hydrogen and nitrogen, in the vacuum environment to improve the quality of steel.
Forming link
Hot rolling forming: The molten steel after steelmaking and refining is first cast into a billet. The steel billet is heated to 1000℃ - 1200℃ in a heating furnace to give it good plasticity and facilitate rolling. The heated steel billet is fed into the rolling mill, where it is gradually deformed by the rotation and extrusion of the rollers and finally rolled into the required shape and size of the carbon steel bar. During the hot rolling process, different rolling processes can be used according to different product requirements, such as continuous rolling, reversible rolling, etc. The continuous rolling process has the advantages of high production efficiency and stable product quality, and can continuously roll the steel billet into carbon steel bars of various specifications. During the hot rolling process, it is also necessary to accurately control parameters such as rolling speed, temperature, and reduction to ensure the dimensional accuracy and internal quality of the carbon steel bar.
Cold rolling forming: Cold rolling forming is to use hot-rolled carbon steel bars as raw materials and roll them at room temperature. Since the steel is not heated during the cold rolling process, the work hardening phenomenon is obvious, so it is necessary to strictly control the equipment accuracy and rolling process of the rolling mill. First, hot-rolled carbon steel bars need to go through pretreatment processes such as pickling to remove the oxide scale and impurities on the surface to ensure the smooth progress of the cold rolling process and the surface quality of the product. Then, the pre-treated hot-rolled bars are sent to the cold rolling mill, and through multiple passes of rolling, the diameter or thickness of the bars is gradually reduced to improve its dimensional accuracy and surface quality. During the cold rolling process, lubricants are also needed to reduce the rolling force, reduce the wear of the rollers, and improve the surface quality of the product. The dimensional accuracy of cold-rolled carbon steel bars can be controlled within ±0.05mm, and the surface roughness can reach Ra0.8 - 1.6μm, which can meet some application scenarios with extremely high requirements for dimensional accuracy and surface quality.
Cold drawing: Cold drawing is to stretch the hot-rolled or cold-rolled carbon steel bars through a die to reduce their cross-sectional size and increase their length. During the cold drawing process, the bars are subjected to tensile stress and plastic deformation occurs.The cold drawing process is usually suitable for the production of some slender carbon steel bars, such as steel bars and steel wires with smaller diameters. During the cold drawing process, the tensile force, tensile speed and dimensional accuracy of the die need to be precisely controlled to ensure the dimensional accuracy and performance requirements of the product. At the same time, in order to ensure the smooth progress of the cold drawing process, the bar needs to be lubricated before cold drawing. Common lubricants include lime milk, soap solution, etc.
Subsequent processing links
Heat treatment: Heat treatment is one of the important means to improve the performance of carbon steel bars. Common heat treatment processes include annealing, normalizing, quenching and tempering. Annealing is the process of heating the carbon steel bar to an appropriate temperature, keeping it for a certain period of time, and then slowly cooling it. Annealing can eliminate the internal stress of the steel, reduce hardness, increase plasticity, and improve cutting performance. Normalizing is a heat treatment process in which the carbon steel bar is heated to above the critical temperature, kept warm for an appropriate period of time, and then cooled in the air. The structure of carbon steel bars after normalizing is finer than that after annealing, and the strength and hardness are improved, while the plasticity is slightly reduced. It is often used to improve the cutting performance of low-carbon steel and medium-carbon steel. Quenching is the process of heating carbon steel bars above the critical temperature, keeping them warm for a certain period of time, and then cooling them rapidly. Quenching can significantly improve the hardness and strength of carbon steel bars, but it will reduce their toughness. In order to eliminate the internal stress of quenching and improve toughness, carbon steel bars after quenching usually need to be tempered. Tempering is the process of heating the quenched carbon steel bars to a temperature range below the critical temperature, keeping them warm for a certain period of time, and then cooling them. By adjusting the tempering temperature and time, different combinations of strength, hardness, toughness and plasticity can be obtained to meet different use requirements.
Surface treatment: In order to improve the corrosion resistance and aesthetics of carbon steel bars, surface treatment is usually performed. Common surface treatment methods include painting, galvanizing, bluing, etc. Painting is a simple and effective surface protection method. By spraying or brushing a layer of paint on the surface of the carbon steel bar, a protective film is formed to prevent oxygen and moisture from contacting the steel, thereby playing a role in rust prevention. There are rich colors and types of paint, which can be selected according to different needs, and it can also play a certain decorative role. Galvanizing is the process of immersing carbon steel rods in molten zinc liquid to form a zinc layer on their surface. The galvanized layer has good corrosion resistance and can protect carbon steel rods from corrosion for a long time in the atmospheric environment. Galvanizing is divided into hot-dip galvanizing and electro-galvanizing. The hot-dip galvanizing layer is thicker and has better corrosion resistance. It is often used in some occasions with high requirements for corrosion resistance, such as galvanized steel pipes for construction, outdoor steel structures, etc.; the electro-galvanizing layer is thinner and has better surface quality. It is often used in some parts with high requirements for appearance. Bluing treatment is to heat the carbon steel rod in a solution containing an oxidant to form a dense oxide film on its surface. This oxide film has a certain corrosion resistance and can also play a decorative role, making the surface of the carbon steel rod present a blue-black luster. Bluing treatment is often used for surface treatment of some mechanical parts, tools, etc.