According to the steel type and steel quality requirements, the smelting of alloy structural steel can use oxygen top-blown converter, open hearth furnace, electric arc furnace; or electroslag remelting and vacuum degassing can be used. Ingot casting can be done by continuous casting or mold casting. The steel ingot should be cooled slowly or sent hot for forging and rolling. When the steel ingot is heated, the temperature should be uniform and there should be sufficient holding time to improve segregation defects and avoid uneven deformation during forging and rolling; steel after forging and rolling should be small in size, especially carburized with a carbon content of about 0.2%. Steel should be cooled quickly when the temperature is above 600°C to avoid aggravating the band structure; for forgings with larger cross-sections, measures should be taken to eliminate internal stress and white spots. Quenched and tempered steel should be quenched into a martensite structure as much as possible, and then tempered into a troostite structure; during the carburizing process of carburized steel, the concentration gradient of the carburized layer should not be too large to avoid the appearance of a continuous network on the grain boundaries of the carburized layer. Carbide; nitrided steel must first be heat treated to obtain the required properties, and then nitrided through final finishing. After the nitriding treatment, there is no further processing except grinding and removing the brittle “white layer”.
Classification A type of carbon steel. The carbon content is about 0.05%~0.70%, and can be as high as 0.90% in some cases. It can be divided into two categories: ordinary carbon structural steel and high-quality carbon structural steel. The former contains more impurities and is cheap. It is used in places where performance requirements are not high. Its carbon content is mostly below 0.30%, its manganese content does not exceed 0.80%, and its strength is low, but its plasticity, toughness, and cold deformation properties are good. good. Except for a few cases, it is generally used directly without heat treatment. It is mostly made into bar steel, special-shaped steel products, steel plates, etc. It has many uses and is used in large quantities. It is mainly used in railways, bridges, and various construction projects to manufacture various metal components that bear static loads, as well as unimportant mechanical parts and general welding parts that do not require heat treatment.
High-quality carbon structural steel is pure steel with few impurities and good mechanical properties. It can be used after heat treatment. According to the manganese content, it is divided into two groups: ordinary manganese content (less than 0.80%) and higher manganese content (0.80%~1.20%). The carbon content is less than 0.25%, and it is mostly used directly without heat treatment, or it is carburized, carbonitrided, etc. to manufacture small and medium gears, shafts, piston pins, etc.; the carbon content is 0.25%~0.60%, typical steel The numbers include 40, 45, 40Mn, 45Mn, etc., which are often quenched and tempered to manufacture various mechanical parts and fasteners. The carbon content exceeds 0.60%, such as 65, 70, 85, 65Mn, 70Mn, etc., which are mostly used as springs. Steel is used.
This type of steel mainly ensures mechanical properties, so its grade reflects its mechanical properties, which is expressed by Q + numbers, where “Q” is the prefix of the Chinese pinyin of the word “Qu” for yield point, and the number indicates the yield point value. For example, Q275 indicates that the yield point is 275Mpa. If the letters A, B, C, and D are marked after the grade, it means that the quality of the steel is different. The content of S and P decreases in sequence, and the quality of the steel increases in sequence. If the letter “F” is marked after the grade, it is boiling steel, if it is marked “b”, it is semi-killed steel, and if it is not marked “F” or “b”, it is killed steel. For example, Q235-A·F represents Grade A boiling steel with a yield point of 235Mpa, and Q235-C represents Grade C killed steel with a yield point of 235Mpa.
Carbon structural steel is generally not heat treated and is used directly as supplied. Generally, Q195, Q215, and Q235 steels have low carbon mass fraction, good welding performance, good plasticity and toughness, and have a certain strength. They are often rolled into thin plates, steel bars, welded steel pipes, etc., and are used in bridges, buildings and other structures and in the manufacture of ordinary screws, Nuts and other parts. Q255 and Q275 steels have a slightly higher carbon mass fraction, higher strength, better plasticity and toughness, and can be welded. They are usually rolled into shaped steel, bar steel and steel plates for structural parts and for manufacturing connecting rods, gears and couplings of simple machines. Sections, pins and other parts.