GB/T699, JIS G4051, JIS G3507 to 3509, SAE J403 standard series of high quality carbon structural steel; GB/T6478 standard series of cold heading and cold extrusion steel used in the largest fastener companies. For such steel fasteners, it still plays an important role in the past and now.
In particular, cold heading steel is used to make bolts, nuts, screws, rivets, pins and other fasteners by cold heading. Cold extrusion steel is used to manufacture automobile parts, bicycles and other mechanical parts through cold extrusion technology, also called riveted steel. Cold heading and cold extrusion are plastic forming at room temperature using metal, instead of machining, not only high efficiency and good quality, but also material saving and low cost (reduced by 10% to 30%).
In modern China, there has been further development in smelting technology. Due to the conservation of resources, the use of scrap steel to smelt steel has increased. 80% or more of molten iron is used in converter steelmaking, and even more than 60% of molten iron can be used in electric arc furnaces. Hot metal pre-desulfurization technology is used for cold heading steel or high purity steel, if not after ladle refining or vacuum degassing The sulfur recovery process is carried out. The sulfur content in the steel is 0.005% to 0.015%, 0.010% to 0.030%, 0.025% to 0.040%, and 0.030% to 0.045%, respectively.
The higher the sulphur content, the better the cutting performance of the steel; the higher the sulphur content, the more serious the forging flow line of the steel piece and the more obvious the decline in the lateral performance. In the cold crucible deformation location, if the metal produces a violent lateral flow, it will lead to the flattening of wire-like and rod-like sulfides, intensifying the role of sulfide in dividing the matrix, causing tiny cracks in the hexagonal part of the bolt head or nut, bearing surface, increasing Quenching cracks or grinding cracking tendency.
In the same performance level of the bolt parts, the remelted steel and the sulfur-reduced steel have less 820-880°C cryogenic austenitic body weight crystallization process than the cold niobium steel. The grain size is coarser than that of the cold niobium steel, which results in poor toughness. Another important reason. Therefore, for high-strength bolts, etc., it is not appropriate to use recycled steel or sulfur-reduced steel.
The GB/T3098.1-2010 standard specifies the materials used for bolts of grades 8.8 to 10.9. For high-strength bolt steels that require heat treatment, the maximum limit of sulfur and phosphorus content is reduced from 0.035% to 0.025%. Carbon steel (such as boron or manganese or chromium), which is an additive element, is not allowed to add calcium or calcium alloys to the steel. All reasonable measures should be taken to prevent the use of scrap or other alloy materials used in production. The elements that affect hardenability, mechanical properties, and service performance.
The heavy use of 45# and 40Cr steels in our country tends to result in excessive residual alloying elements, which makes control very difficult, especially for some impurity elements that are difficult to remove in the smelting process. The contents of S and P are generally 0.025% to 0.035% or more. There are other Cu, Cu, Pb, Sn, and Se selenites. When the harmful elements P and S in the steel increase, the center of loosening, general loosening, and square segregation at the low level of detection will be in the 2 to 3 grade. Defects such as white spots, shrinkage holes, bubbles, and peelings are also visible.
Due to the low price of such steel, many fastener companies still like to adopt it. In the quenching and tempering of bolts, there are often accidents of quenching cracks, which are mainly caused by impurities and non-metallic inclusions. Cracking is unavoidable when any measures are taken during heat treatment. For important products, it is generally recommended that 45# and 40Cr steels be used or not used.
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