Brief introduction of high-strength bolt processing technology

The high-strength bolt processing technology is: hot-rolled wire rod-(cold drawing)-spheroidizing (softening) annealing-mechanical descaling-pickling-cold drawing-cold forging forming-thread processing-heat treatment inspection.

　　 One, steel design

　　 In the manufacturing of fasteners, the correct selection of fastener materials is an important part, because the performance of fasteners is closely related to their materials. If the material is not selected properly or correctly, the performance may not meet the requirements, the service life may be shortened, and even accidents or processing difficulties may occur, and the manufacturing cost may be high. Therefore, the selection of fastener materials is a very important link. Cold heading steel is a steel for fasteners with high interchangeability produced by cold heading forming process. Because it is formed by metal plastic processing at room temperature, the deformation of each part is large, and the deformation speed it withstands is also high. Therefore, the performance requirements of the cold heading steel raw materials are very strict. On the basis of long-term production practice and user research, combined with GB/T6478-2001 'Technical Conditions of Cold Heading and Cold Extrusion Steel' GB/T699-1999 'High-quality Carbon Structural Steel' and the target JISG3507-1991 'Cold Heading' The characteristics of 'Carbon Steel Wire Rod for Steel' take the material requirements of grade 8.8 and grade 9.8 bolts and screws as an example, and the determination of various chemical elements. If the C content is too high, the cold forming performance will be reduced; if it is too low, it will not meet the requirements of the mechanical properties of the parts, so it is set at 0.25%-0.55%. Mn can improve the permeability of steel, but adding too much will strengthen the matrix structure and affect the cold forming performance; it has a tendency to promote the growth of austenite grains during quenching and tempering of parts, so it is appropriately increased on an international basis. It is 0.45%-0.80%. Si can strengthen ferrite and promote the decrease of cold forming performance. The decrease of material elongation is defined as Si less than or equal to 0.30%. S.P. is an impurity element. Their presence will cause segregation along the grain boundary, leading to embrittlement of the grain boundary and damage to the mechanical properties of the steel. It should be reduced as much as possible. P is less than or equal to 0.030%, and S is less than or equal to 0.035%. B. The maximum boron content is 0.005%, because although boron has the effect of significantly improving the permeability of steel, it will also increase the brittleness of steel. Excessive boron content is very detrimental to workpieces such as bolts, screws and studs that require good comprehensive mechanical properties. (Guide: Surface Treatment of Fasteners-Electroplating)

　　 2. Spheroidizing (softening) annealing

　　When countersunk head screws and hexagon socket head bolts are produced by cold heading, the original structure of the steel will directly affect the forming ability during cold heading. The plastic deformation of the local area in the cold heading process can reach 60%-80%. For this reason, the steel must have good plasticity. When the chemical composition of the steel is constant, the metallographic structure is the key factor that determines the plasticity. It is generally believed that the coarse flaky pearlite is not conducive to cold heading, while the fine spherical pearlite can significantly improve the plastic deformation ability of the steel. For medium-carbon steel and medium-carbon alloy steel with a large amount of high-strength fasteners, spheroidizing (softening) annealing is carried out before cold heading in order to obtain uniform and fine spheroidized pearlite to better meet the actual production needs. For the softening annealing of medium carbon steel wire rod, the heating temperature is mostly selected to be kept at the upper and lower critical point of the steel. The heating temperature should not be too high, otherwise it will cause the precipitation of tertiary cementite along the grain boundary, causing cold heading cracking. Medium-carbon alloy steel wire rods are annealed by isothermal spheroidization. After AC1+ (20-30%) heating, the furnace is cooled to slightly lower than Ar1, and the temperature is about 700 degrees Celsius for a period of time, and then the furnace is cooled to about 500 degrees Celsius and air-cooled. The metallographic structure of the steel changes from coarse to fine, from flake to spherical, and the cold heading cracking rate will be greatly reduced. The general softening annealing temperature of 35\45\ML35\SWRCH35K steel is 715-735 degrees Celsius; while the heating temperature of SCM435\40Cr\SCR435 steel spheroidizing annealing is generally 740-770 degrees Celsius, and the isothermal temperature is 680-700 degrees Celsius.

More related hardware stamping die industry news: