Quality Difficulties in Cold Upsetting Steel Processing
The current Cold Upsetting Steel wire rod continent can provide 5.5?~40?, the more mature range is 6.5?~25? High-grade fasteners on the quality of raw materials requirements are as follows: the coil has a high plastic index, high sectional shrinkage and elongation; in the cold plastic deformation, the material deformation resistance is small, low processing hardening rate, the material yield strength ratio is small and the coil hardness HRB appropriate, not too high; the coil has a good surface quality, should be smooth and round, no bump folding, scarring cracks, pockmarks, etc., the steel organization is dense, no Internal defects.
There are two main indicators of the quality of Cold Upsetting Steel
Microstructure: (except for special requirements) should be ferrite plus pearlite (F+P), requirement is 2~4 grade, 3 grade is excellent. Non-metallic inclusions: inclusions are one of the main causes of cold heading cracking, Class B (aluminium oxide) and Class D (spherical oxide) inclusions have the greatest influence, the larger the particles, the easier it is to crack, inclusions within 2mm from the surface should not be greater than 0.15mm. main control defects: there is carbon segregation in the heart of Cold Upsetting Steel, material and surface decarburisation in the annealing process, cold heading cracks, quenching cracking in tempering and deformation.
Optimum cold heading deformation organisation
The most suitable cold heading deformation organization is carbide spheroidization organization, its strength and hardness reduction, plasticity is better, conducive to cold heading deformation. SWRCH35K, ML35 is a typical medium carbon Cold Upsetting Steel coil, when cold heading deformation ≥ 60%, before cold heading should be 700 ℃ -730 ℃ heat preservation 4 ~ 6 levels, to ensure that the finished product processing plasticity. And for medium carbon alloy steel SCM435, ML40Cr using isothermal spheroid annealing, in (Acl above 20 ℃ -40 ℃) 750 ℃ -770 ℃ heating insulation 4-6 hours (note) after the furnace cooling to slightly below the Arl temperature 680 ℃ ~ 700 ℃ isothermal 6-8 hours (note) then cold to about 500 ℃ out of the furnace air cooling. The microstructure of the steel changes from coarse to fine, from flaky to spherical, according to JB/T5074-1991 “low, medium and carbon steel spheroid rating”, controlled at 4-6, the microstructure is characterised by punctiform spheroid and a small amount of spheroid + ferrite or uniformly distributed spheroid + ferrite, the cold heading forming properties can be improved by adjusting the amount of drawing deformation when in use.
Statistics show that 80%-85% of cold heading without cracks are caused by defects such as folding, scratching, dense hairlines, local micro-cracks and scarring on the surface of Cold Upsetting Steel For high strength fasteners of Cold Upsetting Steel intrinsic quality and surface quality requirements are very strict, so should start from five aspects to strengthen quality control, namely: surface quality, microstructure, chemical composition, non-metallic inclusions and grain size.
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