1. Carbon (C) Carbon is the most important element affecting the cold plastic deformation of steel. The higher the carbon content, the higher the strength and lower the plasticity of the steel. For every 0.1% increase in carbon content, the yield strength Re increases by 27.4MPa and the tensile strength Rm increases (58.8-78.4MPa), while the elongation A decreases by 4.3% and the area shrinkage Z decreases by 7.3%. When the carbon content of the steel is <0.5%, the manganese content is <1.2%, and the area shrinkage Z=80%, the approximate relationship between the unit cold deformation force P and the C and Mn content of the steel is as follows:
P=1950C+500Mn+1860 (MPa)……………………(1)
It can be seen that the carbon content in steel has a great influence on the cold plastic deformation properties of steel. In the actual process, when the carbon content of the steel for cold heading extrusion is greater than 0.25%, the steel is required to be annealed into a spherical pearlite structure with the best plastic structure. For cold heading fasteners with a deformation degree of 65%-85% that undergoes three upsetting deformations without intermediate heat treatment, the carbon content should not exceed 0.4%. When upsetting carbon steel with a carbon content exceeding 0.3%-0.5%, it is necessary to add an intermediate complete annealing process or use warm upsetting.
2. Manganese (Mn) Manganese interacts with iron oxide (Mn+FeO+MnO+Fe) in the smelting of steel and is mainly added to deoxidize the steel. Manganese interacts with iron sulfide in steel (Mn+FeS+MnS+Fe), which can reduce the harmfulness of sulfur to steel. The manganese sulfide formed improves the cutting properties of the steel. Manganese increases the strength of steel and reduces its plasticity, which is detrimental to the cold plastic deformation performance of steel. However, the effect of manganese on the deformation force is only about one-quarter that of carbon. Due to the special performance requirements of the finished product, the content of manganese is allowed to be five times that of sulfur. Except for the special requirements of the finished product, it should not exceed 0.9%.
3. Silicon (Si) Silicon is the residue of the deoxidizer in the smelting of steel. When the silicon content in steel increases by 0.1%, it will increase by 13.7mpa. Experience shows that when the silicon content exceeds 0.17% and the carbon content is large, it has a great impact on the reduction of the plasticity of the steel. Appropriately increasing the silicon content in steel is beneficial to the comprehensive mechanical properties of the steel, especially the elastic limit, and can also enhance the corrosion resistance of the steel. However, when the silicon content in steel exceeds 0.15%, non-metallic inclusions will be formed rapidly in the steel. High-silicon steel will not soften even if it is annealed, drastically reducing the cold plastic deformation performance of the steel. If silicon is present in the steel in the form of silicates, the fine particles dispersed in the steel will wear the mold too quickly. Therefore, in addition to the high strength performance requirements of the product, steel used for cold heading always minimizes the silicon content.
4. Sulfur (S) Sulfur is a harmful impurity. The sulfur in steel will cause the crystalline particles of the metal to separate from each other and cause cracks during cold heading. The presence of sulfur also promotes thermal embrittlement and rusting of steel. Therefore, the sulfur content should be less than 0.06%. When heading high-strength fasteners, it should be controlled below 0.04%. Since compounds of sulfur, phosphorus and manganese can improve cutting performance, the sulfur content of steel for cold heading nuts can be relaxed to 0.08%-0.12% to facilitate tapping.
5. Phosphorus (P) Phosphorus has extremely strong solid solution strengthening and work hardening effects. It segregates seriously in steel, increases the cold brittleness and temper brittleness of steel, and makes the steel susceptible to acid erosion. Phosphorus in steel will deteriorate cold plastic deformation properties, causing wire breakage during drawing and cracking of the workpiece during cold heading. The phosphorus content in steel is required to be controlled below 0.045%.
6. Other alloying elements: Chromium (Cr), molybdenum (Mo), nickel (Ni), vanadium (V), tungsten (W) and other alloying elements have far less influence on the cold deformation properties of steel than carbon. Generally speaking, as the alloying elements in steel increase, the mechanical strength index and hardenability of the steel increase, and the cold deformation performance decreases.
