1. Mechanical properties of metal materials for cold heading

According to the characteristics of the cold heading process, the following requirements are imposed on the mechanical properties of steel:

1) The yield strength Re and the deformation resistance are as low as possible, so that the unit deformation force can be correspondingly reduced to prolong the life of the mold;

2) The cold deformation property of the material is good, and the material should have good plasticity, low hardness, and no cracking under large deformation degree. For example, when cold high-strength bolts are used, carbon steel with a higher carbon content can be used, and low-alloy steel with a lower carbon content can be used. If the carbon content is increased, the hardness is increased, the plasticity is lowered, and the cold deformation property is deteriorated. However, adding a small amount of alloying elements (such as adding a small amount of boron 10B21, 10B33 steel) to a steel with a lower carbon content can significantly increase the strength of the steel to meet the performance requirements of the product without damaging its cold deformation properties;

3) The lower the work hardening sensitivity of the material, the better, so that the deformation force during the deformation process is not too great. The work hardening sensitivity of the material can be reflected by the slope of the deformation resistance-strain curve. The greater the slope, the higher the work hardening sensitivity. For example, the slope of the curve of stainless steel 0Cr18Ni9 (SUS304) is the largest. The work hardening sensitivity of this material is more severe, and the deformation resistance increases sharply as the degree of deformation increases. The mechanical properties of the steel not only reflect the Rm, Re, A, Z and hardness of the original billet, but are also affected by the chemical composition, macroscopic structure and microstructure of the raw materials, as well as the drawing and the various processes in the material preparation process. The effect of heat treatment between processes.

2, chemical composition requirements

(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 of the steel and the lower the plasticity. For every 0.1% increase in carbon content, the yield strength Re increased by 27.4 MPa, the tensile strength Rm increased (58.8-78.4 MPa), and the elongation A decreased by 4.3%, and the area shrinkage Z decreased by 7.3%. When the carbon content of steel is <0.5%, the manganese content is <1.2%, and the area shrinkage ratio is Z=80%, the approximate relationship between the unit cold deformation force P and the amount of C and Mn in the steel is as follows: P=1950C+500Mn +1860 (MPa)

(1) It can be seen that the influence of carbon content in steel on the cold plastic deformation property of steel is very large. 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 to have the best plastic structure - spherical pearlite structure. For cold-twisted fasteners with a degree of deformation of 65%-85%, three upset deformations are performed without intermediate heat treatment, and the carbon content should not exceed 0.4%. When upsetting a carbon steel having a carbon content of more than 0.3% to 0.5%, it is necessary to increase the intermediate full annealing process or to use warm enthalpy.

(2) Manganese (Mn) Manganese is added to the iron oxide in the smelting of steel (Mn + FeO + MnO + Fe). Manganese reacts with iron sulfide in steel (Mn+FeS+MnS+Fe), which can reduce the harmfulness of sulfur to steel. The formed manganese sulfide improves the cutting performance of the steel. Manganese increases the strength of the steel and reduces the plasticity, which is unfavorable for the cold plastic deformation of steel, but the effect of manganese on the deformation force is only about a quarter of that of carbon. Due to the special performance requirements of the finished product, the manganese content is allowed to be five times that of sulfur. In addition to the special requirements of the finished product, it should not exceed 0.9%.

(3) Silicon (Si) Silicon is a residue of steel deoxidizer in smelting. When the silicon content in the steel increases by 0.1%, it increases by 13.7mpa. Experience has shown that when the silicon content exceeds 0.17% and the carbon content is large, the plasticity of the steel is greatly reduced. Properly increasing the silicon content in the steel is advantageous for the overall mechanical properties of the steel, particularly the elastic limit, and also enhances the corrosion resistance of the steel. However, when the silicon content in the steel exceeds 0.15%, the steel is sharply formed into non-metallic inclusions, and the high-silicon steel does not soften even if it is annealed, and the cold plastic deformation property of the steel is drastically lowered. If silicon is present in the steel in the form of silicic acid, 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, cold heading steel always minimizes the silicon content.
(4) Sulfur (S) sulfur is a harmful impurity. The sulfur in the steel causes the crystal grains of the metal to separate from each other to cause cracks upon cold heading. The presence of sulfur also causes the steel to be hot brittle and rusty. Therefore, the sulfur content should be less than 0.06%. When tanning high-strength fasteners, it should be controlled below 0.04%. Since sulfur, phosphorus and manganese compounds can improve the cutting performance, the sulfur content of cold heading steel can be relaxed to 0.08%-0.12% to facilitate tapping.

(5) Phosphorus (P) Phosphorus has strong solid solution strengthening and work hardening effect, and segregation is severe in steel, which increases the cold brittleness and temper brittleness of steel, making steel susceptible to acid attack. Phosphorus in the steel deteriorates the cold plastic deformation property, breaks the wire during drawing, and cracks the workpiece in cold heading. The phosphorus content in the steel is required to be controlled below 0.045%.

(6) Other alloying elements Alloying elements such as chromium (Cr), molybdenum (Mo), nickel (Ni), vanadium (V), and tungsten (W) have far less influence on the cold deformation properties of steel than carbon. In general, as the alloying elements in the steel increase, the mechanical strength index and hardenability of the steel increase, and the cold deformation performance decreases.

3. The metallographic organization requires certain requirements for the steel structure, grain size and form, and distribution of non-metallic inclusions in order to better adapt the steel to the cold heading process .

(1) Steel structure structure In addition to ferrite in the steel, there is pearlite. The higher the carbon content, the greater the amount of pearlite. Ferrite is a soft matrix with hard pearlite particles embedded in a soft matrix. The piles of pearlite distribution are detrimental to cold deformation and can form cracks. The organization of the steel should be tight and uniform. For cold heading steel, a grain structure with a uniform distribution and a spherical shape should be used.

(2) Grain size The deformation of the metal occurs due to the slip of the crystal grains and the deformation of the crystal grains themselves. Within a certain volume, the number of grains of fine-grained metal is inevitably larger than that of coarse-grained metal, and there are more grains in the direction of plastic deformation which are favorable for slippage, and the deformation can be more uniformly dispersed to each crystal grain. Correspondingly, the deformation non-uniformity of the fine-grained metal and the stress concentration caused by the deformation non-uniformity are small, the chance of cracking is also small, and the plastic deformation amount that can be withstood before cracking increases, and the plasticity is better reflected externally. . The smaller the grain size, the less stress is generated that excites adjacent grain slip. In order to continue the deformation, the applied stress must be increased, and the deformation resistance is reflected to the outside. Therefore, cold-rolled steel should not be used. The grain is too large, which will make the surface of the workpiece rough and cause obvious scratches and cracks. The work hardening sensitivity of coarse grain steel is larger than that of fine grain steel, the plasticity is poor, and the cold deformation property is also poor. The grain size of the cold heading steel is 4-6, and the grain size is as follows: the average grain diameter is about (0.02-0.06) mm; the number of grains per mm2 is about 250-2300; the average grain size The area is about (400-4000) μm2.

(3) Non-metallic inclusions No matter how the method is used to smelt steel, there will always be more or less non-metallic inclusions. Inclusions such as oxides or sulfides cause discontinuities in the tight crystal structure of the metal. The form, quantity and distribution of inclusions are different, and the effects on the cold deformation properties of steel are also different. The cold-rolled wire is used after cold-rolling steel, and these inclusions are elongated along the deformation direction during rolling and cold drawing. In general, fine, evenly distributed inclusions are not harmful. The fine and dispersed sulfide inclusions can be deformed better with the direction of deformation, and thus are slightly less harmful than other inclusions that are deformed. Particularly harmful are alumina inclusions. Alumina microparticles are not only extremely hard, they can damage the mold; they are also difficult to bond with the steel matrix, often causing tearing of the workpiece during severe cold deformation. Coarse or fine and locally concentrated inclusions have a great influence on the cold heading properties of steel .

4, surface quality requirements The surface state of ordinary hot-rolled steel is mostly not good enough. The surface defects of hot-rolled steel are not removed by cold drawing (if the compression ratio is too small), resulting in surface defects and waste products of the cold-rolled product, which will be severely impossible to produce.

(1) Surface defects of the billet When the steel is smelted, the steel ingot has defects such as bubbles and shrinkage cavities. After hot rolling and cold drawing, the wire has a relatively severe penetrating longitudinal crack, which is obviously exposed to the surface of the product during upsetting. Defects such as folding, ear, segregation, and cracking of raw materials during rolling can cause serious damage in cold heading. Such as: the broken end of the bolt, the crack of the nut; when the workpiece is twisted, the blank is crushed into two halves and so on. Raw materials are improperly treated in pickling, causing pitting and rust on the surface of the steel. If the pitting and rust are slight, after the cold drawing, the pit is elongated, and there is almost no trace on the surface, and the crack does not occur in the cold sputum. If the pit is severe, a crack is formed; the crack is mostly present at the corner of the workpiece with a large amount of deformation. The deeper the defects such as surface cracks on the material, the worse the cold deformation performance. Experiments show that the shape of the crack has little effect on the degree of deformation, regardless of cold drawing or cold rolling, but the effect of crack depth is large. For cold heading materials with a large degree of deformation, the critical depth of surface defects is 0.04-0.10 mm, and deeper defects must be avoided. Heating the steel in a low carbon atmosphere causes decarburization. Although decarburization does not show anything from the appearance quality of the product, any change in the carbon content of the workpiece surface can have a significant impact on the mechanical properties of the workpiece. Especially for steel materials with a carbon content of 0.30% or more, surface decarburization is obviously harmful to the fatigue strength and wear resistance of the workpiece. To prevent the decarburization material from annealing, a protective gas should be used. The opposite of decarburization is carburization. Steel can produce carburization in high temperature and high carbon environments. Although carburizing is acceptable for the finished product to produce a hard shell on the soft core, sometimes it is a method that needs to be used, but it is quite harmful for the cold heading process. The surface of the steel with the carburized layer is as thin and hard as the eggshell. When the material is modified or cold-rolled, cracks or peeling may occur on the surface of the material to reduce the cold deformation property of the steel. Therefore, steel for cold heading should completely avoid decarburization and carburization. Decarburization and carburization of steel can be examined by metallographic microscopy.

(2) Dimensional accuracy requires the dimensional accuracy of the wire to have a great influence on the quality and process of the cold heading product. Cold wire rods and molds are usually processed separately. If the diameter of the wire exceeds the maximum allowable value, the metal of the workpiece head will be excessive when upsetting, which will result in poor flash or bending of the workpiece. Or it is difficult to feed because the diameter of the wire is larger than the diameter of the die hole. And the workpiece rod portion is pulled by the die hole, and a metal tumor is sharply formed in the die hole. If the diameter of the wire is less than the minimum allowable value, the metal cannot completely fill the cavity during upsetting, resulting in an unclear edge of the workpiece. Therefore, the material used for cold heading should be close to the true circle and the diameter should be even. The diameter tolerance of the wire for cold heading is generally 0.20-0.35 mm, and the roundness tolerance is 1/2 of the diameter tolerance.

 

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