Causes of cold rolling cracking of brass strips and their preventive measures
Brass is an important copper alloy material. Because of its "high performance and low cost", it is widely used in various fields of the national economy. Brass strip has beautiful luster, good strength, toughness and corrosion resistance. It is increasingly used in light industry, decoration and other industries, and has broad market prospects.
The world's copper resources are limited. In recent years, with the increase in copper demand, copper prices have risen sharply. Therefore, in the increasingly competitive copper processing industry, especially some small and medium-sized enterprises, they generally use a large amount of old materials to produce brass strips. However, blindly using a large amount of old materials will bring a series of problems to production and product quality. In the production of brass strips in the factory where the author works, rolling cracks or even cracks often appear on the surface of the strips, resulting in an alarming number of product modifications and scraps, seriously affecting production efficiency and causing huge economic losses. This article mainly analyzes the causes of cold rolling cracking of brass strips, and proposes corresponding control and preventive measures, which has certain theoretical and practical significance for production.
1. Brass strip production process
In the production of modern brass plates and strips, horizontal continuous casting is usually used to obtain large coils of heavy billet [2]. After homogenization annealing and surface milling, the billet is cold rolled at a high processing rate, and then intermediate annealing and finish rolling are performed. and other processes. In addition, in order to eliminate internal stress and improve the plate shape, low temperature treatment and stretching, bending and straightening must be carried out. The main process flow is: batching, smelting * horizontal continuous casting, homogenization annealing * milling, cold rough rolling * intermediate annealing - bottom rolling - bottom annealing * finished product rolling * degreasing, cleaning, passivation treatment * stretching Bending and straightening*low temperature treatment*inspection*slitting. Package. By reasonably controlling process parameters, the production of high-quality brass plates and strips can be achieved.
2. Analysis of causes of cold rolling cracking
During the rolling process, when the local deformation of the metal exceeds its ultimate deformation degree, the bonding force between atoms is destroyed and cracks appear. According to the crack propagation mode, cracking can be divided into intergranular cracking and transgranular cracking. The expansion of cracks follows the principle of minimum energy consumption, that is, the expansion of cracks always proceeds along the direction where the atomic bonding force is weakest. Most of the cracks in polycrystalline materials are intergranular fractures, which are caused by the weakening of the grain interface for some reason [st]. These reasons include: the precipitation of a brittle second phase at the grain boundary; the high temperature effect weakens the interface or the segregation of impurity atoms to the grain boundary; the weakening of the interaction between the grain boundary and the environment, such as stress corrosion, etc. The cracking of brass strips mostly belongs to the first two reasons. When harmful elements or harmful phases are present, grain boundary segregation or reverse segregation is caused, which leads to the weakening of the bonding force between grain boundaries, thus causing intergranular cracking or fracture under the action of stress [` ].
2.1 Influence of metallographic structure
Taking 6H5 brass as an example, it can be seen from the Cu-Zn phase diagram that H65 is a single-phase a brass under slow cooling conditions. However, in actual production, the cooling speed is faster and it is non-equilibrium crystallization. The knife phase generated by the peritectic reaction is not available in time. It is completely converted into phase A and remains inside the organization [']. The plasticity of the knife phase at room temperature is different from that of the a phase. During the rolling process, the deformation of the two phases is uneven, which will inevitably lead to the formation of slip dislocations at the interface between the two phases. When the local stress concentration caused by dislocations reaches a certain level At this time, the moon phase matrix is cracked to form a crack source, and then macro cracks are formed under the action of additional tensile stress. Therefore, the number and distribution of Yin phase have a significant impact on the cold rolling cracks of H65 brass.
has an important impact. When there are many moon phases, they are continuously distributed among the dendrites in a network shape. This network structure can withstand greater stress concentration and is less likely to form cracks; when there are few moon phases, due to the large distance between the knife phases, it is not easy to form cracks. Stress concentration is formed, so cracks will not occur. Research shows that v1[, when the volume fraction of the knife phase is greater than 20% or less than 5%, the high-temperature plasticity of H65 brass is relatively good. Although the plasticity of the moon phase is better than that of the a phase under hot rolling conditions, if the stress at the phase boundary is concentrated, cracking will also occur.
The grain size of brass also has a certain impact on its cracking. The larger the grain size, the greater the tendency of cracking. From the analysis of the metallographic structure of brass, it is known that the grains in the outer layer are significantly thicker than the inner layer, and the outer layer is in direct contact with the medium, so it is easy to cause cracking. Research shows that cracking is related to uneven cold deformation; segregation of iron content also has adverse effects.
2.2 Effect of impurities
The effects of several impurities on brass production are as follows [']:
Iron: exists as an impurity and has no significant impact on mechanical properties. The solubility of iron in brass is extremely small, and iron-rich phase impurity points are often distributed in the matrix, which has the effect of refining the grains;
Lead and lead: Lead is a harmful impurity in simple brass and is distributed in granular form on the fusible eutectic at the grain boundary. when. When the lead content of brass is >0.03%, cracking often occurs during the rolling process. The effect of mystery is roughly the same;
Antimony: As the temperature decreases, the solubility of antimony in a brass decreases sharply, and the brittle compound CuZbS precipitates, which is distributed in a network, seriously damaging the cold working performance of brass; Phosphorus: rarely solid solution in Cu-zn alloy, in aPhosphorus content in brass
If it exceeds 0.05% to 0.06%, a brittle phase Cu3P will appear, reducing the plasticity of brass;
Arsenic: The solubility of arsenic in brass at room temperature is <0.1%. Excessive amounts will produce a brittle compound Cu3sA, which is distributed on the grain boundaries and reduces the plasticity of brass. Containing 0.02% to 0.05% As, which can prevent dezincification of brass and improve corrosion resistance.
2.3 Influence of production technology
Generally, brass strips are cracked at the edges but not cracked in the middle. There are two reasons. First, when the actual roll type in production is a flat roll, the edge metal has a tendency to flow sideways, so its longitudinal flow speed is lower than the metal in the middle of the strip. Since the strip is a whole, the deformations of the middle and edge parts are mutually restrained. Therefore, the metal in the middle of the plate is subject to compressive stress, while the metal on both sides is subject to tensile stress. When the tensile stress at the edges exceeds the strength limit of the metal, cracking (cracking) will occur. Secondly, during the rolling process, the temperature rise causes the roll to produce thermal convexity, which makes the middle roll gap smaller and the edge roll gap relatively larger. Therefore, the central reduction amount is large and the edge reduction amount is small. This will further cause the metal flow rate in the middle to be higher than that at the edges, increasing the tendency of edge cracking of the strip. In addition, factors such as improper control of horizontal continuous casting process parameters and excessive pass rolling rates will lead to yellowing. Cracks occur during the rolling process of the copper strip.
3. Preventive measures for cold rolling cracking
3.1 Raw materials
① The impurity composition of old materials changes greatly, so the old materials of the same batch should be mixed evenly before use, which will help the impurity composition of each charge to be consistent. The prerequisite is to control the Pb content of the final ingot within 0.02%. If the bP content is too high, cracking may easily occur;
② Check the bP content of the purchased old materials. When the bP content is very large, it should be used in proportion to reduce the bP content in the ingot;
③When sorting old materials, pay attention to industrial hygiene to prevent other metal impurities and old brass materials from mixing in.
3.2 Production process
① Control the melting and casting process conditions, appropriately lower the casting temperature, increase the cooling intensity, and improve the stop process to reduce the harmful effects of bP, iB and other impurities;
②Reducing the processing rate and increasing intermediate annealing can effectively avoid cracking caused by stress concentration at the phase boundary. This method is simple and easy to implement, and has been verified in actual production;
③ For regular edge cracks, the roll crown can be appropriately reduced or the roll bending force can be adjusted to reduce the edge tensile stress, thereby avoiding or improving edge cracking.
3.3 Control measures for metallographic structure
① It is conducive to the transformation of the casting structure of uneven columnar crystals and equiaxed crystals into a structure with good plasticity and suitable for processing, and the pass processing rate and rolling speed are not easy to be too large;
② Some modifiers should be added appropriately during smelting to achieve the effects of removing impurities, degassing and refining grains. In order to solve the problem of strip edge cracking, it is necessary to reduce the bP content on the grain boundaries, the less the better. To this end, a small amount of rare earth is added. Rare earth elements can form a high melting point compound CePb3 with Pb at temperatures above 1100°C. When the alloy crystallizes, it first precipitates and becomes a non-spontaneous crystal nucleus. The increase in the number of crystal nuclei can refine the grains and increase the number of grain boundaries, thereby reducing the Pb content on the grain boundaries. CebP3 on the grain boundaries can increase the strength of the grain boundaries and help prevent grain boundary cracking.
The key to preventing cold rolling cracking of brass strips is to ensure the quality of raw materials, control the production process and process parameters, and improve the alloy structure.
4. Conclusion
① There are many factors that affect the life of copper alloy extrusion dies. In addition to the factors of the die itself such as die material, structural design, heat treatment process, etc., the use and maintenance of the die are also important factors;
② For large-tonnage copper alloy extrusion presses, especially reverse extrusion presses, great attention must be paid to the cooling of the mold. A reasonable cooling method can maintain the working temperature of the extrusion die below the tempering softening temperature without overcooling the blank and the die, thereby avoiding stuffiness and affecting the quality of the extruded product;
③The current ideal cooling method is liquid nitrogen cooling. By adjusting the liquid nitrogen flow and pressure, the cooling intensity of the mold can be controlled, thereby maximizing the service life of the mold.
