ABSTRACT: The causes of cold rolling cracking in brass strip production due to the use of a large number of used materials are analyzed. ADJUSTING ALLOY COMPOSITION, improving production technology and improving metal structure can effectively reduce or avoid cold rolling crack. 

Key words: Brass Strip; cracking; cause; preventive measures; cold-rolling Chinese picture classification number: T3G39 document identification number: A article number: 1671-6795(2oo8)05-26-03

Brass is an important copper alloy material, because of its "high performance, low cost" , widely used in various fields of the national economy. Brass Strip has beautiful luster, good strength, toughness and corrosion resistance, more and more widely used in light industry and decoration industries, the market prospects are broad. The copper resources in the world are limited. With the increase of copper demand in recent years, the copper price rises sharply. Therefore, in the increasingly competitive copper processing industry, especially some small and medium-sized enterprises generally use a large number of scrap production of brass strip Chuan. However, the indiscriminate use of old materials will bring about a series of problems in production and product quality. In the production of brass strip in our factory, rolling cracks and even cracks often appear on the surface of the Strip, resulting in a tremendous amount of remanufacture and scrap, which seriously affects the production efficiency and causes huge economic losses. In this paper, the causes of cold rolling cracking of brass strip are analyzed, and the corresponding control and prevention measures are put forward.

1. Production process of brass strip

In the production of modern brass plate and strip, the heavy strip billets (2) are usually obtained by horizontal continuous casting. After homogenizing annealing and milling, the billets are cold-rolled with high processing rate, and then the intermediate annealing and finishing rolling are carried out. In addition, in order to eliminate the internal stress and improve the shape of the plate must be low-temperature treatment and tension straightening. The main technological processes are: batching, melting * Horizontal Continuous Casting, homogenizing annealing * Milling Surface, cold rough rolling * Intermediate Annealing * Rolling with one bottom remaining annealing * finished products rolling * degreasing, cleaning, passivation treatment * Tension Straightening * Low Temperature Treatment * Inspection * Cutting. Packing. The production of high quality brass plate and strip can be realized by reasonable control of process parameters.

2. Cause analysis of cold rolling cracking

In the rolling process, when the local deformation of the metal exceeds its limit, the bond force between the atoms is destroyed and a crack appears. According to the mode of crack propagation, the cracking can be divided into intergranular cracking and transgranular cracking. The crack propagation follows the principle of minimum energy consumption, that is, the crack propagation always follows the direction where the atomic bonding force is weakest. The fracture of polycrystalline materials is mostly intergranular fracture, which is caused by the weakening of crystal interface for some reason. These reasons include: the Brittle second phase at the grain boundary, the weak interface or impurity atom segregation to the grain boundary due to high temperature, and the weak interaction between the grain boundary and the environment, such as stress corrosion. The cracking of brass strip mostly belongs to the former two reasons. When there are harmful elements or harmful phases, the segregation or reverse segregation of grain boundaries results in the weakening of the cohesive force between grain boundaries, which results in 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 phase diagram of Cu-Zn that H65 is single-phase a brass under slow cooling condition, the knife phase formed by peritectic reaction does not fully convert to a phase and remains in the structure ['] . The plasticity of the knife phase is different from that of a phase at room temperature, and the deformation of the two phases is not uniform during the rolling process, which will lead to the formation of a plug group at the interface of the two phases. When the local stress concentration caused by the plug accumulation reaches a certain degree, the fracture of the lunar phase matrix will form a crack source, and then a macro crack will form under the action of additional tensile stress. Therefore, the number and distribution of Yin phase have an important influence on the cold rolling crack of H65 brass. When the phase of the moon is more, it is distributed in a network continuously among the dendrites. This network structure can bear greater stress concentration and is not easy to form cracks. The results show that the high temperature plasticity of H65 brass is better when the volume fraction of V 1[ , knife phase is more than 20% or less than 5% . Although the plasticity of phase a is better than that of phase a under hot rolling condition, if the phase boundary stress is concentrated, it will cause cracking. The grain size of brass also has some influence on its cracking. The larger the grain size, the greater the cracking tendency. From the metallographic analysis of brass, it is found that the outer layer is much coarser than the inner layer, and the outer layer is in direct contact with the medium, so it is easy to cause cracking. The results show that the cracking is related to the non-uniformity of cold deformation, and the segregation of iron content is also disadvantageous.

2.2 effects of impurities

The effects of several impurities on brass production are as follows: IRON: the presence of impurities has no significant effect on mechanical properties. The solubility of iron in brass is very small, and the impurity point of iron-rich phase is usually distributed in the Matrix, which has the effect of refining the grain. When... When the lead content of brass is less than 0.03% , cracking often occurs in the rolling process. Antimony: The solubility of antimony in brass a decreases sharply with the decrease of temperature, and the brittle compound CuZbS is netted out, which seriously damages the cold workability of brass; Phosphorus: very little solid solution in Cu-zn alloy, if the content of phosphorus in brass a exceeds 0.05%-0.06% , the brittle phase Cu3P will appear, and the plasticity of brass will be reduced; Arsenic: The solubility of arsenic in brass at room temperature is & Lt; 0.1% , too much arsenic produces the brittle compound Cu3sa, which distributes on the grain boundary and reduces the plasticity of brass. Containing 0.02%-0.05% as, it can prevent the dezincification of brass and improve the corrosion resistance.

2.3 impact of production processes

Generally, the brass strip is edge cracking, central non-cracking. There are two reasons. First, when the actual roll shape is flat, the edge metal tends to flow laterally, so the longitudinal flow velocity is lower than that of the middle metal. As the Strip is a whole, the deformation of the middle part and the edge are restricted each other, so the metal in the middle part of the sheet is subjected to compressive stress, while the metal in the two sides is subjected to tensile stress, when the edge tensile stress exceeds the strength limit of the metal, it will crack (the edge) Second, in the rolling process, the temperature rise causes the roll to produce the thermal crown, which makes the middle gap smaller and the edge gap larger, so the middle gap is larger and the edge gap smaller. This will further cause the metal to flow faster in the middle than in the sides, in addition, the factors such as improper control of horizontal continuous casting process parameters and excessive pass rolling rate will lead to the cracking of brass strip during rolling.

3. Cold rolling cracking precautions

 3.1 raw materials

The impurity composition of the used materials varies greatly, so the same batch of used materials should be mixed evenly before use, which is helpful to the consistency of the impurity composition of each charge. The precondition is to control the PB content of the final ingot within 0.02% . When the content of BP is very high, the content of BP should be used in proportion to reduce the content of BP in the INGOT. 

3 attention should be paid to industrial hygiene when sorting out the used materials, to prevent other metal impurities and brass scrap mixing.

3.2 Production Process 

1. Control melting and casting process conditions, appropriately reduce casting temperature, increase cooling strength and improve drawing and stopping process to reduce harmful influence of impurities such as BP and IB, 

2. Reduce Processing rate and increase intermediate annealing, the cracking caused by the stress concentration at the phase boundary can be effectively avoided. This method is simple and easy, and has been proved in practical production. 3 for regular edge fine crack, roll crown can be reduced or roll bending force can be adjusted to reduce edge tension stress, so as to avoid or improve edge crack.

3.3 The control measure of metallographic structure 1 is that the inhomogeneous columnar crystal and equiaxed crystal can be changed into the plastic one which is suitable for processing, and the pass rate and rolling speed are not easy to be too large; 

2 when melting, some modifier is added to remove impurity, degassing and refining grain. In order to solve the problem of strip edge cracking, it is necessary to reduce the content of BP in grain boundary, the less the better. Therefore, rare earth elements can form high melting point compound CEPB3 with PB above 1100 °C by adding a small amount of rare earth elements. Increasing the number of nuclei can refine the grain and increase the grain boundaries, thus reducing the PB content in the grain boundaries. CebP3 on grain boundary can increase the strength of grain boundary and prevent the crack of grain boundary. The key to prevent the cold rolling cracking of brass strip is to ensure the quality of raw material, 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 die. Besides the die material, structure design and heat treatment process, the use and maintenance of the die are also important factors For the large tonnage copper alloy extrusion machine, especially the reverse extrusion machine, the cooling of the die must be paid great attention to. A reasonable cooling method can keep the working temperature of extrusion die below tempering softening temperature, and at the same time, it will not make the billet and die too cold At present, the ideal cooling mode is liquid nitrogen cooling. The cooling intensity can be controlled by adjusting the flow and pressure of liquid nitrogen so as to prolong the service life of the die. 

Source: Chinanews.com, by Xu Kai

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