ABSTRACT: SEM and EDS were used to analyze the typical peeling parts of H62 copper strip. The results showed that SIO2 and borax covering agent entered the melt and resulted in inclusion and other defects, the cracking of copper strip occurred in the process of stamping and bending. Through the analysis and confirmation of each working procedure in the production and stamping process of the commonly used insert material H62 copper strip, the possible factors affecting the stamping peeling of the copper strip are determined, the production quality control standard of H62 copper strip raw material and the reference standard method of incoming material inspection are formulated to further ensure the production stability of the insert products.

Key Words: H62 Copper Strip; production technology; stamping peeling; bending test

H62 brass is a typical + two-phase brass consisting of a soft phase and a hard phase. H62 brass has good strength and plasticity, can be well cold, hot pressure processing and is widely used in the manufacture of various kinds of flexible connector components, as well as automobile, shipbuilding and precision machinery manufacturing industries, etc. , known as "commercial brass" [2]. Because of its cost advantage, the H62 copper socket has an irreplaceable role in the socket parts of household converters. The socket is the most important part of the socket, which is responsible for carrying current and fixing the plug at the same time. The quality of the socket is directly related to the user's personal safety and use experience. The National Standard [3] has clear requirements on the temperature rise of the socket and the force needed to pull the plug out. H62 Copper Strip is the raw material of socket production. The quality and performance stability of copper strip is related to the quality and efficiency of socket production. Due to the particularity of the production process of H62 copper strip raw material, in the production of H62 copper strip, the defects of Oxide Inclusion and porosity will lead to the delamination of the copper strip, which will lead to various kinds of peeling phenomena in different degrees in the stamping process, it brings great perplexity to production and quality control.

Based on the above situation, it is necessary to analyze the peeling reason of H62 copper strip during the punching process and to establish the control standard for the production and inspection of copper strip. According to the actual situation of each working procedure in the production of H62 copper strip, this paper analyzes the possible influencing factors of the copper strip stamping peeling, and formulates the control standard of each link in the production of H62 copper strip and the method of incoming material inspection for H62 copper strip, to further ensure the stability and efficiency of the production of copper insert products.

1.Test materials and methods

The chemical composition (mass fraction,%) of H62 copper alloy strip used for socket is, cu 60.5 ~ 63.5, Fe ≤0.15, Pb ≤0.08, Zn is residual, and the sum of impurities ≤0.50. The corresponding production process flow is, smelting furnace melting → holding furnace holding → semi-continuous water-cooled casting → ingot sawing → stepping furnace heating → hot rolling → milling surface → blooming → bell jar furnace annealing → Middle Rolling → Bell Jar furnace annealing → finishing rolling → annealing → final washing → stretching straightening → strip storage.

The H62 brass strip produced by a batch of semi-continuous casting has a thickness of 0.5 mm, and there is a typical delamination phenomenon in different parts of the copper insert of the press converter. In view of the situation of stamping peeling, the specific analysis and improvement programs are as follows: (1) through physical observation to determine the appearance of peeling and deduce the reasons for peeling; (2) the peelings morphology and EDS micro-area composition were analyzed by using the EDAX ky2800b scanning electron microscope (SEM) . (3) the processes of copper strip production were analyzed, the key control point and its influence are analyzed and improved, and the production control standard is established. (4) refer to GB/T232-2010 and astme 290-14, the sample size is 0.5 mm (thickness)10 mm (width)50 mm (length) in the direction of 90 ° and 180 ° perpendicular to the rolling direction of the copper strip, the test standard of copper strip is established according to the comparison between the bending results and the actual bending situation of the insert sleeve. (5) the improvement effect after the optimization of the control process is confirmed by collecting the actual data of the peeling ratio before and after the improvement.

2. Test results

2.1 morphological analysis of Peel

Figure 1 for the H62 copper insert case of the typical stamping peeling location and characteristics, can be seen that the occurrence of peeling places there are two major categories. The first is where the bending angle is large (Fig. 1(a)) . White powder is evident in the peeling. This powder may be borax covering agent added during melting. H62 is produced by semi-continuous casting in which Molten borax is deposited on the upper surface of the mold melt. During ingot drawing, part of borax is melted onto the surface of the INGOT, which is hot-rolled, the borax is distributed irregularly in the hot rolled strip. If the milling surface is not complete, part of the sand will remain in the copper strip, and these residual borax will be randomly distributed in the finished strip. The second type is the straight part (Fig. 1(b)) , and there are some defects such as stoma in the peeling part. During melting, some gas remained in the melt due to incomplete degassing, and remained in the ingot during casting, and finally distributed randomly in the finished copper strip with rolling deformation. Under the action of shearing force or bending force, material defects also have peeling phenomenon.

2.2 Sem and eds analysis

Fig. 2 and Fig. 3 are the SEM appearance and energy spectrum of the normal part and the inclusion part of the bent peeling of the copper strip.

As can be seen from Fig. 2, the Matrix elements are mainly Cu and Zn, and there are a few elements such as C, O, etc. . From the analysis of the element content of 3(A)(b) in the inclusion site at the peeling, it can be seen that the matrix elements are mainly Cu and Zn, and a small amount of C, O, SI, etc. . One of the main reasons for the inclusion of Copper Strip substrate is that the brittle phase [7] of CU2O can be formed by the combination of O and Cu in H62 during melting and casting, on the other hand, SI element in the lining material seeps out into the CU melt and results in inclusion. Further analysis of the morphology of the peeling site showed that the peeling originated from inclusions in the copper strip matrix. These inclusions can not deform in coordination with the Copper Matrix during the copper strip bending process, which leads to bending cracking, further increasing of the deformation and more obvious cracking, finally leading to delamination and peeling of the material until it penetrates all the porosity and other defects in the Matrix, and eventually form a curly crust.

3.Analysis and improvement of peeling in stamping process

3.1 critical control points and impacts of production processes

According to the results of the typical peeling of the pressed copper insert, the factors that may cause the peeling of H62 copper strip were analyzed from the production process of H62 copper strip, check and sort out the key control points which may affect the production process of stamping peeling. The possible factors affecting the peeling of H62 copper strip are: (1) raw materials, including source, drying degree and ratio; (2) melting casting, including temperature, covering agent, degassing and slag removal (uncontrollable) , borax addition (uncontrollable) ; (3) ingot processing, including head and Tail Sawing, surface Grinding (not controlled) ; (4) hot rolling, including surface oxide scale removal; (5) milling surface, including cutter replacement cycle, milling surface speed, depth, polished billet (not controlled) .

It can be seen that there are some uncontrollable factors in the copper strip production process. Through the analysis of each production process of H62 copper strip, the control points and influence analysis of the key process causing stamping peeling are as follows: (1) raw materials. Raw materials are divided into new materials and returned waste (factory waste and purchased waste) , raw materials are required to meet the relevant national standards and enterprise standards. In the actual production process, the completely new material and the returned waste material must be invested in a certain proportion, in principle, 20% of the new material (cathode copper, zinc ingots) + 80% of the factory returned waste (if the use of purchased waste, the proportion of its ingredients is 30% of the new material + 70% of the purchased waste) . The content of water and oil in raw materials should be controlled within the acceptable range by proper methods. High content of water and oil will result in too high content of gas in melt during melting and casting process, the phenomenon of rolling delamination is caused by the formation of porosity and other defects in the INGOT. (2) cast. Melting, the furnace temperature is generally observed through the frequency core induction furnace ammeter pointer swing and the number of fire injection to determine. The needle swing is caused by the electric current fluctuation caused by a large amount of zinc vapor evaporating from the inner part of the melting groove, while the flame injection is the phenomenon of zinc vapor ejecting from the melting melt when the melting temperature reaches a certain value. In the actual production process, H62 to spray fire more than 2 times as a furnace sign. Casting temperature is too high, will cause metal strong inhalation and oxidation, casting temperature is too low, easy to appear cold separation, cracks, slag and other surface defects. Casting speed is the speed at which the Ingot is pulled out of the mould. If the casting speed is too fast, the cavity is too deep and the temperature difference between the inside and outside of the Ingot is too large, the defects such as shrinkage hole, crack and looseness will easily appear inside the ingot. The cooling intensity is usually expressed in terms of the primary cooling water pressure of the crystallizer. In the actual production process, the cooling strength of the crystallizer is generally controlled at 0.05 MPA ~ 0.3 MPA. Borax is the main covering agent, it is suitable to cover the surface of melt in mould evenly. Too Much borax may lead to slag entrapment, too little will cause melt oxidation, inhalation and so on. (3) INGOT treatment. The cutting length of the head and tail should be reasonable. If the cutting is too little, there may be some defects such as Air Hole, shrinkage hole and inclusion in the ingots. The surface should be thoroughly polished, leaving no visible defects. When necessary, the cast blank shall be milled. The surface of the cast blank after milled shall be smooth and clean. No chips, pits, copper powder, dirt, milling steps or patterns shall be left, otherwise, the defects such as peeling, black wire and casting lines will be produced after cold-working and annealing. These defects may be pressed into the billet during rolling, resulting in stamping peeling of the finished copper strip. Hot Rolling. The INGOT shall be heated in a protective atmosphere isolated from air to prevent the formation of a surface oxide coating. The hot roll and the delivery table should be cleaned regularly to prevent the falling copper chips from being pressed into the billet and causing the skin forming. (5) milling surface. Should be timely replacement of milling cutter, as a result of milling cutter wear, not timely replacement of milling cutter will lead to uneven milling surface, part of the location of defects. The speed and depth of milling surface should be reasonable, the speed of milling surface is fast and the depth is shallow, which will reduce the quality of milling surface and increase the difficulty of detecting the surface defects after milling surface. Incomplete or non-polished polished polished blank will increase the risk of copper strip stamping skin.

3.2 improvement measures and inspection practices

3.2.1 strict control of key production processes

After analyzing the process and influencing factors of H62 copper strip production, the parameters of key control points are specified and standardized, and the control standard of H62 copper strip production is optimized. The control of key points before and after improvement is shown in Table 1. H62 Copper Strip production control standard upgrade optimization after strict implementation, the formation of enterprise-level controlled standard documents.

3.2.2 development of copper strip inspection specifications

There are some uncontrollable factors in the key control points of the production process of H62 copper strip raw material, so it is far from enough to control the quality of the production process. Therefore, it is very important to check the incoming material of copper strip for stamping and socket. According to the defects of Copper Strip and the feasibility of inspection, the bending tests of H62 copper strip with 90 ° and 180 ° r/t were carried out, and the evaluation standard of bending was established. The bending test method is carried out according to the actual thickness of the Strip with reference to the relevant National Standard and American standard. The copper band samples with different r/t bending were observed under optical microscope as shown in figures 4 and 5. It can be seen that for the 90 ° bending test, there is no orange peel on the bending surface when r/t = 0. For 180 ° bending test, when r/t = 2, the surface of the bend has begun to appear very slight orange peel phenomenon, and when r/t = 1, the surface of the bend has appeared microcrack. The actual condition of the surface after bending with 90 ° and 180 ° perpendicular to the rolling direction of the copper strip in mass production is shown in Fig. 6. According to the results of copper strip bending test in figures 4,5 and 6, and taking into account the roughness of the finished product surface, the qualification criteria of typical 90 ° and 180 ° bending tests for H62 copper strip are: (1)90 ° bending, r/t = 0, no orange peel phenomenon; (2)180 ° bending, r/t = 3, no Orange Peel.

3.3 comparison of key control processes before and after optimization

After the implementation of the quality control standard for H62 copper strip production, the statistics of the number of pilling and the number of incoming materials before and after the improvement of the key control processes of H62 copper strip from three suppliers were collected, and are shown in Table 2. It can be seen that after the optimized implementation of the production control standard, according to the ratio of tons of incoming material and times of peeling, the average ratio of stamping peeling of H62 copper strip is obviously reduced from the average of 294 t/times before the improvement to 819 t/times after the improvement.

4.Conclusion

(1) in the process of semi-continuous casting of H62 copper strip, inclusions such as furnace lining material and Borax melt into the INGOT are randomly distributed in the finished strip. 

(2) the key influencing factors of H62 copper strip stamping peeling are degassing, slag removal, milling surface speed, polished billet and so on. After optimization of production control standard, the average peeling rate of H62 copper strip was reduced from 294 t/times before improvement to 819 t/times after improvement. 

(3) the qualification standards of 90 ° and 180 ° bending test for H62 copper strip are: 90 ° bending, r/t = 0, no orange peel phenomenon, 180 ° bending, r/t = 3, no orange peel phenomenon. 

Source: Chinanews.com