Production technology and performance of sawn steel wire for photovoltaic cells

　　A sharp knife for cutting silicon wafers

　　Silicon wafer thinning technology. Photovoltaic cells used in photovoltaic power generation are components that absorb solar energy and convert it into electrical energy. The raw material is usually monocrystalline silicon or polycrystalline silicon. The silicon ingot is cut into thin slices through a flaking process before use. In the past, when cutting silicon ingots, the cutting method with an inner circular edge was often used, which is to hollow out the middle of the stainless steel sheet into a ring shape, and then attach a sharpened stone to the engraved inner ring, and then use it to cut off individual silicon wafers. Nowadays, a device that uses multiple steel wires to cut silicon wafers is mostly used. The principle of this device is: spread a steel wire between two guide rollers, and saw the silicon ingot into silicon wafers through the running of the steel wire. The steel wire on the guide roller passes through about 500 parallel grooves, so 500 silicon wafers can be sawed at one time. The steel wire is coiled from the vertical cylinder on one side, supplied along the roller groove, and pulled out from the vertical cylinder on the other side, with a total length of about 50 kilometers. In order to produce more silicon wafers from high-priced silicon ingots to reduce costs, measures have been taken to thin the silicon wafers. The thickness is mainly 160 microns to 200 microns, and hundreds of silicon wafers can be produced at a time by sawing.

　　Advances in wire cutting technology. The current method of cutting steel wire is to apply a mixed slurry of oil and abrasive particles on the steel wire to form a free abrasive steel wire for processing. This method can cut multiple silicon wafers at the same time and adapt to the trend of thinner substrates, but the main problem is the large amount of waste generated. The current cutting method is changing to fixed abrasive grain steel wire, that is, diamond abrasive grains are fixed on the steel wire with electroplated nickel. The advantages of this method are short processing time, reduced costs, and a significant reduction in waste output. However, the fine-diameter steel wire used in this method is more expensive.

　　Currently, research and development is underway on sawing devices for multiple steel wires, research on saving steel wire usage and reducing wire load, and a swing-type sawing method that cuts the wire angle while changing it like using a tooth saw to cut wood.

　　Fine diameter and highly intensive cleaning

　　Challenge yourself to cutting the wire continuously. When using sawing steel wire to cut thin sheets, in order to shorten the sawing time and improve the yield of silicon wafers, the diameter of the steel wire is required to be reduced, and the strength of the steel wire is required to ensure the quality of the silicon wafers. Therefore, there is a need to develop and supply extremely high-strength wire rods. At present, eutectoid steel wire rods with ferrite + pearlite (cementite are arranged in layers) structure and high carbon content (up to 0.82%) are generally used and processed into fine wires.

　　The specific process is: after the 5.5 mm hot-rolled wire is removed from the iron oxide scale and treated with the film, it is then subjected to dry stretching, toughening treatment, anode coating and wet stretching to finally produce a saw with a diameter of 0.145 mm to 0.250 mm. Break the wire. The most important requirement for sawing wire is that it must not break during use. For example, when sawing a 156 mm square silicon ingot into 0.3 mm thin slices, the steel wire must be cut up to 10 kilometers long, and the cutting time may take up to several hours. If the wire breaks midway, all the material will be scrapped. In order to ensure the yield of high-purity silicon ingots, it is necessary to ensure that the sawing steel wire is not threaded, and the sawing steel wire must be able to withstand repeated bending stress and tensile stress when sawing the thin sheet.

　　Inclusion control is the focus of production. The main causes of wire breakage during sawing include inclusions, surface cracks and central segregation. In order to prevent wire breakage when sawing, the most important measure taken is to control inclusions. Even though the size of inclusions in the wire is only tens of microns, it can still cause wire breakage, so reducing the inclusions in the sawn steel wire has become an important issue.

　　There are two main categories of channels for inclusions in wires. The first type is mixed with refractory materials during the smelting process, but high-temperature molten steel cannot be separated from refractory materials. Therefore, refractory materials that are refractory can only be used to avoid the formation of inclusions. In particular, the refractory materials in contact with the molten steel should maintain their due Strength and fire resistance. The other type is inclusions analyzed from the components contained in molten steel, represented by hard Al2O3, etc., which also have poor ductility in hot rolling and cold working. Therefore, the content of such elements in molten steel should be controlled as much as possible, and technologies to reduce inclusion content should be further studied.

　　As a measure to detoxify inclusions, technology to lower the melting point of inclusions and improve the ductility of steel wires is also being developed and put into practical use. The inclusions in the wire rod used for sawing steel wire originate from the CaO-SiO2-Al2O3 series in the slag and the MnO-SiO2-Al2O3 series of deoxidation products. After the components of the two series are hot-rolled and extended, the Al2O3 content in the inclusions reaches 20 %about. Therefore, Al2O3 can be made harmless by controlling the components in the slag.

　　At the same time, as a common technology to improve the quality of high-strength wire rods, various technological improvements such as surface defect prevention, continuous process control technology, and improvement of the accuracy of steel quality inspection are also important.

　　The strongest wire among steel products. The tensile strength of sawed steel wire increases as the diameter of the steel wire decreases. The relationship between the tensile strength of sawed steel wire and the diameter of the steel wire is shown in the table. Sawed steel wire is classified as the strongest among steel products, because the tensile strength of the sawed steel wire currently used has been increased to more than 4000MPa, while the tensile strength of the steel wire used in Japan's famous Akashi Kaikyo Bridge is only 1900MPa.

　　The relationship between the tensile strength of sawn steel wire and the diameter of the steel wire

　　The quality of wire rods for sawing steel wire produced in Japan has been generally praised. The main reason is that Japanese steel companies have achieved high-cleanliness steelmaking technology. The market demand for photovoltaic power generation products will continue to develop rapidly in the future, and the competition among sawed steel wire production companies will become increasingly fierce. In order to adapt to the requirements of high strength and thin diameter of sawn steel wires, clean production of wires should be adhered to and the generation of inclusions should be controlled.

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