July 6, 2022
Domestic galvanized sheet consumption increases mainly rely on imports to pull. In recent years, galvanized sheet imports increased year by year, imported varieties are mainly galvanized sheet, but the proportion of galvanized sheet imports decreased year by year; Accordingly, the proportion of hot galvanized sheet imports increased year by year. In 2003, the import of hot galvanized sheet in China exceeded the import of galvanized sheet for the first time.
1, an overview of the
At present, China's iron and steel industry in the variety, quality, cost, service and labor productivity and the world advanced level there is still a certain gap, in the face of increasingly fierce domestic and international market competition, must speed up the pace of structural adjustment, it is not only the need of the survival and development of metallurgical industry in China, also is the development trend of metallurgical industry in the world today. Hot Dip Galavnizing was invented in the middle of the 18th century. It is developed from the Hot Dip galvanizing process. After 50 years of development, the continuous hot dip galvanizing process of steel strip has become a mature and advanced industrial technology. So far, hot dip galvanizing is still the most common and effective process measure used in steel corrosion prevention method. In the construction industry, used in all kinds of industrial and civil buildings of light steel keel, building board, corrugated board, rolling doors, etc. In household appliances and light industry, for all kinds of home appliances shell and base plate; Galvanized steel sheet is widely used in light industry, household appliances, automobile and construction industries. In the automobile industry, used for car body, shell, inner plate, bottom plate, etc. China vigorously develops electric power, transportation, communication, energy and urban infrastructure. With the strong development of construction, household appliances and automobile industries, China's galvanizing industry is facing unprecedented opportunities and a huge market.
In the past twenty years, the continuous hot dip galvanizing technology, equipment and corresponding control means of steel strip have great development, so that galvanized sheet can meet the harsh requirements of automobile panel and household appliances board, and thus to its cheaper than electrogalvanized alloy steel strip, excellent corrosion resistance and have the potential to replace. This topic plans to draw lessons from foreign advanced high performance automobile hot dip galvanized production technology and equipment development technology and experience, through technical innovation, optimization design, precision manufacturing and foreign technical cooperation, to break through the bottleneck restricting China's high performance automobile hot dip galvanized steel production technology and equipment development. At the same time, the significance and prospect of hot dip galvanizing technology development in sustainable development of national economy, energy saving and mineral resources are discussed.
2, automotive high-grade hot galvanized sheet development situation at home and abroad
Automotive industry products with high efficiency, high reliability, high integration requirements, automotive hot dip galvanized steel plate is the best and the best galvanized steel plate. Since 1975, the United States for the first time the hot galvanized sheet used in automobile industry more than 20 years, the world's car production country in an effort to prevent corrosion research and development as the goal, and considering the weldability, coating and formability of hot galvanized steel sheet, high quality and obtained a series of technological progress, including the chemical composition of the steel substrate, the surface of the steel plate rolling quality, A series of technical problems such as thickness uniformity of hot dip galvanizing coating, optimal structure and process control of alloying. After 20 years of research, the above technical problems and the process equipment to complete the technical requirements have been basically solved and began to be applied in a large number of automobiles. The successful application of these technologies can greatly improve the working state of automobiles, prolong their life and save energy.
High surface quality, high strength and Gao Shenchong sex is necessary possess the basic condition of automobile panel, the coating surface quality and the surface quality of plate roughness, cleanliness, strip steel chemical composition and its stamping formability has a direct relationship, while zinc plating process, fluid composition, hot plating equipment are also important factors affecting the quality of coating surface. The high strength and high stamping property are closely related to the chemical composition, physical properties, heat treatment process and hot plating performance of hot-dip galvanized substrate. The following will focus on the development status and trend of coating varieties, substrates, processes and equipment of high-block hot-dip galvanized plate for automobile at home and abroad.
Foreign development
2.1 Development trend of hot galvanized sheet abroad
2.1.1 Development status of hot dip galvanizing coating varieties
1) Lead-free hot dip galvanizing coating
Beautiful zinc flower is the typical characteristic of traditional hot galvanized sheet, which is favored by users and has certain commercial value. But the research shows that the surface of zinc flower is not smooth, the lead content in the coating will affect the processing performance, coating performance and corrosion resistance in the use of the coating plate after processing, so since the 1960s, the production technology of non-zinc flower and small zinc flower has been studied abroad. Heurfey can increase the number of crystal nucleation and control the crystal growth by water spray cooling and zinc powder spraying. Since the 1980s, foreign countries began to study lead-free hot dip zinc technology. At present, new galvanized production lines in Europe and South America all use lead-free galvanizing process. Lead-free galvanizing has become the current development trend of hot dip galvanizing.
2) Alloyed coating of hot galvanized sheet
In the hot-dip galvanized variety, alloyed galvanized sheet was developed by thermal diffusion treatment after plating, and its corrosion resistance was more than one time higher than that of ordinary hot-dip galvanized sheet. Through this alloying treatment, zinc flower on the surface of the coating could be eliminated. At the same time, zinc evaporation during spot welding is reduced, and zinc vapor pollution of copper electrode is reduced, thus the electrode life is improved. To ensure good adhesion of alloying coating of galvanized sheet, for car use the thicker the alloying of galvanized sheet of plate used efficient heating and cooling measures (high burning nozzle, or induction heater), and provide various types of alloying sensor, through analysis instrument and computer instructions in a timely manner, adjust the structure of the coating in the corresponding to the deformation of high production suitable for stamping requirement.
Due to the conditions of automobile stamping molding, in addition to the requirements of steel substrate with necessary excellent mechanical properties, the adhesion of coating (alloying layer) is also very demanding.
In the past 20 years, a lot of research has been carried out on the relationship between the stamping formability of alloy galvanized sheet and the microstructure of the coating. The results show that the key factors to achieve good adhesion of the coating are the phase structure and iron content of the coating. In order to obtain the best coating phase structure and suitable iron content, the coating homogenization technology and alloying control technology have been developed. In order to strictly control the phase structure of the coating, the heating and subsequent cooling processes during alloying must be precisely controlled. The measures are as follows:
(1) The coating reflection factor determination element was set between the heating zone and the holding zone to measure the coating surface alloying and control the temperature of the steel strip to keep within the temperature range of stable δ1 phase formation (laser beam method);
(2) A high thermometer and an emissivity tester are set between the insulation and spray cooling areas to measure the temperature and emissivity of the steel strip at the same time;
(3) After cooling, the steel strip is alloyed with a phase structure sensor at an appropriate position to measure the iron content in the coating and the thickness of δ1, γ and ζ layers in the coating (fluorescence or X-ray diffraction);
(4) The coating thickness (weight) tester (fluorescent X-ray method) is set before the steel strip enters the alloying furnace after the air knife and after the phase structure sensor.
3) Two layer alloyed hot dip galvanized steel plate
Alloy galvanized sheet is prone to bubble and other film defects during coating. Nippon Iron has developed two layer alloy hot dip galvanized steel sheet. In this two-layer alloying hot-dip galvanized steel plate, 10%FeZn alloy layer is hot-plated on the bottom of the steel plate, and the zinc adhesion is 30g/m2 for 5 years and 60g/m2 for 10 years. The upper layer is electroplated with 80% Iron-rich FeZn layer, and the adhesion is 3g/m*2. The experimental results show that the newly developed two-layer alloyed hot-dip galvanized steel plate with 10%FeZn at the bottom and adhesion capacity of 30~60g/m2 can fully meet the requirements of automobile corrosion resistance. The upper layer of the iron rich 80%FeZn layer adhesion 3g/m2, the hardness of the upper layer is improved, not only solves the arc pit defect, but also reduces the impact pressure. For example, when the attachment amount of 80%FeZn layer is 1g/m2, the hardness is 300HV and the impact pressure is 1960Mpa, and there are still arc pit defects. When the adhesion amount is 3g/m2, the hardness is 350HV, the impact pressure is 1770MPA, and there is no arc pit defect, which indicates that the newly developed two-layer alloyed hot dip galvanized steel plate has good processing and forming performance.
4) Difference thickness zinc layer
By controlling the air knife pressure, the thickness difference ratio between the two galvanized layers is 1:3. In this way, the thick side of the zinc layer has good corrosion resistance and the thin side has good weldability. This kind of zinc layer is often used in automobile manufacturing.
5) Single side coating
Single zinc layer is one side of a certain thickness of zinc layer, and the other side is not galvanized galvanized sheet. Production methods of single - sided hot - galvanized sheet are zinc - phobic method, direct method, indirect method and double - layer separation method. Single-sided hot galvanized sheet is mainly used in automobile manufacturing, the surface of zinc layer is anticorrosive, and the surface without zinc layer is conducive to spot welding. Because alloying plate (zinc-ferroalloy) has good weldability, so single-sided galvanized sheet has been gradually eliminated.
Hot dip galvanized substrate development
2.1.2 Development status of hot-dip galvanized substrate
Automobile plate hot dip galvanizing unit focuses on the production of outer plate. The thickness of the production specification plate is mostly between 0.6~1.2mm, the width of the plate is mostly 800~1850mm, and the widest can reach 2000mm. The product level is quite complex, from CQ, DQ, DDQ, EDDQ (IF), to BH, DP, TRIP and other automotive high strength plates. Through the use of a variety of advanced technology, so that the coating morphology, thickness accuracy is precisely controlled, such as coating thickness error is less than 2~3g/m2. Zinc-free sheet steel can be produced by using lead-free zinc plating, which is not easy to produce intergranular corrosion and has a long service life. It is suitable for automobile and electrical industry. Almost every hot-dip galvanizing unit in Japan has set up a zinc-ferroalloy furnace. The standard thickness of zinc-ferroalloy plating plate (Galavneal) is 160~180g/m2 on both sides, and the alloy coating contains Fe8%~12%.
In foreign countries, microalloy steel and phosphorus alloy steel were developed in the 1970s, bidirectional DP steel and bake-hardening steel were developed in the mid-1980s, and TRIP steel with phase change and induced plasticity was developed in the 1990s [3]. At present, the international cooperation project "Ultra-light Steel Body (ULSAB) Development" is well underway. Its goal is to achieve fuel consumption of 34KM/3L (USA) and body weight reduction of 25%. The basic starting point of the car body structure is to use a large number of high and ultra high strength steel plates. The biggest characteristic of ultra-light steel car body through large ULSAB is that it does not sacrifice performance to achieve the purpose of reducing vehicle weight [4]. The ultra-light steel car is designed based on the proportion of high-strength steel in the body structure of more than 90%. During the decade from 1985 to 1995, the proportion of high-strength steel in the body of Japan increased from 10% to 30%. With the improvement of surface treatment performance, the life of the car increased from 6. Seven years extended to nine. In 3 years, after deducting the increase in energy of high performance steel plate manufacturing, 357*104L heavy oil can be saved in a year, almost equivalent to saving a few percent of the energy consumption of the domestic steel industry. In 2000, the use of high strength steel plate in Japanese automobile manufacturing reached 36.3%.
The types and properties of hot-dip galvanized substrates developed at present are described below.
6) IF steel alloyed galvanized sheet
It is difficult to produce zinc-plated sheet suitable for automobile stamping and forming with high quality boiling steel and killed steel. For this reason, in the 1970s, IF steel (non-gap atomic steel) appeared, it has excellent stamping molding, which is suitable for the inside and outside of the car plate. IF steel has excellent deep drawing performance and can meet the stamping requirements of most difficult stamping parts, so it is gradually replacing 08AL series automotive steel. At present, IF steel has been widely used in DDQ pole steel plate abroad. Each high strength steel is also produced on the basis of IF steel by adding Mo, Cu, P and other elements.
The emergence of IF Steel has put forward higher requirements for the technical level and management level of iron and steel enterprises. In terms of technology, it is necessary to use hot iron pretreatment technology to ensure the use of low phosphorus, sulfur and silicon hot iron as raw material for steelmaking. It is necessary to adopt post-furnace refining technology and on-line carbon and oxygen analysis technology to ensure the high accuracy and purity of the composition of molten steel. Must use RH (ultra low carbon steel high-speed decarbonization process), to ensure that molten steel is very low carbon and gas content, must use low carbon covering agent, protection slag and resistant material, to ensure that molten steel does not return to carbon, for the outer plate of the car, must clean the surface of the plate blank; It is necessary to adopt strip temperature control technology from heating to finishing rolling and coiling to ensure uniform mechanical properties of hot rolled strip. It is necessary to use high-precision hot rolling and cold rolling technology to ensure excellent plate shape and high-precision size, as well as to prevent roll printing and coiling defects. A system of production process quality assurance system is very necessary, that is to say, in steelmaking, continuous casting, hot rolling and cold rolling each process, must be strictly in accordance with the final use of the product to organize production, especially in the frequent change of steel varieties, must use the corresponding measures to ensure the quality of the final product. Another major issue is how to link galvanizing production lines with other production processes through an integrated production system to minimize costs. In short, the production of automobile galvanized sheet for every iron and steel enterprises, especially domestic iron and steel enterprises, is a very big challenge.
7) Hot dip galvanizing of solid solution reinforced steel plate
In order to improve the strength of steel, Si, Mn and P are usually added for solution strengthening, and the microstructure of martensite and bainite is used for strengthening. For DP dual-phase steel, martensite microstructure can be obtained by controlling alloying element content and adopting critical cooling rate.
As automotive panel requirements are low yield strength OS < 240Mpa >) and high ￥value. In IF steel, P, Mn, Si and other solid solution strengthening elements are added, because there is no solid solution C and N in this steel, it is basically a non-time-sensitive steel plate. However, without the presence of solution C, it is easy to cause secondary processing embrittlement, so trace B can be added to prevent it. Ob can reach 340~370Mpa when P is the main strengthening element, but when Mn and P are the main elements. Its ob can reach 390~440Mpa.
8) Baking hardening type (BH steel) and heat treatment strengthened steel plate
BH steel means baking hardened steel. IF steel belongs to ultra-low carbon steel, the C content of BH steel is 0.016~0.020%. By using the method of strain aging and strengthening, the residual trace (5ppm) of solid solution C in ultra-low C steel produces dislocations during stamping forming, and fe-C compounds will precipitate and fix the dislocations during coating and baking (170℃, 20min). Thus increasing its strength. BH cold rolled steel sheet produced by adjusting chemical composition also has good deep drawing performance. Add Nb, Ti and other trace elements to form carbon and nitrogen compounds under rolling condition. Its R value is 1.92~2.06. This BH cold-rolled steel plate can increase the tensile strength of auto parts by 40~70MPA at 170 'C paint baking temperature. At present, 340Mpa grade BH cold rolled steel plate has been developed
