1,market situation:

In recent years, China’s galvanized sheet production increased year by year, and the import volume is also growing year by year, the growth rate is even higher than the growth rate of production;After deducting production duplicate statistics, galvanized sheet consumption is about 5.27 million tons.After 1999, galvanized sheet imports exceeded production for the first time, the self-sufficiency rate of domestic products is less than 50%, and there is a trend of decreasing year by year.

(1) galvanized sheet consumption status quo

In recent years, the increase of domestic production of galvanized sheet is mainly the increase of hot galvanized sheet production, 1998-2002, hot galvanized sheet production increased from 970,000 tons to 2.2 million tons;Imports grew even faster, from 370,000 tons in 1998 to 1.56 million tons in 2002.Apparent consumption increased from 1.32 million tons to 3.67 million tons.In 2002, the domestic product self-sufficiency rate of hot galvanized sheet is about 60%, the domestic production and imports of hot galvanized sheet increased than the previous year, especially the growth rate of imports is larger, mainly due to the increase in demand for color coated sheet production with thin specifications hot galvanized sheet, resulting in a decline in the self-sufficiency rate of domestic products.

(2) galvanized sheet consumption status quo

In recent years, with the rapid development of the family car market, the basic popularity of household appliances, personal computers, a large number of exports of electronic products, the development of the anti-theft door market, so that the demand for galvanized sheet growth too fast.At present, the production scale of galvanized sheet in our country, whether from the quantity or from the variety, quality, can not meet the demand of the domestic market.From 1998 to 2002, the domestic production of galvanized sheet increased slowly, from 170,000 tons to 400,000 tons.Imports grew rapidly, from 730,000 tons to 1.78 million tons.Apparent consumption increased from 870,000 tons to 2.16 million tons.The self-sufficiency rate of domestic products is less than 20%.

2,The development situation:

The increase of domestic galvanized sheet consumption mainly depends on import pull.In recent years, galvanized sheet imports are increasing year by year, imported varieties are mainly galvanized sheet, but the proportion of galvanized sheet imports is decreasing year by year;Correspondingly, the proportion of hot galvanized sheet imports increased year by year.In 2003, the import of hot galvanized sheet exceeded the import of electrogalvanized sheet for the first time.

2.1.2 Development status of hot-dip galvanized substrate

The hot dip galvanizing unit for automobile plate focuses on the production of outer plate. The production specification plate thickness is mostly between 0.6~1.2mm, plate width is mostly 800~1850mm, and the widest can reach 2000mm.The product levels are quite complex, ranging from CQ, DQ, DDQ, EDDQ(IF), to high strength automotive steel plates such as BH, DP, TRIP.Through the use of a variety of advanced technology, the coating morphology, thickness precision can be precisely controlled, such as coating thickness error is less than 2~3g/m2.By using lead-free galvanizing can produce zinc-free flower steel plate, this coating is not easy to produce intergranular corrosion, long service life, suitable for automobile and electrical industry.Almost every hot-dip galvanizing unit in Japan has a zinc-ferroalloying furnace. The thickness of the standard galvanizing layer of the zinc-ferroalloying plating plate (Galavneal) is 160~180g/m2 on both sides, and the alloy coating contains Fe8%~12% [2].

Microalloyed and phosphorus alloy steels were developed abroad in the 1970s, bidirectional DP and bake-hardening steels were developed in the mid-1980s, and phase change and induced plasticity TRIP steels were developed in the 1990s [3].Currently, the international project “Ultimate Light Steel Body (ULSAB) Development” is well underway. The goal is to achieve a fuel consumption of 34KM/3L(USA) and a weight reduction of 25%.A large number of high strength and ultra-high strength steel plates are used in the car body structure.The biggest characteristic of the ultra-light steel car body through the large ULSAB is that it does not achieve the purpose of reducing the weight at the expense of performance [4].Ultra-light steel car is designed on the basis that the proportion of high strength steel in the body structure is more than 90%. In Japan, the proportion of high strength steel in the body increases from 10% to 30% in the ten years from 1985 to 1995. With the improvement of surface treatment performance, the service life of the car increases from 6.Seven years to nine.3 years, deducting the high performance steel plate manufacturing energy increase, a year can save 357*104L of heavy oil, almost equivalent to saving domestic steel industry energy consumption a few percent.In 2000, the use of high strength steel plate has reached 36.3% in Japanese automobile manufacturing.

The types and properties of hot dip galvanized substrates are described below.

6) IF steel alloyed galvanized sheet

It is difficult to produce galvanized sheet with high quality rimmed steel and killed steel for automobile stamping forming.For this reason, in the 1970s, the emergence of IF steel (non-gap atomic steel), which has excellent stamping formability, it is suitable for the inside and outside of the automobile plate.IF steel has excellent deep drawing performance, which can meet the stamping requirements of most difficult parts. Therefore, it is gradually replacing 08AL series automotive steel. At present, IF steel has been widely used in DDQ polar steel plate abroad.On the basis of IF steel, the high-strength steels are also produced by adding Mo, Cu, P and other elements.

The emergence of IF steel puts forward higher requirements for the technical level and management level of iron and steel enterprises.In terms of technology, molten iron pretreatment technology must be adopted to ensure that molten iron with low phosphorus, sulfur and silicon is used as raw material for steelmaking.In order to ensure high composition accuracy and purity of molten steel, post-furnace refining technology and online carbon and oxygen analysis technology must be adopted.RH(medium and ultra-low carbon steel high-speed decarburization process) must be used to ensure the very low carbon and gas content of the molten steel; low-carbon covering agent, protective slag and resistant materials must be used to ensure that the molten steel does not return to carbon; for the automobile exterior plate, the surface of the plate blank must be cleaned;The strip temperature control technology from heating to finishing rolling and coiling must be adopted to ensure the uniform mechanical properties of hot rolled strip steel.High precision hot and cold rolling technology must be used to ensure excellent shape and high precision dimensions and to prevent roll marks and coiling defects, etc.In other words, in each process of steelmaking, continuous casting, hot rolling and cold rolling, production must be organized strictly in accordance with the end use of the product. In particular, when the variety of steel is frequently changed, corresponding measures must be adopted to ensure the quality of the final product.Another major issue is how to link the galvanizing line with other production processes through an integrated production system to minimize costs.In short, the production of automotive galvanized sheet for every steel enterprise, especially domestic 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, P and other elements are usually added for solid solution strengthening, and martensite and bainite are used for microstructure strengthening. For DP dual-phase steel, martensite structure can be obtained by controlling the content of alloying elements and adopting critical cooling rate.

The requirements for automotive panels are low yield strength OS < 240MPa >) and high yield value.In IF steel, P, Mn, Si and other solid solution strengthening elements are added. Since there is no solid solution C and N in this steel, it is basically non-time-sensitive steel plate.However, in the absence of solid solution C, it is easy to cause secondary machining embrittlement, so trace amount of B can be added to prevent it.When P is the main strengthening element, OB can reach 340~ 370MPa, while Mn and P are the main addition elements.Its OB can reach 390~ 440MPa [5].

8) bake hardening type (BH steel) and heat treatment strengthening type steel plate

BH steel is bake hardened steel.If steel belongs to ultra-low carbon steel, and the C content of BH steel is 0.016~0.020%. By using the method of strain aging and strengthening, residual trace (5ppm) solid solution C in ultra-low C steel will produce dislocations during pressing and forming, and Fe-C compounds will be precipitated out to fix the dislocations during later coating and baking (170℃, 20min), thus increasing its strength.The BH cold-rolled steel plate produced by adjusting its chemical composition also has good deep drawing performance. The addition of trace elements such as Nb and Ti can make it form carbonitrogen compounds in the rolling state.Its R values ranged from 1.92 to 2.06.This kind of BH cold-rolled steel plate can increase the tensile strength of automobile parts by 40~ 70MPa at the baking temperature of 170 ‘C.At present, 340Mpa grade BH cold-rolled steel plate has been developed [6].

9) Tissue strengthening type high strength steel plate

By changing the microstructure of steel, its strength (440~ 1470MPa) can be improved in a wide range. At present, three kinds of high strength steel plates have been developed, and the representative ones are dual-phase (DP) steel plate and bainite steel plate.This steel has low yield strength, high work hardening index and good formability.In addition, TRIP(metamorphosis induced superplastic section steel) and ultra-high strength steel plates are expected to be applied to automobiles on a large scale.At present, the hot-dip galvanizing and alloying treatment of these steel plates have not been carried out, except for a small amount of researches on hot-dip galvanizing of DP steel plates [7].The hot-dip galvanizing and alloying treatment technology of these steel plates will be quite difficult, which needs to be explored [8].

The research of hot dip galvanizing substrate must be combined with the hot dip galvanizing process, otherwise, even if the substrate reaches the required performance, the galvanizing process can not meet the requirements and can not achieve the purpose.

Development of galvanizing technology

2.1.3 Development of galvanizing technology

In order to obtain high quality surface, in the 90 s foreign new galvanizing production line of steel union method is used in great quantities, steel union method adopts the technology of the high density electric release ester (HCD) can effectively remove the surface of the oil, at the same time in reducing annealing furnace steel unite method of China and the United States use total radiation tube instead of improved sen firing in Jimmy’s method, so it can strictly control the temperature and atmosphere, to ensure the furnace without the oxidizing atmosphere, so as to ensure the quality of the coating adhesion and surface quality.

The process layout of traditional hot dip galvanizing line is to set up a light machine.The modern production of wide strip steel plate galvanized hot rolling line, generally set up in the process layout of two finishing machines, the first finishing is to use a four-roller finishing machine to roll the surface of the strip steel, eliminate the uneven surface appearance;The second finishing is to choose a two-roller finishing machine to give the strip a certain roughness.

The traditional hot dip galvanizing operation line is double looper, arranged at the exit and entrance of the process section.Due to the extremely high requirements of the automobile board to the surface, sometimes 5~10 volumes of the automobile board to replace the smooth roller, in order to achieve non-stop roll change in the process section, improve the operating rate, so some units in front of the light machine to add a looper, that is, to form a line of three looper process layout, the storage time of the new looper is generally 120s.

The production capacity and product quality of newly put into production or old hot-dip galvanized units abroad have been further improved. The newly built production lines in developed countries can produce hot-dip galvanized steel plates with width up to 2100mm. The speed of the unit is up to 200m/min, the alloying speed is 136m/min, and the annual production capacity reaches 600,000 tons.

Folding hot dip galvanizing equipment development

2.1.4 Development status of hot dip galvanizing equipment

The air knife is a key component to control the thickness and uniformity of the coating. The new air knife adopts adjustable integral frame structure, which can reliably and accurately adjust the gap and Angle between the air knife and the steel belt, and the distance between the air knife and the zinc pot. The gap between the knife lip can be automatically controlled to ensure the flatness and uniformity of the coating.

Liquid zinc in zinc slag is the most important factors affecting the quality of the surface of galvanized sheet, in the modern production line, iron, zinc pot, has been replaced with ceramic liner pan with removable or promotion to replace double zinc pan to replace the plating solution, using online measurement probe (Al sensor) and corresponding liquid zinc composition control system can strictly determination of aluminum and the corresponding iron content in liquid zinc, zinc slag can be greatly reduced impact on surface quality of steel sheet.

The vertical zinc pot designed by using the magnetohydrodynamic closure (NHD) created by the electromagnetic moving field can cancel the components in the original zinc pot, and achieve the advantages of high speed, no zinc slag, energy saving and improving the surface quality of zinc layer [9].The provisions of the working principle is: the liquid zinc or Al, ZnAl alloy after from the bottom of the pot and flows into MHD channel up to a certain depth (the depth depends on the metal liquid density, liquid level height, current size etc.), move because ac sensor produced by the external electromagnetic field induced current in liquid zinc, the electric current and the interaction of external electromagnetic field and electromagnetic force, it ACTS on the metal fluid to move up.So the zinc can’t get out of the opening in the bottom of the pot.When the steel strip enters the zinc pot from the bottom channel, the zinc liquid is always in the same position in the channel, and the zinc pot does not need any steel structure.

In order to improve the operation speed and production efficiency of the unit and reduce the formation of surface slag and bottom slag, the related research units abroad have developed “zinc pad” technology and “vertical zinc pot” technology.”Zinc pad” technology works on the principle of liquid metal pad, the steel strip is pressed through the zinc pad by liquid metal pressure, because the submerged roller is not used, thus the product surface quality is improved, the production efficiency is improved, the cost is reduced, especially suitable for high-speed units.

Due to the emergence and application of laser Tilted Welding Technology, the automobile factory can weld the narrow plate together after use, which makes the production of a small amount of wide plate (width of more than 1800mm) is no longer necessary, but the width of the product of cold rolling mill reaches 1550mm, can produce galvanized steel plate for the automobile.

With the continuous development and application of these advanced technology and equipment, the proportion of hot galvanized sheet used in automobile manufacturing is increasing.For example, Japan has been as high as 81%, Canada 76%, Italy 71, France 70%, Germany 60%, the United States 45%.Due to the superiority of alloyed galvanized sheet cost and quality, the Japanese automobile industry decided to use alloyed galvanized sheet completely in 2005, replacing the existing electric galvanized alloy sheet.

Post time: Jan-14-2021