Discuss the advantages and disadvantages of electric furnace steelmaking and blast furnace steelmaking production technology, and analyze the two production products. It shows that in the future development of the iron and steel industry, enterprises need to choose the special steel smelting process reasonably.
Key words: electric furnace steelmaking; converter steelmaking; residue; nitrogen content control
Electric arc furnaces were first successfully developed in the 1920s to melt scrap steel and produce general-purpose steel products such as steel bars. Compared with the blast furnace-converter combined steel plant, the production of the short-process electric furnace steel plant is much simpler and more flexible. It can produce steel of normal quality and cheaper than the blast furnace-converter combined steel plant. All that is required is an adequate supply of scrap steel and electricity.
For a long time, the biggest factor restricting the development of mini-process steel mills is still the inability to produce some high-quality low-carbon steel normally. However, the overall development of technology and the fierce competition between the two processes in recent years have significantly changed the production efficiency. The result is that some world-class electric steel mills have replaced more costly conglomerates as suppliers of most of the high-quality carbon steel such as bar, girder, and SBQ.
In my country, in recent years, faced with the reality of increasing shortage of scrap steel resources, high-quality scrap steel prices and high electricity costs, special steel enterprises that use electric arc furnaces as the main smelting process have also begun to devote themselves to research on the process of smelting special steel with converters technology.
At present, the special steel produced by converter abroad has accounted for about 60% of the total special steel (Japan accounts for about 70%). The main steel types in special steel such as bearing steel, gear steel, spring steel and cold heading steel can be produced by the process of “blast furnace heating hot metal → converter + secondary refining → continuous casting → continuous rolling”.
At present, the converters of key general steel enterprises in my country have been equipped with secondary refining methods such as RH and LF, and a considerable proportion of high-quality special steel products have been occupied by general steel enterprises with the absolute advantages of low cost and low price.
In the future development of the iron and steel industry, the special steel smelting process should be selected scientifically and rationally. It is a subject that every enterprise is very concerned about the analysis of the advantages and disadvantages of electric furnace and converter smelting special steel.
Production process of special steel
At present, there are three main types of special steel production processes:
(1) Electric furnace process: electric furnace→secondary refining→continuous casting-rolling;
(2) Converter process: blast furnace→hot metal pretreatment→converter→secondary refining→continuous casting→rolling;
(3) Special metallurgy: vacuum induction melting, electroslag remelting, vacuum arc remelting, electron beam smelting, plasma smelting, etc. leaf forging or rolling.
Since most of the early special steels were smelted by electric furnaces, it is customary to form special steel factories, that is, electric furnace steel factories. This is mainly due to the characteristics of early electric furnace steelmaking and the product characteristics of special steel itself.
Electric furnace steelmaking relies on physical heat induced by electric energy for smelting, which can melt a large amount of alloys and scrap iron and steel in the furnace. In this way, electric furnace steel has certain advantages over converter steelmaking in terms of alloying. But its disadvantages are long smelting cycle, low production efficiency, expensive electricity price, high cost and small furnace capacity. And because special steel products have the characteristics of high alloy content, multiple varieties, small batches and high added value, the early use of electric furnaces to smelt special steel has achieved the purpose of making use of the strong and the weak. But at the same time, people have always believed that the converter is mainly used to smelt ordinary steel.
With the accumulation of social scrap resources, the development of direct reduction technology, the development of electric power industry, the rapid development of electric furnace steelmaking technology (large-volume electric furnace, ultra-high power electric furnace, etc.), molten iron pretreatment and out-of-furnace refining technology. Electric furnace steel mills are increasingly turning to the production of ordinary steel. Converter steel mills are increasingly producing special steel.
Analysis of advantages and disadvantages of electric furnace and converter production process
The development of hot metal pretreatment and secondary refining technology makes the converter process play an important role in the production of special steel. The converter process to produce special steel technology was first developed by Japan. Japan introduced oxygen converter technology in 1957. In 1961, the converter was used to produce 280,000 tons of special steel, accounting for 10% of the output of special steel at that time; in 1967, it produced 1.56 million tons, accounting for 23%; in 1971, it produced 5.28 million tons, accounting for 51%. ;In 1985, the output reached 12 million tons, and in recent years it has been maintained at a level of more than 10 million tons, accounting for about 70% of the output of special steel.
Advantages and disadvantages of eaf steel making process
(1) The advantage of electric furnace steelmaking combined with LF/ND secondary refining to produce special steel is that the chemically neutral electric arc is used as the heat source. During the smelting process, heating and chemical reactions are independent of each other, thus ensuring the flexibility of the process. And because it allows a larger amount of alloy to be added, it has more obvious advantages in the production of high-alloy steel. The electric furnace steelmaking process is suitable for the production of steel grades with small batches, multiple varieties and high alloy content.
(2) Since the molten steel is heated by the electric furnace, the molten pool will be carbonized, and the composition control of the electric furnace smelting process is relatively easy, so the dominant steel types are medium and high carbon steel and high alloy steel. Since the molten steel in the arc zone absorbs nitrogen, it is difficult to produce products with low nitrogen content.
(3) When the charge is heated by electric arc, most of the physical heat generated is surrounded in the charge, and the heat loss taken away is less, so the thermal efficiency is higher than that of the converter steelmaking method. However, the primary problem of using a small-capacity electric arc furnace to produce special steel is that it cannot be equipped with an effective secondary refining method; secondly, it is difficult to improve the equipment level, which affects the precise control of composition and temperature; The composition fluctuates widely.
(4) High consumption and high production cost. The continuous improvement of modern large-scale ultra-high power electric furnace technology has basically solved the above problems.
Advantages and disadvantages of converter process
(1) The purity and quality stability of molten iron are better than scrap steel.
(2 The deep-use molten iron pretreatment process further improves the purity of molten iron S≤0.005%. P≤0.01%.
(3) The advantages of oxygen converter steelmaking combined with RH secondary refining process are extremely high production rate and excellent purity, so it is suitable for low-carbon/ultra-low-carbon and low-residual element steel grades, especially in large batches. Steel grades with large, low alloy content.
(4) The end point control level of the converter is high, and the slag-steel reaction is closer to equilibrium than that of the electric furnace.
(5) The gas content of molten steel in the converter is as low as N≤20ppm and H≤3ppm.
(6) Since the energy required for the smelting process depends on the supply of chemical energy released by the easily oxidizable elements in the steel and the oxygen interaction surface, because the surface temperature control is closely related to the metallurgical process, the process flexibility is poor.
(7) The converter smelting cycle is short, the production efficiency is high, and it is easy to form high-efficiency production with refining and continuous casting.
(8) The matching of converter and LF furnace can flexibly produce high and medium carbon and low alloy steel with certain alloy content.
(9) Converter smelting uses chemical heat and physical heat. The rich heat can only melt 10~15% scrap steel or alloy, so it is not suitable for smelting high alloy steel, especially for alloy tool steel, refractory tungsten iron and other high melting points. alloy.
Analysis of residue and atmosphere control in electric furnace steelmaking and converter steelmaking
Scrap-based electric furnace steelmaking has its advantages and disadvantages. It is extremely dependent on scrap prices and availability, and electricity is expensive in some places. While EAF production follows market demand better than BOF steelmaking processes, scrap prices follow economic activity. For small electric furnace-based mills to remain competitive, they must ensure adequate supply capacity in the scrap market. and. The quality of scrap steel also has a great influence on the quality of electric furnace steel. The chemical composition of the furnace scrap during smelting should be basically clear. And strive for stability, which is very important for smooth smelting, if a large number of scrap steel briquettes are used. It is difficult to figure out what is inside, and its chemical composition is difficult to grasp, so it is often easy to cause the carbon content of the smelt to be too high or too low, which brings difficulties to normal smelting.
An electric furnace in a steel factory used more briquettes than briquettes, and the quality was not high. There were many impurities and harmful chemical components. The monthly average molten sulfur content was as high as 0.146%, and some furnaces were as high as 0.4% or even 0.48%, and it is not uncommon for the phosphorus content in the smelt to be as high as 0.2% or more, which will greatly prolong the smelting time, increase the amount of lime and reducing agent, and increase the power consumption. As for the fact that the Cu content is too high, the whole furnace may be scrapped from time to time. What is more serious is that the burn-through of the furnace shell is likely to occur due to the prolongation of the smelting time, which brings a direct threat to personal safety and equipment safety. It can be seen that the quality and weight of scrap steel affect the quality of molten steel. It is not advisable to use a large amount of scrap steel with unknown composition, and the consequences and damages caused are also conceivable.
The long-process converter steelmaking uses iron ore as the basic raw material, and the amount of residual metal in the steel is lower than that of the electric furnace process, and it is far below the current standard. For a considerable amount of long products, the tolerance range of the residual metal content is relatively wide, which is acceptable when it does not damage the product properties. Even the residual metal content is quite high. This is the main reason why the products of electric furnace steelmaking plants are basically long products at present, while for flat products, end users have very strict requirements on them, such as deep drawing performance, high impact resistance and surface finish, etc. Therefore, residual Metal content must be low to achieve the required product properties in this field. Long-process converter steelmaking enterprises still occupy an absolute technological advantage.
Nevertheless, in the last 10-15 years, thanks to the development of electric furnace design, production and raw material supply, medium-thick slab continuous casting technology and direct rolling. Some electric furnace steel companies, especially in the United States, have entered the flat product market and produce standard strip steel. Of course, the production of most small steel mills is still concentrated in the field of long products, especially domestic enterprises.
The control of nitrogen content in molten steel is mainly related to process parameters. It is very difficult to control the helium content in the electric furnace. This is the biggest limitation to produce high-quality steel with the electric furnace process. Nitrogen content control has a special relationship with low-carbon killed steel. High helium content in steel will cause strain aging and reduce the ductility of steel. When this steel is generally used for deep drawing products, the nitrogen content is required to be less than 50ppm. In more stringent applications such as ultra-low hardness IV steel, the xenon content is required to be less than 30ppm, and often close to 20ppm. Although these specialized high value-added products account for only a small proportion in the product mix of converter steelmaking integrated steelworks, they express the most stringent production limits. Under normal circumstances, the argon content of the converter steel is only 1/3 of the electric furnace steel, the converter is 15-40ppm, and the electric furnace is 60-100ppm.
The traditional operation of the electric furnace is to complete most of the refining work with the slag door open, which leads to a large amount of air entering the electric furnace, which promotes the high xenon atmosphere in the furnace. Normally, ordinary electric furnaces are not airtight and have many openings in the shell and roof area through which air can pass freely. Therefore, the remarkable feature of the ordinary electric furnace steelmaking process mainly based on smelting scrap steel is that the nitrogen in the furnace atmosphere will be absorbed by the exposed molten steel. This is especially true in arc plasmas, where high temperatures break down helium into atomic or ionic nitrogen. Further promote the absorption of nitrogen.
For this reason, in the electric furnace process, the amount of nitrogen absorption can generally be reduced by effectively eliminating the contact between the gas source and the molten steel. In the production operation, this goal is mainly achieved by forming a large amount of protective foam slag around the arc, enhancing the carbon monoxide atmosphere above the molten pool, and reducing the xenon content of the injection gas.
In addition, “sealing” the electric furnace, that is, closing the slag door, helps to reduce the amount of air infiltration. Recent studies by the European Coal and Steel Community have shown that the nitrogen content of molten steel during EAF tapping varies with the amount of pre-reduced iron in the furnace charge, the amount of pig iron added, or the amount of carbon sprayed during decarburization compared to full scrap charging . Lower nitrogen levels can be achieved with operations that increase carbon monoxide generation during the decarburization period.
However, with the exception of a few cases, modern electric furnace design, operation, furnace feed materials, product specifications, etc. are not optimistic about achieving the lowest xenon tapping. A recent worldwide survey conducted by the World Steel Association shows that for many steel products produced on 53 electric furnaces, the tapping target helium is not lower than 40ppm, and although 40ppm seems to be achievable in basic principles, it can also be achieved. Citations of 20-30 ppm have been gathered, but the minimum achievable nitrogen level has not been established. The low nitrogen content of electric furnace steel is achieved under special circumstances, and low xenon steelmaking is the biggest requirement for the production of high-quality flat products.However, with the exception of a few cases, modern electric furnace design, operation, furnace feed materials, product specifications, etc. are not optimistic about achieving the lowest xenon tapping. A recent worldwide survey conducted by the World Steel Association shows that for many steel products produced on 53 electric furnaces, the tapping target helium is not lower than 40ppm, and although 40ppm seems to be achievable in basic principles, it can also be achieved. Citations of 20-30 ppm have been gathered, but the minimum achievable nitrogen level has not been established. The low nitrogen content of electric furnace steel is achieved under special circumstances, and low xenon steelmaking is the biggest requirement for the production of high-quality flat products.
In the past 20 years, the development of electric furnace steelmaking has been significantly faster than that of blast furnace-converter process. In particular, the number of electric furnace steel mills involved in the production of flat products is increasing, especially new plants with low investment costs and direct reduced iron as raw material are constantly emerging, or factories built on the original site where molten iron can be obtained.
The design of modern electric furnaces is directly related to thin slab and medium-thick slab technology. Coupled with direct rolling technology, its cost is much lower than the traditional sequential batch process. This is because compared with the traditional process, it is the result of simplified operation steps, less process equipment, short production time and low energy consumption.
For flat products, the competition between modern electric furnaces and technologically advanced conventional steelmaking processes has only just begun. This competition is strongly dependent on local raw materials, energy and market conditions.
However, the development trend is more reflected in the product structure, with regional or smaller production concentration, higher automation level and more specialization. That is to produce a certain range of ordinary or high value-added steel products, have their own consumption points, and be able to respond flexibly and quickly to changes in customers or markets. Because this layout is closer to customers, it reduces production and transportation costs by optimizing the distribution of products, and speeds up product development for these smaller scales. The cost competitiveness of plants with low capital intensity has ensured the actions to build new steelmaking capacity in selected regions and develop the market according to the conditions of energy raw materials, labor, market structure, transportation costs, market demand etc. there.
For United Steel Corporation, huge historical assets and long asset life severely restrict their response to the “market factory” production model. Because the new competitors are constantly improving in technology to improve the quality of steel, it is eager to meet at least the quality standards of the United Steel Corporation. Therefore, in the long run, the unfavorable situation of joint iron and steel enterprises is increasing.
Because the optimal allocation of market-oriented enterprises makes it faster, cheaper, and more market-responsive than traditional, centralized, and joint processes. Consolidated iron and steel enterprises can only improve production efficiency by streamlining production links and reducing energy consumption. At the same time, they must continue to develop higher quality standards. Only through these can we remain competitive.
In the field of flat products, the competition between modern electric furnace steelmaking and the combined blast furnace-converter process has just begun.