There are many ways to make steel. The basic principles are the same. The difference is that the sources of oxygen and heat needed in the smelting process are different, and the equipment and operation methods are different. At present, the steelmaking methods used in various countries include bof steelmaking, electric furnace steelmaking and open furnace steelmaking, and the main development trend is pure oxygen bof steelmaking. By 1976, bof steel accounted for 70% of the world’s total steel output.
This method is a new technology developed after 1952. It is a more advanced method adopted in the world at present. Pure oxygen bof steelmaking has the following advantages:
(I) fast production speed due to the use of pure oxygen blowing, it will be high-speed carbon reduction, rapid temperature, greatly shorten the smelting time. A 300T converter blowing time is less than 20min, including auxiliary working time, a total of not more than 1h.
(II) Variety, good quality pure oxygen top blown converter can smelt ordinary steel, but also ordinary low carbon steel. For example, capital Steel Works has successfully tested more than one hundred kinds of steel by this method. Due to the use of pure oxygen blowing, the content of harmful gases such as nitrogen and hydrogen in steel is low.
(iii) Capital investment and production costs The capital investment of low pure oxygen bof is equivalent to 60-70% of that of open hearth workshops with the same production capacity, and the production cost is lower than that of open hearth.
At present, with the success of the development of the porous nozzle of the oxygen gun, the oxygen supply per unit time of the pure oxygen top blown bof is greatly improved, and due to the innovation of operation technology (for example, the use of electronic computing technology to adjust and control the smelting process), no matter the size of the bof capacity, the blowing time is basically similar, even if the 300T bof, Net oxygen blowing time can also be shortened to about 12min. Within a certain limit, the larger the furnace capacity, the better the economic effect, so the top blown converter rapidly to large-scale. At present, the largest converter in the world is 350T, and the construction of 400 ~ 450T converter is being studied.
Electric furnace steelmaking method mainly uses arc heat, in the arc action zone, the temperature is up to 4000℃. Smelting process is generally divided into melting period, oxidation period and reduction period, in the furnace can not only cause oxidation atmosphere, but also cause reduction atmosphere, so dephosphorization, desulfurization efficiency is very high.
Eaf steelmaking using scrap steel as raw material has less investment than bof method. At the same time, due to the development of direct reduction, it provides metallic pellets for eAF to replace most of the scrap steel. Therefore, eAF steelmaking has been greatly promoted. There are about 1400 larger electric furnaces in the world. At present, electric furnaces are developing towards large, ultra-high power and electronic computer automatic control. The maximum capacity of electric furnaces is 400T.
Almost all electric furnaces above 150t abroad are used to smelt ordinary steel, and 60 ~ 80% of the output of electric furnace steel in many countries is low carbon steel. Due to the shortage of electric power and scrap steel in China, it is mainly used for smelting high quality steel and alloy steel.
Before the 1950s, open-hearth steel accounted for 85% of the world’s steel output. In recent years, open hearth steelmaking is still playing its role in addition to casting large castings or large ingot for hydraulic press and other finished materials. Due to the development of pure oxygen top blown bof steelmaking technology, the output of bof steel has increased greatly, and the output of open hearth steel in all countries in the world has decreased year by year. The biggest disadvantages of open hearth steelmaking method are long smelting time (generally 6 ~ 8h), large fuel consumption (only 20 ~ 25% of heat energy utilization), high capital construction investment and production costs. For a steel mill with an annual output of 12 million tons of steel, it is enough to build six 250-300T pure oxygen top blown converter, while 30 to 40 500T large open hearth furnaces are needed to build open hearth furnaces. Although the open hearth furnace still in production in the world has generally adopted oxygen steelmaking, productivity has been greatly improved, but the dust removal system is complex and the investment is high, so the open hearth steelmaking is no longer developed, and there is even a trend of demolition and reconstruction for top or bottom blowing converter.