Scientific research direction of converter steelmaking

Improve the cleanliness of molten steel, that is, greatly reduce the content of various inclusions at the end of blowing, and require S to be lower than 0.005%; Phosphorus is less than 0.005% and nitrogen is less than 20ppm. In order to improve the hit accuracy of chemical composition and temperature in a given range, compound blowing, high-level stirring in molten pool, modern detection means and control model are adopted. Reduce the proportion of supplementary blowing furnace times and reduce the consumption of refractory per ton of steel.

Hot metal pretreatment plays a very important role in improving converter operation index and steel quality. Hot metal pretreatment in the United States and western European countries is limited to desulfurization, while hot metal pretreatment in Japan includes desulfurization, desilication and dephosphorization. For example, 1989, the proportion of pretreated molten iron in Japan is: NKK Keihin Plant 55%, Nippon Steel Junjin Plant 74%, Kobe Plant 85% and Kawasaki Chiba Plant 90%.

All converter steel mills in Japan, dozens of steel mills in the United States and western European countries, and all newly-built steel mills in other countries are equipped with sub-guns for testing. Proper use of this gun within a few minutes before the end of the scheduled blowing time can ensure extremely high carbon content and molten steel temperature hit rate, so that 90%-95% of the heats can be discharged immediately after the blowing is stopped, that is, there is no need to test chemical components, and of course there is no need to supplement blowing. In addition, this also improves the output and greatly reduces the wear of the lining.

Combined blowing can promote the stability of various smelting parameters, so it has been popularized in many countries. The LBE steelmaking process, which was born in Luxembourg and France in the early 1980s, has evolved into a series of derivative processes with more than 20 names, such as STB, LD-KC, BAP, TBM, LD-OTB, LD-CB, K-BOP, K-OBM and LET. Practice has proved that LBE prototype method and derivative method have their own advantages. There is no difference among LBE, LD-KC, BAP and TBM-they all blow oxygen at the top and argon at the bottom. There are other ways to input carbon monoxide, carbon dioxide and oxygen from the bottom of the furnace. Various combined blowing processes can be illustrated by the following figure (number of converter seats). 1983,1988,140,1990,228. All or almost all converters in Austria, Australia, Belgium, Italy, Canada, Luxembourg, Portugal, France, Switzerland, South Korea and other countries adopt combined blowing.

Pure bottom blowing oxygen steelmaking (LWS OBM Q-BOP) has not been popularized. In 1983, there were 26 such converters in operation. In 1990, only 18 were left.

Japan adopts the so-called purging method, that is, argon is blown from the bottom of the furnace at the end of oxygen blowing at the top of the furnace to make the carbon content of molten steel reach 0.0 1%. This is especially important for the smelting of automobile steel, thin steel plate and electrical steel.

It is worth noting that Japan is developing new methods and equipment to adjust the smelting process under the condition of compound blowing. Among them, the optical cable in the oxygen lance at the top of the furnace is used to continuously monitor the manganese content in steel with the blowing process; The temperature of molten steel is continuously monitored by using the information of optical fiber sensor installed at the bottom of the furnace and exhaust gas; The research on splash prediction and prevention is carried out.

The splash prediction developed by Kobe Steel Company is based on the detection of the suspension system of the top oxygen lance. The Jingbin factory of NKK Company in Japan reduces splashing by monitoring the taphole. When the slag rises violently, the video signal gives an instruction to add coal or limestone to the furnace. A better material (in terms of time to stabilize the molten pool) is coal.

The life of converter lining is an extremely important subject. The data analysis of Japan, the United States and western European countries shows that there is no big difference in various smelting parameters that affect lining wear, such as slag oxidation degree, alkalinity and molten steel temperature at the end of blowing. Only when the sub-gun is used for detection can the backblowing time which is most harmful to the furnace lining be reduced from 10- 15 minutes to 1-3 minutes, and backblowing can be eliminated. Each index of converter steelmaking process depends on the chemical composition of molten iron, and the main requirements for molten iron are to optimize the low sulfur content (less than 0.03%), high silicon content (0.7%-0.9%) and high manganese content (0.8%- 1.0%).

The analysis of physical and chemical laws and dynamic characteristics of desulfurization process in each stage of ironmaking and steelmaking shows that desulfurization reaction is easier to ensure dynamically in molten iron than in molten steel because sulfur has higher activity under the conditions of high carbon content and low oxidation degree. However, it is difficult to desulfurization in blast furnace ironmaking, because in a series of complex redox reactions in blast furnace, the energy of various thermodynamic conditions for deep desulfurization will inevitably increase the silicon content, which will lead to the increase of lime and coke consumption and the decrease of output. Therefore, it is necessary to carefully plan the process, use the burden with the least sulfur content, prepare high alkalinity mixed slag and produce low sulfur iron.

Converter blowing desulfurization has no effect, because the steel slag system can not reach the equilibrium state, the oxidation degree of molten pool is high, the carbon content is low, and the sulfur distribution coefficient between slag and steel is only 2-7. Such a low sulfur distribution coefficient makes it difficult to realize deep desulfurization in converter smelting, which leads to huge technical and economic consumption in steelmaking production. No matter in blast furnace ironmaking or converter steelmaking, the thermodynamic conditions required for effective desulfurization of metals cannot be guaranteed, so it is not advisable to study deep desulfurization in blast furnace ironmaking and converter steelmaking. It is reasonable to separate the desulfurization process from blast furnace and converter. This can simplify the production process of sintering-blast furnace-converter and reduce the production cost. Desulfurization is separated from blast furnace and converter, which makes desulfurization outside the blast furnace an important process link in the design of large-scale combined steel mills. While smelting low-silicon iron, it is not necessary to carry out expensive desilication outside the blast furnace to ensure refining inside the converter. Low original silicon content in molten iron can also reduce manganese content. Manganese plays a very important role in oxygen converter steelmaking, which determines the conditions needed for early slagging and regulates the oxidation degree of molten steel before tapping. Long-term practice has proved that manganese in hot metal should be kept at the level of 0.8%- 1.0%, so manganese must be supplemented in sintering mixture, which increases the cost. The analysis of manganese balance in each process of sintering-blast furnace-converter shows that the reduction of manganese in blast furnace and oxidation in converter lead to irreparable huge losses of manganese raw materials and manganese itself, and also add a lot of trouble to the operation of each production process. When blowing is stopped under the condition of low carbon content (0.05%-0.07%), the influence of oxidation degree is so great that the final manganese content will be set in a very narrow range, which is rarely related to the original manganese content of hot metal. Under this condition, although the original manganese content of molten iron is 0.5%- 1.2%, the final manganese content of steel is actually the same (0.07%-0. 1 1%). Therefore, under the current converter steelmaking process conditions (each furnace has blowing operation), it is more reasonable to smelt low-manganese iron without using manganese-containing raw materials in the sintering mixture to improve the original manganese content of hot metal. At the same time, it is of great significance to study the effect of directly using manganese ore for saving the deoxidation of low-manganese iron in converter steelmaking. The comparison of technological indexes obtained by industrial balance calculation of multiple heats shows that manganese ore is not added to molten iron, but added to converter steelmaking. Compared with hot metal steelmaking with 1. 13% manganese, the former can save manganese ore 15.3kg/t per ton of pig iron, in addition to reducing ferromanganese1.3 kg/t.

The content of silicon and manganese in molten iron is low, so desulfurization is not needed. These conditions will change the slagging mechanism and dynamic characteristics, because the amount of lime will decrease, the amount of slag will decrease, and the basicity and oxidation degree of slag will increase. Under this condition, the refining effect of slag is limited to dephosphorization of hot metal. In this way, the slag itself can be used for many times in converter smelting, so that the slag has high refining ability.

According to this principle, a new process of converter steelmaking is developed, that is, converter steelmaking itself uses late slag (circulating slagging) for many times (3-5 times). This process can reduce lime consumption and iron loss in slag. It can provide powerful power for deep-water dephosphorization of steel to generate high alkalinity oxide slag as soon as possible and reduce the content of silicon and manganese.