Research progress on high-efficiency smelting technology of 120t converter

This study compared and analyzed the optimization practices of oxygen supply systems and bottom blowing processes in a 120t converter at a steel plant, as well as the application of intelligent smelting and new smelting technologies. The results showed that: 5 oxygen lance nozzle holes are suitable for a 120t converter; the bottom blowing system of a 120t converter is typically configured with 4–8 bottom blowing pipelines; by increasing the bottom blowing intensity, the FetO content in the slag and the final phosphorus content in the molten steel can be reduced; both the auxiliary lance and the bombardier can be applied to a 120t converter, but the effective distance between the auxiliary lance and the converter mouth must be greater than 400mm. Converter steelmaking is currently the world’s most important steelmaking method. As one of the key links in smelting cost control, controlling converter steelmaking costs and improving smelting efficiency are the most important ways for steel companies to improve their economic benefits. Therefore, the industry has researched and developed a variety of high-efficiency converter smelting processes.

Optimization Practice of Top-blown Oxygen Supply System

The oxygen supply system mainly includes the oxygen lance nozzle structure, oxygen supply pressure, oxygen supply intensity, and oxygen lance position. Optimizing the oxygen supply system is one of the important means to improve converter production efficiency. To address issues such as low oxygen supply intensity, long smelting cycle, high steel material consumption, and high phosphorus content in the converter smelting process, the main plant optimized the oxygen lance nozzles of the 120t converter. The number of nozzle holes was increased from 4 to 5, the Mach number increased from 1.98 to 2.05, the nozzle angle increased from 12.0° to 13.5°, and the throat and outlet diameters were reduced. The design oxygen pressure was also increased, resulting in an oxygen supply intensity increase from 2.98 N·m³/(min·t) to 3.39 N·m³/(min·t). According to research reports from Maanshan Iron & Steel, using the 5-hole oxygen lance nozzles reduced oxygen blowing time from 911s to 783s, increased dephosphorization rate from 81.52% to 85.31%, decreased the total iron content in the final slag from 18.59% to 15.84%, and reduced hot metal consumption from 874 kg/t to 852 kg/t. To address issues such as splashing and slag drying during the 120t converter smelting process, the oxygen lance nozzle parameters were optimized. The Mach number of the oxygen lance nozzle was increased from 1.99 to 2.02, the design oxygen pressure was reduced from 0.92 MPa to 0.85 MPa, the throat diameter and outlet diameter were increased, and the oxygen supply intensity increased from 3.26 N·m³/(min·t) to 3.48 N·m³/(min·t). When using the optimized 5-hole oxygen lance nozzle, the slag formation effect was improved, the blowing time was shortened by 0.16 min, and the total iron content in the slag decreased from 13.88% to 12.59%. To improve the smelting conditions in the 120t converter, the number of oxygen lance nozzle holes was increased from 4 to 5, and the oxygen lance parameters were optimized. After optimization, oxygen consumption decreased from 61.5 m³/t to 59.7 m³/t, oxygen supply time was shortened from 14.7 min to 12.9 min, lime consumption decreased from 31.8 kg/t to 29.7 kg/t, the first-pass dephosphorization rate increased from 79.1% to 84.4%, and the average total iron content in the final slag decreased from 17.2% to 15.6%. The oxygen supply intensity of the 120t converter was increased from 3.15 N·m³/(min·t) to 3.30 N·m³/(min·t), while the throat diameter and outlet diameter of the oxygen lance nozzle were increased, and the nozzle angle was also increased. After the oxygen lance nozzle was optimized, the average oxygen supply time was shortened from 902s to 848s, and the oxygen consumption decreased from 51.7 m³/t to 49.3 m³/t. To improve the smelting efficiency of semi-steel in a 120t converter, the oxygen lance nozzle parameters were optimized. When smelting using the optimized oxygen lance nozzle, the oxygen supply intensity increased from 3.29 N·m³/(min·t) to 3.48 N·m³/(min·t), the oxygen blowing time was shortened by 56 seconds, and the total iron content in the final slag decreased from 20.13% to 19.43%. Through cold-state testing, the straight-hole four-hole casting oxygen lance was replaced with a Laval oxygen lance, and the oxygen lance nozzle parameters were optimized. Production practice shows that after optimization, the oxygen supply time is 15.6 minutes, and the oxygen consumption is 54.69 m³/t. The oxygen lance flow rate was increased from 28,000 N·m³/h to 33,000 N·m³/h, while the outlet diameter and throat diameter were increased, and the nozzle angle was decreased. Simultaneously, the scrap ratio was increased from 17.8% to 25.0%, and the converter oxygen blowing time was shortened from 15.0 min to less than 13.5 min, a reduction of at least 1.5 min. Based on the above data, optimization methods and specific measures for the top-blown oxygen supply system of a 120t converter were obtained. To improve converter blowing efficiency, most enterprises adopt methods such as appropriately increasing the oxygen supply intensity, adjusting the number of oxygen lance nozzle holes from 4 to 5, appropriately increasing the Mach number of the oxygen lance nozzle, increasing the throat diameter and outlet diameter, and increasing the nozzle included angle. The main effects are: (1) Under the condition that other factors remain unchanged, appropriately increasing the oxygen supply intensity can shorten the smelting time; (2) Increasing the number of oxygen lance nozzles from 4 to 5 can significantly increase the impact area of ​​the oxygen jet, promoting slag formation and decarburization reactions; (3) Appropriately increasing the Mach number of the oxygen lance nozzle can increase the oxygen jet velocity and promote the molten pool reaction; (4) Appropriately increasing the included angle of the oxygen lance nozzle can increase the contact area between the oxygen jet and the molten metal. Furthermore, significantly increasing the oxygen flow rate, while shortening the oxygen blowing time, also increases the risk of splashing and the difficulty of dephosphorization. Therefore, under the condition of significantly increasing the oxygen supply intensity (oxygen flow rate increased by 5000 N·m3/h, oxygen supply intensity increased by 0.60 N·m3/(min·t)), Zhongtian Steel reduced the included angle of the oxygen lance nozzle by 0.5°, which not only shortened the oxygen blowing time but also suppressed splashing. The above production practices show that the oxygen lance nozzles suitable for a 120t converter have 5 nozzles, a Mach number of 2.00 to 2.07, a design oxygen pressure of 0.85 to 0.95 MPa, and a maximum oxygen supply intensity of 3.93 N·m3/(min·t). By optimizing the oxygen supply system, the oxygen supply time is shortened by 10 to 120 seconds, and the total iron content in the final slag is reduced by 0.7% to 1.6%.

Bottom blowing process optimization practice

The stirring effect of bottom-blown gas in the converter significantly improves the uniformity of molten pool composition and temperature, reduces splashing, accelerates dephosphorization and decarburization chemical reactions, and lowers the oxygen content of molten steel and the FeO content of slag at the final smelting point. Conventional converters use capillary or circumferential-slit tube bottom-blowing elements. Steel companies have widely implemented bottom-blowing process optimization practices. Optimization of Bottom-blowing Process in 120t Converter: Production Practice The converter’s eight bottom-blowing holes are evenly arranged in a ring at half the radius of the molten pool at the bottom of the furnace, and the capillary brick-type bottom-blowing element has been modified into a double-ring slit bottom-blowing lance. Taking the converter smelting of tire bead wire as an example, the switching time, flow rate, and supply intensity of the bottom-blowing gas source in each smelting stage are shown in Table 8. Production practice shows that the bottom-blowing lance achieves a single-pass permeability of over 7,000 heats; compared with heats with blocked bottom-blowing, the constant oxygen value of heats with smooth bottom-blowing decreased by 8.2 × 10⁻⁶, the total iron content in the final slag decreased by 1.02%, and the manganese recovery rate of the molten iron increased by 2.7%. The 120t converter is equipped with four bottom-blowing holes, all arranged in concentric circles 1804mm from the working surface. The ratio of the circumference diameter of the bottom-blowing element at the bottom of the molten pool to the molten pool diameter D is 0.476D. The four bottom-blowing elements are divided into two groups, distributed on both sides of the line connecting the tapping spout and the molten iron, with an angle of 90° between each pair of bottom-blowing elements. The bottom-blowing elements use circumferential slit lances, and the bottom-blowing process is implemented. By adopting measures such as strictly controlling the final slag composition, strengthening endpoint control, reducing the tapping temperature, optimizing the slag splashing process, and strengthening furnace shape control, the converter achieved 13,000 heats of combined blowing, with an average final slag total iron content of 12% and a final steel residual manganese content of 0.07%–0.12%. To provide kinetic conditions for the dephosphorization process in low-slag steelmaking, the steelmaking and rolling mill replaced the bottom-blown permeable bricks of the 120t converter with annular seam spray guns, developed a bottom-blown process system suitable for dephosphorization, and extended the life of the annular seam bottom-blown elements by taking measures such as furnace bottom maintenance, final slag magnesium oxide control, enhancing slag splashing effect, and enhancing slag splash gun position control, enabling the combined blowing furnace to serve 12,000 heats. The steel plant optimized the bottom-blowing gas supply mode of its 120t converter by removing two bottom-blowing lances from the tapping side and increasing the bottom-blowing flow rate of the remaining six lances, based on the original eight. Production practice shows that after optimizing the bottom-blowing process, the final phosphorus mass fraction decreased from 0.0187% to 0.0175%, and the total iron mass fraction in the slag decreased from 13.81% to 13.10%. The number of gas outlets for the double-ring bottom-blowing gun and the capillary permeable brick is 15% of that for the capillary permeable brick. The total gas outlet area of ​​the former is 2.16 times that of the latter, and the area of ​​a single gas outlet is more than 10 times that of the latter. Production practice shows that a 120t converter bottom blowing system is usually configured with 4 to 8 bottom blowing pipelines. By conducting experiments to optimize the bottom blowing hole layout and improve the bottom blowing intensity of the converter, the bottom blowing intensity during the blowing period can be increased to 0.04 to 0.12 N·m3/(min·t), and the bottom blowing flow rate of a single pipeline can reach 60 to 120 N·m3/h. This can reduce the FetO content in the slag and the final phosphorus content in the molten steel, and increase the residual manganese content in the final molten steel. While increasing the bottom blowing intensity (bottom blowing flow rate), the top blowing oxygen supply system also needs to be optimized. To improve the reblowing effect of the converter, the various process parameters of the oxygen lance nozzles and bottom blowing lances or permeable bricks must be optimized. When the converter adopts a double-ring-slit bottom blowing gas supply element, the gas supply intensity is gradually increased to 0.09 m3/(t·min), the single branch bottom blowing gas flow rate reaches 50-70 N·m3/h, and the oxygen lance nozzle parameters are optimized, thus improving the reblowing effect. Replacing the capillary permeable bricks with double-ring slit bottom-blowing gas supply elements revealed problems such as high maintenance difficulty of the furnace bottom and bottom-blowing elements, significant damage to the slag line and slag pouring surface, and long smelting cycles. By reducing the nozzle angle and increasing the oxygen supply flow rate, while shortening the oxygen supply time, the furnace life of the combined blowing synchronous furnace was increased from 4230 heats to 17182 heats, and the final slag total iron content was reduced by 2.25%. The application of intelligent smelting technology relies on a secondary lance system and a smelting model. The 120t converter is a small to medium-sized converter. Based on the furnace size, a secondary lance is preferred. If the furnace size does not meet the requirements, a bomb-type rapid analyzer (referred to as a bomb analyzer) can be used. In 2005, a secondary lance system and dynamic/static models were introduced to guide the smelting operation of the 120t converter, achieving the effects of shortening the smelting cycle and improving production efficiency. In 2011, secondary lances and dynamic/static control systems were installed on the 120t converter, achieving initial results in improving blowing stability, reducing the number of supplementary blowing furnaces, shortening the converter smelting cycle, increasing labor productivity, and improving the steelmaking working environment. In 2012, the steel plant installed a bomb disposal device on the 120t converter to detect the carbon and phosphorus content and temperature of the first pour of molten steel, and used the data to calibrate the intelligent steelmaking system prediction model, realizing a new mode of steel tapping without tilting the furnace. Compared to the bomb-feeding device, the secondary lance is more energy-efficient and reduces consumption, but it requires more space to install. Under normal blowing conditions, the effective distance between the secondary lance and the converter mouth needs to be greater than 400mm, and the distance between the secondary lance body and the edge of the mouth needs to be greater than 450mm. If the effective distance between the secondary lance body and the mouth is less than 400mm, the secondary lance body needs to be tilted at least 1° backwards towards the furnace when using it, while ensuring that slagging at the mouth is controllable.

Application of new smelting processes

An industrial trial of a post-blown argon stirring process was conducted in a 120t converter. After oxygen blowing in the converter and with the furnace stationary, molten steel was stirred with argon gas at a bottom blowing flow rate of 800 N·m³/h for 5 minutes. The results showed that the average sulfur mass fraction decreased by 0.004%, the average phosphorus mass fraction decreased by 0.002%, the slag iron oxide mass fraction decreased by 4.8%, and the average temperature of the molten steel decreased by 10–20℃. Jiuquan Iron & Steel (JISCO) conducted a high-intensity bottom-blown CO2 test on a 120t combined-blowing converter using a spray gun-type bottom-blowing element, with a maximum bottom-blown CO2 flow rate of 800 m³/h. The results showed that the average carbon-oxygen product at the endpoint decreased by 1.4 × 10⁻⁴. The Iron & Steel Research Institute developed a calcium ferrite-based dephosphorizing agent and completed 13 heat tests on a 120t converter, with results showing a dephosphorization rate of over 90%.

in conclusion

(1) The oxygen lance nozzles suitable for a 120t converter have 5 nozzles, a Mach number of 2.00–2.07, and a design oxygen pressure of 0.85–0.95 MPa. After optimization of the oxygen lance nozzles, the oxygen supply time is shortened by 10–120 s, and the total iron content in the final slag is reduced by 0.7%–1.6%. (2) The bottom blowing system of a 120t converter is typically configured with 4–8 bottom blowing pipelines. Through experiments to optimize the bottom blowing hole arrangement and increase the bottom blowing intensity of the converter, the bottom blowing intensity during the blowing period is increased to 0.04–0.12 N·m³/(min·t), and the bottom blowing flow rate of a single pipeline reaches 60–120 N·m³/h. This can reduce the FeO content in the slag and the final phosphorus content in the molten steel, and increase the residual manganese content in the final molten steel. (3) The auxiliary lance is superior to the bomb-feeder, but under normal blowing conditions, the effective distance between the auxiliary lance and the converter mouth needs to be greater than 400 mm. If the effective distance between the auxiliary lance body and the furnace opening is less than 400mm (Company D), the auxiliary lance body must be tilted at least 1° backwards towards the furnace when using the auxiliary lance, while ensuring that slag formation at the furnace opening is controllable. (4) Intelligent smelting technologies such as automatic feeding technology, automatic blowing technology, automatic control technology at the blowing endpoint, and automatic tapping technology are the development trend of 120t converter smelting technology.