Before and after the secondary refining process, the secondary oxidation of molten steel caused problems in the production of clean steel. The ferrous oxide, manganese oxide, and silicon dioxide contained in the slag or refractory materials in the ladle and tundish will oxidize the aluminum or titanium in the molten steel. The oxygen sources that cause the reoxidation of molten steel are mainly ferrous oxide, manganese oxide, silicon dioxide in the slag and oxygen in the atmosphere. Many scholars have studied the re-oxidation of aluminum by slag with different compositions.
In the production of ultra-low-carbon clean steel, in order to reduce the content of ferrous oxide in the converter slag and improve the clarity of molten steel, a common practice is to add a steel slag deoxidizer during the converter tapping process. However, the addition of steel slag deoxidizer during the tapping process will also cause a series of operational problems, such as the problem of phosphorus recovery, and the problem of insufficient free oxygen content in the vacuum cycle degassing process (RH process). For the problem of phosphorus recovery, double slag operation can be used to solve it. If the free oxygen content in the molten steel is insufficient or the temperature control is not accurate, it is necessary to blow oxygen into the molten steel in the RH process, and the oxygen blowing will deteriorate the cleanliness of the molten steel. In the RH process, in order to blow as little oxygen as possible into the molten steel, the following measures have been taken: the final temperature and free oxygen content of the converter molten iron are established, and the temperature and free oxygen content of the molten steel at the blowing station are controlled in the RH process. . In this experiment, part of the steel slag deoxidizer was added after the molten steel entered the blowing station for blowing, and the other part was added after the RH process decarburization, in order to reduce the iron content in the steel slag.
1. Phosphorus recovery in the converter and
reduction of operating variables are essential for stabilizing the quality of ultra-low carbon steel. For this reason, a process such as double slag operation is used in the converter link. The production process of ultra-low carbon steel at Pohang Gwangyang Plant is as follows: After the converter is tapped, the molten steel is sent to the blowing station; according to the current molten steel temperature and the time to reach the RH process, the molten steel is adjusted by adding coolant or blowing air to the molten steel Temperature: Part of the steel slag deoxidizer is added after blowing, and another part of the steel slag deoxidizer is added after decarburization in RH.
For the converter double slag operation, the process flow is as follows: before the first oxygen blowing, fluxes such as quicklime, iron ore, and recyclable converter slag are added. The amount of ore added mainly depends on the silicon content of the molten steel and the temperature of the molten steel. Stop blowing oxygen when the slag overflows. Tilt the converter after blowing oxygen for the first time and carry out slagging until molten steel overflows. After the slag scraping is completed, oxygen blowing is performed for the second time.
Experiments have confirmed that after the first oxygen blowing, the relationship between the phosphorus content in molten steel and the amount of oxygen blowing is as follows: the amount of oxygen blowing increases, and the phosphorus content in molten steel decreases. The alkalinity mainly depends on the amount of silicon and calcium oxide added in the molten steel. Experiments have confirmed that after the first oxygen blowing, the alkalinity increases and the phosphorus content in the molten steel decreases. Experiments have also confirmed that the relationship between the phosphorus content in the molten steel and the total iron content in the slag is: after the second oxygen blowing, the phosphorus content in the steel is between 0.006% and 0.015%, which makes it feasible to deoxidize the slag after the converter is tapped; When the total iron content in the slag is low, the phosphorus content will exceed 0.015%, and the steel slag cannot be deoxidized.
In order to cooperate with the double slag operation and reduce the operating variables, especially in the later stage of the converter campaign, it is necessary to establish a suitable converter age based on the production cost. The experiment confirmed that the relationship between the oxygen content in the molten steel at the end of the blowing and the age of the converter is: at the end of the converter campaign, the oxygen content increases sharply. This is due to the decrease of the height-diameter ratio of the molten pool at the end of the furnace campaign and the deterioration of the mixing characteristics of the converter. When the converter age is controlled between 5500 and 6000 furnaces, it can reduce the difference of molten steel at the end of the blowing.
2. Steel slag deoxidation operation and vacuum cycle degassing process
at the blowing station In the current experimental process, the deoxidation of steel slag is completed in the blowing station and RH. Compared with the traditional steel slag deoxidation process, the improved steel slag deoxidation process has the following characteristics: At the blowing station, the traditional steel slag deoxidation operation requires 1000kg of calcium oxide (in the converter) and 400-600kg of oxidant, while the improved process requires 800kg of calcium oxide. (In the converter) 400-800kg of oxidant; In RH, the traditional steel slag deoxidizing operation requires 1 to 150kg of steel slag deoxidizer, but after improvement, it needs 0-250kg of deoxidizer and 0-250kg of canned aluminum scrap. At the blowing station, the temperature of the molten steel is first measured after the ladle arrives, and then the blowing time and the amount of coolant added are determined according to the temperature when the molten steel arrives, the temperature of the molten steel required by the RH process, and the start time of the RH process. During the blowing process, calcium oxide is added during tapping to facilitate the reaction with molten steel to form a uniform slag. Nevertheless, calcium oxide and slag cannot be mixed uniformly, and it is necessary to blow for at least 30 seconds after adding calcium oxide to promote uniformity. Add steel slag deoxidizer after blowing. The composition of the deoxidizer is: calcium oxide content 20%-30% (mass fraction), silicon oxide content 10%, aluminum oxide content 5%, total aluminum content 30%-40%, total iron content 2%. The diameter of the deoxidizer is about 5-30mm.
After RH treatment, canned aluminum chips and deoxidizer are added to the molten steel to improve the deoxidation effect of steel slag. The addition amount of canned aluminum chips and deoxidizer mainly depends on the total iron content in the steel slag when the molten steel reaches RH. The deoxidation effect of canned aluminum chips is better than that of aluminum pellets, because canned aluminum chips are lighter and will not penetrate into molten steel.
The titanium content of steel grade A and steel grade B is the same, but steel slag deoxidation is used in the blowing station and RH, and the scrap rate is reduced by 80%. Of course, the scrap rate index is also affected by the composition of steel grades, such as titanium.
Experiments have confirmed that for every increase of 100kg of steel slag deoxidizer, the total iron content in the slag decreases by 1.6%. During the RH treatment process, the total iron content increased, but in the RH process after the deoxidation of the steel slag to the end of the pouring, the total iron content decreased. As for the aluminum oxide in the steel slag, its content has a rising trend from the RH process to the end of pouring. This shows that when the total iron content in the steel slag is higher than a certain value, the steel slag will be re-oxidized and the aluminum oxide content will increase.
In order to study the phenomenon of the molten steel being re-oxidized by the slag in the ladle, the ladle was allowed to stand for 50 minutes after RH treatment, and then the slag and molten steel in the ladle were sampled for experiment. In the experiment, the content of total iron in the steel slag is 3.78% when it reaches RH. After RH treatment, 200kg of steel slag deoxidizer is added. The experimental results show that the dissolved aluminum content in the steel slag does not increase significantly, which indicates that the total iron When the content is relatively low, the oxidation effect of steel slag on molten steel is not obvious.
After RH treatment for 20 minutes, the total iron content in the steel slag remained at 2.0%, basically unchanged, but at the initial stage of the RH treatment, the total iron content in the slag increased slightly due to slag entrapment.
3. The influence of alloying elements on the quality of steel
Titanium is widely used in the production of ultra-low carbon steel, and the total iron content in sponge titanium of different grades (grade A and grade B) has been tested. Brand A contains sintered iron and sponge-like iron, with iron content of 390 ppm and 450 ppm, respectively. The total iron content of sponge titanium of brand B is 100 ppm. Titanium is usually added in the final stage of RH treatment, so the addition of titanium will affect the quality of molten steel. Experiments have confirmed that titanium in brand A leads to a higher nozzle clogging index than titanium in brand B. Correspondingly, the waste rate caused by inclusions in brand A is also higher than that in brand B. In order to reduce the quality problems caused by the addition of alloys, the quality of the alloys should be properly tested.
In order to meet the demand for high-quality ultra-low carbon steel, the converter double slag operation has been developed. Considering the phosphorus content of the molten steel at the end of the blowing can further optimize the double slag operation, thereby improving the deoxidation effect and productivity of steel slag. In order to reduce operating variables, combined with double slag operation, combined with production costs and operating variables, it is recommended that the appropriate converter furnace age is 5500 to 6000 furnaces.
Deoxidizing steel slag in the blowing station and the RH process can reduce the oxidation of molten steel. In the gas blowing station, the lower total iron content in the slag should be considered for deoxidation of steel slag, and proper free oxygen should be left in the slag to avoid blowing oxygen again due to insufficient free oxygen during the RH carbon reduction operation. After deoxidizing the steel slag after RH treatment, adding steel slag deoxidizer and aluminum can powder can reduce the total iron content in the slag.
The titanium content in ferroalloy or molten steel additives also affects the quality of ultra-low carbon steel. If the sponge titanium alloy contains high free oxygen, there will be a high nozzle clogging rate and reject rate for some specific steel grades.
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