Method for improving cleanliness of molten steel

Abstract

The invention relates to a method for improving the cleanliness of molten steel. The method comprises the following steps: 1) controlling the end-point carbon and oxygen content of a converter and the blowing end-point temperature of the converter; (2) molten steel is not deoxidized in the converter tapping process; (3) after converter tapping, top slag and top ash are added into a steel ladle; (4) molten steel obtained after converter tapping is hoisted to an RH refining furnace to be subjected to vacuum decarburization treatment, and oxygen and carbon in the molten steel are removed till the technological requirement is met; (5) after oxygen determination operation is conducted on the molten steel, aluminum particles are added for deoxidation, and the purpose is that the aluminum content in the steel is kept to be 0.020 wt%-0.050 wt%; (6) aluminum particles are added into the vacuum tank, then a steel ladle bottom argon blowing system is opened, and the argon flow in the steel ladle is in a soft blowing state; (7) the molten steel subjected to RH vacuum treatment is hoisted to an LF refining furnace for slagging, heating and other operations; and 8) transporting the molten steel treated by the LF refining furnace to a continuous casting process for pouring. By applying the method for improving the cleanliness of the molten steel, the cleanliness of the molten steel can be improved, and the service life of steel is prolonged.

Description

Technical Field

[0001] The invention belongs to the technical field of metallurgical production, and particularly relates to a method for improving the cleanliness of molten steel. Background Art

[0002] High-quality clean steel places high demands on the quantity, shape, size, composition, and distribution of non-metallic inclusions to ensure quality. For aluminum-deoxidized steels produced in converters, aluminum ingots or cakes are typically added to the molten steel during tapping to pre-deoxidize it. This reaction between the aluminum and oxygen in the steel generates a large number of Al2O3-type inclusions. This is particularly true for low-carbon steels, such as those requiring 0.02% to 0.06% carbon content in the finished continuous casting product. When conventional clean steel production processes are used, the converter must reduce the final carbon content to below 0.02% before tapping to ensure the finished continuous casting product meets the required carbon content. At this point, the oxygen content in the converter molten steel can reach over 1000 ppm. Deoxidation with aluminum in these conditions can generate a large number of Al2O3-type inclusions, severely impacting steel quality. In order to reduce the number of inclusions in molten steel, it needs to be treated in an LF refining furnace. However, a large amount of Al2O3 inclusions will still remain in the steel and are difficult to remove. In order to denature a large number of high-melting-point Al2O3 inclusions in the steel into low-melting-point calcium aluminates and reduce the risk of nodules in the continuous casting nozzle, a large amount of calcium wire needs to be fed to denature it. However, a large amount of calcium treatment will further lead to the introduction of inclusions, which is not conducive to the cleanliness control of the molten steel.

[0003] Therefore, the need to develop methods to improve the cleanliness of molten steel, especially for smelting low-carbon steel grades, has become a key research topic for metallurgical workers. Summary of the Invention

[0004] For aluminum-deoxidized steels smelted in converters, particularly low-carbon steels, the low carbon content at the converter endpoint leads to a high oxygen content in the molten steel at the converter blowing end. Aluminum deoxidation during the converter tapping process generates a large amount of Al2O3 inclusions, which are difficult to remove in subsequent processing and seriously affect the quality of the steel. The present invention is targeted at steels requiring deoxidation and controlled molten steel cleanliness. By applying the present invention, a method for improving molten steel cleanliness can improve molten steel cleanliness and thus increase the service life of the steel.

[0005] In order to achieve the above object, the present invention adopts the following technical solutions:

[0006] A method for improving the cleanliness of molten steel, specifically comprising:

[0007] 1) The carbon content at the converter blowing end point is controlled at 0.035wt% to 0.10wt%, the oxygen content at the converter blowing end point is controlled below 900ppm, and the converter blowing end point temperature is controlled above 1700°C.

[0008] When smelting aluminum-deoxidized clean steel with a carbon content requirement of 0.02wt% to 0.06wt% for the continuous casting product, the carbon content at the converter blowing end point is controlled within the range of 0.035wt% to 0.045wt%, and the oxygen content at the converter blowing end point is controlled within the range of 500 to 800ppm.

[0009] When smelting aluminum-deoxidized clean steel with a carbon content requirement of 0.07wt% to 0.60wt% for continuous casting products, in order to ensure that the phosphorus content of the finished product is less than 0.020wt%, the carbon content at the end of converter blowing is controlled at 0.06wt% to 0.10wt%, and the oxygen content at the end of converter blowing is controlled below 600ppm.

[0010] 2) The molten steel is not deoxidized during the converter tapping process. At the same time, it is only necessary to ensure that the ladle air bricks are permeable during the converter tapping process. It is strictly forbidden to blow argon into the ladle at a large flow rate to avoid excessive reduction in the molten steel temperature during the tapping process.

[0011] 3) After the converter taps the steel, top slag such as calcium aluminate, slag-forming materials such as lime, and flux are added to the ladle. The amount of top slag and lime added is controlled at 1-2 kg / ton of steel to keep the molten steel warm.

[0012] 4) After adding the top slag, appropriately increase the argon flow rate at the bottom of the molten steel (usually set to 30-80NL / min, determined according to the actual state of the ladle) to disperse the top slag.

[0013] 5) The molten steel after tapping the converter is hoisted to the RH refining furnace for vacuum decarburization treatment. The RH refining furnace increases the gas flow rate and dynamically adjusts it according to the vacuum degree and the amount of waste gas. The carbon content in the molten steel is removed to the process requirements by using the principle that carbon and oxygen react to generate gas under vacuum conditions.

[0014] The molten steel after the converter is hoisted to the RH refining furnace for refining. At the beginning of RH refining, the lifting gas flow is set to 1000-2000NL / min and the vacuum degree is set to 5KPa. After the vacuum reaches 5KPa-10KPa and stabilizes, and the waste gas flow is less than 2000kg / h, the vacuum degree is adjusted to the lowest vacuum degree of the equipment and the gas flow is adjusted to 1800-2500NL / min. The carbon content in the molten steel is removed by utilizing the principle that carbon and oxygen react to generate gas under vacuum conditions. The molten steel is sampled after 10min and 15min of vacuum treatment.

[0015] 6) After decarburization, the molten steel is subjected to an oxygen control operation. According to the oxygen content in the molten steel, aluminum particles are added to the vacuum tank for deoxidation, with the goal of maintaining the aluminum content in the steel at 0.020wt% to 0.050wt%.

[0016] When smelting aluminum-deoxidized clean steel with a carbon content requirement of 0.02wt% to 0.06wt% for continuous casting products, when the carbon content in the molten steel drops below 0.005wt% during RH vacuum treatment, the molten steel is subjected to an oxygen determination operation.

[0017] When smelting aluminum-deoxidized clean steel with a carbon content requirement of 0.07wt% to 0.60wt% for continuous casting products, when the oxygen content in the molten steel drops below 150ppm, aluminum particles are added to the vacuum tank for deoxidation. When the production rhythm and molten steel temperature permit, the time for adding aluminum particles to the vacuum tank for deoxidation is delayed as much as possible, thereby extending the vacuum deoxidation treatment time.

[0018] 7) After adding aluminum particles into the vacuum tank, open the argon blowing system at the bottom of the ladle to make the argon flow in the ladle in a soft blowing state (usually the setting value is 10-50NL / min, determined according to the actual state of the ladle), accelerate the uniformity of the aluminum element in the ladle, and at the same time utilize the dynamic conditions of the creep of the slag interface (after blowing argon from the bottom of the ladle, the argon moves upward, driving the flow of the molten steel) to accelerate the transfer of oxygen in the slag to the molten steel, and further remove the oxygen in the slag.

[0019] 8) After adding aluminum particles into the vacuum tank, keep the molten steel circulating under vacuum conditions for 2 to 5 minutes, then determine the oxygen content of the molten steel. After the oxygen content in the molten steel drops below 10 ppm, take a sample and break the vacuum tank.

[0020] 9) The molten steel after RH vacuum treatment is hoisted to the LF refining furnace for slag making and heating operations, reducing the FeO+MnO content in the slag to below 1wt%. The slag-steel reaction is used to further remove inclusions in the molten steel. After the refining furnace slag turns white, the composition of the molten steel is adjusted to the target value.

[0021] 10) The molten steel treated in the LF refining furnace is transported to the continuous casting process for pouring.

[0022] Compared with the prior art, the present invention has the following beneficial effects:

[0023] Through the comprehensive application of the above technical solutions, the carbon content of the continuous casting product can be as low as 0.02wt%; the inclusion index of the finished product can be greatly improved, and the inclusion inspection index can reach the following levels: Class A fine series 0.5 level, Class A coarse series 0 level, Class B fine series 0.5 level, Class B coarse series 0 level, Class C fine series 0 level, Class C coarse series 0 level, Class D fine series 0.5 level, Class D coarse series 0 level, Class DS fine series 0 level, the cleanliness of the molten steel is further improved, and the product performance is more stable. DETAILED DESCRIPTION

[0024] The specific embodiments of the present invention will be further described below in conjunction with examples. The following examples are used to specifically illustrate the content of the present invention. These examples are only general descriptions of the content of the present invention and do not limit the content of the present invention.

[0025] Example 1:

[0026] The following will describe in more detail the implementation method for producing aluminum-deoxidized clean steel with a carbon content requirement of 0.02wt% to 0.06wt% for the smelting and continuous casting finished product.

[0027] After adding molten iron and scrap steel to the converter, high-jet oxygen is blown into the ladle to react carbon, phosphorus and other elements in the molten iron with oxygen to generate gas or oxides; during the converter blowing process, lime, limestone, dolomite, pellets and other auxiliary materials are added to the ladle to form slag, so as to control the temperature of the molten steel in the converter during the converter blowing process, so that the composition and temperature at the end of the blowing meet the converter end process requirements, and at the same time, the phosphorus in the steel reacts with calcium oxide to generate phosphorus-containing compounds that are removed by entering the slag.

[0028] By observing the flame conditions at the converter mouth and combining with auxiliary equipment to judge the carbon content of the converter at the end point, the carbon content at the converter blowing end point is improved and controlled stably. It is strictly forbidden to increase the temperature and dephosphorize by spot blowing to avoid overoxidation of molten steel. The carbon content at the converter end point is kept at 0.04wt%. The oxygen content at the converter blowing end point is controlled at 600ppm~700ppm, which is conducive to removing carbon and oxygen in the steel through carbon-oxygen reaction under RH vacuum conditions; the temperature after the converter blowing is completed is controlled at 1750℃ to ensure that the molten steel after tapping the converter has sufficient temperature for vacuum treatment operation when entering the RH refining furnace.

[0029] The molten steel is not deoxidized during the converter tapping process, which is beneficial to the next process. At the same time, during the converter tapping process, it is only necessary to ensure that the ladle air bricks are permeable. It is strictly forbidden to blow argon into the ladle at a large flow rate to avoid excessive drop in the molten steel temperature during the tapping process.

[0030] After tapping the converter, top slag (such as calcium aluminate), slag-forming materials (such as lime), and flux are added to the ladle. After the top slag is added, the argon flow rate of the molten steel is appropriately increased to disperse the top slag. The amount of top slag and lime added is controlled at 1kg per ton of steel, respectively, to isolate the molten steel from heat transfer to the air, thereby facilitating heat preservation and providing sufficient heat for RH vacuum treatment and treatment time.

[0031] The molten steel after the converter is hoisted to the RH refining furnace for refining. At the beginning of RH refining, the lifting gas flow rate is set to 1500NL / min and the vacuum degree is set to 5KPa. After the vacuum reaches 8KPa and stabilizes, and the waste gas flow rate is 1900kg / h, the vacuum degree is adjusted to the lowest vacuum degree of the equipment and the gas flow rate is adjusted to 2000NL / min. The carbon content in the molten steel is removed by utilizing the principle that carbon and oxygen react to generate gas under vacuum conditions. The molten steel is sampled after 10min and 15min of vacuum treatment.

[0032] The carbon content of the molten steel is less than 0.005wt%. The molten steel is oxygenated and, depending on the oxygen content, aluminum pellets are added to the vacuum tank for deoxidation, with the goal of maintaining the aluminum content at around 0.040wt%. The amount of aluminum pellets required for deoxidation is calculated based on the consumption of 1kg of aluminum pellets for every 5.5-7.5ppm of oxygen removed.

[0033] After adding aluminum particles into the vacuum tank, the ladle bottom blowing argon system is turned on to keep the argon flow in the ladle in a soft blowing state, which accelerates the uniformity of the aluminum element in the ladle. At the same time, the dynamic conditions of the creep of the slag interface are used to accelerate the transfer of oxygen in the slag to the molten steel, and further remove the oxygen in the slag.

[0034] After adding aluminum particles into the vacuum tank, the molten steel is kept circulating under vacuum conditions for 4 minutes, and then the molten steel is oxygenated. After the oxygen content in the molten steel is reduced to below 10ppm, samples are taken and the vacuum tank is emptied.

[0035] The molten steel after RH vacuum treatment is lifted to the LF refining furnace for slag making and heating operations. The slag making materials are lime, calcium aluminate series, and silica. The deoxidizing materials are aluminum particles, silicon carbide (cannot be added when the silicon content is strictly required), and calcium carbide (the amount of calcium carbide added needs to consider the carbon content requirement in the molten steel). The FeO+MnO content in the slag is reduced to below 1wt%. The slag-steel reaction is used to further remove inclusions and sulfur content in the molten steel. After the refining furnace slag turns white, the composition of the molten steel is adjusted to the target value.

[0036] The molten steel treated in the LF refining furnace is hoisted to the continuous casting process for protected pouring to prevent the molten steel from coming into contact with air during the pouring process and affecting the cleanliness of the molten steel.

[0037] In Example 1, the carbon content of the continuous casting product is 0.03wt%; the inspection indicators of the finished product inclusions are as follows: Class A fine series 0.5 level, Class A coarse series 0 level, Class B fine series 0.5 level, Class B coarse series 0 level, Class C fine series 0 level, Class C coarse series 0 level, Class D fine series 0.5 level, Class D coarse series 0 level, Class DS fine series 0 level, the cleanliness of the molten steel is further improved, and the product performance is more stable.

[0038] Example 2:

[0039] The following describes in detail the implementation method for producing aluminum-deoxidized clean steel with a carbon content requirement of 0.07wt% to 0.60wt% for smelting commonly used continuous casting products.

[0040] After adding molten iron and scrap steel to the converter, high-jet oxygen is blown into the ladle to react carbon, phosphorus and other elements in the molten iron with oxygen to generate gas or oxides; during the converter blowing process, lime, limestone, dolomite, pellets and other auxiliary materials are added to the ladle to form slag, so as to control the temperature of the molten steel in the converter during the converter blowing process, so that the composition and temperature at the end of the blowing meet the converter end process requirements, and at the same time, the phosphorus in the steel reacts with calcium oxide to generate phosphorus-containing compounds that are removed by entering the slag.

[0041] By observing the flame conditions at the converter mouth and combining with auxiliary equipment to judge the carbon content of the converter at the end point, the carbon content at the converter blowing end point is improved and controlled stably. It is strictly forbidden to increase the temperature and dephosphorize by spot blowing to avoid overoxidation of molten steel. The carbon content at the converter end point is kept at 0.06wt%, the phosphorus content at the converter end point is kept at 0.015wt%, and the oxygen content at the converter blowing end point is controlled at 400-500ppm, which is conducive to removing carbon and oxygen in the steel through carbon-oxygen reaction under RH vacuum conditions; the temperature after the converter blowing is completed is controlled at 1730℃ to ensure that the molten steel after the converter is tapped has sufficient temperature for vacuum treatment operation when entering the RH refining furnace.

[0042] The molten steel is not deoxidized during the converter tapping process, which is beneficial to the next process. At the same time, during the converter tapping process, it is only necessary to ensure that the ladle air bricks are permeable. It is strictly forbidden to blow argon into the ladle at a large flow rate to avoid excessive drop in the molten steel temperature during the tapping process.

[0043] After tapping the converter, top slag (such as calcium aluminate), slag-forming materials (such as lime), and flux are added to the ladle. After the top slag is added, the argon flow rate of the molten steel is appropriately increased to disperse the top slag. The amount of top slag and lime added is controlled at 1kg per ton of steel, respectively, to isolate the molten steel from heat transfer to the air, thereby facilitating heat preservation and providing sufficient heat for RH vacuum treatment and treatment time.

[0044] The molten steel after the converter is hoisted to the RH refining furnace for refining. At the beginning of RH refining, the lifting gas flow is set to 1800NL / min and the vacuum degree is set to 5KPa. After the vacuum reaches 8KPa and stabilizes, and the waste gas flow is 1950kg / h, the vacuum degree is adjusted to the lowest vacuum degree of the equipment and the gas flow is adjusted to 2200NL / min. The carbon content in the molten steel is removed by utilizing the principle that carbon and oxygen react to generate gas under vacuum conditions. The molten steel is sampled after 10min and 15min of vacuum treatment.

[0045] The molten steel is oxygenated. When the oxygen content in the molten steel reaches 150ppm, aluminum pellets are added to the vacuum tank for deoxidation, with the goal of maintaining the aluminum content in the steel at around 0.040wt%. The amount of aluminum pellets required for deoxidation is calculated based on the consumption of 1kg of aluminum pellets for every 5.5-7.5ppm of oxygen removed.

[0046] After adding aluminum particles into the vacuum tank, the ladle bottom blowing argon system is turned on to keep the argon flow in the ladle in a soft blowing state, which accelerates the uniformity of the aluminum element in the ladle. At the same time, the dynamic conditions of the creep of the slag interface are used to accelerate the transfer of oxygen in the slag to the molten steel, and further remove the oxygen in the slag.

[0047] After adding aluminum particles into the vacuum tank, the molten steel is kept circulating under vacuum conditions for 4 minutes, and then the molten steel is oxygenated. After the oxygen content in the molten steel is reduced to below 10ppm, samples are taken and the vacuum tank is emptied.

[0048] The molten steel after RH vacuum treatment is lifted to the LF refining furnace for slag making operation. The slag making materials are lime, calcium aluminate series, and silica. The deoxidizing materials are aluminum particles, silicon carbide (cannot be added when the silicon content is strictly required), and calcium carbide (the amount of calcium carbide added needs to be based on the carbon content requirements in the molten steel). The FeO+MnO content in the slag is reduced to below 1wt%. The slag-steel reaction is used to further remove inclusions and sulfur content in the molten steel. After the refining furnace slag turns white, the composition of the molten steel is adjusted to the target value.

[0049] The molten steel treated in the LF refining furnace is hoisted to the continuous casting process for protected pouring to prevent the molten steel from coming into contact with air during the pouring process and affecting the cleanliness of the molten steel.

[0050] In Example 2, the carbon content of the continuous casting product is required to be 0.45wt%; the inspection indicators of the finished product inclusions are as follows: Class A fine series 0.5 level, Class A coarse series 0 level, Class B fine series 0.5 level, Class B coarse series 0 level, Class C fine series 0 level, Class C coarse series 0 level, Class D fine series 0.5 level, Class D coarse series 0 level, Class DS fine series 0 level, the cleanliness of the molten steel is further improved, and the product performance is more stable.

Claims

1. A method for improving the cleanliness of molten steel, characterized in that: Specifically include: 1) The carbon content at the converter blowing endpoint is controlled at 0.035wt% to 0.10wt%, the oxygen content at the converter blowing endpoint is controlled below 900ppm, and the converter blowing endpoint temperature is controlled above 1700°C; 2) The molten steel is not deoxidized during the converter tapping process. At the same time, the ladle air bricks are ensured to be permeable during the converter tapping process, and argon blowing is not performed on the ladle; 3) After the converter taps, top slag, lime slag material and flux are added to the ladle. The amount of top slag and lime added is controlled at 1-2 kg / ton of steel respectively to keep the molten steel warm. 4) The molten steel after tapping the converter is hoisted to the RH refining furnace for vacuum decarburization treatment to remove the carbon content in the molten steel to the process requirements; 5) After decarburization, the molten steel is oxygenated. Depending on the oxygen content in the molten steel, aluminum particles are added to the vacuum tank for deoxidation, with the goal of maintaining the aluminum content in the steel at 0.020wt% to 0.050wt%; 6) After adding aluminum particles into the vacuum tank, open the ladle bottom blowing argon system to make the argon flow in the ladle in a soft blowing state; 7) After adding aluminum particles into the vacuum tank, keep the molten steel circulating under vacuum conditions for 2 to 5 minutes, then determine the oxygen content of the molten steel. After the oxygen content in the molten steel drops below 10 ppm, take a sample and pierce the vacuum tank; 8) The molten steel after RH vacuum treatment is hoisted to the LF refining furnace for slag making and temperature raising operations to reduce the FeO+MnO content in the slag to below 1 wt%; 9) The molten steel treated in the LF refining furnace is transported to the continuous casting process for pouring.

2. A method for improving the cleanliness of molten steel according to claim 1, characterized in that: In the step 1), when smelting aluminum-deoxidized clean steel with a carbon content requirement of 0.02wt% to 0.06wt% for the continuous casting product, the carbon content at the converter blowing end point is controlled within the range of 0.035wt% to 0.045wt%, and the oxygen content at the converter blowing end point is controlled within the range of 500 to 800ppm.

3. A method for improving the cleanliness of molten steel according to claim 1, characterized in that: In the step 1), when smelting aluminum-deoxidized clean steel with a carbon content requirement of 0.07wt% to 0.60wt% for the continuous casting product, the carbon content at the converter blowing end point is controlled to be 0.06wt% to 0.10wt%.

4. A method for improving the cleanliness of molten steel according to claim 3, characterized in that: The phosphorus content of the finished steel is less than 0.020wt%.

5. The method for improving the cleanliness of molten steel according to claim 1, wherein: The top slag in step 3) is calcium aluminate top slag.

6. A method for improving the cleanliness of molten steel according to claim 1, characterized in that: In the step 5), when smelting aluminum-deoxidized clean steel with a carbon content requirement of 0.02wt% to 0.06wt% for the continuous casting product, when the carbon content in the molten steel is reduced to below 0.005wt% during the RH vacuum treatment, the molten steel is subjected to an oxygen determination operation.

7. A method for improving the cleanliness of molten steel according to claim 1, characterized in that: In the step 5), when smelting aluminum-deoxidized clean steel with a carbon content requirement of 0.07wt% to 0.60wt% for the continuous casting product, when the oxygen content in the molten steel drops below 150ppm, aluminum particles are added to the vacuum tank for deoxidation.