Method for producing clean steel for railroad bridge through steelmaking whole process control

Abstract

The invention discloses a method for producing clean steel for railroad bridges through steelmaking whole process control, which generally comprises the following processes: desulfurization, converter treatment, LF (ladle furnace) refining, RH (relative humidity) vacuum refining and continuous casting, the method takes an RH vacuum process as the center, and the core is that the inclusion removal capability of the method is fully exerted by prolonging the vacuum treatment time and the soft blowing time; on the premise of a converter working procedure, inclusions are reduced from the source through converter end point C-O accurate control and tapping pre-deoxidation; the LF refining procedure is used as an important means, and the hot slag recovery process is used for rapidly producing white slag to improve the inclusion adsorption capacity; according to the method, the process of a casting machine is taken as a guarantee, protection of casting and elimination of generation of secondary inclusions through large-ladle residual steel operation are taken as cooperation, stable production of the clean steel for the railway bridge is achieved through steelmaking whole-process control, and the steel plate meets the special I-grade requirement of ultrasonic flaw detection of the new energy standard NBT 47013.3-2023 through nondestructive testing.

Description

Technical Field

[0001] The invention belongs to the technical field of steel smelting, and in particular relates to a method for producing clean steel for railway bridges by controlling the entire steelmaking process. Background Art

[0002] Railway bridge steel refers to steel specially used for railway bridge construction. It has the characteristics of high strength, high toughness, good welding performance and corrosion resistance, ensuring the safety and durability of bridges in long-term use, and plays an irreplaceable role in bridge engineering. Its remarkable superiority has made steel bridges develop from their original subordinate position to a dominant position. The United States and Japan, especially Japan, are all steel structure bridges, which also reflects the industrialization level of a country from the side.

[0003] At present, my country's railway bridges are mainly Q370qE and Q420qE, and the domestic market demand for high-performance bridge steel is about 80 billion yuan. Bridge construction is developing towards high speed, heavy load, large span, all-welded nodes, paint-free, high safety, etc. With the accelerated development of infrastructure construction, my country's bridge construction will usher in rapid development, and bridge steel will also usher in a broader market space and development opportunities.

[0004] High development is accompanied by high requirements. In order to adapt to the development trend of the bridge industry, the National Railway Administration recently issued a number of railway industry technical standards such as "Structural Steel for Railway Bridges" (TB / T 3556-2020), which put forward clear standard requirements for the cleanliness of bridge steel, and stipulated that railway steel should meet the non-metallic inclusion inspection according to Method A of GB / T10561, and the results should meet the requirements of Class A ≤ Grade 2.0, Class B ≤ Grade 2.0, Class C ≤ Grade 1.0, Class D ≤ Grade 1.5, and Class Ds ≤ Grade 2.0. Based on the requirements of the above standards, the present invention provides a method for producing clean steel for railway bridges by controlling the entire steelmaking process to produce structural steel plates for railway bridges that meet standards and market requirements, further improves the cleanliness of railway bridge steel, improves the service life and safety of bridges, and enhances market competitiveness, which is of great significance for promoting the internationalization of China's railway standards.

[0005] In order to ensure that the purity of molten steel meets the standard requirements, the present invention takes the RH process as the center, fully ensures that the vacuum treatment and soft blowing time of the RH process can fully exert its function of removing inclusions, and at the same time strengthens the control and execution of key quality points in the entire steelmaking process, improves the cleanliness of molten steel, and ensures that non-metallic inclusions in railway steel meet the standard requirements. Summary of the invention

[0006] The purpose of the present invention is to provide a method for producing clean steel for railway bridges by controlling the entire steelmaking process, which is specifically applied to the production of clean steel for railway bridges in steel plants. The invention can effectively reduce the number of inclusions in the steel, reduce the size of the inclusions, improve the purity of the molten steel, and produce clean bridge steel that meets the requirements, thereby greatly improving the product performance and stability of railway bridge steel.

[0007] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0008] The present invention provides a method for producing clean steel for railway bridges by controlling the entire steelmaking process, including a KR molten iron pretreatment step, a converter smelting step, a LF refining step, a RH refining step, and a continuous casting step; wherein:

[0009] 1) KR desulfurized hot metal pretreatment steps: after pretreatment, [S] in the hot metal should be ≤ 0.005%, the hot metal temperature should be ≮1290℃, and pneumatic double slag removal should be adopted to ensure that the hot metal mirror surface rate is ≥ 95%;

[0010] 2) Converter smelting steps: the ratio of pre-treated molten iron and clean scrap steel is 5:1-7:1, the temperature of molten iron entering the furnace is ≥1290℃, the S content in molten iron is ≤0.005%, and active lime and high-silicon dolomite must be added during the blowing process to ensure the slag basicity; top and bottom composite blowing and bottom blowing are carried out with argon blowing throughout the process, sampling, temperature measurement, and oxygen determination after carbon pulling in the converter, [C] is controlled at ≥0.050%, and the temperature is controlled at ≥1600℃; during the steel tapping process, the steel outlet does not disperse and the steel tapping time is ≮5 minutes, slag is strictly prohibited, and slag washing is carried out during the steel tapping process to ensure that the inclusions are fully gathered and floated, thereby improving the purity of the molten steel;

[0011] 3) LF refining steps: The station adopts hot slag recovery process to quickly make white slag desulfurization, ensure that the slag forming speed is fast and the holding time is ≥15 minutes, and the final slag composition is controlled to be CaO / SiO2=4.5:1~7.0:1; the diameter of the "bright circle" (exposed area of ​​molten steel) during slagging does not exceed 1 / 3 of the diameter of the ladle; the deoxidation and alloying of molten steel requires that Ca treatment soft blowing be carried out for ≥5 minutes after the refining is completed. The slag surface should maintain a slightly fluctuating state during soft blowing. The total [O] content is required to be ≯20ppm and the [S] content is ≯20ppm when leaving the station;

[0012] 4) RH refining step: The temperature of the molten steel at the RH station is 1610-1620°C. It is required that the minimum vacuum degree can reach below 266Pa after evacuation for ≯4 minutes, and the vacuum degree fluctuation rebound during the vacuum treatment is ≤100Pa. During the vacuum treatment, the circulating argon gas flow rate is adjusted to ensure sufficient circulation of the molten steel. After 25 minutes of circulation, the pressure is restored and soft blowing is ≥15 minutes. The total vacuum treatment time is ≥40min. After breaking the air, [H] in the molten steel is ≤1.0ppm, [O] ≤20ppm, and [N] ≤40ppm;

[0013] 5) Continuous casting steps: the casting speed is controlled at 0.8±0.1m / min, the superheat of the tundish is 5℃ lower than the existing process, the nitrogen addition amount of the whole casting is ≤5ppm, the water inlet of the ladle is closed 60s before slag discharge to prevent slag discharge from the ladle, and the parameters of dynamic secondary cooling water distribution, electromagnetic stirring and dynamic light pressure are executed according to computer calculation.

[0014] Furthermore, in the converter smelting step, the converter dry dust removal gas analyzer is used to determine the converter end point C content based on the converter end point flue gas CO change, so as to achieve the converter end point accurate high carbon requirement C: ≥ 0.05%, no spot blowing is allowed, the oxygen setting before the furnace is ≤ 500ppm, and the steel tapping temperature is ≥ 1600℃;

[0015] In the converter smelting step, 500 kg of lime is added after carbon drawing to shake the furnace, and 100 kg of coke is added for pre-deoxidation when tapping steel;

[0016] In the converter smelting step, a slag blocking cap is inserted into the outlet of the converter before steel is tapped. When the tapping is almost finished, the slide plate is closed in advance and slag is strictly prohibited. The phosphorus recovery during the treatment process is required to be ≤0.003%.

[0017] Furthermore, in the converter smelting step, the converter steelmaking lime is ≥800kg, and 150-350kg of synthetic slag is added in the slag washing operation according to actual conditions. The molten steel is deoxidized by aluminum iron. After the converter steelmaking is completed, 30-50kg of aluminum particles are manually sprinkled on the slag surface, and argon is blown and stirred for 2 minutes. The aluminum deoxidation used in the converter process mainly forms coral-like Al2O3, and these clusters are easy to float out, and only a small amount of tight clusters and single Al2O3 particles are retained in the steel.

[0018] Furthermore, in the converter smelting step, slag is added at 1 / 3 of the steel process and completed at 2 / 3. The argon volume is controlled at 1500N l / m 3 The remaining 1 / 3 is controlled by a small flow rate of 200-300N l / m 3 , until the steel is tapped, the stirring of argon is always ensured to promote slag formation and the completion of top slag modification.

[0019] Furthermore, the hot slag recovery process is adopted in the LF refining step, the amount of white ash added is ≥2000kg, and aluminum particles are added during the refining process to promote the production of white slag; 2-5 bags of aluminum particles are added each time the electrode is heated, and when refining to RH steel, all components must be hit;

[0020] Aluminum needs to be replenished once in the LF refining step, and Al s ≥ 0.025% during the entire refining process.

[0021] Furthermore, in the LF refining step, the molten steel after the refining treatment is insulated with a double-layer covering agent. The upper layer is made of carbonized rice husks, which have good porosity and reduce the heat dissipation of the molten steel. The lower layer is made of alkaline covering agent, which is beneficial to the adsorption of inclusions, ensures the thickness of the liquid slag layer, and prevents the secondary oxidation of the molten steel.

[0022] Furthermore, in the RH refining step, the RH vacuum refining sequence is ensured to be ≥40min, the vacuum time is ≥25min, the soft blowing is ≥15min, the process circulation is adjusted to 2500L / min, and the hydrogen determination result is ≤1.0ppm; the immersion tube maintenance work is done in advance, and spraying is avoided as much as possible.

[0023] Furthermore, in the RH refining step, the soft argon blowing volume after re-pressurization is controlled at 100-150 L / min, and the time is more than 15 minutes. Practice has shown that soft argon blowing time of more than 10 minutes can effectively float large particle inclusions.

[0024] Furthermore, in the continuous casting step, the airtightness between the large ladle, the medium ladle and the crystallizer is ensured, a graphite sealing ring is used, and the water change port is put on once to avoid air intake during the continuous casting process. The Al s%-refined Al s% of the medium ladle is required to be ≤40ppm, and the nitrogen increase of the medium ladle is ≤5ppm.

[0025] Furthermore, during the continuous casting step, the casting machine performs the weighing of the empty ladle, creating conditions for the casting machine to accurately control the slag discharge from the ladle.

[0026] Compared with the prior art, the beneficial technical effects of the present invention are:

[0027] The present invention is based on the latest standard requirements for non-metallic inclusions in steel plates for railway bridge steel, with the RH vacuum process as the center, and the core is to give full play to its ability to remove inclusions by extending the vacuum treatment time and soft blowing time; the converter process is the premise, and the inclusions are reduced from the source through precise control of CO at the converter end and pre-deoxidation of steel out; the LF refining process is an important means, and the hot slag recovery process is used to quickly make white slag to improve the ability to adsorb and remove inclusions; the casting machine process is guaranteed, and the generation of secondary inclusions is prevented by protecting the casting and the operation of large-package leftover steel. Through the control of the whole process of steelmaking, a method for producing clean steel for railway bridges through the whole process of steelmaking is successfully developed. By using a method for producing clean steel for railway bridges through the whole process of steelmaking, the non-metallic inclusion inspection of super railway steel plates is realized in industrial production, and the results meet the requirements of Class A ≤ 2.0, Class B ≤ 2.0, Class C ≤ 1.0, Class D ≤ 1.5, and Class Ds ≤ 2.0.

[0028] In summary, a method for producing clean steel for railway bridges by controlling the entire steelmaking process can achieve stable production of railway bridge steel plates. The non-destructive testing of steel plates meets the requirements of the new energy standard NBT 47013.3-2023 ultrasonic flaw detection special level I. The non-metallic inclusions in the steel plates meet the requirements of Class A ≤ Level 2.0, Class B ≤ Level 2.0, Class C ≤ Level 1.0, Class D ≤ Level 1.5, and Class Ds ≤ Level 2.0. The cleanliness of bridge steel fully meets the standards for railway bridge steel and the requirements of market customers. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 It is an example of desulfurization production;

[0030] Figure 2 This is an example of converter production;

[0031] Figure 3 This is an example of LF refining production;

[0032] Figure 4 This is an example of RH vacuum refining production;

[0033] Figure 5 It is an example of casting machine production;

[0034] Figure 6 This is an example of non-metallic inclusion results in steel plates. DETAILED DESCRIPTION

[0035] The present invention will be further described below in conjunction with the embodiments.

[0036] The present invention discloses a method for producing clean steel for railway bridges through full steelmaking process control. The RH vacuum process is the center, and the core is to give full play to its ability to remove inclusions by extending the vacuum treatment time and soft blowing time; the converter process is the premise, and the inclusions are reduced from the source through precise control of the converter end CO and pre-deoxidation of steel; the LF refining process is an important means, and the hot slag recovery process is used to quickly make white slag to improve the ability to absorb inclusions; the casting machine process is guaranteed, and the protection of casting and large ladle leftover steel operation to prevent the generation of secondary inclusions is used as a coordination, and a method for producing clean steel for railway bridges through full steelmaking process control is successfully developed.

[0037] According to the present invention, a method for producing clean steel for railway bridges by controlling the entire steelmaking process is as follows:

[0038] KR desulfurization step → converter smelting step → LF refining step → RH vacuum refining step → continuous casting step. Specifically:

[0039] KR desulfurization step: The hot metal must be pre-treated by KR, and after treatment, the [S] in the hot metal is ≯0.003%, the hot metal temperature is ≮1300℃, and the hot metal must be slag-pulled twice, and the mirror surface rate of the hot metal is ≥95%. See the specific production examples of the steps. Figure 1 .

[0040] Converter smelting steps: The ratio of pre-treated molten iron and clean scrap steel is 5:1-7:1, the temperature of molten iron entering the furnace is ≥1290℃, the S content in molten iron is ≤0.005%, and active lime and high-silicon dolomite must be added during the blowing process to ensure the slag basicity; top and bottom composite blowing and bottom blowing are carried out with argon blowing throughout the process, sampling, temperature measurement, and oxygen determination after carbon pulling in the converter, [C] is controlled at ≥0.050%, and the temperature is controlled at ≥1600℃. During the steel-making process, the steel-making outlet does not disperse and the steel-making time is ≮5 minutes. It is strictly forbidden to slag, and the slag washing operation is carried out during the steel-making process to ensure

[0041] Ensure that inclusions are fully gathered and floated, and improve the purity of molten steel. See the specific production example of the steps Figure 2 .

[0042] LF refining steps: white slag desulfurization at the workstation must be fast and maintained for ≥15 minutes, and the final slag composition must be controlled to CaO / SiO2=4.5:1~7.0:1; the diameter of the "bright circle" (exposed area of ​​molten steel) during slagging should not exceed 1 / 3 of the diameter of the ladle; the deoxidation and alloying of molten steel requires that Ca treatment soft blowing should be carried out after the refining is completed for ≥5 minutes, and the slag surface should maintain a slightly fluctuating state during soft blowing. The total [O] content is required to be ≯20ppm and the [S] content is ≯20ppm when leaving the station. LF refining production examples are shown in Figure 3 .

[0043] RH refining steps: The temperature of the molten steel at the RH station is 1610-1635℃. The minimum vacuum degree must reach below 266Pa within ≯4 minutes of vacuuming and the vacuum degree fluctuation rebound must be ≤100Pa during the vacuum treatment. The RH decarburization endpoint is determined by the CO change. During the vacuum process, the circulating argon flow rate is adjusted to ensure sufficient circulation of the molten steel and decarburization as quickly as possible to ensure the decarburization effect, so that the carbon content in the molten steel is ≤10ppm after 15 minutes. Use low-carbon high-purity alloys such as low-carbon aluminum iron / high-purity silicon steel for deoxidation and alloying, control the carbon increase of the alloy to ≯5ppm, and the pure degassing time after alloying is ≥5 minutes. The molten steel is allowed to stand for ≥5 minutes before loading. The molten steel targets when leaving the site are: [C]≤15ppm, [H]≤1.2ppm, [O]≤15ppm, [N]≤20ppm. See the specific examples of step production for details. Figure 4 .

[0044] Continuous casting steps: the casting speed is controlled at 0.8±0.1m / min, the superheat of the tundish is 5℃ lower than the current process, the nitrogen addition amount of the whole casting protection is ≤5ppm, the water inlet of the large ladle is closed 60s before the slag is discharged to prevent the ladle from slagging, and the parameters of dynamic secondary cooling water distribution, electromagnetic stirring and dynamic light pressure are calculated by computer. Figure 5 .

[0045] This method has achieved stable production of bridge steel plates for industrial railways. The steel plates are tested piece by piece for non-destructive testing and meet the requirements of special grade I of ultrasonic flaw detection of new energy standard NBT 47013.3-2023. The non-metallic inclusions of steel plate samples taken every 60t meet the requirements of Class A ≤ 2.0, Class B ≤ 2.0, Class C ≤ 1.0, Class D ≤ 1.5, and Class Ds ≤ 2.0. The cleanliness of bridge steel fully meets the standards for railway bridge steel and the requirements of market customers. See the results of non-metallic inclusions in steel plates for examples. Figure 6 .

[0046] The embodiments described above are only descriptions of the preferred modes of the present invention, and are not intended to limit the scope of the present invention. Without departing from the design spirit of the present invention, various modifications and improvements made to the technical solutions of the present invention by ordinary technicians in this field should all fall within the protection scope determined by the claims of the present invention.

Claims

1. A method for producing clean steel for railway bridges by controlling the entire steelmaking process, characterized in that: The process comprises a KR molten iron pretreatment step, a converter smelting step, a LF refining step, a RH refining step, and a continuous casting step; wherein: 1) KR desulfurized hot metal pretreatment steps: after pretreatment, [S] in the hot metal should be ≤ 0.005%, the hot metal temperature should be ≮1290℃, and pneumatic double slag removal should be adopted to ensure that the hot metal mirror surface rate is ≥ 95%; 2) Converter smelting steps: the ratio of pre-treated molten iron and clean scrap steel is 5:1-7:1, the temperature of molten iron entering the furnace is ≥1290℃, the S content in molten iron is ≤0.005%, and active lime and high-silicon dolomite must be added during the blowing process to ensure the slag basicity; top and bottom composite blowing and bottom blowing are carried out with argon blowing throughout the process, sampling, temperature measurement, and oxygen determination after carbon pulling in the converter, [C] is controlled at ≥0.050%, and the temperature is controlled at ≥1600℃; during the steel tapping process, the steel outlet does not disperse and the steel tapping time is ≮5 minutes, slag is strictly prohibited, and slag washing is carried out during the steel tapping process to ensure that the inclusions are fully gathered and floated, thereby improving the purity of the molten steel; 3) LF refining steps: The station adopts hot slag recovery process to quickly make white slag desulfurization, ensure that the slag forming speed is fast and the holding time is ≥15 minutes, and the final slag composition is controlled to CaO / SiO2=4.5:1~7.0:1; the diameter of the "bright circle" (exposed area of ​​molten steel) during slagging does not exceed 1 / 3 of the diameter of the ladle; the deoxidation and alloying of molten steel requires that Ca treatment soft blowing be carried out for ≥5 minutes after the refining is completed. The slag surface should maintain a slightly fluctuating state during soft blowing. The total [O] content is required to be ≯20ppm and the [S] content is ≯20ppm when leaving the station; 4) RH refining step: The temperature of the molten steel at the RH station is 1610-1620°C. It is required that the minimum vacuum degree can reach below 266Pa after evacuation for ≯4 minutes, and the vacuum degree fluctuation rebound during the vacuum treatment is ≤100Pa. During the vacuum treatment, the circulating argon gas flow rate is adjusted to ensure sufficient circulation of the molten steel. After 25 minutes of circulation, the pressure is restored and soft blowing is ≥15 minutes. The total vacuum treatment time is ≥40min. After breaking the air, [H] in the molten steel is ≤1.0ppm, [O] ≤20ppm, and [N] ≤40ppm; 5) Continuous casting steps: the casting speed is controlled at 0.8±0.1m / min, the superheat of the tundish is 5℃ lower than the existing process, the nitrogen addition amount of the whole casting is ≤5ppm, the water inlet of the ladle is closed 60s before slag discharge to prevent slag discharge from the ladle, and the parameters of dynamic secondary cooling water distribution, electromagnetic stirring and dynamic light pressure are executed according to computer calculation.

2. The method for producing clean steel for railway bridges by controlling the entire steelmaking process according to claim 1, characterized in that: In the converter smelting step, the converter dry dust removal gas analyzer is used to determine the converter end point C content based on the change of converter end point smoke CO, so as to achieve the converter end point accurate high carbon requirement C: ≥ 0.05%, no spot blowing is allowed, the oxygen setting before the furnace is ≤ 500ppm, and the steel tapping temperature is ≥ 1600℃; In the converter smelting step, 500 kg of lime is added after carbon drawing to shake the furnace, and 100 kg of coke is added for pre-deoxidation when tapping steel; In the converter smelting step, a slag blocking cap is inserted into the outlet of the converter before steel is tapped. When the tapping is almost finished, the slide plate is closed in advance and slag is strictly prohibited. The phosphorus recovery during the treatment process is required to be ≤0.003%.

3. The method for producing clean steel for railway bridges by controlling the entire steelmaking process according to claim 1, characterized in that: In the converter smelting step, the converter steelmaking lime ≥ 800kg, and the slag washing operation adds 150-350kg of synthetic slag according to the actual situation. The molten steel is deoxidized by aluminum iron. After the converter steelmaking is completed, 30-50kg of aluminum particles are manually sprinkled on the slag surface, and argon is blown and stirred for 2 minutes. The aluminum deoxidation in the converter process mainly forms coral-like Al2O3, which are easy to float out, and only a small amount of tight clusters and single Al2O3 particles are retained in the steel.

4. The method for producing clean steel for railway bridges by controlling the entire steelmaking process according to claim 1, characterized in that: In the converter smelting process, slag is added at 1 / 3 of the steel process and completed at 2 / 3. From the beginning of steel tapping to 2 / 3 of the steel tapping, the argon volume is controlled at 1500Nl / m 3 The remaining 1 / 3 is controlled by a small flow rate of 200-300Nl / m 3 , until the steel is tapped, the stirring of argon is always ensured to promote slag formation and the completion of top slag modification.

5. The method for producing clean steel for railway bridges by controlling the entire steelmaking process according to claim 1, characterized in that: In the LF refining step, the hot slag recovery process is adopted, and the amount of white ash added is ≥2000kg. Aluminum particles are added during the refining process to promote the production of white slag; 2-5 bags of aluminum particles are added each time the electrode is heated. When refining to RH steel, all components must be hit; Aluminum needs to be supplemented once in the LF refining step, and Als ≥ 0.025% during the entire refining process.

6. The method for producing clean steel for railway bridges by controlling the entire steelmaking process according to claim 1, characterized in that: In the LF refining step, the molten steel after refining is insulated with a double-layer covering agent. The upper layer is made of carbonized rice husks, which have good porosity and reduce the heat dissipation of the molten steel. The lower layer is made of alkaline covering agent, which is beneficial to the adsorption of inclusions, ensures the thickness of the liquid slag layer, and prevents the secondary oxidation of the molten steel.

7. The method for producing clean steel for railway bridges by controlling the entire steelmaking process according to claim 1, characterized in that: During the RH refining step, ensure that the RH vacuum refining sequence is ≥40min, the vacuum time is ≥25min, the soft blowing is ≥15min, the process circulation is adjusted to 2500L / min, and the hydrogen determination result is ≤1.0ppm; do a good job of immersion tube maintenance in advance and try not to spray.

8. The method for producing clean steel for railway bridges by controlling the entire steelmaking process according to claim 1, characterized in that: In the RH refining step, the soft argon blowing volume after re-pressurization is controlled at 100-150 L / min, and the time is more than 15 minutes. Practice has proved that soft argon blowing time of more than 10 minutes can effectively make large particle inclusions float.

9. The method for producing clean steel for railway bridges by controlling the entire steelmaking process according to claim 1, characterized in that: During the continuous casting step, the airtightness between the large ladle, the medium ladle and the crystallizer is ensured, a graphite sealing ring is used, and the water change port is put on once to avoid air intake during the continuous casting process. The Als% of the medium ladle - the refined Als% is required to be ≤40ppm, and the nitrogen increase of the medium ladle is ≤5ppm.

10. The method for producing clean steel for railway bridges by controlling the entire steelmaking process according to claim 1, characterized in that: During the continuous casting process, the casting machine performs the weighing of the empty ladle, creating conditions for the casting machine to accurately control the slag discharge from the ladle.

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