A refining method for controlling low aluminum content in aluminum-deoxidized heat-resistant steel

Abstract

This invention relates to a refining method for controlling the low aluminum content in aluminum-deoxidized heat-resistant steel, comprising: 1) adding aluminum ingots and alloys for deoxidation and alloying during converter smelting, and adding lime and pre-melted calcium aluminate for top slag modification; 2) adding lime and pre-melted calcium aluminate for slag formation during LF refining, and adding an aluminum-free deoxidizer to the slag surface for rapid deoxidation and slag formation; 3) maintaining a vacuum degree of ≤67Pa for more than 15 minutes during RH refining to control [H] in the steel to ≤1.5ppm, and performing calcium treatment and soft blowing operation after refining. This invention adopts the process of "deep deoxidation of aluminum ingots from converter smelting + low-aluminum or aluminum-free deoxidation in LF refining" to achieve the target of Alt ≤0.010% in heat-resistant steel, and with high steel cleanliness.

Description

Technical Field

[0001] This invention relates to the field of steelmaking technology, and in particular to a refining method for controlling the low aluminum content in aluminum-deoxidized heat-resistant steel. Background Technology

[0002] In heat-resistant steel, aluminum primarily functions as a deoxidizer and alloying element. As a deoxidizer, the aluminum content (acid-soluble aluminum) is generally controlled within the range of 0.010% to 0.050%, ensuring sufficient deoxidation without generating excessive large inclusions. As an alloying element, aluminum preferentially forms a dense, continuous, and strongly adherent Al₂O₃ (alumina) film on the steel surface compared to iron, significantly improving the steel's oxidation resistance at high temperatures. However, on the other hand, excessive aluminum can lead to a decrease in the steel's strength at high temperatures (especially creep strength), increase the tendency for weld cracking, and reduce room-temperature impact toughness.

[0003] Different heat-resistant steels (including austenitic heat-resistant steel, ferritic / martensitic heat-resistant steel, etc.) have different requirements for aluminum content. For most heat-resistant steels used in pressure-bearing components, aluminum is a strictly limited impurity element. For example, high-pressure boiler steel pipes used to manufacture heat-receiving components such as superheaters, reheaters, and main steam pipes in high-pressure boilers can operate at temperatures up to 580℃, requiring the steel to possess good high-temperature mechanical properties, structural stability, corrosion resistance, and oxidation resistance. Because high aluminum content increases the probability of graphitization in the microstructure of high-pressure boiler steel pipes under high-temperature service conditions, leading to their failure during high-temperature service, the industry has clear requirements for the aluminum content in the heat-resistant steel used, generally requiring Alt ≤ 0.015%.

[0004] Currently, steel companies commonly use silicon deoxidation to control the low aluminum content in heat-resistant steel. However, this process generates a large number of silicate inclusions, leading to excessive levels of Class C inclusions in the steel. Furthermore, silicon deoxidation is less effective than aluminum deoxidation, resulting in problems such as excessive oxygen content in the steel. Achieving low aluminum content control in steel using aluminum deoxidation is more challenging, presenting difficulties such as slow slag formation during LF refining due to limitations in the amount of aluminum deoxidizing material added, and unsatisfactory control of molten steel cleanliness. Summary of the Invention

[0005] This invention provides a refining method for controlling the low aluminum content in aluminum-deoxidized heat-resistant steel. The method employs a process of "deep deoxidation of aluminum ingots from converter steelmaking + LF refining with low or no aluminum content" to achieve the target of Alt ≤ 0.010% in heat-resistant steel, while maintaining high steel cleanliness.

[0006] To achieve the above objectives, the present invention employs the following technical solution: A refining method for controlling the low aluminum content in aluminum-deoxidized heat-resistant steel includes the following steps: 1) During the steelmaking process in the converter, aluminum ingots and alloys are added for deoxidation and alloying. After deoxidation and alloying, the Alt content is 0.020wt%~0.030wt%. Lime and pre-melted calcium aluminate are added for top slag modification. Argon gas is introduced throughout the entire process from tapping to ladle hoisting. 2) After the ladle is hoisted to the LF refining station, argon blowing is performed. After heating with electricity, lime and pre-melted calcium aluminate are added to form slag. An aluminum-free deoxidizer is added to the slag surface for rapid deoxidation and slag formation, resulting in a reducing refining slag with FeO + MnO ≤ 1wt%. The white slag is maintained for no less than 20 minutes. Then, the temperature and composition of the molten steel are adjusted. When the steel leaves the LF refining station, the Alt content is controlled between 0.007wt% and 0.011wt%. 3) Hoist the ladle to the RH refining station. The RH refining vacuum degree is ≤67Pa and maintained for more than 15 minutes. Control the [H] in the steel to ≤1.5ppm. After refining, perform calcium treatment and soft blowing operation.

[0007] In step 1), the amount of aluminum ingot added is 1.20 to 2.00 kg / t steel.

[0008] In step 1), the amount of lime added is 2.0 to 3.5 kg / t steel, and the amount of pre-melted calcium aluminate added is 3.5 to 6.0 kg / t steel.

[0009] In step 2), when Alt < 0.015 wt% in the first component sample obtained from LF refining, aluminum wire is fed in to adjust the Alt content to 0.015% to 0.020%.

[0010] In step 2), the amount of lime added is 3.0 to 4.5 kg / t steel, and the amount of pre-melted calcium aluminate added is 0 to 4.0 kg / t steel.

[0011] In step 2), the aluminum-free deoxidizer is composed of ferrosilicon powder and calcium carbide. The amount of ferrosilicon powder added is 0 to 2.0 kg / t steel, and the initial amount of calcium carbide added is 0.2 to 0.6 kg / t steel. The final amount added is dynamically adjusted according to the slag-forming effect and the C content in the steel.

[0012] In step 2), argon gas is first used for strong blowing from the ladle seat to the soft blowing station, with an argon gas flow rate of 600-700 NL / min and a duration of no less than 1 min; after normal gas permeability, the flow rate is adjusted to weak argon gas blowing, with an argon gas flow rate of 200-300 NL / min; after the ladle enters the smelting station, the argon gas flow rate is adjusted to medium blowing, with an argon gas flow rate of 400-500 NL / min; after temperature measurement, the heating rate is controlled, and slag-forming materials are added in batches, with each batch weighing no more than 100 kg; calcium carbide and silicon carbide are mixed in batches and added to the aluminum-free deoxidizer.

[0013] In step 3), after the ladle is degassed at the RH degassing station, the RH is set. During the calcium treatment, a single-layer seamless pure calcium wire is fed. After the wire feeding is completed, the ladle is moved to the soft blowing station to add a covering agent. At the same time, the argon flow rate is adjusted to ensure that the molten steel is not exposed. Then the soft blowing time is started and the soft blowing time is not less than 20 minutes.

[0014] The refined molten steel is continuously cast and hot rolled to obtain finished steel products. The non-metallic inclusion grades in the finished steel products are as follows: Class A fine series ≤ 0.5 grade, Class A coarse series is 0 grade; Class B fine series ≤ 0.5 grade, Class B coarse series ≤ 0.5 grade; Class C fine series ≤ 0.5 grade, Class C coarse series is 0 grade; Class D fine series ≤ 0.5 grade, Class D coarse series is 0 grade; Class Ds is 0 grade; [O] in the steel ≤ 12ppm, Alt ≤ 0.010wt%.

[0015] Compared with the prior art, the beneficial effects of the present invention are: 1) Existing silicon deoxidation processes have poor deoxidation effects and are prone to causing the C-class inclusion rating in steel to exceed the standard. However, the present invention adopts a deep deoxidation process for aluminum ingots produced from converters, which has a better deoxidation effect than silicon deoxidation processes and can ensure that the C-class inclusion rating in steel meets the standard requirements. 2) Existing aluminum deoxidation processes are difficult to control the low aluminum content in steel, and problems such as slow slag formation and unsatisfactory control of molten steel cleanliness are prone to occur during LF refining slag formation. This invention adopts the process of "deep deoxidation of aluminum ingots from converter steelmaking + LF refining with low or no aluminum deoxidation" to achieve the target of Alt≤0.010% in heat-resistant steel and high molten steel cleanliness. 3) The refining method for controlling the low aluminum content of aluminum deoxidized heat-resistant steel described in this invention is simple to operate, easy to implement, and has value for promotion and application. Detailed Implementation

[0016] The refining method for controlling the low aluminum content in aluminum-deoxidized heat-resistant steel according to the present invention includes the following steps: Step 1: Molten iron and scrap steel are smelted into steel in a converter. During the steelmaking process, aluminum ingots and alloys are added for deoxidation and alloying. The amount of aluminum ingots added is 1.20-2.00 kg / t steel. After deoxidation and alloying, the Alt content is 0.020wt%-0.030wt%.

[0017] To achieve rapid slag formation in LF refining, lime and pre-melted calcium aluminate are added during the tapping process to modify the top slag, and argon gas is circulated throughout the entire process from tapping to ladle hoisting.

[0018] The amount of lime added is 2.0 to 3.5 kg / t steel, and the amount of pre-melted calcium aluminate added is 3.5 to 6.0 kg / t steel.

[0019] Step 2: After the ladle is hoisted to the LF refining station, argon blowing is performed. Electricity is supplied to raise the temperature (voltage and current are selected according to temperature and production rhythm). Then, lime and pre-melted calcium aluminate are added to form slag. The lime addition amount is 3.0–4.5 kg / t steel, and the pre-melted calcium aluminate addition amount is 0–4.0 kg / t steel. An aluminum-free deoxidizer is added to the slag surface for rapid deoxidation and slag formation, obtaining a reducing refining slag with FeO + MnO ≤ 1 wt%. The white slag is maintained for at least 20 minutes. Then, the steel temperature and composition are adjusted to the target values.

[0020] From the ladle stand to the soft blowing station, first start strong argon blowing with an argon flow rate of 600-700 NL / min for at least 1 minute; after normal permeability, adjust to weak argon blowing with an argon flow rate of 200-300 NL / min; after the ladle enters the smelting station, adjust the argon blowing to medium blowing with an argon flow rate of 400-500 NL / min; after temperature measurement, control the heating rate and add slag-forming materials in batches, with each batch weighing no more than 100 kg; in the aluminum-free deoxidizer, calcium carbide and silicon carbide are mixed and added in batches.

[0021] To control the aluminum content in steel, the amount of aluminum-containing materials added must be strictly controlled. Specific measures are as follows: 1) When Alt < 0.015wt% in the first component sample obtained from LF refining, feed in aluminum wire to adjust the Alt content to 0.015%~0.020% to ensure more complete deoxidation of molten steel. Subsequent aluminum adjustment is prohibited.

[0022] 2) During refining and slag formation, an aluminum-free deoxidizer composed of ferrosilicon powder and calcium carbide is used to replace the conventional aluminum particle deoxidizer. The amount of ferrosilicon powder added is 0-2.0 kg / t steel; the initial amount of calcium carbide added is 0.2-0.6 kg / t steel. In the specific production process, the amount of calcium carbide added can be dynamically adjusted according to the slag formation effect and the C content in the steel. The core is to achieve a balance between the slag formation effect and the carbon increase of the molten steel, so as to ensure that while obtaining excellent refining effect (white slag, high desulfurization rate), the carbon content of the molten steel is stably controlled within the target range.

[0023] 3) The Alt content in the steel is controlled between 0.007wt% and 0.011wt% when the LF refining station is exiting.

[0024] Step 3: Hoist the ladle to the RH refining station. The RH refining vacuum degree should be ≤67Pa and maintained for at least 15 minutes. After degassing at the RH degassing station, set the H (hydrogen) level to control the [H] in the steel to ≤1.5ppm. After refining, perform calcium treatment and soft blowing. During calcium treatment, feed a single layer of seamless pure calcium wire. After feeding the wire, move the ladle to the soft blowing station and add a covering agent. At the same time, adjust the argon flow rate to ensure that the molten steel is not exposed. Then start the soft blowing timer, which should be no less than 20 minutes.

[0025] The refined molten steel is continuously cast and hot rolled to obtain finished steel products. The non-metallic inclusion grades in the finished steel products are as follows: Class A fine series ≤ 0.5 grade, Class A coarse series is 0 grade; Class B fine series ≤ 0.5 grade, Class B coarse series ≤ 0.5 grade; Class C fine series ≤ 0.5 grade, Class C coarse series is 0 grade; Class D fine series ≤ 0.5 grade, Class D coarse series is 0 grade; Class Ds is 0 grade; [O] in the steel ≤ 12ppm, Alt ≤ 0.010wt%.

[0026] To more intuitively illustrate the present invention, the embodiments of the present invention will be further described in conjunction with the examples. The following examples are merely preferred embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any technical solutions that can be obviously obtained by those skilled in the art within the scope of the technology disclosed in the present invention, including simple variations or equivalent substitutions, are all within the scope of protection of the present invention.

[0027]

Example

[0028] The steel production methods in Examples 1-4 include the following steps: smelting molten steel in a 120t converter → refining molten steel in an LF ladle refining furnace → refining molten steel in an RH vacuum degassing furnace → continuous casting into billets → heating and rolling into round bars → finished product inspection. Details are as follows: 1. Converter smelting Examples 1-4 were carried out in a converter smelting process using conventional methods. After the steel output reached 1 / 4 of the total molten steel, aluminum ingots, alloys, lime, and pre-melted calcium aluminate were added in sequence until the steel output reached 3 / 4 of the total molten steel. The amounts of aluminum ingots, lime, and pre-melted calcium aluminate added are shown in Table 1. Table 1. Amounts of deoxidizer, slag-forming materials, and primary alloy aluminum content during converter tapping in each embodiment. 2. LF Refining: The LF refining process in Examples 1-4 is as follows: From the ladle stand to the soft-blowing station, strong argon blowing (argon flow rate 600-700 NL / min) is initiated for 1 minute; after normal permeability, the argon flow is adjusted to weak blowing (argon flow rate 200-300 NL / min). After the ladle enters the smelting station, the argon flow is adjusted to medium blowing (argon flow rate 400-500 NL / min). Temperature is measured, and voltage and current are selected according to the steel temperature and production rhythm to reasonably control the heating rate. Slag-forming materials (lime and pre-melted calcium aluminate) are added in batches, each batch not exceeding 100 kg to prevent slag agglomeration. An aluminum-free deoxidizer (a mixture of calcium carbide and silicon carbide added in batches) is added to the slag surface for diffusion deoxidation. After the slag turns white, the temperature continues to rise, and the composition is fine-tuned.

[0029] The first component samples taken from the LF refining in Examples 1-4 had Alt contents (mass content) of 0.020%, 0.016%, 0.005%, and 0.015%, respectively. In Example 3, the Alt content was adjusted to 0.016% using aluminum wire feeding. Molten steel with qualified composition and suitable temperature was transported to the RH refining plant for further processing. The amount of slag-forming materials added and the white slag retention time in the LF refining plant are shown in Table 2, and the final slag composition and Alt content at the outlet are shown in Table 3.

[0030] Table 2. Amount of LF refining slag-forming materials added and white slag retention time in each embodiment. Table 3. Composition of the final refined residue sample and the Alt composition of the LF refined product at the station for each embodiment, wt%. 3. RH Refining: The RH refining process in Examples 1-4 is as follows: from the ladle stand to the RH degassing station, after the degassing time meets the requirements, the H content is determined. The H content in the steel of Examples 1-4 is 1.6ppm, 1.3ppm, 0.8ppm, and 0.8ppm respectively. After degassing, a calcium treatment operation is performed, feeding a single-layer seamless pure calcium wire, with a feeding amount of 50m for each layer. After the wire feeding is completed, the process is transferred to the soft blowing station to add a covering agent, while adjusting the argon flow rate to ensure the liquid surface is undulating and the molten steel is not exposed. Soft blowing timing begins, with the soft blowing times for Examples 1-4 being 22min, 24min, 20min, and 22min respectively.

[0031] 4. Continuous casting: The treated molten steel in Examples 1-4 was continuously cast to obtain billets, and the Alt content in the billets was 0.008%, 0.006%, 0.008%, and 0.007%, respectively.

[0032] 5. Heated rolling: The billets obtained in Examples 1-4 were rolled into round bars, and the non-metallic inclusions and oxygen content in the round bars were tested. The test results are shown in Table 4.

[0033] Table 4. Grades of non-metallic inclusions and oxygen content in round steel bars Conclusion: The steel refined according to the method of the present invention in Examples 1-4 has good cleanliness, and the non-metallic inclusion grade and oxygen content of the finished steel meet the requirements of the steel grade.

[0034] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A refining method for controlling the low aluminum content in aluminum-deoxidized heat-resistant steel, characterized in that, Includes the following steps: 1) During the steelmaking process in the converter, aluminum ingots and alloys are added for deoxidation and alloying. After deoxidation and alloying, the Alt content is 0.020wt%~0.030wt%. Lime and pre-melted calcium aluminate are added for top slag modification. Argon gas is introduced throughout the entire process from tapping to ladle hoisting. 2) After the ladle is hoisted to the LF refining station, argon blowing is performed. After heating with electricity, lime and pre-melted calcium aluminate are added to form slag. An aluminum-free deoxidizer is added to the slag surface for rapid deoxidation and slag formation, resulting in a reducing refining slag with FeO + MnO ≤ 1wt%. The white slag is maintained for no less than 20 minutes. Then, the temperature and composition of the molten steel are adjusted. When the steel leaves the LF refining station, the Alt content is controlled between 0.007wt% and 0.011wt%. 3) Hoist the ladle to the RH refining station. The RH refining vacuum degree is ≤67Pa and maintained for more than 15 minutes. Control the [H] in the steel to ≤1.5ppm. After refining, perform calcium treatment and soft blowing operation.

2. The refining method for controlling the low aluminum content of aluminum-deoxidized heat-resistant steel according to claim 1, characterized in that, In step 1), the amount of aluminum ingot added is 1.20 to 2.00 kg / t steel.

3. The refining method for controlling the low aluminum content of aluminum-deoxidized heat-resistant steel according to claim 1, characterized in that, In step 1), the amount of lime added is 2.0 to 3.5 kg / t steel, and the amount of pre-melted calcium aluminate added is 3.5 to 6.0 kg / t steel.

4. The refining method for controlling the low aluminum content of aluminum-deoxidized heat-resistant steel according to claim 1, characterized in that, In step 2), when Alt < 0.015 wt% in the first component sample obtained from LF refining, aluminum wire is fed in to adjust the Alt content to 0.015% to 0.020%.

5. The refining method for controlling the low aluminum content of aluminum-deoxidized heat-resistant steel according to claim 1, characterized in that, In step 2), the amount of lime added is 3.0 to 4.5 kg / t steel, and the amount of pre-melted calcium aluminate added is 0 to 4.0 kg / t steel.

6. The refining method for controlling the low aluminum content of aluminum-deoxidized heat-resistant steel according to claim 1, characterized in that, In step 2), the aluminum-free deoxidizer is composed of ferrosilicon powder and calcium carbide. The amount of ferrosilicon powder added is 0 to 2.0 kg / t steel, and the initial amount of calcium carbide added is 0.2 to 0.6 kg / t steel. The final amount added is dynamically adjusted according to the slag-forming effect and the C content in the steel.

7. The refining method for controlling the low aluminum content of aluminum-deoxidized heat-resistant steel according to claim 1, characterized in that, In step 2), argon gas is first used for strong blowing from the ladle seat to the soft blowing station, with an argon gas flow rate of 600-700 NL / min and a duration of no less than 1 min; after normal gas permeability, the flow rate is adjusted to weak argon gas blowing, with an argon gas flow rate of 200-300 NL / min; after the ladle enters the smelting station, the argon gas flow rate is adjusted to medium blowing, with an argon gas flow rate of 400-500 NL / min; after temperature measurement, the heating rate is controlled, and slag-forming materials are added in batches, with each batch weighing no more than 100 kg; calcium carbide and silicon carbide are mixed in batches and added to the aluminum-free deoxidizer.

8. The refining method for controlling the low aluminum content of aluminum-deoxidized heat-resistant steel according to claim 1, characterized in that, In step 3), after the ladle is degassed at the RH degassing station, the RH is set. During the calcium treatment, a single-layer seamless pure calcium wire is fed. After the wire feeding is completed, the ladle is moved to the soft blowing station to add a covering agent. At the same time, the argon flow rate is adjusted to ensure that the molten steel is not exposed. Then the soft blowing time is started and the soft blowing time is not less than 20 minutes.

9. A refining method for controlling the low aluminum content of aluminum-deoxidized heat-resistant steel according to claim 1, characterized in that, The refined molten steel is continuously cast and hot rolled to obtain finished steel products. The non-metallic inclusion grades in the finished steel products are as follows: Class A fine series ≤ 0.5 grade, Class A coarse series is 0 grade; Class B fine series ≤ 0.5 grade, Class B coarse series ≤ 0.5 grade; Class C fine series ≤ 0.5 grade, Class C coarse series is 0 grade; Class D fine series ≤ 0.5 grade, Class D coarse series is 0 grade; Class Ds is 0 grade; [O] in the steel ≤ 12ppm, Alt ≤ 0.010wt%.

Images