Production method for improving molten steel purity of steel for nuclear power

Abstract

Disclosed in the present invention is a production method for improving molten steel purity of steel for nuclear power. The production method comprises the following step: AOD furnace smelting, involving: in an AOD pre-reduction stage, changing the addition amount calculated for full ferrosilicon pre-reduction into half amount of ferrosilicon and half amount of an aluminum ingot to perform pre-reduction de-oxidation, and controlling the basicity of pre-reduced furnace slag to be 2.5-3.5; and in an AOD reduction stage, adding first-grade lime in an amount of 6.5-8.5 kg per ton of steel, special-grade fluorite in an amount of 5-8 kg per ton of steel and an aluminum ingot in an amount of 1.5-3.0 kg per ton of steel for deep de-oxidation, and controlling the content of Al in molten steel to be 0.035-0.045%. The present invention has the characteristic of improving molten steel purity on the basis of an ultra-low carbon content.

Description

A production method for improving the purity of molten steel for nuclear power plants Technical Field

[0001] This invention relates to a production method for improving the purity of molten steel, and more particularly to a production method for improving the purity of molten steel for nuclear power. Background Technology

[0002] Because nuclear power steel materials generally operate in high-temperature environments (≥300℃) or ultra-low-temperature environments, the special nature of these environments causes carbides to accumulate and precipitate along phase boundaries within the steel at the operating temperatures. This increases the material's brittleness and reduces its plasticity. Due to the harsh operating environment of nuclear power steel materials, extremely high purity is required in the molten steel. To prevent carbide segregation caused by carbon segregation, the carbon content of the steel must be as low as possible, i.e., the C content of the molten steel must be reduced. Simultaneously, the steel must meet the ultimate requirements of a 0.2% yield strength >800MPa at -196℃ and an ultimate tensile strength >1100MPa at -196℃.

[0003] While existing AOD (Alternating Oxygen Deoxidation) furnaces can blow carbon to below 0.01%, the lower the carbon content, the more severe the over-oxidation of the molten steel, resulting in lower steel purity and ultimately, lower steel quality. Existing AOD pre-reduction and reduction deoxidation methods cannot meet the steel quality requirements. Existing LF (Fluorescent Leaching) refining methods are ineffective at flotating inclusions in molten steel, and the inclusion removal effect is unstable.

[0004] Therefore, existing refining methods are insufficient to achieve the high purity of molten steel at ultra-low carbon levels. Summary of the Invention

[0005] The purpose of this invention is to provide a production method for improving the purity of molten steel used in nuclear power plants. This invention features improved purity of molten steel while maintaining ultra-low carbon content.

[0006] The technical solution of the present invention: a production method for improving the purity of molten steel for nuclear power, comprising the following steps: AOD furnace smelting: in the AOD pre-reduction stage, the amount of addition calculated based on the full ferrosilicon pre-reduction is changed to half ferrosilicon and half aluminum ingots for pre-reduction deoxidation, and the alkalinity of the pre-reduction slag is controlled at 2.5 to 3.5.

[0007] During the AOD reduction stage, 6.5–8.5 kg / ton of steel of grade 1 lime, 5–8 kg / ton of steel of grade fluorite, and 1.5–3.0 kg / ton of steel of aluminum ingots are added for deep deoxidation, and the Al content in the molten steel is controlled at 0.035%–0.045%.

[0008] In the aforementioned production method for improving the purity of molten steel for nuclear power, during the AOD furnace smelting process, before the AOD pre-reduction stage, AOD carbon blowing is carried out first. During the oxidation period of the AOD carbon blowing stage, the AOD furnace first adopts full oxygen blowing. When the carbon content is ≤0.10%, the oxygen:argon flow ratio is 3:7 for blowing.

[0009] In the aforementioned production method for improving the purity of molten steel for nuclear power, the amount of lime added during the AOD carbon blowing stage is 70-75 kg / ton.

[0010] In the aforementioned production method for improving the purity of molten steel for nuclear power, AOD pre-reduction begins after carbon blowing until the carbon content in the molten steel is less than 0.01%.

[0011] In the aforementioned production method for improving the purity of molten steel for nuclear power, after the AOD pre-reduction stage is completed, a slag removal operation is performed until the remaining amount of slag in the furnace is ≤600kg.

[0012] In the aforementioned production method for improving the purity of molten steel for nuclear power, after the AOD reduction stage, LF refining is carried out, with the refining time controlled to be >60 min and the silicon-calcium wire feed rate being 0-50 m.

[0013] In the aforementioned production method for improving the purity of molten steel for nuclear power, after LF refining and smelting, IC casting is carried out, and argon gas protection is used for the entire casting process.

[0014] In the aforementioned production method for improving the purity of molten steel for nuclear power, an electroslag remelting operation is performed after IC casting and ingot demolding to obtain the finished steel ingot.

[0015] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0016] This invention improves the carbon blowing stage, pre-reduction stage, slag removal and reduction stage of AOD furnace smelting by adjusting the gas flow rate in the carbon blowing stage, changing the alloys added and their proportions in the pre-reduction stage, reducing the amount of slag remaining after slag removal, and increasing the amount of deep deoxidizing materials used in the reduction stage.

[0017] This invention improves the LF refining and smelting stage by extending the refining time and reducing the amount of silicon-calcium wire feed. As a result, the product of this invention meets the carbon control requirements of ultra-low carbon (carbon content ≤0.02%) while ensuring the purity of molten steel (the overall inclusion control level is ≤1.0 level), improving the inclusion removal effect, saving lime consumption in the reduction stage, and reducing the lime raw material cost in the AOD reduction stage.

[0018] For general steel grades, the pre-reduction basicity is controlled at 1.5–2.5. The pre-reduction adopts full ferrosilicon reduction, and the resulting product has a carbon content of ≤0.03% and inclusions of ≤1.5 grade. Compared with general low-carbon, high-purity steel smelting methods, this method requires a higher basicity of 2.5–3.5 during AOD smelting for a deeper level of deoxidation, adopts a semi-silicon, semi-aluminum deoxidation mode, and adds electroslag treatment to stably control the high purity of molten steel. This results in a product with a carbon content of ≤0.02%, even lower carbon content, inclusions of ≤1.0 grade, and higher purity of molten steel.

[0019] Therefore, this invention has the characteristic of improving the purity of molten steel while maintaining ultra-low carbon content. Detailed Implementation

[0020] The present invention will be further described below with reference to embodiments, but these embodiments are not intended to limit the scope of the invention.

[0021] Example 1:

[0022] A method for improving the purity of molten steel for nuclear power plants includes the following smelting process:

[0023] S1. Primary smelting furnace;

[0024] S2, AOD furnace smelting:

[0025] S201. During the AOD carbon blowing stage, the carbon content is controlled to be ≤0.015% to meet the special requirements of the steel grade. However, blowing the carbon content below 0.015% will lead to severe over-oxidation of the molten steel. To solve this problem, full oxygen blowing is first used in the three-lance AOD furnace during the oxidation period. When the carbon content is ≤0.10%, an oxygen:argon flow ratio of 3:7 is used for blowing, which can minimize the degree of oxidation of the molten steel. The amount of lime added during the AOD carbon blowing stage is increased from 65-60 kg / ton to 70-75 kg / ton, an increase of 15%-25%, which increases the slag basicity and reduces the oxygen content in the slag. AOD pre-reduction begins after carbon blowing reaches a carbon content of less than 0.01% in the molten steel.

[0026] S202. In the AOD pre-reduction stage: Previously, a full ferrosilicon pre-reduction method was used. However, due to the severe oxidation of the molten steel and the weak deoxidation effect of silicon, the slag basicity was controlled at 1.6-1.8, resulting in poor deoxidation and blackening of the pre-reduction slag. This invention adjusts the method to a "half-silicon, half-aluminum" approach for pre-reduction. Half of the amount added is ferrosilicon, and the other half is aluminum ingots, calculated based on the full ferrosilicon pre-reduction amount. For example, if the calculated amount for full ferrosilicon pre-reduction is 900 kg, then 450 kg of ferrosilicon and 450 kg of aluminum ingots are added. The pre-reduction slag basicity is controlled at 2.5-3.5. Using this method, as the pre-reduction slag basicity increases, the pre-reduction deoxidation effect is significantly improved, the deoxidation level is more thorough, and the total content of (FeO+MnO+Cr2O3) in the slag is reduced from 1.5%-3.5% to below 1.5%.

[0027] After the S203 and AOD pre-reduction stages are completed, slag removal is performed: the original process requires 1 to 1.5 tons of slag remaining after slag removal. The low alkalinity and poor deoxidation level of the slag with inclusions ≤2.0 will affect the subsequent slag formation. Therefore, this invention requires that the amount of slag remaining in the furnace after pre-reduction is ≤600kg, which can effectively avoid the impact of pre-reduced slag on subsequent slag formation.

[0028] S204, AOD Reduction Stage: The original process required the addition of 15 kg / ton of ordinary lime and 8.5 kg / ton of ordinary fluorite for slag formation, and 1.0–2.0 kg / ton of aluminum ingots for deep deoxidation, with the Al content in the molten steel controlled at ≤0.010%. The new process of this invention uses 6.5–8.5 kg / ton of first-grade lime and 5–8 kg / ton of premium-grade fluorite, and adds 1.5–3.0 kg / ton of aluminum ingots for further deep deoxidation, controlling the Al content in the molten steel at 0.035%–0.045%. Compared to the original process, the new process not only achieves better deoxidation, but also reduces the total content of (FeO+MnO+Cr2O3) in the slag from 1.0%–2.5% to below 1.0%, and reduces the lime usage in the reduction stage by 43.3–56.7%, thus lowering material costs.

[0029] S3, LF refining: The original process controlled the refining time at around 45 minutes. Due to the poor oxidizability of molten steel, the molten steel was not allowed to stand for long enough after feeding 100 meters of silicon-calcium wire, and the inclusions in the molten steel did not float sufficiently. The new LF smelting process of this invention requires a refining time of >60 minutes and the amount of silicon-calcium wire fed is reduced to 0-50 meters.

[0030] S4, IC casting: Argon gas is used for the entire casting process.

[0031] S5. Electroslag remelting: After the steel ingot is demolded, it undergoes an electroslag remelting operation to obtain the finished steel ingot.

[0032] Through the above operations, the steel ingots produced by this invention have stable quality, meet the requirement of ≤1.0 grade for all inclusions, greatly improve the purity of molten steel, and the C content of the finished steel ingots can be stably controlled between 0.010% and 0.019%.

[0033] Example 2:

[0034] Produced using furnace number FG24C490, nuclear power steel, taking 316LN as an example, has the following composition: C≤0.03%, Si≤0.75%, Mn≤2.0%, P≤0.030%, S≤0.015%, Cr: 16%-18.5%, Ni: 10%-14%, Mo: 2%-3%, N: 0.15%-0.22%.

[0035] A production method for improving the purity of 316LN steel for nuclear power applications includes the following smelting process:

[0036] S1. Primary smelting furnace;

[0037] S2. AOD furnace smelting: During the AOD carbon blowing stage, the C content is controlled to be ≤0.015%. In the oxidation period, the three-gun AOD furnace first adopts full oxygen blowing. When the carbon content is ≤0.10%, the oxygen:argon flow ratio is 3:7 for blowing, and the amount of lime added is 73.8 kg / ton. After carbon blowing until the carbon content in the molten steel is no more than 0.008%, AOD pre-reduction begins.

[0038] In the AOD pre-reduction stage: 415 kg of ferrosilicon and 415 kg of aluminum ingots were added to 30 tons of molten steel for pre-reduction deoxidation. The basicity of the pre-reduction slag was 3.43.

[0039] After the AOD pre-reduction stage is completed, slag is removed, and the remaining amount of slag in the furnace is ≤300kg.

[0040] AOD reduction stage: 6.7 kg / ton of grade 1 lime and 5.8 kg / ton of grade fluorite are added to the molten steel, and 1.8 kg / ton of aluminum ingot is added to carry out deep deoxidation again, and the Al content in the molten steel is controlled to 0.038%.

[0041] S3, LF refining and smelting: refining time is 70 minutes, and the silicon-calcium wire feed rate is 20 meters.

[0042] S4, IC casting: Argon gas is used for the entire casting process.

[0043] S5. Electroslag remelting: After the steel ingot is demolded, an electroslag remelting operation is performed to obtain the finished 316LN steel ingot. Sampling and inspection show that the inclusions are all ≤1.0 level.

[0044] The test results are shown in the table below:

[0045] Table 1. Inclusions in 316LN steel ingots

[0046] It should be understood that the above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit them. Those skilled in the art can modify the technical solutions described in the above embodiments, or make equivalent substitutions for some of the technical features; and all such modifications and substitutions should fall within the protection scope of the appended claims of the present invention.

Claims

1. A production method for improving the purity of molten steel for nuclear power plants, characterized in that: Includes the following steps: AOD furnace smelting: In the AOD pre-reduction stage, the amount of aluminum added calculated based on the full ferrosilicon pre-reduction is changed to half ferrosilicon and half aluminum ingots for pre-reduction deoxidation, and the basicity of the pre-reduction slag is controlled at 2.5 to 3.

5. During the AOD reduction stage, 6.5–8.5 kg / ton of steel of grade 1 lime, 5–8 kg / ton of steel of grade fluorite, and 1.5–3.0 kg / ton of steel of aluminum ingots are added for deep deoxidation, and the Al content in the molten steel is controlled at 0.035%–0.045%.

2. The production method for improving the purity of molten steel for nuclear power plants according to claim 1, characterized in that: During the AOD furnace smelting process, before the AOD pre-reduction stage, AOD carbon blowing is carried out first. During the oxidation period of the AOD carbon blowing stage, the AOD furnace first adopts full oxygen blowing. When the carbon content is ≤0.10%, the oxygen:argon flow ratio is 3:7 for blowing.

3. The production method for improving the purity of molten steel for nuclear power plants according to claim 2, characterized in that: During the AOD carbon blowing stage, the amount of lime added is 70-75 kg / ton.

4. A production method for improving the purity of molten steel for nuclear power plants according to claim 2, characterized in that: After carbon blowing to reduce the carbon content in the molten steel to less than 0.01%, AOD pre-reduction begins.

5. A production method for improving the purity of molten steel for nuclear power plants according to claim 1, characterized in that: After the AOD pre-reduction stage is completed, slag removal is performed until the remaining slag in the furnace is ≤600kg.

6. A production method for improving the purity of molten steel for nuclear power plants according to claim 1, characterized in that: After the AOD reduction stage, LF refining and smelting are carried out, with the refining time controlled to be >60min and the silicon-calcium wire feed rate being 0-50m.

7. A production method for improving the purity of molten steel for nuclear power plants according to claim 5, characterized in that: After LF refining and smelting, IC casting is carried out, and argon gas is used for the entire casting process.

8. A production method for improving the purity of molten steel for nuclear power plants according to claim 6, characterized in that: After IC casting and ingot demolding, electroslag remelting is performed to obtain the finished steel ingot.

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