Production method for improving purity of molten steel for nuclear power

Abstract

The invention discloses a production method for improving purity of molten steel for nuclear power, which comprises the following steps: smelting in an AOD furnace: in an AOD pre-reduction stage, according to the adding amount calculated by full ferrosilicon pre-reduction, carrying out pre-reduction deoxidation on half of ferrosilicon and half of aluminum ingot, and controlling the alkalinity of pre-reduced slag to be 2.5-3.5; in the AOD reduction stage, 6.5-8.5 kg of first-grade lime per ton of steel, 5-8 kg of special-grade fluorite per ton of steel and 1.5-3.0 kg of aluminum ingots per ton of steel are added for deep deoxidation, and the content of Al in molten steel is controlled to be 0.035%-0.045%. The method has the characteristic of improving the purity of the molten steel on the basis of ultralow carbon content.

Description

Technical Field

[0001] The invention relates to a production method for improving the purity of molten steel, in particular to a production method for improving the purity of molten steel for nuclear power. Background Art

[0002] Since nuclear power steel materials are generally used in high temperature environments (≥300℃) or ultra-low temperature environments, due to the particularity of the service environment, at the service temperature, carbides inside the steel will be enriched and precipitated along the phase boundary, which increases the brittleness of the material and reduces the plasticity. Due to the harsh service environment of nuclear power steel materials, the molten steel is required to have extremely high purity. In order to prevent the segregation of carbides caused by the segregation of C elements, the carbon content of the steel should be as low as possible, that is, the C content of the molten steel should be reduced; at the same time, the 0.2% yield strength of the steel at -196℃ needs to be greater than 800MPa, and the ultimate tensile strength at -196℃ needs to be greater than 1100MPa.

[0003] Although the existing AOD argon oxygen refining furnace can blow carbon to below 0.01%, the lower the carbon content is controlled, the more serious the overoxidation of the molten steel, the worse the purity of the molten steel, and the worse the quality of the steel. The existing AOD pre-reduction stage and reduction stage deoxidation methods cannot meet the quality requirements of steel. The existing LF refining method has a poor effect on the floating of inclusions in the molten steel, and the removal effect of inclusions is unstable.

[0004] Therefore, it is difficult to achieve high purity of molten steel at ultra-low carbon using existing refining methods. Summary of the invention

[0005] The purpose of the present invention is to provide a production method for improving the purity of molten steel for nuclear power. The present invention has the characteristics of improving the purity of molten steel on the basis of ultra-low carbon content.

[0006] The technical solution of the present invention is 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 addition amount calculated based on the full ferrosilicon pre-reduction is changed to half of the amount of ferrosilicon and half of the amount of aluminum ingots for pre-reduction and deoxidation, and the basicity of the pre-reduction slag is controlled at 2.5-3.5;

[0007] In the AOD reduction stage, 6.5-8.5 kg / ton of steel of first-grade lime, 5-8 kg / ton of steel of special-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% to 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 first carried out. 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%, oxygen:argon flow ratio = 3:7 is used 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 is started after carbon blowing is performed 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 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 and smelting are carried out, the refining time is controlled to be greater than 60 minutes, and the silicon-calcium wire feeding amount is 0 to 50 meters.

[0013] In the aforementioned production method for improving the purity of molten steel for nuclear power, IC pouring is carried out after LF refining and smelting, and argon gas protection is used throughout the pouring 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 pouring and ingot demoulding to obtain a finished steel ingot.

[0015] Compared with the prior art, the present invention has the following beneficial effects:

[0016] The present invention improves the carbon blowing stage, pre-reduction stage, slag removal and reduction stage of AOD furnace smelting, adjusts the gas flow rate in the carbon blowing stage, changes the alloy added and the proportion in the pre-reduction stage, reduces the amount of slag remaining after slag removal, and increases the amount of deep deoxidized materials in the reduction stage;

[0017] The present invention improves the LF refining and smelting stage, prolongs the refining time, and reduces the feeding amount of silicon-calcium wire, so that the product of the present invention can meet the carbon control requirement of ultra-low carbon (carbon content ≤ 0.02%), while ensuring the purity of molten steel (the control level of all inclusions ≤ 1.0 level), improving the inclusion removal effect, saving the amount of lime used in the reduction stage, and reducing the lime raw material cost in the AOD reduction stage.

[0018] The pre-reduction basicity of general steel grades is controlled at 1.5-2.5, and the pre-reduction adopts all-silicon ferrosilicon reduction. The carbon content of the obtained product is ≤0.03%, and the inclusions are all ≤1.5 level. Compared with the general low-carbon and high-purity steel smelting method, this method uses a higher basicity of 2.5-3.5 during AOD smelting to achieve a deeper deoxidation level, adopts a half-silicon and half-aluminum deoxidation mode, and increases electroslag treatment to stably control the high purity of the molten steel, so that the carbon content of the obtained product is ≤0.02%, the carbon content is lower, the inclusions are all ≤1.0 level, and the molten steel has higher purity.

[0019] Therefore, the present invention has the characteristic of improving the purity of molten steel on the basis of ultra-low carbon content. DETAILED DESCRIPTION

[0020] The present invention will be further described below in conjunction with the embodiments, but they are not intended to limit the present invention.

[0021] Embodiment 1:

[0022] A production method for improving the purity of molten steel for nuclear power generation, comprising the following smelting process:

[0023] S1, primary furnace smelting;

[0024] S2, AOD furnace smelting:

[0025] S201. In the AOD carbon blowing stage, the C content is controlled to be ≤0.015% to meet the special requirements of the steel grade. However, blowing the C content to below 0.015% will cause serious overoxidation of the molten steel. To solve this difficulty, full oxygen blowing is first used in the three-gun AOD furnace during the oxidation period. When the carbon content is ≤0.10%, the oxygen:argon flow ratio = 3:7 is used for blowing, which can minimize the oxidation degree of the molten steel, and increase the amount of lime added in the AOD carbon blowing stage from 65-60kg / ton to 70-75kg / ton. The amount of lime is increased by 15%-25%, which increases the slag basicity and reduces the oxygen content in the slag. After carbon blowing until the carbon content in the molten steel is less than 0.01%, AOD pre-reduction begins.

[0026] S202, in the AOD pre-reduction stage: originally, all-ferrosilicon pre-reduction was adopted. Due to the serious oxidation degree of molten steel and the weak deoxidation effect of silicon element, the slag basicity was controlled at 1.6-1.8, the deoxidation level was poor, and the pre-reduction slag was black. Now the present invention is adjusted to the "half-silicon and half-aluminum" mode for pre-reduction, that is, according to the addition amount calculated for the full-ferrosilicon pre-reduction, half of the amount is added with ferrosilicon, and half of the amount is added with aluminum ingots for pre-reduction deoxidation. For example, if the addition amount calculated for the full-ferrosilicon pre-reduction is 900kg, 450kg of ferrosilicon and 450kg of aluminum ingots are added. The basicity of the pre-reduction slag is controlled at 2.5-3.5. With this method, as the basicity of the pre-reduction slag is increased, 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] S203. After the AOD pre-reduction stage is completed, slag removal is carried out: the original process requires that the remaining slag after slag flow is 1 ton to 1.5 tons. The slag with low basicity and poor deoxidation level formed by inclusions ≤2.0 level will affect the subsequent slag making level. Therefore, the present invention requires that after the pre-reduction is completed, the remaining slag in the furnace is ≤600kg, which can effectively avoid the influence of the pre-reduction slag on the subsequent slag making.

[0028] S204, AOD reduction stage: the original process needs to add 15kg / ton of steel common lime, 8.5kg / ton of steel common fluorite slag, 1.0-2.0kg / ton of steel aluminum ingots for deep deoxidation, and the Al content in the molten steel is controlled at ≤0.010%; the new process of the present invention uses 6.5-8.5kg / ton of steel first-grade lime, 5-8kg / ton of steel special fluorite, and adds 1.5-3.0kg / ton of steel aluminum ingots for deep deoxidation again, and the Al content in the molten steel is controlled at 0.035%-0.045%. Compared with the original process, the new process not only has better deoxidation effect, but also reduces the total content of (FeO+MnO+Cr2O3) in the slag from 1.0%-2.5% to below 1.0%, and the amount of lime used in the reduction stage is reduced by 43.3-56.7%, reducing material costs.

[0029] S3. LF refining and smelting: The refining time of the original process is controlled at about 45 minutes. Due to the poor oxidizability of molten steel, the molten steel is not allowed to stand for enough time after feeding 100 meters of silicon-calcium wire, and the molten steel inclusions do not float up sufficiently. The new LF smelting process of the present invention requires a refining time of more than 60 minutes, and the feeding amount of silicon-calcium wire is reduced to 0-50 meters.

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

[0031] S5. Electroslag remelting: After the steel ingot is demoulded, the electroslag remelting operation is carried out to obtain a finished steel ingot.

[0032] Through the above operation, the quality of the finished steel ingot prepared by the present invention is stable, meets the requirement that the total inclusions are ≤1.0, greatly improves the purity of molten steel, and the C content of the finished steel ingot can be stably controlled at 0.010% to 0.019%.

[0033] Embodiment 2:

[0034] Taking FG24C490 furnace number as an example, 316LN steel for nuclear power generation 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 molten steel for nuclear power, comprising the following smelting process:

[0036] S1, primary furnace smelting;

[0037] S2. AOD furnace smelting: In 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 73.8kg / ton; after blowing carbon until the carbon content in the molten steel is not more than 0.008%, AOD pre-reduction is started.

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

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

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

[0041] S3, LF refining and smelting: the refining time is 70min, and the feeding amount of silicon calcium wire is 20 meters.

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

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

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

[0045] Table 1. Inclusions of 316LN steel ingot

[0046]

[0047] It should be understood that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit it. For those skilled in the art, the technical solutions described in the above embodiments can be modified, or some of the technical features therein can be replaced by equivalents; and all these modifications and replacements should fall within the scope of protection of the claims attached to the present invention.

Claims

1. A production method for improving the purity of molten steel for nuclear power, characterized in that: The following steps are involved: AOD furnace smelting: In the AOD pre-reduction stage, the amount of addition calculated based on full ferrosilicon pre-reduction is changed to half ferrosilicon and half aluminum ingot for pre-reduction and deoxidation, and the basicity of the pre-reduction slag is controlled at 2.5-3.5; In the AOD reduction stage, 6.5-8.5 kg / ton of steel of first-grade lime, 5-8 kg / ton of steel of special-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% to 0.045%.

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

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

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

6. The method for improving the purity of molten steel for nuclear power according to claim 1, characterized in that: After the AOD reduction stage, LF refining and smelting are carried out, the refining time is controlled to be greater than 60 minutes, and the silicon-calcium wire feeding amount is 0 to 50 meters.

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

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

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