A method for improving the recovery rate of rare earth in low-alloy high-strength steel

Abstract

The invention discloses a method for improving the rare earth recovery rate in low-alloy high-strength steel, which comprises adding a rare earth ferrocerium alloy with a grade of 30% for treatment in the late stage of LF refining, and performing calcium treatment after the LF furnace treatment is completed; wherein the amount of the ferrocerium alloy with a grade of 30% is calculated as 20-40 kg based on 260 tons of molten steel or 23-46 ppm in terms of rare earth Ce. The method provided by the invention can achieve a rare earth recovery rate of ≥40% in low-alloy high-strength steel, effectively avoid flocculent flow at the water inlet of the middle package, and make the production stable and smooth.

Description

Technical Field

[0001] The invention belongs to the field of metallurgical materials, and in particular relates to a method for improving the recovery rate of rare earth in low-alloy high-strength steel. Background Art

[0002] The effect of rare earth on the crystal structure of steel is mainly to make the grains finer and increase the equiaxed crystal ratio. The mechanism is that the rare earth compounds act as non-spontaneous cores of crystallization. The effect of rare earth on the crystal structure is mainly manifested in the refinement of the secondary dendrite spacing. The size of the secondary dendrite spacing will affect the micro-segregation, inclusions and looseness, thus affecting the mechanical properties. Rare earth elements form compounds with higher melting points in steel, which precipitate before the molten steel solidifies and are distributed in the molten steel as tiny particles. They act as heterogeneous nucleation centers, reduce the supercooling of the molten steel crystallization, and thus refine the solidification structure of the steel, reduce segregation, and achieve refinement of the solidification structure.

[0003] The patent document "A method for adding rare earth elements to wheel steel (CN114807505A)" adopts the electric furnace-LF-VD-continuous casting process, and adds rare earths at the VD station, which can improve the castability of molten steel, reduce the size of inclusions in the steel, and improve the ultra-detection quality of steel.

[0004] The patent document "A rare earth treatment method for low-alloy wear-resistant steel (CN117660725A)" modifies the refined slag at the end of LF refining, and with the addition of rare earth iron alloy, inhibits the formation of large-sized high-melting point inclusions, avoids nozzle nodules in the continuous casting process, and significantly improves the wear resistance and corrosion resistance of low-alloy wear-resistant steel.

[0005] The patent document "A rare earth treatment method for high-strength wheel steel (CN117683970A)" discloses a rare earth treatment method with slag-metal coordinated control, which greatly improves the recovery rate of rare earth in high-strength wheel steel, refines the grains, and significantly improves the fatigue life of high-strength wheel steel. Summary of the invention

[0006] The present invention aims to provide a method for improving the rare earth recovery rate in low-alloy high-strength steel, which adds a rare earth alloy of a certain grade in the late stage of LF refining. At this time, the O and S contents in the molten steel are low, which can effectively reduce the rare earth and O and S to form inclusions; using a rare earth ferrocerium alloy with a grade of 30%, the rare earth recovery rate is higher than that of 10% and 20% rare earth ferrocerium alloys; it can avoid flocculent flow at the water outlet of the middle package, and the production is stable and smooth. The specific technical scheme of the present invention is as follows.

[0007] The present invention provides a method for improving the rare earth recovery rate in low-alloy high-strength steel, wherein the production process of the low-alloy high-strength steel comprises a converter-LF refining-continuous casting, wherein the method comprises adding a 30% rare earth ferrocerium alloy for treatment in the late stage of LF refining, and performing calcium treatment after the LF furnace treatment.

[0008] The added amount of the 30% rare earth ferrocerium alloy is 20-40 kg calculated based on 260 tons of molten steel or 23-46 ppm in terms of rare earth Ce.

[0009] In some embodiments, in the converter process, the molten iron is subjected to KR desulfurization treatment, and the sulfur content of the molten iron entering the converter is ≤0.020%; the final tapping temperature of the converter is ≥1620°C, and ferroaluminum, ferrosilicon and ferromanganese are added during the converter tapping process for deoxidation and alloying.

[0010] In some embodiments, slag making, deoxidation, and desulfurization are performed in the early stage of the LF refining process, and alloys such as ferromanganese and ferrosilicon are added to adjust the composition of the molten steel.

[0011] In some embodiments, the calcium treatment is completed with a soft blowing time of ≥8 min.

[0012] In some embodiments, in the continuous casting process, the O content in the steel is ≤30 ppm, the S content is ≤100 ppm, the continuous casting superheat is 15-30° C., and the casting speed ranges from 1.0 to 1.6 m / min.

[0013] In some embodiments, the low alloy high strength steel includes Q355B.

[0014] In some embodiments, the chemical composition of the Q355B includes, by mass percentage, C: 0.17-0.19%, Si: 0.14-0.17%, Mn: 0.91-0.94%, P: 0.011-0.022% and S: 0.002-0.006%.

[0015] In some embodiments, the chemical composition of the Q355B is calculated by mass percentage as follows: C: 0.17-0.19%, Si: 0.14-0.17%, Mn: 0.91-0.94%, P: 0.011-0.022%, S: 0.002-0.006%, Ce: 11-20ppm, and the rest is Fe and unavoidable impurities.

[0016] In some embodiments, the rare earth yield of the method is ≥ 40%.

[0017] The beneficial effects of the present invention are as follows: the method provided by the present invention adopts a process of adding 30% CeFe rare earth alloys of different grades in the late refining stage, which can make the rare earth recovery rate in low-alloy high-strength steel ≥40%, effectively avoid the flocculence of the water outlet of the middle package, make the production stable and smooth, and can provide reference for the application research of rare earths in steel. DETAILED DESCRIPTION

[0018] The present invention is described in detail below through specific examples. The examples are intended to help understand the present invention but are not intended to limit the present invention.

[0019] In order to study the improvement effect of rare earth on inclusions in steel and low-magnification structure of ingot, the present invention selects low-alloy high-strength steel Q355B as an example, adds different levels of 30% rare earth CeFe alloy, and compares with the heat without rare earth. In the later stage of LF refining, the O and S content of molten steel is low, the rare earth recovery rate is high, the flocculence of molten steel in the middle package is avoided, the production is stable and smooth, and it provides a reference for further research on the application of rare earth in steel. The technical scheme of the present invention is as follows.

[0020] The low alloy high strength steel Q355B is used as a representative steel grade, and the production process is converter-LF refining-continuous casting. Four heats of steel were tested, the first heat did not add rare earth as a blank comparison heat, and the second to fourth heats added different levels of rare earth in the later stage of LF refining.

[0021] 1. Chemical composition

[0022] Table 1: Chemical composition of steel grades / %

[0023] Steel Type C Si Mn P S Q355B ≤0.24 ≤0.55 ≤1.60 ≤0.035 ≤0.035

[0024] 2. Process conditions

[0025] The molten iron is treated by KR desulfurization, and the sulfur content of the molten iron entering the converter is ≤0.020%. The final tapping temperature of the converter is ≥1620℃, and aluminum iron, ferrosilicon and ferromanganese are added during the tapping process of the converter for deoxidation and alloying. In the early stage of LF refining, slag making, deoxidation and desulfurization are carried out, and alloys such as ferromanganese and ferrosilicon are added to adjust the composition of the molten steel. In the later stage of LF refining, 30% rare earth cerium iron alloy is added. After the LF furnace treatment, calcium treatment is carried out, and the soft blowing time after calcium treatment is completed is ≥8min. The O content in the steel is ≤30ppm, the S content is ≤100ppm, the continuous casting superheat is 15~30℃, and the pulling speed range is 1.0~1.6m / min.

[0026] 3. Rare earth addition amount

[0027] The first furnace is a blank comparison furnace, and different levels of rare earth ferrocerium alloy are added to the second to fourth furnaces, and the added amounts are shown in Table 2 below.

[0028] Table 2: Rare earth alloy addition amount in test heats

[0029]

[0030] Note: The amount of molten steel is calculated based on 260 tons.

[0031] 4. Rare earth content detection

[0032] Take the middle steel sample from the heat with rare earth added, and use ICP-MS to detect the rare earth Ce content in the steel.

[0033] The specific operations of test heats 1-4 are as follows:

[0034] Test heat 1

[0035] The molten iron is treated by KR desulfurization, and the sulfur content of the molten iron entering the converter is 0.001%. The final tapping temperature of the converter is 1620℃. During the tapping process of the converter, aluminum iron, ferrosilicon and ferromanganese are added for deoxidation and alloying. Slag making, deoxidation and desulfurization are carried out in the early stage, and alloys such as ferromanganese and ferrosilicon are added to adjust the composition of the molten steel. Calcium treatment is carried out after the LF furnace treatment. The soft blowing time after calcium treatment is 8min. The steel composition is C: 0.17%, Si: 0.14%, Mn: 0.93%, P: 0.022%, S: 0.006%, continuous casting overheating is 22℃, and the pulling speed range is 1.35m / min.

[0036] Test heat 2

[0037] The molten iron is treated by KR desulfurization, and the sulfur content of the molten iron entering the converter is 0.0077%. The final tapping temperature of the converter is 1625℃. During the tapping process of the converter, aluminum iron, ferrosilicon and ferromanganese are added for deoxidation and alloying. In the early stage, slag making, deoxidation and desulfurization are carried out, and alloys such as ferromanganese and ferrosilicon are added to adjust the composition of the molten steel. In the later stage, 20kg of 30% rare earth cerium iron alloy is added. After the LF furnace treatment, calcium treatment is carried out. The soft blowing time after calcium treatment is 8min. The steel composition is C: 0.19%, Si: 0.17%, Mn: 0.94%, P: 0.015%, S: 0.004%, continuous casting superheat is 26.5℃, and the pulling speed range is 1.35m / min. The rare earth Ce content is 11ppm, and the rare earth recovery rate is 48%.

[0038] Test heat 3

[0039] The molten iron is treated by KR desulfurization, and the sulfur content of the molten iron entering the converter is 0.0049%. The final tapping temperature of the converter is 1622℃. During the tapping process of the converter, aluminum iron, ferrosilicon and ferromanganese are added for deoxidation and alloying. In the early stage, slag making, deoxidation and desulfurization are carried out, and ferromanganese, ferrosilicon and other alloys are added to adjust the composition of the molten steel. In the later stage, 30kg of 30% rare earth cerium iron alloy is added. After the LF furnace treatment, calcium treatment is carried out. The soft blowing time after calcium treatment is 8min. C: 0.18%, Si: 0.14%, Mn: 0.91%, P: 0.011%, S: 0.002%, continuous casting superheat 24℃, pulling speed range 1.34m / min. Rare earth Ce content is 18ppm, and rare earth recovery rate is 51%.

[0040] Test heat 4

[0041] The molten iron is treated by KR desulfurization, and the sulfur content of the molten iron entering the converter is 0.0010%. The final tapping temperature of the converter is 1623℃. During the tapping process of the converter, aluminum iron, ferrosilicon and ferromanganese are added for deoxidation and alloying. In the early stage, slag making, deoxidation and desulfurization are carried out, and ferromanganese, ferrosilicon and other alloys are added to adjust the composition of the molten steel. In the later stage, 40kg of 30% rare earth cerium iron alloy is added. After the LF furnace treatment, calcium treatment is carried out. The soft blowing time after calcium treatment is completed is 11 minutes. C: 0.18%, Si: 0.15%, Mn: 0.92%, P: 0.011%, S: 0.005%, continuous casting superheat 24.5℃, pulling speed range 1.36m / min. Rare earth Ce content 20ppm, rare earth recovery rate 43%.

[0042] Finally, it should be noted that the above is only a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art can still modify the technical solutions described in the aforementioned embodiments or replace some of the technical features therein by equivalents. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for improving the rare earth recovery rate in low alloy high strength steel, wherein the production process of the low alloy high strength steel comprises converter-LF refining-continuous casting, characterized in that: The method comprises adding 30% rare earth ferrocerium alloy for treatment in the late stage of LF refining, and performing calcium treatment after the LF furnace treatment. The added amount of the 30% rare earth ferrocerium alloy is 20-40 kg calculated based on 260 tons of molten steel or 23-46 ppm in terms of rare earth Ce.

2. The method according to claim 1, characterized in that In the converter process, the molten iron is treated with KR desulfurization, and the sulfur content of the molten iron entering the converter is ≤0.020%; the final tapping temperature of the converter is ≥1620°C, and aluminum iron, ferrosilicon and ferromanganese are added during the converter tapping process for deoxidation and alloying.

3. The method according to claim 1 or 2, characterized in that: In the early stage of the LF refining process, slag making, deoxidation and desulfurization are carried out, and alloys such as ferromanganese and ferrosilicon are added to adjust the composition of molten steel.

4. The method according to any one of claims 1 to 3, characterized in that The calcium treatment is completed and the soft blowing time is ≥8 minutes.

5. The method according to any one of claims 1 to 4, characterized in that In the continuous casting process, the O content in the steel is ≤30ppm, the S content is ≤100ppm, the continuous casting overheat is 15-30°C, and the casting speed ranges from 1.0 to 1.6 m / min.

6. The method according to any one of claims 1 to 5, characterized in that The low alloy high strength steel includes Q355B.

7. The method according to claim 6, characterized in that The chemical composition of the Q355B includes, by mass percentage, C: 0.17-0.19%, Si: 0.14-0.17%, Mn: 0.91-0.94%, P: 0.011-0.022% and S: 0.002-0.006%.

8. The method according to claim 7, characterized in that The chemical composition of the Q355B is calculated by mass percentage as follows: C: 0.17-0.19%, Si: 0.14-0.17%, Mn: 0.91-0.94%, P: 0.011-0.022%, S: 0.002-0.006%, Ce: 11-20ppm, and the rest is Fe and unavoidable impurities.

9. The method according to any one of claims 1 to 8, characterized in that The rare earth recovery rate of the method is ≥40%.

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